Mitigation of Flow-Induced Cylinder Noise Through the Control of Sound Diffraction

The sound emitted by the flow past a circular cylinder can be described by a quad rupole placed at the outbreak location of the shedding instability and diffracted surface into the far field by the body with a dipolar directivity. This mechanism is greatly stabilized for a cylinder coated with a porous material, which features a substantial downstream shift of the onset location of the shed vortices that leads, in turn, to a reduction in the efficiency of the sound scattering and consequent noise mitigation. In this research, a novel design for a porous treatment of the cylinder based on the enhancement of this effect is proposed. Far-field acoustics tests were performed at the Delft University of Technology for Reynolds numbers based on the cylinder diameter ranging in the subcritical regime. The outcomes of the analysis demonstrate that, when the aft part of the flow-permeable coating is modified to make the internal flow more streamlined, an additional sound attenuation of up to 10 dB is achieved in comparison with a uniform porous cover. Moreover, a significant noise decrease of up to 10 dB and potential drag reduction are obtained if these components are connected to the bare cylinder without the use of a porous coating. This result can open up interesting opportunities to design disruptive and more optimized sound-mitigation solutions.