A hybrid computational aeroacoustics method to simulate airframe noise

For the prediction of turbulence related noise a hybrid computational two-step approach is considered. In the first step, the unsteady compressible flow field is computed in a computational domain that comprises just the acoustic source region under consideration via a large eddy simulation (LES). Due to the large scale disparity of the acoustic and hydrodynamic fields for low Mach number flows, the acoustic field is computed with linear acoustic equations in a second step in space and time using sources determined from the unsteady compressible flow field. The integration domain of the acoustic computation extends into the near far-field and contains also those parts of a surface geometry, which are not resolved by the local LES. A family of acoustic perturbation equations (APE) is derived for the simulation of flow induced acoustic fields in time and space, which completely prevent the unbounded growth of hydrodynamic instabilities in critical mean flows. The first part of this work deals with the implementation and validation of numerical methods that have been developed in the framework of computational aeroacoustics (CAA) for the discretization of the linearized Euler equations as governing acoustic equations. In the second part, the acoustic perturbation equations are applied to airframe noise problems. The laminar flow over a cylinder at a Mach number M=0.3 and a Reynolds number Re=200 is considered as a test problem to compare different sound source formulations. Furthermore, the hybrid approach is applied to predict trailing 5 edge noise for a flat plate at M=0.15 and Re = 7 ⋅10 exp(5) . The vortex source term of the acoustic perturbation equations is based on Lamb's vector that can be computed easily from an incompressible as well as a compressible LES. The source is identical to the vortex source term of the acoustic analogies of Powell, Howe and Möhring.