Sound generation by a street of vortices in a nonuniform flow

Propagation of acoustic waves originating from periodic vortices deforming in a nonuniform flow about a rigid body is examined numerically using a high-order compact finite-difference approximation. The governing equations are approximated by the linearized Euler equations in terms of disturbances. The aim of the study is to determine the sound directivity and strength as a result of the vortex street interaction with a solid body under subsonic base flow conditions. Both the vortex core diameter and vortex street spacing have a minor influence on the amplitude of the produced sound wave. When low-frequency vortex streets interact with a cylinder, the produced sound waves are very different from those that originated from high-frequency vortex streets. The interaction mechanism, sound generation, and propagation in a nonuniform flow are quite different for Taylor and Vatistas’ vortex streets. In the case of a low-frequency Vatistas vortex street, the root-mean-square (RMS) value of the acoustic pressure has a well-defined sound directivity and amplitude. The former is greatly affected by the Mach number of the mean flow. For a high-frequency Vatistas vortex street, the RMS of acoustic pressure becomes highly nonmonotonic in the angular direction, while the mean flow Mach number has a moderate effect on the RMS angular profile. The striking differences in the sound amplitude and directivity for Taylor and Vatistas vortices are discussed in terms of their vorticity distribution.