Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT. (arXiv:1901.09934v2 [astro-ph.EP] UPDATED)

Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT. (arXiv:1901.09934v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Kegerreis_J/0/1/0/all/0/1">J. A. Kegerreis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eke_V/0/1/0/all/0/1">V. R. Eke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gonnet_P/0/1/0/all/0/1">P. G. Gonnet</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Korycansky_D/0/1/0/all/0/1">D. G. Korycansky</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Massey_R/0/1/0/all/0/1">R. J. Massey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schaller_M/0/1/0/all/0/1">M. Schaller</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Teodoro_L/0/1/0/all/0/1">L. F. A. Teodoro</a>

We perform simulations of giant impacts onto the young Uranus using smoothed
particle hydrodynamics (SPH) with over 100 million particles. This
100–1000$times$ improvement in particle number reveals that simulations with
below 10^7 particles fail to converge on even bulk properties like the
post-impact rotation period, or on the detailed erosion of the atmosphere.
Higher resolutions appear to determine these large-scale results reliably, but
even 10^8 particles may not be sufficient to study the detailed composition of
the debris — finding that almost an order of magnitude more rock is ejected
beyond the Roche radius than with 10^5 particles. We present two software
developments that enable this increase in the feasible number of particles.
First, we present an algorithm to place any number of particles in a spherical
shell such that they all have an SPH density within 1% of the desired value.
Particles in model planets built from these nested shells have a
root-mean-squared velocity below 1% of the escape speed, which avoids the need
for long precursor simulations to produce relaxed initial conditions. Second,
we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT uses
task-based parallelism and other modern algorithmic approaches to take full
advantage of contemporary supercomputer architectures. Both the particle
placement code and SWIFT are publicly released.

We perform simulations of giant impacts onto the young Uranus using smoothed
particle hydrodynamics (SPH) with over 100 million particles. This
100–1000$times$ improvement in particle number reveals that simulations with
below 10^7 particles fail to converge on even bulk properties like the
post-impact rotation period, or on the detailed erosion of the atmosphere.
Higher resolutions appear to determine these large-scale results reliably, but
even 10^8 particles may not be sufficient to study the detailed composition of
the debris — finding that almost an order of magnitude more rock is ejected
beyond the Roche radius than with 10^5 particles. We present two software
developments that enable this increase in the feasible number of particles.
First, we present an algorithm to place any number of particles in a spherical
shell such that they all have an SPH density within 1% of the desired value.
Particles in model planets built from these nested shells have a
root-mean-squared velocity below 1% of the escape speed, which avoids the need
for long precursor simulations to produce relaxed initial conditions. Second,
we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT uses
task-based parallelism and other modern algorithmic approaches to take full
advantage of contemporary supercomputer architectures. Both the particle
placement code and SWIFT are publicly released.

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