Impact of wall oscillation on the drag and flow structure in an open channel at transitional Reynolds number

This study investigates the effects of spanwise wall oscillations (SWOs) on open channel flow at Reτ=85 using direct numerical simulations. The oscillation amplitude is fixed at A+=12, and the period T+ varies from 20 to 400. Results show that SWOs reduce drag, with the highest reduction of 31% at T+=70. The primary mechanisms include an elevated streamwise velocity profile, reduced Reynolds stress, and disruption of near-wall coherent structures. A novel vortex cluster structure emerges, weakening near-wall streaks and reducing skin friction drag. As T+ increases, the spanwise tilt of near-wall streaks becomes more pronounced, and turbulence recovers, leading to drag values closer to the uncontrolled flow. Beyond T+=70, the modulation effect on turbulence fluctuations saturates. SWOs mainly regulate Reynolds shear stress by controlling Q4 events, with shorter periods suppressing large-scale structures and longer periods enhancing velocity fluctuations.