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

Abstract

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 the Cμ range of 0.007 to 0.036. Despite this achievement, the most optimal overall sound pressure level was achieved at θb=180°. It was determined that the dissimilarity in noise reduction among 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 recorded coherence values. The investigation highlighted that surface pressure fluctuations initiated by vortex shedding in the pre- and post-separation regions, particularly at the fundamental vortex shedding frequency, had the most significant impact on far-field noise. The attenuation of such surface pressure fluctuations played a pivotal role in tonal noise reduction by LB, as evidenced by notable reductions in lift fluctuations and the absence of amplitude modulation in both the time and frequency domains.