On the effects of a perforated shroud on turbulence mechanisms in the wake of a cylinder

Abstract

The turbulent wake of a circular cylinder fitted with a perforated shroud is investigated experimentally using time-resolved particle image velocimetry to identify the turbulence mechanisms governing passive wake control. While perforated shrouds are widely used to mitigate aerodynamic loading and vortex-induced vibrations, prior studies have primarily focused on global force and wake-integrated quantities and have generally considered configurations outside the design ranges prescribed by the American Society of Mechanical Engineers in the Standard code for Steel Stacks. Perforation ratios of β=0.3 and 0.5, a shroud-to-cylinder diameter ratio of DS/D≈1.1, and a hole-size ratio of DH/D=0.07 and 0.08 are examined at Reynolds numbers of ReD=23×103, 38×103, and 55×103. Full-field velocity data are used to quantify wake recovery, recirculation dynamics, Reynolds shear stress, turbulent kinetic energy, characteristic temporal scales, and second-order structure functions. Increasing the perforation ratio enlarges the recirculation region while reducing Reynolds shear stress and turbulent kinetic energy and accelerating downstream wake recovery. Despite these changes, normalized temporal statistics and second-order structure functions exhibit a strong collapse across Reynolds number, indicating weak Reynolds-number sensitivity of the normalized multiscale wake organization over the range considered. The primary effect of the shroud is, therefore, to redistribute turbulence production and reduce the spatial coherence of dominant wake structures through distributed momentum exchange. These results provide turbulence measurements linking perforated-shroud performance to wake physics and connecting engineering design guidelines with the underlying mechanisms of passive flow modification.