Interface-resolved direct numerical simulations of the interactions between neutrally buoyant spheroidal particles and turbulent channel flows

Abstract

A parallel direct-forcing fictitious domain method is employed to perform interface-resolved numerical simulations of the interactions between neutrally buoyant finite-size spheroidal particles and turbulent channel flows. The effects of the aspect ratio of the spheroidal particles on the turbulence modulation and the rotation of the particles are investigated at the friction Reynolds number of 180, with the ratio of the particle equivalent diameter to the channel width being 0.1, the particle volume fraction ranging from 0.79% to 14.16%, and the particle aspect ratio ranging from 1/3 to 8. Our results show that the flow friction decreases as the prolate particles become more slender or the oblate particles become flatter and is smaller than that of the single-phase flow for the aspect ratio being 1/3 and 8 at the particle volume fraction of 2.36%. Both effects of the low particle volume fraction in the near-wall region and the relatively small Reynolds stress are important to the occurrence of the drag-reduction by the non-spherical particles, and a lower flow drag for the oblate particles compared to the prolate particles at comparable aspect ratios (e.g., 1/3 vs 4) is mainly caused by a lower Reynolds stress contribution. The prolate particles preferentially align their symmetry axes with the streamwise direction, and the oblate particles preferentially align their symmetry axes with the wall-normal direction. However, the most probable orientation of the major axes of both prolate and oblate particles in the near-wall region is not exactly the streamwise direction but has a positive inclination angle with the streamwise direction. Generally the prolate particles have higher spinning velocities and lower tumbling velocities in the entire channel region, compared to the oblate particles.