Great Physics And Great Responsibility

A hundred years ago, the Kaiser Wilhelm Institute for Flow Research began its work. From it emerged today's Max Planck Institute for Dynamics and Self-Organisation, which continues to explore fundamental questions in the physics of complex dynamic systems - remaining true to its commitment to uncovering the deep interconnections of the physical world. Over the past two decades, fluid dynamics has once again become a central focus of research, with growing relevance for emerging technologies. A look back at the Institute's early years shows how basic research helped drive pioneering technologies, particularly in aviation. But there is a flipside: this field carries a particularly high dual-use risk, and under National Socialism it was quickly steered toward military purposes.

At the start of the 20th century, aerodynamic phenomena were becoming an increasingly pressing challenge in engineering. Thanks to the new steel industry, towers and bridges had reached heights and spans previously unimaginable - but they did not always withstand storms. Gustav Eiffel's iron skeleton tower, proudly unveiled by France at the 1889 Paris World's Fair, demonstrated what was technically possible. Eiffel also used what was then the world's tallest structure - 312 metres - to conduct aerodynamic experiments. But the complex interplay between airflow, obstacles and their shape was neither mathematically predictable nor physically understood. At the same time, aviation pioneers launched themselves skyward with homemade contraptions and a good deal of courage, relying on trial and error to improve their gliders.

Ludwig Prandtl had been professor of applied mechanics at the University of Goettingen since 1905. In 1907, he founded the Model Research Institute for Aviation, which became the Aerodynamic Research Institute (AVA) and joined the Kaiser Wilhelm Society in 1918. Despite AVA's growing status as a world-leading research centre, Prandtl saw a shortfall in addressing fundamental questions beyond aircraft aerodynamics. His wish for a dedicated institute to pursue this goal was fulfilled in 1925 with the founding of the Kaiser Wilhelm Institute for Flow Research.

A dual Institute for theory and practice

What was new - and unusual for the time - was Prandtl's mathematically based approach to solving practical problems faced by aircraft designers and architects dealing with airflow. In 1915, Hugo Junkers had succeeded in building the first fully functional all-metal aircraft in his water heater factory. But at that time, no one could yet explain what forces kept such a craft aloft. Only Prandtl's wing theory - describing the interaction of lift and drag on a tilted wing and incorporating flow velocity - allowed a systematic, physical and calculable explanation. The fact that this theory, developed during the First World War, only reached aircraft designers in 1918 highlights the wide gap that existed between science and engineering, and between theoretical insight and practical application in aerodynamics.

Prandtl's vision was to bring both approaches, practical experimentation and mathematical physics, together in a single entity: the 'Kaiser Wilhelm Institute for Flow Research, in association with the Aerodynamic Research Institute'. His innovative methodology, which came to define a new scientific paradigm, used scale models and highly specialised experimental setups to solve theoretical problems in fluid mechanics - an approach now standard in the automotive, aerospace and civil engineering industries. Yet the research at this new dual institute, as its name suggests, extended far beyond aerodynamics. The focus was on all types of flow in gases and liquids, as well as turbulence.

The new Institute not only made Goettingen a centre of emerging aerodynamic science, but also marked a step toward a more specialised and cooperative model of modern German science. For the first time, state, industry and research formed a dynamic and occasionally tense network of shared interest. Scientific discovery took on unprecedented importance as part of a value chain linking knowledge generation directly to its practical application.

Science as a large-scale operation

The idea that findings from traditional academic disciplines could also benefit technological applications was far from self-evident in the German scientific system at the end of the 19th century. Following the Humboldtian ideal, science was understood as a vehicle for the personal and moral development of individuals - and was meant to be pursued without regard to utility.

But the notion that science might benefit an emerging industrial sector and the state gained increasing traction. In 1910, theologian and imperial science advisor Adolf von Harnack gave this concept institutional form which led to the founding of the Kaiser Wilhelm Society. He envisioned a modern science system as a 'large-scale operation' - on the one hand based on division of labour, and on the other on teamwork and strong interconnection among its separate elements, with research firmly embedded in a shared sphere of interest between science, the state, and the economy. The state welcomed this vision enthusiastically.

In the service of the Third Reich

With the Nazi rise to power, the Institute entered a period of expansion. Among the military projects prioritised by Hitler's government - which began large-scale rearmament from 1935 onward - was aviation. As early as 1933, the Reich Aviation Ministry approved several construction projects at the institute, which expanded rapidly in the years that followed - especially the AVA, which developed into a major research facility employing over 700 people during the Second World War.

Though Prandtl never joined the Nazi Party, he increasingly steered his Institute's basic research towards military applications. The AVA and KWI took on military contracts without needing to radically change their research agendas - their focus on boundary layers and turbulence aligned naturally with the military's interest in novel aircraft and naval vessels. To develop more accurate torpedoes, jet-propelled rockets, faster submarines and more manoeuvrable planes, the arms industry required insights from flow physics. In turn, the AVA and the institute could tackle open research questions posed by real engineering problems - and were generously funded in the process.

In 1935, the KWI set up a dedicated flow channel to investigate the surface roughness of ship plates, working in close cooperation with the naval shipyard in Wilhelmshaven. Since the project failed to deliver quick results, the navy discontinued it in 1941. Surface roughness is highly relevant for flow dynamics - it has a critical impact on drag, flow structure, and energy loss in moving fluids such as air or water. The rougher the surface of a pipe or conduit, for instance, the greater the friction, as even small irregularities generate vortices that extract energy from the flow. The scientific insights from this project remain influential to this day.

Saved from closure

Germany's collapse at the end of the Second World War initially meant the end of the AVA, as the Allies banned all aviation-related research as part of their demilitarization efforts. The KWI for Flow Research, however, was incorporated into the newly founded Max Planck Society in 1948, and its research activities continued with barely a break. After Prandtl's retirement, Albert Betz - formerly head of the AVA - became director, and Prandtl's student Walter Tollmien was appointed to the institute, taking over as director in 1957. Tollmien played a decisive role in establishing flow physics as an interdisciplinary science of great significance. Modern turbulence research - which served as a prototype for chaos theory - is closely linked to his name. One of his doctoral students was Klaus Hasselmann, who earned his PhD under Tollmien in 1957 and later became founding director of the MPI for Meteorology and a Nobel Laureate in Physics (2021).

In the 1970s, the institute expanded its research activities based on a plan proposed by Goettingen Nobel laureate Manfred Eigen. Under the leadership of directors Jan-Peter Toennies, Hans Pauly, and Heinz-Georg Wagner, the focus shifted toward the study of molecular interactions, atomic and molecular physics, and reaction kinetics. Meanwhile, Ernst-August Mueller continued fundamental research in fluid dynamics, with a particular emphasis on aeroacoustics.

Return to flow research

In 1996, Theo Geisel reoriented the Institute's focus toward nonlinear dynamics and its applications in neuroscience and network theory. In 2003, with the arrival of Stephan Herminghaus and Eberhard Bodenschatz, fluid research was reestablished at the Max Planck Institute. The research focus shifted to the study of dynamics and self-organization across a wide array of systems - from cell biology and granular media to turbulence, cloud physics, and even public transportation. In 2004, the institute was renamed the Max Planck Institute for Dynamics and Self-Organization.

With the opening of a new wind tunnel in 2010, the Institute once again gained access to a globally unique large-scale research facility. Among others, Claudia Brunner's group uses it to study how wind turbines behave in turbulent flow conditions. Their goal is to better understand airflow interactions - with the long-term aim of increasing wind farm efficiency. The idea that flow research could play a key role in harnessing wind energy, however, is as old as the institute itself. As early as 1920, Betz calculated the theoretical maximum power output of wind turbines in his doctoral thesis under Prandtl - a result. In 1926, Ludwig Prandtl proposed the establishment of a 'Research and Testing Facility for Wind Power Utilisation', citing a lack of fundamental knowledge for technologies capable of harnessing the 'energy of moving air'. In a remarkably farsighted warning, he noted that 'the two main energy sources that have powered the current industrial age - coal and oil - are by no means inexhaustible.' One had to expect, he wrote, 'that supplies would eventually run out - especially if consumption continues to rise steeply, as it has in recent decades.' At the time, climate change had not yet emerged as a reason to pursue renewable energy.

Prandtl's proposal never moved beyond the planning stage. It wasn't until the 1980s that wind power technologies became commercially viable. Yet his, in hindsight, strikingly visionary memorandum serves as a reminder that technological pathways are shaped by political choices. Basic research can help chart these paths by exploring what is feasible and developing forward-looking visions.