Experimental near-field analysis for flow induced noise of a structured porous-coated cylinder

Porous coating on cylinders reduces the aerodynamic noise due to vortex shedding. A key understanding of how porous media suppress vortex shedding is related to near-field hydrodynamics. In this paper, an experimental study was undertaken over the Reynolds number range of 0.52 x 10(5) <= Re <= 1.74 x 10(5) to measure outer diameter surface pressure fluctuations of a structured porous-coated cylinder (SPCC). The near-field pressure and far-field noise were measured simultaneously to obtain a deep understanding of the noise-reduction mechanism of the SPCC. The results showed that the SPCC causes a delay in the boundary layer separation compared to the bare cylinder. While a substantial reduction in far-field noise was provided by the SPCC, a high-frequency noise attributed to the non-propagating hydrodynamic energy field within the SPCC was observed. The use of an SPCC led to a reduction in the surface pressure fluctuations over the entire frequency range, acting as a noise reduction mechanism. It was shown that the surface pressure fluctuations and acoustic signals are strongly correlated at the vortex shedding frequency for the bare cylinder, while it was insignificant for the SPCC, indicating the strong role of the SPCC in reducing the surface pressure energy content so that it cannot propagate to the far-field. The evaluation of vortex shedding in the time-frequency domain was carried out with the aid of wavelet transform. It was observed that amplitude modulation in time was verified only for the bare cylinder at the fundamental vortex shedding frequency in the pre-and post-separation regions.