Rotational Dynamics of Ellipsoidal Particles in Turbulent Channel Flows

Turbulent flows carrying small non-spherical particles are ubiquitous in both industrial and natural systems. In this study, we employ Direct Numerical Simulations (DNS) coupled with Lagrangian particle tracking to investigate the dynamics of elongated ellipsoidal particles in turbulent channel flows. The Navier-Stokes equations are solved on a fixed Eulerian grid using pseudo-spectral methods, while the motion (translation and rotation) of N_p = 2,000,000 prolate ellipsoids is tracked by solving a set of ordinary differential equations for each particle. The equations describing particle dynamics account solely for the hydrodynamic drag force and torque, consistent with relevant literature. Considering friction Reynolds numbers ranging from Re_τ = 180 to Re_τ = 720, the simulations aim to benchmark and complement the experimental results on the rotational dynamics of microplastic slender fibers recently obtained at the TU Wien Turbulent Water Channel. With a focus on particles with small inertia (St⁺ ~ O(10^-2)) and large aspect ratio (λ ~ O(10²)), we explore the influence of the mean shear and vorticity in the near-wall region on particle rotation rates around their longitudinal axis (spinning rate) and transversal axes (tumbling rates). We further analyze the influence of the Reynolds number on the statistical relation between particle orientation and the local flow topology, specifically examining particle alignment with the local fluid's main directions of compression/expansion and rotation.