Influence of the Boundary Layer State on the Wake of a Wall-Mounted Square Cylinder

The influence of the incoming boundary layer state on the near wake of an AR 4 square cylinder is experimentally investigated. Two cases are considered at Reynolds number ~ 10⁴ and the obstacle protrudes (i) a laminar boundary layer (LBL) with a relative thickness to height of δ/h = 0.05; and (ii) a turbulent boundary layer (TBL) with δ/h = 0.18. Large scale vortex structures are educed through phase-aligning planar PIV measurements using reference pressure measurements on the sides of the obstacle. A mean pair of contra-rotating streamwise vortices, known as dipole, appears in both cases. In the LBL case, the dipole is accompanied by an additional contra-rotating pair of descending vortices, which arises as a result of the interaction between successive Kármán vortices from the opposite sides. Along with the descending vortices, Reynolds stresses and productions terms are significantly higher in the LBL case. Closer to the plate, phase-averaged results show higher interactions of the opposite side shear flow and the forming Kármán vortex in the TBL case, which is evidenced by complementary oil-film visualizations.