The wake of a rectangular flat plate

Abstract

The mean wake of a three-dimensional surface-mounted rectangular flat plate was studied experimentally in a low-speed wind tunnel for four different aspect (height-to-width) ratios, AR = 3, 2, 1, and 0.5. The Reynolds number based on the plate width was Re = 3.8 × 104 and the boundary layer thickness on the ground plane, relative to the plate width, was δ/W = 1.1. The incidence angle of the plate was varied from α = 0° (where the plate is normal to the flow) to α = 90° (where the plate is parallel to the flow). The mean velocity and vorticity fields in the wake were measured using a seven-hole pressure probe. At α = 0°, the length of the recirculation zone behind the plate becomes progressively shorter as the aspect ratio is lowered and follows the same tendency as that of a finite square prism. The wakes of the slenderer flat plates of AR = 3 and 2 are characterised by two pairs of streamwise vortices: a pair of tip vortices in the upper wake and a pair of ground-plane vortices on the lower edges of the wake. With increasing incidence angle, a single tip vortex comes to dominate the wake, secondary vorticity is induced at various locations, a ‘traffic light’ vortex pattern may form, and ultimately a familiar wing-tip (trailing) vortex develops. In contrast, flow downstream of the less slender flat plates of AR = 1 and 0.5 is characterised by a single pair of large streamwise vortices, which become asymmetric with increasing incidence. Close to the flat plate of AR = 0.5, and at small incidence angles only, a unique pair of small inner vorticity concentrations, of opposite sense of rotation to the main streamwise vortices, is found in the upper part of the wake.