Enhancement of drag reduction in particle-laden turbulent flows under span-wise wall oscillation

We report that span-wise sinusoidal wall oscillations can significantly reduce drag in wall-bounded turbulent flows laden with neutrally buoyant, finite-size spherical particles—surpassing the drag reduction observed in corresponding single-phase flows. Direct numerical simulations reveal that this enhanced drag reduction arises from a coupling mechanism between the suspended particles and the wall motion. The span-wise Stokes flow induces particle migration toward the channel center and suppresses near-wall turbulence, thereby reducing the effective viscosity. Quantitatively, we identify an approximately linear relationship linking the coupling-induced drag reduction to both the oscillation-driven variation in near-wall particle volume fraction and the influence of particle size. These findings uncover a previously unrecognized synergistic mechanism for turbulent multiphase drag reduction, bridging Einstein's viscosity model with turbulent boundary layer theory. This insight suggests a approach to actively control wall-bounded multiphase turbulence, with broad implications for fluid mechanics and complex flow systems.