How Do Shark Reduce Drag

How do sharks reduce drag? And what are the engineering applications?
There are three types of drag present on the shark during locomotion. The first type of drag is known as frictional drag, which is the greatest element of drags in the shark. This is arises due to the friction created between the skin and the boundary layer and can be reduced with a condition that the boundary layer maintains a turbulent flow. The second type of drag is pressure drag, which causes by water deflecting off the moving body of a shark and can be minimise if the boundary layer remains stable and in contact with the body along its entire length. The last type of drag is induced drag. This drag is results from the turbulence of the vortices formed along and behind the posterior edges of fins, causing a wake. The wake is formed from the pressure difference between the pressure drag and frictional drag as the boundary layer separates from the body of the shark and interacts with the outer water layer. [1]
Despite the fact that drags can resist a motion of a moving object, shark is still able to swim at a high speed. The reason is because dermal denticles on the shark skin are ribbed with longitudinal grooves which result in water moving more efficiently over their surface. Over the smooth surfaces, fast-moving water begins to break up into eddies, in part because the water flowing at the surface of an object moves slower than water flowing further away from the object. The difference in water speed causes the faster water to get “tripped up” by the adjacent layer of slower water flowing around an object. To reduce the formation of eddy, the grooves in a shark’s scale play an important role. The grooves will reinforce the direction of water flow by channelling it, so they speed up the slower water at the shark’s surface, which lead to the reduction in the difference in speed of surface flow and water just beyond the shark’s surface. On the other hand, the grooves pull the fast moving water towards the shark’s surface so to mix with the slower water, reducing speed differential. By divide up the sheet of water flowing over the shark’s surface, this will results in smaller vortices. [2] The denticls not only just a drag-reducing property, but also alter the structure of flow near the shark skin in a way that enhance thrust. [5]
One of the engineering applications of dermal tenticles of shark skin is on aircraft. The first riblets were machined on flat aluminium sheets and tested in a Langley wind tunnel. When engineers of the 3M Company, St. Paul, Minnesota, learned of the tests, they suggested moulding the riblets into a lightweight plastic film with an adhesive backing. The film could be pressed into place on an airplane, eliminating the need for welding and allowing a relatively inexpensive retrofitting to existing airplanes. Langley accepted 3M 's offer to produce riblet tapes for research and used them in 1986 tests on a Learjet. In flight tests, the film riblets demonstrated a drag reduction capability of about 8% and could represent a fuel saving of 1.5%, similar to the results of wind tunnel tests using the metal sheets. This principle was used on yacht by coating the riblets skin in the hull’s underside that helps the yacht to slide through the sea more smoothly. Riblets also could be used in oil, gas and water transmission lines by reducing the friction inside the pipes, as well as on submarines and jet engine turbine blades, which decrease fuel consumption. [3]
Besides that, the same principle is applied in designing swimsuit. The “shark skin suit” is made of a knitted super-stretch nylon that has V-shaped ridges and a denticle surface print. The fabric makes the water to passes over the swimmer far more effectively and also compresses the body to stop skin vibration and muscle fatigue to save energy and reduce drag up to 4%. The development of the “shark skin suit” involved a team of top experts. CyberFX was involved in the development process from the beginning. Its unique body scanning technology allowed Speedo to create ‘virtual’ and ‘actual’ models of the average male and female swimmer. Digital images of the athletes were taken from eight positions allowing CyberFX to create a 3-D body map. A biomechanist was used to study the co-ordination of muscle groups specific to swimming. The CyberFX ‘virtual’ models were used to create sophisticated Computational Fluid Dynamics (CFD) models to analyse the drag and flow of water around swimmers. The CFD analyses, combined with the flume testing, enabled Speedo to evaluate a huge number of swimsuit design variables and eventually obtain a highly efficient design. The ‘actual’ models were used in over 1,000 flume tests carried out at the most accurate water flume on the planet in Otago, New Zealand. The results supported CFD findings that the “shark skin suit” is the most advanced performance swimwear in the world. [4]
As a conclusion, shark skin is proven to be able to reduce drag and enhance thrust in fluid. It also aids in reduction of fuel consumption and improve the performance of the fluid flow. It has continued to inspire engineers to continue develops technologies for a better future. Therefore, we shall appreciate nature and continue to be inspired so better technologies can be develop.
References
World Wide Web page

[1] Ambrose, T. (2000) Ambrose’s Shark Skin Web Page, [Online], Available: http://www.bio.davidson.edu/Courses/anphys/2000/Tuscano/Tuscano.htm [27th November 2012]

[2] Ask Nature, A Project of The Biomimicry 3.8 Institute (2011), Skin Reduces Drag: Shark [Online], Available: http://www.asknature.org/strategy/038caf2e453c09b3016465cc6ca93605 [27th November 2012]
[3] NASA (1993), NASA Riblets for Stars & Stripes [Online], Available: http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html [27th November 2012]

[4] Science in the News, Swim like a Shark [Online], Available: http://www.scienceinthenews.org.uk/contents/?article=8 [27th November 2012]

Direct quote from Journal Article
[5] “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.” (Oeffner, J. and Lauder, G. V. (2012), The Hydrodynamic Function of Shark Skin and Two Biomimetic Applications. J. Exp. Biol. 215, 785-795.)

References: http://www.bio.davidson.edu/Courses/anphys/2000/Tuscano/Tuscano.htm [27th November 2012] [2] Ask Nature, A Project of The Biomimicry 3.8 Institute (2011), Skin Reduces Drag: Shark [Online], Available: http://www.asknature.org/strategy/038caf2e453c09b3016465cc6ca93605 [27th November 2012] [3] NASA (1993), NASA Riblets for Stars & Stripes [Online], Available: