Active Control of Flow-Induced Acoustic Resonance Through Surface Perturbation

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

“…Meanwhile, an increase in the spectral phase was also noticed (not shown here), suggesting that the vortex shedding had been slowed down by the control action, 25 which is agreeable with the vortex shedding frequency shift phenomena observed in the previous study. 26 Based on the above observation, a mechanism for the fluid-structure alteration in the duct can be proposed. The control action applied on the upper surface of the test model generated the flow perturbation that interacted with the vortex shedding process to alter the generation of the trailing edge vortex shedding.…”

“…Further details have been described in a previous study. 26 Based on those analyses, a possible explanation for the equivalent increase in the thickness of test model is due to the boundary-layer thickening, resulting in a shift in the vortex shedding frequency. Upon perturbation, the flow velocity generated by the vibration plate around the test model might alter the thickness of the boundary layer around the test model, resulting in an increase in the distance between the two shear layers, as observed as an equivalent increase in the overall thickness of the test model seen in Fig.…”

“…Previous work in open-loop control of flow-induced acoustic resonance 26 has observed the vortex shedding frequency shift phenomena, where the shedding frequency is shifted to a lower value under the control action. This frequency shift allows a larger level of noise reduction inside the cavity than in the duct because of the slight mismatch between the vortex shedding and the acoustic resonance frequencies of the downstream cavities, especially when the latter is lightly damped.…”

“…The feasibility of using the proposed technique for the control of flow-induced acoustic resonance with downstream acoustic cavities was investigated by Zhang et al 25 and more recently improved further by Lu et al, 26 both using the open-loop control scheme. This later work provided a comprehensive assessment on the efficiency of the technique by using an improved actuator configuration, and more importantly, offered explanations on the control mechanism of the perturbation technique in attenuating the flow-induced sound.…”

“…This later work provided a comprehensive assessment on the efficiency of the technique by using an improved actuator configuration, and more importantly, offered explanations on the control mechanism of the perturbation technique in attenuating the flow-induced sound. It was demonstrated that the effective reduction in the acoustic resonance is originated not only from a direct impairment of the vortex shedding strength, but also from the perturbation-induced vortex shedding frequency shift, that could be predicted using a simple formula, 26 which described that the shift was linearly related to the equivalent increase of the thickness of the bluff body in a flow duct.…”