Hybrid aeroacoustic methods for broadband noise calculation

The present work investigates hybrid methods for the calculation of broadband noise. Building blocks for these methods are computational fluid dynamics (CFD), high-performance computing (HPC), and the choice of a suitable acoustic propagation equation, with focus on the interface between flow solver and acoustic solver. With respect to CFD, the suitability of different flux discretization schemes, subgrid-scale turbulence models, and meshing requirements were investigated. Regarding the acoustic simulation, different propagation equations and source term formulations are compared, including discretization aspects for the acoustic source terms. Based on these building blocks, a highly scalable and efficient scheme has been implemented. The implemented approach was validated based on various test cases with increasing complexity, including the turbulent flow over a forward-facing step. A direct numerical simulation of the step flow was performed, yielding detailed information about the unsteady flow field, the source term distribution, the acoustic field and the coherence between source region and acoustic far field. These data were used for comparison with results from large eddy simulations. It resulted that especially the discretization of the convective flux and the mesh quality is of major importance. Regarding the mesh quality, it could be shown that not only a good overall resolution but also the local resolution in the acoustic source region is of major importance. Considering the acoustic propagation equations, the best results were obtained using Lighthill's acoustic analogy with a pressure-based source term.