A hybrid method that combines the calculation of unsteady flow by large eddy simulation (LES) and the evaluation of acoustical performance by a compact Green's function is proposed to provide detailed information on the vortices in the flow that contribute to the generation of aeroacoustic sound. This paper clarifies the noise reduction mechanism of pantograph horns with periodic holes numerically, and proves that the intermittent flow through holes suppresses the strength of dipole sound sources at the shear layer and collapses the large two-dimensional structure of sound sources in the spanwise direction.

Copyright (c) 2003 Railway Technical Research Institute