From the Wind Tunnel to Space

At the German Aerospace Center in Göttingen, scientists are conducting research in Europe’s largest and most powerful tube wind tunnel. Through studies of the phenomena of compressible fluid mechanics, they aim to make the aviation and aerospace industries of tomorrow safer and more efficient.

Two storage tubes, each over 80 meters long, run across an open area next to the building and lead from there through the massive exterior wall into the building: The sheer scale of the research facility becomes apparent as soon as you step onto the grounds surrounding the Institute of Aerodynamics and Flow Technology at the German Aerospace Center (DLR) in Göttingen.

Inside stands a huge vacuum chamber with a volume of 50 m³, connected to the tubes. It is here that researchers study the fluid dynamics phenomena that are crucial for accurately predicting the performance of supersonic aircraft. How can the aircraft and spacecraft of tomorrow become more environmentally friendly, safer, and more efficient? And how can supersonic flight be simulated as precisely as possible on a computer to evaluate new configurations as early as the design phase? Scientists hope to provide answers to these and many other questions using the duct wind tunnel. Vacuum technology from Busch plays a key role in these research projects, as reported by the Maulburg-based company. The large-scale research facility was launched in the 1950s. The Göttingen-based physicist and fluid dynamics researcher Prof. Hubert Ludwieg developed a propulsion system—revolutionary for its time—for intermittently operating high-speed wind tunnels. For the first time, it made it possible to conduct investigations involving supersonic and hypersonic flows. He himself called this principle the “tube wind tunnel”—and to this day, it is known worldwide as the “Ludwieg Tube.” In 1968, the Göttingen Tube Wind Tunnel (RWG)—the world’s first large-scale aerodynamic research facility of its kind—went into operation and remains in use at the DLR to this day.

Experiments at Supersonic Speeds

The operating principle of the tube wind tunnel utilizes the interaction between positive and negative pressure: The storage tubes serve as pressure vessels in which the air is compressed. To prevent air condensation in the supersonic nozzle—which occurs due to rapid expansion and the resulting cooling of the air—the storage tubes must be additionally heated when simulating high supersonic speeds.

The storage tubes are connected to the supersonic nozzle via a quick-acting slide valve; the measurement section is located at the end of the nozzle. This is where the experiments are conducted. At the end of the measurement section is the vacuum chamber, to which the vacuum pump is connected. A vacuum valve between the measurement section and the vacuum chamber allows access to the measurement section as needed. The vacuum chamber is evacuated using the vacuum pump. A Cobra NX 0950 A Plus screw vacuum pump from Busch Vacuum Solutions is used for this purpose. It generates a vacuum of approximately 10 to 40 mbar in the vacuum chamber, while the storage tubes are maintained at a pressure of approximately 2 to 40 bar.

For an experiment, the test model is placed in the measurement section using a sliding model holder. These models include, for example, aircraft models, sensors, or material samples. When the quick-slide valve is opened, a continuous rarefaction wave is generated that travels into the storage tube and accelerates the stored air toward the nozzle. Due to the pressure difference between the storage tube and the vacuum chamber, as well as the specially shaped supersonic nozzle, a so-called supersonic flow is established in the RWG’s measurement section. Speeds of up to Mach 7 can be reached—which corresponds to seven times the speed of sound. Measurement times of up to 350–400 ms are achieved in the RWG. According to the announcement, this is a record for wind tunnels of this type and provides researchers with sufficient time to study the flow around the test models. During this time, they can record statistically relevant data or image sequences that enable reliable averaging and analysis of the data.

Clean Vacuum for Technological Progress

Busch’s reliable vacuum system contributes significantly to the success of experiments at the RWG and promotes technical progress, according to the manufacturer’s press release. The DLR makes the results of its basic research available to companies in the aerospace industry to help them develop and refine technology for current and future missions. The scientists in Göttingen also collaborate with international organizations such as NASA, ESA, and other DLR locations on global research projects. For example, in the past, a model of the X-38 space glider was tested in the Göttingen wind tunnel on behalf of NASA and ESA; this glider was intended as a Crew Return Vehicle (CRV) to bring astronauts from the ISS back to Earth in an emergency. Such a space glider must withstand high heat and mechanical loads upon re-entry into Earth’s atmosphere. The researchers were able to simulate these conditions as closely as possible in the wind tunnel.

“Despite enormous advances in computational fluid dynamics, many phenomena in turbulent flows still cannot be predicted adequately and precisely. In our studies, we are creating an important validation database that contributes to improving existing models and developing new numerical calculation methods. We see this as the true purpose of this research facility,” says Schülein. Busch’s vacuum technology plays a key role in this, the report concludes.