Flow past a sphere translating along the axis of a rotating fluid: Revisiting numerically Maxworthy's experiments

TL;DR

This study numerically revisits Maxworthy's experiments on a sphere translating in a rotating fluid, analyzing flow features, drag, and torque, and clarifies the influence of confinement and rotation on these quantities.

Contribution

It provides detailed numerical analysis of flow around a translating sphere in a rotating fluid, reconciling experimental results with theoretical predictions and exploring confinement effects.

Findings

Torque varies under rapid and slow rotation, following different laws.

Drag agrees with inertialess semi-empirical law even with large inertial effects.

Confinement significantly increases drag in rapid rotation, explaining previous discrepancies.

Abstract

We compute the flow induced by the steady translation of a rigid sphere along the axis of a large cylindrical container filled with a low-viscosity fluid set in rigid-body rotation, the sphere being constrained to spin at the same rate as the undisturbed fluid. The parameter range covered by the simulations is similar to that explored experimentally by Maxworthy [\textit{J. Fluid Mech.}, vol. 40, pp. 453-479 (1970)]. We describe the salient features of the flow, especially the internal characteristics of the Taylor columns that form ahead of and behind the body and the inertial wave pattern, and determine the drag and torque acting on the sphere. Torque variations are found to obey two markedly different laws under rapid- and slow-rotation conditions, respectively. The corresponding scaling laws are predicted by examining the dominant balances governing the axial vorticity distribution in the body vicinity. Results for the drag agree well with the semi-empirical law proposed for inertialess regimes by Tanzosh \& Stone [\textit{J. Fluid Mech.}, vol. 275, pp. 225-256 (1994)]. This law is found to apply even in regimes where inertial effects are large, provided rotation effects are also large enough. Influence of axial confinement is shown to increase dramatically the drag in rapidly rotating configurations, and the container length has to be approximately a thousand times larger than the sphere for this influence to become negligibly small. The reported simulations establish that this confinement effect is at the origin of the long-standing discrepancy existing between Maxworthy's results and theoretical predictions.