An Automatic Parallel Octree Grid Generation Software with an Extensible Solver Framework and a Focus on Urban Simulation

The development of an automatic, dynamic, parallel, Cartesian, linear forest-of-octree grid generator and partial differential equation (PDE) solver framework is presented. This research is bundled into an application programmed with C++ which uses MPI for distributed parallelism. The application is named paroswhich stands for PARallel Octree Solver. In its current implementation, the application provides a ‘zeroth’ order representation of the target geometry, and as such, no cutcell algorithm, projection method, or immersed boundary condition are implemented. In this case, ‘zeroth’ ordermeans that no geometry is ever exactly represented in the final computationalmesh: an octree element is either completely in the domain or entirely outside of it. Any element that contains or is intersected by a geometry facet is removed from the final mesh which results in a ‘blocky’ or ‘stepped’ geometry representation and simplifies boundary computations. The computational octree mesh creation is completely parallel and automated. The algorithm is dynamic in the sense that it is repartitioned dynamically throughout the grid generation process to maintain optimal load balancing during all phases of the mesh genesis. A linear octree data structure is used to store the octree mesh elements and is leveraged for optimal load balancing. An additional hierarchical octree is used to significantly improve algorithms that suffer from this linear storage paradigm. This work focuses on, but is not limited to, applications related to urban simulations and may be applied to plume/contaminant propagation. Within the PDE solution framework a cell-centered, incompressible, unsteady, Navier-Stokes solverwith an energy term to account for thermal buoyancy