Active Control of Flow-Induced Cavity Resonance

Grazing flows over open cavities may result in the excitation of self-sustaine d flow oscillations at the acoustic resonance frequency of the cavity system. The associated pressure oscillations and radiated sound are objectionable in situations where they may cause excessive structural vibrations, resulting in acoustic fatigue, or excessive sound radiation in the vicinity of the cavity. Many solutions have been proposed to solve this problem, including for example leading edge spoilers, trailing edge deflectors, and leading edge airmass injection. Most of these control methods are "passive" i.e. they do not require dynamic control systems. Active control methods, which do require dynamic controls, have also been implemented with success for different cases of flow instabilities. Previous investigations of the control of flow-excited cavity resonance have used mainly one or more loudspeakers located within the cavity wall. In the present study, an oscillated spoiler hinged near the leading edge of the cavity orifice is proposed as the actuation device. Experiments were performed using a side-branch cavity installed within the wall of a closed test section wind tunnel. The spoiler was driven using a moving coil loudspeaker. A microphone located within the cavity was used as the feedback sensor. A loop shaping feedback control design methodology was followed in order to ensure robust controller performance over different, varying flow conditions. Cavity pressure level attenuation of up to 20 dB was achieved around the critical velocity, relative to the level in presence of the spoiler held stationary. Actuation effort was low. The spoiler peak displacement was typically smaller than 4% of the mean spoiler angle (i.e. smaller than 1 degree). The control scheme was found to provide robust performance for transient operating conditions (for example when the controller was abruptly turned on, or the flow velocity varied). The potential advantages of oscillated leading edge spoilers relative to loudspeakers for cavity resonance problems are reduced encumbrance (especially for low-frequency applications), and reduced actuation effort.