HIERARCHICAL NUMERICAL COSMOLOGY WITH HYDRODYNAMICS - METHODS AND CODE TESTS

We describe a two-level hierarchical three-dimensional numerical code designed to simulate the formation of galaxies and other larger scale structures in an expanding universe with Newtonian gravity and radiative cooling. The basis of the hierarchical code is a standard particle-mesh (PM) algorithm used to evolve the dark matter constituents and an ideal nonrelativistic MHD fluid solver to evolve the gas components on a cubical Cartesian grid with variable time steps. Within the parent cube we can construct a smaller subgrid (fixed in space) to resolve finer scale structures, providing a larger dynamical range over standard single grid codes without the enormous demand for memory and execution time. The fields from the parent grid are treated as external fields in the subgrid evolution and are passed down only as boundary conditions. The grid structure is a one-way interface so no allowance is made for back-reactions to the parent grid We discuss the details of our methods and present a variety of code tests.