Experiments at supersonic speed

The operating principle of the tube wind tunnel uses the interaction of pressure and vacuum, where the accumulator tubes serve as pressure vessels in which the air is compressed. To prevent air condensation in the ultrasonic nozzle, which occurs due to the strong expansion and the associated cooling of the air, the accumulator tubes must be heated to simulate high supersonic velocities.

The accumulator tubes are connected to the ultrasonic nozzle via a quick-action slide valve. The measuring section is found at the end. This is where experiments are carried out. At the end of the measuring section is the vacuum vessel, which the vacuum pump is connected to. A vacuum slide valve between the measuring section and the vacuum vessel enables access to the measuring section as required. The vacuum vessel is evacuated using the vacuum pump. A COBRA NX screw vacuum pump from Busch Vacuum Solutions is used for this. It generates vacuum of approximately 10 to 40 mbar in the vacuum vessel. In the accumulator tubes, there is overpressure of approximately 2 to 40 bar.

To perform a test, the test model is placed in the measuring section using a movable model holder. Test models include aircraft models, sensors or material samples. Opening the quick-slide valve creates a running dilution wave that flows into the accumulator tube and accelerates the accumulator air towards the nozzle. Due to the differential pressure between the accumulator tube and the vacuum vessel, and thanks to the specially shaped ultrasonic nozzle, an ultrasonic flow is created in the RWG measuring section. Speeds of up to Mach 7 can be achieved – corresponding to seven times the speed of sound. Measurement times of up to 350–400 milliseconds are realized in the RWG. This is a peak value for wind tunnels of this type and gives researchers enough time to study the flow around the test models. During this time period, statistically relevant data or image sequences can be recorded to enable reliable data averaging and analysis.