Flow dynamics in the wakes of low-aspect-ratio wall-mounted obstacles

The influence of shape and relative submergence (the ratio of flow depth to obstacle height, d / H ) on the wake dynamics around low-aspect-ratio wall-mounted obstacles is documented for flows at Re H = 17,800 in a thin boundary layer. The obstacles investigated consisted of semi-ellipsoids with the major axes of the base ellipses aligned in the streamwise and transverse directions, and two cylinders with aspect ratios matching the ellipsoids ( H / D = 0.89 and 0.67, where D is the maximum transverse dimension). Thermal anemometry was employed to provide insight into the wake dynamics, and thus, augment knowledge of the mean structure of these wakes presented by Hajimirzaie et al. (Exp Fluids 53:1943–1962, 2012 ). Weak symmetric shedding was observed at St = 0.183 and 0.188 in the wakes of streamwise and transverse ellipsoids at d / H = 3.9, while cross-spectral measurements confirmed downstream and upstream tilting of arch structures shed by the transverse and streamwise ellipsoids, respectively. Much weaker peaks in the power spectrum were observed for the low- and high-aspect-ratio cylinders at St = 0.084 and 0.065, respectively. While the dominant Strouhal number remained constant as the relative submergence was reduced to d / H = 2.5 for the ellipsoids, it increased significantly at d / H = 1 to St = 0.357 and 0.345 for the streamwise and transverse ellipsoids, respectively, and transitioned to an antisymmetric mode. The shedding behavior was similarly antisymmetric for both cylinders at d / H = 1, with dominant Strouhal numbers of 0.248 and 0.243 for the cylinders of H / D = 0.67 and 0.89, respectively. When calculating Strouhal number using wake width as the length scale, the values for d / H = 1 were found to fall more in line with those typical of two-dimensional bluff bodies.