Cullum, J.
1994-12-31
Plots of the residual norms generated by Galerkin procedures for solving Ax = b often exhibit strings of irregular peaks. At seemingly erratic stages in the iterations, peaks appear in the residual norm plot, intervals of iterations over which the norms initially increase and then decrease. Plots of the residual norms generated by related norm minimizing procedures often exhibit long plateaus, sequences of iterations over which reductions in the size of the residual norm are unacceptably small. In an earlier paper the author discussed and derived relationships between such peaks and plateaus within corresponding Galerkin/Norm Minimizing pairs of such methods. In this paper, through a set of numerical experiments, the author examines connections between peaks, plateaus, numerical instabilities, and the achievable accuracy for such pairs of iterative methods. Three pairs of methods, GMRES/Arnoldi, QMR/BCG, and two bidiagonalization methods are studied.
NASA Astrophysics Data System (ADS)
Perthame, Benoît; Schmeiser, Christian; Tang, Min; Vauchelet, Nicolas
2011-04-01
How can repulsive and attractive forces, acting on a conservative system, create stable travelling patterns or branching instabilities? We have proposed to study this question in the framework of the hyperbolic Keller-Segel system with logistic sensitivity. This is a model system motivated by experiments on cell communities auto-organization, a field which is also called socio-biology. We continue earlier modelling work, where we have shown numerically that branching patterns arise for this system and we have analysed this instability by formal asymptotics for small diffusivity of the chemo-repellent. Here we are interested in the more general situation, where the diffusivities of both the chemo-attractant and the chemo-repellent are positive. To do so, we develop an appropriate functional analysis framework. We apply our method to two cases. Firstly we analyse steady states. Secondly we analyse travelling waves when neglecting the degradation coefficient of the chemo-repellent; the unique wave speed appears through a singularity cancellation which is the main theoretical difficulty. This shows that in different situations the cell density takes the shape of a plateau. The existence of steady states and travelling plateaus are a symptom of how rich the system is and why branching instabilities can occur. Numerical tests show that large plateaus may split into smaller ones, which remain stable.
Numerical analysis of engine instability
NASA Astrophysics Data System (ADS)
Habiballah, M.; Dubois, I.
Following a literature review on numerical analyses of combustion instability, to give the state of the art in the area, the paper describes the ONERA methodology used to analyze the combustion instability in liquid propellant engines. Attention is also given to a model (named Phedre) which describes the unsteady turbulent two-phase reacting flow in a liquid rocket engine combustion chamber. The model formulation includes axial or radial propellant injection, baffles, and acoustic resonators modeling, and makes it possible to treat different engine types. A numerical analysis of a cryogenic engine stability is presented, and the results of the analysis are compared with results of tests of the Viking engine and the gas generator of the Vulcain engine, showing good qualitative agreement and some general trends between experiments and numerical analysis.
The ballistic transport instability in Saturn's rings - III. Numerical simulations
NASA Astrophysics Data System (ADS)
Latter, Henrik N.; Ogilvie, Gordon I.; Chupeau, Marie
2014-07-01
Saturn's inner B-ring and its C-ring support wavetrains of contrasting amplitudes but with similar length-scales, 100-1000 km. In addition, the inner B-ring is punctuated by two intriguing `flat' regions between radii 93 000 and 98 000 km in which the waves die out, whereas the C-ring waves coexist with a forest of plateaus, narrow ringlets, and gaps. In both regions, the waves are probably generated by a large-scale linear instability whose origin lies in the meteoritic bombardment of the rings: the ballistic transport instability. In this paper, the third in a series, we numerically simulate the long-term non-linear evolution of this instability in a convenient local model. Our C-ring simulations confirm that the unstable system forms low-amplitude wavetrains possessing a preferred band of wavelengths. B-ring simulations, on the other hand, exhibit localized non-linear wave `packets' separated by linearly stable flat zones. Wave packets travel slowly while spreading in time, a result that suggests the observed flat regions in Saturn's B-ring are shrinking. Finally, we present exploratory runs of the inner B-ring edge which reproduce earlier numerical results: ballistic transport can maintain the sharpness of a spreading edge while building a `ramp' structure at its base. Moreover, the ballistic transport instability can afflict the ramp region, but only in low-viscosity runs.
NASA Astrophysics Data System (ADS)
Capponi, Sylvain
2017-01-01
We present numerical evidence that the spin-1/2 Heisenberg model on the two-dimensional checkerboard lattice exhibits several magnetization plateaus for m =0 , 1 /4 , 1 /2 , and 3 /4 , where m is the magnetization normalized by its saturation value. These incompressible states correspond to somewhat similar valence-bond crystal phases that break lattice symmetries, though they are different from the already established plaquette phase for m =0 . Our results are based on exact diagonalization as well as density-matrix renormalization-group large-scale simulations and interpreted in terms of simple parameter-free trial wave functions.
NASA Astrophysics Data System (ADS)
Garcia-Castellanos, D.
2011-12-01
Orographic precipitation is thought to exert a significant control on the growth of orogens, namely on the distribution of tectonic uplift and deformation in the multi-million-year time-scale. Computer tectonic models based on simple physical laws predict that the concentration of erosional surface mass removal in either side of an orogen can have a significant impact on the how tectonic deformation propagates within the orogenic building. Proxies or simple formulations of climatic processes such as precipitation and evaporation are key to such coupled tectonic/climatic models. Intramountain high plateaus are extreme cases of climatic control on sediment flow, because they often consist of internally-drained (endorheic) basins that trap most erosional products within the orogen. Such intramountain sediment accumulation leads to high-plateaus characterized by high topography and, in spite of the compressional tectonics, low relief. The development of high-plateaus has been extensively studied in relation to the tectonic setting and to inherited structures in the crust, but in this presentation I will concentrate on the potential climatic controls. Computer modeling results will be shown suggesting that a preexisting dry climate, through its effects on surface transport and orogenic evolution, may significantly modify tectonic deformation and contribute to the later formation of a high plateau. This is due to the following feedback: (1) dry climatic conditions at the early stages of orogenesis favor the tectonic defeat of rivers draining the orogen, promoting internal drainage (endorheism); (2) endorheism extends the life of intramountain basins maximizing the mass trapped within the orogen, rising lithostatic stress underneath and expelling fault deformation towards the flanks of the orogen; and (3) this propagation of deformation further isolates the central parts of the orogen from incoming precipitation, reinforcing sediment trapping and topographic leveling of
A Numerical Study of Feathering Instability
NASA Astrophysics Data System (ADS)
Lee, Wing-Kit; Wang, Hsiang-Hsu
2016-06-01
The stability of a spiral shock of self-gravitating, magnetized interstellar medium is studied by performing two-dimensional numerical simulations of a local patch of tight-winding spiral arm. As previously suggested by the linear studies, two types of instabilities are identified, namely, wiggle instability and feathering instability. The former instability occurs in the hydrodynamics limit and results in short wavelength perturbations. On the other hand, the feathering instability requires both self-gravitating and magnetic fields and results in wider structures.
NASA Astrophysics Data System (ADS)
Pusok, Adina E.; Kaus, Boris; Popov, Anton
2014-05-01
. We also address the question of how large topographic plateaus, such as the Tibetan Plateau, can form in an integrated lithospheric and upper-mantle scale model. Acknowledgements: Funding was provided by the European Research Council under the European Community's Seventh Framework Program (FP7/2007-2013) / ERC Grant agreement #258830. Numerical computations have been performed on MOGON (ZDV Mainz computing center) and JUQUEEN (Jülich high-performance computing center).
Understanding Etna flank instability through numerical models
NASA Astrophysics Data System (ADS)
Apuani, Tiziana; Corazzato, Claudia; Merri, Andrea; Tibaldi, Alessandro
2013-02-01
As many active volcanoes, Mount Etna shows clear evidence of flank instability, and different mechanisms were suggested to explain this flank dynamics, based on the recorded deformation pattern and character. Shallow and deep deformations, mainly associated with both eruptive and seismic events, are concentrated along recognised fracture and fault systems, mobilising the eastern and south-eastern flank of the volcano. Several interacting causes were postulated to control the phenomenon, including gravity force, magma ascent along the feeding system, and a very complex local and/or regional tectonic activity. Nevertheless, the complexity of such dynamics is still an open subject of research and being the volcano flanks heavily urbanised, the comprehension of the gravitative dynamics is a major issue for public safety and civil protection. The present research explores the effects of the main geological features (in particular the role of the subetnean clays, interposed between the Apennine-Maghrebian flysch and the volcanic products) and the role of weakness zones, identified by fracture and fault systems, on the slope instability process. The effects of magma intrusions are also investigated. The problem is addressed by integrating field data, laboratory tests and numerical modelling. A bi- and tri-dimensional stress-strain analysis was performed by a finite difference numerical code (FLAC and FLAC3D), mainly aimed at evaluating the relationship among geological features, volcano-tectonic structures and magmatic activity in controlling the deformation processes. The analyses are well supported by dedicated structural-mechanical field surveys, which allowed to estimate the rock mass strength and deformability parameters. To take into account the uncertainties which inevitably occur in a so complicated model, many efforts were done in performing a sensitivity analysis along a WNW-ESE section crossing the volcano summit and the Valle del Bove depression. This was
Numerical Modelling Of Pumpkin Balloon Instability
NASA Astrophysics Data System (ADS)
Wakefield, D.
Tensys have been involved in the numerical formfinding and load analysis of architectural stressed membrane structures for 15 years. They have recently broadened this range of activities into the `lighter than air' field with significant involvement in aerostat and heavy-lift hybrid airship design. Since early 2004 they have been investigating pumpkin balloon instability on behalf of the NASA ULDB programme. These studies are undertaken using inTENS, an in-house finite element program suite based upon the Dynamic Relaxation solution method and developed especially for the non-linear analysis and patterning of membrane structures. The paper describes the current state of an investigation that started with a numerical simulation of the lobed cylinder problem first studied by Calladine. The influence of material properties and local geometric deformation on stability is demonstrated. A number of models of complete pumpkin balloons have then been established, including a 64-gore balloon with geometry based upon Julian Nott's Endeavour. This latter clefted dramatically upon initial inflation, a phenomenon that has been reproduced in the numerical model. Ongoing investigations include the introduction of membrane contact modelling into inTENS and correlation studies with the series of large-scale ULDB models currently in preparation.
Numerical results for axial flow compressor instability
NASA Technical Reports Server (NTRS)
Mccaughan, F. E.
1988-01-01
Using Cornell's supercomputing facilities, an extensive study of the Moore-Greitzer model was carried out, which gives accurate and reliable information about compressor instability. The bifurcation analysis in the companion paper shows the dependence of the mode of compressor response on the shape of the rotating stall characteristic. The numerical results verify and extend this with a more accurate representation of the characteristic. The effect of the parameters on the shape of the rotating stall characteristic is investigated, and it is found that the parameters with the strongest effects are the inlet length, and the shape of the compressor pressure rise vs. mass flow diagram (i.e. tall diagrams vs. shallow diagrams). The effects of inlet guide vane loss on the characteristic are discussed.
Numerical Analysis of Modal Instability Onset in Fiber Amplifiers
2014-03-11
ABSTRACT 16. SECURITY CLASSIFICATION OF: Numerical analysis of the onset of modal instability in fiber amplifiers is presented. Specifically calculations...analysis of modal instability onset in fiber amplifiers The views, opinions and/or findings contained in this report are those of the author(s) and...linear effects, thermal effects, power scaling, fiber lasers, modal instability, stimulated thermal Rayleigh scattering REPORT DOCUMENTATION PAGE 11
Numerical simulation of instability and transition physics
NASA Technical Reports Server (NTRS)
Streett, C. L.
1990-01-01
The study deals with the algorithm technology used in instability and transition simulations. Discretization methods are outlined, and attention is focused on high-order finite-difference methods and high-order centered-difference formulas. One advantage of finite-difference methods over spectral methods is thought to be in implementation of nonrigorous boundary conditions. It is suggested that the next significant advances in the understanding of transition physics and the ability to predict transition will come with more physically-realistic simulations. Compressible-flow algorithms are discussed, and it is noted that with further development, exploration of bypass mechanism on simple bodies at high speed would be possible.
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
Numerical physics of the Richtmyer-Meshkov instability
NASA Astrophysics Data System (ADS)
Smith, Todd Michael
The Richtmyer-Meshkov instability is a fluid instability of importance to scientists and engineers. This instability occurs when a perturbed fluid interface is crossed by a shock wave and subjected to a rapid acceleration. A central goal of research into the dynamics of this instability is to determine the rate of growth of the boundaries of the resulting mixing layer. In the present work, we report high-quality numerical simulations of the single-mode two-dimensional planer Richtmyer-Meshkov instability across a variety of physical parameters, focusing on the effect of the variation of the Atwood number for Mach two incident shocks. We compare the growth of the mixing layer, bubble and spike fronts to analytical models and examine the coherent structures of the developing mixing layer. We examine the breakdown in symmetry between bubble and spike fronts. We describe a new method of modeling the acceleration terms of traditional bouyancy drag models of mixing layer growth by drawing these terms from the compressible linear theory. As a result, we treat the Richtmyer- Meshkov instability as a Rayleigh-Taylor instability with time-varying acceleration terms. The resulting model gives an excellent fit to the velocities of the spike and bubble fronts for the simulations performed for the present work from shock passage until well into the nonlinear regime. We describe and discuss possible reasons for deviation from the present model.
Numerical Simulations of Instabilities in Single-Hole Office Elements
NASA Technical Reports Server (NTRS)
Ahuja, Vineet; Hosangadi, Ashvin; Hitt, Matthew A.; Lineberry, David M.
2013-01-01
An orifice element is commonly used in liquid rocket engine test facilities either as a flow metering device, a damper for acoustic resonance or to provide a large reduction in pressure over a very small distance in the piping system. While the orifice as a device is largely effective in stepping down pressure, it is also susceptible to a wake-vortex type instability that generates pressure fluctuations that propagate downstream and interact with other elements of the test facility resulting in structural vibrations. Furthermore in piping systems an unstable feedback loop can exist between the vortex shedding and acoustic perturbations from upstream components resulting in an amplification of the modes convecting downstream. Such was the case in several tests conducted at NASA as well as in the Ariane 5 strap-on P230 engine in a static firing test where pressure oscillations of 0.5% resulted in 5% thrust oscillations. Exacerbating the situation in cryogenic test facilities, is the possibility of the formation of vapor clouds when the pressure in the wake falls below the vapor pressure leading to a cavitation instability that has a lower frequency than the primary wake-vortex instability. The cavitation instability has the potential for high amplitude fluctuations that can cause catastrophic damage in the facility. In this paper high-fidelity multi-phase numerical simulations of an orifice element are used to characterize the different instabilities, understand the dominant instability mechanisms and identify the tonal content of the instabilities.
Kuzelev, M. V.; Sepehri Javan, N.
2007-08-15
A kinetic model is proposed that describes the nonlinear dynamics of the instabilities of a transversely nonuniform plasma flow. It is shown that, in the linear approximation, the model yields the familiar boundary-value problem for the scalar potential in plasma. The slipping instability in a plane waveguide is considered as an example. The general dispersion relation for a flow with a stepwise uniform density profile and with a tangential discontinuity in its longitudinal velocity is analyzed qualitatively. The dynamics of the slipping instability is investigated numerically for a flow that is detached from the waveguide walls and whose longitudinal velocity obeys a linear, a sinusoidal, or a discontinuous distribution. In the nonlinear stage of the instability, the flow expands in such a way as to come into contact with the walls, the spread in the longitudinal velocities remains smaller than the initial velocity variation, and the longitudinal velocities of different transverse layers in the flow are not completely equalized.
Tube Feeding Transition Plateaus
ERIC Educational Resources Information Center
Klein, Marsha Dunn
2007-01-01
The journey children make from tube feeding to oral feeding is personal for each child and family. There is a sequence of predictable plateaus that children climb as they move toward orally eating. By better understanding this sequence, parents and children can maximize the development, learning, enjoyment and confidence at each plateau. The…
Can numerical simulations accurately predict hydrodynamic instabilities in liquid films?
NASA Astrophysics Data System (ADS)
Denner, Fabian; Charogiannis, Alexandros; Pradas, Marc; van Wachem, Berend G. M.; Markides, Christos N.; Kalliadasis, Serafim
2014-11-01
Understanding the dynamics of hydrodynamic instabilities in liquid film flows is an active field of research in fluid dynamics and non-linear science in general. Numerical simulations offer a powerful tool to study hydrodynamic instabilities in film flows and can provide deep insights into the underlying physical phenomena. However, the direct comparison of numerical results and experimental results is often hampered by several reasons. For instance, in numerical simulations the interface representation is problematic and the governing equations and boundary conditions may be oversimplified, whereas in experiments it is often difficult to extract accurate information on the fluid and its behavior, e.g. determine the fluid properties when the liquid contains particles for PIV measurements. In this contribution we present the latest results of our on-going, extensive study on hydrodynamic instabilities in liquid film flows, which includes direct numerical simulations, low-dimensional modelling as well as experiments. The major focus is on wave regimes, wave height and wave celerity as a function of Reynolds number and forcing frequency of a falling liquid film. Specific attention is paid to the differences in numerical and experimental results and the reasons for these differences. The authors are grateful to the EPSRC for their financial support (Grant EP/K008595/1).
EFFECTS OF DIFFERENT NUMERICAL INTERFACE METHODS ON HYDRODYNAMICS INSTABILITY
FRANCOIS, MARIANNE M.; DENDY, EDWARD D.; LOWRIE, ROBERT B.; LIVESCU, DANIEL; STEINKAMP, MICHAEL J.
2007-01-11
The authors compare the effects of different numerical schemes for the advection and material interface treatments on the single-mode Rayleigh-Taylor instability, using the RAGE hydro-code. The interface growth and its surface density (interfacial area) versus time are investigated. The surface density metric shows to be better suited to characterize the difference in the flow, than the conventional interface growth metric. They have found that Van Leer's limiter combined to no interface treatment leads to the largest surface area. Finally, to quantify the difference between the numerical methods they have estimated the numerical viscosity in the linear-regime at different scales.
Numerical Simulation of Conductivity Gradient-Induced Electrokinetic Flow Instabilities
NASA Astrophysics Data System (ADS)
Bradford, Stephen; Meinhart, Carl
2006-03-01
This research is focused on the electrokinetic flow instabilities observed in long, thin microchannels with conductivity gradients orthogonal to the streamwise direction and applied potential. This situation often occurs in field amplified sample stacking (FASS) and isoelectric focusing, where control of the instabilities is imperative. Alternatively, the inherently chaotic flow patterns can be leveraged to fabricate an efficient micromixer under specific conditions. These instabilities arise from fluid body forces generated by the action of applied electric fields on electrolyte concentration-based conductivity gradients. A model is developed to describe the phenomena in general and applied specifically to thin microchannels with the conductivity gradient perpendicular to the applied field (both DC and AC). A higher-order, depth averaged correlation is proposed to account for the out of plane effects. Numerical simulations performed using COMSOL 3.2 are compared to 2-D and 3-D simulations as well as experimental data for multiple geometries with good agreement.
2017-04-10
Saturn's C ring isn't uniformly bright. Instead, about a dozen regions of the ring stand out as noticeably brighter than the rest of the ring, while about half a dozen regions are devoid of ring material. Scientists call the bright regions "plateaus" and the devoid regions "gaps." Scientists have determined that the plateaus are relatively bright because they have higher particle density and reflect more light, but researchers haven't solved the trickier puzzle of how the plateaus are created and maintained. This view looks toward the sunlit side of the rings from about 62 degrees above the ring plane. The image was taken Jan. 9, 2017 in green light with the Cassini spacecraft's narrow-angle camera. Cassini obtained the image while approximately 194,000 miles (312,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 67 degrees. Image scale is 1.2 miles (2 kilometers) per pixel. https://photojournal.jpl.nasa.gov/catalog/PIA20529
Numerical Investigation of Galloping Instabilities in Z-Shaped Profiles
Chavez, Miguel; Valero, Eusebio
2014-01-01
Aeroelastic effects are relatively common in the design of modern civil constructions such as office blocks, airport terminal buildings, and factories. Typical flexible structures exposed to the action of wind are shading devices, normally slats or louvers. A typical cross-section for such elements is a Z-shaped profile, made out of a central web and two-side wings. Galloping instabilities are often determined in practice using the Glauert-Den Hartog criterion. This criterion relies on accurate predictions of the dependence of the aerodynamic force coefficients with the angle of attack. The results of a parametric analysis based on a numerical analysis and performed on different Z-shaped louvers to determine translational galloping instability regions are presented in this paper. These numerical analysis results have been validated with a parametric analysis of Z-shaped profiles based on static wind tunnel tests. In order to perform this validation, the DLR TAU Code, which is a standard code within the European aeronautical industry, has been used. This study highlights the focus on the numerical prediction of the effect of galloping, which is shown in a visible way, through stability maps. Comparisons between numerical and experimental data are presented with respect to various meshes and turbulence models. PMID:25054176
Numerical simulations of turbulence and mixing induced by submesoscale instabilities
NASA Astrophysics Data System (ADS)
Stamper, Megan; Taylor, John
2015-11-01
Submesoscale features in the upper ocean with horizontal scales between 1-10km have received significant attention in the oceanography community in recent years. Previous work has found that submesoscales play an important role in setting the stratification of the upper ocean, and these scales are associated with large vertical velocities that modify biological productivity. Submesoscales bridge the dynamical gap between the mesoscale (~100km) where the earth's rotation plays a major role, and turbulent overturning scales (~1-10m) where the earth's rotation is not directly felt. Here, we use very high resolution direct numerical simulations (DNS) to explore the interaction and feedbacks between submesoscales and small scale turbulence. In simulations with submesoscale motions generated via symmetric and baroclinic instability, we find that the emergence of secondary instabilities leads to significant small-scale turbulence and mixing, even in the absence of wind and convective forcing. From the DNS results, we quantify the additional mixing, dissipation, and vertical fluxes induced by small scale turbulence, and its feedback on the primary submesoscale instabilities.
A Numerical Instability in an ADI Algorithm for Gyrokinetics
E.A. Belli; G.W. Hammett
2004-12-17
We explore the implementation of an Alternating Direction Implicit (ADI) algorithm for a gyrokinetic plasma problem and its resulting numerical stability properties. This algorithm, which uses a standard ADI scheme to divide the field solve from the particle distribution function advance, has previously been found to work well for certain plasma kinetic problems involving one spatial and two velocity dimensions, including collisions and an electric field. However, for the gyrokinetic problem we find a severe stability restriction on the time step. Furthermore, we find that this numerical instability limitation also affects some other algorithms, such as a partially implicit Adams-Bashforth algorithm, where the parallel motion operator v{sub {parallel}} {partial_derivative}/{partial_derivative}z is treated implicitly and the field terms are treated with an Adams-Bashforth explicit scheme. Fully explicit algorithms applied to all terms can be better at long wavelengths than these ADI or partially implicit algorithms.
Instabilities in astrophysical jets. II - Numerical simulations of slab jets
NASA Technical Reports Server (NTRS)
Zhao, Jun-Hui; Burns, Jack O.; Norman, Michael L.; Sulkanen, Martin E.
1992-01-01
Numerical simulations of an unstable supersonic slab-symmetric jet are described. The instabilities within the jet are characterized by growing internal body waves and their coupled surface waves that are also predicted in linear perturbation theory. The characteristic theory of fluid dynamics is used to help interpret the wave morphologies. It is demonstrated that these waves can be excited by imposing an arbitrary disturbance. From the numerical simulations, it is found that the sound waves propagating against the flow slow down as they propagate outward, and they grow in amplitude. These waves eventually disrupt the jet at a certain length. This disruption length is related to the jet Mach number and the perturbation intensity. Thus, the Mach number of a jet observed with a radio telescope can be estimated by measuring the disruption length and estimating the perturbation intensity. The jet Mach numbers in radio tailed sources determined in this way agree quite well with estimates from ram pressure bending arguments. The wiggles and flares observed in many extragalactic jets, especially in tailed radio sources, appear to be intimately related to instabilities and the jet disruption process.
A numerical and analytical investigation of Rayleigh-Taylor instability in a solid tungsten plate
Robinson, A.C.; Swegle, J.W.
1987-07-01
The Rayleigh-Taylor instability response of an elastic-plastic tungsten plate is investigated by numerical experiments and an approximate modal analysis. The so-called ''minimum amplitude'' instability criteria derived from plasticity analyses is shown to be incomplete as a general indicator of instability or stability at very large driving pressures. Model equations are derived which are able to reproduce the basic qualitative features of the observed instability response given by the numerical calculations. 11 refs., 29 figs.
Numerical and experimental analysis of instability phenomena in pump turbines
NASA Astrophysics Data System (ADS)
Gentner, Ch; Sallaberger, M.; Widmer, Ch; Braun, O.; Staubli, T.
2012-11-01
Today, utilities operating pump turbines require fast and frequent changes between pumping and generating modes as well as extended operation at off-design conditions. Operation of the units in unstable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. Two main features of unstable behaviour of pump turbines are known. One sometimes occurs in generating mode at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic) and the other one shows in pump operation as a drop in head as the flow is reduced (saddle-type pump instability of head curve). If the stability characteristics of a pump turbine need to be known already at the design stage of the runner, numerical flow simulation (computational fluid dynamics, CFD) is the most promising tool. As the characteristics of the flow near the stability limit are highly unsteady, steady state CFD, as it is usually applied for the runner design, does not deliver the necessary insight into the flow field. In order to analyse the flow field in runner and diffuser of a pump turbine in the unstable areas of operation, ANDRITZ HYDRO has evaluated several CFD procedures with different approaches for the calculation of the described instabilities in pump and turbine operation. The results of the unsteady flow calculation are compared with model test results.
A Cartesian parametrization for the numerical analysis of material instability
Mota, Alejandro; Chen, Qiushi; Foulk, III, James W.; ...
2016-02-25
We examine four parametrizations of the unit sphere in the context of material stability analysis by means of the singularity of the acoustic tensor. We then propose a Cartesian parametrization for vectors that lie a cube of side length two and use these vectors in lieu of unit normals to test for the loss of the ellipticity condition. This parametrization is then used to construct a tensor akin to the acoustic tensor. It is shown that both of these tensors become singular at the same time and in the same planes in the presence of a material instability. Furthermore, themore » performance of the Cartesian parametrization is compared against the other parametrizations, with the results of these comparisons showing that in general, the Cartesian parametrization is more robust and more numerically efficient than the others.« less
A Cartesian parametrization for the numerical analysis of material instability
Mota, Alejandro; Chen, Qiushi; Foulk, III, James W.; Ostien, Jakob T.; Lai, Zhengshou
2016-02-25
We examine four parametrizations of the unit sphere in the context of material stability analysis by means of the singularity of the acoustic tensor. We then propose a Cartesian parametrization for vectors that lie a cube of side length two and use these vectors in lieu of unit normals to test for the loss of the ellipticity condition. This parametrization is then used to construct a tensor akin to the acoustic tensor. It is shown that both of these tensors become singular at the same time and in the same planes in the presence of a material instability. Furthermore, the performance of the Cartesian parametrization is compared against the other parametrizations, with the results of these comparisons showing that in general, the Cartesian parametrization is more robust and more numerically efficient than the others.
Collisional plateaus. [in earth and Venus lithospheres
NASA Technical Reports Server (NTRS)
Morgan, P.; Burke, K.
1985-01-01
Aspects of the geology of collisional plateaus formed by the thickening of continental crust are briefly reviewed. The history of studies of collisional plateaus is summarized, and igneous activity in collisional plateaus is discussed. Isostatic considerations pertaining to these plateaus are addressed, developing models of isostatic support of topography which illustrate the importance of compressional tectonics in the creation of high altitude plateaus. Possible analogous environments on Venus are considered. Finally, the paradox of extension associated with compression in the plateaus is discussed.
Collisional plateaus. [in earth and Venus lithospheres
NASA Technical Reports Server (NTRS)
Morgan, P.; Burke, K.
1985-01-01
Aspects of the geology of collisional plateaus formed by the thickening of continental crust are briefly reviewed. The history of studies of collisional plateaus is summarized, and igneous activity in collisional plateaus is discussed. Isostatic considerations pertaining to these plateaus are addressed, developing models of isostatic support of topography which illustrate the importance of compressional tectonics in the creation of high altitude plateaus. Possible analogous environments on Venus are considered. Finally, the paradox of extension associated with compression in the plateaus is discussed.
Numerical prediction of interfacial instabilities: Sharp interface method (SIM)
NASA Astrophysics Data System (ADS)
Nourgaliev, R. R.; Liou, M.-S.; Theofanous, T. G.
2008-04-01
We introduce a sharp interface method (SIM) for the direct numerical simulation of unstable fluid-fluid interfaces. The method is based on the level set approach and the structured adaptive mesh refinement technology, endowed with a corridor of irregular, cut-cell grids that resolve the interfacial region to third-order spatial accuracy. Key in that regard are avoidance of numerical mixing, and a least-squares interpolation method that is supported by irregular datasets distinctly on each side of the interface. Results on test problems show our method to be free of the spurious current problem of the continuous surface force method and to converge, on grid refinement, at near-theoretical rates. Simulations of unstable Rayleigh-Taylor and viscous Kelvin-Helmholtz flows are found to converge at near-theoretical rates to the exact results over a wide range of conditions. Further, we show predictions of neutral-stability maps of the viscous Kelvin-Helmholtz flows (Yih instability), as well as self-selection of the most unstable wave-number in multimode simulations of Rayleigh-Taylor instability. All these results were obtained with a simple seeding of random infinitesimal disturbances of interface-shape, as opposed to seeding by a complete eigenmode. For other than elementary flows the latter would normally not be available, and extremely difficult to obtain if at all. Sample comparisons with our code adapted to mimic typical diffuse interface treatments were not satisfactory for shear-dominated flows. On the other hand the sharp dynamics of our method would appear to be compatible and possibly advantageous to any interfacial flow algorithm in which the interface is represented as a discrete Heaviside function.
Proposed Ozark Plateaus Province Hydrologic Observatory
NASA Astrophysics Data System (ADS)
Davis, R. K.; Brahana, J. V.; Matlock, M. D.; Chaubey, I.; Pavlowsky, R. T.; Gilzow, F.
2004-12-01
The Upper White River, which drains about 40 percent of the Ozarks Ecoregion, is the main drain for the Ozark Plateaus and is characteristic of rivers draining other karst areas within the United States and the world. The proposed Ozark Plateaus Hydrologic Observatory (OPHO) encompasses twelve 8-digit hydrologic units covering about 67,000 km2 in parts of three states (Arkansas, Missouri, and Oklahoma). Six major U.S. Army Corps of Engineers reservoirs are within the OPHO including four on the main stem of the White River and one on the Illinois River. Karst features are prominent in the Salem, Ozark, and Springfield Plateaus of the OPHO, and include numerous solutionally enlarged fractures, caves, sinkholes, and sinking streams. Within the basin are numerous and diverse biological communities, representing influences from 1) eastern deciduous forest, 2) Great Plains prairies, 3) arid southwest, and 4) relicts of northern species from the Pleistocene Ice Age. Also contain in the OPHO is a diverse and unique array of mussels, an imperiled river organism (38 species), and crayfish. In the extensive karst regions of the OPHO are found largely endemic subterranean organisms also dependent on good water quality: for example, the Ozark Cavefish, Bristly Cave Crayfish and the recently federally- listed Tumbling Creek Cave Snail. Mantled karst aquifers characteristic of the Ozark Plateaus Region represent a coupled atmospheric/surface water/groundwater system that is highly susceptible to external forcing. Little attenuation of contaminants occurs as water moves from surface sources into and through the mantled karst aquifer to discharge naturally at springs and streams throughout the Ozark Plateau Region, and to wells. Because of the very open character of the aquifer, extremely dynamic biogeochemical cycling of nutrients occurs. Upper White River Reservoir development, filling and operation historically have altered and continue to alter the hydrologic and ecosystems
Reckinger, Scott James; Livescu, Daniel; Vasilyev, Oleg V.
2016-09-02
A comprehensive numerical methodology has been developed that handles the challenges introduced by considering the compressive nature of Rayleigh-Taylor instability (RTI) systems, which include sharp interfacial density gradients on strongly stratified background states, acoustic wave generation and removal at computational boundaries, and stratification-dependent vorticity production. The computational framework is used to simulate two-dimensional single-mode RTI to extreme late-times for a wide range of flow compressibility and variable density effects. The results show that flow compressibility acts to reduce the growth of RTI for low Atwood numbers, as predicted from linear stability analysis.
Direct Numerical Simulation of Rayleigh-Taylor Instability
NASA Astrophysics Data System (ADS)
Cook, Andrew W.; Dimotakis, Paul E.
1999-11-01
Direct numerical simulations of Rayleigh-Taylor-Instability flows in a rectangular domain were performed on the ASCI Blue-Pacific computer at Lawrence Livermore National Laboratory. The code solves the Navier-Stokes equations, plus species-continuity equations with Fickian diffusion for two incompressible, miscible fluids with a 3:1 density ratio. The fluids are initially separated by a diffuse interface with broad-banded isotropic perturbations applied to the contact region. Several runs were performed with different initial conditions. Periodic boundary conditions are imposed in the horizontal directions with no-slip walls at the top and bottom of the computational domain. Neumann conditions on pressure are applied on the top/bottom walls. Domain decomposition is used to achieve a high degree of parallelism on 96 processors with a computational mesh of 256x256x512 grid points. The flow Reynolds number, based on the vertical-extent and growth-rate of the mixing region, reaches about 1100 by the end of the calculations. The results indicate that the flow remains sensitive to initial conditions, at least for the time spanned in these simulations.
Numerical investigation on Benard instability in a finite liquid layer
NASA Technical Reports Server (NTRS)
Duh, J. C.
1991-01-01
A numerical procedure for directly simulating the Benard-Marangoni instabilities (B-M-I) in a bounded liquid layer is presented in this paper. The procedure consists of applying a finite amplitude disturbance to the basic static state, and then integrating the Navier-Stokes equations to determine whether the disturbance will die down or will reach a state of finite-strength steady convection. The critical Marangoni number (Mac) for the onset of B-M-I can thus be determined and can be correlated as a function of the aspect ratio (Ar), Prandtl number (Pr), and Rayleigh number (Ra). The Biot number (B) between the liquid and the air is analyzed to approximate the heat transfer condition along the free surface. A 2D calculation is performed to investigate the effect of various initial disturbances, and the Mac is determined for Ar = 2, Ra = 0, and Pr = 0.7. Current results show that disturbances of different nature and amplitude have little effect on Mac. The Mac determined in this study also clearly demonstrates the dominant effect of the sidewalls.
Numerical Study of a Hydrodynamic Instability Driven by Evaporation
NASA Astrophysics Data System (ADS)
Hernandez-Zapata, Sergio; Romo-Cruz, Julio Cesar Ruben; Lopez-Sanchez, Erick Javier; Ruiz-Chavarria, Gerardo
2013-11-01
The study of hydrodynamic instabilities in liquid layers produced by evaporation has several applications on industry and technology. In this work we study numerically the conditions under which a liquid layer becomes unstable when evaporation in the vapor-liquid interphase is present. The evaporation process follows the Hertz-Knudsen law (the evaporation rate is proportional to the difference between the saturated vapor pressure at the liquid layer temperature and the vapor partial pressure in the environment). Additionally to the usual boundary conditions on solid walls (for example, the non-slip condition for the velocity), we analyze the boundary conditions in the vapor-liquid interphase where the momentum and energy balances have to be taken into account and where the evaporation plays a crucial role. To solve this problem the linear theory of stability is used; that is, a small perturbation around the basic solution is applied (flow at rest and a temperature stationary field). The equations are solved using the Chebyshev pseudo-spectral method. The results are compared with the more usual Rayleigh-Bénard and Marangoni mechanisms as well as with some experiments carried out by our team. Authors acknowledge DGAPA-UNAM by support under project IN116312, ``Vorticidad y Ondas no lineales en fluidos.''
Numerical investigation of the transverse instability on the radiation-pressure-driven foil.
Wang, W Q; Yin, Y; Yu, T P; Xu, H; Zou, D B; Shao, F Q
2015-12-01
The development of transverse instability in the radiation-pressure-acceleration dominant laser-foil interaction is numerically examined by two-dimensional particle-in-cell simulations. When a plane laser impinges on a foil with modulated surface, the transverse instability is incited, and periodic perturbations of the proton density develop. The growth rate of the transverse instability is numerically diagnosed. It is found that the linear growth of the transverse instability lasts only a few laser periods, then the instability gets saturated. In order to optimize the modulation wavelength of the target, a method of information entropy is put forward to describe the chaos degree of the transverse instability. With appropriate modulation, the transverse instability shows a low chaos degree, and a quasi-monoenergetic proton beam is produced.
Numerical investigations on the effects of geometrical parameters on free electron laser instability
NASA Astrophysics Data System (ADS)
Sharma, B. S.; Jaiman, N. K.
2008-12-01
In this paper we numerically investigate the effects of various geometrical parameters of a backward wave oscillator (BWO), filled with a magnetized plasma of uniform density and driven by a mild relativistic solid electron beam, on the instability growth rate (Γ) of a free electron laser (FEL). The FEL instability is numerically calculated and the result is compared with the instability growth rate of an annular electron beam for the same set of parameters. The instability growth for a solid electron beam scales inversely to the seventh power of relativistic gamma factor γ0 and directly proportional to the corrugation amplitude.
Improved numerical Cherenkov instability suppression in the generalized PSTD PIC algorithm
NASA Astrophysics Data System (ADS)
Godfrey, Brendan B.; Vay, Jean-Luc
2015-11-01
The family of generalized Pseudo-Spectral Time Domain (including the Pseudo-Spectral Analytical Time Domain) Particle-in-Cell algorithms offers substantial versatility for simulating particle beams and plasmas, and well written codes using these algorithms run reasonably fast. When simulating relativistic beams and streaming plasmas in multiple dimensions, they are, however, subject to the numerical Cherenkov instability. Previous studies have shown that instability growth rates can be reduced substantially by modifying slightly the transverse fields as seen by the streaming particles. Here, we offer an approach which completely eliminates the fundamental mode of the numerical Cherenkov instability while minimizing the transverse field corrections. The procedure, numerically computed residual growth rates (from weaker, higher order instability aliases), and comparisons with simulations using the code Warp are presented. In some instances, there are no numerical instabilities whatsoever, at least in the linear regime.
NASA Astrophysics Data System (ADS)
Reckinger, Scott J.; Livescu, Daniel; Vasilyev, Oleg V.
2016-05-01
An investigation of compressible Rayleigh-Taylor instability (RTI) using Direct Numerical Simulations (DNS) requires efficient numerical methods, advanced boundary conditions, and consistent initialization in order to capture the wide range of scales and vortex dynamics present in the system, while reducing the computational impact associated with acoustic wave generation and the subsequent interaction with the flow. An advanced computational framework is presented that handles the challenges introduced by considering the compressive nature of RTI systems, which include sharp interfacial density gradients on strongly stratified background states, acoustic wave generation and removal at computational boundaries, and stratification dependent vorticity production. The foundation of the numerical methodology described here is the wavelet-based grid adaptivity of the Parallel Adaptive Wavelet Collocation Method (PAWCM) that maintains symmetry in single-mode RTI systems to extreme late-times. PAWCM is combined with a consistent initialization, which reduces the generation of acoustic disturbances, and effective boundary treatments, which prevent acoustic reflections. A dynamic time integration scheme that can handle highly nonlinear and potentially stiff systems, such as compressible RTI, completes the computational framework. The numerical methodology is used to simulate two-dimensional single-mode RTI to extreme late-times for a wide range of flow compressibility and variable density effects. The results show that flow compressibility acts to reduce the growth of RTI for low Atwood numbers, as predicted from linear stability analysis.
Numerical simulation of nonlinear development of instability waves
NASA Technical Reports Server (NTRS)
Mankbadi, Reda R.
1989-01-01
The nonlinear interactions of high amplitude instability waves in turbulent jets are described. In plane shear layers Riley and Metcalf (1980) and Monkewitz (1987) have shown that these interactions are dependent, among other parameters, on the phase-difference between the two instability waves. Therefore, here researchers consider the nonlinear development of both the amplitudes and the phase of the instability waves. The development of these waves are also coupled with the development of the mean flow and the background turbulence. In formulating this model it is assumed that each of the flow components can be characterized by conservation equations supplemented by closure models. Results for the interactions between the two instability waves under high-amplitude forcing at fundamental and subharmonic frequencies are presented here. Qualitative agreements are found between the present predictions and available experimental data.
Lessons Learned from Numerical Simulations of Interfacial Instabilities
NASA Astrophysics Data System (ADS)
Cook, Andrew
2015-11-01
Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM) and Kelvin-Helmholtz (KH) instabilities serve as efficient mixing mechanisms in a wide variety of flows, from supernovae to jet engines. Over the past decade, we have used the Miranda code to temporally integrate the multi-component Navier-Stokes equations at spatial resolutions up to 29 billion grid points. The code employs 10th-order compact schemes for spatial derivatives, combined with 4th-order Runge-Kutta time advancement. Some of our major findings are as follows: The rate of growth of a mixing layer is equivalent to the net mass flux through the equi-molar plane. RT growth rates can be significantly reduced by adding shear. RT instability can produce shock waves. The growth rate of RM instability can be predicted from known interfacial perturbations. RM vortex projectiles can far outrun the mixing region. Thermal fluctuations in molecular dynamics simulations can seed instabilities along the braids in KH instability. And finally, enthalpy diffusion is essential in preserving the second law of thermodynamics. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
On the Numerical Dispersion of Electromagnetic Particle-In-Cell Code : Finite Grid Instability
Meyers, Michael David; Huang, Chengkun; Zeng, Yong; Yi, Sunghwan; Albright, Brian James
2014-07-15
The Particle-In-Cell (PIC) method is widely used in relativistic particle beam and laser plasma modeling. However, the PIC method exhibits numerical instabilities that can render unphysical simulation results or even destroy the simulation. For electromagnetic relativistic beam and plasma modeling, the most relevant numerical instabilities are the finite grid instability and the numerical Cherenkov instability. We review the numerical dispersion relation of the electromagnetic PIC algorithm to analyze the origin of these instabilities. We rigorously derive the faithful 3D numerical dispersion of the PIC algorithm, and then specialize to the Yee FDTD scheme. In particular, we account for the manner in which the PIC algorithm updates and samples the fields and distribution function. Temporal and spatial phase factors from solving Maxwell's equations on the Yee grid with the leapfrog scheme are also explicitly accounted for. Numerical solutions to the electrostatic-like modes in the 1D dispersion relation for a cold drifting plasma are obtained for parameters of interest. In the succeeding analysis, we investigate how the finite grid instability arises from the interaction of the numerical 1D modes admitted in the system and their aliases. The most significant interaction is due critically to the correct representation of the operators in the dispersion relation. We obtain a simple analytic expression for the peak growth rate due to this interaction.
Numerical Modeling of Extensional Necking Instabilities: Application to Ganymede's Grooved Terrain
NASA Technical Reports Server (NTRS)
Bland, M. T.; Showman, A. P.
2005-01-01
Ganymede s pervasive 5-10 km-wavelength grooves have been suggested to result from a necking instability during an epoch of lithospheric extension, but to date few quantitative studies of groove formation have been performed. We present two-dimensional numerical models of necking instabilities under conditions that are appropriate to Ganymede at the time of groove formation. Preliminary simulations indicate that extensional necking instabilities can occur under a range of conditions, many of which may be relevant to Ganymede. The form of the surface topography produced by these instabilities varies as a function of the strain rate, amount of extension, initial topographic perturbation, and rheological parameters.
Numerical study of superradiant instability for charged stringy black hole-mirror system
NASA Astrophysics Data System (ADS)
Li, Ran; Zhao, Junkun
2015-01-01
We numerically study the superradiant instability of charged massless scalar field in the background of charged stringy black hole with mirror-like boundary condition. We compare the numerical result with the previous analytical result and show the dependencies of this instability upon various values of black hole charge Q, scalar field charge q, and mirror radius rm. Especially, we have observed that imaginary part of BQN frequencies grows with the scalar field charge q rapidly.
NASA Astrophysics Data System (ADS)
Dordevic, Mladen; Georgen, Jennifer
2016-03-01
Mantle plumes rising in the vicinity of mid-ocean ridges often generate anomalies in melt production and seafloor depth. This study investigates the dynamical interactions between a mantle plume and a ridge-ridge-ridge triple junction, using a parameter space approach and a suite of steady state, three-dimensional finite element numerical models. The top domain boundary is composed of three diverging plates, with each assigned half-spreading rates with respect to a fixed triple junction point. The bottom boundary is kept at a constant temperature of 1350°C except where a two-dimensional, Gaussian-shaped thermal anomaly simulating a plume is imposed. Models vary plume diameter, plume location, the viscosity contrast between plume and ambient mantle material, and the use of dehydration rheology in calculating viscosity. Importantly, the model results quantify how plume-related anomalies in mantle temperature pattern, seafloor depth, and crustal thickness depend on the specific set of parameters. To provide an example, one way of assessing the effect of conduit position is to calculate normalized area, defined to be the spatial dispersion of a given plume at specific depth (here selected to be 50 km) divided by the area occupied by the same plume when it is located under the triple junction. For one particular case modeled where the plume is centered in an intraplate position 100 km from the triple junction, normalized area is just 55%. Overall, these models provide a framework for better understanding plateau formation at triple junctions in the natural setting and a tool for constraining subsurface geodynamical processes and plume properties.
Inertial Instability and Rossby Wave Breaking in a Numerical Model.
NASA Astrophysics Data System (ADS)
O'Sullivan, Donal J.; Hitchman, Matthew H.
1992-06-01
A mechanistic model of the middle atmosphere is used to study the interaction between Rossby waves forced at the extratropical tropopause and inertial instability in the equatorial lower mesosphere. The impact of cross-equatorial shear strength and Rossby wave forcing amplitude is explored. Model results support the hypothesis, based on satellite temperature observations, that Rossby waves organize regions of equatorial inertial instability into coherent large-scale circulations. Horizontal convergence and divergence maxima are found stacked over the boundaries of regions of anomalous potential vorticity (PV). This supports observational diagnoses and theoretical expectations that parcel inertial accelerations arise in regions of anomalous PV, with divergence and convergence occurring at the boundaries. Although cross-equatorial shear determines the initial volume of inertially unstable air, Rossby waves arriving from the winter hemisphere deform PV contours such that zonally confined regions of anomalous PV extend well into the winter hemisphere and somewhat into the summer hemisphere. Significant parcel inertial accelerations are diagnosed to occur in anomalous PV tongues to 30° latitude in the winter hemisphere. Using a smaller cross-equatorial shear delays the penetration of anomalous PV into the winter hemisphere, while increasing the Rossby wave forcing amplitude causes a more rapid evolution of the low-latitude flow.These results suggest that inertial instability is intimately involved in Rossby wave breaking in the subtropical winter mesosphere. The horizontal scales of inertially unstable regions coevolve with those of PV anomalies, and so affect the breaking process from inception. The vertical scale of inertial circulations is much smaller than that of Rossby waves; hence, Rossby wave PV anomalies are eroded by vertical mixing as well as horizontal mixing. Gravity waves radiate away from the inertially unstable regions, which effectively convert energy
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.
NASA Astrophysics Data System (ADS)
Badjin, D. A.; Glazyrin, S. I.; Manukovskiy, K. V.; Blinnikov, S. I.
2016-06-01
We describe our modelling of the radiatively cooling shocks and their thin shells with various numerical tools in different physical and calculational setups. We inspect structure of the dense shell, its formation and evolution, pointing out physical and numerical factors that sustain its shape and also may lead to instabilities. We have found that under certain physical conditions, the circular shaped shells show a strong bending instability and successive fragmentation on Cartesian grids soon after their formation, while remain almost unperturbed when simulated on polar meshes. We explain this by physical Rayleigh-Taylor-like instabilities triggered by corrugation of the dense shell surfaces by numerical noise. Conditions for these instabilities follow from both the shell structure itself and from episodes of transient acceleration during re-establishing of dynamical pressure balance after sudden radiative cooling onset. They are also easily excited by physical perturbations of the ambient medium. The widely mentioned non-linear thin shell instability, in contrast, in tests with physical perturbations is shown to have only limited chances to develop in real radiative shocks, as it seems to require a special spatial arrangement of fluctuations to be excited efficiently. The described phenomena also set new requirements on further simulations of the radiatively cooling shocks in order to be physically correct and free of numerical artefacts.
Numerical simulations of evaporative instabilities in sessile drops of ethanol on heated substrates
NASA Astrophysics Data System (ADS)
Semenov, Sergey; Carle, Florian; Medale, Marc; Brutin, David
2015-11-01
The work is focussed on numerical simulations of thermo-convective instabilities in evaporating pinned sessile droplets of ethanol on heated substrates. Computed evaporation rate of a droplet is validated against parabolic flight experiments and semi-empirical theory presented here. To the best authors' knowledge, this is the first study which combines theoretical, experimental and computational approaches in convective evaporation of sessile droplets. The influence of gravity level on evaporation rate and contributions of different mechanisms of vapor transport (diffusion, Stefan flow, natural convection) are shown. The qualitative difference (in terms of developing thermo-convective instabilities) between steady-state and unsteady numerical approaches is demonstrated.
On the numerical dispersion of electromagnetic particle-in-cell code: Finite grid instability
NASA Astrophysics Data System (ADS)
Meyers, M. D.; Huang, C.-K.; Zeng, Y.; Yi, S. A.; Albright, B. J.
2015-09-01
The Particle-In-Cell (PIC) method is widely used in relativistic particle beam and laser plasma modeling. However, the PIC method exhibits numerical instabilities that can render unphysical simulation results or even destroy the simulation. For electromagnetic relativistic beam and plasma modeling, the most relevant numerical instabilities are the finite grid instability and the numerical Cherenkov instability. We review the numerical dispersion relation of the Electromagnetic PIC model. We rigorously derive the faithful 3-D numerical dispersion relation of the PIC model, for a simple, direct current deposition scheme, which does not conserve electric charge exactly. We then specialize to the Yee FDTD scheme. In particular, we clarify the presence of alias modes in an eigenmode analysis of the PIC model, which combines both discrete and continuous variables. The manner in which the PIC model updates and samples the fields and distribution function, together with the temporal and spatial phase factors from solving Maxwell's equations on the Yee grid with the leapfrog scheme, is explicitly accounted for. Numerical solutions to the electrostatic-like modes in the 1-D dispersion relation for a cold drifting plasma are obtained for parameters of interest. In the succeeding analysis, we investigate how the finite grid instability arises from the interaction of the numerical modes admitted in the system and their aliases. The most significant interaction is due critically to the correct representation of the operators in the dispersion relation. We obtain a simple analytic expression for the peak growth rate due to this interaction, which is then verified by simulation. We demonstrate that our analysis is readily extendable to charge conserving models.
On the numerical dispersion of electromagnetic particle-in-cell code: Finite grid instability
Meyers, M.D.; Huang, C.-K.; Zeng, Y.; Yi, S.A.; Albright, B.J.
2015-09-15
The Particle-In-Cell (PIC) method is widely used in relativistic particle beam and laser plasma modeling. However, the PIC method exhibits numerical instabilities that can render unphysical simulation results or even destroy the simulation. For electromagnetic relativistic beam and plasma modeling, the most relevant numerical instabilities are the finite grid instability and the numerical Cherenkov instability. We review the numerical dispersion relation of the Electromagnetic PIC model. We rigorously derive the faithful 3-D numerical dispersion relation of the PIC model, for a simple, direct current deposition scheme, which does not conserve electric charge exactly. We then specialize to the Yee FDTD scheme. In particular, we clarify the presence of alias modes in an eigenmode analysis of the PIC model, which combines both discrete and continuous variables. The manner in which the PIC model updates and samples the fields and distribution function, together with the temporal and spatial phase factors from solving Maxwell's equations on the Yee grid with the leapfrog scheme, is explicitly accounted for. Numerical solutions to the electrostatic-like modes in the 1-D dispersion relation for a cold drifting plasma are obtained for parameters of interest. In the succeeding analysis, we investigate how the finite grid instability arises from the interaction of the numerical modes admitted in the system and their aliases. The most significant interaction is due critically to the correct representation of the operators in the dispersion relation. We obtain a simple analytic expression for the peak growth rate due to this interaction, which is then verified by simulation. We demonstrate that our analysis is readily extendable to charge conserving models.
Numerical analysis of electrostatic ion cyclotron instability in an electron-positron-ion plasma
Khorashadizadeh, S. M. Barati Moqadam Niyat, M.; Niknam, A. R.
2016-06-15
This paper presents a theoretical study of the effects of positron density on the electrostatic ion cyclotron instability in an electron-positron-ion plasma using the kinetic theory approach. It is supposed that positrons and electrons can drift parallel to the magnetic field either in the same or the opposite directions. The dispersion relation for the electrostatic ion cyclotron waves in an electron-positron-ion plasma is derived, and the numerical results are investigated. It is found that an increase in positron concentration increases the critical drift velocity for excitation of the instability in both configurations. It is also found that as the positron concentration increases the growth rate of instability decreases. In addition, it is shown that at low velocities the maximum instability growth rate for the unidirectional case is higher than the counter-streaming case; however, after a certain velocity, the maximum growth rate in the counter-streaming case dominates that of the unidirectional case.
A numerical study of Richtmyer{endash}Meshkov instability driven by cylindrical shocks
Zhang, Q.; Graham, M.J. |
1998-04-01
As an incident shock wave hits a material interface between two fluids of different densities, the interface becomes unstable. Small disturbances at the interface start to grow. This interfacial instability is known as a Richtmyer{endash}Meshkov (RM) instability. It plays an important role in the studies of inertial confinement fusion and supernova. The majority of studies of the RM instability were in plane geometry{emdash}namely, plane shocks in Cartesian coordinates. We present a systematic numerical study of the RM instability driven by cylindrical shocks for both the imploding and exploding cases. The imploding (exploding) case refers to a cylindrical shock colliding with the material interface from the outside in (inside out). The phenomenon of reshock caused by the waves reflected from the origin is also studied. A qualitative understanding of this system has been achieved. Detailed studies of the growth rate of the fingers at the unstable interface are presented. {copyright} {ital 1998 American Institute of Physics.}
Numerical analysis of Laser Driven Rayleigh-Tayor instability at short wavelength
NASA Astrophysics Data System (ADS)
Nagatomo, Hideo; Ohnishi, Naofumi; Mima, Kunioki; Nishihara, Katsunobu; Sawada, Keisuke; Takabe, Hideaki
2001-10-01
For the inertial confinement fusion, it is important to simulate and predict the hydrodynamic instabilities. An integrated implosion simulation code was developed in ILE Osaka for IFE and other application usage. This new 2-D implosion code is based on ALE algorithm extended from CIP method which is robust and less numerical dissipation. To validate the code, various simulations of implosion and planner target were performed. In this presentation, the analysis of Rayleigh-Taylor instability will be shown mainly. In recent experiment by GXII Laser, the growth of RT instability at the range of short wavelength of the perturbation below the 30μm were measured with precision. Numerical analysis of the same condition using the new code was performed. The detail result will be shown in this presentation.
Numerical study of self modulation instability of 1 nC electron bunch at ATF
NASA Astrophysics Data System (ADS)
Fang, Yun; Mori, Warren; Muggli, Patric
2012-12-01
The development of self-modulation instability (SMI) is investigated numerically for the 1 nC electron bunch available at Accelerator Test Facility (ATF) of Brookhaven National Laboratory (BNL). Possible experiment based on the simulation results is proposed. All the simulations are performed with the 2D-cylindrically symmetric particle-in-cell code.
Numerical study of self modulation instability of 1 nC electron bunch at ATF
Fang Yun; Mori, Warren; Muggli, Patric
2012-12-21
The development of self-modulation instability (SMI) is investigated numerically for the 1 nC electron bunch available at Accelerator Test Facility (ATF) of Brookhaven National Laboratory (BNL). Possible experiment based on the simulation results is proposed. All the simulations are performed with the 2D-cylindrically symmetric particle-in-cell code.
Mortality plateaus and directionality theory.
Demetrius, L
2001-10-07
Recent large scale studies of senescence in animals and humans have revealed mortality rates that levelled off at advanced ages. These empirical findings are now known to be inconsistent with evolutionary theories of senescence based on the Malthusian parameter as a measure of fitness. This article analyses the incidence of mortality plateaus in terms of directionality theory, a new class of models based on evolutionary entropy as a measure of fitness. We show that the intensity of selection, in the context of directionality theory, is a convex function of age, and we invoke this property to predict that in populations evolving under bounded growth constraints, evolutionarily stable mortality patterns will be described by rates which abate with age at extreme ages. The explanatory power of directionality theory, in contrast with the limitations of the Malthusian model, accords with the claim that evolutionary entropy, rather than the Malthusian parameter, constitutes the operationally valid measure of Darwinian fitness.
Sengupta, M.; Ganesh, R.
2015-07-15
Numerical experiments have been performed to investigate the linear and nonlinear dynamics, and energetics of the ion resonance instability in cylindrically confined nonneutral plasma. The instability is excited on a set of parametrically different unstable equilibria of a cylindrical nonneutral cloud, composed of electrons partially neutralized by a much heavier ion species of single ionization. A particle-in-cell code has been developed and employed to carry out these simulations. The results obtained from the initial exponential growth phase of the instability in these numerical experiments are in agreement with the linearised analytical model of the ion resonance instability. As the simulations delve much further in time beyond the exponential growth phase, very interesting nonlinear phenomena of the ion resonance instability are revealed, such as a process of simultaneous wave breaking of the excited poloidal mode on the ion cloud and pinching of the poloidal perturbations on the electron cloud. This simultaneous nonlinear dynamics of the two components is associated with an energy transfer process from the electrons to the ions. At later stages there is heating induced cross-field transport of the heavier ions and tearing across the pinches on the electron cloud followed by an inverse cascade of the torn sections.
Numerical study on Rayleigh-Taylor instabilities in the lightning return stroke
Chen, Qiang; Chen, Bin Shi, Lihua; Yi, Yun; Wang, Yangyang
2015-09-15
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 the 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.
Experimental and Numerical Investigations of Two Typical Richtmyer-Meshkov instabilities
NASA Astrophysics Data System (ADS)
Bai, Jingsong; Liu, Jinhong; Zou, Liyong; Wang, Tao
2011-06-01
Two typical Richtmyer-Meshkov instabilities are investigated by experiments and simulations. One is the instability with double perturbation interface in nonuniform flows, and the other is the shock-accelerated elliptic heavy gas cylinder instability. The two experiments are conducted in the LSD's horizontal shock tube with 5 m long, 5 ×5 cm2 square cross section and is numerical simulated by our LES code of MVFT. Good agreements have been obtained between simulations and experiment in which the visualizations of mixing interface is tracked by Schlieren photography and multiple dynamics images technology. The results illuminates that the initial nonuniform flow would have a significant effect on the RM instability, and the shape of cylinder also have a significant effect on the cylindrical RM instability. The model of shock-accelerating along the major axis has a stronger convergent effect than the one of shock-accelerating along the minor axis for elliptic gas cylinder instability. The works are supported by the National Science Foundation of China (Grant No. 11072228 and 11002129).
NASA Astrophysics Data System (ADS)
Miura, Hideaki; Zheng, Linjin; Horton, Wendell
2017-09-01
Numerical simulations of interchange/tearing instabilities in a 2D slab with a numerical model for edge plasma resistivity are carried out. Plasma outside the Last Closed Flux Surface (LCFS), or the scrape-off layer (SOL), can be characterized by open magnetic fields which terminate on the divertor plates in the outside. A numerical model with current diffusivity is developed to enforce a low-level saturated current profile in the SOL region as well as a current jump across the LCFS. The numerical simulations show that interchange modes can transform into tearing modes, as the current-interchange tearing modes which has been proposed by Zheng and Furukawa [Phys. Plasmas 17, 052508 (2010)]. An applicability of the model to the tokamak edge stability and ELM studies is discussed.
High order numerical simulations of the Richtmyer- Meshkov instability in a relativistic fluid
NASA Astrophysics Data System (ADS)
Zanotti, O.; Dumbser, M.
2015-07-01
We study the Richtmyer-Meshkov (RM) instability of a relativistic perfect fluid by means of high order numerical simulations with adaptive mesh refinement (AMR). The numerical scheme combines a finite volume reconstruction in space, a local space-time discontinuous Galerkin predictor method, a high order one-step time update scheme, and a "cell-by-cell" space-time AMR strategy with time-accurate local time stepping. In this way, third order accurate (both in space and in time) numerical simulations of the RM instability are performed, spanning a wide parameter space. We present results both for the case in which a light fluid penetrates into a higher density one (Atwood number A > 0) and for the case in which a heavy fluid penetrates into a lower density one (Atwood number A < 0). We find that for large Lorentz factors γs of the incident shock wave, the relativistic RM instability is substantially weakened and ultimately suppressed. More specifically, the growth rate of the RM instability in the linear phase has a local maximum which occurs at a critical value of γs ≈ [1.2, 2]. Moreover, we have also revealed a genuinely relativistic effect, absent in Newtonian hydrodynamics, which arises in three dimensional configurations with a non-zero velocity component tangent to the incident shock front. In particular, in A > 0 models, the tangential velocity has a net magnification effect, while in A < 0 models, the tangential velocity has a net suppression effect.
Numerical investigation of the influence of gravity on the Rayleigh-Plateau jet instability
NASA Astrophysics Data System (ADS)
Rosello, M.; Maîtrejean, G.; Roux, D. C. D.; Jay, P.
2016-12-01
Controlled jet breakup processes are commonly used in many fields of industrial applications, such as inkjet printing, spray painting, and fuel injection. Most of these applications rely on the accurate prediction of the size of droplets resulting from breakup. While many parameters influence the droplets’ size, the present study focuses on the influence of the Bond number. This dimensionless number measures the influence of body forces compared with surface tension, which usually drives the Rayleigh-Plateau instability. The present work uses numerical simulation to assess the influence of the Bond number on both main droplets and satellites issuing from the Rayleigh-Plateau instability jet.
Experimental, Numerical and Analytical Studies of the MHD-driven plasma jet, instabilities and waves
NASA Astrophysics Data System (ADS)
Zhai, Xiang
This thesis describes a series of experimental, numerical, and analytical studies involving the Caltech magnetohydrodynamically (MHD)-driven plasma jet experiment. The plasma jet is created via a capacitor discharge that powers a magnetized coaxial planar electrodes system. The jet is collimated and accelerated by the MHD forces. We present three-dimensional ideal MHD finite-volume simulations of the plasma jet experiment using an astrophysical magnetic tower as the baseline model. A compact magnetic energy/helicity injection is exploited in the simulation analogous to both the experiment and to astrophysical situations. Detailed analysis provides a comprehensive description of the interplay of magnetic force, pressure, and flow effects. We delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. When the experimental jet is sufficiently long, it undergoes a global kink instability and then a secondary local Rayleigh-Taylor instability caused by lateral acceleration of the kink instability. We present an MHD theory of the Rayleigh-Taylor instability on the cylindrical surface of a plasma flux rope in the presence of a lateral external gravity. The Rayleigh-Taylor instability is found to couple to the classic current-driven instability, resulting in a new type of hybrid instability. The coupled instability, produced by combination of helical magnetic field, curvature of the cylindrical geometry, and lateral gravity, is fundamentally different from the classic magnetic Rayleigh-Taylor instability occurring at a two-dimensional planar interface. In the experiment, this instability cascade from macro-scale to micro-scale eventually leads to the failure of MHD. When the Rayleigh-Taylor instability becomes nonlinear, it compresses and pinches the plasma jet to a scale smaller than the ion skin depth and triggers a fast magnetic reconnection. We built a specially designed high-speed 3D magnetic probe and
A semi-numerical algorithm for instability of compressible multilayered structures
NASA Astrophysics Data System (ADS)
Tang, Shan; Yang, Yang; Peng, Xiang He; Liu, Wing Kam; Huang, Xiao Xu; Elkhodary, Khalil
2015-07-01
A computational method is proposed for the analysis and prediction of instability (wrinkling or necking) of multilayered compressible plates and sheets made by metals or polymers under plane strain conditions. In previous works, a basic assumption (or a physical argument) that has been frequently made is that materials are incompressible to simplify mathematical derivations. To account for the compressibility of metals and polymers (the lower Poisson's ratio leads to the more compressible material), we propose a combined semi-numerical algorithm and finite element method for instability analysis. Our proposed algorithm is herein verified by comparing its predictions with published results in literature for thin films with polymer/metal substrates and for polymer/metal systems. The new combined method is then used to predict the effects of compressibility on instability behaviors. Results suggest potential utility for compressibility in the design of multilayered structures.
Development of a computational testbed for numerical simulation of combustion instability
NASA Technical Reports Server (NTRS)
Grenda, Jeffrey; Venkateswaran, Sankaran; Merkle, Charles L.
1993-01-01
A synergistic hierarchy of analytical and computational fluid dynamic techniques is used to analyze three-dimensional combustion instabilities in liquid rocket engines. A mixed finite difference/spectral procedure is employed to study the effects of a distributed vaporization zone on standing and spinning instability modes within the chamber. Droplet atomization and vaporization are treated by a variety of classical models found in the literature. A multi-zone, linearized analytical solution is used to validate the accuracy of the numerical simulations at small amplitudes for a distributed vaporization region. This comparison indicates excellent amplitude and phase agreement under both stable and unstable operating conditions when amplitudes are small and proper grid resolution is used. As amplitudes get larger, expected nonlinearities are observed. The effect of liquid droplet temperature fluctuations was found to be of critical importance in driving the instabilities of the combustion chamber.
NASA Astrophysics Data System (ADS)
Stamper, M. A.; Taylor, J. R.; Adams, K.; Bachman, S.; Hosegood, P. J.; Sallee, J. B.; Torres, R.
2016-02-01
Previous work has suggested that submesoscales play an important role in setting the stratification of the upper ocean; such scales are believed to be associated with large vertical velocities and therefore capable of modifying biological productivity. In this study, the feedbacks between submesoscales and small-scale turbulence are explored using very high resolution numerical simulations. Modelling an idealised domain representative of the upper mixed layer, we find submesoscale motions generated via symmetric and baroclinic instability. These, in turn, via the emergence of secondary instabilities, are able to generate significant small-scale turbulence and mixing even in the absence of surface wind and buoyancy forcing. Using comparison with lower resolution simulations in the same domain we are able to quantify the extent to which fully resolving these submesoscale features and the resulting small scale turbulence impacts on instability growth rates, mixing and dissipation, restratification of the mixed layer and vertical fluxes.
NASA Technical Reports Server (NTRS)
1997-01-01
These two views of Io were acquired by NASA's Galileo spacecraft during its seventh orbit (G7) of Jupiter. The images were designed to view large features on Io at low sun angles when the lighting conditions emphasize the topography or relief of the volcanic satellite. Sun angles are low near the terminator which is the day-night boundary near the left side of the images. These images reveal that the topography is very flat near the active volcanic centers such as Loki Patera (the large dark horseshoe-shaped feature near the terminator in the left-hand image) and that a variety of mountains and plateaus exist elsewhere.
North is to the top of the picture. The resolution is about 6 kilometers per picture element (6.1 for the left hand image and 5.7 for the right). The images were taken on April 4th, 1997 at a ranges of 600,000 kilometers (left image) and 563,000 kilometers (right image) by the solid state imaging (CCD) system on NASA's Galileo spacecraft.
The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC.
This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov. Background information and educational context for the images can be found at URL http://www.jpl.nasa.gov/galileo/sepo
NASA Technical Reports Server (NTRS)
1997-01-01
These two views of Io were acquired by NASA's Galileo spacecraft during its seventh orbit (G7) of Jupiter. The images were designed to view large features on Io at low sun angles when the lighting conditions emphasize the topography or relief of the volcanic satellite. Sun angles are low near the terminator which is the day-night boundary near the left side of the images. These images reveal that the topography is very flat near the active volcanic centers such as Loki Patera (the large dark horseshoe-shaped feature near the terminator in the left-hand image) and that a variety of mountains and plateaus exist elsewhere.
North is to the top of the picture. The resolution is about 6 kilometers per picture element (6.1 for the left hand image and 5.7 for the right). The images were taken on April 4th, 1997 at a ranges of 600,000 kilometers (left image) and 563,000 kilometers (right image) by the solid state imaging (CCD) system on NASA's Galileo spacecraft.
The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC.
This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov. Background information and educational context for the images can be found at URL http://www.jpl.nasa.gov/galileo/sepo
Experimental demonstration of bow-shock instability and its numerical analysis
NASA Astrophysics Data System (ADS)
Kikuchi, Y.; Ohnishi, N.; Ohtani, K.
2017-05-01
An experimental demonstration was carried out in a ballistic range at high Mach numbers with the low specific heat ratio gas hydrofluorocarbon HFC-134a to observe the unstable bow-shock wave generated in front of supersonic blunt objects. The shadowgraph images obtained from the experiments showed instability characteristics, in which the disturbances grow and flow downstream and the wake flow appears wavy because of the shock oscillation. Moreover, the influence of the body shape and specific heat ratio on the instability was investigated for various experimental conditions. Furthermore, the observed features, such as wave structure and disturbance amplitude, were captured by numerical simulations, and it was demonstrated that computational fluid dynamics could effectively simulate the physical instability. In addition, it was deduced that the shock instability is induced by sound emissions from the edge of the object. This inference supports the dependence of the instability on the specific heat ratio and Mach number because the shock stand-off distance is affected by these parameters and limits the sound wave propagation.
Experimental demonstration of bow-shock instability and its numerical analysis
NASA Astrophysics Data System (ADS)
Kikuchi, Y.; Ohnishi, N.; Ohtani, K.
2016-07-01
An experimental demonstration was carried out in a ballistic range at high Mach numbers with the low specific heat ratio gas hydrofluorocarbon HFC-134a to observe the unstable bow-shock wave generated in front of supersonic blunt objects. The shadowgraph images obtained from the experiments showed instability characteristics, in which the disturbances grow and flow downstream and the wake flow appears wavy because of the shock oscillation. Moreover, the influence of the body shape and specific heat ratio on the instability was investigated for various experimental conditions. Furthermore, the observed features, such as wave structure and disturbance amplitude, were captured by numerical simulations, and it was demonstrated that computational fluid dynamics could effectively simulate the physical instability. In addition, it was deduced that the shock instability is induced by sound emissions from the edge of the object. This inference supports the dependence of the instability on the specific heat ratio and Mach number because the shock stand-off distance is affected by these parameters and limits the sound wave propagation.
NASA Astrophysics Data System (ADS)
Brunetti, J.; Massi, F.; Saulot, A.; Renouf, M.; D`Ambrogio, W.
2015-06-01
Mechanical systems present several contact surfaces between deformable bodies. The contact interface can be either static (joints) or in sliding (active interfaces). The sliding interfaces can have several roles and according to their application they can be developed either for maximizing the friction coefficient and the energy dissipation (e.g. brakes) or rather to allow the relative displacement at joints with a maximum efficiency. In both cases the coupling between system and local contact dynamics can bring to system dynamics instabilities (e.g. brake squeal or squeaking of hip prostheses). This results in unstable vibrations of the system, induced by the oscillation of the contact forces. In the literature, a large number of works deal with such kind of instabilities and are mainly focused on applied problems such as brake squeal noise. This paper shows a more general numerical analysis of a simple system constituted by two bodies in sliding contact: a rigid cylinder rotating inside a deformable one. The parametrical Complex Eigenvalue Analysis and the transient numerical simulations show how the friction forces can give rise to in-plane dynamic instabilities due to the interaction between two system modes, even for such a simple system characterized by one deformable body. Results from transient simulations highlight the key role of realistic values of the material damping to have convergence of the model and, consequently, reliable physical results. To this aim an experimental estimation of the material damping has been carried out. Moreover, the simplicity of the system allows for a deeper analysis of the contact instability and a balance of the energy flux among friction, system vibrations and damping. The numerical results have been validated by comparison with experimental ones, obtained by a specific test bench developed to reproduce and analyze the contact friction instabilities.
Numerical Instability in a 2D Gyrokinetic Code Caused by Divergent E × B Flow
NASA Astrophysics Data System (ADS)
Byers, J. A.; Dimits, A. M.; Matsuda, Y.; Langdon, A. B.
1994-12-01
In this paper, a numerical instability first observed in a 2D electrostatic gyrokinetic code is described. The instability should also be present in some form in many versons of particle-in-cell simulation codes that employ guiding center drifts. A perturbation analysis of the instability is given and its results agree quantitatively with the observations from the gyrokinetic code in all respects. The basic mechanism is a false divergence of the E × B flow caused by the interpolation between the grid and the particles as coupled with the specific numerical method for calculating E - ∇φ. Stability or instability depends in detail on the specific choice of particle interpolation method and field method. One common interpolation method, subtracted dipole, is stable. Other commonly used interpolation methods, linear and quadratic, are unstable when combined with a finite difference for the electric field. Linear and quadratic interpolation can be rendered stable if combined with another method for the electric field, the analytic differential of the interpolated potential.
Underlying mechanism of numerical instability in large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Ida, Masato; Taniguchi, Nobuyuki
2004-04-01
This paper extends our recent theoretical work concerning the feasibility of stable and accurate computation of turbulence using a large eddy simulation [
NASA Technical Reports Server (NTRS)
Baum, J. D.; Levine, J. N.
1980-01-01
The selection of a satisfactory numerical method for calculating the propagation of steep fronted shock life waveforms in a solid rocket motor combustion chamber is discussed. A number of different numerical schemes were evaluated by comparing the results obtained for three problems: the shock tube problems; the linear wave equation, and nonlinear wave propagation in a closed tube. The most promising method--a combination of the Lax-Wendroff, Hybrid and Artificial Compression techniques, was incorporated into an existing nonlinear instability program. The capability of the modified program to treat steep fronted wave instabilities in low smoke tactical motors was verified by solving a number of motor test cases with disturbance amplitudes as high as 80% of the mean pressure.
Numerical simulations of magnetic Kelvin-Helmholtz instability at a twisted solar flux tube
NASA Astrophysics Data System (ADS)
Murawski, K.; Chmielewski, P.; Zaqarashvili, T. V.; Khomenko, E.
2016-07-01
The paper aims to study the response of a solar small-scale and weak magnetic flux tube to photospheric twisting motions. We numerically solve three-dimensional ideal magnetohydrodynamic equations to describe the evolution of the perturbation within the initially static flux tube, excited by twists in the azimuthal component of the velocity. These twists produce rotation of the magnetic field lines. Perturbation of magnetic field lines propagates upwardly, driving vertical and azimuthal flow as well as plasma compressions and rarefactions in the form of eddies. We conclude that these eddies result from the sheared azimuthal flow which seeds Kelvin-Helmholtz instability (KHI) between the flux tube and the ambient medium. Numerically obtained properties of the KHI confirm the analytical predictions for the occurrence of the instability.
NASA Astrophysics Data System (ADS)
Mahdaoui, O.; Agassant, J.-F.; Laure, P.; Valette, R.; Silva, L.
2007-04-01
The polymer coextrusion process is a new method of sheet metal lining. It allows to substitute lacquers for steel protection in food packaging industry. The coextrusion process may exhibit flow instabilities at the interface between the two polymer layers. The objective of this study is to check the influence of processing and rheology parameters on the instabilities. Finite elements numerical simulations of the coextrusion allow to investigate various stable and instable flow configurations.
Bai, Jing-Song; Wang, Bing; Wang, Tao; Liu, Kun
2012-12-01
Based on previous instability experiments of the double mode perturbed interface in initially nonuniform flows, we numerically investigate the effect of the nonuniformity of flows on the evolution of instability in a nonlinear regime after reshock by adopting two different nonuniform coefficients (δ_{1} = 0.6162 and δ_{2} = 0.4961) in the Gaussian distribution of the initial nonuniform density. We obtain the evolution of the mixing zone width and vortex structure of the air-SF_{6} interface and compare the circulation discrepancies of the nonuniform and uniform flows before and after reshock. These results indicate that the nonuniformity of the initial flow has great effect on the evolution of instability in the linear regime and the weak nonlinear regime prior to reshock. However, the mixing layer has little dependence on the nonuniformity of the initial flow in the nonlinear regime after reshock; namely, the effect of the nonuniformity is reduced significantly as the instability enters the strongly nonlinear regime after reshock. Although the growth rate of the perturbations has a significant increase, the characteristics of the flow like the mixing width, vorticity, and circulation are close to those of a uniform flow.
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.
NASA Astrophysics Data System (ADS)
Startsev, Edward A.; Davidson, Ronald C.; Qin, Hong
2005-05-01
In neutral plasmas with a uniform magnetic field and strongly anisotropic distribution function (T∥/T⊥≪1) an electrostatic Harris-type collective instability may develop if the plasma is sufficiently dense. Such anisotropies develop naturally in accelerators, and a similar instability may lead to a deterioration of the beam quality in a one-component nonneutral charged particle beam. The instability may also lead to an increase in the longitudinal velocity spread, which would make the focusing of the beam difficult and impose a limit on the minimum spot size achievable in heavy ion fusion experiments. This paper reports the results of recent numerical studies of the temperature anisotropy instability using the newly developed Beam Eigenmodes And Spectra (bEASt) code for space-charge-dominated, low-emittance beams with large tune depression (ν/ν0≪1). Such high-intensity beams are relevant to next-step experiments such as the Integrated Beam Experiment (IBX), which would serve as proof-of-principal experiment for heavy-ion fusion.
Direct numerical simulations of type Ia supernovae flames I: The landau-darrieus instability
Bell, J.B.; Day, M.S.; Rendleman, C.A.; Woosley, S.E.; Zingale, M.
2003-11-24
Planar flames are intrinsically unstable in open domains due to the thermal expansion across the burning front--the Landau-Darrieus instability. This instability leads to wrinkling and growth of the flame surface, and corresponding acceleration of the flame, until it is stabilized by cusp formation. We look at the Landau-Darrieus in stability for C/O thermonuclear flames at conditions relevant to the late stages of a Type Ia supernova explosion. Two-dimensional direct numerical simulations of both single-mode and multi-mode perturbations using a low Mach number hydrodynamics code are presented. We show the effect of the instability on the flame speed as a function of both the density and domain size, demonstrate the existence of the small scale cutoff to the growth of the instability, and look for the proposed breakdown of the non-linear stabilization at low densities. The effects of curvature on the flame as quantified through measurements of the growth rate and computation of the corresponding Markstein number. While accelerations of a few percent are observed, they are too small to have any direct outcome on the supernova explosion.
SRTM Anaglyph: Lava plateaus, Argentina
NASA Technical Reports Server (NTRS)
2001-01-01
All of the major landforms relate to volcanism and/or erosion in this Shuttle Radar Topography Mission scene of Patagonia, near La Esperanza, Argentina. The two prominent plateaus once formed a continuous surface that extended over much of this region. Younger volcanoes have grown through and atop the plateau, and one just south of this scene has sent a long, narrow flow down a stream channel (lower left). The topographic pattern shows that streams dominate the erosion processes in this arid environment even though wind is known to move substantial amounts of sediment here.
This anaglyph was produced by first shading a preliminary SRTM elevation model. The stereoscopic effect was then created by generating two differing perspectives, one for each eye. When viewed through special glasses, the result is a vertically exaggerated view of the Earth's surface in its full three dimensions. Anaglyph glasses cover the left eye with a red filter and cover the right eye with a blue filter.
Elevation data used in this image was acquired by the Shuttle Radar Topography Mission aboard the Space Shuttle Endeavour, launched on February 11, 2000. SRTM used the same radar instrument that comprised the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) that flew twice on the Space Shuttle Endeavour in 1994. SRTM was designed to collect three-dimensional measurements of the Earth's surface. To collect the 3-D data, engineers added a 60-meter-long (200-foot) mast, installed additional C-band and X-band antennas, and improved tracking and navigation devices. The mission is a cooperative project between NASA, the National Imagery and Mapping Agency (NIMA) of the U.S. Department of Defense, and the German and Italian space agencies. It is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Earth Science Enterprise, Washington, DC.
Size: 62.4 by 88.8 kilometers (38.7 by 55.1 miles) Location: 40.0 deg. South lat., 68.6 deg. West lon
NASA Astrophysics Data System (ADS)
Xie, Luo; Yang, Li-jun; Ye, Han-yu
2017-07-01
The instability of gas-surrounded Rayleigh viscous jets is investigated analytically and numerically in this paper. Theoretical analysis is based on a second-order perturbation expansion for capillary jets with surface disturbances, while the axisymmetric two-dimensional, two-phase simulation is conducted by applying the Gerris code for jets subjected to velocity disturbances. The relation between the initial surface and velocity disturbance amplitude was obtained according to the derivation of Moallemi et al. ["Breakup of capillary jets with different disturbances," Phys. Fluids 28, 012101 (2016)], and the breakup lengths resulting from these two disturbances agree well. Analytical and numerical breakup profiles also coincide satisfactorily, except in the vicinity of the breakup point, which shrinks forcefully. The effects of various parameters (i.e., oscillation frequency, Reynolds number, Weber number, and gas-to-liquid density ratio) have also been examined by comparing spatial growth rate, second-order disturbance amplitude, breakup length, and the breakup profiles at low frequency, where obvious satellite droplets form, versus different parameters. In addition, the competition between Rayleigh instability and Kelvin-Helmholtz instability has been examined using an energy approach.
Nonlinear instability and chaos in plasma wave-wave interactions. II. Numerical methods and results
Kueny, C.S.; Morrison, P.J.
1995-05-01
In Part I of this work and Physics of Plasmas, June 1995, the behavior of linearly stable, integrable systems of waves in a simple plasma model was described using a Hamiltonian formulation. It was shown that explosive instability arises from nonlinear coupling between modes of positive and negative energy, with well-defined threshold amplitudes depending on the physical parameters. In this concluding paper, the nonintegrable case is treated numerically. Several sets of waves are considered, comprising systems of two and three degrees of freedom. The time evolution is modelled with an explicit symplectic integration algorithm derived using Lie algebraic methods. When initial wave amplitudes are large enough to support two-wave decay interactions, strongly chaotic motion destroys the separatrix bounding the stable region for explosive triplets. Phase space orbits then experience diffusive growth to amplitudes that are sufficient for explosive instability, thus effectively reducing the threshold amplitude. For initial amplitudes too small to drive decay instability, small perturbations might still grow to arbitrary size via Arnold diffusion. Numerical experiments do not show diffusion in this case, although the actual diffusion rate is probably underestimated due to the simplicity of the model.
NASA Technical Reports Server (NTRS)
Reale, F.; Rosner, R.; Malagoli, A.; Peres, G.; Serio, S.
1991-01-01
The temporal evolution of density perturbations in an initially hydrostatic isothermal atmosphere consisting of an optically thin radiating compressible plasma is studied. Numerical techniques are used to describe the nonlinear evolution of the perturbations, and the relative equilibrium between dynamic and thermal instabilities as governed by three independent control parameters are examined, namely, the initial density contrast of the perturbation, the ratio of the local buoyancy oscillation period to the local radiative cooling time, and the ratio of the perturbation radius to the local scaleheight. Four orders of magnitude of initial density contrasts and ratios of buoyancy and cooling times, and one order of magnitude of the bubble dimensions are explored. Well-defined oscillations were found to occur in a limited parameter range, and thermal instability to occur even within secondary condensations deriving from the bubble fragmentation.
Numerical simulations of thermal-chemical instabilities at the core-mantle boundary
NASA Technical Reports Server (NTRS)
Hansen, Ulrich; Yuen, David A.
1988-01-01
Numerical simulations of thermal-chemical instabilities in the D-double-prime layer at the core-mantle boundary are presented which show that strong lateral heterogeneities in the composition and density fields can be initiated and maintained dynamically if there is continuous replenishment of material from subduced slabs coming from the upper mantle. These chemical instabilities have a tendency to migrate laterally and may help to support core-mantle boundary topography with short and long wavelengths. The thermal-chemical flows produce a relatively stagnant D-double-prime layer with strong lateral and temporal variations in basal heat flux, which gives rise to thermal core-mantle interactions influencing the geodynamo.
Numerical approach to reproduce instabilities of partial cavitation in a Venturi 8° geometry
NASA Astrophysics Data System (ADS)
Charriere, Boris; Goncalves, Eric
2016-11-01
Unsteady partial cavitation is mainly formed by an attached cavity which present periodic oscillations. Under certain conditions, the instabilities are characterized by the formation of vapour clouds, convected downstream the cavity and which collapse in higher pressure region. In order to gain a better understanding of the complex physics involved, many experimental and numerical studies have been carried out. These identified two main mechanisms responsible for the break-off cycles. The development of a liquid re-entrant jet is the most common type of instabilities, but more recently, the role of pressure waves created by the cloud collapses has been highlighted. This paper presents a one-fluid compressible Reynolds- Averaged NavierStokes (RANS) solver closed by two different equations of state (EOS) for the mixture. Based on experimental data, we investigate the ability for our simulations to reproduce the instablities of a self-sustained oscillating cavitation pocket. Two cavitation models are firstly compared. The importance of considering a non-equilibrium state for the vapour phase is also exhibited. To finish, the role played by the added transport equation to compute void ratio is emphasised. In case of partially cavitating flows with detached cavitation clouds, the reproduction of convective mechanisms is clearly improved.
Numerical simulations of the two-dimensional multimode Richtmyer-Meshkov instability
Thornber, B.; Zhou, Y.
2015-03-15
The two-dimensional Richtmyer-Meshkov instability occurs as shock waves pass through a perturbed material interface, triggering transition to an inhomogeneous turbulence variable density flow. This paper presents a series of large-eddy-simulations of the two dimensional turbulent RM instability and compares the results to the fully three dimensional simulations. There are two aims for this paper, the first is to explore what numerical resolution is required for a statistically converged solution for a two dimensional inhomogeneous flow field. The second aim is to elucidate the key differences in flow physics between the two dimensional and three dimensional Richtmyer-Meshkov instabilities, particularly their asymptotic self-similar regime. Convergence is achieved using 64 independent realisations and grid resolutions up to 4096{sup 2} in the plane. It is shown that for narrowband cases the growth rate θ = 0.48 which is substantially higher than the three-dimensional equivalent. Mix measures are consistently lower compared to three-dimensional, and the kinetic energy distribution is homogeneous at late time. The broadband case has a similar initial growth rate as the three-dimensional case, with a marginally lower θ = 0.63. Mix is similar in magnitude, but is reducing at late time. The spectra in both cases exhibit the dual-cascade expected from two-dimensional turbulence.
Numerical Simulations Studies of the Convective Instability Onset in a Supercritical Fluid
NASA Technical Reports Server (NTRS)
Furukawa, A.; Meyer, H.; Onuki, A.
2004-01-01
Numerical simulation studies are reported for the convection of a supercritical fluid, He-3, in a Rayleigh-Benard cell. The calculations provide the temporal profile DeltaT(t) of the temperature drop across the fluid layer. In a previous article, systematic delays in the onset of the convective instability in simulations relative to experiments were reported, as seen from the DeltaT(t) profiles. They were attributed to the smallness of the noise which is needed to start the instability. Therefore i) homogeneous temperature noise and ii) spatial lateral periodic temperature variations in the top plate were programmed into the simulations, and DeltaT(t) compared with that of an experiment with the same fluid parameters. An effective speed-up in the instability onset was obtained, with the best results obtained through the spatial temperature variations with a period of 2L, close to the wavelength of a pair of convections rolls. For a small amplitude of 0.5 micro-K, this perturbation gave a semiquantitative agreement with experimental observations. Results for various noise amplitudes are presented and discussed in relation to predictions by El Khouri and Carl es.
Numerical Investigation of Stratus Cloud Layer Breakup by Cloud Top Instabilities
NASA Astrophysics Data System (ADS)
Witting, Patrick Joseph
1995-01-01
Large scale atmospheric simulations rely on simple boundary layer models because of computational considerations. Cloud coverage predicted by these boundary layer models is greatly affected by their prediction of the entrainment rate, the rate at which the boundary layer rises. Through direct numerical simulations, this dissertation studies the impact that two cloud top instability mechanisms, cloud top entrainment instability and stratified shear layer instability, have on the entrainment rate. Cloud top entrainment instability, CTEI, is possible when mixtures of dry air from above and cloudy air from below the inversion become negatively buoyant due to evaporative cooling. High resolution two-dimensional simulations with grid spacing as small as 0.25m were used to investigate early CTEI development from small disturbances. Simulations revealed CTEI prefers small wavelengths, has an extended period of exponential growth, and establishes a nearly constant rate of global cloud loss during early nonlinear development which continues into the completely nonlinear phases. The growth rates and cloud loss rates were calculated over a wide range of parameter space. Three-dimensional simulations produced more mixing than their two-dimensional counterparts. Simulations also revealed that shear inhibits CTEI development. Shear often accompanies the jumps of density and moisture across the inversion. If enough shear is present relative to the stable stratification, then the shear layer will roll up into periodic arrays of billows, entraining fluid in the process. These billows are unstable to a pairing mechanism which further spreads the layer by entraining fluid and forming new, larger billows. The pairing process leads to an essentially linear spreading rate until the spreading eventually stops because of increased buoyancy effects. Shear layer simulations address the pairing instability being eventually overwhelmed by buoyancy forces, ending further spreading. The
Numerical investigations on the characteristics of thermomagnetic instability in MgB2 bulks
NASA Astrophysics Data System (ADS)
Xia, Jing; Li, Maosheng; Zhou, Youhe
2017-07-01
This paper presents the characteristics of thermomagnetic instability in MgB2 bulks by numerically solving the macroscopic dynamics of thermomagnetic interaction governed by the coupled magnetic and heat diffusion equations in association with a modified E-J power-law relationship. The finite element method is used to discretize the system of partial differential equations. The calculated magnetization loops with flux jumps are consistent with the experimental results for MgB2 slabs bathed in a wide range of ambient temperatures. We reveal the evolution process of the thermomagnetic instability and present the distributions of the magnetic field, temperature, and current density before and after flux jumps. A 2D axisymmetric model is used to study the thermomagnetic instability in cylindrical MgB2 bulks. It is found that the number of flux jumps monotonously reduces as the ambient temperature rises and no flux jump appears when the ambient temperature exceeds a certain value. Moreover, the flux-jump phenomenon exists in a wide range of the ramp rate of the applied external field, i.e. 10-2-102 T s-1. Furthermore, the dependences of the first flux-jump field on the ambient temperature, ramp rate, and bulk thickness are investigated. The critical bulk thicknesses for stability are obtained for different ambient temperatures and sample radii. In addition, the influence of the capability of the interfacial heat transfer on the temporal response of the bulk temperature is discussed. We also find that the prediction of thermomagnetic instability is sensitive to the employment of the flux creep exponent in the simulations.
Numerical study of thermocapillary instabilities in evaporating annular pools and sessile droplets
NASA Astrophysics Data System (ADS)
Sáenz, Pedro J.; Valluri, Prashant; Sefiane, Khellil; Karapetsas, George; Matar, Omar K.
2011-11-01
We investigate thermocapillary flows due to temperature-induced surface tension gradients in annular liquid pools via full two-phase direct numerical simulations in 3D. Phase-change, interface deformation and wettability phenomena are taken into consideration by using a variant of the volume-of-fluid method. The simulation results are validated against experiments (Schwabe et al. 2003 & Riley et al. 1998) and theory (Smith & Davis 1983). The transient results show the evolution of the flow towards an oscillatory state characterized by interfacial hydrothermal waves (HTWs). We present the effects of non-uniform evaporation fluxes and the liquid depths on the linear and non-linear development of these thermocapillary instabilities. The influence on bulk flows, surface temperature patterns and interface deformations are also shown. We finally introduce spontaneously self-excited HTWs in evaporating sessile droplets simulated using novel numerical methods and compare the results against analytical models and experiments. Fundación Caja Madrid & EPSRC DTA.
NASA Astrophysics Data System (ADS)
Wei, Xing; Ji, Hantao; Goodman, Jeremy; Ebrahimi, Fatima; Gilson, Erik; Jenko, Frank; Lackner, Karl
2016-12-01
We investigate numerically the Princeton magnetorotational instability (MRI) experiment and the effect of conducting axial boundaries or endcaps. MRI is identified and found to reach a much higher saturation than for insulating endcaps. This is probably due to stronger driving of the base flow by the magnetically rather than viscously coupled boundaries. Although the computations are necessarily limited to lower Reynolds numbers (Re ) than their experimental counterparts, it appears that the saturation level becomes independent of Re when Re is sufficiently large, whereas it has been found previously to decrease roughly as Re-1 /4 with insulating endcaps. The much higher saturation levels will allow for the positive detection of MRI beyond its theoretical and numerical predictions.
NASA Astrophysics Data System (ADS)
Pham, Van Sang; Li, Zirui; Lim, Kian Meng; White, Jacob K.; Han, Jongyoon
2012-10-01
We present a systematic, multiscale, fully detailed numerical modeling for dynamics of fluid flow and ion transport covering Ohmic, limiting, and overlimiting current regimes in conductance of ion-selective membrane. By numerically solving the Poisson-Nernst-Planck-Navier-Stokes equations, it is demonstrated that the electroconvective instability, arising from the electric field acting upon the extended space charge layer, and the induced strong vortical fluid flow are the dominant factors of the overlimiting current in the planar membrane system. More importantly, at the transition between the limiting and the overlimiting current regimes, hysteresis of electric current is identified. The hysteresis demonstrates the important role of the electroconvective flow in enhancing of current in electrolyte systems with ion-selective membrane.
Numerical investigation of a single-mode chemically reacting Richtmyer-Meshkov instability
NASA Astrophysics Data System (ADS)
Attal, N.; Ramaprabhu, P.
2015-07-01
We report on high-resolution, numerical simulations of a single-mode, chemically reacting, Richtmyer-Meshkov (RM) instability, at different interface thicknesses. The gases on either side of the diffuse interface were Hydrogen (H and Oxygen (O, with a pre-shock Atwood number of 0.5. An incident shock with a Mach number of 1.2 is allowed to traverse from the light (H to the heavy (O medium in the 2D numerical shock tube. The simulations were performed using the astrophysical FLASH code developed at the University of Chicago, with extensive modifications implemented by the authors to describe detailed H-O chemistry, temperature-dependent specific heats, and multi-species equation of state. The interface thickness was systematically varied in the simulations to study the effect of the total mass of fuel burnt and heat added on the hydrodynamic instability growth rates. In the absence of an incident shock, burning results in the formation of so-called combustion waves, which spontaneously trigger RM and Rayleigh-Taylor like instability growth of the interface. We are able to obtain the resulting growth rates of an imposed sinusoidal perturbation, and compare them with the predictions of an impulsive model, with simple modifications to account for the finite thickness of the interface, density changes due to heat addition, and compression of the material line due to the combustion wave. When additionally an incident shock is present, we observe complex interactions between the shock and the aforementioned combustion waves, resulting in significant non-planar distortions of each. When the unstable interface is subjected to a reshock, significant mixing enhancement is observed, accompanied by a dramatic increase in combustion product formation, and combustion efficiency.
A numerical investigation of the barotropic instability on the equatorial β-plane
NASA Astrophysics Data System (ADS)
Kacimi, Abderrahim; Khouider, Boualem
2013-06-01
The barotropic instability of horizontal shear flows is investigated by using two numerical algorithms to solve the equatorial β-plane barotropic equations. The first is the Arakawa Jacobian method (Arakawa, in J Comput Phys 1:119-143, 1966), which is a second-order-centered finite differences scheme that conserves energy and enstrophy, and the second is the fourth-order essentially non-oscillatory scheme for non-linear PDE's of Osher and Shu (SIAM J Numer Anal 28:907-922, 1991), which is designed to track sharp fronts. We are interested in the performance of these two methods in tracking the long-time behavior of the instability, under the influence of the non-linearity, in the simple case of a Helmholtz shear layer. The associated linear problem is solved analytically, and the linear solution is used as an initial condition for the numerical simulations. We run a series of numerical simulations using both methods with various grid refinements and with two different amplitudes of the initial perturbation. A small viscosity term is added to the vorticity equation to damp the grid-scale waves for Arakawa's method. This is not necessary for the high-order ENO-4 scheme, which has its own grid-scale dissipation. At high resolution, the two methods are in good agreement; they yield qualitatively and quantitatively the same solution in the long run: for small disturbances, the total flow stabilizes into a steady-state meridional shear with a smooth profile near the equator, while strong disturbances merge together to form a single large-scale vortex that propagates westward, along the equator, consistent with the African easterly waves and the monsoons trough circulation. At coarse resolution, however, Arakawa's method seems to be much superior to the fourth-order ENO-4 scheme as it provides solutions that are more consistent with the fine resolution one.
Numerical Simulation of Liquid Sheet Instability in a Multiphase Flow Domain
NASA Astrophysics Data System (ADS)
Souvick, Chatterjee; Mahapatra, Soumik; Mukhopadhyay, Achintya; Sen, Swarnendu
2013-11-01
Instability of a liquid sheet leading to the formation of droplets is a classical problem finding a wide range of multi-scale applications like gas turbine engines and inkjet printers. Numerical simulation of such a phenomenon is crucial because of its cost and time effective nature. In this work, the hydrodynamics in a custom designed nozzle is analyzed using Volume of Fluid method in Ansys Fluent. This innovative nozzle design includes an annular liquid sheet sandwiched between two air streams such that the inner air channel is recessed to a certain length. Such a recession leads to interaction between the two multiphase streams inside the atomizer resulting to an increased shear layer instability which augments the disintegration process. The numerical technique employed in this work couples Navier Stokes equation with VoF surface tracking technique. A parametric study with the hydrodynamic parameters involved in the problem, as well as the recession length, is performed while monitoring the axial and tangential exit velocities along with the spray cone angle. Comparison between the full 3D model and two different equivalent 2D axisymmetric models have been shown. The two axisymmetric models vary based on conserving different physical parameters between the 2D and 3D cases.
Numerical modeling of Kelvin-Helmholtz instability by using potential equation
NASA Astrophysics Data System (ADS)
Ahmadi, Somayeh
2012-11-01
This paper presents a potential flow numerical solution for the Kelvin-Helmholtz Instability (KHI) problem of an incompressible two-phase immiscible fluid in a stratified shear flow. As a problem: the two-fluid model becomes illposed when the slip velocity exceeds a critical value, and computations can be quite unstable before the flow reaches the ill-posed condition. In this work, computational stability of various convection schemes together with the potential equation method for the time derivatives in conjunction with the two-fluid model is analyzed. The normal stress balance (with the normal viscous stress) at the interface for the two-fluid model is carefully implemented to minimize its effect on numerical stability. Von Neumann stability analysis shows that: stability condition for two-fluid with equal kinematic viscosity ratio and inviscid flow, supply numerical stability. Excellent agreement has obtained according to analytical result that existing of imaginary part in solution which specialized this method. The numerical algorithm presented in this work can easily handle two-phase fluid flow with various density and viscosity ratios in rectangular channel. Simulation of this model has implemented by writing a code in FORTRAN programming.
Numerical prediction of low frequency combustion instability in a model ramjet combustor
Shang, H.M.; Chen, Y.S.; Shih, M.S.; Farmer, R.C.
1996-12-31
A numerical analysis has been conducted for low-frequency combustion instability in a model ramjet combustor. The facility is two-dimensional, and is comprised of a long inlet duct, a dump combustor chamber, and an exhaust nozzle. The experiments observed that the combustor pressure oscillation under the particular operating condition did not have much cycle-to-cycle variation. The main resonant frequency occurs at about 65 Hz for this case. In the numerical analysis, a time accurate Computational Fluid Dynamics (CFD) code with a pressure-correction algorithm is used, and the combustion process was modeled with a single step chemistry model and a modified eddy breakup model. A high-order upwind scheme with flux limiter is used for convection terms. The convergence of the linear algebraic equations is accelerated through a preconditioned conjugate gradient matrix solver. The numerical predictions show that the flame oscillates in the combustion chamber at the calculation condition and are justified by the experimental schlieren photographs. The numerical analyses correctly predict the chamber pressure oscillation frequency is over-predicted compared with the experimental data. The discrepancy can be explained by the simplified turbulence and combustion model used in this study, and the uncertainty of the inlet boundary conditions.
Direct numerical simulation of instabilities in parallel flow with spherical roughness elements
NASA Technical Reports Server (NTRS)
Deanna, R. G.
1992-01-01
Results from a direct numerical simulation of laminar flow over a flat surface with spherical roughness elements using a spectral-element method are given. The numerical simulation approximates roughness as a cellular pattern of identical spheres protruding from a smooth wall. Periodic boundary conditions on the domain's horizontal faces simulate an infinite array of roughness elements extending in the streamwise and spanwise directions, which implies the parallel-flow assumption, and results in a closed domain. A body force, designed to yield the horizontal Blasius velocity in the absence of roughness, sustains the flow. Instabilities above a critical Reynolds number reveal negligible oscillations in the recirculation regions behind each sphere and in the free stream, high-amplitude oscillations in the layer directly above the spheres, and a mean profile with an inflection point near the sphere's crest. The inflection point yields an unstable layer above the roughness (where U''(y) is less than 0) and a stable region within the roughness (where U''(y) is greater than 0). Evidently, the instability begins when the low-momentum or wake region behind an element, being the region most affected by disturbances (purely numerical in this case), goes unstable and moves. In compressible flow with periodic boundaries, this motion sends disturbances to all regions of the domain. In the unstable layer just above the inflection point, the disturbances grow while being carried downstream with a propagation speed equal to the local mean velocity; they do not grow amid the low energy region near the roughness patch. The most amplified disturbance eventually arrives at the next roughness element downstream, perturbing its wake and inducing a global response at a frequency governed by the streamwise spacing between spheres and the mean velocity of the most amplified layer.
NASA Astrophysics Data System (ADS)
Zhang, Xu; Tan, Duo-Wang
2009-08-01
A novel method is proposed to simulate Rayleigh-Taylor instabilities using a specially-developed unsteady three-dimensional high-order spectral element method code. The numerical model used consists of Navier-Stokes equations and a transport-diffusive equation. The code is first validated with the results of linear stability perturbation theory. Then several characteristics of the Rayleigh-Taylor instabilities are studied using this three-dimensional unsteady code, including instantaneous turbulent structures and statistical turbulent mixing heights under different initial wave numbers. These results indicate that turbulent structures of Rayleigh-Taylor instabilities are strongly dependent on the initial conditions. The results also suggest that a high-order numerical method should provide the capability of simulating small scale fluctuations of Rayleigh-Taylor instabilities of turbulent flows.
Direct numerical simulation of electrokinetic instability and transition to chaotic motion
Demekhin, E. A.; Nikitin, N. V.; Shelistov, V. S.
2013-12-15
A new type of instability—electrokinetic instability—and an unusual transition to chaotic motion near a charge-selective surface (semiselective electric membrane, electrode, or system of micro-/nanochannels) was studied by the numerical integration of the Nernst-Planck-Poisson-Stokes system and a weakly nonlinear analysis near the threshold of instability. A special finite-difference method was used for the space discretization along with a semi-implicit 31/3 -step Runge-Kutta scheme for the integration in time. Two kinds of initial conditions were considered: (a) white-noise initial conditions to mimic “room disturbances” and subsequent natural evolution of the solution, and (b) an artificial monochromatic ion distribution with a fixed wave number to simulate regular wave patterns. The results were studied from the viewpoint of hydrodynamic stability and bifurcation theory. The threshold of electroconvective movement was found by the linear spectral stability theory, the results of which were confirmed by numerical simulation of the entire system. Our weakly nonlinear analysis and numerical integration of the entire system predict possibility of both kinds of bifurcations at the critical point, supercritical and subcritical, depending on the system parameters. The following regimes, which replace each other as the potential drop between the selective surfaces increases, were obtained: one-dimensional steady solution, two-dimensional steady electroconvective vortices (stationary point in a proper phase space), unsteady vortices aperiodically changing their parameters (homoclinic contour), periodic motion (limit cycle), and chaotic motion. The transition to chaotic motion does not include Hopf bifurcation. The numerical resolution of the thin concentration polarization layer showed spike-like charge profiles along the surface, which could be, depending on the regime, either steady or aperiodically coalescent. The numerical investigation confirmed the
Myra, James R.; D'Ippolito, Daniel A.; Russell, David A.; ...
2016-04-11
Sheared flows perpendicular to the magnetic field can be driven by the Reynolds stress or ion pressure gradient effects and can potentially influence the stability and turbulent saturation level of edge plasma modes. On the other hand, such flows are subject to the transverse Kelvin- Helmholtz (KH) instability. Here, the linear theory of KH instabilities is first addressed with an analytic model in the asymptotic limit of long wavelengths compared with the flow scale length. The analytic model treats sheared ExB flows, ion diamagnetism (including gyro-viscous terms), density gradients and parallel currents in a slab geometry, enabling a unified summarymore » that encompasses and extends previous results. In particular, while ion diamagnetism, density gradients and parallel currents each individually reduce KH growth rates, the combined effect of density and ion pressure gradients is more complicated and partially counteracting. Secondly, the important role of realistic toroidal geometry is explored numerically using an invariant scaling analysis together with the 2DX eigenvalue code to examine KH modes in both closed and open field line regions. For a typical spherical torus magnetic geometry, it is found that KH modes are more unstable at and just outside the separatrix as a result of the distribution of magnetic shear. Lastly implications for reduced edge turbulence modeling codes are discussed.« less
Myra, James R.; D'Ippolito, Daniel A.; Russell, David A.; Umansky, Maxim V.; Baver, Derek A.
2016-04-11
Sheared flows perpendicular to the magnetic field can be driven by the Reynolds stress or ion pressure gradient effects and can potentially influence the stability and turbulent saturation level of edge plasma modes. On the other hand, such flows are subject to the transverse Kelvin- Helmholtz (KH) instability. Here, the linear theory of KH instabilities is first addressed with an analytic model in the asymptotic limit of long wavelengths compared with the flow scale length. The analytic model treats sheared ExB flows, ion diamagnetism (including gyro-viscous terms), density gradients and parallel currents in a slab geometry, enabling a unified summary that encompasses and extends previous results. In particular, while ion diamagnetism, density gradients and parallel currents each individually reduce KH growth rates, the combined effect of density and ion pressure gradients is more complicated and partially counteracting. Secondly, the important role of realistic toroidal geometry is explored numerically using an invariant scaling analysis together with the 2DX eigenvalue code to examine KH modes in both closed and open field line regions. For a typical spherical torus magnetic geometry, it is found that KH modes are more unstable at and just outside the separatrix as a result of the distribution of magnetic shear. Lastly implications for reduced edge turbulence modeling codes are discussed.
Collisionless interchange instability 1. Numerical simulations of intermediate-scale irregularities
Zargham, S.; Seyler, C.E. )
1987-09-01
Numerical simulations of the generalized Rayleigh-Taylor instability are presented. The model and simulations are applicable to bottomside and topside spread F, unstable barium cloud dynamics, and collisional interchange instability in general. The principal result is that the evolution of the effective electric field, and shocklike structures propagating perpendicular to E{sub eff}along the extrema of the quasiperiodic structures. The spectral properties of the nonlinear state are analyzed using one-dimensional power spectra calculated along spatial trajectories for selected angles to E{sub eff}. In this way a direct comparison to in situ probe data can be made. The inherent anisotropy of the nonlinear state is reflected in major qualitative differences between the spectra taken parallel to and perpendicular to E{sub eff}. The fundamental finding of the present work is that anisotropy in interchange dynamics is much greater than had been previously reported. This strong anisotropy can explain much of the spectral and spatial structural characteristics of both bottomside and topside spread F. In a companion paper a comparison of the simulation results to various in situ data sets is given.
Numerical Experiments with a Turbulent Single-Mode Rayleigh-Taylor Instability
Cloutman, L.D.
2000-04-01
Direct numerical simulation is a powerful tool for studying turbulent flows. Unfortunately, it is also computationally expensive and often beyond the reach of the largest, fastest computers. Consequently, a variety of turbulence models have been devised to allow tractable and affordable simulations of averaged flow fields. Unfortunately, these present a variety of practical difficulties, including the incorporation of varying degrees of empiricism and phenomenology, which leads to a lack of universality. This unsatisfactory state of affairs has led to the speculation that one can avoid the expense and bother of using a turbulence model by relying on the grid and numerical diffusion of the computational fluid dynamics algorithm to introduce a spectral cutoff on the flow field and to provide dissipation at the grid scale, thereby mimicking two main effects of a large eddy simulation model. This paper shows numerical examples of a single-mode Rayleigh-Taylor instability in which this procedure produces questionable results. We then show a dramatic improvement when two simple subgrid-scale models are employed. This study also illustrates the extreme sensitivity to initial conditions that is a common feature of turbulent flows.
Numerical analysis of gas-dynamic instabilities during the laser drilling process
NASA Astrophysics Data System (ADS)
Khan, A. H.; O'Neill, W.; Tunna, L.; Sutcliffe, C. J.
2006-08-01
The use of high-pressure gas jets in the laser-drilling process has significant influence on the melt ejection mechanism. These jets are highly unstable and this directly relates to the gas pressure and the geometry of the hole being drilled. The evolution of gas-dynamic instabilities during the laser-drilling process was investigated numerically. A minimum length nozzle (MLN) with a 300 μm throat diameter was modelled at various gas pressures, with the gas jet impinging on a range of simulated holes with different aspect ratios. The simulations predict the formation of surface pressure fluctuations that have a broad spectrum due to both the turbulent nature of the jet and the blunt shock oscillation on the surface. The surface pressure variations and the blunt shock oscillation govern the gas dynamic conditions inside the hole, which strongly influence the melt ejection phenomena during the laser-drilling process.
Numerical simulations of fast-axis instability of vector solitons in mode-locked fiber lasers.
Du, Yueqing; Shu, Xuewen; Cheng, Peiyun
2017-01-23
We demonstrate the fast-axis instability in mode-locked fiber lasers numerically for the first time. We find that the energy of the fast mode will be transferred to the slow mode when the strong pump strength makes the soliton period short. A nearly linearly polarized vector soliton along the slow-axis could be generated under certain cavity parameters. The final polarization of the vector soliton is related to the initial polarization of the seed pulse. Two regimes of energy exchanging between the slow mode and the fast mode are explored and the direction of the energy flow between two modes depends on the phase difference. The dip-type sidebands are found to be intrinsic characteristics of the mode-locked fiber lasers under high pulse energy.
Experimental and numerical study of plastic shear instability under high-speed loading conditions
Sokovikov, Mikhail E-mail: naimark@icmm.ru; Chudinov, Vasiliy E-mail: naimark@icmm.ru; Bilalov, Dmitry E-mail: naimark@icmm.ru; Oborin, Vladimir E-mail: naimark@icmm.ru; Uvarov, Sergey E-mail: naimark@icmm.ru; Plekhov, Oleg E-mail: naimark@icmm.ru; Terekhina, Alena E-mail: naimark@icmm.ru; Naimark, Oleg E-mail: naimark@icmm.ru
2014-11-14
The behavior of specimens dynamically loaded during the split Hopkinson (Kolsky) bar tests in a regime close to simple shear conditions was studied. The lateral surface of the specimens was investigated in a real-time mode with the aid of a high-speed infra-red camera CEDIP Silver 450M. The temperature field distribution obtained at different time made it possible to trace the evolution of plastic strain localization. The process of target perforation involving plug formation and ejection was examined using a high-speed infra-red camera and a VISAR velocity measurement system. The microstructure of tested specimens was analyzed using an optical interferometer-profilometer and a scanning electron microscope. The development of plastic shear instability regions has been simulated numerically.
Zig-Zag Thermal-Chemical 3-D Instabilities in the Mantle Wedge: Numerical Study
NASA Astrophysics Data System (ADS)
Zhu, G.; Gerya, T. V.; Arcay, D.; Yuen, D. A.
2008-12-01
To understand the plume initiation and propagation it is important to understand whether small-scale convection is occurring under the back-arc in the Low Viscosity Wedge(LVW) and its implication on the island-arc volcanism. Honda et al. [Honda and Saito, 2003; Honda, et al., 2007]) already deployed small- scale convection in the Low Viscosity Wedge (LVW) above a subducting slab with kinematically imposed velocity boundary condition. They have suggested that a roll (finger)-like pattern of hot and cold anomalies emerges in the mantle wedge above the subducting slab. Here, we perform three-dimensional coupled petrological-thermomechanical numerical simulations of intraoceanic one-sided subduction with spontaneously bending retreating slab characterized by weak hydrated upper interface by using multigrid approach combined with characteristics-based marker-in-cell method with conservative finite difference schemes[Gerya and Yuen, 2003a], to investigate the 3D instabilities above the slab and lateral variation along the arc. Our results show that water released from subducting slab through dehydration reactions may lower the viscosity of the mantle. It allows the existence of wave-like small-scale convection in the LVW, which is shown as roll-like structure in 2D petrological-thermomechanical numerical experiments [Gorczyk et al., 2006] using in-situ rock properties computed on the basis of Gibbs free energy minimization. However, in our 3D cases, the rolls aligning with the arc mainly occur earlier , while zig-zag small-scale thermal-chemical instabilities may episodically form above the slab at later stages, which is different from the aligning finger-like pattern in purely thermal models (Honda et al,2003;2007). Also in contrast to thermal convection chemically buoyant hydrated plumes rising from the slab in our models are actually colder then the mantle wedge [Gerya and Yuen 2003b] which also strongly modify both the convection pattern and the seismic structure in
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.
Numerical simulations of combustion instabilities in gas turbine combustors, with applications
NASA Astrophysics Data System (ADS)
Swenson, Grant Douglas
Recent advances in technology have opened up a potential market for small gas turbine power systems in the 50--100 MW range. In an effort to improve their systems, the gas-turbine industry is interested in understanding and controlling combustion instabilities as well as reducing pollutant production. To understand the dynamics inherent in a combustion system, information about the flow field behavior is required. Because of a scarcity of available experimental or numerical results for full-scale gas-turbine combustors, we decided to use numerical simulations to provide the required information about the flow field dynamics. The ability of the numerical simulations to reproduce unstable behavior in combustion environments will be presented. The investigation of the flow field dynamics has been conducted for three test cases; a planar heat source in a tube, premixed flow in a dump combustor, and premixed and diffusion flames in a full-scale gas turbine combustor. The numerically determined unsteady acoustic modes will be shown to compare well with theory and experiments. An investigation of the local heat release response to an unsteady flow field is conducted for incorporation into an approximate analysis method. The results of including a Helmholtz resonator in a dump combustor as a passive control mechanism will be presented. The production of NOx and CO will be compared between stable and unstable flow configurations. The pollutant results indicate that for the planar flame in a tube and the dump combustor, the NOx levels at the exit plane are reduced when the system is unstable.
How common are ALS plateaus and reversals?
Bedlack, Richard S; Vaughan, Timothy; Wicks, Paul; Heywood, Jamie; Sinani, Ervin; Selsov, Roger; Macklin, Eric A; Schoenfeld, David; Cudkowicz, Merit; Sherman, Alex
2016-03-01
To determine the frequency of amyotrophic lateral sclerosis (ALS) plateaus and reversals in the Pooled Resource Open-Access ALS Clinical Trials (PRO-ACT) database. We analyzed Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS) and ALSFRS-revised (ALSFRS-R) data from PRO-ACT participants. The frequencies of participants experiencing plateaus (periods where scores did not change) were calculated over 6-, 12-, and 18-month epochs. The percentage of participants ever experiencing reversals (periods where scores improved) of different lengths were also calculated and plotted. Over 6 months, 25% of 3,132 participants did not decline. Over 12 months, 16% of 2,105 participants did not decline. Over 18 months, 7% of 1,218 participants did not decline. Small ALS reversals were also common, especially over shorter follow-up intervals; 14% of 1,343 participants had a 180-day interval where their ALSFRS-R slope was greater than zero. Fewer than 1% of participants ever experienced improvements of 4 or more ALSFRS-R points lasting at least 12 months. ALS plateaus and small reversals are common, especially over brief intervals. In light of these data, stable disease, especially for a short period of time, should not be interpreted as an ALS treatment effect. Large sustained ALS reversals, on the other hand, are rare, potentially important, and warrant further study. © 2015 American Academy of Neurology.
Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank
NASA Astrophysics Data System (ADS)
Luan, D. Y.; Lu, J. P.; Bu, Q. X.; Zhang, S. F.; Zheng, S. X.
2016-05-01
The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing.
Numerical Simulation of the Mechanisms Governing the Onset of the BÉNARD-VON KÁRMÁN Instability
NASA Astrophysics Data System (ADS)
Carte, Gilles; Duek, Jan; Fraunié, Philippe
1996-10-01
The onset of the Bénard-von Kármán instability consisting of the selective amplification of the linear unstable mode and yielding finally the well-known saturated state has been described many times on the basis of both numerical and experimental results in various configurations. However, neither the role of the harmonics and their coupling has been examined quantitatively, nor has the spatial structure of the instability been studied in detail. A recently developed numerical method of simulation of quasi-periodic flows makes it possible to integrate the investigation of linear and non-linear characteristics within a single numerical method. The simulation of the 2D afterbody wake presented in this paper allows us to follow the amplification of the instability over many orders of magnitude. It is shown that at all stages of its development the instability is characterized by a series of harmonics, each of them amplified with a multiple of the fundamental amplification rate during the linear regime. The amplification of harmonics results from an energy transfer from the mean flow to harmonics of increasingly higher order. Ultimately the energy losses compensate this transfer and an equilibrium, commonly called saturation of the instability, is reached. It is shown that the coupling between the fundamental harmonic and the mean flow is mainly responsible for the saturation. The convergence rate of the development of the instability into harmonics is investigated. A full description of the spatial structure of all significant harmonics both in the linear regime and at saturation is obtained. The results show that time and space characteristics of the instability can be investigated simultaneously in an efficient way. Such an approach might be particularly important in 3D wakes where the geometry has a strong influence on the behaviour of unstable flows.
NASA Astrophysics Data System (ADS)
Hallo, L.; Olazabal-Loumé, M.; Maire, P. H.; Breil, J.; Morse, R.-L.; Schurtz, G.
2006-06-01
This paper deals with ablation front instabilities simulations in the context of direct drive ICF. A simplified DT target, representative of realistic target on LIL is considered. We describe here two numerical approaches: the linear perturbation method using the perturbation codes Perle (planar) and Pansy (spherical) and the direct simulation method using our Bi-dimensional hydrodynamic code Chic. Numerical solutions are shown to converge, in good agreement with analytical models.
Numerical studies of double-diffusive convection and miscible Rayleigh-Taylor instability
NASA Astrophysics Data System (ADS)
Young, Yuan-Nan
2000-08-01
Slot convection refers to buoyantly driven flow due to the lateral temperature difference. In double-diffusive slot convection the destabilizing temperature gradient (transverse to the gravitational direction) is competing with a stabilizing concentration gradient. For suitable combination of physical parameters, layers form as a result of buoyancy balance between the lateral thermal difference and the stabilizing concentration gradient. In chapter 2, we directly simulate this system using a two- dimensional pseudospectral code. Incompressibility is achieved by the consistent implementation of the tau- correction. We find that layer dynamics depends on the particulars of the imposed boundary conditions for the temperature at the sidewalls and the density stratification ratio (the relative strength of the stabilizing solute gradient to the destabilizing horizontal thermal difference). We demonstrate the effects of the density stratification ratio on the layer dynamics for the constant sidewall temperature case, and we also study the case of constant lateral heat flux in order to understand the effects of the temperature boundary conditions. We apply the argument for layering in turbulent stratified fluids to our problem, and find-despite the tilted nature of cell boundaries in our case-similarities in both the averaged equations and actual layer evolution. Finally, we provide details for both edge mergers and interior mergers. In chapter 3, we investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3 dimensions using direct numerical simulations, where the working fluid is assumed incompressible under the Boussinesq approximation. With a variety of diagnostics, we develop a physical picture for the detailed temporal development of the mixed layer: We identify three distinct evolutionary phases in the development of the mixed layer. Our analysis provides an explanation for the observed differences between two and three-dimensional RT instability; the
NASA Astrophysics Data System (ADS)
Xie, Beibei; Yang, Dong; Xie, Haiyan; Nie, Xin; Liu, Wanyu
2016-08-01
In order to expand the study on flow instability of supercritical circulating fluidized bed (CFB) boiler, a new numerical computational model considering the heat storage of the tube wall metal was presented in this paper. The lumped parameter method was proposed for wall temperature calculation and the single channel model was adopted for the analysis of flow instability. Based on the time-domain method, a new numerical computational program suitable for the analysis of flow instability in the water wall of supercritical CFB boiler with annular furnace was established. To verify the code, calculation results were respectively compared with data of commercial software. According to the comparisons, the new code was proved to be reasonable and accurate for practical engineering application in analysis of flow instability. Based on the new program, the flow instability of supercritical CFB boiler with annular furnace was simulated by time-domain method. When 1.2 times heat load disturbance was applied on the loop, results showed that the inlet flow rate, outlet flow rate and wall temperature fluctuated with time eventually remained at constant values, suggesting that the hydrodynamic flow was stable. The results also showed that in the case of considering the heat storage, the flow in the water wall is easier to return to stable state than without considering heat storage.
Laboratory and numerical simulation of internal wave attractors and their instability.
NASA Astrophysics Data System (ADS)
Brouzet, Christophe; Dauxois, Thierry; Ermanyuk, Evgeny; Joubaud, Sylvain; Sibgatullin, Ilias
2015-04-01
Internal wave attractors are formed as result of focusing of internal gravity waves in a confined domain of stably stratified fluid due to peculiarities of reflections properties [1]. The energy injected into domain due to external perturbation, is concentrated along the path formed by the attractor. The existence of attractors was predicted theoretically and proved both experimentally and numerically [1-4]. Dynamics of attractors is greatly influenced by geometrical focusing, viscous dissipation and nonlinearity. The experimental setup features Schmidt number equal to 700 which impose constraints on resolution in numerical schemes. Also for investigation of stability on large time intervals (about 1000 periods of external forcing) numerical viscosity may have significant impact. For these reasons, we have chosen spectral element method for investigation of this problem, what allows to carefully follow the nonlinear dynamics. We present cross-comparison of experimental observations and numerical simulations of long-term behavior of wave attractors. Fourier analysis and subsequent application of Hilbert transform are used for filtering of spatial components of internal-wave field [5]. The observed dynamics shows a complicated coupling between the effects of local instability and global confinement of the fluid domain. The unstable attractor is shown to act as highly efficient mixing box providing the efficient energy pathway from global-scale excitation to small-scale wave motions and mixing. Acknowledgement, IS has been partially supported by Russian Ministry of Education and Science (agreement id RFMEFI60714X0090) and Russian Foundation for Basic Research, grant N 15-01-06363. EVE gratefully acknowledges his appointment as a Marie Curie incoming fellow at Laboratoire de physique ENS de Lyon. This work has been partially supported by the ONLITUR grant (ANR-2011-BS04-006-01) and achieved thanks to the resources of PSMN from ENS de Lyon 1. Maas, L. R. M. & Lam, F
NASA Astrophysics Data System (ADS)
Yu, Peicheng; Li, Fei; Dalichaouch, Thamine; Fiuza, Frederico; Decyk, Viktor; Davidson, Asher; Tableman, Adam; An, Weiming; Tsung, Frank; Fonseca, Ricardo; Lu, Wei; Vieira, Jorge; Silva, Luis; Mori, Warren
2016-10-01
we present a finite-difference-time-domain (FDTD) Maxwell solver for the particle-in-cell (PIC) algorithm, which is customized to effectively eliminate the numerical Cerenkov instability (NCI) which arises when a plasma (neutral or non-neutral) relativistically drifts on a grid when using the PIC algorithm. We control the EM dispersion curve in the direction of the plasma drift of a FDTD Maxwell solver by using a customized higher order finite difference operator for the spatial derivative along the direction of the drift (1& circ; direction). We show that this eliminates the main NCI modes with moderate | k1 | , while keeps additional main NCI modes well outside the range of physical interest with higher | k1 | . These main NCI modes can be easily filtered out along with first spatial aliasing NCI modes which are also at the edge of the fundamental Brillouin zone. The customized solver has the possible advantage of improved parallel scalability because it can be easily partitioned along 1& circ; which typically has many more cells than other directions for the problems of interest.
Numerical simulations of Richtmyer{endash}Meshkov instabilities in finite-thickness fluid layers
Mikaelian, K.O.
1996-05-01
Direct numerical simulations of Richtmyer{endash}Meshkov instabilities in shocked fluid layers are reported and compared with analytic theory. To investigate new phenomena such as freeze-out, interface coupling, and feedthrough, several new configurations are simulated on a two-dimensional hydrocode. The basic system is an {ital A}/{ital B}/{ital A} combination, where {ital A} is air and {ital B} is a finite-thickness layer of freon, SF{sub 6}, or helium. The middle layer {ital B} has perturbations either on its upstream or downstream side, or on both sides, in which case the perturbations may be in phase (sinuous) or out of phase (varicose). The evolution of such perturbations under a Mach 1.5 shock is calculated, including the effect of a reshock. Recently reported gas curtain experiments [J. M. Budzinski {ital et} {ital al}., Phys. Fluids {bold 6}, 3510 (1994)] are also simulated and the code results are found to agree very well with the experiments. A new gas curtain configuration is also considered, involving an initially sinuous SF{sub 6} or helium layer and a new pattern, opposite mushrooms, is predicted to emerge. Upon reshock a relatively simple sinuous gas curtain is found to evolve into a highly complex pattern of nested mushrooms. {copyright} {ital 1996 American Institute of Physics.}
Numerical simulations of Richtmyer-Meshkov instabilities in finite-thickness fluid layers
NASA Astrophysics Data System (ADS)
Mikaelian, Karnig O.
1996-05-01
Direct numerical simulations of Richtmyer-Meshkov instabilities in shocked fluid layers are reported and compared with analytic theory. To investigate new phenomena such as freeze-out, interface coupling, and feedthrough, several new configurations are simulated on a two-dimensional hydrocode. The basic system is an A/B/A combination, where A is air and B is a finite-thickness layer of freon, SF6, or helium. The middle layer B has perturbations either on its upstream or downstream side, or on both sides, in which case the perturbations may be in phase (sinuous) or out of phase (varicose). The evolution of such perturbations under a Mach 1.5 shock is calculated, including the effect of a reshock. Recently reported gas curtain experiments [J. M. Budzinski et al., Phys. Fluids 6, 3510 (1994)] are also simulated and the code results are found to agree very well with the experiments. A new gas curtain configuration is also considered, involving an initially sinuous SF6 or helium layer and a new pattern, opposite mushrooms, is predicted to emerge. Upon reshock a relatively simple sinuous gas curtain is found to evolve into a highly complex pattern of nested mushrooms.
Direct Numerical Simulation of Richtmeyer-Meshkov Instability Using pWAMR
NASA Astrophysics Data System (ADS)
Grenga, Temistocle; Paolucci, Samuel
2015-11-01
The parallel Wavelet Adaptive Multiresolution Representation (pWAMR) method is used to simulate the Richtmyer-Meshkov instability caused by a shock interacting with a density-stratified interface. The physical problem is studied in several configurations. We present results of numerical studies that investigate the influence of initial condition parameters (amplitude and wavelength of perturbations) on mixing and transition. In addition, the evaluation of turbulence statistics provides a measure of the mixing across the scales and the correlation with the initial condition parameters. The problem is modeled using the compressible reactive Navier-Stokes equations for a gas mixture, including multi-component diffusion, Soret and Dufour effects, and state dependent thermodynamic and transport properties. Since the amplitudes of wavelets provide a direct measure of the local error, the method is able to efficiently capture to any desired accuracy a wide range of spatial scales using a relatively small number of degrees of freedom by evolving the dynamically adaptive grid. In an effective fashion, the multilevel structure of the algorithm provides a simple way to adapt computational refinements to local demands of the solution, thus automatically producing verified solutions. Supported by C-SWARM through the Department of Energy, National Nuclear Security Administration, under Award Number DE-NA0002377.
Tritschler, V K; Zubel, M; Hickel, S; Adams, N A
2014-12-01
In this study we present direct numerical simulation results of the Richtmyer-Meshkov instability (RMI) initiated by Ma=1.05,Ma=1.2, and Ma=1.5 shock waves interacting with a perturbed planar interface between air and SF(6). At the lowest shock Mach number the fluids slowly mix due to viscous diffusion, whereas at the highest shock Mach number the mixing zone becomes turbulent. When a minimum critical Taylor microscale Reynolds number is exceeded, an inertial range spectrum emerges, providing further evidence of transition to turbulence. The scales of turbulent motion, i.e., the Kolmogorov length scale, the Taylor microscale, and the integral length, scale are presented. The separation of these scales is found to increase as the Reynolds number is increased. Turbulence statistics, i.e., the probability density functions of the velocity and its longitudinal and transverse derivatives, show a self-similar decay and thus that turbulence evolving from RMI is not fundamentally different from isotropic turbulence, though nominally being only isotropic and homogeneous in the transverse directions.
NASA Astrophysics Data System (ADS)
Xin, Bo; Sun, Dakun; Jing, Xiaodong; Sun, Xiaofeng
2016-07-01
Lined ducts are extensively applied to suppress noise emission from aero-engines and other turbomachines. The complex noise/flow interaction in a lined duct possibly leads to acoustic instability in certain conditions. To investigate the instability, the full linearized Navier-Stokes equations with eddy viscosity considered are solved in frequency domain using a Galerkin finite element method to compute the sound transmission in shear flow in the lined duct as well as the flow perturbation over the impedance wall. A good agreement between the numerical predictions and the published experimental results is obtained for the sound transmission, showing that a transmission peak occurs around the resonant frequency of the acoustic liner in the presence of shear flow. The eddy viscosity is an important influential factor that plays the roles of both providing destabilizing and making coupling between the acoustic and flow motions over the acoustic liner. Moreover, it is shown from the numerical investigation that the occurrence of the sound amplification and the magnitude of transmission coefficient are closely related to the realistic velocity profile, and we find it essential that the actual variation of the velocity profile in the axial direction over the liner surface be included in the computation. The simulation results of the periodic flow patterns possess the proper features of the convective instability over the liner, as observed in Marx et al.'s experiment. A quantitative comparison between numerical and experimental results of amplitude and phase of the instability is performed. The corresponding eigenvalues achieve great agreement.
Cloutman, L.D.
2000-07-10
Direct numerical simulation and large eddy simulations are powerful tools for studying turbulent flows. Unfortunately, they are computationally demanding in terms of run times, storage, and accuracy of the numerical method used. In particular, high order methods promise high accuracy on a given grid, but they often fail to deliver the expected accuracy due to dispersive truncation errors that appear as unphysical oscillations in the numerical solutions. This report describes a nonlinear flux limiter that has been applied to the second-order tensor viscosity method and markedly reduces the dispersive truncation errors. A Rayleigh-Taylor instability is simulated to show how well the flux limiter works.
NASA Astrophysics Data System (ADS)
Movahed, Pooya
High-speed flows are prone to hydrodynamic interfacial instabilities that evolve to turbulence, thereby intensely mixing different fluids and dissipating energy. The lack of knowledge of these phenomena has impeded progress in a variety of disciplines. In science, a full understanding of mixing between heavy and light elements after the collapse of a supernova and between adjacent layers of different density in geophysical (atmospheric and oceanic) flows remains lacking. In engineering, the inability to achieve ignition in inertial fusion and efficient combustion constitute further examples of this lack of basic understanding of turbulent mixing. In this work, my goal is to develop accurate and efficient numerical schemes and employ them to study compressible turbulence and mixing generated by interactions between shocked (Richtmyer-Meshkov) and accelerated (Rayleigh-Taylor) interfaces, which play important roles in high-energy-density physics environments. To accomplish my goal, a hybrid high-order central/discontinuity-capturing finite difference scheme is first presented. The underlying principle is that, to accurately and efficiently represent both broadband motions and discontinuities, non-dissipative methods are used where the solution is smooth, while the more expensive and dissipative capturing schemes are applied near discontinuous regions. Thus, an accurate numerical sensor is developed to discriminate between smooth regions, shocks and material discontinuities, which all require a different treatment. The interface capturing approach is extended to central differences, such that smooth distributions of varying specific heats ratio can be simulated without generating spurious pressure oscillations. I verified and validated this approach against a stringent suite of problems including shocks, interfaces, turbulence and two-dimensional single-mode Richtmyer-Meshkov instability simulations. The three-dimensional code is shown to scale well up to 4000 cores
NASA Astrophysics Data System (ADS)
Chou, Yi-Ju; Shao, Yun-Chuan
2016-04-01
In this study, we investigate Rayleigh-Taylor instability in which the density stratification is caused by the suspension of particles in liquid flows using the conventional single-phase model and Euler-Lagrange (EL) two-phase model. The single-phase model is valid only when the particles are small and number densities are large, such that the continuum approximation applies. The present single-phase results show that the constant settling of the particle concentration restricts the lateral development of the vortex ring, which results in a decrease of the rising speed of the Rayleigh-Taylor bubbles. The EL model enables the investigation of particle-flow interaction and the influence of particle entrainment, resulting from local non-uniformity in the particle distribution. We compare bubble dynamics in the single-phase and EL cases, and our results show that the deviation between the two cases becomes more pronounced when the particle size increases. The main mechanism responsible for the deviation is particle entrainment, which can only be resolved in the EL model. We provide a theoretical argument for the small-scale local entrainment resulting from the local velocity shear and non-uniformity of the particle concentration. The theoretical argument is supported by numerical evidence. Energy budget analysis is also performed and shows that potential energy is released due to the interphase drag and buoyant effect. The buoyant effect, which results in the transformation of potential energy into kinetic energy and shear dissipation, plays a key role in settling enhancement. We also find that particle entrainment increases the shear dissipation, which in turn enhances the release of potential energy.
NASA Astrophysics Data System (ADS)
Chang, Chih-Hao; Deng, Xiaolong; Theofanous, Theo G.
2013-06-01
We present a conservative and consistent numerical method for solving the Navier-Stokes equations in flow domains that may be separated by any number of material interfaces, at arbitrarily-high density/viscosity ratios and acoustic-impedance mismatches, subjected to strong shock waves and flow speeds that can range from highly supersonic to near-zero Mach numbers. A principal aim is prediction of interfacial instabilities under superposition of multiple potentially-active modes (Rayleigh-Taylor, Kelvin-Helmholtz, Richtmyer-Meshkov) as found for example with shock-driven, immersed fluid bodies (locally oblique shocks)—accordingly we emphasize fidelity supported by physics-based validation, including experiments. Consistency is achieved by satisfying the jump discontinuities at the interface within a conservative 2nd-order scheme that is coupled, in a conservative manner, to the bulk-fluid motions. The jump conditions are embedded into a Riemann problem, solved exactly to provide the pressures and velocities along the interface, which is tracked by a level set function to accuracy of O(Δx5, Δt4). Subgrid representation of the interface is achieved by allowing curvature of its constituent interfacial elements to obtain O(Δx3) accuracy in cut-cell volume, with attendant benefits in calculating cell- geometric features and interface curvature (O(Δx3)). Overall the computation converges at near-theoretical O(Δx2). Spurious-currents are down to machine error and there is no time-step restriction due to surface tension. Our method is built upon a quadtree-like adaptive mesh refinement infrastructure. When necessary, this is supplemented by body-fitted grids to enhance resolution of the gas dynamics, including flow separation, shear layers, slip lines, and critical layers. Comprehensive comparisons with exact solutions for the linearized Rayleigh-Taylor and Kelvin-Helmholtz problems demonstrate excellent performance. Sample simulations of liquid drops subjected to
HOW DO NUMERICAL METHODS EFFECT STATISTICAL DETAILS OF RICHTMYER-MESHKOV INSTABILITIES
W. J. RIDER; J. R. KAMM; C. ZOLDI
2001-05-01
Over the past several years we have presented a less than glowing experimental comparison of hydrodynamic codes with the gas curtain experiment. Here, we discuss the manner in which the various details of the hydrodynamic integration techniques conspire to produce poor results. This also includes some progress in improving the results and agreement with experimental results. Our results are based upon the gas curtain, Richtmyer-Meshkov experiments conducted by Rightley et al. (Rightley et al. 1999) at Los Alamos. We also examine the results of a gas cylinder experiment conducted more recently by Prestridge and Zoldi which includes velocity data obtained via a PIV technique. Traditionally, the integral width of the mixing layer is used as a yardstick to measure the Richtmyer-Meshkov instability. This is also used when investigating the performance of numerical methods. Our focus has been on the details of the mixing below the integral scale. Because the flow is hydrodynamically unstable, we employ statistical measures in our comparisons. This is built upon a parallel effort by the experimentalists investigating the statistical nature of the mixing induced by shock waves. The principle tools we use to measure the spectral structure of the images of these flows are the fractal dimension and the continuous wavelet spectrum. The bottom line is that all the higher order methods used to simulate the gas curtain compare poorly with the experimental data when quantified with these spatial statistics. Moreover, the comparisons degrade under mesh refinement. This occurs despite the fact that the integral scale comparison is acceptable and consistent with the expectations from this class of methods. The most surprising result is that a first-order Godunov method does produce a good comparison relative to the assumed to be higher-order methods. We have examined a broad variety of methodologies associated with the high-order methods to illuminate this problematic result. In the
A Numerical Investigation of the Criterion for Cloud-Top Entrainment Instability.
NASA Astrophysics Data System (ADS)
MacVean, M. K.
1993-08-01
An investigation of cloud-top entrainment instability (CTEI) has been carried out using a fine-resolution two-dimensional numerical model. Initial conditions having specified values of R = cpe/Lqt were used. Here, e, and qt, are the jumps in equivalent potential temperature and total water mixing ratio across cloud top. In order to isolate the effects of entrainment across cloud top, cloud microphysics and surface fluxes were excluded from all the integrations. Radiative processes were generally also excluded, although a number of runs with longwave radiative cooling were performed. Integrations were carried out for specified values of R, using various subgrid models, including several constant values of eddy viscosity. Because the crucial process underlying CTEI is small-scale mixing, which must be parameterized in this model, only those results that are not critically dependent on the precise form of the subgrid model are likely to have any general validity. Fortunately, significant conclusions can still be drawn from the study. At values of R greater than the critical value of about 0.7 recently derived by MacVean and Mason, the cloud layer breaks up and evaporates completely within 1-2 h. On the other hand, for values of R greater than and close to the critical value of about 0.23 derived by earlier authors, no tendency for rapid dissipation of the cloud is observed. The results from the integrations that included longwave cooling at cloud top suggest that the inclusion of this process does not fundamentally modify these conclusions. Furthermore, analysis suggests that the entrainment rate in the simulations is likely to be realistic. It is concluded that CTEI may be an important mechanism governing the rapid dissipation of stratocumulus, although only at much larger values of R than earlier theoretical work had suggested. This conclusion is shown to be consistent with most of the limited, available observational data. These simulations provide strong support
NASA Astrophysics Data System (ADS)
Rivera, Gustavo; Diamessis, Peter
2016-11-01
The shoaling of an internal solitary wave (ISW) of depression over gentle slopes is explored through fully nonlinear and non-hydrostatic simulations based on a high-accuracy deformed spectral multidomain penalty method. As recently observed in the South China Sea, in high-amplitude shoaling ISWs, the along-wave current can exceed the wave celerity resulting in convective instabilities. If the slope is less than 3%, the wave does not disintegrate as in the case of steeper slope shoaling but, instead, maintains its symmetric shape; the above convective instability may drive the formation of a turbulent recirculating core. The sensitivity of convective instabilities in an ISW is examined as a function of the bathymetric slope and wave steepness. ISWs are simulated propagating over both idealized and realistic bathymetry. Emphasis is placed on the structure of the above instabilities, the persistence of trapped cores and their potential for particle entrainment and transport. Additionally, the role of the baroclinic background current on the development of convective instabilities is explored. A preliminary understanding is obtained of the transition to turbulence within a high-amplitude ISW shoaling over progressively varying bathymetry.
NASA Astrophysics Data System (ADS)
Schmidt, Patrick; Ausner, Ilja; Ó Náraigh, Lennon; Lucquiaud, Mathieu; Valluri, Prashant
2016-11-01
The dynamics of vertical counter-current gas-liquid flows are largely determined by interfacial instability, which gives rise to a multitude of complex wave patterns and internal flows. To study the genesis and evolution of the instability in detail, we employ theoretical stability analysis, experiment and a newly developed level set method based in-house solver to carry out direct numerical simulations. Crucial results of these simulations, such as growth rate and phase velocity of interfacial waves, are rigorously compared against linear and weakly nonlinear theory; thereby showing remarkable agreement. The analysis also reveals the spatio-temporal character of the waves, depicting regimes of absolute and convective instability. Complementing the benchmark set by (non-)linear theory, we perform film thickness measurements of a real gas-liquid system (air-silicone oil) by means of a non-intrusive light-induced fluorescence technique to further validate the solver regarding its capability of capturing interfacial dynamics accurately. These measurements are in good agreement with the results of the nonlinear direct numerical simulations with respect to wavelength and wave shape of the most unstable mode.
NASA Astrophysics Data System (ADS)
Lu, C.; Lichtner, P. C.
2007-07-01
CO2 sequestration (capture, separation, and long term storage) in various geologic media including depleted oil reservoirs, saline aquifers, and oceanic sediments is being considered as a possible solution to reduce green house gas emissions. Dissolution of supercritical CO2 in formation brines is considered an important storage mechanism to prevent possible leakage. Accurate prediction of the plume dissolution rate and migration is essential. Analytical analysis and numerical experiments have demonstrated that convective instability (Rayleigh instability) has a crucial effect on the dissolution behavior and subsequent mineralization reactions. Global stability analysis indicates that a certain grid resolution is needed to capture the features of density-driven fingering phenomena. For 3-D field scale simulations, high resolution leads to large numbers of grid nodes, unfeasible for a single workstation. In this study, we investigate the effects of convective instability on geologic sequestration of CO2 by taking advantage of parallel computing using the code PFLOTRAN, a massively parallel 3-D reservoir simulator for modeling subsurface multiphase, multicomponent reactive flow and transport based on continuum scale mass and energy conservation equations. The onset, development and long-term fate of a supercritical CO2 plume will be resolved with high resolution numerical simulations to investigate the rate of plume dissolution caused by fingering phenomena.
An Experimental and Numerical Study of Roughness-Induced Instabilities in a Mach 3.5 Boundary Layer
NASA Technical Reports Server (NTRS)
Kegerise, Michael A.; King, Rudolph A.; Owens, Lewis R.; Choudhari, Meelan M.; Norris, Andrew T.; Li, Fei; Chang, Chau-Layn
2012-01-01
Progress on a joint experimental and numerical study of laminar-to-turbulent transition induced by an isolated roughness element in a high-speed laminar boundary layer is reported in this paper. The numerical analysis suggests that transition is driven by the instability of high- and low-speed streaks embedded in the wake of the isolated roughness element. In addition, spatial stability analysis revealed that the wake flow supports multiple modes (even and odd) of convective instabilities that experience strong enough growth to cause transition. The experimental measurements, which included hot-wire and pitot-probe surveys, confirmed the existence of embedded high- and low-speed streaks in the roughness wake. Furthermore, the measurements indicate the presence of both even and odd modes of instability, although their relative magnitude depends on the specifics of the roughness geometry and flow conditions (e.g., the value of Re(sub kk) or k/delta. For the two test cases considered in the measurements (Re(sub kk) values of 462 and 319), the even mode and the odd mode were respectively dominant and appear to play a primary role in the transition process.
Wilkie, George J.; Dorland, William
2016-05-15
The δf particle-in-cell algorithm has been a useful tool in studying the physics of plasmas, particularly turbulent magnetized plasmas in the context of gyrokinetics. The reduction in noise due to not having to resolve the full distribution function indicates an efficiency advantage over the standard (“full-f”) particle-in-cell. Despite its successes, the algorithm behaves strangely in some circumstances. In this work, we document a fully resolved numerical instability that occurs in the simplest of multiple-species test cases: the electrostatic Ω{sub H} mode. There is also a poorly understood numerical instability that occurs when one is under-resolved in particle number, which may require a prohibitively large number of particles to stabilize. Both of these are independent of the time-stepping scheme, and we conclude that they exist if the time advancement were exact. The exact analytic form of the algorithm is presented, and several schemes for mitigating these instabilities are also presented.
NASA Astrophysics Data System (ADS)
Kasbaoui, Mohamed; Koch, Donald; Desjardins, Olivier
2015-11-01
In a previous study (Kasbaoui et al., J. Fluid Mech. 2015), particle laden homogeneous shear was shown to be subject to an algebraic instability. Initially randomly distributed particles are entrained by wave-like perturbations in the fluid velocity and segregate in a similar wave-like pattern while they sediment under gravity. The preferential concentration mechanism, which is the tendency of particles to exit vortical regions and gather in straining regions, causes the two waves to amplify each other resulting in an algebraic instability. By means of simulations, we compare the perturbations growth to the one yielded by the theory in the limit of small Stokes number particles. The simulations are conducted with an Eulerian model of the particles as well as a Lagrangian model. The two are compared. A secondary Rayleigh-Taylor instability caused by the periodic stacking of heavy layers of concentrated particles on top of depleted lighter layers is analyzed.
Numerical simulations of toroidal Alfven instabilities excited by trapped energetic ions
Zheng, L.-J.; Chen, Liu; Santoro, R. A.
2000-06-01
Extensive magnetohydrodynamic-gyrokinetic hybrid simulations have been carried out to study the excitations of Alfven instabilities by trapped energetic ions in tokamak plasmas. Depending on the parameters, the most unstable mode can be either the toroidal Alfven eigenmode (TAE) or the energetic-particle mode (EPM). In both cases, the dominant instability driving mechanism is the resonance with the trapped-particle magnetic precessional drifts. The mode frequencies of the most unstable modes, meanwhile, tend to be near the toroidal frequency gap in order to minimize the continuum damping. It is also demonstrated that the instabilities can be quenched by reversing the magnetic precessional drifts via either negative magnetic shear or locating the magnetic turning points in the inner (strong B) side of the torus. (c) 2000 American Institute of Physics.
NASA Astrophysics Data System (ADS)
Lee, Wei-Li; Startsev, Edward A.; Davidson, Ronald C.
2004-11-01
In intense charged particle beams with large temperature anisotropy free energy is available to drive a transverse electromagnetic Weibel-type instability. The finite transverse geometry of the confined beam makes a detailed theoretical investigation difficult. In this paper the newly developed bEASt (beam eigenmode and spectra) code which solves the linearized Vlasov-Maxwell equations is used to investigate the detailed properties of the Weibel instability for a long charge bunch propagating through a cylindrical pipe of radius r_w. The stability analysis is carried out for azimuthally symmetric perturbations about a two-temperature thermal equilibrium distribution in the smooth-focusing approximation. To study the nonlinear stage of the instability, the Darwin model is being developed and incorporated into the Beam Equilibrium Stability and Transport(BEST) code.
NASA Technical Reports Server (NTRS)
Matthews, D. A.
1978-01-01
The effects of mesoscale triggering on organized nonsevere convective cloud systems in the High Plains are considered. Two experiments were conducted to determine if a one-dimensional quasi-time dependent model could (1) detect soundings which were sensitive to mesoscale triggering, and (2) discriminate between cases which had mesoscale organized convection and those with no organized convection. The MESOCU model was used to analyze the available potential instability and thermodynamic potential for cloud growth. It is noted that lifting is a key factor in the release of available potential instability on the High Plains.
NASA Technical Reports Server (NTRS)
Matthews, D. A.
1978-01-01
The effects of mesoscale triggering on organized nonsevere convective cloud systems in the High Plains are considered. Two experiments were conducted to determine if a one-dimensional quasi-time dependent model could (1) detect soundings which were sensitive to mesoscale triggering, and (2) discriminate between cases which had mesoscale organized convection and those with no organized convection. The MESOCU model was used to analyze the available potential instability and thermodynamic potential for cloud growth. It is noted that lifting is a key factor in the release of available potential instability on the High Plains.
Lehe, Remi; Kirchen, Manuel; Godfrey, Brendan B; Maier, Andreas R; Vay, Jean-Luc
2016-11-01
Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.
2012-09-13
AFOSR – THEORETICAL, NUMERICAL , AND EXPERIMENTAL INVESTIGATIONS OF THE FUNDAMENTAL PROCESSES THAT DRIVE COMBUSTION INSTABILITIES IN LIQUID ROCKET ...access for optical diagnostics • Injector plate may be interchanged to allow investigation of the driving by different injection systems...Preliminary Results • The “Full scale” LRE tested to identify its instabilities (~170 Hz) • Investigated the acoustics of the ACLRES rig (~250 Hz) • The
Lehe, Remi; Kirchen, Manuel; Godfrey, Brendan B.; Maier, Andreas R.; Vay, Jean -Luc
2016-11-14
Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.
Lehe, Remi; Kirchen, Manuel; Godfrey, Brendan B.; ...
2016-11-14
Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover,more » it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.« less
NASA Astrophysics Data System (ADS)
Lehe, Remi; Kirchen, Manuel; Godfrey, Brendan B.; Maier, Andreas R.; Vay, Jean-Luc
2016-11-01
Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.
NASA Technical Reports Server (NTRS)
Hall, P.; Malik, M. R.
1984-01-01
The instability of a three dimensional attachment line boundary layer is considered in the nonlinear regime. Using weakly nonlinear theory, it is found that, apart from a small interval near the (linear) critical Reynolds number, finite amplitude solutions bifurcate subcritically from the upper branch of the neutral curve. The time dependent Navier-Stokes equations for the attachment line flow have been solved using a Fourier-Chebyshev spectral method and the subcritical instability is found at wavenumbers that correspond to the upper branch. Both the theory and the numerical calculations show the existence of supercritical finite amplitude (equilibrium) states near the lower branch which explains why the observed flow exhibits a preference for the lower branch modes. The effect of blowing and suction on nonlinear stability of the attachment line boundary layer is also investigated.
NASA Astrophysics Data System (ADS)
Keedy, Ryan; Aliseda, Alberto
2016-11-01
Laboratory experiments were performed to understand the effect of viscosity ratio on the development of the round jet when a miscible liquid is injected into another stagnant ambient liquid. Altering the viscosity of the injected liquid jet resulted in noticeable changes in the turbulent/non-turbulent interface in the jet's developing region, including the instability wavelength. The change in the formation of structures at the interface is apparent even when several key non-dimensional numbers (Pe , Re) associated with the flow are kept constant. Large, coherent structures in the turbulent jet resulting from the shear instability of the interface may affect the downstream development of the self-similar profile. Hence, it is important to examine and understand the characteristics of the shear layer instability in order to better understand the role that a viscosity gradient plays in turbulent jet development. The spatial stability equations for a flow in which viscosity varies arbitrarily as a function of scalar concentration are presented. These equations are evaluated at various viscosity ratios and the predicted instability frequencies are compared to experimental results in the range of μjet /μamb = 0 . 5 - 2 and Re 104 . Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's NNSA under contract DE-AC04-94AL85000.
NASA Astrophysics Data System (ADS)
Stelzer, Zacharias; Miralles, Sophie; Cébron, David; Noir, Jérôme; Vantieghem, Stijn; Jackson, Andrew
2015-08-01
We present an investigation of the stability of liquid metal flow under the influence of an imposed magnetic field by means of a laboratory experiment as well as a linear stability analysis of the setup using the finite element method. The experimental device ZUrich Cylindrical CHannel INstability Investigation is a modified cylindrical annulus with electrically driven flow of liquid GaInSn operating at Hartmann and Reynolds numbers up to M = 2022 and Re = 2.6 ṡ 105, respectively. The magnetic field gives rise to a free shear layer at the prominent inner electrode. We identify several flow regimes characterized by the nature of the instabilities. Above a critical current I c = O ( 0 . 1 A ) , the steady flow is destabilized by a Kelvin-Helmholtz mechanism at the free shear layer. The instability consists of counterrotating vortices traveling with the mean flow. For low forcing, the vortices are restricted to the free shear layer. Their azimuthal wave number m grows with M and decreases with Re. At Re/M ≈ 25, the instability becomes container-filling and energetically significant. It enhances the radial momentum transport which manifests itself in a broadening of the free shear layer width δS. We propose that this transition may be related to an unstable Hartmann layer. At R e / M 2 = O ( 1 ) , an abrupt change is observed in the mean azimuthal velocity < u ϕ ¯ > and the friction factor F, which we interpret as the transition between an inertialess and an inertial regime.
Comprehensive experimental and numerical analysis of instability phenomena in pump turbines
NASA Astrophysics Data System (ADS)
Gentner, Ch; Sallaberger, M.; Widmer, Ch; Bobach, B.-J.; Jaberg, H.; Schiffer, J.; Senn, F.; Guggenberger, M.
2014-03-01
The changes in the electricity market have led to changed requirements for the operation of pump turbines. Utilities need to change fast and frequently between pumping and generating modes and increasingly want to operate at off-design conditions for extended periods. Operation of the units in instable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. In turbine operation of pump turbines unstable behaviour can occur at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic). This type of instability may impede the synchronization of the machine in turbine mode and thus increase start-up and switch over times. A pronounced S-shaped instability can also lead to significant drop of discharge in the event of load rejection. Low pressure on the suction side and in the tail-race tunnel could cause dangerous separation of the water column. Understanding the flow features that lead to the instable behaviour of pump turbines is a prerequisite to the design of machines that can fulfil the growing requirements relating to operational flexibility. Flow simulation in these instability zones is demanding due to the complex and highly unsteady flow patterns. Only unsteady simulation methods are able to reproduce the governing physical effects in these operating regions. ANDRITZ HYDRO has been investigating the stability behaviour of pump turbines in turbine operation in cooperation with several universities using simulation and measurements. In order to validate the results of flow simulation of unstable operating points, the Graz University of Technology (Austria) performed detailed experimental investigations. Within the scope of a long term research project, the operating characteristics of several pump turbine runners have been measured and flow patterns in the pump turbine at speed no load and runaway have been examined by 2D Laser particle image velocimetry (PIV
Numerical study of the single-mode Rayleigh-Taylor instability with non-unity Schmidt number
NASA Astrophysics Data System (ADS)
Hutchinson, Maxwell; Rosner, Robert
2014-11-01
Recent experiments and simulations of the single mode Rayleigh-Taylor instability question the assumed existence of a bubble terminal velocity regime, particularly for low Atwood numbers. We present numerical results using the spectral element method and Boussinesq approximation with purely physical viscosity and diffusivity. The Schmidt number is chosen away from unity and boundary conditions are no-slip in an effort to bring the simulations closer to physically realizable conditions. M. Hutchinson acknolwedges the support of the Department of Energy Computational Science Graduate Fellowship.
Direct numerical simulations of type Ia supernovae flames II: The Rayleigh-Taylor instability
Bell, J.B.; Day, M.S.; Rendleman, C.A.; Woosley, S.E.; Zingale, M.
2004-01-12
A Type Ia supernova explosion likely begins as a nuclear runaway near the center of a carbon-oxygen white dwarf. The outward propagating flame is unstable to the Landau-Darrieus, Rayleigh-Taylor, and Kelvin-Helmholtz instabilities, which serve to accelerate it to a large fraction of the speed of sound. We investigate the Rayleigh-Taylor unstable flame at the transition from the flamelet regime to the distributed-burning regime, around densities of 10e7 gm/cc, through detailed, fully resolved simulations. A low Mach number, adaptive mesh hydrodynamics code is used to achieve the necessary resolution and long time scales. As the density is varied, we see a fundamental change in the character of the burning--at the low end of the density range the Rayleigh-Taylor instability dominates the burning, whereas at the high end the burning suppresses the instability. In all cases, significant acceleration of the flame is observed, limited only by the size of the domain we are able to study. We discuss the implications of these results on the potential for a deflagration to detonation transition.
Marxen, Olaf; Magin, Thierry E.; Shaqfeh, Eric S.G.; Iaccarino, Gianluca
2013-12-15
A new numerical method is presented here that allows to consider chemically reacting gases during the direct numerical simulation of a hypersonic fluid flow. The method comprises the direct coupling of a solver for the fluid mechanical model and a library providing the physio-chemical model. The numerical method for the fluid mechanical model integrates the compressible Navier–Stokes equations using an explicit time advancement scheme and high-order finite differences. This Navier–Stokes code can be applied to the investigation of laminar-turbulent transition and boundary-layer instability. The numerical method for the physio-chemical model provides thermodynamic and transport properties for different gases as well as chemical production rates, while here we exclusively consider a five species air mixture. The new method is verified for a number of test cases at Mach 10, including the one-dimensional high-temperature flow downstream of a normal shock, a hypersonic chemical reacting boundary layer in local thermodynamic equilibrium and a hypersonic reacting boundary layer with finite-rate chemistry. We are able to confirm that the diffusion flux plays an important role for a high-temperature boundary layer in local thermodynamic equilibrium. Moreover, we demonstrate that the flow for a case previously considered as a benchmark for the investigation of non-equilibrium chemistry can be regarded as frozen. Finally, the new method is applied to investigate the effect of finite-rate chemistry on boundary layer instability by considering the downstream evolution of a small-amplitude wave and comparing results with those obtained for a frozen gas as well as a gas in local thermodynamic equilibrium.
NASA Astrophysics Data System (ADS)
Marxen, Olaf; Magin, Thierry; Iaccarino, Gianluca; Shaqfeh, Eric S. G.
2011-08-01
Prediction of laminar-turbulent transition is a key factor in the design of the heat shield of vehicles (re-)entering a planetary atmosphere. To investigate the transition by means of numerical simulation, accurate and efficient computational methods are necessary. Here, the compressible Navier-Stokes equations are solved for a gas where properties such as specific heat, thermal conductivity, viscosity, and specific gas constant depend on one or two thermodynamic variables. Our approach models a mixture of perfect gases in local thermodynamic equilibrium. The gas properties are provided either by means of direct calls to a library based on statistical mechanics and kinetic theory or indirectly in the form of look-up tables. In the first part of the paper, our method of handling a high-temperature gas in thermochemical equilibrium is described and verified. In the second part, the method is applied to the investigation of linear and non-linear boundary-layer instability. We carry out numerical simulations for a laminar flat-plate boundary layer at Mach 10 with a small, convectively amplified perturbation for both Earth and Martian atmospheres. Amplification of the perturbations shows favorable agreement with results obtained from linear theory. The secondary instability of the boundary layer in the presence of a large-amplitude two-dimensional wave is investigated. We observe that the non-linear mechanism of fundamental resonance becomes active and leads to a strong increase in amplification of three-dimensional disturbance waves.
NASA Astrophysics Data System (ADS)
Langari, Mostafa; Yang, Zhiyin
2013-07-01
Numerical studies of laminar-to-turbulent transition in a separation bubble subjected to two free-stream turbulence levels (FST) have been performed using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a plate with a semi-circular leading edge at Re = 3450 based on the plate thickness and the uniform inlet velocity. A numerical trip is used to produce the targeted free-stream turbulence levels and the decay of free-stream turbulence is well predicted. A dynamic sub-grid-scale model is employed in the current study and a good agreement has been obtained between the LES results and the experimental data. Detailed analysis of the LES data has been carried out to investigate the primary instability mechanism. The flow visualisations and spectral analysis of the separated shear layer reveal that the 2D Kelvin-Helmholtz instability mode, well known to occur at low FST levels, is bypassed at higher levels leading to earlier breakdown to turbulence.
Numerical simulation of bump-on-tail instability with source and sink
Berk, H.L.; Pekker, M.; Breizman, B.N. |
1995-02-01
This paper presents results of the simulations of the bump-on-tail instability with a weak source and sink. This problem has been posed as a paradigm for the important problem in controlled fusion, that of the unstable excitation of Alfven waves in a tokamak by resonant energetic alpha particles. The source of alpha particles is the controlled fusion reaction produced by the background plasma and the sink is the collisional transport processes that slow down or scatter the energetic particles. The mathematical techniques that are needed to address this applied problem can be demonstrated in the much simpler bump-on-tail problem, which is explained in this paper.
Gas Evolution Dynamics in Godunov-Type Schemes and Analysis of Numerical Shock Instability
NASA Technical Reports Server (NTRS)
Xu, Kun
1999-01-01
In this paper we are going to study the gas evolution dynamics of the exact and approximate Riemann solvers, e.g., the Flux Vector Splitting (FVS) and the Flux Difference Splitting (FDS) schemes. Since the FVS scheme and the Kinetic Flux Vector Splitting (KFVS) scheme have the same physical mechanism and similar flux function, based on the analysis of the discretized KFVS scheme the weakness and advantage of the FVS scheme are closely observed. The subtle dissipative mechanism of the Godunov method in the 2D case is also analyzed, and the physical reason for shock instability, i.e., carbuncle phenomena and odd-even decoupling, is presented.
Theoretical and numerical investigation of diffusive instabilities in multi-component alloys
NASA Astrophysics Data System (ADS)
Lahiri, Arka; Choudhury, Abhik
2017-02-01
Diffusive instabilities of the Mullins-Sekerka type are one of the principal mechanisms through which microstructures form during solidification. In this study, we perform a linear stability analysis for the perturbation of a planar interface, where we derive analytical expressions to characterize the dispersion behavior in multi-component alloys under directional and isothermal solidification conditions. Subsequently, we confirm our calculations using phase-field simulations for different choices of the inter-diffusivity matrices. Thereafter, we highlight the characteristics of the dispersion curves upon change of the diffusivity matrix and the velocity. Finally, we also depict conditions for absolute stability of a planar interface under directional solidification conditions.
Numerical simulations of Rayleigh-Taylor instability in non-premixed flames using detailed chemistry
NASA Astrophysics Data System (ADS)
Attal, Nitesh; Ramaprabhu, Praveen
2016-11-01
The Rayleigh-Taylor (RT) instability occurs at a perturbed interface separating fluids of different densities, when the lighter fluid accelerates the heavier fluid. We examine the occurrence of the RT instability, when the perturbed interface demarcates a light, fuel stream from a heavier air stream at elevated temperatures. The study is conducted using the FLASH code with modifications that include detailed chemistry, temperature-dependent EOS, and diffusive transport. The fuel-air interface is initialized at thermal equilibrium (Tfuel = Tair = 1000K) in a constant background acceleration (g). We vary the density difference across the interface by diluting the H2 fuel stream with inert N2. The non-premixed flame formed across a burning interface alters the underlying density (ρ) stratification, so that an initially RT unstable (stable) interface can be locally stabilized (destabilized). We observe this change in local stability for both single-wavelength and multimode perturbations, and draw some conclusions on the implications of these findings to applications such as ultra-compact combustors. We also make some comparisons of the reacting, non-premixed RT problem with the corresponding inert flow.
NASA Astrophysics Data System (ADS)
Lee, S. Y.; Lee, E.; Kim, K. H.; Lee, D. H.; Seon, J.; Jin, H.
2016-12-01
We investigate effects of magnetic inhomogeneity on the development of whistler instability using a two-dimensional relativistic electromagnetic particle-in-cell (PIC) code. Whistler instability is generated from electron temperature anisotropy, Tperp/Tpara > 1. We apply a gradient of magnetic field intensity perpendicular to background magnetic field. The perpendicular magnetic inhomogeneity with a constant thermal velocity of electrons yields a broad range of electron plasma beta, βe, which determines wave frequency, wave normal angle, and some other important properties of the excited whistler waves. As a result, the waves are excited with a broad range of frequencies and wave normal angles along the perpendicular direction. Interestingly, the whistler waves are separated into two frequency bands at 0.5Ωce, where Ωce is electron gyro-frequency, which is similar to the banded chorus waves observed in the magnetosphere. For the upper band whistler, the wave normal angle is relatively large and the wave power is weak. For the lower band whistler, on the other hand, the wave normal angle is very small and the wave power is strong.
Kelley, M.C.; Seyler, C.E.; Zargham, S. )
1987-09-01
A two-dimensional model applicable to F region ionosphere plasma instabilities has been developed and described in a comparison paper. Here the authors apply the model to equatorial F region irregularities and in particular test the model against rocket and satellite data. As a diagnostic they create simulated data sets similar to the one-dimensional measurements of plasma density performed by space probes and take the Fourier transform of these data in the same manner and used by the space experimentors. To their knowledge this is the first attempt to directly relate a simulation to in situ data in this way, diagnostic technique which resolves the ambiguity inherent to one-dimensional measurements. The comparison is in excellent agreement with the in situ data. In particular, they have been able to resolve the apparent differences between satellite quasi-sinusoidal observations with the shocklike observations made on rockets. Unlike previous simulations of this phenomenon they find an inherent anisotropy in the instability development which is mirrored in the in situ data. They also present evidence that the shallow spectral slopes which often characterize spread F rocket spectra near the F peak may be due to a change in the angle between the rocket velocity vector and the characteristic directions in the medium.
Matsuoka, Chihiro; Nishihara, Katsunobu
2006-12-01
Motion of a fluid interface in the Richtmyer-Meshkov instability in cylindrical geometry is examined analytically and numerically. Nonlinear stability analysis is performed in order to clarify the dependence of growth rates of a bubble and spike on the Atwood number and mode number n involved in the initial perturbations. We discuss differences of weakly and fully nonlinear evolution in cylindrical geometry from that in planar geometry. It is shown that the analytical growth rates coincide well with the numerical ones up to the neighborhood of the break down of numerical computations. Long-time behavior of the fluid interface as a vortex sheet is numerically investigated by using the vortex method and the roll up of the vortex sheet is discussed for different Atwood numbers. The temporal evolution of the curvature of a bubble and spike for several mode numbers is investigated and presented that the curvature of spikes is always larger than that of bubbles. The circulation and the strength of the vortex sheet at the fully nonlinear stage are discussed, and it is shown that their behavior is different for the cases that the inner fluid is heavier than the outer one and vice versa.
Defect dependent multiple magnetization plateaus in frustrated spin-chain cobaltate
NASA Astrophysics Data System (ADS)
Hu, Yong; Wu, Guozhen; Liu, Yan; Yang, Xiaolong; Du, An
2013-07-01
Monte Carlo techniques based on the Glauber and Metropolis algorithms are performed to simulate the influence of nonmagnetic defects on the low-temperature magnetization plateaus in Ca3Co2O6. We combine the microscopic spin-chain energy distributions with the conventional patterns and obtain unambiguously the one-to-one correspondence between magnetization plateaus and microscopic spin-chain configurations. Numerical results indicate that on the basis of the energy competition, nonmagnetic defects in a proper concentration range can activate the new diverse nearest-neighboring configurational cells of spin chains in a microscopic scale, inducing more magnetization plateaus with different heights. Interestingly, this phenomenon is still observed at a relatively high temperature because a little dilution can not only lift the degeneracy of ground state greatly but also prevent the form of commensurate ferrimagnetic state. Theoretical investigation provides a potential for governing such plateau behaviors and this will cast more light on the understanding of multiple magnetization-plateau phenomena observed in the materials with geometrical frustration.
A 3D Numerical Study of Gravitational Instabilities in Young Circumbinary Disks
NASA Astrophysics Data System (ADS)
Cai, Kai; Michael, Scott; Durisen, Richard
2013-07-01
Gravitational instabilities (GIs) in protoplanetary disks have been suggested as one of the major formation mechanisms of giant planets. Theoretical and computational studies have indicated that certain family of GIs can be excited in a circumbinary disk, which could lead to enhanced protoplanet formation (e.g., Sellwood & Lin 1989, Boss 2006). We have carried out a 3D simulation of a gravitationally unstable circumbinary disk around a young Sun-like star and a 0.02-Msun companion, both inside the central hole of the disk. Here we present a preliminary comparison between this simulation and a similarly simulated circumstellar disk around a solar-mass star but without the low-mass companion. The GIs stimulated by the binary and those that arise spontaneously are quite different in structure and strength. However, no fragmentation is observed, even after many orbital periods as measured in the outer disk.
Aslangil, Denis; Banerjee, Arindam; Lawrie, Andrew G W
2016-11-01
The influence of initial conditions on miscible incompressible baroclinically driven Rayleigh-Taylor instability undergoing nonuniform acceleration is explored computationally using an implicit large eddy simulation (ILES) technique. We consider the particular case of evolution during multiple reversals of acceleration direction, where the flow is alternately statically stable or unstable. In the unstable phase, the flow is driven by the baroclinic release of potential energy, whereas in the stable phase, work is done against the density stratification with the energy exchange taking place by wavelike mechanisms. These dynamics are fundamentally different; here, we track the evolution of volume-averaged turbulent statistics that are most sensitive to changes in the distribution of spectral power and bandwidth of the initial conditions as the flow alternates between dynamical regimes due to acceleration reversal.
NASA Astrophysics Data System (ADS)
Aslangil, Denis; Banerjee, Arindam; Lawrie, Andrew G. W.
2016-11-01
The influence of initial conditions on miscible incompressible baroclinically driven Rayleigh-Taylor instability undergoing nonuniform acceleration is explored computationally using an implicit large eddy simulation (ILES) technique. We consider the particular case of evolution during multiple reversals of acceleration direction, where the flow is alternately statically stable or unstable. In the unstable phase, the flow is driven by the baroclinic release of potential energy, whereas in the stable phase, work is done against the density stratification with the energy exchange taking place by wavelike mechanisms. These dynamics are fundamentally different; here, we track the evolution of volume-averaged turbulent statistics that are most sensitive to changes in the distribution of spectral power and bandwidth of the initial conditions as the flow alternates between dynamical regimes due to acceleration reversal.
Fick, Lambert H.; Merzari, Elia; Hassan, Yassin A.
2017-02-20
Computational analyses of fluid flow through packed pebble bed domains using the Reynolds-averaged NavierStokes framework have had limited success in the past. Because of a lack of high-fidelity experimental or computational data, optimization of Reynolds-averaged closure models for these geometries has not been extensively developed. In the present study, direct numerical simulation was employed to develop a high-fidelity database that can be used for optimizing Reynolds-averaged closure models for pebble bed flows. A face-centered cubic domain with periodic boundaries was used. Flow was simulated at a Reynolds number of 9308 and cross-verified by using available quasi-DNS data. During the simulations,more » low-frequency instability modes were observed that affected the stationary solution. Furthermore, these instabilities were investigated by using the method of proper orthogonal decomposition, and a correlation was found between the time-dependent asymmetry of the averaged velocity profile data and the behavior of the highest energy eigenmodes.« less
Avara, Mark J.; Reynolds, Christopher S.; Bogdanovic, Tamara E-mail: chris@astro.umd.edu
2013-08-20
The role played by magnetic fields in the intracluster medium (ICM) of galaxy clusters is complex. The weakly collisional nature of the ICM leads to thermal conduction that is channeled along field lines. This anisotropic heat conduction profoundly changes the instabilities of the ICM atmosphere, with convective stabilities being driven by temperature gradients of either sign. Here, we employ the Athena magnetohydrodynamic code to investigate the local non-linear behavior of the heat-flux-driven buoyancy instability (HBI) relevant in the cores of cooling-core clusters where the temperature increases with radius. We study a grid of two-dimensional simulations that span a large range of initial magnetic field strengths and numerical resolutions. For very weak initial fields, we recover the previously known result that the HBI wraps the field in the horizontal direction, thereby shutting off the heat flux. However, we find that simulations that begin with intermediate initial field strengths have a qualitatively different behavior, forming HBI-stable filaments that resist field-line wrapping and enable sustained vertical conductive heat flux at a level of 10%-25% of the Spitzer value. While astrophysical conclusions regarding the role of conduction in cooling cores require detailed global models, our local study proves that systems dominated by the HBI do not necessarily quench the conductive heat flux.
Ng, E Y; Sudharsan, N M
2001-01-01
The use of engineering in biomedical sciences has opened new facets in research. The present paper deals with problems arising from numerical simulation, in order to develop an expert system for the diagnosis of breast cancer using thermography. A female breast is modelled in three dimensions and the surface temperature pattern is obtained by solving the bioheat equation. This solution will be used in conjunction with a database of thermograms to develop an intelligent diagnostic tool. The focus of the present paper is to build and check the confidence level of the numerical scheme before proceeding to model the actual problem. The parametric study is done along with a check for mesh insensitivity and wiggle free isotherm contours. This process yielded a benchmark nodal distance, with which the 3D model is generated and isotherm pattern analysed. It can be seen that this enhances the accuracy of the surface temperature distribution. The use of this is tested in a close-to-actual numerical breast model and the results compared with the thermographic results. The outcome is very encouraging. Finally, a typical clinical protocol in conjunction with the use of numerical prediction for breast thermographic interpretation is outlined.
NASA Astrophysics Data System (ADS)
Sharma, B. S.; Jaiman, N. K.
2009-02-01
In this paper, we have numerically investigated the effects of various geometrical parameters of a backward wave oscillator, filled with a magnetized plasma of uniform density and driven by a mild relativistic solid electron beam, on the instability growth rate R 0 of a seeded free electron laser. On changing mean radius corrugation amplitude h and corrugation period z 0 of backward wave oscillator; the ponderomotive potential of space charge wave changes. This in turn, changes the coupling strength of TM mode with negative beam space charge mode and hence the growth rate of parametric instability of free electron laser. A dispersion relation is derived and numerically solved for various geometrical parameters of backward wave oscillator and beam profile. A relation for Γ is also derived and computed numerically. The instability growth scales directly to the square root of beam density and inversely as seven power of relativistic gamma factor γ0.
Hindi, Haitham; Prabhakar, Shyam; Fox, John D.; Linscott, Ivan; Teytelman, Dmitri; /SLAC
2011-08-31
We present a technique for the design and verification of efficient bunch-by-bunch controllers for damping longitudinal multibunch instabilities. The controllers attempt to optimize the use of available feedback amplifier power - one of the most expensive components of a feedback system - and define the limits of the closed loop system performance. Our design technique alternates between analytic computation of single bunch optimal controllers and verification on a multibunch numerical simulator. The simulator uses PEP-II parameters and identifies unstable coupled bunch modes, their growth rates and their damping rates with feedback. The results from the simulator are shown to be in reasonable agreement with analytical calculations based on the single bunch model. The technique is then used to evaluate the performance of a variety of controllers proposed for PEP-II.
NASA Astrophysics Data System (ADS)
Nick, Faezeh M.; Hubbard, Alun; van der Veen, Kees; Vieli, Andreas
2010-05-01
Calving of icebergs and bottom melting from ice shelves accounts for roughly half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. Petermann Glacier (north Greenland) with its 16 km wide and 80 km long floating tongue, experiences massive bottom melting. We apply a numerical ice flow model using a physically-based calving criterion based on crevasse depth to investigate the contribution of processes such as bottom melting, sea ice or sikkusak disintegration, surface run off and iceberg calving to the mass balance and instability of Petermann Glacier and its ice shelf. Our modelling study provides insights into the role of ice-ocean interaction, and on how to incorporate calving in ice sheet models, improving our ability to predict future ice sheet change.
NASA Astrophysics Data System (ADS)
Nick, F.; Hubbard, A.; Vieli, A.; van der Veen, C. J.; Box, J. E.; Bates, R.; Luckman, A. J.
2009-12-01
Calving of icebergs and bottom melting from ice shelves accounts for roughly half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. Petermann Glacier (north Greenland) with its 16 km wide and 80 km long floating tongue, experiences massive bottom melting. We apply a numerical ice flow model using a physically-based calving criterion based on crevasse depth to investigate the contribution of processes such as bottom melting, sea ice or sikkusak disintegration, surface run off and iceberg calving to the mass balance and instability of Petermann Glacier and its ice shelf. Our modeling study provides insights into the role of ice-ocean interaction, and on how to incorporate calving in ice sheet models, improving our ability to predict future ice sheet change.
NASA Astrophysics Data System (ADS)
Gilson, Erik; Caspary, Kyle; Ebrahimi, Fatima; Goodman, Jeremy; Ji, Hantao; Nuñez, Tahiri; Wei, Xing
2016-10-01
The liquid metal magnetorotational instability experiment at PPPL is designed to search for the MRI in a controlled laboratory setup. MRI is thought to be the primary mechanism behind turbulence in accretion disks, leading to an enhanced effective viscosity that can explain observed fast accretion rates. The apparatus has several key differences from an accretion disk. The top and bottom surfaces of the vessel exert stresses on the surfaces of the working fluid. There are no surface stresses on an accretion disk, but rather a free-surface. To interpret experimental results, the Spectral Finite Element Maxwell and Navier Stokes (SFEMaNS) code (Guermond et al., 2009) has been used to simulate experiments in the MRI apparatus and study MRI onset in the presence of residual flows induced by the boundaries. These Ekman flows lead to the generation of radial magnetic fields that can obfuscate the MRI signal. Simulation results are presented that show the full spatial distribution of the velocity field and the magnetic field over a range of experimental operating parameters, including both above and below the expected MRI threshold. Both the residual flow and the radial magnetic field at the location of the diagnostics are computed for comparisons with experimental results. This research is supported by DOE, NSF, and NASA.
Numerical 3D Hydrodynamics Study of Gravitational Instabilities in a Circumbinary Disk
NASA Astrophysics Data System (ADS)
Desai, Karna Mahadev; Steiman-Cameron, Thomas Y.; Michael, Scott; Cai, Kai; Durisen, Richard H.
2016-01-01
We present a 3D hydrodynamical study of gravitational instabilities (GIs) in a circumbinary protoplanetary disk around a Solar mass star and a brown dwarf companion (0.02 M⊙). GIs can play an important, and at times dominant, role in driving the structural evolution of protoplanetary disks. The reported simulations were performed employing CHYMERA, a radiative 3D hydrodynamics code developed by the Indiana University Hydrodynamics Group. The simulations include disk self-gravity and radiative cooling governed by realistic dust opacities. We examine the role of GIs in modulating the thermodynamic state of the disks, and determine the strengths of GI-induced density waves, non-axisymmetric density structures, radial mass transport, and gravitational torques. The principal goal of this study is to determine how the presence of the companion affects the nature and strength of GIs. Results are compared with a parallel simulation of a protoplanetary disk without the presence of the brown dwarf binary companion. We detect no fragmentation in either disk. A persistent vortex forms in the inner region of both disks. The vortex seems to be stabilized by the presence of the binary companion.
Numerical Simulation of Multi-Material Mixing in an Inclined Interface Richtmyer-Meshkov Instability
NASA Astrophysics Data System (ADS)
Subramaniam, Akshay; Lele, Sanjiva
2015-11-01
The Richtmyer-Meshkov instability arises when a shock wave interacts with an interface separating two fluids. In this work, high fidelity simulations of shock induced multi-material mixing between N2 and CO2 in a shock tube are performed for a Mach 1.55 shock interacting with a planar material interface that is inclined with respect to the shock propagation direction. In the current configuration, unlike the classical perturbed flat interface case, the evolution of the interface is non-linear from early time onwards. Our previous simulations of this problem at multiple spatial resolutions have shown that very small 3D perturbations have a large effect on vortex breakdown mechanisms and hence fine scale turbulence. We propose a comparison of our simulations to the experiments performed at the Georgia Tech Shock Tube and Advanced Mixing Laboratory (STAML). Results before and after reshock of the interface will be shown. Results from simulations of a second case with a more complex initial interface will also be presented. Simulations shown are conducted with an extended version of the Miranda solver developed by Cook et al. (2007) which combines high-order compact finite differences with localized non-linear artificial properties for shock and interface capturing. This research is part of the Blue Waters sustained-petascale computing project, which is supported by the National Science Foundation (awards OCI-0725070 and ACI-1238993) and the state of Illinois.
NASA Astrophysics Data System (ADS)
Forbes, John C.
Using 0D, 1D, and 3D models of galaxies, I explore different problems in galaxy evolution most suited to each technique. In the simplest case, a galaxy is described by a few numbers integrated via coupled ordinary differential equations. By allowing the galaxies to respond to a stochastic accretion rate, I show a natural way of generating the finite scatter observed in several galaxy scaling relations: the correlation between a galaxy's stellar mass and its star formation rate or metallicity. By comparing this simple model to observations, we constrain the process by which gas accretes onto galaxies, which must occur, but is essentially impossible to observe directly. Adding an additional dimension to the models, we explore the structure of galactic disks as a function of radius. We find that turbulence driven by gravitational instability in the disks and the resulting migration of gas can explain a wide variety of phenomena, including the age-velocity dispersion correlation of stars in the solar neighborhood, the central quenching star formation in disk galaxies, rings of star formation, and the observed radial profile of gas column densities. Finally, we run a set of fully three-dimensional galaxy simulations to try to understand what physics is responsible for basic properties of galaxies, including the rate at which they form stars, and the rate at which they eject mass in large-scale winds. We find that supernovae are capable of driving moderate metal-enhanced winds, but surprisingly they have very little effect on the star formation rates of dwarf galaxies. Instead, ordinary photoelectric heating dominates the star formation law in low-mass galaxies.
NASA Astrophysics Data System (ADS)
Chen, Xiao; Dong, Gang; Jiang, Hua
2017-03-01
The instabilities of a three-dimensional sinusoidally premixed flame induced by an incident shock wave with Mach = 1.7 and its reshock waves were studied by using the Navier-Stokes (NS) equations with a single-step chemical reaction and a high resolution, 9th-order weighted essentially non-oscillatory scheme. The computational results were validated by the grid independence test and the experimental results in the literature. The computational results show that after the passage of incident shock wave the flame interface develops in symmetric structure accompanied by large-scale transverse vortex structures. After the interactions by successive reshock waves, the flame interface is gradually destabilized and broken up, and the large-scale vortex structures are gradually transformed into small-scale vortex structures. The small-scale vortices tend to be isotropic later. The results also reveal that the evolution of the flame interface is affected by both mixing process and chemical reaction. In order to identify the relationship between the mixing and the chemical reaction, a dimensionless parameter, η , that is defined as the ratio of mixing time scale to chemical reaction time scale, is introduced. It is found that at each interaction stage the effect of chemical reaction is enhanced with time. The enhanced effect of chemical reaction at the interaction stage by incident shock wave is greater than that at the interaction stages by reshock waves. The result suggests that the parameter η can reasonably character the features of flame interface development induced by the multiple shock waves.
NASA Astrophysics Data System (ADS)
Chen, Xiao; Dong, Gang; Jiang, Hua
2017-04-01
The instabilities of a three-dimensional sinusoidally premixed flame induced by an incident shock wave with Mach = 1.7 and its reshock waves were studied by using the Navier-Stokes (NS) equations with a single-step chemical reaction and a high resolution, 9th-order weighted essentially non-oscillatory scheme. The computational results were validated by the grid independence test and the experimental results in the literature. The computational results show that after the passage of incident shock wave the flame interface develops in symmetric structure accompanied by large-scale transverse vortex structures. After the interactions by successive reshock waves, the flame interface is gradually destabilized and broken up, and the large-scale vortex structures are gradually transformed into small-scale vortex structures. The small-scale vortices tend to be isotropic later. The results also reveal that the evolution of the flame interface is affected by both mixing process and chemical reaction. In order to identify the relationship between the mixing and the chemical reaction, a dimensionless parameter, η , that is defined as the ratio of mixing time scale to chemical reaction time scale, is introduced. It is found that at each interaction stage the effect of chemical reaction is enhanced with time. The enhanced effect of chemical reaction at the interaction stage by incident shock wave is greater than that at the interaction stages by reshock waves. The result suggests that the parameter η can reasonably character the features of flame interface development induced by the multiple shock waves.
Physical aquatic habitat assessment data, Ozark plateaus, Missouri and Arkansas
Jacobson, Robert B.; Johnson, Harold E.; Reuter, Joanna M.; Wright, Maria Panfil
2004-01-01
This report presents data from two related studies on physical habitat in small streams in the Ozark Plateaus Physiographic Province of Missouri and Arkansas. Seventy stream reaches and their contributing drainage basins were assessed using a physical habitat protocol designed to optimize understanding of how stream reach characteristics relate to drainage-basin characteristics. Drainage-basin characteristics were evaluated using geographic information system (GIS) techniques and datasets designed to evaluate the geologic, physiographic, and land-use characteristics of encompassing drainage basins. Reach characteristics were evaluated using a field-based geomorphology and habitat protocol. The data are intended to complement ecological studies on Ozark Plateaus streams.
Numerical simulation of multi-material mixing in an inclined interface Richtmyer-Meshkov instability
NASA Astrophysics Data System (ADS)
Subramaniam, Akshay; Lele, Sanjiva K.
2017-01-01
In this work, high fidelity simulations of shock induced multi-material mixing between air and SF6 in a shock tube are performed for a Mach 1.5 shock interacting with a planar material interface that is inclined with respect to the shock propagating direction. In the current configuration, unlike the classical perturbed flat interface case, the evolution of the interface is fully non-linear from early time. The simulations attempt to replicate an experiment conducted at the Georgia Tech STAML. Tight coupling between numerics and flow physics and the large range of spatial scales make this a challenging problem to simulate numerically. Often, two dimensional simulations are performed to reduce the computational cost of these simulations. We show here that the effect of small three dimensional perturbations likely to be present in an experimental setting is not negligible. Full 3D simulations would have to be performed to do a proper comparison with experiments. Effect of grid resolution is also studied in the present work. Simulations shown are conducted with an extended version of the Miranda solver developed by Cook et. al [1] which combines high-order compact finite differences [2] with localized non-linear artificial properties for shock and interface capturing [3].
Numerical Study of Three-dimensional Spatial Instability of a Supersonic Flat Plate Boundary Layer
NASA Technical Reports Server (NTRS)
Maestrello, Lucio; Bayliss, A.; Krishnan, R.
1989-01-01
The behavior of spatially growing three-dimensional waves in a supersonic boundary layer was studied numerically by solving the complete Navier-Stokes equations. Satisfactory comparison with linear parallel and non-parallel stability theories, and experiment are obtained when a small amplitude inflow disturbance is used. The three-dimensional unsteady Navier-Stokes equations are solved by a finite difference method which is fourth-order and second-order accurate in the convection and viscous terms respectively, and second-order accurate in time. Spanwise periodicity is assumed. The inflow disturbance is composed of eigenfunctions from linear stability theory. By increasing the amplitude of the inflow disturbance, nonlinear effects in the form of a relaxation type oscillation of the time signal of rho(u) are observed.
Numerical study of instability of nanofluids: the coagulation effect and sedimentation effect.
Ni, Yu; Fan, Jianren; Hu, Yacai
2011-02-28
This study is a numerical study on the coagulation as well as the sedimentation effect of nanofluids using the Brownian dynamics method. Three cases are simulated, focusing on the effects of the sizes, volume fraction, and ζ potentials of nano-particles on the formation of coagulation and sedimentation of nanofluids. The rms fluctuation of the particle number concentration, as well as the flatness factor of it, is employed to study the formation and variation of the coagulation process. The results indicate a superposition of coagulation and sedimentation effect of small nano-particles. Moreover, it is stable of nanofluids with the volume fraction of particles below the limit of "resolution" of the fluids. In addition, the effect of ζ potentials is against the formation of coagulation and positive to the stability of nanofluids.
NASA Astrophysics Data System (ADS)
Scerrato, Daria; Giorgio, Ivan; Rizzi, Nicola Luigi
2016-06-01
In this paper, we determine numerically a large class of equilibrium configurations of an elastic two-dimensional continuous pantographic sheet in three-dimensional deformation consisting of two families of fibers which are parabolic prior to deformation. The fibers are assumed (1) to be continuously distributed over the sample, (2) to be endowed of bending and torsional stiffnesses, and (3) tied together at their points of intersection to avoid relative slipping by means of internal (elastic) pivots. This last condition characterizes the system as a pantographic lattice (Alibert and Della Corte in Zeitschrift für angewandte Mathematik und Physik 66(5):2855-2870, 2015; Alibert et al. in Math Mech Solids 8(1):51-73, 2003; dell'Isola et al. in Int J Non-Linear Mech 80:200-208, 2016; Int J Solids Struct 81:1-12, 2016). The model that we employ here, developed by Steigmann and dell'Isola (Acta Mech Sin 31(3):373-382, 2015) and first investigated in Giorgio et al. (Comptes rendus Mecanique 2016, doi: 10.1016/j.crme.2016.02.009), is applicable to fiber lattices in which three-dimensional bending, twisting, and stretching are significant as well as a resistance to shear distortion, i.e., to the angle change between the fibers. Some relevant numerical examples are exhibited in order to highlight the main features of the model adopted: In particular, buckling and post-buckling behaviors of pantographic parabolic lattices are investigated. The fabric of the metamaterial presented in this paper has been conceived to resist more effectively in the extensional bias tests by storing more elastic bending energy and less energy in the deformation of elastic pivots: A comparison with a fabric constituted by beams which are straight in the reference configuration shows that the proposed concept is promising.
NASA Astrophysics Data System (ADS)
Taffetani, M.; Ciarletta, P.
2015-08-01
Soft cylindrical gels can develop a long-wavelength peristaltic pattern driven by a competition between surface tension and bulk elastic energy. In contrast to the Rayleigh-Plateau instability for viscous fluids, the macroscopic shape in soft solids evolves toward a stable beading, which strongly differs from the buckling arising in compressed elastic cylinders. This work proposes a novel theoretical and numerical approach for studying the onset and the non-linear development of the elasto-capillary beading in soft cylinders, made of neo-Hookean hyperelastic material with capillary energy at the free surface, subjected to axial stretch. Both a theoretical study, deriving the linear and the weakly non-linear stability analyses for the problem, and numerical simulations, investigating the fully non-linear evolution of the beaded morphology, are performed. The theoretical results prove that an axial elongation can not only favour the onset of beading, but also determine the nature of the elastic bifurcation. The fully non-linear phase diagrams of the beading are also derived from finite element numerical simulations, showing two peculiar morphological transitions when varying either the axial stretch or the material properties of the gel. Since the bifurcation is found to be subcritical for very slender cylinders, an imperfection sensitivity analysis is finally performed. In this case, it is shown that a surface sinusoidal imperfection can resonate with the corresponding marginally stable solution, thus selecting the emerging beading wavelength. In conclusion, the results of this study provide novel guidelines for controlling the beaded morphology in different experimental conditions, with important applications in micro-fabrication techniques, such as electrospun fibres.
Numerical study of chiral plasma instability within the classical statistical field theory approach
NASA Astrophysics Data System (ADS)
Buividovich, P. V.; Ulybyshev, M. V.
2016-07-01
We report on a numerical study of real-time dynamics of electromagnetically interacting chirally imbalanced lattice Dirac fermions within the classical statistical field theory approach. Namely, we perform exact simulations of the real-time quantum evolution of fermionic fields coupled to classical electromagnetic fields, which are in turn coupled to the vacuum expectation value of the fermionic electric current. We use Wilson-Dirac Hamiltonian for fermions, and noncompact action for the gauge field. In general, we observe that the backreaction of fermions on the electromagnetic field prevents the system from acquiring chirality imbalance. In the case of chirality pumping in parallel electric and magnetic fields, the electric field is screened by the produced on-shell fermions and the accumulation of chirality is hence stopped. In the case of evolution with initially present chirality imbalance, axial charge tends to transform to helicity of the electromagnetic field. By performing simulations on large lattices we show that in most cases this decay process is accompanied by the inverse cascade phenomenon, which transfers energy from short-wavelength to long-wavelength electromagnetic fields. In some simulations, however, we observe a very clear signature of inverse cascade for the helical magnetic fields that is not accompanied by the axial charge decay. This suggests that the relation between the inverse cascade and axial charge decay is not as straightforward as predicted by the simplest form of anomalous Maxwell equations.
NASA Astrophysics Data System (ADS)
Fernandez, Naiara; Kaus, Boris J. P.
2015-08-01
Many salt diapirs are thought to have formed as a result of down-building, which implies that the top of the diapir remained close to the surface during syn-halokinetic sediment deposition. Down-building is largely a 3-D process and in order to better understand what controls the patterns of the diapirs that form by this process, we here perform 3-D numerical models of down-built diapirs initiated by the gravity instability in linear viscous materials and compare the results with analytical models. We vary several parameters of the numerical models such as initial salt thickness, sedimentation rate, salt viscosity, salt-sediment viscosity ratio as well as the density of sediments. Down-building of 3-D diapirs only occurs for a certain range of parameters and is favoured by lower sediment/salt viscosity contrasts and sedimentation rates in agreement with analytical predictions and findings from previous 2-D models. However, the models show that the sedimentation rate has an additional effect on the formation and evolution of 3-D diapir patterns. At low sedimentation rates, salt ridges that form during early model stages remain preserved at later stages as well. For higher sedimentation rates, the initial salt ridges are covered up and finger-like diapirs form at their junctions, which results in different salt exposure patterns at the surface. Once the initial pattern of diapirs is formed, higher sedimentation rate can also result in covered diapirs if the diapir extrusion velocity is insufficiently large. We quantify the effect of sedimentation rate on the number of diapirs exposed at the surface as well as on their spacing and we explain the observations with analytical predictions using thick-plate analytical models. In some cases, this final pattern is distinctly different from the initial polygonal pattern.
NASA Astrophysics Data System (ADS)
Tritschler, V. K.; Hu, X. Y.; Hickel, S.; Adams, N. A.
2013-07-01
Two-dimensional simulations of the single-mode Richtmyer-Meshkov instability (RMI) are conducted and compared to experimental results of Jacobs and Krivets (2005 Phys. Fluids 17 034105). The employed adaptive central-upwind sixth-order weighted essentially non-oscillatory (WENO) scheme (Hu et al 2010 J. Comput. Phys. 229 8952-65) introduces only very small numerical dissipation while preserving the good shock-capturing properties of other standard WENO schemes. Hence, it is well suited for simulations with both small-scale features and strong gradients. A generalized Roe average is proposed to make the multicomponent model of Shyue (1998 J. Comput. Phys. 142 208-42) suitable for high-order accurate reconstruction schemes. A first sequence of single-fluid simulations is conducted and compared to the experiment. We find that the WENO-CU6 method better resolves small-scale structures, leading to earlier symmetry breaking and increased mixing. The first simulation, however, fails to correctly predict the global characteristic structures of the RMI. This is due to a mismatch of the post-shock parameters in single-fluid simulations when the pre-shock states are matched with the experiment. When the post-shock parameters are matched, much better agreement with the experimental data is achieved. In a sequence of multifluid simulations, the uncertainty in the density gradient associated with transition between the fluids is assessed. Thereby the multifluid simulations show a considerable improvement over the single-fluid simulations.
Livescu, D
2013-11-28
A tentative review is presented of various approaches for numerical simulations of two-fluid gaseous mixtures at high density ratios, as they have been applied to the Rayleigh-Taylor instability (RTI). Systems exhibiting such RTI behaviour extend from atomistic sizes to scales where the continuum approximation becomes valid. Each level of description can fit into a hierarchy of theoretical models and the governing equations appropriate for each model, with their assumptions, are presented. In particular, because the compressible to incompressible limit of the Navier-Stokes equations is not unique and understanding compressibility effects in the RTI critically depends on having the appropriate basis for comparison, two relevant incompressible limits are presented. One of these limits has not been considered before. Recent results from RTI simulations, spanning the levels of description presented, are reviewed in connection to the material mixing problem. Owing to the computational limitations, most in-depth RTI results have been obtained for the incompressible case. Two such results, concerning the asymmetry of the mixing and small-scale anisotropy anomaly, as well as the possibility of a mixing transition in the RTI, are surveyed. New lines for further investigation are suggested and it is hoped that bringing together such diverse levels of description may provide new ideas and increased motivation for studying such flows.
Epistasis-Induced Evolutionary Plateaus in Selection Responses.
Le Rouzic, Arnaud; Álvarez-Castro, José M
2016-12-01
Understanding and predicting evolution is a central challenge in both population and quantitative genetics. The amount of genetic variance for quantitative traits available in a population conditions the particular way in which this population will (or will not) evolve under natural or artificial selection. Here, we explore the potential of gene-gene interactions (epistasis) to induce evolutionary plateaus at which evolutionary change virtually collapses for a number of generations, followed by the release of previously cryptic genetic variation. First, we demonstrate theoretically that a wide range of epistatic interactions has the potential to generate temporary decelerations in the course of response to selection. Second, we perform simulations to show that such microevolutionary plateaus may occur in selection responses under empirically based assumptions. Finally, we show that such events can be traced in artificial selection experiments, thus providing further empirical evidence for this phenomenon.
Continental accretion: From oceanic plateaus to allochthonous terranes
Ben-Avraham, Z.; Nur, A.; Jones, D.; Cox, A.
1981-01-01
Some of the regions of the anomalously high sea-floor topography in today's oceans may be modern allochthonous terranes moving with their oceanic plates. Fated to collide with and be accreted to adjacent continents, they may create complex volcanism, cut off and trap oceanic crust, and cause orogenic deformation. The accretion of plateaus during subduction of oceanic plates may be responsible for mountain building comparable to that produced by the collision of continents. Copyright ?? 1981 AAAS.
Polycyclic aromatic hydrocarbons in cryogenic peat plateaus of northeastern Europe
NASA Astrophysics Data System (ADS)
Pastukhov, A. V.; Kaverin, D. A.; Gabov, D. N.
2017-07-01
The qualitative and quantitative composition of 14 polycyclic aromatic hydrocarbons (PAHs) in peat plateaus at the southern boundary of the permafrost zone in northeastern Europe, where degradation of permafrost occurs because of climate warming, has been studied by high-performance liquid chromatography in gradient mode. PAH concentrations vary from 150 to 3700 ng/g with their average content of about 1500 ± 1000 ng/g. The variation of data is primarily due to the large contribution of heavy PAHs.
NASA Astrophysics Data System (ADS)
Bates, Jason; Schmitt, Andrew; Zalesak, Steve
2015-11-01
The ablative Rayleigh-Taylor (RT) instability is a key factor in the performance of directly-drive inertial-confinement-fusion (ICF) targets. Although this subject has been studied for quite some time, the accurate simulation of the ablative RT instability has proven to be a challenging task for many radiation hydrodynamics codes, particularly when it comes to capturing the ablatively-stabilized region of the linear dispersion spectrum and modeling ab initio perturbations. In this poster, we present results from recent two-dimensional numerical simulations of the ablative RT instability that were performed using the Eulerian code FastRad3D at the U.S. Naval Research Laboratory. We consider both planar and spherical geometries, low and moderate-Z target materials, different laser wavelengths and where possible, compare our findings with experiment data, linearized theory and/or results from other radiation hydrodynamics codes. Overall, we find that FastRad3D is capable of simulating the ablative RT instability quite accurately, although some uncertainties/discrepancies persist. We discuss these issues, as well as some of the numerical challenges associated with modeling this class of problems. Work supported by U.S. DOE/NNSA.
NASA Astrophysics Data System (ADS)
Tetreault, J. L.; Buiter, S. J. H.
2012-08-01
Crustal growth at convergent margins can occur by the accretion of future allochthonous terranes (FATs), such as island arcs, oceanic plateaus, submarine ridges, and continental fragments. Using geodynamic numerical experiments, we demonstrate how crustal properties of FATs impact the amount of FAT crust that is accreted or subducted, the type of accretionary process, and the style of deformation on the overriding plate. Our results show that (1) accretion of crustal units occurs when there is a weak detachment layer within the FAT, (2) the depth of detachment controls the amount of crust accreted onto the overriding plate, and (3) lithospheric buoyancy does not prevent FAT subduction during constant convergence. Island arcs, oceanic plateaus, and continental fragments will completely subduct, despite having buoyant lithospheric densities, if they have rheologically strong crusts. Weak basal layers, representing pre-existing weaknesses or detachment layers, will either lead to underplating of faulted blocks of FAT crust to the overriding plate or collision and suturing of an unbroken FAT crust. Our experiments show that the weak, ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion. The different types of accretionary processes also affect deformation and uplift patterns in the overriding plate, trench migration and jumping, and the dip of the plate interface. The resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes, such as the Wrangellia Terrane and Klamath Mountain terranes in the North American Cordilleran Belt.
Magnetization plateaus and jumps in a frustrated four-leg spin tube under a magnetic field
NASA Astrophysics Data System (ADS)
Gómez Albarracín, F. A.; Arlego, M.; Rosales, H. D.
2014-11-01
We study the ground state phase diagram of a frustrated spin-1/2 four-leg spin tube in an external magnetic field. We explore the parameter space of this model in the regime of all-antiferromagnetic exchange couplings by means of three different approaches: analysis of low-energy effective Hamiltonian, a Hartree variational approach, and density matrix renormalization group for finite clusters. We find that in the limit of weakly interacting plaquettes, low-energy singlet, triplet, and quintuplet states play an important role in the formation of fractional magnetization plateaus. We study the transition regions numerically and analytically, and find that they are described, at first order in a strong-coupling expansion, by an X X Z spin-1/2 chain in a magnetic field; the second-order terms give corrections to the X X Z model. All techniques provide consistent results which allow us to predict the existence of fractional plateaus in an important region in the space of parameters of the model.
Evolution Models with Conditional Mutation Rates: Strange Plateaus in Population Distribution
NASA Astrophysics Data System (ADS)
Saakian, David B.
2017-08-01
Cancer is related to clonal evolution with a strongly nonlinear, collective behavior. Here we investigate a slightly advanced version of the popular Crow-Kimura evolution model, suggested recently, by simply assuming a conditional mutation rate. We investigated the steady-state solution and found a highly intriguing plateau in the distribution. There are selective and nonselective phases, with a rather narrow plateau in the distribution at the peak in the first phase, and a wide plateau for many Hamming classes (a collection of genomes with the same number of mutations from the reference genome) in the second phase. We analytically solved the steady state distribution in the selective and nonselective phases, calculating the widths of the plateaus. Numerically, we also found an intermediate phase with several plateaus in the steady-state distribution, related to large finite-genome-length corrections. We assume that the newly observed phenomena should exist in other versions of evolution dynamics when the parameters of the model are conditioned to the population distribution.
NASA Astrophysics Data System (ADS)
Pan, T. W.; Joseph, D. D.; Glowinski, R.
2001-05-01
In this paper we study the sedimentation of several thousand circular particles in two dimensions using the method of distributed Lagrange multipliers for solid liquid flow. The simulation gives rise to fingering which resembles Rayleigh Taylor instabilities. The waves have a well-defined wavelength and growth rate which can be modelled as a conventional Rayleigh Taylor instability of heavy fluid above light. The heavy fluid is modelled as a composite solid liquid fluid with an effective composite density and viscosity. Surface tension cannot enter this problem and the characteristic shortwave instability is regularized by the viscosity of the solid liquid dispersion. The dynamics of the Rayleigh Taylor instability are studied using viscous potential flow, generalizing work of Joseph, Belanger & Beavers (1999) to a rectangular domain bounded by solid walls; an exact solution is obtained.
The width of the plateaus of the quantum Hall effect
NASA Astrophysics Data System (ADS)
Groshev, Atanas; Scho¨n, Gerd
1994-02-01
We suggest that in high quality samples in the quantum Hall regime the interaction between localized states dominates over disorder effects. It leads to the formation of a Wigner crystal, which melts at a critical value ν c≈0.2 of the filling factor of the localized states. This leads to a finite width of the plateaus of the integer quantum Hall effect Δν=2ν c. This result describes well recent experimental data on single AlGaAs/GaAs heterojunctions (electron and hole gases) and double 2DEG systems.
NASA Astrophysics Data System (ADS)
Fant, Daniel Bartholemew
1987-05-01
Multicellular flow instability due to natural convection between narrow horizontal isothermal cylindrical annuli was investigated numerically and analytically. Bouyancy-induced steady or unsteady flow fields between the annuli are determined using the Boussinesq approximated two-dimensional (2-D) Navier-Stokes (N-S) equations and the viscous dissipated neglected thermal energy equations. The vorticity-stream function formulation of the N-S equations is adopted. Both thermal and hydrodynamic instabilities are explored. An asymptotic expansion theory is applied to the N-S equations in the double-limit of Rayleigh number approaching infinity and gap width approaching zero. Thermal instability of air near the top portions of narrow annuli is considered for various size small gap widths. Numerical solutions of the 2-D N-S equations also yield hysteresis behavior for the two-to-six and two-to four cellular states, with respect to diameter ratios of 1.100 and 1.200. In all cases, finite-differenced solutions to the governing equations are obtained using a stable second-order, fully-implicit time-accurate Gauss-Seidel iterative procedure.
NASA Astrophysics Data System (ADS)
Shvydky, A.; Hohenberger, M.; Radha, P. B.; Rosenberg, M. J.; Craxton, R. S.; Goncharov, V. N.; Marozas, J. A.; Marshall, F. J.; McKenty, P. W.; Regan, S. P.; Sangster, T. C.
2015-11-01
Control of shell nonuniformities imprinted by the laser and amplified by hydrodynamic instabilities in the imploding target is critical to the success of polar-direct-drive ignition at the National Ignition Facility (NIF). To develop a platform for laser-imprint studies, hydrodynamic instability growth experiments in laser-driven implosions were performed on the NIF. The experiments used cone-in-shell targets with sinusoidal modulations of various wavelengths and amplitudes machined on the surface. Throughshell x-ray radiography was used to measure optical depth variations, from which the amplitudes of the shell areal-density modulations were extracted. Results of DRACO simulations of the growth of preimposed modulations and imprint-seeded perturbations will be presented and compared with the experimental data. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.
NASA Astrophysics Data System (ADS)
Wang, Y.-M.; Nepveu, M.; Robertson, J. A.
1984-06-01
An earlier investigation of the nonlinear Rayleigh-Taylor instability for accreting X-ray sources is extended to allow for more realistic initial conditions. The two-dimensional computations show the heavy and light fluids undergoing complementary circulatory motions which result in the formation of alternating inverted and upright 'mushroom' structures along the interface. The structures develop independently of the shape of the initial perturbation. Short wavelength modes have a strong tendency to dominate long ones, with the lower bound being set by viscous damping. A relatively modest vertical magnetic field will act to suppress the vortex motions and produce a 'bubble and spike' structure. A crude simulation of the instability occurring in a radiation-supported accretion column is presented; after a slow start, the magnetically constrained plasma drips down into the photon medium in the form of long narrow fingers, the dominant scale-length being determined by radiative viscosity.
Matsumoto, Jin; Masada, Youhei
2013-07-20
We study the stability of a non-rotating single-component jet using two-dimensional special relativistic hydrodynamic simulations. By assuming translational invariance along the jet axis, we exclude the destabilization effect by Kelvin-Helmholtz mode. The nonlinear evolution of the transverse structure of the jet with a normal jet velocity is highlighted. An intriguing finding in our study is that Rayleigh-Taylor and Richtmyer-Meshkov type instabilities can destroy cylindrical jet configuration as a result of spontaneously induced radial oscillating motion. This is powered by in situ energy conversion between the thermal and bulk kinetic energies. The effective inertia ratio of the jet to the surrounding medium {eta} determines a threshold for the onset of instabilities. The condition {eta} < 1 should be satisfied for the transverse structure of the jet being persisted.
Hydrogeology and hydrologic conditions of the Ozark Plateaus aquifer system
Hays, Phillip D.; Knierim, Katherine J.; Breaker, Brian K.; Westerman, Drew A.; Clark, Brian R.
2016-11-23
The hydrogeology and hydrologic characteristics of the Ozark Plateaus aquifer system were characterized as part of ongoing U.S. Geological Survey efforts to assess groundwater availability across the Nation. The need for such a study in the Ozark Plateaus physiographic province (Ozark Plateaus) is highlighted by increasing demand on groundwater resources by the 5.3 million people of the Ozark Plateaus, water-level declines in some areas, and potential impacts of climate change on groundwater availability. The subject study integrates knowledge gained through local investigation within a regional perspective to develop a regional conceptual model of groundwater flow in the Ozark Plateaus aquifer system (Ozark system), a key phase of groundwater availability assessment. The Ozark system extends across much of southern Missouri and northwestern and north-central Arkansas and smaller areas of southeastern Kansas and northeastern Oklahoma. The region is one of the major karst landscapes in the United States, and karst aquifers are predominant in the Ozark system. Groundwater flow is ultimately controlled by aquifer and confining unit lithologies and stratigraphic relations, geologic structure, karst development, and the character of surficial lithologies and regolith mantle. The regolith mantle is a defining element of Ozark Plateaus karst, affecting recharge, karst development, and vulnerability to surface-derived contaminants. Karst development is more advanced—as evidenced by larger springs, hydraulic characteristics, and higher well yields—in the Salem Plateau and in the northern part of the Springfield Plateau (generally north of the Arkansas-Missouri border) as compared with the southern part of the Springfield Plateau in Arkansas, largely due to thinner, less extensive regolith and purer carbonate lithology.Precipitation is the ultimate source of all water to the Ozark system, and the hydrologic budget for the Ozark system includes inputs from recharge
The Shatsky Rise Supervolcano and the Formation of Oceanic Plateaus
NASA Astrophysics Data System (ADS)
Sager, W. W.; Sano, T.; Korenaga, J.; Sager, W. W.
2011-12-01
Oceanic plateaus are igneous mountains constructed by massive eruptions of basalt and related igneous rocks. Because they are hidden beneath remote parts of the oceans, the structure and evolution of these mountains are poorly known. Shatsky Rise, in the northwest Pacific, is an oceanic plateau that formed ~145-125 Ma near a triple junction. It consists of three large volcanic massifs and a narrow volcanic ridge. Eruptions apparently began with the largest volcanic edifice (Tamu Massif) and waned through time with the formation of two other massifs. The discrete volcanic centers of Shatsky Rise likely resulted from relatively rapid drift of the Pacific plate relative to the melting anomaly. Tamu Massif is a supervolcano, i.e., a huge volcanic edifice with a volcanic center, like a seamount, but much bigger. Its area is similar to Olympus Mons on Mars, the largest volcano in the solar system. Geophysical data show that Tamu Massif has a shape that is symmetric across its axis. A seismic profile across the axis shows that lava flows flowed outward from its center. Seismic profiles in some spots over the axis show normal faulting that implies a volcanic rift zone, which is consistent with it being the major source of lava flows. Flank slopes are low, implying long, low viscosity lava flows. Coring on Integrated Ocean Drilling Program (IODP) Expedition 324 recovered basalt flows of two types: pillows and massive flows. Pillows are indicative of normal seamount volcanism at low effusion rates whereas the massive flows imply high volume lava flows with high effusion rates. Massive flows are typical of continental flood basalts and are also found on other large plateaus. On Shatsky Rise, thick massive flows are found on Tamu Massif, whereas pillows and thin massive flows characterize the other massifs. This trend supports the idea that Tamu Massif was formed by an initial massive eruptive event and afterwards volcanism waned as other massifs were erupted. Shallow water
Geoid height versus topography for oceanic plateaus and swells
NASA Technical Reports Server (NTRS)
Sandwell, David T.; Mackenzie, Kevin R.
1989-01-01
Gridded geoid height data (Marsh et al.l, 1986) and gridded bathymetry data (Van Wykhouse, 1973) are used to estimate the average compensation depths of 53 oceanic swells and plateaus. The relationship between geoid height and topography is examined using Airy and thermal compensation models. It is shown that geoid height is linearly related to topography between wavelengths of 400 and 4000 m as predicted by isostatic compensation models. The geoid/topography ratio is dependent on the average depth of compensation. The intermediate geoid/topography ratios of most thermal swells are interpreted as a linear combination of the decaying thermal swell signature and that of the persisting Airy-compensated volcanic edifice.
Self-Consistant Numerical Modeling of E-Cloud Driven Instability of a Bunch Train in the CERN SPS
Vay, J-L.; Furman, M.A.; Secondo, R.; Venturini, M.; Fox, J.D.; Rivetta, C.H,
2010-09-01
The simulation package WARP-POSINST was recently upgraded for handling multiple bunches and modeling concurrently the electron cloud buildup and its effect on the beam, allowing for direct self-consistent simulation of bunch trains generating, and interacting with, electron clouds. We have used the WARP-POSINST package on massively parallel supercomputers to study the growth rate and frequency patterns in space-time of the electron cloud driven transverse instability for a proton bunch train in the CERN SPS accelerator. Results suggest that a positive feedback mechanism exists between the electron buildup and the e-cloud driven transverse instability, leading to a net increase in predicted electron density. Comparisons to selected experimental data are also given. Electron clouds have been shown to trigger fast growing instabilities on proton beams circulating in the SPS and other accelerators. So far, simulations of electron cloud buildup and their effects on beam dynamics have been performed separately. This is a consequence of the large computational cost of the combined calculation due to large space and time scale disparities between the two processes. We have presented the latest improvements of the simulation package WARP-POSINST for the simulation of self-consistent ecloud effects, including mesh refinement, and generation of electrons from gas ionization and impact at the pipe walls. We also presented simulations of two consecutive bunches interacting with electrons clouds in the SPS, which included generation of secondary electrons. The distribution of electrons in front of the first beam was initialized from a dump taken from a preceding buildup calculation using the POSINST code. In this paper, we present an extension of this work where one full batch of 72 bunches is simulated in the SPS, including the entire buildup calculation and the self-consistent interaction between the bunches and the electrons. Comparisons to experimental data are also given.
NASA Astrophysics Data System (ADS)
Bates, J. W.; Schmitt, A. J.; Karasik, M.; Zalesak, S. T.
2016-12-01
The ablative Rayleigh-Taylor (RT) instability is a central issue in the performance of laser-accelerated inertial-confinement-fusion targets. Historically, the accurate numerical simulation of this instability has been a challenging task for many radiation hydrodynamics codes, particularly when it comes to capturing the ablatively stabilized region of the linear dispersion spectrum and modeling ab initio perturbations. Here, we present recent results from two-dimensional numerical simulations of the ablative RT instability in planar laser-ablated foils that were performed using the Eulerian code FastRad3D. Our study considers polystyrene, (cryogenic) deuterium-tritium, and beryllium target materials, quarter- and third-micron laser light, and low and high laser intensities. An initial single-mode surface perturbation is modeled in our simulations as a small modulation to the target mass density and the ablative RT growth-rate is calculated from the time history of areal-mass variations once the target reaches a steady-state acceleration. By performing a sequence of such simulations with different perturbation wavelengths, we generate a discrete dispersion spectrum for each of our examples and find that in all cases the linear RT growth-rate γ is well described by an expression of the form γ = α [ k g / ( 1 + ɛ k L m ) ] 1 / 2 - β k V a , where k is the perturbation wavenumber, g is the acceleration of the target, Lm is the minimum density scale-length, Va is the ablation velocity, and ɛ is either one or zero. The dimensionless coefficients α and β in the above formula depend on the particular target and laser parameters and are determined from two-dimensional simulation results through the use of a nonlinear curve-fitting procedure. While our findings are generally consistent with those of Betti et al. (Phys. Plasmas 5, 1446 (1998)), the ablative RT growth-rates predicted in this investigation are somewhat smaller than the values previously reported for the
NASA Astrophysics Data System (ADS)
Yu, Peicheng; Xu, Xinlu; Tableman, Adam; Decyk, Viktor K.; Tsung, Frank S.; Fiuza, Frederico; Davidson, Asher; Vieira, Jorge; Fonseca, Ricardo A.; Lu, Wei; Silva, Luis O.; Mori, Warren B.
2015-12-01
A hybrid Maxwell solver for fully relativistic and electromagnetic (EM) particle-in-cell (PIC) codes is described. In this solver, the EM fields are solved in k space by performing an FFT in one direction, while using finite difference operators in the other direction(s). This solver eliminates the numerical Cerenkov radiation for particles moving in the preferred direction. Moreover, the numerical Cerenkov instability (NCI) induced by the relativistically drifting plasma and beam can be eliminated using this hybrid solver by applying strategies that are similar to those recently developed for pure FFT solvers. A current correction is applied for the charge conserving current deposit to ensure that Gauss's Law is satisfied. A theoretical analysis of the dispersion properties in vacuum and in a drifting plasma for the hybrid solver is presented, and compared with PIC simulations with good agreement obtained. This hybrid solver is applied to both 2D and 3D Cartesian and quasi-3D (in which the fields and current are decomposed into azimuthal harmonics) geometries. Illustrative results for laser wakefield accelerator simulation in a Lorentz boosted frame using the hybrid solver in the 2D Cartesian geometry are presented, and compared against results from 2D UPIC-EMMA simulation which uses a pure spectral Maxwell solver, and from OSIRIS 2D lab frame simulation using the standard Yee solver. Very good agreement is obtained which demonstrates the feasibility of using the hybrid solver for high fidelity simulation of relativistically drifting plasma with no evidence of the numerical Cerenkov instability.
Keskinen, M.J.; Ossakow, S.L.; Chaturvedi, P.K.
1980-04-01
Computer simulations of the intermediate wavelength (100--1000 m) collisional Rayleigh-Taylor instability in local unstable regions of the postsunset bottomside (300 km) equatorial F region ionosphere have been performed. For ambient electron density gradient scale lengths L=5, 10, 15 km we find that the linearly unstable horizontal modes saturate by nonlinear generation of linearly damped vertical modes with the result that in the nonlinear regime, power laws are observed in the horizontal P(k/sub x/) proportional k/sub x//sup -n/ and vertical P(k/sub y/) proportional k/sub y//sup -n/ one-dimensional power spectra with n=2--2.5. These results are consistent both with in situ experimental data and with theoretical prediction.
Direct Numerical Modeling of E-Cloud Driven Instability of a Bunch Train in the CERN SPS
Vay, J-L.; Furman, M.A.; Venturini, M.
2011-03-01
The simulation package WARP-POSINST was recently upgraded for handling multiple bunches and modeling concurrently the electron cloud buildup and its effect on the beam, allowing for direct self-consistent simulation of bunch trains generating, and interacting with, electron clouds. We have used the WARP-POSINST package on massively parallel supercomputers to study the buildup and interaction of electron clouds with a proton bunch train in the CERN SPS accelerator. Results suggest that a positive feedback mechanism exists between the electron buildup and the e-cloud driven transverse instability, leading to a net increase in predicted electron density. Electron clouds have been shown to trigger fast growing instabilities on proton beams circulating in the SPS and other accelerators. So far, simulations of electron cloud buildup and their effects on beam dynamics have been performed separately. This is a consequence of the large computational cost of the combined calculation due to large space and time scale disparities between the two processes. We have presented the latest improvements of the simulation package WARP-POSINST for the simulation of self-consistent ecloud effects, including mesh refinement, and generation of electrons from gas ionization and impact at the pipe walls. We also presented simulations of two consecutive bunches interacting with electrons clouds in the SPS, which included generation of secondary electrons. The distribution of electrons in front of the first beam was initialized from a dump taken from a preceding buildup calculation using the POSINST code. In this paper, we present an extension of this work where one full batch of 72 bunches is simulated in the SPS, including the entire buildup calculation and the self-consistent interaction between the bunches and the electrons.
Quantum vs Classical Magnetization Plateaus of S=1/2 Frustrated Heisenberg Chains
NASA Astrophysics Data System (ADS)
Hida, Kazuo; Affleck, Ian
2005-06-01
The competition between quantum and classical magnetization plateaus of S=1/2 frustrated Heisenberg chains with modified exchange couplings is investigated. The conventional S=1/2 frustrated Heisenberg chain is known to exhibit a 3-fold degenerate \\uparrow\\downarrow\\uparrow-type classical plateau at 1/3 of the saturation magnetization accompanied by the spontaneous Z3 translational symmetry breakdown. The stability of this plateau phase against period 3 exchange modulation which favors the \\bullet\\hskip -1pt-\\hskip -1pt\\bullet \\uparrow-type quantum plateau state (\\bullet\\hskip -1pt-\\hskip -1pt\\bullet = singlet dimer) is studied by bosonization, renormalization group and numerical diagonalization methods. The ground state phase diagram and the spin configuration in each phase are numerically determined. The translationally invariant Valence Bond Solid-type model with 4-spin and third neighbor interactions, which has the exact \\bullet\\hskip -1pt-\\hskip -1pt\\bullet \\uparrow-type quantum plateau state, is also presented. The phase transition to the classical \\uparrow\\downarrow\\uparrow-type ground state is also observed by varying the strength of 4-spin and third neighbor interactions. The relation between these two types of models with quantum plateau states is discussed.
The presence of rapidly degrading permafrost plateaus in southcentral Alaska
Jones, Benjamin M.; Baughman, Carson; Romanovsky, Vladimir E.; Parsekian, Andrew D.; Babcock, Esther; Stephani, Eva; Jones, Miriam C.; Grosse, Guido; Berg, Edward E
2016-01-01
Permafrost presence is determined by a complex interaction of climatic, topographic, and ecological conditions operating over long time scales. In particular, vegetation and organic layer characteristics may act to protect permafrost in regions with a mean annual air temperature (MAAT) above 0°C. In this study, we document the presence of residual permafrost plateaus on the western Kenai Peninsula lowlands of southcentral Alaska, a region with a MAAT of 1.5 ± 1°C (1981 to 2010). Continuous ground temperature measurements between 16 September 2012 and 15 September 2015, using calibrated thermistor strings, documented the presence of warm permafrost (−0.04 to −0.08°C). Field measurements (probing) on several plateau features during the fall of 2015 showed that the depth to the permafrost table averaged 1.48 m but was as shallow as 0.53 m. Late winter surveys (drilling, coring, and GPR) in 2016 showed that the average seasonally frozen ground thickness was 0.45 m, overlying a talik above the permafrost table. Measured permafrost thickness ranged from 0.33 to >6.90 m. Manual interpretation of historic aerial photography acquired in 1950 indicates that residual permafrost plateaus covered 920 ha as mapped across portions of four wetland complexes encompassing 4810 ha. However, between 1950 and ca. 2010, permafrost plateau extent decreased by 60%, with lateral feature degradation accounting for 85% of the reduction in area. Permafrost loss on the Kenai Peninsula is likely associated with a warming climate, wildfires that remove the protective forest and organic layer cover, groundwater flow at depth, and lateral heat transfer from wetland surface waters in the summer. Better understanding the resilience and vulnerability of ecosystem-protected permafrost is critical for mapping and predicting future permafrost extent and degradation across all permafrost regions that are currently warming. Further work should focus on reconstructing permafrost history in
NASA Astrophysics Data System (ADS)
Cheng, Yingda; Christlieb, Andrew J.; Zhong, Xinghui
2015-05-01
In this paper, we propose energy-conserving Eulerian solvers for the two-species Vlasov-Ampère (VA) system and apply the methods to simulate current-driven ion-acoustic instability. The two-species VA systems are of practical importance in applications, and they conserve many physical quantities including the particle number of each species and the total energy that is comprised of kinetic energy for both species and the electric energy. The main goal of this paper is to generalize our previous work for the single-species VA system [9] and Vlasov-Maxwell (VM) system [8] to the two-species case. The methodologies proposed involve careful design of temporal discretization and the use of the discontinuous Galerkin (DG) spatial discretizations. We show that the energy-conserving time discretizations for single-species equations [9,8] can also work for the two-species case if extended properly. Compared to other high order schemes, we emphasize that our schemes can preserve the total particle number and total energy on the fully discrete level regardless of mesh size, making them very attractive for long time simulations. We benchmark our algorithms on a test example to check the one-species limit, and the current-driven ion-acoustic instability. To simulate the current-driven ion-acoustic instability, a slight modification for the implicit method is necessary to fully decouple the split equations. This is achieved by a Gauss-Seidel type iteration technique. Numerical results verified the conservation and performance of our methods. Finally, we remark that the schemes in this paper can be readily extended to applications when the models take more general form, such as the multi-species VM equations.
NASA Astrophysics Data System (ADS)
Fan, Zhengfeng; Zhu, Shaoping; Pei, Wenbing; Ye, Wenhua; Li, Meng; Xu, Xiaowen; Wu, Junfeng; Dai, Zhensheng; Wang, Lifeng
2012-09-01
Tritium-hydrogen-deuterium (THD) target is adopted in order to experimentally diagnose the properties of the ignition hot spot and the highly compressed main fusion fuel (Edwards M. J. et al., Phys. Plasmas, 18 (2011) 051003). As compared with deuterium-tritium (DT) target, the thermonuclear alpha particles which are needed to heat the fusion fuel, are much less in the THD target. In the present paper, the effect of alpha particle heating on the deceleration phase Rayleigh-Taylor instability (dp-RTI), which is one of the key problems in hot spot formation, is investigated systematically through numerical simulations. It is found that the mass ablation at the hot spot boundary is greatly increased due to the direct alpha particle heating. As a result, the dp-RTI growth rates are greatly reduced and the cut-off mode number decreases greatly from about 33 to 17. This explains why the hydrodynamic instability in the THD target grows more severely than in the DT ignition target.
Line defects and quantum Hall plateaus in graphene
NASA Astrophysics Data System (ADS)
Dal Lago, V.; Foa Torres, L. E. F.
2015-04-01
Line defects in graphene can be either tailored-growth or arise naturally and are at the center of many discussions. Here we study the multiterminal conductance of graphene with an extended line defect in the quantum Hall regime analyzing the effects of the geometry of the setup, disorder and strain on the quantum Hall plateaus. We show that the defect turns out to affect the local and non-local conductance in very different ways depending on the geometrical configuration. When the defect is parallel to the sample edges one gets an equivalent circuit formed by parallel resistors. In contrast, when the defect bridges opposite edges, the Hall conductance may remain unaltered depending on the geometry of the voltage/current probes. The role of disorder, strain and the microscopic details of the defect in our results is also discussed. We show that the defect provides a realization of the electrical analog of an optical beam splitter. Its peculiar energy dependent inter-edge transmission allows it to be turned on or off at will and it may be used for routing the chiral edge states.
Line defects and quantum Hall plateaus in graphene.
Dal Lago, V; Torres, L E F Foa
2015-04-15
Line defects in graphene can be either tailored-growth or arise naturally and are at the center of many discussions. Here we study the multiterminal conductance of graphene with an extended line defect in the quantum Hall regime analyzing the effects of the geometry of the setup, disorder and strain on the quantum Hall plateaus. We show that the defect turns out to affect the local and non-local conductance in very different ways depending on the geometrical configuration. When the defect is parallel to the sample edges one gets an equivalent circuit formed by parallel resistors. In contrast, when the defect bridges opposite edges, the Hall conductance may remain unaltered depending on the geometry of the voltage/current probes. The role of disorder, strain and the microscopic details of the defect in our results is also discussed. We show that the defect provides a realization of the electrical analog of an optical beam splitter. Its peculiar energy dependent inter-edge transmission allows it to be turned on or off at will and it may be used for routing the chiral edge states.
Report on the geology of the high plateaus of Utah
Dutton, C.E.; Powell, John Wesley
1880-01-01
In the year 1874 my kind friend Prof. J.W. Powell proposed to me that I should undertake, under his direction, the study of a large volcanic tract in the Territory of Utah, provided the consent of proper authority could be entertained. Distrusting my own fitness for the work, I felt that it would be better for him if his proposals were thankfully declined. In 1875, however, he renewed the proposition in such a friendly and complimentary manner that a refusal seemed ungracious. He therefore laid the matter before the Secretary of War, the General of the Army, and the Chief of Ordnance, all of whom gave their cordial approbation; and by order of the Ward Department I was detailed for duty in connection with the survey of the Rocky Mountain Region in charge of Professor Powell. The field which he assigned me to study was the District of the High Plateaus, and the investigations were made during the summers of 1875, 1876, and 1877. The preparation of a report or monograph upon the district has several times between interrupted by the pressure of other official duties to which the writer has been assigned during the last three years.
López Ortega, A; Lombardini, M; Pullin, D I; Meiron, D I
2014-03-01
The Richtmyer-Meshkov instability of interfaces separating elastic-plastic materials from vacuum (heavy-light configuration) is studied by means of computational techniques. A fully Eulerian multimaterial algorithm that solves consistently the Euler equations and the time evolution of the deformations in the material is applied to three distinct materials (copper, aluminum, and stainless steel). If a perfectly plastic constitutive relation is considered, an empirical law is computed that relates the long-term perturbation amplitude of the interface, its maximum growth rate, the initial density, and the yield stress of the material. It is shown that this linear relation can be extended to materials that follow more complex plastic behavior which can account for rate dependency, hardening, and thermal softening, and to situations in which small-perturbation theory is no longer valid. In effect, the yield stress computed from measurements of the long-term amplitude and maximum growth rate closely matches the von Mises stress found at the interface of solid materials for a wide range of cases with different initial parameters.
Naderi, Shadi; Dajani, Iyad; Grosek, Jacob; Madden, Timothy
2016-07-25
Raman fiber lasers have been proposed as potential candidates for scaling beyond the power limitations imposed on near diffraction-limited rare-earth doped fiber lasers. One limitation is the modal instability (MI) and we explore the physics of this phenomenon in Raman fiber amplifiers (RFAs). By utilizing the conservation of number of photons and conservation of energy in the absence of loss, the 3 × 3 governing system of nonlinear equations describing the pump and the signal modal content are decoupled and solved analytically for cladding-pumped RFAs. By comparing the extracted signal at MI threshold for the same step index-fiber, it is found that the MI threshold is independent of the length of the amplifier or whether the amplifier is co-pumped or counter-pumped; dictated by the integrated heat load along the length of fiber. We extend our treatment to gain-tailored RFAs and show that this approach is of limited utility in suppressing MI. Finally, we formulate the physics of MI in core-pumped RFAs where both pump and signal interferences participate in writing the time-dependent index of refraction grating.
Instability of rectangular jets
NASA Technical Reports Server (NTRS)
Tam, Christopher K. W.; Thies, Andrew T.
1992-01-01
The instability of rectangular jets is investigated using a vortex sheet model. It is shown that such jets support four linearly independent families of instability waves. Within each family there are infinitely many modes. A way to classify these modes according to the characteristics of their mode shapes or eigenfunctions is proposed. A parametric study of the instability wave characteristics has been carried out. A sample of the numerical results is reported here. It is found that the first and third modes of each instability wave family are corner modes. The pressure fluctuations associated with these instability waves are localized near the corners of the jet. The second mode, however, is a center mode with maximum fluctuations concentrated in the central portion of the jet flow. The center mode has the largest spatial growth rate. It is anticipated that as the instability waves propagate downstream the center mode would emerge as the dominant instability of the jet.
Instability of rectangular jets
NASA Technical Reports Server (NTRS)
Tam, Christopher K. W.; Thies, Andrew T.
1992-01-01
The instability of rectangular jets is investigated using a vortex sheet model. It is shown that such jets support four linearly independent families of instability waves. Within each family there are infinitely many modes. A way to classify these modes according to the characteristics of their mode shapes or eigenfunctions is proposed. A parametric study of the instability wave characteristics has been carried out. A sample of the numerical results is reported here. It is found that the first and third modes of each instability wave family are corner modes. The pressure fluctuations associated with these instability waves are localized near the corners of the jet. The second mode, however, is a center mode with maximum fluctuations concentrated in the central portion of the jet flow. The center mode has the largest spatial growth rate. It is anticipated that as the instability waves propagate downstream the center mode would emerge as the dominant instability of the jet.
Generalities on combustion instabilities
NASA Astrophysics Data System (ADS)
Kuentzmann, Paul
The main manifestations of combustion instabilities are reviewed, and the specific characteristics of instabilities in solid-propellant rocket engines are analyzed, with the Minuteman III third-stage engine and the SRB engine of Titan 34 D considered as examples. The main approaches for predicting combustion instabilities are discussed, including the linear approach based on the acoustic balance, the nonlinear mode-coupling approach, and the nonlinear approach using numerical calculation. Projected directions for future research are also examined.
Crustal volumes of the continents and of oceanic and continental submarine plateaus
NASA Technical Reports Server (NTRS)
Schubert, G.; Sandwell, D.
1989-01-01
Using global topographic data and the assumption of Airy isostasy, it is estimated that the crustal volume of the continents is 7182 X 10 to the 6th cu km. The crustal volumes of the oceanic and continental submarine plateaus are calculated at 369 X 10 to the 6th cu km and 242 X 10 to the 6th cu km, respectively. The total continental crustal volume is found to be 7581 X 10 to the 6th cu km, 3.2 percent of which is comprised of continental submarine plateaus on the seafloor. An upper bound on the contintental crust addition rate by the accretion of oceanic plateaus is set at 3.7 cu km/yr. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale yields an upper bound to the continental crustal subtraction rate of 2.4 cu km/yr.
Clustering of cochlear oscillations in frequency plateaus as a tool to investigate SOAE generation
NASA Astrophysics Data System (ADS)
Epp, Bastian; Wit, Hero; van Dijk, Pim
2015-12-01
Spontonaeous otoacoustic emissions (SOAE) reflect the net effect of self-sustained activity in the cochlea, but do not directly provide information about the underlying mechanism and place of origin within the cochlea. The present study investigates if frequency plateaus as found in a linear array of coupled oscillators (OAM) [7] are also found in a transmission line model (TLM) which is able to generate realistic SOAEs [2] and if these frequency plateaus can be used to explain the formation of SOAEs. The simulations showed a clustering of oscillators along the simulated basilar membrane Both, the OAM and the TLM show traveling-wave like behavior along the oscillators coupled into one frequency plateau. While in the TLM roughness is required in order to produce SOAEs, no roughness is required to trigger frequency plateaus in the linear array of oscillators. The formation of frequency plateaus as a consequence of coupling between neighbored active oscillators might be the mechanism underlying SOAEs.
Were Oceanic Plateaus Instrumental for Calcareous Nannoplankton Evolution?
NASA Astrophysics Data System (ADS)
Erba, E.; Casellato, C.; Bottini, C.
2011-12-01
The history of calcareous nannoplankton shows a general increase in species richness through the Mesozoic. Fertility and chemistry of the oceans, climate and pCO2 seem instrumental for nannoplankton abundance, diversification and adaptation, but high-resolution chronology of paleobiological and geological events is crucial for the understanding of evolutionary processes relative to ecosystem perturbations. Natural variations in atmospheric CO2 are essentially triggered by igneous activity and the role of ocean crust production in the evolution of seawater composition, nutrient cycling, climate change and, consequently, in calcareous nannoplankton biodiversity, might be more relevant than generally thought. Indeed, two major steps in nannofloral Mesozoic evolution correlate with construction of gigantic oceanic plateaus, namely the Shatsky Rise (SR) (Tithonian/Berriasian boundary interval) and the Ontong Java Plateau (OJP) (Barremian/Aptian boundary interval). During the latest Jurassic calcareous nannoplankton experienced a rapid diversification and rise in abundance of several taxa including heavily calcified nannoliths with consequent major increase in biogenic calcite production. The Tithonian origination of coccoliths and nannoliths suggests ideal paleoecological conditions for calcareous nannoplankton, presumably thriving in stable, relatively oligotrophic and cool oceans under low pCO2. Recent data indicate that this speciation and calcification episode was interrupted during magnetochron CM19r, prior to massive diversification of nannoconids. In the late Barremian-early Aptian interval, the nannoconid decline and crisis are paralleled by a major nannoplankton (mainly coccolith) speciation episode. Such calcification failure and coccolith diversification might reflect disruption of the thermocline, increased fertility and warming under excess CO2 levels. These evolutionary steps show rapid speciation, but differ because nannoliths became dominant in the late
Strong degradation of palsas and peat plateaus in northern Norway during the last 60 years
NASA Astrophysics Data System (ADS)
Borge, Amund F.; Westermann, Sebastian; Solheim, Ingvild; Etzelmüller, Bernd
2017-01-01
Palsas and peat plateaus are permafrost landforms occurring in subarctic mires which constitute sensitive ecosystems with strong significance for vegetation, wildlife, hydrology and carbon cycle. Firstly, we have systematically mapped the occurrence of palsas and peat plateaus in the northernmost county of Norway (Finnmark, ˜ 50 000 km2) by manual interpretation of aerial images from 2005 to 2014 at a spatial resolution of 250 m. At this resolution, mires and wetlands with palsas or peat plateaus occur in about 850 km2 of Finnmark, with the actual palsas and peat plateaus underlain by permafrost covering a surface area of approximately 110 km2. Secondly, we have quantified the lateral changes of the extent of palsas and peat plateaus for four study areas located along a NW-SE transect through Finnmark by utilizing repeat aerial imagery from the 1950s to the 2010s. The results of the lateral changes reveal a total decrease of 33-71 % in the areal extent of palsas and peat plateaus during the study period, with the largest lateral change rates observed in the last decade. However, the results indicate that degradation of palsas and peat plateaus in northern Norway has been a consistent process during the second half of the 20th century and possibly even earlier. Significant rates of areal change are observed in all investigated time periods since the 1950s, and thermokarst landforms observed on aerial images from the 1950s suggest that lateral degradation was already an ongoing process at this time. The results of this study show that lateral erosion of palsas and peat plateaus is an important pathway for permafrost degradation in the sporadic permafrost zone in northern Scandinavia. While the environmental factors governing the rate of erosion are not yet fully understood, we note a moderate increase in air temperature, precipitation and snow depth during the last few decades in the region.
Plebuch, R.O.; Faust, R.J.; Townsend, M.A.
1985-01-01
The Mississippian Plateaus region is the outcrop area of rocks of Mississippian age which extends as a broad arcuate band around the Western Coal Field in westcentral Kentucky. Much of the area is characterized by plains of low relief containing numerous sinkholes, subsurface drainage, and a low density of surface streams. The principal aquifer consists of a thick sequence of limestones extending downward stratigraphically from the base of the Chesterian Series to the black shales at the top of the Devonian rocks. Well yields range from several gallons per minute to as much as 500 gallons per minute in some karst areas where secondary openings are well developed. The potentiometric map indicates that ground-water movement generally conforms to the surface drainage pattern. The actual direction of movement varies from river basin to river basin. Most water from the principal aquifer is a calcium magnesium bicarbonate type and is generally good relative to current drinking water standards. The lower St Louis Limestone, in places, yields a calcium magnesium sulfate water that is corrosive and has a strong hydrogen sulfide odor. The karst areas of the principal aquifer are vulnerable to contamination because of the well-developed subsurface drainage. Urban areas, industries, and agriculture are sources of contaminants that can be easily flushed into the ground-water system. (USGS)
Patterns of orchid bee species diversity and turnover among forested plateaus of central Amazonia.
Antonini, Yasmine; Machado, Carolina de Barros; Galetti, Pedro Manoel; Oliveira, Marcio; Dirzo, Rodolfo; Fernandes, Geraldo Wilson
2017-01-01
The knowledge of spatial pattern and geographic beta-diversity is of great importance for biodiversity conservation and interpreting ecological information. Tropical forests, especially the Amazon Rainforest, are well known for their high species richness and low similarity in species composition between sites, both at local and regional scales. We aimed to determine the effect and relative importance of area, isolation and climate on species richness and turnover in orchid bee assemblages among plateaus in central Brazilian Amazonia. Variance partitioning techniques were applied to assess the relative effects of spatial and environmental variables on bee species richness, phylogeny and composition. We hypothesized that greater abundance and richness of orchid bees would be found on larger plateaus, with a set of core species occurring on all of them. We also hypothesized that smaller plateaus would possess lower phylogenetic diversity. We found 55 bee species distributed along the nine sampling sites (plateaus) with 17 of them being singletons. There was a significant decrease in species richness with decreasing size of plateaus, and a significant decrease in the similarity in species composition with greater distance and climatic variation among sampling sites. Phylogenetic diversity varied among the sampling sites but was directly related to species richness. Although not significantly related to plateau area, smaller or larger PDFaith were observed in the smallest and the largest plateaus, respectively.
Patterns of orchid bee species diversity and turnover among forested plateaus of central Amazonia
Machado, Carolina de Barros; Galetti, Pedro Manoel; Oliveira, Marcio; Dirzo, Rodolfo; Fernandes, Geraldo Wilson
2017-01-01
The knowledge of spatial pattern and geographic beta-diversity is of great importance for biodiversity conservation and interpreting ecological information. Tropical forests, especially the Amazon Rainforest, are well known for their high species richness and low similarity in species composition between sites, both at local and regional scales. We aimed to determine the effect and relative importance of area, isolation and climate on species richness and turnover in orchid bee assemblages among plateaus in central Brazilian Amazonia. Variance partitioning techniques were applied to assess the relative effects of spatial and environmental variables on bee species richness, phylogeny and composition. We hypothesized that greater abundance and richness of orchid bees would be found on larger plateaus, with a set of core species occurring on all of them. We also hypothesized that smaller plateaus would possess lower phylogenetic diversity. We found 55 bee species distributed along the nine sampling sites (plateaus) with 17 of them being singletons. There was a significant decrease in species richness with decreasing size of plateaus, and a significant decrease in the similarity in species composition with greater distance and climatic variation among sampling sites. Phylogenetic diversity varied among the sampling sites but was directly related to species richness. Although not significantly related to plateau area, smaller or larger PDFaith were observed in the smallest and the largest plateaus, respectively. PMID:28410432
NASA Astrophysics Data System (ADS)
Terasawa, T.; Hoshino, M.; Sakai, J.-I.; Hada, T.
1986-04-01
By means of a numerical simulation, the nonlinear evolution of large amplitude dispersive Alfven waves is studied. An energy transfer from the parent wave to two daughter Alfven-like waves and a soundlike wave is observed (a stimulated Brillouin scattering process). The observed growth rates and propagation characteristics of these daughter waves agree with the analytical results, which are obtained by extending the previous treatments by Goldstein (1978), Sakai (1978), and Sonnerup (1983). Ions are first trapped by the electrostatic potential of the daughter soundlike waves. Along with the eventual decay (ion Landau damping) of the soundlike waves, ions are phase-mixed and left heated in the parallel direction. The increased parallel energy of ions is transferred to the perpendicular thermal energy through the nonresonant scattering process in the colliding Alfven waves (parent and daughter waves). It is observed that the daughter Alfven waves, which still have a large amplitude, are also unstable for further decay, and that the wave energy is continuously transferred to the longer wavelength regime (inverse cascading process).
NASA Astrophysics Data System (ADS)
Chen, X.; Qin, G.; Ai, Z.; Ji, Y.
2017-08-01
As an effective and economic method for flow range enhancement, circumferential groove casing treatment (CGCT) is widely used to increase the stall margin of compressors. Different from traditional grooved casing treatments, in which the grooves are always located over the rotor in both axial and radial compressors, one or several circumferential grooves are located along the shroud side of the diffuser passage in this paper. Numerical investigations were conducted to predict the performance of a low flow rate centrifugal compressor with CGCT in diffuser. Computational fluid dynamics (CFD) analysis is performed under stage environment in order to find the optimum location of the circumferential casing groove in consideration of stall margin enhancement and efficiency gain at design point, and the impact of groove number to the effect of this grooved casing treatment configuration in enhancing the stall margin of the compressor stage is studied. The results indicate that the centrifugal compressor with circumferential groove in vaned diffuser can obtain obvious improvement in the stall margin with sacrificing design efficiency a little. Efforts were made to study blade level flow mechanisms to determine how the CGCT impacts the compressor’s stall margin (SM) and performance. The flow structures in the passage, the tip gap, and the grooves as well as their mutual interactions were plotted and analysed.
SRTM Colored Height and Shaded Relief: Lava plateaus in Argentina
NASA Technical Reports Server (NTRS)
2001-01-01
All of the major landforms relate to volcanism and/or erosion in this Shuttle Radar Topography Mission scene of Patagonia, near La Esperanza, Argentina. The two prominent plateaus once formed a continuous surface that extended over much of this region. Younger volcanoes have grown through and atop the plateau, and one just south of this scene has sent a long, narrow flow down a stream channel (lower left). The topographic pattern shows that streams dominate the erosion processes in this arid environment even though wind is known to move substantial amounts of sediment here.
Two visualization methods were combined to produce this image: shading and color coding of topographic height. The shade image was derived by computing topographic slope in the north-south direction. Northern slopes appear bright and southern slopes appear dark, as would be the case at noon at this latitude in the southern hemisphere. Color-coding is directly related to topographic height, with green at the lower elevations, rising through yellow, red, and magenta, to white at the highest elevations.
Elevation data used in this image was acquired by the Shuttle Radar Topography Mission aboard the Space Shuttle Endeavour, launched on February 11, 2000. SRTM used the same radar instrument that comprised the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) that flew twice on the Space Shuttle Endeavour in 1994. SRTM was designed to collect three-dimensional measurements of the Earth's surface. To collect the 3-D data, engineers added a 60-meter-long (200-foot) mast, installed additional C-band and X-band antennas, and improved tracking and navigation devices. The mission is a cooperative project between NASA, the National Imagery and Mapping Agency (NIMA) of the U.S. Department of Defense, and the German and Italian space agencies. It is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Earth Science Enterprise, Washington, DC.
Size: 62.4 by 88
Row, Jeffrey R; Wilson, Paul J; Murray, Dennis L
2014-07-01
cyclic propensity and amplitude, suggesting that changes in this vital rate may contribute to the spatial or temporal variability observed in the cyclic dynamics of both systems. Collectively, our results reveal that the type of density dependence and its effect on different demographic parameters can profoundly influence numeric stability and cyclic propensity and therefore may shift populations across the cyclic-to-noncyclic boundary. © 2014 The Authors. Journal of Animal Ecology © 2014 British Ecological Society.
Franciscan complex calera limestones: Accreted remnants of farallon plate oceanic plateaus
Tarduno, J.A.; McWilliams, M.; Debiche, M.G.; Sliter, W.V.; Blake, M.C.
1985-01-01
The Calera Limestone, part of the Franciscan Complex of northern California, may have formed in a palaeoenvironment similar to Hess and Shatsky Rises of the present north-west Pacific1. We report here new palaeomagnetic results, palaeontological data and recent plate-motion models that reinforce this assertion. The Calera Limestone may have formed on Farallon Plate plateaus, north of the Pacific-Farallon spreading centre as a counterpart to Hess or Shatsky Rises. In one model2, the plateaus were formed by hotspots close to the Farallon_Pacific ridge axis. On accretion to North America, plateau dissection in the late Cretaceous to Eocene (50-70 Myr) could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the Franciscan Complex. Partial subduction of the plateaus could have contributed to Laramide (70-40 Myr) compressional events3. ?? 1985 Nature Publishing Group.
[Voyage to Bahnars country, people living in the high plateaus of central Vietnam].
Rault, J P; Rioux, O; Bellier, L; Verbeek-Hyaoh, C
1995-01-01
Southeast Asia was born from the collision between the Indian subcontinent and mainland China. Vietnam owes its 54 ethnic groups and four languages to a succession of migrations over the milleniums. The high plateaus are inhabited by a multitude of small ethnic groups commonly referred to as the "Moi". Vietnamity and the Association for Aid to the Ethnic Minorities of Vietnam are dedicated to developing and implementing cooperative projects with Vietnam. Living in the remote hinterland, the now less than one million mountain people of the High Plateaus were untouched by Indian and Chinese influences and remained independent until French colonisation which had only minor effects on their culture. During an expedition to the high plateaus, a privileged group sent by Vietenamity encountered one of these peoples, the Bahnars. In the context of a village festival, the authors describe the traditional way of life and beliefs of the Bahnar people.
NASA Astrophysics Data System (ADS)
Li, Fei; Yu, Peicheng; Xu, Xinlu; Fiuza, Frederico; Decyk, Viktor K.; Dalichaouch, Thamine; Davidson, Asher; Tableman, Adam; An, Weiming; Tsung, Frank S.; Fonseca, Ricardo A.; Lu, Wei; Mori, Warren B.
2017-05-01
In this paper we present a customized finite-difference-time-domain (FDTD) Maxwell solver for the particle-in-cell (PIC) algorithm. The solver is customized to effectively eliminate the numerical Cerenkov instability (NCI) which arises when a plasma (neutral or non-neutral) relativistically drifts on a grid when using the PIC algorithm. We control the EM dispersion curve in the direction of the plasma drift of a FDTD Maxwell solver by using a customized higher order finite difference operator for the spatial derivative along the direction of the drift (1 ˆ direction). We show that this eliminates the main NCI modes with moderate |k1 | , while keeps additional main NCI modes well outside the range of physical interest with higher |k1 | . These main NCI modes can be easily filtered out along with first spatial aliasing NCI modes which are also at the edge of the fundamental Brillouin zone. The customized solver has the possible advantage of improved parallel scalability because it can be easily partitioned along 1 ˆ which typically has many more cells than other directions for the problems of interest. We show that FFTs can be performed locally to current on each partition to filter out the main and first spatial aliasing NCI modes, and to correct the current so that it satisfies the continuity equation for the customized spatial derivative. This ensures that Gauss' Law is satisfied. We present simulation examples of one relativistically drifting plasma, of two colliding relativistically drifting plasmas, and of nonlinear laser wakefield acceleration (LWFA) in a Lorentz boosted frame that show no evidence of the NCI can be observed when using this customized Maxwell solver together with its NCI elimination scheme.
Li, Fei; Yu, Peicheng; Xu, Xinlu; ...
2017-01-12
In this study we present a customized finite-difference-time-domain (FDTD) Maxwell solver for the particle-in-cell (PIC) algorithm. The solver is customized to effectively eliminate the numerical Cerenkov instability (NCI) which arises when a plasma (neutral or non-neutral) relativistically drifts on a grid when using the PIC algorithm. We control the EM dispersion curve in the direction of the plasma drift of a FDTD Maxwell solver by using a customized higher order finite difference operator for the spatial derivative along the direction of the drift (1ˆ direction). We show that this eliminates the main NCI modes with moderate |k1|, while keeps additionalmore » main NCI modes well outside the range of physical interest with higher |k1|. These main NCI modes can be easily filtered out along with first spatial aliasing NCI modes which are also at the edge of the fundamental Brillouin zone. The customized solver has the possible advantage of improved parallel scalability because it can be easily partitioned along 1ˆ which typically has many more cells than other directions for the problems of interest. We show that FFTs can be performed locally to current on each partition to filter out the main and first spatial aliasing NCI modes, and to correct the current so that it satisfies the continuity equation for the customized spatial derivative. This ensures that Gauss’ Law is satisfied. Lastly, we present simulation examples of one relativistically drifting plasma, of two colliding relativistically drifting plasmas, and of nonlinear laser wakefield acceleration (LWFA) in a Lorentz boosted frame that show no evidence of the NCI can be observed when using this customized Maxwell solver together with its NCI elimination scheme.« less
NASA Astrophysics Data System (ADS)
Soobbarayen, K.; Sinou, J.-J.; Besset, S.
2014-10-01
This paper presents a numerical study of the influence of loading conditions on the vibrational and acoustic responses of a disc brake system subjected to squeal. A simplified model composed of a circular disc and a pad is proposed. Nonlinear effects of contact and friction over the frictional interface are modelled with a cubic law and a classical Coulomb's law with a constant friction coefficient. The stability analysis of this system shows the presence of two instabilities with one and two unstable modes that lead to friction-induced nonlinear vibrations and squeal noise. Nonlinear time analysis by temporal integration is conducted for two cases of loadings and initial conditions: a static load near the associated sliding equilibrium and a slow and a fast ramp loading. The analysis of the time responses shows that a sufficiently fast ramp loading can destabilize a stable configuration and generate nonlinear vibrations. Moreover, the fast ramp loading applied for the two unstable cases generates higher amplitudes of velocity than for the static load cases. The frequency analysis shows that the fast ramp loading generates a more complex spectrum than for the static load with the appearance of new resonance peaks. The acoustic responses for these cases are estimated by applying the multi-frequency acoustic calculation method based on the Fourier series decomposition of the velocity and the Boundary Element Method. Squeal noise emissions for the fast ramp loading present lower or higher levels than for the static load due to the different amplitudes of velocities. Moreover, the directivity is more complex for the fast ramp loading due to the appearance of new harmonic components in the velocity spectrum. Finally, the sound pressure convergence study shows that only the first harmonic components are sufficient to well describe the acoustic response.
A yellowbrush/grass community type from the Uinta Mountains and Utah Plateaus
Sherel Goodrich; Robert M. Thompson; Allen Huber
2001-01-01
Ecological inventory and vegetation monitoring in the Uinta Mountains and Utah Plateaus on the Ashley National Forest and Manti-LaSal National Forest of the past several years suggest a community type in which yellowbrush (Chrysothamnus viscidiflorus ssp. lanceolatus), and slender wheatgrass (Elymus trachycaulus) are indicator species. This community type is common at...
Imes, Jeffrey L.
1990-01-01
An investigation of the geohydrologic system in the Ozark Plateaus province (index map and Fenneman, 1938) has been made as part of the Central Midwest Regional Aquifer-System Analysis (Jorgenson and Signor, 1981), a major study that encompasses parts of 10 States. The study is one of several by the U.S. Geological Survey that are designed to increase knowledge of the flow regime and geohydrologic properties of regional aquifer systems in the United States. Because a large quantity of fresh groundwater is available in aquifers underlying the Ozark Plateaus province, a subregional project has been established to study the geohydrologic units of this area in more detail than is practical in the regional study. The stratigraphic relationship among the primary geohydrologic units in the Ozark Plateaus province is discussed in Chapter A of this Hydrologic Investigations Atlas series. This chapter focuses on the Ozark aquifer, a geohydrologic unit within the Ozark Plateaus aquifer system underlying the Ozark Plateaus province.
NASA Astrophysics Data System (ADS)
Zhou, Qina; Mathews, John D.; Miller, Clark A.; Seker, Ilgin
2006-07-01
In recent years, all-sky camera airglow observations of evolving nighttime F-region structures have raised questions regarding the formation and apparent motion of these often wave-like structures. We address these issues using a pseudo-spectral method code developed to numerically solve the Perkins (1973. Spread F and ionospheric currents. J. Geophys. Res. 78, 218-226) moment equations modeling F-region electrodynamics. To aid in interpretation of the results, we utilize a Gaussian shape initial condition of the (geomagnetic field, B, parallel) integrated conductivity under the homogeneous TEC ( B-parallel total electron content) condition and a northeastward DC electric field (E-field). We find that the initial Gaussian shape conductivity structure gradually evolves into banded structures oriented along the northwest-southeast direction while the amplitude of the banded structures continues growing and the peak of the structure moves to the northwest due to the E× B drift. The potential distribution corresponding to the initial Gaussian conductivity distribution is more complex but also becomes banded with the same orientation and growing trend as the conductivity. Wave vector domain plots show structure growth in approximately the first and third quadrants and damping in the second and fourth quadrants for both the conductivity and potential, as Perkins predicts - this leads to the orientation of the structures. We note that the evolved banded structures in conductivity and potential are oriented perpendicular to the direction given by half the angle between the DC E-field and east - the direction of maximum instability growth rate predicted by Perkins. The polarization (perturbation) E-field is seen mainly perpendicular to the long axis of the banded structures - i.e., no obvious structure-parallel E-field is observed in the simulation. By tracking the maximum point of the conductivity as a function of time, it is found that the localized structures move
Burke, Brian C.; Freeman, Walter J.; Chang, Hung Jen
1998-04-01
-type power spectra transit earlier to limit cycle activity, usually well before 2000ms. The duration of stationarity is increased by randomizing the terminal bit of the 64-bit words representing the state variables. The 1/f-type solutions are also exquisitely sensitive to parameter truncation; parameter values must be saved in their full binary form for re-starting. The implications in terms of numerical instability, chaos, attractor crowding and the shadowing theorem are discussed.
Erosion and tectonics at the margins of continental plateaus
NASA Technical Reports Server (NTRS)
Masek, Jeffrey G.; Isacks, Bryan L.; Gubbels, Timothy L.; Fielding, Eric J.
1994-01-01
We hypothesize that the steep frontal slope and high peaks of the Beni region and Himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid Pilcomayo region reflects a tectonic landform only slightly modified by erosion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long-wavelength slope. A numerical model coupling orographic precipitation, erosion, and tectonic uplift demonstrates the plausibility of this hypothesis. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya suggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.
Erosion and tectonics at the margins of continental plateaus
NASA Technical Reports Server (NTRS)
Masek, Jeffrey G.; Isacks, Bryan L.; Gubbels, Timothy L.; Fielding, Eric J.
1994-01-01
We hypothesize that the steep frontal slope and high peaks of the Beni region and Himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid Pilcomayo region reflects a tectonic landform only slightly modified by erosion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long-wavelength slope. A numerical model coupling orographic precipitation, erosion, and tectonic uplift demonstrates the plausibility of this hypothesis. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya suggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.
NASA Astrophysics Data System (ADS)
Kuznetsov, V. I.; Firsov, A. A.
2016-11-01
Segments of an almost constant voltage (plateaus) on the V( I) curves of long quasi-one-dimensional superconducting aluminum wires placed in a magnetic field are found slightly below T c, which are unexpected at the parameters and geometry considered in this work. These plateaus are assumingly attributed to subharmonics of the superconducting gap and are due to multiple Andreev reflection and strong quasiparticle heating, which occur in the nonequilibrium region of a wire. The plateaus indicate the coexistence of superconductivity and dissipation in these wires. These results cannot be described by the existing theories.
Oceanic plateaus, the fragmentation of continents, and mountain building
NASA Astrophysics Data System (ADS)
Nur, Amos; Ben-Avraham, Zvi
1982-05-01
Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments, have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent-continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent-continent collision may not be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinities in the Mesozoic-Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle-wide scale, is implied.
Oceanic plateaus, the fragmentation of continents, and mountain building
Nur, A.; Ben-Avraham, Z.
1982-05-10
Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochtronous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent-continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent-continent collision may be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinites in the Mesozoic-Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle-wide scale, is implied.
NASA Technical Reports Server (NTRS)
Mart, Y.
1988-01-01
A system of marine plateaus occurs in the western equatorial Indian Ocean, forming an arcuate series of wide and shallow banks with small islands in places. The oceanic basins that surround the Seychelles - Amirante region are of various ages and reflect a complex seafloor spreading pattern. The structural analysis of the Seychelle - Amirante - Mascarene region reflects the tectonic evolution of the western equatorial Indian Ocean. It is suggested that due to the seafloor spreading during a tectonic stage, the Seychelles continental block drifted southwestwards to collide with the oceanic crust of the Mascarene Basin, forming an elongated folded structure at first, and then a subduction zone. The morphological similarity, the lithological variability and the different origin of the Seychelles Bank, the Mascarene Plateau and the Amirante Arc emphasizes the significant convergent effects of various plate tectonic processes on the development of marine plateaus.
NASA Technical Reports Server (NTRS)
Mart, Y.
1988-01-01
A system of marine plateaus occurs in the western equatorial Indian Ocean, forming an arcuate series of wide and shallow banks with small islands in places. The oceanic basins that surround the Seychelles - Amirante region are of various ages and reflect a complex seafloor spreading pattern. The structural analysis of the Seychelle - Amirante - Mascarene region reflects the tectonic evolution of the western equatorial Indian Ocean. It is suggested that due to the seafloor spreading during a tectonic stage, the Seychelles continental block drifted southwestwards to collide with the oceanic crust of the Mascarene Basin, forming an elongated folded structure at first, and then a subduction zone. The morphological similarity, the lithological variability and the different origin of the Seychelles Bank, the Mascarene Plateau and the Amirante Arc emphasizes the significant convergent effects of various plate tectonic processes on the development of marine plateaus.
Benoit, M H; Nyblade, A A; Pasyanos, M E
2006-01-17
The East African and Ethiopian Plateaus have long been recognized to be part of a much larger topographic anomaly on the African Plate called the African Superswell. One of the few places within the African Superswell that exhibit elevations of less than 1 km is southeastern Sudan and northern Kenya, an area containing both Mesozoic and Cenozoic rift basins. Crustal structure and uppermost mantle velocities are investigated in this area by modeling Rayleigh wave dispersion. Modeling results indicate an average crustal thickness of 25 {+-} 5 km, some 10-15 km thinner than the crust beneath the adjacent East African and Ethiopian Plateaus. The low elevations can therefore be readily attributed to an isostatic response from crustal thinning. Low Sn velocities of 4.1-4.3 km/s also characterize this region.
Distinguishing between yield advances and yield plateaus in historical crop production trends.
Grassini, Patricio; Eskridge, Kent M; Cassman, Kenneth G
2013-01-01
Food security and land required for food production largely depend on rate of yield gain of major cereal crops. Previous projections of food security are often more optimistic than what historical yield trends would support. Many econometric projections of future food production assume compound rates of yield gain, which are not consistent with historical yield trends. Here we provide a framework to characterize past yield trends and show that linear trajectories adequately describe past yield trends, which means the relative rate of gain decreases over time. Furthermore, there is evidence of yield plateaus or abrupt decreases in rate of yield gain, including rice in eastern Asia and wheat in northwest Europe, which account for 31% of total global rice, wheat and maize production. Estimating future food production capacity would benefit from an analysis of past crop yield trends based on a robust statistical analysis framework that evaluates historical yield trajectories and plateaus.
Distinguishing between yield advances and yield plateaus in historical crop production trends
Grassini, Patricio; Eskridge, Kent M.; Cassman, Kenneth G.
2013-01-01
Food security and land required for food production largely depend on rate of yield gain of major cereal crops. Previous projections of food security are often more optimistic than what historical yield trends would support. Many econometric projections of future food production assume compound rates of yield gain, which are not consistent with historical yield trends. Here we provide a framework to characterize past yield trends and show that linear trajectories adequately describe past yield trends, which means the relative rate of gain decreases over time. Furthermore, there is evidence of yield plateaus or abrupt decreases in rate of yield gain, including rice in eastern Asia and wheat in northwest Europe, which account for 31% of total global rice, wheat and maize production. Estimating future food production capacity would benefit from an analysis of past crop yield trends based on a robust statistical analysis framework that evaluates historical yield trajectories and plateaus. PMID:24346131
Czarnecki, John B.; Bolyard, Susan E.; Hart, Rheannon M.; Clark, Jimmy M.
2014-01-01
Digital surfaces and thicknesses of nine hydrogeologic units of the Ozark Plateaus aquifer system from land surface to the top of the Gunter Sandstone in northwestern Arkansas were created using geophysical logs, drillers’ logs, geologist-interpreted formation tops, and previously published maps. The 6,040 square mile study area in the Ozark Plateaus Province includes Benton, Washington, Carroll, Madison, Boone, Newton, Marion, and Searcy Counties. The top of each hydrogeologic unit delineated on geophysical logs was based partly on previously published reports and maps and also from drillers’ logs. These logs were then used as a basis to contour digital surfaces showing the top and thickness of the Fayetteville Shale, the Boone Formation, the Chattanooga Shale, the Everton Formation, the Powell Dolomite, the Cotter Dolomite, the Roubidoux Formation, the Gasconade Dolomite, and the Gunter Sandstone.
K.Y. Ng
2003-08-25
The lecture covers mainly Sections 2.VIII and 3.VII of the book ''Accelerator Physics'' by S.Y. Lee, plus mode-coupling instabilities and chromaticity-driven head-tail instability. Besides giving more detailed derivation of many equations, simple interpretations of many collective instabilities are included with the intention that the phenomena can be understood more easily without going into too much mathematics. The notations of Lee's book as well as the e{sup jwt} convention are followed.
Colaiacomo, M C; Tortora, A; Di Biasi, C; Polettini, E; Casciani, E; Gualdi, G F
2009-01-01
The clinic diagnosis of degenerative lumbar intervertebral instability is a controversial topic and have not yet been clarified clinical criteria for to define this condition with accuracy. Although the lumbar pain is the most common symptom in patients who have lumbar intervertebral instability its clinical presentation is not specific; moreover in patients with lumbar pain there are no agreed signs and symptoms that can be truly attributable to instability. Despite better imaging techniques of testing spinal instability there is not a clear relations between radiologic signs of instability and clinical symptoms. It is, however, still far from unanimous definition of degenerative lumbar intervertebral instability accepted from all specialists involved in diagnosis and treatment of this condition; however, seem there is most agree about suspected vertebral instability. Nevertheless this unresolved topic, it is possible to state that imaging play an increasing role in diagnosis and management of patients with suspected instability. The aim of this study is to investigate the different imaging modalities most indicated in diagnosis if vertebral instability and whether degenerative change can be associated with lower back pain.
Identification and occurrence of uranium and vanadium minerals from the Colorado Plateaus
Weeks, A.D.; Thompson, M.E.
1954-01-01
This report, designed to make available to field geologists and others informa- tion obtained in recent investigations by the Geological Survey on identification and occurrence of uranium minerals of the Colorado Plateaus, contains descriptions of the physical properties, X-ray data, and in some instances results of chemical and spectrographic analysis of 48 uranium and vanadium minerals. Also included is a list of locations of mines from which the minerals have been identified.
Plateaus, Dips, and Leaps: Where to Look for Inventions and Discoveries During Skilled Performance.
Gray, Wayne D; Lindstedt, John K
2017-09-01
The framework of plateaus, dips, and leaps shines light on periods when individuals may be inventing new methods of skilled performance. We begin with a review of the role performance plateaus have played in (a) experimental psychology, (b) human-computer interaction, and (c) cognitive science. We then reanalyze two classic studies of individual performance to show plateaus and dips which resulted in performance leaps. For a third study, we show how the statistical methods of Changepoint Analysis plus a few simple heuristics may direct our focus to periods of performance change for individuals. For the researcher, dips become the marker of exploration where performance suffers as new methods are invented and tested. Leaps mark the implementation of a successful new method and an incremental jump above the path plotted by smooth and steady log-log performance increments. The methods developed during these dips and leaps are the key to surpassing one's teachers and acquiring extreme expertise. Copyright © 2016 Cognitive Science Society, Inc.
Detection of vertebral plateaus in lateral lumbar spinal X-ray images with Gabor filters.
Alvarez Ribeiro, Eduardo; Nogueira-Barbosa, Marcello Henrique; Rangayyan, Rangaraj M; Azevedo-Marques, Paulo M
2010-01-01
A few recent studies have proposed computed-aided methods for the detection and analysis of vertebral bodies in radiographic images. This paper presents a method based on Gabor filters. Forty-one lateral lumbar spinal X-ray images from different patients were included in the study. For each image, a radiologist manually delineated the vertebral plateaus of L1, L2, L3, and L4 using a software tool for image display and mark-up. Each original image was filtered with a bank of 180 Gabor filters. The angle of the Gabor filter with the highest response at each pixel was used to derive a measure of the strength of orientation or alignment. In order to limit the spatial extent of the image data and the derived features in further analysis, a semi-automated procedure was applied to the original image. A neural network utilizing the logistic sigmoid function was trained with pixel intensity from the original image, the result of manual delineation of the plateaus, the Gabor magnitude response, and the alignment image. The average overlap between the results of detection by image processing and manual delineation of the plateaus of L1-L4 in the 41 images tested was 0.917. The results are expected to be useful in the analysis of vertebral deformities and fractures.
Imes, Jeffrey L.
1990-01-01
An investigation of the geohydrologic system in the Ozark Plateaus province (index map and Fenneman, 1938) has been made as part of the Central Midwest Regional Aquifer System Analysis (Jorgensen and Signor, 1981), a major study of the regional aquifer system in parts of 10 States. The study is one of several by the U.S. Geological Survey that are designed to increase knowledge of the flow regime and geohydrologic properties of regional aquifer systems in the United States. Because a large quantity of fresh ground water is available in aquifers underlying the Ozark Plateaus province, a subregional project has been established to study the geohydrologic units of this area in more detail than is practical in the regional study. The stratigraphic and geologic relationships among the primary geohydrologic units in and adjacent to the Ozark Plateaus is depicted in this atlas (Chapter A). This is the first of a series of chapters (A-H) that includes maps of the altitude of the top, thickness, potentiometric surface, and percentage-of-shale content of individual geohydrologic units.
Retired flies, hidden plateaus, and the evolution of senescence in Drosophila melanogaster.
Curtsinger, James W
2016-06-01
Late-life plateaus in age-specific mortality have been an evolutionary and biodemographic puzzle for decades. Although classic theory on the evolution of senescence predicts late-life walls of death, observations in experimental organisms document the opposite trend: a slowing in the rate of increase of mortality at advanced ages. Here, I analyze published life-history data on individual Drosophila melanogaster females and argue for a fundamental change in our understanding of mortality in this important model system. Mortality plateaus are not, as widely assumed, exclusive to late life, and are not explained by population heterogeneity-they are intimately connected to individual fecundity. Female flies begin adult life in the working stage, a period of active oviposition and low but accelerating mortality. Later they transition to the retired stage, a terminal period characterized by limited fecundity and relatively constant mortality. Because ages of transition differ between flies, age-synchronized cohorts contain a mix of working and retired flies. Early- and mid-life plateaus are obscured by the presence of working flies, but can be detected when cohorts are stratified by retirement status. Stage-specificity may be an important component of Drosophila life-history evolution. © 2016 The Author(s). Evolution © 2016 The Society for the Study of Evolution.
NASA Astrophysics Data System (ADS)
Zhang, Guoqing; Yao, Tandong; Piao, Shilong; Bolch, Tobias; Xie, Hongjie; Chen, Deliang; Gao, Yanhong; O'Reilly, Catherine M.; Shum, C. K.; Yang, Kun; Yi, Shuang; Lei, Yanbin; Wang, Weicai; He, You; Shang, Kun; Yang, Xiankun; Zhang, Hongbo
2017-01-01
Asia's high plateaus are sensitive to climate change and have been experiencing rapid warming over the past few decades. We found 99 new lakes and extensive lake expansion on the Tibetan Plateau during the last four decades, 1970-2013, due to increased precipitation and cryospheric contributions to its water balance. This contrasts with disappearing lakes and drastic shrinkage of lake areas on the adjacent Mongolian Plateau: 208 lakes disappeared, and 75% of the remaining lakes have shrunk. We detected a statistically significant coincidental timing of lake area changes in both plateaus, associated with the climate regime shift that occurred during 1997/1998. This distinct change in 1997/1998 is thought to be driven by large-scale atmospheric circulation changes in response to climate warming. Our findings reveal that these two adjacent plateaus have been changing in opposite directions in response to climate change. These findings shed light on the complex role of the regional climate and water cycles and provide useful information for ecological and water resource planning in these fragile landscapes.
NASA Astrophysics Data System (ADS)
Erwin, S. O.; Jacobson, R. B.; Eric, A. B.; Jones, J. C.; Anderson, B. W.
2015-12-01
Perturbations to sediment regimes due to anthropogenic activities may have long lasting effects, especially in systems dominated by coarse sediment where travel times are relatively long. Effectively evaluating management alternatives requires understanding the future trajectory of river response at both the river network and reach scales. The Ozark Plateaus physiographic province is a montane region in the interior US composed primarily of Paleozoic sedimentary rock. Historic land-use practices around the turn of the last century accelerated delivery of coarse sediment to river channels. Effects of this legacy sediment persist in two national parks, Ozark National Scenic Riverways, MO and Buffalo National River, AR, and are of special concern for management of native mussel fauna. These species require stable habitat, yet they occupy inherently dynamic environments: alluvial rivers. At the river-network scale, analysis of historical data reveals the signature of sediment waves moving through river networks in the Ozarks. Channel planform alternates between relatively stable, straight reaches, and wider, multithread reaches which have been more dynamic over the past several decades. These alternate planform configurations route and store sediment differently, and translate into different patterns of bed stability at the reach scale, which in turn affects the distribution and availability of habitat for native biota. Geomorphic mapping and hydrodynamic modeling reveal the complex relations between planform (in)stability, flow dynamics, bed mobility, and aquatic habitat in systems responding to increased sediment supply. Reaches that have a more dynamic planform may provide more hydraulic refugia and habitat heterogeneity compared to stable, homogeneous reaches. This research provides new insights that may inform management of sediment and mussel habitat in rivers subject to coarse legacy sediment.
Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger, Thomas
2012-02-20
The launch of the Hinode satellite led to the discovery of rising plumes, dark in chromospheric lines, that propagate from large ({approx}10 Mm) bubbles that form at the base of quiescent prominences. The plumes move through a height of approximately 10 Mm while developing highly turbulent profiles. The magnetic Rayleigh-Taylor instability was hypothesized to be the mechanism that drives these flows. In this study, using three-dimensional (3D) MHD simulations, we investigate the nonlinear stability of the Kippenhahn-Schlueter prominence model for the interchange mode of the magnetic Rayleigh-Taylor instability. The model simulates the rise of a buoyant tube inside the quiescent prominence model, where the interchange of magnetic field lines becomes possible at the boundary between the buoyant tube and the prominence. Hillier et al. presented the initial results of this study, where upflows of constant velocity (maximum found 6 km s{sup -1}) and a maximum plume width Almost-Equal-To 1.5 Mm which propagate through a height of approximately 6 Mm were found. Nonlinear interaction between plumes was found to be important for determining the plume dynamics. In this paper, using the results of ideal MHD simulations, we determine how the initial parameters for the model and buoyant tube affect the evolution of instability. We find that the 3D mode of the magnetic Rayleigh-Taylor instability grows, creating upflows aligned with the magnetic field of constant velocity (maximum found 7.3 km s{sup -1}). The width of the upflows is dependent on the initial conditions, with a range of 0.5-4 Mm which propagate through heights of 3-6 Mm. These results are in general agreement with the observations of the rising plumes.
NASA Astrophysics Data System (ADS)
Hendy, I. L.; Cosma, T.
2006-12-01
The small, ephemeral Cordilleran Ice Sheet, present in Alaska, British Columbia and northern Washington during the last glacial cycle is believed to have behaved differently than the larger Laurentice Ice Sheet to climate forcing. High quality chronology is required to understand the relationship between this ice sheet and global climate change. Presently MD02-2496 (48°58.47N: 127°02.14W; 1190m water depth) is the highest resolution paleoclimate record available for the last 50 Ka in the Pacific northwest and contains intervals of glacial-marine sedimentation. High resolution dating based on 36 radiocarbon dates provide a chronology that includes radiocarbon age plateaus, while planktonic foraminiferal stable isotopes offer a continuous record of climate change. Glacial-marine sediments collected from this site on the continental slope west of Vancouver Island, British Columbia, document three intervals of iceberg discharge during the last ~50 Ka. Gradually increasing quantities of ice rafted detritus (grains >250μm, g-1; IRD) followed by abrupt cessation within ~500 years is strongly suggestive of catastrophic iceberg discharge. The penultimate event is correlated to marine invasion of the Juan de Fuca Strait, and Puget Sound, while the final IRD event with that of Georgia Strait. We posit that these previously unknown IRD events represent repeated rapid iceberg discharge related to Cordilleran Ice Sheet collapse. The events occur near the end of radiocarbon plateaus at 13.35 ±90 and 14.05 ±70 14C Kyr BP (not reservoir corrected). If these plateaus correlate with the 12.2 and 13.3 14C Kyr BP plateaus recorded in Carriaco Basin and elsewhere, local reservoir ages can be calculated and vary between 1,150 and 1,550 years similar to those derived locally from glacial wood-shell pairs. Furthermore, if the plateaus result from reduced North Atlantic Deep Water export and consequently Heinrich Events, the Cordilleran IRD events are related to North Atlantic iceberg
Transverse instability of a rectangular bunch
Balbekov, V.; /Fermilab
2005-12-01
Transverse instability of a rectangular bunch is investigated. Known theory of bunched beam instability is modified to take into account 100% spread of synchrotron frequency. Series of equations adequately describing the instability is derived and solved analytically and numerically. The theory is applied to the Fermilab Recycler Ring.
Dislocation motion and instability
NASA Astrophysics Data System (ADS)
Zhu, Yichao; Chapman, Stephen Jonathan; Acharya, Amit
2013-08-01
The Peach-Koehler expression for the stress generated by a single (non-planar) curvilinear dislocation is evaluated to calculate the dislocation self stress. This is combined with a law of motion to give the self-induced motion of a general dislocation curve. A stability analysis of a rectilinear, uniformly translating dislocation is then performed. The dislocation is found to be susceptible to a helical instability, with the maximum growth rate occurring when the dislocation is almost, but not exactly, pure screw. The non-linear evolution of the instability is determined numerically, and implications for slip band formation and non-Schmid behavior in yielding are discussed.
Jacobson, Robert B.; Primm, Alexander T.
1997-01-01
Land-use changes have been blamed for creating disturbance in the morphology of streams in the Ozark Plateaus, Missouri (hereafter referred to as the "Ozarks"). Historical evidence and stratigraphic observations document that streams have been aggraded by substantial quantities of gravel beginning sometime at or near the time of European settlement of the Ozarks. Before European settlement, streams were depositing a mixed sediment load of gravel bedload and silty overbank sediment. Observations of early explorers conspicuously lack descriptions of extensive gravel bars; observations of geologists working during the middle to late 1800's before significant landuse disturbance, however, include descriptions of large quantities of gravel in stream banks and beds.The first change in land cover as settlement progressed from the early 1800's to approximately 1880 was replacement of valley-bottom forest with cultivated fields and pastures. At the same time, suppression of wildfires in the uplands caused an increase of woodland with woody understory at the expense of grassland and oak savannah. Valley-bottom clearing probably initiated some direct disturbance of stream channels, but fire suppression would have decreased runoff and sediment yield from uplands.Beginning sometime from 1870 to 1880 and continuing until 1920, commercial timber companies began large operations in the Ozarks to harvest shortleaf pine for sawlogs and oak for railroad ties. Selective cutting of large timber, use of livestock for skidding logs from the forest, and avoidance of the steeper slopes minimized the effect of this phase of logging on runoff and sediment supply of uplands and valley-side slopes. Continued decreases in the erosional resistance of valley bottoms through clearing and road building and the incidence of extreme regional floods from 1895 to 1915 probably caused initiation of moderate stream disturbance. This hypothesis is supported by historical and oral-historical observations
Adamski, James C.; Petersen, James C.; Freiwald, David A.; Davis, Jerri V.
1995-01-01
The environmental and hydrologic setting of the Ozark Plateaus National Water-Quality Assessment (NAWQA) study unit and the factors that affect water quality are described in this report. The primary natural and cultural features that affect water- quality characteristics and the potential for future water-quality problems are described. These environmental features include climate, physio- graphy, geology, soils, population, land use, water use, and surface- and ground-water flow systems. The study-unit area is approximately 47,600 square miles and includes most of the Ozark Plateaus Province and parts of the adjacent Osage Plains and Mississippi Alluvial Plain in parts of Arkansas, Kansas, Missouri, and Oklahoma. The geology is characterized by basement igneous rocks overlain by a thick sequence of dolomites, limestones, sandstones, and shales of Paleozoic age. Land use in the study unit is predominantly pasture and forest in the southeastern part, and pasture and cropland in the northwestern part. All or part of the White, Neosho-lllinois, Osage, Gasconade, Meramec, St. Francis, and Black River Basins are within the study unit. Streams in the Boston Mountains contain the least mineralized water, and those in the Osage Plains contain the most mineralized water. The study unit contains eight hydrogeologic units including three major aquifers--the Springfield Plateau, Ozark, and St. Francois aquifers. Streams and aquifers in the study unit generally contain calcium or calcium-magnesium bicarbonate waters. Ground- and surface-water interactions are greatest in the Salem and Springfield Plateaus and least in the Boston Mountains and Osage Plains. Geology, land use, and population probably are the most important environmental factors that affect water quality.
Water quality in the Ozark Plateaus, Arkansas, Kansas, Missouri, and Oklahoma, 1992-95
Petersen, James C.; Adamski, James C.; Bell, Richard W.; Davis, Jerri V.; Femmer, Suzanne R.; Freiwald, David A.; Joseph, Robert L.
1998-01-01
This report is intended to summarize major findings that emerged between 1992 and 1995 from the water-quality assessment of the Ozark Plateaus Study Unit and to relate these findings to water-quality issues of regional and national concern. The information is primarily intended for those who are involved in water-resource management. Yet, the information contained here may also interest those who simply wish to know more about the quality of water in the rivers and aquifers in the area where they live.
Key wintertime meteorological features of the Grand Canyon and the Colorado Plateaus Basin
Whiteman, C.D.; Allwine, K.J.
1992-06-01
In the winter of 1989--1990 a major meteorological and air pollution experiment was conducted in the Colorado Plateaus Basin (Richards et al., 1991). The focus of the experiment, conducted by Arizona's Soft River Project, was to investigate the influence of three 750-MW coal-fired power plant units at the Navajo Generating Station near Page, Arizona, on visibility at Grand Canyon National Park. As part of the meteorological experiment, surface and upper air data were collected from multiple sites within the basin. This data set is the most comprehensive meteorological data set ever collected within the region, and the purpose of this paper is to briefly summarize the key wintertime meteorological features of the Colorado Plateaus Basin and the Grand Canyon, through which the basin drains, using analyses of the Winter Visibility Study data. Our analyses focused primarily on thermally driven circulations within the basin and the Grand Canyon, but we also investigated the surface energy budget that drives these circulations and the interactions between the thermal circulations and the overlying synoptic-scale flows.
Key wintertime meteorological features of the Grand Canyon and the Colorado Plateaus Basin
Whiteman, C.D.; Allwine, K.J.
1992-06-01
In the winter of 1989--1990 a major meteorological and air pollution experiment was conducted in the Colorado Plateaus Basin (Richards et al., 1991). The focus of the experiment, conducted by Arizona`s Soft River Project, was to investigate the influence of three 750-MW coal-fired power plant units at the Navajo Generating Station near Page, Arizona, on visibility at Grand Canyon National Park. As part of the meteorological experiment, surface and upper air data were collected from multiple sites within the basin. This data set is the most comprehensive meteorological data set ever collected within the region, and the purpose of this paper is to briefly summarize the key wintertime meteorological features of the Colorado Plateaus Basin and the Grand Canyon, through which the basin drains, using analyses of the Winter Visibility Study data. Our analyses focused primarily on thermally driven circulations within the basin and the Grand Canyon, but we also investigated the surface energy budget that drives these circulations and the interactions between the thermal circulations and the overlying synoptic-scale flows.
Ground water in the Springfield-Salem plateaus of southern Missouri and northern Arkansas
Harvey, Edward Joseph
1980-01-01
Average ground-water conditions have not changed significantly in the Springfield-Salem plateaus section of southern Missouri and northern Arkansas in the past 25 years except in the vicinity of well fields. The amount of ground water pumped is approximately 200 cubic feet per second, which is about 5 percent of the total discharge at the 80 percent point on flow-duration curves for major streams. Ground-water recharge is variable and occurs through sinkholes by infiltration in upland areas of good permeability, and through streambeds that lose flow. Main waterbearing zones lie in the Potosi Dolomite and the lower dolomite and sandstone of the Gasconade Dolomite. Cavernous connections from ground surface to depths as great as 1,500 feet occur in the West Plains area, Mo., and result in deep circulation of water. Municipal well-water in the area often becomes turbid after rainstorms, despite well depths of 1 ,500 feet and 950 to 1,000 feet of pressure-grouted casing. Ground-water movement is generaly north and south from the crest of the Springfield-Salem plateaus, which extend across southern Missouri from the St. Francois Mountains to the southwest. Interbasin diversion of surface- and ground-water flow is common. (USGS)
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
Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger, Thomas
2012-09-10
The launch of the Hinode satellite has allowed high-resolution observations of supersonic bright downflows in quiescent prominences, known as prominence knots. We present observations in the Ca II H spectral line using the Solar Optical Telescope on board the Hinode satellite of a descending plasma knot of size {approx}900 km. The knot initially undergoes ballistic motion before undergoing impulsive accelerations at the same time as experiencing increases in intensity. We also present a subset of our three-dimensional magnetohydrodynamic simulations, performed to investigate the nonlinear stability of the Kippenhahn-Shlueter prominence model to the magnetic Rayleigh-Taylor instability in which interchange reconnection occurs. The interchange reconnection in the model breaks the force balance along the field lines which initiates the downflows. The downflows propagate with a downward fluid velocity of {approx}15 km s{sup -1} and a characteristic size of {approx}700 km. We conclude that the observed plasma blob and the simulated downflow are driven by the breaking of the force balance along the magnetic field as a result of a change in magnetic topology caused by reconnection of the magnetic field.
NASA Astrophysics Data System (ADS)
Sugimoto, Takanori; Mori, Michiyasu; Tohyama, Takami; Maekawa, Sadamichi
2015-09-01
The quantum phase transitions induced by a magnetic field are theoretically studied in a frustrated two-leg spin ladder. Using the density-matrix renormalization-group method, we find some magnetic phase transitions and plateaus in two different cases of strong and weak rung couplings. With the strong rung coupling, the three magnetization plateaus are found at 1/3, 1/2, and 2/3 due to the frustration. Those can be understood in terms of a quasispinon reconstructed from the singlet and the triplets of spins on a rung. The plateau at 1/2 corresponds to the valence bond solid of the quasispinons, while the plateaus at 1/3 and 2/3 can be associated with the array of quasispinons such as the soliton lattice. This is different from the usual Bose-Einstein-condensation picture of triplons. Our results will be useful in understanding magnetization curves in BiCu2PO6 .
Simulations of Astrophysical fluid instabilities
NASA Astrophysics Data System (ADS)
Calder, A. C.; Fryxell, B.; Rosner, R.; Dursi, L. J.; Olson, K.; Ricker, P. M.; Timmes, F. X.; Zingale, M.; MacNeice, P.; Tufo, H. M.
2001-10-01
We present direct numerical simulations of mixing at Rayleigh-Taylor unstable interfaces performed with the FLASH code, developed at the ASCI/Alliances Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We present initial results of single-mode studies in two and three dimensions. Our results indicate that three-dimensional instabilities grow significantly faster than two-dimensional instabilities and that grid resolution can have a significant effect on instability growth rates. We also find that unphysical diffusive mixing occurs at the fluid interface, particularly in poorly resolved simulations. .
Instability of a bubble chain.
Zhang, Wenjuan; An, Yu
2013-05-01
Based on the theory of shape instability and diffusive instability for single bubbles, we have studied the instability of an individual bubble in a bubble chain and found that its stable area enlarges the narrower the distance between bubbles. The spatial stability of the bubble chain is due to the secondary Bjerknes force between bubbles. Numerical calculations show the tension of the bubble chain varies with bubble distance and maxima appear at certain distances which could correspond to the stable states of the bubble chain.
NASA Astrophysics Data System (ADS)
Tetreault, J. L.; Buiter, S. J. H.
2014-07-01
Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate during subduction by accretionary processes. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and have 3 distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. In addition many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. And other times we find evidence of collision leaving behind accreted terranes 25 to 40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to
NASA Astrophysics Data System (ADS)
Tetreault, J. L.; Buiter, S. J. H.
2014-12-01
Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT
Electromagnetic ion beam instabilities
NASA Technical Reports Server (NTRS)
Gary, S. P.; Foosland, D. W.; Smith, C. W.; Lee, M. A.; Goldstein, M. L.
1984-01-01
The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T (perpendicular to B)/T (parallel to B) gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.
A mantle plume initiation model for the wrangellia flood basalt and other oceanic plateaus.
Richards, M A; Jones, D L; Duncan, R A; Depaolo, D J
1991-10-11
The vast Wrangellia terrane of Alaska and British Columbia is an accreted oceanic plateau with Triassic strata that contain a 3- to 6-kilometers thick flood basalt, bounded above and below by marine sedimentary rocks. This enormous outpouring of basalt was preceded by rapid uplift and was followed by gradual subsidence of the plateau. The uplift and basalt eruptions occurred in less than approximately 5 million years, and were not accompanied by significant extension or rifting of the lithosphere. This sequence of events is predicted by a mantle plume initiation, or plume head, model that has recently been developed to explain continental flood volcanism. Evidence suggests that other large oceanic basalt plateaus, such as the Ontong-Java, Kerguelen, and Caribbean, were formed as the initial outbursts of the Louisville Ridge, Kerguelen, and Galapagos hot spots, respectively. Such events may play an important role in the creation and development of both oceanic and continental crust.
DO CHILDREN WITH FRAGILE X SYNDROME SHOW DECLINES OR PLATEAUS IN ADAPTIVE BEHAVIOR?
Hahn, Laura J.; Brady, Nancy C.; Warren, Steven F.; Fleming, Kandace K.
2014-01-01
This study explores if children with fragile X syndrome (FXS) show advances, declines, or plateaus in adaptive behavior over time and the relationship of nonverbal cognitive abilities and autistic behavior on these trajectories. Parents of 55 children with FXS completed the Vineland Adaptive Behavior Scales between 3 and 6 times from 2 to 10 years of age. Using raw scores, results indicate that about half of the sample showed advances in adaptive behavior, while the other half showed declines, indicating a regression in skills. Children who were more cognitively advanced and had less autistic behaviors had higher trajectories. Understanding the developmental course of adaptive behavior in FXS has implications for educational planning and intervention, especially for those children showing declines. PMID:26322389
Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene
Alexander-Webber, J. A.; Huang, J.; Maude, D. K.; Janssen, T. J. B. M.; Tzalenchuk, A.; Antonov, V.; Yager, T.; Lara-Avila, S.; Kubatkin, S.; Yakimova, R.; Nicholas, R. J.
2016-01-01
Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology. PMID:27456765
Frey, H.; Semeniuk, A.M.
1985-01-01
The present-day boundary between the cratered highlands and northern lowland plains on Mars represent a major physiographic landform. In some regions there is clear evidence of a southward migration of this boundary. The authors have mapped features which may be relict pieces of highlands; flat-topped, angular mesas which we call detached plateaus, knobby terrain and partial craters to determine where ancient cratered terrain may have been in the past. Most of the detached plateaus and knobby terrain lie along or slightly north of the present-day highland boundary, but significant occurrences are also found at high northern latitudes in several locations. They have also identified, mapped and measured all symmetric knobs visible on the 1:2,000,000 controlled photomosaics with the long dimension greater than or equal to 10 km and classified them by shape. Between +65/sup 0/ and -45/sup 0/ there are 1634 such features, many of which are concentrated along the highland/lowland boundary or around the rims of major impact basins such as Isidis, Argyre and Hellas. The most common shape is elliptical (79%) followed by circular (11%) and triangular or wedge-shaped (6%). Globally, all types share a similar decrease in number with increasing size. Regionally, there are significant differences in the size-frequency distributions that seem to be related to their locations; whether the knobs are associated with impact basin rims, the highland/lowland boundary or volcanic plains. There are similar variations in the shape-size characteristics of the elliptical knobs.
A Three-Dimensional Seismic Velocity Model of the Arabian Plate, Iranian and Turkish Plateaus
NASA Astrophysics Data System (ADS)
Ghalib, Hafidh; Gritto, Roland; Sibol, Matthew; Herrmann, Robert; Aleqabi, Ghassan; Carron, Pierre; Wagner, Robert; Ali, Bakir; Ali, Ali
2010-05-01
Translational and rotational interaction between the Arabian, African and Eurasian plates over time has resulted in a challenging seismotectonic framework that is least understood in the Middle East region, in particular. Sea floor spreading along the Red Sea and Gulf of Aden, transform faulting along the Dead Sea and Own fracture zone, and compressional suture zones form the seismic and tectonic boundaries between the Arabian plate, the Iranian and Turkish plateaus. One objective of this effort is to map the three-dimensional shear-wave velocity variation using surface waves recorded by the broadband stations of North Iraq Seismographic Network (NISN), re-established Iraq Seismographic Network (ISN), and local stations of the Global Seismographic Network (GSN). Analysis of the seismograms netted a new seismicity map for the region consisting of about 2000 well located small to medium size earthquakes using all available phase arrivals including those published by the neighboring Syrian, Iranian and Turkish networks. Analysis of Rayleigh wave pure-path dispersion curves produced detailed maps showing the lateral and vertical variation of seismic velocities throughout the Middle East. These maps show a thick (10-15km) sedimentary layer that overlay the crystalline basement and a Conrad and Moho discontinuities at depths of 20-25km and 45-55km, respectively. The maps also show that the Arabian plate exhibits higher shear-wave velocities than found across the Turkish and Iranian plateaus; imprint of the Zagros Mountain roots extends down as deep as the Moho; and that the tectonic boundaries along the Dead Sea, Taurus and Zagros are more pronounced with depth describing a 60km or thicker Arabian plate. Future plans involving body wave velocity tomography modeling, high frequency wave attenuation, and moment tensor analysis to estimate the focal mechanism and magnitude of events are in preparation.
NASA Technical Reports Server (NTRS)
Greiner, B.; Frederick, R. A., Jr.
1993-01-01
The paper provides a brief review of theoretical and experimental studies concerned with hybrid rocket instability. The instabilities discussed include atomization and mixing instabilities, chuffing instabilities, pressure coupled combustion instabilities, and vortex shedding. It is emphasized that the future use of hybrid motor systems as viable design alternatives will depend on a better understanding of hybrid instability.
NASA Astrophysics Data System (ADS)
Deviren, Seyma Akkaya
2017-02-01
In this research, we have investigated the magnetic properties of the spin-1 Ising model on the Shastry Sutherland lattice with the crystal field interaction by using the effective-field theory with correlations. The effects of the applied field on the magnetization are examined in detail in order to obtain the magnetization plateaus, thus different types of magnetization plateaus, such as 1/4, 1/3, 1/2, 3/5, 2/3 and 7/9 of the saturation, are obtained for strong enough magnetic fields (h). Magnetization plateaus exhibit single, triple, quintuplet and sextuple forms according to the interaction parameters, hence the magnetization plateaus originate from the competition between the crystal field (D) and exchange interaction parameters (J, J‧). The ground-state phase diagrams of the system are presented in three varied planes, namely (h/J, J‧/J), (h/J, D/J) and (D/J, J‧/J) planes. These phase diagrams display the Néel (N), collinear (C) and ferromagnetic (F) phases for certain values of the model parameters. The obtained results are in good agreement with some theoretical and experimental studies.
James F. Jr. Rosson
2008-01-01
E. Lucy Braun coined the term "mixed mesophytic forest" in 1916. These forests are structurally complex and occur extensively across the Appalachian Plateaus Province. This region is considered the epicenter of highest development of the eastern deciduous forest. I used U.S. Forest Service, Forest Inventory and Analysis (FIA) data to study current forest...
Clustering instability of focused swimmers
NASA Astrophysics Data System (ADS)
Lauga, Eric; Nadal, Francois
2016-12-01
One of the hallmarks of active matter is its rich nonlinear dynamics and instabilities. Recent numerical simulations of phototactic algae showed that a thin jet of swimmers, obtained from hydrodynamic focusing inside a Poiseuille flow, was unstable to longitudinal perturbations with swimmers dynamically clustering (Jibuti L. et al., Phys. Rev. E, 90, (2014) 063019). As a simple starting point to understand these instabilities, we consider in this paper an initially homogeneous one-dimensional line of aligned swimmers moving along the same direction, and characterise its instability using both a continuum framework and a discrete approach. In both cases, we show that hydrodynamic interactions between the swimmers lead to instabilities in density for which we compute the growth rate analytically. Lines of pusher-type swimmers are predicted to remain stable while lines of pullers (such as flagellated algae) are predicted to always be unstable.
Libration-driven multipolar instabilities
NASA Astrophysics Data System (ADS)
Cébron, D.; Vantieghem, S.; Herreman, W.
2014-01-01
We consider rotating flows in non-axisymmetric enclosures that are driven by libration, i.e. by a small periodic modulation of the rotation rate. Thanks to its simplicity, this model is relevant to various contexts, from industrial containers (with small oscillations of the rotation rate) to fluid layers of terrestial planets (with length-of-day variations). Assuming a multipolar $n$-fold boundary deformation, we first obtain the two-dimensional basic flow. We then perform a short-wavelength local stability analysis of the basic flow, showing that an instability may occur in three dimensions. We christen it the Libration Driven Multipolar Instability (LDMI). The growth rates of the LDMI are computed by a Floquet analysis in a systematic way, and compared to analytical expressions obtained by perturbation methods. We then focus on the simplest geometry allowing the LDMI, a librating deformed cylinder. To take into account viscous and confinement effects, we perform a global stability analysis, which shows that the LDMI results from a parametric resonance of inertial modes. Performing numerical simulations of this librating cylinder, we confirm that the basic flow is indeed established and report the first numerical evidence of the LDMI. Numerical results, in excellent agreement with the stability results, are used to explore the non-linear regime of the instability (amplitude and viscous dissipation of the driven flow). We finally provide an example of LDMI in a deformed spherical container to show that the instability mechanism is generic. Our results show that the previously studied libration driven elliptical instability simply corresponds to the particular case $n=2$ of a wider class of instabilities. Summarizing, this work shows that any oscillating non-axisymmetric container in rotation may excite intermittent, space-filling LDMI flows, and this instability should thus be easy to observe experimentally.
Magnetohydrodynamic instability
NASA Technical Reports Server (NTRS)
Priest, E. R.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson, R. S.
1986-01-01
There have been major advances in the theory of magnetic reconnection and of magnetic instability, with important implications for the observations, as follows: (1) Fast and slow magnetic shock waves are produced by the magnetohydrodynamics of reconnection and are potential particle accelerators. (2) The impulsive bursty regime of reconnection gives a rapid release of magnetic energy in a series of bursts. (3) The radiative tearing mode creates cool filamentary structures in the reconnection process. (4) The stability analyses imply that an arcade can become unstable when either its height or twist of plasma pressure become too great.
Groundwater withdrawal rates from the Ozark Plateaus aquifer system, 1900 to 2010
Knierim, Katherine J.; Nottmeier, Anna M.; Worland, Scott C.; Westerman, Drew A.; Clark, Brian R.
2016-01-01
Groundwater is an often overlooked freshwater resource compared to surface water, but groundwater is used widely across the United States, especially during periods of drought. If groundwater models can successfully simulate past conditions, they may be used to evaluate potential future pumping scenarios or climate conditions, thus providing a valuable planning tool for water-resource managers. Quantifying the groundwater-use component for a groundwater model is a vital but often challenging endeavor. This dataset includes groundwater withdrawal rates modeled for the Ozark Plateaus aquifer system (Ozark system) from 1900 to 2010 by groundwater model cell (2.6 square kilometers) for five water-use divisions—agriculture (including irrigation and aquaculture), livestock, public supply (including municipal and rural water districts), and non-agriculture (including thermoelectric power generation, mining, commercial, and industrial)—and by country for domestic (self-supplied) use. Two child items are included with the dataset: “Domestic groundwater withdrawal rates from the Ozark Plateaus aquifer system, 1900 to 2010” and “Public supply, non-agriculture, livestock, and agriculture groundwater withdrawal rates from the Ozark Plateaus aquifer system, 1900 to 2010”. The Ozark system is located in the central United States and is composed of interbedded Cambrian to Pennsylvanian clastic and carbonate lithologies. In stratigraphic order, the Ozark system includes the Basement confining unit, St. Francois aquifer, St. Francois confining unit, Ozark aquifer, Ozark confining unit, Springfield Plateau aquifer, and Western Interior Plains confining system. Generally, the lower portion of the Ozark aquifer is the primary source of groundwater across much of Missouri and the Springfield Plateau aquifer is used across northern Arkansas. A full description of the methods used to model groundwater withdrawal rates from the Ozark system are available in Knierim et al., IN
Finite element shell instability analysis
NASA Technical Reports Server (NTRS)
1975-01-01
Formulation procedures and the associated computer program for finite element thin shell instability analysis are discussed. Data cover: (1) formulation of basic element relationships, (2) construction of solution algorithms on both the conceptual and algorithmic levels, and (3) conduction of numerical analyses to verify the accuracy and efficiency of the theory and related programs therein are described.
Extensional instability in electro-osmotic microflows of polymer solutions
NASA Astrophysics Data System (ADS)
Bryce, R. M.; Freeman, M. R.
2010-03-01
Fluid transport in microfluidic systems typically is laminar due to the low Reynolds number characteristic of the flow. The inclusion of suspended polymers imparts elasticity to fluids, allowing instabilities to be excited when substantial polymer stretching occurs. For high molecular weight polymer chains we find that flow velocities achievable by standard electro-osmotic pumping are sufficient to excite extensional instabilities in dilute polymer solutions. We observe a dependence in measured fluctuations on polymer concentration which plateaus at a threshold corresponding to the onset of significant molecular crowding in macromolecular solutions; plateauing occurs well below the overlap concentration. Our results show that electro-osmotic flows of complex fluids are disturbed from the steady regime, suggesting potential for enhanced mixing and requiring care in modeling the flow of complex liquids such as biopolymer suspensions.
2014/2015 Investigations of the Ontong Java and Kerguelen Plateaus
NASA Astrophysics Data System (ADS)
Coffin, M. F.; Whittaker, J. M.
2013-12-01
The two largest oceanic plateaus, Ontong Java in the western Pacific, and Kerguelen in the southern Indian Ocean, will be the focus of scheduled multidisciplinary/interdisciplinary shipboard expeditions in 2014 and 2015. In mid-2014, scientists aboard the Schmidt Ocean Institute's RV Falkor will investigate the origin and evolution of two large atolls, Ontong Java and Nukumanu, surmounting the ca 122 Ma Ontong Java Plateau, as well how Kroenke Canyon, which deeply incises the plateau, formed and evolved. First-ever multibeam bathymetry and sub-bottom profiling data from the atolls and canyon will reveal their submarine and shallow sub-seafloor morphology, and, if combined with geochemical and geochronological analyses of potential igneous basement samples, will yield important information on their origin and evolution. The primary goals of this atoll and canyon project are: to test potential genetic relationships between a) the atolls and the OJP, and b) the atolls and Kroenke Canyon; to understand and model how atolls and canyons form and evolve on oceanic plateaus, isolated from terrestrial influences and subject to sea level fluctuations; and to contribute to understanding tsunami risk on low-lying atolls. In late 2014 and early 2015, researchers aboard Australia's new Marine National Facility, RV Investigator, will investigate active submarine hotspot volcanism on the Kerguelen Plateau and its consequences. The project's overall aim is to test the hypothesis that hydrothermal activity driven by active submarine magmatism fertilizes surface waters with iron that enhances primary biological productivity. Surmounting the Cretaceous plateau, Heard and McDonald Islands are among the world's most active hotspot volcanoes, and new multibeam bathymetry and sub-bottom profiling data will enable identification of candidate active submarine volcanoes, which we will sample. In the overlying water column, we will collect samples to test for the presence or absence of
Presence of rapidly degrading permafrost plateaus in south-central Alaska
NASA Astrophysics Data System (ADS)
Jones, Benjamin M.; Baughman, Carson A.; Romanovsky, Vladimir E.; Parsekian, Andrew D.; Babcock, Esther L.; Stephani, Eva; Jones, Miriam C.; Grosse, Guido; Berg, Edward E.
2016-11-01
Permafrost presence is determined by a complex interaction of climatic, topographic, and ecological conditions operating over long time scales. In particular, vegetation and organic layer characteristics may act to protect permafrost in regions with a mean annual air temperature (MAAT) above 0 °C. In this study, we document the presence of residual permafrost plateaus in the western Kenai Peninsula lowlands of south-central Alaska, a region with a MAAT of 1.5 ± 1 °C (1981-2010). Continuous ground temperature measurements between 16 September 2012 and 15 September 2015, using calibrated thermistor strings, documented the presence of warm permafrost (-0.04 to -0.08 °C). Field measurements (probing) on several plateau features during the fall of 2015 showed that the depth to the permafrost table averaged 1.48 m but at some locations was as shallow as 0.53 m. Late winter surveys (augering, coring, and GPR) in 2016 showed that the average seasonally frozen ground thickness was 0.45 m, overlying a talik above the permafrost table. Measured permafrost thickness ranged from 0.33 to > 6.90 m. Manual interpretation of historic aerial photography acquired in 1950 indicates that residual permafrost plateaus covered 920 ha as mapped across portions of four wetland complexes encompassing 4810 ha. However, between 1950 and ca. 2010, permafrost plateau extent decreased by 60.0 %, with lateral feature degradation accounting for 85.0 % of the reduction in area. Permafrost loss on the Kenai Peninsula is likely associated with a warming climate, wildfires that remove the protective forest and organic layer cover, groundwater flow at depth, and lateral heat transfer from wetland surface waters in the summer. Better understanding the resilience and vulnerability of ecosystem-protected permafrost is critical for mapping and predicting future permafrost extent and degradation across all permafrost regions that are currently warming. Further work should focus on reconstructing
Topological incommensurate magnetization plateaus in quasi-periodic quantum spin chains.
Hu, Hai-Ping; Cheng, Chen; Luo, Hong-Gang; Chen, Shu
2015-02-13
Uncovering topologically nontrivial states in nature is an intriguing and important issue in recent years. While most studies are based on the topological band insulators, the topological state in strongly correlated low-dimensional systems has not been extensively explored due to the failure of direct explanation from the topological band insulator theory on such systems and the origin of the topological property is unclear. Here we report the theoretical discovery of strongly correlated topological states in quasi-periodic Heisenberg spin chain systems corresponding to a series of incommensurate magnetization plateaus under the presence of the magnetic field, which are uniquely determined by the quasi-periodic structure of exchange couplings. The topological features of plateau states are demonstrated by the existence of non-trivial spin-flip edge excitations, which can be well characterized by nonzero topological invariants defined in a two-dimensional parameter space. Furthermore, we demonstrate that the topological invariant of the plateau state can be read out from a generalized Streda formula and the spin-flip excitation spectrum exhibits a similar structure of the Hofstadter's butterfly spectrum for the two-dimensional quantum Hall system on a lattice.
Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D
NASA Astrophysics Data System (ADS)
Shiraki, D.; Commaux, N.; Baylor, L. R.; Cooper, C. M.; Eidietis, N. W.; Paz-Soldan, C.; Hollmann, E. M.; Moyer, R. A.
2016-10-01
Effective runaway electron (RE) mitigation strategies are essential for protecting ITER from the potential damage of a first wall strike. In DIII-D, shattered pellet injection (SPI) with large Ne pellets demonstrates the dissipation of post-disruption RE plateaus by collisions with high-Z impurities, while equivalently sized D2 pellets lead to a reduction of the impurity content of the background plasma, reducing RE dissipation. Varying the relative quantities of Ne /D2 in mixed species pellets shows that the effect of D2 may be dominant in determining the RE/pellet interaction. Compared with injection of the same quantity of Ne by massive gas injection, SPI achieves a similar initial RE current decay rate, but residual RE current remains after SPI. This may be due to the effects of a small quantity of D2 (used as a ``shell'' for firing of the Ne pellets) displacing high-Z impurities. These results will help guide the optimization of injection schemes and pellet compositions for the RE mitigation system in ITER. Work supported by the U.S. DOE under DE-FC02-04ER54698.
Gabli, Abdelhafid; Agabou, Amir; Gabli, Zahra
2015-08-01
A 31-months study was conducted to elucidate the prevalence of brucellosis in nomadic pastoralists and their goats in two provinces of the eastern Algerian high plateaus. Five hundred eight human and 4955 animal sera were screened with the Rose Bengal plate test and the complement fixation test for confirmation. Uterine fluids from aborting goats were subjected to microbiological analyses to determine the biovars responsible for abortions. The overall seroprevalence was 0.98% among animals and 15.84% among herds. A significant correlation was recorded between occurrence of brucellosis and herd size (r = 0.4046, P < 0.0001) as well as age (χ(2) = 5.809, P = 0.0159) and sex of animals (χ(2) = 20.09, P < 0.0001); 89.65% of human cases were related to positive herds and the infection rate was higher in men (7.6%) than in women (6%) and children (0.92%). Brucella melitensis biovar 3 was the only aetiology of brucellosis-associated abortion in goats of the studied region.
The Occurrence of Anomalous Conductance Plateaus and Spin Textures in Quantum Point Contacts
NASA Astrophysics Data System (ADS)
Wan, J.; Cahay, M.; Debray, P.; Newrock, R.
2010-03-01
Recently, we used a NEGF formalism [1] to provide a theoretical explanation for the experimentally observed 0.5G0 (G0=2e^2/h) plateau in the conductance of side-gated quantum point contacts (QPCs) in the presence of lateral spin-orbit coupling (LSOC) [2]. We showed that the 0.5G0 plateau appears in the QPCs without any external magnetic field as a result of three ingredients: an asymmetric lateral confinement, a LSOC, and a strong electron-electron (e-e) interaction. In this report, we present the results of simulations for a wide range of QPC dimensions and biasing parameters showing that the same physics predicts the appearance of other anomalous plateaus at non-integer values of G0, including the well-known 0.7G0 anomaly. These features are related to a plethora of spin textures in the QPC that depend sensitively on material, device, biasing parameters, temperature, and the strength of the e-e interaction. [1] J. Wan, M. Cahay, P. Debray, and R.S. Newrock, Phys. Rev. B 80, 155440 (2009). [2] P. Debray, S.M. Rahman, J. Wan, R.S. Newrock, M. Cahay, A.T. Ngo, S.E. Ulloa, S.T. Herbert, M. Muhammad, and M. Johnson, Nature Nanotech. 4, 759 (2009).
Regional Curves for Bankfull Channel Characteristics in the Appalachian Plateaus, West Virginia
Messinger, Terence
2009-01-01
Streams in the Appalachian Plateaus Physiographic Province in West Virginia were classified as a single region on the basis of bankfull characteristics. Regression lines for annual peak flow and drainage area measured at streamgages in the study area at recurrence intervals between 1.2 and 1.7 years fell within the 99-percent confidence interval of the regression line for bankfull flow. Channel characteristics were intermediate among those from surrounding states and regions where comparable studies have been done. The stream reaches that were surveyed were selected for apparent stability, and to represent gradients of drainage area, elevation, and mean annual precipitation. Profiles of high-water marks left by bankfull and near-bankfull peaks were surveyed, either as part of slope-area flow measurements at ungaged reaches, or to transfer known flow information to cross sections for gaged reaches. The slope-area measurements made it possible to include ungaged sites in the study, but still relate bankfull dimensions to peak flow and frequency.
Changes in Pacific Absolute Plate Motion and Formation of Oceanic Flood Basalt Plateaus
NASA Astrophysics Data System (ADS)
Kroenke, L. W.; Wessel, P.
2006-12-01
The origin of the large oceanic flood basalt plateaus that are prominent features of the central western Pacific Basin remains unclear. Major changes in Pacific Absolute Plate Motion (APM) have been identified as occurring at 145, 125, 96, and 47 Ma. Formation of the Shatsky Rise (~145 Ma), the Ontong Java Plateau (122+ Ma), the Southern Hess Rise (95±5 Ma), and the Louisiade Plateau (~48 Ma) appear to coincide with these changes. A smaller, but still prominent change in Pacific APM also occurred at 110 Ma when the Northern Hess Rise formed. Although these concurrent events may simply be chance occurrences, initiation of plate tectonic reorganizations upon arrival of mantle plume heads also was proposed by Ratcliff et al. (1998), who suggested that the mantle plume head delivery of hot material to produce flood basalts also had the potential to trigger reorganizations of plate motions. It should be noted, however, that Pacific Rim subduction zone development also coincides with these APM changes, and that the actual cause and effect of each change in APM has yet to be clearly established. Here we present a modified Pacific APM model that uses several older seamount chains (Musicians, Ratak-Gilbert-Ellice, the Wake trails, and the Liliuokalani trails) to constrain the oldest Pacific plate motion using the hybrid technique of Wessel et al (2006).
Topological incommensurate magnetization plateaus in quasi-periodic quantum spin chains
Hu, Hai-Ping; Cheng, Chen; Luo, Hong-Gang; Chen, Shu
2015-01-01
Uncovering topologically nontrivial states in nature is an intriguing and important issue in recent years. While most studies are based on the topological band insulators, the topological state in strongly correlated low-dimensional systems has not been extensively explored due to the failure of direct explanation from the topological band insulator theory on such systems and the origin of the topological property is unclear. Here we report the theoretical discovery of strongly correlated topological states in quasi-periodic Heisenberg spin chain systems corresponding to a series of incommensurate magnetization plateaus under the presence of the magnetic field, which are uniquely determined by the quasi-periodic structure of exchange couplings. The topological features of plateau states are demonstrated by the existence of non-trivial spin-flip edge excitations, which can be well characterized by nonzero topological invariants defined in a two-dimensional parameter space. Furthermore, we demonstrate that the topological invariant of the plateau state can be read out from a generalized Streda formula and the spin-flip excitation spectrum exhibits a similar structure of the Hofstadter's butterfly spectrum for the two-dimensional quantum Hall system on a lattice. PMID:25678145
New insights on lithospheric foundering from thermo-mechanically coupled numerical modelling
NASA Astrophysics Data System (ADS)
Pastor-Galán, Daniel; Thieulot, Cedric
2015-04-01
Earth's lithosphere is recycled into the mantle as required by global mass considerations. At least during the latest 1 G.y. the main mechanism of lithospheric foundering into the mantle has been subduction. Yet other mechanisms of mantle removal such as Rayleigh-Taylor-type instability or delamination have significant influence at present as revealed by mantle anomalies, and are considered to be likely candidates for the main recycling mechanisms of lithospheric during the Archean. Although lithospheric mantle removal has been geophysically imaged, e.g. Carpathians, Colorado Plateau, at many other locations geophysical and geological observations also seem to indicate that mantle lithosphere is anomalously thin or absent. Potential places where lithospheric mantle foundering processes took place are The Urals, the Variscides, underneath the Ibero Armorican Orocline in western Europe, and the Tibetan, Puna and Anatolian Plateaus. Lithospheric foundering has been blamed for, among others, cratonization processes, rapid surface uplift, generation of voluminous magmatism, changes in crustal stress from compression to extension and a long etc. However, its triggering mechanisms are not well studied, and a variety of possible explanations have been given for lithospheric foundering processes, including convective instability following orogenic thickening or some other perturbation of thermal boundary layers, development of eclogitic roots, erosion of the lithosphere by a flat-subducting slab or partial melting of the asthenosphere, and partial intruding pyroxenites into the base of lithosphere. To understand the mechanisms, causes and consequences of lithospheric foundering, we explored lithospheric foundering in an assortment of scenarios using the numerical code, ELEFANT, an user-friendly multipurpose geodynamics code. Preliminary results indicate that changes in geometry, thermal state and composition of the lithosphere, associated with mantle flow, can have a first
Direct numerical study of crossflow instability
NASA Astrophysics Data System (ADS)
Spalart, P. R.
Disturbances in the swept Hiemenz flow are calculated by solving the Navier-Stokes equations. The spatially-evolving base flow is treated exactly, allowing a check of the 'local' stability theories. Different types of disturbances such as random noise, waves, and wave packets, are input near the attachment line, develop in space, and exit through an outflow boundary. They all generate streamwise vortices. The effect of the Reynolds number, of the time-dependence of the noise, and of nonlinearity, are investigated.
Direct numerical study of crossflow instability
NASA Technical Reports Server (NTRS)
Spalart, P. R.
1990-01-01
Disturbances in the swept Hiemenz flow are calculated by solving the Navier-Stokes equations. The spatially-evolving base flow is treated exactly, allowing a check of the 'local' stability theories. Different types of disturbances such as random noise, waves, and wave packets, are input near the attachment line, develop in space, and exit through an outflow boundary. They all generate streamwise vortices. The effect of the Reynolds number, of the time-dependence of the noise, and of nonlinearity, are investigated.
Westerman, Drew A.; Gillip, Jonathan A.; Richards, Joseph M.; Hays, Phillip D.; Clark, Brian R.
2016-09-29
A hydrogeologic framework was constructed to represent the altitudes and thicknesses of hydrogeologic units within the Ozark Plateaus aquifer system as part of a regional groundwater-flow model supported by the U.S. Geological Survey Water Availability and Use Science Program. The Ozark Plateaus aquifer system study area is nearly 70,000 square miles and includes parts of Arkansas, Kansas, Missouri, and Oklahoma. Nine hydrogeologic units were selected for delineation within the aquifer system and include the Western Interior Plains confining system, the Springfield Plateau aquifer, the Ozark confining unit, the Ozark aquifer, which was divided into the upper, middle, and lower Ozark aquifers to better capture the spatial variation in the hydrologic properties, the St. Francois confining unit, the St. Francois aquifer, and the basement confining unit. Geophysical and well-cutting logs, along with lithologic descriptions by well drillers, were compiled and interpreted to create hydrologic altitudes for each unit. The final compiled dataset included more than 23,000 individual altitude points (excluding synthetic points) representing the nine hydrogeologic units within the Ozark Plateaus aquifer system.
NASA Astrophysics Data System (ADS)
Labousse, Matthieu
2015-11-01
The interaction of a vortex with a free surface is encountered in a series of experiments, the hydraulic jump, the hydraulic bump, the toroidal Leidenfrost experiment. All these experiments share in common an unstable configuration in which azimuthal perturbations give rise to polygonal patterns. We propose a unified theoretical framework to model the emergence of this instability by investigating the stability of a liquid torus with a poloidal motion. As simple as it is, we show that the model retains the necessary ingredients to account for the experimental observations. In this talk, I will first describe the model and compare it to the existing data. However this model is purely inviscid and reaches its limits when being applied to relatively moderate Reynolds flows. So in a second part, I will present a recent experimental and theoretical investigation in which polygonal patterns are now driven by Marangoni flows. To our great surprise, it extends the range of validity of the initial proposed framework, much more than initially expected.
Carbon Abundance Plateaus among Carbon-Enhanced Metal-Poor Stars
NASA Astrophysics Data System (ADS)
Yoon, Jinmi; He, Siyu; Placco, Vinicius; Carollo, Daniela; Beers, Timothy C.
2016-01-01
A substantial fraction of low-metallicity stars in the Milky Way, the Carbon-Enhanced Metal-Poor (CEMP) stars, exhibit enhancements of their carbon-to-iron relative to the solar value ([C/Fe] > +0.7). They can be divided into several sub-classes, depending on the nature and degree of the observed enhancements of their neutron-capture elements, providing information on their likely progenitors. CEMP-s stars (which exhibit enhanced s-process elements) are thought to be enhanced by mass transfer from an evolved AGB companion, while CEMP-no stars (which exhibit no over-abundances of neutron-capture elements) appear to be associated with explosions of the very first generations of stars. High-resolution spectroscopic analyses are generally required in order to make these sub-classifications.Several recent studies have suggested the existence of bimodality in the distribution of absolute carbon abundances among CEMP stars -- most CEMP-no stars belong to a low-C band ((A(C) ˜ 6.5), while most CEMP-s stars reside on a high-C band (A(C) ˜ 8.25). The number of CEMP stars considered by individual studies is, however, quite small, so we have compiled all available high-resolution spectroscopic data for CEMP stars, in order to further investigate the existence of the claimed carbon bi-modality, and to consider what can be learned about the progenitors of CEMP-s and CEMP-no stars based on the observed distribution of A(C) on the individual plateaus.We acknowledge partial support from the grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation.
NASA Astrophysics Data System (ADS)
Verkholyak, Taras; Strečka, Jozef
2016-10-01
The spin-1/2 Heisenberg orthogonal-dimer chain is considered within the perturbative strong-coupling approach, which is developed from the exactly solved spin-1/2 Ising-Heisenberg orthogonal-dimer chain with the Heisenberg intradimer and the Ising interdimer couplings. Although the spin-1/2 Ising-Heisenberg orthogonal-dimer chain exhibits just intermediate plateaus at zero, one-quarter, and one-half of the saturation magnetization, the perturbative treatment up to second order stemming from this exactly solvable model additionally corroborates the fractional one-third plateau as well as the gapless Luttinger spin-liquid phase. It is evidenced that the approximate results obtained from the strong-coupling approach are in an excellent agreement with the state-of-the-art numerical data obtained for the spin-1/2 Heisenberg orthogonal-dimer chain within the exact diagonalization and density-matrix renormalization group method. The nature of individual quantum ground states is comprehensively studied within the developed perturbation theory.
Instability of EDS maglev systems
Cai, Y.; Chen, S.S.
1993-09-01
Instabilities of an EDS maglev suspension system with 3 D.O.F. and 5 D.O.F. vehicles traveling on a double L-shaped set of guideway conductors have been investigated with various experimentally measured magnetical force data incorporated into the theoretical models. Divergence and flutter are obtained from both analytical and numerical solutions for coupled vibration of the 3 D.O.F. maglev vehicle model. Instabilities of five direction motions (heave, slip, rill, pitch and yaw) are observed for the 4 D.O.F. vehicle model. It demonstrates that system parameters, such as, system damping, vehicle geometry and coupling effects among five different motions play very important roles in the occurrence of dynamic instabilities of maglev vehicles.
Gravitational instabilities in protostellar disks
NASA Technical Reports Server (NTRS)
Tohline, J. E.
1994-01-01
The nonaxisymmetric stability of self-gravitating, geometrically thick accretion disks has been studied for protostellar systems having a wide range of disk-to-central object mass ratios. Global eigenmodes with four distinctly different characters were identified using numerical, nonlinear hydrodynamic techniques. The mode that appears most likely to arise in normal star formation settings, however, resembles the 'eccentric instability' that was identified earlier in thin, nearly Keplerian disks: It presents an open, one-armed spiral pattern that sweeps continuously in a trailing direction through more than 2-pi radians, smoothly connecting the inner and outer edges of the disk, and requires cooperative motion of the point mass for effective amplification. This particular instability promotes the development of a single, self-gravitating clump of material in orbit about the point mass, so its routine appearance in our simulations supports the conjecture that the eccentric instability provides a primary route to the formation of short-period binaries in protostellar systems.
Simulation of the 'negative temperature' instability for line vortices.
NASA Technical Reports Server (NTRS)
Joyce, G.; Montgomery, D.
1972-01-01
In previous numerical solution to the continuum Navier-Stokes equations, a 'negative temperature' instability for the two-dimensional motions of interacting line vortices was observed. The experiment is repeated for a discrete vortex model, thus obtaining a numerical simulation of the 'negative temperature' instability for a large number of discrete line vortices. Typical results which are shown, are thought to lie above and below the energy threshold for negative temperature instability.
Interfacial Instabilities on a Droplet
NASA Astrophysics Data System (ADS)
Jalaal, Maziyar; Mehravaran, Kian
2013-11-01
The fragmentation of droplets is an essential stage of several natural and industrial applications such as fuel atomization and rain phenomena. In spite of its relatively long history, the mechanism of fragmentation is not clear yet. This is mainly due to small length and time scales as well as the non-linearity of the process. In the present study, two and three-dimensional numerical simulations have been performed to understand the early stages of the fragmentation of an initially spherical droplet. Simulations are performed for high Reynolds and a range of relatively high Weber numbers (shear breakup). To resolve the small-scale instabilities generated over the droplet, a second-order adaptive finite volume/volume of fluids (FV/VOF) method is employed, where the grid resolution is increased with the curvature of the gas-liquid interface as well as the vorticity magnitude. The study is focused on the onset and growth of interfacial instabilities. The role of Kelvin-Helmholtz instability (in surface wave formation) and Rayleigh-Taylor instability (in azimuthal transverse modulation) are shown and the obtained results are compared with the linear instability theories for zero and non-zero vorticity layers. Moreover, the analogy between the fragmentation of a single drop and a co-axial liquid jet is discussed. The current results can be used for the further development of the current secondary atomization models.
EXPLOSIVE INSTABILITY AND CORONAL HEATING
Dahlburg, R. B.; Liu, J.-H.; Klimchuk, J. A.; Nigro, G.
2009-10-20
The observed energy-loss rate from the solar corona implies that the coronal magnetic field has a critical angle at which energy is released. It has been hypothesized that at this critical angle an 'explosive instability' would occur, leading to an enhanced conversion of magnetic energy into heat. In earlier investigations, we have shown that a shear-dependent magnetohydrodynamic process called 'secondary instability' has many of the distinctive features of the hypothetical 'explosive instability'. In this paper, we give the first demonstration that this 'secondary instability' occurs in a system with line-tied magnetic fields and boundary shearing-basically the situation described by Parker. We also show that, as the disturbance due to secondary instability attains finite amplitude, there is a transition to turbulence which leads to enhanced dissipation of magnetic and kinetic energy. These results are obtained from numerical simulations performed with a new parallelized, viscoresistive, three-dimensional code that solves the cold plasma equations. The code employs a Fourier collocation-finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization.
Petersen, James C.; Femmer, Suzanne R.
2003-01-01
During August through September of 1993-95, 83 periphyton samples were collected at 51 stream sites in the Ozark Plateaus. These sites were categorized into six land-use categories (20 forest, 18 agriculture, 10 mining, 1 urban, 1 urban/mining, and 1 mix), based on land-use percentages in the basin upstream from the site. Results indicate that periphyton communities of riffles of Ozark streams are affected by natural and land-use related factors. These factors include nutrients, dissolved organic carbon, alkalinity, canopy shading, suspended sediment, embeddedness, stream morphometry, and velocity. For several measures of periphyton communities, statistically significant (p<0.05) differences were found among sites assigned to agriculture, forest, and mining categories. Blue-green algae biovolume, relative abundance of blue-green algae, relative biovolume of diatoms, relative abundance of oligotrophic algae, relative abundance of tolerant taxa, and condition index values were among the measures that differed among land-use categories. Although no environmental factors were significantly correlated with total biovolume, several factors were significantly correlated with biovolume of blue-green algae or biovolume of diatoms. Biovolume of blue-green algae was correlated with percent agriculture land use. Biovolume of diatoms was correlated with orthophosphate, total phosphorus, alkalinity, velocity, embeddedness, and dissolved organic carbon.Diatoms often composed the largest percentage of the biovolume (relative biovolume). Diatom relative biovolume was much higher at mining sites (generally 75 to 90 percent of the total biovolume) than at forest or agriculture sites (generally 15 to 80 percent) and was correlated with several factors, including many land-use related factors. The diatoms Cymbella affinis and Cymbella delicatula and the blue-green algae Calothrix often were the most common (relative abundance and relative biovolume) algae in samples
Progressive Landslides in Uplifted Volcanic Plateaus: Persistent Loci of Channel Perturbation
NASA Astrophysics Data System (ADS)
Safran, E. B.; Anderson, S. W.; Mills-Novoa, M.; Othus, S.; Ely, L.; House, P. K.; O'Connor, J. E.; Grant, G.; Fenton, C.; Beebee, R. A.
2007-12-01
The semi-arid uplifted volcanic plateaus of the southern interior Columbia River basin contain over 300 large landslides or landslide complexes, ranging in area from several tenths of a km2 to several tens of km2. The distribution of these landslides is dominated by the outcropping of key stratigraphic contacts between coherent, volcanic cap rock atop weak sedimentary or volcaniclastic units in areas of >100 m local relief. The morphologies of many of these landslide complexes suggest a progressive mode of mass movement, with rubble-capped failure slices arrayed downslope at intervals of 10s to 100s of meters and deep tension cracks separating incipient failure blocks from the intact headscarp. Field evidence from the Owyhee River in southeastern Oregon indicates that individual landslide complexes can persist for millions of years. In one reach, for example, remnants of a 1.9 million year old intracanyon lava flow are inset against ancient landslide blocks. In the same location, cosmogenic isotope dating of boulders on a likely dam-burst flood deposit reveal a channel-blocking mass movement that may be as young as Holocene in age. The persistence of these landslide complexes has important implications for channel evolution, as it suggests that, in some environments, sediment supply may be chronically elevated at point sources. On the basis of GIS-based mapping of regional landslides and on field study of individual landslide complexes, we hypothesize that: 1) this sediment supply becomes increasingly dominated by fine-grained material as channels progressively incise into the weak units underlying coherent lava caps; and 2) the mass movements that impinge on the channels become correspondingly more earthflow-like. Loci of persistent landsliding are also subject to episodic variations in channel width due to physical constrictions caused by impinging failure masses. The discrete localization of large landslide complexes by particular stratigraphic and topographic
Adamski, James C.
1997-01-01
A total of 229 ground-water samples were collected from 215 sites as part of the Ozark Plateaus study unit of the National Water-Quality Assessment Program. These samples were collected from 1993 through 1995 using a network of springs and wells with three scale-dependent components. The first component, the study-unit survey, consisted of 99 randomly selected springs and domestic wells in the Springfield Plateau and Ozark aquifers. The second component, two land-use studies, consisted of 42 springs and domestic wells in a poultry-dominated agricultural area and 40 springs and domestic wells in a cattle-dominated agricultural area overlying the Springfield Plateau aquifer. The third component, the small-watershed study, consisted of 4 springs, 18 domestic wells, and 11 monitoring wells in a small basin within the poultry land-use study area. Samples were analyzed for major ions, nutrients, dissolved organic carbon, methylene blue active substances, tritium, and 88 pesticides and metabolites.The water-quality data from these samples were analyzed with descriptive and statistical methods. Nitrite plus nitrate, which was detected more often and in greater concentrations than any of the other nutrients, ranged from less than 0.05 to 25 milligrams per liter as nitrogen. Nitrite plus nitrate concentrations positively correlated to percent agricultural land use around each site. Median nitrite plus nitrate concentrations generally were greater in samples from springs than in samples from wells. Concentrations of nitrite, ammonia, and ammonia plus organic nitrogen were also affected by land use and also by concentrations of dissolved oxygen in the ground water. Concentrations of phosphorus and orthophosphate probably were affected by land use and also by phosphorus solubility. Pesticides were detected in 80 of 229 samples from 73 of 215 sites. A total of 20 pesticides were detected with a maximum of 5 pesticides detected in any 1 sample. The most commonly detected
Carbon cycling in the mantled karst of the Ozark Plateaus, central United States
Knierim, Katherine; Pollock, Erik D.; Covington, Matthew D.; Hays, Phillip D.; Brye, Kristofor R.
2017-01-01
The nature of carbon (C) cycling in the unsaturated zone where groundwater is in contact with abundant gas-filled voids is poorly understood. The objective of this study was to trace inorganic-C cycling in a karst landscape using stable-C isotopes, with emphasis on a shallow groundwater flow path through the soil, to an underlying cave, and to the spring outlet of a cave stream in the Ozark Plateaus of northwestern Arkansas. Carbon dioxide (CO2) concentration and isotopic composition (δ13C-CO2) in gas and dissolved inorganic carbon (DIC) concentration and isotopic composition (δ13C-DIC) in water were measured in samples collected from two suction-cup soil samplers above the cave, three sites in the cave, and at the spring outlet of the cave stream. Soil-gas CO2 concentration (median 2,578 ppm) and δ13C-CO2 (median − 21.5‰) were seasonally variable, reflecting the effects of surface temperature changes on soil-CO2 production via respiration and organic-matter decomposition. Cave-air CO2 (median 1,026 ppm) was sourced from the soil zone and the surface atmosphere, with seasonally changing proportions of each source controlled by surface temperature-driven air density gradients. Soil-DIC concentration (median 1.7 mg L− 1) was lower and soil-δ13C-DIC (median − 19.5‰) was lighter compared to the cave (median 23.3 mg L− 1 and − 14.3‰, respectively) because carbonate-bedrock dissolution provided an inorganic source of C to the cave. Carbon species in the soil had a unique, light stable-C isotopic signature compared to the cave. Discrimination of soil-C sources to karst groundwater was achieved, which is critical for developing hydrologic budgets using environmental tracers such as C.
The Isotopic Record of Elevation Thresholds in Continental Plateaus to Atmospheric Circulation
NASA Astrophysics Data System (ADS)
Mulch, A.; Chamberlain, C. P.; Graham, S. A.; Teyssier, C.; Cosca, M. A.
2011-12-01
High-elevation orogenic plateaus and mountain ranges exert a strong control on global climate and precipitation patterns and interact with lithospheric and upper mantle tectonic processes as well as atmospheric circulation. Reconstructing the history of surface elevation thus not only provides a critical link between erosional and tectonic processes but also ties Earth surface processes to the long-term climate history of our planet. This interaction, however, has important implications when using stable isotopes (O, H) as proxies for landscape and terrestrial climate evolution as interacting land surface properties (elevation, relief, vegetation cover) and atmospheric circulation patterns (upstream moisture path) may attain threshold conditions that can cloak or amplify the impact of topography on isotopes in precipitation. A large number of stable isotope studies in lacustrine and pedogenic environments of intermontane basins record the isotopic and sedimentologic fingerprint of the evolving landscape of the Cenozoic western North American Cordillera. In general we observe the onset of strong oxygen isotope in precipitation gradients along the eastward and westward flanks of the Cordilleran orogen and associated 18O-depleted moisture within the Cordilleran hinterland to develop no later than 50-55 Ma in British Columbia and Washington, 49 Ma in Montana, and 39-40 Ma in Nevada. However, some of these shifts to very low oxygen isotope compositions in meteoric water occur at rates that by far exceed those that could be attributed to tectonic surface uplift alone. Here we present a multi-proxy approach from the Elko Basin (NV) that ties stable and radiogenic tracers of landscape evolution with high resolution Ar-Ar geochronology. In pedogenic and lacustrine deposits of the Elko basin we observe a change in oxygen isotope ratios that is far too large (6-8 %) and rapid (<200 000 a) to be solely due to changes in elevation. Rather we suggest that the combined effects of
NASA Astrophysics Data System (ADS)
Sebe, Krisztina; Csillag, Gábor
2015-04-01
On some basalt plateaus of the western Pannonian Basin, Hungary, fields of circular depressions occur. They are traditionally called "basalt karst' and their formation has been attributed either to collapse over karstifying rocks or to anthropogenic action (quarrying); however, both of these theories are questionable. The depressions are situated between elevations of 350-500 m a.s.l. and are characteristically surrounded by circular raised rims or ramparts. They measure a few m-s (up to ~10 m) in diameter, the ramparts emerge 0.5-1.5 m above the surrounding level ground and encircle a depression of 1-2 (-3) m deep in the middle. Depressions cluster in well delineated, high-density groups, with individual fields containing several dozens of these forms. Neighbouring ramparts are tightly packed, often interfere and depressions can thus coalesce creating composite forms. The ramparts are composed of coarse (dm-sized) basalt blocks, whose material is identical to that of the surrounding terrain and seems to originate from the depression. Many of the depressions host ephemeral ponds. Raised rims exclude formation of these landforms by any karstic processes. The anthropogenic theory is opposed by the lack of the remains of any facilities (e.g. roads), of tools and by the very illogical distribution and geometry of depressions from the point of human use. On the contrary, we interpret these ramparted depressions as being of periglacial origin, remnants of cryogenic mounds. The central depression and the emergent rampart can be well explained by the ice core raising the overlying rock and by the radial downsliding of this material on the ice core to the margins. Within cryogenic mounds, clustering and size of the forms fits the characteristics of perennial frost mound without peat cover, i.e. lithalsas or minerogenic palsas. Cryogenic mounds are important paleoclimatic indicators. Based on modern analogs, these lithalsa scars indicate the former presence of discontinuous
Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus
NASA Astrophysics Data System (ADS)
Hoernle, Kaj; Hauff, Folkmar; van den Bogaard, Paul; Werner, Reinhard; Mortimer, Nick; Geldmacher, Jörg; Garbe-Schönberg, Dieter; Davy, Bryan
2010-12-01
Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr-Nd-Pb-Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The 40Ar/ 39Ar age and geochemical data show that the Hikurangi basement lavas (118-96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99-87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (˜52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ˜1% of the Earth's surface was covered with volcanism, resulted from a
2016-07-15
One small section of this image shows boulders that have rolled down the slope of a crater wall. The boulders vary in size, with the largest one approximately 6 meters across. Unlike the boulder in a previous image, this one is not standing on end. We can tell by using the sun angle and shadow length to figure out the height and then comparing that to its other measurements. We can determine the origin of the boulders by tracing their up-slope tracks. They appear to come from one small part of the crater wall that is less stable than surrounding materials. It is likely that there have been numerous rockfall events from this area, as suggested by the many boulders down-slope of this area, some with clear tracks and others with indistinct or no tracks visible. http://photojournal.jpl.nasa.gov/catalog/PIA20744
Combustion Instabilities Modeled
NASA Technical Reports Server (NTRS)
Paxson, Daniel E.
1999-01-01
NASA Lewis Research Center's Advanced Controls and Dynamics Technology Branch is investigating active control strategies to mitigate or eliminate the combustion instabilities prevalent in lean-burning, low-emission combustors. These instabilities result from coupling between the heat-release mechanisms of the burning process and the acoustic flow field of the combustor. Control design and implementation require a simulation capability that is both fast and accurate. It must capture the essential physics of the system, yet be as simple as possible. A quasi-one-dimensional, computational fluid dynamics (CFD) based simulation has been developed which may meet these requirements. The Euler equations of mass, momentum, and energy have been used, along with a single reactive species transport equation to simulate coupled thermoacoustic oscillations. A very simple numerical integration scheme was chosen to reduce computing time. Robust boundary condition procedures were incorporated to simulate various flow conditions (e.g., valves, open ends, and choked inflow) as well as to accommodate flow reversals that may arise during large flow-field oscillations. The accompanying figure shows a sample simulation result. A combustor with an open inlet, a choked outlet, and a large constriction approximately two thirds of the way down the length is shown. The middle plot shows normalized, time-averaged distributions of the relevant flow quantities, and the bottom plot illustrates the acoustic mode shape of the resulting thermoacoustic oscillation. For this simulation, the limit cycle peak-to-peak pressure fluctuations were 13 percent of the mean. The simulation used 100 numerical cells. The total normalized simulation time was 50 units (approximately 15 oscillations), which took 26 sec on a Sun Ultra2.
NASA Astrophysics Data System (ADS)
Wegmann, K. W.; Gallen, S. F.; Frankel, K. L.; Erdenebat, M.; Hopkins, C.
2011-12-01
High mountains with low relief summit surfaces or plateaus are common across western Mongolia and southern Siberia. These flat-topped mountains exist in the Gobi and Mongolian Altai, the Hangay Mountains, and the trans Hovsgol-Sayan Ranges. These surfaces have classically and recently been interpreted as fragments of one or more low-elevation erosion surfaces dating to the Mesozoic and early Cenozoic that have been elevated by late Cenozoic tectonic uplift, yet are still preserved. In this first hypothesis, the low-relief summit surfaces represent the slowest eroding part of the mountain landscape, where lower elevation summits lacking these surfaces must represent regions of faster erosion through the paleoerosion surface (s). We observe a positive correlation between the regional elevation of reconstructed glacial equilibrium line altitudes and mountains with low-gradient summit plateaus. Topographic metrics including hypsometry, relief, and altitudinal variations in surface roughness across these mountain ranges suggest to us that an alternative hypothesis may explain the origin of flat-topped mountains in Central Asia. Specifically, in mountainous regions experiencing slow to moderate rates of uplift, the intersection of topography with the mean elevation of glacial and/or periglacial processes may be limiting elevation and relief production. In this latter scenario, the high summit plateaus would have faster erosion rates than lower elevation mountains that have not yet been uplifted to the point where their summits intersect the glacial-periglacial "sanding block". If correct, the long road to flat mountain-top surfaces found across western Mongolia may be much younger than previously thought, owing their existence to the establishment of a cooler Quaternary climate resulting in more active glacial and periglacial erosional processes.
Wang, Hongsheng; Chen, Tony; Torzilli, Peter; Warren, Russell; Maher, Suzanne
2014-01-01
The spatial distribution and pattern of local contact stresses within the knee joint during activities of daily living have not been fully investigated. The objective of this study was to determine if common contact stress patterns exist on the tibial plateaus of human knees during simulated gait. To test this hypothesis, we developed a novel normalized cross-correlation (NCC) algorithm and applied it to the contact stresses on the tibial plateaus of twelve human cadaveric knees subjected to multi-directional loads mimicking gait. The contact stress profiles at different locations on the tibial plateaus were compared, where regions with similar contact stress patterns were identified across specimens. Three consistent regional patterns were found, among them two most prominent contact stress patterns were shared by 9 to 12 of all the knees and the third pattern was shared by 6 to 8 knees. The first pattern was located at the posterior aspect of the medial tibial plateau and had a single peak stress that occurred during the early stance phase. The second pattern was located at the central-posterior aspects of the lateral plateau and consisted of two peak stresses coincident with the timing of peak axial force at early and late stance. The third pattern was found on the anterior aspect of cartilage-to-cartilage contact region on the medial plateau consisted of double peak stresses. The differences in the location and profile of the contact stress patterns suggest that the medial and lateral menisci function to carry load at different points in the gait cycle: with the posterior aspect of the medial meniscus consistently distributing load only during the early phase of stance, and the posterior aspect of the lateral meniscus consistently distributing load during both the early and late phases of stance. This novel approach can help identify abnormalities in knee contact mechanics and provide a better understanding of the mechanical pathways leading to post
Westerman, Drew A.; Gillip, Jonathan A.; Richards, Joseph M.; Hays, Phillip D.; Clark, Brian R.
2016-01-01
A hydrogeologic framework of the Ozark Plateaus aquifer system was constructed as the base for a groundwater flow model developed as part of the U.S. Geological Survey Water Availability and Use Science Program to aid in the understanding of groundwater availability in select aquifer systems of the United States. The Ozark Plateaus aquifer system study area (hereinafter referred to as the “Ozark system”) is nearly 70,000 square miles and includes parts of Arkansas, Kansas, Missouri, and Oklahoma. A hydrogeologic framework was constructed to represent the altitudes and thicknesses of nine hydrogeologic units within the Ozark Plateaus aquifer system - . the Western Interior Plains confining system, Springfield Plateau aquifer, the Ozark confining unit, Ozark aquifer (divided into the upper, middle, and lower Ozark aquifers to better capture the spatial variation in the hydrologic properties), the St. Francois confining unit, the St. Francois aquifer, and the Basement confining unit. The formations that make up the hydrogeologic units of the Ozark system range from Pennsylvanian to Cambrian age. The scope of effort included the compilation and interpretation of hydrogeologic altitudes from geophysical, lithologic driller description, and well cutting logs. The final compiled dataset included more than 23,000 individual altitude points (excluding synthetic points) representing the nine hydrogeologic units. Shorthand names were used to identify points, extents, and raster surfaces corresponding to each hydrogeologic unit. WIPCS = Western Interior Plains confining system SPA = Springfield Plateau aquifer OCU = Ozark confining unit UOA = upper Ozark aquifer MOA = middle Ozark aquifer LOA = lower Ozark aquifer SFCU = St. Francois confining unit SFA = St. Francois aquifer BCU = Basement confining unit
Electrostatic ion cyclotron velocity shear instability
NASA Technical Reports Server (NTRS)
Lemons, D. S.; Winske, D.; Gary, S. P.
1992-01-01
A local electrostatic dispersion equation is derived for a shear flow perpendicular to an ambient magnetic field, which includes all kinetic effects and involves only one important parameter. The dispersion equation is cast in the form of Gordeyev integrals and is solved numerically. Numerical solutions indicate that an ion cyclotron instability is excited. The instability occurs roughly at multiples of the ion cyclotron frequency (modified by the shear), with the growth rate or the individual harmonics overlapping in the wavenumber. At large values of the shear parameter, the instability is confined to long wavelengths, but at smaller shear, a second distinct branch at shorter wavelengths also appears. The properties of the instability obtained are compared with those obtained in the nonlocal limit by Ganguli et al. (1985, 1988).
NASA Astrophysics Data System (ADS)
Schildgen, T. F.; Yıldırım, C.; Cosentino, D.; Strecker, M. R.
2014-01-01
The Central and Eastern Anatolian plateaus are integral parts of the world's third largest orogenic plateau. In the past decade, geophysical surveys have provided insights into the crust, lithosphere, and mantle beneath Eastern Anatolia. These observations are now accompanied by recent surveys in Central Anatolia and new data constraining the timing and magnitude of uplift along its northern and southern margins. Together with predictions from geodynamic models on the effects of various processes on surface deformation and uplift, the observations can be integrated to identify probable mechanisms of Anatolian Plateau growth.
Nonlinear Dynamics of Single Bunch Instability
Stupakov, G.V.; Breizman, B.N.; Pekker, M.S.; /Texas U.
2011-09-09
A nonlinear equation is derived that governs the evolution of the amplitude of unstable oscillations with account of quantum diffusion effects due to the synchrotron radiation. Numerical solutions to this equation predict a variety of possible scenarios of nonlinear evolution of the instability some of which are in good qualitative agreement with experimental observations. Microwave single bunch instability in circular accelerators has been observed in many machines. The instability usually arises when the number of particles in the bunch exceeds some critical value, Nc, which varies depending on the parameters of the accelerating regime. Recent observations on the SLC damping rings at SLAC with a new low-impedance vacuum chamber revealed new interesting features of the instability. In some cases, after initial exponential growth, the instability eventually saturated at a level that remained constant through the accumulation cycle. In other regimes, relaxation-type oscillations were measured in nonlinear phase of the instability. In many cases, the instability was characterized by a frequency close to the second harmonic of the synchrotron oscillations. Several attempts have been made to address the nonlinear stage of the instability based on either computer simulations or some specific assumptions regarding the structure of the unstable mode. An attempt of a more general consideration of the problem is carried out in this paper. We adopt an approach recently developed in plasma physics for analysis of nonlinear behavior of weakly unstable modes in dynamic systems. Assuming that the growth rate of the instability is much smaller than its frequency, we find a time dependent solution to Vlasov equation and derive an equation for the complex amplitude of the oscillations valid in the nonlinear regime. Numerical solutions to this equation predict a variety of possible scenarios of nonlinear evolution of the instability some of which are in good qualitative agreement
MHD Instabilities in Simple Plasma Configuration
1984-01-01
cause the field lines to break and reconnect. . This work is divided into two parts. Chapters " describe linear theory and Chapters -XV- describe the...details in any non- linear theory can rapidly mushroom out of all proportion. For this reason much work in nonlinear MHD theory is done by numerical...99 IX. INSTABILITIES IN A TOROIDAL PLASMA ........................ 125 X. QUASI- LINEAR THEORY OF MHD INSTABILITIES ........... 133
Computational methods for probability of instability calculations
NASA Technical Reports Server (NTRS)
Wu, Y.-T.; Burnside, O. H.
1990-01-01
This paper summarizes the development of the methods and a computer program to compute the probability of instability of a dynamic system than can be represented by a system of second-order ordinary linear differential equations. Two instability criteria based upon the roots of the characteristics equation or Routh-Hurwitz test functions are investigated. Computational methods based on system reliability analysis methods and importance sampling concepts are proposed to perform efficient probabilistic analysis. Numerical examples are provided to demonstrate the methods.
Relativistically modulational instability by strong Langmuir waves
Liu, X. L.; Liu, S. Q.; Li, X. Q.
2012-09-15
Based on the set of nonlinear coupling equations, which has considered the relativistic effects of electrons, modulational instability by strong Langmuir waves has been investigated in this paper. Both the characteristic scale and maximum growth rate of the Langmuir field will enhance with the increase in the electron relativistic effect. The numerical results indicate that longitudinal perturbations induce greater instability than transverse perturbations do, which will lead to collapse and formation of the pancake-like structure.
Electron heat flux instability
NASA Astrophysics Data System (ADS)
Saeed, Sundas; Sarfraz, M.; Yoon, P. H.; Lazar, M.; Qureshi, M. N. S.
2017-02-01
The heat flux instability is an electromagnetic mode excited by a relative drift between the protons and two-component core-halo electrons. The most prominent application may be in association with the solar wind where drifting electron velocity distributions are observed. The heat flux instability is somewhat analogous to the electrostatic Buneman or ion-acoustic instability driven by the net drift between the protons and bulk electrons, except that the heat flux instability operates in magnetized plasmas and possesses transverse electromagnetic polarization. The heat flux instability is also distinct from the electrostatic counterpart in that it requires two electron species with relative drifts with each other. In the literature, the heat flux instability is often called the 'whistler' heat flux instability, but it is actually polarized in the opposite sense to the whistler wave. This paper elucidates all of these fundamental plasma physical properties associated with the heat flux instability starting from a simple model, and gradually building up more complexity towards a solar wind-like distribution functions. It is found that the essential properties of the instability are already present in the cold counter-streaming electron model, and that the instability is absent if the protons are ignored. These instability characteristics are highly reminiscent of the electron firehose instability driven by excessive parallel temperature anisotropy, propagating in parallel direction with respect to the ambient magnetic field, except that the free energy source for the heat flux instability resides in the effective parallel pressure provided by the counter-streaming electrons.
Double Hall instability: A catalyzer of magnetic energy release
NASA Astrophysics Data System (ADS)
Kitchatinov, L. L.
2017-09-01
A pictorial explanation for shear-Hall instability is suggested and shows that the shear flow is not necessary for the instability because its role can be played by the Hall effect of an inhomogeneous backgroundmagnetic field. Linear stability analysis for a simplemodel of magnetic field varying periodically in space confirms such a "double Hall" instability. Numerical computations show a considerable increase in Ohmic dissipation rate at the nonlinear stage of instability development. Field dissipation has a spiky character associated with magnetic reconnection in current sheets and X-points. Double Hall instability can be significant for magnetic field dissipation in neutron star crusts and, possibly, in the solar corona.
Ground-water quality in the Appalachian Plateaus, Kanawha River basin, West Virginia
Sheets, Charlynn J.; Kozar, Mark D.
2000-01-01
Water samples collected from 30 privately-owned and small public-supply wells in the Appalachian Plateaus of the Kanawha River Basin were analyzed for a wide range of constituents, including bacteria, major ions, nutrients, trace elements, radon, pesticides, and volatile organic compounds. Concentrations of most constituents from samples analyzed did not exceed U.S. Environmental Protection Agency (USEPA) standards. Constituents that exceeded drinking-water standards in at least one sample were total coliform bacteria, Escherichia coli (E. coli), iron, manganese, and sulfate. Total coliform bacteria were present in samples from five sites, and E. coli were present at only one site. USEPA secondary maximum contaminant levels (SMCLs) were exceeded for three constituents -- sulfate exceeded the SMCL of 250 mg/L (milligrams per liter) in samples from 2 of 30 wells; iron exceeded the SMCL of 300 ?g/L (micrograms per liter) in samples from 12 of the wells, and manganese exceeded the SMCL of 50 ?g/L in samples from 17 of the wells sampled. None of the samples contained concentrations of nutrients that exceeded the USEPA maximum contaminant levels (MCLs) for these constituents. The maximum concentration of nitrate detected was only 4.1 mg/L, which is below the MCL of 10 mg/L. Concentrations of nitrate in precipitation and shallow ground water are similar, potentially indicating that precipitation may be a source of nitrate in shallow ground water in the study area. Radon concentrations exceeded the recently proposed maximum contaminant level of 300 pCi/L at 50 percent of the sites sampled. The median concentration of radon was only 290 pCi/L. Radon-222 is a naturally occurring, carcinogenic, radioactive decay product of uranium. Concentrations, however, did not exceed the alternate maximum contaminant level (AMCL) for radon of 4,000 pCi/L in any of the 30 samples. Arsenic concentrations exceeded the proposed MCL of 5?g/L at 4 of the 30 sites. No samples exceeded the
Combustion instability modeling and analysis
Santoro, R.J.; Yang, V.; Santavicca, D.A.; Sheppard, E.J.
1995-12-31
It is well known that the two key elements for achieving low emissions and high performance in a gas turbine combustor are to simultaneously establish (1) a lean combustion zone for maintaining low NO{sub x} emissions and (2) rapid mixing for good ignition and flame stability. However, these requirements, when coupled with the short combustor lengths used to limit the residence time for NO formation typical of advanced gas turbine combustors, can lead to problems regarding unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions, as well as the occurrence of combustion instabilities. The concurrent development of suitable analytical and numerical models that are validated with experimental studies is important for achieving this objective. A major benefit of the present research will be to provide for the first time an experimentally verified model of emissions and performance of gas turbine combustors. The present study represents a coordinated effort between industry, government and academia to investigate gas turbine combustion dynamics. Specific study areas include development of advanced diagnostics, definition of controlling phenomena, advancement of analytical and numerical modeling capabilities, and assessment of the current status of our ability to apply these tools to practical gas turbine combustors. The present work involves four tasks which address, respectively, (1) the development of a fiber-optic probe for fuel-air ratio measurements, (2) the study of combustion instability using laser-based diagnostics in a high pressure, high temperature flow reactor, (3) the development of analytical and numerical modeling capabilities for describing combustion instability which will be validated against experimental data, and (4) the preparation of a literature survey and establishment of a data base on practical experience with combustion instability.
On the convective-absolute nature of river bedform instabilities
NASA Astrophysics Data System (ADS)
Vesipa, Riccardo; Camporeale, Carlo; Ridolfi, Luca; Chomaz, Jean Marc
2014-12-01
River dunes and antidunes are induced by the morphological instability of stream-sediment boundary. Such bedforms raise a number of subtle theoretical questions and are crucial for many engineering and environmental problems. Despite their importance, the absolute/convective nature of the instability has never been addressed. The present work fills this gap as we demonstrate, by the cusp map method, that dune instability is convective for all values of the physical control parameters, while the antidune instability exhibits both behaviors. These theoretical predictions explain some previous experimental and numerical observations and are important to correctly plan flume experiments, numerical simulations, paleo-hydraulic reconstructions, and river works.
Ion-Hose Instability in Long Pulse Induction Accelerators
Caporaso, G J; McCarrick, J F
2000-08-02
The ion-hose (or fast-ion) instability sets limits on the allowable vacuum in a long-pulse, high current accelerator. Beam-induced ionization of the background gas leads to the formation of an ion channel which couples to the transverse motion of the beam. The instability is studied analytically and numerically for several ion frequency distributions. The effects of beam envelope oscillations on the growth of the instability will be discussed. The saturated non-linear growth of the instability is derived analytically and numerically for two different ion frequency distributions.
NASA Astrophysics Data System (ADS)
Margreth, Annina; Gosse, John C.; Dyke, Arthur S.
2016-02-01
Long-term rates of subaerial and episodic subglacial erosion by predominately cold-based ice cover are determined for tors on weathered plateaus on Cumberland Peninsula. By measuring terrestrial cosmogenic nuclide concentrations in differentially weathered surfaces on a given tor, we reconstruct the complex exposure and erosion history involving recurring cold-based ice cover. The style and rate of subaerial and subglacial erosion at multiple tor sites on Cumberland Peninsula are assessed with a Monte Carlo approach that computes plausible exposure histories based on a proxy record of global ice volume. Constant subaerial erosion rates by weathering are low (<2 mm ka-1), corroborated by nuclide concentrations measured on two tors located on coastal ridge crests that have likely never been glaciated. Summit plateaus intermittently covered by cold-based ice throughout the Quaternary have experienced episodic subglacial erosion by plucking of fractured bedrock with rates between 1 and 16 mm ka-1. Variation of episodic erosion rates is associated with topographic position of the sampled tors and bedrock fracture density. Most of the tors were last glacially plucked in pre-ultimate glaciations, not during the Wisconsinan glaciation. Furthermore, the new approach provides evidence for the extent of ice coverage during the late Wisconsinan, which is significant if no erratics are available for exposure dating. Despite late Pleistocene intervals of ice cover and glacial plucking, tor-studded landscapes of Cumberland Peninsula are of considerable antiquity.
NASA Astrophysics Data System (ADS)
Mega, Nabil; Medjerab, Abderrahmane
2016-10-01
The high plateaus of Algeria is a critical region to policymakers in terms of social, economic, and infrastructure development. The main goal of the present work was to monitor the climatic drought and its impact on vegetation health across the Algerian high plateaus using remote sensing techniques. Vegetation health index (VHI) showed a clear drought in the western region of the study area. The results show practically three periods of drought were evident: October to December 2006, November to December 2009, and December 2012. Agreeable correlations among the obtained results using standard precipitation index for 3 months (SPI-3) and other satellite indicators such as temperature vegetation dryness index (TVDI) and VHI were obtained. TVDI and VHI agreed well with the ground-based observations from SPI-3; thus, these may serve as key and easily accessible indicators of drought. The research shows motivating results that decision makers can use to take timely corrective measures to minimize the reduction in agricultural production in drought prone areas.
NASA Astrophysics Data System (ADS)
Alécio, Raphael C.; Lyra, Marcelo L.; Strečka, Jozef
2016-11-01
The ground-state phase diagram, magnetization process and bipartite entanglement of the frustrated spin-1/2 Ising-Heisenberg and Heisenberg triangular tube (three-leg ladder) are investigated in a non-zero external magnetic field. The exact ground-state phase diagram of the spin-1/2 Ising-Heisenberg tube with Heisenberg intra-rung and Ising inter-rung couplings consists of six distinct gapped phases, which manifest themselves in a magnetization curve as intermediate plateaus at zero, one-third and two-thirds of the saturation magnetization. Four out of six available ground states exhibit quantum entanglement between two spins from the same triangular unit evidenced by a non-zero concurrence. Density-matrix renormalization group calculations are used in order to construct the ground-state phase diagram of the analogous but purely quantum spin-1/2 Heisenberg tube with Heisenberg intra- and inter-rung couplings, which consists of four gapped and three gapless phases. The Heisenberg tube shows a continuous change of the magnetization instead of a plateau at zero magnetization, while the intermediate one-third and two-thirds plateaus may be present or not in the zero-temperature magnetization curve.
NASA Astrophysics Data System (ADS)
Morita, Katsuhiro; Shibata, Naokazu
2016-10-01
We study the ground state of the S =1/2 Heisenberg model on the checkerboard lattice in a magnetic field by the density matrix renormalization group method with the sine-square deformation. We obtain magnetization plateaus at M /Msat=0 ,1/4 ,3/8 ,1/2 , and 3/4 , where Msat is the saturated magnetization. The obtained 3/4 plateau state is consistent with the exact result, and the 1/2 plateau is found to have a four-spin resonating loop structure similar to the six-spin loop structure of the 1/3 plateau of the kagome lattice. Different four-spin loop structures are obtained in the 1/4 and 3/8 plateaus but no corresponding states exist in the kagome lattice. The 3/8 plateau has a unique magnetic structure of three types of four-spin local quantum states in a 4 √{2 }×2 √{2 } magnetic unit cell with a 16-fold degeneracy.
NASA Astrophysics Data System (ADS)
Wölbern, I.; Heit, B.; Yuan, X.; Asch, G.; Kind, R.; Viramonte, J.; Tawackoli, S.; Wilke, H.
2009-04-01
Teleseismic data recorded during one and a half years are investigated with the receiver function technique to determine the crustal and upper-mantle structures underneath the highly elevated Altiplano and Puna plateaus in the central Andes. A series of converting interfaces are determined along two profiles at 21°S and 25.5°S, respectively, with a station spacing of approximately 10 km. The data provide the highest resolution gained from a passive project in this area, so far. The oceanic Nazca plate is detected down to 120 km beneath the Altiplano whereas beneath the Puna, the slab can unexpectedly be traced down to 200 km depth at longer periods. A shallow crustal low-velocity zone is determined beneath both plateaus exhibiting segmentation. In the case of the Altiplano, the segments present vertical offsets and are separated by inclined interfaces, which coincide with major fault systems at the surface. An average depth to Moho of about 70 km is determined for the Altiplano plateau. A strong negative velocity anomaly located directly below the Moho along with local crustal thinning is interpreted beneath the volcanic arc of the Altiplano plateau between 67°W and 68.5°W. A deep section of the Puna profile reveals thinning of the mantle transition zone. Although poorly resolved, the detected anomaly may suggest the presence of a mantle plume, which may constitute the origin of the anomalous temperatures at the depth of the upper-mantle discontinuities.
Kinesics of Affective Instability.
ERIC Educational Resources Information Center
Dil, Nasim
1979-01-01
Discusses the rationale of studying kinesics of affective instability, describes the phenonmenon of affective instability, examines the role of kinesics in the overall process of communication, and presents three case studies. (Author/AM)
Turbine instabilities: Case histories
NASA Technical Reports Server (NTRS)
Laws, C. W.
1985-01-01
Several possible causes of turbine rotor instability are discussed and the related design features of a wide range of turbomachinery types and sizes are considered. The instrumentation options available for detecting rotor instability and assessing its severity are also discussed.
NASA Astrophysics Data System (ADS)
Baumgartner, Peter O.; Baumgartner-Mora, Claudia; Andjic, Goran
2016-04-01
The Late Cretaceous-Paleogene sedimentation pattern in space and time along the Middle American convergent margin was controlled by the accretion of Pacific plateaus and seamounts. The accretion of more voluminous plateaus must have caused the temporary extinction of the arc and tectonic uplift, resulting in short lived episodes of both pelagic and neritic biogenic sedimentation. By the Late Eocene, shallow carbonate environments became widespread on a supposed mature arc edifice, that is so far only documented in arc-derived sediments. In northern Costa Rica forearc sedimentation started during the Coniacian-Santonian on the Aptian-Turonian basement of the Manzanillo Terrane. The arrival and collision of the Nicoya Terrane (a CLIP-like, 139-83 Ma Pacific plateau) and the Santa Elena Terrane caused the extinction of the arc during late Campanian- Early Maastrichtian times, indicated by the change to pelagic limestone sedimentation (Piedras Blancas Formation) in deeper areas and shallow-water rudistid - Larger Benthic Foraminfera limestone on tectonically uplifted areas of all terranes. Arc-derived turbidite sedimentation resumed in the Late Maastrichtian and was again interrupted during the Late Paleocene - Early Eocene, perhaps due to the underplating of a yet unknown large seamount. The extinction of the arc resulted in the deposition of the siliceous pelagic Buenavista Formation, as well as the principally Thanetian Barra Honda carbonate platform on a deeply eroded structural high in the Tempisque area. In southern Costa Rica the basement is thought to be the western edge of the CLIP. It is Santonian-Campanian in age and is only exposed in the southwestern corner of Herradura. Cretaceous arc-forearc sequences are unknown, except for the Maastrichtian-Paleocene Golfito Terrane in southeastern Costa Rica. The distribution and age of shallow/pelagic carbonates vs. arc-derived detrital sediments is controlled by the history of accretion of Galápagos hot spot
Numerical Relativistic Quantum Optics
2013-11-08
for Soft Core Potentials 6 C. Scalar Zeeman Effect 7 IV. Time Dependent Simulations 10 A. Numerical Algorithm 10 B. Validation Against Zeeman Effect 12...energies in a Coulomb potential can be found in Ref. [13]. C. Scalar Zeeman Effect Numerical solution of the nonlinear eigenvalue problem (12) allows...instability give Fig. 9(a), while parameters leading to stability give Fig. 9(b). B. Validation Against Zeeman Effect Two codes have been written
Stier, A V; Ellis, C T; Kwon, J; Xing, H; Zhang, H; Eason, D; Strasser, G; Morimoto, T; Aoki, H; Zeng, H; McCombe, B D; Cerne, J
2015-12-11
We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron resonance frequency using highly sensitive Faraday rotation measurements. The sample is electrically gated, allowing the electron density to be changed continuously by more than a factor of 3. We observe clear plateaulike and steplike features in the Faraday rotation angle vs electron density and magnetic field (Landau-level filling factor) even at fields or frequencies very close to cyclotron resonance absorption. These features are the high frequency manifestation of quantum Hall plateaus-a signature of topologically protected edge states. We observe both odd and even filling factor plateaus and explore the temperature dependence of these plateaus. Although dynamical scaling theory begins to break down in the frequency region of our measurements, we find good agreement with theory.
Rogue Waves and Modulational Instability
NASA Astrophysics Data System (ADS)
Zakharov, V. E.; Dyachenko, A.
2015-12-01
The most plausible cause of rogue wave formation in a deep ocean is development of modulational instability of quasimonochromatic wave trains. An adequate model for study of this phenomenon is the Euler equation for potential flow of incompressible fluid with free surface in 2-D geometry. Numerical integration of these equations confirms completely the conjecture of rogue wave formation from modulational instability but the procedure is time consuming for determination of rogue wave appearance probability for a given shape of wave energy spectrum. This program can be realized in framework of simpler model using replacement of the exact interaction Hamiltonian by more compact Hamiltonian. There is a family of such models. The popular one is the Nonlinear Schrodinger equation (NLSE). This model is completely integrable and suitable for numerical simulation but we consider that it is oversimplified. It misses such important phenomenon as wave breaking. Recently, we elaborated much more reliable model that describes wave breaking but is as suitable as NLSE from the point of numerical modeling. This model allows to perform massive numerical experiments and study statistics of rogue wave formation in details.
Combustion instability mitigation by magnetic fields
NASA Astrophysics Data System (ADS)
Jocher, Agnes; Pitsch, Heinz; Gomez, Thomas; Bonnety, Jérôme; Legros, Guillaume
2017-06-01
The present interdisciplinary study combines electromagnetics and combustion to unveil an original and basic experiment displaying a spontaneous flame instability that is mitigated as the non-premixed sooting flame experiences a magnetic perturbation. This magnetic instability mitigation is reproduced by direct numerical simulations to be further elucidated by a flow stability analysis. A key role in the stabilization process is attributed to the momentum and thermochemistry coupling that the magnetic force, acting mainly on paramagnetic oxygen, contributes to sustain. The spatial local stability analysis based on the numerical simulations shows that the magnetic field tends to reduce the growth rates of small flame perturbations.
Combustion instability mitigation by magnetic fields.
Jocher, Agnes; Pitsch, Heinz; Gomez, Thomas; Bonnety, Jérôme; Legros, Guillaume
2017-06-01
The present interdisciplinary study combines electromagnetics and combustion to unveil an original and basic experiment displaying a spontaneous flame instability that is mitigated as the non-premixed sooting flame experiences a magnetic perturbation. This magnetic instability mitigation is reproduced by direct numerical simulations to be further elucidated by a flow stability analysis. A key role in the stabilization process is attributed to the momentum and thermochemistry coupling that the magnetic force, acting mainly on paramagnetic oxygen, contributes to sustain. The spatial local stability analysis based on the numerical simulations shows that the magnetic field tends to reduce the growth rates of small flame perturbations.
Granular Rayleigh-Taylor instability
Vinningland, Jan Ludvig; Johnsen, Oistein; Flekkoey, Eirik G.; Maaloey, Knut Joergen; Toussaint, Renaud
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 Fourier 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.
Circulation in blast driven instabilities
NASA Astrophysics Data System (ADS)
Henry de Frahan, Marc; Johnsen, Eric
2016-11-01
Mixing in many natural phenomena (e.g. supernova collapse) and engineering applications (e.g. inertial confinement fusion) is often initiated through hydrodynamic instabilities. Explosions in these systems give rise to blast waves which can interact with perturbations at interfaces between different fluids. Blast waves are formed by a shock followed by a rarefaction. This wave profile leads to complex time histories of interface acceleration. In addition to the instabilities induced by the acceleration field, the rarefaction from the blast wave decompresses the material at the interface, further increasing the perturbation growth. After the passage of the wave, circulation circulation generated by the blast wave through baroclinic vorticity continues to act upon the interface. In this talk, we provide scaling laws for the circulation and amplitude growth induced by the blast wave. Numerical simulations of the multifluid Euler equations solved using a high-order accurate Discontinuous Galerkin method are used to validate the theoretical results.
NASA Technical Reports Server (NTRS)
Bleacher, Jacob E.; Orr, Tim R.; de Wet, Andrew P.; Zimbelman, James R.; Hamilton, Christopher W.; Garry, W. Brent; Crumpler, Larry S.; Williams, David A.
2017-01-01
The Tharsis Montes rift aprons are composed of outpourings of lava from chaotic terrains to the northeast and southwest flank of each volcano. Sinuous and branching channel networks that are present on the rift aprons suggest the possibility of fluvial processes in their development, or erosion by rapidly emplaced lavas, but the style of lava flow emplacement throughout rift apron development is not clearly understood. To better characterize the style of lava emplacement and role of fluvial processes in rift apron development, we conducted morphological mapping of the Pavonis Mons southwest rift apron and the eastern Tharsis plains using images from the High Resolution Imaging Science Experiment (HiRISE), Mars Orbiter Camera (MOC), Context Camera (CTX), Thermal Emission Imaging System (THEMIS), and High Resolution Stereo Camera (HRSC) along with the Mars Orbiter Laser Altimeter (MOLA) Precision Experiment Data Records (PEDRs) and gridded data. Our approach was to: (1) search for depositional fans at the slope break between the rift apron and adjacent low slope plains; (2) determine if there is evidence that previously formed deposits might have been buried by plains units; (3) characterize the Tharsis plains morphologies east of Pavonis Mons; and (4) assess their relationship to the rift apron units. We have not identified topographically significant depositional fans, nor did we observe evidence to suggest that plains units have buried older rift apron units. Flow features associated with the rift apron are observed to continue across the slope break onto the plains. In this area, the plains are composed of a variety of small fissures and low shield vents around which broad channel-fed and tube-fed flows have been identified. We also find broad, flat-topped plateaus and sinuous ridges mixed among the channels, tubes and vents. Flat-topped plateaus and sinuous ridges are morphologies that are analogous to those observed on the coastal plain of Hawai'i, where lava
Bleacher, Jacob E.; Orr, Tim; de Wet, Andrew P.; Zimbelman, James R.; Hamilton, Christopher W.; Garry, W. Brent; Crumpler, Larry S.; Williams, David A.
2017-01-01
The Tharsis Montes rift aprons are composed of outpourings of lava from chaotic terrains to the northeast and southwest flank of each volcano. Sinuous and branching channel networks that are present on the rift aprons suggest the possibility of fluvial processes in their development, or erosion by rapidly emplaced lavas, but the style of lava flow emplacement throughout rift apron development is not clearly understood. To better characterize the style of lava emplacement and role of fluvial processes in rift apron development, we conducted morphological mapping of the Pavonis Mons southwest rift apron and the eastern Tharsis plains using images from the High Resolution Imaging Science Experiment (HiRISE), Mars Orbiter Camera (MOC), Context Camera (CTX), Thermal Emission Imaging System (THEMIS), and High Resolution Stereo Camera (HRSC) along with the Mars Orbiter Laser Altimeter (MOLA) Precision Experiment Data Records (PEDRs) and gridded data. Our approach was to: (1) search for depositional fans at the slope break between the rift apron and adjacent low slope plains; (2) determine if there is evidence that previously formed deposits might have been buried by plains units; (3) characterize the Tharsis plains morphologies east of Pavonis Mons; and (4) assess their relationship to the rift apron units. We have not identified topographically significant depositional fans, nor did we observe evidence to suggest that plains units have buried older rift apron units. Flow features associated with the rift apron are observed to continue across the slope break onto the plains. In this area, the plains are composed of a variety of small fissures and low shield vents around which broad channel-fed and tube-fed flows have been identified. We also find broad, flat-topped plateaus and sinuous ridges mixed among the channels, tubes and vents. Flat-topped plateaus and sinuous ridges are morphologies that are analogous to those observed on the coastal plain of Hawai‘i, where lava
Nonlocal Studies of the Magnetorotational Instability
NASA Astrophysics Data System (ADS)
Bhattacharjee, A.; Ebrahimi, F.; Lefebvre, B.; Vandenberg, A.
2010-11-01
Viewed from the perspective of nonlocal studies of plasmas with sheared flows, the magnetorotational instability (MRI) is an important member of a larger family of shear- driven instabilities in a magnetized disk. A comprehensive analytical and numerical approach to these instabilities was first developed by Hameiri (1976) and Bondeson and coworkers (1987) with applications to fusion plasmas, and more recently applied by Keppens and cooworkers (2002) to Keplerian disks. The general framework uncovers a number of new features that must be included in our understanding of the linear as well as well as nonlinear evolution of the MRI. These include (1) overstability due the presence of compressibility for non-axisymmetric modes, and (2) the presence of an infinite sequence of discrete unstable modes accumulating toward the edge of the slow wave continuum at the Doppler-shifted frequency, regardless of the pressure gradient. For linear studies of these nonlocal instabilities, we present numerical results from a linear eignemode solver, and compare the predictions with NIMROD. We then use NIMROD to examine the consequences of these nonlocal instabilities for the nonlinear evolution of the MRI, where coupling to non-axisymmetric modes has already been shown to play an important role in the saturation of the instability.
The plasmoid instability during asymmetric inflow magnetic reconnection
Murphy, Nicholas A.; Young, Aleida K.; Shen, Chengcai; Lin, Jun; Ni, Lei
2013-06-15
Theoretical studies of the plasmoid instability generally assume that the reconnecting magnetic fields are symmetric. We relax this assumption by performing two-dimensional resistive magnetohydrodynamic simulations of the plasmoid instability during asymmetric inflow magnetic reconnection. Magnetic asymmetry modifies the onset, scaling, and dynamics of this instability. Magnetic islands develop preferentially into the weak magnetic field upstream region. Outflow jets from individual X-points impact plasmoids obliquely rather than directly as in the symmetric case. Consequently, deposition of momentum by the outflow jets into the plasmoids is less efficient, the plasmoids develop net vorticity, and shear flow slows down secondary merging between islands. Secondary merging events have asymmetry along both the inflow and outflow directions. Downstream plasma is more turbulent in cases with magnetic asymmetry because islands are able to roll around each other after exiting the current sheet. As in the symmetric case, plasmoid formation facilitates faster reconnection for at least small and moderate magnetic asymmetries. However, when the upstream magnetic field strengths differ by a factor of 4, the reconnection rate plateaus at a lower value than expected from scaling the symmetric results. We perform a parameter study to investigate the onset of the plasmoid instability as a function of magnetic asymmetry and domain size. There exist domain sizes for which symmetric simulations are stable but asymmetric simulations are unstable, suggesting that moderate magnetic asymmetry is somewhat destabilizing. We discuss the implications for plasmoid and flux rope formation in solar eruptions, laboratory reconnection experiments, and space plasmas. The differences between symmetric and asymmetric simulations provide some hints regarding the nature of the three-dimensional plasmoid instability.
Engine-Level Simulation of Liquid Rocket Combustion Instabilities
2013-01-01
Chapter 3. DATES COVERED (From - To) January 2013-July 2013 4. TITLE AND SUBTITLE Engine -Level Simulation of Liquid Rocket Combustion Instabilities...ABSTRACT A numerical investigation into combustion instability in liquid rocket engines is undertaken using large eddy simulations (LES). HPCMP resources...have been applied to demonstrate the ability to simulate combustion instability in liquid rocket engines and to gain further understanding of these
Modulational instabilities of periodic traveling waves in deep water
NASA Astrophysics Data System (ADS)
Akers, Benjamin F.
2015-04-01
The spectrum of periodic traveling waves in deep water is discussed. A multi-scale method is used, expanding the spectral data and the Bloch parameter in wave amplitude, to compute the size and location of modulated instabilities. The role of these instabilities in limiting the spectrum's analyticity is explained. Both two-dimensional and three-dimensional instabilities are calculated. The asymptotic predictions are compared to numerical simulations.
Baroclinic instability in stellar radiation zones
Kitchatinov, L. L.
2014-03-20
Surfaces of constant pressure and constant density do not coincide in differentially rotating stars. Stellar radiation zones with baroclinic stratification can be unstable. Instabilities in radiation zones are of crucial importance for angular momentum transport, mixing of chemical species, and, possibly, for magnetic field generation. This paper performs linear analysis of baroclinic instability in differentially rotating stars. Linear stability equations are formulated for differential rotation of arbitrary shape and then solved numerically for rotation nonuniform in radius. As the differential rotation increases, r- and g-modes of initially stable global oscillations transform smoothly into growing modes of baroclinic instability. The instability can therefore be interpreted as stability loss to r- and g-modes excitation. Regions of stellar parameters where r- or g-modes are preferentially excited are defined. Baroclinic instability onsets at a very small differential rotation of below 1%. The characteristic time of instability growth is about 1000 rotation periods. Growing disturbances possess kinetic helicity. Magnetic field generation by the turbulence resulting from baroclinic instability in differentially rotating radiation zones is therefore possible.
Secondary instabilities in compressible boundary layers
NASA Technical Reports Server (NTRS)
Ng, Lian; Erlebacher, Gordon
1990-01-01
Secondary instabilities are examined in compressible boundary layers at Mach numbers M(sub infinity) = 0, 0.8, 1.6, and 4.5. It is found that there is a broad-band of highly unstable 3-d secondary disturbances whose growth rates increase with increasing primary wave amplitude. At M(sub infinity) is less than or equal to 1.6, fundamental resonance dominates at relatively high (2-d) primary disturbance amplitude, while subharmonic resonance is characterized by a low (2-d) primary amplitude. At M(sub infinity) = 4.5, the subharmonic instability which arises from the second mode disturbance is the strongest type of secondary instability. The influence of the inclination, theta, of the primary wave with respect to the mean flow direction on secondary instability is investigated at M(sub infinity) = 1.6 for small to moderate values of theta. It is found that the strongest fundamental instability occurs when the primary wave is inclined at 10 deg to the mean flow direction, although a 2-d primary mode yields the most amplified subharmonic. The subharmonic instability at a high value of theta (namely, theta = 45 deg) is also discussed. Finally, a subset of the secondary instability results are compared against direct numerical simulations.
Parametric instability of two coupled nonlinear oscillators
NASA Astrophysics Data System (ADS)
Denardo, Bruce; Earwood, John; Sazonova, Vera
1999-03-01
One of the two normal modes of a system of two coupled nonlinear oscillators is subject to an instability. Several demonstration apparatus of weakly coupled oscillators that exhibit the instability are described. The effect is due to one normal mode parametrically driving the other, and occurs for the broad range of systems where the nonlinearity has a cubic contribution to the restoring force of each oscillator, which includes pendulums. The instability has an amplitude threshold that increases as the coupling is increased. A naive physical approach predicts that the mode opposite to that observed should be unstable. This is resolved by a weakly nonlinear analysis which reveals that the nonlinearity causes the linear frequency of a normal mode to depend upon the finite amplitude of the other mode. Numerical simulations confirm the theory, and extend the existence of the instability and the accuracy of the theoretical amplitude threshold beyond the regime of weak nonlinearity and weak coupling.
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.
Nguyen, T; de Jonge, L; Smith, S R; Bray, G A
2003-09-01
A robust algorithm for pull-calorimeters that provides a rapid response to changes in respiratory gas exchange has been implemented. Metabolic plateaus (over 20 min), such as that generated by steady treadmill exercise, can be measured accurately (< 2.0% error for an energy expenditure level of 16.7 kJ min(-1)). The time resolution for changes between plateaus can be accurately found with 1 min discrimination. Implementation required only software changes but no structural or instrumentation changes to the chamber. The algorithm was based on the one developed for the push-calorimeter at the Sahlgrenska Hospital in Sweden. The method utilises published equations for the rate of O2 consumption and CO2 production in the chamber, along with techniques for suppressing noise and identifying trends. Using the exact solution of the equations for steady state, the O2 concentrations from the preceding 30 min period are fitted to two connected exponential segments, of variable length, using the least-squares method. The smoothed O2 concentration and associated time derivative are then determined for the time point 15 min earlier and substituted into the respiration equations. The CO2 concentrations are subjected to the same analysis. The process is repeated every minute, and the newly computed rates of O2 consumption and CO2 production, as well as metabolic rate, are then presented. Gas injection tests proved that the chamber can respond instantaneously to a change from one steady state of respiration to another and correctly averages repeated changes in respiration with periods less than 15min (< 1.4% error for simulated, alternating O2 consumption levels of 0.81 min (-1) and 0.01 min). The successful integration of the algorithm into the Pennington chambers allows for traditional 24 h energy expenditure measurements and various metabolic experiments requiring rapid responses.
Topographic-driven instabilities in terrestrial bodies
NASA Astrophysics Data System (ADS)
Vantieghem, S.; Cebron, D.; Herreman, W.; Lacaze, L.
2013-12-01
Models of internal planetary fluid layers (core flows, subsurface oceans) commonly assume that these fluid envelopes have a spherical shape. This approximation however entails a serious restriction from the fluid dynamics point of view. Indeed, in the presence of mechanical forcings (precession, libration, nutation or tides) due to gravitational interaction with orbiting partners, boundary topography (e.g. of the core-mantle boundary) may excite flow instabilities and space-filling turbulence. These phenomena may affect heat transport and dissipation at the main order. Here, we focus on instabilities driven by longitudinal libration. Using a suite of theoretical tools and numerical simulations, we are able to discern a parameter range for which instability may be excited. We thereby consider deformations of different azimuthal order. This study gives the first numerical evidence of the tripolar instability. Furthermore, we explore the non-linear regime and investigate the amplitude as well as the dissipation of the saturated instability. Indeed, these two quantities control the torques on the solid layers and the thermal transport. Furthermore, based on this results, we address the issue of magnetic field generation associated with these flows (by induction or by dynamo process). This instability mechanism applies to both synchronized as non-synchronized bodies. As such, our results show that a tripolar instability might be present in various terrestrial bodies (Early Moon, Gallilean moons, asteroids, etc.), where it could participate in dynamo action. Simulation of a libration-driven tripolar instability in a deformed spherical fluid layer: snapshot of the velocity magnitude, where a complex 3D flow pattern is established.
Symmetry breaking and wake instabilities
NASA Astrophysics Data System (ADS)
Sengupta, Raja
A numerical technique has been developed in the context of spatio-temporal stability analysis. The convective/absolute nature of instability determines the time-asymptotic response of a linearly unstable flow, either in the form an oscillator or in the form of a noise amplifier. This depends on the location of pinch point singularities of the dispersion relations obtained via linear stability analyses. A new and efficient approach to locate such singularities is presented. Local analyticity of the dispersion relations was exploited via the Cauchy-Riemann equations in a quasi-Newton's root- finding procedure employing numerical Jacobians. Initial guesses provided by temporal stability analyses have been shown to converge to the pinch points even in the presence of multiple saddle points for various Falkner- Skan wedge profiles. This effort was motivated by the phenomenon of spontaneous symmetry breaking in flow over a cone. At large enough incidence, a pair of vortices develop on the leeward side of the cone which eventually become asymmetric as the angle of attack is increased further. A conical, thin-layer Navier-Stokes solver was employed to investigate the effect of flowfield saddles in this process. The approximate factorization scheme incorporated in the solver was shown analytically to be symmetric to eliminate possible sources of asymmetry. Local grid resolution studies were performed to demonstrate the importance of correctly computing the leeside saddle point and the secondary separation and reattchment points. Topological studies of the flow field as it loses symmetry agreed well with previous qualitative experimental observations. However, the original goal of this study, to settle an ongoing controversy regarding the nature of the instability responsible for symmetry breaking, could not be realized due to computational inadequacy. It is conjectured that the process is governed by an absolute instability similar to that observed in a flow over a circular
Jeans-Alfvén instability in quantum dusty magnetoplasmas
NASA Astrophysics Data System (ADS)
Jamil, M.; Rasheed, A.; Amir, M.; Abbas, G.; Jung, Young-Dae
2017-02-01
The Jeans instability is examined in quantum dusty magnetoplasmas due to low-frequency magnetosonic perturbations. The fluid model consisting of the momentum balance equation for quantum plasmas, Poisson's equation for the gravitational potential and Maxwell's equations for electromagnetic magnetosonic perturbations is solved. The numerical analysis elaborates the significant contribution of magnetic field, electron number density and variable dust mass to the Jeans instability.
Joint Instability and Osteoarthritis
Blalock, Darryl; Miller, Andrew; Tilley, Michael; Wang, Jinxi
2015-01-01
Joint instability creates a clinical and economic burden in the health care system. Injuries and disorders that directly damage the joint structure or lead to joint instability are highly associated with osteoarthritis (OA). Thus, understanding the physiology of joint stability and the mechanisms of joint instability-induced OA is of clinical significance. The first section of this review discusses the structure and function of major joint tissues, including periarticular muscles, which play a significant role in joint stability. Because the knee, ankle, and shoulder joints demonstrate a high incidence of ligament injury and joint instability, the second section summarizes the mechanisms of ligament injury-associated joint instability of these joints. The final section highlights the recent advances in the understanding of the mechanical and biological mechanisms of joint instability-induced OA. These advances may lead to new opportunities for clinical intervention in the prevention and early treatment of OA. PMID:25741184
Dynamic Instability of Barlike Modes
NASA Astrophysics Data System (ADS)
Durisen, Richard H.; Pickett, Brian K.; Bate, Matthew R.; Imamura, James N.; Brandl, Andreas; Sterzik, Michael F.
Numerical simulations during the 1980's established that prompt binary formation (or ``fission'') through dynamic growth of barlike modes is aborted by gravitational torques. Because these instabilities may occur during star formation and because their outcome over long times is still uncertain, we have combined various linear analyses with simulations by hydrodynamics codes to refine our understanding. We show that it is in fact the torques which cause nonlinear saturation of the mode amplitude. Excellent agreement for the early nonlinear phase is obtained using radically different hydrodynamics codes. However, the ultimate outcome is sensitive to assumptions about dissipative heating and is also somewhat code-dependent.
Ordinary electromagnetic mode instability
NASA Technical Reports Server (NTRS)
Cheng, C. Z.
1974-01-01
The instability of the ordinary electromagnetic mode propagating perpendicular to an external magnetic field is studied for a single-species plasma with ring velocity distribution. The marginal instability boundaries for both the purely growing mode and the propagating growing modes are calculated from the instability criteria. The dispersion characteristics for various sets of plasma parameters are also given. The typical growth rates are of the order of the cyclotron frequency.
Instability in Rotating Machinery
NASA Technical Reports Server (NTRS)
1985-01-01
The proceedings contain 45 papers on a wide range of subjects including flow generated instabilities in fluid flow machines, cracked shaft detection, case histories of instability phenomena in compressors, turbines, and pumps, vibration control in turbomachinery (including antiswirl techniques), and the simulation and estimation of destabilizing forces in rotating machines. The symposium was held to serve as an update on the understanding and control of rotating machinery instability problems.
Jacobson, Robert B.; Primm, Alexander T.
1994-01-01
Ozarks streams have been aggraded by substantial quantities of gravel beginning at or near the time of European settlement. Historical data illustrate multiple, significant changes in land use that may have contributed to stream disturbance. The earliest change was replacement of riparian forest with cultivated fields and pasture, followed by extensive harvesting of shortleaf pine and oak during 1870 to 1920. Selective cutting of timber, use of livestock for skidding logs, and avoidance of steep slopes minimized increases in runoff and sediment supply from logging of uplands. Expanded use of valley bottoms for agriculture and roads, and extreme regional floods from 1895 to 1915 probably initiated significant stream disturbance during this period. The period during 1920-60 included the institution of annual burning of uplands, increased open-range grazing, and increased use of marginal land for row crops. Models for land-use controls on runoff and erosion indicate that this period should have been the most effective in creating stream disturbance. Historical sources corroborate that upland erosion was severe on small areas used for row crops and moderate on large areas subjected to seasonal burning. The most severe effect on streams, however, probably occurred during this period as a result of destruction of riparian vegetation by open-range livestock. From 1960-93, cultivated fields and pasture decreased, while cattle populations increased. Whereas some riparian areas have reverted to bottomland forest, this stabilizing effect occurs on only a small portion of valley- bottom land. Recovery processes aided by riparian vegetation are limited by channel instability and frequent, large floods.
Gravitational instabilities in astrophysical fluids
NASA Astrophysics Data System (ADS)
Tohline, Joel E.
1990-01-01
Over the past decade, the significant advancements that have been made in the development of computational tools and numerical techniques have allowed astrophysicists to begin to model accurately the nonlinear growth of gravitational instabilities in a variety of physical systems. The fragmentation or rotationally driven fission of dynamically evolving, self-gravitating ``drops and bubbles'' is now routinely modeled in full three-dimensional generality as we attempt to understand the behavior of protostellar clouds, rotating stars, galaxies, and even the primordial soup that defined the birth of the universe. A brief review is presented here of the general insights that have been gained from studies of this type, followed by a somewhat more detailed description of work, currently underway, that is designed to explain the process of binary star formation. A short video animation sequence, developed in conjunction with some of the research being reviewed, illustrates the basic-nature of the fission instability in rotating stars and of an instability that can arise in a massive disk that forms in a protostellar cloud.
Soft Dielectrics: Heterogeneity and Instabilities
NASA Astrophysics Data System (ADS)
Rudykh, Stephan; Debotton, Gal; Bhattacharya, Kaushik
2012-02-01
Dielectric Elastomers are capable of large deformations in response to electrical stimuli. Heterogeneous soft dielectrics with proper microstructures demonstrate much stronger electromechanical coupling than their homogeneous constituents. In turn, the heterogeneity is an origin for instability developments leading to drastic change in the composite microstructure. In this talk, the electromechanical instabilities are considered. Stability of anisotropic soft dielectrics is analyzed. Ways to achieve giant deformations and manipulating extreme material properties are discussed. 1. S. Rudykh and G. deBotton, ``Instabilities of Hyperelastic Fiber Composites: Micromechanical Versus Numerical Analyses.'' Journal of Elasticity, 2011. http://dx.doi.org/2010.1007/s10659-011-9313-x 2. S. Rudykh, K. Bhattacharya and G. deBotton, ``Snap-through actuation of thick-wall electroactive balloons.'' International Journal of Non-Linear Mechanics, 2011. http://dx.doi.org/10.1016/j.ijnonlinmec.2011.05.006 3. S. Rudykh and G. deBotton, ``Stability of Anisotropic Electroactive Polymers with Application to Layered Media.'' Zeitschrift f"ur angewandte Mathematik und Physik, 2011. http://dx.doi.org/10.1007/s00033-011-0136-1 4. S. Rudykh, A. Lewinstein, G. Uner and G. deBotton, ``Giant Enhancement of the Electromechanical Coupling in Soft Heterogeneous Dielectrics.'' 2011 http://arxiv.org/abs/1105.4217v1
Roux, G; White, S R; Capponi, S; Poilblanc, D
2006-08-25
The effect of a parallel magnetic field on superconducting two-leg ladders is investigated numerically. The magnetization curve displays an irrational plateau at a magnetization equal to the hole density. Remarkably, its stability is fundamentally connected to the existence of a well-known magnetic resonant mode. Once the zero-field spin gap is suppressed by the field, pairs acquire a finite momentum characteristic of a Fulde-Ferrell-Larkin-Ovchinnikov phase. In addition, Sz = 0 triplet superconducting correlations coexist with singlet ones above the irrational plateau. This provides a simple mechanism in which the Pauli limit is exceeded as suggested by recent experiments.
Morphological instabilities of lamellar eutectics
Karma, A.; Sarkissian, A.
1996-03-01
The authors present the results of a numerical study based on the boundary integral technique of interfacial pattern formation in directional solidification of thin-film lamellar eutectics at low velocity. Microstructure selection maps that identify the stability domains of various steady-state and nonsteady-state growth morphologies in the spacing-composition ({lambda} {minus} C{sub 0}) plane are constructed for the transparent organic alloy CBr{sub 4}-C{sub 2}Cl{sub 6} and for a model eutectic alloy with two solid phases of identical physical properties. In CBr{sub 4}-C{sub 2}Cl{sub 6}, the basic set of instabilities that limit steady-state growth is richer than expected. It consists of three primary instabilities, two of which are oscillatory, which bound the domain of the commonly observed axisymmetric lamellar morphology, and two secondary oscillatory instabilities, which bound the domain of the nonaxisymmetric (tilted) lamellar morphology. Four stable oscillatory microstructures, at least three of which have been seen experimentally, are predicted to occur in unstable regimes. In the model alloy, the structure is qualitatively similar, except that a stable domain of tilted steady-state growth is not found, in agreement with previous random-walk simulations. Furthermore, the composition range of stability of the axisymmetric morphology decreases sharply with increasing spacing away from minimum undercooling but extends further off-eutectic than predicted by the competitive growth criterion. In addition, oscillations with a wavelength equal to two {lambda} lead to lamella termination at a small distance above the onset of instability. The implications of these two features for the eutectic to dendrite transition are examined with the conclusion that in the absence of heterogeneous nucleation, this transition should be histeritic at small velocity and temperature gradient.
Petersen, James C.
1998-01-01
Fish communities from 22 reaches at 18 stations in the Ozark Plateaus were sampled in 1993, 1994, and 1995. The 18 stations were chosen to represent selected combinations of major environmental factors (geology/physiographic area, land use, and basin size). Additional physical, chemical, and biological factors also were measured for each of the 22 reaches and the influence of these factors upon the fish communities was investigated. Fish community samples collected at the 22 reaches identified differences in these communities that can be attributed to differences in land use and related water-quality and habitat characteristics. Communities from agriculture reaches tended to have more species, increased relative abundance of stonerollers and members of the sucker family, and decreased relative abundance of members of the sunfish and darter families. Several groups of environmental factors (concentrations of nutrients, organic carbon, suspended sediment, and dissolved oxygen; measures related to ionic strength; measures related to riparian vegetation; measures related to substrate; and measures related to stream size) appear to be related to land-use differences and fish community differences. Three multivariate analysis techniques (two ordination techniques and a classification technique) yielded similar results when applied to the fish community data. Fish communities from reaches with more similar land use in their basins and with similar drainage areas generally were grouped closer together in the analysis. Water quality, substrate, stream morphology, and riparian measures appear to be affecting fish communities at these reaches. The relations between land use, stream size, and fish communities have implications for waterquality assessments of Ozark streams. Compared to other parts of the United States, many fish species live in the Ozark Plateaus. At least 19 of these species are endemic to the Ozarks area. Many of these species are intolerant of habitat or
Robbins, G M; Masri, B A; Garbuz, D S; Greidanus, N; Duncan, C P
2001-10-01
Instability after total hip arthroplasty is a major source of patient morbidity, second only to aseptic loosening. Certain patient groups have been identified as having a greater risk of instability, including patients undergoing revision arthroplasty as early or late treatment for proximal femoral fractures.
Transverse coherent instability of a bunch in a rectangular potential well
Balbekov, V.; /Fermilab
2006-04-01
Theory of transverse instability of a bunch in a rectangular potential well is developed. Series of equations adequately describing the instability is derived and solved both analytically and numerically. Dependence of the instability increment and threshold on bunch factor is investigated for various beam coupling impedances. The theory is applied to the Fermilab Recycler Ring.
Thermal instability of a radiative and resistive coronal plasma
NASA Technical Reports Server (NTRS)
Sparks, L.; Van Hoven, G.
1988-01-01
Thermal instability is believed to determine the evolution and formation of cool structures in the solar atmosphere such as the transition region and prominences (or filaments). The linear modes that arise in a sheared, force-free, magnetic field due to thermal instability are studied numerically. Previous studies have considered separately modes that arise due to the effects of radiation, compression, anisotropic thermal conduction, and ohmic heating. Here the results of such studies are integrated, first by presenting simple arguments that illustrate the essential physics of ideal, sheared-field, condensation modes, and second by showing numerically how finite resistivity affects the condensational instability in parameter regimes applicable to the solar corona.
Collective Beam Instabilities in the Taiwan Light Source
Chao, Alex W.
2002-08-12
The storage ring at Taiwan Light Source has experienced a strong collective instability since 1994. Various cures have been attempted to suppress this instability, including the use of damping antenna, tunable rf plungers, different filling patterns, and rf gap voltage modulation. So far these cures have improved the beam intensity, but the operation remains to be limited by the instability. The dominant phenomenon is the longitudinal coupled bunch instability. The major source of longitudinal impedance is from rf cavities of Doris type. The high-order modes of the cavity were numerically analyzed using a 3-D code GdfidL. The correlation of the observed phenomenon in user operation with high-order modes of rf cavities will be presented. Results of various attempts to suppress beam instabilities will be summarized. Proposed cures for beam instabilities will be discussed.
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.
Interplay of instabilities in mounded surface growth
Chakrabarti, Buddhapriya; Dasgupta, Chandan
2005-02-01
We numerically study a one-dimensional conserved growth equation with competing linear (Ehrlich-Schwoebel) and nonlinear instabilities. As a control parameter is varied, this model exhibits a nonequilibrium phase transition between two mounded states, one of which exhibits slope selection and the other does not. The coarsening behavior of the mounds in these two phases is studied in detail. In the absence of noise, the steady-state configuration depends crucially on which of the two instabilities dominates the early time behavior.
Resonance instability of axially symmetric magnetostatic equilibria.
Bonanno, Alfio; Urpin, Vadim
2011-11-01
We review the evidence for and against the possibility that a strong enough poloidal field stabilizes an axisymmetric magnetostatic field configuration. We show that there does exist a class of resonant magnetohydrodynamic (MHD) waves which produce instability for any value of the ratio of poloidal and toroidal field strength. We argue that recent investigations of the stability of mixed poloidal and toroidal field configurations based on three-dimensional numerical simulations can miss this instability because of the very large azimuthal wave numbers involved and its resonant character.
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.
Azuma, Takehiro; Morita, Takeshi; Takeuchi, Shingo
2014-08-29
It is expected that the Gregory-Laflamme (GL) instability in the black string in gravity is related to the Rayleigh-Plateau instability in fluid mechanics. Especially, the orders of the phase transitions associated with these instabilities depend on the number of the transverse space dimensions, and they are of first and second order below and above the critical dimension. Through the gauge-gravity correspondence, the GL instability is conjectured to be thermodynamically related to the Hagedorn instability in large-N gauge theories, and it leads to a prediction that the order of the confinement-deconfinement transition associated with the Hagedorn instability may depend on the transverse dimension. We test this conjecture in the D-dimensional bosonic D0-brane model using numerical simulation and the 1/D expansion, and confirm the expected D dependence.
Mixed Pierce-two-stream instability development in an extraction system of a negative ion source
Barminova, H. Y.; Chikhachev, A. S.
2016-02-15
Mixed Pierce-two-stream instability may occur in an extraction system of a negative ion source based on a volume-produced plasma. The reasons for instability development are discussed. Analytically the conditions of unstable beam propagation are determined. The instability threshold is shown to be increased compared with the pure Pierce instability. The influence of inclined perturbations on the instability behavior is investigated. The numerical calculations are performed in COMSOL Multiphysics. The simulation results confirm the existence of such a mixed instability appearance that develops due to both the electrons of the external circuit and the background positive ions.
Mixed Pierce-two-stream instability development in an extraction system of a negative ion source
NASA Astrophysics Data System (ADS)
Barminova, H. Y.; Chikhachev, A. S.
2016-02-01
Mixed Pierce-two-stream instability may occur in an extraction system of a negative ion source based on a volume-produced plasma. The reasons for instability development are discussed. Analytically the conditions of unstable beam propagation are determined. The instability threshold is shown to be increased compared with the pure Pierce instability. The influence of inclined perturbations on the instability behavior is investigated. The numerical calculations are performed in COMSOL Multiphysics. The simulation results confirm the existence of such a mixed instability appearance that develops due to both the electrons of the external circuit and the background positive ions.
Buckling instability in arteries.
Vandiver, Rebecca M
2015-04-21
Arteries can become tortuous in response to abnormal growth stimuli, genetic defects and aging. It is suggested that a buckling instability is a mechanism that might lead to artery tortuosity. Here, the buckling instability in arteries is studied by examining asymmetric modes of bifurcation of two-layer cylindrical structures that are residually stressed. These structures are loaded by an axial force, internal pressure and have nonlinear, anisotropic, hyperelastic responses to stresses. Strain-softening and reduced opening angle are shown to lower the critical internal pressure leading to buckling. In addition, the ratio of the media thickness to the adventitia thickness is shown to have a dramatic impact on arterial instability.
Taylor, Charles J.; Nelson, Hugh L.
2008-01-01
Geospatial data needed to visualize and evaluate the hydrogeologic framework and distribution of karst features in the Interior Low Plateaus physiographic region of the central United States were compiled during 2004-2007 as part of the Ground-Water Resources Program Karst Hydrology Initiative (KHI) project. Because of the potential usefulness to environmental and water-resources regulators, private consultants, academic researchers, and others, the geospatial data files created during the KHI project are being made available to the public as a provisional regional karst dataset. To enhance accessibility and visualization, the geospatial data files have been compiled as ESRI ArcReader data folders and user interactive Published Map Files (.pmf files), all of which are catalogued by the boundaries of surface watersheds using U.S. Geological Survey (USGS) eight-digit hydrologic unit codes (HUC-8s). Specific karst features included in the dataset include mapped sinkhole locations, sinking (or disappearing) streams, internally drained catchments, karst springs inventoried in the USGS National Water Information System (NWIS) database, relic stream valleys, and karst flow paths obtained from results of previously reported water-tracer tests.
NASA Astrophysics Data System (ADS)
Genxu, Wang; Tianxu, Mao; Juan, Chang; Chunlin, Song; Kewei, Huang
2017-07-01
There is a lack of knowledge about how to quantify runoff generation and the hydrological processes operating in permafrost catchments on semi-arid plateaus. To understand how freeze-thaw cycles affect runoff generation processes in permafrost catchments, a typical headwater catchment with continuous permafrost on the Tibetan Plateau was measured. A new approach is presented in this study to account for runoff processes on the spring thawing period and autumn freezing period, when runoff generation clearly differs from that of non-permafrost catchments. This approach introduces a soil temperature-based water saturation function and modifies the soil water storage curve with a soil temperature threshold. The results show that surface soil thawing induced saturation excess runoff and subsurface interflow account for approximately 66-86% and 14-34% of total spring runoff, respectively, and the soil temperature significantly affects the runoff generation pattern, the runoff composition and the runoff coefficient with the enlargement of the active layer. The suprapermafrost groundwater discharge decreases exponentially with active layer frozen processes during autumn runoff recession, whereas the ratio of groundwater discharge to total runoff and the direct surface runoff coefficient simultaneously increase. The bidirectional freezing of the active layer controls and changes the autumn runoff processes and runoff composition. The new approach could be used to further develop hydrological models of cold regions dominated by permafrost.
Introduction to Numerical Methods
Schoonover, Joseph A.
2016-06-14
These are slides for a lecture for the Parallel Computing Summer Research Internship at the National Security Education Center. This gives an introduction to numerical methods. Repetitive algorithms are used to obtain approximate solutions to mathematical problems, using sorting, searching, root finding, optimization, interpolation, extrapolation, least squares regresion, Eigenvalue problems, ordinary differential equations, and partial differential equations. Many equations are shown. Discretizations allow us to approximate solutions to mathematical models of physical systems using a repetitive algorithm and introduce errors that can lead to numerical instabilities if we are not careful.
Turing instabilities on Cartesian product networks
Asllani, Malbor; Busiello, Daniel M.; Carletti, Timoteo; Fanelli, Duccio; Planchon, Gwendoline
2015-01-01
The problem of Turing instabilities for a reaction-diffusion system defined on a complex Cartesian product network is considered. To this end we operate in the linear regime and expand the time dependent perturbation on a basis formed by the tensor product of the eigenvectors of the discrete Laplacian operators, associated to each of the individual networks that build the Cartesian product. The dispersion relation which controls the onset of the instability depends on a set of discrete wavelengths, the eigenvalues of the aforementioned Laplacians. Patterns can develop on the Cartesian network, if they are supported on at least one of its constitutive sub-graphs. Multiplex networks are also obtained under specific prescriptions. In this case, the criteria for the instability reduce to compact explicit formulae. Numerical simulations carried out for the Mimura-Murray reaction kinetics confirm the adequacy of the proposed theory. PMID:26245138
The mirror and ion cyclotron anisotropy instabilities
NASA Technical Reports Server (NTRS)
Gary, S. P.
1992-01-01
The linear dispersion equation for fully electromagnetic waves and instabilities at arbitrary directions of propagation relative to a background magnetic field B(0) in a homogeneous Vlasov plasma is solved numerically for bi-Maxwellian particle distributions. For isotropic plasmas the dispersion and damping of the three modes below the proton cyclotron frequency are studied as functions of Beta(i) and T(e)/T(i). The transport ratios of helicity, cross-helicity, Alfven ratio, compressibility, and parallel compressibility are defined. Under the condition that the proton temperature perpendicular to B(0) is greater than the parallel temperature, the growth rates and transport ratios of the mirror instability and the ion cyclotron anisotropy instability are examined and compared. Both the proton parallel compressibility and the proton Alfven ratio are significantly different for the two growing modes.
Crack instabilities of a heated glass strip
NASA Astrophysics Data System (ADS)
Adda-Bedia, Mokhtar; Pomeau, Yves
1995-10-01
Recently, Yuse and Sano [Nature (London) 362, 329 (1993)] have observed that a crack traveling in a glass strip submitted to a nonuniform thermal diffusion field undergoes numerous instabilities. We study two cases of quasistatic crack propagation. The crack extension condition in straight propagation is determined. An asymptotic analysis of the elastic free energy is introduced and scaling laws are derived. A linear stability analysis of the straight propagation is performed, based on the assumption that the crack tip propagation deviates from the centered straight one as soon as it is submitted to a ``physical'' singular shear stress. It is shown that a straight propagation can become unstable after which a wavy instability appears. The condition for instability as well as the selected wavelength is calculated quantitatively. The results are compared with experiments and the agreement is favorable.
Filtering of non-linear instabilities
NASA Technical Reports Server (NTRS)
Khosla, P. K.; Rubin, S. G.
1978-01-01
For Courant numbers larger than one and cell Reynolds numbers larger than two, oscillations and in some cases instabilities are typically found with implicit numerical solutions of the fluid dynamics equations. This behavior has sometimes been associated with the loss of diagonal dominance of the coefficient matrix. It is shown that these problems can be related to the choice of the spatial differences, with the resulting instability related to aliasing or nonlinear interaction. Appropriate filtering can reduce the intensity of these oscillations and possibly eliminate the instability. These filtering procedures are equivalent to a weighted average of conservation and nonconservation differencing. The entire spectrum of filtered equations retains a three point character as well as second order spatial accuracy. Burgers equation was considered as a model.
Oscillatory interfacial instability between miscible fluids
NASA Astrophysics Data System (ADS)
Shevtsova, Valentina; Gaponenko, Yuri; Mialdun, Aliaksandr; Torregrosa, Marita; Yasnou, Viktar
Interfacial instabilities occurring between two fluids are of fundamental interest in fluid dynamics, biological systems and engineering applications such as liquid storage, solvent extraction, oil recovery and mixing. Horizontal vibrations applied to stratified layers of immiscible liquids may generate spatially periodic waving of the interface, stationary in the reference frame of the vibrated cell, referred to as a "frozen wave". We present experimental evidence that frozen wave instability exists between two ordinary miscible liquids of similar densities and viscosities. At the experiments and at the numerical model, two superimposed layers of ordinary liquids, water-alcohol of different concentrations, are placed in a closed cavity in a gravitationally stable configuration. The density and viscosity of these fluids are somewhat similar. Similar to the immiscible fluids this instability has a threshold. When the value of forcing is increased the amplitudes of perturbations grow continuously displaying a saw-tooth structure. The decrease of gravity drastically changes the structure of frozen waves.
Multidimensional simulations of pair-instability supernovae
NASA Astrophysics Data System (ADS)
Baranov, A. A.; Chardonnet, P.; Chechetkin, V. M.; Filina, A. A.; Popov, M. V.
2013-10-01
According to theoretical models, massive stars with masses within the 100-250 M⊙ range should explode as pair-instability supernovae (PISNe). Since the first stars of the Universe are believed to be very massive, these supernovae should play a significant role in the early stages of its history. But these stars represent the last unobserved population, owing to detection limits of current telescopes. In this work we analyze pair-instability supernovae explosions using various numerical codes. We evolve series of the configurations of oxygen cores to establish a range of masses and initial conditions where this type of explosion is possible. We also study the role of possible instabilities in the propagation of shockwaves during the last stage of the explosion. This investigation could help us to predict the observational properties of PISNe for future space and ground telescopes.
Convective Instabilities in Two Liquid Layers
McFadden, G. B.; Coriell, S. R.; Gurski, K. F.; Cotrell, D. L.
2007-01-01
We perform linear stability calculations for horizontal fluid bilayers, taking into account both buoyancy effects and thermocapillary effects in the presence of a vertical temperature gradient. To help understand the mechanisms driving the instability, we have performed both long-wavelength and short-wavelength analyses. The mechanism for the large wavelength instability is complicated, and the detailed form of the expansion is found to depend on the Crispation and Bond numbers. The system also allows a conventional Rayleigh-Taylor instability if heavier fluid overlies lighter fluid, and the long-wavelength analysis describes this case as well. In addition to the asymptotic analyses for large and small wavelengths, we have performed numerical calculations using materials parameters for a benzene-water system. PMID:27110470
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.
Ladd, David E.
2016-01-01
As part of the U.S. Geological Survey Water Availability and Use Science Program study of Appalachian Plateaus aquifers, bottom elevations and thicknesses were determined for Permian, Upper Pennsylvanian, Lower Pennsylvanian, and Mississippian hydrogeologic framework units in the Appalachian Plateaus, covering parts of Pennsylvania, Maryland, Ohio, West Virginia, Kentucky, Virginia, Tennessee, Georgia, and Alabama. Thickness values for these units were compiled at point locations from available sources (Martens, 1945; McKee and Crosby, 1975; Craig and Connor, 1979; Ryder and others, 2008, 2009, 2012, 2015) and used to interpolate surfaces representing thicknesses, contact elevations, and outcrop elevations using earthVision software (Dynamic Graphics, Inc., 2014). Unit contact and outcrop elevations at cell centers from the earthVision rasters were used as the primary source to create geodatabase rasters representing bottom elevations and thicknesses of the Permian, Upper Pennsylvanian, Lower Pennsylvanian, and Mississippian framework units.
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Evaporatively driven morphological instability
NASA Astrophysics Data System (ADS)
Style, Robert W.; Wettlaufer, J. S.
2007-07-01
Simple observations of evaporating solutions reveal a complex hierarchy of spatiotemporal instabilities. We analyze one such instability suggested by the qualitative observations of Du and Stone and find that it is driven by a variant of the classical morphological instability in alloy solidification. In the latter case a moving solid-liquid interface is accompanied by a solutally enriched boundary layer that is thermodynamically metastable due to constitutional supercooling. Here, we consider the evaporation of an impure film adjacent to a solid composed of the nonvolatile species. In this case, constitutional supercooling within the film is created by evaporation at the solution-vapor interface and this drives the corrugation of the solid-solution interface across the thickness of the film. The principal points of this simple theoretical study are to suggest an instability mechanism that is likely operative across a broad range of technological and natural systems and to focus future quantitative experimental searches.
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
Distal Radioulnar Joint Instability
Mirghasemi, Ali R.; Lee, Daniel J.; Rahimi, Narges; Rashidinia, Shervin
2015-01-01
Distal radioulnar joint (DRUJ) instability is a common clinical condition but a frequently missed diagnosis. Both surgical and nonsurgical treatments are possible for chronic cases of DRUJ instability. Nonsurgical treatment can be considered as the primary therapy in less active patients, while surgery should be considered to recover bone and ligament injuries if nonsurgical treatment fails to restore forearm stability and function. The appropriate choice of treatment depends on the individual patient and specific derangement of the DRUJ PMID:26328241
Rotor internal friction instability
NASA Technical Reports Server (NTRS)
Bently, D. E.; Muszynska, A.
1985-01-01
Two aspects of internal friction affecting stability of rotating machines are discussed. The first role of internal friction consists of decreasing the level of effective damping during rotor subsynchronous and backward precessional vibrations caused by some other instability mechanisms. The second role of internal frication consists of creating rotor instability, i.e., causing self-excited subsynchronous vibrations. Experimental test results document both of these aspects.
State Instability and Terrorism
2010-01-01
terrorism is a form ( Durkheim , 1930 [1951]; Useem, 1998). In addition, different types of instability ought to invite different levels of terrorism...society. The effects of the disruption in controls should be to increase levels of nonroutine collective action, of which terrorism is a form ( Durkheim ...instability at the country-level using a modified breakdown theoretical framework. This framework is based especially upon the work of Emile Durkheim
Prediction of Algebraic Instabilities
NASA Astrophysics Data System (ADS)
Zaretzky, Paula; King, Kristina; Hill, Nicole; Keithley, Kimberlee; Barlow, Nathaniel; Weinstein, Steven; Cromer, Michael
2016-11-01
A widely unexplored type of hydrodynamic instability is examined - large-time algebraic growth. Such growth occurs on the threshold of (exponentially) neutral stability. A new methodology is provided for predicting the algebraic growth rate of an initial disturbance, when applied to the governing differential equation (or dispersion relation) describing wave propagation in dispersive media. Several types of algebraic instabilities are explored in the context of both linear and nonlinear waves.
Equilibrium Electroconvective Instability
NASA Astrophysics Data System (ADS)
Rubinstein, I.; Zaltzman, B.
2015-03-01
Since its prediction 15 years ago, hydrodynamic instability in concentration polarization at a charge-selective interface has been attributed to nonequilibrium electro-osmosis related to the extended space charge which develops at the limiting current. This attribution had a double basis. On the one hand, it has been recognized that neither equilibrium electro-osmosis nor bulk electroconvection can yield instability for a perfectly charge-selective solid. On the other hand, it has been shown that nonequilibrium electro-osmosis can. The first theoretical studies in which electro-osmotic instability was predicted and analyzed employed the assumption of perfect charge selectivity for the sake of simplicity and so did the subsequent studies of various time-dependent and nonlinear features of electro-osmotic instability. In this Letter, we show that relaxing the assumption of perfect charge selectivity (tantamount to fixing the electrochemical potential of counterions in the solid) allows for the equilibrium electroconvective instability. In addition, we suggest a simple experimental test for determining the true, either equilibrium or nonequilibrium, origin of instability in concentration polarization.
Noise-induced instability in self-consistent Monte Carlo calculations
Lemons, D.S.; Lackman, J.; Jones, M.E.; Winske, D.
1995-12-01
We identify, analyze, and propose remedies for a numerical instability responsible for the growth or decay of sums that should be conserved in Monte Carlo simulations of stochastically interacting particles. ``Noisy`` sums with fluctuations proportional to 1/ {radical}{ital n} , where {ital n} is the number of particles in the simulation, provide feedback that drives the instability. Numerical illustrations of an energy loss or ``cooling`` instability in an Ornstein-Uhlenbeck process support our analysis. (c) 1995 The American Physical Society
Baroclinic instability of a buoyant coastal current
NASA Astrophysics Data System (ADS)
Hetland, Robert
2014-05-01
Classic models of baroclinic instability, notably the Eady model, depend on the Rossby (or Richardson) number as the sole non-dimensional parameter. Inclusion of a sloping bottom requires an additional parameter, the slope Burger number, Bu = αNf-1, where α is the bottom slope. Numerical simulations of the evolution of instabilities along the edge of a coastally trapped buoyant flow suggest that the slope may help to stabilize the flow when the deformation radius is similar to or larger than the with of the buoyant flow, that is, the flow is stable when the slope Burger number is larger than about 0.3. In unstable cases, Bu < 0.3, baroclinic instabilities in the flow cause the isopycnals to relax, thereby increasing the local Burger number until the critical condition, Bu ≃ 0.3, is met. At this point the instabilities no longer grow in time, preventing further offshore buoyancy flux by the eddies. This final state corresponds approximately to the case where the slope of the ground is similar to the slope of the mean isopycnal surfaces. The nonlinear, three-dimensional numerical simulations are in basic agreement with one-dimensional linear stability analysis, with a few key exceptions. Notably, numerical simulations suggest that cross-shelf buoyancy fluxes are strongest in within the bottom boundary layer, showing a similar pattern to continental shelf waves in the vertical structure of current and tracer variability. Idealized simulations show a marked similarity to instabilities along the Mississippi/Atchafalaya plume front, as seen in observations and realistic regional models. These eddies have been shown to be important in Lagrangian transport of surface particles, notably oil spill trajectory prediction, and create patchiness in bottom dissolved oxygen distributions during periods of summertime seasonal hypoxia.
Propagating Instabilities in Solids
NASA Astrophysics Data System (ADS)
Kyriakides, Stelios
1998-03-01
Instability is one of the factors which limit the extent to which solids can be loaded or deformed and plays a pivotal role in the design of many structures. Such instabilities often result in localized deformation which precipitates catastrophic failure. Some materials have the capacity to recover their stiffness following a certain amount of localized deformation. This local recovery in stiffness arrests further local deformation and spreading of the instability to neighboring material becomes preferred. Under displacement controlled loading the propagation of the transition fronts can be achieved in a steady-state manner at a constant stress level known as the propagation stress. The stresses in the transition fronts joining the highly deformed zone to the intact material overcome the instability nucleation stresses and, as a result, the propagation stress is usually much lower than the stress required to nucleate the instability. The classical example of this class of material instabilities is L/"uders bands which tend to affect mild steels and other metals. Recent work has demonstrated that propagating instabilities occur in several other materials. Experimental and analytical results from four examples will be used to illustrate this point: First the evolution of L=FCders bands in mild steel strips will be revisited. The second example involves the evolution of stress induced phase transformations (austenite to martensite phases and the reverse) in a shape memory alloy under displacement controlled stretching. The third example is the crushing behavior of cellular materials such as honeycombs and foams made from metals and polymers. The fourth example involves the axial broadening/propagation of kink bands in aligned fiber/matrix composites under compression. The microstructure and, as a result, the micromechanisms governing the onset, localization, local arrest and propagation of instabilities in each of the four materials are vastly different. Despite this
Collisionless shock waves mediated by Weibel Instability
NASA Astrophysics Data System (ADS)
Naseri, Neda; Ruan, Panpan; Zhang, Xi; Khudik, Vladimir; Shvets, Gennady
2015-11-01
Relativistic collisionless shocks are common events in astrophysical environments. They are thought to be responsible for generating ultra-high energy particles via the Fermi acceleration mechanism. It has been conjectured that the formation of collisionless shocks is mediated by the Weibel instability that takes place when two initially cold, unmagnetized plasma shells counter-propagate into each other with relativistic drift velocities. Using a PIC code, VLPL, which is modified to suppress numerical Cherenkov instabilities, we study the shock formation and evolution for asymmetric colliding shells with different densities in their own proper reference frame. Plasma instabilities in the region between the shock and the precursor are also investigated using a moving-window simulation that advances the computational domain at the shock's speed. This method helps both to save computation time and avoid severe numerical Cherenkov instabilities, and it allows us to study the shock evolution in a longer time period. Project is supported by US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945.
On cooperative instabilities of parallel vortex pairs
NASA Astrophysics Data System (ADS)
Bristol, R. L.; Ortega, J. M.; Marcus, P. S.; Savas, Ö.
2004-10-01
We present a combined analytical and numerical study of the instabilities of a pair of parallel unequal-strength vortices. We extend the analyses of a vortex in an external strain field (Crow, AIAA J. vol. 8, 1970, p. 2172; Widnall et al., J. Fluid Mech. vol. 66, 1974, p. 35) to include the orbital motion of the vortex pair. For counter-rotating pairs, the classic Crow-type periodic displacement perturbations are unstable for all vortex strength ratios, with fastest-growing wavelengths several times the vortex spacing. For co-rotating pairs, the orbital motion acts to suppress instability due to displacement perturbations. Instabilities in this case arise for elliptic perturbations at wavelengths that scale with the vortex core size. We also examine the influence of a second vortex pair by extending Crouch's (J. Fluid Mech. vol. 350, 1997, p. 311) analysis. Numerical results from a spectral initial-value code with subgrid-scale modelling agree with the growth rates from the theoretical models. Computations reveal the nonlinear evolution at late times, including wrapping and ring-rejection behaviour observed in experiments. A pair of co-rotating Gaussian vortices perturbed by noise develops elliptic instabilities, leading to the formation of vorticity bridges between the two vortices. The bridging is a prelude to vortex merger. Analytic, computational and experimental results agree well at circulation Reynolds numbers of order 10(5) .
Parametric instabilities of the circularly polarized Alfven waves including dispersion
NASA Astrophysics Data System (ADS)
Wong, H. K.; Goldstein, M. L.
1986-05-01
A class of parametric instabilities of large-amplitude, circularly polarized Alfven waves is considered in which finite frequency (dispersive) effects are included. The dispersion equation governing the instabilities is a sixth-order polynomial which is solved numerically. As a function of K identically equal to k/k-sub-0 (where k-sub-0 and k are the wave number of the 'pump' wave and unstable sound wave, respectively), there are three regionals of instability: a modulation instability at K less than 1, a decay instability at K greater than 1, and a relatively weak and narrow instability at K close to squared divided by v-sub-A squared (where c-sub-s and v-sub-A are the sound and Alfven speeds respectively), the modulational instability occurs when beta is less than 1 (more than 1) for left-hand (right-hand) pump waves, in agreement with the previous results of Sakai and Sonnerup (1983). The growth rate of the decay instability of left-hand waves is greater than the modulational instability at all values of beta. Applications to large-amplitude wave observed in the solar wind, in computer simulations, and in the vicinity of planetary and interplanetary collisionless shocks are discussed.
Combustion instability modeling and analysis
Santoro, R.J.; Yang, V.; Santavicca, D.A.
1995-10-01
It is well known that the two key elements for achieving low emissions and high performance in a gas turbine combustor are to simultaneously establish (1) a lean combustion zone for maintaining low NO{sub x} emissions and (2) rapid mixing for good ignition and flame stability. However, these requirements, when coupled with the short combustor lengths used to limit the residence time for NO formation typical of advanced gas turbine combustors, can lead to problems regarding unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions, as well as the occurrence of combustion instabilities. Clearly, the key to successful gas turbine development is based on understanding the effects of geometry and operating conditions on combustion instability, emissions (including UHC, CO and NO{sub x}) and performance. The concurrent development of suitable analytical and numerical models that are validated with experimental studies is important for achieving this objective. A major benefit of the present research will be to provide for the first time an experimentally verified model of emissions and performance of gas turbine combustors.
Instability mode interactions in a spatially developing plane wake
NASA Technical Reports Server (NTRS)
Maekawa, Hiroshi; Mansour, Nagi N.; Buell, Jeffrey C.
1992-01-01
Direct numerical simulations are used to study the development of various instability modes in a spatially developing 2D wake. Five types of forcing of the inlet are investigated: fundamental mode, fundamental and one or two subharmonics, fundamental mode and random noise, and random noise only. Statistical analyses are carried out, and some numerical results are compared with experimental measurements.
NASA Astrophysics Data System (ADS)
Le Deit, L.; Flahaut, J.; Quantin, C.; Allemand, P.
2009-12-01
The plateaus around Valles Marineris consist in series of mafic rocks suggested to be flood basalts (McEwen et al., 1998), lavas interbedded with sediments (Malin and Edgett, 2000), layered intrusive rocks (Williams et al., 2003), or lava flows dated from the Noachian to the late Hesperian epochs (Scott and Carr, 1978). Recent studies show the occurrence of light layered deposits of hundred meters thick cropping out on plateaus near Ius Chasma, Melas Chasma, Candor Chasma, Juventae Chasma and Ganges Chasma deposited during the Hesperian epoch by fluvio-lacustrine processes (Weitz et al., 2009), or by air-fall processes (Le Deit et al., 2009). These layered deposits are enriched in hydrated minerals including opaline silica (Milliken et al., 2008), hydroxylated ferric sulfates (Bishop et al., 2009), and possibly Al-rich phyllosilicates (Le Deit et al., 2009). We identified another type of formation corresponding to light-toned massive deposits cropping out around Valles Marineris. It appears that these light-toned deposits are associated to bright, rough, and highly cratered terrains, located beneath a dark and thin capping unit. Previous studies report the occurrence of phyllosilicates on few locations around Valles Marineris based on OMEGA data analyses (Gondet et al., 2007; Carter et al., 2009). The analysis of CRISM data show that the light-toned deposits are associated with spectra displaying absorption bands at 1.4 μm, 1.9 μm, and a narrow band at 2.2 μm. These spectral characteristics are consistent with the presence of Al-rich phyllosilicates such as montmorillonite, or illite in the light-toned deposits. They constitute dozens of outcrops located on the plateaus south and east of Coprates Chasma and Capri Chasma, and west of Ganges Chasma. All outcrops investigated so far are present over Noachian terrains mapped as the unit Npl2 by Scott and Tanaka (1986), and Witbeck et al. (1991). These light-toned deposits could result from in situ aqueous alteration
NASA Astrophysics Data System (ADS)
Chapman, J.; Robinson, A. C.; Carrapa, B.; Kapp, P. A.; Worthington, J.; Villarreal, D. P.; Gadoev, M.; Oimahmadov, I.
2016-12-01
The Pamir Mountains form a high-elevation orogenic plateau with 70 km thick crust. Exposures of lower crust within a series of extensional gneiss domes, including the giant Shakhdara-Alichur dome, make the Pamir an ideal laboratory for examining the interrelatedness of upper and lower crustal deformation mechanisms. The Pamir gneiss domes have Eocene to Oligocene prograde metamorphic ages interpreted to reflect crustal thickening associated with the India-Asia collision. We investigated upper crustal deformation in the South Pamir terrane, in the hangingwall of the Shakhdara-Alichur dome. Structural reconstructions indicate <10% shortening in the South Pamir since the Jurassic and detrital zircon U-Pb geochronology and detrital zircon fission track analysis of syn-orogenic strata in the South Pamir suggest that the majority of this deformation is Cretaceous in age and accumulated prior to the India-Asia collision. Moreover, zircon (U-Th)/He thermochronology and field relationships show that the South Pamir has been above sea-level since the early Cretaceous and that there has been no significant exhumation since that time. There is not enough Cenozoic upper crustal shortening to account for the lower crustal prograde metamorphic ages. Lower crustal flow into the South Pamir from adjacent terranes or India is required to account for initial Paleogene crustal thickening as well as to offset Miocene crustal thinning related to gneiss dome extension and exhumation. The results suggest that upper and lower crustal deformation is locally decoupled in orogenic plateaus and that the cool upper crust may not fully record orogenic histories. Prolonged injection of lower crust may lead to vertical extrusion and plateau expansion rather than discrete periods of crustal extension or shortening.
NASA Astrophysics Data System (ADS)
Pastor, Alvar; Babault, Julien; Owen, Lewis A.; Teixell, Antonio; Arboleya, María-Luisa
2015-11-01
The Moulouya river system has intensely eroded the Arhbalou, Missour, and Guercif Neogene foreland basins in northeastern Morocco, having changed from net aggradation during the Miocene-early Pliocene to net incision punctuated by alluvial fan deposition at late Pliocene or early Quaternary time. This region as a whole has experienced mantle-driven, surface uplift (dynamic topography) since the late Cenozoic, being locally affected by uplift due to crustal shortening and thickening of the Middle Atlas too. Knickpoints located along the major streams of the Moulouya fluvial network, appear on both the undeformed margins of the Missour and Guercif foreland basins (High Plateaus), as well as along the thrust mountain front of the southern Middle Atlas, where they reach heights of 800-1000 m. 500-550 m of the knickpoint vertical incision might be explained by long-wavelength mantle-driven dynamic surface uplift, whereas the remaining 450-500 m in the southern Middle Atlas front and 200-300 m in the northeastern Middle Atlas front seem to be thrust-related uplift of the Jebel Bou Naceur. Be-10 terrestrial cosmogenic nuclides have been used to date two Quaternary river terraces in the Chegg Ard valley at 62 ± 14 ka and 411 ± 55 ka. The dated terraces allow the incision rates associated with the frontal structures of the Middle Atlas to be estimated at ~ 0.3 mm yr- 1. Furthermore, these ages have served to evaluate mantle-driven regional surface uplift since the middle Pleistocene in the central Missour basin, yielding values of ~ 0.1-0.2 mm yr- 1.
Cellular detonation - Instability and sub-structure
NASA Astrophysics Data System (ADS)
Sugimura, Tadayoshi; Fujiwara, Toshitaka; Lee, John H.
The present study investigates numerically the instability and irregularity of multidimensional detonations experimentally observed over a lengthy period. By increasing computational resolution significantly, the computed results are able to reproduce most of the salient experimental features like inherent instability of a plane ZND detonation, irregular triple-shock structures seen in self-sustaining detonations, and very fine triple-shock structures superimposed in a macroscopic triple-shock configuration. Under sufficient resolution, the acquired information on the number of triple shocks, regularity and symmetry of wave structure, and detailed substructures of a triple shock configuration are found to be surprisingly incorrect. It is argued that such results would affect the future numerical simulation of detonation initiation, transition, and critical tube diameter problems where the formation of new cells is an essential mechanism.
Theory of instability and transition
NASA Technical Reports Server (NTRS)
Herbert, Thorwald
1990-01-01
The strongly nonlinear area of theory is discussed, as well as linear and weakly nonlinear (i.e., perturbation) theories, and it is noted that the weaknesses of the weakly nonlinear theory are essentially the inappropriate formulation in earlier works and the lack of guidance for the choice of the lowest-order basis. Attention is focused on the areas of theoretical/numerical development contributing to understanding the transition mechanism and new means for analyzing and predicting transition quantitatively. The nonlinear stability of nonparallel flows, linear secondary instability, and nonlinear wave interaction are analyzed. The incompressible flow over a flat plate with zero-pressure gradient is chosen as an example, while applications range to other shear flows including three-dimensional and compressible boundary layers.
2017-01-01
Although Arabic numerals (like ‘2016’ and ‘3.14’) are ubiquitous, we show that in interactive computer applications they are often misleading and surprisingly unreliable. We introduce interactive numerals as a new concept and show, like Roman numerals and Arabic numerals, interactive numerals introduce another way of using and thinking about numbers. Properly understanding interactive numerals is essential for all computer applications that involve numerical data entered by users, including finance, medicine, aviation and science. PMID:28484609
Abbas, Tarek; Keaton, Mignon A.; Dutta, Anindya
2013-01-01
One of the fundamental challenges facing the cell is to accurately copy its genetic material to daughter cells. When this process goes awry, genomic instability ensues in which genetic alterations ranging from nucleotide changes to chromosomal translocations and aneuploidy occur. Organisms have developed multiple mechanisms that can be classified into two major classes to ensure the fidelity of DNA replication. The first class includes mechanisms that prevent premature initiation of DNA replication and ensure that the genome is fully replicated once and only once during each division cycle. These include cyclin-dependent kinase (CDK)-dependent mechanisms and CDK-independent mechanisms. Although CDK-dependent mechanisms are largely conserved in eukaryotes, higher eukaryotes have evolved additional mechanisms that seem to play a larger role in preventing aberrant DNA replication and genome instability. The second class ensures that cells are able to respond to various cues that continuously threaten the integrity of the genome by initiating DNA-damage-dependent “checkpoints” and coordinating DNA damage repair mechanisms. Defects in the ability to safeguard against aberrant DNA replication and to respond to DNA damage contribute to genomic instability and the development of human malignancy. In this article, we summarize our current knowledge of how genomic instability arises, with a particular emphasis on how the DNA replication process can give rise to such instability. PMID:23335075
Posterior Shoulder Instability.
Brelin, Alaina; Dickens, Jonathan F
2017-09-01
Posterior shoulder instability is a relatively uncommon condition, occurring in ∼10% of those with shoulder instability. Because of the rarity of the condition and the lack of knowledge in treatment, it is often misdiagnosed or patients experience a delay in diagnosis. Posterior instability typically affects athletes participating in contact or overhead sports and is usually the result of repetitive microtrauma or blunt force with the shoulder in the provocative position of flexion, adduction, and internal rotation, leading to recurrent subluxation events. Acute traumatic posterior dislocations are rare injuries with an incidence rate of 1.1 per 100,000 person years. This rate is ∼20 times lower than that of anterior shoulder dislocations. Risk factors for recurrent instability are: (1) age below 40 at time of first instability; (2) dislocation during a seizure; (3) a large reverse Hill-Sachs lesion; and (4) glenoid retroversion. A firm understanding of the pathoanatomy, along with pertinent clinical and diagnostic modalities is required to accurately diagnosis and manage this condition.
Scaling the Incompressible Richtmyer-Meshkov Instability
Cotrell, D; Cook, A
2007-01-09
We derive a scaling relation for Richtmyer-Meshkov instability of incompressible fluids. The relation is tested using both numerical simulations and experimental data. We obtain collapse of growth rates for a wide range of initial conditions by using vorticity and velocity scales associated with the interfacial perturbations and the acceleration impulse. A curve fit to the collapsed growth rates yields a fairly universal model for the mixing layer thickness versus time.
Interfacial instabilities and fingering formation in Hele-Shaw flow
NASA Astrophysics Data System (ADS)
Xu, Jian-Jun
1996-10-01
The interfacial instability of Hele-Shaw flow has been a crucial issue for the understanding of the pattern formation of viscous fingers in a Hele-Shaw cell. By using a unified asymptotic approach, we derive two different types of instability mechanisms for slightly' time-dependent finger solutions; namely, (i) the global-trapped-wave (GTW) instability; and (ii) the zero-frequency (null-f) instability. On the basis of these instability mechanisms, the selection of viscous finger formation is clarified; the apparent contradiction between the previous linearstability analysis by Tanveer (1987, Phys. Fluid 30, 1589) and others and the numerical simulations by DeGregoria & Schwartz (1986, J. Fluid Mech. 164, 383)and the experimental evidence is reconciled.
Generalized lower-hybrid-drift instability. [of plasma
NASA Technical Reports Server (NTRS)
Hsia, J. B.; Chiu, S. M.; Hsia, M. F.; Chou, R. L.; Wu, C. S.
1979-01-01
The theory of lower-hybrid-drift instability is extended to include a finite value of the component of wave vector parallel to the ambient magnetic field so that the analysis bridges the usual lower-hybrid-drift instability of flute modes and the modified-two-stream instability. The present theory also includes electromagnetic and ambient magnetic field-gradient effects. It is found that in the cold-electron limit the density and magnetic gradients can qualitatively modify the conclusion obtained in the early theory of the modified-two-stream instability. For example, even if the relative drift far exceeds the Alfven speed of the plasma, the instability may still persist. This result is in contrast to that established in the literature. When the electron temperature is finite, the problem is complicated. Numerical solutions are obtained for a number of cases.
Gravitational instability of an anisotropic and viscoelastic plasma
NASA Astrophysics Data System (ADS)
Sharma, P. K.; Tiwari, Anita; Khan, Nusrat; Argal, Shraddha
2017-05-01
The effect of pressure anisotropy is studied on the growth rate of gravitational instabilities in a viscoelastic medium. The problem is constructed with generalized hydrodynamic fluid model and Chew-Goldberger-Low fluid model for anisotropic pressure then a general dispersion relation for the viscoelastic medium is obtained using the normal mode analysis. The general dispersion relation is reduced for propagation along the magnetic field and propagation perpendicular to the magnetic field. These two modes are discussed for the classical or hydrodynamic and kinetic limits and conditions for jeans instability are obtained. We found that condition of Jeans instability is modified for viscoelastic medium under kinetic limit and depends on compressional viscoelastic mode. Numerical analysis for longitudinal mode for kinetic regime shows that the velocity of compressional viscoelastic mode has a stabilizing effect on the growth rate of Jeans instability. In the transverse mode, the Alfven velocity for kinetic regime has a stabilizing influence on the Jeans instability.
Numerical calculations of flow fields
NASA Technical Reports Server (NTRS)
Anderson, D.; Vogel, J. M.
1973-01-01
Numerical calculations were made of flow fields generated by various aerodynamic configurations. Data cover flow fields generated by a finitely thick lifting three dimensional wing with subsonic tips moving at supersonic speeds, cross flow instability associated with lifting delta wing configurations such as space shuttles, and flow fields produced by a lifting elliptic cone. Finite difference techniques were used to determine elliptic cone flow.
Hydrodynamics of pedestrians' instability in floodwaters
NASA Astrophysics Data System (ADS)
Arrighi, Chiara; Oumeraci, Hocine; Castelli, Fabio
2017-01-01
People's safety is the first objective to be fulfilled by flood risk mitigation measures, and according to existing reports on the causes of casualties, most of the fatalities are due to inappropriate behaviour such as walking or driving in floodwaters. Currently available experimental data on people instability in floodwaters suffer from a large dispersion primarily depending on the large variability of the physical characteristics of the subjects. This paper introduces a dimensionless mobility parameter θP for people partly immersed in flood flows, which accounts for both flood and subject characteristics. The parameter θP is capable of identifying a unique threshold of instability depending on a Froude number, thus reducing the scatter of existing experimental data. Moreover, a three-dimensional (3-D) numerical model describing the detailed geometry of a human body and reproducing a selection of critical pairs of water depth and velocity is presented. The numerical results in terms of hydrodynamic forces and force coefficients are analysed and discussed. Both the mobility parameter θP and the numerical results hint at the crucial role of the Froude number and relative submergence as the most relevant dimensionless numbers to interpret the loss of stability. Finally, the mobility parameter θP is compared with an analogous dimensionless parameter for vehicles' instability in floodwaters, providing a new contribution to support flood risk management and educating people.
Linear analysis of incompressible Rayleigh-Taylor instability in solids
NASA Astrophysics Data System (ADS)
Piriz, A. R.; López Cela, J. J.; Tahir, N. A.
2009-10-01
The study of the linear stage of the incompressible Rayleigh-Taylor instability in elastic-plastic solids is performed by considering thick plates under a constant acceleration that is also uniform except for a small sinusoidal ripple in the horizontal plane. The analysis is carried out by using an analytical model based on the Newton second law and it is complemented with extensive two-dimensional numerical simulations. The conditions for marginal stability that determine the instability threshold are derived. Besides, the boundary for the transition from the elastic to the plastic regime is obtained and it is demonstrated that such a transition is not a sufficient condition for instability. The model yields complete analytical solutions for the perturbation amplitude evolution and reveals the main physical process that governs the instability. The theory is in general agreement with the numerical simulations and provides useful quantitative results. Implications for high-energy-density-physics experiments are also discussed.
Linear analysis of incompressible Rayleigh-Taylor instability in solids.
Piriz, A R; Cela, J J López; Tahir, N A
2009-10-01
The study of the linear stage of the incompressible Rayleigh-Taylor instability in elastic-plastic solids is performed by considering thick plates under a constant acceleration that is also uniform except for a small sinusoidal ripple in the horizontal plane. The analysis is carried out by using an analytical model based on the Newton second law and it is complemented with extensive two-dimensional numerical simulations. The conditions for marginal stability that determine the instability threshold are derived. Besides, the boundary for the transition from the elastic to the plastic regime is obtained and it is demonstrated that such a transition is not a sufficient condition for instability. The model yields complete analytical solutions for the perturbation amplitude evolution and reveals the main physical process that governs the instability. The theory is in general agreement with the numerical simulations and provides useful quantitative results. Implications for high-energy-density-physics experiments are also discussed.
Streaming instability of slime mold amoebae: An analytical model
NASA Astrophysics Data System (ADS)
Höfer, Thomas; Maini, Philip K.
1997-08-01
During the aggregation of amoebae of the cellular slime mould Dictyostelium, the interaction of chemical waves of the signaling molecule cAMP with cAMP-directed cell movement causes the breakup of a uniform cell layer into branching patterns of cell streams. Recent numerical and experimental investigations emphasize the pivotal role of the cell-density dependence of the chemical wave speed for the occurrence of the streaming instability. A simple, analytically tractable, model of Dictyostelium aggregation is developed to test this idea. The interaction of cAMP waves with cAMP-directed cell movement is studied in the form of coupled dynamics of wave front geometries and cell density. Comparing the resulting explicit instability criterion and dispersion relation for cell streaming with the previous findings of model simulations and numerical stability analyses, a unifying interpretation of the streaming instability as a cAMP wave-driven chemotactic instability is proposed.
Nonlinear spacial instability of a fluid sheet
NASA Technical Reports Server (NTRS)
Rangel, R. H.; Hess, C. F.
1990-01-01
The mechanism of nonlinear distortion of a fluid sheet leading to atomization is investigated numerically with the use of vortex dynamics and experimentally by means of holography. The configuration investigated consists of a planar fluid sheet emerging from a rectangular slit with and without coflowing air. The numerical model is two-dimensional, inviscid, and includes surface tension effects. The experimental results indicate the existence of well-defined three-dimensional structures. These are formed mainly by the nonlinear interaction of transverse and streamwise disturbances. The transverse disturbances are associated with the Kelvin-Helmholtz instability while the streamwise disturbances appear related to streamwise vortices possibly originating inside the nozzle.
Nonlinear spacial instability of a fluid sheet
NASA Technical Reports Server (NTRS)
Rangel, R. H.; Hess, C. F.
1990-01-01
The mechanism of nonlinear distortion of a fluid sheet leading to atomization is investigated numerically with the use of vortex dynamics and experimentally by means of holography. The configuration investigated consists of a planar fluid sheet emerging from a rectangular slit with and without coflowing air. The numerical model is two-dimensional, inviscid, and includes surface tension effects. The experimental results indicate the existence of well-defined three-dimensional structures. These are formed mainly by the nonlinear interaction of transverse and streamwise disturbances. The transverse disturbances are associated with the Kelvin-Helmholtz instability while the streamwise disturbances appear related to streamwise vortices possibly originating inside the nozzle.
Oscillatory growth behavior of multistream instabilities
Hou, Y. W.; Chen, M. X.; Yu, M. Y.; Wu, B.
2016-09-15
Multistream instabilities in the one-dimensional Vlasov−Poisson system are studied numerically by using plasmas with multi-humped electron distributions. The evolution of the total wave energy of the plasma oscillations excited by (numerical) noise consists of transient, growth, and nonlinear saturation stages. It is found that, in the growth stage, the total wave energy oscillates for odd number of streams, but it does not oscillate for even number of streams. It is also found that different spectral (Fourier) modes can dominate different stages of the “linear” growth and nonlinear saturation stages.
Entanglement production at Instabilities
NASA Astrophysics Data System (ADS)
Hackl, Lucas; Bianchi, Eugenio; Yokomizo, Nelson
2016-03-01
Instabilities play a major role in various areas of physics. In this talk, I will present some new results on how instabilities produce entanglement between subsystems. We study the asymptotic behavior of the entanglement entropy when we evolve a squeezed vacuum with an unstable quadratic Hamiltonian. We show that in this setting the entanglement entropy always grows linearly with a slope determined by the classical Lyapunov exponents of the system, resembling the classical Kolmogorov-Sinai entropy rate. Our theorem applies to all bosonic quantum field theories with quadratic coupling, including the scalar Schwinger effect, the inverted mass scalar field and various complex field theory models.
Northeastern plateaus bioregion
Gregg M. Riegel; Richard F. Miller; Carl N. Skinner; Sydney E. Smith
2006-01-01
Northeastern California landscape is a mixture of vast arid basins and uplands, and forested mountain ranges interspersed with both fresh water and alkaline wetlands. The entire bioregion is significantly influenced by the rain shadow effect of the Cascade Range to the west. Three ecological unit subsections are treated in this chapter: (1) Modoc Plateau Section (M261G...
Acoustic instability driven by cosmic-ray streaming
NASA Astrophysics Data System (ADS)
Begelman, Mitchell C.; Zweibel, Ellen G.
1994-08-01
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) Pc. At the onset of nonlinearity the fractional amplitude of cosmic-ray pressure perturbations is delta PC/PC approximately (kL) -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
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
Torsional instability in suspension bridges: The Tacoma Narrows Bridge case
NASA Astrophysics Data System (ADS)
Arioli, Gianni; Gazzola, Filippo
2017-01-01
All attempts of aeroelastic explanations for the torsional instability of suspension bridges have been somehow criticised and none of them is unanimously accepted by the scientific community. We suggest a new nonlinear model for a suspension bridge and we perform numerical experiments with the parameters corresponding to the collapsed Tacoma Narrows Bridge. We show that the thresholds of instability are in line with those observed the day of the collapse. Our analysis enables us to give a new explanation for the torsional instability, only based on the nonlinear behavior of the structure.
Electrokinetic instability near charge-selective hydrophobic surfaces
NASA Astrophysics Data System (ADS)
Shelistov, V. S.; Demekhin, E. A.; Ganchenko, G. S.
2014-07-01
The influence of the texture of a hydrophobic surface on the electro-osmotic slip of the second kind and the electrokinetic instability near charge selective surfaces (permselective membranes, electrodes, or systems of microchannels and nanochannels) is investigated theoretically using a simple model based on the Rubinstein-Zaltzman approach. A simple formula is derived to evaluate the decrease in the instability threshold due to hydrophobicity. The study is complemented by numerical investigations both of linear and nonlinear instabilities near a hydrophobic membrane surface. Theory predicts a significant enhancement of the ion flux to the surface and shows a good qualitative agreement with the available experimental data.
Electrokinetic instability near charge-selective hydrophobic surfaces.
Shelistov, V S; Demekhin, E A; Ganchenko, G S
2014-07-01
The influence of the texture of a hydrophobic surface on the electro-osmotic slip of the second kind and the electrokinetic instability near charge selective surfaces (permselective membranes, electrodes, or systems of microchannels and nanochannels) is investigated theoretically using a simple model based on the Rubinstein-Zaltzman approach. A simple formula is derived to evaluate the decrease in the instability threshold due to hydrophobicity. The study is complemented by numerical investigations both of linear and nonlinear instabilities near a hydrophobic membrane surface. Theory predicts a significant enhancement of the ion flux to the surface and shows a good qualitative agreement with the available experimental data.
Raising the mode instability thresholds of fiber amplifiers
NASA Astrophysics Data System (ADS)
Smith, Arlee V.; Smith, Jesse J.
2014-03-01
We use our numerical model of mode instability to analyze the influences of spontaneous thermal Rayleigh scattering (sTRS) and laser gain saturation on instability threshold powers. sTRS is stronger than the quantum noise used as the seed power for stimulated thermal Rayleigh scattering in previous studies, so the threshold is reduced by 15-25% with sTRS seeding. Gain saturation is strong in any efficient amplifier and we show how it can be exploited to raise instability thresholds be a factor of two or more while staying below the stimulated Brillouin threshold.
Coherent betatron instability in the Tevatron
Bogacz, S.A.; Harrison, M.; Ng, K.Y.
1988-06-09
The coherent betatron instability was first observed during the recent 1987-88 Tevatron fixed target run. In this operating mode 1000 consecutive bunches are loaded into the machine at 150 GeV with a bunch spacing of 18.8 /times/ 10/sup -9/ sec (53 MHz). The normalized transverse emittance is typically 15 ..pi.. /times/ 10/sup -6/ m rad in each plane with a longitudinal emittance of about 1.5 eV-sec. The beam is accelerated to 800 GeV in 13 sec. and then it is resonantly extracted during a 23 sec flat top. As the run progressed the bunch intensities were increased until at about 1.4 /times/ 10/sup 10/ppb (protons per bunch) we experienced the onset of a coherent horizontal oscillation taking place in the later stages of the acceleration cycle (>600 GeV). This rapidly developing coherent instability results in a significant emittance growth, which limits machine performance and in a catastrophic scenario it even prevents extraction of the beam. In this paper we will present a simple analytic description of the observed instability. We will show that a combination of a resistive wall coupled bunch effect and a single bunch slow head-tail instability is consistent with the above observations. Finally, a systematic numerical analysis of our model (growth-time vs chromaticity plots) points to the existence of the greater than or equal to1 slow head-tail modes as a plausible mechanism for the observed coherent instability. This last claim, as mentioned before, does not have conclusive experimental evidence, although it is based on a very good agreement between the measured values of the instability growth-time and the ones calculated on the basis of our model. 4 refs., 3 figs.
Double-cross instability in nonuniform plasma
Brizard, A.J.; Kaufman, A.N.; Morehead, J.J.
1996-12-31
One fascinating characteristic associated with the presence of an energetic minority-ion population in a background magnetized plasma is that, if the population is inverted, it can support negative-energy waves (e.g., negative-energy energetic-minority-ion Bernstein waves supported by an inverted alpha-population in tokamak plasmas). In our continuing study of negative-energy waves in nonuniform magnetized plasmas, we investigate a new type of linear absolute instability associated with the resonant interaction between a negative-energy wave and a positive-energy wave in a nonuniform plasma. According to the standard scenario for a linear convective instability associated with the resonant interaction between a negative-energy wave and a positive-energy wave in a one-dimensional background plasma, a negative-energy ray crosses transversely the dispersion surface of a positive-energy wave and resonantly transfers some of its action to create a positive-energy ray. Because of background plasma nonuniformity, however, the two waves interact resonantly only during a finite time interval and, consequently, this linear instability is convective. In this work, we investigate a new scenario for a linear absolute instability in which the negative-energy ray crosses the dispersion surface of the positive-energy wave twice. The new ray topology allows repeated resonant interactions between the two waves if they experience constructive interference at each crossing. This double-cross absolute instability is shown to have a threshold which depends on the area enclosed between the two dispersion curves and the strength of the wave coupling. Numerical and analytical descriptions of this new absolute instability will be presented as well as a discussion of its possible significance in magnetic fusion research.
Control and simulation of thermoacoustic instabilities
NASA Astrophysics Data System (ADS)
Poinsot, Thierry
2014-11-01
Combustion instabilities (CI), due to thermoacoustic coupling between acoustic waves and chemical reaction, constitute a major danger for all combustion systems. They can drive the system to unstable states where the whole combustor can oscillate, vibrate, quench or in extreme cases explode or burn. Such phenomena are commonly observed in the final phases of development programs, leading to major difficulties and significant additional costs. One of the most famous examples of combustion instabilities is the F1 engine of the Apollo program which required more than 1000 engine tests to obtain a stable regime satisfying all other constraints (performance, ignition, etc). CIs constitute one of the most challenging problems in fluid mechanics: they combine turbulence, acoustics, chemistry, unsteady two-phase flow in complex geometries. Since combustion instabilities have been identified (more than hundred years ago), the combustion community has followed two paths: (1) improve our understanding of the phenomena controlling stability to build engines which would be ``stable by design'' and (2) give up on a detailed understanding of mechanisms and add control systems either in open or closed loop devices to inhibit unstable modes. Of course, understanding phenomena driving combustion instabilities to suppress them would be the most satisfying approach but there is no fully reliable theory or numerical method today which can predict whether a combustor will be stable or not before it is fired. This talk will present an overview of combustion instabilities phenomenology before focusing on: (1) active control methods for combustion instabilities and (2) recent methods to predict unstable modes in combustors. These methods are based on recent Large Eddy Simulation codes for compressible reacting flows on HPC systems but we will also describe recent fully analytical methods which provide new insights into unstable modes in annular combustion chambers. Support: European
Adamski, J.C.
2000-01-01
Geochemical data indicate that the Springfield Plateau aquifer, a carbonate aquifer of the Ozark Plateaus Province in central USA, has two distinct hydrochemical zones. Within each hydrochemical zone, water from springs is geochemically and isotopically different than water from wells. Geochemical data indicate that spring water generally interacts less with the surrounding rock and has a shorter residence time, probably as a result of flowing along discrete fractures and solution openings, than water from wells. Water type throughout most of the aquifer was calcium bicarbonate, indicating that carbonate-rock dissolution is the primary geochemical process occurring in the aquifer. Concentrations of calcium, bicarbonate, dissolved oxygen and tritium indicate that most ground water in the aquifer recharged rapidly and is relatively young (less than 40 years). In general, field-measured properties, concentrations of many chemical constituents, and calcite saturation indices were greater in samples from the northern part of the aquifer (hydrochemical zone A) than in samples from the southern part of the aquifer (hydrochemical zone B). Factors affecting differences in the geochemical composition of ground water between the two zones are difficult to identify, but could be related to differences in chert content and possibly primary porosity, solubility of the limestone, and amount and type of cementation between zone A than in zone B. In addition, specific conductance, pH, alkalinity, concentrations of many chemical constituents and calcite saturation indices were greater in samples from wells than in samples from springs in each hydrochemical zone. In contrast, concentrations of dissolved oxygen, nitrite plus nitrate, and chloride generally were greater in samples from springs than in samples from wells. Water from springs generally flows rapidly through large conduits with minimum water-rock interactions. Water from wells flow through small fractures, which restrict
Pair Instability Supernovae of Very Massive Population III Stars
NASA Astrophysics Data System (ADS)
Chen, Ke-Jung; Heger, Alexander; Woosley, Stan; Almgren, Ann; Whalen, Daniel J.
2014-09-01
Numerical studies of primordial star formation suggest that the first stars in the universe may have been very massive. Stellar models indicate that non-rotating Population III stars with initial masses of 140-260 M ⊙ die as highly energetic pair-instability supernovae. We present new two-dimensional simulations of primordial pair-instability supernovae done with the CASTRO code. Our simulations begin at earlier times than previous multidimensional models, at the onset of core contraction, to capture any dynamical instabilities that may be seeded by core contraction and explosive burning. Such instabilities could enhance explosive yields by mixing hot ash with fuel, thereby accelerating nuclear burning, and affect the spectra of the supernova by dredging up heavy elements from greater depths in the star at early times. Our grid of models includes both blue supergiants and red supergiants over the range in progenitor mass expected for these events. We find that fluid instabilities driven by oxygen and helium burning arise at the upper and lower boundaries of the oxygen shell ~20-100 s after core bounce. Instabilities driven by burning freeze out after the SN shock exits the helium core. As the shock later propagates through the hydrogen envelope, a strong reverse shock forms that drives the growth of Rayleigh-Taylor instabilities. In red supergiant progenitors, the amplitudes of these instabilities are sufficient to mix the supernova ejecta.
Pair instability supernovae of very massive population III stars
Chen, Ke-Jung; Woosley, Stan; Heger, Alexander; Almgren, Ann; Whalen, Daniel J.
2014-09-01
Numerical studies of primordial star formation suggest that the first stars in the universe may have been very massive. Stellar models indicate that non-rotating Population III stars with initial masses of 140-260 M {sub ☉} die as highly energetic pair-instability supernovae. We present new two-dimensional simulations of primordial pair-instability supernovae done with the CASTRO code. Our simulations begin at earlier times than previous multidimensional models, at the onset of core contraction, to capture any dynamical instabilities that may be seeded by core contraction and explosive burning. Such instabilities could enhance explosive yields by mixing hot ash with fuel, thereby accelerating nuclear burning, and affect the spectra of the supernova by dredging up heavy elements from greater depths in the star at early times. Our grid of models includes both blue supergiants and red supergiants over the range in progenitor mass expected for these events. We find that fluid instabilities driven by oxygen and helium burning arise at the upper and lower boundaries of the oxygen shell ∼20-100 s after core bounce. Instabilities driven by burning freeze out after the SN shock exits the helium core. As the shock later propagates through the hydrogen envelope, a strong reverse shock forms that drives the growth of Rayleigh-Taylor instabilities. In red supergiant progenitors, the amplitudes of these instabilities are sufficient to mix the supernova ejecta.
[Psychodynamics of childhood instability].
Flavigny, C
1988-01-01
This work focuses on the Anglo-Saxon idea concerning "hyperactivity" and "hyperkinesis" and the French-language idea of "child psycho-motor instability". The author's own personal study (having two separate parts, on the one hand studying the psychic functioning of parent and their interaction with their child, and on the other, studying material gathered on the individual psychotherapy of unstable children), goes along with the French school of thought, highlighting the extent of incestuous sexual advances toward children (especially boys) in the family unit and the sexual nature (in the sense of child sexuality) of this excitement as the source of their instability, justifying a comparison between the unstable child and a Don Juan-type of instability. What comes out is epistemological thinking on Anglo-Saxon and French-language ideas, in particular criticism of the pre-suppositions in the Anglo-Saxon way of seeing things, which seems only to envisage the characterization of a syndromic range, rather than an organic etiology, this being more often implicit; distanced by the idea of psychodynamics, which predominate in the French-language studies, integrating the symptom of "psycho-motor instability" in the general "wholeness" of the child and evaluating ways of parent-child interaction.
NASA Astrophysics Data System (ADS)
Olmedo, Oscar; Zhang, J.
2010-05-01
Flux ropes are now generally accepted to be the magnetic configuration of Coronal Mass Ejections (CMEs), which may be formed prior or during solar eruptions. In this study, we model the flux rope as a current-carrying partial torus loop with its two footpoints anchored in the photosphere, and investigate its instability in the context of the torus instability (TI). Previous studies on TI have focused on the configuration of a circular torus and revealed the existence of a critical decay index. Our study reveals that the critical index is a function of the fractional number of the partial torus, defined by the ratio between the arc length of the partial torus above the photosphere and the circumference of a circular torus of equal radius. We refer to this finding the partial torus instability (PTI). It is found that a partial torus with a smaller fractional number has a smaller critical index, thus requiring a more gradually decreasing magnetic field to stabilize the flux rope. On the other hand, the partial torus with a larger fractional number has a larger critical index. In the limit of a circular torus when the fractional number approaches one, the critical index goes to a maximum value that depends on the distribution of the external magnetic field. We demonstrate that the partial torus instability helps us to understand the confinement, growth, and eventual eruption of a flux rope CME.
Multidirectional Shoulder Instability: Treatment
Ruiz Ibán, Miguel Angel; Díaz Heredia, Jorge; García Navlet, Miguel; Serrano, Francisco; Santos Oliete, María
2017-01-01
Background: The treatment of multidirectional instability of the shoulder is complex. The surgeon should have a clear understanding of the role of hiperlaxity, anatomical variations, muscle misbalance and possible traumatic incidents in each patient. Methods: A review of the relevant literature was performed including indexed journals in English and Spanish. The review was focused in both surgical and conservative management of multidirectional shoulder instability. Results: Most patients with multidirectional instability will be best served with a period of conservative management with physical therapy; this should focus in restoring strength and balance of the dynamic stabilizers of the shoulder. The presence of a significant traumatic incident, anatomic alterations and psychological problems are widely considered to be poor prognostic factors for conservative treatment. Patients who do not show a favorable response after 3 months of conservative treatment seem to get no benefit from further physical therapy. When conservative treatment fails, a surgical intervention is warranted. Both open capsular shift and arthroscopic capsular plication are considered to be the treatment of choice in these patients and have similar outcomes. Thermal or laser capsuloraphy is no longer recommended. Conclusion: Multidirectional instability is a complex problem. Conservative management with focus on strengthening and balancing of the dynamic shoulder stabilizers is the first alternative. Some patients will fare poorly and require either open or arthroscopic capsular plication.
Instabilities of volatile films and drops
NASA Astrophysics Data System (ADS)
Murisic, Nebojsa
2008-12-01
the focus to the gas phase, where the problem of vapor mass diffusion is to be solved, which invokes analogy with the problem of lens-shaped conductor from electrostatics. On the other hand, NEOS model assumes non-equilibrium at the liquid-gas interface and a reaction-limited regime of evaporation; the liquid and gas phases are decoupled using the one-sided assumption, and hence, the problem is to be solved in the liquid phase only. We use lubrication approximation and derive a single governing equation for the evolution of drop thickness, which includes both models. An experimental procedure is described next, which we use in order to estimate the volatility parameter corresponding to each model. We also describe the numerical code, which we use to solve the governing equation for drop thickness, and show how this equation can be used to predict which evaporation model is more appropriate for a particular physical problem. Next, we perform linear stability analysis (LSA) of perturbed thin film configuration. We find excellent agreement between our numerical results and LSA predictions. Furthermore, these results indicate that the IPA/Si configuration is the most unstable one, in direct agreement with experimental results. We perform numerical simulations in the simplified 2d geometry (cross section of the drop) for both planar and radial symmetry and show that our theoretical model reproduces the main features of the experiment, namely, the formation of "octopus"-like features ahead of the contact line of an evaporating drop. Finally, we perform quasi-3d numerical simulations of evaporating drops, where stability to azimuthal perturbations of the contact line is examined. We recover the "octopi" instability for IPA/Si configuration, similarly as seen in the experiments.
NASA Astrophysics Data System (ADS)
Rembiasz, T.; Obergaulinger, M.; Cerdá-Durán, P.; Müller, E.; Aloy, M. A.
2016-03-01
The magnetorotational instability (MRI) can be a powerful mechanism amplifying the magnetic field in core-collapse supernovae. Whether initially weak magnetic fields can be amplified by this instability to dynamically relevant strengths is still a matter of debate. One of the main uncertainties concerns the process that terminates the growth of the instability. Parasitic instabilities of both Kelvin-Helmholtz and tearing-mode type have been suggested to play a crucial role in this process, disrupting MRI channel flows and quenching magnetic field amplification. We perform two-dimensional and three-dimensional sheering-disc simulations of a differentially rotating protoneutron star layer in non-ideal magnetohydrodynamics with unprecedented high numerical accuracy, finding that Kelvin-Helmholtz parasitic modes dominate tearing modes in the regime of large hydrodynamic and magnetic Reynolds numbers, as encountered close to the surface of protoneutron stars. They also determine the maximum magnetic field stress achievable during the exponential growth of the MRI. Our results are consistent with the theory of parasitic instabilities based on a local stability analysis. To simulate the Kelvin-Helmholtz instabilities properly, a very high numerical resolution is necessary. Using ninth-order spatial reconstruction schemes, we find that at least eight grid zones per MRI channel are necessary to simulate the growth phase of the MRI and reach an accuracy of ˜10 per cent in the growth rate, while more than ˜60 zones per channel are required to achieve convergent results for the value of the magnetic stress at MRI termination.
Tidal instability and magnetic field generation
NASA Astrophysics Data System (ADS)
Le Gal, Patrice; Cébron, David; Herreman, Wietze; Le Bars, Michael; Le Dizès, Stéphane
2010-11-01
We are interested in the interaction of the elliptical instability and magnetic fields in liquid metal flows both on laboratory and planetary scales. We first discuss an experimental set-up that realizes an elliptical flow of Galinstan under an imposed field. The presence of a magnetic field is here of double interest. Elliptically excited flows are monitored through the magnetic fields they induce and the instability may be controlled by Joule damping. This study provides some new insight in the nonlinear stages of the elliptical instability. In a planetary context, it is likely that elliptical instability under imposed field occurs in the tidally deformed moon Io of Jupiter. We show how tidally excited flows may significantly deform the imposed field of Jupiter through an induction process. Finally, we also study whether tidally driven flows can be capable of generating and sustaining magnetic fields through the dynamo effect. We present a first numerical study on the possibility of tidally driven dynamo action in triaxial spheroids.
Ultraspinning instability of rotating black holes
Dias, Oscar J. C.; Figueras, Pau; Monteiro, Ricardo; Santos, Jorge E.
2010-11-15
Rapidly rotating Myers-Perry black holes in d{>=}6 dimensions were conjectured to be unstable by Emparan and Myers. In a previous publication, we found numerically the onset of the axisymmetric ultraspinning instability in the singly spinning Myers-Perry black hole in d=7, 8, 9. This threshold also signals a bifurcation to new branches of axisymmetric solutions with pinched horizons that are conjectured to connect to the black ring, black Saturn and other families in the phase diagram of stationary solutions. We firmly establish that this instability is also present in d=6 and in d=10, 11. The boundary conditions of the perturbations are discussed in detail for the first time, and we prove that they preserve the angular velocity and temperature of the original Myers-Perry black hole. This property is fundamental to establishing a thermodynamic necessary condition for the existence of this instability in general rotating backgrounds. We also prove a previous claim that the ultraspinning modes cannot be pure gauge modes. Finally we find new ultraspinning Gregory-Laflamme instabilities of rotating black strings and branes that appear exactly at the critical rotation predicted by the aforementioned thermodynamic criterium. The latter is a refinement of the Gubser-Mitra conjecture.
Perception and self-organized instability
Friston, Karl; Breakspear, Michael; Deco, Gustavo
2012-01-01
This paper considers state-dependent dynamics that mediate perception in the brain. In particular, it considers the formal basis of self-organized instabilities that enable perceptual transitions during Bayes-optimal perception. The basic phenomena we consider are perceptual transitions that lead to conscious ignition (Dehaene and Changeux, 2011) and how they depend on dynamical instabilities that underlie chaotic itinerancy (Breakspear, 2001; Tsuda, 2001) and self-organized criticality (Beggs and Plenz, 2003; Plenz and Thiagarajan, 2007; Shew et al., 2011). Our approach is based on a dynamical formulation of perception as approximate Bayesian inference, in terms of variational free energy minimization. This formulation suggests that perception has an inherent tendency to induce dynamical instabilities (critical slowing) that enable the brain to respond sensitively to sensory perturbations. We briefly review the dynamics of perception, in terms of generalized Bayesian filtering and free energy minimization, present a formal conjecture about self-organized instability and then test this conjecture, using neuronal (numerical) simulations of perceptual categorization. PMID:22783185
Study of Fast Instability in Fermilab Recycler
Antipov, Sergey; Adamson, Philip; Nagaitsev, Sergei; Yang, Ming-Jen
2016-06-01
One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. Various peculiar features of the instability: its occurrence only above a certain intensity threshold, and only in horizontal plane, as well as the rate of the instability, suggest that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beam. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The findings suggest electron cloud trapping in Recycler combined function mag-nets. Bunch-by-bunch measurements of betatron tune show a tune shift towards the end of the bunch train and allow the estimation of the density of electron cloud and the rate of its build-up. The experimental results are in agreement with numerical simulations of electron cloud build-up and its interaction with the beam.
Radiation induced genomic instability in bystander cells
NASA Astrophysics Data System (ADS)
Zhou, H.; Gu, S.; Randers-Pehrson, G.; Hei, T.
There is considerable evidence that exposure to ionizing radiation may induce a heritable genomic instability that leads to a persisting increased frequency of genetic and functional changes in the non-irradiated progeny of a wide variety of irradiated cells Genomic instability is measured as delayed expressions in chromosomal alterations micronucleus formation gene mutations and decreased plating efficiency During the last decade numerous studies have shown that radiation could induce bystander effect in non-irradiated neighboring cells similar endpoints have also been used in genomic instability studies Both genomic instability and the bystander effect are phenomena that result in a paradigm shift in our understanding of radiation biology In the past it seemed reasonable to assume that the production of single- and double-strand DNA breaks are due to direct energy deposition of energy by a charged particle to the nucleus It turns out that biology is not quite that simple Using the Columbia University charged particle microbeam and the highly sensitive human hamster hybrid AL cell mutagenic assay we irradiated 10 of the cells with a lethal dose of 30 alpha particles through the nucleus After overnight incubation the remaining viable bystander cells were replated in dishes for colony formation Clonal isolates were expanded and cultured for 6 consecutive weeks to assess plating efficiency and mutation frequency Preliminary results indicated that there was no significant decrease in plating efficiency among the bystander colonies when compared with
Electrostatic ion cyclotron velocity shear instability
Lemons, D.S.; Winske, D.; Gary, S.P. )
1992-12-01
An electrostatic ion cyclotron instability driven by sheared velocity flow perpendicular to a uniform magnetic field is investigated in the local approximation. The dispersion equation, which includes all kinetic effects and involves only one important parameter, is cast in the form of Gordeyev integrals and solved numerically. The instability occurs roughly at multiples of the ion cyclotron frequency (but modified by the shear) with the growth rate of the individual harmonics overlapping in wavenumber. At small values of the shear parameter, the instability exists in two branches, one at long wavelength, [kappa][rho][sub i] [approximately] 0.5, and one at short wavelength, [kappa][rho][sub i] > 1.5 ([kappa][rho][sub i] is the wavenumber normalized to the ion gyroradius). At larger values of the shear parameter only the longer wavelength branch persists. The growth rate of the long wavelength mode, maximized over wavenumber and frequency, increases monotonically with the shear parameter. Properties of the instability are compared to those of Ganguli et al. obtained in the nonlocal limit.
NASA Astrophysics Data System (ADS)
Dublenych, Yu. I.
2014-11-01
A method for the study of the ground states of lattice-gas models or equivalent spin models with extended-range interactions is proposed. It is shown that effect of longer-range interactions can be studied in terms of the solution of the ground-state problem for a model with short-range interactions. The method is applied to explain the emergence of fractional magnetization plateaus in TmB 4 regarded as a strong Ising magnet on the Shastry-Sutherland lattice with long-range interactions.
Kageyama, H.; Yoshimura, K.; Kato, M.; Kosuge, K.; Kageyama, H.; Mushnikov, N.V.; Onizuka, K.; Goto, T.; Ueda, Y.; Yoshimura, K.; Stern, R.; Slichter, C.P.
1999-04-01
Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline SrCu{sub 2}( BO{sub 3}){sub 2} having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system ({Delta}=30 K ) in which the ground state can be solved {open_quotes}exactly.{close_quotes} Furthermore, in the magnetization, two plateaus corresponding to (1)/(4) and (1)/(8) of the full Cu moment were first observed for 2D quantum spin systems. {copyright} {ital 1999} {ital The American Physical Society}
Testing the gravitational instability hypothesis?
NASA Technical Reports Server (NTRS)
Babul, Arif; Weinberg, David H.; Dekel, Avishai; Ostriker, Jeremiah P.
1994-01-01
We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the methods of gravitational instability theory, implies that the observed large-scale structures and large-scale flows were produced by the action of gravity. This assumption is false, in that there exist nongravitational theories that pass the reconstruction tests and gravitational theories with certain forms of biased galaxy formation that fail them. Gravitational instability theory predicts specific correlations between large-scale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow from homogeneous initial conditions in a way that satisfies the continuity equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate these assertions using analytical arguments and N-body simulations. If large-scale structure is formed by gravitational instability, then the ratio of the galaxy density contrast to the divergence of the velocity field yields an estimate of the density parameter Omega (or, more generally, an estimate of beta identically equal to Omega(exp 0.6)/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations. In nongravitational scenarios, the values of Omega or beta estimated in this way may fail to represent the true cosmological values. However, even if nongravitational forces initiate and shape the growth of structure, gravitationally induced accelerations can dominate the velocity field at late times, long after the action of any nongravitational impulses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstruction tests
Interfacial instabilities in vibrated fluids
NASA Astrophysics Data System (ADS)
Porter, Jeff; Laverón-Simavilla, Ana; Tinao Perez-Miravete, Ignacio; Fernandez Fraile, Jose Javier
2016-07-01
that leads to splitting (fluid separation). We investigate the interaction of these prominent interfacial instabilities in the absence of gravity, concentrating on harmonically vibrated rectangular containers of fluid. We compare vibroequilibria theory with direct numerical simulations and consider the effect of surfaces waves, which can excite sloshing motion of the vibroequilibria. We systematically investigate the saddle-node bifurcation experienced by a symmetric singly connected vibroequilibria solution, for sufficiently deep containers, as forcing is increased. Beyond this instability, the fluid rapidly separates into (at least) two distinct masses. Pronounced hysteresis is associated with this transition, even in the presence of gravity. The interaction of vibroequilibria and frozen waves is investigated in two-fluid systems. Preparations for a parabolic flight experiment on fluids vibrated at high frequencies are discussed.
Testing the gravitational instability hypothesis?
NASA Technical Reports Server (NTRS)
Babul, Arif; Weinberg, David H.; Dekel, Avishai; Ostriker, Jeremiah P.
1994-01-01
We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the methods of gravitational instability theory, implies that the observed large-scale structures and large-scale flows were produced by the action of gravity. This assumption is false, in that there exist nongravitational theories that pass the reconstruction tests and gravitational theories with certain forms of biased galaxy formation that fail them. Gravitational instability theory predicts specific correlations between large-scale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow from homogeneous initial conditions in a way that satisfies the continuity equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate these assertions using analytical arguments and N-body simulations. If large-scale structure is formed by gravitational instability, then the ratio of the galaxy density contrast to the divergence of the velocity field yields an estimate of the density parameter Omega (or, more generally, an estimate of beta identically equal to Omega(exp 0.6)/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations. In nongravitational scenarios, the values of Omega or beta estimated in this way may fail to represent the true cosmological values. However, even if nongravitational forces initiate and shape the growth of structure, gravitationally induced accelerations can dominate the velocity field at late times, long after the action of any nongravitational impulses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstruction tests
Testing the gravitational instability hypothesis?
NASA Astrophysics Data System (ADS)
Babul, Arif; Weinberg, David H.; Dekel, Avishai; Ostriker, Jeremiah P.
1994-05-01
We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the methods of gravitational instability theory, implies that the observed large-scale structures and large-scale flows were produced by the action of gravity. This assumption is false, in that there exist nongravitational theories that pass the reconstruction tests and gravitational theories with certain forms of biased galaxy formation that fail them. Gravitational instability theory predicts specific correlations between large-scale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow from homogeneous initial conditions in a way that satisfies the continuity equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate these assertions using analytical arguments and N-body simulations. If large-scale structure is formed by gravitational instability, then the ratio of the galaxy density contrast to the divergence of the velocity field yields an estimate of the density parameter Omega (or, more generally, an estimate of beta identically equal to Omega0.6/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations. In nongravitational scenarios, the values of Omega or beta estimated in this way may fail to represent the true cosmological values. However, even if nongravitational forces initiate and shape the growth of structure, gravitationally induced accelerations can dominate the velocity field at late times, long after the action of any nongravitational impulses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstruction tests that show
On the transient phase of the Faraday instability
NASA Astrophysics Data System (ADS)
Garih, H.; Estivalezes, J. L.; Casalis, G.
2013-12-01
This study pertains to the three-dimensional direct numerical simulation (DNS) of a vertically oscillating vessel containing an incompressible Newtonian liquid, surrounded by air at rest and ambient conditions. Squire's theorem was extended and shown to apply in this case, allowing for the theory of linear stability to be implemented and a comparison to be made with the DNS results. It was further discovered that the method by which a fluid instability is initiated in the numerical simulation affects the initial development of the instability. This phenomenon was confirmed through an optimal perturbations analysis. A possible physical explanation of this effect is also presented.
Electric field induced lateral instability in a simple autocatalytic front
NASA Astrophysics Data System (ADS)
Horváth, Dezsö; Tóth, Ágota; Yoshikawa, Kenichi
1999-07-01
The effect of ionic drift caused by small constant electric field on autocatalytic reaction fronts of ionic species is studied both theoretically and numerically. Besides varying the velocity of propagation, the electric field parallel to the direction of propagation may induce lateral instability in planar fronts resulting in the emergence of cellular structures. The difference in the diffusivities at the onset of instability are lowered when the electric field tends to separate the species spatially. The predictions of the linear stability analysis based on a thin-front approximation are confirmed by the numerical integration of the full two-dimensional system.
Instability in poroelastic media
NASA Astrophysics Data System (ADS)
Pramanik, Satyajit; Wettlaufer, John
2016-11-01
Fluid flow in deformable porous materials, which play significant role in different biological and geological systems of wide range of scales, is a highly nonlinear problem. Feedback from the elastic deformation of the solid skeleton on the fluid flow and vice-versa gives rise to pattern formation in the porosity structure of the skeleton. We view some of these patterns as instabilities of the coupled fluid-solid system. Due to highly nonlinear nature of the problem, very little has been understood about this instability. Here, we use a minimal poroelastic theory to understand the pattern formation in a fluid-saturated poroelastic material and discuss the similarities/differences with viscous fingering in non-deformable porous media.
Open field lines instabilities
Pozzoli, R. |
1995-09-01
The results of some recent theoretical papers dealing with flute-like instabilities in the scrape-off layer of a tokamak with limiter configuration, where the magnetic field intersects conducting walls, are briefly recalled. Attention is then paid to the instability driven by the electron temperature gradient across the field in conjunction with the formation of the Debye sheath at the boundary, and to the effects due to the inclination of the end walls with respect to the magnetic field. When a divertor configuration is considered, important modifications are found owing to the strong deformations of the flux tubes passing near the {ital x}-point, which contrast the onset of flute-like perturbations, and to the stochasticity of field lines that can be excited by magnetic field perturbations. {copyright} {ital 1995 American Institute of Physics.}
Modulation instability: The beginning
NASA Astrophysics Data System (ADS)
Noskov, Roman; Belov, Pavel; Kivshar, Yuri
2012-11-01
The study of metal nanoparticles plays a central role in the emerging novel technologies employing optics beyond the diffraction limit. Combining strong surface plasmon resonances, high intrinsic nonlinearities and deeply subwavelength scales, arrays of metal nanoparticles offer a unique playground to develop novel concepts for light manipulation at the nanoscale. Here we suggest a novel principle to control localized optical energy in chains of nonlinear subwavelength metal nanoparticles based on the fundamental nonlinear phenomenon of modulation instability. In particular, we demonstrate that modulation instability can lead to the formation of long-lived standing and moving nonlinear localized modes of several distinct types such as bright and dark solitons, oscillons, and domain walls. We analyze the properties of these nonlinear localized modes and reveal different scenarios of their dynamics including transformation of one type of mode to another. We believe this work paves a way towards the development of nonlinear nanophotonics circuitry.
Instabilities in sensory processes
NASA Astrophysics Data System (ADS)
Balakrishnan, J.
2014-07-01
In any organism there are different kinds of sensory receptors for detecting the various, distinct stimuli through which its external environment may impinge upon it. These receptors convey these stimuli in different ways to an organism's information processing region enabling it to distinctly perceive the varied sensations and to respond to them. The behavior of cells and their response to stimuli may be captured through simple mathematical models employing regulatory feedback mechanisms. We argue that the sensory processes such as olfaction function optimally by operating in the close proximity of dynamical instabilities. In the case of coupled neurons, we point out that random disturbances and fluctuations can move their operating point close to certain dynamical instabilities triggering synchronous activity.
Wrist Instability After Injury
Muminagic, Sahib; Kapidzic, Tarik
2012-01-01
Fractures of the bones that make the wrist joint together with injury to the ligaments and joint capsules are frequent traumas. It can cause besides limited movement also the pathological mobility. These mild injuries often do not provide the degree of recognizable symptoms and signs. They are diagnosed by X-ray imaging, stress images. Before arthrography was an important method, but nowadays arthroscopy has the advantage. Fresh bone and ligament injuries can be and should be repaired in the early posttraumatic period. Unrecognized and undiagnosed injuries are leading to instability of the wrist, to motion abnormalities or impingement overload syndrome. In the treatment of instability important place have reconstruction of the ligaments and arthrodesis of the wrist. PMID:23678318
[Orthostatic tremor inducing instability].
Manrique-Huarte, Raquel; Arcocha, Juan; Pérez-Fernández, Nicolás
2012-01-01
Orthostatic tremor (OT) is a neurological disease of unknown aetiology. It is defined by the presence of a 10-20 Hz tremor in the legs while standing still. Symptoms described are dizziness and instability that diminish if the patient sits down or leans on something; drinking small amounts of alcohol significantly reduces OT. Due to the dizziness and/or unsteadiness, these patients are usually referred to the neuro-otology department. We report 4 cases diagnosed with OT. The diagnosis of OT should be considered for patients with instability. The clinical history is a key factor to suspect this entity, and the diagnosis is given by the register of 10-20 Hz contractions on limb electromyography. Treatment for this disease consists of medical treatment; the first option is clonazepam.
Modulation instability: The beginning
NASA Astrophysics Data System (ADS)
Zakharov, V. E.; Ostrovsky, L. A.
2009-03-01
We discuss the early history of an important field of “sturm and drang” in modern theory of nonlinear waves. It is demonstrated how scientific demand resulted in independent and almost simultaneous publications by many different authors on modulation instability, a phenomenon resulting in a variety of nonlinear processes such as envelope solitons, envelope shocks, freak waves, etc. Examples from water wave hydrodynamics, electrodynamics, nonlinear optics, and convection theory are given.
Combustion instability analysis
NASA Technical Reports Server (NTRS)
Chung, T. J.
1990-01-01
A theory and computer program for combustion instability analysis are presented. The basic theoretical foundation resides in the concept of entropy-controlled energy growth or decay. Third order perturbation expansion is performed on the entropy-controlled acoustic energy equation to obtain the first order integrodifferential equation for the energy growth factor in terms of the linear, second, and third order energy growth parameters. These parameters are calculated from Navier-Stokes solutions with time averages performed on as many Navier-Stokes time steps as required to cover at least one peak wave period. Applications are made for a 1-D Navier-Stokes solution for the Space Shuttle Main Engine (SSME) thrust chamber with cross section area variations taken into account. It is shown that instability occurs when the mean pressure is set at 2000 psi with 30 percent disturbances. Instability also arises when the mean pressure is set at 2935 psi with 20 percent disturbances. The system with mean pressures and disturbances more adverse that these cases were shown to be unstable.
Chronic ankle instability: Current perspectives
Al-Mohrej, Omar A.; Al-Kenani, Nader S.
2016-01-01
Ankle sprain is reported to be among the most common recurrent injuries. About 20% of acute ankle sprain patients develop chronic ankle instability. The failure of functional rehabilitation after acute ankle sprain leads to the development of chronic ankle instability. Differentiation between functional and anatomical ankle instability is very essential to guide the proper treatment. Stability testing by varus stress test and anterior drawer test should be carried out. Subtalar instability is an important pathology that is commonly by passed during the assessment of chronic ankle instability. Unlike acute ankle sprain, chronic ankle instability might require surgical intervention. The surgical and conservative management options can be very much developed by in-depth knowledge of the ankle anatomy, biomechanics, and pathology. Anatomical repair, augmentation by tendon, or both are the basic methods of surgical intervention. Arthroscopy is becoming more popular in the management of chronic ankle instability. PMID:27843798
Robust dynamic mitigation of instabilities
Kawata, S.; Karino, T.
2015-04-15
A dynamic mitigation mechanism for instability growth was proposed and discussed in the paper [S. Kawata, Phys. Plasmas 19, 024503 (2012)]. In the present paper, the robustness of the dynamic instability mitigation mechanism is discussed further. The results presented here show that the mechanism of the dynamic instability mitigation is rather robust against changes in the phase, the amplitude, and the wavelength of the wobbling perturbation applied. Generally, instability would emerge from the perturbation of the physical quantity. Normally, the perturbation phase is unknown so that the instability growth rate is discussed. However, if the perturbation phase is known, the instability growth can be controlled by a superposition of perturbations imposed actively: If the perturbation is induced by, for example, a driving beam axis oscillation or wobbling, the perturbation phase could be controlled, and the instability growth is mitigated by the superposition of the growing perturbations.
Patterns and instability of grannular flow
Ecke, Robert E; Borzsonyi, Tamas; Mcelwaine, Jim N
2009-01-01
Dense granular flows are often observed to become unstable and form inhomogeneous structures in nature or industry. Although recently significant advances have been made in understanding simple flows, instabilities are often not understood in detail. We present experimental and numerical results that show the formation of longitudinal stripes. These arise from instability of the uniform flowing state of granular media on a rough inclined plane. The form of the stripes depends critically on the mean density of the flow with a robust form of stripes at high density that consists of fast sliding plug-like regions (stripes) on top of highly agitated boiling material -- a configuration reminiscent of the Leidenfrost effect when a droplet of liquid lifted by its vapor is hovering above a hot surface.
Deployment Instabilities of Lobed-Pumpkin Balloon
NASA Astrophysics Data System (ADS)
Nakashino, Kyoichi
A lobed-pumpkin balloon, currently being developed in ISAS/JAXA as well as in NASA, is a promising vehicle for long duration scientific observations in the stratosphere. Recent ground and flight experiments, however, have revealed that the balloon has deployment instabilities under certain conditions. In order to overcome the instability problems, a next generation SPB called 'tawara' type balloon has been proposed, in which an additional cylindrical part is appended to the standard lobed-pumpkin balloon. The present study investigates the deployment stability of tawara type SPB in comparison to that of standard lobed-pumpkin SPB through eigenvalue analysis on the basis of finite element methods. Our numerical results show that tawara type SPB enjoys excellent deployment performance over the standard lobed-pumpkin SPBs.
Beam ion instability: Measurement, analysis, and simulation
Wang, L.; Safranek, J.; Cai, Y.; Corbett, J.; Hettel, B.; Raubenheimer, T. O.; Schmerge, J.; Sebek, J.; /SLAC
2013-10-03
A weak vertical coupled-bunch instability with oscillation amplitude of the order of a few μ m has been observed in SPEAR3 at nominal vacuum pressure. The instability becomes stronger with increasing neutral gas pressure as observed by turning off vacuum pumps, and becomes weaker when the vertical beam emittance is increased. These observations indicate that the vertical beam motion is driven by ions trapped in the periodic potential of the electron beam. In this paper we present a series of comprehensive beam measurements, impedance-based stability analysis, and numerical simulations of beam-ion interactions in SPEAR3. The effects of vacuum pressure, gas species, beam current, bunch fill pattern, chromaticity, and vertical beam emittance are investigated.
Streak instability in viscoelastic Couette flow
NASA Astrophysics Data System (ADS)
Biancofiore, Luca; Brandt, Luca; Zaki, Tamer
2015-11-01
The secondary instability of streaks and transition to turbulence in viscoelastic Couette flow are studied using direct numerical simulations (DNS). Viscoelasticity is modeled using the FENE-P constitutive equations, and both the polymer concentration and Weissenberg number are varied in order to assess their effect on transition at moderate Reynolds number, Re = 400 .The base streaks are obtained from nonlinear simulations of the Couette flow response to a streamwise vortex, and can be classified as quasi-Newtonian streaks according to the terminology introduced by Page & Zaki (2014). At every streak amplitude of interest, harmonic forcing is introduced to trigger the secondary instability and breakdown to turbulence. The critical amplitude of this forcing decreases at higher Weissenberg number and also with increasing polymer concentration. The results demonstrate the destabilizing effect of elasticity at moderate Reynolds numbers.
Fluctuations and correlations in modulation instability
NASA Astrophysics Data System (ADS)
Solli, D. R.; Herink, G.; Jalali, B.; Ropers, C.
2012-07-01
Stochastically driven nonlinear processes are responsible for spontaneous pattern formation and instabilities in numerous natural and artificial systems, including well-known examples such as sand ripples, cloud formations, water waves, animal pigmentation and heart rhythms. Technologically, a type of such self-amplification drives free-electron lasers and optical supercontinuum sources whose radiation qualities, however, suffer from the stochastic origins. Through time-resolved observations, we identify intrinsic properties of these fluctuations that are hidden in ensemble measurements. We acquire single-shot spectra of modulation instability produced by laser pulses in glass fibre at megahertz real-time capture rates. The temporally confined nature of the gain physically limits the number of amplified modes, which form an antibunched arrangement as identified from a statistical analysis of the data. These dynamics provide an example of pattern competition and interaction in confined nonlinear systems.
NASA Astrophysics Data System (ADS)
Forte, A. M.; Cowgill, E.
2012-12-01
The mechanisms by which orogenic systems and plateaus develop inherently cannibalize stratigraphic records of early orogenic processes within marginal basins. Understanding initial stages of structural growth and feedbacks with surface processes is essential, because these determine subsequent evolution of an orogeny. The Greater Caucasus (GC) form the northern margin of the Arabia-Eurasia collision from 40° to 50°E and are the main locus of ~15 mm/yr NE-SW shortening. Rapid exhumation of the GC starting at 5 Ma and initiation of foreland thrust belts along their southern margin at 1.5 Ma make the GC ideal to investigate early stages of orogeny. Synthesis of our recent structural, stratigraphic, and provenance work in the GC highlight two mechanisms that played primary roles in both the structural evolution of the range and the stratigraphic record of tectonic processes: 1) disruption of the southern, pro-wedge of the GC bivergent orogenic wedge via collision with the Lesser Caucasus (LC) Mountains to the south and 2) extremely high amplitude base-level variations in the large, internally-drained Caspian Sea controlling the stratigraphy of the GC foreland basin. The first order structure of the GC is consistent with bivergent orogenic wedge models, however, the symmetric morphology of the GC and locations of actively propagating fold-thrust belts are inconsistent with these standard models. We suggest that collision between the southern GC and the LC has effectively increased basal friction within the central GC pro-wedge, causing spatially variable accretion of material into both the pro- and retro-wedges. Provenance of sediments within the foreland Kura Basin, southeast of the GC, suggest that wedge disruption in the GC coincides with a large drainage reorganization event in the main range, driven by initiation of new, south-directed thrust systems. While provenance of late Cenozoic Kura Basin sediments indicate a GC source, the stratigraphic architecture is
Double Arc Instability in the Solar Corona
NASA Astrophysics Data System (ADS)
Ishiguro, N.; Kusano, K.
2017-07-01
The stability of the magnetic field in the solar corona is important for understanding the causes of solar eruptions. Although various scenarios have been suggested to date, the tether-cutting reconnection scenario proposed by Moore et al. is one of the widely accepted models to explain the onset process of solar eruptions. Although the tether-cutting reconnection scenario proposes that the sigmoidal field formed by internal reconnection is the magnetic field in the pre-eruptive state, the stability of the sigmoidal field has not yet been investigated quantitatively. In this paper, in order to elucidate the stability problem of the pre-eruptive state, we developed a simple numerical analysis in which the sigmoidal field is modeled by a double arc electric current loop and its stability is analyzed. As a result, we found that the double arc loop is more easily destabilized than the axisymmetric torus, and it becomes unstable even if the external field does not decay with altitude, which is in contrast to the axisymmetric torus instability. This suggests that tether-cutting reconnection may well work as the onset mechanism of solar eruptions, and if so, the critical condition for eruption under a certain geometry may be determined by a new type of instability rather than by the torus instability. Based on them, we propose a new type of instability called double arc instability (DAI). We discuss the critical conditions for DAI and derive a new parameter κ, defined as the product of the magnetic twist and the normalized flux of the tether-cutting reconnection.
The helical decomposition and the instability assumption
NASA Technical Reports Server (NTRS)
Waleffe, Fabian A.
1993-01-01
Direct numerical simulations show that the triadic transfer function T(k,p,q) peaks sharply when q (or p) is much smaller than k. The triadic transfer function T(k,p,q) gives the rate of energy input into wave number k from all interactions with modes of wave number p and q, where k, p, q form a triangle. This observation was thought to suggest that energy is cascaded downscale through non-local interactions with local transfer and that there was a strong connection between large and small scales. Both suggestions were in contradiction with the classical Kolmogorov picture of the energy cascade. The helical decomposition was found useful in distinguishing between kinematically independent interactions. That analysis has gone beyond the question of non-local interaction with local transfer. In particular, an assumption about the statistical direction of triadic energy transfer in any kinematically independent interaction was introduced (the instability assumption). That assumption is not necessary for the conclusions about non-local interactions with local transfer recalled above. In the case of turbulence under rapid rotation, the instability assumption leads to the prediction that energy is transferred in spectral space from the poles of the rotation axis toward the equator. The instability assumption is thought to be of general validity for any type of triad interactions (e.g. internal waves). The helical decomposition and the instability assumption offer detailed information about the homogeneous statistical dynamics of the Navier-Stokes equations. The objective was to explore the validity of the instability assumption and to study the contributions of the various types of helical interactions to the energy cascade and the subgrid-scale eddy-viscosity. This was done in the context of spectral closures of the Direct Interaction or Quasi-Normal type.
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.
Fast instability caused by electron cloud in combined function magnets
Antipov, S. A.; Adamson, P.; Burov, A.; ...
2017-04-10
One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggest that its cause is electron cloud. Here, we studied the phenomena by observing the dynamics of stable and unstable beam, simulating numerically the build-up of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function di-poles. We also found that beam motion can be stabilized by a clearingmore » bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function mag-nets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. Furthermore, in a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth rate of about 30 revolutions and the mode fre-quency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the PEI code. The created instability model allows investigating the beam stability for the future intensity upgrades.« less
Fast Instability Caused by Electron Cloud in Combined Function Magnets
Antipov, S. A.; Adamson, P.; Burov, A.; Nagaitsev, S.; Yang, M. J.
2016-12-12
One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggest that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beam, simulating numerically the build-up of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function di-poles. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function mag-nets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth rate of about 30 revolutions and the mode fre-quency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the PEI code. The created instability model allows investigating the beam stability for the future intensity upgrades.
Instabilities around a rotating ellipsoid in a stratified fluid
NASA Astrophysics Data System (ADS)
Miquel, Benjamin; Meunier, Patrice; Le Dizes, Stephane
2014-11-01
Geosismic observations have revealed the stacking of horizontal layers of water with different densities in the ocean, particularly above and beneath lens-shaped eddies. We present a simplified model together with an experimental setup to reproduce and identify the mechanism responsible for this layering phenomenon: we consider the stably stratified flow around a rotating, solid ellipsoid. Experimentally, a flat oblate rotating ellipsoid reproduces faithfully the boundary condition of an oceanic eddy, whereas the case of a rotating sphere provides an analytically tractable base flow, suitable for a numerical linear analysis. Two instabilities are witnessed experimentally and numerically. The first instability is the classical, inviscid, strato-inertial instability that tends to develop at the equator of the ellipsoid independently of the value of the Schmidt number. The second instability is localised in the vicinity of the poles and appears only if the Schmidt number differs from one. Hence, this instability is reminiscent of the double-diffusive McIntyre instability, a valuable candidate to explain layering in oceanic eddies. Funded by ANR OLA.
Lattice Boltzmann methods for global linear instability analysis
NASA Astrophysics Data System (ADS)
Pérez, José Miguel; Aguilar, Alfonso; Theofilis, Vassilis
2016-11-01
Modal global linear instability analysis is performed using, for the first time ever, the lattice Boltzmann method (LBM) to analyze incompressible flows with two and three inhomogeneous spatial directions. Four linearization models have been implemented in order to recover the linearized Navier-Stokes equations in the incompressible limit. Two of those models employ the single relaxation time and have been proposed previously in the literature as linearization of the collision operator of the lattice Boltzmann equation. Two additional models are derived herein for the first time by linearizing the local equilibrium probability distribution function. Instability analysis results are obtained in three benchmark problems, two in closed geometries and one in open flow, namely the square and cubic lid-driven cavity flow and flow in the wake of the circular cylinder. Comparisons with results delivered by classic spectral element methods verify the accuracy of the proposed new methodologies and point potential limitations particular to the LBM approach. The known issue of appearance of numerical instabilities when the SRT model is used in direct numerical simulations employing the LBM is shown to be reflected in a spurious global eigenmode when the SRT model is used in the instability analysis. Although this mode is absent in the multiple relaxation times model, other spurious instabilities can also arise and are documented herein. Areas of potential improvements in order to make the proposed methodology competitive with established approaches for global instability analysis are discussed.
Transverse instabilities in chemical Turing patterns of stripes
NASA Astrophysics Data System (ADS)
Peña, B.; Pérez-García, C.; Sanz-Anchelergues, A.; Míguez, D. G.; Muñuzuri, A. P.
2003-11-01
We present a theoretical and experimental study of the sideband instabilities in Turing patterns of stripes. We compare numerical computations of the Brusselator model with experiments in a chlorine dioxide iodine malonic acid (CDIMA) reaction in a thin gel layer reactor in contact with a continuously refreshed reservoir of reagents. Spontaneously evolving Turing structures in both systems typically exhibit many defects that break the symmetry of the pattern. Therefore, the study of sideband instabilities requires a method of forcing perfect, spatially periodic Turing patterns with the desired wave number. This is easily achieved in numerical simulations. In experiments, the photosensitivity of the CDIMA reaction permits control and modulation of Turing structures by periodic spatial illumination with a wave number outside the stability region. When a too big wave number is imposed on the pattern, the Eckhaus instability may arise, while for too small wave numbers an instability sets in forming zigzags. By means of the amplitude equation formalism we show that, close to the hexagon-stripe transitions, these sideband instabilities may be preceded by an amplitude instability that grows transient spots locally before reconnecting with stripes. This prediction is tested in both the reaction-diffusion model and the experiment.
Fast instability caused by electron cloud in combined function magnets
NASA Astrophysics Data System (ADS)
Antipov, S. A.; Adamson, P.; Burov, A.; Nagaitsev, S.; Yang, M.-J.
2017-04-01
One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggests that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beams, simulating numerically the buildup of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function dipoles. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function magnets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud buildup is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multiturn accumulation allows the electron cloud to reach final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth time of about 30 revolutions and the mode frequency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the pei code. The created instability model allows investigating the beam stability for the future intensity upgrades.
Davis, Jerri V.; Bell, Richard W.
1998-01-01
Nutrient, bacteria, organic carbon, and suspended- sediment samples were collected from 1993-95 at 43 surface-water-quality sampling sites within the Ozark Plateaus National Water- Quality Assessment Program study unit. Most surface-water-quality sites have small or medium drainage basins, near-homogenous land uses (primarily agricultural or forest), and are located predominantly in the Springfield and Salem Plateaus. The water-quality data were analyzed using selected descriptive and statistical methods to determine factors affecting occurrence in streams in the study unit. Nitrogen and phosphorus fertilizer use increased in the Ozark Plateaus study unit for the period 1965-85, but the application rates are well below the national median. Fertilizer use differed substantially among the major river basins and physiographic areas in the study unit. Livestock and poultry waste is a major source of nutrient loading in parts of the study unit. The quantity of nitrogen and phosphorus from livestock and poultry wastes differed substantially among the river basins of the study unit's sampling network. Eighty six municipal sewage-treatment plants in the study unit have effluents of 0.5 million gallons per day or more (for the years 1985-91). Statistically significant differences existed in surface-water quality that can be attributed to land use, physiography, and drainage basin size. Dissolved nitrite plus nitrate, total phosphorus, fecal coliform bacteria, and dissolved organic carbon concentrations generally were larger at sites associated with agricultural basins than at sites associated with forested basins. A large difference in dissolved nitrite plus nitrate concentrations occurred between streams draining basins with agricultural land use in the Springfield and Salem Plateaus. Streams draining both small and medium agricultural basins in the Springfield Plateau had much larger concentrations than their counterparts in the Salem Plateau. Drainage basin size was not