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.