In an engineering lab not far from Old Campus, a Yale scientist is using super-helium to improve the design of nuclear submarines and aircraft.

Mechanical engineering and physics professor Katepalli Sreenivasan, working with a group of physicists from around the country, will use a $5 million grant to develop a wind tunnel that uses helium to overcome the obstacles of conducting realistic tests in the laboratory.

When engineers test scale models of designs for new aircraft, submarines and other large objects in wind tunnels, they always face a major problem — recreating the flows or fluid resistance that the object would face in real life. Because of this and the large size of the objects, it is nearly impossible for scientists to make them as streamlined and aerodynamic as they could be.

The solution lies in the Reynolds number, a value reached by dividing the product of an object’s velocity and length by the viscosity or stickiness of the fluid it travels through. When the U.S. Navy wants to test the design of a new submarine, for example, it must keep the Reynolds number the same between the model and the prototype. In order to reach Reynolds numbers even half as large as that of a typical submarine or aircraft, engineers have to build massive test facilities costing hundreds of millions of dollars.

Working on a team led by Russell Donnelly GRD ’56, Sreenivasan helped to figure out a way to reach extremely high Reynolds numbers at a low cost. The answer, they discovered, is cryogenic helium, the substance with the lowest viscosity known to man. At temperatures of minus 268 degrees Celsius, helium has a viscosity so low that it becomes “slippery” — which allows scientists to simulate real-life conditions very accurately in a laboratory wind tunnel.

“Using helium instead of water or any other fluid that engineers are currently using is a lot less expensive,” Sreenivasan said. “A lot of tests on submarines are done in large lakes with radio-controlled devices or in other large and costly facilities. But by using helium, we can do these same tests in our lab.”

When Donnelly and Sreenivasan first came up with the idea to use super-cold helium, they had difficulty finding a method for bringing the substance to such low temperatures. In 1995, Donnelly tried obtaining $40 million in unused refrigeration equipment from the failed Superconducting Supercollider particle accelerator project in Texas, but the equipment went to other institutes instead. The team’s luck improved when the National Science Foundation stepped in with a $5 million grant last month for them to build a prototype helium flask.

“We were ecstatic when we received the money from NSF,” Sreenivasan said. “The apparatus that we’re building will be able to measure objects with Reynolds numbers that the Navy would be interested in. Though we won’t actually test their machines, we will develop the technology for them to study their prototypes.”

The flask the team is creating is made of stainless steel and measures a meter high and half a meter wide. Three different layers, including an outer layer of liquid nitrogen, insulate the helium inside the flask. If a regular cup of coffee were as well insulated as the helium inside the team’s flask, Donnelly said, “It would take 75 years for it to cool.”

Donnelly and Sreenivasan also believe the flask has other applications, such as studying convection, the transmission of heat caused by the movement of molecules from cool regions to warmer regions of lower density. Convection is responsible for a variety of weather patterns, including hurricanes, and if better understood could lead to more accurate weather predictions.

Though it will take another year before the flask is completed and the team can start running tests with it, Donnelly and Sreenivasan said they are certain it will be successful.

“I believe that in a few years cryogenic techniques will become widely used in wind tunnel work and for special kinds of surface vessel testing,” Donnelly said. “If so, the credit goes to Sreenivasan, who was the first to recognize the potential.”

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