Closed-loop control of flow-induced sound in a flow duct with downstream resonant cavities.

A closed-loop-controlled surface perturbation technique was developed for controlling the flow-induced sound in a flow duct and acoustic resonance inside downstream cavities. The surface perturbation was created by piezo-ceramic THUNDER (THin layer composite UNimorph Driver and sEnsoR) actuators embedded underneath the surface of a test model with a semi-circular leading edge. A modified closed-loop control scheme based on the down-sampling theory was proposed and implemented due to the practical vibration characteristic limitation of THUNDER actuators. The optimally tuned control achieved a sound pressure reduction of 17.5 dB in the duct and 22.6 dB inside the cavity at the vortex shedding frequency, respectively. Changes brought up by the control in both flow and acoustic fields were analyzed in terms of the spectrum phase shift of the flow field over the upper surface of the test model, and a shift in the vortex shedding frequency. The physical mechanism behind the control was investigated in the view of developing an optimal control strategy.