2D MHD test-particle simulations in modeling geomagnetic storms
NASA Astrophysics Data System (ADS)
Li, Z.; Elkington, S. R.; Hudson, M. K.; Murphy, J. J.; Schmitt, P.; Wiltberger, M. J.
2012-12-01
The effects of magnetic storms on the evolution of the electron radiation belts are studied using MHD test-particle simulations. The 2D guiding center code developed by Elkington et al. (2002) has been used to simulate particle motion in the Solar Magnetic equatorial plane in the MHD fields calculated from the Lyon-Fedder-Mobarry global MHD code. However, our study shows that the B-minimum plane is well off the SM equatorial plane during solstice events. Since 3D test-particle simulation is computationally expensive, we improve the 2D model by pushing particles in the B-minimum surface instead of the SM equatorial plane. Paraview software is used to visualize the LFM data file and to find the B-minimum surface. Magnetic and electric fields on B-minimum surface are projected to the equatorial plane for particle pushing.
2-D MHD numerical simulations of EML plasma armatures with ablation
NASA Astrophysics Data System (ADS)
Boynton, G. C.; Huerta, M. A.; Thio, Y. C.
1993-01-01
We use a 2-D) resistive MHD code to simulate an EML plasma armature. The energy equation includes Ohmic heating, radiation heat transport and the ideal gas equation of state, allowing for variable ionization using the Saha equations. We calculate rail ablation taking into account the flow of heat into the interior of the rails. Our simulations show the development of internal convective flows and secondary arcs. We use an explicit Flux Corrected Transport algorithm to advance all quantities in time.
Resistive MHD and kinetic simulations of 2D magnetotail equilibria leading to reconnection onset
NASA Astrophysics Data System (ADS)
Merkin, V. G.; Sitnov, M. I.; Lyon, J.; Cassak, P.
2013-12-01
Recent progress in theory and fully kinetic particle-in-cell simulations of 2D magnetotail-like configurations has revealed an important class of equilibria, which can be unstable to ion tearing instability and eventually result in explosive dissipation of energy, fast plasma sheet flows, dipolarizations and changes in initial magnetic topology (reconnection). Such configurations are characterized by an increase of magnetic flux at the tailward end of the equilibrium state. While the instability and subsequent reconfiguration of the initial state exhibit kinetic signatures, the question remains, which parts of the process can be reproduced using reduced plasma models, e.g., resistive and Hall MHD. In this presentation we explore the stability of the new class of magnetotail equilibria to the resistive tearing mode and investigate its properties as a function of equilibrium parameters, e.g., the current sheet thickness and the amount of flux accumulation at the tailward end of the equilibrium, as well as other system parameters, e.g., resistivity and Lundquist number. We discuss comparative aspects of the system behavior in kinetic and resistive MHD simulations, in particular, what, if any, parameters of the MHD system lead to similar growth rates of the instability. Since the theoretical onset condition of the kinetic tearing mode can be expressed fully in MHD terms, we also investigate the effects of including this criterion as an additional constraint on the tearing onset in our resistive MHD simulations. This work is a first step toward inclusion of a kinetically-motivated description of reconnection onset in global MHD simulations of the magnetosphere.
The ideal tearing mode: 2D MHD simulations in the linear and nonlinear regimes
NASA Astrophysics Data System (ADS)
Landi, Simone; Del Zanna, Luca; Pucci, Fulvia; Velli, Marco; Papini, Emanuele
2015-04-01
We present compressible, resistive MHD numerical simulations of the linear and nonlinear evolution of the tearing instability, for both Harris sheet and force-free initial equilibrium configurations. We analyze the behavior of a current sheet with aspect ratio S1/3, where S is the Lundquist number. This scaling has been recently recognized to be the threshold for fast reconnection occurring on the ideal Alfvenic timescale, with a maximum growth rate that becomes asymptotically independent on S. Our simulations clearly confirm that the tearing instability maximum growth rate and the full dispersion relation are exactly those predicted by the linear theory, at least for the values of S explored here. In the nonlinear stage, we notice the rapid onset and subsequent coalescence of plasmoids, as observed in previous simulations of the Sweet-Parker reconnection scenario. These findings strongly support the idea that in a fully dynamic regime, as soon as current sheets develop and reach the critical threshold in their aspect ratio of S1/3 (occurring well before the Sweet-Parker configuration is able to form), the tearing mode is able to trigger fast reconnection and plasmoids formation on Alfvenic timescales, as required to explain the violent flare activity often observed in solar and astrophysical plasmas.
NASA Astrophysics Data System (ADS)
Wu, C.; Chang, T.
2010-12-01
A new method in describing the multifractal characteristics of intermittent events was introduced by Cheng and Wu [Chang T. and Wu C.C., Physical Rev, E77, 045401(R), 2008]. The procedure provides a natural connection between the rank-ordered spectrum and the idea of one-parameter scaling for monofractals. This technique has been demonstrated using results obtained from a 2D MHD simulation. It has also been successfully applied to in-situ solar wind observations [Chang T., Wu, C.C. and Podesta, J., AIP Conf Proc. 1039, 75, 2008], and the broadband electric field oscillations from the auroral zone [Tam, S.W.Y. et al., Physical Rev, E81, 036414, 2010]. We take the next step in this procedure. By using the ROMA spectra and the scaled probability distribution functions (PDFs), raw PDFs can be calculated, which can be compared directly with PDFs from observations or simulation results. In addition to 2D MHD simulation results and in-situ solar wind observation, we show clearly using the ROMA analysis the multifractal character of the 3D fluid simulation data obtained from the JHU turbulence database cluster at http://turbulence.pha.jhu.edu. In particular, we show the scaling of the non-symmetrical PDF for the parallel-velocity fluctuations of this 3D fluid data.
NASA Astrophysics Data System (ADS)
Westerhof, E.; de Blank, H. J.; Pratt, J.
2016-03-01
Two dimensional reduced MHD simulations of neoclassical tearing mode growth and suppression by ECCD are performed. The perturbation of the bootstrap current density and the EC drive current density perturbation are assumed to be functions of the perturbed flux surfaces. In the case of ECCD, this implies that the applied power is flux surface averaged to obtain the EC driven current density distribution. The results are consistent with predictions from the generalized Rutherford equation using common expressions for Δ \\text{bs}\\prime and Δ \\text{ECCD}\\prime . These expressions are commonly perceived to describe only the effect on the tearing mode growth of the helical component of the respective current perturbation acting through the modification of Ohm’s law. Our results show that they describe in addition the effect of the poloidally averaged current density perturbation which acts through modification of the tearing mode stability index. Except for modulated ECCD, the largest contribution to the mode growth comes from this poloidally averaged current density perturbation.
3D MHD Simulations of Tokamak Disruptions
NASA Astrophysics Data System (ADS)
Woodruff, Simon; Stuber, James
2014-10-01
Two disruption scenarios are modeled numerically by use of the CORSICA 2D equilibrium and NIMROD 3D MHD codes. The work follows the simulations of pressure-driven modes in DIII-D and VDEs in ITER. The aim of the work is to provide starting points for simulation of tokamak disruption mitigation techniques currently in the CDR phase for ITER. Pressure-driven instability growth rates previously observed in simulations of DIIID are verified; Halo and Hiro currents produced during vertical displacements are observed in simulations of ITER with implementation of resistive walls in NIMROD. We discuss plans to exercise new code capabilities and validation.
2-D skin-current toroidal-MHD-equilibrium code
Feinberg, B.; Niland, R.A.; Coonrod, J.; Levine, M.A.
1982-09-01
A two-dimensional, toroidal, ideal MHD skin-current equilibrium computer code is described. The code is suitable for interactive implementation on a minicomptuer. Some examples of the use of the code for design and interpretation of toroidal cusp experiments are presented.
Extended MHD Simulations of Spheromaks
NASA Astrophysics Data System (ADS)
Howell, E. C.; Sovinec, C. R.
2012-10-01
Nonlinear extended MHD simulations of a spheromak in a cylindrical flux conserver are performed using the NIMROD code (JCP 195, 2004). An idealized series of simulations starting from a Grad-Shafranov equilibrium and small non-axisymmetric perturbations are performed to model the sustained decay phase. The resulting confinement leads to steep resistivity gradients. Strong current gradients develop, driving tearing modes that dominate the evolution of the spheromak. Absent in these simulations are the remains of n=1 fluctuations created during the formation process. A second series of simulations start from vacuum fields and model the full spheromak evolution, including the formation process where the n=1 fluctuations dominate. To understand the role of pressure driven instabilities in the evolution of the spheromak, a numerical diagnostic is developed to calculate the Mercier stability criterion from the axisymmetric fields.
Cusp geometry in MHD simulations
NASA Astrophysics Data System (ADS)
Siscoe, George; Crooker, Nancy; Siebert, Keith; Maynard, Nelson; Weimer, Daniel; White, Willard
2005-01-01
The MHD simulations described here show that the latitude of the high-altitude cusp decreases as the IMF swings from North to South, that there is a pronounced dawn dusk asymmetry at high-altitude associated with a dawn dusk component of the IMF, and that at the same time there is also a pronounced dawn dusk asymmetry at low-altitude. The simulations generate a feature that represents what has been called the cleft. It appears as a tail (when the IMF has a By component) attached to the cusp, extending either toward the dawn flank or the dusk flank depending on the dawn dusk orientation of the IMF. This one-sided cleft connects the cusp to the magnetospheric sash. We compare cusp geometry predicted by MHD simulations against published observations based on Hawkeye and DMSP data. Regarding the high-altitude predictions, the comparisons are not definitive, mainly because the observations are incomplete or mutually inconsistent. Regarding the low-altitude prediction of a strong dawn dusk asymmetry, the observations are unambiguous and are in good qualitative agreement with the prediction.
MHD simulations: Corotating Interaction Regions
NASA Astrophysics Data System (ADS)
Wiengarten, T.; Kleimann, J.; Fichtner, H.; Kühl, P.; Heber, B.; Kissmann, R.
2013-12-01
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
MHD Simulation Heliospheric Magnetic Fields and Turbulence
NASA Technical Reports Server (NTRS)
Roberts, D. Aaron
2005-01-01
This talk will present a summary of our results on simulations of heliospheric structure and dynamics. We use a three-dimensional MHD code in spherical coordinates to produce a solar wind containing a rotating, tilted heliospheric current sheet, fast-slow stream and microstream shear layers, waves, 2-D turbulence, and pressure balanced structures that are input to the inner (superAlfvenic) boundary. The evolution of various combinations of these has led to a deeper understanding of sector structure, magnetic holes, fluctuation anisotropies, and general turbulent evolution. We show how the sectors are likely to be connected, how spiral fields can arise, and how field line diffusion can be caused by waves with transverse structure and microstream shears.
MHD simulations on an unstructured mesh
Strauss, H.R.; Park, W.; Belova, E.; Fu, G.Y.; Longcope, D.W.; Sugiyama, L.E.
1998-12-31
Two reasons for using an unstructured computational mesh are adaptivity, and alignment with arbitrarily shaped boundaries. Two codes which use finite element discretization on an unstructured mesh are described. FEM3D solves 2D and 3D RMHD using an adaptive grid. MH3D++, which incorporates methods of FEM3D into the MH3D generalized MHD code, can be used with shaped boundaries, which might be 3D.
Realistic and efficient 2D crack simulation
NASA Astrophysics Data System (ADS)
Yadegar, Jacob; Liu, Xiaoqing; Singh, Abhishek
2010-04-01
Although numerical algorithms for 2D crack simulation have been studied in Modeling and Simulation (M&S) and computer graphics for decades, realism and computational efficiency are still major challenges. In this paper, we introduce a high-fidelity, scalable, adaptive and efficient/runtime 2D crack/fracture simulation system by applying the mathematically elegant Peano-Cesaro triangular meshing/remeshing technique to model the generation of shards/fragments. The recursive fractal sweep associated with the Peano-Cesaro triangulation provides efficient local multi-resolution refinement to any level-of-detail. The generated binary decomposition tree also provides efficient neighbor retrieval mechanism used for mesh element splitting and merging with minimal memory requirements essential for realistic 2D fragment formation. Upon load impact/contact/penetration, a number of factors including impact angle, impact energy, and material properties are all taken into account to produce the criteria of crack initialization, propagation, and termination leading to realistic fractal-like rubble/fragments formation. The aforementioned parameters are used as variables of probabilistic models of cracks/shards formation, making the proposed solution highly adaptive by allowing machine learning mechanisms learn the optimal values for the variables/parameters based on prior benchmark data generated by off-line physics based simulation solutions that produce accurate fractures/shards though at highly non-real time paste. Crack/fracture simulation has been conducted on various load impacts with different initial locations at various impulse scales. The simulation results demonstrate that the proposed system has the capability to realistically and efficiently simulate 2D crack phenomena (such as window shattering and shards generation) with diverse potentials in military and civil M&S applications such as training and mission planning.
MHD simulation of the Bastille day event
NASA Astrophysics Data System (ADS)
Linker, Jon; Torok, Tibor; Downs, Cooper; Lionello, Roberto; Titov, Viacheslav; Caplan, Ronald M.; Mikić, Zoran; Riley, Pete
2016-03-01
We describe a time-dependent, thermodynamic, three-dimensional MHD simulation of the July 14, 2000 coronal mass ejection (CME) and flare. The simulation starts with a background corona developed using an MDI-derived magnetic map for the boundary condition. Flux ropes using the modified Titov-Demoulin (TDm) model are used to energize the pre-event active region, which is then destabilized by photospheric flows that cancel flux near the polarity inversion line. More than 1033 ergs are impulsively released in the simulated eruption, driving a CME at 1500 km/s, close to the observed speed of 1700km/s. The post-flare emission in the simulation is morphologically similar to the observed post-flare loops. The resulting flux rope that propagates to 1 AU is similar in character to the flux rope observed at 1 AU, but the simulated ICME center passes 15° north of Earth.
MHD Simulations of Core Collapse Supernovae with Cosmos++
NASA Astrophysics Data System (ADS)
Akiyama, Shizuka; Salmonson, Jay
2010-10-01
We performed 2D, axisymmetric, MHD simulations with Cosmos++ in order to examine the growth of the magnetorotational instability (MRI) in core-collapse supernovae. We have initialized a non-rotating 15 Msolar progenitor, infused with differential rotation and poloidal magnetic fields. The collapse of the iron core is simulated with the Shen EOS, and the parametric Ye and entropy evolution. The wavelength of the unstable mode in the post-collapse environment is expected to be only ~200 m. In order to achieve the fine spatial resolution requirement, we employed remapping technique after the iron core has collapsed and bounced. The MRI unstable region appears near the equator and angular momentum and entropy are transported outward. Higher resolution remap run display more vigorous overturns and stronger transport of angular momentum and entropy. Our results are in agreement with the earlier work by Akiyama et al. [1] and Obergaulinger et al. [2].
MHD Simulations of Disruptions in NSTX
NASA Astrophysics Data System (ADS)
Breslau, J. A.; Strauss, H. R.; Paccagnella, R.; Jardin, S. C.
2011-10-01
Research tokamaks such as ITER must be designed to tolerate a limited number of disruptions without sustaining significant damage. It is therefore vital to have numerical tools that can accurately predict the effects of these events. The 3D nonlinear extended MHD code M3D has been augmented with models of the vacuum/halo region and a thin axisymmetric resistive shell that allow it to simulate disruptions and calculate the associated wall currents and forces. Its reliability, however, must be assessed with careful validation studies against disruption databases from existing experiments. Here we compare M3D VDE/kink disruption calculations with data from NSTX. The results of high-resolution numerical simulations at realistic Lundquist numbers show reasonable agreement with experimental data and provide confidence that M3D will be a useful tool for future ITER calculations. The effects of different choices of plasma outflow boundary conditions will also be reported.
Spatially Resolved Synthetic Spectra from 2D Simulations of Stainless Steel Wire Array Implosions
Clark, R. W.; Giuliani, J. L.; Thornhill, J. W.; Chong, Y. K.; Dasgupta, A.; Davis, J.
2009-01-21
A 2D radiation MHD model has been developed to investigate stainless steel wire array implosion experiments on the Z and refurbished Z machines. This model incorporates within the Mach2 MHD code a self-consistent calculation of the non-LTE kinetics and ray trace based radiation transport. Such a method is necessary in order to account for opacity effects in conjunction with ionization kinetics of K-shell emitting plasmas. Here the model is used to investigate multi-dimensional effects of stainless steel wire implosions. In particular, we are developing techniques to produce non-LTE, axially and/or radially resolved synthetic spectra based upon snapshots of our 2D simulations. Comparisons between experimental spectra and these synthetic spectra will allow us to better determine the state of the experimental pinches.
Global MHD simulations of Neptune's magnetosphere
NASA Astrophysics Data System (ADS)
Mejnertsen, L.; Eastwood, J. P.; Chittenden, J. P.; Masters, A.
2016-08-01
A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large-scale reconfiguration of magnetic topology and plasma distribution. During the "pole-on" magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole-on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning "off" and "on" when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations.
MHD simulation of a magnetized target in an imploding conical cavity
NASA Astrophysics Data System (ADS)
Li, Cheng
2015-11-01
Conical imploding magnetic target fusion (MTF) is a new concept, in which the compression comes from both fast mechanic implosion and synchronized theta-pinching or Z-pinching. The compressed magnetized target has a moving end, an increasing external current, and an accumulating high density. Magneto-hydrodynamics (MHD) simulation could help revealing the details of the evolving plasma and finding the parameters (imploding speed, fuel amount, theta-pinch or Z-pinch current profile, etc.) required to reach Lawson Criterion. Preliminary 2D MHD simulation results of a conical imploding theta-pinch are presented.
3D MHD disruptions simulations of tokamaks plasmas
NASA Astrophysics Data System (ADS)
Paccagnella, Roberto; Strauss, Hank; Breslau, Joshua
2008-11-01
Tokamaks Vertical Displacement Events (VDEs) and disruptions simulations in toroidal geometry by means of a single fluid visco-resistive magneto-hydro-dynamic (MHD) model are presented in this paper. The plasma model, implemented in the M3D code [1], is completed with the presence of a 2D homogeneous wall with finite resistivity. This allows the study of the relatively slowly growing magneto-hydro-dynamical perturbation, the resistive wall mode (RWM), which is, in this work, the main drive of the disruptions. Amplitudes and asymmetries of the halo currents pattern at the wall are also calculated and comparisons with tokamak experimental databases and predictions for ITER are given. [1] W. Park, E.V. Belova, G.Y. Fu, X.Z. Tang, H.R. Strauss, L.E. Sugiyama, Phys. Plasmas 6 (1999) 1796.
Simulations of Pulse Detonation Engines with MHD Thrust Augmentation
NASA Astrophysics Data System (ADS)
Zeineh, Christopher; Roth, Timothy; Cole, Lord; Karagozian, Ann; Cambier, Jean-Luc
2008-11-01
Pulse detonation rocket engines (PDREs) have received significant attention in recent years due to their potentially superior performance over constant-pressure engines. Yet unsteady chamber pressures cause the PDRE flow to be either over-expanded or under-expanded for the majority of the cycle, with substantial performance loss in atmospheric flight applications. The present computational studies examine the potential benefits of using magneto-hydrodynamic (MHD) thrust augmentation by extracting energy via a generator in the PDRE nozzle and applying it to a separate, secondary stream. In the present studies, which involve both transient quasi-1D and 2D numerical simulations, the energy extracted from the nozzle flow is directly applied to a by-pass air stream through an MHD accelerator. The air stream is first shocked by the under-expanded nozzle flow and raised to high temperature, allowing thermal ionization. The specific conditions for thrust augmentation are examined. Alternative configurations utilizing a magnetic piston in the PDRE chamber are also explored. Results show potential performance gains but with significant challenges, depending on the operating and flight conditions.
2D and 3D Numerical Simulations of Flux Cancellation
NASA Technical Reports Server (NTRS)
Karpen, Judith T.; DeVore, C.; Antiochos, S. K.; Linton, M. G.
2009-01-01
Cancellation of magnetic flux in the solar photosphere and chromosphere has been linked observationally and theoretically to a broad range of solar activity, from filament channel formation to CME initiation. Because this phenomenon is typically measured at only a single layer in the atmosphere, in the radial (line of sight) component of the magnetic field, the actual processes behind this observational signature are ambiguous. It is clear that reconnection is involved in some way, but the location of the reconnection sites and associated connectivity changes remain uncertain in most cases. We are using numerical modeling to demystify flux cancellation, beginning with the simplest possible configuration: a subphotospheric Lundquist flux tube surrounded by a potential field, immersed in a gravitationally stratified atmosphere, spanning many orders of magnitude in plasma beta. In this system, cancellation is driven slowly by a 2-cell circulation pattern imposed in the convection zone, such that the tops of the cells are located around the beta=1 level (i.e., the photosphere) and the flows converge and form a downdraft at the polarity inversion line; note however that no flow is imposed along the neutral line. We will present the results of 2D and 3D MHD-AMR simulations of flux cancellation, in which the flux at the photosphere begins in either an unsheared or sheared state. In all cases, a low-lying flux rope is formed by reconnection at the polarity inversion line within a few thousand seconds. The flux rope remains stable and does not rise, however, in contrast to models which do not include the presence of significant mass loading.
Global MHD simulations of plasmaspheric plumes
NASA Astrophysics Data System (ADS)
Lyon, J.; Ouellette, J.; Merkin, V. G.
2015-12-01
The plasmasphere represents a separate population from the rest of themagnetosphere, generally high density but cold. When the solar windturns strongly southward this plasma is convected toward the daysidemagnetopause and affects the interaction of the solar wind with themagnetosphere. We have used multi-fluid simulations using the LFMglobal MHD code to model this interaction. The plasmasphere isinitialized as a cold (~1eV) hydrogen plasma in a quiet northward IMFstate with a density distribution appropriate for K_p = 1. Thecorotation potential from the ionosphere spins up the plasmasphereinto rough corotation. After a initialization period of hours, asouthward IMF is introduced and the enhanced convection initiates asurge of plasmaspheric density to the dayside. We discuss two aspectsof this interaction, the effects on dayside reconnection and on theKelvin-Helmholtz instability (KHI). We find that the mass loading ofmagnetospheric flux tubes slows local reconnection rates, though notas much as predicted by Borovsky et al. [2013]. We findthat the total reconnection rate is reduced, although not as much aswould be predicted by just the sub-solar reconnection rate. The KHIis somewhat reduced by the plasmaspheric loading of density in the lowlatitude boundary layer. It has been suggested that the presence ofthe plasmasphere may lead to enhanced ULF wave power in the interiorof the magnetosphere from the KHI waves. We find only a minimal effect during northward IMF. For southward IMF, the situation is complicated by the interaction of KHI with non-steady reconnection.
NASA Astrophysics Data System (ADS)
Griton, Léa; Pantellini, Filippo; Moncuquet, Michel
2016-04-01
We present 3D simulations of the interaction of the solar wind with Mercury's magnetosphere using the magnetohydrodynamic code AMRVAC. A procedure for the identification of standing MHD modes has been applied to these simulations showing that large scale standing slow mode structures may exist in Mercury's magnetosheath. The identification is mostly based on relatively simple approximate analytical solutions to the old problem of determining the family of all standing linear plane MHD waves in a flowing plasma. The question of the identification of standing slow mode structures using in situ measurements such as the future BepiColombo MMO mission to Mercury will be discussed as well.
NASA Technical Reports Server (NTRS)
Wu, S. T.; Song, M. T.; Martens, P. C. H.; Dryer, M.
1991-01-01
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.
NASA Astrophysics Data System (ADS)
Zhai, Cuili; Zhang, Ting
2016-09-01
In this article, we consider the global existence and uniqueness of the solution to the 2D incompressible non-resistive MHD system with non-equilibrium background magnetic field. Our result implies that a strong enough non-equilibrium background magnetic field will guarantee the stability of the nonlinear MHD system. Beside the classical energy method, the interpolation inequalities and the algebraic structure of the equations coming from the incompressibility of the fluid are crucial in our arguments.
2D Radiation MHD K-shell Modeling of Single Wire Array Stainless Steel Experiments on the Z Machine
Thornhill, J. W.; Giuliani, J. L.; Apruzese, J. P.; Chong, Y. K.; Davis, J.; Dasgupta, A.; Whitney, K. G.; Clark, R. W.; Jones, B.; Coverdale, C. A.; Ampleford, D. J.; Cuneo, M. E.; Deeney, C.
2009-01-21
Many physical effects can produce unstable plasma behavior that affect K-shell emission from arrays. Such effects include: asymmetry in the initial density profile, asymmetry in power flow, thermal conduction at the boundaries, and non-uniform wire ablation. Here we consider how asymmetry in the radiation field also contributes to the generation of multidimensional plasma behavior that affects K-shell power and yield. To model this radiation asymmetry, we have incorporated into the MACH2 r-z MHD code a self-consistent calculation of the non-LTE population kinetics based on radiation transport using multi-dimensional ray tracing. Such methodology is necessary for modeling the enhanced radiative cooling that occurs at the anode and cathode ends of the pinch during the run-in phase of the implosion. This enhanced radiative cooling is due to reduced optical depth at these locations producing an asymmetric flow of radiative energy that leads to substantial disruption of large initial diameter (>5 cm) pinches and drives 1D into 2D fluid (i.e., Rayleigh-Taylor like) flows. The impact of this 2D behavior on K-shell power and yield is investigated by comparing 1D and 2D model results with data obtained from a series of single wire array stainless steel experiments performed on the Z generator.
Three Dimensional Simulations of Compressible Hall MHD Plasmas
Shaikh, Dastgeer; Shukla, P. K.
2008-10-15
We have developed three dimensional, time dependent, compressible, non-adiabatic, driven and massively parallelized Hall magnetohydrodynamic (MHD) simulations to investigate turbulent spectral cascades in a regime where characteristic lengthscales associated with plasma fluctuations are smaller than ion gyro radii. Such regime is ubiquitously present in solar wind and many other collisionless space plasmas. Particularly in the solar wind, the high time resolution databases identify a spectral break at the end of MHD inertial range spectrum that corresponds to a high frequency regime. In the regime, turbulent cascades cannot be explained by the usual MHD models. With the help of our 3D Hall MHD code, we find that characteristic turbulent interactions in the high frequency regime evolve typically on kinetic Alfven time scales. The turbulent fluctuation associated with kinetic Alfven interactions are compressive and anisotropic and possess equipartition of kinetic and magnetic energies.
NASA Astrophysics Data System (ADS)
Sertoli, M.; Horváth, L.; Pokol, G. I.; Igochine, V.; Barrera, L.
2013-05-01
A new method for the reconstruction of two-dimensional (2D) electron temperature profiles in the presence of saturated magneto-hydro-dynamic (MHD) modes from the one-dimensional (1D) electron cyclotron emission (ECE) diagnostic is presented. The analysis relies on harmonic decomposition of the electron temperature oscillations through short time Fourier transforms and requires rigid poloidal mode rotation as the only assumption. The method is applicable to any magnetic perturbation as long as the poloidal and toroidal mode numbers m and n are known. Its application to the case of a (m, n) = (1, 1) internal kink mode on ASDEX Upgrade is presented and a new way to estimate the mode displacement is explained. For such modes, it is shown that the higher order harmonics usually visible in the ECE spectrogram arise also for the pure m = n = 1 mode and that they cannot be directly associated with m = n > 1 magnetic perturbations. This method opens up new possibilities for electron heat transport studies in the presence of saturated MHD modes and a way to disentangle the impurity density contributions from electron temperature effects in the analysis of the soft x-ray data.
Kinetic MHD simulation of large 'circ; tearing mode
NASA Astrophysics Data System (ADS)
Cheng, Jianhua; Chen, Yang; Parker, Scott; Uzdensky, Dmitri
2012-03-01
We have developed a second-order accurate semi-implicit δ method for kinetic MHD simulation with Lorentz force ions and fluid electrons. The model has been used to study the resistive tearing mode instability, which involves multiple spatial scales. In small 'circ; cases, the linear growth rate and eigenmode structure are consistent with resistive MHD analysis. The Rutherford stage and saturation are demonstrated, but the simulation exhibits different saturation island widths compared with previous MHD simulations. In large 'circ; cases, nonlinear simulations show multiple islands forming, followed by the islands coalescing at later times. The competition between these two processes strongly influences the reconnection rates and eventually leads to a steady state reconnection. We will present various parameter studies and show that our hybrid results agree with fluid analysis in certain limits (e.g., relatively large resisitivities).
Classical MHD shocks: theory and numerical simulation
Pogorelov, Nikolai V.
2005-08-01
Recent results are surveyed in the investigation of the behavior of shocks in ideal magnetohydrodynamics (MHD) and corresponding structures in dissipative/resistive plasma flows. In contrast to evolutionary shocks, a solution of the problem of the nonevolutionary shock interaction with small perturbations is either nonunique or does not exist. The peculiarity of non-ideal MHD is in that some nonevolutionary shocks have dissipative structures. Since this structure is always non-plane, it can reveal itself in problems where transverse perturbations do not exist due to symmetries restrictions. We discuss the numerical behavior of nonevolutionary shocks and argue that they necessarily disappear once the problem is solved in a genuinely three-dimensional statement.
GMC COLLISIONS AS TRIGGERS OF STAR FORMATION. I. PARAMETER SPACE EXPLORATION WITH 2D SIMULATIONS
Wu, Benjamin; Loo, Sven Van; Tan, Jonathan C.; Bruderer, Simon
2015-09-20
We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model for giant molecular cloud (GMC)–GMC collisions between nearly magnetically critical clouds. The goal is to determine if, and under what circumstances, cloud collisions can cause pre-existing magnetically subcritical clumps to become supercritical and undergo gravitational collapse. We first develop and implement new photodissociation region based heating and cooling functions that span the atomic to molecular transition, creating a multiphase ISM and allowing modeling of non-equilibrium temperature structures. Then in 2D and with ideal MHD, we explore a wide parameter space of magnetic field strength, magnetic field geometry, collision velocity, and impact parameter and compare isolated versus colliding clouds. We find factors of ∼2–3 increase in mean clump density from typical collisions, with strong dependence on collision velocity and magnetic field strength, but ultimately limited by flux-freezing in 2D geometries. For geometries enabling flow along magnetic field lines, greater degrees of collapse are seen. We discuss observational diagnostics of cloud collisions, focussing on {sup 13}CO(J = 2–1), {sup 13}CO(J = 3–2), and {sup 12}CO(J = 8–7) integrated intensity maps and spectra, which we synthesize from our simulation outputs. We find that the ratio of J = 8–7 to lower-J emission is a powerful diagnostic probe of GMC collisions.
2D radiation-magnetohydrodynamic simulations of SATURN imploding Z-pinches
Hammer, J.H.; Eddleman, J.L.; Springer, P.T.
1995-11-06
Z-pinch implosions driven by the SATURN device at Sandia National Laboratory are modeled with a 2D radiation magnetohydrodynamic (MHD) code, showing strong growth of magneto-Rayleigh Taylor (MRT) instability. Modeling of the linear and nonlinear development of MRT modes predicts growth of bubble-spike structures that increase the time span of stagnation and the resulting x-ray pulse width. Radiation is important in the pinch dynamics keeping the sheath relatively cool during the run-in and releasing most of the stagnation energy. The calculations give x-ray pulse widths and magnitudes in reasonable agreement with experiments, but predict a radiating region that is too dense and radially localized at stagnation. We also consider peaked initial density profiles with constant imploding sheath velocity that should reduce MRT instability and improve performance. 2D krypton simulations show an output x-ray power > 80 TW for the peaked profile.
First results from ideal 2-D MHD reconstruction: magnetopause reconnection event seen by Cluster
NASA Astrophysics Data System (ADS)
Teh, W.-L.; Ã-. Sonnerup, B. U.
2008-09-01
We have applied a new reconstruction method (Sonnerup and Teh, 2008), based on the ideal single-fluid MHD equations in a steady-state, two-dimensional geometry, to a reconnection event observed by the Cluster-3 (C3) spacecraft on 5 July 2001, 06:23 UT, at the dawn-side Northern-Hemisphere magnetopause. The event has been previously studied by use of Grad-Shafranov (GS) reconstruction, performed in the deHoffmann-Teller frame, and using the assumption that the flow effects were either negligible or the flow was aligned with the magnetic field. Our new method allows the reconstruction to be performed in the frame of reference moving with the reconnection site (the X-line). In the event studied, this motion is tailward/equatorward at 140 km/s. The principal result of the study is that the new method functions well, generating a magnetic field map that is qualitatively similar to those obtained in the earlier GS-based reconstructions but now includes the reconnection site itself. In comparison with the earlier map by Hasegawa et al. (2004), our new map has a slightly improved ability (cc=0.979 versus cc=0.975) to predict the fields measured by the other three Cluster spacecraft, at distances from C3 ranging from 2132 km (C1) to 2646 km (C4). The new field map indicates the presence of a magnetic X-point, located some 5300 km tailward/equatorward of C3 at the time of its traversal of the magnetopause. In the immediate vicinity of the X-point, the ideal-MHD assumption breaks down, i.e. resistive and/or other effects should be included. We have circumvented this problem by an ad-hoc procedure in which we allow the axial part of convection electric field to be non-constant near the reconnection site. The new reconstruction method also provides a map of the velocity field, in which the inflow into the wedge of reconnected field lines and the plasma jet within it can be seen, and maps of the electric potential and of the electric current distribution. Even though the
Simulating MEMS Chevron Actuator for Strain Engineering 2D Materials
NASA Astrophysics Data System (ADS)
Vutukuru, Mounika; Christopher, Jason; Bishop, David; Swan, Anna
2D materials pose an exciting paradigm shift in the world of electronics. These crystalline materials have demonstrated high electric and thermal conductivities and tensile strength, showing great potential as the new building blocks of basic electronic circuits. However, strain engineering 2D materials for novel devices remains a difficult experimental feat. We propose the integration of 2D materials with MEMS devices to investigate the strain dependence on material properties such as electrical and thermal conductivity, refractive index, mechanical elasticity, and band gap. MEMS Chevron actuators, provides the most accessible framework to study strain in 2D materials due to their high output force displacements for low input power. Here, we simulate Chevron actuators on COMSOL to optimize actuator design parameters and accurately capture the behavior of the devices while under the external force of a 2D material. Through stationary state analysis, we analyze the response of the device through IV characteristics, displacement and temperature curves. We conclude that the simulation precisely models the real-world device through experimental confirmation, proving that the integration of 2D materials with MEMS is a viable option for constructing novel strain engineered devices. The authors acknowledge support from NSF DMR1411008.
Hall MHD Simulations of Comet 67P/Churyumov-Gerasimenko
NASA Astrophysics Data System (ADS)
Shou, Y.; Combi, M. R.; Rubin, M.; Hansen, K. C.; Toth, G.; Gombosi, T. I.
2012-12-01
Comets have highly eccentric orbits and a wide range of gas production rates and thus they are ideal subjects to study the interaction between the solar wind and nonmagnetized bodies. Hansen et al. (2007, Space Sci. Rev. 128, 133) used a fluid-based MHD model and a semi-kinetic hybrid particle model to study the plasma environment of comet 67P/Churyumov-Gerasimenko (CG), the Rosetta mission target comet, at different heliocentric distances. They showed that for such a weak comet at a large heliocentric distance, the length scales of the cometosheath and the bow shock are comparable to or smaller than the ion gyroradius, which violates the underlying assumption for a valid fluid description of the plasma. As a result, the classical ideal MHD model is not able to always give physical results, while the hybrid model, which accounts for the kinetic effects of ions with both cometary and solar wind origin, is more reliable. However, hybrid models are computationally expensive and the results can be noisy. A compromise approach is Hall MHD [Toth et al., 2008], which includes the Hall term in the MHD equations and allows for the decoupling of the ion and electron fluids. We use a single ion species Hall MHD model to simulate the plasma environment of comet 67P/CG and compare the results with the two models mentioned above. We find that the Hall effect is capable of reproducing some features of the hybrid model and thus extends the applicability of MHD. In addition, this study helps to identify the conditions and regions in the cometary plasma where the Hall effect is not negligible. This work is supported by NSF Planetary Astronomy grant AST0707283 and JPL subcontract 1266313 under NASA grant NMO710889.
Simulation of 2D Fields of Raindrop Size Distributions
NASA Astrophysics Data System (ADS)
Berne, A.; Schleiss, M.; Uijlenhoet, R.
2008-12-01
The raindrop size distribution (DSD hereafter) is of primary importance for quantitative applications of weather radar measurements. The radar reflectivity~Z (directly measured by radar) is related to the power backscattered by the ensemble of hydrometeors within the radar sampling volume. However, the rain rate~R (the flux of water to the surface) is the variable of interest for many applications (hydrology, weather forecasting, air traffic for example). Usually, radar reflectivity is converted into rain rate using a power law such as Z=aRb. The coefficients a and b of the Z-R relationship depend on the DSD. The variability of the DSD in space and time has to be taken into account to improve radar rain rate estimates. Therefore, the ability to generate a large number of 2D fields of DSD which are statistically homogeneous provides a very useful simulation framework that nicely complements experimental approaches based on DSD data, in order to investigate radar beam propagation through rain as well as radar retrieval techniques. The proposed approach is based on geostatistics for structural analysis and stochastic simulation. First, the DSD is assumed to follow a gamma distribution. Hence a 2D field of DSDs can be adequately described as a 2D field of a multivariate random function consisting of the three DSD parameters. Such fields are simulated by combining a Gaussian anamorphosis and a multivariate Gaussian random field simulation algorithm. Using the (cross-)variogram models fitted on data guaranties that the spatial structure of the simulated fields is consistent with the observed one. To assess its validity, the proposed method is applied to data collected during intense Mediterranean rainfall. As only time series are available, Taylor's hypothesis is assumed to convert time series in 1D range profile. Moreover, DSD fields are assumed to be isotropic so that the 1D structure can be used to simulate 2D fields. A large number of 2D fields of DSD parameters are
EVIDENCE OF ACTIVE MHD INSTABILITY IN EULAG-MHD SIMULATIONS OF SOLAR CONVECTION
Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul E-mail: strugarek@astro.umontreal.ca
2015-11-10
We investigate the possible development of magnetohydrodynamical instabilities in the EULAG-MHD “millennium simulation” of Passos and Charbonneau. This simulation sustains a large-scale magnetic cycle characterized by solar-like polarity reversals taking place on a regular multidecadal cadence, and in which zonally oriented bands of strong magnetic fields accumulate below the convective layers, in response to turbulent pumping from above in successive magnetic half-cycles. Key aspects of this simulation include low numerical dissipation and a strongly sub-adiabatic fluid layer underlying the convectively unstable layers corresponding to the modeled solar convection zone. These properties are conducive to the growth and development of two-dimensional instabilities that are otherwise suppressed by stronger dissipation. We find evidence for the action of a non-axisymmetric magnetoshear instability operating in the upper portions of the stably stratified fluid layers. We also investigate the possibility that the Tayler instability may be contributing to the destabilization of the large-scale axisymmetric magnetic component at high latitudes. On the basis of our analyses, we propose a global dynamo scenario whereby the magnetic cycle is driven primarily by turbulent dynamo action in the convecting layers, but MHD instabilities accelerate the dissipation of the magnetic field pumped down into the overshoot and stable layers, thus perhaps significantly influencing the magnetic cycle period. Support for this scenario is found in the distinct global dynamo behaviors observed in an otherwise identical EULAG-MHD simulations, using a different degree of sub-adiabaticity in the stable fluid layers underlying the convection zone.
Evidence of Active MHD Instability in EULAG-MHD Simulations of Solar Convection
NASA Astrophysics Data System (ADS)
Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul
2015-11-01
We investigate the possible development of magnetohydrodynamical instabilities in the EULAG-MHD “millennium simulation” of Passos & Charbonneau. This simulation sustains a large-scale magnetic cycle characterized by solar-like polarity reversals taking place on a regular multidecadal cadence, and in which zonally oriented bands of strong magnetic fields accumulate below the convective layers, in response to turbulent pumping from above in successive magnetic half-cycles. Key aspects of this simulation include low numerical dissipation and a strongly sub-adiabatic fluid layer underlying the convectively unstable layers corresponding to the modeled solar convection zone. These properties are conducive to the growth and development of two-dimensional instabilities that are otherwise suppressed by stronger dissipation. We find evidence for the action of a non-axisymmetric magnetoshear instability operating in the upper portions of the stably stratified fluid layers. We also investigate the possibility that the Tayler instability may be contributing to the destabilization of the large-scale axisymmetric magnetic component at high latitudes. On the basis of our analyses, we propose a global dynamo scenario whereby the magnetic cycle is driven primarily by turbulent dynamo action in the convecting layers, but MHD instabilities accelerate the dissipation of the magnetic field pumped down into the overshoot and stable layers, thus perhaps significantly influencing the magnetic cycle period. Support for this scenario is found in the distinct global dynamo behaviors observed in an otherwise identical EULAG-MHD simulations, using a different degree of sub-adiabaticity in the stable fluid layers underlying the convection zone.
3D MHD VDE and disruptions simulations of tokamaks plasmas including some ITER scenarios
NASA Astrophysics Data System (ADS)
Paccagnella, R.; Strauss, H. R.; Breslau, J.
2009-03-01
Tokamaks vertical displacement events (VDEs) and disruptions simulations in toroidal geometry by means of a single fluid visco-resistive magneto-hydro-dynamic (MHD) model are presented in this paper. The plasma model is completed with the presence of a 2D wall with finite resistivity which allows the study of the relatively slowly growing magnetic perturbation, the resistive wall mode (RWM), which is, in this paper, the main drive of the disruption evolution. Amplitudes and asymmetries of the halo currents pattern at the wall are also calculated and comparisons with tokamak experimental databases and predictions for ITER are given.
Coupling MHD Simulations of CMEs to SEP Models
NASA Astrophysics Data System (ADS)
Torok, T.; Gorby, M.; Linker, J.; Schwadron, N.
2015-12-01
Large Solar Energetic Particle events (SEPs) are a main space weather hazard and extremely dangerous to astronauts and electronic equipmentin space. They are typically associated with fast Coronal Mass Ejections (CMEs). Recent results indicate that SEPs can be generated already inthe early phase of CME expansion low in the corona, but the underlyingphysical mechanisms are not yet well understood. State-of-the-artmagnetohydrodynamic (MHD) simulations of CME initiation and evolution,combined with numerical models of particle acceleration and propagation,provide a powerful tool to investigate these mechanisms. In this talk, we present recent developments in the coupling of CORHEL/MAS thermodynamicMHD simulations of fast CMEs to the EPREM particle code, and we discuss the insights that can be gained from such a combined modeling approach.
Simulation of bootstrap current in 2D and 3D ideal magnetic fields in tokamaks
NASA Astrophysics Data System (ADS)
Raghunathan, M.; Graves, J. P.; Cooper, W. A.; Pedro, M.; Sauter, O.
2016-09-01
We aim to simulate the bootstrap current for a MAST-like spherical tokamak using two approaches for magnetic equilibria including externally caused 3D effects such as resonant magnetic perturbations (RMPs), the effect of toroidal ripple, and intrinsic 3D effects such as non-resonant internal kink modes. The first approach relies on known neoclassical coefficients in ideal MHD equilibria, using the Sauter (Sauter et al 1999 Phys. Plasmas 6 2834) expression valid for all collisionalities in axisymmetry, and the second approach being the quasi-analytic Shaing–Callen (Shaing and Callen 1983 Phys. Fluids 26 3315) model in the collisionless regime for 3D. Using the ideal free-boundary magnetohydrodynamic code VMEC, we compute the flux-surface averaged bootstrap current density, with the Sauter and Shaing–Callen expressions for 2D and 3D ideal MHD equilibria including an edge pressure barrier with the application of resonant magnetic perturbations, and equilibria possessing a saturated non-resonant 1/1 internal kink mode with a weak internal pressure barrier. We compare the applicability of the self-consistent iterative model on the 3D applications and discuss the limitations and advantages of each bootstrap current model for each type of equilibrium.
Simulation of bootstrap current in 2D and 3D ideal magnetic fields in tokamaks
NASA Astrophysics Data System (ADS)
Raghunathan, M.; Graves, J. P.; Cooper, W. A.; Pedro, M.; Sauter, O.
2016-09-01
We aim to simulate the bootstrap current for a MAST-like spherical tokamak using two approaches for magnetic equilibria including externally caused 3D effects such as resonant magnetic perturbations (RMPs), the effect of toroidal ripple, and intrinsic 3D effects such as non-resonant internal kink modes. The first approach relies on known neoclassical coefficients in ideal MHD equilibria, using the Sauter (Sauter et al 1999 Phys. Plasmas 6 2834) expression valid for all collisionalities in axisymmetry, and the second approach being the quasi-analytic Shaing-Callen (Shaing and Callen 1983 Phys. Fluids 26 3315) model in the collisionless regime for 3D. Using the ideal free-boundary magnetohydrodynamic code VMEC, we compute the flux-surface averaged bootstrap current density, with the Sauter and Shaing-Callen expressions for 2D and 3D ideal MHD equilibria including an edge pressure barrier with the application of resonant magnetic perturbations, and equilibria possessing a saturated non-resonant 1/1 internal kink mode with a weak internal pressure barrier. We compare the applicability of the self-consistent iterative model on the 3D applications and discuss the limitations and advantages of each bootstrap current model for each type of equilibrium.
Spectral Methods in General Relativistic MHD Simulations
NASA Astrophysics Data System (ADS)
Garrison, David
2012-03-01
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.
3D MHD Simulations of Spheromak Compression
NASA Astrophysics Data System (ADS)
Stuber, James E.; Woodruff, Simon; O'Bryan, John; Romero-Talamas, Carlos A.; Darpa Spheromak Team
2015-11-01
The adiabatic compression of compact tori could lead to a compact and hence low cost fusion energy system. The critical scientific issues in spheromak compression relate both to confinement properties and to the stability of the configuration undergoing compression. We present results from the NIMROD code modified with the addition of magnetic field coils that allow us to examine the role of rotation on the stability and confinement of the spheromak (extending prior work for the FRC). We present results from a scan in initial rotation, from 0 to 100km/s. We show that strong rotational shear (10km/s over 1cm) occurs. We compare the simulation results with analytic scaling relations for adiabatic compression. Work performed under DARPA grant N66001-14-1-4044.
MHD Simulations of Plasma Dynamics with Non-Axisymmetric Boundaries
NASA Astrophysics Data System (ADS)
Hansen, Chris; Levesque, Jeffrey; Morgan, Kyle; Jarboe, Thomas
2015-11-01
The arbitrary geometry, 3D extended MHD code PSI-TET is applied to linear and non-linear simulations of MCF plasmas with non-axisymmetric boundaries. Progress and results from simulations on two experiments will be presented: 1) Detailed validation studies of the HIT-SI experiment with self-consistent modeling of plasma dynamics in the helicity injectors. Results will be compared to experimental data and NIMROD simulations that model the effect of the helicity injectors through boundary conditions on an axisymmetric domain. 2) Linear studies of HBT-EP with different wall configurations focusing on toroidal asymmetries in the adjustable conducting wall. HBT-EP studies the effect of active/passive stabilization with an adjustable ferritic wall. Results from linear verification and benchmark studies of ideal mode growth with and without toroidal asymmetries will be presented and compared to DCON predictions. Simulations of detailed experimental geometries are enabled by use of the PSI-TET code, which employs a high order finite element method on unstructured tetrahedral grids that are generated directly from CAD models. Further development of PSI-TET will also be presented including work to support resistive wall regions within extended MHD simulations. Work supported by DoE.
Multifluid MHD Simulation of Saturn's Interchange Fingers
NASA Astrophysics Data System (ADS)
Lucas, N.; Rajendar, A.; Paty, C. S.
2014-12-01
Saturn's magnetosphere exhibits rich dynamics that have only become apparent through recent missions such as the Cassini mission currently in progress. Examining local time variations in the magnetosphere has shown some interesting phenomena. One of the primary expressions of the dynamics we observe in Saturn's magnetosphere are plasma interchange fingers. These fingers carry hot plasma from the outer magnetosphere to the inner magnetosphere to balance magnetic flux lost due to outward radial transport of cold dense plasma sourced from the neutral cloud. This process leads to a mixing of hot and cold plasma throughout the magnetosphere. Understanding how mass interchange fingers form and quantifying how the plasma they contain is heated and transported will be important for understanding other dynamic processes occurring in the magnetosphere. In this study, we will be using our existing multifluid simulation of Saturn's magnetosphere in combination with data from the Cassini mission in order to investigate the formation of plasma interchange fingers and their dynamics. Our results will be compared with observations as well as previous modeling studies of Saturn's interchange fingers.
Analysis and gyrokinetic simulation of MHD Alfven wave interactions
NASA Astrophysics Data System (ADS)
Nielson, Kevin Derek
The study of low-frequency turbulence in magnetized plasmas is a difficult problem due to both the enormous range of scales involved and the variety of physics encompassed over this range. Much of the progress that has been made in turbulence theory is based upon a result from incompressible magnetohydrodynamics (MHD), in which energy is only transferred from large scales to small via the collision of Alfven waves propagating oppositely along the mean magnetic field. Improvements in laboratory devices and satellite measurements have demonstrated that, while theories based on this premise are useful over inertial ranges, describing turbulence at scales that approach particle gyroscales requires new theory. In this thesis, we examine the limits of incompressible MHD theory in describing collisions between pairs of Alfven waves. This interaction represents the fundamental unit of plasma turbulence. To study this interaction, we develop an analytic theory describing the nonlinear evolution of interacting Alfven waves and compare this theory to simulations performed using the gyrokinetic code AstroGK. Gyrokinetics captures a much richer set of physics than that described by incompressible MHD, and is well-suited to describing Alfvenic turbulence around the ion gyroscale. We demonstrate that AstroGK is well suited to the study of physical Alfven waves by reproducing laboratory Alfven dispersion data collected using the LAPD. Additionally, we have developed an initialization alogrithm for use with AstroGK that allows exact Alfven eigenmodes to be initialized with user specified amplitudes and phases. We demonstrate that our analytic theory based upon incompressible MHD gives excellent agreement with gyrokinetic simulations for weakly turbulent collisions in the limit that k⊥rho i << 1. In this limit, agreement is observed in the time evolution of nonlinear products, and in the strength of nonlinear interaction with respect to polarization and scale. We also examine the
Numerical simulation of rock cutting using 2D AUTODYN
NASA Astrophysics Data System (ADS)
Woldemichael, D. E.; Rani, A. M. Abdul; Lemma, T. A.; Altaf, K.
2015-12-01
In a drilling process for oil and gas exploration, understanding of the interaction between the cutting tool and the rock is important for optimization of the drilling process using polycrystalline diamond compact (PDC) cutters. In this study the finite element method in ANSYS AUTODYN-2D is used to simulate the dynamics of cutter rock interaction, rock failure, and fragmentation. A two-dimensional single PDC cutter and rock model were used to simulate the orthogonal cutting process and to investigate the effect of different parameters such as depth of cut, and back rake angle on two types of rocks (sandstone and limestone). In the simulation, the cutting tool was dragged against stationary rock at predetermined linear velocity and the depth of cut (1,2, and 3 mm) and the back rake angles(-10°, 0°, and +10°) were varied. The simulation result shows that the +10° back rake angle results in higher rate of penetration (ROP). Increasing depth of cut leads to higher ROP at the cost of higher cutting force.
Relativistic MHD simulations of stellar core collapse and magnetars
NASA Astrophysics Data System (ADS)
Font, José A.; Cerdá-Durán, Pablo; Gabler, Michael; Müller, Ewald; Stergioulas, Nikolaos
2011-02-01
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.
MHD simulation studies of z-pinch shear flow stabilization
NASA Astrophysics Data System (ADS)
Paraschiv, I.; Bauer, B. S.; Sotnikov, V. I.; Makhin, V.; Siemon, R. E.
2003-10-01
The development of the m=0 instability in a z-pinch in the presence of sheared plasma flows is investigated with the aid of a two-dimensional magnetohydrodynamic (MHD) simulation code (MHRDR). The linear growth rates are compared to the results obtained by solving the ideal MHD linearized equations [1] and to the results obtained using a 3D hybrid simulation code [2]. The instability development is followed into the nonlinear regime where its growth and saturation are examined. [1] V.I. Sotnikov, I. Paraschiv, V. Makhin, B.S. Bauer, J.-N. Leboeuf, and J.M. Dawson, "Linear analysis of sheared flow stabilization of global magnetohydrodynamic instabilities based on the Hall fluid mode", Phys. Plasmas 9, 913 (2002). [2] V.I. Sotnikov, V. Makhin, B.S. Bauer, P. Hellinger, P. Travnicek, V. Fiala, J.-N. Leboeuf, "Hybrid Simulations of Current-Carrying Instabilities in Z-pinch Plasmas with Sheared Axial Flow", AIP Conference Proceedings, Volume 651, Dense Z-Pinches: 5th International Conference on Dense Z-Pinches, edited by J. Davis et al., page 396, June 2002.
MHD simulations of three-dimensional resistive reconnection in a cylindrical plasma column
NASA Astrophysics Data System (ADS)
Striani, E.; Mignone, A.; Vaidya, B.; Bodo, G.; Ferrari, A.
2016-11-01
Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as S1/2 (where S is the Lundquist number) and yields time-scales several order of magnitude larger than the observed ones. Earlier two-dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of S (`fast reconnection' regime) due to the presence of the secondary tearing instability that takes place for S ≳ 1 × 104. We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at S ≃ 1 × 103.
Understanding Accretion Disks through Three Dimensional Radiation MHD Simulations
NASA Astrophysics Data System (ADS)
Jiang, Yan-Fei
I study the structures and thermal properties of black hole accretion disks in the radiation pressure dominated regime. Angular momentum transfer in the disk is provided by the turbulence generated by the magneto-rotational instability (MRI), which is calculated self-consistently with a recently developed 3D radiation magneto-hydrodynamics (MHD) code based on Athena. This code, developed by my collaborators and myself, couples both the radiation momentum and energy source terms with the ideal MHD equations by modifying the standard Godunov method to handle the stiff radiation source terms. We solve the two momentum equations of the radiation transfer equations with a variable Eddington tensor (VET), which is calculated with a time independent short characteristic module. This code is well tested and accurate in both optically thin and optically thick regimes. It is also accurate for both radiation pressure and gas pressure dominated flows. With this code, I find that when photon viscosity becomes significant, the ratio between Maxwell stress and Reynolds stress from the MRI turbulence can increase significantly with radiation pressure. The thermal instability of the radiation pressure dominated disk is then studied with vertically stratified shearing box simulations. Unlike the previous results claiming that the radiation pressure dominated disk with MRI turbulence can reach a steady state without showing any unstable behavior, I find that the radiation pressure dominated disks always either collapse or expand until we have to stop the simulations. During the thermal runaway, the heating and cooling rates from the simulations are consistent with the general criterion of thermal instability. However, details of the thermal runaway are different from the predictions of the standard alpha disk model, as many assumptions in that model are not satisfied in the simulations. We also identify the key reasons why previous simulations do not find the instability. The thermal
Global MHD Simulation of Mesoscale Structures at the Magnetospheric Boundary
NASA Technical Reports Server (NTRS)
Berchem, Jean
1998-01-01
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.
Quantum Simulation with 2D Arrays of Trapped Ions
NASA Astrophysics Data System (ADS)
Richerme, Philip
2016-05-01
The computational difficulty of solving fully quantum many-body spin problems is a significant obstacle to understanding the behavior of strongly correlated quantum matter. This work proposes the design and construction of a 2D quantum spin simulator to investigate the physics of frustrated materials, highly entangled states, mechanisms potentially underpinning high-temperature superconductivity, and other topics inaccessible to current 1D systems. The effective quantum spins will be encoded within the well-isolated electronic levels of trapped ions, confined in a two-dimensional planar geometry, and made to interact using phonon-mediated optical dipole forces. The system will be scalable to 100+ quantum particles, far beyond the realm of classical intractability, while maintaining individual-ion control, long quantum coherence times, and site-resolved projective spin measurements. Once constructed, the two-dimensional quantum simulator will implement a broad range of spin models on a variety of reconfigurable lattices and characterize their behavior through measurements of spin-spin correlations and entanglement. This versatile tool will serve as an important experimental resource for exploring difficult quantum many-body problems in a regime where classical methods fail.
Coupled simulation of kinetic pedestal growth and MHD ELM crash
Park, G-Y; Cummings, J.; Chang, C S; Podhorszki, Norbert; Klasky, Scott A; Ku, S.; Pankin, A.; Samtaney, Ravi; Shoshani, A.; Snyder, P.; Sugiyama, L.
2009-01-01
Edge pedestal height and the accompanying ELM crash are critical elements of ITER physics yet to be understood and predicted through high performance computing. An entirely self-consistent first principles simulation is being pursued as a long term research goal, and the plan is planned for completion in time for ITER operation. However, a proof-of-principle work has already been established using a computational tool that employs the best first principles physics available at the present time. A kinetic edge equilibrium code XGC0, which can simulate the neoclassically dominant pedestal growth from neutral ionization (using a phenomenological residual turbulence diffusion motion superposed upon the neoclassical particle motion) is coupled to an extended MHD code M3D, which can perform the nonlinear ELM crash. The stability boundary of the pedestal is checked by an ideal MHD linear peeling-ballooning code, which has been validated against many experimental data sets for the large scale (type I) ELMs onset boundary. The coupling workflow and scientific results to be enabled by it are described.
Coupled simulation of kinetic pedestal growth and MHD ELM crash
Park, G.; Cummings, J.; Chang, C. S.; Klasky, Scott A; Ku, S.; Podhorszki, Norbert; Pankin, A.; Samtaney, Ravi; Shoshani, A.; Snyder, P.; Strauss, H.; Sugiyama, L.; CPES Team, the
2007-01-01
Edge pedestal height and the accompanying ELM crash are critical elements of ITER physics yet to be understood and predicted through high performance computing. An entirely self-consistent first principles simulation is being pursued as a long term research goal, and the plan is planned for completion in time for ITER operation. However, a proof-of-principle work has already been established using a computational tool that employs the best first principles physics available at the present time. A kinetic edge equilibrium code XGC0, which can simulate the neoclassically dominant pedestal growth from neutral ionization (using a phenomenological residual turbulence diffusion motion superposed upon the neoclassical particle motion) is coupled to an extended MHD code M3D, which can perform the nonlinear ELM crash. The stability boundary of the pedestal is checked by an ideal MHD linear peeling-ballooning code, which has been validated against many experimental data sets for the large scale (type I) ELMs onset boundary. The coupling workflow and scientific results to be enabled by it are described.
Multi-dimensional computer simulation of MHD combustor hydrodynamics
Berry, G.F.; Chang, S.L.; Lottes, S.A.; Rimkus, W.A.
1991-04-04
Argonne National Laboratory is investigating the nonreacting jet-gas mixing patterns in an MHD second stage combustor by using a two-dimensional multi-phase hydrodynamics computer program and a three-dimensional single-phase hydrodynamics computer program. The computer simulations are intended to enhance the understanding of flow and mixing patterns in the combustor, which in turn may lead to improvement of the downstream MHD channel performance. A two-dimensional steady state computer model, based on mass and momentum conservation laws for multiple gas species, is used to simulate the hydrodynamics of the combustor in which a jet of oxidizer is injected into an unconfined cross-stream gas flow. A three-dimensional code is used to examine the effects of the side walls and the distributed jet flows on the non-reacting jet-gas mixing patterns. The code solves the conservation equations of mass, momentum, and energy, and a transport equation of a turbulence parameter and allows permeable surfaces to be specified for any computational cell. 17 refs., 25 figs.
FLASH MHD simulations of experiments that study shock-generated magnetic fields
NASA Astrophysics Data System (ADS)
Tzeferacos, P.; Fatenejad, M.; Flocke, N.; Graziani, C.; Gregori, G.; Lamb, D. Q.; Lee, D.; Meinecke, J.; Scopatz, A.; Weide, K.
2015-12-01
We summarize recent additions and improvements to the high energy density physics capabilities in FLASH, highlighting new non-ideal magneto-hydrodynamic (MHD) capabilities. We then describe 3D Cartesian and 2D cylindrical FLASH MHD simulations that have helped to design and analyze experiments conducted at the Vulcan laser facility. In these experiments, a laser illuminates a carbon rod target placed in a gas-filled chamber. A magnetic field diagnostic (called a Bdot) employing three very small induction coils is used to measure all three components of the magnetic field at a chosen point in space. The simulations have revealed that many fascinating physical processes occur in the experiments. These include megagauss magnetic fields generated by the interaction of the laser with the target via the Biermann battery mechanism, which are advected outward by the vaporized target material but decrease in strength due to expansion and resistivity; magnetic fields generated by an outward expanding shock via the Biermann battery mechanism; and a breakout shock that overtakes the first wave, the contact discontinuity between the target material and the gas, and then the initial expanding shock. Finally, we discuss the validation and predictive science we have done for this experiment with FLASH.
Alignment of Velocity and Magnetic Fluctuations in Simulations of Anisotropic MHD Turbulence
NASA Astrophysics Data System (ADS)
Ng, C. S.; Bhattacharjee, A.
2007-11-01
There has been recent theoretical interest in the effect of the alignment of velocity and magnetic fluctuations in three-dimensional (3D) MHD turbulence with a large-scale magnetic field [Boldyrev 2005, 2006]. This theory predicts that the angle θ between the velocity and magnetic fluctuation vectors has a scaling of θ&1/4circ;, where λ is the spatial scale of the fluctuations. There have also been simulations on 3D forced MHD turbulence that supports this prediction [Mason et al. 2006, 2007]. The scaling has also been tested against observations of solar wind turbulence [Podesta et al. 2007]. We report here simulation results based on decaying 2D turbulence. The scaling of θ&1/4circ; and Iroshnikov-Kraichnan scaling has also been observed within a range of time interval and spatial scales, despite the fact that Boldyrev's theory was developed for fully 3D turbulence in the presence of a strong external field. As the external field is reduced in magnitude and becomes comparable to the magnitude of magnetic fluctuations or lower, the scale-dependent alignment is weakened. Implications for observations of solar wind turbulence will be discussed.
Final Report: "Large-Eddy Simulation of Anisotropic MHD Turbulence"
Zikanov, Oleg
2008-06-23
To acquire better understanding of turbulence in flows of liquid metals and other electrically conducting fluids in the presence of steady magnetic fields and to develop an accurate and physically adequate LES (large-eddy simulation) model for such flows. The scientific objectives formulated in the project proposal have been fully completed. Several new directions were initiated and advanced in the course of work. Particular achievements include a detailed study of transformation of turbulence caused by the imposed magnetic field, development of an LES model that accurately reproduces this transformation, and solution of several fundamental questions of the interaction between the magnetic field and fluid flows. Eight papers have been published in respected peer-reviewed journals, with two more papers currently undergoing review, and one in preparation for submission. A post-doctoral researcher and a graduate student have been trained in the areas of MHD, turbulence research, and computational methods. Close collaboration ties have been established with the MHD research centers in Germany and Belgium.
The ideal tearing mode: theory and resistive MHD simulations
NASA Astrophysics Data System (ADS)
Del Zanna, L.; Landi, S.; Papini, E.; Pucci, F.; Velli, M.
2016-05-01
Classical MHD reconnection theories, both the stationary Sweet-Parker model and the tearing instability, are known to provide rates which are too slow to explain the observations. However, a recent analysis has shown that there exists a critical threshold on current sheet's thickness, namely a/L ∼ S -1/3, beyond which the tearing modes evolve on fast macroscopic Alfvénic timescales, provided the Lunquist number S is high enough, as invariably found in solar and astrophysical plasmas. Therefore, the classical Sweet-Parker scenario, for which the diffusive region scales as a/L ∼ S -1/2 and thus can be up to ∼ 100 times thinner than the critical value, is likely to be never realized in nature, as the current sheet itself disrupts in the elongation process. We present here two-dimensional, compressible, resistive MHD simulations, with S ranging from 105 to 107, that fully confirm the linear analysis. Moreover, we show that a secondary plasmoid instability always occurs when the same critical scaling is reached on the local, smaller scale, leading to a cascading explosive process, reminiscent of the flaring activity.
A 2D simulation model for urban flood management
NASA Astrophysics Data System (ADS)
Price, Roland; van der Wielen, Jonathan; Velickov, Slavco; Galvao, Diogo
2014-05-01
The European Floods Directive, which came into force on 26 November 2007, requires member states to assess all their water courses and coast lines for risk of flooding, to map flood extents and assets and humans at risk, and to take adequate and coordinated measures to reduce the flood risk in consultation with the public. Flood Risk Management Plans are to be in place by 2015. There are a number of reasons for the promotion of this Directive, not least because there has been much urban and other infrastructural development in flood plains, which puts many at risk of flooding along with vital societal assets. In addition there is growing awareness that the changing climate appears to be inducing more frequent extremes of rainfall with a consequent increases in the frequency of flooding. Thirdly, the growing urban populations in Europe, and especially in the developing countries, means that more people are being put at risk from a greater frequency of urban flooding in particular. There are urgent needs therefore to assess flood risk accurately and consistently, to reduce this risk where it is important to do so or where the benefit is greater than the damage cost, to improve flood forecasting and warning, to provide where necessary (and possible) flood insurance cover, and to involve all stakeholders in decision making affecting flood protection and flood risk management plans. Key data for assessing risk are water levels achieved or forecasted during a flood. Such levels should of course be monitored, but they also need to be predicted, whether for design or simulation. A 2D simulation model (PriceXD) solving the shallow water wave equations is presented specifically for determining flood risk, assessing flood defense schemes and generating flood forecasts and warnings. The simulation model is required to have a number of important properties: -Solve the full shallow water wave equations using a range of possible solutions; -Automatically adjust the time step and
MHD Simulation of the Inverse Pinch Plasma Discharge
Esaulov, A; Bauer, B; Lindemuth, I; Makhin, V; Presura, R; Ryutov, D
2004-07-01
A wall confined plasma in an inverse pinch configuration holds potential as a plasma target for Magnetized Target Fusion (MTF) as well as the simple geometry to study wall-confined plasma. An experiment is planned to study the inverse pinch configuration using the Nevada Terawatt Facility (NTF) at the University of Nevada, Reno (UNR). The dynamics of the discharge formation have been analyzed using analytic models and numerical methods. Strong heating occurs by thermalization of directed energy when an outward moving current sheet (the inverse pinch effect) collides with the outer wall of the experimental chamber. Two dimensional MHD simulations show Rayleigh-Taylor and Richtmyer-Meshkov -like modes of instability, as expected because of the shock acceleration during plasma formation phase. The instabilities are not disruptive, but give rise to a mild level of turbulence. The conclusion from this work is that an interesting experiment relevant to wall confinement for MTF could be done using existing equipment at UNR.
NASA Astrophysics Data System (ADS)
Ju, Wenhua; Stone, James M.; Zhu, Zhaohuan
2016-06-01
We present results from the first global 3D MHD simulations of accretion disks in cataclysmic variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared with that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component of the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states, both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. (1) The linear wave dispersion relation fits the pitch angles of spiral density waves very well. (2) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks. (3) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate {α }{eff} is not accurate when mass accretion is driven by non-axisymmetric shocks. (4) Using the mass accretion rate measured in our simulations to directly measure α defined in standard thin-disk theory, we find 0.02≲ {α }{eff}≲ 0.05 for CV disks, consistent with observed values in quiescent states of dwarf novae. In this regime, the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism. However, we caution that our simulations use unrealistically low Mach numbers in this regime and, therefore, future models with more realistic thermodynamics and non-ideal MHD are warranted.
MHD Simulations of the Initiation of Coronal Mass Ejections
NASA Astrophysics Data System (ADS)
Fan, Yuhong; Chatterjee, Piyali
Using three-dimensional MHD simulations, we model the quasi-static evolution and the onset of eruption of twisted magnetic flux ropes in the solar corona. We present simulations where the eruption is triggered by either the onset of the torus instability or the helical kink instability of the line-tied coronal flux rope. The simulations show that S (or inverse S) shaped current sheets develop along topological structures identified as Quasi Separatrix Layers (QSLs), during the quasi-static phase before the eruption. Reconnections in the current sheets effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically to the critical height for the onset of the torus instability. We examine the thermal features produced by the current sheet formation and the associated reconnections and found that they can explain some of the observed features in coronal prominence cavities as well as in pre-eruption active regions. We also present simulations of the development of a homologous sequence of CMEs caused by the repeated formation and partial eruption of kink unstable flux ropes as a result of continued flux emergence. It is found that such homologous CMEs tend to be cannibalistic, leading to the formation of more energetic, highly twisted ejecta.
A 2D simulation model for urban flood management
NASA Astrophysics Data System (ADS)
Price, Roland; van der Wielen, Jonathan; Velickov, Slavco; Galvao, Diogo
2014-05-01
The European Floods Directive, which came into force on 26 November 2007, requires member states to assess all their water courses and coast lines for risk of flooding, to map flood extents and assets and humans at risk, and to take adequate and coordinated measures to reduce the flood risk in consultation with the public. Flood Risk Management Plans are to be in place by 2015. There are a number of reasons for the promotion of this Directive, not least because there has been much urban and other infrastructural development in flood plains, which puts many at risk of flooding along with vital societal assets. In addition there is growing awareness that the changing climate appears to be inducing more frequent extremes of rainfall with a consequent increases in the frequency of flooding. Thirdly, the growing urban populations in Europe, and especially in the developing countries, means that more people are being put at risk from a greater frequency of urban flooding in particular. There are urgent needs therefore to assess flood risk accurately and consistently, to reduce this risk where it is important to do so or where the benefit is greater than the damage cost, to improve flood forecasting and warning, to provide where necessary (and possible) flood insurance cover, and to involve all stakeholders in decision making affecting flood protection and flood risk management plans. Key data for assessing risk are water levels achieved or forecasted during a flood. Such levels should of course be monitored, but they also need to be predicted, whether for design or simulation. A 2D simulation model (PriceXD) solving the shallow water wave equations is presented specifically for determining flood risk, assessing flood defense schemes and generating flood forecasts and warnings. The simulation model is required to have a number of important properties: -Solve the full shallow water wave equations using a range of possible solutions; -Automatically adjust the time step and
MHD Simulations of the Plasma Flow in the Magnetic Nozzle
NASA Technical Reports Server (NTRS)
Smith, T. E. R.; Keidar, M.; Sankaran, K.; olzin, K. A.
2013-01-01
The magnetohydrodynamic (MHD) flow of plasma through a magnetic nozzle is simulated by solving the governing equations for the plasma flow in the presence of an static magnetic field representing the applied nozzle. This work will numerically investigate the flow and behavior of the plasma as the inlet plasma conditions and magnetic nozzle field strength are varied. The MHD simulations are useful for addressing issues such as plasma detachment and to can be used to gain insight into the physical processes present in plasma flows found in thrusters that use magnetic nozzles. In the model, the MHD equations for a plasma, with separate temperatures calculated for the electrons and ions, are integrated over a finite cell volume with flux through each face computed for each of the conserved variables (mass, momentum, magnetic flux, energy) [1]. Stokes theorem is used to convert the area integrals over the faces of each cell into line integrals around the boundaries of each face. The state of the plasma is described using models of the ionization level, ratio of specific heats, thermal conductivity, and plasma resistivity. Anisotropies in current conduction due to Hall effect are included, and the system is closed using a real-gas equation of state to describe the relationship between the plasma density, temperature, and pressure.A separate magnetostatic solver is used to calculate the applied magnetic field, which is assumed constant for these calculations. The total magnetic field is obtained through superposition of the solution for the applied magnetic field and the self-consistently computed induced magnetic fields that arise as the flowing plasma reacts to the presence of the applied field. A solution for the applied magnetic field is represented in Fig. 1 (from Ref. [2]), exhibiting the classic converging-diverging field pattern. Previous research was able to demonstrate effects such as back-emf at a super-Alfvenic flow, which significantly alters the shape of the
Driving Coronal MHD Simulations with Flux Evolution Models
NASA Astrophysics Data System (ADS)
Linker, J.; Lionello, R.; Mikic, Z.; Riley, P.; Downs, C.; Arge, C. N.; Henney, C. J.
2013-12-01
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.
Supra-Arcade Downflows: Observations and MHD Simulations
NASA Astrophysics Data System (ADS)
Guo, L.; Innes, D.; Bhattacharjee, A.; Huang, Y.
2013-12-01
Supra-arcade downflows (hereafter referred to as SADs) are low-emission, elongated features usually observed above post-eruption coronal arcades. Although SADs have been observed for decades, the mechanism for their formation remains unclear. It has been suggested that SADs are shrinking loops or wakes following shrinking loops in the exhaust region of magnetic reconnection. We have undertaken a detailed comparison between numerical simulation and observation to test these ideas. Using data from SDO/AIA, we study four flare events with SADs, and show distinct differences between reconnected loops and SADs. The observations also exhibit downward moving SADs intertwined with bright upward moving spikes. Whereas SADs are dark, hot voids, spikes are brighter, slightly cooler structures. In our high-Lundquist-number 3D resistive MHD simulations, we demonstrate that secondary ballooning/interchange instabilities develop in the downstream region of a reconnecting current sheet. The instability results in the formation of low-density coherent structures that resemble SADs, and high-density structures that look like the spikes. Comparison between the simulation results and observations suggests that secondary interchange/ballooning instabilities of reconnecting current sheets provide a plausible mechanism for the observed features of SADs and spikes.
MHD Simulation of RF Current Drive in MST
NASA Astrophysics Data System (ADS)
Goetz, J. A.; Hendries, E. R.; Anderson, J. K.; Forest, C. B.; Reusch, J. A.; Seltzman, A. H.; Sovinec, C. R.; Diem, S.; Harvey, R. W.
2013-10-01
Auxiliary current drive using the electron Bernstein wave (EBW) may advance the performance of the RFP. In prior computations, a hypothetical edge-localized current is shown to suppress tearing activity that governs transport in the RFP. Ideal conditions for tearing stabilization include reduced toroidal induction, and precise width and radial position of the current drive. To support MST EBW studies, an integrated modeling scheme incorporates ray tracing and Fokker-Plank predictions of auxiliary current into single fluid MHD. Simulations at low Lundquist number (S ~ 104) agree with the previous work but at MST-like S (S ~ 3 × 106) show unexpected results. The effect on the current profile by the rf-driven force decreases in magnitude and widens considerably as S increases. Simulations reproduce the experimentally observed periodic current profile relaxation events (sawteeth). With rf drive, reduction of tearing mode amplitudes is seen, but is limited to periods between each sawtooth, which persist with up to 10 MW of rf. Prolonged low tearing amplitudes are predicted with the combination of current drive and reduced toroidal loop voltage, consistent with previous conclusions. Finally, these simulations show that the resistivity profile has a strong effect on the optimal current drive profile for mode stabilization. Work supported by US DoE.
MHD Simulation of Plasma Flow through the VASIMR Magnetic Nozzle
NASA Astrophysics Data System (ADS)
Tarditi, A. G.; Shebalin, J. V.
2003-10-01
The VASIMR (Variable Specific Impulse Magnetoplasma Rocket, [1]) concept is currently in the experimental development phase at the Advanced Space Propulsion Laboratory, NASA Johnson Space Center. The current experimental effort is mainly focused on the demonstration of the efficient plasma production (light ion helicon source, [2]) and energy boosting (ion cyclotron resonance heating section). Two other critical issues, the plasma detachment process and the collimation of the plasma plume in the magnetic nozzle, are essential for the near term experimental development and are being addressed through an MHD simulation modeling effort with the NIMROD code [3,4]. The model follows the plasma flow up to few meters from the nozzle throat: at that distance the plasma exhaust parameters reach values comparable with the ionospheric plasma background [5]. Results from two-dimensional simulation runs (cylindrical geometry, assuming azimuthal symmetry) aimed in particular at testing the effectiveness of different open-end boundary condition schemes are presented. [1] F. R. Chang-Diaz, Scientific American, p. 90, Nov. 2000 [2] M. D. Carter, et al., Phys. Plasmas 9, 5097-5110, 2002 [3] http://www.nimrodteam.org [4] A. Tarditi et al., 28th Int. Electric Propulsion Conf., IEPC 2003, Toulouse, France, March 2003 [5] A. V. Ilin et al., Proc. 40th AIAA Aerospace Sciences Meeting, Reno, NV, Jan. 2002
3D MHD simulations of radial wire arrays
NASA Astrophysics Data System (ADS)
Jennings, C.; Ampleford, D.; Ciardi, A.; Chittenden, J.; Bland, S.; Niasse, N.
2008-04-01
We present 3D resistive MHD simulations evaluating multi-MA radial wire arrays as a potential compact, high intensity source for inertial confinement fusion and laboratory astrophysics. A radial wire array consists of wires running radially outwards from a central electrode, and was first investigated at the 1 MA level on the MAGPIE generator at Imperial College. Originally used as a method of producing magnetic tower laboratory jets relevant to astrophysics[1], they have also shown potential as a high power x-ray source. Able to produce x-ray pulses with a rise time and peak power comparable to cylindrical wire arrays, radial arrays occupy a smaller volume and may consequently be able to access higher power densities. We discuss simulation results reproducing radial array experiments performed on the MAGPIE facility as a means of benchmarking our model. This model is then used to evaluate radial wire arrays in the multi-MA regime for planned experiments on the Saturn generator of Sandia National Laboratories. [1] A. Ciardi et al, Phys. Plasmas 14, 056501 (2007)
2D MHD and 1D HD Models of a Solar Flare—a Comprehensive Comparison of the Results
NASA Astrophysics Data System (ADS)
Falewicz, R.; Rudawy, P.; Murawski, K.; Srivastava, A. K.
2015-11-01
Without any doubt, solar flaring loops possess a multithread internal structure that is poorly resolved, and there are no means to observe heating episodes and thermodynamic evolution of the individual threads. These limitations cause fundamental problems in numerical modeling of flaring loops, such as selection of a structure and a number of threads, and an implementation of a proper model of the energy deposition process. A set of one-dimensional (1D) hydrodynamic and two-dimensional (2D) magnetohydrodynamic models of a flaring loop are developed to compare energy redistribution and plasma dynamics in the course of a prototypical solar flare. Basic parameters of the modeled loop are set according to the progenitor M1.8 flare recorded in AR 10126 on 2002 September 20 between 09:21 UT and 09:50 UT. The nonideal 1D models include thermal conduction and radiative losses of the optically thin plasma as energy-loss mechanisms, while the nonideal 2D models take into account viscosity and thermal conduction as energy-loss mechanisms only. The 2D models have a continuous distribution of the parameters of the plasma across the loop and are powered by varying in time and space along and across the loop heating flux. We show that such 2D models are an extreme borderline case of a multithread internal structure of the flaring loop, with a filling factor equal to 1. Nevertheless, these simple models ensure the general correctness of the obtained results and can be adopted as a correct approximation of the real flaring structures.
2D MHD AND 1D HD MODELS OF A SOLAR FLARE—A COMPREHENSIVE COMPARISON OF THE RESULTS
Falewicz, R.; Rudawy, P.; Murawski, K.; Srivastava, A. K. E-mail: rudawy@astro.uni.wroc.pl E-mail: asrivastava.app@iitbhu.ac.in
2015-11-01
Without any doubt, solar flaring loops possess a multithread internal structure that is poorly resolved, and there are no means to observe heating episodes and thermodynamic evolution of the individual threads. These limitations cause fundamental problems in numerical modeling of flaring loops, such as selection of a structure and a number of threads, and an implementation of a proper model of the energy deposition process. A set of one-dimensional (1D) hydrodynamic and two-dimensional (2D) magnetohydrodynamic models of a flaring loop are developed to compare energy redistribution and plasma dynamics in the course of a prototypical solar flare. Basic parameters of the modeled loop are set according to the progenitor M1.8 flare recorded in AR 10126 on 2002 September 20 between 09:21 UT and 09:50 UT. The nonideal 1D models include thermal conduction and radiative losses of the optically thin plasma as energy-loss mechanisms, while the nonideal 2D models take into account viscosity and thermal conduction as energy-loss mechanisms only. The 2D models have a continuous distribution of the parameters of the plasma across the loop and are powered by varying in time and space along and across the loop heating flux. We show that such 2D models are an extreme borderline case of a multithread internal structure of the flaring loop, with a filling factor equal to 1. Nevertheless, these simple models ensure the general correctness of the obtained results and can be adopted as a correct approximation of the real flaring structures.
Preliminary analysis of the dynamic heliosphere by MHD simulations
Washimi, H.; Zank, G. P.; Tanaka, T.
2006-09-26
A preliminary analysis of the dynamic heliosphere to estimate the termination shock (TS) distance from the sun around the time when Voyager 1 passed the termination shock at December 16, 2004 is performed by using MHD simulations. For input to this simulation, we use the Voyager 2 solar-wind data. We first find a stationary solution of the 3-D outer heliosphere by assigning a set of LISM parameters as our outer boundary conditions and then the dynamical analysis is performed. The model TS crossing is within 6 months of the observed date. The TS is pushed outward every time a high ram-pressure solar wind pulse arrives. After the end of the high ram-pressure wind, the TS shock shrinks inward. When the last Halloween event passed through the TS at DOY 250, 2004, the TS began to shrink inward very quickly and the TS crossed V1. The highest inward speed of the TS is over 400 km/s. The high ram-pressure solar wind transmitted through the TS becomes a high thermal-pressure plasma in the heliosheath, acting to push the TS inward. This suggests that the position of the TS is determined not only by the steady-state pressure balance condition between the solar wind ram-pressure and the LISM pressure, but by the dynamical ram pressure too. The period when the high ram-pressure solar wind arrives at the TS shock seems to correspond to the period of the TS particle event (Stone et al, 2005, Decker et al., 2005). The TS crossing date will be revised in future simulations using a more appropriate set of parameters for the LISM. This will enable us to undertake a detailed comparison of the simulation results with the TS particle events.
NASA Astrophysics Data System (ADS)
Choi, M. J.; Park, H. K.; Yun, G. S.; Lee, W.; Luhmann, N. C., Jr.; Lee, K. D.; Ko, W.-H.; Park, Y.-S.; Park, B. H.; In, Y.
2016-06-01
Minor and major disruptions by explosive MHD instabilities were observed with the novel quasi 3D electron cyclotron emission imaging (ECEI) system in the KSTAR plasma. The fine electron temperature (T e) fluctuation images revealed two types of minor disruptions: a small minor disruption is a q∼ 2 localized fast transport event due to a single m/n = 2/1 magnetic island growth, while a large minor disruption is partial collapse of the q≤slant 2 region with two successive fast heat transport events by the correlated m/n = 2/1 and m/n = 1/1 instabilities. The m/n = 2/1 magnetic island growth during the minor disruption is normally limited below the saturation width. However, as the additional interchange-like perturbation grows near the inner separatrix of the 2/1 island, the 2/1 island can expand beyond the limit through coupling with the cold bubble formed by the interchange-like perturbation.
MHD simulation of RF current drive in MST
Hendries, E. R.; Anderson, J. K.; Forest, C. B.; Reusch, J. A.; Seltzman, A. H.; Sovinec, C. R.; Diem, S.; Harvey, R. W.
2014-02-12
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.
Observations and MHD Simulations for a Shocked Magnetotail
NASA Astrophysics Data System (ADS)
Zhou, X.; Zhou, X. Z.; Angelopoulos, V.; Raeder, J.; Oliveira, D.; Shi, Q.
2014-12-01
Recent studies disclosed that interplanetary shocks not only raise global auroral activity, but also cause significant tail disturbances, ranging from ULF wave excitation to abrupt cross-tail current sheet thinning and current density increase, generation of burst bulk flows and dipolarization fronts, and to magnetic reconnection enhancement. In addition, shocks can also cause significant deformation of the magnetotail at ~60 Re and beyond. In this paper we study a shock event using ARTEMIS in situ observations and OpenGGCM MHD simulations. The two ARTEMIS spacecraft were located near the tail current sheet and lobe center at (-60, 1, -5Re_GSM) when the shock arrived and recorded an abrupt tail compression leading to significant enhancements in the plasma density, temperature, magnetic field strength, and cross-tail current density, as well as to tailward flows. However, ~10 min later, the spacecraft entered the sheath solar wind unexpectedly. Two hypotheses are considered: either the tail was cut off by the high solar wind ram pressure (~25-30 nPa), or the compressed tail was pushed aside by the appreciable Vy solar wind flow component imposed by the shock. OpenGGMC simulation results confirmed the second hypothesis and disclosed that for this event the magnetic pressure played a dominant role at X=-60 Re for the compression. In addition to the shock normal direction and shock compression, the anisotropic (transverse) magnetic pressure also contributed to the significant reduction of the lobe Y dimension. Therefore, during this 10 min interval, the lobe center moved dawnward by ~12 Re and the tail width in Y was reduced from 40 to 26 Re, which eventually exposed ARTEMIS to the sheath solar wind. Comparisons of plasma and magnetic parameters between ARTEMIS in situ observations and simulations showed a satisfied consistence.
MHD simulation of RF current drive in MST
NASA Astrophysics Data System (ADS)
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
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.
Global MHD simulation of magnetic erosion and flux transport
NASA Astrophysics Data System (ADS)
Lopez, R. E.; Wiltberger, M. J.
2001-05-01
It has long been known that when the Interplanetary Magnetic Field (IMF) turns from a northward orientation to a southward orientation that the dayside magnetopause moves earthward. This phenomenon is termed "magnetic erosion". Simultaneously, the nightside magnetopause flaring angle changes as the flux removed from the dayside is added to the nightside. A simple picture of erosion is an "onion-peeling" model, in which a newly merged field line is transported by the solar wind to the nightside and the dayside magnetopause is now one field line closer to the Earth. This model however ignores pressure balance considerations, and in fact the agent of erosion is the Region 1 current system. In this presentation we present an analysis of several MHD simulations, driven with real IMF conditions, in which the IMF switched from northward to southward producing erosion. We will show that the increasing Region 1 current system produced a perturbation that reduces the dayside field magnitude, causing the pressure balance condition to be satisfied closer to Earth, while simultaneously increasing the nightside magnetic flux. The increased flaring angle causes the nightside field magnitude to increase as well due to pressure balance considerations.
NASA Astrophysics Data System (ADS)
Yokoyama, Takaaki
Temporal evolution of a current sheet with initial perturbations is studied by using the threedimensional resistive magnetohydrodynamic (MHD) simulations. The magnetic reconnection is considered to be the main engine of the energy rele ase in solar flares. The structure of the diffusion region is, however, not stil l understood under the circumstances with enormously large magnetic Reynolds num ber as the solar corona. In particular, the relationship between the flare's macroscopic physics and the microscopic ones are unclear. It is generally believed that the MHD turbulence s hould play a role in the intermediate scale. The initial current sheet is in an approximately hydromagnetic equilibrium with anti-parallel magnetic field in the y-direction. We imposed a finite-amplitude perturbations (=50ee what happens. Special attention is paid upon the evolution of a three-dimens ional structure in the direction along the initial electric current (z-direction ). Our preliminary results are as follows: (1) In the early phase of the evolut ion, high wavenumber modes in the z-direction are excited and grow. (2) Many "X "-type neutral points (lines) are generated along the magnetic neutral line (pla ne) in the current sheet. When they evolve into the non-linear phase, three-dime nsional structures in the z-direction also evolve. The spatial scale in the z-di rection seems to be almost comparable with that in the xy-plane. (3) The energy release rate is reduced in case of 3D simulations compared with 2D ones probably because of the reduction of the inflow cross sections by the formation of pattc hy structures in the current sheet.
An MHD simulation of plasmoid instability in the dayside ionosphere of an unmagnetized planet
NASA Astrophysics Data System (ADS)
Hitoshi, S.; Terada, N.; Kasaba, Y.
2015-12-01
A numerical simulation of magnetic reconnection in the dayside ionosphere of an unmagnetized planet and a comparison of the size distribution of flux ropes obtained from simulation with that from observation will be reported. Flux ropes have been frequently observed in the dayside ionospheres of Venus and Mars[Russell and Elphic, 1979; Cloutier et al.,1999] and their radius has been found to be between 6 to 12 km near the subsolar location of Venus[Russell et al., 1990]. Dreher et al. [1995] suggested using an MHD simulation that reconnection caused by an IMF rotation can generate flux ropes at the Venus ionopause. However, Dreher et al. [1995] examined only the linear stage of reconnection, so the nonlinear stage that takes into consideration the vertical convection of the reconnection site along the intrinsic convection in the Venus ionosphere has yet to be investigated. In this study, using a 2-D multi-species MHD simulation, the spatiotemporal evolution of reconnection in the ionosphere of Venus is examined. The size distribution of flux ropes is also examined and the validity of the generation mechanism of flux ropes is discussed by comparing the rope size distribution with the observed one. In the ionosphere of Venus, our simulation result shows that plasmoid instability [Loureiro et al., 2007] occurs in a Sweet-Parker (SP) current sheet above the altitude where Lundquist number exceeds 106, and consequently many plasmoids are generated. In the nonlinear stage, secondary reconnections occur in the current sheets, which exist between adjacent pairs of plasmoids, and thus smaller flux ropes are created. It has been suggested that the smaller side of the size distribution increases as a result of hierarchical reconnections in the SP current sheet [Shibata et al., 2001]. The observational size distribution [Vignes et al., 2003] shows that the population of small flux ropes is larger than that of large ones and it is consistent with the simulation result. Through
Substorm features in MHD (magnetohydrodynamics) simulations of magnetotail dynamics
Birn, J.; Hesse, M.
1990-01-01
We present a review and extended analysis of characteristic results from our nonideal three-dimensional MHD simulations of unstable magnetotail evolution, which develops without the necessity of external driving or prescribed localization on nonideal effects. These modes involve magnetic reconnection at a near-Earth site in the tail, consistent with the near-Earth neutral line model of substorms. The evolution tailward of the reconnection site is characterized by plasmoid formation and ejection into the far tail, plasma sheet thinning between the near-Earth neutral line (X line) and the departing plasmoid, and fast tailward flow, which occupies large sections of the plasma sheet at larger distance from the X line, while it occurs only in very limited space and time sections close to the X line. The region earthward of the X line is characterized by dipolarization, propagating from midnight toward the flank regions and, perhaps, tailward. It is associated with the signatures of the substorm current wedge: reduction and diversion of cross-tail current from a region surrounding the reconnection site and increase of Region 1 type field-aligned currents. A mapping of these currents to the Earth on the basis of an empirical magnetic field model shows good agreement of the mapped current system with the observed Region 1 field-aligned current system and its substorm associated changes, including also a nightward and equatorward shift of the peaks of the field-aligned current density. The evolution of the mappings of the boundaries of the closed field line region bears strong resemblance to the formation and expansion of he auroral bulge. The consistency of all of these details with observed substorm features strongly supports the idea that substorm evolution in the tail is that of a large scale nonideal instability.
Reactor2D: A tool for simulation of shock deformation
NASA Astrophysics Data System (ADS)
Kraus, Eugeny I.; Shabalin, Ivan I.
2016-10-01
The basic steps for creating a numerical tool to simulate the deformation and failure processes of complex technical objects (CTO) are presented. Calculations of shock loading of CTO both at low and high speeds, showing the efficiency of the numerical tools created are carried out.
Attempts to Simulate Anisotropies of Solar Wind Fluctuations Using MHD with a Turning Magnetic Field
NASA Technical Reports Server (NTRS)
Ghosh, Sanjoy; Roberts, D. Aaron
2010-01-01
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.
COYOTE: A computer program for 2-D reactive flow simulations
Cloutman, L.D.
1990-04-01
We describe the numerical algorithm used in the COYOTE two- dimensional, transient, Eulerian hydrodynamics program for reactive flows. The program has a variety of options that provide capabilities for a wide range of applications, and it is designed to be robust and relatively easy to use while maintaining adequate accuracy and efficiency to solve realistic problems. It is based on the ICE method, and it includes a general species and chemical reaction network for simulating reactive flows. It also includes swirl, turbulence transport models, and a nonuniform mesh capability. We describe several applications of the program. 33 refs., 4 figs.
Simulation of subgrid orographic precipitation with an embedded 2-D cloud-resolving model
NASA Astrophysics Data System (ADS)
Jung, Joon-Hee; Arakawa, Akio
2016-03-01
By explicitly resolving cloud-scale processes with embedded two-dimensional (2-D) cloud-resolving models (CRMs), superparameterized global atmospheric models have successfully simulated various atmospheric events over a wide range of time scales. Up to now, however, such models have not included the effects of topography on the CRM grid scale. We have used both 3-D and 2-D CRMs to simulate the effects of topography with prescribed "large-scale" winds. The 3-D CRM is used as a benchmark. The results show that the mean precipitation can be simulated reasonably well by using a 2-D representation of topography as long as the statistics of the topography such as the mean and standard deviation are closely represented. It is also shown that the use of a set of two perpendicular 2-D grids can significantly reduce the error due to a 2-D representation of topography.
Simulations of Quantum Spin Models on 2D Frustrated Lattices
NASA Astrophysics Data System (ADS)
Melko, Roger
2006-03-01
Algorithmic advances in quantum Monte Carlo techniques have opened up the possibility of studying models in the general class of the S=1/2 XXZ model (equivalent to hard-core bosons) on frustrated lattices. With an antiferromagnetic diagonal interaction (Jz), these models can be solved exactly with QMC, albeit with some effort required to retain ergodicity in the near-degenerate manifold of states that exists for large Jz. The application of the quantum (ferromagnetic off-diagonal) interaction to this classically degenerate manifold produces a variety of intriguing physics, including an order-by-disorder supersolid phase, novel insulating states, and possible exotic quantum critical phenomena. We discuss numerical results for the triangular and kagome lattices with nearest and next-nearest neighbor exchange interactions, and focus on the relevance of the simulations to related areas of physics, such as experiments of cold trapped atomic gasses and the recent theory of deconfined quantum criticality.
HYBRID AND HALL-MHD SIMULATIONS OF COLLISIONLESS RECONNECTION: EFFECTS OF PLASMA PRESSURE TENSOR
L. YIN; D. WINSKE; ET AL
2001-05-01
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.
Simulations of the C-2/C-2U Field Reversed Configurations with the Q2D code
NASA Astrophysics Data System (ADS)
Onofri, Marco; Dettrick, Sean; Barnes, Daniel; Tajima, Toshiki; TAE Team
2015-11-01
C-2U was built to sustain advanced beam-driven FRCs for 5 + ms. The Q2D transport code is used to simulate the evolution of C-2U discharges and to study sustainment via fast ion current and pressure, with the latter comparable to the thermal plasma pressure. The code solves the MHD equations together with source terms due to neutral beams, which are calculated by a Monte Carlo method. We compare simulations with experimental results obtained in the HPF14 regime of C-2 (6 neutral beams with energy of 20 keV and total power of 4.2 MW). All simulations start from an initial equilibrium and transport coefficients are chosen to match experimental data. The best agreement is obtained when utilizing an enhanced energy transfer between fast ions and the plasma, which may be an indication of anomalous heating due to beneficial beam-plasma instabilities. Similar simulations of C-2U (neutral beam power increased to 10 + MW and angled beam injection) are compared with experimental results, where a steady state has been obtained for 5 + ms, correlated with the neutral beam pulse and limited by engineering constraints.
M3D Simulations of Energetic Particle-driven MHD Mode with Unstructured Mesh
NASA Astrophysics Data System (ADS)
Fu, G. Y.; Park, W.; Strauss, H. R.
2001-10-01
The energetic particle-driven MHD modes are studied using a multi-level extended MHD code M3D(W. Park et al., Phys. Plasmas 6, 1796 (1999)). In a Extended-MHD model, the plasma is divided into the bulk part and the energetic particle component. The bulk plasma is treated as either a single fluid or two fluids. The energetic particles are described by gyrokinetic particles following the self-consistent electromagnetic field. The model is self-consistent, including nonlinear effects of hot particles on the MHD dynamics and the nonlinear MHD mode coupling. Previously we had shown the results of nonlinear saturation of TAEfootnote G.Y. Fu and W. Park, Phys. Rev. Lett. 74, 1594 (1995), energetic particle stabilization of an internal kink and excitation of fishbone^2, and nonlinear saturation of fishbone in circular tokamaks (G.Y. Fu et al, 2000 Sherwood Meeting, Paper 2C2.). In this work, we extend the simulations to general geometry using unstructured mesh(H.R. Strauss and W. Park, Phys. Plasmas 5, 2676 (1998). We also use a gyrofluid model for fishbone in order to study the role of MHD nonlinearity in saturation near the marginal stability. Results of applications to tokamaks and spherical tokamaks will be presented.
2-D simulation of a waveguide free electron laser having a helical undulator
Kim, S.K.; Lee, B.C.; Jeong, Y.U.
1995-12-31
We have developed a 2-D simulation code for the calculation of output power from an FEL oscillator having a helical undulator and a cylindrical waveguide. In the simulation, the current and the energy of the electron beam is 2 A and 400 keV, respectively. The parameters of the permanent-magnet helical undulator are : period = 32 mm, number of periods = 20, magnetic field = 1.3 kG. The gain per pass is 10 and the output power is calculated to be higher than 10 kW The results of the 2-D simulation are compared with those of 1-D simulation.
Forced Reconnection in the Near Magnetotail: Onset and Energy Conversion in PIC and MHD Simulations
NASA Technical Reports Server (NTRS)
Birn, J.; Hesse, Michael
2014-01-01
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.
2D numerical simulation of the MEP energy-transport model with a finite difference scheme
Romano, V. . E-mail: romano@dmi.unict.it
2007-02-10
A finite difference scheme of Scharfetter-Gummel type is used to simulate a consistent energy-transport model for electron transport in semiconductors devices, free of any fitting parameters, formulated on the basis of the maximum entropy principle. Simulations of silicon n{sup +}-n-n{sup +} diodes, 2D-MESFET and 2D-MOSFET and comparisons with the results obtained by a direct simulation of the Boltzmann transport equation and with other energy-transport models, known in the literature, show the validity of the model and the robustness of the numerical scheme.
Plasma wave signatures in the magnetotail reconnection region - MHD simulation and ray tracing
NASA Technical Reports Server (NTRS)
Omura, Yoshiharu; Green, James L.
1993-01-01
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.
The flare position obtained from MHD simulation and comparison with X-ray observations
NASA Astrophysics Data System (ADS)
Podgorny, Alexander; Podgorny, Igor
It was for the first time shown that the position of the current sheet, obtained by numerical MHD simulation, coincides with the position of the thermal X-ray source. In our 3D MHD simulation we do not use any hypotheses about the flare mechanism. Several mechanisms of solar flare production are considered by different authors. Usually the initial conditions at numerical simulation are artificially set such a way that it is required for development of the proposed mechanism. In this approach, the unstable configuration of the magnetic field is set as the initial conditions, and the possibility of forming such an unstable system at the real evolution of the active region before the flare is not considered. Here the flare mechanism is obtained from the numerical MHD simulations in which all the conditions are taken from observations in the active region. It is shown that flare energy accumulation occurs in the current sheet magnetic field created by disturbances focusing in the vicinity of an X-type singular line. According to the developed solar flare electrodynamical model the thermal X-ray emission source appears in a current sheet, where plasma is heated due to magnetic field dissipation. Using 3D MHD numerical simulation the position of source of thermal X-ray emission are found for the flare occurred May 27, 2003 at 02:53. To find positions of sources of thermal X-ray radiation in the corona from MHD simulation results the graphical system is developed. The comparison with RHESSI X-ray observations show the coincidence of current sheet and observed the thermal X-ray emission source.
Using Two-Ribbon Flare Observations and MHD Simulations to Constrain Flare Properties
NASA Astrophysics Data System (ADS)
Kazachenko, Maria D.; Lynch, Benjamin J.; Welsch, Brian
2016-05-01
Flare ribbons are emission structures that are frequently observed during flares in transition-region and chromospheric radiation. These typically straddle a polarity inversion line (PIL) of the radial magnetic field at the photosphere, and move apart as the flare progresses. The ribbon flux - the amount of unsigned photospheric magnetic flux swept out by flare ribbons - is thought to be related to the amount coronal magnetic reconnection, and hence provides a key diagnostic tool for understanding the physical processes at work in flares and CMEs. Previous measurements of the magnetic flux swept out by flare ribbons required time-consuming co-alignment between magnetograph and intensity data from different instruments, explaining why those studies only analyzed, at most, a few events. The launch of the Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA), both aboard the Solar Dynamics Observatory (SDO), presented a rare opportunity to compile a much larger sample of flare-ribbon events than could readily be assembled before. We created a dataset of 363 events of both flare ribbon positions and fluxes, as a function of time, for all C9.-class and greater flares within 45 degrees of disk center observed by SDO from June 2010 till April 2015. For this purpose, we used vector magnetograms (2D magnetic field maps) from HMI and UV images from AIA. A critical problem with using unprocessed AIA data is the existence of spurious intensities in AIA data associated with strong flare emission, most notably "blooming" (spurious smearing of saturated signal into neighboring pixels, often in streaks). To overcome this difficulty, we have developed an algorithmic procedure that effectively excludes artifacts like blooming. We present our database and compare statistical properties of flare ribbons, e.g. evolutions of ribbon reconnection fluxes, reconnection flux rates and vertical currents with the properties from MHD simulations.
Linking 3D and 2D binding kinetics of membrane proteins by multiscale simulations
Xie, Zhong-Ru; Chen, Jiawen; Wu, Yinghao
2014-01-01
Membrane proteins are among the most functionally important proteins in cells. Unlike soluble proteins, they only possess two translational degrees of freedom on cell surfaces, and experience significant constraints on their rotations. As a result, it is currently challenging to characterize the in situ binding of membrane proteins. Using the membrane receptors CD2 and CD58 as a testing system, we developed a multiscale simulation framework to study the differences of protein binding kinetics between 3D and 2D environments. The association and dissociation processes were implemented by a coarse-grained Monte-Carlo algorithm, while the dynamic properties of proteins diffusing on lipid bilayer were captured from all-atom molecular dynamic simulations. Our simulations show that molecular diffusion, linker flexibility and membrane fluctuations are important factors in adjusting binding kinetics. Moreover, by calibrating simulation parameters to the measurements of 3D binding, we derived the 2D binding constant which is quantitatively consistent with the experimental data, indicating that the method is able to capture the difference between 3D and 2D binding environments. Finally, we found that the 2D dissociation between CD2 and CD58 is about 100-fold slower than the 3D dissociation. In summary, our simulation framework offered a generic approach to study binding mechanisms of membrane proteins. PMID:25271078
The simulation of 3D microcalcification clusters in 2D digital mammography and breast tomosynthesis
Shaheen, Eman; Van Ongeval, Chantal; Zanca, Federica; Cockmartin, Lesley; Marshall, Nicholas; Jacobs, Jurgen; Young, Kenneth C.; Dance, David R.; Bosmans, Hilde
2011-12-15
Purpose: This work proposes a new method of building 3D models of microcalcification clusters and describes the validation of their realistic appearance when simulated into 2D digital mammograms and into breast tomosynthesis images. Methods: A micro-CT unit was used to scan 23 breast biopsy specimens of microcalcification clusters with malignant and benign characteristics and their 3D reconstructed datasets were segmented to obtain 3D models of microcalcification clusters. These models were then adjusted for the x-ray spectrum used and for the system resolution and simulated into 2D projection images to obtain mammograms after image processing and into tomographic sequences of projection images, which were then reconstructed to form 3D tomosynthesis datasets. Six radiologists were asked to distinguish between 40 real and 40 simulated clusters of microcalcifications in two separate studies on 2D mammography and tomosynthesis datasets. Receiver operating characteristic (ROC) analysis was used to test the ability of each observer to distinguish between simulated and real microcalcification clusters. The kappa statistic was applied to assess how often the individual simulated and real microcalcification clusters had received similar scores (''agreement'') on their realistic appearance in both modalities. This analysis was performed for all readers and for the real and the simulated group of microcalcification clusters separately. ''Poor'' agreement would reflect radiologists' confusion between simulated and real clusters, i.e., lesions not systematically evaluated in both modalities as either simulated or real, and would therefore be interpreted as a success of the present models. Results: The area under the ROC curve, averaged over the observers, was 0.55 (95% confidence interval [0.44, 0.66]) for the 2D study, and 0.46 (95% confidence interval [0.29, 0.64]) for the tomosynthesis study, indicating no statistically significant difference between real and simulated
An efficient simulation method of a cyclotron sector-focusing magnet using 2D Poisson code
NASA Astrophysics Data System (ADS)
Gad Elmowla, Khaled Mohamed M.; Chai, Jong Seo; Yeon, Yeong H.; Kim, Sangbum; Ghergherehchi, Mitra
2016-10-01
In this paper we discuss design simulations of a spiral magnet using 2D Poisson code. The Independent Layers Method (ILM) is a new technique that was developed to enable the use of two-dimensional simulation code to calculate a non-symmetric 3-dimensional magnetic field. In ILM, the magnet pole is divided into successive independent layers, and the hill and valley shape around the azimuthal direction is implemented using a reference magnet. The normalization of the magnetic field in the reference magnet produces a profile that can be multiplied by the maximum magnetic field in the hill magnet, which is a dipole magnet made of the hills at the same radius. Both magnets are then calculated using the 2D Poisson SUPERFISH code. Then a fully three-dimensional magnetic field is produced using TOSCA for the original spiral magnet, and the comparison of the 2D and 3D results shows a good agreement between both.
Low frequency 2D Raman-THz spectroscopy of ionic solution: A simulation study
NASA Astrophysics Data System (ADS)
Pan, Zhijun; Wu, Tianmin; Jin, Tan; Liu, Yong; Nagata, Yuki; Zhang, Ruiting; Zhuang, Wei
2015-06-01
The 2D Raman-THz spectrum of the MgCl2 solution was simulated using the molecular dynamics simulation and the stability matrix method and compared with that of the pure water. The 2D Raman-THz signal provides more information on the ion effects on the collective water motion than the conventional 1D signal. The presence of MgCl2 suppresses the cross peak of water between the hydrogen bond bending and the other intermolecular vibrational mode, which clearly illustrates that the water hydrogen bending motion is affected by the confining effect of the ions. Our theoretical work thus demonstrates that the 2D Raman-THz technique can become a valuable nonlinear vibrational probe for the molecular dynamics in the ionic solutions.
Global Hall-MHD simulations of magnetorotational instability in a plasma Couette flow experiment
Ebrahimi, F.; Lefebvre, B.; Bhattacharjee, A.; Forest, C. B.
2011-06-15
Global MHD and Hall-MHD numerical simulations relevant to the Madison plasma Couette flow experiment (MPCX) have been performed using the extended MHD code NIMROD. The MPCX has been constructed to study the magnetorotational instability (MRI) in a plasma. The two-fluid Hall effect, which is relevant to some astrophysical situations such as protostellar disks, is also expected to be important in the MPCX. Here, we first derive the local Hall dispersion relation including viscosity, extending earlier work by Balbus and Terquem [Astrophys. J. 552, 235 (2001)]. The predictions of the local analysis are then compared with nonlocal calculations of linear stability of the MRI for a parameter range relevant to the MPCX. It is found that the MHD stability limit and mode structure are altered by the Hall term, and nonlocal analysis is necessary to obtain quantitatively reliable predictions for MPCX. Two-fluid physics also significantly changes the nonlinear evolution and saturation of the axisymmetric MRI. Both the Reynolds and Maxwell stresses contribute significantly to momentum transport. In the Hall regime, when the magnetic field is parallel to the rotation axis, the Maxwell stress is larger than the Reynolds stress (similar to the MHD regime). However, when the magnetic field is antiparallel to the rotation axis in the Hall regime, the Reynolds stress is much larger than the Maxwell stress. To further study the role of non-axisymmetric modes, we have also carried out fully nonlinear MHD computations. Non-axisymmetric modes play an increasingly important role as the magnetic Reynolds number increases and grow to large amplitudes in a saturated turbulent state.
Numerical simulation of ( T 2, T 1) 2D NMR and fluid responses
NASA Astrophysics Data System (ADS)
Tan, Mao-Jin; Zou, You-Long; Zhang, Jin-Yan; Zhao, Xin
2012-12-01
One-dimensional nuclear magnetic resonance (1D NMR) logging technology is limited for fluid typing, while two-dimensional nuclear magnetic resonance (2D NMR) logging can provide more parameters including longitudinal relaxation time ( T 1) and transverse relaxation time ( T 2) relative to fluid types in porous media. Based on the 2D NMR relaxation mechanism in a gradient magnetic field, echo train simulation and 2D NMR inversion are discussed in detail. For 2D NMR inversion, a hybrid inversion method is proposed based on the damping least squares method (LSQR) and an improved truncated singular value decomposition (TSVD) algorithm. A series of spin echoes are first simulated with multiple waiting times ( T W s) in a gradient magnetic field for given fluid models and these synthesized echo trains are inverted by the hybrid method. The inversion results are consistent with given models. Moreover, the numerical simulation of various fluid models such as the gas-water, light oil-water, and vicious oil-water models were carried out with different echo spacings ( T E s) and T W s by this hybrid method. Finally, the influences of different signal-to-noise ratios (SNRs) on inversion results in various fluid models are studied. The numerical simulations show that the hybrid method and optimized observation parameters are applicable to fluid typing of gas-water and oil-water models.
NASA Astrophysics Data System (ADS)
Sirait, S. H.; Edison, R. E.; Baidillah, M. R.; Taruno, W. P.; Haryanto, F.
2016-08-01
The aim of this study is to simulate the potential distribution of 2D brain geometry based on two electrodes ECVT. ECVT (electrical capacitance tomography) is a tomography modality which produces dielectric distribution image of a subject from several capacitance electrodes measurements. This study begins by producing the geometry of 2D brain based on MRI image and then setting the boundary conditions on the boundaries of the geometry. The values of boundary conditions follow the potential values used in two electrodes brain ECVT, and for this reason the first boundary is set to 20 volt and 2.5 MHz signal and another boundary is set to ground. Poisson equation is implemented as the governing equation in the 2D brain geometry and finite element method is used to solve the equation. Simulated Hodgkin-Huxley action potential is applied as disturbance potential in the geometry. We divide this study into two which comprises simulation without disturbance potential and simulation with disturbance potential. From this study, each of time dependent potential distributions from non-disturbance and disturbance potential of the 2D brain geometry has been generated.
NASA Astrophysics Data System (ADS)
Ando, Tsutomu; Ueno, Kazuyuki; Sawada, Keisuke
Numerical simulation at the same condition as an experiment is carried out under the magnetic Stokes approximation for small shielding parameter. Results of the simulation compensate for the information of molten metal flow that we could not directly obtain in the experiment. In this paper, we study the molten metal flow at a starting condition and quasi-steady state. Besides, the energy conversion in the MHD pump is discussed. The simulation result shows that the proposed MHD pump causes the spiral induced current in a molten gallium and produces an axial flow with swirl. At quasi-steady state, it is confirmed that the centrifugal force by the excessive swirl flow produces high pressure at a duct wall and low pressure around the central axis. Since the excessive swirl flow results in large viscous dissipation, the mechanical power output of the pump uses only about 1% of the mechanical energy production in the molten gallium.
The Contribution of Jets to Coronal and Solar Wind Energetics: MHD Simulations
NASA Astrophysics Data System (ADS)
Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Linker, Jon A.; Mikic, Zoran; Leake, James E.; Linton, Mark
2016-05-01
Transient collimated plasma eruptions in the corona, commonly known as coronal jets, are among the most interesting manifestations of solar activity.We use the 3D MHD model with thermodynamics developed at PSI to investigate the origin, dynamics, and plasma properties of coronal jets.Our model is coupled with 3D MHD flux emergence simulations, i.e, we use boundary conditions provided by such simulations to drive a time-dependent coronal evolution. It includes parametric coronal heating, radiative losses, and thermal conduction in the energy equations.This enables us to simulate the energy transfer in coronal jets in a more realistic manner than done so far and to study the amount of energy and mass transported by these phenomena into the higher corona and inner heliosphere. We discuss our results and compare them with previous estimations obtained from observations.
MHD simulation of the solar wind interaction with the magnetosphere of Mercury
NASA Astrophysics Data System (ADS)
Varela, Jacobo; Pantellini, Filippo; Moncuquet, Michel
2014-05-01
We show MHD simulations of the solar wind interaction with the magnetosphere of Mercury. We use the open source codes Pluto and MPI-AMRVAC in 3 dimensional spherical geometry. In order to appreciate the limits of the MHD approach in the context of Mercury's environment we do first compare our simulations with hybrid simulation (e.g. Trávníček et al, Icarus, 209, pp 11-22, 2010). We do also compare magnetic field profiles from the magnetometer on Messenger with profiles sampled along the corresponding spacecraft trajectory in the simulations. These comparisons show that despite the lack of kinetic effects, MHD simulation provide a more than fair description of the interaction of the solar wind with Mercury at low computational cost making it a useful tool to help decrypt data from current and future exploratory missions in the hermean magnetosphere (e.g. Bepi Colombo-MMO). The research leading to these results has received funding from the European Commission's Seventh Framework Programme (FP7/2007-2013) under the grant agreement SHOCK (project number 284515).
The simulation of 3D mass models in 2D digital mammography and breast tomosynthesis
Shaheen, Eman De Keyzer, Frederik; Bosmans, Hilde; Ongeval, Chantal Van; Dance, David R.; Young, Kenneth C.
2014-08-15
Purpose: This work proposes a new method of building 3D breast mass models with different morphological shapes and describes the validation of the realism of their appearance after simulation into 2D digital mammograms and breast tomosynthesis images. Methods: Twenty-five contrast enhanced MRI breast lesions were collected and each mass was manually segmented in the three orthogonal views: sagittal, coronal, and transversal. The segmented models were combined, resampled to have isotropic voxel sizes, triangularly meshed, and scaled to different sizes. These masses were referred to as nonspiculated masses and were then used as nuclei onto which spicules were grown with an iterative branching algorithm forming a total of 30 spiculated masses. These 55 mass models were projected into 2D projection images to obtain mammograms after image processing and into tomographic sequences of projection images, which were then reconstructed to form 3D tomosynthesis datasets. The realism of the appearance of these mass models was assessed by five radiologists via receiver operating characteristic (ROC) analysis when compared to 54 real masses. All lesions were also given a breast imaging reporting and data system (BIRADS) score. The data sets of 2D mammography and tomosynthesis were read separately. The Kendall's coefficient of concordance was used for the interrater observer agreement assessment for the BIRADS scores per modality. Further paired analysis, using the Wilcoxon signed rank test, of the BIRADS assessment between 2D and tomosynthesis was separately performed for the real masses and for the simulated masses. Results: The area under the ROC curves, averaged over all observers, was 0.54 (95% confidence interval [0.50, 0.66]) for the 2D study, and 0.67 (95% confidence interval [0.55, 0.79]) for the tomosynthesis study. According to the BIRADS scores, the nonspiculated and the spiculated masses varied in their degrees of malignancy from normal (BIRADS 1) to highly
NASA Astrophysics Data System (ADS)
Ngwira, Chigomezyo M.; Pulkkinen, Antti; Kuznetsova, Maria M.; Glocer, Alex
2014-06-01
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.
2D-3D hybrid stabilized finite element method for tsunami runup simulations
NASA Astrophysics Data System (ADS)
Takase, S.; Moriguchi, S.; Terada, K.; Kato, J.; Kyoya, T.; Kashiyama, K.; Kotani, T.
2016-09-01
This paper presents a two-dimensional (2D)-three-dimensional (3D) hybrid stabilized finite element method that enables us to predict a propagation process of tsunami generated in a hypocentral region, which ranges from offshore propagation to runup to urban areas, with high accuracy and relatively low computational costs. To be more specific, the 2D shallow water equation is employed to simulate the propagation of offshore waves, while the 3D Navier-Stokes equation is employed for the runup in urban areas. The stabilized finite element method is utilized for numerical simulations for both of the 2D and 3D domains that are independently discretized with unstructured meshes. The multi-point constraint and transmission methods are applied to satisfy the continuity of flow velocities and pressures at the interface between the resulting 2D and 3D meshes, since neither their spatial dimensions nor node arrangements are consistent. Numerical examples are presented to demonstrate the performance of the proposed hybrid method to simulate tsunami behavior, including offshore propagation and runup to urban areas, with substantially lower computation costs in comparison with full 3D computations.
Effects of the driving mechanism in MHD simulations of coronal mass ejections
NASA Technical Reports Server (NTRS)
Linker, J. A.; Van Hoven, G.; Schnack, D. D.
1990-01-01
Results of time-dependent MHD simulations of mass ejections in the solar coronal are presented. Previous authors have shown that results from simulations using a thermal driving mechanism are consistent with the observations only if an elaborate model of the initial corona is used. The first simulation effort, using a simple model of a plasmoid as the driving mechanism and a simple model of the initial corona, produces results that are also consistent with many observational features, suggesting that the nature of the driving mechanism plays an important role in determining the subsequent evolution of mass ejections. First simulations are based on the assumption that mass ejections are driven by magnetic forces.
Numerical Simulation of 3-D Supersonic Viscous Flow in an Experimental MHD Channel
NASA Technical Reports Server (NTRS)
Kato, Hiromasa; Tannehill, John C.; Gupta, Sumeet; Mehta, Unmeel B.
2004-01-01
The 3-D supersonic viscous flow in an experimental MHD channel has been numerically simulated. The experimental MHD channel is currently in operation at NASA Ames Research Center. The channel contains a nozzle section, a center section, and an accelerator section where magnetic and electric fields can be imposed on the flow. In recent tests, velocity increases of up to 40% have been achieved in the accelerator section. The flow in the channel is numerically computed using a new 3-D parabolized Navier-Stokes (PNS) algorithm that has been developed to efficiently compute MHD flows in the low magnetic Reynolds number regime. The MHD effects are modeled by introducing source terms into the PNS equations which can then be solved in a very e5uent manner. To account for upstream (elliptic) effects, the flowfield can be computed using multiple streamwise sweeps with an iterated PNS algorithm. The new algorithm has been used to compute two test cases that match the experimental conditions. In both cases, magnetic and electric fields are applied to the flow. The computed results are in good agreement with the available experimental data.
2D and 3D simulations of damage in 5-grain copper gas gun samples
Tonks, Davis L; Cerreta, Ellen K; Dennis - Koller, Darcie; Escobedo - Diaz, Juan P; Trujillo, Carl P; Luo, Shengian; Bingert, John F
2010-12-16
2D and 3D Hydrocode simulations were done of a gas gun damage experiment involving a 5 grain sample with a polycrystalline flyer with a velocity of about 140 m/s. The simulations were done with the Flag hydrocode and involved explicit meshing of the 5 grains with a single crystal plasticity model and a pressure based damage model. The calculated fields were compared with two cross sections from the recovered sample. The sample exhibited grain boundary cracks at high angle and tilt grain boundaries in the sample but not at a sigma 3 twin boundary. However, the calculation showed large gradients in stress and strain at only the twin boundary, contrary to expectation. This indicates that the twin boundary is quite strong to resist the predicted high gradients and that the calculation needs the addition of a grain boundary fracture mode. The 2D and 3D simulations were compared.
FRANC2D: A two-dimensional crack propagation simulator. Version 2.7: User's guide
NASA Technical Reports Server (NTRS)
Wawrzynek, Paul; Ingraffea, Anthony
1994-01-01
FRANC 2D (FRacture ANalysis Code, 2 Dimensions) is a menu driven, interactive finite element computer code that performs fracture mechanics analyses of 2-D structures. The code has an automatic mesh generator for triangular and quadrilateral elements. FRANC2D calculates the stress intensity factor using linear elastic fracture mechanics and evaluates crack extension using several methods that may be selected by the user. The code features a mesh refinement and adaptive mesh generation capability that is automatically developed according to the predicted crack extension direction and length. The code also has unique features that permit the analysis of layered structure with load transfer through simulated mechanical fasteners or bonded joints. The code was written for UNIX workstations with X-windows graphics and may be executed on the following computers: DEC DecStation 3000 and 5000 series, IBM RS/6000 series, Hewlitt-Packard 9000/700 series, SUN Sparc stations, and most Silicon Graphics models.
An MHD simulation model of time-dependent co-rotating solar wind
NASA Astrophysics Data System (ADS)
Hayashi, K.
2012-08-01
We present a treatment of observation-based time-dependent boundary conditions for the inner boundary sphere in the time-dependent three-dimensional MHD simulations of the global solar wind. With this boundary treatment, we obtain super-Alfvenic MHD solutions of time-dependent co-rotating solar wind structures. The boundary variables on the inner boundary sphere, at 50 solar radii in this study, are assumed to change linearly from one instant to the next. A new feature is that, in order to maintain the divergence-free condition of the magnetic field, the changes of the time-dependent boundary magnetic field are expressed as the potential field in a thin shell volume. The solar magnetic field data from the Wilcox Solar Observatory (WSO) and the solar wind speed data from the interplanetary scintillation (IPS) observations at Nagoya University, Japan, are used as the input boundary data. The solar wind simulated with the time-dependent boundary condition is compared with the near-Earth and Ulysses in situ measurement data and the solar wind simulated with the fixed boundary condition over a 7-month period in 1991. Reasonable agreements with the in situ measurements are obtained. The differences between the two simulations in the interplanetary field line paths are significant. The three-dimensional time-dependent MHD solution of the global solar wind will help enhance space weather models and other fields in heliophysics.
Modeling of substorm development with a kinematic effect by the global MHD simulations
NASA Astrophysics Data System (ADS)
den, Mitsue; Fujita, Shigeru; Tanaka, Takashi; Horiuchi, Ritoku
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.
Simulation of Cardiac Arrhythmias Using a 2D Heterogeneous Whole Heart Model.
Balakrishnan, Minimol; Chakravarthy, V Srinivasa; Guhathakurta, Soma
2015-01-01
Simulation studies of cardiac arrhythmias at the whole heart level with electrocardiogram (ECG) gives an understanding of how the underlying cell and tissue level changes manifest as rhythm disturbances in the ECG. We present a 2D whole heart model (WHM2D) which can accommodate variations at the cellular level and can generate the ECG waveform. It is shown that, by varying cellular-level parameters like the gap junction conductance (GJC), excitability, action potential duration (APD) and frequency of oscillations of the auto-rhythmic cell in WHM2D a large variety of cardiac arrhythmias can be generated including sinus tachycardia, sinus bradycardia, sinus arrhythmia, sinus pause, junctional rhythm, Wolf Parkinson White syndrome and all types of AV conduction blocks. WHM2D includes key components of the electrical conduction system of the heart like the SA (Sino atrial) node cells, fast conducting intranodal pathways, slow conducting atriovenctricular (AV) node, bundle of His cells, Purkinje network, atrial, and ventricular myocardial cells. SA nodal cells, AV nodal cells, bundle of His cells, and Purkinje cells are represented by the Fitzhugh-Nagumo (FN) model which is a reduced model of the Hodgkin-Huxley neuron model. The atrial and ventricular myocardial cells are modeled by the Aliev-Panfilov (AP) two-variable model proposed for cardiac excitation. WHM2D can prove to be a valuable clinical tool for understanding cardiac arrhythmias.
Simulation of Cardiac Arrhythmias Using a 2D Heterogeneous Whole Heart Model
Balakrishnan, Minimol; Chakravarthy, V. Srinivasa; Guhathakurta, Soma
2015-01-01
Simulation studies of cardiac arrhythmias at the whole heart level with electrocardiogram (ECG) gives an understanding of how the underlying cell and tissue level changes manifest as rhythm disturbances in the ECG. We present a 2D whole heart model (WHM2D) which can accommodate variations at the cellular level and can generate the ECG waveform. It is shown that, by varying cellular-level parameters like the gap junction conductance (GJC), excitability, action potential duration (APD) and frequency of oscillations of the auto-rhythmic cell in WHM2D a large variety of cardiac arrhythmias can be generated including sinus tachycardia, sinus bradycardia, sinus arrhythmia, sinus pause, junctional rhythm, Wolf Parkinson White syndrome and all types of AV conduction blocks. WHM2D includes key components of the electrical conduction system of the heart like the SA (Sino atrial) node cells, fast conducting intranodal pathways, slow conducting atriovenctricular (AV) node, bundle of His cells, Purkinje network, atrial, and ventricular myocardial cells. SA nodal cells, AV nodal cells, bundle of His cells, and Purkinje cells are represented by the Fitzhugh-Nagumo (FN) model which is a reduced model of the Hodgkin-Huxley neuron model. The atrial and ventricular myocardial cells are modeled by the Aliev-Panfilov (AP) two-variable model proposed for cardiac excitation. WHM2D can prove to be a valuable clinical tool for understanding cardiac arrhythmias. PMID:26733873
Substorm effects in MHD and test particle simulations of magnetotail dynamics
Birn, J.; Hesse, M.
1998-12-31
Recent magnetohydrodynamic simulations demonstrate that a global tail instability, initiated by localized breakdown of MHD, can cause plasmoid formation and ejection as well as dipolarization and the current diversion of the substorm current wedge. The connection between the reconnection process and the current wedge signatures is provided by earthward flow from the reconnection site. Its braking and diversion in the inner magnetosphere causes dipolarization and the magnetic field distortions of the current wedge. The authors demonstrate the characteristic properties of this process and the current systems involved. The strong localized electric field associated with the flow burst and the dipolarization is also the cause of particle acceleration and energetic particle injections. Test particle simulations of orbits in the MHD fields yield results that are quite consistent with observed injection signatures.
SOLAR WIND TURBULENCE FROM MHD TO SUB-ION SCALES: HIGH-RESOLUTION HYBRID SIMULATIONS
Franci, Luca; Verdini, Andrea; Landi, Simone; Matteini, Lorenzo; Hellinger, Petr
2015-05-10
We present results from a high-resolution and large-scale hybrid (fluid electrons and particle-in-cell protons) two-dimensional numerical simulation of decaying turbulence. Two distinct spectral regions (separated by a smooth break at proton scales) develop with clear power-law scaling, each one occupying about a decade in wavenumbers. The simulation results simultaneously exhibit several properties of the observed solar wind fluctuations: spectral indices of the magnetic, kinetic, and residual energy spectra in the magnetohydrodynamic (MHD) inertial range along with a flattening of the electric field spectrum, an increase in magnetic compressibility, and a strong coupling of the cascade with the density and the parallel component of the magnetic fluctuations at sub-proton scales. Our findings support the interpretation that in the solar wind, large-scale MHD fluctuations naturally evolve beyond proton scales into a turbulent regime that is governed by the generalized Ohm’s law.
Constrained-transport Hall-MHD simulations using CWENO reconstruction with libMRC
NASA Astrophysics Data System (ADS)
Lin, Liwei; Germaschewski, Kai; Abbott, Stephen; Maynard, Kris; Raeder, Jimmy
2013-10-01
We present a new CWENO (Centrally-Weighted Essentially Non-Oscillatory) reconstruction based extended MHD (XMHD) solver that has been built for libMRC. libMRC is a library for creating efficient parallel PDE solvers on structured grids, which is used in the MRC (Magnetic Reconnection Code), OpenGGCM (Open Global Geospace Circulation Model) and PSC (Plasma Simulation Code) codes. The use of libMRC gives us access to its core functionality of providing an automated code generation framework which takes a user provided PDE right hand side in symbolic form to generate an efficient, computer-architecture specific, parallel code. libMRC also supports block-structured adaptive mesh refinement, and implicit-time stepping through integration with the PETSc library. We demonstrate validation of the new CWENO MHD solver against existing solvers both in standard test problems as well as in 3D global magnetosphere simulations.
Gas Core Reactor Numerical Simulation Using a Coupled MHD-MCNP Model
NASA Technical Reports Server (NTRS)
Kazeminezhad, F.; Anghaie, S.
2008-01-01
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.
NASA Technical Reports Server (NTRS)
Tang, H. T.; Hofmann, R.; Yee, G.; Vaughan, D. K.
1980-01-01
Transient, nonlinear soil-structure interaction simulations of an Electric Power Research Institute, SIMQUAKE experiment were performed using the large strain, time domain STEALTH 2D code and a cyclic, kinematically hardening cap soil model. Results from the STEALTH simulations were compared to identical simulations performed with the TRANAL code and indicate relatively good agreement between all the STEALTH and TRANAL calculations. The differences that are seen can probably be attributed to: (1) large (STEALTH) vs. small (TRANAL) strain formulation and/or (2) grid discretization differences.
Momentum Transport: 2D and 3D Cloud Resolving Model Simulations
NASA Technical Reports Server (NTRS)
Tao, Wei-Kuo
2001-01-01
The major objective of this study is to investigate the momentum budgets associated with several convective systems that developed during the TOGA COARE IOP (west Pacific warm pool region) and GATE (east Atlantic region). The tool for this study is the improved Goddard Cumulas Ensemble (GCE) model which includes a 3-class ice-phase microphysical scheme, explicit cloud radiative interactive processes and air-sea interactive surface processes. The model domain contains 256 x 256 grid points (with 2 km resolution) in the horizontal and 38 grid points (to a depth of 22 km) in the vertical. The 2D domain has 1024 grid points. The simulations were performed over a 7-day time period (December 19-26, 1992, for TOGA COARE and September 1-7, 1994 for GATE). Cyclic literal boundary conditions are required for this type of long-term integration. Two well organized squall systems (TOGA, COARE February 22, 1993, and GATE September 12, 1994) were also simulated using the 3D GCE model. Only 9 h simulations were required to cover the life time of the squall systems. the lateral boundary conditions were open for these two squall systems simulations. the following will be examined: (1) the momentum budgets in the convective and stratiform regions, (2) the relationship between momentum transport and cloud organization (i.e., well organized squall lines versus less organized convective), (3) the differences and similarities in momentum transport between 2D and 3D simulated convective systems, and (4) the differences and similarities in momentum budgets between cloud systems simulated with open and cyclic lateral boundary conditions. Preliminary results indicate that there are only small differences between 2D and 3D simulated momentum budgets. Major differences occur, however, between momentum budgets associated with squall systems simulated using different lateral boundary conditions.
Thermodynamic MHD Simulation of the 2000 July 14 "Bastille Day" Eruption
NASA Astrophysics Data System (ADS)
Torok, Tibor; Downs, Cooper; Lionello, Roberto; Linker, Jon A.; Titov, Viacheslav S.; Mikic, Zoran; Riley, Pete
2015-04-01
The "Bastille Day" event that occurred on 2000 July 14 is one of the most extensively studied solar eruptions. It originated in a complex active region close to disk center and produced an X5.7 flare, a fast halo CME, and an intense geomagnetic storm. Accurate numerical simulations of such events, in particular the matching of parameters relevant for space weather such as the CME velocity and magnetic orientation, require a realistic model of the large-scale magnetic field and plasma environment into which the eruption propagates and interacts, as well as a modeling of the pre-eruptive configuration and eruption initiation that are as realistic as possible. Here we present an MHD simulation of the Bastille Day event that complies with these requirements. We first produce a steady-state MHD solution of the background corona that incorporates realistic energy transport ("thermodynamic MHD"), photospheric magnetic field measurements, and the solar wind. In order to model the pre-eruptive magnetic field, we then insert a stable, elongated flux rope that resides above the highly curved polarity inversion line of the active region. Finally, we produce the eruption by imposing photospheric flows that slowly converge towards the polarity inversion line. In this presentation we describe our method, compare the simulation results with the observations, and discuss the challenges and limitations involved in modeling such complex and powerful eruptions.
MHD (magnetohydrodynamic) simulation of a comet magnetosphere. Memorandum report
Fedder, J.A.; Brecht, S.H.; Lyon, J.G.
1984-04-12
This paper presents results of a numerical magnetohydrodynamic simulation of the interaction of the solar wind with a comet. It states that for a steady solar wind and interplanetary magnetic field (IMF) the cometary plasma has a distinctive structure; a spheroidal head and a long ribbon-like tail. Rotational discontinuities in the IMF lead to changes in the tail structure. It is shown how these effects occur and describe ray-like structures as well as a tail disconnection event. The simulation results provide a simple explanation for a number of observable features in cometary plasma tails.
3-D MHD disk wind simulations of protostellar jets
NASA Astrophysics Data System (ADS)
Staff, Jan E.; Koning, Nico; Ouyed, Rachid; Tanaka, Kei; Tan, Jonathan C.
2016-01-01
We present the results of large scale, three-dimensional magnetohydrodynamics simulations of disk winds for different initial magnetic field configurations. The jets are followed from the source to distances, which are resolvable by HST and ALMA observations. Our simulations show that jets are heated along their length by many shocks. The mass of the protostar is a free parameter that can be inserted in the post processing of the data, and we apply the simulations to both low mass and high mass protostars. For the latter we also compute the expected diagnostics when the outflow is photoionized by the protostar. We compute the emission lines that are produced, and find excellent agreement with observations. For a one solar mass protostar, we find the jet width to be between 20 and 30 au while the maximum velocities perpendicular to the jet are found to be 100 km s-1. The initially less open magnetic field configuration simulations result in a wider, two-component jet; a cylindrically shaped outer jet surrounding a narrow and much faster, inner jet. For the initially most open magnetic field configuration the kink mode creates a narrow corkscrew-like jet without a clear Keplerian rotation profile and even regions where we observe rotation opposite to the disk (counter-rotating). This is not seen in the less open field configurations.
PROPERTIES OF UMBRAL DOTS AS MEASURED FROM THE NEW SOLAR TELESCOPE DATA AND MHD SIMULATIONS
Kilcik, A.; Yurchyshyn, V. B.; Abramenko, V.; Goode, P. R.; Cao, W.; Rempel, M.; Kitai, R.; Watanabe, H.
2012-02-01
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.
Two-dimensional magnetohydrodynamic simulations of poloidal flows in tokamaks and MHD pedestal
Guazzotto, L.; Betti, R.
2011-09-15
Poloidal rotation is routinely observed in present-day tokamak experiments, in particular near the plasma edge and in the high-confinement mode of operation. According to the magnetohydrodynamic (MHD) equilibrium theory [R. Betti and J. P. Freidberg, Phys. Plasmas 7, 2439 (2000)], radial discontinuities form when the poloidal velocity exceeds the poloidal sound speed (or rather, more correctly, the poloidal magneto-slow speed). Two-dimensional compressible magnetohydrodynamic simulations show that the transonic discontinuities develop on a time scale of a plasma poloidal revolution to form an edge density pedestal and a localized velocity shear layer at the pedestal location. While such an MHD pedestal surrounds the entire core, the outboard side of the pedestal is driven by the transonic discontinuity while the inboard side is caused by a poloidal redistribution of the mass. The MHD simulations use a smooth momentum source to drive the poloidal flow. Soon after the flow exceeds the poloidal sound speed, the density pedestal and the velocity shear layer form and persist into a quasi steady state. These results may be relevant to the L-H transition, the early stages of the pedestal and edge transport barrier formation.
NASA Astrophysics Data System (ADS)
Hayashi, K.; Zhao, X.; Liu, Y.
2005-12-01
As the proxy of the entire surface distribution of the photospheric magnetic field at a specific time, the synoptic frame of the photospheric magnetic field [Zhao et al., 1999] consists of a magnetogram that contains the real field configuration and newly emerging magnetic features at the observational time, and a part of synoptic charts around the time of interest. Recently we improve the synoptic frame by removing the effect of differential rotation existing in the part of synoptic charts around the specific time. In this work, we will present the results of the time-dependent three-dimensional MHD simulation using this improved synoptic frame of photospheric magnetic field. The MHD simulation code we have developed is based on the TVD-MUSCL strategy, and the projected normal characteristic method is used to deal with the sub-Alfvenic boundary at 1 R_s to obtain the trans-alfvenic nonlinear-MHD solution of the solar wind and corona. The calculated solar coronal quantities are compared with the observations, in terms of the magnetic field topology, such as the shape of the coronal hole.
Nonlinear three-dimensional MHD simulations of tearing modes in tokamak plasmas
NASA Astrophysics Data System (ADS)
Lütjens, H.; Luciani, J. F.; Garbet, X.
2001-12-01
The comprehension of the dynamics of classical and neoclassical tearing modes is a key issue in high-performance tokamak plasmas. Avoiding these instabilities requires a good knowledge of all the physical mechanisms involved in their linear and/or nonlinear onset. Our tridimensional time evolution code XTOR, which solves the full magnetohydrodynamic (MHD) equations including thermal transport, is used to tackle this difficult problem. In this paper, to show the state of art in full-scale nonlinear MHD simulations of tokamak plasmas, we investigate the effect of plasma curvature on the tearing mode dynamics. For a realistic picture of this dynamics, heat diffusion is required in the linear regimes as well, as in the nonlinear regimes. We present a new dispersion relation including perpendicular and parallel transport, and show that it matches the linear and nonlinear regimes. This leads to a new tearing mode island evolution equation including curvature effects, valid for every island size in tokamak plasmas. This equation predicts a nonlinearly unstable regime for tearing instabilities, i.e. a regime which is linearly stable, but where the tearing mode can be destabilized nonlinearly by a finite-size seed island. These theoretical predictions are in good agreement with XTOR simulations. In particular, the nonlinear instability due to curvature effects is reproduced. Our results have an important impact on the onset mechanism of neoclassical tearing modes. They indeed predict that curvature effects lead to a resistive MHD threshold.
Explosive Turbulent Magnetic Reconnection: A New Approach of MHD-Turbulent Simulation
NASA Astrophysics Data System (ADS)
Hoshino, Masahiro; Yokoi, Nobumitsu; Higashimori, Katsuaki
2013-04-01
Turbulent flows are often observed in association with magnetic reconnection in space and astrophysical plasmas, and it is often hypothesized that the turbulence can contribute to the fast magnetic reconnection through the enhancement of magnetic dissipation. In this presentation, we demonstrate that an explosive turbulent reconnection can happen by using a new turbulent MHD simulation, in which the evolution of the turbulent transport coefficients are self-consistently solved together with the standard MHD equations. In our model, the turbulent electromotive force defined by the correlation of turbulent fluctuations between v and B is added to the Ohm's law. We discuss that the level of turbulent can control the topology of reconnection, namely the transition from the Sweet-Parker reconnection to the Petscheck reconnection occurs when the level of fluctuations becomes of order of the ambient physical quantities, and show that the growth of the turbulent Petscheck reconnection becomes much faster than the conventional one.
Penetration of tungsten-alloy rods into composite ceramic targets: Experiments and 2-D simulations
Rosenberg, Z.; Dekel, E.; Hohler, V.; Stilp, A. J.; Weber, K.
1998-07-10
A series of terminal ballistics experiments, with scaled tungsten-alloy penetrators, was performed on composite targets consisting of ceramic tiles glued to thick steel backing plates. Tiles of silicon-carbide, aluminum nitride, titanium-dibroide and boron-carbide were 20-80 mm thick, and impact velocity was 1.7 km/s. 2-D numerical simulations, using the PISCES code, were performed in order to simulate these shots. It is shown that a simplified version of the Johnson-Holmquist failure model can account for the penetration depths of the rods but is not enough to capture the effect of lateral release waves on these penetrations.
2D Spinodal Decomposition in Forced Turbulence
NASA Astrophysics Data System (ADS)
Fan, Xiang; Diamond, Patrick; Chacon, Luis; Li, Hui
2015-11-01
Spinodal decomposition is a second order phase transition for binary fluid mixture, from one thermodynamic phase to form two coexisting phases. The governing equation for this coarsening process below critical temperature, Cahn-Hilliard Equation, is very similar to 2D MHD Equation, especially the conserved quantities have a close correspondence between each other, so theories for MHD turbulence are used to study spinodal decomposition in forced turbulence. Domain size is increased with time along with the inverse cascade, and the length scale can be arrested by a forced turbulence with direct cascade. The two competing mechanisms lead to a stabilized domain size length scale, which can be characterized by Hinze Scale. The 2D spinodal decomposition in forced turbulence is studied by both theory and simulation with ``pixie2d.'' This work focuses on the relation between Hinze scale and spectra and cascades. Similarities and differences between spinodal decomposition and MHD are investigated. Also some transport properties are studied following MHD theories. This work is supported by the Department of Energy under Award Number DE-FG02-04ER54738.
Magnetopause erosion: A global view from MHD simulation
NASA Astrophysics Data System (ADS)
Wiltberger, M.; Lopez, R. E.; Lyon, J. G.
2002-12-01
In this paper we use a global magnetohydrodynamic simulation of the magnetosphere to examine the behavior of the magnetopause position when the interplanetary magnetic field suddenly changes from northward to southward. The inward motion of the magnetopause under the influence of a southward IMF is generally referred to as magnetopause "erosion." Physical models to explain erosion have been proposed, notably the "onion" peeling model, a model based on the effects of fringe fields from the Region 1 Birkeland currents and the nightside cross-tail current. The simulation shows behavior consistent with aspects of these models, but it also shows behavior that is most consistent with the major driver of erosion being the growth of the nightside cross-tail current. In particular, the simulation exhibits a marked delay between the arrival of the southward IMF at the magnetopause and the inward motion of the magnetopause. We attribute this delay to the delay in the growth of the nightside current system in response to the southward turning of the IMF.
Magnetopause erosion: A global view from MHD simulation
NASA Astrophysics Data System (ADS)
Wiltberger, M.; Lopez, R. E.; Lyon, J. G.
2003-06-01
In this paper we use a global magnetohydrodynamic simulation of the magnetosphere to examine the behavior of the magnetopause position when the interplanetary magnetic field suddenly changes from northward to southward. The inward motion of the magnetopause under the influence of a southward IMF is generally referred to as magnetopause "erosion." Physical models to explain erosion have been proposed, notably the "onion" peeling model, a model based on the effects of fringe fields from the Region 1 Birkeland currents and the nightside cross-tail current. The simulation shows behavior consistent with aspects of these models, but it also shows behavior that is most consistent with the major driver of erosion being the growth of the nightside cross-tail current. In particular, the simulation exhibits a marked delay between the arrival of the southward IMF at the magnetopause and the inward motion of the magnetopause. We attribute this delay to the delay in the growth of the nightside current system in response to the southward turning of the IMF.
NASA Astrophysics Data System (ADS)
Hatori, Tomoharu; Ito, Atsushi M.; Nunami, Masanori; Usui, Hideyuki; Miura, Hideaki
2016-08-01
We propose a numerical method to determine the artificial viscosity in magnetohydrodynamics (MHD) simulations with adaptive mesh refinement (AMR) method, where the artificial viscosity is adaptively changed due to the resolution level of the AMR hierarchy. Although the suitable value of the artificial viscosity depends on the governing equations and the model of target problem, it can be determined by von Neumann stability analysis. By means of the new method, "level-by-level artificial viscosity method," MHD simulations of Rayleigh-Taylor instability (RTI) are carried out with the AMR method. The validity of the level-by-level artificial viscosity method is confirmed by the comparison of the linear growth rates of RTI between the AMR simulations and the simple simulations with uniform grid and uniform artificial viscosity whose resolution is the same as that in the highest level of the AMR simulation. Moreover, in the nonlinear phase of RTI, the secondary instability is clearly observed where the hierarchical data structure of AMR calculation is visualized as high resolution region floats up like terraced fields. In the applications of the method to general fluid simulations, the growth of small structures can be sufficiently reproduced, while the divergence of numerical solutions can be suppressed.
THE SUBMILLIMETER BUMP IN Sgr A* FROM RELATIVISTIC MHD SIMULATIONS
Dexter, Jason; Agol, Eric; Fragile, P. Chris; McKinney, Jonathan C.
2010-07-10
Recent high resolution observations of the Galactic center black hole allow for direct comparison with accretion disk simulations. We compare two-temperature synchrotron emission models from three-dimensional, general relativistic magnetohydrodynamic simulations to millimeter observations of Sgr A*. Fits to very long baseline interferometry and spectral index measurements disfavor the monochromatic face-on black hole shadow models from our previous work. Inclination angles {<=}20{sup 0} are ruled out to 3{sigma}. We estimate the inclination and position angles of the black hole, as well as the electron temperature of the accretion flow and the accretion rate, to be i=50{sup o+35o}{sub -15}{sup o}, {xi}=-23{sup o+97o}{sub -22}{sup o}, T{sub e} = (5.4 {+-} 3.0) x 10{sup 10} K, and M-dot =5{sup +15}{sub -2}x10{sup -9} M{sub sun} yr{sup -1}, respectively, with 90% confidence. The black hole shadow is unobscured in all best-fit models, and may be detected by observations on baselines between Chile and California, Arizona, or Mexico at 1.3 mm or .87 mm either through direct sampling of the visibility amplitude or using closure phase information. Millimeter flaring behavior consistent with the observations is present in all viable models and is caused by magnetic turbulence in the inner radii of the accretion flow. The variability at optically thin frequencies is strongly correlated with that in the accretion rate. The simulations provide a universal picture of the 1.3 mm emission region as a small region near the midplane in the inner radii of the accretion flow, which is roughly isothermal and has {nu}/{nu} {sub c} {approx} 1-20, where {nu} {sub c} is the critical frequency for thermal synchrotron emission.
Quantum simulation of 2D topological physics in a 1D array of optical cavities.
Luo, Xi-Wang; Zhou, Xingxiang; Li, Chuan-Feng; Xu, Jin-Shi; Guo, Guang-Can; Zhou, Zheng-Wei
2015-07-06
Orbital angular momentum of light is a fundamental optical degree of freedom characterized by unlimited number of available angular momentum states. Although this unique property has proved invaluable in diverse recent studies ranging from optical communication to quantum information, it has not been considered useful or even relevant for simulating nontrivial physics problems such as topological phenomena. Contrary to this misconception, we demonstrate the incredible value of orbital angular momentum of light for quantum simulation by showing theoretically how it allows to study a variety of important 2D topological physics in a 1D array of optical cavities. This application for orbital angular momentum of light not only reduces required physical resources but also increases feasible scale of simulation, and thus makes it possible to investigate important topics such as edge-state transport and topological phase transition in a small simulator ready for immediate experimental exploration.
MHD simulations of homologous and cannibalistic coronal mass ejections
NASA Astrophysics Data System (ADS)
Fan, Yuhong; Chatterjee, Piyali
2014-06-01
We present magneto-hydrodynamic simulations of the development of a homologous sequence of coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of the continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulations show that a CME erupting into the open magnetic field created by a preceding CME has a higher speed, and therefore tends to be cannibalistic, catching up and merging with the preceding one into a single fast CME. All the CMEs attained speeds of about 1000 km/s as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of sigmoids and “sigmoid-under-cusp” configurations at a low-coronal source of homologous CMEs.
Constrained Transport vs. Divergence Cleanser Options in Astrophysical MHD Simulations
NASA Astrophysics Data System (ADS)
Lindner, Christopher C.; Fragile, P.
2009-01-01
In previous work, we presented results from global numerical simulations of the evolution of black hole accretion disks using the Cosmos++ GRMHD code. In those simulations we solved the magnetic induction equation using an advection-split form, which is known not to satisfy the divergence-free constraint. To minimize the build-up of divergence error, we used a hyperbolic cleanser function that simultaneously damped the error and propagated it off the grid. We have since found that this method produces qualitatively and quantitatively different behavior in high magnetic field regions than results published by other research groups, particularly in the evacuated funnels of black-hole accretion disks where Poynting-flux jets are reported to form. The main difference between our earlier work and that of our competitors is their use of constrained-transport schemes to preserve a divergence-free magnetic field. Therefore, to study these differences directly, we have implemented a constrained transport scheme into Cosmos++. Because Cosmos++ uses a zone-centered, finite-volume method, we can not use the traditional staggered-mesh constrained transport scheme of Evans & Hawley. Instead we must implement a more general scheme; we chose the Flux-CT scheme as described by Toth. Here we present comparisons of results using the divergence-cleanser and constrained transport options in Cosmos++.
Comparison between 2D and 3D Numerical Modelling of a hot forging simulative test
Croin, M.; Ghiotti, A.; Bruschi, S.
2007-04-07
The paper presents the comparative analysis between 2D and 3D modelling of a simulative experiment, performed in laboratory environment, in which operating conditions approximate hot forging of a turbine aerofoil section. The plane strain deformation was chosen as an ideal case to analyze the process because of the thickness variations in the final section and the consequent distributions of contact pressure and sliding velocity at the interface that are closed to the conditions of the real industrial process. In order to compare the performances of 2D and 3D approaches, two different analyses were performed and compared with the experiments in terms of loads and temperatures peaks at the interface between the dies and the workpiece.
Simulation of the flow and mass transfer for KDP crystals undergoing 2D translation during growth
NASA Astrophysics Data System (ADS)
Zhou, Chuan; Li, Mingwei; Hu, Zhitao; Yin, Huawei; Wang, Bangguo; Cui, Qidong
2016-09-01
In this study, a novel motion mode for crystals during growth, i.e., 2D translation, is proposed. Numerical simulations of flow and mass transfer are conducted for the growth of large-scale potassium dihydrogen phosphate (KDP) crystals subjected to the new motion mode. Surface supersaturation and shear stress are obtained as functions of the translational velocity, distance, size, orientation of crystals. The dependence of these two parameters on the flow fields around the crystals is also discussed. The thicknesses of the solute boundary layer varied with translational velocity are described. The characteristics of solution flow and surface supersaturation distribution are summarized, where it suggests that the morphological stability of a crystal surface can be enhanced if the proposed 2D translation is applied to crystal growth.
Global MHD modeling of resonant ULF waves: Simulations with and without a plasmasphere
NASA Astrophysics Data System (ADS)
Claudepierre, S. G.; Toffoletto, F. R.; Wiltberger, M.
2016-01-01
We investigate the plasmaspheric influence on the resonant mode coupling of magnetospheric ultralow frequency (ULF) waves using the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic (MHD) model. We present results from two different versions of the model, both driven by the same solar wind conditions: one version that contains a plasmasphere (the LFM coupled to the Rice Convection Model, where the Gallagher plasmasphere model is also included) and another that does not (the stand-alone LFM). We find that the inclusion of a cold, dense plasmasphere has a significant impact on the nature of the simulated ULF waves. For example, the inclusion of a plasmasphere leads to a deeper (more earthward) penetration of the compressional (azimuthal) electric field fluctuations, due to a shift in the location of the wave turning points. Consequently, the locations where the compressional electric field oscillations resonantly couple their energy into local toroidal mode field line resonances also shift earthward. We also find, in both simulations, that higher-frequency compressional (azimuthal) electric field oscillations penetrate deeper than lower frequency oscillations. In addition, the compressional wave mode structure in the simulations is consistent with a radial standing wave oscillation pattern, characteristic of a resonant waveguide. The incorporation of a plasmasphere into the LFM global MHD model represents an advance in the state of the art in regard to ULF wave modeling with such simulations. We offer a brief discussion of the implications for radiation belt modeling techniques that use the electric and magnetic field outputs from global MHD simulations to drive particle dynamics.
Numerical simulation of MHD shock waves in the solar wind
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.; Dryer, M.
1978-01-01
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.
Application of 2-D simulations to hollow z-pinch implosions
Peterson, D.L.; Bowers, R.L.; Brownell, J.H.
1997-12-01
The application of simulations of z-pinch implosions should have at least two goals: first, to properly model the most important physical processes occurring in the pinch allowing for a better understanding of the experiments and second, provide a design capability for future experiments. Beginning with experiments fielded at Los Alamos on the Pegasus 1 and Pegasus 2 capacitor banks, the authors have developed a methodology for simulating hollow z-pinches in two dimensions which has reproduced important features of the measured experimental current drive, spectrum, radiation pulse shape, peak power and total radiated energy. This methodology employs essentially one free parameter, the initial level of the random density perturbations imposed at the beginning of the 2-D simulation, but in general no adjustments to other parameters are required. Currently the authors are applying this capability to the analysis of recent Saturn and PBFA-Z experiments. The code results provide insight into the nature of the pinch plasma prior to arrival on-axis, during thermalization and development after peak pinch time. Among other things, the simulation results provide an explanation for the production of larger amounts of radiated energy than would be expected from a simple slug-model kinetic energy analysis and the appearance of multiple peaks in the radiation power. The 2-D modeling has also been applied to the analysis of Saturn dynamic hohlraum experiments and is being used in the design of this and other Z-Pinch applications on PBFA-Z.
NASA Astrophysics Data System (ADS)
Tzeferacos, Petros; Daley, Christopher; Fatenejad, Milad; Flocke, Norbert; Graziani, Carlo; Lamb, Donald Q.; Lee, Dongwook; Scopatz, Anthony; Weide, Klaus; Doyle, Hugo; Gregori, Gianluca; Meinecke, Jena; Reville, Brian; Miniati, Francesco
2013-10-01
The process of generation and amplification of Biermann battery magnetic fields is closely linked to the development of turbulence. In an astrophysical environment, a small seed field can be formed in asymmetric supernova remnant blast waves due to misaligned pressure and density gradients. Inhomogeneities in the density distribution can cause the flow to become turbulent and the B-field can be amplified via dynamo action. In this context, the COSMOLAB team will perform experiments using the Omega EP laser at LLE, that represent a scaled-down model of the astrophysical process in a controlled environment. The experiments involve the illumination of a slab-like target, which produces a plasma flow and a Biermann battery field. The flow then propagates through a grid that creates turbulence and amplifies the field. In this study we describe 2D and 3D radiative MHD simulations of the experimental setup, carried out using the FLASH code on Mira (BG/Q) at ALCF. The objective of these simulations is to explore the morphology and strength of the B-fields generated by ablation of target material by the laser, and their amplification due to the grid. This work was supported by DOE NNSA ASC.
Simulation of 3-D Nonequilibrium Seeded Air Flow in the NASA-Ames MHD Channel
NASA Technical Reports Server (NTRS)
Gupta, Sumeet; Tannehill, John C.; Mehta, Unmeel B.
2004-01-01
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.
NASA Technical Reports Server (NTRS)
Klimas, A. J.; Uritsky, V.; Vassiliadis, D.; Baker, D. N.
2005-01-01
Loading and consequent unloading of magnetic flux is an essential element of the substorm cycle in Earth's magnetotail. We are unaware of an available global MHD magnetospheric simulation model that includes a loading- unloading cycle in its behavior. Given the central role that MHD models presently play in the development of our understanding of magnetospheric dynamics, and given the present plans for the central role that these models will play in ongoing space weather prediction programs, it is clear that this failure must be corrected. A 2-dimensional numerical driven current-sheet model has been developed that incorporates an idealized current- driven instability with a resistive MHD system. Under steady loading, the model exhibits a global loading- unloading cycle. The specific mechanism for producing the loading-unloading cycle will be discussed. It will be shown that scale-free avalanching of electromagnetic energy through the model, from loading to unloading, is carried by repetitive bursts of localized reconnection. Each burst leads, somewhat later, to a field configuration that is capable of exciting a reconnection burst again. This process repeats itself in an intermittent manner while the total field energy in the system falls. At the end of an unloading interval, the total field energy is reduced to well below that necessary to initiate the next unloading event and, thus, a loading-unloading cycle results. It will be shown that, in this model, it is the topology of bursty localized reconnection that is responsible for the appearance of the loading-unloading cycle.
Three-dimensional MHD simulation of the Caltech plasma jet experiment: first results
Zhai, Xiang; Bellan, Paul M.; Li, Hui; Li, Shengtai E-mail: pbellan@caltech.edu E-mail: sli@lanl.gov
2014-08-10
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.
Energy storage and dissipation in the magnetotail during substorms. 2. MHD simulations
Steinolfson, R.S. ); Winglee, R.M. )
1993-05-01
The authors present a global MHD simulation of the magnetotail in an effort to study magnetic storm development. They address the question of energy storage in the current sheet in the early phases of storm growth, which previous simulations have not shown. They address this problem by dealing with the variation of the resistivity throughout the magnetosphere. They argue that MHD theory should provide a suitable representation to this problem on a global scale, even if it does not handle all details adequately. For their simulation they use three different forms for the resistivity. First is a uniform and constant resistivity. Second is a resistivity proportional to the current density, which is related to argument that resistivity is driven by wave-particle interactions which should be strongest in regions where the current is the greatest. Thirdly is a model where the resistivity varies with the magnetic field strength, which was suggested by previous results from particle simulations of the same problem. The simulation then gives approximately the same response of the magnetosphere for all three of the models. Each results in the formation and ejection of plasmoids, but the energy stored in the magnetotail, the timing of substorm onset in relation to the appearance of a southward interplanetary magnetic field, and the speed of ejection of the plasmoids formed differ with the resistivity models.
Double Dynamo Signatures in a Global MHD Simulation and Mean-field Dynamos
NASA Astrophysics Data System (ADS)
Beaudoin, Patrice; Simard, Corinne; Cossette, Jean-François; Charbonneau, Paul
2016-08-01
The 11 year solar activity cycle is the most prominent periodic manifestation of the magnetohydrodynamical (MHD) large-scale dynamo operating in the solar interior, yet longer and shorter (quasi-) periodicities are also present. The so-called “quasi-biennial” signal appearing in many proxies of solar activity has been gaining increasing attention since its detection in p-mode frequency shifts, which suggests a subphotospheric origin. A number of candidate mechanisms have been proposed, including beating between co-existing global dynamo modes, dual dynamos operating in spatially separated regions of the solar interior, and Rossby waves driving short-period oscillations in the large-scale solar magnetic field produced by the 11 year activity cycle. In this article, we analyze a global MHD simulation of solar convection producing regular large-scale magnetic cycles, and detect and characterize shorter periodicities developing therein. By constructing kinematic mean-field α 2Ω dynamo models incorporating the turbulent electromotive force (emf) extracted from that same simulation, we find that dual-dynamo behavior materializes in fairly wide regions of the model’s parameters space. This suggests that the origin of the similar behavior detected in the MHD simulation lies with the joint complexity of the turbulent emf and differential rotation profile, rather that with dynamical interactions such as those mediated by Rossby waves. Analysis of the simulation also reveals that the dual dynamo operating therein leaves a double-period signature in the temperature field, consistent with a dual-period helioseismic signature. Order-of-magnitude estimates for the magnitude of the expected frequency shifts are commensurate with helioseismic measurements. Taken together, our results support the hypothesis that the solar quasi-biennial oscillations are associated with a secondary dynamo process operating in the outer reaches of the solar convection zone.
Coronal extension of the MURaM radiative MHD code: From quiet sun to flare simulations
NASA Astrophysics Data System (ADS)
Rempel, Matthias D.; Cheung, Mark
2016-05-01
We present a new version of the MURaM radiative MHD code, which includes a treatment of the solar corona in terms of MHD, optically thin radiative loss and field-aligned heat conduction. In order to relax the severe time-step constraints imposed by large Alfven velocities and heat conduction we use a combination of semi-relativistic MHD with reduced speed of light ("Boris correction") and a hyperbolic formulation of heat conduction. We apply the numerical setup to 4 different setups including a mixed polarity quiet sun, an open flux region, an arcade solution and an active region setup and find all cases an amount of coronal heating sufficient to maintain a corona with temperatures from 1 MK (quiet sun) to 2 MK (active region, arcade). In all our setups the Poynting flux is self-consistently created by photospheric and sub-photospheric magneto-convection in the lower part of our simulation domain. Varying the maximum allowed Alfven velocity ("reduced speed of light") leads to only minor changes in the coronal structure as long as the limited Alfven velocity remains larger than the speed of sound and about 1.5-3 times larger than the peak advection velocity. We also found that varying details of the numerical diffusivities that govern the resistive and viscous energy dissipation do not strongly affect the overall coronal heating, but the ratio of resistive and viscous energy dependence is strongly dependent on the effective numerical magnetic Prandtl number. We use our active region setup in order to simulate a flare triggered by the emergence of a twisted flux rope into a pre-existing bipolar active region. Our simulation yields a series of flares, with the strongest one reaching GOES M1 class. The simulation reproduces many observed properties of eruptions such as flare ribbons, post flare loops and a sunquake.
The Substorm Current Wedge: Further Insights from MHD Simulations
NASA Technical Reports Server (NTRS)
Birn, J.; Hesse, M.
2015-01-01
Using a recent magnetohydrodynamic simulation of magnetotail dynamics, we further investigate the buildup and evolution of the substorm current wedge (SCW), resulting from flow bursts generated by near-tail reconnection. Each flow burst generates an individual current wedge, which includes the reduction of cross-tail current and the diversion to region 1 (R1)-type field-aligned currents (earthward on the dawn and tailward on the duskside), connecting the tail with the ionosphere. Multiple flow bursts generate initially multiple SCW patterns, which at later times combine to a wider single SCW pattern. The standard SCWmodel is modified by the addition of several current loops, related to particular magnetic field changes: the increase of Bz in a local equatorial region (dipolarization), the decrease of |Bx| away from the equator (current disruption), and increases in |By| resulting from azimuthally deflected flows. The associated loop currents are found to be of similar magnitude, 0.1-0.3 MA. The combined effect requires the addition of region 2 (R2)-type currents closing in the near tail through dawnward currents but also connecting radially with the R1 currents. The current closure at the inner boundary, taken as a crude proxy of an idealized ionosphere, demonstrates westward currents as postulated in the original SCW picture as well as North-South currents connecting R1- and R2-type currents, which were larger than the westward currents by a factor of almost 2. However, this result should be applied with caution to the ionosphere because of our neglect of finite resistance and Hall effects.
Fast acceleration of 2D wave propagation simulations using modern computational accelerators.
Wang, Wei; Xu, Lifan; Cavazos, John; Huang, Howie H; Kay, Matthew
2014-01-01
Recent developments in modern computational accelerators like Graphics Processing Units (GPUs) and coprocessors provide great opportunities for making scientific applications run faster than ever before. However, efficient parallelization of scientific code using new programming tools like CUDA requires a high level of expertise that is not available to many scientists. This, plus the fact that parallelized code is usually not portable to different architectures, creates major challenges for exploiting the full capabilities of modern computational accelerators. In this work, we sought to overcome these challenges by studying how to achieve both automated parallelization using OpenACC and enhanced portability using OpenCL. We applied our parallelization schemes using GPUs as well as Intel Many Integrated Core (MIC) coprocessor to reduce the run time of wave propagation simulations. We used a well-established 2D cardiac action potential model as a specific case-study. To the best of our knowledge, we are the first to study auto-parallelization of 2D cardiac wave propagation simulations using OpenACC. Our results identify several approaches that provide substantial speedups. The OpenACC-generated GPU code achieved more than 150x speedup above the sequential implementation and required the addition of only a few OpenACC pragmas to the code. An OpenCL implementation provided speedups on GPUs of at least 200x faster than the sequential implementation and 30x faster than a parallelized OpenMP implementation. An implementation of OpenMP on Intel MIC coprocessor provided speedups of 120x with only a few code changes to the sequential implementation. We highlight that OpenACC provides an automatic, efficient, and portable approach to achieve parallelization of 2D cardiac wave simulations on GPUs. Our approach of using OpenACC, OpenCL, and OpenMP to parallelize this particular model on modern computational accelerators should be applicable to other computational models of
NASA Astrophysics Data System (ADS)
Mendoza-Torres, F.; Diaz-Viera, M. A.
2015-12-01
In many natural fractured porous media, such as aquifers, soils, oil and geothermal reservoirs, fractures play a crucial role in their flow and transport properties. An approach that has recently gained popularity for modeling fracture systems is the Discrete Fracture Network (DFN) model. This approach consists in applying a stochastic boolean simulation method, also known as object simulation method, where fractures are represented as simplified geometric objects (line segments in 2D and polygons in 3D). One of the shortcomings of this approach is that it usually does not consider the dependency relationships that may exist between the geometric properties of fractures (direction, length, aperture, etc), that is, each property is simulated independently. In this work a method for modeling such dependencies by copula theory is introduced. In particular, a nonparametric model using Bernstein copulas for direction-length fracture dependency in 2D is presented. The application of this method is illustrated in a case study for a fractured rock sample from a carbonate reservoir outcrop.
Simulations of two-particle interactions with 2D quantum walks in time
Schreiber, A.; Laiho, K.; Silberhorn, C.; Rohde, P. P.; Štefaňak, M.; Potoček, V.; Hamilton, C.; Jex, I.
2014-12-04
We present the experimental implementation of a quantum walk on a two-dimensional lattice and show how to employ the optical system to simulate the quantum propagation of two interacting particles. Our quantum walk in time transfers the spatial spread of a quantum walk into the time domain, which guarantees a high stability and scalability of the setup. We present with our device quantum walks over 12 steps on a 2D lattice. By changing the properties of the driving quantum coin, we investigate different kinds of two-particle interactions and reveal their impact on the occurring quantum propagation.
Multipacting Simulation Study for 56 MHz Quarter Wave Resonator using 2D Code
Naik,D.; Ben-Zvi, I.
2009-01-02
A beam excited 56 MHz Radio Frequency (RF) Niobium Quarter Wave Resonator (QWR) has been proposed to enhance RHIC beam luminosity and bunching. Being a RF cavity, multipacting is expected; therefore an extensive study was carried out with the Multipac 2.1 2D simulation code. The study revealed that multipacting occurs in various bands up to peak surface electric field 50 kV/m and is concentrated mostly above the beam gap and on the outer conductor. To suppress multipacting, a ripple structure was introduced to the outer conductor and the phenomenon was successfully eliminated from the cavity.
Tuning and simulating a 193-nm resist for 2D applications
NASA Astrophysics Data System (ADS)
Howard, William B.; Wiaux, Vincent; Ercken, Monique; Bui, Bang; Byers, Jeff D.; Pochkowski, Mike
2002-07-01
For some applications, the usefulness of lithography simulation results depends strongly on the matching between experimental conditions and the simulation input parameters. If this matching is optimized and other sources of error are minimized, then the lithography model can be used to explain printed wafer experimental results. Further, simulation can be useful in predicting the results or in choosing the correct set of experiments. In this paper, PROLITH and ProDATA AutoTune were used to systematically vary simulation input parameters to match measured results on printed wafers used in a 193 nm process. The validity of the simulation parameters was then checked using 3D simulation compared to 2D top-down SEM images. The quality of matching was evaluated using the 1D metrics of average gate CD and Line End Shortening (LES). To ensure the most accurate simulation, a new approach was taken to create a compound mask from GDSII contextual information surrounding an accurate SEM image of the reticle region of interest. Corrections were made to account for all metrology offsets.
Passive stabilization in a linear MHD stability code
Todd, A.M.M.
1980-03-01
Utilizing a Galerkin procedure to calculate the vacuum contribution to the ideal MHD Lagrangian, the implementation of realistic boundary conditions are described in a linear stability code. The procedure permits calculation of the effect of arbitrary conducting structure on ideal MHD instabilities, as opposed to the prior use of an encircling shell. The passive stabilization of conducting coils on the tokamak vertical instability is calculated within the PEST code and gives excellent agreement with 2-D time dependent simulations of PDX.
NASA Astrophysics Data System (ADS)
Den, M.; Horiuchi, R.; Fujita, S.; Tanaka, T.
2011-12-01
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
Relativistic modeling capabilities in PERSEUS extended MHD simulation code for HED plasmas
NASA Astrophysics Data System (ADS)
Hamlin, Nathaniel D.; Seyler, Charles E.
2014-12-01
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.
Relativistic modeling capabilities in PERSEUS extended MHD simulation code for HED plasmas
Hamlin, Nathaniel D.; Seyler, Charles E.
2014-12-15
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.
MHD simulation of a propagation of loop-like and bubble-like magnetic clouds
NASA Technical Reports Server (NTRS)
Vandas, M.; Fischer, S.; Pelant, P.; Dryer, M.; Smith, Z.; Detman, T.
1995-01-01
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.
NASA Astrophysics Data System (ADS)
Gao, Donghong
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
NASA Technical Reports Server (NTRS)
Dryer, M.; Steinolfson, R. S.
1976-01-01
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. A second, twin coronal hole is assumed to rotate on the solar disk behind its predecessor. It is shown that the first coronal hole-produced interplanetary shock ensemble is overtaken by the second ensemble because of the higher velocity, lower density environment into which the latter propagates. A number of features predicted by MHD similarity theory are confirmed by the numerical simulation. These features include (1) strong azimuthal magnetic and plasma density compression, accompanied with average temperature depression, at the contact surface between forward and reverse shock ensembles, and (2) increasing spatial separation distance between forward and reverse shocks.
Strain hardening in 2D discrete dislocation dynamics simulations: A new '2.5D' algorithm
NASA Astrophysics Data System (ADS)
Keralavarma, S. M.; Curtin, W. A.
2016-10-01
The two-dimensional discrete dislocation dynamics (2D DD) method, consisting of parallel straight edge dislocations gliding on independent slip systems in a plane strain model of a crystal, is often used to study complicated boundary value problems in crystal plasticity. However, the absence of truly three dimensional mechanisms such as junction formation means that forest hardening cannot be modeled, unless additional so-called '2.5D' constitutive rules are prescribed for short-range dislocation interactions. Here, results from three dimensional dislocation dynamics (3D DD) simulations in an FCC material are used to define new constitutive rules for short-range interactions and junction formation between dislocations on intersecting slip systems in 2D. The mutual strengthening effect of junctions on preexisting obstacles, such as precipitates or grain boundaries, is also accounted for in the model. The new '2.5D' DD model, with no arbitrary adjustable parameters beyond those obtained from lower scale simulation methods, is shown to predict athermal hardening rates, differences in flow behavior for single and multiple slip, and latent hardening ratios. All these phenomena are well-established in the plasticity of crystals and quantitative results predicted by the model are in good agreement with experimental observations.
2D PIC simulations for an EN discharge with magnetized electrons and unmagnetized ions
NASA Astrophysics Data System (ADS)
Lieberman, Michael A.; Kawamura, Emi; Lichtenberg, Allan J.
2009-10-01
We conducted 2D particle-in-cell (PIC) simulations for an electronegative (EN) discharge with magnetized electrons and unmagnetized ions, and compared the results to a previously developed 1D (radial) analytical model of an EN plasma with strongly magnetized electrons and weakly magnetized ions [1]. In both cases, there is a static uniform applied magnetic field in the axial direction. The 1D radial model mimics the wall losses of the particles in the axial direction by introducing a bulk loss frequency term νL. A special (desired) solution was found in which only positive and negative ions but no electrons escaped radially. The 2D PIC results show good agreement with the 1D model over a range of parameters and indicate that the analytical form of νL employed in [1] is reasonably accurate. However, for the PIC simulations, there is always a finite flux of electrons to the radial wall which is about 10 to 30% of the negative ion flux.[4pt] [1] G. Leray, P. Chabert, A.J. Lichtenberg and M.A. Lieberman, J. Phys. D, accepted for publication 2009.
Ion acoustic wave collapse via two-ion wave decay: 2D Vlasov simulation and theory
NASA Astrophysics Data System (ADS)
Chapman, Thomas; Berger, Richard; Banks, Jeffrey; Brunner, Stephan
2015-11-01
The decay of ion acoustic waves (IAWs) via two-ion wave decay may transfer energy from the electric field of the IAWs to the particles, resulting in a significant heating of resonant particles. This process has previously been shown in numerical simulations to decrease the plasma reflectivity due to stimulated Brillouin scattering. Two-ion wave decay is a fundamental property of ion acoustic waves that occurs over most if not all of the parameter space of relevance to inertial confinement fusion experiments, and can lead to a sudden collapse of IAWs. The treatment of all species kinetically, and in particular the electrons, is required to describe the decay process correctly. We present fully kinetic 2D+2V Vlasov simulations of IAWs undergoing decay to a highly nonlinear turbulent state using the code LOKI. The scaling of the decay rate with characteristic plasma parameters and wave amplitude is shown. A new theory describing two-ion wave decay in 2D, that incorporates key kinetic properties of the electrons, is presented and used to explain quantitatively for the first time the observed decay of IAWs. Work performed under auspices of U.S. DoE by LLNL, Contract DE-AC52-07NA2734. Funded by LDRD 15-ERD-038 and supported by LLNL Grand Challenge allocation.
Calibration and simulation of ASM2d at different temperatures in a phosphorus removal pilot plant.
García-Usach, F; Ferrer, J; Bouzas, A; Seco, A
2006-01-01
In this work, an organic and nutrient removal pilot plant was used to study the temperature influence on phosphorus accumulating organisms. Three experiments were carried out at 13, 20 and 24.5 degrees C, achieving a high phosphorus removal percentage in all cases. The ASM2d model was calibrated at 13 and 20 degrees C and the Arrhenius equation constant was obtained for phosphorus removal processes showing that the temperature influences on the biological phosphorus removal subprocesses in a different degree. The 24.5 degrees C experiment was simulated using the model parameters obtained by means of the Arrhenius equation. The simulation results for the three experiments showed good correspondence with the experimental data, demonstrating that the model and the calibrated parameters were able to predict the pilot plant behaviour.
Superclusters of galaxies from the 2dF redshift survey. 2. Comparison with simulations
Einasto, Jaan; Einasto, M.; Saar, E.; Tago, E.; Liivamagi, L.J.; Joeveer, M.J; Suhhonenko, I.; Hutsi, G.; Jaaniste, J.; Heinamaki, P.; Muller, V.; Knebe, A.; Tucker, D.; /Fermilab
2006-04-01
We investigate properties of superclusters of galaxies found on the basis of the 2dF Galaxy Redshift Survey, and compare them with properties of superclusters from the Millennium Simulation.We study the dependence of various characteristics of superclusters on their distance from the observer, on their total luminosity, and on their multiplicity. The multiplicity is defined by the number of Density Field (DF) clusters in superclusters. Using the multiplicity we divide superclusters into four richness classes: poor, medium, rich and extremely rich.We show that superclusters are asymmetrical and have multi-branching filamentary structure, with the degree of asymmetry and filamentarity being higher for the more luminous and richer superclusters. The comparison of real superclusters with Millennium superclusters shows that most properties of simulated superclusters agree very well with real data, the main differences being in the luminosity and multiplicity distributions.
NASA Astrophysics Data System (ADS)
Bezzeccheri, E.; Colasanti, S.; Falco, A.; Liguori, R.; Rubino, A.; Lugli, P.
2016-05-01
Vertical Organic Transistors and Phototransistors have been proven to be promising technologies due to the advantages of reduced channel length and larger sensitive area with respect to planar devices. Nevertheless, a real improvement of their performance is subordinate to the quantitative description of their operation mechanisms. In this work, we present a comparative study on the modeling of vertical and planar Organic Phototransistor (OPT) structures. Computer-based simulations of the devices have been carried out with Synopsys Sentaurus TCAD in a 2D Drift-Diffusion framework. The photoactive semiconductor material has been modeled using the virtual semiconductor approach as the archetypal P3HT:PC61BM bulk heterojunction. It has been found that both simulated devices have comparable electrical and optical characteristics, accordingly to recent experimental reports on the subject.
Highly-resolved 2D HYDRA simulations of Double-Shell Ignition Designs
Milovich, J L; Amendt, P; Hamza, A; Marinak, M; Robey, H
2006-06-30
Double-shell (DS) targets (Amendt, P. A. et al., 2002) offer a complementary approach to the cryogenic baseline design (Lindl, J. et al., 2004) for achieving ignition on the National Ignition Facility (NIF). Among the expected benefits are the ease of room temperature preparation and fielding, the potential for lower laser backscatter and the reduced need for careful shock timing. These benefits are offset, however, by demanding fabrication tolerances, e.g., shell concentricity and shell surface smoothness. In particular, the latter is of paramount importance since DS targets are susceptible to the growth of interface perturbations from impulsive and time-dependent accelerations. Previous work (Milovich, J. L. et al., 2004) has indicated that the growth of perturbations on the outer surface of the inner shell is potentially disruptive. To control this instability new designs have been proposed requiring bimetallic inner shells and material-matching mid-Z nanoporous foam. The challenges in manufacturing such exotic foams have led to a further evaluation of the densities and pore sizes needed to reduce the seeding of perturbations on the outer surface of the inner shell, thereby guiding the ongoing material science research efforts. Highly-resolved 2D simulations of porous foams have been performed to establish an upper limit on the allowable pore sizes for instability growth. Simulations indicate that foams with higher densities than previously thought are now possible. Moreover, while at the present time we are only able to simulate foams with average pore sizes larger than 1 micron (due to computational limitations), we can conclude that these pore sizes are potentially problematic. Furthermore, the effect of low-order hohlraum radiation asymmetries on the growth of intrinsic surface perturbations is also addressed. Highly-resolved 2D simulations indicate that the transverse flows that are set up by these low-order mode features (which can excite Kelvin
NASA Astrophysics Data System (ADS)
Tarditi, Alfonso G.; Shebalin, John V.
2002-11-01
A simulation study with the NIMROD code [1] is being carried on to investigate the efficiency of the thrust generation process and the properties of the plasma detachment in a magnetic nozzle. In the simulation, hot plasma is injected in the magnetic nozzle, modeled as a 2D, axi-symmetric domain. NIMROD has two-fluid, 3D capabilities but the present runs are being conducted within the MHD, 2D approximation. As the plasma travels through the magnetic field, part of its thermal energy is converted into longitudinal kinetic energy, along the axis of the nozzle. The plasma eventually detaches from the magnetic field at a certain distance from the nozzle throat where the kinetic energy becomes larger than the magnetic energy. Preliminary NIMROD 2D runs have been benchmarked with a particle trajectory code showing satisfactory results [2]. Further testing is here reported with the emphasis on the analysis of the diffusion rate across the field lines and of the overall nozzle efficiency. These simulation runs are specifically designed for obtaining comparisons with laboratory measurements of the VASIMR experiment, by looking at the evolution of the radial plasma density and temperature profiles in the nozzle. VASIMR (Variable Specific Impulse Magnetoplasma Rocket, [3]) is an advanced space propulsion concept currently under experimental development at the Advanced Space Propulsion Laboratory, NASA Johnson Space Center. A plasma (typically ionized Hydrogen or Helium) is generated by a RF (Helicon) discharge and heated by an Ion Cyclotron Resonance Heating antenna. The heated plasma is then guided into a magnetic nozzle to convert the thermal plasma energy into effective thrust. The VASIMR system has no electrodes and a solenoidal magnetic field produced by an asymmetric mirror configuration ensures magnetic insulation of the plasma from the material surfaces. By powering the plasma source and the heating antenna at different levels it is possible to vary smoothly of the
Reynolds-Averaged Navier-Stokes Simulation of a 2D Circulation Control Wind Tunnel Experiment
NASA Technical Reports Server (NTRS)
Allan, Brian G.; Jones, Greg; Lin, John C.
2011-01-01
Numerical simulations are performed using a Reynolds-averaged Navier-Stokes (RANS) flow solver for a circulation control airfoil. 2D and 3D simulation results are compared to a circulation control wind tunnel test conducted at the NASA Langley Basic Aerodynamics Research Tunnel (BART). The RANS simulations are compared to a low blowing case with a jet momentum coefficient, C(sub u), of 0:047 and a higher blowing case of 0.115. Three dimensional simulations of the model and tunnel walls show wall effects on the lift and airfoil surface pressures. These wall effects include a 4% decrease of the midspan sectional lift for the C(sub u) 0.115 blowing condition. Simulations comparing the performance of the Spalart Allmaras (SA) and Shear Stress Transport (SST) turbulence models are also made, showing the SST model compares best to the experimental data. A Rotational/Curvature Correction (RCC) to the turbulence model is also evaluated demonstrating an improvement in the CFD predictions.
Spot size variation FCS in simulations of the 2D Ising model
NASA Astrophysics Data System (ADS)
Burns, Margaret C.; Nouri, Mariam; Veatch, Sarah L.
2016-06-01
Spot variation fluorescence correlation spectroscopy (svFCS) was developed to study the movement and organization of single molecules in plasma membranes. This experimental technique varies the size of an illumination area while measuring correlations in time using standard fluorescence correlation methods. Frequently, this data is interpreted using the assumption that correlation measurements reflect the dynamics of single molecule motions, and not motions of the average composition. Here, we explore how svFCS measurements report on the dynamics of components diffusing within simulations of a 2D Ising model with a conserved order parameter. Simulated correlation functions report on both the fast dynamics of single component mobility and the slower dynamics of the average composition. Over a range of simulation conditions, a conventional svFCS analysis suggests the presence of anomalous diffusion even though single molecule motions are nearly Brownian in these simulations. This misinterpretation is most significant when the surface density of the fluorescent label is elevated, therefore we suggest future measurements be made over a range of tracer densities. Some simulation conditions reproduce qualitative features of published svFCS experimental data. Overall, this work emphasizes the need to probe membranes using multiple complimentary experimental methodologies in order to draw conclusions regarding the nature of spatial and dynamical heterogeneity in these systems.
Tropical Oceanic Precipitation Processes over Warm Pool: 2D and 3D Cloud Resolving Model Simulations
NASA Technical Reports Server (NTRS)
Tao, W.- K.; Johnson, D.
1998-01-01
Rainfall is a key link in the hydrologic cycle as well as the primary heat source for the atmosphere, The vertical distribution of convective latent-heat release modulates the large-scale circulations of the tropics, Furthermore, changes in the moisture distribution at middle and upper levels of the troposphere can affect cloud distributions and cloud liquid water and ice contents. How the incoming solar and outgoing longwave radiation respond to these changes in clouds is a major factor in assessing climate change. Present large-scale weather and climate models simulate cloud processes only crudely, reducing confidence in their predictions on both global and regional scales. One of the most promising methods to test physical parameterizations used in General Circulation Models (GCMS) and climate models is to use field observations together with Cloud Resolving Models (CRMs). The CRMs use more sophisticated and physically realistic parameterizations of cloud microphysical processes, and allow for their complex interactions with solar and infrared radiative transfer processes. The CRMs can reasonably well resolve the evolution, structure, and life cycles of individual clouds and cloud systems, The major objective of this paper is to investigate the latent heating, moisture and momenti,im budgets associated with several convective systems developed during the TOGA COARE IFA - westerly wind burst event (late December, 1992). The tool for this study is the Goddard Cumulus Ensemble (CCE) model which includes a 3-class ice-phase microphysical scheme, The model domain contains 256 x 256 grid points (using 2 km resolution) in the horizontal and 38 grid points (to a depth of 22 km depth) in the vertical, The 2D domain has 1024 grid points. The simulations are performed over a 7 day time period. We will examine (1) the precipitation processes (i.e., condensation/evaporation) and their interaction with warm pool; (2) the heating and moisture budgets in the convective and
Jupiter Magnetotail Interaction with a Variable Solar Wind: A 3D MHD Simulation
NASA Astrophysics Data System (ADS)
Ranquist, D. A.; Bagenal, F.; Delamere, P. A.; Ma, X.
2015-12-01
Jupiter's magnetosphere is the largest object within the heliosphere. Voyager 2 detected its influence at Saturn's orbit, 4.3 AU away. It takes considerable time, therefore, for the solar wind to propagate such lengths down the tail. This propagation time is much greater than typical periods between changes in direction of the interplanetary magnetic field (IMF). We expect these variable magnetic fields to create a jumbled structure in Jupiter's magnetotail, resulting in magnetic reconnection and other magnetic processes. We simulate the global interaction of the solar wind with Jupiter's magnetosphere using a 3D magnetohydrodynamics (MHD) code. Delamere & Bagenal (2010) argue that the interaction is largely viscous, so we simulate the jovian magnetosphere as a region where the momentum equation has an added loss term. We also use in situ data gathered by the Ulysses spacecraft near Jupiter's orbit for solar wind input. Here, we report on the simulated dynamics in Jupiter's tail region.
Nonlinear MHD simulation of DC helicity injection in the Pegasus spherical tokamak
NASA Astrophysics Data System (ADS)
Bayliss, Adam; Sovinec, Carl
2006-10-01
DC helicity injection has been successfully employed in spherical tokamaks (ST's) to produce a tokamak-like plasma with either a poloidal-gap voltage known as coaxial helicity injection [HIT-II, NSTX] or a biased cathode gun configuration [CDX, PEGASUS]. In PEGASUS, the tokamak-like plasma which is subsequently ohmically driven is the product of a reversal of vacuum poloidal flux and a merger of gun-injected current filaments. A 3D nonlinear MHD computation using the NIMROD code [Sovinec et al. JCP 195, 355 (2004)] simulates the formation, merger, and relaxation of the gun-injected current filaments to the tokamak-like plasma. The reversal of poloidal flux due to the field induced by the helicity drive is reproduced and the MHD processes leading to the merger and relaxation of the current filaments are described. Over the lifetime of a helically-driven experimental shot (approximately 10ms), the extent to which the merged plasma exhibits amplication of poloidal flux and the injected current in the relaxed state, reported in PEGASUS, is explored. The results are compared with simulations of current drive in NSTX via coaxial helicity injection which exhibit an n=1 open field-line kink [Tang and Boozer, Phys. Plasmas 11, 2679 (2004)].
Chatterjee, Dipankar; Amiroudine, Sakir
2011-02-01
A comprehensive non-isothermal Lattice Boltzmann (LB) algorithm is proposed in this article to simulate the thermofluidic transport phenomena encountered in a direct-current (DC) magnetohydrodynamic (MHD) micropump. Inside the pump, an electrically conducting fluid is transported through the microchannel by the action of an electromagnetic Lorentz force evolved out as a consequence of the interaction between applied electric and magnetic fields. The fluid flow and thermal characteristics of the MHD micropump depend on several factors such as the channel geometry, electromagnetic field strength and electrical property of the conducting fluid. An involved analysis is carried out following the LB technique to understand the significant influences of the aforementioned controlling parameters on the overall transport phenomena. In the LB framework, the hydrodynamics is simulated by a distribution function, which obeys a single scalar kinetic equation associated with an externally imposed electromagnetic force field. The thermal history is monitored by a separate temperature distribution function through another scalar kinetic equation incorporating the Joule heating effect. Agreement with analytical, experimental and other available numerical results is found to be quantitative.
Extended MHD simulations of infernal mode dynamics and coupling to tearing modes
NASA Astrophysics Data System (ADS)
Brunetti, D.; Graves, J. P.; Halpern, F. D.; Luciani, J.-F.; Lütjens, H.; Cooper, W. A.
2015-05-01
A numerical study of pressure driven magnetohydrodynamic (MHD) instabilities in a low-shear tight aspect ratio configuration is presented. When the magnetic shear is sufficiently small over an extended region in the core, enhanced instability occurs due to the coupling to poloidal sidebands, which itself occurs due to toroidicity. Numerical simulations have been performed with the initial value code XTOR-2F both in the ideal and resistive MHD frame. Two-fluid effects (plasma diamagnetic flows) have been retained as well. The predictions of the XTOR-2F code on the amplitude of the growth rate, and on the rotation frequency of the modes, have been compared with analytic linear theory of infernal modes. Qualitative agreement has been found between numerical and analytical results, in spite of the tight aspect ratio configuration. The intermediate scaling γ ˜ S-3/8, predicted by the linear theory (Brunetti et al 2014 Plasma Phys. Control. Fusion 56 075025), is recovered by the numerical results. A study of the nonlinear evolution of the magnetic island of the tearing sideband has been performed and the results from the simulations are compared with Rutherford’s theory.
Numerical simulation of 2D buoyant jets in ice-covered and temperature-stratified water
NASA Astrophysics Data System (ADS)
Gu, Ruochuan
A two-dimensional (2D) unsteady simulation model is applied to the problem of a submerged warm water discharge into a stratified lake or reservoir with an ice cover. Numerical simulations and analyses are conducted to gain insight into large-scale convective recirculation and flow processes in a cold waterbody induced by a buoyant jet. Jet behaviors under various discharge temperatures are captured by directly modeling flow and thermal fields. Flow structures and processes are described by the simulated spatial and temporal distributions of velocity and temperature in various regions: deflection, recirculation, attachment, and impingement. Some peculiar hydrothermal and dynamic features, e.g. reversal of buoyancy due to the dilution of a warm jet by entraining cold ambient water, are identified and examined. Simulation results show that buoyancy is the most important factor controlling jet behavior and mixing processes. The inflow boundary is treated as a liquid wall from which the jet is offset. Similarity and difference in effects of boundaries perpendicular and parallel to flow, and of buoyancy on jet attachment and impingement, are discussed. Symmetric flow configuration is used to de-emphasize the Coanda effect caused by offset.
Rise characteristics of gas bubbles in a 2D rectangular column: VOF simulations vs experiments
Krishna, R.; Baten, J.M. van
1999-10-01
About five centuries ago, Leonardo da Vinci described the sinuous motion of gas bubbles rising in water. The authors have attempted to simulate the rise trajectories of bubbles of 4, 5, 7, 8, 9, 12, and 20 mm in diameter rising in a 2D rectangular column filled with water. The simulations were carried out using the volume-of-fluid (VOF) technique developed by Hirt and Nichols (J. Computational Physics, 39, 201--225 (1981)). To solve the Navier-Stokes equations of motion the authors used a commercial solver, CFX 4.1c of AEA Technology, UK. They developed their own bubble-tracking algorithm to capture sinuous bubble motions. The 4 and 5 mm bubbles show large lateral motions observed by Da Vinci. The 7, 8 and 9 mm bubble behave like jellyfish. The 12 mm bubble flaps its wings like a bird. The extent of lateral motion of the bubbles decreases with increasing bubble size. Bubbles larger than 20 mm in size assume a spherical cap form and simulations of the rise characteristics match experiments exactly. VOF simulations are powerful tools for a priori determination of the morphology and rise characteristics of bubbles rising in a liquid. Bubble-bubble interactions are also properly modeled by the VOF technique.
Simulation of growth normal fault sandbox tests using the 2D discrete element method
NASA Astrophysics Data System (ADS)
Chu, Sheng-Shin; Lin, Ming-Lang; Huang, Wen-Chao; Nien, Wei-Tung; Liu, Huan-Chi; Chan, Pei-Chen
2015-01-01
A fault slip can cause the deformation of shallow soil layers and destroy infrastructures. The Shanchiao Fault on the west side of the Taipei Basin is one such fault. The activities of the Shanchiao Fault have caused the quaternary sediment beneath the Taipei Basin to become deformed, damaging structures, traffic construction, and utility lines in the area. Data on geological drilling and dating have been used to determine that a growth fault exists in the Shanchiao Fault. In an experiment, a sandbox model was built using noncohesive sandy soil to simulate the existence of a growth fault in the Shanchiao Fault and forecast the effect of the growth fault on shear-band development and ground differential deformation. The experimental results indicated that when a normal fault contains a growth fault at the offset of the base rock, the shear band develops upward beside the weak side of the shear band of the original-topped soil layer, and surfaces considerably faster than that of the single-topped layer. The offset ratio required is approximately one-third that of the single-cover soil layer. In this study, a numerical simulation of the sandbox experiment was conducted using a discrete element method program, PFC2D, to simulate the upper-covering sand layer shear-band development pace and the scope of a growth normal fault slip. The simulation results indicated an outcome similar to that of the sandbox experiment, which can be applied to the design of construction projects near fault zones.
2-D LSP Simulations of the Self Magnetic Pinch Radiographic Diode
NASA Astrophysics Data System (ADS)
Threadgold, J.; Crotch, I.; Rose, D. V.
2003-10-01
The Atomic Weapons Establishment (AWE) UK has a number of Pulsed Power driven flash X-ray machines which are used to take transmission radiographs of hydrodynamic experiments. Some of the lower voltage x-ray machines (< 2 MV) use the Self Magnetic (SM) Pinch diode for their source. The SM pinch diode has proved to be a reliable source for providing small diameter radiographic spot sizes. With an emphasis on reduction of the x-ray spot size at higher voltages, one part of the diode research project has been to field SM pinch diodes at higher voltages. The SM pinch diode relies upon the magnitude of its own electron current (> 50 kA) to pinch the electron beam to a small diameter onto a high Z converter target. An electromagnetic PIC code, LSP, has been used to carry out 2-D simulations of the diode to support this project. The code has been used to investigate the effect of different target materials within the diode and to investigate the resultant electron trajectories onto the target. Results of these code simulations will be compared to experimental data The simulations show good agreement with measured experimental data on diode performance. The simulations suggest further improvements in spot size reduction could be achieved with changes in the diode geometry.
2D/3D Monte Carlo Feature Profile Simulator FPS-3D
NASA Astrophysics Data System (ADS)
Moroz, Paul
2010-11-01
Numerical simulation of etching/deposition profiles is important for semiconductor industry, as it allows analysis and prediction of the outcome of materials processing on a micron and sub-micron scale. The difficulty, however, is in making such a simulator a reliable, general, and easy to use tool applicable to different situations, for example, with different ratios of ion to neutral fluxes, different chemistries, different energies of incoming particles, and different angular and energy dependencies for surface reactions, without recompiling the code each time when the parameters change. The FPS-3D simulator [1] does not need recompilation when the features, materials, gases, or plasma are changed -- modifications to input, chemistry, and flux files are enough. The code allows interaction of neutral low-energy species with the surface mono-layer, while considering finite penetration depth into the volume for fast particles and ions. The FPS-3D code can simulate etching and deposition processes, both for 2D and 3D geometries. FPS-3D is using an advanced graphics package from HFS for presenting real-time process and profile evolution. The presentation will discuss the FPS-3D code with examples for different process conditions. The author is thankful to Drs. S.-Y. Kang of TEL TDC and P. Miller of HFS for valuable discussions. [4pt] [1] P. Moroz, URP.00101, GEC, Saratoga, NY, 2009.
Nonlinear MHD simulations of Quiescent H-mode plasmas in DIII-D
NASA Astrophysics Data System (ADS)
Liu, F.; Huijsmans, G. T. A.; Loarte, A.; Garofalo, A. M.; Solomon, W. M.; Snyder, P. B.; Hoelzl, M.; Zeng, L.
2015-09-01
In the Quiescent H-mode (QH-mode) regime, the edge harmonic oscillation (EHO), thought to be a saturated kink-peeling mode (KPM) driven unstable by current and rotation, is found in experiment to provide sufficient stationary edge particle transport to avoid the periodic expulsion of particles and energy by edge localized modes (ELMs). In this paper, both linear and nonlinear MHD modelling of QH-mode plasmas from the DIII-D tokamak have been investigated to understand the mechanism leading to the appearance of the EHO in QH-mode plasmas. For the first time nonlinear MHD simulations with low-n modes both with ideal wall and resistive wall boundary conditions have been carried out with the 3D non-linear MHD code JOREK. The results show, in agreement with the original conjectures, that in the non-linear phase, kink peeling modes are the main unstable modes in QH-mode plasmas of DIII-D and that the kink-peeling modes saturate non-linearly leading to a 3D stationary state. The characteristics of the kink-peeling modes, in terms of mode structure and associated decrease of the edge plasma density associated with them, are in good agreement with experimental measurements of the EHO in DIII-D. The effect of plasma resistivity, the role of plasma parallel rotation as well as the effect of the conductivity of the vacuum vessel wall on the destabilization and saturation of kink-peeling modes have been evaluated for experimental QH-mode plasma conditions in DIII-D.
One year in the Earth's magnetosphere: A global MHD simulation and spacecraft measurements
NASA Astrophysics Data System (ADS)
Facskó, G.; Honkonen, I.; Živković, T.; Palin, L.; Kallio, E.; Ã gren, K.; Opgenoorth, H.; Tanskanen, E. I.; Milan, S.
2016-05-01
The response of the Earth's magnetosphere to changing solar wind conditions is studied with a 3-D Magnetohydrodynamic (MHD) model. One full year (155 Cluster orbits) of the Earth's magnetosphere is simulated using Grand Unified Magnetosphere Ionosphere Coupling simulation (GUMICS-4) magnetohydrodynamic code. Real solar wind measurements are given to the code as input to create the longest lasting global magnetohydrodynamics simulation to date. The applicability of the results of the simulation depends critically on the input parameters used in the model. Therefore, the validity and the variance of the OMNIWeb data are first investigated thoroughly using Cluster measurement close to the bow shock. The OMNIWeb and the Cluster data were found to correlate very well before the bow shock. The solar wind magnetic field and plasma parameters are not changed significantly from the L1 Lagrange point to the foreshock; therefore, the OMNIWeb data are appropriate input to the GUMICS-4. The Cluster SC3 footprints are determined by magnetic field mapping from the simulation results and the Tsyganenko (T96) model in order to compare two methods. The determined footprints are in rather good agreement with the T96. However, it was found that the footprints agree better in the Northern Hemisphere than the Southern one during quiet conditions. If the By is not zero, the agreement of the GUMICS-4 and T96 footprint is worse in longitude in the Southern Hemisphere. Overall, the study implies that a 3-D MHD model can increase our insight of the response of the magnetosphere to solar wind conditions.
NASA Technical Reports Server (NTRS)
Benyo, Theresa L.
2011-01-01
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.
NASA Astrophysics Data System (ADS)
Dessart, L.; Owocki, S. P.
2005-07-01
We present initial attempts to include the multi-dimensional nature of radiation transport in hydrodynamical simulations of the small-scale structure that arises from the line-driven instability in hot-star winds. Compared to previous 1D or 2D models that assume a purely radial radiation force, we seek additionally to treat the lateral momentum and transport of diffuse line-radiation, initially here within a 2D context. A key incentive is to study the damping effect of the associated diffuse line-drag on the dynamical properties of the flow, focusing particularly on whether this might prevent lateral break-up of shell structures at scales near the lateral Sobolev angle of ca. 1^o. Based on 3D linear perturbation analyses that show a viscous diffusion character for the damping at these scales, we first explore nonlinear simulations that cast the lateral diffuse force in the simple, local form of a parallel viscosity. We find, however, that the resulting strong damping of lateral velocity fluctuations only further isolates azimuthal zones, leading again to azimuthal incoherence down to the grid scale. To account then for the further effect of lateral mixing of radiation associated with the radial driving, we next explore models in which the radial force is azimuthally smoothed over a chosen scale, and thereby show that this does indeed translate to a similar scale for the resulting density and velocity structure. Accounting for both the lateral line-drag and the lateral mixing in a more self-consistent way thus requires a multi-ray computation of the radiation transport. As a first attempt, we explore further a method first proposed by Owocki (1999), which uses a restricted 3-ray approach that combines a radial ray with two oblique rays set to have an impact parameter p < Rast within the stellar core. From numerical simulations with various grid resolutions (and p), we find that, compared to equivalent 1-ray simulations, the high-resolution 3-ray models show
NASA Astrophysics Data System (ADS)
Li, Jinghe; Song, Linping; Liu, Qing Huo
2016-02-01
A simultaneous multiple frequency contrast source inversion (CSI) method is applied to reconstructing hydrocarbon reservoir targets in a complex multilayered medium in two dimensions. It simulates the effects of a salt dome sedimentary formation in the context of reservoir monitoring. In this method, the stabilized biconjugate-gradient fast Fourier transform (BCGS-FFT) algorithm is applied as a fast solver for the 2D volume integral equation for the forward computation. The inversion technique with CSI combines the efficient FFT algorithm to speed up the matrix-vector multiplication and the stable convergence of the simultaneous multiple frequency CSI in the iteration process. As a result, this method is capable of making quantitative conductivity image reconstruction effectively for large-scale electromagnetic oil exploration problems, including the vertical electromagnetic profiling (VEP) survey investigated here. A number of numerical examples have been demonstrated to validate the effectiveness and capacity of the simultaneous multiple frequency CSI method for a limited array view in VEP.
Catalog of velocity distributions around a reconnection site in 2D PIC simulations
NASA Astrophysics Data System (ADS)
Lechner, Lukas; Bourdin, Philippe-A.; Nakamura, Takuma K. M.; Nakamura, Rumi; Narita, Yasuhito
2016-04-01
The velocity distribution of electrons and ions are known to be a marker for regions where magnetic reconnection develops. Past theoretical and computational works demonstrated that non-gyrotropic and anisotropic distributions depending on particle meandering motions and accelerations are seen around the reconnection point. The Magnetospheric Multiscale (MMS) mission is expected to resolve such kinetic scale reconnection regions. We present a catalog of velocity distribution functions that can give hints on the location within the current sheet relative to the reconnection point, which is sometimes unclear from pure spacecraft observations. We use 2D PIC simulations of anti-parallel magnetic reconnection to obtain velocity distributions at different locations, like in the center of the reconnection site, the ion and electron diffusion regions, or the reconnection inflow and outflow regions. With sufficiently large number of particles we resolve the distribution functions also in rather small regions. Such catalog may be compared with future MMS observations of the Earth's magnetotail.
Relaxation of ferroelectric states in 2D distributions of quantum dots: EELS simulation
NASA Astrophysics Data System (ADS)
Cortés, C. M.; Meza-Montes, L.; Moctezuma, R. E.; Carrillo, J. L.
2016-06-01
The relaxation time of collective electronic states in a 2D distribution of quantum dots is investigated theoretically by simulating EELS experiments. From the numerical calculation of the probability of energy loss of an electron beam, traveling parallel to the distribution, it is possible to estimate the damping time of ferroelectric-like states. We generate this collective response of the distribution by introducing a mean field interaction among the quantum dots, and then, the model is extended incorporating effects of long-range correlations through a Bragg-Williams approximation. The behavior of the dielectric function, the energy loss function, and the relaxation time of ferroelectric-like states is then investigated as a function of the temperature of the distribution and the damping constant of the electronic states in the single quantum dots. The robustness of the trends and tendencies of our results indicate that this scheme of analysis can guide experimentalists to develop tailored quantum dots distributions for specific applications.
NASA Astrophysics Data System (ADS)
Khuat Duy, B.; Archambeau, P.; Dewals, B. J.; Erpicum, S.; Pirotton, M.
2009-04-01
Following recurrent inundation problems on the Berwinne catchment, in Belgium, a combined hydrologic and hydrodynamic study has been carried out in order to find adequate solutions for the floods mitigation. Thanks to detailed 2D simulations, the effectiveness of the solutions can be assessed not only in terms of discharge and height reductions in the river, but also with other aspects such as the inundated surfaces reduction and the decrease of inundated buildings and roads. The study is carried out in successive phases. First, the hydrological runoffs are generated using a physically based and spatially distributed multi-layer model solving depth-integrated equations for overland flow, subsurface flow and baseflow. Real floods events are simulated using rainfall series collected at 8 stations (over 20 years of available data). The hydrological inputs are routed through the river network (and through the sewage network if relevant) with the 1D component of the modelling system, which solves the Saint-Venant equations for both free-surface and pressurized flows in a unified way. On the main part of the river, the measured river cross-sections are included in the modelling, and existing structures along the river (such as bridges, sluices or pipes) are modelled explicitely with specific cross sections. Two gauging stations with over 15 years of continuous measurements allow the calibration of both the hydrologic and hydrodynamic models. Second, the flood mitigation solutions are tested in the simulations in the case of an extreme flooding event, and their effects are assessed using detailed 2D simulations on a few selected sensitive areas. The digital elevation model comes from an airborne laser survey with a spatial resolution of 1 point per square metre and is completed in the river bed with a bathymetry interpolated from cross-section data. The upstream discharge is extracted from the 1D simulation for the selected rainfall event. The study carried out with this
A new model for two-dimensional numerical simulation of pseudo-2D gas-solids fluidized beds
Li, Tingwen; Zhang, Yongmin
2013-10-11
Pseudo-two dimensional (pseudo-2D) fluidized beds, for which the thickness of the system is much smaller than the other two dimensions, is widely used to perform fundamental studies on bubble behavior, solids mixing, or clustering phenomenon in different gas-solids fluidization systems. The abundant data from such experimental systems are very useful for numerical model development and validation. However, it has been reported that two-dimensional (2D) computational fluid dynamic (CFD) simulations of pseudo-2D gas-solids fluidized beds usually predict poor quantitative agreement with the experimental data, especially for the solids velocity field. In this paper, a new model is proposed to improve the 2D numerical simulations of pseudo-2D gas-solids fluidized beds by properly accounting for the frictional effect of the front and back walls. Two previously reported pseudo-2D experimental systems were simulated with this model. Compared to the traditional 2D simulations, significant improvements in the numerical predictions have been observed and the predicted results are in better agreement with the available experimental data.
Hall-Effect Thruster Simulations with 2-D Electron Transport and Hydrodynamic Ions
NASA Technical Reports Server (NTRS)
Mikellides, Ioannis G.; Katz, Ira; Hofer, Richard H.; Goebel, Dan M.
2009-01-01
A computational approach that has been used extensively in the last two decades for Hall thruster simulations is to solve a diffusion equation and energy conservation law for the electrons in a direction that is perpendicular to the magnetic field, and use discrete-particle methods for the heavy species. This "hybrid" approach has allowed for the capture of bulk plasma phenomena inside these thrusters within reasonable computational times. Regions of the thruster with complex magnetic field arrangements (such as those near eroded walls and magnets) and/or reduced Hall parameter (such as those near the anode and the cathode plume) challenge the validity of the quasi-one-dimensional assumption for the electrons. This paper reports on the development of a computer code that solves numerically the 2-D axisymmetric vector form of Ohm's law, with no assumptions regarding the rate of electron transport in the parallel and perpendicular directions. The numerical challenges related to the large disparity of the transport coefficients in the two directions are met by solving the equations in a computational mesh that is aligned with the magnetic field. The fully-2D approach allows for a large physical domain that extends more than five times the thruster channel length in the axial direction, and encompasses the cathode boundary. Ions are treated as an isothermal, cold (relative to the electrons) fluid, accounting for charge-exchange and multiple-ionization collisions in the momentum equations. A first series of simulations of two Hall thrusters, namely the BPT-4000 and a 6-kW laboratory thruster, quantifies the significance of ion diffusion in the anode region and the importance of the extended physical domain on studies related to the impact of the transport coefficients on the electron flow field.
The role of condensation and heat conduction in the formation of prominences - An MHD simulation
NASA Technical Reports Server (NTRS)
Wu, S. T.; Bao, J. J.; An, C. H.; Tandberg-Hanssen, E.
1990-01-01
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.
The PLX- α project: Radiation-MHD Simulations of Imploding Plasma Liners Using USim
NASA Astrophysics Data System (ADS)
Beckwith, Kristian; Stoltz, Peter; Kundrapu, Madhusudhan; Hsu, Scott; PLX-α Team
2015-11-01
USim is a tool for modeling high energy density plasmas using multi-fluid models coupled to electromagnetics using fully-implicit iterative solvers, combined with finite volume discretizations on unstructured meshes. Prior work has demonstrated application of USim models and algorithms to simulation of supersonic plasma jets relevant to the Plasma Liner Experiment (PLX) and compared synthetic interferometry to that gathered from the experiment. Here, we give an overview of the models and algorithms included in USim; review results from prior modeling campaigns for the PLX; and describe plans for radiation magnetohydrodynamic (MHD) simulation efforts focusing on integrated plasma-liner implosion and target compression in a fusion-relevant regime using USim for the PLX- α project. Supported by ARPA-E's ALPHA program. Original PLX construction supported by OFES. USim development supported in part by Air Force Office of Scientific Research.
Interpreting Irradiance Distributions Using High-Resolution 3D MHD Simulations
NASA Astrophysics Data System (ADS)
Peck, Courtney; Rast, Mark; Criscuoli, Serena; Uitenbroek, Han; Rempel, Matthias D.
2016-05-01
We present initial results of studies aimed at understanding the impact of the unresolved magnetic field distribution on solar spectral irradiance. Using high-resolution 3D MHD simulations (from MURaM code) and spectral synthesis (with the RH code), we examine the emergent spectra of two atmospheres with similar mean field strengths but differing imposed-field conditions at wavelengths spanning from visible to infrared. Comparing the contrast against the magnetic field strength for the two magnetic simulations, we find differences in the distributions of contrasts versus field strength. We repeat the analysis after convolving the images with the PSF of a typical solar telescope (1-meter) and discuss the potential implications for irradiance modeling and future steps.
Simulation of Pyroclastic Flows of Colima Volcano, Mexico, Using the TITAN2D Program
NASA Astrophysics Data System (ADS)
Rupp, B.; Bursik, M.; Patra, A.; Pitman, B.; Bauer, A.; Nichita, C.; Saucedo, R.; Macias, J.
2003-04-01
A new numerical code for simulating granular avalanches, TITAN2D, was used to model block-and-ash flows from the 1991-1999 eruptions of Colima Volcano, Mexico. The block-and-ash flows were simulated on a gridded Digital Elevation Model(DEM), which was prepared and imported using a standard GIS function library (GRASS). The TITAN2D program is based upon a model for an incompressible Coulomb continuum, a 'shallow-water' granular flow. The conservation equations for mass and momentum are solved with a Coulomb-type friction term at the interface between the granular material and the basal surface. It is assumed that conservation of energy can be neglected to first order because the coarse grain size typical of the basal avalanche results in minimal thermal effects on avalanche propagation. The resulting hyperbolic system of equations is solved using a parallel, adaptive mesh, Godunov scheme. The Message Passing Interface (MPI) API allows for computing on multiple processors, which increases computational power, decreases computing time, and allows the use of large data sets. Adaptive gridding allows for the concentration of computing power on regions of special interest. Mesh refinement captures the leading edge of the avalanche, as well as locations where the topography changes rapidly. Mesh unrefinement is applied where solution values are relatively constant or small. There were thousands of rockfalls and numerous block-and-ash flows during the 1991-1999 eruptions of Colima Volcano, with volumes ranging from a few cubic meters to 10^6 m^3. We have records of numerous flows, which include volume, run out distance, deposit area, and in some cases a videotape record of flow propagation. The flows originated from a vent plugging dome, lava flows or minor column collapse. All flows followed cross-slope concavities on the upper edifice, and channels or relative topographic lows on the lower edifice. The flows propagated for distances up to 4 km from the source. We are
NASA Astrophysics Data System (ADS)
Simmons, Daniel; Cools, Kristof; Sewell, Phillip
2016-11-01
Time domain electromagnetic simulation tools have the ability to model transient, wide-band applications, and non-linear problems. The Boundary Element Method (BEM) and the Transmission Line Modeling (TLM) method are both well established numerical techniques for simulating time-varying electromagnetic fields. The former surface based method can accurately describe outwardly radiating fields from piecewise uniform objects and efficiently deals with large domains filled with homogeneous media. The latter volume based method can describe inhomogeneous and non-linear media and has been proven to be unconditionally stable. Furthermore, the Unstructured TLM (UTLM) enables modelling of geometrically complex objects by using triangular meshes which removes staircasing and unnecessary extensions of the simulation domain. The hybridization of BEM and UTLM which is described in this paper is named the Boundary Element Unstructured Transmission-line (BEUT) method. It incorporates the advantages of both methods. The theory and derivation of the 2D BEUT method is described in this paper, along with any relevant implementation details. The method is corroborated by studying its correctness and efficiency compared to the traditional UTLM method when applied to complex problems such as the transmission through a system of Luneburg lenses and the modelling of antenna radomes for use in wireless communications.
Simulation of abrasive flow machining process for 2D and 3D mixture models
NASA Astrophysics Data System (ADS)
Dash, Rupalika; Maity, Kalipada
2015-12-01
Improvement of surface finish and material removal has been quite a challenge in a finishing operation such as abrasive flow machining (AFM). Factors that affect the surface finish and material removal are media viscosity, extrusion pressure, piston velocity, and particle size in abrasive flow machining process. Performing experiments for all the parameters and accurately obtaining an optimized parameter in a short time are difficult to accomplish because the operation requires a precise finish. Computational fluid dynamics (CFD) simulation was employed to accurately determine optimum parameters. In the current work, a 2D model was designed, and the flow analysis, force calculation, and material removal prediction were performed and compared with the available experimental data. Another 3D model for a swaging die finishing using AFM was simulated at different viscosities of the media to study the effects on the controlling parameters. A CFD simulation was performed by using commercially available ANSYS FLUENT. Two phases were considered for the flow analysis, and multiphase mixture model was taken into account. The fluid was considered to be a
NASA Astrophysics Data System (ADS)
Kanki, Takashi; Nagata, Masayoshi; Kagei, Yasuhiro
2011-10-01
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 λ concentrated in the COFC then diffuses to the core region so as to reduce the gradient in λ, relaxing in the direction of the Taylor state.
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics
Klein, R I; Li, P S; McKee, C F; Fisher, R
2008-04-10
Most numerical investigations on the role of magnetic fields in turbulent molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However, MCs are weakly ionized, so that the time scale required for the magnetic field to diffuse through the neutral component of the plasma by ambipolar diffusion (AD) can be comparable to the dynamical time scale. We have performed a series of 256{sup 3} and 512{sup 3} simulations on supersonic but sub-Alfvenic turbulent systems with AD using the Heavy-Ion Approximation developed in Li et al. (2006). Our calculations are based on the assumption that the number of ions is conserved, but we show that these results approximately apply to the case of time-dependent ionization in molecular clouds as well. Convergence studies allow us to determine the optimal value of the ionization mass fraction when using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent systems. We find that ambipolar diffusion steepens the velocity and magnetic power spectra compared to the ideal MHD case. Changes in the density PDF, total magnetic energy, and ionization fraction are determined as a function of the AD Reynolds number. The power spectra for the neutral gas properties of a strongly magnetized medium with a low AD Reynolds number are similar to those for a weakly magnetized medium; in particular, the power spectrum of the neutral velocity is close to that for Burgers turbulence.
MHD SIMULATIONS OF ACCRETION ONTO Sgr A*: QUIESCENT FLUCTUATIONS, OUTBURSTS, AND QUASIPERIODICITY
Chan Chikwan; Liu Siming; Fryer, Christopher L.; Psaltis, Dimitrios; Oezel, Feryal; Melia, Fulvio; Rockefeller, Gabriel
2009-08-10
High-resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations (QPOs), making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudospectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic (MHD) model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme and calibrating the parameters by observations, we show that the MHD turbulence in the environment of Sgr A* can by itself only produce factor two fluctuations in luminosity. These fluctuations cannot explain the magnitude of flares observed in this system. However, we also demonstrate that external forcing of the accretion disk, which may be generated by the 'clumpy material' raining down onto the disk from the large-scale flow, do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and by ground-based facilities in the near infrared. Strong, but short-term QPOs emerge naturally in the simulated light curves. We attribute these to nonaxisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.
NASA Astrophysics Data System (ADS)
Yamada, Susumu; Kitamura, Akihiro; Kurikami, Hiroshi; Machida, Masahiko
2015-04-01
Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March 2011 released significant quantities of radionuclides to atmosphere. The most significant nuclide is radioactive cesium isotopes. Therefore, the movement of the cesium is one of the critical issues for the environmental assessment. Since the cesium is strongly sorbed by soil particles, the cesium transport can be regarded as the sediment transport which is mainly brought about by the aquatic system such as a river and a lake. In this research, our target is the sediment transport on Ogaki dam reservoir which is located in about 16 km northwest from FDNPP. The reservoir is one of the principal irrigation dam reservoirs in Fukushima Prefecture and its upstream river basin was heavily contaminated by radioactivity. We simulate the sediment transport on the reservoir using 2-D river simulation code named Nays2D originally developed by Shimizu et al. (The latest version of Nays2D is available as a code included in iRIC (http://i-ric.org/en/), which is a river flow and riverbed variation analysis software package). In general, a 2-D simulation code requires a huge amount of calculation time. Therefore, we parallelize the code and execute it on a parallel computer. We examine the relationship between the behavior of the sediment transport and the height of the reservoir exit. The simulation result shows that almost all the sand that enter into the reservoir deposit close to the entrance of the reservoir for any height of the exit. The amounts of silt depositing within the reservoir slightly increase by raising the height of the exit. However, that of the clay dramatically increases. Especially, more than half of the clay deposits, if the exit is sufficiently high. These results demonstrate that the water level of the reservoir has a strong influence on the amount of the clay discharged from the reservoir. As a result, we conclude that the tuning of the water level has a possibility for controlling the
Lagrangian MHD Particle-in-Cell simulations of coronal interplanetary shocks driven by observations
NASA Astrophysics Data System (ADS)
Lapenta, Giovanni; Bacchini, Fabio; Bemporad, Alessandro; Susino, Roberto; Olshevskyi, Vyacheslav
2016-04-01
In this work, we compare the spatial distribution of the plasma parameters along the June 11, 1999 CME-driven shock front with the results obtained from a CME-like event simulated with the FLIPMHD3D code, based on the FLIP-MHD Particle-in-Cell (PiC) method. The observational data are retrieved from the combination of white-light (WL) coronagraphic data (for the upstream values) and the application of the Rankine-Hugoniot (RH) equations (for the downstream values). The comparison shows a higher compression ratio X and Alfvénic Mach number MA at the shock nose, and a stronger magnetic field deflection d towards the flanks, in agreement with observations. Then, we compare the spatial distribution of MA with the profiles obtained from the solutions of the shock adiabatic equation relating MA, X, and the angle between the upstream magnetic field and the shock front normal for the special cases of parallel and perpendicular shock, and with a semi-empirical expression for a generically oblique shock. The semi-empirical curve approximates the actual values of MA very well, if the effects of a non-negligible shock thickness and plasma-to magnetic pressure ratio are taken into account throughout the computation. Moreover, the simulated shock turns out to be supercritical at the nose and sub-critical at the flanks. Finally, we develop a new 1D Lagrangian ideal MHD method based on the GrAALE code, to simulate the ion-electron temperature decoupling due to the shock transit. Two models are used, a simple solar wind model and a variable-gamma model. Both produce results in agreement with observations, the second one being capable of introducing the physics responsible for the additional electron heating due to secondary effects (collisions, Alfvén waves, etc.). Work supported by the European Commission under the SWIFF project (swiff.eu)
Simulation and analysis of solute transport in 2D fracture/pipe networks: the SOLFRAC program.
Bodin, Jacques; Porel, Gilles; Delay, Fred; Ubertosi, Fabrice; Bernard, Stéphane; de Dreuzy, Jean-Raynald
2007-01-01
The Time Domain Random Walk (TDRW) method has been recently developed by Delay and Bodin [Delay, F. and Bodin, J., 2001. Time domain random walk method to simulate transport by advection-dispersion and matrix diffusion in fracture networks. Geophys. Res. Lett., 28(21): 4051-4054.] and Bodin et al. [Bodin, J., Porel, G. and Delay, F., 2003c. Simulation of solute transport in discrete fracture networks using the time domain random walk method. Earth Planet. Sci. Lett., 6566: 1-8.] for simulating solute transport in discrete fracture networks. It is assumed that the fracture network can reasonably be represented by a network of interconnected one-dimensional pipes (i.e. flow channels). Processes accounted for are: (1) advection and hydrodynamic dispersion in the channels, (2) matrix diffusion, (3) diffusion into stagnant zones within the fracture planes, (4) sorption reactions onto the fracture walls and in the matrix, (5) linear decay, and (6) mass sharing at fracture intersections. The TDRW method is handy and very efficient in terms of computation costs since it allows for the one-step calculation of the particle residence time in each bond of the network. This method has been programmed in C++, and efforts have been made to develop an efficient and user-friendly software, called SOLFRAC. This program is freely downloadable at the URL (labo.univ-poitiers.fr/hydrasa/intranet/telechargement.htm). It calculates solute transport into 2D pipe networks, while considering different types of injections and different concepts of local dispersion within each flow channel. Post-simulation analyses are also available, such as the mean velocity or the macroscopic dispersion at the scale of the entire network. The program may be used to evaluate how a given transport mechanism influences the macroscopic transport behaviour of fracture networks. It may also be used, as is the case, e.g., with analytical solutions, to interpret laboratory or field tracer test experiments performed
2-D Three Fluid Simulation of Upstreaming Ions Above Auroral Precipitation
NASA Astrophysics Data System (ADS)
Danielides, M. A.; Lummerzheim, D.; Otto, A.; Stevens, R. J.
2006-12-01
The ionosphere is a rich reservoir of charged particles from which a variable fraction is transported to the magnetosphere. An important transport phenomena is the formation of upward ion flow above auroral structure. A primary region of the outflow is not known, but contributions come from polar cap, dayside cusp/cleft region, auroral oval, or even from mid-latitudes. In the past global magnetospheric models and fluid codes were used to simulate large scale ion outflow above, e.g., the polar-cap aurora. However, satellites orbiting at low- altitudes have repeatingly detected localized ion outflow above the auroral oval. Ionosphere-magnetosphere coupling simulations gave first insides into the small-scale dynamics of aurora. The aim of this study is the investigation of coupled plasma and neutral dynamics in smaller scale aurora to explain the generation, structure, and dynamics of vertical ion upstream. We consider auroral electron precipitation at ionospheric heights in a 2-D three fluid ionospheric-magnetospheric coupling code (Otto and Zhu, 2003). Specially we examine the effects of the electron precipitation, heat conduction and heating in field- aligned current through coulomb collisions or turbulence causing: i) electron heating, ii) electron pressure gradients, and iii) upstreaming of ions through a resulting ambipolar electric field. Our first case studies are performed for different boundary conditions and for different auroral electron precipitation parameters (variation in characteristic auroral energy, auroral energy flux and horizontal scale). The results shall clarify how auroral precipitation can drive ions upwards. Finally we discuss the effect of ion drag and the interaction of the upstreaming ions with a stable neutral constituent. Otto, O. and H. Zhu, Fluid plasma simulation of coupled systems: Ionosphere and magnetosphere, Space Plasma Simulation. Edited by J. Buechner, C. Dum, and M. Scholer., Lecture Notes in Physics, vol. 615, p.193
2D IR spectra of cyanide in water investigated by molecular dynamics simulations
Lee, Myung Won; Carr, Joshua K.; Göllner, Michael; Hamm, Peter; Meuwly, Markus
2013-01-01
Using classical molecular dynamics simulations, the 2D infrared (IR) spectroscopy of CN− solvated in D2O is investigated. Depending on the force field parametrizations, most of which are based on multipolar interactions for the CN− molecule, the frequency-frequency correlation function and observables computed from it differ. Most notably, models based on multipoles for CN− and TIP3P for water yield quantitatively correct results when compared with experiments. Furthermore, the recent finding that T 1 times are sensitive to the van der Waals ranges on the CN− is confirmed in the present study. For the linear IR spectrum, the best model reproduces the full widths at half maximum almost quantitatively (13.0 cm−1 vs. 14.9 cm−1) if the rotational contribution to the linewidth is included. Without the rotational contribution, the lines are too narrow by about a factor of two, which agrees with Raman and IR experiments. The computed and experimental tilt angles (or nodal slopes) α as a function of the 2D IR waiting time compare favorably with the measured ones and the frequency fluctuation correlation function is invariably found to contain three time scales: a sub-ps, 1 ps, and one on the 10-ps time scale. These time scales are discussed in terms of the structural dynamics of the surrounding solvent and it is found that the longest time scale (≈10 ps) most likely corresponds to solvent exchange between the first and second solvation shell, in agreement with interpretations from nuclear magnetic resonance measurements.
2D numerical simulation of passive autocatalytic recombiner for hydrogen mitigation
NASA Astrophysics Data System (ADS)
Gera, B.; Sharma, P. K.; Singh, R. K.
2012-04-01
Resolving hydrogen related safety issues, pertaining to nuclear reactor safety has been an important area of research world over for the past decade. The studies on hydrogen transport behavior and development of hydrogen mitigation systems are still being pursued actively in various research labs, including Bhabha Atomic Research Centre (BARC), in India. The passive autocatalytic recombiner (PAR) is one of such hydrogen mitigating device consisting of catalyst surfaces arranged in an open-ended enclosure. In the plate type recombiner design sheets made of stainless steel and coated with platinum catalyst material are arranged in parallel inside a flow channel. The catalyst elements are exposed to a constant flow of a mixture of air, hydrogen and steam, a catalytic reaction occurs spontaneously at the catalyst surfaces and the heat of reaction produces natural convection flow through the enclosure. Numerical simulation and experiments are required for an in-depth knowledge of such plate type PAR. Specific finite volume based in-house 2D computational fluid dynamics (CFD) code has been developed to model and analyse the working of these recombiners and has been used to simulate one literature quoted experiment. The validation results were in good agreement against literature quoted German REKO experiments. Parametric study has been performed for particular recombiner geometry for various inlet conditions. Salient features of the simplified CFD model developed at BARC and results of the present model calculations are presented in this paper.
Observed and simulated power spectra of kinetic and magnetic energy retrieved with 2D inversions
NASA Astrophysics Data System (ADS)
Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R.
2016-10-01
Context. Information on the origin of internetwork magnetic field is hidden at the smallest spatial scales. Aims: We try to retrieve the power spectra with certainty to the highest spatial frequencies allowed by current instrumentation. Methods: To accomplish this, we use a 2D inversion code that is able to recover information up to the instrumental diffraction limit. Results: The retrieved power spectra have shallow slopes that extend further down to much smaller scales than has been found before. They do not seem to show any power law. The observed slopes at subgranular scales agree with those obtained from recent local dynamo simulations. Small differences are found for the vertical component of kinetic energy that suggest that observations suffer from an instrumental effect that is not taken into account. Conclusions: Local dynamo simulations quantitatively reproduce the observed magnetic energy power spectra on the scales of granulation down to the resolution limit of Hinode/SP, within the error bars inflicted by the method used and the instrumental effects replicated.
Some features of auroral electric fields as seen in 2D numerical simulations
NASA Technical Reports Server (NTRS)
Thiemann, H.; Singh, N.; Schunk, R. W.
1984-01-01
Results of 2D plasma simulations are presented and related to auroral observations. The formation of V-shaped potentials is studied with a 2 1/2 dimensional electrostatic particle-in-cell code for a magnetized plasma. It is shown that amplitudes for perpendicular electric fields are larger than for parallel electric fields, and for Te less than 100 eV, the amplitudes are comparable to the electric fields associated with the electrostatic shocks observed from the S3-3 satellite. The excitation of electrostatic ion-cyclotron EIC waves which occurs in the region below the parallel potential drop is discussed. In auroral plasmas EIC waves are observed above the V-shaped double layers in association with ion beams and field-aligned currents. The results also show that oppositely directed electric fields in the center and at the edges of the simulation region produce oppositely directed currents. Precipitating auroral ions in association with electron inverted-V events are seen by the DMSP-F6 satellite.
2D simulation of transport and degradation in the River Rhine.
Teichmann, L; Reuschenbach, P; Müller, B; Horn, H
2002-01-01
A simple 2D model has been developed for the simulation of mass transport and degradation of substances in the river Rhine. The model describes mass transport in the flow direction with a convective and a dispersive term. Transversal transport is described by segmenting the river and formulating a transversal exchange coefficient between the segments. Degradation can be formulated with any kinetics from first order to complex enzyme kinetics. The model was verified with monitoring data from the river Rhine. The hydrodynamic parameters such as dispersion coefficients and exchange coefficients were fitted to the conductivity, which was assumed to be non-degradable. The degradation term was fitted to ammonia values. The model was used to simulate measured concentrations of a readily (Aniline) and a poorly biodegradable substance (1,4-Dioxan) 10 m from the left river bank. It was the objective of this research program to develop a model which allows a realistic estimation of the locally and regionally predicted environmental concentration of chemical substances in the EU risk assessment scheme.
Dayside Proton Aurora: Comparisons between Global MHD Simulations and Image Observations
NASA Technical Reports Server (NTRS)
Berchem, J.; Fuselier, S. A.; Petrinec, S.; Frey, H. U.; Burch, J. L.
2003-01-01
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
A Comparison of 2D to 3D Hydro Simulations of Asteroid Mitigation by a Strong Surface Explosion
NASA Astrophysics Data System (ADS)
Weaver, R.; Dearholdt, W.
2011-12-01
Disruption of a potentially hazardous object (PHO) by an energetic surface or subsurface burst is considered as one possible method of impact-hazard mitigation. This technique of employing surface or subsurface explosions has been popularized in the media but is probably one of the lower priority deflection/disruption methods, unless the warning time is short. In all of our current simulation we use realistic RADAR shape models for the initial geometry, not merely spherical objects. The non-sphericity of the geometry is very important in the resultant shock hydrodynamic evolution. This work is a follow-on to previous 2D simulations with the RAGE hydrocode to simulate the imparted momentum as a function of depth-of-burial (DOB) on a non-spherical "rubble pile" composition. Specifically, here, we have started a full 3D simulation of a 1 Mt surface explosion on a porous (~40% porosity) "rubble pile" model in the shape of asteroid 25143 Itokawa. This simulation has progressed far enough to start comparisons between the 2D and 3D runs of this model. There are significant changes in the 3D geometry that reduce the momentum imparted to the asteroid in these RAGE simulations. I will discuss this set of simulations, give some background results from previous 2D simulations and indicate the differences between 2D and 3D simulations.
NASA Technical Reports Server (NTRS)
Benyo, Theresa L.
2010-01-01
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.
3D MHD SIMULATION OF FLARE SUPRA-ARCADE DOWNFLOWS IN A TURBULENT CURRENT SHEET MEDIUM
Cécere, M.; Zurbriggen, E.; Costa, A.; Schneiter, M.
2015-07-01
Supra-arcade downflows (SADs) are sunward, generally dark, plasma density depletions originated above posteruption flare arcades. In this paper, using 3D MHD simulations we investigate whether the SAD cavities can be produced by a direct combination of the tearing mode and Kelvin–Helmholtz instabilities leading to a turbulent current sheet (CS) medium or if the current sheet is merely the background where SADs are produced, triggered by an impulsive deposition of energy. We find that to give an account of the observational dark lane structures an addition of local energy, provided by a reconnection event, is required. We suggest that there may be a closed relation between characteristic SAD sizes and CS widths that must be satisfied to obtain an observable SAD.
The magnetic topology of the plasmoid flux rope in a MHD-simulation of magnetotail reconnection
NASA Technical Reports Server (NTRS)
Birn, J.; Hesse, M.
1990-01-01
On the basis of a 3D MHD simulation, the magnetic topology of a plasmoid that forms by a localized reconnection process in a magnetotail configuration (including a net dawn-dusk magnetic field component B sub y N is discussed. As a consequence of B sub y N not equalling 0, the plasmoid assumes a helical flux rope structure rather than an isolated island or bubble structure. Initially all field lines of the plasmoid flux rope remain connected with the earth, while at later times a gradually increasing amount of flux tubes becomes separated, connecting to either the distant boundary or to the flank boundaries. In this stage, topologically different flux tubes become tangled and wrapped around each other, consistent with predictions on the basis of an ad hoc plasmoid model.
NASA Astrophysics Data System (ADS)
Gorby, M.; Schwadron, N.; Torok, T.; Downs, C.; Lionello, R.; Linker, J.; Titov, V. S.; Mikic, Z.; Riley, P.; Desai, M. I.; Dayeh, M. A.
2014-12-01
Recent work on the coupling between the Energetic Particle Radiation Environment Module (EPREM, a 3D energetic particle model) and Magnetohydrodynamics Around a Sphere (MAS, an MHD code developed at Predictive Science, Inc.) has demonstrated the efficacy of compression regions around fast coronal mass ejections (CMEs) for particle acceleration low in the corona (˜ 3 - 6 solar radii). These couplings show rapid particle acceleration over a broad longitudinal extent (˜ 80 degrees) resulting from the pile-up of magnetic flux in the compression regions and their subsequent expansion. The challenge for forming large SEP events in such compression-acceleration scenarios is to have enhanced scattering within the acceleration region while also allowing for efficient escape of accelerated particles downstream (away from the Sun) from the compression region. We present here the most recent simulation results including energetic particle and CME plasma profiles, the subsequent flux and dosages at 1AU, and an analysis of the compressional regions as efficient accelerators.
NASA Astrophysics Data System (ADS)
Cheung, M. C.; Schüssler, M.; Moreno-Insertis, F.; Tarbell, T. D.
2007-12-01
With high angular resolution, high temporal cadence and a stable point spread function, the Solar Optical Telescope (SOT) onboard the Hinode satellite is the ideal instrument for the study of magnetic flux emergence and its manifestations on the solar surface. In this presentation, we focus on the development of ephemeral regions and small active regions. In many instances, SOT has been able to capture the entire emergence process from beginning to end: i.e. from the initial stages of flux appearance in granule interiors, through the intermediate stages of G-band bright point formation, and finally to the coalescence of small vertical flux elements to form pores. To investigate the physics of the flux emergence process, we performed 3D numerical MHD simulations with the MURaM code. The models are able to reproduce, and help us explain, various observational signatures of magnetic flux emergence.
Relativistic Modeling Capabilities in PERSEUS Extended-MHD Simulation Code for HED Plasmas
NASA Astrophysics Data System (ADS)
Hamlin, Nathaniel; Seyler, Charles
2015-11-01
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 simulation results. The use of fully relativistic equations enables the model to remain self-consistent in simulations of such relativistic phenomena as hybrid X-pinches and laser-plasma interactions. We have overcome a major challenge of a relativistic fluid implementation, namely the recovery of primitive variables (density, velocity, pressure) from conserved quantities at each time step of a simulation. Our code recovers non-relativistic results along with important features of published Particle-In-Cell simulation results for a laser penetrating a super-critical hydrogen gas with Fast Ignition applications. In particular, we recover the penetration of magnetized relativistic electron jets ahead of the laser. Our code also reveals new physics in the modeling of a laser incident on a thin foil. This work is supported by the National Nuclear Security Administration stewardship sciences academic program under Department of Energy cooperative agreements DE-FOA-0001153 and DE-NA0001836.
2-D spectral element simulations of destructive ground shaking in Catania (Italy)
NASA Astrophysics Data System (ADS)
Priolo, Enrico
This study wants to estimate the strong ground motion in the municipal area of Catania (Italy) for a catastrophic earthquake scenario. It is part of a larger research program funded by the National Research Council - National Group for the Defence Against Earthquakes (CNR-GNDT), The Catania Project, devoted to evaluating the seismic risk of a highly urbanised area, such as that of Catania, located in a seismically active region. The reference earthquake simulates the catastrophic event (M 7.2) of 1693. The ground shaking is computed solving the 2-D full-wave equation by the Chebyshev spectral element method (SPEM). Particular emphasis is given to the construction of realistic structural models, also including the finest local detail, obtained from the geophysical, geological and geotechnical data available. Simulations are performed for several sources, to account for both a change in source position and orientation, and the finite extension of the fault along its dip. Synthetic seismograms and peak ground acceleration (PGA) envelopes, calculated at the surface for four transects across the Catania area, constitute the main result of this study which can be used for practical purposes. Simulations show that ground motion is strongly influenced by both source characteristics and crustal structure. We have found that PGA values range between 0.1 g and 0.5 g, although particular site conditions strongly affect these values locally. For example, the frequencies of maximum interest in civil engineering (1.5-4 Hz) are enhanced selectively by a thick portion of surface sediments (i.e., 30-100 m for an average shear wave velocity of 500-600 m/s). An unexpected feature is the appreciable increase of PGA at large epicentral distances, which contradicts classical attenuation relations. All the results are examined through an analysis of the propagating wavefield.
Using high resolution bathymetric lidar data for a Telemac2D simulation
NASA Astrophysics Data System (ADS)
Dobler, Wolfgang; Baran, Ramona; Steinbacher, Frank; Ritter, Marcel; Aufleger, Markus
2014-05-01
Knowledge about the hydraulic situation in a mountain torrent is relevant to quantify flood risks, to study sediment transport and to assess the waterbodies' ecology. To conduct reliable calculations, high-quality terrain data of riverbeds, riverbanks and floodplains are required. Typically, digital terrain models (DTMs) of floodplains are derived from classical airborne laserscanning (red wavelength) together with terrestrial surveys along riverbeds and riverbanks. Usually, these are restricted to a limited number of cross sections. Terrestrial surveys are required since laser measurement systems cannot penetrate the water column of the observed waterbodies. Consequently, data describing the geometry of riverbeds and bank structures are hardly available at high spatial resolutions and extents, comparable to the airborne-laser scanning derived data for river floodplains. In this study, a newly available, water-penetrating airborne laser system (green wavelength, FFG research project between the University of Innsbruck and Riegl LMS) was used to survey a mountain torrent. Detailed and extensive data (~30 points/m² on topo-bathy side) of the riverbed and the riverbanks were acquired with this single sensor. In order to construct a 2D-Telemac simulation, the point cloud was down-sampled to an appropriate resolution required for the simulation. The creation of the mesh was carried out with the Software HydroVish and imported into Blue Kenue for further boundary treatment. On one hand the calibration of the numerical model was based on a known water discharge-rate and on the other on abundant data points of the water surface. The green laser system demonstrates its great potential for such an analysis. The final results of the numerical simulation show clearly the supremacy of using such a high resolution data basis in contrast to the traditional way of terrestrial surveying of cross sections along riverbeds.
NASA Astrophysics Data System (ADS)
Kivelson, M.; Jia, X.
2013-12-01
In previous work we demonstrated that a magnetohydrodynamic (MHD) simulation of Saturn's magnetosphere in which periodicity is imposed by rotating vortical flows in the ionosphere reproduces many reported periodically varying properties of the system. Here we shall show that previously unreported features of the MHD simulation of Saturn's magnetosphere illuminate additional measured properties of the system. By averaging over a rotation period, we identify a global electric field whose magnitude is a few tenths of a mV/m (see Figure 1). The electric field intensity decreases with radial distance in the middle magnetosphere, consistent with drift speeds v=E/B of a few km/s towards the morning side and relatively independent of radial distance. The electric field within 10 RS in the equatorial plane is oriented from post-noon to post-midnight, in excellent agreement with observations [e.g., Thomsen et al., 2012; Andriopoulou et al., 2012, 2013; Wilson et al., 2013]. By following the electric field over a full rotation phase we identify oscillatory behavior whose magnitude is consistent with the reported fluctuations of measured electric fields. Of particular interest is the nature of the fast mode perturbations that produce periodic displacement of the magnetopause and flapping of the current sheet. Figure (2) shows the total perturbation pressure (the sum of magnetic and thermal pressure) in the equatorial plane at a rotation phase for which the ionospheric flow near noon is equatorward. By following the perturbations over a full rotation period, we demonstrate properties of the fast mode wave launched by the rotating flow structures and thereby characterize the 'cam' signal originally proposed by Espinosa et al. [2003].
MHD simulations of boundary layer formation along the dayside Venus ionopause due to mass loading
NASA Technical Reports Server (NTRS)
Mcgary, J. E.; Pontius, D. H., Jr.
1994-01-01
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 10(exp 5)/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.
MHD simulations of boundary layer formation along the dayside Venus ionopause due to mass loading
NASA Astrophysics Data System (ADS)
McGary, J. E.; Pontius, D. H.
1994-02-01
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.
Towards Simulating the Transverse Ising Model in a 2D Array of Trapped Ions
NASA Astrophysics Data System (ADS)
Sawyer, Brian
2013-05-01
Two-dimensional Coulomb crystals provide a useful platform for large-scale quantum simulation. Penning traps enable confinement of large numbers of ions (>100) and allow for the tunable-range spin-spin interactions demonstrated in linear ion strings, facilitating simulation of quantum magnetism at a scale that is currently intractable on classical computers. We readily confine hundreds of Doppler laser-cooled 9Be+ within a Penning trap, producing a planar array of ions with self-assembled triangular order. The transverse ``drumhead'' modes of our 2D crystal along with the valence electron spin of Be+ serve as a resource for generating spin-motion and spin-spin entanglement. Applying a spin-dependent optical dipole force (ODF) to the ion array, we perform spectroscopy and thermometry of individual drumhead modes. This ODF also allows us to engineer long-range Ising spin couplings of either ferromagnetic or anti-ferromagnetic character whose approximate power-law scaling with inter-ion distance, d, may be varied continuously from 1 /d0 to 1 /d3. An effective transverse magnetic field is applied via microwave radiation at the ~124-GHz spin-flip frequency, and ground states of the effective Ising Hamiltonian may in principle be prepared adiabatically by slowly decreasing this transverse field in the presence of the induced Ising coupling. Long-range anti-ferromagnetic interactions are of particular interest due to their inherent spin frustration and resulting large, near-degenerate manifold of ground states. We acknowledge support from NIST and the DARPA-OLE program.
Icarus: A 2-D Direct Simulation Monte Carlo (DSMC) Code for Multi-Processor Computers
BARTEL, TIMOTHY J.; PLIMPTON, STEVEN J.; GALLIS, MICHAIL A.
2001-10-01
Icarus is a 2D Direct Simulation Monte Carlo (DSMC) code which has been optimized for the parallel computing environment. The code is based on the DSMC method of Bird[11.1] and models from free-molecular to continuum flowfields in either cartesian (x, y) or axisymmetric (z, r) coordinates. Computational particles, representing a given number of molecules or atoms, are tracked as they have collisions with other particles or surfaces. Multiple species, internal energy modes (rotation and vibration), chemistry, and ion transport are modeled. A new trace species methodology for collisions and chemistry is used to obtain statistics for small species concentrations. Gas phase chemistry is modeled using steric factors derived from Arrhenius reaction rates or in a manner similar to continuum modeling. Surface chemistry is modeled with surface reaction probabilities; an optional site density, energy dependent, coverage model is included. Electrons are modeled by either a local charge neutrality assumption or as discrete simulational particles. Ion chemistry is modeled with electron impact chemistry rates and charge exchange reactions. Coulomb collision cross-sections are used instead of Variable Hard Sphere values for ion-ion interactions. The electro-static fields can either be: externally input, a Langmuir-Tonks model or from a Green's Function (Boundary Element) based Poison Solver. Icarus has been used for subsonic to hypersonic, chemically reacting, and plasma flows. The Icarus software package includes the grid generation, parallel processor decomposition, post-processing, and restart software. The commercial graphics package, Tecplot, is used for graphics display. All of the software packages are written in standard Fortran.
Debris Flow Hazard Map Simulation using FLO-2D For Selected Areas in the Philippines
NASA Astrophysics Data System (ADS)
Khallil Ferrer, Peter; Llanes, Francesca; dela Resma, Marvee; Realino, Victoriano, II; Obrique, Julius; Ortiz, Iris Jill; Aquino, Dakila; Narod Eco, Rodrigo; Mahar Francisco Lagmay, Alfredo
2014-05-01
On December 4, 2012, Super Typhoon Bopha wreaked havoc in the southern region of Mindanao, leaving 1,067 people dead and causing USD 800 million worth of damage. Classified as a Category 5 typhoon by the Joint Typhoon Warning Center (JTWC), Bopha brought intense rainfall and strong winds that triggered landslides and debris flows, particularly in Barangay (village) Andap, New Bataan municipality, in the southern Philippine province of Compostela Valley. The debris flow destroyed school buildings and covered courts and an evacuation center. Compostela Valley also suffered the most casualties of any province: 612 out of a total of 1,067. In light of the disaster in Compostela, measures were immediately devised to improve available geohazard maps to raise public awareness about landslides and debris flows. A debris flow is a very rapid to extremely rapid flow of saturated non-plastic debris in a steep channel. They are generated when heavy rainfall saturates sediments, causing them to flow down river channels within an alluvial fan situated at the base of the slope of a mountain drainage network. Many rural communities in the Philippines, such as Barangay Andap, are situated at the apex of alluvial fans and in the path of potential debris flows. In this study, we conducted simulations of debris flows to assess the risks in inhabited areas throughout the Philippines and validated the results in the field, focusing on the provinces of Pangasinan and Aurora as primary examples. Watersheds that drain in an alluvial fan using a 10-m resolution Synthetic Aperture Radar (SAR)-derived Digital Elevation Model (DEM) was first delineated, and then a 1 in 100-year rain return rainfall scenario for the watershed was used to simulate debris flows using FLO-2D, a flood-routing software. The resulting simulations were used to generate debris flow hazard maps which are consistent with danger zones in alluvial fans delineated previously from satellite imagery and available DEMs. The
NASA Technical Reports Server (NTRS)
Shie, Chung-Lin; Tao, Wei-Kuo; Simpson, Joanne
2003-01-01
The 1999 Kwajalein Atoll field experiment (KWAJEX), one of several major TRMM (Tropical Rainfall Measuring Mission) field experiments, has successfully obtained a wealth of information and observation data on tropical convective systems over the western Central Pacific region. In this paper, clouds and convective systems that developed during three active periods (Aug 7-12, Aug 17-21, and Aug 29-Sep 13) around Kwajalein Atoll site are simulated using both 2D and 3D Goddard Cumulus Ensemble (GCE) models. Based on numerical results, the clouds and cloud systems are generally unorganized and short lived. These features are validated by radar observations that support the model results. Both the 2D and 3D simulated rainfall amounts and their stratiform contribution as well as the heat, water vapor, and moist static energy budgets are examined for the three convective episodes. Rainfall amounts are quantitatively similar between the two simulations, but the stratiform contribution is considerably larger in the 2D simulation. Regardless of dimension, fo all three cases, the large-scale forcing and net condensation are the two major physical processes that account for the evolution of the budgets with surface latent heat flux and net radiation solar and long-wave radiation)being secondary processes. Quantitative budget differences between 2D and 3D as well as between various episodes will be detailed.Morover, simulated radar signatures and Q1/Q2 fields from the three simulations are compared to each other and with radar and sounding observations.
ON THE ORIGIN OF THE TYPE II SPICULES: DYNAMIC THREE-DIMENSIONAL MHD SIMULATIONS
MartInez-Sykora, Juan; Hansteen, Viggo; Moreno-Insertis, Fernando E-mail: viggo.hansteen@astro.uio.no
2011-07-20
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.
Flux Emergence In The Solar Photosphere - Diagnostics Based On 3-D Rradiation-MHD Simulations
NASA Astrophysics Data System (ADS)
Yelles Chaouche, L.; Cheung, M.; Lagg, A.; Solanki, S.
2006-08-01
We investigate flux tube emergence in the solar photosphere using a diagnostic procedure based on analyzing Stokes signals from different spectral lines calculated in 3-D radiation-MHD simulations. The simulations include the effects of radiative transport and partial ionization and cover layers both above and below the solar surface. The simulations consider the emergence of a twisted magnetic flux tube through the solar surface. We consider different stages in the emergence process, starting from the early appearance of the flux tube at the solar surface, and following the emergence process until the emerged flux looks similar to a normal bipolar region. At every stage we compute line profiles by numerically solving the Unno-Rachkovsky equations at every horizontal grid point. Then, following observational practice, we apply Milne-Eddington-type inversions to the synthetic spectra in order to retrieve different atmospheric parameters. We include the influence of spatial smearing on the deduced atmospheric parameters to identify signatures of different stages of flux emergence in the solar photosphere.
Time-dependent simulation of oblique MHD cosmic-ray shocks using the two-fluid model
NASA Technical Reports Server (NTRS)
Frank, Adam; Jones, T. W.; Ryu, Dongsu
1995-01-01
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.
Numerical Simulation of Turbulent MHD Flows Using an Iterative PNS Algorithm
NASA Technical Reports Server (NTRS)
Kato, Hiromasa; Tannehill, John C.; Mehta, Unmeel B.
2003-01-01
A new parabolized Navier-Stokes (PNS) algorithm has been developed to efficiently compute magnetohydrodynamic (MHD) flows in the low magnetic Reynolds number regime. In this regime, the electrical conductivity is low and the induced magnetic field is negligible compared to the applied magnetic field. The MHD effects are modeled by introducing source terms into the PNS equation which can then be solved in a very efficient manner. To account for upstream (elliptic) effects, the flowfields are computed using multiple streamwise sweeps with an iterated PNS algorithm. Turbulence has been included by modifying the Baldwin-Lomax turbulence model to account for MHD effects. The new algorithm has been used to compute both laminar and turbulent, supersonic, MHD flows over flat plates and supersonic viscous flows in a rectangular MHD accelerator. The present results are in excellent agreement with previous complete Navier-Stokes calculations.
NASA Technical Reports Server (NTRS)
Kapoor, Kamlesh; Anderson, Bernhard H.; Shaw, Robert J.
1994-01-01
A two-dimensional computational code, PRLUS2D, which was developed for the reactive propulsive flows of ramjets and scramjets, was validated for two-dimensional shock-wave/turbulent-boundary-layer interactions. The problem of compression corners at supersonic speeds was solved using the RPLUS2D code. To validate the RPLUS2D code for hypersonic speeds, it was applied to a realistic hypersonic inlet geometry. Both the Baldwin-Lomax and the Chien two-equation turbulence models were used. Computational results showed that the RPLUS2D code compared very well with experimentally obtained data for supersonic compression corner flows, except in the case of large separated flows resulting from the interactions between the shock wave and turbulent boundary layer. The computational results compared well with the experiment results in a hypersonic NASA P8 inlet case, with the Chien two-equation turbulence model performing better than the Baldwin-Lomax model.
Stability and accuracy of 3D neutron transport simulations using the 2D/1D method in MPACT
Collins, Benjamin; Stimpson, Shane; Kelley, Blake W.; Young, Mitchell T. H.; Kochunas, Brendan; Graham, Aaron; Larsen, Edward W.; Downar, Thomas; Godfrey, Andrew
2016-08-25
We derived a consistent “2D/1D” neutron transport method from the 3D Boltzmann transport equation, to calculate fuel-pin-resolved neutron fluxes for realistic full-core Pressurized Water Reactor (PWR) problems. The 2D/1D method employs the Method of Characteristics to discretize the radial variables and a lower order transport solution to discretize the axial variable. Our paper describes the theory of the 2D/1D method and its implementation in the MPACT code, which has become the whole-core deterministic neutron transport solver for the Consortium for Advanced Simulations of Light Water Reactors (CASL) core simulator VERA-CS. We also performed several applications on both leadership-class and industry-classmore » computing clusters. Results are presented for whole-core solutions of the Watts Bar Nuclear Power Station Unit 1 and compared to both continuous-energy Monte Carlo results and plant data.« less
The Biermann Battery in Cosmological MHD Simulations of Population III Star Formation
NASA Astrophysics Data System (ADS)
Xu, Hao; O'Shea, Brian W.; Collins, David C.; Norman, Michael L.; Li, Hui; Li, Shengtai
2008-12-01
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 simeq 10-9 G in the center of our star-forming halo at z simeq 17.55 at a baryon density of nB ~ 1010 cm-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 β >= 1015 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.
The Biermann Battery In Cosmological Mhd Simulations Of Population III Star Formation
Xu, Hao; O' Shea, Brian W; Li, Hui; Li, Shengtai; Norman, Michael L; Collins, David C
2008-01-01
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.
3D MHD Simulations of Laser Plasma Guiding in Curved Magnetic Field
NASA Astrophysics Data System (ADS)
Roupassov, S.; Rankin, R.; Tsui, Y.; Capjack, C.; Fedosejevs, R.
1999-11-01
The guiding and confinement of laser produced plasma in a curved magnetic field has been investigated numerically. These studies were motivated by experiments on pulsed laser deposition of diamond-like films [1] in which a 1kG magnetic field in a curved solenoid geometry was utilized to steer a carbon plasma around a curved trajectory and thus to separate it from unwanted macroparticles produced by the laser ablation. The purpose of the modeling was to characterize the plasma dynamics during the propagation through the magnetic guide field and to investigate the effect of different magnetic field configurations. A 3D curvilinear ADI code developed on the basis of an existing Cartesian code [2] was employed to simulate the underlying resistive one-fluid MHD model. Issues such as large regions of low background density and nonreflective boundary conditions were addressed. Results of the simulations in a curved guide field will be presented and compared to experimental results. [1] Y.Y. Tsui, D. Vick and R. Fedosejevs, Appl. Phys. Lett. 70 (15), pp. 1953-57, 1997. [2] R. Rankin, and I. Voronkov, in "High Performance Computing Systems and Applications", pp. 59-69, Kluwer AP, 1998.
MHD simulations of magnetized laser-plasma interaction for laboratory astrophysics
NASA Astrophysics Data System (ADS)
Khiar, Benjamin; Ciardi, Andrea; Vinci, Tommaso; Revet, Guilhem; Fuchs, Julien; Higginson, Drew
2015-11-01
Laser-driven plasmas coupled with externally applied strong, steady-state, magnetic fields have applications that range from ICF to astrophysical studies of jet collimation, accretion shock dynamics in young stars and streaming instabilities in space plasmas. We have recently included the modelling of laser energy deposition in our three-dimensional, resistive two-temperature MHD code GORGON. The model assumes linear inverse-bremsstrahlung absorption and the laser propagation is done in the geometrical optics approximation. We present full scale numerical simulations of actual experiments performed on the ELFIE installation at LULI, including plasma generated from single and multiple laser plasmas embedded in a magnetic field of strength up to 20 T, and experiments and astrophysical simulations that have shown the viability of poloidal magnetic fields to directly result in the collimation of outflows and the formation of jets in astrophysical accreting systems, such as in young stellar objects. The authors acknowledge the support from the Ile-de-France DIM ACAV, from the LABEX Plas@par and from the ANR grant SILAMPA.
3D Multifluid MHD simulation for Uranus and Neptune: the seasonal variations of their magnetosphere
NASA Astrophysics Data System (ADS)
Cao, X.; Paty, C. S.
2015-12-01
The interaction between Uranus' intrinsic magnetic field and the solar wind is quite different from the magnetospheric interactions of other planets. Uranus' large obliquity, coupled with the fact that its dipole moment is off-centered and highly tilted relative to the rotation axis, leads to unique and seasonally dependent interaction geometries with the solar wind. We present results from adapting a multifluid MHD simulation to examine these seasonally dependent geometries in terms of the global magnetospheric structure, magnetopause and bow shock location, and magnetotail configuration. The Voyager 2 spacecraft encountered Uranus near solstice, and was able to observe the magnetic field structure and plasma characteristics of a twisted magnetotail [Behannon et al., 1987]. We use such magnetometer and plasma observations as a basis for benchmarking our simulations for the solstice scenario. Auroral observations made by the Hubble Space Telescope during equinox [Lamy et al.,2012] give some indication of the magnetospheric interaction with the solar wind. We also demonstrate the structural difference of the magnetosphere between solstice and equinox seasons. The magnetosphere at equinox is quite distinct due to the orientation and rotation of the magnetic axis relative to the solar wind direction.
MHD simulations of protostellar jets: formation and stability of shock diamonds
NASA Astrophysics Data System (ADS)
Ustamujic, Sabina
2016-07-01
The early stages of a star birth are characterised by a variety of mass ejection phenomena, including outflows and collimated jets, that are strongly related with the accretion process developed in the context of the star-disc interaction. After been ejected, jets move through the ambient medium, interacting and producing shocks and complex structures that are observed at different wavelength bands. In particular, X-ray observations show evidence of strong shocks heating the plasma up to temperatures of a few million degrees. In some cases, the shocked features appear to be stationary and have been interpreted as shock diamonds. We aim at investigating the physical properties of the shocked plasma and the role of the magnetic field on the collimation performing 2.5D MHD simulations, including the effects of the thermal conduction and the radiative losses. We modelled the propagation of a jet ramming with a supersonic speed into an initially isothermal and homogeneous magnetized medium. We studied the physics that guides the formation of a stationary shock (for instance a shock diamond) and compared the results with observations, via the emission measure distribution vs. temperature and the luminosity synthesised from the simulations.
Fast Wave Trains Associated with Solar Eruptions: Insights from 3D Thermodynamic MHD Simulations
NASA Astrophysics Data System (ADS)
Downs, C.; Liu, W.; Torok, T.; Linker, J.; Mikic, Z.; Ofman, L.
2015-12-01
EUV imaging observations during the SDO/AIA era have provided new insights into a variety of wave phenomena occurring in the low solar corona. One example is the observation of quasi-periodic, fast-propagating wave trains that are associated with solar eruptions, including flares and CMEs. While there has been considerable progress in understanding such waves from both an observational and theoretical perspective, it remains a challenge to pin down their physical origin. In this work, we detail our results from a case-study 3D thermodynamic MHD simulation of a coronal mass ejection where quasi-periodic wave trains are generated during the simulated eruption. We find a direct correlation between the onset of non-steady reconnection in the flare current sheet and the generation of quasi-periodic wave train signatures when patchy, collimated downflows interact with the flare arcade. Via forward modeling of SDO/AIA observables, we explore how the appearance of the wave trains is affected by line-of-sight integration and the multi-thermal nature of the coronal medium. We also examine how the wave trains themselves are channeled by natural waveguides formed in 3D by the non-uniform background magnetic field. While the physical association of the reconnection dynamics to the generation of quasi-periodic wave trains appears to be a compelling result, unanswered questions posed from recent observations as well as future prospects will be discussed.
NASA Astrophysics Data System (ADS)
Suzuki, Y.; KOYAGUCHI, T.; OGAWA, M.; Hachisu, I.
2001-05-01
Mixing of eruption cloud and air is one of the most important processes for eruption cloud dynamics. The critical condition of eruption types (eruption column or pyroclastic flow) depends on efficiency of mixing of eruption cloud and the ambient air. However, in most of the previous models (e.g., Sparks,1986; Woods, 1988), the rate of mixing between cloud and air is taken into account by introducing empirical parameters such as entrainment coefficient or turbulent diffusion coefficient. We developed a numerical model of 2-D (axisymmetrical) eruption columns in order to simulate the turbulent mixing between eruption column and air. We calculated the motion of an eruption column from a circular vent on the flat surface of the earth. Supposing that relative velocity of gas and ash particles is sufficiently small, we can treat eruption cloud as a single gas. Equation of state (EOS) for the mixture of the magmatic component (i.e. volcanic gas plus pyroclasts) and air can be expressed by EOS for an ideal gas, because volume fraction of the gas phase is very large. The density change as a function of mixing ratio between air and the magmatic component has a strong non-linear feature, because the density of the mixture drastically decreases as entrained air expands by heating. This non-linear feature can be reproduced by changing the gas constant and the ratio of specific heat in EOS for ideal gases; the molecular weight increases and the ratio of specific heat approaches 1 as the magmatic component increases. It is assumed that the dynamics of eruption column follows the Euler equation, so that no viscous effect except for the numerical viscosity is taken into account. Roe scheme (a general TVD scheme for compressible flow) is used in order to simulate the generation of shock waves inside and around the eruption column. The results show that many vortexes are generated around the boundary between eruption cloud and air, which results in violent mixing. When the size of
Formation and Eruption of an Active Region Sigmoid: NLFFF Modeling and MHD Simulation
NASA Astrophysics Data System (ADS)
Jiang, C.; Wu, S.; Feng, X.; Hu, Q.
2013-12-01
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
NASA Astrophysics Data System (ADS)
Wang, X.; Cai, M.
2016-11-01
A nonlinear velocity model that considers the influence of confinement and rock mass failure on wave velocity is developed. A numerical method, which couples FLAC and SPECFEM2D, is developed for ground motion modeling near excavation boundaries in underground mines. The motivation of developing the FLAC/SPECFEM2D coupled approach is to take merits of each code, such as the stress analysis capability in FLAC and the powerful wave propagation analysis capability in SPECFEM2D. Because stress redistribution and failure of the rock mass around an excavation are considered, realistic non-uniform velocity fields for the SPECFEM2D model can be obtained, and this is a notable feature of this study. Very large differences in wavefields and ground motion are observed between the results from the non-uniform and the uniform velocity models. If the non-uniform velocity model is used, the ground motion around a stope can be amplified up to five times larger than that given by the design scaling law. If a uniform velocity model is used, the amplification factor is only about three. Using the FLAC/SPECFEM2D coupled modeling approach, accurate velocity models can be constructed and this in turn will assist in predicting ground motions accurately around underground excavations.
Matsuda, K.; Terada, N.; Katoh, Y.; Misawa, H.
2011-08-15
There has been a great concern about the origin of the parallel electric field in the frame of fluid equations in the auroral acceleration region. This paper proposes a new method to simulate magnetohydrodynamic (MHD) equations that include the electron convection term and shows its efficiency with simulation results in one dimension. We apply a third-order semi-discrete central scheme to investigate the characteristics of the electron convection term including its nonlinearity. At a steady state discontinuity, the sum of the ion and electron convection terms balances with the ion pressure gradient. We find that the electron convection term works like the gradient of the negative pressure and reduces the ion sound speed or amplifies the sound mode when parallel current flows. The electron convection term enables us to describe a situation in which a parallel electric field and parallel electron acceleration coexist, which is impossible for ideal or resistive MHD.
NASA Astrophysics Data System (ADS)
Reuter, K.; Jenko, F.; Forest, C. B.; Bayliss, R. A.
2008-08-01
A parallel implementation of a nonlinear pseudo-spectral MHD code for the simulation of turbulent dynamos in spherical geometry is reported. It employs a dual domain decomposition technique in both real and spectral space. It is shown that this method shows nearly ideal scaling going up to 128 CPUs on Beowulf-type clusters with fast interconnect. Furthermore, the potential of exploiting single precision arithmetic on standard x86 processors is examined. It is pointed out that the MHD code thereby achieves a maximum speedup of 1.7, whereas the validity of the computations is still granted. The combination of both measures will allow for the direct numerical simulation of highly turbulent cases ( 1500
2-D and 3-D numerical simulation of a supersonic inlet flowfield
NASA Astrophysics Data System (ADS)
Enomoto, Shunji; Arakawa, Chuichi
The 2-D and 3-D steady, Reynolds-averaged Navier-Stokes equations were numerically solved for the flowfields in an experimentally tested inlet model with bleed through a cavity. In the 2-D analysis, a normal shock was located at diffuser inlet instead of the position below the cavity. The normal shock in the middle of the diffuser caused a massive separation of the boundary layer and a large total pressure loss. In the 3-D analysis, the shock wave was distorted by the side wall boundary layer separation, and the complex flow structure was established. The result of the 3-D analysis agreed well with the experiment.
Analysis of Voyager Observed High-Energy Electron Fluxes in the Heliosheath Using MHD Simulations
NASA Technical Reports Server (NTRS)
Washimi, Haruichi; Webber, W. R.; Zank, Gary P.; Hu, Qiang; Florinski, Vladimir; Adams, James; Kubo, Yuki
2011-01-01
The Voyager spacecraft (V1 and V2) observed electrons of 6-14 MeV in the heliosheath which showed several incidences of flux variation relative to a background of gradually increasing flux with distance from the Sun. The increasing flux of background electrons is thought to result from inward radial diffusion. We compare the temporal electron flux variation with dynamical phenomena in the heliosheath that are obtained from our MHD simulations. Because our simulation is based on V2 observed plasma data before V2 crossed the termination shock, this analysis is effective up to late 2008, i.e., about a year after the V2-crossing, during which disturbances, driven prior to the crossing time, survived in the heliosheath. Several electron flux variations correspond to times directly associated with interplanetary shock events. One noteworthy example corresponds to various times associated with the March 2006 interplanetary shock, these being the collision with the termination shock, the passage past the V1 spacecraft, and the collision with the region near the heliopause, as identified by W.R. Webber et al. for proton/helium of 7-200 MeV. Our simulations indicate that all other electron flux variations, except one, correspond well to the times when a shock-driven magneto-sonic pulse and its reflection in the heliosheath either passed across V1/V2, or collided with the termination shock or with the plasma sheet near the heliopause. This result suggests that variation in the electron flux should be due to either direct or indirect effects of magnetosonic pulses in the heliosheath driven by interplanetary shocks
Proceedings of the workshop on nonlinear MHD and extended MHD
1998-12-01
Nonlinear MHD simulations have proven their value in interpreting experimental results over the years. As magnetic fusion experiments reach higher performance regimes, more sophisticated experimental diagnostics coupled with ever expanding computer capabilities have increased both the need for and the feasibility of nonlinear global simulations using models more realistic than regular ideal and resistive MHD. Such extended-MHD nonlinear simulations have already begun to produce useful results. These studies are expected to lead to ever more comprehensive simulation models in the future and to play a vital role in fully understanding fusion plasmas. Topics include the following: (1) current state of nonlinear MHD and extended-MHD simulations; (2) comparisons to experimental data; (3) discussions between experimentalists and theorists; (4) /equations for extended-MHD models, kinetic-based closures; and (5) paths toward more comprehensive simulation models, etc. Selected papers have been indexed separately for inclusion in the Energy Science and Technology Database.
NASA Technical Reports Server (NTRS)
Proffitt, M. H.; Solomon, S.; Loewenstein, M.
1992-01-01
A linear reference relationship between O3 and N2O has been used to estimate polar winter O3 loss from aircraft data taken in the lower stratosphere. Here, this relationship is evaluated at high latitudes by comparing it with a 2D model simulation and with NIMBUS 7 satellite measurements. Although comparisons with satellite measurements are limited to January through May, the model simulations are compared during other seasons. The model simulations and the satellite data are found to be consistent with the winter O3 loss analysis. It is shown that such analyses are likely to be inappropriate during other seasons.
Justification for a 2D versus 3D fingertip finite element model during static contact simulations.
Harih, Gregor; Tada, Mitsunori; Dolšak, Bojan
2016-10-01
The biomechanical response of a human hand during contact with various products has not been investigated in details yet. It has been shown that excessive contact pressure on the soft tissue can result in discomfort, pain and also cumulative traumatic disorders. This manuscript explores the benefits and limitations of a simplified two-dimensional vs. an anatomically correct three-dimensional finite element model of a human fingertip. Most authors still use 2D FE fingertip models due to their simplicity and reduced computational costs. However we show that an anatomically correct 3D FE fingertip model can provide additional insight into the biomechanical behaviour. The use of 2D fingertip FE models is justified when observing peak contact pressure values as well as displacement during the contact for the given studied cross-section. On the other hand, an anatomically correct 3D FE fingertip model provides a contact pressure distribution, which reflects the fingertip's anatomy.
Simulation of multi-steps thermal transition in 2D spin-crossover nanoparticles
NASA Astrophysics Data System (ADS)
Jureschi, Catalin-Maricel; Pottier, Benjamin-Louis; Linares, Jorge; Richard Dahoo, Pierre; Alayli, Yasser; Rotaru, Aurelian
2016-04-01
We have used an Ising like model to study the thermal behavior of a 2D spin crossover (SCO) system embedded in a matrix. The interaction parameter between edge SCO molecules and its local environment was included in the standard Ising like model as an additional term. The influence of the system's size and the ratio between the number of edge molecules and the other molecules were also discussed.
EDGE2D Simulations of JET{sup 13}C Migration Experiments
J.D. Strachan; J.P. Coad; G. Corrigan; G.F. Matthews; J. Spence
2004-06-16
Material migration has received renewed interest due to tritium retention associated with carbon transport to remote vessel locations. Those results influence the desirability of carbon usage on ITER. Subsequently, additional experiments have been performed, including tracer experiments attempting to identify material migration from specific locations. In this paper, EDGE2D models a well-diagnosed JET{sup 13}C tracer migration experiment. The role of SOL flows upon the migration patterns is identified.
NASA Astrophysics Data System (ADS)
Pahud, D. M.; Merkin, V. G.; Arge, C. N.; Hughes, W. J.; McGregor, S. M.
2012-07-01
We present results from a new magnetohydrodynamic (MHD) model of the inner heliosphere. The model is adapted from the well-established Lyon-Fedder-Mobarry (LFM) MHD simulation code, which until recently mostly applied to studies of the terrestrial magnetosphere. We perform quasi steady-state simulations of two Carrington rotations: 2060 and 2068. During both of these periods, the heliosphere remained quiet and undisturbed by transient phenomena, making them well-suited for simulation studies of Corotating Interaction Regions (CIRs). The MHD model of the solar wind is driven at the inner boundary by the Wang-Sheeley-Arge (WSA) model of the corona augmented with empirical relations to infer the solar wind velocity, density, and temperature. Here we report on a validation exercise whereby LFM-helio simulation results are compared with in situ data from the Advanced Composition Explorer (ACE) and MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We find that the model successfully reproduces the large-scale configuration of the inner heliosphere, namely timing and duration of high-speed streams and heliospheric current sheet crossings, as reflected in ACE and MESSENGER observations. Discrepancies between in situ measurements and simulations, such as 1-2 day errors in the time of arrival of a CIR or the strength of the simulated magnetic field at the spacecraft, are attributed to the uncertainty in the specification of the coronal conditions, rather than a poor performance of the solar wind model. More comparisons between different inner heliosphere models driven with identical coronal conditions are suggested as a way to explore their comparative strengths and weaknesses.
Smoothed MHD equations for numerical simulations of ideal quasi-neutral gas dynamic flows
NASA Astrophysics Data System (ADS)
Popov, Mikhail V.; Elizarova, Tatiana G.
2015-11-01
We introduce a mathematical model and related numerical method for numerical modeling of ideal magnetohydrodynamic (MHD) gas flows as an extension of previously known quasi-gasdynamic (QGD) equations. This approach is based on smoothing, or averaging of the original MHD equation system over a small time interval that leads to a new equation system, named quasi-MHD, or QMHD system. The QMHD equations are closely related to the original MHD system except for additional strongly non-linear dissipative τ-terms with a small parameter τ as a factor. The τ-terms depend on the solution itself and decrease in regions with the small space gradients of the solution. In this sense the QMHD system could be regarded as an approach with adaptive artificial dissipation. The QMHD is a generalization of regularized (or quasi-) gas dynamic equation system suggested in last three decades. In the QMHD numerical method the evolution of all physical variables is presented in a non-split divergence form. Divergence-free evolution of the magnetic field provides by using a constrained transport method based on Faraday's law of induction. Accuracy and convergence of the QMHD method is verified on a wide set of standard MHD tests including the 3D Orszag-Tang vortex flow.
Simulations Of 3D MHD Jets: The Effects Of ICM Weather And AGN History On X-ray Cavities
NASA Astrophysics Data System (ADS)
Mendygral, Peter; Jones, T. W.; Dolag, K.
2011-01-01
The powerful jets from AGN produce low density bubbles in the ICM of the host galaxy cluster that are observed as X-ray cavities. The morphology of X-ray cavities is influenced by factors such as AGN history and ICM weather. We present 3D magnetohydrodynamical (MHD) simulations of hypersonic AGN jets in realistic cluster environments that explore the relationship between these factors and cavity properties. We will also discuss the consequences on observations of X-ray cavities with synthetic observations of these simulations. This work is supported by the NSF and by the University of Minnesota Supercomputing Institute.
NASA Astrophysics Data System (ADS)
Elangovan, Premkumar; Warren, Lucy M.; Mackenzie, Alistair; Rashidnasab, Alaleh; Diaz, Oliver; Dance, David R.; Young, Kenneth C.; Bosmans, Hilde; Strudley, Celia J.; Wells, Kevin
2014-08-01
Planar 2D x-ray mammography is generally accepted as the preferred screening technique used for breast cancer detection. Recently, digital breast tomosynthesis (DBT) has been introduced to overcome some of the inherent limitations of conventional planar imaging, and future technological enhancements are expected to result in the introduction of further innovative modalities. However, it is crucial to understand the impact of any new imaging technology or methodology on cancer detection rates and patient recall. Any such assessment conventionally requires large scale clinical trials demanding significant investment in time and resources. The concept of virtual clinical trials and virtual performance assessment may offer a viable alternative to this approach. However, virtual approaches require a collection of specialized modelling tools which can be used to emulate the image acquisition process and simulate images of a quality indistinguishable from their real clinical counterparts. In this paper, we present two image simulation chains constructed using modelling tools that can be used for the evaluation of 2D-mammography and DBT systems. We validate both approaches by comparing simulated images with real images acquired using the system being simulated. A comparison of the contrast-to-noise ratios and image blurring for real and simulated images of test objects shows good agreement ( < 9% error). This suggests that our simulation approach is a promising alternative to conventional physical performance assessment followed by large scale clinical trials.
Elangovan, Premkumar; Warren, Lucy M; Mackenzie, Alistair; Rashidnasab, Alaleh; Diaz, Oliver; Dance, David R; Young, Kenneth C; Bosmans, Hilde; Strudley, Celia J; Wells, Kevin
2014-08-01
Planar 2D x-ray mammography is generally accepted as the preferred screening technique used for breast cancer detection. Recently, digital breast tomosynthesis (DBT) has been introduced to overcome some of the inherent limitations of conventional planar imaging, and future technological enhancements are expected to result in the introduction of further innovative modalities. However, it is crucial to understand the impact of any new imaging technology or methodology on cancer detection rates and patient recall. Any such assessment conventionally requires large scale clinical trials demanding significant investment in time and resources. The concept of virtual clinical trials and virtual performance assessment may offer a viable alternative to this approach. However, virtual approaches require a collection of specialized modelling tools which can be used to emulate the image acquisition process and simulate images of a quality indistinguishable from their real clinical counterparts. In this paper, we present two image simulation chains constructed using modelling tools that can be used for the evaluation of 2D-mammography and DBT systems. We validate both approaches by comparing simulated images with real images acquired using the system being simulated. A comparison of the contrast-to-noise ratios and image blurring for real and simulated images of test objects shows good agreement ( < 9% error). This suggests that our simulation approach is a promising alternative to conventional physical performance assessment followed by large scale clinical trials.
Trapping solids at the inner edge of the dead zone: 3-D global MHD simulations
NASA Astrophysics Data System (ADS)
Dzyurkevich, N.; Flock, M.; Turner, N. J.; Klahr, H.; Henning, Th.
2010-06-01
Context. The poorly-ionized interior of the protoplanetary disk or “dead zone” is the location where dust coagulation processes may be most efficient. However even here, planetesimal formation may be limited by the loss of solid material through radial drift, and by collisional fragmentation of the particles. Both depend on the turbulent properties of the gas. Aims: Our aim here is to investigate the possibility that solid particles are trapped at local pressure maxima in the dynamically evolving disk. We perform the first 3-D global non-ideal magnetohydrodynamical (MHD) calculations of a section of the disk treating the turbulence driven by the magneto-rotational instability (MRI). Methods: We use the ZeusMP code with a fixed Ohmic resistivity distribution. The domain contains an inner MRI-active region near the young star and an outer midplane dead zone, with the transition between the two modeled by a sharp increase in the magnetic diffusivity. Results: The azimuthal magnetic fields generated in the active zone oscillate over time, changing sign about every 150 years. We thus observe the radial structure of the “butterfly pattern” seen previously in local shearing-box simulations. The mean magnetic field diffuses from the active zone into the dead zone, where the Reynolds stress nevertheless dominates, giving a residual α between 10-4 and 10-3. The greater total accretion stress in the active zone leads to a net reduction in the surface density, so that after 800 years an approximate steady state is reached in which a local radial maximum in the midplane pressure lies near the transition radius. We also observe the formation of density ridges within the active zone. Conclusions: The dead zone in our models possesses a mean magnetic field, significant Reynolds stresses and a steady local pressure maximum at the inner edge, where the outward migration of planetary embryos and the efficient trapping of solid material are possible.
Relativistic MHD simulations of core-collapse GRB jets: 3D instabilities and magnetic dissipation
NASA Astrophysics Data System (ADS)
Bromberg, Omer; Tchekhovskoy, Alexander
2016-02-01
Relativistic jets are associated with extreme astrophysical phenomena, like the core collapse of massive stars in gamma-ray bursts (GRBs) and the accretion on to supermassive black holes in active galactic nuclei. It is generally accepted that these jets are powered electromagnetically, by the magnetized rotation of a central compact object (black hole or neutron star). However, how the jets produce the observed emission and survive the propagation for many orders of magnitude in distance without being disrupted by current-driven instabilities is the subject of active debate. We carry out time-dependent 3D relativistic magnetohydrodynamic (MHD) simulations of relativistic, Poynting-flux-dominated jets. The jets are launched self-consistently by the rotation of a strongly magnetized central object. This determines the natural degree of azimuthal magnetic field winding, a crucial factor that controls jet stability. We find that the jets are susceptible to two types of instability: (i) a global, external kink mode that grows on long time-scales. It bodily twists the jet, reducing its propagation velocity. We show analytically that in flat density profiles, like the ones associated with galactic cores, the external mode grows and may stall the jet. In the steep profiles of stellar envelopes the external kink weakens as the jet propagates outward. (ii) a local, internal kink mode that grows over short time-scales and causes small-angle magnetic reconnection and conversion of about half of the jet electromagnetic energy flux into heat. We suggest that internal kink instability is the main dissipation mechanism responsible for powering GRB prompt emission.
Fully Parallel MHD Stability Analysis Tool
NASA Astrophysics Data System (ADS)
Svidzinski, Vladimir; Galkin, Sergei; Kim, Jin-Soo; Liu, Yueqiang
2014-10-01
Progress on full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. It is a powerful tool for studying MHD and MHD-kinetic instabilities and it is widely used by fusion community. Parallel version of MARS is intended for simulations on local parallel clusters. It will be an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, already implemented in MARS. Parallelization of the code includes parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the present MARS algorithm using parallel libraries and procedures. Initial results of the code parallelization will be reported. Work is supported by the U.S. DOE SBIR program.
Fully Parallel MHD Stability Analysis Tool
NASA Astrophysics Data System (ADS)
Svidzinski, Vladimir; Galkin, Sergei; Kim, Jin-Soo; Liu, Yueqiang
2013-10-01
Progress on full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. It is a powerful tool for studying MHD and MHD-kinetic instabilities and it is widely used by fusion community. Parallel version of MARS is intended for simulations on local parallel clusters. It will be an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, already implemented in MARS. Parallelization of the code includes parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the present MARS algorithm using parallel libraries and procedures. Preliminary results of the code parallelization will be reported. Work is supported by the U.S. DOE SBIR program.
Fully Parallel MHD Stability Analysis Tool
NASA Astrophysics Data System (ADS)
Svidzinski, Vladimir; Galkin, Sergei; Kim, Jin-Soo; Liu, Yueqiang
2015-11-01
Progress on full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. It is a powerful tool for studying MHD and MHD-kinetic instabilities and it is widely used by fusion community. Parallel version of MARS is intended for simulations on local parallel clusters. It will be an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, already implemented in MARS. Parallelization of the code includes parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the present MARS algorithm using parallel libraries and procedures. Results of MARS parallelization and of the development of a new fix boundary equilibrium code adapted for MARS input will be reported. Work is supported by the U.S. DOE SBIR program.
A pressure-based high resolution numerical method for resistive MHD
NASA Astrophysics Data System (ADS)
Xisto, Carlos M.; Páscoa, José C.; Oliveira, Paulo J.
2014-10-01
In the paper we describe in detail a numerical method for the resistive magnetohydrodynamic (MHD) equations involving viscous flow and report the results of application to a number of typical MHD test cases. The method is of the finite volume type but mixes aspects of pressure-correction and density based solvers; the algorithm arrangement is patterned on the well-known PISO algorithm, which is a pressure method, while the flux computation makes use of the AUSM-MHD scheme, which originates from density based methods. Five groups of test cases are addressed to verify and validate the method. We start with two resistive MHD cases, namely the Shercliff and Hunt flow problems, which are intended to validate the method for low-speed resistive MHD flows. The remaining three test cases, namely the cloud-shock interaction, the MHD rotor and the MHD blast wave, are standard 2D ideal MHD problems that serve to validate the method under high-speed flow and complex interaction of MHD shocks. Finally, we demonstrate the method with a more complex application problem, and discuss results of simulation for a quasi-bi-dimensional self-field magnetoplasmadynamic (MPD) thruster, for which we study the effect of cathode length upon the electromagnetic nozzle performance.
Multi-layered coarse grid modelling in 2D urban flood simulations
NASA Astrophysics Data System (ADS)
Chen, Albert S.; Evans, Barry; Djordjević, Slobodan; Savić, Dragan A.
2012-11-01
SummaryRegular grids are commonly used in 2D flood modelling due to wide availability of terrain models and low pre-processing required for input preparation. Despite advances in both computing software and hardware, high resolution flood modelling remains computationally demanding when applied to a large study area when the available time and resources are limited. Traditional grid coarsening approach may reduce not only the computing demands, but also the accuracy of results due to the loss of detailed information. To keep key features that affect flow propagation within coarse grid, the approach proposed and tested in this paper adopts multiple layers in flood modelling to reflect individual flow paths separated by buildings within a coarse grid cell. The cell in each layer has its own parameters (elevation, roughness, building coverage ratio, and conveyance reduction factors) to describe itself and the conditions at boundaries with neighbourhood cells. Results of tests on the synthetic case study and the real world urban area show that the proposed multi-layered approach greatly improves the accuracy of coarse grid modelling with an insignificant additional computing cost. The proposed approach has been tested in conjunction with the UIM model by taking the high resolution results as the benchmark. The implementation of the proposed multi-layered methodology to any regular grid based 2D model would be straightforward.
Self-organisation in protoplanetary discs. Global, non-stratified Hall-MHD simulations
NASA Astrophysics Data System (ADS)
Béthune, William; Lesur, Geoffroy; Ferreira, Jonathan
2016-05-01
Context. Recent observations have revealed organised structures in protoplanetary discs, such as axisymmetric rings or horseshoe concentrations, evocative of large-scale vortices. These structures are often interpreted as the result of planet-disc interactions. However, these discs are also known to be unstable to the magneto-rotational instability (MRI) which is believed to be one of the dominant angular momentum transport mechanism in these objects. It is therefore natural to ask whether the MRI itself could produce these structures without invoking planets. Aims: The nonlinear evolution of the MRI is strongly affected by the low ionisation fraction in protoplanetary discs. The Hall effect in particular, which is dominant in dense and weakly ionised parts of these objects, has been shown to spontaneously drive self-organising flows in local, shearing box simulations. Here, we investigate the behaviour of global MRI-unstable disc models dominated by the Hall effect and characterise their dynamics. Methods: We validated our implementation of the Hall effect into the PLUTO code with predictions from a spectral method in cylindrical geometry. We then performed 3D unstratified Hall-MHD simulations of Keplerian discs for a broad range of Hall, Ohmic, and ambipolar Elsasser numbers. Results: We confirm the transition from a turbulent to an organised state as the intensity of the Hall effect is increased. We observe the formation of zonal flows, their number depending on the available magnetic flux and on the intensity of the Hall effect. For intermediate Hall intensity, the flow self-organises into long-lived magnetised vortices. Neither the addition of a toroidal field nor Ohmic or ambipolar diffusion change this picture drastically in the range of parameters we have explored. Conclusions: Self-organisation by the Hall effect is a robust phenomenon in global non-stratified simulations. It is able to quench turbulent transport and spontaneously produce axisymmetric
Karavitis, G.A.
1984-01-01
The SIMSYS2D two-dimensional water-quality simulation system is a large-scale digital modeling software system used to simulate flow and transport of solutes in freshwater and estuarine environments. Due to the size, processing requirements, and complexity of the system, there is a need to easily move the system and its associated files between computer sites when required. A series of job control language (JCL) procedures was written to allow transferability between IBM and IBM-compatible computers. (USGS)
SEM simulation for 2D and 3D inspection metrology and defect review
NASA Astrophysics Data System (ADS)
Levi, Shimon; Schwartsband, Ishai; Khristo, Sergey; Ivanchenko, Yan; Adan, Ofer
2014-03-01
Advanced SEM simulation has become a key element in the ability of SEM inspection, metrology and defect review to meet the challenges of advanced technologies. It grants additional capabilities to the end user, such as 3D height measurements, accurate virtual metrology, and supports Design Based Metrology to bridge the gap between design layout and SEM image. In this paper we present SEM simulations capabilities, which take into consideration all parts of the SEM physical and electronic path, interaction between Electron beam and material, multi perspective SEM imaging and shadowing derived from proximity effects caused by the interaction of the Secondary Electrons signal with neighboring pattern edges. Optimizing trade-off between simulation accuracy, calibration procedures and computational complexity, the simulation is running in real-time with minimum impact on throughput. Experiment results demonstrate Height measurement capacities, and CAD based simulated pattern is compared with SEM image to evaluate simulated pattern fidelity.
Malapaka, Shiva Kumar; Mueller, Wolf-Christian
2013-09-01
Statistical properties of the Sun's photospheric turbulent magnetic field, especially those of the active regions (ARs), have been studied using the line-of-sight data from magnetograms taken by the Solar and Heliospheric Observatory and several other instruments. This includes structure functions and their exponents, flatness curves, and correlation functions. In these works, the dependence of structure function exponents ({zeta}{sub p}) of the order of the structure functions (p) was modeled using a non-intermittent K41 model. It is now well known that the ARs are highly turbulent and are associated with strong intermittent events. In this paper, we compare some of the observations from Abramenko et al. with the log-Poisson model used for modeling intermittent MHD turbulent flows. Next, we analyze the structure function data obtained from the direct numerical simulations (DNS) of homogeneous, incompressible 3D-MHD turbulence in three cases: sustained by forcing, freely decaying, and a flow initially driven and later allowed to decay (case 3). The respective DNS replicate the properties seen in the plots of {zeta}{sub p} against p of ARs. We also reproduce the trends and changes observed in intermittency in flatness and correlation functions of ARs. It is suggested from this analysis that an AR in the onset phase of a flare can be treated as a forced 3D-MHD turbulent system in its simplest form and that the flaring stage is representative of decaying 3D-MHD turbulence. It is also inferred that significant changes in intermittency from the initial onset phase of a flare to its final peak flaring phase are related to the time taken by the system to reach the initial onset phase.
Simulation of Ultra-Small MOSFETs Using a 2-D Quantum-Corrected Drift-Diffusion Model
NASA Technical Reports Server (NTRS)
Biegal, Bryan A.; Rafferty, Connor S.; Yu, Zhiping; Ancona, Mario G.; Dutton, Robert W.; Saini, Subhash (Technical Monitor)
1998-01-01
The continued down-scaling of electronic devices, in particular the commercially dominant MOSFET, will force a fundamental change in the process of new electronics technology development in the next five to ten years. The cost of developing new technology generations is soaring along with the price of new fabrication facilities, even as competitive pressure intensifies to bring this new technology to market faster than ever before. To reduce cost and time to market, device simulation must become a more fundamental, indeed dominant, part of the technology development cycle. In order to produce these benefits, simulation accuracy must improve markedly. At the same time, device physics will become more complex, with the rapid increase in various small-geometry and quantum effects. This work describes both an approach to device simulator development and a physical model which advance the effort to meet the tremendous electronic device simulation challenge described above. The device simulation approach is to specify the physical model at a high level to a general-purpose (but highly efficient) partial differential equation solver (in this case PROPHET, developed by Lucent Technologies), which then simulates the model in 1-D, 2-D, or 3-D for a specified device and test regime. This approach allows for the rapid investigation of a wide range of device models and effects, which is certainly essential for device simulation to catch up with, and then stay ahead of, electronic device technology of the present and future. The physical device model used in this work is the density-gradient (DG) quantum correction to the drift-diffusion model [Ancona, Phys. Rev. B 35(5), 7959 (1987)]. This model adds tunneling and quantum smoothing of carrier density profiles to the drift-diffusion model. We used the DG model in 1-D and 2-D (for the first time) to simulate both bipolar and unipolar devices. Simulations of heavily-doped, short-base diodes indicated that the DG quantum
Direct MD Simulations of Terahertz Absorption and 2D Spectroscopy Applied to Explosive Crystals.
Katz, G; Zybin, S; Goddard, W A; Zeiri, Y; Kosloff, R
2014-03-01
A direct molecular dynamics simulation of the THz spectrum of a molecular crystal is presented. A time-dependent electric field is added to a molecular dynamics simulation of a crystal slab. The absorption spectrum is composed from the energy dissipated calculated from a series of applied pulses characterized by a carrier frequency. The spectrum of crystalline cyclotrimethylenetrinitramine (RDX) and triacetone triperoxide (TATP) were simulated with the ReaxFF force field. The proposed direct method avoids the linear response and harmonic approximations. A multidimensional extension of the spectroscopy is suggested and simulated based on the nonlinear response to a single polarized pulse of radiation in the perpendicular polarization direction. PMID:26274066
NASA Astrophysics Data System (ADS)
Kawamura, E.; Lieberman, M. A.; Graves, D. B.
2014-12-01
A fast 2D axisymmetric fluid-analytical plasma reactor model using the finite elements simulation tool COMSOL is interfaced with a 1D particle-in-cell (PIC) code to study ion energy distributions (IEDs) in multi-frequency capacitive argon discharges. A bulk fluid plasma model, which solves the time-dependent plasma fluid equations for the ion continuity and electron energy balance, is coupled with an analytical sheath model, which solves for the sheath parameters. The time-independent Helmholtz equation is used to solve for the fields and a gas flow model solves for the steady-state pressure, temperature and velocity of the neutrals. The results of the fluid-analytical model are used as inputs to a PIC simulation of the sheath region of the discharge to obtain the IEDs at the target electrode. Each 2D fluid-analytical-PIC simulation on a moderate 2.2 GHz CPU workstation with 8 GB of memory took about 15-20 min. The multi-frequency 2D fluid-analytical model was compared to 1D PIC simulations of a symmetric parallel-plate discharge, showing good agreement. We also conducted fluid-analytical simulations of a multi-frequency argon capacitively coupled plasma (CCP) with a typical asymmetric reactor geometry at 2/60/162 MHz. The low frequency 2 MHz power controlled the sheath width and sheath voltage while the high frequencies controlled the plasma production. A standing wave was observable at the highest frequency of 162 MHz. We noticed that adding 2 MHz power to a 60 MHz discharge or 162 MHz to a dual frequency 2 MHz/60 MHz discharge can enhance the plasma uniformity. We found that multiple frequencies were not only useful for controlling IEDs but also plasma uniformity in CCP reactors.
Numerical Simulations of 2-D Phase-Field Model with Convection
NASA Astrophysics Data System (ADS)
Xu, Ying; McDonough, J. M.; Tagavi, K. A.
2003-11-01
We present a 2-D isotropic phase-field model with convection induced by a flow field applied to freezing into a supercooled melt of pure substance, nickle. Numerical procedures and details of numerical parameters employed are provided, and the convergence of the numerical method is demonstrated by conducting grid-function convergence tests. Dendrite structures, temperature fields, pressure fields, streamlines and velocity vector fields are presented at several different times during the dendrite growth process. Comparisons of dendrites and temperature fields with and without convection indicate that the flow field has a significant effect on the growth rate of the dendrites; in particular, it inhibits the growth. In addition, the flow field influences the dendritic structural morphologies and thickness of the interface. Moreover, the dendrites behave as a solid body in the flow leading to stagnation points and other interesting flow features.
2D Quantum Simulation of MOSFET Using the Non Equilibrium Green's Function Method
NASA Technical Reports Server (NTRS)
Svizhenko, Alexel; Anantram, M. P.; Govindan, T. R.; Yan, Jerry (Technical Monitor)
2000-01-01
The objectives this viewgraph presentation summarizes include: (1) the development of a quantum mechanical simulator for ultra short channel MOSFET simulation, including theory, physical approximations, and computer code; (2) explore physics that is not accessible by semiclassical methods; (3) benchmarking of semiclassical and classical methods; and (4) study other two-dimensional devices and molecular structure, from discretized Hamiltonian to tight-binding Hamiltonian.
Non-equilibrium Helium Ionization in an MHD Simulation of the Solar Atmosphere
NASA Astrophysics Data System (ADS)
Golding, Thomas Peter; Leenaarts, Jorrit; Carlsson, Mats
2016-02-01
The ionization state of the gas in the dynamic solar chromosphere can depart strongly from the instantaneous statistical equilibrium commonly assumed in numerical modeling. We improve on earlier simulations of the solar atmosphere that only included non-equilibrium hydrogen ionization by performing a 2D radiation-magnetohydrodynamics simulation featuring non-equilibrium ionization of both hydrogen and helium. The simulation includes the effect of hydrogen Lyα and the EUV radiation from the corona on the ionization and heating of the atmosphere. Details on code implementation are given. We obtain helium ion fractions that are far from their equilibrium values. Comparison with models with local thermodynamic equilibrium (LTE) ionization shows that non-equilibrium helium ionization leads to higher temperatures in wavefronts and lower temperatures in the gas between shocks. Assuming LTE ionization results in a thermostat-like behavior with matter accumulating around the temperatures where the LTE ionization fractions change rapidly. Comparison of DEM curves computed from our models shows that non-equilibrium ionization leads to more radiating material in the temperature range 11-18 kK, compared to models with LTE helium ionization. We conclude that non-equilibrium helium ionization is important for the dynamics and thermal structure of the upper chromosphere and transition region. It might also help resolve the problem that intensities of chromospheric lines computed from current models are smaller than those observed.
Simulations of the infrared, Raman, and 2D-IR photon echo spectra of water in nanoscale silica pores
Burris, Paul C.; Laage, Damien; Thompson, Ward H.
2016-05-20
Vibrational spectroscopy is frequently used to characterize nanoconfined liquids and probe the effect of the confining framework on the liquid structure and dynamics relative to the corresponding bulk fluid. However, it is still unclear what molecular-level information can be obtained from such measurements. In this Paper, we address this question by using molecular dynamics (MD) simulations to reproduce the linear infrared (IR), Raman, and two-dimensional IR (2D-IR) photon echo spectra for water confined within hydrophilic (hydroxyl-terminated) silica mesopores. To simplify the spectra the OH stretching region of isotopically dilute HOD in D2O is considered. An empirical mapping approach is usedmore » to obtain the OH vibrational frequencies, transition dipoles, and transition polarizabilities from the MD simulations. The simulated linear IR and Raman spectra are in good general agreement with measured spectra of water in mesoporous silica reported in the literature. The key effect of confinement on the water spectrum is a vibrational blueshift for OH groups that are closest to the pore interface. The blueshift can be attributed to the weaker hydrogen bonds (H-bonds) formed between the OH groups and silica oxygen acceptors. Non-Condon effects greatly diminish the contribution of these OH moieties to the linear IR spectrum, but these weaker H-bonds are readily apparent in the Raman spectrum. The 2D-IR spectra have not yet been measured and thus the present results represent a prediction. Lastly, the simulated spectra indicate that it should be possible to probe the slower spectral diffusion of confined water compared to the bulk liquid by analysis of the 2D-IR spectra.« less
Radar Reflectivity Simulated by a 2-D Spectra Bin Model: Sensitivity of Cloud-aerosol Interaction
NASA Technical Reports Server (NTRS)
Li, Kiaowen; Tao, Wei-Kuo; Khain, Alexander; Simpson, Joanne; Johnson, Daniel
2003-01-01
The Goddard Cumulus Ensemble (GCE) model with bin spectra microphysics is used to simulate mesoscale convective systems.The model uses explicit bins to represent size spectra of cloud nuclei, water drops, ice crystals, snow and graupel. Each hydrometeorite category is described by 33 mass bins. The simulations provide a unique data set of simulated raindrop size distribution in a realistic dynamic frame. Calculations of radar parameters using simulated drop size distribution serve as an evaluation of numerical model performance. In addition, the GCE bin spectra modes is a very useful tool to study uncertainties related to radar observations; all the environmental parameters are precisely known. In this presentation, we concentrate on the discussion of Z-R (ZDR-R) relation in the simulated systems. Due to computational limitations, the spectra bin model has been run in two dimensions with 31 stretched vertical layers and 1026 horizontal grid points (1 km resolution). Two different cases, one in midlatitude continent, the other in tropical ocean, have been simulated. The continental case is a strong convection which lasted for two hours. The oceanic case is a persistent system with more than 10 hours' life span. It is shown that the simulated Z-R (ZDR-R) relations generally agree with observations using radar and rain gauge data. The spatial and temporal variations of Z-R relation in different locations are also analyzed. Impact of aerosols on cloud formation and raindrop size distribution was studied. Both clean (low CCN) and dirty (high CCN) cases are simulated. The Z-R relation is shown to vary considerable in the initial CCN concentrations.
Large Scale Earth's Bow Shock with Northern IMF as Simulated by PIC Code in Parallel with MHD Model
NASA Astrophysics Data System (ADS)
Baraka, Suleiman
2016-06-01
In this paper, we propose a 3D kinetic model (particle-in-cell, PIC) for the description of the large scale Earth's bow shock. The proposed version is stable and does not require huge or extensive computer resources. Because PIC simulations work with scaled plasma and field parameters, we also propose to validate our code by comparing its results with the available MHD simulations under same scaled solar wind (SW) and (IMF) conditions. We report new results from the two models. In both codes the Earth's bow shock position is found to be ≈14.8 R E along the Sun-Earth line, and ≈29 R E on the dusk side. Those findings are consistent with past in situ observations. Both simulations reproduce the theoretical jump conditions at the shock. However, the PIC code density and temperature distributions are inflated and slightly shifted sunward when compared to the MHD results. Kinetic electron motions and reflected ions upstream may cause this sunward shift. Species distributions in the foreshock region are depicted within the transition of the shock (measured ≈2 c/ ω pi for Θ Bn = 90° and M MS = 4.7) and in the downstream. The size of the foot jump in the magnetic field at the shock is measured to be (1.7 c/ ω pi ). In the foreshocked region, the thermal velocity is found equal to 213 km s-1 at 15 R E and is equal to 63 km s -1 at 12 R E (magnetosheath region). Despite the large cell size of the current version of the PIC code, it is powerful to retain macrostructure of planets magnetospheres in very short time, thus it can be used for pedagogical test purposes. It is also likely complementary with MHD to deepen our understanding of the large scale magnetosphere.
In vitro construction of 2D and 3D simulations of the murine hematopoietic niche.
Chitteti, Brahmananda Reddy; Bethel, Monique; Voytik-Harbin, Sherry L; Kacena, Melissa A; Srour, Edward F
2013-01-01
Hematopoietic stem cells (HSC) undergo multilineage differentiation or self-renewal to maintain normal hematopoiesis and to sustain the size of the HSC pool throughout life. These processes are determined by a complex interplay of molecular signals between HSC and other cellular components such as osteoblasts (OB), stromal cells, endothelial cells, and a number of extracellular matrix (ECM) proteins. Through changes in its physical properties within the bone marrow (BM) microenvironment, collagen, which is one of the most critical ECM proteins, can modulate HSC function and maintenance of the competence of the hematopoietic niche (HN). At present, there is no consensus as to how different cellular elements of the niche collaborate and interact to promote HSC self-renewal or differentiation to maintain hematopoiesis. Deciphering these interactions and the impact of mechanical properties of the collagen microstructures within the HN has critical clinical implications in the areas of stem cell homing, engraftment, and maintenance of HSC function. In this chapter, we describe several of the in vitro methodologies for establishing and maintaining HSC in vitro including the isolation of OB, stromal cells, and hematopoietic progenitor cells, as well as the establishment of both two-dimensional (2D) and three-dimensional (3D) coculture systems.
Fourier based methodology for simulating 2D-random shapes in heterogeneous materials
NASA Astrophysics Data System (ADS)
Mattrand, C.; Béakou, A.; Charlet, K.
2015-08-01
Gaining insights into the effects of microstructural details on materials behavior may be achieved by incorporating their attributes into numerical modeling. This requires us to make considerable efforts to feature heterogeneity morphology distributions and their spatial arrangement. This paper focuses on modeling the scatter observed in materials heterogeneity geometry. The proposed strategy is based on the development of a 1D-shape signature function representing the 2D-section of a given shape, on Fourier basis functions. The Fourier coefficients are then considered as random variables. This methodology has been applied to flax fibers which are gradually introduced into composite materials as a potential alternative to synthetic reinforcements. In this contribution, the influence of some underlying assumptions regarding the choice of one 1D-shape signature function, its discretization scheme and truncation level, and the best way of modeling the associated random variables is also investigated. Some configurations coming from the combination of these tuning parameters are found to be sufficiently relevant to render efficiently the morphometric factors of the observed fibers statistically speaking.
2014-01-01
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
Monte Carlo simulations of a novel Micromegas 2D array for proton dosimetry.
Dolney, D; Ainsley, C; Hollebeek, R; Maughan, R
2016-02-21
Modern proton therapy affords control of the delivery of radiotherapeutic dose on fine length and temporal scales. The authors have developed a novel detector technology based on Micromesh Gaseous Structure (Micromegas) that is uniquely tailored for applications using therapeutic proton beams. An implementation of a prototype Micromegas detector for Monte Carlo using Geant4 is presented here. Comparison of simulation results with measurements demonstrates agreement in relative dose along the proton longitudinal dose profile to be 1%. The effect of a radioactive calibration source embedded in the chamber gas is demonstrated by measurements and reproduced by simulations, also at the 1% level. Our Monte Carlo simulations are shown to reproduce the time structure of ionization pulses produced by a double-scattering delivery system.
Monte Carlo simulations of a novel Micromegas 2D array for proton dosimetry
NASA Astrophysics Data System (ADS)
Dolney, D.; Ainsley, C.; Hollebeek, R.; Maughan, R.
2016-02-01
Modern proton therapy affords control of the delivery of radiotherapeutic dose on fine length and temporal scales. The authors have developed a novel detector technology based on Micromesh Gaseous Structure (Micromegas) that is uniquely tailored for applications using therapeutic proton beams. An implementation of a prototype Micromegas detector for Monte Carlo using Geant4 is presented here. Comparison of simulation results with measurements demonstrates agreement in relative dose along the proton longitudinal dose profile to be 1%. The effect of a radioactive calibration source embedded in the chamber gas is demonstrated by measurements and reproduced by simulations, also at the 1% level. Our Monte Carlo simulations are shown to reproduce the time structure of ionization pulses produced by a double-scattering delivery system.
MHD simulations of the magnetospheres of Jupiter and Saturn: Application to the Cassini mission
NASA Astrophysics Data System (ADS)
Hansen, Kenneth Calvin
2001-08-01
We have developed global magnetohydrodynamic (MHD) models of the magnetospheres of Jupiter and Saturn motivated by the need to better understand the global structure and dynamics of the magnetospheres, their interaction with the solar wind and the plasma sources internal to them. The models are also used both as planning tools for the Cassini mission to Saturn and to give a global perspective to the measurements. Our model of Jupiter's magnetosphere is the first to include the Io mass loading region and to solve for the plasma flow in the inner magnetosphere. With the model we study the bow shock and magnetopause crossings made by Cassini and Galileo. In addition, we examine the field- aligned currents in Jupiter's inner magnetosphere with a height integrated ionospheric model coupled to the magnetosphere at the inner boundary. We find that the model describes the state of the magnetosphere at the time of Cassini quite well. The models of Saturn's magnetosphere that we present represent the first and only global models of the Kronian magnetosphere to date. With the models we study the effects of different source terms and different solar wind conditions on the configuration of the magnetosphere. Although simpler models are useful for understanding the relative roles of the icy satellite and Titan plasma sources and the configuration for different solar wind conditions, these models cannot fully account for the plasma in the inner magnetosphere of Saturn. The higher source rate gives good agreement with the mass densities measured by Voyager. For this case, we study the general structure of the magnetosphere as well as some applications of the model to the Cassini mission. We have examined the plasma environment at the satellites of Saturn, provided information about the plasma ram direction and extracted data from the model along the Cassini tour. We have carried out two simulations of the two-body, coupled Saturn-Titan system with Titan in super-fast magnetosonic
NASA Astrophysics Data System (ADS)
Michelson, Sara; Bao, Jian-Wen; Grell, Evelyn
2016-04-01
In this study, numerical model simulations of an idealized 2-D squall line are investigated using microphysics budget analysis. Four commonly-used microphysics schemes of various complexity are used in the simulations. Diagnoses of the source and sink terms of the hydrometeor budget equations reveal that the differences related to the assumptions of hydrometeor size-distributions between the schemes lead to the differences in the simulations due to the net effect of various microphysical processes on the interaction between latent heating/evaporative cooling and flow dynamics as the squall line develops. Results from this study also highlight the possibility that the advantage of double-moment formulations can be overshadowed by the uncertainties in the spectral definition of individual hydrometeor categories and spectrum-dependent microphysical processes.
NASA Astrophysics Data System (ADS)
Darvini, G.; Salandin, P.
2009-12-01
To analyze the impact of the hydraulic conductivity K spatial variability in a real field case (as an example to delimitate a well catchment), numerical simulations can be reasonably developed in a two-dimensional vertical average context. Nevertheless the plume evolution is a consequence of a more complex three-dimensional heterogeneous structure whose vertical variability dominates the dispersion phenomena at local scale. In larger domains, the effect of the vertical heterogeneity combines itself with that one due to the horizontal variability of K, and only when the plume has travelled a large number of (horizontal) integral scales, its evolution can be analyzed in a regional context, under the hypothesis that the transmissivity spatial distribution prevails. Until this limit is reached, the vertical and horizontal variability of K are combined to give a fully 3-D dispersion process. In all these situations, to successfully accomplish the 3-D heterogeneous structure of the aquifer in 2-D simulations, more than the planimetric depth-averaged variability of K must be accounted for. To define the uncertainty related to the use of different planimetric schematizations of the real hydraulic conductivity spatial distribution, we present here the results of some numerical experiments that compare the 3-D plume evolution with 2-D simulations developed by tacking into account different hydraulic conductivity distribution schematization, by considering a hierarchical architecture of media also. This description of a sedimentary formation combined with the finite size of the plume requires theoretical and numerical tools able to take into account the flow field inhomogeneity and the ergodicity lack that characterize the transport phenomena. Following this way it will be possible to quantify / reduce the uncertainty related to a 2-D schematization in a large number of real cases where the domain spans between the local and the regional scale and whose dimension may lead to
Kaiglová, Jana; Langhammer, Jakub; Jiřinec, Petr; Janský, Bohumír; Chalupová, Dagmar
2015-03-01
This article used various hydrodynamic and sediment transport models to analyze the potential and the limits of different channel schematizations. The main aim was to select and evaluate the most suitable simulation method for fine-grained sediment remobilization assessment. Three types of channel schematization were selected to study the flow potential for remobilizing fine-grained sediment in artificially modified channels. Schematization with a 1D cross-sectional horizontal plan, a 1D+ approach, splitting the riverbed into different functional zones, and full 2D mesh, adopted in MIKE by the DHI modeling suite, was applied to the study. For the case study, a 55-km stretch of the Bílina River, in the Czech Republic, Central Europe, which has been heavily polluted by the chemical and coal mining industry since the mid-twentieth century, was selected. Long-term exposure to direct emissions of toxic pollutants including heavy metals and persistent organic pollutants (POPs) resulted in deposits of pollutants in fine-grained sediments in the riverbed. Simulations, based on three hydrodynamic model schematizations, proved that for events not exceeding the extent of the riverbed profile, the 1D schematization can provide comparable results to a 2D model. The 1D+ schematization can improve accuracy while keeping the benefits of high-speed simulation and low requirements of input DEM data, but the method's suitability is limited by the channel properties. PMID:25687259
Simulations of chemotaxis and random motility in 2D random porous domains.
Jabbarzadeh, Ehsan; Abrams, Cameron F
2007-02-01
We discuss a generic computational model of eukariotic chemotaxis in 2D random porous domains. The model couples the fully time-dependent finite-difference solution of a reaction-diffusion equation for the concentration field of a chemoattractant to biased random walks representing individual chemotactic cells. We focus in particular on the influence of consumption of chemoattractant by the boundaries of obstacles with irregular shapes which are distributed randomly in the domain on the chemotactic response of the cells. Cells are stimulated to traverse a field of obstacles by a line source of chemoattractant. We find that the reactivity of the obstacle boundaries with respect to the chemoattractant strongly determines the transit time of cells through two primary mechanisms. The channeling effect arises because cells are effectively repelled from surfaces which consume chemoattractant, and opposing surfaces therefore act to keep cells in the middle of channels. This reduces traversal times relative to the case with unreactive boundaries, provided that the appropriate Péclet number relating the strength of reactivity to diffusion in governing chemoattractant transport is neither too low nor too high. The dead-zone effect arises due to a realistic threshold on the chemotactic response, which at steady state results in portions of the domain having no detectable gradient. Of these two, the channeling effect is responsible for 90% of the sensitivity of transit times to boundary reactivity. Based on these results, we speculate that it may be possible to tune the rates of cellular penetration into porous domains by engineering the reactivity of the internal surfaces to cytokines.
Treatment of LBCs in 2D simulation of convection over hills
NASA Astrophysics Data System (ADS)
Tian, Wenshou; Guo, Zhenhai; Yu, Rucong
2004-08-01
A series of idealized model simulations are analyzed to determine the sensitivity of model results to different configurations of the lateral boundary conditions (LBCs) in simulating mesoscale shallow convection over hilly terrain. In the simulations with steady thermal forcing at the model surface, a radiation condition at both boundaries is the best choice under high wind conditions, and the best results are produced when both the normal velocities and the temperature are treated with the radiation scheme in which the phase speed is the same for different variables. When the background wind speed is reasonably small, the LBC configuration with either the radiation or the zero gradient condition at both boundaries tends to make the numerical solution unstable. The choice of a constant condition at the inflow boundary and a radiation outflow boundary condition is appropriate in most cases. In the simulations with diurnal thermal forcing at the model surface, different LBC schemes are combined together to reduce spurious signals induced by the outflow boundary. A specification inflow boundary condition, in which the velocity fields at the inflow boundary are provided using the time-dependent results of a simulation with periodic LBCs over a flat domain, is tested and the results indicate that the specification condition at the inflow boundary makes it possible to use a smaller model domain to obtain reasonable results. The model horizontal domain length should be greater than a critical length, which depends on the domain depth H and the angle between gravity wave phase lines and the vertical. An estimate of minimum domain length is given by[(H - z_i )/π U]sqrt {N^2 L_x^2 - 4π ^2 U^2 } , where N and U are the background stability and wind speed, respectively, L x is the typical gravity wavelength scale, and z i is the convective boundary layer (CBL) depth.
Cyclic thermal signature in a global MHD simulation of solar convection
NASA Astrophysics Data System (ADS)
Cossette, J.; Charbonneau, P.; Smolarkiewicz, P. K.
2013-12-01
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
A mathematical model for a didactic device able to simulate a 2D Newtonian gravitational field
NASA Astrophysics Data System (ADS)
De Marchi, Fabrizio
2015-01-01
In this paper we propose a mathematical model to describe a theoretical device able to simulate an inverse-square force on a test mass moving on a horizontal plane. We use two pulleys, a counterweight, a wire and a smooth rail, in addition to the test mass. The tension of the wire (i.e. the attractive force on the test mass) is determined by the position of a counterweight free to move on a rail placed under the plane. The profile of the rail is calculated in order to obtain the required Newtonian force. Details of this calculation are reported in the paper, and numerical simulations are provided in order to investigate the stability of the orbits under the effect of the main friction forces and other perturbative effects. This work points out that there are some criticalities intrinsic to the apparatus and gives some suggestions about how to minimize their impact.
Mixed-RKDG Finite Element Methods for the 2-D Hydrodynamic Model for Semiconductor Device Simulation
Chen, Zhangxin; Cockburn, Bernardo; Jerome, Joseph W.; Shu, Chi-Wang
1995-01-01
In this paper we introduce a new method for numerically solving the equations of the hydrodynamic model for semiconductor devices in two space dimensions. The method combines a standard mixed finite element method, used to obtain directly an approximation to the electric field, with the so-called Runge-Kutta Discontinuous Galerkin (RKDG) method, originally devised for numerically solving multi-dimensional hyperbolic systems of conservation laws, which is applied here to the convective part of the equations. Numerical simulations showing the performance of the new method are displayed, and the results compared with those obtained by using Essentially Nonoscillatory (ENO) finite difference schemes. Frommore » the perspective of device modeling, these methods are robust, since they are capable of encompassing broad parameter ranges, including those for which shock formation is possible. The simulations presented here are for Gallium Arsenide at room temperature, but we have tested them much more generally with considerable success.« less
2D simulations based on general time-dependent reciprocal relation for LFEIT.
Karadas, Mursel; Gencer, Nevzat Guneri
2015-08-01
Lorentz field electrical impedance tomography (LFEIT) is a newly proposed technique for imaging the conductivity of the tissues by measuring the electromagnetic induction under the ultrasound pressure field. In this paper, the theory and numerical simulations of the LFEIT are reported based on the general time dependent formulation. In LFEIT, a phased array ultrasound probe is used to introduce a current distribution inside a conductive body. The velocity current occurs, due to the movement of the conductive particles under a static magnetic field. In order to sense this current, a receiver coil configuration that surrounds the volume conductor is utilized. Finite Element Method (FEM) is used to carry out the simulations of LFEIT. It is shown that, LFEIT can be used to reconstruct the conductivity even up to 50% perturbation in the initial conductivity distribution. PMID:26736569
Zhou, Y. L.; Wang, Z. H.; Xu, X. Q.; Li, H. D.; Feng, H.; Sun, W. G.
2015-01-15
Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. Transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. It is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.
Zhou, Y. L.; Wang, Z. H.; Xu, X. Q.; Li, H. D.; Feng, H.; Sun, W. G.
2015-01-09
Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Furthermore, two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. In transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. Moreover, it is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.
Zhou, Y. L.; Wang, Z. H.; Xu, X. Q.; Li, H. D.; Feng, H.; Sun, W. G.
2015-01-09
Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Furthermore, two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density,more » heat and momentum transport equations along with neutral density, and momentum transport equations. In transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. Moreover, it is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.« less
High-beta extended MHD simulations of stellarators with Spitzer resistivity
NASA Astrophysics Data System (ADS)
Bechtel, Torrin
2015-11-01
The nonlinear, extended MHD code NIMROD is used to study high-beta, 3D magnetic topology evolution of a toroidal stellarator. The configurations under investigation derive from the geometry of the Compact Toroidal Hybrid (CTH) experiment. However, the vacuum rotational transform profile is artificially raised by modifying applied magnetic fields in an effort to examine the sensitivity of low order rational surfaces and/or magnetic islands. Finite beta plasmas are created using a volumetric heating source and temperature dependent anisotropic heat conduction and resistivity. Flux surface dependent temperature and density profiles are used for the initial condition so that Spitzer resistivity can be applied. The onset of MHD instabilities and nonlinear consequences are monitored as a function of beta as well as the fragility of the magnetic surfaces. Research supported by US DOE under grant no. DE-FG02-99ER54546.
Magnetic fields near spacecraft-explored comets: 3D MHD numerical simulation
NASA Astrophysics Data System (ADS)
Baranov, V. B.; Alexashov, D. B.; Lebedev, M. G.
2015-05-01
The magnetohydrodynamic (MHD) model of the interaction between the solar wind and a cometary ionosphere is presented. The model accounts for photoionization of the neutral component of the cometary outflow and the resonance charge exchange between charged and neutral particles, together with the interplanetary magnetic field. The numerical implementation of the model is performed on the basis of the second-order shock-fitting Godunov method generalized to cover MHD flows. The results of the calculations are analysed with emphasis on the behaviour of the interplanetary magnetic field disturbed by the cometary outflow. The comparison of the numerical results with the spaceboard measured data obtained during the spacecraft flybys near comets Halley and Grigg-Skjellerup shows their good agreement, thus giving promise that the model will be capable to adequately describe the plasma and magnetic surrounding of comet Churyumov-Gerasimenko during the Rosetta spacecraft encounter.
A Three-Dimensional MHD Simulation of the Solar Wind for a Tilted-Dipole Magnetic Field on the Sun
NASA Technical Reports Server (NTRS)
Goldstein, Melvyn L.
2007-01-01
Using a three-dimensional MHD model, we simulate the global steady-state structure of the solar corona and solar wind for a dipole magnetic field on the Sun inclined by 30 degrees to the solar rotation axis. This represents the solar conditions typical for a declining phase of solar cycle. The computations can extend from the coronal base out to 100-AU and at large heliospheric distances includes the effects of interstellar neutral hydrogen and their interaction with solar wind protons. The simulations can model the formation of corotating interaction regions and the heliospheric current sheet. The simulations are also capable of describing very strong rarefaction regions that include embedded sub-Alfvenic regions that form on the trailing edge of a fast flows.
A GPU Simulation Tool for Training and Optimisation in 2D Digital X-Ray Imaging
Gallio, Elena; Rampado, Osvaldo; Gianaria, Elena; Bianchi, Silvio Diego; Ropolo, Roberto
2015-01-01
Conventional radiology is performed by means of digital detectors, with various types of technology and different performance in terms of efficiency and image quality. Following the arrival of a new digital detector in a radiology department, all the staff involved should adapt the procedure parameters to the properties of the detector, in order to achieve an optimal result in terms of correct diagnostic information and minimum radiation risks for the patient. The aim of this study was to develop and validate a software capable of simulating a digital X-ray imaging system, using graphics processing unit computing. All radiological image components were implemented in this application: an X-ray tube with primary beam, a virtual patient, noise, scatter radiation, a grid and a digital detector. Three different digital detectors (two digital radiography and a computed radiography systems) were implemented. In order to validate the software, we carried out a quantitative comparison of geometrical and anthropomorphic phantom simulated images with those acquired. In terms of average pixel values, the maximum differences were below 15%, while the noise values were in agreement with a maximum difference of 20%. The relative trends of contrast to noise ratio versus beam energy and intensity were well simulated. Total calculation times were below 3 seconds for clinical images with pixel size of actual dimensions less than 0.2 mm. The application proved to be efficient and realistic. Short calculation times and the accuracy of the results obtained make this software a useful tool for training operators and dose optimisation studies. PMID:26545097
2D simulations of transport dynamics during tokamak fuelling by supersonic molecular beam injection
NASA Astrophysics Data System (ADS)
Wang, Z. H.; Xu, X. Q.; Xia, T. Y.; Rognlien, T. D.
2014-04-01
Time-dependent transport of both plasma and neutrals is simulated during supersonic molecular beam injection (SMBI) yielding the evolution of edge plasma and neutral profiles. The SMBI model is included as a module, called trans-neut, within the original BOUT++ boundary plasma turbulence code. Results of calculations are reported for the realistic divertor geometry of the HL-2A tokamak. The model can also be used to study the effect of gas puffing. A seven-field fluid model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Collisional interactions between molecules, atoms, and plasma include dissociation, ionization, recombination and charge-exchange effects. Sheath boundary conditions and particle recycling are applied at both the wall and the divertor plates. A localized boundary condition of constant molecular flux (product of density times speed) is applied at the outermost flux surface to model the SMBI. Steady state profiles with and without particle recycling are achieved before SMBI. During SMBI, the simulation shows that neutrals can penetrate several centimetres inside the last closed (magnetic) flux surface (LCFS). Along the SMBI path, plasma density increases while plasma temperature decreases. The molecule penetration depth depends on both the SMBI flux and the initial plasma density and temperature along its path. As the local plasma density increases substantially, molecule and atom penetration depths decrease due to their higher dissociation and ionization rates if the electron temperature does not drop too low. Dynamic poloidal spreading of the enhanced plasma density region is observed due to rapid ion flow along the magnetic field (parallel) driven by a parallel pressure asymmetry during SMBI. Profile relaxation in the radial and poloidal directions is simulated after SMBI termination, showing that the plasma returns to pre-SMBI conditions on
A GPU Simulation Tool for Training and Optimisation in 2D Digital X-Ray Imaging.
Gallio, Elena; Rampado, Osvaldo; Gianaria, Elena; Bianchi, Silvio Diego; Ropolo, Roberto
2015-01-01
Conventional radiology is performed by means of digital detectors, with various types of technology and different performance in terms of efficiency and image quality. Following the arrival of a new digital detector in a radiology department, all the staff involved should adapt the procedure parameters to the properties of the detector, in order to achieve an optimal result in terms of correct diagnostic information and minimum radiation risks for the patient. The aim of this study was to develop and validate a software capable of simulating a digital X-ray imaging system, using graphics processing unit computing. All radiological image components were implemented in this application: an X-ray tube with primary beam, a virtual patient, noise, scatter radiation, a grid and a digital detector. Three different digital detectors (two digital radiography and a computed radiography systems) were implemented. In order to validate the software, we carried out a quantitative comparison of geometrical and anthropomorphic phantom simulated images with those acquired. In terms of average pixel values, the maximum differences were below 15%, while the noise values were in agreement with a maximum difference of 20%. The relative trends of contrast to noise ratio versus beam energy and intensity were well simulated. Total calculation times were below 3 seconds for clinical images with pixel size of actual dimensions less than 0.2 mm. The application proved to be efficient and realistic. Short calculation times and the accuracy of the results obtained make this software a useful tool for training operators and dose optimisation studies. PMID:26545097
A GPU Simulation Tool for Training and Optimisation in 2D Digital X-Ray Imaging.
Gallio, Elena; Rampado, Osvaldo; Gianaria, Elena; Bianchi, Silvio Diego; Ropolo, Roberto
2015-01-01
Conventional radiology is performed by means of digital detectors, with various types of technology and different performance in terms of efficiency and image quality. Following the arrival of a new digital detector in a radiology department, all the staff involved should adapt the procedure parameters to the properties of the detector, in order to achieve an optimal result in terms of correct diagnostic information and minimum radiation risks for the patient. The aim of this study was to develop and validate a software capable of simulating a digital X-ray imaging system, using graphics processing unit computing. All radiological image components were implemented in this application: an X-ray tube with primary beam, a virtual patient, noise, scatter radiation, a grid and a digital detector. Three different digital detectors (two digital radiography and a computed radiography systems) were implemented. In order to validate the software, we carried out a quantitative comparison of geometrical and anthropomorphic phantom simulated images with those acquired. In terms of average pixel values, the maximum differences were below 15%, while the noise values were in agreement with a maximum difference of 20%. The relative trends of contrast to noise ratio versus beam energy and intensity were well simulated. Total calculation times were below 3 seconds for clinical images with pixel size of actual dimensions less than 0.2 mm. The application proved to be efficient and realistic. Short calculation times and the accuracy of the results obtained make this software a useful tool for training operators and dose optimisation studies.
2D fluid simulations of discharges at atmospheric pressure in reactive gas mixtures
NASA Astrophysics Data System (ADS)
Bourdon, Anne
2015-09-01
Since a few years, low-temperature atmospheric pressure discharges have received a considerable interest as they efficiently produce many reactive chemical species at a low energy cost. This potential is of great interest for a wide range of applications as plasma assisted combustion or biomedical applications. Then, in current simulations of atmospheric pressure discharges, there is the need to take into account detailed kinetic schemes. It is interesting to note that in some conditions, the kinetics of the discharge may play a role on the discharge dynamics itself. To illustrate this, we consider the case of the propagation of He-N2 discharges in long capillary tubes, studied for the development of medical devices for endoscopic applications. Simulation results put forward that the discharge dynamics and structure depend on the amount of N2 in the He-N2 mixture. In particular, as the amount of N2 admixture increases, the discharge propagation velocity in the tube increases, reaches a maximum for about 0 . 1 % of N2 and then decreases, in agreement with experiments. For applications as plasma assisted combustion with nanosecond repetitively pulsed discharges, there is the need to handle the very different timescales of the nanosecond discharge with the much longer (micro to millisecond) timescales of combustion processes. This is challenging from a computational point of view. It is also important to better understand the coupling of the plasma induced chemistry and the gas heating. To illustrate this, we present the simulation of the flame ignition in lean mixtures by a nanosecond pulsed discharge between two point electrodes. In particular, among the different discharge regimes of nanosecond repetitively pulsed discharges, a ``spark'' regime has been put forward in the experiments, with an ultra-fast local heating of the gas. For other discharge regimes, the gas heating is much weaker. We have simulated the nanosecond spark regime and have observed shock waves
NASA Astrophysics Data System (ADS)
Xie, Lianghai; Li, Lei; Zhang, Yiteng; Feng, Yongyong; Wang, Xinyue; Zhang, Aibing; Kong, Linggao
2015-08-01
Lunar minimagnetosphere formed by the interaction between the solar wind and a local crustal field often has a scale size comparable to the ion inertia length, in which the Hall effect is very important. In this paper, the general characteristics of lunar minimagnetosphere are investigated by three-dimensional Hall MHD simulations. It is found that the solar wind ions can penetrate across the magnetopause to reduce the density depletion and cause the merging of the shock and magnetopause, but the electrons are still blocked at the boundary. Besides, asymmetric convection occurs, resulting in the magnetic field piles up on one side while the plasma gathers on the other side. The size of the minimagnetosphere is determined by both the solar zenith angle and the magnetosonic Mach number, while the Hall effect is determined by the ratio of the pressure balance distance to the ion inertia length. When the ratio gets small, the shock may disappear. Finally, we present a global Hall MHD simulation for comparison with the observation from Chang'E-2 satellite on 11 October 2010 and confirm that Chang'E-2 flew across compression regions of two separate minimagnetospheres.
NASA Technical Reports Server (NTRS)
Berchem, J.; Raeder, J.; Ashour-Abdalla, M.; Frank, L. A.; Paterson, W. R.; Ackerson, K. L.; Kokubun, S.; Yamamoto, T.; Lepping, R. P.
1998-01-01
Understanding the large-scale dynamics of the magnetospheric boundary is an important step towards achieving the ISTP mission's broad objective of assessing the global transport of plasma and energy through the geospace environment. Our approach is based on three-dimensional global magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere- ionosphere system, and consists of using interplanetary magnetic field (IMF) and plasma parameters measured by solar wind monitors upstream of the bow shock as input to the simulations for predicting the large-scale dynamics of the magnetospheric boundary. The validity of these predictions is tested by comparing local data streams with time series measured by downstream spacecraft crossing the magnetospheric boundary. In this paper, we review results from several case studies which confirm that our MHD model reproduces very well the large-scale motion of the magnetospheric boundary. The first case illustrates the complexity of the magnetic field topology that can occur at the dayside magnetospheric boundary for periods of northward IMF with strong Bx and By components. The second comparison reviewed combines dynamic and topological aspects in an investigation of the evolution of the distant tail at 200 R(sub E) from the Earth.
Real-time 2D floating-point fast Fourier transforms for seeker simulation
NASA Astrophysics Data System (ADS)
Chamberlain, Richard; Lord, Eric; Shand, David J.
2002-07-01
The floating point Fast Fourier Transform (FFT) is one of the most useful basic functions available to the image and signal processing engineer allowing many complex and detailed special functions to be implemented more simply in the frequency domain. In the Hardware-in-the-Loop field an image transformed using FFT would allow the designer to think about accurate frequency based simulation of seeker lens effects, motion blur, detector transfer functions and much more. Unfortunately, the transform requires many hundreds of thousands or millions of floating point operations on a single modest sized image making it impractical for realtime Hardware-in-the-Loop systems. .until now. This paper outlines the development, by Nallatech, of an FPGA based IEEE floating point core. It traces the subsequent use of this core to develop a full 256 X 256 FFT and filter process implemented on COTS hardware at frame rates up to 150Hz. This transform can be demonstrated to model optical transfer functions at a far greater accuracy than the current spatial models. Other applications and extensions of this technique will be discussed such as filtering for image tracking algorithms and in the simulation of radar processing in the frequency domain.
Yielding in a strongly aggregated colloidal gel: 2D simulations and theory
NASA Astrophysics Data System (ADS)
Roy, Saikat; Tirumkudulu, Mahesh
2015-11-01
We investigated the micro-structural details and the mechanical response under uniaxial compression of the strongly aggregating gel starting from low to high packing fraction.The numerical simulations account for short-range inter-particle attractions, normal and tangential deformation at particle contacts,sliding and rolling friction, and preparation history. It is observed that in the absence of rolling resistance(RR),the average coordination number varies only slightly with compaction whereas it is significant in the presence of RR. The particle contact distribution is isotropic throughout the consolidation process. In both cases, the yield strain is constant with the volume fraction. The modulus values are very similar at different attraction, and with and without RR implying that the elastic modulus does not scale with attraction.The modulus was found to be a weak function of the preparation history. The increase in yield stress with volume fraction is a consequence of the increased elastic modulus of the network. However, the yield stress scales similarly both with and without RR. The power law exponent of 5.4 is in good agreement with previous simulation results. A micromechanical theory is also proposed to describe the stress versus strain relation for the gelled network.
NASA Astrophysics Data System (ADS)
Cao, Jiang; Cresti, Alessandro; Esseni, David; Pala, Marco
2016-02-01
We simulate a band-to-band tunneling field-effect transistor based on a vertical heterojunction of single-layer MoS2 and WTe2, by exploiting the non-equilibrium Green's function method and including electron-phonon scattering. For both in-plane and out-of-plane transport, we attempt to calibrate out models to the few available experimental results. We focus on the role of chemical doping and back-gate biasing, and investigate the off-state physics of this device by analyzing the influence of the top-gate geometrical alignment on the device performance. The device scalability as a function of gate length is also studied. Finally, we present two metrics for the switching delay and energy of the device. Our simulations indicate that vertical field-effect transistors based on transition metal dichalcogenides can provide very small values of sub-threshold swing when properly designed in terms of doping concentration and top-gate extension length.
NASA Astrophysics Data System (ADS)
Pérez-Corona, M.; García, J. A.; Taller, G.; Polgár, D.; Bustos, E.; Plank, Z.
2016-02-01
The purpose of geophysical electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters, such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, they have been used for environmental surveys. To obtain a more accurate subsurface model than is possible with a simple 1-D model, a more complex model must be used. In a 2-D model, the resistivity values are allowed to vary in one horizontal direction (usually referred to as the x direction) but are assumed to be constant in the other horizontal (the y) direction. A more realistic model would be a fully 3-D model where the resistivity values are allowed to change in all three directions. In this research, a simulation of the cone penetration test and 2D imaging resistivity are used as tools to simulate the distribution of hydrocarbons in soil.
A hierarchical lattice spring model to simulate the mechanics of 2-D materials-based composites
NASA Astrophysics Data System (ADS)
Brely, Lucas; Bosia, Federico; Pugno, Nicola
2015-07-01
In the field of engineering materials, strength and toughness are typically two mutually exclusive properties. Structural biological materials such as bone, tendon or dentin have resolved this conflict and show unprecedented damage tolerance, toughness and strength levels. The common feature of these materials is their hierarchical heterogeneous structure, which contributes to increased energy dissipation before failure occurring at different scale levels. These structural properties are the key to exceptional bioinspired material mechanical properties, in particular for nanocomposites. Here, we develop a numerical model in order to simulate the mechanisms involved in damage progression and energy dissipation at different size scales in nano- and macro-composites, which depend both on the heterogeneity of the material and on the type of hierarchical structure. Both these aspects have been incorporated into a 2-dimensional model based on a Lattice Spring Model, accounting for geometrical nonlinearities and including statistically-based fracture phenomena. The model has been validated by comparing numerical results to continuum and fracture mechanics results as well as finite elements simulations, and then employed to study how structural aspects impact on hierarchical composite material properties. Results obtained with the numerical code highlight the dependence of stress distributions on matrix properties and reinforcement dispersion, geometry and properties, and how failure of sacrificial elements is directly involved in the damage tolerance of the material. Thanks to the rapidly developing field of nanocomposite manufacture, it is already possible to artificially create materials with multi-scale hierarchical reinforcements. The developed code could be a valuable support in the design and optimization of these advanced materials, drawing inspiration and going beyond biological materials with exceptional mechanical properties.
KEEN and KEEPN wave simulations from 2D to 4D
NASA Astrophysics Data System (ADS)
Mehrenberger, Michel; Afeyan, Bedros; Larson, David; Crouseilles, Nicolas; Casas, Fernando; Faou, Erwan; Dodhy, Adila; Sonnendrucker, Eric; Shoucri, Magdi
2015-11-01
We show for well-driven KEEN (Kinetic Electrostatic Electron Nonlinear) waves and their analogs in pair plasmas KEEPN (Positron) waves, how the dynamics is captured in a variety of complimentary numerical approaches. Symplectic integration and quadrature node based techniques are deployed to achieve satisfactory results in the long time evolution of highly nonlinear, kinetic, non-stationary, self-organized structures in phase space. Fixed and composite velocity grid arbitrary-order interpolation approaches have advantages we highlight. Adaptivity to local phase space density morphological structures will be discussed starting within the framework of the Shape Function Kinetics (SFK) approach. Fine resolution in velocity only in the range affected by KEEN waves makes for more efficient simulations, especially in higher dimensions. We explore the parameter space of unequal electron and positron temperatures as well as the effects of a relative drift velocity in their initial conditions. Ponderomotively driven KEEPN waves have many novelties when compared to KEEN waves, such as double, staggered, vortex structures, which we highlight. Work supported by the AFOSR and OFES.
NASA Technical Reports Server (NTRS)
Zimmerman, Michael I.; Farrell, W. M.; Snubbs, T. J.; Halekas, J. S.
2011-01-01
Anticipating the plasma and electrical environments in permanently shadowed regions (PSRs) of the moon is critical in understanding local processes of space weathering, surface charging, surface chemistry, volatile production and trapping, exo-ion sputtering, and charged dust transport. In the present study, we have employed the open-source XOOPIC code [I] to investigate the effects of solar wind conditions and plasma-surface interactions on the electrical environment in PSRs through fully two-dimensional pattic1e-in-cell simulations. By direct analogy with current understanding of the global lunar wake (e.g., references) deep, near-terminator, shadowed craters are expected to produce plasma "mini-wakes" just leeward of the crater wall. The present results (e.g., Figure I) are in agreement with previous claims that hot electrons rush into the crater void ahead of the heavier ions, fanning a negative cloud of charge. Charge separation along the initial plasma-vacuum interface gives rise to an ambipolar electric field that subsequently accelerates ions into the void. However, the situation is complicated by the presence of the dynamic lunar surface, which develops an electric potential in response to local plasma currents (e.g., Figure Ia). In some regimes, wake structure is clearly affected by the presence of the charged crater floor as it seeks to achieve current balance (i.e. zero net current to the surface).
Ion Dynamics at a Rippled Quasi-parallel Shock: 2D Hybrid Simulations
NASA Astrophysics Data System (ADS)
Hao, Yufei; Lu, Quanming; Gao, Xinliang; Wang, Shui
2016-05-01
In this paper, two-dimensional hybrid simulations are performed to investigate ion dynamics at a rippled quasi-parallel shock. The results show that the ripples around the shock front are inherent structures of a quasi-parallel shock, and the re-formation of the shock is not synchronous along the surface of the shock front. By following the trajectories of the upstream ions, we find that these ions behave differently when they interact with the shock front at different positions along the shock surface. The upstream particles are transmitted more easily through the upper part of a ripple, and the corresponding bulk velocity downstream is larger, where a high-speed jet is formed. In the lower part of the ripple, the upstream particles tend to be reflected by the shock. Ions reflected by the shock may suffer multiple-stage acceleration when moving along the shock surface or trapped between the upstream waves and the shock front. Finally, these ions may escape further upstream or move downstream; therefore, superthermal ions can be found both upstream and downstream.
Origin of energetic ions observed in the terrestrial ion foreshock : 2D full-particle simulations
NASA Astrophysics Data System (ADS)
Savoini, Philippe; Lembege, bertrand
2016-04-01
Collisionless shocks are well-known structures in astrophysical environments which dissipate bulk flow kinetic energy and accelerate large fraction of particle. Spacecrafts have firmly established the existence of the so-called terrestrial foreshock region magnetically connected to the shock and filled by two distinct populations in the quasi-perpendicular shock region (i.e. for 45r{ } ≤ quad θ Bn quad ≤ 90r{ }, where θ Bn is the angle between the shock normal and the upstream magnetic field) : (i) the field-aligned ion beams or `` FAB '' characterized by a gyrotropic distributionsout{,} and (ii) the gyro-phase bunched ions or `` GPB '' characterized by a NON gyrotropic distribution. The present work is based on the use of two dimensional PIC simulation of a curved shock and associated foreshock region where full curvature effects, time of flight effects and both electrons and ions dynamics are fully described by a self consistent approach. Our previous analysis (Savoini et Lembège, 2015) has evidenced that these two types of backstreaming populations can originate from the shock front itself without invoking any local diffusion by ion beam instabilities. Present results are focussed on individual ion trajectories and evidence that "FAB" population is injected into the foreshock mainly along the shock front whereas the "GPB" population penetrates more deeply the shock front. Such differences explain why the "FAB" population loses their gyro-phase coherency and become gyrotropic which is not the case for the "GPB". The impact of these different injection features on the energy gain for each ion population will be presented in détails. Savoini, P. and B. Lembège (2015), `` Production of nongyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock région '', J. Geophys. Res., 120, pp 7154-7171, doi = 10.1002/2015JA021018.
2D conditional simulation of channels on wells using a random walk approach
NASA Astrophysics Data System (ADS)
Wang, Jiahua; Wang, Xiangbo; Ren, Changlin
2009-03-01
Channel modeling is one of the popular topics in the application of geostatistics to fluvial reservoir modeling. This paper presents an approach to designing channels which have a general flow direction through sand well locations and which avoid shale well locations. This approach is named the random walk on graphs of well locations, and is applied to model channel reservoirs. This modeling process consists of two parts: one direction walk modeling and two direction walk modeling. The first model aims to determine each channel location by the use of a transition probability with a random walk essentially in the main flow direction, say the north-south direction, while the second model simulates different channels that can be oriented in both directions, either from north to south or from south to north. In both parts of the model, the transition probability is estimated based on two coefficients: one is the correlation coefficient of channel observations; the other is the obstacle coefficient of non-channel observations. A case study with a dense array of 332 wells is presented using the proposed random walk model. For the purpose of model verification, channel maps created by the random walk are compared to the hand-drawn channel maps made by geologists. The results show a good agreement in both types of maps, but in contrast to the single map supplied by geologists, the random walk model is capable of generating many realizations of channel configuration, hence allowing for uncertainty evaluation. A limitation of this approach, related to the influence of the number of wells, is discussed.
NASA Astrophysics Data System (ADS)
Dages, Cecile; Samouelian, Anatja; Lanoix, Marthe; Dollinger, Jeanne; Chakkour, Sara; Chovelon, Gabrielle; Trabelsi, Khouloud; Voltz, Marc
2015-04-01
Ditches are involved in the transfer of pesticide to surface and groundwaters (e.g. Louchart et al., 2001). Soil horizons underlying ditch beds may present specific soil characteristics compared to neighbouring field soils due to erosion/deposition processes, to the specific biological activities (rooting dynamic and animal habitat) in the ditches (e.g. Vaughan et al., 2008) and to management practices (burning, dredging, mowing,...). Moreover, in contrast to percolation processes in field soils that can be assumed to be mainly 1D vertical, those occurring in the ditch beds are by essence 2D or even 3D. Nevertheless, due to a lake of knowledge, these specific aspects of transfer within ditch beds are generally omitted for hydrological simulation at the catchment scale (Mottes et al., 2014). Accordingly, the aims of this study were i) to characterize subsurface solute transfer through ditch beds and ii) to determine equivalent hydraulic parameters of the ditch beds for use in catchment scale hydrological simulations. A complementary aim was to evaluate the error in predictions performed when percolation in ditches is assumed to be similar to that in the neighbouring field soil. First, bromide transfer experiments were performed on undisturbed soil column (15 cm long with a 15 cm inner-diameter), horizontally and vertically sampled within each soil horizon underlying a ditch bed and within the neighboring field. Columns were sampled at the Roujan catchment (Hérault, France), which belongs to the long term Mediterranean hydrological observatory OMERE (Voltz and Albergel, 2002). Second, for each column, a set of parameters was determined by inverse optimization with mobile-immobile or dual permeability models, with CXTFIT (Toride et al., 1999) or with HYDRUS (Simunek et al., 1998). Third, infiltration and percolation in the ditch was simulated by a 2D flow domain approach considering the 2D variation in hydraulic properties of the cross section of a ditch bed. Last
Schmidt, J. M.; Cairns, Iver H.; Hillan, D. S.
2013-08-20
Type II solar radio bursts are the primary radio emissions generated by shocks and they are linked with impending space weather events at Earth. We simulate type II bursts by combining elaborate three-dimensional MHD simulations of realistic coronal mass ejections (CMEs) at the Sun with an analytic kinetic radiation theory developed recently. The modeling includes initialization with solar magnetic and active region fields reconstructed from magnetograms of the Sun, a flux rope of the initial CME dimensioned with STEREO spacecraft observations, and a solar wind driven with averaged empirical data. We demonstrate impressive accuracy in time, frequency, and intensity for the CME and type II burst observed on 2011 February 15. This implies real understanding of the physical processes involved regarding the radio emission excitation by shocks and supports the near-term development of a capability to predict and track these events for space weather prediction.
NASA Astrophysics Data System (ADS)
Campforts, Benjamin; Vanacker, Veerle; Vanderborght, Jan; Baken, Stijn; Smolders, Erik; Govers, Gerard
2016-04-01
Meteoric 10Be allows for the quantification of vertical and lateral soil fluxes over long time scales (103-105 yr). However, the mobility of meteoric 10Be in the soil system makes a translation of meteoric 10Be inventories into erosion and deposition rates complex. Here, we present a spatially explicit 2D model simulating the behaviour of meteoric 10Be on a hillslope. The model consists of two parts. The first component deals with advective and diffusive mobility of meteoric 10Be within the soil profile, and the second component describes lateral soil and meteoric 10Be fluxes over the hillslope. Soil depth is calculated dynamically, accounting for soil production through weathering as well as downslope fluxes of soil due to creep, water and tillage erosion. Synthetic model simulations show that meteoric 10Be inventories can be related to erosion and deposition across a wide range of geomorphological and pedological settings. Our results also show that meteoric 10Be can be used as a tracer to detect human impact on soil fluxes for soils with a high affinity for meteoric 10Be. However, the quantification of vertical mobility is essential for a correct interpretation of the observed variations in meteoric 10Be profiles and inventories. Application of the Be2D model to natural conditions using data sets from the Southern Piedmont (Bacon et al., 2012) and Appalachian Mountains (Jungers et al., 2009; West et al., 2013) allows to reliably constrain parameter values. Good agreement between simulated and observed meteoric 10Be concentrations and inventories is obtained with realistic parameter values. Furthermore, our results provide detailed insights into the processes redistributing meteoric 10Be at the soil-hillslope scale.
Improvement to Ooty IPS Velocity Maps for Time-Dependent MHD Simulation
NASA Astrophysics Data System (ADS)
Tropf, D.; Pogorelov, N.; Kim, T. K.; Manoharan, P.; Arge, C. N.
2015-12-01
Interplanetary Scintillation (IPS) observations can provide informationon the three-dimensional (3-D) solar wind structure. Manoharan et al.(2015) have recently used IPS velocity data from the Ootacommad RadioTelescope in India directly as time-dependent boundary conditions todrive a 3-D heliospheric MHD model. Their results showed consistentlylarger velocities at Earth compared to in situ data, most likely due tothe assumptions of constant velocity when the IPS velocity data in 3-Dspace are mapped onto the boundary surface. Thus, we improve upontheir results by assuming non-constant velocity when we construct theboundary velocity maps. New boundary conditions are then used to perform solarwind propagation outward from the Sun.
FireStem2D--a two-dimensional heat transfer model for simulating tree stem injury in fires.
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
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.
FireStem2D – A Two-Dimensional Heat Transfer Model for Simulating Tree Stem Injury in Fires
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
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
NASA Astrophysics Data System (ADS)
Humair, F.; Matasci, B.; Carrea, D.; Pedrazzini, A.; Loye, A.; Pedrozzi, G.; Nicolet, P.; Jaboyedoff, M.
2012-04-01
account the results of the experimental testing are performed and compared with the a-priori simulations. 3D simulations were performed using a software that takes into account the effect of the forest cover in the blocky trajectory (RockyFor 3D) and an other that neglects this aspect (Rotomap; geo&soft international). 2D simulation (RocFall; Rocscience) profiles were located in the blocks paths deduced from 3D simulations. The preliminary results show that: (1) high speed movies are promising and allow us to track the blocks using video software, (2) the a-priori simulations tend to overestimate the runout distance which is certainly due to an underestimation of the obstacles as well as the breaking of the failing rocks which is not taken into account in the models, (3) the trajectories deduced from both a-priori simulation and real size experiment highlights the major influence of the channelized slope morphology on rock paths as it tends to follow the flow direction. This indicates that the 2D simulation have to be performed along the line of flow direction.
Theory of MHD Jets and Outflows
NASA Astrophysics Data System (ADS)
Tsinganos, Kanaris
A brief review is given of selected results of our analytical and numerical work on the construction of time-independent and time-dependent MHD models for non relativistic astrophysical outflows and jets. The equations for steady MHD plasma flows are first outlined. Next, 1-D spherically symmetric outflows are briefly discussed, namely the Parker thermally driven nonrotating wind, as the classical prototype of all astrophysical outflows and the Weber-Davis magnetocentrifugally driven wind together with its astrophysical implications for magnetic braking, etc. Then, we turn to the 2-D MHD problem for steady and non steady 2-D axisymmetric magnetized and rotating plasma outflows. The only available exact solutions for such outflows are those in separable coordinates, i.e. those with the symmetry of radial or meridional self-similarity. Physically accepted solutions pass from the fast magnetosonic separatrix surface in order to satisfy MHD causality. An energetic criterion is outlined for selecting radially expanding winds from cylindrically expanding jets. The basics of jet acceleration, collimation, minimum fieldline inclination and angular momentum removal are illustrated in the context of radially self similar models. Numerical simulations of magnetic self-collimation verify several results of analytical steady solutions. The outflow from solar-type inefficient magnetic rotators is very weakly collimated while that from a ten times faster rotating YSO produces a tightly collimated jet. We also propose a two-component model consisting of a wind outflow from the central object and a faster rotating outflow launched from the surrounding accretion disk which plays the role of the flow collimator. We also briefly discuss the problem of shock formation during the magnetic collimation of wind-type outflows into jets.
Analysis of Highly-Resolved Simulations of 2-D Humps Toward Improvement of Second-Moment Closures
NASA Technical Reports Server (NTRS)
Jeyapaul, Elbert; Rumsey Christopher
2013-01-01
Fully resolved simulation data of flow separation over 2-D humps has been used to analyze the modeling terms in second-moment closures of the Reynolds-averaged Navier- Stokes equations. Existing models for the pressure-strain and dissipation terms have been analyzed using a priori calculations. All pressure-strain models are incorrect in the high-strain region near separation, although a better match is observed downstream, well into the separated-flow region. Near-wall inhomogeneity causes pressure-strain models to predict incorrect signs for the normal components close to the wall. In a posteriori computations, full Reynolds stress and explicit algebraic Reynolds stress models predict the separation point with varying degrees of success. However, as with one- and two-equation models, the separation bubble size is invariably over-predicted.
NASA Astrophysics Data System (ADS)
Vela-Vela, Luis; Sanchez, Raul; Reynolds-Barredo, J. Miguel
2015-11-01
Magnetically confined plasmas relevant for fusion scenarios are, to first approximation, well described by ideal and resistive MHD. This includes the description of their equilibrium and stability properties, as well as their medium-to-long term nonlinear evolution under external forcing. In many of these cases, one needs to deal with magnetic topologies that include magnetic islands, stochastic regions or that require the consideration of free-moving boundaries. The present work is part of an on-going effort to develop of a numerical code capable of dealing with these situations by taking advantage of the SPMHD formalism that, although widely used in astrophysical plasmas, is not widespread within the fusion community. SPMHD is a particle (i.e., Lagrangian) method particularly well-suited to deal with complicated boundaries while retaining great parallelization benefits. Here, we will report on the adaptation of the SPMHD equations to the case of magnetically confined plasmas, several benchmarking tests typical for MHD codes, and some preliminary results obtained for more elaborate scenarios. Our results suggest that our new code (EVA) can be very advantageous to deal with problems of current interest for the fusion community, including tokamaks and stellarators.
NASA Astrophysics Data System (ADS)
Pang, Liping; Close, Murray E.; Watt, James P. C.; Vincent, Keith W.
2000-06-01
Two 15 m×15 m field plots, a Te Awa silt loam and a Twyford fine sandy loam, located in Hawkes Bay, New Zealand, were applied with bromide, picloram, atrazine, and simazine. The Te Awa subsoil was a heterogeneous coarse sand and sandy gravel, and the Twyford subsoil was a more homogenous fine sandy loam. The underlying aquifers were composed of alluvial gravels at both sites with the water tables generally between 4-5 m below ground level. The sites were monitored for 2.2-3.5 years at approximately monthly intervals using suction cups in the unsaturated zone and monitoring wells in groundwater. HYDRUS-2D was used to simulate water movement and solute transport in soil and groundwater in a domain with a depth of 10 m and length of 68 m, including a 4.5-m unsaturated zone. The model simulated well the general trend of field observations for soil water content ( θ) and potential ( ψs), and the values matched better for the soils with less heterogeneity. For the soils with significant surface cracks, the simulated θ values were overestimated. On the other hand, for the soil layer perching on top of a less permeable layer, the simulated θ values were underestimated. Simulated pesticide concentrations using the "best available literature values" (BALVs) of organic carbon distribution coefficient ( Koc) and half-life ( T1/2) were generally lower than those observed. At early times in the trails, most simulations using BALVs were still within the same order of magnitude as observed concentrations for the shallow depths. However, at greater depths and later times, there were major differences between observed and simulated concentrations. The model was then calibrated for Koc and T1/2 values using observed data with an aid of the PEST optimisation package. Despite higher organic contents found in the topsoil, optimised Koc values for pesticides were consistently lower for the topsoil than for the subsoil, and were also lower than the BALVs except for picloram, possibly
NASA Astrophysics Data System (ADS)
Shen, F.; Feng, X.; Shen, C.
2013-12-01
Dynamic process of coronal mass ejections (CMEs) in the heliosphere is the key information for us to evaluate the CMEs' geo-effectiveness and to improve the accurate prediction of CME induced Shock Arrival Time (SAT) at Earth's environment. We present a three-dimensional (3D) magnetohydrodynamic (MHD) simulation of the evolution of the CME in a realistic ambient solar wind for the July 12-16, 2012 event by using the 3D COIN-TVD MHD code. The influence of the background solar wind speed to the SAT is analyzed. The influence of the initial position and polarity of the plasma blob to IMF Bz is also studied. In the validation study of this CME event (July 12-16, 2012), we find that this 3D COIN-TVD MHD model, with the magnetized plasma blob as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.
NASA Astrophysics Data System (ADS)
Simão Ferreira, C. J.; Bijl, H.; van Bussel, G.; van Kuik, G.
2007-07-01
The implementation of wind energy conversion systems in the built environment renewed the interest and the research on Vertical Axis Wind Turbines (VAWT), which in this application present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack with the angle of rotation, perceived velocity and consequentially Reynolds number. The phenomenon of dynamic stall is then an intrinsic effect of the operation of a Vertical Axis Wind Turbine at low tip speed ratios, having a significant impact in both loads and power. The complexity of the unsteady aerodynamics of the VAWT makes it extremely attractive to be analyzed using Computational Fluid Dynamics (CFD) models, where an approximation of the continuity and momentum equations of the Navier-Stokes equations set is solved. The complexity of the problem and the need for new design approaches for VAWT for the built environment has driven the authors of this work to focus the research of CFD modeling of VAWT on: •comparing the results between commonly used turbulence models: URANS (Spalart-Allmaras and k-epsilon) and large eddy models (Large Eddy Simulation and Detached Eddy Simulation) •verifying the sensitivity of the model to its grid refinement (space and time), •evaluating the suitability of using Particle Image Velocimetry (PIV) experimental data for model validation. The 2D model created represents the middle section of a single bladed VAWT with infinite aspect ratio. The model simulates the experimental work of flow field measurement using Particle Image Velocimetry by Simão Ferreira et al for a single bladed VAWT. The results show the suitability of the PIV data for the validation of the model, the need for accurate simulation of the large eddies and the sensitivity of the model to grid refinement.
Spontaneous Reconnection Onset in the Magnetotail: Kinetic and MHD Pictures
NASA Astrophysics Data System (ADS)
Sitnov, M. I.; Merkin, V. G.
2014-12-01
The mechanism of the reconnection onset in planetary magnetotails has been a topic of hot debate for more than three decades. At the kinetic level of description the key problem is a seemingly universal stability of the collisionless tearing mode when electrons are magnetized by the magnetic field normal to the current sheet. This effect can be eliminated in 2D equilibria with magnetic flux accumulated at the anti-sunward end of the tail. However, the resulting instability seen in 2D PIC simulations with open boundaries differs from the classical tearing mode because its main effect is the formation of dipolarization fronts, i. e., regions of an enhanced normal magnetic field rather than the reversal of its sign. Strong tailward gradients of the normal magnetic field characteristic of fronts suggest that they can be destroyed in 3D by buoyancy and flapping instabilities. However, 3D PIC simulations show that buoyancy and flapping motions can neither destroy nor change critically the near-2D picture of the front evolution, although they do significantly disturb it. Modeling and understanding of this kinetic picture of the reconnection onset in MHD terms is critically important for incorporating the explosive reconnection physics into global models of the magnetosphere and solar corona. A key to this has become the recognition that tail current sheets with accumulated flux regions can also be unstable with respect to an ideal analog of the tearing mode, which has a similar structure of the electromagnetic field and plasma perturbations but preserves the original magnetic field topology. MHD simulations with high Lundquist number confirm the existence of such "pseudo-tearing" instability regimes. Non-MHD effects, including different motions of electron and ion species as well as the ion Landau dissipation transform these ideal MHD motions into the tearing/slippage instability obtained in PIC simulations.
NASA Astrophysics Data System (ADS)
Jia, Xiaojie; Ai, Bin; Deng, Youjun; Xu, Xinxiang; Peng, Hua; Shen, Hui
2015-08-01
On the basis of perfect PC2D simulation to the measured current density vs voltage (J-V) curve of the best selective emitter (SE) solar cell fabricated by the CSG Company using the screen printing phosphoric paste method, we systematically investigated the effect of the parameters of gridline, base, selective emitter, back surface field (BSF) layer and surface recombination rate on performance of the SE solar cell. Among these parameters, we identified that the base minority carrier lifetime, the front and back surface recombination rate and the ratio of the sheet-resistance of heavily and lightly doped region are the four largest efficiency-affecting factors. If all the parameters have ideal values, the SE solar cell fabricated on a p-type monocrystalline silicon wafer can even obtain the efficiency of 20.45%. In addition, the simulation also shows that fine gridline combining dense gridline and increasing bus bar number while keeping the lower area ratio can offer the other ways to improve the efficiency.
2D simulation of active species and ozone production in a multi-tip DC air corona discharge
NASA Astrophysics Data System (ADS)
Meziane, M.; Eichwald, O.; Sarrette, J. P.; Ducasse, O.; Yousfi, M.
2011-11-01
The present paper shows for the first time in the literature a complete 2D simulation of the ozone production in a DC positive multi-tip to plane corona discharge reactor crossed by a dry air flow at atmospheric pressure. The simulation is undertaken until 1 ms and involves tens of successive discharge and post-discharge phases. The air flow is stressed by several monofilament corona discharges generated by a maximum of four anodic tips distributed along the reactor. The nonstationary hydrodynamics model for reactive gas mixture is solved using the commercial FLUENT software. During each discharge phase, thermal and vibrational energies as well as densities of radical and metastable excited species are locally injected as source terms in the gas medium surrounding each tip. The chosen chemical model involves 10 neutral species reacting following 24 reactions. The obtained results allow us to follow the cartography of the temperature and the ozone production inside the corona reactor as a function of the number of high voltage anodic tips.
NASA Astrophysics Data System (ADS)
Deng, Wei; Li, Hui; Zhang, Bing; Li, Shengtai
2015-06-01
We perform 3D relativistic ideal magnetohydrodynamics (MHD) simulations to study the collisions between high-σ (Poynting-flux-dominated (PFD)) blobs which contain both poloidal and toroidal magnetic field components. This is meant to mimic the interactions inside a highly variable PFD jet. We discover a significant electromagnetic field (EMF) energy dissipation with an Alfvénic rate with the efficiency around 35%. Detailed analyses show that this dissipation is mostly facilitated by the collision-induced magnetic reconnection. Additional resolution and parameter studies show a robust result that the relative EMF energy dissipation efficiency is nearly independent of the numerical resolution or most physical parameters in the relevant parameter range. The reconnection outflows in our simulation can potentially form the multi-orientation relativistic mini jets as needed for several analytical models. We also find a linear relationship between the σ values before and after the major EMF energy dissipation process. Our results give support to the proposed astrophysical models that invoke significant magnetic energy dissipation in PFD jets, such as the internal collision-induced magnetic reconnection and turbulence model for gamma-ray bursts, and reconnection triggered mini jets model for active galactic nuclei. The simulation movies are shown in http://www.physics.unlv.edu/∼deng/simulation1.html.
NASA Astrophysics Data System (ADS)
Vanzo, Davide; Siviglia, Annunziato; Zolezzi, Guido
2014-05-01
In last decades, pushed by an increasing interest in environmental problems and supported by an exponential growth of computational capability, novel numerical methods and models have been developed. Despite the progress in parallel computing, computational time is still one of the main bottlenecks when dealing with long term environmental simulations. To overcome such time constraint in morphodynamic models, artificial acceleration of bed evolution has been implemented with different strategies (e.g. Roelvink 2006). The key idea is to accelerate the morphological evolution increasing the discrete bottom variations of a given "morphological factor" during numerical integration thus considerably speeding up computational time. On the other hand, an artificial alteration of the governing equations is put forward, for which related numerical and physical consequences are not completely known. The present work investigates the role of the morphological factor in numerical simulations of a well-defined, 2D reach-scale process in river morphodynamics, which can be taken as a benchmark for the established knowledge made available from theoretical and physical scale models developed in the past decades. The chosen process is the evolution of free migrating bars in a straight channel. The numerical morphodynamic model used in this work is GIAMT2D (Siviglia et al. 2013), which solves the governing system of shallow water and Exner equations following a fully coupled approach with a finite volume method on unstructured triangular grids. By processing numerical outcomes also through Continuous Wavelet Transform, the differences in free migrating bars properties (temporal evolution and equilibrium values of wavelength, amplitude, celerity) are investigated in simple test cases with different values of the morphological factor. Numerical results are compared with available analytical theories for free bars. The outcomes highlight the consequences of using the morphological
Falvo, Cyril; Zhuang, Wei; Kim, Yung Sam; Axelsen, Paul H.; Hochstrasser, Robin M.; Mukamel, Shaul
2012-01-01
The infrared optical response of Amyloid Fibrils Aβ1–40 is investigated. Simulations of two models corresponding to different protonation states are compared with experiment. The simulations reveal that vibrational frequency distributions inside the fibrils are dominated by sidechain fluctuations. We further confirm earlier suggestions based on 2D-IR measurements that water molecules can be trapped inside the fibrils. PMID:22338639
Deng, Wei
2015-07-21
The question of the energy composition of the jets/outflows in high-energy astrophysical systems, e.g. GRBs, AGNs, is taken up first: Matter-flux-dominated (MFD), σ < 1, and/or Poynting-flux-dominated (PFD), σ >1? The standard fireball IS model and dissipative photosphere model are MFD, while the ICMART (Internal-Collision-induced MAgnetic Reconnection and Turbulence) model is PFD. Motivated by ICMART model and other relevant problems, such as “jets in a jet” model of AGNs, the author investigates the models from the EMF energy dissipation efficiency, relativistic outflow generation, and σ evolution points of view, and simulates collisions between high-σ blobs to mimic the situation of the interactions inside the PFD jets/outflows by using a 3D SRMHD code which solves the conservative form of the ideal MHD equations. σ_{b,f} is calculated from the simulation results (threshold = 1). The efficiency obtained from this hybrid method is similar to the efficiency got from the energy evolution of the simulations (35.2%). Efficiency is nearly σ independent, which is also confirmed by the hybrid method. σ_{b,i} - σ_{b,f} provides an interesting linear relationship. Results of several parameter studies of EMF energy dissipation efficiency are shown.
NASA Astrophysics Data System (ADS)
Hayashi, K.
2013-11-01
We present a model of a time-dependent three-dimensional magnetohydrodynamics simulation of the sub-Alfvenic solar corona and super-Alfvenic solar wind with temporally varying solar-surface boundary magnetic field data. To (i) accommodate observational data with a somewhat arbitrarily evolving solar photospheric magnetic field as the boundary value and (ii) keep the divergence-free condition, we developed a boundary model, here named Confined Differential Potential Field model, that calculates the horizontal components of the magnetic field, from changes in the vertical component, as a potential field confined in a thin shell. The projected normal characteristic method robustly simulates the solar corona and solar wind, in response to the temporal variation of the boundary Br. We conduct test MHD simulations for two periods, from Carrington Rotation number 2009 to 2010 and from Carrington Rotation 2074 to 2075 at solar maximum and minimum of Cycle 23, respectively. We obtained several coronal features that a fixed boundary condition cannot yield, such as twisted magnetic field lines at the lower corona and the transition from an open-field coronal hole to a closed-field streamer. We also obtained slight improvements of the interplanetary magnetic field, including the latitudinal component, at Earth.
NASA Astrophysics Data System (ADS)
Alexakis, A.
2009-04-01
Most astrophysical and planetary systems e.g., solar convection and stellar winds, are in a turbulent state and coupled to magnetic fields. Understanding and quantifying the statistical properties of magneto-hydro-dynamic (MHD) turbulence is crucial to explain the involved physical processes. Although the phenomenological theory of hydro-dynamic (HD) turbulence has been verified up to small corrections, a similar statement cannot be made for MHD turbulence. Since the phenomenological description of Hydrodynamic turbulence by Kolmogorov in 1941 there have been many attempts to derive a similar description for turbulence in conducting fluids (i.e Magneto-Hydrodynamic turbulence). However such a description is going to be based inevitably on strong assumptions (typically borrowed from hydrodynamics) that do not however necessarily apply to the MHD case. In this talk I will discuss some of the properties and differences of the energy and helicity cascades in turbulent MHD and HD flows. The investigation is going to be based on the analysis of direct numerical simulations. The cascades in MHD turbulence appear to be a more non-local process (in scale space) than in Hydrodynamics. Some implications of these results to turbulent modeling will be discussed
The Effects of Differential Rotation on the Magnetic Structure of the Solar Corona: MHD Simulations
NASA Technical Reports Server (NTRS)
Lionello, Roberto; Riley, Pete; Linker, Jon A.; Mikic, Zoran
2004-01-01
Coronal holes are magnetically open regions from which the solar wind streams. Magnetic reconnection has been invoked to reconcile the apparently rigid rotation of coronal holes with the differential rotation of magnetic flux in the photosphere. This mechanism might also be relevant to the formation of the slow solar wind, the properties of which seem to indicate an origin from the opening of closed magnetic field lines. We have developed a global MHD model to study the effect of differential rotation on the coronal magnetic field. Starting from a magnetic flux distribution similar to that of Wang et al., which consists of a bipolar magnetic region added to a background dipole field, we applied differential rotation over a period of 5 solar rotations. The evolution of the magnetic field and of the boundaries of coronal holes are in substantial agreement with the findings of Wang et al.. We identified examples of interchange reconnection and other changes of topology of the magnetic field. Possible consequences for the origin of the slow solar wind are also discussed.
Localized reconnection in the magnetotail driven by lobe flow channels: Global MHD simulation
NASA Astrophysics Data System (ADS)
Nishimura, Y.; Lyons, L. R.
2016-02-01
Recent ionospheric measurements suggest polar cap flow channels often trigger nightside auroral brightening. However, measurements were limited to the ionosphere, and it was not understood if such flow channels can exist in the lobe and can trigger magnetotail reconnection in a localized cross-tail extent. We examined if localized flow channels can form self-consistently in a global MHD regime, and if so, how such flow channels originate and relate to localized magnetotail reconnection. We show that lobe convection became nonuniform with azimuthally narrow flow channels (enhanced dawn-dusk electric fields) of ~3 RE cross-tail width. The flow channels propagated from the dayside toward the plasma sheet as an interplanetary magnetic field (IMF) discontinuity swept tailward. The plasma sheet around the lobe flow channels became thinner with a similar cross-tail extent and then localized reconnection occurred. These results suggest that localized flow channels can propagate tailward across the lobe and drive localized magnetotail reconnection, that the cross-tail width of reconnection and resulting plasma sheet flow channels and dipolarization fronts are related to the width of inflow from the lobe, and that IMF discontinuities drive lobe flow channels.
NASA Technical Reports Server (NTRS)
Li, Xiaofan; Sui, C.-H.; Lau, K.-M.
1999-01-01
The phase relation between the perturbation kinetic energy (K') associated with the tropical convection and the horizontal-mean moist available potential energy (bar-P) associated with environmental conditions is investigated by an energetics analysis of a numerical experiment. This experiment is performed using a 2-D cloud resolving model forced by the TOGA-COARE derived vertical velocity. The imposed upward motion leads to a decrease of bar-P directly through the associated vertical advective cooling, and to an increase of K' directly through cloud related processes, feeding the convection. The maximum K' and its maximum growth rate lags and leads, respectively, the maximum imposed large-scale upward motion by about 1-2 hours, indicating that convection is phase locked with large-scale forcing. The dominant life cycle of the simulated convection is about 9 hours, whereas the time scales of the imposed large-scale forcing are longer than the diurnal cycle. In the convective events, maximum growth of K' leads maximum decay of the perturbation moist available potential energy (P') by about 3 hours through vertical heat transport by perturbation circulation, and perturbation cloud heating. Maximum decay of P' leads maximum decay of bar-P by about one hour through the perturbation radiative, processes, the horizontal-mean cloud heating, and the large-scale vertical advective cooling. Therefore, maximum gain of K' occurs about 4-5 hours before maximum decay of bar-P.
A 2-D FEM thermal model to simulate water flow in a porous media: Campi Flegrei caldera case study
NASA Astrophysics Data System (ADS)
Romano, V.; Tammaro, U.; Capuano, P.
2012-05-01
Volcanic and geothermal aspects both exist in many geologically young areas. In these areas the heat transfer process is of fundamental importance, so that the thermal and fluid-dynamic processes characterizing a viscous fluid in a porous medium are very important to understand the complex dynamics of the these areas. The Campi Flegrei caldera, located west of the city of Naples, within the central-southern sector of the large graben of Campanian plain, is a region where both volcanic and geothermal phenomena are present. The upper part of the geothermal system can be considered roughly as a succession of volcanic porous material (tuff) saturated by a mixture formed mainly by water and carbon dioxide. We have implemented a finite elements approach in transient conditions to simulate water flow in a 2-D porous medium to model the changes of temperature in the geothermal system due to magmatic fluid inflow, accounting for a transient phase, not considered in the analytical solutions and fluid compressibility. The thermal model is described by means of conductive/convective equations, in which we propose a thermal source represented by a parabolic shape function to better simulate an increase of temperature in the central part (magma chamber) of a box, simulating the Campi Flegrei caldera and using more recent evaluations, from literature, for the medium's parameters (specific heat capacity, density, thermal conductivity, permeability). A best-fit velocity for the permeant is evaluated by comparing the simulated temperatures with those measured in wells drilled by Agip (Italian Oil Agency) in the 1980s in the framework of geothermal exploration. A few tens of days are enough to reach the thermal steady state, showing the quick response of the system to heat injection. The increase in the pressure due to the heat transport is then used to compute ground deformation, in particular the vertical displacements characteristics of the Campi Flegrei caldera behaviour. The
TITAN2D simulations of pyroclastic flows at Cerro Machín Volcano, Colombia: Hazard implications
NASA Astrophysics Data System (ADS)
Murcia, H. F.; Sheridan, M. F.; Macías, J. L.; Cortés, G. P.
2010-03-01
Cerro Machín is a dacitic tuff ring located in the central part of the Colombian Andes. It lies at the southern end of the Cerro Bravo-Cerro Machín volcanic belt. This volcano has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have generated pyroclastic flows associated with Plinian activity that have traveled up to 8 km from the crater, and pyroclastic flows associated with Vulcanian activity with shorter runouts of 5 km from the source. Today, some 21,000 people live within a 8 km radius of Cerro Machín. The volcano is active with fumaroles and has shown increasing seismic activity since 2004, and therefore represents a potentially increasing threat to the local population. To evaluate the possible effects of future eruptions that may generate pyroclastic density currents controlled by granular flow dynamics we performed flow simulations with the TITAN2D code. These simulations were run in all directions around the volcano, using the input parameters of the largest eruption reported. The results show that an eruption of 0.3 km 3 of pyroclastic flows from a collapsing Plinian column would travel up to 9 km from the vent, emplacing a deposit thicker than 60 m within the Toche River valley. Deposits >45 m thick can be expected in the valleys of San Juan, Santa Marta, and Azufral creeks, while 30 m thick deposits could accumulate within the drainages of the Tochecito, Bermellón, and Coello Rivers. A minimum area of 56 km 2 could be affected directly by this kind of eruption. In comparison, Vulcanian column-collapse pyroclastic flows of 0.1 km 3 would travel up to 6 km from the vent depositing >45 m thick debris inside the Toche River valley and more than 30 m inside the valleys of San Juan, Santa Marta, and Azufral creeks. The minimum area that could be affected directly by this kind of eruption is 33 km 2. The distribution and thickness of the deposits obtained by these simulations are consistent with the hazard
NASA Astrophysics Data System (ADS)
Park, K.; Ogino, T.; Lee, D.; Walker, R. J.; Kim, K.
2013-12-01
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
NASA Technical Reports Server (NTRS)
Lionello, Roberto; Linker, Jon A.; Mikic, Zoran; Riley, Pete
2006-01-01
Solar energetic particles, which are believed to originate from corotating interacting regions (CIRS) at low heliographic latitude, were observed by the Ulysses spacecraft even as it passed over the Sun's poles. One interpretation of this result is that high-latitude field lines intercepted by Ulysses connect to low-latitude CIRs at much larger heliocentric distances. The Fisk model explains the latitudinal excursion of magnetic field lines in the solar corona and heliosphere as the inevitable consequence of the interaction of a tilted dipole in a differentially rotating photosphere with rigidly rotating coronal holes. We use a time-dependent three-dimensional magnetohydrodynamic (MHD) algorithm to follow the evolution of a simple model of the solar corona in response to the differential rotation of the photospheric magnetic flux. We examine the changes of the coronal-hole boundaries, the redistribution of the line-of-sight magnetic field, and the precession of field lines in the corona. Our results confirm the basic idea of the Fisk model, that differential rotation leads to changes in the heliographic latitude of magnetic field lines. However, the latitudinal excursion of magnetic field lines in this simple "tilted dipole" model is too small to explain the Ulysses observations. Although coronal holes in our model rotate more rigidly than do photospheric features (in general agreement with observations), they do not rotate strictly rigidly as assumed by Fisk. This basic difference between our model and Fisk's will be explored in the future by considering more realistic magnetic flux distributions, as observed during Ulysses polar excursions.
3D Dynamics of Magnetopause Reconnection Using Hall-MHD Global Simulations
NASA Astrophysics Data System (ADS)
Maynard, K.; Germaschewski, K.; Raeder, J.; Bhattacharjee, A.
2011-12-01
Magnetic reconnection at Earth's magnetopause and in the magnetotail is of crucial importance for the dynamics of the global magnetosphere and space weather. Even though the plasma conditions in the magnetosphere are largely in the collisionless regime, most of the existing research using global computational models employ single-fluid magnetohydrodynamics (MHD) with artificial resistivity. Studies of reconnection in simplified, two-dimensional geometries have established that two-fluid and kinetic effects can dramatically alter dynamics and reconnection rates when compared with single-fluid models. These enhanced models also introduce particular signatures, for example a quadrupolar out-of-plane magnetic field component that has already been observed in space by satellite measurements. However, results from simplified geometries cannot be translated directly to the dynamics of three-dimensional magnetospheric reconnection. For instance, magnetic flux originating from the solar wind and arriving at the magnetopause can either reconnect or be advected around the magnetosphere. In this study, we use a new version of the OpenGGCM code that incorporates the Hall term in a Generalized Ohm's Law to study magnetopause reconnection under synthetic solar wind conditions and investigate how reconnection rates and dynamics of flux transfer events depend on the strength of the Hall term. The OpenGGCM, a global model of Earth's magnetosphere, has recently been ported to exploit modern computing architectures like the Cell processor and SIMD capabilities of conventional processors using an automatic code generator. These enhancements provide us with the performance needed to include the computationally expensive Hall physics.
Deng, Wei; Li, Hui; Zhang, Bing; Li, Shengtai
2015-05-29
We perform 3D relativistic ideal MHD simulations to study the collisions between high-σ (Poynting- ux-dominated) blobs which contain both poloidal and toroidal magnetic field components. This is meant to mimic the interactions inside a highly variable Poynting- ux-dominated jet. We discover a significant electromagnetic field (EMF) energy dissipation with an Alfvenic rate with the efficiency around 35%. Detailed analyses show that this dissipation is mostly facilitated by the collision-induced magnetic reconnection. Additional resolution and parameter studies show a robust result that the relative EMF energy dissipation efficiency is nearly independent of the numerical resolution or most physical parameters in themore » relevant parameter range. The reconnection outflows in our simulation can potentially form the multi-orientation relativistic mini-jets as needed for several analytical models. We also find a linear relationship between the σ values before and after the major EMF energy dissipation process. In conclusion, our results give support to the proposed astrophysical models that invoke signi cant magnetic energy dissipation in Poynting- ux-dominated jets, such as the internal collision-induced magnetic reconnection and turbulence (ICMART) model for GRBs, and reconnection triggered mini-jets model for AGNs.« less
Deng, Wei; Li, Hui; Zhang, Bing; Li, Shengtai
2015-05-29
We perform 3D relativistic ideal MHD simulations to study the collisions between high-σ (Poynting- ux-dominated) blobs which contain both poloidal and toroidal magnetic field components. This is meant to mimic the interactions inside a highly variable Poynting- ux-dominated jet. We discover a significant electromagnetic field (EMF) energy dissipation with an Alfvenic rate with the efficiency around 35%. Detailed analyses show that this dissipation is mostly facilitated by the collision-induced magnetic reconnection. Additional resolution and parameter studies show a robust result that the relative EMF energy dissipation efficiency is nearly independent of the numerical resolution or most physical parameters in the relevant parameter range. The reconnection outflows in our simulation can potentially form the multi-orientation relativistic mini-jets as needed for several analytical models. We also find a linear relationship between the σ values before and after the major EMF energy dissipation process. In conclusion, our results give support to the proposed astrophysical models that invoke signi cant magnetic energy dissipation in Poynting- ux-dominated jets, such as the internal collision-induced magnetic reconnection and turbulence (ICMART) model for GRBs, and reconnection triggered mini-jets model for AGNs.
MHD-kinetic Simulation of the Heliospheric Interface and Related 2-3 kHz Radio Emission
NASA Astrophysics Data System (ADS)
Borovikov, S.; Pogorelov, N. V.; Heerikhuisen, J.; Zank, G. P.; Cairns, I. H.
2013-12-01
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.
NASA Astrophysics Data System (ADS)
Fediai, Artem; Ryndyk, Dmitry A.; Cuniberti, Gianaurelio
2016-10-01
Up to now, the electrical properties of the contacts between 3D metals and 2D materials have never been computed at a fully ab initio level due to the huge number of atomic orbitals involved in a current path from an electrode to a pristine 2D material. As a result, there are still numerous open questions and controversial theories on the electrical properties of systems with 3D/2D interfaces—for example, the current path and the contact length scalability. Our work provides a first-principles solution to this long-standing problem with the use of the modular approach, a method which rigorously combines a Green function formalism with the density functional theory (DFT) for this particular contact type. The modular approach is a general approach valid for any 3D/2D contact. As an example, we apply it to the most investigated among 3D/2D contacts—metal/graphene contacts—and show its abilities and consistency by comparison with existing experimental data. As it is applicable to any 3D/2D interface, the modular approach allows the engineering of 3D/2D contacts with the pre-defined electrical properties.
Fediai, Artem; Ryndyk, Dmitry A; Cuniberti, Gianaurelio
2016-10-01
Up to now, the electrical properties of the contacts between 3D metals and 2D materials have never been computed at a fully ab initio level due to the huge number of atomic orbitals involved in a current path from an electrode to a pristine 2D material. As a result, there are still numerous open questions and controversial theories on the electrical properties of systems with 3D/2D interfaces-for example, the current path and the contact length scalability. Our work provides a first-principles solution to this long-standing problem with the use of the modular approach, a method which rigorously combines a Green function formalism with the density functional theory (DFT) for this particular contact type. The modular approach is a general approach valid for any 3D/2D contact. As an example, we apply it to the most investigated among 3D/2D contacts-metal/graphene contacts-and show its abilities and consistency by comparison with existing experimental data. As it is applicable to any 3D/2D interface, the modular approach allows the engineering of 3D/2D contacts with the pre-defined electrical properties.
NASA Astrophysics Data System (ADS)
Sato, Haruo; Fehler, Michael C.
2016-10-01
The envelope broadening and the peak delay of the S-wavelet of a small earthquake with increasing travel distance are results of scattering by random velocity inhomogeneities in the earth medium. As a simple mathematical model, Sato proposed a new stochastic synthesis of the scalar wavelet envelope in 3-D von Kármán type random media when the centre wavenumber of the wavelet is in the power-law spectral range of the random velocity fluctuation. The essential idea is to split the random medium spectrum into two components using the centre wavenumber as a reference: the long-scale (low-wavenumber spectral) component produces the peak delay and the envelope broadening by multiple scattering around the forward direction; the short-scale (high-wavenumber spectral) component attenuates wave amplitude by wide angle scattering. The former is calculated by the Markov approximation based on the parabolic approximation and the latter is calculated by the Born approximation. Here, we extend the theory for the envelope synthesis of a wavelet in 2-D random media, which makes it easy to compare with finite difference (FD) simulation results. The synthetic wavelet envelope is analytically written by using the random medium parameters in the angular frequency domain. For the case that the power spectral density function of the random velocity fluctuation has a steep roll-off at large wavenumbers, the envelope broadening is small and frequency independent, and scattering attenuation is weak. For the case of a small roll-off, however, the envelope broadening is large and increases with frequency, and the scattering attenuation is strong and increases with frequency. As a preliminary study, we compare synthetic wavelet envelopes with the average of FD simulation wavelet envelopes in 50 synthesized random media, which are characterized by the RMS fractional velocity fluctuation ε = 0.05, correlation scale a = 5 km and the background wave velocity V0 = 4 km s-1. We use the radiation
NASA Astrophysics Data System (ADS)
Sato, Haruo; Fehler, Michael C.
2016-07-01
The envelope broadening and the peak delay of the S-wavelet of a small earthquake with increasing travel distance are results of scattering by random velocity inhomogeneities in the earth medium. As a simple mathematical model, Sato (2016) proposed a new stochastic synthesis of the scalar wavelet envelope in 3-D von Kármán type random media when the center wavenumber of the wavelet is in the power-law spectral range of the random velocity fluctuation. The essential idea is to split the random medium spectrum into two components using the center wavenumber as a reference: the long-scale (low-wavenumber spectral) component produces the peak delay and the envelope broadening by multiple scattering around the forward direction; the short-scale (high-wavenumber spectral) component attenuates wave amplitude by wide angle scattering. The former is calculated by the Markov approximation based on the parabolic approximation and the latter is calculated by the Born approximation. Here, we extend the theory for the envelope synthesis of a wavelet in 2-D random media, which makes it easy to compare with finite difference (FD) simulation results. The synthetic wavelet envelope is analytically written by using the random medium parameters in the angular frequency domain. For the case that the power spectral density function of the random velocity fluctuation has a steep roll-off at large wavenumbers, the envelope broadening is small and frequency independent, and scattering attenuation is weak. For the case of a small roll-off, however, the envelope broadening is large and increases with frequency, and the scattering attenuation is strong and increases with frequency. As a preliminary study, we compare synthetic wavelet envelopes with the average of FD simulation wavelet envelopes in 50 synthesized random media, which are characterized by the RMS fractional velocity fluctuation ε=0.05, correlation scale a =5 km and the background wave velocity V0=4 km/s. We use the radiation
NASA Technical Reports Server (NTRS)
Fleming, Eric L.; Jackman, Charles H.; Considine, David B.; Stolarski, Richard S.
1999-01-01
In this study, we examine the sensitivity of long lived tracers to changes in the base transport components in our 2-D model. Changes to the strength of the residual circulation in the upper troposphere and stratosphere and changes to the lower stratospheric K(sub zz) had similar effects in that increasing the transport rates decreased the overall stratospheric mean age, and increased the rate of removal of material from the stratosphere. Increasing the stratospheric K(sub yy) increased the mean age due to the greater recycling of air parcels through the middle atmosphere, via the residual circulation, before returning to the troposphere. However, increasing K(sub yy) along with self-consistent increases in the corresponding planetary wave drive, which leads to a stronger residual circulation, more than compensates for the K(sub yy)-effect, and produces significantly younger ages throughout the stratosphere. Simulations with very small tropical stratospheric K(sub yy) decreased the globally averaged age of air by as much as 25% in the middle and upper stratosphere, and resulted in substantially weaker vertical age gradients above 20 km in the extratropics. We found only very small stratospheric tracer sensitivity to the magnitude of the horizontal mixing across the tropopause, and to the strength of the mesospheric gravity wave drag and diffusion used in the model. We also investigated the transport influence on chemically active tracers and found a strong age-tracer correlation, both in concentration and calculated lifetimes. The base model transport gives the most favorable overall comparison with a variety of inert tracer observations, and provides a significant improvement over our previous 1995 model transport. Moderate changes to the base transport were found to provide modest agreement with some of the measurements. Transport scenarios with residence times ranging from moderately shorter to slightly longer relative to the base case simulated N2O lifetimes
NASA Astrophysics Data System (ADS)
Volpi, D.; Del Zanna, L.; Amato, E.; Bucciantini, N.
2008-07-01
Aims: We develop a set of diagnostic tools for synchrotron-emitting sources, presented in a previous paper, to include a computation of inverse-Compton radiation from the same relativistic particles that give rise to the synchrotron emission. For the first time, we then study the gamma-ray emission properties of Pulsar Wind Nebulae, in the context of the axisymmetric jet-torus scenario. Methods: We evolve the relativistic MHD equations and the maximum energy of the emitting particles, including adiabatic and synchrotron losses along streamlines. The particle energy distribution function is split into two components: one corresponds to radio-emitting electrons, which are interpreted to be a relic population that is born at the outburst of the supernova, and the other is associated with a wind population that is continuously accelerated at the termination shock and emits up to the gamma-ray band. The inverse Compton emissivity is calculated using the general Klein-Nishina differential cross-section and three different photon targets for the relativistic particles are considered: the nebular synchrotron photons, photons associated with the far-infrared thermal excess, and the cosmic microwave background. Results: When the method is applied to the simulations that match the optical and X-ray morphology of the Crab Nebula, the overall synchrotron spectrum can only be fitted assuming an excess of injected particles and a steeper power law (E-2.7) with respect to previous models. The resulting TeV emission has then the correct shape but is in excess of the data. This is related to the magnetic-field structure in the nebula, derived using simulations: in particular, the field is strongly compressed close to the termination shock, but with a lower than expected volume average. The jet-torus structure is also found to be visible clearly in high-resolution gamma-ray synthetic maps. We present a preliminary exploration of time variability in X- and gamma-rays. We find
NASA Technical Reports Server (NTRS)
Denton, R.; Sonnerup, B. U. O.; Swisdak, M.; Birn, J.; Drake, J. F.; Heese, M.
2012-01-01
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
NASA Astrophysics Data System (ADS)
Fernández-Pato, Javier; Caviedes-Voullième, Daniel; García-Navarro, Pilar
2016-05-01
One of the most difficult issues in the development of hydrologic models is to find a rigorous source of data and specific parameters to a given problem, on a given location that enable reliable calibration. In this paper, a distributed and physically based model (2D Shallow Water Equations) is used for surface flow and runoff calculations in combination with two infiltration laws (Horton and Green-Ampt) for estimating infiltration in a watershed. This technique offers the capability of assigning a local and time-dependent infiltration rate to each computational cell depending on the available surface water, soil type or vegetation. We investigate how the calibration of parameters is affected by transient distributed Shallow Water model and the complexity of the problem. In the first part of this work, we calibrate the infiltration parameters for both Horton and Green-Ampt models under flat ponded soil conditions. Then, by means of synthetic test cases, we perform a space-distributed sensitivity analysis in order to show that this calibration can be significantly affected by the introduction of topography or rainfall. In the second part, parameter calibration for a real catchment is addressed by comparing the numerical simulations with two different sets of experimental data, corresponding to very different events in terms of the rainfall volume. We show that the initial conditions of the catchment and the rainfall pattern have a special relevance in the quality of the adjustment. Hence, it is shown that the topography of the catchment and the storm characteristics affect the calibration of infiltration parameters.
NASA Astrophysics Data System (ADS)
Kangliang, Wei; Xiaoyan, Liu; Gang, Du; Ruqi, Han
2010-08-01
We demonstrate a two-dimensional (2D) full-band ensemble Monte-Carlo simulator for heterostructures, which deals with carrier transport in two different semiconductor materials simultaneously as well as at the boundary by solving self-consistently the 2D Poisson and Boltzmann transport equations (BTE). The infrastructure of this simulator, including the energy bands obtained from the empirical pseudo potential method, various scattering mechanics employed, and the appropriate treatment of the carrier transport at the boundary between two different semiconductor materials, is also described. As verification and calibration, we have performed a simulation on two types of silicon-germanium (Si-Ge) heterojunctions with different doping profiles—the p-p homogeneous type and the n-p inhomogeneous type. The current-voltage characteristics are simulated, and the distributions of potential and carrier density are also plotted, which show the validity of our simulator.
NASA Astrophysics Data System (ADS)
Schmidt, J. M.; Cairns, Iver H.; Xie, Hong; St. Cyr, O. C.; Gopalswamy, N.
2016-03-01
Coronal mass ejections (CMEs) are major transient phenomena in the solar corona that are observed with ground-based and spacecraft-based coronagraphs in white light or with in situ measurements by spacecraft. CMEs transport mass and momentum and often drive shocks. In order to derive the CME and shock trajectories with high precision, we apply the graduated cylindrical shell (GCS) model to fit a flux rope to the CME directed toward STEREO A after about 19:00 UT on 29 November 2013 and check the quality of the heliocentric distance-time evaluations by carrying out a three-dimensional magnetohydrodynamic (MHD) simulation of the same CME with the Block Adaptive Tree Solar-Wind Roe Upwind Scheme (BATS-R-US) code. Heliocentric distances of the CME and shock leading edges are determined from the simulated white light images and magnetic field strength data. We find very good agreement between the predicted and observed heliocentric distances, showing that the GCS model and the BATS-R-US simulation approach work very well and are consistent. In order to assess the validity of CME and shock identification criteria in coronagraph images, we also compute synthetic white light images of the CME and shock. We find that the outer edge of a cloud-like illuminated area in the observed and predicted images in fact coincides with the leading edge of the CME flux rope and that the outer edge of a faint illuminated band in front of the CME leading edge coincides with the CME-driven shock front.
NASA Astrophysics Data System (ADS)
Burckel, A.; Sauter, O.; Angioni, C.; Candy, J.; Fable, E.; Lapillonne, X.
2010-11-01
In order to better identify the role of the magnetic topology on ITG and TEM instabilities, different MHD equilibria with increasing complexity are calculated using the CHEASE code [1]. We start from the geometry of the s-α cyclone benchmark case [2], consider the corresponding circular numerical equilibrium, and then successively add a non zero value of a consistent with the kinetic profiles, an elongation of 1.68, a triangularity of 0.15, and finally an up-down asymmetry corresponding to a single-null diverted geometry. This gives the opportunity to study separately the effect of each main characteristics of the equilibrium on microinstabilities in core plasmas. Linear local electrostatic gyrokinetic simulations of these different numerical equilibria and of their corresponding analytical descriptions (Miller-type representations [3]) are performed using the codes GS2 [4, 5] and GYRO[6]. It is observed that each modification of the equilibrium has an influence on the results of gyrokinetic simulations. The effect of the α parameter can compensate the stabilizing effect of an increase in the elongation. A comparison between the up-down symmetric shaped equilibrium and its corresponding diverted configuration show a non negligible effect on the growth rate of ITG and TEM turbulence. The comparison between the local Miller model and using a full equilibrium shows that it is mainly the indirect change of elongation in the plasma core which influences the results. The global aim is to provide well defined benchmark cases including real geometry and kinetic electrons physics, since this is not analyzed by the cyclone case. In addition, the goal is to define a procedure for testing of local simulations inspired by experimental constraints and results.
NASA Astrophysics Data System (ADS)
Levrier, François
2015-08-01
The Planck satellite has mapped the polarized microwave sky (from 30 GHz to 353 GHz) with unprecedented sensitivity and angular resolution. This wealth of data yields the first complete map of polarized thermal emission from dust in our own Galaxy, shedding new light on the formation of dense cold structures within which new stars and planetary systems are born, under the combined effects of gravity, turbulence and magnetic fields. On behalf of the Planck Collaboration, I will present a statistical analysis of this polarized emission from nearby molecular clouds, focusing first on the evolution of the maximum polarization fraction observed as a function of column density, and on the anti-correlation between the polarization fraction and the local dispersion of polarization angles. To interpret this data, I will present numerical simulations of anisotropic MHD turbulence and show the essential role played by the topology of the interstellar magnetic field, in particular its large-scale component. To extend the analysis beyond the polarization fraction and angle, I will present the one-point and two-point statistics of Stokes Q and U, showing in particular how changes in magnetic field direction lead to multimodality in the (Q,U) distributions. Finally, using the results of a numerical experiment with fractional Brownian motion (fBm) fields, I will exhibit the observables best suited to retrieve the spectral index of the turbulent component of the interstellar magnetic field.
NASA Astrophysics Data System (ADS)
Levrier, François
2015-08-01
The Planck satellite has mapped the polarized microwave sky (from 30 GHz to 353 GHz) with unprecedented sensitivity and angular resolution. This wealth of data sheds new light on the polarization of Galactic foregrounds, especially that of thermal emission from dust grains, which dominates the cosmic microwave background (CMB) at the high end of the Planck frequency range. On behalf of the Planck Collaboration, I will present a statistical analysis of this polarized emission from nearby molecular clouds, focusing first on the evolution of the maximum polarization fraction observed as a function of column density, and on the anti-correlation between the polarization fraction and the local dispersion of polarization angles. To interpret this data, I will present numerical simulations of anisotropic MHD turbulence and show the essential role played by the topology of the interstellar magnetic field, in particular its large-scale component. To extend the analysis beyond the polarization fraction and angle, I will present the one-point and two-point statistics of Stokes Q and U, showing in particular how changes in magnetic field direction lead to multimodality in the (Q,U) distributions. Finally, using the results of a numerical experiment with fractional Brownian motion (fBm) fields, I will exhibit the observables best suited to retrieve the spectral index of the turbulent component of the interstellar magnetic field.
NASA Astrophysics Data System (ADS)
McCollough, J. P.; Elkington, S. R.; Baker, D. N.
2008-12-01
On 29 June 2007, EMIC waves were observed on the ground by the CARISMA network of magnetometers and by three THEMIS spacecraft between L=5 and L=6.5. There are two theoretical pictures for the generation for these waves: Olson and Lee [1982] suggested that such waves may be generated by an adiabatic compression of the dayside magnetosphere, which causes the particles to preferentially gain energy perpendicular to the magnetic field. This, in turn, leads to the beam instability generally thought to produce EMIC waves. Summers et al. [1998] put forward the idea that substorm-injected ring current ions driven by enhanced convection excite EMIC waves near the plasmapause. We use the LFM global MHD model to simulate the event, with upstream boundary conditions determined by observations at the ACE (Advanced Composition Explorer) spacecraft. We then use a Lorentz solver to calculate test particle trajectories, and use these results to compute phase space densities of keV protons. We examine temperature anisotropies in ring current ions and compute EMIC wave growth rates for this event. Results are compared with the observations of Usanova et al. [2008], and with results from the two competing theories.
3D Relativistic MHD Simulations of Magnetized Spine-Sheath Relativistic Jets
NASA Technical Reports Server (NTRS)
Mizuno, Yosuke; Hardee, Philip E.; Nishikawa, Ken-Ichi
2007-01-01
We have performed numerical simulations of weakly and strongly magnetized relativistic jets embedded in a weakly and strongly magnetized stationary or mildly relativistic (0.5c) sheath using the RAISHIN code. In the numerical simulations a jet with Lorentz factor gamma=2.5 is precessed to break the initial equilibrium configuration. Results of the numerical simulations are compared to theoretical predictions from a normal mode-analysis of the linearized RMHD equations describing a uniform axially magnetized cylindrical relativistic jet embedded in a uniform axially magnetized moving sheath. The prediction of increased stability of a weakly-magnetized system with mildly relativistic sheath flow to Kelvin-Helmholtz instabilities and the stabilization of a strongly-magnetized system with mildly relativistic sheath flow is confirmed by the numerical simulations.
Numerical linearized MHD model of flapping oscillations
NASA Astrophysics Data System (ADS)
Korovinskiy, D. B.; Ivanov, I. B.; Semenov, V. S.; Erkaev, N. V.; Kiehas, S. A.
2016-06-01
Kink-like magnetotail flapping oscillations in a Harris-like current sheet with earthward growing normal magnetic field component Bz are studied by means of time-dependent 2D linearized MHD numerical simulations. The dispersion relation and two-dimensional eigenfunctions are obtained. The results are compared with analytical estimates of the double-gradient model, which are found to be reliable for configurations with small Bz up to values ˜ 0.05 of the lobe magnetic field. Coupled with previous results, present simulations confirm that the earthward/tailward growth direction of the Bz component acts as a switch between stable/unstable regimes of the flapping mode, while the mode dispersion curve is the same in both cases. It is confirmed that flapping oscillations may be triggered by a simple Gaussian initial perturbation of the Vz velocity.
NASA Astrophysics Data System (ADS)
Lee, Khil-Ha; Kim, Sung-Wook; Kim, Sang-Hyun
2014-05-01
model, called FLO-2D runs to simulate channel routing downstream to give the maximum water level. Once probable inundation areas are identified by the huge volume of water in the caldera lake, the unique geography, and the limited control capability, a potential hazard assessment can be represented. The study will contribute to build a geohazard map for the decision-makers and practitioners. Keywords: Volcanic flood, Caldera lake, Hazard assessment, Magma effusion Acknowledgement This research was supported by a grant [NEMA-BAEKDUSAN-2012-1-2] from the Volcanic Disaster Preparedness Research Center sponsored by National Emergency Management Agency of Korea.
Allain, Ariane; Chauvot de Beauchêne, Isaure; Langenfeld, Florent; Guarracino, Yann; Laine, Elodie; Tchertanov, Luba
2014-01-01
Allostery is a universal phenomenon that couples the information induced by a local perturbation (effector) in a protein to spatially distant regulated sites. Such an event can be described in terms of a large scale transmission of information (communication) through a dynamic coupling between structurally rigid (minimally frustrated) and plastic (locally frustrated) clusters of residues. To elaborate a rational description of allosteric coupling, we propose an original approach - MOdular NETwork Analysis (MONETA) - based on the analysis of inter-residue dynamical correlations to localize the propagation of both structural and dynamical effects of a perturbation throughout a protein structure. MONETA uses inter-residue cross-correlations and commute times computed from molecular dynamics simulations and a topological description of a protein to build a modular network representation composed of clusters of residues (dynamic segments) linked together by chains of residues (communication pathways). MONETA provides a brand new direct and simple visualization of protein allosteric communication. A GEPHI module implemented in the MONETA package allows the generation of 2D graphs of the communication network. An interactive PyMOL plugin permits drawing of the communication pathways between chosen protein fragments or residues on a 3D representation. MONETA is a powerful tool for on-the-fly display of communication networks in proteins. We applied MONETA for the analysis of communication pathways (i) between the main regulatory fragments of receptors tyrosine kinases (RTKs), KIT and CSF-1R, in the native and mutated states and (ii) in proteins STAT5 (STAT5a and STAT5b) in the phosphorylated and the unphosphorylated forms. The description of the physical support for allosteric coupling by MONETA allowed a comparison of the mechanisms of (a) constitutive activation induced by equivalent mutations in two RTKs and (b) allosteric regulation in the activated and non
MHD SIMULATIONS OF ACTIVE GALACTIC NUCLEUS JETS IN A DYNAMIC GALAXY CLUSTER MEDIUM
Mendygral, P. J.; Jones, T. W.; Dolag, K.
2012-05-10
We present a pair of three-dimensional magnetohydrodynamical simulations of intermittent jets from a central active galactic nucleus (AGN) in a galaxy cluster extracted from a high-resolution cosmological simulation. The selected cluster was chosen as an apparently relatively relaxed system, not having undergone a major merger in almost 7 Gyr. Despite this characterization and history, the intracluster medium (ICM) contains quite active 'weather'. We explore the effects of this ICM weather on the morphological evolution of the AGN jets and lobes. The orientation of the jets is different in the two simulations so that they probe different aspects of the ICM structure and dynamics. We find that even for this cluster, which can be characterized as relaxed by an observational standard, the large-scale, bulk ICM motions can significantly distort the jets and lobes. Synthetic X-ray observations of the simulations show that the jets produce complex cavity systems, while synthetic radio observations reveal bending of the jets and lobes similar to wide-angle tail radio sources. The jets are cycled on and off with a 26 Myr period using a 50% duty cycle. This leads to morphological features similar to those in 'double-double' radio galaxies. While the jet and ICM magnetic fields are generally too weak in the simulations to play a major role in the dynamics, Maxwell stresses can still become locally significant.
NASA Technical Reports Server (NTRS)
Raeder, J.; Berchem, J.; Ashour-Abdalla, M.; Frank, L. A.; Paterson, W. R.; Ackerson, K. L.; Kokubun, S.; Yamamoto, T.; Slavin, J. A.
1997-01-01
We present Geotail plasma and field observations from the middle magnetotail near X(sub GSE) = -46 R(sub E) for the time period 1400 to 1800 UT on December 14, 1994. During that period, the Wind satellite monitored the solar wind plasma and interplanetary magnetic field (IMF) upstream of the bow shock. The IMF was northward and the plasma parameters near average. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the plasma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. Comparison of the simulation results with the observational data show very good overall agreement of the magnitudes of the plasma and field parameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal, variations are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which is formed by magnetic magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotail is located very close to the TFBL and may have entered the TFBL proper. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF conditions.
A Global 3D Radiation MHD Simulation of Super-Eddington Accretion Disks
NASA Astrophysics Data System (ADS)
Jiang, Yanfei
2014-10-01
We study how black holes can accrete above the Eddington limit using a global three dimensional radiation magneto-hydrodynamic simulation without ad-hoc assumptions. The simulation reaches an accretion rate ~ 220L_Edd/c^2 and forms a radiation driven outflow along the rotation axis. The radiative luminosity of this flow is ~ 10L_Edd. This yields a radiative efficiency ~ 4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in slim disk model, which neglect vertical advection. The results have important implications for the growth of supermassive black holes in the early universe, tidal disruption events and ultra-luminous X-ray sources.
3D MHD Simulations of accreting neutron stars: evidence of QPO emission from the surface
Bachetti, Matteo; Burderi, Luciano; Romanova, Marina M.; Kulkarni, Akshay; Salvo, Tiziana di
2010-07-15
3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rate M. Moreover, in some cases double QPOs appear, each of them showing the same correlation with M.
SIMULATIONS OF 2D AND 3D THERMOCAPILLARY FLOWS BY A LEAST-SQUARES FINITE ELEMENT METHOD. (R825200)
Numerical results for time-dependent 2D and 3D thermocapillary flows are presented in this work. The numerical algorithm is based on the Crank-Nicolson scheme for time integration, Newton's method for linearization, and a least-squares finite element method, together with a matri...
Numerical simulation of MHD for electromagnetic edge dam in continuous casting.
Chang, F. C.
1999-03-30
A computer model was developed to predict eddy currents and fluid flows in molten steel. The model was verified by comparing predictions with experimental results of liquid-metal containment and fluid flow in electromagnetic (EM) edge dams (EMDs) designed at Inland Steel for twin-roll casting. The model can optimize the EMD design so it is suitable for application, and minimize expensive, time-consuming full-scale testing. Numerical simulation was performed by coupling a three-dimensional (3-D) finite-element EM code (ELEKTRA) and a 3-D finite-difference fluids code (CaPS-EM) to solve heat transfer, fluid flow, and turbulence transport in a casting process that involves EM fields. ELEKTRA is able to predict the eddy-current distribution and the electromagnetic forces in complex geometries. CaPS-EM is capable of modeling fluid flows with free surfaces. Results of the numerical simulation compared measurements obtained from a static test.
CYCLIC THERMAL SIGNATURE IN A GLOBAL MHD SIMULATION OF SOLAR CONVECTION
Cossette, Jean-Francois; Charbonneau, Paul; Smolarkiewicz, Piotr K.
2013-11-10
Global magnetohydrodynamical simulations of the solar convection zone have recently achieved cyclic large-scale axisymmetric magnetic fields undergoing polarity reversals on a decadal time scale. In this Letter, we show that these simulations also display a thermal convective luminosity that varies in-phase with the magnetic cycle, and trace this modulation to deep-seated magnetically mediated changes in convective flow patterns. Within the context of the ongoing debate on the physical origin of the observed 11 yr variations in total solar irradiance, such a signature supports the thesis according to which all, or part, of the variations on decadal time scales and longer could be attributed to a global modulation of the Sun's internal thermal structure by magnetic activity.
Evolution of cosmic filaments and of their galaxy population from MHD cosmological simulations
NASA Astrophysics Data System (ADS)
Gheller, C.; Vazza, F.; Brüggen, M.; Alpaslan, M.; Holwerda, B. W.; Hopkins, A. M.; Liske, J.
2016-10-01
Despite containing about a half of the total matter in the Universe, at most wavelengths the filamentary structure of the cosmic web is difficult to observe. In this work, we use large unigrid cosmological simulations to investigate how the geometrical, thermodynamical and magnetic properties of cosmological filaments vary with mass and redshift (z ≤ 1). We find that the average temperature, length, volume and magnetic field of filaments scales well with their total mass. This reflects the role of self-gravity in shaping their properties and enables statistical predictions of their observational properties based on their mass. We also focus on the properties of the simulated population of galaxy-sized haloes within filaments, and compare their properties to the results obtained from the spectroscopic GAMA survey. Simulated and observed filaments with the same length are found to contain an equal number of galaxies, with very similar distribution of masses. The total number of galaxies within each filament and the total/average stellar mass in galaxies can now be used to predict also the large-scale properties of the gas in the host filaments across tens or hundreds of Mpc in scale. These results are the first steps towards the future use of galaxy catalogues in order to select the best targets for observations of the warm-hot intergalactic medium.
NASA Astrophysics Data System (ADS)
Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.
2007-05-01
Aims:We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere. Methods: To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux. Results: We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of 1019 Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux. Movies are only available in electronic form at http://www.aanda.org
NASA Astrophysics Data System (ADS)
Rajendar, A.; Paty, C. S.; Arridge, C. S.; Jackman, C. M.; Smith, H. T.
2013-12-01
Saturn's magnetosphere is driven externally, by the solar wind, and internally, by the planet's strong magnetic field, rapid rotation rate, and the addition of new plasma created from Saturn's neutral cloud. Externally, the alignment of the rotational and magnetic dipole axes, combined with Saturn's substantial inclination to its plane of orbit result in substantial curvature of the plasma sheet during solstice. Internally, new water group ions are produced in the inner regions of the magnetosphere from photoionization and electron-impact ionization of the water vapor and OH cloud sourced from Enceladus and other icy bodies in Saturn's planetary system. In addition to this, charge-exchange collisions between the relatively fast-moving water group ions and the slower neutrals results in a net loss of momentum from the plasma. In order to study these phenomena, we have made significant modifications to the Saturn multifluid model. This model has been previously used to investigate the external triggering of plasmoids and the interchange process using a fixed internal source rate. In order to improve the fidelity of the model, we have incorporated a physical source of mass- and momentum-loading by including an empirical representation of Saturn's neutral cloud and modifying the multifluid MHD equations to include mass- and momentum-loading terms. Collision cross-sections between ions, electrons, and neutrals are calculated as functions of closure velocity and energy at each grid point and time step, enabling us to simulate the spatially and temporally varying plasma-neutral interactions. In addition to this, by altering the angle of incidence of the solar wind relative to Saturn's rotational axis and applying a realistic latitudinally- and seasonally-varying ionospheric conductivity, we are also able to study seasonal effects on Saturn's magnetosphere. We use the updated multifluid simulation to investigate the dynamics of Saturn's magnetosphere, focusing specifically
Analysis of Helicities and Hall and MHD Dynamo Effects in Two-Fluid Reversed-Field Pinch Simulations
NASA Astrophysics Data System (ADS)
Sauppe, Joshua; Sovinec, Carl
2015-11-01
Relaxation in the RFP is studied numerically with extended-MHD modeling that includes the Hall term and ion gyroviscous stress. Previous results show significant coupling between magnetic relaxation and parallel flow evolution [King PoP 19, 055905]. Computations presented here display quasi-periodic relaxation events with current relaxation through MHD and Hall dynamo drives. The MHD dynamo always relaxes currents while the Hall dynamo may add or subtract from it, but the total dynamo drive is similar to single-fluid MHD computations. Changes in plasma momentum are due to viscous coupling to the wall and fluctuation-induced Maxwell stresses transport momentum radially inward when two-fluid effects are included. The magnetic helicity and hybrid helicity, a two-fluid extension of magnetic helicity that includes cross and kinetic helicity [Turner, 1986], are well-conserved relative to magnetic energy at each event. The cross helicity is well-conserved in single-fluid MHD but is significantly affected by both two-fluid effects and ion gyroviscosity. The plasma parallel current evolves towards the predicted flat profile; however, the plasma flow does not. Work supported through NSF grant PHY-0821899 and DOE grant DE-FG02-06ER54850.
NASA Astrophysics Data System (ADS)
DeGrave, Kyle; Braun, Douglas; Birch, Aaron; Crouch, Ashley D.; Javornik, Brenda; Rempel, Matthias D.
2016-05-01
We test and validate newly-developed, empirically-derived sensitivity kernels for use in helioseismic analysis. These kernels are based on the Born approximation and derived from applying direct measurements to artificial realizations of incoming and scattered wavefields. These kernels are employed in a series of forward and inverse modeling of flows from the near-surface layers of two publicly available magnetohydrodynamic (MURaM-based) solar simulations - a quiet-Sun simulation, and one containing a sunspot. Forward travel times computed using the kernels generally compare favorably in non-magnetic regions. One finding of note is the presence of flow-like artifacts in the sunspot measurements which appear when the spot umbra or penumbra falls within the measurement pupils. Inversions for the horizontal flow components are able to reproduce the large-scale supergranule-sized flows in the upper 3Mm of both domains, but are compromised by noise at greater depths. In spite of the magnetic artifact, the moat flow surrounding the spot is at least qualitatively recovered. This work is supported by the NASA Heliophysics Division through NNH12CF68C, NNH12CF23C, and NNX16AG88G, and by the NSF Solar-Terrestrial Program through grant AGS-1127327.
MHD properties of magnetosheath flow
NASA Astrophysics Data System (ADS)
Siscoe, G. L.; Crooker, N. U.; Erickson, G. M.; Sonnerup, B. U. Ö.; Maynard, N. C.; Schoendorf, J. A.; Siebert, K. D.; Weimer, D. R.; White, W. W.; Wilson, G. R.
2002-04-01
We discuss four aspects of magnetosheath flow that require MHD for their calculation and understanding. We illustrate these aspects with computations using a numerical MHD code that simulates the global magnetosphere and its magnetosheath. The four inherently MHD aspects of magnetosheath flow that we consider are the depletion layer, the magnetospheric sash, MHD flow deflections, and the magnetosheath's slow-mode expansion into the magnetotail. We introduce new details of these aspects or illustrate known details in a new way, including the dependence of the depletion layer on interplanetary magnetic filed clock angle; agreement between the locations of the antiparallel regions of Luhmann et al. (J. Geophys. Res. 89 (1984) 1739) and the magnetospheric sash, and deflections corresponding separately to a stagnation line and magnetic reconnection.
X-Ray Spectra from MHD Simulations of Accreting Black Holes
NASA Technical Reports Server (NTRS)
Schnittman, Jeremy D.; Noble, Scott C.; Krolik, Julian H.
2011-01-01
We present new global calculations of X-ray spectra from fully relativistic magneto-hydrodynamic (MHO) simulations of black hole (BH) accretion disks. With a self consistent radiative transfer code including Compton scattering and returning radiation, we can reproduce the predominant spectral features seen in decades of X-ray observations of stellar-mass BHs: a broad thermal peak around 1 keV, power-law continuum up to >100 keV, and a relativistically broadened iron fluorescent line. By varying the mass accretion rate, different spectral states naturally emerge: thermal-dominant, steep power-law, and low/hard. In addition to the spectral features, we briefly discuss applications to X-ray timing and polarization.
Radiation-MHD Simulations of Black Hole Accretion Flows and Outflows
NASA Astrophysics Data System (ADS)
Ohsuga, K.
2012-08-01
We perform two-dimensional radiation-magnetohydrodynamic simulations of the accretion disks, jets, and disk outflows around black holes. We can reproduce the three distinct inflow-outflow modes, which corresponds to the two-dimensional version of the slim disk model, the standard disk model, and the radiatively inefficient accretion flow, with one numerical code. In the case of the super-Eddington accretion flow, we find that a radiatively driven, magnetically collimated jet is produced around the rotation axis and that a time-dependent, clumpy outflow with larger opening angle forms. Such jet and outflow might resolve the relativistic powerful jets of the luminous compact objects and the ultra fast outflows of active galactic nuclei.
"Bursty" Reconnection Following Solar Eruptions: MHD Simulations and Comparison with Observations
NASA Technical Reports Server (NTRS)
Riley, Pete; Lionello, Roberto; Mikic, zoran; Linker, Jon; Clark, Eric; Lin, Jun; Ko, Yuan-Kuen
2007-01-01
Posteruptive arcades are frequently seen in the aftermath of coronal mass ejections (CMEs). The formation of these loops at successively higher altitudes, coupled with the classic "two-ribbon" flare seen in H-alpha, are interpreted as reconnection of the coronal magnetic field that has been dragged outward by the CME. White-light observations of "rays," which have been interpreted as being coincident with the current sheet at the reconnection site underneath the erupting CME, also provide evidence for its occurrence. "Blobs" occasionally seen within these rays suggest an even richer level of structure. In this report, we present numerical simulations that reproduce both the observed rays and the formation and evolution of the blobs. We compare their properties with SOHO/LASCO observations of similar structures, and relate their formation to standard theories of reconnection,
NASA Astrophysics Data System (ADS)
Romanova, M. M.; Ustyugova, G. V.; Koldoba, A. V.; Lovelace, R. V. E.
2013-03-01
We report results of the first global three-dimensional (3D) magnetohydrodynamic (MHD) simulations of the waves excited in an accretion disc by a rotating star with a dipole magnetic field misaligned from the star's rotation axis (which is aligned with the disc axis). The main results are the following. (1) If the magnetosphere of the star corotates approximately with the inner disc, then we observe a strong one-armed bending wave (a warp). This warp corotates with the star and has a maximum amplitude between corotation radius and the radius of the vertical resonance. The disc's centre of mass can deviate from the equatorial plane up to the distance of zw ≈ 0.1r. However, the effective height of the warp can be larger, hw ≈ 0.3r, due to the finite thickness of the disc. Stars with a range of misalignment angles excite warps. However, the amplitude of the warps is larger for misalignment angles between 15° and 60°. The location and amplitude of the warp do not depend on viscosity, at least for relatively small values of the standard alpha-parameter, up to 0.08. (2) If the magnetosphere rotates slower than the inner disc, then a bending wave is excited at the disc-magnetosphere boundary, but does not form a large-scale warp. Instead, persistent, high-frequency oscillations become strong at the inner region of the disc. These are (a) trapped density waves which form inside the radius where the disc angular velocity has a maximum, and (b) inner bending waves which appear in the case of accretion through magnetic Rayleigh-Taylor instability. These two types of waves are connected with the inner disc and their frequencies will vary with accretion rate. Bending oscillations at lower frequencies are also excited including global oscillations of the disc. In cases where the simulation region is small, slowly precessing warp forms with the maximum amplitude at the vertical resonance. The present simulations are applicable to young stars, cataclysmic variables and
NASA Technical Reports Server (NTRS)
Fairfield, Donald H.; Otto, A.
1999-01-01
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.
Newtonian CAFE: a new ideal MHD code to study the solar atmosphere
NASA Astrophysics Data System (ADS)
González, J. J.; Guzmán, F.
2015-12-01
In this work we present a new independent code designed to solve the equations of classical ideal magnetohydrodynamics (MHD) in three dimensions, submitted to a constant gravitational field. The purpose of the code centers on the analysis of solar phenomena within the photosphere-corona region. In special the code is capable to simulate the propagation of impulsively generated linear and non-linear MHD waves in the non-isothermal solar atmosphere. We present 1D and 2D standard tests to demonstrate the quality of the numerical results obtained with our code. As 3D tests we present the propagation of MHD-gravity waves and vortices in the solar atmosphere. The code is based on high-resolution shock-capturing methods, uses the HLLE flux formula combined with Minmod, MC and WENO5 reconstructors. The divergence free magnetic field constraint is controlled using the Flux Constrained Transport method.
X-Ray Spectra from MHD Simulations of Accreting Black Holes
NASA Technical Reports Server (NTRS)
Schnittman, Jeremy D.; Krolik, Julian H.; Noble, Scott C.
2012-01-01
We present the results of a new global radiation transport code coupled to a general relativistic magneto-hydrodynamic simulation of an accreting, nonrotating black hole. For the first time, we are able to explain from first principles in a self-consistent way the X-ray spectra observed from stellar-mass black holes, including a thermal peak, Compton reflection hump, power-law tail, and broad iron line. Varying only the mass accretion rate, we are able to reproduce the low/hard, steep power-law, and thermal-dominant states seen in most galactic black hole sources. The temperature in the corona is T(sub e) 10 keV in a boundary layer near the disk and rises smoothly to T(sub e) greater than or approximately 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to approximately equal to 6M as the accretion rate decreases, we find that the shape of the Fe Ka line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.
MHD Simulations of Spherical Tori and Compact Stellarators Using M3D
NASA Astrophysics Data System (ADS)
Strauss, H. R.; Park, W.; Tang, X.; Fu, G. Y.; Sugiyama, L.
1999-11-01
The M3D++ unstructured mesh version of the M3D code is being applied to low aspect ratio spherical torus disruption simulations. In addition to spherical tokamaks (ST), similar to NSTX, we also study spherical pinches (SP). It is possible to produce SP equilibria with 1 > q > 1/2, and toroidal current decreasing to zero at the wall. These SP equilibria have reverse magnetic shear except in a narrow layer near the wall. The equilibria are free of the global m=1 modes that plague low q pinches such as RFPs. However there can be unstable (m,n) = (2,3) modes. The M3D++ code has been extended to have the capability of using a 3D mesh in configuration space, suitable for stellarator equilibrium, stability, and nonlinear studies, including resistive effects. Equilibria can be initialized with VMEC output or generated from initial data. Applications to compact stellarator configurations such as the PPPL quasi axisymmetric design will be presented.
Interpretation of solar irradiance monitor measurements through analysis of 3D MHD simulations
Criscuoli, S.; Uitenbroek, H.
2014-06-20
Measurements from the Spectral Irradiance Monitor (SIM) on board the Solar Radiation and Climate Experiment mission indicate that solar spectral irradiance at visible and IR wavelengths varies in counter phase with the solar activity cycle. The sign of these variations is not reproduced by most of the irradiance reconstruction techniques based on variations of surface magnetism employed so far, and it is not yet clear whether SIM calibration procedures need to be improved or if instead new physical mechanisms must be invoked to explain such variations. We employ three-dimensional magnetohydrodynamic simulations of the solar photosphere to investigate the dependence of solar radiance in SIM visible and IR spectral ranges on variations of the filling factor of surface magnetic fields. We find that the contribution of magnetic features to solar radiance is strongly dependent on the location on the disk of the features, which are negative close to disk center and positive toward the limb. If features are homogeneously distributed over a region around the equator (activity belt), then their contribution to irradiance is positive with respect to the contribution of HD snapshots, but decreases with the increase of their magnetic flux for average magnetic flux larger than 50 G in at least two of the visible and IR spectral bands monitored by SIM. Under the assumption that the 50 G snapshots are representative of quiet-Sun regions, we thus find that the Spectral Irradiance can be in counter-phase with the solar magnetic activity cycle.
IRIS observations and MHD simulations of explosive events in the transition region of the Sun
NASA Astrophysics Data System (ADS)
Guo, Lijia; Innes, Davina; Huang, Yi-Min; Bhattacharjee, Amitava
2016-05-01
Small-scale explosive events on the Sun are thought to be related to magnetic reconnection. While Petschek reconnection has been considered as a reconnection mechanism for explosive events on the Sun for quite a long time, the fragmentation of a current sheet in the high-Lundquist-number regime caused by the plasmoid instability has recently been proposed as a possible mechanism for fast reconnection. The actual reconnection sites are too small to be resolved with images but these reconnection mechanisms, Petschek and the plasmoid instability, have very different density and velocity structures and so can be distinguished by high-resolution line profiles observations. We use high-resolution sit-and-stare spectral observations of the Si IV line, obtained by the IRIS spectrometer, to identify sites of reconnection, and follow the development of line profiles. The aim is to obtain a survey of typical line profiles produced by small-scale reconnection events in the transition region and compare them with synthetic line profiles from numerical simulations of a reconnecting current sheet to determine whether reconnection occurs via the plasmoid instabilty or the Petschek mechanism. Direct comparison between IRIS observations and numerical results suggests that the observed Si IV profiles can be reproduced with a fragmented current layer subject to plasmoid instability but not by bi-directional jets that characterise the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.
NASA Technical Reports Server (NTRS)
Tao, W.-K.; Shie, C.-H.; Simpson, J.; Starr, D.; Johnson, D.; Sud, Y.
2003-01-01
Real clouds and clouds systems are inherently three dimensional (3D). Because of the limitations in computer resources, however, most cloud-resolving models (CRMs) today are still two-dimensional (2D). A few 3D CRMs have been used to study the response of clouds to large-scale forcing. In these 3D simulations, the model domain was small, and the integration time was 6 hours. Only recently have 3D experiments been performed for multi-day periods for tropical cloud system with large horizontal domains at the National Center for Atmospheric Research. The results indicate that surface precipitation and latent heating profiles are very similar between the 2D and 3D simulations of these same cases. The reason for the strong similarity between the 2D and 3D CRM simulations is that the observed large-scale advective tendencies of potential temperature, water vapor mixing ratio, and horizontal momentum were used as the main forcing in both the 2D and 3D models. Interestingly, the 2D and 3D versions of the CRM used in CSU and U.K. Met Office showed significant differences in the rainfall and cloud statistics for three ARM cases. The major objectives of this project are to calculate and axamine: (1)the surface energy and water budgets, (2) the precipitation processes in the convective and stratiform regions, (3) the cloud upward and downward mass fluxes in the convective and stratiform regions; (4) cloud characteristics such as size, updraft intensity and lifetime, and (5) the entrainment and detrainment rates associated with clouds and cloud systems that developed in TOGA COARE, GATE, SCSMEX, ARM and KWAJEX. Of special note is that the analyzed (model generated) data sets are all produced by the same current version of the GCE model, i.e. consistent model physics and configurations. Trajectory analyse and inert tracer calculation will be conducted to identify the differences and similarities in the organization of convection between simulated 2D and 3D cloud systems.
NASA Astrophysics Data System (ADS)
Bilski, Bartosz; Frenner, Karsten; Osten, Wolfgang
2010-05-01
Scatterometry is a method commonly used in semiconductor metrology for measuring critical dimension (CD). It relies on measurement of light diffracted on a periodic structure and using it to derive the actual profile by running complex simulations. As CD is getting smaller with next lithography nodes, the Line-Edge Roughness/Line Width Roughness (LER/LWR) are becoming significant fraction of its overall size - therefore there is a need to include them in the simulations. Simulation of the LER/LWR's influence, in its random nature, calls for simulating relatively large fields. These large fields, if treated with rigorous electromagnetic simulations, are either very time-extensive or impossible to conduct, therefore computationally bearable, approximate approach needs to be applied. Our approximate method is "Field-Stitching Method" (FSM). We present its 2D version with varying parameter called "overlap region". We simulate the line grating structure with CD disturbed by LER/LWR and apply Rigorous Coupled Wave Analysis (RCWA) supported by the 2D FSM. We also generate the results obtained exclusively by RCWA, to which we compare the results of the approximate approach. Based on the comparison we discuss the benefits FSM brings and its limitations.
NASA Astrophysics Data System (ADS)
Jia, Xianzhe; Slavin, James; Poh, Gangkai; Toth, Gabor; Gombosi, Tamas
2016-04-01
As the innermost planet, Mercury arguably undergoes the most direct space weathering interactions due to its weak intrinsic magnetic field and its close proximity to the Sun. It has long been suggested that two processes, i.e., erosion of the dayside magnetosphere due to intense magnetopause reconnection and the shielding effect of the induction currents generated at the conducting core, compete against each other in governing the large-scale structure of Mercury's magnetosphere. An outstanding question concerning Mercury's space weather is which of the two processes is more important. To address this question, we have developed a global MHD model in which Mercury's interior is electromagnetically coupled to the surrounding space environment. As demonstrated in Jia et al. (2015), the new modeling capability allows for self-consistently characterizing the dynamical response of the Mercury system to time-varying external conditions. To assess the relative importance of induction and magnetopause reconnection in controlling the magnetospheric configuration, especially under strong solar driving conditions, we have carried out multiple global simulations that adopt a wide range of solar wind dynamic pressure and IMF conditions. We find that, while the magnetopause standoff distance decreases with increasing solar wind pressure, just as expected, its dependence on the solar wind pressure follows closely a power-law relationship with an index of ~ -1/6, rather than a steeper power-law falling-off expected for the case with only induction present. This result suggests that for the range of solar wind conditions examined, the two competing processes, namely induction and reconnection, appear to play equally important roles in determining the global configuration of Mercury's magnetosphere, consistent with the finding obtained by Slavin et al. (2014) based on MESSENGER observations. We also find that the magnetic perturbations produced by the magnetospheric current systems
NASA Astrophysics Data System (ADS)
Ito, Y.; Noborio, K.
2015-12-01
In Japan, soil disinfection with hot water has been popular since the use of methyl bromide was restricted in 2005. Decreasing the amount of hot water applied may make farmers reduce the operation cost. To determine the appropriate amount of hot water needed for soil disinfection, HYDRUS-2D was evaluated. A field experiment was conducted and soil water content and soil temperature were measured at 5, 10, 20, 40, 60, 80 and 100 cm deep when 95oC hot water was applied. Irrigation tubing equipped with drippers every 30 cm were laid at the soil surface, z=0 cm. An irrigation rate for each dripper was 0.83 cm min-1 between t=0 and 120 min, and thereafter it was zero. Temperature of irrigation water was 95oC. Total simulation time with HYDRUS-2D was 720 min for a homogeneous soil. A simulating domain was selected as x=60 cm and z=100 cm. A potential evaporation rate was assumed to be 0 cm min-1 because the soil surface was covered with a plastic sheet. The boundary condition at the bottom was free drainage and those of both sides were no-flux conditions. Hydraulic properties and bulk densities measured at each depth were used for simulation. It was assumed that there was no organic matter contained. Soil thermal properties were adopted from previous study and HYDRUS 2D. Simulated temperatures at 5, 10, 20 and 40 cm deep agreed well with those measured although simulated temperatures at 60, 80, and 100 cm deep were overly estimated. Estimates of volumetric water content at 5 cm deep agreed well with measured values. Simulated values at 10 to 100 cm deep were overly estimated by 0.1 to 0.3 (m3 m-3). The deeper the soil became, the more the simulated wetting front lagged behind the measured one. It was speculated that water viscosity estimated smaller at high temperature might attributed to the slower advances of wetting front simulated with HYDRUS 2-D.
NASA Astrophysics Data System (ADS)
Kim, Kyeong-Hyeon; Kim, Dong-Su; Kim, Tae-Ho; Kang, Seong-Hee; Cho, Min-Seok; Suh, Tae Suk
2015-11-01
The phantom-alignment error is one of the factors affecting delivery quality assurance (QA) accuracy in intensity-modulated radiation therapy (IMRT). Accordingly, a possibility of inadequate use of spatial information in gamma evaluation may exist for patient-specific IMRT QA. The influence of the phantom-alignment error on gamma evaluation can be demonstrated experimentally by using the gamma passing rate and the gamma value. However, such experimental methods have a limitation regarding the intrinsic verification of the influence of the phantom set-up error because experimentally measuring the phantom-alignment error accurately is impossible. To overcome this limitation, we aimed to verify the effect of the phantom set-up error within the gamma evaluation formula by using a Monte Carlo simulation. Artificial phantom set-up errors were simulated, and the concept of the true point (TP) was used to represent the actual coordinates of the measurement point for the mathematical modeling of these effects on the gamma. Using dose distributions acquired from the Monte Carlo simulation, performed gamma evaluations in 2D and 3D. The results of the gamma evaluations and the dose difference at the TP were classified to verify the degrees of dose reflection at the TP. The 2D and the 3D gamma errors were defined by comparing gamma values between the case of the imposed phantom set-up error and the TP in order to investigate the effect of the set-up error on the gamma value. According to the results for gamma errors, the 3D gamma evaluation reflected the dose at the TP better than the 2D one. Moreover, the gamma passing rates were higher for 3D than for 2D, as is widely known. Thus, the 3D gamma evaluation can increase the precision of patient-specific IMRT QA by applying stringent acceptance criteria and setting a reasonable action level for the 3D gamma passing rate.
NASA Astrophysics Data System (ADS)
Fan, Yuhong
2016-05-01
We present a three-dimensional MHD simulation of the initiation of the coronal mass ejection (CME) on 13 December 2006 in the emerging δ-sunspot active region NOAA 10930. The simulation shows that the CME can result from the emergence of a east-west oriented twist flux rope whose positive, following emerging pole corresponds to the observed positive rotating sunspot emerging along the southern edge of the dominant pre-existing negative sunspot. The erupting flux rope resulting in the simulation accelerates to a terminal speed that exceeds 1500 km/s and undergoes a counter-clockwise rotation of nearly 180 degrees in the early phase of the eruption, such that its front and flanks all exhibit southward directed magnetic fields, explaining the observed southward magnetic field in the magnetic cloud impacting the Earth. Our simulation also demonstrates that the source active region is capable of repeated eruptions as a result of continued flux emergence.
NASA Astrophysics Data System (ADS)
Wang, Jin; Ma, Jianyong; Zhou, Changhe
2014-11-01
A 3×3 high divergent 2D-grating with period of 3.842μm at wavelength of 850nm under normal incidence is designed and fabricated in this paper. This high divergent 2D-grating is designed by the vector theory. The Rigorous Coupled Wave Analysis (RCWA) in association with the simulated annealing (SA) is adopted to calculate and optimize this 2D-grating.The properties of this grating are also investigated by the RCWA. The diffraction angles are more than 10 degrees in the whole wavelength band, which are bigger than the traditional 2D-grating. In addition, the small period of grating increases the difficulties of fabrication. So we fabricate the 2D-gratings by direct laser writing (DLW) instead of traditional manufacturing method. Then the method of ICP etching is used to obtain the high divergent 2D-grating.
NASA Technical Reports Server (NTRS)
Montgomery, David
1988-01-01
Three areas of study in MHD turbulence are considered. These are the turbulent relaxation of the toroidal Z pinch, density fluctuations in MHD fluids, and MHD cellular automata. A Boolean computer game that updates a cellular representation in parallel and that has macroscopic averages converging to solutions of the two-dimensional MHD equations is discussed.
NASA Astrophysics Data System (ADS)
Hoelzl, M.; Huijsmans, G. T. A.; Merkel, P.; Atanasiu, C.; Lackner, K.; Nardon, E.; Aleynikova, K.; Liu, F.; Strumberger, E.; McAdams, R.; Chapman, I.; Fil, A.
2014-11-01
The dynamics of large scale plasma instabilities can be strongly influenced by the mutual interaction with currents flowing in conducting vessel structures. Especially eddy currents caused by time-varying magnetic perturbations and halo currents flowing directly from the plasma into the walls are important. The relevance of a resistive wall model is directly evident for Resistive Wall Modes (RWMs) or Vertical Displacement Events (VDEs). However, also the linear and non-linear properties of most other large-scale instabilities may be influenced significantly by the interaction with currents in conducting structures near the plasma. The understanding of halo currents arising during disruptions and VDEs, which are a serious concern for ITER as they may lead to strong asymmetric forces on vessel structures, could also benefit strongly from these non-linear modeling capabilities. Modeling the plasma dynamics and its interaction with wall currents requires solving the magneto-hydrodynamic (MHD) equations in realistic toroidal X-point geometry consistently coupled with a model for the vacuum region and the resistive conducting structures. With this in mind, the non-linear finite element MHD code JOREK [1, 2] has been coupled [3] with the resistive wall code STARWALL [4], which allows us to include the effects of eddy currents in 3D conducting structures in non-linear MHD simulations. This article summarizes the capabilities of the coupled JOREK-STARWALL system and presents benchmark results as well as first applications to non-linear simulations of RWMs, VDEs, disruptions triggered by massive gas injection, and Quiescent H-Mode. As an outlook, the perspectives for extending the model to halo currents are described.
NASA Astrophysics Data System (ADS)
Wendling, A.; Daniel, J. L.; Hivet, G.; Vidal-Sallé, E.; Boisse, P.
2015-12-01
Numerical simulation is a powerful tool to predict the mechanical behavior and the feasibility of composite parts. Among the available numerical approaches, as far as woven reinforced composites are concerned, 3D finite element simulation at the mesoscopic scale leads to a good compromise between realism and complexity. At this scale, the fibrous reinforcement is modeled by an interlacement of yarns assumed to be homogeneous that have to be accurately represented. Among the numerous issues induced by these simulations, the first one consists in providing a representative meshed geometrical model of the unit cell at the mesoscopic scale. The second one consists in enabling a fast data input in the finite element software (contacts definition, boundary conditions, elements reorientation, etc.) so as to obtain results within reasonable time. Based on parameterized 3D CAD modeling tool of unit-cells of dry fabrics already developed, this paper presents an efficient strategy which permits an automated meshing of the models with 3D hexahedral elements and to accelerate of several orders of magnitude the simulation data input. Finally, the overall modeling strategy is illustrated by examples of finite element simulation of the mechanical behavior of fabrics.
NASA Technical Reports Server (NTRS)
Scalapino, D. J.; Sugar, R. L.; White, S. R.; Bickers, N. E.; Scalettar, R. T.
1989-01-01
Numerical simulations on the half-filled three-dimensional Hubbard model clearly show the onset of Neel order. Simulations of the two-dimensional electron-phonon Holstein model show the competition between the formation of a Peierls-CDW state and a superconducting state. However, the behavior of the partly filled two-dimensional Hubbard model is more difficult to determine. At half-filling, the antiferromagnetic correlations grow as T is reduced. Doping away from half-filling suppresses these correlations, and it is found that there is a weak attractive pairing interaction in the d-wave channel. However, the strength of the pair field susceptibility is weak at the temperatures and lattice sizes that have been simulated, and the nature of the low-temperature state of the nearly half-filled Hubbard model remains open.
NASA Astrophysics Data System (ADS)
Suzuki, Akihiro; Maeda, Keiichi; Shigeyama, Toshikazu
2016-07-01
A two-dimensional special relativistic radiation-hydrodynamics code is developed and applied to numerical simulations of supernova shock breakout in bipolar explosions of a blue supergiant. Our calculations successfully simulate the dynamical evolution of a blast wave in the star and its emergence from the surface. Results of the model with spherical energy deposition show a good agreement with previous simulations. Furthermore, we calculate several models with bipolar energy deposition and compare their results with the spherically symmetric model. The bolometric light curves of the shock breakout emission are calculated by a ray-tracing method. Our radiation-hydrodynamic models indicate that the early part of the shock breakout emission can be used to probe the geometry of the blast wave produced as a result of the gravitational collapse of the iron core.
NASA Astrophysics Data System (ADS)
Kononenko, O.; Lopes, N. C.; Cole, J. M.; Kamperidis, C.; Mangles, S. P. D.; Najmudin, Z.; Osterhoff, J.; Poder, K.; Rusby, D.; Symes, D. R.; Warwick, J.; Wood, J. C.; Palmer, C. A. J.
2016-09-01
In this work, two-dimensional (2D) hydrodynamic simulations of a variable length gas cell were performed using the open source fluid code OpenFOAM. The gas cell was designed to study controlled injection of electrons into a laser-driven wakefield at the Astra Gemini laser facility. The target consists of two compartments: an accelerator and an injector section connected via an aperture. A sharp transition between the peak and plateau density regions in the injector and accelerator compartments, respectively, was observed in simulations with various inlet pressures. The fluid simulations indicate that the length of the down-ramp connecting the sections depends on the aperture diameter, as does the density drop outside the entrance and the exit cones. Further studies showed, that increasing the inlet pressure leads to turbulence and strong fluctuations in density along the axial profile during target filling, and consequently, is expected to negatively impact the accelerator stability.
NASA Astrophysics Data System (ADS)
Chen, J.-C.; Chuang, M.-R.; Jeng, C.-J.; Wang, J.-S.
2012-04-01
Taiwan is an island located in the subtropical zone where typhoons often bring heavy rainfall. Heavy rainfall, stream having steep slope, and weak geological condition resulted in a high susceptibility to debris flow. Especially, Typhoon Morakot struck southern Taiwan on August 8, 2009 with high rainfall intensity and accumulated rainfall as high as 2860 mm for 72 hours. Severe landslides and debris flow hazards were induced. In this work, debris-flow events caused by Typhoon Morakot in Shinfa Village of Liouguei District, where resulted in severe impacts to local communities, in southern Taiwan were selected for case study. A two-dimensional model (FLO-2D software) was used to simulate a debris flow, and the accuracy of the simulation, including flow depth, velocity, sediment, and inundation area, was analyzed in the case study. This study consists of three phases. In the first phase, debris flow data, including information on topography, rainfall and rheological parameters were compiled to establish a database of factors that influence debris flow. For the second phase, a numerical simulation was performed using FLO-2D with the results presented as area of debris-flow inundation, maximum deposit depth, and deposit volume. The simulation results were then compared with the aerial photos and the micro geomorphological study. Finally, suitable conditions for using this model and reasonable parameters needed for simulation are presented. In this study, parameters and processes needed for a numerical simulation method for debris flow routing and depositions are formulated to provide a reference for hazard zone mapping or debris-flow hazard mitigation.
Chukalovsky, A. A.; Rakhimova, T. V.; Klopovsky, K. S.; Mankelevich, Yu. A.; Proshina, O. V.
2011-03-15
The kinetic processes occurring in an electric-discharge oxygen-iodine laser are analyzed with the help of a 2D (r, z) gasdynamic model taking into account transport of excited oxygen, singlet oxygen, and radicals from the electric discharge and their mixing with the iodine-containing gas. The main processes affecting the dynamics of the gas temperature and gain are revealed. The simulation results obtained using the 2D model agree well with the experimental data on the mixture gain. A subsonic oxygen-iodine laser in which singlet oxygen is generated by a 350 W transverse RF discharge excited in an oxygen flow at a pressure P = 10 Torr and the discharge tube wall is covered with mercury oxide is simulated. The simulated mixing system is optimized in terms of the flow rate and the degree of preliminary dissociation of the iodine flow. The optimal regime of continuous operation of a subsonic electric-discharge oxygen-iodine laser is found.
System analysis of high performance MHD systems
Chang, S.L.; Berry, G.F.; Hu, N.
1988-01-01
This paper presents the results of an investigation on the upper ranges of performance that an MHD power plant using advanced technology assumptions might achieve and a parametric study on the key variables affecting this high performance. To simulate a high performance MHD power plant and conduct a parametric study, the Systems Analysis Language Translator (SALT) code developed at Argonne National Laboratory was used. The parametric study results indicate that the overall efficiency of an MHD power plant can be further increased subject to the improvement of some key variables such as, the MHD generator inverter efficiency, channel electrical loading factor, magnetic field strength, preheated air temperature, and combustor heat loss. In an optimization calculation, the simulated high performance MHD power plant using advanced technology assumptions can attain an ultra high overall efficiency, exceeding 62%. 12 refs., 5 figs., 4 tabs.
FLIP MHD - A particle-in-cell method for magnetohydrodynamics
NASA Technical Reports Server (NTRS)
Brackbill, J. U.
1991-01-01
The fluid-implicit-particle, or 'FLIP' method presently extended to 2D and 3D MHD flow incorporates a Lagrangian field representation and yields a grid magnetic Reynolds number of up to 16 while preserving contact continuities that retain the Galilean invariance of the MHD flow equations. Analytical arguments and numerical examples demonstrate the conservation of mass, momentum, magnetic flux, and energy; 2D calculation results for the illustrative cases of contact discontinuity convection, Rayleigh-Taylor unstable flow.
Fan, D.; Geng, C.; Chen, L.Q.
1997-03-01
The local kinetics and topological phenomena during normal grain growth were studied in two dimensions by computer simulations employing a continuum diffuse-interface field model. The relationships between topological class and individual grain growth kinetics were examined, and compared with results obtained previously from analytical theories, experimental results and Monte Carlo simulations. It was shown that both the grain-size and grain-shape (side) distributions are time-invariant and the linear relationship between the mean radii of individual grains and topological class n was reproduced. The moments of the shape distribution were determined, and the differences among the data from soap froth. Potts model and the present simulation were discussed. In the limit when the grain size goes to zero, the average number of grain edges per grain is shown to be between 4 and 5, implying the direct vanishing of 4- and 5-sided grains, which seems to be consistent with recent experimental observations on thin films. Based on the simulation results, the conditions for the applicability of the familiar Mullins-Von Neumann law and the Hillert`s equation were discussed.
User`s guide for the casting process simulator software CaPS-2D, Version 1.0
Domanus, H.M.; Schmitt, R.C.; Ahuja, S.
1993-07-01
Most casting defects occur during initial pouring and therefore the design of the running system, which guides the metal from the ladle into the mold, is crucial. Traditionally, the running system and mold filling are designed by trial and error, which is tedious, time consuming. and expensive. The uncertainties that remain can be overcome by a computer simulation that demonstrates the actual process of mold filling and subsequent solidification. Computer simulation of various processes has become more and more common in recent years. The cost-effectiveness of making flawless castings has made the foundry worker more aware of the process of mold filling, identification of hot spots, etc. The macroscopic Casting Process Simulator (CaPS) software combines heat transfer and fluid flow aspects and can describe a variety of solidification aspects, including mold filling. CaPS is a two-dimensional time-dependent computer code involving a finite-volume formulation for the mass, momentum. and energy equations. CaPS has the following characteristics: CaPS uses the PATRAN geometric modeling package for constructing the geometry, generating a neutral file consisting of a list of named components, and post-processing of the simulation results; building the geometry independently of the mesh is a time-saving procedure. A structured mesh generator of structured regular cells is included and is interfaced with the neutral-file output of the solid geometric package. Visual user interfaces have been developed on the basis of the HOOPS package, which contains a hierarchical database of geometric information. The CaPS shell scripts interactively provide a step-by-step procedure to simulate the solidification process, thus making the software very user-friendly.
NASA Astrophysics Data System (ADS)
Tanaka, T.
1994-04-01
A three-dimensional (3D) high-resolution magnetohydrodynamic (MHD) simulation scheme on an unstructured grid system is developed for inhomogeneous systems, including strong background potential fields. The scheme is based on the finite volume method (FVM) with an upwinding numerical flux by the linearized Riemann solver. Upwindings on an unstructured grid system are realized from the fact that the MHD equations are symmetric with the rotation of the space. The equation system is modified to avoid direct inclusions of the background potential field as a dependent variable, through the use of changed dependent variables. Despite such a change of the equation system, the eigenvectors in the mode-synthesis matrix that are necessary for the evaluation of the upwinding numerical flux vectors can still be written analytically. The eigenvalues of the MHD flux Jacobian matrix that are also necessary for the upwinding calculations are derived from the well-known Alfven, fast and slow, velocities. The calculations of the eigenvectors is done with special care when the wave propagations become parallel or perpendicular to the ambient magnetic field, because degeneration of the eigenvalues occurs in these cases. To obtain a higher order of accuracy, the upwinding flux is extended to the second-order TVD numerical flux in the calculation of FVM, through the MUSCL approach and Van Leer's differentiable limiter. In order to show the efficiency of the above scheme, a numerical example is given for the interaction process of high-beta supersonic plasma flow with the region of a strong dipole field, including magnetized low-beta plasma.
3D numerical simulation of laser-generated Lamb waves propagation in 2D acoustic black holes
NASA Astrophysics Data System (ADS)
Yan, Shiling; Lomonosov, Alexey M.; Shen, Zhonghua; Han, Bing
2015-05-01
Acoustic black holes have been widely used in damping structural vibration. In this work, the Lamb waves are utilized to evaluate the specified structure. The three-dimensional numerical model of acoustic black holes with parabolic profile was established. The propagation of laser-generated Lamb wave in two-dimensional acoustic black holes was numerically simulated using the finite element method. The results indicated that the incident wave was trapped by the structure obviously.
2D hybrid simulations of super-diffusion at the magnetopause driven by Kelvin-Helmholtz instability
Cowee, Misa M; Winske, Dan; Gary, S Peter
2009-01-01
This manuscript describes the self-consistent simulation of diffusion at the magnetopause driven by Kelvin-Helmholtz (KH) instability. Two-dimensional hybrid (kinetic ions, fluid electrons) simulations of the most KH-unstable configuration where the shear flow is oriented perpendicular to the uniform magnetic field are carried out. The motion of the simulation particles are tracked during the run and their mean-square displacement normal to the magnetopause is calculated from which diffusion coefficients are determined. The diffusion coefficients are found to be time dependent, with D{sub x} {proportional_to} t{sup {alpha}}, where {alpha} > 1. Additionally, the probability distribution functions (PDF) of the 'jump lengths' the particles make over time are found to be non-gaussian. Such time-dependent diffusion coefficients and non-gaussian PDF's have been associated with so-called 'super-diffusion', in which diffusive mixing of particles is enhanced over classical diffusion. The results indicate that while turbulence associated with the break-down of vortices contributes to this enhanced diffusion, it is the growth of large-scale, coherent vortices is the more important process in facilitating it.
NASA Astrophysics Data System (ADS)
Amaya-Ventura, Gilberto; Rodríguez-Romo, Suemi
2011-09-01
This paper deals with the computational simulation of the reaction-diffusion-advection phenomena emerging in Rayleigh-Bénard (RB) and Poiseuille-Bénard reactive convection systems. We use the Boussinesq's approximation for buoyancy forces and the Lattice Boltzmann method (LBM). The first kinetic mesoscopic model proposed here is based on the discrete Boltzmann equation needed to solve the momentum balance coupled with buoyancy forces. Then, a second lattice Boltzmann algorithm is applied to solve the reaction-diffusion-advection equation to calculate the evolution of the chemical species concentration. We use a reactive system composed by nitrous oxide (so call laughing gas) in air as an example; its spatio-temporal decomposition is calculated. Two cases are considered, a rectangular enclosed cavity and an open channel. The simulations are performed at low Reynolds numbers and in a steady state between the first and second thermo-hydrodynamic instabilities. The results presented here, for the thermo-hydrodynamic behavior, are in good agreement with experimental data; while our| chemical kinetics simulation yields expected results. Some applications of our approach are related to chemical reactors and atmospheric phenomena, among others.
NASA Astrophysics Data System (ADS)
Uchino, H.; Machida, S.
2012-12-01
A physical process of the substorm triggering in the Earth's Magnetotail is thought to be closely related to the magnetic reconnection and the tearing instability. Recently we proposed a new scheme of the substorm onset called "Catapult Current Sheet Relaxation (CCSR) Model " to physically understand the results from GEOTAIL and THEMIS data. The CCSR Model has characters that are the decrease of the total pressure and thinning of the current sheet at the distance about -12Re in the magnetotail a few minutes before the substorm onset, and the simultaneous occurrence of the dipolarization at X~-10Re and the magnetic reconnection at X~-20Re at the time of the onset. In this study, we investigate a stability of the current sheet and the particle acceleration via particle simulation in order to assess the validity of the CCSR model and to clarify the mechanism of substorm onset. We give an initial magnetic field structure which is akin to the Earth's dipole magnetic field together with a stretched magnetic field by thin current sheet, and further add a weak northward magnetic field at the place where Near-Earth Neutral Line is expected to be formed. The results of simulation contain similar features that characterize the CCSR Model. A physically interpretation of the simulation result with the linear instability theory as well as comparison with observations will be given.
NASA Astrophysics Data System (ADS)
Matos, J. R.; Welty, C.; Packman, A.
2005-12-01
The main purpose of the simulations in this research is the analysis of three-dimensional surface-groundwater interchange in heterogeneous systems. The effects of channel pattern, bed forms and aquifer heterogeneity on flow interactions between stream and groundwater systems are examined in order to contribute for a better understanding of the hyporheic process. A two-dimensional approach was also adopted to allow comparisons with the three-dimensional results. The grid was designed using the correlation scales of the heterogeneous fields and the scale of the stream meanders. MODFLOW and MODPATH were used to evaluate magnitude, direction and spatial distribution of the exchange flow. PMWIN and PMPATH were used as pre and post-processors during the construction of the models and analysis of results. Gaining and losing streams as well as parallel flow and flow across streams were simulated as idealized cases intended to describe how properties of the streambed and aquifer in low-gradient lowland streams contribute to hyporheic exchange. At first a straight river was analyzed then meandering streams were created with a sine curve and variations on wavelength and amplitude. Bed forms were simulated assuming a sinusoidal distribution of pressure head in the bed surface. Aspects of the influence of bedforms on mechanisms such as "pumping" and "turnover" are expected to be addressed with simulations. Flow velocities between 20 and 40 cm/s in the channel were tested with the objective of showing the influence of river morphology and natural bed forms on the flow exchange in the hyporheic zone. Several meander cycles and four levels of hydraulic conductivity variance were analyzed. Results of flow variances along the cross-sections and wetted perimeter show the increasing on hyporheic exchange as the degree of heterogeneity increases. Particle tracking was performed to define hyporheic residence time distributions. When comparing the homogeneous fields with all degrees of
NASA Astrophysics Data System (ADS)
Alvarado, M. J.; Prinn, R. G.
2007-12-01
The growth of aerosol particles and production of ozone in young smoke plumes is the result of a complex interaction between the mean flow in the smoke plume, turbulent diffusion, gas-phase oxidation, coagulation, and mass transfer between phases. Models allow us to separate the effects of these processes and predict their impact on the global environment. We present the results of two and three-dimensional Eulerian simulations of the dynamics and chemistry of the smoke plume formed by the Timbavati savannah fire studied during SAFARI 2000 (Hobbs et al., 2003, JGR, doi:10.1029/2002JD002352). The dynamical model is an extension of an Eulerian cloud-resolving model that has previously been used to study the role of deep convective clouds on tropospheric chemistry (Wang and Prinn, 2000, JGR, 105(D17) 22,269-22,297). The model includes a source of sensible heat, gases, and particles at the surface to simulate the savannah fire. The new gas and aerosol chemistry model includes heterogeneous chemistry, kinetic mass transfer, coagulation and the formation of secondary organic and inorganic aerosol. Photolysis rates are calculated based on the solution of the radiative transfer equation within the plume, including the scattering and absorption of radiation by the smoke aerosols. Our preliminary 2D Eulerian results using standard chemistry and UV fluxes show that the model can simulate the lower but not the higher levels of O3 observed. Also, the simulated 2D O3 field shows a wave-like pattern in the downwind direction, even though the emissions from the fire are held constant. This suggests that plume heterogeneity in the downwind direction may account for some of the observed variability in O3. We will present results of runs incorporating higher resolution calculation of photolysis rates, heterogeneous HONO formation, and gas phase reactions involving the uncharacterized organic compounds observed in the gas phase of the Timbavati plume in order to better simulate these
Schaffranek, Raymond W.
2004-01-01
A numerical model for simulation of surface-water integrated flow and transport in two (horizontal-space) dimensions is documented. The model solves vertically integrated forms of the equations of mass and momentum conservation and solute transport equations for heat, salt, and constituent fluxes. An equation of state for salt balance directly couples solution of the hydrodynamic and transport equations to account for the horizontal density gradient effects of salt concentrations on flow. The model can be used to simulate the hydrodynamics, transport, and water quality of well-mixed bodies of water, such as estuaries, coastal seas, harbors, lakes, rivers, and inland waterways. The finite-difference model can be applied to geographical areas bounded by any combination of closed land or open water boundaries. The simulation program accounts for sources of internal discharges (such as tributary rivers or hydraulic outfalls), tidal flats, islands, dams, and movable flow barriers or sluices. Water-quality computations can treat reactive and (or) conservative constituents simultaneously. Input requirements include bathymetric and topographic data defining land-surface elevations, time-varying water level or flow conditions at open boundaries, and hydraulic coefficients. Optional input includes the geometry of hydraulic barriers and constituent concentrations at open boundaries. Time-dependent water level, flow, and constituent-concentration data are required for model calibration and verification. Model output consists of printed reports and digital files of numerical results in forms suitable for postprocessing by graphical software programs and (or) scientific visualization packages. The model is compatible with most mainframe, workstation, mini- and micro-computer operating systems and FORTRAN compilers. This report defines the mathematical formulation and computational features of the model, explains the solution technique and related model constraints, describes the
2D/3D quench simulation using ANSYS for epoxy impregnated Nb3Sn high field magnets
Ryuji Yamada et al.
2002-09-19
A quench program using ANSYS is developed for the high field collider magnet for three-dimensional analysis. Its computational procedure is explained. The quench program is applied to a one meter Nb{sub 3}Sn high field model magnet, which is epoxy impregnated. The quench simulation program is used to estimate the temperature and mechanical stress inside the coil as well as over the whole magnet. It is concluded that for the one meter magnet with the presented cross section and configuration, the thermal effects due to the quench is tolerable. But we need much more quench study and improvements in the design for longer magnets.
Simulation of Ultra-Small MOSFETs Using a 2-D Quantum-Corrected Drift-Diffusion Model
NASA Technical Reports Server (NTRS)
Biegel, Bryan A.; Rafferty, Conor S.; Yu, Zhiping; Dutton, Robert W.; Ancona, Mario G.; Saini, Subhash (Technical Monitor)
1998-01-01
We describe an electronic transport model and an implementation approach that respond to the challenges of device modeling for gigascale integration. We use the density-gradient (DG) transport model, which adds tunneling and quantum smoothing of carrier density profiles to the drift-diffusion model. We present the current implementation of the DG model in PROPHET, a partial differential equation solver developed by Lucent Technologies. This implementation approach permits rapid development and enhancement of models, as well as run-time modifications and model switching. We show that even in typical bulk transport devices such as P-N diodes and BJTs, DG quantum effects can significantly modify the I-V characteristics. Quantum effects are shown to be even more significant in small, surface transport devices, such as sub-0.1 micron MOSFETs. In thin-oxide MOS capacitors, we find that quantum effects may reduce gate capacitance by 25% or more. The inclusion of quantum effects in simulations dramatically improves the match between C-V simulations and measurements. Significant quantum corrections also occur in the I-V characteristics of short-channel MOSFETs due to the gate capacitance correction.
NASA Astrophysics Data System (ADS)
de Garis, Hugo; Korkin, Michael; Guttikonda, Padma; Cooley, Donald
2000-11-01
This paper presents some simulation results of the evolution of 2D visual pattern recognizers to be implemented very shortly on real hardware, namely the 'CAM-Brain Machine' (CBM), an FPGA based piece of evolvable hardware which implements a genetic algorithm (GA) to evolve a 3D cellular automata (CA) based neural network circuit module, of approximately 1,000 neurons, in about a second, i.e. a complete run of a GA, with 10,000s of circuit growths and performance evaluations. Up to 65,000 of these modules, each of which is evolved with a humanly specified function, can be downloaded into a large RAM space, and interconnected according to humanly specified gvdvips -o SPIE-2000.ps SPIE-2000 artificial brain architectures. This RAM, containing an artificial brain with up to 75 million neurons, is then updated by the CBM at a rate of 130 billion CA cells per second. Such speeds will enable real time control of robots and hopefully the birth of a new research field that we call 'brain building.' The first such artificial brain, to be built at STARLAB in 2000 and beyond, will be used to control the behaviors of a life sized kitten robot called 'Robokitty.' This kitten robot will need 2D pattern recognizers in the visual section of its artificial brain. This paper presents simulation results on the evolvability and generalization properties of such recognizers.
Kasinathan, N.; Rajakumar, A.; Vaidyanathan, G.; Chetal, S.C.
1995-09-01
Post shutdown decay heat removal is an important safety requirement in any nuclear system. In order to improve the reliability of this function, Liquid metal (sodium) cooled fast breeder reactors (LMFBR) are equipped with redundant hot pool dipped immersion coolers connected to natural draught air cooled heat exchangers through intermediate sodium circuits. During decay heat removal, flow through the core, immersion cooler primary side and in the intermediate sodium circuits are also through natural convection. In order to establish the viability and validate computer codes used in making predictions, a 1:20 scale experimental model called RAMONA with water as coolant has been built and experimental simulation of decay heat removal situation has been performed at KfK Karlsruhe. Results of two such experiments have been compiled and published as benchmarks. This paper brings out the results of the numerical simulation of one of the benchmark case through a 1D/2D coupled code system, DHDYN-1D/THYC-2D and the salient features of the comparisons. Brief description of the formulations of the codes are also included.
NASA Astrophysics Data System (ADS)
Fan, Yuhong
2016-07-01
We present a three-dimensional MHD simulation of the initiation of the coronal mass ejection (CME) on 13 December 2006 in the emerging δ-sunspot active region NOAA 10930. The setup of the simulation is similar to a previous simulation by Fan (2011), but with a significantly widened simulation domain to accommodate the wide CME. The simulation shows that the CME can result from the emergence of a east-west oriented twist flux rope whose positive, following emerging pole corresponds to the observed positive rotating sunspot emerging against the southern edge of the dominant pre-existing negative sunspot. The erupting flux rope resulting from the simulation accelerates to a terminal speed that exceeds 1500 km/s and undergoes a counter-clockwise rotation of nearly 180 degrees such that its front and flanks all exhibit southward directed magnetic fields, explaining the observed southward magnetic field in the magnetic cloud impacting the Earth. Our simulation also demonstrates that the source region is capable of repeated eruptions as a result of continued flux emergence.
NASA Technical Reports Server (NTRS)
Gnoffo, Peter A.; Berry, Scott A.; VanNorman, John W.
2011-01-01
This paper is one of a series of five papers in a special session organized by the NASA Fundamental Aeronautics Program that addresses uncertainty assessments for CFD simulations in hypersonic flow. Simulations of a shock emanating from a compression corner and interacting with a fully developed turbulent boundary layer are evaluated herein. Mission relevant conditions at Mach 7 and Mach 14 are defined for a pre-compression ramp of a scramjet powered vehicle. Three compression angles are defined, the smallest to avoid separation losses and the largest to force a separated flow engaging more complicated flow physics. The Baldwin-Lomax and the Cebeci-Smith algebraic models, the one-equation Spalart-Allmaras model with the Catrix-Aupoix compressibility modification and two-equation models including Menter SST, Wilcox k-omega 98, and Wilcox k-omega 06 turbulence models are evaluated. Each model is fully defined herein to preclude any ambiguity regarding model implementation. Comparisons are made to existing experimental data and Van Driest theory to provide preliminary assessment of model form uncertainty. A set of coarse grained uncertainty metrics are defined to capture essential differences among turbulence models. Except for the inability of algebraic models to converge for some separated flows there is no clearly superior model as judged by these metrics. A preliminary metric for the numerical component of uncertainty in shock-turbulent-boundary-layer interactions at compression corners sufficiently steep to cause separation is defined as 55%. This value is a median of differences with experimental data averaged for peak pressure and heating and for extent of separation captured in new, grid-converged solutions presented here. This value is consistent with existing results in a literature review of hypersonic shock-turbulent-boundary-layer interactions by Roy and Blottner and with more recent computations of MacLean.
2004-08-01
AnisWave2D is a 2D finite-difference code for a simulating seismic wave propagation in fully anisotropic materials. The code is implemented to run in parallel over multiple processors and is fully portable. A mesh refinement algorithm has been utilized to allow the grid-spacing to be tailored to the velocity model, avoiding the over-sampling of high-velocity materials that usually occurs in fixed-grid schemes.
ERIC Educational Resources Information Center
Kantrowitz, Arthur; Rosa, Richard J.
1975-01-01
Explains the operation of the Magnetohydrodynamic (MHD) generator and advantages of the system over coal, oil or nuclear powered generators. Details the development of MHD generators in the United States and Soviet Union. (CP)
Techniques utilized in the simulated altitude testing of a 2D-CD vectoring and reversing nozzle
NASA Technical Reports Server (NTRS)
Block, H. Bruce; Bryant, Lively; Dicus, John H.; Moore, Allan S.; Burns, Maureen E.; Solomon, Robert F.; Sheer, Irving
1988-01-01
Simulated altitude testing of a two-dimensional, convergent-divergent, thrust vectoring and reversing exhaust nozzle was accomplished. An important objective of this test was to develop test hardware and techniques to properly operate a vectoring and reversing nozzle within the confines of an altitude test facility. This report presents detailed information on the major test support systems utilized, the operational performance of the systems and the problems encountered, and test equipment improvements recommended for future tests. The most challenging support systems included the multi-axis thrust measurement system, vectored and reverse exhaust gas collection systems, and infrared temperature measurement systems used to evaluate and monitor the nozzle. The feasibility of testing a vectoring and reversing nozzle of this type in an altitude chamber was successfully demonstrated. Supporting systems performed as required. During reverser operation, engine exhaust gases were successfully captured and turned downstream. However, a small amount of exhaust gas spilled out the collector ducts' inlet openings when the reverser was opened more than 60 percent. The spillage did not affect engine or nozzle performance. The three infrared systems which viewed the nozzle through the exhaust collection system worked remarkably well considering the harsh environment.
NASA Astrophysics Data System (ADS)
Nikitin, Sergey; Khokhlova, Tatiana; Pelivanov, Ivan
2012-02-01
Dependencies of the optoacoustic (OA) transformation efficiency on tissue temperature were obtained for the application in OA temperature monitoring during thermal therapies. Accurate measurement of the OA signal amplitude versus temperature was performed in different ex-vivo tissues in the temperature range 25°C - 80°C. The investigated tissues were selected to represent different structural components: chicken breast (skeletal muscle), porcine lard (fatty tissue) and porcine liver (richly perfused tissue). Backward mode of the OA signal detection and a narrow probe laser beam were used in the experiments to avoid the influence of changes in light scattering with tissue coagulation on the OA signal amplitude. Measurements were performed in heating and cooling regimes. Characteristic behavior of the OA signal amplitude temperature dependences in different temperature ranges were described in terms of changes in different structural components of the tissue samples. Finally, numerical simulation of the OA temperature monitoring with a linear transducers array was performed to demonstrate the possibility of real-time temperature mapping.
Arnal, B; Pinton, G; Garapon, P; Pernot, M; Fink, M; Tanter, M
2013-10-01
Shear wave imaging (SWI) maps soft tissue elasticity by measuring shear wave propagation with ultrafast ultrasound acquisitions (10 000 frames s(-1)). This spatiotemporal data can be used as an input for an inverse problem that determines a shear modulus map. Common inversion methods are local: the shear modulus at each point is calculated based on the values of its neighbour (e.g. time-of-flight, wave equation inversion). However, these approaches are sensitive to the information loss such as noise or the lack of the backscattered signal. In this paper, we evaluate the benefits of a global approach for elasticity inversion using a least-squares formulation, which is derived from full waveform inversion in geophysics known as the adjoint method. We simulate an acoustic waveform in a medium with a soft and a hard lesion. For this initial application, full elastic propagation and viscosity are ignored. We demonstrate that the reconstruction of the shear modulus map is robust with a non-uniform background or in the presence of noise with regularization. Compared to regular local inversions, the global approach leads to an increase of contrast (∼+3 dB) and a decrease of the quantification error (∼+2%). We demonstrate that the inversion is reliable in the case when there is no signal measured within the inclusions like hypoechoic lesions which could have an impact on medical diagnosis.
Implicit adaptive mesh refinement for 2D reduced resistive magnetohydrodynamics
NASA Astrophysics Data System (ADS)
Philip, Bobby; Chacón, Luis; Pernice, Michael
2008-10-01
An implicit structured adaptive mesh refinement (SAMR) solver for 2D reduced magnetohydrodynamics (MHD) is described. The time-implicit discretization is able to step over fast normal modes, while the spatial adaptivity resolves thin, dynamically evolving features. A Jacobian-free Newton-Krylov method is used for the nonlinear solver engine. For preconditioning, we have extended the optimal "physics-based" approach developed in [L. Chacón, D.A. Knoll, J.M. Finn, An implicit, nonlinear reduced resistive MHD solver, J. Comput. Phys. 178 (2002) 15-36] (which employed multigrid solver technology in the preconditioner for scalability) to SAMR grids using the well-known Fast Adaptive Composite grid (FAC) method [S. McCormick, Multilevel Adaptive Methods for Partial Differential Equations, SIAM, Philadelphia, PA, 1989]. A grid convergence study demonstrates that the solver performance is independent of the number of grid levels and only depends on the finest resolution considered, and that it scales well with grid refinement. The study of error generation and propagation in our SAMR implementation demonstrates that high-order (cubic) interpolation during regridding, combined with a robustly damping second-order temporal scheme such as BDF2, is required to minimize impact of grid errors at coarse-fine interfaces on the overall error of the computation for this MHD application. We also demonstrate that our implementation features the desired property that the overall numerical error is dependent only on the finest resolution level considered, and not on the base-grid resolution or on the number of refinement levels present during the simulation. We demonstrate the effectiveness of the tool on several challenging problems.
A 2D model to design MHD induction pumps
NASA Astrophysics Data System (ADS)
Stieglitz, R.; Zeininger, J.
2006-09-01
Technical liquid metal systems accompanied by a thermal transfer of energy such as reactor systems, metallurgical processes, metal refinement, casting, etc., require a forced convection of the fluid. The increased temperatures and more often the environmental conditions as, e.g., in a nuclear environment, pumping principles are required, in which rotating parts are absent. Additionally, in many applications a controlled atmosphere is indispensable, in order to ensure the structural integrity of the duct walls. An interesting option to overcome the sealing problem of a mechanical pump towards the surrounding is offered by induction systems. Although their efficiency compared to that of turbo machines is quite low, they have several advantages, which are attractive to the specific requirements in liquid metal applications such as: - low maintenance costs due to the absence of sealings, bearings and moving parts; - low degradation rate of the structural material; - simple replacement of the inductor without cut of the piping system; - fine regulation of flow rate by different inductor connections; - change of pump characteristics without change of the mechanical set-up. Within the article, general design requirements of electromagnetic pumps (EMP) are elaborated. The design of two annular linear induction pumps operating with sodium and lead-bismuth are presented and the calculated pump characteristics and experimentally obtained data are compared. In this context, physical effects leading to deviations between the model and the real data are addressed. Finally, the main results are summarized. Tables 4, Figs 4, Refs 12.
NASA Astrophysics Data System (ADS)
Lyra, W.; Johansen, A.; Zsom, A.; Klahr, H.; Piskunov, N.
2009-04-01
Context: As accretion in protoplanetary disks is enabled by turbulent viscosity, the border between active and inactive (dead) zones constitutes a location where there is an abrupt change in the accretion flow. The gas accumulation that ensues triggers the Rossby wave instability, which in turn saturates into anticyclonic vortices. It has been suggested that the trapping of solids within them leads to a burst of planet formation on very short timescales. Aims: We study in the formation and evolution of the vortices in greater detail, focusing on the implications for the dynamics of embedded solid particles and planet formation. Methods: We performed two-dimensional global simulations of the dynamics of gas and solids in a non-magnetized thin protoplanetary disk with the Pencil code. We used multiple particle species of radius 1, 10, 30, and 100 cm. We computed the particles' gravitational interaction by a particle-mesh method, translating the particles' number density into surface density and computing the corresponding self-gravitational potential via fast Fourier transforms. The dead zone is modeled as a region of low viscosity. Adiabatic and locally isothermal equations of state are used. Results: The Rossby wave instability is triggered under a variety of conditions, thus making vortex formation a robust process. Inside the vortices, fast accumulation of solids occurs and the particles collapse into objects of planetary mass on timescales as short as five orbits. Because the drag force is size-dependent, aerodynamical sorting ensues within the vortical motion, and the first bound structures formed are composed primarily of similarly-sized particles. In addition to erosion due to ram pressure, we identify gas tides from the massive vortices as a disrupting agent of formed protoplanetary embryos. We find evidence that the backreaction of the drag force from the particles onto the gas modifies the evolution of the Rossby wave instability, with vortices being
Nibedita, R; Kumar, R A; Majumdar, A; Hosur, R V; Govil, G; Majumder, K; Chauhan, V S
1993-09-01
Solution conformation of a self-complementary 14-mer DNA duplex (d-GGATTGGCCAATCC) containing the GCCAAT recognition motif of several transcription factors has been investigated by NMR spectroscopy. Complete resonance assignment of all the protons (except H5',H5'' protons) has been obtained following standard procedures based on two-dimensional NMR techniques. Three-bond coupling constants have been determined by spectral simulation procedures. New strategies have been described and employed for quantifying NOE intensities from the structural point of view. Approximate ranges of gamma torsion angles have been obtained from a selective NOESY experiment, by estimating the J(4'-5'), J(4'-5''), or their sum in the H1'-H4' cross peaks of the spectrum. Likewise, ranges of epsilon torsion angles have been obtained by monitoring the H3' multiplicities in the H8/H6-H3' cross peaks in selective NOESY spectra. With the help of such a total of 73 coupling constraints, 79 NOE intensity constraints, and 108 H-bond constraints, model building has been carried out to obtain a structure which satisfies the constraints. Starting from such a structure, an expanded distance constraint set has been created which has been used for the distance geometry calculations using the program TANDY. In the best structure thus derived, interesting irregularities similar to a BI-BII transition have been observed in the center. The molecule exhibits a bend. The overall base stacking is different from that in either B- or A-DNA models. The base pairs are tilted with respect to the local helix axes. The observed structural features are likely to have important implications for the recognition mechanism of the GCCAAT motif.
NASA Astrophysics Data System (ADS)
Simard, Corinne; Charbonneau, Paul; Dubé, Caroline
2016-10-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos and Charbonneau (2014). The turbulent electromotive force (emf) operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. (2011), we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed α -tensor shows good agreement with that obtained by Racine et al. (2011), who did not include derivatives of the mean-field in their fit, as well as with the α -tensor extracted by Augustson et al. (2015) from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor β is positive definite and reaches values of 5.0 ×107 m2 s-1 in the middle of the convecting fluid layers. The spatial variations of both αϕϕ and βϕϕ component are well reproduced by expressions obtained under the Second Order Correlation Approximation, with a good matching of amplitude requiring a turbulent correlation time about five times smaller than the estimated turnover time of the small-scale turbulent flow. By segmenting the simulation data into epochs of magnetic cycle minima and maxima, we also measure α - and β -quenching. We find the magnetic quenching of the α -effect to be driven primarily by a reduction of the small-scale flow's kinetic helicity, with variations of the current helicity playing a lesser role in most locations in the simulation domain. Our measurements of turbulent diffusivity quenching are restricted to the βϕϕ component, but indicate a weaker quenching, by a factor of ≃ 1.36, than of the α -effect, which in our simulation drops by a factor of three between
NASA Astrophysics Data System (ADS)
Constantinescu, R.; Thouret, J. C.; Sandri, L.; Irimus, I. A.; Stefanescu, R.
2012-04-01
Pyroclastic density currents, which include pyroclastic surges and pyroclastic flows (PFs), are among the most dangerous volcanic phenomena. We present a probabilistic hazard assessment of the PFs generated from eruptive column collapse at El Misti volcano (5822 m) in South Peru. The high relief of the cone, the location of the city of Arequipa (~1,000,000 people) on two large volcanoclastic fans and the H (3.5 km)/L (17 km) ratio (0.2) between the summit and the city center, make PFs a direct threat. We consider three eruption scenario sizes: small Vulcanian/Phreatomagmatic (VEI 2), medium Sub-Plinian (VEI 3-4), and large Plinian (VEI 4+). We use the Event-Tree approach in a Bayesian scheme with BET_VH (Bayesian Event Tree for Volcanic Hazard) software. Quantitative data that stem from numerical simulations from TITAN2D (termed prior models) and from stratigraphic record (termed past data) are input to BET_VH, which enables us to compute the probabilities (in a 1-year time window) of (i) having an eruption (ii) in a selected location/vent (iii) of a specific size, (iv) and that this eruption will produce PFs (v) that will reach a location of interest around El Misti. TITAN2D simulation runs, expressed as color-coded thicknesses of PDC deposits, fit well the extent of past PFs deposits, including thick confined deposits (0.5-7 m) in the Rio Chili canyon and its tributary ravines (Quebradas San Lazaro, Huarangal and Agua Salada).The unconfined, thinner (≤10cm) deposits, as displayed by simulation runs on the interfluves, is attributed to ash-cloud surges. Such thin, fine ash deposits have not been emphasized in geological maps either because they have been removed away or remain yet unrecognized. The simulated Vulcanian flows, restricted to the upper part of the cone, become confined (0.1-1m thick) in the ravines which converge towards each of the three Quebradas. The simulated Subplinian PF deposits reach 0.1 to 1 m thick in the Quebradas and 1-4 m WNW of El
Sidler, Rolf; Carcione, José M.; Holliger, Klaus
2013-02-15
We present a novel numerical approach for the comprehensive, flexible, and accurate simulation of poro-elastic wave propagation in 2D polar coordinates. An important application of this method and its extensions will be the modeling of complex seismic wave phenomena in fluid-filled boreholes, which represents a major, and as of yet largely unresolved, computational problem in exploration geophysics. In view of this, we consider a numerical mesh, which can be arbitrarily heterogeneous, consisting of two or more concentric rings representing the fluid in the center and the surrounding porous medium. The spatial discretization is based on a Chebyshev expansion in the radial direction and a Fourier expansion in the azimuthal direction and a Runge–Kutta integration scheme for the time evolution. A domain decomposition method is used to match the fluid–solid boundary conditions based on the method of characteristics. This multi-domain approach allows for significant reductions of the number of grid points in the azimuthal direction for the inner grid domain and thus for corresponding increases of the time step and enhancements of computational efficiency. The viability and accuracy of the proposed method has been rigorously tested and verified through comparisons with analytical solutions as well as with the results obtained with a corresponding, previously published, and independently benchmarked solution for 2D Cartesian coordinates. Finally, the proposed numerical solution also satisfies the reciprocity theorem, which indicates that the inherent singularity associated with the origin of the polar coordinate system is adequately handled.
NASA Astrophysics Data System (ADS)
Haberreiter, M.; Guerreiro, N.; Hansteen, V. H.; Schmutz, W. K.
2015-12-01
The physical mechanism that heats the solar corona is one of the still open science questions in solar physics. One of the proposed mechanism for coronal heating are nanoflares. To investigate their role in coronal heating we study the properties of the small-scale heating events in the solar atmosphere using 3D MHD simulations. We present a method to identify and track these heating events in time which allows us to study their life time, energy, and spectral signatures. These spectal signatures will be compared with available spectrosopic observations obtained with IRIS and SUMER. Ultimately, these results will be important for the coordinated scientific exploitation of SPICE and EUI along with other instruments onboard Solar Orbiter to address the coronal heating problem.
NASA Astrophysics Data System (ADS)
Guerreiro, Nuno; Haberreiter, Margit; Hansteen, Viggo; Schmutz, Werner
2016-04-01
Aiming at better understanding the mechanism(s) responsible for the coronal heating and the ubiquitous redshifts observed in the lower transition region we focus on analyzing the properties of small-scale heating events (SSHEs) in the solar atmosphere. We present a comprehensive method to follow SSHEs over time in 3D-MHD simulations of the solar atmosphere. Applying the method we are able to better understand the properties of the SSHEs and how the plasma in their vicinity respond to them. We present results for the lifetime, energy and spectral signatures of the SSHEs. Ultimately, these results will be important for the coordinated scientific exploration of SPICE and EUI along with other interments on board solar orbiter.
NASA Astrophysics Data System (ADS)
Guerreiro, Nuno; Haberreiter, Margit; Schmutz, Werner; Hansteen, Viggo
2016-07-01
Aiming at better understanding the mechanism(s) responsible for the coronal heating we focus on analyzing the properties of the magnetically generated small-scale heating events (SSHEs) in the solar atmosphere. We present a comprehensive method to detect and follow SSHEs over time in 3D-MHD simulations of the solar atmosphere. Applying the method we are able to better understand the properties of the SSHEs and how the plasma in their vicinity respond to them. We study the lifetime, energy and spectral signatures and show that the energy flux dissipated by them is enough to heat the corona. Ultimately, these results will be important for the coordinated scientific exploration of SPICE and EUI along with other instruments on board solar orbiter.
NASA Astrophysics Data System (ADS)
Gürleme, Beran; Tarık Meriç, Hakan; Ulutaş, Ergin; Anunziato, Alessandro
2016-04-01
The aim of this study is the simulation and visualization of the initial and maximum tsunami wave heights in 2D and 3D along the Mediterranean coasts inferred from the five largest earthquakes in history in this region. The earthquakes considered in the study are 21 July 365 Crete, 8 August 1303 Crete, 3 May 1481 Rhodes, 28 December Messina and 21 May 2003 Algeria. All these earthquakes spawned tsunamis and inflicted damage in coastal regions. The study was conducted to explain which could be the potential Tsunami consequences caused by similar earthquakes occurring in the region in the future. The methodology used for the calculation of tsunami wave heights from the earthquakes includes the determination of earthquake parameters, modeling of the initial wave height, simulation of the wave propagation and calculation of the maximum wave heights near coastal areas. The parameters of the earthquakes are based on previously published fault mechanism solutions and known tectonic features of the regions. Static dislocation algorithm for the initial wave height is used from the parameters of focal mechanism solutions. The study was conducted also to understand the reliability of the previously published focal mechanism solutions for the earthquakes by using the principal stress axis in the regions. The 2D and 3D visualized models of tsunamis from the earthquakes include isometric grid representing the sea surface for the purpose of a better understanding of the initial tsunami mechanism compared to 1D visualizations. In many studies, the earthquake locations, tectonic features of the regions, initial heights and tsunami simulations are shown on maps as bird's eye in 1D visualization. However these kinds of features are related in depths and bathymetric features. For that reason, our approaches will contribute to have better understanding where the uplift- subsidence of initial heights and crests-troughs of simulated wave heights and thus provide a better insight of the
Barnes, P.R.; Tesche, F.M.; McConnell, B.W.; Vance, E.F.
1993-09-01
A large nuclear detonation at altitudes of several hundred kilometers above the earth distorts the earth's magnetic field and produces a strong magnetohydrodynamic-electromagnetic pulse (MHD-EMP). MHD-EMP is similar to solar geomagnetic storms in its global and low frequency (less than 1 Hz) nature except that it can be more intense with a shorter duration. It will induce quasi-dc currents in long lines. The MHD-EMP induced currents may cause large voltage fluctuations and severe harmonic distortion in commercial electric power systems. Several MHD-EMP coupling models for predicting the induced current on a wide variety of conducting structures are described, various simulation concepts are summarized, and the results from several MHD-EMP tests are presented. To mitigate the effects of MHD-EMP on a facility, long conductors must be isolated from the building, and the commercial power harmonics and voltage swings must be addressed. It is found that facilities can be protected against MHD-EMP by using methods which are consistent with standard engineering practices. MHD-EMP Interaction Analysis, Power Line Model, MHD-EMP Protection Guidelines, Transformer Test.
NASA Astrophysics Data System (ADS)
Rank, Christopher M.; Heußer, Thorsten; Flach, Barbara; Brehm, Marcus; Kachelrieß, Marc
2015-03-01
We propose a new method for PET/MR respiratory motion compensation, which is based on a 3D-2D registration of strongly undersampled MR data and a) runs in parallel with the PET acquisition, b) can be interlaced with clinical MR sequences, and c) requires less than one minute of the total MR acquisition time per bed position. In our simulation study, we applied a 3D encoded radial stack-of-stars sampling scheme with 160 radial spokes per slice and an acquisition time of 38 s. Gated 4D MR images were reconstructed using a 4D iterative reconstruction algorithm. Based on these images, motion vector fields were estimated using our newly-developed 3D-2D registration framework. A 4D PET volume of a patient with eight hot lesions in the lungs and upper abdomen was simulated and MoCo 4D PET images were reconstructed based on the motion vector fields derived from MR. For evaluation, average SUVmean values of the artificial lesions were determined for a 3D, a gated 4D, a MoCo 4D and a reference (with ten-fold measurement time) gated 4D reconstruction. Compared to the reference, 3D reconstructions yielded an underestimation of SUVmean values due to motion blurring. In contrast, gated 4D reconstructions showed the highest variation of SUVmean due to low statistics. MoCo 4D reconstructions were only slightly affected by these two sources of uncertainty resulting in a significant visual and quantitative improvement in terms of SUVmean values. Whereas temporal resolution was comparable to the gated 4D images, signal-to-noise ratio and contrast-to-noise ratio were close to the 3D reconstructions.
NASA Astrophysics Data System (ADS)
Orlić, Ivica; Mekterović, Darko; Mekterović, Igor; Ivošević, Tatjana
2015-11-01
VIBA-Lab is a computer program originally developed by the author and co-workers at the National University of Singapore (NUS) as an interactive software package for simulation of Particle Induced X-ray Emission and Rutherford Backscattering Spectra. The original program is redeveloped to a VIBA-Lab 3.0 in which the user can perform semi-quantitative analysis by comparing simulated and measured spectra as well as simulate 2D elemental maps for a given 3D sample composition. The latest version has a new and more versatile user interface. It also has the latest data set of fundamental parameters such as Coster-Kronig transition rates, fluorescence yields, mass absorption coefficients and ionization cross sections for K and L lines in a wider energy range than the original program. Our short-term plan is to introduce routine for quantitative analysis for multiple PIXE and XRF excitations. VIBA-Lab is an excellent teaching tool for students and researchers in using PIXE and RBS techniques. At the same time the program helps when planning an experiment and when optimizing experimental parameters such as incident ions, their energy, detector specifications, filters, geometry, etc. By "running" a virtual experiment the user can test various scenarios until the optimal PIXE and BS spectra are obtained and in this way save a lot of expensive machine time.
Supersonic MHD generator system
Rahman, M.A.
1983-11-29
An improved MHD electrical power generating system of the type having a MHD topping cycle and a steam generating bottoming cycle is disclosed. The system typically includes a combustion system, a conventional MHD generator and a first diffuser radiant boiler. The improvement comprises a first supersonic MHD generator and ramjet engine configuration operatively connected in series with each other and with the conventional MHD generator. The first supersonic MHD generator and ramjet engine configuration increase the power output and improve the operating efficiency of the electrical generating system. A diffuser system is also disclosed which is in fluid communication with the supersonic MHD generator and the ramjet engine for collecting bypass plasma gas to be used for heating a second radiant boiler adapted for powering a steam turbine generator.
Simulated annealing applied to two-dimensional low-beta reduced magnetohydrodynamics
Chikasue, Y.; Furukawa, M.
2015-02-15
The simulated annealing (SA) method is applied to two-dimensional (2D) low-beta reduced magnetohydrodynamics (R-MHD). We have successfully obtained stationary states of the system numerically by the SA method with Casimir invariants preserved. Since the 2D low-beta R-MHD has two fields, the relaxation process becomes complex compared to a single field system such as 2D Euler flow. The obtained stationary state can have fine structure. We have found that the fine structure appears because the relaxation processes are different between kinetic energy and magnetic energy.
NASA Astrophysics Data System (ADS)
Tierz, Pablo; Ramona Stefanescu, Elena; Sandri, Laura; Patra, Abani; Marzocchi, Warner; Sulpizio, Roberto
2014-05-01
Probabilistic hazard assessments of Pyroclastic Density Currents (PDCs) are of great interest for decision-making purposes. However, there is a limited number of published works available on this topic. Recent advances in computation and statistical methods are offering new opportunities beyond the classical Monte Carlo (MC) sampling which is known as a simple and robust method but it usually turns out to be slow and computationally intractable. In this work, Titan2D numerical simulator has been coupled to Polynomial Chaos Quadrature (PCQ) to propagate the simulator parametric uncertainty and compute VEI-based probabilistic hazard maps of dense PDCs formed as a result of column collapse at Vesuvius volcano, Italy. Due to the lack of knowledge about the exact conditions under which these PDCs will form, Probability Distribution Functions (PDFs) are assigned to the simulator input parameters (Bed Friction Angle and Volume) according to three VEI sizes. Uniform distributions were used for both parameters since there is insufficient information to assume that any value in the range is more likely that any other value. Reasonable (and compatible) ranges for both variables were constrained according to past eruptions at Vesuvius volcanic system. On the basis of reasoning above a number of quadrature points were taken within those ranges, which resulted in one execution of the TITAN2D code at each quadrature point. With a computational cost several orders of magnitude smaller than MC, exceedance probabilities for a given threshold of flow depth (and conditional to the occurrence of VEI3, VEI4 and VEI5 eruptions) were calculated using PCQ. Moreover, PCQ can be run at different threshold values of the same output variable (flow depth, speed, kinetic energy, …) and, therefore, it can serve to compute Exceedance Probability curves (aka hazard curves) at singular points inside the hazard domain, representing the most important and useful scientific input to quantitative risk
NASA Astrophysics Data System (ADS)
Wyseure, Guido; Chou, Po-Yi
2010-05-01
All hydrological handbooks contain methods for direct runoff and base-flow separation. The semi-log separation method is the most classical one. One can, however, question the physical base for such method. In addition, the water fluxes in the riverbed are important for ecology and water quality. In our study an 2-D cross-section including the river and the surrounding aquifer was set-up in HYDRUS 2D/3D. Initial conditions were a steady-state subsurface flow feeding the river with a recharge from the soil surface. A surface runoff event was simulated by a rise and recession of the water level in the river. Differences between summer and winter situation were explored by given representative temperatures to the different components of the river-aquifer system. The simulations show that the fluxes are very different along the riverbed. Even during steady state baseflow we see that the fluxes through the bottom were 2 to 3 times smaller as compared to the side banks. During the hydrographs the proportion can become up to 5 times. Another interesting result is that within the time frame of the hydrograph and its immediate recession relatively little water, which pentetrated in the aquifer, returns to the river. Most of the water replenishes the aquifer and there is only a very small rise of baseflow. In our simulation we returned to the original level as before the hydrograph, so in reality even less or no rise in baseflow may occur immediately after a hydrograph. Of course, in a longer time-frame the recharge of the aquifer will give a rise to the actual subsurface drainage. The change in seasonal temperatures within the river-aquifer system has a substantial effect. For identical river stage hydrograph changes the hyporheic exchange fluxes are more intense in summer than in winter. If we define the hyporheic zone as the extedn to which the water fluxes from the river can penetrate, then we see that this zone is wider on the sides as compared to the bottom of the
NASA Technical Reports Server (NTRS)
Li, Xiaofan; Sui, C.-H.; Lau, K-M.; Adamec, D.
1999-01-01
A two-dimensional coupled ocean-cloud resolving atmosphere model is used to investigate possible roles of convective scale ocean disturbances induced by atmospheric precipitation on ocean mixed-layer heat and salt budgets. The model couples a cloud resolving model with an embedded mixed layer-ocean circulation model. Five experiment are performed under imposed large-scale atmospheric forcing in terms of vertical velocity derived from the TOGA COARE observations during a selected seven-day period. The dominant variability of mixed-layer temperature and salinity are simulated by the coupled model with imposed large-scale forcing. The mixed-layer temperatures in the coupled experiments with 1-D and 2-D ocean models show similar variations when salinity effects are not included. When salinity effects are included, however, differences in the domain-mean mixed-layer salinity and temperature between coupled experiments with 1-D and 2-D ocean models could be as large as 0.3 PSU and 0.4 C respectively. Without fresh water effects, the nocturnal heat loss over ocean surface causes deep mixed layers and weak cooling rates so that the nocturnal mixed-layer temperatures tend to be horizontally-uniform. The fresh water flux, however, causes shallow mixed layers over convective areas while the nocturnal heat loss causes deep mixed layer over convection-free areas so that the mixed-layer temperatures have large horizontal fluctuations. Furthermore, fresh water flux exhibits larger spatial fluctuations than surface heat flux because heavy rainfall occurs over convective areas embedded in broad non-convective or clear areas, whereas diurnal signals over whole model areas yield high spatial correlation of surface heat flux. As a result, mixed-layer salinities contribute more to the density differences than do mixed-layer temperatures.
Zhang, Y; Yang, J; Liu, H; Liu, D
2014-06-01
Purpose: The purpose of this work is to compare the verification results of three solutions (2D/3D ionization chamber arrays measurement and Monte Carlo simulation), the results will help make a clinical decision as how to do our cervical IMRT verification. Methods: Seven cervical cases were planned with Pinnacle 8.0m to meet the clinical acceptance criteria. The plans were recalculated in the Matrixx and Delta4 phantom with the accurate plans parameters. The plans were also recalculated by Monte Carlo using leaf sequences and MUs for individual plans of every patient, Matrixx and Delta4 phantom. All plans of Matrixx and Delta4 phantom were delivered and measured. The dose distribution of iso slice, dose profiles, gamma maps of every beam were used to evaluate the agreement. Dose-volume histograms were also compared. Results: The dose distribution of iso slice and dose profiles from Pinnacle calculation were in agreement with the Monte Carlo simulation, Matrixx and Delta4 measurement. A 95.2%/91.3% gamma pass ratio was obtained between the Matrixx/Delta4 measurement and Pinnacle distributions within 3mm/3% gamma criteria. A 96.4%/95.6% gamma pass ratio was obtained between the Matrixx/Delta4 measurement and Monte Carlo simulation within 2mm/2% gamma criteria, almost 100% gamma pass ratio within 3mm/3% gamma criteria. The DVH plot have slightly differences between Pinnacle and Delta4 measurement as well as Pinnacle and Monte Carlo simulation, but have excellent agreement between Delta4 measurement and Monte Carlo simulation. Conclusion: It was shown that Matrixx/Delta4 and Monte Carlo simulation can be used very efficiently to verify cervical IMRT delivery. In terms of Gamma value the pass ratio of Matrixx was little higher, however, Delta4 showed more problem fields. The primary advantage of Delta4 is the fact it can measure true 3D dosimetry while Monte Carlo can simulate in patients CT images but not in phantom.
NASA Astrophysics Data System (ADS)
Kitiashvili, Irina; Couvidat, Sebastien
2014-06-01
Observations with the Solar Dynamics Observatory (SDO), and, in particular, Helioseismic and Magnetic Imager (HMI) provide a unique opportunity to investigate various phenomena simultaneously over the whole solar disk. Current state-of-the-art numerical simulations allow us to model the observational data with a high degree of realism, and use the artificial data for interpretation of observed properties ("observables") in terms of the physical conditions, for the testing of new data analysis techniques and the improvement of data calibration. In the current study we use realistic-type 3D radiative MHD simulations of the upper turbulent convective layer and atmosphere of the Sun, obtained with the SolarBox code, and employ the spectro-polarimetric radiative transfer code SPINOR to convert the simulated data into Stokes profiles of the HMI Fe I 6173 A line for different conditions in the solar atmosphere. For testing the HMI calibration the synthetic Stokes profiles are processed through the SDO/JSOC simplified data analysis pipeline. We investigate properties of the HMI observables for various solar features, variations of the line formation height for different angular distances from the disk center, effects of the spatial resolution and iron abundance, and pay particular attention to the center-to-limb variations effects playing important role in local helioseismology measurements.
2-D Imaging of Electron Temperature in Tokamak Plasmas
T. Munsat; E. Mazzucato; H. Park; C.W. Domier; M. Johnson; N.C. Luhmann Jr.; J. Wang; Z. Xia; I.G.J. Classen; A.J.H. Donne; M.J. van de Pol
2004-07-08
By taking advantage of recent developments in millimeter wave imaging technology, an Electron Cyclotron Emission Imaging (ECEI) instrument, capable of simultaneously measuring 128 channels of localized electron temperature over a 2-D map in the poloidal plane, has been developed for the TEXTOR tokamak. Data from the new instrument, detailing the MHD activity associated with a sawtooth crash, is presented.
Fukuyoshi, Shuichi; Kometani, Masaharu; Watanabe, Yurie; Hiratsuka, Masahiro; Yamaotsu, Noriyuki; Hirono, Shuichi; Manabe, Noriyoshi; Takahashi, Ohgi; Oda, Akifumi
2016-01-01
Many natural mutants of the drug metabolizing enzyme cytochrome P450 (CYP) 2D6 have been reported. Because the enzymatic activities of many mutants are different from that of the wild type, the genetic polymorphism of CYP2D6 plays an important role in drug metabolism. In this study, the molecular dynamics simulations of the wild type and mutants of CYP2D6, CYP2D6.1, 2, 10, 14A, 51, and 62 were performed, and the predictions of static and dynamic structures within them were conducted. In the mutant CYP2D6.10, 14A, and 61, dynamic properties of the F-G loop, which is one of the components of the active site access channel of CYP2D6, were different from that of the wild type. The F-G loop acted as the "hatch" of the channel, which was closed in those mutants. The structure of CYP2D6.51 was not converged by the simulation, which indicated that the three-dimensional structure of CYP2D6.51 was largely different from that of the wild type. In addition, the intramolecular interaction network of CYP2D6.10, 14A, and 61 was different from that of the wild type, and it is considered that these structural changes are the reason for the decrease or loss of enzymatic activities. On the other hand, the static and dynamic properties of CYP2D6.2, whose activity was normal, were not considerably different from those of the wild type.
Watanabe, Yurie; Hiratsuka, Masahiro; Yamaotsu, Noriyuki; Hirono, Shuichi; Manabe, Noriyoshi; Takahashi, Ohgi; Oda, Akifumi
2016-01-01
Many natural mutants of the drug metabolizing enzyme cytochrome P450 (CYP) 2D6 have been reported. Because the enzymatic activities of many mutants are different from that of the wild type, the genetic polymorphism of CYP2D6 plays an important role in drug metabolism. In this study, the molecular dynamics simulations of the wild type and mutants of CYP2D6, CYP2D6.1, 2, 10, 14A, 51, and 62 were performed, and the predictions of static and dynamic structures within them were conducted. In the mutant CYP2D6.10, 14A, and 61, dynamic properties of the F-G loop, which is one of the components of the active site access channel of CYP2D6, were different from that of the wild type. The F-G loop acted as the “hatch” of the channel, which was closed in those mutants. The structure of CYP2D6.51 was not converged by the simulation, which indicated that the three-dimensional structure of CYP2D6.51 was largely different from that of the wild type. In addition, the intramolecular interaction network of CYP2D6.10, 14A, and 61 was different from that of the wild type, and it is considered that these structural changes are the reason for the decrease or loss of enzymatic activities. On the other hand, the static and dynamic properties of CYP2D6.2, whose activity was normal, were not considerably different from those of the wild type. PMID:27046024
NASA Astrophysics Data System (ADS)
Martelloni, Gianluca; Bagnoli, Franco; Di Cintio, Pierfrancesco
2015-04-01
We integrate existing soil infiltration modeling with particle based methods in order to simulate two and three-dimensional setups of triggered landslides. Commonly, the infiltration models are based on continuum schemes (e.g. Eulerian approach) by means of which it is possible to define the field of the pore pressure within a soil. By contrast, the particle based methods follow a Lagrangian scheme that allows one to identify the particle trajectories and their dynamical properties. In this work, in order to simulate the triggering mechanism, we apply the classical, fractal and fractional Richards equations and the Mohr-Coulomb failure criterion, adapted to the molecular dynamics technique. In our scheme the (local) positive pore pressure is simply implemented as a perturbation of the rest state of each grain. Therefore, the pore pressure function can be interpreted as a time-space dependent scalar field acting on each particle. To initialize the system we generate, using a molecular dynamics based algorithm, a mechanically stable disk (2D) or sphere (3D) packing simulating the consolidated soil. In this way, we can built the micro and macro pore structure related to different infiltration time scales. The inter-particle interactions are modeled with a Lennard-Jones like potential. The particle positions are updated in time, after and during a rainfall, with standard molecular dynamics. We analyze the sensitivity of the model with respect to the variation of some parameters such as hydraulic conductivity, cohesion, slope and friction angle, soil depth and fractional order of the generalized infiltration model. In addition, we consider both regular and random particle configurations. The results of our simulations are found to be in agreement with real landslides. In particular, the mean velocity patterns of the simulated landslides appear extremely similar to the observed ones. Moreover, it is possible to apply the method of the inverse surface displacement
NASA Astrophysics Data System (ADS)
Hao, Yufei; Lu, Quanming; Lembege, Bertrand; Huang, Can; Wu, Mingyu; Guo, Fan; Shan, Lican; Zheng, Jian; Wang, Shui
2015-04-01
Experimental observations from space missions (including Cluster more recently) have clearly revealed the existence of high speed jets (HSJ) in the downstream region of the quasi-parallel terrestrial bow shock. Presently, two-dimensional (2-D) hybrid simulations are performed to reproduce and investigate the formation of such HSJ through a rippled quasi-parallel shock front. The simulation results show (i) that such shock fronts are strongly nonstationary (self reformation) along the shock normal, and (ii) that ripples are evidenced along the shock front as the upstream ULF waves (excited by interaction between incoming and reflected ions) are convected back to the front by the solar wind and contribute to the rippling formation. Then, these ripples are inherent structures of a quasi-parallel shock and the self reformation of the shock is not synchronous along the surface of the shock front. As a consequence, new incoming solar wind ions interact differently at different locations along the shock surface, and some can be only deflected (instead of being decelerated) at locations where ripples are large enough to play the role of local « secondary » shock. Therefore, the ion bulk velocity is also different locally after ions are transmitted dowstream, and local high-speed jets patterns are formed somewhere downstream. After a short reminder of main quasi-parallel shock features, this presentation will focus (i) on experimental observations of HSJ, (ii) on our preliminary simulation results obtained on HSJ, (iii) on their relationship with local bursty patterns of (turbulent) magnetic field evidenced at the front, and (iv) on the spatial and time scales of HSJ to be compared later on with experimental observations. Such downstream HSJ are shown to be generated by the nonstationary shock front itself and do not require any upstream perturbations (such as tangential/rotational discontinuity, HFA, etc..) to be convected by the solar wind and to interact with the shock
An MHD study of the interaction between the solar wind and the interstellar medium
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.
1993-01-01
The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results will be compared with observations where possible and with the results from previous analytic and numerical studies. The primary progress during the first two years has been to develop codes for 2-D models in both spherical and cylindrical coordinates and to apply them to the solar wind-interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. Work has also been initiated on the development of a 3-D MHD code in spherical coordinates.
Gargett, Maegan Rosenfeld, Anatoly; Oborn, Brad; Metcalfe, Peter
2015-02-15
Purpose: MRI-guided radiation therapy systems (MRIgRT) are being developed to improve online imaging during treatment delivery. At present, the operation of single point dosimeters and an ionization chamber array have been characterized in such systems. This work investigates a novel 2D diode array, named “magic plate,” for both single point calibration and 2D positional performance, the latter being a key element of modern radiotherapy techniques that will be delivered by these systems. Methods: GEANT4 Monte Carlo methods have been employed to study the dose response of a silicon diode array to 6 MV photon beams, in the presence of in-line and perpendicularly aligned uniform magnetic fields. The array consists of 121 silicon diodes (dimensions 1.5 × 1.5 × 0.38 mm{sup 3}) embedded in kapton substrate with 1 cm pitch, spanning a 10 × 10 cm{sup 2} area in total. A geometrically identical, water equivalent volume was simulated concurrently for comparison. The dose response of the silicon diode array was assessed for various photon beam field shapes and sizes, including an IMRT field, at 1 T. The dose response was further investigated at larger magnetic field strengths (1.5 and 3 T) for a 4 × 4 cm{sup 2} photon field size. Results: The magic plate diode array shows excellent correspondence (< ± 1%) to water dose in the in-line orientation, for all beam arrangements and magnetic field strengths investigated. The perpendicular orientation, however, exhibits a dose shift with respect to water at the high-dose-gradient beam edge of jaw-defined fields [maximum (4.3 ± 0.8)% over-response, maximum (1.8 ± 0.8)% under-response on opposing side for 1 T, uncertainty 1σ]. The trend is not evident in areas with in-field dose gradients typical of IMRT dose maps. Conclusions: A novel 121 pixel silicon diode array detector has been characterized by Monte Carlo simulation for its performance inside magnetic fields representative of current prototype and proposed MRI
NASA Astrophysics Data System (ADS)
Leterme, Bertrand; Beerten, Koen
2013-04-01
Climate, soils and vegetation are known to exert strong controls on the water balance in a given area. The role of geomorphological processes, however, is generally overlooked in hydrological studies. In this study, the impact of landscape evolution, including geomorphological processes, is being assessed using HYDRUS 2-D simulations. A realistic sequence of consecutive landscape development stages during the last millennium in the Campine area was taken to investigate the potential role of changing landscapes on the water balance. The sequence is based on a detailed landscape reconstruction of a small interfluve in the Nete basin (Campine area, northern Belgium), following a study of sediment-soil profiles using classical geomorphological techniques, optically stimulated luminescence dating, palynology and historical archives. At least four distinctive phases in the topography-soil-vegetation system have been identified: around ca. 1000 a BP, 500 a BP, 250 a BP and 150 a BP. The sequence is characterised by progressive destruction of the soil catena (podzol profile) and vegetation, and an overall increase in relief intensity due to heavy use of land, until the landscape became stabilized ca. 150 a BP. In parallel, soil hydraulic properties were measured and used for parameterization of the HYDRUS simulations. For each stage of the sequence, a two-dimensional landscape was drawn in HYDRUS-2D using the reconstructed information on vegetation, topography, soil horizons and soil hydraulic properties. The impact of changes in this geomorphological system on water balance was then evaluated by applying a 30-year time series of climate observations. Using the same recent climate data for the different stages allows to focus on the effect of geomorphological and land use changes on evapotranspiration, runoff and groundwater recharge. In general, the results show that soil development and/or erosion alone would have had only very limited impact on the water balance during
NASA Astrophysics Data System (ADS)
Prins, Steven L.; Blatchford, James; Olubuyide, Oluwamuyiwa; Riley, Deborah; Chang, Simon; Hong, Qi-Zhong; Kim, T. S.; Borges, Ricardo; Lin, Li
2009-03-01
As design rules and corresponding logic standard cell layouts continue to shrink node-on-node in accordance with Moore's law, complex 2D interactions, both intra-cell and between cells, become much more prominent. For example, in lithography, lack of scaling of λ/NA implies aggressive use of resolution enhancement techniques to meet logic scaling requirements-resulting in adverse effects such as 'forbidden pitches'-and also implies an increasing range of optical influence relative to cell size. These adverse effects are therefore expected to extend well beyond the cell boundary, leading to lithographic marginalities that occur only when a given cell is placed "in context" with other neighboring cells in a variable design environment [1]. This context dependence is greatly exacerbated by increased use of strain engineering techniques such as SiGe and dual-stress liners (DSL) to enhance transistor performance, both of which also have interaction lengths on the order of microns. The use of these techniques also breaks the formerly straightforward connection between lithographic 'shapes' and end-of-line electrical performance, thus making the formulation of design rules that are robust to process variations and complex 2D interactions more difficult. To address these issues, we have developed a first-principles-based simulation flow to study contextdependent electrical effects in layout, arising not only from lithography, but also from stress and interconnect parasitic effects. This flow is novel in that it can be applied to relatively large layout clips- required for context-dependent analysis-without relying on semi-empirical or 'black-box' models for the fundamental electrical effects. The first-principles-based approach is ideal for understanding contextdependent effects early in the design phase, so that they can be mitigated through restrictive design rules. The lithographic simulations have been discussed elsewhere [1] and will not be presented in detail. The
NASA Astrophysics Data System (ADS)
Ream, J. B.; Walker, R. J.; Kivelson, M.; Ashour-Abdalla, M.; El-Alaoui, M.; Wiltberger, M. J.
2014-12-01
The relationship between the onset of Pi2 pulsations in ground magnetometers and substorm onset has been used extensively over the years to determine when and where substorm onset takes place. However, there are still many questions concerning how and where the Pi2 pulsations themselves are generated, how they propagate through the system, and their relationship to flow bursts in the plasma sheet. In order to investigate Pi2 generation and propagation through the magnetosphere, and their relationship to bursty flows, we have modeled a substorm on 14-Sep-2004 using both the Lyon-Fedder-Mobarry (LFM) and the UCLA 3D global magnetohydrodynamic (MHD) models. By using two different models we show that the perturbations develop as a result of the equations, and do not depend on the set-up or numerical methods used in a particular model. The results from both models show that, in the braking region, the region where flows are slowed as they approach the inner edge of the plasma sheet, Pi2 period pulsations develop in the magnetic field, velocity, and pressure, concurrently with the arrival of fast earthward flows and their associated dipolarization fronts. The disturbance generated by the fast flows at the inner edge of the plasma sheet creates Alfvén waves that map to auroral regions and compressional waves that propagate deeper into the inner magnetosphere. The fast flows also generate Pi2 period fluctuations in the field aligned currents at the inner edge of the plasma sheet and in the inner magnetosphere.
NASA Astrophysics Data System (ADS)
Huang, Zhenguang; Tóth, Gábor; Gombosi, Tamas I.; Jia, Xianzhe; Rubin, Martin; Fougere, Nicolas; Tenishev, Valeriy; Combi, Michael R.; Bieler, Andre; Hansen, Kenneth C.; Shou, Yinsi; Altwegg, Kathrin
2016-05-01
The neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67P/Churyumov-Gerasimenko (CG), the target of the European Space Agency's Rosetta mission. To serve this need and support the Rosetta mission, we have developed a 3-D four-fluid model, which is based on BATS-R-US (Block-Adaptive Tree Solarwind Roe-type Upwind Scheme) within SWMF (Space Weather Modeling Framework) that solves the governing multifluid MHD equations and the Euler equations for the neutral gas fluid. These equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. This model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion-electron recombination, and collisional interactions between different fluids. We simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination-driven neutral gas outflow, and comet CG with a realistic shape model and illumination-driven neutral gas outflow. We compared the results of the three cases and showed that the simulations with illumination-driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature.
Downs, Cooper; Roussev, Ilia I.; Lugaz, Noe; Van der Holst, Bart; Sokolov, Igor V.
2012-05-10
In this work, we present a comprehensive observation and modeling analysis of the 2010 June 13 extreme-ultraviolet (EUV) wave observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). Due to extreme advances in cadence, resolution, and bandpass coverage in the EUV regime, the AIA instrument offers an unprecedented ability to observe the dynamics of large-scale coronal wave-like transients known as EUV waves. To provide a physical analysis and further complement observational insight, we conduct a three-dimensional, time-dependent thermodynamic MHD simulation of the eruption and associated EUV wave, and employ forward modeling of EUV observables to compare the results directly observations. We focus on two main aspects: (1) the interpretation of the stark thermodynamic signatures in the multi-filter AIA data within the propagating EUV wave front, and (2) an in-depth analysis of the simulation results and their implication with respect to EUV wave theories. Multiple aspects, including the relative phases of perturbed variables, suggest that the outer, propagating component of the EUV transient exhibits the behavior of a fast-mode wave. We also find that this component becomes decoupled from the evolving structures associated with the coronal mass ejection that are also visible, providing a clear distinction between wave and non-wave mechanisms at play.
Featured Image: Tests of an MHD Code
NASA Astrophysics Data System (ADS)
Kohler, Susanna
2016-09-01
Creating the codes that are used to numerically model astrophysical systems takes a lot of work and a lot of testing! A new, publicly available moving-mesh magnetohydrodynamics (MHD) code, DISCO, is designed to model 2D and 3D orbital fluid motion, such as that of astrophysical disks. In a recent article, DISCO creator Paul Duffell (University of California, Berkeley) presents the code and the outcomes from a series of standard tests of DISCOs stability, accuracy, and scalability.From left to right and top to bottom, the test outputs shown above are: a cylindrical Kelvin-Helmholtz flow (showing off DISCOs numerical grid in 2D), a passive scalar in a smooth vortex (can DISCO maintain contact discontinuities?), a global look at the cylindrical Kelvin-Helmholtz flow, a Jupiter-mass planet opening a gap in a viscous disk, an MHD flywheel (a test of DISCOs stability), an MHD explosion revealing shock structures, an MHD rotor (a more challenging version of the explosion), a Flock 3D MRI test (can DISCO study linear growth of the magnetorotational instability in disks?), and a nonlinear 3D MRI test.Check out the gif below for a closer look at each of these images, or follow the link to the original article to see even more!CitationPaul C. Duffell 2016 ApJS 226 2. doi:10.3847/0067-0049/226/1/2
NASA Astrophysics Data System (ADS)
Babbick, M.; Dijkstra, C.; Larkin, O. J.; Anthony, P.; Davey, M. R.; Power, J. B.; Lowe, K. C.; Cogoli-Greuter, M.; Hampp, R.
Gravity is an important environmental factor that controls plant growth and development. Studies have shown that the perception of gravity is not only a property of specialized cells, but can also be performed by undifferentiated cultured cells. In this investigation, callus of Arabidopsis thaliana cv. Columbia was used to investigate the initial steps of gravity-related signalling cascades, through altered expression of transcription factors (TFs). TFs are families of small proteins that regulate gene expression by binding to specific promoter sequences. Based on microarray studies, members of the gene families WRKY, MADS-box, MYB, and AP2/EREBP were selected for investigation, as well as members of signalling chains, namely IAA 19 and phosphoinositol-4-kinase. Using qRT-PCR, transcripts were quantified within a period of 30 min in response to hypergravity (8 g), clinorotation [2-D clinostat and 3-D random positioning machine (RPM)] and magnetic levitation (ML). The data indicated that (1) changes in gravity induced stress-related signalling, and (2) exposure in the RPM induced changes in gene expression which resemble those of magnetic levitation. Two dimensional clinorotation resulted in responses similar to those caused by hypergravity. It is suggested that RPM and ML are preferable to simulate microgravity than clinorotation.
NASA Astrophysics Data System (ADS)
Zhao, Yi; Fu, Ceji
2016-10-01
Tailoring the spectrum of thermal emission from the emitter is important for improving the performance of a thermophotovoltaic (TPV) system. In this work, a two-dimensional (2D) layered grating structure made of SiO2 and tungsten (W), which can realize wavelength-selective control of thermal emission, was proposed for a potential emitter in TPV applications. Numerical simulations of the spectral emissivity of the structure from the ultraviolet (UV) to the mid-infrared region reveals that the spectral-normal emissivity of the structure is enhanced to above 0.95 in the wavelength region from 0.55 μm to 1.9 μm for both TE and TM waves, but drops sharply at wavelength larger than 2 μm. Physical mechanisms responsible for the wavelength-selective emissivity were elucidated as due to resonance of magnetic polaritons (MPs) in the SiO2 spacer and in the grooves of the tungsten grating, Wood's anomaly (WA), excitation of surface plasmon polaritons (SPPs) and wave interference. Furthermore, the structure was found to exhibit quasi-diffuse and polarization-insensitive features of thermal emission, suggesting that the proposed structure can serve as the emitter in the design of high performance TPV systems.
NASA Astrophysics Data System (ADS)
Rockwood, Matthew; Green, Melissa
2012-11-01
In experimental, three-dimensional vortex-dominated flows, common particle image velocimetry (PIV) data is often collected in only the plane of interest due to equipment constraints. For flows with significant out of plane velocities or velocity gradients, this can create large discrepancies in Lagrangian analyses that require accurate particle trajectories. A Finite Time Lyapunov Exponent (FTLE) analysis is one such example, and has been shown to be very powerful at examining vortex dynamics and interactions in a variety of aperiodic flows. In this work, FTLE analysis of a turbulent channel simulation was conducted using both full three-dimensional velocity data and modified planar data extracted from the same computational domain. When the out of plane velocity component is neglected the difference in FTLE fields is non-trivial. A quantitative comparison and computation of error is presented for several planes across the width of the channel to determine the efficacy of using 2D analyses on the inherently 3D flows.
Icarus: A 2D direct simulation Monte Carlo (DSMC) code for parallel computers. User`s manual - V.3.0
Bartel, T.; Plimpton, S.; Johannes, J.; Payne, J.
1996-10-01
Icarus is a 2D Direct Simulation Monte Carlo (DSMC) code which has been optimized for the parallel computing environment. The code is based on the DSMC method of Bird and models from free-molecular to continuum flowfields in either cartesian (x, y) or axisymmetric (z, r) coordinates. Computational particles, representing a given number of molecules or atoms, are tracked as they have collisions with other particles or surfaces. Multiple species, internal energy modes (rotation and vibration), chemistry, and ion transport are modelled. A new trace species methodology for collisions and chemistry is used to obtain statistics for small species concentrations. Gas phase chemistry is modelled using steric factors derived from Arrhenius reaction rates. Surface chemistry is modelled with surface reaction probabilities. The electron number density is either a fixed external generated field or determined using a local charge neutrality assumption. Ion chemistry is modelled with electron impact chemistry rates and charge exchange reactions. Coulomb collision cross-sections are used instead of Variable Hard Sphere values for ion-ion interactions. The electrostatic fields can either be externally input or internally generated using a Langmuir-Tonks model. The Icarus software package includes the grid generation, parallel processor decomposition, postprocessing, and restart software. The commercial graphics package, Tecplot, is used for graphics display. The majority of the software packages are written in standard Fortran.
NASA Astrophysics Data System (ADS)
Ocłoń, Paweł; Łopata, Stanisław; Nowak, Marzena
2014-09-01
This study presents a novel, simplified model for the time-efficient simulation of transient conjugate heat transfer in round tubes. The flow domain and the tube wall are modeled in 1D and 2D, respectively and empirical correlations are used to model the flow domain in 1D. The model is particularly useful when dealing with complex physics, such as flow boiling, which is the main focus of this study. The tube wall is assumed to have external fins. The flow is vertical upwards. Note that straightforward computational fluid dynamics (CFD) analysis of conjugate heat transfer in a system of tubes, leads to 3D modeling of fluid and solid domains. Because correlation is used and dimensionality reduced, the model is numerically more stable and computationally more time-efficient compared to the CFD approach. The benefit of the proposed approach is that it can be applied to large systems of tubes as encountered in many practical applications. The modeled equations are discretized in space using the finite volume method, with central differencing for the heat conduction equation in the solid domain, and upwind differencing of the convective term of the enthalpy transport equation in the flow domain. An explicit time discretization with forward differencing was applied to the enthalpy transport equation in the fluid domain. The conduction equation in the solid domain was time discretized using the Crank-Nicholson scheme. The model is applied in different boundary conditions and the predicted boiling patterns and temperature fields are discussed.
NASA Astrophysics Data System (ADS)
Ocłoń, Paweł; Łopata, Stanisław; Nowak, Marzena
2015-04-01
This study presents a novel, simplified model for the time-efficient simulation of transient conjugate heat transfer in round tubes. The flow domain and the tube wall are modeled in 1D and 2D, respectively and empirical correlations are used to model the flow domain in 1D. The model is particularly useful when dealing with complex physics, such as flow boiling, which is the main focus of this study. The tube wall is assumed to have external fins. The flow is vertical upwards. Note that straightforward computational fluid dynamics (CFD) analysis of conjugate heat transfer in a system of tubes, leads to 3D modeling of fluid and solid domains. Because correlation is used and dimensionality reduced, the model is numerically more stable and computationally more time-efficient compared to the CFD approach. The benefit of the proposed approach is that it can be applied to large systems of tubes as encountered in many practical applications. The modeled equations are discretized in space using the finite volume method, with central differencing for the heat conduction equation in the solid domain, and upwind differencing of the convective term of the enthalpy transport equation in the flow domain. An explicit time discretization with forward differencing was applied to the enthalpy transport equation in the fluid domain. The conduction equation in the solid domain was time discretized using the Crank-Nicholson scheme. The model is applied in different boundary conditions and the predicted boiling patterns and temperature fields are discussed.
NASA Astrophysics Data System (ADS)
Tirupathi, S.; Schiemenz, A. R.; Liang, Y.; Parmentier, E.; Hesthaven, J.
2013-12-01
The style and mode of melt migration in the mantle are important to the interpretation of basalts erupted on the surface. Both grain-scale diffuse porous flow and channelized melt migration have been proposed. To better understand the mechanisms and consequences of melt migration in a heterogeneous mantle, we have undertaken a numerical study of reactive dissolution in an upwelling and viscously deformable mantle where solubility of pyroxene increases upwards. Our setup is similar to that described in [1], except we use a larger domain size in 2D and 3D and a new numerical method. To enable efficient simulations in 3D through parallel computing, we developed a high-order accurate numerical method for the magma dynamics problem using discontinuous Galerkin methods and constructed the problem using the numerical library deal.II [2]. Linear stability analyses of the reactive dissolution problem reveal three dynamically distinct regimes [3] and the simulations reported in this study were run in the stable regime and the unstable wave regime where small perturbations in porosity grows periodically. The wave regime is more relevant to melt migration beneath the mid-ocean ridges but computationally more challenging. Extending the 2D simulations in the stable regime in [1] to 3D using various combinations of sustained perturbations in porosity at the base of the upwelling column (which may result from a viened mantle), we show the geometry and distribution of dunite channel and high-porosity melt channels are highly correlated with inflow perturbation through superposition. Strong nonlinear interactions among compaction, dissolution, and upwelling give rise to porosity waves and high-porosity melt channels in the wave regime. These compaction-dissolution waves have well organized but time-dependent structures in the lower part of the simulation domain. High-porosity melt channels nucleate along nodal lines of the porosity waves, growing downwards. The wavelength scales
NASA Astrophysics Data System (ADS)
López-Venegas, Alberto M.; Horrillo, Juan; Pampell-Manis, Alyssa; Huérfano, Victor; Mercado, Aurelio
2015-06-01
The most recent tsunami observed along the coast of the island of Puerto Rico occurred on October 11, 1918, after a magnitude 7.2 earthquake in the Mona Passage. The earthquake was responsible for initiating a tsunami that mostly affected the northwestern coast of the island. Runup values from a post-tsunami survey indicated the waves reached up to 6 m. A controversy regarding the source of the tsunami has resulted in several numerical simulations involving either fault rupture or a submarine landslide as the most probable cause of the tsunami. Here we follow up on previous simulations of the tsunami from a submarine landslide source off the western coast of Puerto Rico as initiated by the earthquake. Improvements on our previous study include: (1) higher-resolution bathymetry; (2) a 3D-2D coupled numerical model specifically developed for the tsunami; (3) use of the non-hydrostatic numerical model NEOWAVE (non-hydrostatic evolution of ocean WAVE) featuring two-way nesting capabilities; and (4) comprehensive energy analysis to determine the time of full tsunami wave development. The three-dimensional Navier-Stokes model tsunami solution using the Navier-Stokes algorithm with multiple interfaces for two fluids (water and landslide) was used to determine the initial wave characteristic generated by the submarine landslide. Use of NEOWAVE enabled us to solve for coastal inundation, wave propagation, and detailed runup. Our results were in agreement with previous work in which a submarine landslide is favored as the most probable source of the tsunami, and improvement in the resolution of the bathymetry yielded inundation of the coastal areas that compare well with values from a post-tsunami survey. Our unique energy analysis indicates that most of the wave energy is isolated in the wave generation region, particularly at depths near the landslide, and once the initial wave propagates from the generation region its energy begins to stabilize.
NASA Astrophysics Data System (ADS)
Federrath, Christoph; Sur, Sharanya; Schleicher, Dominik R. G.; Banerjee, Robi; Klessen, Ralf S.
2011-04-01
Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E sol/E tot ≈ 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficiently resolved. We find that this scale corresponds to about 30 grid cells in the simulations. We thus suggest a new minimum resolution criterion of 30 cells per Jeans length in (magneto)hydrodynamical simulations of self-gravitating gas, in order to resolve turbulence on the Jeans scale, and to capture minimum dynamo amplification of the magnetic field. Due to numerical diffusion, however, any existing simulation today can at best provide lower limits on the physical growth rates. We conclude that a small, initial magnetic field can grow to dynamically important strength on timescales significantly shorter than the free-fall time of the cloud.
Numerical MHD codes for modeling astrophysical flows
NASA Astrophysics Data System (ADS)
Koldoba, A. V.; Ustyugova, G. V.; Lii, P. S.; Comins, M. L.; Dyda, S.; Romanova, M. M.; Lovelace, R. V. E.
2016-05-01
We describe a Godunov-type magnetohydrodynamic (MHD) code based on the Miyoshi and Kusano (2005) solver which can be used to solve various astrophysical hydrodynamic and MHD problems. The energy equation is in the form of entropy conservation. The code has been implemented on several different coordinate systems: 2.5D axisymmetric cylindrical coordinates, 2D Cartesian coordinates, 2D plane polar coordinates, and fully 3D cylindrical coordinates. Viscosity and diffusivity are implemented in the code to control the accretion rate in the disk and the rate of penetration of the disk matter through the magnetic field lines. The code has been utilized for the numerical investigations of a number of different astrophysical problems, several examples of which are shown.
MHD Turbulence and Magnetic Dynamos
NASA Technical Reports Server (NTRS)
Shebalin, John V
2014-01-01
Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implici