Engineers from Penn State's Applied Research Laboratory (ARL) have a developed an approach that smooths out the sometimes bumpy ride experienced in underwater vessels that use supercavitation bubbles to improve their movement through water.

Supercavitation is used to reduce drag and increase the speed of bodies in water. 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 is the source of the instability and noise.

The ideal outcome for supercavitation is that the gas bubble forms, encompasses the entire vehicle and exits behind, dissipating the bubble in twin vortices. Another acceptable gas exit scheme is a re-entrant jet, in which some of the discharged gas reverses and re-enters the cavity without causing pulsation.

The researchers first explored the problem analytically, which suggested a solution, and then sought to verify it experimentally. Ultimately, they decided to use the Garfield Thomas Water Tunnel facility's 12-inch-diameter water tunnel to test their numerical calculations.

A second-order pulsating supercavity as seen in a 12-inch-diameter water tunnel. The circular object is a window-mounted hydrophone. Image credit: ARL/Penn State.Creating a supercavitation bubble, getting it to pulsate and subsequently stopping those pulsations inside a rigid-walled water tunnel tube had not been accomplished before and proved to be a challenge.

Once the engineers could predictably create the phenomenon 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 alternate between increasing and decreasing the air flow in a sinusoidal manner and, in many cases, the pulsation would stop. The amount and rate of air flow variation did not correlate to just one pulsation frequency, but calmed a range of pulsation states.

The researchers report that, despite the fact that modulation of the ventilation rate was effective at suppressing pulsation over a wide range of frequencies, not all modulation frequencies resulted in a transition to the twin vortex closure regime. Up and down air flow rate modulations that did not result in the desired twin vortex did, however, alter the frequency of the pulsation.

The researchers note that successful supercavitation can reduce the drag on underwater vehicles enough to increase their speed about 10 times.

"Supercavitation technology might eventually allow high-speed underwater supersonic transportation," says Michael Moeny, senior research engineer at ARL. "It may be the only way to get the speed. Without the technology, there is no way to control the cavitation that results from those speeds."