Drag modulation by inertial particles in a drag-reduced turbulent channel flow with spanwise wall oscillation

Harmonic oscillations of the walls of a turbulent plane channel flow laden with inertial particles are studied by point-particle direct numerical simulation to improve our understanding of the physical mechanism for friction drag reduction. We specify a high wall oscillation amplitude and choose particle parameters that feature a considerable drag-reduction effect. The particle effect on the drag modulation is investigated by varying the wall oscillation period (T+) across a wide range. We find that particles enhance drag reduction for T+≤30 while attenuating it for T+>30. Specifically, we observe drag increase near the optimal oscillation period, i.e., T+=50 and 75. To explore the coupling mechanism of drag modulation by particles and spanwise wall oscillations, we examine the modifications of turbulence and particle statistics. Moreover, the self-sustaining cycle of near-wall turbulence is modulated by wall oscillations and/or particles. We find that the quasi-streamwise vortices are tilted and weakened by wall oscillations while significantly depopulated by particles. The modulated turbulence also affects the near-wall particle accumulation and clustering patterns, which results in different fluid–particle interactions compared with the non-actuated particle-laden channel flow. The mechanism of drag modulation is governed by the competition between reduced fluid Reynolds shear stress and particle stress. To give a quantitative view of the drag modulation, we compare the contributions of different stress components to the friction drag coefficient. Although the fluid Reynolds shear stress is reduced by particles, which corresponds to reduced turbulent kinetic energy production, the particle stress contribution remains significant, especially for the drag-increase case. Furthermore, the anisotropy invariant maps are provided, which show a striking resemblance of increased near-wall turbulence anisotropy as observed in other drag-reduced flows. This suggests that the constraint of increased turbulence anisotropy might be only a necessary but not sufficient condition for achieving drag reduction since this constraint is satisfied in our drag-increase case.