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Notre Dame unveils largest quiet hypersonic wind tunnel

| Thursday, December 6, 2018

Twenty-three minutes. That is how long it would take a hypersonic aircraft traveling at Mach 6 to travel from Washington, D.C. to Los Angeles.

The country’s largest quiet Mach 6 hypersonic wind tunnel was unveiled on Friday on campus in the hopes of achieving sustained hypersonic flight.

The wind tunnel, located at White Field Research Laboratory on Notre Dame’s campus, is one-of-a-kind. According to professors in the aerospace department, it is currently the largest of only three quiet tunnels in the country and is capable of simulating Mach 6 speeds, which is equivalent to about 4,604 miles per hour. The size of its test chamber, its technical status as a “quiet” tunnel and its capacity to simulate Mach 6 speeds make the new wind tunnel not only one of a kind but also incredibly valuable for aerospace research into hypersonics.

Unlike supersonic flight, which refers to travel at speeds faster than sound, hypersonic travel refers to movement at speeds greater than five times the speed of sound, or Mach 5, and above.

“The space race vehicles are all hypersonic vehicles, so we’ve known the basics for a long time and so we can do one-time-use flight vehicles that need to arrive safely,” Thomas Juliano, assistant professor of aerospace and mechanical engineering, said. “But what we want to be able to do is fly the same vehicle many times.”

Juliano explained that a more complete understanding of the way air flows and distributes heat through friction onto different surfaces and shapes at hypersonic speeds is a key to making sustained hypersonic flight with reusable aircrafts possible. The new wind tunnel will help researchers accomplish this.

The tunnel looks like a metal pipe approximately 300 feet long, Thomas Corke, an engineering professor from the department of aerospace and mechanical engineering, said, though it is vastly more complicated. It contains a section that pumps and heats air to a specific temperature and pressure and a long nozzle leading to the test chamber. The geometry of the nozzle is carefully calculated to allow the air to achieve exactly Mach 6 before reaching the model in the test chamber. This model is placed in the test chamber and observed with various sensors, such as infrared cameras, that allow researchers to collect a vast amount of data about how the air flows over and heats up the surface of different models at hypersonic speeds.

Part of what makes this new wind tunnel unique is the size of its test chamber and nozzle exit.

“For tunnels like this, the key dimension is the diameter of the nozzle exit, because that kind of set the size, both the diameter and the length of the model you can test,” Juliano said.

The new tunnel can accommodate models of up to six feet in length, while other quiet wind tunnels capable of simulating comparable speeds can only accommodate models of up to about 28 inches, Corke said.

Corke and Juliano both said that this capability is very important because the way in which air behaves as it flows over a surface actually changes as a function of distance traveled over the surface. In aerospace engineering, this is known as the difference between laminar and turbulent airflow. Laminar flow is much calmer and occurs as air begins to flow over a surface but gives way to the more energetic and tumultuous turbulent flow as distance increases.

Corke said the larger size of the test chamber allows for larger models with longer surfaces for air to flow over, allowing for a more accurate simulation of the turbulent airflow that a hypersonic aircraft would experience in the atmosphere.

The accuracy of simulation the new wind tunnel can achieve is also aided by the fact that it is known in aerospace engineering as a “quiet” tunnel.

“A quiet tunnel is designed to prevent the airflow along the walls of the nozzle from becoming turbulent,” Corke said. “So it simulates what really happens in the atmosphere for an aircraft.”

Corke said the material composition, geometry and polish of the walls of the nozzle is such that it does not affect airflow through the tunnel before it reaches the model, better simulating the conditions that would be found in the atmosphere. With non-quiet tunnels, in contrast, the nozzle can create additional airflow turbulence, which can affect the model being tested.

“The difference in heating levels between putting the same model in a conventional hypersonic tunnel and a quiet tunnel is about a factor of five,” Corke said.

In short, the new wind tunnel has sophisticated capabilities that will allow researchers to test models for hypersonic flight more accurately than previously possible. This accuracy will allow for a better understanding of the mechanics of hypersonic travel that may one day make sustained flight in reusable hypersonic aircraft a reality, Juliano said.

“The end goal is to get places fast. I think everybody can understand the desire to get there faster, wherever it is that you’re trying to go,” Juliano said. “There are lots of different problems that need to be worked out, and one of them is to get accurate predictions of the heating, and this tunnel is one tool among many others working in concert that are going to make that possible.”

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