Biomimetic spanwise riblet coupled with hydrogel coating for drag reduction

Reducing surface frictional resistance (SFR) on high-speed objects remains a central challenge in engineering research. In this study, a coupled biomimetic surface was fabricated by combining 3D-printed spanwise riblets with a hydrogel coating applied through spraying. Laser scanning confocal microscopy (LSCM) was used to characterize the three-dimensional morphology, while field emission scanning electron microscopy (FESEM) revealed the hydrogel microstructure. Drag reduction effect was assessed in a closed water circulation tunnel and the actual drag reduction performance was measured by model ship in a flume. The coupled surface exhibited a maximum drag reduction rate of 13.2 % at Re=6325. The drag reduction mechanism was investigated using Computational Fluid Dynamics (CFD) and high-speed imaging. Streamwise vortices formed at the riblet valleys transformed sliding friction into rolling friction, significantly reducing resistance. Concurrently, the swelling hydrogel exchanged water within its hydration layer and absorbed near-wall turbulent fluctuations, promoting enhanced boundary slip. Given that hydrogel swelling alters the surface morphology, the impact of coating thickness on drag reduction durability warrants further investigation. The findings contribute to the development of multifunctional surfaces with potential applications in drag reduction and antifouling technologies.