A high-order structured multiblock solver exploiting task-parallelism for compressible multiphase flows*

The solver combines curvilinear coordinates and multiple rectangular blocks, thus maintaining the advantage of structured solvers while not being limited to simulations of simplistic geometries. The solver uses a high order finite difference scheme, which is hybridized with more dissipative schemes close to physical or mesh discontinuities. It builds on top of the HTR solver [Di Renzo et al., Comp. Phys. Comm. 2020], which uses Legion as a runtime for task scheduling and resource utilization. Different from a classic MPI approach, the solver uses Legion's own data structures combined with task requirements to become self-aware of the data communication and synchronization when run in distributed memory. This automation is shown in this work to provide an explicit advantage in the curvilinear multiblock solver, where memory access patterns are often non-trivial and involve geometrical transformations. Such cases arise sometimes when the blocks are (in the physical domain) skewed/rotated to obtain a better mesh quality, leading to the formation of polyjunctions (grid singularities). At those locations, it can be shown that the geometric conservation laws do not hold, hence significant errors causing simulation failure are encountered. Two approaches to accounting for the grid singularities are presented in this work. These are tested against a suite of problems designed to stress each capability of the solver, including accuracy, compressibility, multispecies reacting flows and multiphase flows.