Flow around surface-mounted low-aspect-ratio rectangular flat plates

Abstract The mean flow around surface-mounted low-aspect-ratio rectangular flat plates was studied experimentally for four different aspect (height-to-width) ratios (AR = 1, 0.5, 0.35, 0.2) with the plate oriented normal to flow (at an angle of attack of β = 90°, where the angle of attack represents the rotation of the flat plate about its vertical axis). In addition, the mean wake of the plate of AR = 0.2 was studied at angles of attack from β = 90° to 10°. The Reynolds number was Re = 7.4×104 based on plate width and the boundary layer thickness relative to the plate width was δ/W = 0.83. At β = 90°, the near wake is dominated by the shear layer separating from the top of the plate and a large recirculation zone behind the plate. The far wake is characterised by one or two pairs of streamwise counter-rotating vortex structures. For AR = 1, downwash reaches the ground plane and there is a single pair of dipole structures. For AR = 0.5, 0.35, and 0.2, downwash does not reach the ground plane and there are two pairs of vortex structures: a stronger outer vortex pair and a weaker inner vortex pair. For the plate of AR = 0.2, as β is lowered from 90° to 10°, the mean wake shifts laterally, the outer and inner vortex pairs become asymmetric, and some vortex merging occurs. From β = 45° to 10°, the mean wake is dominated by a wing-tip (trailing) vortex adjacent to the ground plane, which induces additional streamwise vorticity of opposite sign. The maximum vortex strength and greatest peak vorticity occur from β = 30° to 45°, and the greatest vortex generator effectiveness (defined as the ratio of vortex circulation to aerodynamic drag) was found to occur at β = 30°.