The Wake Structure of a Stationary and a Rotary Wall-Mounted Low-Aspect-Ratio Cylinder

Abstract

This paper investigates the wake structure of a low-aspect-ratio rotating cylinder mounted wall-normal to a flat plate at a diameter-based Reynolds number of . The cylinder is driven at a constant rotational rate about its respective axial coordinate. Four nondimensional rotation rates were selected: 0.5, 1, 2, and 3, representing rates based on the static cylinder’s natural shedding frequency and corresponding harmonics. Two aspect ratios were investigated: i) and ii) . The case of represents a cylinder height equal to the local boundary-layer thickness. Both streamwise and cross-stream (transverse) particle image velocimetry (PIV) planes are presented, and the resulting time-averaged flowfield is examined. Results for the rotating cylinder reveal the development of a dominant streamwise vortex that persists for several length scales downstream of the cylinder. Accompanying the streamwise vortex formation is an enhancement in the wall-normal velocity component (downwash) behind the rotating cylinder. Additionally, the maximum downwash occurs immediately behind the rotating cylinder near 0.27 diameters downstream in contradistinction to a stationary cylinder with peak downwash near 2–3 diameters. The maximum downwash, being in line with the formation of the dominant streamwise vortex, shortens the recirculation region in the cylinder’s wake, thus shifting the plate reattachment point further upstream. In the case of , where the cylinder is submerged within the boundary layer, although the length of the recirculation region reflected by the mean flow is lengthened by the rotary motion, the maximum downwash for the rotating cylinder is enhanced by a factor of 10 from that of the maximum downwash experienced by a static counterpart.