A NESTED-GRID PARTICLE-MESH CODE FOR HIGH-RESOLUTION SIMULATIONS OF GRAVITATIONAL-INSTABILITY IN COSMOLOGY

I describe a nested-grid particle-mesh (NGPM) code designed to study g ravitational instability in three dimensions. The code is based upon a standard PM code. Within the parent grid I am able to define smaller subgrids, allowing a substantial extension of the dynamical range in m ass and length. I treat the fields on the parent grid as background fi elds and utilize one-way interactive meshing. Waves on the coarse pare nt grid are allowed to enter and exit the subgrid, but waves from the subgrid are precluded from affecting the dynamics of the parent grid. On the parent grid the potential is computed using a standard multiple Fourier transform technique. On the subgrid a Fourier transform techn ique is used to compute the subgrid potential at high resolution. Quas i-isolated boundary conditions are imposed on the subgrid using the st andard method for generating isolated boundary conditions, but, rather than using the isolated Green function, the Ewald method is used to c ompute a Green function on the subgrid which possesses the full period icity of the parent grid. I present a detailed discussion of my method ology and a series of code tests.