Hierarchical Riblet Structures for Enhanced Drag Reduction and Broader Operational Range in Water Pipelines

In drinking water distribution systems (DWDS), skin-friction drag in turbulent pipe flows contributes significantly to energy losses. Passive drag-reducing surfaces, such as shark-skin-inspired riblets, have shown promise in controlled environments but often underperform under variable flow conditions. This study addresses this limitation by developing and experimentally evaluating a Hierarchical Step-shaped Riblet (HSR) design aimed at sustaining drag reduction under the variable and fluctuating flow conditions typical of water pipelines. Building on conventional riblets (CR) and hierarchical riblets (HR), the HSR configuration introduces progressively tapered riblet tips to the hierarchical design to reduce shear-exposed surface area while maintaining effective interaction with vortices of varying size. Riblet designs were fabricated using high-resolution 3D printing and tested in variable flow conditions, simulating the flows in typical drinking water distribution networks. Drag reduction performance was evaluated across a Reynolds number range of 4200 to 20,000 using friction factor analysis and nondimensional riblet spacing. The HSR design achieved the highest peak drag reduction of 11.2% and sustained favorable performance across a broader range of flow conditions than the CR and HR designs. The results demonstrate that multiscale geometric tuning, combined with reduced shear exposure, enhances drag reduction across a broadened operational range suitable for drinking water distribution pipes.