China's Harbin Institute of Technology's Complex Flow and Heat Transfer Laboratory has made a breakthrough in supercavitation technology that could lead to significant advances in everything from maritime shipping to naval warfare. It has also led to utterly insane claims that go completely overboard.
Before we get to supercavitation, let's discuss plain old cavitation, which is a fluid dynamics phenomenon that occurs when something moves through a liquid so fast that it ends up creating an area of pressure low enough that the liquid actually boils and turns into a small pocket of vapor — in other words, a bubble. A couple things occur as a result: One, the bubble collapses violently, creating a shockwave and a powerful microjet of water. Two, bubbles on the surface of the fast-moving object serve to reduce drag.
Cavitation is either bad news, very bad news, or totally awesome news — it all depends on what you're up to. First, the bad news. When moving parts — like a propeller — cause cavitation, the microjets caused by the collapsing bubbles can do a fair amount of damage, even to metal surfaces, tearing up equipment something fierce. (This is also the phenomenon in play when something is cleaned "ultrasonically." But in ultrasonic cleaning, the idea is that the microjets are powerful enough to knock off the munge, but not so powerful that they damage the thing being cleaned.)
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The very bad news is that the shockwave from a collapsing bubble is noisy. While that's not a concern for some, it's a huge concern for people who spend all of their time trying to be sneaky by hiding in vast quantities of saltwater — sailors on military submarines, for instance. Noisiness is considered worse than physical damage and gets rated as very bad news here because I write about defense issues, not industrial processes. (That said, sub skippers certainly don't want their propellers damaged.)
The theoretically totally awesome news is that if you move something through the water fast enough, the bubble created by cavitation gets enormously large, and it really starts to cut down on the drag experienced by the vessel. Do it fast enough — you can also emit a little bit of gas from the nose of a fast-moving object to create or enlarge a bubble — and the bubble envelops and becomes larger than the object moving through the water. Voilà — supercavitation!
This means the object underwater has no contact with any liquid except for a tiny little area at the front where the bubble is being created. It also means (almost) no hydrodynamic drag is produced, which is basically a license to go mind-blowingly fast.
Imagine racing along in a speedboat and sticking your hand over the edge into the water. That's pretty much how you'd be trying to steer while in a supercavitating bubble.
What the fine folks at Harbin have figured out how to do is reduce the speed at which an object can start supercavitating. By spooing out liquid from the front of a vessel — say, a submarine — they can theoretically get supercavitation to occur at speeds as low as 45 mph.
This revelation has inspired people to daydream about sailing submarines around the world at a zillion miles per hour as supercavitating angels descend from heaven to sing the praises of supercavitation and a new utopian era is ushered in.
Sadly, all of this — supersonic submarines and supercavitating angels — is, from a technical engineering point of view, complete bullshit.
Some articles have expressed excitement about the prospect that a vessel using this phenomenon could cross the Pacific Ocean in a mere 100 minutes. That figure is derived from the fact that the fastest a supercavitating bubble can travel underwater is the speed of sound underwater — about 3,600 mph. But the difference between the speed at which physical laws of nature mandate your destruction and the ability to go anywhere near that fast is rather vast. Would it be possible to ride a submarine across the Pacific in 100 minutes? Sure, in much the same way it would be possible to ride an aircraft traveling at 90 percent the speed of light across the Pacific in a few hundredths of a second. But don't start booking tickets for either quite yet.
Beyond that, there's the matter of steering. Imagine racing along in a speedboat and sticking your hand over the edge into the water. That's pretty much how you'd be trying to steer while in a supercavitating bubble. For a fin or rudder to change the direction the vessel is moving in the water, the steering implement would need to interact with the water. But if you're moving through the water at hundreds — or thousands! — of miles an hour, sticking a fin into the water would tend to get the fin ripped off.
Alternatively, you could fiddle with the way that you're creating the bubble at the front of the vehicle to shift it and change its shape to steer. But even if you could steer successfully, turning too fast could result in the hull touching the side of the bubble. And so instead of just ripping off a fin, you'd rip the whole vessel in two.
There are some smart folks who have been thinking very hard about all this, and who claim to have some neat tricks emerging on this front, but steering and supercavitation are natural enemies in the wild. Currently, applications involving supercavitation mostly focus on figuring out how to not turn.
Experimenting with a combination of steering using fins and a change in the angle of the deflector plate
Which is fine for some applications. The Russians have been doing some leading-edge research to make hella fast torpedoes, because if a torpedo is screaming toward its target at hundreds of miles per hour, there's really no evasive maneuver in the world that will help the target ship. The Russian Shkvall torpedo plows through the water at a brisk 230 mph.
Sounds amazing. But remember: Supercavitation is astonishingly loud. That's why the Shkvall is basically a revenge weapon. If you're not about to die before you launch one, you'll die shortly thereafter because everyone else within 1,000 miles will know you're there the minute you launch the weapon. (In addition, the Shkvall has other practical limitations when it comes to range and guidance.) So a supercavitating torpedo is all about going out in a blaze of glory — firing it when you know you're already dead.
There's one thing that would be even louder than a supercavitating torpedo, and that is a supercavitating submarine. In fact, it might as well tweet its position and heading, and send out e-vites to enemy torpedoes. That's why submariners, who try to be the ninjas of the sea, generally believe that a supercavitating submarine is totally insane.
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There's a final problem with zipping around at a few hundred miles per hour in a supercavitating sub: You can't tell what's in front of you. Sonar is more or less out of the question in part because of all the noise you're already making. The Shkvall torpedo can be steered because it gets guidance signals through a wire connected to the submarine that fired it, but unspooling a 6,000-mile-long wire while traveling hundreds — or thousands! — of miles per hour across an entire ocean is… well, let's just say it's not currently feasible. So a supersonic submarine crossing an entire ocean at thousands of miles per hour is basically just a gigantic underwater harpoon looking for a whale or ship or Great Pacific garbage patch to ram.
Is supercavitation cool? Absolutely. Is it useful? Sort of. It's not ready for primetime, though it might become so if enough people science and engineer at it. But right now, all the clever folks at Harbin have is a strange phenomenon with which to work. The real engineering work hasn't even gotten fully underway. So let's hold off on breaking out the bubbly.
Follow Ryan Faith on Twitter: @Operation_Ryan
Photo via Cavitation and Bubbly Flows Research Group, St. Anthony Falls Laboratory, University of Minnesota