Elad Steinberg

RICH: OPEN-SOURCE HYDRODYNAMIC SIMULATION ON A MOVING VORONOI MESH

We present here RICH, a state of the art 2D hydrodynamic code based on Godunovs method, on an unstructured moving mesh (the acronym stands for Racah Institute Computational Hydrodynamics). This code is largely based on the code AREPO. It differs from AREPO in the interpolation and time advancement scheme as well as a novel parallelization scheme based on Voronoi tessellation. Using our code we study the pros and cons of a moving mesh (in comparison to a static mesh). We also compare its accuracy to other codes. Specifically, we show that our implementation of external sources and time advancement scheme is more accurate and robust than AREPOs, when the mesh is allowed to move. We performed a parameter study of the cell rounding mechanism (Llyod iterations) and it effects. We find that in most cases a moving mesh gives better results than a static mesh, but it is not universally true. In the case where matter moves in one way, and a sound wave is traveling in the other way (such that relative to the grid the wave is not moving) a static mesh gives better results than a moving mesh. Moreover, we show that Voronoi based moving mesh schemes suffer from an error, that is resolution independent, due to inconsistencies between the flux calculation and change in the area of a cell. Our code is publicly available as open source and designed in an object oriented, user friendly way that facilitates incorporation of new algorithms and physical processes.

Instability of Supersonic Cold Streams Feeding Galaxies I: Linear Kelvin-Helmholtz Instability with Body Modes

Massive galaxies at high redshift are predicted to be fed from the cosmic web by narrow, dense, cold streams. These streams penetrate supersonically through the hot medium encompassed by a stable shock near the virial radius of the dark-matter halo. Our long-term goal is to explore the heating and dissipation rate of the streams and their fragmentation and possible breakup, in order to understand how galaxies are fed, and how this affects their star-formation rate and morphology. We present here the first step, where we analyze the linear Kelvin-Helmholtz instability (KHI) of a cold, dense slab or cylinder flowing through a hot, dilute medium in the transonic regime. The current analysis is limited to the adiabatic case with no gravity and assuming equal pressure in the stream and the medium. By analytically solving the linear dispersion relation, we find a transition from a dominance of the familiar rapidly growing surface modes in the subsonic regime to more slowly growing body modes in the supersonic regime. The system is parameterized by three parameters: the density contrast between the stream and the medium, the Mach number of stream velocity with respect to the medium, and the stream width with respect to the halo virial radius. We find that a realistic choice for these parameters places the streams near the mode transition, with the KHI exponential-growth time in the range 0.01-10 virial crossing times for a perturbation wavelength comparable to the stream width. We confirm our analytic predictions with idealized hydrodynamical simulations. Our linear-KHI estimates thus indicate that KHI may in principle be effective in the evolution of streams by the time they reach the galaxy. More definite conclusions await the extension of the analysis to the nonlinear regime and the inclusion of cooling, thermal conduction, the halo potential well, self-gravity and magnetic fields.

BALANCING THE LOAD: A VORONOI BASED SCHEME FOR PARALLEL COMPUTATIONS

The use of numerical simulations in science is ever increasing and with it the computational size. In many cases single processors are no longer adequate and simulations are run on multiple core machines or supercomputers. One of the key issues when running a simulation on multiple CPUs is maintaining a proper load balance throughout the run and minimizing communications between CPUs. We propose a novel method of utilizing a Voronoi diagram to achieve a nearly perfect load balance without the need of any global redistributions of data. As a show case, we implement our method in RICH, a 2D moving mesh hydrodynamical code, but it can be extended trivially to other codes in 2D or 3D. Our tests show that this method is indeed efficient and can be used in a large variety of existing hydrodynamical codes as well as other applications.

Probing the gas density in our Galactic Centre: moving mesh simulations of G2

The G2 object has recently passed its pericenter passage in our Galactic Center. While the

Instability of supersonic cold streams feeding galaxies–II. Non-linear evolution of surface and body modes of Kelvin–Helmholtz instability

Br_\gamma

The Galactic Center Cloud G2 and its Gas Streamer

emission shows clear signs of tidal interaction, the change in the observed luminosity is only of about a factor of 2, in contention with all previous predictions. We present high resolution simulations performed with the moving mesh code, RICH, together with simple analytical arguments that reproduce the observed

BINARY YORP EFFECT AND EVOLUTION OF BINARY ASTEROIDS

Br_\gamma

Sterile and Fertile Planetary Systems - Statistical Analysis of Multi-Planet Systems in Kepler's data

emission. In our model, G2 is a gas cloud that undergoes tidal disruption in a dilute ambient medium. We find that during pericenter passage, the efficient cooling of the cloud results in a vertical collapse, compressing the cloud by a factor of

Grid noise in moving mesh codes: fixing the volume inconsistency problem

\sim5000

SPINS OF LARGE ASTEROIDS: A HINT OF A PRIMORDIAL DISTRIBUTION IN THEIR SPIN RATES

. By properly taking into account the ionization state of the gas, we find that the cloud is UV starved and are able to reproduce the observed