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1

Two-dimensional MHD simulations of accretion disk evaporation

NASA Astrophysics Data System (ADS)

We simulate the accretion disk evaporation to study the nature of spectral transitions in the black hole binaries. We perform 2 dimensional magnetohydrodynamical simulations with heat conduction by electrons. We assume axisymmetric accretion disks. We put cold gas torus in hot hydrostatic halo. Weak magnetic fields are initially threaded vertically. Self-gravity and magnetic resistivity are neglected. We use the numerical codes set "CANS "(Coordinated Astronomical Numerical Software). Heat conduction equation and MHD equations are solved separately according to time splitting method. We adopt Lax-Wendroff method for the MHD part and BiCG stabilized method for heat conduction part. We set an absorbing inner boundary condition. We obtain the result that the hot gas emanates from the disk, since there is the heat flow from hot halo to cool accretion disk. Near central objects, the mass flow rate is large. In inner region the corona gas falls to black hole, otherwise in outer region that goes to far from black hole.

Nakamura, Kenji E.

2007-04-01

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

Simulation of two-dimensional fully developed laminar flow for a magneto-hydrodynamic (MHD) pump.

MHD micro-pumps circumvent the wear and fatigue caused by high pressure-drop across the check valves of mechanical micro-pumps in micro-fluidic systems. Early analyses of the fluid flow for MHD micro-pumps were mostly made possible by the Poiseuille flow theory; however, this conventional laminar approach cannot illustrate the effects of various channel sizes and shapes. This paper, therefore, presents a simplified MHD flow model based upon steady state, incompressible and fully developed laminar flow theory to investigate the characteristics of a MHD pump. Inside the pump, flowing along the channel is the electrically conducting fluid flowing driven by the Lorentz forces in the direction perpendicular to both dc magnetic field and applied electric currents. The Lorentz forces were converted into a hydrostatic pressure gradient in the momentum equations of the MHD channel flow model. The numerical simulations conducted with the explicit finite difference method show that the channel dimensions and the induced Lorentz forces have significant influences on the flow velocity profile. Furthermore, the simulation results agree well with the experimental results published by other researchers. PMID:15142583

Wang, Pei-Jen; Chang, Chia-Yuan; Chang, Ming-Lang

2004-07-30

4

Two-Dimensional MHD Simulations of Tokamak Plasmas with Poloidal Flow

NASA Astrophysics Data System (ADS)

Recent ideal MHD calculations have shown that poloidal flow in a tokamak can result in a pedestal structure across which the velocity and pressure vary strongly. In a low-? tokamak the effective sound speed in the poloidal direction is the sound speed Cs scaled by the ratio of the poloidal to total magnetic field strength, C_s? = C_s(B_?/B). This is a measure of the time scale necessary for a sound wave to propagate from the outer to inner radial edge of the plasma. The poloidal sound speed goes to zero at the center of the plasma by symmetry and gets quite small near the outer radial edge. Hence even small poloidal rotational velocities can result in a supersonic flow. Owing to the finite inverse aspect ratio, the toroidal geometry of the flow behaves as a de Laval nozzle with a throat at the inner midplane. Under the right conditions, a subsonic flow can become supersonic near the inner midplane and remain supersonic as it completes a poloidal revolution. This in turn results in a shock wave that propagates from the outer to the inner midplane, irreversibly heating the plasma and generating a poloidal shear flow. We present time-dependent numerical simulations detailing the formation and evolution of this flow pattern and a comparison with analytic results. This work was supported by the U.S. Department of Energy under Contract No. DE-FG02-93ER54215.

Gardiner, T. A.; Betti, R.; Guazzotto, L.

2001-10-01

5

The problem is formulated and a numerical model is developed for calculating MHD flows in plasma-focus (PF) devices. An implicit (with respect to the magnetic field) difference scheme and a method for its numerical implementation are proposed. The scheme allows one to describe plasma flows with drastically different densities and, therefore, to take into account the presence of a vacuum region behind the PF current sheath. Taking into account this region is important to numerically simulate the focusing process and adequately describe the acceleration mechanism for the generation of fast ions and fusion neutrons. The results of calculations are compared with the experimental data on the plasma dynamics in two different types of PF device. A model of accelerated ions is proposed to estimate the contribution of the acceleration mechanism to the total neutron yield. The neutron yield calculated in the framework of this model for three different geometries of the PF chamber at currents of 0.5-1.5 MA differs from the measured values by less than a factor of 2.

Garanin, S. F.; Mamyshev, V. I. [Russian Federal Nuclear Center-All-Russia Research Institute of Experimental Physics (Russian Federation)

2008-08-15

6

NASA Astrophysics Data System (ADS)

We present two 2D studies of collisionless magnetic reconnection dynamics obtained from our new fully electromagnetic PIC code (ExPIC), and compare them with results obtained earlier from Hall MHD simulations using the same initial conditions. Our studies include realistic values of me/mi. The first study involves the scaling of the maximum electron outflow velocity from the reconnection region in the GEM challenge problem, which, according to Hall MHD models, scales as the electron Alfven speed. Our PIC simulations show flows that are uniformly smaller than the electron Alfven speed, with deviations that increase in magnitude as the mass ratio reaches its actual physical value. The second study involves forced magnetic reconnection in a plasma sheet driven continuously by inward boundary flows. It is observed in the PIC simulations that the reconnection rate in the linear regime increases algebraically in time, and is followed by a sudden impulsive enhancement in the nonlinear regime, qualitatively similar to that seen in Ma and Bhattacharjee's earlier Hall MHD simulation (1996). However, the current sheet produced is more singular and the impulsiveness greater in the Hall MHD simulation than it is in the PIC simulation. Quantitative comparisons between PIC and Hall MHD simulation results will be given, and kinetic mechanisms that produce differences between Hall MHD and PIC models will be discussed.

Bessho, N.; Bhattacharjee, A.; Chandran, B.

2004-12-01

7

Two Dimensional MHD Duct Flow: Numerical Analysis and Measurements.

National Technical Information Service (NTIS)

The problem dealt with is that of the turbulent compressible flow between the insulator walls of an MHD duct, for which a set of equations is derived, accounting for turbulent transport phenomena, nonequilibrium ionization, and ambipolar diffusion. The eq...

H. Snel J. P. F. Lindhout W. Merck J. Houben

1975-01-01

8

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

9

A Two Dimensional Car Crash Victim Simulation

The purpose of this study was to develop a two dimensional mathematical model of an unrestrained, right, front seat, passenger car occupant, subjected to frontal collision. A 10 degrees of freedom linkage system made of 8 rigid segments connected with revolute joints was used as occupant model. Relative rotation between links were constrained by torsional springs, dampers, Coulomb frictions and

M. Batman; R. Seliktar

1990-01-01

10

Global magnetohydrodynamic simulation of the two-dimensional magnetosphere

NASA Technical Reports Server (NTRS)

The time-dependent magnetohydrodynamic interaction of the solar wind with a two-dimensional dipole magnetic field has been simulated using a novel Lagrangian particle type of MHD code that can treat local low density or vacuum regions without numerical instability. This enables one to simulate the time-dependent magnetic tail. When the solar wind field is southward, a magnetic field line topology consistent with Dungey's model emerges in steady state. The tail, however, is short, and the x-points are only slightly shifted from their vacuum locations, because of strong numerical resistivity. Different configurations resulting from different relative orientations of the solar wind magnetic field and dipole axis are also presented. While the magnetic field is relatively steady, the density and flow in the magnetosheath are turbulent, as are the bow shock and magnetopause; the Kelvin-Helmholtz instability may account for these phenomena. We also model a 'substorm' as the passage of a rotational discontinuity in the solar wind over the dipole. Both 90 and 180 deg shifts to a southward solar wind field cause a violent readjustment of the magnetic tail which eventually settles down to the Dungey configuration.

Leboeuf, J. N.; Tajima, T.; Kennel, C. F.; Dawson, J. M.

1979-01-01

11

Two-Dimensional Grain Growth as Simulated by Surface Evolver.

National Technical Information Service (NTIS)

The Surface Evolver program, which is an interactive program for the study of surfaces shaped by surface tension and other energies, has been evaluated for simulation of two-dimensional grain growth. Examples have demonstrated that the grain structure evo...

K. Marthinsen

1994-01-01

12

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

13

Monte Carlo simulation of a two-dimensional magnetic foam

A two-dimensional Ising-like model with spin 1 and long-range interactions is studied numerically through a Monte Carlo simulation. The goal of the simulation is to describe pattern formations and critical temperature of two-dimensional magnetic structures. Three sets of parameters are considered, that give rise to stripes, labyrinths or cellular domain structures. We determine for each configuration the transition ordering temperatures,

J. R. Iglesias; O. A. Nagel; S. Gonçalves; M. Kiwi

2001-01-01

14

Monte Carlo simulation of a two-dimensional magnetic foam

NASA Astrophysics Data System (ADS)

A two-dimensional Ising-like model with spin 1 and long-range interactions is studied numerically through a Monte Carlo simulation. The goal of the simulation is to describe pattern formations and critical temperature of two-dimensional magnetic structures. Three sets of parameters are considered, that give rise to stripes, labyrinths or cellular domain structures. We determine for each configuration the transition ordering temperatures, the relaxation of the energy, the hysteresis cycle, and the average size of the domains.

Iglesias, J. R.; Nagel, O. A.; Gonçalves, S.; Kiwi, M.

2001-05-01

15

On the Transition from Two-Dimensional to Three-Dimensional MHD Turbulence

NASA Technical Reports Server (NTRS)

We report a theoretical investigation of the robustness of two-dimensional inviscid MHD flows at low magnetic Reynolds numbers with respect to three-dimensional perturbations. We analyze three model problems, namely flow in the interior of a triaxial ellipsoid, an unbounded vortex with elliptical streamlines, and a vortex sheet parallel to the magnetic field. We demonstrate that motion perpendicular to the magnetic field with elliptical streamlines becomes unstable with respect to the elliptical instability once the velocity has reached a critical magnitude whose value tends to zero as the eccentricity of the streamlines becomes large. Furthermore, vortex sheets parallel to the magnetic field, which are unstable for any velocity and any magnetic field, are found to emit eddies with vorticity perpendicular to the magnetic field and with an aspect ratio proportional to N(sup 1/2). The results suggest that purely two-dimensional motion without Joule energy dissipation is a singular type of flow which does not represent the asymptotic behaviour of three-dimensional MHD turbulence in the limit of infinitely strong magnetic fields.

Thess, A.; Zikanov, Oleg

2004-01-01

16

Finite Element Simulation of Two-Dimensional Incompressible Magnetohydrodynamic Flows

NASA Astrophysics Data System (ADS)

A new finite element code has been developed for simulation of the dynamics of two-dimensional incompressible magnetohydrodynamic flows. The solution scheme used in spatial discretization is the Galerkin weighted-residual finite-element method, incorporating the mixed interpolation technique, and a combination of the penalty and pseudocompressibility methods for implementing the incompressibility constraint. An implicit and stable theta-weighting finite difference scheme is used for integration in time, and a non-iterative time-level averaging method is employed for treatment of nonlinear terms. The code has been extensively benchmarked against known analytical solutions in magnetohydrodynamics and has been found to produce highly accurate results. The tearing-mode instability of a magnetic-field-reversing current sheet in the presence of coplanar stagnation-point flow, in which the unperturbed equilibrium state is an exact solution of the steady-state dissipative MHD equations, has been examined by use of the code. Simulation results indicate stability for sufficiently small values of the viscous Lundquist number, S_nu, or the resistive Lundquist number, S_eta : a curve in the S_nu -S_eta plane separating the stable and unstable regions has been found. In the unstable regime, the results show occurrence of multiple x-line reconnection along the center of the current sheet at x = 0. Small-scale structures of vorticity and current density near the x-point reconnection sites are observed and are found to be consistent with results obtained by Matthaeus (1982). Average linear growth rates are estimated for modest values of S_eta. In the range S_eta<=500, the number of magnetic islands is found to be independent of Seta, which implies that there exists a single dominant wavelength of the tearing-mode in this range. The stretching of magnetic islands which is present in this configuration but not in the perpendicular flow and field configuration examined by Phan and Sonnerup (1991), caused a substantial decrease in linear growth rate relative to that obtained by those authors. It is of particular interest that, unlike most simulations of the tearing-mode, no symmetry conditions are imposed on the perturbations; nevertheless they develop in an anti -symmetric manner.

Ip, Justin Tsz Ching

17

Particle simulation of a two-dimensional electrostatic plasma.

National Technical Information Service (NTIS)

Computer simulation is a growing field of research and plasma physics is one of the important areas where it is being applied today. This report describes the particle method of simulating a two-dimensional electrostatic plasma. The methods used to discre...

K. Patel

1989-01-01

18

Numerical Simulation of One and Two-Dimensional ESEEM Experiments

Numerical simulation has become an indispensable tool for the interpretation of pulse EPR experiments. In this work it is shown how automatic orientation selection, grouping of operator factors, and direct selection and elimination of coherences can be used to improve the efficiency of time-domain simulations of one- and two-dimensional electron spin echo envelope modulation (ESEEM) spectra. The program allows for

Z. L. Madi; S. Van Doorslaer; A. Schweiger

2002-01-01

19

Two-dimensional numerical simulation of MPD flows

A two-dimensional numerical model has been developed in order to analyze electromagnetic plasma accelerators also called Self-Field Magneto-Plasma-Dynamic Thrusters. This model uses a Magneto-Hydro-Dynamic description of the gas considered as a fully ionized, isothermal plasma, and takes into account the Hall effect (nonlinear conductivity) and the interaction between the magnetic field and the fluid dynamics of the plasma. The system of equations is discretized into finite volumes, and is solved by a Newton-Raphson scheme. Results from the MHD model were calculated for a mass flow rate of 6 g/s of argon and for currents up to ten kilo-Amperes. 29 references.

Martinez-Sanchez, M.; Chanty, J.M.G.

1987-05-01

20

Two-dimensional Simulations of Correlation Reflectometry in Fusion Plasmas

A two-dimensional wave propagation code, developed specifically to simulate correlation reflectometry in large-scale fusion plasmas is described. The code makes use of separate computational methods in the vacuum, underdense and reflection regions of the plasma in order to obtain the high computational efficiency necessary for correlation analysis. Simulations of Tokamak Fusion Test Reactor (TFTR) plasma with internal transport barriers are presented and compared with one-dimensional full-wave simulations. It is shown that the two-dimensional simulations are remarkably similar to the results of the one-dimensional full-wave analysis for a wide range of turbulent correlation lengths. Implications for the interpretation of correlation reflectometer measurements in fusion plasma are discussed.

E.J. Valeo; G.J. Kramer; R. Nazikian

2001-07-05

21

Two-dimensional hybrid simulation of a curved bow shock

Results are presented from two-dimensional hybrid simulations of curved collisionless supercritical shocks, retaining both quasi-perpendicular and quasi-parallel sections of the shock in order to study the character and origin of the foreshock ion population. The simulations demonstrate that the foreshock ion population is dominated by ions impinging upon the quasi-parallel side of the shock, while nonlocal transport from the quasi-perpendicular

V. A. Thomas; D. Winske

1990-01-01

22

Two-dimensional simulation of plasma-based ion implantation

A particle-in-cell simulation is used to study the time-dependent evolution of the potential and the electrical field surrounding two-dimensional objects during a high voltage pulse in the context of plasma immersion ion implantation. The numerical procedure is based on the solution of Poisson's equation on a grid and the determination of the movement of the particles through the grid. Ion

M. Paulus; L. Stals; U. Rüde; B. Rauschenbach

1999-01-01

23

A two-dimensional MHD global coronal model - Steady-state streamers

NASA Technical Reports Server (NTRS)

We describe a two-dimensional time-dependent, numerical, magnetohydrodynamic model for the determination of the physical properties of coronal streamers from the top of the transition zone (solar radius = 1) to 15 solar radii. Four examples are given: for dipole, quadrupole, and hexapole initial field topologies. The computed parameters are density, temperature, velocity, and magnetic field. In addition to the properties of the solutions, their accuracy is discussed. We use the model as the basis for a general discussion of the way boundary conditions are specified in this and similar simulations.

Wang, A.-H.; Wu, S. T.; Suess, S. T.; Poletto, G.

1993-01-01

24

Two dimensional liquid crystal devices and their computer simulations

NASA Astrophysics Data System (ADS)

The main focus of the dissertation is design and optimization two dimensional liquid crystal devices, which means the liquid crystal director configurations vary in two dimensions. Several optimized and designed devices are discussed in the dissertation. They include long-term bistable twisted nematic liquid crystal display (BTN LCD), which is very low power consumption LCD and suitable for E-book application; wavelength tunable liquid crystal Fabry-Perot etalon filter, which is one of the key components in fiber optic telecommunications; high speed refractive index variable devices, which can be used in infrared beam steering and telecommunications; high density polymer wall diffractive liquid crystal on silicon (PWD-LCoS) light valve, which is a promising candidate for larger screen projection display and also can be used in other display applications. Two dimensional liquid crystal director simulation program (relaxation method) and two dimensional optical propagation simulation program (finite-difference time-domain, FDTD method) are developed. The algorithms of these programs are provided. It has been proved that they are the very efficient tools that used in design and optimization the devices described above.

Wang, Bin

25

Two-dimensional hybrid simulation of a curved bow shock

NASA Technical Reports Server (NTRS)

Results are presented from two-dimensional hybrid simulations of curved collisionless supercritical shocks, retaining both quasi-perpendicular and quasi-parallel sections of the shock in order to study the character and origin of the foreshock ion population. The simulations demonstrate that the foreshock ion population is dominated by ions impinging upon the quasi-parallel side of the shock, while nonlocal transport from the quasi-perpendicular side of the shock into the foreshock region is minimal. Further, it is shown that the ions gain energy by drifting significantly in the direction of the convection electric field through multiple shock encounters.

Thomas, V. A.; Winske, D.

1990-01-01

26

Numerical Simulation of One- and Two-Dimensional ESEEM Experiments

NASA Astrophysics Data System (ADS)

Numerical simulation has become an indispensable tool for the interpretation of pulse EPR experiments. In this work it is shown how automatic orientation selection, grouping of operator factors, and direct selection and elimination of coherences can be used to improve the efficiency of time-domain simulations of one- and two-dimensional electron spin echo envelope modulation (ESEEM) spectra. The program allows for the computation of magnetic interactions of any symmetry and can be used to simulate spin systems with an arbitrary number of nuclei with any spin quantum number. Experimental restrictions due to finite microwave pulse lengths are addressed and the enhancement of forbidden coherences by microwave pulse matching is illustrated. A comparison of simulated and experimental HYSCORE (hyperfine sublevel correlation) spectra of ordered and disordered systems with varying complexity shows good qualitative agreement.

Madi, Z. L.; Van Doorslaer, S.; Schweiger, A.

2002-02-01

27

Two-dimensional Vlasov code simulation of magnetic reconnection

NASA Astrophysics Data System (ADS)

There are numerous types of self-consistent simulations that treat plasmas according to some approximations. The fluid codes are used to study global and macroscopic processes in space plasmas. Nonlinear microscopic processes in space plasmas are studied with kinetic simulation codes. Numerical methods for kinetic simulations fall into two groups. One is particle-in-cell (PIC) method which follows motions of individual particles in a self-consistent electromagnetic field. However, a limitation on the number of particles gives rise to numerical thermal fluctuations. Another approach is Vlasov method which follows spatial and temporal development of distribution functions in the position-velocity phase space. In contrast to PIC codes, numerical noise is substantially suppressed. However, Vlasov codes require huge computer resources to represent distribution functions and Vlasov simulation techniques are still developing. Owing to the rapid advancement of recent computer technology, Vlasov code simulation would be more essential in the near future. In the present study, a new two-and-half-dimensional and fully electromagnetic Vlasov simulation code is developed in which phase-space distribution functions are defined in five-dimensional position-velocity phase space (x,y,vx,vy,vz). The Vlasov equation in two-dimensional configuration and three-dimensional velocity spaces is solved with a non-oscillatory and conservative scheme, and the full set of Maxwell’s equations are self-consistently solved based on the implicit Finite Difference Time Domain (FDTD) method. The Geospace Environment Modeling (GEM) magnetic reconnection challenge is chosen as a benchmark test of our two-dimensional Vlasov code. The result is compared with the past simulation results with Darwin-Vlasov, explicit PIC and implicit PIC codes. The present simulation with a very-low spatial resolution gives a high growth rate of magnetic flux, which is in agreement with the results of the GEM reconnection challenge.

Togano, K.; Umeda, T.; Ogino, T.

2009-12-01

28

Numerically simulated two-dimensional auroral double layers

NASA Technical Reports Server (NTRS)

A magnetized 2 1/2-dimensional particle-in-cell system which is periodic in one direction and bounded by reservoirs of Maxwellian plasma in the other is used to numerically simulate electrostatic plasma double layers. For the cases of both oblique and two-dimensional double layers, the present results indicate periodic instability, Debye length rather than gyroradii scaling, and low frequency electrostatic turbulence together with electron beam-excited electrostatatic electron-cyclotron waves. Estimates are given for the thickness of auroral doule layers, as well as the separations within multiple auroral arcs. Attention is given to the temporal modulation of accelerated beams, and the possibilities for ion precipitation and ion conic production by the double layer are hypothesized. Simulations which include the atmospheric backscattering of electrons imply the action of an ionospheric sheath which accelerates ionospheric ions upward.

Borovsky, J. E.; Joyce, G.

1983-01-01

29

One- and two-dimensional simulations of imploding metal shells

We report results of one- and two-dimensional (2D) magnetohydrodynamic simulations of imploding, cylindrical metal shells. One-dimensional simulations are used to calculate implosion velocities of heavy liners driven by 30 MA currents. Accelerated by the j{times}B force, 45 g aluminum/tungsten composite liners achieve velocities on the order of 13 km/s. Used to impact a tungsten target, the liner produces shock pressures of approximately 14 Mbar. The first 2D simulations of these liners are also described. These simulations have focused on two problems: (1) the interaction of the liner with the electrically conducting glide planes, and (2) the effect of realistic surface perturbations on the dynamics of the implosion. The former interaction is confined primarily to the region of the contact point between the liner and glide plane, and does not seriously affect the inner liner surface. However a 0.2 {mu}m surface perturbation has a significant effect on the implosion dynamics. {copyright} {ital 1999 American Institute of Physics.}

Keinigs, R.K.; Atchison, W.L.; Faehl, R.J.; Thomas, V.A.; Mclenithan, K.D.; Trainor, R.J. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)] [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

1999-06-01

30

Two-Dimensional Hydrodynamic Simulations of SSD Laser Imprint

NASA Astrophysics Data System (ADS)

The two-dimensional hydrodynamics code SAGE has been used to model the response of CH-foil targets to realistic equivalent-target-plane (ETP) intensity distributions including smoothing by spectral dispersion (SSD). First the laser propagation code Waasikwa is used to calculate the time-dependent laser ETP distribution, including frequency conversion, for bandwidths up to 1 THz. Up to six such distributions are combined in the target plane. One-dimensional lineouts are then taken in different directions and fed into SAGE. The SAGE simulations, carried out in planar geometry, provide estimates of imprint in the form of transverse mass modulations including the effects of thermal conduction in the evolving plasma profile. While this work is limited by the inclusion of only one transverse direction, it provides qualitative comparisons between different SSD parameters. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

Craxton, R. S.; Marozas, J. A.; Skupsky, S.

2000-10-01

31

Two-dimensional dispersion module for the TOUGH2 simulator.

National Technical Information Service (NTIS)

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...

C. M. Oldenburg K. Pruess

1993-01-01

32

Microscale simulation of bimolecular reaction in two dimensional porous media

NASA Astrophysics Data System (ADS)

Reactive transport in porous media is typically modeled approximating key processes occurring at the pore-scale through a set of continuum- (or Darcy-) scale partial differential equations, the advection dispersion reaction equation (ADRE) being a widely used model. Such formulations hold under a set of assumptions which are not always met in the context of laboratory and/or field scale applications. These hypotheses involve spatial scale separation and restrictions on the magnitude of dimensionless parameters, such as the Damköhler and the Péclet numbers, characterizing the process. In this context, direct measurements and micro-scale numerical simulations are key to (1) assess the validity of upscaled continuum formulations, and (2) quantify the ability of such models to capture the key features of the process dynamics. Here, we focus on the simulation of a homogeneous irreversible bimolecular reaction of the kind A + B ? C. We analyze the evolution of the process in the presence of different pore scale geometrical settings, upon performing numerical pore-scale simulations in ordered and disordered two dimensional arrays of cylinders. The selected pore scale geometries are characterized by different porosities and by the presence of large cavities and regions with different relative importance of diffusive and advective processes. A particle tracking methodology is employed to study the system dynamics and simulations are performed for a wide range of the Péclet and Damköhler numbers. The evolution of the features of the reactive transport process is analyzed on different observation scales. Our results show that the reactive transport process attains an asymptotic regime for which the reaction is limited by (effective) dispersion. The influence of the pore scale geometry on the asymptotic and pre-asymptotic behavior of the reaction rate globally observed in the domain is quantitatively analyzed. Local mixing features and related characteristic scales are also discussed. In particular, the influence of local velocity distributions and medium geometrical setting on the reaction process is documented. These results provide a framework to discuss the appropriateness of continuum scale formulations which can be employed to describe the target geochemical system. The impact of Péclet and Damköhler numbers and of the pore space geometry on the characterization of the parameters introduced in continuum scale formulations of the system is discussed. Recently an alternative continuum formulation of the system has been provided on the basis of a volume averaging analysis of the reactive transport process analyzed. Here the validity of this formulation is discussed, in comparison with the standard ADRE formulation. We assess (i) the influence of the reactive process on the (upscaled) hydrodynamic dispersion coefficient and (ii) the ability of the continuum modeling strategies considered to represent the effect of pore scale incomplete mixing processes.

Porta, G.; Chaynikov, S.; Thovert, J.; Riva, M.; Guadagnini, A.; Adler, P. M.

2012-12-01

33

Two-dimensional Numerical Simulations of Supercritical Accretion Flows Revisited

NASA Astrophysics Data System (ADS)

We study the dynamics of super-Eddington accretion flows by performing two-dimensional radiation-hydrodynamic simulations. Compared with previous works, in this paper we include the T ?phi component of the viscous stress and consider various values of the viscous parameter ?. We find that when T ?phi is included, the rotational speed of the high-latitude flow decreases, while the density increases and decreases at the high and low latitudes, respectively. We calculate the radial profiles of inflow and outflow rates. We find that the inflow rate decreases inward, following a power law form of \\dot{M}_in\\propto r^s. The value of s depends on the magnitude of ? and is within the range of ~0.4-1.0. Correspondingly, the radial profile of density becomes flatter compared with the case of a constant \\dot{M}(r). We find that the density profile can be described by ?(r)vpropr -p and the value of p is almost same for a wide range of ? ranging from ? = 0.1 to 0.005. The inward decrease of inflow accretion rate is very similar to hot accretion flows, which is attributed to the mass loss in outflows. To study the origin of outflow, we analyze the convective stability of the slim disk. We find that depending on the value of ?, the flow is marginally stable (when ? is small) or unstable (when ? is large). This is different from the case of hydrodynamical hot accretion flow, where radiation is dynamically unimportant and the flow is always convectively unstable. We speculate that the reason for the difference is because radiation can stabilize convection. The origin of outflow is thus likely because of the joint function of convection and radiation, but further investigation is required.

Yang, Xiao-Hong; Yuan, Feng; Ohsuga, Ken; Bu, De-Fu

2014-01-01

34

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

35

Simulation of Plastic Deformation in a Two-Dimensional Atomic Glass By Molecular Dynamics - IV.

National Technical Information Service (NTIS)

Plastic deformation in a structurally well-relaxed two-dimensional atomic glass was simulated by a computer molecular dynamics approach. The simulation, which was carried through yielding and to substantial plastic strains, demonstrated that the principal...

A. S. Argon D. Deng S. Yip

1988-01-01

36

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

37

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

Z-pinch implosions driven by the SATURN device [D. D. Bloomquist {ital et} {ital al}., {ital Proceedings} {ital of} {ital the} 6{ital th} {ital Institute} {ital of} {ital Electrical} {ital and} {ital Electronics} {ital Engineers} ({ital IEEE}) {ital Pulsed} {ital Power} {ital Conference}, 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{endash}Taylor (MRT) instability. Modeling of the linear and nonlinear development of MRT modes predicts growth of bubble-spike structures that increase the time span of stagnation and the resulting x-ray pulse width. Radiation is important in the pinch dynamics, keeping the sheath relatively cool during the run-in and releasing most of the stagnation energy. The calculations give x-ray pulse widths and magnitudes in reasonable agreement with experiments, but predict a radiating region that is too dense and radially localized at stagnation. We also consider peaked initial density profiles with constant imploding sheath velocity that should reduce MRT instability and improve performance. Krypton simulations show an output x-ray power {approx_gt}80 TW for the peaked profile. {copyright} {ital 1996 American Institute of Physics.}

Hammer, J.H.; Eddleman, J.L.; Springer, P.T.; Tabak, M.; Toor, A.; Wong, K.L.; Zimmerman, G.B. [Lawrence Livermore National Laboratory, Livermore , California 94526 (United States)] [Lawrence Livermore National Laboratory, Livermore , California 94526 (United States); Deeney, C.; Humphreys, R.; Nash, T.J.; Sanford, T.W.; Spielman, R.B. [Sandia National Laboratory, Albuquerque, New Mexico 87185 (United States)] [Sandia National Laboratory, Albuquerque, New Mexico 87185 (United States); De Groot, J.S. [University of California, Davis, California 95616 (United States)] [University of California, Davis, California 95616 (United States)

1996-05-01

38

Two-dimensional computational simulation of eccentric annular cementing displacements

NASA Astrophysics Data System (ADS)

We consider a two-dimensional Hele-Shaw type model for displacement flows occurring in the primary cementing of an oil well. The fluids are visco-plastic and may get stuck in the annulus if a critical pressure gradient is not exceeded. The model consists of solving a nonlinear elliptic variational inequality equation for the stream function, coupled to an equation for interface advection, or alternatively a concentration equation for the mass fraction of each fluid. The key difficulty is to accurately compute yielded and unyielded zones of the wellbore fluids, which we accomplish by use of an augmented Lagrangian method to solve the stream function equation. We validate the accuracy of our method against analytical solutions for stable steady-state displacements. We study the convergence of the interface to the steady state, showing that the apparent meta-stability is illusory. We then explore the effects of increasing eccentricity, showing that although the interface may remain stable it becomes unsteady. Initially fully mobile flows are found, but as the eccentricity increases further the narrow side fluids fail to move in the far field. The narrow side interface can progress slowly through the static fluids by a burrowing motion, but for still larger eccentricities even the interface becomes static and a narrow-side mud channel forms.

Pelipenko, S.; Frigaard, I. A.

2004-12-01

39

Simulation of two dimensional unilateral contact using a coupled FE\\/EFG method

In this paper, simulation of two dimensional unilateral contact problems using a coupled finite element\\/element free Galerkin method is proposed. For the analysis, the element free Galerkin method and Galerkin formulation for two dimensional elasticity problems are considered. Then, the penalty method for imposition of contact constraint is proposed. The finite element shape functions are used in the penalty term

M. Chehel Amirani; N. Nemati

2011-01-01

40

Computer simulation of two-dimensional impact tests on propellant

Computational simulations were performed of several hazards tests previously carried out by LLNL. The hazards tests consisted of side-on impacts of a propellant cylinder by a massive steel projectile with subsequent blast wave loading from an adjacent explosive charge. The simulations used a dynamic fracture model previously developed by SRI. The computational results showed that whereas the fracture-induced new surface

W. J. Murri; L. Seaman

1983-01-01

41

Simulation of deep one- and two-dimensional redshift surveys

NASA Technical Reports Server (NTRS)

It is shown that slice or pencil-beam redshift surveys of galaxies can be simulated in a box with nonequal sides. This method saves a lot of computer time and memory while providing essentially the same results as from whole-cube simulations. A 2457.6/h Mpc-long rod (out to a redshift z = 0.58 in two opposite directions) is simulated using the standard biased cold dark matter model as an example to mimic the recent deep pencil-beam surveys by Broadhurst et al. (1990). The structures (spikes) seen in these simulated samples occur when the narrow pencil-beam pierces walls, filaments, and clusters appearing randomly along the line-of-sight. A statistical test for goodness of fit to a periodic lattice has been applied to the observations and the simulations. It is found that the statistical significance level (P = 15.4 percent) is not strong enough to reject the null hypothesis that the observations and the simulations were drawn at random from the same set.

Park, Changbom; Gott, J. Richard, III

1991-01-01

42

One and Two Dimensional Simulations of Whistler Chorus in Dipole Geometry

NASA Astrophysics Data System (ADS)

Whistler mode chorus may play an important role in radiation belt electron acceleration and pitch angle scattering loss. Thus far self-consistent simulations have been restricted to one-dimension and propagation parallel to the background magnetic field^1, although observations of the largest amplitude whistlers have been highly oblique^2. We perform two dimensional hybrid code simulations of oblique whistler chorus waves in dipole coordinates to determine their evolution and distribution with respect to space, frequency and wave vector. Our hybrid code uses the particle in cell technique in generalized orthogonal coordinates with various boundary conditions. It has fourth order accuracy in space and second order accuracy in time. The particles are advanced using the full Lorentz force equation neglecting the displacement current to eliminate light waves. The initial particle distribution can be obtained from an anisotropic MHD equilibrium code. We simulate oblique whistler chorus in the limit of large plasma frequency/electron gyrofrequency ratio and small wave frequency/electron gyrofrequency ratio. We use energetic particles (ring current electrons) to stimulate the instability, a higher density population of relatively cold electrons, an even higher density population of cold inertialess electrons, and a fixed background of ions. In the real system, the parallel electric field will be canceled on the timescale of whistler waves by the plasma oscillations, while in our system it is cancelled by the free movement of the inertialess electrons. Our model will be more accurate for lower band chorus (wave frequency smaller than half the electron gyrofrequency). Since lower band chorus is dominant in the space observations, this is not a major limitation. 1 Katoh and Omura, JGR, 2007 2 Cattell et al., GRL, 2008.

Wu, S.; Denton, R. E.; Hudson, M. K.

2011-12-01

43

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

44

Optimisation of interdigitated back contacts solar cells by two-dimensional numerical simulation

In this paper we present the results of the simulation of interdigitated back contacts solar cell on thin-film (?50?m) silicon layer. The influence of several parameters (surface recombination rate, substrate thickness and type, diffusion length, device geometry, doping levels) on device characteristics are simulated using the accurate two-dimensional numerical simulator DESSIS that allows to optimise the cell design.

O. Nichiporuk; A. Kaminski; M. Lemiti; A. Fave; V. Skryshevsky

2005-01-01

45

A flame ball growing into a turbulent mixture exhibits a speed which is proportional to its size- this effect is a consequence of the distorted flame surface and the resulting distributed volume expansion effect. A simple two-dimensional, zero-thickness flame model interacting with defined eddies illustrates how rapidly this flame distortion occurs after a spark ignition. The simulated two-dimensional flow configuration

W. T. ASHURST

1995-01-01

46

Numerical Simulation on Performance of Disk MHD Generator in the Closed-Loop Experimental Facility

A time dependent two-dimensional numerical simulation has been carried out in order to clarify the magnetohydrodynamic (MHD) flow behavior and performance of a disk MHD generator installed in a new closed-loop experimental facility at the Tokyo Institute of Technology. The numerical investigation is not limited to the generator channel only, but also includes an inlet duct and a downstream 90deg-bend

Alessandro Liberati; Tomoyuki Murakami; Yoshihiro Okuno; Hiroyuki Yamasaki

2006-01-01

47

Lekner summation of dipolar interaction in quasi-two-dimensional simulations

The Lekner method for calculation of electrostatic interactions in periodically replicated simulation cells is extended to quasi-two-dimensional systems of particles with dipolar interactions. The electric field, potential energy, forces and torques are expressed through rapidly converging series of modified Bessel functions. The method contains no arbitrary parameters, and has no limitations on the simulation box width.

Sergey V. Lishchuk

2002-01-01

48

Simulations of Spatially Developing Two-Dimensional Shear Layers and Jets

NASA Astrophysics Data System (ADS)

A computational study of spatially evolving two-dimensional free shear flows has been performed using direct numerical simulation of the Navier-Stokes equations in order to investigate the ability of these two-dimensional simulations to predict the overall flow-field quantities of the corresponding three-dimensional ``real'' turbulent flows. The effects of inflow forcing on these two-dimensional flows has also been studied. Simulations were performed of shear layers, as well as weak (large co-flow and relatively weak shear) and strong (small co-flow and relatively strong shear) jets. Several combinations of discrete forcing with and without a broadband background spectrum were used. Although spatially evolving direct simulations of shear layers have been performed in the past, no such simulations of the plane jet have been performed to the best of our knowledge. It was found that, in the two-dimensional shear layers, external forcing led to a strong increase in the initial growth of the shear-layer thickness, followed by a region of decreased growth as in physical experiments. The final downstream growth rate was essentially unaffected by forcing. The mean velocity profile and the naturally evolving growth rate of the shear layer in the case of broadband forcing compare well with experimental data. However, the total and transverse fluctuation intensities are larger in the two-dimensional simulations with respect to experimental data. In the weak-jet simulations it was found that symmetric forcing completely overwhelms the natural tendency to transition to the asymmetric jet column mode downstream. It was observed that two-dimensional simulations of ``strong'' jets with a low speed co-flow led to a fundamentally different flow with large differences even in mean velocity profiles with respect to experimental data for planar jets. This was a result of the dominance of the two-dimensional mechanism of vortex dipole ejection in the flow due to the lack of spanwise instabilities. Experimental studies of planar jets do not show vortex dipole formation and ejection. A three-dimensional ``strong''-jet simulation showed the rapid evolution of three-dimensionality effectively preventing this two-dimensional mechanism, as expected from experimental results.

Stanley, S.; Sarkar, S.

49

MHD simulations of magnetotail dynamics

NASA Astrophysics Data System (ADS)

Resistive MHD simulations of large-scale magnetotail dynamics demonstrate that the same unstable mode causes plasmoid formation and ejection into the far tail and dipolarization and the formation of the substorm current wedge in the inner tail, consistent with the neutral line model of substorms. However, they have also modified some aspects of the model and added details that could not easily be inferred without the self-consistent approach. We review recent results that include the externally driven formation of a thin current sheet in the near tail, which eases the onset of instability and leads to a faster dynamic evolution. In contrast to earlier expectations, the field-aligned current generation and diversion takes place in the inner tail earthward of the reconnection site, resulting from shear and diversion of the earthward flow caused by reconnection farther out. Dipolarization starts most pronounced in the tail-dipole transition region, propagating both tailward and flankward. Strong electric fields and plasma heating also are most prominent in the inner tail. Three-dimensional simulations without mirror symmetries have generalized the picture of plasmoid formation and ejection, demonstrating a tangled geometry of helical flux ropes with different connections that change increasingly from the Earth to the magnetosheath. The interconnection with the magnetosheath may also play a role in generating plasmoid flux ropes with strong core fields. Mass, energy, and momentum gain of plasmoids results mainly from the accumulation of already accelerated plasma rather than from a sling shot effect acting on the entire plasmoid.

Birn, Joachim; Hesse, Michael; Schindler, Karl

1996-06-01

50

Two-dimensional electrostatic simulations of plasma propagation perpendicular to a magnetic field

A two-dimensional electrostatic particle-in-cell code is used to simulate a finite-width plasma streaming across a uniform magnetic field. The simulations show that the plasma polarizes, and non-neutral charge layers develop along its edges. In an electron-ion plasma, the charge layers are asymmetric and the electron layer is unstable to the diocotron mode. The simulations show that this instability has a

Miguel Galvez; Christopher Barnes

1988-01-01

51

Bose condensation of two-dimensional dipolar excitons: Simulation by the quantum Monte Carlo method

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.

Lozovik, Yu. E.; Kurbakov, I. L. [Russian Academy of Sciences, Institute of Spectroscopy (Russian Federation); Astrakharchik, G. E. [Polytechnic University of Catalonia E-08034 (Spain); Willander, M. [Linkoeping University SE-581 83, Institute of Science and Technology (ITN) (Sweden)], E-mail: lozovik@isan.troitsk.ru

2008-02-15

52

A two-dimensional model simulation of the El Chichon volcanic eruption cloud

Using a two-dimensional model of sulfate photochemistry, transport, and aerosol microphysics, we have simulated a 2-year period following the eruptions of El Chichon. Our calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years. The model reproduces the observed optical depth, lidar backscatter, and infrared extinction coefficients if about 10 megatonnes of SO2 are injected.

Louis A. Capone; Owen B. Toon; Robert C. Whitten; Richard P. Turco; Christopher A. Riegel; Krishnamurthy Santhanam

1983-01-01

53

A two-dimensional model simulation of the EL Chichon volcanic eruption cloud

Using a two-dimensional model of sulfate photochemistry, transport, and aerosol microphysics, a 2-year period following the eruptions of El Chichon have been simulated. Present calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years. The model reproduces the observed optical depth, lidar backscatter, and infrared extinction coefficients, if about 10 megatonnes of SO2 are injected.

L. A. Capone; C. A. Riegel; O. B. Toon; R. C. Whitten; R. P. Turco; K. Santhanam

1983-01-01

54

The Application of the Cellular Automata in Simulating and Analyzing Two-Dimensional Traffic Model

Using the BML (Biham, Middleton and Levine) model, a special one of the cellular automata models, the paper simulates and analyzes the two-dimensional traffic system controlled by traffic lights. The relationship among original density, average density and average speed of the traffic flow can be found through computer programs. It can be seen that self-organization phenomenon existed in the processing

Xiuhua Wu; Guokai Sun; Zailin Piao; Zhaoyuan Liu; Ping Yang

2006-01-01

55

Two dimensional mesoscale simulations of projectile instability during penetration in dry sand

To gain insight into the instability and trajectory change in projectiles penetrating dry sand at high velocities, two dimensional plane strain mesoscale simulations were carried out using representative models of a particulate system and of a small projectile. A program, ISP-SAND, was developed and used to generate the representative particulate system with mean grain sizes of 60 and 120 mum

S. K. Dwivedi; R. D. Teeter; C. W. Felice; Y. M. Gupta

2008-01-01

56

Two dimensional mesoscale simulations of projectile instability during penetration in dry sand

To gain insight into the instability and trajectory change in projectiles penetrating dry sand at high velocities, two dimensional plane strain mesoscale simulations were carried out using representative models of a particulate system and of a small projectile. A program, ISP-SAND, was developed and used to generate the representative particulate system with mean grain sizes of 60 and 120 ?m

S. K. Dwivedi; R. D. Teeter; C. W. Felice; Y. M. Gupta

2008-01-01

57

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

58

Numerical simulation of the continuous biomagnetic separation in a two-dimensional channel

Numerical simulation of magnetically mediated separation of labeled biospecies from a native fluid flowing through a two dimensional channel is presented. The transport of the magnetic biospecies is modeled by coupling the fluid flow with an Eulerian advection–convection concentration equation. A magnetic field is imposed in the separator that causes an accumulation of the magnetic labeled species in the vicinity

Saud A. Khashan; Emad Elnajjar; Yousef Haik

2011-01-01

59

The conventional configuration of a radiation driven target for heavy ion fusion is a quasi-cylindrical hohlraum containing a fusion capsule with radiation converters placed at opposite ends of the hohlraum. Ion beams enter each converter from opposite directions and are stopped by the material inside the converters. The first comprehensive two dimensional (2-D) simulations are presented for this configuration using

D. D.-M. Ho; J. A. Harte; M. Tabak

1998-01-01

60

Two dimensional simulation of positive streamer corona under consideration of radical reactions

Corona discharge is useful tool for removal of gaseous pollutants such as NOx. Numerical simulation of chemical process enables us to examine the reaction path. Most of these works have been carried out by assuming uniform space with uniform elecric field, however, it is far from the real column discharges in atmospheric pressure. In this work, we calculated two dimensional

Hideyuki Arai; Fumiyoshi Tochikubo; Tsuneo Watanabe; Timm H. Teich

1998-01-01

61

Two-dimensional RANS simulations of the flow through a compressor cascade with jet flaps

With the aim to reduce the blade count in a stator row, in this paper the application of active blowing is discussed. Based on a high-speed compressor stage blade geometries for a stator cascade with jet flap implementation are developed. Two-dimensional numerical simulations of the cascade flow clarify that a reasonable appliance of the jet is particularly possible at high

S. Fischer; H. Saathoff; R. Radespiel

2008-01-01

62

Nonlinear ultrasonic standing waves: two-dimensional simulations in bubbly liquids.

We present the results of numerical predictions for analyzing the behavior of nonlinear ultrasonic standing waves in two-dimensional cavities filled with bubbly liquids. The model we solve accounts for nonlinearity, dissipation, and dispersion of the two-dimensional media due to the bubbles. The numerical simulations are based on a finite-difference scheme. They consider the bubbles evenly distributed in the liquid. Results are shown for high-amplitude signals. They make it possible to observe how the linear modes turn into multi-frequency nonlinear fields. PMID:20932792

Vanhille, Christian; Campos-Pozuelo, Cleofé

2011-03-01

63

NASA Astrophysics Data System (ADS)

We present simulations of one and two-dimensional infrared (2DIR) and stimulated resonance Raman (SRR) spectra of the dark state (pG) and early red-shifted intermediate (pR) of photoactive yellow protein (PYP). Shifts in the amide I and Glu46 COOH stretching bands distinguish between pG and pR in the IR absorption and 2DIR spectra. The one-dimensional SRR spectra are similar to the spontaneous RR spectra. The two-dimensional SRR spectra show large changes in cross peaks involving the C = O stretch of the two species and are more sensitive to the chromophore structure than 2DIR spectra.

Preketes, Nicholas K.; Biggs, Jason D.; Ren, Hao; Andricioaei, Ioan; Mukamel, Shaul

2013-08-01

64

Insights and applications of two-dimensional simulations to Z-pinch experiments

A two-dimensional (2D) Eulerian radiation-magnetohydrodynamic code has been used to successfully simulate hollow metallic z-pinch experiments fielded on several facilities with a wide variety of drive conditions, time scales, and loads. The 2D simulations of these experiments reproduce important quantities of interest including the radiation pulse energy, power, and pulse width. This match is obtained through the use of an

D. L. Peterson; R. L. Bowers; W. Matuska; K. D. McLenithan; G. A. Chandler; C. Deeney; M. S. Derzon; M. Douglas; M. K. Matzen; T. J. Nash; R. B. Spielman; K. W. Struve; W. A. Stygar; N. F. Roderick

1999-01-01

65

Plasma instabilities are observed in low-pressure inductive discharges in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases are used. A two-dimensional hybrid fluid-analytic simulation is used to determine the space- and time-varying densities of electrons, positive and negative ions, and neutral species, and electron and neutral gas temperatures. The simulation includes both

E Kawamura; M A Lieberman; A J Lichtenberg; D B Graves

2012-01-01

66

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

67

Simulation of wave interactions with MHD

The broad scientific objectives of the SWIM (Simulation of Wave Interaction with MHD) project are twofold: (1) improve our understanding of interactions that both radio frequency (RF) wave and particle sources have on extended-MHD phenomena, and to substantially improve our capability for predicting and optimizing the performance of burning plasmas in devices such as ITER: and (2) develop an integrated computational system for treating multiphysics phenomena with the required flexibility and extensibility to serve as a prototype for the Fusion Simulation Project. The Integrated Plasma Simulator (IPS) has been implemented. Presented here are initial physics results on RF effects on MHD instabilities in tokamaks as well as simulation results for tokamak discharge evolution using the IPS.

Batchelor, Donald B [ORNL; Abla, G [General Atomics, San Diego; Bateman, Glenn [Lehigh University, Bethlehem, PA; Bernholdt, David E [ORNL; Berry, Lee A [ORNL; Bonoli, P. [Massachusetts Institute of Technology (MIT); Bramley, R [Indiana University; Breslau, J. [Princeton Plasma Physics Laboratory (PPPL); Chance, M. [Princeton Plasma Physics Laboratory (PPPL); Chen, J. [Princeton Plasma Physics Laboratory (PPPL); Choi, M. [General Atomics; Elwasif, Wael R [ORNL; Fu, GuoYong [Princeton Plasma Physics Laboratory (PPPL); Harvey, R. W. [CompX, Del Mar, CA; Jaeger, Erwin Frederick [ORNL; Jardin, S. C. [Princeton Plasma Physics Laboratory (PPPL); Jenkins, T [University of Wisconsin; Keyes, David E [Columbia University; Klasky, Scott A [ORNL; Kruger, Scott [Tech-X Corporation; Ku, Long-Poe [Princeton Plasma Physics Laboratory (PPPL); Lynch, Vickie E [ORNL; McCune, Douglas [Princeton Plasma Physics Laboratory (PPPL); Ramos, J. [Massachusetts Institute of Technology (MIT); Schissel, D. [General Atomics; Schnack, [University of Wisconsin; Wright, J. [Massachusetts Institute of Technology (MIT)

2008-07-01

68

Implicit Predictor-Corrector finite difference scheme for the ideal MHD simulations

NASA Astrophysics Data System (ADS)

A innovative simulation code for ideal magnetohydrodynamics (MHD) is developed. We present a multiple-dimensional MHD code based on high-order implicit predictor-corrector finite difference scheme (high-order IPCFD scheme). High-order IPCFD scheme adopts high-order predictor-corrector scheme for the time integration and high-order central difference method as the spatial derivative solver. We use Elimination-of-the-Runoff-Errors (ERE) technology to avoid the numerical oscillations and numerical instability in the simulation results. In one-dimensional MHD problem, our simulation results show good agreement with the Brio & Wu MHD shock tube problem. The divergent B constraint remains fully satisfied, that is the divergent B equals to zero throughout the simulation. When solving the two-dimensional (2D) linear wave in MHD plasma, we clearly obtain the group-velocity Friedrichs diagrams of the MHD waves. Here we demonstrate 2D simulation results of rotor problem, Orszag-Tang vortex system, vortex type K-H instability, and kink type K-H instability by using our IPCFD MHD code and discuss the advantage of our simulation code.

Tsai, T.; Yu, H.; Lai, S.

2012-12-01

69

Multigrid Monte Carlo simulation via XY embedding. II. Two-dimensional SU(3) principal chiral model

We carry out a high-precision simulation of the two-dimensional SU(3) principal chiral model at correlation lengths xi up to ~4×105, using a multigrid Monte Carlo (MGMC) algorithm and approximately one year of Cray C-90 CPU time. We extrapolate the finite-volume Monte Carlo data to infinite volume using finite-size-scaling theory, and we discuss carefully the systematic and statistical errors in this

Gustavo Mana; Andrea Pelissetto; Alan D. Sokal

1997-01-01

70

A two-dimensional model simulation of the EL Chichon volcanic eruption cloud

NASA Technical Reports Server (NTRS)

Using a two-dimensional model of sulfate photochemistry, transport, and aerosol microphysics, a 2-year period following the eruptions of El Chichon have been simulated. Present calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years. The model reproduces the observed optical depth, lidar backscatter, and infrared extinction coefficients, if about 10 megatonnes of SO2 are injected. The major deficiency of the model is a somewhat too rapid transport.

Capone, L. A.; Riegel, C. A.; Toon, O. B.; Whitten, R. C.; Turco, R. P.; Santhanam, K.

1983-01-01

71

A two-dimensional model simulation of the EL Chichon volcanic eruption cloud

NASA Astrophysics Data System (ADS)

Using a two-dimensional model of sulfate photochemistry, transport, and aerosol microphysics, a 2-year period following the eruptions of El Chichon have been simulated. Present calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years. The model reproduces the observed optical depth, lidar backscatter, and infrared extinction coefficients, if about 10 megatonnes of SO2 are injected. The major deficiency of the model is a somewhat too rapid transport.

Capone, L. A.; Riegel, C. A.; Toon, O. B.; Whitten, R. C.; Turco, R. P.; Santhanam, K.

1983-11-01

72

In this paper, two-dimensional (2D) simulation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells is presented using Sentaurus Device, a software package of Synopsys TCAD. A model is established incorporating a distribution of trap states of amorphous-silicon material and thermionic emission across the amorphous-silicon \\/ crystalline-silicon hetero-interface. The 2D nature of IBC-SHJ device is evaluated and current density-voltage (J-V)

Meijun Lu; Ujjwal Das; Stuart Bowden; Steven Hegedus; Robert Birkmire

2009-01-01

73

Two-dimensional, self-consistent, three-moment simulation of RF glow discharge

A two-dimensional self-consistent nonequilibrium fluid model is used to simulate radio frequency (RF) glow discharges to evaluate the quantitative effects of the radial and axial flow dynamics inside a cylindrically symmetric parallel-plate geometry. This model is based on the three moments of the Boltzmann equation and on Poisson's equation. Radial\\/axial flow dynamics of plasma in low-pressure parallel plate RF glow

Fongray Frank Young; ChwanHwa J. Wu

1993-01-01

74

Thermal relaxation of a two dimensional plasma in a dc magnetic field. Part 2: Numerical simulation

NASA Technical Reports Server (NTRS)

The thermal relaxation process for a spatially uniform two dimensional plasma in a uniform dc magnetic field is simulated numerically. Thermal relaxation times are defined in terms of the time necessary for the numerically computer Boltzman H-function to decrease through a given part of the distance to its minimum value. Dependence of relaxation time on two parameters is studied: number of particles per Debye square and ratio of gyrofrequency to plasma frequency.

Hsu, J. Y.; Joyce, G.; Montgomery, D.

1974-01-01

75

In this research, the phase behavior of a lattice-based model for a classical two-dimensional dipolar antiferromagnet on a square lattice is determined using Monte Carlo simulations. Four different systems are investigated, with the magnetic phase diagram for each system being established for both zero and finite temperature. The first model is the Heisenberg system in which the three-dimensional classical spins

Abdel-Rahman Mustafa Abu-Labdeh

2004-01-01

76

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

77

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 Deltaomega=omegap are used, a coherent plasma wave heats the electrons to many megaelectronvolts. In

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

1985-01-01

78

Two-dimensional numerical simulations of shock waves in micro convergent–divergent nozzles

The steady two-dimensional Navier–Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent–divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the

Jinliang Xu; Chuangxin Zhao

2007-01-01

79

Quantitative analysis of voids in percolating structures in two-dimensional N-body simulations

NASA Technical Reports Server (NTRS)

We present in this paper a quantitative method for defining void size in large-scale structure based on percolation threshold density. Beginning with two-dimensional gravitational clustering simulations smoothed to the threshold of nonlinearity, we perform percolation analysis to determine the large scale structure. The resulting objective definition of voids has a natural scaling property, is topologically interesting, and can be applied immediately to redshift surveys.

Harrington, Patrick M.; Melott, Adrian L.; Shandarin, Sergei F.

1993-01-01

80

A Two-dimensional Anisotropic Wet Etching Simulator for Quartz Crystal

NASA Astrophysics Data System (ADS)

We developed an anisotropic wet etching simulator that can predict the etching profile of initial shape, which is two-dimensional and formed by straight lines. New etching rate database of quartz at a special condition was obtained. Improved hull method was used to deal with round corner etching. New programming flow was adopted to avoid improper profile prediction. A friendly graphical user interface was built for user's convenience. The simulation result of this simulator meets well with the experimental results and shows nice accuracy in new emerging faces predicting.

Zhao, Meng; Wang, Jiani; Oigawa, Hiroshi; Ji, Jing; Hayashi, Hisanori; Ueda, Toshitsugu

81

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

82

A bounded two dimensional PIC-MCC code for simulating processing plasmas

The authors have developed a bounded two dimensional particle-in-cell simulation code with a Monte Carlo Collision (MCC) handler to study processing discharges. The MCC package models the collisions, between charged and neutral particles, which are needed to obtain a self sustained plasma and the proper electron and ion energy loss mechanisms. The simulations are aimed at determining uniformity of particle fluxes (magnitude and angle) across a typical target. Some early results are obtained from an x-y model with electrode area ratio of 6:1; a similar r-z model is in progress which can be used to study cylindrical chambers.

Vahedi, V.; Birdsall, C.K.; Lieberman, M.A. [Univ. of California, Berkeley, CA (United States)] [and others

1992-12-01

83

A major long-range goal of theoretical simulations of solar-generated disturbances (transients, coronal holes, etc.) is the realistic modeling of a propagating disturbance from the sun into and throughout interplanetary space. Simulations of this kind, using MHD fluid theory, must always be confronted with observations in order to assess the degree to which one or the other is inadequate. We describe some of this on-going work which is concerned with both one- and two-dimensional, time-dependent MHD simulations. The first example simulates a flare-produced coronal transient. In this case, a sudden current pulse is assumed to produce emerging magnetic flux. This 'magnetic pulse' is sufficient to drive a weak shock wave into the solar wind by virtue of an outwardly-directed Lorentz force. The pulse is assumed to consist of a linearly-increasing (with time) magnetic field of 0.72 G at the base of a closed magnetic topology (initially in hydrostatic equilibrium) to a value ten times larger over a 10-minute period. The shock achieves a velocity of 230 km/s (which would be superimposed upon the existing solar wind). A second example deals with a series of corotating interaction regions observed during a 60-day period by Pioneers 10 and 11 in 1973 prior to, during, and subsequent to the former spacecraft's flyby of Jupiter. An opportunity for a stringent test of our one-dimensional model was made possible by the nearly radial alignment of these two spacecraft.

Dryer, M.; Steinolfson, R.S.; Smith, Z.K.

1982-12-27

84

Magnetotail reconnection, MHD theory and simulations

NASA Technical Reports Server (NTRS)

Magnetotail reconnection leading to plasmoid formation and ejection is discussed, emphasizing three-dimensional structures and deviations from earlier imposed symmetries, based on MHD simulations and topological considerations. In general, the separation of the plasmoid takes a finite amount of time. During this stage the plasmoid is characterized by filamentary structures of interwoven flux tubes with different topological connections.

Birn, J.; Hesse, M.; Schindler, K.

1989-01-01

85

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

86

Relativistic MHD Simulations of Precessed Jets

NASA Technical Reports Server (NTRS)

Relativistic jets are considered to be generated by magnetic fields in a rotating black hole with accretion disk. Consequently, resulting outflows contain magnetic fields in them and control the propagation of jets. We have performed 3D relativistic MHD simulations to investigate the stability and structure of precessed MHD jets with large Lorentz factor by using a newly developed 3D GRMHD code. We have performed simulations of supermagnetosonic jets surrounded by a fast wind. The simulation results reveal complex pressure structure inside the RMHD jet. The structure is produced by a combination of the helical surface and body modes excited by the precession as predicted theoretically. The wavelength of the body mode which occurs in an internal helical twist is much shorter than that of the helical twist surface mode. We will present some comparisons between the RMHD simulations and theoretical predictions, and potential observables and discuss the effect of wind.

Mizuno, Y.; Nishikawa, K.-I.; Hardee, P.; Koide, S.; Fishman, G. J.

2006-01-01

87

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

88

Thermal structure of the ionosphere of Mars - simulations with one- and two-dimensional models

Heat flux saturation effects are included in the present one- and two-dimensional models of the Martian upper ionosphere's thermal structure. The inclusion of small upper boundary and volume heat sources is found to yield satisfactory simulations of the dayside ion temperature observation results obtained by Viking 1's retarding potential analyzers. It is noted that the plasma flow-transport of heat from the dayside to the nightside makes no contribution to the ion and electron temperatures that have been calculated for the nightside. 22 references.

Singhal, R.P.; Whitten, R.C.

1988-05-01

89

Two-dimensional electrostatic simulations of plasma propagation perpendicular to a magnetic field

NASA Astrophysics Data System (ADS)

A two-dimensional electrostatic particle-in-cell code is used to simulate a finite-width plasma streaming across a uniform magnetic field. The simulations show that the plasma polarizes, and non-neutral charge layers develop along its edges. In an electron-ion plasma, the charge layers are asymmetric and the electron layer is unstable to the diocotron mode. The simulations show that this instability has a smaller growth rate for plasma streams that are relatively less dense and wider. For a positive/negative ion plasma with equal mass ions the charge layers are symmetric and the plasma is stable to the diocotron mode. The results show that the diocotron instability leads to a vortex structure when the plasma width is greater than the ion gyroradius, but this instability disrupts the entire plasma when the plasma width is of the order of or smaller than the ion gyroradius.

Galvez, Miguel; Barnes, Christopher

1988-04-01

90

Two-dimensional electrostatic simulations of plasma propagation perpendicular to a magnetic field

A two-dimensional electrostatic particle-in-cell code is used to simulate a finite-width plasma streaming across a uniform magnetic field. The simulations show that the plasma polarizes, and non-neutral charge layers develop along its edges. In an electron--ion plasma, the charge layers are asymmetric and the electron layer is unstable to the diocotron mode. The simulations show that this instability has smaller growth rate for plasma streams that are relatively less dense and wider. For a positive/negative ion plasma with equal mass ions the charge layers are symmetric and the plasma is stable to the diocotron mode. The results show that the diocotron instability leads to vortex structure when the plasma width is greater than the ion gyroradius, but this instability disrupts the entire plasma when the plasma width is of the order of or smaller than the ion gyroradius.

Galvez, M.; Barnes, C.

1988-04-01

91

Two-Dimensional Hybrid Particle-In-Cell Simulation of Solar Wind Plasma Flow around Magnetic Sail

NASA Astrophysics Data System (ADS)

Solar wind plasma flow with interplanetary magnetic field (IMF) and the thrust of the magnetic sail are examined by time-dependent, two-dimensional, X-Y Cartesian, hybrid particle-in-cell (PIC) simulations. The hybrid-PIC simulation model is that the ions are treated kinetically as particles and the electrons are modeled as an inertia-less (mass-less) fluid. In this simulation, the real solar wind parameters around a near-earth orbit are used. The direction and strength of IMF are set to +Y direction which is perpendicular to the solar wind flow (+X direction) and 3 nT. Expressed in rL/L (the ratio of an ion Larmor radius rL of the solar wind at the magnetopause to a representative length of magnetic field L), when rL/L = 0.1 (in the case of MHD scale), magnetopause is formed accompanied by a fast magnetosonic bow shock. When rL/L = 2.0 (in the case of ion inertial scale), the electromagnetic interaction results in the formation of a magnetosphere with standing whistler waves. The drag coefficients, which is the thrust normalized by the solar wind inertial force, of both scales with IMF tend to increase compared with the cases without IMF because the incoming IMF accompanied by the solar wind piles up at upstream of the spacecraft. Also, on the ion inertial scale, the generation mechanism of Whistler wave and the influence of that on the thrust performance are revealed.

Matsumoto, Masaharu; Kajimura, Yoshihiro; Hideyuki Usui; Ikkoh Funaki; Iku Sinohara, And

92

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

93

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 a an algebraic or ordinary differential equation for the potential on the boundary. This scheme allows decomposition of the field solve into a Laplace solver with boundary conditions (e.g., applied potentials) and a Poisson solver with zero boundary conditions. We present the details of the external circuit coupling to an explicit electrostatic planar two -dimensional particle-in-cell code called PDP2, and discuss briefly how the coupling can be done in an implicit electrostatic code. The decomposition replaces the iterative coupling with a direct coupling and reduces the amount of computational time spent in the field solver. We use PDP2 to simulate a dually excited capacitively coupled RF discharge and show how such a system can be used as a plasma processing tool with separate control over ion flux and ion bombarding energy. 29 refs., 14 figs.

Vahedi, V.; DiPeso, G. [Lawrence Livermore National Lab., CA (United States)] [Lawrence Livermore National Lab., CA (United States)

1997-02-01

94

NASA Astrophysics Data System (ADS)

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.

Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo

2012-07-01

95

A two-dimensional simulation of plasma leakage due to dengue infection

NASA Astrophysics Data System (ADS)

Dengue Hemorrhagic Fever (DHF) is a disease caused by Dengue virus infection. One major characteristic in a patient with DHF is the occurrence of plasma leakage. Plasma leakage is a consequence of the immune system mechanism which activates cytokine. As a result, permeability of vascular will increase. Another characteristic in a DHF patient is hypoalbuminea (decreasing of albumin concentration). Plasma leakage can be modelled by constructing mathematical model of albumin concentration in plasma blood due to increasing of cytokine. In this paper, decreasing of albumin concentration in blood plasma is modelled using diffusion equation. In addition, two-dimensional numerical simulations of albumin concentration are also presented. From the simulation, it is found that the greater leakage rate or the wider leakage area, the greater decreasing albumin concentration will be. Furthermore, when time t increases, the albumin concentration decreases to zero.

Nuraini, N.; Windarto, Jayanti, Swarna; Soewono, Edy

2014-03-01

96

Particle-in-Cell Simulations of Two-dimensional Electrostatic Structures

NASA Astrophysics Data System (ADS)

Electrostatic structures have been observed in many regions of space plasmas, including the solar wind, the magnetosphere, 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. Recently we have constructed exact solutions of two-dimensional (2D) BGK modes in a magnetized plasma with finite magnetic field strength in order to gain insights of the ultimate 3D theory [Ng, Bhattacharjee, and Skiff, Phys. Plasmas 13, 055903 (2006)]. Based on the analytic form of these solutions, we have performed Particle-in-Cell (PIC) simulations to study their stability. We have also simulated more general initial conditions, and found 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.

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

2011-11-01

97

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

98

Weakly ionized processing plasmas are studied in two dimensions using a bounded particle-in-cell (PIC) simulation code with a Monte Carlo collision (MCC) package. The MCC package models the collisions between charged and neutral particles, which are needed to obtain a self-sustained plasma and the proper electron and ion energy loss mechanisms. A two-dimensional capacitive radio-frequency (rf) discharge is investigated in detail. Simple frequency scaling laws for predicting the behavior of some plasma parameters are derived and then compared with simulation results, finding good agreements. It is found that as the drive frequency increases, the sheath width decreases, and the bulk plasma becomes more uniform, leading to a reduction of the ion angular spread at the target and an improvement of ion dose uniformity at the driven electrode.

Vahedi, V.; Birdsall, C.K.; Lieberman, M.A. (Department of Electrical Engineering and Computer Science and the Electronics Research Laboratory, University of California, Berkeley, Berkeley, California 94720 (United States)); DiPeso, G.; Rognlien, T.D. (Lawrence Livermore National Laboratory, Livermore, California 94550 (United States))

1993-07-01

99

Simulation study of chiral two dimensional ultraviolet (2DUV) spectroscopy of the protein backbone

Amide n –?* and ?-?* excitations around 200 nm are prominent spectroscopic signatures of the protein backbone, which are routinely used in ultraviolet (UV) circular dichroism for structure characterization. Recently developed ultrafast laser sources may be used to extend these studies to two dimensions (2D). We apply a new algorithm for modelling protein electronic transitions to simulate two-dimensional ultraviolet (2DUV) photon echo signals in this regime and to identify signatures of protein backbone secondary (and tertiary) structure. Simulated signals for a set of globular and fibrillar proteins and their specific regions reveal characteristic patterns of helical and sheet secondary structures. We investigate how these patterns vary and converge with the size of the structural motif. Specific chiral polarization configurations of the UV pulses are found to be sensitive to aspects of the protein structure. This information significantly augments that available from linear circular dichroism.

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

2010-01-01

100

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

101

Numerical simulations of two-dimensional foam by the immersed boundary method

NASA Astrophysics Data System (ADS)

In this paper, we present an immersed boundary (IB) method to simulate a dry foam, i.e., a foam in which most of the volume is attributed to its gas phase. Dry foam dynamics involves the interaction between a gas and a collection of thin liquid-film internal boundaries that partition the gas into discrete cells or bubbles. The liquid-film boundaries are flexible, contract under the influence of surface tension, and are permeable to the gas, which moves across them by diffusion at a rate proportional to the local pressure difference across the boundary. Such problems are conventionally studied by assuming that the pressure is uniform within each bubble. Here, we introduce instead an IB method that takes into account the non-equilibrium fluid mechanics of the gas. To model gas diffusion across the internal liquid-film boundaries, we allow normal slip between the boundary and the gas at a velocity proportional to the (normal) force generated by the boundary surface tension. We implement this method in the two-dimensional case, and test it by verifying the von Neumann relation, which governs the coarsening of a two-dimensional dry foam. The method is further validated by a convergence study, which confirms its first-order accuracy.

Kim, Yongsam; Lai, Ming-Chih; Peskin, Charles S.

2010-07-01

102

Simulations of one- and two-dimensional complex plasmas using a modular, object-oriented code

In a complex plasma, charged microparticles ('dust') are added to a background of ions, electrons, and neutral particles. This dust fully interacts with the surrounding plasma and self-consistently alters the plasma environment leading to the emergence of new plasma behavior. Numerical tools that complement experimental investigations can provide important insights into the properties of complex plasmas. This paper discusses a newly developed code, named DEMON (dynamic exploration of microparticle clouds optimized numerically), for simulating a complex plasma. The DEMON code models the behavior of the charged particle component of a complex plasma in a uniform plasma background. The key feature of the DEMON code is the use of a modular force model that allows a wide variety of experimental configurations to be studied without varying the core code infrastructure. Examples of the flexibility of this modular approach are presented using examples of one- and two-dimensional complex plasmas.

Jefferson, R. A.; Cianciosa, M.; Thomas, E. Jr. [Department of Physics, Auburn University, Alabama 36849-5311 (United States)

2010-11-15

103

Two-dimensional self-consistent fluid simulation of radio frequency inductive sources

The two-dimensional ([ital R][minus][ital Z]) electromagnetic code FMRZ has been used 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 momentum transfer for the ion--neutral reactions. The electrons and ions are treated as 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 time and space scales to resolve only the frequencies and scale lengths of interest. The model retains nonlinear driving terms which give rise to a ponderomotive force that distorts the density profile. Density and power profiles show the physical effects of various terms in the equations. Trends in average density and temperature compare well with an analytic model and other simulation models.

DiPeso, G.; Vahedi, V.; Hewett, D.W.; Rognlien, T.D. (Plasma Physics Research Institute and M Division, Lawrence Livermore National Laboratory, Livermore, California 94550 (United States))

1994-07-01

104

MINBU Distribution of Two-Dimensional Quantum Gravity:. Simulation Result and Semiclassical Analysis

NASA Astrophysics Data System (ADS)

We analyze the MINBU distribution of two-dimensional quantum gravity. New data of R2-gravity by the Monte Carlo simulation and its theoretical analysis by the semiclassical approach are presented. In the distribution, the cross-over phenomenon takes place at some size of the baby universe where the randomness competes with the smoothing (or roughening) force of R2-term. The dependence on the central charge cm and on the R2-coupling are explained for R2-gravity, which includes the ordinary 2d quantum gravity. The R2-Liouville solution plays the central role in the semiclassical analysis. A total derivative term (surface term) and the infrared regularization play important roles. The surface topology is that of a sphere.

Ichinose, S.; Tsuda, N.; Yukawa, T.

105

Two-dimensional and three-dimensional direct numerical simulation of co-rotating vortices

NASA Astrophysics Data System (ADS)

Two-dimensional (2D) and three-dimensional (3D) simulations are presented for co-rotating vortices with distributed vorticity at different values of the separation distance b0. The ratio between the vortices' radius a0 and b0 is smaller than the critical value a0/b0~0.30 for merging in inviscid conditions of blobs of constant vorticity. The dependence of merging on Reynolds number and on a0/b0 has been investigated in 2D simulations. In 2D the resolution can be increased to perform simulations at Re=30000 when very long filaments are generated. These simulations confirm the findings of Brandt and Nomura [L. K. Brandt and K. K. Nomura, Phys. Fluids 18, 51701 (2006)] that the filaments play a minor role in merging. It has been found that a large increase of |P|max (P is the pressure) indicates merging. Time evolving 3D simulations at Re=3000, are similar to the laboratory experiments by Meunier and Leweke [P. Meunier and T. Leweke, J. Fluid Mech. 533, 125 (2005)]. At the same values of a0/b0 as used in 2D, 3D simulations demonstrate that the merging is more complex than in 2D, and that it largely depends on the kind of axial disturbances imposed, at t=0, on the two vortices. 3D simulations of vortices evolving in space and time, without disturbances at x1=0, have shown that at Re=3000 the results, in a convective frame, do not differ from those in 2D simulations. This assumption, attempting to reproduce the evolution of the vortices behind aircrafts, demonstrates that, with the present computational resources, DNS at practical Reynolds numbers are not feasible.

Orlandi, P.

2007-01-01

106

HYBRID AND HALL-MHD SIMULATIONS OF COLLISIONLESS RECONNECTION: EFFECTS OF PLASMA PRESSURE TENSOR

In this study we performed two-dimensional hybrid (particle ions, massless fluid electrons) and Hall-MHD simulations of collisionless reconnection in a thin current sheet. Both calculations include the full electron pressure tensor (instead of a localized resistivity) in the generalized Ohm's law to initiate reconnection, and in both an initial perturbation to the Harris equilibrium is applied. First, electron dynamics from the two calculations are compared, and we find overall agreement between the two calculations in both the reconnection rate and the global configuration. To address the issue of how kinetic treatment for the ions affects the reconnection dynamics, we compared the fluid-ion dynamics from the Hall-MHD calculation to the particle-ion dynamics obtained from the hybrid simulation. The comparison demonstrates that off-diagonal elements of the ion pressure tensor are important in correctly modeling the ion out-of-plane momentum transport from the X point. It is that these effects can be modeled efficiently using a particle Hall-MHD simulation method in which particle ions used in a predictor/corrector to implement the ion gyro-radius corrections. We also investigate the micro- macro-scale coupling in the magnetotail dynamics by using a new integrated approach in which particle Hall-MHD calculations are embedded inside a MHD simulation. Initial results of the simulation concerning current sheet thinning and reconnection dynamics are discussed.

L. YIN; D. WINSKE; ET AL

2001-05-01

107

Interstellar clouds in high-speed, supersonic flows: Two-dimensional simulations

NASA Technical Reports Server (NTRS)

We present a series of gasdynamical simulations of the interaction of a dense, cool interstellar cloud with a high-speed, supersonic wind that confines and accelerates the embedded cloud. Our goal is to attempt to determine if such clouds can survive various potentially disruptive instabilities, that occur at their peripheries, long enough to be accelerated to speeds which are comparable to the wind velocity. These simulations are performed using two-dimensional, Eulerian gas dynamics on both an axisymmetric (about the cloud axis) and 'slab' geometric grid. The spatial and temporal resolutions of the simulations are varied over a wide range to investigate the effects of small-scale instabilities on the overall acceleration of clouds and the development of large-scale, disruptive instabilities. Also, we study the effects of wind/cloud Mach number variations by changing the wind speed constant at about 12 km/s (which corresponds to a cloud temperature of 10,000 K). The current simulations track the evolution of clouds as they are accelerated to speeds approximately 4-5 times greater than their internal sound speeds. Furthermore, the models with the highest resolution were extended far beyond quasi-linear Rayleigh-Taylor growth times reaching 6-7 Rayleigh-Taylor growth times for the largest scale instabilities before being terminated because of the accumulation of errors at the rear grid boundary.

Schiano, A. V. R.; Christiansen, Wayne A.; Knerr, Jeffrey M.

1995-01-01

108

The void spectrum in two-dimensional numerical simulations of gravitational clustering

NASA Technical Reports Server (NTRS)

An algorithm for deriving a spectrum of void sizes from two-dimensional high-resolution numerical simulations of gravitational clustering is tested, and it is verified that it produces the correct results where those results can be anticipated. The method is used to study the growth of voids as clustering proceeds. It is found that the most stable indicator of the characteristic void 'size' in the simulations is the mean fractional area covered by voids of diameter d, in a density field smoothed at its correlation length. Very accurate scaling behavior is found in power-law numerical models as they evolve. Eventually, this scaling breaks down as the nonlinearity reaches larger scales. It is shown that this breakdown is a manifestation of the undesirable effect of boundary conditions on simulations, even with the very large dynamic range possible here. A simple criterion is suggested for deciding when simulations with modest large-scale power may systematically underestimate the frequency of larger voids.

Kauffmann, Guinevere; Melott, Adrian L.

1992-01-01

109

In this paper, two-dimensional (2D) simulation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells is presented using Sentaurus Device, a software package of Synopsys TCAD. A model is established incorporating a distribution of trap states of amorphous-silicon material and thermionic emission across the amorphous-silicon / crystalline-silicon heterointerface. The 2D nature of IBC-SHJ device is evaluated and current density-voltage (J-V) curves are generated. Optimization of IBC-SHJ solar cells is then discussed through simulation. It is shown that the open circuit voltage (VOC) and short circuit current density (JSC) of IBC-SHJ solar cells increase with decreasing front surface recombination velocity. The JSC improves further with the increase of relative coverage of p-type emitter contacts, which is explained by the simulated and measured position dependent laser beam induced current (LBIC) line scan. The S-shaped J-V curves with low fill factor (FF) observed in experiments are also simulated, and three methods to improve FF by modifying the intrinsic a-Si buffer layer are suggested: (i) decreased thickness, (ii) increased conductivity, and (iii) reduced band gap. With all these optimizations, an efficiency of 26% for IBC-SHJ solar cells is potentially achievable.

Lu, Meijun; Das, Ujjwal; Bowden, Stuart; Hegedus, Steven; Birmire, Robert

2009-06-09

110

NASA Astrophysics Data System (ADS)

A new idea to calculate ultracold neutron (UCN) production by using Monte Carlo simulation method to calculate the cold neutron (CN) flux and an analytical approach to calculate the UCN production from the simulated CN flux was given. A super-thermal source (UCN source) was modeled based on an arrangement of D2O and solid D2 (sD2). The D2O was investigated as the neutron moderator, and sD2 as the converter. In order to determine the required parameters, a two-dimensional (2D) neutron balance equation written in Matlab was combined with the MCNPX simulation code. The 2D neutron-transport equation in cylindrical (? - z) geometry was considered for 330 neutron energy groups in the sD2. The 2D balance equation for UCN and CN was solved using simulated CN flux as boundary value. The UCN source dimensions were calculated for the development of the next UCN source. In the optimal condition, the UCN flux and the UCN production rate (averaged over the sD2 volume) equal to 6.79 × 106 cm-2s-1 and 2.20 ×105 cm-3s-1, respectively.

Gheisari, R.; Firoozabadi, M. M.; Mohammadi, H.

2014-01-01

111

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

NASA Astrophysics Data System (ADS)

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 established to simulate stormwater flooding processes in urban areas. With rainfall input, the model simulates 2-D overland flow and 1-D flow in underground stormwater pipes and drainage channels. Drainage channels are treated as special flow paths and arranged along one or more sides of a 2-D computational grid. By using irregular computation grids, the model simulates unsteady flooding and drying processes over urban areas with complex drainage systems. The model results can provide spatial flood risk information (e.g., water depth, inundation time and flow velocity during flooding). The model was applied to the City of Beaumont, Texas, and validated with the recorded rainfall and runoff data from Tropical Storm Allison with good agreement.

Fang, Xing; Su, Dehui

2006-06-01

112

NASA Astrophysics Data System (ADS)

This paper describes the self-consistent simulation of plasma transport across the magnetic field at the magnetopause driven by Kelvin-Helmholtz (KH) instability. Two-dimensional hybrid (kinetic ions, fluid electrons) simulations of the most KH-unstable configuration where the shear flow is oriented perpendicular to the uniform magnetic field are carried out. The motion of the simulation particles is tracked during the run in order to calculate their mean-square displacement normal to the initial magnetopause surface, from which diffusion coefficients may be determined. The diffusion coefficients are found to be time dependent, with D $\\propto$ t ? , where ? > 0. Additionally, the probability distribution functions (PDF) of the “jump lengths” the particles make over time are found to be non-Gaussian. Such time-dependent diffusion coefficients and non-Gaussian PDFs have been associated with so-called “superdiffusion,” in which diffusive mixing of particles is enhanced over classical diffusion. The results indicate that while smaller-scale turbulence associated with the breakdown of vortices contributes to this enhanced diffusion, the growth of large-scale, coherent vortices is the more important process in facilitating it.

Cowee, M. M.; Winske, D.; Gary, S. P.

2009-10-01

113

Two-dimensional viscous flow simulation of a shock accelerated heavy gas cylinder

NASA Astrophysics Data System (ADS)

Numerical simulation of Richtmyer-Meshkov instability (RMI) is conducted using an improved localized artificial diffusivity (LAD) method, which is used to treat discontinuities in the form of material interfaces and shocks in the flow-field. The RMI occurs on a cylindrical interface between air and SF6 accelerated by a Mach 1.2 shock initially in air. Navier-Stokes simulation is conducted to accurately predict the mixing between the two fluids. The initial conditions for the two-dimensional simulations are matched to previous experimental work by C. Tomkins et al. [``An experimental investigation of mixing mechanisms in shock-accelerated flow,'' J. Fluid Mech. 611, 131 (2008)] and good agreement is found between the experimental data and numerical results. The study on initial condition sensitivity indicates that the initial pressure and density gradient are critical parameters that determine the primary vortex generation responsible for the flow development. A grid convergence study is carried out and the relative contribution of the artificial properties introduced by the LAD method is characterized. Novel to this study is the exploration of the effect of the third species (acetone used as a tracer particle in the experiments to obtain contour fields using planar laser induced florescence). The effect of the presence of the third species on the evolution of the RMI and mixing is shown to be non-negligible and an estimate of the amount of the tracer species that was present in the initial experimental set-up is given.

Shankar, Santhosh K.; Kawai, Soshi; Lele, Sanjiva K.

2011-02-01

114

NASA Astrophysics Data System (ADS)

Plasma instabilities are observed in low-pressure inductive discharges in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases are used. A two-dimensional hybrid fluid-analytic simulation is used to determine the space- and time-varying densities of electrons, positive and negative ions, and neutral species, and electron and neutral gas temperatures. The simulation includes both the capacitive and inductive coupling of the source coils to the plasma and the neutral gas dissociation and heating. The plasma is described using the time-dependent fluid equations, along with an analytical sheath model. The simulation is applied to an experiment in Cl2, in which gaps in the electron and positive ion densities versus power curves were observed, with our numerical results indicating the existence of an inductive-capacitive transition instability, corresponding approximately to the observed gaps. The fluid calculation captures various features that are not included in previous global instability models. A method is developed to match the numerical results to the global model formalism, which predicts the existence of the unstable mode, as numerically found. The time and space variations can be used to improve the global model formalism.

Kawamura, E.; Lieberman, M. A.; Lichtenberg, A. J.; Graves, D. B.

2012-08-01

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

General Relativistic MHD Simulations of Jet Formation

NASA Technical Reports Server (NTRS)

We have performed 3-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of jet formation from an accretion disk with/without initial perturbation around a rotating black hole. We input a sinusoidal perturbation (m = 5 mode) in the rotation velocity of the accretion disk. The simulation results show the formation of a relativistic jet from the accretion disk. Although the initial perturbation becomes weakened by the coupling among different modes, it survives and triggers lower modes. As a result, complex non-axisymmetric density structure develops in the disk and the jet. Newtonian MHD simulations of jet formation with a non-axisymmetric mode show the growth of the m = 2 mode but GRMHD simulations cannot see the clear growth of the m = 2 mode.

Mizuno, Y.; Nishikawa, K.-I.; Hardee, P.; Koide, S.; Fishman, G. J.

2005-01-01

117

Insights and applications of two-dimensional simulations to Z-pinch experiments

A two-dimensional (2D) Eulerian radiation-magnetohydrodynamic code has been used to successfully simulate hollow metallic {ital z}-pinch experiments fielded on several facilities with a wide variety of drive conditions, time scales, and loads. The 2D simulations of these experiments reproduce important quantities of interest including the radiation pulse energy, power, and pulse width. This match is obtained through the use of an initial condition: the amplitude of a random density perturbation imposed on the initial plasma shell. The perturbations seed the development of magnetically driven Rayleigh{endash}Taylor instabilities which greatly affect the dynamics of the implosion and the resulting production of radiation. Analysis of such simulations allows insights into the physical processes by which these calculations reproduce the experimental results. As examples, the insights gained from the simulations of Sandia {open_quotes}{ital Z}{close_quotes} accelerator [R. B. Spielman {ital et al.}, Phys. Plasmas {bold 5}, 2105 (1998)] experiments have allowed for the investigation of possible physical processes which produce high powers in {open_quotes}nested array{close_quotes} implosions and high temperatures within {open_quotes}dynamic hohlraum{close_quotes} loads. Building on these insights, the 2D code has been used in designing new experiments to optimize the desired physical conditions and in interpreting the experimental results obtained. These examples and others will be discussed as well as examples of simulation results where improvement is needed and what steps are being taken to make that improvement. {copyright} {ital 1999 American Institute of Physics.}

Peterson, D.L.; Bowers, R.L.; Matuska, W.; McLenithan, K.D. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)] [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Chandler, G.A.; Deeney, C.; Derzon, M.S.; Douglas, M.; Matzen, M.K.; Nash, T.J.; Spielman, R.B.; Struve, K.W.; Stygar, W.A. [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)] [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States); Roderick, N.F. [University of New Mexico, Albuquerque, New Mexico 87131 (United States)] [University of New Mexico, Albuquerque, New Mexico 87131 (United States)

1999-05-01

118

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

119

Brownian dynamics simulations of two-dimensional model for hopping times

NASA Astrophysics Data System (ADS)

Brownian dynamics simulations are used to study the mean first passage time for a hard disk diffusing in a channel. The disk has to hop pass another disk with the same diameter ? but fixed in contact with the wall. This mean first passage time is expected to diverge with an exponent ? as the channel width (2Rp) approaches that of the nonpassing limit (2?) for one disk to hop pass another. Our one disk model is similar to a disk diffusing in a two-dimensional container, shaped as a simplified configuration space of two passing disks in a flat channel of Kalinay. The Kalinay model was proposed recently to elucidate the disagreements between two different theoretical predictions for the exponent ? of the two disk hopping time divergences. For the one disk model, simple transition state theory and a Fick-Jacobs type of dimensional reduction approach predict exponents of 1 and 1/2, respectively. Our Brownian dynamics simulation, results are consistent with a value of 1.

Mon, K. K.

2008-09-01

120

Brownian dynamics simulations of two-dimensional model for hopping times.

Brownian dynamics simulations are used to study the mean first passage time for a hard disk diffusing in a channel. The disk has to hop pass another disk with the same diameter sigma but fixed in contact with the wall. This mean first passage time is expected to diverge with an exponent eta as the channel width (2R(p)) approaches that of the nonpassing limit (2sigma) for one disk to hop pass another. Our one disk model is similar to a disk diffusing in a two-dimensional container, shaped as a simplified configuration space of two passing disks in a flat channel of Kalinay. The Kalinay model was proposed recently to elucidate the disagreements between two different theoretical predictions for the exponent eta of the two disk hopping time divergences. For the one disk model, simple transition state theory and a Fick-Jacobs type of dimensional reduction approach predict exponents of 1 and 12, respectively. Our Brownian dynamics simulation, results are consistent with a value of 1. PMID:19045052

Mon, K K

2008-09-28

121

Two-dimensional simulation of magnetorotational instability with a large Reynolds number

NASA Astrophysics Data System (ADS)

Magnetorotational instability (MRI) is a likely mechanism for enhanced angular momentum transport in accretion disks, whose test has been recently proposed or started in laboratory experiments [1,2]. However, the magnetic Prandtl number (ratio of kinematic viscosity to resistivity) is tiny in those liquid-metal experiments (10-5 - 10-6) while it is supposed of order unity in accretion disks. It is desired to understand the consequence of the disparate magnetic Prandtl number for the interpretation of experimental results. Small Prandtl number, however, leads to extremely high Reynolds number, which has been inhibited the access of direct numerical simulation of the experimental devices. Here we approach realistic values of the Reynolds number by making a two-dimensional spectral simulation in cylindrical geometry with automatic resolution adjustment. By a proper adjustment of the magnetic Reynolds number, we may access the wide range of magnetic Prandtl number from order unity to 10-4. We explore the relation of small Prandtl number and profile relaxation effect to the low saturation level observed in the experiment. Comparison to parasitic instability theory [3] will be also made in the presentation. *This work supported in part by the CMPD, DOE grant DEFC0204ER54784. [1] H.T. Ji et al., Mon. Not. Roy. Astron. Soc. 325, L1 (2001). [2] D.R. Sisan et al., Phys. Rev. Lett. 93, 114502 (2004). [3] J. Goodman and G. Xu, Astrophys. J. 432, 213 (1994).

Tatsuno, T.; Dorland, W.; Tillotson, W. A.

2006-10-01

122

Phase transitions in a two-dimensional vortex lattice with defects: Monte Carlo simulation

NASA Astrophysics Data System (ADS)

Phase diagrams and phase transitions in a two-dimensional vortex lattice with defects have been investigated by Monte Carlo (MC) simulation techniques. It was found that in the presence of defects, the melting of the vortex lattice proceeds in two stages. First, the ideal triangular lattice transforms at low temperature into islands, which are pinned to the defects and rotate around them (“rotating lattice” phase). Then, at a higher temperature, the boundaries of the “vortex lattice islands” become smeared and the system transforms into a vortex liquid. The dependencies of phase transition temperatures on pinning potential have been obtained. The visual pictures of the flux lattice structures have been calculated. The current-voltage characteristics (IVC) of the vortex structures with defects in forms of points, lines and squares were simulated at different temperatures and defects concentration. We have found that the point defects are more efficient than the other type of defects with equivalent specific concentration. It was shown that the dependencies of critical current on temperature and defects' concentrations are directly associated with the phase state of the vortex system.

Rudnev, Igor A.; Kashurnikov, Vladimir A.; Gracheva, Mariya E.; Nikitenko, Oksana A.

2000-05-01

123

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

2012-06-01

124

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

125

NASA Astrophysics Data System (ADS)

In the present study a two-dimensional axisymmetric numerical model is developed for supercritical parametric phase conjugation of ultrasound in a solid active element of cylindrical shape and finite length. The pseudospectral time domain algorithm (PSTD) is used owing to its efficiency to model large-scale problems. PSTD solves elastic wave equation in time-dependent heterogeneous isotropic and axisymmetric anisotropic solids using FFTs for high order approximation of the spatial differential operator on staggered grid, and a fourth-order Adams-Bashforth time integrator. In order to truncate the computational domain absorbing boundary conditions are introduced with complex frequency shifted perfectly matched layers. This procedure is highly effective at absorbing signals of long time-signature. The free surface of the active ceramic rod is introduced through the method of images. A systematic study of the influence of lateral limitations of the active medium on parametric wave phase conjugation of sound is made. It is shown that retro-focusing of the incident pulse takes place even in the case of mode conversions inside the active zone. Nevertheless, amplitude and form of the obtained conjugate pulse depend on the simulated configuration. Numerical simulation correctly describes the parametric amplification and retro-focusing of ultrasound observed in experiments.

Bou Matar, Olivier; Preobrazhensky, Vladimir; Pernod, Philippe

2005-11-01

126

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

NASA Astrophysics Data System (ADS)

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.

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

2012-06-01

127

Two-dimensional hydrodynamical simulations of wind-compressed disks around rapidly rotating B stars

NASA Technical Reports Server (NTRS)

We use a two-dimensional piecewise parabolic method (PPM) code to simulate numerically the hydrodynamics of a radiation-driven stellar wind from a rapidly rotating Be star. The results generally confirm predictions of the semianalytic 'wind-compressed disk' model recently proposed by Bjorkman and Cassinelli to explain the circumstellar disks inferred observationally to exist around such rapidly rotating stars. However, this numerical simulation is able to incorporate several important effects not accounted for in the simple model, including a dynamical treatment of the outward radiative driving and gas pressure, as well as a rotationally distorted, oblate stellar surface. This enables us to model quantitatively the compressed wind and shock that forms the equatorial disk. The simulation results thus do differ in several important details from the simple method, showing, for example, an inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. There is also no evidence for the predicted detachment of the disk that arises in the fixed outflow picture. The peak equatorward velocity in the dynamical models is furthermore about a factor of 2 smaller than the lytically predicted value of approximately 50% of the stellar equatorial rotation speed. As a result, the dynamical disks are somewhat weaker than predicted, with a wider opening angle, lower disk/pole density ratio, and smaller shock velocity jump. The principal cause of these latter differences appears to be an artificially strong equatorward drift of the subsonic outflow in the original analytic model. Much better agreement with the dynamical results can be obtained, however, from a slightly modified, analytic wind-compression model with a more detailed specification of the fixed wind outflow and a lower boundary set to the sonic radius along a rotationally oblate stellar surface. Hence, despite these detailed differences, the general predicted effect of disk formation by wind compression toward the equator is substantially confirmed.

Owocki, Stanley P.; Cranmer, Steven R.; Blondin, John M.

1994-01-01

128

Two-dimensional simulation of organic bulk heterojunction solar cell: influence of the morphology.

Recent developments in organic solar cells show interesting power conversion efficiencies. However, with the use of organic semiconductors and bulk heterojunction cells, many new concepts have to be introduced to understand their characteristics. Only few models investigate these new concepts, and most of them are one-dimensional only. In this work, we present a two-dimensional model based on solving the drift-diffusion equations. The model describes the generation of excitons in the donor phase of the active layer and their diffusion towards an interface between the two separate acceptor and donor domains. Then, when the exciton reaches the interface, it forms a charge transfer state which can split into free charges due to the internal potential. Finally, these free charges are transported toward the electrodes within their respective domains (electrons in acceptor domain, holes in donor domain) before being extracted. In this model, we can follow the distribution of each species and link it to the physical processes taken into account. Using the finite element method to solve the equations of the model, we simulate the effect of the bulk heterojunction morphology on photocurrent curves. We concentrate on the morphology parameters such as the mean acceptor/donor domain sizes and the roughness of,the interface between the donor and acceptor domains. Results are discussed in relation with experimental observations. PMID:23901547

Raba, Adam; Cordan, Anne-Sophie; Leroy, Yann

2013-07-01

129

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

130

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

131

Magnetoconvection dynamics in a stratified layer. 1: Two-dimensional simulations and visualization

NASA Astrophysics Data System (ADS)

To gain insight in the problem of fluid convection below solar photosphere, time-dependent magnetohydrodynamic convection is studied by numerical simulation to the magneto-anelastic equations, a model appropiate for low Mach numbers. Numerical solutions to the equations are generated on a two-dimensional Cartesian mesh by a finite-difference, predictor-corrector algorithm. The thermodynamic properties of the fluid are held constant at the rigid, stress-free top and bottom boundaries of the computational box, while lateral boundaries are treated as periodic. In most runs the background polytropic fluid configuration is held fixed at Rayleigh number R = 5.44 times the critical value, Prandtl number P = 1.8, and aspect ratio a = 1, while the magnetic parameters are allowed to vary. The resulting dynamical behavior is shown to be strongly influenced by a horizontal magnetic field which is imposed at the bottom boundary. As the field strength increases from zero, an initially unsteady 'single-roll' state, featuring complex time dependence is replaced by a steady 'traveling-wave tilted state; then, an oscillatory or 'sloshing' state; then, a steady two-poll state with no tilting; and finally, a stationary state. Because the magnetic field is matched onto a potential field at the top boundary, it can penetrate into the nonconducting region above. By varying a magnetic diffusivity, the concentrations of weak magnetic fields at the top of these flows can be shown to be explainable in terms of an advection-diffusion balance.

Lantz, Steven R.; Sudan, R. N.

1995-03-01

132

NASA Astrophysics Data System (ADS)

The meridional circulation derived from the NCEP/NCAR Reanalysis II data from 1979 to 2000, combined with the solar UV radiation with the 11 year variation, is used to drive the Caltech/JPL two-dimensional (2-D) chemistry and transport model (CTM). The monthly UV fluxes are generated using the monthly solar cycle variation of the 10.7 cm flux data and the solar cycle maximum and minimum flux data measured by UARS/SOLSTICE as a function of wavelength. The wavelength range is from 119.5 nm to 318.5 nm, encompassing the most important wavelengths influencing the production and destruction of ozone. A principal component analysis is applied to the model simulated ozone distribution from 1980 to 2000. The decadal signal is found in the column ozone density, but the signal is smaller than that found in a similar analysis of the merged TOMS/SBUV ozone data. The main source of the decadal variations in the column ozone density is the meridional circulation. However, the direct effect of the UV variation can be seen primarily in the upper stratosphere and above.

Camp, C. D.; Feynman, J.; Jiang, X.; Shia, R.; Walker, C.; Schneider, T.; Allen, M. A.; Yung, Y. L.

2003-12-01

133

Platelet Motion near a Vessel Wall or Thrombus Surface in Two-Dimensional Whole Blood Simulations

Computational simulations using a two-dimensional lattice-Boltzmann immersed boundary method were conducted to investigate the motion of platelets near a vessel wall and close to an intravascular thrombus. Physiological volume fractions of deformable red blood cells and rigid platelet-size elliptic particles were studied under arteriolar flow conditions. Tumbling of platelets in the red-blood-cell depleted zone near the vessel walls was strongly influenced by nearby red blood cells. The thickness of the red-blood-cell depleted zone was greatly reduced near a thrombus, and platelets in this zone were pushed close to the surface of the thrombus to distances that would facilitate their cohesion to it. The distance, nature, and duration of close platelet-thrombus encounters were influenced by the porosity of the thrombus. The strong influence on platelet-thrombus encounters of red-blood-cell motion and thrombus porosity must be taken into account to understand the dynamics of platelet attachment to a growing thrombus.

Skorczewski, Tyler; Erickson, Lindsay Crowl; Fogelson, Aaron L.

2013-01-01

134

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

135

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

136

TWO-DIMENSIONAL BLAST-WAVE-DRIVEN RAYLEIGH-TAYLOR INSTABILITY: EXPERIMENT AND SIMULATION

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 {approx}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 {mu}m and amplitude of 2.5 {mu}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. [University of Michigan, MI (United States); Robey, H. F.; Remington, B. A.; Edwards, M. J.; Miles, A. R.; Perry, T. S. [Lawrence Livermore National Laboratory, University of California, P.O. Box 5508, L-487, Livermore, CA 94550 (United States); Blue, B. E. [General Atomics, San Diego, CA (United States); Plewa, T. [Department of Scientific Computing, Florida State University, Dirac Science Library Tallahassee, FL 32306-4120 (United States); Hearn, N. C. [ASC/Alliances Center for Astrophysical Thermonuclear Flashes, University of Chicago, IL (United States); Knauer, J. P. [Laboratory of Laser Energetics, University of Rochester, Rochester, NY (United States); Arnett, D. [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States); Leibrandt, D. R. [Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)

2009-05-01

137

Movement, deformation, and partitioning of mammalian red blood cells (RBCs) in diverging microvessel bifurcations are simulated using a two-dimensional, flexible-particle model. A set of viscoelastic elements represents the RBC membrane and the cytoplasm. Motion of isolated cells is considered, neglecting cell-to-cell interactions. Center-of-mass trajectories deviate from background flow streamlines due to migration of flexible cells towards the mother vessel centerline upstream of the bifurcation and due to flow perturbations caused by cell obstruction in the neighborhood of the bifurcation. RBC partitioning in the bifurcation is predicted by determining the RBC fraction entering each branch, for a given partition of total flow and for a given upstream distribution of RBCs. Typically, RBCs preferentially enter the higher-flow branch, leading to unequal discharge hematocrits in the downstream branches. This effect is increased by migration toward the centerline but decreased by the effects of obstruction. It is stronger for flexible cells than for rigid circular particles of corresponding size, and decreases with increasing parent vessel diameter. For unequally-sized daughter vessels, partitioning is asymmetric, with RBCs tending to enter the smaller vessel. Partitioning is not significantly affected by branching angles. Model predictions are consistent with previous experimental results.

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

2008-01-01

138

Platelet motion near a vessel wall or thrombus surface in two-dimensional whole blood simulations.

Computational simulations using a two-dimensional lattice-Boltzmann immersed boundary method were conducted to investigate the motion of platelets near a vessel wall and close to an intravascular thrombus. Physiological volume fractions of deformable red blood cells and rigid platelet-size elliptic particles were studied under arteriolar flow conditions. Tumbling of platelets in the red-blood-cell depleted zone near the vessel walls was strongly influenced by nearby red blood cells. The thickness of the red-blood-cell depleted zone was greatly reduced near a thrombus, and platelets in this zone were pushed close to the surface of the thrombus to distances that would facilitate their cohesion to it. The distance, nature, and duration of close platelet-thrombus encounters were influenced by the porosity of the thrombus. The strong influence on platelet-thrombus encounters of red-blood-cell motion and thrombus porosity must be taken into account to understand the dynamics of platelet attachment to a growing thrombus. PMID:23601323

Skorczewski, Tyler; Erickson, Lindsay Crowl; Fogelson, Aaron L

2013-04-16

139

NASA Astrophysics Data System (ADS)

collisionless shocks undergo structural changes with the increase of the Mach number. These changes are related to the increasing role of the reflected ions, which have a highly nongyrotropic distribution. Eventually, it is expected that the shock front becomes nonstationary. At low and moderate Mach numbers, the fraction of reflected ions is small, yet recent observations show the existence of a well-pronounced structure of the postshock magnetic field in the close vicinity of the transition layer. Large amplitude oscillations were earlier interpreted as waves generated by the shock front or passing through the shock in the downstream direction. Here we show, using two-dimensional hybrid simulations of quasi-perpendicular shocks, that the gyration of the directly transmitted ions downstream of the ramp produces the spatial pressure variations, which are accompanied with the observed magnetic oscillations due to the momentum conservation. In a wide range of the upstream ion temperatures, the low and moderate-Mach-number shocks remain stationary and one-dimensional, so that the magnetic and electric field depend only on the coordinate along the shock normal. The downstream ion distributions gradually gyrotropize due to the collisionless mixing of gyrophases. Nonstationary effects in these shocks do not affect noticeably the ion dynamics. However, we find that with the increase of the Mach number, shocks form rippled fronts in the low-? and moderate-? regimes.

Ofman, L.; Gedalin, M.

2013-05-01

140

Formation of polar stratospheric clouds simulated in a two dimensional model of the atmosphere

NASA Technical Reports Server (NTRS)

A microphysics code has been implemented in a two dimensional model of the atmosphere to study formation of polar stratospheric clouds containing HCl or HNO3. The model range is from pole to pole in latitude and from the ground to about 20 km in altitude. Resolution in latitude is 10 deg and about 0.8 km in altitude. This is an Eulerian model with prescribed eddy diffusion coefficients and the circulation obtained from observations. The chemistry of the model follows the family approach for NO(x), Cl(x) and HO(x) while the ozone is fixed and changed seasonally. The aerosol code is based on an assigned population of condensation, coagulation and sedimentation. Aerosol growth is simulated in nine different size bins ranging between 0.01u and 2.56u. The model has been built to study aerosol layers formation in the upper troposphere and lower stratosphere and has been validated for sulfate aerosol resulting from a rather complex sulfur chemistry.

Visconti, Guido; Pitari, Giovanni

1988-01-01

141

Two dimensional mesoscale simulations of projectile instability during penetration in dry sand

NASA Astrophysics Data System (ADS)

To gain insight into the instability and trajectory change in projectiles penetrating dry sand at high velocities, two dimensional plane strain mesoscale simulations were carried out using representative models of a particulate system and of a small projectile. A program, ISP-SAND, was developed and used to generate the representative particulate system with mean grain sizes of 60 and 120 ?m as well as +/-30% uniform size distribution from the mean. Target porosities ranged from 30% to 40%. The penetration of ogive nose steel projectiles with caliber radius head of 3.5 and length-to-diameter (l/d) ratio of 3.85 was simulated using the updated Lagrangian explicit parallel finite element code ISP-TROTP. Deformation of the projectile and individual sand grains was analyzed using a nonlinear elastic-inelastic model for these materials. Grain-grain and grain-projectile interactions were analyzed using a contact algorithm with and without friction. Projectile instability was quantified and compared using the lateral displacement of the center of mass, lateral force acting on the projectile, and its rotational momentum about the center of mass. The main source of projectile instability and the ensuing trajectory change in the penetration simulations was found to be the inhomogeneous loading of the projectile due to the heterogeneities and randomness inherent in a particulate media like sand. The granularity of the media has not been considered explicitly in previous work. Projectile instability increased with impact velocity, as expected. However, it also increased for the case of elastic impactor that preserved the nose shape, with an increase in grain size, and for uniform grain sizes. Moreover, friction, inherently present in geologic materials, was found to be a major contributor to instability. Conclusions derived from one projectile depth simulations were confirmed by two deeper penetration simulations considering up to three full lengths of penetration (requiring a larger sand target). The deep penetration simulation predicted considerable instability with a trajectory change of approximately 45° when friction was considered in the dry sand medium. An overall conclusion of this work is that projectile penetration studies in geologic materials need to explicitly consider the heterogeneous or particulate nature of these materials.

Dwivedi, S. K.; Teeter, R. D.; Felice, C. W.; Gupta, Y. M.

2008-10-01

142

NASA Astrophysics Data System (ADS)

This paper presents initial results based on kinetic extensions of a nonlinear two-dimensional (2D) multi-fluid (three ion species and fluid electrons) MHD model that is designed to study propagation of shear Alfven waves in low-altitude auroral flux tubes. It is intended to use the model for scientific support of the “enhanced polar outflow probe” e-POP/CASSIOPE spacecraft mission (launch scheduled in 2010). Effects of gravity, thermal pressure, and geomagnetic field curvature are included, while the parallel electric field along geomagnetic field lines is calculated under the assumption of plasma quasineutrality. The model has been used successfully to study excitation of eigenmodes of the ionospheric Alfven resonator (IAR) by an Alfven wave packet injected from the magnetospheric end of the simulated plasma region. The formation of density cavities due to the ponderomotive force of standing oscillations in the IAR [Sydorenko, Rankin, and Kabin, 2008], and excitation of double layers and ion-acoustic wave packets, has been demonstrated. The kinetic extension of the multi-fluid code involves replacing the fluid electron model with a kinetic module that solves the simplified drift-kinetic Vlasov equation for the electron velocity distribution function (EVDF). To avoid undue complexity, it is assumed that (i) the electrons move only along geomagnetic field lines and (ii) the electron magnetic moment is conserved. As a result, the evolution of the EVDF is reduced to the problem of advection in 2D phase space “distance along the field line - velocity along the field line”. This problem is solved using a semi-Lagrangian algorithm [Staniforth and Cote, 1991]. The kinetic simulation starts from the initial equilibrium state similar to [Ergun et al., 2000]. The equilibrium assumes that the plasma consists of two electron populations: cold electrons with isotropic EVDF originating from the ionosphere, and hot anisotropic electrons with a loss-cone EVDF coming from the high-altitude end. The loss-cone distribution is prone to strong numerical dispersion, which is compensated by tracing the interface of the EVDF in the coordinate-velocity phase space. Ergun R. E., C. W. Carlson, J. P. McFadden, F. S. Mozer, and R. J. Strangeway (2000), Geophys. Res. Lett., 27, 4053-4056. Staniforth A. and J. Cote (1991), Mon. Wea. Rev., 119, 2206-2223 Sydorenko, D., R. Rankin, and K. Kabin (2008), J. Geophys. Res., 113, A10206, doi:10.1029/2008JA013579.

Sydorenko, D.; Rankin, R.; Kabin, K.

2009-12-01

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

NASA Technical Reports Server (NTRS)

We have developed a new empirically-based transport algorithm for use in our GSFC two-dimensional transport and chemistry model. The new algorithm contains planetary wave statistics, and parameterizations to account for the effects due to gravity waves and equatorial Kelvin waves. As such, this scheme utilizes significantly more information compared to our previous algorithm which was based only on zonal mean temperatures and heating rates. The new model transport captures much of the qualitative structure and seasonal variability observed in long lived tracers, such as: isolation of the tropics and the southern hemisphere winter polar vortex; the well mixed surf-zone region of the winter sub-tropics and mid-latitudes; the latitudinal and seasonal variations of total ozone; and the seasonal variations of mesospheric H2O. The model also indicates a double peaked structure in methane associated with the semiannual oscillation in the tropical upper stratosphere. This feature is similar in phase but is significantly weaker in amplitude compared to the observations. The model simulations of carbon-14 and strontium-90 are in good agreement with observations, both in simulating the peak in mixing ratio at 20-25 km, and the decrease with altitude in mixing ratio above 25 km. We also find mostly good agreement between modeled and observed age of air determined from SF6 outside of the northern hemisphere polar vortex. However, observations inside the vortex reveal significantly older air compared to the model. This is consistent with the model deficiencies in simulating CH4 in the northern hemisphere winter high latitudes and illustrates the limitations of the current climatological zonal mean model formulation. The propagation of seasonal signals in water vapor and CO2 in the lower stratosphere showed general agreement in phase, and the model qualitatively captured the observed amplitude decrease in CO2 from the tropics to midlatitudes. However, the simulated seasonal amplitudes were attenuated too rapidly with altitude in the tropics. Overall, the simulations with the new transport formulation are in substantially better agreement with observations compared with our previous model transport.

Fleming, E. L.; Jackman, C. H.; Stolarski, R. S.; Considine, D. B.

1998-01-01

145

A multiple type-II burst associated with a coronal transient and its MHD simulation

NASA Technical Reports Server (NTRS)

A large coronal transient took place on May 8, 1981. The transient was related to an M7.7/2B flare and was associated with at least two coronal type-II bursts. The velocities of the type-II bursts were in the range 1100-1800 km/s, in excess of the transient velocity of 1000 km/s. Two dimensional positions of the type-II radio sources are available from both the Clark Lake and the Culgoora Radio Observatories. Two dimensional MHD simulations of the event are carried out, taking into account the observed velocity, position, and size of the type-II bursts. The multiple shocks observed during the event and their interaction are simulated, and results of the simulation are discussed.

Gergely, T. E.; Kundu, M. R.; Wu, S. T.; Dryer, M.; Smith, Z.; Stewart, R. T.

1984-01-01

146

NASA Technical Reports Server (NTRS)

A method for deep cumulus cloud simulations is presented, in which the plumes or clouds are simulated using a two-dimensional axisymmetric numerical cloud model with third-moment turbulence closure. This method is based on the assumption that the only uncertainty in the turbulence closure is in the turbulent length scale specification. Therefore, the length scale is varied, and, using different length scales, the simulated plumes are compared to the observed ones, making it possible to optimize the length scale prescription.

Krueger, Steven K.

1988-01-01

147

Two-dimensional network simulation of diffusion driven coarsening of foam inside a porous medium

In order to use foams in subsurface applications, it is necessary to understand their stability in porous media. Diffusion driven coarsening of a stationary or nonflowing foam in a porous medium results in changing gas pressures and a coarsening of the foam texture. A two-dimensional network simulation has been created that predicts the behavior of foam in a porous medium by physically specifying the locations of all the lamellae in the system and by solving the complete set of Young-Laplace and diffusion equations. An hourglass approximates the shape of the pores, and the pore walls are considered to be highly water wet. A singularity arises in the system of differential algebraic equations due to the curvature of the pore walls. This singularity is a signal that the system must undergo oscillations or sudden lamellar rearrangements before the diffusion process can continue. Newton-Raphson iteration is used along with Keller`s method of arc-length continuation and a new jump resolution technique to locate and resolve bifurcations in the system of coupled lamellae. Gas bubbles in pore throats are regions of encapsulated pressure. As gas is released from these bubbles during diffusion, the pressure of the bubbles in the pore bodies increases. When the pressure increase is scaled by the characteristic Young-Laplace pressure, the equilibrium time for the diffusion process is scaled by the ratio of the square of the characteristic length to the gas diffusivity and two dimensionless groups. One describes the ease with which gas can diffuse through a lamella, the second represents the amount of gas encapsulated within the pore throats initially. Given this scaling, the resulting plots of pressure versus time and normalized lamellae positions versus time are universal for all system sizes and characteristics. This is true as long as the initial lamella distribution is the same in each case.

Cohen, D.; Patzek, T.W.; Radke, C.J.

1996-04-01

148

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

149

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

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

2008-01-01

150

Two-dimensional model for the two-phase flow simulation in a viking rocket engine combustion chamber

We have presented here a two-dimensional model for the two-phase flow simulation in a VIKING rocket engine combustion chamber. This model allows the transient flow description and shows the chamber unsteady response to combustion. Several calculations have been performed and the injection parameters influence on the flow pattern has been studied, as the concentration distribution of droplets, frequency and damping

M. Habiballah; H. Monin

1985-01-01

151

MHD Simulation Heliospheric Magnetic Fields and Turbulence

NASA Technical Reports Server (NTRS)

This talk will present a summary of our results on simulations of heliospheric structure and dynamics. We use a three-dimensional MHD code in spherical coordinates to produce a solar wind containing a rotating, tilted heliospheric current sheet, fast-slow stream and microstream shear layers, waves, 2-D turbulence, and pressure balanced structures that are input to the inner (superAlfvenic) boundary. The evolution of various combinations of these has led to a deeper understanding of sector structure, magnetic holes, fluctuation anisotropies, and general turbulent evolution. We show how the sectors are likely to be connected, how spiral fields can arise, and how field line diffusion can be caused by waves with transverse structure and microstream shears.

Roberts, D. Aaron

2005-01-01

152

Geological and geochemical data from six wells, and several seismic profiles from strata in a Cenozoic accretion belt, which occurs between the Aleutian volcanic arc and the Aleutian trench in the Kodiak Shelf, Alaska, have been used in quantitative basin analysis studies. Applying both one-dimensional and two-dimensional fluid flow/compaction models to the basin has enabled a simulation to be made of the history of sedimentation, together with evolutionary behaviors for porosity, permeability, basement subsidence, fluid movement in the strata, formation pressure, paleoheat flux, maturation of organic carbon, and hydrocarbon generation and migration in specific locations and in a two-dimensional cross section of the basin.

Jianchang, Liu; Lerche, I. (Univ. of South Carolina, Columbia (United States))

1991-03-01

153

Simulation of a seawater MHD power generation system

NASA Astrophysics Data System (ADS)

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 exterior of the generator and an anode rod in the interior. Elementary research on a helical-type MHD power generation system has started at Kobe University, and a numerical simulation of the system has been carried out by FEM (Finite Element Method) at the National Institute for Materials Science. By comparing the simulation results with the theoretical and experimental results, we found that the proposed method is valid for simulating the MHD power generation system.

Liu, Xiaojun; Kiyoshi, Tsukasa; Takeda, Minoru

2006-05-01

154

Silicon photodiode-waveguide coupling—Two-dimensional modelling, software simulation and experiments

In the present paper a system composed of a waveguide coupled by leaky waves with a photodiode on the same silicon chip is studied. Maxwell's equations for this system have been solved, with light attenuation and propagation constants being calculated. The light attenuation is determined by the silicon and waveguide parameters. A two-dimensional analysis of current generation in the photodiode

M. Caldararu; F. Craciunoiu; A. Paraschiv; D. Cristea; F. Caldararu; D. Dascalu; C. Klier; G. Mirea; A. Vasile; M. Nicolae

1996-01-01

155

Probing the Mechanism of Fusion in a Two-Dimensional Computer Simulation

A two-dimensional (2D) model of lipid bilayers was developed and used to investigate a possible role of membrane lateral tension in membrane fusion. We found that an increase of lateral tension in contacting monolayers of 2D analogs of liposomes and planar membranes could cause not only hemifusion, but also complete fusion when internal pressure is introduced in the model. With

Alexandr Chanturiya; Puthurapamil Scaria; Oleksandr Kuksenok; Martin C. Woodle

2002-01-01

156

Mixed Finite Element Simulations in Two-Dimensional Groundwater Flow Problems

A computer code of groundwater flow in two-dimensional porous media based on the mixed finite element method was developed for accurate approximations of Darcy velocities in safety evaluation of radioactive waste disposal. The mixed finite element procedure solves for both the Darcy velocities and pressure heads simultaneously in the Darcy equation and continuity equation. Numerical results of a single well

Hideo KIMURA

1989-01-01

157

The Sensitivity of Two-Dimensional Simulations of Tropical Squall Lines to Environmental Profiles

Two dimensional experiments are carried out to determine the effect of various wind and thermodynamic structure on squall line characteristics. Two ideas concerning the effect of shear are found useful in explaining many of the outcomes of the numerical experiments. First, in two dimensions, shear in the absence of vorticity sources and sinks is detrimental to convection (Kuo, Asai). Second,

Melville E. Nicholls; Richard H. Johnson; William R. Cotton

1988-01-01

158

Two dimensional self-consistent fluid simulation of rf inductive sources.

National Technical Information Service (NTIS)

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...

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

1993-01-01

159

A Simulation Model for Two Dimensional Recording on Continuous Granular Media

In this paper, we present analyses of the media response to the write field with the recorded bits each of which involves a small number of grains. Our focus is on the development of a model that can be used to generate speedily the recorded patterns on continuous granular media. A two-dimensional Fourier analysis method is then used to construct

Z. J. Liu; B. J. Chen; H. T. Wang; S. H. Zhang

2010-01-01

160

Two-Dimensional Particle Simulation of Parallel Plate Radio-Frequency (RF) Glow Discharges.

National Technical Information Service (NTIS)

A two-dimensional model of parallel plate RF glow discharges was developed to study discharge phenomena important in plasma assisted processing of materials. The particle-in-cell method is used to calculate the trajectories of computer particles under the...

E. J. Bennett

1992-01-01

161

A proposal for teaching and learning electromagnetic simulation by means of a two-dimensional finite-difference time-domain method is presented. The suggestion has been well received by the academic community since the teaching material is easy to implement and simple to explain in a classroom environment. Two specific codes to simulate microstrip printed dipoles are presented in here but many more can

Alejandro Dueñas Jiménez

2013-01-01

162

Pixel image lag in a 4-T CMOS image sensor is analyzed and simulated in a two-dimensional model. Strategies of reducing image lag are discussed from transfer gate channel threshold voltage doping adjustment, PPD N-type doping dose\\/implant tilt adjustment and transfer gate operation voltage adjustment for signal electron transfer. With the computer analysis tool ISE-TCAD, simulation results show that minimum image

Yu Junting; Li Binqiao; Yu Pingping; Xu Jiangtao; Mou Cun

2010-01-01

163

Dynamic simulation of MHD topping cycles

Combined Cycle MHD (magnetohydrodynamics)\\/steam power plants show promise of achieving high efficiency in coal energy conversion to electricity. The plants are composed of an MHD topping cycle operating in the combustion gas temperature range of 3000-2000 K and a bottoming cycle consisting of a modified steam power plant in the steam temperature range of 810 - 300 K. This paper

R. Johnson; D. Bartle

1980-01-01

164

Two-dimensional tsunami-dynamo simulations using the finite element method in the time domain

NASA Astrophysics Data System (ADS)

Conductive seawater moving in the ocean generates an electromotive force. This effect is called 'Oceanic dynamo effect' and transient tsunamis are also involved in this effect. Toh et al. (2011) reported that the 2006 and 2007 Krill earthquake tsunami induced electromagnetic (EM) variations in the northwest Pacific. Manoj et al. (2011) and Suetsugu et al. (2012) also reported that seafloor EM observations detected tsunami passages. Motional induction due to tsunamis, hereafter called 'Tsunami Dynamo Effect', has been studied mainly in the frequency domain. For example, Larsen (1971) derived an analytical solution considering the conductivity structures and the self-induction term, while Sanford (1971) included effects of bathymetry. Recently, Tyler (2005) analytically expected that the vertical component of the tsunami-induced magnetic field has the same waveform and phase as those of the sea level change in deep oceans. Although these works in the frequency domain are very powerful and useful, they are not able to focus on transient properties of the Tsunami Dynamo Effect. For mitigation of tsunami disasters, it is very important to investigate effects due to the first arrival of tsunamis. We, therefore, developed a two-dimensional finite element (FEM) tsunami dynamo simulation code in the time domain to reproduce magnetic tsunami signals observed in the northwest Pacific at the time of the 2011 off the Tohoku earthquake. We adopted FEM with triangular mesh in order to include realistic bathymetry and arbitrary conductivity structures beneath the seafloor. For the time evolution, the Crank Nikolson method was adopted. As a result, our simulation succeeded in reproducing the magnetic tsunami signals, especially in terms of the first wave. It is noticeable that an initial rise in the horizontal magnetic component as large as 1 nT, parallel to the tsunami propagation direction and observed 5 minutes prior to the tsunami arrival, was also induced by the tsunami. We conducted additional numerical experiments to investigate the initial rise. We laid half-space homogeneous conductor beneath the flat seafloor with a depth of 5km and let a Gaussian waveform soliton propagate with a wave height of 1m and a horizontal extent of 100km. As we assigned 0, 0.01, 0.1, and 1S/m to the conductivity beneath the seafloor, the peak of the initial rise became approximately 3, 2, <1, and ~0 nT, respectively. The initial rise, hence, vanished with the conductivity of 1S/m beneath the seafloor, because an induced current beneath the seafloor generated a horizontal magnetic field opposite to the initial rise. Our numerical results imply that this initial rise may enable us to detect the tsunami passage prior to the tsunami arrival itself under suitable conditions. In the presentation, we will report the initial rise in the horizontal magnetic component observed prior to the tsunami arrival and discuss its applicability to tsunami early warnings.

Minami, T.; Toh, H.

2013-12-01

165

Maze energetics revealed by a large-scale two-dimensional Ginzburg-Landau type simulation

NASA Astrophysics Data System (ADS)

To understand the magnetization process of permanent magnets, the structural analysis of a so-called maze pattern plays a key role, because demagnetized states consist of maze domains. Motivated by this viewpoint, we explore the phase space of a magnetic order parameter in a thin film and clarify the energetics of various nonlinear states including the mazes, based on a two-dimensional Ginzburg-Landau model augmented by long-range dipole-dipole interaction

Iwano, Kaoru; Mitsumata, Chiharu; Ono, Kanta

2014-05-01

166

Computer simulation of the hydrodynamics of a two-dimensional gas-fluidized bed

A first principles model of a gas-fluidized bed has been applied to calculate the hydrodynamics of a two-dimensional (2-D) bed with an orifice in the middle of a porous plate distributor. The advanced hydrodynamic model is based on a two fluid model approach in which both phases are considered to be continuous and fully interpenetrating. Conservation equations for mass, momentum

J. A. M. Kuipers; Duin van KJ; F. P. H. van Beckum; W. P. M. van Swaaij

1993-01-01

167

Two-dimensional self-consistent fluid simulation of radio frequency inductive sources

The two-dimensional ([ital R][minus][ital Z]) electromagnetic code FMRZ has been used 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 momentum transfer for the ion--neutral reactions. The electrons and ions are treated as fluid species and a reduced set of Maxwell's equations is used to advance

G. Dipeso; V. Vahedi; D. W. Hewett; T. D. Rognlien

1994-01-01

168

Numerical simulation of acoustic diffraction of two-dimensional rigid bodies in arbitrary flows

Aeroacoustic perturbation quantities about various two-dimensional rigid bodies have been computed by solving the Euler equations in the complex domain. The governing equations are derived by linearizing the perturbation Euler equations, and by assuming a single frequency disturbance. A pseudo-time variable is introduced, and the entire set of equations is driven to convergence by an explicit, 5-stage Runge-Kutta, time-marching, finite

K. S. Huh; R. K. Agarwal; S. E. Widnall

1990-01-01

169

Two-Dimensional Numerical Simulation of Boiling Two-Phase Flow of Liquid Nitrogen

Two-dimensional characteristics of the boiling two-phase flow of liquid nitrogen in a duct flow are numerically investigated to contribute to the further development of new high-performance cryogenic engineering applications. First, the governing equations of the boiling two-phase flow of liquid nitrogen based on the unsteady drift-flux model are presented and several flow characteristics are numerically calculated taking account the effect

Jun Ishimoto; Mamoru Oike; Kenjiro Kamijo

2001-01-01

170

Two-Dimensional Simulation of Left-Handed Metamaterial Flat Lens Using Remcon XFDTD

NASA Technical Reports Server (NTRS)

Remcom's XFDTD software was used to model the properties of a two-dimensional left-handed metamaterial (LHM) flat lens. The focusing capability and attenuation of the material were examined. The results showed strong agreement with experimental results and theoretical predictions of focusing effects and focal length. The inherent attenuation in the model corresponds well with the experimental results and implies that the code does a reasonably accurate job of modeling the actual metamaterial.

Wilson, Jeffrey D.; Reinert, Jason M.

2006-01-01

171

Two-dimensional Hybrid Simulation of Whistler Excitation by Unstable Particle Distributions

NASA Astrophysics Data System (ADS)

We present results from a two-dimensional hybrid fluid-PIC model applied to the study of whistler wave interaction with energetic electrons. Using parameters appropriate to the equatorial radiation belts, we have studied the excitation of whistlers by unstable particle distributions and the self-consistent modification of the unstable distribution functions. This research has applications for understanding the origin of many types of whistler phenomena, including large-amplitude waves and chorus.

Woodroffe, J. R.; Streltsov, A. V.

2013-12-01

172

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

173

Substorm features in MHD simulations of magnetotail dynamics.

National Technical Information Service (NTIS)

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 effe...

J. Birn M. Hesse

1990-01-01

174

NASA Astrophysics Data System (ADS)

Gas heating in an atmospheric-pressure streamer discharge was analysed by a two-dimensional streamer discharge simulation model describing internal molecular energy transfer. Our two-dimensional streamer simulation model incorporates concepts from the fast gas heating mechanism proposed by Popov (2011 J. Phys. D: Appl. Phys. 44 285201) and our self-developed state-to-state vibrational kinetics. In dry air, gas heating occurs mainly from electron-impact dissociation reactions of O2 molecules and from quenching processes of electronically excited N2(B 3?g, C 3?u) molecules and O(1D) atoms. In humid air, rapid vibration-to-translation transitions of H2O and the exothermicity of the OH formation reactions additionally increase the gas temperature. It is shown that gas heating during the discharge pulse increases with humidity.

Komuro, Atsushi; Ono, Ryo

2014-04-01

175

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

176

The two-dimensional electroconvection between two plates has been studied from the theoretical and numerical point of view. However, the stability of the finite amplitude solution is an open theoretical problem. Here we use two sets of numerical techniques to analyze this problem. On one hand, particle-in-cell techniques for simulating the charge distribution combined with finite elements for computing the velocity

P. A. Vazquez; G. E. Georghiou; A. Castellanos

2008-01-01

177

Free boundary simulations of MHD instabilities with a finite volume based full-MHD code

NASA Astrophysics Data System (ADS)

MHFVSP is a finite volume based code for 3D full-MHD simulations in tokamak plasmas using either structured (rectangular) or unstructured (triangular) mesh in the poloidal plane, and fast Fourier transforms in the toroidal direction. The code is being modified to handle issues on the free boundary configurations related to the growth of MHD instabilities. The pseudo-vacuum model is implemented in the code, and then free boundary MHD simulations are carried out for the equilibrium including the vacuum region which is obtained by the MEUDAS code^1. We describe the algorithm in the MHFVSP code and report on the development and validation of such a modification. The initial results from simulation runs for free boundary configurations are also presented. ^1 T. Takeda and S. Tokuda, J. Comp. Phys. 93, 1 (1991).

Kagei, Yasuhiro; Kishimoto, Yasuaki; Miyoshi, Takahiro; Takechi, Manabu

2006-10-01

178

NASA Astrophysics Data System (ADS)

A novel two-dimensional finite-difference time-domain simulation for treating the interaction of a focused beam with a rewritable optical disk is detailed and experimentally validated. In this simulation, the real material properties of the rewritable multilayer stack and the aperiodic nature of the disk topography are considered. Excellent agreement is obtained between calculated and measured push-pull tracking servosignals for magneto-optical disks with pregrooves and infinite-length preformat pits. To demonstrate the utility of the simulation as a design tool, the design process for a 0.9- mu m track pitch, continuous, composite servoformat magneto-optical disk is given.

Judkins, Justin B.; Ziolkowski, Richard W.; Haggans, Charles W.

1996-05-01

179

Numerical simulations for MHD coronal seismology

NASA Astrophysics Data System (ADS)

Magnetohydrodynamic (MHD) processes are important for the transfer of energy over large scales in plasmas and so are essential to understanding most forms of dynamical activity in the solar atmosphere. The introduction of transverse structuring into models for the corona modifies the behavior of MHD waves through processes such as dispersion and mode coupling. Exploiting our understanding of MHD waves with the diagnostic tool of coronal seismology relies upon the development of sufficiently detailed models to account for all the features in observations. The development of realistic models appropriate for highly structured and dynamical plasmas is often beyond the domain of simple mathematical analysis and so numerical methods are employed. This paper reviews recent numerical results for seismology of the solar corona using MHD.

Pascoe, David James

2014-07-01

180

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.

Dorfman, S.; Ji, H.; Yamada, M. [Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (United States); Daughton, W.; Roytershteyn, V. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Ren, Y. [Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, University of Wisconsin, Madison, Wisconsin 53706 (United States)

2008-10-15

181

The SIMSYS2D two-dimensional water-quality simulation system is a large-scale digital modeling software system used to simulate flow and transport of solutes in freshwater and estuarine environments. Due to the size, processing requirements, and complexity of the system, there is a need to easily move the system and its associated files between computer sites when required. A series of job control language (JCL) procedures was written to allow transferability between IBM and IBM-compatible computers. (USGS)

Karavitis, G. A.

1984-01-01

182

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.

183

MHD simulations of supernova driven ISM turbulence

NASA Astrophysics Data System (ADS)

The dynamic evolution of the (stratified) turbulent interstellar medium (ISM) is simulated utilizing a three-dimensional MHD model including various physical effects. The computational domain covers a box of 0.5x0.5x2.0 kpc at a resolution of typically 128x128x1024 grid cells. The model includes (constant kinematic) viscosity and magnetic diffusivity. The adiabatic equation of state is supplemented by a parameterized heating- and cooling-function allowing for thermal instability (TI). The update due to heating and cooling is implemented implicitly using a Patankar-type discretization. Turbulence is driven by supernova explosions which are modelled as local injections of thermal energy, smeared over three standard-deviations of a Gaussian support with FWHM of 20pc. Supernova rates are adopted for typical cited values. Within our model we make a distinction between Type I and Type II SNe. Latter are statistically clustered by the (artificial) constraint that the density at the explosion site be above average (with respect to a horizontal slab) - former are spatially uncorrelated. The dual-energy feature of the conservative NIRVANA-code is used to tackle the extreme ratio of kinetic to internal energy that arises from the violent energy input. We stress the importance of using a conservative scheme to properly transfer the injected energy to kinetic motion. The model also includes a differentially rotating background (with shearing boundary conditions in radial direction) as well as vertical stratification. The initial density and pressure profiles are in hydrostatic equilibrium with respect to the equation of state given by the radiative equilibrium. Including z-dependent heating rates this leads to a considerable deviation from usual isothermal initial models. The primary focus of this work is on the galactic dynamo and the generation of large-scale magnetic fields. As a secondary target we are also interested in general properties of the ISM that are of importance for star formation.

Gressel, Oliver; Ziegler, Udo

184

Over the pst few years multidimensional self-consistent plasma simulations including complex chemistry have been developed which are promising tools for furthering the understanding of reactive gas plasmas and for reactor design and optimization. These simulations must be benchmarked against experimental data obtained in well-characterized systems such as the Gaseous Electronics Conference (GEC) reference cell. Two-dimensional simulations relevant to the GEC Cell are reviewed in this paper with emphasis on fluid simulations. Important features observed experimentally, such as off-axis maxima in the charge density and hot spots of metastable species density near the electrode edges in capacitively-coupled GEC cells, have been captured by these simulations. Glow discharge plasmas are used extensively in the processing of electronic materials especially for etching and deposition of thin films.

Lymberopoulos, D.P.; Economou, D.J. [Univ. of Houston, TX (United States)

1995-07-01

185

Two-dimensional simulation of a direct-current microhollow cathode discharge

Microhollow cathode discharges (MHCD's) are miniature direct-current discharges that operate at elevated pressures (several tens to hundreds of Torr) with electrode dimensions in the 10-100-{mu}m range. MHCD's have been proposed for a number of applications based on their unique characteristics such as presence of intense excimer radiation and significant gas heating within the submillimeter discharge volume. A two-dimensional, self-consistent fluid model of a helium MHCD in the high-pressure (several hundreds of Torr), high-current ({approx}1 mA) operating regime is presented in this study. Results indicate that the MHCD operates in an abnormal glow discharge mode with charged and excited metastable species with densities of {approx}10{sup 20} m{sup -3}, electron temperatures of approximately tens of eV, and gas temperatures of hundreds of Kelvin above room temperature. Significant discharge activity exists outside of the hollow region. The discharge volume and intensity increases with increasing current and becomes more confined with increasing pressures. Most predictions presented in this paper are in qualitative and quantitative agreement with experimental data for MHCD's under similar conditions.

Kothnur, Prashanth S.; Raja, Laxminarayan L. [Department of Aerospace Engineering and Engineering Mechanics, University of Texas at Austin, Austin, Texas 78712 (United States)

2005-02-15

186

Two-dimensional simulation of a direct-current microhollow cathode discharge

NASA Astrophysics Data System (ADS)

Microhollow cathode discharges (MHCD's) are miniature direct-current discharges that operate at elevated pressures (several tens to hundreds of Torr) with electrode dimensions in the 10-100-?m range. MHCD's have been proposed for a number of applications based on their unique characteristics such as presence of intense excimer radiation and significant gas heating within the submillimeter discharge volume. A two-dimensional, self-consistent fluid model of a helium MHCD in the high-pressure (several hundreds of Torr), high-current (~1 mA) operating regime is presented in this study. Results indicate that the MHCD operates in an abnormal glow discharge mode with charged and excited metastable species with densities of ~1020 m-3, electron temperatures of approximately tens of eV, and gas temperatures of hundreds of Kelvin above room temperature. Significant discharge activity exists outside of the hollow region. The discharge volume and intensity increases with increasing current and becomes more confined with increasing pressures. Most predictions presented in this paper are in qualitative and quantitative agreement with experimental data for MHCD's under similar conditions.

Kothnur, Prashanth S.; Raja, Laxminarayan L.

2005-02-01

187

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

NASA Astrophysics Data System (ADS)

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.

2010-11-01

188

Simulations of spray autoignition and flame establishment with two-dimensional CMC

The unsteady two-dimensional conditional moment closure (CMC) model with first-order closure of the chemistry and supplied with standard models for the conditional convection and turbulent diffusion terms has been interfaced with a commercial engine CFD code and analyzed with two numerical methods, an 'exact' calculation with the method of lines and a faster fractional-step method. The aim was to examine the sensitivity of the predictions to the operator splitting errors and to identify the extent to which spatial transport terms are important for spray autoignition problems. Despite the underlying simplifications, solution of the full CMC equations allows a single model to be used for the autoignition, flame propagation ('premixed mode'), and diffusion flame mode of diesel combustion, which makes CMC a good candidate model for practical engine calculations. It was found that (i) the conditional averages have significant spatial gradients before ignition and during the premixed mode and (ii) that the inclusion of physical-space transport affects the calculation of the autoignition delay time, both of which suggest that volume-averaged CMC approaches may be inappropriate for diesel-like problems. A balance of terms in the CMC equation before and after autoignition shows the relative magnitude of spatial transport and allows conjectures on the structure of the premixed phase of diesel combustion. Very good agreement with available experimental data is found concerning ignition delays and the effect of background air turbulence on them.

Wright, Y.M.; Boulouchos, K. [Aerothermochemistry and Combustion Technology Laboratory, ETH Zurich (Switzerland); De Paola, G.; Mastorakos, E. [Hopkinson Laboratory, Department of Engineering, University of Cambridge, Cambridge CB2 1PZ (United Kingdom)

2005-12-01

189

Kinetic MHD simulation of large 'circ; tearing mode

NASA Astrophysics Data System (ADS)

We have developed a second-order accurate semi-implicit ? method for kinetic MHD simulation with Lorentz force ions and fluid electrons. The model has been used to study the resistive tearing mode instability, which involves multiple spatial scales. In small 'circ; cases, the linear growth rate and eigenmode structure are consistent with resistive MHD analysis. The Rutherford stage and saturation are demonstrated, but the simulation exhibits different saturation island widths compared with previous MHD simulations. In large 'circ; cases, nonlinear simulations show multiple islands forming, followed by the islands coalescing at later times. The competition between these two processes strongly influences the reconnection rates and eventually leads to a steady state reconnection. We will present various parameter studies and show that our hybrid results agree with fluid analysis in certain limits (e.g., relatively large resisitivities).

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

2012-03-01

190

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

191

Surface Evolver simulations of flowing two-dimensional foams are described. These are used for two purposes. Firstly, to extract the location of the T 1s, the changes in bubble topology that occur during plastic flow. It is shown that in linear Couette flow the T1 s are localized in space, becoming more so as the polydispersity of the foam decreases. Secondly, the sedimentation of two circular discs through a foam under gravity is studied. If the discs are sufficiently close, they begin to interact and one moves behind the other during their descent. PMID:18404249

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

2008-01-01

192

Two-dimensional simulations of the population inversion in recombining laser-produced plasma

Simulations of gain in recombining laser-produced plasma of Li-like ions of aluminum were performed using the 2-D code GIDRA-2. Symmetrical 4-beam scheme of fiber irradiation was modeled in accordance with experimental arrangement described in Ref. (1). These simulations show good agreement with earlier 1-D simulations for optically thin plasma. Maximal angle-averaged value of gain on 3d{sub 5/2}-4f{sub 7/2} transition in Li-like aluminum calculated using Sobolev approximation for photon escape probability and the Doppler line profile was 0.95 cm{sup -1}. These 2-D simulations also show more realistic laser energy absorption (25-30% in 2-D simulations compared to nearly 90% in 1-D simulations and 6% in experiments)

Roerich, V. C.; Starostin, A. N.; Stepanov, A. E. [Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow region, 142092 (Russian Federation)

1997-04-15

193

Two-dimensional simulations of the population inversion in recombining laser-produced plasma

Simulations of gain in recombining laser-produced plasma of Li-like ions of aluminum were performed using the 2-D code GIDRA-2. Symmetrical 4-beam scheme of fiber irradiation was modeled in accordance with experimental arrangement described in Ref. (1). These simulations show good agreement with earlier 1-D simulations for optically thin plasma. Maximal angle-averaged value of gain on 3d5\\/2-4f7\\/2 transition in Li-like aluminum

V. C. Roerich; A. N. Starostin; A. E. Stepanov

1997-01-01

194

Two-dimensional simulations of the population inversion in recombining laser-produced plasma

Simulations of gain in recombining laser-produced plasma of Li-like ions of aluminum were performed using the 2-D code GIDRA-2. Symmetrical 4-beam scheme of fiber irradiation was modeled in accordance with experimental arrangement described in Ref. (1). These simulations show good agreement with earlier 1-D simulations for optically thin plasma. Maximal angle-averaged value of gain on 3d5\\/2?4f7\\/2 transition in Li-like aluminum

V. C. Roerich; A. N. Starostin; A. E. Stepanov

1997-01-01

195

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

196

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

197

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

198

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

2006-12-01

199

Global MHD Simulations of Jupiter's Magnetosphere

NASA Astrophysics Data System (ADS)

The Cassini swing-by of Jupiter in December, 2000 for the first time allowed a coordinated observing effort of the Jovian magnetosphere with Cassini acting as an upstream solar wind monitor and with simultaneous Galileo and HST measurements of the magnetosphere and aurora. A global magnetosphere model of the Jovian system would add one more piece of information to this unprecedented data set. We apply our 3D global magnetohydrodynamic (MHD) model to the Jovian system at the time of the Cassini swing-by. The model contains a realistic magnetic field model, mass loading due to Io and the Io torus and an ionosphere model similar to those used for MHD models of Earth. The model is run using Cassini measurements to set the upstream solar wind conditions. We present results showing the global structure of the magnetosphere and the ionosphere.

Hansen, K. C.; Gombosi, T. I.; De Zeeuw, D. L.; Powell, K. G.

2001-05-01

200

Advances in Simulation of Wave Interaction with Extended MHD Phenomena

The Integrated Plasma Simulator (IPS) provides a framework within which some of the most advanced, massively-parallel fusion modeling codes can be interoperated to provide a detailed picture of the multi-physics processes involved in fusion experiments. The presentation will cover four topics: 1) recent improvements to the IPS, 2) application of the IPS for very high resolution simulations of ITER scenarios, 3) studies of resistive and ideal MHD stability in tokamk discharges using IPS facilities, and 4) the application of RF power in the electron cyclotron range of frequencies to control slowly growing MHD modes in tokamaks and initial evaluations of optimized location for RF power deposition.

Batchelor, Donald B [ORNL; Abla, Gheni [ORNL; D'Azevedo, Ed F [ORNL; Bateman, Glenn [Lehigh University, Bethlehem, PA; Bernholdt, David E [ORNL; Berry, Lee A [ORNL; Bonoli, P. [Massachusetts Institute of Technology (MIT); Bramley, R [Indiana University; Breslau, Joshua [ORNL; Chance, M. [Princeton Plasma Physics Laboratory (PPPL); Chen, J. [Princeton Plasma Physics Laboratory (PPPL); Choi, M. [General Atomics; Elwasif, Wael R [ORNL; Foley, S. [Indiana University; Fu, GuoYong [Princeton Plasma Physics Laboratory (PPPL); Harvey, R. W. [CompX, Del Mar, CA; Jaeger, Erwin Frederick [ORNL; Jardin, S. C. [Princeton Plasma Physics Laboratory (PPPL); Jenkins, T [University of Wisconsin; Keyes, David E [Columbia University; Klasky, Scott A [ORNL; Kruger, Scott [Tech-X Corporation; Ku, Long-Poe [Princeton Plasma Physics Laboratory (PPPL); Lynch, Vickie E [ORNL; McCune, Douglas [Princeton Plasma Physics Laboratory (PPPL); Ramos, J. [Massachusetts Institute of Technology (MIT); Schissel, D. [General Atomics; Schnack, [University of Wisconsin; Wright, J. [Massachusetts Institute of Technology (MIT)

2009-01-01

201

Advances in Simulation of Wave Interactions with Extended MHD Phenomena

The Integrated Plasma Simulator (IPS) provides a framework within which some of the most advanced, massively-parallel fusion modeling codes can be interoperated to provide a detailed picture of the multi-physics processes involved in fusion experiments. The presentation will cover four topics: (1) recent improvements to the IPS, (2) application of the IPS for very high resolution simulations of ITER scenarios, (3) studies of resistive and ideal MHD stability in tokamak discharges using IPS facilities, and (4) the application of RF power in the electron cyclotron range of frequencies to control slowly growing MHD modes in tokamaks and initial evaluations of optimized location for RF power deposition.

Batchelor, Donald B [ORNL] [ORNL; D'Azevedo, Eduardo [ORNL] [ORNL; Bateman, Glenn [ORNL] [ORNL; Bernholdt, David E [ORNL] [ORNL; Bonoli, P. [Massachusetts Institute of Technology (MIT)] [Massachusetts Institute of Technology (MIT); Bramley, Randall B [ORNL] [ORNL; Breslau, Joshua [ORNL] [ORNL; Elwasif, Wael R [ORNL] [ORNL; Foley, S. [Indiana University] [Indiana University; Jaeger, Erwin Frederick [ORNL] [ORNL; Jardin, S. C. [Princeton Plasma Physics Laboratory (PPPL)] [Princeton Plasma Physics Laboratory (PPPL); Klasky, Scott A [ORNL] [ORNL; Kruger, Scott E [ORNL] [ORNL; Ku, Long-Poe [ORNL] [ORNL; McCune, Douglas [Princeton Plasma Physics Laboratory (PPPL)] [Princeton Plasma Physics Laboratory (PPPL); Ramos, J. [Massachusetts Institute of Technology (MIT)] [Massachusetts Institute of Technology (MIT); Schissel, David P [ORNL] [ORNL; Schnack, Dalton D [ORNL] [ORNL

2009-01-01

202

NASA Astrophysics Data System (ADS)

The quantum Deutsch-Jozsa algorithm is implemented by using vibrational modes of a two-dimensional double well. The laser fields realizing the different gates (NOT, CNOT, and HADAMARD) on the two-qubit space are computed by the multitarget optimal control theory. The stability of the performance index is checked by coupling the system to an environment. Firstly, the two-dimensional subspace is coupled to a small number Nb of oscillators in order to simulate intramolecular vibrational energy redistribution. The complete (2+Nb)D problem is solved by the coupled harmonic adiabatic channel method which allows including coupled modes up to Nb=5. Secondly, the computational subspace is coupled to a continuous bath of oscillators in order to simulate a confined environment expected to be favorable to achieve molecular computing, for instance, molecules confined in matrices or in a fullerene. The spectral density of the bath is approximated by an Ohmic law with a cutoff for some hundreds of cm-1. The time scale of the bath dynamics (of the order of 10 fs) is then smaller than the relaxation time and the controlled dynamics (2 ps) so that Markovian dissipative dynamics is used.

Ndong, M.; Lauvergnat, D.; Chapuisat, X.; Desouter-Lecomte, M.

2007-06-01

203

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

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. [Sandia National Laboratories, Albuquerque, New Mexico 87185-1186 (United States)

2006-10-15

204

We report an ab-initio simulation study of the ultrafast broad bandwidth ultraviolet (UV) stimulated resonance Raman spectra (SRRS) of L-tyrosine, L-tryptophan and trans-L-tryptophan-L-tyrosine (WY) dipeptide. Two-pulse one-dimensional (1D) SRRS and three-pulse 2D SRRS that reveal inter- and intra-residue vibrational coorelations are simulated using electronically resonant or preresonant pulse configurations that select the Raman signal and discriminate against excited state pathways. Multimode effects are incorporated via the cumulant expansion. The 2D SRRS technique is more sensitive to residue couplings than spontaneous Raman. PMID:23585708

Ren, Hao; Biggs, Jason D; Mukamel, Shaul

2013-04-01

205

We report an ab-initio simulation study of the ultrafast broad bandwidth ultraviolet (UV) stimulated resonance Raman spectra (SRRS) of L-tyrosine, L-tryptophan and trans-L-tryptophan-L-tyrosine (WY) dipeptide. Two-pulse one-dimensional (1D) SRRS and three-pulse 2D SRRS that reveal inter- and intra-residue vibrational coorelations are simulated using electronically resonant or preresonant pulse configurations that select the Raman signal and discriminate against excited state pathways. Multimode effects are incorporated via the cumulant expansion. The 2D SRRS technique is more sensitive to residue couplings than spontaneous Raman.

Ren, Hao; Biggs, Jason D.; Mukamel, Shaul

2013-01-01

206

Double prisms for two-dimensional optical satellite relative-trajectory simulator

This paper presents the development of an ultra precision optical satellite relative-trajectory simulator. It is commonly known that by combining two prisms of equal apex angle in near contact and by independently rotating them about an axis parallel to the normals of their adjacent faces, a ray can be steered in any direction within a limited cone. Due to the

Jianfeng Sun; Liren Liu; Maojin Yun; Lijuan Wang; Nan Xu

2004-01-01

207

Double prisms for two-dimensional optical satellite relative-trajectory simulator

NASA Astrophysics Data System (ADS)

This paper presents the development of an ultra precision optical satellite relative-trajectory simulator. It is commonly known that by combining two prisms of equal apex angle in near contact and by independently rotating them about an axis parallel to the normals of their adjacent faces, a ray can be steered in any direction within a limited cone. Due to the request of high precision, we use the table-looking method rather than the conventional approximately formula to transform the angle between the azimuth and elevation angle of the relative-trajectory and the angle of the servo-motor. To achieve a stable and accurate control of the system, a Proportional-Integral-derivative (PID) controller is used and the controller was optimized use the Genetic Algorithm. Furthermore, we simulated the system with Matlab program under different bandwidth. The results demonstrate that the proposed position control system achieves a good control performance. The simulation indicates that PID controller performed well on the satellite relative-trajectory simulation.

Sun, Jianfeng; Liu, Liren; Yun, Maojin; Wang, Lijuan; Xu, Nan

2004-10-01

208

National Technical Information Service (NTIS)

Weakly ionized processing plasmas are studied in two-dimensions using a bounded particle-in-cell (PIC) simulation code with a Monte Carlo Collision (MCC) package. The MCC package models the collisions between charged and neutral particles, which are neede...

V. Vahedi C. K. Birdsall M. A. Lieberman G. DiPeso T. D. Rognlien

1992-01-01

209

A systematic approach to radome design is presented. The problem is formulated as a global optimization procedure such that the radome performance is optimized by properly adjusting the thickness of the radome layer over the entire radome surface. In this approach the thickness profile is parameterized via B-splines representation. Simulated annealing technique is applied to finding the best thickness profile

Fang Hsu; Po-Rong Chang; Kuan-Kin Chan

1993-01-01

210

Numerical simulation of two-dimensional steady-state and transient forced convective boiling

The study of forced convective boiling has been based on experimental efforts in the past due to the complexity of the flows and lack of analytical tools. With the advancement of the computer hardware and numerical methods, the mathematical simulation of the two-phase flow and heat transfer including the forced convective boiling has become reality. The present mathematical method is

Lai

1992-01-01

211

Two-dimensional simulation of total dose effects on NMOSFET with lateral parasitic transistor

The trapped charge density in the LOCOS bird's beak resulting from irradiating a conventional NMOSFET has been analysed using a 2D finite element simulation. This paper shows a maximum of trapped charge density in the bird's beak region. The resulting voltage shift of the lateral parasitic transistor in the bird's beak region induces a high leakage current, and prevents any

C. Brisset; V. Ferlet-Cavrois; O. Flament; O. Musseau; J. L. Leray; J. L. Pelloie; R. Escoffier; A. Michez; C. Cirba; G. Bordure

1996-01-01

212

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 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.

Radha, P.B.; Goncharov, V.N.; Collins, T.J.B.; Delettrez, J.A.; Elbaz, Y.; Glebov, V.Yu.; 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.P.J.; Stoeckl, C.

2005-02-18

213

Two-dimensional segmentation of small convective patterns in radiation hydrodynamics simulations

NASA Astrophysics Data System (ADS)

Context. Recent results from high-resolution solar granulation observations indicate the existence of a population of small granular cells that are smaller than 600 km in diameter. These small convective cells strongly contribute to the total area of granules and are located in the intergranular lanes, where they form clusters and chains. Aims: We study high-resolution radiation hydrodynamics simulations of the upper convection zone and photosphere to detect small granular cells, define their spatial alignment, and analyze their physical properties. Methods: We developed an automated image-segmentation algorithm specifically adapted to high-resolution simulations to identify granules. The resulting segmentation masks were applied to physical quantities, such as intensity and vertical velocity profiles, provided by the simulation. A new clustering algorithm was developed to study the alignment of small granular cells. Results: Small granules make a distinct contribution to the total area of granules and form clusters of chain-like alignments. The simulation profiles demonstrate a different nature for small granular cells because they exhibit on average lower intensities, lower horizontal velocities, and are located deeper inside of convective layers than regular granules. Their intensity distribution deviates from a normal distribution as known for larger granules, and follows a Weibull distribution.

Lemmerer, B.; Utz, D.; Hanslmeier, A.; Veronig, A.; Thonhofer, S.; Grimm-Strele, H.; Kariyappa, R.

2014-03-01

214

Resistive MHD simulation of edge-localized-modes for double-null discharges in the MAST device

NASA Astrophysics Data System (ADS)

Recent development of the nonlinear magneto hydrodynamic (MHD) code JOREK has enabled the alignment of its two-dimensional finite-element grid along poloidal flux surfaces for double-null Grad-Shafranov equilibria. In previous works with the JOREK code, only single X-point plasmas were studied. The fast-camera diagnostic on MAST, which gives a global view of the pedestal filamentation during an ELM crash, clearly shows filaments travelling far into the scrape-off layer, as far as the first wall. Simulation of such a filament dynamics in MAST double-null plasmas is presented here and compared with experimental observations. In addition to direct comparison with the fast-camera images, general aspects of filaments are studied, such as their radial speed and composition. A qualitative validation of simulations is carried out against other diagnostics, such as the Thomson-scattering profiles or the infra-red camera images. Simulations are found to reproduce experimental edge localized modes in a reasonable manner, with similar energy losses and divertor heat-flux profiles. However, the MHD model used for those simulations is a reduced MHD model, which is likely approaching the limit of its applicability for the MAST device. Also, the absence of diamagnetic drift terms in the present MHD model results in nonlinear simulations being dominated by the highest mode number, and thus coupling with lower mode numbers is not observed.

Pamela, S. J. P.; Huijsmans, G. T. A.; Kirk, A.; Chapman, I. T.; Harrison, J. R.; Scannell, R.; Thornton, A. J.; Becoulet, M.; Orain, F.; the MAST Team

2013-09-01

215

NASA Astrophysics Data System (ADS)

We present here a new analysis in constructing two-dimensional Bernstein-Greene-Kruskal (BGK) modes in a magnetized plasma with finite magnetic field strength. The original method of constructing these modes [Ng, Bhattacharjee, and Skiff, Phys. Plasmas 13, 055903 (2006)], which satisfy the exact electromagnetic Vlasov-Poisson-Ampere system, requires solving them iteratively. An interesting property of these modes is that they can have a strong magnetic component for large electron thermal velocity. Exact solutions are presented using a new method that solves more directly without this iterative step for any electron thermal velocity. We will also present preliminary results on simulating these modes using Particle-in-Cell (PIC) simulations, which is important in studying the stability of these modes.

Ng, C. S.; Soundararajan, S. J.

2010-11-01

216

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

217

NASA Astrophysics Data System (ADS)

Backscattering enhancement of electromagnetic wave scattering from a perfectly conducting two-dimensional random rough surface (three-dimensional scattering problem) is studied with Monte Carlo simulations. The magnetic-field integral equation formulation is used with the method of moments. The solution of the matrix equation is calculated exactly with an efficient method known as the sparse-matrix flat-surface iterative approach. Numerical examples are illustrated with 32,768 surface unknowns, surface areas between 256 and 1024 square wavelengths, rms heights of 0.5 and 1 wavelength, and as many as 1000 realizations. The bistatic scattering simulations show backscattering enhancement for both copolarized and cross-polarized components. Comparisons are made with controlled laboratory experimental data for which the random rough surfaces are fabricated with prescribed properties of a rms height of 1 wavelength and a correlation length equal to 2 wavelengths.

Pak, Kyung; Tsang, Leung; Chan, Chi H.; Johnson, Joel

1995-11-01

218

NASA Astrophysics Data System (ADS)

It has long been suggested that eruptive phenomena such as coronal mass ejections, prominence eruptions, and large flares might be caused by a loss of equilibrium in a coronal flux rope (Van Tend and Kuperus, 1978). Forbes et al. (1994) developed an analytical two-dimensional model in which eruptions occur due to a catastrophic loss of equilibrium and relaxation to a lower-energy state containing a thin current sheet. Magnetic reconnection then intervenes dynamically, leading to the release of magnetic energy and expulsion of a plasmoid. We have carried out high-Lundquist-number simulations to test the loss-of equilibrium mechanism, and demonstrated that it does indeed occur in the quasi-ideal limit. We have studied the subsequent dynamical evolution of the system in resistive and Hall MHD models for single as well as multiple arcades. The typical parallel electric fields are super-Dreicer, which makes it necessary to include collisionless effects via a generalized Ohm's law. It is shown that the nature of the local dissipation mechanism has a significant effect on the global geometry and dynamics of the magnetic configuration. The presence of Hall currents is shown to alter the length of the current sheet and the jets emerging from the reconnection site, directed towards the chromosphere. Furthermore, Hall MHD effects break certain symmetries of resistive MHD dynamics, and we explore their observational consequences.

Yang, H.; Bhattacharjee, A.; Forbes, T. G.

2008-12-01

219

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

220

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

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

1982-01-01

221

Three-phase, two-dimensional numerical simulation of the steam flood process

Steam-drive calculations using numerical simulation techniques constitute a difficult type of problem. The determination of the rate of steam condensation was considerable accuracy is the main difficulty in solving such problem. In this study, the implicit pressure-explicit saturation (IMPES) technique was used to solve the 3-phase fluid flow equations for compressible fluids. A method was developed and applied to determine

Abdalla

1970-01-01

222

Comprehensive comparisons of the numerically simulated results of plasma flow fields in a 100-kW-class 2-D magnetoplasmadynamic thruster with the available experimental data are conducted. The propellant is argon of 1.25 g\\/s, and the discharge current is varied from 8 to 12 kA. The physical model includes a nonequilibrium single level of ionization and a collisional radiative model for argon ion

Kenichi Kubota; Ikkoh Funaki; Yoshihiro Okuno

2009-01-01

223

Two dimensional modeling and simulation of mass transport in microfabricated preconcentrators.

The adsorption and desorption behavior of a planar microfabricated preconcentrator (PC) has been modeled and simulated using the computational fluid dynamics (CFD) package CFDRC-ACE+trade. By comparison with the results of a designed experiment, model parameters were determined. Assuming a first-order reaction for the adsorption of a light hydrocarbon chemical analyte onto the PC adsorbent and a unity-value sticking coefficient, a rate constant of 36,500 s{sup -1} was obtained. This compares favorably with the value of 25,300 s{sup -1} obtained by application of the Modified-Wheeler equation. The modeled rate constant depends on the concentration of adsorbent sites, estimated to be 6.94 ldr 10{sup -8} kmol/m{sup 2} for the Carboxen 1000 adsorbent used. Using the integral method, desorption was found to be first order with an Arrhenius temperature dependence and an activation energy of 30.1 kj/mol. Validation of this model is reported herein, including the use of Aris-Taylor dispersion to predict the influence of fluidics surrounding the PC. A maximum in desorption peak area with flow rate, predicted from a quadratic fit to the results of the designed experiment, was not observed in the 2-D simulation. Either approximations in the simulated model or the nonphysical nature of the quadratic fit are responsible. Despite the apparent simplicity of the model, the simulation is internally self consistent and capable of predicting performance of new device designs. To apply the method to other analytes and other adsorbent materials, only a limited number of comparisons to experiment are required to obtain the necessary rate constants.

Manginell, Ronald Paul; Robinson, Alex Lockwood; Sheriati, Maryam (ESI US R& D, Inc., Huntsville, AL); Radhakrishnan, Sekhar (ESI US R& D, Inc., Huntsville, AL)

2005-07-01

224

A two-dimensional flow model for the process simulation of complex shape composite laminates

A numerical flow-compaction model is developed and implemented in a finite element code to simulate the multiple physical phenomena involved during the autoclave processing of fibre-reinforced composite laminates. The model is based on the e?ective stress formulation coupled with a Darcian flow theory. A Galerkin approach is employed to discretize the weak form of the governing equations. The current formulation

Pascal Hubert; Reza Vaziri; Anoush Poursartip

1999-01-01

225

Numerical simulation of two-dimensional wave propagation in functionally graded materials

The propagation of stress waves in functionally graded materials (FGMs) is studied numerically by means of the composite wave-propagation algorithm. Two distinct models of FGMs are considered: (i) a multilayered metal–ceramic composite with averaged properties within layers; (ii) randomly embedded ceramic particles in a metal matrix with prescribed volume fraction. The numerical simulation demonstrates the applicability of that algorithm to the modelling

Arkadi Berezovski; Juri Engelbrecht; G. A Maugin

2003-01-01

226

Simulation of two dimensional electrophoresis and tandem mass spectrometry for teaching proteomics.

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. The two simulations are integrated together and are designed to teach the concept of proteome analysis of prokaryotic and eukaryotic organisms. 2DE-Tandem MS can be used as a freestanding simulation, or in conjunction with a wet lab, to introduce proteomics in the undergraduate classroom. 2DE Tandem MS is a free program available on Sourceforge at https://sourceforge.net/projects/jbf/. It was developed using Java Swing and functions in Mac OSX, Windows, and Linux, ensuring that every student sees a consistent and informative graphical user interface no matter the computer platform they choose. Java must be installed on the host computer to run 2DE Tandem MS. Example classroom exercises are provided in the Supporting Information. PMID:23166029

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

2012-01-01

227

Two-Dimensional Distribution of Volatiles in the Lunar Regolith from Space Weathering Simulations

NASA Technical Reports Server (NTRS)

We present simulations of space weathering effects on ice deposits in regions of permanent shadow on the Moon. These Monte Carlo simulations follow the effects of space weathering processes on the distribution of the volatiles over time. The model output constrains the coherence of volatile deposits with depth, lateral separation, and time. The results suggest that ice sheets become broken and buried with time. As impacts begin to puncture an initially coherent surficial ice sheet, small areas with a deficit of ice compared to surrounding areas are formed first. As time progresses, holes become prevalent and the anomalous regions are local enhancements of ice concentration in a volume. The 3-D distribution is also heterogeneous because the ice is buried to varying depths in different locations. Analysis of the coherence of ice on 10 cm scales predicts that putative ice sheets in anomalous radar craters are < 100 Myr old. Surface frost becomes homogenized within 20 Myr. The simulations show the data from the LCROSS impact and surrounding region are consistent with the ice deposit in Cabeus being >1000 Myr old. For future in situ analysis of cold trap volatiles, a horizontal range of 10 m is sufficient to acquire surface-based measurements of heterogeneously distributed ice. These results also support previous analyses that Mercury's cold traps are young.

Hurley, Dana M.; Lawrence, David J.; Bussey, D. Benjamin J.; Vondrak, Richard R.; Elphic, Richard C.; Gladstone, G. Randall

2012-01-01

228

The role of condensation and heat conduction in the formation of prominences - An MHD simulation

NASA Technical Reports Server (NTRS)

The effects of condensation and thermal conduction on the formation of Kippenhahn-Schlueter (K-S) type prominences in quiet regions (QP) due to symmetric mass injection are investigated. To implement this investigation a self-consistent, two-dimensional, nonplanar, time-dependent MHD simulation model is developed. In the model, various values of the injection velocity, density, and magnetic field strength are used to determine the most favorable conditions for the QP formation. Based on these simulation results, it is found that the formation of a K-S type field configuration should be considered as a dynamic process which needs both condensation amd mass injection to supply enough mass to maintain such a configuration to complete the quiescent prominence formation process.

Wu, S. T.; Bao, J. J.; An, C. H.; Tandberg-Hanssen, E.

1990-01-01

229

Mesh refinement in a two-dimensional large eddy simulation of a forced shear layer

NASA Technical Reports Server (NTRS)

A series of large eddy simulations are made of a forced shear layer and compared with experimental data. Several mesh densities were examined to separate the effect of numerical inaccuracy from modeling deficiencies. The turbulence model that was used to represent small scale, 3-D motions correctly predicted some gross features of the flow field, but appears to be structurally incorrect. The main effect of mesh refinement was to act as a filter on the scale of vortices that developed from the inflow boundary conditions.

Claus, R. W.; Huang, P. G.; Macinnes, J. M.

1989-01-01

230

Two-dimensional streamflow simulations of the Jordan River, Midvale and West Jordan, Utah

The Jordan River in Midvale and West Jordan, Utah, flows adjacent to two U.S. Environmental Protection Agency Superfund sites: Midvale Slag and Sharon Steel. At both sites, geotechnical caps extend to the east bank of the river. The final remediation tasks for these sites included the replacement of a historic sheet-pile dam and the stabilization of the river banks adjacent to the Superfund sites. To assist with these tasks, two hydraulic modeling codes contained in the U.S. Geological Survey (USGS) Multi-Dimensional Surface-Water Modeling System (MD_SWMS), System for Transport and River Modeling (SToRM) and Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH), were used to provide predicted water-surface elevations, velocities, and boundary shear-stress values throughout the study reach of the Jordan River. A SToRM model of a 0.7 mile subreach containing the sheet-pile dam was used to compare water-surface elevations and velocities associated with the sheet-pile dam and a proposed replacement structure. Maps showing water-surface elevation and velocity differences computed from simulations of the historic sheet-pile dam and the proposed replacement structure topographies for streamflows of 500 and 1,000 cubic feet per second (ft3/s) were created. These difference maps indicated that the velocities associated with the proposed replacement structure topographies were less than or equal to those associated with the historic sheet-pile dam. Similarly, water-surface elevations associated with the proposed replacement structure topographies were all either greater than or equal to water-surface elevations associated with the sheet-pile dam. A FaSTMECH model was developed for the 2.5-mile study reach to aid engineers in bank stabilization designs. Predicted water-surface elevations, velocities and shear-stress values were mapped on an aerial photograph of the study reach to place these parameters in a spatial context. Profile plots of predicted cross-stream average water-surface elevations and cross-stream maximum and average velocities showed how these parameters change along the study reach for two simulated discharges of 1,040 ft3/s and 2,790 ft3/s. The profile plots for the simulated streamflow of 1,040 ft3/s show that the highest velocities are associated with the constructed sheet-pile replacement structure. Results for the simulated streamflow of 2,790 ft3/s indicate that the geometry of the 7800 South Bridge causes more backwater and higher velocities than the constructed sheet-pile replacement structure.

Kenney, Terry A.; Freeman, Michael L.

2011-01-01

231

A FORTRAN-77 computer program code that helps solve a variety of aquifer management problems involving the control of groundwater hydraulics. It is intended for use with any standard mathematical programming package that uses Mathematical Programming System input format. The computer program creates the input files to be used by the optimization program. These files contain all the hydrologic information and management objectives needed to solve the management problem. Used in conjunction with a mathematical programming code, the computer program identifies the pumping or recharge strategy that achieves a user 's management objective while maintaining groundwater hydraulic conditions within desired limits. The objective may be linear or quadratic, and may involve the minimization of pumping and recharge rates or of variable pumping costs. The problem may contain constraints on groundwater heads, gradients, and velocities for a complex, transient hydrologic system. Linear superposition of solutions to the transient, two-dimensional groundwater flow equation is used by the computer program in conjunction with the response matrix optimization method. A unit stress is applied at each decision well and transient responses at all control locations are computed using a modified version of the U.S. Geological Survey two dimensional aquifer simulation model. The program also computes discounted cost coefficients for the objective function and accounts for transient aquifer conditions. (Author 's abstract)

Lefkoff, L. J.; Gorelick, S. M.

1987-01-01

232

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 [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. 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 (l<10) modes due to surface roughness and beam imbalance and the intermediate modes (20{<=}l{<=}50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) 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 1D rates. DRACO simulation results are consistent with experimental observations.

Radha, P.B.; Goncharov, V.N.; Collins, T.J.B.; Delettrez, J.A.; Elbaz, Y.; Glebov, V.Yu.; 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.P.J.; Stoeckl, C. [Laboratory for Laser Energetics, University of Rochester, 250 E. River Road, Rochester, New York 14623 (United States)

2005-03-01

233

An accurate computational method for the classical simulation of the two-dimensional vibrational spectra is presented. The method refines our previous computational method for the third order vibrational response function in the classical limit, and it enables capturing the diagonal elongation and its waiting time (T) dependence widely observed in experimental two-dimensional infrared (2D IR) spectra of intramolecular modes. The improvement is achieved by a series of new developments including (i) a block algorithm for the stability matrix computation, (ii) new equations of motion for the position-perturbed molecular dynamics (MD) trajectory, and (iii) enhanced sampling efficiency by exploiting the time-reversal invariance of MD trajectories. The method is applied to the simulation of 2D IR spectra of the OD stretch mode in a hydrated HOD molecule, employing a hybrid quantum mechanical/molecular mechanical force field. The simulated spectra exhibit diagonal elongation of the 2D IR signal at small T, reflecting the correlation of individual transitions among the inhomogeneously broadened ensemble. The slopes of the nodal lines of the elongated signals are found to decay with a time scale of 1.6 ps as T increases, in reasonable agreement with the frequency correlation decay time of 1.2 ps. The amplitudes of the positive and negative peaks also decay as T increases, due to vibrational population relaxation and molecular rotation. The peak positions tend to blue shift with increasing T, reflecting the different relaxation rates of the strongly and weakly solvated HOD species. These results indicate that the present method can reliably predict the waiting-time-dependent changes of 2D IR spectra of a single vibrational chromophore in solution. PMID:24601590

Jeon, Jonggu; Cho, Minhaeng

2014-07-17

234

Lattice kinetic simulations of 3-D MHD turbulence

A recently proposed lattice Boltzmann kinetic scheme offers a promising tool for simulating complex 3-D MHD flows. The algorithm is based on the BGK modeling of the collision term. The conventional approach for implementing magnetic behavior in LBM methods is based on one tensor-valued distribution function to present both the fluids variables (density and momentum) and the magnetic field. This

G. Breyiannis; D. Valougeorgis

2006-01-01

235

Turbulence prediction in two-dimensional bundle flows using large-eddy simulation

Turbulent flow is characterized by random fluctuations in the fluid velocity and by intense mixing of the fluid. The mixing action is very important in engineering applications. Because of velocity fluctuations, a wide range of eddies exists in the flow field. Because these eddies carry mass, momentum, and energy, this enhanced mixing can lead to serious problems such as tube vibrations in flow bundles. Many engineering systems include fluid-tube bundle combinations. Nuclear fuel bundles, condensers, and heat exchangers are existing examples in nuclear power plants. Problems are often discovered during the operation of such systems because some of the fluid characteristics in tube bundles are not fully understood. For example, fluid-induced vibration has caused tube wear and leakage in many pressurized water reactor steam generators. The purpose of this study is to simulate the flow across tube bundles with various pitch-to-diameter ratios using an accurate numerical technique.

Ibrahim, W.A.; Hassan, Y.A.

1994-12-31

236

Two-Dimensional Simulations of X-Ray Absorption Spectra from Nonuniformly Driven Planar Targets

NASA Astrophysics Data System (ADS)

The effects of the ablative Rayleigh-Taylor instability on simulated absorption spectra in planar targets are examined. Spectra from embedded layers have been used as diagnostics of thermal transport, shock preheat, and other effects. Obtaining consistent temperature values from conventional analysis requires that the signature material remain within bounds that do not exceed a narrow range of conditions. We consider modifications of the spectrum due to, for example, dilution and bias by higher transmission through thin spots in the absorbing layer. We also examine the applicability of one-dimensional turbulent mix modeling to short-scale, multidimensional structure. In this work, the spectral analysis post-processor SPECT3D(Prism Computational Sciences, Inc.) is used in conjunction with the one-dimensional hydrocode LILAC and the multidimensional hydrocode DRACO. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

Epstein, R.; Ling, B.; Delettrez, J. A.; Radha, P. B.; Boehly, T. R.; Yaakobi, B.; Macfarlane, J. J.

2000-10-01

237

FRANC2D: A two-dimensional crack propagation simulator. Version 2.7: User's guide

NASA Technical Reports Server (NTRS)

FRANC 2D (FRacture ANalysis Code, 2 Dimensions) is a menu driven, interactive finite element computer code that performs fracture mechanics analyses of 2-D structures. The code has an automatic mesh generator for triangular and quadrilateral elements. FRANC2D calculates the stress intensity factor using linear elastic fracture mechanics and evaluates crack extension using several methods that may be selected by the user. The code features a mesh refinement and adaptive mesh generation capability that is automatically developed according to the predicted crack extension direction and length. The code also has unique features that permit the analysis of layered structure with load transfer through simulated mechanical fasteners or bonded joints. The code was written for UNIX workstations with X-windows graphics and may be executed on the following computers: DEC DecStation 3000 and 5000 series, IBM RS/6000 series, Hewlitt-Packard 9000/700 series, SUN Sparc stations, and most Silicon Graphics models.

Wawrzynek, Paul; Ingraffea, Anthony

1994-01-01

238

Parametric decay of oblique Alfvén waves in two-dimensional hybrid simulations.

Certain types of plasma waves are known to become parametrically unstable under specific plasma conditions, in which the pump wave will decay into several daughter waves with different wavenumbers and frequencies. In the past, the related plasma instabilities have been treated analytically for various parameter regimes and by use of various numerical methods, yet the oblique propagation with respect to the background magnetic field has rarely been dealt with in two dimensions, mainly because of the high computational demand. Here we present a hybrid-simulation study of the parametric decay of a moderately oblique Alfvén wave having elliptical polarization. It is found that such a compressive wave can decay into waves with higher and lower wavenumbers than the pump. PMID:23005891

Verscharen, D; Marsch, E; Motschmann, U; Müller, J

2012-08-01

239

Two-dimensional hybrid simulations of Alfvénic fluctuations in the expanding solar wind

NASA Astrophysics Data System (ADS)

The supersonic expansion of the solar wind and wave particle interactions which characterize the ion evolution are investigated using a hybrid expanding box model. We present 2D simulations of the interaction between Alfvénic like fluctuations using two different geometries: an in-plane mean magnetic field and an out-of-plane mean magnetic field. The initial conditions for the Alfvénic spectrum are a) a mixed parallel and oblique propagating modes (in-plane) and b) purely perpendicular wavevectors (out-of-plane). For both geometries we consider 2 kinds of initial conditions. 1) the imbalanced case with Alfvén waves propagating in one direction along the guide field, that is an initial condition with maximal correlation between u and b (maximal cross helicity and null residual energy) 2) the balanced case with counter-propagating Alfvén waves and with an initial magnetic energy dominating over kinetic energy (null cross helicity and maximal residual energy). Such characteristics are the most frequently observed in the solar wind. We investigate parallel and perpendicular proton heating properties of the turbulent spectra in the simulations and the influence of the expansion on the evolution of turbulence and its decay. As suggested by solar wind observations, the perpendicular heating and parallel cooling is not strong enough to overcome the expansion-driven anisotropic cooling. Once the expansion drives the system unstable with respect to the fire-hose instability driven by the proton temperature anisotropy, electromagnetic fluctuations are generated. These waves scatter protons and reduce their temperature anisotropy. Consequently, this mechanism constrains the temperature anisotropy and the system evolves along a marginal instability path.

Landi, Simone; Matteini, Lorenzo; Hellinger, Petr; Verdini, Andrea; Travnicek, Pavel M.; Burgess, David

2014-05-01

240

Two-Dimensional Optical Measurement of Waves on Liquid Lithium Jet Simulating IFMIF Target Flow

Waves on a liquid-lithium jet flow, simulating a proposed high-energy beam target design, have been measured using an optical technique based on specular reflection of a single laser beam on the jet surface. The stream-wise and spanwise fluctuations of the local free-surface slope were least-square fitted with a sinusoidal curve to makeup the signals lost due to the constriction in the optical arrangement. The waveform was estimated with an assumption that wave phase speed can be calculated using the dispersion relation for linear capillary-gravity waves. The direction of propagation on the jet surface was also evaluated so that the wave amplitudes, calculated by integral of slope angle signal, agree consistently in stream-wise and spanwise direction. These measurements and analyses show that the waves at the measurement location for a jet velocity of 1.2 m/s can best be represented by oblique waves with an inclination of 1.23 rad, a wavelength of 3.8 mm and a wave amplitude of about 0.05 mm. (authors)

Kazuhiro Itoh; Hiroyuki Koterazawa [University of Hyogo, 1-3-3, Higashikawasaki-cho, Chuo-ku, Kobe-shi, Hyogo (Japan); Taro Itoh; Yutaka Kukita [Nagoya University, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi, 464-8603 (Japan); Hiroo Kondo; Nobuo Yamaoka; Hiroshi Horiike [Osaka University, 1-8 Yamadaoka, Suita, Osaka 565-0871 (Japan); Mizuho Ida; Hideo Nakamura; Hiroo Nakamura [Japan Atomic Energy Agency (Japan); Takeo Muroga [National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, GIFU, 509-5292 (Japan)

2006-07-01

241

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

242

Numerical simulations of two-dimensional saturated granular media. Doctoral thesis

The liquefaction phenomenon in soil has been studied in great detail during the past 20 years. The need to understand this phenomenon has been emphasized by the extent of the damages resulting from soil liquefaction during earthquakes. Although an overall explanation exists for the phenomenon through the concept of effective stress, the basic mechanism of loss of strength of the soil skeleton has not been thoroughly examined and remains unclear. The present study proposes a numerical model for simulations of the behavior of saturated granular media. The model was developed with two main objectives: To represent the mechanical response of an assemblage of discrete particles having the shape of discs; To model and represent the interaction of interstitial pore fluid present with the idealized granular media. The representation of the solid skeleton is based on Cundall and Strack's distinct element model, in which discrete particles are modelled as discs in two dimensions, each obeying Newton's laws. Several validation test cases are presented along with four simple shear tests on dry and saturated granular assemblages. For these last four tests, the numerical results indicate that the model is able to represent qualitatively the behavior of real soil, while at the same time clarifying the processes occurring at the microscale that influence soil response.

Tan, P.

1990-01-01

243

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

NASA Technical Reports Server (NTRS)

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.

Herant, Marc; Benz, Willy; Colgate, Stirling

1992-01-01

244

Can kinetic plasma simulation and MHD talk to each other?

NASA Astrophysics Data System (ADS)

Kinetic simulations show faster tearing mode growth and higher amplitude saturation with electron temperature anisotropy. Implicit simulations in 3D show the lower-hybrid drift instability (LHDI) generates anisotropy and that spontaneous, small-scale tearing evolves into rapid, large-scale reconnection. These results for idealized problems on very small scales need to be tested on larger systems with more realistic boundary conditions, for which we need new methods. We extend implicit simulation to magnetohydrodynamic (MHD) scales by adding a simple but self-consistent collision model to Celeste. An input parameter switches Celeste from a kinetic simulation to Hall-MHD, and can be given different values in different regions so that MHD and kinetic regions interact, flux conservation conditions are rigorously satisfied, and the two plasma populations mix on ion time scales. In 2D, we simulate the LHDI with uniform collisionality, and it grows and saturates normally at low collision rates, but with reduced temperature anisotropy. We model a finite-width current sheet in the direction of current flow, as in the magnetotail, using adjacent collisional and collision-less regions. The resistance to current flow in the collisional region induces an out-of-plane return current flow in the collision-less region. To study embedding a kinetic region in a larger MHD domain, we introduce a collisional region along the magnetic field direction, which can cause localized reconnection unless done carefully. We characterize the transparency of the boundary between kinetic and MHD regions to the propagation of waves and plasma flow, and evaluate the adequacy of our simple collision model.

Brackbill, J. U.; Lapenta, G.

2007-12-01

245

A hybrid model for simulating diffused first reflections in two-dimensional acoustic environments

NASA Astrophysics Data System (ADS)

Although it is widely accepted that the diffusion of early reflections in acoustic spaces intended for music performance greatly improves the perceived quality of sound, current manufacturers of synthetic reverberation engines continue to model reflecting surfaces as having almost perfectly specular characteristics. This dissertation describes a hybrid method of simulating diffusion based on both physical and phenomenological modeling components. In 1979, Manfred Schroeder described a method of designing and constructing diffusing surfaces based on a rather simple mathematical algorithm which provides diffused reflections in predictable frequency bands. This structural device, now known as a "Schroeder diffuser," has become a standard geometry used in constructing diffusive surfaces for spaces intended for music rehearsal, recording and performance. While it is possible to use DSP to model the characteristics of reflections off such a surface, a reflection model based exclusively on a surface constructed of a Schroeder diffuser has proven in informal tests to be as aesthetically inadequate as a perfectly specular model. Control of both the spatial and temporal envelopes of the diffusive reflection are required by an end user in order to tailor the reflection characteristics to the desired impression. In 1974 an empirical model for computing light reflections off objects in a three-dimensional environment was developed by Phong Bui-Toung. This algorithm incorporated both a specular and diffuse component with relationships controlled by an end user. This dissertation describes the adaptation and implementation of the Phong shading algorithm in conjunction with a physical model of components of the Schroeder diffuser for the modeling of diffuse reflections in synthetic acoustic environments. The inclusion of the Phong algorithm provides precise control over the balance between the spectral and diffusive components of the reflection. In addition, directivity functions for sound sources and receivers in the virtual space are described. Analysis and evaluation of the model using mathematical and empirical methodologies are discussed and stereo and multichannel audio examples produced by the system are included.

Martin, Geoffrey Glen

2001-07-01

246

Two-Dimensional Model Simulations of Interannual Variability in the Tropical Stratosphere

NASA Technical Reports Server (NTRS)

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 GSFC 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 03 data is 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 tipper 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 tile 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.; Considine, David B.; Rosenfeld, Joan; Bhartia, P. K. (Technical Monitor)

2001-01-01

247

Non-standard FDTD for elastic wave simulation: two-dimensional P-SV case

NASA Astrophysics Data System (ADS)

A scheme for the non-standard finite-difference method in the time-domain (NS-FDTD), 2-Box scheme, is proposed for elastic wave simulations in two dimensions (P-SV). The method improves the accuracy and efficiently reduces grid dispersion and anisotropy. The proposed non-standard scheme is based on two main operations. The first operation replaces spatial grid spacing and time step by their frequency optimized counterparts, called the correction functions, and the second operation introduces an optimum grid stencil for the finite-difference operator of the 2-D Laplacian. The optimal stencil is obtained by introducing two optimization parameters estimated for a design frequency. Error analysis of the proposed scheme (2-Box scheme) shows that specifying the maximum frequency as the design frequency leads to a significant reduction of the grid dispersion over a wide frequency band. We derive the formulations of grid dispersion and stability condition for the scheme. The grid dispersion is investigated, and it is shown that the proposed scheme reduces not only the grid dispersion but also the grid anisotropy significantly. The grid dispersion is insensitive to the Poisson's ratio and size of the time step within the stability limit. Since the wide spatial stencil of the 2-Box scheme might become difficult to implement at the computational domain boundaries, two additional non-standard schemes-1-Box and 0-Box schemes-are also introduced. The 1-Box scheme uses narrower stencil than the 2-Box scheme, and the 0-Box scheme uses the same stencil as the standard FDTD. Numerical experiments of elastic wave propagation demonstrate the significant superiority of the proposed non-standard schemes over the commonly used standard one. With six grid spacings per minimum wavelength, the 2-Box and 1-Box schemes represent excellent results and the 0-Box scheme has higher accuracy than the standard scheme with seven grid spacings per minimum wavelength.

JafarGandomi, Arash; Takenaka, Hiroshi

2009-07-01

248

We analyzed, based on the theoretical spectroscopic modeling, how the differences in the folding landscapes of two ?-hairpin peptides trpzip2 and trpzip4 are reflected in their thermal unfolding infrared measurements. The isotope-edited equilibrium FTIR and two dimensional infrared spectra of the two peptides were calculated, using the nonlinear exciton propagation method, at a series of temperatures. The spectra calculations were based on the configuration distributions generated using the GB(OBC) implicit solvent MD simulation and the integrated tempering sampling technique. Conformational analysis revealed the different local thermal stabilities for these two peptides, which suggested the different folding landscapes. Our study further suggested that the ellipticities of the isotope peaks in the coherent IR signals are more sensitive to these local stability differences compared with other spectral features such as the peak intensities. Our technique can thus be combined with the relevant experimental measurements to achieve a better understanding of the peptide folding behaviors. PMID:24511982

Wu, Tianmin; Zhang, Ruiting; Li, Huanhuan; Yang, Lijiang; Zhuang, Wei

2014-02-01

249

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

2012-10-16

250

NASA Astrophysics Data System (ADS)

A two-dimensional multiple-histogram method for isothermal-isobaric ensemble is discussed in detail, implemented for isothermal-isobaric Monte Carlo simulations of molecular clusters, and employed in a case study on phase changes in pure water clusters containing 15 through 21 water molecules. Full phase diagrams of these clusters are reported in the temperature-pressure plane over a broad range of temperatures (T=30-800 K) and pressures P=103-109 Pa. The main focus of the work is on the structural transformation occurring in the solid phase of these clusters and leading from cluster structures with all molecules on the cluster surface to cage-like structures with one molecule inside, and on how the transformation is influenced by increased pressure and temperature.

Vítek, Aleš; Kalus, René

2014-06-01

251

NASA Astrophysics Data System (ADS)

We analyzed, based on the theoretical spectroscopic modeling, how the differences in the folding landscapes of two ?-hairpin peptides trpzip2 and trpzip4 are reflected in their thermal unfolding infrared measurements. The isotope-edited equilibrium FTIR and two dimensional infrared spectra of the two peptides were calculated, using the nonlinear exciton propagation method, at a series of temperatures. The spectra calculations were based on the configuration distributions generated using the GBOBC implicit solvent MD simulation and the integrated tempering sampling technique. Conformational analysis revealed the different local thermal stabilities for these two peptides, which suggested the different folding landscapes. Our study further suggested that the ellipticities of the isotope peaks in the coherent IR signals are more sensitive to these local stability differences compared with other spectral features such as the peak intensities. Our technique can thus be combined with the relevant experimental measurements to achieve a better understanding of the peptide folding behaviors.

Wu, Tianmin; Zhang, Ruiting; Li, Huanhuan; Yang, Lijiang; Zhuang, Wei

2014-02-01

252

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

253

NASA Astrophysics Data System (ADS)

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a two-dimensional (2D) hybrid model in an initial Harris sheet equilibrium with flows. The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation. It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and, therefore, modify the growth rate of magnetic reconnection. Furthermore, the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes. Secondary islands are also generated in the process with converting the initial X-point into an O-point.

Wang, Lin; Wang, Xian-Qu; Wang, Xiao-Gang; Liu, Yue

2014-02-01

254

CENTRAL REGIONS OF BARRED GALAXIES: TWO-DIMENSIONAL NON-SELF-GRAVITATING HYDRODYNAMIC SIMULATIONS

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructures may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, a bulge, a central BH, and a bar. To investigate various galactic environments, we vary the gas sound speed, c{sub s} , as well as the mass of the central BH, M{sub BH}. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as c{sub s} increases. Nuclear rings shrink in size with increasing c{sub s} , but are independent of M{sub BH}, suggesting that the ring position is not determined by the Lindblad resonances. Rings in low-c{sub s} models are narrow since they are occupied largely by gas on x{sub 2}-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-c{sub s} models. Nuclear spirals persist only when either c{sub s} is small or M{sub BH} is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on c{sub s} and M{sub BH} such that they are leading if both c{sub s} and M{sub BH} are small, weak trailing if c{sub s} is small and M{sub BH} is large, and strong trailing if both c{sub s} and M{sub BH} are large. While the mass inflow rate toward the nucleus is quite small in low-c{sub s} models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 M{sub Sun} yr{sup -1} when c{sub s} is large, providing a potential explanation of nuclear activity in Seyfert galaxies.

Kim, Woong-Tae; Seo, Woo-Young [Center for the Exploration of the Origin of the Universe (CEOU), Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-742 (Korea, Republic of); Stone, James M. [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States); Yoon, Doosoo [FPRD, Department of Physics and Astronomy, Seoul National University, Seoul 151-742 (Korea, Republic of); Teuben, Peter J., E-mail: wkim@astro.snu.ac.kr [Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)

2012-03-01

255

NASA Astrophysics Data System (ADS)

Aquatic habitat models utilize flow variables which may be predicted with one-dimensional (1D) or two-dimensional (2D) hydrodynamic models to simulate aquatic habitat quality. Studies focusing on the effects of hydrodynamic model dimensionality on predicted aquatic habitat quality are limited. Here we present the analysis of the impact of flow variables predicted with 1D and 2D hydrodynamic models on simulated spatial distribution of habitat quality and Weighted Usable Area (WUA) for fall-spawning Chinook salmon. Our study focuses on three river systems located in central Idaho (USA), which are a straight and pool-riffle reach (South Fork Boise River), small pool-riffle sinuous streams in a large meadow (Bear Valley Creek) and a steep-confined plane-bed stream with occasional deep forced pools (Deadwood River). We consider low and high flows in simple and complex morphologic reaches. Results show that 1D and 2D modeling approaches have effects on both the spatial distribution of the habitat and WUA for both discharge scenarios, but we did not find noticeable differences between complex and simple reaches. In general, the differences in WUA were small, but depended on stream type. Nevertheless, spatially distributed habitat quality difference is considerable in all streams. The steep-confined plane bed stream had larger differences between aquatic habitat quality defined with 1D and 2D flow models compared to results for streams with well defined macro-topographies, such as pool-riffle bed forms. KEY WORDS: one- and two-dimensional hydrodynamic models, habitat modeling, weighted usable area (WUA), hydraulic habitat suitability, high and low discharges, simple and complex reaches

Benjankar, R. M.; Sohrabi, M.; Tonina, D.; McKean, J. A.

2013-12-01

256

NASA Astrophysics Data System (ADS)

Number concentrations, size distributions, and mixing states of aerosols are essential parameters for accurate estimation of aerosol direct and indirect effects. In this study, we develop an aerosol module, designated Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS), that can represent these parameters explicitly by considering new particle formation (NPF), black carbon (BC) aging, and secondary organic aerosol (SOA) processes. A two-dimensional bin representation is used for particles with dry diameters from 40 nm to 10 ?m to resolve both aerosol size (12 bins) and BC mixing state (10 bins) for a total of 120 bins. The particles with diameters from 1 to 40 nm are resolved using an additional 8 size bins to calculate NPF. The ATRAS module is implemented in the WRF-chem model and applied to examine the sensitivity of simulated mass, number, size distributions, and optical and radiative parameters of aerosols to NPF, BC aging and SOA processes over East Asia during the spring of 2009. BC absorption enhancement by coating materials is about 50% over East Asia during the spring, and the contribution of SOA processes to the absorption enhancement is estimated to be 10-20% over northern East Asia and 20-35% over southern East Asia. A clear north-south contrast is also found between the impacts of NPF and SOA processes on cloud condensation nuclei (CCN) concentrations: NPF increases CCN concentrations at higher supersaturations (smaller particles) over northern East Asia, whereas SOA increases CCN concentrations at lower supersaturations (larger particles) over southern East Asia. Application of ATRAS to East Asia also shows that the impact of each process on each optical and radiative parameter depends strongly on the process and the parameter in question. The module can be used in the future as a benchmark model to evaluate the accuracy of simpler aerosol models and examine interactions between NPF, BC aging, and SOA processes under different meteorological conditions and emissions.

Matsui, H.; Koike, M.; Kondo, Y.; Fast, J. D.; Takigawa, M.

2014-04-01

257

NASA Astrophysics Data System (ADS)

There are a number of modeling challenges posed by space weather simulations. Most of them arise from the multiscale and multiphysics aspects of the problem. The multiple scales dramatically increase the requirements, in terms of computational resources, because of the need of performing large scale simulations with the proper small-scales resolution. Lately, several suggestions have been made to overcome this difficulty by using various refinement methods which consist in splitting the domain into regions of different resolutions separated by well defined interfaces. The multiphysics issues are generally treated in a similar way: interfaces separate the regions where different equations are solved. This paper presents an innovative approach based on the coexistence of several levels of description, which differ by their resolutions or, potentially, by their physics. Instead of interacting through interfaces, these levels are entirely simulated and are interlocked over the complete extension of the overlap area. This scheme has been applied to a parallelized, two-dimensional, Implicit Moment Method Particle in Cell code in order to investigate its multiscale description capabilities. Simulations of magnetic reconnection and plasma expansion in vacuum are presented and possible implementation options for this scheme on very large systems are also discussed.

Beck, A.; Innocenti, M. E.; Lapenta, G.; Markidis, S.

2014-08-01

258

Hall MHD Simulations of Comet 67P/Churyumov-Gerasimenko

NASA Astrophysics Data System (ADS)

Comets have highly eccentric orbits and a wide range of gas production rates and thus they are ideal subjects to study the interaction between the solar wind and nonmagnetized bodies. Hansen et al. (2007, Space Sci. Rev. 128, 133) used a fluid-based MHD model and a semi-kinetic hybrid particle model to study the plasma environment of comet 67P/Churyumov-Gerasimenko (CG), the Rosetta mission target comet, at different heliocentric distances. They showed that for such a weak comet at a large heliocentric distance, the length scales of the cometosheath and the bow shock are comparable to or smaller than the ion gyroradius, which violates the underlying assumption for a valid fluid description of the plasma. As a result, the classical ideal MHD model is not able to always give physical results, while the hybrid model, which accounts for the kinetic effects of ions with both cometary and solar wind origin, is more reliable. However, hybrid models are computationally expensive and the results can be noisy. A compromise approach is Hall MHD [Toth et al., 2008], which includes the Hall term in the MHD equations and allows for the decoupling of the ion and electron fluids. We use a single ion species Hall MHD model to simulate the plasma environment of comet 67P/CG and compare the results with the two models mentioned above. We find that the Hall effect is capable of reproducing some features of the hybrid model and thus extends the applicability of MHD. In addition, this study helps to identify the conditions and regions in the cometary plasma where the Hall effect is not negligible. This work is supported by NSF Planetary Astronomy grant AST0707283 and JPL subcontract 1266313 under NASA grant NMO710889.

Shou, Y.; Combi, M. R.; Rubin, M.; Hansen, K. C.; Toth, G.; Gombosi, T. I.

2012-12-01

259

We develop a tricomponent (ternary) hydrodynamic model for multiphase flows of biomass and solvent mixtures, which we employ to simulate biofilm. In this model, the three predominant effective components in biofilms, which are the extracellular polymeric substance (EPS) network, the bacteria, and the effective solvent (consisting of the solvent and nutrient, etc.), are modeled explicitly. The tricomponent fluid mixture is assumed incompressible as a whole, while intercomponent mixing, dissipation, and conversion are allowed among the effective components. Bacterial growth and EPS production due to the growing bacterial population are modeled in the biomass transport equations. Bacterial decay due to starvation and natural causes is accounted for in the bacterial population dynamics to capture the possible bacterial population reduction due to the depletion of the nutrient. In the growth regime for biofilms, the mixture behaves like a multiphase viscous fluid, in which the molecular relaxation is negligible in the corresponding time scale. In this regime, the dynamics of biofilm growth in the solvent (water) are simulated using a two-dimensional finite difference solver that we developed, in which the distribution and evolution of the EPS and bacterial volume fractions are investigated. The hydrodynamic interaction between the biomass and the solvent flow field is also simulated in a shear cell environment, demonstrating the spatially and temporally heterogeneous distribution of the EPS and bacteria under shear. This model together with the numerical codes developed provides a predictive tool for studying biomass-flow interaction and other important biochemical interactions in the biofilm and solvent fluid mixture. PMID:22587124

Lindley, Brandon; Wang, Qi; Zhang, Tianyu

2012-03-01

260

NASA Astrophysics Data System (ADS)

We develop a tricomponent (ternary) hydrodynamic model for multiphase flows of biomass and solvent mixtures, which we employ to simulate biofilm. In this model, the three predominant effective components in biofilms, which are the extracellular polymeric substance (EPS) network, the bacteria, and the effective solvent (consisting of the solvent and nutrient, etc.), are modeled explicitly. The tricomponent fluid mixture is assumed incompressible as a whole, while intercomponent mixing, dissipation, and conversion are allowed among the effective components. Bacterial growth and EPS production due to the growing bacterial population are modeled in the biomass transport equations. Bacterial decay due to starvation and natural causes is accounted for in the bacterial population dynamics to capture the possible bacterial population reduction due to the depletion of the nutrient. In the growth regime for biofilms, the mixture behaves like a multiphase viscous fluid, in which the molecular relaxation is negligible in the corresponding time scale. In this regime, the dynamics of biofilm growth in the solvent (water) are simulated using a two-dimensional finite difference solver that we developed, in which the distribution and evolution of the EPS and bacterial volume fractions are investigated. The hydrodynamic interaction between the biomass and the solvent flow field is also simulated in a shear cell environment, demonstrating the spatially and temporally heterogeneous distribution of the EPS and bacteria under shear. This model together with the numerical codes developed provides a predictive tool for studying biomass-flow interaction and other important biochemical interactions in the biofilm and solvent fluid mixture.

Lindley, Brandon; Wang, Qi; Zhang, Tianyu

2012-03-01

261

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

262

NASA Astrophysics Data System (ADS)

With the rapid increase of GPS/GNSS receivers being deployed and operated in China, real-time GPS data from nearly a thousand sites are available at the National Center for Space Weather, China Meteorology Administration. However, it is challenging to generate a high-quality regional total electron content (TEC) map with the traditional two-dimensional (2-D) retrieval scheme because a large horizontal gradient has been reported over east-south Asia due to the northern equatorial ionization anomaly. We developed an Ionosphere Data Assimilation Analysis System (IDAAS), which is described in this study, using an International Reference Ionosphere (IRI) model as the background and applying a Kalman filter for updated observations. The IDAAS can reconstruct a three-dimensional ionosphere with the GPS slant TEC. The inverse slant TEC correlates well with observations both for GPS sites involved in the reconstruction and sites that are not involved. Based on the IDAAS, simulations were performed to investigate the deviation relative to the slant-to-vertical conversion (STV). The results indicate that the relative deviation induced by slant-to-vertical conversion may be significant in certain instances, and the deviation varies from 0% to 40% when the elevation decreases from 90° to 15°, while the relative IDAAS deviation is much smaller and varies from -5% to 15% without an elevation dependence. Compared with ‘true TEC’ map derived from the model, there is large difference in STV TEC map but no obvious discrepancy in IDAAS map. Generally, the IDAAS TEC map is much closer to the “true TEC” than is STV TEC map is. It is suggested that three-dimensional inversion technique is necessary for GPS observations of low elevation at an equatorial anomaly region, wherein the high horizontal electron density gradient may produce significant slant-to-vertical deviations using the two-dimensional STV inversion method.

Yu, Tao; Mao, Tian; Wang, Yungang; Zeng, Zhongcao; Xia, Chunliang; Wu, Fenglei; Wang, Le

2014-08-01

263

NASA Astrophysics Data System (ADS)

A finite element method was recently designed to model the mechanisms that cause superplastic deformation (A.F. Bower and E. Wininger, A Two-Dimensional Finite Element Method for Simulating the Constitutive Response and Microstructure of Polycrystals during High-Temperature Plastic Deformation, J. Mech. Phys. Solids, 2004, 52, p 1289-1317). The computations idealize the solid as a collection of two-dimensional grains, separated by sharp grain boundaries. The grains may deform plastically by thermally activated dislocation motion, which is modeled using a conventional crystal plasticity law. The solid may also deform by sliding on the grain boundaries, or by stress-driven diffusion of atoms along grain boundaries. The governing equations are solved using a finite element method, which includes a front-tracking procedure to monitor the evolution of the grain boundaries and surfaces in the solid. The goal of this article is to validate these computations by systematically comparing numerical predictions to experimental measurements of the elevated-temperature response of aluminum alloy AA5083 (M.-A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, and T.R. McNelley, Deformation Mechanisms in Superplastic AA5083 materials. Metall. Mater. Trans. A, 2005, 36(5), p 1249-1261). The experimental work revealed that a transition occurs from grain-boundary sliding to dislocation (solute-drag) creep at approximately 0.001/s for temperatures between 425 and 500 °C. In addition, increasing the grain size from 7 to 10 ?m decreased the transition to significantly lower strain rates. Predictions from the finite element method accurately predict the effect of grain size on the transition in deformation mechanisms.

Agarwal, Sumit; Briant, Clyde L.; Krajewski, Paul E.; Bower, Allan F.; Taleff, Eric M.

2007-04-01

264

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

265

Mathematical phantoms are essential for the development and early stage evaluation of image reconstruction algorithms in x-ray computed tomography (CT). This note offers tools for computer simulations using a two-dimensional (2D) phantom that models the central axial slice through the FORBILD head phantom. Introduced in 1999, in response to a need for a more robust test, the FORBILD head phantom is now seen by many as the gold standard. However, the simple Shepp-Logan phantom is still heavily used by researchers working on 2D image reconstruction. Universal acceptance of the FORBILD head phantom may have been prevented by its significantly higher complexity: software that allows computer simulations with the Shepp-Logan phantom is not readily applicable to the FORBILD head phantom. The tools offered here address this problem. They are designed for use with Matlab®, as well as open-source variants, such as FreeMat and Octave, which are all widely used in both academia and industry. To get started, the interested user can simply copy and paste the codes from this PDF document into Matlab® M-files. PMID:22713335

Yu, Zhicong; Noo, Frédéric; Dennerlein, Frank; Wunderlich, Adam; Lauritsch, Günter; Hornegger, Joachim

2012-07-01

266

NASA Astrophysics Data System (ADS)

Monte Carlo computer simulations with variable shape of the periodic box were performed for a few two-dimensional, hard-body models showing elastically isotropic solid phases. Two examples of homo-molecular (i.e. consisting of identical molecules) systems of anisotropic molecules are discussed which form unusual solid phases, showing negative Poisson's ratio and know as auxetic ones, at densities and pressures higher than the elastically isotropic solid phases. Sharp extremes of the Poisson's ratio were observed for both systems at freezing of the orientational degrees of freedom of the molecules: a minimum was observed for the system for which the auxetic phase with frozen rotation is isotropic and a maximum was found for the other system. This indicates that isotropic auxetic phases can have auxetic precursors with much more negative Poisson's ratios than those characterizing the auxetic phases themselves. An example of a hetero-molecular (i.e. mixture consisting of different molecules), elastically isotropic system is also discussed - the polydisperse hard disc system. The simulations revealed a maximum of the Poisson's ratio in the close packing limit at any nonzero polydispersity. Although no manifestation of any auxetic behaviour has been found in this system, the obtained results indicate other unusual effect - a jump (discontinuity) of the Poisson's ratio at close packing when the polydispersity tends to zero. The results obtained in this work demonstrate that the Poisson's ratio can play the role of a sensitive indicator of (at least) some structural changes in solids.

Tretiakov, K. V.; Wojciechowski, K. W.

2005-03-01

267

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

268

Mathematical phantoms are essential for the development and early-stage evaluation of image reconstruction algorithms in x-ray computed tomography (CT). This note offers tools for computer simulations using a two-dimensional (2D) phantom that models the central axial slice through the FORBILD head phantom. Introduced in 1999, in response to a need for a more robust test, the FORBILD head phantom is now seen by many as the gold standard. However, the simple Shepp-Logan phantom is still heavily used by researchers working on 2D image reconstruction. Universal acceptance of the FORBILD head phantom may have been prevented by its significantly-higher complexity: software that allows computer simulations with the Shepp-Logan phantom is not readily applicable to the FORBILD head phantom. The tools offered here address this problem. They are designed for use with Matlab®, as well as open-source variants, such as FreeMat and Octave, which are all widely used in both academia and industry. To get started, the interested user can simply copy and paste the codes from this PDF document into Matlab® M-files.

Yu, Zhicong; Noo, Frederic; Dennerlein, Frank; Wunderlich, Adam; Lauritsch, Gunter; Hornegger, Joachim

2012-01-01

269

NASA Technical Reports Server (NTRS)

Spatial and seasonal distribution of ozone and other trace gases are simulated using a interactive two-dimensional model of the stratosphere updated to include full gas phase chemistry. The model consists of a primitive equation dynamics module, a full radiative transfer scheme, and a comprehensive gas phase chemistry module. The circulation is derived from heating rates in the stratosphere that are calculated using model-generated ozone. In the troposphere, parameterized heating rates are adopted. The eddy momentum flux divergence in the zonal mean momentum equation is given by the eddy fluxes of potential vorticity. Eddy fluxes of potential vorticity and tracers are parameterized using a set of predetermined diffusion coefficients. The adopted values for K(sub yy), show a hemispherical asymmetry in that the values in the lower stratosphere are consistently smaller in the southern hemisphere. The asymmetry in K(sub yy) and in the parameterization of the tropospheric heating rate results in an asymmetry in the circulation giving rise to unique signatures in the trace gas distributions. The model successfully simulates the observed asymmetry in the column abundance of the springtime ozone maxima between northern and southern hemisphere. Results for other trace gases are in agreement with the gross observed features although specific differences exist.

Ko, Malcolm K. W.; Schneider, Hans R.; Shia, Run-Lie; Weisenstein, Debra K.; Sze, Nien-Dak

1993-01-01

270

NASA Astrophysics Data System (ADS)

Electron accelerations at high Mach number collision-less 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 ?e (the ratio of the thermal pressure to the magnetic pressure). We found 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 the 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 et al., Astrophys. J., 755, 109, 2012.

Matsumoto, Y.; Amano, T.; Hoshino, M.

2012-12-01

271

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

272

Spatial and seasonal distribution of ozone and other trace gases are simulated using the interactive two-dimensional model of the stratosphere updated to included full gas phase chemistry. The model consists of a primitive equation dynamics module, a full radiative transfer scheme, and a comprehensive gas phase chemistry module. The circulation is derived from heating rates in the stratosphere that are calculated using model-generated ozone. In the troposphere, parameterized heating rates are adopted. The eddy momentum flux divergence in the zonal mean momentum equation is given by the eddy fluxes of potential vorticity. Eddy fluxes of potential vorticity and tracers are parameterized using a set of predetermined diffusion coefficients. The adopted values for K{sub yy}, which are based on values derived by Newman et al., show a hemispherical asymmetry in that the values in the lower stratosphere are consistently smaller in the southern hemisphere. The asymmetry in K{sub yy} and in the parameterization of the tropospheric heating rate results in an asymmetry in the circulation giving rise to unique signatures in the trace gas distributions. The model successfully simulates the observed asymmetry in the column abundance of the springtime ozone maxima between northern and southern hemisphere. Results for other trace gases are in agreement with the gross observed features although specific differences exist. 37 refs., 12 figs.

Ko, M.K.W.; Shia, R.L.; Weisenstein, D.K. [Atmospheric and Environmental Research, Inc., Cambridge, MA (United States)] [and others

1993-11-20

273

The behavior of two-dimensional binary and ternary amphiphilic fluids under flow conditions is investigated using a hydrodynamic lattice-gas model. After the validation of the model in simple cases (Poiseuille flow, Darcy's law for single component fluids), attention is focused on the properties of binary immiscible fluids in porous media. An extension of Darcy's law which explicitly admits a viscous coupling between the fluids is verified, and evidence of capillary effects is described. The influence of a third component, namely, surfactant, is studied in the same context. Invasion simulations have also been performed. The effect of the applied force on the invasion process is reported. As the forcing level increases, the invasion process becomes faster and the residual oil saturation decreases. The introduction of surfactant in the invading phase during imbibition produces new phenomena, including emulsification and micellization. At very low fluid forcing levels, this leads to the production of a low-resistance gel, which then slows down the progress of the invading fluid. At long times (beyond the water percolation threshold), the concentration of remaining oil within the porous medium is lowered by the action of surfactant, thus enhancing oil recovery. On the other hand, the introduction of surfactant in the invading phase during drainage simulations slows down the invasion process-the invading fluid takes a more tortuous path to invade the porous medium-and reduces the oil recovery (the residual oil saturation increases). PMID:11088774

Maillet; Coveney

2000-08-01

274

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

275

The presence of long-range quantum spin correlations underlies a variety of physical phenomena in condensed-matter systems, potentially including high-temperature superconductivity. However, many properties of exotic, strongly correlated spin systems, such as spin liquids, have proved difficult to study, in part because calculations involving N-body entanglement become intractable for as few as N???30 particles. Feynman predicted that a quantum simulator--a special-purpose 'analogue' processor built using quantum bits (qubits)--would be inherently suited to solving such problems. In the context of quantum magnetism, a number of experiments have demonstrated the feasibility of this approach, but simulations allowing controlled, tunable interactions between spins localized on two- or three-dimensional lattices of more than a few tens of qubits have yet to be demonstrated, in part because of the technical challenge of realizing large-scale qubit arrays. Here we demonstrate a variable-range Ising-type spin-spin interaction, J(i,j), on a naturally occurring, two-dimensional triangular crystal lattice of hundreds of spin-half particles (beryllium ions stored in a Penning trap). This is a computationally relevant scale more than an order of magnitude larger than previous experiments. We show that a spin-dependent optical dipole force can produce an antiferromagnetic interaction J(i,j) proportional variant d(-a)(i,j), where 0???a???3 and d(i,j) is the distance between spin pairs. These power laws correspond physically to infinite-range (a = 0), Coulomb-like (a = 1), monopole-dipole (a = 2) and dipole-dipole (a = 3) couplings. Experimentally, we demonstrate excellent agreement with a theory for 0.05???a???1.4. This demonstration, coupled with the high spin count, excellent quantum control and low technical complexity of the Penning trap, brings within reach the simulation of otherwise computationally intractable problems in quantum magnetism. PMID:22538611

Britton, Joseph W; Sawyer, Brian C; Keith, Adam C; Wang, C-C Joseph; Freericks, James K; Uys, Hermann; Biercuk, Michael J; Bollinger, John J

2012-04-26

276

The hinge region of a mechanical bileaflet valve is implicated in blood damage and initiation of thrombus formation. Detailed fluid dynamic analysis in the complex geometry of the hinge region during the closing phase of the bileaflet valve is the focus of this study to understand the effect of fluid-induced stresses on the activation of platelets. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a two-dimensional geometry of the hinge region of a bi-leaflet mechanical valve. Use of local mesh refinement algorithm provides mesh adaptation based on the gradients of flow in the constricted geometry of the hinge. Leaflet motion is specified from the fluid-structure interaction analysis of the leaflet dynamics during the closing phase from a previous study which focused on the fluid mechanics at the gap between the leaflet edges and the valve housing. A Lagrangian particle tracking method is used to model and track the platelets and to compute the magnitude of the shear stress on the platelets as they pass through the hinge region. Results show that there is a boundary layer separation in the gaps between the leaflet ear and the constricted hinge geometry. Separated shear layers roll up into vortical structures that lead to high residence times combined with exposure to high shear stresses for particles in the hinge region. Particles are preferentially entrained into this re-circulation zone, presenting the possibility of platelet activation, aggregation, and initiation of thrombi.

Govindarajan, V.; Udaykumar, H.S.; Chandran, K.B.

2009-01-01

277

FireStem2D--a two-dimensional heat transfer model for simulating tree stem injury in fires.

FireStem2D, a software tool for predicting tree stem heating and injury in forest fires, is a physically-based, two-dimensional model of stem thermodynamics that results from heating at the bark surface. It builds on an earlier one-dimensional model (FireStem) and provides improved capabilities for predicting fire-induced mortality and injury before a fire occurs by resolving stem moisture loss, temperatures through the stem, degree of bark charring, and necrotic depth around the stem. We present the results of numerical parameterization and model evaluation experiments for FireStem2D that simulate laboratory stem-heating experiments of 52 tree sections from 25 trees. We also conducted a set of virtual sensitivity analysis experiments to test the effects of unevenness of heating around the stem and with aboveground height using data from two studies: a low-intensity surface fire and a more intense crown fire. The model allows for improved understanding and prediction of the effects of wildland fire on injury and mortality of trees of different species and sizes. PMID:23894599

Chatziefstratiou, Efthalia K; Bohrer, Gil; Bova, Anthony S; Subramanian, Ravishankar; Frasson, Renato P M; Scherzer, Amy; Butler, Bret W; Dickinson, Matthew B

2013-01-01

278

FireStem2D - A Two-Dimensional Heat Transfer Model for Simulating Tree Stem Injury in Fires

FireStem2D, a software tool for predicting tree stem heating and injury in forest fires, is a physically-based, two-dimensional model of stem thermodynamics that results from heating at the bark surface. It builds on an earlier one-dimensional model (FireStem) and provides improved capabilities for predicting fire-induced mortality and injury before a fire occurs by resolving stem moisture loss, temperatures through the stem, degree of bark charring, and necrotic depth around the stem. We present the results of numerical parameterization and model evaluation experiments for FireStem2D that simulate laboratory stem-heating experiments of 52 tree sections from 25 trees. We also conducted a set of virtual sensitivity analysis experiments to test the effects of unevenness of heating around the stem and with aboveground height using data from two studies: a low-intensity surface fire and a more intense crown fire. The model allows for improved understanding and prediction of the effects of wildland fire on injury and mortality of trees of different species and sizes.

Chatziefstratiou, Efthalia K.; Bohrer, Gil; Bova, Anthony S.; Subramanian, Ravishankar; Frasson, Renato P. M.; Scherzer, Amy; Butler, Bret W.; Dickinson, Matthew B.

2013-01-01

279

NASA Astrophysics Data System (ADS)

We have examined the wind velocity and the surface stress associated with thermal convection in dust free Martian atmosphere by using a two-dimensional numerical model. Outbreak of dust storm is a striking phenomenon in the Martian atmosphere. However, it has been well recognized that the Martian general circulation model (GCM) does not produce sufficient surface wind stress to raise dust from the surface under dust free condition. Small-scale wind fluctuations which are not represented in GCM may supplement the necessary surface stress. Daily thermal convection is one of the possible contributers. The numerical model is an two-dimensional anelastic model with radiative and surface processes. The computational domain extends horizontally to 51.2 km and vertically to the height of 20 km. Both the horizontal and vertical grid intervals are 100 m except in the lowermost 100 m, where the vertical resolution is enhanced. The solar flux at the top of the model atmosphere changes diurnally with the northern summer condition (Ls=100° ) at 20° N latitude. The model is integrated for 6 days, and the data of the last day is utilized for analyses. The results of the simulation reveal that the thermal convection in the Martian lower atmosphere is km-size; the vertical and horizontal scales of convective cells are 10 km and several km, respectively. The magnitude of wind velocity associated with the km-size thermal convection is quite large. The values of both horizontal and vertical wind velocity often exceeds 20 m/sec. This magnitude can be understood by considering that the work done by the buoyancy force on a plume (about 2 K higher than the surroundings) is converted to the kinetic energy. The instantaneous maximum value of the surface stress associated with the km-size thermal convection reaches 0.04 Pa, which is equal to the threshold value to raise dust from the surface. The result suggests that GCM can represent dust injection into the Martian atmosphere self-consistently by parameterizing the wind fluctuation associated with the km-size thermal convection in the calculation of surface stress.

Odaka, M.; Nakajima, K.; Ishiwatari, M.; Hayashi, Y.

2001-12-01

280

NASA Astrophysics Data System (ADS)

We present the results of two-dimensional calculations of turbulent nuclear burning of hydrogen-rich material accreted onto a white dwarf of 1.0 Msun. The main aim of the present paper is to investigate the question as to whether and how the general properties of the burning are affected by numerical resolution effects. In particular, we want to see whether or not convective overshooting into the surface layers of the C+O white dwarf can lead to self-enrichment of the initially solar composition of the hydrogen-rich envelope with carbon and oxygen from the underlying white dwarf core. Our explicit hydrodynamic code is based on the PPM-method and computes the onset of the thermonuclear runaway on a Cartesian grid. Only part of the white dwarf's surface is covered by the computational grid and curvature effects are ignored. In contrast to previous works we do not observe fast mixing of carbon and oxygen from the white dwarf's surface into the envelope by violent overshooting of large eddies. The main features of the flow fields in our simulations are the appearance of small persistent coherent structures of very high vorticity (and velocity) compared to the background flow. Their typical linear scales are about 10 to 20 grid zones and thus their physical size depends on the numerical resolution, i.e, their size decreases with increasing resolution. For the early phase of the thermonuclear runaway (TNR) they dominate the flow patterns and result in very little overshoot and mixing. Only at late times, after steady slow mixing and with increasing nuclear energy production, do these structures become weak, but show up again once hydrogen has mainly been burnt and the energy generation rate drops. On the other hand, there are no big differences between high and low resolution simulations, as far as the overall properties of the TNR are concerned. The two simulations which are presented here show only moderate differences in spatially integrated quantities such as laterally averaged temperature, energy generation rate, and chemical composition. We have not expanded both simulations equally long, but for the physical time under consideration the major difference seems to be that the highly resolved simulation is a bit less violent. In conclusion, we do find some self-enrichment, but on time-scales much longer than in previous calculations.

Kercek, A.; Hillebrandt, W.; Truran, J. W.

1998-09-01

281

Recently, theoretical investigations of the beamforming capability of two-dimensional (2-D) transducer arrays have characterized the array parameters required to steer a symmetrically focused ultrasound beam up to 45 degrees off-axis. These investigations have also shown that the number of elements in a steered 2-D array can be dramatically reduced by using a sparse set of elements, randomly distributed throughout the aperture of the transducer. The penalty paid for the use of a sparse array is the development of a "pedestal" sidelobe in the beam profile, the amplitude of which increases as the number of elements in the array decreases. In this paper the potential of 2-D arrays for medical imaging is assessed by simulating B-scan images of spherical lesions, both cystic and scattering, embedded in a large random scattering volume. Similar contrast characteristics over a range of cyst sizes are demonstrated for a dense 2-D array and a sparse array with 1/8th the number of elements, both operating at 5 MHz. A 32nd order sparse array is shown to perform at a reduced level, producing unacceptable artifactual echoes within images of cysts. The 8th order sparse array pattern has been fabricated on a fixed-focus poly(vinylidene difluoride) transducer using photolithographic techniques. Experimental images from this transducer are used to verify some of the theoretical predictions made in this paper. Comparisons between simulated B-scan images from linear and 2-D phased arrays are presented in a companion paper. PMID:1296337

Turnbull, D H; Lum, P K; Kerr, A T; Foster, F S

1992-10-01

282

NASA Technical Reports Server (NTRS)

We have developed a new empirically-based transport algorithm for use in our GSFC two-dimensional transport and chemistry assessment model. The new algorithm contains planetary wave statistics, and parameterizations to account for the effects due to gravity waves and equatorial Kelvin waves. We will present an overview of the new algorithm, and show various model-data comparisons of long-lived tracers as part of the model validation. We will also show how the new algorithm gives substantially better agreement with observations compared to our previous model transport. The new model captures much of the qualitative structure and seasonal variability observed methane, water vapor, and total ozone. These include: isolation of the tropics and winter polar vortex, the well mixed surf-zone region of the winter sub-tropics and mid-latitudes, and the propagation of seasonal signals in the tropical lower stratosphere. Model simulations of carbon-14 and strontium-90 compare fairly well with observations in reproducing the peak in mixing ratio at 20-25 km, and the decrease with altitude in mixing ratio above 25 km. We also ran time dependent simulations of SF6 from which the model mean age of air values were derived. The oldest air (5.5 to 6 years) occurred in the high latitude upper stratosphere during fall and early winter of both hemispheres, and in the southern hemisphere lower stratosphere during late winter and early spring. The latitudinal gradient of the mean ages also compare well with ER-2 aircraft observations in the lower stratosphere.

Fleming, Eric L.; Jackman, Charles H.; Stolarski, Richard S.; Considine, David B.

1998-01-01

283

NASA Astrophysics Data System (ADS)

Most lakes worldwide are supersaturated with carbon dioxide (CO2) and consequently act as atmospheric net sources. Since CO2 is a major greenhouse gas (GHG), the accurate estimation of CO2 exchanges at air/water interfaces of aquatic ecosystems is vital in quantifying the carbon budget of aquatic ecosystems overall. To date, lacustrine CO2 emissions are poorly understood, and lake carbon source proportions remain controversial, largely due to a lack of integration between aquatic and terrestrial ecosystems. In this paper a new process-based model (TRIPLEX-Aquatic) is introduced incorporating both terrestrial inputs and aquatic biogeochemical processes to estimate diffusive emissions of CO2 from lake systems. The model was built from a two-dimensional hydrological and water quality model coupled with a new lacustrine CO2 diffusive flux model. For calibration and validation purposes, two years of data collected in the field from two small boreal oligotrophic lakes located in Québec (Canada) were used to parameterize and test the model by comparing simulations with observations for both hydrodynamic and carbon process accuracy. Model simulations were accordant with field measurements in both calibration and verification. Consequently, the TRIPLEX-Aquatic model was used to estimate the annual mean CO2 diffusive flux and predict terrestrial dissolved organic carbon (DOC) impacts on the CO2 budget for both lakes. Results show a significant fraction of the CO2 diffusive flux (~30-45%) from lakes was primarily attributable to the input and mineralization of terrestrial DOC, which indicated terrestrial organic matter was the key player in the diffusive flux of CO2 from oligotropical lake systems in Québec, Canada.

Wu, H.; Peng, C.; Lucotte, M.; Soumis, N.; Gélinas, Y.; Duchemin, É.; Plouhinec, J.-B.; Ouellet, A.; Guo, Z.

2013-06-01

284

NASA Astrophysics Data System (ADS)

The foreshock region is populated by energetic backstreaming particles (electrons and ions) issued from the shock after having interacted with it. Several aspects concerning the origin of these high-energy particles and their corresponding acceleration mechanisms are still unresolved. The present study is focused on a quasi-perpendicular curved shock and associated electron foreshock region (i.e., for 90° ? $\\theta$Bn ? 45°, where $\\theta$Bn is the angle between the shock normal and the upstream magnetostatic field). Two-dimensional full-particle simulation is used in order to include self-consistently the electron and ion dynamics, the full dynamics of the shock, the curvature effects and the time-of-flight effects. All expected salient features of the bow shock are recovered both for particles and for electromagnetic fields. Present simulations evidence that the fast-Fermi acceleration (magnetic mirror) mechanism, which is commonly accepted, is certainly not the unique process responsible for the formation of energetic backstreaming electrons. Other mechanisms also contribute. More precisely, three different classes of backstreaming electrons are identified according to their individual penetration depth within the shock front: (i) “magnetic mirrored” electrons which only suffer a specular reflection at the front, (ii) “trapped” electrons which succeed to penetrate the overshoot region and suffer a local trapping within the parallel electrostatic potential at the overshoot, and (iii) “leaked” electrons which penetrate even much deeper into the downstream region. “Trapped” and “leaked” electrons succeed to find appropriate conditions to escape from the shock and to be reinjected back upstream. All these different types of electrons contribute together to the formation of energetic field-aligned beam. The acceleration mechanisms associated to each electron class and/or escape conditions are analyzed and discussed.

Savoini, P.; Lembége, B.; Stienlet, J.

2010-09-01

285

Quick Time-dependent Ionization Calculations Depending on MHD Simulations

NASA Astrophysics Data System (ADS)

Time-dependent ionization is important in astrophysical environments where the thermodynamic time scale is shorter than ionization time scale. In this work, we report a FORTRAN program that performs fast non-equilibrium ionization calculations based on parallel computing. Using MHD simulation results, we trace the movements of plasma in a Lagrangian framework, and obtain evolutionary history of temperature and electron density. Then the time-dependent ionization equations are solved using the eigenvalue method. For any complex temperature and density histories, we introduce a advanced time-step strategy to improve the computational efficiency. Our tests show that this program has advantages of high numerical stability and high accuracy. In addition, it is also easy to integrate this solver with the other MHD routines.

Shen, Chengcai; Raymond, John C.; Murphy, Nicholas Arnold

2014-06-01

286

MHD simulations of Earth`s bow shock at low Mach numbers: Standoff distances

Global, three-dimensional, ideal MHD simulations of Earth`s bow shock are reported for low Alfven Mach numbers M{sub A} and quasi-perpendicular magnetic field orientations. The simulations use a hard, infinitely conducting magnetopause obstacle, with axisymmetric three-dimensional location given be a scaled standard model, to directly address previous gasdynamic (GD) and field-aligned MHD (FA-MHD) work. Tests of the simulated shocks` density jumps

Iver H. Cairns; J. G. Lyon

1995-01-01

287

NASA Technical Reports Server (NTRS)

Large disturbances in the interplanetary medium were observed by several spacecraft during a period of enhanced solar activity in early February 1986. The locations of six solar flares and the spacecraft considered here encompassed more than 100 deg of heliolongitude. These flares during the minimum of cycle 21 set the stage for an extensive multispacecraft comparison performed with a two-dimensional, MHD numerical experiment. The plasma instruments on the Giotto spacecraft, on its way to encounter Comet Halley in March 1986, made measurements of the solar wind for up to 8 hours/day during February. Solar wind measurements from the Johnstone Plasma Analyzer experiment on Giotto are compared with the MHD simulation of the interplanetary medium throughout these events. Using plasma data obtained by the IMP-8 satellite in addition, it appears that an extended period of high solar wind speed is required as well as the simulated flares to represent the interplanetary medium in this case. The plasma and magnetometer data from Vega-1 is compared with the MHD simulation. This comparison tends to support an interpretation that the major solar wind changes at both Giotto and Vega-1 on February 8, 1986 were due to a shock from a W 05 deg solar flare on February 6, 1986 (06:25 UT). The numerical experiment is considered, qualitatively, to resemble the observations at the former spacecraft, but it has less success at the latter one.

Dryer, M.; Smith, Z. K.; Coates, A. J.; Johnstone, A. D.

1991-01-01

288

NASA Astrophysics Data System (ADS)

Flow resistance, ventilation, and pollutant removal for idealized two-dimensional (2D) street canyons of different building-height to street-width (aspect) ratios AR are examined using the friction factor f, air exchange rate (ACH), and pollutant exchange rate (PCH), respectively, calculated by large-eddy simulation (LES). The flows are basically classified into three characteristic regimes, namely isolated roughness, wake interference, and skimming flow, as functions of the aspect ratios. The LES results are validated by various experimental and numerical datasets available in the literature. The friction factor increases with decreasing aspect ratio and reaches a peak at AR = 0.1 in the isolated roughness regime and decreases thereafter. As with the friction factor, the ACH increases with decreasing aspect ratio in the wake interference and skimming flow regimes, signifying the improved aged air removal for a wider street canyon. The PCH exhibits a behaviour different from its ACH counterpart in the range of aspect ratios tested. Pollutants are most effectively removed from the street canyon with AR = 0.5. However, a minimum of PCH is found nearby at AR = 0.3, at which the pollutant removal is sharply weakened. Besides, the ACH and PCH are partitioned into the mean and turbulent components to compare their relative contributions. In line with our earlier Reynolds-averaged Navier-Stokes calculations (Liu et al., Atmos Environ 45:4763-4769, 2011), the current LES shows that the turbulent components contribute more to both ACH and PCH, consistently demonstrating the importance of atmospheric turbulence in the ventilation and pollutant removal for urban areas.

Chung, Tracy N. H.; Liu, Chun-Ho

2013-07-01

289

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

290

Magnetic reconnection in Hall MHD simulations including electron inertia

NASA Astrophysics Data System (ADS)

Magnetic reconnection is an important physical mechanism of energy conversion in various space plasma physics environments, such as the solar corona or planetary magnetospheres. Theoretical models of magnetic reconnection were first developed within the framework of one-fluid magnetohydrodynamics (MHD), where the Sweet-Parker regime leads to exceedingly low reconnection rates for most space physics problems. Kinetic plasma effects introduce new spatial and temporal scales into the theoretical description, which might significantly increase the reconnection rates. We work within the framework of two-fluid MHD for a fully ionized hydrogen plasma, retaining the effects of the Hall current and electron inertia. The corresponding equations of motion display three ideal invariants: total energy, electron helicity and ion helicity. We performed 2.5 D Hall MHD simulations including electron inertia using a pseudo-spectral code, which yield exponentially fast numerical convergence. Our results show that reconnection takes place in an electron-dominated region, whose spatial scale is given by the electron skin depth. This region is surrounded by a much larger ion-dominated region, with a spatial scale of the order of the ion skin depth. The computed reconnection rates remain a fair fraction of the Alfven velocity, which is much larger than the Sweet-Parker reconnection rate.

Gomez, D. O.; Andres, N.; Martin, L. N.; Dmitruk, P.

2012-12-01

291

NASA Technical Reports Server (NTRS)

Monte Carlo simulation, lattice dynamics in the harmonic approximation, and solution of the hypernetted chain equation were used to study the classical two-dimensional one component plasma. The system consists of a single species of charged particles immersed in a uniform neutralizing background. The particles interact via a l/r potential, where r is the two dimensional separation. Equations of state were calculated for both the liquid and solid phases. Results of calculation of the thermodynamic functions and one and two particle correlation functions are presented.

Gann, R. C.; Chakravarty, S.; Chester, G. V.

1978-01-01

292

Magnetic flux ropes in 3-dimensional MHD simulations

NASA Technical Reports Server (NTRS)

The interaction of the solar wind and the earth's magnetosphere is presently simulated by a 3D, time-dependent, global MHD method in order to model the magnetopause and magnetotail generation of magnetic flux ropes. It is noted that strongly twisted and localized magnetic flux tubes simular to magnetic flux ropes appear at the subpolar magnetopause when the IMF has a large azimuthal component, as well as a southward component. Plasmoids are generated in the magnetotail after the formation of a near-earth magnetic neutral line; the magnetic field lines have a helical structure that is connected from dawn to dusk.

Ogino, Tatsuki; Walker, Raymond J.; Ashour-Abdalla, Maha

1990-01-01

293

MHD Simulations of the Eruption of Coronal Flux Ropes

NASA Astrophysics Data System (ADS)

I present MHD simulations of the eruption of coronal flux ropes and the initiation of coronal mass ejections (CMEs). It is found that the eruption of the flux rope in the corona is triggered when the flux rope rises to a critical height where the corresponding potential field declines with height at a sufficiently steep rate, a mechanism consistent with the onset of the torus instability. The simulations show that S (or inverse S) shaped current sheets develop along topological structures identified as Quasi Separatrix Layers (QSLs), during the quasi-static phase before the eruption. Reconnections in the current sheets effectively add twisted flux to the flux rope, allowing it to rise quasi-statically to the critical height and then the dynamic eruption ensues. It is found that the resulting coronal magnetic field evolution can explain some of the commonly observed features associated CMEs and their pre-cursor structures. Finally I also show an MHD simulation that models qualitatively the magnetic field evolution of the eruptive flare occurred on December 13, 2006 in the emerging ? sunspot region NOAA 10930 observed by the Hinode satellite.

Fan, Yuhong

2012-10-01

294

NASA Astrophysics Data System (ADS)

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations ? i in the streamwise ( ? u ), spanwise ( ? v ) and vertical ( ? w ) directions are located near the roof-level windward corners. Moreover, a second ? w peak is found at z ? 1.5 h ( h is the building height) over the street canyons. Normalizing ? i by the local friction velocity u *, it is found that ? u / u * ? 1.8, ? v / u * ? 1.3 and ? w / u * ? 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u'' w'' shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity R v, x and R w, x drop to zero at a separation larger than h but R u, x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient ? T of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

Cheng, W. C.; Liu, Chun-Ho

2011-06-01

295

Understanding Accretion Disks through Three Dimensional Radiation MHD Simulations

NASA Astrophysics Data System (ADS)

I study the structures and thermal properties of black hole accretion disks in the radiation pressure dominated regime. Angular momentum transfer in the disk is provided by the turbulence generated by the magneto-rotational instability (MRI), which is calculated self-consistently with a recently developed 3D radiation magneto-hydrodynamics (MHD) code based on Athena. This code, developed by my collaborators and myself, couples both the radiation momentum and energy source terms with the ideal MHD equations by modifying the standard Godunov method to handle the stiff radiation source terms. We solve the two momentum equations of the radiation transfer equations with a variable Eddington tensor (VET), which is calculated with a time independent short characteristic module. This code is well tested and accurate in both optically thin and optically thick regimes. It is also accurate for both radiation pressure and gas pressure dominated flows. With this code, I find that when photon viscosity becomes significant, the ratio between Maxwell stress and Reynolds stress from the MRI turbulence can increase significantly with radiation pressure. The thermal instability of the radiation pressure dominated disk is then studied with vertically stratified shearing box simulations. Unlike the previous results claiming that the radiation pressure dominated disk with MRI turbulence can reach a steady state without showing any unstable behavior, I find that the radiation pressure dominated disks always either collapse or expand until we have to stop the simulations. During the thermal runaway, the heating and cooling rates from the simulations are consistent with the general criterion of thermal instability. However, details of the thermal runaway are different from the predictions of the standard alpha disk model, as many assumptions in that model are not satisfied in the simulations. We also identify the key reasons why previous simulations do not find the instability. The thermal instability has many important implications for understanding the observations of both X-ray binaries and Active Galactic Nuclei (AGNs). However, direct comparisons between observations and the simulations require global radiation MHD simulations, which will be the main focus of my future work.

Jiang, Yan-Fei

296

NASA Astrophysics Data System (ADS)

Two-dimensional axisymmetric nonlinear simulations of underground nuclear explosions have been coupled to wave propagation codes to generate synthetic seismograms. The nonlinear models include plastic yielding, pore crushup, cracking, and shock weakening. The effects of gravity and the free surface interaction are explicitly included in the calculations. The calculations provide insight into the effects of spall, depth of burial, and tectonic strain release on seismic signals. A simple, physical model for spall is developed by comparing the complete two-dimensional waveform with the waveform generated by a one-dimensional explosion plus a shallow tension crack in the same layered medium. P-waves generated by the two-dimensional simulation are modeled very well by the simple tension crack plus explosion model, and the resulting parameters for the spall model are consistent with the limited set of near-field observations of spall. Spall is found to be a strong generator of the Lg phase. Comparison of two-dimensional calculations with tectonic prestress with linear models shows that the tectonic source has an effective radius of approximately 80 percent of the elastic radius of the explosion, and is large enough to induce Rayleigh wave reversals with realistic levels of prestress. Rayleigh wave phase shifts and apparent time delays are explained as an interference effect between the Rayleigh waves generated by the explosion and tectonic sources.

Stevens, Jeffry L.; Barker, Terrance G.; Day, Steven M.; McLaughlin, Keith L.; Rimer, Norton; Shkoller, Boris

297

Growth of correlation in compressible two-dimensional magnetofluid turbulence

NASA Technical Reports Server (NTRS)

Spectral transfer has been proposed as the primary mechanism for generating outward-propagating Alfven waves in the solar wind. This process has been investigated extensively for imcompressible magnetofluids, but the issue of whether it occurs in compressible magnetofluids such as the solar wind remains unresolved. The results of direct numerical simulations of nonisentropic-compressible two-dimensional MHD turbulence indicate that, for systems with finite initial cross helicity, the correlation between the fluctuating velocity field and the fluctuating magnetic field grows as a function of time. This growth of correlation can be interpreted as a turbulent process, as shown by examination of modal wavenumber spectra.

Dahlburg, R. B.; Picone, J. M.; Karpen, J. T.

1988-01-01

298

Thermodynamic MHD Simulation of the Bastille Day Event

NASA Astrophysics Data System (ADS)

The "Bastille Day" event on July 14, 2000 is one of the most extensively studied solar eruptions. It originated in a complex active region close to disk center and produced an X5.7 flare, a fast halo CME, and an intense geomagnetic storm. We have recently begun to model this challenging event, with the final goal to simulate its whole evolution, from the pre-eruptive state to the CME's arrival at 1 AU. To this end, we first produce a steady-state MHD solution of the background corona that incorporates realistic energy transport ("thermodynamic MHD"), photospheric magnetic field measurements, and the solar wind. In order to model the pre-eruptive magnetic field, we then insert into this solution a stable, elongated flux rope that resides above the highly curved polarity inversion line of the active region. Finally, we produce an eruption by imposing photospheric flows that slowly converge towards the polarity inversion line. In this presentation we describe our method, compare the simulation results with the observations, and discuss the challenges and limitations involved in modeling such complex and powerful eruptions.

Torok, Tibor; Downs, Cooper; Lionello, Roberto; Linker, Jon A.; Mikic, Zoran; Titov, Viacheslav S.; Riley, Pete

2014-05-01

299

Simulation of combustion processes in an MHD second stage combustor

NASA Astrophysics Data System (ADS)

Computer simulation is used to aid in the design of a magnetohydrodynamic (MHD) second stage combustor. A 2-D steady state computer model, based on mass, momentum, and energy conservation laws for multiple gas species, is used to simulate the hydrodynamics and combustion in a second stage combustor in which opposed jets of oxidizer are injected into a confined cross stream of hot gas/fuel mixture entering from the first stage. The model computes variable distributions (temperature, pressure, velocity, and species concentrations, etc.) in the combustor and predicts reacting flow heat release, mixing patterns, and combustion efficiency. The modeling aids in understanding the complex processes occurring within the combustor and helps in identification of optimum design parameters and operating conditions for the combustor to improve combustor operation and enhance downstream MHD generator performance. Results of the study indicate: (1) co-flow oxidizer jet injection (angle less than 90 degs) yields relatively poor mixing and combustion performance; (2) counter flow injection (angle greater than 90 degs) yields much better mixing and combustion performance, with an optimum injection angle in the range of 120 to 140 degs; (3) the optimum injection angle intervals for combustion efficiency and uniformity of exit temperature profile are the same; and (4) the non-reacting flow optimum injection angle interval for jet/main flow mixing is a relatively good predictor of reacting flow combustor performance in terms of combustion efficiency and uniformity of combustor exit temperature profile.

Lottes, S. A.; Chang, S. L.

300

Coupled simulation of kinetic pedestal growth and MHD ELM crash

NASA Astrophysics Data System (ADS)

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.; Cummings, J.; Chang, C. S.; Podhorszki, N.; Klasky, S.; Ku, S.; Pankin, A.; Samtaney, R.; Shoshani, A.; Snyder, P.; Strauss, H.; Sugiyama, L.; CPES Team

2007-07-01

301

NASA Technical Reports Server (NTRS)

Finite element modeling alternatives as well as the utility and limitations of the two dimensional structural response computer code CIVM-JET 4B for predicting the transient, large deflection, elastic plastic, structural responses of two dimensional beam and/or ring structures which are subjected to rigid fragment impact were investigated. The applicability of the CIVM-JET 4B analysis and code for the prediction of steel containment ring response to impact by complex deformable fragments from a trihub burst of a T58 turbine rotor was studied. Dimensional analysis considerations were used in a parametric examination of data from engine rotor burst containment experiments and data from sphere beam impact experiments. The use of the CIVM-JET 4B computer code for making parametric structural response studies on both fragment-containment structure and fragment-deflector structure was illustrated. Modifications to the analysis/computation procedure were developed to alleviate restrictions.

Stagliano, T. R.; Witmer, E. A.; Rodal, J. J. A.

1979-01-01

302

NASA Technical Reports Server (NTRS)

Theoretical considerations relevant to the rate of thermal relaxation of a two-dimensional plasma in a strong uniform dc magnetic field are developed. The Vahala-Montgomery (1971) kinetic description is completed by providing a cut-off time for the time of interaction of two particles contributing to the collision term. The kinetic equation is shown to predict that thermal relaxation varies as a function of defined dimensionless time.

Hsu, J.-Y.; Joyce, G.; Montgomery, D.

1974-01-01

303

By applying one- and two-dimensional 109Ag NMR, we demonstrate that silver diffusion in silver iodide\\/silver phosphate glasses is governed by a very broad, continuous distribution of correlation times G (lg ?). As a consequence, over a wide temperature range, the 109Ag NMR spectra can be described by a weighted superposition of a Gaussian and a Lorentzian where these line-shape components

M. Vogel; C. Brinkmann; H. Eckert; A. Heuer

2002-01-01

304

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

Benxin Wu; Yung C. Shin

2007-01-01

305

NASA Astrophysics Data System (ADS)

A two-dimensional, user-friendly model of the discharge occurring in a plasma display panel cell was developed. This model was used to study the transient discharges in an alternating current plasma display with a matrix electrode configuration. The space and time variations of the charge particle densities, excitation rates, electric potential, and surface charge densities are described. The model is also used to study the conditions of existence of electrical interaction between adjacent cells and the effects of electrode misalignment.

Punset, C.; Boeuf, J.-P.; Pitchford, L. C.

1998-02-01

306

A comprehensive study of the two-dimensional (2D) compass model on the square lattice is performed for classical and quantum spin degrees of freedom using Monte Carlo and quantum Monte Carlo methods. We employ state-of-the-art implementations using Metropolis, stochastic series expansion, and parallel tempering techniques to obtain the critical ordering temperatures and critical exponents. In a preinvestigation we reconsider the classical

Sandro Wenzel; Wolfhard Janke

2008-01-01

307

MHD Simulations of Earth's Bow Shock at low Mach Numbers: Standoff Distances

NASA Technical Reports Server (NTRS)

Global, three-dimensional, ideal MHD simulations of Earth's bow shock are reported for low Alfven Mach numbers M(sub A) and quasi-perpendicular magnetic field orientations. The simulations use a hard, infinitely conducting magnetopauause obstacle, with axisymmetric three-dimensional location given by scaled standard model, to directly address previous gasdynamic (GD) and field-aligned MHD (FA-MHD) work. Tests of the simulated shocks' density jumps X for 1.4 approx. less than MA approx. less than 10 and the high M(sub A) shock location, and reproduction of the GD relation between magnetosheath thickness and X for quasi-gasdynamic MHD runs with M(sub A) much greater than M(sub s), confirm that the MHD code is working correctly. The MHD simulations show the standoff distance a(sub s), increasing monotonically with decreasing M(sub A). Significantly larger a(sub s), are found at low M(sub A) than predicted by GD and phenomenological MHD models and FA-MHD simulations, as required qualitatively by observations. The GD and FA-MHD predictions err qualitatively, predicting either constant or decreasing a(sub s), with decreasing M(sub A). This qualitative difference between quasi- perpendicular MHD and FA-MHD simulations is direct evidence for a(sub s), depending on the magnetic field orientation Theta. The enhancement factor over the phenomenological MHD predictions at MA approx. 2.4 agrees quantitatively with one observatiorial estimate. A linear relationship is found between the magnetosheath thickness and X, modified both quantitatively and intrinsically by MHD effects from the GD result. The MHD and GD results agree in the high M(sub A) limit. An MHD theory is developed for a(sub s), restricted to sufficiently perpendicular Theta and high sonic Mach numbers M(sub s). It explains the simulation results with excellent accuracy. Observational and further simulation testing of this MHD theory, and of its predicted M(sub A), Theta, and M(sub s) effects, is desirable.

Cairns, Iver H.; Lyon, J. G.

1995-01-01

308

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

309

Final Report: "Large-Eddy Simulation of Anisotropic MHD Turbulence"

To acquire better understanding of turbulence in flows of liquid metals and other electrically conducting fluids in the presence of steady magnetic fields and to develop an accurate and physically adequate LES (large-eddy simulation) model for such flows. The scientific objectives formulated in the project proposal have been fully completed. Several new directions were initiated and advanced in the course of work. Particular achievements include a detailed study of transformation of turbulence caused by the imposed magnetic field, development of an LES model that accurately reproduces this transformation, and solution of several fundamental questions of the interaction between the magnetic field and fluid flows. Eight papers have been published in respected peer-reviewed journals, with two more papers currently undergoing review, and one in preparation for submission. A post-doctoral researcher and a graduate student have been trained in the areas of MHD, turbulence research, and computational methods. Close collaboration ties have been established with the MHD research centers in Germany and Belgium.

Zikanov, Oleg

2008-06-23

310

NASA Technical Reports Server (NTRS)

When analyzing data from an array of spacecraft (such as Cluster or MMS) crossing a site of magnetic reconnection, it is desirable to be able to accurately determine the orientation of the reconnection site. If the reconnection is quasi-two dimensional, there are three key directions, the direction of maximum inhomogeneity (the direction across the reconnection site), the direction of the reconnecting component of the magnetic field, and the direction of rough invariance (the "out of plane" direction). Using simulated spacecraft observations of magnetic reconnection in the geomagnetic tail, we extend our previous tests of the direction-finding method developed by Shi et al. (2005) and the method to determine the structure velocity relative to the spacecraft Vstr. These methods require data from four proximate spacecraft. We add artificial noise and calibration errors to the simulation fields, and then use the perturbed gradient of the magnetic field B and perturbed time derivative dB/dt, as described by Denton et al. (2010). Three new simulations are examined: a weakly three-dimensional, i.e., quasi-two-dimensional, MHD simulation without a guide field, a quasi-two-dimensional MHD simulation with a guide field, and a two-dimensional full dynamics kinetic simulation with inherent noise so that the apparent minimum gradient was not exactly zero, even without added artificial errors. We also examined variations of the spacecraft trajectory for the kinetic simulation. The accuracy of the directions found varied depending on the simulation and spacecraft trajectory, but all the directions could be found within about 10 for all cases. Various aspects of the method were examined, including how to choose averaging intervals and the best intervals for determining the directions and velocity. For the kinetic simulation, we also investigated in detail how the errors in the inferred gradient directions from the unmodified Shi et al. method (using the unperturbed gradient) depended on the amplitude of the calibration errors. For an accuracy of 3 for the maximum gradient direction, the calibration errors could be as large as 3% of reconnection magnetic field, while for the same accuracy for the minimum gradient direction, the calibration errors could only be as large as 0.03% of the reconnection magnetic field. These results suggest that the maximum gradient direction can normally be determined by the unmodified Shi et al. method, while the modified method or some other method must be used to accurately determine the minimum gradient direction. The structure velocity was found with magnitude accurate to 2% and direction accurate to within 5%.

Denton, R.; Sonnerup, B. U. O.; Swisdak, M.; Birn, J.; Drake, J. F.; Heese, M.

2012-01-01

311

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

312

Two-dimensional simulation of high-order laterally-coupled GaAs-AlGaAs DFB laser diodes

NASA Astrophysics Data System (ADS)

Laterally-coupled distributed feedback (LC-DFB) laser diodes made without an epitaxial re-growth process have the advantage of a simple fabrication process. In this paper, two-dimensional optical field distribution of the fundamental quasi TE (transverse electric) mode is calculated by means of a semivectorial finite-difference method (SV-FDM). The dependence of the effective coupling coefficient (kgreff) on the dutycycle of first-, second- and third-order LC-DFB LDs is investigated using modified coupled wave equations.

Zhong, Yuan; Zhu, Xiaopeng; Song, Guofeng; Huang, Yidong; Chen, Lianghui

2004-08-01

313

Formation and Collimation of Relativistic MHD Jets — Simulations and Radio Maps

NASA Astrophysics Data System (ADS)

We present results of magnetohydrodynamic (MHD) simulations of jet formation and propagation, discussing a variety of astrophysical setups. In the first approach we consider simulations of relativistic MHD jet formation, considering jets launched from the surface of a Keplerian disk, demonstrating numerically — for the first time — the self-collimating ability of relativistic MHD jets. We obtain Lorentz factors up to ? 10 while acquiring a high degree of collimation of about 1 degree. We then present synchrotron maps calculated from the intrinsic jet structure derived from the MHD jet formation simulation. We finally present (non-relativistic) MHD simulations of jet lauching, treating the transition between accretion and ejection. These setups include a physical magnetic diffusivity which is essential for loading the accretion material onto the outflow. We find relatively high mass fluxes in the outflow, of the order of 20-40% of the accretion rate.

Fendt, Christian; Porth, Oliver; Sheikhnezami, Somayeh

2014-03-01

314

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. [Center for Laser-based Manufacturing, Purdue University, West Lafayette, Indiana 47907 (United States)

2007-05-15

315

Relative timing of substorm features in MHD simulations

NASA Technical Reports Server (NTRS)

An investigation of the temporal sequence of substorm phenomena based on three dimensional MHD (magnetohydrodynamic) simulations of magnetic reconnection and plasmoid formation is presented. The investigation utilizes a spatially localized resistivity model which leads to a significantly faster evolution than found in previous investigations. The analysis of the results concentrates on substorm features that have received considerable attention in the past. The formation of magnetic neutral lines, the occurrence of fast flows directed both earthward and tailward, and the magnetic field changes leading to the formation of the substorm current wedge, and to the depolarization of the magnetic field earthward of the reconnection region and its dependence on the spatial distribution of resistivity, are discussed. These phenomena are seen as an integral part of the nonlinear evolution of the three dimensional tearing instability.

Hesse, Michael; Birn, Joachim

1992-01-01

316

NASA Technical Reports Server (NTRS)

Numerous studies have been concerned with the possibility of a reduction of the stratospheric ozone layer. Such a reduction could lead to an enhanced penetration of ultraviolet (UV) radiation to the ground, and, as a result, to damage in the case of several biological processes. It is pointed out that the distributions of many trace gases, such as ozone, are governed in part by transport processes. The present investigation presents a two-dimensional photochemistry-transport model using the residual circulation. The global distribution of both ozone and components with ground sources computed in this model is in good agreement with the observations even though slow diffusion is adopted. The agreement is particularly good in the Northern Hemisphere. The results provide additional support for the idea that tracer transport in the stratosphere is mainly of advective nature.

Stordal, F.; Isaksen, I. S. A.; Horntveth, K.

1985-01-01

317

NASA Astrophysics Data System (ADS)

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 to determine the strength of the excited state absorption transition and the relative detuning of electronic states, as well as the dynamics of the single-quantum coherence. To investigate the correlation of spectral fluctuations in different electronically excited states, we have carried out experiments on a solvated dye (Rhodamine 6G) with 23 fs pulses centered at the maximum of the linear absorption spectrum. The 2Q2D spectrum reveals three peaks of alternating signs with the major negative peak located at higher frequencies along the emission axis compared to the single positive peak. The 1Q2D spectrum, on the other hand, shows a negative peak stemming from excited state absorption at lower frequencies along the emission axis. Analysis of the signal in the homogeneous limit fails to account for this observation as well as the number of peaks in the 2Q2D spectrum. Employing a three-level model in which all time correlations of the third-order response function are accounted for via second-order cumulant expansion gives good agreement with both the 1Q2D and 2Q2D data. Furthermore, the analysis shows that the fluctuations of the probed electronic states are highly correlated, reflecting the modulation by a common nuclear bath and similarities in the nature of the electronic transitions.

Nemeth, Alexandra; Milota, Franz; Man?al, Tomáš; Pullerits, Tönu; Sperling, Jaroslaw; Hauer, Jürgen; Kauffmann, Harald F.; Christensson, Niklas

2010-09-01

318

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 to determine the strength of the excited state absorption transition and the relative detuning of electronic states, as well as the dynamics of the single-quantum coherence. To investigate the correlation of spectral fluctuations in different electronically excited states, we have carried out experiments on a solvated dye (Rhodamine 6G) with 23 fs pulses centered at the maximum of the linear absorption spectrum. The 2Q2D spectrum reveals three peaks of alternating signs with the major negative peak located at higher frequencies along the emission axis compared to the single positive peak. The 1Q2D spectrum, on the other hand, shows a negative peak stemming from excited state absorption at lower frequencies along the emission axis. Analysis of the signal in the homogeneous limit fails to account for this observation as well as the number of peaks in the 2Q2D spectrum. Employing a three-level model in which all time correlations of the third-order response function are accounted for via second-order cumulant expansion gives good agreement with both the 1Q2D and 2Q2D data. Furthermore, the analysis shows that the fluctuations of the probed electronic states are highly correlated, reflecting the modulation by a common nuclear bath and similarities in the nature of the electronic transitions. PMID:20831322

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

2010-09-01

319

MHD Simulations of the Plasma Flow in the Magnetic Nozzle

NASA Technical Reports Server (NTRS)

The magnetohydrodynamic (MHD) flow of plasma through a magnetic nozzle is simulated by solving the governing equations for the plasma flow in the presence of an static magnetic field representing the applied nozzle. This work will numerically investigate the flow and behavior of the plasma as the inlet plasma conditions and magnetic nozzle field strength are varied. The MHD simulations are useful for addressing issues such as plasma detachment and to can be used to gain insight into the physical processes present in plasma flows found in thrusters that use magnetic nozzles. In the model, the MHD equations for a plasma, with separate temperatures calculated for the electrons and ions, are integrated over a finite cell volume with flux through each face computed for each of the conserved variables (mass, momentum, magnetic flux, energy) [1]. Stokes theorem is used to convert the area integrals over the faces of each cell into line integrals around the boundaries of each face. The state of the plasma is described using models of the ionization level, ratio of specific heats, thermal conductivity, and plasma resistivity. Anisotropies in current conduction due to Hall effect are included, and the system is closed using a real-gas equation of state to describe the relationship between the plasma density, temperature, and pressure.A separate magnetostatic solver is used to calculate the applied magnetic field, which is assumed constant for these calculations. The total magnetic field is obtained through superposition of the solution for the applied magnetic field and the self-consistently computed induced magnetic fields that arise as the flowing plasma reacts to the presence of the applied field. A solution for the applied magnetic field is represented in Fig. 1 (from Ref. [2]), exhibiting the classic converging-diverging field pattern. Previous research was able to demonstrate effects such as back-emf at a super-Alfvenic flow, which significantly alters the shape of the magnetic field in both the near- and far-field regions. However, in that work the downstream domain was constrained to a channel of constant cross-sectional area. In the present work we seek to address this issue by modeling the downstream region with a domain that permits free expansion of the plasma, permitting a better evaluation of the downstream effects the applied field has on the plasma. The inlet boundary conditions and applied magnetic field values will also be varied to determine the effect the initial plasma energy content and applied magnetic field energy density have on the near- and far-field plasma properties on the MHD code. This will determine the effect of inlet boundary conditions on the results downstream and address issues related to the restrictive numerical domain previously used.

Smith, T. E. R.; Keidar, M.; Sankaran, K.; olzin, K. A.

2013-01-01

320

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

321

MHD Simulation of Periodic Plasmoid Ejections in Saturn

NASA Astrophysics Data System (ADS)

We study large-scale mass loading and plasma loss phenomena in Saturn's magnetosphere by means of global single-fluid MHD simulations (BATS-R-US). The major internal plasma sources, the rings, the icy satellites and Titan are incorporated as mass loading source terms in the MHD equations. We find that even under steady solar wind conditions, large plasmoids are pinched off quasi-periodically along a reconnection line in the dawn side of the magnetotail. Similar plasmoids have been recently observed, with a repetition period of 2-3 days, in Jupiter's magnetotail by the New Horizons spacecraft. In our simulations, the characteristic frequency of plasmoid ejections is decreasing and the size of the plasmoids is increasing with decreasing solar wind dynamic pressure. This can be explained in terms of the Vasyliunas cycle. If the scale size of the magnetosphere is larger due to a lower solar wind dynamic pressure, more time is needed to fill up a recently emptied flux tube with plasma of internal origin. The excessive mass stretches the tailward convected flux tubes until reconnection occurs along an X-line and a plasmoid is pinched off. For medium and low solar wind dynamic pressures, the characteristic period of plasmoid ejections in Saturn's magnetotail varies in the range between 20 and 70 hours. For higher than average solar wind dynamic pressures, however, large plasmoids cannot be observed any more in the simulations, and the loss of plasma in the tail becomes more or less continuous. Interestingly enough, the characteristic frequency of plasmoid ejections becomes lower for larger axial tilts, which means that besides the mass loading rate and the size of the magnetosphere, the geometry of the internal plasma sources also plays an important role in the formation of plasmoids. We demonstrate the periodic formation of plasmoids and the related inner magnetospheric dynamics with 2-D and 3-D animations for different steady solar wind conditions and different axial tilt angles. We conclude that Saturn's magnetosphere behaves Jupiter-like for low and medium solar wind dynamic pressures and more Earth-like for high dynamic pressure.

Zieger, B.; Hansen, K. C.; Gombosi, T. I.

2009-04-01

322

NASA Astrophysics Data System (ADS)

A comprehensive study of the two-dimensional (2D) compass model on the square lattice is performed for classical and quantum spin degrees of freedom using Monte Carlo and quantum Monte Carlo methods. We employ state-of-the-art implementations using Metropolis, stochastic series expansion, and parallel tempering techniques to obtain the critical ordering temperatures and critical exponents. In a preinvestigation we reconsider the classical compass model where we study and contrast the finite-size scaling behavior of ordinary periodic boundary conditions against annealed boundary conditions. It is shown that periodic boundary conditions suffer from extreme finite-size effects which might be caused by closed-loop excitations on the torus. These excitations also appear to have severe effects on the Binder parameter. On this footing we report on a systematic Monte Carlo study of the quantum compass model. Our numerical results are at odds with recent literature on the subject which we trace back to neglecting the strong finite-size effects on periodic lattices. The critical temperatures are obtained as Tc=0.1464(2)J and Tc=0.055(1)J for the classical and quantum versions, respectively, and our data support a transition in the 2D Ising universality class for both cases.

Wenzel, Sandro; Janke, Wolfhard

2008-08-01

323

NASA Astrophysics Data System (ADS)

This paper presents a new dynamic model of equivalent circuit to simulate in the time-domain the effects of saturation and power losses in a nonlinear magnetic component. The parameters of the model are a nonlinear inductance and a nonlinear loss resistance that are computed via two-dimensional finite elements. The effectiveness of the model is analyzed in the case of a soft ferrite inductor excited by a sinusoidal voltage source at frequencies of 500 Hz and 40 kHz. The resulting voltage and current waveforms of the inductor taken in the laboratory are then compared with those computed via the PSIM circuit simulator. PSIM is a simulation software designed for power electronics, motor control, and dynamic system simulation.

Salas, R. A.; Pleite, J.

2010-05-01

324

NASA Astrophysics Data System (ADS)

Results of a new relativistic two-dimensional particle-in-cell code for simulation of laser-driven ion acceleration are presented. It is shown that a complex electric field structure arose during the laser–target interaction as well as the evolution of the ion and electron concentration for the laser-induced cavity pressure acceleration scheme took place. During computation the influence of parameters such as target thickness and width, target starting position inside cavity and its atomic composition, as well as laser pulse length and beam width are examined. Values adopted for the calculation reflect the experimental conditions.

Jab?o?ski, S?awomir

2014-05-01

325

NASA Astrophysics Data System (ADS)

The strength of perpendicular anisotropy is known to drive the spin reorientation in thin magnetic films. Here, we consider the effect different order anisotropies have on two phase transitions: the spin reorientation and the orientational order transitions. We find that the relative magnitude of different order anisotropies can significantly enhance or suppress the degree to which the system reorients. Specifically, Monte Carlo simulations reveal significant changes in the cone angle and planar magnetization. In order to facilitate rapid computation, we have developed a stream processing technique, suitable for use on graphics processing unit (GPU) systems, for computing the transition probabilities in two-dimensional systems with dipole interactions.

Ambrose, M. C.; Stamps, R. L.

2013-05-01

326

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

327

NASA Astrophysics Data System (ADS)

Ventricular fibrillation is a lethal arrhythmia characterized by multiple wavelets usually starting from a single or figure-of-eight re-entrant circuit. Understanding the factors regulating vulnerability to the re-entry is essential for developing effective therapeutic strategies to prevent ventricular fibrillation. In this study, we investigated how pre-existing tissue heterogeneities and electrical restitution properties affect the initiation of re-entry by premature extrastimuli in two-dimensional cardiac tissue models. We studied two pacing protocols for inducing re-entry following the ``sinus'' rhythm (S1) beat: (1) a single premature (S2) extrastimulus in heterogeneous tissue; (2) two premature extrastimuli (S2 and S3) in homogeneous tissue. In the first case, the vulnerable window of re-entry is determined by the spatial dimension and extent of the heterogeneity, and is also affected by electrical restitution properties and the location of the premature stimulus. The vulnerable window first increases as the action potential duration (APD) difference between the inside and outside of the heterogeneous region increases, but then decreases as this difference increases further. Steeper APD restitution reduces the vulnerable window of re-entry. In the second case, electrical restitution plays an essential role. When APD restitution is flat, no re-entry can be induced. When APD restitution is steep, re-entry can be induced by an S3 over a range of S1S2 intervals, which is also affected by conduction velocity restitution. When APD restitution is even steeper, the vulnerable window is reduced due to collision of the spiral tips.

Tran, Diana X.; Yang, Ming-Jim; Weiss, James N.; Garfinkel, Alan; Qu, Zhilin

2007-12-01

328

NASA Astrophysics Data System (ADS)

Cloud Resolving Models (CRMs) which are used increasingly to make operational forecasts, employ Bulk Microphysics Schemes (BMSs) to describe cloud microphysical processes. In this study two BMSs are employed in a new Nonhydrostatic ?-coordinate Model to perform two hour simulations of convection initiated by a warm bubble, using a horizontal grid resolution of 500 m. Different configurations of the two BMSs are applied, to test the effects of the presence of graupel with one scheme (2-configurations) and of changing the cloud droplet sizes in the second scheme (4-configurations), on the simulation of idealised thunderstorms. Maximum updrafts in all the simulations are similar over the first 40 minutes, but start to differ beyond this point. The first scheme simulates the development of a second convective cell that is triggered by the cold pool that develops from the outflow of the first storm. The cold pool is more intense in the simulation with graupel because of melting of graupel particles, which results in relatively large raindrops, decreases the temperature through latent heat absorption, causing stronger downdrafts, which all contribute to the formation of a more intense cold pool. The second scheme simulates the development of a second cell in two of its configurations, while two other configurations do not simulate the redevelopment. Two configurations that simulate the secondary redevelopment produce a slightly stronger cold pool just before redevelopment. Our results show that small differences in the microphysics formulations result in simulations of storm dynamics that diverge, possibly due nonlinearities in the model.

Bopape, Mary-Jane Morongwa; Engelbrecht, Francois Alwyn; Randall, David A.; Landman, Willem Adolf

2014-02-01

329

Time-domain simulation results of two-dimensional (2-D) planar waveguide finite-difference time-domain (FDTD) analysis are normally analyzed using Fourier transform. The introduced method of time series analysis to extract propagation and attenuation constants reduces the desired computation time drastically. Additionally, a nonequidistant discretization together with an adequate excitation technique is used to reduce the number of spatial grid points. Therefore, it is possible to reduce the number of spatial grid points. Therefore, it is possible to simulate normal- and superconducting planar waveguide structures with very thin conductors and small dimensions, as they are used in MMIC technology. The simulation results are compared with measurements and show good agreement.

Hofschen, S.; Wolff, I. [Gerhard Mercator Univ. of Duisburg (Germany). Dept. of Electrical Engineering] [Gerhard Mercator Univ. of Duisburg (Germany). Dept. of Electrical Engineering

1996-08-01

330

Dayside reconnection in 3D global Hall MHD numerical simulations

NASA Astrophysics Data System (ADS)

We investigate magnetic reconnection at the dayside magnetopause using three dimensional global resistive Hall MHD numerical simulations with the new code, Hall OpenGGCM. Runs are performed with constant spatially uniform resistivity and steady southward IMF conditions at various values of Lundquist number and ion-skin depth to determine scaling. Our results show that in the high Lundquist number limit, Hall physics can allow magnetic flux-pileup to be locally suppressed. The pileup scalings obtained are compared with the stagnation point flow solutions of Sonnerup and Priest [J. Plasma Phys., 14, 1975], and the Hall mediated flux pileup analysis of Dorelli [Phys. Plasmas, 10, 2003]. We also investigate how asymmetric reconnection manifests itself in 3D Hall simulations with particular attention to the 2D analysis of Cassak and Shay [Phys. Plasmas, 14, 2007]. While the theory appears to give reasonable predictions for the offset locations of the x-point and stagnation points, the expressions given for the reconnection electric field and outflow velocities do not agree with what we observe and likely require remediation to account for realistic global geometry. Much like what is observed in 2D collisionless reconnection studies, Hall physics in these global simulations gives rise to more compact dissipation regions with bifurcations in current density extending polewards (when viewed in the GSEx-GSEz plane) which bulge outwards into the magnetosheath. We note also that at larger Lundquist numbers, macroscopic dissipation region structures appear to filament along the flanks of the magnetopause due to the development of Kelvin-Helmholtz instability. The bearing of Hall physics on the relative frequency and character of poleward propagating flux transfer events is also discussed.

Lin, L.; Germaschewski, K.; Bhattacharjee, A.; Maynard, K.; Sullivan, B. P.; Raeder, J.

2012-12-01

331

MHD simulation of RF current drive in MST

NASA Astrophysics Data System (ADS)

Auxiliary heating and current drive using RF waves such as the electron Bernstein wave (EBW) promises to advance the performance of the reversed field pinch (RFP). In previous computational work [1], a hypothetical edge-localized current drive is shown to suppress the tearing activity which governs the macroscopic transport properties of the RFP. The ideal conditions for tearing stabilization include a reduced toroidal induction, and precise width and radial position of the Gaussian-shaped external current drive. In support of the EBW experiment on the Madison Symmetric Torus, an integrated modeling scheme now incorporates ray tracing and Fokker-Plank predictions of auxiliary current into single fluid MHD. Simulations at low Lundquist number (S ˜ 104) generally agree with the previous work; significantly more burdensome simulations at MST-like Lundquist number (S ˜ 3×106) show unexpected results. The effect on nonlinearly saturated current profile by a particular RF-driven external force decreases in magnitude and widens considerably as the Lundquist number increases toward experimental values. Simulations reproduce the periodic current profile relaxation events observed in experiment (sawteeth) in the absence of current profile control. Reduction of the tearing mode amplitudes is still observable; however, reduction is limited to periods between the large bursts of magnetic activity at each sawtooth. The sawtoothing pattern persists with up to 10 MW of externally applied RF power. Periods with prolonged low tearing amplitude are predicted with a combination of external current drive and a reduced toroidal loop voltage, consistent with previous conclusions. Finally, the resistivity profile is observed to have a strong effect on the optimal externally driven current profile for mode stabilization.

Hendries, E. R.; Anderson, J. K.; Diem, S.; Forest, C. B.; Harvey, R. W.; Reusch, J. A.; Seltzman, A. H.; Sovinec, C. R.

2014-02-01

332

We have developed a method that enables a fast and exact evaluation of the long-range interaction field by simulating the lattice dipolar systems with periodic boundary conditions. The method is based on the combination of the fast-Fourier-transformation technique and the modified Ewald method for the lattice sum calculation. We have used our algorithm for simulations of the quasistatic remagnetization processes

D. V. Berkov; N. L. Gorn

1998-01-01

333

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

334

NASA Astrophysics Data System (ADS)

Analysis of neutral particle behavior in the GAMMA 10 tandem mirror with open magnetic field and the Heliotron J device with non-axisymmetric plasma confining system is described based on the results of Monte-Carlo neutral transport simulation. Three-dimensional (3-D) Monte-Carlo code DEGAS was applied to both devices in order to investigate precisely the spatial distribution of neutral particle density. In GAMMA 10, a detailed structure of inner components of the central-cell is modeled in the simulation space. In Heliotron J, a carbon limiter inserted into the plasma for investigation of plasma-material interactions is also modeled in addition to the helically twisted vacuum vessel and plasma. In both devices, 2-D images of light-emission from the plasma captured with CCD cameras near the limiters are compared with the simulation results and a good agreement between experiment and simulation is recognized. Comparison of the decay length in the emissivity of H?/D? line-emission also showed that neutral transport in SOL region plays a significant influence on the emission profile.

Nakashima, Y.; Higashizono, Y.; Kawano, H.; Nishino, N.; Kobayashi, S.; Mizuuchi, T.; Shoji, M.; Nagasaki, K.; Okada, H.; Sano, F.; Kondo, K.; Yoneda, Y.; Yonenaga, R.; Yoshikawa, M.; Imai, T.

2009-06-01

335

National Technical Information Service (NTIS)

A 1-D-model has been developed from the 2-D-model of the University of Oxford in order to study the vertical distributions of 23 constituents or groups of constituents. The model is able to simulate the vertical profiles of the trace gases in a reasonable...

D. Goemer

1983-01-01

336

The Lanczos algorithm (LA) is a useful iterative method for the reduction of a large matrix to tridiagonal form. It is a storage efficient procedure requiring only the preceding two Lanczos vectors to compute the next. The quasi-minimal residual (QMR) method is a powerful method for the solution of linear equation systems, Ax = b. In this report we provide another application of the QMR method: we incorporate QMR into the LA to monitor the convergence of the Lanczos projections in the reduction of large sparse matrices. We demonstrate that the combined approach of the LA and QMR can be utilized efficiently for the orthogonal transformation of large, but sparse, complex, symmetric matrices, such as are encountered in the simulation of slow-motional 1D- and 2D-electron spin resonance (ESR) spectra. Especially in the 2D-ESR simulations, it is essential that we store all of the Lanczos vectors obtained in the course of the LA recursions and maintain their orthogonality. In the LA-QMR application, the QMR weight matrix mitigates the problem that the Lanczos vectors lose orthogonality after many LA projections. This enables substantially more Lanczos projections, as required to achieve convergence for the more challenging ESR simulations. It, therefore, provides better accuracy for the eigenvectors and the eigenvalues of the large sparse matrices originating in 2D-ESR simulations than does the previously employed method, which is a combined approach of the LA and the conjugate-gradient (CG) methods, as evidenced by the quality and convergence of the 2D-ESR simulations. Our results show that very slow-motional 2D-ESR spectra at W-band (95 GHz) can be reliably simulated using the LA-QMR method, whereas the LA-CG consistently fails. The improvements due to the LA-QMR are of critical importance in enabling the simulation of high-frequency 2D-ESR spectra, which are characterized by their very high resolution to molecular orientation.

Chiang, Yun-Wei; Freed, Jack H.

2011-01-01

337

A two-dimensional, three-velocity particle-in-cell with Monte Carlo collisions (MCC) gun code is under development at LANSCE to study surface conversion H{sup -} ion source physics from a first-principles approach. The code is electromagnetostatic and is based on a sine transform/tridiagonal matrix Poisson solver implemented in axisymmetric geometry. A filament discharge is simulated by the injection of seed electrons at a biased filament surface. These electrons drive the discharge through a collection of (presently 22) reactions implemented as a MCC package. The simulation region is rectangular with internal electrodes of arbitrary shape. Given the nature of the code, no ad hoc models are necessary to model sheath physics, diffusion across magnetic-field lines, ion extraction, or other plasma phenomena.

Chacon-Golcher, Edwin; Bowers, Kevin J. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2006-03-15

338

NASA Astrophysics Data System (ADS)

Laser–plasma interaction and hot electron generation play a crucial role in the context of inertial confinement fusion and in particular in the shock-ignition concept. Here we present a fully kinetic large-scale two-dimensional simulation studying laser–plasma interaction and hot electron generation in a relatively long and hot coronal plasma. The simulation shows saturation of the reflectivity of an intense spike pulse and absorption taking place close to a quarter critical density in particular, due to cavitation and stimulated Raman scattering. The signatures of steady two-plasmon decay are observed, but the hot electron number produced by this instability is low in comparison with the other two processes. The spectral and angular distribution of the back-scattered light is presented and the energy and angular characteristics of hot electrons due to individual absorption processes are studied.

Klimo, O.; Psikal, J.; Tikhonchuk, V. T.; Weber, S.

2014-05-01

339

NASA Astrophysics Data System (ADS)

We simulate, via a discrete element method, the tapping of a narrow column of disks under gravity. For frictionless disks, this system has a simple analytical expression for the density of states in the Edwards volume ensemble. We compare the predictions of the ensemble at constant compactivity against the results for the steady states obtained in the simulations. We show that the steady states cannot be properly described since the microstates sampled are not in correspondence with the predicted distributions, suggesting that the postulates of flat measure and ergodicity are, either or both, invalid for this simple realization of a static granular system. However, we show that certain qualitative features of the volume fluctuations which are difficult to predict from simple arguments are captured by the theory.

Irastorza, Ramiro M.; Carlevaro, C. Manuel; Pugnaloni, Luis A.

2013-12-01

340

NASA Technical Reports Server (NTRS)

We have adopted the transport scenarios used in Part 1 to examine the sensitivity of stratospheric aircraft perturbations to transport changes in our 2-D model. Changes to the strength of the residual circulation in the upper troposphere and stratosphere and changes to the lower stratospheric K(sub zz) had similar effects in that increasing the transport rates decreased the overall stratospheric residence time and reduced the magnitude of the negative perturbation response in total ozone. Increasing the stratospheric K(sub yy) increased the residence time and enhanced the global scale negative total ozone response. However, increasing K(sub yy) along with self-consistent increases in the corresponding planetary wave drive, which leads to a stronger residual circulation, more than compensates for the K(sub yy)-effect, and results in a significantly weaker perturbation response, relative to the base case, throughout the stratosphere. We found a relatively minor model perturbation response sensitivity to the magnitude of K(sub yy) in the tropical stratosphere, and only a very small sensitivity to the magnitude of the horizontal mixing across the tropopause and to the strength of the mesospheric gravity wave drag and diffusion. These transport simulations also revealed a generally strong correlation between passive NO(sub y) accumulation and age of air throughout the stratosphere, such that faster transport rates resulted in a younger mean age and a smaller NO(y) mass accumulation. However, specific variations in K(sub yy) and mesospheric gravity wave strength exhibited very little NO(sub y)-age correlation in the lower stratosphere, similar to 3-D model simulations performed in the recent NASA "Models and Measurements" II analysis. The base model transport, which gives the most favorable overall comparison with inert tracer observations, simulated a global/annual mean total ozone response of -0.59%, with only a slightly larger response in the northern compared to the southern hemisphere. For transport scenarios which gave tracer simulations within some agreement with measurements, the annual/globally averaged total ozone response ranged from -0.45% to -0.70%. Our previous 1995 model exhibited overly fast transport rates, resulting in a global/annually averaged perturbation total ozone response of -0.25%, which is significantly weaker compared to the 1999 model. This illustrates how transport deficiencies can bias model simulations of stratospheric aircraft.

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

1999-01-01

341

NASA Astrophysics Data System (ADS)

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 is demonstrated for the numerical simulation of the five model flows with a standard high-resolution finite volume numerical MHD code on structured body-fitted grids. In addition, one model flow is presented which is not field-aligned, and it is discussed how grid convergence can be studied for this flow. By formal grid convergence studies of magnetic flux conservation and other flow quantities, it is investigated whether the Powell source term approach to controlling the ?· B constraint leads to correct results for the class of flows under consideration.

De Sterck, H.; Csík, A.; Vanden Abeele, D.; Poedts, S.; Deconinck, H.

2001-01-01

342

In Chap. 5 is given basic information concerning two-dimensional distributions of random variables. Starting from a classical\\u000a problem of the accuracy of artillery fire it is shown that, besides a traditional analytical procedure, components of a covariance\\u000a tensor may be transformed by means of their representation by Mohr circles. It is shown that this representation, used commonly\\u000a in mechanics of solids,

Zbigniew Kotulski; Wojciech Szczepi?ski

343

This article describes a computer simulation of the geometrical and electronic structure of a quasi-two-dimensional carbon layer with a trigonal lattice consisting of fullerenes C36 (1) with topological symmetry D6h. Every polyhedral cluster 1 of this polymeric layer (2) is surrounded by six similar fullerenes and connected with every such a fullerene by two covalent bonds. Atomic coordinates of the repeating unit are estimated on the basis of MNDO/PM3 calculations of hydrocarbon molecule (D6h)-C132H48 (3). The carbon skeleton of 3 coincides with a sufficiently large fragment of the polymeric layer 2. The electronic spectrum of the quasi-two-dimensional layer 2 is calculated by the crystalline orbital method in the EHT approximation. The band gap in the electronic spectrum of 2 was found to be equal to 1.5 eV. The geometric and electronic structure of some oligomers of cluster C36, quasi-linear macromolecule [C36]n, and "hypergraphite" layer is also discussed. PMID:11391867

Gal'pern, E G; Stankevich, I V; Chistyakov, A L; Chernozatonskii, L A

2001-01-01

344

Global MHD Simulations of the Substorm Current Wedge and Dipolarization

NASA Technical Reports Server (NTRS)

This paper presents results from global MHD simulations showing the evolution of the plasma and field in the near-Earth tail during the substorm phases. The late growth phase is characterized by pronounced thinning of the plasma sheet and stretching of the field in the region between approximately -6 R(sub E) to -30 R(sub E). A pre-existing X-line moves tailward to beyond -50 R(sub E). Close to onset, a new X-line forms near -18 R(sub E) in the midnight sector. Earthward flows emanating from this X-line dipolarize the near-Earth field, leading to a reduction of the cross-tail current in the midnight sector, but not elsewhere. The magnetic shear between the dipolarized field near midnight and the stretched field elsewhere is equivalent to currents flowing through the ionosphere in a region I sense, and so forming the current wedge. Later in the expansion phase, the dipolarization spreads in local time at a rate of about 0.3 hours MLT per minute. A strong electric field and a rapid increase of the plasma pressure is associated with the dipolarization. Near midnight the dipolarization appears to occur at all distances between 6.6 and 13 R(sub E) at the same time within the resolution (+/- 2 min) of our model. However, the model results indicate that dipolarization starts before ground onset in the pre-midnight sector and propagates both earthward and eastward.THus, dipolarization may be much more complex than simple earthward/tailward and/or azimuthal expansion.

Raeder, J.; McPherron R. L.

1998-01-01

345

NASA Technical Reports Server (NTRS)

In this study, we examine the sensitivity of long lived tracers to changes in the base transport components in our 2-D model. Changes to the strength of the residual circulation in the upper troposphere and stratosphere and changes to the lower stratospheric K(sub zz) had similar effects in that increasing the transport rates decreased the overall stratospheric mean age, and increased the rate of removal of material from the stratosphere. Increasing the stratospheric K(sub yy) increased the mean age due to the greater recycling of air parcels through the middle atmosphere, via the residual circulation, before returning to the troposphere. However, increasing K(sub yy) along with self-consistent increases in the corresponding planetary wave drive, which leads to a stronger residual circulation, more than compensates for the K(sub yy)-effect, and produces significantly younger ages throughout the stratosphere. Simulations with very small tropical stratospheric K(sub yy) decreased the globally averaged age of air by as much as 25% in the middle and upper stratosphere, and resulted in substantially weaker vertical age gradients above 20 km in the extratropics. We found only very small stratospheric tracer sensitivity to the magnitude of the horizontal mixing across the tropopause, and to the strength of the mesospheric gravity wave drag and diffusion used in the model. We also investigated the transport influence on chemically active tracers and found a strong age-tracer correlation, both in concentration and calculated lifetimes. The base model transport gives the most favorable overall comparison with a variety of inert tracer observations, and provides a significant improvement over our previous 1995 model transport. Moderate changes to the base transport were found to provide modest agreement with some of the measurements. Transport scenarios with residence times ranging from moderately shorter to slightly longer relative to the base case simulated N2O lifetimes that were within the observational estimates of Volk et al. [1997]. However, only scenarios with rather fast transport rates were comparable with the Volk et al. estimates of CFCl3 lifetimes. This is inconsistent with model-measurement comparisons of mean age in which the base model or slightly slower transport rates compared the most favorably with balloon SF6 data. For all comparisons shown, large transport changes away from the base case resulted in simulations that were outside the range of measurements, and in many cases, far outside this range.

Fleming, Eric L.; Jackman, Charles H.; Considine, David B.; Stolarski, Richard S.

1999-01-01

346

NASA Astrophysics Data System (ADS)

We report for the first time on full 2-D radiation-hydrodynamic implosion simulations that explore the impact of highly compressed imposed magnetic fields on the ignition and burn of perturbed spherical implosions of ignition-scale cryogenic capsules. Using perturbations that highly convolute the cold fuel boundary of the hotspot and prevent ignition without applied fields, we impose initial axial seed fields of 20-100 T (potentially attainable using present experimental methods) that compress to greater than 4 × 104 T (400 MG) under implosion, thereby relaxing hotspot areal densities and pressures required for ignition and propagating burn by ~50%. The compressed field is high enough to suppress transverse electron heat conduction, and to allow alphas to couple energy into the hotspot even when highly deformed by large low-mode amplitudes. This might permit the recovery of ignition, or at least significant alpha particle heating, in submarginal capsules that would otherwise fail because of adverse hydrodynamic instabilities.

Perkins, L. J.; Logan, B. G.; Zimmerman, G. B.; Werner, C. J.

2013-07-01

347

The triangular lattice model with nearest-neighbor attraction and third-neighbor repulsion, introduced by P?kalski, Ciach, and Almarza [J. Chem. Phys. 140, 114701 (2014)] is studied by Monte Carlo simulation. Introduction of appropriate order parameters allowed us to construct a phase diagram, where different phases with patterns made of clusters, bubbles or stripes are thermodynamically stable. We observe, in particular, two distinct lamellar phases-the less ordered one with global orientational order and the more ordered one with both orientational and translational order. Our results concern spontaneous pattern formation on solid surfaces, fluid interfaces or membranes that is driven by competing interactions between adsorbing particles or molecules. PMID:24784300

Almarza, N G; P?kalski, J; Ciach, A

2014-04-28

348

We report for the first time on full 2-D radiation-hydrodynamic implosion simulations that explore the impact of highly compressed imposed magnetic fields on the ignition and burn of perturbed spherical implosions of ignition-scale cryogenic capsules. Using perturbations that highly convolute the cold fuel boundary of the hotspot and prevent ignition without applied fields, we impose initial axial seed fields of 20–100 T (potentially attainable using present experimental methods) that compress to greater than 4 × 10{sup 4} T (400 MG) under implosion, thereby relaxing hotspot areal densities and pressures required for ignition and propagating burn by ?50%. The compressed field is high enough to suppress transverse electron heat conduction, and to allow alphas to couple energy into the hotspot even when highly deformed by large low-mode amplitudes. This might permit the recovery of ignition, or at least significant alpha particle heating, in submarginal capsules that would otherwise fail because of adverse hydrodynamic instabilities.

Perkins, L. J.; Logan, B. G.; Zimmerman, G. B.; Werner, C. J. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)] [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)

2013-07-15

349

NASA Astrophysics Data System (ADS)

The critical behavior of long straight rigid rods of length k (k-mers) on square and triangular lattices at intermediate density has been studied. A nematic phase, characterized by a big domain of parallel k-mers, was found. This ordered phase is separated from the isotropic state by a continuous transition occurring at an intermediate density ?c. Two analytical techniques were combined with Monte Carlo simulations to predict the dependence of ?c on k, being ?c(k)~k-1. The first involves simple geometrical arguments, while the second is based on entropy considerations. Our analysis allowed us also to determine the minimum value of k (kmin=7), which allows the formation of a nematic phase on a triangular lattice.

Matoz-Fernandez, D. A.; Linares, D. H.; Ramirez-Pastor, A. J.

2008-06-01

350

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

351

NASA Astrophysics Data System (ADS)

The Earth is 3 dimensional (3-D), but numerical simulations of wave propagation through laterally heterogeneous media are easier to formulate and more efficient to use in 2-D. Schemes to model seismic wave propagation through laterally varying structures with 2-D numerical algorithms are developed and applied to earthquake and explosion problems. 2-D source expressions that have the same radiation patterns as their 3-D counterparts are derived. Synthetic seismograms are constructed for the strong motions of the 1968 Borrego Mountain earthquake recorded at El Centro. A good fit to the data results from using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling. The effect of shallow station structure and lateral velocity variation are investigated for records of the Amchitka blast Milrow. Synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finite difference and Kirchhoff method, for a 515 km deep earthquake. Slabs with only 2 to 3 percent P-wave velocity anomaly or slabs that become considerably thicker with depth are consistent with the data.

Vidale, John Emilio

352

A major concern in the nuclear power industry is failure of the steam generator tubes. Failure of the tubes necessitates the plugging of the failed tubes with the result that nuclear plants are forced to operate at lower, or derated, power levels after expensive repairs. Turbulence-induced vibration is a primary cause of premature and accelerated fretting and wear of the steam generator tubes. An alternative unsteady analysis method for incompressible fluid flow problems is demonstrated. The approach employs large eddy simulation (LES) in conjunction with the finite element method (FEM). A segregated solution technique, solving for each field variable at all nodes, diminishes storage requirements by eliminating the need to solve the globally assembled finite element matrix. A direct benefit is that finer nodalizations can be employed. Equal-order quadrilateral elements are used to facilitate the segregated solution algorithm. The solution scheme is accurate to higher order to mitigate the effects of numerical diffusion in the advection terms. The Smagorinsky-type closure model for the sub-grid scale turbulence is used. The model is easily implemented into this algorithm. This combination of FEM and LES is unique. The time-dependent terms are explicitly treated. The time history of a steam generator tube bundle experiment is studied. The results show the applicability of FEM/LES and determine the prospects for further development of this methodology.

Davis, F.J. Jr.; Hassan, Y.A. (Texas A and M Univ., College Station, TX (United States). Coll. of Engineering)

1994-04-01

353

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

354

NASA Astrophysics Data System (ADS)

The present study addresses the capability of a large set of shock-capturing schemes to recover the basic interactions between acoustic, vorticity and entropy in a direct numerical simulation (DNS) framework. The basic dispersive and dissipative errors are first evaluated by considering the advection of a Taylor vortex in a uniform flow. Two transonic cases are also considered. The first one consists of the interaction between a temperature spot and a weak shock. This test emphasizes the capability of the schemes to recover the production of vorticity through the baroclinic process. The second one consists of the interaction of a Taylor vortex with a weak shock, corresponding to the framework of the linear theory of Ribner. The main process in play here is the production of an acoustic wave. The results obtained by using essentially non-oscillatory (ENO), total variation diminishing (TVD), compact-TVD and MUSCL schemes are compared with those obtained by means of a sixth-order accurate Hermitian scheme, considered as reference. The results are as follows; the ENO schemes agree pretty well with the reference scheme. The second-order accurate Upwind-TVD scheme exhibits a strong numerical diffusion, while the MUSCL scheme behavior is very sensitive to the value on the parameter in the limiter function minmod. The compact-TVD schemes do not yield improvement over the standard TVD schemes. Copyright

Tenaud, C.; Garnier, E.; Sagaut, P.

2000-05-01

355

NASA Astrophysics Data System (ADS)

A hybrid method to model the shear (SH) wave scattering from 2-D fractures embedded in a heterogeneous medium is developed by coupling the boundary element method (BEM) and the finite different method (FDM) in the frequency domain. FDM is used to propagate an SH wave from a source through heterogeneities to fractures embedded in small local homogeneous domains surrounded by artificial boundaries. According to Huygens' Principle, the points at these artificial boundaries can be regarded as `secondary' sources and their amplitudes are calculated by FDM. Given the incident fields from these point sources, BEM is applied to model the scattering from fractures and propagate them back to the artificial boundaries. FDM then continues propagating the scattered field into the heterogeneous medium by taking the scattered field at the boundaries as `secondary' sources. A numerical iterative scheme is also developed to account for the multiple scattering between different sets of fractures. The hybrid method can calculate scattering from different fractures very fast, thus Monte Carlo simulations for characterizing the statistics of fracture attributes can be performed efficiently. To verify the hybrid method, we compared scattering from multiple fractures embedded in a homogeneous space by our method and a pure BEM; also, we compared our method with the time-domain finite-difference method for vertical fractures in a layered medium. Good agreements are found. The hybrid method is also applied to calculate the wave scattered from fractures embedded in complex media.

Chen, Tianrun; Li, Junlun; Toksöz, Nafi

2013-09-01

356

NASA Astrophysics Data System (ADS)

New computational techniques are developed to study conformational equilibrium in helical peptides at finite temperature. Two dimensional umbrella sampling methods involving coordinate systems which incorporate the ?-helical hydrogen bonding distances along the peptide backbone as collective variables are presented. The umbrella sampling techniques are combined with new extended system multiple time step molecular dynamics simulation methods to allow peptide equilibria to be examined on a realistic potential energy surface. In particular, this combination of methods is employed to study a sixteen residue alanine-based peptide, Ac-(AAAAK)3A-NH2, at T=300 K in vacuo and solution, under the all-atom CHARMM22 force field. The efficiency of the multiple time step integration methodology permitted a minimum total run length of 30 ns to be employed in each umbrella sampling calculation reported.

Samuelson, S. O.; Martyna, G. J.

1998-12-01

357

NASA Technical Reports Server (NTRS)

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. (inventor)

1992-01-01

358

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.

359

Daily Coronal MHD Simulation Using HMI Near-Real-Time Magnetograms

NASA Astrophysics Data System (ADS)

SDO/HMI is making full-disk line-of-sight magnetogram measurements with a cadence of 45 seconds. The HMI analysis pipeline regularly generates two types of synoptic map of the solar surface magnetic field. Definitive calibrated data maps are created every Carrington Rotation, about every 27 days and a preliminary synoptic map is updated on a near-real-time basis. As an application of the near-real-time data, we have been running a daily MHD simulation of the global solar corona using the photospheric map as the boundary condition ( http://hmi.stanford.edu/MHD ). The daily MHD model assumes a polytropic gas with the specific heat ratio of 1.05, and the simulation is conducted in a 4-pi spherical grid system with latitudinal and longitudinal grid sizes of pi/64. The output available at hmi.stanford.edu/MHD includes the three-dimensional volume data, the shape of the open-field regions corresponding to the coronal holes, and the LoS-integration of the coronal density mimicking coronagraph observations. For validation, we compare the results of the low-resolution daily MHD simulation and the high-resolution PFSS calculation with SDO/AIA and SOHO/C2 and C3 image data. In the future the simulation region will be extended to 1 AU, and models of coronal heating and acceleration will be applied to allow a timely prediction of solar wind at the Earth for space weather purposes.

Hayashi, Keiji; HMI Team

2012-05-01

360

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

A 2-D resistive MHD (magnetohydrodynamic) code is used 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. Rail ablation is calculated, taking into account the flow of heat into the interior of the rails. Simulations show the development of

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

1993-01-01

361

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

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

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

1993-01-01

362

Cold dense magnetopause boundary layer under northward IMF: Results from THEMIS and MHD simulations

A layer of nearly stagnant cold dense plasma is observed by THEMIS spacecraft in a closed field region immediately inside the dayside magnetopause near the low-latitude boundary layer on 3 June 2007. Using the OpenGGCM global MHD magnetosphere numerical model, we successfully reproduce this observed cold dense plasma layer in the simulation. The simulation results show that reconnection first occurs

Wenhui Li; Joachim Raeder; Marit Øieroset; Tai D. Phan

2009-01-01

363

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, E-mail: maire@celia.u-bordeaux1.fr [CEA/CESTA, 15 Avenue des Sablières, CS 60001 33116 Le Barp cedex (France)] [CEA/CESTA, 15 Avenue des Sablières, CS 60001 33116 Le Barp cedex (France); Abgrall, Rémi, E-mail: remi.abgrall@math.u-bordeau1.fr [INRIA and Univ. Bordeaux, F-33405 Talence (France)] [INRIA and Univ. Bordeaux, F-33405 Talence (France); Breil, Jérôme, E-mail: breil@celia.u-bordeaux1.fr [Univ. Bordeaux, CEA, CNRS, CELIA, UMR5107, F-33400 Talence (France)] [Univ. Bordeaux, CEA, CNRS, CELIA, UMR5107, F-33400 Talence (France); Loubère, Raphaël, E-mail: raphael.loubere@math.univ-toulouse.fr [CNRS Institut de Mathématiques de Toulouse, 31062 Toulouse (France)] [CNRS Institut de Mathématiques de Toulouse, 31062 Toulouse (France); Rebourcet, Bernard, E-mail: bernard.rebourcet@cea.fr [CEA/DAM Ile de France, 91 297 Arpajon cedex (France)] [CEA/DAM Ile de France, 91 297 Arpajon cedex (France)

2013-02-15

364

2D MHD test-particle simulations in modeling geomagnetic storms

NASA Astrophysics Data System (ADS)

The effects of magnetic storms on the evolution of the electron radiation belts are studied using MHD test-particle simulations. The 2D guiding center code developed by Elkington et al. (2002) has been used to simulate particle motion in the Solar Magnetic equatorial plane in the MHD fields calculated from the Lyon-Fedder-Mobarry global MHD code. However, our study shows that the B-minimum plane is well off the SM equatorial plane during solstice events. Since 3D test-particle simulation is computationally expensive, we improve the 2D model by pushing particles in the B-minimum surface instead of the SM equatorial plane. Paraview software is used to visualize the LFM data file and to find the B-minimum surface. Magnetic and electric fields on B-minimum surface are projected to the equatorial plane for particle pushing.

Li, Z.; Elkington, S. R.; Hudson, M. K.; Murphy, J. J.; Schmitt, P.; Wiltberger, M. J.

2012-12-01

365

Two-dimensional modeling of the formation of spheromak configurations

A reduced set of two-dimensional MHD equations have been derived describing the axisymmetric time evolution of a MHD stable plasma evolving slowly due to resistive diffusion and changing boundary conditions. The equations are restricted to low ..beta.. but allow changing topology. They are integrated in time to demonstrate a possible spheromak formation method. External circuit equations are integrated simultaneously with the plasma equations to determine the electromagnetic boundary conditions self consistently. The effects of a finite conductivity vacuum chamber are included.

Jardin, S.C.; Park, W.

1980-09-01

366

Multidimensional MHD Simulations Of DSA Using AstroBEAR

NASA Astrophysics Data System (ADS)

We present a modification to the AstroBEAR (Astronomical Boundary Embedded Adaptive Refinement) MHD code (Cunningham et. al. 2007) that allows it to treat time dependent Diffusive Shock Acceleration (DSA) of cosmic rays in multiple dimensions including dynamical feedback from the cosmic rays. Utilizing the power of Adaptive Mesh Refinement (AMR) in tandem with efficient methods for cosmic ray diffusion and advection, this allows us for the first time to explore the evolution of modified MHD shocks in more than one spatial dimension. Among the early applications of the code will be investigations of colliding and clumpy stellar winds, type II supernova remnants and cosmic ray driven instabilities. This work is supported at the University of Minnesota by NSF, NASA and the Minnesota Supercomputing Institute.

Edmon, Paul; Jones, T.; Mitran, S.; Cunningham, A.; Frank, A.

2009-05-01

367

NASA Astrophysics Data System (ADS)

We employ a two-dimensional numerical model with interacting nested domains to simulate the evolution of a small nonprecipitating cumulus cloud in the absence of shear. Grid nesting permits the use of a realistic boundary layer forcing to initiate cloud growth and, at the same time, the specification of very high spatial resolution in the vicinity of the cloud. The finest mesh employed in this study (5 m) gives about 160 points across the base of the cloud. Initially, the model produces a cloud which has a smooth upper surface. About eight to nine minutes after the onset of condensation, nodes appear on the upper cloud boundary. These nodes have a characteristic tangential length scale which is small compared to the width of the cloud base. In one of our simulations, a down-draft forms above the center of the cloud top and penetrates into the interior of the cloud. The entrainment of this unsaturated air reduces the liquid water content of the cloud below the adiabatic value and curtails growth of the cloud. In the present series of simulations, a penetrative downdraft is observed to form only in a cloud which develops a particular configuration of boundary nodes, a characteristic which is probably due to the assumed environmental conditions. Experiments were performed to assess the role which eddy mixing plays in the formation of the nodes and the entrainment process. It was found that while eddy mixing does not significantly affect the early nodal development, it does tend to inhibit the penetration of the downdraft. Our simulations indicate that entrainment in a growing cumulus is a well-ordered laminar phenomenon driven by inviscid dynamical processes rather than a turbulent phenomenon driven by mixing.

Klaassen, Gary P.; Clark, Terry L.

1985-12-01

368

Possible generation mechanisms of the Pi2 pulsations estimated from a global MHD simulation

NASA Astrophysics Data System (ADS)

The plasmaspheric virtual resonance (PVR) and the transient Alfvén wave bouncing between the ionospheres in both hemispheres (the transient response, TR) are regarded as the possible generation mechanisms of the Pi2 pulsations. However, the global MHD simulation of a substorm (Tanaka et al., 2010) did not reproduce such wave modes because of insufficient ionospheric reflection of the Alfvén wave, numerical transfer of the Alfvén wave across the field lines, and no plasmasphere. Furthermore, it is noted that the substorm current wedge (SCW) which is a driver of the TR is not reproduced in the global MHD simulation. In this study, we search the sources of the Pi2 pulsations in the global MHD simulation, namely, the compressional wave in the inner magnetosphere for the PVR and the Alfvén wave injected to the ionosphere for the TR. In conclusion, there appears a compressional signal in the inner magnetosphere when the high-speed Earthward flow at the substorm onset surges in the inner edge of the plasma sheet. This simulation result suggests that this compressional wave would be trapped in the plasmasphere as the PVR if the model has the plasmasphere. As for TR, the global MHD simulation provides suddenly increasing field-aligned current (the Alfvén wave) associated with sudden appearance of the shear flow which comes from the high-speed flow in the plasma sheet at the onset of the substorm. If the global MHD simulation correctly lets the Alfvén wave be reflected in the ionosphere and transmitted along the field line, the TR would be established. As the ballooning instability is regarded as one of candidates of the Pi2 pulsation sources, we also briefly investigate whether the simulated plasma sheet in the growth phase is unstable or not for the ballooning instability.

Fujita, S.; Tanaka, T.

2013-05-01

369

A 3D Global MHD Simulation of Saturn's Magnetosphere

NASA Astrophysics Data System (ADS)

We present the results of a three dimensional global magnetohydrodynamic (MHD) model of the magnetosphere of Saturn. The model represents the interaction of a magnetized solar wind with a fast rotating, magnetized planet and includes planetary rotation as well as a simplified model of the neutral torus produced by Titan. This neutral cloud acts as a plasma source inside the magnetosphere and can strongly affect plasma densities. We present steady-state solutions for three different interplanetary magnetic field (IMF) conditions and show that the interaction of corotational flow of the inner magnetosphere with the solar wind driven flow depends strongly on the IMF configuration.

Hansen, K. C.; Gombosi, T. I.; Dezeeuw, D. L.; Groth, C. P. T.; Powell, K. G.

370

NASA Astrophysics Data System (ADS)

The influence of an applied magnetic field on plasma-related devices has a wide range of applications. Its effects on a plasma have been studied for years; however, there are still many issues that are not understood well. This paper reports a detailed kinetic study with the two-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo collision method on the role of E×B drift in a capacitive argon discharge, similar to the experiment of You et al. [Thin Solid Films 519, 6981 (2011)]. The parameters chosen in the present study for the external magnetic field are in a range common to many applications. Two basic configurations of the magnetic field are analyzed in detail: the magnetic field direction parallel to the electrode with or without a gradient. With an extensive parametric study, we give detailed influences of the drift on the collective behaviors of the plasma along a two-dimensional domain, which cannot be represented by a 1 spatial and 3 velocity dimensions model. By analyzing the results of the simulations, the occurring collisionless heating mechanism is explained well.

Fan, Yu; Zou, Ying; Sun, Jizhong; Stirner, Thomas; Wang, Dezhen

2013-10-01

371

Two-dimensional unsteady flows of homogeneous density in estuaries and embayments can be described by hyperbolic, quasi-linear partial differential equations involving three dependent and three independent variables. A linear combination of these equations leads to a parametric equation of characteristic form, which consists of two parts: total differentiation along the bicharacteristics and partial differentiation in space. For its numerical solution, the specified-time-interval scheme has been used. The unknown, partial space-derivative terms can be eliminated first by suitable combinations of difference equations, converted from the corresponding differential forms and written along four selected bicharacteristics and a streamline. Other unknowns are thus made solvable from the known variables on the current time plane. The computation is carried to the second-order accuracy by using trapezoidal rule of integration. Means to handle complex boundary conditions are developed for practical application. Computer programs have been written and a mathematical model has been constructed for flow simulation. The favorable computer outputs suggest further exploration and development of model worthwhile. (Woodard-USGS)

Lai, Chintu

1977-01-01

372

NASA Astrophysics Data System (ADS)

The ion entry into the wake behind an obstacle in the solar wind is studied using two-dimensional, electromagnetic full-particle simulations. A significant difference is found between the number of ions and electrons in the near wake, mainly due to the negative electric charge on the nightside surface of the obstacle. The ion acceleration toward the void is observed far from the position of the rarefaction wave expected in the self-similar solution. The velocity profile of the ions in the wake approaches the self-similar solution with finite ion temperature asymptotically until they reach the distance where the ions from both sides of the wake meet. The ions that entered from both sides are accelerated in the opposite directions. They raise the ion temperature and the pressure in the center of the wake, although each component remains cool when treated separately. The electron temperature appears nearly constant, except for the edge of the complete void of electrons. The large-scale obstacle and a slow solar wind are favorable conditions for a detection of well-accelerated ions near the nightside surface of the obstacle, because they have enough time to accelerate. The direction of the electric field in the wake seems consistent with the gradient of the electron pressure.

Nakagawa, Tomoko

2013-05-01

373

NASA Astrophysics Data System (ADS)

In order to study the growth of diamond films over a large area in a traditional hot-filament chemical vapour deposition (HFCVD) reactor, two-dimensional mathematical models were first developed to investigate the temperature fields of the reactor walls, which made significant contributions to thermal round-flow of the reactant gases under different energy transfer systems. The set of partial differential equations involved in the thermal conduction system was solved with different boundary conditions by the finite control volume method. Numerical simulations showed that the temperature space distributions were heterogeneous when thermal radiation was assumed to be the only mechanism in heat transfer from the filaments to the reactor walls. However, taking into account the effects of thermal conduction under adiabatic and different isothermal temperature boundary conditions, the temperature uniformities improved greatly. In addition, thermal convection did not affect the temperature distributions but only increased the total temperature of the reactor wall. These results not only give insight into the dominant reasons resulting in low nucleation density and low growth rate of diamond films, but also provide a basis for the design of industrial HFCVD reactors to obtain high-quality diamond films over a large area.

Wang, A. Y.; Sun, C.; Cao, H. T.; Ji, A. L.; Huang, R. F.; Wen, L. S.

2004-03-01

374

NASA Astrophysics Data System (ADS)

We employ an efficient list-based kinetic Monte Carlo (KMC) method to study traffic flow models on one-dimensional (1D) and two-dimensional (2D) lattices based on the exclusion principle and Arrhenius microscopic dynamics. This model implements stochastic rules for cars' movements based on the configuration of the traffic ahead of each car. In particular, we compare two different look-ahead rules: one is based on the distance from the car under consideration to the car in front of it, and the other one is based on the density of cars ahead. The 1D numerical results of these two rules suggest different coarse-grained macroscopic limits in the form of integro-differential Burgers equations. The 2D results of both rules exhibit a sharp phase transition from freely flowing to fully jammed, as a function of the initial density of cars. However, the look-ahead rule based on the density of the traffic produces more realistic results. The KMC simulations reported in this paper are compared with those from other well-known traffic flow models and the corresponding empirical results from real traffic.

Sun, Yi; Timofeyev, Ilya

2014-05-01

375

At Saturn, both the external (the solar wind) and the internal (the planet's rotation and internal plasma source) conditions play an important role in affecting the global structure and dynamics of the magnetosphere. We have used 3D global MHD simulations to investigate the global configuration and dynamics of Saturn's mass-loaded magnetosphere under different solar wind conditions. Our present model (BATSRUS)

X. Jia; K. C. Hansen; T. I. Gombosi; M. G. Kivelson; G. Toth; D. de Zeeuw; A. J. Ridley

2010-01-01

376

Dynamics of Saturn's magnetotail under different solar wind conditions: 3D global MHD simulations

As revealed by the remote observations from the Hubble Space Telescope and in-situ observations from Cassini, Saturn's magnetosphere responds strongly to the solar wind driving. We have used a 3D global MHD simulation to investigate in detail the global configuration and dynamics of Saturn's magnetosphere in response to changes in the solar wind. Compared with our previously used global models

Xianzhe Jia; Kenneth Hansen; Aaron Ridley; Darren Dezeeuw; Tamas Gombosi

2010-01-01

377

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

378

Four different probabilistic risk assessment methods were compared using the data from the Sangamo Weston\\/Lake Hartwell Superfund site. These were one-dimensional Monte Carlo, two-dimensional Monte Carlo considering uncertainty in the concentration term, two-dimensional Monte Carlo considering uncertainty in ingestion rate, and microexposure event analysis. Estimated high-end risks ranged from 2.0×10 to 3.3×10. Microexposure event analysis produced a lower risk estimate

Ted W. Simon

1999-01-01

379

NASA Astrophysics Data System (ADS)

Geological processes that create sedimentary basins or act during their formation can be simulated using the public domain computer code `BASIN'. For a given set of geological initial and boundary conditions the sedimentary basin evolution is calculated in a forward modeling approach. The basin is represented in a two-dimensional vertical cross section with individual layers. The stratigraphic, tectonic, hydrodynamic and thermal evolution is calculated beginning at an initial state, and subsequent changes of basin geometry are calculated from sedimentation rates, compaction and pore fluid mobilization, isostatic compensation, fault movement and subsidence. The sedimentologic, hydraulic and thermal parameters are stored at discrete time steps allowing the temporal evolution of the basin to be analyzed. A maximum flexibility in terms of geological conditions is achieved by using individual program modules representing geological processes which can be switched on and off depending on the data available for a specific simulation experiment. The code incorporates a module for clastic and carbonate sedimentation, taking into account the impact of clastic sediment supply on carbonate production. A maximum of four different sediment types, which may be mixed during sedimentation, can be defined. Compaction and fluid flow are coupled through the consolidation equation and the nonlinear form of the equation of state for porosity, allowing nonequilibrium compaction and overpressuring to be calculated. Instead of empirical porosity-effective stress equations, a physically consistent consolidation model is applied which incorporates a porosity dependent sediment compressibility. Transient solute transport and heat flow are calculated as well, applying calculated fluid flow rates from the hydraulic model. As a measure for hydrocarbon generation, the Time-Temperature Index (TTI) is calculated. Three postprocessing programs are available to provide graphic output in PostScript format: BASINVIEW is used to display the distribution of parameters in the simulated cross-section of the basin for defined time steps. It is used in conjunction with the Ghostview software, which is freeware and available on most computer systems. AIBASIN provides PostScript output for Adobe Illustrator®, taking advantage of the layer-concept which facilitates further graphic manipulation. BASELINE is used to display parameter distribution at a defined well or to visualize the temporal evolution of individual elements located in the simulated sedimentary basin. The modular structure of the BASIN code allows additional processes to be included. A module to simulate reactive transport and diagenetic reactions is planned for future versions. The program has been applied to existing sedimentary basins, and it has also shown a high potential for classroom instruction, giving the possibility to create hypothetical basins and to interpret basin evolution in terms of sequence stratigraphy or petroleum potential.

Bitzer, Klaus

1999-05-01

380

NASA Astrophysics Data System (ADS)

Cylindrical microcavity plasma devices with diameters (D) in the 100-300 ?m range and a dielectric barrier structure similar to that described by Park et al. [J. Appl. Phys. 99, 026107 (2006)] for Al/Al2O3 devices have been investigated numerically. A two-dimensional fluid simulation of microplasmas in Ne/7% Xe gas mixtures with pD values (where p is the total gas pressure) in the 3-9 Torr cm interval yields the temporal history of the spatially resolved electron and ion number densities in response to a 250 kHz bipolar excitation wave form. Calculations show two distinct regions of plasma development, along the microcavity axis and near the wall, each of which dominates the plasma characteristics in separate pD regions. For low pD values (<4 Torr cm), the negative glow produced at the cavity wall extends to the microcavity axis which, in combination with the strong axial electric field, produces an intense glow discharge on axis. For 4<~pD<~6 Torr cm, a weakened axial discharge is observed early in the life of the plasma but the radial variation of the electron density flattens. Further increases in the gas pressure (to the largest pD values investigated, 6-9 Torr cm) result in the retreat of the negative glow to the vicinity of the microcavity wall, thereby producing a diffuse but annular discharge. Even at the higher pD values, the axial discharge appears to facilitate ignition of the negative glow. The predictions of the simulations are consistent with the behavior of Al/Al2O3 microplasma devices for which D=100-300 ?m.

Seo, Jeong Hyun; Eden, J. Gary

2006-12-01

381

MECHANISMS FOR MHD POYNTING FLUX GENERATION IN SIMULATIONS OF SOLAR PHOTOSPHERIC MAGNETOCONVECTION

We investigate the generation mechanisms of MHD Poynting flux in the magnetized solar photosphere. Using radiative MHD modeling of the solar photosphere with initial magnetic configurations that differ in their field strength and geometry, we show the presence of two different mechanisms for MHD Poynting flux generation in simulations of solar photospheric magnetoconvection. The weaker mechanism is connected to vertical transport of weak horizontal magnetic fields in the convectively stable layers of the upper photosphere, while the stronger is the production of Poynting flux in strongly magnetized intergranular lanes experiencing horizontal vortex motions. These mechanisms may be responsible for the energy transport from the solar convection zone to the higher layers of the solar atmosphere.

Shelyag, S.; Mathioudakis, M.; Keenan, F. P. [Astrophysics Research Centre, School of Mathematics and Physics, Queen's University, Belfast BT7 1NN (United Kingdom)

2012-07-01

382

MHD simulation of direct laser-driven magnetic-flux compression with Nautilus

NASA Astrophysics Data System (ADS)

Direct laser-driven magnetic-flux compression is an innovative approach to achieve magneto-inertial fusion (MIF). A cylindrical target with initial seed magnetic field is compressed by energetic laser beams. The magnetic field that is ``frozen-in'' the plasma gets compressed with the target. The resulting high magnetic field reduces electron thermal conductivity and improves alpha particle confinement, thus providing an additional thermal insulation of the fuel forming the hot spot. Numerical simulations of magneto-inertial fusion implosions require realistic equation of states, thermonuclear fusion energy generation and laser energy deposition coupling with MHD equations. These simulations are important in stability and scaling studies of MIF implosions. Nautilus is a multidimensional shock-capturing MHD simulation framework developed at Tech-X. Incorporated with PROPACEOS equation of states, fusion reaction and laser ray tracing modules, it is utilized to perform direct laser-driven magnetic-flux compression implosions. Simulation results and relevant Nautilus features are discussed.

Zhou, C. D.; Loverich, J.; Hakim, A.

2011-11-01

383

A 3D Global MHD Simulation of the Saturnian Magnetosphere

NASA Astrophysics Data System (ADS)

The recent launch of the Cassini spacecraft and the continued planning of its tour of the Saturnian system makes the development of predictive models and analysis tools very important. We present the results of a 3D global magnetohydrodynamic (MHD) model of the magnetosphere of Saturn as a first step in developing a detailed model which can be used for both prediction and analysis. The present model represents the interaction of a magnetized solar wind with a fast rotating, magnetized planet and includes planetary rotation as well as a simplified model of the neutral torus produced by Titan. Solutions in the steady-state for three different interplanetary magnetic field (IMF) conditions will be shown.

Hansen, K. C.; Dezeeuw, D. L.; Gombosi, T. I.; Groth, C. P. T.; Powell, K. G.

1998-09-01

384

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

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

R. M. Johnson; D. A. Rudberg

1981-01-01

385

Global Hall-MHD simulations of magnetorotational instability in a plasma Couette flow experiment

Global MHD and Hall-MHD numerical simulations relevant to the Madison plasma Couette flow experiment (MPCX) have been performed using the extended MHD code NIMROD. The MPCX has been constructed to study the magnetorotational instability (MRI) in a plasma. The two-fluid Hall effect, which is relevant to some astrophysical situations such as protostellar disks, is also expected to be important in the MPCX. Here, we first derive the local Hall dispersion relation including viscosity, extending earlier work by Balbus and Terquem [Astrophys. J. 552, 235 (2001)]. The predictions of the local analysis are then compared with nonlocal calculations of linear stability of the MRI for a parameter range relevant to the MPCX. It is found that the MHD stability limit and mode structure are altered by the Hall term, and nonlocal analysis is necessary to obtain quantitatively reliable predictions for MPCX. Two-fluid physics also significantly changes the nonlinear evolution and saturation of the axisymmetric MRI. Both the Reynolds and Maxwell stresses contribute significantly to momentum transport. In the Hall regime, when the magnetic field is parallel to the rotation axis, the Maxwell stress is larger than the Reynolds stress (similar to the MHD regime). However, when the magnetic field is antiparallel to the rotation axis in the Hall regime, the Reynolds stress is much larger than the Maxwell stress. To further study the role of non-axisymmetric modes, we have also carried out fully nonlinear MHD computations. Non-axisymmetric modes play an increasingly important role as the magnetic Reynolds number increases and grow to large amplitudes in a saturated turbulent state.

Ebrahimi, F.; Lefebvre, B.; Bhattacharjee, A. [Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, University of New Hampshire, 8 College Road, Durham, New Hampshire 03824 (United States); Forest, C. B. [University of Wisconsin, 1150 University Avenue, Madison, Wisconsin 53706 (United States)

2011-06-15

386

The evaluation of scour at bridges throughout the State of Missouri has been ongoing since 1991, and most of these evaluations have used one-dimensional hydraulic analysis and application of conventional scour depth prediction equations. Occasionally, the complex conditions of a site dictate a more thorough assessment of the stream hydraulics beyond a one-dimensional model. This was the case for structure A-1700, the Interstate 155 bridge crossing the Mississippi River near Caruthersville, Missouri. To assess the complex hydraulics at this site, a two-dimensional hydrodynamic flow model was used to simulate flow conditions on the Mississippi River in the vicinity of the Interstate 155 structure A-1700. The model was used to simulate flow conditions for three discharges: a flood that occurred on April 4, 1975 (the calibration flood), which had a discharge of 1,658,000 cubic feet per second; the 100-year flood, which has a discharge of 1,960,000 cubic feet per second; and the project design flood, which has a discharge of 1,974,000 cubic feet per second. The project design flood was essentially equivalent to the flood that would cause impending overtopping of the mainline levees along the Mississippi River in the vicinity of structure A-1700. Discharge and river-stage readings from the flood of April 4, 1975, were used to calibrate the flow model. The model was then used to simulate the 100-year and project design floods. Hydraulic flow parameters obtained from the three flow simulations were applied to scour depth prediction equations to determine contraction, local pier, and abutment scour depths at structure A-1700. Contraction scour and local pier scour depths computed for the project design discharge generally were the greatest, whereas the depths computed for the calibration flood were the least. The maximum predicted total scour depth (contraction and local pier scour) for the calibration flood was 66.1 feet; for the 100-year flood, the maximum predicted total scour depth was 74.6 feet; for the project design flood, the maximum predicted total scour depth was 93.0 feet. If scour protection did not exist, bent 14 and piers 15 through 21 would be substantially exposed or undermined by the predicted total scour depths in all of the flood simulations. However, piers 18 through 21 have a riprap blanket around the base of each, and the riprap blanket observed on the right bank around bent 14 is thought to extend around the base of pier 15, which would limit the amount of scour that would occur at these piers. Furthermore, the footings and caissons that are not exposed by computed contraction scour may arrest local pier scour, which will limit local pier scour at several bents and piers. Nevertheless, main-channel piers 16 and 17 and all of the bents on the left (as viewed facing downstream) overbank are moderately to substantially exposed by the predicted scour depths from the three flood simulations, and there is no known scour protection at these piers or bents. Abutment scour depths were computed for structure A-1700, but abutment scour is expected to be mitigated by the presence of guidebanks upstream from the bridge abutments, as well as riprap revetment on the abutment and guidebank faces.

Huizinga, Richard J.

2007-01-01

387

MHD simulation of the solar wind interaction with the magnetosphere of Mercury

NASA Astrophysics Data System (ADS)

We show MHD simulations of the solar wind interaction with the magnetosphere of Mercury. We use the open source codes Pluto and MPI-AMRVAC in 3 dimensional spherical geometry. In order to appreciate the limits of the MHD approach in the context of Mercury's environment we do first compare our simulations with hybrid simulation (e.g. Trávní?ek et al, Icarus, 209, pp 11-22, 2010). We do also compare magnetic field profiles from the magnetometer on Messenger with profiles sampled along the corresponding spacecraft trajectory in the simulations. These comparisons show that despite the lack of kinetic effects, MHD simulation provide a more than fair description of the interaction of the solar wind with Mercury at low computational cost making it a useful tool to help decrypt data from current and future exploratory missions in the hermean magnetosphere (e.g. Bepi Colombo-MMO). The research leading to these results has received funding from the European Commission's Seventh Framework Programme (FP7/2007-2013) under the grant agreement SHOCK (project number 284515).

Varela, Jacobo; Pantellini, Filippo; Moncuquet, Michel

2014-05-01

388

Effects of the driving mechanism in MHD simulations of coronal mass ejections

NASA Astrophysics Data System (ADS)

Results of time-dependent MHD simulations of mass ejections in the solar coronal are presented. Previous authors have shown that results from simulations using a thermal driving mechanism are consistent with the observations only if an elaborate model of the initial corona is used. The first simulation effort, using a simple model of a plasmoid as the driving mechanism and a simple model of the initial corona, produces results that are also consistent with many observational features, suggesting that the nature of the driving mechanism plays an important role in determining the subsequent evolution of mass ejections. First simulations are based on the assumption that mass ejections are driven by magnetic forces.

Linker, J. A.; van Hoven, G.; Schnack, D. D.

389

The IMF dependence of the magnetopause from global MHD simulations

NASA Astrophysics Data System (ADS)

Numerical results from a physics-based global magnetohydrodynamic (MHD) model are used to investigate the controlling effects of the interplanetary magnetic field (IMF) components, BY and BZ, on the location and shape of the magnetopause. The subsolar magnetopause is identified by using the plasma density and velocity, the cusp by using the current density, and the other area by streamlines and the current density. These data are fitted with a three-dimensional surface function constructed by Liu et al. (2012), which allows description of the cusp geometry as well as the north-south asymmetry and azimuthal asymmetry of the magnetopause. A new parameter which depends on the IMF BY and BZ is introduced to describe the orientation of the elliptical cross section of the magnetopause. Effects of IMF BY and BZ on the magnetopause configuration parameters are analyzed, and dependence of the magnetopause parameters in the IMF components are obtained. Magnetopause cross section is found to be largely controlled by the IMF clock angle. The stretch direction of the magnetopause cross section is always near the direction of the IMF but is a little closer to the meridional plane than the IMF. Increasing BY or BZincreases the eccentricity of the magnetopause cross section. This effect is larger for southward IMF than for the northward IMF, and the stretching effect of BY is smaller than that of BZ.

Lu, J. Y.; Liu, Z.-Q.; Kabin, K.; Jing, H.; Zhao, M. X.; Wang, Y.

2013-06-01

390

NASA Astrophysics Data System (ADS)

In this paper we present a genuinely two-dimensional HLLC Riemann solver. On logically rectangular meshes, it accepts four input states that come together at an edge and outputs the multi-dimensionally upwinded fluxes in both directions. This work builds on, and improves, our prior work on two-dimensional HLL Riemann solvers. The HLL Riemann solver presented here achieves its stabilization by introducing a constant state in the region of strong interaction, where four one-dimensional Riemann problems interact vigorously with one another. A robust version of the HLL Riemann solver is presented here along with a strategy for introducing sub-structure in the strongly-interacting state. Introducing sub-structure turns the two-dimensional HLL Riemann solver into a two-dimensional HLLC Riemann solver. The sub-structure that we introduce represents a contact discontinuity which can be oriented in any direction relative to the mesh. The Riemann solver presented here is general and can work with any system of conservation laws. We also present a second order accurate Godunov scheme that works in three dimensions and is entirely based on the present multidimensional HLLC Riemann solver technology. The methods presented are cost-competitive with traditional higher order Godunov schemes. The two-dimensional HLLC Riemann solver is shown to work robustly for Euler and Magnetohydrodynamic (MHD) flows. Several stringent test problems are presented to show that the inclusion of genuinely multidimensional effects into higher order Godunov schemes indeed produces some very compelling advantages. For two dimensional problems, we were routinely able to run simulations with CFL numbers of ˜0.7, with some two-dimensional simulations capable of reaching higher CFL numbers. For three dimensional problems, CFL numbers as high as ˜0.6 were found to be stable. We show that on resolution-starved meshes, the scheme presented here outperforms unsplit second order Godunov schemes that are based on conventional one-dimensional Riemann solver technology. Strong discontinuities are shown to propagate very isotropically using the methods presented here. The present Riemann solver provides an elegant resolution to the problem of obtaining multi-dimensionally upwinded electric fields in MHD without resorting to a doubling of the dissipation in each dimension.

Balsara, Dinshaw S.

2012-09-01

391

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

392

Numerical Simulation of 3-D Supersonic Viscous Flow in an Experimental MHD Channel

NASA Technical Reports Server (NTRS)

The 3-D supersonic viscous flow in an experimental MHD channel has been numerically simulated. The experimental MHD channel is currently in operation at NASA Ames Research Center. The channel contains a nozzle section, a center section, and an accelerator section where magnetic and electric fields can be imposed on the flow. In recent tests, velocity increases of up to 40% have been achieved in the accelerator section. The flow in the channel is numerically computed using a new 3-D parabolized Navier-Stokes (PNS) algorithm that has been developed to efficiently compute MHD flows in the low magnetic Reynolds number regime. The MHD effects are modeled by introducing source terms into the PNS equations which can then be solved in a very e5uent manner. To account for upstream (elliptic) effects, the flowfield can be computed using multiple streamwise sweeps with an iterated PNS algorithm. The new algorithm has been used to compute two test cases that match the experimental conditions. In both cases, magnetic and electric fields are applied to the flow. The computed results are in good agreement with the available experimental data.

Kato, Hiromasa; Tannehill, John C.; Gupta, Sumeet; Mehta, Unmeel B.

2004-01-01

393

Two-dimensional magnetic domain patterns

Two-dimensional magnetic garnets exhibit complex and fascinating magnetic domain structures, like stripes, cells, mixed states of stripes and cells and labyrinths, which have challenged theorists for a long time. They change reversibly when the intensity of an externally applied magnetic field is varied. By Monte Carlo simulations we investigate the pattern formation and the thermodynamics of these systems as a

M. Kiwi; J. R. Iglesias; S. Goncalves; O. Nagel

2001-01-01

394

NASA Astrophysics Data System (ADS)

Water flowing through the Sacramento-San Joaquin River Delta supplies drinking water to more than 20 million people in California. Delta water contains elevated concentrations of dissolved organic carbon (DOC) from drainage through the delta peat soils, forming trihalomethanes when the water is chlorinated for drinking. Land subsidence caused by oxidation of the peat soils has led to increased pumping of drainage water from delta islands to maintain arable land. An agricultural field on Twitchell Island was flooded in 1997 to evaluate continuous flooding as a technique to mitigate subsidence. The effects of shallow flooding on DOC loads to the drain water must be determined to evaluate the feasibility of this technique. In this study, heat is used as a nonconservative tracer to determine shallow ground-water flux and calculate DOC loads to an adjacent drainage ditch. Temperature profiles and water levels were measured in 12 wells installed beneath the pond, in the pond, and in an adjacent drainage ditch from May 2000 to June 2001. The range in seasonal temperatures decreased with depth, but seasonal temperature variation was evident in wells screened as deep as 10 to 12 feet below land surface. A constant temperature of 17 degrees C was measured in wells 25 feet beneath the pond. Ground-water flux beneath the pond was quantified in a two-dimensional simulation of water and heat exchange using the SUTRA flow and transport model. The effective vertical hydraulic conductivity of the peat soils underlying the pond was estimated through model calibration. Calibrated hydraulic conductivity is higher (1E-5 m/sec) than estimates from slug tests (2E-6 m/sec). Modeled pond seepage is similar to that estimated from a water budget, although the total seepage determined from the water budget is within the range of error of the instrumentation. Overall, model results indicate that recharge from the pond flows along shallow flow paths and that travel times through the peat to the drainage ditch may be on the order of decades.

Burow, K. R.; Gamble, J. M.; Fujii, R.; Constantz, J.

2001-12-01

395

Direct Numerical Simulations of Nonlinear Evolution of MHD Instability in LHD

NASA Astrophysics Data System (ADS)

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.; Okamoto, M.

2006-11-01

396

Gas Core Reactor Numerical Simulation Using a Coupled MHD-MCNP Model

NASA Technical Reports Server (NTRS)

Analysis is provided in this report of using two head-on magnetohydrodynamic (MHD) shocks to achieve supercritical nuclear fission in an axially elongated cylinder filled with UF4 gas as an energy source for deep space missions. The motivation for each aspect of the design is explained and supported by theory and numerical simulations. A subsequent report will provide detail on relevant experimental work to validate the concept. Here the focus is on the theory of and simulations for the proposed gas core reactor conceptual design from the onset of shock generations to the supercritical state achieved when the shocks collide. The MHD model is coupled to a standard nuclear code (MCNP) to observe the neutron flux and fission power attributed to the supercritical state brought about by the shock collisions. Throughout the modeling, realistic parameters are used for the initial ambient gaseous state and currents to ensure a resulting supercritical state upon shock collisions.

Kazeminezhad, F.; Anghaie, S.

2008-01-01

397

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

398

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

399

Bow shock and magnetopause contributions to the cross-tail current from global MHD simulations

We performed a series of global MHD simulations to study the closure of the cross-tail current in the magnetotail under different solar wind conditions. The cross-tail current closes totally within the magnetopause, forming the classical $\\\\theta$ structure when IMF is set to be zero. The situation changes for southward IMF cases: part of the cross-tail current passes through the magnetosheath

B. B. Tang; X. C. Guo; C. Wang; Y. Q. Hu; J. R. Kan

2009-01-01

400

2D MHD Simulation of the Emergence and Merging of Coherent Structures

NASA Technical Reports Server (NTRS)

A model of sporadic localized merging of coherent structures has recently been proposed by Chang to describe the dynamics of the Earth's magnetotail. Here we report the results of MHD simulations regarding the development and merging of 2D coherent structures. With a magnetic shear. such coherent structures are generated in alignment with the imposed current sheet. The calculated fluctuation spectra suggest long-ranged correlations with power-law characteristics.

Wu, Cheng-Chin; Chang, Tom

2002-01-01

401

3D MHD Simulation of CME Propagation from Solar Corona to 1 AU

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

402

MHD Simulations and Kinetic Analyses of Solar 3He-Rich Events

The origin and magnetic topology of solar impulsive (or 3He-rich) events are numerically simulated by using a three-dimensional axisymmetric time-dependent self-consistent magnetohydrodynamic (MHD) model. The results indicate that, when a magnetic flux of opposite polarity is emerged from the photosphere at the open field line region near a coronal hole boundary, the magnetic topology that leads to solar impulsive events

T. Zhang; S. Wu; A. Tan; A. Winebarger

2007-01-01

403

3D simulations of disc winds extending radially self-similar MHD models

NASA Astrophysics Data System (ADS)

Disc winds originating from the inner parts of accretion discs are considered as the basic component of magnetically collimated outflows. The only available analytical magnetohydrodynamic (MHD) solutions to describe disc-driven jets are those characterized by the symmetry of radial self-similarity. However, radially self-similar MHD jet models, in general, have three geometrical shortcomings: (i) a singularity at the jet axis, (ii) the necessary assumption of axisymmetry and (iii) the non-existence of an intrinsic radial scale, i.e. the jets formally extend to radial infinity. Hence, numerical simulations are necessary to extend the analytical solutions towards the axis, by solving the full three-dimensional equations of MHD and impose a termination radius at finite radial distance. We focus here on studying the effects of relaxing the (ii) assumption of axisymmetry, i.e. of performing full 3D numerical simulations of a disc wind crossing all MHD critical surfaces. We compare the results of these runs with previous axisymmetric 2.5D simulations. The structure of the flow in all simulations shows strong similarities. The 3D runs reach a steady state and stay close to axisymmetry for most of the physical quantities, except for the poloidal magnetic field and the toroidal velocity which slightly deviate from axisymmetry. The latter quantities show signs of instabilities, which, however, are confined to the region inside the fast magnetosonic separatrix surface. The forces present in the flow, both of collimating and accelerating nature, are in good agreement in both the 2.5D and the 3D runs. We conclude that the analytical solution behaves well also after relaxing the basic assumption of axisymmetry.

Stute, Matthias; Gracia, José; Vlahakis, Nektarios; Tsinganos, Kanaris; Mignone, Andrea; Massaglia, Silvano

2014-04-01

404

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 fl