Transition in steady streaming and pumping caused by a sphere oscillating in a viscous incompressible fluid

TL;DR

This paper provides an exact analytical calculation of steady streaming flow caused by an oscillating sphere in a viscous fluid, revealing a flow reversal transition dependent on a dimensionless scale number.

Contribution

It introduces a precise second-order solution for streaming flow around an oscillating sphere, identifying a critical scale number where flow direction reverses.

Findings

Flow pattern depends on a scale number involving sphere size, frequency, and viscosity.

Flow reverses direction at a specific scale number.

The system functions as a bidirectional pump with quantifiable efficiency.

Abstract

The steady streaming flow pattern caused by a no-slip sphere oscillating in an unbounded viscous incompressible fluid is calculated exactly to second order in the amplitude. The pattern depends on a dimensionless scale number, determined by sphere radius, frequency of oscillation, and kinematic viscosity of the fluid. At a particular value of the scale number there is a transition with a reversal of flow. The analytical solution of the flow equations is based on a set of antenna theorems. The flow pattern consists of a boundary layer and an adjacent far-field of long range, falling off with the inverse square distance from the center of the sphere. The boundary layer becomes thin in the limit where inertia dominates over viscosity. The system acts as a pump operating in two directions, depending on the scale number. The efficiency of the pump is estimated from a comparison of the rate of flow with the rate of dissipation.