The aeroacoustic behavior of a cylindrical surface with a small cavity

The aeroacoustic effects of the flow around a cylinder with a small rectangular cavity in its surface are investigated in an acoustic wind tunnel. In different positions, the overflown cavity produces loud tonal whistling noise. In large part, the noise can be explained with the Rossiter model. At a certain position of the cavity, a different aeroacoustic phenomenon occurs, which is in focus of this investigation. Tonal frequencies appear in a narrow band region, which do not scale with different cavities. A sudden onset and a sudden stop of the acoustic radiation are accompanied with a transition of the circulating flow. A strong hysteresis is observable. The separating boundary layer plays a major role in the characterization of the flow in the vicinity of the cavity. Acoustical and various flow measurements at velocities up to 47 m/s as well as a CFD simulation are presented. Consistent results reveal Kelvin–Helmholtz instabilities as the reason for the aeroacoustic phenomenon.