On the Parallelization of a Harmonic Balance Compressible Navier-Stokes Solver for Wind Turbine Aerodynamics

The paper discusses the parallelization of a novel explicit harmonic balance Navier-Stokes solver for wind turbine unsteady aerodynamics. For large three-dimensional problems, the use of a standard MPI parallelization based on the geometric domain decomposition of the physical domain may require an excessive degree of partitioning with respect to that needed when the same aerodynamic analysis is performed with the time-domain solver. This occurrence may penalize the parallel efficiency of the harmonic balance solver due to excessive communication among MPI processes to transfer halo data. In the case of the harmonic balance analysis, the necessity of further grid partitioning may arise because the memory requirement of each block is higher than for the time-domain analysis: it is that of the time-domain analysis multiplied by a variable proportional to the number of complex harmonics used to represent the sought periodic flow field. A hybrid multi-level parallelization paradigm for explicit harmonic balance Navier-Stokes solvers is presented, which makes use of both distributed and shared memory parallelization technologies, and removes the need for further domain decomposition with respect to the case of the time-domain analysis. The discussed parallelization approaches are tested on the multigrid harmonic balance solver being developed by the authors, considering various computational configurations for the CFD analysis of the unsteady flow field past the airfoil of a wind tubine blade in yawed wind.