A new study published by the Journal of Experimental Biology has come to the conclusion that shark skin produces high levels of propulsion by generating thrust, but fabric based on the idea that fish skin removes drag and therefore increases speed may not actually be effective.
Fish are a popular means of inspiration for those engineering swimming aides, including fabric designed to reduce drag and increase a swimmer's speed. A fish's skin is coated in tooth-like scales, known as 'dermal denticles', which are thought to work as a means of disrupting the flow of water across the surface -- therefore reducing drag.
However, does shark skin act in this same manner when in motion? George Lauder from Harvard University and Masters student Johannes Oeffner joined forced in order to discover what specific elements of a shark's skin enable effortless swimming.
'All of the shark skin studies were done on flat shark skin mimics that were held straight and immovable. But shark skin moves', recalled Lauder.
The scientists first had to get hold of fresh shark skin, so they purchased several makos -- short-finned sharks -- from a market in Boston. Sections of the shark's skin were removed and attached to both sides of rigid aluminium foil. Immersing the foil in a flow tank and mimicking the swimming motion of a fish, the researchers measured the foil's speed by matching it with the flow of water moving in the opposite direction.
Once these measures were recorded -- complete with denticles -- the duo sanded away the scales and repeated the test.
However, instead of slowing down movement, as previous theories would have suggested once you remove the drag-reducing properties of shark skin, the results sped up.
Conclusively, that would infer surfaces containing denticles must impede a rigid swimmer. So what happens when the skin is flexed in the manner of a fish?
Building upon their original premise, the researchers glued pieces of skin together to produce a flexible foil, and repeated the test. This time, the intact skin foil swam 12.3 percent faster than the sanded skin.
However, when the duo tested the swimming performance of two shark skin mimics – a sharp-edged riblet design and a Speedo fabric, the former surface improved swimming speed by 7.2 percent, and the latter, dented surface of Speedo fabric had no beneficial improvements at all.
After proving that the rough skin on a shark improves propulsion and then returning the flexible skin foil to the tank, Oeffner and Lauder used laser light to capture the water's motion and realised that in addition to reducing drag, the skin was actively generating thrust.
"That's the number one surprise. It's not just the drag-reducing properties, but the denticles alter the structure of flow near the shark skin in a way that enhances thrust," explained Lauder.
After completing the research, Lauder is now keen to extend upon it by building physical models in order to examine further how altered denticle arrangements affect fluid flows over surfaces, and eventually build a computational model to analyze the resultant thrust and drag reduction.
Citation: Oeffner, J. and Lauder, G. V. (2012). The hydrodynamic function of shark skin and two biomimetic applications. J. Exp. Biol. 215, 785-795.
Image credit: Joi Ito