2,331 research outputs found
Ray Tracing Structured AMR Data Using ExaBricks
Structured Adaptive Mesh Refinement (Structured AMR) enables simulations to
adapt the domain resolution to save computation and storage, and has become one
of the dominant data representations used by scientific simulations; however,
efficiently rendering such data remains a challenge. We present an efficient
approach for volume- and iso-surface ray tracing of Structured AMR data on
GPU-equipped workstations, using a combination of two different data
structures. Together, these data structures allow a ray tracing based renderer
to quickly determine which segments along the ray need to be integrated and at
what frequency, while also providing quick access to all data values required
for a smooth sample reconstruction kernel. Our method makes use of the RTX ray
tracing hardware for surface rendering, ray marching, space skipping, and
adaptive sampling; and allows for interactive changes to the transfer function
and implicit iso-surfacing thresholds. We demonstrate that our method achieves
high performance with little memory overhead, enabling interactive high quality
rendering of complex AMR data sets on individual GPU workstations
Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations
Gas falling into a black hole (BH) from large distances is unaware of BH spin
direction, and misalignment between the accretion disc and BH spin is expected
to be common. However, the physics of tilted discs (e.g., angular momentum
transport and jet formation) is poorly understood. Using our new
GPU-accelerated code H-AMR, we performed 3D general relativistic
magnetohydrodynamic simulations of tilted thick accretion discs around rapidly
spinning BHs, at the highest resolution to date. We explored the limit where
disc thermal pressure dominates magnetic pressure, and showed for the first
time that, for different magnetic field strengths on the BH, these flows launch
magnetized relativistic jets propagating along the rotation axis of the tilted
disc (rather than of the BH). If strong large-scale magnetic flux reaches the
BH, it bends the inner few gravitational radii of the disc and jets into
partial alignment with the BH spin. On longer time scales, the simulated
disc-jet system as a whole undergoes Lense-Thirring precession and approaches
alignment, demonstrating for the first time that jets can be used as probes of
disc precession. When the disc turbulence is well-resolved, our isolated discs
spread out, causing both the alignment and precession to slow down.Comment: MNRAS Letters, accepted. Animations available at
https://www.youtube.com/playlist?list=PL39mDr1uU6a5RYZdXLAjKE1C_GAJkQJN
Simulating streamer discharges in 3D with the parallel adaptive Afivo framework
We present an open-source plasma fluid code for 2D, cylindrical and 3D
simulations of streamer discharges, based on the Afivo framework that features
adaptive mesh refinement, geometric multigrid methods for Poisson's equation,
and OpenMP parallelism. We describe the numerical implementation of a fluid
model of the drift-diffusion-reaction type, combined with the local field
approximation. Then we demonstrate its functionality with 3D simulations of
long positive streamers in nitrogen in undervolted gaps, using three examples.
The first example shows how a stochastic background density affects streamer
propagation and branching. The second one focuses on the interaction of a
streamer with preionized regions, and the third one investigates the
interaction between two streamers. The simulations run on up to grid
cells within less than a day. Without mesh refinement, they would require
grid cells
Numerical simulations of possible finite time singularities in the incompressible Euler equations: comparison of numerical methods
The numerical simulation of the 3D incompressible Euler equation is analyzed
with respect to different integration methods. The numerical schemes we
considered include spectral methods with different strategies for dealiasing
and two variants of finite difference methods. Based on this comparison, a
Kida-Pelz like initial condition is integrated using adaptive mesh refinement
and estimates on the necessary numerical resolution are given. This estimate is
based on analyzing the scaling behavior similar to the procedure in critical
phenomena and present simulations are put into perspective.Comment: Euler equations: 250 years o
Effect of dust on Kelvin-Helmholtz instabilities
Dust is present in a large variety of astrophysical fluids, from tori around
supermassive black holes to molecular clouds, protoplanetary discs, and
cometary outflows. In many such fluids, shearing flows are present, leading to
the formation of Kelvin-Helmholtz instabilities (KHI) and changing the
properties and structures of the fluid through processes such as mixing and
clumping of dust. We investigate how dust changes the growth rates of the KHI
in 2D and 3D and how the it redistributes and clumps dust. We investigate if
similarities can be found between the structures in 3D KHI and those seen in
observations of molecular clouds. We do this by performing numerical
hydrodynamical dust+gas simulations with in addition to gas a number of dust
fluids. Each dust fluid represents a portion of the particle size-distribution.
We study how dust-to-gas mass density ratios between 0.01 and 1 alter the
growth rate in the linear phase of the KHI. We do this for a wide range of
perturbation wavelengths, and compare these values to the analytical gas-only
growth rates. As the formation of high-density dust structures is of interest
in many astrophysical environments, we scale our simulations with physical
quantities similar to values in molecular clouds. Large differences in dynamics
are seen for different grain sizes. We demonstrate that high dust-to-gas ratios
significantly reduce the growth rate of the KHI, especially for short
wavelengths. We compare the dynamics in 2D and 3D simulations, where the latter
demonstrates additional full 3D instabilities during the non-linear phase,
leading to increased dust densities. We compare the structures formed by the
KHI in 3D simulations with those in molecular clouds and see how the column
density distribution of the simulation shares similarities with log-normal
distributions with power-law tails sometimes seen in observations of molecular
clouds.Comment: 14 pages, 20 figure
Adaptive Mesh Refinement for Supersonic Molecular Cloud Turbulence
We performed a series of three-dimensional numerical simulations of
supersonic homogeneous Euler turbulence with adaptive mesh refinement (AMR) and
effective grid resolution up to 1024^3 zones. Our experiments describe
non-magnetized driven supersonic turbulent flows with an isothermal equation of
state. Mesh refinement on shocks and shear is implemented to cover dynamically
important structures with the highest resolution subgrids and calibrated to
match the turbulence statistics obtained from the equivalent uniform grid
simulations.
We found that at a level of resolution slightly below 512^3, when a
sufficient integral/dissipation scale separation is first achieved, the
fraction of the box volume covered by the AMR subgrids first becomes smaller
than unity. At the higher AMR levels subgrids start covering smaller and
smaller fractions of the whole volume, which scale with the Reynolds number as
Re^{-1/4}. We demonstrate the consistency of this scaling with a hypothesis
that the most dynamically important structures in intermittent supersonic
turbulence are strong shocks with a fractal dimension of two. We show that
turbulence statistics derived from AMR simulations and simulations performed on
uniform grids agree surprisingly well, even though only a fraction of the
volume is covered by AMR subgrids. Based on these results, we discuss the
signature of dissipative structures in the statistical properties of supersonic
turbulence and their role in overall flow dynamics.Comment: 5 pages, 5 figures, revised versio
A Multi-Code Analysis Toolkit for Astrophysical Simulation Data
The analysis of complex multiphysics astrophysical simulations presents a
unique and rapidly growing set of challenges: reproducibility, parallelization,
and vast increases in data size and complexity chief among them. In order to
meet these challenges, and in order to open up new avenues for collaboration
between users of multiple simulation platforms, we present yt (available at
http://yt.enzotools.org/), an open source, community-developed astrophysical
analysis and visualization toolkit. Analysis and visualization with yt are
oriented around physically relevant quantities rather than quantities native to
astrophysical simulation codes. While originally designed for handling Enzo's
structure adaptive mesh refinement (AMR) data, yt has been extended to work
with several different simulation methods and simulation codes including Orion,
RAMSES, and FLASH. We report on its methods for reading, handling, and
visualizing data, including projections, multivariate volume rendering,
multi-dimensional histograms, halo finding, light cone generation and
topologically-connected isocontour identification. Furthermore, we discuss the
underlying algorithms yt uses for processing and visualizing data, and its
mechanisms for parallelization of analysis tasks.Comment: 18 pages, 6 figures, emulateapj format. Resubmitted to Astrophysical
Journal Supplement Series with revisions from referee. yt can be found at
http://yt.enzotools.org
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