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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 MHD simulation of the solar wind interaction with magnetic field anomalies on the

of typical solar wind conditions and the surface magnetic field strengths measured by Lunar ProspectorTwo-dimensional MHD simulation of the solar wind interaction with magnetic field anomalies on the Moon when the magnetic anomaly field strength is above 10 nT at 100 km above the surface (for a surface

Harnett , Erika

3

NASA Astrophysics Data System (ADS)

We present results of two new studies on magnetic reconnection dynamics obtained from two-dimensional fully electromagnetic particle-in-cell (PIC) simulations, and compare them with results obtained from earlier Hall MHD theory and 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 reconnnection region in the GEM Reconnection Challenge as a function of the electron mass, which Hall MHD models predict to scale as the electron Alfven speed. (This study has significant implications for particle detectors from the upcoming NASA MMS mission.) The PIC simulations exhibit 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 Harris sheet driven by external electric fields which produce 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 near-explosive enhancement in the nonlinear regime, qualitatively similar to that seen in earlier Hall MHD simulations. Quantitative comparisons between PIC and previous Hall MHD theory and simulations will be reported.

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

2003-10-01

4

The problem is formulated and a numerical model is developed for calculating MHD flows in plasma-focus (PF) devices. An implicit\\u000a (with respect to the magnetic field) difference scheme and a method for its numerical implementation are proposed. The scheme\\u000a allows one to describe plasma flows with drastically different densities and, therefore, to take into account the presence\\u000a of a vacuum

S. F. Garanin; V. I. Mamyshev

2008-01-01

5

Two-dimensional Magnetohydrodynamic Simulations of Relativistic Magnetic Reconnection

NASA Astrophysics Data System (ADS)

It has been recognized that the magnetic reconnection process is of great importance in high-energy astrophysics. We develop a new two-dimensional relativistic resistive magnetohydrodynamic (R2MHD) code and carry out numerical simulations of magnetic reconnection. We find that the outflow velocity reaches the Alfvén velocity in the inflow region and that a higher Alfvén velocity provides a higher reconnection rate. We also find that Lorentz contraction plays an important role in enhancement of the reconnection rate.

Watanabe, Naoyuki; Yokoyama, Takaaki

2006-08-01

6

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

7

Reconstruction of two-dimensional coherent MHD structures in a space plasma: The theory

We develop basic theory for the reconstruction of two-dimensional, time-stationary, ideal, compressible MHD structures in a space plasma from data taken by a single spacecraft as the structures move past it. The MHD equations are solved as a spatial initial-value problem in a manner similar to that used in so-called Grad-Shafranov (GS) reconstruction (e.g., Sonnerup et al., 2006), the difference

Bengt U. Ö. Sonnerup; Wai-Leong Teh

2008-01-01

8

Two-dimensional simulation of the polarization switching in ferroelectrics

NASA Astrophysics Data System (ADS)

The main switching properties in ferroelectrics are simulated within the framework of the extended Ishibashi dipole-lattice model including the dipole-dipole interaction in a two-dimensional case. The mechanism of the polarization reversal is modeled in the two-dimensional case. The results of the modeling are in a good agreement with experimental data for the set of materials.

Bakaleinikov, L. A.; Gordon, A.

2009-12-01

9

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

10

Collisionless reconnection involves both Hall physics and kinetic effects occurring at small scales in thin current sheets. Including these effects in a large scale MHD simulation is essential to correctly model the magnetotail dynamics during substorms. To address the issue of how kinetic treatment for the ions affects the reconnection dynamics, a study of two-dimensional collisionless reconnection in a thin

L. Yin; D. Winske; S. Gary; J. Birn

2001-01-01

11

TWO DIMENSIONAL COMPUTER SIMULATION OF PLASMA IMMERSION

for treating complex shaped objects Â·requires complex target manipulation Â·high production cost #12;Plasma Â·low cost Â·shorter implantation time Disadvantages Â·spatial dose uniformity is a priori not guaranteed. Probabilistic Monte-Carlo collision algorithm is employed to simulate the ionization process. Actual mass of N2

12

TWO DIMENSIONAL IMMERSED BOUNDARY SIMULATIONS OF SWIMMING JELLYFISH

TWO DIMENSIONAL IMMERSED BOUNDARY SIMULATIONS OF SWIMMING JELLYFISH by Haowen Fang B.Eng., Nanjing Simulations Of Swim- ming Jellyfish Examining Committee: Dr. Weiran Sun, Assistant Professor Chair Dr. John iii #12;Abstract The swimming behavior of jellyfish, driven by the periodic contraction of body

Stockie, John

13

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

14

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

15

NASA Astrophysics Data System (ADS)

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

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

2003-05-01

16

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

17

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

NASA Technical Reports Server (NTRS)

A 2D, time-dependent, numerical, MHD model for the simulation of coronal streamers from the solar surface to 15 solar is presented. Three examples are given; for dipole, quadrupole and hexapole (Legendre polynomials P1, P2, and P3) initial field topologies. The computed properties are density, temperature, velocity, and magnetic field. The calculation is set up as an initial-boundary value problem wherein a relaxation in time produces the steady state solution. In addition to the properties of the solutions, their accuracy is discussed. Besides solutions for dipole, quadrupole, and hexapole geometries, the model use of realistic values for the density and Alfven speed while still meeting the requirement that the flow speed be super-Alfvenic at the outer boundary by extending the outer boundary to 15 solar radii.

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

1992-01-01

18

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

19

High order hybrid numerical simulations of two dimensional detonation waves

NASA Technical Reports Server (NTRS)

In order to study multi-dimensional unstable detonation waves, a high order numerical scheme suitable for calculating the detailed transverse wave structures of multidimensional detonation waves was developed. The numerical algorithm uses a multi-domain approach so different numerical techniques can be applied for different components of detonation waves. The detonation waves are assumed to undergo an irreversible, unimolecular reaction A yields B. Several cases of unstable two dimensional detonation waves are simulated and detailed transverse wave interactions are documented. The numerical results show the importance of resolving the detonation front without excessive numerical viscosity in order to obtain the correct cellular patterns.

Cai, Wei

1993-01-01

20

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

21

NON-OSCILLATORY CENTRAL SCHEMES FOR ONE- AND TWO-DIMENSIONAL MHD EQUATIONS. II: HIGH-ORDER SEMI-oscillatory semi-discrete central schemes for the approximate solution of the ideal Magnetohydrodynamics (MHD the simplicity of fully-discrete central schemes while enhancing efficiency and adding versatility. The semi

22

Two dimensional simulation of high power laser-surface interaction

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

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

1998-08-01

23

Two-dimensional Potts model with aperiodic interactions: numerical simulation

NASA Astrophysics Data System (ADS)

The uniform two-dimensional Potts model presents first-order transitions for q (number of states) greater than 4. The introduction of aperiodic modulations on its interactions may change the universality class or the nature of the transition. Previous results for the q=8 Potts model on the square lattice suggest that the Harris-Luck criterion is satisfied also for first-order transitions [1]. However, for random disorder, the new universality class that may emerge depends on the number of states of the Potts model [2]. In order to test this possibility for aperiodic modulations, we have made extensive numerical simulations on the q=6 Potts model on the square lattice, for three aperiodic sequence. Our results show that the Harris-Luck criterion is obeyed and that the new universality class that may emerge is the same as for the q=8 Potts model. Therefore, we stablish that, contrarily to random disorder, the introduction of relevant aperiodic modulation leads the system to a new universality class, irrespective number of states of the Potts model.[4pt] [1] C. Chatelain, B. Berche, Phys. Rev. Lett. 80, 1670 (1998).[0pt] [2] J.L. Jacobsen and J. Cardy, Phys. Rev. Lett. 79, 4063 (1997).

Branco, Nilton; Girardi, Daniel

2010-03-01

24

Two-dimensional simulation of surface wave discharges

NASA Astrophysics Data System (ADS)

Surface wave discharges (SWDs) are sustained by electromagnetic waves propagating along the interface of the plasma and the glass tube containing it. Most models of SWDs describe the surface waves by an approximate semi-analytical solution of Maxwell's equations, neglecting axial variations of the plasma and the tube geometry. Here we present an alternative, fully two-dimensional approach to SWD modeling. We solve Maxwell's equations numerically, self-consistently coupled with a set of fluid equations for the plasma particle species, the electron energy, and the gas temperature. This makes it possible to investigate the effect of geometrical details. As an example, we show simulation results of a SWD in argon (3 Torr), generated by a surfatron launcher (2.45 kHz, 50 W) in a narrow tube (5 mm diameter) and propagating into a wider tube (18 mm). These results reproduce and explain experimentally observed light emission patterns due to the partial reflection of the surface waves against the tube diameter transition.

Hagelaar, Gerjan

2004-09-01

25

Two-dimensional simulation of optical wave propagation through atmospheric turbulence.

A methodology for the two-dimensional simulation of optical wave propagation through atmospheric turbulence is presented. The derivations of common statistical field moments in two dimensions, required for performing and validating simulations, are presented and compared with their traditional three-dimensional counterparts. Wave optics simulations are performed to validate the two-dimensional moments and to demonstrate the utility of performing two-dimensional wave optics simulations so that the results may be scaled to those of computationally prohibitive 3D scenarios. Discussions of the benefits and limitations of two-dimensional atmospheric turbulence simulations are provided throughout. PMID:25679852

Hyde, Milo W; Basu, Santasri; Schmidt, Jason D

2015-01-15

26

Numerical simulation of two?dimensional tsunami runup

The hydrodynamic and mathematical problems connected with discontinuity between wet and dry domains, nonlinearity, friction, and computational instability are the main problems that have to be sorted out in the runup computation. A variety of runup models are analyzed, including the boundary conditions used to move the shoreline. Based on the initial experiments one?dimensional and two?dimensional algorithms are constructed. These

Z. Kowalik; T. S. Murty

1993-01-01

27

Numerical simulation of two-dimensional snowflake growth

We develop an efficient numerical scheme for integrating the equations of two-dimensional dendritic growth in the thermal-diffusion-limiting region. We use a Green's function representation to recast the problem as an essentially one-dimensional integro-differential equation which is solved numerically. We find that anisotropic surface tension is required to produce the stable tip behavior and repeated sidebranching of snowflakelike shapes.

David A. Kessler; Joel Koplik; Herbert Levine

1984-01-01

28

Molecular Dynamics Simulation of a Two-Dimensional Heisenberg Fluid

NASA Astrophysics Data System (ADS)

In this work we use numerical Monte Carlo and molecular dynamics to study a classical two-dimensional compressible magnetic fluid. The magnetic interactions are realized through a Yukawa-like potential while particles interact through Lenard-Jones forces. Our preliminary results point to a very rich phase transition picture. At high density the system seems to undergoes a transition, as suggested by the magnetization and susceptibility results.

Correa, Eberth; Lima, A. B.; Costa, B. V.

2012-04-01

29

Molecular Dynamics Simulation of a Two Dimensional Heisenberg Fluid

NASA Astrophysics Data System (ADS)

In this work we use numerical Monte Carlo and Molecular Dynamics to study a classical two-dimensional compressible magnetic fluid. The magnetic interactions are realized through a Yukawa-like potential while particles interact through Lenard-Jones forces. Our preliminary results point to a very rich phase transition picture. At high density the system seems to undergo a magnetic transition, as suggested by the magnetization and susceptibility results.

Costa, B. V.; Lima, A. B.; Correa, E.

2012-02-01

30

Seabed disposal project two-dimensional axisymmetric penetrometer simulations

Preliminary two-dimensional, one-constituent hole closure analyses of an experimental apparatus and the flow of in situ ocean sediments following a penetrometer explacement have been performed. Boundary conditions associated with the experimental apparatus were found to greatly affect cavity response. Difficulties were encountered in modelling penetrometer-sediment interfaces and in obtaining smooth stress histories. The use of a different computer code in later analyses led to more realistic penetrometer-sediment interface models and to improved success in obtaining stress histories. These results along with some recommendations for future work are presented.

Chavez, P.F.; Dawson, P.R.; Schuler, K.W.

1980-03-01

31

MHD simulations: Corotating Interaction Regions

NASA Astrophysics Data System (ADS)

Corotating Interaction Regions (CIRs) form in the solar wind when parcels of fast-speed wind interact with slow-speed wind due to the rotation of the Sun. The resulting buildup of pressure generates disturbances that, with increasing time (or distance from the Sun), may develop into a so-called forward-reverse shock-pair. During solar-quiet times CIRs can be the dominant force shaping large-scale structures in the heliosphere. Studying CIRs is therefore important because the associated shocks are capable of e.g. accelerating energetic particles or deflecting cosmic rays. The global structure of CIRs can be modeled with an MHD approach that gives the plasma quantities needed to model the transport of particles in the heliosphere (with e.g. stochastic differential equations (SDEs)). Our MHD code CRONOS employs a semi-discrete finite volume scheme with adaptive time-stepping Runge-Kutta integration. The solenoidality of the magnetic field is ensured via constrained transport and the code supports Cartesian, Cylindrical and Spherical coordinates (including coordinate singularities) with the option for non-equidistant grids. The code runs in parallel (MPI) and supports the HDF5 output data format. Here, we show results from 3D-MHD simulations with our code CRONOS for a) analytic boundary conditions where results can be compared to those obtained with a different code and b) boundary conditions derived with the Wang-Sheeley-Arge model from observational data (WSO), which are compared to spacecraft observations. Comparison with Pizzo (1982) for analytic boundary conditions Comparison with STEREO A for Carrington Rotation 2060

Wiengarten, T.; Kleimann, J.; Fichtner, H.; Kühl, P.; Heber, B.; Kissmann, R.

2013-12-01

32

MHD Simulations: Corotating Interaction Regions

NASA Astrophysics Data System (ADS)

Corotating Interaction Regions (CIRs) form in the solar wind when parcels of fast-speed wind interact with slow-speed wind due to the rotation of the Sun. The resulting buildup of pressure generates disturbances that, with increasing time (or distance from the Sun), may develop into a so-called forward-reverse shock pair. During solar-quiet times CIRs can be the dominant force shaping large-scale structures in the heliosphere. Studying CIRs is therefore important because the associated shocks are capable of e.g. accelerating energetic particles or deflecting cosmic rays. The global structure of CIRs can be modeled with an MHD approach that gives the plasma quantities needed to model the transport of particles in the heliosphere with e.g. stochastic differential equations. Here, we show results from 3D-MHD simulations with our code CRONOS for a) analytic boundary conditions where results can be compared to those obtained with a different code and b) boundary conditions derived with the Wang-Sheeley-Arge model from observational data (WSO), which are compared to spacecraft observations.

Wiengarten, T.; Kleimann, J.; Fichtner, H.; Kissmann, R.

2014-09-01

33

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

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

1997-10-01

34

Two-dimensional simulations of the inertial electrostatic confinement device

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

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

2006-01-01

35

Two-dimensional simulations of the inertial electrostatic confinement device

NASA Astrophysics Data System (ADS)

We discuss the application of the CELESTE simulation package to the simulation of the experiments conducted at the Los Alamos Inertial Electrostatic Confinement (IEC) device. Recently considerable experimental advances have been made in understanding of the stability of the virtual cathode and in the physics of POPS. This momentous experimental advance requires a new simulation effort for explaining the new experimental findings, particularly in the area of stability of the configurations obtained experimentally. We have conducted a 2D stability study of the virtual cathode in the IEC device using the DEMOCRITUS package. DEMOCRITUS is a 2D general geometry electrostatic PIC code. In the present study we have done complete stability study and investigate the two-stream instability occuring in the IEC device.

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

2006-04-01

36

Simulation of burn wave propagation on two dimensional distorted meshes

NASA Astrophysics Data System (ADS)

Distorted meshes are often formed at the stagnation phase of direct drive ignition target simulations when using 2-D Lagrangian hydrodynamics codes. A comparison of the subsequent burn propagation through the distorted mesh using both Monte Carlo charged particle tracking and multi-group flux-limited diffusion will be presented. Diffusion theory has limitations in the accurate treatment of the long mean free path charged particle transport and the effects of mesh distortion on burn front uniformity computed using finite difference methods. Monte Carlo has limitations related to statistical noise effects on the burn front and the increasing computational cost to reduce this noise. Simulations will be compared for various L-mode perturbations in laser uniformity for a NIF polar direct drive target.

Moses, Gregory; Yuan, Jiankui

2004-11-01

37

Two-dimensional method for unconditionally stable elastic wave simulations.

The stability of the conventional staggered-grid finite-difference time-domain method for elastic wave simulations is limited by the Courant condition and material heterogeneity. Its computational efficiency is significantly hampered when the mesh size is much smaller than a wavelength (for geometric modeling accuracy) and/or with a high impedance contrast. An unconditionally stable alternating direction implicit method is proposed to overcome this issue. It is based on additive operator splitting that renders tri-diagonal matrices for the implicit update of selected field variables. Theoretical analysis of its stability and grid dispersion error are provided. Numerical examples further demonstrate its accuracy and efficiency. PMID:25324071

Shao, Yu; An, Myoung; Wang, Shumin

2014-10-01

38

Two-dimensional simulations of extreme floods on a large watershed

Two-dimensional simulations of extreme floods on a large watershed John F. England Jr. a,*, Mark L-dimensional, Runoff, Erosion and Export (TREX) model to simulate extreme floods on large watersheds in semi, validation and simulation of extreme storms and floods on the 12,000 km2 Arkansas River watershed above

Julien, Pierre Y.

39

Absence of Electron Surfing Acceleration in a Two-Dimensional Simulation

Electron acceleration in high Mach number perpendicular shocks is investigated through two-dimensional electrostatic particle-in-cell (PIC) simulation. We simulate the shock foot region by modeling particles that consist of three components such as incident protons and electrons and reflected protons in the initial state which satisfies the Buneman instability condition. In contrast to previous one-dimensional simulations in which strong surfing acceleration is realized, we find that surfing acceleration does not occur in two-dimensional simulation. This is because excited electrostatic potentials have a two-dimensional structure that makes electron trapping impossible. Thus, the surfing acceleration does not work either in itself or as an injection mechanism for the diffusive shock acceleration. We briefly discuss implications of the present results on the electron heating and acceleration by shocks in supernova remnants.

Yutaka Ohira; Fumio Takahara

2007-05-16

40

One- and two-dimensional STEALTH simulations of fuel-pin transient response. Final report. [BWR; PWR

This report presents an assessment of the adaptability of EPRI's one- and two-dimensional STEALTH computer codes to perform transient fuel rod analysis. The ability of the STEALTH code to simulate transient mechanical or thermomechanical loss-of-coolant accident is described. Analytic models of one- and two-dimensional formulations and features included in the two-dimensional simulation are discussed.

Wahi, K.K.

1980-08-01

41

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

V. Vahedi; C. K. Birdsall; M. A. Lieberman

1992-01-01

42

Two-dimensional modeling of sodium boiling transients in simulated LMFBR fuel bundles

A two-dimensional code for analysis of sodium boiling in LMFBR fuel assemblies has been developed at ORNL. This code, THORAX, has been used to analyze tests in 19- and 61-pin electrically-heated, simulated LMFBR fuel assemblies in the THORS facility. THORAX has simulated well the transient growth of the two-dimensional boiling region and the resulting static flow instability leading to dryout. Extrapolation of results to a full size fuel pin bundle shows that two-dimensional effects are reduced but still significant. The code will be extended to include a loop model in support of forthcoming tests in the THORS-SHRS Assembly 1 loop, which will include two parallel 19-pin simulated driver bundles.

Rose, S.D.; Dearing, J.F; Carbajo, J.J.; Levin, A.E.; Montgomery, B.H.; Wantland, J.L.

1982-01-01

43

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

44

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

45

1 American Institute of Aeronautics and Astronautics Two-dimensional Simulation of Horseshoe-713 Copyright Â© 2010 by Authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. #12;2 American Institute of Aeronautics and Astronautics I. Introduction lasma that has been used

Roy, Subrata

46

1 Two-Dimensional Computational Fluid Dynamics and Conduction Simulations of Heat Transfer, Shewen et al. 1996, Wright 1996, and Zhao 1998). Less research has been conducted on heat transfer because cavities are a primary area where frame heat transfer can be minimized (the thermal conductivity

47

NANOTCAD2D: Two-dimensional code for the simulation of nanoelectronic devices and structures

In this paper we present NANOTCAD2D, a code for the simulation of the\\u000aelectrical properties of semiconductor-based nanoelectronic devices and\\u000astructures in two-dimensional domains. Such code is based on the solution of\\u000athe Poisson\\/Schr\\\\\\

G. Curatola; G. Iannaccone

2003-01-01

48

Two-dimensional simulation of the fluttering instability using a pseudospectral method with volume Keywords: FluidÂstructure interaction Fluttering instability Volume-penalization method Spectral method a b is realized with a fast explicit staggered scheme. The method is applied to the fluttering instability

Schneider, Kai

49

Simulations of the Two-Dimensional Electronic Spectroscopy of the Photosystem II Reaction Center

Simulations of the Two-Dimensional Electronic Spectroscopy of the Photosystem II Reaction Center K spectroscopy of the Qy band of the D1-D2-Cyt b559 photosystem II reaction center at 77 K. We base models of the photosystem II reaction center and discuss avenues for further refinement of such models

Mukamel, Shaul

50

Theory and simulations of two-dimensional vortex motion driven by a background vorticity gradient

Theory and simulations of two-dimensional vortex motion driven by a background vorticity gradient-dimensional vortex motion in a shear-flow with nonuniform vorticity. Typically, a vortex travels to an extremum, above which the transverse vortex motion is suppressed. A brief account of some of these results has

California at San Diego, University of

51

A structural study of a two-dimensional electrolyte by Monte Carlo simulations.

Properties of superconducting and superfluid thin films, modeled as a two-dimensional classic Coulomb fluid, are connected to the molecular structure of the system. Monte Carlo simulations to explore structural properties and ordering in the classical two-dimensional Coulomb fluid were performed. The density dependence of translational order parameters at various temperatures and cluster distribution below and above the Kosterlitz-Thouless line were studied, and the percolation temperature threshold was determined. Results show that one could detect the insulator-conductor transition by observing the translational order parameters, average cluster number, or mean cluster size besides dielectric constant and dipole moment of the system. PMID:25573571

Aupic, Jana; Urbic, Tomaz

2015-01-01

52

A structural study of a two-dimensional electrolyte by Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Properties of superconducting and superfluid thin films, modeled as a two-dimensional classic Coulomb fluid, are connected to the molecular structure of the system. Monte Carlo simulations to explore structural properties and ordering in the classical two-dimensional Coulomb fluid were performed. The density dependence of translational order parameters at various temperatures and cluster distribution below and above the Kosterlitz-Thouless line were studied, and the percolation temperature threshold was determined. Results show that one could detect the insulator-conductor transition by observing the translational order parameters, average cluster number, or mean cluster size besides dielectric constant and dipole moment of the system.

Aupic, Jana; Urbic, Tomaz

2015-01-01

53

We have recently developed a Buot-Jensen type lattice Weyl-Wigner 2D single band transport computer code which examines two-dimensional transport through a nanoscale quantum device exhibiting circular cylindrical symmetry. By performing the simulation in two dimensions, we are including radial effects in the Wigner function transport equation (WFE) which are not present in any one-dimensional simulation. We will describe the progress

G. Recine; B. Rosen; Hong-Liang Cui

2003-01-01

54

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

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

E. M. Wahba

2009-01-01

55

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

56

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

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

2008-01-01

57

Simulations of the Two-Dimensional Electronic Spectroscopy of the Photosystem II Reaction Center

We report simulations of the two-dimensional electronic spectroscopy of the Qy band of the D1-D2-Cyt b559 photosystem II reaction center at 77 K. We base the simulations on an existing Hamiltonian that was derived by simultaneous fitting to a wide range of linear spectroscopic measurements and described within modified Redfield theory. The model obtains reasonable agreement with most aspects of the two-dimensional spectra, including the overall peak shapes and excited state absorption features. It does not reproduce the rapid equilibration from high energy to low energy excitonic states evident by a strong cross-peak below the diagonal. We explore modifications to the model to incorporate new structural data and improve agreement with the two-dimensional spectra. We find that strengthening the system–bath coupling and lowering the degree of disorder significantly improves agreement with the cross-peak feature, while lessening agreement with the relative diagonal/antidiagonal width of the 2D spectra. We conclude that two-dimensional electronic spectroscopy provides a sensitive test of excitonic models of the photosystem II reaction center and discuss avenues for further refinement of such models. PMID:23210463

Lewis, K. L. M.; Fuller, F. D.; Myers, J. A.; Yocum, C. F.; Mukamel, S.; Abramavicius, D.; Ogilvie, J. P.

2013-01-01

58

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

59

NANOTCAD2D: Two-dimensional code for the simulation of nanoelectronic devices and structures

In this paper we present NANOTCAD2D, a code for the simulation of the electrical properties of semiconductor-based nanoelectronic devices and structures in two-dimensional domains. Such code is based on the solution of the Poisson\\/Schrödinger equation with density functional theory and of the continuity equation of the ballistic current. NANOTCAD2D can be applied to structures fabricated on III–IV, strained-silicon and silicon–germanium

G. Curatola; G. Iannaccone

2003-01-01

60

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

61

A two-dimensional particle-in-cell simulation of stationary plasma thruster discharges

The Hall electric thruster (HET) is an electric propulsion device used for a small satellite that allows a high specific efficiency compared with chemical thrusters. A stationary plasma thruster (SPT), which is a kind of HET, is a popular device that uses a discharge mechanism to ionize and accelerate the ions. A two-dimensional particle-in-cell (PIC) simulation with Monte-Carlo collision (MCC)

Wang Sun Lim; Hae June Lee; Wonho Choe; Jongho Seon

2008-01-01

62

NASA Astrophysics Data System (ADS)

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

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

2008-02-01

63

3D simulation studies of tokamak plasmas using MHD and extended-MHD models

The M3D (Multi-level 3D) tokamak simulation project aims at the simulation of tokamak plasmas using a multi-level tokamak code package. Several current applications using MHD and Extended-MHD models are presented; high-{beta} disruption studies in reversed shear plasmas using the MHD level MH3D code, {omega}{sub *i} stabilization and nonlinear island saturation of TAE mode using the hybrid particle/MHD level MH3D-K code, and unstructured mesh MH3D{sup ++} code studies. In particular, three internal mode disruption mechanisms are identified from simulation results which agree which agree well with experimental data.

Park, W.; Chang, Z.; Fredrickson, E.; Fu, G.Y. [and others

1996-12-31

64

Two-dimensional simulation of temperature distributions inside pressurized fluidized bed combustors

A particular characteristic of Pressurized Fluidized Bed Combustors with stationary fluidized beds (PFBC) is the high rate of heat release in the vicinity of he coal feed points. This may cause local overheating of the bed. Thus a two-dimensional model is presented which describes the temperature distribution inside a PFBC as a function of coal characteristics, operating parameters and geometry. The model is composed of two mass balances describing the carbon and the oxygen concentration inside the stationary bed as well as of an enthalpy balance. Furthermore, a submodel takes into account the feed of coal into the reactor as a coal-water mixture. The numerical calculations yield two-dimensional profiles of the carbon and the oxygen concentration and the temperature distribution, respectively. The simulation results point at and quantify critical parameters which have to be considered when dealing with the scale-up of boilers from the pilot-scale to large-scale size.

Artlich, S.; Mackens, W.; Werther, J. [Technical Univ. Hamburg-Harburg, Hamburg (Germany)

1997-12-31

65

Two-dimensional numerical simulation of boron diffusion for pyramidally textured silicon

NASA Astrophysics Data System (ADS)

Multidimensional numerical simulation of boron diffusion is of great relevance for the improvement of industrial n-type crystalline silicon wafer solar cells. However, surface passivation of boron diffused area is typically studied in one dimension on planar lifetime samples. This approach neglects the effects of the solar cell pyramidal texture on the boron doping process and resulting doping profile. In this work, we present a theoretical study using a two-dimensional surface morphology for pyramidally textured samples. The boron diffusivity and segregation coefficient between oxide and silicon in simulation are determined by reproducing measured one-dimensional boron depth profiles prepared using different boron diffusion recipes on planar samples. The established parameters are subsequently used to simulate the boron diffusion process on textured samples. The simulated junction depth is found to agree quantitatively well with electron beam induced current measurements. Finally, chemical passivation on planar and textured samples is compared in device simulation. Particularly, a two-dimensional approach is adopted for textured samples to evaluate chemical passivation. The intrinsic emitter saturation current density, which is only related to Auger and radiative recombination, is also simulated for both planar and textured samples. The differences between planar and textured samples are discussed.

Ma, Fa-Jun; Duttagupta, Shubham; Shetty, Kishan Devappa; Meng, Lei; Samudra, Ganesh S.; Hoex, Bram; Peters, Ian Marius

2014-11-01

66

Cell-dynamical simulation of magnetic hysteresis in the two-dimensional Ising system

NASA Astrophysics Data System (ADS)

We present results from numerical simulations using a ``cell-dynamical system'' to obtain solutions to the time-dependent Ginzburg-Landau equation for a scalar, two-dimensional (2D), (?2)2 model in the presence of a sinusoidal external magnetic field. Our results confirm a recent scaling law proposed by Rao, Krishnamurthy, and Pandit [Phys. Rev. B 42, 856 (1990)], and are also in excellent agreement with recent Monte Carlo simulations of hysteretic behavior of 2D Ising spins by Lo and Pelcovits [Phys. Rev. A 42, 7471 (1990)].

Sengupta, Surajit; Marathe, Yatin; Puri, S.

1992-04-01

67

Two-dimensional full-wave simulation of microwave reflectometry on Alcator C-Mod

A new two-dimensional full-wave code has been developed to simulate ordinary (O) mode reflectometry signals caused by plasma density fluctuations. The code uses the finite-difference time-domain method with a perfectly matched layer absorption boundary to solve Maxwell's equations. Huygens wave sources are incorporated to generate Gaussian beams. The code has been used to simulate the reflectometer measurement of the quasicoherent mode (60--250 kHz) associated with enhanced D{sub {alpha}} (EDA) H modes in the Alcator C-Mod tokamak. It is found that an analysis of the realistic experimental layout is essential for the quantitative interpretation of the mode amplitude.

Lin, Y.; Irby, J. H.; Nazikian, R.; Marmar, E. S.; Mazurenko, A.

2001-01-01

68

Jet Formation in MHD Simulations

In this talk I review the current status of jet formation in direct numerical simulations of black-hole accretion disks and magnetospheres. I address the following critical questions: What constitutes the jet? What is the launching mechanism? Where is the launching point of the jet? What is the Lorentz factor? What is the opening angle? How is the jet collimated? Just as importantly, I also discuss how dependent the answers to the above questions are on factors such as the initial conditions of the simulation. I end by discussing possible future directions for this research.

P. Chris Fragile

2008-10-02

69

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

70

Computer simulations of a two-dimensional system with competing interactions.

The results and methodology of large scale computer simulations of the two-dimensional dipolar Ising model with long-range interactions are reported. Systems as large as 117,649 particles were studied to elucidate the elementary excitations and phase diagram of two-dimensional systems, such as Langmuir monolayers, thin garnet films, and adsorbed films on solid surfaces, which spontaneously form patterns of stripes, bubbles, and intermediately shaped domains. The challenging numerical investigations of large scale systems with long-range interactions at low temperatures were made possible by combining the fast multipole method and a non-Metropolis Monte Carlo sampling technique. Our simulations provide evidence that, at sufficiently high ratios of the repulsive to the attractive coupling constant for the model, twofold stripe order in the systems of interest is lost through a defect-mediated mechanism. Heat capacity data and the excitations observed in our simulations as the system disorders indicate that it is most likely an instance of a Kosterlitz-Thouless phase transition. The results from simulations with and without external field are in excellent agreement with the predictions of an analytic scaling theory [A. D. Stoycheva and S. J. Singer, Phys. Rev. E 64, 016118 (2001)], confirming the phase diagram furnished by the analytic model. The scaling theory suggests that, under certain conditions, defect-mediated stripe melting may be supplanted by Ising like disordering within stripes for small repulsion strength. A qualitative discussion of a model that supports both disordering mechanisms is presented. PMID:11909306

Stoycheva, Antitsa D; Singer, Sherwin J

2002-03-01

71

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

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

2006-10-15

72

Particle dynamics in two-dimensional random energy landscapes - experiments and simulations

The dynamics of individual colloidal particles in random potential energy landscapes were 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 set-up with a spatial light modulator, and the particle trajectories were followed by video microscopy. The dynamics are 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 are initially diffusive, show an extended sub-diffusive 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.

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

2013-02-13

73

MHD simulations of penumbra fine structure

We present results of numerical 3D MHD simulations with radiative energy transfer of fine structure in a small sunspot of about 4 Mm width. The results show the development of filamentary structure with nearly field-free gaps, interlaced by concentrations of stronger magnetic field at the interface between the umbra and the outer field-free atmosphere. Calculated synthetic images show dark cores like those seen in the observations. They are the result of an elevated $\\tau=1$ surface. The magnetic field in these cores is weaker and more horizontal than for adjacent brighter structures at the photosphere. Higher up in the atmosphere there are only small variations in field strength. Movies made show an inward migration of filamentary structures very similar to the patterns seen in observations of the inner penumbra. Although the filamentary structures in the simulations are too short compared to observations, most other aspects of the simulations appear consistent with observed penumbra filamentary structures.

Heinemann, T; Scharmer, G B; Spruit, H C

2006-01-01

74

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

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

2012-07-10

75

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

76

NASA Astrophysics Data System (ADS)

Finite-size, two-dimensional one-component Coulomb systems (charged rods) with a circular free boundary were simulated by Brownian equation-of-motion dynamics. Positional- and orientational-order correlation functions, site-coordination defects, and particle displacements were examined as a function of the coupling constant ?=q2kBT for samples of 253 and 583 particles. Melting of single-crystal samples occurs as increasing disorder with decreasing ? without a conspicuous feature in the ? dependence of order parameters. A transition between hexaticlike and isotropic fluid phases is identified at ?~148 for the 583-particle sample and ?~153 for the 253-particle sample. Results are compared with Monte Carlo and molecular-dynamics simulations and the theory of dislocation-mediated melting of lattices of quantized vortices.

Fiory, A. T.

1983-07-01

77

Creation of two-dimensional coulomb crystals of ions in oblate Paul traps for quantum simulations

We develop the theory to describe the equilibrium ion positions and phonon modes for a trapped ion quantum simulator in an oblate Paul trap that creates two-dimensional Coulomb crystals in a triangular lattice. By coupling the internal states of the ions to laser beams propagating along the symmetry axis, we study the effective Ising spin-spin interactions that are mediated via the axial phonons and are less sensitive to ion micromotion. We find that the axial mode frequencies permit the programming of Ising interactions with inverse power law spin-spin couplings that can be tuned from uniform to $r^{-3}$ with DC voltages. Such a trap could allow for interesting new geometrical configurations for quantum simulations on moderately sized systems including frustrated magnetism on triangular lattices or Aharonov-Bohm effects on ion tunneling. The trap also incorporates periodic boundary conditions around loops which could be employed to examine time crystals.

Bryce Yoshimura; Marybeth Stork; Danilo Dadic; W. C. Campbell; J. K. Freericks

2014-06-20

78

Creation of two-dimensional coulomb crystals of ions in oblate Paul traps for quantum simulations

We develop the theory to describe the equilibrium ion positions and phonon modes for a trapped ion quantum simulator in an oblate Paul trap that creates two-dimensional Coulomb crystals in a triangular lattice. By coupling the internal states of the ions to laser beams propagating along the symmetry axis, we study the effective Ising spin-spin interactions that are mediated via the axial phonons and are less sensitive to ion micromotion. We find that the axial mode frequencies permit the programming of Ising interactions with inverse power law spin-spin couplings that can be tuned from uniform to $r^{-3}$ with DC voltages. Such a trap could allow for interesting new geometrical configurations for quantum simulations on moderately sized systems including frustrated magnetism on triangular lattices or Aharonov-Bohm effects on ion tunneling. The trap also incorporates periodic boundary conditions around loops which could be employed to examine time crystals.

Yoshimura, Bryce; Dadic, Danilo; Campbell, W C; Freericks, J K

2014-01-01

79

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. PMID:20481498

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

2010-01-01

80

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

81

Two-dimensional simulation of contact surface instabilities in shock-tubes

NASA Astrophysics Data System (ADS)

Numerical simulations of the interaction of Mach 1.3 shock waves and gaseous interfaces between air and helium or SF6 were carried out using EAD, a two-dimensional, second-order non-viscous Eulerian code. The interface, described either by a single sinusoidal wave or a combination of sinusoidal waves of random amplitudes is subjected to Rayleigh-Taylor instability induced by the incident shock and successive waves reflected from the shock tube end wall. The evolution of the subsequent mixing zone thickness and the kinetic energy of the fluctuating velocity field associated with the mixing are extracted from the simulation and compared with theoretical estimates based on a quasi-incompressible analysis of the instability.

Besnard, Didier; Haas, Jean-Francois

82

Forced Reconnection in the Near Magnetotail: Onset and Energy Conversion in PIC and MHD Simulations

NASA Technical Reports Server (NTRS)

Using two-dimensional particle-in-cell (PIC) together with magnetohydrodynamic (MHD) Q1 simulations of magnetotail dynamics, we investigate the evolution toward onset of reconnection and the subsequent energy transfer and conversion. In either case, reconnection onset is preceded by a driven phase, during which magnetic flux is added to the tail at the high-latitude boundaries, followed by a relaxation phase, during which the configuration continues to respond to the driving. The boundary deformation leads to the formation of thin embedded current sheets, which are bifurcated in the near tail, converging to a single sheet farther out in the MHD simulations. The thin current sheets in the PIC simulation are carried by electrons and are associated with a strong perpendicular electrostatic field, which may provide a connection to parallel potentials and auroral arcs and an ionospheric signal even prior to the onset of reconnection. The PIC simulation very well satisfies integral entropy conservation (intrinsic to ideal MHD) during this phase, supporting ideal ballooning stability. Eventually, the current intensification leads to the onset of reconnection, the formation and ejection of a plasmoid, and a collapse of the inner tail. The earthward flow shows the characteristics of a dipolarization front: enhancement of Bz, associated with a thin vertical electron current sheet in the PIC simulation. Both MHD and PIC simulations show a dominance of energy conversion from incoming Poynting flux to outgoing enthalpy flux, resulting in heating of the inner tail. Localized Joule dissipation plays only a minor role.

Birn, J.; Hesse, Michael

2014-01-01

83

Aerodynamic effects of simulated ice shapes on two-dimensional airfoils and a swept finite tail

NASA Astrophysics Data System (ADS)

An experimental study was conducted to investigate the effect of simulated glaze ice shapes on the aerodynamic performance characteristics of two-dimensional airfoils and a swept finite tail. The two dimensional tests involved two NACA 0011 airfoils with chords of 24 and 12 inches. Glaze ice shapes computed with the LEWICE code that were representative of 22.5-min and 45-min ice accretions were simulated with spoilers, which were sized to approximate the horn heights of the LEWICE ice shapes. Lift, drag, pitching moment, and surface pressure coefficients were obtained for a range of test conditions. Test variables included Reynolds number, geometric scaling, control deflection and the key glaze ice features, which were horn height, horn angle, and horn location. For the three-dimensional tests, a 25%-scale business jet empennage (BJE) with a T-tail configuration was used to study the effect of ice shapes on the aerodynamic performance of a swept horizontal tail. Simulated glaze ice shapes included the LEWICE and spoiler ice shapes to represent 9-min and 22.5-min ice accretions. Additional test variables included Reynolds number and elevator deflection. Lift, drag, hinge moment coefficients as well as boundary layer velocity profiles were obtained. The experimental results showed substantial degradation in aerodynamic performance of the airfoils and the swept horizontal tail due to the simulated ice shapes. For the two-dimensional airfoils, the largest aerodynamic penalties were obtained when the 3-in spoiler-ice, which was representative of 45-min glaze ice accretions, was set normal to the chord. Scale and Reynolds effects were not significant for lift and drag. However, pitching moments and pressure distributions showed great sensitivity to Reynolds number and geometric scaling. For the threedimensional study with the swept finite tail, the 22.5-min ice shapes resulted in greater aerodynamic performance degradation than the 9-min ice shapes. The addition of 24-grit roughness to the LEWICE shapes produced greater losses than corresponding smooth ice shapes. Spoiler-ice with constant spanwise height caused larger performance losses than spoiler-ice with height scaled as a function of local chord length. Aerodynamic performance degradation due to the variable height spoiler-ice was similar to that obtained with the corresponding LEWICE shapes.

Alansatan, Sait

84

NASA Technical Reports Server (NTRS)

Investigations of one- and two-dimensional (1- or 2-D) simulations of Stirling machines centered around experimental data generated by the U. of Minnesota Mechanical Engineering Test Rig (METR) are covered. This rig was used to investigate oscillating flows about a zero mean with emphasis on laminar/turbulent flow transitions in tubes. The Space Power Demonstrator Engine (SPDE) and in particular, its heater, were the subjects of the simulations. The heater was treated as a 1- or 2-D entity in an otherwise 1-D system. The 2-D flow effects impacted the transient flow predictions in the heater itself but did not have a major impact on overall system performance. Information propagation effects may be a significant issue in the simulation (if not the performance) of high-frequency, high-pressure Stirling machines. This was investigated further by comparing a simulation against an experimentally validated analytic solution for the fluid dynamics of a transmission line. The applicability of the pressure-linking algorithm for compressible flows may be limited by characteristic number (defined as flow path information traverses per cycle); this warrants further study. Lastly the METR was simulated in 1- and 2-D. A two-parameter k-w foldback function turbulence model was developed and tested against a limited set of METR experimental data.

Goldberg, Louis F.

1990-01-01

85

Percolation analysis of nonlinear structures in scale-free two-dimensional simulations

NASA Technical Reports Server (NTRS)

Results are presented of applying percolation analysis to several two-dimensional N-body models which simulate the formation of large-scale structure. Three parameters are estimated: total area (a(c)), total mass (M(C)), and percolation density (rho(c)) of the percolating structure at the percolation threshold for both unsmoothed and smoothed (with different scales L(s)) nonlinear with filamentary structures, confirming early speculations that this type of model has several features of filamentary-type distributions. Also, it is shown that, by properly applying smoothing techniques, many problems previously considered detrimental can be dealt with and overcome. Possible difficulties and prospects with the use of this method are discussed, specifically relating to techniques and methods already applied to CfA deep sky surveys. The success of this test in two dimensions and the potential for extrapolation to three dimensions is also discussed.

Dominik, Kurt G.; Shandarin, Sergei F.

1992-01-01

86

Two-dimensional dynamic stall as simulated in a varying freestream

NASA Technical Reports Server (NTRS)

A low speed wind tunnel equipped with a axial gust generator to simulate the aerodynamic environment of a helicopter rotor was used to study the dynamic stall of a pitching blade in an effort to ascertain to what extent harmonic velocity perturbations in the freestream affect dynamic stall. The aerodynamic moment on a two dimensional, pitching blade model in both constant and pulsating airstream was measured. An operational analog computer was used to perform on-line data reduction and plots of moment versus angle of attack and work done by the moment were obtained. The data taken in the varying freestream were then compared to constant freestream data and to the results of two analytical methods. These comparisons show that the velocity perturbations have a significant effect on the pitching moment which can not be consistently predicted by the analytical methods, but had no drastic effect on the blade stability.

Pierce, G. A.; Kunz, D. L.; Malone, J. B.

1978-01-01

87

Two-dimensional computational modeling of sodium boiling in simulated LMFBR fuel-pin bundles

Extensive sodium boiling tests have been carried out in two simulated LMFBR fuel pin bundles in the Thermal-Hydraulic Out-of-Reactor Safety (THORS) Facility at Oak Ridge National Laboratory. Experimental results from a 19-pin bundle (THORS Bundle 6A) have been previously reported, and experimental results from a 61-pin bundle (THORS Bundle 9) will be reported soon. The results discussed here are from the 19-pin bundle. Preliminary analysis has shown that the computational methods used and conclusions reached are equally valid for the 61-pin bundle, as well as the 19-pin in-reactor Sodium Loop Safety Facility (SLSF) W-1 experiment. The main result of THORS sodium boiling experimentation is that boiling behavior is determined by two-dimensional effects, i.e., the rates of mass, momentum and energy transfer in the direction perpendicular to the axes of the fuel pins.

Dearing, J.F.

1981-01-01

88

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

89

NASA Astrophysics Data System (ADS)

Global observation of the GPS satellite constellation for ionospheric diagnostics is now a worldwide activity driven by both practical and scientific objectives. Diagnostic methods exploit an induced frequency-dependent phase change, which is proportional to the path-integrated electron density (TEC). However, intermediate-scale structure causes a stochastic modulation of the GPS signals (scintillation), which is a nuisance for data assimilation. Indeed, sufficiently strong propagation disturbances degrade TEC and ultimately disrupt GPS operations altogether. However, the physical processes that generate intermediate-scale structure are intimately part of ionospheric physics. In the best of all possible worlds irregularity identification and classification would be an integral part of ionospheric diagnostics. A two-dimensional propagation model has been used to explore the ramifications of intermediate scale structure on TEC data assimilation and, particularly tomographic reconstruction. Although two-dimensional models confine structure to a planar region, the results are relevant, with caveats, to propagation through highly anisotropic ionospheric structures. In-plane propagation from a source to an array of receivers is amenable to reconstruction with tomographic filtered back-projection algorithms. The angle-dependent Fourier decomposition of the array signal phase identifies a spectral slice. The signals from which the phase is derived are generated with an oblique forward propagation procedure developed by Costa and Basu Costa:02. Under weak-scatter conditions the signal phase is proportional to the ray path integral. The primary challenge for simulation is realistic simulation of the structure environment. Large scale ESF structure can be constructed with physics-based models, but populating the intermediate scale requires untested structure hypotheses. In particular, there is no clear demarcation between quasi-deterministic variation and definitive stochastic variation. Fractional Brownian motion admits trend-like structure variation at large scales that overlaps physics-based realizations. The two-dimensional constraint makes it feasible to explore a broad range of configurations, including meridional field-aligned structure. The results demonstrate the ramifications of diffractive distortion of path-integrated phase as well as unresolved stochastic structure. Anisotropic structure that subtends the distrubed region complicates the reconstruction procedures, particularly under strong scatter conditions. The utility of backpropagation to mitigate propagation disturbances will also be explored.

Rino, C. L.

2013-12-01

90

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

91

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

92

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 D{sub 2}O and solid D{sub 2} (sD{sub 2}). The D{sub 2}O was investigated as the neutron moderator, and sD{sub 2} 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 sD{sub 2}. 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 sD{sub 2} volume) equal to 6.79?×?10{sup 6} cm{sup ?2}s{sup ?1} and 2.20 ×10{sup 5} cm{sup ?3}s{sup ?1}, respectively.

Gheisari, R., E-mail: gheisari@pgu.ac.ir [Physics Department, Persian Gulf University, Bushehr 75169 (Iran, Islamic Republic of); Nuclear Energy Research Center, Persian Gulf University, Bushehr 75169 (Iran, Islamic Republic of); Firoozabadi, M. M.; Mohammadi, H. [Department of Physics, University of Birjand, Birjand 97175 (Iran, Islamic Republic of)] [Department of Physics, University of Birjand, Birjand 97175 (Iran, Islamic Republic of)

2014-01-15

93

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

94

Two-dimensional Monte Carlo simulations of ionic and nonionic silane self-assembly on hydrophilic in MEMS. The Monte Carlo method is used to simulate in two dimensions the self-assembly of silane films, a molecular level study of these films is not available. Atomistic simulation techniques, such as Monte Carlo

Deymier, Pierre

95

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

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

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

2005-06-01

96

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

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

Higashitani; Iimura

1998-08-15

97

High-volume fraction simulations of two-dimensional vesicle suspensions

NASA Astrophysics Data System (ADS)

We consider numerical algorithms for the simulation of the rheology of two-dimensional vesicles suspended in a viscous Stokesian fluid. The vesicle evolution dynamics is governed by hydrodynamic and elastic forces. The elastic forces are due to local inextensibility of the vesicle membrane and resistance to bending. Numerically resolving vesicle flows poses several challenges. For example, we need to resolve moving interfaces, address stiffness due to bending, enforce the inextensibility constraint, and efficiently compute the (non-negligible) long-range hydrodynamic interactions. Our method is based on the work of Rahimian et al. (2010) [33]. It is a boundary integral formulation of the Stokes equations coupled to the interface mass continuity and force balance. We extend the algorithms presented in that paper to increase the robustness of the method and enable simulations with concentrated suspensions. In particular, we propose a scheme in which both intra-vesicle and inter-vesicle interactions are treated semi-implicitly. In addition we use special integration for near-singular integrals and we introduce a spectrally accurate collision detection scheme. We test the proposed methodologies on both unconfined and confined flows for vesicles whose internal fluid may have a viscosity contrast with the bulk medium. Our experiments demonstrate the importance of treating both intra-vesicle and inter-vesicle interactions accurately.

Quaife, Bryan; Biros, George

2014-10-01

98

NASA Astrophysics Data System (ADS)

A new quantum simulation approach has been applied in the present work to the two-dimensional (2D) ferromagnetic and antiferromagnetic Ising lattices to calculate their magnetic structures, magnetizations, free energies and specific heats in the absence of an external magnetic field. Surprisingly, no size effects could be observed in our simulations performed for the Ising lattices of different sizes. Most importantly, our calculated spontaneous thermally averaged spins for the two kinds of systems are exactly same as those evaluated with quantum mean field theory, and the magnetic structures simulated at all chosen temperatures are perfectly ferromagnetic or antiferromagnetic, verifying the correctness and applicability of our quantum model and computational algorithm. On the other hand, if the classical Monte Carlo (CMC) method is applied to the ferromagnetic 2D Ising lattice with S=1, it is able to generate correct magnetization well consistent with Onsager's theory; but in the case of S=1/2, the computational results of CMC are incomparable to those predicted with the quantum mean field theory, giving rise to very much reduced magnetization and considerably underestimated Curie temperature. The difficulty met by the CMC method is mainly caused by its improperly calculated exchange energy of the randomly selected spin in every simulation step, especially immediately below the transition temperature, where the thermal averages of spins are much less than 1/2, however they are assigned to ±1/2 by CMC to evaluate the exchange energies of the spins, such improper manipulation is obviously impossible to lead the code to converge to the right equilibrium states of the spin systems.

Liu, Z.-S.; Sechovský, V.; Diviš, M.

2015-02-01

99

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

NASA Astrophysics Data System (ADS)

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.

1994-04-01

100

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

101

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

102

Two-dimensional Hydrodynamic Simulations of Zero-Age Main-Sequence Convective Cores

NASA Astrophysics Data System (ADS)

Two-dimensional, fully implicit hydrodynamic simulations of the structure and motion of convective cores are made on complete zero-age main-sequence stellar models between 1.2 and 20 Msolar. The calculations present a picture of hydrogen-burning convective cores where the convective speeds are tens of kilometers per second and the superadiabatic excess may be quite large at various locations in the core at any time. However, when averaged over angle, radius, and time the superadiabatic excess is only about 10-3, although the steps to obtain this number involve some uncertainties. Convective core overshooting occurs in all models. In units of the pressure scale height at the convective core boundary, the overshooting calculated is about 0.45Hp for models above 3 Msolar, and it decreases to about 0.3Hp for lower mass models. Part of this decrease arises from the increase in the pressure scale height at the convective core boundary as the convective core boundary approaches the center of the model while the model mass becomes small. However, using the actual e-folding distance for the pressure as the unit of measure, the overshooting still shows a small decrease for lower masses.

Deupree, Robert G.

2000-11-01

103

Simulated two-dimensional electronic spectroscopy of the eight-bacteriochlorophyll FMO complex.

The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire conducting energy between the outer antenna system and the reaction center; it is an important photosynthetic system to study the transfer of excitonic energy. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a) in some of the FMO monomers. To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways: (1) it is directly involved in the first apo form pathway (6 ? 3 ? 1) by passing the excitonic energy to exciton 6; and (2) it facilitates an increase in the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 ? 4,5 ? 2 ? 1) and thus increases the possible downward sampling routes across the BChls. PMID:25527917

Yeh, Shu-Hao; Kais, Sabre

2014-12-21

104

Simulated two-dimensional electronic spectroscopy of the eight-bacteriochlorophyll FMO complex

NASA Astrophysics Data System (ADS)

The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire conducting energy between the outer antenna system and the reaction center; it is an important photosynthetic system to study the transfer of excitonic energy. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a) in some of the FMO monomers. To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways: (1) it is directly involved in the first apo form pathway (6 ? 3 ? 1) by passing the excitonic energy to exciton 6; and (2) it facilitates an increase in the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 ? 4,5 ? 2 ? 1) and thus increases the possible downward sampling routes across the BChls.

Yeh, Shu-Hao; Kais, Sabre

2014-12-01

105

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

106

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.

2013-01-01

107

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

108

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

109

We present two methods for solving the electrostatics of point charges and multipoles on the surface of a sphere, \\textit{i.e.} in the space $\\mathcal{S}_{2}$, with applications to numerical simulations of two-dimensional polar fluids. In the first approach, point charges are associated with uniform neutralizing backgrounds to form neutral pseudo-charges, while, in the second, one instead considers bi-charges, \\textit{i.e.} dumbells of antipodal point charges of opposite signs. We establish the expressions of the electric potentials of pseudo- and bi-charges as isotropic solutions of the Laplace-Beltrami equation in $\\mathcal{S}_{2}$. A multipolar expansion of pseudo- and bi-charge potentials leads to the electric potentials of mono- and bi-multipoles respectively. These potentials constitute non-isotropic solutions of the Laplace-Beltrami equation the general solution of which in spherical coordinates is recast under a new appealing form. We then focus on the case of mono- and bi-dipoles and build the theory of dielectric media in $\\mathcal{S}_{2}$. We notably obtain the expression of the static dielectric constant of a uniform isotropic polar fluid living in $\\mathcal{S}_{2}$ in term of the polarization fluctuations of subdomains of $\\mathcal{S}_{2}$. We also derive the long range behavior of the equilibrium pair correlation function under the assumption that it is governed by macroscopic electrostatics. These theoretical developments find their application in Monte Carlo simulations of the $2D$ fluid of dipolar hard spheres. Some preliminary numerical experiments are discussed with a special emphasis on finite size effects, a careful study of the thermodynamic limit, and a check of the theoretical predictions for the asymptotic behavior of the pair correlation function.

Jean-Michel Caillol

2015-01-22

110

Extended MHD NIMROD Simulations of HIT-SI plasmas

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

111

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

112

Two-dimensional pulsed-plasma simulation of a chlorine discharge Badri Ramamurthi and Demetre J chlorine discharge sustained in an inductively coupled plasma ICP reactor with a planar coil. The self of a pulsed chlorine discharge to achieve significant reduction in charging damage notching compared

Economou, Demetre J.

113

Two-dimensional simulation of red blood cell motion near a wall under a lateral force

NASA Astrophysics Data System (ADS)

The motion of a red blood cell suspended in a linear shear flow adjacent to a fixed boundary subject to an applied lateral force directed toward the boundary is simulated. A two-dimensional model is used that represents the viscous and elastic properties of normal red blood cells. Shear rates in the range of 100 to 600 s-1 are considered, and the suspending medium viscosity is 1 cP. In the absence of a lateral force, the cell executes a tumbling motion. With increasing lateral force, a transition from tumbling to tank-treading is predicted. The minimum force required to ensure tank-treading increases nonlinearly with the shear rate. Transient swinging motions occur when the force is slightly larger than the transition value. The applied lateral force is balanced by a hydrodynamic lift force resulting from the positive orientation of the long axis of the cell with respect to the wall. In the case of cyclic tumbling motions, the orientation angle takes positive values through most of the cycle, resulting in lift generation. These results are used to predict the motion of a cell close to the outer edge of the cell-rich core region that is generated when blood flows in a narrow tube. In this case, the lateral force is generated by shear-induced dispersion, resulting from cell-cell interactions in a region with a concentration gradient. This force is estimated using previous data on shear-induced dispersion. The cell is predicted to execute tank-treading motions at normal physiological hematocrit levels, with the possibility of tumbling at lower hematocrit levels.

Hariprasad, Daniel S.; Secomb, Timothy W.

2014-11-01

114

Two-dimensional simulation of Pinatubo aerosol and its effect on stratospheric ozone

NASA Technical Reports Server (NTRS)

This paper presents time-dependent simulations of the response of the stratosphere to the injection into the atmosphere of massive amounts of sulfur during the eruption of Mt. Pinatubo (The Philippines) in June 1991. The study is based on a coupled two-dimensional chemical-dynamical-radiative model to which a microphysical model for sulfate aerosol formation and fate has been added. The study suggests that, during the first year (July 1991 to June 1992) following the volcanic eruption, the observed changes in the ozone amount integrated between 65 deg S and 65 deg N were caused primarily by changes in the meridional circulation (associated with heating by the volcanic cloud in the tropics) and in the photolysis rate of molecules such as ozone (associated with backscattering of light by the cloud). During the second year after the eruption, as the aerosol was dispersed at all latitudes and, in particular, reached the polar region, the largest contribution to ozone reduction resulted from the heterogeneous chemical conversion of N2O5 and ClONO2 on the surface of the aerosol particles. The conversion of the latter compound, and hence the magnitude of the calculated ozone depletion, is highly dependent on the temperature in the lower stratosphere. Despite the fact that the surface area provided by aerosol particles decreased during the second year following the eruption, the calculated ozone depletion remained significant because the conversion of N2O5 is insensitive to the aerosol surface area density for values larger than 1-10 sq microns/cu cm (depending on latitude). The predicted reduction in ozone at 20 km in March during the third year (July 1993 to June 1994) of the model integration is smaller by a factor of 2 than it was during the second year.

Tie, Xuexi; Brasseur, Guy P.; Briegleb, Bruce; Granier, Claire

1994-01-01

115

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, and 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.; Radke, C.J. [Lawrence Berkeley National Lab., CA (United States)] [Lawrence Berkeley National Lab., CA (United States); Patzek, T.W. [Univ. of California, Berkeley, CA (United States)] [Univ. of California, Berkeley, CA (United States)

1996-05-10

116

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

117

The two-dimensional, transient numerical model of heat and water vapor convection and diffusion during air exfiltration within fiberglass insulation, presented in Part 1 is validated in Part 2, with experimental data for temperature, moisture and frost accumulation, and heat flux. With a few exceptions, the simulation results and experimental data agree within the experimental uncertainty. Exfiltration airflow in the two-dimensional space showed strong entrance and exit effects for temperature, moisture and frost accumulation, and heat flux on the cold side. Between the entrance and exit, the temperature, mass accumulation, and heat flux showed characteristics similar to a boundary layer.

Chen, H.; Besant, R.W. [Univ. of Saskatchewan, Saskatoon, Saskatchewan (Canada). Dept. of Mechanical Engineering; Tao, Y.T. [Tennessee State Univ., Nashville, TN (United States). Mechanical Engineering Dept.

1998-10-01

118

Quantum Monte Carlo simulation of a two-dimensional Bose gas

The equation of state of a homogeneous two-dimensional Bose gas is calculated using quantum Monte Carlo methods. The low-density universal behavior is investigated using different interatomic model potentials, both finite ranged and strictly repulsive and zero ranged, supporting a bound state. The condensate fraction and the pair distribution function are calculated as a function of the gas parameter, ranging from the dilute to the strongly correlated regime. In the case of the zero-range pseudopotential we discuss the stability of the gaslike state for large values of the two-dimensional scattering length, and we calculate the critical density where the system becomes unstable against cluster formation.

Pilati, S. [Dipartimento di Fisica, Universita di Trento and CRS-BEC INFM, I-38050 Povo (Italy); Departament de Fisica i Enginyeria Nuclear, Campus Nord B4-B5, Universitat Politecnica de Catalunya, E-08034 Barcelona (Spain); Boronat, J.; Casulleras, J. [Departament de Fisica i Enginyeria Nuclear, Campus Nord B4-B5, Universitat Politecnica de Catalunya, E-08034 Barcelona (Spain); Giorgini, S. [JILA, University of Colorado, Boulder, Colorado 80309-0440 (United States); Dipartimento di Fisica, Universita di Trento and CRS-BEC INFM, I-38050 Povo (Italy)

2005-02-01

119

MHD simulations of the collapsar model for GRBs

We present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.

D. Proga; A. I. MacFadyen; P. J. Armitage; M. C. Begelman

2003-12-11

120

NASA Technical Reports Server (NTRS)

It can be demonstrated analytically that under certain geometries used in numerical simulations of collisionless shocks in which there is at least one ignorable spatial coordinate, the transport of particles across the magnetic field is essentially zero. This notion is tested using one- and two-dimensional hybrid simulations (kinetic ions/fluid electrons). We find, as the theorem predicts, the particles treated kinetically are tied to the same field line on which they start.

Giacalone, Joe

1994-01-01

121

A two-dimensional particle-in-cell simulation of an electron-cyclotron-resonance etching tool

A particle-in-cell (PIC) simulation of an axisymmetric electron-cyclotron-resonance (ECR) etching tool is developed in which up to 2×106 particles per species are loaded in a two-dimensional spatial computational mesh (r,z), along with three velocity components (vr,v?,vz). An ECR heating scheme based on single-particle trajectories in the resonance zone generates the simulated plasma. Electron- and ion-neutral elastic and inelastic collisions are

K. A. Ashtiani; J. L. Shohet; W. N. G. Hitchon; G.-H. Kim; N. Hershkowitz

1995-01-01

122

The numerical simulation of two-dimensional aluminized composite solid propellent combustion

A numerical framework is presented which examines, for the first time, the burning of two-dimensional aluminized solid propellants. Aluminized composite propellants present a difficult mathematical and numerical challenge because of complex physics and topological changes that occur at the propellant surface. For example, both mathematical models and appropriate numerical solvers must describe the regressing burning surface, aluminium particle detachment and

X. Wang; T. Jackson

2005-01-01

123

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

124

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

125

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

126

Analysis of the two-dimensional simulation results of the near radiation and scattering fields of aircraft reflector-type antenna with sharp nose ogive radome are presented. The method is based on solving of the volume integral equations relatively a full field in a radome layer and integral or integro-differential equations relatively a current density on reflectors. After determination of the current density

S. V. Kukobko; A. Z. Sazonov; I. O. Sukharevsky

2007-01-01

127

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

NASA Astrophysics Data System (ADS)

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 volume scheme. A combination of radiation and characteristic boundary conditions is used in the far-field. Some comparisons are made with known analytic solutions and with previous investigators.

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

1990-10-01

128

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

129

A novel parallel technique for Fourier–Galerkin pseudo-spectral methods with applications to two-dimensional Navier–Stokes equations and inviscid Boussinesq approximation equations is presented. It takes advantage of the programming structure of the phase-shift de-aliased scheme for pseudo-spectral codes, and combines the task-distribution strategy [Z. Yin, H.J.H. Clercx, D.C. Montgomery, An easily implemented task-based parallel scheme for the Fourier pseudo-spectral solver applied to

Z. Yin; Li Yuan; Tao Tang

2005-01-01

130

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

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

2008-11-06

131

Two-dimensional self-consistent microwave argon plasma simulations with experimental verification

NASA Astrophysics Data System (ADS)

Optical emission spectroscopy (OES), absorption measurements, and thermal energy rate analysis were used in tandem with numerical models to characterize microwave argon plasmas. A WAVEMAT (model MPDR-3135) microwave diamond deposition system was used to generate argon plasmas at 5 Torr. Three excited state number densities (4p, 5p, and 5d) were obtained from the OES measurements, and a fourth excited state number density (4s) was obtained from the absorption measurements. Further, power absorbed in the substrate was monitored. A self-consistent two-dimensional argon model coupled with an electromagnetic field model and a 25-level two-dimensional (2D)-collisional-radiative model (CRM) was developed and validated with the experimental measurements. The 2D model provides the gas and electron temperature distributions, and the electron, ion, and 4s state number densities, which are then iteratively fed into the electromagnetic and CRM models. Both the numerically predicted thermal energy rates and excited state densities agreed, within the experimental and numerical uncertainties, with the experimental results.

Li, Y.; Gordon, M. H.; Roe, L. A.; Hassouni, K.; Grotjohn, T.

2003-07-01

132

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

NASA Astrophysics Data System (ADS)

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 of cryogenic flow states. Based on the numerical results, a two-dimensional structure of the boiling two-phase flow of liquid nitrogen is shown in detail, and it is found that the phase change of liquid nitrogen occurs in quite a short time interval compared with that of two-phase pressurized water at high temperature. Next, it is clarified that the distributions of pressure and the void fraction in a two-phase flow show a tendency different from those of fluids at room temperature because of the decrease in sound velocity due to large compressibility and the rapid phase change velocity in a cryogenic two-phase mixture flow. According to these numerical results, the fundamental characteristics of the cryogenic two-phase flow are predicted. The numerical results obtained will contribute to advanced cryogenic industrial applications.

Ishimoto, Jun; Oike, Mamoru; Kamijo, Kenjiro

133

NASA Astrophysics Data System (ADS)

Two-dimensional numerical simulations are used to compare etched geometry and overgrown Si permeable base transistors (PTBs), considering both the etched collector and etched emitter biasing conditions made possible by the asymmetry of the etched structure. In PTB devices, the two-dimensional nature of the depletion region near the Schottky contact base grating results in a smaller electron barrier and, therefore, a larger collector current in the etched than in the overgrown structure. The parasitic feedback effects which result at high base-to-emitter bias levels lead to a deviation from the square-law behavior found in the collector characteristics of the overgrown PBT. These structures also have lower device capacitances and smaller transconductances at high base-to-emitter voltages. As a result, overgrown and etched structures have comparable predicted maximum values of the small signal unity short-circuit current gain frequency and maximum oscillation frequency.

Vojak, B. A.; Alley, G. D.

1983-08-01

134

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

135

Two-dimensional photoelastic simulation of a castable ceramic fixed partial denture.

Two-dimensional photoelastic models were used to determine the optimum design and loading conditions for a three-unit castable ceramic fixed partial denture. Findings relating to design and material variables can be summarized as follows. 1. Higher modulus cements impart more rigidity to the fixed partial denture design and result in lower stresses. 2. A short connector results in higher stress than a long connector; however, the length of the connector should not exceed one half the mesial-to-distal length of the pontic. 3. An intermediate thickness of cement results in the best stress distribution. 4. The male connector is best attached to the bulkier abutment. 5. Occlusal cement eccentricity is less desirable than cervical eccentricity. 6. The most undesirable loading site is directly above the connector. PMID:3276881

Farah, J W; Craig, R G; Eden, G T; Grossman, D G

1988-01-01

136

NASA Astrophysics Data System (ADS)

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

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

2009-10-01

137

MHD Simulation of Magnetically Driven, HyperVelocity Flyer Plates

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

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

2001-01-01

138

MHD solution of interplanetary disturbances generated by simulated velocity perturbations

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

Murray Dryer; R. S. Steinolfson

1976-01-01

139

A two-dimensional dispersion module for the TOUGH2 simulator

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

Oldenburg, C.M.; Pruess, K.

1993-09-01

140

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

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

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

2010-11-15

141

Hybrid Vlasov-Maxwell simulations of two-dimensional turbulence in plasmas

Turbulence in plasmas is a very challenging problem since it involves wave-particle interactions, which are responsible for phenomena such as plasma dissipation, acceleration mechanisms, heating, temperature anisotropy, and so on. In this work, a hybrid Vlasov-Maxwell numerical code is employed to study local kinetic processes in a two-dimensional turbulent regime. In the present model, ions are treated as a kinetic species, while electrons are considered as a fluid. As recently reported in [S. Servidio, Phys. Rev. Lett. 108, 045001 (2012)], nearby regions of strong magnetic activity, kinetic effects manifest through a deformation of the ion velocity distribution function that consequently departs from the equilibrium Maxwellian configuration. Here, the structure of turbulence is investigated in detail in phase space, by evaluating the high-order moments of the particle velocity distribution, i.e., temperature, skewness, and kurtosis. This analysis provides quantitative information about the non-Maxwellian character of the system dynamics. This departure from local thermodynamic equilibrium triggers several processes commonly observed in many astrophysical and laboratory plasmas.

Valentini, F.; Servidio, S.; Veltri, P. [Dipartimento di Fisica, Università della Calabria, I-87036 Rende (CS) (Italy); Perrone, D. [LESIA, Observatoire de Paris, 92190 Meudon (France); Califano, F. [Dipartimento di Fisica and CNISM, Università di Pisa, 56127 Pisa (Italy); Matthaeus, W. H. [Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, Delaware 19716 (United States)

2014-08-15

142

Two-dimensional Monte Carlo simulation of a submicron GaAs MESFET with a nonuniformly doped channel

NASA Astrophysics Data System (ADS)

A two-dimensional-ensemble Monte Carlo program coupled with a program for solving Poisson's equation is used to perform a self-consistent simulation of a GaAs MESFET having a nonuniformly doped (ion-implanted) channel. For Vgs = -0.5 V and Vds = 1 V, the simulation yields Ids = 18 mA/50 ?m, gm = 755 mS/mm and fT = 230 GHz. The results are compared to those obtained from a conventional 2D device-analysis program which uses static velocity-field characteristics and an empirical expression for low-field mobility versus doping concentration. The currents, transconductance, and cutoff frequency obtained from the Monte Carlo simulation are considerably larger than those obtained from the conventional 2D analysis. This difference is explained by the fact that the conventional device analysis program fails to account for transient transport phenomena.

Williams, C. K.; Glisson, T. H.; Hauser, J. R.; Littlejohn, M. A.; Abusaid, M. F.

1985-11-01

143

An automated reliable method for two-dimensional Reynolds-Averaged Navier-Stokes simulations

The development of computational fluid dynamics algorithms and increased computational resources have led to the ability to perform complex aerodynamic simulations. Obstacles remain which prevent autonomous and reliable ...

Modisette, James M

2011-01-01

144

The physical properties of the solar granulation are analyzed on the basis of 2-D fully compressible, radiation-hydrodynamic simulations and the synthetic spectra they produce. The basic physical and numerical treatment of the problem as well as tests of this treatment are described. The simulations are compared with spatially averaged spectral observations made near disk centre and high resolution spectra recorded

A. S. Gadun; S. K. Solanki; A. Johannesson

1999-01-01

145

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

146

Skill Score Analysis of MHD Simulation Global Magnetic Fields

NASA Astrophysics Data System (ADS)

We examine the ability of the Lyon-Fedder-Mobbary (LFM) MHD simulation to reproduce the global magnetic field and current systems in the inner magnetosphere during both quite times and over a range of storm conditions. Using the semi-empirical Tsgyanenko storm model (Tsyganenko, 2003) as a baseline, we quantify the strength and weakness of the MHD simulation through objective comparison with observations. This study extends on our previous related work that focused on the comparison of a single storm event (Huang et al., 2003, Fall AGU meeting). Here, we explore the global magnetic field configuration at various storm phases and as a function of local time, using formal skill score computation as a quantitative measure of model performance.

Huang, C.; Spence, H.; Lyon, J.; Goodrich, C.

2004-05-01

147

NASA Astrophysics Data System (ADS)

Based on the secondary electron emission avalanche (SEEA) model, the SEEA discharge on the vacuum insulator surface is simulated by using a 2D PIC-MCC code developed by ourselves. The evolutions of the number of discharge electrons, insulator surface charge, current, and 2D particle distribution are obtained. The effects of the strength of the applied electric field, secondary electron yield coefficient, rise time of the pulse, length of the insulator on the discharge are investigated. The results show that the number of the SEEA electrons presents a quadratic dependence upon the applied field strength. The SEEA current, which is on the order of Ampere, is directly proportional to the field strength and secondary electron yield coefficient. Finally, the electron-stimulated outgassing is included in the simulation code, and a three-phase discharge curve is presented by the simulation, which agrees with the experimental data.

Cai, Libing; Wang, Jianguo; Zhu, Xiangqin; Wang, Yue; Zhang, Dianhui

2015-01-01

148

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

2013-01-01

149

Numerical Simulations of MHD Turbulence in Accretion Disks

We review numerical simulations of MHD turbulence. The last decade has witnessed fundamental advances both in the technical capabilities of direct numerical simulation, and in our understanding of key physical processes. Magnetic fields tap directly into the free energy sources in a sufficiently ionized gas. The result is that adverse angular velocity and adverse temperature gradients, not the classical angular momentum and entropy gradients, destabilize laminar and stratified flow. This has profound consequences for astrophysical accretion flows, and has opened the door to a new era of numerical simulation experiments.}

Steven A. Balbus; John F. Hawley

2002-03-20

150

The physical properties of the solar granulation are analyzed on the basis of 2-D fully compressible, radiation- hydrodynamic simulations and the synthetic spectra they pro- duce. The basic physical and numerical treatment of the prob- lem as well as tests of this treatment are described. The simula- tions are compared with spatially averaged spectral observations made near disk centre and

A. S. Gadun; S. K. Solanki; A. Johannesson

1999-01-01

151

the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal FFCF . The simulations are also used to examine the number distribution of benzene and CCl4 molecules environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number

Fayer, Michael D.

152

The core helium flash revisited: I. One and two-dimensional hydrodynamic simulations

We investigate the hydrodynamics of the core helium flash near its peak. Past research concerned with the dynamics of this event is inconclusive. However, the most recent multidimensional hydrodynamic studies suggest a quiescent behavior and seem to rule out an explosive scenario. Previous work indicated, that depending on initial conditions, employed turbulence models, grid resolution, and dimensionality of the simulation, the core helium flash leads either to the disruption of a low-mass star or to a quiescent quasi-hydrostatic evolution. We try to clarify this issue by simulating the evolution with advanced numerical methods and detailed microphysics. Assuming spherical or axial symmetry, we simulate the evolution of the helium core of a $1.25 M_{\\odot}$ star with a metallicity Z=0.02 during the core helium flash at its peak with a grid-based hydrodynamics code. We find that the core helium flash neither rips the star apart, nor that it significantly alters its structure, as convection plays a crucial role in keeping the star in hydrostatic equilibrium. In addition, our simulations show the presence of overshooting, which implies new predictions concerning mixing of chemical species in red giants.

M. Mocak; E. Mueller; A. Weiss; K. Kifonidis

2008-05-09

153

Study of the collapse of granular columns using two-dimensional discrete-grain simulation

Numerical simulations of the collapse and spreading of granular columns onto a horizontal plane using the Contact Dynamics method are presented. The results are in agreement with previous experimental work. The final shape of the deposit appears to depend only on the initial aspect ratio a of the column. The normalized runout distance has a power-law dependence on the aspect

L. S TARON

154

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

155

Two-Dimensional Dynamic Simulation of Platelet Activation During Mechanical Heart Valve Closure

A major drawback in the operation of mechanical heart valve prostheses is thrombus formation in the near valve region. Detailed flow analysis in this region during the valve closure phase is of interest in understanding the relationship between shear stress and platelet activation. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a bi-leaflet mechanical

S. Krishnan; H. S. Udaykumar; J. S. Marshall; K. B. Chandran

2006-01-01

156

Hybrid modeling of collisionless reconnection in two-dimensional current sheets: Simulations

The stability of thin current sheets, such as commonly occur in space plasmas, is investigated in the presence of a magnetic normal component. The investigation is based on hybrid simulations using a suitably adapted electron pressure model. In order to properly represent the effects of the electron cyclotron motion in the magnetic normal component, we include the cyclotron part of

Michael Hesse; Dan Winske; Masha M. Kuznetsova

1995-01-01

157

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

158

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

159

Two-dimensional simulation of contact surface instabilities in shock-tubes

NASA Astrophysics Data System (ADS)

Numerical simulations of the interaction of Mach 1.3 shock waves and gaseous interfaces between air and helium or SF6 were carried out using EAD, a 2D, second order non-viscous Eulerian code. The interface, described either by a single sinusoidal wave or a combination of sinusoidal waves of random amplitudes is subjected to Rayleigh-Taylor instability induced by the incident shock and successive waves reflected from the shock tube end wall. The evolution of the subsequent mixing zone thickness and the kinetic energy of the fluctuating velocity field associated with the mixing are extracted from the simulation and compared with theoretical estimates based on an quasi-incompressible analysis of the instability.

Besnard, Didier; Haas, Jean-François

1990-07-01

160

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

161

NASA Astrophysics Data System (ADS)

The physical properties of the solar granulation are analyzed on the basis of 2-D fully compressible, radiation-hydrodynamic simulations and the synthetic spectra they produce. The basic physical and numerical treatment of the problem as well as tests of this treatment are described. The simulations are compared with spatially averaged spectral observations made near disk centre and high resolution spectra recorded near the solar limb. The present simulations reproduce a significant number of observed features, both at the centre of the solar disc and near the solar limb. Reproduced observables include the magnitude of continuum and line-core intensity fluctuations, line bisectors and correlations between different line parameters. Spatially averaged line shifts near disc centre, however, are not so well reproduced, as are individual correlations between line parameters near the solar limb. Possible causes of these discrepancies are discussed. The present models predict the existence of two photospheric layers at which the temperature fluctuations change sign. We point out a diagnostic of the hitherto undetected upper sign reversal based on high spatial resolution spectral observations of a sample of lines formed over a wide range of heights in the photosphere.

Gadun, A. S.; Solanki, S. K.; Johannesson, A.

1999-10-01

162

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

163

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

164

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

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

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

2004-09-27

165

NASA Astrophysics Data System (ADS)

Nanocrystalline silicon thin-film transistors present technological interest in that they combine many of the advantages of amorphous with those of polycrystalline Si structures. Progress in practical implementation of this technology is hampered by limited understanding of the conduction mechanisms in these structures and of the underlying relationship between device behavior and process manufacturing parameters. These mechanisms are explored through detailed simulation that includes model calibration and correlation with experimental results, as well as parametric sensitivity evaluation of this class of devices over the entire range of applied voltage. Through fitting of the tests results, a unique set of density of states was identified that characterizes the particular technology used. The leakage current was attributed to the band to band tunneling and thermal generation-recombination mechanisms. For devices with channel length of less than 20?m, the kink effect was observed in the output characteristics for high drain voltages and the impact ionization coefficient was determined.

Archontas, N.; Georgoulas, N.; Dimitriadis, C. A.; Templier, F.; Oudwan, M.; Kamarinos, G.

2008-05-01

166

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

167

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{sup 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.

Wu, Tianmin; Zhang, Ruiting; Li, Huanhuan; Zhuang, Wei, E-mail: wzhuang@dicp.ac.cn, E-mail: lijiangy@pku.edu.cn [State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning (China)] [State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning (China); Yang, Lijiang, E-mail: wzhuang@dicp.ac.cn, E-mail: lijiangy@pku.edu.cn [College of Chemistry and Molecular Engineering, Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871 (China)] [College of Chemistry and Molecular Engineering, Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871 (China)

2014-02-07

168

NASA Astrophysics Data System (ADS)

Number concentrations, size distributions, and mixing states of aerosols are essential parameters for accurate estimations of aerosol direct and indirect effects. In this study, we develop an aerosol module, designated the Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS), that can explicitly represent these parameters 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 sizes (12 bins) and BC mixing states (10 bins) for a total of 120 bins. The particles with diameters between 1 and 40 nm are resolved using additional eight 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. The 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. The application of ATRAS in 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.

2014-09-01

169

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

170

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 developed 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 µm 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 was 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 was about 50% over East Asia during the spring, and the contribution of SOA processes to the absorption enhancement was estimated to be 10 – 20% over northern East Asia and 20 – 35% over southern East Asia. A clear north-south contrast was also found between the impacts of NPF and SOA processes on cloud condensation nuclei (CCN) concentrations: NPF increased CCN concentrations at higher supersaturations (smaller particles) over northern East Asia, whereas SOA increased CCN concentrations at lower supersaturations (larger particles) over southern East Asia. Application of ATRAS to East Asia also showed that the impact of each process on each optical and radiative parameter depended 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, Makoto; Kondo, Yutaka; Fast, Jerome D.; Takigawa, M.

2014-09-30

171

Spectral Methods in General Relativistic MHD Simulations

NASA Astrophysics Data System (ADS)

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

Garrison, David

2012-03-01

172

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

173

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

174

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

175

NASA Astrophysics Data System (ADS)

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

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

1994-01-01

176

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

177

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

NASA Astrophysics Data System (ADS)

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

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

1994-02-01

178

This paper explores the application of SPH to a Direct Numerical Simulation (DNS) of decaying turbulence in a two-dimensional no-slip wall-bounded domain. In this bounded domain, the inverse energy cascade, and a net torque exerted by the boundary, result in a spontaneous spin up of the fluid, leading to a typical end state of a large monopole vortex that fills the domain. The SPH simulations were compared against published results using a high accuracy pseudo-spectral code. Ensemble averages of the kinetic energy, enstrophy and average vortex wavenumber compared well against the pseudo-spectral results, as did the evolution of the total angular momentum of the fluid. However, while the pseudo-spectral results emphasised the importance of the no-slip boundaries as generators of long lived coherent vortices in the flow, no such generation was seen in the SPH results. Vorticity filaments produced at the boundary were always dissipated by the flow shortly after separating from the boundary layer. The kinetic ene...

Robinson, Martin

2011-01-01

179

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 tau). 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 result from the slow and the fast silver ions in G(lg tau), respectively. For the 109Ag NMR two-time correlation functions F2(t), measured as a stimulated echo, a very stretched decay to F2SS(t(m)) = 0 is observed. When fitting to a Kohlrausch function, exp[-(t/tau)beta], a stretching parameter beta approximately = 0.2 is found. The temperature dependence of the mean correlation time of silver dynamics is described by an Arrhenius law where the activation energy is consistent with the one reported for the dc conductivity sigma(dc). In addition, it is shown that the effect of complex dynamical processes on NMR multi-time correlation functions can easily be calculated when performing random-walk simulations for schematic models such as the random-barrier model and the random-energy model. Based on these models it is possible to simulate various NMR observables and the mean square displacement, thus revealing the information content of multi-dimensional NMR experiments on solid ion conductors. PMID:12469819

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

2002-01-01

180

NASA Astrophysics Data System (ADS)

Spatial and seasonal distribution of ozone and other trace gases are simulated using the interactive two-dimensional model of the stratosphere (Schneider et al., 1989) 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 Kyy, which are based on values derived by Newman et al. (1988), show a hemispherical asymmetry in that the values in the lower stratosphere are consistently smaller in the southern hemisphere. The asymmetry in Kyy and 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

1993-11-01

181

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

182

Relativistic MHD simulations of stellar core collapse and magnetars

NASA Astrophysics Data System (ADS)

We present results from simulations of magneto-rotational stellar core collapse along with Alfvén oscillations in magnetars. These simulations are performed with the CoCoA/CoCoNuT code, which is able to handle ideal MHD flows in dynamical spacetimes in general relativity. Our core collapse simulations highlight the importance of genuine magnetic effects, like the magneto-rotational instability, for the dynamics of the flow. For the modelling of magnetars we use the anelastic approximation to general relativistic MHD, which allows for an effective suppression of fluid modes and an accurate description of Alfvén waves. We further compute Alfvén oscillation frequencies along individual magnetic field lines with a semi-analytic approach. Our work confirms previous results based on perturbative approaches regarding the existence of two families of quasi-periodic oscillations (QPOs), with harmonics at integer multiples of the fundamental frequency. Additional material is presented in the accompanying contribution by Gabler et al (2010b) in these proceedings.

Font, José A.; Cerdá-Durán, Pablo; Gabler, Michael; Müller, Ewald; Stergioulas, Nikolaos

2011-02-01

183

Global MHD Simulation of Mesoscale Structures at the Magnetospheric Boundary

NASA Technical Reports Server (NTRS)

The research carried out for this protocol was focused on the study of mesoscales structures at the magnetospheric boundary. We investigated three areas: (1) the structure of the magnetospheric boundary for steady solar wind conditions; (2) the dynamics of the dayside magnetospheric boundary and (3) the dynamics of the distant tail magnetospheric boundary. Our approach was to use high resolution three-dimensional global magnetohydrodynamic (MHD) simulations of the interaction of the solar wind with the Earth's magnetosphere. We first considered simple variations of the interplanetary conditions to obtain generic cases that helped us in establishing the basic cause and effect relationships for steady solar wind conditions. Subsequently, we used actual solar wind plasma and magnetic field parameters measured by an upstream spacecraft as input to the simulations and compared the simulation results with sequences of events observed by another or several other spacecraft located downstream the bow shock. In particular we compared results with observations made when spacecraft crossed the magnetospheric boundary.

Berchem, Jean

1998-01-01

184

Two-dimensional Vlasov simulations of nonlinear electron plasma waves are presented, in which the interplay of linear and nonlinear kinetic effects is evident. The plasma wave is created with an external traveling wave potential with a transverse envelope of width {Delta}y such that thermal electrons transit the wave in a ''sideloss'' time, t{sub sl{approx}{Delta}}y/v{sub e}. Here, v{sub e} is the electron thermal velocity. The quasisteady distribution of trapped electrons and its self-consistent plasma wave are studied after the external field is turned off. In cases of particular interest, the bounce frequency, {omega}{sub be}=k{radical}(e{phi}/m{sub e}), satisfies the trapping condition {omega}{sub be}t{sub sl}>2{pi} such that the wave frequency is nonlinearly downshifted by an amount proportional to the number of trapped electrons. Here, k is the wavenumber of the plasma wave and {phi} is its electric potential. For sufficiently short times, the magnitude of the negative frequency shift is a local function of {phi}. Because the trapping frequency shift is negative, the phase of the wave on axis lags the off-axis phase if the trapping nonlinearity dominates linear wave diffraction. In this case, the phasefronts are curved in a focusing sense. In the opposite limit, the phasefronts are curved in a defocusing sense. Analysis and simulations in which the wave amplitude and transverse width are varied establish criteria for the development of each type of wavefront. The damping and trapped-electron-induced focusing of the finite-amplitude electron plasma wave are also simulated. The damping rate of the field energy of the wave is found to be about the sideloss rate, {nu}{sub e{approx}}t{sub sl}{sup -1}. For large wave amplitudes or widths {Delta}y, a trapping-induced self-focusing of the wave is demonstrated.

Banks, J. W.; Berger, R. L.; Cohen, B. I.; Hittinger, J. A. F. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); Brunner, S. [Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Federale de Lausanne, Lausanne (Switzerland)

2011-05-15

185

We propose an alternative method for the calculation of the phase-matched contributions, which are responsible for the third-order optical signals measured in four-wave mixing experiments. In particular, we extend the strong field dissipation theory of Meier and Tannor [J. Chem. Phys. 111, 3365 (1999)] to the case of a perturbative treatment with respect to the exciting laser fields. Our approach is based on an analytical expression of the third-order density matrix and hence it does not require to verify numerically the irrelevance of higher order terms or the calculation of a spatial Fourier transform. In order to illustrate this method, we simulate the experimental signal measured in femtosecond two-dimensional infrared (2D-IR) vibrational spectroscopy. We consider an intramolecular anharmonic vibrational mode modeled by a Morse potential and coupled to a dissipative bath of harmonic oscillators. We calculate the 2D-IR correlation spectrum and we discuss the influence of the population decay on the line shapes. In particular, we compare two situations, one where only pure dephasing processes are considered, and another one where phase losses due to population relaxation are also taken into account. We show that the shape of the peaks observed in a 2D-IR correlation spectrum differs in these two cases, and therefore this difference appears as a signature of population decay and gives information on the importance of pure dephasing processes in phase loss mechanisms. PMID:20568850

Lavoine, J P

2009-10-21

186

The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire between the outer antenna system and the reaction center (RC); it is an important model system to study the excitonic energy transfer. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a). To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the difference between the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways, these being: (1) directly involve in the first pathway 6 $\\rightarrow$ 3 $\\rightarrow$ 1 of the apo form model by passing the excitonic energy to exciton 6; and (2) increase the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 $\\rightarrow$ 4,5 $\\rightarrow$ 2 $\\rightarrow$ ...

Yeh, Shu-Hao

2014-01-01

187

Two-dimensional (2D) optical spectroscopy techniques based on ultrashort laser pulses have been recently extended to the optical domain in the ultraviolet (UV) spectral region. UV-active aromatic side chains can thus be used as local highly specific markers for tracking dynamics and structural rearrangements of proteins. Here we demonstrate that 2D electronic spectra of a model proteic system, a tetrapeptide with two aromatic side chains, contain enough structural information to distinguish between two different configurations with distant and vicinal side chains. For accurate simulations of the 2DUV spectra in solution, we combine a quantum mechanics/molecular mechanics approach based on wave function methods, accounting for interchromophores coupling and environmental effects, with nonlinear response theory. The proposed methodology reveals effects, such as charge transfer between vicinal aromatic residues that remain concealed in conventional exciton Hamiltonian approaches. Possible experimental setups are discussed, including multicolor experiments and signal manipulation techniques for limiting undesired background contributions and enhancing 2DUV signatures of specific electronic couplings. PMID:24803989

2015-01-01

188

Human Papillomavirus (HPV) has been known as one of the cause of virus-induced cancer such as cervical cancer and carcinoma. Among other types of cancer, this type has higher chance in being prevented earlier. The main idea is to eradicate the virus as soon as it enters the body by marking it with antibodies; signaling the immune system to dispose of it. However, the antibodies must be trained to recognize the virus. They can be trained by inserting an object similar to the virus allowing them to learn to recognize and surround the inserted object. In response to this, molecular dynamics simulation was chosen to study the antibody-virus interaction. In this work, two-dimensional case that involves HPV and immunoglobulin (Ig) was studied and observed. Two types of objects will be defined; one stands for HPV while another stands for antibodies. The interaction between the two objects will be governed by two forces; Coulomb force and repulsive contact force. Through the definition of some rules and condition, th...

Haris, Luman; Khotimah, Siti Nurul; Haryanto, Freddy; Viridi, Sparisoma

2013-01-01

189

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

190

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. PMID:19154061

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

2009-01-01

191

Center for Simulation of Wave Interactions with MHD (SWIM) PASCI PAC meeting, May, 2007

Center for Simulation of Wave Interactions with MHD (SWIM) PASCI PAC meeting, May, 2007 E. D and particle sources have on extended-MHD phenomena thereby improving our capability for predicting of a computational platform referred to as the Integrated Plasma Simulator (IPS) that will allow efficient coupling

192

NASA Technical Reports Server (NTRS)

Over the southern California coastal region, observations of the vertical distributions of pollutants show that maximum concentrations can occur within temperature inversion layers well above the surface. A mesoscale model is used to study the dynamical phenomena that cause such layers, including sea breezes and mountain flows, and to study the characteristics of air pollutant transport in a coastal environment capped by a temperature inversion. The mathematical and physical structure of the model is described. Two-dimensional simulations corresponding to four configurations of coastal plains and mountains are discussed. The simulations reveal that pollutant transport over a coastal plain is strongly influenced by the topographic configuration, including the height of coastal mountains and their distance from the coastline. Sea breezes induced by land-sea thermal contrasts, as well as upslope winds induced along mountain flanks, both create vertical transport that can lead to the formation of elevated pollution layers. The sea-breeze circulation generates pollution layers by undercutting the mixed layer and lofting pollutants into the stable layer. Heating of mountain slopes acts to vent pollutants above the mountain ridge during the day; during the evening, pollutants can be injected directly into the inversion layer from the decaying upslope flows. In a land-sea configuration with mountains close to the coastline, the sea breeze and heated-mountain flow are strongly coupled. In the afternoon, this interaction can produce upslope flow from which polluted air is detrained into the inversion layer as a return circulation. When the mountains lie farther inland, however, pollutants may be trapped aloft when the mixed layer stabilizes in the late afternoon. As the nocturnal boundary layer forms over the coast in the evening, polluted mixed-layer air is effectively left behind in the inversion layer. In the Los Angeles Basin, the formation mechanism for elevated polluted layers is most similar to our cases with inland mountains.

Lu, Rong; Turco, Richard P.

1994-01-01

193

Resistive MHD simulations of the Parker instability in galactic disks

Parker instability leads to the formation of tangential discontinuities in a magnetic field and subsequent magnetic reconnection due to a numerical and/or an explicit resistivity. In this paper we investigate the role of the uniform, localized and numerical resistivity on the diffusion of magnetic field lines during the growth phase of Parker instability modes. We propose a new method to quantify the diffusion of magnetic field lines which is attributed to the presence of resistivity in ideal and non-ideal MHD codes. The method relies (1) on integration of magnetic lines in between periodic boundaries, (2) on measurements of the dispersion of magnetic field lines with the left and the right periodic boundaries and (3) on a statistical analysis of shifts of a large set of magnetic lines. The proposed method makes it possible to detect topological evolution of magnetic field. We perform a series of resistive MHD simulations of the Parker instability in uniformly rotating galactic disks. We follow the topological evolution of the magnetic field evolving due to the Parker instability and relate it to the ratio of total to uniform magnetic field in galactic disks. We find that after the onset of the Parker instability, the magnetic field becomes first tangled and later on it evolves toward a uniform state due to the presence of resistivity. A similar effect of a varying contribution of a turbulent magnetic field is observed in arms and inter-arm regions of galaxies.

Grzegorz Kowal; Michal Hanasz; Katarzyna Otmianowska-Mazur

2003-04-11

194

Gravitational waves from 3D MHD core collapse simulations

We present the gravitational wave analysis from rotating (model s15g) and nearly non-rotating (model s15h) 3D MHD core collapse supernova simulations at bounce and the first couple of ten milliseconds afterwards. The simulations are launched from 15M_{\\odot} progenitor models stemming from stellar evolution calculations. Gravity is implemented by a spherically symmetric effective general relativistic potential. The input physics uses the Lattimer-Swesty equation of state for hot, dense matter and a neutrino parametrisation scheme that is accurate until the first few ms after bounce. The 3D simulations allow us to study features already known from 2D simulations as well as nonaxisymmetric effects. In agreement with recent results we find only type I gravitational wave signals at core bounce. In the later stage of the simulations, one of our models (s15g) shows nonaxisymmetric gravitational wave emission caused by a low T/|W| dynamical instability, while the other model radiates gravitational waves due to a convective instability in the protoneutron star. The total energy released in gravitational waves within the considered time intervals is 1.52\\times10^{-7}M_{\\odot} (s15g) and 4.72\\times10^{-10}M_{\\odot} (s15h). Both core collapse simulations indicate that corresponding events in our Galaxy would be detectable either by the LIGO or Advanced LIGO detector.

S. Scheidegger; T. Fischer; S. C. Whitehouse; M. Liebendoerfer

2007-09-03

195

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

196

NASA Astrophysics Data System (ADS)

We provide a dynamics-based comparison on the results from three-dimensional and two-dimensional simulations of hurricane storm surge. We begin with the question, What may have occurred in the Tampa Bay, Florida vicinity had Hurricane Ivan made landfall there instead of at the border between Alabama and Florida? This question is explored using a three-dimensional, primitive equation, finite volume coastal ocean model. The results show that storm surges are potentially disastrous for the Tampa Bay area, especially for landfalls located to the north of the bay mouth. The worst case among the simulations considered is for landfall at Tarpon Springs, such that the maximum wind is positioned at the bay mouth. Along with such regional aspects of storm surge, we then consider the dynamical balances to assess the importance of using a three-dimensional model instead of the usual, vertically integrated, two-dimensional approach to hurricane storm surge simulation. With hurricane storm surge deriving from the vertically integrated pressure gradient force tending to balance the difference between the surface and bottom stresses, we show that three-dimensional structure is intrinsically important. Two-dimensional models may overestimate (or underestimate) bottom stress, necessitating physically unrealistic parameterizations of surface stress or other techniques for model calibration. Our examination of the dynamical balances inherent to storm surges over complex coastal topography suggests that three-dimensional models are preferable over two-dimensional models for simulating storm surges.

Weisberg, Robert H.; Zheng, Lianyuan

2008-12-01

197

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

198

MHD Simulations of CME Acceleration and Impulsive Magnetic Reconnection

NASA Astrophysics Data System (ADS)

MHD simulations of flux rope motion and magnetic reconnection are presented. In particular, we discuss the role of nonuniform anomalous resistivity on the time scale of flux rope acceleration and reconnection rate. The simulation results show that the flux rope's accelerated rising motion is associated with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. By choosing a particular dependence of resistivity on the current density, the simulation results are in good quantitative agreement with the observed temporal profile of the filament-CME acceleration. Moreover, the impulsive rise of the magnetic reconnection rate obtained from the simulations are also in good quantitative agreement with those obtained from the magnetogram data and horizontally expanding two-ribbon emissions for CME-flare events. For the X-class flare events the peak reconnection electric field is ˜ O(103 V/m) or larger, enough to accelerate electrons to over 100 keV in a field-aligned distance of 0.1 km and produce an impulsive hard X-ray emission observed during the flare rise phase. Thus, the simulation results are consistent with CME-flare events that show a temporal correlation among the peak filament-CME acceleration, the impulsive flare non-thermal emissions, and the peak rate of GOES X-ray emission. We will discuss the implications of the empirical anomalous resistivity model on the microscopic reconnection and particle acceleration processes in the current sheet.

Cheng, C. Z.; Choe, G. S.; Ren, Y.; Qiu, J.; Moon, Y. J.

2004-05-01

199

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

200

A Molecular Dynamics Simulation of the Two-Dimensional Cu--O Chain Structure in YBa_2Cu_3O_6+x

A molecular dynamics (MD) simulation was used to study the two-dimensional (2D) structure of the copper-oxygen chains in YBa_2Cu_3O_6+x. The simulation was performed on an arbitrary Cu--O chain plane in a cell, taking into account interactions with atoms located on nearest neighbor planes. A layered 2D screened Coulomb potential developed by Fan and Malozovsky(Y.M. Malozovsky and J.D. Fan, Phys. Rev.

T. Edis; J. D. Fan; D. Bagayoko; G. L. Zhao; Y. M. Malozovsky

1998-01-01

201

MHD Simulations on Parallel Computers R.F. Stein \\Lambda Klaus Galsgaard y Ake Nordlund y

MHD Simulations on Parallel Computers R.F. Stein \\Lambda Klaus Galsgaard y Å¡ Ake Nordlund y to the study of driven magnetic reconnection. 1 Data Parallel Model and the Code HD & MHD calculations, except the performance and lay out our arrays in an efficient manner. We believe that compiler efficiency

Stein, Robert

202

MAGNETOHYDRODYNAMICS Vol. 00 (1964), No. 00, pp. 1?? Numerical simulation of unsteady MHD flows and

MAGNETOHYDRODYNAMICS Vol. 00 (1964), No. 00, pp. 1Â?? Numerical simulation of unsteady MHD flows- dimensional unsteady MHD model. The constraint Â· B = 0 is enforced by the use a Generalized Lagrange-order accurate at the converged state. To show the efficiency of our method, we will then comment some 2D results

Paris-Sud XI, UniversitÃ© de

203

Three-Dimensional MHD on Cubed-Sphere Grids: Parallel Solution-Adaptive Simulation Framework

Three-Dimensional MHD on Cubed-Sphere Grids: Parallel Solution-Adaptive Simulation Framework L cubed-sphere grid framework is described for simu- lation of magnetohydrodynamic (MHD) space and improves convergence efficiency of the iterative method. The Schwarz preconditioning and block-based data

De Sterck, Hans

204

Global hybrid MHD-GK simulation of collective mode excitations by fast

Global hybrid MHD-GK simulation of collective mode excitations by fast ions in ITER G. Vlad;Outline Â· Introduction (AlfvÃ©n waves and global modes, relevance for burning plasmas) Â· Hybrid MHD to collective modes (e.g., shear AlfvÃ©n waves), thus preserving the (thermonuclear) heating efficiency Âto

Vlad, Gregorio

205

A Flux-Limited Numerical Method for the MHD Equations to Simulate Propulsive Plasma Flows

For numerical simulations to be effective tools in plasma propulsion research, a higher order accu- rate solver that captures MHD shocks monotoni- cally and works reliably for strong magnetic fields is needed. For this purpose, a characteristics based scheme for the MHD equations, with flux limiters to improve spatial accuracy, has been developed. In this method, the symmetric form of

K. Sankaran; L. Martinelli; E. Y. Choueiri

2000-01-01

206

Direct numerical simulation of Alfv'enwave filamentation in HallMHD

Direct numerical simulation of Alfv'enÂwave filamentation in HallÂMHD D. Laveder, T. Passot and P along an ambient field. When subject to purely transverse perturbations, smallÂamplitude waves can the Hall term in a generalized Ohm's law. This leads to the HallÂMHD equations @ t ae +r \\Delta (aeu) = 0

Passot, Thierry

207

Magnetosheath dynamics and lobe reconnection as seen from a global beyond-MHD simulation Vlasiator

NASA Astrophysics Data System (ADS)

Global magnetohydrodynamic (MHD) simulations have been successful in describing systems where the important spatial scales are larger than ion gyro radii and the plasma has a well-defined temperature. The weakness of global MHD simulations is their inability to model the multi-temperature, multi-component plasmas in the inner magnetosphere, where most of space-borne technology, including communication and navigation systems reside. We are developing a global Vlasov-hybrid simulation called the Vlasiator, where electrons are massless MHD fluid, but protons are modelled as distribution functions evolved in time using the Vlasov equation. This approach does not include the noise present in kinetic-hybrid simulations, but is computationally extremely challenging requiring petascale computations with thousands of cores. Here, we briefly review the status of the new six-dimensional Vlasiator. We carry out a test particle simulation and propagate the distribution functions using the electromagnetic fields of the GUMICS-4 global MHD simulation. We test and validate Vlasiator in a global setup by comparing the results from the test particle simulation against the standalone GUMICS-4 global MHD simulation. We find that the magnetosheath and magnetopause plasma properties from the test particle simulation are in rough agreement with the results from the GUMICS-4 simulation; however, also important differences arising from the kinetic treatment of plasma are observed. These beyond-MHD effect include the magnetosheath flow pattern changes after a newly established lobe reconnection within one hemisphere.

Palmroth, M.; Honkonen, I.; Kempf, Y.; von Alfthan, S.; Sandroos, A.

2012-04-01

208

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

209

Driving Coronal MHD Simulations with Flux Evolution Models

NASA Astrophysics Data System (ADS)

The solar corona and solar wind strongly influences space weather at Earth. While coronal mass ejections (CMEs) are the most obvious source of this influence, the structure and dynamics of the ambient solar corona and solar wind also play an important role. Coronal structure leads to the partitioning of the solar wind into fast and slow streams, which are the source of recurrent geomagnetic activity. The geo-effectiveness of CMEs is in part determined by their interaction with the ambient wind, and the connection of the ambient interplanetary magnetic field to CME-related shocks and impulsive solar flares determines where solar energetic particles propagate. MHD simulations of the solar corona based on maps of the solar magnetic field have been demonstrated to describe many aspects of coronal structure. However, these models are typically integrated to steady state, using synoptic or daily-updated magnetic maps to derive the boundary conditions. The Sun's magnetic flux is always evolving, and these changes in the flux affect the structure and dynamics of the corona and heliosphere. In this presentation, we describe an approach to evolutionary models of the corona and so wind, using time-dependent boundary conditions. A key aspect of our approach is the use of the Air Force Data Assimilative Photospheric flux Transport (ADAPT) model to develop time-evolving boundary conditions for the magnetic field. ADAPT incorporates data assimilation techniques into the Worden and Harvey (2000) flux evolution model, making it an especially suitable candidate for providing boundary conditions to MHD models. We describe initial results and compare them with more traditional approaches. Research supported by AFOSR, NASA, and NSF.

Linker, J.; Lionello, R.; Mikic, Z.; Riley, P.; Downs, C.; Arge, C. N.; Henney, C. J.

2013-12-01

210

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

211

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

212

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

Huizinga, Richard J.

2008-01-01

213

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

214

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

Alexandros Avdis; Sylvain Lardeau; Michael Leschziner

2009-01-01

215

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

Robert H. Weisberg; Lianyuan Zheng

2008-01-01

216

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

217

MHD simulation of RF current drive in MST

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 ? 10{sup 4}) generally agree with the previous work; significantly more burdensome simulations at MST-like Lundquist number (S ? 3×10{sup 6}) 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.; Forest, C. B.; Reusch, J. A.; Seltzman, A. H.; Sovinec, C. R. [University of Wisconsin, Madison WI (United States); Diem, S. [Oak Ridge National Laboratory, Oak Ridge TN (United States); Harvey, R. W. [CompX, Del Mar, CA (United States)

2014-02-12

218

MHD Simulations on Relaxation in Helicity-driven Spheromaks

NASA Astrophysics Data System (ADS)

The FACT/HIST experiments demonstrated that the poloidal magnetic flux of spheromak/spherical torus plasmas is sustained against its resistive decay by injecting current along open field lines. We have carried out three dimensional magnetohydrodynamic (MHD) simulations on electrostatic helicity injection current drive (HICD) in spheromak plasmas. We have examined effects of varying geometry, the magnetic Reynolds number, and an external toroidal field (TF) on HICD. On spheromaks in a simple domain with HICD, typical numerical results exhibit amplification and sustainment of the poloidal magnetic flux. The safety factor q, initially less than 1.0 at every poloidal flux surface, becomes larger with time due to injection of external current into open flux. At the time when q=1 appears in the closed poloidal flux region, the toroidal mode number n=1 rapidly grows and then decreases through relaxation process. In the periodic relaxation cycle, the helical kink deformation of the central column of open flux play a major role in the flux conversion from toroidal to poloidal magnetic flux, although the relaxation does not make the helically deformed central column to relax completely to an axisymmetric state. This relaxation process agrees well with the FACT experimental result (M. Nagata et al. Phys. Rev. Lett. 71, 4342 (1993)) .

Kagei, Y.; Nagata, M.; Fukumoto, N.; Uyama, T.

2000-10-01

219

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

220

MHD Simulations of a Moving Subclump with Heat Conduction

High resolution observations of cluster of galaxies by Chandra have revealed the existence of an X-ray emitting comet-like galaxy C153 in the core of cluster of galaxies A2125. The galaxy C153 moving fast in the cluster core has a distinct X-ray tail on one side, obviously due to ram pressure stripping, since the galaxy C153 crossed the central region of A2125. The X-ray emitting plasma in the tail is substantially cooler than the ambient plasma. We present results of two-dimensional magnetohydrodynamic simulations of the time evolution of a subclump like C153 moving in magnetized intergalactic matter. Anisotropic heat conduction is included. We found that the magnetic fields are essential for the existence of the cool X-ray tail, because in non-magnetized plasma the cooler subclump tail is heated up by isotropic heat conduction from the hot ambient plasma and does not form such a comet-like tail.

Naoki Asai; Naoya Fukuda; Ryoji Matsumoto

2004-12-15

221

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

222

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

223

Finite-difference time-domain simulation of two-dimensional photonic crystal surface-emitting laser.

Using the three-dimensional (3D) finite-difference time-domain (FDTD) method, we have investigated in detail the optical properties of a two-dimensional (2D) photonic crystal (PC) surface-emitting laser having a square-lattice structure. In this study we perform the 3D-FDTD calculation for the structure of an actual fabricated device. The device is based on bandedge resonance, and four band edges are present at the corresponding band edge point. For these band edges, we calculate the quality (Q) factor. The results show that the Q factor of a resonant mode labeled A1 is larger than that of other resonant modes; that is, lasing occurs easily in mode A1. The device can thus achieve single-mode lasing oscillation. To increase the Q factor, we also consider the optimization of device parameters. The results provide important guidelines for device fabrication. PMID:19495182

Yokoyama, Mitsuru; Noda, Susumu

2005-04-18

224

NASA Astrophysics Data System (ADS)

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

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

2011-04-01

225

NASA Astrophysics Data System (ADS)

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

Huang, Huaxiong; Takagi, Shu

2003-08-01

226

Ionospheric power consumption in global MHD Simulation predicted from solar wind measurements

Ionospheric power consumption in a global magnetohydrodynamic (MHD) simulation is investigated. The sum of Joule heating and precipitation power integrated over both hemispheres is calculated in four simulation runs. The simulation results of the total ionospheric dissipation are correlated with solar wind density, velocity, and magnetic field using a linear multi-variable fit and a power law. The fitting procedure yields

Minna Palmroth; Hannu E. J. Koskinen; Tuija I. Pulkkinen; Pekka Janhunen

2004-01-01

227

The kinetic processes accompanying plasma column formation in an inhomogeneous discharge in a Ne/Xe/HCl gas mixture at a pressure of 4 atm were investigated by using a two-dimensional model. Two cathode spots spaced by 0.7 cm were initiated by distorting the cathode surface at local points, which resulted in an increase in the field strength in the cathode region. Three regimes differing in the charging voltage, electric circuit inductance, and electric field strength at the local cathode points were considered. The spatiotemporal distributions of the discharge current; the electron density; and the densities of excited xenon atoms, HCl(v = 0) molecules in the ground state, and HCl(v > 0) molecules in vibrational levels were calculated. The development of the discharge with increasing the electron density from 10{sup 4} to 10{sup 16} cm{sup -3} was analyzed, and three characteristic stages in the evolution of the current distribution were demonstrated. The width of the plasma column was found to depend on the energy deposited in the discharge. The width of the plasma column was found to decrease in inverse proportion to the deposited energy due to spatiotemporal variations in the rates of electron production and loss. The calculated dependences of the cross-sectional area of the plasma column on the energy deposited in the discharge agree with the experimental results.

Bychkov, Yu. I., E-mail: bych-yu@yandex.ru; Yampolskaya, S. A.; Yastremskii, A. G. [Russian Academy of Sciences, Institute of High Current Electronics, Siberian Branch (Russian Federation)

2013-05-15

228

Electronic excitations and the optical properties of the photosynthetic complex PSI are analyzed using an effective exciton model developed by Vaitekonis et al. [Photosynth. Res. 2005, 86, 185]. States of the reaction center, the linker states, the highly delocalized antenna states and the red states are identified and assigned in absorption and circular dichroism spectra by taking into account the spectral distribution of density of exciton states, exciton delocalization length, and participation ratio in the reaction center. Signatures of exciton cooperative dynamics in nonchiral and chirality-induced two-dimensional (2D) photon-echo signals are identified. Nonchiral signals show resonances associated with the red, the reaction center, and the bulk antenna states as well as transport between them. Spectrally overlapping contributions of the linker and the delocalized antenna states are clearly resolved in the chirality-induced signals. Strong correlations are observed between the delocalized antenna states, the linker states, and the RC states. The active space of the complex covering the RC, the linker, and the delocalized antenna states is common to PSI complexes in bacteria and plants. PMID:19351124

Abramavicius, Darius; Mukamel, Shaul

2009-04-30

229

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

230

NASA Astrophysics Data System (ADS)

Microdischarges have gained much attention in the plasma process community for a variety of applications. Recently, microdischarges have been proposed in applications that incorporate bulk fluid flow. For example, in one recent study, microhollow electrode plasma devices have been proposed for use as microflow reactors. Our motivation for this study comes from our proposed application in microthrusters for small satellite propulsion. The phenomena of intense, localized, controllable gas heating in microdischarges enables the development of an electrothermal class device capable of thrust levels in the mN range. While some estimates of properties of microdischarges are available, a detailed understanding of the plasma dynamics, chemistry and coupling with bulk flow is lacking. The focus of this talk is to explore the fundamental characteristics of microdischarges, and investigate interactions between plasma dynamics and bulk fluid flow using two-dimensional computational modelling. In view of our proposed microthruster application, the gas-heating aspect of microdischarges is explored in detail. The model incorporates a self-consistent representation of the plasma that includes a description of multi-species transport and chemistry, electric field, electron and heavy species energy distributions in the microdischarge. The Poisson's equation is solved for the electric field and species conservation equations for generation and transport of species in the discharge. The electron energy equation is solved to determine the electron temperature distribution and the heavy species energy equation is used to determine the gas temperature. Our studies indicate that the structure of the microdischarge is highly inhomogeneous, and electron temperatures of order several tens of electron volts are possible. Significant gas heating is predicted, with typical gas temperatures in the range 600-1000 K. The gas temperature is found to vary strongly with discharge current, but shows a relatively weaker dependence on the operating pressure. It is observed that the plasma dynamics are relatively insensitive to bulk fluid flow, confirming that these two phenomena are decoupled.

Kothnur, Prashanth; Raja, Laxminarayan

2004-09-01

231

2-D MHD and Hall-MHD local simulations of the Kelvin-Helmholtz Instability at the Ionopause of Mars

NASA Astrophysics Data System (ADS)

Our previous simulation results and comparison with Cluster spacecraft data have shown that reconnection can be generated in thin current layers produced by the Kelvin-Helmholtz Instability (KHI) at the Earth's magnetosphere. Our simulation studies of the KHI for the Earth's magnetospheric parameters have shown that reconnection due to KHI can facilitate efficient plasma transport from the solar wind to the magnetosphere during northward IMF conditions producing diffusion coefficients of the order 109 m2/s. When the magnetic fields are initially parallel and the asymmetry is small across the initial shear flow layer the reconnection inside KH vortices occurs always first at the high-density spine of the vortex -leading to plasma transport from high-density magnetosheath to low density magnetosphere. We expect that in Mars and Venus this process may be reversed because the plasma inside the ionopause is more dense that in the solar wind. Although, Mars and Venus are un-magnetized the IMF will have a strong impact on the evolution of the KHI. We will present a systematic parameter study of the Kelvin-Helmholtz instability at the ionopause of Mars using 2-D local MHD and Hall-MHD approximations. The efficiency and direction of the plasma transport will be investigated for the typical plasma and magnetic field profiles observed in the vicinity of the Martian ionopause.

Beales, J.; Nykyri, K.; Otto, A.; Eastwood, J.; Brain, D.

2008-12-01

232

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

233

3D MHD Free Surface Fluid Flow Simulation Based on Magnetic-Field Induction Equations

1 3D MHD Free Surface Fluid Flow Simulation Based on Magnetic-Field Induction Equations H.L. HUANG, a penalty factor is introduced in order to force the local divergence free condition of the magnetic fields boundaries is null. These simulation results for lithium film free surface flows under NSTX outboard mid

California at Los Angeles, University of

234

3D MHD free surface fluid flow simulation based on magnetic-field induction equations

3D MHD free surface fluid flow simulation based on magnetic-field induction equations H.L. Huang 1 in order to force the local divergence free condition of the magnetic fields. The second is that we extend. These simulation results for lithium film free surface flows under NSTX outboard mid-plane magnetic field

Abdou, Mohamed

235

Device Simulation of Trap-Assisted Generation-Recombination Noise Under Periodic Large-Signal Conditions Bosman, Senior Member, IEEE, and Mark E. Law, Fellow, IEEE Abstract--The simulation of generation-recombination (GR) noise under periodic large-signal conditions in a partial differen- tial equation-based silicon

Florida, University of

236

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

237

NASA Astrophysics Data System (ADS)

The existence of small-scale plasmoids associated with the Hall effect has been often observed in the magnetotail. They are considered as the signature of multiple X-line collisionless reconnection. To study these plasmoids structures, we present some Cluster observations and Hall MHD simulations of their features. In this study, the observation survey is divided into two types. The first one is the isolated plasmoid with two typical plasmoid events the flux-rope-like plasmoid on 3 August 2001 and the closed-loop-like plasmoid on 22 August 2001. The second type contains multiple successive plasmoids, on 12 September 2001 with three neighboring plasmoids structures observed during a substorm. Especially for the second plasmoid, three main features were observed, including a core field in the plasmoid, a quadrupole magnetic field near the X line, and a local plasma convection within the plasmoid. The Grad-Shafranov reconstruction method was used to recover the two-dimensional magnetic field maps for this plasmoid. These results may provide evidence that the small-scale plasmoids frequently observed in the magnetotail may be produced by multiple X-line collisionless reconnection. To study the impact of crosstail magnetic field on the structures of small-scale plasmoids, a 2.5-D Hall MHD simulation was performed. In the case with a guide field By0, the in-plane plasma inflows carrying Byflux enter into the plasmoid due to magnetic reconnection. However, there is no such By flux transport process for the case without guide field. These results demonstrate that a crosstail magnetic field is an important factor in the formation of flux-rope-like plasmoids.

Liu, Chaoxu; Feng, Xueshang; Guo, Jianpeng; Ye, Yudong

2013-05-01

238

To improve the reliability of two-dimensional information, codes that can correct two-dimensional bursts (or spots) may be useful. In this paper a class of two-dimensional burst-correcting codes, called two-dimensional Fire codes, is proposed. The definition of these codes is a natural extension of that of the conventional Fire codes. The two-dimensional Fire code is a two-dimensional cyclic code designed for

H. Imai

1973-01-01

239

with experimental data Richard S. Wise, Dimitris P. Lymberopoulos, and Demetre J. Economoua) Plasma Processing of arbitrary geometry and with arbitrary plasma and surface chemistries. In this modular finite element fluid distribution function.12 Both Paranjpe and Kortschagen et al. simulated an argon plasma no plasma chemistry

Economou, Demetre J.

240

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

C. Tenaud; E. Garnier; P. Sagaut

2000-01-01

241

This paper assesses the accuracy of the simplified frame cavity conduction\\/convection and radiation models presented in ISO 15099 and used in software for rating and labeling window products. Temperatures and U-factors for typical horizontal window frames with internal cavities are compared; results from Computational Fluid Dynamics (CFD) simulations with detailed radiation modeling are used as a reference. Four different frames

Arlid Gustavsen; Christian Kohler; Arvid Dalehaug; Dariush Arasteh

2008-01-01

242

Two-Fluid 2.5D MHD-Code for Simulations in the Solar Atmosphere

NASA Astrophysics Data System (ADS)

We investigate magnetic reconnection due to the evolution of magnetic flux tubes in the solar chromosphere. We developed a new numerical two-fluid magnetohydrodynamic (MHD) code which will perform a 2.5D simulation of the dynamics from the upper convection zone up to the transition region. Our code is based on the Total Variation Diminishing Lax-Friedrichs scheme and makes use of an alternating-direction implicit method, in order to accommodate the two spatial dimensions. Since we apply a two-fluid model for our simulations, the effects of ion-neutral collisions, ionization/recombination, thermal/resistive diffusivity and collisional/resistive heating are included in the code. As initial conditions for the code we use analytically constructed vertically open magnetic flux tubes within a realistic stratified atmosphere. Initial MHD tests have already shown good agreement with known results of numerical MHD test problems like e.g. the Orszag-Tang vortex test.

Piantschitsch, I.; Amerstorfer, U.; Thalmann, J.; Utz, D.; Hanslmeier, A.; Bárta, M.; Thonhofer, S.; Lemmerer, B.

243

An approximate two-dimensional solver of the nonlinear Fokker-Planck-Landau collision operator has been developed using the assumption that the particle probability distribution function is independent of gyroangle in the limit of strong magnetic field. The isotropic one-dimensional scheme developed for nonlinear Fokker-Planck-Landau equation by Buet and Cordier [J. Comput. Phys. 179, 43 (2002)] and for linear Fokker-Planck-Landau equation by Chang and Cooper [J. Comput. Phys. 6, 1 (1970)] have been modified and extended to two-dimensional nonlinear equation. In addition, a method is suggested to apply the new velocity-grid based collision solver to Lagrangian particle-in-cell simulation by adjusting the weights of marker particles and is applied to a five dimensional particle-in-cell code to calculate the neoclassical ion thermal conductivity in a tokamak plasma. Error verifications show practical aspects of the present scheme for both grid-based and particle-based kinetic codes.

Yoon, E. S.; Chang, C. S., E-mail: cschang@pppl.gov [Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States); Korea Advanced Institute of Science and Technology, Yuseong-gu, DaeJeon 305-701 (Korea, Republic of)

2014-03-15

244

NASA Astrophysics Data System (ADS)

An approximate two-dimensional solver of the nonlinear Fokker-Planck-Landau collision operator has been developed using the assumption that the particle probability distribution function is independent of gyroangle in the limit of strong magnetic field. The isotropic one-dimensional scheme developed for nonlinear Fokker-Planck-Landau equation by Buet and Cordier [J. Comput. Phys. 179, 43 (2002)] and for linear Fokker-Planck-Landau equation by Chang and Cooper [J. Comput. Phys. 6, 1 (1970)] have been modified and extended to two-dimensional nonlinear equation. In addition, a method is suggested to apply the new velocity-grid based collision solver to Lagrangian particle-in-cell simulation by adjusting the weights of marker particles and is applied to a five dimensional particle-in-cell code to calculate the neoclassical ion thermal conductivity in a tokamak plasma. Error verifications show practical aspects of the present scheme for both grid-based and particle-based kinetic codes.

Yoon, E. S.; Chang, C. S.

2014-03-01

245

Statistical properties of the X-band sea clutter are studied using 2-D direct numerical simulations. Surfaces are modeled as realizations of a Gaussian random process with the Pierson-Moskowitz or Elfouhaily spectrum. The Creamer transform is further applied to account for the lowest-order surface nonlinearities. Backscattered field at a given frequency is found using the first-principles boundary integral equation (BIE) technique. Calculations

Jakov V. Toporkov; Mark A. Sletten

2007-01-01

246

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

247

The triangular lattice model with nearest-neighbor attraction and third-neighbor repulsion, introduced by Pekalski, 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.

Almarza, N. G. [Instituto de Química Física Rocasolano, CSIC, Serrano 119, E-28006 Madrid (Spain)] [Instituto de Química Física Rocasolano, CSIC, Serrano 119, E-28006 Madrid (Spain); Pekalski, J.; Ciach, A. [Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warszawa (Poland)] [Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warszawa (Poland)

2014-04-28

248

NASA Astrophysics Data System (ADS)

Global scale magnetohydrodynamic simulations have been used to successfully study the evolution of the magnetosphere-ionosphere system under a variety of solar wind conditions. Early studies with the Lyon-Fedder-Mobarry (LFM) model show the presence of flow channels in substorm simulations that had characteristics similar to those seen in observations of bursty bulk flows (BBFs) observed by numerous spacecraft, such as AMPTE and Geotail. More recently the THEMIS constellation has provided a unique opportunity to track the evolution of dipolarization fronts (DFs) from the mid-tail into the inner magnetosphere. Additionally, advances in high performance computing capability make it possible to conduct ultra-high resolution global simulations. In this paper we present comparisons between these ultra-high resolution simulations and the observations of THEMIS. The comparisons include a case study for a DF that was well observed on February 27, 2009 and statistical properties of the flow and electromagnetic field signatures seen in observations and MHD simulations with idealized solar wind conditions. In addition to these comparisons we will present results of using test-particle simulations of electrons driven by the simulated fields to study particle energization in regions around DFs.

Wiltberger, M. J.; Lyon, J.; Elkington, S. R.; Merkin, V. G.

2013-12-01

249

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

250

NASA Astrophysics Data System (ADS)

In this paper we report on two-dimensional numerical simulations of heating of a rotating, wheel shaped target impacted by the full intensity of the ion beam that will be delivered by the SPIRAL2 facility at Caen, France. The purpose of this work is to study heating of solid targets that will be used to strip the fast ions of SPIRAL2 to the required high charge state for the FISIC (Fast Ion-Slow Ion Collision) experiments. Strippers of aluminum with different emissivities and of carbon are exposed to high beam current of different ion species as oxygen, neon and argon. These studies show that carbon, due to its much higher sublimation temperature and much higher emissivity, is more favorable compared to aluminum. For the highest beam intensities, an aluminum stripper does not survive. However, problem of the induced thermal stresses and long term material fatigue needs to be investigated before a final conclusion can be drawn.

Tahir, N. A.; Kim, V.; Lamour, E.; Lomonosov, I. V.; Piriz, A. R.; Rozet, J. P.; Stöhlker, Th.; Sultanov, V.; Vernhet, D.

2012-11-01

251

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

252

Plasma wave signatures in the magnetotail reconnection region - MHD simulation and ray tracing

NASA Technical Reports Server (NTRS)

An MHD simulation was performed to obtain a self-consistent model of magnetic field and plasma density near the X point reconnection region. The MHD model was used to perform extensive ray tracing calculations in order to clarify the propagation characteristics of the plasma waves near the X point reconnection region. The dynamic wave spectra possibly observed by the Geotail spacecraft during a typical cross-tail trajectory are reconstructed. By comparing the extensive ray tracing calculations with the plasma wave data from Geotail, it is possible to perform a kind of 'remote sensing' to identify the location and structure of potential X point reconnection regions.

Omura, Yoshiharu; Green, James L.

1993-01-01

253

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

254

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

255

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

256

NASA Astrophysics Data System (ADS)

Gravel-bed braided rivers are characterized by shallow, branching flow across low relief, complex, and mobile bed topography. These conditions present a major challenge for the application of higher dimensional hydraulic models, the predictions of which are nevertheless vital to inform flood risk and ecosystem management. This paper demonstrates how high-resolution topographic survey and hydraulic monitoring at a density commensurate with model discretization can be used to advance hydrodynamic simulations in braided rivers. Specifically, we detail applications of the shallow water model, Delft3d, to the Rees River, New Zealand, at two nested scales: a 300 m braid bar unit and a 2.5 km reach. In each case, terrestrial laser scanning was used to parameterize the topographic boundary condition at hitherto unprecedented resolution and accuracy. Dense observations of depth and velocity acquired from a mobile acoustic Doppler current profiler (aDcp), along with low-altitude aerial photography, were then used to create a data-rich framework for model calibration and testing at a range of discharges. Calibration focused on the estimation of spatially uniform roughness and horizontal eddy viscosity, ?H, through comparison of predictions with distributed hydraulic data. Results revealed strong sensitivity to ?H, which influenced cross-channel velocity and localization of high shear zones. The high-resolution bed topography partially accounts for form resistance, and the recovered roughness was found to scale by 1.2-1.4 D84 grain diameter. Model performance was good for a range of flows, with minimal bias and tight error distributions, suggesting that acceptable predictions can be achieved with spatially uniform roughness and ?H.

Williams, R. D.; Brasington, J.; Hicks, M.; Measures, R.; Rennie, C. D.; Vericat, D.

2013-09-01

257

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

258

NASA Technical Reports Server (NTRS)

A situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere undergoes symmetrical shear motion at the footpoints is investigated via a 2D (nonplanar) MHD simulation. It was found that the vertical plasma flow velocities grow exponentially, leading to a new type of global MHD instability. The growth rate increases almost linearly until it reaches the same order of magnitude as the Alfven speed. Then a nonlinear MHD instability occurs beyond this point. It was found that the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. The nonlinear dynamical shearing instability is illustrated by a numerical example that is given for three different values of the plasma beta that span several orders of magnitude.

Wu, S. T.; Song, M. T.; Martens, P. C. H.; Dryer, M.

1991-01-01

259

Two-Dimensional Colloidal Alloys

NASA Astrophysics Data System (ADS)

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

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

2011-03-01

260

Two-dimensional colloidal alloys.

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

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

2011-03-25

261

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

262

NASA Astrophysics Data System (ADS)

The dynamics of proton transfer (PT) through hydrogen bonds in a two-dimensional water layer confined between two graphene sheets at room temperature are investigated through ab initio and quantum-classical simulations. The excess proton is found to be mostly solvated as an Eigen cation where the hydronium ion donates three hydrogen bonds to the neighboring water molecules. In the solvation shell of the hydronium ion, the three coordinated water molecules with two donor hydrogen bonds are found to be properly presolvated to accept a proton. Although no hydrogen bond needs to be broken for transfer of a proton to such presolvated water molecules from the hydronium ion, the PT rate is still found to be not as fast as it is for one-dimensional chains. Here, the PT is slowed down as the probability of finding a water with two donor hydrogen bonds in the solvation shell of the hydronium ion is found to be only 25%-30%. The hydroxide ion is found to be solvated mainly as a complex anion where it accepts four H-bonds through its oxygen atom and the hydrogen atom of the hydroxide ion remains free all the time. Here, the presolvation of the hydroxide ion to accept a proton requires that one of its hydrogen bonds is broken and the proton comes from a neighboring water molecule with two acceptor and one donor hydrogen bonds. The coordination number reduction by breaking of a hydrogen bond is a slow process, and also the population of water molecules with two acceptor and one donor hydrogen bonds is only 20%-25% of the total number of water molecules. All these factors together tend to slow down the hydroxide ion migration rate in two-dimensional water layers compared to that in three-dimensional bulk water.

Bankura, Arindam; Chandra, Amalendu

2015-01-01

263

In some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolaï-Busquet [G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000)] has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies.

Marocchino, A.; Atzeni, S.; Schiavi, A. [Dipartimento SBAI, Università di Roma “La Sapienza” and CNISM, Roma 00161 (Italy)] [Dipartimento SBAI, Università di Roma “La Sapienza” and CNISM, Roma 00161 (Italy)

2014-01-15

264

Two-dimensional magnetohydrodynamic studies of implosion modes of nested wire array z-pinches

Implosion dynamics of nested wire arrays in (r, ?) geometry was studied with two-dimensional magnetohydrodynamic (2D MHD) simulations. Three different implosion modes are obtained by just changing the wire number of the outer array, when the other conditions, such as the initial radius, length, mass of each array, the wire number of the inner array, and the discharge voltage waveform, are fixed. Simulation results show that the effect of discrete wires, which cannot be described by the thin shell inductive model, will influence the distribution of current between the outer and inner arrays at the early stage, and the discrepancy between results from MHD and thin shell model increases with the interwire gap of the outer array.

Huang, Jun; Ding, Ning, E-mail: ding-ning@iapcm.ac.cn; Xue, Chuang; Sun, Shunkai [Institute of Applied Physics and Computational Mathematics, Beijing 100088 (China)

2014-07-15

265

NASA Astrophysics Data System (ADS)

We developed a two-dimensional gradiometer that detects the gradient of a magnetic field in two orthogonal directions to measure the biomagnetic signal in an unshielded environment. We based the gradiometer on a low-Tc superconducting quantum interference device (SQUID) and wire-wound pickup coil. The gradiometer we developed detects both the axial-second-order and planar-first-order gradient of a magnetic field. The experimental results revealed that its noise-reduction ratio (NRR) was 54 dB from 0.5 to 49 Hz and 14 dB (5 times) larger than that of the axial-second-order gradiometer. Moreover, by using the new gradiometer, we obtained a clear magnetocardiography (MCG) waveform in real time without averaging under an unshielded environment (noise level: 3.8 nT/\\sqrt{Hz} at 1 Hz; 150 pT/\\sqrt{Hz} at 10 Hz).

Seki, Yusuke; Kandori, Akihiko

2007-06-01

266

NASA Astrophysics Data System (ADS)

Interest in utilizing liquid metal film flows to protect the plasma-facing solid structures places increasing demand on understanding the magnetohydrodynamics (MHD) of such flows in a magnetic field with spatial variation. The field gradient effect is studied by a two-dimensional (2D) model in Cartesian coordinates. The thin film flow down an inclined plane in spanwise (z-direction) magnetic field with constant streamwise gradient and applied current is analyzed. The solution to the equilibrium flow shows forcefully the M-shaped velocity profile and dependence of side layer thickness on Ha-1/2 whose definition is based on field gradient. The major part of the dissertation is the numerical simulation of free surface film flows and understanding the results. The VOF method is employed to track the free surface, and the CSF model is combined with VOF method to account for surface dynamics condition. The code is validated with respect to Navier-Stokes solver and MHD implementation by computations of ordinary wavy films, MHD flat films and a colleague proposed film flow. The comparisons are performed against respective experimental, theoretical or numerical solutions, and the results are well matched with them. It is found for the ordinary water falling films, at low frequency and high flowrate, the small forcing disturbance at inlet flowrate develops into big roll waves preceded by small capillary bow waves; at high frequency and low Re, it develops into nearly sinusoidal waves with small amplitude and without fore-running capillary waves. The MHD surface instability is investigated for two kinds of film flows in constant streamwise field gradient: one with spatial disturbance and without surface tension, the other with inlet forcing disturbance and with surface tension. At no surface tension condition, the finite amplitude disturbance is rapidly amplified and degrades to irregular shape. With surface tension to maintain smooth interface, finite amplitude regular waves can be established only on near inlet region and they decay to nearly zero amplitude ripple on the far downstream region. At both film conditions, the wave traveling velocity is reduced by the MHD drag from field gradient. The code is also used to explore the exit-pipe and first wall conceptual designs for fusion reactor being proposed in the APEX program. It is seen that the field gradient restrains and lifts up the flow to the whole channel in the exit-pipe high field gradient condition, but an applied streamwise current can propel the flow through the gradient region. The Sn jet flow with high inertia is able to overcome the inverted gravity and MHD induction to form the desired protection liquid layer on top of the first wall.

Gao, Donghong

267

Comparison of global MHD simulations with AMIE simulations for the events of May 19-20, 1996

NASA Astrophysics Data System (ADS)

Using WIND-measured solar wind data, we have simulated the magnetosphere for the time between 1200 UT May 19 and 0200 UT May 20, 1996, with a three-dimensional MHD model. This time period has been chosen as an International Solar-Terrestrial Physics/Global Geospace Science event for community study, and there is a large set of data with which to compare. In this paper we will compare the simulation predictions with results from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) analysis. We show comparisons for the convection, the auroral precipitation, the ionospheric conductances, the field-aligned currents, and the Joule heating distribution. The results concentrate on four time periods when the two DMSP spacecraft, F12 and F13, and the POLAR spacecraft were passing over the northern (summer) polar region. The comparisons show excellent agreement with the F13 electric field measurements. The ionospheric convection patterns agree well between the simulation and the AMIE analysis with the cross polar potential drop somewhat higher in the MHD model. The auroral electron precipitation energy flux from the MHD model is too low, particularly in the late morning, when compared with the POLAR UVI data because of the lack of electron drift physics in the model. We show how the MHD auroral input can be improved by adjusting the parameters in the auroral precipitation model.

Slinker, S. P.; Fedder, J. A.; Emery, B. A.; Baker, K. B.; Lummerzheim, D.; Lyon, J. G.; Rich, F. J.

268

NASA Astrophysics Data System (ADS)

There is a growing concern over possible severe societal consequences related to adverse space weather impacts on man-made technological infrastructure. In the last two decades, significant progress has been made toward the first-principles modeling of space weather events, and three-dimensional (3-D) global magnetohydrodynamics (MHD) models have been at the forefront of this transition, thereby playing a critical role in advancing our understanding of space weather. However, the modeling of extreme space weather events is still a major challenge even for the modern global MHD models. In this study, we introduce a specially adapted University of Michigan 3-D global MHD model for simulating extreme space weather events with a Dst footprint comparable to the Carrington superstorm of September 1859 based on the estimate by Tsurutani et. al. (2003). Results are presented for a simulation run with "very extreme" constructed/idealized solar wind boundary conditions driving the magnetosphere. In particular, we describe the reaction of the magnetosphere-ionosphere system and the associated induced geoelectric field on the ground to such extreme driving conditions. The model setup is further tested using input data for an observed space weather event of Halloween storm October 2003 to verify the MHD model consistence and to draw additional guidance for future work. This extreme space weather MHD model setup is designed specifically for practical application to the modeling of extreme geomagnetically induced electric fields, which can drive large currents in ground-based conductor systems such as power transmission grids. Therefore, our ultimate goal is to explore the level of geoelectric fields that can be induced from an assumed storm of the reported magnitude, i.e., Dst˜=-1600 nT.

Ngwira, Chigomezyo M.; Pulkkinen, Antti; Kuznetsova, Maria M.; Glocer, Alex

2014-06-01

269

Modeling of substorm development with a kinematic effect by the global MHD simulations

NASA Astrophysics Data System (ADS)

Magnetic reconnection is considered to play an important role in space phenomena such as substorm in the Earth's magnetosphere. Recently, Tanaka and Fujita reproduced substorm evoution process by numerical simulation with the global MHD code. In the MHD framework, the dissipation model is used for modeling of the kinetic effects. They found that the normalized reconnection viscosity, one of the dessipation model employed there, gave a large effect for the substorm development though that viscosity was assumed to be a constant parameter. It is well known that magnetric reconnection is controlled by microscopic kinetic mechanism. Horiuchi et al. investigated the roles of microscopic plasma instabilities on the violation of the frozen-in condition by examining the force balance equation based on explicit electromagnetic particle simulation for an ion-scale current sheet, and concluded that the growth of drift kink instability can create anomalous resistivity leading to the excitation of collisionless reconnection. They estimated the effective resistivity based on the particle simulation data. In this paper, we perform substorm simulation by using the global MHD code with this anomalous resistivity obtained in their microscopic approach istead of the emprical resistivity model, and investigate the relationship between the substorm development and the anomalous resistivity model.

den, Mitsue; Fujita, Shigeru; Tanaka, Takashi; Horiuchi, Ritoku

270

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

271

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

NASA Astrophysics Data System (ADS)

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

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

2012-05-01

272

2D Radiative MHD Simulations of the Importance of Partial Ionization in the Chromosphere

The solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the plasma. We investigate the importance of introducing neutral particles using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-grey and NLTE radiative transfer, and thermal conduction. The effects of partial ionization are implemented using the generalized Ohm's law. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic diffusion, the 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 VAL-C model as a basis for estimates. In addition, ...

Martinez-Sykora, Juan; Hansteen, Viggo

2012-01-01

273

NASA Astrophysics Data System (ADS)

In this paper, a simple cavitation model is developed under the framework of homogeneousone-fluid model, in which the perfect mixture of liquid and vapor phases is assumed. In most of conventional models, the vapor phase is considered as a dispersed phase against the liquid phase as a continuous phase, while in the present model, two extreme conditions are considered: for low void fraction, dispersed vapor bubbles in continuous liquid phase, while for high void fraction, dispersed droplets in continuous vapor phase. The growth of bubbles and droplets are taken into account in the mass transfer between vapor and liquid phases, and are switched according to the local void fraction. The model is applied for the simulation of cavitating flow in a two-dimensional convergent-divergent nozzle, and the result is compared with that using a conventional model. To enhance the unsteadiness of cavitation due to the instability at the cavity interphase, the turbulent shear stress is modified depending upon the continuous phases in combination with the proposed cavitation model, which drastically reduces the turbulent viscosity for high void fraction region. As a result, the unsteadiness of cavitation observed in experiments is well reproduced.

Yamamoto, Y.; Watanabe, S.; Tsuda, S. I.

2015-01-01

274

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

275

NASA Astrophysics Data System (ADS)

A two-dimensional computational model of a single red blood cell (RBC) floating in a plasma-alcohol solution through a microchannel with stenosis was created using the Arbitrary Lagrangian-Eulerian (ALE) method with moving mesh for a fluid structure interaction problem. Cell deformability and stability were studied in a plasma-alcohol solution at different fluid flow conditions during movement through the channel with stenosis. Different results were obtained for different input parameters. Motion through 45% and 70% stenoses with the high and law velocities of the RBC and different viscosities was analyzed and successfully simulated. Results show that changes in RBC deformability were due to the effects of alcohol. Changes in behavior during motion were also observed. At low shear rate and high surrounding fluid viscosity the RBC showed a tendency to rotate during movement. The proposed model with its coupling of structural and fluid analysis techniques could be useful to understand the effect of alcohol on the RBC passing through stenosis.

Ni, Aleksey; Cheema, Taqi Ahmad; Kwak, Moon Kyu; Park, Cheol Woo

2014-08-01

276

Background Stereopsis is believed to be advantageous for surgical tasks that require precise hand-eye coordination. We investigated the effects of short-term and long-term absence of stereopsis on motor task performance in three-dimensional (3D) and two-dimensional (2D) viewing conditions. Methods 30 participants with normal stereopsis and 15 participants with absent stereopsis performed a simulated surgical task both in free space under direct vision (3D) and via a monitor (2D), with both eyes open and one eye covered in each condition. Results The stereo-normal group scored higher, on average, than the stereo-absent group with both eyes open under direct vision (p<0.001). Both groups performed comparably in monocular and binocular monitor viewing conditions (p=0.579). Conclusions High-grade stereopsis confers an advantage when performing a fine motor task under direct vision. However, stereopsis does not appear advantageous to task performance under 2D viewing conditions, such as in video-assisted surgery. PMID:25185439

Bloch, Edward; Uddin, Nabil; Gannon, Laura; Rantell, Khadija; Jain, Saurabh

2015-01-01

277

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

Hu Zhanghu; Song Yuanhong; Wang Younian [School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024 (China)

2010-08-15

278

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; Wang, Dezhen [Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024 (China)] [Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024 (China); Stirner, Thomas [Department of Electronic Engineering, University of Applied Sciences Deggendorf, Edlmairstr. 6-8, D-94469 Deggendorf (Germany)] [Department of Electronic Engineering, University of Applied Sciences Deggendorf, Edlmairstr. 6-8, D-94469 Deggendorf (Germany)

2013-10-15

279

NASA Astrophysics Data System (ADS)

A two-dimensional numerical code is developed to model the effect of grains boundaries (g.b.) on the capacitance-voltage (C-V) characteristics of polysilicon diodes. The test structures are lateral polysilicon P+N diodes where the thickness of the film tf deposited by the low-pressure chemical vapour deposition method is 700 or 450 nm. The P+ and N dopings were performed by ion implantation using, respectively, boron in a dose of 2 × 1015 cm-2 and phosphorus in two doses of 1014 cm-2 for tf = 700 nm and 5 × 1014 cm-2 for tf = 450 nm. Using scanning electron microscopy, the presence of one grain boundary (g.b.) perpendicular to the metallurgic junction and localized at 100 nm of the interface deposition has been observed. In this work, we particularly investigate the effect of this g.b. on the C-V characteristics. The measured C-V characteristics at 100 kHz and 1 MHz show that the frequency effect is more important in the case of the weakly doped film (tf = 700 nm). A determination of the series resistance gives the profile doping concentration: abrupt (ND = 5.5 × 1018 cm-3) for tf = 700 nm and gradual (slope = 5 × 1025 cm-4) for tf = 450 nm. Using the previous experimental parameters in the two-dimensional simulation, we show that the presence of the perpendicular g.b. can reduce by up to 25% the capacitance of the diode and decreases considerably the VRP voltage that corresponds to the realizing-pinning of the electrostatic potential at the first parallel g.b. This effect is more important when the doping is gradual. The fit of the experimental curves gives, in the weak doping case (tf = 700 nm), the position of the first parallel g.b. (LG1 = 37.5 nm) and the density of the inter-granular trap states (NT = 3.2 × 1012 cm-2). On the other hand, when the doping is relatively strong (tf = 450 nm), the fit shows that the C-V characteristic is dominated much more by the doping profile than by the position of the first grain boundary and the density of the inter-granular trap states.

Amrani, M.; Benamara, Z.; Menezla, R.; Boudissa, A.; Chellali, M.; Brahim, T. Mohammed; Raoult, F.

2005-02-01

280

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

NASA Astrophysics Data System (ADS)

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

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

1997-09-01

281

Cylindrical microcavity plasma devices with diameters (D) in the 100-300 {mu}m range and a dielectric barrier structure similar to that described by Park et al. [J. Appl. Phys. 99, 026107 (2006)] for Al/Al{sub 2}O{sub 3} 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(less-or-similar sign)pD(less-or-similar sign)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/Al{sub 2}O{sub 3} microplasma devices for which D=100-300 {mu}m.

Seo, Jeong Hyun; Eden, J. Gary [Laboratory for Optical Physics and Engineering, Department of Electrical and Computer Engineering, University of Illinois, 1406 West Green Street, Urbana, Illinois 61801 (United States)

2006-12-15

282

Kinetic modeling of the exciton migration in the cyanobacterial photosystem I core complex from Synechococcus sp. was performed by an exact solution of the Pauli master equation for exciton motion. A square two-dimensional 10 x 10 pigment lattice and a Förster dipole-dipole coupling between chromophores was assumed. We calculated decay-associated spectra and lifetimes and compared them to the corresponding experimental data from picosecond fluorescence and transient absorption obtained by global analysis. Seven spectral chlorophyll(Chl) forms, identical in shape but shifted in their absorption maximums, were used to describe the non-homogeneous broadening of the PS I-100 particle absorption spectrum. The optimized Chl lattice arrangement best reproducing the experimental decay-associated spectra as well as the steady-state fluorescence spectrum indicated the long-wavelength-absorbing Chls forming a cluster in the corner of the lattice with the reaction center (RC) placed apart at a distance of two lattice constants. The variable parameters, i.e., the charge separation rate in the RC and the lattice constant a, were found to be optimal at kRC = 2.3 ps-1 and a = 1.14 nm, respectively. The surprising conclusions of the simulations is that Chls with absorption maxima as long a 724 nm have to be taken into account to describe the time-resolved spectra of this PS I particle properly. The dependencies of the exciton decay in the model PS I particle on the excitation wavelength and on the temperature are discussed. We also show that the excited state decay of similar PS I particles that lack the long-wavelength absorbing Chls is nearly mono-exponential. Various critical factors that limit the general reliability of the conclusions of such simulations are discussed in detail. PMID:8161695

Trinkunas, G; Holzwarth, A R

1994-01-01

283

On March 24, 1995, the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the low-latitude boundary layer about 15RE tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the Bz component showed strong short-duration fluctuations in which Bz could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations

A. Otto; D. H. Fairfield

2000-01-01

284

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

NASA Astrophysics Data System (ADS)

Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17°, supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation.

Riethmüller, T. L.; Solanki, S. K.; Berdyugina, S. V.; Schüssler, M.; Martínez Pillet, V.; Feller, A.; Gandorfer, A.; Hirzberger, J.

2014-08-01

285

NASA Astrophysics Data System (ADS)

In order to describe the behavior of tokamak plasmas in both core and peripheral regions self-consistently, two-dimensional transport simulation is desirable and becoming feasible. We have formulated transport equations with poloidal-angle dependence from Braginskii equations for two-dimensional transport analysis. Following assumptions have been made to derive these equations; axisymmetry, MHD equilibrium, transport process much slower than the Alfvén velocity, and weak time dependence of basis vectors. The set of transport equations is composed of continuity equation, equation for velocity including the neoclassical viscosity, and equation of energy transport for each species. These equations are coupled with equations for electromagnetic field; Grad-Shaftranov equation, magnetic diffusion equation, and Poisson equation for electrostatic potential. Preliminary numerical results of two-dimensional transport analysis will be presented.

Seto, H.; Fukuyama, A.

2011-11-01

286

Extragalactic jets with helical magnetic fields: relativistic MHD simulations

Context: Extragalactic jets are judged to harbor dynamically important, organized magnetic fields that presumably aid in the collimation of the relativistic jet flows. Aims: We here explore the morphology of AGN jets pervaded by helical field and flow topologies by means of grid-adaptive, high-resolution numerical simulations. We concentrate on morphological features of the bow shock and the jet beam behind

R. Keppens; Z. Meliani; B. van der Holst; F. Casse

2008-01-01

287

High Lundquist Number Resistive MHD Simulations of Magnetic

instability of the Sweet-Parker current sheet was identified as a possible mechanism that can lead to fast of 104. Our simulations are based on a fairly standard pseudo spectral code, which has been well tested is supported by a NASA grant NNX08BA71G and a NSF grant AGS-0962477. #12;"Slow" Sweet-Parker reconnection From

Ng, Chung-Sang

288

PROPERTIES OF UMBRAL DOTS AS MEASURED FROM THE NEW SOLAR TELESCOPE DATA AND MHD SIMULATIONS

We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statistical properties to recent MHD models. The study is based on high-resolution data recorded by the New Solar Telescope at the Big Bear Solar Observatory and three-dimensional (3D) MHD simulations of sunspots. Observed UDs, living longer than 150 s, were detected and tracked in a 46 minute long data set, using an automatic detection code. A total of 1553 (620) UDs were detected in the photospheric (low chromospheric) data. Our main findings are (1) none of the analyzed UDs is precisely circular, (2) the diameter-intensity relationship only holds in bright umbral areas, and (3) UD velocities are inversely related to their lifetime. While nearly all photospheric UDs can be identified in the low chromospheric images, some small closely spaced UDs appear in the low chromosphere as a single cluster. Slow-moving and long-living UDs seem to exist in both the low chromosphere and photosphere, while fast-moving and short-living UDs are mainly detected in the photospheric images. Comparison to the 3D MHD simulations showed that both types of UDs display, on average, very similar statistical characteristics. However, (1) the average number of observed UDs per unit area is smaller than that of the model UDs, and (2) on average, the diameter of model UDs is slightly larger than that of observed ones.

Kilcik, A.; Yurchyshyn, V. B.; Abramenko, V.; Goode, P. R.; Cao, W. [Big Bear Solar Observatory, Big Bear City, CA 92314 (United States); Rempel, M. [High Altitude Observatory, NCAR, Boulder, CO 80307-3000 (United States); Kitai, R.; Watanabe, H. [Kwasan and Hida Observatories, Kyoto University, Kyoto 607-8417 (Japan)

2012-02-01

289

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

Klaus Bitzer

1999-01-01

290

General relativistic MHD simulations of monopole magnetospheres of black holes

In this paper we report the results of the first ever time-dependent general relativistic magnetohydrodynamic simulations of the magnetically dominated monopole magnetospheres of black holes. It is found that the numerical solution evolves towards a stable steady-state solution which is very close to the corresponding force-free solution found by Blandford and Znajek. Contrary to the recent claims, the particle inertia does not become dynamically important near the event horizon and the force-free approximation provides a proper framework for magnetically dominated magnetospheres of black holes. For the first time, our numerical simulations show the development of an ultra-relativistic particle wind from a rotating black hole. However, the flow remains Poynting dominated all the way up to the fast critical point. This suggests that the details of the so-called ``astrophysical load'', where the electromagnetic energy is transferred to particles, may have no effect on the efficiency of the Blandford-Znajek mechanism.

S. S. Komissarov

2004-02-18

291

3D MHD Simulations of Large-Scale Structures of Magnetic Jets

NASA Astrophysics Data System (ADS)

Extragalactic radio jets represent a significant amount of magnetic energy (and perhaps magnetic flux) flow from supermassive black holes inside massive galaxies to the intergalactic medium (IGM). We will present 3D MHD simulations of the formation of large scale magnetic jets/``towers,'' evolved from an isolated and idealized initial state where magnetic fields are injected in a small volume. We will present a detailed analysis of the ``tower'' structure, collimation mechanisms, instabilities, and flux conversion processes. We will also compare our simulation results with astrophysical jet observations.

Li, Hui; Nakamura, Masanori; Li, Shengtai

2006-04-01

292

Two-Dimensional Modulation Codes

A new class of run-length-limited codes in introduced. These codes are called two-dimensional or multitrack modulation codes. Two-dimensional modulation codes provide substantial data storage density increases for multitrack recording systems by operating on multiple tracks in parallel. Procedures for computing the capacity of these new codes are given along with fast algorithms for implementing these procedures. Examples of two-dimensional codes

Michael W. Marcellin; Harold J. Weber

1992-01-01

293

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

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

1996-01-01

294

Three-dimensional MHD Simulation of the Caltech Plasma Jet Experiment: First Results

NASA Astrophysics Data System (ADS)

Magnetic fields are believed to play an essential role in astrophysical jets with observations suggesting the presence of helical magnetic fields. Here, we present three-dimensional (3D) ideal MHD simulations of the Caltech plasma jet experiment using a magnetic tower scenario as the baseline model. Magnetic fields consist of an initially localized dipole-like poloidal component and a toroidal component that is continuously being injected into the domain. This flux injection mimics the poloidal currents driven by the anode-cathode voltage drop in the experiment. The injected toroidal field stretches the poloidal fields to large distances, while forming a collimated jet along with several other key features. Detailed comparisons between 3D MHD simulations and experimental measurements provide a comprehensive description of the interplay among magnetic force, pressure, and flow effects. In particular, we delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. With suitably chosen parameters that are derived from experiments, the jet in the simulation agrees quantitatively with the experimental jet in terms of magnetic/kinetic/inertial energy, total poloidal current, voltage, jet radius, and jet propagation velocity. Specifically, the jet velocity in the simulation is proportional to the poloidal current divided by the square root of the jet density, in agreement with both the experiment and analytical theory. This work provides a new and quantitative method for relating experiments, numerical simulations, and astrophysical observation, and demonstrates the possibility of using terrestrial laboratory experiments to study astrophysical jets.

Zhai, Xiang; Li, Hui; Bellan, Paul M.; Li, Shengtai

2014-08-01

295

Numerical simulation of MHD shock waves in the solar wind

NASA Technical Reports Server (NTRS)

The effects of the interplanetary magnetic field on the propagation speed of shock waves through an ambient solar wind are examined by numerical solutions of the time-dependent nonlinear equations of motion. The magnetic field always increases the velocity of strong shocks. Although the field may temporarily slow down weak shocks inside 1 AU, it eventually also causes weak shocks to travel faster than they would without the magnetic field at larger distances. Consistent with the increase in the shock velocity, the gas pressure ratio across a shock is reduced considerably in the presence of the magnetic field. The numerical method is used to simulate (starting at 0.3 AU) the large deceleration of a shock observed in the lower corona by ground-based radio instrumentation and the more gradual deceleration of the shock in the solar wind observed by the Pioneer 9 and Pioneer 10 spacecraft.

Steinolfson, R. S.; Dryer, M.

1978-01-01

296

Simulation of 3-D Nonequilibrium Seeded Air Flow in the NASA-Ames MHD Channel

NASA Technical Reports Server (NTRS)

The 3-D nonequilibrium seeded air flow in the NASA-Ames experimental MHD channel has been numerically simulated. 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 us ing a 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 efficient manner. The algorithm has been extended in the present study to account for nonequilibrium seeded air flows. The electrical conductivity of the flow is determined using the program of Park. 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 seeded flow. The computed results are in good agreement with the experimental data.

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

2004-01-01

297

The Evolution and Efficiency of Oblique MHD Cosmic-Ray Shocks: Two-Fluid Simulations

Using a new, second-order accurate numerical method we present dynamical simulations of oblique MHD cosmic ray (CR) modified plane shock evolution using the two-fluid model for diffusive particle acceleration. The numerical shocks evolve to published analytical steady state properties. In order to probe the dynamical role of magnetic fields we have explored for these time asymptotic states the parameter space of upstream fast mode Mach number, $M_f$, and plasma $\\beta$, compiling the results into maps of dynamical steady state CR acceleration efficiency, $\\epsilon_c$. These maps, along with additional numerical experiments, show that $\\epsilon_c$ is reduced through the action of compressive work on tangential magnetic fields in CR-MHD shocks. Thus $\\epsilon_c$ in low $\\beta$, moderate $M_f$ shocks tends to be smaller in quasi perpendicular shocks than it would be high $\\beta$ shocks of the same $M_f$. This result supports earlier conclusions that strong, oblique magnetic fields inhibit diffusive shock acceleration. For quasi parallel shocks with $\\beta efficiencies for high and low $\\beta$ having comparable sonic Mach numbers are more similar. Time evolution of CR-MHD shocks is qualitatively similar to CR-gasdynamical shocks. However, several potentially interesting differences are apparent.

Adam Frank; T. W. Jones; Dongsu Ryu

1994-04-29

298

NASA Astrophysics Data System (ADS)

Milne-Eddington (M-E) inversion codes for the radiative transfer equation are the most widely used tools to infer the magnetic field from observations of the polarization signals in photospheric and chromospheric spectral lines. Unfortunately, a comprehensive comparison between the different M-E codes available to the solar physics community is still missing, and so is a physical interpretation of their inferences. In this contribution we offer a comparison between three of those codes (VFISV, ASP/HAO, and HeLIx+). These codes are used to invert synthetic Stokes profiles that were previously obtained from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD) simulations. The results of the inversion are compared with each other and with those from the MHD simulations. In the first case, the M-E codes retrieve values for the magnetic field strength, inclination and line-of-sight velocity that agree with each other within ?B ? 35 (Gauss), ?? ? 1.2°, and ?v ? 10 m s-1, respectively. Additionally, M-E inversion codes agree with the numerical simulations, when compared at a fixed optical depth, within ?B ? 130 (Gauss), ?? ? 5°, and ?v ? 320 m s-1. Finally, we show that employing generalized response functions to determine the height at which M-E codes measure physical parameters is more meaningful than comparing at a fixed geometrical height or optical depth. In this case the differences between M-E inferences and the 3D MHD simulations decrease to ?B ? 90 (Gauss), ?? ? 3°, and ?v ? 90 m s-1.

Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M.

2014-12-01

299

Two-dimensional rotating heat pipe analysis

A detailed transient numerical simulation of rotating heat pipes is presented. This two-dimensional, axisymmetric formulation accounts for the thin liquid condensate film on the inner surface of the rotating pipe wall, the vapor flow in the vapor space, and the unsteady heat conduction in the pipe wall. The thin liquid film is coupled to the vapor velocity at the liquid-vapor

C. Harley; A. Faghri

1995-01-01

300

Relativistic modeling capabilities in PERSEUS extended MHD simulation code for HED plasmas

NASA Astrophysics Data System (ADS)

We discuss the incorporation of relativistic modeling capabilities into the PERSEUS extended MHD simulation code for high-energy-density (HED) plasmas, and present the latest hybrid X-pinch simulation results. The use of fully relativistic equations enables the model to remain self-consistent in simulations of such relativistic phenomena as X-pinches and laser-plasma interactions. By suitable formulation of the relativistic generalized Ohm's law as an evolution equation, we have reduced the recovery of primitive variables, a major technical challenge in relativistic codes, to a straightforward algebraic computation. Our code recovers expected results in the non-relativistic limit, and reveals new physics in the modeling of electron beam acceleration following an X-pinch. Through the use of a relaxation scheme, relativistic PERSEUS is able to handle nine orders of magnitude in density variation, making it the first fluid code, to our knowledge, that can simulate relativistic HED plasmas.

Hamlin, Nathaniel D.; Seyler, Charles E.

2014-12-01

301

NASA Astrophysics Data System (ADS)

Magnetic reconnection is considered to play an important role in space phenomena such as substorm in the Earth's magnetosphere. Tanaka and Fujita reproduced substorm evolution process by numerical simulation with the global MHD code [1]. In the MHD framework, the dissipation model is introduced for modeling of the kinetic effects. They found that the normalized reconnection viscosity, one of the dissipation model employed there, gave a large effect for the dipolarization, central phenomenon in the substorm development process, though that viscosity was assumed to be a constant parameter. It is well known that magnetic reconnection is controlled by microscopic kinetic mechanism. Frozen-in condition is broken due to particle kinetic effects and collisionless reconnection is triggered when current sheet is compressed as thin as ion kinetic scales under the influence of external driving flow [2, 3]. Horiuchi and his collaborators showed that reconnection electric field generated by microscopic physics evolves inside ion meandering scale so as to balance the flux inflow rate at the inflow boundary, which is controlled by macroscopic physics [2]. That is, effective resistivity generated through this process can be expressed by balance equation between micro and macro physics. In this paper, we perform substorm simulation by using the global MHD code developed by Tanaka [3] with this effective resistivity instead of the empirical resistivity model. We obtain the AE indices from simulation data, in which substorm onset can be seen clearly, and investigate the relationship between the substorm development and the effective resistivity model. [1] T. Tanaka, A, Nakamizo, A. Yoshikawa, S. Fujita, H. Shinagawa, H. Shimazu, T. Kikuchi, and K. K. Hashimoto, J. Geophys. Res. 115 (2010) A05220,doi:10.1029/2009JA014676. [2] W. Pei, R. Horiuchi, and T. Sato, Physics of Plasmas,Vol. 8 (2001), pp. 3251-3257. [3] A. Ishizawa, and R. Horiuchi, Phys. Rev. Lett., Vol. 95, 045003 (2005). [4] T. Tanaka, J. Comp. Phys. 111 (1994) 381.

Den, M.; Horiuchi, R.; Fujita, S.; Tanaka, T.

2011-12-01

302

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

303

MHD simulation of a propagation of loop-like and bubble-like magnetic clouds

NASA Technical Reports Server (NTRS)

Propagation and evolution of magnetic clouds in the ambient solar wind flow is studied self-consistently using ideal MHD equations in three dimensions. Magnetic clouds as ideal force-free objects (cylinders or spheres) are ejected near the Sun and followed beyond the Earth's orbit. We investigate the influence of various initial parameters like the injection velocity, magnetic field strength, magnetic helicity, orientation of the clouds' axis, etc., on their propagation and evolution. We demonstrate that the injection velocity and magnetic field strength have a major influence on propagation. Simulation results are compared with analytical solutions of magnetic cloud evolution.

Vandas, M.; Fischer, S.; Pelant, P.; Dryer, M.; Smith, Z.; Detman, T.

1995-01-01

304

Particle escape in the interplanetary medium: Link between CME observations and MHD simulations

NASA Astrophysics Data System (ADS)

Among the more hazardous forms of space weather at Earth and in the heliosphere are the intense solar energetic particle (SEP) bursts associated with fast coronal mass ejections (CMEs) and eruptive flares. A fundamental question to understand the origin and the evolution of solar energetic particles is: How do solar energetic particles escape the Sun? Answering this question is critical for understanding how the corona couples dynamically to the heliosphere during explosive events, and is fundamental to developing any future forecasting capability for SEP events. The release onto open field lines of energetic particles originating in the low corona is the bridge connecting the acceleration site to the interplanetary propagation and is, therefore, the key to reconciling remote and in-situ observations of energetic particles. Recent multi-instrument studies showed that CMEs are important factors that determine whether the energetic particles escape into the heliosphere and partly define the spatial distribution of particle flux. In order to understand how and why CMEs play a crucial role in the particle escape, we must understand the dynamics of the corona disturbed by a CME ejection. The details of the dynamics can be studied through MHD simulations. To advance understanding, it is pertinent to combine observations and simulations to develop models that respect the observational constraints. Thus, first we will describe the observational results, then discuss how MHD simulations help demonstrate why CMEs are important for particle release.

Masson, Sophie; Antiochos, Spiro; DeVore, C. Richard

305

NASA Astrophysics Data System (ADS)

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

Watanabe, Tadashi; Kaburaki, Hideo; Yokokawa, Mitsuo

1995-04-01

306

Comparing resistive MHD simulations and experiments of Coaxial Helicity Injection (CHI) in NSTX

NASA Astrophysics Data System (ADS)

NSTX generates CHI plasmas with current, density, and temperature appropriate for ST startup.ootnotetextR. Raman, et al., PRL 104, 095003 (2010). Whole-device simulations of CHI using the NIMROD MHD codeootnotetextC.R. Sovinec, et al., J. Comp. Phys 195, 355 (2004). extend the HIT-II model.ootnotetextA. Bayliss, et al., submitted for publication. A model power supply generates time-dependent voltage and current at the injection gap. Absorber gap voltage maintains a constant vacuum toroidal flux. Simulation physics includes ohmic heating and thermal conductivity along and across the magnetic field and generation of nonaxisymmetric fields and flows. A flux bubble expands in the simulation with current and plasma temperature similar to experiment; an n=1 mode is observed to generate an helical ribbon of current and velocity vortices on the flux bubble surface. Time-dependent poloidal-field boundary conditions for interesting NSTX discharges are used for quantitative comparisons with experiments.

Hooper, E. B.; Sovinec, C. R.; Raman, R.; Menard, J. E.

2011-11-01

307

Beaver Lake is considered a primary watershed of concern in the State of Arkansas. As such, information is needed to assess water quality, especially nutrient enrichment, nutrient-algal relations, turbidity, and sediment issues within the system. A previously calibrated two-dimensional, laterally averaged model of hydrodynamics and water quality was used for the evaluation of changes in input nutrient and sediment concentrations on the water quality of the reservoir for the period of April 2001 to April 2003. Nitrogen and phosphorus concentrations were increased and decreased and tested independently and simultaneously to examine the nutrient concentrations and algal response in the reservoir. Suspended-solids concentrations were increased and decreased to identify how solids are distributed in the reservoir, which can contribute to decreased water clarity. The Beaver Lake model also was evaluated using a conservative tracer. A conservative tracer was applied at various locations in the reservoir model to observe the fate and transport and how the reservoir might react to the introduction of a conservative substance, or a worst-case spill scenario. In particular, tracer concentrations were evaluated at the locations of the four public water-supply intakes in Beaver Lake. Nutrient concentrations in Beaver Lake increased proportionally with increases in loads from the three main tributaries. An increase of 10 times the calibrated daily input nitrogen and phosphorus in the three main tributaries resulted in daily mean total nitrogen concentrations in the epilimnion that were nearly 4 times greater than the calibration concentrations at site L2 and more than 2 times greater than the calibrated concentrations at site L5. Increases in daily input nitrogen in the three main tributaries independently did not correspond in substantial increases in concentrations of nitrogen in Beaver Lake. The greatest proportional increase in phosphorus occurred in the epilimnion at sites L3 and L4 and the least increase occurred at sites L2 and L5 when calibrated daily input phosphorus concentrations were increased. When orthophosphorus was increased in all three tributaries simultaneously by a factor of 10, daily mean orthophosphorus concentrations in the epilimnion of the reservoir were almost 11 times greater than the calibrated concentrations at sites L2 and L5, and 15 times greater in the epilimnion of the reservoir at sites L3 and L4. Phosphorus concentrations in Beaver Lake increased less when nitrogen and phosphorus were increased simultaneously than when phosphorus was increased independently. The greatest simulated increase in algal biomass (represented as chlorophyll a) occurred when nitrogen and phosphorus were increased simultaneously in the three main tributaries. On average, the chlorophyll a values only increased less than 1 microgram per liter when concentrations of nitrogen or phosphorous were increased independently by a factor of 10 at all three tributaries. In comparison, when nitrogen and phosphorus were increased simultaneously by a factor of 10 for all three tributaries, the chlorophyll a concentration increased by about 10 micrograms per liter on average, with a maximum increase of about 57 micrograms per liter in the epilimnion at site L3 in Beaver Lake. Changes in algal biomass with changes in input nitrogen and phosphorus were variable through time in the Beaver Lake model from April 2001 to April 2003. When calibrated daily input nitrogen and phosphorus concentrations were increased simultaneously for the three main tributaries, the increase in chlorophyll a concentration was the greatest in late spring and summer of 2002. Changes in calibrated daily input inorganic suspended solids concentrations were examined because of the effect they may have on water clarity in Beaver Lake. The increase in total suspended solids was greatest in the hypolimnion at the upstream end of Beaver Lake, and negligible changes

Galloway, Joel M.; Green, W. Reed

2007-01-01

308

NASA Astrophysics Data System (ADS)

In global modeling, magnetosphere-ionosphere (MI) coupling physically connects a global magnetospheric (GM) model and a global ionospheric-thermospheric (GIT) model. The field-aligned current from the GM model and the conductance distributions from the GIT model are used in a Poisson equation derived from the ionospheric Ohm's law combined with current continuity to determine the electrostatic potential in the ionosphere. In current GM models, this electrostatic potential is mapped to the inner boundary of the GM simulation to determine electrostatic boundary conditions on the electric field and MHD velocity there. Inductive effects and the finite Alfven transit time between the low-altitude GM boundary and the high-altitude GIT boundary (MI gap region) are neglected in this formulation of MI coupling. Using fields and currents derived from Lyon-Fedder-Mobarry GM simulations, and conductance distributions derived from its standalone empirical conductance model in the MI coupling Poisson equation, we have computed the fast Fourier transform of the electrostatic field at the low-altitude LFM simulation boundary as described above, and the FFT of the inductive electric field at the boundary under the assumption that ? 0 ? P vA ? 1, where ? P is the ionospheric Pedersen conductance and vA is the smallest value of the Alfven speed in the MI gap region. In this regime, the complete electric field at the low-altitude simulation boundary includes the usual mapped electrostatic field with an inductive addition for which the finite Alfven transit time and the diversion of field-aligned into polarization currents in the gap region are negligible (Lotko, 2004). By comparing the boundary-averaged spectra of the electrostatic and so-determined inductive fields, we confirm that the purely electrostatic formulation of MI coupling is valid when the MHD state varies on times scales exceeding about 200 s. For faster MHD time variations, the inductive electric field is shown to be larger than the electrostatic field at the low-altitude boundary and is thus non-negligible. For example, inductive corrections are expected to be important for sudden impulse events and substorm-related Pi2 fluctuations. We are currently implementing the inductive MI coupling algorithm of Lotko (2004) in the LFM global simulation. Lotko, W. (2004), Inductive magnetosphere-ionosphere coupling, JASTP 66, 1443-1456.

Xi, S.; Lotko, W.; Zhang, B.; Brambles, O.; Wiltberger, M. J.; Lyon, J.; Merkin, V. G.

2010-12-01

309

LARGE EDDY SIMULATION FOR TURBULENT MHD FLOWS A. LABOVSKY AND C. TRENCHEA

], electromagnetic turbulence control in induction furnaces [54], electromagnetic damping of buoyancy- driven flow dedicated to both experimental and theoretical investigations on the influence of electromagnetic force reactors, electromagnetic casting of metals, MHD sea water propulsion. The MHD effects arising from

Trenchea, Catalin

310

NASA Technical Reports Server (NTRS)

Flow matching has been successfully achieved for an MHD energy bypass system on a supersonic turbojet engine. The Numerical Propulsion System Simulation (NPSS) environment helped perform a thermodynamic cycle analysis to properly match the flows from an inlet employing a MHD energy bypass system (consisting of an MHD generator and MHD accelerator) on a supersonic turbojet engine. Working with various operating conditions (such as the applied magnetic field, MHD generator length and flow conductivity), interfacing studies were conducted between the MHD generator, the turbojet engine, and the MHD accelerator. This paper briefly describes the NPSS environment used in this analysis. This paper further describes the analysis of a supersonic turbojet engine with an MHD generator/accelerator energy bypass system. Results from this study have shown that using MHD energy bypass in the flow path of a supersonic turbojet engine increases the useful Mach number operating range from 0 to 3.0 Mach (not using MHD) to a range of 0 to 7.0 Mach with specific net thrust range of 740 N-s/kg (at ambient Mach = 3.25) to 70 N-s/kg (at ambient Mach = 7). These results were achieved with an applied magnetic field of 2.5 Tesla and conductivity levels in a range from 2 mhos/m (ambient Mach = 7) to 5.5 mhos/m (ambient Mach = 3.5) for an MHD generator length of 3 m.

Benyo, Theresa L.

2011-01-01

311

3D MHD Simulations of Large-Scale Structures of Magnetic Jets

NASA Astrophysics Data System (ADS)

Extragalactic radio jets represent a significant amount of magnetic energy (and perhaps magnetic flux) flow from supermassive black holes inside massive galaxies to the intergalactic medium (IGM). Magnetic fields are believed to play an important role in determining the overall structure of astrophysical jets, though many fundamental questions remain, such as what collimation mechanisms are, what determines the lobe formation, etc. We will present 3D MHD simulations of the formation of large scale magnetic jets/``towers,'' evolved from an isolated and idealized initial state where magnetic fields are injected in a small volume. We will present a detailed analysis of the ``tower'' structure, collimation mechanisms, instabilities, and flux conversion processes. We will also compare our simulation results with astrophysical jet observations.

Li, Hui

2005-10-01

312

Broken Ergodicity in Two-Dimensional Homogeneous Magnetohydrodynamic Turbulence

NASA Technical Reports Server (NTRS)

Two-dimensional (2-D) homogeneous magnetohydrodynamic (MHD) turbulence has many of the same qualitative features as three-dimensional (3-D) homogeneous MHD turbulence.The se features include several ideal invariants, along with the phenomenon of broken ergodicity. Broken ergodicity appears when certain modes act like random variables with mean values that are large compared to their standard deviations, indicating a coherent structure or dynamo.Recently, the origin of broken ergodicity in 3-D MHD turbulence that is manifest in the lowest wavenumbers was explained. Here, a detailed description of the origins of broken ergodicity in 2-D MHD turbulence is presented. It will be seen that broken ergodicity in ideal 2-D MHD turbulence can be manifest in the lowest wavenumbers of a finite numerical model for certain initial conditions or in the highest wavenumbers for another set of initial conditions.T he origins of broken ergodicity in ideal 2-D homogeneous MHD turbulence are found through an eigen analysis of the covariance matrices of the modal probability density functions.It will also be shown that when the lowest wavenumber magnetic field becomes quasi-stationary, the higher wavenumber modes can propagate as Alfven waves on these almost static large-scale magnetic structures

Shebalin, John V.

2010-01-01

313

Interaction of Cometary Material With the Solar Corona: EUV Observations and MHD Simulations

NASA Astrophysics Data System (ADS)

Extreme ultraviolet (EUV) emission from two recent sun-grazing comets, C/2011 N3 and C/2011 W3 (Lovejoy), has been observed in the solar corona for the first time by the SDO/AIA and STEREO/EUVI instruments (Schrijver et al. 2011). These observations provided a unique opportunity to investigate the interaction of the cometary material with the solar corona and probe their physical conditions. We present here EUV observations and MHD simulations on this subject, focusing on the deceleration of the cometary tail material within the corona. We found that despite their different local coronal environments, the two comets exhibited quite similar characteristics. The initial EUV emitting tail had a projected velocity of 100-200 km/s, which was much lower than the orbital velocity of 500-600 km/s in the plane-of-sky. This indicates that significant deceleration had taken place while the tail material was heated to coronal temperatures on the order of 1 MK before it started to emit in EUV (Bryans & Pesnell 2012). After its initial appearance, the tail further experienced a projected deceleration of ~1 km/s^2 (or 4 g_Sun). In particular, in the Lovejoy case, the tail appeared as clusters of bright parallel striations roughly at right angles to the orbit direction, suggestive of magnetic field lines illuminated by the plasma frozen onto them. These striations came to a stop and then accelerated in an opposite direction (seen in projection), approaching a constant velocity of ~50 km/s. These observations suggest that a Lorentz force from the coronal magnetic field was operating on the newly ionized cometary plasma. To test this hypothesis and understand tail deceleration mechanisms, we adopted a multi-fluid MHD model (Jia et al. 2012) to simulate the interaction between charged particles and the magnetized coronal plasma. We used potential extrapolation (Schrijver & DeRosa 2003) and a more sophisticated global MHD model (Lionello et al. 2009) to infer the magnetic field and plasma conditions of the corona along the comet's orbit as inputs to the simulations. We will compare the observations and simulation results, and discuss the implications for using sun-grazing comets as probes to the solar corona in the context of NASA's future Solar Probe Plus mission.

Liu, W.; Jia, Y.; Downs, C.; Schrijver, C.; Saint-Hilaire, P.; Battams, K.; Tarbell, T. D.; Shine, R. A.

2012-12-01

314

NASA Astrophysics Data System (ADS)

The evolution of the inner heliosphere over a 9-year period from 2001 to 2009, from 50 Rs to 1250 Rs, is simulated by means of the time-dependent three-dimensional MHD simulation with a new treatment handling the observation-based temporally evolving boundary data. The inner boundary magnetic field is calculated from the solar surface magnetic field data made at Wilcox Solar Observatory (WSO), and the plasma speed data is from the maps constructed by means of the MHD tomography analysis using the interplanetary scintillation (IPS) data. The density and temperatures are determined by empirical relation functions that are derived from Helios data. In this way, all inner boundary values of the solar wind flowing outward are from measurements. The two ground-based observations, WSO magnetic field and IPS solar wind data, have been long conducted, and we used these two dataset to drive the simulation system over the 9-year span well covering the solar cycle 23. The simulated interplanetary MHD variables are compared with the in-situ measurements, OMNI (nearby the Earth) and the Ulysses. Overall, except the poloidal component of the magnetic field, the simulated MHD variables derived agree very well with the in-situ data.

Hayashi, Keiji; Washimi, H.; Tokumaru, M.

2012-05-01

315

NASA Astrophysics Data System (ADS)

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

Kanki, Takashi; Nagata, Masayoshi; Kagei, Yasuhiro

2011-11-01

316

Two-Species, 3D, MHD Simulation of Europa's Interaction with Jupiter's Magnetosphere

NASA Technical Reports Server (NTRS)

The interaction of Europa with the Jovian a magnetosphere has been studied by using a two species in ideal magnetohydrodynamic (MHD) numerical model. This model considers the upstream plasma in the Jovian magnetosphere and the molecular oxygen ions in the ionosphere of Europa, separately. We present results a from simulation studies, which take into account impact ionization, recombination, and the effect of a possible induced dipole magnetic field of Europa. The total mass loading of the magnetospheric flow and the ionization frequency used in the model are consistent with the estimates of Europa's ionosphere and atmosphere. The multi-species MHD equations are solved by using a finite volume, high-order, Godunov-type method on an adoptively refined unstructured grid, which allows a detailed modeling of the region near Europa's surface, while still resolving both the upstream region and the satellite's wake. We have paid special attention to the wake of Europa, in order to be able to make comparisons with the Galileo's E4 flyby observations, as well as other model calculations. The calculated escape flux of a O2+ down the tail was found to be about 5.6 x 10(exp 25) s(sup -1).

Liu, Yifan; Nagy, Andrew F.; Kabin, Konstantin; Combi, Michael R.; DeZeeuw, Darren L.; Gombosi, Tamas I.; Powell, Kenneth G.

2000-01-01

317

MHD simulation of relaxation transition to a flipped relaxed state in spherical torus

NASA Astrophysics Data System (ADS)

Recently, it has been demonstrated in the HIST device that in spite of the violation of the Kruskal-Shafranov stability condition, a normal spherical torus (ST) plasma has relaxed to a flipped ST state through a transient reversed-field pinch-like state when the vacuum toroidal field is decreased and its direction is reversed [1]. It has been also observed during this relaxation transition process that not only the toroidal field but also the poloidal field reverses polarity spontaneously and that the ion flow velocity is strongly fluctuated and abruptly increased up to > 50 km/s. The purpose of the present study is to investigate the plasma flows and the relevant MHD relaxation phenomena to elucidate this transition mechanism by using three-dimensional MHD simulations [2]. It is found from the numerical results that the magnetic reconnection between the open and closed field lines occurs due to the non-linear growth of the n=1 kink instability of the central open flux, generating the toroidal flow ˜ 60 km/s in the direction of the toroidal current. The n=1 kink instability and the plasma flows driven by the magnetic reconnection are consider to be responsible for the self-reversal of the magnetic fields. [1] M. Nagata el al., Phys. Rev. Lett. 90, 225001 (2003). [2] Y. Kagei el al., Plasma. Phys. Control. Fusion 45, L17 (2003).

Kanki, Takashi; Nagata, Masayoshi; Kagei, Yasuhiro

2008-11-01

318

Hybrid simulations of the interaction of hot gyrokinetic particles with MHD waves

A self-consistent study of the interaction of energetic ions with low-frequency MHD waves is performed using hybrid MHD-gyrokinetic particle simulations. In particular, the excitation of magnetospheric hydromagnetic waves by magnetic drift-bounce resonance with energetic ring current ions is investigated. In the model, energetic ions are treated as gyrokinetic particles using fully electromagnetic gyro-center equations, while the cold background plasma is treated as a fluid. The particles are coupled to the fluid equations through their current which appear in the bulk plasma momentum equation: where {rho}{sub b}, V{sub b} and p{sub b} are bulk plasma density, velocity and pressure, n{sub h} and j{sub h} axe hot ion density and current density. Other equations for the bulk plasma axe that of the MHD equations including E = - V{sub b} x B/c. It is assumed that n{sub h} {much_lt} n{sub b}. Spatial gyroaveraging in the gyro-center equations of motion as well as transformation to physical space axe performed by using four or eight point gyroangle distribution, in order to include the finite Larmor radius effects. In test runs, good conservation of the total energy was obtained and the finite Larmor radius effects were well reproduced for k{sub {perpendicular}}{rho}{sub h} {approximately} 1. Since magnetic drift-bounce resonant instability is driven by radial pressure gradients and requires resonance between azimuthal ion drift motion and bounce motion along magnetic field line, 3-D simulations are necessary for its investigation. The use of a multiple spatial scale expansion method enables to separate the equilibrium spatial scale lengths from those of the perturbations. In this case the zero-order ion pressure and magnetic field gradients become input parameters for the 2-D simulation. The 2-D numerical model with fixed background inhomogeneity was developed and it is used to study the drift-bounce resonant instability in 2-D box geometry.

Belova, E.V.; Denton, R.E.; Hudson, M.K. [Dartmouth College, Hanover, NH (United States); Chan, A.A. [Rice Univ., Houston, TX (United States)

1996-12-31

319

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

NASA Astrophysics Data System (ADS)

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

Schlutt, Mark

2012-10-01

320

A Forecast of the Heliospheric Termination-Shock Position by Three-dimensional MHD Simulations

NASA Astrophysics Data System (ADS)

The effects of heliospheric disturbances on the position of the termination shock (TS) are examined using a three-dimensional MHD model. Variations in the solar wind ram pressure due to the interplanetary shock waves drive the TS away from its steady state equilibrium position and emit shocks and waves downstream. Transmitted/emitted disturbances propagating from the TS to the heliopause (HP) are partially reflected at the HP, and the reflected waves return and collide with the TS. Thus, besides upstream solar wind disturbances, the TS location RTS changes in response to incident downstream disturbances associated with waves reflected from the HP produced by earlier supersonic solar wind disturbances. To determine the time-varying RTS, we incorporate Voyager 2 (V2) plasma data as a boundary condition into our 3D MHD simulations, which allows us to forecast the termination shock movement for nearly a year after the present V2 data. Our simulations indicate that the TS was at ~90 AU along the Sun-V1 line on 2007 August 14, the last tentative available date of the V2 data. After this, our simulation forecasts that RTS will decrease to a minimum distance in late 2007 or early 2008. This decrease will be mainly caused by the heliosheath returned pulse driven by the 2006 March event. Whether V2 will cross the TS or not in this period depends on the future solar wind ram pressure and also on the degree of the north-south asymmetry of the heliospheric structure. Some quantitative discussions are given.

Washimi, Haruichi; Zank, Gary P.; Hu, Qiang; Tanaka, Takashi; Munakata, Kazuoki

2007-12-01

321

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

NASA Astrophysics Data System (ADS)

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

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

2001-05-01

322

Dayside Proton Aurora: Comparisons between Global MHD Simulations and Image Observations

NASA Technical Reports Server (NTRS)

The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12 and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the iMF changes in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree with the antiparallel merging model. though locally component merging might broaden the merging region, especially in the region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause. as well as the intricate connection between the bow shock and the auroral region.

Berchem, J.; Fuselier, S. A.; Petrinec, S.; Frey, H. U.; Burch, J. L.

2003-01-01

323

Two-dimensional material nanophotonics

NASA Astrophysics Data System (ADS)

Two-dimensional materials exhibit diverse electronic properties, ranging from insulating hexagonal boron nitride and semiconducting transition metal dichalcogenides such as molybdenum disulphide, to semimetallic graphene. In this Review, we first discuss the optical properties and applications of various two-dimensional materials, and then cover two different approaches for enhancing their interactions with light: through their integration with external photonic structures, and through intrinsic polaritonic resonances. Finally, we present a narrow-bandgap layered material -- black phosphorus -- that serendipitously bridges the energy gap between the zero-bandgap graphene and the relatively large-bandgap transition metal dichalcogenides. The plethora of two-dimensional materials and their heterostructures, together with the array of available approaches for enhancing the light-matter interaction, offers the promise of scientific discoveries and nanophotonics technologies across a wide range of the electromagnetic spectrum.

Xia, Fengnian; Wang, Han; Xiao, Di; Dubey, Madan; Ramasubramaniam, Ashwin

2014-12-01

324

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

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

2008-01-01

325

The recent developments of power generation experiments and numerical simulations for closed cycle MHD power generation performed at Tokyo Institute of Technology are explained and discussed. The FUJI-1 experiments realize 18.4% of enthalpy extraction, and the 38.1 and 30.1% of enthalpy extraction are obtained by shock tunnel facility for He/Cs and Ar/Cs working gases, respectively. The author can succeed with the 3-dimensional calculations of two temperature model equation for nonequilibrium plasma in a disk generator. The experimental and numerical results promise a high efficiency MHD power generation system, and a typical system which realizes the total efficiency of 60% is proposed.

Kabashima, S.

1998-07-01

326

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

NASA Astrophysics Data System (ADS)

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 is formed. Magnetic reconnections at the coronal base due to emergences of photospheric magnetic flux strongly disturb the magnetic fields in the solar corona and interplanetary space, and generate fast jet-like plasma outflows (or non- flux-rope coronal mass ejections). The magnetic field line disturbances scatter charged particles and therefore accelerate them to high energies through the Fermi acceleration mechanism. In terms of our previously developed two-stage acceleration model, the Fermi acceleration of a certain species (e.g. 3He) of ions can lead to the abundance enhancement of this species of ions in high-energy particles relative to the solar corona, if they are preferentially heated. The magnetic reconnections at the coronal base can generate the high-frequency Alfvén waves, which can heat heavy ions with low charge-mass ratios, especially ultraheavy ions with Z > 50. The electrostatic and electromagnetic ion-cyclotron waves including 4He-cyclotron waves and H-cyclotron waves generated by electron beams, electric currents, and the low-frequency global MHD modes can significantly heat 3He, electrons, and heavy ions with appropriate charge states via the harmonic cyclotron and Landau resonances. In this presentation, we will present our new MHD simulations for solar impulsive events including the origin; magnetic topology, reconnection, and fluctuations; jet-like fast plasma outflows and non-flux-rope coronal mass ejections (CMEs); properties of CME-driven quasi-parallel shocks; and mechanisms of acceleration. We will also overview the kinetic analyses in accord with the two-stage acceleration model for the impulsive (or 3He-rich) events including plasma wave excitations, heating of particles by waves, acceleration of particles via the Fermi acceleration mechanism; and abundance enhancements and energy spectra of high- energy particles.

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

2007-12-01

327

NASA Technical Reports Server (NTRS)

Preliminary flow matching has been demonstrated for a MHD energy bypass system on a supersonic turbojet engine. The Numerical Propulsion System Simulation (NPSS) environment was used to perform a thermodynamic cycle analysis to properly match the flows from an inlet to a MHD generator and from the exit of a supersonic turbojet to a MHD accelerator. Working with various operating conditions such as the enthalpy extraction ratio and isentropic efficiency of the MHD generator and MHD accelerator, interfacing studies were conducted between the pre-ionizers, the MHD generator, the turbojet engine, and the MHD accelerator. This paper briefly describes the NPSS environment used in this analysis and describes the NPSS analysis of a supersonic turbojet engine with a MHD generator/accelerator energy bypass system. Results from this study have shown that using MHD energy bypass in the flow path of a supersonic turbojet engine increases the useful Mach number operating range from 0 to 3.0 Mach (not using MHD) to an explored and desired range of 0 to 7.0 Mach.

Benyo, Theresa L.

2010-01-01

328

3D MHD Simulations of the Nonlinear Kelvin-Helmholtz Instability

NASA Astrophysics Data System (ADS)

We have studied new 3D simulations of the MHD Kelvin-Helmholtz instability in a periodic section of a strongly sheared flow. Our focus is on the role of magnetic fields that are too weak to stabilize the linear growth of the instability, but that may still influence its nonlinear evolution. We consider media that are initially uniform except for the shear layer, which contains a velocity transition equal to the local sound speed. Alfvenic Mach numbers from about 4 to over 10(3) were examined. The initial magnetic field lies parallel to the shear plane, but is oblique to the flow direction, at an angle of 30 degrees. This work extends previous 2D simulations reported in the Astrophysical Journal (Frank etal 1995, vol 460, p 777; Jones etal 1997, vol 482, p 230). Consistent with the earlier results, we find that even initially very weak magnetic fields have the ability to dynamically modify the flows through tension developed by vortex formation and stretching. During reconnection the magnetic and velocity fields are dynamically aligned, so that the flow is self-organized. In the absense of magnetic fields, or for extremely weak fields the outcome of the instability is turbulence. However, for Alfvenic Mach numbers less than about 50 (plasma beta < 2000) we find that a broadened shear layer exists to the end of our simulations. This work is supported by KOSEF in Korea, and by the NSF, NASA and the Minnesota Supercomputing Institute in the USA.

Ryu, D.; Jones, T. W.; Frank, A.

1998-05-01

329

ON THE ORIGIN OF THE TYPE II SPICULES: DYNAMIC THREE-DIMENSIONAL MHD SIMULATIONS

Recent high temporal and spatial resolution observations of the chromosphere have forced the definition of a new type of spicule, 'type II's', that are characterized by rising rapidly, having short lives, and by fading away at the end of their lifetimes. Here, we report on features found in realistic three-dimensional simulations of the outer solar atmosphere that resemble the observed type II spicules. These features evolve naturally from the simulations as a consequence of the magnetohydrodynamical evolution of the model atmosphere. The simulations span from the upper layer of the convection zone to the lower corona and include the emergence of a horizontal magnetic flux. The state-of-art Oslo Staggered Code is used to solve the full MHD equations with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field lines. We describe in detail the physics involved in a process which we consider a possible candidate for the driver mechanism that produces type II spicules. The modeled spicule is composed of material rapidly ejected from the chromosphere that rises into the corona while being heated. Its source lies in a region with large field gradients and intense electric currents, which lead to a strong Lorentz force that squeezes the chromospheric material, resulting in a vertical pressure gradient that propels the spicule along the magnetic field, as well as Joule heating, which heats the jet material, forcing it to fade.

MartInez-Sykora, Juan [Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304 (United States); Hansteen, Viggo [Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo (Norway); Moreno-Insertis, Fernando, E-mail: j.m.sykora@astro.uio.no, E-mail: viggo.hansteen@astro.uio.no, E-mail: fmi@iac.es [Instituto de Astrofisica de Canarias, 38200 La Laguna (Tenerife) (Spain)

2011-07-20

330

Time-dependent simulation of oblique MHD cosmic-ray shocks using the two-fluid model

NASA Technical Reports Server (NTRS)

Using a new, second-order accurate numerical method we present dynamical simulations of oblique MHD cosmic-ray (CR)-modified plane shock evolution. Most of the calculations are done with a two-fluid model for diffusive shock acceleration, but we provide also comparisons between a typical shock computed that way against calculations carried out using the more complete, momentum-dependent, diffusion-advection equation. We also illustrate a test showing that these simulations evolve to dynamical equilibria consistent with previously published steady state analytic calculations for such shocks. In order to improve understanding of the dynamical role of magnetic fields in shocks modified by CR pressure we have explored for time asymptotic states the parameter space of upstream fast mode Mach number, M(sub f), and plasma beta. We compile the results into maps of dynamical steady state CR acceleration efficiency, epsilon(sub c). We have run simulations using constant, and nonisotropic, obliquity (and hence spatially) dependent forms of the diffusion coefficient kappa. Comparison of the results shows that while the final steady states achieved are the same in each case, the history of CR-MHD shocks can be strongly modified by variations in kappa and, therefore, in the acceleration timescale. Also, the coupling of CR and MHD in low beta, oblique shocks substantially influences the transient density spike that forms in strongly CR-modified shocks. We find that inside the density spike a MHD slow mode wave can be generated that eventually steepens into a shock. A strong layer develops within the density spike, driven by MHD stresses. We conjecture that currents in the shear layer could, in nonplanar flows, results in enhanced particle accretion through drift acceleration.

Frank, Adam; Jones, T. W.; Ryu, Dongsu

1995-01-01

331

Two-dimensional thermofield bosonization

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

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

2005-12-15

332

Two-Dimensional Thermofield Bosonization

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

R. L. P. G. Amaral; L. V. Belvedere; K. D. Rothe

2005-04-01

333

Two-dimensional flexible nanoelectronics

NASA Astrophysics Data System (ADS)

2014/2015 represents the tenth anniversary of modern graphene research. Over this decade, graphene has proven to be attractive for thin-film transistors owing to its remarkable electronic, optical, mechanical and thermal properties. Even its major drawback--zero bandgap--has resulted in something positive: a resurgence of interest in two-dimensional semiconductors, such as dichalcogenides and buckled nanomaterials with sizeable bandgaps. With the discovery of hexagonal boron nitride as an ideal dielectric, the materials are now in place to advance integrated flexible nanoelectronics, which uniquely take advantage of the unmatched portfolio of properties of two-dimensional crystals, beyond the capability of conventional thin films for ubiquitous flexible systems.

Akinwande, Deji; Petrone, Nicholas; Hone, James

2014-12-01

334

Two-dimensional river modeling

conveyance of the section would not reflect the inertial effects influencing this variation in the channel and overbank velocity. The water surface elevation is presented as water surface contours in Figure 12 and as a three-dimensional surface in Figure... OF FIGURES INTRODUCTION One-Dimensional and Two-Dimensional River Modeling Description of the Modeling System GOVERNING EQUATIONS Three-Dimensional Equations of Motion Effects of Turbulence Depth-Averaged Equations of Motion Non-conservative Equations...

Thompson, James Cameron

1988-01-01

335

Two-dimensional optical storage

With storage capacities increasing much faster than data rates, fast read-out of content is becoming a bottleneck for the convenient use of optical storage devices. Two-dimensional optical storage (TwoDOS) is a new concept that solves this data-rate problem by using a multi-spot parallel readout system. In addition, the storage capacity is increased with a factor of at least 2. Using

W. M. J. Coene; D. M. Bruls; A. H. J. Immink; A. M. van der Lee; A. P. Hekstra; J. Riani; S. van Beneden; M. Ciacci; J. W. M. Bergmans; M. Furuki

2005-01-01

336

Observability for two dimensional systems

NASA Technical Reports Server (NTRS)

Sufficient conditions that a two-dimensional system with output is locally observable are presented. Known results depend on time derivatives of the output and the inverse function theorem. In some cases, no informaton is provided by these theories, and one must study observability by other methods. The observability problem is dualized to the controllability problem, and the deep results of Hermes on local controllability are applied to prove a theorem concerning local observability.

Hunt, L. R.; Su, R.

1981-01-01

337

NASA Astrophysics Data System (ADS)

Earthward-propagating dipolarization fronts (DFs) are often found to be associated with magnetic reconnection and bursty bulk flows (BBFs) in the magnetotail. Recent THEMIS (Time History of Events and Macroscale Interactions During Substorms) probe observations have shown a DF propagating over 10 RE from the mid-tail region to the near-Earth tail region, and THEMIS All-Sky Imager data show a north-south auroral form and intensification of westward auroral zone currents. In this study, we examine THEMIS in situ observations of DFs in the magnetotail and simultaneous observations of the proton aurora from ground-based CANOPUS (the Canadian Auroral Network for the OPEN Program Unified Study) Meridian Scanning Photometers (MSPs). We find that earthward-moving DFs are often associated with intensification of proton aurora when the THEMIS probes are conjugate to the meridian of the MSP. The proton auroral intensifications are transient and in some cases detached from the background proton precipitation. Just before the DFs, the ion distribution is anisotropic in the field-aligned direction (mostly earthward) and the ion energy increases. These observations suggest that plasma sheet protons can be reflected and energized by earthward-moving DFs as they propagate through the magnetotail. We postulate that this population of ions is the source of the proton auroral intensification observed on the ground. This conjecture is tested using our global MHD simulation results, where the proton precipitation is calculated with the field-line curvature (FLC) model. The MHD simulation results show that proton precipitation enhancement can be caused by compression of plasma by approaching DFs/BBFs, which is consistent with ion reflection at DFs. Thus, using the conjugate observations from THEMIS spacecraft and MSP in this study, we are able to directly link the magnetotail dynamics, i.e., dipolarization fronts, with ground auroral activities. However, understanding of DF-associated ion energization requires detailed test-particle simulations with an analytical magnetotail model, such as those in our companion paper.

Ge, Y. S.; Zhou, X.-Z.; Liang, J.; Raeder, J.; Gilson, M. L.; Donovan, E.; Angelopoulos, V.; Runov, A.

2012-10-01

338

Attempts to Simulate Anisotropies of Solar Wind Fluctuations Using MHD with a Turning Magnetic Field

NASA Technical Reports Server (NTRS)

We examine a "two-component" model of the solar wind to see if any of the observed anisotropies of the fields can be explained in light of the need for various quantities, such as the magnetic minimum variance direction, to turn along with the Parker spiral. Previous results used a 3-D MHD spectral code to show that neither Q2D nor slab-wave components will turn their wave vectors in a turning Parker-like field, and that nonlinear interactions between the components are required to reproduce observations. In these new simulations we use higher resolution in both decaying and driven cases, and with and without a turning background field, to see what, if any, conditions lead to variance anisotropies similar to observations. We focus especially on the middle spectral range, and not the energy-containing scales, of the simulation for comparison with the solar wind. Preliminary results have shown that it is very difficult to produce the required variances with a turbulent cascade.

Ghosh, Sanjoy; Roberts, D. Aaron

2010-01-01

339

The Biermann Battery In Cosmological Mhd Simulations Of Population III Star Formation

We report the results of the first self-consistent three-dimensional adaptive mesh refinement magnetohydrodynamical simulations of Population III star formation including the Biermann battery effect. We find that the Population III stellar cores formed including this effect are both qualitatively and quantitatively similar to those from hydrodynamics-only (non-MHD) cosmological simulations. We observe peak magnetic fields of {approx_equal} 10{sup -9} G in the center of our star-forming halo at z {approx_equal} 17.55 at a baryon density of n{sub B} {approx} 10{sup 10} cm{sup -3}. The magnetic fields created by the Biermann battery effect are predominantly formed early in the evolution of the primordial halo at low density and large spatial scales, and then grow through compression and by shear flows. The fields seen in this calculation are never large enough to be dynamically important (with {beta} {ge} 10{sup 15} at all times before the termination of our calculation), and should be considered the minimum possible fields in existence during Population III star formation. The lack of magnetic support lends credibility to assumptions made in previous calculations regarding the lack of importance of magnetic fields in Population III star formation. In addition, these magnetic fields may be seed fields for the stellar dynamo or the magnetorotational instability at higher densities and smaller spatial scales.

Xu, Hao [Los Alamos National Laboratory; O' Shea, Brian W [Los Alamos National Laboratory; Li, Hui [Los Alamos National Laboratory; Li, Shengtai [Los Alamos National Laboratory; Norman, Michael L [UCSD; Collins, David C [UCSD

2008-01-01

340

Global MHD Simulations of Magnetospheric and Ionospheric Responses to the 5th June 1998 Event

NASA Astrophysics Data System (ADS)

Using WIND and ACE solar wind data, we have simulated the magnetospheric and ionospheric responses to the 5th June 1998 event with global MHD model SWMF. The chosen time period in our calculations includes a sharp northward turning and a southward turning in the IMF, followed by a period of relatively stead condition, respectively. We first investigate the calculation influence of running the code in different ways: (1) with or without RICE convection model, and (2) time-accurate run or local time stepping or some combinations of both to drive to a steady state before time-varying solar wind. We show that the simulated magnetospheric and ionospheric responses, such as the location of the polar cap boundary and ionospheric currents, can be significantly affected by the treatment of the steady conditions. We conclude that, with appropriate treatment of the true magnetospheric configuration, the modelling framework SWMF can accurately reflect the dynamic features of the solar wind-magnetosphere-ionosphere coupling and provide reliable results for estimating magnetic field topology and ionospheric responses.

Lu, Jianyong; Rae, Ian; Rankin, Robert; Zhang, Jichun; Kabin, Konstantin; Gombosi, T.; de Zeeuw, D. L.; Toth, G.

341

Through a direct comparison between numerical simulations in two and three dimensions, we investigate topological effects in reconnection. A simple estimate on increase in reconnection rate in three dimensions by a factor of $\\sqrt{2}$, when compared with a two-dimensional case, is confirmed in our simulations. We also show that both the reconnection rate and the fraction of magnetic energy in the simulations depend linearly on the height of the reconnection region. The degree of structural complexity of a magnetic field and the underlying flow is measured by current helicity and cross-helicity. We compare results in simulations with different computational box heights.

?emelji?, Miljenko

2014-01-01

342

Formation and Eruption of an Active Region Sigmoid: NLFFF Modeling and MHD Simulation

NASA Astrophysics Data System (ADS)

We present a magnetic analysis of the formation and eruption of an active region sigmoid in AR 11283 from 2011 September 4 to 6, which is jointly based on observations, static nonlinear force-free field (NLFFF) extrapolation and dynamic MHD simulation. A time sequence of NLFFF model's outputs are used to reproduce the evolution of the magnetic field of the region over three days leading to a X-class flare near the end of 2011 September 6. In the first day, a new bipolar emerges into the negative polarity of a pre-existing mature bipolar, forming a magnetic topology with a coronal null on the magnetic separatrix surface between the two flux system, while the field is still near potential at the end of the day. After then photospheric shearing and twisting build up non-potentiality in the embedded core region, with a flux rope (FR) formed there above the polarity inversion line by tether-cutting reconnection between the strongly sheared field lines. Within this duration, the core field has gained a magnetic free energy of ˜ 1032 erg. In this core a sigmoid is observed distinctly at 22:00 UT on September 6, closely before its eruption at 22:12 UT. Comparison of the SDO/AIA observations with coronal magnetic field suggests that the sigmoid is formed by emission due to enhanced current sheet along the BPSS (bald-patch separatrix surface, in which the field lines graze the line-tied photosphere at the neutral line) that separates the FR from the ambient flux. Quantitative inspection of the pre-eruption field on 22:00 UT suggests a mechanism for the eruption: tether cutting at the null triggers a torus instability of the FR--overlying field system. This pre-eruption NLFFF is then input into a time-dependent MHD model to simulate the fast magnetic evolution during eruption, which successfully reproduces the observations. The highly asymmetric magnetic environment along with the lateral location of the null leads to a strongly inclined non-radial direction of the eruption. The study of this kind provides important insights in a quantitative way to many open issues on the formation and eruption of sigmoidal FR.

Jiang, C.; Wu, S.; Feng, X.; Hu, Q.

2013-12-01

343

Collisionless magnetic reconnection under anisotropic MHD approximation

NASA Astrophysics Data System (ADS)

We study the formation of slow-mode shocks in collisionless magnetic reconnection by using one- and two-dimensional collisionless magneto-hydro-dynamic (MHD) simulations based on the double adiabatic approximation, which is an important step to bridge the gap between the Petschek-type MHD reconnection model accompanied by a pair of slow shocks and the observational evidence of the rare occasion of in-situ slow shock observation. According to our results, a pair of slow shocks does form in the reconnection layer. The resultant shock waves, however, are quite weak compared with those in an isotropic MHD from the point of view of the plasma compression and the amount of the magnetic energy released across the shock. Once the slow shock forms, the downstream plasma are heated in highly anisotropic manner and a firehose-sense (P_{||}>P_{?}) pressure anisotropy arises. The maximum anisotropy is limited by the marginal firehose criterion, 1-(P_{||}-P_{?})/B(2) =0. In spite of the weakness of the shocks, the resultant reconnection rate is kept at the same level compared with that in the corresponding ordinary MHD simulations. It is also revealed that the sequential order of propagation of the slow shock and the rotational discontinuity, which appears when the guide field component exists, changes depending on the magnitude of the guide field. Especially, when no guide field exists, the rotational discontinuity degenerates with the contact discontinuity remaining at the position of the initial current sheet, while with the slow shock in the isotropic MHD. Our result implies that the slow shock does not necessarily play an important role in the energy conversion in the reconnection system and is consistent with the satellite observation in the Earth's magnetosphere.

Hirabayashi, Kota; Hoshino, Masahiro

344

Forced Magnetic Reconnection at an X-point: Particle-In-Cell and Ten-Moment Extended MHD Simulations

NASA Astrophysics Data System (ADS)

We will present comparative numerical studies of current sheet formation and forced magnetic reconnection at an X-point, beginning from a potential field. The problem will be simulated by the fully kinetic Particle Simulation Code (PSC) [1] and an extended ten-moment MHD code Gkeyll [2] that retains important kinetic physics, particularly, electron inertia and full electron/ion pressure tensors. Our goals are to investigate the similarities and differences between the two models, and to seek suitable parameterization of kinetic effects in the fluid models. The simulation domain is restrained in 2-D and is closed by conducting wall boundaries. The reconnection is forced by in-plane flows imposed on two opposite boundaries, where the forcing flows converge at the two boundary centers, and are slow compared to the characteristic Alfvén speed. We will compare results on the time-dependence of the reconnecting electric field (suitably normalized), as well as the structure of current sheets from PSC, Gkeyll, and an MHD code, varying ion-to-electron mass ratio and domain size. This study is carried out under the auspices of a Focus Topic in the NASA Living With a Star Targeted Research and Technology Program. [1] Fox, W., A. Bhattacharjee, and K. Germaschewski. "Magnetic reconnection in high-energy-density laser-produced plasmas." Physics of Plasmas 19 (2012): 056309. [2] Hakim, Ammar H. "Extended MHD modelling with the ten-moment equations." Journal of Fusion Energy 27.1-2 (2008): 36-43.

Wang, L.; Bessho, N.; Bhattacharjee, A.; Germaschewski, K.; Hakim, A.

2013-12-01

345

MHD simulations of jet acceleration from Keplerian accretion disks: the effects of disk resistivity

Accretion disks and astrophysical jets are used to model many active astrophysical objects, viz., young stars, relativistic stars, and active galactic nuclei. In this paper we present self-consistent time-dependent simulations of supersonic jets launched from magnetized accretion disks, using high resolution numerical techniques. In particular we study the effects of the disk magnetic resistivity, parametrized through an alpha-prescription, in determining the properties of the inflow-outflow system. Moreover we analyze under which conditions steady state solutions of the type proposed in the self similar models of Blandford and Payne can be reached and maintained in a self consistent nonlinear stage. We use the resistive MHD FLASH code with adaptive mesh refinement, allowing us to follow the evolution of the structure for a time scale long enough to reach steady state. A detailed analysis of the initial configuration state is given. We obtain the expected solutions in the axisymmetric (2.5D) limit. Assuming a magnetic field around equipartition with the thermal pressure of the disk, we show how the characteristics of the disk jet system, as the ejection efficiency and the energetics, are affected by the anomalous resistivity acting inside the disk.

C. Zanni; A. Ferrari; R. Rosner; G. Bodo; S. Massaglia

2007-03-05

346

Modeling the relative magnetic helicity in MHD simulations and its application to solar activity

NASA Astrophysics Data System (ADS)

Magnetic helicity is a key geometrical parameter to describe the structure and evolution of solar coronal magnetic fields. For a better understanding of solar magnetic field evolution it is appropriate to evaluate the magnetic helicity based on observations and to compare it with numerical simulation results. We have developed a method for calculating the vector potential of a magnetic field given in a finite volume. Our method uses a fast Laplace/Poisson solver to obtain the vector potentials for a given magnetic field and for the corresponding potential (current-free) field. This allows an efficient calculation of the relative magnetic helicity in a finite 3D volume. We tested our approach on a theoretical model (Low and Lou, Astrophys. J. 352, 343, 1990) and also applied our method to the magnetic field above active region NOAA 8210 obtained by a photospheric-data-driven MHD model. We found that the amount of accumulated relative magnetic helicity coincides well with the relative helicity inflow through the boundaries in the ideal and non-ideal cases. The temporal evolution of relative magnetic helicity is consistent with that of magnetic energy. The maximum value of normalized helicity when a drastic energy release by magnetic reconnection is close to the corresponding value inferred from the formula that connects the magnetic flux and the accumulated magnetic helicity based on the observations of solar active regions.

Yang, Shangbin; Büchner, Jörg; Carlo Santos, Jean; Zhang, Hongqi

347

3D MHD Simulations of Planet Migration in Turbulent Stratified Disks

We performed 3D MHD simulations of planet migration in stratified disks using the Godunov code PLUTO, where the disk is turbulent due to the magnetorotational instability. We study the migration for planets with different planet-star mass ratios $q=M_{p}/M_{s}$. In agreement with previous studies, for the low-mass planet cases ($q=5\\times10^{-6}$ and $10^{-5}$), migration is dominated by random fluctuations in the torque. For a Jupiter-mass planet $(q=M_{p}/M_{s}=10^{-3}$ for $M_{s}=1M_{\\odot})$, we find a reduction of the magnetic stress inside the orbit of the planet and around the gap region. After an initial stage where the torque on the planet is positive, it reverses and we recover migration rates similar to those found in disks where the turbulent viscosity is modelled by an $\\alpha$ viscosity. For the intermediate-mass planets ($q=5\\times10^{-5}, 10^{-4}$ and $2\\times10^{-4}$) we find a new and so far unexpected behavior. In some cases they experience sustained and systematic outwards migration for th...

Uribe, Ana; Flock, Mario; Henning, Thomas

2011-01-01

348

Counter equatorial electrojet and overshielding after substorm onset: Global MHD simulation study

NASA Astrophysics Data System (ADS)

performing a global magnetohydrodynamic (MHD) simulation, we have demonstrated for the first time that an electrojet at the dayside magnetic equator can be reversed and an overshielding condition can be established in the inner magnetosphere after substorm onset without northward turning of the interplanetary magnetic field. Near the substorm onset, the plasma pressure is highly enhanced in the inner magnetosphere on the nightside. The Region 2 field-aligned current diverges from the diamagnetic current on the surface of the dayside extension of the high-pressure region, which is connected to the ionosphere in the relatively low-conductivity region a few degrees equatorward of the main auroral oval that is formed as the projection of the plasma sheet. The separation of the equatorward boundary of the auroral region and the equatorward boundary of the Region 2 current results in dusk-dawn electric fields that generate a counter electrojet (CEJ) at the dayside magnetic equator. Poleward electric fields in a narrow latitudinal width, which may be regarded as subauroral ion drift and subauroral polarization stream, are simultaneously intensified. The dusk-dawn electric fields may propagate to the inner magnetosphere along a field line as shear Alfvén waves. Then, the inner magnetosphere is completely constrained by the overshielding condition. The intensity and polarity of the CEJ depend largely on at least the ionospheric conductivity that is related to the plasma pressure (probably associated with diffuse aurora). This may explain the observational fact that overshielding does not always occur after onset.

Ebihara, Y.; Tanaka, T.; Kikuchi, T.

2014-09-01

349

Solar wind-magnetosphere energy coupling function fitting: Results from a global MHD simulation

NASA Astrophysics Data System (ADS)

Quantitatively estimating the energy input from the solar wind into the magnetosphere on a global scale is still an observational challenge. We perform three-dimensional magnetohydrodynamic (MHD) simulations to derive the energy coupling function. Based on 240 numerical test runs, the energy coupling function is given by Ein=3.78×107nsw0.24Vsw1.47BT0.86[sin2.70(?/2)+0.25]. We study the correlations between the energy coupling function and a wide variety of magnetospheric activity, such as the indices of Dst, Kp, ap, AE, AU, AL, the polar cap index, and the hemispheric auroral power. The results indicate that this energy coupling function gives better correlations than the ? function. This result is also applied to a storm event under northward interplanetary magnetic field conditions. About 13% of the solar wind kinetic energy is transferred into the magnetosphere and about 35% of the input energy is dissipated in the ionosphere, consistent with previous studies.

Wang, C.; Han, J. P.; Li, H.; Peng, Z.; Richardson, J. D.

2014-08-01

350

MHD simulations of near-surface convection in cool main-sequence stars

The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected. We carried out 3D local-box MHD simulations of unipolar magnetized regions (average fields of 20, 100, and 500G) with parameters corresponding to six main-sequence stars (spectral types F3V to M2V). The influence of the magnetic field on the convection and the local thermodynamical structure were analyzed in detail. For three spectral lines, we determined the impact of the magnetic field on the disc-integrated Stokes-I profiles. Line weakening has i...

Beeck, Benjamin; Reiners, Ansgar

2014-01-01

351

Hot electrons may significantly influence interaction of ultra short laser pulses with solids. Accurate consideration of resonant absorption of laser energy and hot electrons generation at a critical surface was achieved through the developed physical and mathematical models. 2D ray tracing algorithm has been developed to simulate laser beam refraction and Bremsstrahlung absorption with allowance for non-linear influence of a

Ivan G. Lebo; Alexey B. Iskakov; Nikolai N. Demchenko; Jiri Limpouch; Vladislav B. Rozanov; Vladimir F. Tishkin

2004-01-01

352

We study the properties of MHD turbulence driven by the magnetorotational instability (MRI) in accretion disks. We adopt the local shearing box model and focus on the special case for which the initial magnetic flux threading the disk vanishes. We employ the finite difference code ZEUS to evolve the ideal MHD equations. Performing a set of numerical simulations in a fixed computational domain with increasing resolution, we demonstrate that turbulent activity decreases as resolution increases. We quantify the turbulent activity by measuring the rate of angular momentum transport through evaluating the standard alpha parameter. We find alpha=0.004 when (N_x,N_y,N_z)=(64,100,64), alpha=0.002 when (N_x,N_y,N_z)=(128,200,128) and alpha=0.001 when (N_x,N_y,N_z)=(256,400,256). This steady decline is an indication that numerical dissipation, occurring at the grid scale is an important determinant of the saturated form of the MHD turbulence. Analysing the results in Fourier space, we demonstrate that this is due to the MRI forcing significant flow energy all the way down to the grid dissipation scale. We also use our results to study the properties of the numerical dissipation in ZEUS. Its amplitude is characterised by the magnitude of an effective magnetic Reynolds number Re_M which increases from 10^4 to 10^5 as the number of grid points is increased from 64 to 256 per scale height. The simulations we have carried out do not produce results that are independent of the numerical dissipation scale, even at the highest resolution studied. Thus it is important to use physical dissipation, both viscous and resistive, and to quantify contributions from numerical effects, when performing numerical simulations of MHD turbulence with zero net flux in accretion disks at the resolutions normally considered.

S. Fromang; J. Papaloizou

2007-10-04

353

NASA Astrophysics Data System (ADS)

We apply the Bammann inelastic internal state variable model (BIISV) to a mantle convection code TERRA2D to investigate the influence of a history dependent solid mechanics model on mantle convection. We compare and contrast the general purpose BIISV model to the commonly used power-law model. We implemented the BIISV model using a radial return algorithm and tested it against previously published mantle convection simulation results for verification. Model constants for the BIISV are used based on experimental stress-strain behaviour found in the literature. After implementation we give illustrative simulation examples were the BIISV produces hardened areas on the cold thermal boundary layer that the power-law model cannot produce. The hardened boundary layers divert material downward giving a plausible reason for the current subduction zones that are present on the Earth.

Sherburn, J. A.; Horstemeyer, M. F.; Bammann, D. J.; Baumgardner, J. R.

2011-09-01

354

Two-Dimensional Computational Fluid Dynamics and Conduction

Two-Dimensional Computational Fluid Dynamics and Conduction Simulations of Heat Transfer were performed, two with a CFD code and four with a building- component thermal-simulation tool using) simulations with detailed radiation modeling are used as a reference. Four different frames were studied. Two

355

NASA Technical Reports Server (NTRS)

A computational code for solving two-dimensional, time-dependent MHD equations by the method of characteristics is presented. Its capabilities are demonstrated by solving two very different problems for which analytical solutions exist: linearized, standing MHD wave motions in a magnetized cylindrical plasma, and nonlinear self-similar expansion of a magnetized plasma ball. The nonlinear development of standing MHD wave solutions in a cylindrical plasma is also studied. The method can be naturally embedded in the computational architecture of massively parallel processors.

Lou, Y. Q.; Rosner, R.; Ulmschneider, P.

1987-01-01

356

NASA Astrophysics Data System (ADS)

The Kelvin-Helmholtz Instability (KHI) has long been suggested to operate on the magnetospheric boundary, where the magnetosheath plasma streams past the magnetosphere. The instability is thought to be responsible for inducing various wave populations in the magnetosphere and for mass, momentum and energy transport across the magnetospheric boundary. Waves attributed to the KHI have been observed at the Earth's magnetosphere flanks as well as at Saturn and Mercury during spacecraft crossings, and remotely at boundaries of Coronal Mass Ejections (CMEs). Recent high-resolution global 3D magnetohydrodynamic (MHD) simulations of the magnetosphere confirm the existence of pronounced perturbations of the magnetospheric boundary, which are thought to be due to KHI. Such global simulations had been challenging in the past because of the need to encompass the entire magnetosphere, while sufficiently resolving the boundary layer. Here we present results of such a high-resolution simulation of the magnetosphere, using the Lyon-Fedder-Mobarry (LFM) model, under steady northward Interplanetary Magnetic Field (IMF) conditions. We find the magnetospheric boundary to be globally unstable, including the high-latitude boundary layer (meridional plane), where magnetic tension is apparently not sufficient to stabilize the growth of oscillations. Roughly beyond the terminator, global modes, coupled into the surface modes, become apparent, so that the entire body of the magnetosphere is engaged in an oscillatory motion. The wave vector of the surface oscillations has a component perpendicular to the background flow and tangential to the shear layer (in the equatorial plane, k_z component of the wave vector), which is consistent with the generation of field-aligned currents that flow on closed field lines between the inner portion of the boundary layer and the ionosphere. We calculate the distribution of wave power in the equatorial plane and find it consistent with the existence of a double-vortex sheet, with vortex trains propagating along the inner and outer edges of the boundary layer. The double-vortex sheet is most apparent in the simulation past the terminator plane, but is transient and appears to be unstable, and is most likely a consequence of non-linear development of the velocity shear layer with a finite width. We compute the salient characteristics of the KH waves, including phase speeds, spectra and growth rates. The latter are compared with linear theory and found to be in excellent agreement. Finally, we find that the plasma compressibility is a key factor in controlling the growth rate of the KHI at the magnetosphere flanks in our simulations.

Merkin, V. G.; Lyon, J.; Claudepierre, S. G.

2013-12-01

357

Shell Model of Two-dimensional Turbulence in Polymer Solutions

We address the effect of polymer additives on two dimensional turbulence, an issue that was studied recently in experiments and direct numerical simulations. We show that the same simple shell model that reproduced drag reduction in three-dimensional turbulence reproduces all the reported effects in the two-dimensional case. The simplicity of the model offers a straightforward understanding of the all the major effects under consideration.

Roberto Benzi; Nizan Horesh; Itamar Procaccia

2003-10-13

358

In this paper we report the results of axisymmetric relativistic MHD simulations for the problem of Kerr black hole immersed into a rarefied plasma with ''uniform'' magnetic field. The long term solution shows properties which are significantly different from those of the initial transient phase studied recently by Koide(2003). The topology of magnetic field lines within the ergosphere is similar to that of the split-monopole model with a strong current sheet in the equatorial plane. Closer inspection reveals a system of isolated magnetic islands inside the sheet and ongoing magnetic reconnection. No regions of negative hydrodynamic ''energy at infinity'' are seen inside the ergosphere and the so-called MHD Penrose process does not operate. Yet, the rotational energy of the black hole continues to be extracted via purely electromagnetic mechanism of Blandford and Znajek(1977). However, this is not followed by development of strong relativistic outflows from the black hole. Combined with results of other recent simulations this signals a potential problem for the standard MHD model of relativistic astrophysical jets should they still be observed at distances as small as few tens of gravitational radii from the central black hole.

S. S. Komissarov

2005-01-27

359

This is a sequel to a recent work of Gaudin, who studied the classical equilibrium statistical mechanics of the two-dimensional Coulomb gas on a lattice at a special value of the coupling constant Gamma such that the model is exactly solvable. This model is briefly reviewed, and it is shown that the correlation functions obey the sum rules that characterize a conductive phase. A related model in which the particles are constrained to move on an array of equidistant parallel lines has simpler mathematics, and the asymptotic behavior of its correlation functions is studied in some detail. In the low-density limit, the lattice model is expected to have the same properties as a system of charged, hard disks; the correlation functions, internal energy, and specific heat of the latter are discussed.

Cornu, F.; Jancovici, B.

1987-10-01

360

Two-dimensional NMR spectroscopy

Written for chemists and biochemists who are not NMR spectroscopists, but who wish to use the new techniques of two-dimensional NMR spectroscopy, this book brings together for the first time much of the practical and experimental data needed. It also serves as information source for industrial, academic, and graduate student researchers who already use NMR spectroscopy, but not yet in two dimensions. The authors describe the use of 2-D NMR in a wide variety of chemical and biochemical fields, among them peptides, steroids, oligo- and poly-saccharides, nucleic acids, natural products (including terpenoids, alkaloids, and coal-derived heterocyclics), and organic synthetic intermediates. They consider throughout the book both the advantages and limitations of using 2-D NMR.

Croasmun, W.R.; Carlson, R.M.K.

1987-01-01

361

NASA Technical Reports Server (NTRS)

On March 24, 1995 the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the Low Latitude Boundary Layer (LLBL) about 15 R(sub E) tailward of the dusk meridian. Although the magnetospheric and the magnetosheath field were strongly northward, the B(sub z) component showed strong short duration fluctuations in which B(sub z) could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this. Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin-Helmholtz wavelength of about 5 R(sub E) with a vortex size of close to 2 R(sub E) for an average repetition time of 2.5 minutes. The growth time for these waves implies a source region of about 10 to 16 R(sub E) upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.

Fairfield, Donald H.; Otto, A.

1999-01-01

362

NASA Technical Reports Server (NTRS)

The influence of dynamics on model results is investigated via a 2D photochemical model. Three representations of atmospheric transport are tested in order to simulate total ozone and C-14 amounts after nuclear tests in the early 1960s. Three scenarios of NO(x) injections from a proposed fleet of stratospheric aircraft and their effects on ozone are simulated. The three dynamical formulations used are Dynamics A, a base dynamics used in previous work with this model; Dynamics B, a strong circulation dynamics discussed by Jackman et al. (1989); and Dynamics C, the dynamics used by Shia et al. (1989). The advective component of the stratosphere to the troposphere mass exchange rate is largest for Dynamics B (5.8 x 10 exp 17 kg/yr) and smallest for Dynamics C (1.4 x 10 exp 17 kg/yr), with the advective strat/trop exchange rate for Dynamics A (2.4 x 10 exp 17 kg/yr) being these two extremes. Ozone depletion from NO(x) injections of stratospheric aircraft showed a strong sensitivity to dynamics.

Jackman, Charles H.; Douglas, Anne R.; Brueske, Kurt F.; Klein, Stephen A.

1991-01-01

363

The HPM Applied to MHD Nanofluid Flow over a Horizontal Stretching Plate

The nonlinear two-dimensional forced-convection boundary-layer magneto hydrodynamic (MHD) incompressible flow of nanofluid over a horizontal stretching flat plate with variable magnetic field including the viscous dissipation effect is solved using the homotopy perturbation method (HPM). In the present work, our results of the HPM are compared with the results of simulation using the finite difference method, Keller's box-scheme. The comparisons

S. S. Nourazar; M. Habibi Matin; M. Simiari

2011-01-01

364

Collage of two-dimensional words

We consider a new operation on one-dimensional (resp. two-dimensional) word lan- guages, obtained by piling up, one on top of the other, words of a given recognizable language (resp. two-dimensional recognizable language) on a previously empty one- dimensional (resp. two-dimensional) array. The resulting language is the set of words \\

Christian Choffrut; Berke Durak

2005-01-01

365

The Delta x B = 0 Constraint Versus Minimization of Numerical Errors in MHD Simulations

NASA Technical Reports Server (NTRS)

The MHD equations are a system of non-strictly hyperbolic conservation laws. The non-convexity of the inviscid flux vector resulted in corresponding Jacobian matrices with undesirable properties. It has previously been shown by Powell et al. (1995) that an 'almost' equivalent MHD system in non-conservative form can be derived. This non-conservative system has a better conditioned eigensystem. Aside from Powell et al., the MHD equations can be derived from basic principles in either conservative or non-conservative form. The Delta x B = 0 constraint of the MHD equations is only an initial condition constraint, it is very different from the incompressible Navier-Stokes equations in which the divergence condition is needed to close the system (i.e., to have the same number of equations and the same number of unknown). In the MHD formulations, if Delta x B = 0 initially, all one needs is to construct appropriate numerical schemes that preserve this constraint at later time evolutions. In other words, one does not need the Delta x B condition to close the MHD system. We formulate our new scheme together with the Cargo & Gallice (1997) form of the MHD approximate Riemann solver in curvilinear grids for both versions of the MHD equations. A novel feature of our new method is that the well-conditioned eigen-decomposition of the non-conservative MHD equations is used to solve the conservative equations. This new feature of the method provides well-conditioned eigenvectors for the conservative formulation, so that correct wave speeds for discontinuities are assured. The justification for using the non-conservative eigen-decomposition to solve the conservative equations is that our scheme has a better control of the numerical error associated with the divergence of the magnetic condition. Consequently, computing both forms of the equations with the same eigen-decomposition is almost equivalent. It will be shown that this approach, using the non-conservative eigensystem when solving the conservative equations, also works well in the context of standard shock-capturing schemes.

Yee, H. C.; Sjoegreen, Bjoern; Mansour, Nagi (Technical Monitor)

2002-01-01

366

Two-Dimensional Melting under Quenched Disorder

NASA Astrophysics Data System (ADS)

We study the influence of quenched disorder on the two-dimensional melting behavior of superparamagnetic colloidal particles, using both video microscopy and computer simulations of repulsive parallel dipoles. Quenched disorder is introduced by pinning a fraction of the particles to an underlying substrate. We confirm the occurrence of the Kosterlitz-Thouless-Halperin-Nelson-Young scenario and observe an intermediate hexatic phase. While the fluid-hexatic transition remains largely unaffected by disorder, the hexatic-solid transition shifts to lower temperatures with increasing disorder. This results in a significantly broadened stability range of the hexatic phase. In addition, we observe spatiotemporal critical(like) fluctuations, which are consistent with the continuous character of the phase transitions. Characteristics of first-order transitions are not observed.

Deutschländer, Sven; Horn, Tobias; Löwen, Hartmut; Maret, Georg; Keim, Peter

2013-08-01

367

ANALYSIS AND FINITE ELEMENT SIMULATION OF MHD FLOWS, WITH AN APPLICATION TO SEAWATER DRAG REDUCTION1

Much research effort has recently been devoted to the electromagnetic control of saltwater flows, exploiting the macroscopic interaction of saltwater with electric currents and magnetic fields. This interaction is governed by the equations of viscous incompressible MHD, essentially, the Navier-Stokes equations coupled to Maxwell's equations. A major problem in the analysis and numerical solution of these equations is the fact

P. G. Schmidt

368

Computer simulations of two dimensional quasicrystals

sized tiling would more than extend beyond the bounds of available small computer memory. Thus we must look at how the hexagon flip is accomplished. First, notice that the central vertex of either type of hexagon may have only three bonds. Next, when... 21. Disallowed configurations in a. hard-disk tiling. tiling (where any hexagon flip which svould create a vertex type other than 1, 2, 3, or 4 would be rejected), and the hard-disk tiling (a 4-vertex tiling with additional forbidden configurations...

Johnson, Steven Lee

1989-01-01

369

The present work is devoted to study the numerical simulation for unsteady MHD flow and heat transfer of a couple stress fluid over a rotating disk. A similarity transformation is employed to reduce the time dependent system of nonlinear partial differential equations (PDEs) to ordinary differential equations (ODEs). The Runge-Kutta method and shooting technique are employed for finding the numerical solution of the governing system. The influences of governing parameters viz. unsteadiness parameter, couple stress and various physical parameters on velocity, temperature and pressure profiles are analyzed graphically and discussed in detail. PMID:24835274

2014-01-01

370

We study the influence of the choice of transport coefficients (viscosity and resistivity) on MHD turbulence driven by the magnetorotational instability (MRI) in accretion disks. We follow the methodology described in paper I: we adopt an unstratified shearing box model and focus on the case where the net vertical magnetic flux threading the box vanishes. For the most part we use the finite difference code ZEUS, including explicit transport coefficients in the calculations. However, we also compare our results with those obtained using other algorithms (NIRVANA, the PENCIL code and a spectral code) to demonstrate both the convergence of our results and their independence of the numerical scheme. We find that small scale dissipation affects the saturated state of MHD turbulence. In agreement with recent similar numerical simulations done in the presence of a net vertical magnetic flux, we find that turbulent activity (measured by the rate of angular momentum transport) is an increasing function of the magnetic Prandtl number Pm for all values of the Reynolds number Re that we investigated. We also found that turbulence disappears when the Prandtl number falls below a critical value Pm_c that is apparently a decreasing function of Re. For the limited region of parameter space that can be probed with current computational resources, we always obtained Pm_c>1. We conclude that the magnitudes of the transport coefficients are important in determining the properties of MHD turbulence in numerical simulations in the shearing box with zero net flux, at least for Reynolds numbers and magnetic Prandtl numbers that are such that transport is not dominated by numerical effects and thus can be probed using current computational resources.

S. Fromang; J. Papaloizou; G. Lesur; T. Heinemann

2007-10-04

371

An MHD simulation study of the poloidal mode feld line resonance in the Earth`s dipole magnetosphere

The poloidal mode field line resonance in the Earth`s dipole magnetic field is investigated using cold plasma ideal MHD simulations in dipole geometry. In order to excite the poloidal mode resonance, the authors use either an initial or a continuous velocity perturbation to drive the system. The perturbation is localized at magnetic shell L=7 with plasma flow in the radial direction (electric field component in the azimuthal direction). It is found that with the initial perturbation alone, no poloidal mode resonance can be obtained and the initially localized perturbation spreads out across all magnetic L shells. With the continuous perturbation, oscillating near the poloidal resonance frequency, a global-scale poloidal cavity mode can be obtained. For the first time, a localized guided poloidal mode resonance is obtained when a radial component of electric field is added to the initial perturbation such that the curl of the electric field is everywhere perpendicular to the background dipole magnetic field. During the localized poloidal resonance, plasma vortices parallel/antiparallel to the background dipole magnetic field B{sub o} lead to circular plasma flow perpendicular to B{sub o}. This circular flow, elongated radially, results in twisting of magnetic field flux tubes, which, in turn, leads to the slowdown of the circular plasma flow and reversal of the plasma vortices. The energy associated with the localized poloidal resonance is conserved as it shifts back and forth between the oscillating plasma vortices and the alternatively twisted magnetic flux tubes. In the simulations the eigenfunctions associated with the localized poloidal resonance are grid-scale singular functions. This result indicates that ideal MHD is inadequate to describe the underlying problem and nonideal MHD effects are needed for mode broadening. 39 refs., 10 figs.

Ding, D.Q.; Denton, R.E.; Hudson, M.K. [Dartmouth College, Hanover, NH (United States)] [Dartmouth College, Hanover, NH (United States); Lysak, R.L. [Univ. of Minnesota, Minneapolis, MN (United States)] [Univ. of Minnesota, Minneapolis, MN (United States)

1995-01-01

372

Cyclic thermal signature in a global MHD simulation of solar convection

NASA Astrophysics Data System (ADS)

Space-based observations have clearly established that total solar irradiance (TSI) varies on time scales from minutes to days and months as well as on the longer time scale of the 11-year solar cycle. The most conspicuous of these variations is arguably the slight increase of TSI (0.1%) at solar maxima relative to solar minima. Models that include contributions from surface solar magnetism alone (i.e. sunspots, faculae and magnetic network) have been very successful at reproducing the observed TSI fluctuations on time scales shorter than a year, but leave some doubts as to the origin of the longer decadal fluctuations. In particular, one school of thought argues that surface magnetism alone can explain the entire TSI variance; see (Lean & al. 1998, ApJ, 492, 390), whereas; the other emphasizes on taking into account the effect of a global modulation of solar thermal structure by magnetic activity; see (Li & al. 2003, ApJ, 591, 1267). Observationally, the potential for the occurrence of magnetically-modulated global structural changes is supported by a positive correlation between p-mode oscillation frequencies and the TSI cycle as well as by recent evidence for a long-term trend in the TSI record that is not seen in indicators of surface magnetism; see (Bhatnagar & al. 1999, ApJ, 521, 885; Fröhlich 2013, Space Sci Rev,176, 237). Additionally, 1D structural solar models have demonstrated that the inclusion of a magnetically-modulated turbulent mechanism could explain the observed p-mode oscillation frequency changes with great accuracy. However, these models relied upon an ad-hoc parametrization of the alleged process and therefore obtaining a complete physical picture of the modulating mechanism requires solving the equations governing the self-consistent evolution of the solar plasma. Here we present a global magnetohydrodynamical (MHD) simulation of solar convection extending over more than a millennium that produces large-scale solar-like axisymmetric magnetic fields undergoing polarity reversals on a decadal time scale. Most importantly, we find that the convective heat flux in this simulation varies in phase with the strength of the cyclic magnetic field, which is consistent with the enhanced value of TSI observed at solar maxima. The impact of the observed modulation on the amplitude of TSI fluctuations remains yet to be understood, since the domain of our simulation stops at 0.96 R, which is slightly below the photosphere. Nevertheless, the fact that we observe a positive correlation between convective energy transport and magnetic activity suggests that global structural changes may indeed affect the amplitude of long-term TSI variations. Knowing whether or not such a global thermal modulation operates independently from the mechanism responsible for the emergence of sunspots at the solar surface is therefore important for assessing possible connections between periods of quiet surface magnetism and the Earth's climate, such as the postulated relationship between the Maunder Minimum and the Little Ice Age.

Cossette, J.; Charbonneau, P.; Smolarkiewicz, P. K.

2013-12-01

373

Two-dimensional nozzle plume characteristics

NASA Technical Reports Server (NTRS)

Future high performance aircraft will likely feature asymmetric or two-dimensional nozzles with or without ejectors. In order to design two-dimensional nozzle/ejector systems of minimum size and weight, the plume decay and spreading characteristics of basic two-dimensional nozzles must first be established. The present work deals with the experimental analyses of these plume characteristics and includes the effects of nozzle aspect ratio and flow conditions (jet Mach number and temperature) on the plume decay and spreading of two-dimensional nozzles. Correlations including these variables are developed in a manner similar to those previously developed successfully for conic and dual-flow plumes.

Von Glahn, Uwe H.

1987-01-01

374

Fully three-dimensional time-resolved MHD simulations in wall bounded geometry

NASA Astrophysics Data System (ADS)

We present a new method for computing 3D viscous-resistive MHD turbulence in wall bounded geometries of arbitrary shape [1]. The numerical scheme is based on a classical Fourier pseudo-spectral solver combined with a volume penalization method to impose the boundary condition of the velocity and magnetic field. The new code is validated using different test cases, such as three-dimensional Taylor-Couette flow and MHD in cylindrical geometry. Imposing helical magnetic boundary conditions in the latter geometry, the flow shows a self-organization to a chaotic state for elevated Hartmann number. In toroidal geometry, this self-organization leads to toroidal velocity fields. [4pt] [1] Morales, Leroy, Bos and Schneider (submitted).

Bos, Wouter; Leroy, Matthieu; Morales, Jorge; Schneider, Kai

2012-10-01

375

Matrix dynamics as conservative discrete MHD

NASA Astrophysics Data System (ADS)

Two dimensional Fourier codes conserve quadratic invariants of ideal fluid motion but destroy an infinity of Casimirs. As high-mode dynamics are inevitably distorted by truncation and dealiasing, it makes sense to further deliberately alter the equations for large k in order to restore some of the lost integrals. For 2D ideal fluid, Zeitlin(V. Zeitlin, Physica D 49, 353 (1991).) demonstrated an O(N^2)-mode ODE system conserving O(N) invariants. Here the same is done for 2D ideal MHD system for the magnetic flux dot a^k=C^k_pq?^pa^q and the fluid vorticity dot ?^k=C^k_pq(?^p?^q+j^pa^q), where j^k=k^2a^k, ?^k=k^2?^k, and the structure constants C^k_pq=-(N/2?)sin(2? p× q/N)?(p+q-k) reproduce correct MHD for N??. The conservative properties follow from the existence(D. B. Fairly and C. K. Zachos, Phys. Lett. B 224, 101 (1989).) of N× N Hermitean traceless matrices Lk with the commutation property [L_p,L_q]=C^k_pqL_k. Thus the MHD equations can be formulated in terms of N× N matrices A=a^kL_k, ?=?^kL_k, etc., dot A=[?,A], dot ?=[?,?]+[J,A]. This system conserves the 2(N-1) independent Casimir invariants I_n=Tr A^n and J_n=Tr ? A^n for n=2,ldots,N. The usefulness of this discrete system for MHD turbulence simulation will be investigated.

Isichenko, Michael B.

1996-11-01

376

The Evolution and Efficiency of Oblique MHD Cosmic-Ray Shocks: Two-Fluid Simulations

Using a new, second-order accurate numerical method we present dynamical\\u000asimulations of oblique MHD cosmic ray (CR) modified plane shock evolution using\\u000athe two-fluid model for diffusive particle acceleration. The numerical shocks\\u000aevolve to published analytical steady state properties. In order to probe the\\u000adynamical role of magnetic fields we have explored for these time asymptotic\\u000astates the parameter space

Adam Frank; T. W. Jones; Dongsu Ryu

1994-01-01

377

A two-dimensional finite-difference model was developed for simulation of steady-state ground-water flow in the Floridan aquifer throughout a 932-square-mile area, which contains nine municipal well fields. The overlying surficial aquifer contains a constant-head water table and is coupled to the Floridan aquifer by a leakage term that represents flow through a confining layer separating the two aquifers. Under the steady-state condition, all storage terms are set to zero. Utilization of the head-controlled flux condition allows head and flow to vary at the model-grid boundaries. Procedures are described to calibrate the model, test its sensitivity to input-parameter errors, and verify its accuracy for predictive purposes. Also included are attachments that describe setting up and running the model. An example model-interrogation run shows anticipated drawdowns that should result from pumping at the newly constructed Cross Bar Ranch and Morris Bridge well fields. (USGS)

Hutchinson, C.B.; Johnson, Dale M.; Gerhart, James M.

1981-01-01

378

Phase Transitions in Two-Dimensional Superconductors

In this thesis, we describe a number of experiments which are designed to explore the theoretically predicted phase transitions for two-dimensional superconductors. We first examine the behavior of a two-dimensional superconductor in the absence of a magnetic field, where the fluctuations in the phase of the superconducting order parameter results in the reduction of the superconducting transition temperature for a

Ali Yazdani

1995-01-01

379

Scanning of two-dimensional space groups.

Tables of the scanning of two-dimensional space groups are presented to determine the frieze-group symmetry of lines that transect two-dimensional crystals. It is shown how these tables can be used to predict the (001) projection symmetries of migration-related segments of coincidence site lattice tilt boundaries with [001] tilt axis. PMID:25537394

Litvin, Daniel B

2015-01-01

380

A survey of two-dimensional codes for optical CDMA

The paper presents the implementation of an optical CDMA system with two-dimensional coding using wavelength channels and time chips to create the codeword. Computer simulations have been performed in order to analyze and optimize the electronic circuits. The experimental work has been designed in order to investigate and evaluate the capabilities ofthe implemented optical CDMA system.

Petre Stroica; Marian Vladescu; Ovidiu Iancu

2007-01-01

381

Image transmission by two-dimensional contour coding

The results of a computer simulation of an image transmission system are reported. A reduction in the total number of bits required to describe a picture by a factor of 4 to 23 is possible as compared with 6-bit PCM. In this system an image is treated as a two-dimensional signal of the spatial coordinates x and y. The large

DONALD NORMAN GRAHAM

1967-01-01

382

Two-dimensional coded classification schemes in wireless sensor networks

This work proposes a novel fault-tolerant classification system based on distributed detection and two-dimensional channel coding. A rule is then derived to reduce the search space such that the optimal code matrix can be found. Simulation results reveal that the proposed scheme has higher classification reliability and better capability of fault tolerance than previous methods. Moreover, a code matrix using

Hung-ta Pai; Yunghsiang S. Han; Jing-tian Sung

2008-01-01

383

Numerical Modelling of induction heating for two dimensional geometries.

Numerical Modelling of induction heating for two dimensional geometries. P. Dreyfuss J. Rappaz Summary We present both a mathematical model and a numerical method for simulating induction heating and infinite in one direction. Thus the induction heating problem can be studied in a plane perpendicular

Dreyfuss, Pierre

384

[A two-dimensional double dispersed hadamard transform spectrometer].

A kind of two-dimensional hadamard transform spectrometer was developed. A grating was used for chromatic dispersion of orders and a prism was used for spectral dispersion. Quite different from traditional CCD detection method, a digital micromirror device (DMD) was applied for optical modulation, and a simple point detector was used as the sensor. Compared with traditional two-dimensional spectrometer, it has the advantage of high resolution and signal-noise-ratio, which was proved by theoretical calculation and computer simulation. PMID:22870674

Liu, Jia; Shi, Lei; Li, Kai; Zheng, Xin-Wen; Zeng, Li-Bo; Wu, Qiong-Shui

2012-06-01

385

Two-dimensional convective turbulence

We show that 2D {bold E{times}B} ionospheric turbulence of the electron density in the equatorial electrojet is isomorphic to the viscous convection of an ordinary fluid in a porous medium due to temperature gradients. Numerical simulations reveal the strong anisotropy in the turbulence, which consists of rising hot bubbles and falling cool bubbles. These bubbles break up into fingers leading to the formation of stable shear flows. After reaching a quasisteady state, the omnidirectional energy spectrum approaches a {ital k}{sup {minus}2} behavior, rather than {ital k}{sup {minus}5/3} as expected from isotropic turbulence. Physical mechanisms that lead to anisotropy are analyzed. {copyright} {ital 1996 The American Physical Society.}

Gruzinov, A.V. [Department of Physics, University of California at San Diego, La Jolla, California 92093 (United States)] [Department of Physics, University of California at San Diego, La Jolla, California 92093 (United States); Kukharkin, N. [Fluid Dynamics Research Center, Princeton University, Princeton, New Jersey 08544 (United States)] [Fluid Dynamics Research Center, Princeton University, Princeton, New Jersey 08544 (United States); Sudan, R.N. [Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 (United States)] [Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 (United States)

1996-02-01

386

Two-dimensional quantum mechanical modeling of nanotransistors

Quantization in the inversion layer and phase coherent transport are anticipated to have significant impact on device performance in ``ballistic'' nanoscale transistors. While the role of some quantum effects have been analyzed qualitatively using simple one-dimensional ballistic models, two-dimensional (2D) quantum mechanical simulation is important for quantitative results. In this paper, we present a framework for 2D quantum mechanical simulation

A. Svizhenko; M. P. Anantram; T. R. Govindan; B. Biegel; R. Venugopal

2002-01-01

387

Nonlinear tunneling in two-dimensional lattices

We present a thorough analysis of the nonlinear tunneling of Bose-Einstein condensates in static and accelerating two-dimensional lattices within the framework of the mean-field approximation. We deal with nonseparable lattices, considering different initial atomic distributions in highly symmetric states. For an analytical description of the condensate before instabilities develop, we derive several few-mode models, analyzing essentially both nonlinear and quasilinear regimes of tunneling. By direct numerical simulations, we show that two-mode models provide an accurate description of tunneling when either initially two states are populated or tunneling occurs between two stable states. Otherwise, a two-mode model may give only useful qualitative hints for understanding tunneling, but does not reproduce many features of the phenomenon. This reflects the crucial role of instabilities developed due to two-body interactions resulting in a non-negligible population of the higher bands. This effect becomes even more pronounced in the case of accelerating lattices. In the latter case we show that the direction of the acceleration is a relevant physical parameter which affects the tunneling by changing the atomic rates at different symmetric states and by changing the numbers of bands involved in the atomic transfer.

Brazhnyi, V. A. [Centro de Fisica Teorica e Computacional, Universidade de Lisboa, Complexo Interdisciplinar, Avenida Professor Gama Pinto 2, Lisbon 1649-003 (Portugal); Konotop, V. V. [Centro de Fisica Teorica e Computacional, Universidade de Lisboa, Complexo Interdisciplinar, Avenida Professor Gama Pinto 2, Lisbon 1649-003 (Portugal); Departamento de Fisica, Faculdade de Ciencias, Universidade de Lisboa, Campo Grande, Ed. C8, Piso 6, Lisbon 1749-016 (Portugal); Kuzmiak, V. [Institute of Photonics and Electronics, v.v.i., Czech Academy of Sciences, Chaberska 57, 182 51 Prague 8 (Czech Republic); Shchesnovich, V. S. [Instituto de Fisica, Universidade Federal de Alagoas, Maceio AL 57072-970 (Brazil)

2007-08-15

388

MHD simulations of the flapping instability in tail-like magnetic configurations with guide field

NASA Astrophysics Data System (ADS)

The flapping (kink) mode developing in the magnetotail-like magnetic configuration with tailward growing normal magnetic component and finite guide field is studied by means of linearized 2-dimensional and non-linear 3-dimensional MHD modeling. We consider a particular case of a weak normal magnetic component (that is, small radius of the magnetic field line curvature), which makes the configuration unstable to a special branch of ballooning instability known as "double-gradient" mode, introduced recently to describe the magnetotail flapping oscillations. The initial tail-like equilibrium is provided by conventional Grad-Shafranov equation. The results of the 2D linearized MHD code are in agreement with the analytical predictions, and the growth rate is found to be close to the peak value provided by an analytical estimate. Both 2D and 3D calculations confirm that the double-gradient mode is excited in a region of large curvature of the magnetic field lines. In agreement with the analytical predictions, non-zero guide field reduces the growth rate significantly for large (compare to the current sheet width L) wave numbers k, hence the modes kL ~ 1 are the fastest growing. Thus, the non-zero guide field introduces a characteristic wavelength corresponding to the dispersion curve peak. For the guide field of ~ 0.5 (in the lobe magnetic field units), the mode decays totally.

Korovinskiy, Daniil; Divin, Andrey; Ivanov, Ivan; Semenov, Vladimir; Erkaev, Nikolay; Artemiev, Anton; Markidis, Stefano; Lapenta, Giovanni; Ivanova, Viktoria; Kubyshkina, Darya

2014-05-01

389

Force-freeness of a solar magnetic field is a key to reconstructing invisible coronal magnetic structure of an emerging flux region on the Sun where active phenomena such as flares and coronal mass ejections frequently occur. We have performed magnetohydrodynamic (MHD) simulations which are adjusted to investigate force-freeness of an emerging magnetic field by using the virial theorem. Our focus is on how the force-free range of an emerging flux region develops and how it depends on the twist of a pre-emerged magnetic field. As an emerging flux region evolves, the upper limit of the force-free range continuously increases while the lower limit is asymptotically reduced to the order of a photospheric pressure scale height above the solar surface. As the twist becomes small the lower limit increases and then seems to be saturated. We also discuss the applicability of the virial theorem to an evolving magnetic structure on the Sun.

Kang, Jihye

2014-01-01

390

Mars Pathfinder Two-Dimensional Model

NSDL National Science Digital Library

This activity is about the Mars Pathfinder spacecraft. Using cardboard and other materials, learners will create a two dimensional model of the spacecraft. A diagram with approximate measures of each component is provided.

2012-08-03

391

Two-Dimensional Planetary Surface Lander

NASA Astrophysics Data System (ADS)

A systems engineering study was conducted to leverage a new two-dimensional (2D) lander concept with a low per unit cost to enable scientific study at multiple locations with a single entry system as the delivery vehicle.

Hemmati, H.; Sengupta, A.; Castillo, J.; McElrath, T.; Roberts, T.; Willis, P.

2014-06-01

392

Two-dimensional order and disorder thermofields

The main objective of this paper was to obtain the two-dimensional order and disorder thermal operators using the Thermofield Bosonization formalism. We show that the general property of the two-dimensional world according with the bosonized Fermi field at zero temperature can be constructed as a product of an order and a disorder variables which satisfy a dual field algebra holds at finite temperature. The general correlation functions of the order and disorder thermofields are obtained.

Belvedere, L. V. [Instituto de Fisica - Universidade Federal Fluminense, Av. Litora circumflex nea S/N, Boa Viagem Niteroi, CEP 24210-340 Rio de Janeiro (Brazil)

2006-11-15

393

Two-Dimensional Optical Proximity Effects

NASA Astrophysics Data System (ADS)

In projection printing the proximity effects between adjacent two-dimensional features such as concentric elbows can be the limiting factor in designing layout rules. An aerial image simulation code based on the imaging algorithms in SAMPLE has been developed and used to investigate these proximity effects. The program accepts arbitrary polygonal shapes constructed of rectangular and triangular patches. The image is calculated using Hopkins transmission cross coefficient formulation and uses rapid integral evaluation techniques. The cpu time for this FORTRAN F77 program depends on the size of the mask and the partial coherence factor as 0.25[(1 + ?) 2A(NA/?)2]2 seconds on a DEC VAX 11/780 using double precision, where A is the mask area, ? the coherence factor, NA the numerical aperture and ? the wavelength. The output intensity can be displayed with graphics tools such as UNIGRAFIX or cross-sectioned for input to SAMPLE development simulation along critical paths. Proximity effects in critical regions between features such as nested elbows, contacts near contacts and lines, and lines near large pads are studied. For small contacts studies show that a contact hole can be placed as close as 0.5?/NA microns to another contact hole. For nested elbows the critical effect is the variation in intensity in the straight regions just adjacent to the corner. This undesirable variation is primarily due to the intrafeature intensity interactions and is not greatly influenced by the proximity of another nested elbow. For general feature shapes the proximity effects are reduced by increasing the partial coherence factor to 0.5 or higher but at the cost of reducing contrast and peak intensity. For contact masks a partial coherence of 0.3 is recommended for higher edge slope and peak intensities. Proximity effects of small defects are also illustrated.

Flanner, Philip D.; Subramanian, Shankar; Neureuther, Andrew R.

1986-08-01

394

Mechanisms that generate the field-aligned current (FAC) systems in the magnetosphere-ionosphere coupling scheme by virtue of the solar wind-magnetosphere interaction are investigated with a three-dimensional magnetohydrodynamic (MHD) simulation. As a simulation scheme, the finite volume total variation diminishing (TVD) scheme on an unstructured grid system is employed for precise calculations of the ionospheric region. In the ionosphere, the divergence of

T. Tanaka

1995-01-01

395

Two-Dimensional Fourier Transform Analysis of Helicopter Flyover Noise

NASA Technical Reports Server (NTRS)

A method to separate main rotor and tail rotor noise from a helicopter in flight is explored. Being the sum of two periodic signals of disproportionate, or incommensurate frequencies, helicopter noise is neither periodic nor stationary. The single Fourier transform divides signal energy into frequency bins of equal size. Incommensurate frequencies are therefore not adequately represented by any one chosen data block size. A two-dimensional Fourier analysis method is used to separate main rotor and tail rotor noise. The two-dimensional spectral analysis method is first applied to simulated signals. This initial analysis gives an idea of the characteristics of the two-dimensional autocorrelations and spectra. Data from a helicopter flight test is analyzed in two dimensions. The test aircraft are a Boeing MD902 Explorer (no tail rotor) and a Sikorsky S-76 (4-bladed tail rotor). The results show that the main rotor and tail rotor signals can indeed be separated in the two-dimensional Fourier transform spectrum. The separation occurs along the diagonals associated with the frequencies of interest. These diagonals are individual spectra containing only information related to one particular frequency.

SantaMaria, Odilyn L.; Farassat, F.; Morris, Philip J.

1999-01-01

396

A system of numerical hydraulic modeling, geographic information system processing, and Internet map serving, supported by new data sources and application automation, was developed that generates inundation maps for forecast floods in near real time and makes them available through the Internet. Forecasts for flooding are generated by the National Weather Service (NWS) River Forecast Center (RFC); these forecasts are retrieved automatically by the system and prepared for input to a hydraulic model. The model, TrimR2D, is a new, robust, two-dimensional model capable of simulating wide varieties of discharge hydrographs and relatively long stream reaches. TrimR2D was calibrated for a 28-kilometer reach of the Snoqualmie River in Washington State, and is used to estimate flood extent, depth, arrival time, and peak time for the RFC forecast. The results of the model are processed automatically by a Geographic Information System (GIS) into maps of flood extent, depth, and arrival and peak times. These maps subsequently are processed into formats acceptable by an Internet map server (IMS). The IMS application is a user-friendly interface to access the maps over the Internet; it allows users to select what information they wish to see presented and allows the authors to define scale-dependent availability of map layers and their symbology (appearance of map features). For example, the IMS presents a background of a digital USGS 1:100,000-scale quadrangle at smaller scales, and automatically switches to an ortho-rectified aerial photograph (a digital photograph that has camera angle and tilt distortions removed) at larger scales so viewers can see ground features that help them identify their area of interest more effectively. For the user, the option exists to select either background at any scale. Similar options are provided for both the map creator and the viewer for the various flood maps. This combination of a robust model, emerging IMS software, and application interface programming should allow the technology developed in the pilot study to be applied to other river systems where NWS forecasts are provided routinely.

Jones, Joseph L.; Fulford, Janice M.; Voss, Frank D.

2002-01-01

397

A Two-Dimensional Lattice Model with Exact Supersymmetry

Starting from a simple discrete model which exhibits a supersymmetric invariance we construct a local, interacting, two-dimensional Euclidean lattice theory which also admits an exact supersymmetry. This model is shown to correspond to the Wess-Zumino model with extended N=2 supersymmetry in the continuum. We have performed dynamical fermion simulations to check the spectrum and supersymmetric Ward identities and find good agreement with theory.

S. Catterall; S. Karamov

2001-10-12

398

Dynamics of Clusters in Two-dimensional Potts Model

Dynamical behavior of the clusters during relaxation is studied in two-dimensional Potts model using cluster algorithm. Average cluster size and cluster formation velocity are calculated on two different lattice sizes for different number of states during initial stages of the Monte Carlo simulation. Dependence of these quantities on the order of the transition provides an efficient method to study nature of the phase transitions occuring in similar models.

Yigit Gunduc; Meral Aydin

1996-05-11

399

Highly directional thermal emission from two-dimensional silicon structures.

We simulate, fabricate, and characterize near perfectly absorbing two-dimensional grating structures in the thermal infrared using heavily doped silicon (HdSi) that supports long wave infrared surface plasmon polaritons (LWIR SPP's). The devices were designed and optimized using both finite difference time domain (FDTD) and rigorous coupled wave analysis (RCWA) simulation techniques to satisfy stringent requirements for thermal management applications requiring high thermal radiation absorption over a narrow angular range and low visible radiation absorption over a broad angular range. After optimization and fabrication, characterization was performed using reflection spectroscopy and normal incidence emissivity measurements. Excellent agreement between simulation and experiment was obtained. PMID:23546065

Ribaudo, Troy; Peters, David W; Ellis, A Robert; Davids, Paul S; Shaner, Eric A

2013-03-25

400

NASA Astrophysics Data System (ADS)

A complete anisotropic, inhomogeneous electrical conductivity tensor, which includes Spitzer, Pedersen, and Hall conductivities is included in an MHD simulation to describe how MHD shock waves may form, propagate, and resistively heat the atmosphere from the photosphere through the chromosphere. The MHD model includes an energy equation. The initial state is defined by FAL density, pressure, and temperature profiles, and by a magnetic field that decreases with height z. The initial magnetic field strength at the photosphere is 500 G. A harmonic magnetic field perturbation with amplitude 250 G and period 30 seconds is applied at the photosphere. Smooth waves are generated at the photosphere that propagate upward and begin to form shock waves near z=350 km. This is the height near which electrons first become magnetized. The shocks become fully formed near the FAL temperature minimum at z=500 km. This is the height where the product of the electron and proton magnetizations first exceeds unity, causing the Pedersen resistivity to begin to rapidly exceed the Spitzer resistivity by orders of magnitude with increasing height. This is also the height at which heating by proton Pedersen current dissipation rapidly increases with height, and rapidly becomes large enough to balance the radiative losses from the chromosphere. The onset of this strong heating is triggered by the onset of electron and proton magnetization near the temperature minimum. The shock thicknesses are ~ ~ 5 km. The shocks are the sites of resistive heating rates as large as 3-10 ergs-cm-3-sec-1 in the chromosphere. The time averaged heating rate over an interval of 162 seconds corresponds to a chromospheric heating flux ~ 2-3 × 106 ergs-cm-2-sec-1. The heating rate increases with driving frequency, and is ? B2. These results support the proposition of Goodman (e.g. Goodman 2000, ApJ, 533, 501; Goodman 2004, A&A, 424,691; Kazeminezhad & Goodman 2006, ApJ, 166, 613) that the onset of electron and proton magnetization near the local temperature minimum, and their rapid increase with height causes the rate of proton Pedersen current dissipation to rapidly increase by orders of magnitude with height, creating and maintaining the solar chromosphere, and the chromospheres of solar type stars. This mechanism is not restricted to shock waves. It operates on any current generating MHD process. Such a process must involve currents driven by a combination of induction and convection generated electric fields. Examples are linear waves, and steady convection across magnetic field lines. It is the weakly ionized, strongly magnetized nature of the chromosphere that allows this heating mechanism to be so effective, and that distinguishes the chromosphere from the weakly ionized, weakly magnetized photosphere, and the strongly ionized, strongly magnetized corona. The dominance of proton-neutral H collisions in determining the proton collision frequency is necessary for this Pedersen current dissipation mechanism to be an effective heating mechanism in the chromosphere. This work was supported by Grant ATM 0650443 from the National Science Foundation to the West Virginia High Technology Consortium Foundation. class="ab'>

Kazeminezhad, F.; Goodman, M. L.

2008-12-01

401

NASA Astrophysics Data System (ADS)

One of the significant problems in magnetospheric physics concerns the nature and properties of the processes which occur at the magnetopause boundary; in particular how energy, momentum, and plasma the magnetosphere receives from the solar wind. Basic processes are magnetic reconnection [Dungey, 1961] and viscouslike interaction, such as Kelvin-Helmholtz instability [Dungey 1955, Miura, 1984] and pressure-pulse driven [Sibeck et al. 1989]. In generally, magnetic reconnection occurs efficiently when the IMF is southward and the rate is largest where the magnetosheath magnetic field is antiparallel to the geomagnetic field. [Sonnerup, 1974; Crooker, 1979; Luhmann et al., 1984; Park et al., 2006, 2009]. The Kelvin-Helmholtz instability is driven by the velocity shear at the boundary, which occur frequently when the IMF is northward. Also variation of the magnetic field and the plasma properties is reported to be quasi-periodic with 2-3min [Otto and Fairfield, 2000] and period of vortex train with 3 to 4 minutes by global MHD simulation [Ogino, 2011]. The pressure-pulse is driven by the solar wind. And the observations of the magnetospheric magnetic field response show quasi-periodic with a period of 8 minutes [Sibeck et al., 1989; Kivelson and Chen, 1995]. There have been few studies of the vortices in the magnetospheric boundary under southward IMF condition. However it is not easy to find the generation mechanism and characteristic for vortices in complicated 3-dimensional space. Thus we have performed global MHD simulation for the steady solar wind and southward IMF conditions. From the simulation results, we find that the vortex occurs at R= 11.7Re (IMF Bz = -2 nT) and R= 10.2Re (IMF Bz = -10 nT) in the dayside magnetopause boundary. Also the vortex rotates counterclockwise in duskside magnetopause (clockwise in dawnside) and propagates tailward. Across the vortex, magnetic field and plasma properties clearly show quasi-periodic fluctuations with a period of 8~10 minutes under the weak southward IMF and 4~8 minutes for strong southward IMF conditions. Magnetic reconnection favorably occurs in anti-parallel field region with slower shear velocity in the magnetosheath. The magnetic field lines are highly bent by parallel vorticity (Omega||) in the flanks of the magnetopause boundary. Also, similar vortices are formed in a grid spacing of 0.3Re and 0.2Re. A small structure vortices are generated in higher resolution (0.1Re) and two vortices are mixed after 1m30s We suggest that the reconnection is a mechanism of generating vortex with a periodicity in the dayside during the southward IMF.

Park, K.; Ogino, T.; Lee, D.; Walker, R. J.; Kim, K.

2013-12-01

402

Numerical prediction on performance of closed cycle disk MHD generator

Numerical calculations were made on nonequilibrium MHD flows in a disk MHD generator in a strong magnetic field. The main objectives of the present study are to develop a numerical simulator for the authors' present closed-cycle disk MHD generator and to obtain knowledge for the design of a new disk MHD generator with high performance-especially high adiabatic efficiency. The basic

Masaaki OKUBO; Hiroyuki YAMASAKI

1996-01-01

403

Extended MHD Simulations of the Formation, Merging, and Heating of Compact Tori

NASA Astrophysics Data System (ADS)

We examine the formation, compression, merging, and stability of compact tori (CT) for magnetic field generation and heating by use of the 3D extended MHD code, NIMROD [C.R. Sovinec et al. J. Comp. Phys. 355, 195, (2004)]. Recent advances in the NIMROD code allow us to study plasmas, including the effects of Hall physics and highly anisotropic and field dependent transport. The physics of CT formation and acceleration requires numerical models that can effectively treat plasma flows in systems that are often far from equilibrium. The formation of plasmas with strong magnetic fields by use of a low power source still remains a critical issue. Recently, a novel means for generating strong B from a low current source was developed, and relies on the repetitive injection of plasma from a coaxial gun, leading to the step-wise increase in both total circulating current and core plasma temperature. A natural limit is encountered much as in CT injection for fueling into tokamaks, namely the injected plasma must penetrate the target plasma. To reach high fields, this then will require compression before injection. Stability of the configuration to fluid (e.g. Rayleigh-Taylor) and ideal modes (e.g. tilt/shift) are examined.

Macnab, Angus; Woodruff, Simon

2008-11-01

404

Magnetosphere simulations with a high-performance 3D AMR MHD Code

NASA Astrophysics Data System (ADS)

BATS-R-US is a high-performance 3D AMR MHD code for space physics applications running on massively parallel supercomputers. In BATS-R-US the electromagnetic and fluid equations are solved with a high-resolution upwind numerical scheme in a tightly coupled manner. The code is very robust and it is capable of spanning a wide range of plasma parameters (such as ?, acoustic and Alfvénic Mach numbers). Our code is highly scalable: it achieved a sustained performance of 233 GFLOPS on a Cray T3E-1200 supercomputer with 1024 PEs. This talk reports results from the BATS-R-US code for the GGCM (Geospace General Circularculation Model) Phase 1 Standard Model Suite. This model suite contains 10 different steady-state configurations: 5 IMF clock angles (north, south, and three equally spaced angles in- between) with 2 IMF field strengths for each angle (5 nT and 10 nT). The other parameters are: solar wind speed =400 km/sec; solar wind number density = 5 protons/cc; Hall conductance = 0; Pedersen conductance = 5 S; parallel conductivity = ?.

Gombosi, Tamas; Dezeeuw, Darren; Groth, Clinton; Powell, Kenneth; Song, Paul

1998-11-01

405

Particle simulation algorithms with short-range forces in MHD and fluid flow

Attempts are made to develop numerical algorithms for handling fluid flows involving liquids and liquid-gas mixtures. In these types of systems, the short-range intermolecular interactions are important enough to significantly alter behavior predicted on the basis of standard fluid mechanics and magnetohydrodynamics alone. We have constructed a particle-in-cell (PIC) code for the purpose of studying the effects of these interactions. Of the algorithms considered, the one which has been successfully implemented is based on a MHD particle code developed by Brunel et al. In the version presented here, short range forces are included in particle motion by, first, calculating the forces between individual particles and then, to prevent aliasing, interpolating these forces to the computational grid points, then interpolating the forces back to the particles. The code has been used to model a simple two-fluid Rayleigh-Taylor instability. Limitations to the accuracy of the code exist at short wavelengths, where the effects of the short-range forces would be expected to be most pronounced.

Cable, S.; Tajima, T.; Umegaki, K.

1992-07-01

406

Shear viscosity and shear thinning in two-dimensional Yukawa , J. Goree2

Shear viscosity and shear thinning in two-dimensional Yukawa liquids Z. DonkÂ´o1 , J. Goree2 , P using two different nonequi- librium molecular dynamics simulation methods. Shear viscosity values.e., the viscosity diminishes with increasing shear rate. It is expected that two-dimensional dusty plasmas

Goree, John

407

Singularity-free two-dimensional cosmologies

NASA Astrophysics Data System (ADS)

We present a class of theories of two-dimensional gravity which admits homogeneous and isotropic solutions that are nonsingular and asymptotically approach a FRW matter-dominated universe at late times. These models are generalizations of two-dimensional dilaton gravity and both vacuum solutions and those including conformally coupled matter are investigated. In each case our construction leads to an inflationary stage driven by the gravitational sector. Our work comprises a simple example of the ``nonsingular universe'' constructions of Brandenberger and co-workers.

Moessner, R.; Trodden, M.

1995-03-01

408

Singularity-free two-dimensional cosmologies

We present a class of theories of two-dimensional gravity which admits homogeneous and isotropic solutions that are nonsingular and asymptotically approach a FRW matter-dominated universe at late times. These models are generalizations of two-dimensional dilaton gravity and both vacuum solutions and those including conformally coupled matter are investigated. In each case our construction leads to an inflationary stage driven by the gravitational sector. Our work comprises a simple example of the nonsingular universe'' constructions of Brandenberger and co-workers.

Moessner, R.; Trodden, M. (Department of Physics, Brown University, Providence, Rhode Island 02912 (United States))

1995-03-15

409

Phase Transitions in Two-Dimensional Superconductors

NASA Astrophysics Data System (ADS)

In this thesis, we describe a number of experiments which are designed to explore the theoretically predicted phase transitions for two-dimensional superconductors. We first examine the behavior of a two-dimensional superconductor in the absence of a magnetic field, where the fluctuations in the phase of the superconducting order parameter results in the reduction of the superconducting transition temperature for a two-dimensional superconductor below the mean field transition temperature. The experimental results presented show the importance of the vortex unbinding mechanism; however they challenge some of the predictions of the current theoretical model for the normal/superconducting transition. We then turn our attention to the behavior of two-dimensional superconductors in the presence of a magnetic field. In this case, our experimental results show that vortices in weakly disordered two-dimensional superconducting films freeze into locally correlated areas at temperatures below the mean field transition temperature. The experimentally measured phase boundary between the liquid phase and the locally ordered solid phase for the vortices in our samples shows good agreement with that predicted by the dislocation unbinding melting mechanism for the two-dimensional vortex lattice. For this transition, we have also done a detailed study of the correlations in the vortex lattices as a function of the strength of pinning in our samples and established a connection between the melting theory and collective pinning theory for vortices in two dimensions. To fully examine the phase diagram of a two-dimensional superconductor we have extended our studies of the properties of this system to the limit of strong disorder, where superconductivity is weak. In this limit, we report experimental evidence for a zero temperature field-tuned superconducting-insulating phase transition driven by quantum fluctuation of the phase of the superconducting order parameter. Overall, the results of this thesis provide an experimental bases for a number of phase transitions in two-dimensional superconductors, thereby establishing a rich phase diagram for this system as a function of the temperature, the magnetic field, and the amount of disorder.

Yazdani, Ali

1995-01-01

410

Crossflow in two-dimensional asymmetric nozzles

NASA Technical Reports Server (NTRS)

An experimental investigation of the crossflow effects in three contoured, two-dimensional asymmetric nozzles is described. The data were compared with theoretical predictions of nozzle flow by using an inviscid method of characteristics solution and two-dimensional turbulent boundary-layer calculations. The effect of crossflow as a function of the nozzle maximum expansion angle was studied by use of oil-flow techniques, static wall-pressure measurements, and impact-pressure surveys at the nozzle exit. Reynolds number effects on crossflow were investigated.

Sebacher, D. I.; Lee, L. P.

1975-01-01

411

Two-Dimensional Clifford Windowed Fourier Transform

NASA Astrophysics Data System (ADS)

Recently several generalizations to higher dimension of the classical Fourier transform (FT) using Clifford geometric algebra have been introduced, including the two-dimensional (2D) Clifford-Fourier transform (CFT). Based on the 2D CFT, we establish the two-dimensional Clifford windowed Fourier transform (CWFT). Using the spectral representation of the CFT, we derive several important properties such as shift, modulation, a reproducing kernel, isometry, and an orthogonality relation. Finally, we discuss examples of the CWFT and compare the CFT and CWFT.

Bahri, Mawardi; Hitzer, Eckhard M. S.; Adji, Sriwulan

412

MHD-kinetic Simulation of the Heliospheric Interface and Related 2-3 kHz Radio Emission

NASA Astrophysics Data System (ADS)

We present the results of our analysis of the three-dimensional, MHD-kinetic simulation of the solar wind (SW) interaction with the local interstellar medium (LISM). The choice of the LISM quantities is in the range that agrees with the IBEX observations of the bright ribbon of enhanced energetic neutral atom flux. The LISM velocity vector and temperature are taken from the new IBEX data analysis and are somewhat different from the previously accepted based on Ulysses data. We use our Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS) to perform adaptive mesh refinement calculations for the analysis of the bow shock/wave structure in unprecedented details. It is shown that a bow wave may have only a tiny subshock even for the LISM magnetic field strength of 2.5 microG. Since the 2-3 kHz radio emission criteria depend on the LISM properties at the heliopause, we used the obtained results to determine the distribution of radio emission sources and compare them with Voyager observations.

Borovikov, S.; Pogorelov, N. V.; Heerikhuisen, J.; Zank, G. P.; Cairns, I. H.

2013-12-01

413

A nonsingular two dimensional black hole

NASA Astrophysics Data System (ADS)

We construct a model of gravity in 1 + 1 spacetime dimensions in which the solutions approach the Schwarzschild metric at large r and de Sitter space far inside the horizon. Our model may be viewed as a two dimensional application of the ``Limiting Curvature Construction'' of Mukhanov and Brandenberger [Phys. Rev. Lett. 68 (1992) 1969].

Trodden, M.; Mukhanov, V. F.; Brandenberger, R. H.

1993-10-01

414

High Capacity Colored Two Dimensional Codes

Barcodes enable automated work processes without human intervention, and are widely deployed because they are fast and accurate, eliminate many errors and often save time and money. In order to increase the data capacity of barcodes, two dimensional (2D) code were developed; the main challenges of 2D codes lie in their need to store more information and more character types

Antonio Grillo; Alessandro Lentini; Marco Querini; Giuseppe F. Italiano

2010-01-01

415

Spatial resource analysis of two dimensional barcodes

It cannot be denied that documents in the form of hardcopy are still being used, especially important documents such as land titles, application forms, contracts and tickets. However there are reports of forgery cases over the years and as such, it is imperative to have a mechanism for integrity verification of hardcopy document. This research proposed the usage of two-dimensional

Teoh Chin Yew; S. Mazleena; I. Subariah

2008-01-01

416

On the two-dimensional associativity equation

We consider the two-dimensional associativity (or WDVV) equation u_yyy - u_xxy^2 + u_xxx u_xyy=0 and describe all integrable structures related to it (i.e., Hamiltonian, symplectic, and recursion operators). Infinite hierarchies of symmetries and conservation laws are constructed as well.

Kersten, Paul; Verbovetsky, Alexander; Vitolo, Raffaele

2011-01-01

417

Fractal two-dimensional electromagnetic bandgap structures

Fractal two-dimensional electromagnetic bandgap (EBG) materials are proposed and studied by means of a full-wave method developed for diffraction gratings. Such technique allows us to characterize, In an accurate and rapidly convergent way, the transmission and reflection properties of periodic fractal structures with an arbitrary geometry in the unit cell. Both polarization cases can be treated. A validation of the

Fabrizio Frezza; Lara Pajewski; Giuseppe Schettini

2004-01-01

418

Two-Dimensional Turbulence in Magnetized Plasmas

ERIC Educational Resources Information Center

In an inhomogeneous magnetized plasma the transport of energy and particles perpendicular to the magnetic field is in general mainly caused by quasi two-dimensional turbulent fluid mixing. The physics of turbulence and structure formation is of ubiquitous importance to every magnetically confined laboratory plasma for experimental or industrial…

Kendl, A.

2008-01-01

419

Two dimensional gas turbine engine exhaust nozzle

A two-dimensional variable area gas turbine engine exhaust nozzle is described having thrust reversing capability, the nozzle including spaced apart side wall means and upper and lower flap assemblies connected to the side wall means defining an exhaust gas flow path wihtin the nozzle, the nozzle having a centerline.

Thayer, E.B.; McLafferty, G.H.

1988-06-28

420

Overlap distributions in two-dimensional spin glasses

NASA Astrophysics Data System (ADS)

Numerical results are presented for overlaps of configurations of two-dimensional Ising spin glasses. At low temperatures, the correlation length greatly exceeds the system size, so that spin-spin correlations are relatively long range and domain wall energies exhibit sensitive dependence to temperature,as seen in the low temperature phase of three-dimensional spin glasses. Exact sampling algorithms are used so that there is no doubt of equilibration. High statistics runs are carried out, with tens of thousands of samples of size L^2=256^2 simulated. The results of the size-dependent spin overlap distribution P(q) are evaluated using statistics recently developed by Yucesoy, Katzgraber and Machta. The statistics for two-dimensional models at low temperature are found to be quite similar to those of three-dimensional spin glasses at finite temperatures below the spin-glass transition.

Middleton, A. Alan

2013-03-01

421

Context: The context of this paper is buoyant toroidal magnetic flux ropes, which is a part of flux tube dynamo theory and the framework of solar-like magnetic activity. Aims: The aim is to investigate how twisted magnetic flux ropes interact with a simple magnetized stellar model envelope--a magnetic "convection zone"--especially to examine how the twisted magnetic field component of a flux rope interacts with a poloidal magnetic field in the convection zone. Method: Both the flux ropes and the atmosphere are modelled as idealized 2.5-dimensional concepts using high resolution numerical magneto-hydrodynamic (MHD) simulations. Results: It is illustrated that twisted toroidal magnetic flux ropes can interact with a poloidal magnetic field in the atmosphere to cause a change in both the buoyant rise dynamics and the flux rope's geometrical shape. The details of these changes depend primarily on the polarity and strength of the atmospheric field relative to the field strength of the flux rope. It is suggested that the effects could be verified observationally.

S. B. F. Dorch

2006-09-22

422