838 research outputs found
Computer Simulations of Cosmic Reionization
The cosmic reionization of hydrogen was the last major phase transition in
the evolution of the universe, which drastically changed the ionization and
thermal conditions in the cosmic gas. To the best of our knowledge today, this
process was driven by the ultra-violet radiation from young, star-forming
galaxies and from first quasars. We review the current observational
constraints on cosmic reionization, as well as the dominant physical effects
that control the ionization of intergalactic gas. We then focus on numerical
modeling of this process with computer simulations. Over the past decade,
significant progress has been made in solving the radiative transfer of
ionizing photons from many sources through the highly inhomogeneous
distribution of cosmic gas in the expanding universe. With modern simulations,
we have finally converged on a general picture for the reionization process,
but many unsolved problems still remain in this young and exciting field of
numerical cosmology.Comment: Invited Review to appear on Advanced Science Letters (ASL), Special
Issue on Computational Astrophysics, edited by Lucio Maye
A Moving Frame Algorithm for High Mach Number Hydrodynamics
We present a new approach to Eulerian computational fluid dynamics that is
designed to work at high Mach numbers encountered in astrophysical hydrodynamic
simulations. The Eulerian fluid conservation equations are solved in an
adaptive frame moving with the fluid where Mach numbers are minimized. The
moving frame approach uses a velocity decomposition technique to define local
kinetic variables while storing the bulk kinetic components in a smoothed
background velocity field that is associated with the grid velocity.
Gravitationally induced accelerations are added to the grid, thereby minimizing
the spurious heating problem encountered in cold gas flows. Separately tracking
local and bulk flow components allows thermodynamic variables to be accurately
calculated in both subsonic and supersonic regions. A main feature of the
algorithm, that is not possible in previous Eulerian implementations, is the
ability to resolve shocks and prevent spurious heating where both the preshock
and postshock Mach numbers are high. The hybrid algorithm combines the high
resolution shock capturing ability of the second-order accurate Eulerian TVD
scheme with a low-diffusion Lagrangian advection scheme. We have implemented a
cosmological code where the hydrodynamic evolution of the baryons is captured
using the moving frame algorithm while the gravitational evolution of the
collisionless dark matter is tracked using a particle-mesh N-body algorithm.
The MACH code is highly suited for simulating the evolution of the IGM where
accurate thermodynamic evolution is needed for studies of the Lyman alpha
forest, the Sunyaev-Zeldovich effect, and the X-ray background. Hydrodynamic
and cosmological tests are described and results presented. The current code is
fast, memory-friendly, and parallelized for shared-memory machines.Comment: 19 pages, 5 figure
Planar shape manipulation using approximate geometric primitives
We present robust algorithms for set operations and Euclidean transformations
of curved shapes in the plane using approximate geometric primitives. We use a
refinement algorithm to ensure consistency. Its computational complexity is
\bigo(n\log n+k) for an input of size with k=\bigo(n^2) consistency
violations. The output is as accurate as the geometric primitives. We validate
our algorithms in floating point using sequences of six set operations and
Euclidean transforms on shapes bounded by curves of algebraic degree~1 to~6. We
test generic and degenerate inputs.
Keywords: robust computational geometry, plane subdivisions, set operations
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