Experimental investigation of active local blowing on the aerodynamic noise reduction of a circular cylinder

The strategic implementation of local blowing (LB) around a circular cylinder within a uniform
flow has demonstrated its capacity to effectively suppress aerodynamic noise under specific blowing
conditions. This study aimed to comprehend the underlying mechanism driving noise reduction
through the synchronisation of far-field noise with surface pressure fluctuations, which were measured at various peripheral angles. The parameters under examination for LB were the angle of blowing in relation to the freestream flow (θb) and the equivalent momentum coefficient (Cµ). A dedicated series of chambers were employed to facilitate LB at θb = ±41◦, ±90◦, ±131◦, and 180◦across the ranges of Cµ = 0.007–0.036 (Re = 0.7 × 105) and Cµ = 0.003–0.016 (Re = 1.04 × 105).Notably, LB at θb = ±41◦ and 180◦exhibited a remarkable reduction in tonal noise within theCµ range of 0.007 to 0.036. Despite this achievement, the most optimal overall sound pressurelevel was achieved at θb = 180◦. It was determined that the dissimilarity in noise reductionamong these LB cases was attributed to additional high-frequency noise generated by the blowing technique. The connection between the near- and far-field signals was established through recordedcoherence values. The investigation highlighted that surface pressure fluctuations initiated byvortex shedding in the pre- and post-separation regions, particularly at the fundamental vortexshedding frequency, had the most significant impact on far-field noise. The attenuation of suchsurface pressure fluctuations played a pivotal role in tonal noise reduction by LB, as evidenced bynotable reductions in lift fluctuations and the absence of amplitude modulation in both the time and frequency domains.