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Hydrodynamic and N-body Schemes On An Unstructured, Adaptive Mesh with Applications to Cosmological Simulations

The theory and application of numerical methods for unstructured meshes have been improved significantly in recent years. Because the grids can be place arbitrarily in space, unstructured meshes can provide much higher spatial resolution than regular meshes. The built-in nature of mesh adaptivity for unstructured meshes gives one way to simulate highly dynamic, hierarchical problems involving both collisionless dark matter and collisional gas dynamics. In this paper, we describe algorithms to construct unstructured meshes from a set of points with periodic boundary conditions through Delaunay triangulation, and algorithms to solve hydrodynamic and N-body problems on an unstructured mesh . A combination of a local transformation algorithm and the traditional Bowyer-Watson algorithm gives an efficient approach to perform Delaunay triangulation. A novel algorithm to solve N-body equations of motion on an unstructured mesh is described. Poisson's equation is solved using the conjugate gradient method. A gas-kinetic scheme based on the BGK model to solve Euler equations is used to evolve the hydrodynamic equations. We apply these algorithms to solve cosmological settings, which involve both dark and baryonic matter. Various cooling and heating processes for primordial baryonic matter are included in the code. The numerical results show that the N-body and hydrodynamic algorithms based on unstructured meshes with mesh refinement are well-suited for hierarchical structure formation problems.