In supercavitation, a bubble of gas encompasses an underwater vehicle reducing friction drag and allowing high rates of speed through the water.
"Basically supercavitation is used to significantly reduce drag and increase the speed of bodies in water," said Grant M. Skidmore, recent Penn State Ph.D. recipient in aerospace engineering. "However, sometimes these bodies can get locked into a pulsating mode."
To create the bubble around a vehicle, air is introduced in the front and expands back to encase the entire object. However, sometimes the bubble will contract, allowing part of the vehicle to get wet. The periodic expansion and contraction of the bubble is known as pulsation and might cause instability.
"Shrinking and expanding is not good," said Timothy A. Brungart, senior research associate at ARL and associate professor of acoustics."We looked at the problem on paper first and then experimentally."
The researchers first explored the problem analytically, which suggested a solution, but then verifying with an experiment was not simple. The ideal outcome for supercavitation is that the gas bubble forms, encompasses the entire vehicle and exits behind, dissipating the bubble without pulsation. The researchers report the results of their analytic analysis and experimentation online in the International journal of Multiphase Flow.
"It is easier to study this problem in the lab than in open water," said Michael J. Moeny, senior research engineer at ARL. "There are tow basins where you can pull models along, but it is harder to observe what is happening than in a water tunnel and the experimental runs are short because of the basin sizes."
The ARL researchers decided to use the Garfield Thomas Water Tunnel facility's 12-inch diameter water tunnel to test their numerical calculations.
"The water tunnel was the easiest way to observe the experiment," said Brungart. "But not the easiest place to create the pulsation."
Creating a supercavitation bubble and getting it to pulsate in order to stop the pulsations inside a rigid-walled water tunnel tube had not been done.
"Eventually we ramped up the gas really high and then way down to get pulsation," said Jules W. Lindau, senior research associate at ARL and associate professor of aerospace engineering. "It was a challenge because the walls of the tunnel are really close. Others couldn't get pulsation in a closed tunnel. That's what we did."
Once they could predictably create the phenomena in the water tunnel, they then had to apply their numerical solution to the experimental model. They found that once they had supercavitation with pulsation, they could moderate the air flow and, in some cases, stop pulsation.
"Supercavitation technology might eventually allow high speed underwater supercavitation transportation," said Moeney.
Developed in the 1970s, Russia's Shkval torpedo is equipped with a bubble generator in the nose that envelops the torpedo in a gas membrane while a solid rocket fuel engine provides thrust. The Shkval is capable of speeds in excess of 200 knots—up to five times faster than conventional torpedoes.
In 2004, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, Barracuda. According to Diehl, it reaches more than 400 kilometres per hour (250 mph).
Supercavitation technology has faced two major problems. First, the submerged vessel has needed to be launched at high speeds, approaching 100km/h, to generate and maintain the air bubble.
Second, it is extremely difficult - if not impossible - to steer the vessel using conventional mechanisms, such as a rudder, which are inside the bubble without any direct contact with water.
As a result, its application has been limited to unmanned vessels, such as torpedoes, but nearly all of these torpedoes were fired in a straight line because they had limited ability to turn.
Chinese scientists had found an innovative means of addressing both problems.
Once in the water, the team's supercavitation vessel would constantly "shower" a special liquid membrane on its own surface. Although this membrane would be worn off by water, in the meantime it could significantly reduce the water drag on the vessel at low speed.
After its speed had reached 75km/h or more the vessel would enter the supercavitation state. The man-made liquid membrane on the vessel surface could help with steering because, with precise control, different levels of friction could be created on different parts of the vessel.
"Our method is different from any other approach, such as vector propulsion," or thrust created by an engine, Li said. "By combining liquid-membrane technology with supercavitation, we can significantly reduce the launch challenges and make cruising control easier."
However, Li said many problems still needed to be solved before supersonic submarine travel became feasible. Besides the control issue, a powerful underwater rocket engine still had to be developed to give the vessel a longer range. The effective range of the Russian supercavitation torpedoes, for example, was only between 11 km and 15 km.
SOURCES - Penn State, South China Morning Post