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An experimental study was conducted on the wakes around three different obstacles confined between two parallel walls: semi-ellipsoids with the major axes of the base ellipses aligned in the streamwise and transverse directions, and a sphere. The Reynolds number was 1.78 × 10 4 based on obstacle height, H. Common to each of these geometries is a convex surface of the obstacle intersecting one or more boundaries, which was shown to result in a wall jet in the vicinity of the junction. Large streamwise counter-rotating vortex structures were found to be a robust feature of the wakes, which are referred to as lobes, due to their resemblance to previously coined lobe structures identified in the wake of a deeply submerged sphere. Inner lobe structures, of opposite sense to the primary lobes, were also observed in all cases, and it is proposed that the primary and inner lobe structures are formed by the wall jet deforming vorticity shed by the obstacle into a hairpin-like loop. Wavelet analysis of velocity time series on both sides of the wake revealed approximately antisymmetric vortex shedding in the ellipsoid wakes; however, intermittent in-phase shedding was observed in the wake of the sphere. This study showed that the shape of the obstacle junction (convex vs. flat) plays an important role in the behavior of the wake. [ABSTRACT FROM AUTHOR]

The mean wake structures of a cube (square cylinder) and circular cylinder of height-to-width aspect ratio 1.0, at a Reynolds number of 1.78 × 104 based on the obstacle width, were investigated experimentally. The boundary-layer thickness was 0.14 of the obstacle height. The study was performed using thermal anemometry and two-dimensional digital particle image velocimetry (DPIV). Streamwise structures observed in the mean wake for both cylinders included well-known tip- and horseshoe (HS)-,vortex pairs as well as additional structures akin to the base vortices. In addition to tip-, base-, and HS-vortices, in the near wake of the cube, two more counter-rotating pairs of streamwise structures, including upper and inboard vortices, were observed. The existence of base vortices formed in the near wake for both obstacles is a unique observation and has not been previously reported for such low-aspect-ratio obstacles in thin boundary-layers. A model of arch-vortex evolution was proposed, in which arch structures were deformed by the external shear-flow to explain the observed base-vortices in the cylinder wake. A weak dominant-frequency of St = f0D/U∞ = 0.114 was observed across the height for the cube, while no discernible spectral peaks were apparent in the wake of the cylinder. Cross-spectral analysis revealed the shedding to be symmetric (in-phase) arch-type for the cylinder and predominantly anti-symmetric (out-of-phase) Karman-type for the cube. The study makes fundamental contributions to the understanding of the flow-field surrounding low-aspect-ratio cylinders. [ABSTRACT FROM AUTHOR]