Experimental investigation of yaw-angle effects on drag reduction rate for trapezoidal riblets

Effects of the yaw angle on the drag reduction rate of riblets were investigated experimentally for trapezoidal riblets with ridge angle of 30^∘ 30 ∘ and height-to-spacing ratio of 0.5 in a turbulent wind channel at Reynolds numbers (based on the friction velocity and channel half depth) Re_τ = 520 - 2100 Re τ = 520 - 2100 . Drag reduction rates were estimated carefully by comparing the streamwise pressure gradients between turbulent channel flows with and without riblets. The maximum drag reduction rate which was about 7% for the streamwise riblets was maintained without any attenuation for the yaw angle φ ϕ below 10^∘ 10 ∘ . When φ ϕ exceeded 10^∘ 10 ∘ , the drag reduction rate was attenuated steeply and the performance of riblets was almost lost at φ = 15^∘ ϕ = 15 ∘ . The yaw angle at which the drag turned to increase was by 10º smaller than the value reported for the saw-tooth riblets. The yaw angle effect on the riblet performance was further examined by detailed comparisons of mean velocity profiles and turbulent structures near the ribbed surfaces for φ = 10^∘ ϕ = 10 ∘ and 15^∘ 15 ∘ . Although no appreciable difference in the turbulent intensity and pre-multiplied spectrum of near-wall turbulence was found between these cases ( φ = 10^∘ ϕ = 10 ∘ and 15^∘ 15 ∘ ), the mean velocity profile for the case of φ = 15^∘ ϕ = 15 ∘ indicated an evidence of flow separation at the ridges of riblets, leading to the attenuation of drag-reduction rate due to increase in the pressure drag.