Flow organization in the near wake of isolated and sheltered two-dimensional bar roughness elements

Planar particle image velocimetry and volumetric particle tracking velocimetry were used to investigate the flow over isolated and sheltered two-dimensional bar roughness elements immersed within a turbulent boundary layer. Flow measurements were made for bars occupying up to 17% of the boundary layer thickness at a Reynolds number of 68 000, based on the boundary layer thickness and free stream velocity. In addition to an isolated bar case, the flow was investigated over two bars positioned at streamwise spacings of 0.8h2, 1.6h2, 2.4h2, and 4.8h2, where h2 is the height of the downstream bar. By varying the height of the upstream bar h1, three height ratios h1/h2 = 0.5, 0.75, and 1 were considered at each spacing. The results highlight the effects of the spacing and height ratio on the mean flow field and turbulence past the downstream bar. Specifically, sheltering by an upstream bar reduces the reattachment length and velocity deficit past the downstream bar. The introduction of an upstream bar with h1/h2 < 1 lessens the upwash experienced by the two-bar unit and the overall perturbation to the incoming turbulent boundary layer. The Reynolds shear stress past the downstream bar is also reduced, by up to 45% in some cases, due to sheltering. Overall, the flow organization past the downstream bar is influenced by the flow deflection over the two-bar unit, the upstream bar shear layer, and the structure of flow recirculation between the two bars. Moreover, visualizations of the vortical structures past the isolated bar highlight the growth and evolution of coherent spanwise vortices. Sheltering by an upstream bar enhances the three-dimensionality of the vortical structures in the wake of the downstream bar. The effects of the bar(s) on the boundary layer turbulence structure are investigated through two-point correlations and proper orthogonal decomposition, which suggest a weakening of the large-scale flow structures of the incoming flow.

Automated summary

Planar particle image velocimetry and volumetric particle tracking velocimetry were used to study the flow over 2D bar roughness elements in a turbulent boundary layer. The effects of spacing and height ratio on the flow field and turbulence past the downstream bar were investigated. Results showed that sheltering by an upstream bar reduced the reattachment length and velocity deficit past the downstream bar, and also reduced the Reynolds shear stress. The vortical structures past the isolated bar were visualized, and the three-dimensionality of the vortical structures in the wake of the downstream bar was enhanced by sheltering.