Shark skin-inspired surface designs for drag reduction in drinking water distribution pipes

In drinking water distribution systems (DWDS), drag caused by turbulent flow results in significant energy losses and increased energy consumption. Biomimetic riblet surfaces, inspired by shark skin, are a widely explored solution for reducing drag and enhancing efficiency. However, their behavior in circular pipes under turbulent flow has received limited attention due to fabrication challenges and assumptions of similarity with channel flows. In this study, we experimentally investigated the drag reduction performance of blade riblets in circular pipes of different diameters (12, 20, and 28 mm). Riblet structures were 3D printed, and their drag reduction capabilities were evaluated using a water flow experimental setup. Results showed that the optimal riblet spacing (s_opt⁺) for maximum drag reduction varied significantly with pipe diameter (D) and height to spacing ratio (h/s). Lower D resulted in Lower s_opt⁺, while increasing D shifted s_opt⁺​ to higher s⁺ values, approaching behavior observed in channel flow studies. The transition from drag reduction to drag increase was also effected by both D and h/s ratio, with larger D shifting the transition to higher s⁺ values and larger h/s ratios shifting the transition to lower s⁺ values. Riblets with h/s ratios of 0.4 and 0.5 demonstrated the highest drag reduction capabilities, achieving up to 6 % reduction across all tested pipe diameters. A practical correlation was developed to predict s_opt⁺ based on riblet geometry and pipe diameter, which was validated against experimental data with <5 % error across all tested cases. Furthermore, a conceptual model based on vortex-riblet interactions was proposed to explain the results. These findings underscore the necessity of tailoring riblet designs to specific pipe dimensions and flow conditions to maximize drag reduction in DWDS.