Session G46: Particle-Laden Flows: Deformable Particles

Trapping of heavy flexible discs in a vortical flow

During the course of experiments in a water tank, thin heavy flexible silicone discs are surprisingly observed orbiting at a quasi-stable radial position from the axis of a strong vortex. Given that the discs are denser than the fluid, their persistent orbit around the vortex is notable because it would have been expected that the discs, similarly to other types of particles, would be centrifuged away from the vortex core. To investigate this curiosity, referred to as a trapping event, a process was developed to obtain the 3D reconstruction of the thin flexible discs in the flow. It is observed that the discs’ physical properties in relation to the vortex strength (Re_Γ ∼ 10⁵) are linked to the radial positions of the discs’ orbit. The 3D reconstruction process and theoretical approaches to understanding the mechanism behind the trapping event will be discussed.   

The Settling Behavior of Viscoelastic Gel Polymer Particles in Weak Turbulence

Particle transport in natural waterways is very important to the overall health of the environment. This transport mechanism is responsible for the movement of sediment, particulate pollutants such as microplastics, and organic material such as microalgae, which play a major role in the global carbon cycle. Particle transport in nature is highly influenced by biology, where the presence of sticky and viscoelastic biofilms and extracellular polymer exudates influence the size, porosity, and shape of both biotic (e.g. microalgae) and abiotic (e.g. microplastic, sediment) particles. Thus, the ability to understand and predict the settling dynamics of these complex, biologically-influenced particles is essential for monitoring particulate material in aquatic environments. In this investigation we examined the effects of viscoelasticity, characterized by the Deborah number, on the settling dynamics of model biological gel polymer particles in turbulence. We performed experiments in a turbulence tank designed to generate homogeneous and isotropic turbulence at low dissipation rates common in natural aquatic environments (ε <= O(10-4 ) m2 /s3 ). We determined the particle size, morphology, and settling velocity through Lagrangian particle tracking. We found that highly-deformable particles have unique settling characteristics compared to traditionally-studied, non-deformable particles. We discuss the implications of these findings on particle transport in the natural environment.   

Settling dynamics of Brownian elastic filaments in centrifuge systems

The settling dynamics of filaments in viscous fluids is a fundamental problem in fluid mechanics with important implications in different fields of industry and science. For instance, the ultracentrifuge technique has been widely used to separate DNA filaments based on their length. In the present study, we investigate the dynamics of a single Brownian elastic filament in centrifuge systems. For the numerical scheme, we discretized the filament in the bead-spring fashion. We implement the Brownian Dynamics method accounting for hydrodynamic interactions to evolve the beads' positions in time and capture the dynamics of the filament. We show that as one increases the centrifuge rotational velocity, the filament's configuration tends to collapse into dense structures leading to higher sedimentation factors. For strong centrifuge forces, the filament tends to acquire a stable configuration composed of a condensed head with a trailing stretched tail. The intense drag at the tail induces the reduction of sedimentation. We find that the stability of such a configuration is due to the field-induced knot-tightening mechanism caused by the tension introduced by the drag difference between the tail and head structures. Also, we find hydrodynamically induced super-diffusive-like dynamics of the filament in the plane perpendicular to the settling direction.   

Signatures of cross-streamline migration of elastic fibers in microscale flows

The complex dynamics of elastic fibers in viscous fluids are vital in many biological and industrial systems. Understanding these dynamics can help elucidate the behavior in confined suspension systems and tune flow properties in applications such as printing and clogging. In this work, we use slender-body theory in numerical simulations to study microscale dynamics that contribute to cross-streamline migration. In unidirectional flows, a buckled fiber can make a tank-treading motion, resulting in the fiber drifting away from its original position. The magnitude of the net drift between an upward and downward motion differs if the shear rate spatially varies. We then perform simulations of a Brownian fiber to show the fiber’s ability to move from regions of low to high shear. In Poiseuille flow, this results in selective migration towards walls, where the steady-state position is determined by the hydrodynamic drift towards the walls and the steric repulsion from them. We develop scaling laws for the extent of this depletion layer. Collectively, these results highlight the importance of the fiber’s microscale dynamics towards its macroscopic behavior, providing the groundwork to connect elastohydrodynamics and fiber suspensions in confined microfluidic geometries.   

Flexible fibres in turbulent channel flows

Turbulent suspensions of long slender fibres have wide-ranging applications in industry and nature, e.g. reducing drag in oil pipelines or microplastic pollution. We study the dynamics of flexible fibres in turbulent channel flow by performing DNS in an Euler-Lagrange framework. Jeffery (1922) gave the expression describing the rotation executed by ellipsoids suspended in a viscous simple shear flow devoid of fluid inertia. Di Giusto & Marchioli (2022) used the rod-chain model to construct fibres by linking sub-Kolmogorov rods, each undergoing Jeffery orbits, whilst neglecting fluid inertia effects on the forces and torques experienced by the fibres. However, these effects may significantly alter the orientation dynamics of the fibres as they drift away from Jeffery orbits and may strengthen the influence of local stretching. We account for fluid inertia by using the model by Dabade et al. (2016) and study how incorporating fluid-inertial forces and torques affects the collective dynamics of the fibres. We focus on the conditions under which the inertial contribution becomes relevant given the intermittent nature of the flow, and we do that by comparing the drift time (of typical order of a few periods of rotation) with the typical time of the flow velocity fluctuations.