| Aeronautics

Making the invisible visible

Chris Willert vom Institut für Antriebstechnik macht Strömungen sichtbar.

Chris Willert deals with what the human eye cannot perceive. He works on the airflows that occur in and around turbines, aircraft wings, combustion chambers and compressors. And also, behind the blades of wind turbines or on road vehicles. Wherever fluids such as air or water flow around objects, measurement technologies must make the invisible visible. To do this, Chris Willert and his team in the Engine Measurement Systems Department at the Institute of Propulsion Technology have mastered a variety of methods. “Optical measurement technology as a field is very heterogeneous and has many disciplines,” he says. However, all measurements have one thing in common. The environment in which they are carried out is always very complex and usually not routine.

Test objects from turbines to champagne glasses

The camera on the windowsill is facing outwards. On Willert’s screen there is only noise against a black background. Then, the body of a man appears. On the footpath in front of the Institute building, a member of staff is walking towards the canteen and his outline can be seen as moving dots on the camera image. The camera is an ‘event camera’ and registers the change in light incidence for each individual pixel – a new technology. And for Willert, it is one of those ‘flashes of inspiration’ that sometimes hit him and make his brain run at full speed and keep him busy until he has an idea for an implementation. After reading about the new camera technology in an online blog, it quickly became clear to him: “I must have that!” How can the camera be used to record measurement data? And for which types is it best? At home, the measurement technician first tried out what the camera could do in a very pragmatic way. The raindrops outside the window, which the wind drives diagonally in front of the camera lens. The air bubbles in the sparkling champagne that was actually meant for Christmas dinner. During the Covid-19 lockdown, all these became test objects in his improvised laboratory at home as he tried to find out how the new technology could be used for tests on turbines, combustion chambers or compressors.

Pioneer in flow measurement technologies

Ohne Messtechnik gibt es keine Daten für die Wissenschaft.

It worked in a similar way back when Willert was still living in San Diego and doing his doctorate in experimental fluid mechanics after studying applied mechanics. There, he got into image recording and digitisation more by chance as a career change. Photographs show him at the end of the 1980s with bulky computers and TV screens – he was one of the first to conduct research in the field of digital image processing for flow measurement. Electronics and programming were part of it, even though he had not studied these disciplines. But when Willert is working on a topic and the ‘flash of ideas’ has struck, he stays on it. “The measurement technology we developed back then within a very short time is considered standard in experimental fluid mechanics today,” he says. And that is despite the fact that fluid mechanics was not actually his best subject at university. Today, he is a globally recognised expert in Particle Image Velocimetry (PIV), in which a laser illuminates the minute particles in the flows and their movement is recorded by a high-speed camera. In June 2023, he was also awarded a prize for this achievement, together with his doctoral supervisor. “For my life’s work, so to speak.”

From the laboratory to the large test stands

Chris Willert has now been working at DLR for 28 years. After his post-doc work at Caltech in Pasadena, the then 30-year-old came to the DLR Institute of Aerodynamics and Flow Technology in Göttingen as a visiting scholar, initially for just one year. The field of experimental procedures had lured him to Germany. Willert decided to stay and, together with his team, transferred his digital PIV measurement technology to the large wind tunnels for use in industry-related applications. In 1997, he moved to the DLR Institute of Propulsion Technology in Cologne, where he now leads a 12-member team that collects measurement data from experiments in the test stands for DLR researchers and external partners such as Rolls Royce, General Electric, Siemens and MTU Aero Engines. The team also provides reference data for the Institute’s numerical analysts. “We develop and operate optical measurement technology that has to function precisely and reliably under extremely high sound pressures, in harsh, sooty environments and at high temperatures,” explains Willert. Lasers and cameras have to detect the flow between rapidly rotating engine blades or observe complex processes in combustion chambers.

Service provider and idea generator

Leidenschaft und Begeisterung für neue Ideen - Chris Willert ist immer offen für neue Technologien.

In addition to this work as a ‘service provider’ for the other departments in the Institute, Willert also sees himself as a generator of ideas. “I often put a lot of my heart and soul into the development of a measurement technique,” he says. He does not give up until a solution is found. An example of this was when it came to using LEDs instead of lasers. And again, he asked himself: ‘How can the more cost-effective high-performance LEDs be used for flow measurement technology?’ In the meantime, this idea is also being used for research. Willert takes a playful and creative approach to new techniques. “I often trust my gut feeling, but behind that there is also a lot of experience.” The fact that he had not focused directly on fluid mechanics in his studies in the US is not an obstacle. On the contrary: “Among physicists, I’m the engineer, and among engineers, I’m the physicist. I see it as an advantage to be able to do both – I know the entire process from the initial idea to the finished product.”