The dispersion of a tracer in a fluid flow is influenced by the Lagrangian motion of fluid elements. Even in laminar regimes, the irregular chaotic behavior of a fluid flow can lead to effective stirring that rapidly redistributes a tracer throughout the domain. For flows with arbitrary time-dependence, the modern approach of Lagrangian Coherent Structures (LCSs) provide a method for identifying the key material lines that organize flow transport [Peacock and Haller (2013)]. When the advected tracer particles possess a finite size and nontrivial shape, however, their dynamics can differ markedly from passive tracers, thus affecting the dispersion phenomena [Sapsis and Haller (2008), Parsa et al. (2011)]. I investigated the behavior of finite size particles in 2-dimensional chaotic flows, combining numerical simulations and laboratory experiments. We showed that the shape and the size of neutrally buoyant particles can alter the underlying LCSs, affecting the dispersion of tracers with different shape within the same flow field.
Experiments performed in a two-dimensional cellular flow [Hackborn et al. (1997)], exhibited a focusing effect in vortex core of anisotropic particles. Neutrally buoyant, rod shape particles display markedly different trajectories and overall organization than spherical particles. The observed clustering phenomena in vortices, changes accordingly with the anisotropic particle’s aspect ratio. Our numerical model suggests that this focusing effect is driven by an orientation dependent drag force. We combine individual particle tracking with particle image velocimetry (PIV) to reconstruct the particles trajectories and the flow velocity field within the same experimental setup. A laser sheet illuminates a horizintal plan of the fluid and microscopic particles, seeded in the fluid, are detected from the reflected light and used to determine the local velocity. The macroscopic inertiel particles position are directly recorded with a camera and fluorecent light. This allows us to performe numerical simulations using the flow field measured experimentally. I developped an effective model to predict the dynamics of anisotropic and neutrally buoyant particles in low Reynolds flows. The predicted trajectories exhibit a similar focusing effect and suggest the existence of an atttracting orbit in the field for particles with large Stokes number.
Publications
. Anisotropic Particles Focusing Effect in Complex Flows preprint, arxiv:2101.04896
. Laboratory Investigations of a Chaotic Flow Using Braid Theory PRF 5 054504 (2020)
