Rising and Sinking in Resonance: Probing the critical role of rotational dynamics for buoyancy driven spheres

TL;DR

This study experimentally investigates how rotational dynamics, influenced by center of mass offset, critically affect the oscillation, drag, and overall motion of buoyant spheres in a fluid, revealing a resonance phenomenon linked to rotational and vortex shedding timescales.

Contribution

It introduces a method to control rotational dynamics via center of mass offset and demonstrates the significant impact of rotational-vortex interactions on sphere motion and drag.

Findings

Resonance occurs at specific ratios of rotational to vortex shedding timescales.

Rotational effects significantly influence translational oscillations and drag.

Small offsets can lead to large changes in sphere dynamics.

Abstract

We present experimental results for spherical particles rising and settling in a still fluid. Imposing a well-controlled center of mass offset enables us to vary the rotational dynamics selectively by introducing an intrinsic rotational timescale to the problem. Results are highly sensitive even to small degrees of offset, rendering this a practically relevant parameter by itself. We further find that for a certain ratio of the rotational to a vortex shedding timescale (capturing a Froude-type similarity) a resonance phenomenon sets in. Even though this is a rotational effect in origin, it also strongly affects translational oscillation frequency and amplitude, and most importantly the drag coefficient. This observation equally applies to both heavy and light spheres, albeit with slightly different characteristics for which we offer an explanation. Our findings highlight the need to consider rotational parameters when trying to understand and classify path properties of rising and settling spheres.