DIRECT NUMERICAL-SIMULATION OF TURBULENT-FLOW OVER RIBLETS

Direct numerical simulations of turbulent flows over riblet-mounted su rfaces are performed to educe the mechanism of drag reduction by rible ts. The computed drag on the riblet surfaces is in good agreement with the existing experimental data. The mean-velocity profiles show upwar d and downward shifts in the log-law for drag-decreasing and drag-incr easing cases, respectively. Turbulence statistics above the riblets ar e computed and compared with those above a flat plate. Differences in the mean-velocity profile and turbulence quantities are found to be li mited to the inner region of the boundary layer. Velocity and vorticit y fluctuations as well as the Reynolds shear stresses above the riblet s are reduced in drag-reducing configurations. Quadrant analysis indic ates that riblets mitigate the positive Reynolds-shear-stress-producin g events in drag-reducing configurations. From examination of the inst antaneous flow fields, a drag reduction mechanism by riblets is propos ed: riblets with small spacings reduce viscous drag by restricting the location of the streamwise vortices above the wetted surface such tha t only a limited area of the riblets is exposed to the downwash of hig h-speed fluid that the vortices induce.