On the Propulsion of a Rigid Body in a Viscous Liquid Under the Action of a Time-Periodic Force

TL;DR

This paper investigates the conditions under which a rigid body in a viscous fluid can achieve net motion under a small, time-periodic force, revealing key criteria and providing explicit examples within a weak solution framework.

Contribution

It establishes necessary and sufficient conditions for net propulsion of a rigid body driven by a periodic force in a viscous liquid, including the case when spinning is prevented.

Findings

Net motion occurs if and only if a specific condition involving force and body is satisfied.

Prevention of spinning guarantees net motion under the force.

Explicit examples demonstrate when the condition holds or fails.

Abstract

A rigid body $\mathcal{B}$ moves in an otherwise quiescent viscous liquid filling the whole space outside $\mathcal{B}$, under the action of a time-periodic force $\boldsymbol{\mathsf{f}}$ of period $T$ applied to a given point of $\mathcal{B}$ and of fixed direction. We assume that the average of $\boldsymbol{\mathsf{f}}$ over an interval of length $T$ does not not vanish, and that the amplitude, $\delta$, of $\boldsymbol{\mathsf{f}}$ is sufficiently small. Our goal is to investigate when $\mathcal{B}$ executes a non-zero net motion; that is, $\mathcal{B}$ is able to cover any prescribed distance in a finite time. We show that, at the order $\delta$, this happens if and only if $\boldsymbol{\mathsf{f}}$ and $\mathcal{B}$ satisfy a certain condition. We also show that this is always the case if $\mathcal{B}$ is prevented from spinning. Finally, we provide explicit examples where the condition above is satisfied or not. All our analysis is performed in a general class of weak solutions to the coupled system body-liquid problem.