On the separation mechanics of accelerating spheres

Abstract

The instantaneous drag forces and wake mechanics of an accelerating sphere have been investigated experimentally. Drag forces are first compared to the circular flat plate, which is characterized by stable and Reynolds-number independent vortex-ring formation during accelerations from rest. For the sphere, vortex-ring formation is shown to be greatly suppressed by the time-dependent movement of the separation line during start-up towards the steady-state position, which induces strong vortexbody interactions. Next, inviscid theory is used to predict the state of the pressure-gradient field around a sphere during accelerations from a non-zero initial velocity. The azimuthal point of separation extracted from experimental data for the subcritical cases is found to be strongly correlated with the start of the adverse pressure gradient predicted by theory. For the supercritical cases, the point of separation is unaffected by the imposed accelerations and remains at the steady-state position. The results suggest that accelerations can only be exploited to delay flow separation at Reynolds numbers with steady separation points near the apex, where the tangential accelerations are the largest.

Discrete shedding of secondary vortices along a modified Kaden spiral

“…The life of vortices around bluff bodies often begins with a shear layer (Jeon & Gharib 2004;Fernando & Rival 2016;Fernando et al 2017;Rosi & Rival 2017;Corkery, Babinsky & Graham 2019). When a bluff body moves relative to a fluid flow, a thin layer of fluid emerges at the edge of the body where non-zero shear flow gradients are present.…”

Modeling Transverse Gusts Using Pitching, Plunging, and Surging Airfoil Motions

“…Recently Fernando et al [10] and Mulleners et al [11] used surging motions to simulate a streamwise gust. In a streamwise gust, the differences between a moving-model and a moving-fluid case can be resolved by adding/subtracting buoyancy and added-mass forces as done by Granlund et al [12].…”

Vortex ring formation coupled with a translating bluff body

“…Although many studies have been carried out to understand diverse aspects of vortex ring formation and their mutual interaction, the coupling of a vortex ring ejected from a cylinder with a bluff body propagating ahead of the vortex ring remains unexplored. The translation of a bluff body in a fluid medium develops a wake structure behind the body, which undergoes roll-up and pinch-off processes similar to those of a piston-generated vortex ring (Fernando & Rival 2016;Fernando et al 2017). Hence, in the configuration of a vortex ring with a forward-moving bluff body in front of it, the vortex ring formation may be strongly affected by the wake produced from the bluff body.…”

“…The translation of a bluff body in a fluid medium develops a wake structure behind the body, which undergoes roll-up and pinch-off processes similar to those of a piston-generated vortex ring (Fernando & Rival 2016; Fernando et al. 2017). Hence, in the configuration of a vortex ring with a forward-moving bluff body in front of it, the vortex ring formation may be strongly affected by the wake produced from the bluff body.…”