Aeolian tones generated by a square cylinder with a detached flat plate

Abstract

The case of a rigid flat plate placed in the wake of a square cylinder for passive sound control is investigated numerically at a Reynolds number of 150 and a Mach number of 0.2. Two flow regimes are observed, and these are shown to affect the radiated sound. An overall sound reduction at an observer directly above the cylinder is observed for a small gap distance G between the cylinder and the plate (0≤G≤2.3D, in which D is the cylinder height), where a 2.9 dB reduction in the sound-pressure level is obtained when there is no gap between the two bodies. In contrast, the emitted sound-pressure level increases by at least 8.0 dB for large gap distances (2.4D≤G≤7D). Despite this, a 6.3 dB reduction in the sound-pressure level is obtained due to a sound-cancellation mechanism when the plate length is reduced to 0.26D and placed 5.6D downstream of the cylinder. The maximum sound reduction attainable is shown to be limited by the nonlinear unsteady stall process on the plate.