Why a shark is like a golf ball

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New research has revealed startling similarities between a shark and a golf ball, uncovering the secrets behind the sharks' capability to make quick turns at high speeds, which makes them such lethal hunters.

In research presented last week at the American Physical Society's Division of Fluid Dynamics meeting in Long Beach, California, aerospace engineer Amy Lang of the University of Alabama showed that it is their scales that enable sharks to make rapid changes in direction without the need to slow down.

Lang collaborated with a team of biologists from the University of South Florida to study the Shortfin mako shark (Isurus oxirhynchus), one of the fastest shark species swimming around. 

The team of researchers combined computer models with observations of the shark scales in the laboratory, discovering that the Shortfin mako’s scales differed in size and flexibility over its body. 

Some scales are wider at the top than at their bases (where they attach to the skin).  This tapered design enables them to be easily manipulated to angles of 60 degrees or more, endowing them with movement called 'denticle bristling'  This particular feature reduces what physicists call '‘flow separation', where the fluid flow at the edge of the boundary layer (this is the layer of fluid closest to the surface of an object moving through fluid) breaks up and forms eddies and vortices that create drag, slowing the object down.

Lang likens this mechanism to the dimples on a golf ball. "Imagine a stream of flow going over the ball," she said. "You get a low-velocity wake behind the body, but the dimples help to decrease the size of the wake—that's what we think the scales are helping to do with the shark."

These special scales, known as mako scales, are not found all over the body, but only in areas where flow separation is likely to be the most significant (such as behind the gills).
This innovation can have practical applications in the field of aerodynamics. 

"As we investigate further, we imagine applications of controlling flow separation in design of aircraft, helicopters, wind turbines — anywhere flow separation is an issue," added Lang.