An Experimental Study of the Near-Wake Region of Low-Aspect-Ratio Surface-Mounted Flat Plates

A surface-mounted flat plate is an important fundamental shape in fluid dynamics, due to the interaction of the plate with the boundary layer on the surface (or ground plane), and the vortex structures produced in the wake. The plate acts as a bluff body when oriented normal to the flow. When angled with respect to the flow, a low-aspect-ratio flat plate may be used as a vortex generator. In the present study, the flow over a surface-mounted flat plate is examined experimentally in a low-speed wind tunnel using a seven-hole pressure probe and particle image velocimetry, with the aim of better understanding the behaviour of the flow in the near wake. This study investigates the wake region for four low-aspect-ratio rectangular flat plates with aspect (height-to-width) ratios of AR = 0.2, 0.35, 0.5, and 1. In each test, the plate was oriented normal to the flow, with a boundary layer thickness to plate width ratio of δ/D = 0.6 and a Reynolds number of Re = 7.5 × 104. For the three lowest aspect ratios, there were two streamwise vortex pairs within the wake, leading to upwards flow at the centerline and one region of downwash on either side. Additionally, in the vertical symmetry plane, these three aspect ratios had a saddle point near the downstream edge of the recirculation zone that defines the flow field and leads to regions of upward and downward directed flow at the edge of the mean recirculation zone. In contrast, for the plate with AR = 1, there is only downwash on the wake centerline, a single pair of streamwise vorticity concentrations, and the saddle point is absent in the vertical symmetry plane.