Observations of the frequencies in a sphere wake and of drag increase by acoustic excitation

Vortex shedding and instability wave frequencies have been measured in the wakes of spheres in the Reynolds number range 500<Re<60 000. The effect of acoustic excitation was examined and an interaction between the two frequency modes was found at the lower Reynolds numbers; through this interaction, external forcing at the instability frequency could change the vortex shedding frequency. The development of the mean wake was manipulated by forcing near to the dominant shear layer instability frequency. With this forcing, the separated shear layer moved closer to the surface of the sphere and the reversed flow region of the wake was shortened. Concomitantly, the base pressure decreased and drag increased.