Flow Around Surface-Mounted Low-Aspect-Ratio Square Blocks

When compared with more slender square prisms, relatively little is known about the flow around surface-mounted square blocks with very low aspect ratios (AR = H/D, where H and D refer to the height and width of the block, respectively). This research investigates the mean flow behavior behind and around surface-mounted low-aspect-ratio square blocks by performing systematic low-speed wind tunnel experiments. Using a seven-hole pressure probe and two-dimensional particle image velocimetry (PIV), the mean velocity fields were obtained for blocks of AR = 0.2, 0.4, 0.6, and 0.8, which were fully submerged in a turbulent boundary layer on the ground plane. The experiments were conducted at a Reynolds number (based on D) of Re = 7.4 × 104. Two-dimensional PIV was used to obtain measurements in the vertical symmetry plane upstream, above, and in the wake of the blocks at an incidence angle of α = 0°. Using a seven-hole probe, measurements were made in the vertical transverse plane at three locations downstream of the blocks. Additionally, for AR = 0.2 and 0.4, α was varied from 0 to 45° in increments of 5°. The mean wake of the blocks is characterized by two pairs of time-averaged counter-rotating streamwise vortices. The presence of the second pair of vortices affects the downwash in the wake of block in a manner unique to low-aspect-ratio prisms. A horseshoe vortex, a free-end vortex, and a near-wake vortex have been identified as structures of interest in the vertical symmetry plane of the blocks at α = 0°.