Drag reduction in a turbulent boundary layer with sinusoidal riblets

We report on an experimental investigation on the effect of sinusoidal riblets on the near-wall characteristics of a turbulent boundary layer. The investigated riblets are characterized by a fixed wavelength and two different values of the amplitude. We comment on the flow field organization via hot wire anemometry, planar and stereoscopic particle image velocimetry experiments; furthermore, we infer on the friction drag, directly measured with a load cell, comparing the sinusoidal riblets to the reference case of riblets aligned with the mean flow (longitudinal riblets) and the Smooth case.We show that the sinusoidal riblets generally yield higher drag reduction, attaining values as large as 10%, compared with the longitudinal riblets that are limited to 8% under the same conditions. We demonstrate that the drag reduction is associated with an overall attenuation of the turbulence intensity in the buffer layer. Furthermore, we provide statistical evidence of the fact that the sinusoidal riblets are responsible for an attenuation of the Reynolds shear stresses that contribute the most to turbulence production. From the detection of the accelerated events in the buffer layer, we show that the sinusoidal riblets lead to a weakening of the intensity of the events in the streamwise plane and an enhancement of the spanwise induced motion. We relate this mechanism to that responsible for drag reduction when using spanwise wall oscillations, suggesting a possible effect of a secondary alternating vorticity in the grooves of the sinusoidal riblets.