SPF Enhances Wind Tunnel Technology


By Jessica A. Baris

Who would have thought that spray polyurethane foam (SPF) insulation could help out a team of scientists in need of a high-tech wind tunnel? This story takes us to the Department of Aerospace Engineering & Engineering Mechanics at the University of Texas at Austin. A research professor was in the middle of building an open-jet wind tunnel with an anechoic chamber to study aeroacoustics and experimental fluid dynamics. For the non-engineer, that means sound testing.

That professor, Dr. Charles E. Tinney, spearheaded the construction of the wind tunnel. He is collaborating with organizations such as NASA (National Aeronautics and Space Administration) and AFOSR (Air-Force Office of Scientific Research) to better understand the noise generated by rocket engines, high-speed aircraft, and rotorcraft (helicopters). To say the least, having a high-performance wind tunnel is necessary for Dr. Tinney to successfully execute his research.

“When the [NASA] shuttle is fired up, the sound and vibrations are so intense that they can adversely affect critical systems on board the spacecraft, such as sensitive payload and even astronauts,” says Dr. Tinney. “Think of it this way: A church is a reverberating structure—that’s the opposite of what we need. We need to know what’s producing the sound and to be able to isolate that sound so that we can understand its source. The wind tunnel and anechoic chamber are designed with special walls that trap the sound so that it does not reflect back and contaminate our measurements.”

To build the perfect wind tunnel, Dr. Tinney needed an insulation that would help isolate the sounds they would test. As he researched his options, he came upon spray polyurethane foam (SPF) insulation.

“We wanted something that would fill the wall space and completely seal it off,” says Dr. Tinney in reference to the wind tunnel. “The foam seals in all the cracks, so it gives a full coverage of the area I’m insulating. For a wind tunnel, an airtight seal is critical. Cotton fiber doesn’t give you the seal you get with spray foam. [The foam is] lightweight as well and provides us with sufficient sound absorption qualities.”

The Man for the Job

Dan Amon of BioTex Foam Insulation was the man for the job. Dr. Tinney gave him the rough calculations for the wind tunnel’s board feet, and Amon gave him a loose bid over the phone. A few months later, the wind tunnel’s framing was complete, and Amon got a call back. “They wanted me to come down and take a look at it, and that’s when I did a formal proposal,” he says.

The anechoic chamber is approximately a 26 x 20 x 18-foot cube with 19-inch-thick walls. Amon said that he wanted to achieve “maximum foam depth without doing a lot of scraping and throwing away a lot of bags of waste.”

When figuring what the cost of the application would be, Amon goes by board footage.

“I have a working margin. I ask how many board feet are in the job. I figure out the area that’s going to get foamed and at what thickness. Length by height by depth gives you board feet, which is the measurement of volume. Since we’re working with liquid, that’s what you need to know to figure out how much material is required to cover it. I charge per board foot.”

Numbers taken care of, it was time to get down to the foam.

Foam the Cube

Amon’s three-man crew finished the spray foam application in one day’s time with a Gooseneck Trailer, a Graco E-20 proportioner, and a Fusion AP spray gun. The crew put down plastic and taped off surfaces. They also set up a rolling scaffold to reach the top of the 20-foot wind tunnel. Once the spray site was prepped, Amon heated the chemicals.  

“We took off from there,” says Amon. The application was straightforward—11 inches of BioBased 501 open-cell foam was applied to the wind tunnel’s sidewalls and roof.

“The wind tunnel is made of 2 x 12 studs with 3-quarter plywood screwed and glued together,” describes Amon. “This thing was a giant cube. You could roll it down a hill, and it wouldn’t lose its shape. The forces of wind that were going to be exerted on it would be tremendous, so it needed to be built strong.”


SPF? Brilliant!

When Amon was hired to do the job, he thought, “How brilliant is that?”

The spray foam in the wind tunnel will help scientists solve problems that most people are not aware of, one of which is rotorcraft noise testing.

“When helicopters maneuver or are descending to land, they make a ‘wop wop wop’ noise, which results from an interaction of the rotor blade with the wake from its preceding blade. This also produces unwanted vibrations in the helicopter,” explains Dr. Tinney. “Communities don’t want these sounds. We are trying to understand the basic physics of that problem, and that’s one thing we’ll be testing.”

A second project in store is to “replicate space shuttle main engine startup,” adds Dr. Tinney. “Rocket engines produce massive vibrations during startup. We want to identify what is causing the fluid-induced vibration so that it can be accounted for in the design of the next generation of space vehicles.”

Dr. Tinney could not be more satisfied with the outcome of the SPF application. “Spray foam allows us to achieve an acoustically treated environment in which we can conduct our tests.”

The wind tunnel was definitely an atypical project, “but that’s what made it so interesting to the crew and me,” Amon says. “It was a lot of fun.” Amon’s enthusiasm for spray foam was apparent when he added, “Even if it was a 100 percent strange project, I would still be all over it.”

To learn more about the wind tunnel and see more photos of its construction, visit: http://www.ae.utexas.edu/facultysites/tinney/index.php?option=com_content&view=article&id=34&Itemid=38.


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