arXiv subjects

H. M. P. Couchman

Publications and source records attributed to H. M. P. Couchman.

78 records · Page 5Linked to original sources

arXiv1997

Hydra: A Parallel Adaptive Grid Code

We describe the first parallel implementation of an adaptive particle-particle, particle-mesh code with smoothed particle hydrodynamics. Parallelisation of the serial code, ``Hydra'', is achieved by using CRAFT, a Cray proprietary language which allows rapid implementation of a serial code on a parallel machine by allowing global addressing of distributed memory. The collisionless variant of the code has already completed several 16.8 million particle cosmological simulations on a 128 processor Cray T3D whilst the full hydrodynamic code has completed several 4.2 million particle combined gas and dark matter runs. The efficiency of the code now allows parameter-space explorations to be performed routinely using $64^3$ particles of each species. A complete run including gas cooling, from high redshift to the present epoch requires approximately 10 hours on 64 processors. In this paper we present implementation details and results of the performance and scalability of the CRAFT version of Hydra under varying degrees of particle clustering.

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arXiv1997

The Virgo Consortium: The evolution and formation of galaxy clusters

We report on work done by the Virgo consortium, an international collaboration set up in order to study the formation and evolution of Large Scale Structure using N-body simulations on the latest generation of parallel supercomputers. We show results of 256^3 particle simulations of the formation of clusters in four Dark Matter models with different cosmological parameters. Normalizing the models such that one obtains the correct abundance of rich clusters yields an interesting result: The peculiar velocities of the clusters are almost independent of Ω, and depend only weakly on Γ, the shape parameter of the power spectrum. Thus, it is nearly impossible to distinguish between high and low Ωmodels on the basis of the peculiar velocities.

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arXiv1996

The Virgo consortium: simulations of dark matter and galaxy clustering

We report on work in progress by the Virgo consortium, a collaboration set up to carry out large simulations of the formation of galaxies and large-scale structure exploiting the latest generation of parallel supercomputers. We show results of $256^3$ particle N-body simulations of the clustering evolution of dark matter in four cold dark matter models with different cosmological parameters. The high resolution and large volume of these simulations allows us to determine reliably the mass autocorrelation function for pair separations in the range $40\hkpc$ to $20\hmpc$. Comparison of these with the observed galaxy correlation function shows that for any of these models to be viable, the distribution of galaxies must be biased relative to the distribution of mass in a non-trivial, scale-dependent fashion. In particular, low $Ω_0$ models require the galaxies to be more ``weakly'' clustered than the mass at small and intermediate pair separations. Simulations which include the evolution of gas show that cold gas knots form with approximately the abundance expected on theoretical grounds, although a few excessively massive objects grow near the centres of rich clusters. The locations where these cold gas knots form are, in general, biased relative to the distribution of mass in a scale-dependent way. Some of these biases have the required sign but they are, for the most part, weaker than is necessary for agreement with observations. The antibias present in our low $Ω_0$ N-body/SPH simulation appears to be related to the merging and disruption of galaxies in rich clusters.

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arXiv1996

Release of data from cosmological N-body simulations

We intend to make sets of cosmological simulations available, with a large number of different output times that may be placed side-by-side to produce a complete history of the universe stretching back to high redshift. Currently there is only one series of runs under preparation as a trial. Gauging by the response, we will consider further releases. The first series consists of N-body simulations with CDM-like spectra (Gamma=0.25) in a 100/h Mpc box, in three different cosmologies: Einstein-de Sitter, low-density flat and low-density open. The parameters of each run are: 1) Run 501: Omega=1.0, lambda=0.0, sigma_8=0.64. N=128**3. 38 time-slices back to z=5.8 (a further 11 back to z=23.4 on request). 2) Run 502: Omega=0.3, lambda=0.7, sigma_8=1.22. N=86**3. 49 time-slices back to z=3.6. 3) Run 503: Omega=0.3, lambda=0.0, sigma_8=1.06. N=86**3. 49 time-slices back to z=6.2 Check out our Web pages for more details: http://coho.astro.uwo.ca/pub/consort.html

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arXiv1994

Hydra: An Adaptive--Mesh Implementation of PPPM--SPH

We present an implementation of Smoothed Particle Hydrodynamics (SPH) in an adaptive-mesh PPPM algorithm. The code evolves a mixture of purely gravitational particles and gas particles. The code retains the desirable properties of previous PPPM--SPH implementations; speed under light clustering, naturally periodic boundary conditions and accurate pairwise forces. Under heavy clustering the cycle time of the new code is only 2--3 times slower than for a uniform particle distribution, overcoming the principal disadvantage of previous implementations\dash a dramatic loss of efficiency as clustering develops. A 1000 step simulation with 65,536 particles (half dark, half gas) runs in one day on a Sun Sparc10 workstation. The choice of time integration scheme is investigated in detail. A simple single-step Predictor--Corrector type integrator is most efficient. A method for generating an initial distribution of particles by allowing a a uniform temperature gas of SPH particles to relax within a periodic box is presented. The average SPH density that results varies by $\sim\pm1.3$\%. We present a modified form of the Layzer--Irvine equation which includes the thermal contribution of the gas together with radiative cooling. Tests of sound waves, shocks, spherical infall and collapse are presented. Appropriate timestep constraints sufficient to ensure both energy and entropy conservation are discussed. A cluster simulation, repeating Thomas and

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arXiv1993

Mergers of Systems Containing Gas

Several simple mergers between model galaxy clusters containing a mixture of gas and dark matter are examined, testing the coupling of the gas to the underlying collisionless material. The gas is shocked, irreversibly dissipating the energy fed into it by the collisionless component and forms a resolved constant-density core. For the dark matter, however, admixture of phase space vacuum is not very efficient and a constant-density core is not produced. In the final state the central gas has little residual kinetic energy, indicating that streaming motions do not help to support the gas.

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