SUBSONIC AND SUPERSONIC WIND TUNNELS
Department of Mechanical and Aerospace Engineering
It’s a sound that may be familiar to students attending class in Engineering Building III (EB III) on NC State’s Centennial Campus — a roar that’s reminiscent of a commercial jet preparing for takeoff.
In reality, it is the sound of the Department of Mechanical and Aerospace Engineering’s (MAE) supersonic wind tunnel, located in an annex building on the back side of EB III. The building is also home to a closed-loop subsonic wind tunnel operated by the department.
The two wind tunnels are used to simulate flight conditions so that research data can be gathered on everything from unmanned aerial vehicles (UAVs) to military jets. The slower subsonic tunnel can also be used to simulate storm conditions that test the durability of structures such as buildings or bridges.
While the subsonic tunnel only reaches wind speeds of 90 mph, the supersonic tunnel can hit anywhere from Mach 1.5 to Mach 4.5. The supersonic tunnel uses blasts of compressed air that only last about six seconds. The subsonic tunnel, which came to NC State by way of NASA in the 1950s, is powered by a propeller from a World War II-era aircraft.
The subsonic tunnel utilizes a four-foot by 3.5-foot testing area. Because of the high pressure used in the supersonic tunnel, the testing area is a mere six inches tall and 1.5 feet wide.
The subsonic tunnel is used to test UAVs, cars and some smaller aircraft, and the supersonic tunnel is ideal for fighter jets and the next generation of supersonic commercial aircraft to replace the Concorde.
Dr. Srinath Ekkad, professor and head of MAE, hopes to install a transonic tunnel for the department to fill in the gap between the two existing facilities. Transonic tunnels, which reach wind speeds of Mach 1 or a little more, would be useful for research on commercial aircraft.
Both facilities stay busy, said Dr. Shreyas Narsipur, a teaching assistant professor who manages both facilities for the department. Along with faculty members and graduate students, the tunnels are used by sophomore, junior and senior undergraduate aerospace engineering students as part of the three Experimental Aerodynamics labs all are required to take as part of their coursework.
Tests look at how air flows around an aircraft and how components hold up under high wind speeds. So, an experiment might use a scaled-down model of a jet fighter or just a section of wing from a real plane.
Will a wing made of a new material flutter under flight conditions? Will components produced by additive manufacturing hold up as well as those made using traditional techniques? Using load sensors, a faculty member can determine how and where force impacts a material.
Airflow can be measured with high-speed cameras using particle image velocimetry, in which particles released into the flow pass through a laser sheet. A similar technique uses smoke released into airflow. Sometimes airplane models and components are painted with a fluorescent dye, and data can be gleaned by the way in which the dye is blown away.
Though much relevant data can be arrived at computationally, Narsipur said, wind tunnel experiments are still needed because those methods are not able to model flow dynamics with complete accuracy.
“Computations need experimental data to validate against,” he said. “That process is always going to be there.”