Large scale EMF in current sheets induced by tearing modes

NASA Astrophysics Data System (ADS)

Mizerski, Krzysztof A.

2018-02-01

An extension of the analysis of resistive instabilities of a sheet pinch from a famous work by Furth et al (1963 Phys. Fluids 6 459) is presented here, to study the mean electromotive force (EMF) generated by the developing instability. In a Cartesian configuration and in the presence of a current sheet first the boundary layer technique is used to obtain global, matched asymptotic solutions for the velocity and magnetic field and then the solutions are used to calculate the large-scale EMF in the system. It is reported, that in the bulk the curl of the mean EMF is linear in {{j}}0\\cdot {{B}}0, a simple pseudo-scalar quantity constructed from the large-scale quantities.

Characteristics and transport effects of the electron drift instability in Hall-effect thrusters

NASA Astrophysics Data System (ADS)

Lafleur, T.; Baalrud, S. D.; Chabert, P.

2017-02-01

The large electron {E}× {B} drift (relative to the ions) in the azimuthal direction of Hall-effect thrusters is well known to excite a strong instability. In a recent paper (Lafleur et al 2016 Phys. Plasmas 23 053503) we demonstrated that this instability leads to an enhanced electron-ion friction force that increases the electron cross-field mobility to levels similar to those seen experimentally. Here we extend this work by considering in detail the onset criteria for the formation of this instability (both in xenon, and other propellants of interest), and identify a number of important characteristics that it displays within Hall-effect thrusters (HETs): including the appearance of an additional non-dimensionalized scaling parameter (the instability growth-to-convection ratio), which controls the instability evolution and amplitude. We also investigate the effect that the instability has on electron and ion heating in HETs, and show that it leads to an ion rotation in the azimuthal direction that is in agreement with that seen experimentally.

An Instability in Narrow Planetary Rings

NASA Astrophysics Data System (ADS)

Weiss, J. W.; Stewart, G. R.

2003-08-01

We will present our work investigating the behavior of narrow planetary rings with low dispersion velocities. Such narrow a ring will be initially unstable to self-gravitational collapse. After the collapse, the ring is collisionally very dense. At this stage, it is subject to a new instability. Waves appear on the inner and outer edges of the ring within half of an orbital period. The ring then breaks apart radially, taking approximately a quarter of an orbital period of do so. As clumps of ring particles expand radially away from the dense ring, Kepler shear causes these clumps to stretch out azimuthally, and eventually collapse into a new set of dense rings. Small-scale repetitions of the original instability in these new rings eventually leads to a stabilized broad ring with higher dispersion velocities than the initial ring. Preliminary results indicate that this instability may be operating on small scales in broad rings in the wake-like features seen by Salo and others. Some intriguing properties have been observed during this instability. The most significant is a coherence in the epicyclic phases of the particles. Both self-gravity and collisions in the ring operated to create and enforce this coherence. The coherence might also be responsible for the instability to radial expansion. We also observe that guiding centers of the particles do not migrate to the center of the ring during the collapse phase of the ring. In fact, guiding centers move radially away from the core of the ring during this phase, consistent with global conservation of angular momentum. We will show the results of our simulations to date, including movies of the evolution of various parameters. (Audiences members wanting popcorn are advised to bring their own.) This work is supported by a NASA Graduate Student Research Program grant and by the Cassini mission.

Rayleigh-Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. I

NASA Astrophysics Data System (ADS)

Zhou, Ye

2017-12-01

Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities play an important role in a wide range of engineering, geophysical, and astrophysical flows. They represent a triggering event that, in many cases, leads to large-scale turbulent mixing. Much effort has been expended over the past 140 years, beginning with the seminal work of Lord Rayleigh, to predict the evolution of the instabilities and of the instability-induced mixing layers. The objective of Part I of this review is to provide the basic properties of the flow, turbulence, and mixing induced by RT, RM, and Kelvin-Helmholtz (KH) instabilities. Historical efforts to study these instabilities are briefly reviewed, and the significance of these instabilities is discussed for a variety of flows, particularly for astrophysical flows and for the case of inertial confinement fusion. Early experimental efforts are described, and analytical attempts to model the linear, and nonlinear regimes of these mixing layers are examined. These analytical efforts include models for both single-mode and multi-mode initial conditions, as well as multi-scale models to describe the evolution. Comparisons of these models and theories to experimental and simulation studies are then presented. Next, attention is paid to the issue of the influence of stabilizing mechanisms (e.g., viscosity, surface tension, and diffuse interface) on the evolution of these instabilities, as well as the limitations and successes of numerical methods. Efforts to study these instabilities and mixing layers using group-theoretic ideas, as well as more formal notions of turbulence cascade processes during the later stages of the induced mixing layers, are inspected. A key element of the review is the discussion of the late-time self-similar scaling for the RT and RM growth factors, α and θ. These parameters are influenced by the initial conditions and much of the observed variation can be explained by this. In some cases, these instabilities induced flows can transition to turbulence. Both the spatial and temporal criteria to achieve the transition to turbulence have been examined. Finally, a description of the energy-containing scales in the mixing layers, including energy "injection" and cascade processes are presented in greater detail. Part II of this review is designed to provide a much broader and in-depth understanding of this critical area of research (Zhou, 2017. Physics Reports, 723-725, 1-160).

Transient and diffusion analysis of HgCdTe

NASA Technical Reports Server (NTRS)

Clayton, J. C.

1982-01-01

Solute redistribution during directional solidification of HgCdTe is addressed. Both one-dimensional and two-dimensional models for solute redistribution are treated and model results compared to experiment. The central problem studied is the cause of radial inhomogeneities found in directionally solidified HgCdTe. A large scale gravity-driven interface instability, termed shape instability, is postulated to be the cause of radial inhomogeneities. Recommendations for future work, along with appropriate computer programs, are included.

Numerical Simulation of Self-gravitational Instability of Isothermal Gaseous Slab Under High External Pressure

NASA Astrophysics Data System (ADS)

Miyaji, S.; Umekawa, M.; Matsumoto, R.; Yoshida, T.

1996-05-01

Gaseous slab is formed with shock waves from super novae, collision of interstellar clouds, etc. When the mass in the Jeans scale is more than Jeans mass, the slab fragments into many clumps by gravitational instability. But in high external pressure environment, even the slab which is stable against Jeans mode can fragment(Elmegreen and Elmegreen 1978).This phenomenon results from incompressible mode instability(Lubow and Pringle 1993). These works are by linear analysis. We study numerically this isothermal gaseous slab which is formed by high external pressure and whose thickness is much smaller than its scale height. We assume self-gravitational fluid, and use two dimensional flux split method. Our model size is taken about the scale of linear maximum growth rate wave length and its five times length, which is an example of much longer than the maximum growth rate wave length. When the incompressible mode instability takes place, it becomes clumps. Each mass of the clumps is less than the Jeans mass. Then the clumps approach each other by gravitational interaction to form bigger clumps. In the presentation we will show results of numerical simulation and discuss about the interaction of fragments on star formation or initial mass function.

Analytical scalings of the linear Richtmyer-Meshkov instability

NASA Astrophysics Data System (ADS)

Cobos, Francisco; Wouchuk, Juan Gustavo

2017-11-01

In the linear Richtmyer-Meshkov instability (RMI), hydrodynamic perturbations are generated behind the transmitted and reflected rippled fronts. The contact surface reaches an asymptotic normal velocity and two different tangential velocities at each side, which are always different for moderate to strong levels of compression, depending on the amount of vorticity generated by the corrugated shocks. We show analytical scaling laws for the ripple velocity (δvi∞)in different physical limits and approximate formulas are provided, valid for arbitrary initial pre-shock parameters. An asymptotic growth for the contact surface ripple of the form ψi(t) ψ∞ + δ vi∞t is obtained. The quantity ψ∞ is in general different from the initial post-shock ripple amplitude, in agreement with the early finding of. Comparison to simulations and experimental work is shown. F.C. acknowledges support from UCLM for a predoctoral fellowship. This work has received support from MINECO, JCCM, and UCLM (Spain).

A powerful local shear instability in weakly magnetized disks. I - Linear analysis. II - Nonlinear evolution

NASA Technical Reports Server (NTRS)

Balbus, Steven A.; Hawley, John F.

1991-01-01

A broad class of astronomical accretion disks is presently shown to be dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field, an insight with consequently broad applicability to gaseous, differentially-rotating systems. In the first part of this work, a linear analysis is presented of the instability, which is local and extremely powerful; the maximum growth rate, which is of the order of the angular rotation velocity, is independent of the strength of the magnetic field. Fluid motions associated with the instability directly generate both poloidal and toroidal field components. In the second part of this investigation, the scaling relation between the instability's wavenumber and the Alfven velocity is demonstrated, and the independence of the maximum growth rate from magnetic field strength is confirmed.

Nonlinear saturation of the Weibel instability

DOE PAGES

Cagas, P.; Hakim, A.; Scales, W.; ...

2017-11-21

The growth and saturation of magnetic fields due to the Weibel instability (WI) have important implications for laboratory and astrophysical plasmas, and this has drawn significant interest recently. Since the WI can generate a large magnetic field from no initial field, the maximum magnitudes achieved can have significant consequences for a number of applications. Hence, an understanding of the detailed dynamics driving the nonlinear saturation of the WI is important. This work considers the nonlinear saturation of the WI when counter-streaming populations of initially unmagnetized electrons are perturbed by a magnetic field oriented perpendicular to the direction of streaming. Previousmore » works have found magnetic trapping to be important and connected electron skin depth spatial scales to the nonlinear saturation of the WI. The results presented in this work are consistent with these findings for a high-temperature case. However, using a high-order continuum kinetic simulation tool, this work demonstrates that when the electron populations are colder, a significant electrostatic potential develops that works with the magnetic field to create potential wells. The electrostatic field develops due to transverse flows induced by the WI and in some cases is strengthened by a secondary instability. This field plays a key role in saturation of the WI for colder populations. In conclusion, the role of the electrostatic potential in Weibel instability saturation has not been studied in detail previously.« less

Nonlinear saturation of the Weibel instability

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

Cagas, P.; Hakim, A.; Scales, W.

The growth and saturation of magnetic fields due to the Weibel instability (WI) have important implications for laboratory and astrophysical plasmas, and this has drawn significant interest recently. Since the WI can generate a large magnetic field from no initial field, the maximum magnitudes achieved can have significant consequences for a number of applications. Hence, an understanding of the detailed dynamics driving the nonlinear saturation of the WI is important. This work considers the nonlinear saturation of the WI when counter-streaming populations of initially unmagnetized electrons are perturbed by a magnetic field oriented perpendicular to the direction of streaming. Previousmore » works have found magnetic trapping to be important and connected electron skin depth spatial scales to the nonlinear saturation of the WI. The results presented in this work are consistent with these findings for a high-temperature case. However, using a high-order continuum kinetic simulation tool, this work demonstrates that when the electron populations are colder, a significant electrostatic potential develops that works with the magnetic field to create potential wells. The electrostatic field develops due to transverse flows induced by the WI and in some cases is strengthened by a secondary instability. This field plays a key role in saturation of the WI for colder populations. In conclusion, the role of the electrostatic potential in Weibel instability saturation has not been studied in detail previously.« less

Upstream and Downstream Influence in STBLI Instability

NASA Astrophysics Data System (ADS)

Martin, Pino; Priebe, Stephan; Helm, Clara

2016-11-01

Priebe and Martín (JFM, 2012) show that the low-frequency unsteadiness in shockwave and turbulent boundary layer interactions (STBLI) is governed by an inviscid instability. Priebe, Tu, Martín and Rowley (JFM, 2016) show that the instability is an inviscid centrifugal one, i.e Görtlerlike vortices. Previous works had given differing conclusions as to whether the low-frequency unsteadiness in STBLI is caused by an upstream or downstream mechanism. In this paper, we reconcile these opposite views and show that upstream and downstream correlations co-exist in the context of the nature of Görtler vortices. We find that the instability is similar to that in separated subsonic and laminar flows. Since the turbulence is modulated but passive to the global mode, the turbulent separated flows are amenable to linear global analysis. As such, the characteristic length and time scales, and the receptivity of the global mode might be determined, and low-order models that represent the low-frequency dynamics in STBLI might be developed. The centrifugal instability persists even under hypersonic conditions. This work is funded by the AFOSR Grant Number AF9550-15-1-0284 with Dr. Ivett Leyva.

The Occurrence of Small-scale Irregularities in the Mid-latitude Ionosphere from SuperDARN HF Radar Observations

NASA Astrophysics Data System (ADS)

Ruohoniemi, J. M.; Baker, J. B.; Maimaiti, M.; Oksavik, K.; Erickson, P. J.; Scales, W.; Eltrass, A.

2017-12-01

The mid-latitude radars of the SuperDARN network routinely observe backscatter from nighttime decameter-scale F region irregularities at latitudes well equatorward of the auroral boundary. This Sub-Auroral Ionospheric Scatter (SAIS) is strongly distinguished from auroral and SAPS backscatter by low Doppler velocities ( tens m/s) and stable, long-lived ( hours) occurrence in discrete events that are extended in both latitude and longitude. Statistical and event studies of SAIS with the SuperDARN radars indicate that the subauroral F region ionosphere is replete with irregularities during events, at least poleward of the 50° Λ horizon of the North American mid-latitude radars, and that radar observation of SAIS backscatter is then primarily limited by the magnetic aspect condition. Joint experiments with incoherent scatter radar have furnished sets of plasma measurements suitable for testing theories of plasma instability. Modeling work stimulated by the observations has explored the temperature-gradient instability (TGI) and the gradient drift instability (GDI) as possible sources of the irregularities. In this talk we review the findings on the occurrence of the SAIS category of mid-latitude F region irregularities, summarize the results of the modeling work, and discuss future research directions.

Transverse electron-scale instability in relativistic shear flows.

PubMed

Alves, E P; Grismayer, T; Fonseca, R A; Silva, L O

2015-08-01

Electron-scale surface waves are shown to be unstable in the transverse plane of a sheared flow in an initially unmagnetized collisionless plasma, not captured by (magneto)hydrodynamics. It is found that these unstable modes have a higher growth rate than the closely related electron-scale Kelvin-Helmholtz instability in relativistic shears. Multidimensional particle-in-cell simulations verify the analytic results and further reveal the emergence of mushroomlike electron density structures in the nonlinear phase of the instability, similar to those observed in the Rayleigh Taylor instability despite the great disparity in scales and different underlying physics. This transverse electron-scale instability may play an important role in relativistic and supersonic sheared flow scenarios, which are stable at the (magneto)hydrodynamic level. Macroscopic (≫c/ωpe) fields are shown to be generated by this microscopic shear instability, which are relevant for particle acceleration, radiation emission, and to seed magnetohydrodynamic processes at long time scales.

The Work Instability Scale for rheumatoid arthritis predicts arthritis-related work transitions within 12 months.

PubMed

Tang, Kenneth; Beaton, Dorcas E; Gignac, Monique A M; Lacaille, Diane; Zhang, Wei; Bombardier, Claire

2010-11-01

Among people with arthritis, the need for work transitions may signal a risk for more adverse work outcomes in the future, such as permanent work loss. Our aim was to evaluate the ability of the Work Instability Scale for Rheumatoid Arthritis (RA-WIS) to predict arthritis-related work transitions within a 12-month period. Workers with osteoarthritis or rheumatoid arthritis (n = 250) from 3 clinical sites participated in self-administered surveys that assessed the impact of health on employment at multiple time points over 12 months. Multivariable logistic regressions were conducted to assess the ability of the RA-WIS (range 0-23, where 23 = highest work instability) to predict 4 types of work transition: reductions in work hours, disability leaves of absence, changes in job/occupation, or temporary unemployment, assembled as a composite outcome. Covariates assessed include age, sex, education, marital status, income, pain intensity, disease duration, and the Health Assessment Questionnaire. Areas under the receiver operating characteristic curves (AUROCCs) were also assessed to further examine the predictive ability of the RA-WIS and to determine optimal cut points for predicting specific work transitions. After 12 months, 21.7% (n = 50 of 230) of the participants had indicated at least one arthritis-related work transition. Higher baseline RA-WIS was predictive of such an outcome (relative risk [RR] 1.05 [95% confidence interval (95% CI) 1.00-1.11]), particularly at >17 (RR 2.30 [95% CI 1.11-4.77]). The RA-WIS cut point of >13 was found to be most accurate for prediction (AUROCC 0.68 [95% CI 0.58-0.78]). The RA-WIS demonstrated the ability to predict arthritis-related work transitions within a short timeframe, and could be a promising measurement candidate for risk prognostication where work disability outcomes are of concern. Copyright © 2010 by the American College of Rheumatology.

Some effects of horizontal discretization on linear baroclinic and symmetric instabilities

NASA Astrophysics Data System (ADS)

Barham, William; Bachman, Scott; Grooms, Ian

2018-05-01

The effects of horizontal discretization on linear baroclinic and symmetric instabilities are investigated by analyzing the behavior of the hydrostatic Eady problem in ocean models on the B and C grids. On the C grid a spurious baroclinic instability appears at small wavelengths. This instability does not disappear as the grid scale decreases; instead, it simply moves to smaller horizontal scales. The peak growth rate of the spurious instability is independent of the grid scale as the latter decreases. It is equal to cf /√{Ri} where Ri is the balanced Richardson number, f is the Coriolis parameter, and c is a nondimensional constant that depends on the Richardson number. As the Richardson number increases c increases towards an upper bound of approximately 1/2; for large Richardson numbers the spurious instability is faster than the Eady instability. To suppress the spurious instability it is recommended to use fourth-order centered tracer advection along with biharmonic viscosity and diffusion with coefficients (Δx) 4 f /(32√{Ri}) or larger where Δx is the grid scale. On the B grid, the growth rates of baroclinic and symmetric instabilities are too small, and converge upwards towards the correct values as the grid scale decreases; no spurious instabilities are observed. In B grid models at eddy-permitting resolution, the reduced growth rate of baroclinic instability may contribute to partially-resolved eddies being too weak. On the C grid the growth rate of symmetric instability is better (larger) than on the B grid, and converges upwards towards the correct value as the grid scale decreases.

Electron Debye scale Kelvin-Helmholtz instability: Electrostatic particle-in-cell simulations

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

Lee, Sang-Yun; Lee, Ensang, E-mail: eslee@khu.ac.kr; Kim, Khan-Hyuk

2015-12-15

In this paper, we investigated the electron Debye scale Kelvin-Helmholtz (KH) instability using two-dimensional electrostatic particle-in-cell simulations. We introduced a velocity shear layer with a thickness comparable to the electron Debye length and examined the generation of the KH instability. The KH instability occurs in a similar manner as observed in the KH instabilities in fluid or ion scales producing surface waves and rolled-up vortices. The strength and growth rate of the electron Debye scale KH instability is affected by the structure of the velocity shear layer. The strength depends on the magnitude of the velocity and the growth ratemore » on the velocity gradient of the shear layer. However, the development of the electron Debye scale KH instability is mainly determined by the electric field generated by charge separation. Significant mixing of electrons occurs across the shear layer, and a fraction of electrons can penetrate deeply into the opposite side fairly far from the vortices across the shear layer.« less

Zonal flow evolution and overstability in accretion discs

NASA Astrophysics Data System (ADS)

Vanon, R.; Ogilvie, G. I.

2017-04-01

This work presents a linear analytical calculation on the stability and evolution of a compressible, viscous self-gravitating (SG) Keplerian disc with both horizontal thermal diffusion and a constant cooling time-scale when an axisymmetric structure is present and freely evolving. The calculation makes use of the shearing sheet model and is carried out for a range of cooling times. Although the solutions to the inviscid problem with no cooling or diffusion are well known, it is non-trivial to predict the effect caused by the introduction of cooling and of small diffusivities; this work focuses on perturbations of intermediate wavelengths, therefore representing an extension to the classical stability analysis on thermal and viscous instabilities. For density wave modes, the analysis can be simplified by means of a regular perturbation analysis; considering both shear and thermal diffusivities, the system is found to be overstable for intermediate and long wavelengths for values of the Toomre parameter Q ≲ 2; a non-SG instability is also detected for wavelengths ≳18H, where H is the disc scale-height, as long as γ ≲ 1.305. The regular perturbation analysis does not, however, hold for the entropy and potential vorticity slow modes as their ideal growth rates are degenerate. To understand their evolution, equations for the axisymmetric structure's amplitudes in these two quantities are analytically derived and their instability regions obtained. The instability appears boosted by increasing the value of the adiabatic index and of the Prandtl number, while it is quenched by efficient cooling.

Nucleation versus instability race in strained films

NASA Astrophysics Data System (ADS)

Liu, Kailang; Berbezier, Isabelle; David, Thomas; Favre, Luc; Ronda, Antoine; Abbarchi, Marco; Voorhees, Peter; Aqua, Jean-Noël

2017-10-01

Under the generic term "Stranski-Krastanov" are grouped two different growth mechanisms of SiGe quantum dots. They result from the self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. While these regimes are well known, we elucidate here the origin of the transition between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the time scales for ATG instability development and three-dimensional (3D) nucleation onset, we demonstrate that the transition between these two regimes is simply explained by the crossover between their divergent evolutions. Nucleation exhibits a strong exponential deviation at low strain while ATG behaves only algebraically. The associated time scale varies with exp(1 /x4) for nucleation, while it only behaves as 1 /x8 for the ATG instability. Consequently, at high (low) strain, nucleation (instability) occurs faster and inhibits the alternate evolution. It is then this different kinetic evolution which explains the transition from one regime to the other. Such a kinetic view of the transition between these two 3D growth regimes was not provided before. The crossover between nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50% in the experimental conditions used here. Varying the experimental conditions and/or the system parameters does not allow us to suppress the transition. This means that the SiGe quantum dots always grow via ATG instability at low strain and nucleation at high strain. This result is important for the self-organization of quantum dots.

Vortex survival in 3D self-gravitating accretion discs

NASA Astrophysics Data System (ADS)

Lin, Min-Kai; Pierens, Arnaud

2018-04-01

Large-scale, dust-trapping vortices may account for observations of asymmetric protoplanetary discs. Disc vortices are also potential sites for accelerated planetesimal formation by concentrating dust grains. However, in 3D discs vortices are subject to destructive `elliptic instabilities', which reduces their viability as dust traps. The survival of vortices in 3D accretion discs is thus an important issue to address. In this work, we perform shearing box simulations to show that disc self-gravity enhances the survival of 3D vortices, even when self-gravity is weak in the classic sense (e.g. with a Toomre Q ≃ 5). We find a 3D, self-gravitating vortex can grow on secular timescales in spite of the elliptic instability. The vortex aspect-ratio decreases as it strengthens, which feeds the elliptic instability. The result is a 3D vortex with a turbulent core that persists for ˜103 orbits. We find when gravitational and hydrodynamic stresses become comparable, the vortex may undergo episodic bursts, which we interpret as interaction between elliptic and gravitational instabilities. We estimate the distribution of dust particles in self-gravitating, turbulent vortices. Our results suggest large-scale vortices in protoplanetary discs are more easily observed at large radii.

From crater functions to partial differential equations: a new approach to ion bombardment induced nonequilibrium pattern formation.

PubMed

Norris, Scott A; Brenner, Michael P; Aziz, Michael J

2009-06-03

We develop a methodology for deriving continuum partial differential equations for the evolution of large-scale surface morphology directly from molecular dynamics simulations of the craters formed from individual ion impacts. Our formalism relies on the separation between the length scale of ion impact and the characteristic scale of pattern formation, and expresses the surface evolution in terms of the moments of the crater function. We demonstrate that the formalism reproduces the classical Bradley-Harper results, as well as ballistic atomic drift, under the appropriate simplifying assumptions. Given an actual set of converged molecular dynamics moments and their derivatives with respect to the incidence angle, our approach can be applied directly to predict the presence and absence of surface morphological instabilities. This analysis represents the first work systematically connecting molecular dynamics simulations of ion bombardment to partial differential equations that govern topographic pattern-forming instabilities.

Vortex Dynamics and Shear-Layer Instability in High-Intensity Cyclotrons.

PubMed

Cerfon, Antoine J

2016-04-29

We show that the space-charge dynamics of high-intensity beams in the plane perpendicular to the magnetic field in cyclotrons is described by the two-dimensional Euler equations for an incompressible fluid. This analogy with fluid dynamics gives a unified and intuitive framework to explain the beam spiraling and beam breakup behavior observed in experiments and in simulations. Specifically, we demonstrate that beam breakup is the result of a classical instability occurring in fluids subject to a sheared flow. We give scaling laws for the instability and predict the nonlinear evolution of beams subject to it. Our work suggests that cyclotrons may be uniquely suited for the experimental study of shear layers and vortex distributions that are not achievable in Penning-Malmberg traps.

Heating, Cooling, and Gravitational Instabilities in Protostellar and Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Pickett, B. K.; Mejia, A. C.; Durisen, R. H.

2001-12-01

We present three-dimensional hydrodynamic simulations of protostellar disk models, in order to explore how the interplay between heating and cooling regulates significant gravitational instabilities. Artificial viscosity is used to treat irreversible heating, such as would occur in shocks; volumetric cooling at several different rates is also applied throughout a broad radial region of the disk. We study the evolution of a disk that is already unstable (due to the low value of the Toomre Q parameter), and a marginally unstable disk that is cooled towards instability. The evolutions have implications for the transport of mass and angular momentum in protostellar disks, the effects of gravitational instabilities on the vertical structure of the disks, and the formation of stellar and substellar companions on dynamic time scales due to disk instabilties. This work is supported by grants from the NASA Planetary Geology and Geophysics and Origins of Solar Systems Programs.

The Segmented Aperture Interferometric Nulling Testbed (SAINT) I: Overview and Air-side System Description

NASA Technical Reports Server (NTRS)

Hicks, Brian A.; Lyon, Richard G.; Petrone, Peter, III; Bolcar, Matthew R.; Bolognese, Jeff; Clampin, Mark; Dogoda, Peter; Dworzanski, Daniel; Helmbrecht, Michael A.; Koca, Corina;

Self-reported knee joint instability is related to passive mechanical stiffness in medial knee osteoarthritis.

PubMed

Creaby, Mark W; Wrigley, Tim V; Lim, Boon-Whatt; Hinman, Rana S; Bryant, Adam L; Bennell, Kim L

2013-11-20

Self-reported knee joint instability compromises function in individuals with medial knee osteoarthritis and may be related to impaired joint mechanics. The purpose of this study was to evaluate the relationship between self-reported instability and the passive varus-valgus mechanical behaviour of the medial osteoarthritis knee. Passive varus-valgus angular laxity and stiffness were assessed using a modified isokinetic dynamometer in 73 participants with medial tibiofemoral osteoarthritis. All participants self-reported the absence or presence of knee instability symptoms and the degree to which instability affected daily activity on a 6-point likert scale. Forward linear regression modelling identified a significant inverse relationship between passive mid-range knee stiffness and symptoms of knee instability (r = 0.27; P < 0.05): reduced stiffness was indicative of more severe instability symptoms. Angular laxity and end-range stiffness were not related to instability symptoms (P > 0.05). Conceivably, a stiffer passive system may contribute toward greater joint stability during functional activities. Importantly however, net joint stiffness is influenced by both active and passive stiffness, and thus the active neuromuscular system may compensate for reduced passive stiffness in order to maintain joint stability. Future work is merited to examine the role of active stiffness in symptomatic joint stability.

Self-reported knee joint instability is related to passive mechanical stiffness in medial knee osteoarthritis

PubMed Central

2013-01-01

Background Self-reported knee joint instability compromises function in individuals with medial knee osteoarthritis and may be related to impaired joint mechanics. The purpose of this study was to evaluate the relationship between self-reported instability and the passive varus-valgus mechanical behaviour of the medial osteoarthritis knee. Methods Passive varus-valgus angular laxity and stiffness were assessed using a modified isokinetic dynamometer in 73 participants with medial tibiofemoral osteoarthritis. All participants self-reported the absence or presence of knee instability symptoms and the degree to which instability affected daily activity on a 6-point likert scale. Results Forward linear regression modelling identified a significant inverse relationship between passive mid-range knee stiffness and symptoms of knee instability (r = 0.27; P < 0.05): reduced stiffness was indicative of more severe instability symptoms. Angular laxity and end-range stiffness were not related to instability symptoms (P > 0.05). Conclusions Conceivably, a stiffer passive system may contribute toward greater joint stability during functional activities. Importantly however, net joint stiffness is influenced by both active and passive stiffness, and thus the active neuromuscular system may compensate for reduced passive stiffness in order to maintain joint stability. Future work is merited to examine the role of active stiffness in symptomatic joint stability. PMID:24252592

Active suppression of vortex-driven combustion instability using controlled liquid-fuel injection

NASA Astrophysics Data System (ADS)

Pang, Bin

Combustion instabilities remain one of the most challenging problems encountered in developing propulsion and power systems. Large amplitude pressure oscillations, driven by unsteady heat release, can produce numerous detrimental effects. Most previous active control studies utilized gaseous fuels to suppress combustion instabilities. However, using liquid fuel to suppress combustion instabilities is more realistic for propulsion applications. Active instability suppression in vortex-driven combustors using a direct liquid fuel injection strategy was theoretically established and experimentally demonstrated in this dissertation work. Droplet size measurements revealed that with pulsed fuel injection management, fuel droplet size could be modulated periodically. Consequently, desired heat release fluctuation could be created. If this oscillatory heat release is coupled with the natural pressure oscillation in an out of phase manner, combustion instabilities can be suppressed. To identify proper locations of supplying additional liquid fuel for the purpose of achieving control, the natural heat release pattern in a vortex-driven combustor was characterized in this study. It was found that at high Damkohler number oscillatory heat release pattern closely followed the evolving vortex front. However, when Damkohler number became close to unity, heat release fluctuation wave no longer coincided with the coherent structures. A heat release deficit area was found near the dump plane when combustor was operated in lean premixed conditions. Active combustion instability suppression experiments were performed in a dump combustor using a controlled liquid fuel injection strategy. High-speed Schlieren results illustrated that vortex shedding plays an important role in maintaining self-sustained combustion instabilities. Complete combustion instability control requires total suppression of these large-scale coherent structures. The sound pressure level at the excited dominant frequency was reduced by more than 20 dB with controlled liquid fuel injection method. Scaling issues were also investigated in this dump combustor to test the effectiveness of using pulsed liquid fuel injection strategies to suppress instabilities at higher power output conditions. With the liquid fuel injection control method, it was possible to suppress strong instabilities with initial amplitude of +/-5 psi down to the background noise level. The stable combustor operating range was also expanded from equivalence ratio of 0.75 to beyond 0.9.

A non-linear theory of the parallel firehose and gyrothermal instabilities in a weakly collisional plasma

NASA Astrophysics Data System (ADS)

Rosin, M. S.; Schekochihin, A. A.; Rincon, F.; Cowley, S. C.

2011-05-01

Weakly collisional magnetized cosmic plasmas have a dynamical tendency to develop pressure anisotropies with respect to the local direction of the magnetic field. These anisotropies trigger plasma instabilities at scales just above the ion Larmor radius ρi and much below the mean free path λmfp. They have growth rates of a fraction of the ion cyclotron frequency, which is much faster than either the global dynamics or even local turbulence. Despite their microscopic nature, these instabilities dramatically modify the transport properties and, therefore, the macroscopic dynamics of the plasma. The non-linear evolution of these instabilities is expected to drive pressure anisotropies towards marginal stability values, controlled by the plasma beta βi. Here this non-linear evolution is worked out in an ab initio kinetic calculation for the simplest analytically tractable example - the parallel (k⊥= 0) firehose instability in a high-beta plasma. An asymptotic theory is constructed, based on a particular physical ordering and leading to a closed non-linear equation for the firehose turbulence. In the non-linear regime, both the analytical theory and the numerical solution predict secular (∝t) growth of magnetic fluctuations. The fluctuations develop a k-3∥ spectrum, extending from scales somewhat larger than ρi to the maximum scale that grows secularly with time (∝t1/2); the relative pressure anisotropy (p⊥-p∥)/p∥ tends to the marginal value -2/βi. The marginal state is achieved via changes in the magnetic field, not particle scattering. When a parallel ion heat flux is present, the parallel firehose mutates into the new gyrothermal instability (GTI), which continues to exist up to firehose-stable values of pressure anisotropy, which can be positive and are limited by the magnitude of the ion heat flux. The non-linear evolution of the GTI also features secular growth of magnetic fluctuations, but the fluctuation spectrum is eventually dominated by modes around a maximal scale ˜ρilT/λmfp, where lT is the scale of the parallel temperature variation. Implications for momentum and heat transport are speculated about. This study is motivated by our interest in the dynamics of galaxy cluster plasmas (which are used as the main astrophysical example), but its relevance to solar wind and accretion flow plasmas is also briefly discussed.

The physics of transverse mode instability-induced nonlinear phase distortions in large area optical fiber amplifiers and their mitigation with applications in scaling of pulsed and continuous wave high-energy lasers

DTIC Science & Technology

2016-12-13

plate and novel all-fiber fused coupler. Such work has laid the platform to demonstrate the mitigation of thermal mode instability through vortex beam...at IIT Madras to experimentally validate the above results as well as to explore the generation of vortex modes through a spiral phase plate and...modes through spiral phase plates and novel all-fiber fused couplers. We have demonstrated the excitation of a vortex mode with charge 1 through a

Scaling Relationships for ELM Diverter Heat Flux on DIII D

NASA Astrophysics Data System (ADS)

Peters, E. A.; Makowski, M. A.; Leonard, A. W.

2015-11-01

Edge Localized Modes (ELMs) are periodic plasma instabilities that occur during H-mode operation in tokamaks. Left unmitigated, these instabilities result in concentrated particle and heat fluxes at the divertor and stand to cause serious damage to the plasma facing components of tokamaks. The purpose of this research is to find scaling relationships that predict divertor heat flux due to ELMs based on plasma parameters at the time of instability. This will be accomplished by correlating characteristic ELM parameters with corresponding plasma measurements and analyzing the data for trends. One early assessment is the effect of the heat transmission coefficient ? on the in/out asymmetry of the calculated ELM heat fluxes. Using IR camera data, further assessments in this study will continue to emphasize in/out asymmetry in ELMs, as this has important implications for ITER operation. Work supported in part by the US DOE, DE-AC52-07NA27344, DE-FC02-04ER54698, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internships Program (SULI).

Investigation of instability of displacement front in non-isothermal flow problems

NASA Astrophysics Data System (ADS)

Syulyukina, Natalia; Pergament, Anna

2012-11-01

In this paper, we investigate the issues of front instability arising in non-isothermal flow displacement processes. The problem of two-phase flow of immiscible fluids, oil and water, is considered, including sources and dependence of viscosity on temperature. Three-dimensional problem with perturbation close to the injection well was considered to find the characteristic scale of the instability. As a result of numerical calculations, theoretical studies on the development of the instability due to the fact that the viscosity of the displacing fluid is less than the viscosity of the displaced have been confirmed. The influence of temperature on the evolution of the instability was considered. For this purpose, the dependence of oil viscosity on temperature has been added to the problem. Numerical calculations were carried out for different values of temperature and it was shown that with increasing of production rate. Thus, it has been demonstrated that the selection of the optimal temperature for injected fluids a possible way for stimulation of oil production also delaying the field water-flooding. This work was supporting by the RFBR grant 12-01-00793-a.

An enstrophy-based linear and nonlinear receptivity theory

NASA Astrophysics Data System (ADS)

Sengupta, Aditi; Suman, V. K.; Sengupta, Tapan K.; Bhaumik, Swagata

2018-05-01

In the present research, a new theory of instability based on enstrophy is presented for incompressible flows. Explaining instability through enstrophy is counter-intuitive, as it has been usually associated with dissipation for the Navier-Stokes equation (NSE). This developed theory is valid for both linear and nonlinear stages of disturbance growth. A previously developed nonlinear theory of incompressible flow instability based on total mechanical energy described in the work of Sengupta et al. ["Vortex-induced instability of an incompressible wall-bounded shear layer," J. Fluid Mech. 493, 277-286 (2003)] is used to compare with the present enstrophy based theory. The developed equations for disturbance enstrophy and disturbance mechanical energy are derived from NSE without any simplifying assumptions, as compared to other classical linear/nonlinear theories. The theory is tested for bypass transition caused by free stream convecting vortex over a zero pressure gradient boundary layer. We explain the creation of smaller scales in the flow by a cascade of enstrophy, which creates rotationality, in general inhomogeneous flows. Linear and nonlinear versions of the theory help explain the vortex-induced instability problem under consideration.

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

HERRON, ISOM H

Executive summary Two important general problems of interest in plasma physics that may be addressed successfully by Magnetohydrodynamics (MHD) are: (1) Find magnetic field configurations capable of confining a plasma in equilibrium. (2) Study the stability properties of each such an equilibrium. It is often found that the length scale of many instabilities and waves that are able to grow or propagate in a system, are comparable with plasma size, such as in magnetically confined thermonuclear plasmas or in astrophysical accretion disks. Thus MHD is able to provide a good description of such large-scale disturbances. The Magnetorotational instability (MRI) ismore » one particular instance of a potential instability. The project involved theoretical work on fundamental aspects of plasma physics. Researchers at the Princeton Plasma Physics Laboratory (PPPL) began to perform a series of liquid metal Couette flow experiments between rotating cylinders. Their purpose was to produce MRI, which they had predicted theoretically 2002, but was only observed in the laboratory since this project began. The personnel on the project consisted of three persons: (1) The PI, who was partially supported on the budget during each of four summers 2005-2008. (2) Two graduate research assistants, who worked consecutively on the project throughout the years 2005-2009. As a result, the first student, Fritzner Soliman, obtained an M.S. degree in 2006; the second student, Pablo Suarez obtained the Ph.D. degree in 2009. The work was in collaboration with scientists in Princeton, periodic trips were made by the PI as part of the project. There were 4 peer-reviewed publications and one book produced.« less

Experimental Observation of Nonlinear Mode Coupling In the Ablative Rayleigh-Taylor Instability on the NIF

NASA Astrophysics Data System (ADS)

Martinez, David

2015-11-01

We investigate on the National Ignition Facility (NIF) the ablative Rayleigh-Taylor (RT) instability in the transition from linear to highly nonlinear regimes. This work is part of the Discovery Science Program on NIF and of particular importance to indirect-drive inertial confinement fusion (ICF) where careful attention to the form of the rise to final peak drive is calculated to prevent the RT instability from shredding the ablator in-flight and leading to ablator mixing into the cold fuel. The growth of the ablative RT instability was investigated using a planar plastic foil with pre-imposed two-dimensional broadband modulations and diagnosed using x-ray radiography. The foil was accelerated for 12ns by the x-ray drive created in a gas-filled Au radiation cavity with a radiative temperature plateau at 175 eV. The dependence on initial conditions was investigated by systematically changing the modulation amplitude, ablator material and the modulation pattern. For each of these cases bubble mergers were observed and the nonlinear evolution of the RT instability showed insensitivity to the initial conditions. This experiment provides critical data needed to validate current theories on the ablative RT instability for indirect drive that relies on the ablative stabilization of short-scale modulations for ICF ignition. This paper will compare the experimental data to the current nonlinear theories. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC.

The Work Instability Scale for Rheumatoid Arthritis (RA-WIS): Does it work in osteoarthritis?

PubMed

Tang, Kenneth; Beaton, Dorcas E; Lacaille, Diane; Gignac, Monique A M; Zhang, Wei; Anis, Aslam H; Bombardier, Claire

2010-09-01

To validate the 23-item Work Instability Scale for Rheumatoid Arthritis (RA-WIS) for use in osteoarthritis (OA) using both classical test theory and item response theory approaches. Baseline and 12-month follow-up data were collected from workers with OA recruited from community and clinical settings (n = 130). Fit of RA-WIS data to the Rasch model was evaluated by item- and person-fit statistics (size of residual, chi-sq), assessments of differential item functioning, and tests of unidimensionality and local independence. Internal consistency was assessed by KR-20. Convergent construct validity (Spearman r, known-groups) was evaluated against theoretical constructs that assess impact of health on work. Responsiveness to global indicators of change was assessed by standardized response means (SRM) and area under the receiver operating characteristic curves. Data structure of the RA-WIS showed adequate fit to the Rasch model (chi-sq = 83.2, P = 0.03) after addressing local dependency in three item pairs by creating testlets. High internal consistency (KR-20 = 0.93) and convergent validity with work-oriented constructs (|r| = 0.55-0.77) were evident. The RA-WIS correlated most strongly with the concept of illness intrusiveness (r = 0.77) and was highly responsive to changes (SRM = 1.05 [deterioration]; -0.78 [improvement]). Although developed for RA, the RA-WIS is psychometrically sound for OA and demonstrates interval-level property.

Electrokinetic Particle Aggregation and Flow Instabilities in Non-Dilute Colloidal Suspensions

NASA Astrophysics Data System (ADS)

Navaneetham, Guru; Posner, Jonathan

2007-11-01

An experimental investigation of electrokinetic particle aggregation and flow instabilities of non-dilute colloidal suspensions in microfabricated channels is presented. The addition of charged colloidal particles can alter the solution's conductivity, permittivity as well as the average particle electrophoretic mobility. In this work, a colloid volume fraction gradient is achieved at the intersection of a Y-shaped PDMS microchannel. The solution conductivity and the particle mobility as a function of the particle (500 nm polystyrene) volume fraction are presented. The critical conditions required for particle aggregation and flow instability are given along with a scaling analysis which shows that the flow becomes unstable at a critical electric Rayleigh number for a wide range of applied electric fields and colloid volume fractions. Electrokinetic particle aggregation and instabilities of non-dilute colloidal suspensions may be important for applications such as the electrophoretic deposition of particles to form micropatterned colloidal assemblies, electrorheological devices, and on-chip, electrokinetic manipulation of colloids.

Gyrokinetic theory of slab universal modes and the non-existence of the gradient drift coupling (GDC) instability

NASA Astrophysics Data System (ADS)

Rogers, Barrett N.; Zhu, Ben; Francisquez, Manaure

2018-05-01

A gyrokinetic linear stability analysis of a collisionless slab geometry in the local approximation is presented. We focus on k∥=0 universal (or entropy) modes driven by plasma gradients at small and large plasma β. These are small scale non-MHD instabilities with growth rates that typically peak near k⊥ρi˜1 and vanish in the long wavelength k⊥→0 limit. This work also discusses a mode known as the Gradient Drift Coupling (GDC) instability previously reported in the gyrokinetic literature, which has a finite growth rate γ=√{β/[2 (1 +β)] }Cs/|Lp| with Cs2=p0/ρ0 for k⊥→0 and is universally unstable for 1 /Lp≠0 . We show that the GDC instability is a spurious, unphysical artifact that erroneously arises due to the failure to respect the total equilibrium pressure balance p0+B02/(8 π)=constant , which renders the assumption B0'=0 inconsistent if p0'≠0 .

Electron inertia and quasi-neutrality in the Weibel instability

NASA Astrophysics Data System (ADS)

Camporeale, Enrico; Tronci, Cesare

2017-06-01

While electron kinetic effects are well known to be of fundamental importance in several situations, the electron mean-flow inertia is often neglected when length scales below the electron skin depth become irrelevant. This has led to the formulation of different reduced models, where electron inertia terms are discarded while retaining some or all kinetic effects. Upon considering general full-orbit particle trajectories, this paper compares the dispersion relations emerging from such models in the case of the Weibel instability. As a result, the question of how length scales below the electron skin depth can be neglected in a kinetic treatment emerges as an unsolved problem, since all current theories suffer from drawbacks of different nature. Alternatively, we discuss fully kinetic theories that remove all these drawbacks by restricting to frequencies well below the plasma frequency of both ions and electrons. By giving up on the length scale restrictions appearing in previous works, these models are obtained by assuming quasi-neutrality in the full Vlasov-Maxwell system.

Bandwidth Dependence of Laser Plasma Instabilities Driven by the Nike KrF Laser

NASA Astrophysics Data System (ADS)

Weaver, J. L.; Oh, J.; Seely, J.; Kehne, D.; Brown, C. M.; Obenschain, S.; Serlin, V.; Schmitt, A. J.; Phillips, L.; Lehmberg, R. H.; McLean, E.; Manka, C.; Feldman, U.

2011-10-01

The Nike krypton-fluoride (KrF) laser at the Naval Research Laboratory operates in the deep UV (248 nm) and employs beam smoothing by induced spatial incoherence (ISI). In the first ISI studies at longer wavelengths (1054 nm and 527 nm) [Obenschain, PRL 62, 768(1989);Mostovych, PRL, 59, 1193(1987); Peyser, Phys. Fluids B 3, 1479(1991)], stimulated Raman scattering, stimulated Brillouin scattering, and the two plasmon decay instability were reduced when wide bandwidth ISI (δν / ν ~ 0.03-0.19%) pulses irradiated targets at moderate to high intensities (1014-1015W/cm2) . Recent Nike work showed that the threshold for quarter critical instabilities increased with the expected wavelength scaling, without accounting for the large bandwidth (δν ~ 1-3 THz). New experiments will compare laser plasma instabilities (LPI) driven by narrower bandwidth pulses to those observed with the standard operation. The bandwidth of KrF lasers can be reduced by adding narrow filters (etalons or gratings) in the initial stages of the laser. This talk will discuss the method used to narrow the output spectrum of Nike, the laser performance for this new operating mode, and target observations of LPI in planar CH targets. Work supported by DoE/NNSA.

The Effect of External Magnetic Fields on the MRT Instability in MagLIF

NASA Astrophysics Data System (ADS)

Hess, Mark; Peterson, Kyle; Weis, Matthew; Lau, Yue Ying

2014-10-01

Recent experiments on MagLIF which incorporate an external B-field suggest that the MRT instability within the liner has a different behavior than without the B-field. Previous work by Chandrasekhar and Harris have illustrated how the MRT growth rate, assuming fixed liner density and fixed acceleration, can change due to the presence of an external B-field. In this work, we show how the growth rate of the MRT instability is dynamically affected by the rapidly varying acceleration, liner density, and surface magnetic field, which is composed of the external B-field and the drive B-field of the liner in the MagLIF experiments. In addition, we also examine the effects of finite liner resistivity on MRT growth, which gives rise to an additional time scale corresponding to magnetic diffusion. We discuss the implications of this result for future MagLIF designs. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

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

Sen, Amiya K.

The goal of this grant has been to study the basic physics of various sources of anomalous transport in tokamaks. Anomalous transport in tokamaks continues to be one of the major problems in magnetic fusion research. As a tokamak is not a physics device by design, direct experimental observation and identification of the instabilities responsible for transport, as well as physics studies of the transport in tokamaks, have been difficult and of limited value. It is noted that direct experimental observation, identification and physics study of microinstabilities including ITG, ETG, and trapped electron/ion modes in tokamaks has been very difficultmore » and nearly impossible. The primary reasons are co-existence of many instabilities, their broadband fluctuation spectra, lack of flexibility for parameter scans and absence of good local diagnostics. This has motivated us to study the suspected tokamak instabilities and their transport consequences in a simpler, steady state Columbia Linear Machine (CLM) with collisionless plasma and the flexibility of wide parameter variations. Earlier work as part of this grant was focused on both ITG turbulence, widely believed to be a primary source of ion thermal transport in tokamaks, and the effects of isotope scaling on transport levels. Prior work from our research team has produced and definitively identified both the slab and toroidal branches of this instability and determined the physics criteria for their existence. All the experimentally observed linear physics corroborate well with theoretical predictions. However, one of the large areas of research dealt with turbulent transport results that indicate some significant differences between our experimental results and most theoretical predictions. Latter years of this proposal were focused on anomalous electron transport with a special focus on ETG. There are several advanced tokamak scenarios with internal transport barriers (ITB), when the ion transport is reduced to neoclassical values by combined mechanisms of ExB and diamagnetic flow shear suppression of the ion temperature gradient (ITG) instabilities. However, even when the ion transport is strongly suppressed, the electron transport remains highly anomalous. The most plausible physics scenario for the anomalous electron transport is based on electron temperature gradient (ETG) instabilities. This instability is an electron analog of and nearly isomorphic to the ITG instability, which we had studied before extensively. However, this isomorphism is broken nonlinearily. It is noted that as the typical ETG mode growth rates are larger (in contrast to ITG modes) than ExB shearing rates in usual tokamaks, the flow shear suppression of ETG modes is highly unlikely. This motivated a broader range of investigations of other physics scenarios of nonlinear saturation and transport scaling of ETG modes.« less

Slowly-growing gap-opening planets trigger weaker vortices

NASA Astrophysics Data System (ADS)

Hammer, Michael; Kratter, Kaitlin M.; Lin, Min-Kai

2017-04-01

The presence of a giant planet in a low-viscosity disc can create a gap edge in the disc's radial density profile sharp enough to excite the Rossby wave instability. This instability may evolve into dust-trapping vortices that might explain the 'banana-shaped' features in recently observed asymmetric transition discs with inner cavities. Previous hydrodynamical simulations of planet-induced vortices have neglected the time-scale of hundreds to thousands of orbits to grow a massive planet to Jupiter size. In this work, we study the effect of a giant planet's runaway growth time-scale on the lifetime and characteristics of the resulting vortex. For two different planet masses (1 and 5 Jupiter masses) and two different disc viscosities (α = 3 × 10-4 and 3 × 10-5), we compare the vortices induced by planets with several different growth time-scales between 10 and 4000 planet orbits. In general, we find that slowly-growing planets create significantly weaker vortices with lifetimes and surface densities reduced by more than 50 per cent. For the higher disc viscosity, the longest growth time-scales in our study inhibit vortex formation altogether. Additionally, slowly-growing planets produce vortices that are up to twice as elongated, with azimuthal extents well above 180° in some cases. These unique, elongated vortices likely create a distinct signature in the dust observations that differentiates them from the more concentrated vortices that correspond to planets with faster growth time-scales. Lastly, we find that the low viscosities necessary for vortex formation likely prevent planets from growing quickly enough to trigger the instability in self-consistent models.

Explosion of comet Shoemaker-Levy 9 on entry into the Jovian atmosphere

NASA Technical Reports Server (NTRS)

Mac Low, Mordecai-Mark; Zahnle, Kevin

1994-01-01

We use the astrophysical hydrocode ZEUS to compute high-resolution models of the disruption and deceleration of cometary fragments striking Jupiter. We find that simple analytic and semianalytic models work well for kilometer-size impactors. We show that previous numerical models that placed the explosion much deeper in the atmosphere failed to fully resolve important gasdynamical instabilities. These instabilities tear the comet apart, greatly increase its effective cross section, and bring it to an abrupt halt. A 1 km diameter fragment loses over 90% of its kinetic energy within a single scale height at an atmospheric pressure of order 10 bars. For all practical purposes, it explodes.

A modern approach to superradiance

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

Endlich, Solomon; Penco, Riccardo

In this paper, we provide a simple and modern discussion of rotational super-radiance based on quantum field theory. We work with an effective theory valid at scales much larger than the size of the spinning object responsible for superradiance. Within this framework, the probability of absorption by an object at rest completely determines the superradiant amplification rate when that same object is spinning. We first discuss in detail superradiant scattering of spin 0 particles with orbital angular momentum ℓ = 1, and then extend our analysis to higher values of orbital angular momentum and spin. Along the way, we providemore » a simple derivation of vacuum friction — a ''quantum torque'' acting on spinning objects in empty space. Our results apply not only to black holes but to arbitrary spinning objects. We also discuss superradiant instability due to formation of bound states and, as an illustration, we calculate the instability rate Γ for bound states with massive spin 1 particles. For a black hole with mass M and angular velocity Ω, we find Γ ~ (GMμ) 7Ω when the particle’s Compton wavelength 1/μ is much greater than the size GM of the spinning object. This rate is parametrically much larger than the instability rate for spin 0 particles, which scales like (GM μ) 9Ω. This enhanced instability rate can be used to constrain the existence of ultralight particles beyond the Standard Model.« less

A modern approach to superradiance

DOE PAGES

Endlich, Solomon; Penco, Riccardo

2017-05-10

In this paper, we provide a simple and modern discussion of rotational super-radiance based on quantum field theory. We work with an effective theory valid at scales much larger than the size of the spinning object responsible for superradiance. Within this framework, the probability of absorption by an object at rest completely determines the superradiant amplification rate when that same object is spinning. We first discuss in detail superradiant scattering of spin 0 particles with orbital angular momentum ℓ = 1, and then extend our analysis to higher values of orbital angular momentum and spin. Along the way, we providemore » a simple derivation of vacuum friction — a ''quantum torque'' acting on spinning objects in empty space. Our results apply not only to black holes but to arbitrary spinning objects. We also discuss superradiant instability due to formation of bound states and, as an illustration, we calculate the instability rate Γ for bound states with massive spin 1 particles. For a black hole with mass M and angular velocity Ω, we find Γ ~ (GMμ) 7Ω when the particle’s Compton wavelength 1/μ is much greater than the size GM of the spinning object. This rate is parametrically much larger than the instability rate for spin 0 particles, which scales like (GM μ) 9Ω. This enhanced instability rate can be used to constrain the existence of ultralight particles beyond the Standard Model.« less

Magnetic Flux Concentrations in Stratified Turbulent Plasma Due to Negative Effective Magnetic Pressure Instability

NASA Astrophysics Data System (ADS)

Jabbari, S.; Brandenburg, A.

2014-12-01

Recent studies have suggested a new mechanism that can be used to explain the formation of magnetic spots or bipolar regions in highly stratified turbulent plasmas. According to this model, a large-scale magnetic field suppresses the turbulent pressure, which leads to a negative contribution of turbulence to the effective magnetic pressure. Direct numerical simulations (DNS) have confirmed that the negative contribution is large enough so that the effective magnetic pressure becomes negative and leads to a large-scale instability, which we refer to as negative effective magnetic pressure Instability (NEMPI). NEMPI was used to explain the formation of active regions and sunspots on the solar surface. One step toward improving this model was to combine dynamo in- stability with NEMPI. The dynamo is known to be responsible for the solar large-scale magnetic field and to play a role in solar activity. In this context, we studied stratified turbulent plasmas in spherical geometry, where the background field was generated by alpha squared dynamo. For NEMPI to be excited, the initial magnetic field should be in a proper range, so we used quenching function for alpha. Using the Pencil Code and mean field simulations (MFS), we showed that in the presence of dynamo-generated magnetic fields, we deal with a coupled system, where both instabilities, dynamo and NEMPI, work together and lead to the formation of magnetic structures (Jabbari et al. 2013). We also studied a similar system in plane geometry in the presence of rotation and confirmed that for slow rotation NEMPI works, but as the Coriolis number increases, the rotation suppresses NEMPI. By increasing the Coriolis number even further, the combination of fast rotation and high stratification excites a dynamo, which leads again to a coupled system of dynamo and NEMPI (Jabbari et al. 2014). Another important finding concerning NEMPI is the case where the instability is excited by a vertical magnetic field (Brandenburg et al. 2013). When the field is vertical, the resulting magnetic flux concentrations lead to the magnetic spots and can be of equipartition field strength. DNS, MFS, and implicit large eddy simulations (ILES) confirm that in a proper parameter regime, vertical imposed fields lead to the formation of circular magnetic spots (Brandenburg et al. 2014).

Relativistic thermal electron scale instabilities in sheared flow plasma

NASA Astrophysics Data System (ADS)

Miller, Evan D.; Rogers, Barrett N.

2016-04-01

> The linear dispersion relation obeyed by finite-temperature, non-magnetized, relativistic two-fluid plasmas is presented, in the special case of a discontinuous bulk velocity profile and parallel wave vectors. It is found that such flows become universally unstable at the collisionless electron skin-depth scale. Further analyses are performed in the limits of either free-streaming ions or ultra-hot plasmas. In these limits, the system is highly unstable in the parameter regimes associated with either the electron scale Kelvin-Helmholtz instability (ESKHI) or the relativistic electron scale sheared flow instability (RESI) recently highlighted by Gruzinov. Coupling between these modes provides further instability throughout the remaining parameter space, provided both shear flow and temperature are finite. An explicit parameter space bound on the highly unstable region is found.

Predictive values and other quality criteria of the German version of the Nurse-Work Instability Scale (Nurse-WIS) – follow-up survey findings of a prospective study of a cohort of geriatric care workers

PubMed Central

2014-01-01

Background Until now there has been a lack of effective screening instruments for health care workers at risk. To counteract the forecast shortage for health care workers, the offer of early interventions to maintain their work ability will become a central concern. The Nurse-Work Instability Scale (Nurse-WIS) seems to be suitable as a screening instrument and therefore a prospective study of a cohort of nursing staff from nursing homes was undertaken to validate the Nurse-Work Instability Scale (Nurse-WIS). Methods The follow-up data was used to test the sensitivity, specificity and the predictive values of the Nurse-WIS. The participants answered a questionnaire in the baseline investigation (T1) and in a follow-up 12 month after baseline. The hypothesis was that geriatric care workers with an increased risk according to the Nurse-WIS in T1 would be more likely to have taken long-term sick leave or drawn a pension for reduced work capacity in T2. Results 396 persons took part in T1 (21.3% response), 225 in T2 (42.3% loss-to-follow-up). In T1, 28.4% indicated an increased risk according to the Nurse-WIS. In T2, 10.2% had taken long-term sick leave or had drawn a pension for reduced work capacity. The sensitivity is 73.9% (95%-CI 55.7%–92.3%), the specificity is 76.7% (95%-CI 71.2%–82.8%). The ROC AUC indicated a moderate precision for the scale, at 0.74 (95%-CI 0.64–0.84). The PPV of the Nurse-WIS is 26.6%, and the NPV is 96.3%. For those with an increased risk according to the Nurse-WIS, the probability in T2 of long-term sick leave or a pension for reduced work capacity is around eight times higher (OR 8.3, 95%-CI 2.90–23.07). Persons who had indicated a long-term sick leave or made an application for a pension for reduced work capacity in T1 had a 17 times higher risk (OR 17.4, 95%-CI 3.34–90.55). Conclusion The German version of the Nurse-WIS appears to be a valid instrument with satisfactory predictive capabilities for recording an impending long-term sick leave. Whether the Nurse-WIS can be used as a screening tool which helps to design risk adjusted prevention programs for the afflicted nurse should be studied. PMID:25516765

The Inhibition of the Rayleigh-Taylor Instability by Rotation.

PubMed

Baldwin, Kyle A; Scase, Matthew M; Hill, Richard J A

2015-07-01

It is well-established that the Coriolis force that acts on fluid in a rotating system can act to stabilise otherwise unstable flows. Chandrasekhar considered theoretically the effect of the Coriolis force on the Rayleigh-Taylor instability, which occurs at the interface between a dense fluid lying on top of a lighter fluid under gravity, concluding that rotation alone could not stabilise this system indefinitely. Recent numerical work suggests that rotation may, nevertheless, slow the growth of the instability. Experimental verification of these results using standard techniques is problematic, owing to the practical difficulty in establishing the initial conditions. Here, we present a new experimental technique for studying the Rayleigh-Taylor instability under rotation that side-steps the problems encountered with standard techniques by using a strong magnetic field to destabilize an otherwise stable system. We find that rotation about an axis normal to the interface acts to retard the growth rate of the instability and stabilise long wavelength modes; the scale of the observed structures decreases with increasing rotation rate, asymptoting to a minimum wavelength controlled by viscosity. We present a critical rotation rate, dependent on Atwood number and the aspect ratio of the system, for stabilising the most unstable mode.

The Inhibition of the Rayleigh-Taylor Instability by Rotation

PubMed Central

Baldwin, Kyle A.; Scase, Matthew M.; Hill, Richard J. A.

2015-01-01

It is well-established that the Coriolis force that acts on fluid in a rotating system can act to stabilise otherwise unstable flows. Chandrasekhar considered theoretically the effect of the Coriolis force on the Rayleigh-Taylor instability, which occurs at the interface between a dense fluid lying on top of a lighter fluid under gravity, concluding that rotation alone could not stabilise this system indefinitely. Recent numerical work suggests that rotation may, nevertheless, slow the growth of the instability. Experimental verification of these results using standard techniques is problematic, owing to the practical difficulty in establishing the initial conditions. Here, we present a new experimental technique for studying the Rayleigh-Taylor instability under rotation that side-steps the problems encountered with standard techniques by using a strong magnetic field to destabilize an otherwise stable system. We find that rotation about an axis normal to the interface acts to retard the growth rate of the instability and stabilise long wavelength modes; the scale of the observed structures decreases with increasing rotation rate, asymptoting to a minimum wavelength controlled by viscosity. We present a critical rotation rate, dependent on Atwood number and the aspect ratio of the system, for stabilising the most unstable mode. PMID:26130005

Shock heating in numerical simulations of kink-unstable coronal loops

PubMed Central

Bareford, M. R.; Hood, A. W.

2015-01-01

An analysis of the importance of shock heating within coronal magnetic fields has hitherto been a neglected area of study. We present new results obtained from nonlinear magnetohydrodynamic simulations of straight coronal loops. This work shows how the energy released from the magnetic field, following an ideal instability, can be converted into thermal energy, thereby heating the solar corona. Fast dissipation of magnetic energy is necessary for coronal heating and this requirement is compatible with the time scales associated with ideal instabilities. Therefore, we choose an initial loop configuration that is susceptible to the fast-growing kink, an instability that is likely to be created by convectively driven vortices, occurring where the loop field intersects the photosphere (i.e. the loop footpoints). The large-scale deformation of the field caused by the kinking creates the conditions for the formation of strong current sheets and magnetic reconnection, which have previously been considered as sites of heating, under the assumption of an enhanced resistivity. However, our simulations indicate that slow mode shocks are the primary heating mechanism, since, as well as creating current sheets, magnetic reconnection also generates plasma flows that are faster than the slow magnetoacoustic wave speed. PMID:25897092

Nonlinear Excitation of the Ablative Rayleigh-Taylor Instability for All Wave Numbers

NASA Astrophysics Data System (ADS)

Zhang, H.; Betti, R.; Gopalaswamy, V.; Aluie, H.; Yan, R.

2017-10-01

Small-scale modes of the ablative Rayleigh-Taylor instability (ARTI) are often neglected because they are linearly stable when their wavelength is shorter than a linear cutoff. Using 2-D and 3-D numerical simulations, it is shown that linearly stable modes of any wavelength can be destabilized. This instability regime requires finite amplitude initial perturbations. Compared to 2-D, linearly stable ARTI modes are more easily destabilized in 3-D and the penetrating bubbles have a higher density because of enhanced vorticity. It is shown that for conditions found in laser fusion targets, short-wavelength ARTI modes are more efficient at driving mixing of ablated material throughout the target since the nonlinear bubble density increases with the wave number and small-scale bubbles carry a larger mass flux of mixed material. This work was supported by the Office of Fusion Energy Sciences Nos. DE-FG02-04ER54789, DE-SC0014318, the Department of Energy National Nuclear Security Administration under Award No. DE-NA0001944, the Ministerio de Ciencia e Innovacion of Spain (Grant No. ENE2011-28489), and the NANL LDRD program through Project Number 20150568ER.

A low-altitude mechanism for mesoscale dynamics, structure, and current filamentation in the discrete aurora

NASA Technical Reports Server (NTRS)

Keskinen, M. J.; Chaturvedi, P. K.; Ossakow, S. L.

1992-01-01

The 2D nonlinear evolution of the ionization-driven adiabatic auroral arc instability is studied. We find: (1) the adiabatic auroral arc instability can fully develop on time scales of tens to hundreds of seconds and on spatial scales of tens to hundreds of kilometers; (2) the evolution of this instability leads to nonlinear 'hook-shaped' conductivity structures: (3) this instability can lead to parallel current filamentation over a wide range of scale sizes; and (4) the k-spectra of the density, electric field, and parallel current develop into inverse power laws in agreement with satellite observations. Comparison with mesoscale auroral phenomenology and current filamentation structures is made.

Electroweak vacuum instability and renormalized Higgs field vacuum fluctuations in the inflationary universe

NASA Astrophysics Data System (ADS)

Kohri, Kazunori; Matsui, Hiroki

2017-08-01

In this work, we investigated the electroweak vacuum instability during or after inflation. In the inflationary Universe, i.e., de Sitter space, the vacuum field fluctuations < δ phi 2 > enlarge in proportion to the Hubble scale H2. Therefore, the large inflationary vacuum fluctuations of the Higgs field < δ phi 2 > are potentially catastrophic to trigger the vacuum transition to the negative-energy Planck-scale vacuum state and cause an immediate collapse of the Universe. However, the vacuum field fluctuations < δ phi 2 >, i.e., the vacuum expectation values have an ultraviolet divergence, and therefore a renormalization is necessary to estimate the physical effects of the vacuum transition. Thus, in this paper, we revisit the electroweak vacuum instability from the perspective of quantum field theory (QFT) in curved space-time, and discuss the dynamical behavior of the homogeneous Higgs field phi determined by the effective potential V eff( phi ) in curved space-time and the renormalized vacuum fluctuations < δ phi 2 >ren via adiabatic regularization and point-splitting regularization. We simply suppose that the Higgs field only couples the gravity via the non-minimal Higgs-gravity coupling ξ(μ). In this scenario, the electroweak vacuum stability is inevitably threatened by the dynamical behavior of the homogeneous Higgs field phi, or the formations of AdS domains or bubbles unless the Hubble scale is small enough H< ΛI .

Unraveling the mechanisms underlying postural instability in Parkinson's disease using dynamic posturography.

PubMed

Nonnekes, Jorik; de Kam, Digna; Geurts, Alexander C H; Weerdesteyn, Vivian; Bloem, Bastiaan R

2013-12-01

Postural instability, one of the cardinal symptoms of Parkinson's disease (PD), has devastating consequences for affected patients. Better strategies to prevent falls are needed, but this calls for an improved understanding of the complex mechanisms underlying postural instability. We must also improve our ability to timely identify patients at risk of falling. Dynamic posturography is a promising avenue to achieve these goals. The latest moveable platforms can deliver 'real-life' balance perturbations, permitting study of everyday fall circumstances. Dynamic posturography studies have shown that PD patients have fundamental problems in scaling their postural responses in accordance with the need of the actual balance task at hand. On-going studies evaluate the predictive ability of impaired posturography performance for daily life falls. We also review recent work aimed at exploring balance correcting steps in PD, and the presumed interaction between startle pathways and postural responses.

Radiative Hydrodynamics and the Formation of Gas Giant Planets

NASA Astrophysics Data System (ADS)

Durisen, Richard H.

2009-05-01

Gas giant planets undoubtedly form from the orbiting gas and dust disks commonly observed around young stars, and there are two principal mechanisms proposed for how this may occur. The core accretion plus gas capture model argues that a solid core forms first and then accretes gas from the surrounding disk once the core becomes massive enough (about 10 Earth masses). The gas accumulation process is comparatively slow but becomes hydrodynamic at later times. The disk instability model alternatively suggests that gas giant planet formation is initiated by gas-phase gravitational instabilities (GIs) that fragment protoplanetary disks into bound gaseous protoplanets rapidly, on disk orbit period time scales. Solid cores then form more slowly by accretion of solid planetesimals and settling. The overall formation time scales for these two mechanisms can differ by orders of magnitude. Both involve multidimensional hydrodynamic flows at some phase, late in the process for core accretion and early on for disk instability. The ability of cores to accrete gas and the ability of GIs to produce bound clumps depend on how rapidly gas can lose energy by radiation. This regulatory process, while important for controlling the time scale for core accretion plus gas capture, turns out to be absolutely critical for disk instability to work at all. For this reason, I will focus in my talk on the use of radiation hydrodynamics simulations to determine whether and where disk instability can actually form gas giant planets in disks. Results remain controversial, but simulations by several different research groups support analytic arguments that disk instability leading to fragmentation probably cannot occur in disks around Sun-like stars at orbit radii of 10's of Earth-Sun distances or less. On the other hand, very recent simulations suggest that very young, rapidly accreting disks with much larger radii (100's of times the Sun-Earth distance) can indeed readily fragment by disk instability into super-Jupiters and brown dwarfs. It is possible that there are two distinct modes of gas giant planet formation in Nature which operate at different times and in different regions of disks around young stars. The application of more radiative hydrodynamics codes with better numerical techniques could play an important role in future theoretical developments.

Experimental Replication of an Aeroengine Combustion Instability

NASA Technical Reports Server (NTRS)

Cohen, J. M.; Hibshman, J. R.; Proscia, W.; Rosfjord, T. J.; Wake, B. E.; McVey, J. B.; Lovett, J.; Ondas, M.; DeLaat, J.; Breisacher, K.

2000-01-01

Combustion instabilities in gas turbine engines are most frequently encountered during the late phases of engine development, at which point they are difficult and expensive to fix. The ability to replicate an engine-traceable combustion instability in a laboratory-scale experiment offers the opportunity to economically diagnose the problem (to determine the root cause), and to investigate solutions to the problem, such as active control. The development and validation of active combustion instability control requires that the causal dynamic processes be reproduced in experimental test facilities which can be used as a test bed for control system evaluation. This paper discusses the process through which a laboratory-scale experiment was designed to replicate an instability observed in a developmental engine. The scaling process used physically-based analyses to preserve the relevant geometric, acoustic and thermo-fluid features. The process increases the probability that results achieved in the single-nozzle experiment will be scalable to the engine.

Generation of Plasma Density Irregularities in the Midlatitude/Subauroral F Region

NASA Astrophysics Data System (ADS)

Mishin, E. V.

2017-12-01

A concise review is given of the current state of the theoretical understanding of the creation of small- and meso-scale plasma density irregularities in the midlatitude/subauroral F region during quiet and disturbed periods. The former are discussed in terms of the temperature gradient instability (TGI) in the vicinity of the ionospheric projection of the plasmapause and the Perkins instability. During active conditions some part of the midlatitude ionosphere becomes the subauroral region dominated by enhanced westward flows (SAPS and SAID) driven by poleward electric fields. Their irregular, often nonlinear wave structure leads to the formation of plasma density irregularities in the plasmasphere and conjugate ionosphere. Here, meso-scale irregularities are due to the positive feedback magnetosphere-ionosphere coupling instability, while small scales resulted from the gradient drift instability (GDI), temperature GDI, and the ion frictional heating instability. The theoretical predictions are compared with satellite observations in the perturbed subauroral geospace.

The Shoulder Instability-Return to Sport after Injury (SIRSI): a valid and reproducible scale to quantify psychological readiness to return to sport after traumatic shoulder instability.

PubMed

Gerometta, Antoine; Klouche, Shahnaz; Herman, Serge; Lefevre, Nicolas; Bohu, Yoann

2018-01-01

The main goal of this study was to propose and validate a tool to quantify the psychological readiness of athletes to return to sport following traumatic shoulder instability and conservative or surgical management. «Knee» was replaced by the term «shoulder» in the Anterior Cruciate Ligament-Return to Sport after Injury scale. This pilot test of the Shoulder Instability-Return to Sport after Injury scale (SIRSI) was performed in a group of athletes who underwent surgery for post-traumatic chronic anterior shoulder instability. The final version was then validated according to the international COSMIN methodology. A retrospective study was performed including all rugby players who had reported an episode of instability between 2012 and 2013. The WOSI and the Walch-Duplay scales were used as reference questionnaires. Sixty-two patients were included, mean age 26 ± 5.2 years old, 5 women/57 men. Patients were professional or competitive athletes (70.9%) and followed-up for 4.6 ± 1.6 years after the first episode of shoulder instability. Shoulder surgery was performed in 30/62 (48.4%) patients, a mean 1.6 ± 1.2 years after the first episode of instability. The SIRSI was strongly correlated with the reference questionnaires (r = 0.80, p < 10 -5 ). The mean SIRSI score was significantly higher in patients who returned to play rugby (60.9 ± 26.6% vs 38.1 ± 25.6%, p = 0.001). The internal consistency of the scale was high (α = 0.96). Reproducibility of the test-retest was excellent (ρ = 0.93, 95% CI [0.89-0.96], p < 10 -5 ). No ceiling/floor effects were found. The SIRSI is a valid, reproducible scale that identifies patients who are ready to return to the same sport after an episode of shoulder instability, whether they undergo surgery or not. III.

Azimuthal magnetorotational instability with super-rotation

NASA Astrophysics Data System (ADS)

Rüdiger, G.; Schultz, M.; Gellert, M.; Stefani, F.

2018-02-01

It is demonstrated that the azimuthal magnetorotational instability (AMRI) also works with radially increasing rotation rates contrary to the standard magnetorotational instability for axial fields which requires negative shear. The stability against non-axisymmetric perturbations of a conducting Taylor-Couette flow with positive shear under the influence of a toroidal magnetic field is considered if the background field between the cylinders is current free. For small magnetic Prandtl number the curves of neutral stability converge in the (Hartmann number,Reynolds number) plane approximating the stability curve obtained in the inductionless limit . The numerical solutions for indicate the existence of a lower limit of the shear rate. For large the curves scale with the magnetic Reynolds number of the outer cylinder but the flow is always stable for magnetic Prandtl number unity as is typical for double-diffusive instabilities. We are particularly interested to know the minimum Hartmann number for neutral stability. For models with resting or almost resting inner cylinder and with perfectly conducting cylinder material the minimum Hartmann number occurs for a radius ratio of \\text{in}=0.9$ . The corresponding critical Reynolds numbers are smaller than 4$ .

Turbulent mixing noise from supersonic jets

NASA Technical Reports Server (NTRS)

Tam, Christopher K. W.; Chen, Ping

1994-01-01

There is now a substantial body of theoretical and experimental evidence that the dominant part of the turbulent noise of supersonic jets is generated directly by the large turbulence structures/instability waves of the jet flow. Earlier, Tam and Burton provided a description of the physical mechanism by which supersonically traveling instability waves can generate sound efficiently. They used the method of matched asymptotic expansions to construct an instability wave solution which is valid in the far field. The present work is an extension of the theory of Tam and Burton. It is argued that the instability wave spectrum of the jet may be regarded as generated by stochastic white noise excitation at the nozzle lip region. The reason why the excitation has white noise characteristics is that near the nozzle lip region the flow in the jet mixing layer has no intrinsic length and time scales. The present stochastic wave model theory of supersonic jet noise contains a single unknown multiplicative constant. Comparisons between the calculated noise directivities at selected Strouhal numbers and experimental measurements of a Mach 2 jet at different jet temperatures have been carried out. Favorable agreements are found.

Emergence of power-law scalings in shock-driven mixing transition

NASA Astrophysics Data System (ADS)

Vorobieff, Peter; Wayne, Patrick; Olmstead, Dell; Simons, Dylan; Truman, C. Randall; Kumar, Sanjay

2016-11-01

We present an experimental study of transition to turbulence due to shock-driven instability evolving on an initially cylindrical, diffuse density interface between air and a mixture of sulfur hexafluoride (SF6) and acetone. The plane of the shock is at an initial angle θ with the axis of the heavy-gas cylinder. We present the cases of planar normal (θ = 0) and oblique (θ =20°) shock interaction with the initial conditions. Flow is visualized in two perpendicular planes with planar laser-induced fluorescence (PLIF) triggered in acetone with a pulsed ultraviolet laser. Statistics of the flow are characterized in terms of the second-order structure function of the PLIF intensity. As instabilities in the flow evolve, the structure functions begin to develop power-law scalings, at late times manifesting over a range of scales spanning more than two orders of magnitude. We discuss the effects of the initial conditions on the emergence of these scalings, comparing the fully three-dimensional case (oblique shock interaction) with the quasi-two-dimensional case (planar normal shock interaction). We also discuss the flow anisotropy apparent in statistical differences in data from the two visualization planes. This work is funded by NNSA Grant DE-NA0002913.

Wing Wake Vortices and Temporal Vortex Pair Instabilities

NASA Astrophysics Data System (ADS)

Williamson, C. H. K.; Leweke, T.; Miller, G. D.

In this presentation we include selected results which have originated from vortex dynamics studies conducted at Cornell, in collaboration with IRPHE, Marseille. These studies concern, in particular, the spatial development of delta wing trailing vortices, and the temporal development of counter-rotating vortex pairs. There are, as might be expected, similarities in the instabilities of both of these basic flows, as shown in our laboratory-scale studies. In the case of the spatial development of vortex pairs in the wake of a delta wing, either in free flight or towed from an XY carriage system in a towing tank, we have found three distinct instability length scales as the trailing vortex pair travels downstream. The first (smallest-scale) instability is found immediately behind the delta wing, and this scales on the thickness of the two shear layers separating from the wing trailing edge. The second (short-wave) instability, at an intermediate distance downstream, scales on the primary vortex core dimensions. The third (long-wave) instability far downstream represents the classical "Crow" instability (Crow, 1970), scaling on the distance between the two primary vortices. By imposing disturbances on the delta wing incident velocity, we find that the long-wave instability is receptive to a range of wavelengths. Our experimental measurements of instability growth rates are compared with theoretical predictions, which are based on the theory of Widnall et al. (1971), and which require, as input, DPIV measurements of axial and circumferential velocity profiles. This represents the first time that theoretical and experimental growth rates have been compared, without the imposition of ad-hoc assumptions regarding the vorticity distribution. The agreement with theory appears to be good. The ease with which a Delta wing may be flown in free flight was demonstrated at the Symposium, using a giant polystyrene triangular wing, launched from the back of the auditorium, and ably caught by Professor Sid Leibovich, in whose honour the Symposium was held. In the case of the temporal growth of vortex pairs, formed by the closing of a pair of long flaps underwater, we find two principal instabilities; namely, a longwavelength Crow instability, and a short-wavelength "elliptic" instability. Comparisons between experiment and theory for the growth rates of the long-wave instability, over a range of perturbed wavelengths, appears to be very good. The vortex pair "pinches off", or reconnects, to form vortex rings in the manner assumed to occur in contrails behind jet aircraft. We discover a symmetry-breaking phase relationship for the short wave disturbances growing in the two vortices, which we 380 C.H.K. Williamson et al. show to be consistent with a kinematic matching condition between the two disturbances. Further results demonstrate that this instability is a manifestation of an elliptic instability, which is here identified for the first time in a real open flow. We therefore refer to this flow as a "cooperative elliptic" instability. The long-term evolution of the flow involves the inception of secondary miniscule vortex pairs, which are perpendicular to the primary vortex pair.

Comparing kinetic Monte Carlo and thin-film modeling of transversal instabilities of ridges on patterned substrates

NASA Astrophysics Data System (ADS)

Tewes, Walter; Buller, Oleg; Heuer, Andreas; Thiele, Uwe; Gurevich, Svetlana V.

2017-03-01

We employ kinetic Monte Carlo (KMC) simulations and a thin-film continuum model to comparatively study the transversal (i.e., Plateau-Rayleigh) instability of ridges formed by molecules on pre-patterned substrates. It is demonstrated that the evolution of the occurring instability qualitatively agrees between the two models for a single ridge as well as for two weakly interacting ridges. In particular, it is shown for both models that the instability occurs on well defined length and time scales which are, for the KMC model, significantly larger than the intrinsic scales of thermodynamic fluctuations. This is further evidenced by the similarity of dispersion relations characterizing the linear instability modes.

Advanced Design Concepts for Dense Plasma Focus Devices at LLNL

NASA Astrophysics Data System (ADS)

Povilus, Alexander; Podpaly, Yuri; Cooper, Christopher; Shaw, Brian; Chapman, Steve; Mitrani, James; Anderson, Michael; Pearson, Aric; Anaya, Enrique; Koh, Ed; Falabella, Steve; Link, Tony; Schmidt, Andrea

2017-10-01

The dense plasma focus (DPF) is a z-pinch device where a plasma sheath is accelerated down a coaxial railgun and ends in a radial implosion, pinch phase. During the pinch phase, the plasma generates intense, transient electric fields through physical mechanisms, similar to beam instabilities, that can accelerate ions in the plasma sheath to MeV-scale energies on millimeter length scales. Using kinetic modeling techniques developed at LLNL, we have gained insight into the formation of these accelerating fields and are using these observations to optimize the behavior of the generated ion beam for producing neutrons via beam-target interactions for kilojoule to megajoule-scale devices. Using a set of DPF's, both in operation and in development at LLNL, we have explored critical aspects of these devices, including plasma sheath formation behavior, power delivery to the plasma, and instability seeding during the implosion in order to improve the absolute yield and stability of the device. Prepared by LLNL under Contract DE-AC52-07NA27344. Computing support for this work came from the LLNL Institutional Computing Grand Challenge program.

Characterization of long-scale-length plasmas produced from plastic foam targets for laser plasma instability (LPI) research

NASA Astrophysics Data System (ADS)

Oh, Jaechul; Weaver, J. L.; Serlin, V.; Obenschain, S. P.

2017-10-01

We report on an experimental effort to produce plasmas with long scale lengths for the study of parametric instabilities, such as two plasmon decay (TPD) and stimulated Raman scattering (SRS), under conditions relevant to fusion plasma. In the current experiment, plasmas are formed from low density (10-100 mg/cc) CH foam targets irradiated by Nike krypton fluoride laser pulses (λ = 248 nm, 1 nsec FWHM) with energies up to 1 kJ. This experiment is conducted with two primary diagnostics: the grid image refractometer (Nike-GIR) to measure electron density and temperature profiles of the coronas, and time-resolved spectrometers with absolute intensity calibration to examine scattered light features of TPD or SRS. Nike-GIR was recently upgraded with a 5th harmonic probe laser (λ = 213 nm) to access plasma regions near quarter critical density of 248 nm light (4.5 ×1021 cm-3). The results will be discussed with data obtained from 120 μm scale-length plasmas created on solid CH targets in previous LPI experiments at Nike. Work supported by DoE/NNSA.

PIV and LDA measurements of the wake behind a wind turbine model

NASA Astrophysics Data System (ADS)

Naumov, I. V.; Mikkelsen, R. F.; Okulov, V. L.; Sørensen, J. N.

2014-06-01

In the present work we review the results of a series of measurements of the flow behind a model scale of a horizontal axis wind turbine rotor carried out at the water flume at Technical University of Denmark (DTU). The rotor is three-bladed and designed using Glauert theory for tip speed ratio λ =5 with a constant design lift coefficient along the span, CLdesign= 0.8. The measurements include dye visualization, Particle Image Velocimetry and Laser Doppler Anemometry. The wake instability has been studied in the range λ =3 - 9 at different cross-sections from the very near wake up to 10 rotor diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different processes and instabilities in the rotor wake. In the far wake a constant frequency corresponding to the Strouhal number was found for the long-scale instabilities. This Strouhal number is in good agreement with the well-known constant that usually characterizes the oscillation in wakes behind bluff bodies. From associated visualizations and reconstructions of the flow field, it was found that the dynamics of the far wake is associated with the precession (rotation) of a helical vortex core. The data indicate that Strouhal number of this precession is independent of the rotor angular speed.

Experimental investigation of the transverse modal instabilities onset in high power fully-aperiodic-large-pitch fiber lasers

NASA Astrophysics Data System (ADS)

Malleville, Marie-Alicia; Benoît, Aurélien; Dauliat, Romain; Leconte, Baptiste; Darwich, Dia; du Jeu, Rémi; Jamier, Raphaël.; Schwuchow, Anka; Schuster, Kay; Roy, Philippe

2018-02-01

Over the last decade, significant work has been carried out in order to increase the energy/peak power provided by fiber lasers. Indeed, new microstructured fibers with large (or very large) mode area cores (LMA) such as Distributed Mode Filtering (DMF) fibers and Large-Pitch Fibers (LPF) have been developed to address this concern. These technologies have allowed diffraction-limited emission with core diameters higher than 80 μm, and have state-of-the-art performances in terms of pulse energy or peak power while keeping an excellent spatial beam quality. Although these fibers were designed to reach high power levels while maintaining a single transverse mode propagation, power scaling becomes quickly limited by the onset of transverse modal instabilities (TMI). This effect suddenly arises when a certain average power threshold is exceeded, drastically degrading the emitted beam quality. In this work, we investigate the influence of the core dimensions and the refractive index mismatch between the active core and the background cladding material, on the TMI power threshold in rod-type Fully-Aperiodic-LPF. This fiber structure was specifically designed to enhance the higher-order modes (HOMs) delocalization out of the gain region and thus push further the onset of modal instabilities. Using a 400W pump diode at 976 nm, the power scaling, as well as the spatial beam quality and its temporal behavior were investigated in laser configuration, which theoretically provides a lower TMI power threshold than the amplifier one due to the lack of selective excitation of the fundamental mode.

Theory for the anomalous electron transport in Hall-effect thrusters

NASA Astrophysics Data System (ADS)

Lafleur, Trevor; Baalrud, Scott; Chabert, Pascal

2016-09-01

Using insights from particle-in-cell (PIC) simulations, we develop a kinetic theory to explain the anomalous cross-field electron transport in Hall-effect thrusters (HETs). The large axial electric field in the acceleration region of HETs, together with the radially applied magnetic field, causes electrons to drift in the azimuthal direction with a very high velocity. This drives an electron cyclotron instability that produces large amplitude oscillations in the plasma density and azimuthal electric field, and which is convected downstream due to the large axial ion drift velocity. The frequency and wavelength of the instability are of the order of 5 MHz and 1 mm respectively, while the electric field amplitude can be of a similar magnitude to axial electric field itself. The instability leads to enhanced electron scattering many orders of magnitude higher than that from standard electron-neutral or electron-ion Coulomb collisions, and gives electron mobilities in good agreement with experiment. Since the instability is a strong function of almost all plasma properties, the mobility cannot in general be fitted with simple 1/B or 1/B2 scaling laws, and changes to the secondary electron emission coefficient of the HET channel walls are expected to play a role in the evolution of the instability. This work received financial support from a CNES postdoctoral research award.

Distinguishing the Magnetorotational Instability (MRI) from Magnetized Ekman Flows in the PPPL MRI Experiment

NASA Astrophysics Data System (ADS)

Gilson, Erik; Caspary, Kyle; Goodman, Jeremy; Ji, Hantao; Schartman, Ethan; Wei, Xing

2015-11-01

Results are presented from initial experiments on the upgraded Magnetorotational Instability (MRI) experiment that uses GaInSn as the working fluid and now operates with conductive end caps to improve the coupling of angular momentum to the fluid to increase the saturation amplitude of the MRI signal. Measurements of the fluid velocity field and perturbed magnetic field over a range of magnetic Reynolds numbers, Rm , and Lundquist numbers, S, are compared with results from the SFEMaNS code in order to separate the effects of MRI on the system from effects such as Ekman flows and Shercliff layer instabilities. The MRI can be identified by observing its growth rate, noting the relative magnitudes and spatial distributions of the perturbed radial flow velocity ur and radial magnetic field Br, and measuring the scaling of ur and Br with Rm . The clear identification of the onset of MRI in the apparatus is complicated by the geometry and boundary conditions creating an imperfect supercritical pitchfork bifurcation. Nevertheless, a stability diagram can be created that shows that MRI is a weak-field instability that occurs only below a certain value of the normalized magnetic field S / Rm but above a threshold where viscous effects damps the growth of the instability.

Translation into Brazilian Portuguese, cross-cultural adaptation and validation of the Stanford presenteeism scale-6 and work instability scale for ankylosing spondylitis.

PubMed

Frauendorf, Renata; de Medeiros Pinheiro, Marcelo; Ciconelli, Rozana Mesquita

2014-12-01

Loss of productivity at work, as a result of health problems, is becoming an issue of interest due to the high burden it represents in society. The measurement of such phenomenon can be made using generic and specific scales for certain diseases such as the Stanford Presenteeism Scale (SPS-6) and the Work Instability Scale for Ankylosing Spondylitis (AS-WIS), specific for patients with ankylosing spondylitis (AS). The aim of this study was to translate and perform a cross-cultural adaptation of SPS-6 and AS-WIS into Portuguese and check their psychometric properties. The study also aimed to evaluate the relationship between the general scores of the scales and the main sociodemographic and clinical data, lifestyles, and absenteeism in patients with AS and correlate these variables with SPS-6 and AS-WIS scales. A sample of 120 patients with AS and 80 workers at a university hospital was evaluated. The processes for the translation and cross-cultural adaptation of the instruments followed preestablished steps and rules presented in the literature. For the evaluation of measurement properties and correlations between scales, intra-class correlation coefficient (reproducibility analysis), Cronbach alpha (internal consistency), and Pearson correlation coefficient (validity) were employed. The inter-observer (0.986) and intra-observer (0.992) reproducibilities of the AS-WIS were shown to be high as well as the internal consistency (0.995). Similarly, the inter-observer reliability of SPS-6 was considered good (0.890), although it showed a poorer performance when considering the same observer (Pearson correlation coefficient = 0.675 and intra-class correlation = 0.656). Internal consistency, for the total number of items, as measured by Cronbach alpha, was 0.889. The validity of the scales was evaluated thru the comparison of the achieved scores with the results of the WLQ, SF-36, ASQoL, BASFI, BASDAI, HAQ-S, and SRQ-20 instruments. Correlations between loss of productivity at work, worse quality of life, presence of emotional disturbances, and worse health conditions were positive. The process of translation, cross-cultural adaptation, and validation of the SPS-6 as a generic measurement for the loss of productivity at work and of the AS-WIS as a specific measurement for patients with AS are valid, reproducible, and specific instruments to be used in Brazil. In both scales, productivity at work was associated to advanced age, higher rate of absenteeism in the last month and year, presence of peripheral arthritis, and a larger number of comorbidities in patients with AS. The AS-WIS and SPS-6 showed a good correlation among them although they are not mutually exclusive but supplementary.

Electroweak vacuum instability and renormalized Higgs field vacuum fluctuations in the inflationary universe

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

Kohri, Kazunori; Matsui, Hiroki, E-mail: kohri@post.kek.jp, E-mail: matshiro@post.kek.jp

In this work, we investigated the electroweak vacuum instability during or after inflation. In the inflationary Universe, i.e., de Sitter space, the vacuum field fluctuations < δ φ {sup 2} > enlarge in proportion to the Hubble scale H {sup 2}. Therefore, the large inflationary vacuum fluctuations of the Higgs field < δ φ {sup 2} > are potentially catastrophic to trigger the vacuum transition to the negative-energy Planck-scale vacuum state and cause an immediate collapse of the Universe. However, the vacuum field fluctuations < δ φ {sup 2} >, i.e., the vacuum expectation values have an ultraviolet divergence, andmore » therefore a renormalization is necessary to estimate the physical effects of the vacuum transition. Thus, in this paper, we revisit the electroweak vacuum instability from the perspective of quantum field theory (QFT) in curved space-time, and discuss the dynamical behavior of the homogeneous Higgs field φ determined by the effective potential V {sub eff}( φ ) in curved space-time and the renormalized vacuum fluctuations < δ φ {sup 2} >{sub ren} via adiabatic regularization and point-splitting regularization. We simply suppose that the Higgs field only couples the gravity via the non-minimal Higgs-gravity coupling ξ(μ). In this scenario, the electroweak vacuum stability is inevitably threatened by the dynamical behavior of the homogeneous Higgs field φ, or the formations of AdS domains or bubbles unless the Hubble scale is small enough H < Λ {sub I} .« less

Magnetic Rayleigh-Taylor instability in radiative flows

NASA Astrophysics Data System (ADS)

Yaghoobi, Asiyeh; Shadmehri, Mohsen

2018-06-01

We present a linear analysis of the radiative Rayleigh-Taylor (RT) instability in the presence of magnetic field for both optically thin and thick regimes. When the flow is optically thin, magnetic field not only stabilizes perturbations with short wavelengths, but also growth rate of the instability at long wavelengths is reduced compared to a non-magnetized case. Then, we extend our analysis to the optically thick flows with a conserved total specific entropy, and properties of the unstable perturbations are investigated in detail. Growth rate of the instability at short wavelengths is suppressed due to the presence of the magnetic field; however, growth rate is nearly constant at long wavelengths because of the radiation field. Since the radiative bubbles around massive protostars are subject to the RT instability, we also explore implications of our results in this context. In the non-magnetized case, the growth time-scale of the instability for a typical bubble is found to be less than 1000 yr, which is very short compared to the typical star formation time-scale. Magnetic field with a reasonable strength significantly increases the growth time-scale to more than hundreds of thousand years. The instability, furthermore, is more efficient at large wavelengths, whereas in the non-magnetized case, growth rate at short wavelengths is more significant.

Minimal model for a hydrodynamic fingering instability in microroller suspensions

NASA Astrophysics Data System (ADS)

Delmotte, Blaise; Donev, Aleksandar; Driscoll, Michelle; Chaikin, Paul

2017-11-01

We derive a minimal continuum model to investigate the hydrodynamic mechanism behind the fingering instability recently discovered in a suspension of microrollers near a floor [M. Driscoll et al., Nat. Phys. 13, 375 (2017), 10.1038/nphys3970]. Our model, consisting of two continuous lines of rotlets, exhibits a linear instability driven only by hydrodynamic interactions and reproduces the length-scale selection observed in large-scale particle simulations and in experiments. By adjusting only one parameter, the distance between the two lines, our dispersion relation exhibits quantitative agreement with the simulations and qualitative agreement with experimental measurements. Our linear stability analysis indicates that this instability is caused by the combination of the advective and transverse flows generated by the microrollers near a no-slip surface. Our simple model offers an interesting formalism to characterize other hydrodynamic instabilities that have not been well understood, such as size scale selection in suspensions of particles sedimenting adjacent to a wall, or the recently observed formations of traveling phonons in systems of confined driven particles.

Learning from the Past, Looking to the Future: Modeling Social Unrest in Karachi, Pakistan

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

Olson, Jarrod; Kurzrok, Andrew J.; Hund, Gretchen

Social unrest represents a major challenge for policy makers around the globe, as it can quickly escalate from small scale disturbances to highly public protests, riots and even civil war. This research was motivated by a need to understand social instability and to unpack the comments made during a spring 2013 conference hosted by Pacific Northwest National Laboratory’s Center for Global Security and the U.S. Institute for Peace, where policymakers noted that models considering social instability are often not suitable for decision-making. This analysis shows that existing state level models of instability could be improved in spatial scale to themore » city level, even without significantly improved data access. Better data would make this analysis more complete and likely improve the quality of the model. Another challenge with incorporating modeling into decision-making is the need to understand uncertainty in a model. Policy makers are frequently tasked with making decisions without a clear outcome, so characterization of uncertainty is critical. This report describes the work and findings of the project. It took place in three phases: a literature review of social stability research, a “hindsight scan” that looked at historical data, and a “foresight scan” looking at future scenarios.« less

The relationship between ADHD symptoms, mood instability, and self-reported offending.

PubMed

Gudjonsson, Gisli H; Sigurdsson, Jon Fridrik; Adalsteinsson, Tomas F; Young, Susan

2013-05-01

To investigate the relative importance of ADHD symptoms, mood instability, and antisocial personality disorder traits in predicting self-reported offending. A total of 295 Icelandic students completed two scales of offending behavior and measures of ADHD symptoms, mood instability, and antisocial personality traits. Self-reported offending from the two independent scales correlated significantly with ADHD symptoms, mood instability, and antisocial personality traits with medium to large effect size. Multiple regressions showed that ADHD symptoms contributed to the two outcome measures beyond that of age and gender with a medium effect size. The ADHD effects were only partly mediated by mood instability and antisocial personality traits for general offending but were almost completely mediated by the more reactive measure of antisocial behavior. ADHD appears to be a potential risk factor for general offending in its own right irrespective of the presence of comorbidity, whereas mood instability is more important with regard to reactive behavior.

Global Scale Observations of Ionospheric Instabilities from GPS in Low Earth Orbit

NASA Technical Reports Server (NTRS)

Kramer, Leonard; Goodman, John L.

2003-01-01

The GPS receiver used for navigation on the Space Shuttle exhibits range rate noise which appears to result from scintillation of the satellite signals by irregularities in ionospheric plasma. The noise events cluster in geographic regions previously identified as susceptible to instability and disturbed ionospheric conditions. These mechanisms are reviewed in the context of the GPS observations. Range rate data continuously monitored during the free orbiting phase of several space shuttle missions reveals global scale distribution of ionospheric irregularities. Equatorial events cluster +/- 20 degrees about the magnetic equator and polar events exhibit hemispheric asymmetry suggesting influence of off axis geomagnetic polar oval system. The diurnal, seasonal and geographic distribution is compared to previous work concerning equatorial spread F, Appleton anomaly and polar oval. The observations provide a succinct demonstration of the utility of space based ionospheric monitoring using GPS. The susceptability of GPS receivers to scintillation represents 'an unanticipated technical risk not factored into the selection of receivers for the United States space program.

How Satisfied Are Patients with Arthroscopic Bankart Repair? A 2-Year Follow-up on Quality-of-Life Outcome.

PubMed

Saier, Tim; Plath, Johannes E; Waibel, Sabrina; Minzlaff, Philipp; Feucht, Matthias J; Herschbach, Peter; Imhoff, Andreas B; Braun, Sepp

2017-10-01

To report general life and health satisfaction after arthroscopic Bankart repair in patients with post-traumatic recurrent anterior glenohumeral instability and to investigate postoperative time lost to return to work at 2-year follow-up. Between 2011 and 2013 patients treated with arthroscopic Bankart repair in the beach chair position for acute shoulder instability were included in this study. Questions on Life Satisfaction Modules (FLZ M ) and the Short Form 12 (SF-12) were used as quality-of-life outcome scales. Oxford Instability Score (OIS), Quick Disabilities of the Arm, Shoulder and Hand questionnaire (QuickDASH), and self-reported American Shoulder and Elbow Surgeons (ASES) shoulder index were used as functional outcome scales. Return to work (months) was monitored and analyzed depending on physical workload. Data were assessed the day before surgery and prospectively monitored until 24 months postoperatively. Quality-of-life outcome was correlated with functional shoulder outcome and compared with normative age-adjusted data. Paired t-test, Wilcoxon test, Mann-Whitney U-Test, and Spearman's correlation coefficient were used for statistical analysis. Fifty-three patients were prospectively included. The mean age at surgery was 29.4 years. Satisfaction with general life and satisfaction with health (FLZ M ) as well as physical component scale (SF-12) improved significantly to values above normative data within 6 to 12 months after surgery (each P < .001). OIS, QuickDASH, and ASES improved significantly from baseline until 24 months after surgery (each P < .001). For ASES, improvement above minimal clinically important difference was shown. There was a positive correlation between quality of life and functional outcome scores (P < .05; rho, 0.3-0.4). Mean time to return to work was 2 months (range, 0-10; standard deviation, 1.9), with significantly longer time intervals observed in patients with heavy physical workload (3.1 months; range, 0 to 10; standard deviation, 2.4; P = .002). Following arthroscopic Bankart repair, quality of life was impaired during early course after surgery and increased significantly above preoperative levels within 6 to 12 months after the procedure. A steady state of excellent quality-of-life and functional outcomes was noted after 12 months of follow-up. Quality-of-life outcome scales correlated significantly with the functional outcome. Heavy physical workload must be considered as a risk factor for prolonged time lost to return to work. Level III, prospective noncomparative therapeutic case series. Copyright © 2017 Arthroscopy Association of North America. Published by Elsevier Inc. All rights reserved.

Instability in dynamic fracture

NASA Astrophysics Data System (ADS)

Fineberg, J.; Marder, M.

1999-05-01

The fracture of brittle amorphous materials is an especially challenging problem, because the way a large object shatters is intimately tied to details of cohesion at microscopic scales. This subject has been plagued by conceptual puzzles, and to make matters worse, experiments seemed to contradict the most firmly established theories. In this review, we will show that the theory and experiments fit within a coherent picture where dynamic instabilities of a crack tip play a crucial role. To accomplish this task, we first summarize the central results of linear elastic dynamic fracture mechanics, an elegant and powerful description of crack motion from the continuum perspective. We point out that this theory is unable to make predictions without additional input, information that must come either from experiment, or from other types of theories. We then proceed to discuss some of the most important experimental observations, and the methods that were used to obtain the them. Once the flux of energy to a crack tip passes a critical value, the crack becomes unstable, and it propagates in increasingly complicated ways. As a result, the crack cannot travel as quickly as theory had supposed, fracture surfaces become rough, it begins to branch and radiate sound, and the energy cost for crack motion increases considerably. All these phenomena are perfectly consistent with the continuum theory, but are not described by it. Therefore, we close the review with an account of theoretical and numerical work that attempts to explain the instabilities. Currently, the experimental understanding of crack tip instabilities in brittle amorphous materials is fairly detailed. We also have a detailed theoretical understanding of crack tip instabilities in crystals, reproducing qualitatively many features of the experiments, while numerical work is beginning to make the missing connections between experiment and theory.

Hall thruster microturbulence under conditions of modified electron wall emission

NASA Astrophysics Data System (ADS)

Tsikata, S.; Héron, A.; Honoré, C.

2017-05-01

In recent numerical, theoretical, and experimental papers, the short-scale electron cyclotron drift instability (ECDI) has been studied as a possible contributor to the anomalous electron current observed in Hall thrusters. In this work, features of the instability, in the presence of a zero-electron emission material at the thruster exit plane, are analyzed using coherent Thomson scattering. Limiting the electron emission at the exit plane alters the localization of the accelerating electric field and the expected drift velocity profile, which in turn modifies the amplitude and localization of the ECDI. The resulting changes to the standard thruster operation are expected to favor an increased contribution by the ECDI to electron current. Such an operation is associated with a degradation of thruster performance and stability.

Investigation of natural circulation instability and transients in passively safe novel modular reactor

NASA Astrophysics Data System (ADS)

Shi, Shanbin

The Purdue Novel Modular Reactor (NMR) is a new type small modular reactor (SMR) that belongs to the design of boiling water reactor (BWR). Specifically, the NMR is one third the height and area of a conventional BWR reactor pressure vessel (RPV) with an electric output of 50 MWe. The fuel cycle length of the NMR-50 is extended up to 10 years due to optimized neutronics design. The NMR-50 is designed with double passive engineering safety system. However, natural circulation BWRs (NCBWR) could experience certain operational difficulties due to flow instabilities that occur at low pressure and low power conditions. Static instabilities (i.e. flow excursion (Ledinegg) instability and flow pattern transition instability) and dynamic instabilities (i.e. density wave instability and flashing/condensation instability) pose a significant challenge in two-phase natural circulation systems. In order to experimentally study the natural circulation flow instability, a proper scaling methodology is needed to build a reduced-size test facility. The scaling analysis of the NMR uses a three-level scaling method, which was developed and applied for the design of the Purdue Multi-dimensional Integral Test Assembly (PUMA). Scaling criteria is derived from dimensionless field equations and constitutive equations. The scaling process is validated by the RELAP5 analysis for both steady state and startup transients. A new well-scaled natural circulation test facility is designed and constructed based on the scaling analysis of the NMR-50. The experimental facility is installed with different equipment to measure various thermal-hydraulic parameters such as pressure, temperature, mass flow rate and void fraction. Characterization tests are performed before the startup transient tests and quasi-steady tests to determine the loop flow resistance. The controlling system and data acquisition system are programmed with LabVIEW to realize the real-time control and data storage. The thermal-hydraulic and nuclear coupled startup transients are performed to investigate the flow instabilities at low pressure and low power conditions. Two different power ramps are chosen to study the effect of power density on the flow instability. The experimental startup transient tests show the existence of three different flow instability mechanisms during the low pressure startup transients, i.e., flashing instability, condensation induced instability, and density wave oscillations. Flashing instability in the chimney section of the test loop and density wave oscillation are the main flow instabilities observed when the system pressure is below 0.5 MPa. They show completely different type of oscillations, i.e., intermittent oscillation and sinusoidal oscillation, in void fraction profile during the startup transients. In order to perform nuclear-coupled startup transients with void reactivity feedback, the Point Kinetics model is utilized to calculate the transient power during the startup transients. In addition, the differences between the electric resistance heaters and typical fuel element are taken into account. The reactor power calculated shows some oscillations due to flashing instability during the transients. However, the void reactivity feedback does not have significant influence on the flow instability during the startup procedure for the NMR-50. Further investigation of very small power ramp on the startup transients is carried out for the thermal-hydraulic startup transients. It is found that very small power density can eliminate the flashing oscillation in the single phase natural circulation and stabilize the flow oscillations in the phase of net vapor generation. Furthermore, initially pressurized startup procedure is investigated to eliminate the main flow instabilities. The results show that the pressurized startup procedure can suppress the flashing instability at low pressure and low power conditions. In order to have a deep understanding of natural circulation flow instability, the quasi-steady tests are performed using the test facility installed with preheater and subcooler. The effects of system pressure, core inlet subcooling, core power density, inlet flow resistance coefficient, and void reactivity feedback are investigated in the quasi-steady state tests. The stability boundaries are determined between unstable and stable flow conditions in the dimensionless stability plane of inlet subcooling number and Zuber number. In order to predict the stability boundary theoretically, linear stability analysis in the frequency domain is performed at four sections of the loop. The flashing in the chimney is considered as an axially uniform heat source. The dimensionless characteristic equation of the pressure drop perturbation is obtained by considering the void fraction effect and outlet flow resistance in the chimney section. The flashing boundary shows some discrepancies with previous experimental data from the quasi-steady state tests. In the future, thermal non-equilibrium is recommended to improve the accuracy of flashing instability boundary.

Cavitation instability as a trigger of aneurysm rupture.

PubMed

Volokh, K Y

2015-10-01

Aneurysm formation and growth is accompanied by microstructural alterations in the arterial wall. Particularly, the loss of elastin may lead to tissue disintegration and appearance of voids or cavities at the micron scale. Unstable growth and coalescence of voids may be a predecessor and trigger for the onset of macroscopic cracks. In the present work, we analyze the instability of membrane (2D) and bulk (3D) voids under hydrostatic tension by using two experimentally calibrated constitutive models of abdominal aortic aneurysm enhanced with energy limiters. The limiters provide the saturation value for the strain energy, which indicates the maximum energy that can be stored and dissipated by an infinitesimal material volume. We find that the unstable growth of voids can start when the critical stress is considerably less than the aneurysm strength. Moreover, this critical stress may even approach the arterial wall stress in the physiological range. This finding suggests that cavitation instability can be a rational indicator of the aneurysm rupture.

Magnetorotational instability and dynamo action in gravito-turbulent astrophysical discs

NASA Astrophysics Data System (ADS)

Riols, A.; Latter, H.

2018-02-01

Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertically stratified shearing-box simulations, combining self-gravity and magnetic fields. We show that gravito-turbulence greatly weakens the zero-net-flux MRI. In the limit of efficient cooling (and thus enhanced GI), the MRI is completely suppressed, and yet strong magnetic fields are sustained by the gravito-turbulence. This turbulent `spiral wave' dynamo may have widespread application, especially in galactic discs. Finally, we present preliminary work showing that a strong net-vertical-flux revives the MRI and supports a magnetically dominated state in which the GI is secondary.

The role of zonal flows in disc gravito-turbulence

NASA Astrophysics Data System (ADS)

Vanon, R.

2018-07-01

The work presented here focuses on the role of zonal flows in the self-sustenance of gravito-turbulence in accretion discs. The numerical analysis is conducted using a bespoke pseudo-spectral code in fully compressible, non-linear conditions. The disc in question, which is modelled using the shearing sheet approximation, is assumed to be self-gravitating, viscous, and thermally diffusive; a constant cooling time-scale is also considered. Zonal flows are found to emerge at the onset of gravito-turbulence and they remain closely linked to the turbulent state. A cycle of zonal flow formation and destruction is established, mediated by a slow mode instability (which allows zonal flows to grow) and a non-axisymmetric instability (which disrupts the zonal flow), which is found to repeat numerous times. It is in fact the disruptive action of the non-axisymmetric instability to form new leading and trailing shearing waves, allowing energy to be extracted from the background flow and ensuring the self-sustenance of the gravito-turbulent regime.

Development and Application of Plasma Actuators for Active Control of High-Speed and High Reynolds Number Flows

NASA Technical Reports Server (NTRS)

Sammy, Mo

2010-01-01

Active flow control is often used to manipulate flow instabilities to achieve a desired goal (e.g. prevent separation, enhance mixing, reduce noise, etc.). Instability frequencies normally scale with flow velocity scale and inversely with flow length scale (U/l). In a laboratory setting for such flow experiments, U is high, but l is low, resulting in high instability frequency. In addition, high momentum and high background noise & turbulence in the flow necessitate high amplitude actuation. Developing a high amplitude and high frequency actuator is a major challenge. Ironically, these requirements ease up in application (but other issues arise).

Plasmoid Instability in Forming Current Sheets

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

Comisso, L.; Lingam, M.; Huang, Y. -M.

The plasmoid instability has revolutionized our understanding of magnetic reconnection in astrophysical environments. By preventing the formation of highly elongated reconnection layers, it is crucial in enabling the rapid energy conversion rates that are characteristic of many astrophysical phenomena. Most previous studies have focused on Sweet–Parker current sheets, which are unattainable in typical astrophysical systems. Here we derive a general set of scaling laws for the plasmoid instability in resistive and visco-resistive current sheets that evolve over time. Our method relies on a principle of least time that enables us to determine the properties of the reconnecting current sheet (aspect ratio and elapsed time) and the plasmoid instability (growth rate, wavenumber, inner layer width) at the end of the linear phase. After this phase the reconnecting current sheet is disrupted and fast reconnection can occur. The scaling laws of the plasmoid instability are not simple power laws, and they depend on the Lundquist number (S), the magnetic Prandtl number (P m), the noise of the system (more » $${\\psi }_{0}$$), the characteristic rate of current sheet evolution ($$1/\\tau $$), and the thinning process. We also demonstrate that previous scalings are inapplicable to the vast majority of astrophysical systems. Furthermore, we explore the implications of the new scaling relations in astrophysical systems such as the solar corona and the interstellar medium. In both of these systems, we show that our scaling laws yield values for the growth rate, wavenumber, and aspect ratio that are much smaller than the Sweet–Parker–based scalings.« less

Plasmoid Instability in Forming Current Sheets

DOE PAGES

Comisso, L.; Lingam, M.; Huang, Y. -M.; ...

2017-11-28

The plasmoid instability has revolutionized our understanding of magnetic reconnection in astrophysical environments. By preventing the formation of highly elongated reconnection layers, it is crucial in enabling the rapid energy conversion rates that are characteristic of many astrophysical phenomena. Most previous studies have focused on Sweet–Parker current sheets, which are unattainable in typical astrophysical systems. Here we derive a general set of scaling laws for the plasmoid instability in resistive and visco-resistive current sheets that evolve over time. Our method relies on a principle of least time that enables us to determine the properties of the reconnecting current sheet (aspect ratio and elapsed time) and the plasmoid instability (growth rate, wavenumber, inner layer width) at the end of the linear phase. After this phase the reconnecting current sheet is disrupted and fast reconnection can occur. The scaling laws of the plasmoid instability are not simple power laws, and they depend on the Lundquist number (S), the magnetic Prandtl number (P m), the noise of the system (more » $${\\psi }_{0}$$), the characteristic rate of current sheet evolution ($$1/\\tau $$), and the thinning process. We also demonstrate that previous scalings are inapplicable to the vast majority of astrophysical systems. Furthermore, we explore the implications of the new scaling relations in astrophysical systems such as the solar corona and the interstellar medium. In both of these systems, we show that our scaling laws yield values for the growth rate, wavenumber, and aspect ratio that are much smaller than the Sweet–Parker–based scalings.« less

Health coverage instability for mothers in working families.

PubMed

Anderson, Steven G; Eamon, Mary Keegan

2004-07-01

Using data from the National Longitudinal Survey of Youth, the authors examined the health insurance coverage stability of 1,667 women in working families over a three-year period (1995-1997). Findings revealed that coverage instability is common. Nearly one-half of low-income women experienced health coverage instability over the three-year study period, and low-income women with poor education, single marital status, low work hours, and frequent job changes were at even greater risk of coverage instability. The findings also imply that women affected by recent welfare reforms are likely to experience widespread health coverage problems. The implications for health care policy development, social work administration, and social work practice are discussed.

Traveling Crossow Instability for HIFiRE-5 in a Quiet Hypersonic Wind Tunnel (Postprint)

DTIC Science & Technology

2013-06-01

scale model of the 2:1 elliptic cone HIFiRE-5 flight vehicle was used to investigate the traveling crossflow instability at Mach 6 in Purdue...Force Research Laboratory, Air Vehicles Directorate 2130 8th St., WPAFB, OH 45433-7542, USA Abstract A scale model of the 2:1 elliptic cone HIFiRE-5...flight vehicle was used to investigate the traveling crossflow instability at Mach 6 in Purdue University’s Mach-6 quiet wind tunnel. Traveling crossflow

Self-similarity in high Atwood number Rayleigh-Taylor experiments

NASA Astrophysics Data System (ADS)

Mikhaeil, Mark; Suchandra, Prasoon; Pathikonda, Gokul; Ranjan, Devesh

2017-11-01

Self-similarity is a critical concept in turbulent and mixing flows. In the Rayleigh-Taylor instability, theory and simulations have shown that the flow exhibits properties of self-similarity as the mixing Reynolds number exceeds 20000 and the flow enters the turbulent regime. Here, we present results from the first large Atwood number (0.7) Rayleigh-Taylor experimental campaign for mixing Reynolds number beyond 20000 in an effort to characterize the self-similar nature of the instability. Experiments are performed in a statistically steady gas tunnel facility, allowing for the evaluation of turbulence statistics. A visualization diagnostic is used to study the evolution of the mixing width as the instability grows. This allows for computation of the instability growth rate. For the first time in such a facility, stereoscopic particle image velocimetry is used to resolve three-component velocity information in a plane. Velocity means, fluctuations, and correlations are considered as well as their appropriate scaling. Probability density functions of velocity fields, energy spectra, and higher-order statistics are also presented. The energy budget of the flow is described, including the ratio of the kinetic energy to the released potential energy. This work was supported by the DOE-NNSA SSAA Grant DE-NA0002922.

Viscous-resistive layer in Rayleigh-Taylor instability

NASA Astrophysics Data System (ADS)

Silveira, F. E. M.; Orlandi, H. I.

2017-03-01

In this work, new scaling laws of the time growth rate γ of the Rayleigh-Taylor instability with the plasma resistivity η, kinematic viscosity ν, and electron number density ne are derived. A viscosity scale is defined in terms of the time decay of the perturbative fluid flow perpendicular to the equilibrium magnetic field, at the quasi-static approximation. Such a scale provides the identification of a viscous layer that can be combined with the resistive layer to produce a viscous-resistive layer. The latter, in turn, is found to satisfy an algebraic biquadratic equation. When viscous effects are negligible, it is shown that the viscous-resistive layer is given by the resistive layer. Somewhat surprisingly, when viscous effects cannot be neglected, it is shown that the viscous-resistive layer is given by the geometric mean of the resistive and viscous layers. A dispersion relation for the time growth rate is derived in terms of the viscous-resistive layer. When viscous effects cannot be neglected, two new scaling laws are found. At the quasi-static approximation, it is shown that γ ˜ (ην)1/4. However, on account of a finite electron mass, it is shown that γ˜(ν/ne ) 1 /3 . Further developments of our formulation are addressed in connection with a finite compressibility in the perturbative flow.

Assessing a hydrodynamic description for instabilities in highly dissipative, freely cooling granular gases.

PubMed

Mitrano, Peter P; Garzó, Vicente; Hilger, Andrew M; Ewasko, Christopher J; Hrenya, Christine M

2012-04-01

An intriguing phenomenon displayed by granular flows and predicted by kinetic-theory-based models is the instability known as particle "clustering," which refers to the tendency of dissipative grains to form transient, loose regions of relatively high concentration. In this work, we assess a modified-Sonine approximation recently proposed [Garzó, Santos, and Montanero, Physica A 376, 94 (2007)] for a granular gas via an examination of system stability. In particular, we determine the critical length scale associated with the onset of two types of instabilities--vortices and clusters--via stability analyses of the Navier-Stokes-order hydrodynamic equations by using the expressions of the transport coefficients obtained from both the standard and the modified-Sonine approximations. We examine the impact of both Sonine approximations over a range of solids fraction φ<0.2 for small restitution coefficients e = 0.25-0.4, where the standard and modified theories exhibit discrepancies. The theoretical predictions for the critical length scales are compared to molecular dynamics (MD) simulations, of which a small percentage were not considered due to inelastic collapse. Results show excellent quantitative agreement between MD and the modified-Sonine theory, while the standard theory loses accuracy for this highly dissipative parameter space. The modified theory also remedies a high-dissipation qualitative mismatch between the standard theory and MD for the instability that forms more readily. Furthermore, the evolution of cluster size is briefly examined via MD, indicating that domain-size clusters may remain stable or halve in size, depending on system parameters.

Rayleigh-Taylor-instability evolution in colliding-plasma-jet experiments with magnetic and viscous stabilization

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

Adams, Colin Stuart

The Rayleigh-Taylor instability causes mixing in plasmas throughout the universe, from micron-scale plasmas in inertial confinement fusion implosions to parsec-scale supernova remnants. The evolution of this interchange instability in a plasma is influenced by the presence of viscosity and magnetic fields, both of which have the potential to stabilize short-wavelength modes. Very few experimental observations of Rayleigh-Taylor growth in plasmas with stabilizing mechanisms are reported in the literature, and those that are reported are in sub-millimeter scale plasmas that are difficult to diagnose. Experimental observations in well-characterized plasmas are important for validation of computational models used to make design predictionsmore » for inertial confinement fusion efforts. This dissertation presents observations of instability growth during the interaction between a high Mach-number, initially un-magnetized plasma jet and a stagnated, magnetized plasma. A multi-frame fast camera captures Rayleigh-Taylor-instability growth while interferometry, spectroscopy, photodiode, and magnetic probe diagnostics are employed to estimate plasma parameters in the vicinity of the collision. As the instability grows, an evolution to longer mode wavelength is observed. Comparisons of experimental data with idealized magnetohydrodynamic simulations including a physical viscosity model suggest that the observed instability evolution is consistent with both magnetic and viscous stabilization. These data provide the opportunity to benchmark computational models used in astrophysics and fusion research.« less

Study of the Transition from MRI to Magnetic Turbulence via Parasitic Instability by a High-order MHD Simulation Code

NASA Astrophysics Data System (ADS)

Hirai, Kenichiro; Katoh, Yuto; Terada, Naoki; Kawai, Soshi

2018-02-01

Magnetic turbulence in accretion disks under ideal magnetohydrodynamic (MHD) conditions is expected to be driven by the magneto-rotational instability (MRI) followed by secondary parasitic instabilities. We develop a three-dimensional ideal MHD code that can accurately resolve turbulent structures, and carry out simulations with a net vertical magnetic field in a local shearing box disk model to investigate the role of parasitic instabilities in the formation process of magnetic turbulence. Our simulations reveal that a highly anisotropic Kelvin–Helmholtz (K–H) mode parasitic instability evolves just before the first peak in turbulent stress and then breaks large-scale shear flows created by MRI. The wavenumber of the enhanced parasitic instability is larger than the theoretical estimate, because the shear flow layers sometimes become thinner than those assumed in the linear analysis. We also find that interaction between antiparallel vortices caused by the K–H mode parasitic instability induces small-scale waves that break the shear flows. On the other hand, at repeated peaks in the nonlinear phase, anisotropic wavenumber spectra are observed only in the small wavenumber region and isotropic waves dominate at large wavenumbers unlike for the first peak. Restructured channel flows due to MRI at the peaks in nonlinear phase seem to be collapsed by the advection of small-scale shear structures into the restructured flow and resultant mixing.

Modulated electron cyclotron drift instability in a high-power pulsed magnetron discharge.

PubMed

Tsikata, Sedina; Minea, Tiberiu

2015-05-08

The electron cyclotron drift instability, implicated in electron heating and anomalous transport, is detected in the plasma of a planar magnetron. Electron density fluctuations associated with the mode are identified via an adapted coherent Thomson scattering diagnostic, under direct current and high-power pulsed magnetron operation. Time-resolved analysis of the mode amplitude reveals that the instability, found at MHz frequencies and millimeter scales, also exhibits a kHz-scale modulation consistent with the observation of larger-scale plasma density nonuniformities, such as the rotating spoke. Sharply collimated axial fluctuations observed at the magnetron axis are consistent with the presence of escaping electrons in a region where the magnetic and electric fields are antiparallel. These results distinguish aspects of magnetron physics from other plasma sources of similar geometry, such as the Hall thruster, and broaden the scope of instabilities which may be considered to dictate magnetron plasma features.

Mirror Instability in the Turbulent Solar Wind

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

Hellinger, Petr; Landi, Simone; Verdini, Andrea

2017-04-01

The relationship between a decaying strong turbulence and the mirror instability in a slowly expanding plasma is investigated using two-dimensional hybrid expanding box simulations. We impose an initial ambient magnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly polarized, random-phase Alfvénic fluctuations that have energy equipartition between kinetic and magnetic fluctuations and a vanishing correlation between the two fields. A turbulent cascade rapidly develops, magnetic field fluctuations exhibit a Kolmogorov-like power-law spectrum at large scales and a steeper spectrum at sub-ion scales. The imposed expansion (taking a strictly transverse ambient magnetic field) leadsmore » to the generation of an important perpendicular proton temperature anisotropy that eventually drives the mirror instability. This instability generates large-amplitude, nonpropagating, compressible, pressure-balanced magnetic structures in a form of magnetic enhancements/humps that reduce the perpendicular temperature anisotropy.« less

Microphase separation in solid lipid dosage forms as the cause of drug release instability.

PubMed

Lopes, Diogo Gomes; Koutsamanis, Ioannis; Becker, Karin; Scheibelhofer, Otto; Laggner, Peter; Haack, Detlev; Stehr, Michael; Zimmer, Andreas; Salar-Behzadi, Sharareh

2017-01-30

Although lipid excipients are of increasing interest for development of taste-masked and modified release formulations, the drug release instability and the lack of mechanistic understanding in that regard still prevent their larger-scale application. In this work, we investigated the physical stability of a binary (tripalmitin/polysorbate 65) lipid coating formulation with a known stable polymorphism. The coating composition was characterized using DSC to construct the phase diagram of binary system and polarized light microscopy to display the microstructure organization. The water uptake and the erosion of slabs cast from the coating formulations were investigated post-production and after storage. Subsequently, N-acetylcysteine particles were coated with the selected formulations and the drug release stability was investigated. Additionally, microstructure characterization was performed via SEM and X-ray diffraction. The drug release instability was explained by polysorbate 65 and tripalmitin phase growth during storage, especially at 40°C, suggesting that polysorbate 65 can leak out of tripalmitin spherulitic structures, creating lipophilic and impermeable tripalmitin regions. The growth of polysorbate 65 phase leads to larger hydrophilic channels with reduced tortuosity. This work indicates that for obtaining stable drug release profiles from advanced lipid formulations, microphase separation should be prevented during storage. Copyright © 2016 Elsevier B.V. All rights reserved.

Laboratory simulation of space plasma phenomena*

NASA Astrophysics Data System (ADS)

Amatucci, B.; Tejero, E. M.; Ganguli, G.; Blackwell, D.; Enloe, C. L.; Gillman, E.; Walker, D.; Gatling, G.

2017-12-01

Laboratory devices, such as the Naval Research Laboratory's Space Physics Simulation Chamber, are large-scale experiments dedicated to the creation of large-volume plasmas with parameters realistically scaled to those found in various regions of the near-Earth space plasma environment. Such devices make valuable contributions to the understanding of space plasmas by investigating phenomena under carefully controlled, reproducible conditions, allowing for the validation of theoretical models being applied to space data. By working in collaboration with in situ experimentalists to create realistic conditions scaled to those found during the observations of interest, the microphysics responsible for the observed events can be investigated in detail not possible in space. To date, numerous investigations of phenomena such as plasma waves, wave-particle interactions, and particle energization have been successfully performed in the laboratory. In addition to investigations such as plasma wave and instability studies, the laboratory devices can also make valuable contributions to the development and testing of space plasma diagnostics. One example is the plasma impedance probe developed at NRL. Originally developed as a laboratory diagnostic, the sensor has now been flown on a sounding rocket, is included on a CubeSat experiment, and will be included on the DoD Space Test Program's STP-H6 experiment on the International Space Station. In this presentation, we will describe several examples of the laboratory investigation of space plasma waves and instabilities and diagnostic development. *This work supported by the NRL Base Program.

On the feeding zone of planetesimal formation by the streaming instability

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

Yang, Chao-Chin; Johansen, Anders, E-mail: ccyang@astro.lu.se, E-mail: anders@astro.lu.se

2014-09-10

The streaming instability is a promising mechanism to overcome the barriers in direct dust growth and lead to the formation of planetesimals. Most previous studies of the streaming instability, however, were focused on a local region of a protoplanetary disk with a limited simulation domain such that only one filamentary concentration of solids has been observed. The characteristic separation between filaments is therefore not known. To address this, we conduct the largest-scale simulations of the streaming instability to date, with computational domains up to 1.6 gas scale heights both horizontally and vertically. The large dynamical range allows the effect ofmore » vertical gas stratification to become prominent. We observe more frequent merging and splitting of filaments in simulation boxes of high vertical extent. We find multiple filamentary concentrations of solids with an average separation of about 0.2 local gas scale heights, much higher than the most unstable wavelength from linear stability analysis. This measures the characteristic separation of planetesimal forming events driven by the streaming instability and thus the initial feeding zone of planetesimals.« less

Finite-temperature mechanical instability in disordered lattices.

PubMed

Zhang, Leyou; Mao, Xiaoming

2016-02-01

Mechanical instability takes different forms in various ordered and disordered systems and little is known about how thermal fluctuations affect different classes of mechanical instabilities. We develop an analytic theory involving renormalization of rigidity and coherent potential approximation that can be used to understand finite-temperature mechanical stabilities in various disordered systems. We use this theory to study two disordered lattices: a randomly diluted triangular lattice and a randomly braced square lattice. These two lattices belong to two different universality classes as they approach mechanical instability at T=0. We show that thermal fluctuations stabilize both lattices. In particular, the triangular lattice displays a critical regime in which the shear modulus scales as G∼T(1/2), whereas the square lattice shows G∼T(2/3). We discuss generic scaling laws for finite-T mechanical instabilities and relate them to experimental systems.

Kelvin-Helmholtz instability of the Dirac fluid of charge carriers on graphene

NASA Astrophysics Data System (ADS)

Coelho, Rodrigo C. V.; Mendoza, Miller; Doria, Mauro M.; Herrmann, Hans J.

2017-11-01

We provide numerical evidence that a Kelvin-Helmholtz instability occurs in the Dirac fluid of electrons in graphene and can be detected in current experiments. This instability appears for electrons in the viscous regime passing though a micrometer-scale obstacle and affects measurements on the time scale of nanoseconds. A possible realization with a needle-shaped obstacle is proposed to produce and detect this instability by measuring the electric potential difference between contact points located before and after the obstacle. We also show that, for our setup, the Kelvin-Helmholtz instability leads to the formation of whirlpools similar to the ones reported in Bandurin et al. [Science 351, 1055 (2016), 10.1126/science.aad0201]. To perform the simulations, we develop a lattice Boltzmann method able to recover the full dissipation in a fluid of massless particles.

Planetesimal Formation through the Streaming Instability

NASA Astrophysics Data System (ADS)

Yang, Chao-Chin; Johansen, Anders; Schäfer, Urs

2015-12-01

The streaming instability is a promising mechanism to circumvent the barriers in direct dust growth and lead to the formation of planetesimals, as demonstrated by many previous studies. In order to resolve the thin layer of solids, however, most of these studies were focused on a local region of a protoplanetary disk with a limited simulation domain. It remains uncertain how the streaming instability is affected by the disk gas on large scales, and models that have sufficient dynamical range to capture both the thin particle layer and the large-scale disk dynamics are required.We hereby systematically push the limits of the computational domain up to more than the gas scale height, and study the particle-gas interaction on large scales in the saturated state of the streaming instability and the initial mass function of the resulting planetesimals. To overcome the numerical challenges posed by this kind of models, we have developed a new technique to simultaneously relieve the stringent time step constraints due to small-sized particles and strong local solid concentrations. Using these models, we demonstrate that the streaming instability can drive multiple radial, filamentary concentrations of solids, implying that planetesimals are born in well separated belt-like structures. We also find that the initial mass function of planetesimals via the streaming instability has a characteristic exponential form, which is robust against computational domain as well as resolution. These findings will help us further constrain the cosmochemical history of the Solar system as well as the planet formation theory in general.

Physics and Novel Schemes of Laser Radiation Pressure Acceleration for Quasi-monoenergetic Proton Generation

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

Liu, Chuan S.; Shao, Xi

2016-06-14

The main objective of our work is to provide theoretical basis and modeling support for the design and experimental setup of compact laser proton accelerator to produce high quality proton beams tunable with energy from 50 to 250 MeV using short pulse sub-petawatt laser. We performed theoretical and computational studies of energy scaling and Raleigh--Taylor instability development in laser radiation pressure acceleration (RPA) and developed novel RPA-based schemes to remedy/suppress instabilities for high-quality quasimonoenergetic proton beam generation as we proposed. During the project period, we published nine peer-reviewed journal papers and made twenty conference presentations including six invited talks onmore » our work. The project supported one graduate student who received his PhD degree in physics in 2013 and supported two post-doctoral associates. We also mentored three high school students and one undergraduate student of physics major by inspiring their interests and having them involved in the project.« less

Stability and instability of hydromagnetic Taylor-Couette flows

NASA Astrophysics Data System (ADS)

Rüdiger, Günther; Gellert, Marcus; Hollerbach, Rainer; Schultz, Manfred; Stefani, Frank

2018-04-01

Decades ago S. Lundquist, S. Chandrasekhar, P. H. Roberts and R. J. Tayler first posed questions about the stability of Taylor-Couette flows of conducting material under the influence of large-scale magnetic fields. These and many new questions can now be answered numerically where the nonlinear simulations even provide the instability-induced values of several transport coefficients. The cylindrical containers are axially unbounded and penetrated by magnetic background fields with axial and/or azimuthal components. The influence of the magnetic Prandtl number Pm on the onset of the instabilities is shown to be substantial. The potential flow subject to axial fields becomes unstable against axisymmetric perturbations for a certain supercritical value of the averaged Reynolds number Rm bar =√{ Re ṡ Rm } (with Re the Reynolds number of rotation, Rm its magnetic Reynolds number). Rotation profiles as flat as the quasi-Keplerian rotation law scale similarly but only for Pm ≫ 1 while for Pm ≪ 1 the instability instead sets in for supercritical Rm at an optimal value of the magnetic field. Among the considered instabilities of azimuthal fields, those of the Chandrasekhar-type, where the background field and the background flow have identical radial profiles, are particularly interesting. They are unstable against nonaxisymmetric perturbations if at least one of the diffusivities is non-zero. For Pm ≪ 1 the onset of the instability scales with Re while it scales with Rm bar for Pm ≫ 1. Even superrotation can be destabilized by azimuthal and current-free magnetic fields; this recently discovered nonaxisymmetric instability is of a double-diffusive character, thus excluding Pm = 1. It scales with Re for Pm → 0 and with Rm for Pm → ∞. The presented results allow the construction of several new experiments with liquid metals as the conducting fluid. Some of them are described here and their results will be discussed together with relevant diversifications of the magnetic instability theory including nonlinear numerical studies of the kinetic and magnetic energies, the azimuthal spectra and the influence of the Hall effect.

Nonlinear calculations of the time evolution of black hole accretion disks

NASA Technical Reports Server (NTRS)

Luo, C.

1994-01-01

Based on previous works on black hole accretion disks, I continue to explore the disk dynamics using the finite difference method to solve the highly nonlinear problem of time-dependent alpha disk equations. Here a radially zoned model is used to develop a computational scheme in order to accommodate functional dependence of the viscosity parameter alpha on the disk scale height and/or surface density. This work is based on the author's previous work on the steady disk structure and the linear analysis of disk dynamics to try to apply to x-ray emissions from black candidates (i.e., multiple-state spectra, instabilities, QPO's, etc.).

Student Motivation and Program Participation

ERIC Educational Resources Information Center

Reardon, Robert C.; Bertoch, Sara Cummings

2010-01-01

The present study investigated the relationship between goal instability and participation in a career planning program (course) using the goal instability scale (GIS) and a completed course performance contract. Specifically, this study addressed the following question: What is the relationship between motivation, as measured by goal instability,…

Rotordynamic Instability Problems in High-Performance Turbomachinery

NASA Technical Reports Server (NTRS)

1982-01-01

Rotor dynamic instability problems in high performance turbomachinery are reviewed. Mechanical instability mechanisms are discussed. Seal forces and working fluid forces in turbomachinery are discussed. Control of rotor instability is also investigated.

Spiral-arm instability: giant clump formation via fragmentation of a galactic spiral arm

NASA Astrophysics Data System (ADS)

Inoue, Shigeki; Yoshida, Naoki

2018-03-01

Fragmentation of a spiral arm is thought to drive the formation of giant clumps in galaxies. Using linear perturbation analysis for self-gravitating spiral arms, we derive an instability parameter and define the conditions for clump formation. We extend our analysis to multicomponent systems that consist of gas and stars in an external potential. We then perform numerical simulations of isolated disc galaxies with isothermal gas, and compare the results with the prediction of our analytic model. Our model describes accurately the evolution of the spiral arms in our simulations, even when spiral arms dynamically interact with one another. We show that most of the giant clumps formed in the simulated disc galaxies satisfy the instability condition. The clump masses predicted by our model are in agreement with the simulation results, but the growth time-scale of unstable perturbations is overestimated by a factor of a few. We also apply our instability analysis to derive scaling relations of clump properties. The expected scaling relation between the clump size, velocity dispersion, and circular velocity is slightly different from that given by the Toomre instability analyses, but neither is inconsistent with currently available observations. We argue that the spiral-arm instability is a viable formation mechanism of giant clumps in gas-rich disc galaxies.

Mixing the Solar Wind Proton and Electron Scales: Effects of Electron Temperature Anisotropy on the Oblique Proton Firehose Instability

NASA Technical Reports Server (NTRS)

Maneva, Y.; Lazar, M.; Vinas, A.; Poedts, S.

2016-01-01

The double adiabatic expansion of the nearly collisionless solar wind plasma creates conditions for the firehose instability to develop and efficiently prevent the further increase of the plasma temperature in the direction parallel to the interplanetary magnetic field. The conditions imposed by the firehose instability have been extensively studied using idealized approaches that ignore the mutual effects of electrons and protons. Recently, more realistic approaches have been proposed that take into account the interplay between electrons and protons,? unveiling new regimes of the parallel oscillatory modes. However, for oblique wave propagation the instability develops distinct branches that grow much faster and may therefore be more efficient than the parallel firehose instability in constraining the temperature anisotropy of the plasma particles. This paper reports for the first time on the effects of electron plasma properties on the oblique proton firehose (PFH) instability and provides a comprehensive vision of the entire unstable wave-vector spectrum, unifying the proton and the smaller electron scales. The plasma ß and temperature anisotropy regimes considered here are specific for the solar wind and magnetospheric conditions, and enable the electrons and protons to interact via the excited electromagnetic fluctuations. For the selected parameters, simultaneous electron and PFH instabilities can be observed with a dispersion spectrum of the electron firehose (EFH) extending toward the proton scales. Growth rates of the PFH instability are markedly boosted by the anisotropic electrons, especially in the oblique direction where the EFH growth rates are orders of magnitude higher.

MIXING THE SOLAR WIND PROTON AND ELECTRON SCALES: EFFECTS OF ELECTRON TEMPERATURE ANISOTROPY ON THE OBLIQUE PROTON FIREHOSE INSTABILITY

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

Maneva, Y.; Lazar, M.; Poedts, S.

2016-11-20

The double adiabatic expansion of the nearly collisionless solar wind plasma creates conditions for the firehose instability to develop and efficiently prevent the further increase of the plasma temperature in the direction parallel to the interplanetary magnetic field. The conditions imposed by the firehose instability have been extensively studied using idealized approaches that ignore the mutual effects of electrons and protons. Recently, more realistic approaches have been proposed that take into account the interplay between electrons and protons, unveiling new regimes of the parallel oscillatory modes. However, for oblique wave propagation the instability develops distinct branches that grow much fastermore » and may therefore be more efficient than the parallel firehose instability in constraining the temperature anisotropy of the plasma particles. This paper reports for the first time on the effects of electron plasma properties on the oblique proton firehose (PFH) instability and provides a comprehensive vision of the entire unstable wave-vector spectrum, unifying the proton and the smaller electron scales. The plasma β and temperature anisotropy regimes considered here are specific for the solar wind and magnetospheric conditions, and enable the electrons and protons to interact via the excited electromagnetic fluctuations. For the selected parameters, simultaneous electron and PFH instabilities can be observed with a dispersion spectrum of the electron firehose (EFH) extending toward the proton scales. Growth rates of the PFH instability are markedly boosted by the anisotropic electrons, especially in the oblique direction where the EFH growth rates are orders of magnitude higher.« less

Interplay between protons and electrons in a firehose-unstable plasma: Particle-in-cell simulations

NASA Astrophysics Data System (ADS)

Bourdin, Philippe-A.; Maneva, Yana

2017-04-01

Kinetic plasma instabilities originating from unstable, non-Maxwellian shapes of the velocity distribution functions serve as internal degrees of freedom in plasma dynamics, and play an important role near solar current sheets and in solar wind plasmas. In the presence of strong temperature anisotropy (different thermal spreads in the velocity space with respect to the mean magnetic field), plasmas are unstable either to the firehose mode or to the mirror mode in the case of predominant parallel and perpendicular temperatures, respectively. The growth rates of these instabilities and their thresholds depend on plasma properties, such as the temperature anisotropy and the plasma beta. The physics of the temperature anisotropy-driven instabilities becomes even more diverse for various shapes of velocity distribution functions and the particle species of interest. Recent studies based on a linear instability analysis show an interplay in the firehose instability between protons and electrons when the both types of particle species are prone to unstable velocity distribution functions and their instability thresholds. In this work we perform for the first time 3D nonlinear PIC (particle-in-cell) numerical simulations to test for the linear-theory prediction of the simultaneous proton-electron firehose instability. The simulation setup allows us not only to evaluate the growth rate of each firehose instability, but also to track its nonlinear evolution and the related wave-particle interactions such as the pitch-angle scattering or saturation effects. The specialty of our simulation is that the magnetic and electric fields have a low numerical noise level by setting a sufficiently large number of super-particles into the simulation box and enhancing the statistical significance of the velocity distribution functions. We use the iPIC3D code with fully periodic boundaries under various conditions of the electron-to-proton mass ratio, which gives insight into the instability interplay at the intermediate electron-proton and on the scaling of our results towards more realistic particle settings.

Recent and future liquid metal experiments on homogeneous dynamo action and magnetic instabilities

NASA Astrophysics Data System (ADS)

Stefani, Frank; Gerbeth, Gunter; Giesecke, Andre; Gundrum, Thomas; Kirillov, Oleg; Seilmayer, Martin; Gellert, Marcus; Rüdiger, Günther; Gailitis, Agris

2011-10-01

The present status of the Riga dynamo experiment is summarized and the prospects for its future exploitation are evaluated. We further discuss the plans for a large-scale precession driven dynamo experiment to be set-up in the framework of the new installation DRESDYN (DREsden Sodium facility for dynamo and thermohydraulic studies) at Helmholtz-Zentrum Dresden-Rossendorf. We report recent investigations of the magnetorotational instability and the Tayler instability and sketch the plans for another large-scale liquid sodium facility devoted to the combined study of both effects.

Rayleigh-Taylor instability in an equal mass plasma

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

Adak, Ashish, E-mail: ashish-adak@yahoo.com; Ghosh, Samiran, E-mail: sran-g@yahoo.com; Chakrabarti, Nikhil, E-mail: nikhil.chakrabarti@saha.ac.in

The Rayleigh-Taylor (RT) instability in an inhomogeneous pair-ion plasma has been analyzed. Considering two fluid model for two species of ions (positive and negative), we obtain the possibility of the existence of RT instability. The growth rate of the RT instability as usual depends on gravity and density gradient scale length. The results are discussed in context of pair-ion plasma experiments.

Exploring Model Assumptions Through Three Dimensional Mixing Simulations Using a High-order Hydro Option in the Ares Code

NASA Astrophysics Data System (ADS)

White, Justin; Olson, Britton; Morgan, Brandon; McFarland, Jacob; Lawrence Livermore National Laboratory Team; University of Missouri-Columbia Team

2015-11-01

This work presents results from a large eddy simulation of a high Reynolds number Rayleigh-Taylor instability and Richtmyer-Meshkov instability. A tenth-order compact differencing scheme on a fixed Eulerian mesh is utilized within the Ares code developed at Lawrence Livermore National Laboratory. (LLNL) We explore the self-similar limit of the mixing layer growth in order to evaluate the k-L-a Reynolds Averaged Navier Stokes (RANS) model (Morgan and Wickett, Phys. Rev. E, 2015). Furthermore, profiles of turbulent kinetic energy, turbulent length scale, mass flux velocity, and density-specific-volume correlation are extracted in order to aid the creation a high fidelity LES data set for RANS modeling. Prepared by LLNL under Contract DE-AC52-07NA27344.

Inertio-elastic mixing in a straight microchannel with side wells

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

Hong, Sun Ok; Cooper-White, Justin J.; School of Chemical Engineering, University of Queensland, St Lucia, 4072 QLD

Mixing remains a challenging task in microfluidic channels because of their inherently small length scale. In this work, we propose an efficient microfluidic mixer based on the chaotic vortex dynamics of a viscoelastic flow in a straight channel with side wells. When the inertia and elasticity of a dilute polymer solution are balanced (i.e., the Reynolds number Re and Weissenberg number Wi are both on the order of 10{sup 1}), chaotic vortices appear in the side wells (inertio-elastic flow instability), enhancing the mixing of adjacent fluid streams. However, there is no chaotic vortex motion in Newtonian flows for any flowmore » rate. Efficient mixing by such an inertio-elastic instability is found to be relevant for a wide range of Re values.« less

Filament instability under constant loads

NASA Astrophysics Data System (ADS)

Monastra, A. G.; Carusela, M. F.; D’Angelo, M. V.; Bruno, L.

2018-04-01

Buckling of semi-flexible filaments appears in different systems and scales. Some examples are: fibers in geophysical applications, microtubules in the cytoplasm of eukaryotic cells and deformation of polymers freely suspended in a flow. In these examples, instabilities arise when a system’s parameter exceeds a critical value, being the Euler force the most known. However, the complete time evolution and wavelength of buckling processes are not fully understood. In this work we solve analytically the time evolution of a filament under a constant compressive force in the small amplitude approximation. This gives an insight into the variable force scenario in terms of normal modes. The evolution is highly sensitive to the initial configuration and to the magnitude of the compressive load. This model can be a suitable approach to many different real situations.

Suppression of AGN-Driven G-Mode Turbulence by Magnetic Fields in a Magnetohydrodynamic Model of the Intracluster Medium

NASA Astrophysics Data System (ADS)

Bambic, Christopher J.; Morsony, Brian J.; Reynolds, Christopher S.

2017-08-01

We investigate the role of AGN feedback in turbulent heating of galaxy clusters. X-ray measurements of the Perseus Cluster intracluster medium (ICM) by the Hitomi Mission found a velocity dispersion measure of σ ˜ 164 km/s, indicating a large-scale turbulent energy of approximately 4% of the thermal energy. If this energy is transferred to small scales via a turbulent cascade and dissipated as heat, radiative cooling can be offset and the cluster can remain in the observed thermal equilibrium. Using 3D ideal MHD simulations and a plane-parallel model of the ICM, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work has shown that this process is inefficient, with less than 1% of the injected energy ending up in turbulence. Hydrodynamic instabilities shred the bubbles apart before they can excite sufficiently strong g-modes. We examine the role of a large-scale magnetic field which is able to drape around these rising bubbles, preserving them from instabilities. We show that a helical magnetic field geometry is able to better preserve bubbles, driving stronger g-modes; however, the production of turbulence is still inefficient. Magnetic tension acts to stabilize g-modes, preventing the nonlinear transition to turbulence. In addition, the magnetic tension force acts along the field lines to suppress the formation of small-scale vortices. These two effects halt the turbulent cascade. Our work shows that ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects. In addition, other mechanisms of heating the ICM plasma such as sound waves or cosmic rays may be responsible to account for observed feedback in galaxy clusters.

Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet

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

Li, C. K.; Tzeferacos, P.; Lamb, D.

X-ray images from the Chandra X-ray Observatory show that the South-East jet in the Crab nebula changes direction every few years. This remarkable phenomenon is also observed in jets associated with pulsar wind nebulae and other astrophysical objects, and therefore is a fundamental feature of astrophysical jet evolution that needs to be understood. Theoretical modeling and numerical simulations have suggested that this phenomenon may be a consequence of magnetic fields (B) and current-driven magnetohydrodynamic (MHD) instabilities taking place in the jet, but until now there has been no verification of this process in a controlled laboratory environment. Here we reportmore » the first such experiments, using scaled laboratory plasma jets generated by high-power lasers to model the Crab jet and monoenergetic-proton radiography to provide direct visualization and measurement of magnetic fields and their behavior. The toroidal magnetic field embedded in the supersonic jet triggered plasma instabilities and resulted in considerable deflections throughout the jet propagation, mimicking the kinks in the Crab jet. We also demonstrated that these kinks are stabilized by high jet velocity, consistent with the observation that instabilities alter the jet orientation but do not disrupt the overall jet structure. We successfully modeled these laboratory experiments with a validated three-dimensional (3D) numerical simulation, which in conjunction with the experiments provide compelling evidence that we have an accurate model of the most important physics of magnetic fields and MHD instabilities in the observed, kinked jet in the Crab nebula. The experiments initiate a novel approach in the laboratory for visualizing fields and instabilities associated with jets observed in various astrophysical objects, ranging from stellar to extragalactic systems. We expect that future work along this line will have important impact on the study and understanding of such fundamental astrophysical phenomena.« less

Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet

DOE PAGES

Li, C. K.; Tzeferacos, P.; Lamb, D.; ...

2016-10-07

X-ray images from the Chandra X-ray Observatory show that the South-East jet in the Crab nebula changes direction every few years. This remarkable phenomenon is also observed in jets associated with pulsar wind nebulae and other astrophysical objects, and therefore is a fundamental feature of astrophysical jet evolution that needs to be understood. Theoretical modeling and numerical simulations have suggested that this phenomenon may be a consequence of magnetic fields (B) and current-driven magnetohydrodynamic (MHD) instabilities taking place in the jet, but until now there has been no verification of this process in a controlled laboratory environment. Here we reportmore » the first such experiments, using scaled laboratory plasma jets generated by high-power lasers to model the Crab jet and monoenergetic-proton radiography to provide direct visualization and measurement of magnetic fields and their behavior. The toroidal magnetic field embedded in the supersonic jet triggered plasma instabilities and resulted in considerable deflections throughout the jet propagation, mimicking the kinks in the Crab jet. We also demonstrated that these kinks are stabilized by high jet velocity, consistent with the observation that instabilities alter the jet orientation but do not disrupt the overall jet structure. We successfully modeled these laboratory experiments with a validated three-dimensional (3D) numerical simulation, which in conjunction with the experiments provide compelling evidence that we have an accurate model of the most important physics of magnetic fields and MHD instabilities in the observed, kinked jet in the Crab nebula. The experiments initiate a novel approach in the laboratory for visualizing fields and instabilities associated with jets observed in various astrophysical objects, ranging from stellar to extragalactic systems. We expect that future work along this line will have important impact on the study and understanding of such fundamental astrophysical phenomena.« less

THE EFFECTS OF KINETIC INSTABILITIES ON SMALL-SCALE TURBULENCE IN EARTH’S MAGNETOSHEATH

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

Breuillard, H.; Yordanova, E.; Vaivads, A.

2016-09-20

The Earth's magnetosheath is the region delimited by the bow shock and the magnetopause. It is characterized by highly turbulent fluctuations covering all scales from MHD down to kinetic scales. Turbulence is thought to play a fundamental role in key processes such as energy transport and dissipation in plasma. In addition to turbulence, different plasma instabilities are generated in the magnetosheath because of the large anisotropies in plasma temperature introduced by its boundaries. In this study we use high-quality magnetic field measurements from Cluster spacecraft to investigate the effects of such instabilities on the small-scale turbulence (from ion down tomore » electron scales). We show that the steepening of the power spectrum of magnetic field fluctuations in the magnetosheath occurs at the largest characteristic ion scale. However, the spectrum can be modified by the presence of waves/structures at ion scales, shifting the onset of the small-scale turbulent cascade toward the smallest ion scale. This cascade is therefore highly dependent on the presence of kinetic instabilities, waves, and local plasma parameters. Here we show that in the absence of strong waves the small-scale turbulence is quasi-isotropic and has a spectral index α ≈ −2.8. When transverse or compressive waves are present, we observe an anisotropy in the magnetic field components and a decrease in the absolute value of α . Slab/2D turbulence also develops in the presence of transverse/compressive waves, resulting in gyrotropy/non-gyrotropy of small-scale fluctuations. The presence of both types of waves reduces the anisotropy in the amplitude of fluctuations in the small-scale range.« less

Kinetic simulation of edge instability in fusion plasmas

NASA Astrophysics Data System (ADS)

Fulton, Daniel Patrick

In this work, gyrokinetic simulations in edge plasmas of both tokamaks and field reversed. configurations (FRC) have been carried out using the Gyrokinetic Toroidal Code (GTC) and A New Code (ANC) has been formulated for cross-separatrix FRC simulation. In the tokamak edge, turbulent transport in the pedestal of an H-mode DIII-D plasma is. studied via simulations of electrostatic driftwaves. Annulus geometry is used and simulations focus on two radial locations corresponding to the pedestal top with mild pressure gradient and steep pressure gradient. A reactive trapped electron instability with typical ballooning mode structure is excited in the pedestal top. At the steep gradient, the electrostatic instability exhibits unusual mode structure, peaking at poloidal angles theta=+- pi/2. Simulations find this unusual mode structure is due to steep pressure gradients in the pedestal but not due to the particular DIII-D magnetic geometry. Realistic DIII-D geometry has a stabilizing effect compared to a simple circular tokamak geometry. Driftwave instability in FRC is studied for the first time using gyrokinetic simulation. GTC. is upgraded to treat realistic equilibrium calculated by an MHD equilibrium code. Electrostatic local simulations in outer closed flux surfaces find ion-scale modes are stable due to the large ion gyroradius and that electron drift-interchange modes are excited by electron temperature gradient and bad magnetic curvature. In the scrape-off layer (SOL) ion-scale modes are excited by density gradient and bad curvature. Collisions have weak effects on instabilities both in the core and SOL. Simulation results are consistent with density fluctuation measurements in the C-2 experiment using Doppler backscattering (DBS). The critical density gradients measured by the DBS qualitatively agree with the linear instability threshold calculated by GTC simulations. One outstanding critical issue in the FRC is the interplay between turbulence in the FRC. core and SOL regions. While the magnetic flux coordinates used by GTC provide a number of computational advantages, they present unique challenges at the magnetic field separatrix. To address this limitation, a new code, capable of coupled core-SOL simulations, is formulated, implemented, and successfully verified.

Localized multi-scale energy and vorticity analysis. II. Finite-amplitude instability theory and validation

NASA Astrophysics Data System (ADS)

San Liang, X.; Robinson, Allan R.

2007-12-01

A novel localized finite-amplitude hydrodynamic stability analysis is established in a unified treatment for the study of real oceanic and atmospheric processes, which are in general highly nonlinear, and intermittent in space and time. We first re-state the classical definition using the multi-scale energy and vorticity analysis (MS-EVA) developed in Liang and Robinson [Liang, X.S., Robinson, A.R., 2005. Localized multiscale energy and vorticity analysis. I. Fundamentals. Dyn. Atmos. Oceans 38, 195-230], and then manipulate certain global operators to achieve the temporal and spatial localization. The key of the spatial localization is transfer-transport separation, which is made precise with the concept of perfect transfer, while relaxation of marginalization leads to the localization of time. In doing so the information of transfer lost in the averages is retrieved and an easy-to-use instability metric is obtained. The resulting metric is field-like (Eulerian), conceptually generalizing the classical formalism, a bulk notion over the whole system. In this framework, an instability has a structure, which is of particular use for open flow processes. We check the structure of baroclinic instability with the benchmark Eady model solution, and the Iceland-Faeroe Frontal (IFF) intrusion, a highly localized and nonlinear process occurring frequently in the region between Iceland and Faeroe Islands. A clear isolated baroclinic instability is identified around the intrusion, which is further found to be characterized by the transition from a spatially growing mode to a temporally growing mode. We also check the consistency of the MS-EVA dynamics with the barotropic Kuo model. An observation is that a local perturbation burst does not necessarily imply an instability: the perturbation energy could be transported from other processes occurring elsewhere. We find that our analysis yields a Kuo theorem-consistent mean-eddy interaction, which is not seen in a conventional Reynolds stress framework. Using the techniques of marginalization and localization, this work sets up an example for the generalization of certain geophysical fluid dynamics theories for more generic purposes.

The Relationship between ADHD Symptoms, Mood Instability, and Self-Reported Offending

ERIC Educational Resources Information Center

Gudjonsson, Gisli H.; Sigurdsson, Jon Fridrik; Adalsteinsson, Tomas F.; Young, Susan

2013-01-01

Objective: To investigate the relative importance of ADHD symptoms, mood instability, and antisocial personality disorder traits in predicting self-reported offending. Method: A total of 295 Icelandic students completed two scales of offending behavior and measures of ADHD symptoms, mood instability, and antisocial personality traits. Results:…

Health Coverage Instability for Mothers in Working Families

ERIC Educational Resources Information Center

Anderson, Steven G.; Eamon, Mary Keegan

2004-01-01

Using data from the National Longitudinal Survey of Youth, the authors examined the health insurance coverage stability of 1,667 women in working families over a three-year period (1995-1997). Findings revealed that coverage instability is common. Nearly one-half of low-income women experienced health coverage instability over the three-year study…

Perceived Instability in Emerging Adulthood: The Protective Role of Identity Capital

ERIC Educational Resources Information Center

Luyckx, Koen; De Witte, Hans; Goossens, Luc

2011-01-01

Developmental instability characterizes the late teens and twenties in today's late-modern society. The present study (a) focused on the link between instability and both general (i.e., self-esteem and depressive symptoms) and work-related outcomes (i.e., work engagement and burnout) and (b) investigated the possible moderating role of identity…

Diffusive instabilities in a hyperbolic activator-inhibitor system with superdiffusion

NASA Astrophysics Data System (ADS)

Mvogo, Alain; Macías-Díaz, Jorge E.; Kofané, Timoléon Crépin

2018-03-01

We investigate analytically and numerically the conditions for wave instabilities in a hyperbolic activator-inhibitor system with species undergoing anomalous superdiffusion. In the present work, anomalous superdiffusion is modeled using the two-dimensional Weyl fractional operator, with derivative orders α ∈ [1,2]. We perform a linear stability analysis and derive the conditions for diffusion-driven wave instabilities. Emphasis is placed on the effect of the superdiffusion exponent α , the diffusion ratio d , and the inertial time τ . As the superdiffusive exponent increases, so does the wave number of the Turing instability. Opposite to the requirement for Turing instability, the activator needs to diffuse sufficiently faster than the inhibitor in order for the wave instability to occur. The critical wave number for wave instability decreases with the superdiffusive exponent and increases with the inertial time. The maximum value of the inertial time for a wave instability to occur in the system is τmax=3.6 . As one of the main results of this work, we conclude that both anomalous diffusion and inertial time influence strongly the conditions for wave instabilities in hyperbolic fractional reaction-diffusion systems. Some numerical simulations are conducted as evidence of the analytical predictions derived in this work.

Investigation on thermo-acoustic instability dynamic characteristics of hydrocarbon fuel flowing in scramjet cooling channel based on wavelet entropy method

NASA Astrophysics Data System (ADS)

Zan, Hao; Li, Haowei; Jiang, Yuguang; Wu, Meng; Zhou, Weixing; Bao, Wen

2018-06-01

As part of our efforts to find ways and means to further improve the regenerative cooling technology in scramjet, the experiments of thermo-acoustic instability dynamic characteristics of hydrocarbon fuel flowing have been conducted in horizontal circular tubes at different conditions. The experimental results indicate that there is a developing process from thermo-acoustic stability to instability. In order to have a deep understanding on the developing process of thermo-acoustic instability, the method of Multi-scale Shannon Wavelet Entropy (MSWE) based on Wavelet Transform Correlation Filter (WTCF) and Multi-Scale Shannon Entropy (MSE) is adopted in this paper. The results demonstrate that the developing process of thermo-acoustic instability from noise and weak signals is well detected by MSWE method and the differences among the stability, the developing process and the instability can be identified. These properties render the method particularly powerful for warning thermo-acoustic instability of hydrocarbon fuel flowing in scramjet cooling channels. The mass flow rate and the inlet pressure will make an influence on the developing process of the thermo-acoustic instability. The investigation on thermo-acoustic instability dynamic characteristics at supercritical pressure based on wavelet entropy method offers guidance on the control of scramjet fuel supply, which can secure stable fuel flowing in regenerative cooling system.

Characteristics of aeroelastic instabilities in turbomachinery - NASA full scale engine test results

NASA Technical Reports Server (NTRS)

Lubomski, J. F.

1979-01-01

Several aeromechanical programs were conducted in the NASA/USAF Joint Engine System Research Programs. The scope of these programs, the instrumentation, data acquisition and reduction, and the test results are discussed. Data pertinent to four different instabilities were acquired; two types of stall flutter, choke flutter and a system mode instability. The data indicates that each instability has its own unique characteristics. These characteristics are described.

On the tertiary instability formalism of zonal flows in magnetized plasmas

NASA Astrophysics Data System (ADS)

Rath, F.; Peeters, A. G.; Buchholz, R.; Grosshauser, S. R.; Seiferling, F.; Weikl, A.

2018-05-01

This paper investigates the so-called tertiary instabilities driven by the zonal flow in gyro-kinetic tokamak core turbulence. The Kelvin Helmholtz instability is first considered within a 2D fluid model and a threshold in the zonal flow wave vector kZF>kZF,c for instability is found. This critical scale is related to the breaking of the rotational symmetry by flux-surfaces, which is incorporated into the modified adiabatic electron response. The stability of undamped Rosenbluth-Hinton zonal flows is then investigated in gyro-kinetic simulations. Absolute instability, in the sense that the threshold zonal flow amplitude tends towards zero, is found above a zonal flow wave vector kZF,cρi≈1.3 ( ρi is the ion thermal Larmor radius), which is comparable to the 2D fluid results. Large scale zonal flows with kZF

Generation of mesoscale magnetic fields and the dynamics of Cosmic Ray acceleration

NASA Astrophysics Data System (ADS)

Diamond, P. H.; Malkov, M. A.

The problem of the cosmic ray origin is discussed in connection with their acceleration in supernova remnant shocks. The diffusive shock acceleration mechanism is reviewed and its potential to accelerate particles to the maximum energy of (presumably) galactic cosmic rays (1018eV ) is considered. It is argued that to reach such energies, a strong magnetic field at scales larger than the particle gyroradius must be created as a result of the acceleration process, itself. One specific mechanism suggested here is based on the generation of Alfven wave at the gyroradius scale with a subsequent transfer to longer scales via interaction with strong acoustic turbulence in the shock precursor. The acoustic turbulence in turn, may be generated by Drury instability or by parametric instability of the Alfven waves. The generation mechanism is modulational instability of CR generated Alfven wave packets induced, in turn, by scattering off acoustic fluctuations in the shock precursor which are generated by Drury instability.

A Numerical Model for Two-Plasmon-Decay Hot-Electron Production and Mitigation in Direct-Drive Implosions

NASA Astrophysics Data System (ADS)

Myatt, J. F.; Shaw, J. G.; Solodov, A. A.; Maximov, A. V.; Short, R. W.; Seka, W.; Follett, R. K.; Edgell, D. H.; Froula, D. H.; Goncharov, V. N.

2015-11-01

Hot-electron preheat, caused by laser-plasma instabilities, can impair the performance of inertial confinement fusion implosions. It is therefore imperative to understand processes that can generate hot electrons and to design mitigation strategies should preheat be found to be excessive at the ignition scale (laser-plasma interactions do not follow hydrodynamic scaling). For this purpose, a new 3-D model [laser-plasma simulation environment (LPSE)] has been constructed that computes hot-electron generation in direct-drive plasmas based on the assumption that two-plasmon decay is the dominant, hot-electron-producing instability. It uses an established model of TPD-driven turbulence together with a new GPU based hybrid particle method of hot-electron production. The time-dependent hot-electron power, total energy, and energy spectrum are computed and compared with data from recent OMEGA implosion experiments that have sought to mitigate TPD by the use of multilayered (mid- Z) ablators. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.

On the abrupt growth dynamics of nonlinear resistive tearing mode and the viscosity effects

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

Ali, A.; Li, Jiquan, E-mail: lijq@energy.kyoto-u.ac.jp; Kishimoto, Y.

2014-05-15

The nonlinear evolution of the resistive tearing mode exhibits an abrupt growth after an X-point collapse once the magnetic island exceeds a certain critical width Δ′w{sub c} for large instability parameter Δ′, leading to a current sheet formation [N. F. Loureiro et al., Phys. Rev. Lett. 95, 235003 (2005)]. In this work, we investigate the underlying mechanism of the X-point collapse as well as the current sheet formation including the viscosity effects, based on a secondary instability analysis. The secondary instability is excited due to the quasilinear current modification by the zonal current. In particular, it is identified that themore » current peaking effect is plausibly responsible for the onset of the X-point collapse and the current sheet formation, leading to the explosive growth of reconnected flux. In the presence of finite viscosity, the Δ′w{sub c} scaling with the resistivity gets modified. A transition behavior is revealed at P{sub r}≈1 for the viscosity dependence of Δ′w{sub c} and the linear tearing instability. However, the explosive growth seems to be independent of the viscosity in the magnetic Prandtl number P{sub r}<1 regime, while large viscosity plays a strong dissipation role.« less

Relativistic electromagnetic ion cyclotron instabilities

NASA Astrophysics Data System (ADS)

Chen, K. R.; Huang, R. D.; Wang, J. C.; Chen, Y. Y.

2005-03-01

The relativistic instabilities of electromagnetic ion cyclotron waves driven by MeV ions are analytically and numerically studied. As caused by wave magnetic field and in sharp contrast to the electrostatic case, interesting characteristics such as Alfvénic behavior and instability transition are discovered and illuminated in detail. The instabilities are reactive and are raised from the coupling of slow ions’ first-order resonance and fast ions’ second-order resonance, that is an essential extra mechanism due to relativistic effect. Because of the wave magnetic field, the nonresonant plasma dielectric is usually negative and large, that affects the instability conditions and scaling laws. A negative harmonic cyclotron frequency mismatch between the fast and slow ions is required for driving a cubic (and a coupled quadratic) instability; the cubic (square) root scaling of the peak growth rate makes the relativistic effect more important than classical mechanism, especially for low fast ion density and Lorentz factor being close to unity. For the cubic instability, there is a threshold (ceiling) on the slow ion temperature and density (the external magnetic field and the fast ion energy); the Alfvén velocity is required to be low. This Alfvénic behavior is interesting in physics and important for its applications. The case of fast protons in thermal deuterons is numerically studied and compared with the analytical results. When the slow ion temperature or density (the external magnetic field or the fast ion energy) is increased (reduced) to about twice (half) the threshold (ceiling), the same growth rate peak transits from the cubic instability to the coupled quadratic instability and a different cubic instability branch appears. The instability transition is an interesting new phenomenon for instability.

Tibiofemoral Instability After Primary Total Knee Arthroplasty: Posterior-Stabilized Implants for Obese Patients.

PubMed

Can, Ata; Erdogan, Fahri; Erdogan, Ayse Ovul

2017-09-01

Tibiofemoral instability is a common complication after total knee arthroplasty (TKA), accounting for up to 22% of all revision procedures. Instability is the second most common cause of revision in the first 5 years after primary TKA. In this study, 13 knees with tibiofemoral instability after TKA were identified among 693 consecutive primary TKA procedures. Patient demographics, body mass index, clinical symptoms, previous deformity, previous knee surgery, complications, interval between index TKA and first tibiofemoral instability, causes of instability, and interval between index TKA and revision TKA were retrospectively reviewed. Clinical outcomes were assessed with the Lysholm Knee Scoring Scale. All patients were women, and mean body mass index was 37.7 kg/m 2 (range, 27.2-52.6 kg/m 2 ). Mean interval between index TKA and first tibiofemoral instability was 23.4 months (range, 9-45 months), and mean interval between index TKA and revision TKA was 25.6 months (range, 14-48 months). All patients had posterior cruciate ligament-retaining implants. Of the 13 knees, 11 had flexion instability and 2 had global instability. In all patients, instability was caused by incompetence of the posterior cruciate ligament; additionally, 1 patient had undersized and malpositioned implants. In 4 knees, the polyethylene insert was broken as well. All patients underwent revision TKA. Lysholm Knee Scoring Scale score had improved from a mean of 35.8 (range, 30-46) to a mean of 68.3 (range, 66-76). All patients included in this study were female and obese. The main cause of instability was secondary posterior cruciate ligament rupture and incompetence. The use of posterior-stabilized implants for primary TKA may prevent secondary instability in obese patients. [Orthopedics. 2017; 40(5):e812-e819.]. Copyright 2017, SLACK Incorporated.

MHD thermal instabilities in cool inhomogeneous atmospheres

NASA Technical Reports Server (NTRS)

Bodo, G.; Ferrari, A.; Massaglia, S.; Rosner, R.

1983-01-01

The formation of a coronal state in a stellar atmosphere is investigated. A numerical code is used to study the effects of atmospheric gradients and finite loop dimension on the scale of unstable perturbations, solving for oscillatory perturbations as eigenfunctions of a boundary value problem. The atmosphere is considered as initially isothermal, with density and pressure having scale heights fixed by the hydrostatic equations. Joule mode instability is found to be an efficient mechanism for current filamentation and subsequent heating in initially cool atmospheres. This instability is mainly effective at the top of magnetic loops and is not suppressed by thermal conduction.

Perception of Job Instability in Europe

ERIC Educational Resources Information Center

Bockerman, Petri

2004-01-01

The perception of job instability is an important measure of subjective wellbeing of individuals, because most people derive their income from selling their labour services. The study explores the determination of perception of job instability in Europe. The study is based on a large-scale survey from the year 1998. There are evidently large…

Applications of Analytical Self-Similar Solutions of Reynolds-Averaged Models for Instability-Induced Turbulent Mixing

NASA Astrophysics Data System (ADS)

Hartland, Tucker; Schilling, Oleg

2017-11-01

Analytical self-similar solutions to several families of single- and two-scale, eddy viscosity and Reynolds stress turbulence models are presented for Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instability-induced turbulent mixing. The use of algebraic relationships between model coefficients and physical observables (e.g., experimental growth rates) following from the self-similar solutions to calibrate a member of a given family of turbulence models is shown. It is demonstrated numerically that the algebraic relations accurately predict the value and variation of physical outputs of a Reynolds-averaged simulation in flow regimes that are consistent with the simplifying assumptions used to derive the solutions. The use of experimental and numerical simulation data on Reynolds stress anisotropy ratios to calibrate a Reynolds stress model is briefly illustrated. The implications of the analytical solutions for future Reynolds-averaged modeling of hydrodynamic instability-induced mixing are briefly discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Probes, Moons, and Kinetic Plasma Wakes

NASA Astrophysics Data System (ADS)

Hutchinson, I. H.; Malaspina, D.; Zhou, C.

2017-10-01

Nonmagnetic objects as varied as probes in tokamaks or moons in space give rise to flowing plasma wakes in which strong distortions of the ion and electron velocity distributions cause electrostatic instabilities. Non-linear phenomena such as electron holes are then produced. Historic probe theory largely ignores the resulting unstable character of the wake, but since we can now simulate computationally the non-linear wake phenomena, a timely challenge is to reassess the influence of these instabilities both on probe measurements and on the wakes themselves. Because the electron instability wavelengths are very short (typically a few Debye-lengths), controlled laboratory experiments face serious challenges in diagnosing them. That is one reason why they have long been neglected as an influence in probe interpretation. Space-craft plasma observations, by contrast, easily obtain sub-Debye-length resolution, but have difficulty with larger-scale reconstruction of the plasma spatial variation. In addition to surveying our developing understanding of wakes in magnetized plasmas, ongoing analysis of Artemis data concerning electron holes observed in the solar-wind lunar wake will be featured. Work partially supported by NASA Grant NNX16AG82G.

Cold-Flow Study of Low Frequency Pressure Instability in Hybrid Rocket Motors

NASA Technical Reports Server (NTRS)

Jenkins, Rhonald M.

1997-01-01

Past experience with hybrid rockets has shown that certain motor operating conditions are conducive to the formation of low frequency pressure oscillations, or flow instabilities, within the motor. Both past and present work in the hybrid propulsion community acknowledges deficiencies in the understanding of such behavior, though it seems probable that the answer lies in an interaction between the flow dynamics and the combustion heat release. Knowledge of the fundamental flow dynamics is essential to the basic understanding of the overall stability problem. A first step in this direction was a study conducted at NASA Marshall Space Flight Center (MSFC), centered around a laboratory-scale two dimensional water flow model of a hybrid rocket motor. Principal objectives included: (1) visualization of flow and measurement of flow velocity distributions: (2) assessment of the importance of shear layer instabilities in driving motor pressure oscillations; (3) determination of the interactions between flow induced shear layers with the mainstream flow, the secondary (wall) throughflow, and solid boundaries; (4) investigation of the interactions between wall flow oscillations and the mainstream flow pressure distribution.

Paleoclimate: A fresh look at glacial floods

USGS Publications Warehouse

Colman, S. M.

2002-01-01

Over the last 20 years, it has become clear that ice ages are characterized by glacial as well as climatic instability on millennial time scales. In his Perspective, Colman highlights two recent papers investigating the role of glacial meltwater and continental drainage in this instability. The results suggest a fundamental instability feedback between ocean circulation and ice sheet dynamics and provides an explanation for why instability was greatest at times of intermediate ice volume.

A fresh look at glacial foods

USGS Publications Warehouse

Colman, Steven M.

2002-01-01

Over the last 20 years, it has become clear that ice ages are characterized by glacial as well as climatic instability on millennial time scales. In his Perspective, Colman highlights two recent papers investigating the role of glacial meltwater and continental drainage in this instability. The results suggest a fundamental instability feedback between ocean circulation and ice sheet dynamics and provides an explanation for why instability was greatest at times of intermediate ice volume.

Long-wave instabilities of two interlaced helical vortices

NASA Astrophysics Data System (ADS)

Quaranta, H. U.; Brynjell-Rahkola, M.; Leweke, T.; Henningson, D. S.

2016-09-01

We present a comparison between experimental observations and theoretical predictions concerning long-wave displacement instabilities of the helical vortices in the wake of a two-bladed rotor. Experiments are performed with a small-scale rotor in a water channel, using a set-up that allows the individual triggering of various instability modes at different azimuthal wave numbers, leading to local or global pairing of successive vortex loops. The initial development of the instability and the measured growth rates are in good agreement with the predictions from linear stability theory, based on an approach where the helical vortex system is represented by filaments. At later times, local pairing develops into large-scale distortions of the vortices, whereas for global pairing the non-linear evolution returns the system almost to its initial geometry.

An analytic model for limiting high density LH transition by the onset of the tertiary instability

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

Singh, Raghvendra, E-mail: rsingh129@gmail.com; Jhang, Hogun; Kaang, Helen H.

2016-07-15

We perform an analytic study of the tertiary instability driven by a strong excitation of zonal flows during high density low to high (LH) mode transition. The drift resistive ballooning mode is assumed to be a dominant edge turbulence driver. The analysis reproduces main qualitative features of early computational results [Rogers and Drake, Phys. Rev. Lett. 81, 4396 (1998); Guzdar et al., Phys. Plasmas 14, 020701 (2007)], as well as new characteristics of the maximum edge density due to the onset of the tertiary instability. An analytical scaling indicates that the density scaling of LH transition power may be determinedmore » by the onset condition of the tertiary instability when the operating density approaches to the Greenwald density.« less

Charge-Induced Saffman-Taylor Instabilities in Toroidal Droplets

NASA Astrophysics Data System (ADS)

Fragkopoulos, A. A.; Aizenman, A.; Fernández-Nieves, A.

2017-06-01

We show that charged toroidal droplets can develop fingerlike structures as they expand due to Saffman-Taylor instabilities. While these are commonly observed in quasi-two-dimensional geometries when a fluid displaces another fluid of higher viscosity, we show that the toroidal confinement breaks the symmetry of the problem, effectively making it quasi-two-dimensional and enabling the instability to develop in this three-dimensional situation. We control the expansion speed of the torus with the imposed electric stress and show that fingers are observed provided the characteristic time scale associated with this instability is smaller than the characteristic time scale associated with Rayleigh-Plateau break-up. We confirm our interpretation of the results by showing that the number of fingers is consistent with expectations from linear stability analysis in radial Hele-Shaw cells.

Streak instability as an initiating mechanism of the large-scale motions in a turbulent channel flow

NASA Astrophysics Data System (ADS)

de Giovanetti, Matteo; Sung, Hyung Jin; Hwang, Yongyun

2016-11-01

The large-scale motions (or bulges) have often been believed to be formed via merge and/or growth of the near-wall hairpin vortical structures. Here, we report our observation that they can be directly generated by an instability of the amplified streaky motions in the outer region (i.e. very-large-scale motions) through the self-sustaining process. We design a LES-based numerical experiment in turbulent channel flow for Reτ = 2000 where a body forcing is implemented to artificially drive an infinitely long streaky motion in the outer layer. As the forcing amplitude is increased, it is found that a new energetic structure emerges at λx 3 4 h of the streamwise length (h is the half height of channel) particularly in the wall-normal and spanwise velocities. A careful statistical examination reveals that this structure is likely to be linked with the sinuous-mode streak instability of the amplified streak, consistent with previous theoretical studies. Application of dynamic mode decomposition to this instability further shows that the phase speed of this structure scales with the outer velocity and it is initiated around the critical layer of the streaky flow.

Contact Line Instability Caused by Air Rim Formation under Nonsplashing Droplets.

PubMed

Pack, Min; Kaneelil, Paul; Kim, Hyoungsoo; Sun, Ying

2018-05-01

Drop impact is fundamental to various natural and industrial processes such as rain-induced soil erosion and spray-coating technologies. The recent discovery of the role of air entrainment between the droplet and the impacting surface has produced numerous works, uncovering the unique physics that correlates the air film dynamics with the drop impact outcomes. In this study, we focus on the post-failure air entrainment dynamics for We numbers well below the splash threshold under different ambient pressures and elucidate the interfacial instabilities formed by air entrainment at the wetting front of impacting droplets on perfectly smooth, viscous films of constant thickness. A high-speed total internal reflection microscopy technique accounting for the Fresnel reflection at the drop-air interface allows for in situ measurements of an entrained air rim at the wetting front. The presence of an air rim is found to be a prerequisite to the interfacial instability which is formed when the capillary pressure in the vicinity of the contact line can no longer balance the increasing gas pressure near the wetting front. A critical capillary number for the air rim formation is experimentally identified above which the wetting front becomes unstable where this critical capillary number inversely scales with the ambient pressure. The contact line instabilities at relatively low We numbers ( We ∼ O(10)) observed in this study provide insight into the conventional understanding of hydrodynamic instabilities under drop impact which usually require We ≫ 10.

Electron acceleration via magnetic island coalescence

NASA Astrophysics Data System (ADS)

Shinohara, I.; Yumura, T.; Tanaka, K. G.; Fujimoto, M.

2009-06-01

Electron acceleration via fast magnetic island coalescence that happens as quick magnetic reconnection triggering (QMRT) proceeds has been studied. We have carried out a three-dimensional full kinetic simulation of the Harris current sheet with a large enough simulation run for two magnetic islands coalescence. Due to the strong inductive electric field associated with the non-linear evolution of the lower-hybrid-drift instability and the magnetic island coalescence process observed in the non-linear stage of the collisionless tearing mode, electrons are significantly accelerated at around the neutral sheet and the subsequent X-line. The accelerated meandering electrons generated by the non-linear evolution of the lower-hybrid-drift instability are resulted in QMRT, and QMRT leads to fast magnetic island coalescence. As a whole, the reconnection triggering and its transition to large-scale structure work as an effective electron accelerator.

Studies on Equatorial Shock Formation During Plasmaspheric Refilling

NASA Technical Reports Server (NTRS)

Singh, Nagendra

1995-01-01

During the grant period from August 1, 1994 to October 31, 1995 we have continued to investigate the effects of plasma wave instabilities on the early stage plasmaspheric refilling. Since ion beams are the primary feature of the interhemispheric plasma flows during the early stage refilling, ion-beam driven instabilities and associated waves are of primary interest. The major findings of this research are briefly summarized here. After a systematic examination of the relevant plasma instabilities, we realized that when the interhemispheric plasma flows begin to interpenetrate at the equator, the most relevant plasma instability is the electrostatic ion cyclotron wave instability. Only at later stages the ion-acoustic instability may be affecting the plasma flow. An interesting property of the electrostatic ion cyclotron wave is that it heats ions perpendicular to the magnetic field. When the ions in the field-aligned flows are transversely heated, they are trapped in the magnetic flux tube, thus affecting the refilling process. The eic wave instability is a microprocess with scale length of the order of ion Larmor radius and the corresponding time scale is the ion cyclotron period. We have attempted to tackle the problem for the plasmaspheric refilling by incorporating the effects of eic wave instability on the mesoscale plasma flow when the properties of the latter exceeds the critical conditions for the former. We have compared the results on refilling from the model with and without the eic instability effects.

Farley-Buneman Instability in the Solar Chromosphere

NASA Astrophysics Data System (ADS)

Gogoberidze, G.; Voitenko, Y.; Poedts, S.; Goossens, M.

2009-11-01

The Farley-Buneman instability (FBI) is studied in the partially ionized plasma of the solar chromosphere taking into account the finite magnetization of the ions and Coulomb collisions. We obtain the threshold value for the relative velocity between ions and electrons necessary for the instability to develop. It is shown that Coulomb collisions play a destabilizing role in the sense that they enable the instability even in the regions where the ion magnetization is larger than unity. By applying these results to chromospheric conditions, we show that the FBI cannot be responsible for the quasi-steady heating of the solar chromosphere. However, we do not exclude the instability development locally in the presence of strong cross-field currents and/or strong small-scale magnetic fields. In such cases, FBI should produce locally small-scale, ~0.1-3 m, density irregularities in the solar chromosphere. These irregularities can cause scintillations of radio waves with similar wave lengths and provide a tool for remote chromospheric sensing.

FARLEY-BUNEMAN INSTABILITY IN THE SOLAR CHROMOSPHERE

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

Gogoberidze, G.; Voitenko, Y.; Poedts, S.

2009-11-20

The Farley-Buneman instability (FBI) is studied in the partially ionized plasma of the solar chromosphere taking into account the finite magnetization of the ions and Coulomb collisions. We obtain the threshold value for the relative velocity between ions and electrons necessary for the instability to develop. It is shown that Coulomb collisions play a destabilizing role in the sense that they enable the instability even in the regions where the ion magnetization is larger than unity. By applying these results to chromospheric conditions, we show that the FBI cannot be responsible for the quasi-steady heating of the solar chromosphere. However,more » we do not exclude the instability development locally in the presence of strong cross-field currents and/or strong small-scale magnetic fields. In such cases, FBI should produce locally small-scale, approx0.1-3 m, density irregularities in the solar chromosphere. These irregularities can cause scintillations of radio waves with similar wave lengths and provide a tool for remote chromospheric sensing.« less

Radiation from Relativistic Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Mizuno, Y.; Hardee, P.; Sol, H.; Medvedev, M.; Zhang, B.; Nordlund, A.; Frederiksen, J. T.; Fishman, G. J.; Preece, R.

2008-01-01

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the presence of relativistic jets, instabilities such as the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability create collisionless shocks, which are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The 'jitter' radiation from deflected electrons in small-scale magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation, a case of diffusive synchrotron radiation, may be important to understand the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Status of NASA full-scale engine aeroelasticity research

NASA Technical Reports Server (NTRS)

Lubomski, J. F.

1980-01-01

Data relevant to several types of aeroelastic instabilities were obtained using several types of turbojet and turbofan engines. In particular, data relative to separated flow (stall) flutter, choke flutter, and system mode instabilities are presented. The unique characteristics of these instabilities are discussed, and a number of correlations are presented that help identify the nature of the phenomena.

Active flow control for a blunt trailing edge profiled body

NASA Astrophysics Data System (ADS)

Naghib Lahouti, Arash

Flow in the wake of nominally two-dimensional bluff bodies is dominated by vortex shedding, beyond a very small threshold Reynolds number. Vortex shedding poses challenges in the design of structures, due to its adverse effects such as cyclic aerodynamic loads and fatigue. The wake vortices are often accompanied by large- and small-scale secondary instabilities, which manifest as dislocations in the primary wake vortices, and/or pairs of counter-rotating streamwise vortices, depending on the dominant instability mode(s), which in turn depends on the profile geometry and Reynolds number. The secondary instabilities interact with the wake vortices through several mechanisms. Therefore, manipulation of the secondary instabilities can be used as a means to alter the wake vortices, in order to reduce their adverse effects. In the present study, flow in the wake of a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, has been studied at Reynolds numbers ranging from Re(d) = 500 to 2150 where d is thickness of the body, to identify the secondary instabilities. Various tools, including numerical simulations, Laser Induced Fluorescence (LIF), and Particle Image Velocimetry (PIV) have been used for this study. Proper Orthogonal Decomposition (POD) has been applied to analyze the velocity field data. The results indicate the existence of small-scale instabilities with a spanwise wavelength of 2.0d to 2.5d in the near wake. The mechanism of the instability is similar to the Mode-A instability of a circular cylinder; however, it displays features that are specific to the blunt trailing edge profiled body. An active three-dimensional flow control mechanism based on the small-scale instabilities has been designed and evaluated. The mechanism comprises a series of trailing edge injection ports, with a spanwise spacing equal to the wavelength of the small-scale instabilities. Following preliminary evaluation of the control mechanism through numerical simulations, and experimental study of the effect of injection flow rate, extensive PIV experiments have been conducted to investigate the effectiveness of the flow control mechanism, and its effects on the wake flow structure, at Reynolds numbers ranging from Re(d ) = 700 to 1980. Measurements have been carried out at multiple spanwise locations, to establish a comprehensive image of the effect of the flow control mechanism on parameters such as drag force, wake width, and formation length. POD analysis and frequency spectrums are used to describe the process by which the mechanism affects the wake parameters and drag force. The results indicate that the flow control mechanism is able to reduce drag force by 10%. It is also shown that the best effectiveness in terms of suppression of the drag component resulting from velocity fluctuations is achieved when the flow control actuation wavelength closely matches the wavelength of the small-scale instabilities. KEYWORDS: Blunt Trailing Edge Profiled Body, Vortex Shedding, Wake Instability, Streamwise Vortex, Flow Control, Drag Reduction, Particle Image Velocimetry (PIV), Laser Induced Fluorescence (LIF), Flow Visualization, Numerical Simulation

Characterization of beam-driven instabilities and current redistribution in MST plasmas

NASA Astrophysics Data System (ADS)

Parke, E.

2015-11-01

A unique, high-rep-rate (>10 kHz) Thomson scattering diagnostic and a high-bandwidth FIR interferometer-polarimeter on MST have enabled characterization of beam-driven instabilities and magnetic equilibrium changes observed during high power (1 MW) neutral beam injection (NBI). While NBI leads to negligible net current drive, an increase in on-axis current density observed through Faraday rotation is offset by a reduction in mid-radius current. Identification of the phase flip in temperature fluctuations associated with tearing modes provides a sensitive measure of rational surface locations. This technique strongly constrains the safety factor for equilibrium reconstruction and provides a powerful new tool for measuring the equilibrium magnetic field. For example, the n = 6 temperature structure is observed to shift inward 1.1 +/- 0.6 cm, with an estimated reduction of q0 by 5%. This is consistent with a mid-radius reduction in current, and together the Faraday rotation and Thomson scattering measurements corroborate an inductive redistribution of current that compares well with TRANSP/MSTFit predictions. Interpreting tearing mode temperature structures in the RFP remains challenging; the effects of multiple, closely-spaced tearing modes on the mode phase measurement require further verification. In addition to equilibrium changes, previous work has shown that the large fast ion population drives instabilities at higher frequencies near the Alfvén continuum. Recent observations reveal a new instability at much lower frequency (~7 kHz) with strongly chirping behavior. It participates in extensive avalanches of the higher frequency energetic particle and Alfvénic modes to drive enhanced fast ion transport. Internal structures measured from Te and ne fluctuations, their dependence on the safety factor, as well as frequency scaling motivate speculation about mode identity. Work supported by U.S. DOE.

Papaloizou-Pringle instability suppression by the magnetorotational instability in relativistic accretion discs

NASA Astrophysics Data System (ADS)

Bugli, M.; Guilet, J.; Müller, E.; Del Zanna, L.; Bucciantini, N.; Montero, P. J.

2018-03-01

Geometrically thick tori with constant specific angular momentum have been widely used in the last decades to construct numerical models of accretion flows on to black holes. Such discs are prone to a global non-axisymmetric hydrodynamic instability, known as Papaloizou-Pringle instability (PPI), which can redistribute angular momentum and also lead to an emission of gravitational waves. It is, however, not clear yet how the development of the PPI is affected by the presence of a magnetic field and by the concurrent development of the magnetorotational instability (MRI). We present a numerical analysis using three-dimensional GRMHD simulations of the interplay between the PPI and the MRI considering, for the first time, an analytical magnetized equilibrium solution as initial condition. In the purely hydrodynamic case, the PPI selects as expected the large-scale m = 1 azimuthal mode as the fastest growing and non-linearly dominant mode. However, when the torus is threaded by a weak toroidal magnetic field, the development of the MRI leads to the suppression of large-scale modes and redistributes power across smaller scales. If the system starts with a significantly excited m = 1 mode, the PPI can be dominant in a transient phase, before being ultimately quenched by the MRI. Such dynamics may well be important in compact star mergers and tidal disruption events.

Direct Numerical Simulations of Small-Scale Gravity Wave Instability Dynamics in Variable Stratification and Shear

NASA Astrophysics Data System (ADS)

Mixa, T.; Fritts, D. C.; Laughman, B.; Wang, L.; Kantha, L. H.

2015-12-01

Multiple observations provide compelling evidence that gravity wave dissipation events often occur in multi-scale environments having highly-structured wind and stability profiles extending from the stable boundary layer into the mesosphere and lower thermosphere. Such events tend to be highly localized and thus yield local energy and momentum deposition and efficient secondary gravity wave generation expected to have strong influences at higher altitudes [e.g., Fritts et al., 2013; Baumgarten and Fritts, 2014]. Lidars, radars, and airglow imagers typically cannot achieve the spatial resolution needed to fully quantify these small-scale instability dynamics. Hence, we employ high-resolution modeling to explore these dynamics in representative environments. Specifically, we describe numerical studies of gravity wave packets impinging on a sheet of high stratification and shear and the resulting instabilities and impacts on the gravity wave amplitude and momentum flux for various flow and gravity wave parameters. References: Baumgarten, Gerd, and David C. Fritts (2014). Quantifying Kelvin-Helmholtz instability dynamics observed in noctilucent clouds: 1. Methods and observations. Journal of Geophysical Research: Atmospheres, 119.15, 9324-9337. Fritts, D. C., Wang, L., & Werne, J. A. (2013). Gravity wave-fine structure interactions. Part I: Influences of fine structure form and orientation on flow evolution and instability. Journal of the Atmospheric Sciences, 70(12), 3710-3734.

Validation of the German version of the Nurse-Work Instability Scale: baseline survey findings of a prospective study of a cohort of geriatric care workers

PubMed Central

2013-01-01

Background A prospective study of a cohort of nursing staff from nursing homes was undertaken to validate the Nurse-Work Instability Scale (Nurse-WIS). Baseline investigation data was used to test reliability, construct validity and criterion validity. Method A survey of nursing staff from nursing homes was conducted using a questionnaire containing the Nurse-WIS along with other survey instruments (including SF-12, WAI, SPE). The self-reported number of days’ sick leave taken and if a pension for reduced work capacity was drawn were recorded. The reliability of the scale was checked by item difficulty (P), item discrimination (rjt) and by internal consistency according to Cronbach’s coefficient. The hypotheses for checking construct validity were tested on the basis of correlations. Pearson’s chi-square was used to test concurrent criterion validity; discriminant validity was tested by means of binary logistic regression. Results 396 persons answered the questionnaire (21.3% response rate). More than 80% were female and mostly work full-time in a rotating shift pattern. Following the test for item discrimination, two items were removed from the Nurse-WIS test. According to Cronbach’s (0.927) the scale provides a high degree of measuring accuracy. All hypotheses and assumptions used to test validity were confirmed: As the Nurse-WIS risk increases, health-related quality of life, work ability and job satisfaction decline. Depressive symptoms and a poor subjective prognosis of earning capacity are also more frequent. Musculoskeletal disorders and impairments of psychological well-being are more frequent. Age also influences the Nurse-WIS result. While 12.0% of those below the age of 35 had an increased risk, the figure for those aged over 55 was 50%. Conclusion This study is the first validation study of the Nurse-WIS to date. The Nurse-WIS shows good reliability, good validity and a good level of measuring accuracy. It appears to be suitable for recording prevention and rehabilitation needs among health care workers. If, in the follow-up, the Nurse-WIS likewise proves to be a reliable screening instrument with good predictive validity, it could ensure that suitable action is taken at an early stage, thereby helping to counteract early retirement and the anticipated shortage of health care workers. PMID:24330532

Effects of energetic particle phase space modifications by instabilities on integrated modeling

NASA Astrophysics Data System (ADS)

Podestà, M.; Gorelenkova, M.; Fredrickson, E. D.; Gorelenkov, N. N.; White, R. B.

2016-11-01

Tokamak plasmas can feature a large population of energetic particles (EP) from neutral beam injection or fusion reactions. In turn, energetic particles can drive instabilities, which affect the driving EP population leading to a distortion of the original EP distribution function and of quantities that depend on it. The latter include, for example, neutral beam (NB) current drive and plasma heating through EP thermalization. Those effects must be taken into account to enable reliable and quantitative simulations of discharges for present devices as well as predictions for future burning plasmas. Reduced models for EP transport are emerging as an effective tool for long time-scale integrated simulations of tokamak plasmas, possibly including the effects of instabilities on EP dynamics. Available models differ in how EP distribution properties are modified by instabilities, e.g. in terms of gradients in real or phase space. It is therefore crucial to assess to what extent different assumptions in the transport models affect predicted quantities such as EP profile, energy distribution, NB driven current and energy/momentum transfer to the thermal populations. A newly developed kick model, which includes modifications of the EP distribution by instabilities in both real and velocity space, is used in this work to investigate these issues. Coupled to TRANSP simulations, the kick model is used to analyze NB-heated NSTX and DIII-D discharges featuring unstable Alfvén eigenmodes (AEs). Results show that instabilities can strongly affect the EP distribution function, and modifications propagate to macroscopic quantities such as NB-driven current profile and NB power transferred to the thermal plasma species. Those important aspects are only qualitatively captured by simpler fast ion transport models that are based on radial diffusion of energetic ions only.

Effects of energetic particle phase space modifications by instabilities on integrated modeling

DOE PAGES

Podesta, M.; Gorelenkova, M.; Fredrickson, E. D.; ...

2016-07-22

Tokamak plasmas can feature a large population of energetic particles (EP) from neutral beam injection or fusion reactions. In turn, energetic particles can drive instabilities, which affect the driving EP population leading to a distortion of the original EP distribution function and of quantities that depend on it. The latter include, for example, neutral beam (NB) current drive and plasma heating through EP thermalization. Those effects must be taken into account to enable reliable and quantitative simulations of discharges for present devices as well as predictions for future burning plasmas. Reduced models for EP transport are emerging as an effectivemore » tool for long time-scale integrated simulations of tokamak plasmas, possibly including the effects of instabilities on EP dynamics. Available models differ in how EP distribution properties are modified by instabilities, e.g. in terms of gradients in real or phase space. It is therefore crucial to assess to what extent different assumptions in the transport models affect predicted quantities such as EP profile, energy distribution, NB driven current and energy/momentum transfer to the thermal populations. A newly developed kick model, which includes modifications of the EP distribution by instabilities in both real and velocity space, is used in this work to investigate these issues. Coupled to TRANSP simulations, the kick model is used to analyze NB-heated NSTX and DIII-D discharges featuring unstable Alfvén eigenmodes (AEs). Results show that instabilities can strongly affect the EP distribution function, and modifications propagate to macroscopic quantities such as NB-driven current profile and NB power transferred to the thermal plasma species. Furthermore, those important aspects are only qualitatively captured by simpler fast ion transport models that are based on radial diffusion of energetic ions only.« less

Modeling Hohlraum-Based Laser Plasma Instability Experiments

NASA Astrophysics Data System (ADS)

Meezan, N. B.

2005-10-01

Laser fusion targets must control laser-plasma instabilities (LPI) in order to perform as designed. We present analyses of recent hohlraum LPI experiments from the Omega laser facility. The targets, gold hohlraums filled with gas or SiO2 foam, are preheated by several 3φ beams before an interaction beam (2φ or 3φ) is fired along the hohlraum axis. The experiments are simulated in 2-D and 3-D using the code hydra. The choice of electron thermal conduction model in hydra strongly affects the simulated plasma conditions. This work is part of a larger effort to systematically explore the usefulness of linear gain as a design tool for fusion targets. We find that the measured Raman and Brillouin backscatter scale monotonically with the peak linear gain calculated for the target; however, linear gain is not sufficient to explain all trends in the data. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

Full-Scale Direct Numerical Simulation of Two- and Three-Dimensional Instabilities and Rivulet Formulation in Heated Falling Films

NASA Technical Reports Server (NTRS)

Krishnamoorthy, S.; Ramaswamy, B.; Joo, S. W.

1995-01-01

A thin film draining on an inclined plate has been studied numerically using finite element method. Three-dimensional governing equations of continuity, momentum and energy with a moving boundary are integrated in an arbitrary Lagrangian Eulerian frame of reference. Kinematic equation is solved to precisely update interface location. Rivulet formation based on instability mechanism has been simulated using full-scale computation. Comparisons with long-wave theory are made to validate the numerical scheme. Detailed analysis of two- and three-dimensional nonlinear wave formation and spontaneous rupture forming rivulets under the influence of combined thermocapillary and surface-wave instabilities is performed.

Magnetic Shear Damped Polar Convective Fluid Instabilities

NASA Astrophysics Data System (ADS)

Atul, Jyoti K.; Singh, Rameswar; Sarkar, Sanjib; Kravchenko, Oleg V.; Singh, Sushil K.; Chattopadhyaya, Prabal K.; Kaw, Predhiman K.

2018-01-01

The influence of the magnetic field shear is studied on the E × B (and/or gravitational) and the Current Convective Instabilities (CCI) occurring in the high-latitude F layer ionosphere. It is shown that magnetic shear reduces the growth rate of these instabilities. The magnetic shear-induced stabilization is more effective at the larger-scale sizes (≥ tens of kilometers) while at the scintillation causing intermediate scale sizes (˜ a few kilometers), the growth rate remains largely unaffected. The eigenmode structure gets localized about a rational surface due to finite magnetic shear and has broken reflectional symmetry due to centroid shift of the mode by equilibrium parallel flow or current.

Turbulent transport regimes and the SOL heat flux width

NASA Astrophysics Data System (ADS)

Myra, J. R.; D'Ippolito, D. A.; Russell, D. A.

2014-10-01

Understanding the responsible mechanisms and resulting scaling of the scrape-off layer (SOL) heat flux width is important for predicting viable operating regimes in future tokamaks, and for seeking possible mitigation schemes. Simulation and theory results using reduced edge/SOL turbulence models have produced SOL widths and scalings in reasonable accord with experiments in many cases. In this work, we attempt to qualitatively and conceptually understand various regimes of edge/SOL turbulence and the role of turbulent transport in establishing the SOL heat flux width. Relevant considerations include the type and spectral characteristics of underlying instabilities, the location of the gradient drive relative to the SOL, the nonlinear saturation mechanism, and the parallel heat transport regime. Recent SOLT turbulence code results are employed to understand the roles of these considerations and to develop analytical scalings. We find a heat flux width scaling with major radius R that is generally positive, consistent with older results reviewed in. The possible relationship of turbulence mechanisms to the heuristic drift mechanism is considered, together with implications for future experiments. Work supported by US DOE grant DE-FG02-97ER54392.

Can a numerically stable subgrid-scale model for turbulent flow computation be ideally accurate?: a preliminary theoretical study for the Gaussian filtered Navier-Stokes equations.

PubMed

Ida, Masato; Taniguchi, Nobuyuki

2003-09-01

This paper introduces a candidate for the origin of the numerical instabilities in large eddy simulation repeatedly observed in academic and practical industrial flow computations. Without resorting to any subgrid-scale modeling, but based on a simple assumption regarding the streamwise component of flow velocity, it is shown theoretically that in a channel-flow computation, the application of the Gaussian filtering to the incompressible Navier-Stokes equations yields a numerically unstable term, a cross-derivative term, which is similar to one appearing in the Gaussian filtered Vlasov equation derived by Klimas [J. Comput. Phys. 68, 202 (1987)] and also to one derived recently by Kobayashi and Shimomura [Phys. Fluids 15, L29 (2003)] from the tensor-diffusivity subgrid-scale term in a dynamic mixed model. The present result predicts that not only the numerical methods and the subgrid-scale models employed but also only the applied filtering process can be a seed of this numerical instability. An investigation concerning the relationship between the turbulent energy scattering and the unstable term shows that the instability of the term does not necessarily represent the backscatter of kinetic energy which has been considered a possible origin of numerical instabilities in large eddy simulation. The present findings raise the question whether a numerically stable subgrid-scale model can be ideally accurate.

INTRODUCTION: Award of the 2004 Hannes Alfvén Prize of the European Physical Society to J W Connor, R J Hastie and J B Taylor

NASA Astrophysics Data System (ADS)

Lister, Jo, Dr

2004-12-01

Jack Connor, Jim Hastie and Bryan Taylor The Hannes Alfvén Prize of the European Physical Society for Outstanding Contributions to Plasma Physics (2004) has been awarded to Jack Connor, Jim Hastie and Bryan Taylor `for their seminal contributions to a wide range of issues of fundamental importance to the success of magnetic confinement fusion, including: the development of gyro-kinetic theory; the prediction of the bootstrap current; dimensionless scaling laws; pressure-limiting instabilities, and micro-stability and transport theory'. Jack Connor, Jim Hastie and Bryan Taylor form one of the most successful teams of theoretical physicists in the history of magnetic confinement fusion. They have made important contributions individually, but their greatest discoveries have mostly been accomplished jointly, either in pairs or as a team involving all three. Their early work, in the 1960s, included the development of the gyro-kinetic theory for fine-scale plasma instabilities, which today forms the basis of the most advanced turbulence simulation codes in tokamak and stellarator research. The theoretical prediction of the bootstrap current, made in 1970-71 was not confirmed experimentally for over a decade but is now regarded as crucial to the success of the tokamak as a steady-state fusion power source. Their work on collisional transport also included the prediction of impurity ion accumulation, which is observed in internal transport barriers and is a key concern for long-pulse tokamak operation. The relativistic threshold for runaway electrons, identified in 1975, forms the basis of the most recent tokamak disruption mitigation schemes. In the late 1970s, the team developed the theory for ballooning instabilities, which provided an important ingredient in the `Troyon-Sykes' β-limit—an expression that is still used as a guide to the performance of tokamaks and in the design of ITER. Ballooning mode theory has also contributed to the understanding of instabilities in space plasmas such as magnetospheres and the solar corona. Finally, coming right up to date, the ballooning mode is thought to be a key ingredient in edge-localized modes (ELMs), which are a main issue for ITER, and ballooning stability is an important feature of modern stellarators. In the late 1970s and through the 1980s, the concept of dimensionless scaling laws was introduced and developed (following work by Kadomtsev), enabling scalings for transport coefficients to be derived without tackling all the details of the plasma turbulence. The same ideas are still used today to provide various constraints on confinement scaling laws, for example, on which the ITER design is largely based. The linear theory of toroidal drift waves was also developed by the team during this period, and into the 1990s. Key results on the role of shear damping in toroidal geometry, the identification of modes with extended radial correlation lengths, and the role of flow shear in reducing these correlation lengths (and hence transport) were deduced. All of these are key ideas that are often components in theoretical models for tokamak confinement and the generation of transport barriers. This laudation can only address a small number of the areas in which this formidable team of theoretical plasma physicists have made great contributions to our understanding of magnetically confined plasmas. It is appropriate and timely that their contributions are recognized as they approach the end of their careers.

The Segmented Aperture Interferometric Nulling Testbed (SAINT) I: overview and air-side system description

NASA Astrophysics Data System (ADS)

Hicks, Brian A.; Lyon, Richard G.; Petrone, Peter; Ballard, Marlin; Bolcar, Matthew R.; Bolognese, Jeff; Clampin, Mark; Dogoda, Peter; Dworzanski, Daniel; Helmbrecht, Michael A.; Koca, Corina; Shiri, Ron

2016-07-01

This work presents an overview of the Segmented Aperture Interferometric Nulling Testbed (SAINT), a project that will pair an actively-controlled macro-scale segmented mirror with the Visible Nulling Coronagraph (VNC). SAINT will incorporate the VNC's demonstrated wavefront sensing and control system to refine and quantify end-to-end high-contrast starlight suppression performance. This pathfinder testbed will be used as a tool to study and refine approaches to mitigating instabilities and complex diffraction expected from future large segmented aperture telescopes.

Effects of laser-plasma instabilities on hydro evolution in an OMEGA-EP long-scale-length experiment

DOE PAGES

Li, J.; Hu, S. X.; Ren, C.

2017-02-28

Laser-plasma instabilities and hydro evolution of the coronal plasma in an OMEGA EP long-scale-length experiment with planar targets were studied with particle-in-cell (PIC) and hydrodynamic simulations. Plasma and laser conditions were first obtained in a two-dimensional DRACO hydro simulation with only inverse-bremsstrahlung absorption. Using these conditions, an OSIRIS PIC simulation was performed to study laser absorption and hot-electron generation caused by laser-plasma instabilities (LPIs) near the quarter-critical region. The obtained PIC information was subsequently coupled to another DRACO simulation to examine how the LPIs affect the overall hydrodynamics. Lastly, the results showed that the LPI-induced laser absorption increased the electronmore » temperature but did not significantly change the density scale length in the corona.« less

Effects of laser-plasma instabilities on hydro evolution in an OMEGA-EP long-scale-length experiment

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

Li, J.; Hu, S. X.; Ren, C.

Laser-plasma instabilities and hydro evolution of the coronal plasma in an OMEGA EP long-scale-length experiment with planar targets were studied with particle-in-cell (PIC) and hydrodynamic simulations. Plasma and laser conditions were first obtained in a two-dimensional DRACO hydro simulation with only inverse-bremsstrahlung absorption. Using these conditions, an OSIRIS PIC simulation was performed to study laser absorption and hot-electron generation caused by laser-plasma instabilities (LPIs) near the quarter-critical region. The obtained PIC information was subsequently coupled to another DRACO simulation to examine how the LPIs affect the overall hydrodynamics. Lastly, the results showed that the LPI-induced laser absorption increased the electronmore » temperature but did not significantly change the density scale length in the corona.« less

Gravity Waves in the Atmosphere: Instability, Saturation, and Transport.

DTIC Science & Technology

1995-11-13

role of gravity wave drag in the extratropical QBO , destabilization of large-scale tropical waves by deep moist convection, and a general theory of equatorial inertial instability on a zonally nonuniform, nonparallel flow.

Angular Momentum Transport in Thin Magnetically Arrested Disks

NASA Astrophysics Data System (ADS)

Marshall, Megan D.; Avara, Mark J.; McKinney, Jonathan C.

2018-05-01

In accretion disks with large-scale ordered magnetic fields, the magnetorotational instability (MRI) is marginally suppressed, so other processes may drive angular momentum transport leading to accretion. Accretion could then be driven by large-scale magnetic fields via magnetic braking, and large-scale magnetic flux can build-up onto the black hole and within the disk leading to a magnetically-arrested disk (MAD). Such a MAD state is unstable to the magnetic Rayleigh-Taylor (RT) instability, which itself leads to vigorous turbulence and the emergence of low-density highly-magnetized bubbles. This instability was studied in a thin (ratio of half-height H to radius R, H/R ≈ 0.1) MAD simulation, where it has a more dramatic effect on the dynamics of the disk than for thicker disks. Large amounts of flux are pushed off the black hole into the disk, leading to temporary decreases in stress, then this flux is reprocessed as the stress increases again. Throughout this process, we find that the dominant component of the stress is due to turbulent magnetic fields, despite the suppression of the axisymmetric MRI and the dominant presence of large-scale magnetic fields. This suggests that the magnetic RT instability plays a significant role in driving angular momentum transport in MADs.

Control of Thermo-Acoustics Instabilities: The Multi-Scale Extended Kalman Approach

NASA Technical Reports Server (NTRS)

Le, Dzu K.; DeLaat, John C.; Chang, Clarence T.

2003-01-01

"Multi-Scale Extended Kalman" (MSEK) is a novel model-based control approach recently found to be effective for suppressing combustion instabilities in gas turbines. A control law formulated in this approach for fuel modulation demonstrated steady suppression of a high-frequency combustion instability (less than 500Hz) in a liquid-fuel combustion test rig under engine-realistic conditions. To make-up for severe transport-delays on control effect, the MSEK controller combines a wavelet -like Multi-Scale analysis and an Extended Kalman Observer to predict the thermo-acoustic states of combustion pressure perturbations. The commanded fuel modulation is composed of a damper action based on the predicted states, and a tones suppression action based on the Multi-Scale estimation of thermal excitations and other transient disturbances. The controller performs automatic adjustments of the gain and phase of these actions to minimize the Time-Scale Averaged Variances of the pressures inside the combustion zone and upstream of the injector. The successful demonstration of Active Combustion Control with this MSEK controller completed an important NASA milestone for the current research in advanced combustion technologies.

Effect of collisions on neutrino flavor inhomogeneity in a dense neutrino gas

DOE PAGES

Cirigliano, Vincenzo; Paris, Mark W.; Shalgar, Shashank

2017-09-25

We investigate the stability, with respect to spatial inhomogeneity, of a two-dimensional dense neutrino gas. The system exhibits growth of seed inhomogeneity due to nonlinear coherent neutrino self-interactions. In the absence of incoherent collisional effects, we also observe a dependence of this instability growth rate on the neutrino mass spectrum: the normal neutrino mass hierarchy exhibits spatial instability over a larger range of neutrino number density compared to that of the inverted case. Furthermore, we consider the effect of elastic incoherent collisions of the neutrinos with a static background of heavy, nucleon-like scatterers. At small scales, the growth of flavormore » instability can be suppressed by collisions. At large length scales we find, perhaps surprisingly, that for inverted neutrino mass hierarchy incoherent collisions fail to suppress flavor instabilities, independent of the coupling strength.« less

Dynamics of Mesoscale Magnetic Field in Diffusive Shock Acceleration

NASA Astrophysics Data System (ADS)

Diamond, P. H.; Malkov, M. A.

2007-01-01

We present a theory for the generation of mesoscale (krg<<1, where rg is the cosmic-ray gyroradius) magnetic fields during diffusive shock acceleration. The decay or modulational instability of resonantly excited Alfvén waves scattering off ambient density perturbations in the shock environment naturally generates larger scale fields. For a broad spectrum of perturbations, the physical mechanism of energy transfer is random refraction, represented by the diffusion of Alfvén wave packets in k-space. The scattering field can be produced directly by the decay instability or by the Drury instability, a hydrodynamic instability driven by the cosmic-ray pressure gradient. This process is of interest to acceleration since it generates waves of longer wavelength, and so enables the confinement and acceleration of higher energy particles. This process also limits the intensity of resonantly generated turbulent magnetic fields on rg scales.

Influence of self-gravity on the runaway instability of black-hole-torus systems.

PubMed

Montero, Pedro J; Font, José A; Shibata, Masaru

2010-05-14

Results from the first fully general relativistic numerical simulations in axisymmetry of a system formed by a black hole surrounded by a self-gravitating torus in equilibrium are presented, aiming to assess the influence of the torus self-gravity on the onset of the runaway instability. We consider several models with varying torus-to-black-hole mass ratio and angular momentum distribution orbiting in equilibrium around a nonrotating black hole. The tori are perturbed to induce the mass transfer towards the black hole. Our numerical simulations show that all models exhibit a persistent phase of axisymmetric oscillations around their equilibria for several dynamical time scales without the appearance of the runaway instability, indicating that the self-gravity of the torus does not play a critical role favoring the onset of the instability, at least during the first few dynamical time scales.

Buckling instability in amorphous carbon films

NASA Astrophysics Data System (ADS)

Zhu, X. D.; Narumi, K.; Naramoto, H.

2007-06-01

In this paper, we report the buckling instability in amorphous carbon films on mirror-polished sapphire (0001) wafers deposited by ion beam assisted deposition at various growth temperatures. For the films deposited at 150 °C, many interesting stress relief patterns are found, which include networks, blisters, sinusoidal patterns with π-shape, and highly ordered sinusoidal waves on a large scale. Starting at irregular buckling in the centre, the latter propagate towards the outer buckling region. The maximum length of these ordered patterns reaches 396 µm with a height of ~500 nm and a wavelength of ~8.2 µm. However, the length decreases dramatically to 70 µm as the deposition temperature is increased to 550 °C. The delamination of the film appears instead of sinusoidal waves with a further increase of the deposition temperature. This experimental observation is correlated with the theoretic work of Crosby (1999 Phys. Rev. E 59 R2542).

Burn Control Mechanisms in Tokamaks

NASA Astrophysics Data System (ADS)

Hill, M. A.; Stacey, W. M.

2015-11-01

Burn control and passive safety in accident scenarios will be an important design consideration in future tokamak reactors, in particular fusion-fission hybrid reactors, e.g. the Subcritical Advanced Burner Reactor. We are developing a burning plasma dynamics code to explore various aspects of burn control, with the intent to identify feedback mechanisms that would prevent power excursions. This code solves the coupled set of global density and temperature equations, using scaling relations from experimental fits. Predictions of densities and temperatures have been benchmarked against DIII-D data. We are examining several potential feedback mechanisms to limit power excursions: i) ion-orbit loss, ii) thermal instability density limits, iii) MHD instability limits, iv) the degradation of alpha-particle confinement, v) modifications to the radial current profile, vi) ``divertor choking'' and vii) Type 1 ELMs. Work supported by the US DOE under DE-FG02-00ER54538, DE-FC02-04ER54698.

Contribution of peculiar shear motions to large-scale structure

NASA Technical Reports Server (NTRS)

Mueler, Hans-Reinhard; Treumann, Rudolf A.

1994-01-01

Self-gravitating shear flow instability simulations in a cold dark matter-dominated expanding Einstein-de Sitter universe have been performed. When the shear flow speed exceeds a certain threshold, self-gravitating Kelvin-Helmoholtz instability occurs, forming density voids and excesses along the shear flow layer which serve as seeds for large-scale structure formation. A possible mechanism for generating shear peculiar motions are velocity fluctuations induced by the density perturbations of the postinflation era. In this scenario, short scales grow earlier than large scales. A model of this kind may contribute to the cellular structure of the luminous mass distribution in the universe.

Multiple-Scale Physics During Magnetic Reconnection

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

Jara-Almonte, Jonathan

Magnetic reconnection is a key fundamental process in magnetized plasmas wherein the global magnetic topology is modified and stored energy is transferred from fields to particles. Reconnection is an inherently local process, and mechanisms to couple global-scale dynamics are not well understood. This dissertation explores two different mechanisms for cross-scale coupling during magnetic reconnection. As one example, we theoretically examine reconnection in a collisionless plasma using particle-in-cell simulations and demonstrate that large scale reconnection physics can couple to and drive microscopic instabilities, even in two-dimensional systems if significant scale separation exists between the Debye length and the electron skin depth.more » The physics underlying these instabilities is explained using simple theoretical models, and their potential connection to existing discrepancies between laboratory experiments and numerical simulations is explored. In three-dimensional systems, these instabilities are shown to generate anomalous resistivity that balances a substantial fraction of the electric field. In contrast, we also use experiments to investigate cross-scale couplings during reconnection in a collisional plasma. A leading candidate for coupling global and local scales is the hierarchical breakdown of elongated, reconnecting current sheets into numerous smaller current sheets -– the plasmoid instability. In the Magnetic Reconnection Experiment (MRX), recent hardware improvements have extended the accessible parameter space allowing for the study of long-lived, elongated current sheets. Moreover, by using Argon, reproducible and collisional plasmas are produced, which allow for a detailed statistical study of collisional reconnection. As a result, we have conclusively measured the onset of sub-ion-scale plasmoids during resistive, anti-parallel reconnection for the first time. The current sheet thickness is intermediate between ion and electron kinetic scales such that the plasma is in the Hall-MHD regime. Surprisingly, plasmoids are observed at Lundquist numbers < 100 well below theoretical predictions (> 10,000). The number of plasmoids scales with both Lundquist number and current sheet aspect ratio. The Hall quadrupolar fields are shown to suppress plasmoids. Finally, plasmoids are shown to couple local and global physics by enhancing the reconnection rate. These results are compared with prior studies of tearing and plasmoid instability, and implications for astrophysical plasmas, laboratory experiments, and theoretical studies of reconnection are discussed.« less

Paternal Multiple Partner Fertility and Environmental Chaos Among Unmarried Nonresident Fathers

PubMed Central

Petren, Raymond E.

2017-01-01

This study examined the association between paternal multiple partner fertility (MPF; having children with two or more partners) and indicators of environmental chaos (partnership instability, residential instability, work stability, material hardship, and perceived social support) among unmarried, non-resident fathers. Survey data from the Fragile Families and Child Wellbeing Study (N = 873) were used to compare unmarried non-resident fathers who experienced MPF to those who had children with one partner. Results show that paternal MPF is associated with most indicators of environmental chaos (greater partnership instability, residential instability, work instability, material hardship), but not social support. Results suggest that fathers who experience MPF face challenges beyond those of other non-resident fathers. Policies and interventions should address aspects of instability and hardship that are unique to paternal MPF in order to encourage fathers’ positive contributions to children and families. Directions for future research are discussed. PMID:29755155

Structural instability of shell-like assemblies of a keplerate-type polyoxometalate induced by ionic strength.

PubMed

Veen, Sandra J; Kegel, Willem K

2009-11-19

We demonstrate a new structural instability of shell-like assemblies of polyoxometalates. Besides the colloidal instability, that is, the formation of aggregates that consist of many single layered POM-shells, these systems also display an instability on a structural scale within the shell-like assemblies. This instability occurs at significantly lower ionic strength than the colloidal stability limit and only becomes evident after a relatively long time. For the polyoxometalate, abbreviated as {Mo(72)Fe(30)}, it is shown that the structural stability limit of POM-shells lies between a NaCl concentration of 1.00 and 5.00 mM in aqueous solution.

Progress towards Continental River Dynamics modeling

NASA Astrophysics Data System (ADS)

Yu, Cheng-Wei; Zheng, Xing; Liu, Frank; Maidment, Daivd; Hodges, Ben

2017-04-01

The high-resolution National Water Model (NWM), launched by U.S. National Oceanic and Atmospheric Administration (NOAA) in August 2016, has shown it is possible to provide real-time flow prediction in rivers and streams across the entire continental United States. The next step for continental-scale modeling is moving from reduced physics (e.g. Muskingum-Cunge) to full dynamic modeling with the Saint-Venant equations. The Simulation Program for River Networks (SPRNT) provides a computational approach for the Saint-Venant equations, but obtaining sufficient channel bathymetric data and hydraulic roughness is seen as a critical challenge. However, recent work has shown the Height Above Nearest Drainage (HAND) method can be applied with the National Elevation Dataset (NED) to provide automated estimation of effective channel bathymetry suitable for large-scale hydraulic simulations. The present work examines the use of SPRNT with the National Hydrography Dataset (NHD) and HAND-derived bathymetry for automated generation of rating curves that can be compared to existing data. The approach can, in theory, be applied to every stream reach in the NHD and thus provide flood guidance where none is available. To test this idea we generated 2000+ rating curves in two catchments in Texas and Alabama (USA). Field data from the USGS and flood records from an Austin, Texas flood in May 2015 were used as validation. Large-scale implementation of this idea requires addressing several critical difficulties associated with numerical instabilities, including ill-posed boundary conditions generated in automated model linkages and inconsistencies in the river geometry. A key to future progress is identifying efficient approaches to isolate numerical instability contributors in a large time-space varying solution. This research was supported in part by the National Science Foundation under grant number CCF-1331610.

Wave Turning and Flow Angle in the E-Region Ionosphere

NASA Astrophysics Data System (ADS)

Young, M.; Oppenheim, M. M.; Dimant, Y. S.

2016-12-01

This work presents results of particle-in-cell (PIC) simulations of Farley-Buneman (FB) turbulence at various altitudes in the high-latitude E-region ionosphere. In that region, the FB instability regularly produces meter-scale plasma irregularities. VHF radars observe coherent echoes via Bragg scatter from wave fronts parallel or anti-parallel to the radar line of sight (LoS) but do not necessarily measure the mean direction of wave propagation. Haldoupis (1984) conducted a study of diffuse radar aurora and found that the spectral width of back-scattered power depends critically on the angle between the radar LoS and the true flow direction, called the flow angle. Knowledge of the flow angle will allow researchers to better interpret observations of coherent back-scatter. Experiments designed to observe meter-scale irregularities in the E-region ionosphere created by the FB instability typically assume that the predominant flow direction is the E×B direction. However, linear theory of Dimant and Oppenheim (2004) showed that FB waves should turn away from E×B and particle-in-cell simulations by Oppenheim and Dimant (2013) support the theory. The present study comprises a quantitative analysis of the dependence of back-scattered power, flow velocity, and spectral width as functions of the flow angle. It also demonstrates that the mean direction of meter-scale wave propagation may differ from the E×B direction by tens of degrees. The analysis includes 2-D and 3-D simulations at a range of altitudes in the auroral ionosphere. Comparison between 2-D and 3-D simulations illustrates the relative importance to the irregularity spectrum of a small but finite component in the direction parallel to B. Previous work has shown this small parallel component to be important to turbulent electron heating and nonlinear transport.

Development of Computational Aeroacoustics Code for Jet Noise and Flow Prediction

NASA Astrophysics Data System (ADS)

Keith, Theo G., Jr.; Hixon, Duane R.

2002-07-01

Accurate prediction of jet fan and exhaust plume flow and noise generation and propagation is very important in developing advanced aircraft engines that will pass current and future noise regulations. In jet fan flows as well as exhaust plumes, two major sources of noise are present: large-scale, coherent instabilities and small-scale turbulent eddies. In previous work for the NASA Glenn Research Center, three strategies have been explored in an effort to computationally predict the noise radiation from supersonic jet exhaust plumes. In order from the least expensive computationally to the most expensive computationally, these are: 1) Linearized Euler equations (LEE). 2) Very Large Eddy Simulations (VLES). 3) Large Eddy Simulations (LES). The first method solves the linearized Euler equations (LEE). These equations are obtained by linearizing about a given mean flow and the neglecting viscous effects. In this way, the noise from large-scale instabilities can be found for a given mean flow. The linearized Euler equations are computationally inexpensive, and have produced good noise results for supersonic jets where the large-scale instability noise dominates, as well as for the tone noise from a jet engine blade row. However, these linear equations do not predict the absolute magnitude of the noise; instead, only the relative magnitude is predicted. Also, the predicted disturbances do not modify the mean flow, removing a physical mechanism by which the amplitude of the disturbance may be controlled. Recent research for isolated airfoils' indicates that this may not affect the solution greatly at low frequencies. The second method addresses some of the concerns raised by the LEE method. In this approach, called Very Large Eddy Simulation (VLES), the unsteady Reynolds averaged Navier-Stokes equations are solved directly using a high-accuracy computational aeroacoustics numerical scheme. With the addition of a two-equation turbulence model and the use of a relatively coarse grid, the numerical solution is effectively filtered into a directly calculated mean flow with the small-scale turbulence being modeled, and an unsteady large-scale component that is also being directly calculated. In this way, the unsteady disturbances are calculated in a nonlinear way, with a direct effect on the mean flow. This method is not as fast as the LEE approach, but does have many advantages to recommend it; however, like the LEE approach, only the effect of the largest unsteady structures will be captured. An initial calculation was performed on a supersonic jet exhaust plume, with promising results, but the calculation was hampered by the explicit time marching scheme that was employed. This explicit scheme required a very small time step to resolve the nozzle boundary layer, which caused a long run time. Current work is focused on testing a lower-order implicit time marching method to combat this problem.

Curvature-Induced Instabilities of Shells

NASA Astrophysics Data System (ADS)

Pezzulla, Matteo; Stoop, Norbert; Steranka, Mark P.; Bade, Abdikhalaq J.; Holmes, Douglas P.

2018-01-01

Induced by proteins within the cell membrane or by differential growth, heating, or swelling, spontaneous curvatures can drastically affect the morphology of thin bodies and induce mechanical instabilities. Yet, the interaction of spontaneous curvature and geometric frustration in curved shells remains poorly understood. Via a combination of precision experiments on elastomeric spherical shells, simulations, and theory, we show how a spontaneous curvature induces a rotational symmetry-breaking buckling as well as a snapping instability reminiscent of the Venus fly trap closure mechanism. The instabilities, and their dependence on geometry, are rationalized by reducing the spontaneous curvature to an effective mechanical load. This formulation reveals a combined pressurelike term in the bulk and a torquelike term in the boundary, allowing scaling predictions for the instabilities that are in excellent agreement with experiments and simulations. Moreover, the effective pressure analogy suggests a curvature-induced subcritical buckling in closed shells. We determine the critical buckling curvature via a linear stability analysis that accounts for the combination of residual membrane and bending stresses. The prominent role of geometry in our findings suggests the applicability of the results over a wide range of scales.

Optimal Transient Growth of Submesoscale Baroclinic Instabilities

NASA Astrophysics Data System (ADS)

White, Brian; Zemskova, Varvara; Passaggia, Pierre-Yves

2016-11-01

Submesoscale instabilities are analyzed using a transient growth approach to determine the optimal perturbation for a rotating Boussinesq fluid subject to baroclinic instabilities. We consider a base flow with uniform shear and stratification and consider the non-normal evolution over finite-time horizons of linear perturbations in an ageostrophic, non-hydrostatic regime. Stone (1966, 1971) showed that the stability of the base flow to normal modes depends on the Rossby and Richardson numbers, with instabilities ranging from geostrophic (Ro -> 0) and ageostrophic (finite Ro) baroclinic modes to symmetric (Ri < 1 , Ro > 1) and Kelvin-Helmholtz (Ri < 1 / 4) modes. Non-normal transient growth, initiated by localized optimal wave packets, represents a faster mechanism for the growth of perturbations and may provide an energetic link between large-scale flows in geostrophic balance and dissipation scales via submesoscale instabilities. Here we consider two- and three-dimensional optimal perturbations by means of direct-adjoint iterations of the linearized Boussinesq Navier-Stokes equations to determine the form of the optimal perturbation, the optimal energy gain, and the characteristics of the most unstable perturbation.

Parametric dependence of density limits in the Tokamak Experiment for Technology Oriented Research (TEXTOR): Comparison of thermal instability theory with experiment

NASA Astrophysics Data System (ADS)

Kelly, F. A.; Stacey, W. M.; Rapp, J.

2001-11-01

The observed dependence of the TEXTOR [Tokamak Experiment for Technology Oriented Research: E. Hintz, P. Bogen, H. A. Claassen et al., Contributions to High Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994), p. 373] density limit on global parameters (I, B, P, etc.) and wall conditioning is compared with the predicted density limit parametric scaling of thermal instability theory. It is necessary first to relate the edge parameters of the thermal instability theory to n¯ and the other global parameters. The observed parametric dependence of the density limit in TEXTOR is generally consistent with the predicted density limit scaling of thermal instability theory. The observed wall conditioning dependence of the density limit can be reconciled with the theory in terms of the radiative emissivity temperature dependence of different impurities in the plasma edge. The thermal instability theory also provides an explanation of why symmetric detachment precedes radiative collapse for most low power shots, while a multifaceted asymmetric radiation from the edge MARFE precedes detachment for most high power shots.

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

NASA Astrophysics Data System (ADS)

Weaver, J. L.; Oh, J.; Phillips, L.; Afeyan, B.; Seely, J.; Kehne, D.; Brown, C. M.; Obenschain, S. P.; Serlin, V.; Schmitt, A. J.; Feldman, U.; Lehmberg, R. H.; Mclean, E.; Manka, C.

2013-02-01

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ =248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ =351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for instabilities in the quarter critical density region.

Methylphenidate Efficacy: Immediate versus Extended Release at Short Term in Mexican Children with ADHD Assessed by Conners Scale and EEG

PubMed Central

Alatorre-Miguel, Efren; Zambrano-Sánchez, Elizabeth; Reyes-Legorreta, Celia

2015-01-01

Attention deficit hyperactivity disorder (ADHD) affects 5-6% of school aged children worldwide. Pharmacological therapy is considered the first-line treatment and methylphenidate (MPH) is considered the first-choice medication. There are two formulations: immediate release (IR) MPH and long-acting (or extended release) formulation (MPH-ER). In this work, we measure the efficacy of treatment for both presentations in one month with Conners' scales and electroencephalography (EEG). Results. for IR group, in parents and teachers Conners test, all items showed significant differences, towards improvement, except for teachers in perfectionism and emotional instability. For ER group in parent's Conners test, the items in which there were no significant differences are psychosomatic and emotional instability. For teachers, there were no significant differences in: hyperactivity and perfectionism. Comparing the Conners questionnaires (parents versus teachers) we find significant differences before and after treatment in hyperactivity, perfectionism, psychosomatics, DSM-IV hyperactive-impulsive, and DSM-IV total. In the EEG the Wilcoxon test showed a significant difference (P < 0.0001). As we can see, both presentations are suitable for managing the ADHD and have the same effect on the symptomatology and in the EEG. PMID:25838946

Methylphenidate Efficacy: Immediate versus Extended Release at Short Term in Mexican Children with ADHD Assessed by Conners Scale and EEG.

PubMed

Durand-Rivera, Alfredo; Alatorre-Miguel, Efren; Zambrano-Sánchez, Elizabeth; Reyes-Legorreta, Celia

2015-01-01

Attention deficit hyperactivity disorder (ADHD) affects 5-6% of school aged children worldwide. Pharmacological therapy is considered the first-line treatment and methylphenidate (MPH) is considered the first-choice medication. There are two formulations: immediate release (IR) MPH and long-acting (or extended release) formulation (MPH-ER). In this work, we measure the efficacy of treatment for both presentations in one month with Conners' scales and electroencephalography (EEG). Results. for IR group, in parents and teachers Conners test, all items showed significant differences, towards improvement, except for teachers in perfectionism and emotional instability. For ER group in parent's Conners test, the items in which there were no significant differences are psychosomatic and emotional instability. For teachers, there were no significant differences in: hyperactivity and perfectionism. Comparing the Conners questionnaires (parents versus teachers) we find significant differences before and after treatment in hyperactivity, perfectionism, psychosomatics, DSM-IV hyperactive-impulsive, and DSM-IV total. In the EEG the Wilcoxon test showed a significant difference (P < 0.0001). As we can see, both presentations are suitable for managing the ADHD and have the same effect on the symptomatology and in the EEG.

Magnetic Fields in the Interstellar Medium

NASA Astrophysics Data System (ADS)

Clark, Susan

2017-01-01

The Milky Way is magnetized. Invisible magnetic fields thread the Galaxy on all scales and play a vital but still poorly understood role in regulating flows of gas in the interstellar medium and the formation of stars. I will present highlights from my thesis work on magnetic fields in the diffuse interstellar gas and in accretion disks. At high Galactic latitudes, diffuse neutral hydrogen is organized into an intricate network of slender linear features. I will show that these neutral hydrogen “fibers” are extremely well aligned with the ambient magnetic field as traced by both starlight polarization (Clark et al. 2014) and Planck 353 GHz polarized dust emission (Clark et al. 2015). The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. Because the orientation of neutral hydrogen is an independent predictor of the local dust polarization angle, our work provides a new tool in the search for inflationary gravitational wave B-mode polarization in the cosmic microwave background, which is currently limited by dust foreground contamination. Magnetic fields also drive accretion in astrophysical disks via the magnetorotational instability (MRI). I analytically derive the behavior of this instability in the weakly nonlinear regime and show that the saturated state of the instability depends on the geometry of the background magnetic field. The analytical model describes the behavior of the MRI in a Taylor-Couette flow, a set-up used by experimentalists in the ongoing quest to observe MRI in the laboratory (Clark & Oishi 2016a, 2016b).

Strain E?ect on the Instability of Island Formation in Submonolayer Heteroepitaxy

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

Yao, yugui; Li, Maozhi; Wu, Biao

2009-01-01

Strain e ect on the instability of island formation in submonolayer heteroepitaxy is studied in both thermodynamic and kinetic regimes. By using linear stability analysis, the energy change of an island due to small perturbations is analyzed. A phase diagram is constructed to illustrate the interplay between kinetic processes and strain e ect on the shape instability. Critical island sizes beyond which islands grow unstable are also derived for various growth conditions and can be used to estimate energy parameters. The scaling forms of shape instability are also discussed.

How high energy fluxes may affect Rayleigh-Taylor instability growth in young supernova remnants

NASA Astrophysics Data System (ADS)

Kuranz, C. C.; Park, H.-S.; Huntington, C. M.; Miles, A. R.; Remington, B. A.; Drake, R. P.; Tranthan, M. A.; Handy, T. A.; Shvarts, D.; Malamud, G.; Shimony, A.; Shvarts, D.; Kline, J.; Flippo, K. A.; Doss, F. W.; Plewa, T.

2017-10-01

Energy-transport effects can alter the structure that develops as a supernova evolves into a supernova remnant. The Rayleigh Taylor instability is thought to produce structure at the interface between the stellar ejecta and the circumstellar matter, based on simple models and hydrodynamic simulations. Simulations predict that RT produces structures at this interface, having a range of spatial scales. When the CSM is dense enough, as in the case of SN 1993J, the hot shocked matter can produce significant radiative fluxes that affect the emission from the SNR. Here we report experimental results from the National Ignition Facility to explore how large energy fluxes, which are present in supernovae such as SN 1993J, might affect this structure. We present data and simulations from Rayleigh-Taylor instability experiments in high- and low- energy flux experiments performed at the National Ignition Facility. We also will discuss the apparent, larger role of heat conduction when we closely examined the comparison between the experimental results, and the SNR observations and models. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, Grant Number DE-NA0002956.

Onset of magnetic reconnection in a weakly collisional, high- β plasma

NASA Astrophysics Data System (ADS)

Alt, Andrew; Kunz, Matthew

2017-10-01

In a magnetized, weakly collisional plasma, the magnetic moment of the constituent particles is an adiabatic invariant. An increase of the magnetic-field strength in such a plasma thus leads to an increase in the thermal pressure perpendicular to the field lines. Above a β-dependent threshold, this pressure anisotropy drives the mirror instability, which produces strong distortions in the field lines and traps particles on ion-Larmor scales. The impact of this instability on magnetic reconnection is investigated using simple analytical and numerical models for the formation of a current sheet and the associated production of pressure anisotropy. The difficulty in maintaining an isotropic, Maxwellian particle distribution during the formation and subsequent thinning of a current sheet in a weakly collisional plasma, coupled with the low threshold for the mirror instability in a high- β plasma, imply that the topology of reconnecting magnetic fields can radically differ from the standard Harris-sheet profile often used in kinetic simulations of collisionless reconnection. Depending on the rate of current-sheet formation, this mirror-induced disruption may occur before standard tearing modes are able to develop. This work was supported by U.S. DOE contract DE-AC02-09CH11466.

Kinetic Effects in Parametric Instabilities of Finite Amplitude Alfven Waves in a Drifting Multi-Species Plasma

NASA Astrophysics Data System (ADS)

Maneva, Y. G.; Araneda, J. A.; Poedts, S.

2014-12-01

We consider parametric instabilities of finite-amplitude large-scale Alfven waves in a low-beta collisionless multi-species plasma, consisting of fluid electrons, kinetic protons and a drifting population of minor ions. Complementary to many theoretical studies, relying on fluid or multi-fluid approach, in this work we present the solutions of the parametric instability dispersion relation, including kinetic effects in the parallel direction, along the ambient magnetic field. This provides us with the opportunity to predict the importance of some wave-particle interactions like Landau damping of the daughter ion-acoustic waves for the given pump wave and plasma conditions. We apply the dispersion relation to plasma parameters, typical for low-beta collisionless solar wind close to the Sun. We compare the analytical solutions to the linear stage of hybrid numerical simulations and discuss the application of the model to the problems of preferential heating and differential acceleration of minor ions in the solar corona and the fast solar wind. The results of this study provide tools for prediction and interpretation of the magnetic field and particles data as expected from the future Solar Orbiter and Solar Probe Plus missions.

Changes to nurses' practice environment over time.

PubMed

Roche, Michael A; Duffield, Christine; Friedman, Sarah; Twigg, Di; Dimitrelis, Sofia; Rowbotham, Samantha

2016-07-01

To examine changes in the nursing practice environment, retention-related factors, unit stability and patient care tasks delayed or left undone, over two periods between 2004 and 2013. Positive nurse practice environments have been linked to nurse retention and care quality outcomes. The collection of the Practice Environment Scale of the Nursing Work Index, job satisfaction, intent to leave, unit instability and tasks delayed or not done at six acute-care hospitals across three Australian states, in two waves between 2004 and 2013; results from the two waves are compared. On average, practice environment scores declined slightly; nurses reported a greater difficulty in finding another nursing position, lower intent to leave their current job and greater instability in their current position. Rates of delayed tasks increased over the period, whereas rates of tasks left undone have decreased over the period. The decline in nurses' perceptions of the quality of the practice environment is disappointing, particularly given the protracted workforce shortages that have persisted. Significant organisational restructuring and turnover of nurse executives may have contributed to this decline. Managers need to apply existing evidence to improve nurse practice environments and manage instability. © 2016 John Wiley & Sons Ltd.

Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

NASA Technical Reports Server (NTRS)

Vishniac, Ethan T.; Diamond, Patrick

1992-01-01

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.

Generation of large-scale vorticity in rotating stratified turbulence with inhomogeneous helicity: mean-field theory

NASA Astrophysics Data System (ADS)

Kleeorin, N.

2018-06-01

We discuss a mean-field theory of the generation of large-scale vorticity in a rotating density stratified developed turbulence with inhomogeneous kinetic helicity. We show that the large-scale non-uniform flow is produced due to either a combined action of a density stratified rotating turbulence and uniform kinetic helicity or a combined effect of a rotating incompressible turbulence and inhomogeneous kinetic helicity. These effects result in the formation of a large-scale shear, and in turn its interaction with the small-scale turbulence causes an excitation of the large-scale instability (known as a vorticity dynamo) due to a combined effect of the large-scale shear and Reynolds stress-induced generation of the mean vorticity. The latter is due to the effect of large-scale shear on the Reynolds stress. A fast rotation suppresses this large-scale instability.

Working in dissonance: experiences of work instability in workers with common mental disorders.

PubMed

Danielsson, Louise; Bertilsson, Monica; Holmgren, Kristina; Hensing, Gunnel

2017-05-18

Common mental disorders have a negative impact on work functioning, but less is known about the process when the functioning starts to destabilize. This study explores experiences of work instability in workers with common mental disorders. A grounded theory study using a theoretical sampling frame, individual in-depth interviews and a constant comparative analysis conducted by a multidisciplinary research team. The sample involved 27 workers with common mental disorders, currently working full or part time, or being on sick leave not more than 6 months. They were women and men of different ages, representing different occupations and illness severity. A general process of work instability was conceptualized by the core category Working in dissonance: captured in a bubble inside the work stream. The workers described that their ordinary fluency at work was disturbed. They distanced themselves from other people at and outside work, which helped them to regain their flow but simultaneously made them feel isolated. Four categories described sub-processes of the dissonance: Working out of rhythm, Working in discomfort, Working disconnected and Working in a no man's land. The experience of work instability in CMDs was conceptualized as "working in dissonance", suggesting a multifaceted dissonance at work, characterized by a sense of being caught up, as if in a bubble. Focusing on how the worker can re-enter their flow at work when experiencing dissonance is a new approach to explore in occupational and clinical settings.

Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Hardee, P.; Richardson, G.; Preece, R.; Sol, H.; Fishman, G. J.

2004-01-01

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The non-linear fluctuation amplitudes of densities, currents, electric, and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. Additionally, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by the Weibel instability scale proportional to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform, small-scale magnetic fields which contribute to the electron's (positron's) transverse deflection behind the jet head. This small scale magnetic field structure is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a a uniform magnetic field. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks

NASA Technical Reports Server (NTRS)

Nishikawa, K.-L.; Hardee, P.; Richardson, G.; Preece, R.; Sol, H.; Fishman, G. J.

2004-01-01

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The non-linear fluctuation amplitudes of densities, currents, electric, and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper at the comparable simulation time. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. Additionally, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by the Weibel instability scale proportional to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform: small-scale magnetic fields which contribute to the electron's (positron's) transverse deflection behind the jet head. This small scale magnetic field structure is appropriate to the generation of jitter radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a a uniform magnetic field. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Hardee, P.; Richardson, G.; Preece, R.; Sol, H.; Fishman, G. J.

2005-01-01

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel, and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a three-dimensional relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. New simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. Furthermore, the nonlinear fluctuation amplitudes of densities, currents, and electric and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper at a comparable simulation time. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. In addition, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by tine Weibel instability scale proportionally to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform, small-scale magnetic fields, which contribute to the electron s (positron s) transverse deflection behind the jet head. This small- scale magnetic field structure is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a uniform magnetic field. The jitter radiation resulting from small-scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Gravitational Instabilities in Circumstellar Disks

NASA Astrophysics Data System (ADS)

Kratter, Kaitlin; Lodato, Giuseppe

2016-09-01

Star and planet formation are the complex outcomes of gravitational collapse and angular momentum transport mediated by protostellar and protoplanetary disks. In this review, we focus on the role of gravitational instability in this process. We begin with a brief overview of the observational evidence for massive disks that might be subject to gravitational instability and then highlight the diverse ways in which the instability manifests itself in protostellar and protoplanetary disks: the generation of spiral arms, small-scale turbulence-like density fluctuations, and fragmentation of the disk itself. We present the analytic theory that describes the linear growth phase of the instability supplemented with a survey of numerical simulations that aim to capture the nonlinear evolution. We emphasize the role of thermodynamics and large-scale infall in controlling the outcome of the instability. Despite apparent controversies in the literature, we show a remarkable level of agreement between analytic predictions and numerical results. In the next part of our review, we focus on the astrophysical consequences of the instability. We show that the disks most likely to be gravitationally unstable are young and relatively massive compared with their host star, Md/M*≥0.1. They will develop quasi-stable spiral arms that process infall from the background cloud. Although instability is less likely at later times, once infall becomes less important, the manifestations of the instability are more varied. In this regime, the disk thermodynamics, often regulated by stellar irradiation, dictates the development and evolution of the instability. In some cases the instability may lead to fragmentation into bound companions. These companions are more likely to be brown dwarfs or stars than planetary mass objects. Finally, we highlight open questions related to the development of a turbulent cascade in thin disks and the role of mode-mode coupling in setting the maximum angular momentum transport rate in thick disks.

Hydrodynamical processes in planet-forming accretion disks

NASA Astrophysics Data System (ADS)

Lin, Min-Kai

Understanding the physics of accretion flows in circumstellar disk provides the foundation to any theory of planet formation. The last few years have witnessed dramatic a revision in the fundamental fluid dynamics of protoplanetary accretion disks. There is growing evidence that the key to answering some of the most pressing questions, such as the origin of disk turbulence, mass transport, and planetesimal formation, may lie within, and intimately linked to, purely hydrodynamical processes in protoplanetary disks. Recent studies, including those from the proposal team, have discovered and highlighted the significance of several new hydrodynamical instabilities in the planet-forming regions of these disks. These include, but not limited to: the vertical shear instability, active between 10 to 100 AU; the zombie vortex instability, operating in regions interior to about 1AU; and the convective over-stability at intermediate radii. Secondary Rossbywave and elliptic instabilities may also be triggered, feeding off the structures that emerge from the above primary instabilities. The result of these hydrodynamic processes range from small-scale turbulence that transports angular momentum, to large-scale vortices that concentrate dust particles and enhance planetesimal formation. Hydrodynamic processes pertain to a wide range of disk conditions, meaning that at least one of these processes are active at any given disk location and evolutionary epoch. This remains true even after planet formation, which affects their subsequent orbital evolution. Hydrodynamical processes also have direct observable consequences. For example, vortices have being invoked to explain recent ALMA images of asymmetric `dust-traps' in transition disks. Hydrodynamic activities thus play a crucial role at every stage of planet formation and disk evolution. We propose to develop theoretical models of the above hydrodynamic processes under physical disk conditions by properly accounting for disk thermodynamics, dust dynamics, disk self-gravity and three-dimensional effects. By including these effects, we go wellbeyond previous works based on idealized disk models. This effort is necessary to understand how these instabilities operate and interact in realistic protoplanetary disks. This will enable us to provide a unified picture of how various hydrodynamic activities fit together to drive global disk evolution. We will address key questions including the strength of the resulting hydrodynamic turbulence, the lifetime of large-scale vortices under realistic disk conditions, and their impact on the evolution of solids within the disk. Inclusion of these additional physics will likely uncover new, yet-unknown hydrodynamic processes. Our generalized models enables a direct link between theory and observations. For example, a self-consistent incorporation of dust dynamics into the theory of hydrodynamic instabilities is particularly important, since it is the dust component that is usually observed. We will also establish the connection between the properties of large-scale, observable structures such as vortices, to the underlying disk properties, such as disk mass, and vertical structure, which are difficult to infer directly from observations. We also propose to study, for the first time, the dynamical interaction between hydrodynamic turbulence and proto-planets, as well as the influence of largescale vortices on disk-planet interaction. This is necessary towards a realistic modeling of the orbital evolution of proto planets, and thus in predicting the final architecture of planetary systems. The proposal team's expertise and experience, ranging from mathematical analyses to state-of the-art numerical simulations in astrophysical fluid dynamics, provides a multi-method approach to these problems. This is necessary towards establishing a rigorous understanding of these fundamental hydrodynamical processes in protoplanetary accretion disks.

Winter westerly disturbance dynamics and precipitation in the western Himalaya and Karakoram: a wave-tracking approach

NASA Astrophysics Data System (ADS)

Cannon, Forest; Carvalho, Leila M. V.; Jones, Charles; Norris, Jesse

2016-07-01

Extratropical cyclones, including winter westerly disturbances (WWD) over central Asia, are fundamental features of the atmosphere that maintain energy, momentum, and moisture at global scales while intimately linking large-scale circulation to regional-scale meteorology. Within high mountain Asia, WWD are the primary contributor to regional precipitation during winter. In this work, we present a novel WWD tracking methodology, which provides an inventory of location, timing, intensity, and duration of events, allowing for a comprehensive study of the factors that relate WWD to orographic precipitation, on an individual event basis and in the aggregate. We identify the relationship between the strength of disturbances, the state of the background environment during their propagation, and precipitation totals in the Karakoram/western Himalaya. We observe significant differences in convective and mechanical instability contributions to orographic precipitation as a function of the relationship between the intensity of WWD and the background temperature and moisture fields, which exhibit strong intraseasonal variability. Precipitation is primarily orographically forced during intense WWD with strong cross-barrier winds, while weaker WWD with similar precipitation totals are observed to benefit from enhanced instability due to high moisture content and temperature at low levels, occurring primarily in the late winter/premonsoon. The contribution of these factors is observed to fluctuate on a per-case basis, indicating important influences of intraseasonal oscillations and tropical-extratropical interactions on regional precipitation.

Ion-temperature-gradient sensitivity of the hydrodynamic instability caused by shear in the magnetic-field-aligned plasma flow

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

Mikhailenko, V. V., E-mail: vladimir@pusan.ac.kr; Mikhailenko, V. S.; Faculty of Transportation Systems, Kharkiv National Automobile and Highway University, 61002 Kharkiv

2014-07-15

The cross-magnetic-field (i.e., perpendicular) profile of ion temperature and the perpendicular profile of the magnetic-field-aligned (parallel) plasma flow are sometimes inhomogeneous for space and laboratory plasma. Instability caused either by a gradient in the ion-temperature profile or by shear in the parallel flow has been discussed extensively in the literature. In this paper, (1) hydrodynamic plasma stability is investigated, (2) real and imaginary frequency are quantified over a range of the shear parameter, the normalized wavenumber, and the ratio of density-gradient and ion-temperature-gradient scale lengths, and (3) the role of inverse Landau damping is illustrated for the case of combinedmore » ion-temperature gradient and parallel-flow shear. We find that increasing the ion-temperature gradient reduces the instability threshold for the hydrodynamic parallel-flow shear instability, also known as the parallel Kelvin-Helmholtz instability or the D'Angelo instability. We also find that a kinetic instability arises from the coupled, reinforcing action of both free-energy sources. For the case of comparable electron and ion temperature, we illustrate analytically the transition of the D'Angelo instability to the kinetic instability as (a) the shear parameter, (b) the normalized wavenumber, and (c) the ratio of density-gradient and ion-temperature-gradient scale lengths are varied and we attribute the changes in stability to changes in the amount of inverse ion Landau damping. We show that near a normalized wavenumber k{sub ⊥}ρ{sub i} of order unity (i) the real and imaginary values of frequency become comparable and (ii) the imaginary frequency, i.e., the growth rate, peaks.« less

Analysis of Self-Excited Combustion Instabilities Using Decomposition Techniques

DTIC Science & Technology

2016-07-05

are evaluated for the study of self-excited longitudinal combustion instabilities in laboratory-scaled single-element gas turbine and rocket...Air Force Base, California 93524 DOI: 10.2514/1.J054557 Proper orthogonal decomposition and dynamic mode decomposition are evaluated for the study of...instabilities. In addition, we also evaluate the capabilities of the methods to deal with data sets of different spatial extents and temporal resolution

On the instability of wave-fields with JONSWAP spectra to inhomogeneous disturbances, and the consequent long-time evolution

NASA Astrophysics Data System (ADS)

Ribal, A.; Stiassnie, M.; Babanin, A.; Young, I.

2012-04-01

The instability of two-dimensional wave-fields and its subsequent evolution in time are studied by means of the Alber equation for narrow-banded random surface-waves in deep water subject to inhomogeneous disturbances. A linear partial differential equation (PDE) is obtained after applying an inhomogeneous disturbance to the Alber's equation and based on the solution of this PDE, the instability of the ocean wave surface is studied for a JONSWAP spectrum, which is a realistic ocean spectrum with variable directional spreading and steepness. The steepness of the JONSWAP spectrum depends on γ and α which are the peak-enhancement factor and energy scale of the spectrum respectively and it is found that instability depends on the directional spreading, α and γ. Specifically, if the instability stops due to the directional spreading, increase of the steepness by increasing α or γ can reactivate it. This result is in qualitative agreement with the recent large-scale experiment and new theoretical results. In the instability area of α-γ plane, a long-time evolution has been simulated by integrating Alber's equation numerically and recurrent evolution is obtained which is the stochastic counterpart of the Fermi-Pasta-Ulam recurrence obtained for the cubic Schrödinger equation.

Two-dimensional studies of relativistic electron beam plasma instabilities in an inhomogeneous plasma

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

Shukla, Chandrasekhar; Das, Amita, E-mail: amita@ipr.res.in; Patel, Kartik

Relativistic electron beam propagation in plasma is fraught with several micro instabilities like two stream, filamentation, etc., in plasma. This results in severe limitation of the electron transport through a plasma medium. Recently, however, there has been an experimental demonstration of improved transport of Mega Ampere of electron currents (generated by the interaction of intense laser with solid target) in a carbon nanotube structured solid target [G. Chatterjee et al., Phys. Rev. Lett. 108, 235005 (2012)]. This then suggests that the inhomogeneous plasma (created by the ionization of carbon nanotube structured target) helps in containing the growth of the beammore » plasma instabilities. This manuscript addresses this issue with the help of a detailed analytical study and 2-D Particle-In-Cell simulations. The study conclusively demonstrates that the growth rate of the dominant instability in the 2-D geometry decreases when the plasma density is chosen to be inhomogeneous, provided the scale length 1/k{sub s} of the inhomogeneous plasma is less than the typical plasma skin depth (c/ω{sub 0}) scale. At such small scale lengths channelization of currents is also observed in simulation.« less

Alternative Splicing of CHEK2 and Codeletion with NF2 Promote Chromosomal Instability in Meningioma1

PubMed Central

Yang, Hong Wei; Kim, Tae-Min; Song, Sydney S; Shrinath, Nihal; Park, Richard; Kalamarides, Michel; Park, Peter J; Black, Peter M; Carroll, Rona S; Johnson, Mark D

2012-01-01

Mutations of the NF2 gene on chromosome 22q are thought to initiate tumorigenesis in nearly 50% of meningiomas, and 22q deletion is the earliest and most frequent large-scale chromosomal abnormality observed in these tumors. In aggressive meningiomas, 22q deletions are generally accompanied by the presence of large-scale segmental abnormalities involving other chromosomes, but the reasons for this association are unknown. We find that large-scale chromosomal alterations accumulate during meningioma progression primarily in tumors harboring 22q deletions, suggesting 22q-associated chromosomal instability. Here we show frequent codeletion of the DNA repair and tumor suppressor gene, CHEK2, in combination with NF2 on chromosome 22q in a majority of aggressive meningiomas. In addition, tumor-specific splicing of CHEK2 in meningioma leads to decreased functional Chk2 protein expression. We show that enforced Chk2 knockdown in meningioma cells decreases DNA repair. Furthermore, Chk2 depletion increases centrosome amplification, thereby promoting chromosomal instability. Taken together, these data indicate that alternative splicing and frequent codeletion of CHEK2 and NF2 contribute to the genomic instability and associated development of aggressive biologic behavior in meningiomas. PMID:22355270

Transition scenario and transition control of the flow over a semi-infinite square leading-edge plate

NASA Astrophysics Data System (ADS)

Huang, Yadong; Zhou, Benmou; Tang, Zhaolie; Zhang, Fei

2017-07-01

In recent investigations of the flow over a square leading-edge flat plate, elliptic instability and transient growth of perturbations are proposed to explain the turbulent transition mechanism of the separating and reattaching flow reported in early experimental visualizations. An original transition scenario as well as a transition control method is presented by a detailed numerical study in this paper. The transient growth of perturbations in the separation bubble induces the primary instability that causes the 2D unsteady flow consisting of Kelvin-Helmholtz (KH) vortices. The pairing instability of the KH vortices induces the subharmonic secondary instability, and then resonance transition occurs. The streamwise Lorentz force as the control input is applied in the recirculation region where the separation bubble generates. The maximum energy amplification magnitude of perturbations takes a linear attenuation with the interaction number; thus, the primary instability is reduced under control. The interaction number represents the strength of the streamwise Lorentz force relative to the inertial force of the fluid. The reduced primary instability is not strong enough to induce the secondary instability, so the flow is globally stable under control. Three-dimensional direct numerical simulation confirms the results of the linear stability analysis. Although the growth rate of the convectively unstable secondary instability is limited by the flow field scale, the feedback loop of the energy transfer promotes the resonance transition. However, as the separation bubble scale is reduced and the feedback loop is broken by the streamwise Lorentz force, the three-dimensional transition is suppressed and a skin-friction drag reduction is achieved.

High Frequency Adaptive Instability Suppression Controls in a Liquid-Fueled Combustor

NASA Technical Reports Server (NTRS)

Kopasakis, George

2003-01-01

This effort extends into high frequency (>500 Hz), an earlier developed adaptive control algorithm for the suppression of thermo-acoustic instabilities in a liquidfueled combustor. The earlier work covered the development of a controls algorithm for the suppression of a low frequency (280 Hz) combustion instability based on simulations, with no hardware testing involved. The work described here includes changes to the simulation and controller design necessary to control the high frequency instability, augmentations to the control algorithm to improve its performance, and finally hardware testing and results with an experimental combustor rig developed for the high frequency case. The Adaptive Sliding Phasor Averaged Control (ASPAC) algorithm modulates the fuel flow in the combustor with a control phase that continuously slides back and forth within the phase region that reduces the amplitude of the instability. The results demonstrate the power of the method - that it can identify and suppress the instability even when the instability amplitude is buried in the noise of the combustor pressure. The successful testing of the ASPAC approach helped complete an important NASA milestone to demonstrate advanced technologies for low-emission combustors.

Tertiary instability of zonal flows within the Wigner-Moyal formulation of drift turbulence

NASA Astrophysics Data System (ADS)

Zhu, Hongxuan; Ruiz, D. E.; Dodin, I. Y.

2017-10-01

The stability of zonal flows (ZFs) is analyzed within the generalized-Hasegawa-Mima model. The necessary and sufficient condition for a ZF instability, which is also known as the tertiary instability, is identified. The qualitative physics behind the tertiary instability is explained using the recently developed Wigner-Moyal formulation and the corresponding wave kinetic equation (WKE) in the geometrical-optics (GO) limit. By analyzing the drifton phase space trajectories, we find that the corrections proposed in Ref. to the WKE are critical for capturing the spatial scales characteristic for the tertiary instability. That said, we also find that this instability itself cannot be adequately described within a GO formulation in principle. Using the Wigner-Moyal equations, which capture diffraction, we analytically derive the tertiary-instability growth rate and compare it with numerical simulations. The research was sponsored by the U.S. Department of Energy.

General theory of the plasmoid instability

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

Comisso, L.; Lingam, M.; Huang, Y. -M.

2016-10-05

In a general theory of the onset and development of the plasmoid instability is formulated by means of a principle of least time. We derive and show the scaling relations for the final aspect ratio, transition time to rapid onset, growth rate, and number of plasmoids that depend on the initial perturbation amplitude (more » $$\\hat{w}$$ 0), the characteristic rate of current sheet evolution (1/τ), and the Lundquist number (S). They are not simple power laws, and are proportional to S ατ β[ln f(S,τ,$$\\hat{w}$$ 0)] σ. Finally, the detailed dynamics of the instability is also elucidated, and shown to comprise of a period of quiescence followed by sudden growth over a short time scale.« less

Dynamics of flows, fluctuations, and global instability under electrode biasing in a linear plasma device

NASA Astrophysics Data System (ADS)

Desjardins, T. R.; Gilmore, M.

2016-05-01

Grid biasing is utilized in a large-scale helicon plasma to modify an existing instability. It is shown both experimentally and with a linear stability analysis to be a hybrid drift-Kelvin-Helmholtz mode. At low magnetic field strengths, coherent fluctuations are present, while at high magnetic field strengths, the plasma is broad-band turbulent. Grid biasing is used to drive the once-coherent fluctuations to a broad-band turbulent state, as well as to suppress them. There is a corresponding change in the flow shear. When a high positive bias (10Te) is applied to the grid electrode, a large-scale ( n ˜/n ≈50 % ) is excited. This mode has been identified as the potential relaxation instability.

A solution to Rayleigh-Taylor instabilities. Bubbles, spikes, and their scalings

DOE PAGES

Mikaelian, Karnig O.

2014-05-12

A fluid that pushes on and accelerates a heavier fluid, small perturbations at their interface grows with time and lead. to turbulent mixing. The same instability, known as the Rayleigh-Taylor instability, operates when a heavy fluid is supported by a lighter fluid in a gravitational field. Moreover, it has a particularly deleterious effect on inertial-confinement-fusion implosions and is known to operate over 18 orders of magnitude in dimension. We propose analytic expressions for the bubble and spike amplitudes and mixing widths in the linear, nonlinear, and turbulent regimes. They cover arbitrary density ratios and accelerations that are constant or changingmore » relatively slowly with time. Here, we discuss their scalings and compare them with simulations and experiments.« less

Combustion instability and active control: Alternative fuels, augmentors, and modeling heat release

NASA Astrophysics Data System (ADS)

Park, Sammy Ace

Experimental and analytical studies were conducted to explore thermo-acoustic coupling during the onset of combustion instability in various air-breathing combustor configurations. These include a laboratory-scale 200-kW dump combustor and a 100-kW augmentor featuring a v-gutter flame holder. They were used to simulate main combustion chambers and afterburners in aero engines, respectively. The three primary themes of this work includes: 1) modeling heat release fluctuations for stability analysis, 2) conducting active combustion control with alternative fuels, and 3) demonstrating practical active control for augmentor instability suppression. The phenomenon of combustion instabilities remains an unsolved problem in propulsion engines, mainly because of the difficulty in predicting the fluctuating component of heat release without extensive testing. A hybrid model was developed to describe both the temporal and spatial variations in dynamic heat release, using a separation of variables approach that requires only a limited amount of experimental data. The use of sinusoidal basis functions further reduced the amount of data required. When the mean heat release behavior is known, the only experimental data needed for detailed stability analysis is one instantaneous picture of heat release at the peak pressure phase. This model was successfully tested in the dump combustor experiments, reproducing the correct sign of the overall Rayleigh index as well as the remarkably accurate spatial distribution pattern of fluctuating heat release. Active combustion control was explored for fuel-flexible combustor operation using twelve different jet fuels including bio-synthetic and Fischer-Tropsch types. Analysis done using an actuated spray combustion model revealed that the combustion response times of these fuels were similar. Combined with experimental spray characterizations, this suggested that controller performance should remain effective with various alternative fuels. Active control experiments validated this analysis while demonstrating 50-70% reduction in the peak spectral amplitude. A new model augmentor was built and tested for combustion dynamics using schlieren and chemiluminescence techniques. Novel active control techniques including pulsed air injection were implemented and the results were compared with the pulsed fuel injection approach. The pulsed injection of secondary air worked just as effectively for suppressing the augmentor instability, setting up the possibility of more efficient actuation strategy.

Experimental investigation of large-scale vortices in a freely spreading gravity current

NASA Astrophysics Data System (ADS)

Yuan, Yeping; Horner-Devine, Alexander R.

2017-10-01

A series of laboratory experiments are presented to compare the dynamics of constant-source buoyant gravity currents propagating into laterally confined (channelized) and unconfined (spreading) environments. The plan-form structure of the spreading current and the vertical density and velocity structures on the interface are quantified using the optical thickness method and a combined particle image velocimetry and planar laser-induced fluorescence method, respectively. With lateral boundaries, the buoyant current thickness is approximately constant and Kelvin-Helmholtz instabilities are generated within the shear layer. The buoyant current structure is significantly different in the spreading case. As the current spreads laterally, nonlinear large-scale vortex structures are observed at the interface, which maintain a coherent shape as they propagate away from the source. These structures are continuously generated near the river mouth, have amplitudes close to the buoyant layer thickness, and propagate offshore at speeds approximately equal to the internal wave speed. The observed depth and propagation speed of the instabilities match well with the fastest growing mode predicted by linear stability analysis, but with a shorter wavelength. The spreading flows have much higher vorticity, which is aggregated within the large-scale structures. Secondary instabilities are generated on the leading edge of the braids between the large-scale vortex structures and ultimately break and mix on the lee side of the structures. Analysis of the vortex dynamics shows that lateral stretching intensifies the vorticity in the spreading currents, contributing to higher vorticity within the large-scale structures in the buoyant plume. The large-scale instabilities and vortex structures observed in the present study provide new insights into the origin of internal frontal structures frequently observed in coastal river plumes.

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

Chen, Qiang, E-mail: cq0405@126.com; Luoyang Electronic Equipment Testing Center, Luoyang 471000; Chen, Bin, E-mail: emcchen@163.com

The Rayleigh-Taylor (R-T) instabilities are important hydrodynamics and magnetohydrodynamics (MHD) phenomena that are found in systems in high energy density physics and normal fluids. The formation and evolution of the R-T instability at channel boundary during back-flow of the lightning return stroke are analyzed using the linear perturbation theory and normal mode analysis methods, and the linear growth rate of the R-T instability in typical condition for lightning return stroke channel is obtained. Then, the R-T instability phenomena of lightning return stroke are simulated using a two-dimensional Eulerian finite volumes resistive radiation MHD code. The numerical results show that themore » evolution characteristics of the R-T instability in the early stage of back-flow are consistent with theoretical predictions obtained by linear analysis. The simulation also yields more evolution characteristics for the R-T instability beyond the linear theory. The results of this work apply to some observed features of the return stroke channel and further advance previous theoretical and experimental work.« less

High Frequency Backscatter from the Polar and Auroral E-Region Ionosphere

NASA Astrophysics Data System (ADS)

Forsythe, Victoriya V.

The Earth's ionosphere contains collisional and partially-ionized plasma. The electric field, produced by the interaction between the Earth's magnetosphere and the solar wind, drives the plasma bulk motion, also known as convection, in the F-region of the ionosphere. It can also destabilize the plasma in the E-region, producing irregularities or waves. Intermediate-scale waves with wavelengths of hundreds of meters can cause scintillation and fading of the Global Navigation Satellite System (GNSS) signals, whereas the small-scale waves (lambda < 100 m) can scatter radar signals, making possible detection of these plasma structures and measurements of their characteristics such as their phase velocity and intensity. In this work, production of the decameter-scale (lambda ≈ 10 m) irregularities in the ionospheric E-region (100-120 km in altitude) at high latitudes is investigated both theoretically, using linear fluid theory of plasma instability processes that generate small-scale plasma waves, and experimentally, by analyzing data collected with the newly-deployed high-southern-latitude radars within the Super Dual Auroral Radar Network (SuperDARN). The theoretical part of this work focuses on symmetry properties of the general dispersion relation that describes wave propagation in the collisional plasma in the two-stream and gradient-drift instability regimes. The instability growth rate and phase velocity are examined under the presence of a background parallel electric field, whose influence is demonstrated to break the spatial symmetry of the wave propagation patterns. In the observational part of this thesis, a novel dual radar setup is used to examine E-region irregularities in the magnetic polar cap by probing the E-region along the same line from opposite directions. The phase velocity analysis together with raytracing simulations demonstrated that, in the polar cap, the radar backscatter is primarily controlled by the plasma density conditions. In particular, when the E-region layer is strong and stratified, the radar backscatter properties are controlled by the convection velocity, whereas for a tilted E-layer, the height and aspect angle conditions are more important. Finally, the fundamental dependence of the E-region irregularity phase velocity on the component of the plasma convection is investigated using two new SuperDARN radars at high southern latitudes where plasma convection estimates are accurately deduced from all SuperDARN radars in the southern hemisphere. Statistical analysis is presented showing that the predominance of the E-region echoes of a particular polarity is strongly dictated by the orientation of the convection plasma flow which itself has a significant asymmetry towards westward zonal flow.

Nano Liquid Crystal Droplet Impact on Solid Surfaces

NASA Astrophysics Data System (ADS)

Zhang, Rui; de Pablo, Juan; dePablo Team

2015-03-01

Liquid droplet impaction on solid surfaces is an important problem with a wide range of applications in everyday life. Liquid crystals (LCs) are anisotropic liquids whose internal structure gives rise to rich optical and morphological phenomena. In this work we study the liquid crystal droplet impaction on solid surfaces by molecular dynamics simulations. We employ a widely used Gay-Berne model to describe the elongated liquid crystal molecules and their interactions. Our work shows that, in contrast to isotropic liquids, drop deformation is symmetric unless an instability kicks in, in which case a nano scale liquid crystal droplet exhibits distinct anisotropic spreading modes that do not occur in simple liquids. The drop prefers spreading along the low viscosity direction, but inertia can in some cases overcome that bias. The effects of the director field of the droplet, preferred anchoring direction and the anchoring strength of the wall are investigated. Large scale (0.1 micron) simulations are performed to connect our nano scale results to the experiments. Our studies indicate that LCs could provide an interesting alternative for development of next-generation printing inks.

Richtmyer-Meshkov instability in shock-flame interactions

NASA Astrophysics Data System (ADS)

Massa, Luca; Pallav Jha Collaboration

2011-11-01

Shock-flame interactions occur in supersonic mixing and detonation formation. Therefore, their analysis is important to explosion safety, internal combustion engine performance, and supersonic combustor design. The fundamental process at the basis of the interaction is the Richtmyer-Meshkov instability supported by the density difference between burnt and fresh mixtures. In the present study we analyze the effect of reactivity on the Richtmyer- Meshkov instability with particular emphasis on combustion lengths that typify the scaling between perturbation growth and induction. The results of the present linear analysis study show that reactivity changes the perturbation growth rate by developing a non-zero pressure gradient at the flame surface. The baroclinic torque based on the density gradient across the flame acts to slow down the instability growth for high wave numbers. A non-hydrodynamic flame representation leads to the definition of an additional scaling Peclet number, the effects of which are investigated. It is found that an increased flame-contact separation destabilizes the contact discontinuity by augmenting the tangential shear.

On ballooning instability in current sheets

NASA Astrophysics Data System (ADS)

Leonovich, Anatoliy; Kozlov, Daniil

2015-06-01

The problem of instability of the magnetotail current sheet to azimuthally small-scale Alfvén and slow magnetosonic (SMS) waves is solved. The solutions describe unstable oscillations in the presence of a current sheet and correspond to the region of stretched closed field lines of the magnetotail. The spectra of eigen-frequencies of several basic harmonics of standing Alfvén and SMS waves are found in the local and WKB approximation, which are compared. It is shown that the oscillation properties obtained in these approximations differ radically. In the local approximation, the Alfvén waves are stable in the entire range of magnetic shells. SMS waves go into the aperiodic instability regime (the regime of the "ballooning" instability), on magnetic shells crossing the current sheet. In the WKB approximation, both the Alfvén and SMS oscillations go into an unstable regime with a non-zero real part of their eigen-frequency, on magnetic shells crossing the current sheet. The structure of azimuthally small-scale Alfvén waves across magnetic shells is determined.

Particle acceleration and magnetic field generation in SNR shocks

NASA Astrophysics Data System (ADS)

Suslov, M.; Diamond, P. H.; Malkov, M. A.

2006-04-01

We discuss the diffusive acceleration mechanism in SNR shocks in terms of its potential to accelerate CRs to 10^18 eV, as observations imply. One possibility, currently discussed in the literature, is to resonantly generate a turbulent magnetic field via accelerated particles in excess of the background field. We analyze some problems of this scenario and suggest a different mechanism, which is based on the generation of Alfven waves at the gyroradius scale at the background field level, with a subsequent transfer to longer scales via interaction with strong acoustic turbulence in the shock precursor. The acoustic turbulence in turn, may be generated by Drury instability or by parametric instability of the Alfven (A) waves. The essential idea is an A->A+S decay instability process, where one of the interacting scatterers (i.e. the sound, or S-waves) are driven by the Drury instability process. This rapidly generates longer wavelength Alfven waves, which in turn resonate with high energy CRs thus binding them to the shock and enabling their further acceleration.

Directionally asymmetric self-assembly of cadmium sulfide nanotubes using porous alumina nanoreactors: need for chemohydrodynamic instability at the nanoscale.

PubMed

Varghese, Arthur; Datta, Shouvik

2012-05-01

We explore nanoscale hydrodynamical effects on synthesis and self-assembly of cadmium sulfide nanotubes oriented along one direction. These nanotubes are synthesized by horizontal capillary flow of two different chemical reagents from opposite directions through nanochannels of porous anodic alumina which are used primarily as nanoreactors. We show that uneven flow of different chemical precursors is responsible for directionally asymmetric growth of these nanotubes. On the basis of structural observations using scanning electron microscopy, we argue that chemohydrodynamic convective interfacial instability of multicomponent liquid-liquid reactive interface is necessary for sustained nucleation of these CdS nanotubes at the edges of these porous nanochannels over several hours. However, our estimates clearly suggest that classical hydrodynamics cannot account for the occurrence of such instabilities at these small length scales. Therefore, we present a case which necessitates further investigation and understanding of chemohydrodynamic fluid flow through nanoconfined channels in order to explain the occurrence of such interfacial instabilities at nanometer length scales.

The Zombie Instability: Using Numerical Simulation to Design a Laboratory Experiment

NASA Astrophysics Data System (ADS)

Wang, Meng; Pei, Suyang; Jiang, Chung-Hsiang; Hassanzadeh, Pedram; Marcus, Philip

2014-11-01

A new type of finite amplitude-instability has been found in numerical simulations of stratified, rotating, shear flows. The instability occurs via baroclinic critical layers that create linearly unstable vortex layers, which roll-up into vortices. Under the right conditions, those vortices can form a new generation of vortices, resulting in ``vortex self-replication'' that fills the fluid with vortices. Creating this instability in a laboratory would provide further evidence for the existence of the instability, which we first found in numerical simulations of protoplanetary disks. To design a laboratory experiment we need to know how the flow parameters-- shear, rotation and stratification, etc. affect the instability. To build an experiment economically, we also need to know how the finite-amplitude trigger of the instability scales with viscosity and the size of the domain. In this talk, we summarize our findings. We present a map, in terms of the experimentally controllable parameters, that shows where the instability occurs and whether the instability creates a few isolated transient vortices, a few long-lived vortices, or long-lived, self-replicating vortices that fill the entire flow.

Flow Instability and Flow Control Scaling Laws

NASA Astrophysics Data System (ADS)

van Ness, Daniel; Corke, Thomas; Morris, Scott

2006-11-01

A flow instability that is receptive to perturbations is present in the tip clearance leakage flow over the tip of a turbine blade. This instability was investigated through the introduction of active flow control in the viscous flow field. Control was implemented in the form of a dielectric barrier discharge created by a weakly-ionized plasma actuation arrangement. The experimental setup consisted of a low-speed linear turbine cascade made up of an array of nine Pratt & Whitney ``PakB'' turbine blades. This idealized cascade configuration was used to examine the tip clearance leakage flow that exists within the low pressure turbine stage of a gas-turbine engine. The center blade of the cascade array had a variable tip clearance up to five percent chord. Reynolds numbers based on axial blade chord varied from 10^4 to 10^5. Multi-port pressure probe measurements, as well as Stereo Particle Image Velocimetry were used to document the dependence of the instability on the frequency and amplitude of flow control perturbations. Scaling laws based on the variation of blade tip clearance height and inflow conditions were investigated. These results permitted an improved understanding of the mechanism of flow instability.

Nonlinear Interaction of Detuned Instability Waves in Boundary-Layer Transition: Resonant-Triad Interaction

NASA Technical Reports Server (NTRS)

Lee, Sang Soo

1998-01-01

The non-equilibrium critical-layer analysis of a system of frequency-detuned resonant-triads is presented using the generalized scaling of Lee. It is shown that resonant-triads can interact nonlinearly within the common critical layer when their (fundamental) Strouhal numbers are different by a factor whose magnitude is of the order of the growth rate multiplied by the wavenumber of the instability wave. Since the growth rates of the instability modes become larger and the critical layers become thicker as the instability waves propagate downstream, the frequency-detuned resonant-triads that grow independently of each other in the upstream region can interact nonlinearly in the later downstream stage. In the final stage of the non-equilibrium critical-layer evolution, a wide range of instability waves with the scaled frequencies differing by almost an Order of (l) can nonlinearly interact. Low-frequency modes are also generated by the nonlinear interaction between oblique waves in the critical layer. The system of partial differential critical-layer equations along with the jump equations are presented here. The amplitude equations with their numerical solutions are given in Part 2. The nonlinearly generated low-frequency components are also investigated in Part 2.

Reconfinement and loss of stability in jets from active galactic nuclei

NASA Astrophysics Data System (ADS)

Gourgouliatos, Konstantinos N.; Komissarov, Serguei S.

2018-02-01

Jets powered by active galactic nuclei appear impressively stable compared with their terrestrial and laboratory counterparts—they can be traced from their origin to distances exceeding their injection radius by up to a billion times1,2. However, some less energetic jets get disrupted and lose their coherence on the scale of their host galaxy1,3. Quite remarkably, on the same scale, these jets are expected to become confined by the thermal pressure of the intra-galactic gas2. Motivated by these observations, we have started a systematic study of active galactic nuclei jets undergoing reconfinement via computer simulations. Here, we show that in the case of unmagnetized relativistic jets, the reconfinement is accompanied by the development of an instability and transition to a turbulent state. During their initial growth, the perturbations have a highly organized streamwise-oriented structure, indicating that it is not the Kelvin-Helmholtz instability, the instability which has been the main focus of the jet stability studies so far4,5. Instead, it is closely related to the centrifugal instability6. This instability is likely to be behind the division of active galactic nuclei jets into two morphological types in the Fanaroff-Riley classification7.

A Correlation between the Higgs Mass and Dark Matter

DOE PAGES

Hertzberg, Mark P.

2017-07-27

Depending on the value of the Higgs mass, the Standard Model acquires an unstable region at large Higgs field values due to RG running of couplings, which we evaluate at 2-loop order. For currently favored values of the Higgs mass, this renders the electroweak vacuum only metastable with a long lifetime. We argue on statistical grounds that the Higgs field would be highly unlikely to begin in the small field metastable region in the early universe, and thus some new physics should enter in the energy range of order of, or lower than, the instability scale to remove the largemore » field unstable region. We assume that Peccei-Quinn (PQ) dynamics enters to solve the strong CP problem and, for a PQ-scale in this energy range, may also remove the unstable region. We allow the PQ-scale to scan and argue, again on statistical grounds, that its value in our universe should be of order of the instability scale, rather than (significantly) lower. Since the Higgs mass determines the instability scale, which is argued to set the PQ-scale, and since the PQ-scale determines the axion properties, including its dark matter abundance, we are led to a correlation between the Higgs mass and the abundance of dark matter. We thus find the correlation to be in good agreement with current data.« less

A Correlation between the Higgs Mass and Dark Matter

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

Hertzberg, Mark P.

Depending on the value of the Higgs mass, the Standard Model acquires an unstable region at large Higgs field values due to RG running of couplings, which we evaluate at 2-loop order. For currently favored values of the Higgs mass, this renders the electroweak vacuum only metastable with a long lifetime. We argue on statistical grounds that the Higgs field would be highly unlikely to begin in the small field metastable region in the early universe, and thus some new physics should enter in the energy range of order of, or lower than, the instability scale to remove the largemore » field unstable region. We assume that Peccei-Quinn (PQ) dynamics enters to solve the strong CP problem and, for a PQ-scale in this energy range, may also remove the unstable region. We allow the PQ-scale to scan and argue, again on statistical grounds, that its value in our universe should be of order of the instability scale, rather than (significantly) lower. Since the Higgs mass determines the instability scale, which is argued to set the PQ-scale, and since the PQ-scale determines the axion properties, including its dark matter abundance, we are led to a correlation between the Higgs mass and the abundance of dark matter. We thus find the correlation to be in good agreement with current data.« less

Multi-scale phenomena of rotation-modified mode-2 internal waves

NASA Astrophysics Data System (ADS)

Deepwell, David; Stastna, Marek; Coutino, Aaron

2018-03-01

We present high-resolution, three-dimensional simulations of rotation-modified mode-2 internal solitary waves at various rotation rates and Schmidt numbers. Rotation is seen to change the internal solitary-like waves observed in the absence of rotation into a leading Kelvin wave followed by Poincaré waves. Mass and energy is found to be advected towards the right-most side wall (for a Northern Hemisphere rotation), leading to increased amplitude of the leading Kelvin wave and the formation of Kelvin-Helmholtz (K-H) instabilities on the upper and lower edges of the deformed pycnocline. These fundamentally three-dimensional instabilities are localized within a region near the side wall and intensify in vigour with increasing rotation rate. Secondary Kelvin waves form further behind the wave from either resonance with radiating Poincaré waves or the remnants of the K-H instability. The first of these mechanisms is in accord with published work on mode-1 Kelvin waves; the second is, to the best of our knowledge, novel to the present study. Both types of secondary Kelvin waves form on the same side of the channel as the leading Kelvin wave. Comparisons of equivalent cases with different Schmidt numbers indicate that while adopting a numerically advantageous low Schmidt number results in the correct general characteristics of the Kelvin waves, excessive diffusion of the pycnocline and various density features precludes accurate representation of both the trailing Poincaré wave field and the intensity and duration of the Kelvin-Helmholtz instabilities.

Baroclinic instability in the solar tachocline

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

Gilman, Peter; Dikpati, Mausumi, E-mail: gilman@ucar.edu, E-mail: dikpati@ucar.edu

2014-05-20

The solar tachocline is likely to be close to a geostrophic 'thermal wind', for which the Coriolis force associated with differential rotation is closely balanced by a latitudinal pressure gradient, leading to a tight relation between the vertical gradient of rotation and the latitudinal entropy gradient. Using a hydrostatic but nongeostrophic spherical shell model, we examine baroclinic instability of the tachocline thermal wind. We find that both the overshoot and radiative parts of the tachocline should be baroclinicly unstable at most latitudes. Growth rates are roughly five times higher in middle and high latitudes compared to low latitudes, and muchmore » higher in the overshoot than in the radiative tachocline. They range in e-folding amplification from 10 days in the high latitude overshoot tachocline, down to 20 yr for the low latitude radiative tachocline. In the radiative tachocline only, longitudinal wavenumbers m = 1, 2 are unstable, while in the overshoot tachocline a much broader range of m are unstable. At all latitudes and with all stratifications, the longitudinal scale of the most unstable mode is comparable to the Rossby deformation radius, while the growth rate is set by the local latitudinal entropy gradient. Baroclinic instability in the tachocline competing with instability of the latitude rotation gradient established in earlier studies should be important for the workings of the solar dynamo and should be expected to be found in most stars that contain an interface between radiative and convective domains.« less

Wrinkle-to-fold transition in soft layers under equi-biaxial strain: A weakly nonlinear analysis

NASA Astrophysics Data System (ADS)

Ciarletta, P.

2014-12-01

Soft materials can experience a mechanical instability when subjected to a finite compression, developing wrinkles which may eventually evolve into folds or creases. The possibility to control the wrinkling network morphology has recently found several applications in many developing fields, such as scaffolds for biomaterials, stretchable electronics and surface micro-fabrication. Albeit much is known of the pattern initiation at the linear stability order, the nonlinear effects driving the pattern selection in soft materials are still unknown. This work aims at investigating the nature of the elastic bifurcation undertaken by a growing soft layer subjected to a equi-biaxial strain. Considering a skin effect at the free surface, the instability thresholds are found to be controlled by a characteristic length, defined by the ratio between capillary energy and bulk elasticity. For the first time, a weakly nonlinear analysis of the wrinkling instability is performed here using the multiple-scale perturbation method applied to the incremental theory in finite elasticity. The Ginzburg-Landau equations are derived for different superposing linear modes. This study proves that a subcritical pitchfork bifurcation drives the observed wrinkle-to-fold transition in swelling gels experiments, favoring the emergence of hexagonal creased patterns, albeit quasi-hexagonal patterns might later emerge because of an expected symmetry break. Moreover, if the surface energy is somewhat comparable to the bulk elastic energy, it has the same stabilizing effect as for fluid instabilities, driving the formation of stable wrinkles, as observed in elastic bi-layered materials.

The HyperV Full-Scale Contoured-Gap Coaxial Plasma Railgun

NASA Astrophysics Data System (ADS)

Brockington, Samuel; Case, Andrew; Messer, Sarah; Bomgardner, Richard; Elton, Raymond; Wu, Linchun; Witherspoon, F. Douglas

2009-11-01

HyperV has been developing pulsed plasma injected coaxial railguns with a contoured gap profile designed to mitigate the blowby instability. Previous work using half-scale guns has been successful in launching 150 μg plasmas at 90 km/s [1]. In order to meet the original goal of 200 μg at 200 km/s the full-scale coaxial plasma gun has been constructed, and initial testing is beginning. This new plasma gun consists of two machined aluminum electrodes and a UHMW polyethylene breech insulator. The gun is breech fed by 64 ablative polyethylene capillary discharge units identical to the half-scale gun units. Maximum accelerator energy storage has also been increased 50%. Refractory coatings may be necessary to allow full current (˜800 kA) operation. The outer electrode includes 24 small diagnostic ports for optical and magnetic probe access to the plasma inside the gun to allow direct measurement of the plasma armature dynamics. Initial test data from the full-scale coax gun will be presented along with plans for future testing. Work supported by the U.S. DOE Office of Fusion Energy Sciences.[4pt] [1] F. D. Witherspoon, A. Case, S. Messer, R. Bomgardner, M. Phillips, S. Brockington, R. Elton, ``Contoured Gap Coaxial Plasma Gun with Injected Plasma Armature'' Rev. Sci. Instr. submitted (2009)

Higher-order modulation instability in nonlinear fiber optics.

PubMed

Erkintalo, Miro; Hammani, Kamal; Kibler, Bertrand; Finot, Christophe; Akhmediev, Nail; Dudley, John M; Genty, Goëry

2011-12-16

We report theoretical, numerical, and experimental studies of higher-order modulation instability in the focusing nonlinear Schrödinger equation. This higher-order instability arises from the nonlinear superposition of elementary instabilities, associated with initial single breather evolution followed by a regime of complex, yet deterministic, pulse splitting. We analytically describe the process using the Darboux transformation and compare with experiments in optical fiber. We show how a suitably low frequency modulation on a continuous wave field induces higher-order modulation instability splitting with the pulse characteristics at different phases of evolution related by a simple scaling relationship. We anticipate that similar processes are likely to be observed in many other systems including plasmas, Bose-Einstein condensates, and deep water waves. © 2011 American Physical Society

Superconductivity, phase separation, and charge-transfer instability in the U = infinity limit of the three-band model of the CuO sub 2 planes

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

Grilli, M.; Raimondi, R.; Castellani, C.

1991-07-08

The {ital U}={infinity} limit of the three-band Hubbard model with nearest-neighbor repulsion {ital V} is studied using the slave-boson approach and the large-{ital N} expansion technique to order 1/{ital N}. A charge-transfer instability is found as in weak-coupling theory. The charge-transfer instability is always associated with a diverging compressibility leading to a phase separation. Near the phase-separation, charge-transfer-instability region we find superconducting instabilities in the {ital s}- and {ital d}-wave channel. The requirement for superconductivity is that {ital V} be on the scale of the Cu-O hopping as suggested by Varma, Schmitt-Rink, and Abrahams.

On the parameterization of interleaving and turbulent mixing using CTD data from the Azores Frontal Zone

NASA Astrophysics Data System (ADS)

Kuzmina, N. P.

2000-01-01

CTD-data obtained in the Azores Frontal Zone using a towed undulating vehicle are analyzed to study the relationship between characteristics of intrusions and mean parameters of the thermohaline field. A self-similar dependence between intrusion intensity and hydrological parameters is obtained. The most well-founded interpretation of the empirical dependence is as follows: (a) the main source supporting intrusive layering is the salt finger convection; (b) the abrupt decrease of intrusion intensity with the reduction of geostrophic Richardson number obtained from the analysis is explained by the beginning of turbulence when salt fingers do not work any longer, so the "driving force" for intrusive motion disappears. These results are consistent with the conclusions of the paper [Kuzmina N.P., Rodionov V.B., 1992. About the influence of baroclinicity upon generation of the thermohaline intrusions in the oceanic frontal zones. Izvestiya Akad. Nauk SSSR, Atmosperic and Oceanic Physics 28 (10-11), 1077-1086]. These conclusions imply that there are three main mechanisms of intrusive layering at oceanic fronts, namely the 2D baroclinic instability of geostrophic flow, the vertical shear instability and the thermohaline instability where the driving source of intrusive motion is double diffusive convection. The baroclinic and thermohaline instabilities can generate intrusions of large vertical scale, while vertical shear instability usually gives rise to thin turbulent layers. Turbulence in these thin layers can prevent salt finger convection and thus destroy the energy source of the intrusive motion conditioned by thermoclinicity. Therefore, the baroclinicity plays two parts in the processes of the intrusive layering: (1) it prevents double-diffusion interleaving by means of turbulence, and (2) it generates intrusions due to the 2D baroclinic instability of geostrophic current. Using features of thermohaline interleaving as a specific tracer of turbulent mixing, we have estimated turbulent mixing coefficient as k t˜ Ri-0.8 ( Ri>1), where Ri is the geostrophic Richardson number. Application of the proposed approach to other frontal zones is discussed.

Radiation-induced genomic instability: radiation quality and dose response

NASA Technical Reports Server (NTRS)

Smith, Leslie E.; Nagar, Shruti; Kim, Grace J.; Morgan, William F.

2003-01-01

Genomic instability is a term used to describe a phenomenon that results in the accumulation of multiple changes required to convert a stable genome of a normal cell to an unstable genome characteristic of a tumor. There has been considerable recent debate concerning the importance of genomic instability in human cancer and its temporal occurrence in the carcinogenic process. Radiation is capable of inducing genomic instability in mammalian cells and instability is thought to be the driving force responsible for radiation carcinogenesis. Genomic instability is characterized by a large collection of diverse endpoints that include large-scale chromosomal rearrangements and aberrations, amplification of genetic material, aneuploidy, micronucleus formation, microsatellite instability, and gene mutation. The capacity of radiation to induce genomic instability depends to a large extent on radiation quality or linear energy transfer (LET) and dose. There appears to be a low dose threshold effect with low LET, beyond which no additional genomic instability is induced. Low doses of both high and low LET radiation are capable of inducing this phenomenon. This report reviews data concerning dose rate effects of high and low LET radiation and their capacity to induce genomic instability assayed by chromosomal aberrations, delayed lethal mutations, micronuclei and apoptosis.

Self-organized kilometer-scale shoreline sand wave generation: Sensitivity to model and physical parameters

NASA Astrophysics Data System (ADS)

Idier, Déborah; Falqués, Albert; Rohmer, Jérémy; Arriaga, Jaime

2017-09-01

The instability mechanisms for self-organized kilometer-scale shoreline sand waves have been extensively explored by modeling. However, while the assumed bathymetric perturbation associated with the sand wave controls the feedback between morphology and waves, its effect on the instability onset has not been explored. In addition, no systematic investigation of the effect of the physical parameters has been done yet. Using a linear stability model, we investigate the effect of wave conditions, cross-shore profile, closure depth, and two perturbation shapes (P1: cross-shore bathymetric profile shift, and P2: bed level perturbation linearly decreasing offshore). For a P1 perturbation, no instability occurs below an absolute critical angle θc0≈ 40-50°. For a P2 perturbation, there is no absolute critical angle: sand waves can develop also for low-angle waves. In fact, the bathymetric perturbation shape plays a key role in low-angle wave instability: such instability only develops if the curvature of the depth contours offshore the breaking zone is larger than the shoreline one. This can occur for the P2 perturbation but not for P1. The analysis of bathymetric data suggests that both curvature configurations could exist in nature. For both perturbation types, large wave angle, small wave period, and large closure depth strongly favor instability. The cross-shore profile has almost no effect with a P1 perturbation, whereas large surf zone slope and gently sloping shoreface strongly enhance instability under low-angle waves for a P2 perturbation. Finally, predictive statistical models are set up to identify sites prone to exhibit either a critical angle close to θc0 or low-angle wave instability.

Saturation of the Hosing Instability in Quasilinear Plasma Accelerators

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

Lehe, R.; Schroeder, C. B.; Vay, J. -L.

The beam hosing instability is analyzed theoretically for a witness beam in the quasilinear regime of plasma accelerators. In this regime, the hosing instability saturates, even for a monoenergetic bunch, at a level much less than standard scalings predict. Analytic expressions are derived for the saturation distance and amplitude and are in agreement with numerical results. Saturation is due to the natural head-to-tail variations in the focusing force, including the self-consistent transverse beam loading.

Saturation of the Hosing Instability in Quasilinear Plasma Accelerators

DOE PAGES

Lehe, R.; Schroeder, C. B.; Vay, J. -L.; ...

2017-12-13

The beam hosing instability is analyzed theoretically for a witness beam in the quasilinear regime of plasma accelerators. In this regime, the hosing instability saturates, even for a monoenergetic bunch, at a level much less than standard scalings predict. Analytic expressions are derived for the saturation distance and amplitude and are in agreement with numerical results. Saturation is due to the natural head-to-tail variations in the focusing force, including the self-consistent transverse beam loading.

Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter

NASA Astrophysics Data System (ADS)

Espinosa, J. R.; Racco, D.; Riotto, A.

2018-03-01

For the current central values of the Higgs boson and top quark masses, the standard model Higgs potential develops an instability at a scale of the order of 1 011 GeV . We show that a cosmological signature of such instability could be dark matter in the form of primordial black holes seeded by Higgs fluctuations during inflation. The existence of dark matter might not require physics beyond the standard model.

Direct Measurements of the Baroclinic Instability in the Ocean

NASA Astrophysics Data System (ADS)

Sadek, Mahmoud; Aluie, Hussein; Hecht, Matthew; Vallis, Geoffrey

2016-11-01

The ocean is mechanically driven by wind and buoyancy at the surface which produce sloping isopycnals with a reservoir of available potential energy (APE). Large scale APE can be converted to kinetic energy via the baroclinic instability, which produces mesoscale eddies. Mesoscale eddies are ubiquitous in mid- and high-latitudes, and play a primary role in determining the strength and trajectories of currents and in generating intrinsic climate variability. The widespread belief that mesoscale eddies are generated through baroclinic instability is based on general accord between observations and linear stability analysis and the predicted behavior of nonlinear models. However, these models are unable to give us quantitative evidence of the extent to which the instability is responsible for eddy generation at various locations in the ocean. To this end, we implement a new coarse-graining framework, recently developed to study flow on a sphere, to directly analyze the baroclinic instability as a function of scale and geographic location, and implement it using strongly eddying high-resolution simulations in the North Atlantic and in the Southern Ocean. The results give us new information about location and intensity of the instability in both physical and spectral space. Partial support was provided by National Science Foundation (NSF) Grant OCE-1259794, US Department of Energy (US DOE) Grant DE-SC0014318, and the LANL LDRD program through Project Number 20150568ER.

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

Beresnyak, Andrey; Li, Hui

Nonresonant current instability was identified by Bell as an important mechanism for magnetic field amplification in supernova remnants. In this paper we focus on studying the nonlinear stage of this instability using the incompressible MHD formulation. We demonstrate that the evolution of magnetic turbulence driven by the Bell instability resembles turbulence driven on large scales. More importantly, we demonstrate that the energy-containing scale for magnetic fields is proportional to the square root of the magnetic energy density. Given the observational constraints of the possible field amplification, this new relation allows us to directly estimate the maximum energy of particles scatteredmore » by such fields, and this estimate is normally below the average particle energy. This implies that, without taking into account the feedback to cosmic rays (CRs), the typical scales of Bell fields, in either the linear or nonlinear regime, will be too small to affect high-energy particle acceleration. We mention several scenarios of back reaction to CRs that could be important.« less

Vortex/Flame Interactions in Microgravity Pulsed Jet Diffusion Flames

NASA Technical Reports Server (NTRS)

Bahadori, M. Y.; Hegde, U.; Stocker, D. P.

1999-01-01

The problem of vortex/flame interaction is of fundamental importance to turbulent combustion. These interactions have been studied in normal gravity. It was found that due to the interactions between the imposed disturbances and buoyancy induced instabilities, several overall length scales dominated the flame. The problem of multiple scales does not exist in microgravity for a pulsed laminar flame, since there are no buoyancy induced instabilities. The absence of buoyant convection therefore provides an environment to study the role of vortices interacting with flames in a controlled manner. There are strong similarities between imposed and naturally occurring perturbations, since both can be described by the same spatial instability theory. Hence, imposing a harmonic disturbance on a microgravity laminar flame creates effects similar to those occurring naturally in transitional/turbulent diffusion flames observed in microgravity. In this study, controlled, large-scale, axisymmetric vortices are imposed on a microgravity laminar diffusion flame. The experimental results and predictions from a numerical model of transient jet diffusion flames are presented and the characteristics of pulsed flame are described.

Extension of continental lithosphere - A model for two scales of basin and range deformation

NASA Technical Reports Server (NTRS)

Zuber, M. T.; Parmentier, E. M.; Fletcher, R. C.

1986-01-01

The development of a model for deformation in an extending continental lithosphere that is stratified in density and strength is described. The lithosphere model demonstrates that the necking instabilities at two wavelengths originate due to a strong upper crust, a mantle layer, and a weak lower crust. It is observed that the dominant wavelengths of necking are controlled by layer thickness and the strength of the layers control the amplitude of the instabilities. The model is applied to the Basin and Range Province of the western U.S. where deformations in ranges and tile domains are detected. The relation between the Bouguer gravity anomaly and the deformations is studied. The data reveal that the horizontal scale of short wavelength necking correlates with the spacings of individual basins and ranges, and the longer wavelength corresponds to the width of tilt domains. The control of the Basin and Range deformation by two scales of extensional instability is proposed.

Turbulent mass transfer caused by vortex induced reconnection in collisionless magnetospheric plasmas.

PubMed

Nakamura, T K M; Hasegawa, H; Daughton, W; Eriksson, S; Li, W Y; Nakamura, R

2017-11-17

Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth's magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic shear is too small for spontaneous reconnection, the Kelvin-Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin-Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed by the Magnetospheric Multiscale (MMS) spacecraft. Here, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin-Helmholtz instability.

Nonlinear excitation of the ablative Rayleigh-Taylor instability for all wave numbers

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

Zhang, H.; Betti, R.; Gopalaswamy, V.

Small-scale perturbations in the ablative Rayleigh-Taylor instability (ARTI) are often neglected because they are linearly stable when their wavelength is shorter than a linear cutoff. Using 2D and 3D numerical simulations, it is shown that linearly stable modes of any wavelength can be destabilized. This instability regime requires finite amplitude initial perturbations and linearly stable ARTI modes are more easily destabilized in 3D than in 2D. In conclusion, it is shown that for conditions found in laser fusion targets, short wavelength ARTI modes are more efficient at driving mixing of ablated material throughout the target since the nonlinear bubble densitymore » increases with the wave number and small scale bubbles carry a larger mass flux of mixed material.« less

Granular Rayleigh-Taylor instability

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

Vinningland, Jan Ludvig; Johnsen, Oistein; Flekkoey, Eirik G.

2009-06-18

A granular instability driven by gravity is studied experimentally and numerically. The instability arises as grains fall in a closed Hele-Shaw cell where a layer of dense granular material is positioned above a layer of air. The initially flat front defined by the grains subsequently develops into a pattern of falling granular fingers separated by rising bubbles of air. A transient coarsening of the front is observed right from the start by a finger merging process. The coarsening is later stabilized by new fingers growing from the center of the rising bubbles. The structures are quantified by means of Fouriermore » analysis and quantitative agreement between experiment and computation is shown. This analysis also reveals scale invariance of the flow structures under overall change of spatial scale.« less

On Instability of Geostrophic Current with Linear Vertical Shear at Length Scales of Interleaving

NASA Astrophysics Data System (ADS)

Kuzmina, N. P.; Skorokhodov, S. L.; Zhurbas, N. V.; Lyzhkov, D. A.

2018-01-01

The instability of long-wave disturbances of a geostrophic current with linear velocity shear is studied with allowance for the diffusion of buoyancy. A detailed derivation of the model problem in dimensionless variables is presented, which is used for analyzing the dynamics of disturbances in a vertically bounded layer and for describing the formation of large-scale intrusions in the Arctic basin. The problem is solved numerically based on a high-precision method developed for solving fourth-order differential equations. It is established that there is an eigenvalue in the spectrum of eigenvalues that corresponds to unstable (growing with time) disturbances, which are characterized by a phase velocity exceeding the maximum velocity of the geostrophic flow. A discussion is presented to explain some features of the instability.

Studies of small-scale plasma inhomogeneities in the cusp ionosphere using sounding rocket data

NASA Astrophysics Data System (ADS)

Chernyshov, Alexander A.; Spicher, Andres; Ilyasov, Askar A.; Miloch, Wojciech J.; Clausen, Lasse B. N.; Saito, Yoshifumi; Jin, Yaqi; Moen, Jøran I.

2018-04-01

Microprocesses associated with plasma inhomogeneities are studied on the basis of data from the Investigation of Cusp Irregularities (ICI-3) sounding rocket. The ICI-3 rocket is devoted to investigating a reverse flow event in the cusp F region ionosphere. By numerical stability analysis, it is demonstrated that inhomogeneous-energy-density-driven (IEDD) instability can be a mechanism for the excitation of small-scale plasma inhomogeneities. The Local Intermittency Measure (LIM) method also applied the rocket data to analyze irregular structures of the electric field during rocket flight in the cusp. A qualitative agreement between high values of the growth rates of the IEDD instability and the regions with enhanced LIM is observed. This suggests that IEDD instability is connected to turbulent non-Gaussian processes.

Nonlinear excitation of the ablative Rayleigh-Taylor instability for all wave numbers

DOE PAGES

Zhang, H.; Betti, R.; Gopalaswamy, V.; ...

2018-01-16

Small-scale perturbations in the ablative Rayleigh-Taylor instability (ARTI) are often neglected because they are linearly stable when their wavelength is shorter than a linear cutoff. Using 2D and 3D numerical simulations, it is shown that linearly stable modes of any wavelength can be destabilized. This instability regime requires finite amplitude initial perturbations and linearly stable ARTI modes are more easily destabilized in 3D than in 2D. In conclusion, it is shown that for conditions found in laser fusion targets, short wavelength ARTI modes are more efficient at driving mixing of ablated material throughout the target since the nonlinear bubble densitymore » increases with the wave number and small scale bubbles carry a larger mass flux of mixed material.« less

Investigation of Natural Circulation Instability and Transients in Passively Safe Small Modular Reactors

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

Ishii, Mamoru

The NEUP funded project, NEUP-3496, aims to experimentally investigate two-phase natural circulation flow instability that could occur in Small Modular Reactors (SMRs), especially for natural circulation SMRs. The objective has been achieved by systematically performing tests to study the general natural circulation instability characteristics and the natural circulation behavior under start-up or design basis accident conditions. Experimental data sets highlighting the effect of void reactivity feedback as well as the effect of power ramp-up rate and system pressure have been used to develop a comprehensive stability map. The safety analysis code, RELAP5, has been used to evaluate experimental results andmore » models. Improvements to the constitutive relations for flashing have been made in order to develop a reliable analysis tool. This research has been focusing on two generic SMR designs, i.e. a small modular Simplified Boiling Water Reactor (SBWR) like design and a small integral Pressurized Water Reactor (PWR) like design. A BWR-type natural circulation test facility was firstly built based on the three-level scaling analysis of the Purdue Novel Modular Reactor (NMR) with an electric output of 50 MWe, namely NMR-50, which represents a BWR-type SMR with a significantly reduced reactor pressure vessel (RPV) height. The experimental facility was installed with various equipment to measure thermalhydraulic parameters such as pressure, temperature, mass flow rate and void fraction. Characterization tests were performed before the startup transient tests and quasi-steady tests to determine the loop flow resistance. The control system and data acquisition system were programmed with LabVIEW to realize the realtime control and data storage. The thermal-hydraulic and nuclear coupled startup transients were performed to investigate the flow instabilities at low pressure and low power conditions for NMR-50. Two different power ramps were chosen to study the effect of startup power density on the flow instability. The experimental startup transient results showed the existence of three different flow instability mechanisms, i.e., flashing instability, condensation induced flow instability, and density wave oscillations. In addition, the void-reactivity feedback did not have significant effects on the flow instability during the startup transients for NMR-50. ii Several initial startup procedures with different power ramp rates were experimentally investigated to eliminate the flow instabilities observed from the startup transients. Particularly, the very slow startup transient and pressurized startup transient tests were performed and compared. It was found that the very slow startup transients by applying very small power density can eliminate the flashing oscillations in the single-phase natural circulation and stabilize the flow oscillations in the phase of net vapor generation. The initially pressurized startup procedure was tested to eliminate the flashing instability during the startup transients as well. The pressurized startup procedure included the initial pressurization, heat-up, and venting process. The startup transient tests showed that the pressurized startup procedure could eliminate the flow instability during the transition from single-phase flow to two-phase flow at low pressure conditions. The experimental results indicated that both startup procedures were applicable to the initial startup of NMR. However, the pressurized startup procedures might be preferred due to short operating hours required. In order to have a deeper understanding of natural circulation flow instability, the quasi-steady tests were performed using the test facility installed with preheater and subcooler. The effect of system pressure, core inlet subcooling, core power density, inlet flow resistance coefficient, and void reactivity feedback were investigated in the quasi-steady state tests. The experimental stability boundaries were determined between unstable and stable flow conditions in the dimensionless stability plane of inlet subcooling number and Zuber number. To predict the stability boundary theoretically, linear stability analysis in the frequency domain was performed at four sections of the natural circulation test loop. The flashing phenomena in the chimney section was considered as an axially uniform heat source. And the dimensionless characteristic equation of the pressure drop perturbation was obtained by considering the void fraction effect and outlet flow resistance in the core section. The theoretical flashing boundary showed some discrepancies with previous experimental data from the quasi-steady state tests. In the future, thermal non-equilibrium was recommended to improve the accuracy of flashing instability boundary. As another part of the funded research, flow instabilities of a PWR-type SMR under low pressure and low power conditions were investigated experimentally as well. The NuScale reactor design was selected as the prototype for the PWR-type SMR. In order to experimentally study the natural circulation behavior of NuScale iii reactor during accidental scenarios, detailed scaling analyses are necessary to ensure that the scaled phenomena could be obtained in a laboratory test facility. The three-level scaling method is used as well to obtain the scaling ratios derived from various non-dimensional numbers. The design of the ideally scaled facility (ISF) was initially accomplished based on these scaling ratios. Then the engineering scaled facility (ESF) was designed and constructed based on the ISF by considering engineering limitations including laboratory space, pipe size, and pipe connections etc. PWR-type SMR experiments were performed in this well-scaled test facility to investigate the potential thermal hydraulic flow instability during the blowdown events, which might occur during the loss of coolant accident (LOCA) and loss of heat sink accident (LOHS) of the prototype PWR-type SMR. Two kinds of experiments, normal blowdown event and cold blowdown event, were experimentally investigated and compared with code predictions. The normal blowdown event was experimentally simulated since an initial condition where the pressure was lower than the designed pressure of the experiment facility, while the code prediction of blowdown started from the normal operation condition. Important thermal hydraulic parameters including reactor pressure vessel (RPV) pressure, containment pressure, local void fraction and temperature, pressure drop and natural circulation flow rate were measured and analyzed during the blowdown event. The pressure and water level transients are similar to the experimental results published by NuScale [51], which proves the capability of current loop in simulating the thermal hydraulic transient of real PWR-type SMR. During the 20000s blowdown experiment, water level in the core was always above the active fuel assemble during the experiment and proved the safety of natural circulation cooling and water recycling design of PWR-type SMR. Besides, pressure, temperature, and water level transient can be accurately predicted by RELAP5 code. However, the oscillations of natural circulation flow rate, water level and pressure drops were observed during the blowdown transients. This kind of flow oscillations are related to the water level and the location upper plenum, which is a path for coolant flow from chimney to steam generator and down comer. In order to investigate the transients start from the opening of ADS valve in both experimental and numerical way, the cold blow-down experiment is conducted. For the cold blowdown event, different from setting both reactor iv pressure vessel (RPV) and containment at high temperature and pressure, only RPV was heated close to the highest designed pressure and then open the ADS valve, same process was predicted using RELAP5 code. By doing cold blowdown experiment, the entire transients from the opening of ADS can be investigated by code and benchmarked with experimental data. Similar flow instability observed in the cold blowdown experiment. The comparison between code prediction and experiment data showed that the RELAP5 code can successfully predict the pressure void fraction and temperature transient during the cold blowdown event with limited error, but numerical instability exists in predicting natural circulation flow rate. Besides, the code is lack of capability in predicting the water level related flow instability observed in experiments.« less

Large-scale circulation patterns, instability factors and global precipitation modeling as influenced by external forcing

NASA Astrophysics Data System (ADS)

Bundel, A.; Kulikova, I.; Kruglova, E.; Muravev, A.

2003-04-01

The scope of the study is to estimate the relationship between large-scale circulation regimes, various instability indices and global precipitation with different boundary conditions, considered as external forcing. The experiments were carried out in the ensemble-prediction framework of the dynamic-statistical monthly forecast scheme run in the Hydrometeorological Research Center of Russia every ten days. The extension to seasonal intervals makes it necessary to investigate the role of slowly changing boundary conditions among which the sea surface temperature (SST) may be defined as the most effective factor. Continuous integrations of the global spectral T41L15 model for the whole year 2000 (starting from January 1) were performed with the climatic SST and the Reynolds Archive SSTs. Monthly values of the SST were projected on the year days using spline interpolation technique. First, the global precipitation values in experiments were compared to the GPCP (Global Precipitation Climate Program) daily observation data. Although the global mean precipitation is underestimated by the model, some large-scale regional amounts correspond to the real ones (e.g. for Europe) fairly well. On the whole, however, anomaly phases failed to be reproduced. The precipitation averaged over the whole land revealed a greater sensitivity to the SSTs than that over the oceans. The wavelet analysis was applied to separate the low- and high-frequency signal of the SST influence on the large-scale circulation and precipitation. A derivative of the Wallace-Gutzler teleconnection index for the East-Atlantic oscillation was taken as the circulation characteristic. The daily oscillation index values and precipitation amounts averaged over Europe were decomposed using wavelet approach with different “mother wavelets” up to approximation level 3. It was demonstrated that an increase in the precipitation amount over Europe was associated with the zonal flow intensification over the Northern Atlantic when the real SSTs were used. Blocking structures in the circulation caused decreasing precipitation amounts. The wavelet approach gave a more distinctive discrimination in the modeled circulation and precipitation patterns versus different external forcing than a number of other statistical techniques. Several atmospheric instability indices (e.g. the Phillips like parameters, Richardson number etc) were additionally used in post-processing for a more detailed validation of the modeled large-scale and total precipitation amounts. It was shown that a reasonable variety of instability indices must be used for such validations and for precipitation output corrections. Their statistical stability may be substantiated only on the ensemble modeling basis. This work was performed with the financial support of the Russian Foundation for Basic Research (02-05-64655).

Parametric Instability Rates in Periodically Driven Band Systems

NASA Astrophysics Data System (ADS)

Lellouch, S.; Bukov, M.; Demler, E.; Goldman, N.

2017-04-01

In this work, we analyze the dynamical properties of periodically driven band models. Focusing on the case of Bose-Einstein condensates, and using a mean-field approach to treat interparticle collisions, we identify the origin of dynamical instabilities arising from the interplay between the external drive and interactions. We present a widely applicable generic numerical method to extract instability rates and link parametric instabilities to uncontrolled energy absorption at short times. Based on the existence of parametric resonances, we then develop an analytical approach within Bogoliubov theory, which quantitatively captures the instability rates of the system and provides an intuitive picture of the relevant physical processes, including an understanding of how transverse modes affect the formation of parametric instabilities. Importantly, our calculations demonstrate an agreement between the instability rates determined from numerical simulations and those predicted by theory. To determine the validity regime of the mean-field analysis, we compare the latter to the weakly coupled conserving approximation. The tools developed and the results obtained in this work are directly relevant to present-day ultracold-atom experiments based on shaken optical lattices and are expected to provide an insightful guidance in the quest for Floquet engineering.

Plasma Irregularity Production in the Polar Cap F-Region Ionosphere

NASA Astrophysics Data System (ADS)

Lamarche, Leslie

Plasma in the Earth's ionosphere is highly irregular on scales ranging between a few centimeters and hundreds of kilometers. Small-scale irregularities or plasma waves can scatter radio waves resulting in a loss of signal for navigation and communication networks. The polar region is particularly susceptible to strong disturbances due to its direct connection with the Sun's magnetic field and energetic particles. In this thesis, factors that contribute to the production of decameter-scale plasma irregularities in the polar F region ionosphere are investigated. Both global and local control of irregularity production are studied, i.e. we consider global solar control through solar illumination and solar wind as well as much more local control by plasma density gradients and convection electric field. In the first experimental study, solar control of irregularity production is investigated using the Super Dual Auroral Radar Network (SuperDARN) radar at McMurdo, Antarctica. The occurrence trends for irregularities are analyzed statistically and a model is developed that describes the location of radar echoes within the radar's field-of-view. The trends are explained through variations in background plasma density with solar illumination affecting radar beam propagation. However, it is found that the irregularity occurrence during the night is higher than expected from ray tracing simulations based on a standard ionospheric density model. The high occurrence at night implies an additional source of plasma density and it is proposed that large-scale density enhancements called polar patches may be the source of this density. Additionally, occurrence maximizes around the terminator due to different competing irregularity production processes that favor a more or less sunlit ionosphere. The second study is concerned with modeling irregularity characteristics near a large-scale density gradient reversal, such as those expected near polar patches, with a particular focus on the asymmetry of the irregularity growth rate across the gradient reversal. Directional dependencies on the plasma density gradient, plasma drift, and wavevector are analyzed in the context of the recently developed general fluid theory of the gradient-drift instability. In the ionospheric F region, the strongest asymmetry is found when an elongated structure is oriented along the radar's boresight and moving perpendicular to its direction of elongation. These results have important implications for finding optimal configurations for oblique-scanning ionospheric radars such as SuperDARN to observe gradient reversals. To test the predictions of the developed model and the general theory of the gradient-drift instability, an experimental investigation is presented focusing on decameter-scale irregularities near a polar patch and the previously uninvestigated directional dependence of irregularity characteristics. Backscatter power and occurrence of irregularities are analyzed using measurements from the SuperDARN radar at Rankin Inlet, Canada, while background density gradients and convection electric fields are found from the north face of the Resolute Bay Incoherent Scatter Radar. It is shown that irregularity occurrence tends to follow the expected trends better than irregularity power, suggesting that while the gradient-drift instability may be a dominant process in generating small-scale irregularities, other mechanisms such as a shear-driven instability or nonlinear process may exert greater control over their intensity. It is concluded from this body of work that the production of small-scale plasma irregularities in the polar F-region ionosphere is controlled both by global factors such as solar illumination as well as local plasma density gradients and electric fields. In general, linear gradient-drift instability theory describes small-scale irregularity production well, particularly for low-amplitude perturbations. The production of irregularities is complex, and while ground-based radars are invaluable tools to study the ionosphere, care must be taken to interpret results correctly.

Gravitational Effects on Flow Instability and Transition in Low Density Jets

NASA Technical Reports Server (NTRS)

Agrawal A. K.; Parthasarathy, K.; Pasumarthi, K.; Griffin, D. W.

2000-01-01

Recent experiments have shown that low-density gas jets injected into a high-density ambient gas undergo an instability mode, leading to highly-periodic oscillations in the flow-field for certain conditions. The transition from laminar to turbulent flow in these jets is abrupt, without the gradual change in scales. Even the fine scale turbulent structure repeats itself with extreme regularity from cycle to cycle. Similar observations were obtained in buoyancy-dominated and momentum-dominated jets characterized by the Richardson numbers, Ri = [gD(rho(sub a)-rho(sub j))/rho(sub j)U(sub j)(exp 2) ] where g is the gravitational acceleration, D is the jet diameter, rho(sub a) and rho(sub a) are, respectively, the free-stream and jet densities, and U(sub j) is the mean jet exit velocity. At high Richardson numbers, the instability is presumably caused by buoyancy since the flow-oscillation frequency (f) or the Strouhal number, St = [fD/U(sub j)] scales with Ri. In momentum-dominated jets, however, the Strouhal number of the oscillating flow is relatively independent of the Ri. In this case, a local absolute instability is predicted in the potential core of low-density jets with S [= rho(sub j)/rho(sub a)] < 0.7, which agrees qualitatively with experiments. Although the instability in gas jets of high Richardson numbers is attributed to buoyancy, direct physical evidence has not been acquired in experiments. If the instability is indeed caused by buoyancy, the near-field flow structure of the jet will change significantly when the buoyancy is removed, for example, in the microgravity environment. Thus, quantitative data on the spatial and temporal evolutions of the instability, length and time scale of the oscillating mode and its effects on the mean flow and breakdown of the potential core are needed in normal and microgravity to delineate gravitational effects in buoyant jets. In momentum dominated low-density jets, the instability is speculated to originate in the potential core. However, experiments have not succeeded in identifying the direct physical cause of the instability. For example, the theory predicts an oscillating mode for S<0.62 in the limit of zero momentum thickness, which contradicts with the experimental findings of Kyle and Sreenivasan. The analyses of momentum-dominated jets neglect buoyancy effects because of the small Richardson number. Although this assumption is appropriate in the potential core, the gravitational effects are important in the annular region surrounding the jet, where the density and velocity gradients are large. This reasoning provides basis for the hypothesis that the instability in low Richardosn number jets studied by Kyle and Sreenivasan and Monkewitz et al. is caused by buoyancy. The striking similarity in characteristics of the instability and virtually the identical conclusions reached by Subbarao and Cantwell in buoyant (Ri>0.5) helium jets on one hand and by Kyle and Sreenivasan in momentum-dominated (Ri<1x10(exp -3)) helium jets on the other support this hypothesis. However, quantitative experiments in normal and microgravity are necessary to obtain direct physical evidence of buoyancy effects on the flow instability and structure of momentum-dominated low-density jets. The primary objective of this new research project is to quantify how buoyancy affects the flow instability and structure in the near field of low-density jets. The flow will be described by the spatial and temporal evolutions of the instability, length and time scales of the oscillating mode, and the mean and fluctuating concentration fields. To meet this objective, concentration measurements will be obtained across the whole field using quantitative Rainbow Schlieren Deflectometry, providing spatial resolution of 0.1mm and temporal resolution of 0.017s to 1ms. The experimental effort will be supplemented with linear stability analysis of low-density jets by considering buoyancy. The first objective of this research is to investigate the effects of gravity on the flow instability and structure of low-density jets. The flow instability in these jets has been attributed to buoyancy. By removing buoyancy in our experiments, we seek to obtain the direct physical evidence of the instability mechanism. In the absence of the instability, the flow structure will undergo a significant change. We seek to quantify these changes by mapping the flow field (in terms of the concentration profiles) of these jets at non-buoyant conditions. Such information is presently lacking in the existing literature. The second objective of this research is to determine if the instability in momentum-driven, low-density jets is caused by buoyancy. At these conditions, the buoyancy effects are commonly ignored because of the small Richardson based on global parameters. By eliminating buoyancy in our experiments, globally as well as locally, we seek to examine the possibility that the instability mechanism in self-excited, buoyant or momentum-driven jets is the same. To meet this objective, we would quantify the jet flow in normal and microgravity, while systematically decreasing the Richardson number from buoyancy-driven to momentum driven flow regime. The third objective of this research is to perform a linear stability analysis of low-density gas jets by including the gravitational effects. The flow oscillations in these jets are attributed to an absolute instability, whereby the disturbance grows exponentially at the site to ultimately contaminate the entire flow field. We seek to study the characteristics of both convective and absolute instabilities and demarcate the boundary between them.

What drives gravitational instability in nearby star-forming spirals? The impact of CO and H I velocity dispersions

NASA Astrophysics Data System (ADS)

Romeo, Alessandro B.; Mogotsi, Keoikantse Moses

2017-07-01

The velocity dispersion of cold interstellar gas, σ, is one of the quantities that most radically affect the onset of gravitational instabilities in galaxy discs, and the quantity that is most drastically approximated in stability analyses. Here we analyse the stability of a large sample of nearby star-forming spirals treating molecular gas, atomic gas and stars as three distinct components, and using radial profiles of σCO and σ _{H I} derived from HERA CO-Line Extragalactic Survey (HERACLES) and The H I Nearby Galaxy Survey (THINGS) observations. We show that the radial variations of σCO and σ _{H I} have a weak effect on the local stability level of galaxy discs, which remains remarkably flat and well above unity, but is low enough to ensure (marginal) instability against non-axisymmetric perturbations and gas dissipation. More importantly, the radial variation of σCO has a strong impact on the size of the regions over which gravitational instabilities develop, and results in a characteristic instability scale that is one order of magnitude larger than the Toomre length of molecular gas. Disc instabilities are driven, in fact, by the self-gravity of stars at kiloparsec scales. This is true across the entire optical disc of every galaxy in the sample, with a few exceptions. In the linear phase of the disc-instability process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas.

Formation of matter-wave soliton trains by modulational instability

NASA Astrophysics Data System (ADS)

Nguyen, Jason H. V.; Luo, De; Hulet, Randall G.

2017-04-01

Nonlinear systems can exhibit a rich set of dynamics that are inherently sensitive to their initial conditions. One such example is modulational instability, which is believed to be one of the most prevalent instabilities in nature. By exploiting a shallow zero-crossing of a Feshbach resonance, we characterize modulational instability and its role in the formation of matter-wave soliton trains from a Bose-Einstein condensate. We examine the universal scaling laws exhibited by the system and, through real-time imaging, address a long-standing question of whether the solitons in trains are created with effectively repulsive nearest-neighbor interactions or rather evolve into such a structure.

Onset of dissolution-driven instabilities in fluids with nonmonotonic density profile

NASA Astrophysics Data System (ADS)

Jafari Raad, Seyed Mostafa; Hassanzadeh, Hassan

2015-11-01

Analog systems have recently been used in several experiments in the context of convective mixing of C O2 . We generalize the nonmonotonic density dependence of the growth of instabilities and provide a scaling relation for the onset of instability. The results of linear stability analysis and direct numerical simulations show that these fluids do not resemble the dynamics of C O2 -water convective instabilities. A typical analog system, such as water-propylene glycol, is found to be less unstable than C O2 -water. These results provide a basis for further research and proper selection of analog systems and are essential to the interpretation of experiments.

Common origin of kinetic scale turbulence and the electron halo in the solar wind – Connection to nanoflares

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

Che, Haihong; Goddard Space Flight Center, NASA, Greenbelt, MD, 20771

2016-03-25

We summarize our recent studies on the origin of solar wind kinetic scale turbulence and electron halo in the electron velocity distribution function. Increasing observations of nanoflares and microscopic type III radio bursts strongly suggest that nanoflares and accelerated electron beams are common in the corona. Based on particle-in-cell simulations, we show that both the core-halo feature and kinetic scale turbulence observed in the solar wind can be produced by the nonlinear evolution of electron two-stream instability driven by nanoflare accelerated electron beams. The energy exchange between waves and particles reaches equilibrium in the inner corona and the key featuresmore » of the turbulence and velocity distribution are preserved as the solar wind escapes into interplanetary space along open magnetic field lines. Observational tests of the model and future theoretical work are discussed.« less

Overview of the FuZE Fusion Z-Pinch Experiment

NASA Astrophysics Data System (ADS)

Shumlak, U.; Nelson, B. A.; Claveau, E. L.; Forbes, E. G.; Golingo, R. P.; Stepanov, A. D.; Weber, T. R.; Zhang, Y.; McLean, H. S.; Higginson, D. P.; Schmidt, A.; Tummel, K. K.

2017-10-01

Successful results of the sheared flow stabilized (SFS) Z-pinch from ZaP and ZaP-HD have motivated the new FuZE project to scale the plasma performance to fusion conditions. The SFS Z-pinch is immune to the instabilities that plague the conventional Z-pinch yet maintains the same favorable radial scaling. The plasma density and temperature increase rapidly with decreasing plasma radius, which naturally leads to a compact configuration at fusion conditions. The SFS Z-pinch is being investigated as a novel approach to a compact fusion device in a collaborative ARPA-E ALPHA project with the University of Washington and Lawrence Livermore National Laboratory. The project includes an experimental effort coupled with high-fidelity physics modeling using kinetic and fluid simulations. Along with scaling law analysis, computational and experimental results from the FuZE device are presented. This work is supported by an award from US ARPA-E.

Kinetic Interactions Between the Solar Wind and Lunar Magnetic Fields

NASA Astrophysics Data System (ADS)

Halekas, J. S.; Poppe, A. R.; Fatemi, S.; Turner, D. L.; Holmstrom, M.

2016-12-01

Despite their relatively weak strength, small scale, and incoherence, lunar magnetic anomalies can affect the incoming solar wind flow. The plasma interaction with lunar magnetic fields drives significant compressions of the solar wind plasma and magnetic field, deflections of the incoming flow, and a host of plasma waves ranging from the ULF to the electrostatic range. Recent work suggests that the large-scale features of the solar wind-magnetic anomaly interactions may be driven by ion-ion instabilities excited by reflected ions, raising the possibility that they are analogous to ion foreshock phenomena. Indeed, despite their small scale, many of the phenomena observed near lunar magnetic anomalies appear to have analogues in the foreshock regions of terrestrial planets. We discuss the charged particle distributions, fields, and waves observed near lunar magnetic anomalies, and place them in a context with the foreshocks of the Earth, Mars, and other solar system objects.

2D OR NOT 2D: THE EFFECT OF DIMENSIONALITY ON THE DYNAMICS OF FINGERING CONVECTION AT LOW PRANDTL NUMBER

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

Garaud, Pascale; Brummell, Nicholas

2015-12-10

Fingering convection (otherwise known as thermohaline convection) is an instability that occurs in stellar radiative interiors in the presence of unstable compositional gradients. Numerical simulations have been used in order to estimate the efficiency of mixing induced by this instability. However, fully three-dimensional (3D) computations in the parameter regime appropriate for stellar astrophysics (i.e., low Prandtl number) are prohibitively expensive. This raises the question of whether two-dimensional (2D) simulations could be used instead to achieve the same goals. In this work, we address this issue by comparing the outcome of 2D and 3D simulations of fingering convection at low Prandtlmore » number. We find that 2D simulations are never appropriate. However, we also find that the required 3D computational domain does not have to be very wide: the third dimension only needs to contain a minimum of two wavelengths of the fastest-growing linearly unstable mode to capture the essentially 3D dynamics of small-scale fingering. Narrow domains, however, should still be used with caution since they could limit the subsequent development of any large-scale dynamics typically associated with fingering convection.« less

Pair-instability supernovae of fast rotating stars

NASA Astrophysics Data System (ADS)

Chen, Ke-Jung

2015-01-01

We present 2D simulations of pair-instability supernovae considering rapid rotation during their explosion phases. Recent studies of the Population III (Pop III) star formation suggested that these stars could be born with a mass scale about 100 M⊙ and with a strong rotation. Based on stellar evolution models, these massive Pop III stars might have died as highly energetic pair-instability supernovae. We perform 2D calculations to investigate the impact of rotation on pair-instability supernovae. Our results suggest that rotation leads to an aspherical explosion due to an anisotropic collapse. If the first stars have a 50% of keplerian rotational rate of the oxygen core before their pair-instability explosions, the overall 56Ni production can be significantly reduced by about two orders of magnitude. An extreme case of 100% keplerian rotational rate shows an interesting feature of fluid instabilities along the equatorial plane caused by non-synchronized and non-isotropic ignitions of explosions, so that the shocks run into the in-falling gas and generate the Richtmyer-Meshkov instability.

Secondary instability in boundary-layer flows

NASA Technical Reports Server (NTRS)

Nayfeh, A. H.; Bozatli, A. N.

1979-01-01

The stability of a secondary Tollmien-Schlichting wave, whose wavenumber and frequency are nearly one half those of a fundamental Tollmien-Schlichting instability wave is analyzed using the method of multiple scales. Under these conditions, the fundamental wave acts as a parametric exciter for the secondary wave. The results show that the amplitude of the fundamental wave must exceed a critical value to trigger this parametric instability. This value is proportional to a detuning parameter which is the real part of k - 2K, where k and K are the wavenumbers of the fundamental and its subharmonic, respectively. For Blasius flow, the critical amplitude is approximately 29% of the mean flow, and hence many other secondary instabilities take place before this parametric instability becomes significant. For other flows where the detuning parameter is small, such as free-shear layer flows, the critical amplitude can be small, thus the parametric instability might play a greater role.

The stability of a thin water layer over a rotating disk revisited

NASA Astrophysics Data System (ADS)

Poncet, Sébastien

2014-08-01

The flow driven by a rotating disk of a thin fluid layer in a fixed cylindrical casing is studied by direct numerical simulation and experimental flow visualizations. The characteristics of the flow are first briefly discussed but the focus of this work is to understand the transition to the primary instability. The primary bifurcation is 3D and appears as spectacular sharp-cornered polygonal patterns located along the shroud. The stability diagram is established experimentally in a ( Re, G plane, where G is the aspect ratio of the cavity and Re the rotational Reynolds number and confirmed numerically. The number of vortices scales well with the Ekman number based on the water depth, which confirms the existence of a Stewartson layer along the external cylinder. The critical mixed Reynolds number is found to be constant as in other rotating flows involving a shear-layer instability. Hysteresis cycles are observed highlighting the importance of the spin-up and spin-down processes. In some particular cases, a crossflow instability appears under the form of high azimuthal wave number spiral patterns, similar to those observed in a rotor-stator cavity with throughflow and coexists with the polygons. The DNS calculations confirm the experimental results under the flat free surface hypothesis.

High-Strain-Rate Material Behavior and Adiabatic Material Instability in Impact of Micron-Scale Al-6061 Particles

NASA Astrophysics Data System (ADS)

Chen, Qiyong; Alizadeh, Arash; Xie, Wanting; Wang, Xuemei; Champagne, Victor; Gouldstone, Andrew; Lee, Jae-Hwang; Müftü, Sinan

2018-04-01

Impact of spherical particles onto a flat sapphire surface was investigated in 50-950 m/s impact speed range experimentally and theoretically. Material parameters of the bilinear Johnson-Cook model were determined based on comparison of deformed particle shapes from experiment and simulation. Effects of high-strain-rate plastic flow, heat generation due to plasticity, material damage, interfacial friction and heat transfer were modeled. Four distinct regions were identified inside the particle by analyzing temporal variation of material flow. A relatively small volume of material near the impact zone becomes unstable due to plasticity-induced heating, accompanied by severe drop in the flow stress for impact velocity that exceeds 500 m/s. Outside of this region, flow stress is reduced due to temperature effects without the instability. Load carrying capacity of the material degrades and the material expands horizontally leading to jetting. The increase in overall plastic and frictional dissipation with impact velocity was found to be inherently lower than the increase in the kinetic energy at high speeds, leading to the instability. This work introduces a novel method to characterize HSR (109 s-1) material properties and also explains coupling between HSR material behavior and mechanics that lead to extreme deformation.

Deep Landslides in flysch formations

NASA Astrophysics Data System (ADS)

Marinos, Vassilis

2017-04-01

Flysch, linked with the tectonic development of an area, has suffered from compressional forces being highly deformed by thrust faults and folds, containing thus often tectonically pre-sheared zones of various size. These geological characteristics may produce weak to very weak rock masses which may present instability and landslides in both mountain and local slope scale. The paper mainly discusses the "mountain" scale phenomena. The size of these masses can reach hundreds of meters in both depth and width on the valley sides. A brief presentation of the flysch formation is presented. A typology is presented with 11 types of flysch, depending on the persistence and participation or not of the strong members (as sandstones) against the weak ones (as siltstones, shales) and the degree and scale of tectonic disturbance. These rock mass types are connected with the landslide mechanism. In all cases the tectonic conditions of a broader area are responsible and the establishment of the tectonic-paleogeographic model is necessary before the conceptual study and design of any major infrastructure work and the choice of its alignment or location. Given the size of such instability areas remedial measures are in most cases not feasible and the realignment or relocation from the initial plans are often the only solution. Cases from highways and pipelines in Greek and Albanian territory are presented. A large number of information from lab tests, geotechnical classifications and back analyses collected from a wide variety of flysch formations is presented and discussed.

Grain scale observations of stick-slip dynamics in fluid saturated granular fault gouge

NASA Astrophysics Data System (ADS)

Johnson, P. A.; Dorostkar, O.; Guyer, R. A.; Marone, C.; Carmeliet, J.

2017-12-01

We are studying granular mechanics during slip. In the present work, we conduct coupled computational fluid dynamics (CFD) and discrete element method (DEM) simulations to study grain scale characteristics of slip instabilities in fluid saturated granular fault gouge. The granular sample is confined with constant normal load (10 MPa), and sheared with constant velocity (0.6 mm/s). This loading configuration is chosen to promote stick-slip dynamics, based on a phase-space study. Fluid is introduced in the beginning of stick phase and characteristics of slip events i.e. macroscopic friction coefficient, kinetic energy and layer thickness are monitored. At the grain scale, we monitor particle coordination number, fluid-particle interaction forces as well as particle and fluid kinetic energy. Our observations show that presence of fluids in a drained granular fault gouge stabilizes the layer in the stick phase and increases the recurrence time. In saturated model, we observe that average particle coordination number reaches higher values compared to dry granular gouge. Upon slip, we observe that a larger portion of the granular sample is mobilized in saturated gouge compared to dry system. We also observe that regions with high particle kinetic energy are correlated with zones of high fluid motion. Our observations highlight that spatiotemporal profile of fluid dynamic pressure affects the characteristics of slip instabilities, increasing macroscopic friction coefficient drop, kinetic energy release and granular layer compaction. We show that numerical simulations help characterize the micromechanics of fault mechanics.

Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter.

PubMed

Espinosa, J R; Racco, D; Riotto, A

2018-03-23

For the current central values of the Higgs boson and top quark masses, the standard model Higgs potential develops an instability at a scale of the order of 10^{11}  GeV. We show that a cosmological signature of such instability could be dark matter in the form of primordial black holes seeded by Higgs fluctuations during inflation. The existence of dark matter might not require physics beyond the standard model.

Baroclinic instability with variable static stability - A design study for a spherical atmospheric model experiment. [for Spacelab flight

NASA Technical Reports Server (NTRS)

Giere, A. C.; Fowlis, W. W.

1980-01-01

The effect of a radially-variable, dielectric body force, analogous to gravity on baroclinic instability for the design of a spherical, synoptic-scale, atmospheric model experiment in a Spacelab flight is investigated. Exact solutions are examined for quasi-geostrophic baroclinic instability in which the rotational Froude number is a linear function of the height. Flow in a rotating rectilinear channel with a vertically variable body force without horizontal shear of the basic state is also discussed.

Scalings of Alfvén-cyclotron and ion Bernstein instabilities on temperature anisotropy of a ring-like velocity distribution in the inner magnetosphere

DOE PAGES

Min, Kyungguk; Liu, Kaijun; Gary, S. Peter

2016-03-18

Here, a ring-like proton velocity distribution with ∂f p(v ⊥)/∂v ⊥>0 and which is sufficiently anisotropic can excite two distinct types of growing modes in the inner magnetosphere: ion Bernstein instabilities with multiple ion cyclotron harmonics and quasi-perpendicular propagation and an Alfvén-cyclotron instability at frequencies below the proton cyclotron frequency and quasi-parallel propagation. Recent particle-in-cell simulations have demonstrated that even if the maximum linear growth rate of the latter instability is smaller than the corresponding growth of the former instability, the saturation levels of the fluctuating magnetic fields can be greater for the Alfvén-cyclotron instability than for the ion Bernsteinmore » instabilities. In this study, linear dispersion theory and two-dimensional particle-in-cell simulations are used to examine scalings of the linear growth rate and saturation level of the two types of growing modes as functions of the temperature anisotropy T ⊥/T || for a general ring-like proton distribution with a fixed ring speed of 2v A, where v A is the Alfvén speed. For the proton distribution parameters chosen, the maximum linear theory growth rate of the Alfvén-cyclotron waves is smaller than that of the fastest-growing Bernstein mode for the wide range of anisotropies (1≤T ⊥/T ||≤7) considered here. Yet the corresponding particle-in-cell simulations yield a higher saturation level of the fluctuating magnetic fields for the Alfvén-cyclotron instability than for the Bernstein modes as long as inline image. Since fast magnetosonic waves with ion Bernstein instability properties observed in the magnetosphere are often not accompanied by electromagnetic ion cyclotron waves, the results of the present study indicate that the ring-like proton distributions responsible for the excitation of these fast magnetosonic waves should not be very anisotropic.« less

Numerical methods for large eddy simulation of acoustic combustion instabilities

NASA Astrophysics Data System (ADS)

Wall, Clifton T.

Acoustic combustion instabilities occur when interaction between the combustion process and acoustic modes in a combustor results in periodic oscillations in pressure, velocity, and heat release. If sufficiently large in amplitude, these instabilities can cause operational difficulties or the failure of combustor hardware. In many situations, the dominant instability is the result of the interaction between a low frequency acoustic mode of the combustor and the large scale hydrodynamics. Large eddy simulation (LES), therefore, is a promising tool for the prediction of these instabilities, since both the low frequency acoustic modes and the large scale hydrodynamics are well resolved in LES. Problems with the tractability of such simulations arise, however, due to the difficulty of solving the compressible Navier-Stokes equations efficiently at low Mach number and due to the large number of acoustic periods that are often required for such instabilities to reach limit cycles. An implicit numerical method for the solution of the compressible Navier-Stokes equations has been developed which avoids the acoustic CFL restriction, allowing for significant efficiency gains at low Mach number, while still resolving the low frequency acoustic modes of interest. In the limit of a uniform grid the numerical method causes no artificial damping of acoustic waves. New, non-reflecting boundary conditions have also been developed for use with the characteristic-based approach of Poinsot and Lele (1992). The new boundary conditions are implemented in a manner which allows for significant reduction of the computational domain of an LES by eliminating the need to perform LES in regions where one-dimensional acoustics significantly affect the instability but details of the hydrodynamics do not. These new numerical techniques have been demonstrated in an LES of an experimental combustor. The new techniques are shown to be an efficient means of performing LES of acoustic combustion instabilities and are shown to accurately predict the occurrence and frequency of the dominant mode of the instability observed in the experiment.

Darrieus-Landau instability of premixed flames enhanced by fuel droplets

NASA Astrophysics Data System (ADS)

Nicoli, Colette; Haldenwang, Pierre; Denet, Bruno

2017-07-01

Recent experiments on spray flames propagating in a Wilson cloud chamber have established that spray flames are much more sensitive to wrinkles or corrugations than single-phase flames. To propose certain elements of explanation, we numerically study the Darrieus-Landau (or hydrodynamic) instability (DL-instability) developing in premixtures that contain an array of fuel droplets. Two approaches are compared: numerical simulation starting from the general conservation laws in reactive media, and the numerical computation of Sivashinsky-type model equations for DL-instability. Both approaches provide us with results in deep agreement. It is first shown that the presence of droplets in fuel-air premixtures induces initial perturbations which are large enough to trigger the DL-instability. Second, the droplets are responsible for additional wrinkles when the DL-instability is developed. The latter wrinkles are of length scales shorter than those of the DL-instability, in such a way that the DL-unstable spray flames have a larger front surface and therefore propagate faster than the single-phase ones when subjected to the same instability.

Plasma Instability Growth Rates in the F-Region Cusp Ionosphere

NASA Astrophysics Data System (ADS)

Moen, J. I.; Daabakk, Y.; Oksavik, K.; Clausen, L.; Bekkeng, T. A.; Abe, T.; Saito, Y.; Baddeley, L. J.; Lorentzen, D. A.; Sigernes, F.; Yeoman, T. K.

2014-12-01

There are at least two different micro-instability processes that applies to the F-region cusp/polar cap ionosphere. These are the Gradient Drift Instability (GDI) and the Kelvin Helmholtz Instability (KHI). Due to space weather effects on radio communication and satellite signals it is of practical interest to assess the relative importance of these two instability modes and to quantify their growth rates. The Investigation of Cusp Irregularities (ICI) rocket program has been developed to investigate these plasma instabilities and formation scintillation irregularities. High resolution measurements are critical to get realistic quantities on the growth rates. The results achieved so far demonstrates that cusp ionosphere precipitation can give rise to km scale plasma structures on which grow rates are down to a few tens of seconds compared to earlier measures of ten minutes based on ground observations. This has to do with the spatial resolution required for these measurements. Growth rates for the KHI instability is found to be of the same order, which is consistent with growth rates calculated from the EISCAT Svalbard Radar. I.e. both instability modes can be highly efficient in the cusp ionosphere.

Psychological determinants of job retention in multiple sclerosis.

PubMed

Ford, Helen L; Wicks, Charlotte R; Stroud, Amanda; Tennant, Alan

2018-01-01

Maintaining paid work is a key issue for people with multiple sclerosis (PwMS). Different factors, including psychological attributes, can influence job retention. Understanding their role should inform potential interventions to help PwMS retain employment. The aim of this study was to identify the key factors which improve job retention in an employed cohort of PwMS. This three-year longitudinal study used validated self-completed measures of physical and psychological factors at four time points over 28 months. Of 208 employed PwMS, just over 1 in 10 was no longer working at the end of the study. Three variables were predictive of continuing employment; low 'work instability' at baseline increased the odds of job retention by a factor of 12.76; high levels of self-efficacy by a factor of 4.66 and being less than 50 years of age increased the odds of job retention by a factor of 3.90. Path analysis demonstrated the mediating role of self-efficacy between the physical impact of MS and the level of work instability at exit. Screening for work instability and self-efficacy in a clinical setting followed by appropriate interventions to increase self-efficacy and reduce work instability could aid job retention in MS.

Distribution of Plasmoids in Post-Coronal Mass Ejection Current Sheets

NASA Astrophysics Data System (ADS)

Bhattacharjee, A.; Guo, L.; Huang, Y.

2013-12-01

Recently, the fragmentation of a current sheet in the high-Lundquist-number regime caused by the plasmoid instability has been proposed as a possible mechanism for fast reconnection. In this work, we investigate this scenario by comparing the distribution of plasmoids obtained from Large Angle and Spectrometric Coronagraph (LASCO) observational data of a coronal mass ejection event with a resistive magnetohydrodynamic simulation of a similar event. The LASCO/C2 data are analyzed using visual inspection, whereas the numerical data are analyzed using both visual inspection and a more precise topological method. Contrasting the observational data with numerical data analyzed with both methods, we identify a major limitation of the visual inspection method, due to the difficulty in resolving smaller plasmoids. This result raises questions about reports of log-normal distributions of plasmoids and other coherent features in the recent literature. Based on nonlinear scaling relations of the plasmoid instability, we infer a lower bound on the current sheet width, assuming the underlying mechanism of current sheet broadening is resistive diffusion.

Seeking simplicity for the understanding of multiphase flows

NASA Astrophysics Data System (ADS)

Stone, Howard A.

2017-10-01

Fluid mechanics is a discipline with rich phenomena, with motions occurring over an enormous range of length scales, and spanning a wide range of laminar and turbulent flows, instabilities, and applications in industry, nature, biology, and medicine. The subfield of complex fluids typically refers to those flows where the complexity is introduced, for example, by the presence of suspended particles, multiple phases, soft boundaries, and electrokinetic effects; several distinct multiphase flows of Newtonian fluids make up the examples in this article. Interfaces play a significant role and modify the flow with feedback that further changes the shapes of the interfaces. I will provide examples of our work highlighting (i) new features of classical instabilities triggered by changes in geometry, (ii) multiphase flows relevant to the design of liquid-infused substrates exhibiting effective slip while retaining the trapped liquid, and (iii) unexpected dynamics in flow at a T-junction. The interplay of experiments and mathematical models and/or simulations is critical to the new understanding developed.

Universal relations for differentially rotating relativistic stars at the threshold to collapse

NASA Astrophysics Data System (ADS)

Bozzola, Gabriele; Stergioulas, Nikolaos; Bauswein, Andreas

2018-03-01

A binary neutron star merger produces a rapidly and differentially rotating compact remnant whose lifespan heavily affects the electromagnetic and gravitational emissions. Its stability depends on both the equation of state (EOS) and the rotation law and it is usually investigated through numerical simulations. Nevertheless, by means of a sufficient criterion for secular instability, equilibrium sequences can be used as a computational inexpensive way to estimate the onset of dynamical instability, which, in general, is close to the secular one. This method works well for uniform rotation and relies on the location of turning points: stellar models that are stationary points in a sequence of equilibrium solutions with constant rest mass or angular momentum. Here, we investigate differentially rotating models (using a large number of EOSs and different rotation laws) and find that several universal relations between properly scaled gravitational mass, rest mass and angular momentum of the turning-point models that are valid for uniform rotation are insensitive to the degree of differential rotation, to high accuracy.

The Kelvin-Helmhotz instability and thin current sheets in the MHD and Hall MHD formalisms

NASA Astrophysics Data System (ADS)

Chacon, L.; Knoll, D.

2005-12-01

Sheared magnetic fields and sheared flows co-exist in many space, astrophysical, and laboratory plasmas. In such situations the evolution of the Kelvin-Helmhotz instability (KHI) can have a significant impact on the topology of the magnetic field. In particular, it can result in current sheet thinning [2,3], which may allow Hall scales to become relevant and result in fast reconnection rates [1]. There are a number of interesting applications of this phenomena in the magnetosphere. We will discuss some of our recent work in this area [1,2,3] with special focus on Hall MHD effects on the KHI [1]. As an example, we will discuss the parameter regime in which the 2-D parallel KHI can evolve for sub-Alfvenic flows [1]. This may have important implication for dayside reconnection in the magnetopause. [1] Chacon, Knoll, and Finn, Phys. Lett. A, vol. 308, 2003 [2] Knoll and Chacon, PRL, vol. 88, 2002 [3] Brackbill and Knoll, PRL, vol. 86, 2001

Laser plasma coupling with moderate Z, long scalelength underdense plasma

NASA Astrophysics Data System (ADS)

Kruer, William; Berger, Richard; Meezan, Nathaniel; Suter, Larry; Moody, John; Glenzer, Siegfried; Stevenson, R. M.; Oades, K.

2004-11-01

Recent experiments1,2 have focussed new attention on the coupling of laser light with moderate Z, long scalelength underdense plasmas. We discuss some intriguing features of these experiments, including a significant reduction of stimulated Raman and Brillouin scattering in higher Z plasmas, such as Krypton and Xenon. Threshold conditions for various instabilities are discussed, and potential consequences of thermal filamentation and self-focussing are explored. The presence of significant temperature modulations in the plasma can lead to a number of interesting effects not usually taken into account, such as ion wave refraction out of hot spots and instability reduction by the long wavelength modulations. We also consider the extrapolation of these results to the higher temperature regimes more relevant to ignition-scale hohlraums. 1. R. M. Stevenson, et. al, Phys. Plasmas 11, 2709 (2004) 2. J. Moody (to be published) Work performed under the auspices of the U.S. DOE by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.

Comorbid anxious signs and symptoms in major depression: impact on functional work capacity and comparative treatment outcomes.

PubMed

Tollefson, G D; Souetre, E; Thomander, L; Potvin, J H

1993-01-01

Psychological distress is a driver both of direct and indirect health care costs. Depression compromises functional well-being, such as work productivity. Comorbid anxious features often complicate the recognition of depression and may herald a poor prognosis. We report the results of a cross-sectional naturalistic study to determine the impact of three interventions (no antidepressant, fluoxetine, or tricyclic antidepressant therapy) on relative risk of work days lost in 454 French outpatients with either major or minor depression. Most depressed patients also manifested anxious features (76% with a Hamilton Rating Scale for Anxiety score > or = 12). The presence of anxiety was related to the severity of depression, work absenteeism, and current social instability. Depression severity (Hamilton Rating Scale for Depression score > or = 26, including the contributions of anxious symptoms), psychiatric comorbidity, and psychomotor retardation best predicted continued work absenteeism. Patients with major depression were more likely to receive an antidepressant if they had a past history of depressive episodes and/or previous work disability. Patients with minor depression were less likely to receive drug therapy than patients with major depression, despite their current work disability. Among patients who received fluoxetine or a tricyclic antidepressant for at least 8 weeks, fluoxetine was associated with statistically significantly lower mean anxiety and depression scores and fewer work days missed.

Challenges in Scale-Resolving Simulations of turbulent wake flows with coherent structures

NASA Astrophysics Data System (ADS)

Pereira, Filipe S.; Eça, Luís; Vaz, Guilherme; Girimaji, Sharath S.

2018-06-01

The objective of this work is to investigate the challenges encountered in Scale-Resolving Simulations (SRS) of turbulent wake flows driven by spatially-developing coherent structures. SRS of practical interest are expressly intended for efficiently computing such flows by resolving only the most important features of the coherent structures and modelling the remainder as stochastic field. The success of SRS methods depends upon three important factors: i) ability to identify key flow mechanisms responsible for the generation of coherent structures; ii) determine the optimum range of resolution required to adequately capture key elements of coherent structures; and iii) ensure that the modelled part is comprised nearly exclusively of fully-developed stochastic turbulence. This study considers the canonical case of the flow around a circular cylinder to address the aforementioned three key issues. It is first demonstrated using experimental evidence that the vortex-shedding instability and flow-structure development involves four important stages. A series of SRS computations of progressively increasing resolution (decreasing cut-off length) are performed. An a priori basis for locating the origin of the coherent structures development is proposed and examined. The criterion is based on the fact that the coherent structures are generated by the Kelvin-Helmholtz (KH) instability. The most important finding is that the key aspects of coherent structures can be resolved only if the effective computational Reynolds number (based on total viscosity) exceeds the critical value of the KH instability in laminar flows. Finally, a quantitative criterion assessing the nature of the unresolved field based on the strain-rate ratio of mean and unresolved fields is examined. The two proposed conditions and rationale offer a quantitative basis for developing "good practice" guidelines for SRS of complex turbulent wake flows with coherent structures.

3D Stagnation instabilities in MagLIF loads on the Z Generator

NASA Astrophysics Data System (ADS)

Jennings, Christopher

2017-10-01

Experiments with Magnetized Liner Inertial Fusion (MagLIF) loads have successfully demonstrated the premise of magnetized fusion. While these experiments are increasingly well diagnosed, many of the measurements (particularly during stagnation) are time integrated, limited in spatial resolution or require additional assumptions to interpret in the context of a structured, rapidly evolving system. As such, there is some ambiguity over what may be limiting performance. Poor laser coupling in preheating the fuel prior to implosion has been suggested as a mechanism. Mix of high Z contaminants that cool the fuel is also a significant concern. In addition, time integrated crystal imaging has shown significant structure in the final fuel assembly indicating potential disruption from instabilities. Understanding the balance between these degradation mechanisms is vital to progress with MagLif. We compare several sets of experimental data with synthetically generated data from systematically varied 3D resistive-MHD simulations to gain insight into the relative contributions of different degradation mechanisms. We demonstrate how some measurements strongly indicate disruption from liner material penetrating into the fuel at stagnation, and discuss the implications this has for how MagLif targets work and scale to larger drive currents. We then explore the extent to which different combinations of instability development, current delivery, high-Z mix into the fuel and initial laser deposition can be differentiated in our existing measurements. Better determining the dominant degradation mechanisms can directly influence the direction we take to improve performance, or our confidence in scaling these targets to higher currents. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DoE's NNSA under contract DE-NA0003525.

Modeling self-excited combustion instabilities using a combination of two- and three-dimensional simulations

NASA Astrophysics Data System (ADS)

Harvazinski, Matthew Evan

Self-excited combustion instabilities have been studied using a combination of two- and three-dimensional computational fluid dynamics (CFD) simulations. This work was undertaken to assess the ability of CFD simulations to generate the high-amplitude resonant combustion dynamics without external forcing or a combustion response function. Specifically, detached eddy simulations (DES), which allow for significantly coarser grid resolutions in wall bounded flows than traditional large eddy simulations (LES), were investigated for their capability of simulating the instability. A single-element laboratory rocket combustor which produces self-excited longitudinal instabilities is used for the configuration. The model rocket combustor uses an injector configuration based on practical oxidizer-rich staged-combustion devices; a sudden expansion combustion section; and uses decomposed hydrogen peroxide as the oxidizer and gaseous methane as the fuel. A better understanding of the physics has been achieved using a series of diagnostics. Standard CFD outputs like instantaneous and time averaged flowfield outputs are combined with other tools, like the Rayleigh index to provide additional insight. The Rayleigh index is used to identify local regions in the combustor which are responsible for driving and damping the instability. By comparing the Rayleigh index to flowfield parameters it is possible to connect damping and driving to specific flowfield conditions. A cost effective procedure to compute multidimensional local Rayleigh index was developed. This work shows that combustion instabilities can be qualitatively simulated using two-dimensional axisymmetric simulations for fuel rich operating conditions. A full three-dimensional simulation produces a higher level of instability which agrees quite well with the experimental results. In addition to matching the level of instability the three-dimensional simulation also predicts the harmonic nature of the instability that is observed in experiments. All fuel rich simulations used a single step global reaction for the chemical kinetic model. A fuel lean operating condition is also studied and has a lower level of instability. The two-dimensional results are unable to provide good agreement with experimental results unless a more expensive four-step chemical kinetic model is used. The three-dimensional simulation is able to predict the harmonic behavior but fails to capture the amplitude of the instability observed in the companion experiment, instead predicting lower amplitude oscillations. A detailed analysis of the three-dimensional results on a single cycle shows that the periodic heat release commonly associated with combustion instability can be interpreted to be a result of the time lag between the instant the fuel is injected and when it is burned. The time lag is due to two mechanisms. First, methane present near the backstep can become trapped and transported inside shed vortices to the point of combustion. The second aspect of the time lag arises due to the interaction of the fuel with upstream-running pressure waves. As the wave moves past the injection point the flow is temporarily disrupted, reducing the fuel flow into the combustor. A comparison between the fuel lean and fuel rich cases shows several differences. Whereas both cases can produce instability, the fuel-rich case is measurably more unstable. Using the tools developed differences in the location of the damping, and driving regions are evident. By moving the peak driving area upstream of the damping region the level of instability is lower in the fuel lean case. The location of the mean heat release is also important; locating the mean heat release adjacent to the vortex impingement point a higher level of instability is observed for the fuel rich case. This research shows that DES instability modeling has the ability to be a valuable tool in the study of combustion instability. The lower grid size requirement makes the use of DES based modeling a potential candidate in the modeling of full-scale rocket engines. Whereas three-dimensional simulations may be necessary for very good agreement, two-dimensional simulations allow efficient parametric investigation and tool development. The insights obtained from the simulations offer the possibility that their results can be used in the design of future engines to exploit damping and reduce driving.

Parameter scaling toward high-energy density in a quasi-steady flow Z-pinch

NASA Astrophysics Data System (ADS)

Hughes, M. C.; Shumlak, U.; Nelson, B. A.; Golingo, R. P.; Claveau, E. L.; Doty, S. A.; Forbes, E. G.; Kim, B.; Ross, M. P.

2016-10-01

Sheared axial flows are utilized by the ZaP Flow Z-Pinch Experiment to stabilize MHD instabilities. The pinches formed are 50 cm long with radii ranging from 0.3 to 1.0 cm. The plasma is generated in a coaxial acceleration region, similar to a Marshall gun, which provides a steady supply of plasma for approximately 100 us. The power to the plasma is partially decoupled between the acceleration and pinch assembly regions through the use of separate power supplies. Adiabatic scaling of the Bennett relation gives targets for future devices to reach high-energy density conditions or fusion reactors. The applicability of an adiabatic assumption is explored and work is done experimentally to clarify the plasma compression process, which may be more generally polytropic. The device is capable of a much larger parameter space than previous machine iterations, allowing flexibility in the initial conditions of the compression process to preserve stability. This work is supported by DoE FES and NNSA.

Erosion waves: Transverse instabilities and fingering

NASA Astrophysics Data System (ADS)

Malloggi, F.; Lanuza, J.; Andreotti, B.; Clément, E.

2006-09-01

Two laboratory scale experiments of dry and underwater avalanches of non-cohesive granular materials are investigated. We trigger solitary waves and study the conditions under which the front is transversally stable. We show the existence of a linear instability followed by a coarsening dynamics and finally the onset of a fingering pattern. Due to the different operating conditions, both experiments strongly differ by the spatial and time scales involved. Nevertheless, the quantitative agreement between the stability diagram, the wavelengths selected and the avalanche morphology suggest a common scenario for an erosion/deposition process.

Cloud-Top Entrainment in Stratocumulus Clouds

NASA Astrophysics Data System (ADS)

Mellado, Juan Pedro

2017-01-01

Cloud entrainment, the mixing between cloudy and clear air at the boundary of clouds, constitutes one paradigm for the relevance of small scales in the Earth system: By regulating cloud lifetimes, meter- and submeter-scale processes at cloud boundaries can influence planetary-scale properties. Understanding cloud entrainment is difficult given the complexity and diversity of the associated phenomena, which include turbulence entrainment within a stratified medium, convective instabilities driven by radiative and evaporative cooling, shear instabilities, and cloud microphysics. Obtaining accurate data at the required small scales is also challenging, for both simulations and measurements. During the past few decades, however, high-resolution simulations and measurements have greatly advanced our understanding of the main mechanisms controlling cloud entrainment. This article reviews some of these advances, focusing on stratocumulus clouds, and indicates remaining challenges.

Rayleigh convective instability in a cloud medium

NASA Astrophysics Data System (ADS)

Shmerlin, B. Ya.; Shmerlin, M. B.

2017-09-01

The problem of convective instability of an atmospheric layer containing a horizontally finite region filled with a cloud medium is considered. Solutions exponentially growing with time, i.e., solitary cloud rolls or spatially localized systems of cloud rolls, have been constructed. In the case of axial symmetry, their analogs are convective vortices with both ascending and descending motions on the axis and cloud clusters with ring-shaped convective structures. Depending on the anisotropy of turbulent exchange, the scale of vortices changes from the tornado scale to the scale of tropical cyclones. The solutions with descending motions on the axis can correspond to the formation of a tornado funnel or a hurricane eye in tropical cyclones.

Lateral ankle instability and revision surgery alternatives in the athlete.

PubMed

Schenck, Robert C; Coughlin, Michael J

2009-06-01

Ankle instability in the athlete is a common problem that is routinely treated non-operatively, with a 90% success rate. With proprioceptive training, preventive equipment (bracing/taping), and closed kinetic chain strengthening, surgery for ankle instability is uncommon. Nonetheless, some athletes present with recurrent ankle instability that, despite work-up and conservative treatment, requires surgical correction. The use of a primary ligament repair (Brostrom procedure) versus augmented (anatomic) reconstructions is discussed in detail in this article.

Three dimensional instabilities of an electron scale current sheet in collisionless magnetic reconnection

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

Jain, Neeraj; Büchner, Jörg; Max Planck Institute for Solar System Research, Justus-Von-Liebig-Weg-3, Göttingen

In collisionless magnetic reconnection, electron current sheets (ECS) with thickness of the order of an electron inertial length form embedded inside ion current sheets with thickness of the order of an ion inertial length. These ECS's are susceptible to a variety of instabilities which have the potential to affect the reconnection rate and/or the structure of reconnection. We carry out a three dimensional linear eigen mode stability analysis of electron shear flow driven instabilities of an electron scale current sheet using an electron-magnetohydrodynamic plasma model. The linear growth rate of the fastest unstable mode was found to drop with themore » thickness of the ECS. We show how the nature of the instability depends on the thickness of the ECS. As long as the half-thickness of the ECS is close to the electron inertial length, the fastest instability is that of a translational symmetric two-dimensional (no variations along flow direction) tearing mode. For an ECS half thickness sufficiently larger or smaller than the electron inertial length, the fastest mode is not a tearing mode any more and may have finite variations along the flow direction. Therefore, the generation of plasmoids in a nonlinear evolution of ECS is likely only when the half-thickness is close to an electron inertial length.« less

Rayleigh-Taylor and Richtmyer-Meshkov Instabilities in Turbulent Regime

NASA Astrophysics Data System (ADS)

Dimonte, G.

1998-11-01

The Rayleigh-Taylor instability (RTI) and its shock driven analog, the Richtmyer-Meshkov instability (RMI), affect a wide variety of important phenomena from sub-terrainian to astrophysical environments. The ``fluids" are equally varied from plasmas and magnetic fields to elastic-plastic solids. In most applications, the instabilities occur with a complex acceleration history and evolve to a highly nonlinear state, making the theoretical description formidable. We will link the fluid and plasma regimes while describing the theoretical issues and basic experiments in different venues to isolate key physics issues. RMI experiments on the Nova laser investigate the affects of compressibility with strong radiatively driven shocks (Mach > 10) in near solid density plasmas of sub-millimeter scale. The growth of single sinusoidal and random 3-D perturbations are measured using backlit radiography. RTI experiments with the Linear Electric Motor (LEM) are conducted with a variety of acceleration (<< 10^4 m/s^2) histories and fluids of 10 cm scale. Turbulent RTI experiments with high Reynolds number liquids show self-similar growth which is characterized with laser induced fluorescence. LEM experiments with an elastic-plastic material (yogurt) exhibit a critical wavelength and amplitude for instability. The experimental results will be compared with linear and nonlinear theories and hydrodynamic simulations.

Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet.

PubMed

Li, C K; Tzeferacos, P; Lamb, D; Gregori, G; Norreys, P A; Rosenberg, M J; Follett, R K; Froula, D H; Koenig, M; Seguin, F H; Frenje, J A; Rinderknecht, H G; Sio, H; Zylstra, A B; Petrasso, R D; Amendt, P A; Park, H S; Remington, B A; Ryutov, D D; Wilks, S C; Betti, R; Frank, A; Hu, S X; Sangster, T C; Hartigan, P; Drake, R P; Kuranz, C C; Lebedev, S V; Woolsey, N C

2016-10-07

The remarkable discovery by the Chandra X-ray observatory that the Crab nebula's jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet.

Active Surfaces and Interfaces of Soft Materials

NASA Astrophysics Data System (ADS)

Wang, Qiming

A variety of intriguing surface patterns have been observed on developing natural systems, ranging from corrugated surface of white blood cells at nanometer scales to wrinkled dog skins at millimeter scales. To mimetically harness functionalities of natural morphologies, artificial transformative skin systems by using soft active materials have been rationally designed to generate versatile patterns for a variety of engineering applications. The study of the mechanics and design of these dynamic surface patterns on soft active materials are both physically interesting and technologically important. This dissertation starts with studying abundant surface patterns in Nature by constructing a unified phase diagram of surface instabilities on soft materials with minimum numbers of physical parameters. Guided by this integrated phase diagram, an electroactive system is designed to investigate a variety of electrically-induced surface instabilities of elastomers, including electro-creasing, electro-cratering, electro-wrinkling and electro-cavitation. Combing experimental, theoretical and computational methods, the initiation, evolution and transition of these instabilities are analyzed. To apply these dynamic surface instabilities to serving engineering and biology, new techniques of Dynamic Electrostatic Lithography and electroactive anti-biofouling are demonstrated.

Plasma Irregularities on the Leading and Trailing Edges of Polar Cap Patches

NASA Astrophysics Data System (ADS)

Lamarche, L. J.; Varney, R. H.; Gillies, R.; Chartier, A.; Mitchell, C. N.

2017-12-01

Plasma irregularities in the polar cap have often been attributed to the gradient drift instability (GDI). Traditional fluid theories of GDI predicts irregularity growth only on the trailing edge of polar patches, where the plasma density gradient is parallel to the plasma drift velocity, however many observations show irregularities also form on the leading edge of patches. We consider decameter-scale irregularities detected by polar-latitude SuperDARN (Super Dual Auroral Radar Network) radars with any relationship between the background density gradients and drift velocity. Global electron density from the Multi-Instrument Data Analysis System (MIDAS), a GPS tomography routine, is used to provide context for where irregularities are observed relative to polar patches and finer-scale background density gradients are found from 3D imaging from both the North and Canada faces of the Resolute Bay Incoherent Scatter Radars (RISR-N and RISR-C) jointly. Shear-based instabilities are considered as mechanisms by which plasma irregularities could form on the leading edge of patches. Theoretical predictions of instability growth from both GDI and shear instabilities are compared with irregularity observations for the October 13, 2016 storm.

A New Mechanism of Magnetic Field Generation in Supernova Shock Waves and its Implication for Cosmic Ray Acceleration

NASA Astrophysics Data System (ADS)

Diamond, Patrick

2005-10-01

SNR shocks are the most probable source of galactic cosmic rays. We discuss the diffusive acceleration mechanism in terms of its potential to accelerate CRs to 10^18 eV, as observations imply. One possibility, currently discussed in the literature, is to resonantly generate a turbulent magnetic field via accelerated particles in excess of the background field. We indicate some difficulties of this scenario and suggest a different possibility, which is based on the generation of Alfven waves at the gyroradius scale at the background field level, with a subsequent transfer to longer scales via interaction with strong acoustic turbulence in the shock precursor. The acoustic turbulence in turn, may be generated by Drury instability or by parametric instability of the Alfven (A) waves. The essential idea is an A-->A+S decay instability process, where one of the interacting scatterers (i.e. the sound, or S-waves) are driven by the Drury instability process. This rapidly generates longer wavelength Alfven waves, which in turn resonate with high energy CRs thus binding them to the shock and enabling their further acceleration.

Supersonic Coaxial Jets: Noise Predictions and Measurements

NASA Technical Reports Server (NTRS)

Dahl, Milo D.; Papamoschou, Dimitri; Hixon, Ray

1998-01-01

The noise from perfectly expanded coaxial jets was measured in an anechoic chamber for different operating conditions with the same total thrust, mass flow, and exit area. The shape of the measured noise spectrum at different angles to the jet axis was found to agree with spectral shapes for single, axisymmetric jets. Based on these spectra, the sound was characterized as being generated by large turbulent structures or fine-scale turbulence. Modeling the large scale structures as instability waves, a stability analysis was conducted for the coaxial jets to identify the growing and decaying instability waves in each shear layer and predict their noise radiation pattern outside the jet. When compared to measured directivity, the analysis identified the region downstream of the outer potential core, where the two shear layers were merging, as the source of the peak radiated noise where instability waves, with their origin in the inner shear layer, reach their maximum amplitude. Numerical computations were also performed using a linearized Euler equation solver. Those results were compared to both the results from the instability wave analysis and to measured data.

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Pair Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K. I.; Hardee, P.; Hededal, C. B.; Richardson, G.; Sol, H.; Preece, R.; Fishman, G. J.

2004-01-01

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma. We find that the growth times depend on the Lorenz factors of jets. The jets with larger Lorenz factors grow slower. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The small scale magnetic field structure generated by the Weibel instability is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Wavelet-based multiscale window transform and energy and vorticity analysis

NASA Astrophysics Data System (ADS)

Liang, Xiang San

A new methodology, Multiscale Energy and Vorticity Analysis (MS-EVA), is developed to investigate sub-mesoscale, meso-scale, and large-scale dynamical interactions in geophysical fluid flows which are intermittent in space and time. The development begins with the construction of a wavelet-based functional analysis tool, the multiscale window transform (MWT), which is local, orthonormal, self-similar, and windowed on scale. The MWT is first built over the real line then modified onto a finite domain. Properties are explored, the most important one being the property of marginalization which brings together a quadratic quantity in physical space with its phase space representation. Based on MWT the MS-EVA is developed. Energy and enstrophy equations for the large-, meso-, and sub-meso-scale windows are derived and their terms interpreted. The processes thus represented are classified into four categories: transport; transfer, conversion, and dissipation/diffusion. The separation of transport from transfer is made possible with the introduction of the concept of perfect transfer. By the property of marginalization, the classical energetic analysis proves to be a particular case of the MS-EVA. The MS-EVA developed is validated with classical instability problems. The validation is carried out through two steps. First, it is established that the barotropic and baroclinic instabilities are indicated by the spatial averages of certain transfer term interaction analyses. Then calculations of these indicators are made with an Eady model and a Kuo model. The results agree precisely with what is expected from their analytical solutions, and the energetics reproduced reveal a consistent and important aspect of the unknown dynamic structures of instability processes. As an application, the MS-EVA is used to investigate the Iceland-Faeroe frontal (IFF) variability. A MS-EVA-ready dataset is first generated, through a forecasting study with the Harvard Ocean Prediction System using the data gathered during the 1993 NRV Alliance cruise. The application starts with a determination of the scale window bounds, which characterize a double-peak structure in either the time wavelet spectrum or the space wavelet spectrum. The resulting energetics, when locally averaged, reveal that there is a clear baroclinic instability happening around the cold tongue intrusion observed in the forecast. Moreover, an interaction analysis shows that the energy released by the instability indeed goes to the meso-scale window and fuel the growth of the intrusion. The sensitivity study shows that, in this case, the key to a successful application is a correct decomposition of the large-scale window from the meso-scale window.

Evidence of Active MHD Instability in EULAG-MHD Simulations of Solar Convection

NASA Astrophysics Data System (ADS)

Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul

2015-11-01

We investigate the possible development of magnetohydrodynamical instabilities in the EULAG-MHD “millennium simulation” of Passos & Charbonneau. This simulation sustains a large-scale magnetic cycle characterized by solar-like polarity reversals taking place on a regular multidecadal cadence, and in which zonally oriented bands of strong magnetic fields accumulate below the convective layers, in response to turbulent pumping from above in successive magnetic half-cycles. Key aspects of this simulation include low numerical dissipation and a strongly sub-adiabatic fluid layer underlying the convectively unstable layers corresponding to the modeled solar convection zone. These properties are conducive to the growth and development of two-dimensional instabilities that are otherwise suppressed by stronger dissipation. We find evidence for the action of a non-axisymmetric magnetoshear instability operating in the upper portions of the stably stratified fluid layers. We also investigate the possibility that the Tayler instability may be contributing to the destabilization of the large-scale axisymmetric magnetic component at high latitudes. On the basis of our analyses, we propose a global dynamo scenario whereby the magnetic cycle is driven primarily by turbulent dynamo action in the convecting layers, but MHD instabilities accelerate the dissipation of the magnetic field pumped down into the overshoot and stable layers, thus perhaps significantly influencing the magnetic cycle period. Support for this scenario is found in the distinct global dynamo behaviors observed in an otherwise identical EULAG-MHD simulations, using a different degree of sub-adiabaticity in the stable fluid layers underlying the convection zone.

Mexican-Origin Women's Employment Instability. Working Paper No. 51.

ERIC Educational Resources Information Center

De Anda, Roberto M.

This paper compares the causes and consequences of employment instability among Mexican-origin women, White women, and White men. Data came from the work experience supplement in the March 1995 file of the Current Population Survey for a sample that included 1,399 Mexican-origin women, 17,092 White women, and 24,440 White men. All were experienced…

Coping with Complexity and Instability in the UK Vocational Training System

ERIC Educational Resources Information Center

Halasz, Gabor

2011-01-01

Vocational training systems that take the needs of the word of work seriously and maintain strong and dynamic connections with it are faced to growing complexity and instability. Some countries try to cope with this through creating new mediation mechanisms between the systems of training and work that allow higher level complexity while…

Nonstandard Work and Marital Instability: Evidence from the National Longitudinal Survey of Youth

ERIC Educational Resources Information Center

Kalil, Ariel; Ziol-Guest, Kathleen M.; Epstein, Jodie Levin

2010-01-01

This article replicated and extended Harriet Presser's (2000) investigation of the linkages between nonstandard work and marital instability. We reexplored this question using data from a sample of 2,893 newlywed couples from the National Longitudinal Survey of Youth (NLSY) and using different analytic techniques. In contrast to Presser, we found…

Nurses with sensory disabilities: their perceptions and characteristics.

PubMed

Neal-Boylan, Leslie; Fennie, Kristopher; Baldauf-Wagner, Sara

2011-01-01

A survey design was used to explore the perceptions and characteristics of registered nurses (RNs) with sensory disabilities and their risk for leaving their jobs. An earlier study found that nurses with disabilities are leaving nursing and that employers do not appear to support these nurses. Work instability and the mismatch between a nurse's perceptions of his or her ability and the demands of their work increase risk for job retention problems. This study's convenience sample of U.S. RNs had hearing, vision, or communication disabilities. Participants completed a demographic form, three U.S. Census questions, and the Nurse-Work Instability Survey. Hospital nurses were three times more likely to be at risk for retention problems. Nurses with hearing disabilities were frustrated at work. Hearing difficulties increased with years spent working as a nurse. Many nurses with sensory disabilities have left nursing. Early intervention may prevent work instability and increase retention, and rehabilitation nurses are ideally positioned to lead early intervention programs.

Rotordynamic Instability Problems in High-Performance Turbomachinery

NASA Technical Reports Server (NTRS)

1980-01-01

Diagnostic and remedial methods concerning rotordynamic instability problems in high performance turbomachinery are discussed. Instabilities due to seal forces and work-fluid forces are identified along with those induced by rotor bearing systems. Several methods of rotordynamic control are described including active feedback methods, the use of elastometric elements, and the use of hydrodynamic journal bearings and supports.

Multiple time scale analysis of pressure oscillations in solid rocket motors

NASA Astrophysics Data System (ADS)

Ahmed, Waqas; Maqsood, Adnan; Riaz, Rizwan

2018-03-01

In this study, acoustic pressure oscillations for single and coupled longitudinal acoustic modes in Solid Rocket Motor (SRM) are investigated using Multiple Time Scales (MTS) method. Two independent time scales are introduced. The oscillations occur on fast time scale whereas the amplitude and phase changes on slow time scale. Hopf bifurcation is employed to investigate the properties of the solution. The supercritical bifurcation phenomenon is observed for linearly unstable system. The amplitude of the oscillations result from equal energy gain and loss rates of longitudinal acoustic modes. The effect of linear instability and frequency of longitudinal modes on amplitude and phase of oscillations are determined for both single and coupled modes. For both cases, the maximum amplitude of oscillations decreases with the frequency of acoustic mode and linear instability of SRM. The comparison of analytical MTS results and numerical simulations demonstrate an excellent agreement.

The Exchange-Correlation Field Effect over the Magnetoacoustic-Gravitational Instability in Plasmas

NASA Astrophysics Data System (ADS)

Rasheed, A.; Jamil, M.; Jung, Young-Dae; Sahar, A.; Asif, M.

2017-09-01

Jeans instability with magnetosonic perturbations is discussed in quantum dusty magnetoplasmas. The quantum and smaller thermal effects are associated only with electrons. The quantum characteristics include exchange-correlation potential, recoil effect, and Fermi degenerate pressure. The multifluid model of plasmas is used for the analytical study of this problem. The significant contribution of electron exchange is noticed on the threshold value of wave vector and Jeans instability. The presence of electron exchange and correlation effects reduce the time to stabilise the phenomenon of self-gravitational collapse of massive species. The results of Jeans instability by magnetosonic perturbations at quantum scale help to disclose the details of the self-gravitating dusty magnetoplasma systems.

A photoionization instability in the early intergalactic medium

NASA Technical Reports Server (NTRS)

Hogan, Craig J.

1992-01-01

It is argued that any fairly uniform source of ionizing photons can be the cause of an instability in the pregalactic medium on scales larger than a photon path length. Underdense regions receive more ionizing energy per atom and reach higher temperature and entropy, driving the density down still further. Fluctuations created by this instability can lead to the formation of structures resembling protogalaxies and intergalactic clouds, obviating the need for gas clouds or density perturbations of earlier cosmological provenance, as is usually assumed in theories of galaxy and structure formation. Characteristic masses for clouds produced by the instability, with log mass in solar units plotted against log radius in kpc, are illustrated.

Observation of instability-induced current redistribution in a spherical-torus plasma.

PubMed

Menard, J E; Bell, R E; Gates, D A; Kaye, S M; LeBlanc, B P; Levinton, F M; Medley, S S; Sabbagh, S A; Stutman, D; Tritz, K; Yuh, H

2006-09-01

A motional Stark effect diagnostic has been utilized to reconstruct the parallel current density profile in a spherical-torus plasma for the first time. The measured current profile compares favorably with neoclassical theory when no large-scale magnetohydrodynamic instabilities are present in the plasma. However, a current profile anomaly is observed during saturated interchange-type instability activity. This apparent anomaly can be explained by redistribution of neutral beam injection current drive and represents the first observation of interchange-type instabilities causing such redistribution. The associated current profile modifications contribute to sustaining the central safety factor above unity for over five resistive diffusion times, and similar processes may contribute to improved operational scenarios proposed for ITER.

Spatial Instability of the Linearly Polarized Plane Wave in a Cubic Crystal

NASA Astrophysics Data System (ADS)

Kuz'mina, M. S.; Khazanov, E. A.

2016-12-01

We study theoretically the development of a small-scale spatial instability of a plane wave in a cubic crystal with [111], [001] and [101] orientations. It is shown that in the [111] oriented crystals the instability develops at lower intensities than in the [001] and [101] oriented crystals. In the latter two crystals, the instability can significantly be suppressed by choosing the optimal radiation polarization. It is found that in the case of a small B integral, the method of temporal contrast enhancement of laser pulses by generating an orthogonal polarization achieves the largest efficiency with the [101] orientation, while the [001] orientation is more preferable for B > 3.

Simulating the frontal instability of lock-exchange density currents with dissipative particle dynamics

NASA Astrophysics Data System (ADS)

Li, Yanggui; Geng, Xingguo; Wang, Heping; Zhuang, Xin; Ouyang, Jie

2016-06-01

The frontal instability of lock-exchange density currents is numerically investigated using dissipative particle dynamics (DPD) at the mesoscopic particle level. For modeling two-phase flow, the “color” repulsion model is adopted to describe binary fluids according to Rothman-Keller method. The present DPD simulation can reproduce the flow phenomena of lock-exchange density currents, including the lobe-and-cleft instability that appears at the head, as well as the formation of coherent billow structures at the interface behind the head due to the growth of Kelvin-Helmholtz instability. Furthermore, through the DPD simulation, some small-scale characteristics can be observed, which are difficult to be captured in macroscopic simulation and experiment.

The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades

NASA Astrophysics Data System (ADS)

Vafin, S.; Rafighi, I.; Pohl, M.; Niemiec, J.

2018-04-01

This work revisits the electrostatic instability for blazar-induced pair beams propagating through the intergalactic medium (IGM) using linear analysis and PIC simulations. We study the impact of the realistic distribution function of pairs resulting from the interaction of high-energy gamma-rays with the extragalactic background light. We present analytical and numerical calculations of the linear growth rate of the instability for the arbitrary orientation of wave vectors. Our results explicitly demonstrate that the finite angular spread of the beam dramatically affects the growth rate of the waves, leading to the fastest growth for wave vectors quasi-parallel to the beam direction and a growth rate at oblique directions that is only a factor of 2–4 smaller compared to the maximum. To study the nonlinear beam relaxation, we performed PIC simulations that take into account a realistic wide-energy distribution of beam particles. The parameters of the simulated beam-plasma system provide an adequate physical picture that can be extrapolated to realistic blazar-induced pairs. In our simulations, the beam looses only 1% of its energy, and we analytically estimate that the beam would lose its total energy over about 100 simulation times. An analytical scaling is then used to extrapolate the parameters of realistic blazar-induced pair beams. We find that they can dissipate their energy slightly faster by the electrostatic instability than through inverse-Compton scattering. The uncertainties arising from, e.g., details of the primary gamma-ray spectrum are too large to make firm statements for individual blazars, and an analysis based on their specific properties is required.

Stability and dewetting of metal nanoparticle filled thin polymer films: control of instability length scale and dynamics.

PubMed

Mukherjee, Rabibrata; Das, Soma; Das, Anindya; Sharma, Satinder K; Raychaudhuri, Arup K; Sharma, Ashutosh

2010-07-27

We investigate the influence of gold nanoparticle addition on the stability, dewetting, and pattern formation in ultrathin polymer-nanoparticle (NP) composite films by examining the length and time scales of instability, morphology, and dynamics of dewetting. For these 10-50 nm thick (h) polystyrene (PS) thin films containing uncapped gold nanoparticles (diameter approximately 3-4 nm), transitions from complete dewetting to arrested dewetting to absolute stability were observed depending on the concentration of the particles. Experiments show the existence of three distinct stability regimes: regime 1, complete dewetting leading to droplet formation for nanoparticle concentration of 2% (w/w) or below; regime 2, partial dewetting leading to formation of arrested holes for NP concentrations in the range of 3-6%; and regime 3, complete inhibition of dewetting for NP concentrations of 7% and above. Major results are (a) length scale of instability, where lambdaH approximately hn remains unchanged with NP concentration in regime 1 (n approximately 2) but increases in regime 2 with a change in the scaling relation (n approximately 3-3.5); (b) dynamics of instability and dewetting becomes progressively sluggish with an increase in the NP concentration; (c) there are distinct regimes of dewetting velocity at low NP concentrations; (d) force modulation AFM, as well as micro-Raman analysis, shows phase separation and aggregation of the gold nanoparticles within each dewetted polymer droplet leading to the formation of a metal core-polymer shell morphology. The polymer shell could be removed by washing in a selective solvent, thus exposing an array of bare gold nanoparticle aggregates.

Is a Higgs vacuum instability fatal for high-scale inflation?

DOE PAGES

Kearney, John; Yoo, Hojin; Zurek, Kathryn M.

2015-06-25

We study the inflationary evolution of a scalar field h with an unstable potential for the case where the Hubble parameter H during inflation is larger than the instability scale Λ I of the potential. Quantum fluctuations in the field of size δh ~ H/2π imply that the unstable part of the potential is sampled during inflation. We investigate the evolution of these fluctuations to the unstable regime and in particular whether they generate cosmological defects or even terminate inflation. We apply the results of a toy scalar model to the case of the Standard Model (SM) Higgs boson, themore » quartic of which evolves to negative values at high scales, and extend previous analyses of Higgs dynamics during inflation utilizing statistical methods to a perturbative and fully gauge-invariant formulation. We show that the dynamics are controlled by the renormalization group-improved quartic coupling λ(μ) evaluated at a scale μ = H, such that Higgs fluctuations are enhanced by the instability if H > Λ I. Even if H > Λ I, the instability in the Standard Model Higgs potential does not end inflation; instead the universe slowly sloughs off crunching patches of space that never come to dominate the evolution. As inflation proceeds past 50 e-folds, a significant proportion of patches exits inflation in the unstable vacuum, and as much as 1% of the spacetime can rapidly evolve to a defect. Depending on the nature of these defects, however, the resulting universe could still be compatible with ours.« less

IRIS Si iv LINE PROFILES: AN INDICATION FOR THE PLASMOID INSTABILITY DURING SMALL-SCALE MAGNETIC RECONNECTION ON THE SUN

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

Innes, D. E.; Guo, L.-J.; Huang, Y.-M.

Our understanding of the process of fast reconnection has undergone a dramatic change in the last 10 years driven, in part, by the availability of high-resolution numerical simulations that have consistently demonstrated the break-up of current sheets into magnetic islands, with reconnection rates that become independent of Lundquist number, challenging the belief that fast magnetic reconnection in flares proceeds via the Petschek mechanism which invokes pairs of slow-mode shocks connected to a compact diffusion region. The reconnection sites are too small to be resolved with images, but these reconnection mechanisms, Petschek and the plasmoid instability, have reconnection sites with verymore » different density and velocity structures and so can be distinguished by high-resolution line-profile observations. Using IRIS spectroscopic observations we obtain a survey of typical line profiles produced by small-scale events thought to be reconnection sites on the Sun. Slit-jaw images are used to investigate the plasma heating and re-configuration at the sites. A sample of 15 events from 2 active regions is presented. The line profiles are complex with bright cores and broad wings extending to over 300 km s{sup −1}. The profiles can be reproduced with the multiple magnetic islands and acceleration sites that characterize the plasmoid instability but not by bi-directional jets that characterize the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.« less

A Classification of Mediterranean Cyclones Based on Global Analyses

NASA Technical Reports Server (NTRS)

Reale, Oreste; Atlas, Robert

2003-01-01

The Mediterranean Sea region is dominated by baroclinic and orographic cyclogenesis. However, previous work has demonstrated the existence of rare but intense subsynoptic-scale cyclones displaying remarkable similarities to tropical cyclones and polar lows, including, but not limited to, an eye-like feature in the satellite imagery. The terms polar low and tropical cyclone have been often used interchangeably when referring to small-scale, convective Mediterranean vortices and no definitive statement has been made so far on their nature, be it sub-tropical or polar. Moreover, most of the classifications of Mediterranean cyclones have neglected the small-scale convective vortices, focusing only on the larger-scale and far more common baroclinic cyclones. A classification of all Mediterranean cyclones based on operational global analyses is proposed The classification is based on normalized horizontal shear, vertical shear, scale, low versus mid-level vorticity, low-level temperature gradients, and sea surface temperatures. In the classification system there is a continuum of possible events, according to the increasing role of barotropic instability and decreasing role of baroclinic instability. One of the main results is that the Mediterranean tropical cyclone-like vortices and the Mediterranean polar lows appear to be different types of events, in spite of the apparent similarity of their satellite imagery. A consistent terminology is adopted, stating that tropical cyclone- like vortices are the less baroclinic of all, followed by polar lows, cold small-scale cyclones and finally baroclinic lee cyclones. This classification is based on all the cyclones which occurred in a four-year period (between 1996 and 1999). Four cyclones, selected among all the ones which developed during this time-frame, are analyzed. Particularly, the classification allows to discriminate between two cyclones (occurred in October 1996 and in March 1999) which both display a very well-defined eye-like feature in the satellite imagery. According to our classification system, the two events are dynamically different and can be categorized as being respectively a tropical cyclone-like vortex and well-developed polar low.

Gravitomagnetic Instabilities in Anisotropically Expanding Fluids

NASA Astrophysics Data System (ADS)

Kleidis, Kostas; Kuiroukidis, Apostolos; Papadopoulos, Demetrios B.; Vlahos, Loukas

Gravitational instabilities in a magnetized Friedman-Robertson-Walker (FRW) universe, in which the magnetic field was assumed to be too weak to destroy the isotropy of the model, are known and have been studied in the past. Accordingly, it became evident that the external magnetic field disfavors the perturbations' growth, suppressing the corresponding rate by an amount proportional to its strength. However, the spatial isotropy of the FRW universe is not compatible with the presence of large-scale magnetic fields. Therefore, in this paper we use the general-relativistic version of the (linearized) perturbed magnetohydrodynamic equations with and without resistivity, to discuss a generalized Jeans criterion and the potential formation of density condensations within a class of homogeneous and anisotropically expanding, self-gravitating, magnetized fluids in curved space-time. We find that, for a wide variety of anisotropic cosmological models, gravitomagnetic instabilities can lead to subhorizontal, magnetized condensations. In the nonresistive case, the power spectrum of the unstable cosmological perturbations suggests that most of the power is concentrated on large scales (small k), very close to the horizon. On the other hand, in a resistive medium, the critical wave-numbers so obtained, exhibit a delicate dependence on resistivity, resulting in the reduction of the corresponding Jeans lengths to smaller scales (well bellow the horizon) than the nonresistive ones, while increasing the range of cosmological models which admit such an instability.

A large-scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae.

PubMed

Mösta, Philipp; Ott, Christian D; Radice, David; Roberts, Luke F; Schnetter, Erik; Haas, Roland

2015-12-17

Magnetohydrodynamic turbulence is important in many high-energy astrophysical systems, where instabilities can amplify the local magnetic field over very short timescales. Specifically, the magnetorotational instability and dynamo action have been suggested as a mechanism for the growth of magnetar-strength magnetic fields (of 10(15) gauss and above) and for powering the explosion of a rotating massive star. Such stars are candidate progenitors of type Ic-bl hypernovae, which make up all supernovae that are connected to long γ-ray bursts. The magnetorotational instability has been studied with local high-resolution shearing-box simulations in three dimensions, and with global two-dimensional simulations, but it is not known whether turbulence driven by this instability can result in the creation of a large-scale, ordered and dynamically relevant field. Here we report results from global, three-dimensional, general-relativistic magnetohydrodynamic turbulence simulations. We show that hydromagnetic turbulence in rapidly rotating protoneutron stars produces an inverse cascade of energy. We find a large-scale, ordered toroidal field that is consistent with the formation of bipolar magnetorotationally driven outflows. Our results demonstrate that rapidly rotating massive stars are plausible progenitors for both type Ic-bl supernovae and long γ-ray bursts, and provide a viable mechanism for the formation of magnetars. Moreover, our findings suggest that rapidly rotating massive stars might lie behind potentially magnetar-powered superluminous supernovae.

Rayleigh-Bénard convection with uniform vertical magnetic field.

PubMed

Basak, Arnab; Raveendran, Rohit; Kumar, Krishna

2014-09-01

We present the results of direct numerical simulations of Rayleigh-Bénard convection in the presence of a uniform vertical magnetic field near instability onset. We have done simulations in boxes with square as well as rectangular cross sections in the horizontal plane. We have considered the horizontal aspect ratio η=L(y)/L(x)=1 and 2. The onset of the primary and secondary instabilities are strongly suppressed in the presence of the vertical magnetic field for η=1. The Nusselt number Nu scales with the Rayleigh number Ra close to the primary instability as [{Ra-Ra(c)(Q)}/Ra(c)(Q)](0.91), where Ra(c)(Q) is the threshold for onset of stationary convection at a given value of the Chandrasekhar number Q. Nu also scales with Ra/Q as (Ra/Q)(μ). The exponent μ varies in the range 0.39≤μ≤0.57 for Ra/Q≥25. The primary instability is stationary as predicted by Chandrasekhar. The secondary instability is temporally periodic for Pr=0.1 but quasiperiodic for Pr=0.025 for moderate values of Q. Convective patterns for higher values of Ra consist of periodic, quasiperiodic, and chaotic wavy rolls above the onset of the secondary instability for η=1. In addition, stationary as well as time-dependent cross rolls are observed, as Ra is further raised. The ratio r(o)/Pr is independent of Q for smaller values of Q. The delay in the onset of the oscillatory instability is significantly reduced in a simulation box with η=2. We also observe inclined stationary rolls for smaller values of Q for η=2.

Thermal effects in rapid directional solidification - Linear theory

NASA Technical Reports Server (NTRS)

Huntley, D. A.; Davis, S. H.

1993-01-01

We study the morphological instability of the planar solid/liquid interface for a unidirectionally-solidified dilute binary mixture. We use a model developed by Boettinger et al. (1985, 1986), Aziz (1982), and Jackson et al. (1980), which allows for nonequilibrium effects on the interface through velocity-dependent segregation and attachment kinetics. Two types of instabilities are found in the linear stability analysis: (1) a cellular instability, and (2) an oscillatory instability driven by disequilibrium effects. Merchant and Davis (1990) characterized these instabilities subject to the frozen-temperature approximation (FTA). The present work relaxes the FTA by including the effects of latent heat and the full temperature distribution. Thermal effects slightly postpone the onset of the cellular instability but dramatically postpone the onset of the oscillatory instability; however, the absolute-stability conditions, at which at high speed the cellular and oscillatory instabilities are suppressed, remain unchanged from the FTA.

Methods of Optimal Control of Laser-Plasma Instabilities Using Spike Trains of Uneven Duration and Delay (STUD Pulses)

NASA Astrophysics Data System (ADS)

Afeyan, Bedros

2013-10-01

We have recently introduced and extensively studied a new adaptive method of LPI control. It promises to extend the effectiveness of laser as inertial fusion drivers by allowing active control of stimulated Raman and Brillouin scattering and crossed beam energy transfer. It breaks multi-nanosecond pulses into a series of picosecond (ps) time scale spikes with comparable gaps in between. The height and width of each spike as well as their separations are optimization parameters. In addition, the spatial speckle patterns are changed after a number of successive spikes as needed (from every spike to never). The combination of these parameters allows the taming of parametric instabilities to conform to any desired reduced reflectivity profile, within the bounds of the performance limitations of the lasers. Instead of pulse shaping on hydrodynamical time scales, far faster (from 1 ps to 10 ps) modulations of the laser profile will be needed to implement the STUD pulse program for full LPI control. We will show theoretical and computational evidence for the effectiveness of the STUD pulse program to control LPI. The physics of why STUD pulses work and how optimization can be implemented efficiently using statistical nonlinear optical models and techniques will be explained. We will also discuss a novel diagnostic system employing STUD pulses that will allow the boosted measurement of velocity distribution function slopes on a ps time scale in the small crossing volume of a pump and a probe beam. Various regimes from weak to strong coupling and weak to strong damping will be treated. Novel pulse modulation schemes and diagnostic tools based on time-lenses used in both microscope and telescope modes will be suggested for the execution of the STUD pule program. Work Supported by the DOE NNSA-OFES Joint Program on HEDLP and DOE OFES SBIR Phase I Grants.

PIC Simulations of Hypersonic Plasma Instabilities

NASA Astrophysics Data System (ADS)

Niehoff, D.; Ashour-Abdalla, M.; Niemann, C.; Decyk, V.; Schriver, D.; Clark, E.

2013-12-01

The plasma sheaths formed around hypersonic aircraft (Mach number, M > 10) are relatively unexplored and of interest today to both further the development of new technologies and solve long-standing engineering problems. Both laboratory experiments and analytical/numerical modeling are required to advance the understanding of these systems; it is advantageous to perform these tasks in tandem. There has already been some work done to study these plasmas by experiments that create a rapidly expanding plasma through ablation of a target with a laser. In combination with a preformed magnetic field, this configuration leads to a magnetic "bubble" formed behind the front as particles travel at about Mach 30 away from the target. Furthermore, the experiment was able to show the generation of fast electrons which could be due to instabilities on electron scales. To explore this, future experiments will have more accurate diagnostics capable of observing time- and length-scales below typical ion scales, but simulations are a useful tool to explore these plasma conditions theoretically. Particle in Cell (PIC) simulations are necessary when phenomena are expected to be observed at these scales, and also have the advantage of being fully kinetic with no fluid approximations. However, if the scales of the problem are not significantly below the ion scales, then the initialization of the PIC simulation must be very carefully engineered to avoid unnecessary computation and to select the minimum window where structures of interest can be studied. One method of doing this is to seed the simulation with either experiment or ion-scale simulation results. Previous experiments suggest that a useful configuration for studying hypersonic plasma configurations is a ring of particles rapidly expanding transverse to an external magnetic field, which has been simulated on the ion scale with an ion-hybrid code. This suggests that the PIC simulation should have an equivalent configuration; however, modeling a plasma expanding radially in every direction is computationally expensive. In order to reduce the computational expense, we use a radial density profile from the hybrid simulation results to seed a self-consistent PIC simulation in one direction (x), while creating a current in the direction (y) transverse to both the drift velocity and the magnetic field (z) to create the magnetic bubble observed in experiment. The simulation will be run in two spatial dimensions but retain three velocity dimensions, and the results will be used to explore the growth of micro-instabilities present in hypersonic plasmas in the high-density region as it moves through the simulation box. This will still require a significantly large box in order to compare with experiment, as the experiments are being performed over distances of 104 λDe and durations of 105 ωpe-1.

Turbulent mass transfer caused by vortex induced reconnection in collisionless magnetospheric plasmas

DOE PAGES

Nakamura, T. K. M.; Hasegawa, H.; Daughton, William Scott; ...

2017-11-17

Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth’s magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic shear is too small for spontaneous reconnection, the Kelvin–Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin–Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed bymore » the Magnetospheric Multiscale (MMS) spacecraft. Here in this paper, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin–Helmholtz instability.« less

Particle Energization via Tearing Instability with Global Self-Organization Constraints

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

Sarff, John; Guo, Fan

The presentation reviews how tearing magnetic reconnection leads to powerful ion energization in reversed field pinch (RFP) plasmas. A mature MHD model for tearing instability has been developed that captures key nonlinear dynamics from the global to intermediate spatial scales. A turbulent cascade is also present that extends to at least the ion gyroradius scale, within which important particle energization mechanisms are anticipated. In summary, Ion heating and acceleration associated with magnetic reconnection from tearing instability is a powerful process in the RFP laboratory plasma (gyro-resonant and stochastic processes are likely candidates to support the observed rapid heating and othermore » features, reconnection-driven electron heating appears weaker or even absent, energetic tail formation for ions and electrons). Global self-organization strongly impacts particle energization (tearing interactions that span to core to edge, global magnetic flux change produces a larger electric field and runaway, correlations in electric and magnetic field fluctuations needed for dynamo feedback, impact of transport processes (which can be quite different for ions and electrons), inhomogeneity on the system scale, e.g., strong edge gradients).« less

Turbulent mass transfer caused by vortex induced reconnection in collisionless magnetospheric plasmas

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

Nakamura, T. K. M.; Hasegawa, H.; Daughton, William Scott

Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth’s magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic shear is too small for spontaneous reconnection, the Kelvin–Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin–Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed bymore » the Magnetospheric Multiscale (MMS) spacecraft. Here in this paper, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin–Helmholtz instability.« less

An asymptotically exact reduced PDE model for the magnetorotational instability: derivation and numerical simulations

NASA Astrophysics Data System (ADS)

Jamroz, Ben; Julien, Keith; Knobloch, Edgar

2008-12-01

Taking advantage of disparate spatio-temporal scales relevant to astrophysics and laboratory experiments, we derive asymptotically exact reduced partial differential equation models for the magnetorotational instability. These models extend recent single-mode formulations leading to saturation in the presence of weak dissipation, and are characterized by a back-reaction on the imposed shear. Numerical simulations performed for a broad class of initial conditions indicate an initial phase of growth dominated by the optimal (fastest growing) magnetorotational instability fingering mode, followed by a vertical coarsening to a box-filling mode.

Scale effect challenges in urban hydrology highlighted with a distributed hydrological model

NASA Astrophysics Data System (ADS)

Ichiba, Abdellah; Gires, Auguste; Tchiguirinskaia, Ioulia; Schertzer, Daniel; Bompard, Philippe; Ten Veldhuis, Marie-Claire

2018-01-01

Hydrological models are extensively used in urban water management, development and evaluation of future scenarios and research activities. There is a growing interest in the development of fully distributed and grid-based models. However, some complex questions related to scale effects are not yet fully understood and still remain open issues in urban hydrology. In this paper we propose a two-step investigation framework to illustrate the extent of scale effects in urban hydrology. First, fractal tools are used to highlight the scale dependence observed within distributed data input into urban hydrological models. Then an intensive multi-scale modelling work is carried out to understand scale effects on hydrological model performance. Investigations are conducted using a fully distributed and physically based model, Multi-Hydro, developed at Ecole des Ponts ParisTech. The model is implemented at 17 spatial resolutions ranging from 100 to 5 m. Results clearly exhibit scale effect challenges in urban hydrology modelling. The applicability of fractal concepts highlights the scale dependence observed within distributed data. Patterns of geophysical data change when the size of the observation pixel changes. The multi-scale modelling investigation confirms scale effects on hydrological model performance. Results are analysed over three ranges of scales identified in the fractal analysis and confirmed through modelling. This work also discusses some remaining issues in urban hydrology modelling related to the availability of high-quality data at high resolutions, and model numerical instabilities as well as the computation time requirements. The main findings of this paper enable a replacement of traditional methods of model calibration by innovative methods of model resolution alteration based on the spatial data variability and scaling of flows in urban hydrology.

Turnover Begets Turnover: An Examination of Job and Occupational Instability among Child Care Center Staff.

ERIC Educational Resources Information Center

Whitebook, Marcy; Sakai, Laura

2003-01-01

Examined job/occupational instability among child care teachers and directors. Found that over half of teachers and a third of directors had left their centers 4 years after initial interviews. Highly trained teachers were more likely to leave jobs if they earned lower wages, worked in less stable climate, or worked with greater percentage of…

State Instability and Terrorism

DTIC Science & Technology

2010-01-01

instability at the country-level using a modified breakdown theoretical framework. This framework is based especially upon the work of Emile Durkheim ...Quantitative Criminology, ed. Alex R. Piquero and David Weisburd. New York: Springer New York. 225 Durkheim , Emile . 1930 [1951]. Suicide: A...terrorism is a form ( Durkheim , 1930 [1951]; Useem, 1998). In addition, different types of instability ought to invite different levels of terrorism

A validated non-linear Kelvin-Helmholtz benchmark for numerical hydrodynamics

NASA Astrophysics Data System (ADS)

Lecoanet, D.; McCourt, M.; Quataert, E.; Burns, K. J.; Vasil, G. M.; Oishi, J. S.; Brown, B. P.; Stone, J. M.; O'Leary, R. M.

2016-02-01

The non-linear evolution of the Kelvin-Helmholtz instability is a popular test for code verification. To date, most Kelvin-Helmholtz problems discussed in the literature are ill-posed: they do not converge to any single solution with increasing resolution. This precludes comparisons among different codes and severely limits the utility of the Kelvin-Helmholtz instability as a test problem. The lack of a reference solution has led various authors to assert the accuracy of their simulations based on ad hoc proxies, e.g. the existence of small-scale structures. This paper proposes well-posed two-dimensional Kelvin-Helmholtz problems with smooth initial conditions and explicit diffusion. We show that in many cases numerical errors/noise can seed spurious small-scale structure in Kelvin-Helmholtz problems. We demonstrate convergence to a reference solution using both ATHENA, a Godunov code, and DEDALUS, a pseudo-spectral code. Problems with constant initial density throughout the domain are relatively straightforward for both codes. However, problems with an initial density jump (which are the norm in astrophysical systems) exhibit rich behaviour and are more computationally challenging. In the latter case, ATHENA simulations are prone to an instability of the inner rolled-up vortex; this instability is seeded by grid-scale errors introduced by the algorithm, and disappears as resolution increases. Both ATHENA and DEDALUS exhibit late-time chaos. Inviscid simulations are riddled with extremely vigorous secondary instabilities which induce more mixing than simulations with explicit diffusion. Our results highlight the importance of running well-posed test problems with demonstrated convergence to a reference solution. To facilitate future comparisons, we include as supplementary material the resolved, converged solutions to the Kelvin-Helmholtz problems in this paper in machine-readable form.

Convective Electrokinetic Instability With Conductivity Gradients

NASA Astrophysics Data System (ADS)

Chen, Chuan-Hua; Lin, Hao; Lele, Sanjiva; Santiago, Juan

2003-11-01

Electrokinetic flow instability has been experimentally identified and quantified in a glass T-junction microchannel system with a cross section of 11 um x 155 um. In this system, buffers of different conductivities were electrokinetically driven into a common mixing channel by a DC electric field. A convective instability was observed with a threshold electric field of 0.45 kV/cm for a 10:1 conductivity ratio. A physical model has been developed which consists of a modified Ohmic model formulation for electrolyte solutions and the Navier-Stokes equations with an electric body force term. The model and experiments show that bulk charge accumulation in regions of conductivity gradients is the key mechanism of such instabilities. A linear stability analysis was performed in a convective framework, and Briggs-Bers criteria were applied to determine the nature of instability. The analysis shows the instability is governed by two key parameters: the ratio of molecular diffusion to electroviscous time scale which governs the onset of instability, and the ratio of electroviscous to electroosmotic velocity which governs whether the instability is convective or absolute. The model predicted critical electric field, growth rate, wavelength, and phase speed which were comparable to experimental data.

Particle acceleration magnetic field generation, and emission in Relativistic pair jets

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Ramirez-Ruiz, E.; Hardee, P.; Hededal, C.; Kouveliotou, C.; Fishman, G. J.

2005-01-01

Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) are responsible for particle acceleration in relativistic pair jets. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic pair jet propagating through a pair plasma. Simulations show that the Weibel instability created in the collisionless shock accelerates particles perpendicular and parallel to the jet propagation direction. Simulation results show that this instability generates and amplifies highly nonuniform, small-scale magnetic fields, which contribute to the electron's transverse deflection behind the jet head. The "jitter' I radiation from deflected electrons can have different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants. The growth rate of the Weibel instability and the resulting particle acceleration depend on the magnetic field strength and orientation, and on the initial particle distribution function. In this presentation we explore some of the dependencies of the Weibel instability and resulting particle acceleration on the magnetic field strength and orientation, and the particle distribution function.

Dynamics Evolution Investigation of Mack Mode Instability in a Hypersonic Boundary Layer by Bicoherence Spectrum Analysis

NASA Astrophysics Data System (ADS)

Han, Jian; Jiang, Nan

2012-07-01

The instability of a hypersonic boundary layer on a cone is investigated by bicoherence spectrum analysis. The experiment is conducted at Mach number 6 in a hypersonic wind tunnel. The time series signals of instantaneous fluctuating surface-thermal-flux are measured by Pt-thin-film thermocouple temperature sensors mounted at 28 stations on the cone surface along streamwise direction to investigate the development of the unstable disturbances. The bicoherence spectrum analysis based on wavelet transform is employed to investigate the nonlinear interactions of the instability of Mack modes in hypersonic laminar boundary layer transition. The results show that wavelet bicoherence is a powerful tool in studying the unstable mode nonlinear interaction of hypersonic laminar-turbulent transition. The first mode instability gives rise to frequency shifts to higher unstable modes at the early stage of hypersonic laminar-turbulent transition. The modulations subsequently lead to the second mode instability occurrence. The second mode instability governs the last stage of instability and final breakdown to turbulence with multi-scale disturbances growth.

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Pair Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Ramirez-Ruiz, E.; Hardee, P.; Hededal, C.; Mizuno, Y.

2005-01-01

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created by relativistic pair jets are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet propagating through an ambient plasma with and without initial magnetic fields. The growth rates of the Weibel instability depends on the distribution of pair jets. Simulations show that the Weibel instability created in the collisionless shock accelerates particles perpendicular and parallel to the jet propagation direction. The simulation results show that this instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields, which contribute to the electron's transverse deflection behind the jet head. The "jitter" radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Adaptive and Nonadaptive Feedback Control of Global Instabilities with Application to a Heated 2-D Jet

DTIC Science & Technology

1992-04-01

Hannemann & Oertel (1989) and many others. If the the mean flow is weakly nonparallel, i.e. evolves slowly on the scale of a typical instability wave... HANNEMANN , K. & OERTEL, H. Jr. 1989 Numerical simulation of the absolutely and convectively unstable wake. J. Fluid Mech. 199, 55-88. HUERRE, P. & MONKEWITZ

Rayleigh-Taylor instability in soft elastic layers

NASA Astrophysics Data System (ADS)

Riccobelli, D.; Ciarletta, P.

2017-04-01

This work investigates the morphological stability of a soft body composed of two heavy elastic layers attached to a rigid surface and subjected only to the bulk gravity force. Using theoretical and computational tools, we characterize the selection of different patterns as well as their nonlinear evolution, unveiling the interplay between elastic and geometric effects for their formation. Unlike similar gravity-induced shape transitions in fluids, such as the Rayleigh-Taylor instability, we prove that the nonlinear elastic effects saturate the dynamic instability of the bifurcated solutions, displaying a rich morphological diagram where both digitations and stable wrinkling can emerge. The results of this work provide important guidelines for the design of novel soft systems with tunable shapes, with several applications in engineering sciences. This article is part of the themed issue 'Patterning through instabilities in complex media: theory and applications.'

Radiation from particles moving in small-scale magnetic fields created in solid-density laser-plasma laboratory experiments

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

Keenan, Brett D., E-mail: bdkeenan@ku.edu; Medvedev, Mikhail V.

2015-11-15

Plasmas created by high-intensity lasers are often subject to the formation of kinetic-streaming instabilities, such as the Weibel instability, which lead to the spontaneous generation of high-amplitude, tangled magnetic fields. These fields typically exist on small spatial scales, i.e., “sub-Larmor scales.” Radiation from charged particles moving through small-scale electromagnetic (EM) turbulence has spectral characteristics distinct from both synchrotron and cyclotron radiation, and it carries valuable information on the statistical properties of the EM field structure and evolution. Consequently, this radiation from laser-produced plasmas may offer insight into the underlying electromagnetic turbulence. Here, we investigate the prospects for, and demonstrate themore » feasibility of, such direct radiative diagnostics for mildly relativistic, solid-density laser plasmas produced in lab experiments.« less

Cross-scale transport processes in the three-dimensional Kelvin-Helmholtz instability

NASA Astrophysics Data System (ADS)

Delamere, P. A.; Burkholder, B. L.; Ma, X.; Nykyri, K.

2017-12-01

The Kelvin-Helmholtz (KH) instability is a crucial aspect of the solar wind interaction with the giant magnetospheres. Rapid internal rotation of the magnetodisc produces conditions favorable for the growth of KH vortices along much of the equatorial magnetopause boundary. Pronounced dawn/dusk asymmetries at Jupiter and Saturn indicate a robust interaction with the solar wind. Using three-dimensional hybrid simulations we investigate the transport processes associated with the flow shear-driven KH instability. Of particular importance is small-scale and intermittent reconnection generated by the twisting of the magnetic field into configurations with antiparallel components. In three-dimensions strong guide field reconnection can occur even for initially parallel magnetic field configurations. Often the twisting motion leads to pairs of reconnection sites that can operate asynchronously, generating intermittent open flux and Maxwell stresses at the magnetopause boundary. We quantify the generation of open flux using field line tracing methods, determine the Reynolds and Maxwell stresses, and evaluate the mass transport as functions of magnetic shear, velocity shear, electron pressure and plasma beta. These results are compared with magnetohydrodynamic simulations (Ma et al., 2017). In addition, we present preliminary results for the role of cross-scale coupling processes, from fluid to ion scales. In particular, we characterize small-scale waves and the their role in mixing, diffusing and heating plasma at the magnetopause boundary.

Vorticity imbalance and stability in relation to convection

NASA Technical Reports Server (NTRS)

Read, W. L.; Scoggins, J. R.

1977-01-01

A complete synoptic-scale vorticity budget was related to convection storm development in the eastern two-thirds of the United States. The 3-h sounding interval permitted a study of time changes of the vorticity budget in areas of convective storms. Results of analyses revealed significant changes in values of terms in the vorticity equation at different stages of squall line development. Average budgets for all areas of convection indicate systematic imbalance in the terms in the vorticity equation. This imbalance resulted primarily from sub-grid scale processes. Potential instability in the lower troposphere was analyzed in relation to the development of convective activity. Instability was related to areas of convection; however, instability alone was inadequate for forecast purposes. Combinations of stability and terms in the vorticity equation in the form of indices succeeded in depicting areas of convection better than any one item separately.

Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet

PubMed Central

Li, C. K.; Tzeferacos, P.; Lamb, D.; Gregori, G.; Norreys, P. A.; Rosenberg, M. J.; Follett, R. K.; Froula, D. H.; Koenig, M.; Seguin, F. H.; Frenje, J. A.; Rinderknecht, H. G.; Sio, H.; Zylstra, A. B.; Petrasso, R. D.; Amendt, P. A.; Park, H. S.; Remington, B. A.; Ryutov, D. D.; Wilks, S. C.; Betti, R.; Frank, A.; Hu, S. X.; Sangster, T. C.; Hartigan, P.; Drake, R. P.; Kuranz, C. C.; Lebedev, S. V.; Woolsey, N. C.

2016-01-01

The remarkable discovery by the Chandra X-ray observatory that the Crab nebula's jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet. PMID:27713403

A cosmic double helix in the archetypical quasar 3C273.

PubMed

Lobanov, A P; Zensus, J A

2001-10-05

Finding direct evidence for plasma instability in extragalactic jets is crucial for understanding the nature of relativistic outflows from active galactic nuclei. Our radio interferometric observations of the quasar 3C273 made with the orbiting radio telescope, HALCA, and an array of ground telescopes have yielded an image in which the emission across the jet is resolved, revealing two threadlike patterns that form a double helix inside the jet. This double helical structure is consistent with a Kelvin-Helmholtz instability, and at least five different instability modes can be identified and modeled by a light jet with a Lorentz factor of 2 and Mach number of 3.5. The model reproduces in detail the internal structure of the jet on scales of up to 30 milli-arc seconds ( approximately 300 parsecs) and is consistent with the general morphology of the jet on scales of up to 1 kiloparsec.

Electron Jet of Asymmetric Reconnection

NASA Technical Reports Server (NTRS)

Khotyaintsev, Yu. V.; Graham, D. B.; Norgren, C.; Eriksson, E.; Li, W.; Johlander, A.; Vaivads, A.; Andre, M.; Pritchett, P. L.; Retino, A.;

The Growth of Instabilities in Annular Liquid Sheets

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

Duke, Daniel J.; Honnery, Damon R; Soria, Julio

An annular liquid sheet surrounded by parallel co-flowing gas is an effective atomiser. However, the initial instabilities which determine the primary break-up of the liquid sheet are not well understood. Lack of agreement on the influence of the boundary conditions and the non-dimension scaling of the initial instability persists between theoretical stability analyses and experiments. To address this matter, we have undertaken an experimental parametric study of an aerodynamically-driven, non-swirling annular water sheet. The effects of sheet thickness, inner and outer gas-liquid momentum ratio were investigated over an order of magnitude variation in Reynolds and Weber number. From high-speed imagemore » correlation measurements in the near-nozzle region, we propose new empirical correlations for the frequency of the instability as a function of the total gas-liquid momentum ratio, with good non-dimensional collapse. From analysis of the instability velocity probability densities, we find two persistent and distinct superimposed instabilities with different growth rates. The first is a short-lived, rapidly saturating sawtooth-like instability. The second is a slower-growing stochastic instability which persists through the break-up of the sheet. The presence of multiple instabilities whose growth rates do not strongly correlate with the shear velocities may explain some of the discrepancies between experiments and stability analyses.« less

Bead lightning formation

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

Ludwig, G.O.; Saba, M.M.F.; Division of Space Geophysics, National Space Research Institute, 12227-010, Sao Jose dos Campos, SP

2005-09-15

Formation of beaded structures in triggered lightning discharges is considered in the framework of both magnetohydrodynamic (MHD) and hydrodynamic instabilities. It is shown that the space periodicity of the structures can be explained in terms of the kink and sausage type instabilities in a cylindrical discharge with anomalous viscosity. In particular, the fast growth rate of the hydrodynamic Rayleigh-Taylor instability, which is driven by the backflow of air into the channel of the decaying return stroke, dominates the initial evolution of perturbations during the decay of the return current. This instability is responsible for a significant enhancement of the anomalousmore » viscosity above the classical level. Eventually, the damping introduced at the current channel edge by the high level of anomalous viscous stresses defines the final length scale of bead lightning. Later, during the continuing current stage of the lightning flash, the MHD pinch instability persists, although with a much smaller growth rate that can be enhanced in a M-component event. The combined effect of these instabilities may explain various aspects of bead lightning.« less

Elliptical Instability of Rotating Von Karman Street

NASA Astrophysics Data System (ADS)

Stegner, A.; Pichon, T.; Beunier, M.

Clouds often reveal a meso-scale vortex shedding in the wake of mountainous islands. Unlike the classical bi-dimensional Von-Karman street, these observed vortex street are affected by the earth rot ation and vertical stratification. Theses effects could induce a selective destabilization of anticyclonic vortices. It is well known that inertial instability (also called centrifugal instability) induce a three- dimensional destabilization of anticyclonic structures when the absolute vorticity is larger than the local Coriolis parameter. However, we have shown, by the mean of laboratory experiments, that it is a different type of instability which is mainly responsible for asymmetric rotating Von-Karman street. A serie of experiments were performed to study the wake of a cylinder in a rotating fluid, at medium Reynolds number and order one Rossby number. We have shown that the vertical structure of unstable anticyclonic vortices is characteristic of an elliptical instability. Besides, unlike the inertial instability, the vertical unstable wavelength depends on the Rossby number.

Interfacial fluid instabilities and Kapitsa pendula.

PubMed

Krieger, Madison S

2017-07-01

The onset and development of instabilities is one of the central problems in fluid mechanics. Here we develop a connection between instabilities of free fluid interfaces and inverted pendula. When acted upon solely by the gravitational force, the inverted pendulum is unstable. This position can be stabilized by the Kapitsa phenomenon, in which high-frequency low-amplitude vertical vibrations of the base creates a fictitious force which opposes the gravitational force. By transforming the dynamical equations governing a fluid interface into an appropriate pendulum-type equation, we demonstrate how stability can be induced in fluid systems by properly tuned vibrations. We construct a "dictionary"-type relationship between various pendula and the classical Rayleigh-Taylor, Kelvin-Helmholtz, Rayleigh-Plateau and the self-gravitational instabilities. This makes several results in control theory and dynamical systems directly applicable to the study of tunable fluid instabilities, where the critical wavelength depends on the external forces or the instability is suppressed entirely. We suggest some applications and instances of the effect ranging in scale from microns to the radius of a galaxy.

Perturbation solutions of combustion instability problems

NASA Technical Reports Server (NTRS)

Googerdy, A.; Peddieson, J., Jr.; Ventrice, M.

1979-01-01

A method involving approximate modal analysis using the Galerkin method followed by an approximate solution of the resulting modal-amplitude equations by the two-variable perturbation method (method of multiple scales) is applied to two problems of pressure-sensitive nonlinear combustion instability in liquid-fuel rocket motors. One problem exhibits self-coupled instability while the other exhibits mode-coupled instability. In both cases it is possible to carry out the entire linear stability analysis and significant portions of the nonlinear stability analysis in closed form. In the problem of self-coupled instability the nonlinear stability boundary and approximate forms of the limit-cycle amplitudes and growth and decay rates are determined in closed form while the exact limit-cycle amplitudes and growth and decay rates are found numerically. In the problem of mode-coupled instability the limit-cycle amplitudes are found in closed form while the growth and decay rates are found numerically. The behavior of the solutions found by the perturbation method are in agreement with solutions obtained using complex numerical methods.

Characterising Dynamic Instability in High Water-Cut Oil-Water Flows Using High-Resolution Microwave Sensor Signals

NASA Astrophysics Data System (ADS)

Liu, Weixin; Jin, Ningde; Han, Yunfeng; Ma, Jing

2018-06-01

In the present study, multi-scale entropy algorithm was used to characterise the complex flow phenomena of turbulent droplets in high water-cut oil-water two-phase flow. First, we compared multi-scale weighted permutation entropy (MWPE), multi-scale approximate entropy (MAE), multi-scale sample entropy (MSE) and multi-scale complexity measure (MCM) for typical nonlinear systems. The results show that MWPE presents satisfied variability with scale and anti-noise ability. Accordingly, we conducted an experiment of vertical upward oil-water two-phase flow with high water-cut and collected the signals of a high-resolution microwave resonant sensor, based on which two indexes, the entropy rate and mean value of MWPE, were extracted. Besides, the effects of total flow rate and water-cut on these two indexes were analysed. Our researches show that MWPE is an effective method to uncover the dynamic instability of oil-water two-phase flow with high water-cut.

Suppression of small-scale self-focusing of high-power laser beams due to their self-filtration during propagation in free space

NASA Astrophysics Data System (ADS)

Ginzburg, V. N.; Kochetkov, A. A.; Potemkin, A. K.; Khazanov, E. A.

2018-04-01

It has been experimentally confirmed that self-cleaning of a laser beam from spatial noise during propagation in free space makes it possible to suppress efficiently the self-focusing instability without applying spatial filters. Measurements of the instability increment by two independent methods have demonstrated quantitative agreement with theory and high efficiency of small-scale self-focusing suppression. This opens new possibilities for using optical elements operating in transmission (frequency doublers, phase plates, beam splitters, polarisers, etc.) in beams with intensities on the order of a few TW cm‑2.

A light Higgs boson would invite supersymmetry

NASA Astrophysics Data System (ADS)

Ellis, J.; Ross, D.

2001-05-01

If the Higgs boson weighs about 115 GeV, the effective potential of the Standard Model becomes unstable above a scale of about 106 GeV. This instability may be rectified only by new bosonic particles such as stop squarks. However, avoiding the instability requires fine-tuning of the model couplings, in particular if the theory is not to become non-perturbative before the Planck scale. Such fine-tuning is automatic in a supersymmetric model, but is lost if there are no higgsinos. A light Higgs boson would be prima facie evidence for supersymmetry in the top-quark and Higgs sectors.

Statistical simulation of the magnetorotational dynamo.

PubMed

Squire, J; Bhattacharjee, A

2015-02-27

Turbulence and dynamo induced by the magnetorotational instability (MRI) are analyzed using quasilinear statistical simulation methods. It is found that homogenous turbulence is unstable to a large-scale dynamo instability, which saturates to an inhomogenous equilibrium with a strong dependence on the magnetic Prandtl number (Pm). Despite its enormously reduced nonlinearity, the dependence of the angular momentum transport on Pm in the quasilinear model is qualitatively similar to that of nonlinear MRI turbulence. This demonstrates the importance of the large-scale dynamo and suggests how dramatically simplified models may be used to gain insight into the astrophysically relevant regimes of very low or high Pm.

Hydrodynamic scaling of the deceleration-phase Rayleigh–Taylor instability

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

Bose, A., E-mail: abos@lle.rochester.edu; Woo, K. M.; Betti, R.

2015-07-15

The scaling of the deceleration phase of inertial fusion direct-drive implosions is investigated for OMEGA and National Ignition Facility (NIF)-size targets. It is shown that the deceleration-phase Rayleigh–Taylor instability (RTI) does not scale hydro-equivalently with implosion size. This is because ablative stabilization resulting from thermal conduction and radiation transport in a spherically converging geometry is different on the two scales. As a consequence, NIF-scale implosions show lower hot-spot density and mass ablation velocity, allowing for higher RTI growth. On the contrary, stabilization resulting from density-gradient enhancement, caused by reabsorption of radiation emitted from the hot spot, is higher on NIFmore » implosions. Since the RTI mitigation related to thermal conduction and radiation transport scale oppositely with implosion size, the degradation of implosion performance caused by the deceleration RTI is similar for NIF and OMEGA targets. It is found that a minimum threshold for the no-α Lawson ignition parameter of χ{sub Ω} ≈ 0.2 at the OMEGA scale is required to demonstrate hydro-equivalent ignition at the NIF scale for symmetric direct-drive implosions.« less

Hydrodynamic scaling of the deceleration-phase Rayleigh–Taylor instability

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

Bose, A.; Woo, K. M.; Nora, R.

2015-07-02

The scaling of the deceleration phase of inertial fusion direct-drive implosions is investigated for OMEGA and National Ignition Facility (NIF)-size targets. It is shown that the deceleration-phase Rayleigh–Taylor instability (RTI) does not scale hydro-equivalently with implosion size. This is because ablative stabilization resulting from thermal conduction and radiation transport in a spherically converging geometry is different on the two scales. As a consequence, NIF-scale implosions show lower hot-spot density and mass ablation velocity, allowing for higher RTI growth. On the contrary, stabilization resulting from density-gradient enhancement, caused by reabsorption of radiation emitted from the hot spot, is higher on NIFmore » implosions. Since the RTI mitigation related to thermal conduction and radiation transport scale oppositely with implosion size, the degradation of implosion performance caused by the deceleration RTI is similar for NIF and OMEGA targets. It is found that a minimum threshold for the no-α Lawson ignition parameter of ΧΩ ≈ 0.2 at the OMEGA scale is required to demonstrate hydro-equivalent ignition at the NIF scale for symmetric direct-drive implosions.« less

Influence of beta instabilities on the early stages of nucleation and growth of alpha in beta titanium alloys

NASA Astrophysics Data System (ADS)

Nag, Soumya

Microstructural evolution in beta Titanium alloys is an important factor that governs the properties exhibited by them. Intricate understanding of complex phase transformations in these alloys is vital to tailor their microstructures and in turn their properties to our advantage. One such important subject of study is the nucleation and growth of alpha precipitates triggered by the compositional instabilities in the beta matrix, instilled in them during non equilibrium heat treatments. The present work is an effort to investigate such a phenomenon. Here studies have been conducted primarily on two different beta-Titanium alloys of commercial relevance- Ti5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe), an alloy used in the aerospace industry for landing gear applications and, TNZT (Ti-35Nb-7Zr-5Ta), a potential load bearing orthopedic implant alloy. Apart from the effect of thermal treatment on these alloys, the focus of this work is to study the interplay between different alpha and beta stabilizers present in them. For this, advanced nano-scale characterization tools such as High Resolution STEM, High Resolution TEM, EFTEM and 3D Atom Probe have been used to determine the structure, distribution and composition of the non equilibrium instabilities such as beta' and o, and also to investigate the subsequent nucleation of stable alpha. Thus in this work, very early stages of phase separation via spinodal decomposition and second phase nucleation in titanium alloys are successfully probed at an atomic resolution. For the first time, atomically resolved HRSTEM 'Z'-contrast image is recorded showing modulated structures within the as-quenched beta matrix. Also in the same condition HRTEM results showed the presence of nanoscale alpha regions. These studies are revalidated by conventional selected area diffraction and 3D atom probe reconstruction results. Also TEM dark field and selected are diffraction studies are conducted to understand the effect of quenching and subsequent aging of o precipitates. Using 3D atom probe tomography, the elemental partitioning involved in coarsening of o is investigated in detail. Finally by performing a series of well planned heat treatments, an effort is made to reason out the influence of these instabilities on the morphology, volume fraction and nucleation site of alpha.

Magnetorotational Dynamo Action in the Shearing Box

NASA Astrophysics Data System (ADS)

Walker, Justin; Boldyrev, Stanislav

2017-10-01

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disk plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale nonzero azimuthal magnetic fluxes, facilitating the instability. NSF AGS-1261659, Vilas Associates Award, NSF-Teragrid Project TG-PHY110016.

Modifying mixing and instability growth through the adjustment of initial conditions in a high-energy-density counter-propagating shear experiment on OMEGA

DOE PAGES

Merritt, E. C.; Doss, F. W.; Loomis, E. N.; ...

2015-06-24

Counter-propagating shear experiments conducted at the OMEGA Laser Facility have been evaluating the effect of target initial conditions, specifically the characteristics of a tracer foil located at the shear boundary, on Kelvin-Helmholtz instability evolution and experiment transition toward nonlinearity and turbulence in the high-energy-density (HED) regime. Experiments are focused on both identifying and uncoupling the dependence of the model initial turbulent length scale in variable-density turbulence models of k-ϵ type on competing physical instability seed lengths as well as developing a path toward fully developed turbulent HED experiments. We present results from a series of experiments controllably and independently varyingmore » two initial types of scale lengths in the experiment: the thickness and surface roughness (surface perturbation scale spectrum) of a tracer layer at the shear interface. We show that decreasing the layer thickness and increasing the surface roughness both have the ability to increase the relative mixing in the system, and thus theoretically decrease the time required to begin transitioning to turbulence in the system. In addition, we also show that we can connect a change in observed mix width growth due to increased foil surface roughness to an analytically predicted change in model initial turbulent scale lengths.« less

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

Kim, Hoyoung; Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon, Gyeongnam 642-831; Kang, Jun-Yun, E-mail: firice@kims.re.kr

This study aimed to present the complete history of carbide evolution in a cold-work tool steel along its full processing route for fabrication and application. A sequence of processes from cast to final hardening heat treatment was conducted on an 8% Cr-steel to reproduce a typical commercial processing route in a small scale. The carbides found at each process step were then identified by electron diffraction with energy dispersive spectroscopy in a scanning or transmission electron microscope. After solidification, MC, M{sub 7}C{sub 3} and M{sub 2}C carbides were identified and the last one dissolved during hot compression at 1180 °C.more » In a subsequent annealing at 870 °C followed by slow cooling, M{sub 6}C and M{sub 23}C{sub 6} were added, while they were dissolved in the following austenitization at 1030 °C. After the final tempering at 520 °C, fine M{sub 23}C{sub 6} precipitated again, thus the final microstructure was the tempered martensite with MC, M{sub 7}C{sub 3} and M{sub 23}C{sub 6} carbide. The transient M{sub 2}C and M{sub 6}C originated from the segregation of Mo and finally disappeared due to attenuated segregation and the consequent thermodynamic instability. - Highlights: • The full processing route of a cold-work tool steel was simulated in a small scale. • The carbides in the tool steel were identified by chemical–crystallographic analyses. • MC, M{sub 7}C{sub 3}, M{sub 2}C, M{sub 6}C and M{sub 23}C{sub 6} carbides were found during the processing of the steel. • M{sub 2}C and M{sub 6}C finally disappeared due to thermodynamic instability.« less

Is the American Society of Shoulder and Elbow Therapists' rehabilitation guideline better than standard care when applied to Bankart-operated patients? A controlled study.

PubMed

Damkjær, Lars; Petersen, Tom; Juul-Kristensen, Birgit

2015-02-01

To determine whether there is a difference in shoulder-related physical function and quality of life between postoperative rehabilitation patients receiving standard care and those receiving care according to the American Society of Shoulder and Elbow Therapists' rehabilitation guideline for arthroscopic anterior capsulolabral repair of the shoulder. Descriptive studies with comparison between a retrospective and a prospective cohort. Municipal outpatient rehabilitation centre. A total of 96 arthroscopic Bankart-operated patients. A total of 52 patients received standard care; 44 patients underwent rehabilitation according to the American Society of Shoulder and Elbow Therapists' rehabilitation guideline. Primary outcome variable was Western Ontario Shoulder Instability Index. Secondary outcome measures were Patient-Specific Functional Scale, shoulder range of motion, return to work, return to sports, and costs. There was no significant difference in adjusted mean change scores between the standard care group and the guideline group in the primary outcome variable (Western Ontario Shoulder Instability Index total = 574.85 vs. 644.48) or the secondary outcomes (Patient-Specific Functional Scale = 4.6 vs. 5.0; range of motion in forward flexion = 46.49° vs. 49.58°; external rotation in adduction = 28.58° vs. 34.18°; external rotation in abduction = 51.29° vs. 47.55°; weeks until return to work = 5.2 vs. 6.9; weeks until return to sports =13.9 vs. 13.1; costs = number of visits; 18.5 vs. 15.9). There were no significant between-group differences in shoulder-related physical function and quality of life between the standard care group and the guideline group, following Bankart operations. © The Author(s) 2014.

Nonlinear evolution of three-dimensional instabilities of thin and thick electron scale current sheets: Plasmoid formation and current filamentation

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

Jain, Neeraj; Büchner, Jörg; Max Planck Institute for Solar System Research, Justus-Von-Liebig-Weg-3, Göttingen

Nonlinear evolution of three dimensional electron shear flow instabilities of an electron current sheet (ECS) is studied using electron-magnetohydrodynamic simulations. The dependence of the evolution on current sheet thickness is examined. For thin current sheets (half thickness =d{sub e}=c/ω{sub pe}), tearing mode instability dominates. In its nonlinear evolution, it leads to the formation of oblique current channels. Magnetic field lines form 3-D magnetic spirals. Even in the absence of initial guide field, the out-of-reconnection-plane magnetic field generated by the tearing instability itself may play the role of guide field in the growth of secondary finite-guide-field instabilities. For thicker current sheetsmore » (half thickness ∼5 d{sub e}), both tearing and non-tearing modes grow. Due to the non-tearing mode, current sheet becomes corrugated in the beginning of the evolution. In this case, tearing mode lets the magnetic field reconnect in the corrugated ECS. Later thick ECS develops filamentary structures and turbulence in which reconnection occurs. This evolution of thick ECS provides an example of reconnection in self-generated turbulence. The power spectra for both the thin and thick current sheets are anisotropic with respect to the electron flow direction. The cascade towards shorter scales occurs preferentially in the direction perpendicular to the electron flow.« less

Reliability of sternal instability scale (SIS) for transverse sternotomy in lung transplantation (LTX).

PubMed

Fuller, Louise M; El-Ansary, Doa; Button, Brenda; Bondarenko, Janet; Marasco, Silvana; Snell, Greg; Holland, Anne E

2018-01-25

A surgical incision for bilateral sequential lung transplantation (BSLTX) is the "clam shell" (CSI) approach via bilateral anterior thoracotomies and a transverse sternotomy to allow for sequential replacement of the lungs. This can be associated with significant post-operative pain, bony overriding or sternal instability. The sternal instability scale (SIS) is a non-invasive manual assessment tool that can be used to detect early bony non-union or instability following CSI; however, its reliability is unknown. This prospective blinded reliability study aimed to assess intra-rater and inter-rater reliability of the SIS following lung transplantation. Participants post BSLTX aged older than 18 years underwent sternal assessment utilizing the SIS. Two assessors examined the sternum using a standardized protocol at two separate time points with a test-re-test time of 48 hours. The outcome measure was SIS tool using four categories from 0 (clinically stable) to 3 (separated sternum with overriding). In total, 20 participants (75% female) with a mean age of 48 years (SD 17) and mean pain score of 3 out of 10 were included, 60% having well healed wounds and 25% reporting symptoms of sternal clicking. The most painful self-reported painful activity was coughing. The SIS demonstrated excellent reliability with a kappa = 0.91 by different assessors on the same day, and kappa = 0.83 for assessments by the same assessor on different days. The SIS is a reliable manual assessment tool for evaluation of sternal instability after CSI following BSLTX and may facilitate the timely detection and management of sternal instability.

EVIDENCE OF ACTIVE MHD INSTABILITY IN EULAG-MHD SIMULATIONS OF SOLAR CONVECTION

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

Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul, E-mail: nicolas.laws@gmail.ca, E-mail: strugarek@astro.umontreal.ca, E-mail: paulchar@astro.umontreal.ca

We investigate the possible development of magnetohydrodynamical instabilities in the EULAG-MHD “millennium simulation” of Passos and Charbonneau. This simulation sustains a large-scale magnetic cycle characterized by solar-like polarity reversals taking place on a regular multidecadal cadence, and in which zonally oriented bands of strong magnetic fields accumulate below the convective layers, in response to turbulent pumping from above in successive magnetic half-cycles. Key aspects of this simulation include low numerical dissipation and a strongly sub-adiabatic fluid layer underlying the convectively unstable layers corresponding to the modeled solar convection zone. These properties are conducive to the growth and development of two-dimensionalmore » instabilities that are otherwise suppressed by stronger dissipation. We find evidence for the action of a non-axisymmetric magnetoshear instability operating in the upper portions of the stably stratified fluid layers. We also investigate the possibility that the Tayler instability may be contributing to the destabilization of the large-scale axisymmetric magnetic component at high latitudes. On the basis of our analyses, we propose a global dynamo scenario whereby the magnetic cycle is driven primarily by turbulent dynamo action in the convecting layers, but MHD instabilities accelerate the dissipation of the magnetic field pumped down into the overshoot and stable layers, thus perhaps significantly influencing the magnetic cycle period. Support for this scenario is found in the distinct global dynamo behaviors observed in an otherwise identical EULAG-MHD simulations, using a different degree of sub-adiabaticity in the stable fluid layers underlying the convection zone.« less

Reduced modeling of the magnetorotational instability

NASA Astrophysics Data System (ADS)

Jamroz, Ben F.

2009-06-01

Accretion describes the process by which matter in an astrophysical disk falls onto a central massive object. Accretion disks are present in many astrophysical situations including binary star systems, young stellar objects, and near black holes at the center of galaxies. Measurements from observations of these disks have shown that viscous processes are unable to transport the necessary levels of angular momentum needed for accretion. Therefore, accretion requires an efficient mechanism of angular momentum transport. Mixing by turbulent processes greatly enhances the level of angular momentum transport in a turbulent fluid. Thus, the generation of turbulence in these disks may provide the mechanism needed for accretion. A classical result of hydrodynamic theory is that typical accretion disks are hydrodynamically stable to shear instabilities, since the specific angular momentum increases outwards. Other processes of generating hydrodynamic turbulence (barotropic instability, baroclinic instability, sound wave, shock waves, finite amplitude instabilities) may be present in these disks, however, none of these mechanisms has been shown to produce the level of angular momentum transport needed for accretion. Hydrodynamical turbulence does not produce enough angular momentum transport to produce the level of accretion observed in astrophysical accretion disks. The leading candidate for the source of turbulence leading to the transport of angular momentum is the magnetorotational instability, a linear axisymmetric instability of electrically conducting fluid in the presence of an imposed magnetic field and shear (or differential rotation). This instability is an efficient mechanism of angular momentum transport generating the level of transport needed for accretion. The level of effective angular momentum transport is determined by the saturated state of sustained turbulence generated by the instability. The mechanism of nonlinear saturation of this instability is not well understood. Many recent numerical investigations of this problem are performed in a local domain, where the global cylindrical background state is projected onto a local Cartesian domain. The resulting system is then numerically modeled within a "shearing box" framework to obtain estimates of angular momentum transport and therefore accretion. However, the simplified geometry of the local domain, and the projection of global quantities leads to a model where the instability is able to grow unboundedly. Utilizing disparate characteristic scales, this thesis presents a reduced asymptotic model for the magnetorotational instability that allows a large scale feedback of local stresses (Reynolds, Maxwell and mixed) onto the projected background state. This system is investigated numerically to determine the impact of allowing this feedback on the saturated level of angular momentum transport.

Linear study of the precessional fishbone instability

NASA Astrophysics Data System (ADS)

Idouakass, M.; Faganello, M.; Berk, H. L.; Garbet, X.; Benkadda, S.

2016-10-01

The precessional fishbone instability is an m = n = 1 internal kink mode destabilized by a population of trapped energetic particles. The linear phase of this instability is studied here, analytically and numerically, with a simplified model. This model uses the reduced magneto-hydrodynamics equations for the bulk plasma and the Vlasov equation for a population of energetic particles with a radially decreasing density. A threshold condition for the instability is found, as well as a linear growth rate and frequency. It is shown that the mode frequency is given by the precession frequency of the deeply trapped energetic particles at the position of strongest radial gradient. The growth rate is shown to scale with the energetic particle density and particle energy while it is decreased by continuum damping.

Open and Closed Loop Stability of Hingeless Rotor Helicopter Air and Ground Resonance

NASA Technical Reports Server (NTRS)

Young, M. I.; Bailey, D. J.; Hirschbein, M. S.

1974-01-01

The air and ground resonance instabilities of hingeless rotor helicopters are examined on a relatively broad parametric basis including the effects of blade tuning, virtual hinge locations, and blade hysteresis damping, as well as size and scale effects in the gross weight range from 5,000 to 48,000 pounds. A special case of a 72,000 pound helicopter air resonance instability is also included. The study shows that nominal to moderate and readily achieved levels of blade inertial hysteresis damping in conjunction with a variety of tuning and/or feedback conditions are highly effective in dealing with these instabilities. Tip weights and reductions in pre-coning angles are also shown to be effective means for improving the air resonance instability.

Bacterial Genome Instability

PubMed Central

Darmon, Elise

2014-01-01

SUMMARY Bacterial genomes are remarkably stable from one generation to the next but are plastic on an evolutionary time scale, substantially shaped by horizontal gene transfer, genome rearrangement, and the activities of mobile DNA elements. This implies the existence of a delicate balance between the maintenance of genome stability and the tolerance of genome instability. In this review, we describe the specialized genetic elements and the endogenous processes that contribute to genome instability. We then discuss the consequences of genome instability at the physiological level, where cells have harnessed instability to mediate phase and antigenic variation, and at the evolutionary level, where horizontal gene transfer has played an important role. Indeed, this ability to share DNA sequences has played a major part in the evolution of life on Earth. The evolutionary plasticity of bacterial genomes, coupled with the vast numbers of bacteria on the planet, substantially limits our ability to control disease. PMID:24600039

Nonlinear evolution of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities

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

Dimonte, G

Scaled experiments on the nonlinear evolution of the Rayleigh- Taylor (RT) and Richtmyer-Meshkov (RM) instabilities are described under a variety, of conditions that occur in nature. At high Reynolds number, the mixing layer grows self-similarly - {alpha}{sub i}Agt{sup 2} for a constant acceleration (g), and as a power law t{sup {theta}{sub i}} for impulsive accelerations U{delta}(t) at low and high Mach numbers. The growth coefficients {alpha}{sub i} and {theta}{sub i} exponents are measured over a comprehensive range of Atwood numbers A. The RT instability is also investigated with Non- Newtonian materials which are independently characterized. A critical wavelength and amplitudemore » for instability is observed associated with the shear modulus and tensile yield of the material. The results are applicable from supernova explosions to geophysical flows subject to these hydrodynamic instabilities.« less

Mapping Shallow Landslide Slope Inestability at Large Scales Using Remote Sensing and GIS

NASA Astrophysics Data System (ADS)

Avalon Cullen, C.; Kashuk, S.; Temimi, M.; Suhili, R.; Khanbilvardi, R.

2015-12-01

Rainfall induced landslides are one of the most frequent hazards on slanted terrains. They lead to great economic losses and fatalities worldwide. Most factors inducing shallow landslides are local and can only be mapped with high levels of uncertainty at larger scales. This work presents an attempt to determine slope instability at large scales. Buffer and threshold techniques are used to downscale areas and minimize uncertainties. Four static parameters (slope angle, soil type, land cover and elevation) for 261 shallow rainfall-induced landslides in the continental United States are examined. ASTER GDEM is used as bases for topographical characterization of slope and buffer analysis. Slope angle threshold assessment at the 50, 75, 95, 98, and 99 percentiles is tested locally. Further analysis of each threshold in relation to other parameters is investigated in a logistic regression environment for the continental U.S. It is determined that lower than 95-percentile thresholds under-estimate slope angles. Best regression fit can be achieved when utilizing the 99-threshold slope angle. This model predicts the highest number of cases correctly at 87.0% accuracy. A one-unit rise in the 99-threshold range increases landslide likelihood by 11.8%. The logistic regression model is carried over to ArcGIS where all variables are processed based on their corresponding coefficients. A regional slope instability map for the continental United States is created and analyzed against the available landslide records and their spatial distributions. It is expected that future inclusion of dynamic parameters like precipitation and other proxies like soil moisture into the model will further improve accuracy.

Suppression of AGN-Driven Turbulence by Magnetic Fields in a Magnetohydrodynamic Model of the Intracluster Medium

NASA Astrophysics Data System (ADS)

Bambic, Christopher J.; Morsony, Brian; Reynolds, Christopher S.

2018-01-01

We investigate the role of AGN feedback in turbulent heating of galaxy clusters. Specifically, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work which neglects magnetic fields has shown that this process is inefficient, with less than 1% of the injected energy ending up in turbulence. This inefficiency is primarily due to the fact that the bubbles are shredded apart by hydrodynamic instabilities before they can excite sufficiently strong g-modes. Using a plane-parallel model of the ICM and 3D ideal MHD simulations, we examine the role of a large-scale magnetic field which is able to drape around these rising bubbles, preserving them from hydrodynamic instabilities. We find that, while magnetic draping appears better able to preserve AGN-driven bubbles, the driving of g-modes and the resulting production of turbulence is still inefficient. The magnetic tension force prevents g-modes from transitioning into the nonlinear regime, suppressing turbulence in our model ICM below levels measured in hydrodynamic simulations. Our work highlights the ways in which ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects. These results suggest the hypothesis that other mechanisms of heating the ICM plasma such as sound waves or cosmic rays may be responsible for observed feedback in galaxy clusters.

Suppression of AGN-driven Turbulence by Magnetic Fields in a Magnetohydrodynamic Model of the Intracluster Medium

NASA Astrophysics Data System (ADS)

Bambic, Christopher J.; Morsony, Brian J.; Reynolds, Christopher S.

2018-04-01

We investigate the role of active galactic nucleus (AGN) feedback in turbulent heating of galaxy clusters. Specifically, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work that neglects magnetic fields has shown that this process is inefficient, with less than 1% of the injected energy ending up in turbulence. This inefficiency primarily arises because the bubbles are shredded apart by hydrodynamic instabilities before they can excite sufficiently strong g-modes. Using a plane-parallel model of the intracluster medium (ICM) and 3D ideal magnetohydrodynamics (MHD) simulations, we examine the role of a large-scale magnetic field that is able to drape around these rising bubbles, preserving them from hydrodynamic instabilities. We find that while magnetic draping appears better able to preserve AGN-driven bubbles, the driving of g-modes and the resulting production of turbulence is still inefficient. The magnetic tension force prevents g-modes from transitioning into the nonlinear regime, suppressing turbulence in our model ICM. Our work highlights the ways in which ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects. These results suggest the hypothesis that other mechanisms of heating the ICM plasma such as sound waves or cosmic rays may be responsible for the observed feedback in galaxy clusters.

Current Driven Instabilities and Anomalous Mobility in Hall-effect Thrusters

NASA Astrophysics Data System (ADS)

Tran, Jonathan; Eckhardt, Daniel; Martin, Robert

2017-10-01

Due to the extreme cost of fully resolving the Debye length and plasma frequency, hybrid plasma simulations utilizing kinetic ions and quasi-steady state fluid electrons have long been the principle workhorse methodology for Hall-effect thruster (HET) modeling. Plasma turbulence and the resulting anomalous electron transport in HETs is a promising candidate for developing predictive models for the observed anomalous transport. In this work, we investigate the implementation of an anomalous electron cross field transport model for hybrid HET simulations such a HPHall. A theory for anomalous transport in HETs and current driven instabilities has been recently studied by Lafleur et al. This work has shown collective electron-wave scattering due to large amplitude azimuthal fluctuations of the electric field. We will further adapt the previous results for related current driven instabilities to electric propulsion relevant mass ratios and conduct a preliminary study of resolving this instability with a modified hybrid (fluid electron and kinetic ion) simulation with the hope of integration with established hybrid HET simulations. This work is supported by the Air Force Office of Scientific Research award FA9950-17RQCOR465.

C/NOFS Satellite Electric Field and Plasma Density Observations of Plasma Instabilities Below the Equatorial F-Peak -- Evidence for Approximately 500 km-Scale Spread-F "Precursor" Waves Driven by Zonal Shear Flow and km-Scale, Narrow-Banded Irregularities

NASA Technical Reports Server (NTRS)

Pfaff, R.; Freudenreich, H.; Klenzing, J.; Liebrecht, C.; Valladares, C.

2011-01-01

As solar activity has increased, the ionosphere F-peak has been elevated on numerous occasions above the C/NOFS satellite perigee of 400km. In particular, during the month of April, 2011, the satellite consistently journeyed below the F-peak whenever the orbit was in the region of the South Atlantic anomaly after sunset. During these passes, data from the electric field and plasma density probes on the satellite have revealed two types of instabilities which had not previously been observed in the C/NOFS data set (to our knowledge): The first is evidence for 400-500km-scale bottomside "undulations" that appear in the density and electric field data. In one case, these large scale waves are associated with a strong shear in the zonal E x B flow, as evidenced by variations in the meridional (outward) electric fields observed above and below the F-peak. These undulations are devoid of smaller scale structures in the early evening, yet appear at later local times along the same orbit associated with fully-developed spread-F with smaller scale structures. This suggests that they may be precursor waves for spread-F, driven by a collisional shear instability, following ideas advanced previously by researchers using data from the Jicamarca radar. A second new result (for C/NOFS) is the appearance of km-scale irregularities that are a common feature in the electric field and plasma density data that also appear when the satellite is below the F -peak at night. The vector electric field instrument on C/NOFS clearly shows that the electric field component of these waves is strongest in the zonal direction. These waves are strongly correlated with simultaneous observations of plasma density oscillations and appear both with, and without, evidence of larger-scale spread-F depletions. These km-scale, quasi-coherent waves strongly resemble the bottomside, sinusoidal irregularities reported in the Atmosphere Explorer satellite data set by Valladares et al. [JGR, 88, 8025, 1983]. We interpret these new observations in terms of fundamental plasma instabilities associated with the unstable, nighttime equatorial ionosphere.

Effect of Magnetic Twist on Nonlinear Transverse Kink Oscillations of Line-tied Magnetic Flux Tubes

NASA Astrophysics Data System (ADS)

Terradas, J.; Magyar, N.; Van Doorsselaere, T.

2018-01-01

Magnetic twist is thought to play an important role in many structures of the solar atmosphere. One of the effects of twist is to modify the properties of the eigenmodes of magnetic tubes. In the linear regime standing kink solutions are characterized by a change in polarization of the transverse displacement along the twisted tube. In the nonlinear regime, magnetic twist affects the development of shear instabilities that appear at the tube boundary when it is oscillating laterally. These Kelvin–Helmholtz instabilities (KHI) are produced either by the jump in the azimuthal component of the velocity at the edge of the sharp boundary between the internal and external part of the tube or by the continuous small length scales produced by phase mixing when there is a smooth inhomogeneous layer. In this work the effect of twist is consistently investigated by solving the time-dependent problem including the process of energy transfer to the inhomogeneous layer. It is found that twist always delays the appearance of the shear instability, but for tubes with thin inhomogeneous layers the effect is relatively small for moderate values of twist. On the contrary, for tubes with thick layers, the effect of twist is much stronger. This can have some important implications regarding observations of transverse kink modes and the KHI itself.

Integration of GB-InSAR, laser scanning and in situ monitoring on the rockslope instability of Mannen/Børa (western Norway)

NASA Astrophysics Data System (ADS)

Rouyet, Line; Kristensen, Lene; Derron, Marc-Henri; Michoud, Clément; Harald, Blikra Lars; Michel, Jaboyedoff

2013-04-01

This work is part of a master thesis about the use of Ground-Based InSAR for the monitoring of rock instabilities (University of Lausanne in cooperation with the Åknes/Tafjord Early Warning Centre in Norway). Main goals are (1) the evaluation of the GB-InSAR potential to investigate different kinds of instabilities, (2) the combination of data from GB-InSAR, conventional in situ devices and laser scanning to get information about instability behavior and geometry. The rockslope instability of Mannen/Børa is located in Møre of Romsdal County (western Norway). Mannen is a complex rockslide of 15-25 mill. m3 of volume, affecting the left side of the Romsdalen valley. Børa is a large plateau directly located on its south-eastern side and showing signs of activity. In this case, the analysis included GB-InSAR data of 2011 and 2012 campaigns in Børa compared with results of a permanent GB-InSAR in Mannen. The results of continuous monitoring in Mannen (GPS, extensometers, laser-reflectors and tiltmeters) since end of 2009, as well as periodical GPS campaigns on Børa plateau were integrated. The analysis showed a quite regular inter-annual velocity with seasonal effects in Mannen site and a slower movement in Børa. Moreover, it allowed highlighting an area in mid-slope, affected by high variations and periodical inversions of movement in the overlap sector between the two GB-InSAR. The first interpretation of this pattern involves networks of water flow across the slope. A novel point of this site is to have two GB-InSAR systems (one permanent and one temporary) imaging the rockslope with an overlap of views. GB-InSAR results were compared to other types of monitoring data, in terms of spatial coverage (punctual vs. large area), temporal scale (continuous monitoring vs. periodical campaigns) or recorded information (eg. 3D vs. 1D along the LOS). Moreover, a structural geology analysis based on terrestrial and airborne laser scanning data provided information about the geometry of rock instabilities and sliding surfaces.

Acoustic instability driven by cosmic-ray streaming

NASA Technical Reports Server (NTRS)

Begelman, Mitchell C.; Zweibel, Ellen G.

1994-01-01

We study the linear stability of compressional waves in a medium through which cosmic rays stream at the Alfven speed due to strong coupling with Alfven waves. Acoustic waves can be driven unstable by the cosmic-ray drift, provided that the streaming speed is sufficiently large compared to the thermal sound speed. Two effects can cause instability: (1) the heating of the thermal gas due to the damping of Alfven waves driven unstable by cosmic-ray streaming; and (2) phase shifts in the cosmic-ray pressure perturbation caused by the combination of cosmic-ray streaming and diffusion. The instability does not depend on the magnitude of the background cosmic-ray pressure gradient, and occurs whether or not cosmic-ray diffusion is important relative to streaming. When the cosmic-ray pressure is small compared to the gas pressure, or cosmic-ray diffusion is strong, the instability manifests itself as a weak overstability of slow magnetosonic waves. Larger cosmic-ray pressure gives rise to new hybrid modes, which can be strongly unstable in the limits of both weak and strong cosmic-ray diffusion and in the presence of thermal conduction. Parts of our analysis parallel earlier work by McKenzie & Webb (which were brought to our attention after this paper was accepted for publication), but our treatment of diffusive effects, thermal conduction, and nonlinearities represent significant extensions. Although the linear growth rate of instability is independent of the background cosmic-ray pressure gradient, the onset of nonlinear eff ects does depend on absolute value of DEL (vector differential operator) P(sub c). At the onset of nonlinearity the fractional amplitude of cosmic-ray pressure perturbations is delta P(sub C)/P(sub C) approximately (kL) (exp -1) much less than 1, where k is the wavenumber and L is the pressure scale height of the unperturbed cosmic rays. We speculate that the instability may lead to a mode of cosmic-ray transport in which plateaus of uniform cosmic-ray pressure are separated by either laminar or turbulent jumps in which the thermal gas is subject to intense heating.

Experimental investigation on thermo-magnetic convection inside cavities.

PubMed

Gontijo, R G; Cunha, F R

2012-12-01

This paper presents experimental results on thermo-magnetic convection inside cavities. We examine the flow induced by convective currents inside a cavity with aspect ratio near the unity and the heat transfer rates measurements inside a thin cavity with aspect ratio equal to twelve. The convective unstable currents are formed when a magnetic suspension is subjected to a temperature gradient combined with a gradient of an externally imposed magnetic field. Under these conditions, stratifications in the suspension density and susceptibility are both important effects to the convective motion. We show a comparison between flow patterns of magnetic and gravitational convections. The impact of the presence of a magnetic field on the amount of heat extracted from the system when magnetic and gravitational effects are combined inside the test cell is evaluated. The convection state is largely affected by new instability modes produced by stratification in susceptibility. The experiments reveal that magnetic field enhances the instability in the convective flow leading to a more effective mixing and consequently to a more statistically homogenous temperature distribution inside the test cell. The experimental results allow the validation of the scaling law proposed in a previous theoretical work that has predicted that the Nusselt number scales with the magnetic Rayleigh number to the power of 1/3, in the limit in which magnetic force balances viscous force in the convective flow.

Parametric instability, inverse cascade and the range of solar-wind turbulence

NASA Astrophysics Data System (ADS)

Chandran, Benjamin D. G.

2018-02-01

In this paper, weak-turbulence theory is used to investigate the nonlinear evolution of the parametric instability in three-dimensional low- plasmas at wavelengths much greater than the ion inertial length under the assumption that slow magnetosonic waves are strongly damped. It is shown analytically that the parametric instability leads to an inverse cascade of Alfvén wave quanta, and several exact solutions to the wave kinetic equations are presented. The main results of the paper concern the parametric decay of Alfvén waves that initially satisfy +\\gg e-$ , where +$ and -$ are the frequency ( ) spectra of Alfvén waves propagating in opposite directions along the magnetic field lines. If +$ initially has a peak frequency 0$ (at which +$ is maximized) and an `infrared' scaling p$ at smaller with , then +$ acquires an -1$ scaling throughout a range of frequencies that spreads out in both directions from 0$ . At the same time, -$ acquires an -2$ scaling within this same frequency range. If the plasma parameters and infrared +$ spectrum are chosen to match conditions in the fast solar wind at a heliocentric distance of 0.3 astronomical units (AU), then the nonlinear evolution of the parametric instability leads to an +$ spectrum that matches fast-wind measurements from the Helios spacecraft at 0.3 AU, including the observed -1$ scaling at -4~\\text{Hz}$ . The results of this paper suggest that the -1$ spectrum seen by Helios in the fast solar wind at -4~\\text{Hz}$ is produced in situ by parametric decay and that the -1$ range of +$ extends over an increasingly narrow range of frequencies as decreases below 0.3 AU. This prediction will be tested by measurements from the Parker Solar Probe.

In search of late time evolution self-similar scaling laws of Rayleigh-Taylor and Richtmyer-Meshkov hydrodynamic instabilities - recent theorical advance and NIF Discovery-Science experiments

NASA Astrophysics Data System (ADS)

Shvarts, Dov

2017-10-01

Hydrodynamic instabilities, and the mixing that they cause, are of crucial importance in describing many phenomena, from very large scales such as stellar explosions (supernovae) to very small scales, such as inertial confinement fusion (ICF) implosions. Such mixing causes the ejection of stellar core material in supernovae, and impedes attempts at ICF ignition. The Rayleigh-Taylor instability (RTI) occurs at an accelerated interface between two fluids with the lower density accelerating the higher density fluid. The Richtmyer-Meshkov (RM) instability occurs when a shock wave passes an interface between the two fluids of different density. In the RTI, buoyancy causes ``bubbles'' of the light fluid to rise through (penetrate) the denser fluid, while ``spikes'' of the heavy fluid sink through (penetrate) the lighter fluid. With realistic multi-mode initial conditions, in the deep nonlinear regime, the mixing zone width, H, and its internal structure, progress through an inverse cascade of spatial scales, reaching an asymptotic self-similar evolution: hRT =αRT Agt2 for RT and hRM =αRM tθ for RM. While this characteristic behavior has been known for years, the self-similar parameters αRT and θRM and their dependence on dimensionality and density ratio have continued to be intensively studied and a relatively wide distribution of those values have emerged. This talk will describe recent theoretical advances in the description of this turbulent mixing evolution that sheds light on the spread in αRT and θRM. Results of new and specially designed experiments, done by scientists from several laboratories, were performed recently using NIF, the only facility that is powerful enough to reach the self-similar regime, for quantitative testing of this theoretical advance, will be presented.

Classically Stable Nonsingular Cosmological Bounces

NASA Astrophysics Data System (ADS)

Ijjas, Anna; Steinhardt, Paul J.

2016-09-01

One of the fundamental questions of theoretical cosmology is whether the Universe can undergo a nonsingular bounce, i.e., smoothly transit from a period of contraction to a period of expansion through violation of the null energy condition (NEC) at energies well below the Planck scale and at finite values of the scale factor such that the entire evolution remains classical. A common claim has been that a nonsingular bounce either leads to ghost or gradient instabilities or a cosmological singularity. In this Letter, we consider a well-motivated class of theories based on the cubic Galileon action and present a procedure for explicitly constructing examples of a nonsingular cosmological bounce without encountering any pathologies and maintaining a subluminal sound speed for comoving curvature modes throughout the NEC violating phase. We also discuss the relation between our procedure and earlier work.

Independent and collective roles of surface structures at different length scales on pool boiling heat transfer

PubMed Central

Li, Calvin H.; Rioux, Russell P.

2016-01-01

Spherical Cu nanocavity surfaces are synthesized to examine the individual role of contact angles in connecting lateral Rayleigh-Taylor wavelength to vertical Kevin-Helmholtz wavelength on hydrodynamic instability for the onset of pool boiling Critical Heat Flux (CHF). Solid and porous Cu pillar surfaces are sintered to investigate the individual role of pillar structure pitch at millimeter scale, named as module wavelength, on hydrodynamic instability at CHF. Last, spherical Cu nanocavities are coated on the porous Cu pillars to create a multiscale Cu structure, which is studied to examine the collective role and relative significance of contact angles and module wavelength on hydrodynamic instability at CHF, and the results indicate that module wavelength plays the dominant role on hydrodynamic instability at CHF when the height of surface structures is equal or above ¼ Kelvin-Helmholtz wavelength. Pool boiling Heat Transfer Coefficient (HTC) enhancements on spherical Cu nanocavity surfaces, solid and porous Cu pillar surfaces, and the integrated multiscale structure have been investigated, too. The experimental results reveal that the nanostructures and porous pillar structures can be combined together to achieve even higher enhancement of HTC than that of individual structures. PMID:27841322

Linear analysis of the Richtmyer-Meshkov instability in shock-flame interactions

NASA Astrophysics Data System (ADS)

Massa, L.; Jha, P.

2012-05-01

Shock-flame interactions enhance supersonic mixing and detonation formation. Therefore, their analysis is important to explosion safety, internal combustion engine performance, and supersonic combustor design. The fundamental process at the basis of the interaction is the Richtmyer-Meshkov instability supported by the density difference between burnt and fresh mixtures. In the present study we analyze the effect of reactivity on the Richtmyer-Meshkov instability with particular emphasis on combustion lengths that typify the scaling between perturbation growth and induction. The results of the present linear analysis study show that reactivity changes the perturbation growth rate by developing a pressure gradient at the flame surface. The baroclinic torque based on the density gradient across the flame acts to slow down the instability growth of high wave-number perturbations. A gasdynamic flame representation leads to the definition of a Peclet number representing the scaling between perturbation and thermal diffusion lengths within the flame. Peclet number effects on perturbation growth are observed to be marginal. The gasdynamic model also considers a finite flame Mach number that supports a separation between flame and contact discontinuity. Such a separation destabilizes the interface growth by augmenting the tangential shear.

Studies on the dynamic stability of an axially moving nanobeam based on the nonlocal strain gradient theory

NASA Astrophysics Data System (ADS)

Wang, Jing; Shen, Huoming; Zhang, Bo; Liu, Juan

2018-06-01

In this paper, we studied the parametric resonance issue of an axially moving viscoelastic nanobeam with varying velocity. Based on the nonlocal strain gradient theory, we established the transversal vibration equation of the axially moving nanobeam and the corresponding boundary condition. By applying the average method, we obtained a set of self-governing ordinary differential equations when the excitation frequency of the moving parameters is twice the intrinsic frequency or near the sum of certain second-order intrinsic frequencies. On the plane of parametric excitation frequency and excitation amplitude, we can obtain the instability region generated by the resonance, and through numerical simulation, we analyze the influence of the scale effect and system parameters on the instability region. The results indicate that the viscoelastic damping decreases the resonance instability region, and the average velocity and stiffness make the instability region move to the left- and right-hand sides. Meanwhile, the scale effect of the system is obvious. The nonlocal parameter exhibits not only the stiffness softening effect but also the damping weakening effect, while the material characteristic length parameter exhibits the stiffness hardening effect and damping reinforcement effect.

Subcritical saturation of the magnetorotational instability through mean magnetic field generation

NASA Astrophysics Data System (ADS)

Xie, Jin-Han; Julien, Keith; Knobloch, Edgar

2018-03-01

The magnetorotational instability is widely believed to be responsible for outward angular momentum transport in astrophysical accretion discs. The efficiency of this transport depends on the amplitude of this instability in the saturated state. We employ an asymptotic expansion based on an explicit, astrophysically motivated time-scale separation between the orbital period, Alfvén crossing time and viscous or resistive dissipation time-scales, originally proposed by Knobloch and Julien, to formulate a semi-analytical description of the saturated state in an incompressible disc. In our approach a Keplerian shear flow is maintained by the central mass but the instability saturates via the generation of a mean vertical magnetic field. The theory assumes that the time-averaged angular momentum flux and the radial magnetic flux are constant and determines both self-consistently. The results predict that, depending on parameters, steady saturation may be supercritical or subcritical, and in the latter case that the upper (lower) solution branch is always stable (unstable). The angular momentum flux is always outward, consistent with the presence of accretion, and for fixed wavenumber peaks in the subcritical regime. The limit of infinite Reynolds number at large but finite magnetic Reynolds number is also discussed.

Anelastic and Compressible Simulation of Moist Dynamics at Planetary Scales

NASA Astrophysics Data System (ADS)

Kurowski, M.; Smolarkiewicz, P. K.; Grabowski, W.

2015-12-01

Moist anelastic and compressible numerical solutions to the planetary baroclinic instability and climate benchmarks are compared. The solutions are obtained applying a consistent numerical framework for dis- crete integrations of the various nonhydrostatic flow equations. Moist extension of the baroclinic instability benchmark is formulated as an analog of the dry case. Flow patterns, surface vertical vorticity and pressure, total kinetic energy, power spectra, and total amount of condensed water are analyzed. The climate bench- mark extends the baroclinic instability study by addressing long-term statistics of an idealized planetary equilibrium and associated meridional transports. Short-term deterministic anelastic and compressible so- lutions differ significantly. In particular, anelastic baroclinic eddies propagate faster and develop slower owing to, respectively, modified dispersion relation and abbreviated baroclinic vorticity production. These eddies also carry less kinetic energy, and the onset of their rapid growth occurs later than for the compressible solutions. The observed differences between the two solutions are sensitive to initial conditions as they di- minish for large-amplitude excitations of the instability. In particular, on the climatic time scales, the anelastic and compressible solutions evince similar zonally averaged flow patterns with the matching meridional transports of entropy, momentum, and moisture.

A Fuzzy Cognitive Model of aeolian instability across the South Texas Sandsheet

NASA Astrophysics Data System (ADS)

Houser, C.; Bishop, M. P.; Barrineau, C. P.

2014-12-01

Characterization of aeolian systems is complicated by rapidly changing surface-process regimes, spatio-temporal scale dependencies, and subjective interpretation of imagery and spatial data. This paper describes the development and application of analytical reasoning to quantify instability of an aeolian environment using scale-dependent information coupled with conceptual knowledge of process and feedback mechanisms. Specifically, a simple Fuzzy Cognitive Model (FCM) for aeolian landscape instability was developed that represents conceptual knowledge of key biophysical processes and feedbacks. Model inputs include satellite-derived surface biophysical and geomorphometric parameters. FCMs are a knowledge-based Artificial Intelligence (AI) technique that merges fuzzy logic and neural computing in which knowledge or concepts are structured as a web of relationships that is similar to both human reasoning and the human decision-making process. Given simple process-form relationships, the analytical reasoning model is able to map the influence of land management practices and the geomorphology of the inherited surface on aeolian instability within the South Texas Sandsheet. Results suggest that FCMs can be used to formalize process-form relationships and information integration analogous to human cognition with future iterations accounting for the spatial interactions and temporal lags across the sand sheets.

Dynamics of large-scale brain activity in normal arousal states and epileptic seizures

NASA Astrophysics Data System (ADS)

Robinson, P. A.; Rennie, C. J.; Rowe, D. L.

2002-04-01

Links between electroencephalograms (EEGs) and underlying aspects of neurophysiology and anatomy are poorly understood. Here a nonlinear continuum model of large-scale brain electrical activity is used to analyze arousal states and their stability and nonlinear dynamics for physiologically realistic parameters. A simple ordered arousal sequence in a reduced parameter space is inferred and found to be consistent with experimentally determined parameters of waking states. Instabilities arise at spectral peaks of the major clinically observed EEG rhythms-mainly slow wave, delta, theta, alpha, and sleep spindle-with each instability zone lying near its most common experimental precursor arousal states in the reduced space. Theta, alpha, and spindle instabilities evolve toward low-dimensional nonlinear limit cycles that correspond closely to EEGs of petit mal seizures for theta instability, and grand mal seizures for the other types. Nonlinear stimulus-induced entrainment and seizures are also seen, EEG spectra and potentials evoked by stimuli are reproduced, and numerous other points of experimental agreement are found. Inverse modeling enables physiological parameters underlying observed EEGs to be determined by a new, noninvasive route. This model thus provides a single, powerful framework for quantitative understanding of a wide variety of brain phenomena.

New Model for Ionospheric Irregularities at Mars

NASA Astrophysics Data System (ADS)

Keskinen, M. J.

2018-03-01

A new model for ionospheric irregularities at Mars is presented. It is shown that wind-driven currents in the dynamo region of the Martian ionosphere can be unstable to the electromagnetic gradient drift instability. This plasma instability can generate ionospheric density and magnetic field irregularities with scale sizes of approximately 15-20 km down to a few kilometers. We show that the instability-driven magnetic field fluctuation amplitudes relative to background are correlated with the ionospheric density fluctuation amplitudes relative to background. Our results can explain recent observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the Martian ionosphere dynamo region.

Jeans instability with exchange effects in quantum dusty magnetoplasmas

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

Jamil, M., E-mail: jamil.gcu@gmail.com; Rasheed, A.; Rozina, Ch.

2015-08-15

Jeans instability is examined in magnetized quantum dusty plasmas using the quantum hydrodynamic model. The quantum effects are considered via exchange-correlation potential, recoil effect, and Fermi degenerate pressure, in addition to thermal effects of plasma species. It is found that the electron exchange and correlation potential have significant effects over the threshold value of wave vector and Jeans instability. The presence of electron exchange and correlation effect shortens the time of dust sound that comparatively stabilizes the self gravitational collapse. The results at quantum scale are helpful in understanding the collapse of the self-gravitating dusty plasma systems.

Simulations of Instabilities in Complex Valve and Feed Systems

NASA Technical Reports Server (NTRS)

Ahuja, Vineet; Hosangadi, Ashvin; Shipman, Jeremy; Cavallo, Peter A.

2006-01-01

CFD analyses are playing an increasingly important role in identifying and characterizing flow induced instabilities in rocket engine test facilities and flight systems. In this paper, we analyze instability mechanisms that range from turbulent pressure fluctuations due to vortex shedding in structurally complex valve systems to flow resonance in plug cavities to large scale pressure fluctuations due to collapse of cavitation induced vapor clouds. Furthermore, we discuss simulations of transient behavior related to valve motion that can serve as guidelines for valve scheduling. Such predictions of valve response to varying flow conditions is of crucial importance to engine operation and testing.

Must is a Four Letter Word: The Role of Plasma Instabilities in the Intergalactic Magnetic Field Story

NASA Astrophysics Data System (ADS)

Broderick, Avery

2014-06-01

The detection of inverse Compton halos from cosmological TeV sources provide a direct means to constrain the putative intergalactic magnetic field. However, the converse may not be the case! The fate of the pairs generated by TeV gamma rays annihilating on the extragalactic background light is presently unclear, clouded by the possibility that cosmological scale plasma instabilities may dominate their energetic evolution. I will briefly motivate these plasma instabilities theoretically, summarize some empirical evidence that they may be occurring in practice, and assess their potential impact upon studies of intergalactic magnetic fields.

Assessment of demographic and pathoanatomic risk factors in recurrent patellofemoral instability.

PubMed

Hiemstra, Laurie Anne; Kerslake, Sarah; Lafave, Mark

2017-12-01

The WARPS/STAID classification employs clinical assessment of presenting features and anatomic characteristics to identify two distinct subsets of patients within the patellofemoral instability population. The purpose of this study was to further define the specific demographics and the prevalence of risky pathoanatomies in patients classified as either WARPS or STAID presenting with recurrent patellofemoral instability. A secondary purpose was to further validate the WARPS/STAID classification with the Banff Patella Instability Instrument (BPII), the Marx activity scale and the Patellar Instability Severity Score (ISS). A convenience sample of 50 patients with recurrent patellofemoral instability, including 25 WARPS and 25 STAID subtype patients, were assessed. Clinical data were collected including assessment of demographic risk factors (sex, BMI, bilaterality of symptoms, affected limb side and age at first dislocation) and pathoanatomic risk factors (TT-TG distance, patella height, patellar tilt, grade of trochlear dysplasia, Beighton score and rotational abnormalities of the tibia or femur). Patients completed the BPII and the Marx activity scale. The ISS was calculated from the clinical assessment data. Patients were stratified into the WARPS or STAID subtypes for comparative analysis. An independent t test was used to compare demographics, the pathoanatomic risk factors and subjective measures between the groups. Convergent validity was tested with a Pearson r correlation coefficient between the WARPS/STAID and ISS scores. Demographic risk factors statistically associated with a WARPS subtype included female sex, age at first dislocation and bilaterality. Pathoanatomic risk factors statistically associated with a WARPS subtype included trochlear dysplasia, TT-TG distance, generalized ligamentous laxity, patellar tilt and rotational abnormalities. The independent t test revealed a significant difference between the ISS scores: WARPS subtype (M = 4.4, SD = 1.1) and STAID subtype (M = 2.5, SD = 1.5); t(48) = 5.2, p < 0.001. The relationship between the WARPS/STAID and the ISS scores, measured using a Pearson r correlation coefficient, demonstrated a strong relationship: r = -0.61, n = 50, p < 0.001. This study has demonstrated statistically significant evidence that certain demographics and pathoanatomies are more prevalent in each of the WARPS and STAID patellofemoral instability subtypes. There was no difference in quality-of-life or activity level between the subtypes. The WARPS/STAID score demonstrated convergent validity to the ISS and divergent validity to the BPII score and the Marx activity scale. This study has further validated both the WARPS/STAID classification and the ISS of patients that present with recurrent patellofemoral instability. III.

Arthroscopic Bankart Repair Versus Open Bristow-Latarjet for Shoulder Instability: A Matched-Pair Multicenter Study Focused on Return to Sport.

PubMed

Blonna, Davide; Bellato, Enrico; Caranzano, Francesco; Assom, Marco; Rossi, Roberto; Castoldi, Filippo

2016-12-01

The arthroscopic Bankart repair and open Bristow-Latarjet procedure are the 2 most commonly used techniques to treat recurrent shoulder instability. To compare in a case control-matched manner the 2 techniques, with particular emphasis on return to sport after surgery. Cohort study; Level of evidence, 3. A study was conducted in 2 hospitals matching 60 patients with posttraumatic recurrent anterior shoulder instability with a minimum follow-up of 2 years (30 patients treated with arthroscopic Bankart procedure and 30 treated with open Bristow-Latarjet procedure). Patients with severe glenoid bone loss and revision surgeries were excluded. In one hospital, patients were treated with arthroscopic Bankart repair using anchors; in the other, patients underwent the Bristow-Latarjet procedure. Patients were matched according to age at surgery, type and level of sport practiced before shoulder instability (Degree of Shoulder Involvement in Sports [DOSIS] scale), and number of dislocations. The primary outcomes were return to sport (Subjective Patient Outcome for Return to Sports [SPORTS] score), rate of recurrent instability, Oxford Shoulder Instability Score (OSIS), Subjective Shoulder Value (SSV), Western Ontario Shoulder Instability Index (WOSI), and range of motion (ROM). After a mean follow-up of 5.3 years (range, 2-9 years), patients who underwent arthroscopic Bankart repair obtained better results in terms of return to sport (SPORTS score: 8 vs 6; P = .02) and ROM in the throwing position (86° vs 79°; P = .01), and they reported better subjective perception of the shoulder (SSV: 86% vs 75%; P = .02). No differences were detectable using the OSIS or WOSI. The rate of recurrent instability was not statistically different between the 2 groups (Bankart repair 10% vs Bristow-Latarjet 0%; P = .25), although the study may have been underpowered to detect a clinically important difference in this parameter. The multiple regression analysis showed that the independent variables associated with return to sport were preoperative DOSIS scale, type of surgery, and recurrent dislocations after surgery. Patients who played sports with high upper extremity involvement (eg, swimming, rugby, martial arts) at a competitive level (DOSIS scale 9 or 10) had a lower level of return to sport with both repair techniques. Arthroscopic stabilization using anchors provided better return to sport and subjective perception of the shoulder compared with the open Bristow-Latarjet procedure in the population studied. Recurrence may be higher in the arthroscopic Bankart group; further study is needed on this point. © 2016 The Author(s).

A classical instability of Reissner-Nordstrom solutions and the fate of magnetically charged black holes

NASA Technical Reports Server (NTRS)

Lee, Kimyeong; Nair, V. P.; Weinberg, Erick J.

1991-01-01

Working in the context of spontaneously broken gauge theories, we show that the magnetically charged Reissner-Nordstrom solution develops a classical instability if the horizon is sufficiently small. This instability has significant implications for the evolution of a magnetically charged black hole. In particular, it leads to the possibility that such a hole could evaporate completely, leaving in its place a nonsingular magnetic monopole.

Large Eddy Simulations of Transverse Combustion Instability in a Multi-Element Injector

DTIC Science & Technology

2016-07-27

plagued the development of liquid rocket engines and remains a large riskin the development and acquisition of new liquid rocket engines. Combustion...simulations to better understand the physics that can lead combustion instability in liquid rocket engines. Simulations of this type are able to...instabilities found in liquid rocket engines are transverse. The motivating of the experiment behind the current work is to subject the CVRC injector

SIMULATIONS OF THE KELVIN–HELMHOLTZ INSTABILITY DRIVEN BY CORONAL MASS EJECTIONS IN THE TURBULENT CORONA

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

Gómez, Daniel O.; DeLuca, Edward E.; Mininni, Pablo D.

Recent high-resolution Atmospheric Imaging Assembly/Solar Dynamics Observatory images show evidence of the development of the Kelvin–Helmholtz (KH) instability, as coronal mass ejections (CMEs) expand in the ambient corona. A large-scale magnetic field mostly tangential to the interface is inferred, both on the CME and on the background sides. However, the magnetic field component along the shear flow is not strong enough to quench the instability. There is also observational evidence that the ambient corona is in a turbulent regime, and therefore the criteria for the development of the instability are a priori expected to differ from the laminar case. To studymore » the evolution of the KH instability with a turbulent background, we perform three-dimensional simulations of the incompressible magnetohydrodynamic equations. The instability is driven by a velocity profile tangential to the CME–corona interface, which we simulate through a hyperbolic tangent profile. The turbulent background is generated by the application of a stationary stirring force. We compute the instability growth rate for different values of the turbulence intensity, and find that the role of turbulence is to attenuate the growth. The fact that KH instability is observed sets an upper limit on the correlation length of the coronal background turbulence.« less

Genomic instability in human cancer: Molecular insights and opportunities for therapeutic attack and prevention through diet and nutrition

PubMed Central

Ferguson, Lynnette R.; Chen, Helen; Collins, Andrew R.; Connell, Marisa; Damia, Giovanna; Dasgupta, Santanu; Malhotra, Meenakshi; Meeker, Alan K.; Amedei, Amedeo; Amin, Amr; Ashraf, S. Salman; Aquilano, Katia; Azmi, Asfar S.; Bhakta, Dipita; Bilsland, Alan; Boosani, Chandra S.; Chen, Sophie; Ciriolo, Maria Rosa; Fujii, Hiromasa; Guha, Gunjan; Halicka, Dorota; Helferich, William G.; Keith, W. Nicol; Mohammed, Sulma I.; Niccolai, Elena; Yang, Xujuan; Honoki, Kanya; Parslow, Virginia R.; Prakash, Satya; Rezazadeh, Sarallah; Shackelford, Rodney E.; Sidransky, David; Tran, Phuoc T.; Yang, Eddy S.; Maxwell, Christopher A.

2015-01-01

Genomic instability can initiate cancer, augment progression, and influence the overall prognosis of the affected patient. Genomic instability arises from many different pathways, such as telomere damage, centrosome amplification, epigenetic modifications, and DNA damage from endogenous and exogenous sources, and can be perpetuating, or limiting, through the induction of mutations or aneuploidy, both enabling and catastrophic. Many cancer treatments induce DNA damage to impair cell division on a global scale but it is accepted that personalized treatments, those that are tailored to the particular patient and type of cancer, must also be developed. In this review, we detail the mechanisms from which genomic instability arises and can lead to cancer, as well as treatments and measures that prevent genomic instability or take advantage of the cellular defects caused by genomic instability. In particular, we identify and discuss five priority targets against genomic instability: (1) prevention of DNA damage; (2) enhancement of DNA repair; (3) targeting deficient DNA repair; (4) impairing centrosome clustering; and, (5) inhibition of telomerase activity. Moreover, we highlight vitamin D and B, selenium, carotenoids, PARP inhibitors, resveratrol, and isothiocyanates as priority approaches against genomic instability. The prioritized target sites and approaches were cross validated to identify potential synergistic effects on a number of important areas of cancer biology. PMID:25869442

Elastic instabilities in planar elongational flow of monodisperse polymer solutions

PubMed Central

Haward, Simon J.; McKinley, Gareth H.; Shen, Amy Q.

2016-01-01

We investigate purely elastic flow instabilities in the almost ideal planar stagnation point elongational flow field generated by a microfluidic optimized-shape cross-slot extensional rheometer (OSCER). We use time-resolved flow velocimetry and full-field birefringence microscopy to study the behavior of a series of well-characterized viscoelastic polymer solutions under conditions of low fluid inertia and over a wide range of imposed deformation rates. At low deformation rates the flow is steady and symmetric and appears Newtonian-like, while at high deformation rates we observe the onset of a flow asymmetry resembling the purely elastic instabilities reported in standard-shaped cross-slot devices. However, for intermediate rates, we observe a new type of elastic instability characterized by a lateral displacement and time-dependent motion of the stagnation point. At the onset of this new instability, we evaluate a well-known dimensionless criterion M that predicts the onset of elastic instabilities based on geometric and rheological scaling parameters. The criterion yields maximum values of M which compare well with critical values of M for the onset of elastic instabilities in viscometric torsional flows. We conclude that the same mechanism of tension acting along curved streamlines governs the onset of elastic instabilities in both extensional (irrotational) and torsional (rotational) viscoelastic flows. PMID:27616181

Combustion instability investigations on the BR710 jet engine

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

Konrad, W.; Brehm, N.; Kameier, F.

1998-01-01

During the development of the BR710 jet engine, audible combustor instabilities (termed rumble) occurred. Amplitudes measured with test cell microphones were up to 130 dB at around 100 Hz. Disturbances of this amplitude are clearly undesirable, even if only present during start-up, and a research program was initiated to eliminate the problem. Presented here is the methodical and structured approach used to identify, understand, and remove the instability. Some reference is made to theory, which was used for guidance, but the focus of the work is on the research done to find the cause of the problem and to correctmore » it. The investigation followed two separate, but parallel, paths--one looking in detail at individual components of the engine to identify possible involvement in the instability and the other looking at the pressure signals from various parts of a complete engine to help pinpoint the source of the disturbance. The main cause of the BR710 combustor rumble was found to be a self-excited aerodynamic instability arising from the design of the fuel injector head. In the end, minor modifications lead to spray pattern changes, which greatly reduced the combustor noise. As a result of this work, new recommendation are made for reducing the risk of combustion instabilities in jet engines.« less

TURBULENT MAGNETOHYDRODYNAMIC RECONNECTION MEDIATED BY THE PLASMOID INSTABILITY

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

Huang, Yi-Min; Bhattacharjee, A., E-mail: yiminh@princeton.edu

2016-02-10

It has been established that the Sweet–Parker current layer in high Lundquist number reconnection is unstable to the super-Alfvénic plasmoid instability. Past two-dimensional magnetohydrodynamic simulations have demonstrated that the plasmoid instability leads to a new regime where the Sweet–Parker current layer changes into a chain of plasmoids connected by secondary current sheets, and the averaged reconnection rate becomes nearly independent of the Lundquist number. In this work, a three-dimensional simulation with a guide field shows that the additional degree of freedom allows plasmoid instabilities to grow at oblique angles, which interact and lead to self-generated turbulent reconnection. The averaged reconnectionmore » rate in the self-generated turbulent state is of the order of a hundredth of the characteristic Alfvén speed, which is similar to the two-dimensional result but is an order of magnitude lower than the fastest reconnection rate reported in recent studies of externally driven three-dimensional turbulent reconnection. Kinematic and magnetic energy fluctuations both form elongated eddies along the direction of the local magnetic field, which is a signature of anisotropic magnetohydrodynamic turbulence. Both energy fluctuations satisfy power-law spectra in the inertial range, where the magnetic energy spectral index is in the range from −2.3 to −2.1, while the kinetic energy spectral index is slightly steeper, in the range from −2.5 to −2.3. The anisotropy of turbulence eddies is found to be nearly scale-independent, in contrast with the prediction of the Goldreich–Sridhar theory for anisotropic turbulence in a homogeneous plasma permeated by a uniform magnetic field.« less

Impact of oceanic-scale interactions on the seasonal modulation of ocean dynamics by the atmosphere.

PubMed

Sasaki, Hideharu; Klein, Patrice; Qiu, Bo; Sasai, Yoshikazu

2014-12-15

Ocean eddies (with a size of 100-300 km), ubiquitous in satellite observations, are known to represent about 80% of the total ocean kinetic energy. Recent studies have pointed out the unexpected role of smaller oceanic structures (with 1-50 km scales) in generating and sustaining these eddies. The interpretation proposed so far invokes the internal instability resulting from the large-scale interaction between upper and interior oceanic layers. Here we show, using a new high-resolution simulation of the realistic North Pacific Ocean, that ocean eddies are instead sustained by a different process that involves small-scale mixed-layer instabilities set up by large-scale atmospheric forcing in winter. This leads to a seasonal evolution of the eddy kinetic energy in a very large part of this ocean, with an amplitude varying by a factor almost equal to 2. Perspectives in terms of the impacts on climate dynamics and future satellite observational systems are briefly discussed.

Impact of oceanic-scale interactions on the seasonal modulation of ocean dynamics by the atmosphere

PubMed Central

Sasaki, Hideharu; Klein, Patrice; Qiu, Bo; Sasai, Yoshikazu

2014-01-01

Ocean eddies (with a size of 100–300 km), ubiquitous in satellite observations, are known to represent about 80% of the total ocean kinetic energy. Recent studies have pointed out the unexpected role of smaller oceanic structures (with 1–50 km scales) in generating and sustaining these eddies. The interpretation proposed so far invokes the internal instability resulting from the large-scale interaction between upper and interior oceanic layers. Here we show, using a new high-resolution simulation of the realistic North Pacific Ocean, that ocean eddies are instead sustained by a different process that involves small-scale mixed-layer instabilities set up by large-scale atmospheric forcing in winter. This leads to a seasonal evolution of the eddy kinetic energy in a very large part of this ocean, with an amplitude varying by a factor almost equal to 2. Perspectives in terms of the impacts on climate dynamics and future satellite observational systems are briefly discussed. PMID:25501039

Ground State Structure Search of Fluoroperovskites through Lattice Instability

NASA Astrophysics Data System (ADS)

Mei, W. N.; Hatch, D. M.; Stokes, H. T.; Boyer, L. L.

2002-03-01

Many Fluoroperovskite are capable of a ferroelectric transition from a cubic to a tetragonal and even lower-symmetry structures. In this work, we studied systematically the structural phase transitions of several fluoroperovskites ABF3 where A= Na, K and B= Ca, Sr. Combining the Self-Consistent Atom Deformation (SCAD) -- a density-functional method using localized densities -- and the frozen-phonon method which utilizes the isotropy subgroup operations, we calculate the phonon energies and find instabilities which lower the symmetry of the crystal. Following this scheme, we work down to lower symmetry structures until we no longer find instabilities. The final results are used to compare with those obtained from molecular dynamics based on Gordon-Kim potentials.

MHD Instability and Turbulence in the Tachocline

NASA Technical Reports Server (NTRS)

Werne, Joe; Wagner, William J. (Technical Monitor)

2003-01-01

The focus of this project was to study the physical processes that govern tachocline dynamics and structure. Specific features explored included stratification, shear, waves, and toroidal and poloidal background fields. In order to address recent theoretical work on anisotropic mixing and dynamics in the tachocline, we were particularly interested in such anisotropic mixing for the specific tachocline processes studied. Transition to turbulence often shapes the largest-scale features that appear spontaneously in a flow during the development of turbulence. The resulting large-scale straining field can control the subsequent dynamics; therefore, anticipation of the large-scale straining field that results for individual realizations of the transition to turbulence can be important for subsequent dynamics, flow morphology, and transport characteristics. As a result, we paid particular attention to the development of turbulence in the stratified and sheared environment of the tachocline. This is complicated by the fact that the linearly stability of sheared MHD flows is non-self-adjoint, implying that normal asymptotic linear stability theory may not be relevant.

The Alfvénic nature of energy transfer mediation in localized, strongly nonlinear Alfvén wavepacket collisions

NASA Astrophysics Data System (ADS)

Verniero, J. L.; Howes, G. G.

2018-02-01

In space and astrophysical plasmas, violent events or instabilities inject energy into turbulent motions at large scales. Nonlinear interactions among the turbulent fluctuations drive a cascade of energy to small perpendicular scales at which the energy is ultimately converted into plasma heat. Previous work with the incompressible magnetohydrodynamic (MHD) equations has shown that this turbulent energy cascade is driven by the nonlinear interaction between counterpropagating Alfvén waves - also known as Alfvén wave collisions. Direct numerical simulations of weakly collisional plasma turbulence enables deeper insight into the nature of the nonlinear interactions underlying the turbulent cascade of energy. In this paper, we directly compare four cases: both periodic and localized Alfvén wave collisions in the weakly and strongly nonlinear limits. Our results reveal that in the more realistic case of localized Alfvén wave collisions (rather than the periodic case), all nonlinearly generated fluctuations are Alfvén waves, which mediates nonlinear energy transfer to smaller perpendicular scales.

Measuring precarious employment in times of crisis: the revised Employment Precariousness Scale (EPRES) in Spain.

PubMed

Vives, Alejandra; González, Francisca; Moncada, Salvador; Llorens, Clara; Benach, Joan

2015-01-01

This study examines the psychometric properties of the revised Employment Precariousness Scale (EPRES-2010) in a context of economic crisis and growing unemployment. Data correspond to salaried workers with a contract (n=4,750) from the second Psychosocial Work Environment Survey (Spain, 2010). Analyses included acceptability, scale score distributions, Cronbach's alpha coefficient and exploratory factor analysis. Response rates were 80% or above, scores were widely distributed with reductions in floor effects for temporariness among permanent workers and for vulnerability. Cronbach's alpha coefficients were 0.70 or above; exploratory factor analysis confirmed the theoretical allocation of 21 out of 22 items. The revised version of the EPRES demonstrated good metric properties and improved sensitivity to worker vulnerability and employment instability among permanent workers. Furthermore, it was sensitive to increased levels of precariousness in some dimensions despite decreases in others, demonstrating responsiveness to the context of the economic crisis affecting the Spanish labour market. Copyright © 2015 SESPAS. Published by Elsevier Espana. All rights reserved.

Monitoring of the stability of underground workings in Polish copper mines conditions

NASA Astrophysics Data System (ADS)

Fuławka, Krzysztof; Mertuszka, Piotr; Pytel, Witold

2018-01-01

One of the problems associated with the excavation of deposit in underground mines is the local disturbance in a state of unstable equilibrium results in the sudden release of energy, mainly in the form of roof falls. The scale and intensity of this type of events depends on a number of factors. To minimize the risk of instability occurrence, continuous observations of the roof strata condition are recommended. Different roof strata observation methods used in the Polish copper mines have been analysed within the framework of presented paper. In addition, selected prospective methods, which could significantly increase efficiency of rock fall prevention are presented.

On the origin of the HLX-1 outbursts

NASA Astrophysics Data System (ADS)

Sun, Mouyuan; Gu, Wei-Min; Yan, Zhen; Wu, Qingwen; Liu, Tong

2016-11-01

HLX-1, currently the best intermediate-mass black hole candidate, has undergone seven violent outbursts, each with a peak X-ray luminosity of Lpeak,X ˜ 1042 erg s-1. Interestingly, the properties of the HLX-1 outbursts evolve with time. In this work, we aim to constrain the physical parameters of the central engine of the HLX-1 outbursts in the framework of the black hole accretion. We find that the physical properties of the HLX-1 outbursts are consistent with being driven by the radiation pressure instability. This scenario can explain the evolution of the recurrent time-scales of the HLX-1 outbursts as a function of the durations.

The influence of cosmic rays on the stability and large-scale dynamics of the interstellar medium

NASA Astrophysics Data System (ADS)

Kuznetsov, V. D.

1986-06-01

The diffusion-convection formulation is used to study the influence of galactic cosmic rays on the stability and dynamics of the interstellar medium which is supposedly kept in equilibrium by the gravitational field of stars. It is shown that the influence of cosmic rays on the growth rate of MHD instability depends largely on a dimensionless parameter expressing the ratio of the characteristic acoustic time scale to the cosmic-ray diffusion time. If this parameter is small, the cosmic rays will decelerate the build-up of instabilities, thereby stabilizing the system; in contrast, if the parameter is large, the system will be destabilized.

Simple Model of Macroscopic Instability in XeCl Discharge Pumped Lasers

NASA Astrophysics Data System (ADS)

Ahmed, Belasri; Zoheir, Harrache

2003-10-01

The aim of this work is to study the development of the macroscopic non uniformity of the electron density of high pressure discharge for excimer lasers and eventually its propagation because of the medium kinetics phenomena. This study is executed using a transverse mono-dimensional model, in which the plasma is represented by a set of resistance's in parallel. This model was employed using a numerical code including three strongly coupled parts: electric circuit equations, electron Boltzmann equation, and kinetics equations (chemical kinetics model). The time variations of the electron density in each plasma element are obtained by solving a set of ordinary differential equations describing the plasma kinetics and external circuit. The use of the present model allows a good comprehension of the halogen depletion phenomena, which is the principal cause of laser ending and allows a simple study of a large-scale non uniformity in preionization density and its effects on electrical and chemical plasma properties. The obtained results indicate clearly that about 50consumed at the end of the pulse. KEY WORDS Excimer laser, XeCl, Modeling, Cold plasma, Kinetic, Halogen depletion, Macroscopic instability.

Parametrically excited helicopter ground resonance dynamics with high blade asymmetries

NASA Astrophysics Data System (ADS)

Sanches, L.; Michon, G.; Berlioz, A.; Alazard, D.

2012-07-01

The present work is aimed at verifying the influence of high asymmetries in the variation of in-plane lead-lag stiffness of one blade on the ground resonance phenomenon in helicopters. The periodical equations of motions are analyzed by using Floquet's Theory (FM) and the boundaries of instabilities predicted. The stability chart obtained as a function of asymmetry parameters and rotor speed reveals a complex evolution of critical zones and the existence of bifurcation points at low rotor speed values. Additionally, it is known that when treated as parametric excitations; periodic terms may cause parametric resonances in dynamic systems, some of which can become unstable. Therefore, the helicopter is later considered as a parametrically excited system and the equations are treated analytically by applying the Method of Multiple Scales (MMS). A stability analysis is used to verify the existence of unstable parametric resonances with first and second-order sets of equations. The results are compared and validated with those obtained by Floquet's Theory. Moreover, an explanation is given for the presence of unstable motion at low rotor speeds due to parametric instabilities of the second order.

Global Surrogates for the Upshift of the Critical Threshold in the Gradient for ITG Driven Turbulence

NASA Astrophysics Data System (ADS)

Michoski, Craig; Janhunen, Salomon; Faghihi, Danial; Carey, Varis; Moser, Robert

2017-10-01

The suppression of micro-turbulence and ultimately the inhibition of large-scale instabilities observed in tokamak plasmas is partially characterized by the onset of a global stationary state. This stationary attractor corresponds experimentally to a state of ``marginal stability'' in the plasma. The critical threshold that characterizes the onset in the nonlinear regime is observed both experimentally and numerically to exhibit an upshift relative to the linear theory. That is, the onset in the stationary state is up-shifted from those predicted by the linear theory as a function of the ion temperature gradient R0 /LT . Because the transition to this state with enhanced transport and therefore reduced confinement times is inaccessible to the linear theory, strategies for developing nonlinear reduced physics models to predict the upshift have been ongoing. As a complement to these effort, the principle aim of this work is to establish low-fidelity surrogate models that can be used to predict instability driven loss of confinement using training data from high-fidelity models. DE-SC0008454 and DE-AC02-09CH11466.

On the Evolution From Micrometer-Scale Inhomogeneity to Global Overheated Structure During the Intense Joule Heating of a z-Pinch Rod

DOE PAGES

Awe, T. J.; Yu, E. P.; Yates, K. C.; ...

2017-02-21

Ultrafast optical microscopy of metal z-pinch rods pulsed with megaampere current is contributing new data and critical insight into what provides the fundamental seed for the magneto-Rayleigh-Taylor (MRT) instability. A two-frame near infrared/visible intensified-charge-coupled device gated imager with 2-ns temporal resolution and 3-μm spatial resolution captured emissions from the nonuniformly Joule heated surfaces of ultrasmooth aluminum (Al) rods. Nonuniform surface emissions are consistently first observed from discrete, 10-μm scale, subelectronvolt spots. Aluminum 6061 alloy, with micrometer-scale nonmetallic resistive inclusions, forms several times more spots than 99.999% pure Al 5N; 5-10 ns later, azimuthally stretched elliptical spots and distinct strata (40-100more » μm wide by 10 μm tall) are observed on Al 6061, but not on Al 5N. In such overheat strata, aligned parallel to the magnetic field, we find that they are highly effective seeds for MRT instability growth. Our data give credence to the hypothesis that early nonuniform Joule heating, such as the electrothermal instability, may provide the dominant seed for MRT.« less

Two-length-scale turbulence model for self-similar buoyancy-, shock-, and shear-driven mixing

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

Morgan, Brandon E.; Schilling, Oleg; Hartland, Tucker A.

The three-equation k-L-a turbulence model [B. Morgan and M. Wickett, Three-equation model for the self-similar growth of Rayleigh-Taylor and Richtmyer-Meshkov instabilities," Phys. Rev. E 91 (2015)] is extended by the addition of a second length scale equation. It is shown that the separation of turbulence transport and turbulence destruction length scales is necessary for simultaneous prediction of the growth parameter and turbulence intensity of a Kelvin-Helmholtz shear layer when model coeficients are constrained by similarity analysis. Constraints on model coeficients are derived that satisfy an ansatz of self-similarity in the low-Atwood-number limit and allow the determination of model coeficients necessarymore » to recover expected experimental behavior. The model is then applied in one-dimensional simulations of Rayleigh-Taylor, reshocked Richtmyer-Meshkov, Kelvin{Helmholtz, and combined Rayleigh-Taylor/Kelvin-Helmholtz instability mixing layers to demonstrate that the expected growth rates are recovered numerically. Finally, it is shown that model behavior in the case of combined instability is to predict a mixing width that is a linear combination of Rayleigh-Taylor and Kelvin-Helmholtz mixing processes.« less

Core turbulence behavior moving from ion-temperature-gradient regime towards trapped-electron-mode regime in the ASDEX Upgrade tokamak and comparison with gyrokinetic simulation

NASA Astrophysics Data System (ADS)

Happel, T.; Navarro, A. Bañón; Conway, G. D.; Angioni, C.; Bernert, M.; Dunne, M.; Fable, E.; Geiger, B.; Görler, T.; Jenko, F.; McDermott, R. M.; Ryter, F.; Stroth, U.

2015-03-01

Additional electron cyclotron resonance heating (ECRH) is used in an ion-temperature-gradient instability dominated regime to increase R / L Te in order to approach the trapped-electron-mode instability regime. The radial ECRH deposition location determines to a large degree the effect on R / L Te . Accompanying scale-selective turbulence measurements at perpendicular wavenumbers between k⊥ = 4-18 cm-1 (k⊥ρs = 0.7-4.2) show a pronounced increase of large-scale density fluctuations close to the ECRH radial deposition location at mid-radius, along with a reduction in phase velocity of large-scale density fluctuations. Measurements are compared with results from linear and non-linear flux-matched gyrokinetic (GK) simulations with the gyrokinetic code GENE. Linear GK simulations show a reduction of phase velocity, indicating a pronounced change in the character of the dominant instability. Comparing measurement and non-linear GK simulation, as a central result, agreement is obtained in the shape of radial turbulence level profiles. However, the turbulence intensity is increasing with additional heating in the experiment, while gyrokinetic simulations show a decrease.

Two-length-scale turbulence model for self-similar buoyancy-, shock-, and shear-driven mixing

DOE PAGES

Morgan, Brandon E.; Schilling, Oleg; Hartland, Tucker A.

2018-01-10

The three-equation k-L-a turbulence model [B. Morgan and M. Wickett, Three-equation model for the self-similar growth of Rayleigh-Taylor and Richtmyer-Meshkov instabilities," Phys. Rev. E 91 (2015)] is extended by the addition of a second length scale equation. It is shown that the separation of turbulence transport and turbulence destruction length scales is necessary for simultaneous prediction of the growth parameter and turbulence intensity of a Kelvin-Helmholtz shear layer when model coeficients are constrained by similarity analysis. Constraints on model coeficients are derived that satisfy an ansatz of self-similarity in the low-Atwood-number limit and allow the determination of model coeficients necessarymore » to recover expected experimental behavior. The model is then applied in one-dimensional simulations of Rayleigh-Taylor, reshocked Richtmyer-Meshkov, Kelvin{Helmholtz, and combined Rayleigh-Taylor/Kelvin-Helmholtz instability mixing layers to demonstrate that the expected growth rates are recovered numerically. Finally, it is shown that model behavior in the case of combined instability is to predict a mixing width that is a linear combination of Rayleigh-Taylor and Kelvin-Helmholtz mixing processes.« less

A Pedagogical Look at Jeans' Density Scale

ERIC Educational Resources Information Center

Chu, Kwang-Hua W.

2007-01-01

We illustrate the derivations of Jeans' criteria for the gravitational instabilities in a static homogeneous Newtonian system for pedagogical objectives. The critical Jeans density surface is presented in terms of dimensionless sound speeds and (characteristic) length scales. (Contains 1 figure.)

Turbulence sources, character, and effects in the stable boundary layer: Insights from multi-scale direct numerical simulations and new, high-resolution measurements

NASA Astrophysics Data System (ADS)

Fritts, Dave; Wang, Ling; Balsley, Ben; Lawrence, Dale

2013-04-01

A number of sources contribute to intermittent small-scale turbulence in the stable boundary layer (SBL). These include Kelvin-Helmholtz instability (KHI), gravity wave (GW) breaking, and fluid intrusions, among others. Indeed, such sources arise naturally in response to even very simple "multi-scale" superpositions of larger-scale GWs and smaller-scale GWs, mean flows, or fine structure (FS) throughout the atmosphere and the oceans. We describe here results of two direct numerical simulations (DNS) of these GW-FS interactions performed at high resolution and high Reynolds number that allow exploration of these turbulence sources and the character and effects of the turbulence that arises in these flows. Results include episodic turbulence generation, a broad range of turbulence scales and intensities, PDFs of dissipation fields exhibiting quasi-log-normal and more complex behavior, local turbulent mixing, and "sheet and layer" structures in potential temperature that closely resemble high-resolution measurements. Importantly, such multi-scale dynamics differ from their larger-scale, quasi-monochromatic gravity wave or quasi-horizontally homogeneous shear flow instabilities in significant ways. The ability to quantify such multi-scale dynamics with new, very high-resolution measurements is also advancing rapidly. New in-situ sensors on small, unmanned aerial vehicles (UAVs), balloons, or tethered systems are enabling definition of SBL (and deeper) environments and turbulence structure and dissipation fields with high spatial and temporal resolution and precision. These new measurement and modeling capabilities promise significant advances in understanding small-scale instability and turbulence dynamics, in quantifying their roles in mixing, transport, and evolution of the SBL environment, and in contributing to improved parameterizations of these dynamics in mesoscale, numerical weather prediction, climate, and general circulation models. We expect such measurement and modeling capabilities to also aid in the design of new and more comprehensive future SBL measurement programs.

Propulsion and Instability of a Flexible Helical Rod Rotating in a Viscous Fluid

NASA Astrophysics Data System (ADS)

Jawed, M. K.; Khouri, N. K.; Da, F.; Grinspun, E.; Reis, P. M.

2015-10-01

We combine experiments with simulations to investigate the fluid-structure interaction of a flexible helical rod rotating in a viscous fluid, under low Reynolds number conditions. Our analysis takes into account the coupling between the geometrically nonlinear behavior of the elastic rod with a nonlocal hydrodynamic model for the fluid loading. We quantify the resulting propulsive force, as well as the buckling instability of the originally helical filament that occurs above a critical rotation velocity. A scaling analysis is performed to rationalize the onset of this instability. A universal phase diagram is constructed to map out the region of successful propulsion and the corresponding boundary of stability is established. Comparing our results with data for flagellated bacteria suggests that this instability may be exploited in nature for physiological purposes.

Volume nanograting formation in laser-silica interaction as a result of the 1D plasma-resonance ionization instability

NASA Astrophysics Data System (ADS)

Gildenburg, V. B.; Pavlichenko, I. A.

2016-08-01

The initial stage of the small-scale ionization-induced instability developing inside the fused silica volume exposed to the femtosecond laser pulse is studied as a possible initial cause of the self-organized nanograting formation. We have calculated the spatial spectra of the instability with the electron-hole diffusion taken into account for the first time and have found that it results in the formation of some hybrid (diffusion-wave) 1D structure with the spatial period determined as the geometrical mean of the laser wavelength and characteristic diffusion length of the process considered. Near the threshold of the instability, this period occurs to be approximately equal to the laser half-wavelength in the silica, close to the one experimentally observed.

Elimination of Gravity Influence on Flame Propagation Via Enhancement of the Saffman-Taylor Instability

NASA Technical Reports Server (NTRS)

Aldredge, R. C.

2003-01-01

In this analytical work the influence of the Saffman-Taylor instability on flame propagation is formulated for computational investigation. Specifically, it is of interest to examine the influence of this instability as a potential means of eliminating the effect of gravitational acceleration on the development of thermoacoustic instability. Earlier experimental investigations of thermoacoustic instability employed tubes of large circular or annular cross-section, such that neither heat loss nor viscosity at the burner walls was of significant importance in influencing flame behavior. However, it has been demonstrated recently that flames propagating between closely spaced walls, may be subject to long-wavelength wrinkling associated with the Saffman-Taylor instability, known to be relevant when a less-viscous fluid pushes a more-viscous fluid through a porous medium or between two closely spaced walls.

Sawtooth Stabilization and Onset of Alfvenic Instabilities

NASA Astrophysics Data System (ADS)

Nishimura, Y.; Cheng, C. Z.

2011-10-01

Tokamak sawtooth instabilities can be stabilized by high energy particles as a consequence of conservation of the third adiabatic invariant.On the other hand, termination of the stabilized period is reported due to the onset of Alfvenic instabilities (and thus the absence of the stabilizing mechanism). In this work, employing a kinetic-fluid model, the interaction of m=1 resistive kink mode and high energy particles is investigated. The onset of Alfvenic instabilities is examined as a function of the inversion radius location. D.J. Campbell et al., Phys. Rev. Lett. 60, 2148 (1988); F. Porcelli, Plasma Phys. Controlled Fusion 33, 1601 (1991).

3D DNS and LES of Breaking Inertia-Gravity Waves

NASA Astrophysics Data System (ADS)

Remmler, S.; Fruman, M. D.; Hickel, S.; Achatz, U.

2012-04-01

As inertia-gravity waves we refer to gravity waves that have a sufficiently low frequency and correspondingly large horizontal wavelength to be strongly influenced by the Coriolis force. Inertia-gravity waves are very active in the middle atmosphere and their breaking is potentially an important influence on the circulation in this region. The parametrization of this process requires a good theoretical understanding, which we want to enhance with the present study. Primary linear instabilities of an inertia-gravity wave and "2.5-dimensional" nonlinear simulations (where the spatial dependence is two dimensional but the velocity and vorticity fields are three-dimensional) with the wave perturbed by its leading primary instabilities by Achatz [1] have shown that the breaking differs significantly from that of high-frequency gravity waves due to the strongly sheared component of velocity perpendicular to the plane of wave-propagation. Fruman & Achatz [2] investigated the three-dimensionalization of the breaking by computing the secondary linear instabilities of the same waves using singular vector analysis. These secondary instabilities are variations perpendicular to the direction of the primary perturbation and the wave itself, and their wavelengths are an order of magnitude shorter than both. In continuation of this work, we carried out fully three-dimensional nonlinear simulations of inertia-gravity waves perturbed by their leading primary and secondary instabilities. The direct numerical simulation (DNS) was made tractable by restricting the domain size to the dominant scales selected by the linear analyses. The study includes both convectively stable and unstable waves. To the best of our knowledge, this is the first fully three-dimensional nonlinear direct numerical simulation of inertia-gravity waves at realistic Reynolds numbers with complete resolution of the smallest turbulence scales. Previous simulations either were restricted to high frequency gravity waves (e. g. Fritts et al. [3]), or the ratio N/f was artificially reduced (e. g. Lelong & Dunkerton [4]). The present simulations give us insight into the three-dimensional breaking process as well as the emerging turbulence. We assess the possibility of reducing the computational costs of three-dimensional simulations by using an implicit turbulence subgrid-scale parametrization based on the Adaptive Local Deconvolution Method (ALDM) for stratified turbulence [5]. In addition, we have performed ensembles of nonlinear 2.5-dimensional DNS, like those in Achatz [1] but with a small amount of noise superposed to the initial state, and compared the results with coarse-resolution simulations using either ALDM as well as with standard LES schemes. We found that the results of the models with parametrized turbulence, which are orders of magnitude more computationally economical than the DNS, compare favorably with the DNS in terms of the decay of the wave amplitude with time (the quantity most important for application to gravity-wave drag parametrization) suggesting that they may be trusted in future simulations of gravity wave breaking.

Marangoni instability in a thin film heated from below: Effect of nonmonotonic dependence of surface tension on temperature

NASA Astrophysics Data System (ADS)

Sarma, Rajkumar; Mondal, Pranab Kumar

2018-04-01

We investigate Marangoni instability in a thin liquid film resting on a substrate of low thermal conductivity and separated from the surrounding gas phase by a deformable free surface. Considering a nonmonotonic variation of surface tension with temperature, here we analytically derive the neutral stability curve for the monotonic and oscillatory modes of instability (for both the long-wave and short-wave perturbations) under the framework of linear stability analysis. For the long-wave instability, we derive a set of amplitude equations using the scaling k ˜(Bi) 1 /2 , where k is the wave number and Bi is the Biot number. Through this investigation, we demonstrate that for such a fluid layer upon heating from below, both monotonic and oscillatory instability can appear for a certain range of the dimensionless parameters, viz., Biot number (Bi ) , Galileo number (Ga ) , and inverse capillary number (Σ ) . Moreover, we unveil, through this study, the influential role of the above-mentioned parameters on the stability of the system and identify the critical values of these parameters above which instability initiates in the liquid layer.

Modulational instability and discrete breathers in a nonlinear helicoidal lattice model

NASA Astrophysics Data System (ADS)

Ding, Jinmin; Wu, Tianle; Chang, Xia; Tang, Bing

2018-06-01

We investigate the problem on the discrete modulation instability of plane waves and discrete breather modes in a nonlinear helicoidal lattice model, which is described by a discrete nonlinear Schrödinger equation with the first-, second-, and third-neighbor coupling. By means of the linear stability analysis, we present an analytical expression of the instability growth rate and identify the regions of modulational instability of plane waves. It is shown that the introduction of the third-neighbor coupling will affect the shape of the areas of modulational instability significantly. Based on the results obtained by the modulational instability analysis, we predict the existence conditions for the stationary breather modes. Otherwise, by making use of the semidiscrete multiple-scale method, we obtain analytical solutions of discrete breather modes and analyze their properties for different types of nonlinearities. Our results show that the discrete breathers obtained are stable for a long time only when the system exhibits the repulsive nonlinearity. In addition, it is found that the existence of the stable bright discrete breather closely relates to the presence of the third-neighbor coupling.

Asymptotic Laws of Thermovibrational Convecton in a Horizontal Fluid Layer

NASA Astrophysics Data System (ADS)

Smorodin, B. L.; Myznikova, B. I.; Keller, I. O.

2017-02-01

Theoretical study of convective instability is applied to a horizontal layer of incompressible single-component fluid subjected to the uniform steady gravity, longitudinal vibrations of arbitrary frequency and initial temperature difference. The mathematical model of thermovibrational convection has the form of initial boundary value problem for the Oberbeck-Boussinesq system of equations. The problems are solved using different simulation strategies, like the method of averaging, method of multiple scales, Galerkin approach, Wentzel-Kramers-Brillouin method and Floquet technique. The numerical analysis has shown that the effect of vibrations on the stability threshold is complex: vibrations can either stabilize or destabilize the basic state depending on values of the parameters. The influence of the Prandtl number on the instability thresholds is investigated. The asymptotic behaviour of critical values of the parameters is studied in two limiting cases: (i) small amplitude and (ii) low frequency of vibration. In case (i), the instability is due to the influence of thermovibrational mechanism on the classical Rayleigh-Benard convective instability. In case (ii), the nature of the instability is related to the instability of oscillating counter-streams with a cubic profile.

Non-thermal electron distribution functions through 3D magnetic reconnection instabilities in the solar wind

NASA Astrophysics Data System (ADS)

Alejandro Munoz Sepulveda, Patricio; Buechner, Joerg

2017-04-01

The effects of kinetic instabilities on the solar wind electron velocity distribution functions (eVDFs) are mostly well understood under local homogeneous and stationary conditions. But the solar wind also contains current sheets, which affect the local properties of instabilities, turbulence and thus the observed non-maxwellian features in the eVDFs. Those processes are vastly unexplored. Therefore, we aim to investigate the influence of self-consistently generated turbulence via electron-scale instabilities in reconnecting current sheets on the formation of suprathermal features in the eVDFs. For this sake, we carry out 3D fully-kinetic Particle-in-Cell code numerical simulations of force free current sheets with a guide magnetic field. We find extended tails, anisotropic plateaus and non-gyrotropic features in the eVDFs, correlated with the locations and time where micro-turbulence is enhanced in the current sheet due to current-aligned streaming instabilities. We also discuss the influence of the plasma parameters, such as the ion to electron temperature ratio, on the excitation of current sheet instabilities and their effect on the properties of the eVDFs.

Wave generation by contaminant ions near a large spacecraft

NASA Technical Reports Server (NTRS)

Singh, N.

1993-01-01

Measurements from the space shuttle flights have revealed that a large spacecraft in a low earth orbit is accompanied by an extensive gas cloud which is primarily made up of water. The charge exchange between the water molecule and the ionospheric O(+) ions produces a water ion beam traversing downstream of the spacecraft. In this report we present results from a study on the generation of plasma waves by the interaction of the water ion beams with the ionospheric plasma. Since velocity distribution function is key to the understanding of the wave generation process, we have performed a test particle simulation to determine the nature of H2O(+) ions velocity distribution function. The simulations show that at the time scales shorter than the ion cyclotron period tau(sub c), the distribution function can be described by a beam. On the other hand, when the time scales are larger than tau(sub c), a ring distribution forms. A brief description of the linear instabilities driven by an ion beam streaming across a magnetic field in a plasma is presented. We have identified two types of instabilities occurring in low and high frequency bands; the low-frequency instability occurs over the frequency band from zero to about the lower hybrid frequency for a sufficiently low beam density. As the beam density increases, the linear instability occurs at decreasing frequencies below the lower-hybrid frequency. The high frequency instability occurs near the electron cyclotron frequency and its harmonics.

A classical instability of Reissner-Nordstrom solutions and the fate of magnetically charged black holes

NASA Technical Reports Server (NTRS)

Lee, Kimyeong; Nair, V. P.; Weinberg, Erick J.

1992-01-01

Working in the context of spontaneously broken gauge theories, it is shown that the magnetically charged Reissner-Nordstrom solution develops a classical instability if the horizon is sufficiently small. This instability has significant implications for the evolution of a magnetically charged black hole. In particular, it leads to the possibility that such a hole could evaporate completely, leaving in its place a nonsingular magnetic monopole.

Advanced specialty fiber designs for high power fiber lasers

NASA Astrophysics Data System (ADS)

Gu, Guancheng

The output power of fiber lasers has increased rapidly over the last decade. There are two major limiting factors, namely nonlinear effects and transverse mode instability, prohibiting the power scaling capability of fiber lasers. The nonlinear effects, originating from high optical intensity, primarily limit the peak power scaling. The mode instability, on the other hand, arises from quantum-defect driven heating, causing undesired mode coupling once the power exceeds the threshold and degradation of beam quality. The mode instability has now become the bottleneck for average output power scaling of fiber lasers. Mode area scaling is the most effective way to mitigate nonlinear effects. However, the use of large mode area may increase the tendency to support multiple modes in the core, resulting in lower mode instability threshold. Therefore, it is critical to maintain single mode operation in a large mode area fiber. Sufficient higher order mode suppression can lead to effective single-transverse-mode propagation. In this dissertation, we explore the feasibility of using specialty fiber to construct high power fiber lasers with robust single-mode output. The first type of fiber discussed is the resonantly-enhanced leakage channel fiber. Coherent reflection at the fiber outer boundary can lead to additional confinement especially for highly leaky HOM, leading to lower HOM losses than what are predicted by conventional finite element mothod mode solver considering infinite cladding. In this work, we conducted careful measurements of HOM losses in two leakage channel fibers (LCF) with circular and rounded hexagonal boundary shapes respectively. Impact on HOM losses from coiling, fiber boundary shapes and coating indexes were studied in comparison to simulations. This work demonstrates the limit of the simulation method commonly used in the large-mode-area fiber designs and the need for an improved approach. More importantly, this work also demonstrates that a deviation from circular fiber outer shape may be an effective method to mitigate HOM loss reduction from coherent reflection from fiber outer boundary. In an all-solid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses, which is essential for lasing far from the gain peak and suppression of stimulated Raman scattering. We will show that all-solid photonic bandgap fibers with effective mode area of 920microm2 can be made with excellent higher order mode suppression. We then demonstrate a 50microm-core-diameter Yb-doped all-solid photonic bandgap fiber laser. 75W output power has been generated with a diffraction-limited beam and an efficiency of 70% relative to the launched pump power. We have also experimentally confirmed that a robust single-mode regime exists near the high frequency edge of the bandgap. It is well known that incorporation of additional smaller cores in the cladding can be used to resonantly out-couple higher-order modes from a main core to suppress higher-order-mode propagation in the main core. Using a novel design with multiple coupled smaller cores in the cladding, we further scaled up the mode area and have successfully demonstrated a single-mode photonic bandgap fiber with record effective mode area of 2650microm2. Detailed numeric studies have been conducted for multiple cladding designs. For the optimal designs, the simulated minimum higher-order-mode losses are well over two orders of magnitudes higher than that of fundamental mode when expressed in dBs. We have also experimentally validated one of the designs. M 2<1.08 across the transmission band was demonstrated. Lowering quantum defect heating is another approach to mitigate mode instability. Highly-efficient high-power fiber lasers operating at wavelength below 1020nm are critical for tandem-pumping in >10kW fiber lasers to provide high pump brightness and low thermal loading. Using an ytterbium-doped-phosphosilicate double-clad leakage-channel fiber with 50microm core and 420microm cladding, we have achieved 70% optical-to-optical efficiency at 1018nm. The much larger cladding than those in previous reports demonstrates the much lower required pump brightness, a key for efficient kW operation. The demonstrated 1018nm fiber laser has ASE suppression of 41dB. This is higher than previous reports and further demonstrates the advantages of the fiber used. Limiting factors to efficiency are also systematically studied.

Magnetorotational dynamo action in the shearing box

NASA Astrophysics Data System (ADS)

Walker, Justin; Boldyrev, Stanislav

2017-09-01

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disc plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale non-zero azimuthal magnetic fluxes, facilitating the instability. In agreement with this picture, the cases when the dynamo is efficient are characterized by a strong intermittency of the local azimuthal magnetic fluxes.

Multiple frequency radar observations of high-latitude E region irregularities in the HF modified ionosphere

NASA Technical Reports Server (NTRS)

Noble, S. T.; Gordon, W. E.; Djuth, F. T.; Jost, R. J.; Hedberg, A.

1987-01-01

This paper discusses the results of the September 1983 observations of artificial field-aligned irregularities (AFAIs) in the Tromso, Norway region, made by backscatter radars operating at 46.9, 143.8, 21.4, and 140.0 MHz. Four classes of resonant instability processes at work in the E and F regions are examined in detail: (1) the coupling of parametric decay instability waves across geomagnetic field lines, (2) thermal parametric instability, (3) four-wave interaction thermal parametric instability, and (4) the resonance instability. The characteristics of the AFAI scatter are described, with special attention given to the growth and decay time constants, functional dependence on the heater power and polarization, and the scattering cross sections of the irregularities.

Modification of electron beam ion source instability by longitudinal kinetic effects

NASA Astrophysics Data System (ADS)

Krafft, G. A.; Mark, J. W.-K.

1982-07-01

The electron beam ion source (EBIS) was proposed and subsequently realized by Donets. Further development has been done by the Orsay group and by the Berkeley group. Much theoretical work has been done on EBIS and, in particular, Litwin, Vella, and Sessler find that an electrostatic mode of the electron beam-ion system is unstable using a fluid calculation. The present work indicates that the inclusion of a longitudinal velocity spread in the electron beam decreases the instability growth rate, but their conclusions are not significantly modified. Numerically it is shown that substantial linear instability remains, even when sizeable longitudinal velocity spread is included. The principal result of the work is fig. 1; it gives the complex frequency of the mode as a function of σ/ υ0 z, σ being the spread and υ0 z being the average electron axial velocity.

Study of solar wind spectra by nonlinear waves interaction

NASA Astrophysics Data System (ADS)

Dwivedi, Navin; Sharma, Rampal; Narita, Yasuhito

2014-05-01

The nature of small-scale turbulent fluctuations in the solar wind (SW) turbulence is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations predict the evidence of the dominance of kinetic Alfvén waves (KAW) at sub-ion scales with frequency below than ion cyclotron frequency, while other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode is more relevant. In the present work, nonlinear interaction of kinetic Alfvén wave with whistler wave is considered as one of the possible cause responsible for the solar wind turbulence. A set of coupled dimensionless equations are derived for the intermediate beta plasmas and the nonlinear interaction between these two wave modes has been studied. As a consequence of ponderomotive nonlinearity, the pump KAW becomes filamented when its power exceeds the threshold for the filamentation instability. Whistler is considered to be weak and thus doesn't have enough intensity to initiate its own localization. It gets localized while propagating through the density channel created by KAW localization. In addition, spectral scales of power spectra of KAW and whistler are also calculated. The steeper spectra are found with scaling greater than -5/3. This type of nonlinear interaction between different wave modes and steeper spectra is one of the reasons for the solar wind turbulence and particles acceleration. This work is partially supported by DST (India) and FP7/STORM (313038)

Buckling Instability Causes Inertial Thrust for Spherical Swimmers at All Scales

NASA Astrophysics Data System (ADS)

Djellouli, Adel; Marmottant, Philippe; Djeridi, Henda; Quilliet, Catherine; Coupier, Gwennou

2017-12-01

Microswimmers, and among them aspirant microrobots, generally have to cope with flows where viscous forces are dominant, characterized by a low Reynolds number (Re). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities. Here, we propose a swimming mechanism which uses the buckling instability triggered by pressure waves to propel a spherical, hollow shell. With a macroscopic experimental model, we show that a net displacement is produced at all Re regimes. An optimal displacement caused by nontrivial history effects is reached at intermediate Re. We show that, due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high-frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm /s for a remote-controlled microrobot, a suitable value for biological applications such as drug delivery.

Flow and acoustic properties of low Reynolds number supersonic underexpanded jets

NASA Technical Reports Server (NTRS)

Hu, T. F.; Mclaughlin, D. K.

1981-01-01

Flow and acoustic measurements are made of cold model jets exhausting from a choked nozzle at pressure conditions corresponding to those of Mach 1.4 and 2.1 jets to investigate noise production properties of underexpanded supersonic jets. Mean flow measurements are made using pitot and static pressure probes, with flow fluctuation measurements made with a hot-wire probe and acoustic measurements made with a transversing microphone. Two convergent nozzles with exit diameters of 7.0 and 7.9 mm are used with an exciter consisting of a 0.8 mm tungsten electrode positioned 2 mm from the exit. Shock structure is observed as having a significant effect on the development of the flow field, while large-scale instabilities have higher growth rates in the shock containing underexpanded jets. The role of the asymmetric n = + or - 1 sinusoidal instability is clarified, and results suggest that the broadband shock associated noise of conventional high Reynolds number jets is not related to large-scale jet instability.

Instability of confined water films between elastic surfaces.

PubMed

de Beer, Sissi; 't Mannetje, Dieter; Zantema, Sietske; Mugele, Frieder

2010-03-02

We investigated the dynamics of nanometer thin water films at controlled ambient humidity adsorbed onto two atomically smooth mica sheets upon rapidly bringing the surfaces into contact. Using a surface forces apparatus (SFA) in imaging mode, we found that the water films break up into a distribution of drops with a typical thickness of a few nanometers and a characteristic lateral size and spacing of several micrometers. Whereas the characteristic length is found to be independent of the ambient humidity, the characteristic time of the breakup decreases from approximately 1 to 0.01 s with increasing humidity. The existence of characteristic length and time scales shows that this breakup is controlled by an instability rather than a conventional nucleation and growth mechanism for SFA experiments. These findings cannot be explained by a dispersion-driven instability mechanism. In contrast, a model involving the elastic energies for the deformation of both the mica sheets and the underlying glue layer correctly reproduces the scaling of the characteristic length and time with humidity.

Spoil pile instabilities with reference to a strip coal mine in Turkey: mechanisms and assessment of deformations

NASA Astrophysics Data System (ADS)

Kasmer, Ozgu; Ulusay, Resat; Gokceoglu, Candan

2006-02-01

With the increasing adoption of the surface mining of coal, problems associated with spoil pile instability, which affects resource recovery, mining cost, and safety and presents environmental hazards, have become a matter of prime concern to mine planners and operators. The study of geotechnical aspects is thus very important in the rational planning for the disposal, reclamation, treatment and utilization of spoil material. A strip coal mine, one of the largest open pit mines in Turkey, is located in Central Anatolia and provides coal to a thermal power station. Coal production is carried out in two adjacent open pits, the Central Pit and South Pit. A large-scale spoil pile instability over an area of 0.3 km2 occurred within the dumping area of the Central pit. In addition, small-scale movement occurred in the outside dumping area. This paper outlines the results of field and laboratory investigations to describe the mechanisms of the spoil pile instabilities and to assess deformations monitored over a long period following the failure. Shear test results indicate that the interface between the floor and spoil material dumped by dragline has a negligible cohesion and is the most critical plane of weakness for spoil pile instability. Back analyses based on the method of limit equilibrium and the numerical modelling technique, and observations in the pit revealed that failure occurred along a combined sliding surface consisting of a circular surface through the spoil material itself and a planar surface passing along the interface between the spoil piles and floor. The analyses also indicated that pore water pressure ratios of about 0.25 satisfy limiting equilibrium condition and that rainfall about one month before the failure may be a contributing factor to the instability. Movement monitoring data obtained following the failure over a 1.5-year period suggested that the ongoing deformations were mainly due to compaction of the spoil material. Based on the monitoring data and the results of the analyses, the failure mode of the local instability occurring at the outside dumping area was considerably similar to that of the large instability.

2-Shock layered tuning campaign

NASA Astrophysics Data System (ADS)

Masse, Laurent; Dittrich, T.; Khan, S.; Kyrala, G.; Ma, T.; MacLaren, S.; Ralph, J.; Salmonson, J.; Tipton, R.; Los Alamos Natl Lab Team; Lawrence Livermore Natl Lab Team

2016-10-01

The 2-Shock platform has been developed to maintain shell sphericity throughout the compression phase of an indirect-drive target implosion and produce a stagnating hot spot in a quasi 1D-like manner. A sub-scale, 1700 _m outer diameter, and thick, 200 _m, uniformly Silicon doped, gas-filled plastic capsule is driven inside a nominal size 5750 _m diameter ignition hohlraum. The hohlraum fill is near vacuum to reduce back-scatter and improve laser/drive coupling. A two-shock pulse of about 1 MJ of laser energy drives the capsule. The thick capsule prevents ablation front feed-through to the imploded core. This platform has demonstrated its efficiency to tune a predictable and reproducible 1-D implosion with a nearly round shape. It has been shown that the high foot performance was dominated by the local defect growth due to the ablation front instability and by the hohlraum radiation asymmetries. The idea here is to take advantage of this 2-Shock platform to design a 1D-like layered implosion and eliminates the deleterious effects of radiation asymmetries and ablation front instability growth. We present the design work and our first experimental results of this near one-dimensional 2-Shock layered design. This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344.

How plasmas dissipate: cascade and the production of internal energy and entropy in weakly collisional plasma turbulence

NASA Astrophysics Data System (ADS)

Matthaeus, W. H.; Yang, Y.; Servidio, S.; Parashar, T.; Chasapis, A.; Roytershteyn, V.

2017-12-01

Turbulence cascade transfers energy from large scale to small scale but what happens once kinetic scales are reached? In a collisional medium, viscosity and resistivity remove fluctuation energy in favor of heat. In the weakly collisional solar wind, (or corona, m-sheath, etc.), the sequence of events must be different. Heating occurs, but through what mechanisms? In standard approaches, dissipation occurs though linear wave modes or instabilities and one seeks to identify them. A complementary view is that cascade leads to several channels of energy conversion, interchange and spatial rearrangement that collectively leads to production of internal energy. Channels may be described using compressible MHD & multispecies Vlasov Maxwell formulations. Key steps are: Conservative rearrangement of energy in space; Parallel incompressible and compressible cascades - conservative rearrangment in scale; electromagnetic work on particles that drives flows, both macroscopic and microscopic; and pressure-stress interactions, both compressive and shear-like, that produces internal energy. Examples given from MHD, PIC simulations and MMS observations. A more subtle issue is how entropy is related to this degeneration (or, "dissipation") of macroscopic, fluid-scale fluctuations. We discuss this in terms of Boltzmann and thermodynamic entropies, and velocity space effects of collisions.

Nonlinear Waves, Instabilities and Singularities in Plasma and Hydrodynamics

NASA Astrophysics Data System (ADS)

Silantyev, Denis Albertovich

Nonlinear effects are present in almost every area of science as soon as one tries to go beyond the first order approximation. In particular, nonlinear waves emerge in such areas as hydrodynamics, nonlinear optics, plasma physics, quantum physics, etc. The results of this work are related to nonlinear waves in two areas, plasma physics and hydrodynamics, united by concepts of instability, singularity and advanced numerical methods used for their investigation. The first part of this work concentrates on Langmuir wave filamentation instability in the kinetic regime of plasma. In Internal Confinement Fusion Experiments (ICF) at National Ignition Facility (NIF), where attempts are made to achieve fusion by compressing a small target by many powerful lasers to extremely high temperatures and pressures, plasma is created in the first moments of the laser reaching the target and undergoes complicated dynamics. Some of the most challenging difficulties arise from various plasma instabilities that occur due to interaction of the laser beam and a plasma surrounding the target. In this work we consider one of such instabilities that describes a decay of nonlinear plasma wave, initially excited due to interaction of the laser beam with the plasma, into many filaments in direction perpendicular to the laser beam, therefore named Langmuir filamentation instability. This instability occurs in the kinetic regime of plasma, klambda D > 0.2, where k is the wavenumber and lambda D is the Debye length. The filamentation of Langmuir waves in turn leads to the saturation of the stimulated Raman scattering (SRS) in laser-plasma interaction experiments which plays an essential role in ICF experiments. The challenging part of this work was that unlike in hydrodynamics we needed to use fully kinetic description of plasma to capture the physics in question properly, meaning that we needed to consider the distribution function of charged particles and its evolution in time not only with respect to spatial coordinates but with respect to velocities as well. To study Langmuir filamentation instability in its simplest form we performed 2D+2V numerical simulations. Taking into account that the distribution function in question was 4-dimensional function, making these simulation quite challenging, we developed an efficient numerical method making these simulations possible on modern desktop computers. Using the developed numerical method we studied how Langmuir wave filamentation instability depends on the parameters of the Langmuir wave such as wave length and amplitude that are relevant to ICF experiments. We considered several types of Langmuir waves, including nonlinear Langmuir waves exited by external electric field as well as an idealized approximation of such Langmuir waves by a particular family of Bernstein-Greene-Kruskal (BGK) modes that bifurcates from the linear Langmuir wave. The results of these simulations were compared to the theoretical predictions in our recent papers. An alternative approach to overcome computational difficulty of this problem was considered by our research group in Ref. It involves reducing the number of transverse direction in the model therefore lowering computational difficulty at a cost of lesser accuracy of the model. The second part of this work concentrates on 2D free surface hydrodynamics and in particular on computing Stokes waves with high-precision using conformal maps and spectral methods. Stokes waves are fully nonlinear periodic gravity waves propagating with the constant velocity on a free surface of two-dimensional potential flow of the ideal incompressible fluid of infinite depth. The increase of the scaled wave height H/lambda, where H is the wave height and lambda is the wavelength, from H/lambda = 0 to the critical value Hmax/lambda marks the transition from almost linear wave to a strongly nonlinear limiting Stokes wave. The Stokes wave of the greatest height H = Hmax has an angle of 120° at the crest. To obtain Stokes wave fully nonlinear Euler equations describing the flow can be reformulated in terms of conformal map of the fluid domain into the complex lower half-plane, with fluid free surface mapped into the real line. This description is convenient for analysis and numerical simulations since the whole problem is then reduced to a single nonlinear equation on the real line. Having computed solutions on the real line we extend them to the rest of the complex plane to analyze the singularities above real line. The distance vc from the closest singularity in the upper half-plane to the real line goes to zero as we approach the limiting Stokes wave with maximum hight Hmax/lambda, which is the reason for the widening of the solution's Fourier spectrum. (Abstract shortened by ProQuest.).

Instability of liquid crystal elastomers

NASA Astrophysics Data System (ADS)

An, Ning; Li, Meie; Zhou, Jinxiong

2016-01-01

Nematic liquid crystal elastomers (LCEs) contract in the director direction but expand in other directions, perpendicular to the director, when heated. If the expansion of an LCE is constrained, compressive stress builds up in the LCE, and it wrinkles or buckles to release the stored elastic energy. Although the instability of soft materials is ubiquitous, the mechanism and programmable modulation of LCE instability has not yet been fully explored. We describe a finite element method (FEM) scheme to model the inhomogeneous deformation and instability of LCEs. A constrained LCE beam working as a valve for microfluidic flow, and a piece of LCE laminated with a nanoscale poly(styrene) (PS) film are analyzed in detail. The former uses the buckling of the LCE beam to occlude the microfluidic channel, while the latter utilizes wrinkling or buckling to measure the mechanical properties of hard film or to realize self-folding. Through rigorous instability analysis, we predict the critical conditions for the onset of instability, the wavelength and amplitude evolution of instability, and the instability patterns. The FEM results are found to correlate well with analytical results and reported experiments. These efforts shed light on the understanding and exploitation of the instabilities of LCEs.

White-light parametric instabilities in plasmas.

PubMed

Santos, J E; Silva, L O; Bingham, R

2007-06-08

Parametric instabilities driven by partially coherent radiation in plasmas are described by a generalized statistical Wigner-Moyal set of equations, formally equivalent to the full wave equation, coupled to the plasma fluid equations. A generalized dispersion relation for stimulated Raman scattering driven by a partially coherent pump field is derived, revealing a growth rate dependence, with the coherence width sigma of the radiation field, scaling with 1/sigma for backscattering (three-wave process), and with 1/sigma1/2 for direct forward scattering (four-wave process). Our results demonstrate the possibility to control the growth rates of these instabilities by properly using broadband pump radiation fields.

Majority of Solar Wind Intervals Support Ion-Driven Instabilities

NASA Astrophysics Data System (ADS)

Klein, K. G.; Alterman, B. L.; Stevens, M. L.; Vech, D.; Kasper, J. C.

2018-05-01

We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist's instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He2 + temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He2 + components on stability. Of 309 randomly selected spectra from the Wind spacecraft, 53.7% are unstable when the ion components are modeled as drifting bi-Maxwellians; only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates γ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large He2 + drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.

Microscopic Processes in Relativistic Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.; Hardee, P.; Mizuno, Y.; Medvedev, M.; Zhang, B.; Nordlund, A.; Fredricksen, J.; Sol, H.; Niemiec, J.; Lyubarsky, Y.;

Quasilinear saturation of the aperiodic ordinary mode streaming instability

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

Stockem Novo, A., E-mail: anne@tp4.rub.de; Schlickeiser, R.; Yoon, P. H.

2015-09-15

In collisionless plasmas, only kinetic instabilities and fluctuations are effective in reducing the free energy and scatter plasma particles, preventing an increase of their anisotropy. Solar energetic outflows into the interplanetary plasma give rise to important thermal anisotropies and counterstreaming motions of plasma shells, and the resulting instabilities are expected to regulate the expansion of the solar wind. The present paper combines quasilinear theory and kinetic particle-in-cell simulations in order to study the weakly nonlinear saturation of the ordinary mode in hot counter-streaming plasmas with a temperature anisotropy as a follow-up of the paper by Seough et al. [Phys. Plasmasmore » 22, 082122 (2015)]. This instability provides a plausible mechanism for the origin of dominating, two-dimensional spectrum of transverse magnetic fluctuations observed in the solar wind. Stimulated by the differential motion of electron counterstreams the O mode instability may convert their free large-scale energy by nonlinear collisionless dissipation on plasma particles.« less

The use of thermal capsulorrhaphy in the treatment of multidirectional instability.

PubMed

Fitzgerald, Brian T; Watson, B Thomas; Lapoint, John M

2002-01-01

The purpose of this study is to report on our experience with thermal capsulorrhaphy in the treatment of multidirectional instability of the shoulder. Thirty-three consecutive patients (33 shoulders) with multidirectional instability were treated with arthroscopic thermal capsulorrhaphy. Twelve patients had a history of traumatic dislocation. Three patients had been previously treated with open inferior capsular shift procedures. At a mean follow-up of 36 months (range, 24-40 months), results were available for 30 patients. On the basis of the UCLA rating scale, out of a possible 35 points, the mean preoperative score was 16.7 points and postoperatively it was 30.1 points, with 3 excellent, 20 good, and 7 poor results. Twenty-three patients (76%) were returned to full activity. In our experience the majority of patients with multidirectional instability were able to return to their previous occupations in the armed forces following thermal capsulorrhaphy. More information further defining the biomechanical pathology of capsular laxity and the specific role of electrothermal shrinkage in treating shoulder instability is needed.

Majority of Solar Wind Intervals Support Ion-Driven Instabilities.

PubMed

Klein, K G; Alterman, B L; Stevens, M L; Vech, D; Kasper, J C

2018-05-18

We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist's instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He^{2+} temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He^{2+} components on stability. Of 309 randomly selected spectra from the Wind spacecraft, 53.7% are unstable when the ion components are modeled as drifting bi-Maxwellians; only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates γ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large He^{2+} drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.

Particle Acceleration, Magnetic Field Generation, and Associated Emission in Collisionless Relativistic Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K.-I.

2007-01-01

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations using injected relativistic electron-ion (electro-positron)jets show that acceleration occurs within the downstream jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, other two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The "jitter" radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Particle Acceleration, Magnetic Field Generation and Associated Emission in Collisionless Relativistic Jets

NASA Technical Reports Server (NTRS)

Nishikawa, K. I.; Ramirez-Ruiz, E.; Hardee, P.; Mizuno, Y.; Fishman. G. J.

2007-01-01

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations using injected relativistic electron-ion (electro-positron) jets show that acceleration occurs within the downstream jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, other two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The "jitter" radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Earthquake nucleation on faults with rate-and state-dependent strength

USGS Publications Warehouse

Dieterich, J.H.

1992-01-01

Dieterich, J.H., 1992. Earthquake nucleation on faults with rate- and state-dependent strength. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 115-134. Faults with rate- and state-dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability. Calculations with a plane-strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch. The dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip. The critical length is a function of normal stress, loading conditions and constitutive parameters which include Dc, the characteristic slip distance. If slip starts on a patch that exceeds the critical size, the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches. Solutions have been obtained for a uniform, fixed-patch model that are in good agreement with results from the plane-strain model. Over a wide range of conditions, above the steady-state stress, the logarithm of the time to instability linearly decreases as the initial stress increases. Because nucleation patch length and premonitory displacement are proportional to Dc, the moment of premonitory slip scales by D3c. The scaling of Dc is currently an open question. Unless Dc for earthquake faults is significantly greater than that observed on laboratory faults, premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods. Excluding the possibility that Dc in the nucleation zone controls the magnitude of the subsequent earthquake, then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch. ?? 1992.

Kelvin-Helmholtz instability of counter-rotating discs

NASA Astrophysics Data System (ADS)

Quach, Dan; Dyda, Sergei; Lovelace, Richard V. E.

2015-01-01

Observations of galaxies and models of accreting systems point to the occurrence of counter-rotating discs where the inner part of the disc (r < r0) is corotating and the outer part is counter-rotating. This work analyses the linear stability of radially separated co- and counter-rotating thin discs. The strong instability found is the supersonic Kelvin-Helmholtz instability. The growth rates are of the order of or larger than the angular rotation rate at the interface. The instability is absent if there is no vertical dependence of the perturbation. That is, the instability is essentially three dimensional. The non-linear evolution of the instability is predicted to lead to a mixing of the two components, strong heating of the mixed gas, and vertical expansion of the gas, and annihilation of the angular momenta of the two components. As a result, the heated gas will free-fall towards the disc's centre over the surface of the inner disc.

Prediction of severe thunderstorms over Sriharikota Island by using the WRF-ARW operational model

NASA Astrophysics Data System (ADS)

Papa Rao, G.; Rajasekhar, M.; Pushpa Saroja, R.; Sreeshna, T.; Rajeevan, M.; Ramakrishna, S. S. V. S.

2016-05-01

Operational short range prediction of Meso-scale thunderstorms for Sriharikota(13.7°N ,80.18°E) has been performed using two nested domains 27 & 9Km configuration of Weather Research & Forecasting-Advanced Research Weather Model (WRF- ARW V3.4).Thunderstorm is a Mesoscale system with spatial scale of few kilometers to a couple of 100 kilometers and time scale of less than an one hour to several hours, which produces heavy rain, lightning, thunder, surface wind squalls and down-bursts. Numerical study of Thunderstorms at Sriharikota and its neighborhood have been discussed with its antecedent thermodynamic stability indices and Parameters that are usually favorable for the development of convective instability based on WRF ARW model predictions. Instability is a prerequisite for the occurrence of severe weather, the greater the instability, the greater will be the potential of thunderstorm. In the present study, K Index, Total totals Index (TTI), Convective Available Potential Energy (CAPE), Convective Inhibition Energy (CINE), Lifted Index (LI), Precipitable Water (PW), etc. are the instability indices used for the short range prediction of thunderstorms. In this study we have made an attempt to estimate the skill of WRF ARW predictability and diagnosed three thunderstorms that occurred during the late evening to late night of 31st July, 20th September and 2nd October of 2015 over Sriharikota Island which are validated with Local Electric Field Mill (EFM), rainfall observations and Chennai Doppler Weather Radar products. The model predicted thermodynamic indices (CAPE, CINE, K Index, LI, TTI and PW) over Sriharikota which act as good indicators for severe thunderstorm activity.

Experimental Investigations in a Reactor Cavity Cooling System with Advanced Instrumentation for the Study of Instabilities, Oscillations, and Transients

NASA Astrophysics Data System (ADS)

Tompkins, Casey A.

A research team at University of Wisconsin - Madison designed and constructed a 1/4 height scaled experimental facility to study two-phase natural circulation cooling in a water-based reactor cavity cooling system (WRCCS) for decay heat removal in an advanced high temperature reactor. The facility is capable of natural circulation operation scaled for simulated decay heat removal (up to 28.5 kW m-2 (45 kW) input power, which is equivalent to 14.25 kW m-2 (6.8 MW) at full scale) and pressurized up to 2 bar. The UW-WRCCS facility has been used to study instabilities and oscillations observed during natural circulation flow due to evaporation of the water inventory. During two-phase operation, the system exhibits flow oscillations and excursions, which cause thermal oscillations in the structure. This can cause degradation in the mechanical structure at welds and limit heat transfer to the coolant. The facility is equipped with wire mesh sensors (WMS) that enable high-resolution measurements of the void fraction and steam velocities in order to study the instability's and oscillation's growth and decay during transient operation. Multiple perturbations to the system's operating point in pressure and inlet throttling have shown that the oscillatory behavior present under normal two-phase operating conditions can be damped and removed. Furthermore, with steady-state modeling it was discovered that a flow regime transition instability is the primary cause of oscillations in the UW-WRCCS facility under unperturbed conditions and that proper orifice selection can move the system into a stable operating regime.

Double-Diffusive Convection at Low Prandtl Number

NASA Astrophysics Data System (ADS)

Garaud, Pascale

2018-01-01

This work reviews present knowledge of double-diffusive convection at low Prandtl number obtained using direct numerical simulations, in both the fingering regime and the oscillatory regime. Particular emphasis is given to modeling the induced turbulent mixing and its impact in various astrophysical applications. The nonlinear saturation of fingering convection at low Prandtl number usually drives small-scale turbulent motions whose transport properties can be predicted reasonably accurately using a simple semi-analytical model. In some instances, large-scale internal gravity waves can be excited by a collective instability and eventually cause layering. The nonlinear saturation of oscillatory double-diffusive convection exhibits much more complex behavior. Weakly stratified systems always spontaneously transition into layered convection associated with very efficient mixing. More strongly stratified systems remain dominated by weak wave turbulence unless they are initialized into a layered state. The effects of rotation, shear, lateral gradients, and magnetic fields are briefly discussed.

Power Scaling Fiber Amplifiers Using Very-Large-Mode-Area Fibers

DTIC Science & Technology

2016-02-23

fiber lasers are limited to below 1kW due to limited mode size and thermal issues, particularly thermal mode instability (TMI). Two comprehensive models...accurately modeling very- large-mode-area fiber amplifiers while simultaneously including thermal lensing and TMI. This model was applied to investigate...expected resilience to TMI. 15. SUBJECT TERMS Fiber amplifier, high power laser, thermal mode instability, large-mode-area fiber, ytterbium-doped

Nonlinear growth of zonal flows by secondary instability in general magnetic geometry

DOE PAGES

Plunk, G. G.; Navarro, A. Banon

2017-02-23

Here we present a theory of the nonlinear growth of zonal flows in magnetized plasma turbulence, by the mechanism of secondary instability. The theory is derived for general magnetic geometry, and is thus applicable to both tokamaks and stellarators. The predicted growth rate is shown to compare favorably with nonlinear gyrokinetic simulations, with the error scaling as expected with the small parameter of the theory.

Particle Simulations of Magnetospheric Plasmas

DTIC Science & Technology

1989-03-14

scale vortices. 2 2. Beam Instability in the Foreshock As an application of the simulation method used in the proposed research (Broadband...electrostatic noise), the beam instability in the foreshock has been investigated. Electrons backstreaming into the Earth’s foreshock generate waves near the...narrowband waves near the foreshock boundary may be between 0.9wp and 0.98wpe, rather than being above w., as previously believed. 3 3. Whistler Mode

Proto-jet configurations in RADs orbiting a Kerr SMBH: symmetries and limiting surfaces

NASA Astrophysics Data System (ADS)

Pugliese, D.; Stuchlík, Z.

2018-05-01

Ringed accretion disks (RADs) are agglomerations of perfect-fluid tori orbiting around a single central attractor that could arise during complex matter inflows in active galactic nuclei. We focus our analysis to axi-symmetric accretion tori orbiting in the equatorial plane of a supermassive Kerr black hole; equilibrium configurations, possible instabilities, and evolutionary sequences of RADs were discussed in our previous works. In the present work we discuss special instabilities related to open equipotential surfaces governing the material funnels emerging at various regions of the RADs, being located between two or more individual toroidal configurations of the agglomerate. These open structures could be associated to proto-jets. Boundary limiting surfaces are highlighted, connecting the emergency of the jet-like instabilities with the black hole dimensionless spin. These instabilities are observationally significant for active galactic nuclei, being related to outflows of matter in jets emerging from more than one torus of RADs orbiting around supermassive black holes.

Review of Combustion-acoustic Instabilities

NASA Technical Reports Server (NTRS)

Oyediran, Ayo; Darling, Douglas; Radhakrishnan, Krishnan

1995-01-01

Combustion-acoustic instabilities occur when the acoustic energy increase due to the unsteady heat release of the flame is greater than the losses of acoustic energy from the system. The problem of combustion-acoustic instability is a concern in many devices for various reasons, as each device may have a unique mechanism causing unsteady heat release rates and many have unique boundary conditions. To accurately predict and quantify combustion-acoustic stabilities, the unsteady heat release rate and boundary conditions need to be accurately determined. The present review brings together work performed on a variety of practical combustion devices. Many theoretical and experimental investigations of the unsteady heat release rate have been performed, some based on perturbations in the fuel delivery system particularly for rocket instabilities, while others are based on hydrodynamic processes as in ramjet dump combustors. The boundary conditions for rocket engines have been analyzed and measured extensively. However, less work has been done to measure acoustic boundary conditions in many other combustion systems.

Nonlinear hydrodynamic stability and transition; Proceedings of the IUTAM Symposium, Nice, France, Sept. 3-7, 1990

NASA Astrophysics Data System (ADS)

Theoretical and experimental research on nonlinear hydrodynamic stability and transition is presented. Bifurcations, amplitude equations, pattern in experiments, and shear flows are considered. Particular attention is given to bifurcations of plane viscous fluid flow and transition to turbulence, chaotic traveling wave covection, chaotic behavior of parametrically excited surface waves in square geometry, amplitude analysis of the Swift-Hohenberg equation, traveling wave convection in finite containers, focus instability in axisymmetric Rayleigh-Benard convection, scaling and pattern formation in flowing sand, dynamical behavior of instabilities in spherical gap flows, and nonlinear short-wavelength Taylor vortices. Also discussed are stability of a flow past a two-dimensional grid, inertia wave breakdown in a precessing fluid, flow-induced instabilities in directional solidification, structure and dynamical properties of convection in binary fluid mixtures, and instability competition for convecting superfluid mixtures.

Volume nanograting formation in laser-silica interaction as a result of the 1D plasma-resonance ionization instability

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

Gildenburg, V. B., E-mail: gil@appl.sci-nnov.ru; Pavlichenko, I. A.; Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod 603950

2016-08-15

The initial stage of the small-scale ionization-induced instability developing inside the fused silica volume exposed to the femtosecond laser pulse is studied as a possible initial cause of the self-organized nanograting formation. We have calculated the spatial spectra of the instability with the electron-hole diffusion taken into account for the first time and have found that it results in the formation of some hybrid (diffusion-wave) 1D structure with the spatial period determined as the geometrical mean of the laser wavelength and characteristic diffusion length of the process considered. Near the threshold of the instability, this period occurs to be approximatelymore » equal to the laser half-wavelength in the silica, close to the one experimentally observed.« less

Effects of geometry and fluid elasticity during polymeric droplet pinch-off in microfluidic environments

NASA Astrophysics Data System (ADS)

Steinhaus, Ben; Shen, Amy; Sureshkumar, Radhakrishna

2006-11-01

We investigate the effects of fluid elasticity and channel geometry on polymeric droplet pinch-off by performing systematic experiments using viscoelastic polymer solutions which possess practically shear rate-independent viscosity (Boger fluids). Four different geometric sizes (width and depth are scaled up proportionally at the ratio of 0.5, 1, 2, 20) are used to study the effect of the length scale, which in turn influences the ratio of elastic to viscous forces as well as the Rayleigh time scale associated with the interfacial instability of a cylindrical column of liquid. We observe a power law relationship between the dimensionless (scaled with respect to the Rayleigh time scale) capillary pinch-off time, T, and the elasticity number, E, defined as the ratio of the fluid relaxation time to the time scale of viscous diffusion. In general, T increases dramatically with increasing E. The inhibition of ``bead-on-a-string'' formation is observed for flows with effective Deborah number, De, defined as the ratio of the fluid relaxation time to the Rayleigh time scale becomes greater than 10. For sufficiently large values of De, the Rayleigh instability may be modified substantially by fluid elasticity.

Scaling and universality in heart rate variability distributions

NASA Technical Reports Server (NTRS)

Rosenblum, M. G.; Peng, C. K.; Mietus, J. E.; Havlin, S.; Stanley, H. E.; Goldberger, A. L.

1998-01-01

We find that a universal homogeneous scaling form describes the distribution of cardiac variations for a group of healthy subjects, which is stable over a wide range of time scales. However, a similar scaling function does not exist for a group with a common cardiopulmonary instability associated with sleep apnea. Subtle differences in the distributions for the day- and night-phase dynamics for healthy subjects are detected.

Scaling and universality in heart rate variability distributions

NASA Astrophysics Data System (ADS)

Ivanov, P. Ch; Rosenblum, M. G.; Peng, C.-K.; Mietus, J. E.; Havlin, S.; Stanley, H. E.; Goldberger, A. L.

We find that a universal homogeneous scaling form describes the distributions of cardiac variations for a group of healthy subjects, which is stable over a wide range of time scales. However, a similar scaling function does not exist for a group with a common cardiopulmonary instability associated with sleep apnea. Subtle differences in the distributions for the day- and night-phase dynamics for healthy subjects are detected.

Hot-spot mix in ignition-scale inertial confinement fusion targets.

PubMed

Regan, S P; Epstein, R; Hammel, B A; Suter, L J; Scott, H A; Barrios, M A; Bradley, D K; Callahan, D A; Cerjan, C; Collins, G W; Dixit, S N; Döppner, T; Edwards, M J; Farley, D R; Fournier, K B; Glenn, S; Glenzer, S H; Golovkin, I E; Haan, S W; Hamza, A; Hicks, D G; Izumi, N; Jones, O S; Kilkenny, J D; Kline, J L; Kyrala, G A; Landen, O L; Ma, T; MacFarlane, J J; MacKinnon, A J; Mancini, R C; McCrory, R L; Meezan, N B; Meyerhofer, D D; Nikroo, A; Park, H-S; Ralph, J; Remington, B A; Sangster, T C; Smalyuk, V A; Springer, P T; Town, R P J

2013-07-26

Mixing of plastic ablator material, doped with Cu and Ge dopants, deep into the hot spot of ignition-scale inertial confinement fusion implosions by hydrodynamic instabilities is diagnosed with x-ray spectroscopy on the National Ignition Facility. The amount of hot-spot mix mass is determined from the absolute brightness of the emergent Cu and Ge K-shell emission. The Cu and Ge dopants placed at different radial locations in the plastic ablator show the ablation-front hydrodynamic instability is primarily responsible for hot-spot mix. Low neutron yields and hot-spot mix mass between 34(-13,+50)  ng and 4000(-2970,+17 160)  ng are observed.

Thermal instability in post-flare plasmas

NASA Technical Reports Server (NTRS)

Antiochos, S. K.

1976-01-01

The cooling of post-flare plasmas is discussed and the formation of loop prominences is explained as due to a thermal instability. A one-dimensional model was developed for active loop prominences. Only the motion and heat fluxes parallel to the existing magnetic fields are considered. The relevant size scales and time scales are such that single-fluid MHD equations are valid. The effects of gravity, the geometry of the field and conduction losses to the chromosphere are included. A computer code was constructed to solve the model equations. Basically, the system is treated as an initial value problem (with certain boundary conditions at the chromosphere-corona transition region), and a two-step time differencing scheme is used.

Hot-spot mix in ignition-scale inertial confinement fusion targets

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

Regan, S. P.; Epstein, R.; Hammel, B. A.

Mixing of plastic ablator material, doped with Cu and Ge dopants, deep into the hot spot of ignition-scale inertial confinement fusion implosions by hydrodynamic instabilities is diagnosed with x-ray spectroscopy on the National Ignition Facility. The amount of hot-spot mix mass is determined from the absolute brightness of the emergent Cu and Ge K-shell emission. The Cu and Ge dopants placed at different radial locations in the plastic ablator show the ablation-front hydrodynamic instability is primarily responsible for hot-spot mix. As a result, low neutron yields and hot-spot mix mass between 34(–13,+50) ng and 4000(–2970,+17 160) ng are observed.

Hot-spot mix in ignition-scale inertial confinement fusion targets

DOE PAGES

Regan, S. P.; Epstein, R.; Hammel, B. A.; ...

2013-07-22

Mixing of plastic ablator material, doped with Cu and Ge dopants, deep into the hot spot of ignition-scale inertial confinement fusion implosions by hydrodynamic instabilities is diagnosed with x-ray spectroscopy on the National Ignition Facility. The amount of hot-spot mix mass is determined from the absolute brightness of the emergent Cu and Ge K-shell emission. The Cu and Ge dopants placed at different radial locations in the plastic ablator show the ablation-front hydrodynamic instability is primarily responsible for hot-spot mix. As a result, low neutron yields and hot-spot mix mass between 34(–13,+50) ng and 4000(–2970,+17 160) ng are observed.

Scaling and Instabilities in Bubble Pinch-Off

NASA Astrophysics Data System (ADS)

Burton, J. C.; Waldrep, R.; Taborek, P.

2005-05-01

We have used a 100 000 frame-per-second video to analyze the pinch-off of nitrogen gas bubbles in fluids with a wide range of viscosity. If the external fluid is highly viscous (ηext>100 cP), the bubble neck radius is proportional to the time before break, τ, and decreases smoothly to zero. If the external fluid has low viscosity (ηext<10 cP), the radius scales as τ1/2 until an instability develops in the gas bubble, which causes the neck to rupture and tear apart. Finally, if the viscosity of the external fluid is in an intermediate range, an elongated thread is formed, which breaks apart into micron-sized bubbles.

Contact lines are unstable even under non-splashing droplets

NASA Astrophysics Data System (ADS)

Pack, Min; Kaneelil, Paul; Sun, Ying

2017-11-01

Drop impact is fundamental to natural and industrial processes such as rain-induced soil erosion and spray coating technologies. In this study, we elucidate the interfacial instabilities formed by air entrainment at the wetting front of impacting droplets on atomically smooth, viscous silicone oil films of constant thickness with varying droplet velocity, viscosity, surface tension, and ambient pressures. A high-speed total internal reflection microscopy technique accounting for the Fresnel relations at the droplet interface allowed for in-situ measurements of an entrained air rim at the wetting front. The growth of the air rim is a prerequisite to the instability which is formed when the gas pressure balances the capillary pressure near the wetting front. A critical capillary number, which inversely scales as the ambient pressure, is predicted and the result agrees well with the experiments. The wavenumber in the instability is shown to increase with viscosity and velocity but decrease with surface tension of the impacting drop. We thus conclude that the instability mechanism is in qualitative agreement with the Saffman-Taylor instability - where the low viscosity air is displacing the higher viscosity droplet. The low We contact line instabilities observed in this study provide a paradigm shift in the conventional understanding of hydrodynamic instabilities under drop impact which usually require We >>10.

Stripes instability of an oscillating non-Brownian iso-dense suspension of spheres

NASA Astrophysics Data System (ADS)

Roht, Y. L.; Ippolito, I.; Hulin, J. P.; Salin, D.; Gauthier, G.

2018-03-01

We analyze experimentally the behavior of a non-Brownian, iso-dense suspension of spheres submitted to periodic square wave oscillations of the flow in a Hele-Shaw cell of gap H. We do observe an instability of the initially homogeneous concentration in the form of concentration variation stripes transverse to the flow. The wavelength of these regular spatial structures scales roughly as the gap of the cell and is independent of the particle concentration and of the period of oscillation. This instability requires large enough particle volume fractions φ≥ 0.25 and a gap large enough compared to the sphere diameter (H/d ≥ 8) . Mapping the domain of the existence of this instability in the space of the control parameters shows that it occurs only in a limited range of amplitudes of the fluid displacement. The analysis of the concentration distribution across the gap supports a scenario of particle migration towards the wall followed by an instability due to a particle concentration gradient with a larger concentration at the walls. In order to account for the main features of this stripes instability, we use the theory of longitudinal instability due to normal stresses difference and recent observations of a dependence of the first normal stresses difference on the particle concentration.

Determination Of Slope Instability Using Spatially Integrated Mapping Framework

NASA Astrophysics Data System (ADS)

Baharuddin, I. N. Z.; Omar, R. C.; Roslan, R.; Khalid, N. H. N.; Hanifah, M. I. M.

2016-11-01

The determination and identification of slope instability are often rely on data obtained from in-situ soil investigation work where it involves the logistic of machineries and manpower, thus these aspects may increase the cost especially for remote locations. Therefore a method, which is able to identify possible slope instability without frequent ground walkabout survey, is needed. This paper presents the method used in prediction of slope instability using spatial integrated mapping framework which applicable for remote areas such as tropical forest and natural hilly terrain. Spatial data such as geology, topography, land use map, slope angle and elevation were used in regional analysis during desktop study. Through this framework, the occurrence of slope instability was able to be identified and was validate using a confirmatory site- specific analysis.

A statistical study of current-sheet formation above solar active regions based on selforganized criticality

NASA Astrophysics Data System (ADS)

Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M.; Anastasiadis, A.; Toutountzi, A.

2013-09-01

We treat flaring solar active regions as physical systems having reached the self-organized critical state. Their evolving magnetic configurations in the low corona may satisfy an instability criterion, related to the excession of a specific threshold in the curl of the magnetic field. This imposed instability criterion implies an almost zero resistivity everywhere in the solar corona, except in regions where magnetic-field discontinuities and. hence, local currents, reach the critical value. In these areas, current-driven instabilities enhance the resistivity by many orders of magnitude forming structures which efficiently accelerate charged particles. Simulating the formation of such structures (thought of as current sheets) via a refined SOC cellular-automaton model provides interesting information regarding their statistical properties. It is shown that the current density in such unstable regions follows power-law scaling. Furthermore, the size distribution of the produced current sheets is best fitted by power laws, whereas their formation probability is investigated against the photospheric magnetic configuration (e.g. Polarity Inversion Lines, Plage). The average fractal dimension of the produced current sheets is deduced depending on the selected critical threshold. The above-mentioned statistical description of intermittent electric field structures can be used by collisional relativistic test particle simulations, aiming to interpret particle acceleration in flaring active regions and in strongly turbulent media in astrophysical plasmas. The above work is supported by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme.

Implication of Tsallis entropy in the Thomas–Fermi model for self-gravitating fermions

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

Ourabah, Kamel; Tribeche, Mouloud, E-mail: mouloudtribeche@yahoo.fr

The Thomas–Fermi approach for self-gravitating fermions is revisited within the theoretical framework of the q-statistics. Starting from the q-deformation of the Fermi–Dirac distribution function, a generalized Thomas–Fermi equation is derived. It is shown that the Tsallis entropy preserves a scaling property of this equation. The q-statistical approach to Jeans’ instability in a system of self-gravitating fermions is also addressed. The dependence of the Jeans’ wavenumber (or the Jeans length) on the parameter q is traced. It is found that the q-statistics makes the Fermionic system unstable at scales shorter than the standard Jeans length. -- Highlights: •Thomas–Fermi approach for self-gravitatingmore » fermions. •A generalized Thomas–Fermi equation is derived. •Nonextensivity preserves a scaling property of this equation. •Nonextensive approach to Jeans’ instability of self-gravitating fermions. •It is found that nonextensivity makes the Fermionic system unstable at shorter scales.« less

Self-induced flavor conversion of supernova neutrinos on small scales

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

Chakraborty, S.; Hansen, R. S.; Izaguirre, I.

2016-01-15

Self-induced flavor conversion of supernova (SN) neutrinos is a generic feature of neutrino-neutrino dispersion. The corresponding run-away modes in flavor space can spontaneously break the original symmetries of the neutrino flux and in particular can spontaneously produce small-scale features as shown in recent schematic studies. However, the unavoidable “multi-angle matter effect” shifts these small-scale instabilities into regions of matter and neutrino density which are not encountered on the way out from a SN. The traditional modes which are uniform on the largest scales are most prone for instabilities and thus provide the most sensitive test for the appearance of self-inducedmore » flavor conversion. As a by-product we clarify the relation between the time evolution of an expanding neutrino gas and the radial evolution of a stationary SN neutrino flux. Our results depend on several simplifying assumptions, notably stationarity of the solution, the absence of a “backward” neutrino flux caused by residual scattering, and global spherical symmetry of emission.« less

Self-induced flavor conversion of supernova neutrinos on small scales

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

Chakraborty, S.; Izaguirre, I.; Raffelt, G.G.

2016-01-01

Self-induced flavor conversion of supernova (SN) neutrinos is a generic feature of neutrino-neutrino dispersion. The corresponding run-away modes in flavor space can spontaneously break the original symmetries of the neutrino flux and in particular can spontaneously produce small-scale features as shown in recent schematic studies. However, the unavoidable ''multi-angle matter effect'' shifts these small-scale instabilities into regions of matter and neutrino density which are not encountered on the way out from a SN. The traditional modes which are uniform on the largest scales are most prone for instabilities and thus provide the most sensitive test for the appearance of self-inducedmore » flavor conversion. As a by-product we clarify the relation between the time evolution of an expanding neutrino gas and the radial evolution of a stationary SN neutrino flux. Our results depend on several simplifying assumptions, notably stationarity of the solution, the absence of a ''backward'' neutrino flux caused by residual scattering, and global spherical symmetry of emission.« less

Altered characteristics of balance control in obese older adults.

PubMed

Melzer, Itshak; Oddsson, Lars I E

2016-01-01

Obesity is one of the most significant epidemiological trends of the last decades. Recently it was found that obese individuals show postural instability. Balance control mechanisms in obese older adults were less studied. Therefore we aimed to investigate the effect of obesity on balance control mechanisms in older adults. Parameters from Stabilogram-Diffusion Analysis (SDA) and measures from summary statistics of foot centre-of-pressure (COP) displacements along the anterior-posterior (AP) and mediolateral (ML) directions in eyes open and eyes closed conditions were used to characterize postural control in 22 obese (30-<35kg/m(2)), 26 overweight (25-<30kg/m(2)), and 18 normal weight subjects (18.5-<25kg/m(2)). Obese group subjects demonstrated significantly greater transition displacement, transition time interval, and short-term scaling exponent in the ML-direction compared with the normal weight group (eyes open and closed). In the AP-direction the obese group showed greater transition displacement (eyes open) and short-term scaling exponent (eyes open and closed). Average AP-COP and ML-COP ranges of COP sway were higher in the obese group compared with the normal weight group (eyes open and closed). This work indicates an altered postural control process in obese older adults. A greater sway displacement before closed-loop feedback mechanisms are called into play was seen in the ML direction that may lead to a higher risk of instability and fall events. Copyright © 2015 Asia Oceania Association for the Study of Obesity. Published by Elsevier Ltd. All rights reserved.

Mechanical instability and percolation of deformable particles through porous networks

NASA Astrophysics Data System (ADS)

Benet, Eduard; Lostec, Guillaume; Pellegrino, John; Vernerey, Franck

2018-04-01

The transport of micron-sized particles such as bacteria, cells, or synthetic lipid vesicles through porous spaces is a process relevant to drug delivery, separation systems, or sensors, to cite a few examples. Often, the motion of these particles depends on their ability to squeeze through small constrictions, making their capacity to deform an important factor for their permeation. However, it is still unclear how the mechanical behavior of these particles affects collective transport through porous networks. To address this issue, we present a method to reconcile the pore-scale mechanics of the particles with the Darcy scale to understand the motion of a deformable particle through a porous network. We first show that particle transport is governed by a mechanical instability occurring at the pore scale, which leads to a binary permeation response on each pore. Then, using the principles of directed bond percolation, we are able to link this microscopic behavior to the probability of permeating through a random porous network. We show that this instability, together with network uniformity, are key to understanding the nonlinear permeation of particles at a given pressure gradient. The results are then summarized by a phase diagram that predicts three distinct permeation regimes based on particle properties and the randomness of the pore network.

Dynamo-driven plasmoid formation from a current-sheet instability

DOE PAGES

Ebrahimi, F.

2016-12-15

Axisymmetric current-carrying plasmoids are formed in the presence of nonaxisymmetric fluctuations during nonlinear three-dimensional resistive MHD simulations in a global toroidal geometry. In this study, we utilize the helicity injection technique to form an initial poloidal flux in the presence of a toroidal guide field. As helicity is injected, two types of current sheets are formed from the oppositely directed field lines in the injector region (primary reconnecting current sheet), and the poloidal flux compression near the plasma edge (edge current sheet). We first find that nonaxisymmetric fluctuations arising from the current-sheet instability isolated near the plasma edge have tearingmore » parity but can nevertheless grow fast (on the poloidal Alfven time scale). These modes saturate by breaking up the current sheet. Second, for the first time, a dynamo poloidal flux amplification is observed at the reconnection site (in the region of the oppositely directed magnetic field). This fluctuation-induced flux amplification increases the local Lundquist number, which then triggers a plasmoid instability and breaks the primary current sheet at the reconnection site. Finally, the plasmoids formation driven by large-scale flux amplification, i.e., a large-scale dynamo, observed here has strong implications for astrophysical reconnection as well as fast reconnection events in laboratory plasmas.« less

Lithospheric Instability beneath the Southeast Carpathians

NASA Astrophysics Data System (ADS)

Houseman, G. A.; Lorinczi, P.; Ren, Y.; Stuart, G. W.

2012-12-01

The South Carpathian Project, a major seismological experiment carried out during 2009-2011 by the University of Leeds, the National Institute of Earth Physics in Bucharest, the Eötvös Loránd Geophysical Institute in Budapest, and the Seismological Survey of Serbia in Belgrade, has resulted in the most detailed tomographic images yet obtained of the upper mantle structure beneath the Pannonian - Carpathian region (Ren et al., EPSL, 2012). These images illuminate the upper mantle over a wide region, but they specifically shed new light on the unique geological structure which is responsible for the damaging earthquakes that occur in the upper mantle beneath the Vrancea Zone of the South-east Carpathians. The earthquakes occur at the NE end of an asymmetric high velocity structure that extends upward to the SW, oblique to the southern edge of the South Carpathians. This sub-vertical high-velocity body is bounded by slow anomalies to the NW and SE, which extend down to the top of the Mantle Transition Zone. With increasing depth, the fast region becomes more circular in cross-section until about 400 km where the fast anomaly fades out. The main mass of fast (presumably dense) material is located directly beneath the seismic activity. The earthquakes are all characterised by near-vertical T-axes, which means they are caused by vertical stretching. The seismic moment release rate can be used to estimate vertical strain rates; these strain-rates imply that the mantle at 200 km is moving downward at about 20 mm/yr relative to the surface. The depth distribution of seismic-moment release rate follows a characteristic pattern that is most easily explained if this high velocity structure is produced by a Rayleigh-Taylor instability acting on an unstable stratification of mantle lithosphere above asthenosphere. Three-dimensional numerical experiments assuming viscous flow confirm that the drip-like structure that we image may be a natural consequence of a Rayleigh-Taylor instability triggered by recent convergence of Adria and Europe. One property of Rayleigh-Taylor instability is that rates of deformation increase with time during the peak development phase of the instability. The present high rate of tectonic activity in this region therefore is probably short-lived on a geological scale (< 1 Myr) but is likely to increase in energy before it is abates. These unique tomographic and seismological datasets provide clear evidence of lithospheric gravitational instability occurring on a short length-scale and fast time-scale via a mode that is best described as a form of Rayleigh-Taylor instability.

Helical instability in film blowing process: Analogy to buckling instability

NASA Astrophysics Data System (ADS)

Lee, Joo Sung; Kwon, Ilyoung; Jung, Hyun Wook; Hyun, Jae Chun

2017-12-01

The film blowing process is one of the most important polymer processing operations, widely used for producing bi-axially oriented film products in a single-step process. Among the instabilities observed in this film blowing process, i.e., draw resonance and helical motion occurring on the inflated film bubble, the helical instability is a unique phenomenon portraying the snake-like undulation motion of the bubble, having the period on the order of few seconds. This helical instability in the film blowing process is commonly found at the process conditions of a high blow-up ratio with too low a freezeline position and/or too high extrusion temperature. In this study, employing an analogy to the buckling instability for falling viscous threads, the compressive force caused by the pressure difference between inside and outside of the film bubble is introduced into the simulation model along with the scaling law derived from the force balance between viscous force and centripetal force of the film bubble. The simulation using this model reveals a close agreement with the experimental results of the film blowing process of polyethylene polymers such as low density polyethylene and linear low density polyethylene.