Slashdot Mirror
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."
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.
They actually increase it! What they do is exploit the Bernoulli effect and create lift so the ball goes further away before coming down again.
While I don't question the conclusions of the researchers, I do doubt that comparing sharks to golf balls is a good analogy. I haven't read TFA so I don't know if the golf ball analogy is by the submitter or the scientists but at any rate, it seems wrong.
Not the same thing. Sharkskin was indeed known for a while to present an extremely slippery surface. What these researchers found is an additional effect: that the sharkskin can raise the individual "teeth" so that a turbulent layer gets created. This turbulent water layer prevents flow dissociation, which in turn reduces drag.
There was a 2-man boat at the last Olympics that was using dimples in its coating rather than the fairly standard sharkskin approach - it didn't win, but it was noted due to its novely.
Those who can, do. Those who can't, sue.
No, the principal is still in your school. He's asking you to take your grammar and spelling more seriously.
Not at all. Supercavitation puts a small high frequency oscillator at the front of the submersible. While cavitation creates small air bubbles which form and collapse, supercavitation produces an entire cavity of air in the water which the submersible now flies through with reduced drag. It's reduction of drag through reduction of medium density.
This method energizes the flow, and induces a premature shift from laminar to turbulent flow. When a laminar flow encounters an adverse pressure gradient (large cross section to small cross section), it detaches from the surface creating large drag inducing vorticies. Think of the suction behind a semi, or other flat backed truck. By inducing turbulence in the flow, it has sufficient kinetic energy to remain attached to the surface, preventing drag. So called 'vortex generators' have been used for decades on aircraft to improve airflow over wings, allowing higher lift and lower drag.
I initially dismissed this, thinking the flow should have become turbulent, but at those speeds, water becomes turbulent after about 5 meters. However that means it would be of marginal benefit for anything but the control surfaces on a submarine.
In fact a sharkskin like surface was added to Stars and Stripes racing yacht. Stars and Stripes scored a 4-0 sweep in America Cup in 1987.
The technology provided such a tremendous advantage that it was banned in subsequent years of America Cup
http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html