Shark-inspired riblet design and optimization for drag reduction in drinking water distribution pipes across varying flow rates

Abstract

Drinking water distribution systems (DWDS) experience significant energy losses due to turbulence-induced drag. While shark-inspired riblet surfaces have been shown to reduce drag in controlled conditions, their effectiveness in DWDS remains uncertain, particularly under the dynamic flow variations. This experimental study explores biomimetic riblet designs as a potential solution for drag reduction in such environments. Two riblet configurations were evaluated: one designed after the shortfin mako shark (MSI), with smaller, tightly spaced riblets, and another based on the blacktip shark (BSI), with larger, widely spaced riblets. Riblet structures were 3D-printed and tested in a water flow loop system. The results show that although MSI and BSI achieved similar maximum drag reduction of approximately 6 % near a nondimensional spacing of s⁺ ≈ 14.5, their performance differed significantly versus Reynolds numbers. The MSI design sustained drag reduction over a wider range (2500 < Re < 20,000), while the BSI design was effective only within 2500 < Re < 8500. However, beyond these ranges, both designs began to experience drag increase. In addition, a comparison of geometric descriptors revealed that the square root of the groove cross-sectional area ($l_g^{+}$), provided the most consistent predictor for optimal riblet performance in pipe flow. However, the mean optimal value of $l_g^{+}$ was approximately 8.45, which is lower than the reference value of 10.7 reported for channel flows. This deviation likely results from confinement and curvature effects in pipe geometries, which modify vortex–riblet interactions compared to planar flows. These findings highlight the need to tailor riblet design to pipe-specific conditions and show that combining geometric and flow parameters improves performance evaluation in DWDS.