Spanwise Coherence Measurements of Porous Coated Cylinders in Uniform Flow

Abstract

A porous coated cylinder (PCC) generates weaker vortex shedding noise than a smooth cylinder placed in uniform flow. A key understanding of how the porous media relates to vortex shedding reduction is linked to the spanwise coherence length of the PCC at the near-wall region. To date, few studies estimate the PCC spanwise coherence to be five-to-six outer diameters in length, yet a more comprehensive study is needed. This paper presents an experimental investigation conducted in an anechoic wind tunnel using a set of polyurethane PCCs tested with varying porosity, thickness and at several Reynolds numbers. Spanwise coherence is estimated at various circumferential angles using (1) flush-mounted microphones distributed along the bare cylinder span, beneath the porous layer, to record surface pressure fluctuations, and (2) two hot-wire anemometry probes in the near-wall flow field, where one probe remains in place and the other is shifted along the span. A transfer function of the porous layer is estimated based on the surface pressure and velocity fluctuations. The results presented here give some new visions of the spatial structure of vortex shedding caused by porous materials, noise generating mechanisms and a fitting formula of spanwise coherence length of PCCs.