Controlling an unsteady separating boundary layer on a cylinder with an active compliant wall

An active compliant wall, consisting of an array of nominally two-dimensional actuating and sensing transducers, has been used to control the unsteady separating flow over a circular cylinder. The amplitudes of wall displacements were in the order of 10" of the boundary layer thickness. The control was effective for a Reynolds number range of 1.2 x 10 < Re,, < 1.5 x 10. The smallamplitude forced vibration of the compliant wall modified the separation characteristics of the near-parallel oscillatory flow. The compliant wall actively interacts with the naturally occurring instabilities of the flow. The singlefrequency wall actuation was found to delay unsteady flow separation on the cylinder. It also resulted in the reduction of velocity fluctuations, a modification of the wake velocity profiles, and a reduction in mean pressure drag.