Scientists Discover Why Sharks Can Swim So Fast

MediaSight writes "Shortfin mako sharks can shoot through the ocean at up to 50 miles per hour (80 kilometres an hour). Now a trick that helps them to reach such speeds has been discovered — the sharks can raise their scales to create tiny wells across the surface of their skin, reducing drag like the dimples on a golf ball."

Comments

Nitpicking

[Anonymous Coward]

The article mistates several things

First, the turbulent layer formed by the raised scales does not act as a buffer and will actually cause more surface drag on the shark than a smoother layer (if the scales were flat, for example).

Second, the scales do not prevent a turbulent wake, they create it.

The way this reduces drag is reasonably straightforward and has to do with the boundary layer.

In an idealized model (no friction) you would calculate that any object has zero drag, or net force from the air acting on it. You would integrate the force of the air pressure acting on all sides of the object and get zero. If you are looking at a circular cross section object, you have high pressure at the leading point, very low pressures at the top and bottom, and high pressure again at the trailing point, for a net of zero drag.

However, what happens (aside from the usually small effect of friction) is that the boundary layer "seperates" from the object, so (back to our circle) you have high pressure in front, low pressure on the sides, and then the boundary layer seperates from the object and you wind up with low pressure in the back, too. So, high pressure in front, low pressure everywhere else, you have drag.

The way that golf balls (and sharks, apparently) attack this problen is to screw with the boundary layer flow. They "trip" the flow (using dimples or raised scales) into a turbulent boundary layer. This boundary layer creates more friction drag than a viscous (smooth) boundary layer, but because the particles in the boundary layer are moving every which way (it's a higher energy boundary layer) it will remain attached to more varying geometries than a viscous boundary layer will, so it won't seperate (or at least it won't seperate as early) from a shape like a golf ball or a shark, so you've reduced pressure drag by increasing viscous drag.

This usually works out in your favor, viscous drag is usually nearly negligible next to pressure drag.

I think the whole thing is very cool.

Re:WTF?? - Ah!

[tick-tock-atona]

There's still a problem with this - The Kolmogorov scale is all about the smallest scales at which turbulence can occur in a fluid. It is effectively a fundamental constant in a fluid (can fluctuate in time/space, but usually treated as a field constant). Now, according to this source, the K-scale in the ocean is in the order of 1 mm. This means that while vortices may form easily behind 2cm high "scales", they probably do not form so easily behind real shark scales which are an order of magnitude or two below 2cm in length. I believe this is what TFA meant in this part at the end:

Sergei Chernyshenko, an aeronautical engineer from Imperial College London, UK, describes the research as fascinating. However, he points out that while the team have shown the existence of vortices, they haven't yet quantified the extent of the effect on the shark's drag, which he thinks could be minimal.