Effect of boundary layer state on the wake of a cantilevered square cylinder of aspect ratio 4

A comparative experimental study is conducted on the effect of the incoming boundary layer state on the near-wake of a surface-mounted square cylinder with a height-to-width aspect ratio (h/d) of 4. Two cases are considered: (i) an incoming laminar boundary layer (LBL) with relative thickness δ/h=0.053 at a nominal Reynolds number (Re) of 10 500, and (ii) an incoming turbulent boundary layer (TBL) with δ/h=0.18 at Re=12000. Planar particle image velocimetry synchronized with surface pressure measurements enable a phase-averaged characterization of the wake dynamics. The mean field description is complemented using oil-film visualizations of the surface flow patterns. For the TBL case, the obstacle-wall-junction region is characterized by the interaction of the von Kármán vortices with a junction vortex, which results in distinctive mean surface flow patterns not observed for the LBL case. The mean wake structure for both cases shows a dipole consisting of a counter-rotating pair of streamwise vortices extending from the recirculation region. However, the LBL wake contains an additional vortex pair descending from the dipole. The descending vortices coincide with the locus of points along which successive von Kármán vortices of opposite sign connect via vortical strands. These connection sites are associated with regions of high Reynolds stresses and turbulence production. The higher fluctuation levels and reconnection process observed in the LBL wake can be related to the strength of the connector strands enhancing the energy transfer between coherent and incoherent fluctuating fields.