  Previous studies on nonspherical particle-fluid interaction were mostly
confined to elongated fiber-like particles, which were observed to induce
turbulence drag reduction. However, with the presence of tiny disk-like
particles how wall turbulence is modulated and whether drag reduction occurs
are still unknown. Motivated by those open questions, we performed two-way
coupled direct numerical simulations of inertialess spheroids in turbulent
channel flow by an Eulerian-Lagrangian approach. The additional stress accounts
for the feedback from inertialess spheroids on the fluid phase. The results
demonstrate that both rigid elongated fibers (prolate spheroids) and thin disks
(oblate spheroids) can lead to significant turbulence modulations and drag
reduction. However, the disk-induced drag reduction is less pronounced than
that of rigid fibers with the same volume fraction. Typical features of
drag-reduced flows by additives are observed in both flow statistics and
turbulence coherent structures. Moreover, in contrast to one-way simulations,
the two-way coupled results of spheroidal particles exhibit stronger
preferential alignments and lower rotation rates. At the end we propose a drag
reduction mechanism by inertialess spheroids and explain the different
performance for drag reduction by fibers and disks. We find that the spheroidal
particles weaken the quasistreamwise vortices through negative work and,
therefore, the Reynolds shear stress is reduced. However, the mean shear stress
generated by particles, which is 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 in wall turbulence.
