Turbulence modulations and drag reduction by inertialess spheroids in turbulent channel flow

Previous studies on nonspherical particle-fluid interaction were mostly confined to tiny fiber-like particles, which were observed to induce turbulence drag reduction. Nevertheless, with the presence of disk-like particles how wall turbulence is modulated and whether drag reduction occurs are still unknown. Motivated by those open questions, we perform two-way coupled direct numerical simulations of inertialess spheroids in a turbulent channel flow with additional particle stresses to account for the feedback from spheroids on the fluid phase. The results demonstrate that tiny disk-like particles can also lead to significant turbulence modulations and drag reduction in wall turbulence. Moreover, the attenuation of turbulent activities and the alteration of turbulence anisotropy are observed in the laden flows. To explain the different performances of drag reduction by fibers and disks in the channel flow, we propose a mechanism by analyzing the modulated near-wall turbulence structures and particle shear stress. We find that the spheroidal particles weaken the quasistreamwise vortices through negative work and, therefore, attenuate the Reynolds shear stress. However, the mean shear stress generated by particles, which is particle shape-dependent, partly compensates for the reduction of Reynolds shear stress and thus affects the efficiency of drag reduction. The present study implies that tiny disk-like particles can be an alternative drag reduction agent applied in wall turbulence.