Vortical structures in the near wake of tabs with various geometries

The vortical structures and turbulence statistics in the near wake of rectangular, trapezoidal, triangular and ellipsoidal tabs were experimentally studied in a refractiveindex- matching channel. The tabs share the same bulk dimensions, including a 17 mm height, a 28 mm base width and a 24.5 degrees inclination angle. Measurements were performed at two Reynolds numbers based on the tab height, Re-h similar or equal to 2000 (laminar incoming flow) and 13 000 (turbulent incoming flow). Three-dimensional, three-component particle image velocimetry (PIV) was used to study the mean flow distribution and dominant large-scale vortices, while complementary high-spatialresolution planar PIV measurements were used to quantify high-order statistics. Instantaneous three-dimensional fields revealed the coexistence of a coherent counter-rotating vortex pair (CVP) and hairpin structures. The CVP and hairpin vortices (the primary structures) exhibit distinctive characteristics and strength across Reh and tab geometries. The CVP is coherently present in the mean flow field and grows in strength over a significantly longer distance at the low Reh due to the lower turbulence levels and the delayed shedding of the hairpin vortices. These features at the low Reh are associated with the presence of Kelvin-Helmholtz instability that develops over three tab heights downstream of the trailing edge. Moreover, a secondary CVP with an opposite sense of rotation resides below the primary one for the four tabs at the low Reh. The interaction between the hairpin structures and the primary CVP is experimentally measured in three dimensions and shows complex coexistence. Although the CVP undergoes deformation and splitting at times, it maintains its presence and leads to significant mean spanwise and wall-normal flows.