Towards a Generic Non-Reflective Characteristic Boundary Condition for Aeroacoustic Simulations

A blended zonal characteristic boundary condition is proposed following a quantitative investigation of the performance of several non-reective boundary conditions. Two test cases are considered that investigate the effects of acoustic and vortical plane waves impinging on the domain outflow region. A third test case investigates the effects of broad- band turbulent flow impinging on a non-reective outflow boundary condition. From these studies, two non-reective boundary conditions based on a zonal characteristic method are found to provide a minimal acoustic response for impinging acoustic and vortical disturbances, respectively. These methods both make use of the transverse characteristic terms to improve performance, although each method uses a different inclusion of these terms. A final boundary condition is proposed that blends the performance of the two zonal characteristic methods. A blending function is used that switches the characteristic boundary condition smoothly between regions dominated by acoustic, or vortical, disturbances. The feasibility of this novel method is demonstrated on a test case where broadband turbulence impinges on a small section of an outow region.