Control of interior pressure fluctuations due to flow over vehicle openings

Grazing flows over open windows or sunroofs may result in "flow buffeting," i.e. self-sustained flow oscillations at the Helmholtz acoustic resonance frequency of the vehicle. The associated pressure fluctuations may cause passenger fatigue and discomfort. Many solutions have been proposed to solve this problem, including for example leading edge spoilers, trailing edge deflectors, and leading edge flow diffusers. Most of these control devices are "passive" i.e. they do not involve dynamic control systems. Active control methods, which do require dynamic controls, have 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 this study, oscillated spoilers hinged near the leading edge of the cavity orifice were used. Experiments were performed using a cavity installed within the test section wall of a wind tunnel. The cavity dimensions were approximately those of a one-fifth scale model sedan. The spoiler was driven using a moving coil linear actuator. A microphone located within the cavity was used as the feedback sensor. A loop shaping feedback control design methodology was used in order to ensure robust controller performance over 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. The required actuation effort was small. The spoiler peak displacement was typically only 4% of the mean spoiler angle (approximately one 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 was varied). Oscillated leading edge spoilers have potential advantages over loudspeakers for cavity resonance problems, including a reduced encumbrance (especially for low-frequency applications), and a smaller actuation effort. These devices may find useful applications for the reduction of buffeting due to flow over sunroofs and open windows.