Shark Skin Surface for Fluid Drag Reduction in Turbulent Flow
Nature has created ways of reducing drag in fluid flow, evident in the efficient movement of fish, dolphins, and sharks. The mucus secreted by fish causes a reduction in drag as they move through water, protects the fish from abrasion by making the fish slide across objects rather than scrape, and prevents disease by making the surface of the fish difficult for microscopic organisms to adhere to (Shephard, 1994) [Accumulation of unwanted biological matter on surfaces with biofilms created by microorganisms is referred to as biofouling (Bixler and Bhushan, 2012) It has been known for many years that by adding as little as a few hundred parts per million guar, a naturally occurring polymer, friction in pipe flow, can be reduced by up to two thirds. Other synthetic polymers provide an even larger benefit (Hoyt, 1975) The compliant skin of the dolphin has also been studied for drag reducing properties. By responding to the pressure fluctuations across the surface, a compliant material on the surface of an object in a fluid flow has been shown to be beneficial. Though early studies showed dramatic drag reduction benefits, later studies have only been able to confirm 7% drag reduction (Choi et al. 1997)