Accumulation and alignment of elongated gyrotactic swimmers in turbulence

TL;DR

This study investigates how gyrotactic swimmers behave in turbulence, focusing on their spatial accumulation and alignment influenced by swimming speed and gyrotactic stability, using direct numerical simulations and Voronoï analysis.

Contribution

The paper introduces a detailed analysis of gyrotactic swimmer dynamics in turbulence, highlighting the effects of swimming velocity and gyrotaxis strength on clustering and alignment patterns.

Findings

Clustering duration varies with swimming speed and gyrotaxis strength.

Preferential sampling of clusters and voids is more pronounced than for ensemble particles.

Lagrangian alignment is affected by swimming velocity and gyrotaxis, being enhanced or reduced accordingly.

Abstract

We study the dynamics of gyrotactic swimmers in turbulence, whose orientation is governed by gravitational torque and local fluid velocity gradient. The gyrotaxis strength is measured by the ratio of the Kolmogorov time scale to the reorientation time scale due to gravity, and a large value of this ratio means the gyrotaxis is strong. By means of direct numerical simulations, we investigate the effects of swimming velocity and gyrotactic stability on spatial accumulation and alignment. Three-dimensional Vorono{\"\i} analysis is used to study the spatial distribution and time evolution of the particle concentration. We study spatial distribution by examing the overall preferential sampling and where clusters and voids (subsets of particles that have small and large Vorono{\"\i} volumes respectively) form. Compared with the ensemble particles, the preferential sampling of clusters and voids is found to be more pronounced. The clustering of fast swimmers lasts much longer than slower swimmers when the gyrotaxis is strong and intermediate, but an opposite trend emerges when the gyrotaxis is weak. In addition, we study the preferential alignment with the Lagrangian stretching direction, with which passive slender rods have been known to align. We show that the Lagrangian alignment is reduced by the swimming velocity when the gyrotaxis is weak, while the Lagrangian alignment is enhanced for the regime in which gyrotaxis is strong.