On the flow dynamics of wall-mounted low-aspect-ratio rotating cylinders

Abstract

The flow physics and vortical features attributed to a low-aspect-ratio circular cylinder cantilevered on a laminar boundary layer plate and rotated about its respective axial coordinate were investigated experimentally using Stereo Particle Image Velocimetry. The investigation documents the three-dimensional spatial evolution of on-body dynamics to deep-wake phenomena. In disseminating the more salient global characteristics of the resultant flowfield, the analysis focuses on the particular case of an aspect ratio of 1 driven in continuous rotary motion at a speed ratio of 3. The two key findings are (i) vortex source: a wall-normal swirl develops and reorients into a streamwise vortex at the cylinder's mid-diameter location due to the heavy influence of the incoming boundary layer, local fluid entrainment, and proximity to the wall. (ii) Enhanced downwash: in return, the streamwise vortex primarily drives the production of additional downwash in the wake, which appends to the downwash generated by the cylinder's free end. A two-dimensional planar particle image velocimetry campaign in service of a parametric survey reveals the sensitivity of streamwise vortex production and downwash to rotational rate and cylinder height relative to boundary layer thickness. The streamwise vortex production tends to deteriorate when the cylinder is immersed in the boundary layer due to reduced downwash.