Impact of multi-scale riblet design on turbulent boundary layer control and drag reduction effectiveness*

This research explored the impact of incorporating secondary blade riblet structures on flow statistics and the effectiveness on friction drag reduction. Turbulent flow behaviors and drag reduction capabilities were assessed in a flow visualization channel with varying non-dimensional riblet spacing s⁺. The findings indicated that although both riblet surfaces showed comparable drag reduction at low s⁺, the secondary riblet blade structure notably extended the drag reduction effectiveness to s⁺=32, achieving approximately 10% lower friction drag compared to the single-scale surface when s⁺ increases to 44.2. Additionally, the average number of uniform momentum zones on the multi-scale blade riblet surface decreased by approximately 9% compared to the single-scale riblet, indicating a reduction in strong shear layers within the turbulent boundary layer. Near-wall flow analysis revealed that at higher s⁺, the multi-scale riblet surface resulted in reduced wall-normal velocity fluctuations and Reynolds shear stresses. Quadrant analysis further showed that this design suppressed both sweep and ejection events. These experimental results illustrate that surfaces with spanwise riblet height variations can sustain drag reduction efficiency over a broader range of flow speeds.