Miyamoto, T., Yokoyama, H., and Iida, A., "Suppression of Aerodynamic Tonal Noise from an Automobile Bonnet Using a Plasma Actuator," SAE Int. J. Passeng. Cars - Mech. Syst. 10(3):712-720, 2017.
Intense aeroacoustic feedback noises may radiate from flow around an airfoil, rearview mirror with small gaps and so on. Reductions of these noises are important issues in the development of industrial application. The intense noise from a bonnet of the automobile is one of the typical problems of acoustic feedback noise. In order to reduce this noise, plasma actuator (PA) was utilized to control flow and acoustic fields. The aim of this investigation is to clarify the effects of flow control by the PA on noise reduction and the noise reduction mechanism. Wind tunnel experiments were conducted with a half scale bonnet model and a low noise wind tunnel. Simultaneous measurements of flow and noise fields were conducted to understand the generation mechanism of the bonnet noise. Coherent output power (COP) of the velocity fluctuations with reference to far-field sound pressure was measured to visualize noise source distribution. The result showed the velocity fluctuations around the re-attachment point are strongly related with the tonal acoustic radiation. Moreover, the velocity fluctuation rapidly increased at the downstream of the kink shape with the influence of radiated sound. The measurement result of COP reflects this state and we can found that COP increased from the kink shape to the re-attachment point. This result indicated that the separated flow just behind the kink and reattached flow are key factors to control the bonnet noise. Based on this knowledge, experiments of flow and noise control with PA were conducted. As a result, intense tonal noise can be effectively reduced by the PA control. The mean velocity profile showed that the separation around the kink was suppressed by PA control. COP also showed the velocity fluctuations which were related with the tonal noise become weaker than that of the flow field without PA control.