739 research outputs found
Two-Fluid MHD Simulations of Converging HI Flows in the Interstellar Medium. I: Methodology and Basic Results
We develop an unconditionally stable numerical method for solving the
coupling between two fluids (frictional forces/heatings, ionization, and
recombination), and investigate the dynamical condensation process of thermally
unstable gas that is provided by the shock waves in a weakly ionized and
magnetized interstellar medium by using two-dimensional two-fluid
magnetohydrodynamical simulations. If we neglect the effect of magnetic field,
it is known that condensation driven by thermal instability can generate high
density clouds whose physical condition corresponds to molecular clouds
(precursor of molecular clouds). In this paper, we study the effect of magnetic
field on the evolution of supersonic converging HI flows and focus on the case
in which the orientation of magnetic field to converging flows is orthogonal.
We show that the magnetic pressure gradient parallel to the flows prevents the
formation of high density and high column density clouds, but instead generates
fragmented, filamentary HI clouds. With this restricted geometry, magnetic
field drastically diminishes the opportunity of fast molecular cloud formation
directly from the warm neutral medium, in contrast to the case without magnetic
field.Comment: ApJ accepte
High Order Upwind Schemes for Multidimensional Magnetohydrodynamics
A general method for constructing high order upwind schemes for
multidimensional magnetohydrodynamics (MHD), having as a main built-in
condition the divergence-free constraint \divb=0 for the magnetic field
vector \bb, is proposed. The suggested procedure is based on {\em
consistency} arguments, by taking into account the specific operator structure
of MHD equations with respect to the reference Euler equations of gas-dynamics.
This approach leads in a natural way to a staggered representation of the \bb
field numerical data where the divergence-free condition in the cell-averaged
form, corresponding to second order accurate numerical derivatives, is exactly
fulfilled. To extend this property to higher order schemes, we then give
general prescriptions to satisfy a order accurate \divb=0
relation for any numerical \bb field having a order interpolation
accuracy. Consistency arguments lead also to a proper formulation of the upwind
procedures needed to integrate the induction equations, assuring the exact
conservation in time of the divergence-free condition and the related
continuity properties for the \bb vector components. As an application, a
third order code to simulate multidimensional MHD flows of astrophysical
interest is developed using ENO-based reconstruction algorithms. Several test
problems to illustrate and validate the proposed approach are finally
presented.Comment: 34 pages, including 14 figure
Analysis of shock capturing methods for chemical species transport in unsteady compressible flow
This paper presents a chemical species transport model to account for variable composition and gas properties along the flow path in internal combustion engines. The numerical solution to adapt the gas dynamic model to chemical species transport in boundary conditions by means of the Method of Characteristics and in volumes by means of a filling and emptying model is described. The performance for chemical species transport in 1D elementsof shock-capturing methods, such as the two-step Lax Wendroff method and the Sweby s TVD scheme considering several flux limiter definitions, is carried out by means of shocktube tests. The influence of the fluid modelling as perfect or non-perfect gas on the numerical methods features and the flow characteristics on shock-tube results are analysed.This work has been partially supported by the Spanish Ministerio de Ciencia e Innovacion through grant number DPI2010-20891-C02-02.Serrano Cruz, JR.; Climent, H.; Piqueras, P.; García Afonso, Ó. (2013). Analysis of shock capturing methods for chemical species transport in unsteady compressible flow. Mathematical and Computer Modelling. 57(7-8):1751-1759. https://doi.org/10.1016/j.mcm.2011.11.026S17511759577-
The interaction of a giant planet with a disc with MHD turbulence I: The initial turbulent disc models
This is the first of a series of papers aimed at developing and interpreting
simulations of protoplanets interacting with turbulent accretion discs. Here we
study the disc models prior to the introduction of a protoplanet.We study
models in which a Keplerian domain is unstable to the magnetorotational
instability (MRI). Various models with B-fields having zero net flux are
considered.We relate the properties of the models to classical viscous disc
theory.All models attain a turbulent state with volume averaged stress
parameter alpha ~ 0.005. At any particular time the vertically and azimuthally
averaged value exhibited large fluctuations in radius. Time averaging over
periods exceeding 3 orbital periods at the outer boundary of the disc resulted
in a smoother quantity with radial variations within a factor of two or so. The
vertically and azimuthally averaged radial velocity showed much larger spatial
and temporal fluctuations, requiring additional time averaging for 7-8 orbital
periods at the outer boundary to limit them. Comparison with the value derived
from the averaged stress using viscous disc theory yielded schematic agreement
for feasible averaging times but with some indication that the effects of
residual fluctuations remained. The behaviour described above must be borne in
mind when considering laminar disc simulations with anomalous Navier--Stokes
viscosity. This is because the operation of a viscosity as in classical viscous
disc theory with anomalous viscosity coefficient cannot apply to a turbulent
disc undergoing rapid changes due to external perturbation. The classical
theory can only be used to describe the time averaged behaviour of the parts of
the disc that are in a statistically steady condition for long enough for
appropriate averaging to be carried out.Comment: 10 pages, 23 figures, accepted for publication in MNRAS. A gzipped
postscript version including high resolution figures is available at
http://www.maths.qmul.ac.uk/~rp
Recommended from our members
Observation and analysis of in situ carbonaceous matter in Nakhla: part II
Analysis of in situ carbonaceous matter in the Nakhla SNC meteorite has been carried out using a variety of techniques. Laser raman data shows the carbonaceous matter to be highly complex and static mass spectrometry has shown it to have an isotopic composition of '18 to '20' C
An Euler Solver Based on Locally Adaptive Discrete Velocities
A new discrete-velocity model is presented to solve the three-dimensional
Euler equations. The velocities in the model are of an adaptive nature---both
the origin of the discrete-velocity space and the magnitudes of the
discrete-velocities are dependent on the local flow--- and are used in a finite
volume context. The numerical implementation of the model follows the
near-equilibrium flow method of Nadiga and Pullin [1] and results in a scheme
which is second order in space (in the smooth regions and between first and
second order at discontinuities) and second order in time. (The
three-dimensional code is included.) For one choice of the scaling between the
magnitude of the discrete-velocities and the local internal energy of the flow,
the method reduces to a flux-splitting scheme based on characteristics. As a
preliminary exercise, the result of the Sod shock-tube simulation is compared
to the exact solution.Comment: 17 pages including 2 figures and CMFortran code listing. All in one
postscript file (adv.ps) compressed and uuencoded (adv.uu). Name mail file
`adv.uu'. Edit so that `#!/bin/csh -f' is the first line of adv.uu On a unix
machine say `csh adv.uu'. On a non-unix machine: uudecode adv.uu; uncompress
adv.tar.Z; tar -xvf adv.ta
On the evolution of eccentric and inclined protoplanets embedded in protoplanetary disks
Young planets embedded in their protoplanetary disk interact gravitationally
with it leading to energy and angular momentum exchange. This interaction
determines the evolution of the planet through changes to the orbital
parameters. We investigate changes in the orbital elements of a 20 Earth--mass
planet due to the torques from the disk. We focus on the non-linear evolution
of initially non-vanishing eccentricity and/or inclination . We treat
the disk as a two- or three-dimensional viscous fluid and perform
hydrodynamical simulations with an embedded planet. We find rapid exponential
decay of the planet orbital eccentricity and inclination for small initial
values of and , in agreement with linear theory. For larger values of the decay time increases and the decay rate scales as , consistent with existing theoretical models. For large inclinations
( > 6 deg) the inclination decay rate shows an identical scaling . We find an interesting dependence of the migration on the
eccentricity. In a disk with aspect ratio the migration rate is
enhanced for small non-zero eccentricities (), while for larger values
we see a significant reduction by a factor of . We find no indication
for a reversal of the migration for large , although the torque experienced
by the planet becomes positive when . This inward migration is
caused by the persisting energy loss of the planet.
For non gap forming planets, eccentricity and inclination damping occurs on a
time scale that is very much shorter than the migration time scale. The results
of non linear hydrodynamic simulations are in very good agreement with linear
theory for small and .Comment: accepted for Astronomy & Astrophysics, 16 pages, 16 figures,
animations under:
http://www.tat.physik.uni-tuebingen.de/~kley/publ/paper/eccp.htm
The impact of the dark matter-gas interaction on the collapse behaviour of spherical symmetric systems
If the gas in the evolving cosmic halos is dissipating energy (cooling) then
due to the variation of the gravitational potential the dark matter halo also
undergoes a compactification. This is well-known as Adiabatic contraction (AC).
Complementary to the AC we investigate the resulting dynamical behaviour of the
whole system if the backreaction of the AC of DM onto the gas is taken into
account. In order to achieve sufficient high resolution also within the central
halo region, we use a crude fluid approximation for the DM obeying the
adiabatic contraction behaviour. Further, we restrict ourself to spherical
symmetry and vanishing angular momentum of the studied matter configurations.
The computations are done using a first-order Godunov type scheme. Our results
show that the dynamical interaction between gas and DM may lead to significant
shorter collapse times. If the gas cools the dynamical behaviour of the whole
system depends strongly on the shape of the initial density profile. Our
findings indicate that for a certain mass range of halo configurations the
dynamical interaction between gas and DM might be important for the halo
evolution and must be taken into account.Comment: 9 pages, 13 figures, accepted for publication in A&A, added
reference
On the Divergence-Free Condition in Godunov-Type Schemes for Ideal Magnetohydrodynamics: the Upwind Constrained Transport Method
We present a general framework to design Godunov-type schemes for
multidimensional ideal magnetohydrodynamic (MHD) systems, having the
divergence-free relation and the related properties of the magnetic field B as
built-in conditions. Our approach mostly relies on the 'Constrained Transport'
(CT) discretization technique for the magnetic field components, originally
developed for the linear induction equation, which assures div(B)=0 and its
preservation in time to within machine accuracy in a finite-volume setting. We
show that the CT formalism, when fully exploited, can be used as a general
guideline to design the reconstruction procedures of the B vector field, to
adapt standard upwind procedures for the momentum and energy equations,
avoiding the onset of numerical monopoles of O(1) size, and to formulate
approximate Riemann solvers for the induction equation. This general framework
will be named here 'Upwind Constrained Transport' (UCT). To demonstrate the
versatility of our method, we apply it to a variety of schemes, which are
finally validated numerically and compared: a novel implementation for the MHD
case of the second order Roe-type positive scheme by Liu and Lax (J. Comp.
Fluid Dynam. 5, 133, 1996), and both the second and third order versions of a
central-type MHD scheme presented by Londrillo and Del Zanna (Astrophys. J.
530, 508, 2000), where the basic UCT strategies have been first outlined
A New Computational Fluid Dynamics Code I: Fyris Alpha
A new hydrodynamics code aimed at astrophysical applications has been
developed. The new code and algorithms are presented along with a comprehensive
suite of test problems in one, two, and three dimensions.
The new code is shown to be robust and accurate, equalling or improving upon
a set of comparison codes. Fyris Alpha will be made freely available to the
scientific community.Comment: 59 pages, 27 figures For associated code see
http://www.mso.anu.edu.au/fyri
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