Flow development over isolated droplet-inspired shapes

Highlights: • PIV study of the flow around droplet-inspired objects in a laminar boundary layer. • Sessile and depinning droplet shapes with height equal to boundary layer thickness. • Horseshoe vortex system more complex for the sessile model. • More consistent merging of shear layer vortices shed from the depinning model shape. • All models induce a turbulent wedge, reminiscent of late stage bypass transition. Flow development over isolated, surface-mounted, droplet-inspired three-dimensional obstacles submerged in a laminar boundary layer is investigated at a Reynolds number based on obstacle height of ${\mathrm{Re}}_h=2070$ using particle image velocimetry. Three geometries are considered, a sessile droplet; a droplet on the verge of runback (depinning); and a spherical cap, which serves as a first order approximation of a sessile droplet. For all three models a horseshoe vortex system forms at the leading edge and wraps around the obstacle, with the sessile model producing the most prominent horseshoe system due to its relatively high fore-body bluntness. Shear layer vortices shed from the objects form arch vortices, which produce mean streamwise vortices in wakes of the models. Downstream of the objects, turbulent fluctuations grow in a wedge shape. The spreading mechanisms in the wall-normal and lateral directions are explored through proper orthogonal decomposition of the velocity fluctuations. The analysis shows that wall-normal growth is associated with shear layer vortex breakdown, while lateral spreading is influenced by interactions between the horseshoe vortex system and streamwise wake vortices. Overall, similarity between the chopped and sessile model flow fields suggest the spherical cap is a reasonable first order approximation of a sessile droplet from a vortex dynamics perspective. Shear layer vortices shed from the runback model exhibit more complete merging than for the other two models, perhaps due to the smaller radius of curvature at the object peak. This model also exhibits a more muted horsheshoe vortex system and narrower near-field wake than the sessile model, suggesting less disruption to the incoming boundary layer flow.