Aeroacoustic investigation of active base blowing applied to a structured porous-coated cylinder

The influence of local blowing (LB) through a structured porous-coated cylinder (SPCC) as a hybrid method (HM) for vortex-induced noise suppression of a circular cylinder was investigated experimentally. The air blowing was applied at the base of the cylinder from the front stagnation point (���� = 180○) over a long span length (seven times the outer diameter of the cylinder). To better understand the underlying noise reduction mechanism of HM compared to the other methods tested here (i.e., SPCC and LB without SPCC at ���� = 180○), near-field pressure was synchronised with far-field noise measurements. In addition, planar particle image velocimetry measurements were carried out to visualise the flow field and wake development back of the cylinder under the influence of different methods. The results showed that HM causes a significant reduction in the tonal and broadband noises produced by the baseline, specifically relative to LB at ���� = 180○, yet the HM performance was very similar to that of the SPCC. The coherence between near- and far-field pressure demonstrated that the surface pressure fluctuations imposed at the shoulders of the cylinder (approximately in the pre- and post-separation regions) at the fundamental vortex shedding frequency have the most contribution to the tonal noise produced by the baseline. It was observed that SPCC and HM suppress vertical velocity fluctuations close to the cylinder significantly with a very similar delay in rolling up the shear layers into vortex structures and pushing the recirculation bubbles further downstream in comparison to the baseline and LB at ���� = 180○. In these cases, the energy content of surface pressure fluctuations is reduced as a result of vertical flow movement during vortex shedding, and the acoustic pressure field cannot propagate as sound waves to the far-field.