On the active control of shear layer oscillations across a cavity in the presence of pipeline acoustic resonance

This paper presents an experimental study of shear layer oscillations past an axisymmetric cavity in a pipeline in the presence of acoustic resonance and active control. The acoustic pressure and velocity fluctuations were measured using a flush-mounted microphone in the pipe wall and a hot-wire probe located in the cavity, respectively. Control was achieved by passing such signals through a phase shifter and amplifier to a loudspeaker at the pipeline entrance. It was found that the shear layer instability could be driven to higher sound pressure levels than naturally occurring at resonance or entirely eliminated by the controller, depending on the phase shift given to the feedback signal. The flow visualization photographs clearly show the first three modes of shear layer instability across the cavity and their elimination. The accompanying hot-wire spectra show the frequency content of the flow in each case.