Separation control over a grooved surface inspired by dolphin skin

Abstract

Over many decades the biological surfaces of aquatic swimmers have been studied for their potential as drag reducing surfaces. The hydrodynamic benefit of riblets, or grooves embedded parallel to the flow which appear on surfaces such as shark skin, have been well documented. However the skin of dolphins is embedded with sinusoidal grooves that run perpendicular or transverse to the flow over their bodies. It is theorized that the transverse grooves present on dolphin skin trap vortices between them, creating a partial slip condition over the surface and inducing turbulence augmentation in the boundary layer, thus acting as a potential mechanism to reduce flow separation and thus pressure drag. In an attempt to test this hypothesis and study these effects, an adverse pressure gradient was induced above a flat plate resulting in a controlled region of flow separation occurring within a tripped, turbulent boundary layer. Small transverse grooves of both rectangular and sinusoidal shape were 3D printed and mounted to the plate to measure their effect on the boundary layer flow. The results were compared to a flat plate without grooves using digital particle image velocimetry (DPIV). The strength of the adverse pressure gradient was varied, and the observed control in flow separation and other effects upon the boundary layer are discussed.

Experimental investigations on drag-reduction characteristics of bionic surface with water-trapping microstructures of fish scales

“…In the last decades, some researchers have made great efforts to address issues concerning the innovation in technology to be inspired by organisms in nature 3 – 7 . After millions of years of continuous adaption and evolution, bodies of creatures in nature have formed delicate biological surface structures with drag-reducing characteristics 8 – 11 . For instance, convex hull structures of the head of the dung beetle 12 – 14 , the herringbone riblet textures inspired by the bird feathers 15 , 16 , ridged structures distributed on the mouthpart of the honeybee 17 , 18 , and the super-hydrophobic surface of the skins of many creatures 19 – 22 .…”

“…The bionic jet surface, inspired by the jet characteristics of shark-gill slits, was established in order to produce a great drag-reduction performance through the method of changing the fluid friction resistance on the surface 29 . And then, it is well-known that the surfaces of dolphins exhibited self-adaptive characteristics, which could reduce the velocity gradient of the boundary layer, making the wall shear force decrease, to realize great drag-reduction effect 11 , 30 – 32 . Also, the flippers of humpback whales had some spherical non-smooth structures, which could reduce the flow resistance of its movement 33 – 35 .…”

Hierarchical Surface Patterns via Global Wrinkling on Curved Substrate for Fluid Drag Control

“…The previous investigation on the dolphin skin, which is coved by transverse sinusoidal shallow grooves, suggests the sinusoidal grooves probably could delay flow separation resulting in decreased drag pressure (same principle as golf ball). [ 50 ] However, this is not a possible reason for the observed drag reduction in our case, since no transition to turbulence was involved in this study.…”

Dermal Denticles of Three Slowly Swimming Shark Species: Microscopy and Flow Visualization

“…The putative drag-reducing effect of some of these engineered structures has, however, been shown to be limited [7] and for some conditions even drag increasing [7]. In recent studies, surfaces that were replicas of the natural structures were investigated [8,9], and discussions of the design include the importance of the flexibility of the substrate in which the dermal denticles reside [8,10] and the ability of the dermal denticles to bristle [11,12,13,14]. Also, the spacing and modifications of the patterning of the denticle-like structures are a topic of investigations related to the drag-reducing properties of the structured surface [10,15,16].…”