Influence of the Boundary Layer State on the Wake of a Cantilevered Square Cylinder

Differences in the mean wake topology of a cantilevered square cylinder of aspect ratio 4 are related to the influence of the thin, on-coming boundary layer state on interaction involving periodically shed vortices. Surface pressure and particle-image velocimetry measurements are conducted for a laminar boundary layer (LBL), of thickness-to-obstacle height ratio of δ/h≈0.05 , and a turbulent boundary layer (TBL) of δ/h≈0.18 at Reynolds number ∼10⁴, based on the obstacle width.  A dipole, consisting of a pair of counter-rotating streamwise vortices extending from the obstacle free-end region, characterizes the TBL wake. An additional vortex pair descending from the dipole exists for the LBL wake.  The descending vortices arise due to the interaction between successive shed vortices from opposing sides.  Along the descending vortices, the Reynolds stress and turbulence production rates are significantly higher than observed for the TBL case. In contrast, the interaction between the horseshoe and shed vortices in the obstacle-ground junction region are only observed for the TBL case.  Complementary oil-film visualisations of the surface flow patterns show evidence of the mean signature of these interactions and differences in the attachment point topology between the two wakes.