Gyrotactic swimmers in turbulence: shape effects and role of the large-scale flow

TL;DR

This study uses numerical simulations to analyze how the shape and swimming behavior of microorganisms affect their distribution and clustering in turbulent aquatic environments.

Contribution

It provides new insights into how shape and swimming parameters influence microorganism distribution and clustering in turbulence, especially comparing spherical and elongated shapes.

Findings

Spherical swimmers prefer downwelling flows.

Elongated swimmers transition to upwelling regions at high speeds.

Both shapes exhibit fractal spatial distributions at small scales.

Abstract

We study the dynamics and the statistics of dilute suspensions of gyrotactic swimmers, a model for many aquatic motile microorganisms. By means of extensive numerical simulations of the Navier-Stokes equations at different Reynolds numbers, we investigate preferential sampling and small scale clustering as a function of the swimming (stability and speed) and shape parameters, considering in particular the limits of spherical and rod-like particles. While spherical swimmers preferentially sample local downwelling flow, for elongated swimmers we observe a transition from downwelling to upwelling regions at sufficiently high swimming speed. The spatial distribution of both spherical and elongated swimmers is found to be fractal at small scales in a wide range of swimming parameters. The direct comparison between the different shapes shows that spherical swimmers are more clusterized at small stability and speed numbers, while for large values of the parameters elongated cells concentrate more. The relevance of our results for phytoplankton swimming in the ocean is briefly discussed.