Abstract
It has long been suspected that the denticles on shark skin reduce hydrodynamic drag during locomotion, and a number of man-made materials have been produced that purport to use shark-skin-like surface roughness to reduce drag during swimming. But no studies to date have tested these claims of drag reduction under dynamic and controlled conditions in which the swimming speed and hydrodynamics of shark skin and skin-like materials can be quantitatively compared with those of controls lacking surface ornamentation or with surfaces in different orientations. We use a flapping foil robotic device that allows accurate determination of the self-propelled swimming (SPS) speed of both rigid and flexible membrane-like foils made of shark skin and two biomimetic models of shark skin to measure locomotor performance. We studied the SPS speed of real shark skin, a silicone riblet material with evenly spaced ridges and a Speedo® 'shark skin-like' swimsuit fabric attached to rigid flat-plate foils and when made into flexible membrane-like foils. We found no consistent increase in swimming speed with Speedo® fabric, a 7.2% increase with riblet material, whereas shark skin membranes (but not rigid shark skin plates) showed a mean 12.3% increase in swimming speed compared with the same skin foils after removing the denticles. Deformation of the shark skin membrane is thus crucial to the drag-reducing effect of surface denticles. Digital particle image velocimetry (DPIV) of the flow field surrounding moving shark skin foils shows that skin denticles promote enhanced leading-edge suction, which might have contributed to the observed increase in swimming speed. Shark skin denticles might thus enhance thrust, as well as reduce drag.
Highlights
In recent decades, the skin of sharks has achieved a certain biomimetic status among both science popularizers and in research circles for the notion that the specialized skin surface structure could reduce drag and enhance the efficiency of locomotion
Manufactured body suits have been loosely modeled on shark skin with various ridges and dents, to induce surface roughness, that purportedly enhance swimming performance in humans, and researchers have long suspected that the special surface structure of shark skin contributes to the efficiency of locomotion [shark skin structure has been comprehensively reviewed (e.g. Applegate, 1967; Lang et al, 2008; Reif, 1982; Reif, 1985); see images in Castro (Castro, 2011)]
We avoided chemical removal methods in order not to affect skin flexural stiffness. This method did not remove every denticle, and small nubs of the denticle bases were left on the surface, we found that more-aggressive denticle removal methods damaged the skin surface
Summary
The skin of sharks has achieved a certain biomimetic status among both science popularizers and in research circles for the notion that the specialized skin surface structure could reduce drag and enhance the efficiency of locomotion. A variety of ‘shark-inspired’ engineered materials have been produced to reduce drag when applied to the surface of submerged bodies. Experiments with an adjustable surface with longitudinal blade ribs and slits revealed the highest stiff-body drag reduction of 9.9%, with a groove depth of half the size of lateral riblet spacing (Bechert et al, 1997). Somewhat similar to the ridges in shark denticles, produce a maximal stiff-body drag reduction of approximately 7% (Bechert et al, 1985)
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