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 $(4\times {10}^4$ to $6\times {10}^5)$. While the dynamics of rising buoyant spheres and bubbles has been thoroughly investigated for Reynolds numbers below ${10}^4$, 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.

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• Boundary layer receptivity, stability & separation
• Boundary layers
• Interfacial flows
• Transition to turbulence
• Vortex flows