Bristled shark skin: a microgeometry for boundary layer control?

Abstract

There exists evidence that some fast-swimming shark species may have the ability to bristle their scales during fast swimming. Experimental work using a water tunnel facility has been performed to investigate the flow field over and within a bristled shark skin model submerged within a boundary layer to deduce the possible boundary layer control mechanisms being used by these fast-swimming sharks. Fluorescent dye flow visualization provides evidence of the formation of embedded cavity vortices within the scales. Digital particle image velocimetry (DPIV) data, used to evaluate the cavity vortex formation and boundary layer characteristics close to the surface, indicate increased momentum in the slip layer forming above the scales. This increase in flow velocity close to the shark's skin is indicative of boundary layer control mechanisms leading to separation control and possibly transition delay for the bristled shark skin microgeometry.

The hydrodynamic function of shark skin and two biomimetic applications

“…Lang et al, 2008). We compared the measured surface curvatures of our flexible shark skin foils with those we measured in live sharks swimming in our laboratory flow tank, and the values of 0.17-0.25cm -1 for foils accord well with measured maximal mid-body values from live spiny dogfish swimming at 1.0BLs -1 (0.14-0.20cm…”

“…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); also see images in Castro (Castro, 2011)].…”

Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review

“…Three-dimensional riblets, which include segmented two-dimensional riblets as well as shark-skin mouldings and replicas have also been studied. Riblet types characterized include aligned segmented-blade riblets (Wilkinson & Lazos 1987), offset segmented-blade riblets (Bechert et al 2000a), offset-threedimensional blade riblets (Bechert et al 2000a) and three-dimensional shark-skin replicas (Bechert et al 2000b;Lang et al 2008;Jung & Bhushan 2010). Most studies are done by changing the non-dimensionalized spacing, s + , by varying only fluid velocity and collecting shear-stress data from a shearstress balance in a wind tunnel or fluid-flow channel.…”

“…In an experiment simulating the bristling of flexible riblet covered scales into the boundary layer as a possible mechanism of control and drag reduction, no drag-reduction benefit was achieved through extreme scale bristling. However, notable flow phenomena were found to occur as a result of scale bristling, including the formation of three distinct vortex shapes (Lang et al 2008). …”

Biofouling: lessons from nature

“…Banerjee et al [40] cayenne pepper deterrent pepper with silicone grease Manov et al [130] shark skin replicas [135][136][137]. A riblet optimization review paper suggests that a blade riblet height divided by the spacing equalling 0.5 is optimal for drag reduction, regardless of riblet length [34].…”