A COMBINED BOUNDARY INTEGRAL FORMULATION FOR WAVE PROPAGATION IN WEAKLY NON-UNIFORM FLOWS

A combined Helmholtz integral equation formulation (CHIEF) for wave propagation in a weakly non-uniform subsonic potential mean flow is presented. This approach allows the irregular frequency issue, affecting boundary element solutions of external noise propagation, to be overcome. The CHIEF method is a well-established approach for low-frequency long-wavelength sound propagation in a quiescent media. For wave propagation in a non-uniform flow, however, it has yet to be extended to boundary element solutions of an integral equation in the physical space. A CHIEF method is proposed for a boundary integral formulation based on a combination of the physical models associated with the Taylor and Lorentz transformations. Numerical examples are used to assess the proposed formulation. It is shown that the irregular frequency issue deteriorates with an increase of the mean flow Mach number. For uniform flow Mach numbers M-infinity <= 0.3, the proposed CHIEF method is effective in removing the fictitious resonances at low frequency. Assuming a uniform mean flow at any over-determination point seems to provide better results than considering a quiescent media.