For example, Collicott has developed highly sensitive optical devices to detect, measure and track instabilities in the airflow. The air flows over a test piece at Mach 4 for only about 3.5 seconds, but Schneider says that's plenty of time for the instruments to collect data. Purdue's wind tunnel uses a pipe, closed on one end, to supply special dust-free air. The other end is connected to a huge tank sitting just outside Schneider's laboratory. Between the tank and the test area is a special diaphragm.
Pumps remove the air from the tank, creating a vacuum between the tank and the diaphragm.
When the diaphragm is broken, air flows at Mach 4 through the pipe and the test area and into the tank. This setup allows us to get great data at relatively little cost. The new Mach 6 tunnel, to be built in the Aerospace Science Lab, also will rely on this type of design, called a Ludwieg tube. The major advantage of the new tunnel is that it will be 10 to 100 times quieter than other tunnels, which is critical in studying noise-sensitive phenomena. As the air flows through the experimental area of a wind tunnel, it flows across the tunnel's interior wall, where it also experiences laminar-turbulent transition, Schneider explains.