347 research outputs found
An Intercomparison Between Divergence-Cleaning and Staggered Mesh Formulations for Numerical Magnetohydrodynamics
In recent years, several different strategies have emerged for evolving the
magnetic field in numerical MHD. Some of these methods can be classified as
divergence-cleaning schemes, where one evolves the magnetic field components
just like any other variable in a higher order Godunov scheme. The fact that
the magnetic field is divergence-free is imposed post-facto via a
divergence-cleaning step. Other schemes for evolving the magnetic field rely on
a staggered mesh formulation which is inherently divergence-free. The claim has
been made that the two approaches are equivalent. In this paper we
cross-compare three divergence-cleaning schemes based on scalar and vector
divergence-cleaning and a popular divergence-free scheme. All schemes are
applied to the same stringent test problem. Several deficiencies in all the
divergence-cleaning schemes become clearly apparent with the scalar
divergence-cleaning schemes performing worse than the vector
divergence-cleaning scheme. The vector divergence-cleaning scheme also shows
some deficiencies relative to the staggered mesh divergence-free scheme. The
differences can be explained by realizing that all the divergence-cleaning
schemes are based on a Poisson solver which introduces a non-locality into the
scheme, though other subtler points of difference are also catalogued. By using
several diagnostics that are routinely used in the study of turbulence, it is
shown that the differences in the schemes produce measurable differences in
physical quantities that are of interest in such studies
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
Second Order Accurate Schemes for Magnetohydrodynamics With Divergence-Free Reconstruction
In this paper we study the problem of divergence-free numerical MHD and show
that the work done so far still has four key unresolved issues. We resolve
those issues in this paper. The problem of reconstructing MHD flow variables
with spatially second order accuracy is also studied. The other goal of this
paper is to show that the same well-designed second order accurate schemes can
be formulated for more complex geometries such as cylindrical and spherical
geometry. Being able to do divergence-free reconstruction in those geometries
also resolves the problem of doing AMR in those geometries. The resulting MHD
scheme has been implemented in Balsara's RIEMANN framework for parallel,
self-adaptive computational astrophysics. The present work also shows that
divergence-free reconstruction and the divergence-free time-update can be done
for numerical MHD on unstructured meshes. All the schemes designed here are
shown to be second order accurate. Several stringent test problems are
presented to show that the methods work, including problems involving high
velocity flows in low plasma-b magnetospheric environments.Comment: 85 pages, 6 figure
Modelling the Interfacial Flow of Two Immiscible Liquids in Mixing Processes
This paper presents an interface tracking method for modelling the flow of immiscible metallic liquids in mixing processes. The methodology can provide an insight into mixing processes for studying the fundamental morphology development mechanisms for immiscible interfaces. The volume-of-fluid (VOF) method is adopted in the present study, following a review of various modelling approaches for immiscible fluid systems. The VOF method employed here utilises the piecewise linear for interface construction scheme as well as the continuum surface force algorithm for surface force modelling. A model coupling numerical and experimental data is established. The main flow features in the mixing process are investigated. It is observed that the mixing of immiscible metallic liquids is strongly influenced by the viscosity of the system, shear forces and turbulence. The numerical results show good qualitative agreement with experimental results, and are useful for optimisating the design of mixing casting processes
Investigation of transition frequencies of two acoustically coupled bubbles using a direct numerical simulation technique
The theoretical results regarding the ``transition frequencies'' of two
acoustically interacting bubbles have been verified numerically. The theory
provided by Ida [Phys. Lett. A 297 (2002) 210] predicted the existence of three
transition frequencies per bubble, each of which has the phase difference of
between a bubble's pulsation and the external sound field, while
previous theories predicted only two natural frequencies which cause such phase
shifts. Namely, two of the three transition frequencies correspond to the
natural frequencies, while the remaining does not. In a subsequent paper [M.
Ida, Phys. Rev. E 67 (2003) 056617], it was shown theoretically that transition
frequencies other than the natural frequencies may cause the sign reversal of
the secondary Bjerknes force acting between pulsating bubbles. In the present
study, we employ a direct numerical simulation technique that uses the
compressible Navier-Stokes equations with a surface-tension term as the
governing equations to investigate the transition frequencies of two coupled
bubbles by observing their pulsation amplitudes and directions of translational
motion, both of which change as the driving frequency changes. The numerical
results reproduce the recent theoretical predictions, validating the existence
of the transition frequencies not corresponding to the natural frequency.Comment: 18 pages, 8 figures, in pres
2-D Magnetohydrodynamic Simulations of Induced Plasma Dynamics in the Near-Core Region of a Galaxy Cluster
We present results from numerical simulations of the cooling-core cluster
A2199 produced by the two-dimensional (2-D) resistive magnetohydrodynamics
(MHD) code MACH2. In our simulations we explore the effect of anisotropic
thermal conduction on the energy balance of the system. The results from
idealized cases in 2-D axisymmetric geometry underscore the importance of the
initial plasma density in ICM simulations, especially the near-core values
since the radiation cooling rate is proportional to . Heat conduction
is found to be non-effective in preventing catastrophic cooling in this
cluster. In addition we performed 2-D planar MHD simulations starting from
initial conditions deliberately violating both thermal balance and hydrostatic
equilibrium in the ICM, to assess contributions of the convective terms in the
energy balance of the system against anisotropic thermal conduction. We find
that in this case work done by the pressure on the plasma can dominate the
early evolution of the internal energy over anisotropic thermal conduction in
the presence of subsonic flows, thereby reducing the impact of the magnetic
field. Deviations from hydrostatic equilibrium near the cluster core may be
associated with transient activity of a central active galactic nucleus and/or
remnant dynamical activity in the ICM and warrant further study in three
dimensions.Comment: 16 pages, 13 figures, accepted for publication in MNRA
Respiratory Protective Equipment, Mask Use, and Respiratory Outcomes among World Trade Center Rescue and Recovery Workers
Background
Serious respiratory illnesses have been reported among rescue/recovery workers (RRW) following the World Trade Center (WTC) attacks.
Methods
We studied RRW enrolled in the WTC Health Registry to assess the effects of different respiratory protection equipment (RPE) types on respiratory outcomes, such as recurrent respiratory symptoms and diseases possibly associated with 9/11 exposures. We performed descriptive and multivariate analyses adjusting for demographics and exposure variables.
Results
A total of 9,296 RRW met inclusion criteria. The strongest predictors of using adequate RPE were being affiliated with construction, utilities or environmental remediation organizations and having received RPE training. Workers who used respirators were less likely to report adverse respiratory outcomes compared to those who reported no/lower levels of respiratory protection.
Conclusions
Level of respiratory protection was associated with the odds of reporting respiratory symptoms and diseases. Training, selection, fit testing, and consistent use of RPE should be emphasized among emergency responders
Local Turbulence Simulations for the Multiphase ISM
In this paper we show results of numerical simulations for the turbulence in
the interstellar medium. These results were obtained using a Riemann
solver-free numerical scheme for high-Mach number hyperbolic equations. Here we
especially concentrate on the physical properties of the ISM. That is, we do
not present turbulence simulations trimmed to be applicable to the interstellar
medium. The simulations are rather based on physical estimates for the relevant
parameters of the interstellar gas.
Applying our code to simulate the turbulent plasma motion within a typical
interstellar molecular cloud, we investigate the influence of different
equations of state (isothermal and adiabatic) on the statistical properties of
the resulting turbulent structures. We find slightly different density power
spectra and dispersion maps, while both cases yield qualitatively similar
dissipative structures, and exhibit a departure from the classical Kolmogorov
case towards a scaling described by the She-Leveque model.
Solving the full energy equation with realistic heating/cooling terms
appropriate for the diffuse interstellar gas, we are able to reproduce a
realistic two-phase distribution of cold and warm plasma. When extracting maps
of polarised intensity from our simulation data, we find encouraging similarity
to actual observations. Finally, we compare the actual magnetic field strength
of our simulations to its value inferred from the rotation measure. We find
these to be systematically different by a factor of about 1.5, thus
highlighting the often underestimated influence of varying line-of-sight
particle densities on the magnetic field strength derived from observed
rotation measures
Electron acceleration and heating in collisionless magnetic reconnection
We discuss electron acceleration and heating during collisionless magnetic
reconnection by using the results of implicit kinetic simulations of Harris
current sheets. We consider and compare electron dynamics in plasmas with
different \beta values and perform simulations up to the physical mass ratio.
We analyze the typical trajectory of electrons passing through the reconnection
region, we study the electron velocity, focusing on the out-of-plane velocity,
and we discuss the electron heating along the in-plane and out-of-plane
directions
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