Collisions of droplets on spherical particles

NASA Astrophysics Data System (ADS)

Charalampous, Georgios; Hardalupas, Yannis

2017-10-01

Head-on collisions between droplets and spherical particles are examined for water droplets in the diameter range between 170 μm and 280 μm and spherical particles in the diameter range between 500 μm and 2000 μm. The droplet velocities range between 6 m/s and 11 m/s, while the spherical particles are fixed in space. The Weber and Ohnesorge numbers and ratio of droplet to particle diameter were between 92 < We < 1015, 0.0070 < Oh < 0.0089, and 0.09 < Ω < 0.55, respectively. The droplet-particle collisions are first quantified in terms of the outcome. In addition to the conventional deposition and splashing regimes, a regime is observed in the intermediate region, where the droplet forms a stable crown, which does not breakup but propagates along the particle surface and passes around the particle. This regime is prevalent when the droplets collide on small particles. The characteristics of the collision at the onset of rim instability are also described in terms of the location of the film on the particle surface and the orientation and length of the ejected crown. Proper orthogonal decomposition identified that the first 2 modes are enough to capture the overall morphology of the crown at the splashing threshold.

Planetary science

NASA Technical Reports Server (NTRS)

Marshall, John R.; Bridges, Frank; Gault, Donald; Greeley, Ronald; Houpis, Harry; Lin, Douglas; Weidenschilling, Stuart

1987-01-01

The following types of experiments for a proposed Space Station Microgravity Particle Research Facility are described: (1) low velocity collisions between fragile particles; (2) low velocity collisions of ice particles; (3) plasma-dust interaction; and (4) aggregation of finely-comminuted geological materials. The required capabilities and desired hardware for the facility are detailed.

Microgravity collisions of dust aggregates as an analogue to early planetesimal formation

NASA Astrophysics Data System (ADS)

Whizin, Akbar; Blum, Jürgen; Colwell, Joshua

2014-11-01

During the early stages of planet formation the dusty progenitors of planetesimals collided with each other continuously to form the seeds of planets. These collisions could result in growth or disruption depending on the individual impact velocities. Based on input from solar nebula models a laboratory-based microgravity dust collision experiment was developed for a drop tower at the Technische Universität Braunschweig, Germany. We collided 1.0 - 1.6 mm SiO2 dust aggregates with clusters of these aggregates at a range of velocities and mass ratios to determine the thresholds between bouncing, sticking, and fragmentation. Presented here are the results of 264 microgravity collisions occurring at velocities of 1 - 160 cm/s with target-impactor mass ratios of 5:1 to 400:1. We also present the coefficient of restitutions for low-velocity collisions and we find the specific collision energy of fragmentation Q* for aggregates of this size. We find sticking occurs at mass ratios larger than 40:1, but only for low velocities ≤ 3 cm/s, clear boundaries exist for bouncing up to 30 cm/s, and fragmentation at ~50 cm/s and up, with total disruption occurring above 1 m/s.

The dynamics of milk droplet-droplet collisions

NASA Astrophysics Data System (ADS)

Finotello, Giulia; Kooiman, Roeland F.; Padding, Johan T.; Buist, Kay A.; Jongsma, Alfred; Innings, Fredrik; Kuipers, J. A. M.

2018-01-01

Spray drying is an important industrial process to produce powdered milk, in which concentrated milk is atomized into small droplets and dried with hot gas. The characteristics of the produced milk powder are largely affected by agglomeration, combination of dry and partially dry particles, which in turn depends on the outcome of a collision between droplets. The high total solids (TS) content and the presence of milk proteins cause a relatively high viscosity of the fed milk concentrates, which is expected to largely influence the collision outcomes of drops inside the spray. It is therefore of paramount importance to predict and control the outcomes of binary droplet collisions. Only a few studies report on droplet collisions of high viscous liquids and no work is available on droplet collisions of milk concentrates. The current study therefore aims to obtain insight into the effect of viscosity on the outcome of binary collisions between droplets of milk concentrates. To cover a wide range of viscosity values, three milk concentrates (20, 30 and 46% TS content) are investigated. An experimental set-up is used to generate two colliding droplet streams with consistent droplet size and spacing. A high-speed camera is used to record the trajectories of the droplets. The recordings are processed by Droplet Image Analysis in MATLAB to determine the relative velocities and the impact geometries for each individual collision. The collision outcomes are presented in a regime map dependent on the dimensionless impact parameter and Weber ( We) number. The Ohnesorge ( Oh) number is introduced to describe the effect of viscosity from one liquid to another and is maintained constant for each regime map by using a constant droplet diameter ( d ˜ 700 μ m). In this work, a phenomenological model is proposed to describe the boundaries demarcating the coalescence-separation regimes. The collision dynamics and outcome of milk concentrates are compared with aqueous glycerol solutions experiments. While milk concentrates have complex chemical composition and rheology, glycerol solutions are Newtonian fluids and therefore easy to characterize. The collision morphologies of glycerol solutions and milk concentrates are similar, and the regime maps can be described by the same phenomenological model developed in this work. The regime of bouncing, however, was not observed for any of the milk concentrates.

Driven fragmentation of granular gases.

PubMed

Cruz Hidalgo, Raúl; Pagonabarraga, Ignacio

2008-06-01

The dynamics of homogeneously heated granular gases which fragment due to particle collisions is analyzed. We introduce a kinetic model which accounts for correlations induced at the grain collisions and analyze both the kinetics and relevant distribution functions these systems develop. The work combines analytical and numerical studies based on direct simulation Monte Carlo calculations. A broad family of fragmentation probabilities is considered, and its implications for the system kinetics are discussed. We show that generically these driven materials evolve asymptotically into a dynamical scaling regime. If the fragmentation probability tends to a constant, the grain number diverges at a finite time, leading to a shattering singularity. If the fragmentation probability vanishes, then the number of grains grows monotonously as a power law. We consider different homogeneous thermostats and show that the kinetics of these systems depends weakly on both the grain inelasticity and driving. We observe that fragmentation plays a relevant role in the shape of the velocity distribution of the particles. When the fragmentation is driven by local stochastic events, the long velocity tail is essentially exponential independently of the heating frequency and the breaking rule. However, for a Lowe-Andersen thermostat, numerical evidence strongly supports the conjecture that the scaled velocity distribution follows a generalized exponential behavior f(c) approximately exp(-cn) , with n approximately 1.2 , regarding less the fragmentation mechanisms.

NanoRocks: A Long-Term Microgravity Experiment to Stydy Planet Formation and Planetary Ring Particles

NASA Astrophysics Data System (ADS)

Brisset, J.; Colwell, J. E.; Dove, A.; Maukonen, D.; Brown, N.; Lai, K.; Hoover, B.

2015-12-01

We report on the results of the NanoRocks experiment on the International Space Station (ISS), which simulates collisions that occur in protoplanetary disks and planetary ring systems. A critical stage of the process of early planet formation is the growth of solid bodies from mm-sized chondrules and aggregates to km-sized planetesimals. To characterize the collision behavior of dust in protoplanetary conditions, experimental data is required, working hand in hand with models and numerical simulations. In addition, the collisional evolution of planetary rings takes place in the same collisional regime. The objective of the NanoRocks experiment is to study low-energy collisions of mm-sized particles of different shapes and materials. An aluminum tray (~8x8x2cm) divided into eight sample cells holding different types of particles gets shaken every 60 s providing particles with initial velocities of a few cm/s. In September 2014, NanoRocks reached ISS and 220 video files, each covering one shaking cycle, have already been downloaded from Station. The data analysis is focused on the dynamical evolution of the multi-particle systems and on the formation of cluster. We track the particles down to mean relative velocities less than 1 mm/s where we observe cluster formation. The mean velocity evolution after each shaking event allows for a determination of the mean coefficient of restitution for each particle set. These values can be used as input into protoplanetary disk and planetary rings simulations. In addition, the cluster analysis allows for a determination of the mean final cluster size and the average particle velocity of clustering onset. The size and shape of these particle clumps is crucial to understand the first stages of planet formation inside protoplanetary disks as well as many a feature of Saturn's rings. We report on the results from the ensemble of these collision experiments and discuss applications to planetesimal formation and planetary ring evolution.

Collisional effects on the numerical recurrence in Vlasov-Poisson simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pezzi, Oreste; Valentini, Francesco; Camporeale, Enrico

The initial state recurrence in numerical simulations of the Vlasov-Poisson system is a well-known phenomenon. Here, we study the effect on recurrence of artificial collisions modeled through the Lenard-Bernstein operator [A. Lenard and I. B. Bernstein, Phys. Rev. 112, 1456–1459 (1958)]. By decomposing the linear Vlasov-Poisson system in the Fourier-Hermite space, the recurrence problem is investigated in the linear regime of the damping of a Langmuir wave and of the onset of the bump-on-tail instability. The analysis is then confirmed and extended to the nonlinear regime through an Eulerian collisional Vlasov-Poisson code. It is found that, despite being routinely used,more » an artificial collisionality is not a viable way of preventing recurrence in numerical simulations without compromising the kinetic nature of the solution. Moreover, it is shown how numerical effects associated to the generation of fine velocity scales can modify the physical features of the system evolution even in nonlinear regime. This means that filamentation-like phenomena, usually associated with low amplitude fluctuations contexts, can play a role even in nonlinear regime.« less

Low-frequency instabilities and plasma turbulence

NASA Technical Reports Server (NTRS)

Ilic, D. B.

1973-01-01

A theoretical and experimental study is reported of steady-state and time-dependent characteristics of the positive column and the hollow cathode discharge (HCD). The steady state of a non-isothermal, cylindrical positive column in an axial magnetic field is described by three moment equations in the plasma approximation. Volume generation of electron-ion pairs by single-stage ionization, the presence of axial current, and collisions with neutrals are considered. The theory covers the range from the low pressure, collisionless regime to the intermediate pressure, collisional regime. It yields radial profiles of the charged particle velocities, density, potential, electron and ion temperatures, and demonstrates similarity laws for the positive column. The results are compared with two moment theories and with experimental data on He, Ar and Hg found in the literature for a wide range of pressures. A simple generalization of the isothermal theory for an infinitely long cylinder in an axial magnetic field to the case of a finite column with axial current flow is also demonstrated.

Aftermath of early Hit-and-Run collisions in the Inner Solar System

NASA Astrophysics Data System (ADS)

Sarid, Gal; Stewart, Sarah T.; Leinhardt, zoe M.

2015-08-01

Planet formation epoch, in the terrestrial planet region and the asteroid belt, was characterized by a vigorous dynamical environment that was conducive to giant impacts among planetary embryos and asteroidal parent bodies, leading to diverse outcomes. Among these the greatest potential for producing diverse end-members lies is the erosive Hit-and-Run regime (small mass ratios, off-axis oblique impacts and non-negligible ejected mass), which is also more probable in terms of the early dynamical encounter configuration in the inner solar system. This collision regime has been invoked to explain outstanding issues, such as planetary volatile loss records, origin of the Moon and mantle stripping from Mercury and some of the larger asteroids (Vesta, Psyche).We performed and analyzed a set of simulations of Hit-and-Run events, covering a large range of mass ratios (1-20), impact parameters (0.25-0.96, for near head-on to barely grazing) and impact velocities (~1.5-5 times the mutual escape velocity, as dependent on the mass ratio). We used an SPH code with tabulated EOS and a nominal simlated time >1 day, to track the collisional shock processing and the provenance of material components. of collision debris. Prior to impact runs, all bodies were allowed to initially settle to negligible particle velocities in isolation, within ~20 simulated hrs. The total number of particles involved in each of our collision simulations was between (1-3 x 105). Resulting configurations include stripped mantles, melting/vaporization of rock and/or iron cores and strong variations of asteroid parent bodies fromcanonical chondritic composition.In the context of large planetary formation simulations, velocity and impact angle distributions are necessary to asses impact probabilities. The mass distribution and interaction within planetary embryo and asteroid swarms depends both on gravitational dynamics and the applied fragmentation mechanism. We will present results pertaining to general projectile remnant scaling relations, constitution of ejected unbound material and the composition of variedcollision remnants, which become available to seed the asteroid belt.

A theory of local and global processes which affect solar wind electrons. I - The origin of typical 1 AU velocity distribution functions - Steady state theory

NASA Technical Reports Server (NTRS)

Scudder, J. D.; Olbert, S.

1979-01-01

A kinetic theory for the velocity distribution of solar wind electrons which illustrates the global and local properties of the solar wind expansion is proposed. By means of the Boltzmann equation with the Krook collision operator accounting for Coulomb collisions, it is found that Coulomb collisions determine the population and shape of the electron distribution function in both the thermal and suprathermal energy regimes. For suprathermal electrons, the cumulative effects of Coulomb interactions are shown to take place on the scale of the heliosphere itself, whereas the Coulomb interactions of thermal electrons occur on a local scale near the point of observation (1 AU). The bifurcation of the electron distribution between thermal and suprathermal electrons is localized to the deep solar corona (1 to 10 solar radii).

Experimental observations of low-velocity collisional systems

NASA Astrophysics Data System (ADS)

Jorges, Jeffery; Dove, Adrienne; Colwell, Joshua

Low-velocity collisions in systems of centimeter-sized objects may result in particle growth by accretion, rebounding, or erosive processes that result in the production of additional smaller particles. Numerical simulations of these systems are limited by a need to understand the collisional parameters governing the outcomes of these collisions over a range of conditions. Here, we present the results from laboratory experiments designed to explore low-velocity collisions by conducting experiments in a vacuum chamber in our 0.8-sec drop tower apparatus. These experiments utilize a variety of impacting spheres, including glass, Teflon, aluminum, stainless steel, and brass. These spheres are either used in their natural state or are ``mantled'' - coated with a few-mm thick layer of a cohesive powder. A high-speed, high-resolution video camera is used to record the motion of the colliding bodies. These videos are then processed and we track the particles to determine impactor speeds before and after collision and the collisional outcome. We determine how the coefficient of restitution varies as a function of material type, morphology, and impact velocity. For impact velocities in the range from about 20-100 cm/s we observe that mantling of particles has the most significant effect, reducing the coefficients of restitution.

MD Simulation on Collision Behavior Between Nano-Scale TiO₂ Particles During Vacuum Cold Spraying.

PubMed

Yao, Hai-Long; Yang, Guan-Jun; Li, Chang-Jiu

2018-04-01

Particle collision behavior influences significantly inter-nano particle bonding formation during the nano-ceramic coating deposition by vacuum cold spraying (or aerosol deposition method). In order to illuminate the collision behavior between nano-scale ceramic particles, molecular dynamic simulation was applied to explore impact process between nano-scale TiO2 particles through controlling impact velocities. Results show that the recoil efficiency of the nano-scale TiO2 particle is decreased with the increase of the impact velocity. Nano-scale TiO2 particle exhibits localized plastic deformation during collision at low velocities, while it is intensively deformed by collision at high velocities. This intensive deformation promotes the nano-particle adhesion rather than rebounding off. A relationship between the adhesion energy and the rebound energy is established for the bonding formation of the nano-scale TiO2 particle. The adhesion energy required to the bonding formation between nano-scale ceramic particles can be produced by high velocity collision.

Transition from normal to ballistic diffusion in a one-dimensional impact system

NASA Astrophysics Data System (ADS)

Livorati, André L. P.; Kroetz, Tiago; Dettmann, Carl P.; Caldas, Iberê L.; Leonel, Edson D.

2018-03-01

We characterize a transition from normal to ballistic diffusion in a bouncing ball dynamics. The system is composed of a particle, or an ensemble of noninteracting particles, experiencing elastic collisions with a heavy and periodically moving wall under the influence of a constant gravitational field. The dynamics lead to a mixed phase space where chaotic orbits have a free path to move along the velocity axis, presenting a normal diffusion behavior. Depending on the control parameter, one can observe the presence of featured resonances, known as accelerator modes, that lead to a ballistic growth of velocity. Through statistical and numerical analysis of the velocity of the particle, we are able to characterize a transition between the two regimes, where transport properties were used to characterize the scenario of the ballistic regime. Also, in an analysis of the probability of an orbit to reach an accelerator mode as a function of the velocity, we observe a competition between the normal and ballistic transport in the midrange velocity.

Transition from normal to ballistic diffusion in a one-dimensional impact system.

PubMed

Livorati, André L P; Kroetz, Tiago; Dettmann, Carl P; Caldas, Iberê L; Leonel, Edson D

2018-03-01

We characterize a transition from normal to ballistic diffusion in a bouncing ball dynamics. The system is composed of a particle, or an ensemble of noninteracting particles, experiencing elastic collisions with a heavy and periodically moving wall under the influence of a constant gravitational field. The dynamics lead to a mixed phase space where chaotic orbits have a free path to move along the velocity axis, presenting a normal diffusion behavior. Depending on the control parameter, one can observe the presence of featured resonances, known as accelerator modes, that lead to a ballistic growth of velocity. Through statistical and numerical analysis of the velocity of the particle, we are able to characterize a transition between the two regimes, where transport properties were used to characterize the scenario of the ballistic regime. Also, in an analysis of the probability of an orbit to reach an accelerator mode as a function of the velocity, we observe a competition between the normal and ballistic transport in the midrange velocity.

The importance of momentum transfer in collision-induced breakups in low Earth orbit

NASA Technical Reports Server (NTRS)

Reynolds, Robert C.; Lillie, Brian J.

1991-01-01

Although there is adequate information on larger objects in low Earth orbit, specifically those objects larger than about 10 cm in diameter, there is little direct information on objects from this size down to 1 mm. Yet, this is the sized regime where objects acting as projectiles represent the ability to seriously damage or destroy a functioning spacecraft if they collide with it. The observed consequences of known collisional breakups in orbit indicates no significant momentum transfer in the resulting debris cloud. The position taken in this paper is that this is an observational selection effect: what is seen in these events is an explosion-like breakup of the target structure arising from shock waves introduced into the structure by the collision, but one that occurs significantly after the collision processes are completed; the collision cloud, in which there is momentum transfer, consists of small, unobserved fragments. Preliminary computations of the contribution of one known collisional breakup, Solwind at 500 km in 1985, and Cosmos 1275 in 1981, assume no momentum transfer on breakup and indicate that these two events are the dominant contributors to the current millimeter and centimeter population. A different story would emerge if momentum transfer was taken into account. The topics covered include: (1) observation of on-orbit collisional breakups; (2) a model for momentum transfer; and (3) velocity space representation of breakup clouds.

Collision of Dual Aggregates (CODA): Experimental observations of low-velocity collisions

NASA Astrophysics Data System (ADS)

Jorges, Jeffery; Dove, Adrienne; Colwell, Josh E.

2016-10-01

Low-velocity collisions are one of the driving factors that determine the particle size distribution and particle size evolution in planetary ring systems and in the early stages of planet formation. Collisions of sub-micron to decimeter-sized objects may result in particle growth by accretion, rebounding, or erosive processes that result in the production of additional smaller particles. Numerical simulations of these systems are limited by a need to understand these collisional parameters over a range of conditions. We present the results of a sequence of laboratory experiments designed to explore collisions over a range of parameter space . We are able to observe low-velocity collisions by conducting experiments in vacuum chambers in our 0.8-sec drop tower apparatus. Initial experiments utilize a variety of impacting spheres, including glass, Teflon, aluminum, stainless steel, and brass. These spheres are either used in their natural state or are "mantled" - coated with a few-mm thick layer of a cohesive powder. A high-speed, high-resolution video camera is used to record the motion of the colliding bodies. We track the particles to determine impactor speeds before and after collision, the impact parameter, and the collisional outcome. In the case of the mantled impactors, we can assess how much rotation is generated by the collision and estimate how much powder is released (i.e. how much mass is lost) due to the collision. We also determine how the coefficient of restitution varies as a function of material type, morphology, and impact velocity. With impact velocities ranging from about 20-100 cm/s we observe that mantling of particles significantly reduces their coefficients of restitution, but we see basically no dependence of the coefficient of restitution on the impact velocity, impact parameter, or system mass. The results of this study will contribute to a better empirical model of collisional outcomes that will be refined with numerical simulation of the experiment to improve our understanding of the collisional evolution of ring systems and early planet formation.

A fast iterative scheme for the linearized Boltzmann equation

NASA Astrophysics Data System (ADS)

Wu, Lei; Zhang, Jun; Liu, Haihu; Zhang, Yonghao; Reese, Jason M.

2017-06-01

Iterative schemes to find steady-state solutions to the Boltzmann equation are efficient for highly rarefied gas flows, but can be very slow to converge in the near-continuum flow regime. In this paper, a synthetic iterative scheme is developed to speed up the solution of the linearized Boltzmann equation by penalizing the collision operator L into the form L = (L + Nδh) - Nδh, where δ is the gas rarefaction parameter, h is the velocity distribution function, and N is a tuning parameter controlling the convergence rate. The velocity distribution function is first solved by the conventional iterative scheme, then it is corrected such that the macroscopic flow velocity is governed by a diffusion-type equation that is asymptotic-preserving into the Navier-Stokes limit. The efficiency of this new scheme is assessed by calculating the eigenvalue of the iteration, as well as solving for Poiseuille and thermal transpiration flows. We find that the fastest convergence of our synthetic scheme for the linearized Boltzmann equation is achieved when Nδ is close to the average collision frequency. The synthetic iterative scheme is significantly faster than the conventional iterative scheme in both the transition and the near-continuum gas flow regimes. Moreover, due to its asymptotic-preserving properties, the synthetic iterative scheme does not need high spatial resolution in the near-continuum flow regime, which makes it even faster than the conventional iterative scheme. Using this synthetic scheme, with the fast spectral approximation of the linearized Boltzmann collision operator, Poiseuille and thermal transpiration flows between two parallel plates, through channels of circular/rectangular cross sections and various porous media are calculated over the whole range of gas rarefaction. Finally, the flow of a Ne-Ar gas mixture is solved based on the linearized Boltzmann equation with the Lennard-Jones intermolecular potential for the first time, and the difference between these results and those using the hard-sphere potential is discussed.

Water radiolysis by low-energy carbon projectiles from first-principles molecular dynamics

PubMed Central

Kohanoff, Jorge

2017-01-01

Water radiolysis by low-energy carbon projectiles is studied by first-principles molecular dynamics. Carbon projectiles of kinetic energies between 175 eV and 2.8 keV are shot across liquid water. Apart from translational, rotational and vibrational excitation, they produce water dissociation. The most abundant products are H and OH fragments. We find that the maximum spatial production of radiolysis products, not only occurs at low velocities, but also well below the maximum of energy deposition, reaching one H every 5 Å at the lowest speed studied (1 Bohr/fs), dissociative collisions being more significant at low velocity while the amount of energy required to dissociate water is constant and much smaller than the projectile’s energy. A substantial fraction of the energy transferred to fragments, especially for high velocity projectiles, is in the form of kinetic energy, such fragments becoming secondary projectiles themselves. High velocity projectiles give rise to well-defined binary collisions, which should be amenable to binary approximations. This is not the case for lower velocities, where multiple collision events are observed. H secondary projectiles tend to move as radicals at high velocity, as cations when slower. We observe the generation of new species such as hydrogen peroxide and formic acid. The former occurs when an O radical created in the collision process attacks a water molecule at the O site. The latter when the C projectile is completely stopped and reacts with two water molecules. PMID:28267804

Effects of perturbations on space debris in supersynchronous storage orbits

NASA Astrophysics Data System (ADS)

Luu, Khanh Kim

1998-12-01

Accumulation of space debris in the geosynchronous region (GEO) has raised attention among spacefaring nations. The current mitigation measure supported is to boost satellites into supersynchronous orbits in the time before station-keeping fuel is expected to be exhausted. Because this solution does not remove mass from space, debris generation by fragmentation events remains a possibility. The collision hazard between inactive satellites in the supersynchronous region raises questions about the consequences of collisions in this regime and possible interaction with GEO. In considering the use of supersynchronous orbits for satellite disposal, the first concern is to determine the minimum safe distance above GEO such that objects in the disposal orbits will not interfere with the GEO population in the future. This involves defining the useful GEO area and studying the perturbation effects on objects in supersynchronous orbits. Thus far, research has focused on propagating the orbits of intact objects. However, in the aftermath of a collision, pieces of varying sizes and shapes can be found in orbits quite different from the parent objects' orbits. This document summarizes background information on debris in the GEO region, sources and management strategies, and then addresses the problem: Will orbits of fragments from a collision in a storage orbit occupy GEO altitudes at some time after the collision? If so, at what altitude should the storage orbit occupy such that collision fragments will not interfere with the GEO population? The methods and tools by which the effects of collisions in the supersynchronous region can be analyzed are discussed. A low-velocity collision model is employed to provide delta-velocities imparted to the fragments. An analytical study of perturbation effects, including solar and lunar third body gravitation, Earth oblateness through degree and order four, and solar radiation pressure, follows in order to evaluate the magnitude of these disturbing forces on the fragmentation debris. Validation of these results by numerical analysis using proven numerical and semianalytical orbit propagators is discussed. The results show that currently practiced reorbiting distances above GEO do not isolate debris from GEO after the occurrence of collisions in the storage orbit.

Surface Collisions Involving Particles and Moisture (SCIP'M)

NASA Technical Reports Server (NTRS)

Davis, Robert H.

2005-01-01

Experiments were performed on the collision of a solid sphere with a nearly horizontal flat surface covered with a thin layer of viscous liquid. High-speed collisions were obtained by dropping the ball onto the surface from various heights, using gravitational acceleration. Low-speed collisions were obtained using pendulums with long strings or by launching the balls at low velocities in the reduced-gravity environment of parabolic flight. The sphere bounces only when the impact velocity exceeds a critical value. The coefficient of restitution (ratio of rebound velocity to impact velocity) increases with increasing impact velocity above the critical value, indicating the increasing relative importance of elastic deformation to viscous dissipation. The critical impact velocity increases, and the coefficient of restitution decreases, with increasing viscosity or thickness of the liquid layer and with decreasing density or size of the sphere. The ratio of the wet and dry coefficients is expressed as a function of the Stokes number (ratio of particle inertia and viscous forces), showing good agreement between theory and experiment. Similar experiments were performed with the flat surface inclined at various angles to the approaching sphere. A modified Stokes number, which is a measure of the ratio of inertia of the sphere in the normal direction to the viscous forces exerted by the fluid layer, was used for the analysis of oblique collisions. Even for these oblique collisions, it was found that no rebound of the ball was observed below a certain critical Stokes number. The coefficient of normal restitution, defined as a ratio of normal rebound velocity to normal approach velocity, was found to increase beyond the critical Stokes number and even out as it approaches the value for dry restitution at high Stokes numbers. It was also found that, for smooth spheres like steel, the normal restitution at the same modified Stokes number is independent of the angle of impact. The tangential coefficient of restitution, defined as the ratio of tangential rebound velocity to tangential approach velocity, is found to be nearly unity, except for very low approach velocities. Thus, as a first approximation, the theories that predict the coefficient of restitution for head-on wet collisions can be extended to predict the coefficient of normal restitution for oblique wet collisions. Additional experiments were performed with soft surfaces in which a porous cloth or sponge layer was placed over the hard, flat surface. In these experiments, the coefficient of restitution was found to decrease with increasing impact velocity, due to inelastic losses in the soft material. A model combining inelastic deformation and flow through porous media was developed to describe these findings.

Nonequilibrium fluctuations in a frictional granular motor: Experiments and kinetic theory

NASA Astrophysics Data System (ADS)

Gnoli, Andrea; Sarracino, Alessandro; Puglisi, Andrea; Petri, Alberto

2013-05-01

We report the study of an experimental granular Brownian motor, inspired by the one published in Eshuis [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.104.248001 104, 248001 (2010)], but different in some ingredients. As in that previous work, the motor is constituted by a rotating blade, the surfaces of which break the rotation-inversion symmetry through alternated patches of different inelasticity, immersed in a gas of granular particles. The main difference of our experimental setup is in the orientation of the main axis, which is parallel to the (vertical) direction of shaking of the granular fluid, guaranteeing an isotropic distribution for the velocities of colliding grains, characterized by a variance v02. We also keep the granular system diluted, in order to compare with Boltzmann-equation-based kinetic theory. In agreement with theory, we observe the crucial role of Coulomb friction which induces two main regimes: (i) rare collisions, with an average angular velocity <ω>˜v03, and (ii) frequent collisions (FC), with <ω>˜v0. We also study the fluctuations of the angle spanned in a large-time interval Δθ, which in the FC regime is proportional to the work done upon the motor. We observe that the fluctuation relation is satisfied with a slope which weakly depends on the relative collision frequency.

Dependence of enhanced asymmetry-induced transport on collision frequency

NASA Astrophysics Data System (ADS)

Eggleston, D. L.

2014-07-01

A single-particle code with collisional effects is used to study how asymmetry-induced radial transport in a non-neutral plasma depends on collision frequency. For asymmetries of the form ϕ1(r) cos(kz) cos(ωt-lθ), two sources for the transport have been identified: resonant particles and axially trapped particles. The simulation shows that this latter type, which occurs near the radius where ω matches the azimuthal rotation frequency ωR, is usually dominant at low collision frequency ν but becomes negligible at higher ν. This behavior can be understood by noting that axially trapped particles have a lower trapping frequency than resonant particles. In the low ν (banana) regime, the radial oscillations have amplitude Δr ≈ vr/ωT, so axially trapped particles dominate, and the transport may even exceed the resonant particle plateau regime level. As ν increases, collisions start to interrupt the slower axially trapped particle oscillations, while the resonant particles are still in the banana regime, so the axially trapped particle contribution to the transport decreases. At the largest ν values, axially trapped particle transport is negligible and the observed diffusion coefficient matches that given by plateau regime resonant particle theory. Heuristic models based on these considerations give reasonable agreement with the observed scaling laws for the value of the collision frequency where axially trapped particle transport starts to decrease and for the enhancement of the diffusion coefficient produced by axially trapped particles.

Gas-phase hydrogen atom abstraction reactions of S- with H2, CH4, and C2H6

NASA Astrophysics Data System (ADS)

Angel, Laurence A.; Dogbevia, Moses K.; Rempala, Katarzyna M.; Ervin, Kent M.

2003-11-01

Reaction cross sections, product axial velocity distributions, and potential energy surfaces are presented for the hydrogen atom abstraction reactions S-+RH→R+HS- (R=H, CH3, C2H5) as a function of collision energy. The observed threshold energy, E0, for S-+H2→H+HS- agrees with the reaction endothermicity, ΔrH0. At low collision energies, the H+HS- products exhibit symmetric, low-recoil-velocity scattering, consistent with statistical reaction behavior. The S-+CH4→CH3+HS- and S-+C2H6→C2H5+HS reactions, in contrast, show large excess threshold energies when compared to ΔrH0. The excess energies are partly explained by a potential energy barrier separating products from reactants. However, additional dynamical constraints must account for more than half of the excess threshold energy. The observed behavior seems to be general for collisional activation of anion-molecule reactions that proceed through a tight, late transition state. For RH=CH4 and C2H6, the HS- velocity distributions show anisotropic backward scattering at low collision energies indicating small impact parameters and a direct rebound reaction mechanism. At higher collision energies, there is a transition to HS- forward scattering and high velocities consistent with grazing collisions and a stripping mechanism.

Toward a Physical Characterization of Raindrop Collision Outcome Regimes

NASA Technical Reports Server (NTRS)

Testik, F. Y.; Barros, Ana P.; Bilven, Francis L.

2011-01-01

A comprehensive raindrop collision outcome regime diagram that delineates the physical conditions associated with the outcome regimes (i.e., bounce, coalescence, and different breakup types) of binary raindrop collisions is proposed. The proposed diagram builds on a theoretical regime diagram defined in the phase space of collision Weber numbers We and the drop diameter ratio p by including critical angle of impact considerations. In this study, the theoretical regime diagram is first evaluated against a comprehensive dataset for drop collision experiments representative of raindrop collisions in nature. Subsequently, the theoretical regime diagram is modified to explicitly describe the dominant regimes of raindrop interactions in (We, p) by delineating the physical conditions necessary for the occurrence of distinct types of collision-induced breakup (neck/filament, sheet, disk, and crown breakups) based on critical angle of impact consideration. Crown breakup is a subtype of disk breakup for lower collision kinetic energy that presents distinctive morphology. Finally, the experimental results are analyzed in the context of the comprehensive collision regime diagram, and conditional probabilities that can be used in the parameterization of breakup kernels in stochastic models of raindrop dynamics are provided.

Collisional disruption of gravitational aggregates in the tidal environment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hyodo, Ryuki; Ohtsuki, Keiji

2014-05-20

The degree of disruption in collisions in free space is determined by specific impact energy, and the mass fraction of the largest remnant is a monotonically decreasing function of impact energy. However, it has not been shown whether such a relationship is applicable to collisions under the influence of a planet's tidal force, which is important in ring dynamics and satellite accretion. Here we examine the collisional disruption of gravitational aggregates in the tidal environment by using local N-body simulations. We find that outcomes of such a collision largely depend on the impact velocity, the direction of impact, and themore » radial distance from the planet. In the case of a strong tidal field corresponding to Saturn's F ring, collisions in the azimuthal direction are much more destructive than those in the radial direction. Numerical results of collisions sensitively depend on the impact velocity, and a complete disruption of aggregates can occur even in impacts with velocity much lower than their escape velocity. In such low-velocity collisions, the deformation of colliding aggregates plays an essential role in determining collision outcomes, because the physical size of the aggregate is comparable to its Hill radius. On the other hand, the dependence of collision outcomes on impact velocity becomes similar to the case in free space when the distance from the planet is sufficiently large. Our results are consistent with Cassini observations of the F ring, which suggest ongoing creation and disruption of aggregates within the ring.« less

From pebbles to dust: experiments to observe low-velocity collisional outcomes

NASA Astrophysics Data System (ADS)

Dove, A.; Jorges, J.; Colwell, J. E.

2015-12-01

Particle size evolution in planetary ring systems can be driven by collisions at relatively low velocities (<1 m/s) occurring between objects with a range of sizes from very fine dust to decimeter-sized objects. In these complex systems, collisions between centimeter-sized objects may result in particle growth by accretion, rebounding, or erosive processes that result in the production of additional smaller particles. The outcomes of these collisions are dependent on factors such as collisional energy, particle size, and particle morphology. Numerical simulations are limited by a need to understand these collisional parameters over a range of conditions. We present the results of a sequence of laboratory experiments designed to explore collisions over a range of these parameters. We are able to observe low-velocity collisions by conducting experiments in vacuum chambers in our 0.8-sec drop tower apparatus. Initial experiments utilize a variety of impacting spheres, including glass, Teflon, aluminum, stainless steel, and brass. These spheres are either used in their natural state or are "mantled" - coated with a few-mm thick layer of a cohesive powder. A high-speed, high-resolution video camera is used to record the motion of the colliding bodies. These videos are then processed and we track the particles to determine impactor speeds before and after collision and the collisional outcome; in the case of the mantled impactors, we can assess how much of the powder was released in the collision. We also determine how the coefficient of restitution varies as a function of material type, morphology, and impact velocity. Impact velocities range from about 20-60 cm/s, and we observe that mantling of particles significantly reduces their coefficients of restitution. These results will contribute to an empirical model of collisional outcomes that can help refine our understanding of dusty ring system collisional evolution.

Molecular three-body Brauner-Briggs-Klar theory for ion-impact ionization of molecules

NASA Astrophysics Data System (ADS)

Ghanbari-Adivi, E.

2016-12-01

Molecular three-body Brauner-Briggs-Klar (M3BBK) theory is developed to study the single ionization of diatomic molecules by ion impact. The orientation-averaged molecular orbital (OAMO) approximation is used to reduce the required computer time without sacrificing the performance of the method. The post-collision interaction (PCI) between the scattered projectile and the ejected electron is included. The theory is applied to collision of protons with hydrogen molecules. Results are obtained for two different kinematical regimes: i) fast collisions and low emission energies, and ii) not so fast collisions and higher emission energies. For both considered regimes, experimental fully differential cross-sections as well as different theoretical calculations are available for comparison. These comparisons are carried out and discussed.

Friction properties and deformation mechanisms of halite(-mica) gouges from low to high sliding velocities

NASA Astrophysics Data System (ADS)

Buijze, Loes; Niemeijer, André R.; Han, Raehee; Shimamoto, Toshihiko; Spiers, Christopher J.

2017-01-01

The evolution of friction as a function of slip rate is important in understanding earthquake nucleation and propagation. Many laboratory experiments investigating friction of fault rocks are either conducted in the low velocity regime (10-8-10-4 ms-1) or in the high velocity regime (0.01-1 m s-1). Here, we report on the evolution of friction and corresponding operating deformation mechanisms in analog gouges deformed from low to high slip rates, bridging the gap between these low and high velocity regimes. We used halite and halite-muscovite gouges to simulate processes, governing friction, active in upper crustal quartzitic fault rocks, at conditions accessible in the laboratory. The gouges were deformed over a 7 orders of magnitude range of slip rate (10-7-1 m s-1) using a low-to-high velocity rotary shear apparatus, using a normal stress of 5 MPa and room-dry humidity. Microstructural analysis was conducted to study the deformation mechanisms. Four frictional regimes as a function of slip rate could be recognized from the mechanical data, showing a transitional regime and stable sliding (10-7-10-6 m s-1), unstable sliding and weakening (10-6-10-3 m s-1), hardening (10-2-10-1 m s-1) and strong weakening (10-1-1 m s-1). Each of the four regimes can be associated with a distinct microstructure, reflecting a transition from mainly brittle deformation accompanied by pressure solution healing to temperature activated deformation mechanisms. Additionally, the frictional response of a sliding gouge to a sudden acceleration of slip rate to seismic velocities was investigated. These showed an initial strengthening, the amount of which depended on the friction level at which the step was made, followed by strong slip weakening.

Monte Carlo simulation of nonadiabatic expansion in cometary atmospheres - Halley

NASA Astrophysics Data System (ADS)

Hodges, R. R.

1990-02-01

Monte Carlo methods developed for the characterization of velocity-dependent collision processes and ballistic transports in planetary exospheres form the basis of the present computer simulation of icy comet atmospheres, which iteratively undertakes the simultaneous determination of velocity distribution for five neutral species (water, together with suprathermal OH, H2, O, and H) in a flow regime varying from the hydrodynamic to the ballistic. Experimental data from the neutral mass spectrometer carried by Giotto for its March, 1986 encounter with Halley are compared with a model atmosphere.

Low-Energy Impacts onto Lunar Regolith Simulant

NASA Astrophysics Data System (ADS)

Seward, Laura M.; Colwell, J.; Mellon, M.; Stemm, B.

2012-10-01

Low-Energy Impacts onto Lunar Regolith Simulant Laura M. Seward1, Joshua E. Colwell1, Michael T. Mellon2, and Bradley A. Stemm1, 1Department of Physics, University of Central Florida, Orlando, Florida, 2Southwest Research Institute, Boulder, Colorado. Impacts and cratering in space play important roles in the formation and evolution of planetary bodies. Low-velocity impacts and disturbances to planetary regolith are also a consequence of manned and robotic exploration of planetary bodies such as the Moon, Mars, and asteroids. We are conducting a program of laboratory experiments to study low-velocity impacts of 1 to 5 m/s into JSC-1 lunar regolith simulant, JSC-Mars-1 Martian regolith simulant, and silica targets under 1 g. We use direct measurement of ejecta mass and high-resolution video tracking of ejecta particle trajectories to derive ejecta mass velocity distributions. Additionally, we conduct similar experiments under microgravity conditions in a laboratory drop tower and on parabolic aircraft with velocities as low as 10 cm/s. We wish to characterize and understand the collision parameters that control the outcome of low-velocity impacts into regolith, including impact velocity, impactor mass, target shape and size distribution, regolith depth, target relative density, and crater depth, and to experimentally determine the functional dependencies of the outcomes of low-velocity collisions (ejecta mass and ejecta velocities) on the controlling parameters of the collision. We present results from our ongoing study showing the positive correlation between impact energy and ejecta mass. The total ejecta mass is also dependent on the packing density (porosity) of the regolith. We find that ejecta mass velocity fits a power-law or broken power-law distribution. Our goal is to understand the physics of ejecta production and regolith compaction in low-energy impacts and experimentally validate predictive models for dust flow and deposition. We will present our results from one-g and microgravity impact experiments.

Growth properties of protoplanetary dust in a long-term microgravity experiment

NASA Astrophysics Data System (ADS)

Brisset, Julie; Kothe, Stefan; Weidling, Rene; Heisselmann, Daniel; Blum, Juergen

2014-11-01

In the very first steps of the formation of a new planetary system, dust agglomerates and grows inside the protoplanetary disk that rotates around the newly formed star. In this disk, collisions between the dust particles, induced by interactions with the surrounding gas, lead to sticking. Aggregates start growing until their sizes and relative velocities are high enough for collisions to result in bouncing or fragmentation. As part of a series of microgravity experiments aiming at the investigation of the transitions between sticking, bouncing and fragmentation of colliding dust aggregates, the Suborbital Particle and Aggregation Experiment (SPACE) was designed, built and operated both at the drop tower in Bremen (August 2011) and on the REXUS 12 suborbital rocket (March 2012). The SPACE experiment allowed for the observation of collisions between aggregates of sizes of a few 100 µm that were composed of SiO2, a commonly used protoplanetary dust analog material. At velocities below 10 cm/s, clusters composed of a high number of aggregates (more than 10^4) formed and grew to sizes of up to 5 mm. The analysis of these collisions delivered valuable input to a current dust collision model, which maps the outcome of collisions depending on the aggregate sizes and their relative velocities. The sticking probability of sub-mm-sized dust aggregates could directly be measured during the suborbital rocket flight, over a velocity range covering the transition between the sticking and bouncing regimes. In addition, the evolution of clusters formed from sub-mm-sized aggregates during the different experiments could be observed and some of their intrinsic properties derived. The measured characteristics were the cluster fractal dimensions, the tensile strength of their outer aggregate layer and the effective surface energy of their constituents. Threshold energies for cluster restructuring and fragmentation could also be determined. All these cluster properties are important input parameters for molecular dynamics or numerical simulations investigating the behavior of macroscopic clusters (>1 mm in size) in protoplanetary disks.

Gas-kinetic unified algorithm for hypersonic flows covering various flow regimes solving Boltzmann model equation in nonequilibrium effect

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Zhihui; Ma, Qiang; Wu, Junlin

2014-12-09

Based on the Gas-Kinetic Unified Algorithm (GKUA) directly solving the Boltzmann model equation, the effect of rotational non-equilibrium is investigated recurring to the kinetic Rykov model with relaxation property of rotational degrees of freedom. The spin movement of diatomic molecule is described by moment of inertia, and the conservation of total angle momentum is taken as a new Boltzmann collision invariant. The molecular velocity distribution function is integrated by the weight factor on the internal energy, and the closed system of two kinetic controlling equations is obtained with inelastic and elastic collisions. The optimization selection technique of discrete velocity ordinatemore » points and numerical quadrature rules for macroscopic flow variables with dynamic updating evolvement are developed to simulate hypersonic flows, and the gas-kinetic numerical scheme is constructed to capture the time evolution of the discretized velocity distribution functions. The gas-kinetic boundary conditions in thermodynamic non-equilibrium and numerical procedures are studied and implemented by directly acting on the velocity distribution function, and then the unified algorithm of Boltzmann model equation involving non-equilibrium effect is presented for the whole range of flow regimes. The hypersonic flows involving non-equilibrium effect are numerically simulated including the inner flows of shock wave structures in nitrogen with different Mach numbers of 1.5-Ma-25, the planar ramp flow with the whole range of Knudsen numbers of 0.0009-Kn-10 and the three-dimensional re-entering flows around tine double-cone body.« less

Existence regimes for the formation of nonlinear dissipative structures in inhomogeneous magnetoplasmas with non-Maxwellian electrons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Masood, W.; National Centre for Physics, Shahdara Valley Road, Islamabad; Zahoor, Sara

2016-09-15

Nonlinear dissipative structures are studied in one and two dimensions in nonuniform magnetized plasmas with non-Maxwellian electrons. The dissipation is incorporated in the system through ion-neutral collisions. Employing the drift approximation, nonlinear drift waves are derived in 1D, whereas coupled drift-ion acoustic waves are derived in 2D in the weak nonlinearity limit. It is found that the ratio of the diamagnetic drift velocity to the velocity of nonlinear structure determines the nature (compressive or rarefactive) of the shock structure. The upper and lower bounds for velocity of the nonlinear shock structures are also found. It is noticed that the existencemore » regimes for the drift shock waves in one and two dimensions for Cairns distributed electrons are very distinct from those with kappa distributed electrons. Interestingly, it is found that both compressive and rarefactive shock structures could be obtained for the one dimensional drift waves with kappa distributed electrons.« less

Existence regimes for the formation of nonlinear dissipative structures in inhomogeneous magnetoplasmas with non-Maxwellian electrons

NASA Astrophysics Data System (ADS)

Masood, W.; Zahoor, Sara; Gul-e-Ali, Ahmad, Ali

2016-09-01

Nonlinear dissipative structures are studied in one and two dimensions in nonuniform magnetized plasmas with non-Maxwellian electrons. The dissipation is incorporated in the system through ion-neutral collisions. Employing the drift approximation, nonlinear drift waves are derived in 1D, whereas coupled drift-ion acoustic waves are derived in 2D in the weak nonlinearity limit. It is found that the ratio of the diamagnetic drift velocity to the velocity of nonlinear structure determines the nature (compressive or rarefactive) of the shock structure. The upper and lower bounds for velocity of the nonlinear shock structures are also found. It is noticed that the existence regimes for the drift shock waves in one and two dimensions for Cairns distributed electrons are very distinct from those with kappa distributed electrons. Interestingly, it is found that both compressive and rarefactive shock structures could be obtained for the one dimensional drift waves with kappa distributed electrons.

A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces

NASA Astrophysics Data System (ADS)

Sunday, C.; Murdoch, N.; Cherrier, O.; Morales Serrano, S.; Valeria Nardi, C.; Janin, T.; Avila Martinez, I.; Gourinat, Y.; Mimoun, D.

2016-08-01

This work presents an experimental design for studying low-velocity collisions into granular surfaces in low-gravity. In the experiment apparatus, reduced-gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use of an Atwood machine, or a system of pulleys and counterweights. The starting height of the surface container and the initial separation distance between the projectile and surface are variable and chosen to accommodate collision velocities up to 20 cm/s and effective accelerations of ˜0.1 to 1.0 m/s2. Accelerometers, placed on the surface container and inside the projectile, provide acceleration data, while high-speed cameras capture the collision and act as secondary data sources. The experiment is built into an existing 5.5 m drop tower frame and requires the custom design of all components, including the projectile, surface sample container, release mechanism, and deceleration system. Data from calibration tests verify the efficiency of the experiment's deceleration system and provide a quantitative understanding of the performance of the Atwood system.

A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces.

PubMed

Sunday, C; Murdoch, N; Cherrier, O; Morales Serrano, S; Valeria Nardi, C; Janin, T; Avila Martinez, I; Gourinat, Y; Mimoun, D

2016-08-01

This work presents an experimental design for studying low-velocity collisions into granular surfaces in low-gravity. In the experiment apparatus, reduced-gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use of an Atwood machine, or a system of pulleys and counterweights. The starting height of the surface container and the initial separation distance between the projectile and surface are variable and chosen to accommodate collision velocities up to 20 cm/s and effective accelerations of ∼0.1 to 1.0 m/s(2). Accelerometers, placed on the surface container and inside the projectile, provide acceleration data, while high-speed cameras capture the collision and act as secondary data sources. The experiment is built into an existing 5.5 m drop tower frame and requires the custom design of all components, including the projectile, surface sample container, release mechanism, and deceleration system. Data from calibration tests verify the efficiency of the experiment's deceleration system and provide a quantitative understanding of the performance of the Atwood system.

Low velocity collisions of porous planetesimals in the early solar system

NASA Astrophysics Data System (ADS)

de Niem, D.; Kührt, E.; Hviid, S.; Davidsson, B.

2018-02-01

The ESA Rosetta mission has shown that Comet 67P/Churuymov-Gerasimenko is bi-lobed, has a high average porosity of around 70%, does not have internal cavities on size scales larger than 10 m, the lobes could have individual sets of onion shell-like layering, and the nucleus surface contains 100 m-scale cylindrical pits. It is currently debated whether these properties are consistent with high-velocity collisional evolution or if they necessarily are surviving signatures of low-velocity primordial accretion. We use an Eulerian hydrocode to study collisions between highly porous bodies of different sizes, material parameters and relative velocities with emphasis on 5-100 m/s to characterize the effects of collisions in terms of deformation, compaction, and heating. We find that accretion of 1 km cometesimals by 3 km nuclei at 13.5 m/s flattens and partially buries the cometesimal with ∼ 1% reduction of the bulk porosity. This structure locally becomes more dense but the global effect of compaction is minor, suggesting that low-velocity accretion does not lead to a 'bunch of grapes' structure with large internal cavities but a more homogeneous interior, consistent with Rosetta findings. The mild local compaction associated with accretion is potentially the origin of the observed nucleus layering. In 2D axially symmetric impacts hit-and-stick collisions of similarly-sized nuclei are possible at velocities up to 30 m/s where deformation becomes severe. The bulk porosity is reduced significantly, even at 30-50 m/s relative velocity. To avoid hit-and-run collisions the impact angle must be less than 35°-45° from the surface normal at 10 m/s, and even smaller at higher velocities. Impact heating is insignificant. We find that the small cross section of the 67P neck may require a ≤ 5 m/s impact, unless the cohesion exceeds 10 kPa. We conclude that bi-lobe nucleus formation is possible at velocities typically discussed in hierarchical growth scenarios. Impacts of a 7 m projectile at 100-500 m/s create a rimless cylindrical shaft with vertical walls, up to 50 m wide and 70 m deep. These shafts bear some resemblance with the pits on 67P, particularly if the depth-to-width ratio is reduced by nucleus erosion. Collisions between similarly-sized nuclei above 100 m/s lead to complete disintegration, and even small fragments suffer different degrees of compaction. Thus, we strongly doubt that 67P has been subjected to high-velocity collisions by projectiles larger than those that might have formed the pits, or is the fragment of a larger parent body. We suggest that the observed properties of 67P are more consistent with primordial accretion.

Planet formation in binary systems: simulating coagulation using analytically determined collision velocities.

NASA Astrophysics Data System (ADS)

Silsbee, Kedron; Rafikov, Roman

2017-06-01

The existence of planets in tight binary systems presents an interesting puzzle. It is thought that cores of giant planets form via agglomeration of planetesimals in mutual collisions. However, in tight binary systems, one would naïvely expect the collision velocities between planetesimals to be so high that even 100 km bodies would be destroyed, rather than growing in mutual collisions. In these systems, planetesimals are perturbed by gravity from the companion star, and gravity and gas drag from a massive eccentric gas disk. There is a damaging secular resonance that occurs due to the combination of disk gravity and gravity from the binary companion, however the disk gravity can also create locations of low relative eccentricity between planetesimals of different sizes that would not exist if the disk gravity were ignored. Because the gas drag acts more strongly on smaller planetesimals, orbital eccentricity and apsidal angle depend on planetesimal size. Consequently, planetesimal collision velocities depend on the sizes of the collision partners. Same-size bodies collide at low velocity because their orbits are apsidally aligned. Therefore, often in a given environment some collisions will lead to planetesimal growth, and some to erosion or destruction. This variety of collisional outcomes makes it difficult to determine whether any planetesimals can grow to large sizes. We run a multi-annulus coagulation/fragmentation simulation that also includes the effect of size-dependent radial drift of planetesimals to determine the minimum size of initial planetesimal necessary for growth to large sizes in collisions. The minimum initial size of planetesimal necessary for growth depends greatly on the disk mass, eccentricity and the degree of apsidal alignment with the binary. We find that in a wide variety of situations, it is a reasonable approximation that growth occurs as long as there are no collisions capable of completely destroying a planetesimal, but erosion by moderately damaging collisions can also prevent growth from occurring.

Magnetic trapping of cold bromine atoms.

PubMed

Rennick, C J; Lam, J; Doherty, W G; Softley, T P

2014-01-17

Magnetic trapping of bromine atoms at temperatures in the millikelvin regime is demonstrated for the first time. The atoms are produced by photodissociation of Br2 molecules in a molecular beam. The lab-frame velocity of Br atoms is controlled by the wavelength and polarization of the photodissociation laser. Careful selection of the wavelength results in one of the pair of atoms having sufficient velocity to exactly cancel that of the parent molecule, and it remains stationary in the lab frame. A trap is formed at the null point between two opposing neodymium permanent magnets. Dissociation of molecules at the field minimum results in the slowest fraction of photofragments remaining trapped. After the ballistic escape of the fastest atoms, the trapped slow atoms are lost only by elastic collisions with the chamber background gas. The measured loss rate is consistent with estimates of the total cross section for only those collisions transferring sufficient kinetic energy to overcome the trapping potential.

Collisional dissipation in Vlasov turbulence

NASA Astrophysics Data System (ADS)

Pezzi, O.; Perrone, D.; Servidio, S.; Valentini, F.; Sorriso-Valvo, L.; Zouganelis, Y.; Veltri, P.

2017-12-01

A puzzling aspect of solar-wind dynamics consists in the empirical evidence that it is hotter than expected for an adiabatic expanding gas. The cooling of the expanding solar wind is less efficient than it should be, then a key question is how does the solar wind energy turn into heat and keep it hot. Understanding the mechanisms of energy dissipation into heat from the Sun in such a collision-free system represents a key challenge not only in space plasma physics but also from a general thermodynamic perspective. Indeed, any mechanism which does not take into account collisions lacks the final part of the heating process description, related to the irreversible degradation of information. In the solar wind collisions are considered far too weak to produce significant effects on plasma behavior. However, the presence of strong out-of-equilibrium phase space structures, whose signature has been highlighted by in-situ spacecraft measurements and by means of kinetic numerical simulations, could enhance the inter-particle collisions and convert the non-equilibrium features into heat. Here, by focusing on a spatially homogeneous force-free weakly collisional plasma, it is shown that several characteristic times are recovered during the collisional relaxation of fine velocity structures and, hence, fine velocity structures are dissipated by collisions in a time much shorter compared to global non-Maxwellian features, as temperature anisotropies. This indicates that plasma collisionality can locally increase due to the strong velocity space deformation of the particle velocity distribution function (VDF). To quantify the effect of collisions in a turbulent scenario, a hybrid Vlasov-Maxwell simulation has been performed to generate the typical turbulent kinetic plasma regime, characterized by the presence of coherent structures, such as vortices and current sheets, where the ion distribution function is found to be strongly deformed. A direct measure of the collisional dissipation confirms that VDF deformations are significantly related to the enhancement of the plasma collisionality. Finally, the use of the collisional operator in an already developed turbulence allows us to investigate the inter-play of collisions, which tend to restore the thermal equilibrium, and other collisionless physical processes.

Neotectonic Deformation in Central Eurasia: A Geodynamic Model Approach

NASA Astrophysics Data System (ADS)

Tunini, Lavinia; Jiménez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume; Bird, Peter

2017-11-01

Central Eurasia hosts wide orogenic belts of collision between India and Arabia with Eurasia, with diffuse or localized deformation occurring up to hundreds of kilometers from the primary plate boundaries. Although numerous studies have investigated the neotectonic deformation in central Eurasia, most of them have focused on limited segments of the orogenic systems. Here we explore the neotectonic deformation of all of central Eurasia, including both collision zones and the links between them. We use a thin-spherical sheet approach in which lithosphere strength is calculated from lithosphere structure and its thermal regime. We investigate the contributions of variations in lithospheric structure, rheology, boundary conditions, and fault friction coefficients on the predicted velocity and stress fields. Results (deformation pattern, surface velocities, tectonic stresses, and slip rates on faults) are constrained by independent observations of tectonic regime, GPS, and stress data. Our model predictions reproduce the counterclockwise rotation of Arabia and Iran, the westward escape of Anatolia, and the eastward extrusion of the northern Tibetan Plateau. To simulate the observed extensional faults in the Tibetan Plateau, a weaker lithosphere is required, provided by a change in the rheological parameters. The southward movement of the SE Tibetan Plateau can be explained by the combined effects of the Sumatra trench retreat, a thinner lithospheric mantle, and strik-slip faults in the region. This study offers a comprehensive model for regions with little or no data coverage, like the Arabia-India intercollision zone, where the surface velocity is northward showing no deflection related to Arabia and India indentations.

Lateral variations in the crustal structure of the Indo-Eurasian collision zone

NASA Astrophysics Data System (ADS)

Gilligan, Amy; Priestley, Keith

2018-05-01

The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on Earth have formed. As the largest and highest orogenic plateau on Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh Wave group velocity maps for periods from 5-70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone. The Moho occurs at lower shear velocities below north eastern Tibet than it does beneath western and southern Tibet and below India. The east-west difference across Tibet is particularly apparent in the elevated velocities observed west of 84° E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west, but possibly not in the east. At depths of 20-40 km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (˜10% decrease in velocity, Vs <3.4 km/s) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements (Acton et al., 2010), we find that the low velocity layer has a radial anisotropic signature with Vsh > Vsv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.

Studying low-velocity impacts in reduced gravity: an asteroid landing experiment

NASA Astrophysics Data System (ADS)

Murdoch, Naomi; Calandry, Alexis; Sunday, Cecily; Avila Martinez, Iris; Cherrier, Olivier; Cadu, Alexandre; Zenou, Emmanuel; Gourinat, Yves

2016-10-01

Several current and future small body missions include lander components e.g., MASCOT and the MINERVA rovers on-board JAXA's Hayabusa-2 mission [1], MASCOT2 and possibly AGEX on board ESA's AIM mission [2,3]. The understanding of surface-lander interactions is important for all such landers as these considerations influence the deployment strategy, the mission design and operations, and even the choice of payload. The dynamics of low-velocity interactions with granular material in reduced gravity are also important for other missions, such as OSIRIS-REx (NASA), that will interact directly with the asteroid's surface in order to retrieve a regolith sample.In our experiment, reduced gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use of an Atwood machine, or a system of pulleys and counterweights. The experiment is built into an existing 5.5 m drop-tower frame and has required the custom design of all components, including the projectiles, surface sample container and release mechanism [4].Previous experiments using similar methods have demonstrated the important role of gravity in the peak accelerations and collision timescales during low velocity granular impacts [5,6]. The design of our experiment accommodates collision velocities and effective accelerations that are lower than in previous experiments (<20 cm/s and ~0.1-1.0 m/s2 respectively), allowing us to come closer to the conditions that may be encountered by current and future small body missions.Here we will present the results of our experimental trials and discuss the implications for small body missions. The unique experimental data obtained in these trials may also be valuable to benchmark different numerical simulation approaches. These simulations can then subsequently be used to extrapolate the results to even lower gravity regimes.[1] Tsuda, Y. et al., Acta Astronaut, 2013; [2] Michel, P. et al., Adv Space Res, 2016; [3] Karatekin, O., et al., EGU, 2016; [4] Sunday, C., et al, RSI accepted, 2016; [5] Altshuler, E, arxiv 2013; [6] Goldman, D. I. and Umbanhowar, P., PRE, 2008.

Low-Energy Collisions of Protonated Enantiopure Amino Acids with Chiral Target Gases

NASA Astrophysics Data System (ADS)

Kulyk, K.; Rebrov, O.; Ryding, M.; Thomas, R. D.; Uggerud, E.; Larsson, M.

2017-12-01

Here we report on the gas-phase interactions between protonated enantiopure amino acids ( l- and d-enantiomers of Met, Phe, and Trp) and chiral target gases [( R)- and ( S)-2-butanol, and ( S)-1-phenylethanol] in 0.1-10.0 eV low-energy collisions. Two major processes are seen to occur over this collision energy regime, collision-induced dissociation and ion-molecule complex formation. Both processes were found to be independent of the stereo-chemical composition of the interacting ions and targets. These data shed light on the currently debated mechanisms of gas-phase chiral selectivity by demonstrating the inapplicability of the three-point model to these interactions, at least under single collision conditions. [Figure not available: see fulltext.

Development of a Nonlinear Probability of Collision Tool for the Earth Observing System

NASA Technical Reports Server (NTRS)

McKinley, David P.

2006-01-01

The Earth Observing System (EOS) spacecraft Terra, Aqua, and Aura fly in constellation with several other spacecraft in 705-kilometer mean altitude sun-synchronous orbits. All three spacecraft are operated by the Earth Science Mission Operations (ESMO) Project at Goddard Space Flight Center (GSFC). In 2004, the ESMO project began assessing the probability of collision of the EOS spacecraft with other space objects. In addition to conjunctions with high relative velocities, the collision assessment method for the EOS spacecraft must address conjunctions with low relative velocities during potential collisions between constellation members. Probability of Collision algorithms that are based on assumptions of high relative velocities and linear relative trajectories are not suitable for these situations; therefore an algorithm for handling the nonlinear relative trajectories was developed. This paper describes this algorithm and presents results from its validation for operational use. The probability of collision is typically calculated by integrating a Gaussian probability distribution over the volume swept out by a sphere representing the size of the space objects involved in the conjunction. This sphere is defined as the Hard Body Radius. With the assumption of linear relative trajectories, this volume is a cylinder, which translates into simple limits of integration for the probability calculation. For the case of nonlinear relative trajectories, the volume becomes a complex geometry. However, with an appropriate choice of coordinate systems, the new algorithm breaks down the complex geometry into a series of simple cylinders that have simple limits of integration. This nonlinear algorithm will be discussed in detail in the paper. The nonlinear Probability of Collision algorithm was first verified by showing that, when used in high relative velocity cases, it yields similar answers to existing high relative velocity linear relative trajectory algorithms. The comparison with the existing high velocity/linear theory will also be used to determine at what relative velocity the analysis should use the new nonlinear theory in place of the existing linear theory. The nonlinear algorithm was also compared to a known exact solution for the probability of collision between two objects when the relative motion is strictly circular and the error covariance is spherically symmetric. Figure I shows preliminary results from this comparison by plotting the probabilities calculated from the new algorithm and those from the exact solution versus the Hard Body Radius to Covariance ratio. These results show about 5% error when the Hard Body Radius is equal to one half the spherical covariance magnitude. The algorithm was then combined with a high fidelity orbit state and error covariance propagator into a useful tool for analyzing low relative velocity nonlinear relative trajectories. The high fidelity propagator is capable of using atmospheric drag, central body gravitational, solar radiation, and third body forces to provide accurate prediction of the relative trajectories and covariance evolution. The covariance propagator also includes a process noise model to ensure realistic evolutions of the error covariance. This paper will describe the integration of the nonlinear probability algorithm and the propagators into a useful collision assessment tool. Finally, a hypothetical case study involving a low relative velocity conjunction between members of the Earth Observation System constellation will be presented.

Onset of radial flow in p + p collisions

DOE PAGES

Jiang, Kun; Zhu, Yinying; Liu, Weitao; ...

2015-02-23

It has been debated for decades whether hadrons emerging from p+p collisions exhibit collective expansion. The signal of the collective motion in p+p collisions is not as clear as in heavy-ion collisions because of the low multiplicity and large fluctuation in p+p collisions. Tsallis Blast-Wave (TBW) model is a thermodynamic approach, introduced to handle the overwhelming correlation and fluctuation in the hadronic processes. We have systematically studied the identified particle spectra in p+p collisions from RHIC to LHC using TBW and found no appreciable radial flow in p+p collisions below √s = 900 GeV. At LHC higher energy of 7more » TeV in p+p collisions, the radial flow velocity achieves an average of (β) = 0.320 ± 0.005. This flow velocity is comparable to that in peripheral (40-60%) Au+Au collisions at RHIC. In addition, breaking of the identified particle spectra m T scaling was also observed at LHC from a model independent test.« less

Environmental dependence of star formation induced by cloud collisions in a barred galaxy

NASA Astrophysics Data System (ADS)

Fujimoto, Yusuke; Tasker, Elizabeth J.; Habe, Asao

2014-11-01

Cloud collision has been proposed as a way to link the small-scale star formation process with the observed global relation between the surface star formation rate and gas surface density. We suggest that this model can be improved further by allowing the productivity of such collisions to depend on the relative velocity of the two clouds. Our adjustment implements a simple step function that results in the most successful collisions being at the observed velocities for triggered star formation. By applying this to a high-resolution simulation of a barred galaxy, we successfully reproduce the observational result that the star formation efficiency (SFE) in the bar is lower than that in the spiral arms. This is not possible when we use an efficiency dependent on the internal turbulence properties of the clouds. Our results suggest that high-velocity collisions driven by the gravitational pull of the clouds are responsible for the low bar SFE.

Mobility in a strongly coupled dusty plasma with gas.

PubMed

Liu, Bin; Goree, J

2014-04-01

The mobility of a charged projectile in a strongly coupled dusty plasma is simulated. A net force F, opposed by a combination of collisional scattering and gas friction, causes projectiles to drift at a mobility-limited velocity up. The mobility μp=up/F of the projectile's motion is obtained. Two regimes depending on F are identified. In the high-force regime, μp∝F0.23, and the scattering cross section σs diminishes as up-6/5. Results for σs are compared with those for a weakly coupled plasma and for two-body collisions in a Yukawa potential. The simulation parameters are based on microgravity plasma experiments.

Mobility in a strongly coupled dusty plasma with gas

NASA Astrophysics Data System (ADS)

Liu, Bin; Goree, J.

2014-04-01

The mobility of a charged projectile in a strongly coupled dusty plasma is simulated. A net force F, opposed by a combination of collisional scattering and gas friction, causes projectiles to drift at a mobility-limited velocity up. The mobility μp=up/F of the projectile's motion is obtained. Two regimes depending on F are identified. In the high-force regime, μp∝F0.23, and the scattering cross section σs diminishes as up-6/5. Results for σs are compared with those for a weakly coupled plasma and for two-body collisions in a Yukawa potential. The simulation parameters are based on microgravity plasma experiments.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). III. Possible evidence for formation of NGC 6618 cluster in M 17 by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Nishimura, Atsushi; Minamidani, Tetsuhiro; Umemoto, Tomofumi; Fujita, Shinji; Matsuo, Mitsuhiro; Hattori, Yusuke; Kohno, Mikito; Yamagishi, Mitsuyoshi; Tsuda, Yuya; Kuriki, Mika; Kuno, Nario; Torii, Kazufumi; Tsutsumi, Daichi; Okawa, Kazuki; Sano, Hidetoshi; Tachihara, Kengo; Ohama, Akio; Fukui, Yasuo

2018-05-01

We present 12CO (J = 1-0), 13CO (J = 1-0), and C18O (J = 1-0) images of the M 17 giant molecular clouds obtained as part of the FUGIN (FOREST Ultra-wide Galactic Plane Survey In Nobeyama) project. The observations cover the entire area of the M 17 SW and M 17 N clouds at the highest angular resolution (˜19″) to date, which corresponds to ˜0.18 pc at the distance of 2.0 kpc. We find that the region consists of four different velocity components: a very low velocity (VLV) clump, a low velocity component (LVC), a main velocity component (MVC), and a high velocity component (HVC). The LVC and the HVC have cavities. Ultraviolet photons radiated from NGC 6618 cluster penetrate into the N cloud up to ˜5 pc through the cavities and interact with molecular gas. This interaction is correlated with the distribution of young stellar objects in the N cloud. The LVC and the HVC are distributed complementarily after the HVC is displaced by 0.8 pc toward the east-southeast direction, suggesting that collision of the LVC and the HVC created the cavities in both clouds. The collision velocity and timescale are estimated to be 9.9 km s-1 and 1.1 × 105 yr, respectively. The high collision velocity can provide a mass accretion rate of up to 10^{-3} M_{⊙} yr-1, and the high column density (4 × 1023 cm-2) might result in massive cluster formation. The scenario of cloud-cloud collision likely explains well the stellar population and the formation history of the NGC 6618 cluster proposed by Hoffmeister et al. (2008, ApJ, 686, 310).

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). III. Possible evidence for formation of NGC 6618 cluster in M 17 by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Nishimura, Atsushi; Minamidani, Tetsuhiro; Umemoto, Tomofumi; Fujita, Shinji; Matsuo, Mitsuhiro; Hattori, Yusuke; Kohno, Mikito; Yamagishi, Mitsuyoshi; Tsuda, Yuya; Kuriki, Mika; Kuno, Nario; Torii, Kazufumi; Tsutsumi, Daichi; Okawa, Kazuki; Sano, Hidetoshi; Tachihara, Kengo; Ohama, Akio; Fukui, Yasuo

2018-05-01

We present 12CO (J = 1-0), 13CO (J = 1-0), and C18O (J = 1-0) images of the M 17 giant molecular clouds obtained as part of the FUGIN (FOREST Ultra-wide Galactic Plane Survey In Nobeyama) project. The observations cover the entire area of the M 17 SW and M 17 N clouds at the highest angular resolution (˜19″) to date, which corresponds to ˜0.18 pc at the distance of 2.0 kpc. We find that the region consists of four different velocity components: a very low velocity (VLV) clump, a low velocity component (LVC), a main velocity component (MVC), and a high velocity component (HVC). The LVC and the HVC have cavities. Ultraviolet photons radiated from NGC 6618 cluster penetrate into the N cloud up to ˜5 pc through the cavities and interact with molecular gas. This interaction is correlated with the distribution of young stellar objects in the N cloud. The LVC and the HVC are distributed complementarily after the HVC is displaced by 0.8 pc toward the east-southeast direction, suggesting that collision of the LVC and the HVC created the cavities in both clouds. The collision velocity and timescale are estimated to be 9.9 km s-1 and 1.1 × 105 yr, respectively. The high collision velocity can provide a mass accretion rate of up to 10^{-3} M_{⊙}yr-1, and the high column density (4 × 1023 cm-2) might result in massive cluster formation. The scenario of cloud-cloud collision likely explains well the stellar population and the formation history of the NGC 6618 cluster proposed by Hoffmeister et al. (2008, ApJ, 686, 310).

The Physics of Protoplanetesimal Dust Agglomerates. VII. The Low-velocity Collision Behavior of Large Dust Agglomerates

NASA Astrophysics Data System (ADS)

Schräpler, Rainer; Blum, Jürgen; Seizinger, Alexander; Kley, Wilhelm

2012-10-01

We performed micro-gravity collision experiments in our laboratory drop tower using 5 cm sized dust agglomerates with volume filling factors of 0.3 and 0.4, respectively. This work is an extension of our previous experiments reported in Beitz et al. to aggregates of more than one order of magnitude higher masses. The dust aggregates consisted of micrometer-sized silica particles and were macroscopically homogeneous. We measured the coefficient of restitution for collision velocities ranging from 1 cm s-1 to 0.5 m s-1, and determined the fragmentation velocity. For low velocities, the coefficient of restitution decreases with increasing impact velocity, in contrast to findings by Beitz et al. At higher velocities, the value of the coefficient of restitution becomes constant, before the aggregates break at the onset of fragmentation. We interpret the qualitative change in the coefficient of restitution as the transition from a solid-body-dominated to a granular-medium-dominated behavior. We complement our experiments by molecular-dynamics simulations of porous aggregates and obtain a reasonable match to the experimental data. We discuss the importance of our experiments for protoplanetary disks, debris disks, and planetary rings. This work is an extension to the previous work of our group and gives new insight into the velocity dependency of the coefficient of restitution due to improved measurements, better statistics, and a theoretical approach.

Mass Transfer via Low-Velocity Rebound in a Microgravity Environment

NASA Astrophysics Data System (ADS)

Jarmak, S. G.; Colwell, J. E.; Brisset, J.; Dove, A.; Brown, A. Q.

2017-12-01

Observations of low-velocity collisions (< 1 m/s) between μm to cm-size particles in a microgravity environment are crucial to an understanding of the surface properties of small, airless bodies as well as the processes that lead to their formation. The COLLIDE (Collisions Into Dust Experiment) and PRIME (Physics of Regolith Impacts in Microgravity Experiment) programs created impacts into simulated planetary regolith with cm-scale impactors to observe ejecta production and coefficients of restitution in microgravity. These experiments were carried out on orbit (COLLIDE, COLLIDE-2), in suborbital space (COLLIDE-3), and on parabolic airplane flights (PRIME) under vacuum. Some impacts at speeds less than 40 cm/s resulted in mass transfer from the target regolith onto the impactor. To study these mass-transfer collisions in more detail without the cost or time requirements of spaceflight or parabolic flights, we developed an experimental apparatus in a laboratory drop tower (free-fall time 0.75 s) and performed experiments at standard pressure. The impactor is suspended from a spring and remains in contact with the bed of regolith until free-fall allows the spring to retract and pull the impactor upwards. This method allowed us to simulate the rebound portion of a low-velocity collision in a laboratory microgravity environment. We achieved rebound velocities of 10 - 60 cm/s, and we observed mass transfer events with rebound speeds below 40 cm/s. The amount of mass transfer produced was more significant than a monolayer of granular material, but less than the amount observed in the COLLIDE and PRIME experiments. These mass-transfer collisions may play a role in the growth of planetesimals. We will present the results of our laboratory-based studies where we vary impact velocity and target material, and discuss implications for collisional evolution in the protoplanetary disk and planetary rings.

Mixing water ice into regolith in low-velocity impact experiments

NASA Astrophysics Data System (ADS)

Brisset, J.; Colwell, J. E.; Dove, A.; Rascon, A. N.; Mohammed, N.; Cox, C.

2016-12-01

Collisions between dust and ice grains of different sizes lead to particle growth both in Saturn's rings and in the protoplanetary disk (PPD). Low-velocity collisions (a few m/s or less) among ring or PPD particles produce ejecta and play an important role in this growth process as ejected particles accrete on larger grains. We report on the results of a series of experiments to study the ejecta mass-velocity distribution from impacts of cm-scale particles into granular media at speeds below 3 m/s. These experiments were performed using the lunar regolith simulant JSC-1 in both microgravity and 1-g conditions, under vacuum and at room temperature. As most planetesimal formation occurred beyond the frost line and as Satrun's rings particles are mostly composed of water ice, we proceeded to perform impact experiments at 1-g into JSC-1 lunar regolith simulant mixed with water ice particles at low temperatures (<150 K). We will present the results of the cryogenic impacts and compare them to the study performed at room temperature without water ice. The inclusion of water ice into the target sample is a first step towards better understanding the influence of the presence of water ice in the production of ejecta in response to low-velocity impacts. We will discuss the implications of our results for planetary ring particle collisions as well as planetesimal formation.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Terai, Tsuyoshi; Takahashi, Jun; Itoh, Yoichi, E-mail: tsuyoshi.terai@nao.ac.jp

Main-belt asteroids have been continuously colliding with one another since they were formed. Their size distribution is primarily determined by the size dependence of asteroid strength against catastrophic impacts. The strength scaling law as a function of body size could depend on collision velocity, but the relationship remains unknown, especially under hypervelocity collisions comparable to 10 km s{sup –1}. We present a wide-field imaging survey at an ecliptic latitude of about 25° for investigating the size distribution of small main-belt asteroids that have highly inclined orbits. The analysis technique allowing for efficient asteroid detections and high-accuracy photometric measurements provides sufficientmore » sample data to estimate the size distribution of sub-kilometer asteroids with inclinations larger than 14°. The best-fit power-law slopes of the cumulative size distribution are 1.25 ± 0.03 in the diameter range of 0.6-1.0 km and 1.84 ± 0.27 in 1.0-3.0 km. We provide a simple size distribution model that takes into consideration the oscillations of the power-law slope due to the transition from the gravity-scaled regime to the strength-scaled regime. We find that the high-inclination population has a shallow slope of the primary components of the size distribution compared to the low-inclination populations. The asteroid population exposed to hypervelocity impacts undergoes collisional processes where large bodies have a higher disruptive strength and longer lifespan relative to tiny bodies than the ecliptic asteroids.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Richard, P.

The study of inelastic collision phenomena with highly charged projectile ions and the interpretation of spectral features resulting from these collisions remain as the major focal points in the atomic physics research at the J.R. Macdonald Laboratory, Kansas State University, Manhattan, Kansas. The title of the research project, ``Atomic Physics with Highly Charged Ions,`` speaks to these points. The experimental work in the past few years has divided into collisions at high velocity using the primary beams from the tandem and LINAC accelerators and collisions at low velocity using the CRYEBIS facility. Theoretical calculations have been performed to accurately describemore » inelastic scattering processes of the one-electron and many-electron type, and to accurately predict atomic transition energies and intensities for x rays and Auger electrons. Brief research summaries are given for the following: (1) electron production in ion-atom collisions; (2) role of electron-electron interactions in two-electron processes; (3) multi-electron processes; (4) collisions with excited, aligned, Rydberg targets; (5) ion-ion collisions; (6) ion-molecule collisions; (7) ion-atom collision theory; and (8) ion-surface interactions.« less

Distinct turbulence sources and confinement features in the spherical tokamak plasma regime

DOE PAGES

Wang, W. X.; Ethier, S.; Ren, Y.; ...

2015-10-30

New turbulence contributions to plasma transport and confinement in the spherical tokamak (ST) regime are identified through nonlinear gyrokinetic simulations. The drift wave Kelvin-Helmholtz (KH) mode characterized by intrinsic mode asymmetry is shown to drive significant ion thermal transport in strongly rotating national spherical torus experiment (NSTX) L-modes. The long wavelength, quasi-coherent dissipative trapped electron mode (TEM) is destabilized in NSTX H-modes despite the presence of strong E x B shear, providing a robust turbulence source dominant over collisionless TEM. Dissipative trapped electron mode (DTEM)-driven transport in the NSTX parametric regime is shown to increase with electron collision frequency, offeringmore » one possible source for the confinement scaling observed in experiments. There exists a turbulence-free regime in the collision-induced collisionless trapped electron mode to DTEM transition for ST plasmas. In conclusion, this predicts a natural access to a minimum transport state in the low collisionality regime that future advanced STs may cover.« less

Non-Parametric Collision Probability for Low-Velocity Encounters

NASA Technical Reports Server (NTRS)

Carpenter, J. Russell

2007-01-01

An implicit, but not necessarily obvious, assumption in all of the current techniques for assessing satellite collision probability is that the relative position uncertainty is perfectly correlated in time. If there is any mis-modeling of the dynamics in the propagation of the relative position error covariance matrix, time-wise de-correlation of the uncertainty will increase the probability of collision over a given time interval. The paper gives some examples that illustrate this point. This paper argues that, for the present, Monte Carlo analysis is the best available tool for handling low-velocity encounters, and suggests some techniques for addressing the issues just described. One proposal is for the use of a non-parametric technique that is widely used in actuarial and medical studies. The other suggestion is that accurate process noise models be used in the Monte Carlo trials to which the non-parametric estimate is applied. A further contribution of this paper is a description of how the time-wise decorrelation of uncertainty increases the probability of collision.

pT spectra in pp and AA collisions at RHIC and LHC energies using the Tsallis-Weibull approach

NASA Astrophysics Data System (ADS)

Dash, Sadhana; Mahapatra, D. P.

2018-04-01

The Tsallis q -statistics have been incorporated in the Weibull model of particle production, in the form of q-Weibull distribution, to describe the transverse momentum (pT) distribution of charged hadrons at mid-rapidity, measured at RHIC and LHC energies. The q-Weibull distribution is found to describe the observed pT distributions over all ranges of measured pT. Below 2.2 GeV/c, while going from peripheral to central collisions, the parameter q is found to decrease systematically towards unity, indicating an evolution from a non-equilibrated system in peripheral collisions, towards a more thermalized system in central collisions. However, the trend is reversed in the all inclusive pT regime. This can be attributed to an increase in relative contribution of hard pQCD processes in central collisions. The λ-parameter is found to be associated with the mean pT or the collective expansion velocity of the produced hadrons, which shows an expected increase with centrality of collisions. The k parameter is observed to increase with the onset of hard QCD scatterings, initial fluctuations, and other processes leading to non-equilibrium conditions.

Dependence of enhanced asymmetry-induced transport on collision frequency

DOE Office of Scientific and Technical Information (OSTI.GOV)

Eggleston, D. L.

A single-particle code with collisional effects is used to study how asymmetry-induced radial transport in a non-neutral plasma depends on collision frequency. For asymmetries of the form ϕ{sub 1}(r) cos(kz) cos(ωt−lθ), two sources for the transport have been identified: resonant particles and axially trapped particles. The simulation shows that this latter type, which occurs near the radius where ω matches the azimuthal rotation frequency ω{sub R}, is usually dominant at low collision frequency ν but becomes negligible at higher ν. This behavior can be understood by noting that axially trapped particles have a lower trapping frequency than resonant particles. In the lowmore » ν (banana) regime, the radial oscillations have amplitude Δr ≈ v{sub r}/ω{sub T}, so axially trapped particles dominate, and the transport may even exceed the resonant particle plateau regime level. As ν increases, collisions start to interrupt the slower axially trapped particle oscillations, while the resonant particles are still in the banana regime, so the axially trapped particle contribution to the transport decreases. At the largest ν values, axially trapped particle transport is negligible and the observed diffusion coefficient matches that given by plateau regime resonant particle theory. Heuristic models based on these considerations give reasonable agreement with the observed scaling laws for the value of the collision frequency where axially trapped particle transport starts to decrease and for the enhancement of the diffusion coefficient produced by axially trapped particles.« less

An evaluation of collision models in the Method of Moments for rarefied gas problems

NASA Astrophysics Data System (ADS)

Emerson, David; Gu, Xiao-Jun

2014-11-01

The Method of Moments offers an attractive approach for solving gaseous transport problems that are beyond the limit of validity of the Navier-Stokes-Fourier equations. Recent work has demonstrated the capability of the regularized 13 and 26 moment equations for solving problems when the Knudsen number, Kn (where Kn is the ratio of the mean free path of a gas to a typical length scale of interest), is in the range 0.1 and 1.0-the so-called transition regime. In comparison to numerical solutions of the Boltzmann equation, the Method of Moments has captured both qualitatively, and quantitatively, results of classical test problems in kinetic theory, e.g. velocity slip in Kramers' problem, temperature jump in Knudsen layers, the Knudsen minimum etc. However, most of these results have been obtained for Maxwell molecules, where molecules repel each other according to an inverse fifth-power rule. Recent work has incorporated more traditional collision models such as BGK, S-model, and ES-BGK, the latter being important for thermal problems where the Prandtl number can vary. We are currently investigating the impact of these collision models on fundamental low-speed problems of particular interest to micro-scale flows that will be discussed and evaluated in the presentation. Engineering and Physical Sciences Research Council under Grant EP/I011927/1 and CCP12.

Upper mantle beneath foothills of the western Himalaya: subducted lithospheric slab or a keel of the Indian shield?

NASA Astrophysics Data System (ADS)

Vinnik, L.; Singh, A.; Kiselev, S.; Kumar, M. Ravi

2007-12-01

The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60° at 80-160 km depths to 150° at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 +/- 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s-1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya.

Global and local re-impact and velocity regime of ballistic ejecta of boulder craters on Ceres

NASA Astrophysics Data System (ADS)

Schulzeck, F.; Schröder, S. E.; Schmedemann, N.; Stephan, K.; Jaumann, R.; Raymond, C. A.; Russell, C. T.

2018-04-01

Imaging by the Dawn-spacecraft reveals that fresh craters on Ceres below 40 km often exhibit numerous boulders. We investigate how the fast rotating, low-gravity regime on Ceres influences their deposition. We analyze size-frequency distributions of ejecta blocks of twelve boulder craters. Global and local landing sites of boulder crater ejecta and boulder velocities are determined by the analytical calculation of elliptic particle trajectories on a rotating body. The cumulative distributions of boulder diameters follow steep-sloped power-laws. We do not find a correlation between boulder size and the distance of a boulder to its primary crater. Due to Ceres' low gravitational acceleration and fast rotation, ejecta of analyzed boulder craters (8-31 km) can be deposited across the entire surface of the dwarf planet. The particle trajectories are strongly influenced by the Coriolis effect as well as the impact geometry. Fast ejecta of high-latitude craters accumulate close to the pole of the opposite hemisphere. Fast ejecta of low-latitude craters wraps around the equator. Rotational effects are also relevant for the low-velocity regime. Boulders are ejected at velocities up to 71 m/s.

Collision and subduction structure of the Izu-Bonin arc, central Japan: Recent studies from refraction/wide-angle reflection analysis and seismic tomography

NASA Astrophysics Data System (ADS)

Arai, R.; Iwasaki, T.; Sato, H.; Abe, S.; Hirata, N.

2009-12-01

Since the middle Miocene, the Izu-Bonin arc has been colliding from south with the Honshu arc in central Japan associated with subduction of the Philippine Sea plate. This process is responsible for forming a complex crustal structure called the Izu collision zone. Geological studies indicate the several geological blocks derived from the Izu-Bonin arc, such as the Misaka Mountains (MM), the Tanzawa Mountains (TM) and the Izu Peninsula (IP), were accreted onto the Honshu crust in the course of the collision, forming several tectonic boundaries in and around this collision zone (e.g. Amano, 1991). Recent seismic experiments succeeded in revealing the deep crustal structure in the eastern part of the Izu collision zone by reflection analysis (Sato et al., 2005) and refraction/wide-angle reflection analysis (Arai et al., 2009). Although these studies delineate the collision boundary between the Honshu crust and TM, and the upper surface of the subducting Philippine Sea plate, the southern part of the profile including the Kozu-Matsuda Fault (KMF, the tectonic boundary between TM and IP) is not well constrained due to the poor ray coverage. Moreover, clear images of tectonic boundaries are not obtained for the central or western part of the collision zone. In order to construct the structure model dominated by collision and subduction for the whole part of the collision zone, we carried out the following two analyses: (1) refraction tomography of active source data including another profile line in the western part of the collision zone (Sato et al., 2006), and (2) seismic tomography combining active and passive source data. In the analysis (1), we applied first arrival seismic tomography (Zelt and Barton, 1998) to the refraction data .We inverted over 39,000 travel times to construct a P wave velocity model for the 75-km-long transect, and a fine-scale structure with strong lateral heterogeneity was recovered. We conducted checkerboard resolution test to evaluate a spatial resolution, and confirmed that the final model has an enough resolution down to the depth of 5 km. We also performed a Monte Carlo uncertainty analysis (Korenaga et al, 2000) to estimate the posteriori model variance, showing that most velocities are well constrained with standard deviation of less than 0.20 km/s. Our result strongly indicates the existences of low velocity zones (< 6.0 km/s) along the tectonic boundaries and high velocity bodies (> 6.0 km/s) just beneath MM and TM, which correspond to the middle crust of the Izu-Bonin arc (Kodaira et al., 2007). In the analysis (2), hypocenters and velocity structure were simultaneously determined based on the double-difference method (Zhang and Thurber, 2003). The hypocenter distribution and final velocity structure obtained indicate several interesting features, including low velocity sedimentary layer (< 6.0 km/s) along the KMF and prominent seismic activity in the middle-lower crust (6.0-6.8 km/s) in the Izu-Bonin arc (10-25 km depth beneath TM). These results give us very important constraints for the collision process ongoing in our research area.

Chiral Anomalous Dispersion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sadofyev, Andrey; Sen, Srimoyee

The linearized Einstein equation describing graviton propagation through a chiral medium appears to be helicity dependent. We analyze features of the corresponding spectrum in a collision-less regime above a flat background. In the long wave-length limit, circularly polarized metric perturbations travel with a helicity dependent group velocity that can turn negative giving rise to a new type of an anomalous dispersion. We further show that this chiral anomalous dispersion is a general feature of polarized modes propagating through chiral plasmas extending our result to the electromagnetic sector.

Chiral Anomalous Dispersion

DOE PAGES

Sadofyev, Andrey; Sen, Srimoyee

2018-02-16

The linearized Einstein equation describing graviton propagation through a chiral medium appears to be helicity dependent. We analyze features of the corresponding spectrum in a collision-less regime above a flat background. In the long wave-length limit, circularly polarized metric perturbations travel with a helicity dependent group velocity that can turn negative giving rise to a new type of an anomalous dispersion. We further show that this chiral anomalous dispersion is a general feature of polarized modes propagating through chiral plasmas extending our result to the electromagnetic sector.

Acceleration to High Velocities and Heating by Impact Using Nike KrF laser

NASA Astrophysics Data System (ADS)

Karasik, Max

2009-11-01

Shock ignition, impact ignition, as well as higher intensity conventional hot spot ignition designs reduce driver energy requirement by pushing the envelope in laser intensity and target implosion velocities. This talk will describe experiments that for the first time reach target velocities in the range of 700 -- 1000 km/s. The highly accelerated planar foils of deuterated polystyrene, some with bromine doping, are made to collide with a witness foil to produce extreme shock pressures and result in heating of matter to thermonuclear temperatures. Target acceleration and collision are diagnosed using large field of view monochromatic x-ray imaging with backlighting as well as bremsstrahlung self-emission. The impact conditions are diagnosed using DD fusion neutron yield, with over 10^6 neutrons produced during the collision. Time-of-flight neutron detectors are used to measure the ion temperature upon impact, which reaches 2 -- 3 keV. The experiments are performed on the Nike facility, reconfigured specifically for high intensity operation. The short wavelength and high illumination uniformity of Nike KrF laser uniquely enable access to this new parameter regime. Intensities of (0.4 -- 1.2) x 10^15 W/cm^2 and pulse durations of 0.4 -- 2 ns were utilized. Modeling of the target acceleration, collision, and neutron production is performed using the FAST3D radiation hydrodynamics code with a non-LTE radiation model. Work is supported by US Department of Energy.

Low-velocity impacts into cryogenic icy regolith

NASA Astrophysics Data System (ADS)

Brisset, Julie; Colwell, Josh E.; Dove, Adrienne; Rascon, Allison; Mohammed, Nadia; Cox, Christopher

2016-10-01

The first stages of planet formation take place in the protoplanetary disk (PPD), where µm-sized dust grains accrete into km-sized planetesimals. In the current discussion on the processes involved in accretion beyond the cm scale, the size distribution of the particles colliding at low speeds (a few m/s) inside the PPD is thought to play an important role. A few larger bodies that survived bouncing and fragmentation collisions accumulate the fine dust residue of the erosion and fragmentation of other particles that were destroyed in more energetic collisions. A significant component of this dust on bodies farther out in the PPD will be composed of ices.We have carried out a series of experiments to study the ejecta mass-velocity distribution from impacts of cm-scale particles into granular media at speeds below 3 m/s in both microgravity and 1-g conditions in vacuo and room temperature. Aggregate-aggregate collision experiments have shown bouncing and fragmentation at speeds above ~ 1 m/s. However, most planetesimal formation occurred beyond the frost line and at much lower temperatures than our earlier experiments. We have performed impact experiments at 1-g into JSC-1 lunar regolith simulant at low temperatures (<150 K) with water ice particles mixed into the JSC-1 sample. We varied the impact energy and the water ice content of the sample and measured the ejecta mass-velocity distribution as well as the final crater size. Our goal is to determine if the cryogenic temperature and the presence of water ice in the regolith affects the dynamic response to low-velocity impacts and the production of regolith. We will present the results of the cryogenic impacts and compare them to the study performed at room temperature without water ice. The inclusion of water ice into the target sample is a first step towards better understanding the influence of the presence of water ice in the production of ejecta in response to low-velocity impacts. We will discuss the implications of our results for planetary ring particle collisions as well as planetesimal formation.

Outcome regimes of binary raindrop collisions

NASA Astrophysics Data System (ADS)

Testik, Firat Y.

2009-11-01

This study delineates the physical conditions that are responsible for the occurrence of main outcome regimes (i.e., bounce, coalescence, and breakup) for binary drop collisions with a precipitation microphysics perspective. Physical considerations based on the collision kinetic energy and the surface energies of the colliding drops lead to the development of a theoretical regime diagram for the drop/raindrop collision outcomes in the We- p plane ( We — Weber number, p — raindrop diameter ratio). This theoretical regime diagram is supported by laboratory experimental observations of drop collisions using high-speed imaging. Results of this fundamental study bring in new insights into the quantitative understanding of drop dynamics, applications of which extend beyond precipitation microphysics. In particular, results of this drop collision study are expected to give impetus to the physics-based dynamic modeling of the drop size distributions that is essential for various typical modern engineering applications, including numerical modeling of evolution of raindrop size distribution in rain shaft.

Investigation of Crustal Thickness in Eastern Anatolia Using Gravity, Magnetic and Topographic Data

NASA Astrophysics Data System (ADS)

Pamukçu, Oya Ankaya; Akçığ, Zafer; Demirbaş, Şevket; Zor, Ekrem

2007-12-01

The tectonic regime of Eastern Anatolia is determined by the Arabia-Eurasia continent-continent collision. Several dynamic models have been proposed to characterize the collision zone and its geodynamic structure. In this study, change in crustal thickness has been investigated using gravity, magnetic and topographic data of the region. In the first stage, two-dimensional low-pass filter and upward analytical continuation techniques were applied to the Bouguer gravity data of the region to investigate the behavior of the regional gravity anomalies. Next the moving window power spectrum method was used, and changes in the probable structural depths from 38 to 52 km were determined. The changes in crustal thickness where free air gravity and magnetic data have inversely correlated and the type of the anomaly resources were investigated applying the Euler deconvolution method to Bouguer gravity data. The obtained depth values are consistent with the results obtained using the power spectrum method. It was determined that the types of anomaly resources are different in the west and east of the 40° E longitude. Finally, using the obtained findings from this study and seismic velocity models proposed for this region by previous studies, a probable two-dimensional crust model was constituted.

Size dependence of the disruption threshold: laboratory examination of millimeter-centimeter porous targets

NASA Astrophysics Data System (ADS)

Nakamura, Akiko M.; Yamane, Fumiya; Okamoto, Takaya; Takasawa, Susumu

2015-03-01

The outcome of collision between small solid bodies is characterized by the threshold energy density Q*s, the specific energy to shatter, that is defined as the ratio of projectile kinetic energy to the target mass (or the sum of target and projectile) needed to produce the largest intact fragment that contains one half the target mass. It is indicated theoretically and by numerical simulations that the disruption threshold Q*s decreases with target size in strength-dominated regime. The tendency was confirmed by laboratory impact experiments using non-porous rock targets (Housen and Holsapple, 1999; Nagaoka et al., 2014). In this study, we performed low-velocity impact disruption experiments on porous gypsum targets with porosity of 65-69% and of three different sizes to examine the size dependence of the disruption threshold for porous material. The gypsum specimens were shown to have a weaker volume dependence on static tensile strength than do the non-porous rocks. The disruption threshold had also a weaker dependence on size scale as Q*s ∝D-γ , γ ≤ 0.25 - 0.26, while the previous laboratory studies showed γ=0.40 for the non-porous rocks. The measurements at low-velocity lead to a value of about 100 J kg-1 for Q*s which is roughly one order of magnitude lower than the value of Q*s for the gypsum targets of 65% porosity but impacted by projectiles with higher velocities. Such a clear dependence on the impact velocity was also shown by previous studies of gypsum targets with porosity of 50%.

Evaluation of minimum coverage size and orbital accuracy at different orbital regimes for one order of magnitude reduction of the catastrophic collision risk

NASA Astrophysics Data System (ADS)

Sánchez-Ortiz, Noelia; Domínguez-González, Raúl; Krag, Holger

2015-03-01

One of the main objectives of Space Surveillance and Tracking (SST) systems is to support space collision avoidance activities. This collision avoidance capability aims to significantly reduce the catastrophic collision risk of space objects. In particular, for the case of the future European SST, the objective is translated into a risk reduction of one order of magnitude whilst keeping a low number of false alarm events. In order to translate this aim into system requirements, an evaluation of the current catastrophic collision risk for different orbital regimes is addressed. The reduction of such risk depends on the amount of catalogued objects (coverage) and the knowledge of the associated orbits in the catalogue (accuracy). This paper presents an analysis of the impact of those two aspects in the capability to reduce the catastrophic collision risk at some orbital regimes. A reliable collision avoidance support depends on the accuracy of the predicted miss-events. The assessment of possible conjunctions is normally done by computing the estimated miss-distances between objects (which is compared with a defined distance threshold) or by computing the associated collision risk (which is compared with the corresponding accepted collision probability level). This second method is normally recommended because it takes into account the reliability of the orbits and allows reducing false alarm events. The collision risk depends on the estimated miss-distance, the object sizes and the accuracy of the two orbits at the time of event. This accuracy depends on the error of the orbits at the orbit determination epoch and the error derived from the propagation from that epoch up to the time of event. The modified DRAMA ARES (Domínguez-González et al., 2012, 2013a,b; Gelhaus et al., 2014) provides information on the expected number of encounters for a given mission and year. It also provides information on the capacity to reduce the risk of collision by means of avoidance manoeuvres as a function of the accepted collision probability level and the cataloguing performance of the surveillance system (determined by the limiting coverage size-altitude function and the orbital data accuracy). The assessment of avoidance strategies takes into account statistical models of the space object environment, as provided by ESA's MASTER-2009 model, and a mathematical framework for the collision risk estimation as used in satellite operations. In this papers, results are provided for some orbit types, covering different orbital regimes. The analysis is done for different cataloguing capacity levels (accuracy and coverage), concluding that 5 cm are to be covered at LEO for diminishing the catastrophic collision risk by one order of magnitude. For MEO and GEO regime, coverage down to 40 and 100 cm respectively allow similar reduction of risk.

Colliding-Droplet Microreactor: Rapid On-Demand Inertial Mixing and Metal-Catalyzed Aqueous Phase Oxidation Processes.

PubMed

Davis, Ryan D; Jacobs, Michael I; Houle, Frances A; Wilson, Kevin R

2017-11-21

In-depth investigations of the kinetics of aqueous chemistry occurring in microdroplet environments require experimental techniques that allow a reaction to be initiated at a well-defined point in time and space. Merging microdroplets of different reactants is one such approach. The mixing dynamics of unconfined (airborne) microdroplets have yet to be studied in detail, which is an essential step toward widespread use and application of merged droplet microreactors for monitoring chemical reactions. Here, we present an on-demand experimental approach for initiating chemical reactions in and characterizing the mixing dynamics of colliding airborne microdroplets (40 ± 5 μm diameter) using a streak-based fluorescence microscopy technique. The advantages of this approach include the ability to generate two well-controlled monodisperse microdroplet streams and collide (and thus mix) the microdroplets with high spatial and temporal control while consuming small amounts of sample (<0.1 μL/s). Mixing times are influenced not only by the velocity at which microdroplets collide but also the geometry of the collision (i.e., head-on vs off-center collision). For head-on collisions, we achieve submillisecond mixing times ranging from ∼900 μs at a collision velocity of 0.1 m/s to <200 μs at ∼6 m/s. For low-velocity (<1 m/s) off-center collisions, mixing times were consistent with the head-on cases. For high-velocity (i.e., > 1 m/s) off-center collisions, mixing times increased by as much as a factor of 6 (e.g., at ∼6 m/s, mixing times increased from <200 μs for head-on collisions to ∼1200 μs for highly off-center collisions). At collision velocities >7 m/s, droplet separation and fragmentation occurred, resulting in incomplete mixing. These results suggest a limited range of collision velocities over which complete and rapid mixing can be achieved when using airborne merged microdroplets to, e.g., study reaction kinetics when reaction times are short relative to typical bulk reactor mixing times. We benchmark our reactor using an aqueous-phase oxidation reaction: iron-catalyzed hydroxyl radical production from hydrogen peroxide (Fenton's reaction) and subsequent aqueous-phase oxidation of organic species in solution. Kinetic simulations of our measurements show that quantitative agreement can be obtained using known bulk-phase kinetics for bimolecular reactions in our colliding-droplet microreactor.

Colliding-Droplet Microreactor: Rapid On-Demand Inertial Mixing and Metal-Catalyzed Aqueous Phase Oxidation Processes

DOE PAGES

Davis, Ryan D.; Jacobs, Michael I.; Houle, Frances A.; ...

2017-10-30

In-depth investigations of the kinetics of aqueous chemistry occurring in microdroplet environments require experimental techniques that allow a reaction to be initiated at a well-defined point in time and space. Merging microdroplets of different reactants is one such approach. The mixing dynamics of unconfined (airborne) microdroplets have yet to be studied in detail, which is an essential step toward widespread use and application of merged droplet microreactors for monitoring chemical reactions. Here, we present an on-demand experimental approach for initiating chemical reactions in and characterizing the mixing dynamics of colliding airborne microdroplets (40 ± 5 μm diameter) using a streak-basedmore » fluorescence microscopy technique. The advantages of this approach include the ability to generate two well-controlled monodisperse microdroplet streams and collide (and thus mix) the microdroplets with high spatial and temporal control while consuming small amounts of sample (<0.1 μL/s). Mixing times are influenced not only by the velocity at which microdroplets collide but also the geometry of the collision (i.e., head-on vs off-center collision). For head-on collisions, we achieve submillisecond mixing times ranging from ~900 μs at a collision velocity of 0.1 m/s to <200 μs at ~6 m/s. For low-velocity (<1 m/s) off-center collisions, mixing times were consistent with the head-on cases. For high-velocity (i.e., > 1 m/s) off-center collisions, mixing times increased by as much as a factor of 6 (e.g., at ~6 m/s, mixing times increased from <200 μs for head-on collisions to ~1200 μs for highly off-center collisions). At collision velocities >7 m/s, droplet separation and fragmentation occurred, resulting in incomplete mixing. These results suggest a limited range of collision velocities over which complete and rapid mixing can be achieved when using airborne merged microdroplets to, e.g., study reaction kinetics when reaction times are short relative to typical bulk reactor mixing times. We benchmark our reactor using an aqueous-phase oxidation reaction: iron-catalyzed hydroxyl radical production from hydrogen peroxide (Fenton's reaction) and subsequent aqueous-phase oxidation of organic species in solution. In conclusion, kinetic simulations of our measurements show that quantitative agreement can be obtained using known bulk-phase kinetics for bimolecular reactions in our colliding-droplet microreactor.« less

Colliding-Droplet Microreactor: Rapid On-Demand Inertial Mixing and Metal-Catalyzed Aqueous Phase Oxidation Processes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davis, Ryan D.; Jacobs, Michael I.; Houle, Frances A.

In-depth investigations of the kinetics of aqueous chemistry occurring in microdroplet environments require experimental techniques that allow a reaction to be initiated at a well-defined point in time and space. Merging microdroplets of different reactants is one such approach. The mixing dynamics of unconfined (airborne) microdroplets have yet to be studied in detail, which is an essential step toward widespread use and application of merged droplet microreactors for monitoring chemical reactions. Here, we present an on-demand experimental approach for initiating chemical reactions in and characterizing the mixing dynamics of colliding airborne microdroplets (40 ± 5 μm diameter) using a streak-basedmore » fluorescence microscopy technique. The advantages of this approach include the ability to generate two well-controlled monodisperse microdroplet streams and collide (and thus mix) the microdroplets with high spatial and temporal control while consuming small amounts of sample (<0.1 μL/s). Mixing times are influenced not only by the velocity at which microdroplets collide but also the geometry of the collision (i.e., head-on vs off-center collision). For head-on collisions, we achieve submillisecond mixing times ranging from ~900 μs at a collision velocity of 0.1 m/s to <200 μs at ~6 m/s. For low-velocity (<1 m/s) off-center collisions, mixing times were consistent with the head-on cases. For high-velocity (i.e., > 1 m/s) off-center collisions, mixing times increased by as much as a factor of 6 (e.g., at ~6 m/s, mixing times increased from <200 μs for head-on collisions to ~1200 μs for highly off-center collisions). At collision velocities >7 m/s, droplet separation and fragmentation occurred, resulting in incomplete mixing. These results suggest a limited range of collision velocities over which complete and rapid mixing can be achieved when using airborne merged microdroplets to, e.g., study reaction kinetics when reaction times are short relative to typical bulk reactor mixing times. We benchmark our reactor using an aqueous-phase oxidation reaction: iron-catalyzed hydroxyl radical production from hydrogen peroxide (Fenton's reaction) and subsequent aqueous-phase oxidation of organic species in solution. In conclusion, kinetic simulations of our measurements show that quantitative agreement can be obtained using known bulk-phase kinetics for bimolecular reactions in our colliding-droplet microreactor.« less

Particle-in-cell simulation of an electronegative plasma under direct current bias studied in a large range of electronegativity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oudini, N.; Laboratoire des plasmas de Decharges, Centre de Developement des Technologies Avancees, Cite du 20 Aout BP 17 Baba Hassen, 16081 Algiers; Raimbault, J.-L.

2013-04-15

A one-dimensional electronegative plasma situated between two symmetrical parallel electrodes under DC bias is studied by Particle-In-Cell simulation with Monte Carlo Collisions. By varying the electronegativity {alpha}{identical_to}n{sub -}/n{sub e} from the limit of electron-ion plasmas (negative ion free) to ion-ion plasmas (electron free), the sheaths formation, the negative ion flux flowing towards the electrodes, and the particle velocities at the sheath edges are investigated. Depending on {alpha}, it is shown that the electronegative plasma behavior can be described by four regimes. In the lowest regime of {alpha}, i.e., {alpha} < 50, negative ions are confined by two positive sheaths withinmore » the plasma, while in the higher regimes of {alpha}, a negative sheath is formed and the negative ion flux can be extracted from the bulk plasma. In the two intermediate regimes of {alpha}, i.e., 50 < {alpha} < 10{sup 5}, both the electron and the negative ion fluxes are involved in the neutralization of the positive ions flux that leaves the plasma. In particular, we show that the velocity of the negative ions entering the negative sheath is affected by the presence of the electrons, and is not given by the modified Bohm velocity generally accepted for electronegative plasmas. For extremely high electronegativity, i.e., {alpha} > 10{sup 5}, the presence of electrons in the plasma is marginal and the electronegative plasma can be considered as an ion-ion plasma (electron free).« less

Anomalous inverse bremsstrahlung heating of laser-driven plasmas

NASA Astrophysics Data System (ADS)

Kundu, Mrityunjay

2016-05-01

Absorption of laser light in plasma via electron-ion collision (inverse bremsstrahlung) is known to decrease with the laser intensity as I 0 -3/2 or with the electron temperature as T e -3/2 where Coulomb logarithm ln Λ = 0.5ln(1 + k 2 min/k 2 max) in the expression of electron-ion collision frequency v ei is assumed to be independent of ponderomotive velocity v 0 = E0/ω which is unjustified. Here k -1 min = v th/max(ω, ω p), and k -1 max = Z/v 2 th are maximum and minimum cut-off distances of the colliding electron from the ion, v th = √T e is its thermal velocity, ω, ω p are laser and plasma frequency. Earlier with a total velocity v = (v 2 0 + v 2 th)1/2 dependent ln Λ(v) it was reported that v ei and corresponding fractional laser absorption (α) initially increases with increasing intensity, reaches a maximum value, and then fall according to the conventional I 0 -3/2 scaling. This anomalous increase in v ei and α may be objected due to an artifact introduced in ln Λ(v) through k-1 min ∝ v. Here we show similar anomalous increase of v ei and α versus I 0 (in the low temperature and under-dense density regime) with quantum and classical kinetic models of v ei without using ln Λ, but a proper choice of the total velocity dependent inverse cut-off length kmax -1 ∝ v 2 (in classical case) or kmax ∝ v (in quantum case). For a given I 0 < 5 × 1014Wcm-2, v ei versus T e also exhibits so far unnoticed identical anomalous increase as v ei versus Io, even if the conventional k max ∝ v2 th, or k max ∝ v th is chosen. However, for higher T e > 15 eV, anomalous growth of vei and a disappear. The total velocity dependent k max in kinetic models, as proposed here, may explain anomalous increase of a with I 0 measured in some earlier laser-plasma experiments. This work may be important to understand collisional absorption in the under-dense pre-plasma region due to low intensity pre-pulses and amplified spontaneous emission (ASE) pedestal in the context of laser induced inertial confinement fusion.

Thermophoretic force on nonspherical particles in the free-molecule regime

NASA Astrophysics Data System (ADS)

Yu, Song; Wang, Jun; Xia, Guodong; Zong, Luxiang

2018-05-01

The present paper is devoted to studying the thermophoresis of a nonspherical convex particle suspended in a gas with nonuniform temperature distribution in the free-molecule regime. Based on the gas kinetic theory and the assumption of a rigid-body collision for the gas-particle interaction, analytical expressions for the thermophoretic forces are obtained for several typical nonspherical bodies, including cylinders, spheroids, needles, disks, and cuboids. The orientation dependences of the thermophoretic forces and thermophoretic velocities are evaluated based on these expressions. It is found that the influence of the pitching effect of the nonspheres can be significant. The expressions for the orientation-averaged thermophoretic forces are also obtained under the assumption of a uniform particle orientation distribution.

Effect of velocity-dependent friction on multiple-vehicle collisions in traffic flow

NASA Astrophysics Data System (ADS)

Nagatani, Takashi

2017-01-01

We present the dynamic model for the multiple-vehicle collisions to take into account the velocity-dependent friction force. We study the effect of the velocity-dependent friction on the chain-reaction crash on a road. In the traffic situation, drivers brake according to taillights of the forward vehicle and the friction force depends highly on the vehicular speed. The first crash may induce more collisions. We investigate whether or not the first collision induces the multiple-vehicle collisions, numerically and analytically. The dynamic transitions occur from no collisions, through a single collision and double collisions, to multiple collisions with decreasing the headway. We explore the effect of the velocity-dependent friction on the dynamic transitions and the region maps in the multiple-vehicle collisions.

When Moons Collide

NASA Astrophysics Data System (ADS)

Rufu, Raluca; Aharonson, Oded

2017-10-01

Impacts between two orbiting satellites is a natural consequence of Moon formation. Mergers between moonlets are especially important for the newly proposed multiple-impact hypothesis as these moonlets formed from different debris disks merge together to form the final Moon. However, this process is relevant also for the canonical giant impact, as previous work shows that multiple moonlets are formed from the same debris disk.The dynamics of impacts between two orbiting bodies is substantially different from previously heavily studied planetary-sized impacts. Firstly, the impact velocities are smaller and limited to, thus heating is limited. Secondly, both fragments have similar mass therefore, they would contribute similarly and substantially to the final satellite. Thirdly, this process can be more erosive than planetary impacts as the velocity of ejected material required to reach the mutual Hill sphere is smaller than the escape velocity, altering the merger efficiency. Previous simulations show that moonlets inherit different isotopic signatures from their primordial debris disk, depending on the parameters of the collision with the planet. We therefore, evaluate the degree of mixing in moonlet-moonlet collisions in the presence of a planetary gravitational field, using Smooth Particle Hydrodynamics (SPH). Preliminary results show that the initial thermal state of the colliding moonlets has only a minor influence on the amount of mixing, compared to the effects of velocity and impact angle over their likely ranges. For equal mass bodies in accretionary collisions, impact angular momentum enhances mixing. In the hit-and-run regime, only small amounts of material are transferred between the bodies therefore mixing is limited. Overall, these impacts can impart enough energy to melt ~15-30% of the mantle extending the magma ocean phase of the final Moon.

Scattering of the double sine-Gordon kinks

NASA Astrophysics Data System (ADS)

Gani, Vakhid A.; Marjaneh, Aliakbar Moradi; Askari, Alidad; Belendryasova, Ekaterina; Saadatmand, Danial

2018-04-01

We study the scattering of kink and antikink of the double sine-Gordon model. There is a critical value of the initial velocity v_{{cr}} of the colliding kinks, which separates different regimes of the collision. At v_{in}>v_{cr} we observe kinks reflection, while at v_{in}

A Monte Carlo simulation of the effect of ion self-collisions on the ion velocity distribution function in the high-latitude F-region

NASA Technical Reports Server (NTRS)

Barghouthi, I. A.; Barakat, A. R.; Schunk, R. W.

1994-01-01

Non-Maxwellian ion velocity distribution functions have been theoretically predicted and confirmed by observations, to occur at high latitudes. These distributions deviate from Maxwellian due to the combined effect of the E x B drift and ion-neutral collisions. At high altitude and/or for solar maximum conditions, the ion-to-neutral density ratio increases and, hence, the role of ion self-collisions becomes appreciable. A Monte Carlo simulation was used to investigate the behavior of O(+) ions that are E x B-drifting through a background of neutral O, with the effect of O(+) (Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n(sub i)/n(sub n) and the electrostatic field E were considered in order to investigate the change of ion behavior with solar cycle and with altitude. For low altitudes and/or solar minimum (n(sub i)/n(sub n) less than or equal to 10(exp -5)), the effect of self-collisions is negligible. For higher values of n(sub i)/n(sub n), the effect of self-collisions becomes significant and, hence, the non-Maxwellian features of the O(+) distribution are reduced. The Monte Carlo results were compared to those that used simplified collision models in order to assess their validity. In general, the simple collision models tend to be more accurate for low E and for high n(sub i)/n(sub n).

Blowing in the Wind: I. Velocities of Chondrule-sized Particles in a Turbulent Protoplanetary Nebula

NASA Technical Reports Server (NTRS)

Cuzzi, Jeffrey N.; Hogan, Robert C.; Fonda, Mark (Technical Monitor)

2003-01-01

Small but macroscopic particles - chondrules, higher temperature mineral inclusions, metal grains, and their like - dominate the fabric of primitive meteorites. The properties of these constituents, and their relationship to the fine dust grains which surround them, suggest that they led an extended existence in a gaseous protoplanetary nebula prior to their incorporation into their parent primitive bodies. In this paper we explore in some detail the velocities acquired by such particles in a turbulent nebula. We treat velocities in inertial space (relevant to diffusion), velocities relative to the gas and entrained microscopic dust (relevant to accretion of dust rims), and velocities relative to each other (relevant to collisions). We extend previous work by presenting explicit, closed-form solutions for the magnitude and size dependence of these velocities in this important particle size regime, and compare these expressions with new numerical calculations. The magnitude and size dependence of these velocities have immediate applications to chondrule and CAI rimming by fine dust, and to their diffusion in the nebula, which we explore separately.

Impact experiments of porous gypsum-glass bead mixtures simulating parent bodies of ordinary chondrites: Implications for re-accumulation processes related to rubble-pile formation

NASA Astrophysics Data System (ADS)

Yasui, M.; Arakawa, M.

2011-12-01

Most of asteroids are expected to be impact fragments produced by collisions among planetesimals or rubble-pile bodies produced by re-accumulation of fragments. In order to study the formation processes of asteroids, it is necessary to examine the collisional disruption and re-accumulation conditions of planetesimals. Most of meteorites recovered on the Earth are ordinary chondrites (OCs). The OCs have the components of millimeter-sized round grains (chondrules) and submicron-sized dusts (matrix). So, the planetesimals forming the parent bodies of OCs could be mainly composed of chondrules and matrix. Therefore, we conducted impact experiments with porous gypsum mixed with glass beads having the spherical shape with various diameters simulating chondrules, and examined the effect of chondrules on the ejecta velocity and the impact strength. The targets included glass beads with a diameter ranging from 100 μm to 3 mm and the volume fraction was 0.6, similar to that of ordinary chondrites, which is about 0.65-0.75. We also prepared the porous gypsum sample without glass bead to examine the effect of volume fraction. Nylon projectiles with the diameters of 10 mm and 2 mm were impacted at 60-180 m/s by a single-stage gas gun and at about 4 km/s by a two-stage light gas gun, respectively. After the shot, we measured the mass of the recovered fragments to calculate the impact strength Q defined by Q=mpVi^2/2(mp+Mt), where Vi is the impact velocity, and mp and Mt are the mass of projectile and target, respectively. The collisional disruption of the target was observed by a high-speed video camera to measure the ejecta velocity. The antipodal velocity Va increased with the increase of Q, irrespective of glass bead size and volume fraction. However, the Va for low-velocity collisions at 60-180 m/s was an order magnitude larger than that for high-velocity collisions at 4 km/s. The velocities of fragments ejected from two corners on the impact surface of the target Vc-g measured in the center of the mass system, were independent on the target materials. The impact strength of the mixture target was found to range from 56 to 116 J/kg depending on the glass bead size, and was several times smaller than that of the gypsum target, 446 J/kg in low-velocity collisions. The impact strengths of the 100 μm bead target and the gypsum target strongly depended on the impact velocity: those obtained in high-velocity collisions were several times greater than those obtained in low-velocity collisions. The obtained results of Vc-g were compared to the escape velocity of chondrule-including planetesimals (CiPs) to study the conditions for the formation of rubble-pile bodies after the catastrophic disruption. The fragments of CiPs for catastrophic disruption could be re-accumulated at the radius of a body larger than 3 km, irrespective of chondrule size included in the CiPs, which is rather smaller than that for basalt bodies. Thus, we suggested that there were more parent bodies of OCs having a rubble-pile structure.

Influence of velocity effects on the shape of N2 (and air) broadened H2O lines revisited with classical molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Ngo, N. H.; Tran, H.; Gamache, R. R.; Bermejo, D.; Domenech, J.-L.

2012-08-01

The modeling of the shape of H2O lines perturbed by N2 (and air) using the Keilson-Storer (KS) kernel for collision-induced velocity changes is revisited with classical molecular dynamics simulations (CMDS). The latter have been performed for a large number of molecules starting from intermolecular-potential surfaces. Contrary to the assumption made in a previous study [H. Tran, D. Bermejo, J.-L. Domenech, P. Joubert, R. R. Gamache, and J.-M. Hartmann, J. Quant. Spectrosc. Radiat. Transf. 108, 126 (2007)], 10.1016/j.jqsrt.2007.03.009, the results of these CMDS show that the velocity-orientation and -modulus changes statistically occur at the same time scale. This validates the use of a single memory parameter in the Keilson-Storer kernel to describe both the velocity-orientation and -modulus changes. The CMDS results also show that velocity- and rotational state-changing collisions are statistically partially correlated. A partially correlated speed-dependent Keilson-Storer model has thus been used to describe the line-shape. For this, the velocity changes KS kernel parameters have been directly determined from CMDS, while the speed-dependent broadening and shifting coefficients have been calculated with a semi-classical approach. Comparisons between calculated spectra and measurements of several lines of H2O broadened by N2 (and air) in the ν3 and 2ν1 + ν2 + ν3 bands for a wide range of pressure show very satisfactory agreement. The evolution of non-Voigt effects from Doppler to collisional regimes is also presented and discussed.

Electron-positron pair production in ion collisions at low velocity beyond Born approximation

NASA Astrophysics Data System (ADS)

Lee, R. N.; Milstein, A. I.

2016-10-01

We derive the spectrum and the total cross section of electromagnetic e+e- pair production in the collisions of two nuclei at low relative velocity β. Both free-free and bound-free e+e- pair production is considered. The parameters ηA,B =ZA,B α are assumed to be small compared to unity but arbitrary compared to β (ZA,B are the charge numbers of the nuclei and α is the fine structure constant). Due to a suppression of the Born term by high power of β, the first Coulomb correction to the amplitude appears to be important at ηA,B ≳ β. The effect of a finite nuclear mass is discussed. In contrast to the result obtained in the infinite nuclear mass limit, the terms ∝M-2 are not suppressed by the high power of β and may easily dominate at sufficiently small velocities.

Velocity-dependent quantum phase slips in 1D atomic superfluids.

PubMed

Tanzi, Luca; Scaffidi Abbate, Simona; Cataldini, Federica; Gori, Lorenzo; Lucioni, Eleonora; Inguscio, Massimo; Modugno, Giovanni; D'Errico, Chiara

2016-05-18

Quantum phase slips are the primary excitations in one-dimensional superfluids and superconductors at low temperatures but their existence in ultracold quantum gases has not been demonstrated yet. We now study experimentally the nucleation rate of phase slips in one-dimensional superfluids realized with ultracold quantum gases, flowing along a periodic potential. We observe a crossover between a regime of temperature-dependent dissipation at small velocity and interaction and a second regime of velocity-dependent dissipation at larger velocity and interaction. This behavior is consistent with the predicted crossover from thermally-assisted quantum phase slips to purely quantum phase slips.

A cryofuge for cold-collision experiments with slow polar molecules

NASA Astrophysics Data System (ADS)

Wu, Xing; Gantner, Thomas; Koller, Manuel; Zeppenfeld, Martin; Chervenkov, Sotir; Rempe, Gerhard

2017-11-01

Ultracold molecules represent a fascinating research frontier in physics and chemistry, but it has proven challenging to prepare dense samples at low velocities. Here, we present a solution to this goal by means of a nonconventional approach dubbed cryofuge. It uses centrifugal force to bring cryogenically cooled molecules to kinetic energies below 1 K × kB in the laboratory frame, where kB is the Boltzmann constant, with corresponding fluxes exceeding 1010 per second at velocities below 20 meters per second. By attaining densities higher than 109 per cubic centimeter and interaction times longer than 25 milliseconds in samples of fluoromethane as well as deuterated ammonia, we observed cold dipolar collisions between molecules and determined their collision cross sections.

Collisions of energetic particles with atoms, molecules & solids: A theoretical study

NASA Astrophysics Data System (ADS)

Quashie, Edwin Exam

The detailed knowledge of the accurate ion-solid interaction is at the heart of many technological applications such as nuclear safety, applied material science, medical physics and fusion and fission applications. Its accurate evaluation poses an enormous challenge due to the need of incorporating electronic structure, bound states, size effects, basis sets, and the quantum classical aspects of the problem. Most recent approaches relying on the fitting to experimental data or phenomenological model, fail to describe the ion-solid interaction properly (see [S. N. Markin, D. Primetzhofer, M. Spitz, and P. Bauer, Phys. Rev. B 80 (2009)]) for slow ions. A general Time-Dependent Density Functional Theory (TDDFT) is used in this thesis to evaluate electron-dynamics easily. For the first time a unified theory is proposed to describe the ion-solid interaction accurately over several orders of magnitude in the ion velocities, unveiling different regimes that before were only partially seen by separate experiments and rarely by any level of existing theory. We identified an electronic stopping which in the band-regime produces a quantum friction that is nonlinear with a power-law with an exponent ˜1.5. At low velocity this nonlinear effect will provide a new impetus for experimental investigations and an improve microscopic models of electron-ion dissipative dynamics. Our study will potentially impact both the experimental and theoretical research in condensed matter. We have applied our developed theory to study stopping of H+ in Cu. The target Cu comprises complicated band structure and this system will help to understand radiation of matter, both in its experimental understanding and also in the modeling of the process, for example in the context of damped molecular dynamics for the simulation of radiation cascades. At this present stage in the field of ion-solid interactions and quantum dissipative dynamics, our findings remain very significant. The same techniques are used in studying the ion-molecule interactions at lower ion velocities. We reported here H+ + CH4 collision dynamics at E = 30 eV. Different exchange-correlation (XC) approximations were implemented and their important roles are studied systematically. For a single orientation of CH4 our rainbow angle at E = 30 eV agrees well with experimental and other theoretical values.

Pn tomography with Moho depth correction from eastern Europe to western China

NASA Astrophysics Data System (ADS)

Lü, Yan; Ni, Sidao; Chen, Ling; Chen, Qi-Fu

2017-02-01

We proposed a modified Pn velocity and anisotropy tomography method by considering the Moho depth variations using the Crust 1.0 model and obtained high-resolution images of the uppermost mantle Pn velocity and anisotropy structure from eastern Europe to western China. The tomography results indicate that the average Pn velocities are approximately 8.0 and 8.1 km/s under the western and eastern parts of the study area, respectively, with maximum velocity perturbations of 3%-4%. We observed high Pn velocities under the Adriatic Sea, Black Sea, Caspian Sea, Arabian Plate, Indian Plate, and in the Tarim and Sichuan Basins but low Pn velocities under the Apennine Peninsula, Dead Sea fault zone, Anatolia, Caucasus, Iranian Plateau, Hindu Kush, and in the Yunnan and Myanmar regions. Generally, regions with stable structures and low lithospheric temperatures exhibit high Pn velocities. Low Pn velocities provide evidence for the upwelling of hot material, which is associated with plate subduction and continental collision processes. Our Pn velocity and anisotropy imaging results indicate that the Adriatic microplate dives to the east and west, the hot material upwelling caused by subduction beneath the Tibetan Plateau is not as significant as that in the Caucasus and Myanmar regions, the lithosphere exhibits coupled rotational movement around the Eastern Himalayan syntaxes, and the areas to the north and south of 26°N in the Yunnan region are affected by different geodynamic processes. Our newly captured images of the uppermost mantle velocity and anisotropy structure provide further information about continental collision processes and associated dynamic mechanisms.

A comprehensive assessment of collision likelihood in Geosynchronous Earth Orbit

NASA Astrophysics Data System (ADS)

Oltrogge, D. L.; Alfano, S.; Law, C.; Cacioni, A.; Kelso, T. S.

2018-06-01

Knowing the likelihood of collision for satellites operating in Geosynchronous Earth Orbit (GEO) is of extreme importance and interest to the global community and the operators of GEO spacecraft. Yet for all of its importance, a comprehensive assessment of GEO collision likelihood is difficult to do and has never been done. In this paper, we employ six independent and diverse assessment methods to estimate GEO collision likelihood. Taken in aggregate, this comprehensive assessment offer new insights into GEO collision likelihood that are within a factor of 3.5 of each other. These results are then compared to four collision and seven encounter rate estimates previously published. Collectively, these new findings indicate that collision likelihood in GEO is as much as four orders of magnitude higher than previously published by other researchers. Results indicate that a collision is likely to occur every 4 years for one satellite out of the entire GEO active satellite population against a 1 cm RSO catalogue, and every 50 years against a 20 cm RSO catalogue. Further, previous assertions that collision relative velocities are low (i.e., <1 km/s) in GEO are disproven, with some GEO relative velocities as high as 4 km/s identified. These new findings indicate that unless operators successfully mitigate this collision risk, the GEO orbital arc is and will remain at high risk of collision, with the potential for serious follow-on collision threats from post-collision debris when a substantial GEO collision occurs.

Finite-Frequency Seismic Imaging of Upper-Mantle Velocity Structures Beneath the South China Continent

NASA Astrophysics Data System (ADS)

Qu, P.; Chen, Y. J.; Yu, Y.

2017-12-01

South China Continent is major formed from the Paleo-South China plate. The continent has experienced complicated tectonic history after Neoproterozoic. Previous studies suggested some possible model for the collision between South China Continent and North China Continent. Body wave tomography and surface wave tomography are widely used to inverse upper mantle velocity structure. In our study, finite frequency tomography were carried on to get explanation more correctly. We gathered nearly 60000 pieces of teleseismic event records by 166 broad band seismic stations with Mw > 5.5. Here sensitive kernel of ak135 velocity structure was calculated, which is based on Born approximation, and then we applied multi-channel cross-correlation to pick arrival time difference under 3 frequency band. Combining with crust thickness correct from receiver function, we solve the inversion matrix by LSQR method, and get accurate upper mantle structure of P, S velocity. For more accurate results, we apply a method to calculate Vp/Vs ratio, to help to verify the velocity anomaly. The result in this research shows: 1. A strong velocity anomaly exists in the northern of South China Continent, in an area 31°N between 112°-118°E. The anomaly is about . We suggest that, this anomaly is related to the collision from North China Continent. It implies the collision underthrusted to southward. 2. A clearly slow velocity anomaly exists in the northern of Cathaysia block. This low velocity anomaly exist on the boundary of Yangtz block and Cathysian block, it is related to the left over of block collision in early phanerozoic. 3. We recognized some little velocity anomaly exit in the research area. Comparing these velocity anomaly with U-Pb zircon ages, we suggest complicated orogenesis in Phanerozoic is the cause of the formation of these little anomaly. The result in our study support the collision model, which shows the underthrust direction is southward, on the south of Qinling-Dabie Orogen. The anomaly mass is larger than the composite orogenic in Yangtze block.

Molecular Collision Processes in Gases and at Surfaces.

DTIC Science & Technology

1987-12-12

expansion and reached the collisionless flow regime so that no further changes in state (due to collision) can occur; (2) Those in which two free jets...this free jet experiment seems to exhibit. (5) To us these results fairly cry out for further investigation. In another variation on the corona...produce I high dimer concentrations is to combine high pressures with low temperatures, in diametric opposition to the conventional practice we have just

An adaptive, conservative 0D-2V multispecies Rosenbluth–Fokker–Planck solver for arbitrarily disparate mass and temperature regimes

DOE PAGES

Taitano, William; Chacon, Luis; Simakov, Andrei Nikolaevich

2016-04-25

In this paper, we propose an adaptive velocity-space discretization scheme for the multi-species, multidimensional Rosenbluth–Fokker–Planck (RFP) equation, which is exactly mass-, momentum-, and energy-conserving. Unlike most earlier studies, our approach normalizes the velocity-space coordinate to the temporally varying individual plasma species' local thermal velocity, v th (t), and explicitly considers the resulting inertial terms in the Fokker–Planck equation. Our conservation strategy employs nonlinear constraints to enforce discretely the conservation properties of these inertial terms and the Fokker–Planck collision operator. To deal with situations of extreme thermal velocity disparities among different species, we employ an asymptotic v th -ratio-based expansion ofmore » the Rosenbluth potentials that only requires the computation of several velocity-space integrals. Numerical examples demonstrate the favorable efficiency and accuracy properties of the scheme. Specifically, we show that the combined use of the velocity-grid adaptivity and asymptotic expansions delivers many orders-of-magnitude savings in mesh resolution requirements compared to a single, static uniform mesh.« less

A cloud collision model for water maser excitation.

PubMed

Tarter, J C; Welch, W J

1986-06-01

High-velocity collisions between small, dense, neutral clouds or between a dense cloud and a dense shell can provide the energy source required to excite H2O maser emission. The radiative precursor from the surface of the collisional shock front rapidly diffuses through the cloud, heating the dust grains but leaving the H2 molecules cool. Transient maser emission occurs as the conditions for the Goldreich and Kwan "hot-dust cold-gas" maser pump scheme are realized locally within the cloud. In time the local maser action quenches due to the heating of the H2 molecules by collisions against the grains. Although this model cannot explain the very long-lived steady maser features, it is quite successful in explaining a number of the observed properties of the high-velocity features in such sources as Orion, W51, and W49. In particular, it provides a natural explanation for the rapid time variations, the narrow line widths, juxtaposition of high- and low-velocity features, and the short lifetimes which are frequently observed for the so-called high-velocity maser "bullets" thought to be accelerated by strong stellar winds.

Ultrasonic waves in classical gases

NASA Astrophysics Data System (ADS)

Magner, A. G.; Gorenstein, M. I.; Grygoriev, U. V.

2017-12-01

The velocity and absorption coefficient for the plane sound waves in a classical gas are obtained by solving the Boltzmann kinetic equation, which describes the reaction of the single-particle distribution function to a periodic external field. Within the linear response theory, the nonperturbative dispersion equation valid for all sound frequencies is derived and solved numerically. The results are in agreement with the approximate analytical solutions found for both the frequent- and rare-collision regimes. These results are also in qualitative agreement with the experimental data for ultrasonic waves in dilute gases.

Experimental observation of strong coupling effects on the dispersion of dust acoustic waves in a plasma

NASA Astrophysics Data System (ADS)

Bandyopadhyay, P.; Prasad, G.; Sen, A.; Kaw, P. K.

2007-09-01

The dispersion properties of low frequency dust acoustic waves in the strong coupling regime are investigated experimentally in an argon plasma embedded with a mixture of kaolin and MnO2 dust particles. The neutral pressure is varied over a wide range to change the collisional properties of the dusty plasma. In the low collisional regime the turnover of the dispersion curve at higher wave numbers and the resultant region of ∂ω/∂k<0 are identified as signatures of dust dust correlations. In the high collisional regime dust neutral collisions produce a similar effect and prevent an unambiguous identification of strong coupling effects.

Integrated structural model for active arc-continental collision from southern Taiwan to central Taiwan inferred from seismogenic views

NASA Astrophysics Data System (ADS)

Nagai, S.; Wang, Y.; Ma, K.; Wu, Y.; Huang, H.

2010-12-01

The Taiwan Island is located in ongoing arc-continent collision zone between the Philippine Sea Plate and the Eurasian Plate. Numerous geophysical and geological studies have explained the tectonic processes and developed various models. There are two end-member models for Taiwan collision; Thin-skinned model [e.g. Suppe, 1987] and Lithospheric collision model [Wu et al., 1997]. One of most important issue is that collision in Taiwan involves and contributes deformation to what depth. We have presented on this point in central Taiwan through seismological views, including both observed facts and results of tomographic inversion [Nagai et al., 2010 in WPGM 2010]. We have concluded that orogenic process should involve and contribute to depth of 30 km at least in central Taiwan, and suggested ’Upper Crustal Stacking Model’. We have indicated that existence of low-velocity blocks under Central Mountain Range (CMR) is one of the most important factors to understand Taiwan arc-continental collision process. We extend this idea to southern and northern Taiwan and to connect with balanced cross-sections proposed in Ustaszewski et al. [2010]. In this talk, we focused on the structural variation from southern Taiwan to central Taiwan. We have been performing the local double-difference tomography [Zhang and Thurber, 2003] in southern and northern Taiwan using the Central Weather Bureau Seismic Network with temporary array observations. These results in seismic tomography show variation of seismic velocity under the CMR like in central Taiwan. Low-velocity anomalies are also detected. Although it should be checked carefully, velocity blocks segmented some parts and seismic activities seem to be located on their boundaries. The Jiaxian earthquake on 4th Mar, 2010 occurred on one of these segmentation boundaries. The tectonic process in Taiwan arc-continental collision consists of different-scale structures, which means finer resolution structures may be nested with the larger ones. The thin-skinned model is the first step and the finest structural model in collision process, dominated in south and western Taiwan. After thin-skinned process, processes in the UCS model deformed and pushed up these finest structures in thin-skinned model, and then eroded on the CMR. Although this idea for integrated model for Taiwan orogeny should be verified more through geological and geophysical views both, it is pretty simple and can explain many features among various models between two end-members. Previous presentation for central Taiwan Nagai et al., (2010), Integrated structural model for arc-continent collision in Taiwan inferred from seismic velocity, relocated seismicity, and attenuation inverted by seismic tomography, Eos Trans. AGU, 91(26), West. Pac. Geophys. Meet. Suppl., Abstract T22A-06.

Galaxy Cluster Bulk Flows and Collision Velocities in QUMOND

NASA Astrophysics Data System (ADS)

Katz, Harley; McGaugh, Stacy; Teuben, Peter; Angus, G. W.

2013-07-01

We examine the formation of clusters of galaxies in numerical simulations of a QUMOND cosmogony with massive sterile neutrinos. Clusters formed in these exploratory simulations develop higher velocities than those found in ΛCDM simulations. The bulk motions of clusters attain ~1000 km s-1 by low redshift, comparable to observations whereas ΛCDM simulated clusters tend to fall short. Similarly, high pairwise velocities are common in cluster-cluster collisions like the Bullet Cluster. There is also a propensity for the most massive clusters to be larger in QUMOND and to appear earlier than in ΛCDM, potentially providing an explanation for "pink elephants" like El Gordo. However, it is not obvious that the cluster mass function can be recovered.

Explosion of a supernova with a red giant companion

NASA Technical Reports Server (NTRS)

Livne, E.; Tuchman, Y.; Wheeler, J. C.

1992-01-01

Two-dimensional numerical simulations of the collision between spherical ejecta from a supernova and a red giant companion are presented. In contrast to previous numerical studies, in which the companion was a main-sequence star or a compact object, the collision consequences are found to have a dramatic impact upon the red giant. In most cases the red giant companion loses most of its envelope in a time scale of 10 exp 7 s with typical velocities about an order of magnitude less than those of the expanding velocity of the supernova shell. We confirm the conclusion of Chugai (1986) that the stripped hydrogen tends to come off as a low-velocity component interior to the supernova ejecta. Possible observational consequences of the results are discussed.

Experimental investigation of the wake behind a rotating sphere

NASA Astrophysics Data System (ADS)

Skarysz, M.; Rokicki, J.; Goujon-Durand, S.; Wesfreid, J. E.

2018-01-01

The wake behind a sphere, rotating about an axis aligned with the streamwise direction, has been experimentally investigated in a low-velocity water tunnel using laser-induced fluorescence visualizations and particle image velocimetry measurements. The measurements focused on the evolution of the flow regimes that appear depending on two control parameters: the Reynolds number Re and the dimensionless rotation or swirl rate Ω , which is the ratio of the maximum azimuthal velocity of the body to the free-stream velocity. In the present investigation, we cover the range of Re smaller than 400 and Ω from 0 and 4. Different wakes regimes such as an axisymmetric flow, a low helical state, and a high helical mode are represented in the (Re, Ω ) parameter plane.

Analytic non-Maxwellian electron velocity distribution function in a Hall discharge plasma

NASA Astrophysics Data System (ADS)

Shagayda, Andrey; Tarasov, Alexey

2017-10-01

The electron velocity distribution function in the low-pressure discharges with the crossed electric and magnetic fields, which occur in magnetrons, plasma accelerators, and Hall thrusters with a closed electron drift, is not Maxwellian. A deviation from equilibrium is caused by a large electron mean free path relative to the Larmor radius and the size of the discharge channel. In this study, we derived in the relaxation approximation the analytical expression of the electron velocity distribution function in a weakly ionized Lorentz plasma with the crossed electric and magnetic fields in the presence of the electron density and temperature gradients in the direction of the electric field. The solution was obtained in the stationary approximation far from boundary surfaces, when diffusion and mobility are determined by the classical effective collision frequency of electrons with ions and atoms. The moments of the distribution function including the average velocity, the stress tensor, and the heat flux were calculated and compared with the classical hydrodynamic expressions. It was shown that a kinetic correction to the drift velocity stems from a contribution of the off-diagonal component of the stress tensor. This correction becomes essential if the drift velocity in the crossed electric and magnetic fields would be comparable to the thermal velocity of electrons. The electron temperature has three different components at a nonzero effective collision frequency and two different components in the limit when the collision frequency tends to zero. It is shown that, in the presence of ionization collisions, the components of the heat flux have additives that are not related to the temperature gradient, and arise because of the electron drift.

Formation of hollow nanoshells in solution-based reactions via collision coalescence of nanobubble-particle systems

NASA Astrophysics Data System (ADS)

Vongehr, Sascha; Tang, Shaochun

2016-06-01

Research on hollow nanoshells has, for years, claimed to involve free, pre-existing nanobubbles as soft templates. It is a challenge to demonstrate this due to the difficulty of in situ observation during solution-based reactions. We show that no available free-bubble theory can describe the mysterious behavior of the bubble number density n. A new mechanism of collision coalescence of bubble-particle systems is suggested to form hollow nanoshells. By approximating relative velocity as ˜R -z (R is bubble radius), numerical simulations can reproduce the counterintuitive observations in the regime 1 < z < 2. We discuss the mechanism based on successful synthesis of grain-monolayer thin, fractal-like incomplete, multi-metallic nanoshells with superior catalytic activity. The behaviors of n, R, and shell thickness h are closely reproduced by z = 1.6.

SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects

NASA Astrophysics Data System (ADS)

Emsenhuber, Alexandre; Jutzi, Martin; Benz, Willy

2018-02-01

We model large-scale ( ≈ 2000 km) impacts on a Mars-like planet using a Smoothed Particle Hydrodynamics code. The effects of material strength and of using different Equations of State on the post-impact material and temperature distributions are investigated. The properties of the ejected material in terms of escaping and disc mass are analysed as well. We also study potential numerical effects in the context of density discontinuities and rigid body rotation. We find that in the large-scale collision regime considered here (with impact velocities of 4 km/s), the effect of material strength is substantial for the post-impact distribution of the temperature and the impactor material, while the influence of the Equation of State is more subtle and present only at very high temperatures.

Rheology of serpentinite in high-temperature and low-slip-velocity regime

NASA Astrophysics Data System (ADS)

Takahashi, M.; Uehara, S.; Mizoguchi, K.; Takeda, N.; Masuda, K.

2009-12-01

This study was designed to clarify the rheology of serpentinite experimentally, related both the sliding velocity and the temperature. The frictional behavior of serpentinite is of particular interest in the study of earthquake generation processes along subducting plates and transform faults. Previous studies [Reinen et al., 1991-93] revealed that the serpentinites indicated two-mechanical behaviors at velocity-step test: ‘state-variable dominated behavior’ at relatively higher velocity (0.1-10 μm/sec) and ‘flow-dominated behavior’ at lower velocity (less than 0.1 μm/sec). Such complexity on the frictional behavior could make it complicated to forecast on the slip acceleration process from the plate motion velocity to the earthquake. Even under the room-temperature condition, those multiple behavior could be observed, thus, serpentinite can be a model substance to present a new constitutive law at the brittle-ductile transition regime. We, therefore, focus to discuss the transient behaviors of serpentinite at the velocity-step test. We used a gas-medium, high-pressure, and high-temperature triaxial testing machine belonging to the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Sliding deformation was applied on the thin zone of the serpentinite gouge (1.0 g of almost pure antigorite powder) sandwiched between two alumina blocks with oblique surfaces at 30° to the axis. All experiments were carried out under a set of constant conditions, 100 MPa of the confining pressure (Ar-gas) and 30 MPa of the pore pressure (distilled water). The temperature conditions were varied from the room-temperature to 500° C, and three sliding velocity-regimes were adopted: low (0.0115 - 0.115 μm/sec), middle (0.115 - 1.15 μm/sec) and high (1.15 - 11.5 μm/sec) velocity regimes. In each velocity regime, the sliding velocity was increased or decreased in a stepwise fashion, and then we observed the transient behaviors until it reached the new steady-state frictional strength. Most results showed velocity-strengthening and flow-like transient behavior. Roughly said, the degree of the velocity dependence became larger with increasing the temperature until 400° C, and became larger with decreasing the velocity. At the temperature condition from 300° C to 400° C, the increasing of the velocity dependence became conspicuous with decreasing the velocity. Moreover, just after the dehydration of the antigorite started (450° - 500° C), the friction behaved unstable sliding. The rheology of the serpentinite seemed to be “not simple” at this experimental conditions on this study.

Existence regimes for shocks in inhomogeneous magneto-plasmas having entropy

NASA Astrophysics Data System (ADS)

Iqbal, Javed; Yaqub Khan, M.

2018-04-01

The finding of connection of plasma density and temperature with entropy gives an incitement to study different plasma models with respect to entropy. Nonlinear dissipative one- and two-dimensional structures (shocks) are investigated in nonuniform magnetized plasma with respect to entropy. The dissipation comes in the medium through ion-neutral collisions. The linear dispersion relation is derived. The Korteweg-deVries-Burgers and Kadomtsev-Petviashvili-Burgers equations are derived for nonlinear drift waves in 1-D and 2-D by employing the drift approximation. It is found that vd/u ( vd is the diamagnetic drift velocity and u is the velocity of nonlinear structure) plays a significant role in the shock formation. It is also found that entropy has a significant effect on the strength of shocks. It is noticed that v d/u determines the rarefactive and compressive nature of the shocks. It is observed that upper and lower bounds exist for the shock velocity. It is also observed that the existing regimes for both one- and two-dimensional shocks for kappa distributed electrons are different from shocks with Cairns distributed electrons. Both rarefactive and compressive shocks are found for the 1-D drift waves with kappa distributed electrons. Interestingly, it is noticed that entropy enhances the strength of one- and two-dimensional shocks.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Syed, M. Bukhari; Blum, J.; Jansson, K. Wahlberg

Previous work on protoplanetary dust growth shows a halt at centimeter sizes owing to the occurrence of bouncing at velocities of ≳0.1 m s{sup −1} and fragmentation at velocities ≳1 m s{sup −1}. To overcome these barriers, spatial concentration of centimeter-sized dust pebbles and subsequent gravitational collapse have been proposed. However, numerical investigations have shown that dust aggregates may undergo fragmentation during the gravitational collapse phase. This fragmentation in turn changes the size distribution of the solids and thus must be taken into account in order to understand the properties of the planetesimals that form. To explore the fate of dustmore » pebbles undergoing fragmenting collisions, we conducted laboratory experiments on dust-aggregate collisions with a focus on establishing a collision model for this stage of planetesimal formation. In our experiments, we analyzed collisions of dust aggregates with masses between 0.7 and 91 g mass ratios between target and projectile from 1 to 126 at a fixed porosity of 65%, within the velocity range of 1.5–8.7 m s{sup −1}, at low atmospheric pressure of ∼10{sup −3} mbar, and in free-fall conditions. We derived the mass of the largest fragment, the fragment size/mass distribution, and the efficiency of mass transfer as a function of collision velocity and projectile/target aggregate size. Moreover, we give recipes for an easy-to-use fragmentation and mass-transfer model for further use in modeling work. In a companion paper, we use the experimental findings and the derived dust-aggregate collision model to investigate the fate of dust pebbles during gravitational collapse.« less

Coagulation processes of kaolinite and montmorillonite in calm, saline water

NASA Astrophysics Data System (ADS)

Zhang, Jin-Feng; Zhang, Qing-He; Maa, Jerome P.-Y.

2018-03-01

A three dimensional numerical model for simulating the coagulation processes of colloids has been performed by monitoring the time evolution of particle number concentration, the size distribution of aggregates, the averaged settling velocity, the collision frequency, and the collision efficiency in quiescent water with selected salinities. This model directly simulates all interaction forces between particles based on the lattice Boltzmann method (LBM) and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and thus, can reveal the collision and coagulation processes of colloidal suspensions. Although using perfect spherical particles in the modeling, the results were compared with those for kaolinite and montmorillonite suspensions to demonstrate the capability of simulating the responses of these particles with highly irregular shape. The averaged settling velocity of kaolinite aggregates in quiescent saline water reached a maximum of 0.16 mm/s when the salinity increasing to about 3, and then, exhibited little dependence on salinity thereafter. Model simulations results (by choosing specific values that represent kaolinite's characteristics) indicate a similar trend: rapid decrease of the particle number concentration (i.e., rapidly flocculated, and thus, settling velocity also increases rapidly) when salinity increases from 0 to 2, and then, only increased slightly when salinity was further increased from 5 to 20. The collision frequency for kaolinite only decreases slightly with increasing salinity because that the fluid density and viscosity increase slightly in sea water. It suggests that the collision efficiency for kaolinite rises rapidly at low salinities and levels off at high salinity. For montmorillonite, the settling velocity of aggregates in quiescent saline water continuedly increases to 0.022 mm/s over the whole salinity range 0-20, and the collision efficiency for montmorillonite rises with increasing salinities.

The formation of a Spitzer bubble RCW 79 triggered by a cloud-cloud collision

NASA Astrophysics Data System (ADS)

Ohama, Akio; Kohno, Mikito; Hasegawa, Keisuke; Torii, Kazufumi; Nishimura, Atsushi; Hattori, Yusuke; Hayakawa, Takahiro; Inoue, Tsuyoshi; Sano, Hidetoshi; Yamamoto, Hiroaki; Tachihara, Kengo; Fukui, Yasuo

2018-05-01

Understanding the mechanism of O-star formation is one of the most important current issues in astrophysics. Also an issue of keen interest is how O stars affect their surroundings and trigger secondary star formation. An H II region RCW 79 is one of the typical Spitzer bubbles alongside RCW 120. New observations of CO J = 1-0 emission with Mopra and NANTEN2 revealed that molecular clouds are associated with RCW 79 in four velocity components over a velocity range of 20 km s-1. We hypothesize that two of the clouds collided with each other and the collision triggered the formation of 12 O stars inside the bubble and the formation of 54 low-mass young stellar objects along the bubble wall. The collision is supported by observational signatures of bridges connecting different velocity components in the colliding clouds. The whole collision process happened over a timescale of ˜3 Myr. RCW 79 has a larger size by a factor of 30 in the projected area than RCW 120 with a single O star, and the large size favored formation of the 12 O stars due to the greater accumulated gas in the collisional shock compression.

Impact experiments of porous gypsum-glass bead mixtures simulating parent bodies of ordinary chondrites: Implications for re-accumulation processes related to rubble-pile formation

NASA Astrophysics Data System (ADS)

Yasui, Minami; Arakawa, Masahiko

2011-08-01

Laboratory impact experiments were conducted for gypsum-glass bead targets simulating the parent bodies of ordinary chondrites. The effects of the chondrules included in the parent bodies on impact disruption were experimentally investigated in order to determine the impact conditions for the formation of rubble-pile bodies after catastrophic disruption. The targets included glass beads with a diameter ranging from 100 μm to 3 mm and the volume fraction was 0.6, similar to that of ordinary chondrites, which is about 0.65-0.75. Nylon projectiles with diameters of 10 mm and 2 mm were impacted at 60-180 m s -1 by a single-stage gas gun and at 4 km s -1 by a two-stage light gas gun, respectively. The impact strength of the gypsum-glass bead target was found to range from 56 to 116 J kg -1 depending on the glass bead size, and was several times smaller than that of the porous gypsum target, 446 J kg -1 in low-velocity collisions. The impact strengths of the 100 μm bead target and the porous gypsum target strongly depended on the impact velocity: those obtained in high-velocity collisions were several times greater than those obtained in low-velocity collisions. The velocities of fragments ejected from two corners on the impact surface of the target, measured in the center of the mass system, were slightly dependent on the target materials, irrespective of impact velocity. These results suggest that chondrule-including planetesimals (CiPs) can reconstruct rubble-pile bodies in catastrophic disruptions at the size of the planetesimal smaller than that of planetesimals without chondrules.

Zonal wind observations during a geomagnetic storm

NASA Technical Reports Server (NTRS)

Miller, N. J.; Spencer, N. W.

1986-01-01

In situ measurements taken by the Wind and Temperature Spectrometer (WATS) onboard the Dynamics Explorer 2 spacecraft during a geomagnetic storm display zonal wind velocities that are reduced in the corotational direction as the storm intensifies. The data were taken within the altitudes 275 to 475 km in the dusk local time sector equatorward of the auroral region. Characteristic variations in the value of the Dst index of horizontal geomagnetic field strength are used to monitor the storm evolution. The detected global rise in atmospheric gas temperature indicates the development of thermospheric heating. Concurrent with that heating, reductions in corotational wind velocities were measured equatorward of the auroral region. Just after the sudden commencement, while thermospheric heating is intense in both hemispheres, eastward wind velocities in the northern hemisphere show reductions ranging from 500 m/s over high latitudes to 30 m/s over the geomagnetic equator. After 10 hours storm time, while northern thermospheric heating is diminishing, wind velocity reductions, distinct from those initially observed, begin to develop over southern latitudes. In the latter case, velocity reductions range from 300 m/s over the highest southern latitudes to 150 m/s over the geomagnetic equator and extend into the Northern Hemisphere. The observations highlight the interhemispheric asymmetry in the development of storm effects detected as enhanced gas temperatures and reduced eastward wind velocities. Zonal wind reductions over high latitudes can be attributed to the storm induced equatorward spread of westward polar cap plasma convection and the resulting plasma-neutral collisions. However, those collisions are less significant over low latitudes; so zonal wind reductions over low latitudes must be attributed to an equatorward extension of a thermospheric circulation pattern disrupted by high latitude collisions between neutrals transported via eastward winds and ions convecting westward.

Generalized Jeans' Escape of Pick-Up Ions in Quasi-Linear Relaxation

NASA Technical Reports Server (NTRS)

Moore, T. E.; Khazanov, G. V.

2011-01-01

Jeans escape is a well-validated formulation of upper atmospheric escape that we have generalized to estimate plasma escape from ionospheres. It involves the computation of the parts of particle velocity space that are unbound by the gravitational potential at the exobase, followed by a calculation of the flux carried by such unbound particles as they escape from the potential well. To generalize this approach for ions, we superposed an electrostatic ambipolar potential and a centrifugal potential, for motions across and along a divergent magnetic field. We then considered how the presence of superthermal electrons, produced by precipitating auroral primary electrons, controls the ambipolar potential. We also showed that the centrifugal potential plays a small role in controlling the mass escape flux from the terrestrial ionosphere. We then applied the transverse ion velocity distribution produced when ions, picked up by supersonic (i.e., auroral) ionospheric convection, relax via quasi-linear diffusion, as estimated for cometary comas [1]. The results provide a theoretical basis for observed ion escape response to electromagnetic and kinetic energy sources. They also suggest that super-sonic but sub-Alfvenic flow, with ion pick-up, is a unique and important regime of ion-neutral coupling, in which plasma wave-particle interactions are driven by ion-neutral collisions at densities for which the collision frequency falls near or below the gyro-frequency. As another possible illustration of this process, the heliopause ribbon discovered by the IBEX mission involves interactions between the solar wind ions and the interstellar neutral gas, in a regime that may be analogous [2].

Dynamical study of low Earth orbit debris collision avoidance using ground based laser

NASA Astrophysics Data System (ADS)

Khalifa, N. S.

2015-06-01

The objective of this paper was to investigate the orbital velocity changes due to the effect of ground based laser force. The resulting perturbations of semi-major axis, miss distance and collision probability of two approaching objects are studied. The analytical model is applied for low Earth orbit debris of different eccentricities and area to mass ratio and the numerical test shows that laser of medium power ∼5 kW can perform a small change Δ V ‾ of an average magnitude of 0.2 cm/s which can be accumulated over time to be about 3 cm/day. Moreover, it is confirmed that applying laser Δ V ‾ results in decreasing collision probability and increasing miss distance in order to avoid collision.

Comet formation

NASA Astrophysics Data System (ADS)

Blum, J.

2014-07-01

There has been vast progress in our understanding of planetesimal formation over the past decades, owing to a number of laboratory experiments as well as to refined models of dust and ice agglomeration in protoplanetary disks. Coagulation rapidly forms cm-sized ''pebbles'' by direct sticking in collisions at low velocities (Güttler et al. 2010; Zsom et al. 2010). For the further growth, two model approaches are currently being discussed: (1) Local concentration of pebbles in nebular instabilities until gravitational instability occurs (Johansen et al. 2007). (2) A competition between fragmentation and mass transfer in collisions among the dusty bodies, in which a few ''lucky winners'' make it to planetesimal sizes (Windmark et al. 2012a,b; Garaud et al. 2013). Predictions of the physical properties of the resulting bodies in both models allow a distinction of the two formation scenarios of planetesimals. In particular, the tensile strength (i.e, the inner cohesion) of the planetesimals differ widely between the two models (Skorov & Blum 2012; Blum et al. 2014). While model (1) predicts tensile strengths on the order of ˜ 1 Pa, model (2) results in rather compactified dusty bodies with tensile strengths in the kPa regime. If comets are km-sized survivors of the planetesimal-formation era, they should in principle hold the secret of their formation process. Water ice is the prime volatile responsible for the activity of comets. Thermophysical models of the heat and mass transport close to the comet-nucleus surface predict water-ice sublimation temperatures that relate to maximum sublimation pressures well below the kPa regime predicted for formation scenario (2). Model (1), however, is in agreement with the observed dust and gas activity of comets. Thus, a formation scenario for cometesimals involving gravitational instability is favored (Blum et al. 2014).

New feature of low pT charm quark hadronization in pp collisions at √{s}=7 TeV

NASA Astrophysics Data System (ADS)

Song, Jun; Li, Hai-hong; Shao, Feng-lan

2018-04-01

Treating the light-flavor constituent quarks and antiquarks whose momentum information is extracted from the data of soft light-flavor hadrons in pp collisions at √{s}=7 TeV as the underlying source of chromatically neutralizing the charm quarks of low transverse momenta (pT), we show that the experimental data of pT spectra of single-charm hadrons D^{0,+}, D^{*+} Ds+, Λ c+ and Ξc0 at mid-rapidity in the low pT range (2≲ pT≲ 7 GeV/ c) in pp collisions at √{s}=7 TeV can be well understood by the equal-velocity combination of perturbatively created charm quarks and those light-flavor constituent quarks and antiquarks. This suggests a possible new scenario of low pT charm quark hadronization, in contrast to the traditional fragmentation mechanism, in pp collisions at LHC energies. This is also another support for the exhibition of the soft constituent quark degrees of freedom for the small parton system created in pp collisions at LHC energies.

Acoustic Emission Signals in Thin Plates Produced by Impact Damage

NASA Technical Reports Server (NTRS)

Prosser, William H.; Gorman, Michael R.; Humes, Donald H.

1999-01-01

Acoustic emission (AE) signals created by impact sources in thin aluminum and graphite/epoxy composite plates were analyzed. Two different impact velocity regimes were studied. Low-velocity (less than 0.21 km/s) impacts were created with an airgun firing spherical steel projectiles (4.5 mm diameter). High-velocity (1.8 to 7 km/s) impacts were generated with a two-stage light-gas gun firing small cylindrical nylon projectiles (1.5 mm diameter). Both the impact velocity and impact angle were varied. The impacts did not penetrate the aluminum plates at either low or high velocities. For high-velocity impacts in composites, there were both impacts that fully penetrated the plate as well as impacts that did not. All impacts generated very large amplitude AE signals (1-5 V at the sensor), which propagated as plate (extensional and/or flexural) modes. In the low-velocity impact studies, the signal was dominated by a large flexural mode with only a small extensional mode component detected. As the impact velocity was increased within the low velocity regime, the overall amplitudes of both the extensional and flexural modes increased. In addition, a relative increase in the amplitude of high-frequency components of the flexural mode was also observed. Signals caused by high-velocity impacts that did not penetrate the plate contained both a large extensional and flexural mode component of comparable amplitudes. The signals also contained components of much higher frequency and were easily differentiated from those caused by low-velocity impacts. An interesting phenomenon was observed in that the large flexural mode component, seen in every other case, was absent from the signal when the impact particle fully penetrated through the composite plates.

How to quantitatively evaluate safety of driver behavior upon accident? A biomechanical methodology

PubMed Central

Zhang, Wen; Cao, Jieer

2017-01-01

How to evaluate driver spontaneous reactions in various collision patterns in a quantitative way is one of the most important topics in vehicle safety. Firstly, this paper constructs representative numerical crash scenarios described by impact velocity, impact angle and contact position based on finite element (FE) computation platform. Secondly, a driver cabin model is extracted and described in the well validated multi-rigid body (MB) model to compute the value of weighted injury criterion to quantitatively assess drivers’ overall injury under certain circumstances. Furthermore, based on the coupling of FE and MB, parametric studies on various crash scenarios are conducted. It is revealed that the WIC (Weighted Injury Criteria) value variation law under high impact velocities is quite distinct comparing with the one in low impact velocities. In addition, the coupling effect can be elucidated by the fact that the difference of WIC value among three impact velocities under smaller impact angles tends to be distinctly higher than that under larger impact angles. Meanwhile, high impact velocity also increases the sensitivity of WIC under different collision positions and impact angles. Results may provide a new methodology to quantitatively evaluate driving behaviors and serve as a significant guiding step towards collision avoidance for autonomous driving vehicles. PMID:29240789

How to quantitatively evaluate safety of driver behavior upon accident? A biomechanical methodology.

PubMed

Zhang, Wen; Cao, Jieer; Xu, Jun

2017-01-01

How to evaluate driver spontaneous reactions in various collision patterns in a quantitative way is one of the most important topics in vehicle safety. Firstly, this paper constructs representative numerical crash scenarios described by impact velocity, impact angle and contact position based on finite element (FE) computation platform. Secondly, a driver cabin model is extracted and described in the well validated multi-rigid body (MB) model to compute the value of weighted injury criterion to quantitatively assess drivers' overall injury under certain circumstances. Furthermore, based on the coupling of FE and MB, parametric studies on various crash scenarios are conducted. It is revealed that the WIC (Weighted Injury Criteria) value variation law under high impact velocities is quite distinct comparing with the one in low impact velocities. In addition, the coupling effect can be elucidated by the fact that the difference of WIC value among three impact velocities under smaller impact angles tends to be distinctly higher than that under larger impact angles. Meanwhile, high impact velocity also increases the sensitivity of WIC under different collision positions and impact angles. Results may provide a new methodology to quantitatively evaluate driving behaviors and serve as a significant guiding step towards collision avoidance for autonomous driving vehicles.

Elastic turbulence in entangled semi-dilute DNA solutions measured with optical coherence tomography velocimetry.

PubMed

Malm, A V; Waigh, T A

2017-04-26

The flow instabilities of solutions of high molecular weight DNA in the entangled semi-dilute concentration regime were investigated using optical coherence tomography velocimetry, a technique that provides high spatial (probe volumes of 3.4 pL) and temporal resolution (sub μs) information on the flow behaviour of complex fluids in a rheometer. The velocity profiles of the opaque DNA solutions (high and low salt) were measured as a function of the distance across the gap of a parallel plate rheometer, and their evolution over time was measured. At lower DNA concentrations and low shear rates, the velocity fluctuations were well described by Gaussian functions and the velocity gradient was uniform across the rheometer gap, which is expected for Newtonian flows. As the DNA concentration and shear rate were increased there was a stable wall slip regime followed by an evolving wall slip regime, which is finally followed by the onset of elastic turbulence. Strain localization (shear banding) is observed on the boundaries of the flows at intermediate shear rates, but decreases in the high shear elastic turbulence regime, where bulk strain localization occurs. A dynamic phase diagram for non-linear flow was created to describe the different behaviours.

GALAXY CLUSTER BULK FLOWS AND COLLISION VELOCITIES IN QUMOND

DOE Office of Scientific and Technical Information (OSTI.GOV)

Katz, Harley; McGaugh, Stacy; Teuben, Peter

We examine the formation of clusters of galaxies in numerical simulations of a QUMOND cosmogony with massive sterile neutrinos. Clusters formed in these exploratory simulations develop higher velocities than those found in {Lambda}CDM simulations. The bulk motions of clusters attain {approx}1000 km s{sup -1} by low redshift, comparable to observations whereas {Lambda}CDM simulated clusters tend to fall short. Similarly, high pairwise velocities are common in cluster-cluster collisions like the Bullet Cluster. There is also a propensity for the most massive clusters to be larger in QUMOND and to appear earlier than in {Lambda}CDM, potentially providing an explanation for ''pink elephants''more » like El Gordo. However, it is not obvious that the cluster mass function can be recovered.« less

Mass-energy and momentum extraction by gravitational wave emission in the merger of two colliding black holes: The non-head-on case

NASA Astrophysics Data System (ADS)

Aranha, R. F.; Soares, I. Damião; Tonini, E. V.

2012-01-01

We examine numerically the post-merger regime of two nonspining holes in non-head-on collisions in the realm of nonaxisymmetric Robinson-Trautman spacetimes. Characteristic initial data for the system are constructed and evolved via the Robinson-Trautman equation. The numerical integration is performed using a Galerkin spectral method which is sufficiently stable to reach the final configuration of the remnant black hole, when the gravitational wave emission ceases. The initial data contains three independent parameters, the ratio mass α of the individual colliding black holes, their initial premerger infalling velocity and the incidence angle of collision ρ0. The remnant black hole is characterized by its final boost parameter, rest mass and scattering angle. The motion of the remnant black hole is restricted to the plane determined by the directions of the two initial colliding black holes, characterizing a planar collision. The net momentum fluxes carried out by gravitational waves are confined to this plane. We evaluate the efficiency of mass-energy extraction, the total energy and momentum carried out by gravitational waves and the momentum distribution of the remnant black hole for a large domain of initial data parameters. Our analysis is based on the Bondi-Sachs four-momentum conservation laws. The process of mass-energy extraction is shown to be less efficient as the initial data departs from the head-on configuration. Head-on collisions (ρ0=0o) and orthogonal collisions (ρ0=90°) constitute, respectively, upper and lower bounds to the power emission and to the efficiency of mass-energy extraction. On the contrary, head-on collisions and orthogonal collisions constitute, respectively, lower and upper bounds for the momentum of the remnant. Distinct regimes of gravitational wave emission (bursts or quiescent emission) are characterized by the analysis of the time behavior of the gravitational wave power as a function of α. In particular, the net gravitational wave flux is nonzero for equal-mass colliding black holes in non-head-on collisions. The momentum extraction and the patterns of the momentum fluxes, as a function of the incidence angle, are examined. The relation between the incidence angle and the scattering angle closely approximates a relation for the inelastic collision of classical particles in Newtonian dynamics.

Detailed seismic velocity structure beneath the Hokkaido corner, NE Japan: Collision process of the forearc sliver

NASA Astrophysics Data System (ADS)

Kita, S.; Hasegawa, A.; Okada, T.; Nakajima, J.; Matsuzawa, T.; Katsumata, K.

2010-12-01

1. Introduction In south-eastern Hokkaido, the Kuril forearc sliver is colliding with the northeastern Japan arc due to the oblique subduction of the Pacific plate. This collision causes the formation of the Hidaka mountain range since the late Miocene (Kimura, 1986) and delamination of the lower-crust materials of the Kuril forearc sliver, which would be expected to descend into the mantle wedge below (e.g., Ito 2000; Ito and Iwasaki, 2002). In this study, we precisely investigated the three-dimensional seismic velocity structure beneath the Hokkaido corner to examine the collision of two forearcs in this area by using both of data from a dense temporary seismic network deployed in this area (Katsumata et al. [2006]) and those from the Kiban observation network, which covers the entire Japanese Islands with a station separation of 15-20 km. 2. Data and method The double-difference tomography method (Zhang and Thurber, 2003; 2006) was applied to a large number of arrival time data of 201,527 for P-waves and 150,963 for S-waves that were recorded at 125 stations from 10,971 earthquakes that occurred from 1999 to 2010. Grid intervals were set at 10 km in the along-arc direction, 12.5 km perpendicular to it, and 5-10 km in the vertical direction. 3. Results and discussion Inhomogeneous seismic velocity structure was clearly imaged in the Hokkaido corner at depths of 0-120 km. A high-velocity anomaly of P- and S- waves with a volume of 20 km x 90 km x 35km was detected just beneath the main zone of the Hidaka metamorphic belt at depths of 0-35 km. This high-velocity anomaly is continuously distributed from the depths of the mantle wedge to the surface. The western edge of the anomaly exactly corresponds to the Hidaka main thrust (HMT) at the surface. The highest velocity value in the anomaly corresponds to those of the uppermost mantle material (e.g. peridotite). The location of them at depths of 0-35km is also consistent with that of the Horoman-Peridotite belt, which is located at the southwestern edge of the main zone of the Hidaka metamorphic belt.On the other hand, a low-velocity anomaly of P- and S- waves with a volume of 80 km x 100 km x 50 km is distributed to the west of the Hidaka metamorphic belt at depths of 35-90km. This low-velocity anomaly seems to be continuously distributed from the continental crust of the NE Japan forearc. The velocity values of this low-V anomaly correspond to those of crustal materials, which is consistent with results of the tomographic study of Kita et al. [2010, EPSL] and Takanami et al. [1982]. Comparison with the results of seismic reflection surveys of Ito [2000] shows that the boundary between anomalous high-velocity mantle materials and low-velocity continental crustal materials just beneath the Hidaka main thrust (HMT) presently obtained is exactly consistent with the locations of reflection planes of their study. Moreover, our study also suggests that the anomalous low-velocity crustal materials at the mantle wedge depth appears to belong to the NE Japan forearc crust, which does not support the expectation of the previous studies that the delaminated lower-crust materials of the Kuril forearc sliver descends into the mantle wedge due to the collision.

Role of E-cadherin in membrane-cortex interaction probed by nanotube extrusion.

PubMed

Tabdanov, Erdem; Borghi, Nicolas; Brochard-Wyart, Françoise; Dufour, Sylvie; Thiery, Jean-Paul

2009-03-18

This study aims to define the role of E-cadherin (Ecad) engagement in cell-cell contact during membrane-cortex interaction. As a tool, we used a hydrodynamic membrane tube extrusion technique to characterize the mechanical interaction between the plasma membrane and the underlying cortical cytoskeleton. Cells were anchored on 4.5 microm beads coated with polylysine (PL) to obtain nonspecific cell adhesion or with an antibody against Ecad to mimic specific Ecad-mediated cell adhesion. We investigated tube length dynamics L(t) over time and through successive extrusions applied to the cell at regular time intervals. A constant slow velocity was observed for the first extrusion, for PL-attached cells. Subsequent extrusions had two phases: an initial high-velocity regime followed by a low-velocity regime. Successive extrusions gradually weakened the binding of the membrane around the tube neck to the underlying cortical cytoskeleton. Cells specifically attached via Ecad first exhibited a very low extrusion velocity regime followed by a faster extrusion regime similar to nonspecific extrusion. This indicates that Ecad strengthens the membrane-cortical cytoskeleton interaction, but only in a restricted area corresponding to the site of contact between the cell and the bead. Occasional giant "cortex" tubes were extruded with specifically anchored cells, demonstrating that the cortex remained tightly bound to the membrane through Ecad-mediated adhesion at the contact site.

Role of E-Cadherin in Membrane-Cortex Interaction Probed by Nanotube Extrusion

PubMed Central

Tabdanov, Erdem; Borghi, Nicolas; Brochard-Wyart, Françoise; Dufour, Sylvie; Thiery, Jean-Paul

2009-01-01

This study aims to define the role of E-cadherin (Ecad) engagement in cell-cell contact during membrane-cortex interaction. As a tool, we used a hydrodynamic membrane tube extrusion technique to characterize the mechanical interaction between the plasma membrane and the underlying cortical cytoskeleton. Cells were anchored on 4.5 μm beads coated with polylysine (PL) to obtain nonspecific cell adhesion or with an antibody against Ecad to mimic specific Ecad-mediated cell adhesion. We investigated tube length dynamics L(t) over time and through successive extrusions applied to the cell at regular time intervals. A constant slow velocity was observed for the first extrusion, for PL-attached cells. Subsequent extrusions had two phases: an initial high-velocity regime followed by a low-velocity regime. Successive extrusions gradually weakened the binding of the membrane around the tube neck to the underlying cortical cytoskeleton. Cells specifically attached via Ecad first exhibited a very low extrusion velocity regime followed by a faster extrusion regime similar to nonspecific extrusion. This indicates that Ecad strengthens the membrane-cortical cytoskeleton interaction, but only in a restricted area corresponding to the site of contact between the cell and the bead. Occasional giant “cortex” tubes were extruded with specifically anchored cells, demonstrating that the cortex remained tightly bound to the membrane through Ecad-mediated adhesion at the contact site. PMID:19289070

Dynamic Obstacle Avoidance for Unmanned Underwater Vehicles Based on an Improved Velocity Obstacle Method

PubMed Central

Zhang, Wei; Wei, Shilin; Teng, Yanbin; Zhang, Jianku; Wang, Xiufang; Yan, Zheping

2017-01-01

In view of a dynamic obstacle environment with motion uncertainty, we present a dynamic collision avoidance method based on the collision risk assessment and improved velocity obstacle method. First, through the fusion optimization of forward-looking sonar data, the redundancy of the data is reduced and the position, size and velocity information of the obstacles are obtained, which can provide an accurate decision-making basis for next-step collision avoidance. Second, according to minimum meeting time and the minimum distance between the obstacle and unmanned underwater vehicle (UUV), this paper establishes the collision risk assessment model, and screens key obstacles to avoid collision. Finally, the optimization objective function is established based on the improved velocity obstacle method, and a UUV motion characteristic is used to calculate the reachable velocity sets. The optimal collision speed of UUV is searched in velocity space. The corresponding heading and speed commands are calculated, and outputted to the motion control module. The above is the complete dynamic obstacle avoidance process. The simulation results show that the proposed method can obtain a better collision avoidance effect in the dynamic environment, and has good adaptability to the unknown dynamic environment. PMID:29186878

Summary talk: Experiments at low energies

NASA Astrophysics Data System (ADS)

Leifels, Yvonne

2016-01-01

In heavy-ion collisions at beam energies √sNN between 1 and 150A GeV highest baryonic densities are reached at rather moderate temperatures. By varying the beam energy and the system size a broad range of the QCD phase diagram is scanned where several interesting phenomena are predicted by theoretical models. Apart from possible phase transitions and existence of a critical point in this regime, the production of strangeness and the interaction of strange particles with the surrounding hot and dense nuclear medium constitutes a prominent probe not only to address the underlying reaction mechanisms and production processes but in particular to constrain densities and temperatures reached in the course of the collision. Recent results on heavy-ion collisions in this beam energy regime obtained by various experimental collaborations are summarized, with special emphasis on strangeness production, rare probes, and critical phenomena. The importance of data on elementary reactions (i.e., pp, p+nucleus, and π+nucleus) as a bench mark for theoretical models and their relevance for understanding the underlying mechanisms of heavy-ion collisions are being discussed. Several interesting observables have been presented in various contributions, which give further motivation for the construction of high-rate experiments at new accelerator facilities.

Human Kinematics During Non-Collinear Low Velocity Rear End Collisions

PubMed Central

McConnell, Whitman E.; Guzman, Herbert M.; Krenrich, Scott W.; Bomar, John B.; Harding, Richard M.; Raddin, James H.; Funk, James R.; Smith, Darrin A.

2003-01-01

Non-collinear low velocity rear end (LVRE) collision human kinematics have not previously been studied. Occupant head and neck motions during twenty similar non-collinear (15 and 30 degree angle) left rear end collisions were analyzed for five male test subjects alternately positioned in the left and right front seats of the struck vehicle. Displacement-time and acceleration data for occupant, seat, and vehicles were determined by 3D motion analyses and linear accelerometer outputs. The dynamics of the struck vehicle at 6.0 to 9.3 kph (3.8 to 5.8 mph) delta-V showed an initial period of yaw, even when the rear tires did not lose traction with the pavement. The brief yaw seen during the 15 degree impacts was accompanied by early relative rightward movement of the vehicle’s seat and seatback behind the stationary test subject: the subjects subsequently engaged the left region of the seatback and head restraint. A more pronounced yaw accompanied the loss of rear tire traction during the 30 degree tests, and resulted in occupant contact/loading further toward the left edge of the seat back and head restraint. For a given striking vehicle velocity, the impact severity in terms of head acceleration and changes in head velocity were significantly lower (p<0.05) at vehicle impact angles of 30 degrees compared with 15 degrees. Clinically, there were only minor short-term symptoms and no long-term symptoms observed in these angled impacts. PMID:12941242

The use of the energy flow change theorem in solving the problem of perfectly elastic collision of three mass points

NASA Astrophysics Data System (ADS)

Kolyari I., G.

2018-05-01

The proposed theoretical model allows for the perfectly elastic collision of three bodies (three mass points) to calculate: 1) the definite value of the three bodies' projected velocities after the collision with a straight line, along which the bodies moved before the collision; 2) the definite value of the scattering bodies' velocities on the plane and the definite value of the angles between the bodies' momenta (or velocities), which the bodies obtain after the collision when moving on the plane. The proposed calculation model of the velocities of the three collided bodies is consistent with the dynamic model of the same bodies' interaction during the collision, taking into account that the energy flow is conserved for the entire system before and after the collision. It is shown that under the perfectly elastic interaction during the collision of three bodies the energy flow is conserved in addition to the momentum and energy conservation.

Effect of collision duration on the chaotic dynamics of a ball bouncing on a vertically vibrating plate

NASA Astrophysics Data System (ADS)

Jiang, Z. H.; Liang, Z. J.; Wu, A. C.; Zheng, R. H.

2018-03-01

Experiments have been performed to study the chaotic dynamics of a ball bouncing on a vertically vibrating plate. The velocity dependence of collision duration and coefficient of restitution is determined, and phase portraits of chaotic structures for the flight time and the relative collision velocities are obtained. Numerical calculations are carried out to examine the effects of velocity-dependent collision duration on the ball dynamics. It is revealed that when the collision is instantaneous, sticking solutions are always observed, whereas when the collision duration is taken into account, sticking solutions are destroyed and thereby chaos behaviors are induced.

Contact control for advanced applications of light weight arms

NASA Technical Reports Server (NTRS)

Book, Wayne J.; Kwon, Dong-Soo

1991-01-01

Many applications of robotic and teleoperated manipulator arms require operation in contact and non-contact regimes. This paper deals with both regimes and the transition between them with special attention given to problems of flexibility in the links and drives. This is referred to as contact control. Inverse dynamics is used to plan the tip motion of the flexible link so that the free motion can stop very near the contact surface without collision due to overshoot. Contact must occur at a very low speed since the high frequency impact forces are too sudden to be affected by any feedback generated torques applied to a joint at the other end of the link. The effect of approach velocity and surface properties are discussed. Force tracking is implemented by commands to the deflection states of the link and the contact force. This enables a natural transition between tip position and tip force control that is not possible when the arm is treated as rigid. The effect of feedback gain, force trajectory, and desired final force are of particular interest and are studied. Experimental results are presented on a one link arm and the system performance in the overall contact task is analyzed. Extension to multi-link cases with potential applications are discussed.

Numerical Calculation of Neoclassical Distribution Functions and Current Profiles in Low Collisionality, Axisymmetric Plasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

B.C. Lyons, S.C. Jardin, and J.J. Ramos

2012-06-28

A new code, the Neoclassical Ion-Electron Solver (NIES), has been written to solve for stationary, axisymmetric distribution functions (f ) in the conventional banana regime for both ions and elec trons using a set of drift-kinetic equations (DKEs) with linearized Fokker-Planck-Landau collision operators. Solvability conditions on the DKEs determine the relevant non-adiabatic pieces of f (called h ). We work in a 4D phase space in which Ψ defines a flux surface, θ is the poloidal angle, v is the total velocity referenced to the mean flow velocity, and λ is the dimensionless magnetic moment parameter. We expand h inmore » finite elements in both v and λ . The Rosenbluth potentials, φ and ψ, which define the integral part of the collision operator, are expanded in Legendre series in cos χ , where χ is the pitch angle, Fourier series in cos θ , and finite elements in v . At each ψ , we solve a block tridiagonal system for hi (independent of fe ), then solve another block tridiagonal system for he (dependent on fi ). We demonstrate that such a formulation can be accurately and efficiently solved. NIES is coupled to the MHD equilibrium code JSOLVER [J. DeLucia, et al., J. Comput. Phys. 37 , pp 183-204 (1980).] allowing us to work with realistic magnetic geometries. The bootstrap current is calculated as a simple moment of the distribution function. Results are benchmarked against the Sauter analytic formulas and can be used as a kinetic closure for an MHD code (e.g., M3D-C1 [S.C. Jardin, et al ., Computational Science & Discovery, 4 (2012).]).« less

Temporomandibular joint injury potential imposed by the low-velocity extension-flexion maneuver.

PubMed

Howard, R P; Hatsell, C P; Guzman, H M

1995-03-01

It has been proposed that significant temporomandibular joint injury can occur as a result of rapid extension-flexion motion of the neck (whip-lash). This motion, which is experienced by passengers in vehicles that undergo rear-end collisions, has been described as causing rapid protrusion and opening of the mandible. It has been speculated that this relative motion between the mandible and the cranium produces forces at the temporomandibular joint (TMJ) that injure the articular elements. The objective of this study was to measure these forces by an experimental method. Accelerometer sensor and high-speed cinematographic data were obtained from the kinematic responses of live human test subjects positioned as occupants in motor vehicles that underwent staged low-velocity rear-end collisions. Linear and moment forces generated at the TMJs were obtained from the resultant acceleration pulse at the craniomandibular complex, estimation of the mass properties of the mandible and its appended soft tissues, and the application of Newton's Second Law of motion. The maximum linear forces generated at the TMJ in a rear-end collision resulting in a velocity change of the test subject of 8 km/h (5 mph) were in the 7 to 10 N (1.6 to 2.2 lb) range. Moment forces at the joint peaked briefly at 0.55 N.m (4.81 lb-in). These force magnitudes generated at the TMJ constitute a minor fraction of the forces experienced at the joint during normal physiologic function. It is a conclusion of this study that injuries to the TMJ attributed to low-velocity "whiplash" cannot be accounted for by the joint forces produced by this maneuver.

Characteristic electron variations across simple high-speed solar wind streams

NASA Technical Reports Server (NTRS)

Feldman, W. C.; Asbridge, J. R.; Bame, S. J.; Gosling, J. T.; Lemons, D. S.

1978-01-01

The paper deals with electron variations across simple high-speed streams. Comprehensive scans of the shapes of electron distributions measured at the highest bulk speeds confirm the results of Rosenbauer et al. (1976, 1977) and show that the electron velocity distributions can be broken down into a low-energy or core component and a high-energy strongly beamed component. The low-energy component displays many characteristics expected from a fluid: the internal particle coupling necessary to maintain this state must result from both binary Coulomb collisions and wave-particle interactions. The high-energy or halo component displays many characteristics expected to develop in the absence of collisions beyond a certain base radius. These electrons appear to evolve under the primary influence of static interplanetary magnetic and electric fields and, therefore, develop very anisotropic velocity distributions.

Cold chemistry with cold molecules

NASA Astrophysics Data System (ADS)

Shagam, Yuval

Low temperature chemistry has been predicted to be dominated by quantum effects, such as shape resonances, where colliding particles exhibit wave-like behavior and tunnel through potential barriers. Observation of these quantum effects provides valuable insight into the microscopic mechanism that governs scattering processes. Our recent advances in the control of neutral supersonic molecular beams, namely merged beam experiments, have enabled continuous tuning of collision energies from the classical regime at room temperature down to 0.01 kelvin, where a quantum description of the dynamics is necessary. I will discuss our use of this technique to study how the dynamics change when molecules participate in collisions, demonstrating the crucial role the molecular quantum rotor plays. We have found that at low temperatures rotational state of the molecule can strongly affect collision dynamics considerably changing reaction rates, due to the different symmetries of the molecular wavefunction.

TURBULENCE-INDUCED RELATIVE VELOCITY OF DUST PARTICLES. IV. THE COLLISION KERNEL

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pan, Liubin; Padoan, Paolo, E-mail: lpan@cfa.harvard.edu, E-mail: ppadoan@icc.ub.edu

Motivated by its importance for modeling dust particle growth in protoplanetary disks, we study turbulence-induced collision statistics of inertial particles as a function of the particle friction time, τ{sub p}. We show that turbulent clustering significantly enhances the collision rate for particles of similar sizes with τ{sub p} corresponding to the inertial range of the flow. If the friction time, τ{sub p,} {sub h}, of the larger particle is in the inertial range, the collision kernel per unit cross section increases with increasing friction time, τ{sub p,} {sub l}, of the smaller particle and reaches the maximum at τ{sub p,}more » {sub l} = τ{sub p,} {sub h}, where the clustering effect peaks. This feature is not captured by the commonly used kernel formula, which neglects the effect of clustering. We argue that turbulent clustering helps alleviate the bouncing barrier problem for planetesimal formation. We also investigate the collision velocity statistics using a collision-rate weighting factor to account for higher collision frequency for particle pairs with larger relative velocity. For τ{sub p,} {sub h} in the inertial range, the rms relative velocity with collision-rate weighting is found to be invariant with τ{sub p,} {sub l} and scales with τ{sub p,} {sub h} roughly as ∝ τ{sub p,h}{sup 1/2}. The weighting factor favors collisions with larger relative velocity, and including it leads to more destructive and less sticking collisions. We compare two collision kernel formulations based on spherical and cylindrical geometries. The two formulations give consistent results for the collision rate and the collision-rate weighted statistics, except that the spherical formulation predicts more head-on collisions than the cylindrical formulation.« less

Precise seismic velocity structure beneath the Hokkaido corner, northern Japan: Arc-arc collision and the 1970 M 6.7 Hidaka region earthquake and the 1982 M 7.1 Urakawa-oki earthquake

NASA Astrophysics Data System (ADS)

Kita, S.; Hasegawa, A.; Nakajima, J.; Okada, T.; Matsuzawa, T.; Katsumata, K.

2011-12-01

Using arrival-time data both from the nationwide Kiban seismic network and from a dense temporary seismic network covering the area of the Hokkaido corner [Katsumata et al., 2002a; 2003, JGR], we precisely determined three-dimensional seismic velocity structure beneath this area to understand the collision process between the Kuril and northeasetern Japan forearcs. Tomographic inversions were performed with smaller grid spacing [5 x 10 x 5 km] than our previous study [Kita et al., 2010b, EPSL] by using the double-difference tomography method [Zhang and Thurber, 2003; 2006]. Inhomogeneous seismic velocity structure was more precisely imaged in the Hokkaido corner at depths of 0-120 km. A broad low-velocity zone of P- and S- waves having velocities of crust materials with a total volume of 80 km x 100 km x 50 km is distributed to the west of the Hidaka metamorphic belt (the Hidaka main thrust) at depths of 30-90km. On the other hand, several small-scale high-velocity zones having velocities of mantle materials were detected at depths of 0-35 km), inclined east-northeastward at a high angle of 40-60 degrees. All of these anomaly high velocity zones are respectively located in the deeper extension of the Neogene thrust faults, striking almost N-S direction and dipping 40-50 degrees at depths of 0-10km [e.g. Ito 2000]. The largest high-velocity zone is located in the deeper extension of the Hidaka main thrust, being in contact with the eastern edge of the low-V zone. This high-V zone reaches near the surface at the Hidaka metamorphic belt and its southern edge is located just beneath the Horoman-peridotite, which is one of the most famous peridotite outcrops. Moreover, the boundary of the high-V zone with the broad low-V zone corresponds to the fault plane of the 1970 Mj 6.7 Hidaka region earthquake [Moriya 1972]. Another high-V zone is located within the broad low-V zone at depths of 20-30km and in the deeper extension of thrust, which belongs to the Ishikari Low land eastern edge fault groups. The western boundary of this small high-V zone corresponds to the fault plane of the 1982 Mj 7.1 Urakawa-oki earthquake [Moriya et al, 1983]. Both of the hanging walls of the fault planes of two M 7 class big earthquakes have anomalously high velocities, while both of the foot walls have low velocities. A considerable number of earthquakes, including aftershocks of these two big earthquake and, occur in the broad low-V zone at depths of 0-80 km (even at depths of the mantle wedge), whereas seismicity is very low in other areas. These present observations provide important in formation to deepen our understanding of the ongoing arc-arc collision process and earthquake generation mechanism in the Hokkaido corner.

An experimental study on low-velocity low-gravity collisions into granular surfaces

NASA Astrophysics Data System (ADS)

Sunday, C.; Murdoch, N.; Mimoun, D.

2014-07-01

The Japanese Space Agency (JAXA) is scheduled to launch the asteroid sample-return mission, Hayabusa-2, to target body 1999 JU_3 in December 2014 [1]. The spacecraft will arrive at the C-type near-Earth asteroid in mid-2018 and deploy several science payloads to its surface. Among these payloads is a 10-kg lander, the Mobile Asteroid Surface Scout (MASCOT), provided by the German Space Agency (DLR) with cooperation from the Centre National d'Etudes Spatiales (CNES). MASCOT will reach the asteroid's surface with an anticipated impact speed of 10--20 cm/s. In addition to housing four instruments for in-situ science investigation, MASCOT contains a mobility mechanism that will correct its orientation and enable it to ''hop'' to various measurement sites [2]. Based on thermal infrared observations [3,4,5] and previous space missions [6,7], it is strongly believed that 1999 JU_3 is covered by loose regolith. The asteroid's granular surface, in combination with the low surface gravity, makes it difficult to predict the lander's collision behavior from existing theoretical models. However, to ensure that MASCOT can successfully fulfill its mission, it is vital to understand the rebound dynamics of the lander in the asteroid surface environment. The objective of this work, derived from the needs of current and future asteroid missions, is to present an experiment designed to study low-velocity, low-gravity collisions into granular surfaces. The experiment measures the amount of energy lost during impact via a projectile's coefficient of restitution and also the acceleration profile of the projectile during collision. The key challenge to designing an asteroid collision experiment is finding a way to simulate reduced gravity conditions on the Earth so that the prevailing forces in micro-gravity collisions can be reflected in the experimental results. The proposed way to achieve this goal is to let a free-falling projectile impact a surface with a constant downward acceleration, or an acceleration less than that of gravity, so that the effective surface acceleration felt by the grains at impact is very small. In reducing the effective surface acceleration of the granular material, the medium's inter-grain cohesion forces will become more important compared to its weight force [8], and the properties of the granular material will become more representative of those on an asteroid's surface. The concept of effective acceleration drives the design of this experiment and results in the following key features: First, the granular surface is given a constant downward acceleration using an Atwood machine, or a system of pulleys and counterweights. Next, the projectile and surface are simultaneously released from rest using a magnetic solenoid and hook assembly. The starting height of the surface container and the initial separation distance between the projectile and surface are variable and chosen to accommodate collision velocities of 10--20 cm/s and effective accelerations of 0.3--1.0 m/s^2. Finally, wireless accelerometers, placed on the surface container and in the projectile, provide acceleration data, while high-speed cameras capable of recording 100,000 frames per second capture the collision and act as secondary data sources. The experiment is built into an existing 6-m drop-tower frame and requires the custom design of all components, including the projectile, surface sample container, release mechanism, and deceleration system. This work will present the detailed design of the asteroid-collision experiment as well as a discussion on the planned experimental trials. The experimental results, once obtained, will be used to create a scaling law that will help predict a projectile's rebound and acceleration behavior during a low-velocity collision into a granular surface in micro-gravity conditions.

RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Sano, Hidetoshi; Enokiya, Rei; Hayashi, Katsuhiro; Yamagishi, Mitsuyoshi; Saeki, Shun; Okawa, Kazuki; Tsuge, Kisetsu; Tsutsumi, Daichi; Kohno, Mikito; Hattori, Yusuke; Yoshiike, Satoshi; Fujita, Shinji; Nishimura, Atsushi; Ohama, Akio; Tachihara, Kengo; Torii, Kazufumi; Hasegawa, Yutaka; Kimura, Kimihiro; Ogawa, Hideo; Wong, Graeme F.; Braiding, Catherine; Rowell, Gavin; Burton, Michael G.; Fukui, Yasuo

2018-02-01

A collision between two molecular clouds is one possible candidate for high-mass star formation. The H II region RCW 36, located in the Vela molecular ridge, contains a young star cluster (˜ 1 Myr old) and two O-type stars. We present new CO observations of RCW 36 made with NANTEN2, Mopra, and ASTE using 12CO(J = 1-0, 2-1, 3-2) and 13CO(J = 2-1) emission lines. We have discovered two molecular clouds lying at the velocities VLSR ˜ 5.5 and 9 km s-1. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of ˜ 0.6-1.2 for CO J = 3-2/1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s-1. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45° relative to the line-of-sight. We estimate the collision timescale to be ˜ 105 yr. It is probable that the cluster age found by Ellerbroek et al. (2013b, A&A, 558, A102) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud-cloud collision

NASA Astrophysics Data System (ADS)

Sano, Hidetoshi; Enokiya, Rei; Hayashi, Katsuhiro; Yamagishi, Mitsuyoshi; Saeki, Shun; Okawa, Kazuki; Tsuge, Kisetsu; Tsutsumi, Daichi; Kohno, Mikito; Hattori, Yusuke; Yoshiike, Satoshi; Fujita, Shinji; Nishimura, Atsushi; Ohama, Akio; Tachihara, Kengo; Torii, Kazufumi; Hasegawa, Yutaka; Kimura, Kimihiro; Ogawa, Hideo; Wong, Graeme F.; Braiding, Catherine; Rowell, Gavin; Burton, Michael G.; Fukui, Yasuo

2018-05-01

A collision between two molecular clouds is one possible candidate for high-mass star formation. The H II region RCW 36, located in the Vela molecular ridge, contains a young star cluster (˜ 1 Myr old) and two O-type stars. We present new CO observations of RCW 36 made with NANTEN2, Mopra, and ASTE using 12CO(J = 1-0, 2-1, 3-2) and 13CO(J = 2-1) emission lines. We have discovered two molecular clouds lying at the velocities VLSR ˜ 5.5 and 9 km s-1. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of ˜ 0.6-1.2 for CO J = 3-2/1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s-1. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45° relative to the line-of-sight. We estimate the collision timescale to be ˜ 105 yr. It is probable that the cluster age found by Ellerbroek et al. (2013b, A&A, 558, A102) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

NASA Astrophysics Data System (ADS)

Zhu, Feng; Lu, Zhanbin; Wang, Shuangfeng

2016-05-01

Flame spread and extinction phenomena over a thick PMMA in purely opposed and concurrent flows are investigated by conducting systematical experiments in a narrow channel apparatus. The present tests focus on low-velocity flow regime and hence complement experimental data previously reported for high and moderate velocity regimes. In the flow velocity range tested, the opposed flame is found to spread much faster than the concurrent flame at a given flow velocity. The measured spread rates for opposed and concurrent flames can be correlated by corresponding theoretical models of flame spread, indicating that existing models capture the main mechanisms controlling the flame spread. In low-velocity gas flows, however, the experimental results are observed to deviate from theoretical predictions. This may be attributed to the neglect of radiative heat loss in the theoretical models, whereas radiation becomes important for low-intensity flame spread. Flammability limits using oxygen concentration and flow velocity as coordinates are presented for both opposed and concurrent flame spread configurations. It is found that concurrent spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode only. The lowest oxygen concentration allowing concurrent flame spread in forced flow is estimated to be approximately 14 % O2, substantially below that for opposed spread (18.5 % O2).

DOE Office of Scientific and Technical Information (OSTI.GOV)

Friedman, B.; DuCharme, G.

We present a semi-empirical scaling law for non-resonant ion–atom single charge exchange cross sections for collisions with velocities frommore » $${10}^{7}\\,{\\rm{t}}{\\rm{o}}\\,{10}^{9}\\,\\mathrm{cm}\\,{{\\rm{s}}}^{-1}$$ and ions with positive charge $$q\\lt 8$$. Non-resonant cross sections tend to have a velocity peak at collision velocities $$v\\lesssim 1\\ {\\rm{a}}{\\rm{u}}$$ with exponential decay around this peak. We construct a scaling formula for the location of this peak then choose a functional form for the cross section curve and scale it. The velocity at which the cross section peaks, v m, is proportional to the energy defect of the collision, $${\\rm{\\Delta }}E$$, which we predict with the decay approximation. The value of the cross section maximum is proportional to the charge state q, inversely proportional to the target ionization energy I T, and inversely proportional to v m. For the shape of the cross section curve, we use a function that decays exponentially asymptotically at high and low velocities. We scale this function with parameters $${v}_{{\\rm{m}}},{I}_{{\\rm{T}}},{Z}_{{\\rm{T}}},\\mathrm{and}\\ {Z}_{{\\rm{P}}}$$, where the $${Z}_{{\\rm{T}},{\\rm{P}}}$$ are the target and projectile atomic numbers. In conclusion, for the more than 100 cross section curves that we use to find the scaling rules, the scaling law predicts cross sections within a little over a factor of 2 on average.« less

COLLIDE: Collisions into Dust Experiment

NASA Technical Reports Server (NTRS)

Colwell, Joshua E.

1999-01-01

The Collisions Into Dust Experiment (COLLIDE) was completed and flew on STS-90 in April and May of 1998. After the experiment was returned to Earth, the data and experiment were analyzed. Some anomalies occurred during the flight which prevented a complete set of data from being obtained. However, the experiment did meet its criteria for scientific success and returned surprising results on the outcomes of very low energy collisions into powder. The attached publication, "Low Velocity Microgravity Impact Experiments into Simulated Regolith," describes in detail the scientific background, engineering, and scientific results of COLLIDE. Our scientific conclusions, along with a summary of the anomalies which occurred during flight, are contained in that publication. We offer it as our final report on this grant.

Analysis and Consequences of the Iridium 33-Cosmos 2251 Collision

NASA Technical Reports Server (NTRS)

Anz-Meador, P. D.; Liou, Jer-Chi

2010-01-01

The collision of Iridium 33 and Cosmos 2251, on 10 February 2009, was the first known unintentional hypervelocity collision in space of intact satellites. Iridium 33 was an active commercial telecommunications satellite, while Cosmos 2251 was a derelict communication satellite of the Strela-2M class. The collision occurred at a relative velocity of 11.6 km/s at an altitude of approximately 790 km over the Great Siberian Plain and near the northern apex of Cosmos 2251 s orbit. This paper describes the physical and orbital characteristics of the relevant spacecraft classes and reports upon our analysis of the resulting debris clouds size, mass, area-to-mass ratio, and relative velocity/directionality distributions. We compare these distributions to those predicted by the NASA breakup model and notable recent fragmentation events; in particular, we compare the area-to-mass ratio distribution for each spacecraft to that exhibited by the FY-1C debris cloud for the purpose of assessing the relative contribution of modern aerospace materials to debris clouds resulting from energetic collisions. In addition, we examine the long-term consequences of this event for the low Earth orbit (LEO) environment. Finally, we discuss "lessons learned", which may be incorporated into NASA s environmental models.

Incoherent radar spectra in the auroral ionosphere in the presence of a large electric field: The effect of O+-O+ Coulomb collisions

NASA Astrophysics Data System (ADS)

Barghouthi, I. A.

2005-06-01

We have used Monte Carlo simulations of O+ velocity distributions in the high latitude F- region to improve the calculation of incoherent radar spectra in auroral ionosphere. The Monte Carlo simulation includes ionneutral, O+-O collisions (resonant charge exchange and polarization interaction) as well as O+-O+ Coulomb self-collisions. At high altitudes, atomic oxygen O and atomic oxygen ion O+ dominate the composition of the auroral ionosphere and consequently, the influence of O+-O+ Coulomb collisions becomes significant. In this study we consider the effect of O+-O+ Coulomb collisions on the incoherent radar spectra in the presence of large electric field (100 mVm-1). As altitude increases (i.e. the ion-to-neutral density ratio increases) the role of O+-O+ Coulomb self-collisions becomes significant, therefore, the one-dimensional, 1-D, O+ ion velocity distribution function becomes more Maxwellian and the features of the radar spectrum corresponding to non-Maxwellian ion velocity distribution (e.g. baby bottle and triple hump shapes) evolve to Maxwellian ion velocity distribution (single and double hump shapes). Therefore, O+-O+ Coulomb self-collisions act to isotropize the 1-D O+ velocity distribution by transferring thermal energy from the perpendicular direction to the parallel direction, however the convection electric field acts to drive the O+ ions away from equilibrium and consequently, non-Maxwellian O+ ion velocity distributions appeared. Therefore, neglecting O+-O+ Coulomb self-collisions overestimates the effect of convection electric field.

Transverse-velocity scaling of femtoscopy in \\sqrt{s}=7\\,{TeV} proton–proton collisions

NASA Astrophysics Data System (ADS)

Humanic, T. J.

2018-05-01

Although transverse-mass scaling of femtoscopic radii is found to hold to a good approximation in heavy-ion collision experiments, it is seen to fail for high-energy proton–proton collisions. It is shown that if invariant radius parameters are plotted versus the transverse velocity instead, scaling with the transverse velocity is seen in \\sqrt{s}=7 TeV proton–proton experiments. A simple semi-classical model is shown to qualitatively reproduce this transverse velocity scaling.

Low-frequency fluid waves in fractures and pipes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Korneev, Valeri

2010-09-01

Low-frequency analytical solutions have been obtained for phase velocities of symmetrical fluid waves within both an infinite fracture and a pipe filled with a viscous fluid. Three different fluid wave regimes can exist in such objects, depending on the various combinations of parameters, such as fluid density, fluid viscosity, walls shear modulus, channel thickness, and frequency. Equations for velocities of all these regimes have explicit forms and are verified by comparisons with the exact solutions. The dominant role of fractures in rock permeability at field scales and the strong amplitude and frequency effects of Stoneley guided waves suggest the importancemore » of including these wave effects into poroelastic theories.« less

DSMC Simulations of Apollo Capsule Aerodynamics for Hypersonic Rarefied Conditions

NASA Technical Reports Server (NTRS)

Moss, James N.; Glass, Christopher E.; Greene, Francis A.

2006-01-01

Direct simulation Monte Carlo DSMC simulations are performed for the Apollo capsule in the hypersonic low density transitional flow regime. The focus is on ow conditions similar to that experienced by the Apollo Command Module during the high altitude portion of its reentry Results for aerodynamic forces and moments are presented that demonstrate their sensitivity to rarefaction that is for free molecular to continuum conditions. Also aerodynamic data are presented that shows their sensitivity to a range of reentry velocity encompasing conditions that include reentry from low Earth orbit lunar return and Mars return velocities to km/s. The rarefied results are anchored in the continuum regime with data from Navier Stokes simulations

Collision rates and impact velocities in the Main Asteroid Belt

NASA Technical Reports Server (NTRS)

Farinella, Paolo; Davis, Donald R.

1992-01-01

Wetherill's (1967) algorithm is presently used to compute the mutual collision probabilities and impact velocities of a set of 682 asteroids with large-than-50-km radius representative of a bias-free sample of asteroid orbits. While collision probabilities are nearly independent of eccentricities, a significant decrease is associated with larger inclinations. Collisional velocities grow steeply with orbital eccentricity and inclination, but with curiously small variation across the asteroid belt. Family asteroids are noted to undergo collisions with other family members 2-3 times more often than with nonmembers.

Active spectroscopy upgrades and neutral beam injection on LTX- β

NASA Astrophysics Data System (ADS)

Elliott, Drew; Biewer, Theodore; Canik, John; Reinke, Matthew; Bell, Ronald; Boyle, Dennis; Guttenfelder, Walter; Kaita, Robert; Kozub, Thomas; Majeski, Richard; Merino, Enrique

2017-10-01

The LTX- β upgrade includes the addition of neutral beam injection (NBI) and increased active spectroscopy. Typical plasmas have been and are expected to remain inboard limited, at 14 cm with minor radii of 18-23 cm. The NBI, 35 Amps of 20 keV particles, will enable active diagnosis of ion velocity distribution profiles through charge exchange (CHERS). 18 CHERS views will cover more than a full minor radius, each sampling 2 cm of major radius. The system has both a set of beam directed ``active'' views and a symmetric set of views pointing away from the beam for stray light subtraction. Along with measuring ion temperatures and impurity transport, the CHERS diagnostic will measure the plasma rotation profiles. The recently described low recycling regime is predicted to allow for high rotational velocities due to the low neutral drag. The planned NBI has been predicted to give on axis velocities near 100 km/s. Flow shear is expected to increase confinement in this regime by suppressing trapped electron mode and other microturbulence enhanced transport. Upgrades to the Thomson scattering system, including an array of polychromators and a new camera, will assist in diagnosing the low density hot edge in this low recycling regime. Supported by US DOE contracts DE-AC02-09CH11466 and DE-AC05-00OR22725.

Applications of the discrete Enskog-Boltzmann approach

NASA Astrophysics Data System (ADS)

Chu, Kwang-Hua Rainer

1998-08-01

The continuous progress of micromachining technology has led to a growing interest in MicroElectroMechanical System (MEMS) for applications ranging from simple microsensors and microactuators to sophisticated microsystems. The characteristic length scale of these microdevices will be of the order of sub-microns so that the gas flow in this environment is within the rarified gas (RG) regime. In this PhD work, the mass/momentum/energy transport of the monatomic gases along the microchannel and the dispersion/attenuation of 1-D ultrasound propagation (plane wave) of RG are investigated by using the Discrete Enskog-Boltzmann approaches. We applied the 4-velocity coplanar model to plane Poiseuille flow of RG in microchannels. Firstly we reported a steady-state solution for this flow with a final-stage uniform density distribution. Then, we modified the model by introducing a density ratio to accomodate the density variations along the microchannel and to include the grazing-collision effects. We also borrowed thee idea from the Extended Irreversible/Reversible Thermodynamics to derive the pressure-gradient for the dimensional velocity field. Our results show the Knudsen minimum of the non- dimensional volume flow rate for Knudsen number (Kn) around 1.5. Using the macroscopic velocity fields, with Cercignani's comments for the 'Kinetic Temperature', we can calculate the related temperature distribution across the microchannel. We also checked the thermodynamic or equilibrium properties of 4-, 6-, and 8-velocity models, by calculating the dispersion relation of 1-D plane ultrasound wave propagation in the RG regime which has large Kn of O(1). The results (after comparison with the measurements) confirmed that the 4-velocity model is the most suitable model for our applications.

Non-Lorentzian ion cyclotron resonance line shapes arising from velocity-dependent ion-neutral collision frequencies

NASA Technical Reports Server (NTRS)

Whealton, J. H.; Mason, E. A.

1973-01-01

An asymptotic solution of the Boltzmann equation is developed for ICR absorption, without restrictions on the ion-neutral collision frequency or mass ratio. Velocity dependence of the collision frequency causes deviations from Lorentzian line shape.

Energy efficient engine, low-pressure turbine boundary layer program

NASA Technical Reports Server (NTRS)

Gardner, W. B.

1981-01-01

A study was conducted to investigate development of boundary layers under the influence of velocity distributions simulating the suction side of two state-of-the-art turbine airfoils: a forward loaded airfoil (squared-off design) and an aft loaded airfoil (aft-loaded design). These velocity distributions were simulated in a boundary layer wind tunnel. Detailed measurements of boundary layer mean velocity and turbulence intensity profiles were obtained for an inlet turbulence level of 2.4 percent and an exit Reynolds number of 800,000. Flush-mounted hot film probes identified the boundary layer transition regimes in the adverse pressure gradient regions for both velocity distributions. Wall intermittency data showed good agreement with the correlations of Dhawan and Narasimha for the intermittency factor distribution in transitional flow regimes.

Can the synchronization regime for an elastically-mounted cylinder be infinitely wide ?

NASA Astrophysics Data System (ADS)

Govardhan, R.; Williamson, C. H. K.

2000-11-01

One would expect an elastically-mounted cylinder, in a free-stream, to resonate when flow velocities are such that the vortex shedding frequency (f_v) is close to the natural frequency of the structure (f_N), which corresponds to a normalized velocity U = U/f_ND ~ 1/S ~ 5. This was shown by Feng (1968), and by others, yielding typically large amplitude oscillations over a regime of velocities, U^*, from 5 to 7. At low mass ratios, m^* (defined as oscillating mass / displaced fluid mass), Ramberg & Griffin (1981), and Khalak & Williamson (1999), find an increase in this regime of synchronization up to U^* ≈ 12, for mass ratios of around 3. We now ask the question : as mass ratios are further reduced, what limits exist, if any, for the extent of this synchronization regime ? Directly relevant to this question, we discover the existence of a critical mass ratio (m^*_crit), whose value is determined to be: ; m^*_crit = 0.54. In essence, for heavy bodies where m^* >> m^*_crit, the classical ``resonance'' is found close to U^* ~ 5 only. On the other hand, for very light structures, where m^* < m^*_crit, a ``resonance'', with large amplitude vibrations, persists for all velocities, up to extremely high values of flow speed - in fact, such a ``resonant'' vibration regime has no upper limit in flow speed. This infinitely-wide synchronization regime is an unusual result, in sharp contrast to vortex-induced vibration phenomena for bodies of moderate or large mass.

Precollisional velocity correlations in a hard-disk fluid with dissipative collisions.

PubMed

Soto, R; Piasecki, J; Mareschal, M

2001-09-01

Velocity correlations are studied in granular fluids, modeled by the inelastic hard sphere gas. Making a density expansion of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for the evolution of the reduced distributions, we predict the presence of precollisional velocity correlations. They are created by the propagation through correlated sequences of collisions (ring events) of the velocity correlations generated after dissipative collisions. The correlations have their origin in the dissipative character of collisions, being always present in granular fluids. The correlations, that manifest microscopically as an alignment of the velocities of a colliding pair produce modifications of collisional averages, in particular, the virial pressure. The pressure shows a reduction with respect to the elastic case as a consequence of the velocity alignment. Good qualitative agreement is obtained for the comparison of the numerical evaluations of the obtained analytical expressions and molecular dynamics results that showed evidence of precollisional velocity correlations [R. Soto and M. Mareschal, Phys. Rev. E 63, 041303 (2001)].

Collision Based Blood Cell Distribution of the Blood Flow

NASA Astrophysics Data System (ADS)

Cinar, Yildirim

2003-11-01

Introduction: The goal of the study is the determination of the energy transferring process between colliding masses and the application of the results to the distribution of the cell, velocity and kinetic energy in arterial blood flow. Methods: Mathematical methods and models were used to explain the collision between two moving systems, and the distribution of linear momentum, rectilinear velocity, and kinetic energy in a collision. Results: According to decrease of mass of the second system, the velocity and momentum of constant mass of the first system are decreased, and linearly decreasing mass of the second system captures a larger amount of the kinetic energy and the rectilinear velocity of the collision system on a logarithmic scale. Discussion: The cause of concentration of blood cells at the center of blood flow an artery is not explained by Bernoulli principle alone but the kinetic energy and velocity distribution due to collision between the big mass of the arterial wall and the small mass of blood cells must be considered as well.

Universal Behavior of the Initial Stage of Drop Impact

NASA Astrophysics Data System (ADS)

Klaseboer, Evert; Manica, Rogerio; Chan, Derek Y. C.

2014-11-01

During the early stages of the impact of a drop on a solid surface, pressure builds up in the intervening thin lubricating air layer and deforms the drop. The extent of the characteristic deformation is determined by the competition between capillary, gravitational, and inertial forces that has been encapsulated in a simple analytic scaling law. For millimetric drops, variations of the observed deformation with impact velocity V exhibit a maximum defined by the Weber and Eötvös numbers: We =1 +Eo . The deformation scales as V1 /2 at the low-velocity capillary regime and as V-1 /2 at the high-velocity inertia regime, in excellent agreement with a variety of experimental systems.

An experimental study of low velocity impacts into granular material in reduced gravity

NASA Astrophysics Data System (ADS)

Murdoch, Naomi; Avila Martinez, Iris; Sunday, Cecily; Cherrier, Olivier; Zenou, Emanuel; Janin, Tristan; Cadu, Alexandre; Gourinat, Yves; Mimoun, David

2016-04-01

The granular nature of asteroid surfaces, in combination with the low surface gravity, makes it difficult to predict lander - surface interactions from existing theoretical models. Nonetheless, an understanding of such interactions is particularly important for the deployment of a lander package. This was demonstrated by the Philae lander, which bounced before coming to rest roughly 1 kilometer away from its intended landing site on the surface of comet 67P/Churyumov-Gerasimenko before coming to rest (Biele et al., 2015). In addition to being important for planning the initial deployment, information about the acceleration profile upon impact is also important in the choice of scientific payloads that want to exploit the initial landing to study the asteroid surface mechanical properties (e.g., Murdoch et al., 2016). Using the ISAE-SUPAERO drop tower, we have performed a series of low-velocity collisions into granular material in low gravity. Reduced-gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use an Atwood machine, or a system of pulleys and counterweights. In reducing the effective surface acceleration of the granular material, the confining pressure will be reduced, and the properties of the granular material will become more representative of those on an asteroid's surface. In addition, since both the surface and projectile are falling, the projectile requires a minimum amount of time to catch the surface before the collision begins. This extended free-fall increases the experiment duration, making it easier to use accelerometers and high-speed cameras for data collection. The experiment is built into an existing 5.5 m drop-tower frame and has required the custom design of all components, including the projectile, surface sample container, release mechanism and deceleration system (Sunday et al., 2016). Previous experiments using similar methods have demonstrated the important role of gravity in the peak accelerations and collision timescales during low velocity granular impacts (Goldman and Umbanhower, 2007; Alsthuler et al., 2013). The design of our experiment accommodates collision velocities and effective accelerations that are lower than in previous experiments (<20 cm/s and ˜0.1 - 1.0 m/s2, respectively), allowing us to come closer to the conditions that may be encountered by current and future small body missions. [1] Altshuler, E., et al., "Extraterrestrial sink dynamics in granular matter", arXiv 1305.6796, 2013. [2] Biele, J., et al., "The landing(s) of Philae and inferences about comet surface mechanical properties", Science, 349 (6247), 2015. [3] Goldman, D. I., Umbanhowar, P., Scaling and dynamics of sphere and disk impact into granular media, Physics Review E 77 (2), (2008) 021308. [4] Murdoch, N., et al. "Investigating the surface and subsurface properties of the Didymos binary asteroid with a landed CubeSat", EGU, 2016. [5] Sunday, C., et al., "An original facility for reduced-gravity testing: a set-up for studying low-velocity collisions into granular surfaces", Submitted to the Review of Scientific Instruments, 2016.

Maintenance of Austral Summertime Upper-Tropospheric Circulation over Tropical South America: The Bolivian High-Nordeste Low System.

NASA Astrophysics Data System (ADS)

Chen, Tsing-Chang; Weng, Shu-Ping; Schubert, Siegfried

1999-07-01

Using the NASA/GEOS reanalysis data for 1980-95, the austral-summer stationary eddies in the tropical-subtropical Southern Hemisphere are examined in two wave regimes: long and short wave (wave 1 and waves 2-6, respectively). The basic structure of the Bolivian high-Nordeste low (BH-NL) system is formed by a short-wave train across South America but modulated by the long-wave regime. The short-wave train exhibits a monsoonlike vertical phase reversal in the midtroposphere and a quarter-wave phase shift relative to the divergent circulation. As inferred from (a) the spatial relationship between the streamfunction and velocity potential and (b) the structure of the divergent circulation, the short-wave train forming the BH-NL system is maintained by South American local heating and remote African heating, while the long-wave regime is maintained by western tropical Pacific heating.The maintenance of the stationary waves in the two wave regimes is further illustrated by a simple diagnostic scheme that includes the velocity-potential maintenance equation (which links velocity potential and diabatic heating) and the streamfunction budget (which is the inverse Laplacian transform of the vorticity equation). Some simple relationships between streamfunction and velocity potential for both wave regimes are established to substantiate the links between diabatic heating and streamfunction; of particular interest is a Sverdrup balance in the short-wave regime. This simplified vorticity equation explains the vertical structure of the short-wave train associated with the BH-NL system and its spatial relationship with the divergent circulation.Based upon the diagnostic analysis of its maintenance a simple forced barotropic model is adopted to simulate the BH-NL system with idealized forcings, which imitates the real 200-mb divergence centers over South America, Africa, and the tropical Pacific. Numerical simulations demonstrate that the formation of the BH-NL system is affected not only by the African remote forcing, but also by the tropical Pacific forcing.

Rydberg Molecules for Ion-Atom Scattering in the Ultracold Regime

NASA Astrophysics Data System (ADS)

Schmid, T.; Veit, C.; Zuber, N.; Löw, R.; Pfau, T.; Tarana, M.; Tomza, M.

2018-04-01

We propose a novel experimental method to extend the investigation of ion-atom collisions from the so far studied cold, essentially classical regime to the ultracold, quantum regime. The key aspect of this method is the use of Rydberg molecules to initialize the ultracold ion-atom scattering event. We exemplify the proposed method with the lithium ion-atom system, for which we present simulations of how the initial Rydberg molecule wave function, freed by photoionization, evolves in the presence of the ion-atom scattering potential. We predict bounds for the ion-atom scattering length from ab initio calculations of the interaction potential. We demonstrate that, in the predicted bounds, the scattering length can be experimentally determined from the velocity of the scattered wave packet in the case of 6Li+ = 6Li and from the molecular ion fraction in the case of 7Li+ - 7Li. The proposed method to utilize Rydberg molecules for ultracold ion-atom scattering, here particularized for the lithium ion-atom system, is readily applicable to other ion-atom systems as well.

Rydberg Molecules for Ion-Atom Scattering in the Ultracold Regime.

PubMed

Schmid, T; Veit, C; Zuber, N; Löw, R; Pfau, T; Tarana, M; Tomza, M

2018-04-13

We propose a novel experimental method to extend the investigation of ion-atom collisions from the so far studied cold, essentially classical regime to the ultracold, quantum regime. The key aspect of this method is the use of Rydberg molecules to initialize the ultracold ion-atom scattering event. We exemplify the proposed method with the lithium ion-atom system, for which we present simulations of how the initial Rydberg molecule wave function, freed by photoionization, evolves in the presence of the ion-atom scattering potential. We predict bounds for the ion-atom scattering length from ab initio calculations of the interaction potential. We demonstrate that, in the predicted bounds, the scattering length can be experimentally determined from the velocity of the scattered wave packet in the case of ^{6}Li^{+}-^{6}Li and from the molecular ion fraction in the case of ^{7}Li^{+}-^{7}Li. The proposed method to utilize Rydberg molecules for ultracold ion-atom scattering, here particularized for the lithium ion-atom system, is readily applicable to other ion-atom systems as well.

Cold Collisions in a Molecular Synchrotron

NASA Astrophysics Data System (ADS)

van der Poel, Aernout P. P.; Zieger, Peter C.; van de Meerakker, Sebastiaan Y. T.; Loreau, Jérôme; van der Avoird, Ad; Bethlem, Hendrick L.

2018-01-01

We study collisions between neutral, deuterated ammonia molecules (ND3 ) stored in a 50 cm diameter synchrotron and argon atoms in copropagating supersonic beams. The advantages of using a synchrotron in collision studies are twofold: (i) By storing ammonia molecules many round-trips, the sensitivity to collisions is greatly enhanced; (ii) the collision partners move in the same direction as the stored molecules, resulting in low collision energies. We tune the collision energy in three different ways: by varying the velocity of the stored ammonia packets, by varying the temperature of the pulsed valve that releases the argon atoms, and by varying the timing between the supersonic argon beam and the stored ammonia packets. These give consistent results. We determine the relative, total, integrated cross section for ND3+Ar collisions in the energy range of 40 - 140 cm-1 , with a resolution of 5 - 10 cm-1 and an uncertainty of 7%-15%. Our measurements are in good agreement with theoretical scattering calculations.

Comparison of collision operators for the geodesic acoustic mode

NASA Astrophysics Data System (ADS)

Li, Yang; Gao, Zhe

2015-04-01

The collisional damping rate and real frequency of the geodesic acoustic mode (GAM) are solved from a drift kinetic model with different collision operators. As the ion collision rate increases, the damping rate increases at low collision rate but decays at high ion collision rate. Different collision operators do not change the overall trend but influence the magnitude of the damping rate. The collision damping is much overestimated with the number-conserving-only Krook operator; on the other hand, using the Lorentz operator with a constant collision rate, the damping is overestimated at low collision rate but underestimated at high collision rate. The results from the Krook operator with both number and energy conservation terms, the Lorentz operator with an energy-dependent collision rate and the full Hirshman-Sigmar-Clarke collision operator are very close. Meanwhile, as the ion collision rate increases, the GAM frequency decreases from the collisionless value, \\sqrt {7/4+τ} {vti}/R , to \\sqrt {1+τ} {vti}/R for the number-conserving-only Krook operator, but to \\sqrt {5/3+τ} {vti}/R for the other four operators, which conserve both number and energy, where τ, vti and R are the ratio of electron temperature to ion temperature, the ion thermal velocity and the major radius, respectively. The results imply that the property of energy conservation of the collision operator is important to the dynamics of the GAM as well as that of number conservation, which may provide guidance in choosing collision operators in further study of the zonal flow (ZF) dynamics, such as the nonlinear simulation of the ZF-turbulence system.

Systematization of material consumption norms in spray-coating

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lelyukh, I.M.

1995-03-01

Regulating the consumption of materials is particularly important in the economics and organization of spray-coating operations. Three main factors are taken into account when establishing norms for the consumption of the materials of the coating: the physicomechanical and chemical properties of the particles; the shape of the substrate; the dimensions of the substrate. The most important parameters of the spraying regime are the velocity and temperature of the particles. Given the same velocity, the optimum particle kinetic energy for producing a strong bond with the substrate depends on particle shape and size and the density of the materials being spray-coated.more » These parameters determine the heating of the particles in the plasma jet or, in the case of the use of a detonation gun, during collision with the surface of the part. Powders of fragmented or drop shape are used to obtain coatings by spraying.« less

Collision velocity of dust grains in self-gravitating protoplanetary discs

PubMed Central

Booth, Richard A.; Clarke, Cathie J.

2016-01-01

We have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments. The viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient spiral features (Stokes numbers, St ∼ 1); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation. We show that, for a single-sized dust population, velocity driving by the disc's gravitational perturbations is only effective for St > 3, while coupling to the gas velocity dominates otherwise. We develop a criterion for understanding this result in terms of the stopping distance being of the order of the disc scaleheight. Nevertheless, the relative velocities induced by differential radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond St ∼ 10− 2 or 10−1 (10 cm to m sizes at 30 au), such Stokes numbers being insufficient to allow concentration of solids in spiral features. However, for icy solids (which may survive collisions up to several 10 m s−1), growth to St ∼ 1 (10 m size) may be possible beyond 30 au from the star. Such objects would be concentrated in spiral features and could potentially produce larger icy planetesimals/comets by gravitational collapse. These planetesimals would acquire moderate eccentricities and remain unmodified over the remaining lifetime of the disc. PMID:27346980

The viscous to brittle transition in eruptions of clay suspensions

NASA Astrophysics Data System (ADS)

Schmid, Diana; Scheu, Bettina; Wadsworth, Fabian B.; Kennedy, Ben; Jolly, Art; Dingwell, Donald B.

2017-04-01

The research is motivated by the early 2013 activity of White Island, New Zealand, which was characterized by frequent small phreatic activity through a fine grained mud rich shallow crater lake. Field observations demonstrate that the small eruptions were driven by bubble-burst events. Additionally, during the ongoing eruption, water vigorously evaporated, causing a shift in rheology of the crater lake liquid-solid suspension. Yet, the effect of water content on the eruptive behaviour of clay-bearing liquid suspensions is poorly understood. Here we investigate the influence of the solid to water ratio of the clay material erupted on the eruption characteristics. Kaolin was used as an analogue for the clay and was mixed with water in different proportions. We conducted experiments with different kaolin/water mixtures held at 120°C, in which they were decompressed from 2-4 bars to ambient conditions in a few milliseconds. During an experimental eruption, the velocity of the ejected material decreased, resulting in shifts in behaviour. Based on our experimental observations we established five different regimes that depend on the particle velocity relative to the gas velocity, and on the kaolin to water ratio of the mixture. In all experiments and for all kaolin to water ratios, regime 1 is one in which particles are ejected rapidly in an expanding high velocity gas jet. In the liquid-dominated system (low kaolin to water ratios), the jet phase evolves to the ejection of elongate fluidal structures (regime 2) and then to discrete droplets (regime 3) as the ejection velocity wanes. Contrastingly, in the solid-dominated system, the jet phase (regime 1) transitions to a mixed solid-fluid structures (regime 4) and then to individual angular ejecta (regime 5). On the basis of high speed image analysis, we establish a phase diagram separating these regimes based on kaolin/water mixing rations and the ejecta velocities observed. The dominant transition between fluidal and solid-like behaviour is a viscous to brittle transition and occurs between a kaolin mass fraction of 0.48 and 0.65, which is consistent with previous observations of the liquid and plastic rheological limits, respectively. We find that a Stokes' number balances the timescale of flow with the timescale of particle motion opposing flow. We suggest that the transition from regime 1 to regime 2 occurs when the relative velocity between the ejected material and the gas phase increases and the Stokes' number exceeds 1, leading to decoupling and shear-stresses at the ejected fluid interfaces. A capillary number characterizes the transition from elongated liquid structures (regime 2) to individual droplets (regime 3) in the liquid-dominated system when the relative velocity drops to a value at which surface tension can restore the droplets to spherical. Our results emphasize that the different rheology of muddy material exhibit different characteristic eruption styles and offers a way to classify them.

Dewetting of low-viscosity films at solid/liquid interfaces.

PubMed

Péron, Nicolas; Brochard-Wyart, Françoise; Duval, Hervé

2012-11-13

We report new experimental results on the dewetting of a mercury film (A) intercalated between a glass slab and an external nonmiscible liquid phase (B) under conditions of a large equilibrium contact angle. The viscosity of the external phase, ηB, was varied over 7 orders of magnitude. We observe a transition between two regimes of dewetting at a threshold viscosity of η(B)* ≈ (ρ(A)e|S̃|)(1/2), where ρ(A) is the mercury density, e is the film thickness, and |S̃| is the effective spreading coefficient. For η(B) < η(B)*, the regime is inertial. The velocity of dewetting is constant and ruled by Culick’s law, V ≈ (|S̃|/(ρ(A)e))(1/2). Capillary waves were observed at high dewetting velocities: they are a signature of hydraulic shock. For η(B) > η(B)*, the regime is viscous. The dewetting velocity is constant and scales as V ≈ |S̃|/η(B) in the limit of large η(B). We interpret this regime by a balance between the surface energy released during dewetting and the viscous dissipation in the surrounding liquid.

CFD simulations of the flow control performance applied for inlet of low drag high-bypass turbofan engine at cross flow regimes

NASA Astrophysics Data System (ADS)

Kursakov, I. A.; Kazhan, E. V.; Lysenkov, A. V.; Savelyev, A. A.

2016-10-01

Paper describes the optimization procedure for low cruise drag inlet of high-bypass ratio turbofan engine (HBRE). The critical cross-flow velocity when the flow separation on the lee side of the inlet channel occurs is determined. The effciency of different flow control devices used to improve the flow parameters at inlet section cross flow regime is analyzed. Boundary layer suction, bypass slot and vortex generators are considered. It is shown that flow control devices enlarge the stability range of inlet performance at cross flow regimes.

Impact cratering on porous targets in the strength regime

NASA Astrophysics Data System (ADS)

Nakamura, Akiko M.

2017-12-01

Cratering on small bodies is crucial for the collision cascade and also contributes to the ejection of dust particles into interplanetary space. A crater cavity forms against the mechanical strength of the surface, gravitational acceleration, or both. The formation of moderately sized craters that are sufficiently larger than the thickness of the regolith on small bodies, in which mechanical strength plays the dominant role rather than gravitational acceleration, is in the strength regime. The formation of microcraters on blocks on the surface is also within the strength regime. On the other hand, the formation of a crater of a size comparable to the thickness of the regolith is affected by both gravitational acceleration and cohesion between regolith particles. In this short review, we compile data from the literature pertaining to impact cratering experiments on porous targets, and summarize the ratio of spall diameter to pit diameter, the depth, diameter, and volume of the crater cavity, and the ratio of depth to diameter. Among targets with various porosities studied in the laboratory to date, based on conventional scaling laws (Holsapple and Schmidt, J. Geophys. Res., 87, 1849-1870, 1982) the cratering efficiency obtained for porous sedimentary rocks (Suzuki et al., J. Geophys. Res. 117, E08012, 2012) is intermediate. A comparison with microcraters formed on a glass target with impact velocities up to 14 km s-1 indicates a different dependence of cratering efficiency and depth-to-diameter ratio on impact velocity.

Effect of Particle Size and Impact Velocity on Collision Behaviors Between Nano-Scale TiN Particles: MD Simulation.

PubMed

Yao, Hai-Long; Hu, Xiao-Zhen; Yang, Guan-Jun

2018-06-01

Inter-particle bonding formation which determines qualities of nano-scale ceramic coatings is influenced by particle collision behaviors during high velocity collision processes. In this study, collision behaviors between nano-scale TiN particles with different diameters were illuminated by using Molecular Dynamics simulation through controlling impact velocities. Results show that nano-scale TiN particles exhibit three states depending on particle sizes and impact velocities, i.e., bonding, bonding with localized fracturing, and rebounding. These TiN particles states are summarized into a parameter selection map providing an overview of the conditions in terms of particle sizes and velocities. Microstructure results show that localized atoms displacement and partial fracture around the impact region are main reasons for bonding formation of nano-scale ceramic particles, which shows differences from conventional particles refining and amorphization. A relationship between the adhesion energy and the rebound energy is established to understand bonding formation mechanism for nano-scale TiN particle collision. Results show that the energy relationship is depended on the particle sizes and impact velocities, and nano-scale ceramic particles can be bonded together as the adhesion energy being higher than the rebound energy.

Using Collision Cones to Asses Biological Deconiction Methods

NASA Astrophysics Data System (ADS)

Brace, Natalie

For autonomous vehicles to navigate the world as efficiently and effectively as biological species, improvements are needed in terms of control strategies and estimation algorithms. Reactive collision avoidance is one specific area where biological systems outperform engineered algorithms. To better understand the discrepancy between engineered and biological systems, a collision avoidance algorithm was applied to frames of trajectory data from three biological species (Myotis velifer, Hirundo rustica, and Danio aequipinnatus). The algorithm uses information that can be sensed through visual cues (relative position and velocity) to define collision cones which are used to determine if agents are on a collision course and if so, to find a safe velocity that requires minimal deviation from the original velocity for each individual agent. Two- and three-dimensional versions of the algorithm with constant speed and maximum speed velocity requirements were considered. The obstacles provided to the algorithm were determined by the sensing range in terms of either metric or topological distance. The calculated velocities showed good correlation with observed velocities over the range of sensing parameters, indicating that the algorithm is a good basis for comparison and could potentially be improved with further study.

The Physics of Protoplanetesimal Dust Agglomerates. VIII. Microgravity Collisions between Porous SiO{sub 2} Aggregates and Loosely Bound Agglomerates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Whizin, Akbar D.; Colwell, Joshua E.; Blum, Jürgen, E-mail: Akbar.Whizin@ucf.edu

2017-02-10

We performed laboratory experiments colliding 0.8–1.0 mm and 1.0–1.6 mm SiO{sub 2} dust aggregates with loosely bound centimeter-sized agglomerates of those aggregates in microgravity. This work builds on previous microgravity laboratory experiments examining the collisional properties of porous loosely bound dust aggregates. In centimeter-sized aggregates, surface forces dominate self-gravity and may play a large role in aggregate growth beyond this size range. We characterize the properties of protoplanetary aggregate analogs to help place constraints on initial formation mechanisms and environments. We determined several important physical characteristics of these aggregates in a large number of low-velocity collisions. We observed low coefficientsmore » of restitution and fragmentation thresholds near 1 m s{sup −1} for 1–2 cm agglomerates, which are in good agreement with previous findings in the literature. We find the accretion efficiency for agglomerates of loosely bound aggregates to be higher than that for just aggregates themselves. We find sticking thresholds of 6.6 ± 2 cm s{sup −1}, somewhat higher than those in similar studies, which have observed few aggregates stick at speeds of under 3 cm s{sup −1}. Even with highly dissipative collisions, loosely bound agglomerates have difficulty accreting beyond centimeter-sized bodies at typical collision speeds in the disk. Our results indicate agglomerates of porous aggregates have slightly higher sticking thresholds than previously thought, allowing possible growth to decimeter-sized bodies if velocities are low enough.« less

Microgravity experiments on vibrated granular gases in a dilute regime: non-classical statistics

NASA Astrophysics Data System (ADS)

Leconte, M.; Garrabos, Y.; Falcon, E.; Lecoutre-Chabot, C.; Palencia, F.; Évesque, P.; Beysens, D.

2006-07-01

We report on an experimental study of a dilute gas of steel spheres colliding inelastically and excited by a piston performing sinusoidal vibration, in low gravity. Using improved experimental apparatus, here we present some results concerning the collision statistics of particles on a wall of the container. We also propose a simple model where the non-classical statistics obtained from our data are attributed to the boundary condition playing the role of a 'velostat' instead of a thermostat. The significant differences from the kinetic theory of usual gas are related to the inelasticity of collisions.

A particle-particle collision strategy for arbitrarily shaped particles at low Stokes numbers

NASA Astrophysics Data System (ADS)

Daghooghi, Mohsen; Borazjani, Iman

2016-11-01

We present a collision strategy for particles with any general shape at low Stokes numbers. Conventional collision strategies rely upon a short -range repulsion force along particles centerline, which is a suitable choice for spherical particles and may not work for complex-shaped particles. In the present method, upon the collision of two particles, kinematics of particles are modified so that particles have zero relative velocity toward each other along the direction in which they have the minimum distance. The advantage of this novel technique is that it guaranties to prevent particles from overlapping without unrealistic bounce back at low Stokes numbers, which may occur if repulsive forces are used. This model is used to simulate sedimentation of many particles in a vertical channel and suspensions of non-spherical particles under simple shear flow. This work was supported by the American Chemical Society (ACS) Petroleum Research Fund (PRF) Grant Number 53099-DNI9. The computational resources were partly provided by the Center for Computational Research (CCR) at the University at Buffalo.

Charge transfer during individual collisions in ice growing by riming

NASA Technical Reports Server (NTRS)

Avila, Eldo E.; Caranti, Giorgio M.

1991-01-01

The charging of a target by riming in the wind was studied in the temperature range of (-10, -18 C). For each temperature, charge transfers of both signs are observed and, according to the environmental conditions, one of them prevails. The charge is more positive as the liquid water concentration is increased at any particular temperature. It is found that even at the low impact velocities used (5 m/s) there is abundant evidence of fragmentation following the collision.

Effects of Mean Flow Profiles on the Instability of a Low-Density Gas Jet Injected into a High-Density Gas

NASA Technical Reports Server (NTRS)

Vedantam, NandaKishore; Parthasarathy, Ramkumar N.

2004-01-01

The effects of the mean velocity profiles on the instability characteristics in the near-injector region of axisymmetric low density gas jets injected vertically upwards into a high-density gas medium were investigated using linear inviscid stability analysis. The flow was assumed to be isothermal and locally parallel. Three velocity profiles, signifying different changes in the mean velocity in the shear layer, were used in the analysis. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the instability for each set of mean profiles were delineated. At a large Froude number (negligible gravity), a critical density ratio was found for the three profiles at which the jet became absolutely unstable. The critical density ratio for each velocity profile was increased as the Froude number was reduced. A critical Froude number was found for the three sets of profiles, below which the jet was absolutely unstable for all the density ratios less than unity, which demarcated the jet flow into the momentum-driven regime and the buoyancy-driven regime.

Imaging chemical reactions - 3D velocity mapping

NASA Astrophysics Data System (ADS)

Chichinin, A. I.; Gericke, K.-H.; Kauczok, S.; Maul, C.

Visualising a collision between an atom or a molecule or a photodissociation (half-collision) of a molecule on a single particle and single quantum level is like watching the collision of billiard balls on a pool table: Molecular beams or monoenergetic photodissociation products provide the colliding reactants at controlled velocity before the reaction products velocity is imaged directly with an elaborate camera system, where one should keep in mind that velocity is, in general, a three-dimensional (3D) vectorial property which combines scattering angles and speed. If the processes under study have no cylindrical symmetry, then only this 3D product velocity vector contains the full information of the elementary process under study.

Peculiarities of structural transformations in metal nanoparticles at high speed collisions

NASA Astrophysics Data System (ADS)

Zolnikov, K. P.; Kryzhevich, D. S.; Korchuganov, A. V.

2018-01-01

A molecular dynamics simulation of nanosized particle collision under the electrical explosion of metal wires of different types was conducted. Interatomic interactions were described on the base of the embedded atom method. Used potentials allowed describing with high accuracy many mechanical and physical properties which are very important for the simulations of nanoparticle collisions with high velocities. The dynamics of the nanosized particle formation at the electric pulse explosion of metal wires of different types was studied. Features of particle collisions on the example of nanoscale particles of copper and nickel, whose velocities varied from 50 to 1500 m/s were investigated. The peculiarities of structural transformations in the colliding particles depending on the velocity of collision were determined. The intervals of collision velocities in which interaction between particles is elastic or leads to the formation of structural defects or melting were calculated. The analysis of the structure and distribution of chemical elements over the cross section of the particles which were synthesized under simultaneous explosions of different metal wires was carried out.

Dipole saturated absorption modeling in gas phase: Dealing with a Gaussian beam

NASA Astrophysics Data System (ADS)

Dupré, Patrick

2018-01-01

With the advent of new accurate and sensitive spectrometers, cf. combining optical cavities (for absorption enhancement), the requirement for reliable molecular transition modeling is becoming more pressing. Unfortunately, there is no trivial approach which can provide a definitive formalism allowing us to solve the coupled systems of equations associated with nonlinear absorption. Here, we propose a general approach to deal with any spectral shape of the electromagnetic field interacting with a molecular species under saturation conditions. The development is specifically applied to Gaussian-shaped beams. To make the analytical expressions tractable, approximations are proposed. Finally, two or three numerical integrations are required for describing the Lamb-dip profile. The implemented model allows us to describe the saturated absorption under low pressure conditions where the broadening by the transit-time may dominate the collision rates. The model is applied to two specific overtone transitions of the molecular acetylene. The simulated line shapes are discussed versus the collision and the transit-time rates. The specific collisional and collision-free regimes are illustrated, while the Rabi frequency controls the intermediate regime. We illustrate how to recover the input parameters by fitting the simulated profiles.

Deterministic Role of Collision Cascade Density in Radiation Defect Dynamics in Si

NASA Astrophysics Data System (ADS)

Wallace, J. B.; Aji, L. B. Bayu; Shao, L.; Kucheyev, S. O.

2018-05-01

The formation of stable radiation damage in solids often proceeds via complex dynamic annealing (DA) processes, involving point defect migration and interaction. The dependence of DA on irradiation conditions remains poorly understood even for Si. Here, we use a pulsed ion beam method to study defect interaction dynamics in Si bombarded in the temperature range from ˜-30 ° C to 210 °C with ions in a wide range of masses, from Ne to Xe, creating collision cascades with different densities. We demonstrate that the complexity of the influence of irradiation conditions on defect dynamics can be reduced to a deterministic effect of a single parameter, the average cascade density, calculated by taking into account the fractal nature of collision cascades. For each ion species, the DA rate exhibits two well-defined Arrhenius regions where different DA mechanisms dominate. These two regions intersect at a critical temperature, which depends linearly on the cascade density. The low-temperature DA regime is characterized by an activation energy of ˜0.1 eV , independent of the cascade density. The high-temperature regime, however, exhibits a change in the dominant DA process for cascade densities above ˜0.04 at.%, evidenced by an increase in the activation energy. These results clearly demonstrate a crucial role of the collision cascade density and can be used to predict radiation defect dynamics in Si.

Deterministic Role of Collision Cascade Density in Radiation Defect Dynamics in Si.

PubMed

Wallace, J B; Aji, L B Bayu; Shao, L; Kucheyev, S O

2018-05-25

The formation of stable radiation damage in solids often proceeds via complex dynamic annealing (DA) processes, involving point defect migration and interaction. The dependence of DA on irradiation conditions remains poorly understood even for Si. Here, we use a pulsed ion beam method to study defect interaction dynamics in Si bombarded in the temperature range from ∼-30 °C to 210 °C with ions in a wide range of masses, from Ne to Xe, creating collision cascades with different densities. We demonstrate that the complexity of the influence of irradiation conditions on defect dynamics can be reduced to a deterministic effect of a single parameter, the average cascade density, calculated by taking into account the fractal nature of collision cascades. For each ion species, the DA rate exhibits two well-defined Arrhenius regions where different DA mechanisms dominate. These two regions intersect at a critical temperature, which depends linearly on the cascade density. The low-temperature DA regime is characterized by an activation energy of ∼0.1  eV, independent of the cascade density. The high-temperature regime, however, exhibits a change in the dominant DA process for cascade densities above ∼0.04 at.%, evidenced by an increase in the activation energy. These results clearly demonstrate a crucial role of the collision cascade density and can be used to predict radiation defect dynamics in Si.

Numerical Simulation of Droplet Breakup and Collision in the Solution Precursor Plasma Spraying

NASA Astrophysics Data System (ADS)

Shan, Y.; Coyle, T. W.; Mostaghimi, J.

2007-12-01

Finely structured ceramic coatings can be obtained by solution precursor plasma spraying. The final structure of the coating highly depends on the droplet size and velocity distribution at the injection, the evolution of the spray in the jet, and droplet breakup and collision within the spray. This article describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. O’Rourke’s droplet collision model is used to take into account the influence of droplet collision. The influence of droplet breakup is also considered by implementing TAB droplet breakup models into the plasma jet model. The effects of droplet collisions and breakup on the droplet size, velocity, and temperature distribution of the solution spray are investigated. The results indicate that droplet breakup and collision play an important role in determining the final particle size and velocity distributions on the substrate.

Low-energy collisions of helium clusters with size-selected cobalt cluster ions

NASA Astrophysics Data System (ADS)

Odaka, Hideho; Ichihashi, Masahiko

2017-04-01

Collisions of helium clusters with size-selected cobalt cluster ions, Com+ (m ≤ 5), were studied experimentally by using a merging beam technique. The product ions, Com+Hen (cluster complexes), were mass-analyzed, and this result indicates that more than 20 helium atoms can be attached onto Com+ at the relative velocities of 103 m/s. The measured size distributions of the cluster complexes indicate that there are relatively stable complexes: Co2+Hen (n = 2, 4, 6, and 12), Co3+Hen (n = 3, 6), Co4+He4, and Co5+Hen (n = 3, 6, 8, and 10). These stabilities are explained in terms of their geometric structures. The yields of the cluster complexes were also measured as a function of the relative velocity (1 × 102-4 × 103 m/s), and this result demonstrates that the main interaction in the collision process changes with the increase of the collision energy from the electrostatic interaction, which includes the induced deformation of HeN, to the hard-sphere interaction. Supplementary material in the form of one pdf file available from the Journal web page at http://https://doi.org/10.1140/epjd/e2017-80015-0

A comparison of the temperature and density structure in high and low speed thermal proton flows

NASA Technical Reports Server (NTRS)

Raitt, W. J.; Schunk, R. W.; Banks, P. M.

1975-01-01

Steady-state altitude profiles of H(+) density, drift velocity, and temperature and O(+) density and temperature were deduced for a wide range of H(+) outflow velocities from subsonic to supersonic flow for plasma densities typical of both undisturbed and trough regions of the ionsophere. Allowance was made for the effects of inertia, parallel stress, and the velocity dependence of the H(+) collision frequencies. It was found that at supersonic outflow velocities there is a decrease in H(+) temperature with increasing outflow velocity. The H(+) temperatures are substantially increased above the O(+) temperatures when H(+) is flowing, with T(H+)/T(O+) reaching a maximum ratio of about 3:1.

Self-consistent gyrokinetic modeling of neoclassical and turbulent impurity transport

NASA Astrophysics Data System (ADS)

Estève, D.; Sarazin, Y.; Garbet, X.; Grandgirard, V.; Breton, S.; Donnel, P.; Asahi, Y.; Bourdelle, C.; Dif-Pradalier, G.; Ehrlacher, C.; Emeriau, C.; Ghendrih, Ph.; Gillot, C.; Latu, G.; Passeron, C.

2018-03-01

Trace impurity transport is studied with the flux-driven gyrokinetic GYSELA code (Grandgirard et al 2016 Comput. Phys. Commun. 207 35). A reduced and linearized multi-species collision operator has been recently implemented, so that both neoclassical and turbulent transport channels can be treated self-consistently on an equal footing. In the Pfirsch-Schlüter regime that is probably relevant for tungsten, the standard expression for the neoclassical impurity flux is shown to be recovered from gyrokinetics with the employed collision operator. Purely neoclassical simulations of deuterium plasma with trace impurities of helium, carbon and tungsten lead to impurity diffusion coefficients, inward pinch velocities due to density peaking, and thermo-diffusion terms which quantitatively agree with neoclassical predictions and NEO simulations (Belli et al 2012 Plasma Phys. Control. Fusion 54 015015). The thermal screening factor appears to be less than predicted analytically in the Pfirsch-Schlüter regime, which can be detrimental to fusion performance. Finally, self-consistent nonlinear simulations have revealed that the tungsten impurity flux is not the sum of turbulent and neoclassical fluxes computed separately, as is usually assumed. The synergy partly results from the turbulence-driven in-out poloidal asymmetry of tungsten density. This result suggests the need for self-consistent simulations of impurity transport, i.e. including both turbulence and neoclassical physics, in view of quantitative predictions for ITER.

New Orogenic Model for Taiwan Collision Zone Inferred From Three-dimensional P- and S-wave Velocity Structures and Seismicity

NASA Astrophysics Data System (ADS)

Nagai, S.; Hirata, N.; Sato, H.

2008-12-01

The island of Taiwan is located in the site of ongoing arc-continent collision zone between the Philippine Sea Plate (PSP) and the Eurasian Plate (EUP). Numerous geophysical and geological studies are done in and around Taiwan to develop various models to explain the tectonic processes in the Taiwan region. However, their details have not been known enough, especially under the Central Range. We suggest a new orogenic model for Taiwan orogeny, named 'Upper Crustal Stacking Model', inferred from our tomographic images using three temporary seismic networks with the Central Weather Bureau Seismic Network. These three temporary networks are the aftershock observation after the 1999 Chi-Chi Taiwan earthquake and two dense array observations across central and southern Taiwan, respectively. Tomographic images by the double-difference tomography [Zhang and Thurber, 2003] show a lateral alternate variation of high- and low-velocity, which are well correlated to surface geology and separated by east-dipping boundaries. These images have reliable high-resolution by dense arrays to be able to discuss this alternate variation. We found three high-velocity zones (> 6.0km/s). The westernmost zone corresponds to the subducting EUP. Other two zones are located beneath the Hsuehshan Range and the Eastern Central Range with trends of eastward dipping, respectively. And, we could image low-velocity zone located beneath Backbone Range between the two high-velocity zones clearly. We interpret that these east-dipping high- and low-velocity zones can be divided into two layered blocks and the subducting EUP, each of which consists of a high-velocity body under low-velocity one. Layered blocks can be interpreted as stacked thrust sheets between the subducting EUP and the Northern Luzon Arc, a part of PSP. These thrust sheets are parts of upper- and mid-crust detached from the subducting EUP. The model of continental subduction followed by buoyancy-driven exhumation can explain the existence of stacked thrust sheets. Thus we propose a new orogenic model, as referred to as the 'Upper Crustal Stacking Model'.

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

NASA Astrophysics Data System (ADS)

Wang, Chun-Yong; Huangfu, Gang

2004-02-01

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling-Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian-Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.

Satellite Collision Modeling with Physics-Based Hydrocodes: Debris Generation Predictions of the Iridium-Cosmos Collision Event and Other Impact Events

NASA Astrophysics Data System (ADS)

Springer, H.; Miller, W.; Levatin, J.; Pertica, A.; Olivier, S.

2010-09-01

Satellite collision debris poses risks to existing space assets and future space missions. Predictive models of debris generated from these hypervelocity collisions are critical for developing accurate space situational awareness tools and effective mitigation strategies. Hypervelocity collisions involve complex phenomenon that spans several time and length-scales. We have developed a satellite collision debris modeling approach consisting of a Lagrangian hydrocode enriched with smooth particle hydrodynamics (SPH), advanced material failure models, detailed satellite mesh models, and massively parallel computers. These computational studies enable us to investigate the influence of satellite center-of-mass (CM) overlap and orientation, relative velocity, and material composition on the size, velocity, and material type distributions of collision debris. We have applied our debris modeling capability to the recent Iridium 33-Cosmos 2251 collision event. While the relative velocity was well understood in this event, the degree of satellite CM overlap and orientation was ill-defined. In our simulations, we varied the collision CM overlap and orientation of the satellites from nearly maximum overlap to partial overlap on the outermost extents of the satellites (i.e, solar panels and gravity boom). As expected, we found that with increased satellite overlap, the overall debris cloud mass and momentum (transfer) increases, the average debris size decreases, and the debris velocity increases. The largest predicted debris can also provide insight into which satellite components were further removed from the impact location. A significant fraction of the momentum transfer is imparted to the smallest debris (< 1-5mm, dependent on mesh resolution), especially in large CM overlap simulations. While the inclusion of the smallest debris is critical to enforcing mass and momentum conservation in hydrocode simulations, there seems to be relatively little interest in their disposition. Based on comparing our results to observations, it is unlikely that the Iridium 33-Cosmos 2251 collision event was a large mass-overlap collision. We also performed separate simulations studying the debris generated by the collision of 5 and 10 cm spherical projectiles on the Iridium 33 satellite at closing velocities of 5, 10, and 15 km/s. It is important to understand the vulnerability of satellites to small debris threats, given their pervasiveness in orbit. These studies can also be merged with probabilistic conjunction analysis to better understand the risk to space assets. In these computational studies, we found that momentum transfer, kinetic energy losses due to dissipative mechanisms (e.g., fracture), fragment number, and fragment velocity increases with increasing velocity for a fixed projectile size. For a fixed velocity, we found that the smaller projectile size more efficiently transfers momentum to the satellite. This latter point has an important implication: Eight (spaced) 5 cm debris objects can impart more momentum to the satellite, and likely cause more damage, than a single 10 cm debris object at the same velocity. Further studies are required to assess the satellite damage induced by 1-5 cm sized debris objects, as well as multiple debris objects, in this velocity range.

A mass census of the nearby universe with the RESOLVE survey

NASA Astrophysics Data System (ADS)

Eckert, Kathleen

The galaxy mass function, i.e., the distribution of galaxies as a function of mass, is a useful way to characterize the galaxy population. In this work, we examine the stellar and baryonic mass function, and the velocity function of galaxies and galaxy groups for two volume-limited surveys of the nearby universe. Stellar masses are estimated from multi-band photometry, and we add cold atomic gas from measurements and a newly calibrated estimator to obtain baryonic mass. Velocities are measured from the internal motions of galaxies and groups and account for all matter within the system. We compare our observed mass and velocity functions with the halo mass function from theoretical simulations of dark matter, which predict a much more steeply rising low-mass slope than is normally observed for the galaxy mass function. We show that taking into account the cold gas mass, which dominates the directly detectable mass of low-mass galaxies, steepens the low-mass slope of the galaxy mass function. The low- mass slope of the baryonic mass function, however, is still much shallower than that of the halo mass function. The discrepancy in low-mass slope persists when examining the velocity function, which accounts for all matter in galaxies (detectable or not), suggesting that some mechanism must reduce the mass in halos or destroy them completely. We investigate the role of environment by performing group finding and examining the mass and velocity functions as a function of group halo mass. Broken down by halo mass regime, we find dips and varying low-mass slopes in the mass and velocity functions, suggesting that group formation processes such as merging and stripping, which destroy and lower the mass of low-mass satellites respectively, potentially contribute to the discrepancy in low-mass slope. In particular, we focus on the nascent group regime, groups of mass 10 11.4-12 [solar mass] with few members, which has a depressed and flat low-mass slope in the galaxy mass and velocity function. We find that nascent groups are at the peak baryonic collapse efficiency (group-integrated cold baryonic mass divided by the group halo mass), while isolated dwarfs in lower mass halos are rapidly growing in their collapsed baryonic mass and larger groups are increasingly dominated by their hot halo gas. Scatter in this collapsed baryon efficiency could indicate varying hot gas fractions in nascent groups, suggestive of a wide variety of group formation processes occurring at these scales. We point to this nascent group regime as a period of transition in group evolution, where merging and stripping remove galaxies from the population, contributing to the discrepancy in low-mass slope between observations and dark matter simulations.

Edge-to-center plasma density ratios in two-dimensional plasma discharges

NASA Astrophysics Data System (ADS)

Lucken, R.; Croes, V.; Lafleur, T.; Raimbault, J.-L.; Bourdon, A.; Chabert, P.

2018-03-01

Edge-to-center plasma density ratios—so-called h factors—are important parameters for global models of plasma discharges as they are used to calculate the plasma losses at the reactor walls. There are well-established theories for h factors in the one-dimensional (1D) case. The purpose of this paper is to establish h factors in two-dimensional (2D) systems, with guidance from a 2D particle-in-cell (PIC) simulation. We derive analytical solutions of a 2D fluid theory that includes the effect of ion inertia, but assumes a constant (independent of space) ion collision frequency (using an average ion velocity) across the discharge. Predicted h factors from this 2D fluid theory have the same order of magnitude and the same trends as the PIC simulations when the average ion velocity used in the collision frequency is set equal to the ion thermal velocity. The best agreement is obtained when the average ion velocity varies with pressure (but remains independent of space), going from half the Bohm velocity at low pressure, to the thermal velocity at high pressure. The analysis also shows that a simple correction of the widely-used 1D heuristic formula may be proposed to accurately incorporate 2D effects.

Vibrationally enhanced charge transfer and mode/bond-specific H{sup +} and D{sup +} transfer in the reaction of HOD{sup +} with N{sub 2}O

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bell, David M.; Anderson, Scott L.

2013-09-21

The reaction of HOD{sup +} with N{sub 2}O was studied over the collision energy (E{sub col}) range from 0.20 eV to 2.88 eV, for HOD{sup +} in its ground state and in each of its fundamental vibrational states: bend (010), OD stretch (100), and OH stretch (001). The dominant reaction at low E{sub col} is H{sup +} and D{sup +} transfer, but charge transfer becomes dominant for E{sub col} > 0.5 eV. Increasing E{sub col} enhances charge transfer only in the threshold region (E{sub col} < 1 eV), but all modes of HOD{sup +} vibrational excitation enhance this channel overmore » the entire energy range, by up to a factor of three. For reaction of ground state HOD{sup +}, the H{sup +} and D{sup +} transfer channels have similar cross sections, enhanced by increasing collision energy for E{sub col} < 0.3 eV, but suppressed by E{sub col} at higher energies. OD stretch excitation enhances D{sup +} transfer by over a factor of 2, but has little effect on H{sup +} transfer, except at low E{sub col} where a modest enhancement is observed. Excitation of the OH stretch enhances H{sup +} transfer by up to a factor of 2.5, but actually suppresses D{sup +} transfer over most of the E{sub col} range. Excitation of the bend mode results in ∼60% enhancement of both H{sup +} and D{sup +} transfer at low E{sub col} but has little effect at higher energies. Recoil velocity distributions at high E{sub col} are strongly backscattered in the center-of-mass frame, indicating direct reaction dominated by large impact parameter collisions. At low E{sub col} the distributions are compatible with mediation by a short-lived collision complex. Ab initio calculations find several complexes that may be important in this context, and RRKM calculations predict lifetimes and decay branching that is consistent with observations. The recoil velocity distributions show that HOD{sup +} vibrational excitation enhances reactivity in all collisions at low E{sub col}, while for high E{sub col} with enhancement comes entirely from the subset of collisions that generate strongly back-scattered product ions.« less

The footprint of cometary dust analogues - I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data

NASA Astrophysics Data System (ADS)

Ellerbroek, L. E.; Gundlach, B.; Landeck, A.; Dominik, C.; Blum, J.; Merouane, S.; Hilchenbach, M.; Bentley, M. S.; Mannel, T.; John, H.; van Veen, H. A.

2017-07-01

Cometary dust provides a unique window on dust growth mechanisms during the onset of planet formation. Measurements by the Rosetta spacecraft show that the dust in the coma of comet 67P/Churyumov-Gerasimenko has a granular structure at size scales from sub-μmup to several hundreds of μm, indicating hierarchical growth took place across these size scales. However, these dust particles may have been modified during their collection by the spacecraft instruments. Here, we present the results of laboratory experiments that simulate the impact of dust on the collection surfaces of the COSIMA (Cometary Secondary Ion Mass Anaylzer) and MIDAS (Micro-Imaging Dust Analysis System) instruments onboard the Rosetta spacecraft. We map the size and structure of the footprints left by the dust particles as a function of their initial size (up to several hundred μm) and velocity (up to 6 m s-1). We find that in most collisions, only part of the dust particle is left on the target; velocity is the main driver of the appearance of these deposits. A boundary between sticking/bouncing and fragmentation as an outcome of the particle-target collision is found at v ˜ 2 m s-1. For velocities below this value, particles either stick or leave a single deposit on the target plate, or bounce, leaving a shallow footprint of monomers. At velocities >2 m s-1and sizes >80 μm, particles fragment upon collision, transferring up to 50 per cent of their mass in a rubble-pile-like deposit on the target plate. The amount of mass transferred increases with the impact velocity. The morphologies of the deposits are qualitatively similar to those found by the COSIMA instrument.

Onset of the radial electric field oscillations in the neoclassical plasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, C.S.; Novakovskii, S.V.; Sagdeev, R.Z.

1996-12-31

It is shown that the relaxation of the radial electric field in the tokomak plasmas towards its neoclassical value is accompanied by the fast oscillations of the order of the ion transient frequency V{sub T}/qR. This happens during the transition from the Pfirsch-Schluter collisional regime to the plateau regime at v{sub c}qR/V{sub T} {le} c{sub cr} {le} 1. The investigation has been performed with the help of the specially developed numerical code for solution of the nonsteady-state drift kinetic equation with the exact collisional term in the Hirshman-Sigmar-Clarke form. Comparison with the analytical results, corresponding to the regime of themore » very low collisions as well as with previous approximate models for the plateau regime will also be reported.« less

Collision induced broadening and shifting of the H and K lines of Ca+ at low temperature

NASA Astrophysics Data System (ADS)

Wang, Xin; Zhang, Rui; Shen, Yong; Liu, Qu; Zou, Hongxin; Yan, Bing

2017-09-01

Multireference configuration interaction method was used to compute the potential energy curves of Λ-S states correlating with lowest three atomic limits in Ca+-He molecular collision system. The potential energy curves of nine Ω states were obtained with inclusion of spin-orbit coupling. And the electric dipole and quadrupole moment matrix elements between excited states and ground state were also computed. Furthermore, with aid of the Anderson-Talman theory we calculated the broadening and shifting coefficients for Ca+-He spectral lines in the low temperature regime. For H line, α = 0.303 × 10-20 cm-1/cm-3, β = -0.0527 × 10-20cm-1/cm-3; For K line, α = 0.233 × 10-20cm-1/cm-3, β = -0.0402 × 10-20cm-1/cm-3 These results are helpful to understand the collision effects induced by He atom in further spectra investigations of cold Ca+ ions.

Pinning in the flux-line-cutting regime of Bi 2Sr 2Ca 1Cu 2O 8 single crystals at high field

NASA Astrophysics Data System (ADS)

D'Anna, G.; André, M.-O.; Indenbom, M. V.; Benoit, W.

1994-09-01

Using a low-frequency torsion pendulum we show that in a Bi 2Sr 2Ca 1Cu 2O 8 single crystal the irreversibility line Birr( T) is frequency dependent down to 10 -5 Hz in the high-field regime. The activation energy has a logarithmic field dependence, U0( B)= U∗ 1n( B∗/ B). A microscopic model for flux-line-cutting and pancake collision yields quantitative expressions for U0 and for Birr( T)= B∗ exp(- T/T∗), which reproduce the experimental data very well.

High-Reynolds-number turbulent-boundary-layer wall-pressure fluctuations with dilute polymer solutions

NASA Astrophysics Data System (ADS)

Elbing, Brian R.; Winkel, Eric S.; Ceccio, Steven L.; Perlin, Marc; Dowling, David R.

2010-08-01

Wall-pressure fluctuations were investigated within a high-Reynolds-number turbulent boundary layer (TBL) modified by the addition of dilute friction-drag-reducing polymer solutions. The experiment was conducted at the U.S. Navy's Large Cavitation Channel on a 12.9 m long flat-plate test model with the surface hydraulically smooth (k+<0.2) and achieving downstream-distance-based Reynolds numbers to 220×106. The polymer (polyethylene oxide) solution was injected into the TBL through a slot in the surface. The primary flow diagnostics were skin-friction drag balances and an array of flush-mounted dynamic pressure transducers 9.8 m from the model leading edge. Parameters varied included the free-stream speed (6.7, 13.4, and 20.2 m s-1) and the injection condition (polymer molecular weight, injection concentration, and volumetric injection flux). The behavior of the pressure spectra, convection velocity, and coherence, regardless of the injection condition, were determined primarily based on the level of drag reduction. Results were divided into two regimes dependent on the level of polymer drag reduction (PDR), nominally separated at a PDR of 40%. The low-PDR regime is characterized by decreasing mean-square pressure fluctuations and increasing convection velocity with increasing drag reduction. This shows that the decrease in the pressure spectra with increasing drag reduction is due in part to the moving of the turbulent structures from the wall. Conversely, with further increases in drag reduction, the high-PDR regime has negligible variation in the mean-squared pressure fluctuations and convection velocity. The convection velocity remains constant at approximately 10% above the baseline-flow convection velocity, which suggests that the turbulent structures no longer move farther from the wall with increasing drag reduction. In light of recent numerical work, the coherence results indicate that in the low-PDR regime, the turbulent structures are being elongated in the streamwise direction and occurring at decreasing frequency. In the high-PDR regime, the rate of occurrence continues to decrease until large-scale coherent turbulent structures are potentially no longer present.

The effect of DC voltage polarity on ionic wind in ambient air for cooling purposes

NASA Astrophysics Data System (ADS)

Chen, She; van den Berg, R. G. W.; Nijdam, S.

2018-05-01

Gas flows can be induced by gas discharges like DC coronas because neutral molecules gain momentum by ion-neutral collisions. This can be used for active cooling and has advantages over mechanical fans. We investigate ionic wind by a DC corona discharge under different conditions with an emphasis on the effects of voltage polarity and the transition between different discharge regimes. We also consider the gas temperature of a DC corona which is important when it is to be used for cooling purposes. Although DC coronas are usually characterized as low temperature plasmas, gas heating can have a significant impact on flow generation, especially at higher operating voltages. In this paper, a 5–20 kV DC voltage of positive and negative polarity is applied to a needle–cylinder electrode. The ionic wind velocity at the exit of the cylinder electrode is measured by hot wire anemometry and the emission spectrum is used to study the gas temperature. It is found that the flow velocity induced by positive coronas is higher than that by negative coronas for voltages above 10–15 kV, which is also demonstrated by a phenomenological EHD force model. Furthermore, a heated column is observed by Schlieren technique for both voltage polarities. An improved self-consistent ionic wind model considering heat transfer is built to study the temperature distribution. The simulation results indicate that the gas flow velocity is lower on the symmetry axis when the temperature gradient is taken into account, something which is usually ignored in ionic wind simulations.

MULTI-FLUID APPROACH TO HIGH-FREQUENCY WAVES IN PLASMAS. I. SMALL-AMPLITUDE REGIME IN FULLY IONIZED MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Martínez-Gómez, David; Soler, Roberto; Terradas, Jaume, E-mail: david.martinez@uib.es

Ideal magnetohydrodynamics (MHD) provides an accurate description of low-frequency Alfvén waves in fully ionized plasmas. However, higher-frequency waves in many plasmas of the solar atmosphere cannot be correctly described by ideal MHD and a more accurate model is required. Here, we study the properties of small-amplitude incompressible perturbations in both the low- and the high-frequency ranges in plasmas composed of several ionized species. We use a multi-fluid approach and take into account the effects of collisions between ions and the inclusion of Hall’s term in the induction equation. Through the analysis of the corresponding dispersion relations and numerical simulations, wemore » check that at high frequencies ions of different species are not as strongly coupled as in the low-frequency limit. Hence, they cannot be treated as a single fluid. In addition, elastic collisions between the distinct ionized species are not negligible for high-frequency waves, since an appreciable damping is obtained. Furthermore, Coulomb collisions between ions remove the cyclotron resonances and the strict cutoff regions, which are present when collisions are not taken into account. The implications of these results for the modeling of high-frequency waves in solar plasmas are discussed.« less

Isolating signatures of major cloud-cloud collisions using position-velocity diagrams

NASA Astrophysics Data System (ADS)

Haworth, T. J.; Tasker, E. J.; Fukui, Y.; Torii, K.; Dale, J. E.; Shima, K.; Takahira, K.; Habe, A.; Hasegawa, K.

2015-06-01

Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position-velocity diagrams from models of cloud-cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high-velocity cloud-cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5 Myr after the formation of the first massive (ionizing) star. However for a head-on 10 km s-1 collision, we find that this will only be observable from 20 to 30 per cent of viewing angles. Such broad-bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud-cloud collision by Torii et al., and also towards star formation in the outer Milky Way by Izumi et al.

Microgravity experiments on the effect of internal flow on solidification of Fe-Cr-Ni stainless steels.

PubMed

Hanlon, Alaina B; Matson, Douglas M; Hyers, Robert W

2006-09-01

A new hypothesis has been developed to explain the effect of internal fluid flow on the lifetime of a metastable phase in solidifying Fe-Cr-Ni alloys. The hypothesis shows excellent agreement with available experimental results, but microgravity experiments are required for complete validation. Certain Fe-Cr-Ni stainless steel alloys solidify from an undercooled melt by a two-step process in which the metastable ferrite phase forms first followed by the stable austenite phase. Recent experiments using containerless processing techniques have shown that the lifetime of the metastable phase is strongly influenced by flow within the molten sample. Simulations using a commercial computational fluid dynamics (CFD) package, FIDAP, were performed to determine the time required for collision of dendrites and compared to experimental delay time. If the convective velocities are strong enough to bend the primary arms, then the secondary arms of adjacent dendrites can touch. The points of collision form low-angle boundaries and result in high-energy sites that can serve as nuclei for the transformation to the stable phase. It has been determined that the convective velocities in electrostatic levitation (ESL) are not strong enough to cause collision. However, in ground-based electromagnetic levitation (EML), the convective velocities are strong enough to cause the dendrites to deflect so that the secondary arms of adjacent dendrites collide. There is quantitative agreement between the numerically determined time to collision and the experimentally observed delay time in EML. The strong internal velocity due to convection within the EML samples is the reason for the observed difference in delay times between ESL and EML. Microgravity testing is essential because the significant change in nucleation behavior occurs between the ranges accessible by ground-based ESL and EML. Testing in microgravity using EML will permit a large range of internal convective velocities including those that are inaccessible in 1 g.

Exploring Granular Flows at Intermediate Velocities

NASA Astrophysics Data System (ADS)

Brodsky, E. E.; van der Elst, N.

2012-12-01

Geophysical and geomorphological flows often encompass a wide range of strain rates. Landslides accelerate from nearly static conditions to velocities in the range of meters/seconds. The rheology of granular flows for the end-members is moderately well-understood, but the constitutive low at intermediate velocities is largely unexplored. Here we present evidence that granular flows transition through a regime in which internally generated acoustic waves play a critical role in controlling rheology. In laboratory experiments on natural sand under shear in a commercial rheometer, we observe that the steady-state flows at intermediate velocities are compacted relative to the end members. In a confined volume, this compaction results in a decrease in stress on the boundaries. We establish the key role of the acoustic waves by measuring the noise generated by the shear flows with an accelerometer and then exciting the flow with similar amplitude noise under lower shear rate conditions. The observed compaction for a given amplitude noise is the same in both cases, regardless of whether the noise is generated internally by the grains colliding or artificially applied externally. The boundaries of this acoustically controlled regime can be successfully predicted through non-dimensional analysis balancing the overburden, acoustic pressure and granular inertial terms. In our laboratory experiments, this regime corresponds to 0.1 to 10 cm/s. The controlling role of acoustic waves in intermediate velocities is significant because: (1) Geological systems must pass through this regime on their route to instability. (2) Acoustic waves are much more efficiently generated by angular particles, likely to be found in natural samples, than by perfectly spherical particles, which are more tractable for laboratory and theoretical studies. Therefore, this regime is likely to be missed in many analog and computational approaches. (3) Different mineralogies and shapes result in different noise generation. Therefore, there is a potential to extrapolate and predict rheological behavior of an active flow through studies of the recoverable granular products.Steady-state thickness vs. shear rate for angular sand and glass beads. Individual curves represent multiple up-going and down-going velocity ramps, and thick error bars show means and standard deviations between runs. Thickness is independent of shear rate at low shear rates, and strongly dependent on shear rate for intermediate and high shear rates. Compaction is observed at intermediate shear rates for angular sand, but not for smooth glass beads.

Combustion Characteristics in a Non-Premixed Cool-Flame Regime of n-Heptane in Microgravity

NASA Technical Reports Server (NTRS)

Takahashi, Fumiaki; Katta, Viswanath R.; Hicks, Michael C.

2015-01-01

A series of distinct phenomena have recently been observed in single-fuel-droplet combustion tests performed on the International Space Station (ISS). This study attempts to simulate the observed flame behavior numerically using a gaseous n-heptane fuel source in zero gravity and a time-dependent axisymmetric (2D) code, which includes a detailed reaction mechanism (127 species and 1130 reactions), diffusive transport, and a radiation model (for CH4, CO, CO2, H2O, and soot). The calculated combustion characteristics depend strongly on the air velocity around the fuel source. In a near-quiescent air environment (< or = 2 mm/s), with a sufficiently large fuel injection velocity (1 cm/s), a growing spherical diffusion flame extinguishes at ˜1200 K due to radiative heat losses. This is typically followed by a transition to the low-temperature (cool-flame) regime with a reaction zone (at ˜700 K) in close proximity to the fuel source. The 'cool flame' regime is formed due to the negative temperature coefficient in the low-temperature chemistry. After a relatively long period (˜18 s) of the cool flame regime, a flash re-ignition occurs, associated with flame-edge propagation and subsequent extinction of the re-ignited flame. In a low-speed (˜3 mm/s) airstream (which simulates the slight droplet movement), the diffusion flame is enhanced upstream and experiences a local extinction downstream at ˜1200 K, followed by steady flame pulsations (˜0.4 Hz). At higher air velocities (4-10 mm/s), the locally extinguished flame becomes steady state. The present axisymmetric computational approach helps in revealing the non-premixed 'cool flame' structure and 2D flame-flow interactions observed in recent microgravity droplet combustion experiments.

Experimental Observations on a Low Strain Counter-Flow Diffusion Flame: Flow and Bouyancy Effects

NASA Technical Reports Server (NTRS)

Sutula, J. A.; Torero, J. L.; Ezekoye, O. A.

1999-01-01

Diffusion flames are of great interest in fire safety and many industrial processes. The counter-flow configuration provides a constant strain flow, and therefore is ideal to study the structure of diffusion flames. Most studies have concentrated on the high velocity, high strain limit, since buoyantly induced instabilities will disintegrate the planar flame as the velocity decreases. Only recently, experimental studies in microgravity conditions have begun to explore the low strain regimes. Numerical work has shown the coupling between gas phase reaction rates, soot reaction rates, and radiation. For these programs, size, geometry and experimental conditions have been chosen to keep the flame unaffected by the physical boundaries. When the physical boundaries can not be considered infinitely far from the reaction zone discrepancies arise. A computational study that includes boundary effects and accounts for the deviations occurring when the major potential flow assumptions are relaxed was presented by Borlik et al. This development properly incorporates all heat loss terms and shows the possibility of extinction in the low strain regime. A major constraint of studying the low strain regime is buoyancy. Buoyant instabilities have been shown to have a significant effect on the nature of reactants and heat transport, and can introduce instabilities on the flow that result in phenomena such as flickering or fingering. The counter-flow configuration has been shown to provide a flame with no symmetry disrupting instabilities for inlet velocities greater than 50 mm/s. As the velocity approaches this limit, the characteristic length of the experiment has to be reduced to a few millimetres so as to keep the Rayleigh number (Ra(sub L) = (Beta)(g(sub 0))(L(exp 3) del T)/(alpha(v))) below 2000. In this work, a rectangular counter-flow burner was used to study a two-dimensional counter-flow diffusion flame. Flow visualisation and Particle Image Velocimetry served to describe the nature of the stagnation plane for strain rates smaller than 100 (1/s). These experiments were conducted with a non-reacting flow. Video images of a propane air diffusion flame were used to describe the behaviour of a diffusion flame in this regime. Flame geometry and pulsation frequency are described.

Aeolian Sand Transport with Collisional Suspension

NASA Technical Reports Server (NTRS)

Jenkins, James T.; Pasini, Jose Miguel; Valance, Alexandre

2004-01-01

Aeolian transport is an important mechanism for the transport of sand on Earth and on Mars. Dust and sand storms are common occurrences on Mars and windblown sand is responsible for many of the observed surface features, such as dune fields. A better understanding of Aeolian transport could also lead to improvements in pneumatic conveying of materials to be mined for life support on the surface of the Moon and Mars. The usual view of aeolian sand transport is that for mild winds, saltation is the dominant mechanism, with particles in the bed being dislodged by the impact of other saltating particles, but without in-flight collisions. As the wind becomes stronger, turbulent suspension keeps the particles in the air, allowing much longer trajectories, with the corresponding increase in transport rate. We show here that an important regime exists between these two extremes: for strong winds, but before turbulent suspension becomes dominant, there is a regime in which in-flight collisions dominate over turbulence as a suspension mechanism, yielding transport rates much higher than those for saltation. The theory presented is based on granular kinetic theory, and includes both turbulent suspension and particle-particle collisions. The wind strengths for which the calculated transport rates are relevant are beyond the published strengths of current wind tunnel experiments, so these theoretical results are an invitation to do experiments in the strong-wind regime. In order to make a connection between the regime of saltation and the regime of collisional suspension, it is necessary to better understand the interaction between the bed and the particles that collide with it. This interaction depends on the agitation of the particles of the bed. In mild winds, collisions with the bed are relatively infrequent and the local disturbance associated with a collision can relax before the next nearby collision. However, as the wind speed increases, collision become more frequent and the agitation need not decay completely. In the regime of collisional suspension, the particles near the surface of the bed are assumed to be in a state of constant agitation. We indicate the conditions at the bed corresponding to the limits of saltation and collisional suspension and outline experiments, simulations, and modeling that have been undertaken to bridge these limits.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Springer, H K; Miller, W O; Levatin, J L

Satellite collision debris poses risks to existing space assets and future space missions. Predictive models of debris generated from these hypervelocity collisions are critical for developing accurate space situational awareness tools and effective mitigation strategies. Hypervelocity collisions involve complex phenomenon that spans several time- and length-scales. We have developed a satellite collision debris modeling approach consisting of a Lagrangian hydrocode enriched with smooth particle hydrodynamics (SPH), advanced material failure models, detailed satellite mesh models, and massively parallel computers. These computational studies enable us to investigate the influence of satellite center-of-mass (CM) overlap and orientation, relative velocity, and material composition onmore » the size, velocity, and material type distributions of collision debris. We have applied our debris modeling capability to the recent Iridium 33-Cosmos 2251 collision event. While the relative velocity was well understood in this event, the degree of satellite CM overlap and orientation was ill-defined. In our simulations, we varied the collision CM overlap and orientation of the satellites from nearly maximum overlap to partial overlap on the outermost extents of the satellites (i.e, solar panels and gravity boom). As expected, we found that with increased satellite overlap, the overall debris cloud mass and momentum (transfer) increases, the average debris size decreases, and the debris velocity increases. The largest predicted debris can also provide insight into which satellite components were further removed from the impact location. A significant fraction of the momentum transfer is imparted to the smallest debris (< 1-5mm, dependent on mesh resolution), especially in large CM overlap simulations. While the inclusion of the smallest debris is critical to enforcing mass and momentum conservation in hydrocode simulations, there seems to be relatively little interest in their disposition. Based on comparing our results to observations, it is unlikely that the Iridium 33-Cosmos 2251 collision event was a large mass-overlap collision. We also performed separate simulations studying the debris generated by the collision of 5 and 10 cm spherical projectiles on the Iridium 33 satellite at closing velocities of 5, 10, and 15 km/s. It is important to understand the vulnerability of satellites to small debris threats, given their pervasiveness in orbit. These studies can also be merged with probabilistic conjunction analysis to better understand the risk to space assets. In these computational studies, we found that momentum transfer, kinetic energy losses due to dissipative mechanisms (e.g., fracture), fragment number, and fragment velocity increases with increasing velocity for a fixed projectile size. For a fixed velocity, we found that the smaller projectile size more efficiently transfers momentum to the satellite. This latter point has an important implication: Eight (spaced) 5 cm debris objects can impart more momentum to the satellite, and likely cause more damage, than a single 10 cm debris object at the same velocity. Further studies are required to assess the satellite damage induced by 1-5 cm sized debris objects, as well as multiple debris objects, in this velocity range.« less

Flow Regime Study in a Circulating Fluidized Bed Riser with an Abrupt Exit: [1] High Density Suspension

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mei, J.S.; Lee, G.T.; Seachman, S.M.

2008-05-13

Flow regime study was conducted in a 0.3 m diameter, 15.5 m tall circulating fluidized bed (CFB) riser with an abrupt exit at the National Energy Technology Laboratory of the U. S. Department of Energy. A statistical designed test series was conducted including four (4) operating set points and a duplicated center point (therefore a total of 6 operating set points). Glass beads of mean diameter 200 μm and particle density of 2,430 kg/m3 were used as bed material. The CFB riser was operated at various superficial gas velocities ranging from 5.6 to 7.6 m/s and solid mass flux frommore » a low of 86 to a high of 303 kg/m2-s. Results of the apparent solids fraction profile as well as the radial particle velocity profile were analyzed in order to identify the presence of Dense Suspension Upflow (DSU) conditions. DSU regime was found to exist at the bottom of the riser, while the middle section of the riser was still exhibiting core-annular flow structure. Due to the abrupt geometry of the exit, the DSU regime was also found at the top of the riser. In addition the effects of the azimuthal angle, riser gas velocity, and mass solids flux on the particle velocity were investigated and are discussed in this paper.« less

Low-speed impacts between rubble piles modeled as collections of polyhedra

NASA Astrophysics Data System (ADS)

Korycansky, D. G.; Asphaug, Erik

2006-04-01

We present results of modeling rubble piles as collections of polyhedra. The use of polyhedra allows more realistic (irregular) shapes and interactions (e.g. collisions), particularly for objects of different sizes. Rotational degrees of freedom are included in the modeling, which may be important components of the motion. We solved the equations of rigid-body dynamics, including frictional/inelastic collisions, for collections of up to several hundred elements. As a demonstration of the methods and to compare with previous work by other researchers, we simulated low-speed collisions between km-scale bodies with the same general parameters as those simulated by Leinhardt et al. [Leinhardt, Z.M., Richardson, D.C., Quinn, T., 2000. Icarus 146, 133-151]. High-speed collisions appropriate to present-day asteroid encounters require additional treatment of shock effects and fragmentation and are the subject of future work; here we study regimes appropriate to planetesimal accretion and re-accretion in the aftermath of catastrophic events. Collisions between equal-mass objects at low speeds ( <10 cms) were simulated for both head-on and off-center collisions between rubble piles made of a power-law mass spectrum of sub-elements. Very low-speed head-on collisions produce single objects from the coalescence of the impactors. For slightly higher speeds, extensive disruption occurs, but re-accretion produces a single object with most of the total mass. For increasingly higher speeds, the re-accreted object has smaller mass, finally resulting in complete catastrophic disruption with all sub-elements on escape trajectories and only small amounts of mass in re-accreted bodies. Off-center collisions at moderately low speeds produce two re-accreted objects of approximately equal mass, separating at greater than escape speed. At high speed, complete disruption occurs as with the high-speed head-on collisions. Head-on collisions at low to moderate speeds result in objects of mostly oblate shape, while higher speed collisions produce mostly prolate objects, as do off-center collisions at moderate and high speeds. Collisions carried out with the same dissipative coefficients (coefficient of restitution ɛ=0.8, zero friction) as used by Leinhardt et al. [Leinhardt, Z.M., Richardson, D.C., Quinn, T., 2000. Icarus 146, 133-151] result in a value for specific energy for disruption QD∗≈1.4 J/kg, somewhat lower than the value of 2 J/kg found by them, while collisions with a lower coefficient of restitution and friction [ ɛ=0.5, ɛ=0, μ=0.5, similar to those used by Michel, et al. [Michel, P., Benz, W., Richardson, D.C., 2004. Planet. Space Sci. 52, 1109-1117] for SPH + N-body calculations] yield QD∗≈4.5 J/kg.

Rayleigh wave tomography in South China from ambient seismic noise cross-correlation

NASA Astrophysics Data System (ADS)

Xu, S.; Song, X.; Wang, L.; Xu, M.

2012-12-01

South China is a composite of continental blocks with relics of Proterozoic and late-Paleozoic sutures, and underwent complex transformations in geological history. The connection of Yangtze Block (YZB) and Cathaysian Block (CTB) around 1 Ga led to Neoproterozoic rift systems in the central South China. Subsequently, strong folding and thrusting with intracontinental shortening occurred in the central South China during Mesozoic as a response to the collision of China-Indochina with Philippine Plate. Post orogenesis during Jurassic and Cretaceous, accompanied by extension and delamination, led to the thinning of lithosphere and granite intrusion, which becomes the main mineralization process in South China. The mechanism of the intracontinental collision and boundary of YZB with CTB have been debated for a long time. Detailed lithospheric structures are important for us to understand characteristics of the continental blocks, their boundaries, and the evolution processes. We use ambient noise cross-correlation method to obtain high resolution 3-D velocity images of South China. Continuous records of 298 stations from January 2010 to June 2011 are used to retrieve Rayleigh wave Green functions between possible station pairs. Waveform and dispersion curve comparisons with earthquake-emitted Rayleigh wave indicate that the ambient noise method is efficient and reliable. Both group and phase velocity images of 0.5°×0.5° grid from 8 to 40 s are estimated. Then, shear velocities are inversed in each grid, using a recent regional S-velocity model as a constraint for deeper structures. The surface wave maps in short periods clearly delineate basins and mountains. Deeper Moho in west of South China is observed, which can be interpreted as a result of strata folding and shortening in Mesozoic or long-range effects from Indian-Asian collision in Cenozoic. The collision mode between YZB and CTB is different for Lower-YZB (L-YZB) and Upper-YZB (U-YZB): the L-YZB is a simple attachment while the CTB overthrusts over U-YZB. Strong extension and delamination in the eastern part of South China result in thinner and weaker lithosphere than in the west. Remnant materials of partial melting induced by Paleo-Pacific plate subduction form the origin of Late-Mesozoic Volcanic Belt in the Southeast China. Low velocity along the fault implies that Tanlu is a lithosphere-scale structure. We also observe a vertical low velocity zone under the previously-claimed "Hainan Island mantle plume".

Qualitative numerical studies of the modification of the pitch angle distribution of test particles by alfvènic wave activity

NASA Astrophysics Data System (ADS)

Keilbach, D.; Drews, C.; Berger, L.; Marsch, E.; Wimmer-Schweingruber, R. F.

2017-12-01

Using a test particle approach we have investigated, how an oxygen pickup ion torus velocity distribution is modified by continuous and intermittent alfvènic waves on timescales, where the gyro trajectory of each particle can be traced.We have therefore exposed the test particles to mono frequent waves, which expanded through the whole simulation in time and space. The general behavior of the pitch angle distribution is found to be stationary and a nonlinear function of the wave frequency, amplitude and the initial angle between wave elongation and field-perpendicular particle velocity vector. The figure shows the time-averaged pitch angle distributions as a function of the Doppler shifted wave frequency (where the Doppler shift was calculated with respect to the particles initial velocity) for three different wave amplitudes (labeled in each panel). The background field is chosen to be 5 nT and the 500 test particles were initially distributed on a torus with 120° pitch angle at a solar wind velocity of 450 km/s. Each y-slice of the histogram (which has been normalized to it's respective maximum) represents an individual run of the simulation.The frequency-dependent behavior of the test particles is found to be classifiable into the regimes of very low/high frequencies and frequencies close to first order resonance. We have found, that only in the latter regime the particles interact strongly with the wave, where in the time averaged histograms a branch structure is found, which was identified as a trace of particles co-moving with the wave phase. The magnitude of pitch angle change of these particles is as well as the frequency margin, where the branch structure is found, an increasing function with the wave amplitude.We have also investigated the interaction with mono frequent intermittent waves. Exposed to such waves a torus distribution is scattered in pitch angle space, whereas the pitch angle distribution is broadened systematically over time similar to pitch angle diffusion.The framework of our simulations is a first step toward understanding wave particle interactions at the most basic level and is readily expandable to e.g. the inclusion of multiple wave frequencies, intermittent wave activity, gradients in the background magnetic field or collisions with solar wind particles.

Surface drifter derived circulation in the northern and middle Adriatic Sea: Response to wind regime and season

USGS Publications Warehouse

Ursella, L.; Poulain, P.-M.; Signell, R.P.

2007-01-01

More than 120 satellite-tracked drifters were deployed in the northern and middle Adriatic (NMA) Sea between September 2002 and November 2003, with the purpose of studying the surface circulation at mesoscale to seasonal scale in relation to wind forcing, river runoff, and bottom topography. Pseudo-Eulerian and Lagrangian statistics were calculated from the low-pass-filtered drifter velocity data between September 2002 and December 2003. The structure of the mean circulation is determined with unprecedented high horizontal resolution by the new data. In particular, mean currents, velocity variance, and kinetic energy levels are shown to be maximal in the Western Adriatic Current (WAC). Separating data into seasons, we found that the mean kinetic energy is maximal in fall, with high values also in winter, while it is significantly weaker in summer. High-resolution Local Area Model Italy winds were used to relate the drifter velocities to the wind fields. The surface currents appear to be significantly influenced by the winds. The mean flow during the northeasterly bora regime shows an intensification of the across-basin recirculating currents. In addition, the WAC is strongly intensified both in intensity and in its offshore lateral extension. In the southeasterly sirocco regime, northward flow without recirculation dominates in the eastern half of the basin, while during northwesterly maestro the WAC is enhanced. Separating the data into low and high Po River discharge rates for low-wind conditions shows that the WAC and the velocity fluctuations in front of the Po delta are stronger for high Po River runoff. Lagrangian covariance, diffusivity, and integral time and space scales are larger in the along-basin direction and are maximal in the southern portion of the WAC. Copyright 2006 by the American Geophysical Union.

Exploring load, velocity, and surface disorder dependence of friction with one-dimensional and two-dimensional models.

PubMed

Dagdeviren, Omur E

2018-08-03

The effect of surface disorder, load, and velocity on friction between a single asperity contact and a model surface is explored with one-dimensional and two-dimensional Prandtl-Tomlinson (PT) models. We show that there are fundamental physical differences between the predictions of one-dimensional and two-dimensional models. The one-dimensional model estimates a monotonic increase in friction and energy dissipation with load, velocity, and surface disorder. However, a two-dimensional PT model, which is expected to approximate a tip-sample system more realistically, reveals a non-monotonic trend, i.e. friction is inert to surface disorder and roughness in wearless friction regime. The two-dimensional model discloses that the surface disorder starts to dominate the friction and energy dissipation when the tip and the sample interact predominantly deep into the repulsive regime. Our numerical calculations address that tracking the minimum energy path and the slip-stick motion are two competing effects that determine the load, velocity, and surface disorder dependence of friction. In the two-dimensional model, the single asperity can follow the minimum energy path in wearless regime; however, with increasing load and sliding velocity, the slip-stick movement dominates the dynamic motion and results in an increase in friction by impeding tracing the minimum energy path. Contrary to the two-dimensional model, when the one-dimensional PT model is employed, the single asperity cannot escape to the minimum energy minimum due to constraint motion and reveals only a trivial dependence of friction on load, velocity, and surface disorder. Our computational analyses clarify the physical differences between the predictions of the one-dimensional and two-dimensional models and open new avenues for disordered surfaces for low energy dissipation applications in wearless friction regime.

The Relation between Relaxation Time, Mean Free Path, Collision Time and Drift Velocity--Pitfalls and a Proposal for an Approach Illustrating the Essentials

ERIC Educational Resources Information Center

Jakoby, Bernhard

2009-01-01

The collision model is frequently introduced to describe electronic conductivity in solids. Depending on the chosen approach, the introduction of the collision time can lead to erroneous results for the average velocity of the electrons, which enters the expression for the electrical conductivity. In other textbooks, correct results are obtained…

Hemodynamic transition driven by stent porosity in sidewall aneurysms.

PubMed

Bouillot, Pierre; Brina, Olivier; Ouared, Rafik; Lovblad, Karl-Olof; Farhat, Mohamed; Pereira, Vitor Mendes

2015-05-01

The healing process of intracranial aneurysms (IAs) treated with flow diverter stents (FDSs) depends on the IA flow modifications and on the epithelization process over the neck. In sidewall IA models with straight parent artery, two main hemodynamic regimes with different flow patterns and IA flow magnitude were broadly observed for unstented and high porosity stented IA on one side, and low porosity stented IA on the other side. The hemodynamic transition between these two regimes is potentially involved in thrombosis formation. In the present study, CFD simulations and multi-time lag (MTL) particle imaging velocimetry (PIV) measurements were combined to investigate the physical nature of this transition. Measurable velocity fields and non-measurable shear stress and pressure fields were assessed experimentally and numerically in the aneurysm volume in the presence of stents with various porosities. The two main regimes observed in both PIV and CFD showed typical flow features of shear and pressure driven regimes. In particular, the waveform of the averaged IA velocities was matching both the shear stress waveform at IA neck or the pressure gradient waveform in parent artery. Moreover, the transition between the two regimes was controlled by stent porosity: a decrease of stent porosity leads to an increase (decrease) of pressure differential (shear stress) through IA neck. Finally, a good PIV-CFD agreement was found except in transitional regimes and low motion eddies due to small mismatch of PIV-CFD running conditions. Copyright © 2015 Elsevier Ltd. All rights reserved.

High velocity collisions of nanoparticles

NASA Astrophysics Data System (ADS)

Johnson, Donald F.; Mattson, William D.

2017-01-01

Nanoparticles (NPs) are a unique class of material with highly functionalizable surfaces and exciting applications. With a large surface-to-volume ratio and potentially high surface tension, shocked nanoparticles might display unique materials behavior. Using density functional theory, we have simulated high-velocity NP collisions under a variety of conditions. NPs composed of diamond-C, cubic-BN, and diamond-Si were considered with particle sizes up to 3.5 nm diameter. Additional simulations involved NPs that were destabilized by incorporating internal strain. The initial spherical NP structures were carved out of bulk crystals while the NPs with internal strain were constructed as a dense core (compressive strain) encompassed by a thin shell (tensile strain). Both on-axis and off-axis collisions were simulated at 10 km/s relative velocity. The amount of internal strain was artificially increased by creating a dense inner core with bond lengths compressed up to 8%. Collision dynamics, shock propagation, and fragmentation will be analyzed, but the simulation are ongoing and results are not finalized. The effect of material properties, internal strain, and collision velocity will be discussed.

Conformational dynamics and internal friction in homopolymer globules: equilibrium vs. non-equilibrium simulations.

PubMed

Einert, T R; Sing, C E; Alexander-Katz, A; Netz, R R

2011-12-01

We study the conformational dynamics within homopolymer globules by solvent-implicit Brownian dynamics simulations. A strong dependence of the internal chain dynamics on the Lennard-Jones cohesion strength ε and the globule size N (G) is observed. We find two distinct dynamical regimes: a liquid-like regime (for ε < ε(s) with fast internal dynamics and a solid-like regime (for ε > ε(s) with slow internal dynamics. The cohesion strength ε(s) of this freezing transition depends on N (G) . Equilibrium simulations, where we investigate the diffusional chain dynamics within the globule, are compared with non-equilibrium simulations, where we unfold the globule by pulling the chain ends with prescribed velocity (encompassing low enough velocities so that the linear-response, viscous regime is reached). From both simulation protocols we derive the internal viscosity within the globule. In the liquid-like regime the internal friction increases continuously with ε and scales extensive in N (G) . This suggests an internal friction scenario where the entire chain (or an extensive fraction thereof) takes part in conformational reorganization of the globular structure.

On the overriding issue of train front end collision in rail vehicle dynamics

NASA Astrophysics Data System (ADS)

Yang, Chao; Li, Qiang; Xiao, Shoune; Wang, Xi

2018-04-01

A three-dimensional dynamic model of crashed vehicles coupled with moving tracks is developed to research the dynamic behaviour of the train front end collision on tangent tracks. The three-dimensional dynamic model consists of a crashed vehicle model, moving track models, a simple wheel-rail contact model, a velocity-based coupler model and the model of energy absorption and anti-climbing devices. The vector method dealing with the nonlinear wheel-rail geometry is put forward in the paper. The developed model is applicable in the scope that central collisions occur on tangent tracks at low speeds. The examples of the vehicle impacting with a rigid wall and the train front end collision are carried out to obtain the dynamic responses of vehicles. The overriding issue is studied on the basis of the wheel rise in train collisions. The results show that the second bogie of the first colliding vehicle possesses the maximal wheel rise. The wheel rise increases with the increase of vehicles. However, the number of vehicles has tiny influence on the overriding in train collisions at low speeds. On the contrary, the impact speed has significant influence on the overriding in train collisions. The wheel rise increases rapidly if the impact speed is close to the critical speed of overriding. The large wheel rise is principally generated by the great coupler force related to the rigid impact in the axial direction.

Modelling droplet collision outcomes for different substances and viscosities

NASA Astrophysics Data System (ADS)

Sommerfeld, Martin; Kuschel, Matthias

2016-12-01

The main objective of the present study is the derivation of models describing the outcome of binary droplet collisions for a wide range of dynamic viscosities in the well-known collision maps (i.e. normalised lateral droplet displacement at collision, called impact parameter, versus collision Weber number). Previous studies by Kuschel and Sommerfeld (Exp Fluids 54:1440, 2013) for different solution droplets having a range of solids contents and hence dynamic viscosities (here between 1 and 60 mPa s) revealed that the locations of the triple point (i.e. coincidence of bouncing, stretching separation and coalescence) and the critical Weber number (i.e. condition for the transition from coalescence to separation for head-on collisions) show a clear dependence on dynamic viscosity. In order to extend these findings also to pure liquids and to provide a broader data basis for modelling the viscosity effect, additional binary collision experiments were conducted for different alcohols (viscosity range 1.2-15.9 mPa s) and the FVA1 reference oil at different temperatures (viscosity range 3.0-28.2 mPa s). The droplet size for the series of alcohols was around 365 and 385 µm for the FVA1 reference oil, in each case with fixed diameter ratio at Δ= 1. The relative velocity between the droplets was varied in the range 0.5-3.5 m/s, yielding maximum Weber numbers of around 180. Individual binary droplet collisions with defined conditions were generated by two droplet chains each produced by vibrating orifice droplet generators. For recording droplet motion and the binary collision process with good spatial and temporal resolution high-speed shadow imaging was employed. The results for varied relative velocity and impact angle were assembled in impact parameter-Weber number maps. With increasing dynamic viscosity a characteristic displacement of the regimes for the different collision scenarios was also observed for pure liquids similar to that observed for solutions. This displacement could be described on a physical basis using the similarity number and structure parameter K which was obtained through flow process evaluation and optimal proportioning of momentum and energy by Naue and Bärwolff (Transportprozesse in Fluiden. Deutscher Verlag für Grundstoffindustrie GmbH, Leipzig 1992). Two correlations including the structure parameter K could be derived which describe the location of the triple point and the critical We number. All fluids considered, pure liquids and solutions, are very well fitted by these physically based correlations. The boundary model of Jiang et al. (J Fluid Mech 234:171-190, 1992) for distinguishing between coalescence and stretching separation could be adapted to go through the triple point by the two involved model parameters C a and C b, which were correlated with the relaxation velocity u_{{relax}} = {σ/μ}. Based on the predicted critical Weber number, denoting the onset of reflexive separation, the model of Ashgriz and Poo (J Fluid Mech 221:183-204, 1990) was adapted accordingly. The proper performance of the new generalised models was validated based on the present and previous measurements for a wide range of dynamic viscosities (i.e. 1-60 mPa s) and liquid properties. Although the model for the lower boundary of bouncing (Estrade et al. in J Heat Fluid Flow 20:486-491, 1999) could be adapted through the shape factor, it was found not suitable for the entire range of Weber numbers and viscosities.

United theory of planet formation (i): Tandem regime

NASA Astrophysics Data System (ADS)

Ebisuzaki, Toshikazu; Imaeda, Yusuke

2017-07-01

The present paper is the first one of a series of papers that present the new united theory of planet formation, which includes magneto-rotational instability and porous aggregation of solid particles in an consistent way. We here describe the ;tandem; planet formation regime, in which a solar system like planetary systems are likely to be produced. We have obtained a steady-state, 1-D model of the accretion disk of a protostar taking into account the magneto-rotational instability (MRI) and and porous aggregation of solid particles. We find that the disk is divided into an outer turbulent region (OTR), a MRI suppressed region (MSR), and an inner turbulent region (ITR). The outer turbulent region is fully turbulent because of MRI. However, in the range, rout(= 8 - 60 AU) from the central star, MRI is suppressed around the midplane of the gas disk and a quiet area without turbulence appears, because the degree of ionization of gas becomes low enough. The disk becomes fully turbulent again in the range rin(= 0.2 - 1 AU), which is called the inner turbulent region, because the midplane temperature become high enough (>1000 K) due to gravitational energy release. Planetesimals are formed through gravitational instability at the outer and inner MRI fronts (the boundaries between the MRI suppressed region (MSR) and the outer and inner turbuent regions) without particle enhancement in the original nebula composition, because of the radial concentration of the solid particles. At the outer MRI front, icy particles grow through low-velocity collisions into porous aggregates with low densities (down to ∼10-5 gcm-3). They eventually undergo gravitational instability to form icy planetesimals. On the other hand, rocky particles accumulate at the inner MRI front, since their drift velocities turn outward due to the local maximum in gas pressure. They undergo gravitational instability in a sub-disk of pebbles to form rocky planetesimals at the inner MRI front. They are likely to be volatile-free because of the high temperature (>1000 K) at this formation site. Such water-free rocky particles may explain the formation of enstatite chondrites, of which the Earth is likely to be primarily composed of. It is also consistent with the model in which the Earth was initially formed as a completely volatile-free planet. The water and other volatile elements came later through the accretion of icy particles by the occasional scatterings in the outer regions. Our new proposed tandem planet formation regime shows that planetesimals are formed at two distinct sites (outer and inner edges of the MRI suppressed region). The former is likely to be the source of outer gas giants and the latter inner rocky planets. The tandem regime also explains the gap in the distribution of solid components (2-4 AU), which is necessary to form a ;solar-system-like; planetary system, which has a relatively small Mars and a very small mass in the main asteroid belt. We found that this tandem regime dose not take place when the vertical magnetic field of the disk five times weaker compared with that we assumed in the present paper, since the outer MRI front shift outward beyond 100 AU. This suggests that yet other regimes exists in our united theory. It may explain the variation observed in exsoplanetary systems by variations in magnetic field and probably angular momentum of the parent molecular cloud.

Parallel Fokker–Planck-DSMC algorithm for rarefied gas flow simulation in complex domains at all Knudsen numbers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Küchlin, Stephan, E-mail: kuechlin@ifd.mavt.ethz.ch; Jenny, Patrick

2017-01-01

A major challenge for the conventional Direct Simulation Monte Carlo (DSMC) technique lies in the fact that its computational cost becomes prohibitive in the near continuum regime, where the Knudsen number (Kn)—characterizing the degree of rarefaction—becomes small. In contrast, the Fokker–Planck (FP) based particle Monte Carlo scheme allows for computationally efficient simulations of rarefied gas flows in the low and intermediate Kn regime. The Fokker–Planck collision operator—instead of performing binary collisions employed by the DSMC method—integrates continuous stochastic processes for the phase space evolution in time. This allows for time step and grid cell sizes larger than the respective collisionalmore » scales required by DSMC. Dynamically switching between the FP and the DSMC collision operators in each computational cell is the basis of the combined FP-DSMC method, which has been proven successful in simulating flows covering the whole Kn range. Until recently, this algorithm had only been applied to two-dimensional test cases. In this contribution, we present the first general purpose implementation of the combined FP-DSMC method. Utilizing both shared- and distributed-memory parallelization, this implementation provides the capability for simulations involving many particles and complex geometries by exploiting state of the art computer cluster technologies.« less

Temperature Variations and N+/O+ in the Orion Nebula II. The Collision Strengths

NASA Astrophysics Data System (ADS)

Rubin, R. H.; Dufour, R. J.; Martin, P. G.; Ferland, G. J.; Baldwin, J. A.; Ortiz, C. O.; Walter, D. K.

2001-03-01

We continue an investigation of electron temperature (T[e]), mean-square T[e] variation (t2), and the N+/O+ abundance ratio. Our previous analysis of HST spectra of the Orion Nebula used collision strengths for N+ by Stafford et al. (1994). Here we examine the consequences of changing just these collision strengths by using those of Lennon & Burke (1994). Rather than utilize the standard analytical, low electron density (N[e]) regime treatment for the analysis, we develop a numerical technique that is valid at any density. With Stafford et al. collision strengths, we find the average N[e] for the (N+, O+)-zone is 7500 cm-3, the average T[e] is 9160 K, t2 is 0.045, and N+/O+ is 0.14. Using Lennon & Burke values, the ``best" solution is found when these respective quantities are: 9000 cm-3, 9920 K, 0.00073, and 0.15. The value for t2 is dramatically lower than that found using Stafford et al. data.

Monte Carlo Calculations of F-region Incoherent Radar Spectra at High Latitudes: the Effect of O+-O+ Coulomb Collisions

NASA Astrophysics Data System (ADS)

Barghouthi, I.; Barakat, A.

We have used Monte Carlo simulations of O+ velocity distributions in the high latitude F-region to improve the calculation of incoherent radar spectra in auroral ionosphere. The Monte Carlo simulation includes ion-neutral O+ -- O resonant charge exchange and polarization interactions as well as Coulomb self-collisions O+ -- O+. At a few hundreds kilometers of altitude, atomic oxygen O and atomic oxygen ion O+ dominate the composition of the auroral ionosphere and, consequently, the influence of O+ -- O+ Coulomb collisions becomes significant. In this study we consider the effect of O+ -- O+ collisions on the incoherent radar spectra in the presence of large electric field (˜ 100 mVm-1). As altitude increases, (i.e. the role of O+ -- O+ becomes significant), the 1-D O+ ion velocity distribution function becomes more Maxwellian and the features of the radar spectrum corresponding to non-Maxwellian ion velocity distribution (e.g. baby bottle and triple hump shapes) evolve to Maxwellian ion velocity distribution (single and double hump shapes). Therefore, O+ -- O+ Coulomb collisions act to istropize the 1-D O+ velocity distribution, and modify the radar spectrum accordingly, by transferring thermal energy from the perpendicular direction to the parallel direction.

Measurement of the velocity of neutral fragments by the "correlated ion and neutral time of flight" method combined with "velocity-map imaging"

NASA Astrophysics Data System (ADS)

Berthias, F.; Feketeová, L.; Della Negra, R.; Dupasquier, T.; Fillol, R.; Abdoul-Carime, H.; Farizon, B.; Farizon, M.; Märk, T. D.

2017-08-01

In the challenging field of imaging molecular dynamics, a novel method has been developed and implemented that allows the measurement of the velocity of neutral fragments produced in collision induced dissociation experiments on an event-by-event basis. This has been made possible by combining a correlated ion and neutral time of flight method with a velocity map imaging technique. This new method relies on a multiparametric correlated detection of the neutral and charged fragments from collision induced dissociation on one single detector. Its implementation on the DIAM device (Device for irradiation of biomolecular clusters) (Dispositif d'Irradiation d'Agrégats bioMoléculaires) allowed us to measure the velocity distribution of water molecules evaporated from collision induced dissociation of mass- and energy-selected protonated water clusters.

Cumulative Damage in Strength-Dominated Collisions of Rocky Asteroids: Rubble Piles and Brick Piles

NASA Technical Reports Server (NTRS)

Housen, Kevin

2009-01-01

Laboratory impact experiments were performed to investigate the conditions that produce large-scale damage in rock targets. Aluminum cylinders (6.3 mm diameter) impacted basalt cylinders (69 mm diameter) at speeds ranging from 0.7 to 2.0 km/s. Diagnostics included measurements of the largest fragment mass, velocities of the largest remnant and large fragments ejected from the periphery of the target, and X-ray computed tomography imaging to inspect some of the impacted targets for internal damage. Significant damage to the target occurred when the kinetic energy per unit target mass exceeded roughly 1/4 of the energy required for catastrophic shattering (where the target is reduced to one-half its original mass). Scaling laws based on a rate-dependent strength were developed that provide a basis for extrapolating the results to larger strength-dominated collisions. The threshold specific energy for widespread damage was found to scale with event size in the same manner as that for catastrophic shattering. Therefore, the factor of four difference between the two thresholds observed in the lab also applies to larger collisions. The scaling laws showed that for a sequence of collisions that are similar in that they produce the same ratio of largest fragment mass to original target mass, the fragment velocities decrease with increasing event size. As a result, rocky asteroids a couple hundred meters in diameter should retain their large ejecta fragments in a jumbled rubble-pile state. For somewhat larger bodies, the ejection velocities are sufficiently low that large fragments are essentially retained in place, possibly forming ordered "brick-pile" structures.

Low- and high-order accurate boundary conditions: From Stokes to Darcy porous flow modeled with standard and improved Brinkman lattice Boltzmann schemes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silva, Goncalo, E-mail: goncalo.nuno.silva@gmail.com; Talon, Laurent, E-mail: talon@fast.u-psud.fr; Ginzburg, Irina, E-mail: irina.ginzburg@irstea.fr

The present contribution focuses on the accuracy of reflection-type boundary conditions in the Stokes–Brinkman–Darcy modeling of porous flows solved with the lattice Boltzmann method (LBM), which we operate with the two-relaxation-time (TRT) collision and the Brinkman-force based scheme (BF), called BF-TRT scheme. In parallel, we compare it with the Stokes–Brinkman–Darcy linear finite element method (FEM) where the Dirichlet boundary conditions are enforced on grid vertices. In bulk, both BF-TRT and FEM share the same defect: in their discretization a correction to the modeled Brinkman equation appears, given by the discrete Laplacian of the velocity-proportional resistance force. This correction modifies themore » effective Brinkman viscosity, playing a crucial role in the triggering of spurious oscillations in the bulk solution. While the exact form of this defect is available in lattice-aligned, straight or diagonal, flows; in arbitrary flow/lattice orientations its approximation is constructed. At boundaries, we verify that such a Brinkman viscosity correction has an even more harmful impact. Already at the first order, it shifts the location of the no-slip wall condition supported by traditional LBM boundary schemes, such as the bounce-back rule. For that reason, this work develops a new class of boundary schemes to prescribe the Dirichlet velocity condition at an arbitrary wall/boundary-node distance and that supports a higher order accuracy in the accommodation of the TRT-Brinkman solutions. For their modeling, we consider the standard BF scheme and its improved version, called IBF; this latter is generalized in this work to suppress or to reduce the viscosity correction in arbitrarily oriented flows. Our framework extends the one- and two-point families of linear and parabolic link-wise boundary schemes, respectively called B-LI and B-MLI, which avoid the interference of the Brinkman viscosity correction in their closure relations. The performance of LBM and FEM is thoroughly evaluated in three benchmark tests, which are run throughout three distinctive permeability regimes. The first configuration is a horizontal porous channel, studied with a symbolic approach, where we construct the exact solutions of FEM and BF/IBF with different boundary schemes. The second problem refers to an inclined porous channel flow, which brings in as new challenge the formation of spurious boundary layers in LBM; that is, numerical artefacts that arise due to a deficient accommodation of the bulk solution by the low-accurate boundary scheme. The third problem considers a porous flow past a periodic square array of solid cylinders, which intensifies the previous two tests with the simulation of a more complex flow pattern. The ensemble of numerical tests provides guidelines on the effect of grid resolution and the TRT free collision parameter over the accuracy and the quality of the velocity field, spanning from Stokes to Darcy permeability regimes. It is shown that, with the use of the high-order accurate boundary schemes, the simple, uniform-mesh-based TRT-LBM formulation can even surpass the accuracy of FEM employing hardworking body-fitted meshes.« less

Low- and high-order accurate boundary conditions: From Stokes to Darcy porous flow modeled with standard and improved Brinkman lattice Boltzmann schemes

NASA Astrophysics Data System (ADS)

Silva, Goncalo; Talon, Laurent; Ginzburg, Irina

2017-04-01

The present contribution focuses on the accuracy of reflection-type boundary conditions in the Stokes-Brinkman-Darcy modeling of porous flows solved with the lattice Boltzmann method (LBM), which we operate with the two-relaxation-time (TRT) collision and the Brinkman-force based scheme (BF), called BF-TRT scheme. In parallel, we compare it with the Stokes-Brinkman-Darcy linear finite element method (FEM) where the Dirichlet boundary conditions are enforced on grid vertices. In bulk, both BF-TRT and FEM share the same defect: in their discretization a correction to the modeled Brinkman equation appears, given by the discrete Laplacian of the velocity-proportional resistance force. This correction modifies the effective Brinkman viscosity, playing a crucial role in the triggering of spurious oscillations in the bulk solution. While the exact form of this defect is available in lattice-aligned, straight or diagonal, flows; in arbitrary flow/lattice orientations its approximation is constructed. At boundaries, we verify that such a Brinkman viscosity correction has an even more harmful impact. Already at the first order, it shifts the location of the no-slip wall condition supported by traditional LBM boundary schemes, such as the bounce-back rule. For that reason, this work develops a new class of boundary schemes to prescribe the Dirichlet velocity condition at an arbitrary wall/boundary-node distance and that supports a higher order accuracy in the accommodation of the TRT-Brinkman solutions. For their modeling, we consider the standard BF scheme and its improved version, called IBF; this latter is generalized in this work to suppress or to reduce the viscosity correction in arbitrarily oriented flows. Our framework extends the one- and two-point families of linear and parabolic link-wise boundary schemes, respectively called B-LI and B-MLI, which avoid the interference of the Brinkman viscosity correction in their closure relations. The performance of LBM and FEM is thoroughly evaluated in three benchmark tests, which are run throughout three distinctive permeability regimes. The first configuration is a horizontal porous channel, studied with a symbolic approach, where we construct the exact solutions of FEM and BF/IBF with different boundary schemes. The second problem refers to an inclined porous channel flow, which brings in as new challenge the formation of spurious boundary layers in LBM; that is, numerical artefacts that arise due to a deficient accommodation of the bulk solution by the low-accurate boundary scheme. The third problem considers a porous flow past a periodic square array of solid cylinders, which intensifies the previous two tests with the simulation of a more complex flow pattern. The ensemble of numerical tests provides guidelines on the effect of grid resolution and the TRT free collision parameter over the accuracy and the quality of the velocity field, spanning from Stokes to Darcy permeability regimes. It is shown that, with the use of the high-order accurate boundary schemes, the simple, uniform-mesh-based TRT-LBM formulation can even surpass the accuracy of FEM employing hardworking body-fitted meshes.

Collisions of dark matter axion stars with astrophysical sources

DOE PAGES

Eby, Joshua; Leembruggen, Madelyn; Leeney, Joseph; ...

2017-04-18

If QCD axions form a large fraction of the total mass of dark matter, then axion stars could be very abundant in galaxies. As a result, collisions with each other, and with other astrophysical bodies, can occur. We calculate the rate and analyze the consequences of three classes of collisions, those occurring between a dilute axion star and: another dilute axion star, an ordinary star, or a neutron star. In all cases we attempt to quantify the most important astrophysical uncertainties; we also pay particular attention to scenarios in which collisions lead to collapse of otherwise stable axion stars, and possible subsequent decay through number changing interactions. Collisions between two axion stars can occur with a high total rate, but the low relative velocity required for collapse to occur leads to a very low total rate of collapses. On the other hand, collisions between an axion star and an ordinary star have a large rate,more » $$\\Gamma_\\odot \\sim 3000$$ collisions/year/galaxy, and for sufficiently heavy axion stars, it is plausible that most or all such collisions lead to collapse. We identify in this case a parameter space which has a stable region and a region in which collision triggers collapse, which depend on the axion number ($N$) in the axion star, and a ratio of mass to radius cubed characterizing the ordinary star ($$M_s/R_s^3$$). Finally, we revisit the calculation of collision rates between axion stars and neutron stars, improving on previous estimates by taking cylindrical symmetry of the neutron star distribution into account. Finally, collapse and subsequent decay through collision processes, if occurring with a significant rate, can affect dark matter phenomenology and the axion star mass distribution.« less

A Low Velocity 0.30-cal. Gun System

DTIC Science & Technology

2015-02-01

control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) February 2015 2. REPORT TYPE Final 3. DATES COVERED...298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 iii Contents List of Figures iv List of Tables iv 1. Introduction 1 2. Approach 4 3...the speed of sound in air. It is exceedingly difficult to get precise velocity control of gun-fired FSPs in this velocity regime using a

NanoRocks: Design and performance of an experiment studying planet formation on the International Space Station

NASA Astrophysics Data System (ADS)

Brisset, Julie; Colwell, Joshua; Dove, Adrienne; Maukonen, Doug

2017-07-01

In an effort to better understand the early stages of planet formation, we have developed a 1.5U payload that flew on the International Space Station (ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016. This payload, named NanoRocks, ran a particle collision experiment under long-term microgravity conditions. The objectives of the experiment were (a) to observe collisions between mm-sized particles at relative velocities of < 1 cm/s and (b) to study the formation and disruption of particle clusters for different particle types and collision velocities. Four types of particles were used: mm-sized acrylic, glass, and copper beads and 0.75 mm-sized JSC-1 lunar regolith simulant grains. The particles were placed in sample cells carved out of an aluminum tray. This tray was attached to one side of the payload casing with three springs. Every 60 s, the tray was agitated, and the resulting collisions between the particles in the sample cells were recorded by the experiment camera. During the 18 months the payload stayed on ISS, we obtained 158 videos, thus recording a great number of collisions. The average particle velocities in the sample cells after each shaking event were around 1 cm/s. After shaking stopped, the inter-particle collisions damped the particle kinetic energy in less than 20 s, reducing the average particle velocity to below 1 mm/s, and eventually slowing them to below our detection threshold. As the particle velocity decreased, we observed the transition from bouncing to sticking collisions. We recorded the formation of particle clusters at the end of each experiment run. This paper describes the design and performance of the NanoRocks ISS payload.

NanoRocks: Design and performance of an experiment studying planet formation on the International Space Station.

PubMed

Brisset, Julie; Colwell, Joshua; Dove, Adrienne; Maukonen, Doug

2017-07-01

In an effort to better understand the early stages of planet formation, we have developed a 1.5U payload that flew on the International Space Station (ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016. This payload, named NanoRocks, ran a particle collision experiment under long-term microgravity conditions. The objectives of the experiment were (a) to observe collisions between mm-sized particles at relative velocities of < 1 cm/s and (b) to study the formation and disruption of particle clusters for different particle types and collision velocities. Four types of particles were used: mm-sized acrylic, glass, and copper beads and 0.75 mm-sized JSC-1 lunar regolith simulant grains. The particles were placed in sample cells carved out of an aluminum tray. This tray was attached to one side of the payload casing with three springs. Every 60 s, the tray was agitated, and the resulting collisions between the particles in the sample cells were recorded by the experiment camera. During the 18 months the payload stayed on ISS, we obtained 158 videos, thus recording a great number of collisions. The average particle velocities in the sample cells after each shaking event were around 1 cm/s. After shaking stopped, the inter-particle collisions damped the particle kinetic energy in less than 20 s, reducing the average particle velocity to below 1 mm/s, and eventually slowing them to below our detection threshold. As the particle velocity decreased, we observed the transition from bouncing to sticking collisions. We recorded the formation of particle clusters at the end of each experiment run. This paper describes the design and performance of the NanoRocks ISS payload.

Flute Instability of Expanding Plasma Cloud

NASA Astrophysics Data System (ADS)

Dudnikova, Galina; Vshivkov, Vitali

2000-10-01

The expansion of plasma against a magnetized background where collisions play no role is a situation common to many plasma phenomena. The character of interaction between expanding plasma and background plasma is depending of the ratio of the expansion velocity to the ambient Alfven velocity. If the expansion speed is greater than the background Alfven speed (super-Alfvenic flows) a collisionless shock waves are formed in background plasma. It is originally think that if the expansion speed is less than Alfvenic speed (sub-Alfvenic flows) the interaction of plasma flows will be laminar in nature. However, the results of laboratory experiments and chemical releases in magnetosphere have shown the development of flute instability on the boundary of expanding plasma (Rayleigh-Taylor instability). A lot of theoretical and experimental papers have been devoted to study the Large Larmor Flute Instability (LLFI) of plasma expanding into a vacuum magnetic field. In the present paper on the base of computer simulation of plasma cloud expansion in magnetizied background plasma the regimes of development and stabilization LLFI for super- and sub-Alfvenic plasma flows are investigated. 2D hybrid numerical model is based on kinetic Vlasov equation for ions and hydrodynamic approximation for electrons. The similarity parameters characterizing the regimes of laminar flows are founded. The stabilization of LLFI takes place with the transition from sub- to super-Alfvenic plasma cloud expansion. The results of the comparision between computer simulation and laboratory simulation are described.

Accommodation by Varying Strain Regimes along the Northern Luzon Arc (Coastal Range, Taiwan) - Insights from Focal Mechanism Strain Inversions

NASA Astrophysics Data System (ADS)

O'Hara, D.; Lee, J.; Lewis, J. C.; Rau, R.

2013-12-01

Taiwan is the product of modern subduction polarity reversal coupled with arc-continent collision. The NW-moving Philippine Sea plate (PSP) subducts beneath the Eurasian plate (EUR) to the northeast of Taiwan at the Ryukyu trench, while overriding EUR south of Taiwan at the Manila trench, bringing the Luzon volcanic arc into collision with the deforming sediments of the Eurasian passive margin. The obliquity between the N-S trending Luzon Arc (LA) and NE-SW trending passive margin is causing the southward, temporal propagation of collision since ~6 Ma. The collided forearc and clastic sediments accreted by the advancing arc created the Coastal Range (CR), whose western-most extent lies at the suture zone between the two plates, the NNE-SSW trending Longitudinal Valley Fault (LVF). In order to understand the change in stress along the northern LA as it docks onto EUR, we inverted over 1900 relocated earthquake focal mechanism solutions within the on-land CR and offshore LA regions for spatial strain tensors. The focal mechanisms cover seismicity from 1991-2013, ranging in depths 0-112 km and magnitudes 2.22-6.92. For our analyses, we grouped the focal mechanisms based on 15' Latitudinal intervals along the study area and inverted the data for best-fit strain tensors using a micropolar continuum model of crustal deformation. Results suggest dominant compression in all regions with accommodation occurring through oblique reverse faults of varying dips. Trends of σ1 rotate clockwise (CW) from 100° in the south to 155° in the north. This CW rotation is also observed in the preferred nodal plane slip vector trends - from E-W orientation in the south to NW-SE in the north. The rotation of σ1 and slip vector trends creates varying degrees of obliquity with the direction normal (DN) to CR (112°). The trends in the southern part of the study area show obliquity counterclockwise (CCW) to DN; trends in the central part are near parallel to DN; and trends in the northern part show obliquity CW to DN. GPS vectors from 2008-2012 using an ITRF reference frame show similar changes in obliquity with GPS velocity trends oblique CCW and CW to DN in the southern and northern areas, respectively, and near parallel to DN in the central area. Our results suggest the accommodation of three varying strain regimes along the northern LA (CR) system - (1) strain partitioning within the Manila forearc basin due to the obliquity between N-S trending LA and NW trending convergence vector; (2) convergence-related strain in the central LVF and CR along NNE trending major thrust faults with a small oblique component due to the obliquity between DN and the convergence vector; and (3) strain partitioning along the Ryukyu trench and forearc basin due to the obliquity between northward subducting plate along the WNW- ESE trending Ryukyu trench and NW convergence vector. As a result, the different subducting characteristics of strain regimes correspond to the different stages of arc accretion/collision, from south to north: pre-collision, present collision, waning collision, and subduction.

Negative ion productions in high velocity collision between small carbon clusters and Helium atom target

NASA Astrophysics Data System (ADS)

M, Chabot; K, Béroff; T, Pino; G, Féraud; N, Dothi; Padellec A, Le; G, Martinet; S, Bouneau; Y, Carpentier

2012-11-01

We measured absolute double capture cross section of Cn+ ions (n=1,5) colliding, at 2.3 and 2.6 a.u velocities, with an Helium target atom and the branching ratios of fragmentation of the so formed electronically excited anions Cn-*. We also measured absolute cross section for the electronic attachment on neutral Cn clusters colliding at same velocities with He atom. This is to our knowledge the first measurement of neutral-neutral charge exchange in high velocity collision.

Flow Regime Study in a High Density Circulating Fluidized Bed Riser with an Abrupt Exit

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mei, J.S.; Shadle, L.J.; Yue, P.C.

2007-01-01

Flow regime study was conducted in a 0.3 m diameter, 15.5 m height circulating fluidized bed (CFB) riser with an abrupt exit at the National Energy Technology Laboratory of the U.S. Department of Energy. Local particle velocities were measured at various radial positions and riser heights using an optical fiber probe. On-line measurement of solid circulating rate was continuously recorded by the Spiral. Glass beads of mean diameter 61 μm and particle density of 2,500 kg/m3 were used as bed material. The CFB riser was operated at various superficial gas velocities ranging from 3 to 7.6 m/s and solid massmore » flux from 20 to 550 kg/m2-s. At a constant riser gas velocity, transition from fast fluidization to dense suspension upflow (DSU) regime started at the bottom of the riser with increasing solid flux. Except at comparatively low riser gas velocity and solid flux, the apparent solid holdup at the top exit region was higher than the middle section of the riser. The solid fraction at this top region could be much higher than 7% under high riser gas velocity and solid mass flux. The local particle velocity showed downward flow near the wall at the top of the riser due to its abrupt exit. This abrupt geometry reflected the solids and, therefore, caused solid particles traveling downward along the wall. However, at location below, but near, the top of the riser the local particle velocities were observed flowing upward at the wall. Therefore, DSU was identified in the upper region of the riser with an abrupt exit while the fully developed region, lower in the riser, was still exhibiting core-annular flow structure. Our data were compared with the flow regime boundaries proposed by Kim et al. [1] for distinguishing the dilute pneumatic transport, fast fluidization, and DSU.« less

Negative differential mobility and trapping in active matter systems

NASA Astrophysics Data System (ADS)

Reichhardt, C.; Reichhardt, C. J. O.

2018-01-01

Using simulations, we examine the average velocity as a function of applied drift force for active matter particles moving through a random obstacle array. We find that for low drift force, there is an initial flow regime where the mobility increases linearly with drive, while for higher drift forces a regime of negative differential mobility appears in which the velocity decreases with increasing drive due to the trapping of active particles behind obstacles. A fully clogged regime exists at very high drift forces when all the particles are permanently trapped behind obstacles. We find for increasing activity that the overall mobility is nonmonotonic, with an enhancement of the mobility for small levels of activity and a decrease in mobility for large activity levels. We show how these effects evolve as a function of disk and obstacle density, active run length, drift force, and motor force.

Collisional damping rates for electron plasma waves reassessed

DOE Office of Scientific and Technical Information (OSTI.GOV)

Banks, J. W.; Brunner, S.; Berger, R. L.

Collisional damping of electron plasma waves, the primary damping for high phase velocity waves, is proportional to the electron-ion collision rate, ν ei,th. Here in this work, it is shown that the damping rate normalized to ν ei,th depends on the charge state, Z, on the magnitude of ν ei,th and the wave number k in contrast with the commonly used damping rate in plasma wave research. Only for weak collision rates in low-Z plasmas for which the electron self-collision rate is comparable to the electron-ion collision rate is the damping rate given by the commonly accepted value. The resultmore » presented here corrects the result presented in textbooks at least as early as 1973. Lastly, the complete linear theory requires the inclusion of both electron-ion pitch-angle and electron-electron scattering, which itself contains contributions to both pitch-angle scattering and thermalization.« less

Collisional damping rates for electron plasma waves reassessed

DOE PAGES

Banks, J. W.; Brunner, S.; Berger, R. L.; ...

2017-10-13

Collisional damping of electron plasma waves, the primary damping for high phase velocity waves, is proportional to the electron-ion collision rate, ν ei,th. Here in this work, it is shown that the damping rate normalized to ν ei,th depends on the charge state, Z, on the magnitude of ν ei,th and the wave number k in contrast with the commonly used damping rate in plasma wave research. Only for weak collision rates in low-Z plasmas for which the electron self-collision rate is comparable to the electron-ion collision rate is the damping rate given by the commonly accepted value. The resultmore » presented here corrects the result presented in textbooks at least as early as 1973. Lastly, the complete linear theory requires the inclusion of both electron-ion pitch-angle and electron-electron scattering, which itself contains contributions to both pitch-angle scattering and thermalization.« less

Characteristics of Whipple Shield Performance in the Shatter Regime

NASA Technical Reports Server (NTRS)

Ryan, Shannon; Bjorkman, Michael; Christiansen, Eric L.

2009-01-01

Between the onset of projectile fragmentation and the assumption of rear wall failure due to an impulsive load, multi-wall ballistic limit equations are linearly interpolated to provide reasonable yet conservative predictions of perforation thresholds with conveniently simple mathematics. Although low velocity and hypervelocity regime predictions are based on analytical expressions, there is no such scientific foundation for predictions in the intermediate (or shatter) regime. As the debris flux in low earth orbit (LEO) becomes increasingly dominated by manmade pollution, the profile of micrometeoroid and orbital debris (MMOD) risk shifts continually towards lower velocities. For the International Space Station (ISS), encounter velocities below 7 km/s now constitute approximately 50% of the penetration risk. Considering that the transition velocity from shatter to hypervelocity impact regimes described by common ballistic limit equations (e.g. new non-optimum Whipple shield equation [1]) occurs at 7 km/s, 50% of station risk is now calculated based on failure limit equations with little analytical foundation. To investigate projectile and shield behavior for impact conditions leading to projectile fragmentation and melt, a series of hypervelocity impact tests have been performed on aluminum Whipple shields. In the experiments projectile diameter, bumper thickness, and shield spacing were kept constant, while rear wall thickness was adjusted to determine spallation and perforation limits at various impact velocities and angles. The results, shown in Figure 1 for normal and 45 impacts, demonstrated behavior that was not sufficiently described by the simplified linear interpolation of the NNO equation (also shown in Figure 1). Hopkins et al. [2] investigated the performance of a nominally-identical aluminum Whipple shield, identifying the effects of phase change in the shatter regime. The results (conceptually represented in Figure 2) were found to agree well with those obtained in this study at normal incidence, suggesting that shielding performance in the shatter regime could be well described by considering more complex phase conditions than currently implemented in most BLEs. Furthermore, evidence of these phase effects were found in the oblique test results, providing the basis for an empirical description of these effects that can be applied in MMOD risk assessment software. In this paper, results of the impact experiments are presented, and characteristics of target damage are evaluated. A comparison of intermediate velocity impact failure mechanisms in current BLEs are discussed and compared to the findings of the experimental study. Risk assessment calculations have been made on a simplified structure using currently implemented penetration equations and predicted limits from the experimental program, and the variation in perceived mission risk is discussed. It was found that ballistic limit curves that explicitly incorporated phase change effects within the intermediate regime lead to a decrease in predicted MMOD risk for ISS-representative orbits. When considered for all Whipple-based shielding configurations onboard the ISS, intermediate phase change effects could lead to significant variations in predicted mission risk.

Molecular modelling investigations on the possibility of phenanthrene dimers to be the primary nuclei of soot

NASA Astrophysics Data System (ADS)

Wei, Mingrui; Wu, Sheng; Li, Fan; Zhang, Dongju; Zhang, Tingting; Guo, Guanlun

2017-11-01

Pyrene dimerisation was successfully used to model the beginning of soot nucleation in some simulation models. However, the quantum mechanics (QM) calculations proved that the binding energy of a PAH dimer with three six-member rings was similar to that of a pyrene dimer. Meanwhile, the high concentration of phenanthrene at flame conditions indicated high probability of collisions among them. The small difference of the binding energy and high concentration indicated that PAHs structurally smaller than pyrene also could be involved in soot inception. Hence, binary collisions of phenanthrene were simulated to find out whether phenanthrene dimers can serve as soot primary nuclei or not by using non-equilibrium molecular dynamics (MD). Three temperatures, six collision orientations and 155 initial translational velocities (ITVs) were considered. The results indicated that the number of dimers with lifetime over 10 ps which can serve as soot nuclei decreased from 52 at 1000 K to 17 at 1600 K, and further to 6 at 2400 K, which means that low temperature was more favourable for phenanthrene to form soot nuclei. Meanwhile, no soot nuclei were formed at the high velocity region (HVR), compared to 43 and 9 at low and middle velocity regions (LVR and MVR), respectively, when temperature was 1000 K. Also, no soot nuclei were formed at HVR when the temperature was raised to 1600 K and 2400 K. This indicated that HVR was unfavourable for phenanthrene to form soot nuclei. The results computationally further illustrated that small PAHs such as phenanthrene could serve as soot primary nuclei, since they have similar mole fractions in some flames. This may be useful for future soot simulation models.

Limitations on Inferring 3D Architecture and Dynamics From Surface Velocities in the India-Eurasia Collision Zone

NASA Astrophysics Data System (ADS)

Flesch, L.; Bendick, R.; Bischoff, S.

2018-02-01

Surface velocities derived from Global Positioning System observations and Quaternary fault slip rates measured throughout an extended region of high topography in South Asia vary smoothly over thousands of kilometers and are broadly symmetrical, with components of both north-south shortening and east-west extension relative to stable Eurasia. The observed velocity field does not contain discontinuities or steep gradients attributable to along-strike differences in collision architecture, despite the well-documented presence of a lithospheric slab beneath the Pamir but not the Tibetan Plateau. We use a modified Akaike information criterion (AICc) to show that surface velocities do not efficiently constrain 3D rheology, geometry, or force balance. Therefore, although other geophysical and geological observations may indicate the presence of mechanical or dynamic heterogeneities within the Indian-Asian collision, the surface Global Positioning System velocities contain little or no usable information about them.

Solar wind helium ions - Observations of the Helios solar probes between 0.3 and 1 AU

NASA Technical Reports Server (NTRS)

Marsch, E.; Rosenbauer, H.; Schwenn, R.; Muehlhaeuser, K.-H.; Neubauer, F. M.

1982-01-01

A Helios solar probe survey of solar wind helium ion velocity distributions and derived parameters between 0.3 and 1 AU is presented. Distributions in high-speed wind are found to generally have small total anisotropies, with some indication that, in the core part, the temperatures are greater parallel rather than perpendicular to the magnetic field. The anisotropy tends to increase with heliocentric radial distance, and the average dependence of helium ion temperatures on radial distance from the sun is described by a power law. Differential ion speeds with values of more than 150 km/sec are observed near perihelion, or 0.3 AU. The role of Coulomb collisions in limiting differential ion speeds and the ion temperature ratio is investigated, and it is found that collisions play a distinct role in low-speed wind, by limiting both differential ion velocity and temperature.

Double pendulum model for a tennis stroke including a collision process

NASA Astrophysics Data System (ADS)

Youn, Sun-Hyun

2015-10-01

By means of adding a collision process between the ball and racket in the double pendulum model, we analyzed the tennis stroke. The ball and the racket system may be accelerated during the collision time; thus, the speed of the rebound ball does not simply depend on the angular velocity of the racket. A higher angular velocity sometimes gives a lower rebound ball speed. We numerically showed that the proper time-lagged racket rotation increased the speed of the rebound ball by 20%. We also showed that the elbow should move in the proper direction in order to add the angular velocity of the racket.

Extension of the Viscous Collision Limiting Direct Simulation Monte Carlo Technique to Multiple Species

NASA Technical Reports Server (NTRS)

Liechty, Derek S.; Burt, Jonathan M.

2016-01-01

There are many flows fields that span a wide range of length scales where regions of both rarefied and continuum flow exist and neither direct simulation Monte Carlo (DSMC) nor computational fluid dynamics (CFD) provide the appropriate solution everywhere. Recently, a new viscous collision limited (VCL) DSMC technique was proposed to incorporate effects of physical diffusion into collision limiter calculations to make the low Knudsen number regime normally limited to CFD more tractable for an all-particle technique. This original work had been derived for a single species gas. The current work extends the VCL-DSMC technique to gases with multiple species. Similar derivations were performed to equate numerical and physical transport coefficients. However, a more rigorous treatment of determining the mixture viscosity is applied. In the original work, consideration was given to internal energy non-equilibrium, and this is also extended in the current work to chemical non-equilibrium.

Aging in freely evolving granular gas with impact velocity dependent coefficient of restitution

NASA Astrophysics Data System (ADS)

Kumari, Shikha; Ahmad, Syed Rashid

2018-05-01

The evolution of granular system is governed by the concept of coefficient of restitution that gives a relationship between normal component of relative velocities before and after collision. Most of the studies consider a simplified collision model where particles interact through coefficient of restitution which is a constant while in reality, the coefficient of restitution must be a variable that depends on the impact velocity of colliding particles. In this work, we have considered the aging in the velocity autocorrelation function, A(τw, τ) for a granular gas of realistic particles interacting through coefficient of restitution that is depending on impact velocity. Molecular dynamics simulation is used to study granular gas that is evolving freely in absence of any external force. From the simulation results, we observe that A(τw, τ) depends explicitly on waiting time τw and collision time τ. Initially, the function decays exponentially but as the waiting time increases the decay of function becomes slow due to correlations that emerge in velocity field.

Slow dynamics and aging of a confined granular flow

NASA Astrophysics Data System (ADS)

Clement, Eric

2004-03-01

We present experimental results on slow flow properties of granular assemblies confined in a vertical column and driven upwards at a constant velocity V [1]. The wall roughness is much lower than the typical grain size. For monodisperse assemblies this study evidences at low velocities (1

Progress with the COGENT Edge Kinetic Code: Collision operator options

DOE PAGES

Dorf, M. A.; Cohen, R. H.; Compton, J. C.; ...

2012-06-27

In this study, COGENT is a continuum gyrokinetic code for edge plasmas being developed by the Edge Simulation Laboratory collaboration. The code is distinguished by application of the fourth order conservative discretization, and mapped multiblock grid technology to handle the geometric complexity of the tokamak edge. It is written in v∥-μ (parallel velocity – magnetic moment) velocity coordinates, and making use of the gyrokinetic Poisson equation for the calculation of a self-consistent electric potential. In the present manuscript we report on the implementation and initial testing of a succession of increasingly detailed collision operator options, including a simple drag-diffusion operatormore » in the parallel velocity space, Lorentz collisions, and a linearized model Fokker-Planck collision operator conserving momentum and energy (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)« less

Trench-parallel variations in Pacific and Indo-Australian crustal velocity structure due to Louisville Ridge seamount subduction

NASA Astrophysics Data System (ADS)

Stratford, W. R.; Knight, T. P.; Peirce, C.; Watts, A. B.; Grevemeyer, I.; Paulatto, M.; Bassett, D.; Hunter, J.; Kalnins, L. M.

2012-12-01

Variations in trench and forearc morphology, and lithospheric velocity structure are observed where the Louisville Ridge seamount chain subducts at the Tonga-Kermadec Trench. Subduction of these seamounts has affected arc and back-arc processes along the trench for the last 5 Myr. High subduction rates (80 mm/yr in the north, 55 mm/yr in the south), a fast southwards migrating collision zone (~180 km/myr), and the obliquity of the subducting plate and the seamount chain to the trench, make this an ideal location to study the effects of seamount subduction on lithospheric structure. The "before and after" subduction regions have been targeted by several large-scale geophysical projects in recent years; the most recent being the R/V Sonne cruise SO215 in 2011. The crust and upper mantle velocity structure observed in profiles along strike of the seamount chain and perpendicular to the trench from this study, are compared to a similar profile from SO195, recorded ~100 km to the north. The affects of the passage of the seamounts through the subduction system are indicated by velocity anomalies in the crust and mantle of the overriding plate. Preliminary results indicate that in the present collision zone, mantle velocities (Pn) are reduced by ~5%. Around 100 km to the north, where seamounts are inferred to have subducted ~1 Myr ago, a reduction of 7% in mantle P-wave velocity is observed. The width of the trench slope and elevation of the forearc also vary along strike. At the collision zone a >100 km wide collapse region of kilometre-scale block faults comprise the trench slope, while the forearc is elevated. The elevated forearc has a 5 km think upper crust with a Vp of 2.5-5.5 km/s and the collapse zone also has upper crustal velocities as low as 2.5 km/s. To the east in the Pacific Plate, lower P-wave velocities are also observed and attributed to serpentinization due to deep fracturing in the outer trench high. Large bending faults permeate the crust and the Osbourn Seamount, currently on the verge of subduction, is fractured stepwise down into the trench. Pn velocities in the hinge zone of the Pacific Plate are as low as 7.3 km/s indicating that fracturing and serpentinization may also extend to sub-crustal depths. Finally, trench-parallel variations in subduction zone velocity structure are used to infer the degree to which seamount subduction has altered the physical state of the Pacific and Indo-Australian plates both pre- and post subduction.

Continent-arc collision in the Banda Arc imaged by ambient noise tomography

NASA Astrophysics Data System (ADS)

Porritt, Robert W.; Miller, Meghan S.; O'Driscoll, Leland J.; Harris, Cooper W.; Roosmawati, Nova; Teofilo da Costa, Luis

2016-09-01

The tectonic configuration of the Banda region in southeast Asia captures the spatial transition from subduction of Indian Ocean lithosphere to subduction and collision of the Australian continental lithosphere beneath the Banda Arc. An ongoing broadband seismic deployment funded by NSF is aimed at better understanding the mantle and lithospheric structure in the region and the relationship of the arc-continent collision to orogenesis. Here, we present results from ambient noise tomography in the region utilizing this temporary deployment of 30 broadband instruments and 39 permanent stations in Indonesia, Timor Leste, and Australia. We measure dispersion curves for over 21,000 inter-station paths resulting in good recovery of the velocity structure of the crust and upper mantle beneath the Savu Sea, Timor Leste, and the Nusa Tenggara Timur (NTT) region of Indonesia. The resulting three dimensional model indicates up to ∼25% variation in shear velocity throughout the plate boundary region; first-order velocity anomalies are associated with the subducting oceanic lithosphere, subducted Australian continental lithosphere, obducted oceanic sediments forming the core of the island of Timor, and high velocity anomalies in the Savu Sea and Sumba. The structure in Sumba and the Savu Sea is consistent with an uplifting forearc sliver. Beneath the island of Timor, we confirm earlier inferences of pervasive crustal duplexing from surface mapping, and establish a link to underlying structural features in the lowermost crust and uppermost mantle that drive upper crustal shortening. Finally, our images of the volcanic arc under Flores, Wetar, and Alor show high velocity structures of the Banda Terrane, but also a clear low velocity anomaly at the transition between subduction of oceanic and continental lithosphere. Given that the footprint of the Banda Terrane has previously been poorly defined, this model provides important constraints on tectonic reconstructions that formerly have lacked information on the lower crust and uppermost mantle.

QCD studies in ep collisions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smith, W.H.

1997-06-01

These lectures describe QCD physics studies over the period 1992--1996 from data taken with collisions of 27 GeV electrons and positrons with 820 GeV protons at the HERA collider at DESY by the two general-purpose detectors H1 and ZEUS. The focus of these lectures is on structure functions and jet production in deep inelastic scattering, photoproduction, and diffraction. The topics covered start with a general introduction to HERA and ep scattering. Structure functions are discussed. This includes the parton model, scaling violation, and the extraction of F{sub 2}, which is used to determine the gluon momentum distribution. Both low andmore » high Q{sup 2} regimes are discussed. The low Q{sup 2} transition from perturbative QCD to soft hadronic physics is examined. Jet production in deep inelastic scattering to measure {alpha}{sub s}, and in photoproduction to study resolved and direct photoproduction, is also presented. This is followed by a discussion of diffraction that begins with a general introduction to diffraction in hadronic collisions and its relation to ep collisions, and moves on to deep inelastic scattering, where the structure of diffractive exchange is studied, and in photoproduction, where dijet production provides insights into the structure of the Pomeron. 95 refs., 39 figs.« less

Nonlinear optical conductivity and subharmonic instabilities of graphene in a strong electromagnetic field

NASA Astrophysics Data System (ADS)

Sun, Zhiyuan; Basov, Dimitri; Fogler, Michael

We study theoretically the second-order nonlinear optical conductivity σ (2) of graphene as a function of frequency and momentum. We distinguish two regimes. At frequencies ω higher than the temperature-dependent electron-electron collision rate γee- 1 , the conductivity σ (2) can be derived from the semiclassical kinetic equation. The calculation requires taking into account the photon drag (Lorentz force) due to the ac magnetic field. In the low-frequency hydrodynamic regime ω <<γee- 1 , the nonlinear conductivity has a different form and the photon drag effect is suppressed. As a consequence of the nonlinearity, a strong enough photoexcitation can cause spontaneous generation of collective modes in a graphene strip: plasmons in the high-frequency regime and energy waves (demons) in the hydrodynamic one. The dominant instability occurs at frequency ω / 2 .

Temperature and excitation power influence on the velocity-selective optical pumping resonances of 133Cs atoms confined in an extremely thin cell

NASA Astrophysics Data System (ADS)

Vartanyan, T.; Polishchuk, V.; Sargsyan, A.; Krasteva, A.; Cartaleva, St.; Todorov, G.

2018-03-01

Linear and nonlinear absorption spectra of 133Cs vapor confined in an extremely thin cell were computed via iterations with respect to the resonance radiation intensity. When the incident radiation intensity is low, the transient polarization of the atoms that undergo frequent collisions with the cell walls leads to sub-Doppler features in the absorption spectra. Higher incident radiation intensities result in the appearance of velocity-selective optical pumping resonances. The theory developed agrees quantitatively with the experimental findings.

High velocity collisions between large dust aggregates at the limit for growing planetesimals

NASA Astrophysics Data System (ADS)

Wurm, G.; Teiser, J.; Paraskov, G.

2007-08-01

Planetesimals are km-size bodies supposed to be formed in protoplanetary disks as planetary precursors [1]. The most widely considered mechanism for their formation is based on mutual collisions of smaller bodies, a process which starts with the aggregation of (sub)-micron size dust particles. In the absence of events that lithify the growing dust aggregates, only the surface forces between dust particles provide adhesion and internal strength of the objects. It has been assumed that this might be a disadvantage as dust aggregates are readily destroyed by rather weak collisions. In fact, experimental research on dust aggregation showed that for collisions in the m/s range (sub)-mm size dust aggregates impacting a larger body do show a transition from sticking to rebound and/or fragmentation in collisions and no growth occurs at the large velocities [2, 3]. This seemed to be incompatible with typical collision velocities of small dust aggregates with m-size bodies which are expected to be on the order 50 m/s in protoplanetary disks [4]. We recently found that the experimental results cannot be scaled from m/s to tens of m/s collisions. In contrast to the assumptions and somewhat counterintuitive, it is the fragility of dust aggregates that allows growth at higher collision velocities. In impact experiments Wurm et al. [5] showed that between 13 m/s and 25 m/s a larger compact (target) body consisting of micron-size SiO2 dust particles accreted 50 % of the mass of a 1 cm dust projectile consisting of the same dust. For slower impacts the projectile only rebounded or fragmented slightly.

Toward Two-Color Sub-Doppler Saturation Recovery Kinetics in CN (x, v = 0, J)

NASA Astrophysics Data System (ADS)

Xu, Hong; Forthomme, Damien; Sears, Trevor; Hall, Gregory; Dagdigian, Paul

2015-06-01

Collision-induced rotational energy transfer among rotational levels of ground state CN (X 2σ+, v = 0) radicals has been probed by saturation recovery experiments, using high-resolution, polarized transient FM spectroscopy to probe the recovery of population and the decay of alignment following ns pulsed laser depletion of selected CN rotational levels. Despite the lack of Doppler selection in the pulsed depletion and the thermal distribution of collision velocities, the recovery kinetics are found to depend on the probed Doppler shift of the depleted signal. The observed Doppler-shift-dependent recovery rates are a measure of the velocity dependence of the inelastic cross sections, combined with the moderating effects of velocity-changing elastic collisions. New experiments are underway, in which the pulsed saturation is performed with sub-Doppler velocity selection. The time evolution of the spectral hole bleached in the initially thermal CN absorption spectrum can characterize speed-dependent inelastic collisions along with competing elastic velocity-changing collisions, all as a function of the initially bleached velocity group and rotational state. The initial time evolution of the depletion recovery spectrum can be compared to a stochastic model, using differential cross sections for elastic scattering as well as speed-dependent total inelastic cross sections, derived from ab initio scattering calculations. Progress to date will be reported. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 and DE-SC0012704 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.

Optical observation of metal jet generated by high speed inclined collision

NASA Astrophysics Data System (ADS)

Mori, A.; Tanaka, S.; Hokamoto, K.

2017-02-01

Explosive welding, one of the high energy rate material processing, is known the technique to weld strongly for the dissimilar metal combinations. When a metal is collided to the other metal at high velocity with a certain angle, good welding is achieved in this technique. Important parameters of the explosive welding method are the collision velocity and the collision angle. And it is necessary to know these parameters to obtain the explosively welded materials of several metals combinations. However, the optical observation for the collision of metal plate accelerated by the explosive is difficult because of the obstruction by the spreading of detonation gas. In the present work a single-stage powder gun and high speed video camera were used to observe the inclined collision of metals at the high velocity. Projectile consisted by a metal disc and sabot was accelerated by the deflagration of a gunpowder and was collided to another metal disc set with a certain angle. Metal jet was generated at the collision point when the projectile was collided to the target disc in the range of suitable conditions. By using this observation system, a series of the flow from the high speed collision to the generation of metal jet could be taken photographs clearly. This investigation shows the experimental results of the similar and dissimilar metal collision, with comparing the visualization of a metal jet simulated numerically.

A solution algorithm for fluid–particle flows across all flow regimes

DOE PAGES

Kong, Bo; Fox, Rodney O.

2017-05-12

Many fluid–particle flows occurring in nature and in technological applications exhibit large variations in the local particle volume fraction. For example, in circulating fluidized beds there are regions where the particles are closepacked as well as very dilute regions where particle–particle collisions are rare. Thus, in order to simulate such fluid–particle systems, it is necessary to design a flow solver that can accurately treat all flow regimes occurring simultaneously in the same flow domain. In this work, a solution algorithm is proposed for this purpose. The algorithm is based on splitting the free-transport flux solver dynamically and locally in themore » flow. In close-packed to moderately dense regions, a hydrodynamic solver is employed, while in dilute to very dilute regions a kinetic-based finite-volume solver is used in conjunction with quadrature-based moment methods. To illustrate the accuracy and robustness of the proposed solution algorithm, it is implemented in OpenFOAM for particle velocity moments up to second order, and applied to simulate gravity-driven, gas–particle flows exhibiting cluster-induced turbulence. By varying the average particle volume fraction in the flow domain, it is demonstrated that the flow solver can handle seamlessly all flow regimes present in fluid–particle flows.« less

A solution algorithm for fluid-particle flows across all flow regimes

NASA Astrophysics Data System (ADS)

Kong, Bo; Fox, Rodney O.

2017-09-01

Many fluid-particle flows occurring in nature and in technological applications exhibit large variations in the local particle volume fraction. For example, in circulating fluidized beds there are regions where the particles are close-packed as well as very dilute regions where particle-particle collisions are rare. Thus, in order to simulate such fluid-particle systems, it is necessary to design a flow solver that can accurately treat all flow regimes occurring simultaneously in the same flow domain. In this work, a solution algorithm is proposed for this purpose. The algorithm is based on splitting the free-transport flux solver dynamically and locally in the flow. In close-packed to moderately dense regions, a hydrodynamic solver is employed, while in dilute to very dilute regions a kinetic-based finite-volume solver is used in conjunction with quadrature-based moment methods. To illustrate the accuracy and robustness of the proposed solution algorithm, it is implemented in OpenFOAM for particle velocity moments up to second order, and applied to simulate gravity-driven, gas-particle flows exhibiting cluster-induced turbulence. By varying the average particle volume fraction in the flow domain, it is demonstrated that the flow solver can handle seamlessly all flow regimes present in fluid-particle flows.

A solution algorithm for fluid–particle flows across all flow regimes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kong, Bo; Fox, Rodney O.

Many fluid–particle flows occurring in nature and in technological applications exhibit large variations in the local particle volume fraction. For example, in circulating fluidized beds there are regions where the particles are closepacked as well as very dilute regions where particle–particle collisions are rare. Thus, in order to simulate such fluid–particle systems, it is necessary to design a flow solver that can accurately treat all flow regimes occurring simultaneously in the same flow domain. In this work, a solution algorithm is proposed for this purpose. The algorithm is based on splitting the free-transport flux solver dynamically and locally in themore » flow. In close-packed to moderately dense regions, a hydrodynamic solver is employed, while in dilute to very dilute regions a kinetic-based finite-volume solver is used in conjunction with quadrature-based moment methods. To illustrate the accuracy and robustness of the proposed solution algorithm, it is implemented in OpenFOAM for particle velocity moments up to second order, and applied to simulate gravity-driven, gas–particle flows exhibiting cluster-induced turbulence. By varying the average particle volume fraction in the flow domain, it is demonstrated that the flow solver can handle seamlessly all flow regimes present in fluid–particle flows.« less

Kinetic description of large-scale low pressure glow discharges

NASA Astrophysics Data System (ADS)

Kortshagen, Uwe; Heil, Brian

1997-10-01

In recent years the so called ``nonlocal approximation'' to the solution of the electron Boltzmann equation has attracted considerable attention as an extremely efficient method for the kinetic modeling of low pressure discharges. However, it appears that modern discharges, which are optimized to provide large-scale plasma uniformity, are explicitly designed to work in a regime, in which the nonlocal approximation is no longer strictly valid. In the presentation we discuss results of a hybrid model, which is based on the natural division of the electron distribution function into a nonlocal body, which is determined by elastic collisions only, and a high energy part which requires a more complete treatment due to the action of inelastic collisions and wall losses of electrons. The method is applied to an inductively coupled low pressure discharge. We discuss the transition from plasma density profiles maximal on the discharge axis to plasma density profiles with off-center maxima, which has been observed in experiments. A positive feedback mechanism involved in this transition is pointed out.

A New Look at the Molecular Gas in M42 and M43: Possible Evidence for Cloud–Cloud Collision that Triggered Formation of the OB Stars in the Orion Nebula Cluster

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Torii, Kazufumi; Hattori, Yusuke; Nishimura, Atsushi; Ohama, Akio; Shimajiri, Yoshito; Shima, Kazuhiro; Habe, Asao; Sano, Hidetoshi; Kohno, Mikito; Yamamoto, Hiroaki; Tachihara, Kengo; Onishi, Toshikazu

2018-06-01

The Orion Nebula Cluster toward the H II region M42 is the most outstanding young cluster at the smallest distance (410 pc) among the rich high-mass stellar clusters. By newly analyzing the archival molecular data of the 12CO(J = 1–0) emission at 21″ resolution, we identified at least three pairs of complementary distributions between two velocity components at 8 and 13 km s‑1. We present a hypothesis that the two clouds collided with each other and triggered formation of the high-mass stars, mainly toward two regions including the nearly 10 O stars in M42 and the B star, NU Ori, in M43. The timescale of the collision is estimated to be ∼0.1 Myr by a ratio of the cloud size and velocity corrected for projection, which is consistent with the age of the youngest cluster members less than 0.1 Myr. The majority of the low-mass cluster members were formed prior to the collision in the last Myr. We discuss the implications of the present hypothesis and the scenario of high-mass star formation by comparing with the other eight cases of triggered O-star formation via cloud–cloud collision.

Flow and axial dispersion in a sinusoidal-walled tube: Effects of inertial and unsteady flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

Richmond, Marshall C.; Perkins, William A.; Scheibe, Timothy D.

2013-12-01

Dispersion in porous media flows has been the subject of much experimental, theoretical and numerical study. Here we consider a wavy-walled tube (a three-dimensional tube with sinusoidally-varying diameter) as a simplified conceptualization of flow in porous media, where constrictions represent pore throats and expansions pore bodies. A theoretical model for effective (macroscopic) longitudinal dispersion in this system has been developed by volume averaging the microscale velocity field. Direct numerical simulation using computational fluid dynamics (CFD) methods was used to compute velocity fields by solving the Navier-Stokes equations, and also to numerically solve the volume averaging closure problem, for a rangemore » of Reynolds numbers (Re) spanning the low-Re to inertial flow regimes, including one simulation at Re = 449 for which unsteady flow was observed. Dispersion values were computed using both the volume averaging solution and a random walk particle tracking method, and results of the two methods were shown to be consistent. Our results are compared to experimental measurements of dispersion in porous media and to previous theoretical results for the low-Re, Stokes flow regime. In the steady inertial regime we observe an power-law increase in effective longitudinal dispersion (DL) with Re, consistent with previous results. This rapid rate of increase is caused by trapping of solute in expansions due to flow separation (eddies). For the unsteady case (Re = 449), the rate of increase of DL with Re was smaller than that observed at lower Re. Velocity fluctuations in this regime lead to increased rates of solute mass transfer between the core flow and separated flow regions, thus diminishing the amount of tailing caused by solute trapping in eddies and thereby reducing longitudinal dispersion.« less

Magnetic merging in colliding flux tubes

NASA Technical Reports Server (NTRS)

Zweibel, Ellen G.; Rhoads, James E.

1995-01-01

We develop an analytical theory of reconnection between colliding, twisted magnetic flux tubes. Our analysis is restricted to direct collisions between parallel tubes and is based on the collision dynamics worked out by Bogdan (1984). We show that there is a range of collision velocities for which neutral point reconnection of the Parker-Sweet type can occur, and a smaller range for which reconnection leads to coalescence. Mean velocities within the solar convection zone are probably significantly greater than the upper limit for coalescence. This suggests that the majority of flux tube collisions do not result in merging, unless the frictional coupling of the tubes to the background flow is extremely strong.

Seismic evidence for central Taiwan magnetic low and deep-crustal deformation caused by plate collision

NASA Astrophysics Data System (ADS)

Cheng, Win-Bin

2018-01-01

Crustal seismic velocity structure was determined for the northern Taiwan using seismic travel-time data to investigate the northeastern extension of the northern South China Sea's high-magnetic belt. In order to increase the model resolution, a joint analysis of gravity anomaly and seismic travel-time data have been conducted. A total of 3385 events had been used in the inversion that was collected by the Central Weather Bureau Seismological Network from 1990 to 2015. The main features of the obtained three-dimensional velocity model are: (1) a relatively high Vp zone with velocity greater than 6.5 km/s is observed in the middle to lower crust, (2) the high Vp zone generally parallels to the north-south structural trending of the Chuchih fault and Hsuehshan Range, (3) at 25 km depth-slice, the high Vp zone shows structural trends change from northeastward to northward in central Taiwan, where the values of high-magnetic anomalies are rapidly decreasing to low values. A combination of seismic, GPS, and structural interpretations suggests that the entire crust has been deformed and demagnetized in consequence of the collision between the Philippine Sea plate and the Asian continental margin. We suggest that the feature of sharp bending of the high Vp zone would migrate southwestward and cause further crustal deformation of the Peikang High in the future.

A Hybrid Method for Accelerated Simulation of Coulomb Collisions in a Plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Caflisch, R; Wang, C; Dimarco, G

2007-10-09

If the collisional time scale for Coulomb collisions is comparable to the characteristic time scales for a plasma, then simulation of Coulomb collisions may be important for computation of kinetic plasma dynamics. This can be a computational bottleneck because of the large number of simulated particles and collisions (or phase-space resolution requirements in continuum algorithms), as well as the wide range of collision rates over the velocity distribution function. This paper considers Monte Carlo simulation of Coulomb collisions using the binary collision models of Takizuka & Abe and Nanbu. It presents a hybrid method for accelerating the computation of Coulombmore » collisions. The hybrid method represents the velocity distribution function as a combination of a thermal component (a Maxwellian distribution) and a kinetic component (a set of discrete particles). Collisions between particles from the thermal component preserve the Maxwellian; collisions between particles from the kinetic component are performed using the method of or Nanbu. Collisions between the kinetic and thermal components are performed by sampling a particle from the thermal component and selecting a particle from the kinetic component. Particles are also transferred between the two components according to thermalization and dethermalization probabilities, which are functions of phase space.« less

A numerical investigation of continental collision styles

NASA Astrophysics Data System (ADS)

Ghazian, Reza Khabbaz; Buiter, Susanne J. H.

2013-06-01

Continental collision after closure of an ocean can lead to different deformation styles: subduction of continental crust and lithosphere, lithospheric thickening, folding of the unsubducted continents, Rayleigh-Taylor (RT) instabilities and/or slab break-off. We use 2-D thermomechanical models of oceanic subduction followed by continental collision to investigate the sensitivity of these collision styles to driving velocity, crustal and lithospheric temperature, continental rheology and the initial density difference between the oceanic lithosphere and the asthenosphere. We find that these parameters influence the collision system, but that driving velocity, rheology and lithospheric (rather than Moho and mantle) temperature can be classified as important controls, whereas reasonable variations in the initial density contrast between oceanic lithosphere and asthenosphere are not necessarily important. Stable continental subduction occurs over a relatively large range of values of driving velocity and lithospheric temperature. Fast and cold systems are more likely to show folding, whereas slow and warm systems can experience RT-type dripping. Our results show that a continent with a strong upper crust can experience subduction of the entire crust and is more likely to fold. Accretion of the upper crust at the trench is feasible when the upper crust has a moderate to weak strength, whereas the entire crust can be scraped-off in the case of a weak lower crust. We also illustrate that weakening of the lithospheric mantle promotes RT-type of dripping in a collision system. We use a dynamic collision model, in which collision is driven by slab pull only, to illustrate that adjacent plates can play an important role in continental collision systems. In dynamic collision models, exhumation of subducted continental material and sediments is triggered by slab retreat and opening of a subduction channel, which allows upward flow of buoyant materials. Exhumation continues after slab break-off by reverse motion of the subducting plate (`eduction') caused by the reduced slab pull. We illustrate how a simple force balance of slab pull, slab push, slab bending, viscous resistance and buoyancy can explain the different collision styles caused by variations in velocity, temperature, rheology, density differences and the interaction with adjacent plates.

Characteristics of Whipple Shield Performance in the Shatter Regime

NASA Technical Reports Server (NTRS)

Ryan, S.; Bjorkman, M.; Christiansen, E. L.

2010-01-01

Ballistic limit equations define the failure of metallic Whipple shields in three parts: low velocity, shatter, and hypervelocity. Failure limits in the shatter regime are based on a linear interpolation between the onset of projectile fragmentation, and impulsive rupture of the shield rear wall. A series of hypervelocity impact tests have been performed on aluminum alloy Whipple shields to investigate failure mechanisms and performance limits in the shatter regime. Test results demonstrated a more rapid increase in performance than predicted by the latest iteration of the JSC Whipple shield BLE following the onset of projectile fragmentation. This increase in performance was found to level out between 4.0-5.0 km/s, with a subsequent decrease in performance for velocities up to 6.0 km/s. For a detached spall failure criterion, the failure limit was found to continually decrease up to a velocity of 7.0 km/s, substantially varying from the BLE, while for perforation-based failure an increase in performance was observed. An existing phenomenological ballistic limit curve was found to provide a more accurate reproduction of shield behavior that the BLE, however a number of underlying assumptions such as the occurrence of complete projectile fragmentation and the effect on performance of incipient projectile melt were found to be inaccurate. A cratering relationship based on the largest residual fragment size has been derived for application at velocities between 3.0-4.0 km/s, and was shown to accurately reproduce the trends of the experimental data. Further investigation is required to allow a full analytical description of shatter regime performance for metallic Whipple shields.

Trajectory description of the quantum–classical transition for wave packet interference

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chou, Chia-Chun, E-mail: ccchou@mx.nthu.edu.tw

2016-08-15

The quantum–classical transition for wave packet interference is investigated using a hydrodynamic description. A nonlinear quantum–classical transition equation is obtained by introducing a degree of quantumness ranging from zero to one into the classical time-dependent Schrödinger equation. This equation provides a continuous description for the transition process of physical systems from purely quantum to purely classical regimes. In this study, the transition trajectory formalism is developed to provide a hydrodynamic description for the quantum–classical transition. The flow momentum of transition trajectories is defined by the gradient of the action function in the transition wave function and these trajectories follow themore » main features of the evolving probability density. Then, the transition trajectory formalism is employed to analyze the quantum–classical transition of wave packet interference. For the collision-like wave packet interference where the propagation velocity is faster than the spreading speed of the wave packet, the interference process remains collision-like for all the degree of quantumness. However, the interference features demonstrated by transition trajectories gradually disappear when the degree of quantumness approaches zero. For the diffraction-like wave packet interference, the interference process changes continuously from a diffraction-like to collision-like case when the degree of quantumness gradually decreases. This study provides an insightful trajectory interpretation for the quantum–classical transition of wave packet interference.« less

A model for accretion of the terrestrial planets

NASA Technical Reports Server (NTRS)

Weidenschilling, S. J.

1974-01-01

One possible origin of the terrestrial planets involves their formation by gravitational accretion of particles originally in Keplerian orbits about the sun. Some implications of this theory are considered. A formal expression for the rate of mass accretion by a planet is developed. The formal singularity of the gravitational collision cross section for low relative velocities is shown to be without physical significance when the accreting bodies are in heliocentric orbits. The distribution of particle velocities relative to an accreting planet is considered; the mean velocity increases with time. The internal temperature of an accreting planet is shown to depend simply on the accretion rate. A simple and physically reasonable approximate expression for a planetary accretion rate is proposed.

Mediterranean extension and the Africa-Eurasia collision

NASA Astrophysics Data System (ADS)

Jolivet, Laurent; Faccenna, Claudio

2000-12-01

A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30-25 Myr ago. These events are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa. Geological observations in the Neogene extensional basins of the Mediterranean region reveal that extension started synchronously from west to east 30-25 Myr ago. In the western Mediterranean it started in the Gulf of Lion, Valencia trough, and Alboran Sea as well as between the Maures massif and Corsica between 33 and 27 Ma ago. It then propagated eastward and southward to form to Liguro-Provençal basin and the Tyrrhenian Sea. In the eastern Mediterranean, extension started in the Aegean Sea before the deposition of marine sediments onto the collapsed Hellenides in the Aquitanian and before the cooling of high-temperature metamorphic core complexes between 20 and 25 Ma. Foundering of the inner zones of the Carpathians and extension in the Panonnian basin also started in the late Oligocene-early Miocene. The body of the Afro-Arabian plate first collided with Eurasia in the eastern Mediterranean region progressively from the Eocene to the Oligocene. Extensional tectonics was first recorded in the Gulf of Aden, Afar triple junction, and Red Sea region also in the Oligocene. A general magmatic surge occurred above all African hot spots, especially the Afar one. We explore the possibility that these drastic changes in the stress regime of the Mediterranean region and Middle East and the contemporaneous volcanic event were triggerred by the Africa/Arabia-Eurasia collision, which slowed down the motion of Africa. The present-day Mediterranean Sea was then locked between two collision zones, and the velocity of retreat of the African slab increased and became larger than the velocity of convergence leading to backarc extension. East of the Caucasus and northern Zagros collision zone the Afro-Arabian plate was still pulled by the slab pull force in the Zagros subduction zone, which created extensional stresses in the northeast corner of the Afro-Arabian plate. The Arabian plate was formed by propagation of a crack from the Carlsberg ridge westward toward the weak part of the African lithosphere above the Afar plume.

Viscosity effects in wind wave generation

NASA Astrophysics Data System (ADS)

Paquier, A.; Moisy, F.; Rabaud, M.

2016-12-01

We investigate experimentally the influence of the liquid viscosity on the problem of the generation of waves by a turbulent wind at the surface of a liquid, extending the results of Paquier et al. [A. Paquier et al., Phys. Fluids 27, 122103 (2015), 10.1063/1.4936395] over nearly three decades of viscosity. The surface deformations are measured with micrometer accuracy using the free-surface synthetic schlieren method. We recover the two regimes of surface deformations previously identified: the wrinkle regime at small wind velocity, resulting from the viscous imprint on the liquid surface of the turbulent fluctuations in the boundary layer, and the regular wave regime at large wind velocity. Below the wave threshold, we find that the characteristic amplitude of the wrinkles scales as ν-1 /2u*3 /2 over nearly the whole range of viscosities, whereas their size is essentially unchanged. We propose a simple model for this scaling, which compares well with the data. We show that the critical friction velocity u* for the onset of regular waves slowly increases with viscosity as ν0.2. Whereas the transition between wrinkles and waves is smooth at low viscosity, including for water, it becomes rather abrupt at high viscosity. A third wave regime is found at ν >(100 -200 ) ×10-6m2s-1 , characterized by a slow, nearly periodic emission of large-amplitude isolated fluid bumps.

Effects of Cocos Ridge Collision on the Western Caribbean: Is there a Panama Block?

NASA Astrophysics Data System (ADS)

Kobayashi, D.; La Femina, P. C.; Geirsson, H.; Chichaco, E.; Abrego M, A. A.; Fisher, D. M.; Camacho, E. I.

2011-12-01

It has been recognized that the subduction and collision of the Cocos Ridge, a 2 km high aseismic ridge standing on >20 km thick oceanic crust of the Cocos plate, drives upper plate deformation in southern Central America. Recent studies of Global Positioning System (GPS) derived horizontal velocities relative to the Caribbean Plate showed a radial pattern centered on the Cocos Ridge axis where Cocos-Caribbean convergence is orthogonal, and margin-parallel velocities to the northwest. Models of the full three-dimensional GPS velocity field and earthquake slip vectors demonstrate low mechanical coupling along the Middle America subduction zone in Nicaragua and El Salvador, and a broad zone of high coupling beneath the Osa Peninsula, where the Cocos Ridge intersects the margin. These results suggest that Cocos Ridge collision may be the main driver for trench-parallel motion of the fore arc to the northwest and for uplift and shortening of the outer fore arc in southern Central America, whereby thickened and hence buoyant Cocos Ridge crust acts as an indenter causing the tectonic escape of the fore arc. These studies, however, were not able to constrain well the pattern of surface deformation east-southeast of the ridge axis due to a lack of GPS stations, and Cocos Ridge collision may be responsible for the kinematics and deformation of the proposed Panama block. Recent reinforcement of the GPS network in southeastern Costa Rica and Panama has increased the spatial and temporal resolution of the network and made it possible to further investigate surface deformation of southern Central America and the Panama block. We present a new regional surface velocity field for Central America from geodetic GPS data collected at 11 recently-installed and 178 existing episodic, semi-continuous, and continuous GPS sites in Nicaragua, Costa Rica, and Panama. We investigate the effects of Cocos Ridge collision on the Panama block through kinematic block modeling. Published earthquake relocation and geologic data are used to define block boundaries and fault geometries. We invert the three-dimensional GPS velocity vectors and earthquake slip vectors to estimate the magnitude and spatial distribution of interplate mechanical coupling on active plate and block boundaries around the Panama block; the Middle America Trench - South Panama Deformed Belt, the Central Costa Rican Deformed Belt, and the North Panama Deformed Belt in particular, and the rates of relative plate motion between the Panama block and the adjacent Cocos, Nazca, and Caribbean plates. This study tests whether the Panama block responds to the ridge collision as a rigid tectonic block or as a deforming zone consisting of multiple blocks.

Return currents in solar flares - Collisionless effects

NASA Technical Reports Server (NTRS)

Rowland, H. L.; Vlahos, L.

1985-01-01

If the primary, precipitating electrons in a solar flare are unstable to beam plasma interactions, it is shown that strong Langmuir turbulence can seriously modify the way in which a return current is carried by the background plasma. In particular, the return (or reverse) current will not be carried by the bulk of the electrons, but by a small number of high velocity electrons. For beam/plasma densities greater than 0.01, this can reduce the effects of collisions on the return current. For higher density beams where the return current could be unstable to current driven instabilities, the effects of strong turbulence anomalous resistivity is shown to prevent the appearance of such instabilities. Again in this regime, how the return current is carried is determined by the beam generated strong turbulence.

A low noise discrete velocity method for the Boltzmann equation with quantized rotational and vibrational energy

NASA Astrophysics Data System (ADS)

Clarke, Peter; Varghese, Philip; Goldstein, David

2018-01-01

A discrete velocity method is developed for gas mixtures of diatomic molecules with both rotational and vibrational energy states. A full quantized model is described, and rotation-translation and vibration-translation energy exchanges are simulated using a Larsen-Borgnakke exchange model. Elastic and inelastic molecular interactions are modeled during every simulated collision to help produce smooth internal energy distributions. The method is verified by comparing simulations of homogeneous relaxation by our discrete velocity method to numerical solutions of the Jeans and Landau-Teller equations, and to direct simulation Monte Carlo. We compute the structure of a 1D shock using this method, and determine how the rotational energy distribution varies with spatial location in the shock and with position in velocity space.

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Gao, Yuan; Li, Aibing; Li, Lun; Chen, Anguo

2017-06-01

Lithospheric shear wave velocity beneath the southeastern margin of the Tibetan Plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20-100 s are determined and used to construct the 3-D shear wave velocity model. Low-velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low-velocity column beneath the Tengchong Volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid-induced partial melting due to the subduction of the Burma slab. The western Yangtze Craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low-velocity zone is imaged at the depths of 70-150 km beneath the eastern part of the Yangtze Craton, which could be caused by small-scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Scaling Relations of Starburst-driven Galactic Winds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tanner, Ryan; Cecil, Gerald; Heitsch, Fabian, E-mail: rytanner@augusta.edu

2017-07-10

Using synthetic absorption lines generated from 3D hydrodynamical simulations, we explore how the velocity of a starburst-driven galactic wind correlates with the star formation rate (SFR) and SFR density. We find strong correlations for neutral and low ionized gas, but no correlation for highly ionized gas. The correlations for neutral and low ionized gas only hold for SFRs below a critical limit set by the mass loading of the starburst, above which point the scaling relations flatten abruptly. Below this point the scaling relations depend on the temperature regime being probed by the absorption line, not on the mass loading.more » The exact scaling relation depends on whether the maximum or mean velocity of the absorption line is used. We find that the outflow velocity of neutral gas can be up to five times lower than the average velocity of ionized gas, with the velocity difference increasing for higher ionization states. Furthermore, the velocity difference depends on both the SFR and mass loading of the starburst. Thus, absorption lines of neutral or low ionized gas cannot easily be used as a proxy for the outflow velocity of the hot gas.« less

Rebound mechanics of micrometre-scale, spherical particles in high-velocity impacts.

PubMed

Yildirim, Baran; Yang, Hankang; Gouldstone, Andrew; Müftü, Sinan

2017-08-01

The impact mechanics of micrometre-scale metal particles with flat metal surfaces is investigated for high-velocity impacts ranging from 50 m s -1 to more than 1 km s -1 , where impact causes predominantly plastic deformation. A material model that includes high strain rate and temperature effects on the yield stress, heat generation due to plasticity, material damage due to excessive plastic strain and heat transfer is used in the numerical analysis. The coefficient of restitution e is predicted by the classical work using elastic-plastic deformation analysis with quasi-static impact mechanics to be proportional to [Formula: see text] and [Formula: see text] for the low and moderate impact velocities that span the ranges of 0-10 and 10-100 m s -1 , respectively. In the elastic-plastic and fully plastic deformation regimes the particle rebound is attributed to the elastic spring-back that initiates at the particle-substrate interface. At higher impact velocities (0.1-1 km s -1 ) e is shown to be proportional to approximately [Formula: see text]. In this deeply plastic deformation regime various deformation modes that depend on plastic flow of the material including the time lag between the rebound instances of the top and bottom points of particle and the lateral spreading of the particle are identified. In this deformation regime, the elastic spring-back initiates subsurface, in the substrate.

Sound velocity and absorption in a coarsening foam.

PubMed

Mujica, Nicolás; Fauve, Stéphan

2002-08-01

We present experimental measurements of sound velocity and absorption in a commercial shaving foam. We observe that both quantities evolve with time as the foam coarsens increasing its mean bubble radius . By varying the acoustic frequency we probe the foam from the large wavelength regime, lambda approximately 1500, down to the scale lambda approximately 20. Sound absorption alpha varies significantly with both the foam age and the excitation frequency. After an initial transition time of 20 min, the attenuation per wavelength, alphalambda, varies linearly with the foam age. In addition, for evolution times smaller than approximately 90 min, we observe that alphalambda scales linearly with both foam age and frequency. From these scalings we show that the thermal dissipation mechanism is the dominant one. Sound velocity c is initially frequency independent but the medium becomes slightly dispersive as the foam coarsens. We observe that sound velocity depends on the evolution of the structure of the foam, even in the large wavelength regime. After 2 h of foam coarsening, c decreases at least by a factor of 20%, due to the softening of the foam. These facts are explained by considering the liquid matrix elasticity, due to the presence of surfactant molecules. A simple model of foam structure, combined with results of Biot's theory for porous media, gives both good qualitative and quantitative agreement with our experimental results in the low frequency regime.

Investigation and visualization of liquid-liquid flow in a vertically mounted Hele-Shaw cell: flow regimes, velocity and shape of droplets

NASA Astrophysics Data System (ADS)

Shad, S.; Gates, I. D.; Maini, B. B.

2009-11-01

The motion and shape of a liquid drop flowing within a continuous, conveying liquid phase in a vertical Hele-Shaw cell were investigated experimentally. The continuous phase was more viscous and wetted the bounding walls of the Hele-Shaw cell. The gap between the Hele-Shaw plates was set equal to 0.0226 cm. Four different flow regimes were observed: (a) small-droplet flow, (b) elongated-droplet flow, (c) churn flow and (d) channel flow. At low capillary number, that is, when capillary forces are larger than viscous forces, the droplet shape was irregular and changed with time and distance, and it moved with lower velocity than that of the conveying phase. At higher capillary number, several different shapes of stabilized elongated and flattened drops were observed. In contrast to gas-liquid systems, the velocities of droplets are higher than that of conveying liquid. New correlations derived from dimensionless analysis and fitted to the experimental data were generated to predict the elongated-drop velocity and aspect ratio.

Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos

NASA Astrophysics Data System (ADS)

Burger, C.; Maindl, T. I.; Schäfer, C. M.

2018-01-01

Collisions between large, similar-sized bodies are believed to shape the final characteristics and composition of terrestrial planets. Their inventories of volatiles such as water are either delivered or at least significantly modified by such events. Besides the transition from accretion to erosion with increasing impact velocity, similar-sized collisions can also result in hit-and-run outcomes for sufficiently oblique impact angles and large enough projectile-to-target mass ratios. We study volatile transfer and loss focusing on hit-and-run encounters by means of smooth particle hydrodynamics simulations, including all main parameters: impact velocity, impact angle, mass ratio and also the total colliding mass. We find a broad range of overall water losses, up to 75% in the most energetic hit-and-run events, and confirm the much more severe consequences for the smaller body also for stripping of volatile layers. Transfer of water between projectile and target inventories is found to be mostly rather inefficient, and final water contents are dominated by pre-collision inventories reduced by impact losses, for similar pre-collision water mass fractions. Comparison with our numerical results shows that current collision outcome models are not accurate enough to reliably predict these composition changes in hit-and-run events. To also account for non-mechanical losses, we estimate the amount of collisionally vaporized water over a broad range of masses and find that these contributions are particularly important in collisions of ˜ Mars-sized bodies, with sufficiently high impact energies, but still relatively low gravity. Our results clearly indicate that the cumulative effect of several (hit-and-run) collisions can efficiently strip protoplanets of their volatile layers, especially the smaller body, as it might be common, e.g., for Earth-mass planets in systems with Super-Earths. An accurate model for stripping of volatiles that can be included in future planet formation simulations has to account for the peculiarities of hit-and-run events and track compositional changes in both large post-collision fragments.

The recent breakup of an asteroid in the main-belt region.

PubMed

Nesvorný, David; Bottke, William F; Dones, Luke; Levison, Harold F

2002-06-13

The present population of asteroids in the main belt is largely the result of many past collisions. Ideally, the asteroid fragments resulting from each impact event could help us understand the large-scale collisions that shaped the planets during early epochs. Most known asteroid fragment families, however, are very old and have therefore undergone significant collisional and dynamical evolution since their formation. This evolution has masked the properties of the original collisions. Here we report the discovery of a family of asteroids that formed in a disruption event only 5.8 +/- 0.2 million years ago, and which has subsequently undergone little dynamical and collisional evolution. We identified 39 fragments, two of which are large and comparable in size (diameters of approximately 19 and approximately 14 km), with the remainder exhibiting a continuum of sizes in the range 2-7 km. The low measured ejection velocities suggest that gravitational re-accumulation after a collision may be a common feature of asteroid evolution. Moreover, these data can be used to check numerical models of larger-scale collisions.

Catastrophic global-avalanche of a hollow pressure filament

NASA Astrophysics Data System (ADS)

van Compernolle, B.; Poulos, M. J.; Morales, G. J.

2017-10-01

New results are presented of a basic heat transport experiment performed in the Large Plasma Device at UCLA. A ring-shaped electron beam source injects low energy electrons along a strong magnetic field into a preexisting, large and cold plasma. The injected electrons are thermalized by Coulomb collisions within a short distance and provide an off-axis heat source that results in a long, hollow, cylindrical region of elevated plasma pressure. The off-axis source is active for a period long compared to the density decay time, i.e., as time progresses the power per particle increases. Two distinct regimes are observed to take place, an early regime dominated by multiple avalanches, identified as a sudden intermittent rearrangement of the pressure profile that repeats under sustained heating, and a second regime dominated by broadband drift-Alfvén fluctuations. The transition between the two regimes is sudden and global, both radially and axially. The initial regime is characterized by peaked density and temperature profiles, while only the peaked temperature profile survives in the second regime. Recent measurements at multiple axial locations provide new insight into the axial dynamics of the global avalanche. Sponsored by NSF Grant 1619505 and by DOE/NSF at BaPSF.

Severity of child pedestrian injuries due to bonnet-type-vehicle collision.

PubMed

Hitosugi, Masahito; Kawato, Hitoshi; Gomei, Sayaka; Mizuno, Koji; Tokudome, Shogo

2013-10-01

The aim of this study was to clarify the pattern of child pedestrian injury, injury severity, and its relation to collision velocity in bonnet-type-vehicle collision. In-depth data were retrospectively collected from the Institute for Traffic Accident Research and Data Analysis on pedestrians younger than 13 years old with any bodily injuries from collisions with bonnet-type vehicles between 1993 and 2004. Forty-seven patients from 43 collisions with a mean age of 6.9 ± 2.5 years were included in the study. Injury severity was not significantly different between patients who were hit by the front of the vehicle and those who were hit by the side of the vehicle. In front collisions, impact with the vehicle was associated with significantly higher Abbreviated Injury Scale (AIS) scores than those for impact with the road, especially for the lower extremities (mean: 1.2 vs 0.2, P < 0.001). Injury severity of the lower extremities and collision velocity were examined. The estimated collision velocity of the vehicle was not significantly different between patients with lower extremity AIS scores of 0 or 1 and those of 2 or 3. Some pediatric pedestrians suffer from collisions with bonnet-type vehicles without lower extremity fractures owing to the characteristics of child pedestrians. Providing injury prevention programs for children in communities and schools, developing active safety devices in the vehicle, and modifying the vehicle body to a pediatric pedestrian-friendly structure may increase pedestrian protection. © 2013 The Authors. Pediatrics International © 2013 Japan Pediatric Society.

Triggered O Star Formation in M20 via Cloud-Cloud Collision: Comparisons between High-resolution CO Observations and Simulations

NASA Astrophysics Data System (ADS)

Torii, K.; Hattori, Y.; Hasegawa, K.; Ohama, A.; Haworth, T. J.; Shima, K.; Habe, A.; Tachihara, K.; Mizuno, N.; Onishi, T.; Mizuno, A.; Fukui, Y.

2017-02-01

Understanding high-mass star formation is one of the top-priority issues in astrophysics. Recent observational studies have revealed that cloud-cloud collisions may play a role in high-mass star formation in several places in the Milky Way and the Large Magellanic Cloud. The Trifid Nebula M20 is a well-known Galactic H II region ionized by a single O7.5 star. In 2011, based on the CO observations with NANTEN2, we reported that the O star was formed by the collision between two molecular clouds ˜0.3 Myr ago. Those observations identified two molecular clouds toward M20, traveling at a relative velocity of 7.5 {km} {{{s}}}-1. This velocity separation implies that the clouds cannot be gravitationally bound to M20, but since the clouds show signs of heating by the stars there they must be spatially coincident with it. A collision is therefore highly possible. In this paper we present the new CO J = 1-0 and J = 3-2 observations of the colliding clouds in M20 performed with the Mopra and ASTE telescopes. The high-resolution observations revealed that the two molecular clouds have peculiar spatial and velocity structures, I.e., a spatially complementary distribution between the two clouds and a bridge feature that connects the two clouds in velocity space. Based on a new comparison with numerical models, we find that this complementary distribution is an expected outcome of cloud-cloud collisions, and that the bridge feature can be interpreted as the turbulent gas excited at the interface of the collision. Our results reinforce the cloud-cloud collision scenario in M20.

Visco-instability of shear viscoelastic collisional dusty plasma systems

NASA Astrophysics Data System (ADS)

Mahdavi-Gharavi, M.; Hajisharifi, K.; Mehidan, H.

2018-04-01

In this paper, the stability of Newtonian and non-Newtonian viscoelastic collisional shear-velocity dusty plasmas is studied, using the framework of a generalized hydrodynamic (GH) model. Motivated by Banerjee et al.'s work (Banerjee et al., New J. Phys., vol. 12 (12), 2010, p. 123031), employing linear perturbation theory as well as the local approximation method in the inhomogeneous direction, the dispersion relations of the Fourier modes are obtained for Newtonian and non-Newtonian dusty plasma systems in the presence of a dust-neutral friction term. The analysis of the obtained dispersion relation in the non-Newtonian case shows that the inhomogeneous viscosity force depending on the velocity shear profile can be the genesis of a free energy source which leads the shear system to be unstable. Study of the dust-neutral friction effect on the instability of the considered systems using numerical analysis of the dispersion relation in the Newtonian case demonstrates that the maximum growth rate decreases considerably by increasing the collision frequency in the hydrodynamic regime, while this reduction can be neglected in the kinetic regime. Results show a more significant stabilization role of the dust-neutral friction term in the non-Newtonian cases, through decreasing the maximum growth rate at any fixed wavenumber and construction of the instable wavenumber region. The results of the present investigation will greatly contribute to study of the time evolution of viscoelastic laboratory environments with externally applied shear; where in these experiments the dust-neutral friction process can play a considerable role.

Dynamic analysis of a circulating fluidized bed riser

DOE Office of Scientific and Technical Information (OSTI.GOV)

Panday, Rupen; Shadle, Lawrence J.; Guenther, Chris

2012-01-01

A linear state model is proposed to analyze dynamic behavior of a circulating fluidized bed riser. Different operating regimes were attained with high density polyethylene beads at low and high system inventories. The riser was operated between the classical choking velocity and the upper transport velocity demarcating fast fluidized and transport regimes. At a given riser superficial gas velocity, the aerations fed at the standpipe were modulated resulting in a sinusoidal solids circulation rate that goes into the riser via L-valve. The state model was derived based on the mass balance equation in the riser. It treats the average solidsmore » fraction across the entire riser as a state variable. The total riser pressure drop was modeled using Newton’s second law of motion. The momentum balance equation involves contribution from the weight of solids and the wall friction caused by the solids to the riser pressure drop. The weight of solids utilizes the state variable and hence, the riser inventory could be easily calculated. The modeling problem boils down to estimating two parameters including solids friction coefficient and time constant of the riser. It has been shown that the wall friction force acts in the upward direction in fast fluidized regime which indicates that the solids were moving downwards on the average with respect to the riser wall. In transport regimes, the friction acts in the opposite direction. This behavior was quantified based on a sign of Fanning friction factor in the momentum balance equation. The time constant of the riser appears to be much higher in fast fluidized regime than in transport conditions.« less

Multiple-vehicle collision induced by a sudden stop in traffic flow

NASA Astrophysics Data System (ADS)

Sugiyama, Naoki; Nagatani, Takashi

2012-04-01

We study the dynamic process of the multiple-vehicle collision when a vehicle stops suddenly in a traffic flow. We apply the optimal-velocity model to the vehicular motion. If a vehicle does not decelerate successfully, it crashes into the vehicle ahead with a residual speed. The collision criterion is presented by vi(t)/Δxi(t)→∞ if Δxi(t)→0 where vi(t) and Δxi(t) are the speed and headway of vehicle i at time t. The number of crumpled vehicles depends on the initial velocity, the sensitivity, and the initial headway. We derive the region map (or phase diagram) for the multiple-vehicle collision.

The collisional drift mode in a partially ionized plasma. [in the F region

NASA Technical Reports Server (NTRS)

Hudson, M. K.; Kennel, C. F.

1974-01-01

The structure of the drift instability was examined in several density regimes. Let sub e be the total electron mean free path, k sub z the wave-vector component along the magnetic field, and the ratio of perpendicular ion diffusion to parallel electron streaming rates. At low densities (k sub z lambda 1) the drift mode is isothermal and should be treated kineticly. In the finite heat conduction regime square root of m/M k sub z Lambda sub 1) the drift instability threshold is reduced at low densities and increased at high densities as compared to the isothermal threshold. Finally, in the energy transfer limit (k sub z kambda sub e square root of m/M) the drift instability behaves adiabatically in a fully ionized plasma and isothermally in a partially ionized plasma for an ion-neutral to Coulomb collision frequency ratio.

Electromagnetic drift waves dispersion for arbitrarily collisional plasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Wonjae, E-mail: wol023@ucsd.edu; Krasheninnikov, Sergei I., E-mail: skrash@mae.ucsd.edu; Angus, J. R.

2015-07-15

The impacts of the electromagnetic effects on resistive and collisionless drift waves are studied. A local linear analysis on an electromagnetic drift-kinetic equation with Bhatnagar-Gross-Krook-like collision operator demonstrates that the model is valid for describing linear growth rates of drift wave instabilities in a wide range of plasma parameters showing convergence to reference models for limiting cases. The wave-particle interactions drive collisionless drift-Alfvén wave instability in low collisionality and high beta plasma regime. The Landau resonance effects not only excite collisionless drift wave modes but also suppress high frequency electron inertia modes observed from an electromagnetic fluid model in collisionlessmore » and low beta regime. Considering ion temperature effects, it is found that the impact of finite Larmor radius effects significantly reduces the growth rate of the drift-Alfvén wave instability with synergistic effects of high beta stabilization and Landau resonance.« less

A zero-gravity demonstration of the collision and coalescence of water droplets

NASA Technical Reports Server (NTRS)

Hung, R. J.; Vaughan, O. H.; Smith, R. E.

1974-01-01

The mechanics of the collision and coalescence of liquid droplets is one of the main research areas in the fields of nuclear physics, astrophysics, meteorology and fluid mechanics. The crew members on the Skylab 3 and 4 missions were requested to perform demonstrations of the collision and coalescence of water droplets under the low gravity environment at orbital altitude. In Skylab 4 two water droplets with equal volumes, 30 cu cm each, were used. A dark colored droplet (contaminated with grape drink) moving with a velocity of 3.14 cm/sec collided with a stationary pink colored droplet (contaminated with strawberry drink) and coalescence occurred. Theoretical models are proposed to study the various stages of the collision-coalescence processes. Special considerations are concentrated in the investigation of the bounce-coalescence and coalescence-instability processes. The surface tension of the coalesced droplets was calculated to be 52 dynes/cm in perfect agreement with laboratory measurements made after the flight using a reproduction of the liquids.

Angular momentum transfer in low velocity oblique impacts - Implications for asteroids

NASA Technical Reports Server (NTRS)

Yanagisawa, Masahisa; Eluszkiewicz, Janusz; Ahrens, Thomas J.

1991-01-01

An experimental study has been conducted for the low-velocity oblique impact efficiency of angular momentum transfer, which is defined as that fraction of incident angular momentum that is transferred to the rotation of a target. The results obtained suggest that more energetic impacts are able to transfer angular momentum more efficiently. In the cases of ricochetted projectiles, the fraction of angular momentum carried off by the ejecta was noted to be less than 30 percent. It is suggested that, if asteroid spin rates are due to mutual noncatastrophic collisions and the taxonomic classes are indicative of bulk properties, the differences between corresponding spin rates will be smaller than expected from a consideration of relative strength and density alone.

Tabletop Traffic Jams: Modeling Traffic Jams using Self Propelled Particles

NASA Astrophysics Data System (ADS)

Yadav, Vikrant; Kudrolli, Arshad

2015-03-01

We model behavior of traffic using Self Propelled Particles (SPPs). Granular rods with asymmetric mass distribution confined to move in a circular channel on a vibrated substrate and interact with each other through inelastic collision serve as our model vehicle. Motion of a single vehicle is observed to be composed of 2 parts, a linear velocity in the direction of lighter end of particle and a non-Gaussian random velocity. We find that the collective mean speed of the SPPs is constant over a wide range of line densities before decreasing rapidly as the maximum packing is approached indicating the spontaneous formation of Phantom jams. This decrease in speed is observed to be far greater than any small differences in the mean drift speed of individual SPPs , and occurs as the collision frequency between SPPs increase exponentially with line density. However the random velocity component of SPPs remain super-diffusive over entire range of line densities. While the collective motion at low densities is characterized by caravan following behind the slowest particle leading to clustering, at higher densities we see formation of jamming waves travelling in direction opposite to that of motion of particles.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

PubMed Central

Prieto, Germán A.; Froment, Bérénice; Yu, Chunquan; Poli, Piero; Abercrombie, Rachel

2017-01-01

Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere. PMID:28345055

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle.

PubMed

Prieto, Germán A; Froment, Bérénice; Yu, Chunquan; Poli, Piero; Abercrombie, Rachel

2017-03-01

Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

Blunt Body Aerodynamics for Hypersonic Low Density Flows

NASA Technical Reports Server (NTRS)

Moss, James N.; Glass, Christopher E.; Greene, Francis A.

2006-01-01

Numerical simulations are performed for the Apollo capsule from the hypersonic rarefied to the continuum regimes. The focus is on flow conditions similar to those experienced by the Apollo 6 Command Module during the high altitude portion of its reentry. The present focus is to highlight some of the current activities that serve as a precursor for computational tool assessments that will be used to support the development of aerodynamic data bases for future capsule flight environments, particularly those for the Crew Exploration Vehicle (CEV). Results for aerodynamic forces and moments are presented that demonstrate their sensitivity to rarefaction; that is, free molecular to continuum conditions. Also, aerodynamic data are presented that shows their sensitivity to a range of reentry velocities, encompassing conditions that include reentry from low Earth orbit, lunar return, and Mars return velocities (7.7 to 15 km/s). The rarefied results obtained with direct simulation Monte Carlo (DSMC) codes are anchored in the continuum regime with data from Navier-Stokes simulations.

Large-scale CO J = 1-0 observations of the giant molecular cloud associated with the infrared ring N35 with the Nobeyama 45 m telescope

NASA Astrophysics Data System (ADS)

Torii, Kazufumi; Fujita, Shinji; Matsuo, Mitsuhiro; Nishimura, Atsushi; Kohno, Mikito; Kuriki, Mika; Tsuda, Yuya; Minamidani, Tetsuhiro; Umemoto, Tomofumi; Kuno, Nario; Hattori, Yusuke; Yoshiike, Satoshi; Ohama, Akio; Tachihara, Kengo; Shima, Kazuhiro; Habe, Asao; Fukui, Yasuo

2018-05-01

We report an observational study of the giant molecular cloud (GMC) associated with the Galactic infrared ring-like structure N35 and two nearby H II regions G024.392+00.072 (H II region A) and G024.510-00.060 (H II region B), using the new CO J = 1-0 data obtained as a part of the FOREST Unbiased Galactic Plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project at a spatial resolution of 21″. Our CO data reveals that the GMC, with a total molecular mass of 2.1 × 106 M⊙, has two velocity components of over ˜10-15 km s-1. The majority of molecular gas in the GMC is included in the lower-velocity component (LVC) at ˜110-114 km s-1, while the higher-velocity components (HVCs) at ˜118-126 km s-1 consist of three smaller molecular clouds which are located near the three H II regions. The LVC and HVCs show spatially complementary distributions along the line-of-sight, despite large velocity separations of ˜5-15 km s-1, and are connected in velocity by the CO emission with intermediate intensities. By comparing the observations with simulations, we discuss a scenario where collisions of the three HVCs with the LVC at velocities of ˜10-15 km s-1 can provide an interpretation of these two observational signatures. The intermediate-velocity features between the LVC and HVCs can be understood as broad bridge features, which indicate the turbulent motion of the gas at the collision interfaces, while the spatially complementary distributions represent the cavities created in the LVC by the HVCs through the collisions. Our model indicates that the three H II regions were formed after the onset of the collisions, and it is therefore suggested that the high-mass star formation in the GMC was triggered by the collisions.

Collision Induced Velocity Changes from Molecular Dynamic Simulations. Application to the Spectral Shape of the Q(1) Raman Lines of H{_2}/H{_2}

NASA Astrophysics Data System (ADS)

Tran, H.; Hartmann, J. M.

2011-06-01

Collision induced velocity changes for pure H{_2} have been computed from classical dynamic simulations. The results have been compared with the Keilson-Storer model from four different points of view. The first involves various autocorrelation functions associated with the velocity. The second and third give more detailed information, and are time evolutions of some conditional probabilities for changes of the velocity modulus and orientation and the collision kernels themselves. The fourth considers the evolutions, with density, of the half widths of the Q(1) lines of the isotropic Raman (1-0) fundamental band and of the (2-0) overtone quadrupole band. These spectroscopic data enable an indirect test of the models since velocity changes translate into line-shape modifications through the speed dependence of collisional parameters and the Dicke narrowing of the Doppler contribution to the profile. The results indicate that, while the KS approach gives a poor description of detailed velocity-to-velocty changes, it leads to accurate results for the correlation functions and spectral shapes, quantities related to large averages over the velocity. It is also shown that the use of collision kernels directly derived from MDS lead to an almost perfect prediction of all considered quantities (correlation functions, conditional probabilities, and spectral shapes). Finally, the results stress the need for very accurate calculations of line-broadening and -shifting coefficients from the intermolecular potential to obviate the need for experimental data and permit fully meaningful tests of the models. H. Tran, J.M. Hartmann J. Chem. Phys. 130, 094301, 2009.

Jet dynamics post drop impact on a deep pool

NASA Astrophysics Data System (ADS)

Michon, Guy-Jean; Josserand, Christophe; Séon, Thomas

2017-02-01

We investigate experimentally the jet formed by the collapse of a cavity created by the impact of a drop on a pool of the same aqueous liquid. We show that jets can emerge with very different shapes and velocities, depending on the impact parameters, thus generating droplets with various initial sizes and velocities. After presenting the jet velocity and top drop radius variation as a function of the impact parameters, we discuss the influence of the liquid parameters on the jet velocity. This allows us to define two different regimes: the singular jet and the cavity jet regimes, where the mechanisms leading to the cavity retraction and subsequent jet dynamics are drastically different. In particular, we demonstrate that in the first regime, a singular capillary wave collapse sparks the whole jet dynamics, making the jet's fast, thin, liquid parameters dependent and barely reproducible. On the contrary, in the cavity jet regime, defined for higher impact Froude numbers, the jets are fat and slow. We show that jet velocity is simply proportional to the capillary velocity √{γ /ρlDd }, where γ is the liquid surface tension, ρl the liquid density, and Dd the impacting drop diameter, and it is in particular independent of viscosity, impact velocity, and gravity, even though the cavity is larger than the capillary length. Finally, we demonstrate that capillary wave collapse and cavity retraction are correlated in the singular regime and decorrelated in the cavity jet regime.

Validity of Binary Collision Theory in Ion-Surface Interactions at 50-500 eV

NASA Astrophysics Data System (ADS)

Gordon, Michael; Giapis, Kostas

2003-10-01

Ion-surface interactions in the 50-500 eV regime have become increasingly important in plasma processing. Concerns exist in literature about the validity of the binary collision approximation (BCA) at low impact energies because peculiarities are frequently seen in the scattered ion energy distribution. Sub-surface processes, multiple bouncing, and super-elastic phenomena have all been hypothesized. This talk will explore the usefulness of BCA theory in predicting energy transfer during ion-surface collisions in the 50-500 eV energy range. Well-defined beams of rare gas ions (Ne, Ar, Kr) were scattered off semiconductor (Si, Ge) and metal surfaces (Ag, Au, Ni, Nb) to measure energy loss upon impact. The ion beams were produced from a floating ICP reactor coupled to a small accelerator beamline for transport and mass filtering. Exit channel energies were measured using a 90 gegree electrostatic sector coupled to a quadrupole mass filter with single ion detection capability. Although the BCA presents an over-simplified picture of the collision process, our results demonstrate that it is remarkably accurate in the low energy range for a variety of projectile-target combinations. In addition, reactive ion scattering of O2+ and O+ on inert and reactive surfaces (Au vs. Ag, Pt) suggests there may be rather high energy threshold processes which determine exit channel selectivity.

Numerical simulation of velocity and temperature fields in natural circulation loop

NASA Astrophysics Data System (ADS)

Sukomel, L. A.; Kaban'kov, O. N.

2017-11-01

Low flow natural circulation regimes are realized in many practical applications and the existence of the reliable engineering and design calculation methods of flows driven exclusively by buoyancy forces is an actual problem. In particular it is important for the analysis of start up regimes of passive safety systems of nuclear power plants. In spite of a long year investigations of natural circulation loops no suitable predicting recommendations for heat transfer and friction for the above regimes have been proposed for engineering practice and correlations for forced flow are commonly used which considerably overpredicts the real flow velocities. The 2D numerical simulation of velocity and temperature fields in circular tubes for laminar flow natural circulation with reference to the laboratory experimental loop has been carried out. The results were compared with the 1D modified model and experimental data obtained on the above loop. The 1D modified model was still based on forced flow correlations, but in these correlations the physical properties variability and the existence of thermal and hydrodynamic entrance regions are taken into account. The comparison of 2D simulation, 1D model calculations and the experimental data showed that even subject to influence of liquid properties variability and entrance regions on heat transfer and friction the use of 1D model with forced flow correlations do not improve the accuracy of calculations. In general, according to 2D numerical simulation the wall shear stresses are mainly affected by the change of wall velocity gradient due to practically continuous velocity profiles deformation along the whole heated zone. The form of velocity profiles and the extent of their deformation in its turn depend upon the wall heat flux density and the hydraulic diameter.

On the frictional (in) stability of clay-bearing faults

NASA Astrophysics Data System (ADS)

Violay, M.; Orellana, F.; Scuderi, M. M.; Collettini, C.

2016-12-01

Opalinus clay (OPA) is shale rock studied under the context of deep geological disposal by The Mont Terri Laboratory research program in Switzerland. Despite its favorable hydro-mechanical properties, the presence of a large tectonic fault system intersecting the rock formation arises questions over the long-term safety performance of a nuclear waste repository, in terms of possible leakages and the possibility of earthquakes triggered by fault instability. To study the frictional stability of OPA, we have performed velocity steps (1-300 μm/s) and slide-hold-slide tests (1-10000 s) on simulated gouge and intact samples - sheared parallel and perpendicular to foliation - at different normal stresses (4 - 30 MPa). To understand the deformation mechanisms, we have analyzed the microstructures of the sheared samples trough optical and SE microscopy. Results reported peak and steady state friction values ranging from 0.21 to 0.52 and from 0.14 to 0.39 respectively. Consistently, samples with well-developed layering showed lower friction values than gouge samples even though they have the same mineralogical composition. At all normal stresses, velocity dependence tests on gouge showed a velocity strengthening regime, whereas, intact samples developed both velocity-strengthening and velocity-weakening regimes. Finally, we have recorded near zero healing values for both intact and powdered samples at different normal stress. However, a complex evolution from negative to positive frictional healing rate, with an inflexion holding time of 300 s, has been observed. In conclusion, our data suggests that both the velocity strengthening regime and the near zero healing for the simulated gouge, are consistent with aseismic creep. We have also reported the possibility of unstable sliding outside the fault core accompanied by low capacity of contact regeneration, and low capacity to sustain future stress drops compared to evidence showed by experiments on simulated gouge. Moreover, microstructure analysis revealed different deformation patterns due to anisotropy of the material. Thus, the complex frictional behavior of OPA highlights the need for further experiments in order to better evaluate the seismic risk during long-term nuclear waste disposal within the OPA clay formation.

Quenching of internally 'hot' H2 and N2 gases by collisions with ultracold electrons: a computational 'experiment'

NASA Astrophysics Data System (ADS)

Gianturco, F. A.

2008-11-01

Quantum mechanical methods are employed to obtain superelastic cross sections involving H2 and N2 molecules, in excited rotational states, colliding with electrons at the very low collision energies which are encountered in cold trap experiments. This computational analysis intends to explore the feasibility of cold electrons for the collisional quenching of molecular gases down to the nanokelvin regimes. The results are obtained using rigorous coupled-channel (CC) calculations in the laboratory frame of reference which allows one to correctly describe the cross section behaviour at ultralow energies. The results are analysed down to the ultracold region of validity of Wigner's law, where it is found that electron-driven collisions exhibit substantial efficiency for the quenching of rotational populations in molecular gases involving the title systems. This work is affectionately dedicated to Anna Giardini, a creative experimentalist and a long-time friend, on the occasion of her 'official' retirement.

Investigation of Body Force Effects on Flow Boiling Critical Heat Flux

NASA Technical Reports Server (NTRS)

Zhang, Hui; Mudawar, Issam; Hasan, Mohammad M.

2002-01-01

The bubble coalescence and interfacial instabilities that are important to modeling critical heat flux (CHF) in reduced-gravity systems can be sensitive to even minute body forces. Understanding these complex phenomena is vital to the design and safe implementation of two-phase thermal management loops proposed for space and planetary-based thermal systems. While reduced gravity conditions cannot be accurately simulated in 1g ground-based experiments, such experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1g flow boiling experiments at different orientations. FC-72 liquid was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface at conditions approaching CHF. High-speed video imaging was employed to capture dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed great sensitivity to orientation for flow velocities below 0.2 m/s, where very small CHF values where measured, especially with downflow and downward-facing heated wall orientations. High flow velocities dampened the effects of orientation considerably. Figure I shows representative images for the different CHF regimes. The Wavy Vapor Layer regime was dominant for all high velocities and most orientations, while all other regimes were encountered at low velocities, in the downflow and/or downward-facing heated wall orientations. The Interfacial Lift-off model was modified to predict the effects of orientation on CHF for the dominant Wavy Vapor Layer regime. The photographic study captured a fairly continuous wavy vapor layer travelling along the heated wall while permitting liquid contact only in wetting fronts, located in the troughs of the interfacial waves. CHF commenced when wetting fronts near the outlet were lifted off the wall. The Interfacial Lift-off model is shown to be an effective tool for predicting the effects of body force on CHF at high velocities.

Research on slow electron collision processes in gases. Final report, September 15, 1970--December 31, 1972

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baldwin, G C

1974-04-30

Research on low energy electron collisions in gases by the time-of- flight velocity selection technique included, as a preliminary to total cross section measurements, investigations of the statistical and systematic errors inherent in the technique. In particular, thermal transpiration and instrumental fluctuation errors in manometry were investigated, and the results embodied in computer programs for data reduction. The instrumental system was improved to permit extended periods of data accumulation without manual attention. Total cross section measurements in helium, made prior to, and in molecular nitrogen, made after the supporting work was completed, are reported. The total cross sec tion ofmore » helium is found to be higher than reported in previous beam determinations. That of nitrogen is found to be structureless at low energies. (auth)« less

Asteroid collisions: Target size effects and resultant velocity distributions

NASA Technical Reports Server (NTRS)

Ryan, Eileen V.

1993-01-01

To study the dynamic fragmentation of rock to simulate asteroid collisions, we use a 2-D, continuum damage numerical hydrocode which models two-body impacts. This hydrocode monitors stress wave propagation and interaction within the target body, and includes a physical model for the formation and growth of cracks in rock. With this algorithm we have successfully reproduced fragment size distributions and mean ejecta speeds from laboratory impact experiments using basalt, and weak and strong mortar as target materials. Using the hydrocode, we have determined that the energy needed to fracture a body has a much stronger dependence on target size than predicted from most scaling theories. In addition, velocity distributions obtained indicate that mean ejecta speeds resulting from large-body collisions do not exceed escape velocities.

Elastic instability in stratified core annular flow.

PubMed

Bonhomme, Oriane; Morozov, Alexander; Leng, Jacques; Colin, Annie

2011-06-01

We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic--it is caused by the viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation supplemented with a kinematic criterion captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure the rheological properties of dilute polymer solutions that are difficult to assess otherwise.

Penetration of spherical projectiles into wet granular media.

PubMed

Birch, S P D; Manga, M; Delbridge, B; Chamberlain, M

2014-09-01

We measure experimentally the penetration depth d of spherical particles into a water-saturated granular medium made of much smaller sand-sized grains. We vary the density, size R, and velocity U of the impacting spheres, and the size δ of the grains in the granular medium. We consider velocities between 7 and 107 m/s, a range not previously addressed, but relevant for impacts produced by volcanic eruptions. We find that d∝R(1/3)δ(1/3)U(2/3). The scaling with velocity is similar to that identified in previous, low-velocity collisions, but it also depends on the size of the grains in the granular medium. We develop a model, consistent with the observed scaling, in which the energy dissipation is dominated by the work required to rearrange grains along a network of force chains in the granular medium.

Deep structure of the Alborz Mountains by joint inversion of P receiver functions and dispersion curves

NASA Astrophysics Data System (ADS)

Rastgoo, Mehdi; Rahimi, Habib; Motaghi, Khalil; Shabanian, Esmaeil; Romanelli, Fabio; Panza, Giuliano F.

2018-04-01

The Alborz Mountains represent a tectonically and seismically active convergent boundary in the Arabia - Eurasia collision zone, in western Asia. The orogenic belt has undergone a long-lasted tectono-magmatic history since the Cretaceous. The relationship between shallow and deep structures in this complex tectonic domain is not straightforward. We present a 2D velocity model constructed by the assemblage of 1D shear wave velocity (Vs) models from 26 seismic stations, mainly distributed along the southern flank of the Alborz Mountains. The shear wave velocity structure has been estimated beneath each station using joint inversion of P-waves receiver functions and Rayleigh wave dispersion curves. A substantiation of the Vs inversion results sits on the modeling of Bouguer gravity anomaly data. Our velocity and density models show low velocity/density anomalies in uppermost mantle of western and central Alborz at a depth range of ∼50-100 km. In deeper parts of the uppermost mantle (depth range of 100-150 km), a high velocity/density anomaly is located beneath most of the Mountain range. The spatial pattern of these low and high velocity/density structures in the upper mantle is interpreted as the result of post collisional delamination of lower part of the western and central Alborz lithosphere.

Ortho-para H₂ conversion by proton exchange at low temperature: an accurate quantum mechanical study.

PubMed

Honvault, P; Jorfi, M; González-Lezana, T; Faure, A; Pagani, L

2011-07-08

We report extensive, accurate fully quantum, time-independent calculations of cross sections at low collision energies, and rate coefficients at low temperatures for the H⁺ + H₂(v = 0, j) → H⁺ + H₂(v = 0, j') reaction. Different transitions are considered, especially the ortho-para conversion (j = 1 → j' = 0) which is of key importance in astrophysics. This conversion process appears to be very efficient and dominant at low temperature, with a rate coefficient of 4.15 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 10 K. The quantum mechanical results are also compared with statistical quantum predictions and the reaction is found to be statistical in the low temperature regime (T < 100 K).

Low velocity impact of 6082-T6 aluminum plates

NASA Astrophysics Data System (ADS)

Mocian, Oana Alexandra; Constantinescu, Dan Mihai; Sandu, Marin; Sorohan, Ştefan

2018-02-01

The low velocity domain covers vehicle impacts, ship collisions and even accidentally tool drops. Even though more and more research is needed into these fields, most of the papers concerning impact problems focus on impact at medium and high velocities. Understanding the behavior of structures subjected to low velocity impact is of major importance when referring to impact resistance and damage tolerance. The paper presents an experimental and numerical investigation on the low velocity behavior of 6082-T6 aluminum plates. Impact tests were performed using an Instron Ceast 9340 drop-weight testing machine. In the experimental procedure, square plates were mounted on a circular support, fixed with a pneumatic clamping system and impacted with a hemispherical steel projectile. Specimens were impacted at constant weight and different impact velocities. The effect of different impact energies was investigated. The impact event was then simulated using the nonlinear finite element code LS_DYNA in order to determine the effect of strain rate upon the mechanical behavior of the aluminum plates. Moreover, in order to capture the exact behavior of the material, a special attention has been given to the selection of the correct material model and its parameters, which, in large extent, depend on the observed behavior of the aluminum plate during the test and the actual response of the plate under simulation. The numerical predictions are compared with the experimental observations and the applicability of the numerical model for further researches is analyzed.

Shear localization in a mature mylonitic rock analog during fast slip

NASA Astrophysics Data System (ADS)

Takahashi, M.; van den Ende, M. P. A.; Niemeijer, A. R.; Spiers, C. J.

2017-02-01

Highly localized slip zones developed within ductile shear zones, such as pseudotachylyte bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large changes in velocity on a brine-saturated, 80:20 (wt %) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional-viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity-step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low-velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (˜20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate toward seismic velocities almost instantly and without much loss of fracture energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle-ductile transitional regime.

Giant molecular cloud collisions as triggers of star formation. VI. Collision-induced turbulence

NASA Astrophysics Data System (ADS)

Wu, Benjamin; Tan, Jonathan C.; Nakamura, Fumitaka; Christie, Duncan; Li, Qi

2018-05-01

We investigate collisions between giant molecular clouds (GMCs) as potential generators of their internal turbulence. Using magnetohydrodynamic (MHD) simulations of self-gravitating, magnetized, turbulent GMCs, we compare kinematic and dynamic properties of dense gas structures formed when such clouds collide compared to those that form in non-colliding clouds as self-gravity overwhelms decaying turbulence. We explore the nature of turbulence in these structures via distribution functions of density, velocity dispersions, virial parameters, and momentum injection. We find that the dense clumps formed from GMC collisions have higher effective Mach number, greater overall velocity dispersions, sustain near-virial equilibrium states for longer times, and are the conduit for the injection of turbulent momentum into high density gas at high rates.

Giant molecular cloud collisions as triggers of star formation. VI. Collision-induced turbulence

NASA Astrophysics Data System (ADS)

Wu, Benjamin; Tan, Jonathan C.; Nakamura, Fumitaka; Christie, Duncan; Li, Qi

2018-01-01

We investigate collisions between giant molecular clouds (GMCs) as potential generators of their internal turbulence. Using magnetohydrodynamic (MHD) simulations of self-gravitating, magnetized, turbulent GMCs, we compare kinematic and dynamic properties of dense gas structures formed when such clouds collide compared to those that form in non-colliding clouds as self-gravity overwhelms decaying turbulence. We explore the nature of turbulence in these structures via distribution functions of density, velocity dispersions, virial parameters, and momentum injection. We find that the dense clumps formed from GMC collisions have higher effective Mach number, greater overall velocity dispersions, sustain near-virial equilibrium states for longer times, and are the conduit for the injection of turbulent momentum into high density gas at high rates.

Part 1: Classical laser. Part 2: The effect of velocity changing collisions on the output of a gas laser. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Borenstein, M.

1972-01-01

A classical model for laser action is discussed, in which an active medium consisting of anharmonic oscillators interacts with an electromagnetic field in a resonant cavity. Comparison with the case of a medium consisting of harmonic oscillators shows the significance of nonlinearities for producing self-sustained oscillations in the radiation field. A theoretical model is presented for the pressure dependence of the intensity of a gas laser, in which only velocity-changing collisions with foreign gas atoms are included. A collision model for hard sphere, repulsive interactions was derived. Collision theory was applied to a third-order expansion of the polarization in powers of the cavity electric field (weak signal theory).

Effects of velocity-changing collisions on two-photon and stepwise-absorption spectroscopic line shapes

NASA Astrophysics Data System (ADS)

Liao, P. F.; Bjorkholm, J. E.; Berman, P. R.

1980-06-01

We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the 3S12-->3P12-->4D12 transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the 3S12-->3P12 transition frequency and excites a nonthermal longitudinal velocity distribution of excited 3P12 atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-Störer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the 3P12 state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium 3P12 atoms and helium, neon, and krypton perturbers.

Investigation of the effect of impeller speed on granules formed using a PMA-1 high shear granulator.

PubMed

Logan, R; Briens, L

2012-11-01

Impeller speed was varied from 300 to 1500 rpm during the wet high shear granulation of a placebo formulation using a new vertical shaft PharmaMATRIX-1 granulator. The resulting granules were extensively analysed for differences caused by the varying impeller speed with emphasis on flowability. Microscopy showed that initial granules were formed primarily from microcrystalline cellulose at all tested impeller speeds. At low impeller speed of 300 rpm in the "bumpy" flow regime, forces from the impeller were insufficient to incorporate all the components of the formulation into the granules and to promote granule growth to a size that significantly improved flowability. The "roping" flow regime at higher impeller speeds promoted granule growth to a median particle size of at least 100 µm that improved the flowability of the mixture. Particle size distribution measurements and advanced indicators based on avalanching behavior, however, showed that an impeller speed of 700 rpm produced the largest fraction of optimal granules with the best flowability potential. This impeller speed allowed good development of "roping" flow for sufficient mixing, collision rates and kinetic energy for collisions while minimizing excessive centrifugal forces that promote buildup around the bowl perimeter.

Adiabatic description of long range frequency sweeping

NASA Astrophysics Data System (ADS)

Breizman, Boris; Nyqvist, Robert; Lilley, Matthew

2012-10-01

A theoretical framework is developed to describe long range frequency sweeping events in the 1D electrostatic bump-on-tail model with fast particle sources and collisions. The model includes three collision operators (Krook, drag (dynamical friction) and velocity space diffusion), and allows for a general shape of the fast particle distribution function. The behavior of phase space holes and clumps is analyzed, and the effect of particle trapping due to separatrix expansion is discussed. With a fast particle distribution function whose slope decays above the resonant phase velocity, hooked frequency sweeping is found for holes in the presence of drag collisions alone.

Target Z dependence of Xe L x-ray emission in heavy ion-atom collision near the Bohr velocity: influence of level matching

NASA Astrophysics Data System (ADS)

Ren, Jieru; Zhao, Yongtao; Zhou, Xianming; Cheng, Rui; Lei, Yu; Sun, Yuanbo; Wang, Xing; Xu, Ge; Wang, Yuyu; Liu, Shidong; Yu, Yang; Li, Yongfeng; Zhang, Xiaoan; Xu, Zhongfeng; Xiao, Guoqing

2013-09-01

X-ray yields for the projectile L-shell have been measured for collisions between Xe20+ and thick solid targets throughout the periodic table with incident energies near the Bohr velocity. The yields show a very pronounced cyclic dependence on the target atomic number. This result indicates that Xe L x-ray emission intensity is greatly enhanced either in near-symmetric collisions or if the binding energy of the Xe M-shell matches the L- or N-shell binding energy of the target.

Atomic data from the IRON Project. XXXII. On the accuracy of the effective collision strength for the electron impact excitation of the quadrupole transition in AR III

NASA Astrophysics Data System (ADS)

Galavís, M. E.; Mendoza, C.; Zeippen, C. J.

1998-12-01

Since te[Burgess et al. (1997)]{bur97} have recently questioned the accuracy of the effective collision strength calculated in the IRON Project for the electron impact excitation of the 3ssp23p sp4 \\ sp1 D -sp1 S quadrupole transition in Ar iii, an extended R-matrix calculation has been performed for this transition. The original 24-state target model was maintained, but the energy regime was increased to 100 Ryd. It is shown that in order to ensure convergence of the partial wave expansion at such energies, it is necessary to take into account partial collision strengths up to L=30 and to ``top-up'' with a geometric series procedure. By comparing effective collision strengths, it is found that the differences from the original calculation are not greater than 25% around the upper end of the common temperature range and that they are much smaller than 20% over most of it. This is consistent with the accuracy rating (20%) previously assigned to transitions in this low ionisation system. Also the present high-temperature limit agrees fairly well (15%) with the Coulomb-Born limit estimated by Burgess et al., thus confirming our previous accuracy rating. It appears that Burgess et al., in their data assessment, have overextended the low-energy behaviour of our reduced effective collision strength to obtain an extrapolated high-temperature limit that appeared to be in error by a factor of 2.

Observation of correlated excitations in bimolecular collisions

NASA Astrophysics Data System (ADS)

Gao, Zhi; Karman, Tijs; Vogels, Sjoerd N.; Besemer, Matthieu; van der Avoird, Ad; Groenenboom, Gerrit C.; van de Meerakker, Sebastiaan Y. T.

2018-02-01

Although collisions between atoms and molecules are largely understood, collisions between two molecules have proven much harder to study. In both experiment and theory, our ability to determine quantum-state-resolved bimolecular cross-sections lags behind their atom-molecule counterparts by decades. For many bimolecular systems, even rules of thumb—much less intuitive understanding—of scattering cross sections are lacking. Here, we report the measurement of state-to-state differential cross sections on the collision of state-selected and velocity-controlled nitric oxide (NO) radicals and oxygen (O2) molecules. Using velocity map imaging of the scattered NO radicals, the full product-pair correlations of rotational excitation that occurs in both collision partners from individual encounters are revealed. The correlated cross sections show surprisingly good agreement with quantum scattering calculations using ab initio NO-O2 potential energy surfaces. The observations show that the well-known energy-gap law that governs atom-molecule collisions does not generally apply to bimolecular excitation processes, and reveal a propensity rule for the vector correlation of product angular momenta.

A Mass Census of the Nearby Universe with RESOLVE and ECO

NASA Astrophysics Data System (ADS)

Eckert, Kathleen D.; Kannappan, Sheila; Stark, David; Moffett, Amanda J.; Norris, Mark A.; Berlind, Andreas A.; Hall, Kirsten; Baker, Ashley; Snyder, Elaine M.; Bittner, Ashley; Hoversten, Erik A.; Lagos, Claudia; Nasipak, Zachary; RESOVE Team

2017-01-01

The low-mass slope of the galaxy stellar mass function is significantly shallower than that of the theoretical dark matter halo mass function, leading to several possible interpretations including: 1) stellar mass does not fully represent galaxy mass, 2) galaxy formation becomes increasingly inefficient in lower mass halos, and 3) environmental effects, such as stripping and merging, may change the mass function. To investigate these possible scenarios, we present the census of stellar, baryonic (stars + cold gas), and dynamical masses of galaxies and galaxy groups for the RESOLVE and ECO surveys. RESOLVE is a highly complete volume-limited survey of ~1500 galaxies, enabling direct measurement of galaxy mass functions without statistical completeness corrections down to baryonic mass Mb ~ 10^9 Msun. ECO provides a larger data set (~10,000 galaxies) complete down to Mb ~ 10^9.4 Msun. We show that the baryonic mass function has a steeper low-mass slope than the stellar mass function due to the large population of low-mass, gas-rich galaxies. The baryonic mass function’s low-mass slope, however, is still significantly shallower than that of the dark matter halo mass function. A more direct probe of total galaxy mass is its characteristic velocity, and we present RESOLVE’s preliminary galaxy velocity function, which combines ionized-gas rotation curves, stellar velocity dispersions, and estimates from scaling relations. The velocity function also diverges from the dark matter halo velocity function at low masses. To study the effect of environment, we break the mass functions into different group halo mass bins, finding complex substructure, including a depressed and flat low-mass slope for groups with halo masses ~10^11.4-12 Msun, which we refer to as the nascent group regime, with typical membership of 2-4 galaxies. This substructure is suggestive of efficient merging or gas stripping in nascent groups, which we find also have large scatter in their cold-baryon fractions, possibly pointing to diversity in hot halo gas content in this regime. This work is supported by NSF grant AST-0955368, the NC Space Grant Graduate Research Fellowship Program, and a UNC Royster Society Dissertation Completion Fellowship.

Motion of discrete solitons assisted by nonlinearity management.

PubMed

Cuevas, Jesús; Malomed, Boris A; Kevrekidis, P G

2005-06-01

We demonstrate that time-periodic modulation of the nonlinearity coefficient in the discrete nonlinear Schrödinger equation strongly facilitates creation of traveling solitons in the lattice. We predict this possibility in a semi-qualitative form analytically, and test it in direct numerical simulations. Systematic computations reveal several generic dynamical regimes, depending on the amplitude and frequency of the time modulation, and on the initial thrust which sets the soliton in motion. These regimes include irregular motion of the soliton, regular motion of a decaying one, and regular motion of a stable soliton. The motion may occur in both the straight and reverse directions, relative to the initial thrust. In the case of stable motion, extremely long simulations in a lattice with periodic boundary conditions demonstrate that the soliton keeps moving indefinitely long without any visible loss. Velocities of moving stable solitons are in good agreement with the analytical prediction, which is based on requiring a resonance between the ac drive and motion of the soliton through the periodic lattice. The generic dynamical regimes are mapped in the model's parameter space. Collisions between moving stable solitons are briefly investigated too, with a conclusion that two different outcomes are possible: elastic bounce, or bounce with mass transfer from one soliton to the other. The model can be realized experimentally in a Bose-Einstein condensate trapped in a deep optical lattice.

Planetesimal Collisions as a Chondrule Forming Event

NASA Astrophysics Data System (ADS)

Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi; Hasegawa, Yasuhiro

2017-01-01

Chondritic meteorites contain unique spherical materials named chondrules: sub-mm sized silicate grains once melted in a high temperature condition in the solar nebula. We numerically explore one of the chondrule forming processes—planetesimal collisions. Previous studies have found that impact jetting via protoplanet-planetesimal collisions can make chondrules with 1% of the impactors’ mass, when the impact velocity exceeds 2.5 km s-1. Based on the mineralogical data of chondrules, undifferentiated planetesimals would be more suitable for chondrule-forming collisions than potentially differentiated protoplanets. We examine planetesimal-planetesimal collisions using a shock physics code and find two things: one is that planetesimal-planetesimal collisions produce nearly the same amount of chondrules as protoplanet-planetesimal collisions (˜1%). The other is that the amount of produced chondrules becomes larger as the impact velocity increases when two planetesimals collide with each other. We also find that progenitors of chondrules can originate from deeper regions of large targets (planetesimals or protoplanets) than small impactors (planetesimals). The composition of targets is therefore important, to fully account for the mineralogical data of currently sampled chondrules.

Precisely relocated seismicity using 3-D seismic velocity model by double-difference tomography method and orogenic processes in central and southern Taiwan

NASA Astrophysics Data System (ADS)

Nagai, S.; Wu, Y.; Suppe, J.; Hirata, N.

2009-12-01

The island of Taiwan is located in the site of ongoing arc-continent collision zone between the Philippine Sea Plate and the Eurasian Plate. Numerous geophysical and geological studies are done in and around Taiwan to develop various models to explain the tectonic processes in the Taiwan region. The active and young tectonics and the associated high seismicity in Taiwan provide us with unique opportunity to explore and understand the processes in the region related to the arc-continent collision. Nagai et al. [2009] imaged eastward dipping alternate high- and low-velocity bodies at depths of 5 to 25 km from the western side of the Central Mountain Range to the eastern part of Taiwan, by double-difference tomography [Zhang and Thurber, 2003] using three temporary seismic networks with the Central Weather Bureau Seismic Network(CWBSN). These three temporary networks are the aftershock observation after the 1999 Chi-Chi Taiwan earthquake and two dense linear array observations; one is across central Taiwan in 2001, another is across southern Taiwan in 2005, respectively. We proposed a new orogenic model, ’Upper Crustal Stacking Model’ inferred from our tomographic images. To understand the detailed seismic structure more, we carry on relocating earthquakes more precisely in central and southern Taiwan, using three-dimensional velocity model [Nagai et al., 2009] and P- and S-wave arrival times both from the CWBSN and three temporary networks. We use the double-difference tomography method to improve relative and absolute location accuracy simultaneously. The relocated seismicity is concentrated and limited along the parts of boundaries between low- and high-velocity bodies. Especially, earthquakes occurred beneath the Eastern Central Range, triggered by 1999 Chi-Chi earthquake, delineate subsurface structural boundaries, compared with profiles of estimated seismic velocity. The relocated catalog and 3-D seismic velocity model give us some constraints to reconstruct the orogenic model in Taiwan. We show these relocated seismicity with P- and S-wave velocity profiles, with focal mechanisms [e.g. Wu et al., 2008] and spatio-temporal variation, in central and southern Taiwan and discuss tectonic processes in Taiwan.

Aerodynamic characteristics of the upper stages of a launch vehicle in low-density regime

NASA Astrophysics Data System (ADS)

Oh, Bum Seok; Lee, Joon Ho

2016-11-01

Aerodynamic characteristics of the orbital block (remaining configuration after separation of nose fairing and 1st and 2nd stages of the launch vehicle) and the upper 2-3stage (configuration after separation of 1st stage) of the 3 stages launch vehicle (KSLV-II, Korea Space Launch Vehicle) at high altitude of low-density regime are analyzed by SMILE code which is based on DSMC (Direct Simulation Monte-Carlo) method. To validating of the SMILE code, coefficients of axial force and normal forces of Apollo capsule are also calculated and the results agree very well with the data predicted by others. For the additional validations and applications of the DSMC code, aerodynamic calculation results of simple shapes of plate and wedge in low-density regime are also introduced. Generally, aerodynamic characteristics in low-density regime differ from those of continuum regime. To understand those kinds of differences, aerodynamic coefficients of the upper stages (including upper 2-3 stage and the orbital block) of the launch vehicle in low-density regime are analyzed as a function of Mach numbers and altitudes. The predicted axial force coefficients of the upper stages of the launch vehicle are very high compared to those in continuum regime. In case of the orbital block which flies at very high altitude (higher than 250km), all aerodynamic coefficients are more dependent on velocity variations than altitude variations. In case of the upper 2-3 stage which flies at high altitude (80km-150km), while the axial force coefficients and the locations of center of pressure are less changed with the variations of Knudsen numbers (altitudes), the normal force coefficients and pitching moment coefficients are more affected by variations of Knudsen numbers (altitude).

High Velocity Absorption during Eta Car B's Periastron Passage

NASA Technical Reports Server (NTRS)

Nielsen, Krister E.; Groh, J. H.; Hillier, J.; Gull, Theodore R.; Owocki, S. P.; Okazaki, A. T.; Damineli, A.; Teodoro, M.; Weigelt, G.; Hartman, H.

2010-01-01

Eta Car is one of the most luminous massive stars in the Galaxy, with repeated eruptions with a 5.5 year periodicity. These events are caused by the periastron passage of a massive companion in an eccentric orbit. We report the VLT/CRIRES detection of a strong high-velocity, (<1900 km/s) , broad absorption wing in He I at 10833 A during the 2009.0 periastron passage. Previous observations during the 2003.5 event have shown evidence of such high-velocity absorption in the He I 10833 transition, allowing us to conclude that the high-velocity gas is crossing the line-of-sight toward Eta Car over a time period of approximately 2 months. Our analysis of HST/STlS archival data with observations of high velocity absorption in the ultraviolet Si IV and C IV resonance lines, confirm the presence of a high-velocity material during the spectroscopic low state. The observations provide direct detection of high-velocity material flowing from the wind-wind collision zone around the binary system, and we discuss the implications of the presence of high-velocity gas in Eta Car during periastron

Variable Weight Fractional Collisions for Multiple Species Mixtures

DTIC Science & Technology

2017-08-28

DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED; PA #17517 6 / 21 VARIABLE WEIGHTS FOR DYNAMIC RANGE Continuum to Discrete ...Representation: Many Particles →̃ Continuous Distribution Discretized VDF Yields Vlasov But Collision Integral Still a Problem Particle Methods VDF to Delta...Function Set Collisions between Discrete Velocities But Poorly Resolved Tail (Tail Critical to Inelastic Collisions) Variable Weights Permit Extra DOF in

- and Frequency-Domain Signatures of Velocity Changing Collisions in Sub-Doppler Saturation Spectra and Pressure Broadening

NASA Astrophysics Data System (ADS)

Hall, Gregory; Xu, Hong; Forthomme, Damien; Dagdigian, Paul; Sears, Trevor

2017-06-01

We have combined experimental and theoretical approaches to the competition between elastic and inelastic collisions of CN radicals with Ar, and how this competition influences time-resolved saturation spectra. Experimentally, we have measured transient, two-color sub-Doppler saturation spectra of CN radicals with an amplitude chopped saturation laser tuned to selected Doppler offsets within rotational lines of the A-X (2-0) band, while scanning a frequency modulated probe laser across the hyperfine-resolved saturation features of corresponding rotational lines of the A-X (1-0) band. A steady-state depletion spectrum includes off-resonant contributions ascribed to velocity diffusion, and the saturation recovery rates depend on the sub-Doppler detuning. The experimental results are compared with Monte Carlo solutions to the Boltzmann equation for the collisional evolution of the velocity distributions of CN radicals, combined with a pressure-dependent and speed-dependent lifetime broadening. Velocity changing collisions are included by appropriately sampling the energy resolved differential cross sections for elastic scattering of selected rotational states of CN (X). The velocity space diffusion of Doppler tagged molecules proceeds through a series of small-angle scattering events, eventually terminating in an inelastic collision that removes the molecule from the coherently driven ensemble of interest. Collision energy-dependent total cross sections and differential cross sections for elastic scattering of selected CN rotational states with Ar were computed with Hibridon quantum scattering calculations, and used for sampling in the Monte Carlo modeling. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences.

Symmetrical collision of multiple vortex rings

NASA Astrophysics Data System (ADS)

Hernández, R. H.; Reyes, T.

2017-10-01

In this work, we investigate the motion, interaction, and simultaneous collision between many initially stable vortex rings arranged symmetrically in two initial configurations, three and six rings making an angle of 60 and 120° between their straight path lines, respectively. We report results for laminar vortex rings in air obtained through numerical simulations of the ring velocity, pressure, and vorticity fields, both in free flight and during the entire collision. Each collision was studied for small Reynolds numbers R e <1 03 based on both the self-induced velocity and diameter of the ring. The case of three rings produces secondary vortical structures formed by laterally expanding dipolar arms with top and bottom secondary vortex rings. The case of six colliding rings produces, as secondary structures, two big rings moving in opposite directions, a process that reminds us of the head-on collision of two rings [T. T. Lim and T. B. Nickels, "Instability and reconnection in the head-on collision of two vortex rings," Nature 357, 225-227 (1992)] under a hypothetical time reversal transformation. Both collisions display a characteristic kinetic energy evolution where mean collision stages can be identified within the range of Reynolds numbers investigated here.

Optimization of edge state velocity in the integer quantum Hall regime

NASA Astrophysics Data System (ADS)

Sahasrabudhe, H.; Novakovic, B.; Nakamura, J.; Fallahi, S.; Povolotskyi, M.; Klimeck, G.; Rahman, R.; Manfra, M. J.

2018-02-01

Observation of interference in the quantum Hall regime may be hampered by a small edge state velocity due to finite phase coherence time. Therefore designing two quantum point contact (QPCs) interferometers having a high edge state velocity is desirable. Here we present a new simulation method for designing heterostructures with high edge state velocity by realistically modeling edge states near QPCs in the integer quantum Hall effect (IQHE) regime. Using this simulation method, we also predict the filling factor at the center of QPCs and their conductance at different gate voltages. The 3D Schrödinger equation is split into 1D and 2D parts. Quasi-1D Schrödinger and Poisson equations are solved self-consistently in the IQHE regime to obtain the potential profile, and quantum transport is used to solve for the edge state wave functions. The velocity of edge states is found to be /B , where is the expectation value of the electric field for the edge state. Anisotropically etched trench gated heterostructures with double-sided delta doping have the highest edge state velocity among the structures considered.

The kinetic regime of the Vicsek model

NASA Astrophysics Data System (ADS)

Chepizhko, A. A.; Kulinskii, V. L.

2009-12-01

We consider the dynamics of the system of self-propelling particles modeled via the Vicsek algorithm in continuum time limit. It is shown that the alignment process for the velocities can be subdivided into two regimes: "fast" kinetic and "slow" hydrodynamic ones. In fast kinetic regime the alignment of the particle velocity to the local neighborhood takes place with characteristic relaxation time. So, that the bigger regions arise with the velocity alignment. These regions align their velocities thus giving rise to hydrodynamic regime of the dynamics. We propose the mean-field-like approach in which we take into account the correlations between density and velocity. The comparison of the theoretical predictions with the numerical simulations is given. The relation between Vicsek model in the zero velocity limit and the Kuramoto model is stated. The mean-field approach accounting for the dynamic change of the neighborhood is proposed. The nature of the discontinuity of the dependence of the order parameter in case of vectorial noise revealed in Gregorie and Chaite, Phys. Rev. Lett., 92, 025702 (2004) is discussed and the explanation of it is proposed.

Compensating for velocity truncation during subaperture polishing by controllable and time-variant tool influence functions.

PubMed

Dong, Zhichao; Cheng, Haobo; Tam, Hon-Yuen

2015-02-10

The velocity-varying regime used in deterministic subaperture polishing employs a time-invariant tool influence function (TIF) to figure localized surface errors by varying the transverse velocities of polishing tools. Desired transverse velocities have to be truncated if they exceed the maximal velocity of computer numerical control (CNC) machines, which induces excessive material removal and reduces figuring efficiency (FE). A time-variant (TV) TIF regime is presented, in which a TIF serves as a variable to compensate for excessive material removal when the transverse velocities are truncated. Compared with other methods, the TV-TIF regime exhibits better performance in terms of convergence rate, FE, and versatility; its operability can also be strengthened by a TIF library. Comparative experiments were conducted on a magnetorheological finishing machine to validate the effectiveness of the TV-TIF regime. Without a TV-TIF, the tool made an unwished dent (depth of 76 nm) at the center because of the velocity truncation problem. Through compensation with a TV-TIF, the dent was completely removed by the second figuring process, and a TV-TIF improved the FE from 0.029 to 0.066  mm(3)/h.

Effects of target plasma electron-electron collisions on correlated motion of fragmented protons.

PubMed

Barriga-Carrasco, Manuel D

2006-02-01

The objective of the present work is to examined the effects of plasma target electron-electron collisions on H2 + protons traversing it. Specifically, the target is deuterium in a plasma state with temperature Te=10 eV and density n=10(23) cm(-3), and proton velocities are vp=vth, vp=2vth, and vp=3vth, where vth is the electron thermal velocity of the target plasma. Proton interactions with plasma electrons are treated by means of the dielectric formalism. The interactions among close protons through plasma electronic medium are called vicinage forces. It is checked that these forces always screen the Coulomb explosions of the two fragmented protons from the same H2 + ion decreasing their relative distance. They also align the interproton vector along the motion direction, and increase the energy loss of the two protons at early dwell times while for longer times the energy loss tends to the value of two isolated protons. Nevertheless, vicinage forces and effects are modified by the target electron collisions. These collisions enhance the calculated self-stopping and vicinage forces over the collisionless results. Regarding proton correlated motion, when these collisions are included, the interproton vector along the motion direction overaligns at slower proton velocities (vp=vth) and misaligns for faster ones (vp=2vth, vp=3vth). They also contribute to a great extend to increase the energy loss of the fragmented H2 + ion. This later effect is more significant in reducing projectile velocity.

Influence of damping on proton energy loss in plasmas of all degeneracies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barriga-Carrasco, Manuel D.

2007-07-15

The purpose of the present paper is to describe the effects of electron-electron collisions on the stopping power of plasmas of any degeneracy. Plasma targets are considered fully ionized so electronic stopping is only due to the free electrons. We focus our analysis on plasmas which electronic density is around solid values n{sub e}{approx_equal}10{sup 23} cm{sup -3} and which temperature is around T{approx_equal}10 eV; these plasmas are in the limit of weakly coupled plasmas. This type of plasma has not been studied extensively though it is very important for inertial confinement fusion. The electronic stopping is obtained from an exactmore » quantum mechanical evaluation, which takes into account the degeneracy of the target plasma, and later it is compared with common classical and degenerate approximations. Differences are around 30% in some cases which can produce bigger mistakes in further energy deposition and projectile range studies. Then we consider electron-electron collisions in the exact quantum mechanical electronic stopping calculation. Now the maximum stopping occurs at velocities smaller than for the calculations without considering collisions for all kinds of plasmas analyzed. The energy loss enhances for velocities smaller than the velocity at maximum while decreases for higher velocities. Latter effects are magnified with increasing collision frequency. Differences with the same results for the case of not taking into account collisions are around 20% in the analyzed cases.« less

Distributed combustion in a cyclonic burner

NASA Astrophysics Data System (ADS)

Sorrentino, Giancarlo; Sabia, Pino; de Joannon, Mara; Cavaliere, Antonio; Ragucci, Raffaele

2017-11-01

Distributed combustion regime occurs in several combustion technologies were efficient and environmentally cleaner energy conversion are primary tasks. For such technologies (MILD, LTC, etc…), working temperatures are enough low to boost the formation of several classes of pollutants, such as NOx and soot. To access this temperature range, a significant dilution as well as preheating of reactants is required. Such conditions are usually achieved by a strong recirculation of exhaust gases that simultaneously dilute and pre-heat the fresh reactants. However, the intersection of low combustion temperatures and highly diluted mixtures with intense pre-heating alters the evolution of the combustion process with respect to traditional flames, leading to significant features such as uniformity and distributed ignition. The present study numerically characterized the turbulence-chemistry and combustion regimes of propane/oxygen mixtures, highly diluted in nitrogen, at atmospheric pressure, in a cyclonic combustor under MILD Combustion operating conditions. The velocity and mixing fields were obtained using CFD with focus on mean and fluctuating quantities. The flow-field information helped differentiate between the impact of turbulence levels and dilution ones. The integral length scale along with the fluctuating velocity is critical to determine Damköhler and Karlovitz numbers. Together these numbers identify the combustion regime at which the combustor is operating. This information clearly distinguishes between conventional flames and distributed combustion. The results revealed that major controllers of the reaction regime are dilution and mixing levels; both are significantly impacted by lowering oxygen concentration through entrainment of hot reactive species from within the combustor, which is important in distributed combustion. Understanding the controlling factors of distributed regime is critical for the development and deployment of these novel combustion technologies for near zero emissions from high intensity combustors and energy savings using fossil and biofuels for sustainable energy conversion.

ALMA Images of the Host Cloud of the Intermediate-mass Black Hole Candidate CO‑0.40–0.22*: No Evidence for Cloud–Black Hole Interaction, but Evidence for a Cloud–Cloud Collision

NASA Astrophysics Data System (ADS)

Tanaka, Kunihiko

2018-06-01

This paper reports a reanalysis of archival ALMA data of the high velocity(-width) compact cloud CO‑0.40–0.22, which has recently been hypothesized to host an intermediate-mass black hole (IMBH). If beam-smearing effects, difference in beam sizes among frequency bands, and Doppler shift due to the motion of the Earth are considered accurately, none of the features reported as evidence for an IMBH in previous studies are confirmed in the reanalyzed ALMA images. Instead, through analysis of the position–velocity structure of the HCN J = 3–2 data cube, we have found kinematics typical of a cloud–cloud collision (CCC), namely, two distinct velocity components bridged by broad emission features with elevated temperatures and/or densities. One velocity component has a straight filamentary shape with approximately constant centroid velocities along its length but with a steep, V-shaped velocity gradient across its width. This contradicts the IMBH scenario but is consistent with a collision between two dissimilar-sized clouds. From a non-LTE analysis of the multitransition methanol lines, the volume density of the post-shock gas has been measured to be ≳106 cm‑3, indicating that the CCC shock can compress gas in a short timescale to densities typical of star-forming regions. Evidence for star formation has not been found, possibly because the cloud is in an early phase of CCC-triggered star formation or because the collision is nonproductive.

A Quantum Simulation on the Emergence of Lorentz Invariance

NASA Astrophysics Data System (ADS)

Zueco, David; Quijandría, Fernando; Blas, Diego; Pujòlas, Oriol

2014-03-01

Lorentz invariance (LI) is one of the best tested symmetries of Nature. It is natural to think that LI is a fundamental property. However, this does not need to be so. In fact, it could be an emergent symmetry in the low energy world. One motivation on Lorentz-violating theories may come from consistent non-relativistic models of gravity, where LI appears at low energies. The basic approach is by taking two interacting quantum fields. The bare (uncoupled fields) have different light velocities, say v1 and v2. The coupling tends to ``synchronize'' those velocities providing a common light velocity: the LI emergence. So far, only perturbative calculations are available. In this perturbative regime the emergence of LI is too slow. Therefore it is mandatory going beyond perturbative calculations. In this talk I will discuss that such models for emergent Lorentz Invariance can be simulated in an analog quantum simulator. In 1+1 dimensions two transmission lines coupled trough Josephson Junctions do the job. We show that the emergence can be checked by measuring photon correlations. Everything within the state of the art in circuit QED. We show that our proposal can provide a definite answer about the LI emergence hypothesis in the strong coupling regime.

Three-thrust fault system at the plate suture of arc-continent collision in the southernmost Longitudinal Valley, eastern Taiwan

NASA Astrophysics Data System (ADS)

Lee, J.; Chen, H.; Hsu, Y.; Yu, S.

2013-12-01

Active faults developed into a rather complex three-thrust fault system at the southern end of the narrow Longitudinal Valley in eastern Taiwan, a present-day on-land plate suture between the Philippine Sea plate and Eurasia. Based on more than ten years long geodetic data (including GPS and levelling), field geological investigation, seismological data, and regional tomography, this paper aims at elucidating the architecture of this three-thrust system and the associated surface deformation, as well as providing insights on fault kinematics, slip behaviors and implications of regional tectonics. Combining the results of interseismic (secular) horizontal and vertical velocities, we are able to map the surface traces of the three active faults in the Taitung area. The west-verging Longitudinal Valley Fault (LVF), along which the Coastal Range of the northern Luzon arc is thrusting over the Central Range of the Chinese continental margin, braches into two active strands bounding both sides of an uplifted, folded Quaternary fluvial deposits (Peinanshan massif) within the valley: the Lichi fault to the east and the Luyeh fault to the west. Both faults are creeping, to some extent, in the shallow surface level. However, while the Luyeh fault shows nearly pure thrust type, the Lichi fault reveals transpression regime in the north and transtension in the south end of the LVF in the Taitung plain. The results suggest that the deformation in the southern end of the Longitudinal Valley corresponds to a transition zone from present arc-collision to pre-collision zone in the offshore SE Taiwan. Concerning the Central Range, the third major fault in the area, the secular velocities indicate that the fault is mostly locked during the interseismic period and the accumulated strain would be able to produce a moderate earthquake, such as the example of the 2006 M6.1 Peinan earthquake, expressed by an oblique thrust (verging toward east) with significant left-lateral strike slip component. Taking into account of the recent study on the regional seismic Vp tomography, it shows a high velocity zone with steep east-dipping angle fills the gap under the Longitudinal Valley between the opposing verging LVF and the Central Range fault, implying a possible rolled-back forearc basement under the Coastal Range.

Kolmogorov and scalar spectral regimes in numerical turbulence

NASA Technical Reports Server (NTRS)

Kerr, R. M.

1985-01-01

Velocity and passive-scalar spectra for turbulent fields generated by a forced three-dimensional simulation and Taylormicroscale Reynolds numbers up to 83 are shown to have distinct spectral regimes, including a Kolmogorov inertial subrange. Both one- and three-dimensional spectra are shown for comparison with experiment and theory, respectively. When normalized by the Kolmogorov dissipation scales velocity spectra collapse to a single curve and a high-wavenumber bulge is seen. The bulge leads to an artificially high Kolmogorov constant, but is consistent with recent measurements of the velocity spectrum in the dissipation regime and the velocity-derivative skewness. Scalar spectra, when normalized by the Oboukov-Corrsin scales, collapse to curves which depend only on Prandtl number and show a universal inertial-convective subrange, independent of Prandtl number. When normalized by the Batchelor scales, the scalar spectra show a universal dissipation regime which is independent of Prandtl numbers from 0.1 to 1.0. The time development of velocity spectra is illustrated by energy-transfer spectra in which distinct pulses propagate to high wavenumbers.

Evaluation of new collision-pair selection models in DSMC

NASA Astrophysics Data System (ADS)

Akhlaghi, Hassan; Roohi, Ehsan

2017-10-01

The current paper investigates new collision-pair selection procedures in a direct simulation Monte Carlo (DSMC) method. Collision partner selection based on the random procedure from nearest neighbor particles and deterministic selection of nearest neighbor particles have already been introduced as schemes that provide accurate results in a wide range of problems. In the current research, new collision-pair selections based on the time spacing and direction of the relative movement of particles are introduced and evaluated. Comparisons between the new and existing algorithms are made considering appropriate test cases including fluctuations in homogeneous gas, 2D equilibrium flow, and Fourier flow problem. Distribution functions for number of particles and collisions in cell, velocity components, and collisional parameters (collision separation, time spacing, relative velocity, and the angle between relative movements of particles) are investigated and compared with existing analytical relations for each model. The capability of each model in the prediction of the heat flux in the Fourier problem at different cell numbers, numbers of particles, and time steps is examined. For new and existing collision-pair selection schemes, the effect of an alternative formula for the number of collision-pair selections and avoiding repetitive collisions are investigated via the prediction of the Fourier heat flux. The simulation results demonstrate the advantages and weaknesses of each model in different test cases.

Lava Fountaining Discharge Regime driven by Slug-to-Churn Flow Transition. (Invited)

NASA Astrophysics Data System (ADS)

Ripepe, M.; Pioli, L.; Marchetti, E.; Ulivieri, G.

2013-12-01

Lava fountaining episodes at Etna volcano appear characterized by the transition between Strombolian and Hawaiian end-member eruptive styles. There is no evidence for this transition in the seismic (i.e. seismic tremor) signal. However, infrasonic records provide unprecedented evidence on this flow transition. Each eruptive episode is characterized by distinctive common trend in the amplitude, waveform and frequency content of the infrasonic wavefield, which evidences the shift from discrete, and transient, strombolian to sustained, and oscillatory, lava fountain dynamics. Large scale experiments on the dynamics of two-phase flow of basaltic magmas show how the transition between different regimes mainly depends on gas volume flow, which in turn controls pressure distribution within the conduit and also magma vesicularity. In particular, while regular large bubble bursting is associated with slug flow regime, large amplitude and low frequency column oscillations are associated with churn flow. In large pipes, transition from slug to churn flow regime is independent on conduit diameter and it is reached at high superficial gas velocity. Lava fountaining episodes at Etna can be thus interpreted as induced by the transition from the slug (discrete strombolian) to churn flow (sustained lava fountain) regimes that is reflecting an increase in the gas discharge rate. Based on laboratory experiments, we calculate that transition between these two end-member explosive regimes at Etna occurs when gas superficial velocity is 76 m/s for near-the-vent stagnant magma conditions.

Edge gyrokinetic theory and continuum simulations

NASA Astrophysics Data System (ADS)

Xu, X. Q.; Xiong, Z.; Dorr, M. R.; Hittinger, J. A.; Bodi, K.; Candy, J.; Cohen, B. I.; Cohen, R. H.; Colella, P.; Kerbel, G. D.; Krasheninnikov, S.; Nevins, W. M.; Qin, H.; Rognlien, T. D.; Snyder, P. B.; Umansky, M. V.

2007-08-01

The following results are presented from the development and application of TEMPEST, a fully nonlinear (full-f) five-dimensional (3d2v) gyrokinetic continuum edge-plasma code. (1) As a test of the interaction of collisions and parallel streaming, TEMPEST is compared with published analytic and numerical results for endloss of particles confined by combined electrostatic and magnetic wells. Good agreement is found over a wide range of collisionality, confining potential and mirror ratio, and the required velocity space resolution is modest. (2) In a large-aspect-ratio circular geometry, excellent agreement is found for a neoclassical equilibrium with parallel ion flow in the banana regime with zero temperature gradient and radial electric field. (3) The four-dimensional (2d2v) version of the code produces the first self-consistent simulation results of collisionless damping of geodesic acoustic modes and zonal flow (Rosenbluth-Hinton residual) with Boltzmann electrons using a full-f code. The electric field is also found to agree with the standard neoclassical expression for steep density and ion temperature gradients in the plateau regime. In divertor geometry, it is found that the endloss of particles and energy induces parallel flow stronger than the core neoclassical predictions in the SOL.

Kinetic theory of Lennard-Jones fluids

NASA Astrophysics Data System (ADS)

Leegwater, Jan A.

1991-12-01

A kinetic theory that describes the time evolution of a fluid consisting of Lennard-Jones particles at all densities is proposed. The kinetic equation assumes binary collisions, but takes into account the finite time duration of a collision. Furthermore, it is an extension of a kinetic equation for the square well fluid as well as the hard sphere Enskog theory. In the low density limit, the Boltzmann theory is obtained. It is shown that the proposed theory obeys all the conservation laws. The exchange of potential and kinetic energies is studied and it is shown that at high density this is a fast process. The dominant mechanism for energy exchange is found to be collisions at the strongly repulsive part of the potential that are disturbed by third particles. The kinetic equation is also used to calculate the Green-Kubo integrands for shear viscosity and heat conductivity. The major structures found in molecular dynamics simulations are reproduced at intermediate densities quantitatively and at high density semiquantitatively. It is found that at high density, not only correlated collisions have to be taken into account, but that even the concept of collisions in the sense of sudden changes in the velocity is no longer useful.

BRIEF COMMUNICATION: A note on the Coulomb collision operator in curvilinear coordinates

NASA Astrophysics Data System (ADS)

Goncharov, P. R.

2010-10-01

The dynamic friction force, diffusion tensor, flux density in velocity space and Coulomb collision term are expressed in curvilinear coordinates via Trubnikov potential functions corresponding to each species of a background plasma. For comparison, explicit formulae are given for the dynamic friction force, diffusion tensor and collisional flux density in velocity space in curvilinear coordinates via Rosenbluth potential functions summed over all species of the background plasma.

SPH simulations of high-speed collisions

NASA Astrophysics Data System (ADS)

Rozehnal, Jakub; Broz, Miroslav

2016-10-01

Our work is devoted to a comparison of: i) asteroid-asteroid collisions occurring at lower velocities (about 5 km/s in the Main Belt), and ii) mutual collisions of asteroids and cometary nuclei usually occurring at significantly higher relative velocities (> 10 km/s).We focus on differences in the propagation of the shock wave, ejection of the fragments and possible differences in the resultingsize-frequency distributions of synthetic asteroid families. We also discuss scaling with respect to the "nominal" target diameter D = 100 km, projectile velocity 3-7 km/s, for which a number of simulations were done so far (Durda et al. 2007, Benavidez et al. 2012).In the latter case of asteroid-comet collisions, we simulate the impacts of brittle or pre-damaged impactors onto solid monolithic targets at high velocities, ranging from 10 to 15 km/s. The purpose of this numerical experiment is to better understand impact processes shaping the early Solar System, namely the primordial asteroid belt during during the (late) heavy bombardment (as a continuation of Broz et al. 2013).For all hydrodynamical simulations we use a smoothed-particle hydrodynamics method (SPH), namely the lagrangian SPH3D code (Benz & Asphaug 1994, 1995). The gravitational interactions between fragments (re-accumulation) is simulated with the Pkdgrav tree-code (Richardson et al. 2000).

An integrated collision prediction and avoidance scheme for mobile robots in non-stationary environments

NASA Technical Reports Server (NTRS)

Kyriakopoulos, K. J.; Saridis, G. N.

1993-01-01

A formulation that makes possible the integration of collision prediction and avoidance stages for mobile robots moving in general terrains containing moving obstacles is presented. A dynamic model of the mobile robot and the dynamic constraints are derived. Collision avoidance is guaranteed if the distance between the robot and a moving obstacle is nonzero. A nominal trajectory is assumed to be known from off-line planning. The main idea is to change the velocity along the nominal trajectory so that collisions are avoided. A feedback control is developed and local asymptotic stability is proved if the velocity of the moving obstacle is bounded. Furthermore, a solution to the problem of inverse dynamics for the mobile robot is given. Simulation results verify the value of the proposed strategy.

The dynamics of head-on collisions of spherical stellar systems

NASA Astrophysics Data System (ADS)

Narasimhan, K. S. V. S.; Alladin, Saleh Mohammed

1986-12-01

Energy changes in a head-on collision between two unequal Plummer model stellar systems (galaxies) are studied analytically under the impulsive approximation. The variation of the disruptive effects within and the mass escape from systems widely differing in mass and scalelength ratios are determined, and some physical implications regarding the dynamical stability of the systems undergoing head-on collisions are indicated. It is found that if two systems differ considerably in size, both systems generally survive the collision if (1) the mass of the bigger is greater than about six times the mass of the smaller and (2) the density of the smaller is more than about twenty-five times the entity of the bigger system, when the velocity at minimum separation is equal to the parabolic velocity of escape.

Experimental results of a hydrodynamic friction behaviour of a linear contact at low sliding velocity

NASA Astrophysics Data System (ADS)

Bouzana, A.; Guermat, A.; Belarifi, F.

2018-01-01

We propose in this work the experimental results of the lubricated friction behavior of linear contact (finite length) in isoviscous hydrodynamic regime. This study was made on a tribometer Plint - Cameron TE77, using a pure mineral oil lubricant (N175). without additives for three loads 20, 40 and 80 Newton. and a velocity, range varying from 0.05 to 0.4 ms-1, trials are held in pure sliding mode for a total distance of displacement L = 15mm. The studied contact is a cylinder/cylinder. The geometry of test pieces is part of a piston ring and a liner of a real engine. The first cylinder represents the male part with material of MKJet nuance having undergoes a surface coating by thermal projection (HVOF). the second cylinder represents the female part whose material is cast iron of nuance FGL, without surface treatment, and whose dimensions were adapted to minimize the computational error on the speed of sliding and the force of friction which is lower than 5%. Processing the results recorded for ten cycles with four hundred points per cycle to the extraction of average curves, enables us to plot the curves of friction according to velocity and thereafter the curve of Stribeck. The results show that we can get a total isoviscous regime for loads 20 and 40N, however for load 80N, this regime is partial, as it comes off the final curve from a speed value 0.1 m / s. the values of the friction coefficient varies for the three loads used between 0.004 and 0.017. These results show the possibility of obtaining a hydrodynamic regime with high load and low speed, with treatments suitable surfaces and are made to reduce wear and increase the lifetime of the mechanism.

Time domain characteristics of hoof-ground interaction at the onset of stance phase.

PubMed

Burn, J F

2006-11-01

Little is known about the interaction of the hoof with the ground at the onset of stance phase although is it widely believed that high power collisions are involved in the aetiopathology of several conditions causing lameness. To answer 3 questions regarding the fundamental nature of hoof-ground collision: (1) is the collision process deterministic for ground surfaces that present a consistent mechanical interface (2) do collision forces act on the hoof in a small or large range of directions and (3) Is the hoof decelerated to near-zero velocity by the initial deceleration peak following ground contact? Hoof acceleration during the onset of stance phase was recorded using biaxial accelerometry for horses trotting on a tarmac surface and on a sand surface. Characteristics of the collision process were identified both from vector plots and time series representations of hoof acceleration, velocity and displacement. The response of the hoof to collision with smooth tarmac was predominantly deterministic and consistent with the response of a spring-damper system following shock excitation. The response to collision with sand was predominantly random. The deceleration peak following ground contact did not decelerate the hoof to near-zero velocity on tarmac but appeared to on sand. On both surfaces, collision forces acted on the hoof in a wide range of directions. The study suggests the presence of stiff, viscoelastic structures within the foot that may act as shock absorbers isolating the limb from large collision forces. The study indicates objectives for future in vivo and in vitro research into the shock absorbing mechanism within the equine foot; and the effects of shoe type and track surface properties on the collision forces experienced during locomotion. Studies of this nature should help to establish a link between musculoskeletal injury, hoof function and hoof-ground interaction if, indeed, one exists.

Analytical study of the critical behavior of the nonlinear pendulum

NASA Astrophysics Data System (ADS)

Lima, F. M. S.

2010-11-01

The dynamics of a simple pendulum consisting of a small bob and a massless rigid rod has three possible regimes depending on its total energy E: Oscillatory (when E is not enough for the pendulum to reach the top position), "perpetual ascent" when E is exactly the energy needed to reach the top, and nonoscillatory for greater energies. In the latter regime, the pendulum rotates periodically without velocity inversions. In contrast to the oscillatory regime, for which an exact analytic solution is known, the other two regimes are usually studied by solving the equation of motion numerically. By applying conservation of energy, I derive exact analytical solutions to both the perpetual ascent and nonoscillatory regimes and an exact expression for the pendulum period in the nonoscillatory regime. Based on Cromer's approximation for the large-angle pendulum period, I find a simple approximate expression for the decrease of the period with the initial velocity in the nonoscillatory regime, valid near the critical velocity. This expression is used to study the critical slowing down, which is observed near the transition between the oscillatory and nonoscillatory regimes.

ECRH and its effects on neoclassical transport in a stellarator

NASA Astrophysics Data System (ADS)

Seol, Jaechun

The banana center orbit deviates significantly from the magnetic surface due to the symmetry-breaking term in the magnetic field configuration. Energetic electrons can escape the plasma without collision, since the drift speed is proportional to the perpendicular energy of electron and the collision frequency is reduced as the electron energy goes up. A direct loss flux can be generated from energetic electron population in a stellarator. Thus energetic electron populations can substantially modify the neoclassical transport properties in stellarators. A model accounting for this change in transport is developed assuming the presence of electron cyclotron resonance heating (ECRH). The quasilinear diffusion coefficient for second harmonic X-mode ECRH is developed for a bumpy stellarator. Care is taken in accounting for the pitch-angle dependence of the quasilinear diffusion coefficient since application to experiments with narrow resonance zones is of interest. Weakly relativistic effects are considered through the mass effect on the cyclotron frequency. For trapped particles in a three dimensional configuration, collisionless loss zones exist in velocity space. Radio-frequency (rf) waves accelerate trapped electrons into the direct loss zone in bumpy stellarators and produce a direct loss flux. An analytic expression for this loss flux is derived; it is proportional to the rf field strength and the value of the zeroth order distribution function at the minimum speed for collisionless loss. The direct loss flux of electrons is another source of a non-ambipolar particle flux in bumpy stellarators. This additional non-ambipolar flux modifies the ambipolarity equation which generally has multiple roots for the radial electric field. An electron root (large positive Er) is easily obtained if the electrons are in the 1/nu regime and the ions are in the nu regime.

The collision forces and lower-extremity inter-joint coordination during running.

PubMed

Wang, Li-I; Gu, Chin-Yi; Wang, I-Lin; Siao, Sheng-Wun; Chen, Szu-Ting

2018-06-01

The purpose of this study was to compare the lower extremity inter-joint coordination of different collision forces runners during running braking phase. A dynamical system approach was used to analyse the inter-joint coordination parameters. Data were collected with six infra-red cameras and two force plates. According to the impact peak of the vertical ground reaction force, twenty habitually rearfoot-strike runners were categorised into three groups: high collision forces runners (HF group, n = 8), medium collision forces runners (MF group, n = 5), and low collision forces runners (LF group, n = 7). There were no significant differences among the three groups in the ankle and knee joint angle upon landing and in the running velocity (p > 0.05). The HF group produced significantly smaller deviation phase (DP) of the hip flexion/extension-knee flexion/extension during the braking phase compared with the MF and LF groups (p < 0.05). The DP of the hip flexion/extension-knee flexion/extension during the braking phase correlated negatively with the collision force (p < 0.05). The disparities regarding the flexibility of lower extremity inter-joint coordination were found in high collision forces runners. The efforts of the inter-joint coordination and the risk of running injuries need to be clarified further.

Perpetual motion and driven dynamics of a mobile impurity in a quantum fluid

NASA Astrophysics Data System (ADS)

Lychkovskiy, O.

2015-04-01

We study the dynamics of a mobile impurity in a quantum fluid at zero temperature. Two related settings are considered. In the first setting, the impurity is injected in the fluid with some initial velocity v0, and we are interested in its velocity at infinite time, v∞. We derive a rigorous upper bound on | v0-v∞| for initial velocities smaller than the generalized critical velocity. In the limit of vanishing impurity-fluid coupling, this bound amounts to v∞=v0 , which can be regarded as a rigorous proof of the Landau criterion of superfluidity. In the case of a finite coupling, the velocity of the impurity can drop, but not to zero; the bound quantifies the maximal possible drop. In the second setting, a small constant force is exerted upon the impurity. We argue that two distinct dynamical regimes exist—backscattering oscillations of the impurity velocity and saturation of the velocity without oscillations. For fluids with vc L=vs (where vc L and vs are the Landau critical velocity and sound velocity, respectively), the latter regime is realized. For fluids with vc L

Fragment properties at the catastrophic disruption threshold: The effect of the parent body’s internal structure

NASA Astrophysics Data System (ADS)

Jutzi, Martin; Michel, Patrick; Benz, Willy; Richardson, Derek C.

2010-05-01

Numerical simulations of asteroid breakups, including both the fragmentation of the parent body and the gravitational interactions between the fragments, have allowed us to reproduce successfully the main properties of asteroid families formed in different regimes of impact energy, starting from a non-porous parent body. In this paper, using the same approach, we concentrate on a single regime of impact energy, the so-called catastrophic threshold usually designated by QD*, which results in the escape of half of the target's mass. Thanks to our recent implementation of a model of fragmentation of porous materials, we can characterize QD* for both porous and non-porous targets with a wide range of diameters. We can then analyze the potential influence of porosity on the value of QD*, and by computing the gravitational phase of the collision in the gravity regime, we can characterize the collisional outcome in terms of the fragment size and ejection speed distributions, which are the main outcome properties used by collisional models to study the evolutions of the different populations of small bodies. We also check the dependency of QD* on the impact speed of the projectile. In the strength regime, which corresponds to target sizes below a few hundreds of meters, we find that porous targets are more difficult to disrupt than non-porous ones. In the gravity regime, the outcome is controlled purely by gravity and porosity in the case of porous targets. In the case of non-porous targets, the outcome also depends on strength. Indeed, decreasing the strength of non-porous targets make them easier to disrupt in this regime, while increasing the strength of porous targets has much less influence on the value of QD*. Therefore, one cannot say that non-porous targets are systematically easier or more difficult to disrupt than porous ones, as the outcome highly depends on the assumed strength values. In the gravity regime, we also confirm that the process of gravitational reaccumulation is at the origin of the largest remnant's mass in both cases. We then propose some power-law relationships between QD* and both target's size and impact speed that can be used in collisional evolution models. The resulting fragment size distributions can also be reasonably fitted by a power-law whose exponent ranges between -2.2 and -2.7 for all target diameters in both cases and independently on the impact velocity (at least in the small range investigated between 3 and 5 km/s). Then, although ejection velocities in the gravity regime tend to be higher from porous targets, they remain on the same order as the ones from non-porous targets.

Gas holdup and flow regime transition in spider-sparger bubble column: effect of liquid phase properties

NASA Astrophysics Data System (ADS)

Besagni, G.; Inzoli, F.; De Guido, G.; Pellegrini, L. A.

2017-01-01

This paper discusses the effects of the liquid velocity and the liquid phase properties on the gas holdup and the flow regime transition in a large-diameter and large-scale counter-current two-phase bubble column. In particular, we compared and analysed the experimental data obtained in our previous experimental studies. The bubble column is 5.3 m in height, has an inner diameter of 0.24 m, it was operated with gas superficial velocities in the range of 0.004-0.20 m/s and, in the counter-current mode, the liquid was recirculated up to a superficial velocity of -0.09 m/s. Air was used as the dispersed phase and various fluids (tap water, aqueous solutions of sodium chloride, ethanol and monoethylene glycol) were employed as liquid phases. The experimental dataset consist in gas holdup measurements and was used to investigate the global fluid dynamics and the flow regime transition between the homogeneous flow regime and the transition flow regime. We found that the liquid velocity and the liquid phase properties significantly affect the gas holdup and the flow regime transition. In this respect, a possible relationship (based on the lift force) between the flow regime transition and the gas holdup was proposed.

Physiological breakdown of Jeffrey six constant nanofluid flow in an endoscope with nonuniform wall

NASA Astrophysics Data System (ADS)

Nadeem, S.; Shaheen, A.; Hussain, S.

2015-12-01

This paper analyse the endoscopic effects of peristaltic nanofluid flow of Jeffrey six-constant fluid model in the presence of magnetohydrodynamics flow. The current problem is modeled in the cylindrical coordinate system and exact solutions are managed (where possible) under low Reynolds number and long wave length approximation. The influence of emerging parameters on temperature and velocity profile are discussed graphically. The velocity equation is solved analytically by utilizing the homotopy perturbation technique strongly, while the exact solutions are computed from temperature equation. The obtained expressions for velocity , concentration and temperature is sketched during graphs and the collision of assorted parameters is evaluate for transform peristaltic waves. The solution depend on thermophoresis number Nt, local nanoparticles Grashof number Gr, and Brownian motion number Nb. The obtained expressions for the velocity, temperature, and nanoparticles concentration profiles are plotted and the impact of various physical parameters are investigated for different peristaltic waves.

Dynamic conductivity and plasmon profile of aluminum in the ultra-fast-matter regime

NASA Astrophysics Data System (ADS)

Dharma-wardana, M. W. C.

2016-06-01

We use an explicitly isochoric two-temperature theory to analyze recent x-ray laser scattering data for aluminum in the ultra-fast-matter (UFM) regime up to 6 eV. The observed surprisingly low conductivities are explained by including strong electron-ion scattering effects using the phase shifts calculated via the neutral-pseudo-atom model. The difference between the static conductivity for UFM-Al and equilibrium aluminum in the warm-dense matter state is clearly brought out by comparisons with available density-fucntional+molecular-dynamics simulations. Thus the applicability of the Mermin model to UFM is questioned. The static and dynamic conductivity, collision frequency, and the plasmon line shape, evaluated within the simplest Born approximation for UFM aluminum, are in good agreement with experiment.

OH-LIF measurement of H2/O2/N2 flames in a micro flow reactor with a controlled temperature profile

NASA Astrophysics Data System (ADS)

Shimizu, T.; Nakamura, H.; Tezuka, T.; Hasegawa, S.; Maruta, K.

2014-11-01

This paper presents combustion and ignition characteristic of H2/O2/N2 flames in a micro flow reactor with a controlled temperature profile. OH-LIF measurement was conducted to capture flame images. Flame responses were investigated for variable inlet flow velocity, U, and equivalence ratio, phi. Three kinds of flame responses were experimentally observed for the inlet flow velocities: stable flat flames (normal flames) in the high inlet flow velocity regime; unstable flames called Flames with Repetitive Extinction and Ignition (FREI) in the intermediate flow velocity regime; and stable weak flames in the low flow velocity regime, at phi = 0.6, 1.0 and 1.2. On the other hand, weak flame was not observed at phi = 3.0 by OH-LIF measurement. Computational OH mole fractions showed lower level at the rich conditions than those at stoichiometric and lean conditions. To examine this response of OH signal to equivalence ratio, rate of production analysis was conducted and four kinds of major contributed reaction for OH production: R3(O + H2 <=> H + OH); R38(H + O2 <=> O + OH); R46(H + HO2 <=> 2OH); and R86(2OH <=> O + H2O), were found. Three reactions among them, R3, R38 and R46, did not showed significant difference in rate of OH production for different equivalence ratios. On the other hand, rate of OH production from R86 at phi = 3.0 was extremely lower than those at phi = 0.6 and 1.0. Therefore, R86 was considered to be a key reaction for the reduction of the OH production at phi = 3.0.

Consistent lattice Boltzmann modeling of low-speed isothermal flows at finite Knudsen numbers in slip-flow regime: Application to plane boundaries

NASA Astrophysics Data System (ADS)

Silva, Goncalo; Semiao, Viriato

2017-07-01

The first nonequilibrium effect experienced by gaseous flows in contact with solid surfaces is the slip-flow regime. While the classical hydrodynamic description holds valid in bulk, at boundaries the fluid-wall interactions must consider slip. In comparison to the standard no-slip Dirichlet condition, the case of slip formulates as a Robin-type condition for the fluid tangential velocity. This makes its numerical modeling a challenging task, particularly in complex geometries. In this work, this issue is handled with the lattice Boltzmann method (LBM), motivated by the similarities between the closure relations of the reflection-type boundary schemes equipping the LBM equation and the slip velocity condition established by slip-flow theory. Based on this analogy, we derive, as central result, the structure of the LBM boundary closure relation that is consistent with the second-order slip velocity condition, applicable to planar walls. Subsequently, three tasks are performed. First, we clarify the limitations of existing slip velocity LBM schemes, based on discrete analogs of kinetic theory fluid-wall interaction models. Second, we present improved slip velocity LBM boundary schemes, constructed directly at discrete level, by extending the multireflection framework to the slip-flow regime. Here, two classes of slip velocity LBM boundary schemes are considered: (i) linear slip schemes, which are local but retain some calibration requirements and/or operation limitations, (ii) parabolic slip schemes, which use a two-point implementation but guarantee the consistent prescription of the intended slip velocity condition, at arbitrary plane wall discretizations, further dispensing any numerical calibration procedure. Third and final, we verify the improvements of our proposed slip velocity LBM boundary schemes against existing ones. The numerical tests evaluate the ability of the slip schemes to exactly accommodate the steady Poiseuille channel flow solution, over distinct wall slippage conditions, namely, no-slip, first-order slip, and second-order slip. The modeling of channel walls is discussed at both lattice-aligned and non-mesh-aligned configurations: the first case illustrates the numerical slip due to the incorrect modeling of slippage coefficients, whereas the second case adds the effect of spurious boundary layers created by the deficient accommodation of bulk solution. Finally, the slip-flow solutions predicted by LBM schemes are further evaluated for the Knudsen's paradox problem. As conclusion, this work establishes the parabolic accuracy of slip velocity schemes as the necessary condition for the consistent LBM modeling of the slip-flow regime.

Consistent lattice Boltzmann modeling of low-speed isothermal flows at finite Knudsen numbers in slip-flow regime: Application to plane boundaries.

PubMed

Silva, Goncalo; Semiao, Viriato

2017-07-01

The first nonequilibrium effect experienced by gaseous flows in contact with solid surfaces is the slip-flow regime. While the classical hydrodynamic description holds valid in bulk, at boundaries the fluid-wall interactions must consider slip. In comparison to the standard no-slip Dirichlet condition, the case of slip formulates as a Robin-type condition for the fluid tangential velocity. This makes its numerical modeling a challenging task, particularly in complex geometries. In this work, this issue is handled with the lattice Boltzmann method (LBM), motivated by the similarities between the closure relations of the reflection-type boundary schemes equipping the LBM equation and the slip velocity condition established by slip-flow theory. Based on this analogy, we derive, as central result, the structure of the LBM boundary closure relation that is consistent with the second-order slip velocity condition, applicable to planar walls. Subsequently, three tasks are performed. First, we clarify the limitations of existing slip velocity LBM schemes, based on discrete analogs of kinetic theory fluid-wall interaction models. Second, we present improved slip velocity LBM boundary schemes, constructed directly at discrete level, by extending the multireflection framework to the slip-flow regime. Here, two classes of slip velocity LBM boundary schemes are considered: (i) linear slip schemes, which are local but retain some calibration requirements and/or operation limitations, (ii) parabolic slip schemes, which use a two-point implementation but guarantee the consistent prescription of the intended slip velocity condition, at arbitrary plane wall discretizations, further dispensing any numerical calibration procedure. Third and final, we verify the improvements of our proposed slip velocity LBM boundary schemes against existing ones. The numerical tests evaluate the ability of the slip schemes to exactly accommodate the steady Poiseuille channel flow solution, over distinct wall slippage conditions, namely, no-slip, first-order slip, and second-order slip. The modeling of channel walls is discussed at both lattice-aligned and non-mesh-aligned configurations: the first case illustrates the numerical slip due to the incorrect modeling of slippage coefficients, whereas the second case adds the effect of spurious boundary layers created by the deficient accommodation of bulk solution. Finally, the slip-flow solutions predicted by LBM schemes are further evaluated for the Knudsen's paradox problem. As conclusion, this work establishes the parabolic accuracy of slip velocity schemes as the necessary condition for the consistent LBM modeling of the slip-flow regime.

Analyzing collision processes with the smartphone acceleration sensor

NASA Astrophysics Data System (ADS)

Vogt, Patrik; Kuhn, Jochen

2014-02-01

It has been illustrated several times how the built-in acceleration sensors of smartphones can be used gainfully for quantitative experiments in school and university settings (see the overview in Ref. 1). The physical issues in that case are manifold and apply, for example, to free fall,2 radial acceleration,3 several pendula, or the exploitation of everyday contexts.6 This paper supplements these applications and presents an experiment to study elastic and inelastic collisions. In addition to the masses of the two impact partners, their velocities before and after the collision are of importance, and these velocities can be determined by numerical integration of the measured acceleration profile.

Adiabatic description of long range frequency sweeping

NASA Astrophysics Data System (ADS)

Nyqvist, R. M.; Lilley, M. K.; Breizman, B. N.

2012-09-01

A theoretical framework is developed to describe long range frequency sweeping events in the 1D electrostatic bump-on-tail model with fast particle sources and collisions. The model includes three collision operators (Krook, drag (dynamical friction) and velocity space diffusion), and allows for a general shape of the fast particle distribution function. The behaviour of phase space holes and clumps is analysed in the absence of diffusion, and the effect of particle trapping due to separatrix expansion is discussed. With a fast particle distribution function whose slope decays above the resonant phase velocity, hooked frequency sweeping is found for holes in the presence of drag collisions alone.

Isotropic matrix elements of the collision integral for the Boltzmann equation

NASA Astrophysics Data System (ADS)

Ender, I. A.; Bakaleinikov, L. A.; Flegontova, E. Yu.; Gerasimenko, A. B.

2017-09-01

We have proposed an algorithm for constructing matrix elements of the collision integral for the nonlinear Boltzmann equation isotropic in velocities. These matrix elements have been used to start the recurrent procedure for calculating matrix elements of the velocity-nonisotropic collision integral described in our previous publication. In addition, isotropic matrix elements are of independent interest for calculating isotropic relaxation in a number of physical kinetics problems. It has been shown that the coefficients of expansion of isotropic matrix elements in Ω integrals are connected by the recurrent relations that make it possible to construct the procedure of their sequential determination.

Low-velocity collision behaviour of clusters composed of sub-millimetre sized dust aggregates

NASA Astrophysics Data System (ADS)

Brisset, J.; Heißelmann, D.; Kothe, S.; Weidling, R.; Blum, J.

2017-07-01

Context. The experiment results presented apply to the very first stages of planet formation, when small dust aggregates collide in the protoplanetary disc and grow into bigger clusters. In 2011, before flying on the REXUS 12 suborbital rocket in 2012, the Suborbital Particle and Aggregation Experiment (SPACE) performed drop tower flights. We present the results of this first microgravity campaign. Aims: The experiments presented aim to measure the outcome of collisions between sub-mm sized protoplanetary dust aggregate analogues. We also observed the clusters formed from these aggregates and their collision behaviour. Methods: The experiments were performed at the drop tower in Bremen. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO2 particles prepared into aggregates with sizes between 120 μm and 250 μm. One of the dust samples contained aggregates that were previously compacted through repeated bouncing. During three flights of 9 s of microgravity each, individual collisions between aggregates and the formation of clusters of up to a few millimetres in size were observed. In addition, the collisions of clusters with the experiment cell walls leading to compaction or fragmentation were recorded. Results: We observed collisions amongst dust aggregates and collisions between dust clusters and the cell aluminium walls at speeds ranging from about 0.1 cm s-1 to 20 cm s-1. The velocities at which sticking occurred ranged from 0.18 to 5.0 cm s-1 for aggregates composed of monodisperse dust, with an average value of 2.1 ± 0.9 cm s-1 for reduced masses ranging from 1.2 × 10-6 to 1.8 × 10-3 g with an average value of 2.2+16-2.1 × 10-4 g. The velocities at which bouncing occurred ranged from 1.9 to 11.9 cm s-1 for the same aggregates with an average of 5.9 ± 3.2 cm s-1 for reduced masses ranging from 2.1 × 10-6 to 2.4 × 10-4 with an average of 7.8 ± 2.4 × 10-5 g. The velocities at which fragmentation occurred ranged from 4.9 to 23.8 cm s-1 for the same aggregates with an average of 10.1 ± 3.2 cm s-1 for reduced masses ranging from 1.2 × 10-5 to 1.2 × 10-3 with an average value of 4.2 ± 2.4 × 10-4 g. From the restructuring and fragmentation of clusters composed of dust aggregates colliding with the aluminium cell walls, we derived a collision recipe for dust aggregates ( 100 μm) following the model of Dominik & Tielens (1997, ApJ, 480, 647) developed for microscopic particles. We measured a critical rolling energy of 1.8 ± 0.9 × 10-13 J and a critical breaking energy of 3.5 ± 1.5 × 10-13 J for 100 μm-sized non-compacted aggregates. A movie associated to Fig. 3 is available at http://www.aanda.org

Ultrasonic velocity dispersion in ethane-argon mixtures.

NASA Technical Reports Server (NTRS)

Amme, R. C.; Warren, B. E.

1968-01-01

Ultrasonic interferometry to measure velocity dispersion in ethane-Ar mixtures, discussing ethane relaxation characteristics and relaxation characteristics and relaxation times for particle collisions

Collision energy dependent cross section and rotational alignment of NO (A 2Σ+) in the energy-transfer reaction of N2 (A 3Σu+) + NO (X 2Π) → N2 (X 1Σg+) + NO (A 2Σ+).

PubMed

Ohoyama, H

2014-10-16

We have studied the collision energy dependent cross section and alignment of NO (A (2)Σ(+)) rotation in the energy-transfer reaction of N2 (A (3)Σ(u)(+)) + NO (X (2)Π) → N2 (X (1)Σ(g)(+)) + NO (A (2)Σ(+)) at the collision energy (E) region of 0.03-0.2 eV. NO (A (2)Σ(+)) emission in two linear polarization directions in the collision frame (parallel (∥) and perpendicular (⊥) with respect to the relative velocity vector (vR)) has been measured as a function of collision energy. NO (A (2)Σ(+)) rotation (J-vector) turns out to be aligned perpendicular to vR. In addition, collision energy is found to enhance the degree of alignment of NO (A (2)Σ(+)) rotation. The collision energy dependent cross sections σ(∥,(⊥))(E) (excitation functions) show a rapid fall-off following an initial rise with a threshold less than 0.02 eV. The excitation function at the parallel alignment of NO (A (2)Σ(+)) rotation, σ(J∥v(R), (E), is slightly shifted to the low collision energy region as compared with σ(J ⊥ vR, E). We propose that the rapid fall-off feature in the excitation function is attributed to the multidimensional nonadiabatic transitions.

Electron transport fluxes in potato plateau regime

NASA Astrophysics Data System (ADS)

Shaing, K. C.; Hazeltine, R. D.

1997-12-01

Electron transport fluxes in the potato plateau regime are calculated from the solutions of the drift kinetic equation and fluid equations. It is found that the bootstrap current density remains finite in the region close to the magnetic axis, although it decreases with increasing collision frequency. This finite amount of the bootstrap current in the relatively collisional regime is important in modeling tokamak startup with 100% bootstrap current.

Erosion and the limits to planetesimal growth

NASA Astrophysics Data System (ADS)

Krijt, S.; Ormel, C. W.; Dominik, C.; Tielens, A. G. G. M.

2015-02-01

Context. The coagulation of microscopic dust into planetesimals is the first step towards the formation of planets. The composition, size, and shape of the growing aggregates determine the efficiency of this early growth. In particular, it has been proposed that fluffy ice aggregates can grow very efficiently in protoplanetary disks, suffering less from the bouncing and radial drift barriers. Aims: While the collision velocity between icy aggregates of similar size is thought to stay below the fragmentation threshold, they may nonetheless lose mass from collisions with much smaller projectiles. As a result, erosive collisions have the potential to terminate the growth of pre-planetesimal bodies. We investigate the effect of these erosive collisions on the ability of porous ice aggregates to cross the radial drift barrier. Methods: We develop a Monte Carlo code that calculates the evolution of the masses and porosities of growing aggregates, while resolving the entire mass distribution at all times. The aggregate's porosity is treated independently of its mass, and is determined by collisional compaction, gas compaction, and eventually self-gravity compaction. We include erosive collisions and study the effect of the erosion threshold velocity on aggregate growth. Results: For erosion threshold velocities of 20-40 m s-1, high-velocity collisions with small projectiles prevent the largest aggregates from growing when they start to drift. In these cases, our local simulations result in a steady-state distribution, with most of the dust mass in particles with Stokes numbers close to unity. Only for the highest erosion threshold considered (60 m s-1) do porous aggregates manage to cross the radial drift barrier in the inner 10 AU of MMSN-like disks. Conclusions: Erosive collisions are more effective in limiting the growth than fragmentary collisions between similar-size particles. Conceivably, erosion limits the growth before the radial drift barrier, although the robustness of this statement depends on uncertain material properties of icy aggregates. If erosion inhibits planetesimal formation through direct sticking, the sea of ~109 g, highly porous particles appears suitable for triggering streaming instability.

Liquid rims collisions and the formation of fines

NASA Astrophysics Data System (ADS)

Néel, Baptiste; Villermaux, Emmanuel

2017-11-01

As an elementary mechanism for the formation of drops from liquid sheets, we investigate the collision of liquid cylinders. This results from the opening of two nearby holes on a liquid film, growing at a constant speed while collecting liquid into two rims, eventually colliding with each other. In this surface tension driven phenomenon, a unique Weber number We = ρ(2 V) 2 2 a / σ controls a variety of behaviors (ρ , σ are the liquid density and surface tension, and 2 V the relative velocity of the impinging rims, each of individual radius a). At low We , the rims merge through an inelastic, dissipative collision which produces a corrugated ligament, finally breaking into drops of size scaling like a, on average. Above a critical Wec 60 , the collision leads to a splash, with the formation of a thin transverse liquid sheet. We will describe the expansion-retraction dynamics of this secondary sheet and its destabilization, responsible for the production of a mist of finer droplets. These alter sensibly the mean, and overall drops size distribution, thus weighted by a substantial fraction of so-called fines.

Ion imaging studies of product rotational alignment in collisions of NO ( X2Π1/2, j=0.5) with Ar

NASA Astrophysics Data System (ADS)

Wade, Elisabeth A.; Thomas Lorenz, K.; Chandler, David W.; Barr, James W.; Barnes, George L.; Cline, Joseph I.

2004-06-01

The collision-induced rotational alignment of NO ( X2Π1/2, v=0, j=4.5 , 8.5, 11.5, 12.5, and 15.5) is measured for rotationally inelastic scattering of NO ( X2Π1/2, v=0, j=0.5) with Ar at 520 ± 70 cm -1 of center-of-mass collision energy. The experiments are performed by velocity-mapped ion imaging with polarized 1+1 ' REMPI of the scattered NO product. Differential cross-sections (DCSs), corrected for alignment effects, are also reported. While the alignment correction is important, it does not change the positions of the observed rotational rainbows. The alignment moments and DCSs are compared with calculations using Alexander's CCSD(T) PESs. The theoretical and experimental DCSs show excellent agreement, as do the theoretical and experimental alignment moments for low Δ j. For high Δ j collisions and back-scattered trajectories, which sample the hard wall of the PES, the theoretical and experimental alignment moments show less agreement.

Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC

NASA Astrophysics Data System (ADS)

Mäntysaari, Heikki; Schenke, Björn; Shen, Chun; Tribedy, Prithwish

2017-09-01

Results for particle production in √{ s} = 5.02TeV p + Pb collisions at the Large Hadron Collider within a combined classical Yang-Mills and relativistic viscous hydrodynamic calculation are presented. We emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients vn, demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean transverse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury-Brown-Twiss radii.

Spin entanglement in elastic electron scattering from quasi-one electron atoms

NASA Astrophysics Data System (ADS)

Fonseca Dos Santos, Samantha; Bartschat, Klaus

2017-04-01

We have extended our work on e-Li collisions to investigate low-energy elastic electron collisions with atomic hydrogen and other alkali targets (Na,K,Rb). These systems have been suggested for the possibility of continuously varying the degree of entanglement between the elastically scattered projectile and the valence electron. In order to estimate how well such a scheme may work in practice, we carried out overview calculations for energies between 0 and 10 eV and the full range of scattering angles 0° -180° . In addition to the relative exchange asymmetry parameter that characterizes the entanglement, we present the differential cross section in order to estimate whether the count rates in the most interesting energy-angle regimes are sufficient to make such experiments feasible in practice. Work supported by the NSF under PHY-1403245.

Determination of equilibrium electron temperature and times using an electron swarm model with BOLSIG+ calculated collision frequencies and rate coefficients

DOE PAGES

Pusateri, Elise N.; Morris, Heidi E.; Nelson, Eric M.; ...

2015-08-04

Electromagnetic pulse (EMP) events produce low-energy conduction electrons from Compton electron or photoelectron ionizations with air. It is important to understand how conduction electrons interact with air in order to accurately predict EMP evolution and propagation. An electron swarm model can be used to monitor the time evolution of conduction electrons in an environment characterized by electric field and pressure. Here a swarm model is developed that is based on the coupled ordinary differential equations (ODEs) described by Higgins et al. (1973), hereinafter HLO. The ODEs characterize the swarm electric field, electron temperature, electron number density, and drift velocity. Importantmore » swarm parameters, the momentum transfer collision frequency, energy transfer collision frequency, and ionization rate, are calculated and compared to the previously reported fitted functions given in HLO. These swarm parameters are found using BOLSIG+, a two term Boltzmann solver developed by Hagelaar and Pitchford (2005), which utilizes updated cross sections from the LXcat website created by Pancheshnyi et al. (2012). We validate the swarm model by comparing to experimental effective ionization coefficient data in Dutton (1975) and drift velocity data in Ruiz-Vargas et al. (2010). In addition, we report on electron equilibrium temperatures and times for a uniform electric field of 1 StatV/cm for atmospheric heights from 0 to 40 km. We show that the equilibrium temperature and time are sensitive to the modifications in the collision frequencies and ionization rate based on the updated electron interaction cross sections.« less

THE REBOUND CONDITION OF DUST AGGREGATES REVEALED BY NUMERICAL SIMULATION OF THEIR COLLISIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wada, Koji; Tanaka, Hidekazu; Yamamoto, Tetsuo

2011-08-10

Collisional growth of dust aggregates is a plausible root of planetesimals forming in protoplanetary disks. However, a rebound of colliding dust aggregates prevents dust from growing into planetesimals. In fact, rebounding aggregates are observed in laboratory experiments but not in previous numerical simulations. Therefore, the condition of rebound between dust aggregates should be clarified to better understand the processes of dust growth and planetesimal formation. We have carried out numerical simulations of aggregate collisions for various types of aggregates and succeeded in reproducing a rebound of colliding aggregates under specific conditions. Our finding is that in the rebound process, themore » key factor of the aggregate structure is the coordination number, namely, the number of particles in contact with a particle. A rebound is governed by the energy dissipation along with restructuring of the aggregates and a large coordination number inhibits the restructuring at collisions. Results of our numerical simulation for various aggregates indicate that they stick to each other when the mean coordination number is less than 6, regardless of their materials and structures, as long as their collision velocity is less than the critical velocity for fragmentation. This criterion of the coordination number would correspond to a filling factor of {approx}0.3, which is somewhat larger than that reported in laboratory experiments. In protoplanetary disks, dust aggregates are expected to have low bulk densities (<0.1 g cm{sup -3}) during their growth, which would prevent dust aggregates from rebounding. This result supports the formation of planetesimals with direct dust growth in protoplanetary disks.« less

Transition to Turbulent Dynamo Saturation

NASA Astrophysics Data System (ADS)

Seshasayanan, Kannabiran; Gallet, Basile; Alexakis, Alexandros

2017-11-01

While the saturated magnetic energy is independent of viscosity in dynamo experiments, it remains viscosity dependent in state-of-the-art 3D direct numerical simulations (DNS). Extrapolating such viscous scaling laws to realistic parameter values leads to an underestimation of the magnetic energy by several orders of magnitude. The origin of this discrepancy is that fully 3D DNS cannot reach low enough values of the magnetic Prandtl number Pm. To bypass this limitation and investigate dynamo saturation at very low Pm, we focus on the vicinity of the dynamo threshold in a rapidly rotating flow: the velocity field then depends on two spatial coordinates only, while the magnetic field consists of a single Fourier mode in the third direction. We perform numerical simulations of the resulting set of reduced equations for Pm down to 2 ×10-5. This parameter regime is currently out of reach to fully 3D DNS. We show that the magnetic energy transitions from a high-Pm viscous scaling regime to a low-Pm turbulent scaling regime, the latter being independent of viscosity. The transition to the turbulent saturation regime occurs at a low value of the magnetic Prandtl number, Pm ≃10-3 , which explains why it has been overlooked by numerical studies so far.

Transition to Turbulent Dynamo Saturation.

PubMed

Seshasayanan, Kannabiran; Gallet, Basile; Alexakis, Alexandros

2017-11-17

While the saturated magnetic energy is independent of viscosity in dynamo experiments, it remains viscosity dependent in state-of-the-art 3D direct numerical simulations (DNS). Extrapolating such viscous scaling laws to realistic parameter values leads to an underestimation of the magnetic energy by several orders of magnitude. The origin of this discrepancy is that fully 3D DNS cannot reach low enough values of the magnetic Prandtl number Pm. To bypass this limitation and investigate dynamo saturation at very low Pm, we focus on the vicinity of the dynamo threshold in a rapidly rotating flow: the velocity field then depends on two spatial coordinates only, while the magnetic field consists of a single Fourier mode in the third direction. We perform numerical simulations of the resulting set of reduced equations for Pm down to 2×10^{-5}. This parameter regime is currently out of reach to fully 3D DNS. We show that the magnetic energy transitions from a high-Pm viscous scaling regime to a low-Pm turbulent scaling regime, the latter being independent of viscosity. The transition to the turbulent saturation regime occurs at a low value of the magnetic Prandtl number, Pm≃10^{-3}, which explains why it has been overlooked by numerical studies so far.

The “Main-Belt Comets” are not comets, nor active asteroids; they are temporary shaken asteroids

NASA Astrophysics Data System (ADS)

Tancredi, Gonzalo

2015-08-01

Several objects in asteroidal orbits have presented comaes and tails similar to the ones presented by comets for short period of times. There are at present 16 objects in this group. Several hypotheses have been proposed to explain the activity of this object [Jewitt 2012]. Among them, the most accepted scenario for many objects is the ice sublimation and the ejection of dust, in a similar way as the cometary activity. Therefore several authors have coined these objects “Main Belt Comets” [Hsieh & Jewitt 2006]. Nevertheless, in some cases, some authors have concluded that the ejection of dust must be due to an impact.We propose an alternative model for the formation of the dusty comaes and tails.The impact of a small body against a larger one initially produces a crater and the ejection of dust at high velocity (>100 m/s). The dust is rapidly dispersed and it should be only observable just after the impact. In addition the impact generates a shock wave, which propagates to the body interior. The asteroid is globally shaken. Material is ejected at low velocities from the entire surface, similar to the low escape velocities at the surface. The particles move away from the asteroid due to the solar radiation pressure, forming the thin tails aligned with the orbital plane. These tails could persist for various months, as they have been seen in these objects.In addition, chunks of rock could be ejected in suborbital flights lasting for days; which, at return they would induce a new low-velocity ejection of particles. This process can explain some of the long-lasting events.The recurrence of the activity for some objects could be explained due to the collision with a dense meteor shower present in the main-belt.The so-called “Main Belt Comets” could be explained with a hypothesis that does not require the presence of ice on the surface of these objects. We also do not favor the term “Activated asteroids”, because it implies some kind of endogenous process. The objects are plain asteroids that suffered a recent collision, and the entire body is shaken, ejecting dust from the surface at low velocities.

Four-way coupled simulations of small particles in turbulent channel flow: The effects of particle shape and Stokes number

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, F.; Wachem, B. G. M. van, E-mail: berend.van.wachem@gmail.com; George, W. K.

2015-08-15

This paper investigates the effects of particle shape and Stokes number on the behaviour of non-spherical particles in turbulent channel flow. Although there are a number of studies concerning spherical particles in turbulent flows, most important applications occurring in process, energy, and pharmaceutical industries deal with non-spherical particles. The computation employs a unique and novel four-way coupling with the Lagrangian point-particle approach. The fluid phase at low Reynolds number (Re{sub τ} = 150) is modelled by direct numerical simulation, while particles are tracked individually. Inter-particle and particle-wall collisions are also taken into account. To explore the effects of particles onmore » the flow turbulence, the statistics of the fluid flow such as the fluid velocity, the terms in the turbulence kinetic energy equation, the slip velocity between the two phases and velocity correlations are analysed considering ellipsoidal particles with different inertia and aspect ratio. The results of the simulations show that the turbulence is considerably attenuated, even in the very dilute regime. The reduction of the turbulence intensity is predominant near the turbulence kinetic energy peak in the near wall region, where particles preferentially accumulate. Moreover, the elongated shape of ellipsoids strengthens the turbulence attenuation. In simulations with ellipsoidal particles, the fluid-particle interactions strongly depend on the orientation of the ellipsoids. In the near wall region, ellipsoids tend to align predominantly within the streamwise (x) and wall-normal (y) planes and perpendicular to the span-wise direction, whereas no preferential orientation in the central region of the channel is observed. Important conclusions from this work include the effective viscosity of the flow is not affected, the direct dissipation by the particles is negligible, and the primary mechanism by which the particles affect the flow is by altering the turbulence structure around the turbulence kinetic energy peak.« less

Temporal and spatial intermittencies within Newtonian turbulence

NASA Astrophysics Data System (ADS)

Kushwaha, Anubhav; Graham, Michael

2015-11-01

Direct numerical simulations of a pressure driven turbulent flow are performed in a large rectangular channel. Intermittent high- and low-drag regimes within turbulence that have earlier been found to exist temporally in minimal channels have been observed both spatially and temporally in full-size turbulent flows. These intermittent regimes, namely, ''active'' and ''hibernating'' turbulence, display very different structural and statistical features. We adopt a very simple sampling technique to identify these intermittent intervals, both temporally and spatially, and present differences between them in terms of simple quantities like mean-velocity, wall-shear stress and flow structures. By conditionally sampling of the low wall-shear stress events in particular, we show that the Maximum Drag Reduction (MDR) velocity profile, that occurs in viscoelastic flows, can also be approached in a Newtonian-fluid flow in the absence of any additives. This suggests that the properties of polymer drag reduction are inherent to all flows and their occurrence is just enhanced by the addition of polymers. We also show how the intermittencies within turbulence vary with Reynolds number. The work was supported by AFOSR grant FA9550-15-1-0062.

Obseration of flow regime transition in CFB riser using an LDV

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yue, Paul C.; Mei, Joseph S.; Shadle, Lawrence J.

2011-01-01

The solids flow in a circulating fluidized bed (CFB) riser is often described to have a core-annular structure. For a given superficial gas velocity, at the initial introduction of solids into a riser a flow structure of dilute upflow regime exists. Continuing to increase the solids flow in the riser transitions the flow structure to the core-annular flow regime. However, with further increase of solids flow a condition is reached, depending on the superficial gas velocity, where all the solids across the riser cross section flow upwards, even those at the wall. When the solids flux, solids fraction and gasmore » velocity are relatively high, such a condition is described as the dense phase suspense upflow (DSU) regime. In this paper we report our observations of these flow regime transitions by using a laser Doppler velocimeter (LDV) to monitor the upward and downward particle flow velocities at and near the riser wall of the National Energy Technology Laboratory’s 30.4 centimeters diameter CFB cold flow model. The particles were high density polyethylene (PPE) spheres with a Sauter mean diameter of 861 micron and a density of 800 kg/m3. Three superficial gas velocities of 6.55 m/s, 10.67 m/s and 13.72 m/s were used in this study. For the case of superficial gas velocity 6.55 m/s, the experimental data show that the transition from dilute upflow to core-annular flow occurred when the solids flux was about 7 kg/m{sup 2}-s and the transition from core-annular flow to dense suspension upflow was about 147 kg/m{sup 2}-s. As the superficial gas velocity was increased to 10.67 m/s the corresponding flow regime transitions were at 34 kg/m{sup 2}-s and 205 kg/m{sup 2}-s, respectively. For the case of superficial gas velocity of 13.72 m/s the data showed no distinct transition of flow regimes. The particles were all upflow for the range of solids fluxes from 10 kg/m{sup 2}-s to 286 kg/m{sup 2}-s.« less

Noise fluctuations and drive dependence of the skyrmion Hall effect in disordered systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reichhardt, Charles; Olson Reichhardt, Cynthia Jane

Using a particle-based simulation model, we show that quenched disorder creates a drive-dependent skyrmion Hall effect as measured by the change in the ratiomore » $$R={V}_{\\perp }/{V}_{| | }$$ of the skyrmion velocity perpendicular (V ⊥) and parallel ($${V}_{| | }$$) to an external drive. R is zero at depinning and increases linearly with increasing drive, in agreement with recent experimental observations. At sufficiently high drives where the skyrmions enter a free flow regime, R saturates to the disorder-free limit. In addition, this behavior is robust for a wide range of disorder strengths and intrinsic Hall angle values, and occurs whenever plastic flow is present. For systems with small intrinsic Hall angles, we find that the Hall angle increases linearly with external drive, as also observed in experiment. In the weak pinning regime where the skyrmion lattice depins elastically, R is nonlinear and the net direction of the skyrmion lattice motion can rotate as a function of external drive. We show that the changes in the skyrmion Hall effect correlate with changes in the power spectrum of the skyrmion velocity noise fluctuations. The plastic flow regime is associated with $1/f$ noise, while in the regime in which R has saturated, the noise is white with a weak narrow band signal, and the noise power drops by several orders of magnitude. Finally, at low drives, the velocity noise in the perpendicular and parallel directions is of the same order of magnitude, while at intermediate drives the perpendicular noise fluctuations are much larger.« less

Noise fluctuations and drive dependence of the skyrmion Hall effect in disordered systems

DOE PAGES

Reichhardt, Charles; Olson Reichhardt, Cynthia Jane

2016-09-29

Using a particle-based simulation model, we show that quenched disorder creates a drive-dependent skyrmion Hall effect as measured by the change in the ratiomore » $$R={V}_{\\perp }/{V}_{| | }$$ of the skyrmion velocity perpendicular (V ⊥) and parallel ($${V}_{| | }$$) to an external drive. R is zero at depinning and increases linearly with increasing drive, in agreement with recent experimental observations. At sufficiently high drives where the skyrmions enter a free flow regime, R saturates to the disorder-free limit. In addition, this behavior is robust for a wide range of disorder strengths and intrinsic Hall angle values, and occurs whenever plastic flow is present. For systems with small intrinsic Hall angles, we find that the Hall angle increases linearly with external drive, as also observed in experiment. In the weak pinning regime where the skyrmion lattice depins elastically, R is nonlinear and the net direction of the skyrmion lattice motion can rotate as a function of external drive. We show that the changes in the skyrmion Hall effect correlate with changes in the power spectrum of the skyrmion velocity noise fluctuations. The plastic flow regime is associated with $1/f$ noise, while in the regime in which R has saturated, the noise is white with a weak narrow band signal, and the noise power drops by several orders of magnitude. Finally, at low drives, the velocity noise in the perpendicular and parallel directions is of the same order of magnitude, while at intermediate drives the perpendicular noise fluctuations are much larger.« less

Discrete Velocity Models for Polyatomic Molecules Without Nonphysical Collision Invariants

NASA Astrophysics Data System (ADS)

Bernhoff, Niclas

2018-05-01

An important aspect of constructing discrete velocity models (DVMs) for the Boltzmann equation is to obtain the right number of collision invariants. Unlike for the Boltzmann equation, for DVMs there can appear extra collision invariants, so called spurious collision invariants, in plus to the physical ones. A DVM with only physical collision invariants, and hence, without spurious ones, is called normal. The construction of such normal DVMs has been studied a lot in the literature for single species, but also for binary mixtures and recently extensively for multicomponent mixtures. In this paper, we address ways of constructing normal DVMs for polyatomic molecules (here represented by that each molecule has an internal energy, to account for non-translational energies, which can change during collisions), under the assumption that the set of allowed internal energies are finite. We present general algorithms for constructing such models, but we also give concrete examples of such constructions. This approach can also be combined with similar constructions of multicomponent mixtures to obtain multicomponent mixtures with polyatomic molecules, which is also briefly outlined. Then also, chemical reactions can be added.

Excess velocity of magnetic domain walls close to the depinning field

NASA Astrophysics Data System (ADS)

Caballero, Nirvana B.; Fernández Aguirre, Iván; Albornoz, Lucas J.; Kolton, Alejandro B.; Rojas-Sánchez, Juan Carlos; Collin, Sophie; George, Jean Marie; Diaz Pardo, Rebeca; Jeudy, Vincent; Bustingorry, Sebastian; Curiale, Javier

2017-12-01

Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been measured using polar magneto-optic Kerr effect microscopy. The low field results are shown to be consistent with the universal creep regime of domain wall motion, characterized by a stretched exponential growth of the velocity with the inverse of the applied field. Approaching the depinning field from below results in an unexpected excess velocity with respect to the creep law. We analyze these results using scaling theory to show that this speeding up of domain wall motion can be interpreted as due to the increase of the size of the deterministic relaxation close to the depinning transition. We propose a phenomenological model to accurately fit the observed excess velocity and to obtain characteristic values for the depinning field Hd, the depinning temperature Td, and the characteristic velocity scale v0 for each sample.

Collision-free motion planning for fiber positioner robots: discretization of velocity profiles

NASA Astrophysics Data System (ADS)

Makarem, Laleh; Kneib, Jean-Paul; Gillet, Denis; Bleuler, Hannes; Bouri, Mohamed; Hörler, Philippe; Jenni, Laurent; Prada, Francisco; Sánchez, Justo

2014-07-01

The next generation of large-scale spectroscopic survey experiments such as DESI, will use thousands of fiber positioner robots packed on a focal plate. In order to maximize the observing time with this robotic system we need to move in parallel the fiber-ends of all positioners from the previous to the next target coordinates. Direct trajectories are not feasible due to collision risks that could undeniably damage the robots and impact the survey operation and performance. We have previously developed a motion planning method based on a novel decentralized navigation function for collision-free coordination of fiber positioners. The navigation function takes into account the configuration of positioners as well as their envelope constraints. The motion planning scheme has linear complexity and short motion duration (2.5 seconds with the maximum speed of 30 rpm for the positioner), which is independent of the number of positioners. These two key advantages of the decentralization designate the method as a promising solution for the collision-free motion-planning problem in the next-generation of fiber-fed spectrographs. In a framework where a centralized computer communicates with the positioner robots, communication overhead can be reduced significantly by using velocity profiles consisting of a few bits only. We present here the discretization of velocity profiles to ensure the feasibility of a real-time coordination for a large number of positioners. The modified motion planning method that generates piecewise linearized position profiles guarantees collision-free trajectories for all the robots. The velocity profiles fit few bits at the expense of higher computational costs.

Quantum turbulence and correlations in Bose-Einstein condensate collisions

NASA Astrophysics Data System (ADS)

Norrie, A. A.; Ballagh, R. J.; Gardiner, C. W.

2006-04-01

We investigate numerically simulated collisions between experimentally realistic Bose-Einstein condensate wave packets, within a regime where highly populated scattering haloes are formed. The theoretical basis for this work is the truncated Wigner method, for which we present a detailed derivation, paying particular attention to its validity regime for colliding condensates. This paper is an extension of our previous Letter [A. A. Norrie, R. J. Ballagh, and C. W. Gardiner, Phys. Rev. Lett. 94, 040401 (2005)], and we investigate both single-trajectory solutions, which reveal the presence of quantum turbulence in the scattering halo, and ensembles of trajectories, which we use to calculate quantum-mechanical correlation functions of the field.

Collective emission of matter-wave jets from driven Bose-Einstein condensates.

PubMed

Clark, Logan W; Gaj, Anita; Feng, Lei; Chin, Cheng

2017-11-16

Scattering is used to probe matter and its interactions in all areas of physics. In ultracold atomic gases, control over pairwise interactions enables us to investigate scattering in quantum many-body systems. Previous experiments on colliding Bose-Einstein condensates have revealed matter-wave interference, haloes of scattered atoms, four-wave mixing and correlations between counter-propagating pairs. However, a regime with strong stimulation of spontaneous collisions analogous to superradiance has proved elusive. In this regime, the collisions rapidly produce highly correlated states with macroscopic population. Here we find that runaway stimulated collisions in Bose-Einstein condensates with periodically modulated interaction strength cause the collective emission of matter-wave jets that resemble fireworks. Jets appear only above a threshold modulation amplitude and their correlations are invariant even when the number of ejected atoms grows exponentially. Hence, we show that the structures and atom occupancies of the jets stem from the quantum fluctuations of the condensate. Our findings demonstrate the conditions required for runaway stimulated collisions and reveal the quantum nature of matter-wave emission.

Coagulation of grains in static and collapsing protostellar clouds

NASA Technical Reports Server (NTRS)

Weidenschilling, S. J.; Ruzmaikina, T. V.

1994-01-01

We simulate collisional evolution of grains in dense turbulent molecular cloud cores (or Bok globules) in static equilibrium and free-fall collapse, assuming spherical symmetry. Relative velocities are due to thermal motions, differential settling, and turbulence, with the latter dominant for sonic turbulence with an assumed Kolmogorov spectrum. Realistic criteria are used to determine outcomes of collisions (coagulation vs. destruction) as functions of particle size and velocity. Results are presented for a variety of cloud parameters (radial density profile, turbulent velocity) and particle properties (density, impact strength). Results are sensitive to the assumed mechanical properties (density and impact strength) of grain aggregates. Particle growth is enhanced if aggregates have low density or fractal structures. On a timescale of a few Myr, an initial population of 0.1 micrometers grains may produce dense compact particles approximately 1 micrometer in size, or fluffy aggregates approximately 100 micrometers. For impact strengths less than or equal to 10(exp 6) ergs/g, a steady state is reached between coagulation of small grains and collisional disruption of larger aggregates. Formation of macroscopic aggregates requires high mechanical strengths and low aggregate densities. We assume sonic turbulence during collapse, with varied eddy size scales determining the dissipation rate or turbulence strength. The degree of collisional evolution during collapse is sensitive to the assumed small-scale structure (inner sc ale) of the turbulence. Weak turbulence results in few collisions and preserves the precollapse particle size distribution with little change. Strong turbulence tends to produce net destruction, rather than particle growth, during infall, unless inpact strengths are greater than 10(exp 6)ergs/g.

Sharp Transition in the Lift Force of a Fluid Flowing Past Nonsymmetrical Obstacles: Evidence for a Lift Crisis in the Drag Crisis Regime.

PubMed

Bot, Patrick; Rabaud, Marc; Thomas, Goulven; Lombardi, Alessandro; Lebret, Charles

2016-12-02

Bluff bodies moving in a fluid experience a drag force which usually increases with velocity. However in a particular velocity range a drag crisis is observed, i.e., a sharp and strong decrease of the drag force. This counterintuitive result is well characterized for a sphere or a cylinder. Here we show that, for an object breaking the up-down symmetry, a lift crisis is observed simultaneously to the drag crisis. The term lift crisis refers to the fact that at constant incidence the time-averaged transverse force, which remains small or even negative at low velocity, transitions abruptly to large positive values above a critical flow velocity. This transition is characterized from direct force measurements as well as from change in the velocity field around the obstacle.

Intra-jet shocks in two counter-streaming, weakly collisional plasma jets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ryutov, D. D.; Kugland, N. L.; Park, H.-S.

2012-07-15

Counterstreaming laser-generated plasma jets can serve as a test-bed for the studies of a variety of astrophysical phenomena, including collisionless shock waves. In the latter problem, the jet's parameters have to be chosen in such a way as to make the collisions between the particles of one jet with the particles of the other jet very rare. This can be achieved by making the jet velocities high and the Coulomb cross-sections correspondingly low. On the other hand, the intra-jet collisions for high-Mach-number jets can still be very frequent, as they are determined by the much lower thermal velocities of themore » particles of each jet. This paper describes some peculiar properties of intra-jet hydrodynamics in such a setting: the steepening of smooth perturbations and shock formation affected by the presence of the 'stiff' opposite flow; the role of a rapid electron heating in shock formation; ion heating by the intrajet shock. The latter effect can cause rapid ion heating which is consistent with recent counterstreaming jet experiments by Ross et al.[Phys. Plasmas 19, 056501 (2012)].« less

Kinetic theory of fermions in curved spacetime

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fidler, Christian; Pitrou, Cyril, E-mail: christian.fidler@uclouvain.be, E-mail: pitrou@iap.fr

We build a statistical description of fermions, taking into account the spin degree of freedom in addition to the momentum of particles, and we detail its use in the context of the kinetic theory of gases of fermions particles. We show that the one-particle distribution function needed to write a Liouville equation is a spinor valued operator. The degrees of freedom of this function are covariantly described by an intensity function and by a polarisation vector which are parallel transported by free streaming. Collisions are described on the microscopic level and lead to a Boltzmann equation for this operator. Wemore » apply our formalism to the case of weak interactions, which at low energies can be considered as a contact interaction between fermions, allowing us to discuss the structure of the collision term for a few typical weak-interaction mediated reactions. In particular we find for massive particles that a dipolar distribution of velocities in the interacting species is necessary to generate linear polarisation, as opposed to the case of photons for which linear polarisation is generated from the quadrupolar distribution of velocities.« less

Thermodynamic and Turbulence Characteristics of the Southern Great Plains Nocturnal Boundary Layer Under Differing Turbulent Regimes

NASA Astrophysics Data System (ADS)

Bonin, Timothy A.; Blumberg, William G.; Klein, Petra M.; Chilson, Phillip B.

2015-12-01

The nocturnal stable boundary layer (SBL) can generally be classified into the weakly stable boundary layer (wSBL) and very stable boundary layer (vSBL). Within the wSBL, turbulence is relatively continuous, whereas in the vSBL, turbulence is intermittent and not well characterized. Differentiating characteristics of each type of SBL are still unknown. Herein, thermodynamic and kinematic data collected by a suite of instruments in north central Oklahoma in autumn 2012 are analyzed to better understand both SBL regimes and their differentiating characteristics. Many low-level jets were observed during the experiment, as it took place near a climatological maximum. A threshold wind speed, above which bulk shear-generated turbulence develops, is found to exist up to 300 m. The threshold wind speed must also be exceeded at lower heights (down to the surface) in order for strong turbulence to develop. Composite profiles, which are normalized using low-level jet scaling, of potential temperature, wind speed, vertical velocity variance, and the third-order moment of vertical velocity (overline{w'^3}) are produced for weak and moderate/strong turbulence regimes, which exhibit features of the vSBL and wSBL, respectively. Within the wSBL, turbulence is generated at the surface and transported upward. In the vSBL, values of vertical velocity variance are small throughout the entire boundary layer, likely due to the fact that a strong surface inversion typically forms after sunset. The temperature profile tends to be approximately isothermal in the lowest portions of the wSBL, and it did not substantially change over the night. Within both types of SBL, stability in the residual layer tends to increase as the night progresses. It is thought that this stability increase is due to differential warm air advection, which frequently occurs in the southern Great Plains when southerly low-level jets and a typical north-south temperature gradient are present. Differential radiative flux divergence also contributes to this increase in stability.

Experiments on 1,000 km/s flyer acceleration and collisions

NASA Astrophysics Data System (ADS)

Karasik, Max; Weaver, J. L.; Aglitskiy, Y.; Kehne, D. M.; Zalesak, S. T.; Velikovich, A. L.; Oh, J.; Serlin, V.; Obenschain, S. P.

2012-10-01

We will present results from follow-on experiments to the record-high velocities achieved using the ultra-uniform deep-uv drive of the Nike KrF laser [Karasik et al, Phys. Plasmas 17, 056317 (2010)], in which highly accelerated planar foils of deuterated polystyrene were made to collide with a witness foil to produce ˜1 Gbar shock pressures and result in heating of matter to thermonuclear temperatures. Such velocities may indicate a path to lower minimum energy required for central ignition. Still higher velocities and higher target densities are required for impact fast ignition. New results give velocity of >1,100 km/s achieved through improvements in pulseshaping. Variation of second foil parameters results in significant change in fusion neutron production on impact. In-flight target density is inferred from target heating upon collision via DD neutron time-of-flight ion temperature measurement. Availability of pressures generated by collisions of such highly accelerated flyers may provide an experimental platform for study of matter at extreme conditions. Work is supported by US DOE (NNSA).

Different stages of collision zones on examples of Gujarat province (India) and Caucasus

NASA Astrophysics Data System (ADS)

Zabelina, Irina; Koulakov, Ivan; Ranjan Kayal, Jnana; Pratap Singh, Ajay; Kumar, Santosh; Kukarina, Ekaterina; Amanatashvili, Iason

2016-04-01

In this study we present seismic structures of the crust and upper mantle beneath two regions: Kachchh Gujarat region (India), and Caucasus that may represent different stages of the collisional processes. In both cases, the 3D seismic models were obtained based on tomography inversion of arrival times of P and S seismic waves from local and regional earthquakes. Collisional processes in the Caucasus region began 35 million years ago with the closure of the Tethys Ocean, and continues to this day. The rate of shortening between the Scythian and the Arabian plate is currently 1-2.2 mm/year. The tomography inversion used the dataset provided by several seismic agencies of the Caucasus region that contained 23,071 P- and 21,598 S-picks from 1374 events. The obtained P and S velocity models clearly delineate major tectonic units in the study area. A high velocity anomaly in Transcaucasian separating the Great and Lesser Caucasus possibly represents a rigid crustal block corresponding to the remnant oceanic lithosphere of Tethys. Another high-velocity pattern coincides with the southern edge of the Scythian Plate. Strongly deformed areas of Great and Lesser Caucasus are mostly associated with low-velocity patterns representing thickened felsic part of the crust and strong fracturing of rocks. Most Cenozoic volcanic centers of Caucasus match to the low-velocity seismic anomalies in the crust. We propose that the mantle part of the Arabian and Eurasian Plates has been delaminated due to the continental collision in the Caucasus region. As a result, overheated asthenosphere appeared nearly the bottom of the crust and facilitated melting of the crustal material that caused the origin of recent volcanism in Great and Lesser Caucasus. The Kachchh province, in contrast to the Caucasus, is far from any boundaries of major lithospheric plates. However, this area is one of the most seismically active in India. It is suggested that it may be a site of the lithosphere rupture and initiation of a new collision zone. For the tomography inversion of the Kachchh region we selected the data of 4105 earthquakes with arrival times 29660 P and 30278 S waves. Based on the obtained seismic anomalies, we identify the left-lateral displacement to approximately 70 km along a hidden fault. We suggest that this fault can be associated with a series of ridges having the SW-NE direction, which are clearly seen on the bathymetry of the Indian Ocean bottom. Northwards displacement of the Indo-Australian Plate and contraction with Asia causes strong compression deformations in the broad areas of the Indian Plate. The curved geometry of the western boundary of the Indo-Australian plate and orientations of the fracture zones presume both shear and compressional displacements along faults. The presence of both thrust and strike-slip mechanisms of earthquakes in the Kachchh province may support the existence of such combined deformations leading to initiation of a new collision belt.

Study on collision resistance characteristics of the side tanks with water inside

NASA Astrophysics Data System (ADS)

Liu, Yuxi; Hu, Jinwen; Liu, Ting; Wu, Can

2018-05-01

When we evaluate the safety performance of ships against external events, one of the most important indicator is the collision resistance to which water inside the side tanks also make some contributions because of the water effect. To further analyze the interaction mechanism, different collision velocities and side tank waterlines are set for the analysis model. Results indicate the outside shell and the inner shell of the side structure significantly enhanced the collision resistance performance to a certain extension. The water effect on the failure of the outside shell is unobvious, while, it performs a great influence on the destructive reaction force of the inner shell. When the velocity of the coming bulbous bow gradually increases, the destructive reaction forces of the outside shell and the inner shell increase with a decreasing rate. Besides, water influence the collision characteristics of the inner shell a lot when the waterlines are below the upper rib of the strong frame.

Collision detection for spacecraft proximity operations

NASA Technical Reports Server (NTRS)

Vaughan, Robin M.; Bergmann, Edward V.; Walker, Bruce K.

1991-01-01

A new collision detection algorithm has been developed for use when two spacecraft are operating in the same vicinity. The two spacecraft are modeled as unions of convex polyhedra, where the resulting polyhedron many be either convex or nonconvex. The relative motion of the two spacecraft is assumed to be such that one vehicle is moving with constant linear and angular velocity with respect to the other. Contacts between the vertices, faces, and edges of the polyhedra representing the two spacecraft are shown to occur when the value of one or more of a set of functions is zero. The collision detection algorithm is then formulated as a search for the zeros (roots) of these functions. Special properties of the functions for the assumed relative trajectory are exploited to expedite the zero search. The new algorithm is the first algorithm that can solve the collision detection problem exactly for relative motion with constant angular velocity. This is a significant improvement over models of rotational motion used in previous collision detection algorithms.

A fast conservative spectral solver for the nonlinear Boltzmann collision operator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gamba, Irene M.; Haack, Jeffrey R.; Hu, Jingwei

2014-12-09

We present a conservative spectral method for the fully nonlinear Boltzmann collision operator based on the weighted convolution structure in Fourier space developed by Gamba and Tharkabhushnanam. This method can simulate a broad class of collisions, including both elastic and inelastic collisions as well as angularly dependent cross sections in which grazing collisions play a major role. The extension presented in this paper consists of factorizing the convolution weight on quadrature points by exploiting the symmetric nature of the particle interaction law, which reduces the computational cost and memory requirements of the method to O(M{sup 2}N{sup 4}logN) from the O(N{supmore » 6}) complexity of the original spectral method, where N is the number of velocity grid points in each velocity dimension and M is the number of quadrature points in the factorization, which can be taken to be much smaller than N. We present preliminary numerical results.« less

Boundary Between Stable and Unstable Regimes of Accretion

NASA Astrophysics Data System (ADS)

Blinova, A. A.; Lovelace, R. V. E.; Romanova, M. M.

2014-01-01

We investigated the boundary between stable and unstable regimes of accretion and its dependence on different parameters. Simulations were performed using a "cubed sphere" code with high grid resolution (244 grid points in the azimuthal direction), which is twice as high as that used in our earlier studies. We chose a very low viscosity value, with alpha-parameter α=0.02. We observed from the simulations that the boundary strongly depends on the ratio between magnetospheric radius rm (where the magnetic stress in the magnetosphere matches the matter stress in the disk) and corotation radius rcor (where the Keplerian velocity in the disk is equal to the angular velocity of the star). For a small misalignment angle of the dipole field, Θ = 5°, accretion is unstable if rcor/rm> 1.35, and is stable otherwise. In cases of a larger misalignment angle of the dipole, Θ = 20°, instability occurs at slightly larger values, rcor/rm> 1.41

Fermi acceleration in time-dependent billiards: theory of the velocity diffusion in conformally breathing fully chaotic billiards

NASA Astrophysics Data System (ADS)

Batistić, Benjamin; Robnik, Marko

2011-09-01

We study aspects of the Fermi acceleration (the unbounded growth of the energy) in a certain class of time-dependent 2D billiards. Specifically, we look at the conformally breathing billiards (periodic oscillation of the boundary which preserves the shape of the billiard at all times), which are fully chaotic as static (frozen) billiards, and we show that for large velocities around v0 and for not too long times, we observe just normal diffusion of the velocity as a function of the physical (continuous) time, around v0. However, the diffusion is not homogeneous, as the diffusion constant D depends on v0 as a power law D∝1/v30. Taking this into account, we show that to the leading order the average velocity v(n) as a function of the number of collisions n obeys a power law v∝n1/6 thus, the Fermi acceleration exponent is β = 1/6, which is in excellent agreement with the numerical calculations of the fully chaotic oval billiard, the Sinai billiard and the cardioid billiard. The error of the velocity estimates is of the order 1/v2. Thus, the higher the velocity, the better our analytic approximation. Moreover, we derive the underlying universal equation of the velocity dynamics of the time-dependent conformally breathing billiards, correct up to and including the order 1/v in the regime of the large velocity of the particle v. This universal equation does not depend on the dynamical properties of the system (integrability, ergodicity, chaoticity). We present the results of the numerical simulations for three billiards in complete agreement with the theory. We believe that this is a first step towards theoretical understanding of the power law growth and the Fermi acceleration exponents in 2D billiards, although our theory is so far specialized to the conformally breathing fully chaotic billiards.

Applied Impact Physics Research

NASA Astrophysics Data System (ADS)

Wickert, Matthias

2013-06-01

Applied impact physics research is based on the capability to examine impact processes for a wide range of impact conditions with respect to velocity as well as mass and shape of the projectile. For this reason, Fraunhofer EMI operates a large variety of launchers that address velocities up to ordnance velocities as single stage powder gun but which can also be operated as two-stage light gas guns achieving the regime of low earth orbital velocity. Thereby for projectile masses of up to 100 g hypervelocity impact phenomena up to 7.8 km/s can be addressed. Advanced optical diagnostic techniques like microsecond video are used as commercial systems but - since impact phenomena are mostly related with debris or dust - specialized diagnostics are developed in-house like x-ray cinematography and x-ray tomography. Selected topics of the field of applied impact physics will be presented like the interesting behavior of long rods penetrating low-density materials or experimental findings at hypervelocity for this class of materials as well as new x-ray diagnositic techniques.

Many-body effects and ultraviolet renormalization in three-dimensional Dirac materials

NASA Astrophysics Data System (ADS)

Throckmorton, Robert E.; Hofmann, Johannes; Barnes, Edwin; Das Sarma, S.

2015-09-01

We develop a theory for electron-electron interaction-induced many-body effects in three-dimensional Weyl or Dirac semimetals, including interaction corrections to the polarizability, electron self-energy, and vertex function, up to second order in the effective fine-structure constant of the Dirac material. These results are used to derive the higher-order ultraviolet renormalization of the Fermi velocity, effective coupling, and quasiparticle residue, revealing that the corrections to the renormalization group flows of both the velocity and coupling counteract the leading-order tendencies of velocity enhancement and coupling suppression at low energies. This in turn leads to the emergence of a critical coupling above which the interaction strength grows with decreasing energy scale. In addition, we identify a range of coupling strengths below the critical point in which the Fermi velocity varies nonmonotonically as the low-energy, noninteracting fixed point is approached. Furthermore, we find that while the higher-order correction to the flow of the coupling is generally small compared to the leading order, the corresponding correction to the velocity flow carries an additional factor of the Dirac cone flavor number (the multiplicity of electron species, e.g. ground-state valley degeneracy arising from the band structure) relative to the leading-order result. Thus, for materials with a larger multiplicity, the regime of velocity nonmonotonicity is reached for modest values of the coupling strength. This is in stark contrast to an approach based on a large-N expansion or the random phase approximation (RPA), where higher-order corrections are strongly suppressed for larger values of the Dirac cone multiplicity. This suggests that perturbation theory in the coupling constant (i.e., the loop expansion) and the RPA/large-N expansion are complementary in the sense that they are applicable in different parameter regimes of the theory. We show how our results for the ultraviolet renormalization of quasiparticle properties can be tested experimentally through measurements of quantities such as the optical conductivity or dielectric function (with carrier density or temperature acting as the scale being varied to induce the running coupling). Although experiments typically access the finite-density regime, we show that our zero-density results still capture clear many-body signatures that should be visible at higher temperatures even in real systems with disorder and finite doping.

Modelling the inner debris disc of HR 8799

NASA Astrophysics Data System (ADS)

Contro, B.; Horner, J.; Wittenmyer, R. A.; Marshall, J. P.; Hinse, T. C.

2016-11-01

In many ways, the HR 8799 planetary system strongly resembles our own. It features four giant planets and two debris belts, analogues to the Asteroid and Edgeworth-Kuiper belts. Here, we present the results of dynamical simulations of HR8799's inner debris belt, to study its structure and collisional environment. Our results suggest that HR 8799's inner belt is highly structured, with gaps between regions of dynamical stability. The belt is likely constrained between sharp inner and outer edges, located at ˜6 and ˜8 au, respectively. Its inner edge coincides with a broad gap cleared by the 4:1 mean-motion resonance with HR 8799e. Within the belt, planetesimals are undergoing a process of collisional attrition like that observed in the Asteroid belt. However, whilst the mean collision velocity in the Asteroid belt exceeds 5 km s-1, the majority of collisions within HR 8799's inner belt occur with velocities of order 1.2 km s-1, or less. Despite this, they remain sufficiently energetic to be destructive - giving a source for the warm dust detected in the system. Interior to the inner belt, test particles remain dynamically unstirred, aside from narrow bands excited by distant high-order resonances with HR 8799e. This lack of stirring is consistent with earlier thermal modelling of HR 8799's infrared excess, which predicted little dust inside 6 au. The inner system is sufficiently stable and unstirred that the formation of telluric planets is feasible, although such planets would doubtless be subject to a punitive impact regime, given the intense collisional grinding required in the inner belt to generate the observed infrared excess.

Current collection from an unmagnetized plasma: A tutorial

NASA Technical Reports Server (NTRS)

Whipple, Elden C.

1990-01-01

The current collected by a body in an unmagnetized plasma depends in general on: (1) the properties of the plasma; (2) the properties of the body; and (3) the properties of any neutral species that are present. The important plasma properties are the velocity distributions of the plasma particles at a location remote from the body (at infinity), and the Debye length which determines the importance of plasma space charge effects. The important body properties are its surface characteristics, namely the conductivity and secondary yield coefficients. The neutral species affect the current through collisions which impede the flow of current and possibly through ionization of the neutrals which can enhance the current. The technique for calculating the current collected by a body in a plasma is reviewed with special attention given to the distinction between orbit limited and space charge limited regimes, the asymptotic variation of the potential with distance from a body, and the concept of a sheath.

Nonlinear chiral plasma transport in rotating coordinates

NASA Astrophysics Data System (ADS)

Dayi, Ömer F.; Kilinçarslan, Eda

2017-08-01

The nonlinear transport features of inhomogeneous chiral plasma in the presence of electromagnetic fields, in rotating coordinates are studied within the relaxation time approach. The chiral distribution functions up to second order in the electric field in rotating coordinates and the derivatives of chemical potentials are established by solving the Boltzmann transport equation. First, the vector and axial current densities in the weakly ionized chiral plasma for vanishing magnetic field are calculated. They involve the rotational analogues of the Hall effect as well as several new terms arising from the Coriolis and fictitious centrifugal forces. Then in the short relaxation time regime the angular velocity and electromagnetic fields are treated as perturbations. The current densities are obtained by retaining the terms up to second order in perturbations. The time evolution equations of the inhomogeneous chemical potentials are derived by demanding that collisions conserve the particle number densities.

New Insights on Coastal Foredune Growth: The Relative Contributions of Marine and Aeolian Processes

NASA Astrophysics Data System (ADS)

Cohn, Nicholas; Ruggiero, Peter; de Vries, Sierd; Kaminsky, George M.

2018-05-01

Coastal foredune growth is typically associated with aeolian sediment transport processes, while foredune erosion is associated with destructive marine processes. New data sets collected at a high energy, dissipative beach suggest that total water levels in the collision regime can cause dunes to accrete—requiring a paradigm shift away from considering collisional wave impacts as unconditionally erosional. From morphologic change data sets, it is estimated that marine processes explain between 9% and 38% of annual dune growth with aeolian processes accounting for the remaining 62% to 91%. The largest wind-driven dune growth occurs during the winter, in response to high wind velocities, but out of phase with summertime beach growth via intertidal sandbar welding. The lack of synchronization between maximum beach sediment supply and wind-driven dune growth indicates that aeolian transport at this site is primarily transport, rather than supply, limited, likely due to a lack of fetch limitations.

Rarefied gas flow through two-dimensional nozzles

NASA Technical Reports Server (NTRS)

De Witt, Kenneth J.; Jeng, Duen-Ren; Keith, Theo G., Jr.; Chung, Chan-Hong

1989-01-01

A kinetic theory analysis is made of the flow of a rarefied gas from one reservoir to another through two-dimensional nozzles with arbitrary curvature. The Boltzmann equation simplified by a model collision integral is solved by means of finite-difference approximations with the discrete ordinate method. The physical space is transformed by a general grid generation technique and the velocity space is transformed to a polar coordinate system. A numerical code is developed which can be applied to any two-dimensional passage of complicated geometry for the flow regimes from free-molecular to slip. Numerical values of flow quantities can be calculated for the entire physical space including both inside the nozzle and in the outside plume. Predictions are made for the case of parallel slots and compared with existing literature data. Also, results for the cases of convergent or divergent slots and two-dimensional nozzles with arbitrary curvature at arbitrary knudsen number are presented.

Diffusion of test particles in stochastic magnetic fields for small Kubo numbers.

PubMed

Neuer, Marcus; Spatschek, Karl H

2006-02-01

Motion of charged particles in a collisional plasma with stochastic magnetic field lines is investigated on the basis of the so-called A-Langevin equation. Compared to the previously used A-Langevin model, here finite Larmor radius effects are taken into account. The A-Langevin equation is solved under the assumption that the Lagrangian correlation function for the magnetic field fluctuations is related to the Eulerian correlation function (in Gaussian form) via the Corrsin approximation. The latter is justified for small Kubo numbers. The velocity correlation function, being averaged with respect to the stochastic variables including collisions, leads to an implicit differential equation for the mean square displacement. From the latter, different transport regimes, including the well-known Rechester-Rosenbluth diffusion coefficient, are derived. Finite Larmor radius contributions show a decrease of the diffusion coefficient compared to the guiding center limit. The case of small (or vanishing) mean fields is also discussed.

Micropropulsion devices based on molecular acceleration by pulsed optical lattices

NASA Astrophysics Data System (ADS)

Shneider, Mikhail N.; Gimelshein, Sergey F.; Barker, Peter F.

2006-03-01

The ability of a traveling periodic optical potential to increase the thrust and specific impulse of microthrusters is investigated. Two flow regimes, high density and low density, are considered. The thrust from a micronozzle, with a stagnation pressure of 1 atm and temperature of 300 K, can be increased by more than an order of magnitude. These conditions can be achieved for a constant velocity lattice, produced by two near counterpropagating optical fields that are focused into the nozzle throat. A propulsion system that operates in low-density regime and is driven by molecules trapped by an accelerating optical lattice is proposed. It is shown that such a system has a potential to achieve a specific impulse of thousands of seconds.

Dynamics of mode-coupling-induced microresonator frequency combs in normal dispersion

NASA Astrophysics Data System (ADS)

Jang, Jae K.; Okawachi, Yoshitomo; Yu, Mengjie; Luke, Kevin; Ji, Xingchen; Lipson, Michal; Gaeta, Alexander L.

2016-12-01

We experimentally and theoretically investigate the dynamics of microresonator-based frequency comb generation assisted by mode coupling in the normal group-velocity dispersion (GVD) regime. We show that mode coupling can initiate intracavity modulation instability (MI) by directly perturbing the pump-resonance mode. We also observe the formation of a low-noise comb as the pump frequency is tuned further into resonance from the MI point. We determine the phase-matching conditions that accurately predict all the essential features of the MI and comb spectra, and extend the existing analogy between mode coupling and high-order dispersion to the normal GVD regime. We discuss the applicability of our analysis to the possibility of broadband comb generation in the normal GVD regime.

Zero-G experiments in two-phase fluids flow regimes

NASA Technical Reports Server (NTRS)

Heppner, D. B.; King, C. D.; Littles, J. W.

1975-01-01

The two-phase flows studied were liquid and gas mixtures in a straight flow channel of circular cross-section. Boundaries between flow regimes have been defined for normogravity on coordinates of gas quality and total mass velocity; and, when combined with boundary expressions having a Froude number term, an analytical model was derived predicting boundary shifts with changes in gravity level. Experiments with air and water were performed, first in the normogravity environment of a ground laboratory and then in 'zero gravity' aboard a KC-135 aircraft flying parabolic trajectories. Data reduction confirmed regime boundary shifts in the direction predicted, although the magnitude was a little less than predicted. Pressure drop measurements showed significant increases for the low gravity condition.

Influence of the Metal Volume Fraction on the maximum deflection and impact load of GLARE plates subjected to low velocity impact

NASA Astrophysics Data System (ADS)

Bikakis, GSE; Savaidis, A.; Zalimidis, P.; Tsitos, S.

2016-11-01

Fiber-metal laminates are hybrid composite materials, consisting of alternating metal layers bonded to fiber-reinforced prepreg layers. GLARE (GLAss REinforced) belongs to this new family of materials. GLARE is the most successful fiber-metal laminate up to now and is currently being used for the construction of primary aerospace structures, such as the fuselage of the Airbus A380 air plane. Impact properties are very important in aerospace structures, since impact damage is caused by various sources, such as maintenance damage from dropped tools, collision between service cars or cargo and the structure, bird strikes and hail. The principal objective of this article is to evaluate the influence of the Metal Volume Fraction (MVF) on the low velocity impact response of GLARE fiber-metal laminates. Previously published differential equations of motion are employed for this purpose. The low velocity impact behavior of various circular GLARE plates is predicted and characteristic values of impact variables, which represent the impact phenomenon, are evaluated versus the corresponding MVF of the examined GLARE material grades. The considered GLARE plates are subjected to low velocity impact under identical impact conditions. A strong effect of the MVF on the maximum impact load and a significant effect on the maximum plate deflection of GLARE plates has been found.

Influence of the Metal Volume Fraction on the permanent dent depth and energy absorption of GLARE plates subjected to low velocity impact

NASA Astrophysics Data System (ADS)

Bikakis, GSE; Savaidis, A.; Zalimidis, P.; Tsitos, S.

2016-11-01

Fiber-metal laminates are hybrid composite materials, consisting of alternating metal layers bonded to fiber-reinforced prepreg layers. GLARE (GLAss REinforced) belongs to this new family of materials. GLARE is the most successful fiber-metal laminate up to now and is currently being used for the construction of primary aerospace structures, such as the fuselage of the Airbus A380 air plane. Impact properties are very important in aerospace structures, since impact damage is caused by various sources, such as maintenance damage from dropped tools, collision between service cars or cargo and the structure, bird strikes and hail. The principal objective of this article is to evaluate the influence of the Metal Volume Fraction (MVF) on the low velocity impact response of GLARE fiber-metal laminates. Previously published differential equations of motion are employed for this purpose. The low velocity impact behavior of various circular GLARE plates is predicted and characteristic values of impact variables, which represent the impact phenomenon, are evaluated versus the corresponding MVF of the examined GLARE material grades. The considered GLARE plates are subjected to low velocity impact under identical impact conditions. A strong effect of the MVF on the maximum impact load and a significant effect on the maximum plate deflection of GLARE plates has been found.

Information entropy method and the description of echo hologram formation in gaseous media

NASA Astrophysics Data System (ADS)

Garnaeva, G. I.; Nefediev, L. A.; Akhmedshina, E. N.

2018-02-01

The effect of collisions with a change in velocity of gas particles, on the value of information entropy, is associated with the spectral structure of the echo hologram’s response, where its temporal form is considered. It is shown that collisions with a change in gas particle velocity increase the ‘parasitical’ information, on the background of which the information contained in the temporary shape of the object laser pulse is lost.

GNSS/Electronic Compass/Road Segment Information Fusion for Vehicle-to-Vehicle Collision Avoidance Application

PubMed Central

Cheng, Qi; Xue, Dabin; Wang, Guanyu; Ochieng, Washington Yotto

2017-01-01

The increasing number of vehicles in modern cities brings the problem of increasing crashes. One of the applications or services of Intelligent Transportation Systems (ITS) conceived to improve safety and reduce congestion is collision avoidance. This safety critical application requires sub-meter level vehicle state estimation accuracy with very high integrity, continuity and availability, to detect an impending collision and issue a warning or intervene in the case that the warning is not heeded. Because of the challenging city environment, to date there is no approved method capable of delivering this high level of performance in vehicle state estimation. In particular, the current Global Navigation Satellite System (GNSS) based collision avoidance systems have the major limitation that the real-time accuracy of dynamic state estimation deteriorates during abrupt acceleration and deceleration situations, compromising the integrity of collision avoidance. Therefore, to provide the Required Navigation Performance (RNP) for collision avoidance, this paper proposes a novel Particle Filter (PF) based model for the integration or fusion of real-time kinematic (RTK) GNSS position solutions with electronic compass and road segment data used in conjunction with an Autoregressive (AR) motion model. The real-time vehicle state estimates are used together with distance based collision avoidance algorithms to predict potential collisions. The algorithms are tested by simulation and in the field representing a low density urban environment. The results show that the proposed algorithm meets the horizontal positioning accuracy requirement for collision avoidance and is superior to positioning accuracy of GNSS only, traditional Constant Velocity (CV) and Constant Acceleration (CA) based motion models, with a significant improvement in the prediction accuracy of potential collision. PMID:29186851

GNSS/Electronic Compass/Road Segment Information Fusion for Vehicle-to-Vehicle Collision Avoidance Application.

PubMed

Sun, Rui; Cheng, Qi; Xue, Dabin; Wang, Guanyu; Ochieng, Washington Yotto

2017-11-25

The increasing number of vehicles in modern cities brings the problem of increasing crashes. One of the applications or services of Intelligent Transportation Systems (ITS) conceived to improve safety and reduce congestion is collision avoidance. This safety critical application requires sub-meter level vehicle state estimation accuracy with very high integrity, continuity and availability, to detect an impending collision and issue a warning or intervene in the case that the warning is not heeded. Because of the challenging city environment, to date there is no approved method capable of delivering this high level of performance in vehicle state estimation. In particular, the current Global Navigation Satellite System (GNSS) based collision avoidance systems have the major limitation that the real-time accuracy of dynamic state estimation deteriorates during abrupt acceleration and deceleration situations, compromising the integrity of collision avoidance. Therefore, to provide the Required Navigation Performance (RNP) for collision avoidance, this paper proposes a novel Particle Filter (PF) based model for the integration or fusion of real-time kinematic (RTK) GNSS position solutions with electronic compass and road segment data used in conjunction with an Autoregressive (AR) motion model. The real-time vehicle state estimates are used together with distance based collision avoidance algorithms to predict potential collisions. The algorithms are tested by simulation and in the field representing a low density urban environment. The results show that the proposed algorithm meets the horizontal positioning accuracy requirement for collision avoidance and is superior to positioning accuracy of GNSS only, traditional Constant Velocity (CV) and Constant Acceleration (CA) based motion models, with a significant improvement in the prediction accuracy of potential collision.

Mass Transfer and Rheology of Fiber Suspensions

NASA Astrophysics Data System (ADS)

Wang, Jianghui

Rheological and mass transfer properties of non-Brownian fiber suspensions are affected by fiber characteristics, fiber interactions, and processing conditions. In this thesis we develop several simulation methods to study the dynamics of single fibers in simple shear flow, as well as the rheology and mass transfer of fiber suspensions. Isolated, rigid, neutrally-buoyant, non-Brownian, slightly curved, nonchiral fibers in simple shear flow of an incompressible Newtonian fluid at low Reynolds number can drift steadily in the gradient direction without external forces or torques. The average drift velocity and direction depend on the fiber aspect ratio, curvature and initial orientation. The drift results from the coupling of rotational and translational dynamics, and the combined effects of flipping, scooping, and spinning motions of the fiber. Irreversible fiber collisions in the suspensions cause shear-induced diffusion. The shear-induced self-diffusivity of dilute suspensions of fibers increases with increasing concentration and increasing static friction between contacts. The diffusivities in both the gradient and vorticity directions are larger for suspensions of curved fibers than for suspensions of straight fibers. For suspensions of curved fibers, significant enhancements in the diffusivity in the gradient direction are attributed to fiber drift in the gradient direction. The shear-induced self-diffusivity of concentrated suspensions of fibers increases with increasing concentration before fiber networks or flocs are formed, after which the diffusivity decreases with increasing concentration. The diffusivity increases with increasing fiber equilibrium bending angle, effective stiffness, coefficient of static friction, and rate of collisions. The specific viscosity of fiber suspensions increases with increasing fiber curvature, friction coefficient between mechanical contacts, and solids concentration. The specific viscosity increases linearly with concentration in the dilute regime, and increases with the cube of the concentration in the semi-dilute regime. Concentrated fiber suspensions are highly viscous, shear thinning, and exhibit significant yield stresses and normal stress differences. Yield stresses scale with volume concentration and fiber aspect ratio in the same way as that observed in experiments. The first normal stress difference increases linearly with shear rate. The shear-induced diffusivity increases linearly with the derivative of the particle contribution to stress for dilute suspensions with respective to concentration. This correlation between rheology and shear-induced diffusion makes it possible to predict diffusivity from easily measured rheological properties.

Rotationally inelastic collisions of excited NaK and NaCs molecules with noble gas and alkali atom perturbers.

PubMed

Jones, J; Richter, K; Price, T J; Ross, A J; Crozet, P; Faust, C; Malenda, R F; Carlus, S; Hickman, A P; Huennekens, J

2017-10-14

We report measurements of rate coefficients at T ≈ 600 K for rotationally inelastic collisions of NaK molecules in the 2(A) 1 Σ + electronic state with helium, argon, and potassium atom perturbers. Several initial rotational levels J between 14 and 44 were investigated. Collisions involving molecules in low-lying vibrational levels (v = 0, 1, and 2) of the 2(A) 1 Σ + state were studied using Fourier-transform spectroscopy. Collisions involving molecules in a higher vibrational level, v = 16, were studied using pump/probe, optical-optical double resonance spectroscopy. In addition, polarization spectroscopy measurements were carried out to study the transfer of orientation in these collisions. Many, but not all, of the measurements were carried out in the "single-collision regime" where more than one collision is unlikely to occur within the lifetime of the excited molecule. The analysis of the experimental data, which is described in detail, includes an estimate of effects of multiple collisions on the reported rate coefficients. The most significant result of these experiments is the observation of a strong propensity for ΔJ = even transitions in collisions involving either helium or argon atoms; the propensity is much stronger for helium than for argon. For the initial rotational levels studied experimentally, almost all initial orientation is preserved in collisions of NaK 2(A) 1 Σ + molecules with helium. Roughly between 1/3 and 2/3 of the orientation is preserved in collisions with argon, and almost all orientation is destroyed in collisions with potassium atoms. Complementary measurements on rotationally inelastic collisions of NaCs 2(A) 1 Σ + with argon do not show a ΔJ = even propensity. The experimental results are compared with new theoretical calculations of collisions of NaK 2(A) 1 Σ + with helium and argon. The calculations are in good agreement with the absolute magnitudes of the experimentally determined rate coefficients and accurately reproduce the very strong propensity for ΔJ = even transitions in helium collisions and the less strong propensity for ΔJ = even transitions in argon collisions. The calculations also show that collisions with helium are less likely to destroy orientation than collisions with argon, in agreement with the experimental results.

Rotationally inelastic collisions of excited NaK and NaCs molecules with noble gas and alkali atom perturbers

NASA Astrophysics Data System (ADS)

Jones, J.; Richter, K.; Price, T. J.; Ross, A. J.; Crozet, P.; Faust, C.; Malenda, R. F.; Carlus, S.; Hickman, A. P.; Huennekens, J.

2017-10-01

We report measurements of rate coefficients at T ≈ 600 K for rotationally inelastic collisions of NaK molecules in the 2(A)1Σ+ electronic state with helium, argon, and potassium atom perturbers. Several initial rotational levels J between 14 and 44 were investigated. Collisions involving molecules in low-lying vibrational levels (v = 0, 1, and 2) of the 2(A)1Σ+ state were studied using Fourier-transform spectroscopy. Collisions involving molecules in a higher vibrational level, v = 16, were studied using pump/probe, optical-optical double resonance spectroscopy. In addition, polarization spectroscopy measurements were carried out to study the transfer of orientation in these collisions. Many, but not all, of the measurements were carried out in the "single-collision regime" where more than one collision is unlikely to occur within the lifetime of the excited molecule. The analysis of the experimental data, which is described in detail, includes an estimate of effects of multiple collisions on the reported rate coefficients. The most significant result of these experiments is the observation of a strong propensity for ΔJ = even transitions in collisions involving either helium or argon atoms; the propensity is much stronger for helium than for argon. For the initial rotational levels studied experimentally, almost all initial orientation is preserved in collisions of NaK 2(A)1Σ+ molecules with helium. Roughly between 1/3 and 2/3 of the orientation is preserved in collisions with argon, and almost all orientation is destroyed in collisions with potassium atoms. Complementary measurements on rotationally inelastic collisions of NaCs 2(A)1Σ+ with argon do not show a ΔJ = even propensity. The experimental results are compared with new theoretical calculations of collisions of NaK 2(A)1Σ+ with helium and argon. The calculations are in good agreement with the absolute magnitudes of the experimentally determined rate coefficients and accurately reproduce the very strong propensity for ΔJ = even transitions in helium collisions and the less strong propensity for ΔJ = even transitions in argon collisions. The calculations also show that collisions with helium are less likely to destroy orientation than collisions with argon, in agreement with the experimental results.

Mantle Discontinuities under Iranian Plateau and Turan Shield from the Modeling of Seismic Triplications

NASA Astrophysics Data System (ADS)

Tseng, Tai-Lin; Chi, Hui-Ching; Huang, Bor-Shouh; Godoladze, Tea; Javakhishvili, Zurab; Karakhanyan, Arkadi

2015-04-01

Recent studies of seismic tomography show velocity anomalies in the mantle transition zone (TZ) under Zagros and Iranian Plateau, which are created by active collision between Africa and Eurasia. Remnants of Neo-Tethys slab that subducted before the collision might had experienced a break-off and likely be rested in the deep mantle. In this study, we utilize triplicate arrivals of high-resolution P waveforms to investigate the velocity structure of mantle beneath this continental collision zone and the surroundings. By combining several broadband arrays in eastern Turkey and Caucasus, we construct a fan of profiles, each about 800 km long, which consist of triplicate waveforms generated from the 410- and 660-km discontinuities. The method is particularly sensitive to the size of the velocity contrast for the sampled regions, including the central Iranian Plateau, Turan shield and part of South Caspian basin. Our results show that the lower TZ under the stable Turan shield is fast. The corresponding 660-km contrast is about 4.5% only, smaller than the value in global average model IASP91, but fairly close to that under the northern Indian shield in Precambrian age. For profiles sampling Iran, we observe azimuthal changes in the waveforms which require further data division or grouping. The preliminary analysis suggests that the velocity near the bottom of the TZ is comparable to model appropriate for Turan and probably has a slightly shallower 660-km discontinuity. We hope the comparisons between velocity structures under different terranes can improve our understandings to the lithosphere-mantle dynamics under the process of continental collision.

Visual guidance of forward flight in hummingbirds reveals control based on image features instead of pattern velocity.

PubMed

Dakin, Roslyn; Fellows, Tyee K; Altshuler, Douglas L

2016-08-02

Information about self-motion and obstacles in the environment is encoded by optic flow, the movement of images on the eye. Decades of research have revealed that flying insects control speed, altitude, and trajectory by a simple strategy of maintaining or balancing the translational velocity of images on the eyes, known as pattern velocity. It has been proposed that birds may use a similar algorithm but this hypothesis has not been tested directly. We examined the influence of pattern velocity on avian flight by manipulating the motion of patterns on the walls of a tunnel traversed by Anna's hummingbirds. Contrary to prediction, we found that lateral course control is not based on regulating nasal-to-temporal pattern velocity. Instead, birds closely monitored feature height in the vertical axis, and steered away from taller features even in the absence of nasal-to-temporal pattern velocity cues. For vertical course control, we observed that birds adjusted their flight altitude in response to upward motion of the horizontal plane, which simulates vertical descent. Collectively, our results suggest that birds avoid collisions using visual cues in the vertical axis. Specifically, we propose that birds monitor the vertical extent of features in the lateral visual field to assess distances to the side, and vertical pattern velocity to avoid collisions with the ground. These distinct strategies may derive from greater need to avoid collisions in birds, compared with small insects.

Charmonium interaction in nuclear matter at FAIR

NASA Astrophysics Data System (ADS)

Pratim Bhaduri, Partha; Deveaux, Michael; Toia, Alberica

2018-05-01

We have studied the dissociation of J/ψ mesons in low energy proton-nucleus (p + A) collisions in the energy range of the future SIS100 accelerator at Facility for Anti-proton and Ion Research (FAIR). According to the results of our calculations, various scenarios of J/ψ absorption in nuclear matter show very distinct suppression patterns in the kinematic regime to be probed at FAIR. This suggests that the SIS100 energies are particularly suited to shed light on the issue of interaction of J/ψ resonance in nuclear medium.

Complexified boost invariance and holographic heavy ion collisions

DOE PAGES

Gubser, Steven S.; van der Schee, Wilke

2015-01-08

At strong coupling holographic studies have shown that heavy ion collisions do not obey normal boost invariance. Here we study a modified boost invariance through a complex shift in time, and show that this leads to surprisingly good agreement with numerical holographic computations. When including perturbations the agreement becomes even better, both in the hydrodynamic and the far-from-equilibrium regime. Finally, one of the main advantages is an analytic formulation of the stress-energy tensor of the longitudinal dynamics of holographic heavy ion collisions.

Experimental Study on the Flow Regimes and Pressure Gradients of Air-Oil-Water Three-Phase Flow in Horizontal Pipes

PubMed Central

Al-Hadhrami, Luai M.; Shaahid, S. M.; Tunde, Lukman O.; Al-Sarkhi, A.

2014-01-01

An experimental investigation has been carried out to study the flow regimes and pressure gradients of air-oil-water three-phase flows in 2.25 ID horizontal pipe at different flow conditions. The effects of water cuts, liquid and gas velocities on flow patterns and pressure gradients have been studied. The experiments have been conducted at 20°C using low viscosity Safrasol D80 oil, tap water and air. Superficial water and oil velocities were varied from 0.3 m/s to 3 m/s and air velocity varied from 0.29 m/s to 52.5 m/s to cover wide range of flow patterns. The experiments were performed for 10% to 90% water cuts. The flow patterns were observed and recorded using high speed video camera while the pressure drops were measured using pressure transducers and U-tube manometers. The flow patterns show strong dependence on water fraction, gas velocities, and liquid velocities. The observed flow patterns are stratified (smooth and wavy), elongated bubble, slug, dispersed bubble, and annular flow patterns. The pressure gradients have been found to increase with the increase in gas flow rates. Also, for a given superficial gas velocity, the pressure gradients increased with the increase in the superficial liquid velocity. The pressure gradient first increases and then decreases with increasing water cut. In general, phase inversion was observed with increase in the water cut. The experimental results have been compared with the existing unified Model and a good agreement has been noticed. PMID:24523645

Experimental study on the flow regimes and pressure gradients of air-oil-water three-phase flow in horizontal pipes.

PubMed

Al-Hadhrami, Luai M; Shaahid, S M; Tunde, Lukman O; Al-Sarkhi, A

2014-01-01

An experimental investigation has been carried out to study the flow regimes and pressure gradients of air-oil-water three-phase flows in 2.25 ID horizontal pipe at different flow conditions. The effects of water cuts, liquid and gas velocities on flow patterns and pressure gradients have been studied. The experiments have been conducted at 20 °C using low viscosity Safrasol D80 oil, tap water and air. Superficial water and oil velocities were varied from 0.3 m/s to 3 m/s and air velocity varied from 0.29 m/s to 52.5 m/s to cover wide range of flow patterns. The experiments were performed for 10% to 90% water cuts. The flow patterns were observed and recorded using high speed video camera while the pressure drops were measured using pressure transducers and U-tube manometers. The flow patterns show strong dependence on water fraction, gas velocities, and liquid velocities. The observed flow patterns are stratified (smooth and wavy), elongated bubble, slug, dispersed bubble, and annular flow patterns. The pressure gradients have been found to increase with the increase in gas flow rates. Also, for a given superficial gas velocity, the pressure gradients increased with the increase in the superficial liquid velocity. The pressure gradient first increases and then decreases with increasing water cut. In general, phase inversion was observed with increase in the water cut. The experimental results have been compared with the existing unified Model and a good agreement has been noticed.

Discovery of a Molecular Collision Front in Interacting Galaxies NGC 4567/4568 with ALMA

NASA Astrophysics Data System (ADS)

Kaneko, Hiroyuki; Kuno, Nario; Saitoh, Takayuki R.

2018-06-01

We present results of 12CO(J = 1–0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we find clear evidence of a molecular collision front with a velocity dispersion that is 16.8 ± 1.4 km s‑1 at the overlapping region, owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc × 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark lane seen in the R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 106 M ⊙ the radii matching a typical value for giant molecular clouds (GMCs) and the masses corresponding to those between GMCs and giant molecular associations (GMAs). All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.

VLTI-AMBER Velocity-Resolved Aperture-Synthesis Imaging of Eta Carinae with a Spectral Resolution of 12 000: Studies of the Primary Star Wind and Innermost Wind-Wind Collision Zone

NASA Technical Reports Server (NTRS)

Weigelt, G.; Hofmann, K.-H.; Schertl, D.; Clementel, N.; Corcoran, M. F.; Damineli, A.; de Wit, W.-J.; Grellmann, R.; Groh, J.; Guieu, S.;

Ocean acidification hampers sperm-egg collisions, gamete fusion, and generation of Ca2+ oscillations of a broadcast spawning bivalve, Tegillarca granosa.

PubMed

Shi, Wei; Han, Yu; Guo, Cheng; Zhao, Xinguo; Liu, Saixi; Su, Wenhao; Wang, Yichen; Zha, Shanjie; Chai, Xueliang; Liu, Guangxu

2017-09-01

Although the effect of ocean acidification on fertilization success of marine organisms is increasingly well documented, the underlying mechanisms are not completely understood. The fertilization success of broadcast spawning invertebrates depends on successful sperm-egg collisions, gamete fusion, and standard generation of Ca 2+ oscillations. Therefore, the realistic effects of future ocean pCO 2 levels on these specific aspects of fertilization of Tegillarca granosa were investigated in the present study through sperm velocity trials, fertilization kinetics model analysis, and intracellular Ca 2+ assays, respectively. Results obtained indicated that ocean acidification significantly reduced the fertilization success of T. granosa, which could be accountable by (i) decreased sperm velocity hence reducing the probability for sperm-egg collisions; (ii) lowered probability of gamete fusion for each gamete collision event; and (iii) disrupted intracellular Ca 2+ oscillations. Copyright © 2017 Elsevier Ltd. All rights reserved.

Multiple impacts of dusty projectiles

NASA Astrophysics Data System (ADS)

Kothe, Stefan; Güttler, Carsten; Blum, Jurgen

In the context of early stages of planetesimal formation we performed laboratory and drop tower experiments to study multiple impacts of small dust-aggregate projectiles into solid sintered dust targets. Both collision partners consisted of 1.5 µm monodisperse spherical SiO2 monomers with volume filling factors of 0.15 (projectiles) and 0.35 (targets), respectively. The fragile projectiles were accelerated by a solenoid accelerator with a linear projectile magazine, which enabled us to perform 25 impacts within 4.5 s of microgravity time in the Bremen drop tower. We measured the mass-accretion efficiency for different impact velocities between 3 and 5 m s-1 , using an analytical balance and imaging methods. Furthermore, we observed random collisions among small dust aggregates with sizes around 1 mm and collision velocities of the order of 0.25 m s-1 and used them to improve the dust-aggregate collision model of Güttler et al. (2010). u

Underplating along the northern portion of the Zagros suture zone, Iran

NASA Astrophysics Data System (ADS)

Motaghi, K.; Shabanian, E.; Kalvandi, F.

2017-07-01

A 2-D absolute shear wave velocity model has been resolved beneath a seismic profile across the northeastern margin of the Arabian Plate-Central Iran by simultaneously inverting data from P receiver functions and fundamental mode Rayleigh wave phase velocity. The data were gathered by a linear seismic array crossing the Zagros fold and thrust belt, Urmia-Dokhtar magmatic arc and Central Iran block assemblage as three major structural components of the Arabia-Eurasia collision. Our model shows a low-velocity tongue protruding from upper to lower crust which, north of the Zagros suture, indicates the signature of an intracontinent low-strength shear zone between the underthrusting and overriding continents. The velocity model confirms the presence of a significant crustal root as well as a thick high-velocity lithosphere in footwall of the suture, continuing northwards beneath the overriding continent for at least 200 km. These features are interpreted as underthrusting of Arabia beneath Central Iran. Time to depth migration of P receiver functions reveals an intracrustal flat interface at ∼17 km depth south of the suture; we interpret it as a significant decoupling within the upper crust. All these crustal scale structural features coherently explain different styles and kinematics of deformation in northern Zagros (Lorestan zone) with respect to its southern part (Fars zone).

Crustal shear wave velocity structure in the northeastern Tibet based on the Neighbourhood algorithm inversion of receiver functions

NASA Astrophysics Data System (ADS)

Wu, Zhenbo; Xu, Tao; Liang, Chuntao; Wu, Chenglong; Liu, Zhiqiang

2018-03-01

The northeastern (NE) Tibet records and represents the far-field deformation response of the collision between the Indian and Eurasian plates in the Cenozoic time. Over the past two decades, studies have revealed the existence of thickened crust in the NE Tibet, but the thickening mechanism is still in debate. We deployed a passive-source seismic profile with 22 temporary broad-band seismic stations in the NE Tibet to investigate the crustal shear wave velocity structure in this region. We selected 288 teleseismic events located in the west Pacific subduction zone near Japan with similar ray path to calculate P-wave receiver functions. Neighbourhood algorithm method is applied to invert the shear wave velocity beneath stations. The inversion result shows a low-velocity zone (LVZ) is roughly confined to the Songpan-Ganzi block and Kunlun mountains and extends to the southern margin of Gonghe basin. Considering the low P-wave velocity revealed by the wide-angle reflection-refraction seismic experiment and high ratio of Vp/Vs based on H-κ grid searching of the receiver functions in this profile, LVZ may be attributed to partial melting induced by temperature change. This observation appears to be consistent with the crustal ductile deformation in this region derived from other geophysical investigations.

Upper-mantle tectonic compartmentalization beneath Amazonian Craton from P-wave seismic tomography

NASA Astrophysics Data System (ADS)

Rocha, M. P.; Azevedo, P. A. D.

2017-12-01

The Amazonian Craton (AC) is one of the largest cratonic areas in the world, with more than 4.4 million square meters, defined as a cratonic nucleus composed of Archean to Mesoproterozoic provinces. Among the questions that remain open about AC, is that of its formation. Geochronological studies suggest that smaller blocks would have assembled in successive collisions until they stabilize in the current configuration, being AC formed by six provinces with different ages. Recent results using the P-wave seismic tomography method suggest that the geochronological boundaries between the Archean blocks exist and reach the upper mantle. These limits appear as low-velocity anomalies with NW-SE direction inside AC, differently from the expected velocity signatures for cratonic regions (high-velocities). The Archean blocks can be interpreted as high-velocity anomalies between the low-velocity anomalies, and are consistent with previous geochronological models. These results were achieved mainly by the installation of the stations of the Brazilian Seismographic Network in recent years, improving coverage especially in the northern region of Brazil. However, they are still preliminary since the seismographic stations in AC region are very distant from each other, which impairs the resolution of such structures. New stations would need to be installed in the region to confirm these results.

A Teachable Moment Uncovered by Video Analysis

NASA Astrophysics Data System (ADS)

Gates, Joshua

2011-05-01

Early in their study of one-dimensional kinematics, my students build an algebraic model that describes the effects of a rolling ball's (perpendicular) collision with a wall. The goal is for the model to predict the ball's velocity when it returns to a fixed point approximately 50-100 cm from the wall as a function of its velocity as it passes this point initially. They are told to assume that the ball's velocity does not change while it rolls to or from the wall—that the velocity change all happens very quickly and only at the wall. In order to evaluate this assumption following the data collection, I have the students analyze one such collision using video analysis. The results uncover an excellent teachable moment about assumptions and their impact on models and error analysis.

Effects of group velocity and multiplasmon resonances on the modulation of Langmuir waves in a degenerate plasma

NASA Astrophysics Data System (ADS)

Misra, Amar P.; Chatterjee, Debjani; Brodin, Gert

2017-11-01

We study the nonlinear wave modulation of Langmuir waves (LWs) in a fully degenerate plasma. Using the Wigner-Moyal equation coupled to the Poisson equation and the multiple scale expansion technique, a modified nonlocal nonlinear Schrödinger (NLS) equation is derived which governs the evolution of LW envelopes in degenerate plasmas. The nonlocal nonlinearity in the NLS equation appears due to the group velocity and multiplasmon resonances, i.e., resonances induced by the simultaneous particle absorption of multiple wave quanta. We focus on the regime where the resonant velocity of electrons is larger than the Fermi velocity and thereby the linear Landau damping is forbidden. As a result, the nonlinear wave-particle resonances due to the group velocity and multiplasmon processes are the dominant mechanisms for wave-particle interaction. It is found that in contrast to classical or semiclassical plasmas, the group velocity resonance does not necessarily give rise the wave damping in the strong quantum regime where ℏ k ˜m vF with ℏ denoting the reduced Planck's constant, m the electron mass, and vF the Fermi velocity; however, the three-plasmon process plays a dominant role in the nonlinear Landau damping of wave envelopes. In this regime, the decay rate of the wave amplitude is also found to be higher compared to that in the modest quantum regime where the multiplasmon effects are forbidden.

Unusual phonon behavior and ultra-low thermal conductance of monolayer InSe.

PubMed

Zhou, Hangbo; Cai, Yongqing; Zhang, Gang; Zhang, Yong-Wei

2017-12-21

Monolayer indium selenide (InSe) possesses numerous fascinating properties, such as high electron mobility, quantum Hall effect and anomalous optical response. However, its phonon properties, thermal transport properties and the origin of its structural stability remain unexplored. Using first-principles calculations, we show that the atoms in InSe are highly polarized and such polarization causes strong long-range dipole-dipole interaction (DDI). For acoustic modes, DDI is essential for maintaining its structural stability. For optical modes, DDI causes a significant frequency shift of its out-of-phase vibrations. Surprisingly, we observed that there were two isolated frequency regimes, which were completely separated from other frequency regimes with large frequency gaps. Within each frequency regime, only a single phonon mode exists. We further reveal that InSe possesses the lowest thermal conductance among the known two-dimensional materials due to the low cut-off frequency, low phonon group velocities and the presence of large frequency gaps. These unique behaviors of monolayer InSe can enable the fabrication of novel devices, such as thermoelectric module, single-mode phonon channel and phononic laser.

Solution of non-continuum flows using BGK-type model with enforced relaxation of moments

NASA Astrophysics Data System (ADS)

Alekseenko, Alexander; Gimelshein, Sergey; Nguyen, Truong; Vedula, Prakash

2016-11-01

A BGK-type model with velocity dependent collision frequency and enforced relaxation rates for selected moments is applied to simulation of one- and two-dimensional super sonic flows. Relaxation rates of the moments are estimated by evaluating the full Boltzmann collision integral several times during the simulation. The solutions show improvements in velocity and temperature profiles as compared to the classical ES-BGK model. However, enforcement of relaxation rates for high order moments increases stiffness of the model.

Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mantysaari, Heikki; Schenke, Bjorn; Shen, Chun

Results for particle production inmore » $$\\sqrt{s}$$ = 5.02 TeV p+Pb collisions at the Large Hadron Collider within a combined classical Yang-Mills and relativistic viscous hydrodynamic calculation are presented. We emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients v n, demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean trans-verse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury-Brown-Twiss radii.« less

Imprints of fluctuating proton shapes on flow in proton-lead collisions at the LHC

DOE PAGES

Mantysaari, Heikki; Schenke, Bjorn; Shen, Chun; ...

2017-07-21

Results for particle production inmore » $$\\sqrt{s}$$ = 5.02 TeV p+Pb collisions at the Large Hadron Collider within a combined classical Yang-Mills and relativistic viscous hydrodynamic calculation are presented. We emphasize the importance of sub-nucleon scale fluctuations in the proton projectile to describe the experimentally observed azimuthal harmonic coefficients v n, demonstrating their sensitivity to the proton shape. We stress that the proton shape and its fluctuations are not free parameters in our calculations. Instead, they have been constrained using experimental data from HERA on exclusive vector meson production. Including temperature dependent shear and bulk viscosities, as well as UrQMD for the low temperature regime, we present results for mean trans-verse momenta, harmonic flow coefficients for charged hadrons and identified particles, as well as Hanbury-Brown-Twiss radii.« less

Demonstration of optical parametric gain generation in the 1 μm regime based on a photonic crystal fiber pumped by a picosecond mode-locked ytterbium-doped fiber laser

NASA Astrophysics Data System (ADS)

Zhang, Lei; Yang, Si-Gang; Wang, Xiao-Jian; Gou, Dou-Dou; Chen, Hong-Wei; Chen, Ming-Hua; Xie, Shi-Zhong

2014-01-01

We report the experimental demonstration of the optical parametric gain generation in the 1 μm regime based on a photonic crystal fiber (PCF) with a zero group velocity dispersion (GVD) wavelength of 1062 nm pumped by a homemade tunable picosecond mode-locked ytterbium-doped fiber laser. A broad parametric gain band is obtained by pumping the PCF in the anomalous GVD regime with a relatively low power. Two separated narrow parametric gain bands are observed by pumping the PCF in the normal GVD regime. The peak of the parametric gain profile can be tuned from 927 to 1038 nm and from 1099 to 1228 nm. This widely tunable parametric gain band can be used for a broad band optical parametric amplifier, large span wavelength conversion or a tunable optical parametric oscillator.

Less efficient oculomotor performance is associated with increased incidence of head impacts in high school ice hockey.

PubMed

Kiefer, Adam W; DiCesare, Christopher; Nalepka, Patrick; Foss, Kim Barber; Thomas, Staci; Myer, Gregory D

2018-01-01

To evaluate associations between pre-season oculomotor performance on visual tracking tasks and in-season head impact incidence during high school boys ice hockey. Prospective observational study design. Fifteen healthy high school aged male hockey athletes (M=16.50±1.17years) performed two 30s blocks each of a prosaccade and self-paced saccade task, and two trials each of a slow, medium, and fast smooth pursuit task (90°s -1 ; 180°s -1 ; 360°s -1 ) during the pre-season. Regular season in-game collision data were collected via helmet-mounted accelerometers. Simple linear regressions were used to examine relations between oculomotor performance measures and collision incidence at various impact thresholds. The variability of prosaccade latency was positively related to total collisions for the 20g force cutoff (p=0.046, adjusted R 2 =0.28). The average self-paced saccade velocity (p=0.020, adjusted R 2 =0.37) and variability of smooth pursuit gaze velocity (p=0.012, adjusted R 2 =0.47) were also positively associated with total collisions for the 50g force cutoff. These results provide preliminary evidence that less efficient oculomotor performance on three different oculomotor tasks is associated with increased incidence of head impacts during a competitive ice hockey season. The variability of prosaccade latency, the average self-paced saccade velocity and the variability of gaze velocity during predictable smooth pursuit all related to increased head impacts. Future work is needed to further understand player initiated collisions, but this is an important first step toward understanding strategies to reduce incidence of injury risk in ice hockey, and potentially contact sports more generally. Copyright © 2017 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.

Steady state plasma operation in RF dominated regimes on EAST

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, X. J.; Zhao, Y. P.; Gong, X. Z.

Significant progress has recently been made on EAST in the 2014 campaign, including the enhanced CW H&CD system over 20MW heating power (LHCD, ICRH and NBI), more than 70 diagnostics, ITER-like W-monoblock on upper divertor, two inner cryo-pumps and RMP coils, enabling EAST to investigate long pulse H mode operation with dominant electron heating and low torque to address the critical issues for ITER. H-mode plasmas were achieved by new H&CD system or 4.6GHz LHCD alone for the first time. Long pulse high performance H mode has been obtained by LHCD alone up to 28s at H{sub 98}∼1.2 or bymore » combing of ICRH and LHCD, no or small ELM was found in RF plasmas, which is essential for steady state operation in the future Tokamak. Plasma operation in low collision regimes were implemented by new 4.6GHz LHCD with core Te∼4.5keV. The non-inductive scenarios with high performance at high bootstrap current fraction have been demonstrated in RF dominated regimes for long pulse operation. Near full non-inductive CD discharges have been achieved. In addition, effective heating and decoupling method under multi-transmitter for ICRF system were developed in this campaign, etc. EAST could be in operation with over 30MW CW heating and current drive power (LHCD ICRH NBI and ECRH), enhanced diagnostic capabilities and full actively-cooled metal wall from 2015. It will therefore allow to access new confinement regimes and to extend these regimes towards to steady state operation.« less

Collisional redistribution of radiation. III - The equation of motion for the correlation function and the scattered spectrum

NASA Technical Reports Server (NTRS)

Burnett, K.; Cooper, J.

1980-01-01

Computations were made of the scattering of monochromatic radiation by a degenerate atom in the binary-collision approximation for field strengths whose products of the Rabi frequency for atomic transition and the duration of a strong collision are much less than 1. An expression of motion for the correlation function is derived which does not exclude the region where thermal correlations may be neglected; the equation is valid outside the quantum-regression regime, and has a straightforward solution for practical cases. Solutions for the weak-field linear response regime are presented in terms of generalized absorption and emission profiles which depend on the indices of the atomic multipoles.

On the rates of type Ia supernovae originating from white dwarf collisions in quadruple star systems

NASA Astrophysics Data System (ADS)

Hamers, Adrian S.

2018-04-01

We consider the evolution of stellar hierarchical quadruple systems in the 2+2 (two binaries orbiting each other's barycentre) and 3+1 (triple orbited by a fourth star) configurations. In our simulations, we take into account the effects of secular dynamical evolution, stellar evolution, tidal evolution and encounters with passing stars. We focus on type Ia supernovae (SNe Ia) driven by collisions of carbon-oxygen (CO) white dwarfs (WDs). Such collisions can arise from several channels: (1) collisions due to extremely high eccentricities induced by secular evolution, (2) collisions following a dynamical instability of the system, and (3) collisions driven by semisecular evolution. The systems considered here have initially wide inner orbits, with initial semilatus recti larger than 12 {au}, implying no interaction if the orbits were isolated. However, taking into account dynamical evolution, we find that ≈0.4 (≈0.6) of 2+2 (3+1) systems interact. In particular, Roche Lobe overflow can be triggered possibly in highly eccentric orbits, dynamical instability can ensue due to mass-loss-driven orbital expansion or secular evolution, or a semisecular regime can be entered. We compute the delay-time distributions (DTDs) of collision-induced SNe Ia, and find that they are flatter compared to the observed DTD. Moreover, our combined SNe Ia rates are (3.7± 0.7) × 10^{-6} M_⊙^{-1} and (1.3± 0.2) × 10^{-6} M_⊙^{-1} for 2+2 and 3+1 systems, respectively, three orders of magnitude lower compared to the observed rate, of order 10^{-3} M_⊙^{-1}. The low rates can be ascribed to interactions before the stars evolve to CO WDs. However, our results are lower limits given that we considered a subset of quadruple systems.

On the rates of Type Ia supernovae originating from white dwarf collisions in quadruple star systems

NASA Astrophysics Data System (ADS)

Hamers, Adrian S.

2018-07-01

We consider the evolution of stellar hierarchical quadruple systems in the 2+2 (two binaries orbiting each other's barycentre) and 3+1 (triple orbited by a fourth star) configurations. In our simulations, we take into account the effects of secular dynamical evolution, stellar evolution, tidal evolution, and encounters with passing stars. We focus on Type Ia supernovae (SNe Ia) driven by collisions of carbon-oxygen (CO) white dwarfs (WDs). Such collisions can arise from several channels: (1) collisions due to extremely high eccentricities induced by secular evolution, (2) collisions following a dynamical instability of the system, and (3) collisions driven by semisecular evolution. The systems considered here have initially wide inner orbits, with initial semilatus recti larger than 12 au, implying no interaction if the orbits were isolated. However, taking into account dynamical evolution, we find that ≈0.4 (≈0.6) of 2+2 (3+1) systems interact. In particular, Roche lobe overflow can be triggered possibly in highly eccentric orbits, dynamical instability can ensue due to mass-loss-driven orbital expansion or secular evolution, or a semisecular regime can be entered. We compute the delay-time distributions (DTDs) of collision-induced SNe Ia, and find that they are flatter compared to the observed DTD. Moreover, our combined SNe Ia rates are (3.7± 0.7) × 10^{-6} M_{⊙}^{-1} and (1.3± 0.2) × 10^{-6} M_{⊙}^{-1} for 2+2 and 3+1 systems, respectively, three orders of magnitude lower compared to the observed rate, of the order of 10^{-3} M_{⊙}^{-1}. The low rates can be ascribed to interactions before the stars evolve to CO WDs. However, our results are lower limits given that we considered a subset of quadruple systems.

Metamorphism, Plate Tectonics, and the Supercontinent Cycle

NASA Astrophysics Data System (ADS)

Brown, Michael

Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; G-UHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Medium-temperature eclogite, high-pressure granulite metamorphism (E-HPGM), is also first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E-HPGM belts are complementary to G-UHTM belts and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (high-pressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although perhaps counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a 'Proterozoic plate tectonics regime', although the style of tectonics likely involved differences. The 'Proterozoic plate tectonics regime' evolved during a Neoproterozoic transition to the 'modern plate tectonics regime' characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPM-UHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).

Ultra-clean Layers (UCLs) and Low Albedo Clouds ("gray clouds") in the Marine Boundary Layer - CSET aircraft data, 2-D bin spectral cloud parcel model, large eddy simulation and satellite observations from CALIPSO, MODIS and COSMIC

NASA Astrophysics Data System (ADS)

O, K. T.; Wood, R.; Bretherton, C. S.; Eastman, R. M.; Tseng, H. H.

2016-12-01

During the 2015 Cloud System Evolution in the Trades (CSET) field program (CSET, Jul-Aug 2015, subtropical NE Pacific), the NSF/NCAR G-V aircraft frequently encountered ultra clean layers (hereafter UCLs) with extremely low accumulation mode aerosol (i.e. diameter da> 100nm) concentration (hereafter Na), and low albedo ( 0.2) warm clouds (termed "gray clouds" in our study) with low droplet concentration (hereafter Nd). The analysis of CSET aircraft data shows that (1) UCLs and gray clouds are mostly commonly found at a height of 1.5-2km, typically close to the top of the MBL, (2) UCLs and gray cloud coverage as high as 40-60% between 135W and 155W (i.e. Sc-Cu transition region) but occur very infrequently east of 130W (i.e. shallow, near-coastal stratocumulus region), and (3) UCLs and gray clouds exhibit remarkably low turbulence compared with non-UCL clear sky and clouds. It should be noted that most previous aircraft sampling of low clouds occurred close to the Californian coast, so the prevalence of UCLs and gray clouds has not been previously noted. Based on the analysis of aircraft data, we hypothesize that gray clouds result from detrainment of cloud close to the top of precipitating trade cumuli, and UCLs are remnants of these layers when gray clouds evaporates. The simulations in our study are performed using 2-D bin spectral cloud parcel model and version 6.9 of the System for Atmospheric Modeling (SAM). Our idealized simulations suggest that collision-coalescence plays a crucial role in reducing Nd such that gray clouds can easily form via collision-coalescence in layers detrained from the cloud top at trade cumulus regime, but can not form at stratocumulus regime. Upon evaporation of gray clouds, only few accumulation mode aerosols are returned to the clear sky, leaving horizontally-extensive UCLs (i.e. clean clear sky). Analysis of CSET flight data and idealized model simulations both suggest cloud top/PBL height may play an important role in the formation of UCLs and gray clouds. In our satellite observation study, the comparison between PBL height (from COSMIC and MODIS) and fraction of low optical depth cloud (from MODIS and CALIPSO) at NEP trade cumulus regime (20-35N, 140-155W) also suggest a strong positive correlation.

A unified gas-kinetic scheme for continuum and rarefied flows IV: Full Boltzmann and model equations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Chang, E-mail: cliuaa@ust.hk; Xu, Kun, E-mail: makxu@ust.hk; Sun, Quanhua, E-mail: qsun@imech.ac.cn

Fluid dynamic equations are valid in their respective modeling scales, such as the particle mean free path scale of the Boltzmann equation and the hydrodynamic scale of the Navier–Stokes (NS) equations. With a variation of the modeling scales, theoretically there should have a continuous spectrum of fluid dynamic equations. Even though the Boltzmann equation is claimed to be valid in all scales, many Boltzmann solvers, including direct simulation Monte Carlo method, require the cell resolution to the order of particle mean free path scale. Therefore, they are still single scale methods. In order to study multiscale flow evolution efficiently, themore » dynamics in the computational fluid has to be changed with the scales. A direct modeling of flow physics with a changeable scale may become an appropriate approach. The unified gas-kinetic scheme (UGKS) is a direct modeling method in the mesh size scale, and its underlying flow physics depends on the resolution of the cell size relative to the particle mean free path. The cell size of UGKS is not limited by the particle mean free path. With the variation of the ratio between the numerical cell size and local particle mean free path, the UGKS recovers the flow dynamics from the particle transport and collision in the kinetic scale to the wave propagation in the hydrodynamic scale. The previous UGKS is mostly constructed from the evolution solution of kinetic model equations. Even though the UGKS is very accurate and effective in the low transition and continuum flow regimes with the time step being much larger than the particle mean free time, it still has space to develop more accurate flow solver in the region, where the time step is comparable with the local particle mean free time. In such a scale, there is dynamic difference from the full Boltzmann collision term and the model equations. This work is about the further development of the UGKS with the implementation of the full Boltzmann collision term in the region where it is needed. The central ingredient of the UGKS is the coupled treatment of particle transport and collision in the flux evaluation across a cell interface, where a continuous flow dynamics from kinetic to hydrodynamic scales is modeled. The newly developed UGKS has the asymptotic preserving (AP) property of recovering the NS solutions in the continuum flow regime, and the full Boltzmann solution in the rarefied regime. In the mostly unexplored transition regime, the UGKS itself provides a valuable tool for the non-equilibrium flow study. The mathematical properties of the scheme, such as stability, accuracy, and the asymptotic preserving, will be analyzed in this paper as well.« less

Shear velocity structure of central Eurasia from inversion of surface wave velocities

NASA Astrophysics Data System (ADS)

Villaseñor, A.; Ritzwoller, M. H.; Levshin, A. L.; Barmin, M. P.; Engdahl, E. R.; Spakman, W.; Trampert, J.

2001-04-01

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2°×2° grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish-Iranian Plateaus, structures originated by continent-continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

Sequential evaporation of water molecules from protonated water clusters: measurement of the velocity distributions of the evaporated molecules and statistical analysis.

PubMed

Berthias, F; Feketeová, L; Abdoul-Carime, H; Calvo, F; Farizon, B; Farizon, M; Märk, T D

2018-06-22

Velocity distributions of neutral water molecules evaporated after collision induced dissociation of protonated water clusters H+(H2O)n≤10 were measured using the combined correlated ion and neutral fragment time-of-flight (COINTOF) and velocity map imaging (VMI) techniques. As observed previously, all measured velocity distributions exhibit two contributions, with a low velocity part identified by statistical molecular dynamics (SMD) simulations as events obeying the Maxwell-Boltzmann statistics and a high velocity contribution corresponding to non-ergodic events in which energy redistribution is incomplete. In contrast to earlier studies, where the evaporation of a single molecule was probed, the present study is concerned with events involving the evaporation of up to five water molecules. In particular, we discuss here in detail the cases of two and three evaporated molecules. Evaporation of several water molecules after CID can be interpreted in general as a sequential evaporation process. In addition to the SMD calculations, a Monte Carlo (MC) based simulation was developed allowing the reconstruction of the velocity distribution produced by the evaporation of m molecules from H+(H2O)n≤10 cluster ions using the measured velocity distributions for singly evaporated molecules as the input. The observed broadening of the low-velocity part of the distributions for the evaporation of two and three molecules as compared to the width for the evaporation of a single molecule results from the cumulative recoil velocity of the successive ion residues as well as the intrinsically broader distributions for decreasingly smaller parent clusters. Further MC simulations were carried out assuming that a certain proportion of non-ergodic events is responsible for the first evaporation in such a sequential evaporation series, thereby allowing to model the entire velocity distribution.

Ion velocity distribution functions in argon and helium discharges: detailed comparison of numerical simulation results and experimental data

NASA Astrophysics Data System (ADS)

Wang, Huihui; Sukhomlinov, Vladimir S.; Kaganovich, Igor D.; Mustafaev, Alexander S.

2017-02-01

Using the Monte Carlo collision method, we have performed simulations of ion velocity distribution functions (IVDF) taking into account both elastic collisions and charge exchange collisions of ions with atoms in uniform electric fields for argon and helium background gases. The simulation results are verified by comparison with the experiment data of the ion mobilities and the ion transverse diffusion coefficients in argon and helium. The recently published experimental data for the first seven coefficients of the Legendre polynomial expansion of the ion energy and angular distribution functions are used to validate simulation results for IVDF. Good agreement between measured and simulated IVDFs shows that the developed simulation model can be used for accurate calculations of IVDFs.

From the Boltzmann to the Lattice-Boltzmann Equation:. Beyond BGK Collision Models

NASA Astrophysics Data System (ADS)

Philippi, Paulo Cesar; Hegele, Luiz Adolfo; Surmas, Rodrigo; Siebert, Diogo Nardelli; Dos Santos, Luís Orlando Emerich

In this work, we present a derivation for the lattice-Boltzmann equation directly from the linearized Boltzmann equation, combining the following main features: multiple relaxation times and thermodynamic consistency in the description of non isothermal compressible flows. The method presented here is based on the discretization of increasingly order kinetic models of the Boltzmann equation. Following a Gross-Jackson procedure, the linearized collision term is developed in Hermite polynomial tensors and the resulting infinite series is diagonalized after a chosen integer N, establishing the order of approximation of the collision term. The velocity space is discretized, in accordance with a quadrature method based on prescribed abscissas (Philippi et al., Phys. Rev E 73, 056702, 2006). The problem of describing the energy transfer is discussed, in relation with the order of approximation of a two relaxation-times lattice Boltzmann model. The velocity-step, temperature-step and the shock tube problems are investigated, adopting lattices with 37, 53 and 81 velocities.

Linear momentum, angular momentum and energy in the linear collision between two balls

NASA Astrophysics Data System (ADS)

Hanisch, C.; Hofmann, F.; Ziese, M.

2018-01-01

In an experiment of the basic physics laboratory, kinematical motion processes were analysed. The motion was recorded with a standard video camera having frame rates from 30 to 240 fps the videos were processed using video analysis software. Video detection was used to analyse the symmetric one-dimensional collision between two balls. Conservation of linear and angular momentum lead to a crossover from rolling to sliding directly after the collision. By variation of the rolling radius the system could be tuned from a regime in which the balls move away from each other after the collision to a situation in which they re-collide.

Dynamics of landslide model with time delay and periodic parameter perturbations

NASA Astrophysics Data System (ADS)

Kostić, Srđan; Vasović, Nebojša; Franović, Igor; Jevremović, Dragutin; Mitrinovic, David; Todorović, Kristina

2014-09-01

In present paper, we analyze the dynamics of a single-block model on an inclined slope with Dieterich-Ruina friction law under the variation of two new introduced parameters: time delay Td and initial shear stress μ. It is assumed that this phenomenological model qualitatively simulates the motion along the infinite creeping slope. The introduction of time delay is proposed to mimic the memory effect of the sliding surface and it is generally considered as a function of history of sliding. On the other hand, periodic perturbation of initial shear stress emulates external triggering effect of long-distant earthquakes or some non-natural vibration source. The effects of variation of a single observed parameter, Td or μ, as well as their co-action, are estimated for three different sliding regimes: β < 1, β = 1 and β > 1, where β stands for the ratio of long-term to short-term stress changes. The results of standard local bifurcation analysis indicate the onset of complex dynamics for very low values of time delay. On the other side, numerical approach confirms an additional complexity that was not observed by local analysis, due to the possible effect of global bifurcations. The most complex dynamics is detected for β < 1, with a complete Ruelle-Takens-Newhouse route to chaos under the variation of Td, or the co-action of both parameters Td and μ. These results correspond well with the previous experimental observations on clay and siltstone with low clay fraction. In the same regime, the perturbation of only a single parameter, μ, renders the oscillatory motion of the block. Within the velocity-independent regime, β = 1, the inclusion and variation of Td generates a transition to equilibrium state, whereas the small oscillations of μ induce oscillatory motion with decreasing amplitude. The co-action of both parameters, in the same regime, causes the decrease of block's velocity. As for β > 1, highly-frequent, limit-amplitude oscillations of initial stress give rise to oscillatory motion. Also for β > 1, in case of perturbing only the initial shear stress, with smaller amplitude, velocity of the block changes exponentially fast. If the time delay is introduced, besides the stress perturbation, within the same regime, the co-action of Td (Td < 0.1) and small oscillations of μ induce the onset of deterministic chaos.

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

NASA Astrophysics Data System (ADS)

Liang, Xiaofeng; Shen, Yang; Chen, Yongshun John; Ren, Yong

2011-02-01

Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Transport of biocolloids in water saturated columns packed with sand: Effect of grain size and pore water velocity

NASA Astrophysics Data System (ADS)

Syngouna, Vasiliki I.; Chrysikopoulos, Constantinos V.

2012-03-01

The main objective of this study was to evaluate the combined effects of grain size and pore water velocity on the transport in water saturated porous media of three waterborne fecal indicator organisms (Escherichia coli, MS2, and ΦX174) in laboratory-scale columns packed with clean quartz sand. Three different grain sizes and three pore water velocities were examined and the attachment behavior of Escherichia coli, MS2, and ΦX174 onto quartz sand was evaluated. The mass recoveries of the biocolloids examined were shown to be highest for Escherichia coli and lowest for MS2. However, no obvious relationships between mass recoveries and water velocity or grain size could be established from the experimental results. The observed mean dispersivity values for each sand grain size were smaller for bacteria than coliphages, but higher for MS2 than ΦX174. The single collector removal and collision efficiencies were quantified using the classical colloid filtration theory. Furthermore, theoretical collision efficiencies were estimated only for E. coli by the Interaction-Force-Boundary-Layer, and Maxwell approximations. Better agreement between the experimental and Maxwell theoretical collision efficiencies were observed.

Transport of biocolloids in water saturated columns packed with sand: Effect of grain size and pore water velocity

NASA Astrophysics Data System (ADS)

Syngouna, Vasiliki I.; Chrysikopoulos, Constantinos V.

2011-11-01

The main objective of this study was to evaluate the combined effects of grain size and pore water velocity on the transport in water saturated porous media of three waterborne fecal indicator organisms ( Escherichia coli, MS2, and ΦX174) in laboratory-scale columns packed with clean quartz sand. Three different grain sizes and three pore water velocities were examined and the attachment behavior of Escherichia coli, MS2, and ΦX174 onto quartz sand was evaluated. The mass recoveries of the biocolloids examined were shown to be highest for Escherichia coli and lowest for MS2. However, no obvious relationships between mass recoveries and water velocity or grain size could be established from the experimental results. The observed mean dispersivity values for each sand grain size were smaller for bacteria than coliphages, but higher for MS2 than ΦX174. The single collector removal and collision efficiencies were quantified using the classical colloid filtration theory. Furthermore, theoretical collision efficiencies were estimated only for E. coli by the Interaction-Force-Boundary-Layer, and Maxwell approximations. Better agreement between the experimental and Maxwell theoretical collision efficiencies were observed.

Refinement of the probability density function model for preferential concentration of aerosol particles in isotropic turbulence

NASA Astrophysics Data System (ADS)

Zaichik, Leonid I.; Alipchenkov, Vladimir M.

2007-11-01

The purposes of the paper are threefold: (i) to refine the statistical model of preferential particle concentration in isotropic turbulence that was previously proposed by Zaichik and Alipchenkov [Phys. Fluids 15, 1776 (2003)], (ii) to investigate the effect of clustering of low-inertia particles using the refined model, and (iii) to advance a simple model for predicting the collision rate of aerosol particles. The model developed is based on a kinetic equation for the two-point probability density function of the relative velocity distribution of particle pairs. Improvements in predicting the preferential concentration of low-inertia particles are attained due to refining the description of the turbulent velocity field of the carrier fluid by including a difference between the time scales of the of strain and rotation rate correlations. The refined model results in a better agreement with direct numerical simulations for aerosol particles.

Robust low-bias negative differential resistance in graphene superlattices

NASA Astrophysics Data System (ADS)

Sattari-Esfahlan, S. M.; Fouladi-Oskuei, J.; Shojaei, S.

2017-06-01

In this work, we present a detailed theoretical study on the low bias current-voltage (I-V) characteristic of biased planar graphene superlattice (PGSL), provided by a heterostructured substrate and a series of grounded metallic planes placed over a graphene sheet, which induce a periodically modulated Dirac gap and Fermi velocity barrier, respectively. We investigate the effect of PGSL parameters on the I-V characteristic and the appearance of multipeak negative differential resistance (NDR) in the proposed device within the Landauer-Buttiker formalism and adopted transfer matrix method. Moreover‚ we propose a novel venue to control the NDR in PGSL with Fermi velocity barrier. Different regimes of NDR have been recognized, based on the PGSL parameters and external bias. From this viewpoint‚ we obtain multipeak NDR through miniband aligning in PGSL. The maximum pick to valley ratio (PVR) up to 167 obtained for ~{{\\upsilon}c} , the Fermi velocity correlation (ratio of Fermi velocity in barrier and well region), is 1.9 at bias voltages between 70-130 mV. Our findings have good agreement with experiments and can be considered in designing multi-valued memory‚ functional circuit, low power and high-speed nanoelectronic device applications.

Kinetic theory of binary particles with unequal mean velocities and non-equipartition energies

NASA Astrophysics Data System (ADS)

Chen, Yanpei; Mei, Yifeng; Wang, Wei

2017-03-01

The hydrodynamic conservation equations and constitutive relations for a binary granular mixture composed of smooth, nearly elastic spheres with non-equipartition energies and different mean velocities are derived. This research is aimed to build three-dimensional kinetic theory to characterize the behaviors of two species of particles suffering different forces. The standard Enskog method is employed assuming a Maxwell velocity distribution for each species of particles. The collision components of the stress tensor and the other parameters are calculated from the zeroth- and first-order approximation. Our results demonstrate that three factors, namely the differences between two granular masses, temperatures and mean velocities all play important roles in the stress-strain relation of the binary mixture, indicating that the assumption of energy equipartition and the same mean velocity may not be acceptable. The collision frequency and the solid viscosity increase monotonously with each granular temperature. The zeroth-order approximation to the energy dissipation varies greatly with the mean velocities of both species of spheres, reaching its peak value at the maximum of their relative velocity.

Manipulating time and space: Collision prediction in peripersonal and extrapersonal space.

PubMed

Iachini, Tina; Ruotolo, Francesco; Vinciguerra, Michela; Ruggiero, Gennaro

2017-09-01

Being able to predict potential collisions is a necessary survival prerequisite for all moving species. Temporal and spatial information is fundamental for this purpose. However, it is not clear yet if the peripersonal (i.e. near) and extrapersonal (i.e. far) distance between our body and the moving objects affects the way in which we can predict possible collisions. In order to assess this, we manipulated independently velocity and path of two balls moving one towards the other in such a way as to collide or not in peripersonal and extrapersonal space. In two experiments, participants had to judge if these balls were to collide or not. The results consistently showed a lower discrimination capacity and a more liberal tendency to predict collisions when the moving balls were in peripersonal space and their velocity was different rather than equal. This did not happen in extrapersonal space. Therefore, peripersonal space was particularly affected by temporal information. The possible link between the motor and anticipatory adaptive function of peripersonal space and collision prediction mechanisms is discussed. Copyright © 2017 Elsevier B.V. All rights reserved.

Formation of bi-lobed 67P/C-G-like shapes by sub-catastrophic collisions

NASA Astrophysics Data System (ADS)

Jutzi, Martin; Benz, Willy

2016-10-01

Small bodies with a bi-lobe shape such as comet 67P/C-G have been argued to form as the result of low velocity (v ≈ vesc) collisional mergers of similar-sized bodies very early on in the history of the solar system [1]. However, the recent analysis of the subsequent collisional survival of the global structure and shape of comet 67P/C-G strongly suggests that such a structure will not survive until today [2]. Hence, the comet must have acquired its present characteristics as a result of a collision occurring at later time when the relative velocities between small bodies are much higher (v >> vesc).One possible scenario would be that 67P/C-G-like bi-lobe structures form as the result of collisional disruptions of larger parent bodies [3]. Whether the internal properties of such larger parent bodies, the timing of such a collision, and the subsequent survival of the shape produced is compatible with observations will remain to be seen.Here, we propose a scenario in which the final bi-lobe shapes result from low-energy, sub-catastrophic impacts. We start with bodies of about the same mass as comet 67P/C-G, which are rotating and are slightly elongated (i.e., with properties which are consistent with the outcome of the disruptions of larger bodies). We use a SPH shock physics code to model the impacts, the subsequent re-accumulation of material and the reconfiguration into a stable final shape. Our modelling results suggest that these kind of collisions result in "splitting" events which frequently lead to formation of bi-lobe 67P/C-G-like shapes.The frequency of such small-scale impact events is consistent with a young (less than 1 Gy) age of the shape of comet 67P/C-G [2]. Equally important, the probability for such a shape-forming event to take place without a subsequent shape-destroying event occurring until today is reasonably high.Although the collisions considered in this work can alter the global shape, their respective energy is small enough not to lead to any substantial global-scale heating or compaction, consistent with the observed primordial characteristics of comets.[1] Jutzi&Asphaug, 2015, Science 348. [2] Jutzi et al., submitted. [3] Schwartz et al., in prep.

Effective collision strengths for fine-structure forbidden transitions among the 3s^23p^3 levels of AR IV

NASA Astrophysics Data System (ADS)

Ramsbottom, C. A.; Bell, K. L.; Keenan, F. P.

1997-01-01

The multichannel R-matrix method is used to compute electron impact excitation collision strengths in Ar iv for all fine-structure transitions among the ^4S^o, ^2D^o and ^2P^o levels in the 3s^23p^3 ground configuration. Included in the expansion of the total wavefunction are the lowest 13 LS target eigenstates of Ar iv formed from the 3s^23p^3, 3s3p^4 and 3s^23p^23d configurations. The effective collision strengths, obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities, are presented for all 10 fine-structure transitions over a wide range of electron temperatures of astrophysical interest (T_e=2000-100 000K). Comparisons are made with an earlier 7-state close-coupling calculation by Zeippen, Butler & Le Bourlot, and significant differences are found to occur for many of the forbidden transitions considered, in particular those involving the ^4S^o ground state, where discrepancies of up to a factor of 3 are found in the low-temperature region.

Is Hidden Crossings Theory a New MOCC Method?

NASA Astrophysics Data System (ADS)

Krstić, Predrag; Schultz, David

1998-05-01

We find un unitary transformation of the scaled adiabatic Hamiltonian of a two-center, one-electron collision system which yields a new representation for the matrix elements of nonadiabatic radial coupling, valid for low-to-intermediate collision velocities. These are given in analytic form once the topology of the branch points of the adiabatic Hamiltonian in the plane of complex internuclear distance R is known. The matrix elements do not depend on origin of electronic coordinates and properly vanish at large internuclear distances. The role of the rotational couplings in the new representation is also discussed. The aproach is appropriately extended and compared with the PSS treatment in the fully quantal description of the collision. We apply new radial and rotational matrix elements in the standard Molecular Orbital Close Coupling (MOCC) approach to describe excitation and ionization in collisions of antiprotons with He^+ and of alpha-particles with hydrogen(P.S. Krstić et al, J. Phys. B. 31, in press (1998).). The results are compared with those obtained from the standard MOCC method and from the direct solutions of the Schrödinger equation on lattice (LTDSE)(D.R. Schultz et al, Phys. Rev. A 56, 3710 (1997)).

Interfacial mixing in high-energy-density matter with a multiphysics kinetic model

NASA Astrophysics Data System (ADS)

Haack, Jeffrey R.; Hauck, Cory D.; Murillo, Michael S.

2017-12-01

We have extended a recently developed multispecies, multitemperature Bhatnagar-Gross-Krook model [Haack et al., J. Stat. Phys. 168, 822 (2017), 10.1007/s10955-017-1824-9], to include multiphysics capabilities that enable modeling of a wider range of physical conditions. In terms of geometry, we have extended from the spatially homogeneous setting to one spatial dimension. In terms of the physics, we have included an atomic ionization model, accurate collision physics across coupling regimes, self-consistent electric fields, and degeneracy in the electronic screening. We apply the model to a warm dense matter scenario in which the ablator-fuel interface of an inertial confinement fusion target is heated, but for larger length and time scales and for much higher temperatures than can be simulated using molecular dynamics. Relative to molecular dynamics, the kinetic model greatly extends the temperature regime and the spatiotemporal scales over which we are able to model. In our numerical results we observe hydrogen from the ablator material jetting into the fuel during the early stages of the implosion and compare the relative size of various diffusion components (Fickean diffusion, electrodiffusion, and barodiffusion) that drive this process. We also examine kinetic effects, such as anisotropic distributions and velocity separation, in order to determine when this problem can be described with a hydrodynamic model.

Scaling laws for mixing and dissipation in unforced rotating stratified turbulence

NASA Astrophysics Data System (ADS)

Pouquet, A.; Rosenberg, D.; Marino, R.; Herbert, C.

2018-06-01

We present a model for the scaling of mixing in weakly rotating stratified flows characterized by their Rossby, Froude and Reynolds numbers Ro, Fr, Re. It is based on quasi-equipartition between kinetic and potential modes, sub-dominant vertical velocity and lessening of the energy transfer to small scales as measured by the ratio rE of kinetic energy dissipation to its dimensional expression. We determine their domains of validity for a numerical study of the unforced Boussinesq equations mostly on grids of 10243 points, with Ro/Fr> 2.5 and with 1600< Re<1.9x104; the Prandtl number is one, initial conditions are either isotropic and at large scale for the velocity, and zero for the temperature {\\theta}, or in geostrophic balance. Three regimes in Fr are observed: dominant waves, eddy-wave interactions and strong turbulence. A wave-turbulence balance for the transfer time leads to rE growing linearly with Fr in the intermediate regime, with a saturation at ~0.3 or more, depending on initial conditions for larger Froude numbers. The Ellison scale is also found to scale linearly with Fr, and the flux Richardson number Rf transitions for roughly the same parameter values as well. Putting together the 3 relationships of the model allows for the prediction of mixing efficiency scaling as Fr-2~RB-1 in the low and intermediate regimes, whereas for higher Fr, it scales as RB-1/2, as already observed: as turbulence strengthens, rE~1, the velocity is isotropic and smaller buoyancy fluxes altogether correspond to a decoupling of velocity and temperature fluctuations, the latter becoming passive.

Electron heat flux dropouts in the solar wind - Evidence for interplanetary magnetic field reconnection?

NASA Technical Reports Server (NTRS)

Mccomas, D. J.; Gosling, J. T.; Phillips, J. L.; Bame, S. J.; Luhmann, J. G.; Smith, E. J.

1989-01-01

An examination of ISEE-3 data from 1978 reveal 25 electron heat flux dropout events ranging in duration from 20 min to over 11 hours. The heat flux dropouts are found to occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. It is suggested that the heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the sun and instead are connected to the outer heliosphere at both ends.

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

NASA Astrophysics Data System (ADS)

Bhatti, Zahid Imran; Zhao, Junmeng; Khan, Nangyal Ghani; Shah, Syed Tallataf Hussain

2018-08-01

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Discrete velocity computations with stochastic variance reduction of the Boltzmann equation for gas mixtures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clarke, Peter; Varghese, Philip; Goldstein, David

We extend a variance reduced discrete velocity method developed at UT Austin [1, 2] to gas mixtures with large mass ratios and flows with trace species. The mixture is stored as a collection of independent velocity distribution functions, each with a unique grid in velocity space. Different collision types (A-A, A-B, B-B, etc.) are treated independently, and the variance reduction scheme is formulated with different equilibrium functions for each separate collision type. The individual treatment of species enables increased focus on species important to the physics of the flow, even if the important species are present in trace amounts. Themore » method is verified through comparisons to Direct Simulation Monte Carlo computations and the computational workload per time step is investigated for the variance reduced method.« less

The hydrogen coma of Comet P/Halley observed in Lyman-alpha using sounding rockets

NASA Technical Reports Server (NTRS)

Mccoy, R. P.; Meier, R. R.; Keller, H. U.; Opal, C. B.; Carruthers, G. R.

1992-01-01

Hydrogen Lyman-alpha (121.6 nm) images of Comet P/Halley were obtained using sounding rockets launched from White Sands Missile Range on 24.5 February and 13.5 March 1986. The second rocket was launched 13 hours before the fly-by of the Giotto spacecraft. An electrographic camera on both flights provided Lyman-alpha images covering a 20 field of view with 3 arcmin resolution. The data from both flights have been compared with a time-dependent model of hydrogen kinetics. To match the measured isophote contours, hydrogen sources with velocity components of 8 km/s and 20 km/s (from OH and H2O respectively) as well as a low velocity component (about 2 km/s) are required. This low velocity component is thought to result from thermalization of fast hydrogen atoms within the collision zone, providing an important diagnostic of temperature and density near the nucleus. Hydrogen production rates of 3.8 x 10 exp 30/s and 1.7 x 10 exp 30/s have been obtained for the two observations.

On the Development of Turbulent Wakes from Vortex Streets

NASA Technical Reports Server (NTRS)

Roshko, Anatol

1954-01-01

Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hot-wire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At r=40 to 150, called the stable range, regular vortex streets are formed and no turbulent velocity fluctuations accompany the periodic formation of vortices. The range r=150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. The transition first occurs in the range r=150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations.

Covariance Between Arctic Sea Ice and Clouds Within Atmospheric State Regimes at the Satellite Footprint Level

NASA Technical Reports Server (NTRS)

Taylor, Patrick C.; Kato, Seiji; Xu, Kuan-Man; Cai, Ming

2015-01-01

Understanding the cloud response to sea ice change is necessary for modeling Arctic climate. Previous work has primarily addressed this problem from the interannual variability perspective. This paper provides a refined perspective of sea ice-cloud relationship in the Arctic using a satellite footprint-level quantification of the covariance between sea ice and Arctic low cloud properties from NASA A-Train active remote sensing data. The covariances between Arctic low cloud properties and sea ice concentration are quantified by first partitioning each footprint into four atmospheric regimes defined using thresholds of lower tropospheric stability and mid-tropospheric vertical velocity. Significant regional variability in the cloud properties is found within the atmospheric regimes indicating that the regimes do not completely account for the influence of meteorology. Regional anomalies are used to account for the remaining meteorological influence on clouds. After accounting for meteorological regime and regional influences, a statistically significant but weak covariance between cloud properties and sea ice is found in each season for at least one atmospheric regime. Smaller average cloud fraction and liquid water are found within footprints with more sea ice. The largest-magnitude cloud-sea ice covariance occurs between 500m and 1.2 km when the lower tropospheric stability is between 16 and 24 K. The covariance between low cloud properties and sea ice is found to be largest in fall and is accompanied by significant changes in boundary layer temperature structure where larger average near-surface static stability is found at larger sea ice concentrations.

Covariance between Arctic sea ice and clouds within atmospheric state regimes at the satellite footprint level

PubMed Central

Kato, Seiji; Xu, Kuan‐Man; Cai, Ming

2015-01-01

Abstract Understanding the cloud response to sea ice change is necessary for modeling Arctic climate. Previous work has primarily addressed this problem from the interannual variability perspective. This paper provides a refined perspective of sea ice‐cloud relationship in the Arctic using a satellite footprint‐level quantification of the covariance between sea ice and Arctic low cloud properties from NASA A‐Train active remote sensing data. The covariances between Arctic low cloud properties and sea ice concentration are quantified by first partitioning each footprint into four atmospheric regimes defined using thresholds of lower tropospheric stability and midtropospheric vertical velocity. Significant regional variability in the cloud properties is found within the atmospheric regimes indicating that the regimes do not completely account for the influence of meteorology. Regional anomalies are used to account for the remaining meteorological influence on clouds. After accounting for meteorological regime and regional influences, a statistically significant but weak covariance between cloud properties and sea ice is found in each season for at least one atmospheric regime. Smaller average cloud fraction and liquid water are found within footprints with more sea ice. The largest‐magnitude cloud‐sea ice covariance occurs between 500 m and 1.2 km when the lower tropospheric stability is between 16 and 24 K. The covariance between low cloud properties and sea ice is found to be largest in fall and is accompanied by significant changes in boundary layer temperature structure where larger average near‐surface static stability is found at larger sea ice concentrations. PMID:27818851

Covariance between Arctic sea ice and clouds within atmospheric state regimes at the satellite footprint level.

PubMed

Taylor, Patrick C; Kato, Seiji; Xu, Kuan-Man; Cai, Ming

2015-12-27

Understanding the cloud response to sea ice change is necessary for modeling Arctic climate. Previous work has primarily addressed this problem from the interannual variability perspective. This paper provides a refined perspective of sea ice-cloud relationship in the Arctic using a satellite footprint-level quantification of the covariance between sea ice and Arctic low cloud properties from NASA A-Train active remote sensing data. The covariances between Arctic low cloud properties and sea ice concentration are quantified by first partitioning each footprint into four atmospheric regimes defined using thresholds of lower tropospheric stability and midtropospheric vertical velocity. Significant regional variability in the cloud properties is found within the atmospheric regimes indicating that the regimes do not completely account for the influence of meteorology. Regional anomalies are used to account for the remaining meteorological influence on clouds. After accounting for meteorological regime and regional influences, a statistically significant but weak covariance between cloud properties and sea ice is found in each season for at least one atmospheric regime. Smaller average cloud fraction and liquid water are found within footprints with more sea ice. The largest-magnitude cloud-sea ice covariance occurs between 500 m and 1.2 km when the lower tropospheric stability is between 16 and 24 K. The covariance between low cloud properties and sea ice is found to be largest in fall and is accompanied by significant changes in boundary layer temperature structure where larger average near-surface static stability is found at larger sea ice concentrations.

Dirac bubble potential for He-He and inadequacies in the continuum: Comparing an analytic model with elastic collision experiments

NASA Astrophysics Data System (ADS)

Chrysos, Michael

2017-01-01

We focus on the long-pending issue of the inadequacy of the Dirac bubble potential model in the description of He-He interactions in the continuum [L. L. Lohr and S. M. Blinder, Int. J. Quantum Chem. 53, 413 (1995)]. We attribute this failure to the lack of a potential wall to mimic the onset of the repulsive interaction at close range separations. This observation offers the explanation to why this excessively simple model proves incapable of quantitatively reproducing previous experimental findings of glory scattering in He-He, although being notorious for its capability of reproducing several distinctive features of the atomic and isotopic helium dimers and trimers [L. L. Lohr and S. M. Blinder, Int. J. Quantum Chem. 90, 419 (2002)]. Here, we show that an infinitely high, energy-dependent potential wall of properly calculated thickness rc(E) taken as a supplement to the Dirac bubble potential suffices for agreement with variable-energy elastic collision cross section experiments for 4He-4He, 3He-4He, and 3He-3He [R. Feltgen et al., J. Chem. Phys. 76, 2360 (1982)]. In the very low energy regime, consistency is found between the Dirac bubble potential (to which our extended model is shown to reduce) and cold collision experiments [J. C. Mester et al., Phys. Rev. Lett. 71, 1343 (1993)]; this consistency, which in this regime lends credence to the Dirac bubble potential, was never noticed by its authors. The revised model being still analytic is of high didactical value while expected to increase in predictive power relative to other appraisals.

Velocity renormalization in graphene: The role of trigonal warping and electron-phonon coupling effects

NASA Astrophysics Data System (ADS)

Kandemir, B. S.; Gökçek, N.

2017-12-01

We investigate the combined effects of trigonal warping and electron-phonon interactions on the renormalization of the Fermi velocity in graphene. We present an analytical solution to the associated Fröhlich Hamiltonian describing the interaction of doubly degenerate-optical phonon modes of graphene with electrons in the presence of trigonal warp within the framework of Lee-Low-Pines theory. On the basis of our model, it is analytically shown that in addition to its renormalization, Fermi velocity exhibits strong anisotropy due to the trigonal warping. It is also found that in the regime where the trigonal warp starts, distortion of energy bands emerges due to electron-phonon coupling, and the bands exhibit strong anisotropy.

Linear instability of compound liquid threads in the presence of surfactant

NASA Astrophysics Data System (ADS)

Ye, Han-yu; Yang, Li-jun; Fu, Qing-fei

2017-08-01

This paper investigates the linear instability of compound liquid threads in the presence of surfactant. The limitation of the one-dimensional approximation in previous work [Craster, Matar, and Papageorgiou, Phys. Fluids 15, 3409 (2003), 10.1063/1.1611879] is removed; hence the radial dependence of the axial velocity can be taken into account. Therefore both the stretching and the squeezing modes can be investigated. The disturbance growth rate is reduced with an increase of the dimensionless surface-tension gradient (whether in the stretching or squeezing mode). For the parameter range investigated, it is found that the squeezing mode is much more sensitive to the Marangoni effect than the stretching mode. The disturbance axial velocity and disturbance surfactant concentration for a typical case is investigated. It is found that the disturbance axial velocity is close to uniform in the stretching mode when the dimensionless surface-tension gradient and the wave number are small. In contrast, for wave numbers close to cutoff, or a large dimensionless surface-tension gradient, or in the squeezing mode, the disturbance axial velocity is not uniform. Analytical relations between growth rate and wave number valid in the long-wave limit are derived. In the stretching mode, the flow moves from an extension-dominated regime to a shear-dominated regime when β1+R σ β2 increases through 1 +R σ , where β1 and β2 are the dimensionless surface-tension gradient of the inner and outer interface, respectively, R is the radius ratio, and σ is the surface tension ratio. In the squeezing mode, whatever the values of β1 and β2, the flow is always in the shear-dominated regime. The expressions of the leading-order axial perturbation velocity in the long-wave limit are derived and they explain the applicability of one-dimensional models. It is found that the leading-order axial velocity in the extension-dominated regime is always uniform and one-dimensional models work well in this regime. For the shear-dominated regime, the leading-order axial velocity can be either nonuniform or close to uniform, depending on the ratio between the dimensionless surfactant diffusivity d1 and the Laplace number La : when d1≫La the velocity profile is close to uniform and one-dimensional models work well; otherwise the velocity profile is nonuniform and one-dimensional models fail.

Ion-Atom Cold Collisions and Atomic Clocks

NASA Technical Reports Server (NTRS)

Prestage, John D.; Maleki, Lute; Tjoelker, Robert L.

1997-01-01

Collisions between ultracold neutral atoms have for some time been the subject of investigation, initially with hydrogen and more recently with laser cooled alkali atoms. Advances in laser cooling and trapping of neutral atoms in a Magneto-Optic Trap (MOT) have made cold atoms available as the starting point for many laser cooled atomic physics investigations. The most spectacularly successful of these, the observation of Bose-Einstein Condensation (BEC) in a dilute ultra-cold spin polarized atomic vapor, has accelerated the study of cold collisions. Experimental and theoretical studies of BEC and the long range interaction between cold alkali atoms is at the boundary of atomic and low temperature physics. Such studies have been difficult and would not have been possible without the development and advancement of laser cooling and trapping of neutral atoms. By contrast, ion-atom interactions at low temperature, also very difficult to study prior to modern day laser cooling, have remained largely unexplored. But now, many laboratories worldwide have almost routine access to cold neutral atoms. The combined technologies of ion trapping, together with laser cooling of neutrals has made these studies experimentally feasible and several very important, novel applications might come out of such investigations . This paper is an investigation of ion-atom interactions in the cold and ultra-cold temperature regime. Some of the collisional ion-atom interactions present at room temperature are very much reduced in the low temperature regime. Reaction rates for charge transfer between unlike atoms, A + B(+) approaches A(+) + B, are expected to fall rapidly with temperature, approximately as T(sup 5/2). Thus, cold mixtures of atoms and ions are expected to coexist for very long times, unlike room temperature mixtures of the same ion-atom combination. Thus, it seems feasible to cool ions via collisions with laser cooled atoms. Many of the conventional collisional interactions, exploited as a useful tool at room temperature and higher, are greatly enhanced at low energy. For example, collisional spin transfer from one species of polarized atoms to another has long been a useful method for polarizing a sample of atoms where no other means was available. Because optical pumping cannot be used to polarize the nuclear spin of Xe-129 or He-3 (for use in nmr imaging of the lungs), the nuclear spins are polarized via collisions with an optically pumped Rb vapor in a cell containing both gases. In another case, a spin polarized thermal Cs beam was used to polarize the hyperfine states of trapped He(+)-3 ions in order to measure their hyperfine clock transition frequency. The absence of an x-ray light source to optically pump the ground state of the He(+)-3 ion necessitated this alternative state preparation. Similarly, Cd(+) and Sr(+) ions were spin-oriented via collisions in a cell with optically pumped Rb vapor. Resonant RF spin changing transitions in the ground state of the ions were detected by changes in the Rb resonance light absorption. Because cold collision spin exchange rates scale with temperature as T(sup -1/2) this technique is expected to be a far more powerful tool than the room temperature counterpart. This factor of 100 or more enhancement in spin exchange reaction rates at low temperatures is the basis for a novel trapped ion clock where laser cooled neutrals will cool, state select and monitor the ion clock transition. The advantage over conventional direct laser cooling of trapped ions is that the very expensive and cumbersome UV laser light sources, required to excite the ionic cooling transition, are effectively replaced by simple diode lasers.

From P-T-age to secular change and global tectonic regimes (or Essene in reverse - from granulites to blueschists and eclogites over time)

NASA Astrophysics Data System (ADS)

Brown, M.

2006-12-01

Essene's contributions began pre-plate tectonics more than 40 years ago; they range from mineralogy to tectonics, from experiments and thermobarometry to elements and isotopes, and from the Phanerozoic to the Precambrian. Eric is a true polymath! Assessing the P-T conditions and age distribution of crustal metamorphism is an important step in evaluating secular change in tectonic regimes and geodynamics. In general, Archean rocks exhibit moderate-P - moderate-to-high-T facies series metamorphism (greenstone belts and granulite terranes); neither blueschists nor any record of deep continental subduction and return are documented and only one example of granulite facies ultrahigh-temperature metamorphism is reported. Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian, although G-UHTM facies series rocks may be inferred at depth in younger orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the formation and breakup of supercontinents, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those around the modern Pacific rim. Medium-temperature eclogite - high-pressure granulite metamorphism (E-HPGM) also is first recognized in the Neoarchean rock record, and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E- HPGM belts are complementary to G-UHTM belts, and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; lawsonite blueschists and eclogites (high-pressure metamorphism, HPM), and ultrahigh pressure metamorphism (UHPM) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts - reflecting a duality of thermal regimes - appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G- UHTM and E-HPGM belts since the Neoarchean manifests the onset of a `Proterozoic plate tectonics regime', although the style of tectonics likely involved differences from modern Earth. Although the style of Proterozoic subduction remains cryptic, the change in tectonic regime whereby interactions between discrete lithospheric plates generated tectonic settings with contrasting thermal regimes was a landmark event in Earth history. The `Proterozoic plate tectonics regime' evolved during a Neoproterozoic transition to the `modern plate tectonics regime' characterized by colder subduction, and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of blueschists and HPM-UHPM in the rock record.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter.

PubMed

Cayzac, W; Frank, A; Ortner, A; Bagnoud, V; Basko, M M; Bedacht, S; Bläser, C; Blažević, A; Busold, S; Deppert, O; Ding, J; Ehret, M; Fiala, P; Frydrych, S; Gericke, D O; Hallo, L; Helfrich, J; Jahn, D; Kjartansson, E; Knetsch, A; Kraus, D; Malka, G; Neumann, N W; Pépitone, K; Pepler, D; Sander, S; Schaumann, G; Schlegel, T; Schroeter, N; Schumacher, D; Seibert, M; Tauschwitz, An; Vorberger, J; Wagner, F; Weih, S; Zobus, Y; Roth, M

2017-06-01

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

NASA Astrophysics Data System (ADS)

Cayzac, W.; Frank, A.; Ortner, A.; Bagnoud, V.; Basko, M. M.; Bedacht, S.; Bläser, C.; Blažević, A.; Busold, S.; Deppert, O.; Ding, J.; Ehret, M.; Fiala, P.; Frydrych, S.; Gericke, D. O.; Hallo, L.; Helfrich, J.; Jahn, D.; Kjartansson, E.; Knetsch, A.; Kraus, D.; Malka, G.; Neumann, N. W.; Pépitone, K.; Pepler, D.; Sander, S.; Schaumann, G.; Schlegel, T.; Schroeter, N.; Schumacher, D.; Seibert, M.; Tauschwitz, An.; Vorberger, J.; Wagner, F.; Weih, S.; Zobus, Y.; Roth, M.

2017-06-01

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

PubMed Central

Cayzac, W.; Frank, A.; Ortner, A.; Bagnoud, V.; Basko, M. M.; Bedacht, S.; Bläser, C.; Blažević, A.; Busold, S.; Deppert, O.; Ding, J.; Ehret, M.; Fiala, P.; Frydrych, S.; Gericke, D. O.; Hallo, L.; Helfrich, J.; Jahn, D.; Kjartansson, E.; Knetsch, A.; Kraus, D.; Malka, G.; Neumann, N. W.; Pépitone, K.; Pepler, D.; Sander, S.; Schaumann, G.; Schlegel, T.; Schroeter, N.; Schumacher, D.; Seibert, M.; Tauschwitz, An.; Vorberger, J.; Wagner, F.; Weih, S.; Zobus, Y.; Roth, M.

2017-01-01

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions. PMID:28569766

Magnetic Reconnection Processes Involving Modes Propagating in the Ion Diamagnetic Velocity Direction

NASA Astrophysics Data System (ADS)

Buratti, P.; Coppi, B.; Pucella, G.; Zhou, T.

2013-10-01

Experiments in weakly collisional plasma regimes, (e.g. neutral beam heated plasmas in the H-regime), measuring the Doppler shift associated with the plasma local rotation, have shown that the toroidal mode phase velocity vph in the frame with Er = 0 is in the direction of the ion diamagnetic velocity. For ohmically heated plasmas, with higher collisionalities, vph in the laboratory frame is in the direction of the electron diamagnetic velocity, but plasma rotation is reversed as well, and vph, in the Er = 0 frame, is in the ion diamagnetic velocity direction. Theoretically, two classes of reconnecting modes should emerge: drift-tearing modes and ``inductive modes'' that depend on the effects of a finite plasma inductivity. The former modes, with vph in the direction of the electron diamagnetic velocity, require the pre-excitation of a different kind of mode in order to become unstable in weakly collisional regimes. The second kind of modes has a growth rate associated with the relevant finite ion viscosity. A comprehensive theory is presented. Sponsored in part by the US DOE.

The Effect of a Potentially Low Solar Cycle #24 on Orbital Lifetimes of Fengyun 1-C Debris

NASA Technical Reports Server (NTRS)

Whitlock, David; Johnson, Nicholas; Matney, Mark; Krisko, Paula

2008-01-01

The magnitude of Solar Cycle #24 will have a non-trivial impact on the lifetimes of debris pieces that resulted from the intentional hypervelocity impact of the Fengyun 1-C satellite in January 2007. Recent solar flux measurements indicate Solar Cycle #24 has begun in the last few months, and will continue until approximately 2019. While there have been differing opinions on whether the intensity of this solar cycle will be higher or lower than usual, the Space Weather Prediction Center within the National Oceanic Atmospheric Administration (NOAA/SWPC) has recently forecast unusually low solar activity, which would result in longer orbital lifetimes. Using models for both the breakup of Fengyun 1-C and the propagation of the resultant debris cloud, the Orbital Debris Program Office at NASA Johnson Space Center conducted a study to better understand the impact of the solar cycle on lifetimes for pieces as small as 1 mm. Using a modified collision breakup model and PROP3D propagation software, the orbits of nearly 2 million objects 1 mm and larger were propagated for up to 200 years. By comparing a normal solar cycle with that of the NOAA/SWPC forecast low cycle, the effect of the solar flux on the lifetimes of the debris pieces is evaluated. The modeling of the low solar cycle shows an additional debris count of 12% for pieces larger than 10 cm by 2019 when compared to the resultant debris count using a normal cycle. The difference becomes more exaggerated (over 15%) for debris count in the smaller size regimes. However, in 50 years, the models predict the differences in debris count from differing models of Solar Cycle #24 to be less than 10% for all size regimes, with less variance in the smaller sizes. Understanding the longevity of the debris cloud will affect collision probabilities for both operational spacecraft and large derelict objects over the next century and beyond.

Reflection, transmutation, annihilation, and resonance in two-component kink collisions

NASA Astrophysics Data System (ADS)

Alonso-Izquierdo, A.

2018-02-01

In this paper, the study of collisions between kinks arising in the family of MSTB models is addressed. Phenomena such as elastic kink reflection, mutual annihilation, kink-antikink transmutation and inelastic reflection are found and depend on the impact velocity.

A Laboratory Impact Study of Simulated Edgeworth-Kuiper Belt Objects

NASA Astrophysics Data System (ADS)

Ryan, Eileen V.; Davis, Donald R.; Giblin, Ian

1999-11-01

This paper reports on a series of laboratory impact experiments designed to provide basic data on how simulated Edgeworth-Kuiper belt objects (EKOs) fragment in an impact event. In September-October 1997 we carried out 20 low-velocity airgun shots at the Ames Vertical Gun Range into porous and homogeneous ice spheres using aluminum, fractured ice, and solid ice projectiles. We found that the porous ice targets behaved as strongly as solid ice in collision. Energy is apparently well dissipated by the void spaces within the target, such that these fragile ice structures respond as if they were strong in impacts. Therefore, it would appear that if EKOs are porous, they are not collisionally weak. Also, our data show that collisional outcomes for low-velocity impacts into ice targets depend on the type of projectile used as well as the properties of the target. We observed that the degree of fragmentation for a given type of target increases as the strength of the projectile increases. Aluminum projectiles are far more damaging to the target at the same collisional energy than are solid ice projectiles, which, in turn, are more damaging than fractured ice projectiles. One possible explanation for this behavior is the variable depth of penetration of the projectile for the different cases—stronger projectiles penetrate more deeply and couple more energy into the target than do weak projectiles. Based on this, if we assume that there has not been significant heating or differentiation in the Edgeworth-Kuiper (E-K) belt, the most applicable impact strength for the low-velocity E-K belt collisions is likely to be that derived from similar target/projectile materials impacting each other. The laboratory data from this analysis indicate that a value for impact strength>5×10 5 erg/cm 3 is appropriate for porous ice targets impacted with solid/porous ice projectiles.