Fluid-solid Floquet stability analysis of self-propelled heaving foils

TL;DR

This paper uses fluid-solid Floquet stability analysis to understand how linear mechanisms lead to nonlinear self-propelled states in a heaving foil, revealing the stability conditions for different motion regimes.

Contribution

It introduces a fluid-solid Floquet stability framework that accurately predicts the onset of self-propelled states, incorporating solid-fluid interactions beyond purely hydrodynamic analysis.

Findings

Unstable synchronous mode correlates with unidirectional propulsion.

Unstable asynchronous mode relates to back & forth motion.

Analysis of large density ratios shows convergence of modes.

Abstract

We investigate the role of linear mechanisms in the emergence of nonlinear horizontal self-propelled states of a heaving foil in a quiescent fluid. Two states are analyzed: a periodic state of unidirectional motion and a quasi-periodic state of slow back & forth motion around a mean horizontal position. The states emergence is explained through a fluid-solid Floquet stability analysis of the non-propulsive symmetric base solution. Unlike a purely-hydrodynamic analysis, our analysis accurately determine the locomotion states onset. An unstable synchronous mode is found when the unidirectional propulsive solution is observed. The obtained mode has a propulsive character, featuring a mean horizontal velocity and an asymmetric flow that generates a horizontal force accelerating the foil. An unstable asynchronous mode, also featuring flow asymmetry and a non-zero velocity, is found when the back & forth state is observed. Its associated complex multiplier introduces a slow modulation of the flapping period, agreeing with the quasi-periodic nature of the back & forth regime. The temporal evolution of this perturbation shows how the horizontal force exerted by the flow is alternatively propulsive or resistive over a slow period. For both modes, an analysis of the velocity and force perturbation time-averaged over the flapping period is used to establish physical instability criteria. The behaviour for large solid-to-fluid density ratio of the modes is thus analyzed. The asynchronous fluid-solid mode converges towards the purely-hydrodynamic one, whereas the synchronous mode becomes marginally unstable in our analysis not converging to the purely-hydrodynamic analysis where it is never destabilised.