Flow and geometrical effects on radiated noise from exhaust pipes computed by the Fourier pseudospectral time-domain method

The current work initializes a systematic numerical study on the combined effects of the geometry surrounding an automotive exhaust pipe, i.e. the ground surface and the automotive body, as well as the influence of the non-uniform velocity and temperature from the exhaust jet, on the acoustic radiation pattern of the pipe. The problem is treated by first solving the RANS equations to obtain the mean velocity and temperature solution of the exhaust jet. The acoustic radiation is computed by solving the linearized Euler (LEE) equations. For an efficient solution of the LEE, a recently developed multi-domain extended Fourier pseudospectral time-domain (PSTD) methodology is used. This method combines the favorable spectral accuracy of the PSTD method with a local grid refinement in the region with high gradients of the mean flow and temperature fields. A filter was found necessary for numerical stability of the PSTD method for this application. For radiation of noise from an exhaust pipe in free field, results from PSTD show a good agreement with reference results, both for cases with and without a jet flow. Results show that the presence of a rigid ground surface and simplified automotive body increases the radiated sound power by 6 dB for the lower frequency region, and the effect of the body on the directivity is largest for the higher frequencies. Flow effects slightly increase the shielding effect of the body for all frequencies, but have a main impact behind the exhaust pipe, where low frequencies experience higher levels and a cone of low sound levels characterizes the high frequencies.