This experimental work will study a new and unique passive boundary-layer separation control methodology derived from shark skin, functioning at the micro-scale level. The skin and denticles (scales) of sharks represent over 400 million years of natural selection for swimming efficiency. Evolutionary adaptations in the morphological structure of the shark skin, to develop unique boundary layer control (BLC) mechanisms, stem from the ensuing decrease in drag, probable increase in fin performance (e.g. thrust production) and enhanced turning agility for fast-swimming sharks. Previous work, confirmed by the PIs, has shown the capability for shark denticles to bristle. The PI discovered that a bristled microgeometry results in the formation of a system of interlocking embedded cavity vortices. Three mechanisms are hypothesized which lead to boundary layer control via deterrence of separation over the shark skin. The first mechanism is the formation of embedded micro-vortices that increase momentum in the very near-wall region due to the partial slip condition resulting on the outer boundary layer flow. The second mechanism is that the preferential flow direction inherent in the surface geometry inhibits global flow reversal. The third mechanism, occurring during transitioning and turbulent boundary layer conditions, involves an exchange of flow with the cavities resulting in turbulence augmentation, or an additional energizing of flow in the near-wall region and cavities. The study involves engineers, working together with biologists, to fully comprehend the morphological bristling mechanism of shark denticles. This study will provide the first comprehensive characterization of the morphological mechanism resulting in denticle bristling and will classify the scope and degree (or angle) of bristling, yielding data for the building of shark skin models for hydrodynamic testing. The three passive BLC mechanisms will be evaluated through flow visualization and measurement using Time-Resolved Digital Particle Image Velocimetry (TR-DPIV). Innovations in the field of BLC are needed to provide efficient methodologies to decrease drag (resulting in increased payload, range or fuel savings), improve performance of control surfaces and enhance turning agility of modern technologies (e.g., submarines, aircraft). Dissemination of results will occur in journals/conference proceedings and the public media (e.g. Discovery Channel Canada). Undergraduate student involvement will take place through participation with two NSF REU programs (University of Alabama and Mote Marine Laboratory) with a focus on involving underrepresented groups; an REU supplement will also be sought to involve additional underrepresented undergraduates. Finally, the results from this research will be incorporated into educational outreach programs/exhibits at the Mote Marine Laboratory on sharks by the co-PIs and at the McWane Science Center in Birmingham, AL by the PI. Outreach through these two outlets alone should educate over 700,000 people each year about the drag-reducing properties of shark skin.
The shortfin mako shark Isurus oxyrinchus is perhaps the fastest swimming shark. This shark has a variety of adaptations to reduce drag and increase swimming speed. Rapid burst swimming is extremely important for catching prey and avoiding predation. Some of these modifications involve a very fusiform body with pointed head, narrow region just in front of the sickle-shaped caudal fin, powerful and warmed body muscles, and supposedly modified and flexible body scales. Shark scales (placoid scales) on active, faster swimming sharks are fusiform in shape and have minute elevated ridges (riblets) than lie in the longitudinal plane of the scale. The spacing and height of these ridges has been experimentally determined to reduce cross-flow over the scale and thereby reduce drag. It has also been speculated and to some degree tested, that flexible scales can erect during burst swimming reducing drag much as the dimples on a golf ball do. Our study was the first to quantify and map the regions of highly flexible scales on any shark. We quantified placoid scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and compared it to the slower swimming blacktip shark Carcharhinus limbatus. The mako shark has shorter scales with narrowly spaced riblets. Furthermore, the mako shark has a region of highly flexible scales along the flank on each side, with scales being able to easily erect to at least 50 degrees from their resting position. Such flexibility is not found on the blacktip shark. We further demonstrated that the flexible scales have longer upper crowns, yet smaller and narrow bases, allowing them to pivot about their bases. Similar to other scales on sharks, they are anchored to the upper regions of the dermis of the skin. We also document regions of highly flexible scales on the fins. Working with engineers on this collaborative grant has allowed them to mount sections of mako shark flank skin on aerodynamic wings and run these in water tunnels. Analysis of water flow over the mako skin surfaces indicates that the scales do bristle and erect, affecting water flow over the surface. Further research continues, holding promise for a man-made surface that could be applied to underwater vehicles thereby reducing drag and perhaps fuel costs. Our funded research provides the first step in our understanding of shark scale structure and function in fast swimming sharks.
