AN EXPERIMENTAL STUDY OF THE FLOW AROUND SURFACE-MOUNTED RECTANGULAR FLAT PLATES

Abstract

An experimental study was carried out to study the mean wake of a surface-mounted finite-height three-dimensional rectangular flat plate. Four different flat plates were used in the experiments, with aspect ratios of AR = 0.5, 1, 2 and 3. For each aspect ratio, the incidence angle α was varied from α = 0° to 90° in increments as low as 5°. The experiments were conducted in a low-speed wind tunnel, at a Reynolds number of Re = 3.8×104 and with a boundary layer thickness of δ/W = 1.14 (where W is the width of the plate). Time-averaged velocity measurements in the wake of the plate were made with a seven-hole pressure probe. Wake measurements were performed in the vertical cross-stream plane (normal to the freestream) at x/W = 6 downstream of the plate, and in the vertical plane on the wake centreline (parallel to the freestream). The data were compared with results from similar experiments for surface-mounted finite-height square prisms and cylinders at similar flow conditions and of similar aspect ratios.

When the flat plate is located normal to the flow, with α = 0º, the streamwise length of the mean recirculation zone and the strength of the downwash on the wake centreline both increase with AR. The recirculation zone of the flat plate is longer than those of the finite-height square prism and the finite cylinder, owing to differences in the body shape and flow separation.

Flat plates of AR = 0.5 and 1 have a single pair of time-averaged counter-rotating streamwise vortex structures in the wake, referred to as ground plane vortex structures. As the incidence angle increases, these vortex structures become asymmetric and of different size, strength, and location. The streamwise vortex produced by the leading edge of the plate located further upstream overtakes the opposing one and at sufficiently high at α starts behaving like a wing-tip vortex.

Flat plates of AR = 2 and 3 have two pairs of time-averaged counter-rotating streamwise vortex structures, with a set of tip vortices and a set of ground plane vortices. Some additional induced vorticity is also seen for the flat plate of AR = 3. At high incidence angles, the wakes of these flat plates have a single wing-tip vortex type structure.

A comparison between the wakes of the flat plate of AR = 0.5 and a finite square prism of AR = 0.5 showed that induced vorticity, with opposite rotation to the ground plane vortices near the centerline, was only present for the square prism. This feature is attributed to the interaction of the main vortex structures with the top x-y face (free end) of the afterbody. A comparison between the wakes of the flat plate of AR = 3 and a finite square prism of AR = 3 showed a dual wake boundary peak for the square prism while the flat plate shows a single wake boundary. The differences are attributed to the presence of the afterbody on the square prism. Both wakes denote a different behaviour when varying α. The finite square prism shows its largest asymmetry between α = 10º - 15º and regaining its symmetric features at α = 45º while the flat plate shows an increasingly asymmetric wake behaviour up to α = 90º.