Biomimetic Riblets Inspired by Sharkskin Denticles: Digitizing, Modeling and Flow Simulation

While sharkskin surface roughness in terms of denticle morphology has been hypothesized but remains yet controversial to be capable of achieving turbulent flow control and drag reduction, sharkskin-inspired “riblets” have been reported to be an effective biomimetic design. Here we address an integrated study of biomimetic riblets inspired by sharkskin denticles by combining 3D digitizing and modeling of “fresh” denticles and computational fluid dynamic modeling of turbulent flows on a rough surface with staggered denticles and hound-tooth-patterned grooves. Realistic microstructures of denticles in five shark species of Galapagos, great white, whitetip reef, blacktip reef, and hammerhead sharks were first measured and digitized in three fold: (1) 2D imaging of lubricated sharkskin in a wet state by means of a “nano-suit” technique with a Field-Emission Scanning Electron Microscope (FE-SEM); (2) 3D structures of sharkskin denticles with a micro-focus X-ray CT; and (3) single denticles of the five shark species in a 3D manner with 3D-CAD. The denticles at mid-body location in the five species were observed to have a structure of five non-uniform-ridges (herein termed “non-uniform grooves”) with Angles Of Inclination (AOI) ranging over 20°–32°. Hydrodynamics associated with the unique five-ridge denticles were then investigated through modeling turbulent flow past a denticle-staggered skin surface. We further constructed a biomimetic riblet model inspired by the non-uniform grooves and investigated the hydrodynamic effects of height-to-spacing ratios of mid-ridge and side-ridges. Our results indicate that the morphological non-uniformity in sharkskin denticles likely plays a critical role in passively controlling local turbulent flow and point to the potential of denticle-inspired biomimetic riblets for turbulent-flow control in aquatic vehicles as well as other fluid machinery.