Vortex dynamics of a boundary layer disturbed by cylinder wakes: effects of the gap ratio and the relative boundary layer thickness

The vortical structures in a flat-plate boundary layer disturbed by circular cylinder wakes are investigated using particle image velocimetry and hydrogen bubble visualization techniques. The cylinder diameter ( D ) and the gap between the cylinder and flat plate ( G ) are systematically varied to establish an investigated matrix of Reynolds number Re D = 530–1600, gap ratio upper G divided by upper D G / D $G/D$ = 0.33–6 and relative boundary layer thickness delta 0 divided by upper D δ 0 / D $\\delta_0/D$ = 0.35–1.06 ( δ 0 is boundary layer thickness at the installation position of cylinder). An unexpected large critical value of upper G divided by upper D G / D $G/D$ = 4 is captured to divide the wake-vortices–boundary-layer interaction into two modes for delta 0 divided by upper D δ 0 / D $\\delta_0/D$ = 1.06: the wake vortices trigger secondary vortices (SV) for upper G divided by upper D G / D $G/D$ ≤ 4; no secondary vortex is observed inside the boundary layer for upper G divided by upper D G / D $G/D$ > 4. The flow physics behind this mode switching is elaborated. An increase in delta 0 divided by upper D δ 0 / D $\\delta_0/D$ is found