46,003 research outputs found
Fast and accurate frequency-dependent radiation transport for hydrodynamics simulations in massive star formation
Context: Radiative feedback plays a crucial role in the formation of massive
stars. The implementation of a fast and accurate description of the proceeding
thermodynamics in pre-stellar cores and evolving accretion disks is therefore a
main effort in current hydrodynamics simulations.
Aims: We introduce our newly implemented three-dimensional frequency
dependent radiation transport algorithm for hydrodynamics simulations of
spatial configurations with a dominant central source.
Methods: The module combines the advantage of the speed of an approximate
Flux Limited Diffusion (FLD) solver with the high accuracy of a frequency
dependent first order ray-tracing routine.
Results: We prove the viability of the scheme in a standard radiation
benchmark test compared to a full frequency dependent Monte-Carlo based
radiative transfer code. The setup includes a central star, a circumstellar
flared disk, as well as an envelope. The test is performed for different
optical depths. Considering the frequency dependence of the stellar
irradiation, the temperature distributions can be described precisely in the
optically thin, thick, and irradiated transition regions. Resulting radiative
forces onto dust grains are reproduced with high accuracy. The achievable
parallel speedup of the method imposes no restriction on further radiative
(magneto-) hydrodynamics simulations.
Conclusions: The proposed approximate radiation transport method enables
frequency dependent radiation hydrodynamics studies of the evolution of
pre-stellar cores and circumstellar accretion disks around an evolving massive
star in a highly efficient and accurate manner.Comment: 16 pages, 11 figure
A public code for general relativistic, polarised radiative transfer around spinning black holes
Ray tracing radiative transfer is a powerful method for comparing theoretical
models of black hole accretion flows and jets with observations. We present a
public code, grtrans, for carrying out such calculations in the Kerr metric,
including the full treatment of polarised radiative transfer and parallel
transport along geodesics. The code is written in Fortran 90 and efficiently
parallelises with OpenMP, and the full code and several components have Python
interfaces. We describe several tests which are used for verifiying the code,
and we compare the results for polarised thin accretion disc and semi-analytic
jet problems with those from the literature as examples of its use. Along the
way, we provide accurate fitting functions for polarised synchrotron emission
and transfer coefficients from thermal and power law distribution functions,
and compare results from numerical integration and quadrature solutions of the
polarised radiative transfer equations. We also show that all transfer
coefficients can play an important role in predicted images and polarisation
maps of the Galactic center black hole, Sgr A*, at submillimetre wavelengths.Comment: 22 pages, 12 figures, submitted to MNRAS. code available at:
github.com/jadexter/grtran
TRAPHIC - Radiative Transfer for Smoothed Particle Hydrodynamics Simulations
We present TRAPHIC, a novel radiative transfer scheme for Smoothed Particle
Hydrodynamics (SPH) simulations. TRAPHIC is designed for use in simulations
exhibiting a wide dynamic range in physical length scales and containing a
large number of light sources. It is adaptive both in space and in angle and
can be employed for application on distributed memory machines. The commonly
encountered computationally expensive scaling with the number of light sources
in the simulation is avoided by introducing a source merging procedure. The
(time-dependent) radiative transfer equation is solved by tracing individual
photon packets in an explicitly photon-conserving manner directly on the
unstructured grid traced out by the set of SPH particles. To accomplish
directed transport of radiation despite the irregular spatial distribution of
the SPH particles, photons are guided inside cones. We present and test a
parallel numerical implementation of TRAPHIC in the SPH code GADGET-2,
specified for the transport of mono-chromatic hydrogen-ionizing radiation. The
results of the tests are in excellent agreement with both analytic solutions
and results obtained with other state-of-the-art radiative transfer codes.Comment: 31 pages, 20 figures. Accepted for publication in MNRAS. Revised
version includes many clarifications and a new time-dependent radiative
transfer calculation (fig. 19
The Failure of Monte Carlo Radiative Transfer at Medium to High Optical Depths
Computer simulations of photon transport through an absorbing and/or
scattering medium form an important research tool in astrophysics. Nearly all
software codes performing such simulations for three-dimensional geometries
employ the Monte Carlo radiative transfer method, including various forms of
biasing to accelerate the calculations. Because of the probabilistic nature of
the Monte Carlo technique, the outputs are inherently noisy, but it is often
assumed that the average values provide the physically correct result. We show
that this assumption is not always justified. Specifically, we study the
intensity of radiation penetrating an infinite, uniform slab of material that
absorbs and scatters the radiation with equal probability. The basic Monte
Carlo radiative transfer method, without any biasing mechanisms, starts to
break down for transverse optical depths above ~20 because so few of the
simulated photon packets reach the other side of the slab. When including
biasing techniques such as absorption/scattering splitting and path length
stretching, the simulated photon packets do reach the other side of the slab
but the biased weights do not necessarily add up to the correct solution. While
the noise levels seem to be acceptable, the average values sometimes severely
underestimate the correct solution. Detecting these anomalies requires the
judicious application of statistical tests, similar to those used in the field
of nuclear particle transport, possibly in combination with convergence tests
employing consecutively larger numbers of photon packets. In any case, for
transverse optical depths above ~75 the Monte Carlo methods used in our study
fail to solve the one-dimensional slab problem, implying the need for
approximations such as a modified random walk.Comment: Accepted for publication in the ApJ; 13 pages, 6 figure
Adaptive multiscale methods for 3D streamer discharges in air
We discuss spatially and temporally adaptive implicit-explicit (IMEX) methods
for parallel simulations of three-dimensional fluid streamer discharges in
atmospheric air. We examine strategies for advancing the fluid equations and
elliptic transport equations (e.g. Poisson) with different time steps,
synchronizing them on a global physical time scale which is taken to be
proportional to the dielectric relaxation time. The use of a longer time step
for the electric field leads to numerical errors that can be diagnosed, and we
quantify the conditions where this simplification is valid. Likewise, using a
three-term Helmholtz model for radiative transport, the same error diagnostics
show that the radiative transport equations do not need to be resolved on time
scales finer than the dielectric relaxation time. Elliptic equations are
bottlenecks for most streamer simulation codes, and the results presented here
potentially provide computational savings. Finally, a computational example of
3D branching streamers in a needle-plane geometry that uses up to 700 million
grid cells is presented.Comment: 17 pages, 5 figure
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