Finite-size Anisotropic Particles in Two-dimensional Turbulent Flows

Abstract

Dispersion in chaotic flows characterises the mechanisms of the transport of mass, heat or momentum in a fluid. It is related to random and chaotic time-dependent fluid motion. Dispersion of particles in fluids governs numerous processes in nature and in industrial flows, where particles vary in size and shape. This is a significant physics problem in which it is necessary to account for the interactions between particles and the structure of the flow. The main aim of this thesis is to study the dispersion of finite-size anisotropic particles in two-dimensional turbulent flows. This work focuses on the coupling between translational and rotational motion of finite-size particles, as well as on the impact of the inhomogeneity of turbulence on the particle motion.

By analysing the dispersion of ellipsoidal particles in 2D turbulence, we find that particles have preferential directions of motion, either transversely (perpendicular to the major axis) or longitudinally (along the major axis). This preferred direction can be changed by altering the ratio of the size of the ellipsoidal particle and the forcing scale of the underlying 2D turbulence. These features of the turbulent transport of ellipsoids are attributed here to the interaction of these anisotropic objects with the structure of 2D turbulent flows made of meandering coherent bundles. Large ellipsoids interact with many bundles and thus diffuse faster in the direction parallel to their major axes, a behavior reminiscent of the dynamics of an ellipsoid undergoing a Brownian motion. By contrast, small ellipsoids diffuse faster in the direction transverse to their major axes. We demonstrate that the coupling between translational and rotational diffusion arises as a result of the advection of the small ellipsoid by a single coherent bundle.

We also study the motion of floating discs with a cut-out sector in 2D turbulent flows. Such model anisotropic particles are suitable for capturing the essential features of their interaction with underlying turbulence. In particular, we find that the discs modify the surrounding flow by converting turbulent eddies into persistent bundles of fluid particles, causing the 'rectification' of the turbulent fluctuations and generating unidirectional propulsion of the discs. The randomisation of such a motion is caused by the turbulent rotation of the anisotropic particle. It is found that translational and rotational motion of particles are coupled. This coupling depends on the particle's shape and size relative to the characteristic scale of the underlying turbulence. We also show that such a coupling can be effectively controlled by engineering the shape and size of the anisotropic particle to match the characteristic scale of turbulence.

The results offer a novel method of engineering the dispersion of anisotropic particles in chaotic and turbulent flows. Our conclusion will provide better understanding of the motion of anisotropic particles in applications including pollutant dispersion in the atmosphere, the motion of ocean buoys, the drift of sea ice and may also help in clarifying the mechanisms behind the enhanced swimming abilities of some aquatic organisms.