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.
This study examined the role that shark scales can have on controlling flow so the shark can swim faster or have increased maneuverability. Sharks date back about 400 million years and have evolved a skin made up of tiny teeth all over their body. The shortfin mako, the speediest of sharks, has scales only 0.2 mm in size (see image). Biologists working on this project found that these scales are flexible to the touch, and on the gray shaded areas shown in the image can bristle easily to 50 degrees. You could think of them like loose teeth just sitting in their skin. We think these scales will be actuated by the water flow around the shark’s body to keep the flow from separating; this is similar to why a golf ball has dimples. Controlling the flow to keep it from separating reduces drag. Next, we tested real shark skin samples from a shortfin mako in our water tunnel facility at The University of Alabama. The first test took a shark pectoral fin and orientated it at various angles to the flow, as the shark would experience during swimming. The image shows that we calculated backflow, or the percentage of time the flow was reversed during a time interval in which we measured the flow velocity, with the fin orientated at an angle of 18 degrees. We also painted over the fin to remove the effect of the scales to control the flow. For this particular case it was found that the scales controlled the flow separation as there was almost zero backflow over the top of the fin in the vicinity of the trailing edge. However, with the scales painted over the flow was completely separated. For the shark the advantage of the flexible scales on its fin is to use the fin as a means of controlling its body motion during maneuvers, and flow separation would cause the shark to lose that control. Two other experiments tested skin pieces from the flank region of the shark, and in both cases we also saw evidence that the scales worked to control flow separation. Our hypothesis as to how the scales work to control the flow consists of two mechanisms. First, when the flow begins to separate the fluid right next to the skin of the shark begins to reverse (go in the direction opposite to the arrow shown in the image). This is due to the fact that on the back half of the body, after the point of maximum girth, the pressure over the body is increasing in the direction of the flow. But like sucking on a straw, fluid always wants to move towards a region of lower pressure. The fluid closest to the skin is most susceptible to this suction pressure upstream and begins to reverse. This reversing flow causes the scales to bristle, and this bristling then inhibits the flow from reversing any further. Secondly, when the scales are bristled suddenly cavities form between the scales (see image). These cavities trap fluid and cause a mixing to occur that brings higher speed flow closer to the skin. This also helps to keep the flow attached. For a swimming shark this process may be taking place at very small timescales and in various regions in the gray-shaded areas on its body, including its tail where it generates thrust. But it is not something the shark is actively doing to control the flow, thus we refer to this as a passive flow control mechanism. Our ultimate goal as engineers is to fully understand the mechanism of these flexible scales so that it can inspire man-made surfaces that work in the same way to control flow separation. For instance, one application has to do with helicopters. When a helicopter flies forward too fast the flow over the top of the rotor blades begins to separate which means it loses the lift force needed to fly. If engineers could design a shark skin surface to apply to rotor blades it could allow helicopters to fly faster. Other applications include aircraft control surfaces, turbine blades, submarines and even internal ducted flows where flow separation is detrimental to the performance of manmade vehicles or machines.
