A moving-mesh hydrodynamic solver for ChaNGa

We describe the structure and implementation of a moving-mesh (MM) hydrodynamics (HD) solver in the large-scale parallel code, Charm N-body GrAvity solver (ChaNGa). While largely based on the algorithm implemented in AREPO, our implementation differs a few aspects. We describe our use of the Voronoi tessellation library, VORO++, to compute the Voronoi tessellation directly. We also incorporate some recent advances in gradient estimation and reconstruction that gives better accuracy in HD solutions at minimal computational cost. We validate this module with a small battery of test problems against the smooth particle HD solver included in ChaNGa. Finally, we study one example of a scientific problem involving the mergers of two main-sequence stars and highlight the small quantitative differences between smooth particle and MM HD. We close with a discussion of anticipated future improvements and advancements.