An Experimental Study of the Flow Above the Free Ends of Surface-Mounted Bluff Bodies

Abstract

The flow around surface-mounted finite-height bluff bodies is more complex than the flow around a two-dimensional or “infinite” cylinder. The flow over the free end and the boundary layer flow around the body-wall junction strongly influence the near-wake flow pattern. Streamwise tip vortex structures interact in a complex manner with Kármán vortex shedding from the sides of the body, and are responsible for a downward-directed local velocity field in the upper part of the wake known as “downwash.” A second pair of streamwise vortex structures, known as the base vortices, is found close to the ground plane. Upstream of the body the familiar horseshoe vortex is found. The interactions between the tip vortices, base vortices, and Kármán vortex shedding are strongly influenced by the aspect ratio, AR = H/D (for height, H, and width, D), the Reynolds number, Re, and the relative thickness of the boundary layer, δ/D. The flow above the free ends of surface-mounted finite-height circular cylinders and square prisms was studied in a low-speed wind tunnel using particle image velocimetry (PIV). Cylinders and prisms of AR = 9, 7, 5, and 3 were tested at Re = 4.2 × 104. The bodies were mounted normal to a ground plane and were partially immersed in a turbulent flat-plate boundary layer with δ/D = 1.7. PIV measurements were made above the free ends in three vertical planes at different cross-stream locations (y/D = 0, 0.25, and 0.375). The ensemble-averaged streamlines, turbulence intensity and Reynolds shear stress fields were obtained in these planes. The PIV results provide insight into the separated flow above the free ends, including the effects of AR and body shape. For the finite square prism, the large, separated, recirculating flow region extends into the near-wake. For the finite circular cylinder, this region is smaller and the separated flow reattaches onto the free-end surface. For the square prism of AR = 3, considerable difference is seen in the free-end flow pattern compared to the more slender prisms of AR = 9, 7 and 5. In particular, a cross-stream vortex is formed due to interaction between the separated flow from the leading edge of the prism and the reverse flow over the free end. This vortex is seen in all three planes for AR = 3 but only in the symmetry plane for AR = 9, while for the finite circular cylinder the flow pattern above the free end seems to be the same in all three planes for all aspect ratios, consisting of a cross-stream vortex at approximately x/D = 0.