Buoyant spheres released below the free surface can rise well above the surface in a phenomenon known as pop-up. Contrary to intuition, increasing the release depth sometimes results in a lower pop-up height. We present the pop-up height of rising buoyant spheres over a range of release depths (1–12.5 diameters) and Reynolds numbers (4×104 to 6×105). While the dynamics of rising buoyant spheres and bubbles has been thoroughly investigated for Reynolds numbers below 104, pop-up in these larger-Reynolds-number regimes has not been studied. Yet the underwater motions of the sphere for the Reynolds numbers we study are the key to understanding the pop-up height. Two major regimes are apparent: vertical and oscillatory. The vertical regime exhibits a nearly vertical underwater trajectory and results in the largest pop-up heights. The oscillatory regime exhibits an underwater trajectory with periodic lateral motions and results in lower pop-up heights; this periodic lateral motion is modulated by unsteady vortex shedding in the wake of the sphere. Despite these complex fluid structure interactions, the experiments presented herein yield extremely repeatable results.

Citation and Link:

Tadd T. Truscott, Brenden P. Epps, and Randy H. Munns, “Water exit dynamics of buoyant spheres.” Physical Review Fluids 1, 074501, 10.1103/PhysRevFluids.1.074501, November 2016.