The mechanism of unsteady wake transition behind large depth-ratio wall-mounted prisms*

The onset of wake unsteadiness for small aspect-ratio (height-to-width) wall-mounted prisms depends on both flow and geometrical parameters, i.e., depth-ratio (length-to-width) and Reynolds number (Re). While past studies have characterized the onset of unsteady wake for infinitely-span and large aspect-ratio (finite span) wall-mounted prisms, the effect of depth-ratio remains unexplored. As such, we numerically investigate the onset unsteady flow in the wake of wall-mounted finite prisms with an aspect-ratio of 1 and varying depth-ratios between 0.016 and 4 at Re=50-2500. The minimum depth-ratio considered here represents the special case of a wall-mounted thin flat plate. Preliminary results indicate that the onset of unsteady wake for wall-mounted thin flat plate occurs at Re=190, while that of a wall-mounted cube occurs at Re=400. Transition to unsteady wake for prisms with large depth-ratio occurs at Re=600. The onset of unsteady wake is characterized by symmetric hairpin-like vortex shedding, which leads to an oscillatory drag force and separating side-edge shear-layers. Moreover, threshold Reynolds number of transition to unsteady wake for wall-mounted prisms is significantly larger than that of finite-suspended prisms and thin flat plates. It is known that transition to unsteady wake occurs due to Hopf-bifurcation for suspended bluff bodies, whereas the present results indicate a different mechanism that governs this phenomenon for wall-mounted prisms. Thus, we aim to identify and characterize the mechanism of transition to unsteady wake for wall-mounted prisms with changing depth-ratio. Furthermore, we aim to characterize the various regimes of unsteady wake to fully understand the transition-to-turbulence wake phenomenon induced by the increasing depth-ratio in wall-mounted prisms.