Investigations of a Half-Stepped Cylinder in a Turbulent Boundary Layer

Abstract

Abstract Experimental investigations of a single half-stepped cylinder and a half-stepped cylinder sheet placed on a flat plate inside a turbulent boundary layer at zero pressure gradient have been performed. The goal of the study was to investigate whether a half-stepped cylinder could be used to control the low-speed streaks similar to riblets in reducing turbulent shear stress. The half-cylinder diameter (d) was 2 mm and the step-spacing (s) was 1.5 mm. The width (w) of the cylinder between the steps was 2 mm. The structure of the bump and valley repeated itself the entire length of the cylinder. The single cylinder row or sheet was placed in the spanwise direction where the bump round leading edge was facing the incoming axial flow. The flat plate had 7 rows of staggered round pegs at the leading edge to ensure a fully developed turbulent boundary layer at the cylinder’s location. The experiments were performed at 10 m/sec. in the closed-circuit CSULB-Boing low-speed wind tunnel. The ratio of the cylinder’s radius to the local boundary layer thickness (r/δ0) was 4% and the Reynolds number based on flat plate length was 5.8 × 105. An estimate of the skin friction coefficient was made using the Clauser chart and turbulence measurements were made using a hot wire anemometer. Results showed that, with the stepped cylinder in place, the local skin friction coefficient drops downstream of the half-stepped cylinder with the maximum drops exceeding 12% and 35% respectively for the single row and cylinder sheet at x/d = 13 and then gradually recovers to the corresponding value for an unperturbed flat plate at x/d = 26. At x/d = 13 and further downstream the spanwise variations of the boundary layer parameters behind the stepped cylinder were insignificant. Above the stepped cylinder, the turbulence intensity is reduced, however, in the near wake downstream, there is an increase in axial turbulence intensity attributed to the wake of the cylinder.