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1

The nonlinear evolution of large-scale turbulent boundary layer flow with magnetosphere-ionosphere coupling is investigated by a two-dimensional, forced, time-dependent MHD model of the disturbed flux tube. It is suggested that the nonlinear effect, especially the current nonlinear effect, plays an important role in breaking large-scale vortices and currents into medium and small ones. Spectral simulation results show that the large-scale

Y. Song; R. L. Lysak

1988-01-01

2

Two-dimensional magnetohydrodynamic simulations of poloidal flows in tokamaks and MHD pedestal

Poloidal rotation is routinely observed in present-day tokamak experiments, in particular near the plasma edge and in the high-confinement mode of operation. According to the magnetohydrodynamic (MHD) equilibrium theory [R. Betti and J. P. Freidberg, Phys. Plasmas 7, 2439 (2000)], radial discontinuities form when the poloidal velocity exceeds the poloidal sound speed (or rather, more correctly, the poloidal magneto-slow speed). Two-dimensional compressible magnetohydrodynamic simulations show that the transonic discontinuities develop on a time scale of a plasma poloidal revolution to form an edge density pedestal and a localized velocity shear layer at the pedestal location. While such an MHD pedestal surrounds the entire core, the outboard side of the pedestal is driven by the transonic discontinuity while the inboard side is caused by a poloidal redistribution of the mass. The MHD simulations use a smooth momentum source to drive the poloidal flow. Soon after the flow exceeds the poloidal sound speed, the density pedestal and the velocity shear layer form and persist into a quasi steady state. These results may be relevant to the L-H transition, the early stages of the pedestal and edge transport barrier formation.

Guazzotto, L. [Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (United States); Betti, R. [Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (United States); Princeton Plasma Physics Laboratory, Princeton, New Jersey 08540 (United States)

2011-09-15

3

Two-dimensional MHD generator model

NASA Astrophysics Data System (ADS)

A steady state, two dimensional MHD generator code, GEN, is presented. The code solves the equations of conservation of mass, momentum, and energy, using a Von Mises transformation and a local linearization of the equations. By splitting the source terms into a part proportional to the axial pressure gradient and a part independent of the gradient, the pressure distribution along the channel is easily obtained to satisfy various criteria. Thus, the code can run effectively in both design modes, where the channel geometry is determined, and analysis modes, where the geometry is previously known. The code also employs a mixing length concept for turbulent flows, Cebeci and Chang's wall roughness model, and an extension of that model to the effective thermal diffusivities. Results on code validation, as well as comparisons of skin friction and Stanton number calculations with experimental results, are presented.

Geyer, H. K.; Ahluwalia, R. K.; Doss, E. D.

1980-09-01

4

Two-dimensional magnetohydrodynamic simulations of the solar wind interaction with the magnetized regions on the surface of the Moon suggest ``mini-magnetospheres'' can form around the regions on the Moon when the magnetic anomaly field strength is above 10 nT at 100 km above the surface (for a surface field strength of 290 nT) and when the solar wind ion density is

Erika M. Harnett; Robert Winglee

2000-01-01

5

Magnetic reconnection in two-dimensional MHD turbulence

NASA Astrophysics Data System (ADS)

The nonlinear dynamics of magnetic reconnection in broadband turbulence is investigated through direct numerical simulations of decaying, compressible and incompressible, two-dimensional magnetohydrodynamics (MHD). Complex processes of reconnection between magnetic islands (or magnetic vortices), of different size and field strengths, locally occur. Matching classical turbulence analysis with the Sweet-Parker theory, the main statistical features of these multi-scale reconnection events are identified. Locally, magnetic reconnection can be described through a steady state, asymmetric, Sweet-Parker model, in which parameters controlling reconnection rate are determined by the turbulence. Research supported in part by NASA (NNX07AR48G, NNG06GD47G, NNX08AI47G) and NSF (ATM0752135 ATM 0539995).

Servidio, S.; Matthaeus, W. H.; Cassak, P.; Shay, M.; Dmitruk, P.

2008-12-01

6

Energetic Charged Particle Transport in Two-dimensional MHD Solar Wind Turbulence

In recent years, accumulative evidence from nearly incompressible MHD theory, simulations, and solar wind observations pointed to the possibility of the strong presence of two-dimensional (2D) MHD turbulence in the solar wind. However, not much has been done to investigate theoretically the consequences of 2D turbulence for energetic particle transport. Within the framework of quasi-linear kinetic theory we will discuss

J. A. Le Roux; G. P. Zank; W. H. Matthaeus; L. J. Milano

2002-01-01

7

Two-dimensional MHD generator model. [GEN code

A steady state, two-dimensional MHD generator code, GEN, is presented. The code solves the equations of conservation of mass, momentum, and energy, using a Von Mises transformation and a local linearization of the equations. By splitting the source terms into a part proportional to the axial pressure gradient and a part independent of the gradient, the pressure distribution along the channel is easily obtained to satisfy various criteria. Thus, the code can run effectively in both design modes, where the channel geometry is determined, and analysis modes, where the geometry is previously known. The code also employs a mixing length concept for turbulent flows, Cebeci and Chang's wall roughness model, and an extension of that model to the effective thermal diffusities. Results on code validation, as well as comparisons of skin friction and Stanton number calculations with experimental results, are presented.

Geyer, H.K.; Ahluwalia, R.K.; Doss, E.D.

1980-09-01

8

Two-dimensional simulations of magnetically-driven instabilities

A two-dimensional Eulerian MHD code is used to study the evolution of magnetically-driven instabilities in cylindrical geometry. The code incorporates an equation of state, resistivity, and radiative cooling model appropriate for an aluminum plasma. The simulations explore the effects of initial perturbations, electrical resistivity, and radiative cooling on the growth and saturation of the instabilities. Comparisons are made between the 2-D simulations, previous 1-D simulations, and results from the Pioneer experiments of the Los Alamos foil implosion program.

Peterson, D.; Bowers, R.; Greene, A.E.; Brownell, J.

1986-01-01

9

MHD Simulation of Columbia Hbt

The plasma of Columbia High Beta Tokamak (HBT) is studied numerically by using the two dimensional resistive MHD model. The main object of this work is to understand the high beta formation process of HBT plasma and to compare the simulation with the experiments. After its z-pinch phase, the filling gas of helium in HBT is partially ionized and the

Xiao Lin Li

1987-01-01

10

MHD simulation of Columbia HBT

The plasma of Columbia High Beta Tokamak (HBT) is studied numerically by using the two dimensional resistive MHD model. The main object of this work is to understand the high beta formation process of HBT plasma and to compare the simulation with the experiments. 21 refs., 48 figs., 2 tabs.

Li, X.L.

1987-01-01

11

Energetic Charged Particle Transport in Two-dimensional MHD Solar Wind Turbulence

NASA Astrophysics Data System (ADS)

In recent years, accumulative evidence from nearly incompressible MHD theory, simulations, and solar wind observations pointed to the possibility of the strong presence of two-dimensional (2D) MHD turbulence in the solar wind. However, not much has been done to investigate theoretically the consequences of 2D turbulence for energetic particle transport. Within the framework of quasi-linear kinetic theory we will discuss the theoretical implications of 2D MHD turbulence for large-scale energetic charged particle transport in the heliosphere during quiet solar wind conditions. We will deal with two cases: (i) Anisotropic particle distributions where we will focus on pickup ion pitch angle scattering and stochastic acceleration in the quiet slow low-latitude solar wind, and (ii) near-isotropic particle distributions where we will discuss cosmic-ray transport in terms of spatial diffusion parallel and perpendicular to the large-scale magnetic field. The discussion will include topics such as the importance of turbulent motional electric field fluctuations for particle transport in the solar wind, and how a new kind of cosmic-ray transport equation arises when 2D turbulence is important for parallel diffusion.

Le Roux, J. A.; Zank, G. P.; Matthaeus, W. H.; Milano, L. J.

2002-12-01

12

Hall MHD Modeling of Two-dimensional Reconnection: Application to MRX Experiment

Two-dimensional resistive Hall magnetohydrodynamics (MHD) code is used to investigate the dynamical evolution of driven reconnection in the Magnetic Reconnection Experiment (MRX). The initial conditions and dimensionless parameters of the simulation are set to be similar to the experimental values. We successfully reproduce many features of the time evolution of magnetic configurations for both co- and counter-helicity reconnection in MRX. The Hall effect is shown to be important during the early dynamic X-phase of MRX reconnection, while effectively negligible during the late ''steady-state'' Y-phase, when plasma heating takes place. Based on simple symmetry considerations, an experiment to directly measure the Hall effect in MRX configuration is proposed and numerical evidence for the expected outcome is given.

V.S. Lukin; S.C. Jardin

2003-01-09

13

An effective two-dimensional model for MHD flows with transverse magnetic field

This paper presents a model for quasi-two-dimensional MHD flows between two planes with small magnetic Reynolds number and constant transverse magnetic field orthogonal to the planes. A method is presented that allows three-dimensional effects to be taken into account in a two-dimensional equation of motion thanks to a model for the transverse velocity profile. This model is obtained by using

A. Pothérat; J. Sommeria; R. Moreau

2000-01-01

14

Linear Two-Dimensional MHD of Accretion Disks:. Crystalline Structure and Nernst Coefficient

We analyse the two-dimensional MHD configurations characterising the steady state of the accretion disk on a highly magnetised neutron star. The model we describe has a local character and represents the extension of the crystalline structure outlined in Ref. 1, dealing with a local model too, when a specific accretion rate is taken into account. We limit our attention to

Giovanni Montani; Riccardo Benini

2009-01-01

15

Proton temperature anisotropy (T$\\\\perp$p\\/T$\\\\parallel$p > 1) with sufficiently high ?$\\\\parallel$p will drive the electromagnetic ion cyclotron (EMIC) instability. A two-dimensional hybrid code is employed to simulate the EMIC waves in a dipole magnetic field. We initialize the electron-proton plasma such that the MHD equilibrium J × B = ?$\\\\bf\\\

Y. Hu; R. E. Denton

2009-01-01

16

Two-dimensional Magnetohydrodynamic Simulations of Barred Galaxies

NASA Astrophysics Data System (ADS)

Barred galaxies are known to possess magnetic fields that may affect the properties of bar substructures such as dust lanes and nuclear rings. We use two-dimensional high-resolution magnetohydrodynamic (MHD) simulations to investigate the effects of magnetic fields on the formation and evolution of such substructures, as well as on the mass inflow rates to the galaxy center. The gaseous medium is assumed to be infinitesimally thin, isothermal, non-self-gravitating, and threaded by initially uniform, azimuthal magnetic fields. We find that there exists an outermost x 1-orbit relative to which gaseous responses to an imposed stellar bar potential are completely different between inside and outside. Inside this orbit, gas is shocked into dust lanes and infalls to form a nuclear ring. Magnetic fields are compressed in dust lanes, reducing their peak density. Magnetic stress removes further angular momentum of the gas at the shocks, temporarily causing the dust lanes to bend into an "L" shape and eventually leading to a smaller and more centrally distributed ring than in unmagnetized models. The mass inflow rates in magnetized models correspondingly become larger, by more than two orders of magnitude when the initial fields have an equipartition value with thermal energy, than in the unmagnetized counterparts. Outside the outermost x 1-orbit, on the other hand, an MHD dynamo due to the combined action of the bar potential and background shear operates near the corotation and bar-end regions, efficiently amplifying magnetic fields. The amplified fields shape into trailing magnetic arms with strong fields and low density. The base of the magnetic arms has a thin layer in which magnetic fields with opposite polarity reconnect via a tearing-mode instability. This produces numerous magnetic islands with large density that propagate along the arms to turn the outer disk into a highly chaotic state.

Kim, Woong-Tae; Stone, James M.

2012-06-01

17

Two-dimensional beat wave acceleration simulation

Finite laser beam particle simulations of beat wave acceleration show that a coherent plasma wave excited by two-colinear laser beams at a difference frequency equal to the plasma frequency can produce maximum electron energies as predicted by simple one-dimensional theory. The time to saturation and the saturation amplitude of the plasma wave electric field agrees with the Rosenbluth-Liu theory. Stimulated Raman scattering does not appear to degrade the electron acceleration process. Eventually self-focussing and filamentation limit the lifetime of the coherent plasma wave to tens of picoseconds for an intense CO/sub 2/ laser beam.

Kindel, J.M.; Forslund, D.W.; Mori, W.B.; Joshi, C.; Dawson, J.M.

1984-01-01

18

Two-dimensional radiation MHD modeling of stainless steel and Cu wire array Z-pinch implosions

Summary form only given. A two-dimensional radiation MHD model was recently developed and employed to investigate large diameter wire array Z-pinch experiments performed on the refurbished Z generator. This model incorporates into the Mach2 MHD code a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray trace based radiation transport. This level of detail is necessary in order to model

J. W. Thornhill; J. L. Giuliani; J. P. Apruzese; Y. K. Chong; J. Davis; A. Dasgupta; B. Jones; D. J. Ampleford; C. A. Coverdale; M. E. Cuneo

2009-01-01

19

Granular dynamics simulations of two-dimensional heap formation

Granular dynamics simulations have been carried out of vertical feed two-dimensional heap formation by a freefall method using a more realistic granule interaction law than has been employed in previous studies to permit prolonged contacts between adjacent granules. Stable heaps are found to form only on a geometrically rough base comprised of discrete particles, and heap formation is only weakly

J. Baxter; U. Tüautzüautn; J. Burnell; D. M. Heyes

1997-01-01

20

An asymptotic method for calculating the spectrum of high-frequency acoustic normal modes in the plane perpendicular to the magnetic field is developed in application to nonuniform subsonic MHD flow. The method is based on a representation of the two-dimensional eigenfunctions of the nonuniform gasdynamic tract, which is treated as an open resonator, in the form of a superposition of waveguide

I. M. Rutkevich; P. M. Tokar

1985-01-01

21

Two-dimensional theory and simulation of free electron lasers

NASA Astrophysics Data System (ADS)

Two-dimensional homogeneous theory of free-electron lasers with a wiggler magnetic field of constrant wavelength was formulated. It was found that waves propagating obliquely with respect to the electron beam are always unstable with appreciable growth rates; therefore, mode competition among the on-axis and off-six modes is an important consideration in the design of the free-electron laser. absolutely Due to long nonlinear saturation levels of the low frequency absolute instability, the dynamics of the electron beam, and the generation of the high frequency electromagnetic radiation can be severely affected. Two-dimensional particle simulations show that efficiency of genration of the on-axis high-frequency of the off-axis modes. In addition, complete disruption of the electron beam and laser oscillation due to the onset of the absolute instability were observed in simulations.

Kwan, T. J. T.; Cary, J. R.

1981-07-01

22

The effect of two-dimensional turbulence on resistive-MHD reconnection

NASA Astrophysics Data System (ADS)

Two-dimensional numerical simulations of the effect of background turbulence on 2D resistive magnetic reconnection are presented. For sufficiently small values of the resistivity (?) and moderate values of the turbulent power (?), the reconnection rate is found to have a much weaker dependence on ? than the Sweet-Parker scaling of &1/2circ; and is even consistent with an ?-independent value. For a given value of ?, the dependence of the reconnection rate on the turbulent power exhibits a critical threshold in ? above which the reconnection rate is significantly enhanced.

Loureiro, Nuno; Uzdensky, Dmitri; Schekochihin, Alexander; Cowley, Stephen; Yousef, Tarek

2009-11-01

23

Two-Dimensional Simulation of Richtmyer-Meshkov Instability

Two-dimensional simulation of a single-mode Richtmyer--Meshkov instability (RMI) is undertaken to validate GASP, which is a structured, multi-block flow solver based on Reynolds-averaged Navier-Stokes equations (RANS). The fluid configuration is comprised of a diffuse SF6 cylinder immersed in air and accelerated by a Mach 1.2 planar shock. Good overall agreement is achieved between our results and the experiment (Tomkins phet

Amol Palekar; Charles R. Truman; Peter Vorobieff

2004-01-01

24

NASA Astrophysics Data System (ADS)

The intense magnetic field generated by the 20 megaampere Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories is being used as a pressure source for material science studies. An application we have studied in great detail involves using the intense magnetic field to accelerate flyer plates (small metal disks) to very high velocities (>20 km/s) for use in shock loading experiments. We have used two-dimensional (2D) magnetohydrodynamic (MHD) simulation to investigate the physics of accelerating flyer plates using multi-megabar magnetic drive pressures. A typical shock physics load is comprised of conducting electrodes that are highly compressible at multi-megabar pressures. Electrode deformation that occurs during the rise time of the current pulse causes significant inductance increase, which reduces the peak current (drive pressure) relative to a static geometry. This important dynamic effect is modeled self-consistently by driving the MHD simulation with an accurate circuit model of Z. Self-consistent, 2D, MHD simulations are able to produce and predict time resolved velocity interferometry measurements when the drive circuit includes models of current losses and short circuiting in Z. Simulation results elucidate the phenomena contributing to the flyer velocity history, and show that electrical and hydrodynamic optimization of the load are necessary to minimize effects of time varying inductance. Details of the modeling, the physics, and comparisons with experiment are presented.

Lemke, R. W.; Knudson, M. D.; Robinson, A. C.; Haill, T. A.; Struve, K. W.; Asay, J. R.; Mehlhorn, T. A.

2003-05-01

25

Granular dynamics simulations of two-dimensional heap formation

NASA Astrophysics Data System (ADS)

Granular dynamics simulations have been carried out of vertical feed two-dimensional heap formation by a freefall method using a more realistic granule interaction law than has been employed in previous studies to permit prolonged contacts between adjacent granules. Stable heaps are found to form only on a geometrically rough base comprised of discrete particles, and heap formation is only weakly sensitive to the value of the contact friction coefficient. The appearance of avalanches, the pressure distribution on the base, and the voidage distribution are sensitive to the analytic form of the elastic component of the normal interaction, with a soft-sphere r-36 potential giving more realistic behavior than an equivalent Hooke law interaction with the same apparent spring constant. The r-36 interaction gives more realistic assembly dynamics as it introduces medium range collective motion caused by particle roughness and shape found in typical granular materials, without having to model anisotropic particles.

Baxter, J.; Tüautzüautn, U.; Burnell, J.; Heyes, D. M.

1997-03-01

26

Two dimensional simulation of high power laser-surface interaction

For laser intensities in the range of 10{sup 8}--10{sup 9} W/cm{sup 2}, and pulse lengths of order 10 {micro}sec or longer, the authors have modified the inertial confinement fusion code Lasnex to simulate gaseous and some dense material aspects of the laser-matter interaction. The unique aspect of their treatment consists of an ablation model which defines a dense material-vapor interface and then calculates the mass flow across this interface. The model treats the dense material as a rigid two-dimensional mass and heat reservoir suppressing all hydrodynamic motion in the dense material. The computer simulations and additional post-processors provide predictions for measurements including impulse given to the target, pressures at the target interface, electron temperatures and densities in the vapor-plasma plume region, and emission of radiation from the target. The authors will present an analysis of some relatively well diagnosed experiments which have been useful in developing their modeling. The simulations match experimentally obtained target impulses, pressures at the target surface inside the laser spot, and radiation emission from the target to within about 20%. Hence their simulational technique appears to form a useful basis for further investigation of laser-surface interaction in this intensity, pulse-width range. This work is useful in many technical areas such as materials processing.

Goldman, S.R.; Wilke, M.D.; Green, R.E.L.; Johnson, R.P. [Los Alamos National Lab., NM (United States); Busch, G.E. [KMS Fusion, Inc., Ann Arbor, MI (United States)

1998-08-01

27

One-and-Two-Dimensional Simulations of Liner Performance at Atlas Parameters

The authors report results of one-and-two-dimensional MHD simulations of an imploding heavy liner in Z-pinch geometry. The driving current has a pulse shape and peak current characteristic of the Atlas pulsed-power facility being constructed at Los Alamos National Laboratory. One-dimensional simulations of heavy composite liners driven by 30 MA currents can achieve velocities on the order of 14 km/sec. Used to impact a tungsten target, the liner produces shock pressures of approximately fourteen megabars. The first 2-D simulations of imploding liners driven at Atlas current parameters are also described. These simulations have focused on the interaction of the liner with the glide planes, and the effect of realistic surface perturbations on the dynamics of the pinch. It is found that the former interaction does not seriously affect the inner liner surface. Results from the second problem indicate that a surface perturbation having amplitude as small as 0.2 {micro}m can have a significant effect on the implosion dynamics.

Keinigs, R.K.; Atchison, W.L.; Faehl, R.J.; Mclenithan, K.D.; Trainor, R.J.

1998-10-18

28

Two-dimensional radiation-magnetohydrodynamic simulations of SATURN imploding Z pinches

Z-pinch implosions driven by the SATURN device [D. D. Bloomquist etal., Proceedingsofthe 6thInstituteofElectricalandElectronicsEngineers (IEEE) PulsedPowerConference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] at Sandia National Laboratory are modeled with a two-dimensional radiation magnetohydrodynamic (MHD) code, showing strong growth of the magneto-Rayleigh–Taylor (MRT) instability. Modeling of the linear and nonlinear development

James H. Hammer; James L. Eddleman; Paul T. Springer; Max Tabak; Arthur Toor; Keith L. Wong; George B. Zimmerman; Chris Deeney; Russ Humphreys; Thomas J. Nash; Thomas W. L. Sanford; Rick B. Spielman; John S. de Groot

1996-01-01

29

The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in a variety of astrophysical accretion disks. In systems accreting at well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A*), the plasma in the disk is essentially collisionless. We present a nonlinear study of the collisionless MRI using first-principles particle-in-cell plasma simulations. We focus on local two-dimensional (axisymmetric) simulations, deferring more realistic three-dimensional simulations to future work. For simulations with net vertical magnetic flux, the MRI continuously amplifies the magnetic field, B, until the Alfven velocity, v{sub A} , is comparable to the speed of light, c (independent of the initial value of v{sub A} /c). This is consistent with the lack of saturation of MRI channel modes in analogous axisymmetric MHD simulations. The amplification of the magnetic field by the MRI generates a significant pressure anisotropy in the plasma (with the pressure perpendicular to B being larger than the parallel pressure). We find that this pressure anisotropy in turn excites mirror modes and that the volume-averaged pressure anisotropy remains near the threshold for mirror mode excitation. Particle energization is due to both reconnection and viscous heating associated with the pressure anisotropy. Reconnection produces a distinctive power-law component in the energy distribution function of the particles, indicating the likelihood of non-thermal ion and electron acceleration in collisionless accretion disks. This has important implications for interpreting the observed emission-from the radio to the gamma-rays-of systems such as Sgr A*.

Riquelme, Mario A.; Quataert, Eliot [Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720 (United States); Sharma, Prateek [Department of Physics, Indian Institute of Science, Banglore 560012 (India); Spitkovsky, Anatoly, E-mail: marh@astro.berkeley.edu, E-mail: eliot@astro.berkeley.edu, E-mail: prateek@physics.iisc.ernet.in, E-mail: anatoly@astro.princeton.edu [Department of Astrophysical Sciences, Princeton University, Princeton, NJ (United States)

2012-08-10

30

Monte Carlo simulation of the two-dimensional planar model

The authors present results for the two-dimensional planar model on the square lattice. They have developed a Monte Carlo routine which is more efficient than the single-spin-flip algorithms used previously. They report on the variation of the following quantities with temperature: specific heat, energy, magnetization, susceptibility, correlation function, helicity modulus, the density of vortex/antivortex pairs, the average distance between a vortex and its nearest antivortex, and the average distance between a vortex/antivortex pair and the nearest pair. The results are in excellent agreement with the reliable results reported in the literature and are in accord with the general features of the Kosterlitz-Thouless theory.

Bowen, C.; Hunter, D.L.; Jan, N. (St. Francis Xavier Univ., Antigonish, Nova Scotia (Canada))

1992-12-01

31

Coupling of MHD and PIC codes for simulations of the Earth's Magnetoshpere.

NASA Astrophysics Data System (ADS)

We performed multiphysics simulations of the Earth's magnetosphere by coupling magnetohydrodynamic and particle-in-cell solvers. In our simulations, two-dimensional MHD model of solar wind-magnetosphere interaction was created within the COOLFluiD framework, a component-based environment for multi-disciplinary research. Implicit moment particle-in-cell code iPIC3D was used to model the magnetotail region. We have implemented one-way coupling, in which MHD solution provides boundary conditions for the PIC simulation. Our results suggest that joint PIC-MHD simulations are promising for global magnetospheric modeling.

Olshevsky, Vyacheslav; Sarp Yalim, Mehmet; Lani, Andrea; Markidis, Stefano; Lapenta, Giovanni

2013-04-01

32

Two-dimensional magnetohydrodynamics simulations of young Type Ia supernova remnants

NASA Astrophysics Data System (ADS)

Using two-dimensional magnetohydrodynamics (MHD) simulations, we investigate the dynamical properties of Type Ia supernova remnants (SNRs) evolved either in a uniform ambient medium or from an interaction with a dense clump. The initial conditions assume that the expansion of the supernova ejecta is of free inertia with a power-law density distribution in the outer part of the ejecta. To include the effects of the diffusive shock acceleration process and the escape of the accelerated particles from the shock front, we use different adiabatic indices in the simulations to study the dynamical evolution of the Type Ia SNRs. Moreover, we investigate the interactions of a SNR with either a small or a large clump. A double-shock structure with a contact discontinuity is produced as the ejecta flow supersonically in the ambient medium; Rayleigh-Taylor instability is clearly shown as fingers near the contact discontinuity in the contour maps of density, and a high density and a high magnetic field can be triggered because of the instability around the Rayleigh-Taylor fingers. We perform simulations with different adiabatic indices, and the results show that a narrower intershock region is produced with a smaller adiabatic index because a larger compression ratio for the SNR shock is induced. The influence of the Rayleigh-Taylor instability on the morphologies of both the forward and reverse shocks is more significant with a smaller adiabatic index. Finally, the simulations of a SNR interacting with a dense clump show that the morphology of the remnant is greatly twisted after the collision, and a filament with a high density and a high magnetic field can be produced as a SNR colliding with a large dense clump.

Fang, Jun; Zhang, Li

2012-08-01

33

Two-dimensional simulations of the inertial electrostatic confinement device

We discuss the application of the CELESTE simulation package to the simulation of the experiments conducted at the Los Alamos Inertial Electrostatic Confinement (IEC) device. Recently considerable experimental advances have been made in understanding of the stability of the virtual cathode and in the physics of POPS. This momentous experimental advance requires a new simulation effort for explaining the new

Alberto Marocchino; Giovanni Lapenta; Evstati Evstatiev; Richard Nebel; Jaeyoung Park

2006-01-01

34

Experiments being conducted at the Los Alamos National Laboratory Pegasus facility are examining stability issues for driving an aluminum liner with a pulsed magnetic field. The Pegasus facility provides a current of 5 to 8 Megamperes to compress a cylindrical liner. Liners of various size and thickness are used, depending on the specific experimental objectives. In several of these experiments, the outer surface clearly develops perturbations in the mass distribution. These perturbations are strongest when the aluminum is suspected to have melted and in some cases partially vaporized. A series of specific experiments was designed to examine the growth rate of these instabilities. These experiments involved machining a sine wave onto the outer surface of the liner to seed a given wavelength. Two-dimensional MHD calculations, using the measured current profile, were performed to model the behavior of the liner under magnetic field compression. These predictions were made with a 2D Eulerian code complete with a Steinburg-Guinan strength model. The results of these calculations will be discussed in this paper. The density contours at specific times will be compared with the processed radiography.

Atchison, W.L.; Faehl, R.J.; Morgan, D.V.; Reinovsky, R.E.

1997-10-01

35

Preliminary Two-Dimensional Area Navigation Terminal Simulation.

National Technical Information Service (NTIS)

The simulation study analyzed selected controller workload and system performance measures for two route configurations representing two time frames and various percentage mixes of area navigation (RNAV) and non-RNAV operations, referred to in this report...

J. Maurer P. J. O'Brien W. Crimbring

1975-01-01

36

Stochastic simulation of chemical exchange in two dimensional infrared spectroscopy.

The stochastic Liouville equations are employed to investigate the combined signatures of chemical exchange (two-state jump) and spectral diffusion (coupling to an overdamped Brownian oscillator) in the coherent response of an anharmonic vibration to three femtosecond infrared pulses. Simulations reproduce the main features recently observed in the OD stretch of phenol in benzene. PMID:16863316

Sanda, Frantisek; Mukamel, Shaul

2006-07-01

37

Two-dimensional magnetic flux shell model for magnetohydrodynamic simulations

A magnetic flux shell model is described which is suitable for numerical simulations of cylindrically symmetric plasmas. The magnetohydrodynamic equations in magnetic field line coordinates are derived including the effects of finite electrical conductivity and anisotropic heat conductivity. Subsequently, the equations of motion of coaxial plasma shells of constant density, temperature, axial velocity, and axial magnetic field are derived including

J. N. McMullin; R. D. Milroy; C. E. Capjack

1979-01-01

38

Two-dimensional bioluminescence tomography: numerical simulations and phantom experiments

NASA Astrophysics Data System (ADS)

The reconstruction of internal light sources in bioluminescence tomography (BLT) is a challenging inverse problem because of the limited amount of information available compared with that for other kinds of tomography such as fluorescence tomography in which external illumination sources are used. We demonstrated previously, using phantom experiments, that a target containing luciferases could be detected tomographically when the target was located relatively close to the imaging boundary. Here we describe an improved BLT reconstruction method that can detect luciferase-containing targets located anywhere within an imaging domain. The method is tested with numerical simulations and further confirmed with several phantom experiments.

Li, Senhu; Zhang, Qizhi; Jiang, Huabei

2006-05-01

39

Indiana University Southeast Physics Applets: Two Dimensional Diatomic Model Simulation

NSDL National Science Digital Library

This item is a simulation on the movement and energy of a diatomic molecule made up of two atoms with a mass ratio of 1:4. Users can observe the kinetic energy in both atoms, as well as the energy stored in the spring connecting them. Context-rich questions are included on the average kinetic energy of the atoms, their thermal equilibrium with each other, and how their speed changes as the spring gains energy. This page is part of a collection of Java applets with tutorial questions developed for students of undergraduate physics.

Forinash, Kyle

2008-08-31

40

Dynamical mesh adaption for two-dimensional reactive flow simulations

NASA Astrophysics Data System (ADS)

The most recent developments on an adaptive refinement-unrefinement method aimed at constructing a time-dependent unstructured finite-element triangulation for the simulation of stiff unsteady 2D flows are presented. The main features of the adaptive procedure and address the question of choosing the criterion which governs the refinement and unrefinement decisions are described. Then the basic conservative method used to discretize the reactive flow equations on the adaptive triangulation is presented and the problem of defining a numerical scheme which preserves the positivity of all mass fractions on the adaptive mesh is discussed.

Maman, N.; Larrouturou, B.

41

Quasi-two dimensional simulations of electron thermal transport

NASA Astrophysics Data System (ADS)

There exist two leading theoretical turbulent transport models which are driven by the electron temperature gradient and induce anomalous electron transport in tokamaks: (1) electron temperature gradient (ETG) and (2) trapped-electron mode (TEM) turbulence [1,2]. In ETG turbulence, the instability leads to the formation of small scale (on the order of q ?e R/L_Te and c / ?_pe) vortices which provide transport via the toroidal curvature analog of Rayleigh-Benard convection. In contrast, the larger scale (on the order of ?_s) TEM/ITG turbulence has no clear critical electron temperature gradient and can be driven by the density gradient alone or the ion temperature gradient. Thus, we present 2D psuedo-spectral simulations of these two models and constrast their electron transport properties. Particular attention is given to the scaling of the anomalous heat flux with the electron temperature gradient, holding other parameters fixed. 1. G. G.Craddock, et al., Phys. Plasmas 1 (6), 1877 (1994). 2. D.A. Baver, P.W. Terry, and R. Gatto, Phys. Plasmas 9 (8), 3318 (2002).

Holder, B.; Horton, W.

2003-10-01

42

Nonlinear evolution of the electron two-stream instability: Two-dimensional particle simulations

We studied nonlinear evolution of the electron two-stream instability in a two-dimensional system. Electron two-stream and bump-on-tail instabilities are considered to be the most probable generation mechanisms for electrostatic solitary waves and electron holes observed in various regions of the Earth's magnetosphere. We performed two-dimensional particle-in-cell simulations for various sets of electron cyclotron frequencies and initial electron thermal velocities. We

Takayuki Umeda; Yoshiharu Omura; Taketoshi Miyake; Hiroshi Matsumoto; Maha Ashour-Abdalla

2006-01-01

43

Simulation of switched reluctance motor drives using two-dimensional bicubic spline

In this paper, a novel simulation algorithm of switched reluctance motor drives is presented. With the proposed algorithm the two-dimensional (2-D) bicubic spline interpolation is used to describe the nonlinear magnetic characteristics in switched reluctance motors. The corresponding computational method of 2-D bicubic spline function is described in detail. The simulation results are also compared with and validated by experimental

Xiang-Dang Xue; K. W. E. Cheng; S. L. Ho

2002-01-01

44

Strip detector design for ATLAS and HERA-B using two-dimensional device simulation

Irradiation scenarios were simulated in order to evaluate different technology and design options for silicon strip detectors exposed to a high luminosity environment. Two-dimensional process and device simulations were performed to get an insight into the device behaviour. The boundary condition of the free oxide regions between the strips was evaluated thoroughly to obtain correct field distributions. Using these results

R. H. Richter; L. Andricek; T. Gebhart; D. Hauff; J. Kemmer; G. Lutz; R. Weiß; A. Rolf

1996-01-01

45

Numerical simulation of two-dimensional Faraday waves with phase-field modelling

NASA Astrophysics Data System (ADS)

Fully nonlinear numerical simulation of two dimensional Faraday waves between two incompressible and immiscible fluids is performed by adopting the phase-field method with the Cahn-Hilliard equation due to Jacqmin (1999) [J. Comput. Phys., v.155, 96]. Its validation is checked against the linear theory. In a nonlinear regime, qualitative comparison is made with an earlier vortex-sheet simulation of two dimensional Faraday waves by Wright et al. (2000) [J. Fluid Mech., v.400, 1]. The vorticity outside the interface region is studied in this comparison. The period tripling state, which is observed in the quasi-two dimensional experiment by Jiang et al. (1998) [J. Fluid Mech., v.369, 273], is successfully simulated with the present phase-field method.

Takagi, Kentaro; Matsumoto, Takeshi

2011-11-01

46

Multi-dimensional computer simulation of MHD combustor hydrodynamics

Argonne National Laboratory is investigating the nonreacting jet-gas mixing patterns in an MHD second stage combustor by using a two-dimensional multi-phase hydrodynamics computer program and a three-dimensional single-phase hydrodynamics computer program. The computer simulations are intended to enhance the understanding of flow and mixing patterns in the combustor, which in turn may lead to improvement of the downstream MHD channel performance. A two-dimensional steady state computer model, based on mass and momentum conservation laws for multiple gas species, is used to simulate the hydrodynamics of the combustor in which a jet of oxidizer is injected into an unconfined cross-stream gas flow. A three-dimensional code is used to examine the effects of the side walls and the distributed jet flows on the non-reacting jet-gas mixing patterns. The code solves the conservation equations of mass, momentum, and energy, and a transport equation of a turbulence parameter and allows permeable surfaces to be specified for any computational cell. 17 refs., 25 figs.

Berry, G.F.; Chang, S.L.; Lottes, S.A.; Rimkus, W.A.

1991-04-04

47

Simultaneous Potential and Circuit Solution for Two-Dimensional Bounded Plasma Simulation Codes

An algorithm for coupling external circuit elements to be bounded two-dimensional electrostatic plasma simulation codes is developed. In general, the external circuit equations provide a mixture of Dirichlet and Neumann boundary conditions for the Poisson equation, which is solved each time step for the internal plasma potential. We rewrite the coupling between the plasma and the external circuit parameters as

Vahid Vahedi; G. DiPeso

1997-01-01

48

Simulation of two-dimensional contaminant transport with dual reciprocity boundary elements

Here the dual reciprocity boundary element method is used for the simulation of two-dimensional contaminant transport problems in porous media. In the governing equation, the time-dependent and convective terms are approximated using the dual reciprocity method and Green's theorem is used to get a boundary only solution. Linear elements are used for boundary discretization. The model is verified with some

T. I. Eldho; B. Vasudeva Rao

1997-01-01

49

Three-phase fluid migration with solubilities in a two-dimensional basin simulation model

Three-phase fluid migration with solubilities is examined in a two-dimensional basin simulation model. Darcy's law is applied with the driving forces of excess pressure from disequilibrium compaction, buoyancy and capillary pressure controlling the flow of different fluid phases. Relative permabilities are related to the saturation fractions taken by each fluid phase in the pore space. The test results show that

Z. Yu; I. Lerche

1995-01-01

50

Monte Carlo Simulation of Growth Process of Two-Dimensional Quasicrystal

Simulations of the growth process of quasicrystal and square crystal lattice have been performed by the canonical Monte Carlo method. By varying the deposition rate and dimensionless temperature, the conditions for obtaining defect-free single crystal were investigated. The two-dimensional Penrose pattern was adopted as a model of the quasicrystal lattice where the positions of particles are restricted to the lattice

Yasushi Sasajima; Takahiro Ohtsuka; Katsumi Adachi; Minoru Ichimura; Satoru Ozawa

1995-01-01

51

Monte Carlo simulations of a two-dimensional hard dimer system.

National Technical Information Service (NTIS)

Monte Carlo simulations of a system of two-dimensional hard, homonuclear dimers are reported. The equation-of-state, structural and orientational properties, and the free energy were computed for the fluid phase and several crystalline and non-crystalline...

K. W. Wojciechowski A. C. Branka D. Frenkel

1992-01-01

52

Understanding turbulence degradation of electromagnetic wave propagation is essential for efficient operation of laser weapons, target designators, and imaging systems. Random atmospheric refractive index inhomogeneities alter the phase and amplitude of electromagnetic waves. This thesis attempts to model atmospheric turbulence effects by using filtered Gaussian phase screens to represent the random nature of refractive index changes. The simulation uses two-dimensional

Jeffrey L. Turner

1989-01-01

53

Langevin simulation approach to a two-dimensional coupled flashing ratchet

NASA Astrophysics Data System (ADS)

We study the directed motion of a Brownian particle moving in a two-dimensional coupled flashing ratchet driven by Gaussian white noise. The temperature dependence of the current is evaluated numerically in terms of Langevin simulation and discussed by means of a heuristic analytical ad hoc approximation. The current can be enhanced or reversed due to the coupling effects between two degrees of freedom. Our results show that the noise is rectified even in the absence of a true activation energy barrier and the two-dimensional ratchet current does not vanish as the temperature approaches infinity.

Bao, Jing-Dong; Zhuo, Yi-Zhong

1998-03-01

54

A two-dimensional hybrid simulation of the magnetotail reconnection layer

Two-dimensional (2-D) hybrid simulations are carried out to study the structure of the reconnection layer in the distant magnetotail. In the simulation an initial current sheet separates the two lobes with antiparallel magnetic field components in the x direction. The current sheet normal is along the z direction. It is found that a leading bulge-like magnetic configuration and a trailing,

Y. Lin; D. W. Swift

1996-01-01

55

Incompressible extensions of the lattice Boltzmann method [Zou Q, Hou S, Chen S, Doolen GD. J Stat Phys 1995;81:35, He X, Luo LS. J Stat Phys 1997;88:927, Fang HP, Wan RZ, Lin ZF. Phys Rev E 2002;66:036314] are studied simulating two-dimensional decaying turbulence in a double-periodic domain. The simulation results are compared with reference calculations obtained by a pseudo-spectral code.

G. Házi; C. Jiménez

2006-01-01

56

Numerical simulation of relative dispersion in two-dimensional, homogeneous, decaying turbulence

Lagrangian statistical results are presented from numerical simulations of an ensemble of fluid particles which were generated from a two-dimensional pseudospectral code. The single-particle results are in qualitative agreement with previous simulations on a lower-resolution grid. The two-particle, relative velocity correlations were found to fall off more rapidly than the single-particle correlations for short to intermediate times due to large-scale

A. D. Kowalski; R. L. Peskin

1981-01-01

57

Turbulence modeling for two-dimensional water hammer simulations in the low Reynolds number range

Modeling turbulence in two-dimensional water hammer simulations is considered in the present study. The Baldwin–Lomax turbulence model is implemented, both in quasi-steady and frozen forms. Numerical simulations using both forms agree well with experimental data for lower Reynolds numbers (Re=5600) and the attenuation of the transient is adequately captured. However, for higher Reynolds numbers (Re=15,800), the frozen form overpredicts the

E. M. Wahba

2009-01-01

58

The Monte Carlo simulation on multiferroic behaviors of the two-dimensional MnO2 lattice in multiferroic manganites is performed based on the model [I. A. Sergienko and E. Dagotto, Phys. Rev. B 73, 094434 (2006)] associated with the Dzyaloshinskii-Moriya interaction. The simulated ferroelectric polarization induced by the spiral spin ordering and its response to external magnetic field agree well with reported experimental

Qichang Li; Shuai Dong; J.-M. Liu

2008-01-01

59

Two-Dimensional, Time-Domain Simulation of Klystrons and Inductive Output Tubes

We report the development of two-dimensional, time-domain simulation tools for modeling klystrons and inductive output tubes (IOTs). The present development is based upon the techniques developed for the GATOR (Freund et al., 2002) simulation code for coupled-cavity traveling wave tubes. This technique relies on the integration of equivalent circuit equations in time coupled with the Lorentz force equations for particle

H. P. Freund; J. Verboncoeufi; J. Pasour

2006-01-01

60

NASA Astrophysics Data System (ADS)

The Monte Carlo simulation on multiferroic behaviors of the two-dimensional MnO2 lattice in multiferroic manganites is performed based on the model [I. A. Sergienko and E. Dagotto, Phys. Rev. B 73, 094434 (2006)] associated with the Dzyaloshinskii-Moriya interaction. The simulated ferroelectric polarization induced by the spiral spin ordering and its response to external magnetic field agree well with reported experimental observations. Furthermore, the coexistence of clamped ferroelectric domains and spiral spin domains is revealed in our simulation.

Li, Qichang; Dong, Shuai; Liu, J.-M.

2008-02-01

61

Two-dimensional numerical simulation of radical generation in the positive corona discharge

By a two-dimensional (2-D) numerical simulation of positive corona with extensive chemical reactions, two parameters such as E\\/Neff (effective electric field for electrical energy transfer) and G-factor are calculated. E\\/Neff weakly depends on the humidity but strongly depends on the applied voltage. As the applied voltage increases, the corona formation and propagation time decrease, but the other parameters show very

Youn Taeg Kim; Ki Woong Whang

2000-01-01

62

Simulating Flood Propagation in Urban Areas using a Two-Dimensional Numerical Model

A two-dimensional numerical model (RiverFLO-2D) has been enhanced to simulate flooding of urban areas by developing an innovative wet and dry surface algorithm, accounting for variable rainfall, and recoding the model computer program for parallel computing. The model formulation is based on the shallow water equations solved with an explicit time-stepping element-by-element finite element method. The dry-wet surface algorithm is

Noemi Gonzalez-Ramirez

2010-01-01

63

Two-dimensional simulation of a miniaturized inductively coupled plasma reactor

A two-dimensional self-consistent simulation of a miniaturized inductively coupled plasma (mICP) reactor was developed. The coupled equations for plasma power deposition, electron temperature, and charged and neutral species densities, were solved to obtain the spatial distribution of an argon discharge. The effect of control parameters, such as power and pressure, on the evolution of plasma density and electron temperature was

Sang Ki Nam; Demetre J. Economou

2004-01-01

64

Movement, deformation, and partitioning of mammalian red blood cells (RBCs) in diverging microvessel bifurcations are simulated\\u000a using a two-dimensional, flexible-particle model. A set of viscoelastic elements represents the RBC membrane and the cytoplasm.\\u000a Motion of isolated cells is considered, neglecting cell-to-cell interactions. Center-of-mass trajectories deviate from background\\u000a flow streamlines due to migration of flexible cells towards the mother vessel centerline

Jared O. Barber; Jonathan P. Alberding; Juan M. Restrepo; Timothy W. Secomb

2008-01-01

65

This article reports a computer simulation study of the microstructures produced by martensitic transformations. In the present\\u000a work, the transformation strain is dyadic, and the transformation is athermal and irreversible. The transformation occurs\\u000a in a two-dimensional crystal that is constrained in a matrix that has no net transformation strain and may be subject to external\\u000a stress. The crystal is divided

Ping Xu; J. W. Jr. Morris

1993-01-01

66

An Integrated One-Dimensional and Two-Dimensional Urban Stormwater Flood Simulation Model

Flash flooding is the rapid flooding of low lying areas caused by the stormwater of intense rainfall associated with thunderstorms. Flash flooding occurs in many urban areas with relatively flat terrain and can result in severe property damage as well as the loss of lives. In this paper, an integrated one-dimensional (1-D) and two-dimensional (2-D) hydraulic simulation model has been

Xing Fang; Dehui Su

2006-01-01

67

Two-dimensional simulations of single-frequency and beat-wave laser-plasma heating

Finite-beam, two-dimensional particle simulations of single- and double-frequency laser-plasma heating are presented. In the single-frequency case, Raman backscatter and side scatter initially heat the plasma. Even in the absence of strong forward Raman scattering, strong subsequent electron heating is observed. When two collinear laser beams with ..delta omega.. = ..omega..\\/sub p\\/ are used, a coherent plasma wave heats the electrons

D. W. Forslund; J. M. Kindel; W. B. Mori; C. Joshi; J. M. Dawson

1985-01-01

68

MHD simulation studies of z-pinch shear flow stabilization

NASA Astrophysics Data System (ADS)

The development of the m=0 instability in a z-pinch in the presence of sheared plasma flows is investigated with the aid of a two-dimensional magnetohydrodynamic (MHD) simulation code (MHRDR). The linear growth rates are compared to the results obtained by solving the ideal MHD linearized equations [1] and to the results obtained using a 3D hybrid simulation code [2]. The instability development is followed into the nonlinear regime where its growth and saturation are examined. [1] V.I. Sotnikov, I. Paraschiv, V. Makhin, B.S. Bauer, J.-N. Leboeuf, and J.M. Dawson, "Linear analysis of sheared flow stabilization of global magnetohydrodynamic instabilities based on the Hall fluid mode", Phys. Plasmas 9, 913 (2002). [2] V.I. Sotnikov, V. Makhin, B.S. Bauer, P. Hellinger, P. Travnicek, V. Fiala, J.-N. Leboeuf, "Hybrid Simulations of Current-Carrying Instabilities in Z-pinch Plasmas with Sheared Axial Flow", AIP Conference Proceedings, Volume 651, Dense Z-Pinches: 5th International Conference on Dense Z-Pinches, edited by J. Davis et al., page 396, June 2002.

Paraschiv, I.; Bauer, B. S.; Sotnikov, V. I.; Makhin, V.; Siemon, R. E.

2003-10-01

69

MHD Simulation of the Inverse Pinch Plasma Discharge

A wall confined plasma in an inverse pinch configuration holds potential as a plasma target for Magnetized Target Fusion (MTF) as well as the simple geometry to study wall-confined plasma. An experiment is planned to study the inverse pinch configuration using the Nevada Terawatt Facility (NTF) at the University of Nevada, Reno (UNR). The dynamics of the discharge formation have been analyzed using analytic models and numerical methods. Strong heating occurs by thermalization of directed energy when an outward moving current sheet (the inverse pinch effect) collides with the outer wall of the experimental chamber. Two dimensional MHD simulations show Rayleigh-Taylor and Richtmyer-Meshkov -like modes of instability, as expected because of the shock acceleration during plasma formation phase. The instabilities are not disruptive, but give rise to a mild level of turbulence. The conclusion from this work is that an interesting experiment relevant to wall confinement for MTF could be done using existing equipment at UNR.

Esaulov, A; Bauer, B; Lindemuth, I; Makhin, V; Presura, R; Ryutov, D

2004-07-01

70

A simulation formalism for the nonlinear response of vibrational excitons is presented and applied to the OH stretching vibrations of neat liquid H2O. The method employs numerical integration of the Schrödinger equation and allows explicit treatment of fluctuating transition frequencies, vibrational couplings, dipole moments, and the anharmonicities of all these quantities, as well as nonadiabatic effects. The split operator technique greatly increases computational feasibility and performance. The electrostatic map for the OH stretching vibrations in liquid water employed in our previous study [A. Paarmann et al., J. Chem. Phys. 128, 191103 (2008)] is presented. The two-dimensional spectra are in close agreement with experiment. The fast 100 fs dynamics are primarily attributed to intramolecular mixing between states in the two-dimensional OH stretching potential. Small intermolecular couplings are sufficient to reproduce the experimental energy transfer time scales. Interference effects between Liouville pathways in excitonic systems and their impact on the analysis of the nonlinear response are discussed.

Paarmann, A.; Hayashi, T.; Mukamel, S.; Miller, R. J. D.

2009-01-01

71

NASA Astrophysics Data System (ADS)

The magnetization recovery of a two-dimensional ferromagnetic system after excitation by intensive ultrafast pump pulses is investigated by a Monte Carlo method with a focus on the evolution of domain structure and the magnetization in the equilibrium state. Our simulations can explain the effect of the pumping fluence on the recovery process. In particular, they reveal the importance of domain formation in the recently reported accumulation effect already found by the pump--probe magnetization hysteresis loop measurement of a metallic ferromagnetic film. In the framework of the model, it is predicted that by repeating pump pulses a sufficient number of times, the magnetization of the two-dimensional ferromagnetic system can be eliminated when the pump fluence is above a critical value.

Fang, WenXiao; En, YunFei; Zhou, Bin; Huang, QinWen; Liu, Xin; Chen, YiQiang

2012-12-01

72

Two-dimensional Green`s function Poisson solution appropriate for cylindrical-symmetry simulations

This report describes the numerical procedure used to implement the Green`s function method for solving the Poisson equation in two-dimensional (r,z) cylindrical coordinates. The procedure can determine the solution to a problem with any or all of the applied voltage boundary conditions, dielectric media, floating (insulated) conducting media, dielectric surface charging, and volumetric space charge. The numerical solution is reasonably fast, and the dimension of the linear problem to be solved is that of the number of elements needed to represent the surfaces, not the whole computational volume. The method of solution is useful in the simulation of plasma particle motion in the vicinity of complex surface structures as found in microelectronics plasma processing applications. This report is a stand-alone supplement to the previous Sandia Technical Report SAND98-0537 presenting the two-dimensional Cartesian Poisson solver.

Riley, M.E.

1998-04-01

73

NASA Astrophysics Data System (ADS)

Highly sheared poloidal flows are present across the H-mode pedestal. Transonic equilibria produce a radial jump (tangential discontinuity in ideal MHD) in density and poloidal velocity. The tangential discontinuity, required by equilibrium force balance and mass and energy conservation, is not a shock, and there is no flow across the discontinuity. In transonic equilibria edge poloidal velocity exceeds the poloidal sound speed Csp?CsBp/B˜ 10s km/s. In the present work, we simulate the time evolution of tokamak plasmas in the presence of a smooth source of poloidal velocity with the 2D resistive-MHD code SIM2D. Simulations include a cold halo region (resistive plasma) outside the plasma, and realistic magnetic configurations, including X-points. Simulations show the formation of a discontinuous profile for velocity and density. Plasma core and magnetic field are not modified by the transient. Remarkably, and differently from equilibrium theory, time-dependent simulations show the formation of a pedestal at all angular locations, due to mass redistribution during the transient.

Guazzotto, Luca; Betti, Riccardo

2011-11-01

74

Simulation and theory for two-dimensional beam-plasma instability

A comparative study of the dynamics of the electron beam-plasma system in two spatial dimensions is carried out by means of particle-in-cell (PIC) simulation and quasilinear theory. In the literature, the beam-plasma instability is usually studied with one-dimensional assumption. Among the few works that include higher-dimensional effects are two- and three-dimensional quasilinear theory and two-dimensional PIC simulations. However, no efforts were made to compare the theory and simulation side by side. The present paper carries out a detailed comparative study of two-dimensional simulation and quasilinear theory. It is found that the quasilinear theory quite adequately accounts for most important features associated with the simulation result. For instance, the particle diffusion time scale, the maximum wave intensity, dynamical development of the electron distribution function, and the change in the wave spectrum all agree quantitatively. However, certain nonlinear effects such as the Langmuir condensation phenomenon are not reproduced by the quasilinear theory. Nevertheless, the present paper verifies that the simple quasilinear theory is quite effective for the study of beam-plasma instability for the present choice of parameters.

Yi, Sumin [National Fusion Research Institute, Eoeun-dong, Yuseong-gu, Daejeon 305-333 (Korea, Republic of); Rhee, Tongnyeol [National Fusion Research Institute, Eoeun-dong, Yuseong-gu, Daejeon 305-333 (Korea, Republic of); Department of Physics, Pohang University of Science and Technology, Pohang 790-751 (Korea, Republic of); Ryu, Chang-Mo [Department of Physics, Pohang University of Science and Technology, Pohang 790-751 (Korea, Republic of); Yoon, Peter H. [Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742 (United States)

2010-12-15

75

Study of shock nonstationarity with one-and two-dimensional hybrid simulations

NASA Astrophysics Data System (ADS)

Shock reformation involves regions of the shock undergoing periodic collapse and redevelopment on a time scale close to the ion cyclotron period and characteristic transverse spatial scales of order several upstream ion gyroradii. This phenomenon is often observed in computer simulations of high Mach number (MA ), low plasma beta (? ? 0.4), one-dimensional collisionless shocks. Strong support for the shock reformation seen in the one-dimensional simulations was provided by recent space observations. However,some recent two-dimensional hybrid and PIC simulations suggest that reformation does not occur. In this work we use a standard oneand two-dimensional hybrid simulation code to examine the shock reformation processes. The mechanism of excitation of whistler waves at the shock foot and in the ramp region, and their role in the shock reformation processes, are studied in detail. It is found that the twodimensional hybrid shocks can indeed reform periodically and that large amplitude whistler waves are generated in the shock foot. The whistlers have almost zero phase speeds in the shock frame and oblique wave vectors with respect to the upstream magnetic field. The shock reformation processes are significantly affected by the whistler waves and are quite different from the one-dimensional simulation results.

Yuan, Xingqiu; Cairns, Iver; Trichtchenko, Larisa; Rankin, Robert

76

NASA Astrophysics Data System (ADS)

Fractional differential equations have attracted considerable interest because of their ability to model anomalous transport phenomena. Space fractional diffusion equations with a nonlinear reaction term have been presented and used to model many problems of practical interest. In this paper, a two-dimensional Riesz space fractional diffusion equation with a nonlinear reaction term (2D-RSFDE-NRT) is considered. A novel alternating direction implicit method for the 2D-RSFDE-NRT with homogeneous Dirichlet boundary conditions is proposed. The stability and convergence of the alternating direction implicit method are discussed. These numerical techniques are used for simulating a two-dimensional Riesz space fractional Fitzhugh-Nagumo model. Finally, a numerical example of a two-dimensional Riesz space fractional diffusion equation with an exact solution is given. The numerical results demonstrate the effectiveness of the methods. These methods and techniques can be extended in a straightforward method to three spatial dimensions, which will be the topic of our future research.

Liu, Fawang; Chen, Shiping; Turner, Ian; Burrage, Kevin; Anh, Vo

2013-10-01

77

Numerical Simulation of Dendritic Solidification with Convection: Two-Dimensional Geometry

NASA Astrophysics Data System (ADS)

A front tracking method is presented for simulations of dendritic growth of pure substances in the presence of flow. The liquid-solid interface is explicitly tracked and the latent heat released during solidification is calculated using the normal temperature gradient near the interface. A projection method is used to solve the Navier-Stokes equations. The no-slip condition on the interface is enforced by setting the velocities in the solid phase to zero. The method is validated through a comparison with an exact solution for a Stefan problem, a grid refinement test, and a comparison with a solution obtained by a boundary integral method. Three sets of two-dimensional simulations are presented: a comparison with the simulations of Beckermann et al. (J. Comput. Phys.154, 468, 1999); a study of the effect of different flow velocities; and a study of the effect of the Prandtl number on the growth of a group of dendrites growing together. The simulations show that on the upstream side the dendrite tip velocity is increased due to the increase in the temperature gradient and the formation of side branches is promoted. The flow has the opposite effect on the downstream side. The results are in good qualitative agreement with published experimental results, even though only the two-dimensional aspects are examined here.

Al-Rawahi, Nabeel; Tryggvason, Gretar

2002-08-01

78

Two-dimensional micromagnetic simulation of domain structures in films with combined anisotropy

NASA Astrophysics Data System (ADS)

The transformation of the domain structure of micrometer-thick films with variations in the induced uniaxial anisotropy constant with the easy magnetization axis perpendicular to the film surface has been investigated using numerical micromagnetic simulation in the framework of a two-dimensional model of the magnetization distribution. The case where the tetra-axial crystallographic anisotropy exists in the film with uniaxial magnetic anisotropy has been considered. The transformation of the open domain structure into the structure with a magnetic flux closed inside the sample has been investigated in detail, and new types of 109-degree and 90-degree vortex-like domain walls and periodic domain structures have been obtained.

Dubovik, M. N.; Zverev, V. V.; Filippov, B. N.

2013-10-01

79

Two-dimensional simulation of nanocluster formation and comparison with experiments

NASA Astrophysics Data System (ADS)

The CIP method is used to calculate macroscopic plume expansion combined with the Zeldovich-Raiser theory for cluster formation process such as nucleation and growth. The effect of background gas and latent heat is examined in one-dimensional case. The latent heats keep the plume temperature at 2500K for a long period and this explains the delayed photoluminescence. Two contradicting experiments on the size dependence on ambient pressure are clearly explained and are attributed to the difference of laser energy. In two dimensional simulation, mushroom-like plume shape is replicated consistent with experimental results.

Ohkubo, Tomomasa; Kuwata, Masahiro; Luk'yanchuk, Boris S.; Yabe, Takashi

2002-09-01

80

Numerical simulation of reflecting structures by solution of the two-dimensional Helmholtz equation

A method is described for modeling two-dimensional reflecting structures based on a solution of the scalar Helmholtz equation. The equation is solved by use of an alternating-direction-implicit iterative method together with a semioptimum sequence of acceleration parameters similar to those introduced decades ago for the solution of elliptic equations with positive-definite operators. The resulting technique is efficient and simple to program, permits the simulation of complex structures with modest storage requirements, and is of very general applicability.

Hadley, G.R. (Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 (United States))

1994-01-15

81

Computer simulations of two-dimensional quasiperiodic crystal and random tiling models

NASA Astrophysics Data System (ADS)

The recent discovery of quasicrystalline materials exhibiting resolution-limited diffraction peaks focusses attention on those quasicrystalline models which predict long-range translational order. Two such models have been suggested: the quasiperiodic crystal model (for which the Penrose tiling is the prototype) and the random tiling model (analogous to allowing arbitrary spacefilling arrangements of the Penrose rhombuses). Physically, the two models are distinguished by primary dependence on energy and entropy, respectively, in stabilizing the quasicrystal phase. Here we describe computer simulation results for simple two-dimensional realizations of each model.

Strandburg, Katherine J.

1989-08-01

82

Computer simulations of two-dimensional quasiperiodic crystal and random tiling models

The recent discovery of quasicrystalline materials exhibiting resolution-limited diffraction peaks focusses attention on those quasicrystalline models which predict long-range translational order. Two such models have been suggested: the quasiperiodic crystal model (for which the Penrose tiling is the prototype) and the random tiling model (analogous to allowing arbitrary spacefilling arrangements of the Penrose rhombuses). Physically, the two models are distinguished by primary dependence on energy and entropy, respectively, in stabilizing the quasicrystal phase. Here we describe computer simulation results for simple two-dimensional realizations of each model. 24 refs., 13 figs.

Strandburg, K.J.

1989-08-01

83

A sheet-shaped thermal lithium beam probe has been developed for two-dimensional density measurements in the edge region of the torus plasma. A numerical simulation was carried out to confirm the validity of the diagnostics for fast and transient phenomena such as edge localized modes or blobs, etc., where the velocity of blobs is faster than that of the probe beam. It was found in the simulation that the density of the blob itself is reconstructed to be low and unexpected ghosts appear in the reconstructed density profile near the blob, if the conventional reconstruction method is employed. These results invite our attention to the numerical errors in the density reconstruction process. On the other hand, the errors can be corrected by using the simulation results.

Tsuchiya, H.; Morisaki, T.; Komori, A.; Motojima, O. [Graduate University for Advanced Studies, Toki 509-5292 (Japan); National Institute for Fusion Science, Toki 509-5292 (Japan)

2006-10-15

84

Two-Dimensional Radiation-Hydrodynamical Simulations of Core Collapse Supernova Progenitors

NASA Astrophysics Data System (ADS)

We performed simulations using two dimensional EVH-1(Enhanced Virginia Hydrodynamics One) hydrodynamics and radial ray MGFLD (multigroup flux-limited diffusion) neutrino transport in order to simulate doubly diffusive instabilities below the neutrinospheres of core collapse supernova progenitors. The aim is to compare these simulations with the results from our semi-analytical investigation (Bruenn, S. W., Raley, E. A. ,and Mezzacappa, A. submitted to the ApJ). These indicate that below the neutrinosphere a new type of instability, we call Lepto-Entropy Fingers, is present and may play a role in the supernova mechanism. This work was partially funded by a grant from the DOE Office of Science, Scientific Discovery through Advanced Computing Program.

Raley, E. A.; Bruenn, S. W.

2004-05-01

85

Numerical Simulations of Turbulent MHD Reconnection

NASA Astrophysics Data System (ADS)

Magnetic reconnection is a very important process in a large number of laboratory, space, and astrophysical plasmas. It is usually believed that in the resistive MHD regime, the reconnection rate obeys the classical Sweet-Parker scaling, proportional to the square root of the resistivity, and is thus very slow. Whether reconnection can be significantly accelerated in the presence of MHD turbulence is still an unresolved question. In this study, we use high-resolution incompressible resistive magnetohydrodynamic numerical simulations to investigate the effect of externally imposed small-scale turbulence on the reconnection of a large-scale magnetic field. We characterize the turbulent enhancement of the reconnection rate over the laminar Sweet-Parker rate as a function of the resistivity, turbulent driving scale and amplitude. A. Lazarian & E. Vishniac, ApJ, 517, 700 (1999).

Loureiro, Nuno; Uzdensky, Dmitri; Schekochihin, Alexander; Yousef, Tarek

2008-11-01

86

3-D Relativistic MHD Simulations

NASA Astrophysics Data System (ADS)

We present 3-D numerical simulations of moderately hot, supersonic jets propagating initially along or obliquely to the field lines of a denser magnetized background medium with Lorentz factors of W = 4.56 and evolving in a four-dimensional spacetime. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently in the simulations. This effect is analogous to pushing Japanese ``noren'' or vertical Venetian blinds out of the way while the slats are allowed to bend in 3-D space rather than as a 2-D slab structure.

Nishikawa, K.-I.; Frank, J.; Koide, S.; Sakai, J.-I.; Christodoulou, D. M.; Sol, H.; Mutel, R. L.

1998-12-01

87

Particle dynamics in two-dimensional random-energy landscapes: Experiments and simulations

NASA Astrophysics Data System (ADS)

The dynamics of individual colloidal particles in random potential energy landscapes was investigated experimentally and by Monte Carlo simulations. The value of the potential at each point in the two-dimensional energy landscape follows a Gaussian distribution. The width of the distribution, and hence the degree of roughness of the energy landscape, was varied and its effect on the particle dynamics studied. This situation represents an example of Brownian dynamics in the presence of disorder. In the experiments, the energy landscapes were generated optically using a holographic setup with a spatial light modulator, and the particle trajectories were followed by video microscopy. The dynamics is characterized using, e.g., the time-dependent diffusion coefficient, the mean squared displacement, the van Hove function, and the non-Gaussian parameter. In both experiments and simulations the dynamics is initially diffusive, showing an extended subdiffusive regime at intermediate times before diffusive motion is recovered at very long times. The dependence of the long-time diffusion coefficient on the width of the Gaussian distribution agrees with theoretical predictions. Compared to the dynamics in a one-dimensional potential energy landscape, the localization at intermediate times is weaker and the diffusive regime at long times reached earlier, which is due to the possibility to avoid local maxima in two-dimensional energy landscapes.

Evers, Florian; Zunke, Christoph; Hanes, Richard D. L.; Bewerunge, Jörg; Ladadwa, Imad; Heuer, Andreas; Egelhaaf, Stefan U.

2013-08-01

88

Particle dynamics in two-dimensional random-energy landscapes: Experiments and simulations.

The dynamics of individual colloidal particles in random potential energy landscapes was investigated experimentally and by Monte Carlo simulations. The value of the potential at each point in the two-dimensional energy landscape follows a Gaussian distribution. The width of the distribution, and hence the degree of roughness of the energy landscape, was varied and its effect on the particle dynamics studied. This situation represents an example of Brownian dynamics in the presence of disorder. In the experiments, the energy landscapes were generated optically using a holographic setup with a spatial light modulator, and the particle trajectories were followed by video microscopy. The dynamics is characterized using, e.g., the time-dependent diffusion coefficient, the mean squared displacement, the van Hove function, and the non-Gaussian parameter. In both experiments and simulations the dynamics is initially diffusive, showing an extended subdiffusive regime at intermediate times before diffusive motion is recovered at very long times. The dependence of the long-time diffusion coefficient on the width of the Gaussian distribution agrees with theoretical predictions. Compared to the dynamics in a one-dimensional potential energy landscape, the localization at intermediate times is weaker and the diffusive regime at long times reached earlier, which is due to the possibility to avoid local maxima in two-dimensional energy landscapes. PMID:24032793

Evers, Florian; Zunke, Christoph; Hanes, Richard D L; Bewerunge, Jörg; Ladadwa, Imad; Heuer, Andreas; Egelhaaf, Stefan U

2013-08-14

89

Evaluation of a two-dimensional numerical model for air quality simulation in a street canyon

NASA Astrophysics Data System (ADS)

For many urban areas, the most severe air pollution caused by automobile emissions appears along a road surrounded by tall buildings: the so=called street canyon. A practical two-dimensional numerical model has been developed to be applied to this kind of road structure. This model contains two submodels: a wind-field model and a diffusion model based on a Monte Carlo particle scheme. In order to evaluate the predictive performance of this model, an air quality simulation was carried out at three trunk roads in the Tokyo metropolitan area: Nishi-Shimbashi, Aoyama and Kanda-Nishikicho (using SF 6 as a tracer and NO x measurement). Since this model has two-dimensional properties and cannot be used for the parallel wind condition, the perpendicular wind condition was selected for the simulation. The correlation coefficients for the SF 6 and NO x data in Aoyama were 0.67 and 0.62, respectively. When predictive performance of this model is compared with other models, this model is comparable to the SRI model, and superior to the APPS three-dimensional numerical model.

Okamoto, Shin `Ichi; Lin, Fu Chi; Yamada, Hiroaki; Shiozawa, Kiyoshige

90

3-D Relativistic MHD Simulations

NASA Astrophysics Data System (ADS)

We present 3-D numerical simulations of moderately hot, supersonic jets propagating initially along or obliquely to the field lines of a denser magnetized background medium with Lorentz factors of W=4.56 and evolving in a four-dimensional spacetime. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently in the simulations. This effect is analogous to pushing Japanese ``noren'' or vertical Venetian blinds out of the way while the slats are allowed to bend in 3-D space rather than as a 2-D slab structure. We also simulate jets with the more realistic initial conditions for injecting jets for helical mangetic field, perturbed density, velocity, and internal energy, which are supposed to be caused in the process of jet generation. Three possible explanations for the observed variability are (i) tidal disruption of a star falling into the black hole, (ii) instabilities in the relativistic accretion disk, and (iii) jet-related PRocesses. New results will be reported at the meeting.

Nishikaw, K.-I.; Frank, J.; Christodoulou, D. M.; Koide, S.; Sakai, J.-I.; Sol, H.; Mutel, R. L.

1998-12-01

91

The bulk of the solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the chromospheric plasma. We investigate the importance of introducing neutral particles into the MHD equations using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span the solar atmosphere from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-gray and non-LTE radiative transfer, and thermal conduction along the magnetic field. The effects of partial ionization are implemented using the generalized Ohm's law, i.e., we consider the effects of the Hall term and ambipolar diffusion in the induction equation. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic diffusion, Hall term, and ambipolar diffusion show strong variations in the chromosphere. These strong variations of the various magnetic diffusivities are absent or significantly underestimated when, as has been common for these types of studies, using the semi-empirical VAL-C model as a basis for estimates. In addition, we find that differences in estimating the magnitude of ambipolar diffusion arise depending on which method is used to calculate the ion-neutral collision frequency. These differences cause uncertainties in the different magnetic diffusivity terms. In the chromosphere, we find that the ambipolar diffusion is of the same order of magnitude or even larger than the numerical diffusion used to stabilize our code. As a consequence, ambipolar diffusion produces a strong impact on the modeled atmosphere. Perhaps more importantly, it suggests that at least in the chromospheric domain, self-consistent simulations of the solar atmosphere driven by magnetoconvection can accurately describe the impact of the dominant form of resistivity, i.e., ambipolar diffusion. This suggests that such simulations may be more realistic in their approach to the lower solar atmosphere (which directly drives the coronal volume) than previously assumed.

Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo, E-mail: j.m.sykora@astro.uio.no [Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304 (United States)

2012-07-10

92

Two-dimensional (r, z) magnetohydrodynamic simulations with nonlocal thermodynamic equilibrium ionization and radiation transport are used to investigate the K-shell radiation output from doubly nested large-diameter (> 60 mm) stainless-steel arrays fielded on the refurbished Z pulsed-power generator. The effects of the initial density perturbations, wire ablation rate, and current loss near the load on the total power, K-shell power, and

J. Ward Thornhill; John L. Giuliani; Arati Dasgupta; John P. Apruzese; Jack Davis; Young K. Chong; Christopher A. Jennings; Dave A. Ampleford; Brent Jones; Christine A. Coverdale; M. C. Jones; Michael E. Cuneo; W. A. Stygar

2010-01-01

93

NASA Astrophysics Data System (ADS)

The kinetic electron firehose instability (EFI) is thought to be a crucial mechanism for constraining the observed electron anisotropy in expanding astrophysical plasmas, such as the solar wind. The EFI arises in a bi-Maxwellian plasma when the parallel temperature is greater than the perpendicular one, and its effect is to reduce anisotropy. We study this mechanism via kinetic linear theory, extending and refining previous results, and by new two-dimensional particle-in-cell (PIC) simulations with physical mass ratio. The results of PIC simulations show under which conditions the EFI can indeed be regarded as a constraint for electron distribution function. The detailed electron physics near marginal stability condition is discussed, with emphasis on the competition between growing and damping modes and on wave patterns formed at the nonlinear stage. The results also suggest an observational signature that the EFI has operated, namely the appearance of low-frequency, quasiperpendicular whistler/electron-cyclotron waves.

Camporeale, Enrico; Burgess, David

2008-07-01

94

Two-dimensional simulation of a miniaturized inductively coupled plasma reactor

NASA Astrophysics Data System (ADS)

A two-dimensional self-consistent simulation of a miniaturized inductively coupled plasma (mICP) reactor was developed. The coupled equations for plasma power deposition, electron temperature, and charged and neutral species densities, were solved to obtain the spatial distribution of an argon discharge. The effect of control parameters, such as power and pressure, on the evolution of plasma density and electron temperature was investigated. Strong ion density gradients were observed which can make spatially resolved Langmuir probe measurements particularly challenging. Simulation results were in reasonable agreement with available experimental data. The neutral gas temperature was predicted to be close to the wall temperature, due to the small length scale of the mICP, allowing for efficient heat transfer.

Nam, Sang Ki; Economou, Demetre J.

2004-03-01

95

Reactive dynamics on two-dimensional supports: Monte Carlo simulations and mean-field theory

Monte Carlo simulations and mean-field models are used for the study of nonequilibrium reactions taking place on the surface of a catalyst. The model represents the catalytic reduction of NO with H{sub 2} on a Pt surface. Both Monte Carlo simulations and mean-field results predict the existence of a critical surface in the parameter space where the catalyst remains active for long times. Outside this critical region the catalyst remains active for finite times only. A discrete version of the mean-field model is proposed that takes into account the discrete, two-dimensional nature of the catalyst. For homogeneous initial conditions this improved model provides better quantitative agreement with the Monte Carlo results.

Kalosakas, G.; Provata, A.

2001-06-01

96

Reactive dynamics on two-dimensional supports: Monte Carlo simulations and mean-field theory.

Monte Carlo simulations and mean-field models are used for the study of nonequilibrium reactions taking place on the surface of a catalyst. The model represents the catalytic reduction of NO with H2 on a Pt surface. Both Monte Carlo simulations and mean-field results predict the existence of a critical surface in the parameter space where the catalyst remains active for long times. Outside this critical region the catalyst remains active for finite times only. A discrete version of the mean-field model is proposed that takes into account the discrete, two-dimensional nature of the catalyst. For homogeneous initial conditions this improved model provides better quantitative agreement with the Monte Carlo results. PMID:11415192

Kalosakas, G; Provata, A

2001-05-25

97

Two-dimensional fluid model simulation of bell jar top inductively coupled plasma

In the present paper, argon (Ar) plasmas in a bell jar inductively coupled plasma (ICP) source are systematically studied over pressures from 5 to 20 mtorr and power inputs from 0.2 to 0.5 kW. In this study, both a two-dimensional (2-D) fluid model simulation and global model calculation are compared. The 2-D fluid model simulation with a self-consistent power deposition is developed to describe the Ar plasma behavior as well as predict the plasma parameter distributions. Finally, a quantitative comparison between the global model and the fluid model is made to test their validity. Low-pressure ICP has been employed for etching processing for the last few years.

Wu, H.M.; Yu, B.W. [CFD Research Corp., Huntsville, AL (United States); Li, M. [Univ. of California, Berkeley, CA (United States). Dept. of Chemical Engineering; Yang, Y. [Chinese Academy of Sciences, Beijing (China). Inst. of Mechanics

1997-02-01

98

Two-dimensional full-wave code for reflectometry simulations in TJ-II

A two-dimensional full-wave code in the extraordinary mode has been developed to simulate reflectometry in TJ-II. The code allows us to study the measurement capabilities of the future correlation reflectometer that is being installed in TJ-II. The code uses the finite-difference-time-domain technique to solve Maxwell's equations in the presence of density fluctuations. Boundary conditions are implemented by a perfectly matched layer to simulate free propagation. To assure the stability of the code, the current equations are solved by a fourth-order Runge-Kutta method. Density fluctuation parameters such as fluctuation level, wave numbers, and correlation lengths are extrapolated from those measured at the plasma edge using Langmuir probes. In addition, realistic plasma shape, density profile, magnetic configuration, and experimental setup of TJ-II are included to determine the plasma regimes in which accurate information may be obtained.

Blanco, E.; Heuraux, S.; Estrada, T.; Sanchez, J.; Cupido, L. [Laboratorio Nacional de Fusion por Confinamiento Magnetico, Asociacion Euratom-CIEMAT, Av. Complutense 22, 28040 Madrid (Spain); LPMIA UMR 7040, UHP Nancy I, BP 239, 54506 Vandoeuvre Cedex (France); Laboratorio Nacional de Fusion por Confinamiento Magnetico, Asociacion Euratom-CIEMAT, Av. Complutense 22, 28040 Madrid (Spain); Associacao Euratom-IST, CFN, Instituto Superior Tecnico, 1096 Lisbon (Portugal)

2004-10-01

99

Phonon mediated quantum spin simulator made from a two-dimensional Wigner crystal in Penning traps

NASA Astrophysics Data System (ADS)

Motivated by recent advances in quantum simulations in a Penning trap, we give a theoretical description for the use of two-dimensional cold ions in a rotating trap as a quantum emulator. The collective axial phonon modes and planar modes are studied in detail, including all effects of the rotating frame. We show the character of the phonon modes and spectrum, which is crucial for engineering exotic spin interactions. In the presence of laser-ion coupling with these coherent phonon excitations, we show theoretically how the spin-spin Hamiltonian can be generated. Specifically, we notice certain parameter regimes in which the level of frustration between spins can be engineered by the coupling to the planar modes. This may be relevant to the quantum simulation of spin-glass physics or other disordered problems.

Wang, Joseph; Keith, Adam; Freericks, J. K.

2013-03-01

100

Large-Scale Monte Carlo Simulation of Two-Dimensional Classical XY Model Using Multiple GPUs

NASA Astrophysics Data System (ADS)

We study the two-dimensional classical XY model by the large-scale Monte Carlo simulation of the Swendsen-Wang multi-cluster algorithm using multiple GPUs on the open science supercomputer TSUBAME 2.0. Simulating systems up to the linear system size L=65536, we investigate the Kosterlitz--Thouless (KT) transition. Using the generalized version of the probability-changing cluster algorithm based on the helicity modulus, we locate the KT transition temperature in a self-adapted way. The obtained inverse KT temperature ?KT is 1.11996(6). We estimate the exponent to specify the multiplicative logarithmic correction, -2r, and precisely reproduce the theoretical prediction -2r=1/8.

Komura, Yukihiro; Okabe, Yutaka

2012-11-01

101

Numerical simulations of MHD dynamos

NASA Astrophysics Data System (ADS)

The generation of magnetic fields in space plasmas and in astrophysics is usually described within the framework of magnetohydrodynamics. Turbulent helical flows produce magnetic fields very efficiently, with correlation length scales larger than those characterizing the flow. Within the context of the solar magnetic cycle, a turbulent dynamo is responsible for the so-called alpha effect, while the Omega effect is associated to the differential rotation of the Sun. We present direct numerical simulations of turbulent magnetohydrodynamic dynamos including two-fluid effects such as the Hall current. More specifically, we study the evolution of an initially weak and small-scale magnetic field in a system maintained in a stationary regime of hydrodynamic turbulence, and explore the conditions for exponential growth of the magnetic energy. In all the cases considered, we find that the dynamo saturates at the equipartition level between kinetic and magnetic energy, and the total energy reaches a Kolmogorov power spectrum.

Gómez, Daniel O.; Mininni, Pablo

2005-12-01

102

Simulation of two-dimensional diffusive barrier crossing with a curved reaction path

NASA Astrophysics Data System (ADS)

The crossing of a potential energy barrier by Brownian particles in a system having a curved reaction coordinate is studied by solving numerically the two-dimensional Langevin equation. The activated trajectory method of Northrup and McCammon is used to determine appropriate initial conditions for the simulations and to extract rate constants from the raw data. The calculated crossing rates are in good agreement with those predicted by a formula derived from the two-dimensional Fokker-Planck equation; in particular, they confirm that the reaction path curvature gives rise to an increase in the rate constant for moderate and high degrees of coupling to the heat bath. Examination of individual trajectories shows that this increase results from a tendency for particles to bypass the saddle point of the potential energy surface in favor of a shorter path between the reactant and product states. In the strong-coupling limit, the preferred crossing point agrees well with previous predictions. Current address: Thermal and Fluid Mechanics Division, Sandia National Laboratories, Livermore, CA 94550, USA.

Larson, Richard S.

1986-07-01

103

Topological events in two-dimensional grain growth: Experiments and simulations

Grain growth in polycrystals is a process that occurs as a result of the vanishing of small grains. The mean topological class of vanishing two-dimensional (2-D) grains was found experimentally to be about 4.5. This result suggests that most vanishing grains are either 4- or 5-sided. A recent theory of 2-D grain growth is explicitly based on this fact, treating the switching as random events. The process of shrinking of 4- and 5-sided two-dimensional grains was observed experimentally on polycrystalline films of transparent, pure succinonitrile (SCN). Grain shrinking was studied theoretically and simulated by computer (both dynamic and Monte Carlo). It was found that most shrinking grains are topologically stable and remain within their topological class until they are much smaller than their neighbors. They discuss differences which were found with respect to the behavior of 2-D polycrystals, a 2-D ideal soap froth, and a 2-D section of a 3-D grain structure.

Fradkov, V.E.; Glicksman, M.E.; Palmer, M.; Rajan, K. (Rensselaer Polytechnic Inst., Troy, NY (United States). Materials Engineering Dept.)

1994-08-01

104

Simulation Study of Seemingly Fickian but Heterogeneous Dynamics of Two Dimensional Colloids

NASA Astrophysics Data System (ADS)

A two-dimensional (2D) solid lacks long-range positional order and is diffusive by means of the cooperative motion of particles. We find from molecular dynamics simulations of hard discs that 2D colloids in solid and hexatic phases show seemingly Fickian but strongly heterogeneous dynamics. Beyond translational relaxation time, the mean-square displacement is linear with time, t, implying that discs would undergo Brownian diffusion and the self-part of the van Hove correlation function [Gs(r,t)] might be Gaussian. But dynamics is still heterogeneous and Gs(r,t) is exponential at large r and oscillatory with multiple peaks at intermediate length. We attribute the existence of several such peaks to the observation that there are several clusters of discs with discretized mobility. The cluster of marginally mobile discs grows with time and begins to percolate around translational relaxation time while clusters of fast discs emerge in the middle of the marginally mobile cluster.

Kim, Jeongmin; Kim, Chanjoong; Sung, Bong June

2013-01-01

105

Domain growth in computer simulations of segregating two-dimensional binary fluids

NASA Astrophysics Data System (ADS)

We studied phase segregation kinetics with hydrodynamic interactions, following a quench, in the two-dimensional binary fluid lattice gas model of Rothman and Keller. Carrying out computer simulations at different overall fluid densities d, with equal volume fractions of the two components, we find that the growth of domain sizes R(t) at different d has a scaling behavior with all data well fitted by R(t)/Rs=a+b(t/ts)2/3. The characteristic lengths Rs(d) and times ts(d) are related in a simple way to the viscosity and surface tension of the system at different values of d. We also discuss the growth exponents expected in the general case of phase segregation with hydrodynamic interactions.

Bastea, S.; Lebowitz, J. L.

1995-10-01

106

Using atomistic computer simulations based on analytical potential and density-functional theory models, we study effects of ion irradiation on graphene. We identify the types and concentrations of defects which appear in graphene under impacts of various ions with energies ranging from tens of electron volts to mega-electron volts. For two-dimensional targets, defects beyond single and double vacancies are formed via in-plane recoils. We demonstrate that the conventional approach based on binary-collision approximation and stochastic algorithms developed for bulk solids cannot be applied to graphene and other low-dimensional systems. Finally, taking into account the gas-holding capacity of graphene, we suggest the use of graphene as the ultimate membrane for ion-beam analysis of gases and other volatile systems which cannot be put in the high vacuum required for the operation of ion beams.

Lehtinen, O.; Kotakoski, J.; Tolvanen, A.; Nordlund, K.; Keinonen, J. [Materials Physics Division, University of Helsinki, P.O. Box 43, FI-00014 Helsinki (Finland); Krasheninnikov, A. V. [Materials Physics Division, University of Helsinki, P.O. Box 43, FI-00014 Helsinki (Finland); Department of Applied Physics, Aalto University, P.O. Box 1100, FI-00076 Aalto (Finland)

2010-04-15

107

Packings and defects of strongly coupled two-dimensional Coulomb clusters: Numerical simulation

NASA Astrophysics Data System (ADS)

The packings and defects of the strongly coupled two-dimensional Coulomb clusters with particle number N from a few to a few hundred with different forms of mutual repulsion and central confining potentials at zero temperature are investigated using molecular-dynamics simulation through many annealing cycles. The circular symmetry of the confining potential and the interplay with the mutual repulsion lead to the strong competition between the outer circular shells and the inner triangular lattice. Generic packing behaviors, such as the concentric shells with the classical periodic packing sequence at small N, and the triangular latticelike inner core surrounded by a few outer circular shells at large N are observed. The effects of changing the interaction and confining potentials on the detailed packing sequence, the radial variation of packing density, and the positions of the shell-triangular core interface are investigated with a detailed study of the cluster structures along with the formation and distribution of topological defects.

Lai, Ying-Ju; I, Lin

1999-10-01

108

Shear-flow driven current filamentation: Two-dimensional magnetohydrodynamic-simulations

NASA Astrophysics Data System (ADS)

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional magnetohydrodynamic-simulations. This situation is typical for astrophysical systems like jets, and the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus, one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.

Konz, C.; Wiechen, H.; Lesch, H.

2000-12-01

109

Monte Carlo Simulation of Growth Process of Two-Dimensional Quasicrystal

NASA Astrophysics Data System (ADS)

Simulations of the growth process of quasicrystal and square crystal lattice have been performed by the canonical Monte Carlo method. By varying the deposition rate and dimensionless temperature, the conditions for obtaining defect-free single crystal were investigated. The two-dimensional Penrose pattern was adopted as a model of the quasicrystal lattice where the positions of particles are restricted to the lattice sites and the interaction range between particles was assumed to be only within the nearest-neighbor distance. It was found that the diffusivity of quasicrystal was higher than that of square crystal lattice if attractive interaction was assumed. This tendency is opposite to that in the case of free particles. The conditions for obtaining a defect-free single crystal were that the deposition rate was low, under 6.57×10-6 atom/(site MCS) and the dimensionless temperature was as low as possible but higher than that for polynuclear growth.

Sasajima, Yasushi; Ohtsuka, Takahiro; Adachi, Katsumi; Ichimura, Minoru; Ozawa, Satoru

1995-07-01

110

This report describes the numerical procedure used to implement the Green`s function method for solving the Poisson equation in two-dimensional Cartesian coordinates. The procedure can determine the solution to a problem with any or all of applied voltage boundary conditions, dielectric media, floating (insulated) conducting media, dielectric surface charging, periodic (reflective) boundary conditions, and volumetric space charge. The numerical solution is reasonably fast, and the dimension of the linear problem to be solved is that of the number of elements needed to represent the surfaces, not the whole computational volume. The method of solution is useful in the simulation of plasma particle motion in the vicinity of complex surface structures as found in microelectronics plasma processing applications. A FORTRAN implementation of this procedure is available from the author.

Riley, M.E.

1998-03-01

111

NASA Astrophysics Data System (ADS)

Magneto-optical phenomena in quasi two-dimensional magnetophotonic crystals (Q-2D MPCs) have been investigated experimentally and numerically. Multiple Bragg diffraction was shown to be responsible for enhancement of the Faraday rotation and for alteration of its sign in spectra of experimental samples fabricated from bismuth-substituted yttrium iron garnet. It was shown that, for the magnetic superprism effect-control of the light propagation direction by the external magnetic, materials with much larger magneto-optical activity than that of experimental samples are necessary. Spectra obtained by numerical simulations were in a good agreement with experimental ones. Magnetic superprism effect was demonstrated for model samples and parameters necessary for alteration flow of light in quasi-2D MPCs were obtained.

Baek, Seungmin; Baryshev, Alexander V.; Inoue, Mitsuteru

2012-04-01

112

Helioseismology of a Realistic MHD Sunspot Simulation

NASA Astrophysics Data System (ADS)

We have recently measured travel times and absorption of p modes propagating through a realistic numerical model of solar convection in the presence of a sunspot-like structure. Both the mean travel-time perturbations and the absorption in the simulation are remarkably similar to those observed in typical sunspots. Therefore, simulations of this type provide both the means to understand the physics behind the helioseismic observations and the opportunity to validate existing and future models of the subsurface structure of sunspots. We will compare helioseismic measurements made with the simulation with those of a typical sunspot observed with MDI. We will discuss the implications of these comparisons for structural inversions of sunspots and understanding the role of MHD mode conversion in interpreting helioseismic observations. This work is supported by NASA contracts NNH09CE41C and NNG07EI51C.

Braun, Douglas; Birch, A. C.; Rempel, M.

2009-05-01

113

NASA Astrophysics Data System (ADS)

Molecular dynamics (MD) simulations and quantum mechanical electronic structure calculations are used to investigate the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal equilibrium conditions, the complexes are continuously dissociating and forming. The MD simulations are used to calculate the experimental observables related to the phenol hydroxyl stretching mode, i.e., the two dimensional infrared vibrational echo spectrum as a function of time, which directly displays the formation and dissociation of the complex through the growth of off-diagonal peaks, and the linear absorption spectrum, which displays two hydroxyl stretch peaks, one for the complex and one for the free phenol. The results of the simulations are compared to previously reported experimental data and are found to be in quite reasonable agreement. The electronic structure calculations show that the complex is T shaped. The classical potential used for the phenol-benzene interaction in the MD simulations is in good accord with the highest level of the electronic structure calculations. A variety of other features is extracted from the simulations including the relationship between the structure and the projection of the electric field on the hydroxyl group. The fluctuating electric field is used to determine the hydroxyl stretch frequency-frequency correlation function (FFCF). The simulations are also used to examine the number distribution of benzene and CCl4 molecules in the first solvent shell around the phenol. It is found that the distribution is not that of the solvent mole fraction of benzene. There are substantial probabilities of finding a phenol in either a pure benzene environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number of benzene molecules in phenol's first solvent shell.

Kwac, Kijeong; Lee, Chewook; Jung, Yousung; Han, Jaebeom; Kwak, Kyungwon; Zheng, Junrong; Fayer, M. D.; Cho, Minhaeng

2006-12-01

114

Molecular dynamics (MD) simulations and quantum mechanical electronic structure calculations are used to investigate the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal equilibrium conditions, the complexes are continuously dissociating and forming. The MD simulations are used to calculate the experimental observables related to the phenol hydroxyl stretching mode, i.e., the two dimensional infrared vibrational echo spectrum as a function of time, which directly displays the formation and dissociation of the complex through the growth of off-diagonal peaks, and the linear absorption spectrum, which displays two hydroxyl stretch peaks, one for the complex and one for the free phenol. The results of the simulations are compared to previously reported experimental data and are found to be in quite reasonable agreement. The electronic structure calculations show that the complex is T shaped. The classical potential used for the phenol-benzene interaction in the MD simulations is in good accord with the highest level of the electronic structure calculations. A variety of other features is extracted from the simulations including the relationship between the structure and the projection of the electric field on the hydroxyl group. The fluctuating electric field is used to determine the hydroxyl stretch frequency-frequency correlation function (FFCF). The simulations are also used to examine the number distribution of benzene and CCl4 molecules in the first solvent shell around the phenol. It is found that the distribution is not that of the solvent mole fraction of benzene. There are substantial probabilities of finding a phenol in either a pure benzene environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number of benzene molecules in phenol's first solvent shell. PMID:17199356

Kwac, Kijeong; Lee, Chewook; Jung, Yousung; Han, Jaebeom; Kwak, Kyungwon; Zheng, Junrong; Fayer, M D; Cho, Minhaeng

2006-12-28

115

Laser-shock-wave simulation of two-dimensional nuclear shock waves

Results from experiments that used shock waves generated by a high-power laser to simulate multi-dimensional nuclear shocks are described. The shocks were produced in 50 torr air by irradiating hollow plastic shell targets with 30J, 300 ps Nd-glass laser pulses. The individual near-spherical shocks were investigated to determine over what range the shock radius, R/sub s/, obeyed the Taylor-von-Neumann-Sedov expansion law, R/sub s/..cap alpha..t/sup 2/5/. The relationship was found to hold for 0.9 cm less than or equal to R/sub s/ less than or equal to 2.0 cm. Also, the shocks were modeled with the nuclear effects code RADFLO and good agreement was found between calculation and data for R/sub s/ vs t and also gas and electron densities determined from two-wavelength interferograms of the shock waves. Based on the understanding of the individual shocks, two experiments were designed to investigate two-dimensional shock waves. The first experiment consisted of reflecting a spherical shock off a plastic block suspended 0.9 cm above the target. In the second experiment, two identical spherical shocks were simultaneously generated approx. 1.8 cm apart and allowed to collide. The reflected shocks were compared through scaling laws to the Teapot/Met shock wave generated from a 22 KT nuclear explosion 122 M above the ground. The Mach structures were found to be similar. Then the reflecting and interacting shocks were modeled with a two-dimensional effects code using the one-dimensional RADFLO output to start the problem. Calculation and data for Mach angles and triple point propagation were found to be in good agreement.

Wilke, M.D.; Stone, S.N.; Barasch, G.E.

1982-01-01

116

MHD simulations of jets from accretion disks

NASA Astrophysics Data System (ADS)

We present the MHD simulation including accretion flows in disks, acceleration of outflows from disks, and collimation of the outflows self-consistently. Although it was considered that this kind of simulations only shows the transient phenomena of jets, we found that the outflow and accretion flow reached a quasi-steady state by performing a long-term calculation in a large calculation region. Though the final stage is not exactly the steady state, the acceleration and collimation mechanisms of the outflow were the same as those of the steady theory. The scale of the calculation is approaching to the scale that was observed by the VLBI technique, which provides the current highest resolution for YSO jets.

Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari

117

Two-dimensional simulations of the neutron yield in cryogenic deuterium-tritium implosions on OMEGA

Maximizing the neutron yield to obtain energy gain is the ultimate goal for inertial confinement fusion. Nonuniformities seeded by target and laser perturbations can disrupt neutron production via the Rayleigh-Taylor instability growth. To understand the effects of perturbations on the neutron yield of cryogenic DT implosions on the Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], two-dimensional DRACO[P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)] simulations have been performed to systematically investigate each perturbation source and their combined effects on the neutron-yield performance. Two sources of nonuniformity accounted for the neutron-yield reduction in DRACO simulations: target offset from the target chamber center and laser imprinting. The integrated simulations for individual shots reproduce the experimental yield-over-clean (YOC) ratio within a factor of 2 or better. The simulated neutron-averaged ion temperatures

Hu, S. X.; Goncharov, V. N.; Radha, P. B.; Marozas, J. A.; Skupsky, S.; Boehly, T. R.; Sangster, T. C.; Meyerhofer, D. D.; McCrory, R. L. [Laboratory for Laser Energetics, University of Rochester, 250 E. River Road, Rochester, New York 14623 (United States)

2010-10-15

118

NASA Astrophysics Data System (ADS)

A two-dimensional (2-D) Eulerian Radiation-Magnetohydrodynamic (RMHD) code has been used to simulate imploding z pinches for three experiments fielded on the Los Alamos Pegasus II capacitor bank [J. C. Cochrane et al., Dense Z-Pinches, Third International Conference, London, United Kingdom 1993 (American Institute of Physics, New York, 1994), p. 381] and the Sandia Saturn accelerator [R. B. Spielman et al., Dense Z-Pinches, Second International Conference, Laguna Beach, 1989 (American Institute of Physics, New York, 1989), p. 3] and Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. These simulations match the experimental results closely and illustrate how the code results may be used to track the flow of energy in the simulation and account for the amount of total radiated energy. The differences between the calculated radiated energy and power in 2-D simulations and those from zero-dimensional (0-D) and one-dimensional (1-D) Lagrangian simulations (which typically underpredict the total radiated energy and overpredict power) are due to the radially extended nature of the plasma shell, an effect which arises from the presence of magnetically driven Rayleigh-Taylor instabilities. The magnetic Rayleigh-Taylor instabilities differ substantially from hydrodynamically driven instabilities and typical measures of instability development such as e-folding times and mixing layer thickness are inapplicable or of limited value. A new measure of global instability development is introduced, tied to the imploding plasma mass, termed ``fractional involved mass.'' Examples of this quantity are shown for the three experiments along with a discussion of the applicability of this measure.

Peterson, D. L.; Bowers, R. L.; McLenithan, K. D.; Deeney, C.; Chandler, G. A.; Spielman, R. B.; Matzen, M. K.; Roderick, N. F.

1998-09-01

119

Simulations of the Interaction of Energetic Particles with MHD Waves

A self-consistent study of the interaction of energetic ions with low-frequency MHD waves is performed using hybrid MHD-gyrokinetic particle simulations. In the code, energetic ions are treated as gyrokinetic particles using fully electromagnetic gyrokinetic equations, while the rest of the plasma is treated as a fluid, using MHD description. The particles are coupled to the fluid equations through their density

E. V. Belova; R. E. Denton; M. K. Hudson; A. A. Chan

1996-01-01

120

Two-dimensional lattice Boltzmann model for magnetohydrodynamics.

We present a lattice Boltzmann model for the simulation of two-dimensional magnetohydro dynamic (MHD) flows. The model is an extension of a hydrodynamic lattice Boltzman model with 9 velocities on a square lattice resulting in a model with 17 velocities. Earlier lattice Boltzmann models for two-dimensional MHD used a bidirectional streaming rule. However, the use of such a bidirectional streaming rule is not necessary. In our model, the standard streaming rule is used, allowing smaller viscosities. To control the viscosity and the resistivity independently, a matrix collision operator is used. The model is then applied to the Hartmann flow, giving reasonable results. PMID:12443375

Schaffenberger, Werner; Hanslmeier, Arnold

2002-10-09

121

Two-dimensional simulations of foil implosion experiments on the Los Alamos capacitor bank

A number of z-pinch experiments have been conducted at Los Alamos on the Pegasus capacitor bank in which 2-cm high, 5-cm radius, thin foil loads were imploded with currents in excess of 3 MA. Two-dimensional radiation magnetohydrodynamic (RMHD) simulations of these implosions have been performed to model the implosion dynamics and subsequent generation of an x-ray pulse. Comparison of the simulation instability development with visible light framing camera photographs show good agreement and illustrate the instability evolution from short to long wavelengths and a final disruption of the imploding plasma shell. The calculations also show good agreement with experimental timing and measured current and voltage waveforms, and also reproduce features characteristic of the x-ray output. These include a broad pulsewidth, and thepresence of multiple peaks and small time scale structures, features which cannot be reproduced by one-dimensional models. X-ray spectra obtained from the calculated pinch also reproduce qualitative features inthe measured spectra.

Peterson, D.L.; Bowers, R.L.; Brownell, J.H.; Greene, A.E.; Lee, H.; Matuska, W.

1993-05-01

122

Anatomy of the petroleum geology in Chukchi Sea basin: Two-dimensional simulation

The Chukchi Sea basin is located offshore from the National Petroleum Reserve in Alaska (NPRA). The petroleum exploration history of the Chukchi Sea basin goes back to 1969. Although several wells were drilled, none of them revealed encouraging amounts of oil and gas accumulations. Exploration efforts have been limited mainly to geophysical exploratory work. Increasing recent interest in this area has led to a basin analysis study using available data acquired over the past two decades, in relation to petroleum evolution. This study applies a two-dimensional computer simulation model to the Chukchi Sea basin. An automatic procedure, termed dynamical tomography, uses available measured data to search for the best parameters within a specified range. In an integrated manner the model then simulates (1) geohistory and structural development, (2) thermal history, and (3) organic matter evolution. The outputs include both data tables and plots (in both one and two dimensions). These outputs provide detailed information on the spatial evolution with time of fluid pressure, formation temperature, thermal indicator indices (like Waples' TTI and vitrinite reflectance), porosity, and hydrocarbon generation, migration, and accumulation. In this way the hydrocarbon proneness of various parts of the basin can be evaluated.

Wei Zengpu; Lerche, I. (Univ. of South Carolina, Columbia (United States))

1991-03-01

123

Stress results from two-dimensional granular shear flow simulations using various collision models.

Collision resolution is one of the key elements in a discrete element method algorithm for modeling granular flows. Several collision models have been proposed for this process. The hard-particle collision approach is typically used for dilute systems, or for those in which the assumption of binary and instantaneous particle-particle contact remains valid. As the solids fraction increases, however, multiple, enduring collisions can occur and a soft-particle approach is more appropriate for resolving the collision dynamics. In this work, the delineation between dilute and dense systems and the suitability of contact models are explored for a range of solid fractions. Stress results for two-dimensional shear flow simulations are compared using several collision models including an event-driven hard-particle model, a hysteretic spring soft-particle collision model following Walton and Braun [J. Rheol. 30, 949 (1986)], and a hybrid hard-particle-with-overlap model following Hopkins and Louge [Phys. Fluids A 3, 47 (1991)]. Results show that stresses are accurately predicted for a range of solids fractions, coefficients of restitution, and friction coefficients by both the hard-particle-with-overlap and soft-particle models so long as a sufficiently large loading stiffness is used for the soft-particle model. Additional results investigating the accuracy of the collision models and the amount of collisional overlap are presented as functions of the simulation time step and model parameters. PMID:16089735

Ketterhagen, William R; Curtis, Jennifer S; Wassgren, Carl R

2005-06-30

124

Two-Dimensional Simulation of the Breakup Process of Aggregates in Shear and Elongational Flows.

A modified discrete element method in which the hydrodynamic contribution is taken into account is proposed to simulate the deformation and breakup process of coagulated particles in two-dimensional shear and elongational flows. The simulation was performed for aggregates of various sizes, constitutive particles and fractal dimensions, and the followings were found: (i) the average number of particles in broken fragments * is related with the intensity of flow field Gamma by ~ Gamma-P, where the value of P for aggregates of fractal dimension 1.8 is about 0.86 in the shear flow and about 1.0 in the elongational flow, (ii) aggregates are fragmented in the same fashion if their fractal dimension is the same, and a scaling law for fragmentation will hold if their fractal dimension, particle number and ratio of the minimum gap between neighboring particles to the particle size are the same among aggregates, (iii) aggregates in flow fields are broken by splitting into the smaller fragments but not by eroding particles one by one from their surface, and (iv) the elongational flow is more effective to break up aggregates than the shear flow under usual flow conditions. Copyright 1998 Academic Press. PMID:9698410*

Higashitani; Iimura

1998-08-15

125

Langevin dynamics simulations of a two-dimensional colloidal crystal under confinement and shear.

Langevin dynamics simulations are used to study the effect of shear on a two-dimensional colloidal crystal (with implicit solvent) confined by structured parallel walls. When walls are sheared very slowly, only two or three crystalline layers next to the walls move along with them, while the inner layers of the crystal are only slightly tilted. At higher shear velocities, this inner part of the crystal breaks into several pieces with different orientations. The velocity profile across the slit is reminiscent of shear banding in flowing soft materials, where liquid and solid regions coexist; the difference, however, is that in the latter case the solid regions are glassy while here they are crystalline. At even higher shear velocities, the effect of the shearing becomes smaller again. Also the effective temperature near the walls (deduced from the velocity distributions of the particles) decreases again when the wall velocity gets very large. When the walls are placed closer together, thereby introducing an incommensurability between the periodicity of the confined crystal and the walls, a structure containing a soliton staircase arises in simulations without shear. Introducing shear increases the disorder in these systems until no solitons are visible anymore. Instead, similar structures like in the case without mismatch result. At high shear rates, configurations where the incommensurability of the crystalline structure is compensated by the creation of holes become relevant. PMID:23005095

Wilms, D; Virnau, P; Sengupta, S; Binder, K

2012-06-18

126

NASA Astrophysics Data System (ADS)

In this study, the evolution of the flow stress for grain sizes ranging from about 16 to 2 ?m under shear deformation was simulated using two-dimensional discrete dislocation dynamics. The analyses were confined to a single slip system and to the collective behaviour of a large number of edge dislocations, modelled as line defects in elastic medium. A superposition technique, combined with boundary element method, was used to obtain the solution resulting from the dislocation microstructures and kinematic boundary conditions. The long-range interactions of dislocations were fully accounted for with the multi-pole algorithm without introducing an artificial cut-off radius. The dynamic behaviour of the dislocations, including lattice resistance to dislocation motion, dislocation nucleation and annihilation, were described by a set of constitutive rules in the simulation. Flow stress values increased with decreasing grain size and correlated with grain size in the form of classical Hall-Petch relationship (d)-1/2. However, a similar correlation was also observed between the flow stress and grain size in the form of (d)-1. The flow stress values for different grain sizes unified to a single curve when expressed as a function of the dislocation density normalized by the grain size. It was observed that dislocation pile-ups can both activate neighbouring dislocation sources and also shut down the active dislocation sources.

Biner, S. B.; Morris, J. R.

2002-11-01

127

Two-dimensional nonsteady viscous flow simulation on the Navier-Stokes computer miniNode

The needs of large-scale scientific computation are outpacing the growth in performance of mainframe supercomputers. In particular, problems in fluid mechanics involving complex flow simulations require far more speed and capacity than that provided by current and proposed Class VI supercomputers. To address this concern, the Navier-Stokes Computer (NSC) was developed. The NSC is a parallel-processing machine, comprised of individual Nodes, each comparable in performance to current supercomputers. The global architecture is that of a hypercube, and a 128-Node NSC has been designed. New architectural features, such as a reconfigurable many-function ALU pipeline and a multifunction memory-ALU switch, have provided the capability to efficiently implement a wide range of algorithms. Efficient algorithms typically involve numerically intensive tasks, which often include conditional operations. These operations may be efficiently implemented on the NSC without, in general, sacrificing vector-processing speed. To illustrate the architecture, programming, and several of the capabilities of the NSC, the simulation of two-dimensional, nonsteady viscous flows on a prototype Node, called the miniNode, is presented. 8 references.

Nosenchuck, D.M.; Littman, M.G.; Flannery, W.

1986-01-01

128

We have constructed a lattice Wigner-Weyl code to expand the Buot-Jensen algorithm to calculation of electron transport in two-dimensional cylindrically symmetric structures. Almost all of the numerical simulations to date have dealt with the restricted problem of one-dimensional transport. In real devices, electrons are not confined to a single transport dimension and the coulombic potential is fully present and felt in three dimensions. We show the derivation of the 2D equation in cylindrical coordinates as well as approximations employed in the calculation of the four-dimensional convolution integral of the Wigner function and the potential. We work under the assumption that longitudinal transport is more dominant than radial transport and employ parallel processing techniques. The total transport is calculated in two steps: (1) transport the particles in the longitudinal direction in each shell separately, then (2) each shell exchanges particles with its nearest neighbor. Most of this work is concerned with the former step: A 1D space and 2D momentum transport problem. Time evolution simulations based on these method are presented for three different cases. Each case lead to numerical results consistent with expectations. Discussions of future improvements are discussed.

Recine, Greg [Applied Electronics Laboratory, Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, NJ 07030 (United States)]. E-mail: gjr5y@virginia.edu; Rosen, Bernard [Applied Electronics Laboratory, Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, NJ 07030 (United States); Cui, H.-L. [Applied Electronics Laboratory, Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, NJ 07030 (United States)

2005-11-01

129

We present two-dimensional MHD simulations of the evolution of a young Type\\u000aIa supernova remnant during its interaction with an interstellar cloud of\\u000acomparable size at impact. We include for the first time in such simulations\\u000aexplicit relativistic electron transport, including spectral information using\\u000aa simple but effective scheme that follows their acceleration at shocks and\\u000asubsequent transport. From this

Byung-Il Jun; T. W. Jones

1998-01-01

130

NASA Astrophysics Data System (ADS)

The structure and dynamics of diffuse gas in the Milky Way and other disk galaxies may be strongly influenced by thermal and magnetorotational instabilities (TI and MRI, respectively) on scales ~1-100 pc. We initiate a study of these processes, using two-dimensional numerical hydrodynamic and magnetohydrodynamic simulations with conditions appropriate for the atomic interstellar medium (ISM). Our simulations incorporate thermal conduction and adopt local ``shearing-periodic'' equations of motion and boundary conditions to study dynamics of a (100 pc) 2 radial-vertical section of the disk. We demonstrate, consistent with previous work, that nonlinear development of ``pure TI'' produces a network of filaments that condense into cold clouds at their intersections, yielding a distinct two-phase warm/cold medium within ~20 Myr. TI-driven turbulent motions of the clouds and warm intercloud medium are present but saturate at quite subsonic amplitudes for uniform initial P/k=2000 K cm -3. MRI has previously been studied in near-uniform media; our simulations include both TI+MRI models, which begin from uniform-density conditions, and cloud+MRI models, which begin with a two-phase cloudy medium. Both the TI+MRI and cloud+MRI models show that MRI develops within a few galactic orbital times, just as for a uniform medium. The mean separation between clouds can affect which MRI mode dominates the evolution. Provided intercloud separations do not exceed half the MRI wavelength, we find the MRI growth rates are similar to those for the corresponding uniform medium. This opens the possibility that if low cloud volume filling factors increase MRI dissipation times compared to those in a uniform medium, then MRI-driven motions in the ISM could reach amplitudes comparable to observed H I turbulent line widths.

Piontek, Robert A.; Ostriker, Eve C.

2004-02-01

131

NASA Astrophysics Data System (ADS)

Argon Z-pinch experiments are to be performed on the refurbished Z machine (which we will refer to as ZR here in order to distinguish between pre-refurbishment Z) at Sandia National Laboratories with a new 8 cm diameter double-annulus gas puff nozzle constructed by Alameda Applied Sciences Corporation (AASC). The gas exits the nozzle from an outer and inner annulus and a central jet. The amount of gas present in each region can be varied. Here a two-dimensional radiation MHD (2DRMHD) model, MACH2-TCRE, with tabular collisional radiative equilibrium atomic kinetics is used to theoretically investigate stability and K-shell emission properties of several measured (interferometry) initial gas distributions emanating from this new nozzle. Of particular interest is to facilitate that the distributions employed in future experiments have stability and K-shell emission properties that are at least as good as the Titan nozzle generated distribution that was successfully fielded in earlier experiments on the Z machine before it underwent refurbishment. The model incorporates a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray-trace based radiation transport. This level of detail is necessary in order to model opacity effects, non-local radiation effects, and the high temperature state of K-shell emitting Z-pinch loads. Comparisons of radiation properties and stability of measured AASC gas profiles are made with that of the distribution used in the pre-refurbished Z experiments. Based on these comparisons, an optimal K-shell emission producing initial gas distribution is determined from among the AASC nozzle measured distributions and predictions are made for K-shell yields attainable from future ZR experiments.

Thornhill, J. W.; Giuliani, J. L.; Chong, Y. K.; Velikovich, A. L.; Dasgupta, A.; Apruzese, J. P.; Jones, B.; Ampleford, D. J.; Coverdale, C. A.; Jennings, C. A.; Waisman, E. M.; Lamppa, D. C.; McKenney, J. L.; Cuneo, M. E.; Krishnan, M.; Coleman, P. L.; Madden, R. E.; Elliott, K. W.

2012-09-01

132

A two-dimensional electromagnetic particle-in-cell simulation with the realistic ion-to-electron mass ratio of 1836 is carried out to investigate the electrostatic collisionless shocks in relatively high-speed (approx3000 km s{sup -1}) plasma flows and also the influence of both electrostatic and electromagnetic instabilities, which can develop around the shocks, on the shock dynamics. It is shown that the electrostatic ion-ion instability can develop in front of the shocks, where the plasma is under counterstreaming condition, with highly oblique wave vectors as was shown previously. The electrostatic potential generated by the electrostatic ion-ion instability propagating obliquely to the shock surface becomes comparable with the shock potential and finally the shock structure is destroyed. It is also shown that in front of the shock the beam-Weibel instability gradually grows as well, consequently suggesting that the magnetic field generated by the beam-Weibel instability becomes important in long-term evolution of the shock and the Weibel-mediated shock forms long after the electrostatic shock vanished. It is also observed that the secondary electrostatic shock forms in the reflected ions in front of the primary electrostatic shock.

Kato, Tsunehiko N.; Takabe, Hideaki [Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871 (Japan)

2010-03-15

133

NASA Astrophysics Data System (ADS)

We have carried out molecular dynamics (MD) simulations for monolayers of smectic A and C liquid crystal (LC) phases in order to investigate the in-plane molecular diffusion from the microscopic point of view. In contrast to similar complex two-dimensional systems (e.g., biomembranes) whose molecular diffusion is anomalous, in-plane mean square displacements (MSDs) for both phases increase linearly with passing time similar to typical fluids on the nanosecond time scale. By following the relation between the diffusion and the viscosity in the fluids, we estimated the viscosity coefficients for both LC monolayers, and the obtained values indicate that the smectic A monolayer has a higher viscosity than the smectic C one. Moreover, we investigate the in-plane self-diffusion anisotropy D\\|/D\\bot for smectic C and found that the diffusion parallel to the molecular tilt is 1.5 times larger than that in the perpendicular direction. This anisotropic diffusion property in the smectic C monolayer has not been clearly confirmed thus far.

Watanabe, Go; Saito, Jun-ichi; Fujita, Yusuke; Tabe, Yuka

2013-08-01

134

NASA Astrophysics Data System (ADS)

Two-dimensional (2D) symmetric flapping flight is investigated by an immersed boundary-lattice Boltzmann method (IB-LBM). In this method, we can treat the moving boundary problem efficiently on the Cartesian grid. We consider a model consisting of 2D symmetric flapping wings without mass connected by a hinge with mass. Firstly, we investigate the effect of the Reynolds number in the range of 40-200 on flows around symmetric flapping wings under no gravity field and find that for high Reynolds numbers (Re ? 55), asymmetric vortices with respect to the horizontal line appear and the time-averaged lift force is induced on the wings, whereas for low Reynolds numbers (Re ? 50), only symmetric vortices appear around the wings and no lift force is induced. Secondly, the effect of the initial position of the wings is investigated, and the range of the initial phases where the upward flight is possible is found. The effects of the mass and flapping amplitude are also studied. Finally, we carry out free flight simulations under gravity field for various Reynolds numbers in the range 60 ? Re ? 300 and Froude numbers in the range 3 ? Fr ? 60 and identify the region where upward flight is possible.

Ota, Keigo; Suzuki, Kosuke; Inamuro, Takaji

2012-08-01

135

Simulation of a two-dimensional model for colloids in a uniaxial electric field.

We perform Monte Carlo simulations of a simplified two-dimensional model for colloidal hard spheres in an external uniaxial ac electric field. Experimentally, the external field induces dipole moments in the colloidal particles, which in turn form chains. We therefore approximate the system as composed of well-formed chains of dipolar hard spheres of a uniform length. The dipolar interaction between colloidal spheres gives rise to an effective interaction between the chains, which we treat as disks in a plane, that includes a short-range attraction and long-range repulsion. Hence, the system favors finite clustering over bulk phase separation, and indeed we observe at low temperature and density that the system does form a cluster phase. As the density increases, percolation is accompanied by a pressure anomaly. The percolated phase, despite being composed of connected, locally crystalline domains, does not bear the typical signatures of a hexatic phase. At very low densities, we find no indication of a "void phase" with a cellular structure seen recently in experiments. PMID:21867166

Almudallal, Ahmad M; Saika-Voivod, Ivan

2011-07-25

136

As a generic model system for phase separation in polymer solutions, a coarse-grained model for hexadecane/carbon dioxide mixtures has been studied in two-dimensional geometry. Both the phase diagram in equilibrium (obtained from a finite size scaling analysis of Monte Carlo data) and the kinetics of state changes caused by pressure jumps (studied by large scale molecular dynamics simulations) are presented. The results are compared to previous work where the same model was studied in three-dimensional geometry and under confinement in slit geometry. For deep quenches the characteristic length scale ?(t) of the formed domains grows with time t according to a power law close to [Formula: see text]. Since in this problem both the polymer density ?(p) and the solvent density ?(s) matter, the time evolution of the density distribution P(L)(?(p),?(s),t) in L × L subboxes of the system is also analyzed. It is found that in the first stage of phase separation the system separates locally into low density carbon dioxide regions that contain no polymers and regions of high density polymer melt that are supersaturated with this solvent. The further coarsening proceeds via the growth of domains of rather irregular shapes. A brief comparison of our findings with results of other models is given. PMID:22301356

Reith, Daniel; Bucior, Katarzyna; Yelash, Leonid; Virnau, Peter; Binder, Kurt

2012-02-03

137

NASA Astrophysics Data System (ADS)

As a generic model system for phase separation in polymer solutions, a coarse-grained model for hexadecane/carbon dioxide mixtures has been studied in two-dimensional geometry. Both the phase diagram in equilibrium (obtained from a finite size scaling analysis of Monte Carlo data) and the kinetics of state changes caused by pressure jumps (studied by large scale molecular dynamics simulations) are presented. The results are compared to previous work where the same model was studied in three-dimensional geometry and under confinement in slit geometry. For deep quenches the characteristic length scale ?(t) of the formed domains grows with time t according to a power law close to \\ell (t)\\propto \\sqrt{t}. Since in this problem both the polymer density ?p and the solvent density ?s matter, the time evolution of the density distribution PL(?p,?s,t) in L × L subboxes of the system is also analyzed. It is found that in the first stage of phase separation the system separates locally into low density carbon dioxide regions that contain no polymers and regions of high density polymer melt that are supersaturated with this solvent. The further coarsening proceeds via the growth of domains of rather irregular shapes. A brief comparison of our findings with results of other models is given.

Reith, Daniel; Bucior, Katarzyna; Yelash, Leonid; Virnau, Peter; Binder, Kurt

2012-03-01

138

Simulation of a two-dimensional model for colloids in a uniaxial electric field

NASA Astrophysics Data System (ADS)

We perform Monte Carlo simulations of a simplified two-dimensional model for colloidal hard spheres in an external uniaxial ac electric field. Experimentally, the external field induces dipole moments in the colloidal particles, which in turn form chains. We therefore approximate the system as composed of well-formed chains of dipolar hard spheres of a uniform length. The dipolar interaction between colloidal spheres gives rise to an effective interaction between the chains, which we treat as disks in a plane, that includes a short-range attraction and long-range repulsion. Hence, the system favors finite clustering over bulk phase separation, and indeed we observe at low temperature and density that the system does form a cluster phase. As the density increases, percolation is accompanied by a pressure anomaly. The percolated phase, despite being composed of connected, locally crystalline domains, does not bear the typical signatures of a hexatic phase. At very low densities, we find no indication of a “void phase” with a cellular structure seen recently in experiments.

Almudallal, Ahmad M.; Saika-Voivod, Ivan

2011-07-01

139

Two-dimensional simulation of a low-current dielectric barrier discharge in atmospheric helium

A two-dimensional computational study is presented to unravel radial structure of a dielectric barrier discharge in atmospheric helium when the gas voltage exceeds slightly the breakdown voltage and the discharge current is low to retain a repetitive dynamic pattern of one discharge event every half cycle of the applied voltage. Simulation results reveal that during each half cycle of the applied voltage gas breakdown occurs first in a central region around the electrode axis. After it is extinguished, a second breakdown is triggered in the boundary region near the radial edge of the two electrodes as confirmed by the dynamic evolution of the radial profile of the electric field, the current density and the charged particles. These predictions are consistent with relevant experimental observations in literature. It is also shown that an increase in the applied voltage or in the excitation frequency reduces the time delay between the two breakdown events and the difference between their corresponding current densities. This offers a route to improve the uniformity of atmospheric dielectric barrier discharges for their intended applications.

Zhang Yuantao; Wang Dezhen; Kong, Michael G. [State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Department of Physics, Dalian University of Technology, Dalian (China); Department of Electronic and Electrical Engineering, Loughborough University, Leicestershire LE11 3TU (United Kingdom)

2005-12-01

140

Simulation of Two Dimensional Ultraviolet (2DUV) Spectroscopy of Amyloid Fibrils

Revealing the structure and aggregation mechanism of amyloid fibrils is essential for the treatment of over 20 diseases related to protein misfolding. Coherent two dimensional (2D) infrared spectroscopy is a novel tool that provides a wealth of new insight into the structure and dynamics of biomolecular systems. Recently developed ultrafast laser sources are extending multidimensional spectroscopy into the ultraviolet (UV) region, and this opens up new opportunities for probing fibrils. In a simulation study, we show that 2DUV spectra of the backbone of a 32-residue ?-amyloid (A?9–40) fibril associated with Alzheimer’s disease, and two intermediate prefibrillar structures carry characteristic signatures of fibril size and geometry that could be used to monitor its formation kinetics. The dependence of these signals on the fibril size and geometry is explored. We demonstrate that the dominant features of the ?-amyloid fibril spectra are determined by intramolecular interactions within a single A?9–40, while intermolecular interactions at the “external interface” have clear signatures in the fine details of these signals.

Jiang, Jun; Abramavicius, Darius; Falvo, Cyril; Bulheller, Benjamin M.; Hirst, Jonathan D.; Mukamel, Shaul

2010-01-01

141

In the following work, we analyze one-dimensional (1D) and two-dimensional (2D) full particle-in-cell simulations of stimulated Raman scattering (SRS) and study the evolution of Langmuir waves (LWs) in the kinetic regime. It is found that SRS reflectivity becomes random due to a nonlinear frequency shift and that the transverse modulations of LWs are induced by (i) the Weibel instability due to the current of trapped particles and (ii) the trapped particle modulational instability (TPMI) [H. Rose, Phys. Plasmas 12, 12318 (2005)]. Comparisons between 1D and 2D cases indicate that the nonlinear frequency shift is responsible for the first saturation of SRS. After this transient interval of first saturation, 2D effects become important: a strong side-scattering of the light, caused by these transverse modulations of the LW and the presence of a nonlinear frequency shift, is observed together with a strong transverse diffusion. This leads to an increase of the Landau damping rate of the LW, contributing to the limiting of Raman backscattering. A model is developed that reproduces the transverse evolution of the magnetic field due to trapped particles. Based on a simple 1D hydrodynamic model, the growth rate for the Weibel instability of the transverse electrostatic mode and magnetic field is estimated and found to be close to the TPMI growth rate [H. Rose et al., Phys. Plasmas 15, 042311 (2008)].

Masson-Laborde, P. E.; Casanova, M.; Loiseau, P. [CEA, DAM, DIF, F-91297 Arpajon (France); Rozmus, W.; Peng, Z. [Department of Physics, Theoretical Physics Institute, University of Alberta, Edmonton, Alberta T6G 2G7 (Canada); Pesme, D.; Hueller, S.; Chapman, T. [Centre de Physique Theorique, CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex (France); Bychenkov, V. Yu. [P. N. Lebedev Physics Institute, Russian Academy of Science, Leninskii pr. 53, 11991 Moscow (Russian Federation)

2010-09-15

142

Two-Dimensional Blast-Wave-Driven Rayleigh-Taylor Instability: Experiment and Simulation

NASA Astrophysics Data System (ADS)

This paper shows results from experiments diagnosing the development of the Rayleigh-Taylor instability with two-dimensional initial conditions at an embedded, decelerating interface. Experiments are performed at the Omega Laser and use ~5 kJ of energy to create a planar blast wave in a dense, plastic layer that is followed by a lower density foam layer. The single-mode interface has a wavelength of 50 ?m and amplitude of 2.5 ?m. Some targets are supplemented with additional modes. The interface is shocked then decelerated by the foam layer. This initially produces the Richtmyer-Meshkov instability followed and then dominated by Rayleigh-Taylor growth that quickly evolves into the nonlinear regime. The experimental conditions are scaled to be hydrodynamically similar to SN1987A in order to study the instabilities that are believed to occur at the He/H interface during the blast-wave-driven explosion phase of the star. Simulations of the experiment were performed using the FLASH hydrodynamics code.

Kuranz, C. C.; Drake, R. P.; Harding, E. C.; Grosskopf, M. J.; Robey, H. F.; Remington, B. A.; Edwards, M. J.; Miles, A. R.; Perry, T. S.; Blue, B. E.; Plewa, T.; Hearn, N. C.; Knauer, J. P.; Arnett, D.; Leibrandt, D. R.

2009-05-01

143

Anisotropic propagation of cardiac action potential was studied using a two-dimensional hexagonal model which consists of single cells interconnected by gap junctional conduction (ggap) with the six neighbouring cells. To examine the structural difference between the models for propagation properties, a one-dimensional model and a two-dimensional lattice model were also constructed. They have two and four gap connections, respectively. The

M. Shirakawa; T. Izawa; S. Usui; A. Taniguchi; T. Anno; J. Toyama

1992-01-01

144

MHD Simulations of Spheromaks and HIT-SI

NASA Astrophysics Data System (ADS)

The PSI Center's tetrahedral mesh MHD equilibrium code, PSI-TET, has been upgraded to solve the time-dependent ideal and resistive MHD equations with either bare conductor or insulated conductor boundary conditions. This poster will describe the numerical methods for solving both the linear and non-linear MHD equations and the numerical technique for implementing the insulated conductor boundary condition. Solutions will be presented for: (1.) Ideal and resistive MHD simulations of a cylindrical spheromak tilting mode with both bare conductor and insulated conductor boundary conditions, which are compared to highlight the effect of an insulated conducting wall and: (2.) Resistive MHD simulations of an inductively formed and sustained spheromak in the HIT-SI experiment with insulated conducting walls and specified time varying injector fluxes and currents.

Marklin, George; Hansen, Chris; Jarboe, Tom

2011-11-01

145

Two-dimensional model simulations of the QBO in ozone and tracers in the tropical stratosphere

NASA Astrophysics Data System (ADS)

Meteorological data from the United Kingdom Meteorological Office (UKMO) and constituent data from the Upper Atmospheric Research Satellite (UARS) are used to construct yearly zonal mean dynamical fields for the 1990s for use in the NASA/Goddard Space Flight Center (GSFC) two-dimensional (2-D) chemistry and transport model. This allows for interannual dynamical variability to be included in the model constituent simulations. In this study, we focus on the tropical stratosphere. We find that the phase of quasi-biennial oscillation (QBO) signals in equatorial CH4 and profile and total column O3 data are resolved quite well using this empirically based 2-D model transport framework. However, the QBO amplitudes in the model constituents are systematically underestimated relative to the observations at most levels. This deficiency is probably due in part to the limited vertical resolutions of the 2-D model and the UKMO and UARS input data sets. We find that using different heating rate calculations in the model affects the interannual and QBO amplitudes in the constituent fields, but has little impact on the phase. Sensitivity tests reveal that the QBO in transport dominates the ozone interannual variability in the lower stratosphere, with the effect of the temperature QBO being dominant in the upper stratosphere via the strong temperature dependence of the ozone loss reaction rates. We also find that the QBO in odd nitrogen radicals, which is caused by the QBO modulated transport of NOy, plays a significant but not dominant role in determining the ozone QBO variability in the middle stratosphere. The model mean age of air is in good overall agreement with that determined from tropical lower-middle stratospheric OMS balloon observations of SF6 and CO2. The interannual variability of the equatorial mean age in the model increases with altitude and maximizes near 40 km, with a range of 4-5 years over the 1993-2000 time period.

Fleming, Eric L.; Jackman, Charles H.; Rosenfield, Joan E.; Considine, David B.

2002-12-01

146

Two-dimensional FSI simulation of closing dynamics of a tilting disc mechanical heart valve

The fluid dynamics during valve closure resulting in high shear flows and large residence times of particles has been implicated in platelet activation and thrombus formation in mechanical heart valves. Our previous studies with bi-leaflet valves have shown that large shear stresses induced in the gap between the leaflet edge and the valve housing results in relatively high platelet activation levels whereas flow between the leaflets results in shed vortices not conducive to platelet damage. In this study we compare the result of closing dynamics of a tilting disc valve with that of a bi-leaflet valve. The two-dimensional fluid-structure interaction analysis of a tilting disc valve closure mechanics is performed with a fixed grid Cartesian mesh flow solver with local mesh refinement, and a Lagrangian particle dynamic analysis for computation of potential for platelet activation. Throughout the simulation the flow remains in the laminar regime and the flow through the gap width is marked by the development of a shear layer which separates from the leaflet downstream of the valve. Zones of re-circulation are observed in the gap between the leaflet edge and the valve housing on the major orifice region of the tilting disc valve and are seen to be migrating towards the minor orifice region. Jet flow is observed at the minor orifice region and a vortex is formed which sheds in the direction of fluid motion as observed in experiments using PIV measurements. The activation parameter computed for the tilting disc valve, at the time of closure was found to be 2.7 times greater than that of the bi-leaflet mechanical valve and was found to be in the vicinity of the minor orifice region mainly due to the migration of vortical structures from the major to the minor orifice region during the leaflet rebound of the closing phase.

Govindarajan, V.; Udaykumar, H.S.; Herbertson, L. H.; Deutsch, S.; Manning, K. B.; Chandran, K.B.

2010-01-01

147

Simulation of melting of two-dimensional Lennard-Jones solids

NASA Astrophysics Data System (ADS)

We study the nature of melting of a two-dimensional (2D) Lennard-Jones solid using large-scale Monte Carlo simulation. We use systems of up to 102 400 particles to capture the decay of the correlation functions associated with translational order (TO) as well as the bond-orientational (BO) order. We study the role of dislocations and disclinations and their distribution functions. We computed the temperature dependence of the second moment of the TO parameter (?G) as well as of the order parameter ?6 associated with BO order. By applying finite-size scaling of these second moments, we determined the anomalous dimension critical exponents ?(T) and ?6(T) associated with power-law decay of the ?G and ?6 correlation functions. We also computed the temperature-dependent distribution of the order parameters ?G and ?6 on the complex plane that supports a two-stage melting with a hexatic phase as an intermediate phase. From the correlation functions of ?G and ?6, we extracted the corresponding temperature-dependent correlation lengths ?(T) and ?6(T). The analysis of our results leads to a consistent picture strongly supporting a two-stage melting scenario as predicted by the Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY) theory where melting occurs via two continuous phase transitions, first from solid to a hexatic fluid at temperature Tm, and then from the hexatic fluid to an isotropic fluid at a critical temperature Ti. We find that ?(T) and ?6(T) have a distinctly different temperature dependence, each diverging at different temperature, and that their finite-size scaling properties are consistent with the KTHNY theory. We also used the temperature dependence of ? and ?6 and their theoretical bounds to provide estimates for the critical temperatures Tm and Ti, which can also be estimated using the Binder ratio. Our results are within error bars, the same as those extracted from the divergence of the correlation lengths.

Wierschem, Keola; Manousakis, Efstratios

2011-06-01

148

This work explores the use of a tree tensor network ansatz to simulate the ground state of a local Hamiltonian on a two-dimensional lattice. By exploiting the entropic area law, the tree tensor network ansatz seems to produce quasiexact results in systems with sizes well beyond the reach of exact diagonalization techniques. We describe an algorithm to approximate the ground

L. Tagliacozzo; G. Evenbly; G. Vidal

2009-01-01

149

A new decision support system has been developed for integrated flood management within the framework of ArcGIS based on realistic two dimensional flood simulations. This system has the ability to interact with and use classified Remote Sensing (RS) image layers and other GIS feature layers like zoning layer, survey database and census block boundaries for flood damage calculations and loss

Honghai Qi; M. S. Altinakar

2011-01-01

150

MHD Simulations of Disruptions in NSTX

NASA Astrophysics Data System (ADS)

Research tokamaks such as ITER must be designed to tolerate a limited number of disruptions without sustaining significant damage. It is therefore vital to have numerical tools that can accurately predict the effects of these events. The 3D nonlinear extended MHD code M3D [1] has been augmented with models of the vacuum/halo region and a thin axisymmetric resistive shell that allow it to simulate disruptions and calculate the associated wall currents and forces [2]. Its reliability, however, must be assessed with careful validation studies against disruption databases from existing experiments. Here we compare M3D VDE/kink disruption calculations with data from NSTX. The results of high-resolution numerical simulations at realistic Lundquist numbers show reasonable agreement with experimental data and provide confidence that M3D will be a useful tool for future ITER calculations. The effects of different choices of plasma outflow boundary conditions will also be reported. [4pt] [1] W. Park, et al., Phys. Plasmas 6 (1999) 1796.[0pt] [2] H.R. Strauss, et al., Phys. Plasmas 17 (2010) 082505.

Breslau, J. A.; Strauss, H. R.; Paccagnella, R.; Jardin, S. C.

2011-11-01

151

The substorm current wedge in MHD simulations

NASA Astrophysics Data System (ADS)

Using magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the build-up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail. Three different scenarios are considered: the propagation of magnetic flux ropes of artificially reduced entropy (bubbles), and the formation and propagation of bubbles resulting from magnetic reconnection in the near and far tail. The simulations confirm the important role of the entropy reduction in the earthward penetration of bubbles, as well as in the build-up of field-aligned current signatures attributed to the SCW. Low-entropy flow channels can indeed propagate close to the Earth from the distant tail, as suggested recently. However, this requires substantial entropy reduction, presumably from progression of reconnection into the lobes. The major SCW and pressure build-up occurred when the low-entropy flow channels were braked and the flow diverted azimuthally in the near-Earth region. The flows commonly exhibit multiple narrow channels, separated in space and time, whereas the associated increases in Bz (dipolarization) accumulate over a wider spatial range, spreading both azimuthally and radially. This suggests a picture of the SCW as being composed of multiple smaller "wedgelets," rather than one big wedge.

Birn, J.; Hesse, M.

2013-06-01

152

Extended MHD NIMROD Simulations of HIT-SI plasmas

NASA Astrophysics Data System (ADS)

We present results from zero ? two-fluid MHD (2fl-MHD) NIMROD calculations of the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI). HIT-SI uses two semi-toroidal helicity injectors oscillated out of phase to generate and sustain toroidal plasmas via steady inductive helicity injection (SIHI). All the plasma-facing walls of the experiment are coated with an insulating material to guarantee an inductive discharge. The helicity injectors are simulated as oscillating normal magnetic and parallel electric field boundary conditions with odd toroidal symmetry. A highly resistive edge-layer approximates the insulating walls. The Prandtl number (Pm = 10), and Lundquist number (S = 10^3-10^4) closely match the experimental values. 2fl-MHD calculations produce more toroidal current (Itor) and faster growth rates than their resistive MHD (rMHD) counterparts. An energetics analysis indicates 2fl-MHD dynamo channels more energy into the axisymmetric mode than the MHD dynamo. The simulation results show good agreement with internal and surface magnetic measurements. Singular value decomposition indicates the calculations mostly capture the spatial eigenmode structure of the experiment. Simulation output is also comped with chord-averaged ion velocities.

Akcay, Cihan; Kim, Charlson; Jarboe, Tom; Nelson, Brian

2012-10-01

153

We have developed a coupled two-dimensional dynamical\\/chemical\\/microphysical model to study the global distribution of stratospheric sulfate aerosols. In particular, we use this model to simulate the global distribution of volcanic aerosols after the eruption of El Chichón in Mexico in April 1982. The simulated background aerosol distributions are highly dispersed, while a slight latitudinal gradient is also noticed. The calculated

XueXi Tie; Xing Lin; Guy Brasseur

1994-01-01

154

Computer simulation of two-dimensional electrolyte flow in electrochemical machining

A mathematical model and a method for numerical modeling of two-dimensional electrolyte flow in the interelectrode gap during electrochemical machining (ECM) are presented. Programs for modeling electrolyte flow and observing the distribution of ECM parameters are designed. The modeling results are compared with experimental data on continuous ECM and ECM with a vibrating tool.

L. Dabrowski; T. Paczkowski

2005-01-01

155

This report describes the numerical procedure used to implement the Green`s function method for solving the Poisson equation in two-dimensional Cartesian coordinates. The procedure can determine the solution to a problem with any or all of applied voltage boundary conditions, dielectric media, floating (insulated) conducting media, dielectric surface charging, periodic (reflective) boundary conditions, and volumetric space charge. The numerical solution

Riley

1998-01-01

156

A numerical simulation model for the study of magnetohydrodynamic (MHD) waves in a structured medium

NASA Astrophysics Data System (ADS)

This dissertation contains two basic parts: a formal development of a numerical simulation model for the study of magnetohydrodynamic (MHD) waves in a structured medium, and an application of the model to the investigation of the propagation of MHD waves in a magnetic slab and their interactions with non-magnetic surroundings. The numerical model is a time-dependent, two-dimensional, and nonlinear MHD model with gravity and radiative energy loss. The corresponding numerical code is based on the newly developed SINIL (Semi-Implicit-Non-Iterative-Lagrangian) scheme. The MHD governing equations are discretized on a Lagrangian rid, using the control-volume method. The gas dynamic properties are solved explicitly, and the magnetic field is solved implicitly without using numerical iterations. Using this numerical model, three kinds of slab waves are studied, namely, kink type slab waves, sausage type slab waves, and kink type single interface waves (which are considered as kink type slab waves in the limit of infinite slab width). In the present study, external acoustic waves can only be excited by internal body waves. The excitation of external acoustic waves represents the energy leakage from the internal magnetized region to the external field-free region. The condition of the excitation is the relative compressibility (the ratio of the internal body wave speed to the external sound speed) being greater than one.

Xiao, Yingcai

157

A plasma discharge due to a sustaining pulse of an alternating current plasma display panel was analyzed using a two-dimensional particle-in-cell code, and basic characteristics of the plasma discharge were calculated. The characteristics of the plasma discharge due to a sustaining pulse are as follows. (i) A large amount of space charge remained after drawing the discharge current. This excess

Y. Ikeda; K. Suzuki; H. Fukumoto; J. P. Verboncoeur; P. J. Christenson; C. K. Birdsall; M. Shibata; M. Ishigaki

2000-01-01

158

A two-dimensional dispersion module for the TOUGH2 simulator

A standard model for hydrodynamic dispersion has been added to TOUGH2--the dispersion model, intended for use with the EOS7 fluid properties module, accounts for the effects of hydrodynamic dispersion and molecular diffusion in two-dimensional rectangular domains. Because the model requires Darcy velocity and species concentration gradient vectors at all connections, known vector components (perpendicular to the grid block interfaces) from

C. M. Oldenburg; K. Pruess

1993-01-01

159

Two-dimensional simulation of streamers using the FE-FCT algorithm

The improved finite-element flux-corrected transport method (FE-FCT), developed by the authors, has been applied in its full two-dimensional form to the numerical solution of streamer development and propagation in air at atmospheric pressure. The numerical algorithm used employs the FE-FCT method for the solution of transport equations of charged species under the action of space-charge electric field, with the field

G E Georghiou; R Morrow; A C Metaxas

2000-01-01

160

Two-dimensional Green`s function Poisson solution appropriate for cylindrical-symmetry simulations

This report describes the numerical procedure used to implement the Green`s function method for solving the Poisson equation in two-dimensional (r,z) cylindrical coordinates. The procedure can determine the solution to a problem with any or all of the applied voltage boundary conditions, dielectric media, floating (insulated) conducting media, dielectric surface charging, and volumetric space charge. The numerical solution is reasonably

Riley

1998-01-01

161

Numerical simulations of Hall MHD small-scale dynamos

NASA Astrophysics Data System (ADS)

Much of the progress in our understanding of dynamo mechanisms, has been made within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect eventually becomes non-negligible. We present results from three dimensional simulations of the Hall-MHD equations subjected to random non-helical forcing. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter, using a pseudospectral code to achieve exponentially fast convergence.

Gómez, Daniel O.; Mininni, Pablo D.; Dmitruk, Pablo

2010-11-01

162

NASA Astrophysics Data System (ADS)

Transient, numerical simulations of the de-icing of composite aircraft components by electrothermal heating were performed for a two dimensional rectangular geometry. The implicit Crank-Nicolson formulation was used to insure stability of the finite-difference heat conduction equations and the phase change in the ice layer was simulated using the Enthalpy method. The Gauss-Seidel point iterative method was used to solve the system of difference equations. Numerical solutions illustrating de-icer performance for various composite aircraft structures and environmental conditions are presented. Comparisons are made with previous studies. The simulation can also be used to solve a variety of other heat conduction problems involving composite bodies.

Chao, D. F. K.

1983-11-01

163

Two-dimensional Vlasov simulations of nonlinear electron plasma waves are presented, in which the interplay of linear and nonlinear kinetic effects is evident. The plasma wave is created with an external traveling wave potential with a transverse envelope of width Deltay such that thermal electrons transit the wave in a ``sideloss'' time, tsl~Deltay\\/ve. Here, ve is the electron thermal velocity. The

J. W. Banks; R. L. Berger; S. Brunner; B. I. Cohen; J. A. F. Hittinger

2011-01-01

164

Two-dimensional simulation and measurement of high-performance MOSFET's made on a very thin SOI film

Thinning effects on the device characteristics of silicon-on-insulator (SOI) MOSFETs are discussed. Two-dimensional\\/two-carrier device simulation revealed the following advantages. An n-channel MOSFET with 500-A-SOI thickness exhibited a high-punchthrough resistance as well as an improved subthreshold swing down to a deep submicrometer region, even if the film was nearly intrinsic. A capacitance coupling model has been proposed to explain these subthreshold

Makoto Yoshimi; Hiroaki Hazama; Minoru Takahashi; Shigeru Kambayashi; Tetsunori Wada; H. Tango

1989-01-01

165

High-temperature steam electrolysis (HTSE) is a promising method for highly efficient large-scale hydrogen production. The HTSE process not only reduces the amount of thermodynamic electrical energy requirement but also decreases the polarization losses, which improves the overall efficiency of hydrogen production.In this paper, a two-dimensional simulation method of the efficiency of the HTSE system integrated with high-temperature gas-cooled nuclear reactor

Liu Mingyi; Yu Bo; Xu Jingming; Chen Jing

2008-01-01

166

Two-dimensional axisymmetric particle-in-cell simulations with Monte Carlo collision calculations (PIC-MCC) have been conducted to investigate argon microplasma characteristics of a miniature inductively coupled plasma source with a 5-mm-diameter planar coil, where the radius and length are 5 mm and 6 mm, respectively. Coupling the rf-electromagnetic fields to the plasma is carried out based on a collisional model and a kinetic

Yoshinori Takao; Naoki Kusaba; Koji Eriguchi; Kouichi Ono

2010-01-01

167

Two-dimensional particle-in-cell simulations of laser-plasma interaction using a plane-wave geometry show strong bursty stimulated Brillouin backscattering, rapid filamentation, and subsequent plasma cavitation. It is shown that the cavitation is not induced by self-focusing. The electromagnetic fields below the plasma frequency that are excited are related to transient soliton-like structures. At the origin of these solitons is a three-wave decay process

C. Riconda; S. Weber; V. T. Tikhonchuk; J.-C. Adam; A. Heron

2006-01-01

168

. Surface Evolver simulations of flowing two-dimensional foams are described. These are used for two purposes. Firstly, to extract\\u000a the location of the T\\u000a 1 s, the changes in bubble topology that occur during plastic flow. It is shown that in linear Couette flow the T\\u000a 1 s are localized in space, becoming more so as the polydispersity of the foam

A. Wyn; I. T. Davies; S. J. Cox

2008-01-01

169

NASA Astrophysics Data System (ADS)

The effect of shock impingement on the mixing and combustion of a reacting shear-layer is numerically simulated. Hydrogen fuel is injected at sonic velocity behind a backward facing step in a direction parallel to a supersonic freestream vitiated with H2O. The two-dimensional Navier-Stokes equations are solved and explicitly coupled to a chemistry 'package' employing a global, two-step combustion model. The results show that shock impingement enhances the mixing and combustion.

Domel, N. D.; Thompson, D. S.

1991-01-01

170

The National Aero-Space Plane (NASP) was conceptually designed for flight at hypersonic speeds. Because its airframe configuration was determined by the need for good high-speed flight performance, its low-speed and ground effect characteristics were poor. The purpose of this investigation was to characterize the ground effect aerodynamic performance of a two-dimensional model of the NASP with thrust simulation. This study

Robert E. Bond

1997-01-01

171

Numerical simulations of thermal conductivity in dissipative two-dimensional Yukawa systems.

Numerical data on the heat transfer constants in two-dimensional Yukawa systems were obtained. Numerical study of the thermal conductivity and diffusivity was carried out for the equilibrium systems with parameters close to conditions of laboratory experiments with dusty plasma. For calculations of heat transfer constants the Green-Kubo formulas were used. The influence of dissipation (friction) on the heat transfer processes in nonideal systems was investigated. The approximation of the coefficient of thermal conductivity is proposed. Comparison of the obtained results to the existing experimental and numerical data is discussed. PMID:22680584

Khrustalyov, Yu V; Vaulina, O S

2012-04-04

172

Heat transfer coefficients in two-dimensional Yukawa systems (numerical simulations)

NASA Astrophysics Data System (ADS)

New data on heat transfer in two-dimensional Yukawa systems have been obtained. The results of a numerical study of the thermal conductivity for equilibrium systems with parameters close to the conditions of laboratory experiments in dusty plasma are presented. The Green-Kubo relations are used to calculate the heat transfer coefficients. The influence of dissipation (internal friction) on the heat transfer processes in nonideal systems is studied. New approximations are proposed for the thermal conductivity and diffusivity for nonideal dissipative systems. The results obtained are compared with the existing experimental and numerical data.

Khrustalyov, Yu. V.; Vaulina, O. S.

2013-05-01

173

Two-dimensional fully kinetic simulations are performed using global boundary conditions relevant to model the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys Plasmas 4, 1936 (1997)]. The geometry is scaled in terms of the ion kinetic scales in the experiment, and a reconnection layer is created by reducing the toroidal current in the flux cores in a manner similar to the actual experiment. The ion-scale features in these kinetic simulations are in remarkable agreement with those observed in MRX, including the reconnection inflow rate and quadrupole field structure. In contrast, there are significant discrepancies in the simulated structure of the electron layer that remain unexplained. In particular, the measured thickness of the electron layers is 3?5 times thicker in MRX than in the kinetic simulations. The layer length is highly sensitive to downstream boundary conditions as well as the time over which the simulation is driven. However, for a fixed set of chosen boundary conditions, an extrapolation of the scaling with the ion to electron mass ratio implies that at realistic mass ratio both the length and width will be too small compared to the experiment. This discrepancy implies that the basic electron layer physics may differ significantly between MRX and the two-dimensional, collisionless simulations. The two leading possibilities to explain the discrepancy are weak Coulomb collisions and three-dimensional effects that are present in the experiment but not included in the simulation model.

S. Dorfman, W. Daughton, V. Roytershteyn, H. Ji, Y. Ren, and M. Yamada

2008-11-06

174

Simulation of Charged Particle Diffusion in MHD plasmas

Magnetohydrodynamical simulations of turbulent plasmas have been performed to study the transport of energetic test particles. Several parameters of the underlying MHD simulation have been varied to gain insight into the main processes governing transport. Here also the distinct effects of wave-particle resonance and field line wandering shall be studied.

F. Spanier; M. Wisniewski

2011-01-01

175

NASA Astrophysics Data System (ADS)

The National Aero-Space Plane (NASP) was conceptually designed for flight at hypersonic speeds. Because its airframe configuration was determined by the need for good high-speed flight performance, its low-speed and ground effect characteristics were poor. The purpose of this investigation was to characterize the ground effect aerodynamic performance of a two-dimensional model of the NASP with thrust simulation. This study included a comparison between experimental and numerical results. The experiments were conducted in the WVU subsonic wind tunnel, while numerical results were obtained using FLUENT, a commercial finite volume CFD code. The aerodynamic characteristics of the two-dimensional NASP model were reported as functions of various parameters such as: model configuration, angle of attack, ground plane proximity and thrust coefficient. In this research program, it was found that the characteristic ground effect lift loss, associated with previously studied three-dimensional hypersonic models operating at low, subsonic speed was related to that measured with a two-dimensional model. In an attempt to improve low-speed take-off and landing performance, the simulated engine exhaust was ducted to the upper aft surface and to the trailing edge of the model in two different configurations. Those two in addition to the base configuration were modeled using CFD to compare with experimental data. The test results demonstrated that exhaust ducting can be used to achieve dramatically enhanced lift coefficient characteristics while in ground effect. Of these configurations, the use of external ducting yielded the most favorable ground effect characteristics for the two-dimensional geometry. Additionally, this was the only configuration which produced positive lift in the take-off configuration. The computational fluid dynamic code used for this research failed to produce accurate coefficient data; however, it was found to estimate the lift curve slopes of each of the configurations with reasonable accuracy. Additionally, trends in the lift, drag and pressure coefficients were also predicted with reasonable accuracy.

Bond, Robert E.

176

Reconnection in a dipole-dominated magnetosphere: A two-dimensional model

We study the problem of onset of reconnection and the corresponding nonlinear evolution of reconnection in the near-Earth plasma sheet, where the influence of the Earth's dipole field is essential. This is done by the help of two-dimensional resistive MHD simulations. The simulations start from a two-dimensional equilibrium model for the near-Earth plasma sheet, including the Earth's dipole field explicitly.

H. Wiechen; J. Buechner; A. Otto

1995-01-01

177

NASA Astrophysics Data System (ADS)

Boundary conditions (BCs) play an essential role in lattice Boltzmann (LB) simulations. This paper investigates several most commonly applied BCs by evaluating the relative L2-norm errors of the LB simulations for two-dimensional (2-D) Poiseuille flow. It is found that the relative L2-norm error resulting from FHML's BC is smaller than that from other BCs as a whole. Then, based on the FHML's BC, it formulates an LB model for simulating fluid flows in 2-D channel with complex geometries. Afterwards, the flows between two inclined plates, in a pulmonary blood vessel and in a blood vessel with local expansion region, are simulated. The numerical results are in good agreement with the analytical predictions and clearly show that the model is effective. It is expected that the model can be extended to simulate some real biologic flows, such as blood flows in arteries, vessels with stenosises, aneurysms and bifurcations, etc.

Wen, Bing-Hai; Liu, Hai-Yan; Zhang, Chao-Ying; Wang, Qiang

2009-10-01

178

The numerical MHD simulation of solar flares

NASA Astrophysics Data System (ADS)

The 3D MHD calculations carried out above the active region AR 0365 before the flare on May 27, 2003 show solar flare energy accumulation with current sheet (CS) creation in the singular line (SL) vicinity. The maximal radio-emission intensity measured with the SSRT radio telescope (Irkutsk) coincides with the current density maximum in CS. The obtained results confirm the solar flare electrodynamical model and open the possibility for improving the solar flare prognosis.

Podgorny, A. I.; Podgorny, I. M.; Meshalkina, N. S.

2008-02-01

179

The numerical simulation of two-dimensional fluid flow with strong shocks

Many numerical schemes have been devised for the simulation of compressible gas dynamics on digital computers. One major difficulty regarding such a simulation is related to the representation of the shock and contact discontinuities which arise in these simulations. Von Neumann and Richtmyer (1950) have proposed a solution to this problem which with minor modifications is still in general use

P. Woodward; P. Colella

1984-01-01

180

Two-dimensional particle-in-cell simulations of transport in a magnetized electronegative plasma

Particle transport in a uniformly magnetized electronegative plasma is studied in two-dimensional (2D) geometry with insulating (dielectric) boundaries. A 2D particle-in-cell (PIC) code is employed, with the results compared to analytic one-dimensional models that approximate the end losses as volume losses. A modified oxygen reaction set is used to scale to the low densities used in PIC codes and also to approximately model other gases. The principal study is the limiting of the transverse electron flow due to strong electron magnetization. The plasma in the PIC calculation is maintained by axial currents that vary across the transverse dimension. For a cosine current profile nearly uniform electron temperature is obtained, which at the B-fields studied (600-1200 G) give a small but significant fraction (0.25 or less) of electron to negative ion transverse loss. For a more transverse-confined current, and approximating the higher mass and attachment reaction rate of iodine, the fraction of electron to negative ion transverse loss can be made very small. The models which have been constructed reasonably approximate the PIC results and indicate that the cross-field transport is nearly classical.

Kawamura, E.; Lichtenberg, A. J.; Lieberman, M. A. [Department of Electrical Engineering, University of California, Berkeley, California 94720-1770 (United States)

2010-11-15

181

NASA Astrophysics Data System (ADS)

A plasma discharge due to a sustaining pulse of an alternating current plasma display panel was analyzed using a two-dimensional particle-in-cell code, and basic characteristics of the plasma discharge were calculated. The characteristics of the plasma discharge due to a sustaining pulse are as follows. (i) A large amount of space charge remained after drawing the discharge current. This excess space charge did not contribute to wall charge formation or ultraviolet radiation. (ii) The electron energy distribution function was evaluated and could be well fitted by the Druyvesteyn distribution in the high-energy region. The Druyvesteyn distribution was a consequence of the cross section for electron-Ne elastic scattering. (iii) The calculated ultraviolet radiation efficiency ? of the plasma discharge due to a sustaining pulse was between 5.51% and 30.7%. Examination of the sensitivity of the efficiency to the electron temperature showed that reducing the electron temperature was a key to improving the efficiency. (iv) A detailed understanding of the conditions for a stable plasma discharge, memory margin, firing voltage, and electrode voltage of the sustaining pulse were obtained from the voltage transfer curve. The voltage transfer curve could be used to optimize the discharge cell design.

Ikeda, Y.; Suzuki, K.; Fukumoto, H.; Verboncoeur, J. P.; Christenson, P. J.; Birdsall, C. K.; Shibata, M.; Ishigaki, M.

2000-12-01

182

A two-dimensional dispersion module for the TOUGH2 simulator

A standard model for hydrodynamic dispersion has been added to TOUGH2- The dispersion model, intended for use with the EOS7 fluid properties module, accounts for the effects of hydrodynamic dispersion and molecular diffusion in two-dimensional rectangular domains. Because the model.requires Darcy velocity and species concentration gradient vectors at all connections, known vector components (perpendicular to the grid block interfaces) from neighboring connections are interpolated to form the unknown components (parallel to the grid block interfaces) at each connection. Thus the dispersive fluxes depend not only on the primary variables of the two connected grid blocks but on all p variables of the six neighbor grid blocks of each interface. This gives rise to added terms in the Jacobian matrix relative to standard TOUGH2 where fluxes depend only on primary variables in the two connected grid blocks. For flexibility in implementing boundary conditions, the model allows the user to define a flow domain that is a subset of the calculation domain. The PARAM and SELEC blocks of the TOUGH2 input file are used to specify parameters and boundary condition options for the dispersion module. The dispersion module has been verified by comparing computed results to analytical solutions. As an introduction to applications, we demonstrate the solution of a difficult twodimensional flow problem with variable salinity and strong coupling between dispersive and advective flow.

Oldenburg, C.M.; Pruess, K.

1993-09-01

183

Two dimensional self-consistent fluid simulation of rf inductive sources

The two-dimensional (R - Z) electromagnetic code FMRZ has been written to model inductive sources self-consistently in time. The code models an argon plasma with momentum-transfer, excitation and ionization as electron-neutral reactions and scattering and charge-exchange for the ion-neutral reactions. The electrons and ions are treated as Maxwellian fluid species and a reduced set of Maxwell`s equations is used to advance the electromagnetic fields. The set of equations used in FMRZ is not subject to typical numerical constraints present in many time dynamic codes allowing one to choose appropriate the and space scales to resolve only the frequencies and scale lengths of interest. The model retains nonlinear driving terms which give rise to a pondermotive force that distorts the density profile. Density and power profiles will be used to illustrate the physical effects of various terms in the equations. Trends in average density and temperature compare well with an analytic model.

DiPeso, G.; Vahedi, V.; Hewett, D.W.; Rognlien, T.D.

1993-11-17

184

Two-dimensional numerical simulations of the inertial electrostatic confinement device (IEC)

NASA Astrophysics Data System (ADS)

The theoretical works of Barnes and Nebel [1] have shown that a small ion cloud immersed in a uniform background electron density may undergo self-similar oscillations while being in local thermal equilibrium at all times. During the collapse phase of this oscillation the density and temperature of the ion cloud my reach extremely large values thus making this scheme particularly attractive for fusion application. One main purpose of the present experimental and theoretical work is to understand the stability properties of the electron background. Recent experimental results [2] indicate that the formation of a uniform electron background and a parabolic potential of up to 60% of the grid potential is possible. However increasing of the grid potential past a certain value, while keeping all other parameters unchanged, leads to the destruction of the potential well. This instability is studied with the help of the two-dimensional particle-in-cell code CELESTE2D [3]. Results from different injection schemes will be shown and their influence on the stability of the background are presented. [1] R.A. Nebel, D. C. Barnes, Fusion Technology 34, 28 (1998); Physics of Plasmas 5, 2498 (1998). [2] R.A. Nebel et al. - Phys. Plasma 12, 12701 (2005). [3] G. Lapenta, Phys. Plsmas, 6, 1442 (1999); J.Computat. Phys., 181, 317 (2002).

Evstatiev, Evstati G.

2005-10-01

185

The effect of shock impingement on the mixing and combustion of a reacting shear-layer is numerically simulated. Hydrogen fuel is injected at sonic velocity behind a backward facing step in a direction parallel to a supersonic freestream vitiated with H{sub 2}O. The two-dimensional Navier-Stokes equations are solved and explicitly coupled to a chemistry package employing a global, two-step combustion model. The results show that shock impingement enhances the mixing and combustion. 17 refs.

Domel, N.D.; Thompson, D.S. (Texas Univ., Arlington (USA))

1991-01-01

186

NASA Astrophysics Data System (ADS)

Strongly localized, magnetic field structures oriented perpendicular to two-dimensional electron gases, also known as magnetic barriers, are shifted in B space by homogeneous, perpendicular magnetic fields. The magnetoresistance across the barrier shows a characteristic asymmetric dip in the regime where the polarity of the homogeneous magnetic field is opposite to that of the magnetic barrier. The measurements are in quantitative agreement with semiclassical simulations, which reveal that the magnetoresistance originates from the interplay of snake orbits with E×B drift at the edges of the Hall bar and with elastic scattering.

Hugger, S.; Cerchez, M.; Xu, H.; Heinzel, T.

2007-11-01

187

Methods for simulation of two-dimensional signals with arbitrary power spectral densities and signal amplitude probability density functions are disclosed. The method relies on initially transforming a white noise sample set of random Gaussian distributed numbers into a corresponding set with the desired spectral distribution, after which this colored Gaussian probability distribution is transformed via an inverse transform into the desired probability distribution. In most cases the method provides satisfactory results and can thus be considered an engineering approach. Several illustrative examples with relevance for optics are given. PMID:22505116

Yura, Harold T; Hanson, Steen G

2012-04-01

188

MHD simulations with resistive wall and magnetic separatrix

A number of problems in resistive MHD magnetic fusion simulations describe plasmas with three regions: the core, the halo region, and the resistive boundary. Treating these problems requires maintenance of an adequate resistivity contrast between the core and halo. This can be helped by the presence of a magnetic separatrix, which in any case is required for reasons of realistic

H. R. Strauss; A. Pletzer; Stephen C. Jardin; J. Breslau; L. Sugiyama

2004-01-01

189

Numerical simulation of two-dimensional fluid flow with strong shocks

Results of an extensive comparison of numerical methods for simulating hydrodynamics are presented and discussed. This study focuses on the simulation of fluid flows with strong shocks in two dimensions. By ''strong shocks,'' we here refer to shocks in which there is substantial entropy production. For the case of shocks in air, we therefore refer to Mach numbers of three

P. Woodward; P. Colella

1984-01-01

190

Two-Dimensional Mixed Mode Crack Simulation Using the Material Point Method

The material point method (MPM) has demonstrated its capabilities in the simulation of impact\\/contact\\/penetration and interfacial crack growth problems. Because of the use of material points in the description of a continuum, consistent with the particle description (atoms) using molecular dynamics (MD), it is natural to couple MPM with MD for simulation from atomistic to continuum levels. However, in addressing

B. Wang; V. Karuppiah; H. Lu; R. Komanduri; S. Roy

2005-01-01

191

NASA Astrophysics Data System (ADS)

We performed simulations using two-dimensional EVH-1(Enhanced Virginia Hydrodynamics One) hydrodynamics and radial ray MGFLD (multigroup flux-limited diffusion) neutrino transport code in order to simulate doubly diffusive instabilities below the neutrinospheres of core collapse supernova progenitors. The results of these simulations are compared with the results of our semi-analytical investigation (Bruenn, S. W., Raley, E. A., and Mezzacappa, A., ApJ in press), which indicate that below the neutrinosphere a new type of instability, we call Lepto-Entropy Fingers, is present and may play a role in the supernova mechanism. This work was partially funded by a grant from the DOE Office of Science, Scientific Discovery though Advanced Computing Program.

Raley, E.; Bruenn, S.

2004-12-01

192

NASA Astrophysics Data System (ADS)

Two-dimensional optical model for simulation of thin-film solar cells with periodical textured interfaces is presented. The model is based on finite element method and uses triangular discrete elements for the structure description. The advantages of the model in comparison to other existing models are highlighted. After validation and verification of the developed simulator, simulations of a microcrystalline silicon solar cell with a sinusoidal grating texture applied to the interfaces are carried out. The analysis and optimization of the two grating parameters-period and height of the grooves-are performed with respect to the maximal short-circuit current density of the cell. Up to 45% increase in the current density is identified for the optimized structure, compared to that of the cell with flat interfaces. Optical losses in the periodically textured silver back reflector are determined.

?ampa, Andrej; Kr?, Janez; Topi?, Marko

2009-04-01

193

The fluid flow and heat transfer of a liquid metal past a circular cylinder in a rectangular duct (width-to-height aspect ratio of 2) under a strong transverse magnetic field is studied numerically using a quasi-two-dimensional model. Transition from steady to unsteady flow regimes is determined as a function of Hartmann number and blockage ratio, as are Strouhal number, and the

Wisam K. Hussam; Mark C. Thompson; Gregory J. Sheard

2011-01-01

194

A two-dimensional domain decomposition technique for the simulation of quantum-scale devices

NASA Astrophysics Data System (ADS)

The simulation of realistically sized devices under the Non-Equilibrium Greens Function (NEGF) formalism typically requires prohibitive amounts of memory and computation time. In order to meet the rising computational challenges associated with quantum-scale device simulation we offer a 2-D domain decomposition technique. This technique is applicable to a large class of atomistic and spatial simulation problems. Considering a decomposition along both the cross section and length of the device, the framework presented in this work ensures efficient distribution of both memory and computation based upon the underlying device structure. As an illustration we stably generate the density of states and transmission, under the NEGF formalism, for the atomistic-based simulation of square 5 nm cross section silicon nanowires consisting of over one million atomic orbitals.

Cauley, Stephen; Balakrishnan, Venkataramanan; Klimeck, Gerhard; Koh, Cheng-Kok

2012-02-01

195

Monte Carlo-molecular dynamics simulations for two-dimensional magnets

A combined Monte Carlo-molecular dynamics simulation technique is used to study the dynamic structure factor on a square lattice for isotropic Heisenberg and planar classical ferromagnetic spin Hamiltonians.

Kawabata, C.; takeuchi, M.; Bishop, A.R.

1985-01-01

196

Reactive dynamics on two-dimensional supports: Monte Carlo simulations and mean-field theory

Monte Carlo simulations and mean-field models are used for the study of nonequilibrium reactions taking place on the surface of a catalyst. The model represents the catalytic reduction of NO with H2 on a Pt surface. Both Monte Carlo simulations and mean-field results predict the existence of a critical surface in the parameter space where the catalyst remains active for

G. Kalosakas; A. Provata

2001-01-01

197

Computer simulations of two-dimensional and three-dimensional ideal grain growth.

We developed an efficient computation scheme for the phase-field simulation of grain growth, which allows unlimited number of the orientation variables and high computational efficiency independent of them. Large-scale phase-field simulations of the ideal grain growth in two-dimensions (2D) and three-dimensions (3D) were carried out with holding the coalescence-free condition, where a few tens of thousands grains evolved into a few thousand grains. By checking the validity of the von Neumann-Mullins law for individual grains, it could be shown that the present simulations were correctly carried out under the conditions of the ideal grain growth. The steady-state grain size distribution in 2D appeared as a symmetrical shape with a plateau slightly inclined to the small grain side, which was quite different from the Hillert 2D distribution. The existence of the plateau stems from the wide separation of the peaks in the size distributions of the grains with five, six, and seven sides. The steady-state grain size distribution in 3D simulation of the ideal grain growth appeared to be very close to the Hillert 3D distribution, independent of the initial average grain size and size distribution. The mean-field assumption, the Lifshitz-Slyozov stability condition, and all resulting predictions in the Hillert 3D theory were in excellent agreement with the present 3D simulation. Thus the Hillert theory can be regarded as an accurate description for the 3D ideal grain growth. The dependence of the growth rate in 3D simulations on the grain topology were discussed. The large-scale phase-field simulation confirms the 3D growth law obtained from the Surface Evolver simulations in smaller scales. PMID:17280076

Kim, Seong Gyoon; Kim, Dong Ik; Kim, Won Tae; Park, Yong Bum

2006-12-27

198

Integrated two-dimensional simulations of dynamic hohlraum driven inertial fusion capsule implosions

NASA Astrophysics Data System (ADS)

Simulations have been useful for improving the design of dynamic hohlraums for the purpose of imploding inertial fusion capsules [S. A. Slutz, J. E. Bailey, G. A. Chandler et al., Phys. Plasmas 10, 1875 (2003)]. These design changes, which have resulted in capsule implosions with hot dense cores [J. E. Bailey, G. A. Chandler, S. A. Slutz et al., Phys. Rev. Lett. 92, 085002 (2004)] and the production of thermonuclear neutrons [C. L. Ruiz, G. Cooper, S. A. Slutz et al., Phys. Rev. Lett. 93, 015001 (2005)], were based primarily on a series of one-dimensional numerical simulations, which treated the dynamic hohlraum and the capsule implosion separately. In this paper we present simulations which are fully integrated to include the implosion of wire arrays onto foam convertors, the implosion of the capsule imbedded in the foam, and the absorption of radiation into the electrodes. These simulations yield predictions that are in remarkably good agreement with measured values considering the complexity of the problem, which spans more than 100 ns of wire implosion with the subsequent capsule implosion on a few ns timescale. For example, the predicted neutron yields are less than a factor of 2 higher than the measured values, while the predicted shock velocity is about 30% higher than the measured value. The spectroscopically inferred imploded capsule gas core temperatures are somewhat lower than predicted by the simulations, while the gas densities are about a factor of 2 higher. Simulations indicate that a more slowly rising radiation drive temperature yields higher core densities and lower temperatures and thus better agreement with experimental measurements. Possible reasons for a more slowly rising radiation drive are discussed.

Slutz, S. A.; Peterson, K. J.; Vesey, R. A.; Lemke, R. W.; Bailey, J. E.; Varnum, W.; Ruiz, C. L.; Cooper, G. W.; Chandler, G. A.; Rochau, G. A.; Mehlhorn, T. A.

2006-10-01

199

Laser heating of finite two-dimensional dust clusters: B. Simulations

NASA Astrophysics Data System (ADS)

Laser heating of monolayer dusty plasmas is investigated theoretically by Langevin dynamics simulations. The laser radiation pressure is used to externally control the dust temperature without changing the plasma properties. We show that the laser scanning pattern has a major influence on both the velocity distribution function and the stationary structure of the cluster. Furthermore, the heating effect is found to be enhanced when the laser spots move with slightly higher frequencies than the trap frequency. The simulations confirm that a proper thermodynamic excitation of the dust particles is possible.

Thomsen, Hauke; Kählert, Hanno; Bonitz, Michael; Schablinski, Jan; Block, Dietmar; Piel, Alexander; Melzer, André

2012-02-01

200

MHD solution of interplanetary disturbances generated by simulated velocity perturbations

An MHD time-dependent numerical simulation, restricted to the solar equatorial plane, is used to demonstrate the interplanetary disturbances caused by several simplified coronal holes. Each 'hole' is assumed to have a configuration such that the higher solar wind velocity produced within their longitudinal extent is Gaussian over a 7-day period at the inner boundary (0.3 AU) of the numerical simulation.

Murray Dryer; R. S. Steinolfson

1976-01-01

201

MHD Simulation of Magnetically Driven, HyperVelocity Flyer Plates

The intense magnetic field generated in the 20 MA Z-machine is used to accelerate flyer plates to high velocity for EOS experiments. Velocities up to 20 km\\/s have been achieved for aluminum (Al) flyers. We are using magneto-hydrodynamic (MHD) simulation, in conjunction with highly accurate velocity interferometry measurements (error < 1understand flyer dynamics. We present comparisons of simulation results with

R. W. Lemke; M. D. Knudson; C. A. Hall; J. R. Asay; M. P. Desjarlais; T. A. Haill; T. A. Mehlhorn; D. B. Hayes

2001-01-01

202

Three-dimensional MHD simulations of Ganymede’s magnetosphere

Ganymede is unique among planetary moons because it has its own magnetic field strong enough to form a magnetosphere within Jupiter’s magnetospheric environment. Here we report on our three-dimensional global magnetohydrodynamic (MHD) simulations that model the interaction between Ganymede’s magnetosphere and the corotating Jovian plasma. We use the measured field and particle properties to define our boundary conditions. Our simulations

Xianzhe Jia; Raymond J. Walker; Margaret G. Kivelson; Krishan K. Khurana; Jon A. Linker

2008-01-01

203

Two-dimensional particle-in-cell simulation of the expansion of a plasma into a rarefied medium

NASA Astrophysics Data System (ADS)

The expansion of a dense plasma through a more rarefied ionized medium has been studied by means of two-dimensional particle-in-cell simulations. The initial conditions involve a density jump by a factor of 100, located in the middle of an otherwise equally dense electron-proton plasma with uniform proton and electron temperatures of 10 eV and 1 keV, respectively. Simulations show the creation of a purely electrostatic collisionless shock together with an ion-acoustic soliton tied to its downstream region. The shock front is seen to evolve in filamentary structures consistently with the onset of the ion-ion instability. Meanwhile, an un-magnetized drift instability is triggered in the core part of the dense plasma. Such results explain recent experimental laser-plasma experiments, carried out in similar conditions, and are of intrinsic relevance to non-relativistic shock scenarios in the solar and astrophysical systems.

Sarri, G.; Murphy, G. C.; Dieckmann, M. E.; Bret, A.; Quinn, K.; Kourakis, I.; Borghesi, M.; Drury, L. O. C.; Ynnerman, A.

2011-07-01

204

In supernova remnants, the nonlinear amplification of magnetic fields upstream of collisionless shocks is essential for the acceleration of cosmic rays to the energy of the 'knee' at 10{sup 15.5} eV. A nonresonant instability driven by the cosmic ray current is thought to be responsible for this effect. We perform two-dimensional, particle-in-cell simulations of this instability. We observe an initial growth of circularly polarized nonpropagating magnetic waves as predicted in linear theory. It is demonstrated that in some cases the magnetic energy density in the growing waves can grow to at least 10 times its initial value. We find no evidence of competing modes, nor of significant modification by thermal effects. At late times, we observe saturation of the instability in the simulation, but the mechanism responsible is an artifact of the periodic boundary conditions and has no counterpart in the supernova-shock scenario.

Ohira, Yutaka; Takahara, Fumio [Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 (Japan); Reville, Brian; Kirk, John G. [Max-Planck-Institut fuer Kernphysik, Heidelberg 69029 (Germany)], E-mail: yutaka@vega.ess.sci.osaka-u.ac.jp

2009-06-10

205

NASA Astrophysics Data System (ADS)

Electrostatic structures have been observed in many regions of space plasmas, including the solar wind, the magnetosphere, and the auroral acceleration region. One possible theoretical description of some of these structures is the concept of Bernstein-Greene-Kruskal (BGK) modes, which are exact nonlinear steady-state solutions of the Vlasov-Poisson system of equations in collisionless kinetic theory. We will present latest Particle-in-Cell (PIC) simulations using exact solutions of two-dimensional (2D) BGK modes in a magnetized plasma with finite magnetic field strength [Ng, Bhattacharjee, and Skiff, Phys. Plasmas 13, 055903 (2006)] as initial conditions to study their stability. We will also present simulations using more general initial conditions, which shows that while these are not steady-state solutions, they still keep their overall structures with modulations having frequency of the order of electron cyclotron frequency. This work is supported by a National Science Foundation grant PHY-1004357.

Ng, C.; Soundararajan, S.; Yasin, E. S.

2011-12-01

206

NASA Astrophysics Data System (ADS)

This paper presents a new approach to the direct numerical simulation of particle flows. The basic idea is to use a local analytic representation valid near the particle to ``transfer'' the no-slip condition from the particle surface to the adjacent grid nodes. In this way the geometric complexity arising from the irregular relation between the particle boundary and the underlying mesh is avoided and fast solvers can be used. The results suggest that the computational effort increases very slowly with the number of particles so that the method is efficient for large-scale simulations. The focus here is on the two-dimensional case (cylindrical particles), but the same procedure, to be developed in forthcoming papers, applies to three dimensions (spherical particles). Several extensions are briefly discussed.

Takagi, S.; Og~Uz, H. N.; Zhang, Z.; Prosperetti, A.

2003-05-01

207

Simulation of Two Dimensional Electrophoresis and Tandem Mass Spectrometry for Teaching Proteomics

ERIC Educational Resources Information Center

|In proteomics, complex mixtures of proteins are separated (usually by chromatography or electrophoresis) and identified by mass spectrometry. We have created 2DE Tandem MS, a computer program designed for use in the biochemistry, proteomics, or bioinformatics classroom. It contains two simulations--2D electrophoresis and tandem mass spectrometry.…

Fisher, Amanda; Sekera, Emily; Payne, Jill; Craig, Paul

2012-01-01

208

Purpose – The purpose of this paper is to investigate the formation process of linear-shaped charge jet using the smoothed particle hydrodynamics (SPH). Different material yield models are embed to test the performance of SPH method in the simulation of explosive driven metal liner. The effects of different ignition model to the formation of metal jet have also been studied.

Yang Gang; Han Xu; Hu Dean

2011-01-01

209

New mixing algorithms to model the vertical mixing processes in stratified lakes have been developed for the Dynamic Reservoir Simulations Model, DYRESM, and have been validated using five lakes of different size, shape and wind forcing characteristics. An analysis of temperature profiles from Lake Kinneret, Canning Reservoir and Mundaring Reservoir, were used to develop a strong inverse relationship between the

P. S. Yeates; J. Imberger

2003-01-01

210

Two-dimensional particle simulation of plasma expansion between plane parallel electrodes

We simulate in two dimensions the expansion of a plasma between biased plane parallel electrodes using the particle-in-cell method. Such a plasma is frequently created in many experiments by the interaction of a pulsed laser with atomic vapor or gas stream. We describe the motion of the electrons and ions and reproduce the experimentally observed bulk drift of the plasma

Kartik Patel; V. K. Mago

1995-01-01

211

Two-dimensional simulations with the BZOHAR [B. I. Cohen, B. F. Lasinski, A. B. Langdon, and E. A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability, including the effects of ion-ion collisions and inhomogeneity. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models the collisions are functions of the local ion temperature and density, but the collisions have no velocity dependence in the first model. In the second model the collisions are also functions of the energy of the ion that is being scattered so as to represent a more physical Fokker-Planck collision operator. Collisions decorrelate the ions from the acoustic waves in SBS, which disrupts ion trapping in the acoustic wave. Nevertheless, ion trapping leading to a hot ion tail and two-dimensional physics that allows the SBS ion waves to nonlinearly scatter, remain important saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities decrease with increasing collisionality in the simulations for velocity-independent collisions and very weakly decrease for the range of Fokker-Planck collisionality considered. SBS backscatter in the presence of a spatially nonuniform plasma flow is also investigated. Simulations show that, depending on the sign of the spatial gradient of the flow relative to the backscatter, ion trapping effects that produce a nonlinear frequency shift can enhance (autoresonance) reflectivities relative to anti-autoresonant configurations, in agreement with theoretical arguments.

Cohen, B.I.; Divol, L.; Langdon, A.B.; Williams, E.A. [University of California, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551 (United States)

2006-02-15

212

Two-dimensional simulations with the BZOHAR [B.I. Cohen, B.F. Lasinski, A.B. Langdon, and E.A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability including the effects of ion-ion collisions and inhomogeneity. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities tend to decrease with increasing collisionality in the simulations. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models used the collisions are functions of the local ion temperature and density, but the collisions have no velocity dependence in the first model. In the second model, the collisions are also functions of the energy of the ion that is being scattered so as to represent a Fokker-Planck collision operator. Collisions decorrelate the ions from the acoustic waves in SBS, which disrupts ion trapping in the acoustic wave. Nevertheless, ion trapping leading to a hot ion tail and two-dimensional physics that allows the SBS ion waves to nonlinearly scatter remain robust saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. SBS backscatter in the presence of a spatially nonuniform plasma flow is also investigated. Simulations show that depending on the sign of the spatial gradient of the flow relative to the backscatter, ion trapping effects that produce a nonlinear frequency shift can enhance (auto-resonance) or decrease (anti-auto-resonance) reflectivities in agreement with theoretical arguments.

Cohen, B I; Divol, L; Langdon, A B; Williams, E A

2005-10-17

213

Two-dimensional simulations of the tsunami dynamo effect using the finite element method

NASA Astrophysics Data System (ADS)

Conductive seawater moving in the geomagnetic main field generates electromotive force in the ocean. This effect is well known as the "oceanic dynamo effect." Recently, it has been reported that tsunamis are also associated with the oceanic dynamo effect, and tsunami-induced electromagnetic field variations were actually observed on the seafloor. For instance, our research group succeeded in observing tsunami-induced magnetic variations on the seafloor in the northwest Pacific at the time of the 2011 Tohoku earthquake. In this study, we developed a time domain tsunami dynamo simulation code using the finite element method to explain the tsunami-induced electromagnetic variations observed on the seafloor. Our simulations successfully reproduced the observed seafloor magnetic variations as large as 3 nT. It was also revealed that an initial rise in the horizontal magnetic component prior to the tsunami arrival as large as 1 nT was induced by the tsunami.

Minami, Takuto; Toh, Hiroaki

2013-09-01

214

Hybrid kinetic-MHD simulations in general geometry

NASA Astrophysics Data System (ADS)

We present a hybrid kinetic-MHD model consisting of 3 species, the bulk fluid ions and electrons, and a kinetic minority hot particle species. The 3 species equations are derived from moments of the Vlasov Equation and then reduced using the usual hot particle assumption of nh<

Kim, Charlson C.; Sovinec, Carl R.; Parker, Scott E.

2004-12-01

215

Two different numerical techniques have been applied to the numerical integration of equations modelling gas discharges: a finite-difference flux corrected transport (FD-FCT) technique and a particle-in-cell (PIC) technique. The PIC technique here implemented has been specifically designed for the simulation of 2D electrical discharges using cylindrical coordinates. The development and propagation of a streamer between two parallel electrodes has been

C. Soria-Hoyo; F. Pontiga; A. Castellanos

2008-01-01

216

Direct numerical simulations of helical dynamo action: MHD and beyond

NASA Astrophysics Data System (ADS)

Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.

Gómez, D. O.; Mininni, P. D.

2004-12-01

217

Population transfer between vibrational eigenstates is important for many phenomena in chemistry. In solution, this transfer is induced by fluctuations in molecular conformation as well as in the surrounding solvent. We develop a joint electrostatic density functional theory map that allows us to connect the mixing of and thereby the relaxation between the amide I and amide II modes of the peptide building block N-methyl acetamide. This map enables us to extract a fluctuating vibrational Hamiltonian from molecular dynamics trajectories. The linear absorption spectrum, population transfer, and two-dimensional infrared spectra are then obtained from this Hamiltonian by numerical integration of the Schrodinger equation. We show that the amide I/amide II cross peaks in two-dimensional infrared spectra in principle allow one to follow the vibrational population transfer between these two modes. Our simulations of N-methyl acetamide in heavy water predict an efficient relaxation between the two modes with a time scale of 790 fs. This accounts for most of the relaxation of the amide I band in peptides, which has been observed to take place on a time scale of 450 fs in N-methyl acetamide. We therefore conclude that in polypeptides, energy transfer to the amide II mode offers the main relaxation channel for the amide I vibration. PMID:18698926

Bloem, Robbert; Dijkstra, Arend G; Jansen, Thomas la Cour; Knoester, Jasper

2008-08-01

218

Two Dimensional Schrodinger Equation

NSDL National Science Digital Library

The Two Dimensional Schrodinger Equation model simulates the time evolution of a two-dimensional wave packet as it moves towards a slit with an obstacle in it, both with variable widths. By changing three parameters via sliders provided, slit width, obstacle width, and initial position of the wave packet, different behaviors can be explored. These phenomena include interference, diffraction produced by a slit, a corner, and an obstacle, and bouncing of the wave packet. In addition, the angle of propagation for the diffracted part of the wave packet can be measured. This simulation is described by a paper in the European Journal of Physics, "A versatile applet to explore the wave behaviour of particles, " J I FernÃ¡ndez Palop, 2009 Eur. J. Phys. 30 771, which outlines the simulation and how the usefulness of the simulation has been tested in the subject of quantum physics. The Two Dimensional Schrodinger Equation model was created using the Easy Java Simulations (EJS) modeling tool. It is distributed as a ready-to-run (compiled) Java archive. Double clicking the ejs_qm_schrodinger2d.jar file will run the program if Java is installed.

Palop, Jose I.

2010-07-16

219

Two Dimensional Simulations of Plastic-Shell, Direct-Drive Implosions on OMEGA

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength ({ell} < 10) modes due to surface roughness and beam imbalance and the intermediate modes (20 {le} {ell} {le} 50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1-D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths ({lambda} {approx} {Delta}, where {Delta} is the in-flight shell thickness). The neutron-rate curves for the thinner shells have significantly lower amplitudes and a fall-off that is less steep than 1-D rates. DRACO simulation results are consistent with experimental observations.

Radha, P B; Goncharov, V N; Collins, T B; Delettrez, J A; Elbaz, Y; Glebov, V Y; Keck, R L; Keller, D E; Knauer, J P; Marozas, J A; Marshall, F J; McKenty, P W; Meyerhofer, D D; Regan, S P; Sangster, T C; Shvarts, D; Skupsky, S; Srebro, Y; Town, R J; Stoeckl, C

2004-09-27

220

Radiative MHD simulation of an Emerging Flux Region

NASA Astrophysics Data System (ADS)

We present a radiation magnetohydrodynamics (MHD) simulation of the birth of an active region. The simulation models the rise of a magnetic flux bundle from the convection zone into the solar photosphere. Observational properties of the simulation are consistent with recent, high-cadence and high spatial resolution observations of emerging flux regions taken by Hinode/SOT. Observational properties common to both simulation and observation include the hierarchical formation of progressively larger photospheric magnetic structures, the formation and disappearance of light bridges, umbral dots as well as penumbral filaments.

Cheung, C.; Rempel, M.; Title, A. M.; Schuessler, M.

2009-12-01

221

One-dimensional and Pseudo-Two-dimensional Hydrodynamic Simulations of Solar X-Ray Jets

NASA Astrophysics Data System (ADS)

We present results of one-dimensional hydrodynamic simulations of the chromospheric evaporation produced by a microflare in a large-scale loop as a model of X-ray jets. The initial conditions of the simulations are based on the observations of X-ray jets. We deposit thermal energy (~1×1028 ergs) in the corona. The deposited energy is rapidly transported to the chromosphere by conduction, which heats the dense plasma in the upper chromosphere. As a result, the gas pressure is increased and drives a strong upflow of dense, hot plasma along the magnetic loop. We found the following features of evaporation in the results of our simulations: (1) the maximum temperature of the evaporating plasma is determined by the balance between the conductive flux and the heating flux; (2) the total mass of evaporating plasma is controlled by the balance between the conductive flux and enthalpy flux; (3) the relationship between the density neva, height of energy deposition sflare, and heating rate Fh is described as neva~F4/7h/s3/7flare (4) the X-ray intensity along the evaporation-flow plasma decreases exponentially with distance from the footpoint, and that exponential intensity distribution holds from the early phase to the decay phase; (5) in the single-loop model, the temperature decreases with distance from the energy deposition site (on the other hand, a hot region is present in front of the evaporation front in the multiple-loop model); (6) we compare the physical parameters of the evaporation flow with the observations of the X-ray jet that occurred on 1992 September 3 and find that the physical parameters of evaporating plasma are similar to those of the Yohkoh-observed X-ray jet. Since these properties of the evaporation flow are similar to the observed properties of X-ray jets, we suggest that an X-ray jet is the evaporation flow produced by a flare near the footpoint of a large-scale loop. Furthermore, according to the X-ray intensity distribution along the evaporation flow, we suggest that a multiple-loop model based on the magnetic reconnection mechanism can reproduce the properties of an X-ray jet better than the single-loop model.

Shimojo, Masumi; Shibata, Kazunari; Yokoyama, Takaaki; Hori, Kuniko

2001-04-01

222

Amide I IR absorption and two-dimensional (2D) IR photon echo spectra of a model beta hairpin in aqueous solution are theoretically studied and simulated by combining semiempirical quantum chemistry calculations and molecular dynamics simulation methods. The instantaneous normal-mode analysis of the beta hairpin in solution is performed to obtain the density of states and the inverse participation ratios of the one-exciton states. The motional and exchange narrowing processes are taken into account by employing the time-correlation function theory for the linear and nonlinear response functions. Numerically simulated IR absorption and 2D spectra are then found to be determined largely by the amide I normal modes delocalized on the peptides in the two strands. The site-specific isotope-labeling effects on the IR and 2D IR spectra are discussed. The simulation results for the ideal (A17) beta hairpin are directly compared with those of the realistic 16-residue (GB1) beta hairpin from an immunoglobulin G-binding protein. It was found that the characteristic features in IR and 2D spectra of both the ideal (A17) beta hairpin and the GB1 beta hairpin are the same. The simulated IR spectrum of the GB1 beta hairpin is found to be in good agreement with experiment, which demonstrates that the present computational method is quantitatively reliable. PMID:16852448

Hahn, Seungsoo; Ham, Sihyun; Cho, Minhaeng

2005-06-16

223

Two-dimensional slope wind simulations in the finite element approximation

The hydrostatic fluid dynamics model developed at LLL has been used to simulate the development of katabatic winds. This model solves the Navier-Stokes equations in the Boussinesq approximation by the finite element method. Preliminary results indicate that to obtain physically reasonable results one has to choose unequal diffusion parameters in the horizontal (K/sub x/) and vertical (K/sub z/). The maximum velocities obtained with K/sub z/ = 1 m/sup 2//sec and K/sub x/ = 100 m/sup 2//sec are of the order of 2.5 m/sec for a slope of .2. Profiles of the downslope velocities will be presented at different points in the flow. As expected, the magnitude of the vertical diffusion coefficient K/sub z/ controls the depth of the flow which seems to increase only slightly with downhill distance, and the magnitude of the flow increases with cooling rate and slope.

Tuerpe, D.R.

1980-06-01

224

Lattice Boltzmann simulation of the rise and dissolution of two-dimensional immiscible droplets

NASA Astrophysics Data System (ADS)

We used a coupled multiphase lattice Boltzmann (LB) model to simulate the dissolution of immiscible liquid droplets in another liquid during the rising process resulting from buoyancy. It was found that there existed a terminal rise velocity for each droplet, and there was a power law relationship between the Eötvös (Eo) number and the terminal Reynolds (Re) number. Our simulation results were in agreement with the empirical correlation derived for predicting bubble rise. When more than two identical droplets rose simultaneously in a close proximity, the average terminal rise velocity was lower than that of a single droplet with the same size because of the mutual resistant interactions. The droplet trajectories at the noncentral positions were not straight because of the nonzero net horizontal forces acting on the droplets. The Damkohler (Da) and Peclet (Pe) numbers were varied to investigate the coupling between droplet size, flow field, dissolution at the interface, and solute transport. For a given Pe, increasing Da led to a higher dissolution rate. For a given Da, increasing Pe led to a higher dissolution rate. For a large Da and a small Pe, the process near the interface was diffusion limited, and the advective flow relative to the droplet resulting from droplet rise was unable to move the accumulated solute away from the interface quickly. In this case, it was favorable to split the single droplet into as many small ones as possible in order to increase the interface area per unit mass and consequently enhance the whole dissolution process. For a small Da and a large Pe, the process was dissolution limited near the interface. The mass of accumulated solute near the interface was little, so the advective flow at the top side of the droplet was able to clean the solute quickly. In this case it was favorable to keep the droplet as a single one in order to obtain a high rise velocity and consequently enhance the whole dissolution process. By studying the coupling between Da and Pe, we qualitatively proposed to construct a Da-Pe phase plane and found the interface dividing the plane into regions 1 and 2. Region 1 was the collection of points where it was favorable to break down the droplet into as many small ones as possible in order to accelerate dissolution, while region 2 was the collection of points where it was favorable to keep the droplet in a single one for the same purpose. Based on our LB simulations, we found that the interface was an increasing function of Pe. Region 1 was the portion above the interface, while region 2 was the portion below it. In real applications, if both Pe and Da are obtained, it will be easy to judge whether it is favorable to break down the droplet or not in order to accelerate dissolution by checking whether (Pe, Da) falls in region 1 or 2.

Chen, Cheng; Zhang, Dongxiao

2009-10-01

225

The effect of parametric wave phase conjugation (WPC) in application to ultrasound or acoustic waves in magnetostrictive solids has been addressed numerically by Ben Khelil et al. [J. Acoust. Soc. Am. 109, 75-83 (2001)] using 1-D unsteady formulation. Here the numerical method presented by Voinovich et al. [Shock waves 13(3), 221-230 (2003)] extends the analysis to the 2-D effects. The employed model describes universally elastic solids and liquids. A source term similar to Ben Khelil et al.'s accounts for the coupling between deformation and magnetostriction due to external periodic magnetic field. The compatibility between the isotropic constitutive law of the medium and the model of magnetostriction has been considered. Supplementary to the 1-D simulations, the present model involves longitudinal/transversal mode conversion at the sample boundaries and separate magnetic field coupling with dilatation and shear stress. The influence of those factors in a 2-D geometry on the potential output of a magneto-elastic wave phase conjugator is analyzed in this paper. The process under study includes propagation of a wave burst of a given frequency from a point source in a liquid into the active solid, amplification of the waves due to parametric resonance, and formation of time-reversed waves, their radiation into liquid, and focusing. The considered subject is particularly important for ultrasonic applications in acoustic imaging, nondestructive testing, or medical diagnostics and therapy. PMID:16419796

Voinovich, Peter; Merlen, Alain

2005-12-01

226

Confined binary two-dimensional colloidal crystals: Monte Carlo simulation of crack formation.

Binary mixtures (A, B) of colloidal particles of different sizes in two dimensions may form crystals with square lattice structure (the A-particles occupying the white sites and the B-particles the black sites of a checkerboard). Confining such a system by two parallel 'walls' a distance D apart, long-range order in the direction parallel to the walls is stabilized by 'corrugated walls' that are commensurate with the lattice structure but destabilized by structureless 'hard walls', even if there is no misfit between the strip width D and the crystal lattice spacing. The crossover to quasi-one-dimensional behavior is studied by Monte Carlo simulations, analyzing Lindemann parameters and displacement correlation functions. When D is reduced and thus a misfit created, the stress in the crystal increases up to a critical value, at which the stress jumps to much smaller values due to the formation of an (almost periodic) crack pattern. These cracks typically have a width of several particle diameters, and are mostly disordered, although sometimes small domains with hexagonal order can be identified. At very large misfits, glass-like structures appear. We discuss various methods to characterize order and disorder in such systems. PMID:21406860

Medina, Stefan; Virnau, Peter; Binder, Kurt

2010-12-21

227

Postcollapse hydrodynamics of SN 1987A - Two-dimensional simulations of the early evolution

The first few seconds of the explosion of SN 1987A are modeled here using a 2D cylindrical geometry smooth particle hydrodynamics code. The success of the explosion is determined to be sensitive to the duration of the infall, the timing of the bounce, and the subsequent neutrino heating. A semianalytical model for the initial structure of the collapsed object is used to present two simulations that differ by the mass that has been allowed to collapse into a neutron star prior to the bounce. In the case of a short initial infall, the explosion fails due to excessive cooling. For a longer initial infall, the cooling is less and the explosion is successful although relatively weak. It is shown that in this case, a successful explosion is brought about by the presence of an entropy gradient which, combined with the gravitational pull of the neutron star, leads to extremely strong instabilities. The critical importance of the global circulation for the success of the explosion is demonstrated. 43 refs.

Herant, M.; Benz, W.; Colgate, S. (Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States) Steward Observatory, Tucson, AZ (United States) Los Alamos National Laboratory, NM (United States))

1992-08-01

228

NASA Astrophysics Data System (ADS)

The parametric instability of a monochromatic shear Alfvén wave in oblique propagation with respect the ambient magnetic field is investigated in a kinetic regime, performing one-dimensional (1-D) and two-dimensional (2-D) hybrid simulations. The parallel component of the mother wave is found to be subject to a parametric decay which excites an ion-acoustic wave along the magnetic field and a backward propagating daughter shear Alfvén wave, as in the instability for a purely parallel mother wave. At the same time, the acoustic wave generation supports the acceleration of a velocity beam in the ion distribution function, due to the non-linear trapping of protons. Moreover, the instability leads to the generation of broad band oblique spectra of coupled Alfvénic and compressive modes with variable perpendicular wavevectors, and, as a consequence, the magnetic field after saturation is characterized by a strong transverse modulation.

Matteini, Lorenzo; Landi, Simone; Del Zanna, Luca; Velli, Marco; Hellinger, Petr

2010-10-01

229

We use a two-dimensional volatility basis set (2D-VBS) box model to simulate secondary organic aerosol (SOA) mass yields of linear oxygenated molecules: n-tridecanal, 2- and 7-tridecanone, 2- and 7-tridecanol, and n-pentadecane. A hybrid model with explicit, a priori treatment of the first-generation products for each precursor molecule, followed by a generic 2D-VBS mechanism for later-generation chemistry, results in excellent model-measurement agreement. This strongly confirms that the 2D-VBS mechanism is a predictive tool for SOA modeling but also suggests that certain important first-generation products for major primary SOA precursors should be treated explicitly for optimal SOA predictions. PMID:22970932

Chacon-Madrid, Heber J; Murphy, Benjamin N; Pandis, Spyros N; Donahue, Neil M

2012-09-25

230

A fast and accurate method is developed to compute the natural frequencies and scattering characteristics of arbitrary-shape two-dimensional dielectric resonators. The problem is formulated in terms of a uniquely solvable set of second-kind boundary integral equations and discretized by the Galerkin method with angular exponents as global test and trial functions. The log-singular term is extracted from one of the kernels, and closed-form expressions are derived for the main parts of all the integral operators. The resulting discrete scheme has a very high convergence rate. The method is used in the simulation of several optical microcavities for modern dense wavelength-division-multiplexed systems. PMID:15005404

Boriskina, Svetlana V; Sewell, Phillip; Benson, Trevor M; Nosich, Alexander I

2004-03-01

231

A two-dimensional simulation of tritium transport in the vadose zone at the Nevada Test site

The site of a 0.75-kiloton underground nuclear explosion, the Cambric event, was selected for the study of radionuclide transport in the hydrologic environment. Water samples from RNM-2S, a well located 91 m from Cambric, have been analyzed for tritium and other radionuclides since the initiation of pumping. Water from RNM-2S flows to Frenchman Lake via an unlined canal. Flume data indicate canal transmission losses of approximately 2m{sup 3}/day/meter of canal. To determine if infiltrating canal water might be recirculated by RNM-2S, and therefore provide an additional radionuclide input to water samples collected at RNM-2S, a two-dimensional variably saturated solute transport computer model (SATURN, Huyakorn et al., 1983) was used to simulate the movement of tritium from the canal to the water table. Results indicate that recirculated canal water has not had a significant effect on the breakthrough of tritium at RNM-2S.

Ross, W.C.; Wheatcraft, S.W.

1994-09-01

232

MHD instabilities in accretion mounds - II. 3D simulations

NASA Astrophysics Data System (ADS)

We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on neutron stars by 3D magnetohydrodynamic (MHD) simulations using the PLUTO MHD code. Our results confirm that for mounds beyond a threshold mass, instabilities form finger-like channels at the periphery, resulting in mass-loss from the magnetically confined mound. Ring-like mounds with hollow interior show the instabilities at the inner edge as well. We perform the simulations for mounds of different sizes to investigate the effect of the mound mass on the growth rate of the instabilities. We also investigate the effect of such instabilities on observables such as cyclotron resonant scattering features and timing properties of such systems.

Mukherjee, Dipanjan; Bhattacharya, Dipankar; Mignone, Andrea

2013-10-01

233

Magnetocentrifugally Driven Winds: Comparison of MHD Simulations with Theory

Stationary MHD outflows from a rotating accretion disk are investigated numerically by time-dependent axisymmetric simulations. The initial magnetic field is taken to be a split-monopole poloidal field configuration frozen into the disk. The disk is treated as a perfectly conducting, time-independent density boundary [rho(r)] in Keplerian rotation. The outflow velocity from this surface is not specified but rather is determined

G. V. Ustyugova; A. V. Koldoba; M. M. Romanova; V. M. Chechetkin; R. V. E. Lovelace

1999-01-01

234

3D MHD Simulations of Flux Rope Driven CMEs

We present a three-dimensional (3D) numerical ideal magnetohydrodynamics (MHD) model describing the time-dependent expulsion of a CME from the solar corona propagating all the way to 1 A.U.. The simulations are performed using the BATS-R-US (Block Adaptive Tree Solarwind Roe Upwind Scheme) code. We begin by developing a global steady-state model of the corona that possesses high-latitude coronal holes and

W. B. Manchester; I. Roussev; M. Opher; T. Gombosi; D. DeZeeuw; G. Toth; I. Sokolov; K. Powell

2002-01-01

235

Kinetic MHD simulation of large &'circ; tearing mode instability

NASA Astrophysics Data System (ADS)

We have developed a second-order accurate semi-implicit ?f method for kinetic MHD simulation with Lorentz force ion and fluid electron. The model has been implemented in GEM code and benchmarked on Alfvén waves, ion sound waves and whistler waves against analytical dispersion relation in a uniform plasma. We have also studied the resistive tearing mode instability by adding a resistive term in the generalized Ohm's law using the Harris sheet equilibrium. For small &'circ;, the linear growth rate and eigenmode structure are comparable with resistive MHD analysis. The Rutherford stage and saturation are demonstrated, though the simulation exhibits different behaviors than previous MHD simulations. For large &'circ;, the tearing mode develops multiple islands in the nonlinear regime and the islands start to coalesce later on. The competition between the two processes strongly influences the reconnection rates and eventually leads the reconnection to a steady state. We will identify the role played by particle ions in the process using detailed ion diagnostics.

Cheng, Jianhua; Chen, Yang; Parker, Scott; Uzdensky, Dmitri

2011-11-01

236

NASA Astrophysics Data System (ADS)

In June 2008, the state of Iowa experienced an unprecedented flood event which resulted in an economic loss of approximately $2.88 billion. Flooding in the Iowa River corridor, which exceeded the previous flood of record by 3 feet, devastated several communities, including Coralville and Iowa City, home to the University of Iowa. Recognizing an opportunity to capture a unique dataset detailing the impacts of the historic flood, the investigators contacted the National Center for Airborne Laser Mapping (NCALM), which performed an aerial Light Detection and Ranging (LiDAR) survey along the Iowa River. The survey, conducted immediately following the flood peak, provided coverage of a 60-mile reach. The goal of the present research is to develop a process by which flood extents and water surface elevations can be accurately extracted from the LiDAR data set and to evaluate the benefit of such data in calibrating one- and two-dimensional hydraulic models. Whereas data typically available for model calibration include sparsely distributed point observations and high water marks, the LiDAR data used in the present study provide broad-scale, detailed, and continuous information describing the spatial extent and depth of flooding. Initial efforts were focused on a 10-mile, primarily urban reach of the Iowa River extending from Coralville Reservoir, a United States Army Corps of Engineers flood control project, downstream through the Coralville and Iowa City. Spatial extent and depth of flooding were estimated from the LiDAR data. At a given cross-sectional location, river channel and floodplain measurements were compared. When differences between floodplain and river channel measurements were less than a standard deviation of the vertical uncertainty in the LiDAR survey, floodplain measurements were classified as flooded. A flood water surface DEM was created using measurements classified as flooded. A two-dimensional, depth-averaged numerical model of a 10-mile reach of the Iowa River corridor was developed using the United States Bureau of Reclamation SRH-2D hydraulic modeling software. The numerical model uses an unstructured numerical mesh and variable surface roughness, assigned according to observed land use and cover. The numerical model was calibrated using inundation extents and water surface elevations derived from the LiDAR data. It was also calibrated using high water marks and land survey data collected daily during the 2008 flood. The investigators compared the two calibrations to evaluate the benefit of high-resolution LiDAR data in improving the accuracy of a two-dimensional urban flood simulation.

Piotrowski, J.; Goska, R.; Chen, B.; Krajewski, W. F.; Young, N.; Weber, L.

2009-12-01

237

NASA Astrophysics Data System (ADS)

This dissertation presents original research into the melting process of a downward facing cryogenic solid hydrogen surface subject to a two dimensional axisymmetric jet impingement flow of superheated hydrogen vapor. The motivation for the study is to investigate concepts of storing rocket propellants as a solid and rapidly melting the solid for liquid propellant delivery to a rocket engine. The present study considers a more favorable liquid removal arrangement than prior (1970s) experiments which melted solid hydrogen at the bottom of a cryostat. This is a numerical study that involves computation fluid dynamic (CFD) simulation of four distinct physical phenomena: (1) melting, (2) jet impingement heat transfer (JIHT), (3) multiphase transport, and (4) film breakup/droplet formation. The volume of fluid (VOF) method is used with the V2F turbulence model in a commercial CFD Navier-Stokes solver (FLUENT) to investigate the multiphase nature of melt transport and its interaction with the vapor stream; i.e., the phenomena relevant to effective heat transfer between the vapor and the melting interface. The goal of the research is: (1) to develop a numerical method to study the problem and (2) evaluate several simple configurations to begin investigating relevant phenomena for the purpose of enhancing melting rate. Many options exist for the vapor to interact with the solid surface. The scope of this initial research is limited to a steady jet of single phase superheated hydrogen vapor at fixed jet exit conditions (T = 525 R and Re = 11,000) at a fixed jet standoff ( H/D = 1.0). Condensation/vaporization are not considered. Although film breakup/droplet formation is a phenomenon where two dimensional features evolve into three dimensional events, this phenomenon is approximated as two dimensional to allow a computationally tractable problem for this initial study. Calculations are performed validating the numerical method for melting and JIHT against known results. Validation of film breakup/droplet formation is cited in the literature. A numerical method is developed to model the four physical phenomena. Four simple configurations are evaluated and a fundamental understanding is obtained of the multiphase melt transport and vapor interaction.

Baran, Adam J.

238

NASA Astrophysics Data System (ADS)

Experiments on the excitation of counterpropagating zonal flows by the magnetohydrodynamic (MHD) method in a rotating cylindrical vessel with a conic bottom have been performed. Flows appear in a conducting fluid layer in the field of ring magnets under the action of a radial electric field. The velocity fields have been reconstructed by the particle image velocimetry (PIV) method. In the fast rotation regimes with a thin fluid layer, where the Rossby-Obukhov scale does not exceed the characteristic sizes of the vessel, the system of perturbations appears with almost immobile blocked anticyclones in the outer part of the flow and rapidly moving cyclones in the main stream. The diagram of regimes is plotted in the variables of the relative angular velocities of the averaged zonal flow and transfer of vortices about the system rotation axis. Attention is focused on the results for the regions of the diagram with slow motion of vortices with respect to the rotating coordinate system near the parameters for stationary Rossby waves (blocking of circulation). The results are compared to the results previously obtained in similar experiments using the source-sink method.

Gledzer, A. E.; Gledzer, E. B.; Khapaev, A. A.; Chkhetiani, O. G.

2013-05-01

239

NASA Astrophysics Data System (ADS)

Two metal-organic ?-octamolybdate supported compounds containing nitrogen-heterocyclic ligands, [Co(dpq)2 H2O]2[?-Mo8O26] 0.5H2O 1, (H3biim)4[?-Mo8O26] 2 (dpq = 1,10-phenanthroline-5,6-dione; H2bimm = 2,2?-biimidazole), have been hydrothermally synthesized and characterized by X-ray crystal structural analysis, elemental analysis, IR spectrum, UV-VIS DRIS, 2D-IR COS (two-dimensional infrared correlation spectroscopy), PXRD, TG analysis, simulated IR spectrum and theoretical calculations from HOMO-LUMO. X-ray crystal structural analyses reveal that compound 1 has the 1-D supramolecular structure, which is formed via hydrogen bonds between water molecules coordinated to Co2+. In compound 2, the 2-D supramolecular structure is built by aromatic-aromatic stacking interactions and H-bonds between ligands. Simulated IR spectra gained by quantum calculation are used for structure discussion, which is the first time that simulated IR results are used to reveal the relationship between bond vibrations of ?-octamolybdate compounds and their related IR peak responses. Additionally, by using 2D-IR COS, structural properties like thermal and magnetic sensitivities have also been analyzed.

Chen, Xiang-Yi; Chen, Yi-Ping; Chai, Feng; Sun, Yan-Qiong; Huang, Bi-Hua

2013-03-01

240

For some potentially useful and emerging in situ bioremediation technologies it is important to control bacterial attachment to subsurface materials during the injection of microbial cell suspensions. In this study the attachment patterns of Methylosinus trichosporium OB3b were measured after horizontal injections into a two-dimensional miniature aquifer simulator containing a wet homogeneous sand. In preliminary sand column assays, bacterial attachment to the sand was increased nearly 2 orders of magnitude compared to attachment in the presence of distilled water by raising the concentration of a pH 7.0 sodium-potassium phosphate buffer to 10 mM. The maximal concentration of attached cells was [approximately]4 [times] 10[sup 8] cells/g dry sand with both sand minicolumns and the wet sand aquifer simulator. For the latter this occurred on streamlines directly between the horizontal injector and withdrawal ports, where injection-withdrawal velocities were the highest. The effects of a simulated groundwater cross flow during suspension injection on bacterial attachment to the aquifer simulator sands were also studied, and a peristaltic pumping method to counteract these groundwater flow effects resulted in a more localized pattern, i.e., without extensive downstream skewing of the bacterial attachment zone. Phenol red was utilized as a nonbinding, red-colored tracer compound. It proved to be very convenient for quantitatively measuring the earlier breakthroughs of cells versus an inert tracer during the aquifer simulator and subsequent capillary tube cell injection experiments and also for visualizing the anticipated boundaries of cell attachment in the aquifer simulator. The effect of injection velocity on the observed bacterial attachment patterns in these experiments appears to be accounted for by colloid filtration theory. 45 refs., 9 figs.

Shonnard, D.R.; Taylor, R.T.; Hanna, M.L.; Boro, C.O.; Duba, A.G. (Lawrence Livermore National Lab., CA (United States))

1994-01-01

241

NASA Astrophysics Data System (ADS)

For some potentially useful and emerging in situ bioremediation technologies it is important to control bacterial attachment to subsurface materials during the injection of microbial cell suspensions. In this study the attachment patterns of Methylosinus trichosporium OB3b were measured after horizontal injections into a two-dimensional miniature aquifer simulator containing a wet homogeneous sand. In preliminary sand column assays, bacterial attachment to the sand was increased nearly 2 orders of magnitude compared to attachment in the presence of distilled water by raising the concentration of a pH 7.0 sodium-potassium phosphate buffer to 10 mM. The maximal concentration of attached cells was ˜4×108 cells/g dry sand with both sand minicolumns and the wet sand aquifer simulator. For the latter this occurred on streamlines directly between the horizontal injector and withdrawal ports, where injection-withdrawal velocities were the highest. The effects of a simulated groundwater cross flow during suspension injection on bacterial attachment to the aquifer simulator sands were also studied, and a peristaltic pumping method to counteract these groundwater flow effects resulted in a more localized pattern, i.e., without extensive downstream skewing of the bacterial attachment zone. Phenol red was utilized as a nonbinding, red-colored tracer compound. It proved to be very convenient for quantitatively measuring the earlier breakthroughs of cells versus an inert tracer during the aquifer simulator and subsequent capillary tube cell injection experiments and also for visualizing the anticipated boundaries of cell attachment in the aquifer simulator. The effect of injection velocity on the observed bacterial attachment patterns in these experiments appears to be accounted for by colloid filtration theory.

Shonnard, D. R.; Taylor, R. T.; Hanna, M. L.; Boro, C. O.; Duba, A. G.

1994-01-01

242

A two-dimensional (2-D) Eulerian Radiation-Magnetohydrodynamic (RMHD) code has been used to simulate imploding {ital z} pinches for three experiments fielded on the Los Alamos Pegasus II capacitor bank [J. C. Cochrane {ital et al.}, {ital Dense Z-Pinches, Third International Conference, London, United Kingdom 1993} (American Institute of Physics, New York, 1994), p. 381] and the Sandia Saturn accelerator [R. B. Spielman {ital et al.}, {ital Dense Z-Pinches, Second International Conference, Laguna Beach, 1989} (American Institute of Physics, New York, 1989), p. 3] and {ital Z} accelerator [R. B. Spielman {ital et al.}, Phys. Plasmas {bold 5}, 2105 (1998)]. These simulations match the experimental results closely and illustrate how the code results may be used to track the flow of energy in the simulation and account for the amount of total radiated energy. The differences between the calculated radiated energy and power in 2-D simulations and those from zero-dimensional (0-D) and one-dimensional (1-D) Lagrangian simulations (which typically underpredict the total radiated energy and overpredict power) are due to the radially extended nature of the plasma shell, an effect which arises from the presence of magnetically driven Rayleigh{endash}Taylor instabilities. The magnetic Rayleigh{endash}Taylor instabilities differ substantially from hydrodynamically driven instabilities and typical measures of instability development such as {ital e}-folding times and mixing layer thickness are inapplicable or of limited value. A new measure of global instability development is introduced, tied to the imploding plasma mass, termed {open_quotes}fractional involved mass.{close_quotes} Examples of this quantity are shown for the three experiments along with a discussion of the applicability of this measure. {copyright} {ital 1998 American Institute of Physics.}

Peterson, D.L.; Bowers, R.L.; McLenithan, K.D. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Deeney, C.; Chandler, G.A.; Spielman, R.B.; Matzen, M.K. [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States); Roderick, N.F. [University of New Mexico, Albuquerque, New Mexico 87131 (United States)

1998-09-01

243

NASA Astrophysics Data System (ADS)

Two-dimensional hybrid (particle ions, massless fluid electrons) simulations of quasi-parallel collisionless shocks are carried out in order to investigate the upstream wave properties, the shock re-formation process, and the downstream turbulence. The two-dimensional simulations confirm the results of earlier one-dimensional simulations. When backstreaming diffuse ions are retained re-formation of a shock with an upstream magnetic field - shock normal angle of thetaBno = 30 deg occurs as a result of upstream low-frequency waves which steepen, become pulsation-like structures and take over as the re-formed shock. The upstream waves are initially aligned with the shock normal; later in the run the waves becomes more and more aligned with the upstream magnetic field. However, when approaching the shock, the wave vectors are refracted in the region of increasing diffuse ion density into the shock normal direction so that shock re-formation is again coherent along the shock surface. In addition, re-formation on a smaller scale and out of phase along the shock front is due to more or less specularly reflected ions. Re-formation of a ThetaBno = 10 deg shock is due to locally at the shock ramp emerging waves. These are attributed to the so-called interface instability in the region of partial overlap between the incident cold solar wind and part of the hot downstream distribution. These waves emerge in phase along the shock surface and thus re-formation is in this more parallel case also coherent along the shock. At medium Alfven Mach number (MA approx. 5) shocks, upstream wave which are aligned with the upstream magnetic field are convected into the shock and produce ripples on the shock surface. At higher Mach number (MA approx. 9) the shock surface becomes less coherent and the local value of the shock normal - magnetic field angle varies greatly. The re-formation length scale is larger than in the lower Mach number case. The turbulence downstream reflects the two mechanisms of shock reformation: in the Theta(Bno) = 30 deg case the upstream pulsations are mode converted when convected through the shock layer. In the ThetaBno = 30 deg case the upstream pulsations are mode converted when convected through the shock layer. In the ThetaBno = 10 deg case the downstream turbulence results from the local instability at the shock front.

Scholer, M.; Fujimoto, M.; Kucharek, H.

1993-11-01

244

Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfven Mach numbers, ion-to-electron mass ratios, and the upstream electron {beta}{sub e} (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (M{sub A} {approx} 30) with a mass ratio of M/m = 100 and {beta}{sub e} = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.

Matsumoto, Yosuke [Department of Physics, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522 (Japan); Amano, Takanobu; Hoshino, Masahiro, E-mail: ymatumot@astro.s.chiba-u.ac.jp [Department of Earth and Planetary Science, University of Tokyo, Hongo 1-33, Bunkyo-ku, Tokyo 113-0033 (Japan)

2012-08-20

245

Two-dimensional axisymmetric particle-in-cell simulations with Monte Carlo collision calculations (PIC-MCC) have been conducted to investigate argon microplasma characteristics of a miniature inductively coupled plasma source with a 5-mm-diameter planar coil, where the radius and length are 5 mm and 6 mm, respectively. Coupling the rf-electromagnetic fields to the plasma is carried out based on a collisional model and a kinetic model. The former employs the cold-electron approximation and the latter incorporates warm-electron effects. The numerical analysis has been performed for pressures in the range 370-770 mTorr and at 450 MHz rf powers below 3.5 W, and then the PIC-MCC results are compared with available experimental data and fluid simulation results. The results show that a considerably thick sheath structure can be seen compared with the plasma reactor size and the electron energy distribution is non-Maxwellian over the entire plasma region. As a result, the distribution of the electron temperature is quite different from that obtained in the fluid model. The electron temperature as a function of rf power is in a reasonable agreement with experimental data. The pressure dependence of the plasma density shows different tendency between the collisional and kinetic model, implying noncollisional effects even at high pressures due to the high rf frequency, where the electron collision frequency is less than the rf driving frequency.

Takao, Yoshinori; Kusaba, Naoki; Eriguchi, Koji; Ono, Kouichi [Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501 (Japan)

2010-11-15

246

NASA Astrophysics Data System (ADS)

Two-dimensional axisymmetric particle-in-cell simulations with Monte Carlo collision calculations (PIC-MCC) have been conducted to investigate argon microplasma characteristics of a miniature inductively coupled plasma source with a 5-mm-diameter planar coil, where the radius and length are 5 mm and 6 mm, respectively. Coupling the rf-electromagnetic fields to the plasma is carried out based on a collisional model and a kinetic model. The former employs the cold-electron approximation and the latter incorporates warm-electron effects. The numerical analysis has been performed for pressures in the range 370-770 mTorr and at 450 MHz rf powers below 3.5 W, and then the PIC-MCC results are compared with available experimental data and fluid simulation results. The results show that a considerably thick sheath structure can be seen compared with the plasma reactor size and the electron energy distribution is non-Maxwellian over the entire plasma region. As a result, the distribution of the electron temperature is quite different from that obtained in the fluid model. The electron temperature as a function of rf power is in a reasonable agreement with experimental data. The pressure dependence of the plasma density shows different tendency between the collisional and kinetic model, implying noncollisional effects even at high pressures due to the high rf frequency, where the electron collision frequency is less than the rf driving frequency.

Takao, Yoshinori; Kusaba, Naoki; Eriguchi, Koji; Ono, Kouichi

2010-11-01

247

NASA Astrophysics Data System (ADS)

Two-dimensional scattering of elastic waves in a medium containing a circular heterogeneity is investigated with an analytical solution and numerical wave propagation simulations. Different combinations of finite difference methods (FDM) and finite element methods (FEM) are used to numerically solve the elastodynamic wave equations. Finite difference and finite element techniques are applied to approximate both the time and space derivatives and are combined in various ways to provide different numerical algorithms for modeling elastic wave propagation. The results of the different numerical algorithms are compared for simulations of an incident plane P-wave that is scattered by a mechanically weak circular inclusion whereby the diameter of the inclusion is of the same order than the P-wave's wavelength. For this scattering problem an analytical solution is available and used as the reference solution in the comparison of the different numerical algorithms. Staircase-like spatial discretization of the inclusion's circular shape with the finite difference method using a rectangular grid provides accurate velocity and displacement fields close to the inclusion boundary only for very high spatial resolutions. Implicit time integration based on either finite differences or finite elements does not provide computational advantages compared to explicit schemes. The best numerical algorithm in terms of accuracy and computation time for the investigated scattering problem consists of a finite element method in space using an unstructured mesh combined with an explicit finite difference method in time. The computational advantages and disadvantages of the different numerical algorithms are discussed.

Frehner, Marcel; Schmalholz, Stefan M.; Saenger, Erik H.; Steeb, Holger

2008-12-01

248

NASA Astrophysics Data System (ADS)

Magnetron sputtering has been commonly used for the deposition of a wide range of industrial thin film coating. This method has almost no restrictions in the target materials and the magnetic field enables lower pressures operation. Conventional flat type sputters generate ion-bombardment sputtering in only a localized region of target where electric fields and magnetic fields are perpendicular to each other. Therefore, the utilization efficiency of the target material is about 20˜30%. To overcome this drawback, a rotating cylindrical target is devised to make uniform sputtering on the target. In this paper, the difference of the physical effects of ions on the targets between the flat-type and the cylindrical-type sputters is investigated using a two-dimensional particle-in-cell (PIC) simulation with Monte Carlo collisions. Especially for the calculation of cylindrical field solver with a finite difference method, an image charge method is introduced instead of solving the Poisson equation directly. Simulation Diagnostics include the plasma density, the distributions of energy and angle of incident ions on the target, and the deposition profiles on the substrate calculated by a ray-trace particle deposition model. The analysis for the rotating speed and the magnet structure is

Hur, Min Young; Bae, Hyowon; Song, In Cheol; Lee, Ho-Jun; Lee, Hae June

2012-10-01

249

Evaluation of subgrid-scale models in large eddy simulation of flow past a two-dimensional block

NASA Astrophysics Data System (ADS)

Large eddy simulations (LES) are performed to study flow past a two-dimensional (2D) block. An immersed boundary method (IBM) is developed and implemented to model the block in the simulation. The accuracy of the IBM method and the performance of four subgrid-scale (SGS) models are examined by comparing the results with wind tunnel experimental data from the literature. The SGS models that are tested include (a) the Smagorinsky model, (b) the Lagrangian dynamc model, (c) the scale-dependent Largrangian dynamic model, and (d) the modulated gradient model. Good agreement is observed between the experiments and the results from the scale-dependent Lagrangian dynamic model and the modulated gradient model. These models are able to reproduce the mean wind and turbulence statistics around the block. Moreover, the values of the eddy viscosity coefficient and scale-dependence coefficient obtained with the Lagrangian scale-dependent dynamic model are found to have strong spatial variability. Lower values of the eddy viscosity coefficient are found in regions of the flow with higher shear in order to account for the reduced length scales of the turbulence.

Cheng, W. C.; Porté-Agel, F.

2012-04-01

250

NASA Astrophysics Data System (ADS)

Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfvén Mach numbers, ion-to-electron mass ratios, and the upstream electron ? e (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (MA ~ 30) with a mass ratio of M/m = 100 and ? e = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.

Matsumoto, Yosuke; Amano, Takanobu; Hoshino, Masahiro

2012-08-01

251

NASA Astrophysics Data System (ADS)

Starting as highly relativistic collimated jets, gamma-ray burst outflows gradually slow down and become nonrelativistic spherical blast waves. Although detailed analytical solutions describing the afterglow emission received by an on-axis observer during both the early and late phases of the outflow evolution exist, a calculation of the received flux during the intermediate phase and for an off-axis observer requires either a more simplified analytical model or direct numerical simulations of the outflow dynamics. In this paper, we present light curves for off-axis observers covering the long-term evolution of the blast wave, calculated from a high-resolution two-dimensional relativistic hydrodynamics simulation using a synchrotron radiation model. We compare our results to earlier analytical work and calculate the consequence of the observer angle with respect to the jet axis both for the detection of orphan afterglows and for jet break fits to the observational data. We confirm earlier results in the literature finding that only a very small number of local type Ibc supernovae can harbor an orphan afterglow. For off-axis observers, the observable jet break can be delayed up to several weeks, potentially leading to overestimation of the beaming-corrected total energy. In addition we find that, when using our off-axis light curves to create synthetic Swift X-ray data, jet breaks are likely to remain hidden in the data.

van Eerten, Hendrik; Zhang, Weiqun; MacFadyen, Andrew

2010-10-01

252

NASA Astrophysics Data System (ADS)

We present computer simulation study of two-dimensional infrared spectroscopy (2D-IR) of water confined in reverse micelles (RMs) of various sizes. The present study is motivated by the need to understand the altered dynamics of confined water by performing layerwise decomposition of water, with an aim to quantify the relative contributions of different layers water molecules to the calculated 2D-IR spectrum. The 0-1 transition spectra clearly show substantial elongation, due to inhomogeneous broadening and incomplete spectral diffusion, along the diagonal in the surface water layer of different sized RMs. Fitting of the frequency fluctuation correlation functions reveal that the motion of the surface water molecules is sub-diffusive and indicate the constrained nature of their dynamics. This is further supported by two peak nature of the angular analogue of van Hove correlation function. With increasing system size, the water molecules become more diffusive in nature and spectral diffusion almost completes in the central layer of the larger size RMs. Comparisons between experiments and simulations establish the correspondence between the spectral decomposition available in experiments with the spatial decomposition available in simulations. Simulations also allow a quantitative exploration of the relative role of water, sodium ions, and sulfonate head groups in vibrational dephasing. Interestingly, the negative cross correlation between force on oxygen and hydrogen of O-H bond in bulk water significantly decreases in the surface layer of each RM. This negative cross correlation gradually increases in the central water pool with increasing RMs size and this is found to be partly responsible for the faster relaxation rate of water in the central pool.

Biswas, Rajib; Furtado, Jonathan; Bagchi, Biman

2013-10-01

253

We present computer simulation study of two-dimensional infrared spectroscopy (2D-IR) of water confined in reverse micelles (RMs) of various sizes. The present study is motivated by the need to understand the altered dynamics of confined water by performing layerwise decomposition of water, with an aim to quantify the relative contributions of different layers water molecules to the calculated 2D-IR spectrum. The 0-1 transition spectra clearly show substantial elongation, due to inhomogeneous broadening and incomplete spectral diffusion, along the diagonal in the surface water layer of different sized RMs. Fitting of the frequency fluctuation correlation functions reveal that the motion of the surface water molecules is sub-diffusive and indicate the constrained nature of their dynamics. This is further supported by two peak nature of the angular analogue of van Hove correlation function. With increasing system size, the water molecules become more diffusive in nature and spectral diffusion almost completes in the central layer of the larger size RMs. Comparisons between experiments and simulations establish the correspondence between the spectral decomposition available in experiments with the spatial decomposition available in simulations. Simulations also allow a quantitative exploration of the relative role of water, sodium ions, and sulfonate head groups in vibrational dephasing. Interestingly, the negative cross correlation between force on oxygen and hydrogen of O-H bond in bulk water significantly decreases in the surface layer of each RM. This negative cross correlation gradually increases in the central water pool with increasing RMs size and this is found to be partly responsible for the faster relaxation rate of water in the central pool. PMID:24116645

Biswas, Rajib; Furtado, Jonathan; Bagchi, Biman

2013-10-14

254

Spectral Methods in General Relativistic MHD Simulations

NASA Astrophysics Data System (ADS)

In this talk I discuss the use of spectral methods in improving the accuracy of a General Relativistic Magnetohydrodynamic (GRMHD) computer code. I introduce SpecCosmo, a GRMHD code developed as a Cactus arrangement at UHCL, and show simulation results using both Fourier spectral methods and finite differencing. This work demonstrates the use of spectral methods with the FFTW 3.3 Fast Fourier Transform package integrated with the Cactus Framework to perform spectral differencing using MPI.

Garrison, David

2012-03-01

255

Simulations of MHD flows with moving interfaces

We report on the numerical simulation of a two-fluid magnetohydrodynamics problem arising in the industrial production of aluminium. The motion of the two non-miscible fluids is modeled through the incompressible Navier–Stokes equations coupled with the Maxwell equations. Stabilized finite elements techniques and an arbitrary Lagrangian–Eulerian formulation (for the motion of the interface separating the two fluids) are used in the

J.-F. Gerbeau; T. Lelièvre; C. Le Bris

2003-01-01

256

Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon

NASA Astrophysics Data System (ADS)

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.

Michioka, Takenobu; Sato, Ayumu; Takimoto, Hiroshi; Kanda, Manabu

2011-02-01

257

NASA Astrophysics Data System (ADS)

We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, ?0, we can reproduce three distinct modes of accretion flow. In model A, which has the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of mild beaming, the apparent (isotropic) photon luminosity is ~22L E (where L E is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B, which has moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than ~7 R S (where R S is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C, the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, together with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

Ohsuga, Ken; Mineshige, Shin

2011-07-01

258

NASA Astrophysics Data System (ADS)

A chemical oxidation scheme where CH3D produces HDO has been incorporated into a two-dimensional model to simulate the transport and isotopic composition chemistry of stratospheric methane and water. The model results show that deuterium ratios in water and methane are good tracers of stratospheric dynamics. Comparisons with measurements by the ATMOS instrument suggest that the modeled methane isotopic ratio is too low in the upper stratosphere and mesosphere. The very low amount of CH4 and CH3D in the middle to upper stratosphere makes the isotopic ratio for methane more sensitive to model uncertainties than the isotopic ratio of water vapor. One reason for the too low isotopic ratio values may be that the reaction rates in the oxidation of CH3D are slower than assumed. The isotopic ratio for methane was very sensitive to changes in the rate constants for the reactions of CH3D with OH, O(1D) and Cl, while the water vapor isotopic ratio only shows small changes.

Ridal, Martin; Siskind, David E.

2002-12-01

259

NASA Astrophysics Data System (ADS)

We introduce a regularization procedure to define electrostatic energies and forces in a slab system of thickness h that is periodic in two dimensions and carries a net charge. The regularization corresponds to a neutralization of the system by two charged walls and can be viewed as the extension to the two-dimensional (2D)+h geometry of the neutralization by a homogeneous background in the standard three-dimensional Ewald method. The energies and forces can be computed efficiently by using advanced methods for systems with 2D periodicity, such as MMM2D or P3M/ELC, or by introducing a simple background-charge correction to the Yeh-Berkowitz approach of slab systems. The results are checked against direct lattice sum calculations on simple systems. We show, in particular, that the Madelung energy of a 2D square charge lattice in a uniform compensating background is correctly reproduced to high accuracy. A molecular dynamics simulation of a sodium ion close to an air/water interface is performed to demonstrate that the method does indeed provide consistent long-range electrostatics. The mean force on the ion reduces at large distances to the image-charge interaction predicted by macroscopic electrostatics. This result is used to determine precisely the position of the macroscopic dielectric interface with respect to the true molecular surface.

Ballenegger, V.; Arnold, A.; Cerdà, J. J.

2009-09-01

260

Absorption or fluorescence-based two-dimensional (2-D) optical imaging is widely employed in functional brain imaging. The image is a weighted sum of the real signal from the tissue at different depths. This weighting function is defined as “depth sensitivity.” Characterizing depth sensitivity and spatial resolution is important to better interpret the functional imaging data. However, due to light scattering and absorption in biological tissues, our knowledge of these is incomplete. We use Monte Carlo simulations to carry out a systematic study of spatial resolution and depth sensitivity for 2-D optical imaging methods with configurations typically encountered in functional brain imaging. We found the following: (i) the spatial resolution is <200 ?m for NA ?0.2 or focal plane depth ?300 ?m. (ii) More than 97% of the signal comes from the top 500 ?m of the tissue. (iii) For activated columns with lateral size larger than spatial resolution, changing numerical aperature (NA) and focal plane depth does not affect depth sensitivity. (iv) For either smaller columns or large columns covered by surface vessels, increasing NA and?or focal plane depth may improve depth sensitivity at deeper layers. Our results provide valuable guidance for the optimization of optical imaging systems and data interpretation.

Tian, Peifang; Devor, Anna; Sakadzic, Sava; Dale, Anders M.; Boas, David A.

2011-01-01

261

NASA Astrophysics Data System (ADS)

In the present paper we investigate the transport of accreting plasma across the magnetopause onto a strongly magnetized massive star (i.e. white dwarf or neutron star) by magnetic reconnection. A simplified axisymmetric magnetic field model of an aligned rotator is used to study the reconnection process. To be able to separate effects caused by instabilities of the system from intrinsic time-dependent behaviour, we first construct self-consistent stationary states of the magnetosphere-disk system. We include a rigid magnetospheric rotation and Keplerian rotation of the magnetized disk plasma. The stationary states are computed numerically with a relaxation method which conserves the magnetic topology. Therefore we can prescribe an initial condition of the relaxation process using a magnetic field consisting of a dipole of the compact object and a homogeneous field threading the disk. The magnetopause then separates the regions of closed field lines with corotating plasma from open field lines with plasma in Keplerian motion. The resistive stability of the stationary states is examined by two-dimensional magnetohydrodynamic simulations. We find that magnetic reconnection leads to mass transport across the magnetopause onto closed magnetic field lines The accretion disk material is accelerated along the magnetic field lines that are connected to the magnetic poles of the compact object and will eventually be accreted by the star at its polar caps.

Rastaetter, L.; Neukirch, T.

1997-07-01

262

MHD simulations of boundary layer formation along the dayside Venus ionopause due to mass loading

NASA Astrophysics Data System (ADS)

A two-dimensional magnetohydrodynamic (MHD) simulation of mass-loaded solar wind flow around the dayside of Venus is presented. For conditions appropriate to a low-altitude ionopause the simulations show that mass loading from the pickup of oxygen ions produces a boundary layer of finite thickness along the ionopause. Within this layer the temperatures exhibit strong gradients normal to and away from the ionopause. Furthermore, there is a shear in the bulk flow velocity across the boundary layer, such that the (predominantly tangential) flow decreases in speed as the ionopause is approached and remains small along the ionopause, consistent with Pioneer Venus observations. The total mass density increases significantly as the flow approaches the ionopause, where the contribution of O(+) to the total number density is a few percent. Numerical simulations are carried out for various mass addition rates and demonstrate that the boundary layer develops when oxygen ion production exceeds approximately 2 x 105/cu m/s. For the upstream solar wind parameters and mass loading rates chosen for these simulations, the results are consistent with observations made on the dayside of Venus for average ionopause conditions near 300 km.

McGary, J. E.; Pontius, D. H.

1994-02-01

263

MHD-Calibrated ELM Model in Simulations of ITER

NASA Astrophysics Data System (ADS)

Simulations of ITER have been carried out using the JETTO integrated modeling code in which theory motivated models are used for the H-mode pedestal and for the stability criteria that lead to the ELM crashes. In the simulations, ELM crashes are triggered either by ballooning or peeling modes. The equilibrium and MHD stability analyses codes, HELENA and MISHKA, are used to evaluate the edge stability of the plasma just prior to an ELM crash in order to calibrate and confirm the validity of the stability criteria used to trigger ELMs in the JETTO simulations. In the simulations, core transport is calculated using an anomalous transport model such as the Mixed Bohm/gyro-Bohm model, while ion thermal neoclassical transport is used for the pedestal region. Studies are carried out varying the auxiliary heating power and the width of the pedestal in order to examine sensitivity of fusion Q to these parameters.

Onjun, T.; Kritz, A. H.; Bateman, G.; Parail, V.; Wilson, H.; Lönnroth, J.; Huysmans, G.; Dnestrovskij, A.

2004-11-01

264

Double-quantum coherence two-dimensional (2Q2D) electronic spectroscopy is utilized to probe the dynamic fluctuations of electronic states in a solvated molecule at approximately twice the energy of the ground state bleach transition. The 2Q2D spectrum gives insight into the energetic position and spectral fluctuations (system-bath interaction) of the probed excited states. Combining it with single-quantum two-dimensional (1Q2D) electronic spectroscopy enables one

Alexandra Nemeth; Franz Milota; Tomás Mancal; Tönu Pullerits; Jaroslaw Sperling; Jürgen Hauer; Harald F. Kauffmann; Niklas Christensson

2010-01-01

265

The influence of reduced dimensionality (two-dimensional (2-D) versus 3-D) on predictions of dense nonaqueous phase liquid (DNAPL) infiltration and entrapment in statistically homogeneous, nonuniform permeability fields was investigated using the University of Texas Chemical Compositional Simulator (UTCHEM), a 3-D numerical multiphase simulator. Hysteretic capillary pressure–saturation and relative permeability relationships implemented in UTCHEM were benchmarked against those of another lab-tested simulator,

John A. Christ; Lawrence D. Lemke; Linda M. Abriola

2005-01-01

266

3D MHD Simulations of Stratified Accretion Disks

NASA Astrophysics Data System (ADS)

We investigate the growth and nonlinear saturation of a powerful local shear instability in weakly magnetised accretion disks using three dimensional magnetohydrodynamic (MHD) simulations. To achieve a sufficiently high numerical resolution, we use a local approximation for the disk and carry out the simulations on massively parallel supercomputers. Here we investigate the linear growth and nonlinear saturation of the instability in a vertically stratified, intially isothermal disk. A variety of initial field configurations and strengths are considered. The simulations allow a quantitative analysis of the role of bouyancy as a saturation mechanism, and possible dynamo action in the disk. This work is partially supported by NSF grant PHY-9018251 and NASA grants NAGW-1510 and NAGW-2376. Code development is supported by the NASA HPCC Initiative through grant NAG5-2202. Computations were carried out on the CM200 system of the National Center for Supercomputing Applications.

Stone, James M.; Hawley, John F.; Gammie, Charles

1993-12-01

267

In a recent paper, Watanabe, {ital et. al.} used direct simulation Monte Carlo to study Rayleigh-B{acute e}nard convection. They reported that, using stress-free boundary conditions, the onset of convection in the simulation occurred at a Rayleigh number much larger than the critical Rayleigh number predicted by linear stability analysis. We show that the source of their discrepancy is their failure to include the temperature jump effect in the calculation of Rayleigh number.

Garcia, A.L. [Lawrence Livermore National Lab., CA (United States); Baras, F.; Mansour, M.M. [Universite Libre de Bruxelles (Belgium)

1994-06-30

268

NASA Astrophysics Data System (ADS)

1-D simulations based on the quasi-one-dimensional equations of fluid motion plus an ignition delay model and 2-D numerical simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations have been performed for two different scramjet combustors. The combustor configurations at DLR and NASA's SCHOLAR Supersonic Combustor have been used as test cases for the 1-D and 2-D simulations. Comparisons between the published 3-D computational and experimental results and quasi-one-dimensional and 2-D simulations have been performed. The quasi-one dimensional modeling of NASA's SCHOLAR supersonic combustor captures the trends in Mach number, static pressure and static temperature for both cold flow and combustion case. The comparison with experimental result for combustion case reveals a close agreement with the pressure peak and the presence of an ignition delay. Thus, 1-D simulation very closely predicts the flow evolution within the combustor. On the other hand, for DLR supersonic combustor, due to the lack of oblique wave (i.e. shock waves and expansion waves) and shear dominated viscous flow simulation, 1-D model severely fails to predict the trend followed by the experimental result along the centerline of the combustor. However, the 1-D model is able to match the overall flow velocity achieved within the combustor downstream of the wedge at approximately six wedge chord lengths.

Tourani, Chandraprakash

269

A grid resolution sensitivity analysis using a two-dimensional flood inundation model has been presented in this paper. Simulations for 6 dam breaches located randomly in the United States were run at 10,30,60,90, and 120 meter resolutions. The dams represent a range of topographic conditions, ranging from 0% slope to 1.5% downstream of the dam. Using 10 meter digital elevation model (DEM) simulation results as the baseline, the coarser simulation results were compared in terms of flood inundation area, peak depths, flood wave travel time, daytime and nighttime population in flooded area, and economic impacts. The results of the study were consistent with previous grid resolution studies in terms of inundated area, depths, and velocity impacts. The results showed that as grid resolution is decreased, the relative fit of inundated area between the baseline and coarser resolution decreased slightly. This is further characterized by increasing over prediction as well as increasing under prediction with decreasing resolution. Comparison of average peak depths showed that depths generally decreased as resolution decreased, as well as the velocity. It is, however, noted that the trends in depth and velocity showed less consistency than the inundation area metrics. This may indicate that for studies in which velocity and depths must be resolved more accurately (urban environments when flow around buildings is important in the calculation of drag effects), higher resolution DEM data should be used. Perhaps the most significant finding from this study is the perceived insensitivity of socio-economic impacts to grid resolution. The difference in population at risk (PAR) and economic cost generally remained within 10% of the estimated impacts using the high resolution DEM. This insensitivity has been attributed to over estimated flood area and associated socio-economic impacts compensating for under estimated flooded area and associated socio-economic impacts. The United States has many dams that are classified as high-hazard potential that need an emergency action plan (EAP). It has been found that the development of EAPs for all high-hazard dams is handicapped due to funding limitations. The majority of the cost associated with developing an EAP is determining the flooded area. The results of this study have shown that coarse resolution dam breach studies can be used to provide an acceptable estimate of the inundated area and economic impacts, with very little computational cost. Therefore, the solution to limited funding may be to perform coarse resolution dam breach studies on high-hazard potential dams and use the results to help prioritize the order in which detailed EAPs should be developed.

Judi, David R [Los Alamos National Laboratory; Mcpherson, Timothy N [Los Alamos National Laboratory; Burian, Steven J [UNIV OF UTAH

2009-01-01

270

We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, {rho}{sub 0}, we can reproduce three distinct modes of accretion flow. In model A, which has the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of mild beaming, the apparent (isotropic) photon luminosity is {approx}22L{sub E} (where L{sub E} is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B, which has moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than {approx}7 R{sub S} (where R{sub S} is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C, the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, together with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

Ohsuga, Ken [National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588 (Japan); Mineshige, Shin [Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502 (Japan)

2011-07-20

271

Experimental investigations indicate that colloidal particles are transported more rapidly than soluble tracers through porous and fractured media. The prevailing in- terpretation is that colloids are confined to larger pores, larger channels or regions within channels where flow is more rapid. A lattice-Boltzmann modeling approach was used to analyze how size-dependent exclusion from low velocity fields in chan- nels can lead to accelerated transport of an inert non-Brownian colloidal particle in low-Reynolds number flows in two-dimensional smooth-walled and rough-walled channels. The simulations were based on pore-scale particle-fluid hydrodynamics without direct interactions between the particle surface and the channel surface. For the smooth-walled channel, the particle consistently drifted towards the center- line and traveled faster than the average fluid velocity. In rough-walled channels, differences between the velocity of the particle and the average velocity of the fluid displayed stronger variations than in the smooth-walled channel. Surface roughness increased the residence time of the particle in the flow channel and modified its trajectories differently in each flow regime. The final position (at the channel exit) and the average velocity of the particle in the rough-walled flow channel were sen- sitive to the release location of the particle, the flow strength, and the magnitude of the surface roughness in the channel. Under all flow conditions investigated, a large particle had a longer residence time in rough-walled flow channels, but drifted Preprint submitted to Elsevier Science 19 September 2006 toward the centerline more strongly than a smaller particle as it approached the channel walls.

H. Basagaoglu; Meakin, P.; S. Succi; Redden, George D; T.R. Ginn

2008-05-01

272

Groundwater transport of tritium from the Cheshire (U-20n) site at the Nevada Test Site (NTS) is investigated using a stochastic approach and numerical simulations. The hydrogeologic system is modeled as a horizontal two dimensional fractured-rhyolite aquifer intersected by a rubble chimney that extends upward from the shot cavity. The chimney, which is considered to be more permeable than surrounding rocks,

K. Pohlmann; R. Andricevic; V. Vallikat

1993-01-01

273

Two-dimensional combustion processes in a spark ignition engine with and without an unscavenged horizontal prechamber are calculated numerically using a Îº-Îµ turbulence model, a flame kernel ignition model and an irreversible reaction model to obtain a better understanding of the spatial and temporal distributions of flow and combustion. The simulation results are compared with the measured results under the same

H. Ryu; T. Asanuma

1989-01-01

274

MHD simulation of a fast forward shock event

NASA Astrophysics Data System (ADS)

Following the propagation of interplanetary shocks (IPS) in the solar wind and throughout the Earth's magnetosphere is vital to understand the processes associated with their interaction with the magnetosphere. A case study using multiple spacecraft analysis and GUMICS-4 global MHD simulation shows that stronger fast forward shocks with a higher shock speed cause a greater response in the magnetosphere. Additionally, a statistical analysis of a set of fast forward shock events shows that the solar wind speed and dynamic pressure play an important role in the geomagnetic activity: Our results show that higher interplanetary shock speeds cause a higher response in the AE index. Furthermore, we have used the GUMICS-4 global MHD simulation results to interpret the unusual double step structure observed at geostationary orbit. The temporal variations in the velocity and electric field in the dayside magnetosphere near geostationary orbit strongly suggest that the second part of the double step structure is a reflected disturbance from the inner boundary of the magnetosphere.

Andreeova, K.; Pulkkinen, T. I.; Palmroth, M.

2008-12-01

275

Coupled simulation of kinetic pedestal growth and MHD ELM crash

Edge pedestal height and the accompanying ELM crash are critical elements of ITER physics yet to be understood and predicted through high performance computing. An entirely self-consistent first principles simulation is being pursued as a long term research goal, and the plan is planned for completion in time for ITER operation. However, a proof-of-principle work has already been established using a computational tool that employs the best first principles physics available at the present time. A kinetic edge equilibrium code XGC0, which can simulate the neoclassically dominant pedestal growth from neutral ionization (using a phenomenological residual turbulence diffusion motion superposed upon the neoclassical particle motion) is coupled to an extended MHD code M3D, which can perform the nonlinear ELM crash. The stability boundary of the pedestal is checked by an ideal MHD linear peeling-ballooning code, which has been validated against many experimental data sets for the large scale (type I) ELMs onset boundary. The coupling workflow and scientific results to be enabled by it are described.

Park, G. [New York University; Cummings, J. [California Institute of Technology, University of California, Davis; Chang, C. S. [New York University; Klasky, Scott A [ORNL; Ku, S. [New York University; Podhorszki, Norbert [University of California, Davis; Pankin, A. [Lehigh University, Bethlehem, PA; Samtaney, Ravi [Princeton Plasma Physics Laboratory (PPPL); Shoshani, A. [Lawrence Berkeley National Laboratory (LBNL); Snyder, P. [General Atomics, San Diego; Strauss, H. [New York University; Sugiyama, L. [Massachusetts Institute of Technology (MIT); CPES Team, the [SciDAC Prototype FSP Center for Plasma Edge Simulation

2007-01-01

276

NASA Astrophysics Data System (ADS)

Context: The physics of the pulsar magnetosphere near the neutron star surface remains poorly constrained by observations. Indeed, little is known about its emission mechanism, from radio to high-energy X-ray and gamma-rays. Nevertheless, it is believed that large vacuum gaps exist in this magnetosphere, and a non-neutral plasma partially fills the neutron star surroundings to form an electrosphere in differential rotation. Aims: According to several of our previous works, the equatorial disk in this electrosphere is diocotron and magnetron unstable, at least in the linear regime. To better assess the long term evolution of these instabilities, we study the behavior of the non-neutral plasma using particle simulations. Methods: We designed a two-dimensional electrostatic particle-in-cell (PIC) code in cylindrical coordinates, solving Poisson equation for the electric potential. In the diocotron regime, the equation of motion for particles obeys the electric drift approximation. As in the linear study, the plasma is confined between two conducting walls. Moreover, in order to simulate a pair cascade in the gaps, we add a source term feeding the plasma with charged particles having the same sign as those already present in the electrosphere. Results: First we checked our code by looking for the linear development of the diocotron instability in the same regime as the one used in our previous work, for a plasma annulus and for a typical electrosphere with differential rotation. To very good accuracy, we retrieve the same growth rates, supporting the correctness of our PIC code. Next, we consider the long term non-linear evolution of the diocotron instability. We found that particles tend to cluster together to form a small vortex of high charge density rotating around the axis of the cylinder with only little radial excursion of the particles. This grouping of particles generates new low density or even vacuum gaps in the plasma column. Finally, in more general initial configurations, we show that particle injection into the plasma can drastically increase the diffusion of particles across the magnetic field lines. The newly formed vacuum gaps cannot be replenished by simply invoking diocotron instability. Conclusions: Diocotron instability offers a new possibility to solve the current closure problem in a pulsar magnetosphere. It is a promising mechanism leading to highly unstable flows in the pulsar inner magnetosphere. When flowing towards the light cylinder, some relativistic and particle inertia effects (not included in this study) appear. Nevertheless, the system should remain unstable because of the relativistic diocotron or the magnetron instability. Therefore, we expect an electric current to circulate in the closed magnetosphere and to feed the base of the wind with charged particles.

Pétri, J.

2009-08-01

277

MHD and Hall MHD simulations of 3-D turbulence lead by the Kelvin-Helmholtz instability

NASA Astrophysics Data System (ADS)

The entry process of the solar wind plasma into the magnetosphere during the northward IMF condition has been controversial in contrast to the Dungey's reconnection model for the southward IMF case. The major candidate processes are the double lobe reconnection model [Song et al., 1999], in which newly closed magnetic field lines on the dayside magnetopause capture the solar wind plasma, and the turbulent transport by the Kelvin- Helmholtz instability (KHI) driven by the fast solar wind flow. We have shown by simulation studies that the strong flow turbulence is a natural consequence of the nonlinear development of the KHI through the secondary instability [Matsumoto and Hoshino, 2004, 2006], which significantly contribute to the formation of a large scale mixing area (e.g., LLBL). Recently, we have studied the 3-D nonlinear evolution of the KHI by performing MHD simulations [Matsumoto and Seki, 2007]. The KH vortex is also susceptible to "the 3-D secondary instability" which converts the rotating energy into the magnetic energy by generating large amplitude magnetic fluctuations which finally lead the system to turbulent state. The fundamental mechanism is similar to the magneto-rotational instability (MRI) which has usually been applied to the accretion disk. Sano and Stone [2002] showed that the Hall term (ion kinetic) effect is important in the nonlinear saturation of the MRI as well as in the linear growth [Balbus and Terquem, 2001]; the direction of the initial magnetic field with respect to the angular velocity separates the fate of the instability. By analogy with their studies on the MRI, we have also examined an ion kinetic effect on the 3-D nonlinear evolution of the KHI. 3-D Hall MHD simulation showed a faster and more turbulent evolution of the secondary instability when the magnetic field directed opposite to the angular velocity of the vortex. On the other hand, it was inhibited when the magnetic field was set in the same direction. The results indicate importance of the ion dynamics in rapidly rotating plasma in which a vortex finally collapses into turbulence. The detailed mechanism which separates the natures of the secondary instability is also addressed in this presentation.

Matsumoto, Y.; Seki, K.

2007-12-01

278

MHD simulation study of the interplanetary shocks under different conditions

NASA Astrophysics Data System (ADS)

Simulation study of interplanetary shocks is important in order to describe dynamic process of their propagation and evolution during their passage through the Earth's magnetosphere as a whole in order to improve our observational knowledge of geospace. Our statistical analysis of a set of fast forward shocks shows that disturbance speed in the Earth's magnetosphere is higher than the original shock speed in the solar wind and differs according to the cone angle between the shock front normal in the solar wind and Sun-Earth line. Disturbance speed seems to be higher in all perpendicular cases than for oblique cases. For the purpose to describe this difference, GUMICS-4 global MHD simulation is used to calculate shock/disturbance propagation and evolution in different regions of the magnetosphere under different original conditions. Several methods are used to derive the propagation speeds and directions, such as Rankine-Hugoniot, velocity and magnetic coplanarity, and minimum variance analysis.

Andreeova, K.; Pulkkinen, T. I.; Juusola, L.; Palmroth, M. M.; Santolik, O.

2009-12-01

279

Simulation of a seawater MHD power generation system

MHD (magnetohydrodynamics) power generation systems are expected to become popular with the development of superconducting technology because of their low cost and high efficiency. MHD power generation directly utilizes electromotive force, which arises when seawater crosses a magnetic field. The helical-type MHD generator is composed mainly of a helical partition board and electrodes, which include a cathode pipe on the

Xiaojun Liu; Tsukasa Kiyoshi; Minoru Takeda

2006-01-01

280

We have investigated the cross-sectional electric field and potential distribution of a cleaved n+-InP\\/InGaAsP\\/p+-InP p-i-n laser diode using Kelvin probe force microscopy (KFM) with a lateral resolution reaching 50 nm. The powerful characterization capabilities of KFM were compared with two-dimensional (2D) physics-based simulations. The agreement between simulations and KFM measurements regarding the main features of the electric field and potential

F. Robin; H. Jacobs; O. Homan; A. Stemmer; W. Bächtold

2000-01-01

281

Simulations of Pulse Detonation Engines with MHD Thrust Augmentation

NASA Astrophysics Data System (ADS)

Pulse detonation rocket engines (PDREs) have received significant attention in recent years due to their potentially superior performance over constant-pressure engines. Yet unsteady chamber pressures cause the PDRE flow to be either over-expanded or under-expanded for the majority of the cycle, with substantial performance loss in atmospheric flight applications. The present computational studies examine the potential benefits of using magneto-hydrodynamic (MHD) thrust augmentation by extracting energy via a generator in the PDRE nozzle and applying it to a separate, secondary stream. In the present studies, which involve both transient quasi-1D and 2D numerical simulations, the energy extracted from the nozzle flow is directly applied to a by-pass air stream through an MHD accelerator. The air stream is first shocked by the under-expanded nozzle flow and raised to high temperature, allowing thermal ionization. The specific conditions for thrust augmentation are examined. Alternative configurations utilizing a magnetic piston in the PDRE chamber are also explored. Results show potential performance gains but with significant challenges, depending on the operating and flight conditions.

Zeineh, Christopher; Roth, Timothy; Cole, Lord; Karagozian, Ann; Cambier, Jean-Luc

2008-11-01

282

Nonlinear MHD simulations of edge-localized-modes in JET

NASA Astrophysics Data System (ADS)

Nonlinear magneto-hydrodynamic (MHD) simulations with the JOREK code may be used to improve our understanding of edge-localized-modes (ELMs) (Huysmans and Czarny 2007 Nucl. Fusion 47 659-66, Huysmans et al 2009 Plasma Phys. Control. Fusion 51 124012, Pamela et al 2010 Plasma Phys. Control. Fusion 52 075006). These H-mode related instabilities may cause some damage to the tungsten divertor of ITER (Bazylev et al 2007 Phys. Scr. T128 229-33), and it was demonstrated experimentally that the ELM energy losses increase with both machine size and decreasing collisionality (ITER Physics Basis Editors and ITER EDA 1999 Nucl. Fusion 39 2175, Loarte et al 2003 Plasma Phys. Control. Fusion 45 1549-69). In sight of producing simulations of ELMs in ITER, in order to give some predictions of ELM size and divertor heat fluxes in the future device, simulations first need to be quantitatively validated against the experimental data of present machines. This paper presents simulations of ELMs in the JET tokamak for the low-collosionality type-I ELMy H-mode pulse #73569. The simulation results are compared with experimental data to provide a qualitative validation of the simulations. This comparison comprises the dynamics of filaments and divertor heat fluxes, the effect of resistivity and collisionality on ELM energy losses and the observation of ELM precursors prior to the pedestal collapse.

Pamela, S. J. P.; Huysmans, G. T. A.; Beurskens, M. N. A.; Devaux, S.; Eich, T.; Benkadda, S.; EFDA contributors, JET

2011-05-01

283

Reconnection events in two-dimensional Hall magnetohydrodynamic turbulence

The statistical study of magnetic reconnection events in two-dimensional turbulence has been performed by comparing numerical simulations of magnetohydrodynamics (MHD) and Hall magnetohydrodynamics (HMHD). The analysis reveals that the Hall term plays an important role in turbulence, in which magnetic islands simultaneously reconnect in a complex way. In particular, an increase of the Hall parameter, the ratio of ion skin depth to system size, broadens the distribution of reconnection rates relative to the MHD case. Moreover, in HMHD the local geometry of the reconnection region changes, manifesting bifurcated current sheets and quadrupolar magnetic field structures in analogy to laminar studies, leading locally to faster reconnection processes in this case of reconnection embedded in turbulence. This study supports the idea that the global rate of energy dissipation is controlled by the large scale turbulence, but suggests that the distribution of the reconnection rates within the turbulent system is sensitive to the microphysics at the reconnection sites.

Donato, S.; Servidio, S.; Carbone, V. [Dipartimento di Fisica, Universita della Calabria, I-87036 Cosenza (Italy); Dmitruk, P. [Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires and Instituto de Fisica de Buenos Aires, CONICET, Buenos Aires (Argentina); Shay, M. A.; Matthaeus, W. H. [Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716 (United States); Cassak, P. A. [Department of Physics, West Virginia University, Morgantown, West Virginia 26506 (United States)

2012-09-15

284

MHD simulation of deuterium-fiber-initiated Z-pinches with two-fluid effects

Two-dimensional ``cold-start`` resistive MHD computations of formation and evolution of deuterium-fiber-initiated Z-pinches have been extended to include separate ion and electron energy equations and finite-Larmor-radius ordered terms. In the Ohm`s Law (magnetic field evolution) equation, Hall and diamagnetic pressure terms have been added, and corresponding terms have been added to the energy equations. Comparison is made of the results of these computations with previous computations and with experiments.

Sheehey, P. [California Univ., Los Angeles, CA (United States). Dept. of Physics; Lindemuth, I.R. [Los Alamos National Lab., NM (United States)

1993-05-01

285

MHD simulation of deuterium-fiber-initiated Z-pinches with two-fluid effects

Two-dimensional cold-start'' resistive MHD computations of formation and evolution of deuterium-fiber-initiated Z-pinches have been extended to include separate ion and electron energy equations and finite-Larmor-radius ordered terms. In the Ohm's Law (magnetic field evolution) equation, Hall and diamagnetic pressure terms have been added, and corresponding terms have been added to the energy equations. Comparison is made of the results of these computations with previous computations and with experiments.

Sheehey, P. (California Univ., Los Angeles, CA (United States). Dept. of Physics); Lindemuth, I.R. (Los Alamos National Lab., NM (United States))

1993-01-01

286

In cooperation with the Missouri Department of Transportation, the U.S. Geological Survey determined hydrologic and hydraulic parameters for the Gasconade River at the site of a proposed bridge replacement and highway realignment of State Highway 17 near Waynesville, Missouri. Information from a discontinued streamflow-gaging station on the Gasconade River near Waynesville was used to determine streamflow statistics for analysis of the 25-, 50-, 100-, and 500-year floods at the site. Analysis of the streamflow-gaging stations on the Gasconade River upstream and downstream from Waynesville indicate that flood peaks attenuate between the upstream gaging station near Hazelgreen and the Waynesville gaging station, such that the peak discharge observed on the Gasconade River near Waynesville will be equal to or only slightly greater (7 percent or less) than that observed near Hazelgreen. A flood event occurred on the Gasconade River in March 2008, and a flood measurement was obtained near the peak at State Highway 17. The elevation of high-water marks from that event indicated it was the highest measured flood on record with a measured discharge of 95,400 cubic feet per second, and a water-surface elevation of 766.18 feet near the location of the Waynesville gaging station. The measurements obtained for the March flood resulted in a shift of the original stage-discharge relation for the Waynesville gaging station, and the streamflow statistics were modified based on the new data. A two-dimensional hydrodynamic flow model was used to simulate flow conditions on the Gasconade River in the vicinity of State Highway 17. A model was developed that represents existing (2008) conditions on State Highway 17 (the 'model of existing conditions'), and was calibrated to the floods of March 20, 2008, December 4, 1982, and April 14, 1945. Modifications were made to the model of existing conditions to create a model that represents conditions along the same reach of the Gasconade River with preliminary proposed replacement bridges and realignment of State Highway 17 (the 'model of proposed conditions'). The models of existing and proposed conditions were used to simulate the 25-, 50-, 100-, and 500-year recurrence floods, as well as the March 20, 2008 flood. Results from the model of proposed conditions show that the proposed replacement structures and realignment of State Highway 17 will result in additional backwater upstream from State Highway 17 ranging from approximately 0.18 foot for the 25-year flood to 0.32 foot for the 500-year flood. Velocity magnitudes in the proposed overflow structures were greater than in the existing structures [by as much as 4.9 feet per second in the left (west) overflow structure for the 500-year flood], and shallow, high-velocity flow occurs at the upstream edges of the abutments of the proposed overflow structures in the 100- and 500-year floods where flow overtops parts of the existing road embankment that will be left in place in the proposed scenario. Velocity magnitude in the main channel of the model of proposed conditions increased by a maximum of 1.2 feet per second over the model of existing conditions, with the maximum occurring approximately 1,500 feet downstream from existing main channel structure J-802.

Huizinga, Richard J.

2008-01-01

287

This paper adresses the construction and study of a Crank-Nicol- son-type discretization of the two-dimensional linear Schrodinger equation in a bounded domain with articial boundary conditions set on the arbitrarily shaped boundary of . These conditions present the features of being dier- ential in space and nonlocal in time since their denition involves some time fractional operators. After having proved

Xavier Antoine; Christophe Besse; Vincent Mouysset

2004-01-01

288

CHEM2D is a two-dimensional, three-phase, nine-component finite-difference chemical flood simulator. It can simulate waterfloods, polymer floods, and micellar/polymer floods using heterogeneous one- or two-dimensional (areal or cross-sectional) reservoir models. At present only one injection and four production wells are available. The user may specify well performance as either pressure or rate constrained. A constant time step size or a variable time step size determined by extrapolation of concentration changes may be specified. Volume I of this report provides a discussion of the formulation and algorithms used within CHEM2D. Included in Volume I are a number of validation and illustrative examples, as well as the FORTRAN code. The CHEM2D user's manual, Volume II, contains both the input data sets for the examples presented in Volume I and an example output. All appendices and a phase behavior calculation program are collected in Volume III.

Fanchi, J.R.

1985-04-01

289

NASA Astrophysics Data System (ADS)

Focusing of an intense laser pulse produced by backward Raman pulse amplification (BRA) has been numerically studied using a two-dimensional, axisymmetric kinetic model. The two-dimensional averaged particle-in-cell (aPIC) simulation assumes slowly varying field envelopes and is comprised of one-dimensional sub-models that are coupled radially through laser diffraction. A converging 33 TW seed pulse was amplified up to 1 PW. The focusing of the seed pulse, even when particle trapping was important, was maintained. It was also found that the focusing properties of the pulse tail can lead to some rewidening of the longitudinal pulse duration and some ideas for eliminating this effect were suggested. Simulations performed for various plasma densities and temperatures exhibited robust amplification and pulse shortening.

Hur, Min Sup; Wurtele, Jonathan S.

2009-04-01

290

An Object-Oriented Kinetic MHD Simulation Code using the POOMA C++ Class Library

A kinetic MHD simulation code has been developed for the study of trapped energetic particle effects on Alfvén-ballooning modes in a dipole magnetic configuration with anisotropic equilibria. The simulation code treats the core plasma as an MHD fluid, while the energetic particles are modeled using a gyrokinetic delta-f scheme, with finite-orbit-width effects taken into account. This simulation has been applied

Julian Cummings; Linjin Zheng; Liu Chen

2000-01-01

291

We provide a dynamics-based comparison on the results from three-dimensional and two-dimensional simulations of hurricane storm surge. We begin with the question, What may have occurred in the Tampa Bay, Florida vicinity had Hurricane Ivan made landfall there instead of at the border between Alabama and Florida? This question is explored using a three-dimensional, primitive equation, finite volume coastal ocean

Robert H. Weisberg; Lianyuan Zheng

2008-01-01

292

In this paper the meshless local radial point interpolation method (LRPIM) is adopted to simulate the two-dimensional nonlinear sine-Gordon (S-G) equation. The meshless LRPIM is one of the “truly meshless” methods since it does not require any background integration cells. In this case, all integrations are carried out locally over small quadrature domains of regular shapes, such as circles or

Mehdi Dehghan; Arezou Ghesmati

2010-01-01

293

Large Eddy Simulations for two flows separating from a two-dimensional hump in a duct are reported and discussed. The flows\\u000a differ through the presence or absence of a synthetic slot-jet injected in a sinusoidal manner, i.e. at zero net mass-flow\\u000a rate, close to the location of separation and intended to reduce (“control”) the extent of the separated region. Results reported

Alexandros Avdis; Sylvain Lardeau; Michael Leschziner

2009-01-01

294

Using a hydrodynamic simulation, we have studied the two-dimensional (symmetry in the azimuthal direction) evolution of a fast, high-pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. The latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (>20Â°) the solar

Pete Riley; J. T. Gosling; V. J. Pizzo

1997-01-01

295

Nonlinear MHD simulation of DC helicity injection in spherical tokamaks

NASA Astrophysics Data System (ADS)

3-D nonlinear MHD computations using the NIMROD code have been performed to study DC helicity injection in the HIT-II and Pegasus spherical tokamaks. Current drive via DC helicity injection has been successfully employed with either a poloidal-gap voltage known as coaxial helicity injection (CHI) [used in HIT-II and NSTX] or a biased miniature plasma gun [used in CDX and Pegasus]. Numerical studies of CHI in a simplified geometry with ?=0 reproduce the ``bubble-burst" formation and the subsequent excitation and saturation (characterized in HIT-II by amplification of poloidal flux) of a line-tied internal kink-mode. The computed strength of saturated fluctuations and poloidal flux are in quantitative agreement with data obtained in the HIT-II experiment. Results from ?!=0 simulations with an experimentally accurate geometry will also be presented. Cases driven by a numerical representation of miniature plasma gun self-consistently evolve pressure and anisotropic thermal transport and simulate the formation, merger, and relaxation of the current filaments to a tokamak-like plasma. The results are compared to experimental data from the Pegasus ST. In both injection scenarios the simulations permit a detailed description of the 3-D equilibria exhibited by the helicity-injected driven plasma and reproduce the observations made in the Pegasus ST and HIT-II of amplified poloidal flux and generation of toroidal current.

Bayliss, R. A.; Sovinec, C. R.

2007-11-01

296

3D MHD Simulations of Flux Rope Driven CMEs

NASA Astrophysics Data System (ADS)

We present a three-dimensional (3D) numerical ideal magnetohydrodynamics (MHD) model describing the time-dependent expulsion of a CME from the solar corona propagating all the way to 1 A.U.. The simulations are performed using the BATS-R-US (Block Adaptive Tree Solarwind Roe Upwind Scheme) code. We begin by developing a global steady-state model of the corona that possesses high-latitude coronal holes and a helmet streamer structure with a current sheet at the equator. The Archimedian spiral topology of the interplanetary magnetic field is reproduced along with fast and slow speed solar wind at high and low latitudes respectively. Within this model system,we drive a CME to erupt by the introduction of a twisted magnetic flux rope that is anchored at both ends in the photosphere and embedded in the helmet streamer. The flux rope configuration that we employ was first developed by Gibson and Low as part of a 3D self-similar model of a CME. In this case, the flux rope has the form of a spherical ball of twisted magnetic field distorted to a tear shape by a stretching transformation. The stretch transformation produces an outward radially directed Lorentz force within the flux rope that rapidly accelerates the leading edge of the rope to speeds of 1800 km/sec, driving a strong shock as part of the CME. We follow the evolution of the CME from the low corona as it makes its way through the heliosphere. We explore the dynamics of the expanding flux rope as it interacts with the rotating, bi-modal solar wind to determine significant MHD effects. Finally we present synthetic white-light coronagraph images of the model CME which show a three-part structure that can be compared with observations of CME structure.

Manchester, W. B.; Roussev, I.; Opher, M.; Gombosi, T.; DeZeeuw, D.; Toth, G.; Sokolov, I.; Powell, K.

2002-05-01

297

Three-dimensional MHD simulations of Ganymede's magnetosphere

NASA Astrophysics Data System (ADS)

Ganymede is unique among planetary moons because it has its own magnetic field strong enough to form a magnetosphere within Jupiter's magnetospheric environment. Here we report on our three-dimensional global magnetohydrodynamic (MHD) simulations that model the interaction between Ganymede's magnetosphere and the corotating Jovian plasma. We use the measured field and particle properties to define our boundary conditions. Our simulations show that, in addition to the familiar structures such as the magnetopause and equatorial current sheet, Ganymede's magnetosphere extends into an Alfvén wing that mediates the interaction of Ganymede with the plasma and ionosphere of Jupiter. The field-aligned currents in the Alfvén wing close on themselves not only through the moon and its ionosphere. They also close through the magnetopause and tail current sheets. The pattern of the field-aligned currents varies according to the orientation of the external magnetic field and asymmetries in the intensities of the parallel currents are organized by the clock angle of the ambient field in the plane perpendicular to the incident flow. The simulations reproduce quite closely the magnetic field structure measured by the Galileo magnetometer for all six close encounters. The magnetopause currents are well resolved in our high resolution simulations, producing sharp rotations in the field orientation consistent with the observations. However, the discrepancies between our model results and the data, such as the weaker field strength near closest approach in multiple simulated flybys, suggest the possibility that Ganymede's intrinsic magnetic field may be stronger than the accepted value. The magnetosphere produced in our simulations can provide us with realistic estimates of the moon's magnetic environment thereby enabling us to refine our determination of Ganymede's internal magnetic field and to better understand the energetic particle behavior.

Jia, Xianzhe; Walker, Raymond J.; Kivelson, Margaret G.; Khurana, Krishan K.; Linker, Jon A.

2008-06-01

298

NASA Astrophysics Data System (ADS)

A self-consistent approach is proposed to determine the temperature dependent thermal conductivity k(T) of fused silica, for a range of temperatures up to material evaporation using a CO2 laser irradiation. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was linked step by step as the laser power was increased with experimental measurements using infrared thermography. We show that previously reported k(T) does not reproduce the temporal profile as well as our adaptive fit which shows that k(T) evolves with slope discontinuities at the annealing temperature and the softening temperature.

Combis, Patrick; Cormont, Philippe; Gallais, Laurent; Hebert, David; Robin, Lucile; Rullier, Jean-Luc

2012-11-01

299

A self-consistent approach is proposed to determine the temperature dependent thermal conductivity k(T) of fused silica, for a range of temperatures up to material evaporation using a CO{sub 2} laser irradiation. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was linked step by step as the laser power was increased with experimental measurements using infrared thermography. We show that previously reported k(T) does not reproduce the temporal profile as well as our adaptive fit which shows that k(T) evolves with slope discontinuities at the annealing temperature and the softening temperature.

Combis, Patrick [CEA DAM Ile-de-France, F-91297 ARPAJON (France); Cormont, Philippe; Hebert, David; Robin, Lucile; Rullier, Jean-Luc [CEA CESTA, F-33114 LE BARP (France); Gallais, Laurent [Institut Fresnel, CNRS, Aix-Marseille Universite, Ecole Centrale Marseille, 13013 Marseille (France)

2012-11-19

300

NASA Astrophysics Data System (ADS)

In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascals, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. For axisymmetric laser spots with finite size, the pressure generation near the water-coating interface is a two dimensional process in nature. This is in particular the case for microscale LSP performed with very small laser spots, which is a very promising technique to improve the reliability performance of microdevices. However, models capable of predicting two dimensional (2D) spatial distributions of the induced pressures near the coating-water interface in LSP have rarely been reported in literature. In this paper, a predictive 2D axisymmetric model is developed by numerically solving the hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can produce 2D spatial distributions of material responses near the water-coating interface in LSP, and is verified through comparisons with experimental measurements. The model calculation shows that the effect of radial release wave on pressure spatial distributions becomes more significant as the laser spot size decreases, indicating the importance of a 2D model, particularly for microscale LSP.

Wu, Benxin; Shin, Yung C.

2007-05-01

301

3D-MHD Simulations of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Growth, saturation and turbulent evolution of the Madison dynamo experiment is investigated numerically using a 3-D pseudo-spectral simulation of the MHD equations; results of the simulations are used to predict behavior of the experiment. The code solves the self-consistent full evolution of the magnetic and velocity fields. The code uses a spectral representation via spherical harmonic basis functions of the vector fields in longitude and latitude, and fourth order finite differences in the radial direction. The magnetic field evolution has been benchmarked against the laminar kinematic dynamo predicted by M.L. Dudley and R.W. James [Proc. R. Soc. Lond. A 425. 407-429 (1989)]. Initial results indicate that saturation of the magnetic field occurs so that the resulting perturbed backreaction of the induced magnetic field changes the velocity field such that it would no longer be linearly unstable, suggesting non-linear terms are necessary for explaining the resulting state. Saturation and self-excitation depend in detail upon the magnetic Prandtl number.

Bayliss, R. A.; Forest, C. B.; Wright, J. C.; O'Connell, R.

2003-10-01

302

Simulations and Theory of Resistive MHD in Spheromaks*

NASA Astrophysics Data System (ADS)

Three-dimensional nonlinear resistive MHD equations addressing spheromak evolution are solved in simulations with the NIMROD code [1] and analytically with a Connor-Taylor scaling analysis[2]. The NIMROD simulations explore (1) the quality of spheromak magnetic surfaces in the presence of pulsed electrostatic drive, (2) the 3D evolution of spheromak equilibria that are initially axisymmetric, and (3) the formation and sustainment of spheromak plasmas with an external circuit to model a constant current source using parameters approaching experimental values with attention given to convergence and div(B) errors. The Connor-Taylor analysis is used to explore spheromak scaling of plasma conditions and transport coefficients with respect to parameters, e.g., plasma radius, local magnetic Lundquist number and q value, mass ratio, etc. *Work performed by Univ. Calif. LLNL under U.S. DOE contract W-7405-ENG-48 and by U. Wisc. under contract DE-FC02ER54668. [1] C.R. Sovinec, J.M. Finn, and D. Del-Castillo-Negrete, Phys. Plasmas 8, 475 (2001). [2] J.W. Connor and J.B. Taylor, Nuc. Fusion 17, 1047 (1977) and Phys. Fluids 27, 2676 (1984); R.H. Cohen and L.L. LoDestro, 2002 Sherwood Conf., paper 1D7, Rochester, NY.

Cohen, B. I.; Cohen, R. H.; Lodestro, L. L.; Sovinec, C. R.

2002-11-01

303

3-D Relativistic MHD Simulations of Extragalactic Jets

NASA Astrophysics Data System (ADS)

We present the numerical simulations of relativistic jets propagating initially oblique to the field lines of a magnetized ambient medium. Our simulations incorporate relativistic MHD in a four-dimensional spacetime and clearly show that (a) relatively weak, oblique fields (at 1/16 of the equipartition value) have only a negligible influence on the propagating jet and they are passively pushed away by the relativistically moving head; (b) oblique fields in equipartition with the ambient plasma provide more resistance and cause bending at the jet head, but the magnitude of this deflection and the associated backflow are small compared to those identified by previous studies with a 2-D slab model. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently during the simulations. The effect is analogous to pushing Japanese ``noren'' or vertical Venetian blinds out of the way while the slats are allowed to bend in 3-D space rather than as a 2-D slab structure. Applied to relativistic extragalactic jets from blazars, the new results are encouraging since superluminal outflows exhibit bending near their sources and their environments are profoundly magnetized---but observations do not provide support for irregular kinematics such as large-scale vortical motions and pronounced reverse flows near the points of origin.

Nishikawa, K.-I.; Koide, S.; Sakai, J.-I.; Frank, J.; Christodoulou, D. M.; Sol, H.; Mutel, R. L.

1997-12-01

304

3D Solar Null Point Reconnection MHD Simulations

NASA Astrophysics Data System (ADS)

Numerical MHD simulations of 3D reconnection events in the solar corona have improved enormously over the last few years, not only in resolution, but also in their complexity, enabling more and more realistic modeling. Various ways to obtain the initial magnetic field, different forms of solar atmospheric models as well as diverse driving speeds and patterns have been employed. This study considers differences between simulations with stratified and non-stratified solar atmospheres, addresses the influence of the driving speed on the plasma flow and energetics, and provides quantitative formulas for mapping electric fields and dissipation levels obtained in numerical simulations to the corresponding solar quantities. The simulations start out from a potential magnetic field containing a null-point, obtained from a Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) magnetogram magnetogram extrapolation approximately 8 hours before a C-class flare was observed. The magnetic field is stressed with a boundary motion pattern similar to - although simpler than - horizontal motions observed by SOHO during the period preceding the flare. The general behavior is nearly independent of the driving speed, and is also very similar in stratified and non-stratified models, provided only that the boundary motions are slow enough. The boundary motions cause a build-up of current sheets, mainly in the fan-plane of the magnetic null-point, but do not result in a flare-like energy release. The additional free energy required for the flare could have been partly present in non-potential form at the initial state, with subsequent additions from magnetic flux emergence or from components of the boundary motion that were not represented by the idealized driving pattern.

Baumann, G.; Galsgaard, K.; Nordlund, Å.

2013-06-01

305

NASA Astrophysics Data System (ADS)

A four-quadrant transducer (4QT) electric machine is an integrated electric machine used for hybrid electric vehicles. In this article a 4QT prototype machine that is radially integrated by two permanent-magnet synchronous machines is analyzed. Skewed slots are adopted for the two machines, and the multislice two-dimensional time-stepping finite element method is used to calculate the performance. The dividing method of the 4QT, the choice of calculated cutplanes and the determination of the number of slices are discussed. The no-load and load performance of the 4QT are calculated. The 4QT prototype machine was tested. The tested no-load line-to-line back electromotive force curves are in good agreement with the calculated ones, which verifies the feasibility of the proposed method.

Zheng, Ping; Thelin, Peter; Nordlund, Erik; Sadarangani, Chandur

2006-04-01

306

NASA Astrophysics Data System (ADS)

We demonstrate results of a numerical analysis on functionality of waveguide circulators working in the spectral range of optical communication. Studied circulators were silicon-based three-port waveguide splitters where working elements were slabs of two-dimensional magnetophotonic crystals (2D MPCs) with different structural parameters. An optimized circulator had a 2D MPC slab-hexagonal array of cylindrical air holes in silicon with a magneto-optical defect made of bismuth-substituted yttrium iron garnet and demonstrated an insertion loss of 9 dB and an isolation of 16 dB. If compared to circulators reported in Z. Wan and S. Fan [Appl. Phys. B 81, 369 (2005)], the studied circulators are attractive for realization due to the relative simplicity of their designs.

Yayoi, Kazuo; Tobinaga, Kazuma; Kaneko, Yusuke; Baryshev, Alexander V.; Inoue, Mitsuteru

2011-04-01

307

NASA Astrophysics Data System (ADS)

In this paper, we study the convergence property of PHYSALIS when it is applied to incompressible particle flows in two-dimensional space. PHYSALIS is a recently proposed iterative method which computes the solution without imposing the boundary conditions on the particle surfaces directly. Instead, a consistency equation based on the local (near particle) representation of the solution is used as the boundary conditions. One of the important issues needs to be addressed is the convergence properties of the iterative procedure. In this paper, we present the convergence analysis using Laplace and biharmonic equations as two model problems. It is shown that convergence of the method can be achieved but the rate of convergence depends on the relative locations of the cages. The results are directly related to potential and Stokes flows. However, they are also relevant to Navier-Stokes flows, heat conduction in composite media, and other problems.

Huang, Huaxiong; Takagi, Shu

2003-08-01

308

On the propagation of blobs in the magnetotail: MHD simulations

NASA Astrophysics Data System (ADS)

Using three-dimensional magnetohydrodynamic (MHD) simulations of the magnetotail, we investigate the fate of entropy-enhanced localized magnetic flux tubes ("blobs"). Such flux tubes may be the result of a slippage process that also generates entropy-depleted flux tubes ("bubbles") or of a rapid localized energy increase, for instance, from wave absorption. We confirm the expectation that the entropy enhancement leads to a tailward motion and that the speed and distance traveled into the tail increase with the entropy enhancement, even though the blobs tend to break up into pieces. The vorticity on the outside of the blobs twists the magnetic field and generates field-aligned currents predominantly of region-2 sense (earthward on the dusk side and tailward on the dawn side), which might provide a possibility for remote identification from the ground. The breakup, however, leads to more turbulent flow patterns, associated with opposite vorticity and the generation of region-1 sense field-aligned currents of lower intensity but approximately equal integrated magnitude.

Birn, J.; Nakamura, R.; Hesse, M.

2013-09-01

309

NASA Astrophysics Data System (ADS)

This work is part of an attempt to quantify the relationship between the permeability tensor ( K) and the micro-structure of natural porous media. A brief account is first provided of popular theories used to relate the micro-structure to K. Reasons for the lack of predictive power and restricted generality of current models are discussed. An alternative is an empirically based implicit model wherein K is expressed as a consequence of a few “pore-types” arising from the dynamics of depositional processes. The analytical form of that implicit model arises from evidence of universal association between pore-type and throat size in sandstones and carbonates. An explicit model, relying on the local change of scale technique is then addressed. That explicit model allows, from knowledge of the three-dimensional micro-geometry to calculate K explicitly without having recourse to any constitutive assumptions. The predictive and general character of the explicit model is underlined. The relevance of the change of scale technique is recalled to be contingent on the availability of rock-like three-dimensional synthetic media. A random stationary ergodic process is developed, that allows us to generate three-dimensional synthetic media from a two-dimensional autocorrelation function r(? x ,? y ) and associated probability density function ? ? measured on a single binary image. The focus of this work is to ensure the rock-like character of those synthetic media. This is done first through a direct approach: n two-dimensional synthetic media, derived from single set ( ? ? , r(? x ,? y )) yield n permeability tensors K {/i-1,n i} (calculated by the local change of scale) of the same order. This is a necessary condition to ensure that r(? x ,? y ) and ? ? carry all structural information relevant to K. The limits of this direct approach, in terms of required Central Process Unit time and Memory is underlined, raising the need for an alternative. This is done by comparing the pore-type content of a sandstone sample and n synthetic media derived from r(? x ,? y ) and ? ? measured on that sandstone-sample. Achievement of a good match ensures that the synthetic media comprise the fundamental structural level of all natural sandstones, that is a domainal structure of well-packed clusters of grains bounded by loose-packed pores.

Anguy, Yannick; Bernard, Dominique; Ehrlich, Robert

1996-05-01

310

An adaptive MHD method for global space weather simulations

A 3D parallel adaptive mesh refinement (AMR) scheme is described for solving the partial-differential equations governing ideal magnetohydrodynamic (MHD) flows. This new algorithm adopts a cell-centered upwind finite-volume discretization procedure and uses limited solution reconstruction, approximate Riemann solvers, and explicit multi-stage time stepping to solve the MHD equations in divergence form, providing a combination of high solution accuracy and computational

Darren L. De Zeeuw; Tamas I. Gombosi; Clinto P. T. Groth; Kenneth G. Powell; Quentin F. Stout

2000-01-01

311

NASA Astrophysics Data System (ADS)

We present the GPU calculation with the common unified device architecture (CUDA) for the Swendsen-Wang multi-cluster algorithm of two-dimensional classical spin systems. We adjust the two connected component labeling algorithms recently proposed with CUDA for the assignment of the cluster in the Swendsen-Wang algorithm. Starting with the q-state Potts model, we extend our implementation to the system of vector spins, the q-state clock model, with the idea of embedded cluster. We test the performance, and the calculation time on GTX580 is obtained as 2.51 nsec per a spin flip for the q=2 Potts model (Ising model) and 2.42 nsec per a spin flip for the q=6 clock model with the linear size L=4096 at the critical temperature, respectively. The computational speed for the q=2 Potts model on GTX580 is 12.4 times as fast as the calculation speed on a current CPU core. That for the q=6 clock model on GTX580 is 35.6 times as fast as the calculation speed on a current CPU core.

Komura, Yukihiro; Okabe, Yutaka

2012-06-01

312

Substorm features in MHD (magnetohydrodynamics) simulations of magnetotail dynamics

We present a review and extended analysis of characteristic results from our nonideal three-dimensional MHD simulations of unstable magnetotail evolution, which develops without the necessity of external driving or prescribed localization on nonideal effects. These modes involve magnetic reconnection at a near-Earth site in the tail, consistent with the near-Earth neutral line model of substorms. The evolution tailward of the reconnection site is characterized by plasmoid formation and ejection into the far tail, plasma sheet thinning between the near-Earth neutral line (X line) and the departing plasmoid, and fast tailward flow, which occupies large sections of the plasma sheet at larger distance from the X line, while it occurs only in very limited space and time sections close to the X line. The region earthward of the X line is characterized by dipolarization, propagating from midnight toward the flank regions and, perhaps, tailward. It is associated with the signatures of the substorm current wedge: reduction and diversion of cross-tail current from a region surrounding the reconnection site and increase of Region 1 type field-aligned currents. A mapping of these currents to the Earth on the basis of an empirical magnetic field model shows good agreement of the mapped current system with the observed Region 1 field-aligned current system and its substorm associated changes, including also a nightward and equatorward shift of the peaks of the field-aligned current density. The evolution of the mappings of the boundaries of the closed field line region bears strong resemblance to the formation and expansion of he auroral bulge. The consistency of all of these details with observed substorm features strongly supports the idea that substorm evolution in the tail is that of a large scale nonideal instability.

Birn, J.; Hesse, M.

1990-01-01

313

This paper assesses the accuracy of the simplified frame cavity conduction/convection and radiation models presented in ISO 15099 and used in software for rating and labeling window products. Temperatures and U-factors for typical horizontal window frames with internal cavities are compared; results from Computational Fluid Dynamics (CFD) simulations with detailed radiation modeling are used as a reference. Four different frames were studied. Two were made of polyvinyl chloride (PVC) and two of aluminum. For each frame, six different simulations were performed, two with a CFD code and four with a building-component thermal-simulation tool using the Finite Element Method (FEM). This FEM tool addresses convection using correlations from ISO 15099; it addressed radiation with either correlations from ISO 15099 or with a detailed, view-factor-based radiation model. Calculations were performed using the CFD code with and without fluid flow in the window frame cavities; the calculations without fluid flow were performed to verify that the CFD code and the building-component thermal-simulation tool produced consistent results. With the FEM-code, the practice of subdividing small frame cavities was examined, in some cases not subdividing, in some cases subdividing cavities with interconnections smaller than five millimeters (mm) (ISO 15099) and in some cases subdividing cavities with interconnections smaller than seven mm (a breakpoint that has been suggested in other studies). For the various frames, the calculated U-factors were found to be quite comparable (the maximum difference between the reference CFD simulation and the other simulations was found to be 13.2 percent). A maximum difference of 8.5 percent was found between the CFD simulation and the FEM simulation using ISO 15099 procedures. The ISO 15099 correlation works best for frames with high U-factors. For more efficient frames, the relative differences among various simulations are larger. Temperature was also compared, at selected locations on the frames. Small differences was found in the results from model to model. Finally, the effectiveness of the ISO cavity radiation algorithms was examined by comparing results from these algorithms to detailed radiation calculations (from both programs). Our results suggest that improvements in cavity heat transfer calculations can be obtained by using detailed radiation modeling (i.e. view-factor or ray-tracing models), and that incorporation of these strategies may be more important for improving the accuracy of results than the use of CFD modeling for horizontal cavities.

Gustavsen, Arlid; Kohler, Christian; Dalehaug, Arvid; Arasteh, Dariush

2008-12-01

314

Real-time simulation of MHD/steam power plants by digital parallel processors

NASA Astrophysics Data System (ADS)

Attention is given to a large FORTRAN coded program which simulates the dynamic response of the MHD/steam plant on either a SEL 32/55 or VAX 11/780 computer. The code realizes a detailed first-principle model of the plant. Quite recently, in addition to the VAX 11/780, an AD-10 has been installed for usage as a real-time simulation facility. The parallel processor AD-10 is capable of simulating the MHD/steam plant at several times real-time rates. This is desirable in order to develop rapidly a large data base of varied plant operating conditions. The combined-cycle MHD/steam plant model is discussed, taking into account a number of disadvantages. The disadvantages can be overcome with the aid of an array processor used as an adjunct to the unit processor. The conversion of some computations for real-time simulation is considered.

Johnson, R. M.; Rudberg, D. A.

315

Simulations of MHD Turbulence in the Interstellar Medium

NASA Astrophysics Data System (ADS)

We have considered through simulation a wide range of phenomena for magnetohydrodynamic (MHD) turbulence in the interstellar medium (ISM). The focus is on Alfven wave turbulence in the high plasma ? limit, and where the fluctuating field amplitudes are substantially less than the background magnetic field. Our results are presented in the context of radio scintillation observations. Summarizing, we have found that: 1) The anisotropic Fourier space structure for the Alfven wave cascade predicted by Goldreich and Sridhar is correct, and we have evaluated the dimensionless cascade constants and extended it to include the structure of a passive scalar. 2) At small scales, Alfven wavemodes evolve independently of other wavemodes, and the slow wavemode cascade rate is determined by the energy in the Alfven wavemodes. The fast wavemodes are essentially independent of the Alfven and slow wavemodes. 3) The Alfven wave cascade is unstable to imbalance, where the wave polarization with greater energy more quickly cascades the wavemodes in the subdominant direction than it is itself cascaded. The dominant polarization remains and does not further evolve after the subdominant polarization has been dissipated. 4) Gradients of the dynamical variables (velocity, magnetic field, and passive scalar) are focused into thin sheets aligned with the magnetic field, while the rest of the volume is quiescent. Furthermore, the locations of sheets among the two Alfven polarizations are uncorrelated. 5) Fluctuations in the electron density constitute a spatially varying index of refraction responsible for the distortion of radio waves. We modeled the electron density as a passive scalar in our simulations and studied its effect on radio wave propagation. The scattering angle distribution for short paths is extra-Gaussian due to the presence of spatially coherent structure (the gradient sheets). This leads to an amplification of the RMS scattering compared to turbulence with the same power spectrum but without the spatial coherence (random-phase). A second scattering enhancement arises from the spatial coherence along the magnetic field due to the anisotropic nature of the cascade. 6) An anisotropic small-scale cascade develops from isotropic outer-scale turbulence. Furthermore, at the outer scale, V tends to either align or anti-align with B. The space is divided into zones where it is either one case or the other, with sharp boundaries in between. 7) Whistler wave turbulence has been found to be anisotropic in the same manner as Alfven wave turbulence.

Maron, Jason; Goldreich, Peter

1999-11-01

316

MHD simulation of preflare energy storage in the corona using photospheric magnetic charts

Current sheet creation in the corona and energy accumulation has been demonstrated in 3D MHD numerical experiments. The two methods of boundary magnetic field settings are developed for simulations. Using photospheric magnetic charts for setting initial and boundary conditions permits to simulate more accurate preflare situation comparing to approximation of sunspots magnetic field by vertical dipoles. For reproduction of real

A. I. Podgorny; I. M. Podgorny

2004-01-01

317

NASA Astrophysics Data System (ADS)

Stress waves, known as acoustic emissions (AEs), are released by localized inelastic deformation events during the progressive failure of brittle rocks. Although several numerical models have been developed to simulate the deformation and damage processes of rocks, such as non-linear stress-strain behaviour and localization of failure, only a limited number have been capable of providing quantitative information regarding the associated seismicity. Moreover, the majority of these studies have adopted a pseudo-static approach based on elastic strain energy dissipation that completely disregards elastodynamic effects. This paper describes a new AE modelling technique based on the combined finite-discrete element method (FEM/DEM), a numerical tool that simulates material failure by explicitly considering fracture nucleation and propagation in the modelling domain. Given the explicit time integration scheme of the solver, stress wave propagation and the effect of radiated seismic energy can be directly captured. Quasi-dynamic seismic information is extracted from a FEM/DEM model with a newly developed algorithm based on the monitoring of internal variables (e.g. relative displacements and kinetic energy) in proximity to propagating cracks. The AE of a wing crack propagation model based on this algorithm are cross-analysed by traveltime inversion and energy estimation from seismic recordings. Results indicate a good correlation of AE initiation times and locations, and scaling of energies, independently calculated with the two methods. Finally, the modelling technique is validated by simulating a laboratory compression test on a granite sample. The micromechanical parameters of the heterogeneous model are first calibrated to reproduce the macroscopic stress-strain response measured during standard laboratory tests. Subsequently, AE frequency-magnitude statistics, spatial clustering of source locations and the evolution of AE rate are investigated. The distribution of event magnitude tends to decay as power law while the spatial distribution of sources exhibits a fractal character, in agreement with experimental observations. Moreover, the model can capture the decrease of seismic b value associated with the macrorupture of the rock sample and the transition of AE spatial distribution from diffuse, in the pre-peak stage, to strongly localized at the peak and post-peak stages, as reported in a number of published laboratory studies. In future studies, the validated FEM/DEM-AE modelling technique will be used to obtain further insights into the micromechanics of rock failure with potential applications ranging from laboratory-scale microcracking to engineering-scale processes (e.g. excavations within mines, tunnels and caverns, petroleum and geothermal reservoirs) to tectonic earthquakes triggering.

Lisjak, A.; Liu, Q.; Zhao, Q.; Mahabadi, O. K.; Grasselli, G.

2013-10-01

318

The assembly of peptides into ordered nanostructures is increasingly recognized as both a bioengineering tool for generating new materials and a critical aspect of aggregation processes that underlie neurological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. There is a major problem in understanding how extremely subtle sequence changes can lead to profound and often unexpected differences in self-assembly behavior. To better delineate the complex interplay of different microscopic driving forces in such cases, we develop a methodology to quantify and compare the propensity of different peptide sequences to form small oligomers during early self-assembly stages. This umbrella-sampling replica exchange molecular dynamics method performs a replica exchange molecular dynamics simulation along peptide association reaction coordinates using umbrella restraints. With this method, we study a set of sequence-similar peptides that differ in net charge: K+TVIIE?, K+TVIIE, and +K+TVIIE. Interestingly, experiments show that only the monovalent peptide, K+TVIIE, forms fibrils, whereas the others do not. We examine dimer, trimer, and tetramer formation processes of these peptides, and compute high-accuracy potential of mean force association curves. The potential of mean forces recapitulate a higher stability and equilibrium constant of the fibril-forming peptide, similar to experiment, but reveal that entropic contributions to association free energies can play a surprisingly significant role. The simulations also show behavior reminiscent of experimental aggregate polymorphism, revealed in multiple stable conformational states and association pathways. Our results suggest that sequence changes can have significant effects on self-assembly through not only direct peptide-peptide interactions but conformational entropies and degeneracies as well.

Jeon, Joohyun; Shell, M. Scott

2012-01-01

319

NASA Astrophysics Data System (ADS)

The biodegradation of oxidizable hydrocarbon contaminants in the subsurface requires the presence of compatible microbial communities as well as sufficient amounts of electron acceptors and nutrients. In this context, transverse mixing, driven by dispersion and diffusion, is one of the main mechanisms governing the availability of dissolved electron acceptors at a hydrocarbon plume fringe. Aerobic and anaerobic biodegradation of hydrocarbons limited by transverse mixing has been studied experimentally in 2D bench-scale flow-through tanks, filled with a saturated porous medium. Flow of groundwater through the tanks was induced by pumping water at one side through injection ports, and simultaneously extracting water at the other side of the tank. An ethylbenzene plume was established by injection through the central inlet port. A mixture of unlabeled and fully deuterium-labeled isotopomers was used in order to investigate the spatial distribution of degradation processes via monitoring of compound-specific stable isotope fractionation. In the first phase of the experiment, aerobic biodegradation was studied. For this purpose, the tank was recharged with water containing oxygen as a dissolved electron acceptor and the aerobic strain Pseudomonas putida F1 was inoculated. Later, nitrate was added to the recharge water as an additional electron acceptor and the denitrifying strain Aromatoleum aromaticum EbN1 was amended to study competitive aerobic/anaerobic biodegradation. A numerical reactive transport model of the experiment was set up for a model based interpretation of the observed degradation patterns. In a sensitivity analysis, the influence of the relevant hydrodynamic parameters on the observable distributions of ethylbenzene isotopomers, oxygen and nitrate was studied. Subsequent model calibration allowed for a good agreement with ethylbenzene concentrations measured at the tank outlet ports as well as oxygen concentrations, which were measured at several profiles perpendicular to the flow direction along the plume. Simulated microbial growth was strongest near the central tank inlet, where both, oxygen and ethylbenzene were available at high concentrations, and along the transverse mixing zone at the fringe of the developed ethylbenzene plume. Model based interpretation of the aerobic/anaerobic phase with competitive biodegradation proved to be ambiguous due to uncertainties regarding the actual stoichiometry of the specific denitrification reaction. Also, the simulated isotopic patterns were very sensitive to the assumed initial distribution of the A. aromaticum EbN1 biomass. Ethylbenzene concentrations and isotopic patterns predicted by the numerical model match the measurements quite well for the first half of the aerobic/anaerobic phase. A distinct increase in biodegradation dynamics later on hints at a change in biodegradation dynamics during the course of the experiment.

Beyer, C.; Ballarini, E.; Bauer, R.; Griebler, C.; Bauer, S.

2011-12-01

320

NASA Astrophysics Data System (ADS)

The aeroacoustic sound generated from the flow around two NACA four-digit airfoils is investigated numerically, at relatively low Reynolds numbers that do not prompt boundary-layer transition. By using high-order finite-difference schemes to discretize compressible Navier-Stokes equations, the sound scattered on airfoil surface is directly resolved as an unsteady pressure fluctuation. As the wavelength of an emitted noise is shortened compared to the airfoil chord, the diffraction effect on non-compact chord length appears more noticeable, developing multiple lobes in directivity. The instability mechanism that produces sound sources, or unsteady vortical motions, is quantitatively examined, also by using a linear stability theory. While the evidence of boundary-layer instability waves is captured in the present result, the most amplified frequency in the boundary shear layer does not necessarily agree with the primary frequency of a trailing-edge noise, when wake instability is dominant in laminar flow. This contradicts the observation of other trailing-edge noise studies at higher Reynolds numbers. However, via acoustic disturbances, the boundary-layer instability may become more significant, through the resonance with the wake instability, excited by increasing a base-flow Mach number. Evidence suggests that this would correspond to the onset of an acoustic feedback loop. The wake-flow frequencies derived by an absolute-instability analysis are compared with the frequencies realized in flow simulations, to clarify the effect of an acoustic feedback mechanism, at a low Reynolds number.

Ikeda, Tomoaki; Atobe, Takashi; Takagi, Shohei

2012-01-01

321

NASA Astrophysics Data System (ADS)

An innovative pumped loop concept for 600 K space power system radiators utilizing direct contact heat transfer, which facilitates repeated startup/shutdown of the power system without complex and time-consuming coolant thawing during power startup, is under development. The heat transfer process with melting/freezing of Li in an NaK flow was studied through two-dimensional time-dependent numerical simulations to characterize and predict the Li/NaK radiator performance during startup (thawing) and shutdown (cold-trapping). Effects of system parameters and the criteria for the plugging domain are presented together with temperature distribution patterns in solid Li and subsequent melting surface profile variations in time.

Rhee, Hyop S.; Begg, Lester L.; Wetch, Joseph R.; Jang, Jong H.; Juhasz, Albert J.

322

NASA Astrophysics Data System (ADS)

In this paper the meshless local radial point interpolation method (LRPIM) is adopted to simulate the two-dimensional nonlinear sine-Gordon (S-G) equation. The meshless LRPIM is one of the “truly meshless” methods since it does not require any background integration cells. In this case, all integrations are carried out locally over small quadrature domains of regular shapes, such as circles or squares in two dimensions and spheres or cubes in three dimensions. A technique is proposed to construct shape functions using radial basis functions. These shape functions which are constructed by point interpolation method using the radial basis functions have delta function property. The time derivatives are approximated by the time-stepping method. In order to eliminate the nonlinearity, a simple predictor-corrector scheme is performed. Numerical results are obtained for various cases involving line and ring solitons. Also the conservation of energy in undamped sine-Gordon equation is investigated.

Dehghan, Mehdi; Ghesmati, Arezou

2010-04-01

323

Development of Real-Time Earth's Magnetosphere Simulator with 3-Dimensional MHD Code

NASA Astrophysics Data System (ADS)

The super computing system is a powerful equipment to realize physical models of the Sun-Earth connection system for Space Weather forecast. Adopting the three-dimensional (3D) magneto-hydrodynamical (MHD) simulation code developed by Tanaka, we have constructed the real-time numerical simulator of interplanetary space-magnetosphere-ionosphere coupling system. By using the real-time one-minute solar wind data of a density, a flow speed and interplanetary magnetic field (IMF) from the ACE spacecraft as boundary conditions, this MHD simulation system can reproduced numerically the global structure of magnetosphere at the same time with the real world. Our real-time MHD simulator can calculate the dynamical response of the magnetosphere and can predict the timing when the geomagnetic disturbances occur and how they develop. The real-time MHD simulator have been developed by using the super computer system (SX-6) in Communications Research Labortory (CRL). To realize a real-time speed in simulations, 8 CPUs are being used adapting the newly developed HPF (High Performance Fortran) language that is an appropriate language for the calculations in multiprocessor parallel system. By now we have achieved a real-time speed in a 3D simulation of interplanetary space-magnetosphere-ionosphere coupling system with the 44x56x60 mesh size. Visualization of dynamical response of the Earth's magnetosphere is also done at the same time to numerical calculations by using RVSLIB (Real Time Visualization System Library), which is developed by NEC. This real-time MHD simulator will be run routinely on the CRL's super computer system. In this paper, we will explain the system configuration in detail and some examples will be presented to show how the geomagnetic disturbances occur responding to a solar wind change. This project will be one of the essential approaches to perform Space Weather forecast.

Tanaka, T.; den, M.; Obara, T.; Shimazu, H.; Amo, H.; Hayashi, Y.; Nakano, E.; Seo, Y.; Suehiro, K.; Takahara, H.; Takei, T.

2003-12-01

324

The soft X-ray telescope on the Yohkoh mission enabled us to observe the evolution of emerging flux regions (EFR) in coronal X-rays with high spatial and temportal resolution. Futhermore, we now have enough computing capability to perform three-dimensional MHD simulation of EFRs with sufficient spacial resolution to study details of the flux emergence process. These new tools provide the opportunity to investigate the physics involved in the formation of coronal loops in much more detail. We carried out 3D MHD simulations of emerging magnetic flux regions under various initial conditions; (1) a horizontal magnetic flux sheet, (2) a bundle of horizontal flux tubes, and (3) a flux sheet with sheared magnetic fields. Numerical results show that coronal magnetic loops are formed due to the enhanced bouyancy resulting from gas precipitating along magnetic field lines. The interchange modes help to produce a fine fibrous structure perpendicular to the magnetic field direction in the linear stage, while the undular modes determine the overall loop structure. We observe in 3D simulations that during the ascendance of loops the bundle of flux tubes, or even the flux sheet, developes into dense filaments pinched between magnetic loops. We also find that magnetic field lines are twisted by the vortex motion produced by the horizontal expansion of magnetic loops. Our numerical results may explain the observed signatures such as (1) the spacial relation between soft X-ray loops and H[alpha] arch filaments obtained by coordinated observation between Yohkoh and ground-based observatories (Kawai et al. 1992), (2) the rate of increase in size of soft X-ray loops in EFRs (Ishido et al. 1992), (3) emergence of twisted magnetic loops, and (4) the threshold flux for formation of chromospheric arch filament systems (AFS).

Matsumoto, R.; Tajima, T.; Kaisig, M.; Shibata, K.; Ishido, Y.; Tsuneta, S.; Kawai, G; Kurokawa, H.; Akioka, M.; Acton, L.; Strong, K.; Nitta, N.

1992-01-01

325

NASA Astrophysics Data System (ADS)

A two-dimensional vernier scale is disclosed utilizing a cartesian grid on one plate member with a polar grid on an overlying transparent plate member. The polar grid has multiple concentric circles at a fractional spacing of the spacing of the cartesian grid lines. By locating the center of the polar grid on a location on the cartesian grid, interpolation can be made of both the X and Y fractional relationship to the cartesian grid by noting which circles coincide with a cartesian grid line for the X and Y direction.

Juday, Richard D.

1992-01-01

326

Two-dimensional colloidal alloys.

We study the structure of mixed monolayers of large (3 ?m diameter) and small (1 ?m diameter) very hydrophobic silica particles at an octane-water interface as a function of the number fraction of small particles ?. We find that a rich variety of two-dimensional hexagonal super-lattices of large (A) and small (B) particles can be obtained in this system due to strong and long-range electrostatic repulsions through the nonpolar octane phase. The structures obtained for the different compositions are in good agreement with zero temperature calculations and finite temperature computer simulations. PMID:21517357

Law, Adam D; Buzza, D Martin A; Horozov, Tommy S

2011-03-24

327

NASA Astrophysics Data System (ADS)

A two-dimensional particle-in-cell simulation is performed to investigate weakly magnetized perpendicular shocks with a magnetization parameter of ? = 6 × 10-5, which is equivalent to a high Alfvén Mach number M A of ~130. It is shown that current filaments form in the foot region of the shock due to the ion-beam-Weibel instability (or the ion filamentation instability) and that they generate a strong magnetic field there. In the downstream region, these current filaments also generate a tangled magnetic field that is typically 15 times stronger than the upstream magnetic field. The thermal energies of electrons and ions in the downstream region are not in equipartition and their temperature ratio is T e/T i ~ 0.3-0.4. Efficient electron acceleration was not observed in our simulation, although a fraction of the ions are accelerated slightly on reflection at the shock. The simulation results agree very well with the Rankine-Hugoniot relations. It is also shown that electrons and ions are heated in the foot region by the Buneman instability (for electrons) and the ion-acoustic instability (for both electrons and ions). However, the growth rate of the Buneman instability is significantly reduced due to the relatively high temperature of the reflected ions. For the same reason, ion-ion streaming instability does not grow in the foot region.

Kato, Tsunehiko N.; Takabe, Hideaki

2010-09-01

328

NASA Astrophysics Data System (ADS)

Oxygenic photosynthesis is key to life on this planet, and photosystem II is key to oxygenic photosynthesis. The only natural molecule capable of splitting water, it has been studied extensively with a wide range of linear and nonlinear spectroscopic methods. Still, the energy and charge transfer pathways remain poorly understood. Two dimensional electronic spectroscopy (2DES) extends previous non-linear spectroscopics into an additional frequency axis, uncovering information about electronic coupling and energy transfer that is difficult to discern in other methods. This thesis presents technical advances to 2DES with a pulse shaper in the pump-probe geometry, particularly phase-cycling for isolating signals of interest and for reducing scatter signals. This method is applied to the first 2DES measurements of the Qy band of the D1D2-cyt. b559 reaction center of photosystem II (PSII RC). A new method for extracting kinetic information from such a rich data set is presented: two dimensional decay associated spectra. The 2DES data directly reveal excitonic coupling between blue and red states within the band. The rapid growth of a cross-peak below the diagonal provides unambiguous evidence for energy equilibration within the reaction center on the order of 100 fs. Spectrally dependent lifetimes of 2--3 ps are observed, in agreement with a recent model in which charge separation occurs along two distinct pathways. Slower time constants of ˜7 ps and ˜50 ps are consistent with slow energy transfer from peripheral chlorophylls and secondary charge transfer, respectively. The first simulations of the PSII RC are presented and compared to experiment. The simulations examine a well-tested model for the excitonic structure of the PSII RC, which provides a good description for linear absorption, linear dichroism, circular dichroism, steady-state fluorescence, triplet-minus-singlet as well as Stark spectra. The resulting simulations match neither the experimental lineshapes nor the observed kinetics, revealing the power of 2DES for constraining theoretical models. An improved version of this model is proposed that gives qualitatively better lineshapes, although still fails to predict the observed kinetics. The thesis concludes with a brief discussion of future experimental and simulation work that is needed that builds on the work presented here.

Lewis, Kristin Lee Morgenstern

329

NASA Astrophysics Data System (ADS)

In this paper, we describe a cell-centered Lagrangian scheme devoted to the numerical simulation of solid dynamics on two-dimensional unstructured grids in planar geometry. This numerical method, utilizes the classical elastic-perfectly plastic material model initially proposed by Wilkins [M.L. Wilkins, Calculation of elastic–plastic flow, Meth. Comput. Phys. (1964)]. In this model, the Cauchy stress tensor is decomposed into the sum of its deviatoric part and the thermodynamic pressure which is defined by means of an equation of state. Regarding the deviatoric stress, its time evolution is governed by a classical constitutive law for isotropic material. The plasticity model employs the von Mises yield criterion and is implemented by means of the radial return algorithm. The numerical scheme relies on a finite volume cell-centered method wherein numerical fluxes are expressed in terms of sub-cell force. The generic form of the sub-cell force is obtained by requiring the scheme to satisfy a semi-discrete dissipation inequality. Sub-cell force and nodal velocity to move the grid are computed consistently with cell volume variation by means of a node-centered solver, which results from total energy conservation. The nominally second-order extension is achieved by developing a two-dimensional extension in the Lagrangian framework of the Generalized Riemann Problem methodology, introduced by Ben-Artzi and Falcovitz [M. Ben-Artzi, J. Falcovitz, Generalized Riemann Problems in Computational Fluid Dynamics, Cambridge Monogr. Appl. Comput. Math. (2003)]. Finally, the robustness and the accuracy of the numerical scheme are assessed through the computation of several test cases.

Maire, Pierre-Henri; Abgrall, Rémi; Breil, Jérôme; Loubère, Raphaël; Rebourcet, Bernard

2013-02-01

330

Gas stream composition and temperature determination in a coal-fired MHD simulation facility

A minicomputer controlled and monitored test stand for simulation of the gas stream conditions which will exist in various components of a coal-fired baseline MHD power plant and for evaluation of the substructures is described. Emphasis is devoted to the thermal aspects of the design and operation of this facility. A comprehensive thermal model of the system is described, and

R. E. Powe

1978-01-01

331

This paper proposes diagnosis methods to trace the magnetic merging line and to calculate the electric potential along it for Earth's magnetospheric magnetic fields obtained by global magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere-ionosphere (SMI) system. The points with minimum magnetic field strength along last closed magnetic field lines and properly selected closed field lines are combined to trace the

Y. Q. Hu; Z. Peng; C. Wang; J. R. Kan

2009-01-01

332

Numerical Simulation of Free Surface MHD Flows: Richtmyer - Meshkov Instability and Applications

Abstract: Numerical methods for free surface MHD ows have beendeveloped and numerical simulations of the Richtmyer - Meshkov typeinstability in liquid jets in strong magneticelds caused by an externalenergy deposition have been performed. Numerical results shed light onthe evolution of the proposed Muon Collider target which will be designedas a pulsed jet of mercury interacting with strong proton beamsin a

Roman Samulyak; James Glimm; Oh Wonho; Harold Kirk; Kirk Mcdonald

2003-01-01

333

Dynamo Action and its Temporal Variation Inside the Tangent Cylinder in MHD Dynamo Simulations

We have been performing numerical simulations of MHD dynamos. In this paper, we present the results of our parameter survey for this problem. We consider a spherical shell rotating about its rotation axis (z-axis) and it is filled with electrically conducting Boussinesq fluid. The solid inner core has the same electrical conductivity as that of the fluid outer core, and

F. Takahashi; M. Matsushima; Y. Honkura

2002-01-01

334

Phase Transition-like Behavior of Magnetospheric Substorms: Global MHD Simulation Results

NASA Astrophysics Data System (ADS)

Because of their relevance to massive global energy loading and unloading, lots of observations and studies have been made for magnetic substorm events. Using nonlinear dynamical techniques, we investigate whether the simulated substorms from global MHD models have the non-equilibrium phase transition-like features revealed by \\markcite{Sitnov et al. [2000]}. We simulated 6 intervals of total duration of 240 hours from the same data set used in Sitnov et al. [2000]. We analyzed the input-output (vBs--pseudo-AL index) system obtained from the global MHD model and compared the results to those in \\markcite{Sitnov et al. [2000, 2001]}. The analysis of the coupled vBs--pseudo-AL index system shows the first-order phase transition map, which is consistent with the map obtained for the vBs--observed-AL index system from Sitnov et al. [2000]. The explanation lies in the cusp catastrophe model proposed by Lewis [1991]. Although, the comparison between observation and individual global MHD simulations may vary, the overall global transition pattern during the substorm cycle revealed by Singular Spectrum Analysis (SSA) is consistent between simulations and observations. This is an important validation of the global MHD simulations of the magnetosphere. The coupled vBs--pseudo-AL index system shows multi-scale behavior (scale-invarianet power-law dependence) in singular power spectrum. We found critical exponents of the non-equilibrium transitions in the magnetosphere, which reflect the multi-scale aspect of the substorm activity, different from power-law frequency of autonomous systems. The exponents relate input and output parameters of the magnetosphere and distinguish the second order phase transition model from the self-organized criticality model. We also discuss the limitations of the global MHD model in reproducing the multi-scale behavior when compared to the real system.

Shao, X.; Sitnov, M.; Sharma, A. S.; Papadopoulos, K.; Guzdar, P. N.; Goodrich, C. C.; Milikh, G. M.; Wiltberger, M. J.; Lyon, J. G.

2001-12-01

335

Two-dimensional (2D) photon-echo spectra of a single subunit of the Fenna-Matthews-Olson (FMO) bacteriochlorophyll trimer of Chlorobium tepidum are simulated, employing the equation-of-motion phase-matching approach (EOM-PMA). We consider a slightly extended version of the previously proposed Frenkel exciton model, which explicitly accounts for exciton coherences in the secular approximation. The study is motivated by a recent experiment reporting long-lived coherent oscillations in 2D transients [Engel et al., Nature 446, 782 (2007)] and aims primarily at accurate simulations of the spectroscopic signals, with the focus on oscillations of 2D peak intensities with population time. The EOM-PMA accurately accounts for finite pulse durations as well as pulse-overlap effects and does not invoke approximations apart from the weak-field limit for a given material system. The population relaxation parameters of the exciton model are taken from the literature. The effects of various dephasing mechanisms on coherence lifetimes are thoroughly studied. It is found that the experimentally detected multiple frequencies in peak oscillations cannot be reproduced by the employed FMO model, which calls for the development of a more sophisticated exciton model of the FMO complex.

Sharp, Leah Z.; Egorova, Dassia; Domcke, Wolfgang [Department of Chemistry, Technische Universitaet Muenchen, D-85747 Garching (Germany)

2010-01-07

336

2-D MHD numerical simulations of EML plasma armatures with ablation

NASA Astrophysics Data System (ADS)

We use a 2-D) resistive MHD code to simulate an EML plasma armature. The energy equation includes Ohmic heating, radiation heat transport and the ideal gas equation of state, allowing for variable ionization using the Saha equations. We calculate rail ablation taking into account the flow of heat into the interior of the rails. Our simulations show the development of internal convective flows and secondary arcs. We use an explicit Flux Corrected Transport algorithm to advance all quantities in time.

Boynton, G. C.; Huerta, M. A.; Thio, Y. C.

1993-01-01

337

A numerical study of soot formation in the near-field of a strongly radiating, nonpremixed, acetylene-air planar jet flame is conducted using Large Eddy Simulation in two dimensions to examine coupled turbulence, soot chemistry, and radiation effects. The two-dimensional, Favre-filtered, compressible Navier-Stokes, total sensible energy and mixture fraction equations are closed using the Smagorinsky subgrid-scale (SGS) turbulence model. Major species of gas-phase combustion are obtained using a laminar flamelet model by employing experimentally obtained laminar flame state relationships for the major species mass fractions as a function of gas-phase mixture fraction. A combination of a presumed Beta filtered density function and a scale-similarity model are used to account for SGS mixture fraction and scalar dissipation fluctuations on the filtered composition and heat release rate. A soot transport and finite-rate kinetics model accounting for soot nucleation, surface growth, agglomeration, and oxidation is used. Radiation is modeled by integrating the filtered radiative transfer equation using the discrete ordinates method. Both instantaneous and time-averaged results are presented in order to highlight physical and numerical modeling issues and to examine turbulence, soot chemistry, and radiation interactions. Qualitative comparisons are made to precious numerical results and experimental data.

Desjardin, P.E.; Frankel, S.H.

1999-10-01

338

A two-dimensional particle-in-cell (PIC) model is proposed to study the wake field and stopping power induced by a nonrelativistic charged particle moving perpendicular to the external magnetic field in two-component plasmas. The effects of the magnetic field on the wake potential and the stopping due to the polarization of both the plasma ions and electrons are discussed. The velocity fields of plasma ions and electrons are investigated, respectively, in the weak and strong magnetic field cases. Our simulation results show that in the case of weak magnetic field and high ion velocity, the wakes exhibit typical V-shaped cone structures and the opening cone angles decrease with the increasing ion velocity. As the magnetic field becomes strong, the wakes lose their typical V-shaped structures and become highly asymmetrical. Similar results can be obtained in the case of low ion velocity and strong magnetic field. In addition, stopping power is calculated and compared with previous one-dimensional and full three-dimensional PIC results. PMID:20866923

Hu, Zhang-Hu; Song, Yuan-Hong; Wang, You-Nian

2010-08-16

339

NASA Astrophysics Data System (ADS)

The critical behavior of adsorbed monomers that reversibly polymerize into linear chains with restricted orientations relative to the substrate has been studied. In the model considered here, which is known as self-assembled rigid rods (SARRs) model, the surface is represented by a two-dimensional lattice and a continuous orientational transition occurs as a function of temperature and coverage. The phase diagrams were obtained for the square, triangular, and honeycomb lattices by means of Monte Carlo simulations and finite-size scaling analysis. The numerical results were compared with Bethe-Peierls analytical predictions about the orientational transition for the square and triangular lattices. The analysis of the phase diagrams, along with the behavior of the critical average rod lengths, showed that the critical properties of the model do not depend on the structure of the lattice at low temperatures (coverage), revealing a quasi-one-dimensional behavior in this regime. Finally, the universality class of the SARRs model, which has been subject of controversy, has been revisited.

López, L. G.; Linares, D. H.; Ramirez-Pastor, A. J.; Stariolo, D. A.; Cannas, S. A.

2013-06-01

340

NASA Astrophysics Data System (ADS)

We have performed an analysis of fluid instabilities below the neutrinospheres of the collapsed cores of supernova progenitors using a methodology introduced by Bruenn and Dineva (Bruenn, S. W. and Dineva, T. S. 1996, ApJ, 458:L71-L74). In an extensive survey we found that the rate of lepton diffusion always exceeds the rate of thermal diffusion and as a result we do not anywhere see the neutron finger instability as described by the Livermore group (e.g. Wilson, J. R. and Mayle, R. W. 1993, Physics Reports 227:97-111). A new instability, lepto-entropy fingers, extending from a radius of 10-15 km out to the vicinity of the neutrinosphere, driven by the cross-response functions (i.e. the dependence of lepton transport on entropy perturbations and vice versa) was discovered. This instability has a maximum growth rate of the order of 100 s-1 with a scale of approximately 1/20 the distance of a perturbed fluid element from the core center (Bruenn, S. W., Raley, E. A., and Mezzacappa, A. to be submitted to the ApJ). This instability has probably already been seen in some multi-dimensional core collapse calculations. Using two dimensional hydrodynamics with radial ray multigroup flux-limited neutrino transport we have simulated doubly diffusive instabilities below the neutrinosphere. This work was partially funded by a grant from the DOE Office of Science, Scientific Discovery through Advanced Computing Program.

Raley, E. A.; Bruenn, S. W.

2003-12-01

341

Alternating Direction Implicit Techniques for Two-Dimensional Magnetohydrodynamic Calculations

Alternating direction implicit numerical techniques for solving time-; dependent, two-dimensional, two-fluid MHD equations are presented. The ; techniques are illustrated with applications to the dynamics of a theta pinch and ; the expansion of a laserproduced plasma, and important features of both cases are ; demonstrated. The enhanced numerical stability of the method is discussed. ; (auth);

Irvin Lindemuth; John Killeen

1973-01-01

342

The scaling properties of two-dimensional compressible magnetohydrodynamic turbulence

Understanding the phenomenology captured in direct numerical simulation (DNS) of magnetohydrodynamic (MHD) turbulence rests upon models and assumptions concerning the scaling of field variables and dissipation. Here compressible MHD turbulence is simulated in two spatial dimensions by solving the isothermal equations of resistive MHD on a periodic square grid. In these simulations it is found that the energy spectrum decreases more slowly with k, and the viscous cutoff length is larger, than would be expected from the 1941 phenomenology of Kolmogorov (K41). Both these effects suggest that the cascade time is modified by the presence of Alfven waves as in the phenomenology of Iroshnikov and Kraichnan (IK). Motivated by this, these scaling exponents are compared with those of the IK-based model of Politano and Pouquet [Phys. Rev. E 52, 636 (1995)], which is an extension of the model of She and Leveque [Phys. Rev. Lett. 72, 336 (1994)]. However, the scaling exponents from these simulations are not consistent with the model of Politano and Pouquet, so that neither IK nor K41 models would appear to describe the simulations. The spatial intermittency of turbulent activity in such simulations is central to the observed phenomenology and relates to the geometry of structures that dissipate most intensely via the scaling of the local rate of dissipation. The framework of She and Leveque implies a scaling relation that links the scaling of the local rate of dissipation to the scaling exponents of the pure Elsaesser field variables (z{sup {+-}}=v{+-}B/{radical}({mu}{sub o}{rho})). This scaling relation is conditioned by the distinct phenomenology of K41 and IK. These distinct scaling relations are directly tested using these simulations and it is found that neither holds. This deviation suggests that additional measures of the character of the dissipation may be required to fully capture the turbulent scaling, for example, pointing towards a refinement of the phenomenological models. It may also explain why previous attempts to predict the scaling exponents of the pure Elsaesser fields in two-dimensional magnetohydrodynamic turbulence by extending the theory of She and Leveque have proved unsuccessful.

Merrifield, J.A.; Arber, T.D.; Chapman, S.C.; Dendy, R.O. [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom); UKAEA Culham Division, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB (United Kingdom)

2006-01-15

343

Groundwater transport of tritium from the Cheshire (U-20n) site at the Nevada Test Site (NTS) is investigated using a stochastic approach and numerical simulations. The hydrogeologic system is modeled as a horizontal two dimensional fractured-rhyolite aquifer intersected by a rubble chimney that extends upward from the shot cavity. The chimney, which is considered to be more permeable than surrounding rocks, is thought to act as a conduit for flow of tritiated groundwater from the cavity to the aquifer under a vertically upward hydraulic gradient. Heterogeneity of hydraulic conductivity in the aquifer is described as a random function with the mean, variance, and isotropic correlation length estimated from available field data. Realizations of the conductivity field having these statistical characteristics are generated with a numerical implementation of the turning bands algorithm. Several realizations of the spatially correlated random groundwater velocity field are then produced for each realization of the conductivity field using a numerical finite-element code. Inclusion of groundwater flux from the chimney produces a {open_quotes}groundwater mound{close_quotes} in the aquifer near the chimney, with a significant component of flow directed opposite to regional flow, thereby introducing highly non-uniform flow to the otherwise uniform flow field. Transport of tritium in the flow field away from the chimney is simulated with the particle tracking random walk (PTRW) technique, which is suitable for treating the processes of advection and dispersion. Effects on the distribution of tritium caused by flux from the chimney, aquifer porosity, and size of the tritium source area are analyzed. Results indicate that the magnitude of chimney flux is an important control on tritium plume behavior because the nonuniform flow conditions in the chimney region greatly increase dispersion of the plume when compared to the case without chimney flux.

Pohlmann, K.; Andricevic, R.; Vallikat, V.

1993-09-01

344

The model of a multivelocity heterogeneous medium is used for one- and two-dimensional numerical calculations of the deflagration-to-detonation\\u000a transition for charges of a porous explosive enclosed in a casing. Calculation results are compared with experimental data.\\u000a Depending on the charge diameter, different explosion regimes — detonation and low-velocity explosive transformation — are\\u000a registered in both the two-dimensional calculations and experiments.

O. A. Dibirov; S. V. Tsikin; Yu. V. Yanilkin

2000-01-01

345

Flux-Tube Texture of the Solar Wind: Weakly Compressible MHD Theory and Direct Numerical Simulations

NASA Astrophysics Data System (ADS)

Over the years, there has been a steady accumulation of observational evidence that the solar wind may be thought of as a network of individual magnetic flux tubes each with its own magnetic and plasma characteristics [Bartley et al. 1966, Marliani et al. 1973, Tu and Marsch 1990, Bruno et al. 2001, Borovsky 2008]. The weakly compressible MHD (WC-MHD) model [Bhattacharjee et al., 1998], which incorporates the effect of background spatial inhomogeneities, has been used recently to characterize the anisotropic magnetic fluctuation spectra (the so-called variance anisotropy) observed by ACE spacecraft. For a model of local pressure-driven interchange turbulence in a generic solar wind flux tube, the WC-MHD theory uses the Invariance Principle approach [Connor and Taylor 1997, Bhattacharjee and Hameiri 1988] to calculate explicitly the scaling of magnetic field fluctuations with plasma beta and other background plasma parameters. We test these theoretical predictions by direct numerical simulations of interchange turbulence in a flux tube using the DEBS MHD code. Synthetic variance anisotropy within a generic flux tube is computed in the high-Lundquist-number regime, and shows remarkable similarity with ACE observations.

Bhattacharjee, A.; Sarkar, A.; Ebrahimi, F.

2012-10-01

346

A note on using thermally driven solar wind models in MHD space weather simulations

NASA Astrophysics Data System (ADS)

One of the challenges in constructing global magnetohydrodynamic (MHD) models of the inner heliosphere for, e.g., space weather forecasting purposes, is to correctly capture the acceleration and expansion of the solar wind. In many current models, the solar wind is driven by varying the polytropic index so that a desired heating is obtained. While such schemes can yield solar wind properties consistent with observations, they are not problem-free. In this work, we demonstrate by performing MHD simulations that altering the polytropic index affects the properties of propagating shocks significantly, which in turn affect the predicted space weather conditions. Thus, driving the solar wind with such a mechanism should be used with care in simulations where correctly capturing the shock physics is essential. As a remedy, we present a simple heating function formulation by which the polytropic wind can be used while still modeling the shock physics correctly.

Pomoell, Jens; Vainio, Rami

2011-06-01

347

Direct Numerical Simulations of Nonlinear Evolution of MHD Instability in LHD

Nonlinear evolutions of MHD instabilities in the large helical device are studied by means of direct numerical simulations under the vacuum configuration with the magnetic axis position R = 3.6m, including effects of its full three-dimensional geometry, flows parallel to the magnetic field lines and the fluid compressibility. The linear growth of the pressure-driven modes and their nonlinear saturations are observed. The linear growth brings about the flows parallel to the magnetic field lines as strong as the perpendicular flows. The fluid compressibility reduces the linear growth rate significantly. In the nonlinear saturation process, a qualitative difference is found in the behaviors of the parallel and perpendicular flows. The plasma appears to approach to a near-equilibrium state, keeping finite amplitudes of the parallel flow. Our numerical results highlight important roles of the parallel flow and compressibility in nonlinear MHD simulations in the large helical device.

Miura, H.; Nakajima, N.; Hayashi, T. [National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292 (Japan); Department of Fusion Science, The Graduate University for Advanced Studies (SOKENDAI), 322-6 Oroshi, Toki, Gifu 509-5292 (Japan); Okamoto, M. [Department of Information Science, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501 (Japan)

2006-11-30

348

Substorm effects in MHD and test particle simulations of magnetotail dynamics

Recent magnetohydrodynamic simulations demonstrate that a global tail instability, initiated by localized breakdown of MHD, can cause plasmoid formation and ejection as well as dipolarization and the current diversion of the substorm current wedge. The connection between the reconnection process and the current wedge signatures is provided by earthward flow from the reconnection site. Its braking and diversion in the inner magnetosphere causes dipolarization and the magnetic field distortions of the current wedge. The authors demonstrate the characteristic properties of this process and the current systems involved. The strong localized electric field associated with the flow burst and the dipolarization is also the cause of particle acceleration and energetic particle injections. Test particle simulations of orbits in the MHD fields yield results that are quite consistent with observed injection signatures.

Birn, J. [Los Alamos National Lab., NM (United States); Hesse, M. [National Aeronautics and Space Administration, Greenbelt, MD (United States). Goddard Space Flight Center

1998-12-31

349

Parametric decay of large-amplitude Alfvén waves: MHD and hybrid simulations

NASA Astrophysics Data System (ADS)

Parametric instabilities have often been invoked to explain some of the features of Alfvénic turbulence as observed in the (fast) solar wind plasma, namely the gradual reduction of cross helicity and its final saturation with heliocentric distance. Moreover, this instability could also be responsible for direct plasma heating and creation of transverse small-scale Alfvénic fluctuations, as required by recent models for coronal heating and solar wind acceleration. Here we discuss numerical simulations of the long-term nonlinear evolution of Alfvén waves, both in the MHD and hybrid regimes, performed by the plasma theory group in Florence in the last decade and we show, for the first time, 2-D MHD simulations of the parametric decay of oblique arc-polarized waves.

Del Zanna, L.; Matteini, L.; Landi, S.; Velli, M.

2012-05-01

350

NO\\/SUB\\/x formation experiments in an MHD simulation facility

A laboratory test facility was used to simulate the conditions present in a coal-fired MHD powerplant. Various quantities of oil, oil-flyash, and oil-flyash-KâSOâ were burned with preheated air in order to examine the effects of primary combustion stoichiometry and several secondary combustion parameters on the generation of nitrogen oxides. Experimental results show that the effect of changing the primary combustion

A. G. Wehr; R. Tang

1983-01-01

351

Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction

We perform a suite of simulations of cooling cores in clusters of galaxies in\\u000aorder to investigate the effect of the recently discovered heat flux buoyancy\\u000ainstability (HBI) on the evolution of cores. Our models follow the\\u000a3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture\\u000athe effects of anisotropic heat conduction along the lines of magnetic field,\\u000abut do

Tamara Bogdanovi; Christopher S. Reynolds; Steven A. Balbus; Ian J. Parrish

2009-01-01

352

Experimental study of MHD effects on turbulent flow of Flibe simulant fluid in circular pipe

An investigation of MHD effects on a Flibe (Li2BeF4) simulant fluid has been conducted under the U.S.–Japan JUPITER-II collaboration program using the “FLIHY” pipe flow facility at UCLA. The present paper reports experimental results on turbulent pipe flow of an aqueous potassium hydroxide solution under magnetic field using particle image velocimetry (PIV) technique. The modification of turbulence was investigated by

Junichi Takeuchi; Shin-ichi Satake; Neil B. Morley; Tomoaki Kunugi; Takehiko Yokomine; Mohamed A. Abdou

2008-01-01

353

NASA Astrophysics Data System (ADS)

We have performed an analysis of fluid instabilities below the neutrinospheres of the collapsed cores of supernova progenitors using a methodology introduced by Bruenn and Dineva [28, 29, 31]. In an extensive survey we found that the rate of lepton diffusion always exceeds the rate of thermal diffusion and as a result we do not anywhere see the neutron finger instability as described by the Livermore group [16, 17]. A new instability, lepto-entropy fingers, extending from a radius of 10 15 km out to the vicinity of the neutrinosphere, driven by the cross-response functions (i.e. the dependence of lepton transport on entropy perturbations and vice versa) was discovered. This instability has a maximum growth rate of the order of 100 s-1 with a scale of approximately 1/20 the distance of a perturbed fluid element from the core center [18]. This instability has probably already been seen in some multi-dimensional core collapse calculations. To test our results predicting the presence of doubly diffusive instabilities below the neutrinosphere of a proto-supernova, we have performed two dimensional hydrodynamic simulations with radial ray neutrino transport. This entailed rewriting RadHyd, which is the merger of EVH-1 hydrodynamics and MGFLD neutrino transport developed by Bruenn and DiNisco [43], for two dimensions. In particular, hydrodynamic evolution along angular arrays was included, as was MPI message passing capabilities, in order to utilize massively parallel computer platform such as FAU's BOCA4 Beowulf cluster. This work was partially funded by a grant from the DOE Office of Science, Scientific Discovery through Advanced Computing Program.

Raley, Elizabeth

2004-12-01

354

MHD Simulation of Comets: The Plasma Environment of Comet Hale-Bopp

NASA Astrophysics Data System (ADS)

MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvénic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H2O+, CO+ and soft x-ray images for observing conditions on April 11, 1997.

Gombosi, Tamas I.; Hansen, Kenneth C.; Dezeeuw, Darren L.; Combi, Michael R.; Powell, Kenneth G.

1997-09-01

355

NASA Astrophysics Data System (ADS)

Energy transport on the Kelvin-Helmholtz (K-H) instability is studied by the two-dimensional magnetohydrodynamic (MHD) simulation. In the two-dimensional simulation, the ridges of the surface waves are the line sources that can perturb the ambient plasma. Then, the slow-mode, the intermediate-mode and the fast-mode waves will be continually radiated from these line sources. The fast-mode waves will propagate away from these line sources. When the fast-mode Mach number of the surface wave is greater than or equal to one, the K-H instability will develop into the fast-mode Mach-cone-like plane waves generated by the constructive interferences of the fast-mode waves emitted from the ridges of the surface waves. In contrast with the surface waves, these Mach-cone-like plane waves can expand away from the velocity shear layer. The fast-mode wave should play an important role in transporting the wave energy away from the velocity shear layer. Our results indicate that the energy transport on the K-H instability will not take place only in the vicinity of the boundary layer since the fast-mode waves emitted from the ridges of the surface waves will continually extract a part of the energy of the surface waves and transport the energy away from the velocity shear layer. For the K-H instability characterized by the small eddies or the vortex structures, a small amount of the energy can be transported away from the boundary layer by the fast-mode waves. On the other hand, a huge amount of the energy can be transported away from the velocity shear layer with the expanding of the fast-mode Mach-cone-like plane waves. We conclude that the occurrences of the fast-mode Mach-cone-like plane waves generated by the K-H instability can provide an efficient way to transfer the wave energy deep into the medium far from the velocity shear layer. In the study, we also calculate the energy transport velocity and the theoretical group velocity of the fast-mode waves. These results are also compared with the results obtained by the linear wave analysis.

Lai, S.; Lyu, L.

2009-12-01

356

Comparison of local energy transfer estimates from Cluster with global MHD simulations

NASA Astrophysics Data System (ADS)

We have used several magnetopause crossings by the multi-spacecraft mission Cluster to make observational estimates of the local energy transfer across the magnetopause. During one occasion we could identify a load region on the dayside high-latitude magnetopause, where the energy was found to be transferred from the magnetic field to the particles as a probable consequence of the reconnection process during southward IMF. During another crossing further towards the dawn flank of the magnetosphere we could identify a generator region, where solar wind kinetic energy was transferred to the magnetic field as a result of magnetic stresses along the magnetopause during dominantly duskward IMF. These results from real spacecraft crossings have at the same locations been compared to the BATS-R-US global MHD simulation results, using the actual ACE solar wind plasma and magnetic field measurements from the individual events as model input. The local characteristics of the global model results correctly predict the regions crossed by Cluster during the two different cases as a load and a generator region, correspondingly. Furthermore, the magnitude of the energy transfer across the magnetopause as deduced from Cluster and the model are in reasonable agreement. These results may help to validate the use of global MHD simulations to estimate the total energy input to the magnetosphere, which is of considerable importance for Space Weather considerations. Also, it appears that any local Cluster observations at the magnetopause can be used to scale the MHD model results to obtain better global estimates.

Rosenqvist, L.; Opgenoorth, H.

2007-05-01

357

Simulation of Radiation Belt Wave-Particle Interactions Using MHD-SDE Methods

NASA Astrophysics Data System (ADS)

Interactions with a variety of plasma waves, including whistler-mode waves, EMIC waves and MHD waves, can cause significant local acceleration and loss, and/or radial transport of radiation belt particles, especially relativistic electrons. In this paper we report on the development of a new computational model designed to comprehensively simulate local acceleration/loss and radial transport in the radiation belts. The model uses bounce-averaged relativistic equations of motion to follow marker particles in the Lyon-Fedder-Mobarry global MHD code. The marker particles are subjected to energy and pitch-angle diffusion by a variety of high-frequency waves, including whistler waves and other ELF/VLF waves, by using stochastic differential equation (SDE) methods to diffusively adjust the marker-particle phase-space coordinates. The marker trajectories are used to construct phase-space densities and particle fluxes using Liouville-theorem weighting techniques similar to particle-in-cell methods. The new simulation model will calculate effects of (i) local acceleration and loss due to interactions with chorus, hiss, and EMIC waves, and magnetosonic equatorial noise, including quasilinear and non-quasilinear nonlinear interactions, and (ii) radial transport, including (but not restricted to) radial diffusion caused by MHD waves, all in a dynamic global magnetosphere.

Chan, A. A.; Elkington, S. R.; Albert, J. M.

2010-12-01

358

Explosive Turbulent Magnetic Reconnection: A New Approach of MHD-Turbulent Simulation

NASA Astrophysics Data System (ADS)

Turbulent flows are often observed in association with magnetic reconnection in space and astrophysical plasmas, and it is often hypothesized that the turbulence can contribute to the fast magnetic reconnection through the enhancement of magnetic dissipation. In this presentation, we demonstrate that an explosive turbulent reconnection can happen by using a new turbulent MHD simulation, in which the evolution of the turbulent transport coefficients are self-consistently solved together with the standard MHD equations. In our model, the turbulent electromotive force defined by the correlation of turbulent fluctuations between v and B is added to the Ohm's law. We discuss that the level of turbulent can control the topology of reconnection, namely the transition from the Sweet-Parker reconnection to the Petscheck reconnection occurs when the level of fluctuations becomes of order of the ambient physical quantities, and show that the growth of the turbulent Petscheck reconnection becomes much faster than the conventional one.

Hoshino, Masahiro; Yokoi, Nobumitsu; Higashimori, Katsuaki

2013-04-01

359

Neoclassical viscous stress tensor for non-linear MHD simulations with XTOR-2F

NASA Astrophysics Data System (ADS)

The neoclassical viscous stress tensor is implemented in the non-linear MHD code XTOR-2F (Lütjens and Luciani 2010 J. Comput. Phys. 229 8130-43), allowing consistent bi-fluid simulations of MHD modes, including the metastable branch of neoclassical tearing modes (NTMs) (Carrera et al 1986 Phys. Fluids 29 899-902). Equilibrium flows and bootstrap current from the neoclassical theory are formally recovered in this Chew-Goldberger-Low formulation. The non-linear behaviour of the new model is verified on a test case coming from a Tore Supra non-inductive discharge. A NTM threshold that is larger than with the previous model is obtained. This is due to the fact that the velocity is now part of the bootstrap current and that it differs from the theoretical neoclassical value.

Mellet, N.; Maget, P.; Lütjens, H.; Meshcheriakov, D.; the Tore Supra Team

2013-04-01

360

The shape of a two-dimensional viscous drop deforming in several time-dependent flow elds, including that due to a potential vortex, has been studied. Vortex flow was approximated by linearizing the induced velocity eld at the drop centre, giving rise to an extensional flow with rotating axes of stretching. A generalization of the potential vortex, a flow we have called rotating

K AUSIK S ARKAR; WILLIAM R. S CHOWALTER

2001-01-01

361

3D MHD simulations of planet migration in turbulent stratified disks

NASA Astrophysics Data System (ADS)

We performed 3D MHD numerical simulations of planet migration in stratified disks using the Godunov code PLUTO (Mignone et al. 2007). The disk is invaded by turbulence generated by the magnetorotational instability (MRI). We study the migration for planets with different mass to primary mass ratio. The migration of the low-mass planet (q=Mp/Ms=10-5) is dominated by random fluctuations in the torque and there is no defined direction of migration on timescales of 100 orbits. The intermediate-mass planet (q=Mp/Ms=10-4) can experience systematic outwards migration that was sustained for the times we were able to simulate.

Uribe, Ana; Klahr, Hubert; Flock, Mario; Henning, Thomas

2011-11-01

362

Radiation-MHD Simulations of HII Region Expansion in Turbulent Molecular Clouds

NASA Astrophysics Data System (ADS)

We use numerical simulations to investigate how the expansion of an HII region is affected by an ambient magnetic field. First we consider the test problem of expansion in a uniform medium with a unidirectional magnetic field. We then describe the expansion of an HII region in a turbulent medium, taking as our initial conditions the results of and MHD turbulence simulation. We find that although in the uniform medium case the magnetic field does produce interesting effects over long length and timescales, in the turbulent medium case the main effect of the magnetic field is to reduce the efficiency of fragmentation of the molecular gas.

Arthur, S. J.; Henney, W. J.; Mellema, G.; de Colle, F.; Vázquez-Semadeni, E.

2011-04-01

363

A fully three-dimensional (3D), time-dependent, MHD interplanetary global model (3D IGM) has been used, for the first time, to study the relationship between different forms of solar activity and transient variations of the north-south component, Bz, of the interplanetary magnetic field (IMF) at 1 AU. One form of solar activity, the flare, is simulated by using a pressure pulse at

C.-C. Wu; M. Dryer; S. T. Wu

1996-01-01

364

Five model flows of increasing complexity belonging to the class of stationary two-dimensional planar field-aligned magnetohydrodynamic (MHD) flows are presented which are well suited to the quantitative evaluation of MHD codes. The physical properties of these five flows are investigated using characteristic theory. Grid convergence criteria for flows belonging to this class are derived from characteristic theory, and grid convergence

H. De Sterck; A. Cs??k; D. Vanden Abeele; S. Poedts; H. Deconinck

2001-01-01

365

MHD-PIC Interlocked Simulation Model for Magnetic Reconnection

Although many kinds of simulation models have been developed to understand the complex plasma systems, the physical process and the spatial-temporal scales must be restricted by the fundamental assumption of each model. However, the interaction across multiple scales may play a crucial role in some plasma phenomena. Magnetic reconnection process is the typical example, where the kinetic process in the

T. Sugiyama; K. Kusano

2006-01-01

366

The use of one-dimensional hydraulic models currently is the standard method for estimating velocity fields through a bridge opening for scour computations and habitat assessment. Flood-flow contraction through bridge openings, however, is hydrodynamically two dimensional and often three dimensional. Although there is awareness of the utility of two-dimensional models to predict the complex hydraulic conditions at bridge structures, little guidance is available to indicate whether a one- or two-dimensional model will accurately estimate the hydraulic conditions at a bridge site. The U.S. Geological Survey, in cooperation with the North Carolina Department of Transportation, initiated a study in 2004 to compare one- and two-dimensional model results with field measurements at complex riverine and tidal bridges in North Carolina to evaluate the ability of each model to represent field conditions. The field data consisted of discharge and depth-averaged velocity profiles measured with an acoustic Doppler current profiler and surveyed water-surface profiles for two high-flow conditions. For the initial study site (U.S. Highway 13 over the Tar River at Greenville, North Carolina), the water-surface elevations and velocity distributions simulated by the one- and two-dimensional models showed appreciable disparity in the highly sinuous reach upstream from the U.S. Highway 13 bridge. Based on the available data from U.S. Geological Survey streamgaging stations and acoustic Doppler current profiler velocity data, the two-dimensional model more accurately simulated the water-surface elevations and the velocity distributions in the study reach, and contracted-flow magnitudes and direction through the bridge opening. To further compare the results of the one- and two-dimensional models, estimated hydraulic parameters (flow depths, velocities, attack angles, blocked flow width) for measured high-flow conditions were used to predict scour depths at the U.S. Highway 13 bridge by using established methods. Comparisons of pier-scour estimates from both models indicated that the scour estimates from the two-dimensional model were as much as twice the depth of the estimates from the one-dimensional model. These results can be attributed to higher approach velocities and the appreciable flow angles at the piers simulated by the two-dimensional model and verified in the field. Computed flood-frequency estimates of the 10-, 50-, 100-, and 500-year return-period floods on the Tar River at Greenville were also simulated with both the one- and two-dimensional models. The simulated water-surface profiles and velocity fields of the various return-period floods were used to compare the modeling approaches and provide information on what return-period discharges would result in road over-topping and(or) pressure flow. This information is essential in the design of new and replacement structures. The ability to accurately simulate water-surface elevations and velocity magnitudes and distributions at bridge crossings is essential in assuring that bridge plans balance public safety with the most cost-effective design. By compiling pertinent bridge-site characteristics and relating them to the results of several model-comparison studies, the framework for developing guidelines for selecting the most appropriate model for a given bridge site can be accomplished.

Wagner, Chad R.

2007-01-01

367

NASA Astrophysics Data System (ADS)

Studies of the magnetosphere during substorms based on the observational data of the solar wind and the geomagnetic indices have shown clear features of phase transition-like behavior [Sitnov et al., 2000]. The global MHD simulations of the events in the Bargatze et al. [1985] database are used to study the non-equilibrium phase transition-like features of substorms. We simulated 7 intervals of total duration of 280 hours from the same data set used in Sitnov et al. [2000]. From the simulations the AL index is computed from the maximum of the westward Hall current and is referred to as the pseudo-AL index. We analyzed the input-output (vBs-pseudo-AL index) system obtained from the global MHD model and compare the results to those in Sitnov et al. [2000, 2001]. The analysis of the coupled vBs-pseudo-AL index system shows the first-order phase transition characterizing global beahavior, similar to the case of vBs-observed-AL index [Sitnov et al., 2000]. Although, the comparison between observations and global MHD simulations for individual events may vary, the overall global transition pattern during the substorm cycle revealed by singular spectrum snalysis is statistically consistent between simulations and observations. The coupled vBs-pseudo-AL index system shows multi-scale behavior (scale-invariant power-law dependence) in singular power spectrum. We find critical exponents of the non-equilibrium transitions in the magnetosphere, which reflect the multi-scale aspect of the substorm activity, different from power-law frequency of autonomous systems. The exponents relate input and output parameters of the magnetosphere.

Shao, X.; Sitnov, M. I.; Sharma, A. S.; Papadopoulos, K.; Goodrich, C. C.; Guzdar, P. N.; Milikh, G. M.; Wiltberger, M. J.; Lyon, J. G.

2002-05-01

368

Two-dimensional nanolithography using atom interferometry

We propose a scheme for the lithography of arbitrary, two-dimensional nanostructures via matter-wave interference. The required quantum control is provided by a {pi}/2-{pi}-{pi}/2 atom interferometer with an integrated atom lens system. The lens system is developed such that it allows simultaneous control over the atomic wave-packet spatial extent, trajectory, and phase signature. We demonstrate arbitrary pattern formations with two-dimensional {sup 87}Rb wave packets through numerical simulations of the scheme in a practical parameter space. Prospects for experimental realizations of the lithography scheme are also discussed.

Gangat, A.; Pradhan, P.; Pati, G.; Shahriar, M.S. [Department of Electrical and Computer Engineering, Northwestern University, Evanston, Illinois 60208 (United States)

2005-04-01

369

NASA Astrophysics Data System (ADS)

In response to the preceding Comment by Garcia, Baras, and Mansour [Phys. Rev. E 51, 3784 (1995)], we evaluate the Rayleigh number by taking the temperature jump at the wall into consideration. It is shown that a good agreement between the direct simulation Monte Carlo results and the linear stability theory is obtained by using the diffuse boundary condition, while there is a slight discrepancy in the case of the semislip boundary condition.

Watanabe, Tadashi; Kaburaki, Hideo; Yokokawa, Mitsuo

1995-04-01

370

Reconnection in a dipole-dominated magnetosphere: A two-dimensional model

NASA Astrophysics Data System (ADS)

We study the problem of onset of reconnection and the corresponding nonlinear evolution of reconnection in the near-Earth plasma sheet, where the influence of the Earth's dipole field is essential. This is done by the help of two-dimensional resistive MHD simulations. The simulations start from a two-dimensional equilibrium model for the near-Earth plasma sheet, including the Earth's dipole field explicitly. The stability properties of this start configuration differ significantly from the stability of two-dimensional tail equilibria, including both plasma sheet and lobe. This allows us to study onset and evolution of resistive modes that are linearly stable but non-linearly unstable. We find qualitatively significant differences, as compared with simulations of tail configurations. For instance, we find sunward flow both tailward and earthward of the reconnection X line. The corresponding reconnection rates come out to be about an order of magnitude lower compared with reconnection as a result of a large-scale tearing mode in tail configurations. We show that the reconnection rates can be significantly increased by suitable modifications of the start configuration. .

Wiechen, H.; Büchner, J.; Otto, A.

1995-10-01

371

THE SUBMILLIMETER BUMP IN Sgr A* FROM RELATIVISTIC MHD SIMULATIONS

Recent high resolution observations of the Galactic center black hole allow for direct comparison with accretion disk simulations. We compare two-temperature synchrotron emission models from three-dimensional, general relativistic magnetohydrodynamic simulations to millimeter observations of Sgr A*. Fits to very long baseline interferometry and spectral index measurements disfavor the monochromatic face-on black hole shadow models from our previous work. Inclination angles {<=}20{sup 0} are ruled out to 3{sigma}. We estimate the inclination and position angles of the black hole, as well as the electron temperature of the accretion flow and the accretion rate, to be i=50{sup o+35o}{sub -15}{sup o}, {xi}=-23{sup o+97o}{sub -22}{sup o}, T{sub e} = (5.4 {+-} 3.0) x 10{sup 10} K, and M-dot =5{sup +15}{sub -2}x10{sup -9} M{sub sun} yr{sup -1}, respectively, with 90% confidence. The black hole shadow is unobscured in all best-fit models, and may be detected by observations on baselines between Chile and California, Arizona, or Mexico at 1.3 mm or .87 mm either through direct sampling of the visibility amplitude or using closure phase information. Millimeter flaring behavior consistent with the observations is present in all viable models and is caused by magnetic turbulence in the inner radii of the accretion flow. The variability at optically thin frequencies is strongly correlated with that in the accretion rate. The simulations provide a universal picture of the 1.3 mm emission region as a small region near the midplane in the inner radii of the accretion flow, which is roughly isothermal and has {nu}/{nu} {sub c} {approx} 1-20, where {nu} {sub c} is the critical frequency for thermal synchrotron emission.

Dexter, Jason [Department of Physics, University of Washington, Seattle, WA 98195-1560 (United States); Agol, Eric [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States); Fragile, P. Chris [Department of Physics and Astronomy, College of Charleston, Charleston, SC 29424 (United States); McKinney, Jonathan C., E-mail: jdexter@u.washington.ed [Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305-4060 (United States)

2010-07-10

372

Constrained Transport vs. Divergence Cleanser Options in Astrophysical MHD Simulations

NASA Astrophysics Data System (ADS)

In previous work, we presented results from global numerical simulations of the evolution of black hole accretion disks using the Cosmos++ GRMHD code. In those simulations we solved the magnetic induction equation using an advection-split form, which is known not to satisfy the divergence-free constraint. To minimize the build-up of divergence error, we used a hyperbolic cleanser function that simultaneously damped the error and propagated it off the grid. We have since found that this method produces qualitatively and quantitatively different behavior in high magnetic field regions than results published by other research groups, particularly in the evacuated funnels of black-hole accretion disks where Poynting-flux jets are reported to form. The main difference between our earlier work and that of our competitors is their use of constrained-transport schemes to preserve a divergence-free magnetic field. Therefore, to study these differences directly, we have implemented a constrained transport scheme into Cosmos++. Because Cosmos++ uses a zone-centered, finite-volume method, we can not use the traditional staggered-mesh constrained transport scheme of Evans & Hawley. Instead we must implement a more general scheme; we chose the Flux-CT scheme as described by Toth. Here we present comparisons of results using the divergence-cleanser and constrained transport options in Cosmos++.

Lindner, Christopher C.; Fragile, P.

2009-01-01

373

This report describes the computer model NRFL02 which has been developed to calculate the near-field stratified turbulent flow driven by the intakes and outflows of an ocean thermal power plant. A two-dimensional geometry is assumed, with the power plant intakes and jet outflows modelled by boundary conditions on the left boundary of a rectangular domain. Horizontal flow through the right

G. O. Roberts; S. A. Piacsek; J. Toomre

1977-01-01

374

Purpose – The purpose of this paper is to propose an efficient\\/robust numerical algorithm for solving the two-dimensional laminar mixed-convection in a lid-driven cavity using the mixed finite element (FE) technique. Design\\/methodology\\/approach – A numerical algorithm was based on the so-called consistent splitting scheme, which improved the numerical accuracy of the primitive variables. In order to obtain a stable solution,

Jeff C.-F. Wong

2007-01-01

375

Two Dimensional Unstable Scar Statistics.

National Technical Information Service (NTIS)

This report examines the localization of time harmonic high frequency modal fields in two dimensional cavities along periodic paths between opposing sides of the cavity. The cases where these orbits lead to unstable localized modes are known as scars. Thi...

J. D. Kotulski K. S. H. Lee L. K. Warne R. E. Jorgensen

2006-01-01

376

Two-Dimensional Critical Phenomena.

National Technical Information Service (NTIS)

Two dimensional critical systems are studied using transformation to free fields and conformal invariance methods. The relations between the two approaches are also studied. The analytical results obtained generally depend on universality hypotheses or on...

H. Saleur

1987-01-01

377

Pressure in MHD/Brinkman flow past a stretching sheet

NASA Astrophysics Data System (ADS)

Obtained pressure in a MHD/Brinkman flow of viscous fluid over a stretching sheet. In the MHD case, the pressure distribution is finite. The pressure for the two-dimensional and axisymmetric cases have a significant difference.

Tamizharasi, R.; Kumaran, V.

2011-12-01

378

NASA Astrophysics Data System (ADS)

Delcourt and Sauvaud [1999] trace energetic particles in a statistical magnetospheric model [Mead and Fairfield, 1975] and show that the existence of a magnetic field minimum in the outer cusp region causes transport of energetic (hundreds of keV) equatorial particles toward high latitudes at the dayside magnetopause. It is shown in Delcourt and Sauvaud [1999] that this process may be significant for popoulating the cusp region by energetic particles during substorms. Recently, we developed a 3D particle tracing code which traces particles in the electric and magnetic field obtained from the Lyon-Fedder-Mobbary (LFM) [Fedder and Lyon, 1987] global MHD simulation. We observed the large-scale transport of the equatorial particles to the cusp region and the pseudo-trapping at the cusp region. Two global MHD simulations are carried out. In the first case, the solar wind interplanetary magnetic field (IMF) turns from northward to southward and an idealized substorm is simulated. In the second case, the solar wind ram pressure is enhanced by 4 times and a shock compression event is simulated . We performed 3D high energy proton tracing with the global MHD simulation outputs for these two cases. We will also consider electrons. In the first case, We investigate the effects of the transport process (transport of the equatorial particles to high latitudes) on particle energization associated with substorms. In the second case, we test the idea on the dayside injection of the cusp-originated relativistic electrons (CORE) due to the motion of the dayside magnetopause [Tan and Fung, 2002].

Shao, X.; Fung, S. F.; Tan, L. C.; Papadopoulos, K.; Fok, M. C.; Wiltberger, M.; Lyon, J. G.; Goodrich, C. C.

2003-04-01

379

Realistic MHD Simulations of Formation of Sunspot-like Structures and Comparison with Observations

NASA Astrophysics Data System (ADS)

The process of formation of magnetic structures such as sunspot and pores in the turbulent convection zone is still enigma. However, the recent progress in numerical radiative MHD simulations provides clues about the possible mechanism of magnetic field accumulation in spontaneously formed stable structures. Implementation of sub-grid turbulent models in our "SolarBox" code, gives us the possibility to model more accurately turbulent properties, and reproduce the dynamics of the magnetized plasma. The code takes into account non-ideal (tabular) EOS, effects of ionization, chemical composition, radiation, turbulence and magnetic field. Our simulation results show an important role of vortices, which create local cavity of pressure and are associated with strong converging flows under the surface, during the initial stage of the spontaneous structure formation. The resulting structure represents a compact self-organized concentration of strong magnetic field, reaching ~6 kG in the interior, and ~1.5 kG on the surface. It has a cluster-like internal structurization, and is maintained by strong downdrafts extending into the deep layers. We discuss the role of turbulent MHD dynamics in this mechanism, and compare the simulation results with observations of the sunspot formation process during a magnetic flux emergence, from the Solar Dynamics Observatory and Hinode.

Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray, A. A.

2010-12-01

380

2--D Resistive MHD Simulations of Merging Co-- and Counter--Helicity Spheromaks

NASA Astrophysics Data System (ADS)

Studies of the global equilibrium properties of merging spheromaks as well as of the local properties of the reconnection boundary layer in the MHD limit will be presented. The Princeton Tokamak Simulation Code (TSC)(S.C. Jardin, et. al., J. Comp. Phys. 66) (1986) 481, a free--boundary, axisymmetric resistive MHD code, is modified to resolve the two spatial scales of the merging spheromak problem, the equilibrium scale and the boundary layer, and to include convective terms in the coded set of scalar momentum equations. The simulations reported here are performed with parameters T ~ 15 eV, ne ~ 10^14 cm-3, Bz ~ 2 kG. We present comparisons of our simulation results with results from recent analytic(R. Kulsrud, D. Uzdensky, personal communication) and experimental work(M. Yamada, et. al., Phys. Rev. Lett. 78) (1997) 3117 and Y. Ono, et. al., Phys. Rev. Lett. 76 (1996) 3328. In particular we emphasize merging rates for co-- and counter--helicity merging, boundary layer geometry, scaling of merging rate with dimensionless parameters, and formation of spheromak and Field Reversed Configuration (FRC) plasmas as a result of co-- and counter--helicity merging.

Carter, T. A.; Jardin, S. C.

1997-11-01

381

Overview of the Simulation of Wave Interactions with MHD Project (SWIM)

NASA Astrophysics Data System (ADS)

The SWIM center has the scientific objectives of: improving our understanding of interactions that both RF wave and particle sources have on extended-MHD phenomena, improving our capability for predicting and optimizing the performance of burning plasmas, developing an integrated computational system for treating multi-physics phenomena with the required flexibility and extensibility to serve as a prototype for the Fusion Simulation Project, addressing mathematics issues related to the multi-scale, coupled physics of RF waves and extended MHD, and optimizing the integrated system on high performance computers. Our Center has now built an end-to-end computational system that allows existing physics codes to be able to function together in a parallel environment and connects them to utility software components and data management systems. We have used this framework to couple together state-of-the-art fusion energy codes to produce a unique and world-class simulation capability. A physicist's overview of the Integrated Plasma Simulator (IPS) will be given and applications described. For example the IPS is being employed to support ITER with operational scenario studies.

Batchelor, Donald

2010-11-01

382

Impurity Mixing Due to MHD in MGI Simulations of DIII-D

NASA Astrophysics Data System (ADS)

Simulations of massive gas injection (MGI) into DIII-D are performed with the NIMROD code. An impurity source that is strongly localized to the edge is used to initiate the simulated MGI shutdown, based on observations that gas jets do not penetrate deeply into the plasma. When the neutral impurity source is poloidally symmetric, it is found that the distribution of impurity ions does not remain poloidally symmetric as the ions diffuse toward the core; instead 2D flows tend to concentrate the impurities on the low-field-side (LFS). Later, 3D flows associated with an m=1/n=1 MHD instability, efficiently mix the impurities into the core, causing a rapid increase in the central electron density simultaneous with a rapid decrease in the central temperature. Further simulations are carried out in which the impurity source is concentrated only on the LFS or the high-field-side (HFS). The effects of varying the poloidal impurity distribution on the efficiency of the MHD mixing are investigated.

Izzo, V. A.

2012-03-01

383

Realistic MHD simulations of magnetic self-organization in solar plasma

NASA Astrophysics Data System (ADS)

Filamentary structure is a fundamental property of the magnetized solar plasma. Recent high-resolution observations and numerical simulations have revealed close links between the filamentary structures and plasma dynamics in large-scale solar phenomena, such as sunspots and magnetic network. A new emerging paradigm is that the mechanisms of the filamentary structuring and large-scale organization are natural consequences of turbulent magnetoconvection on the Sun. We present results of 3D radiative MHD large-eddy simulations (LES) of magnetic structures in the turbulent convective boundary layer of the Sun. The results show how the initial relatively weak and uniformly distributed magnetic field forms the filamentary structures, which under certain conditions gets organized on larger scales, creating stable long-living magnetic structures. We discuss the physics of magnetic self-organization in the turbulent solar plasma, and compare the simulation results with observations.

Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour, N. N.

2011-06-01

384

MHD simulations of coronal mass ejections: Importance of the driving mechanism

The authors have investigated the importance of the form of the driving mechanism in MHD simulations of coronal mass ejections. Previous authors have performed simulations using a thermal driving mechanism, and have found that this mechanism cannot reproduce the observed features of mass ejections unless an elaborate model of the initial corona is used. They have devised a model simulation problem and have found that the use of a simple form for the initial corona, with an upward moving parcel of cold, dense plasma as the driving mechanism, can produce results that are consistent with many of the features observed by coronagraphs. Their results imply that the nature of the driving mechanism may play an important role in determining the dynamical evolution of mass ejections.

Linker, J.A.; Van Hoven, G. (Univ. of California, Irvine (United States)); Schnack, D.D. (Science Applications International Corp., San Diego, CA (United States))

1990-04-01

385

MHD simulation of preflare energy storage in the corona using photospheric magnetic charts

NASA Astrophysics Data System (ADS)

Current sheet creation in the corona and energy accumulation has been demonstrated in 3D MHD numerical experiments. The two methods of boundary magnetic field settings are developed for simulations. Using photospheric magnetic charts for setting initial and boundary conditions permits to simulate more accurate preflare situation comparing to approximation of sunspots magnetic field by vertical dipoles. For reproduction of real boundary condition (that is very important for flare prognoses) the magnetic flux between spots should be taken into account. The evolution of active region is considered during several days before the flare. The tilt of solar surface and deflection of line of sight magnetic field from the normal to the solar surface is taken into account. These calculations and results of observation support the electrodynamical solar flare model that explains solar flares and CME appearance during current sheet disruption due to fast reconnection. The model includes also all phenomena observed in flares. The calculations have been carried out for several big flares. The energy accumulated in the current sheet magnetic field exceeds 1032 erg. The Peresvet code is improved, it uses the absolute implicit difference scheme for MHD equations solving, which is conservative relative to magnetic flux. Examples of calculation for the real flare are demonstrated.

Podgorny, A. I.; Podgorny, I. M.

386

MHD simulations of jet formation - protostellar jets & applications to AGN jets

NASA Astrophysics Data System (ADS)

Jet formation MHD simulations are presented considering a variety of model setups. The first approach investigates the interrelation between the disk magnetisation profile and jet collimation. Our results suggest (and quantify) that outflows launched from a very concentrated region at the inner disk tend to be weakly collimated. In the second approach, jet formation is investigated from a magnetic field configuration consisting of a stellar dipole superposed by a strong disk field. We find that the central dipole considerably de-collimates the disk wind. In addition, reconnection flares are launched in the interaction region of disk and stellar magnetic field, subsequently changing the outflow mass flux by factors of two. The time interval between flare ejection is about 1000 Keplerian periods - surprisingly similar to the observed time lag between jet knots. The third approach considers radiative pressure effects on jet collimation - an environment which is interesting mainly for outflows from massive young stars (but also for relativistic jets). Finally we present relativistic MHD simulations of jet formation from accretion disks extenting the previous non-relativistic approaches.

Fendt, Christian; Vaidya, Bhargav; Porth, Oliver; Nezami, Somayeh Sheikh

2011-02-01

387

Intermittency, dissipation, and scaling in two-dimensional magnetohydrodynamic turbulence

Direct numerical simulations (DNS) provide a means to test phenomenological models for the scaling properties of intermittent MHD turbulence. The well-known model of She and Leveque, when generalized to MHD, is in good agreement with the DNS in three dimensions, however, it does not coincide with DNS in two dimensions (2D). This is resolved here using the results of recent DNS of driven MHD turbulence in 2D which directly determine the scaling of the rate of dissipation. Specifically, a simple modification to generalized refined similarity is proposed that captures the results of the 2D MHD simulations. This leads to a new generalization of She and Leveque in MHD that is coincident with the DNS results in 2D. A key feature of this model is that the most intensely dissipating structures, which are responsible for the intermittency, are thread-like in 2D, independent of whether the underlying phenomenology of the cascade is Kolmogorov or Iroshnikov Kraichnan.

Merrifield, J. A.; Chapman, S. C.; Dendy, R. O. [Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry, CV4 7AL (United Kingdom); UKAEA Culham Division, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom and Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry, CV4 7AL (United Kingdom)

2007-01-15

388

Stochastic models of two-dimensional fracture

NASA Astrophysics Data System (ADS)

Two statistical models of (strictly two-dimensional) layer destruction are presented. The first is built as a strict percolation model with an added ``conservation law'' (conservation of mass) as physical constraint. The second allows for damped or limited fracture. Two successive fracture crack thresholds are considered. Percolation (i.e., fracture) probability and cluster distributions are studied by use of numerical simulations. Different fractal dimension, critical exponents for cluster distribution, and universality laws characterize both models.

Ausloos, M.; Kowalski, J. M.

1992-06-01

389

Superdiffusion in two-dimensional Yukawa liquids

Superdiffusion of two-dimensional (2D) liquids was studied using an equilibrium molecular dynamics simulation. At intermediate temperatures, the mean-squared displacement, probability distribution function (PDF), and velocity autocorrelation function (VACF) all indicate superdiffusion; the VACF has a long-time tail; and the PDF indicates no Levy flights. These effects are predicted to occur in 2D dusty plasmas and other 2D liquids that can be modeled with a long-range repulsive potential.

Liu Bin; Goree, J. [Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242 (United States)

2007-01-15

390

Cloaking two-dimensional fermions

A cloaking theory for a two-dimensional spin-(1/2) fermion is proposed. It is shown that the spinor of the two-dimensional fermion can be cloaked perfectly through controlling the fermion's energy and mass in a specific manner moving in an effective vector potential inside a cloaking shell. Different from the cloaking of three-dimensional fermions, the scaling function that determines the invisible region is uniquely determined by a nonlinear equation. It is also shown that the efficiency of the cloaking shell is unaltered under the Aharonov-Bohm effect.

Lin, De-Hone [Department of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan (China)

2011-09-15

391

We apply the Liouville space hierarchical equations of motion method to calculate the linear and two-dimensional (2D) electronic spectra of the Fenna-Matthews-Olson (FMO) protein complex from Chlorobium tepidum, using a widely used model Hamiltonian. The absorption and linear dichroism spectra of the FMO complex, as well as the main features of the 2D spectra are well reproduced. However, comparison with the recent experimental 2D spectra reveals several limitations of the current model: (1) The homogeneous and inhomogeneous broadening seems to be overestimated for the first exciton peak, but may be underestimated for several other exciton peaks. (2) The calculated oscillations of the diagonal and off-diagonal peaks in the 2D spectra are much weaker than the experimental observations, which indicates that an improved model is needed for the excitonic dynamics of the FMO complex. PMID:21599074

Chen, Liping; Zheng, Renhui; Jing, Yuanyuan; Shi, Qiang

2011-05-21

392

A two-dimensional, steady-state ground-water-flow model of the shallow ground-water-flow system near the community of New Post, Sawyer County, Wis., was refined from an existing model of the area. Hydraulic-conductivity and recharge values were not changed from the existing model for the scenario simulations described in this report. Rather, the model was refined by adding detail along the Chippewa Flowage and then was used to simulate contributing areas for three potential replacement wells pumping 30,000 gallons per day. The model also was used to simulate potential surface-water leakage out of the Chippewa Flowage captured by replacement-well pumping. A range in resistance to vertical ground-water flow was simulated along the Chippewa Flowage for each potential replacement-well location to bound the potential effects of representing three-dimensional flow with a two-dimensional model. Results indicate that pumping from a replacement well sited about 130 feet from the Chippewa Flowage could capture as much as 39 percent of the total pumping from the flowage. Pumping from either of two potential replacement wells sited at least 400 feet from the Chippewa Flowage did not induce surface-water leakage out of the flowage regardless of the resistance applied along the flowage for simulations described in this report.

Juckem, Paul F.; Hunt, Randall J.

2008-01-01

393

Two-dimensional solvable chaos

NASA Astrophysics Data System (ADS)

Two methods are proposed to construct two-dimensional chaotic maps. Several examples of exactly solvable chaotic maps and their invariant measures are obtained. They are isomorphic maps of square to square, plane to plane and circle to circle having various symmetry such as uniform, rotational and the quartic rotational symmetry.

Sogo, Kiyoshi; Masumizu, Atsushi

2011-09-01

394

The closure of region-1 field-aligned current in MHD simulation

NASA Astrophysics Data System (ADS)

We have studied the closure path of Region-1 field-aligned current system in our global MHD simulation and found some interesting results. After leaving the ionosphere on the eveningside the current deviates from the field-aligned direction rather early and bends towards the Sun. After passing through the velocity shear region near the magnetopause, which is a generator region, it climbs up along the magnetopause, being more or less field-aligned. Because of this it again bends sunward. When approaching the noon-midnight meridian plane (Y=0 plane in GSM coordinates), however, the current becomes more perpendicular to the magnetic field. It passes through the Y=0 plane at about X=2±1, Z=11±1. On the morningside the closure path is a mirror reflection of that on the evening side. Thus the closure path seems to reside more on the dayside than perhaps usually thought. We give a simple physical explanation for the dayside bending, which is based on the MHD pressure balance equation.

Janhunen, P.; Koskinen, H. E. J.

395

NASA Astrophysics Data System (ADS)

The dynamics of structures of magnetic field, current density, and plasma flow generated during multi-pulsed coaxial helicity injection in spherical torus is investigated by 3-D nonlinear MHD simulations. During the driven phase, the flux and current amplifications occur due to the merging and magnetic reconnection between the preexisting plasma in the confinement region and the ejected plasma from the gun region involving the n=1 helical kink distortion of the central open flux column (COFC). Interestingly, the diamagnetic poloidal flow which tends toward the gun region is then observed due to the steep pressure gradients of the COFC generated by ohmic heating through an injection current winding around the inboard field lines, resulting in the formation of the strong poloidal flow shear at the interface between the COFC and the core region. This result is consistent with the flow shear observed in the HIST. During the decay phase, the configuration approaches the axisymmetric MHD equilibrium state without flow because of the dissipation of magnetic fluctuation energy to increase the closed flux surfaces, suggesting the generation of ordered magnetic field structure. The parallel current density ? concentrated in the COFC then diffuses to the core region so as to reduce the gradient in ?, relaxing in the direction of the Taylor state.

Kanki, Takashi; Nagata, Masayoshi; Kagei, Yasuhiro

2011-11-01

396

Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics

Most numerical investigations on the role of magnetic fields in turbulent molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However, MCs are weakly ionized, so that the time scale required for the magnetic field to diffuse through the neutral component of the plasma by ambipolar diffusion (AD) can be comparable to the dynamical time scale. We have performed a series of 256{sup 3} and 512{sup 3} simulations on supersonic but sub-Alfvenic turbulent systems with AD using the Heavy-Ion Approximation developed in Li et al. (2006). Our calculations are based on the assumption that the number of ions is conserved, but we show that these results approximately apply to the case of time-dependent ionization in molecular clouds as well. Convergence studies allow us to determine the optimal value of the ionization mass fraction when using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent systems. We find that ambipolar diffusion steepens the velocity and magnetic power spectra compared to the ideal MHD case. Changes in the density PDF, total magnetic energy, and ionization fraction are determined as a function of the AD Reynolds number. The power spectra for the neutral gas properties of a strongly magnetized medium with a low AD Reynolds number are similar to those for a weakly magnetized medium; in particular, the power spectrum of the neutral velocity is close to that for Burgers turbulence.

Klein, R I; Li, P S; McKee, C F; Fisher, R

2008-04-10

397

MHD SIMULATIONS OF ACCRETION ONTO Sgr A*: QUIESCENT FLUCTUATIONS, OUTBURSTS, AND QUASIPERIODICITY

High-resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations (QPOs), making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudospectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic (MHD) model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme and calibrating the parameters by observations, we show that the MHD turbulence in the environment of Sgr A* can by itself only produce factor two fluctuations in luminosity. These fluctuations cannot explain the magnitude of flares observed in this system. However, we also demonstrate that external forcing of the accretion disk, which may be generated by the 'clumpy material' raining down onto the disk from the large-scale flow, do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and by ground-based facilities in the near infrared. Strong, but short-term QPOs emerge naturally in the simulated light curves. We attribute these to nonaxisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.

Chan Chikwan [Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States); Liu Siming [Department of Physics and Astronomy, University of Glasgow, Kelvin Bldg Rm 620, Glasgow G12 8QQ (United Kingdom); Fryer, Christopher L.; Psaltis, Dimitrios; Oezel, Feryal; Melia, Fulvio [Department of Physics, the University of Arizona, Tucson, AZ 85721 (United States); Rockefeller, Gabriel [Computational Computer Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2009-08-10

398

MHD waves in small magnetic elements: comparing IMaX observations to simulations.

NASA Astrophysics Data System (ADS)

Small-scale magnetic fields are thought to play an important role in the heating of the outer solar atmosphere. By exploiting the high-spatial and temporal resolution of IMaX, the bidimensional spectropolarimeter on board the Sunrise balloon-borne observatory, we study the excitation of MHD waves in small magnetic elements, providing clues on the interaction of the magnetic structures with the photospheric forcing and the ambient acoustic field. The large fraction of magnetic features observed by IMaX made it possible to study the interaction between the photospheric granulation and the flux tubes from a statistical point-of-view. In particular we find a 90 degree phase lag with an high confidence level between the horizontal displacements of the flux tubes and the velocity perturbations measured inside them. We also find that the observational results are in excellent agreement with MHD simulations. This result suggests that the horizontal displacement of small-scale magnetic features by the surrounding granulation excites longitudinal waves within the magnetic elements.

Stangalini, M.; Solanki, S. K.; Cameron, R.

399

Large eddy simulations in MHD: The rise of counter-rotating vortices at the magnetopause

NASA Astrophysics Data System (ADS)

A study of the magnetohydrodinamic (MHD) development of coherent structures in compressible, inhomogeneous, mixing layers due to the velocity shear instability is reported. The non-linear evolution of the original vorticity sheet is computed with 3-D large eddy simulations (LES) of temporal mixing layers tailored to represent distinctive conditions at the terrestrial magnetopause. We find that the boundary layer is characterized by the growth of large-scale vortices and becomes a site of mass mixing and enhanced plasma diffusion. In MHD the Lorentz force and its associated baroclinic term, together with the ordinary baroclinic term, and stratified entropy across the mixing layer, conspire to hinder vorticity flux conservation. In our LES the non-conservation of vorticity becomes manifest after ~ one rollover time when in addition to vortices with positive rotation (the same as the original vorticity sheet) other coherent structures with strong negative vorticity also arise, a noteworthy effect examined here. It is found that the vorticity is concentrated in cores of both signs with absolute values ~ 4-5 × ?i, (maximum vorticity of the initial shear layer). Concomitant with 3-D vortex stretching, the kinetic helicity also rises at vorticity cores. Furthermore, high temperature occurs in the cores, ~ 3xTi (magnetospheric temperature) correlated with local density depletion, ~ 0.4xni (magnetospheric density), while gas and magnetic pressure remain close to surrounding values. The study is intended as a contribution to the understanding of solar wind interaction with the magnetosphere during periods of northward interplanetary magnetic field.

Gratton, F. T.; Bilbao, L. E.; Farrugia, C. J.; Gnavi, G.

2009-05-01

400

This study uses two conjunctions between Cluster and Double Star TC-1 spacecraft together with global magnetohydrodynamic (MHD) simulations to investigate the large-scale configuration of magnetic reconnection at the dayside magnetopause. Both events involve southward interplanetary magnetic fields with significant B y components. The first event occurred on 8 May 2004, while both spacecraft were exploring the dawn flank of the

J. Berchem; A. Marchaudon; M. Dunlop; C. P. Escoubet; J. M. Bosqued; H. Reme; I. Dandouras; A. Balogh; E. Lucek; C. Carr; Z. Pu

2008-01-01

401

We present three-dimensional magnetohydrodynamic (MHD) simulations of superbubbles, to study the importance of MHD effects in the interpretation of images from recent surveys of the Galactic plane. These simulations focus mainly on atmospheres defined by an exponential density distribution and the Dickey and Lockman density distribution. In each case, the magnetic field is parallel to the Galactic plane and we investigate cases with either infinite scale height (constant magnetic field) or a constant ratio of gas pressure to magnetic pressure. The three-dimensional structure of superbubbles in these simulations is discussed with emphasis on the axial ratio of the cavity as a function of magnetic field strength and the age of the bubble. We investigate systematic errors in the age of the bubble and scale height of the surrounding medium that may be introduced by modeling the data with purely hydrodynamic models. Age estimates derived with symmetric hydrodynamic models fitted to an asymmetric magnetized superbubble can differ by up to a factor of 4, depending on the direction of the line of sight. The scale height of the surrounding medium based on the Kompaneets model may be up to 50% lower than the actual scale height. We also present the first ever predictions of Faraday rotation by a magnetized superbubble based on three-dimensional MHD simulations. We emphasize the importance of MHD effects in the interpretation of observations of superbubbles.

Stil, Jeroen; Wityk, Nicole; Ouyed, Rachid; Taylor, A. R. [Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4 (Canada)

2009-08-10

402

Application of a 3D, Adaptive, Parallel, MHD Code to Supernova Remnant Simulations

NASA Astrophysics Data System (ADS)

We at Michigan have a computational model, BATS-R-US, which incorporates several modern features that make it suitable for calculations of supernova remnant evolution. In particular, it is a three-dimensional MHD model, using a method called the Multiscale Adaptive Upwind Scheme for MagnetoHydroDynamics (MAUS-MHD). It incorporates a data structure that allows for adaptive refinement of the mesh, even in massively parallel calculations. Its advanced Godunov method, a solution-adaptive, upwind, high-resolution scheme, incorporates a new, flux-based approach to the Riemann solver with improved numerical properties. This code has been successfully applied to several problems, including the simulation of comets and of planetary magnetospheres, in the 3D context of the Heliosphere. The code was developed under a NASA computational grand challenge grant to run very rapidly on parallel platforms. It is also now being used to study time-dependent systems such as the transport of particles and energy from solar coronal mass ejections to the Earth. We are in the process of modifying this code so that it can accommodate the very strong shocks present in supernova remnants. Our test case simulates the explosion of a star of 1.4 solar masses with an energy of 1 foe, in a uniform background medium. We have performed runs of 250,000 to 1 million cells on 8 nodes of an Origin 2000. These relatively coarse grids do not allow fine details of instabilities to become visible. Nevertheless, the macroscopic evolution of the shock is simulated well, with the forward and reverse shocks visible in velocity profiles. We will show our work to date. This work was supported by NASA through its GSRP program.

Kominsky, P.; Drake, R. P.; Powell, K. G.

2001-05-01

403

NASA Astrophysics Data System (ADS)

We present global observations of Birkeland currents associated with two magnetic clouds passing by the Earth's magnetosphere. During the first event on 19--21 March 2001, the interplanetary magnetic field (IMF) is initially oriented southward with a negative B {y} component. After a sign reversal of B {y}, the IMF turns increasingly northward over the course of 36 hours. For the second event on 17--19 August 2003, the IMF is initially directed due northward and subsequently turns strongly southward while sustaining a negative IMF B {y} component. Following a brief interval of positive B {y}, the B {y} component turns negative once again, and the IMF transitions beyond its initial northward direction toward positive B {y} within a 24-hour period. The events were specifically chosen because of the slow transitions in the IMF orientation and because of the differences in the IMF rotation between the two events. The transitions are slow enough to derive global maps of Birkeland currents from the 70 satellites of the Iridium constellation. The Birkeland current distributions observed by Iridium show characteristic variations in response to changes in IMF orientation, and the most equatorward latitude of the Birkeland currents expands from 60o for northward IMF to 40o during southward IMF orientation. MHD simulations, driven by solar wind conditions observed during the magnetic cloud events, exhibit variations similar to the ones observed by Iridium. We compare the Iridium Birkeland current distributions obtained during the two events with the MHD simulation results and examine the temporal development of the Birkeland currents between observations and simulations.

Korth, H.; Anderson, B. J.; Lyon, J. G.; Wiltberger, M. J.

2004-12-01

404

Self-consistent simulations of nonlinear MHD and profile evolution in stellarator configurations

NASA Astrophysics Data System (ADS)

Self-consistent MHD equilibrium and nonlinear stability of 3D magnetic configurations are investigated using the extended MHD code NIMROD. In these calculations, initial conditions are given by 3D vacuum solutions with robust magnetic surfaces. We examine two classes of problems: those with current-driven instabilities and those with pressure-driven instabilities. Ohmic discharges in the Compact Toroidal Hybrid (CTH) are simulated [1]. The vacuum magnetic field of CTH is initialized and current is driven by specifying a toroidal electric field at the vessel boundary. The driven current penetrates toward the core and raises the rotational transform profile. Island formation is observed that is linked to the n=5 periodicity of the device. A prominent feature of these simulations is the coalescence of n/m=5/10 islands to n/m=1/2 islands when the rotational transform exceeds 0.5. At high levels of current drive, complete flux surface destruction is observed. Comparison with CTH data will be presented. Finite beta discharges in a straight stellarator are simulated. Vacuum magnetic fields are applied to produce stellarator-like rotational transform profiles with iota(0)<=0.5 and iota(0)>=0.5. The vacuum magnetic fields are either helically symmetric or spoiled by the presence of magnetic harmonics of incommensurate helicity. As heat is added to the system, pressure-driven instabilities are excited when a critical ? is exceeded. These instabilities may cause disruption, or they may saturate nonlinearly as the equilibrium evolves. In all of these studies, anisotropic heat conduction is allowed with kpar/kperp = 10^5--10^7. Due to the finite parallel heat conduction, in some cases an equilibrium state persists that has a stochastic edge region which supports a pressure gradient. [4pt] [1] M.G. Schlutt, et al., submitted to Nuclear Fusion (2012).

Schlutt, Mark

2012-10-01

405

Cooperative two-dimensional directed transport

NASA Astrophysics Data System (ADS)

A mechanism for the cooperative directed transport in two-dimensional ratchet potentials is proposed. With the aid of mutual couplings among particles, coordinated unidirectional motion along the ratchet direction can be achieved by transforming the energy from the transversal rocking force (periodic or stochastic) to the work in the longitude direction. Analytical predictions on the relation between the current and other parameters for the ac-driven cases are given, which are in good agreement with numerical simulations. Stochastic driving forces can give rise to the resonant directional transport. The effect of the free length, which has been explored in experiments on the motility of bipedal molecular motors, is investigated for both the single- and double-channel cases. The mechanism and results proposed in this letter may both shed light on the collective locomotion of molecular motors and open ways on studies in two-dimensional collaborative ratchet dynamics.

Zheng, Zhigang; Chen, Hongbin

2010-11-01

406

Two-dimensional thermofield bosonization

The main objective of this paper was to obtain an operator realization for the bosonization of fermions in 1 + 1 dimensions, at finite, non-zero temperature T. This is achieved in the framework of the real-time formalism of Thermofield Dynamics. Formally, the results parallel those of the T = 0 case. The well-known two-dimensional Fermion-Boson correspondences at zero temperature are shown to hold also at finite temperature. To emphasize the usefulness of the operator realization for handling a large class of two-dimensional quantum field-theoretic problems, we contrast this global approach with the cumbersome calculation of the fermion-current two-point function in the imaginary-time formalism and real-time formalisms. The calculations also illustrate the very different ways in which the transmutation from Fermi-Dirac to Bose-Einstein statistics is realized.

Amaral, R.L.P.G. [Instituto de Fisica, Universidade Federal Fluminense, Av. Litoranea S/N, Boa Viagem, Niteroi, CEP, 24210-340 Rio de Janeiro (Brazil)]. E-mail: rubens@if.uff.br; Belvedere, L.V. [Instituto de Fisica, Universidade Federal Fluminense, Av. Litoranea S/N, Boa Viagem, Niteroi, CEP, 24210-340 Rio de Janeiro (Brazil); Rothe, K.D. [Institut fuer Theoretische Physik, Universitaet Heidelberg, Philosophenweg 16, D-69120 Heidelberg (Germany)

2005-12-15

407

Comparison of local energy conversion estimates from Cluster with global MHD simulations

NASA Astrophysics Data System (ADS)

The local energy conversion across the magnetopause has been estimated with Cluster for two magnetopause crossings. A load region, conversion from magnetic to particle energy, was identified on the dayside high-latitude magnetopause during south/dawnward IMF. Another crossing of the dawn flank magnetotail during dominantly duskward IMF was identified as a generator region where the magnetosphere is loaded with magnetic energy. The observations have been compared to results of the BATS-R-US global MHD simulation based on observed IMF conditions. BATS-R-US reproduced the magnetopause regions crossed by Cluster as a load and a generator region, correspondingly. The magnitude of the estimated energy conversion from Cluster and the model are in quite good agreement. BATS-R-US cannot reproduce the observed sharp magnetopause and some topological differences between the observations and the model occur.

Rosenqvist, L.; Opgenoorth, H. J.; Rastaetter, L.; Vaivads, A.; Dandouras, I.; Buchert, S.

2008-11-01

408

X-ray Spectra from MHD Simulations of Accreting Black Holes

NASA Astrophysics Data System (ADS)

We present new global calculations of X-ray spectra from fully relativistic magneto-hydrodynamic (MHD) simulations of black hole (BH) accretion disks. With a self-consistent radiative transfer code including Compton scattering and returning radiation, we can reproduce the predominant spectral features seen in decades of X-ray observations of stellar-mass BHs: a broad thermal peak around 1 keV, power-law continuum up to >100 keV, and a relativistically broadened iron fluorescent line. By varying the mass accretion rate, different spectral states naturally emerge: thermal-dominant, steep power-law, and low/hard. In addition to the spectral features, we briefly discuss applications to X-ray timing and polarization.

Schnittman, Jeremy; Noble, S.; Krolik, J.

2011-05-01

409

MHD simulations of solar wind interaction with Mars and test particle injection

NASA Astrophysics Data System (ADS)

The solar wind interaction with Mars is very different to that of Earth because Mars lacks a global magnetic field. Despite this fact, an induced magnetic boundary is formed due to the interaction of the solar wind with the ionosphere, an a bow shock forms ahead of the planet, much closer than in the case of our planet. Solar radiation ionizes Mars upper atmosphere and later, due to the dynamical interaction between the solar wind and the newly ionized material, some ions are swept away from the planet, i.e. picked up into the solar flow. These ions form ring-beam distributions in velocity space making the plasma unstable to the generation of ion cyclotron waves that have been previously reported. In this work use an MHD code to simulate the solar wind interaction with Mars under various IMF geometries. We also inject test particles, follow their trajectories, and build velocity distributions to sudy possible wave growth due to these distributions.

Matias Schneiter, Ernesto; Velazquez, Pablo F.; Esquivel, Alejandro; Blanco-Cano, Xochitl

410

Two-dimensional plasmonic metamaterials

The fabrication of three-dimensional photonic metamaterials faces numerous technological challenges. Many new concepts and\\u000a ideas in the optics of metamaterials may be more easily tested in two spatial dimensions using the planar optics of surface\\u000a plasmon polaritons. In this paper we review recent progress in this direction. Two-dimensional photonic crystals exhibiting\\u000a either positive or negative refraction, and strongly anisotropic metamaterials,

I. I. Smolyaninov

2007-01-01

411

Two-dimensional generalization of the original peak finding algorithm suggested earlier is given. The ideology of the algorithm\\u000a emerged from the well-known quantum mechanical tunneling property which enables small bodies to penetrate through narrow potential\\u000a barriers. We merge this “quantum” ideology with the philosophy of Particle Swarm Optimization to get the global optimization\\u000a algorithm which can be called Quantum Swarm Optimization.

Z. K. Silagadze

2007-01-01

412

MHD simulations of upflows in the Kippenhahn-Schlueter prominence model

NASA Astrophysics Data System (ADS)

The launch of SOT on the Hinode satellite, with it's previously unprecedented high resolution, high cadence images of solar prominences, led to the discovery of small scale, highly dynamic flows in quiescent prominences. Berger et al. (2008) reported dark upflows that propagated from the base of the prominence through a height of approximately 10 Mm before ballooning into the familiar mushroom shape often associated with the Rayleigh-Taylor instability. Whether such phenomena can be driven by instabilities and, if so, how the instability evolve is yet to be fully investigated. In this study, we use the Kippenhahn-Schlueter (K-S) prominence model as the base for 3D numerical MHD simulations. The K-S prominence model is linearly stable for ideal MHD perturbationss, but can be made unstable through nonlinear perturbations, which we impose through inserting a low density (high temperature) tube through the centre of the prominence. Our simulations follow the linear and nonlinear evolution of upflows propagating from the hot tube through the K-S prominence model. We excited Rayleigh-Taylor like modes inside the K-S model with a wave along the contact discontinuity created between the hot tube and the K-S prominence, and solved the pertur-bations of this system. For such a complex setting, the linear evolution of the instability has 0.7 not been studied, and we found the growth rate to be ˜ ( ?+ -?- - 0.05)k 0.22 . The most ?+ +?- unstable wavelength was ˜ 100 km which, through the inverse cascade process, created upflows of ˜ 300 km. The rising plumes obtained a constant rise velocity in the nonlinear stage due to the creation of adverse magnetic and gas pressure gradients at the top of the plume.

Hillier, Andrew; Shibata, Kazunari; Isobe, Hiroaki; Berger, Thomas

413

Global evolution of Birkeland currents on 10 min timescales: MHD simulations and observations

NASA Astrophysics Data System (ADS)

In this paper we compare time-dependent global ionospheric field-aligned current (FAC) patterns on 10 min timescales inferred from the Active Magnetosphere and Polar Electrodynamics Response Experiment (AMPERE) with the high-resolution Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic (MHD) model. The improved LFM model yields temporally varying FAC patterns with a fine structure on the sub-100 km scale. The goal of the study is to explore the responses of observed and simulated FAC patterns and underlying magnetic perturbations to a succession of rapid transitions in the solar wind and Interplanetary Magnetic Field (IMF) parameters. To drive the simulations, we use the upstream Wind and Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements recorded on 3 August 2010. For the time interval of interest (˜40 min following the impact of an interplanetary shock), the IMF is characterized by a BZ rotation from southward to northward direction under negative BY conditions. Through this case study analysis, it is found that the simulations have generally reproduced the salient characteristics of both the morphology and dynamics of the AMPERE FAC patterns. Due to the high resolution of the global model, the peak current densities are found to significantly (by a factor of 2-4) exceed those obtained from AMPERE. As a further quantitative analysis, the low-altitude magnetic perturbations measured by Iridium spacecraft and used to derive the AMPERE 2-D FAC patterns are also compared with the magnetic field variations calculated from the simulations. It is found that outside of localized regions of peak current densities, which mainly occur on the dayside and can fall between the Iridium tracks, the simulated magnetic perturbations closely follow the Iridium measurements. This demonstrates, in particular, that there is no systematic bias in the simulations to overestimate the magnetic perturbations and corresponding FAC densities. Overall, our results demonstrate that given sufficient resolution, contemporary global MHD models are capable of reproducing observed features of global ionospheric FAC distributions. This, in particular, suggests the feasibility of potential efforts to assimilate AMPERE observations in global magnetospheric models.

Merkin, V. G.; Anderson, B. J.; Lyon, J. G.; Korth, H.; Wiltberger, M.; Motoba, T.

2013-08-01

414

Time-dependent global MHD simulations of Cassini T32 flyby: From magnetosphere to magnetosheath

NASA Astrophysics Data System (ADS)

When the Cassini spacecraft flew by Titan on 13 June 2007, at 13.6 Saturn local time, Titan was directly observed to be outside Saturn's magnetopause. Cassini observations showed dramatic changes of magnetic field orientation as well as other plasma flow parameters during the inbound and outbound segments. In this paper, we study Titan's ionospheric responses to such a sudden change in the upstream plasma conditions using a sophisticated multispecies global MHD model. Simulation results of three different cases (steady state, simple current sheet crossing, and magnetopause crossing) are presented and compared against Cassini Magnetometer, Langmuir Probe, and Cassini Plasma Spectrometer observations. The simulation results provide clear evidence for the existence of a fossil field that was induced in the ionosphere. The main interaction features, as observed by the Cassini spacecraft, are well reproduced by the time-dependent simulation cases. Simulation also reveals how the fossil field was trapped during the interaction and shows the coexistence of two pileup regions with opposite magnetic orientation, as well as the formation of a pair of new Alfven wings and tail disconnection during the magnetopause crossing process.

Ma, Y. J.; Russell, C. T.; Nagy, A. F.; Toth, G.; Bertucci, C.; Dougherty, M. K.; Neubauer, F. M.; Wellbrock, A.; Coates, A. J.; Garnier, P.; Wahlund, J.-E.; Cravens, T. E.; Crary, F. J.

2009-03-01

415

Dynamical Properties of Two-Dimensional Josephson Junction Arrays.

National Technical Information Service (NTIS)

We have investigated the dynamical properties of two dimensional (2D) Josephson junction arrays at temperatures below the Kosterlitz-Thouless phase transition. We have completed a comprehensive study, based on experiments and simulations, of the effects o...

S. P. Benz

1990-01-01

416

Two Dimensional Stochastic Model of a Heterogeneous Geologic System.

National Technical Information Service (NTIS)

Models based on probabilistic laws are increasingly being employed to simulate heterogeneous geologic systems. One such model, the 'Poisson lines' model, is discussed. This is a two dimensional stochastic model with Markovian properties which is generated...

M. J. Lippmann

1973-01-01

417

ON THE ORIGIN OF THE TYPE II SPICULES: DYNAMIC THREE-DIMENSIONAL MHD SIMULATIONS

Recent high temporal and spatial resolution observations of the chromosphere have forced the definition of a new type of spicule, 'type II's', that are characterized by rising rapidly, having short lives, and by fading away at the end of their lifetimes. Here, we report on features found in realistic three-dimensional simulations of the outer solar atmosphere that resemble the observed type II spicules. These features evolve naturally from the simulations as a consequence of the magnetohydrodynamical evolution of the model atmosphere. The simulations