Vacuum technology is important not only for accelerating, but also for slowing down the high flow velocity. The air from the accumulator tube is collected in the vacuum vessel during the test and then discharged outside as normal ambient air. Dr. Erich Schülein, group leader and scientific supervisor of the RWG at the Institute of Aerodynamics and Flow Technology, explains: “Thanks to vacuum technology, we can carry out the tests much more efficiently. Without it, we would not only have to significantly increase the boost pressure in the accumulator tube, but also the requirements for the stability of the entire system and the testing technology in order to achieve the required pressure ratio in the ultrasonic nozzle at all. The technical effort required for this would be enormous. The vacuum pump does this work for us. The combined application of pressure and vacuum accumulators makes it easy to change the pressure level and thus the Reynolds number of the flow.”

The tube wind tunnel in Göttingen has been in use since 1968 together with an old rotary vane vacuum pump. In 2021, it was high time to replace it. Busch was able to win the contract as part of a tender. The company’s experts then provided support with their expertise in selecting and dimensioning the system to find a suitable solution. A solution was quickly found with the dry COBRA NX. Karsten Pfeiffer, technical manager of the RWG, explains: "For us, it is crucial that the vacuum pump used works reliably, because the flow generated in the tube wind tunnel must be clean. The tests are often carried out several times, and it is important that the conditions are reproducible at all times – which is why nothing should interfere with the flow." The performance of the COBRA also made a very positive impression. Compared to the previous rotary vane pump, the Busch screw vacuum pump evacuates the vacuum vessel twice as fast. The most commonly required pressure of 50 mbar in the vessel is now reached after just 15 minutes instead of half an hour. These shorter run times have a very positive effect on the energy consumption of the plant. In addition, a frequency converter adapts the vacuum pump's rotational speed to the required pressure conditions.  The institute’s employees are also pleased with the new vacuum solution. Previously, they were unable to use the working rooms directly above the plant during the tests due to the high noise level and vibrations generated by the old vacuum pump. With the COBRA, this is no longer a problem, as it operates very quietly and with low vibration. "You can't hear anything except a slight buzz," laughs Pfeiffer. Another major benefit is oil-free operation. "In the past, I had to do hands-on work regularly and change the oil – and then also change my resulting dirty clothes. This is no longer necessary. Maintenance is carried out by a Busch service technician as part of a maintenance contract. All I have to do is switch on the pump and it starts running," says Pfeiffer happily. Above all, the proximity of Busch to customers is a key benefit. Thanks to the company's extensive service network, the local contact person can be on site very quickly when needed.