Reexamining the hydrodynamics of sharkskin

Los Angeles Times : Covering surfaces with a sharkskin-like material called riblets has long been thought to reduce drag through a surrounding fluid. Until recently, however, it’s been hard to evaluate the actual benefits of such a structured material. Now researchers have used data on shortfin mako shark denticles to develop a computer model that compares the fluid flow over those natural structures with the flow over the manmade riblets. Whereas the riblets were found to reduce drag by 5.2%, the denticles actually increased drag. Moreover, when rows of denticles were aligned, they increased drag by 44%; when staggered, they increased drag by 50%. The researchers attribute the difference to the fact that riblets are two-dimensional structures, while denticles are three dimensional. The denticles’ thickness causes the flows to become more turbulent and complicated. The researchers conclude that further study will be needed to determine whether denticles can indeed reduce drag or whether they work better on the body of a living, moving creature than attached to an inanimate object.