Mirvahid Mohammadpour Chehrghani, Doekle Yntema, David Matthews, Matthijn de Rooij, Jamal Seyyed Monfared Zanjani

Abstract: This study examines the role of riblet tip curvature in controlling drag in pipe flow and identifies how subtle geometric variations can alter performance. Three tip shapes were investigated: conventional flat tips, concave tips and convex tips. The riblets were manufactured using masked stereolithography 3D printing and systematically tested across a broad range of Reynolds numbers. The results show that tip curvature shows little influence under laminar conditions (Re 2200). Convex tips consistently produced weaker drag reduction than flat tips, indicating that outward curvature may disrupt near-wall vortex organization. In contrast, concave tips enhanced drag reduction, yielding up to 30 percent drag reduction at Re ≈ 6000. However, the benefit diminished at both lower and higher Re, indicating strong sensitivity to flow–scale interactions. These findings demonstrate that riblet effectiveness is dependent on tip curvature and flow regime, and they provide new design principles for engineering advanced riblet surfaces that can reduce frictional losses and energy consumption in pipelines.

Keywords: Drag reduction; Riblets; Turbulent flow; Pipe flow; Shark-skin; Curved Riblet.

Date Published: December 1, 2025
DOI: 10.11159/jffhmt.2025.039

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