MMaterialsgateNEWS 2016/06/20

Innovative approach makes for a smoother ride

Moving through water can be a drag, but the use of supercavitation bubbles can reduce that drag and increase the speed of underwater vehicles.

Sometimes these bubbles produce a bumpy ride, but now a team of engineers from Penn State's Applied Research Laboratory have an approach that smooths out the ride and stabilizes the bubble.

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, causing a problem with stability and noise."

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.

"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 in twin vortices. Another acceptable gas exit scheme is a re-entrant jet where some of the discharged gas reverses and re-enters the cavity, but pulsation is to be avoided. 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 alternate increasing the air flow 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 could calm a range of pulsation states.

The researchers reported 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 alter the frequency of the pulsation.

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

"Supercavitation technology might eventually allow high speed underwater supersonic transportation," said Moeny. "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."

Source: Penn State – 16.06.2016.

Investigated and edited by:

Dr.-Ing. Christoph Konetschny, Inhaber und Gründer von Materialsgate
Büro für Material- und Technologieberatung
The investigation and editing of this document was performed with best care and attention.
For the accuracy, validity, availability and applicability of the given information, we take no liability.
Please discuss the suitability concerning your specific application with the experts of the named company or organization.

You want additional material or technology investigations concerning this subject?

Materialsgate is leading in material consulting and material investigation.
Feel free to use our established consulting services

MMore on this topic

Harvard Microrobotics Lab develops first insect-size robot capable of flying and swimming

In 1939, a Russian engineer proposed a "flying submarine" -- a vehicle that can seamlessly transition from air to water and back again. While it may sound like something out of a James Bond film, engineers have been trying to design functional aerial-aquatic vehicles for decades with little success. Now, engineers may be one step closer to the elusive flying submarine. The biggest challenge is conflicting design requirements: aerial vehicles require large airfoils like wings or sails to generate lift while underwater vehicles need to minimize surface area to reduce drag. To solve this engineers at the Harvard John A. Paulson School of Engineering and Applied Science (SEAS) took... more read more

Research team is first to identify surface 'roughness' required to achieve amazing feat

Imagine staying dry underwater for months. Now Northwestern University engineers have examined a wide variety of surfaces that can do just that - and, better yet, they know why. The research team is the first to identify the ideal "roughness" needed in the texture of a surface to keep it dry for a long period of time when submerged in water. The valleys in the surface roughness typically need to be less than one micron in width, the researchers found. That's really small -- less than one millionth of a meter -- but these nanoscopic valleys have macroscopic impact. Understanding how the surfaces deflect water so well means the valuable feature could be reproduced in other... more read more

Researchers from the University of Twente have succeeded in clearly identifying why droplets on soft, squishy surfaces react differently than on hard surfaces.

A water droplet, for example, moves very differently over jelly than over glass, but the science of how this works has never been investigated. Better understanding of this phenomenon is of importance for a variety of applications where droplets come into contact with extremely soft, deformable materials, as is the case in 3D printing, soft contact lenses or sauces such as mayonnaise. The result was published in the renowned scientific journal Nature Communications. Due to the surface tension in the liquid, minuscule 'ridges' arise at the edge of the droplet on the soft, jellylike surface. "That little ridge is always there, even if the droplet is motionless", explains... more read more

Modeling structures that trap air under water and could one day lead to more energy-efficient ships described in the journal 'Physics of Fluids'

From the sleek hulls of racing yachts to Michael Phelps' shaved legs, most objects that move through the water quickly are also smooth. But researchers from UCLA have found that bumpiness can sometimes be better. "A properly designed rough surface, contrary to our intuition, can reduce skin-friction drag," said John Kim, a professor in the mechanical and aerospace engineering department at UCLA. Kim and his colleagues modeled the fluid flow between two surfaces covered with tiny ridges. They found that even in turbulent conditions the rough surface reduced the drag created by the friction of flowing water. The researchers report their findings in the journal Physics of Fluids... more read more

MaterialsgateNEWSLETTER

Partner of the Week

Search in MaterialsgateNEWS

Books and products

MaterialsgateFAIR:
LET YOURSELF BE INSPIRED