Experimental investigation of local blowing for noise and flow control of a circular cylinder

An experimental study was conducted to investigate the effectiveness of local blowing (LB)to suppress vortex-induced noise of a circular cylinder at𝑅𝑒=0.7×105. The far-field noise wassynchronised with surface pressure fluctuations and velocity field in the wake to understand theunderlying mechanism of LB noise reduction. A series of specially designed chambers wereused to apply LB in the boundary layer (𝜃𝑏=±41○) and separated region (𝜃𝑏=±90○and±131○)within𝐶𝜇=0.007−0.036. It was observed that all LB cases except the LB at𝜃𝑏=±90○arecapable to reduce tonal noise from the baseline over the range of𝐶𝜇yet the application of LB inthe boundary layer presented a superior reduction. It was proved that vortex shedding is theprimary mechanism for inducing surface pressure fluctuations and propagating sound wavesinto the far field. While the vortex shedding as a hydrodynamic pressure field is prone to radiateacoustic waves into the far field, the coherence between the near and far field pressure signalsconfirmed that the pressure fluctuations induced on the cylinder surface specifically in the pre-and post-separation regions are mostly responsible for tonal noise. It was found that the LB at𝜃𝑏=±41○pushes vortex shedding further downstream in comparison to other LB cases andthere is a region of low turbulent kinetic energy close to the cylinder, which is consistent withsuppressing vertical flow movement and thus surface pressure fluctuations