GRAPESPH: cosmological SPH simulations with the special purpose hardware GRAPE

A combined N-body/hydrodynamical code is presented. Hydrodynamical properties are determined using smoothed particle hydrodynamics (SPH). The gravitational interaction of gas and collisionless particles is treated in a direct summation approach which benefits from the high speed of the special purpose hardware GRAPE (GRavity PipE). Besides gravitational forces, GRAPE also returns the list of neighbours and can, therefore, be used to speed up the hydrodynamical part, too. After the interaction list has been passed, density, pressure forces, propagation and interpolation of particles etc. are calculated on the front end, a 50 MHz SUN SPARC 10. In order to combine SPH and GRAPE, possible limitations due to the hardware design of GRAPE are carefully analyzed and modifications compared to current SPH codes are discussed. The resulting code, GRAPESPH is similarly flexible as TREESPH. 50-55% of the CPU time is spent to calculate the densities and the pressure forces on the front end, 15-20% to calculate gravity, and about 10% in miscellaneous subroutines. Another 20% are required by communication, mainly to read out the neighbour list via the VME interface. The main shortcoming is the inflexible, hardwired force law (a Plummer law), which makes it difficult to include periodic boundary conditions. Also because of the limited dynamic range, GRAPESPH seems to be less suitable to perform large scale structure simulations, where the resolution should be high everywhere in the simulation volume. The resulting code seems, however, especially well suited to investigate the formation of individual objects in a large scale structure environment, like e.g., galaxies or clusters. Such simulations require a very high spatial resolution, but only within a relatively small subvolume. By means of a multiple time step scheme, time step constraints due to local stability criteria can almost be avoided. The total performance is at least half as good as TREESPH on a CRAY and for most applications it seems to be even better. The CPU time per time step is only slighly dependent on the clustering state. GRAPESPH, therefore, provides a very attractive alternative to the use of supercomputers in cosmology. (orig.)Available from TIB Hannover: RR 4697(872) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman