Scientists Discover Why Sharks Can Swim So Fast 103
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."
This has been known for many years (Score:5, Interesting)
Back in the 80s we switched from polishing the bottom of our race boat to a glass like finish to spraying it with a gel mixture (as in gel coat, not jello) full of small oblong granules. We found that by spraying it a certain way we could get the particles to more or less line up in the orientation we needed. Careful polishing after the fact gave us the finish we were looking for without destroying this new, textured surface. We did this directly in response to an article I had read about how a sharks skin allows it to move quickly through the water. The article went further to say that this also applied to most all scaled fish.
This modification allowed the boat to break the surface tension of the water more easily when launching from a standing start and added several miles an hour to our top end speed. In a game where every mile an hour might cost 1000s or 10s of thousands of dollars this was *the* most effective modification we had ever done to the boat and one that to this day we joke about because it took our competition many years to figure out.
What about Kolmogorov? (Score:2, Interesting)
This is at least a full order of magnitude larger than the scales on a shark's skin.
According to this source [google.com.au], the kolmogorov scale [wikipedia.org] in the ocean is in the order of 1mm. Therefore, is the effect described in TFA going to actually be present for shark's skin? It seems to me that the effect will be minimal, if it is present at all..
WTF?? - Ah! (Score:5, Interesting)
I was dubious about this science when I read the article, but I learned something in the end.
From the article:
Shark scales are tiny - the crown is barely visible to the naked eye. So these scientists have scaled them up (so to speak) at least 2 orders of magnitude. With fluid dynamics the scale of a model can change everything, especially in the range of sizes they are working with here. I thought they should have substituted a more viscous fluid for the water in order to get a useful model. I thought maybe this was just preliminary work and they'd do a better study if their results suggested that it could be valuable."
But before flaming the Slashdot editors for trumpeting this study as a "discovery", I did a little Googling and quickly wound up at Wikepedia learning about Reynolds numbers. Turns out you can model turbulence pretty accurately as long as the Reynolds number stays the same. In this case the Reynolds number is proportional to both the size of the shark scales and the velocity of the water flow, so it can be preserved while the scales are made larger if the velocity is reduced proportionally.
Which is exactly what they did. They're studying sharks swimming at 80 km/hr.
80km/hr = 8,000,000 / 3600 cm/sec = 2200 cm/sec
Or, about 100 times faster than the flow rate they used in their model. Neat.
Re:No walking on the submarines of tomorrow! (Score:3, Interesting)
Wouldn't the new shape be more likely to create noise? Subs want no noise at all. If the shape could change on the fly, smooth and slower to sneak up while rough and faster to get away.
Re:This has been known for many years (Score:2, Interesting)
http://www.californiaskicompany.com/Ski_tune.html [californiaskicompany.com] (step 7)
Step Seven- New Structure on the base Much like a car tire, your ski has a tread pattern we call the structure. What type of structure works best depends on what kind of snow you typically ski in. Utah shops will run a different pattern on their skis than we do here in California. In any case this is the part that puts the pattern into the base. A very big stone is actually used for this process. This is the stone that makes us call the machine a stone grinder. We use a diamond to score a pattern into the stone. The stone is then used to cut this pattern into the ski. In addition, the stone is very precise and ensures that the ski base is perfectly flat and even.
Re:No walking on the submarines of tomorrow! (Score:3, Interesting)
Re:Dimpling on race cars? (Score:2, Interesting)
Well, if you're talking about devices that cause turbulence for the sake of boundary-layer adhesion, vortex generators have been in use on aircraft for years. More recently, they have been adapted to automotive use. Take a look at the trailing edge of the Lancer Evolution IX's roof... It has a line of 8-9 (if you count the antenna) vortex generators.
Re:This has been known for many years (Score:1, Interesting)
The placebo effect is powerful, yes?
In all honesty, without knowing the flow conditions existing on the hull surface, there is just as good a chance that you increased drag (increased skin friction with a fully attached flow) as decreased it (tripped the laminar boundary layer into turbulence and delayed flow separation). Without a carefully controlled experiment, you probably just attributed speed increases from outside sources to your new, expensive hull finish.
You probably would've done better with keel winglets, but again, that's something that must be designed with knowledge of the existing flow conditions.
Re:No walking on the submarines of tomorrow! (Score:5, Interesting)
The story is so oversimplified that raising questions from it is just pointless.
The facts are as follow:
1. Roughness tends to increase drag because makes boundary layers turbulent.
2. Turbulent boundary layers do stand higher adverse pressure gradients prior to separation
3. Separation increases drag much more than turbulent boundary layers.
Then, there are some applications where you would have a separated flow, and promoting turbulence through roughness would reduce the drag. This is not the case of aviation. It is not the case for sure of sharks when they are not moving their tails. It may be the case of sharks when they are moving their tails to obtain propulsion.