Application of local blowing to a structured porous-coated cylinder for flow and noise control

A comprehensive experimental study was carried out to investigate the effectiveness of a local blowing through a structured porous-coated cylinder (SPCC) as a novel technique for vortex-induced noise suppression in subcritical Reynolds number. The active flow control method was applied using a special design chamber with some holes distributed along two spanwise lines at angles 0 = ±130.9 . The near-and far-field noise measurements were simultaneously carried out to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the application of the blowing to the SPCC. A highly instrumented cylinder with several pressure taps distributed around the circumference of the cylinder at midspan was used to analyze the near-field noise using remote-sensing techniques of the fluctuating pressure fields. Although quite similar acoustic spectra were demonstrated by the SPCC with and without blowing, it was found that applying the local blowing to the SPCC causes a significant reduction of the vortex shedding tone produced by the bare cylinder, for any of the non-dimensional equivalent momentum coefficient considered in this study. A substantial reduction in energy content of the surface pressure fluctuations over the whole frequency range has also been observed for the SPCC with and without blowing especially in the post-separation region. The SPCC, with or without the local blowing resulted in higher frequency contributions that were shown to be independent of the surface pressure fluctuations. The near-to far-field coherence revealed that the increased noise observed for the SPCC with and without blowing configurations is solely due to the near-field non-propagating hydrodynamic field.