Port Angeles, WA —When does a bad thing become a good thing?
When water flows quickly around an object, its pressure drops. If the pressure drops enough, cavitation can cause erosion in metal propellers. But it becomes a good thing when, instead of creating a quickly collapsing vapor bubble, cavitation causes a lasting bubble that keeps a body dry as it travels through the water. It even gets a different name: supercavitation.
The U.S. Navy and several defense subcontractors, including C Tech Defense Corp., are developing very fast underwater weapons, like those rumored to have been tested by the Russian submarine Kursk at the time it sank. Instead of moving at the maximum torpedo speed of 80 mph, they would be capable of traveling faster than the speed of sound. These weapons already exist at the test stage. So does a controversy about the very nature of supercavitation.
"Two questions have to be answered," says Rod Daebelliehn, president of C Tech Defense Corp. "Is there any fundamental speed limit under water? And how is a cavity formed?"
The cavity forms around the nose of a projectile pushing through water. "If the projectile moves fast enough, the water flowing around the nose turns 90°, and creates a cavity. That is, it forms a cone-shaped bubble in front of the projectile, attached to the nose. As the projectile gains speed, the bubble elongates and, if it goes fast enough, the bubble gets longer and the water never touches the projectile at all, except for the nose tip," Daebelliehn says.
Controversy surrounds what happens when the water jets around 90°. Daebelliehn believes the pressure forms a true vacuum that causes the water to boil and fill the cavity with water vapor. Other researchers believe that an initial pressure drop makes the water boil first, forming water vapor, which in turn creates a vacuum. "If the vapor comes first, you can only go so fast before the speed makes the cavity collapse," Daebelliehn says. "If a vacuum causes vapor, there's no speed limit."
Researchers at Purdue University have attempted to model supercavitation with a home-grown CFD (computational fluid dynamics) program, constructed from a modified version of an aeroballistics code applied to six-degrees-of-freedom modeling. But they've had limited success: "So far, CFD has only been useful to show the thickness of the bubble's boundary layer," Daebelliehn says.
To test a projectile, C Tech fires it from a 20-mm cannon at about 3,500 ft/sec—over Mach 3 in air. "The speed of sound in water is 4,800 ft/sec," Daebelliehn says, "so the projectile is sub-sonic in water, but still fast enough to create supercavitation, which is a function of the length of the projectile and the thickness of the nose. The smallest nose is optimal."