Alignment of elongated swimmers in a laminar and turbulent Kolmogorov flow

TL;DR

This study investigates how elongated microswimmers align with flow directions and accumulate in both steady and turbulent Kolmogorov flows, revealing general alignment behavior and effects of flow fluctuations.

Contribution

It introduces a mechanical model and combines simulations with analytical calculations to analyze microswimmer alignment and accumulation in turbulent shear flows.

Findings

Alignment of swimming direction with local flow is a general phenomenon.

Flow fluctuations modify microorganism accumulation patterns.

Steady and turbulent flows influence microswimmer behavior differently.

Abstract

Many aquatic microorganisms are able to swim. In natural environments they typically do so in the presence of flows. In recent years it has been shown that the interplay of swimming and flows can give rise to interesting and biologically relevant phenomena, such as accumulation of microorganisms in specific flow regions and local alignment with the flow properties. Here, we consider a mechanical model for elongated microswimmers in a Kolmogorov flow, a prototypic shear flow, both in steady and in turbulent conditions. By means of direct numerical simulations, supported by analytical calculation in a simplified stochastic setting, we find that the alignment of the swimming direction with the local velocity is a general phenomenon. We also explore how the accumulation of microorganisms, typically observed in steady flows, is modified by the presence of unsteady fluctuations.