Enhanced upswing in immersed collisions of tethered spheres

Experiments and simplified computations are performed to study the influence of liquid motions on the dynamics of immersed pendulums. The pendulums consist of tethered spheres, swinging alone or colliding with one another in liquid initially at rest. Surprisingly, oscillations initiated in identical fashion are found to be damped less rapidly when spheres collide. Using refractive-index-matched materials and laser-illuminated tracers to visualize the flow between and around the spheres, we link this effect to the response of wake vortices. Set in motion by impact, a sphere emerges nearly wake-free from the collision, leading to reduced drag and enhanced upswing.