An investigation of wake flows produced by asymmetrically structured porous coated cylinders

Abstract

The vortex shedding tone of a cylinder in uniform flow can be reduced by applying a porous coating yet this mechanism is not fully understood. An experimental investigation of asymmetric structured porous coated cylinders (SPCCs) was conducted in a small anechoic wind tunnel using a hot-wire anemometry probe placed in the boundary layer, separated shear layer and wake, in conjunction with a microphone in the far-field. Tests were conducted at Reynolds numbers 105, 1.53×105, and 1.66×105. Each SPCC revealed a widened and deeper wake and reduced turbulent kinetic energy levels in the separated shear layer than the bare baseline cylinder. Furthermore, each SPCC revealed two tones that were a multiple of two apart in both the velocity and acoustic power spectral densities. It was shown that the higher frequency tone is generated by localized flow behaviors in the separated shear layer, independent of the vortex shedding tone and its magnitude is inversely related to the SPCC windward surface porosity. Applying a more densely spaced porous region on the cylinder windward side resulted in higher frequency broadband contributions that were shown to be independent of the velocity fluctuations in the wake region. Time-averaged velocity profiles in the wake revealed that the leeward side porosity strongly influences the drag coefficient. Linear stability analysis revealed that the SPCCs develop absolute instabilities in the near wake.