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