Local flow field of a surface-mounted finite circular cylinder

Abstract

The local flow field and near-wake region of a surface-mounted finite circular cylinder were studied experimentally in a low-speed wind tunnel using particle image velocimetry (PIV). The cylinder was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer. Four finite circular cylinders of aspect ratios AR=9, 7, 5 and 3 were tested at a Reynolds number of ReD=4.2×104. At the location of the cylinder, the boundary layer thickness relative to the cylinder diameter was δ/D=1.6. PIV velocity field measurements were made in a vertical plane parallel to the mean flow direction on the flow centerline (the symmetry plane), within five diameters upstream and downstream of the cylinder, and also above the free end. Above the free end of the cylinder, flow separation from the leading edge leads to the formation of a mean recirculation zone on the free-end surface. The point of reattachment of the flow onto the free-end surface moves towards the trailing edge as the cylinder aspect ratio is decreased. Large regions of elevated turbulence intensity and Reynolds shear stress are found above the free end. In the near-wake region, the large recirculation zone contains a vortex immediately behind and below the free end; this vortex was found for all four aspect ratios. A second vortex is found behind the cylinder near the cylinder-wall junction; this vortex was not observed for the cylinder of AR=3, indicating a distinct wake structure for this cylinder.