The fluid mechanics community has lost Dietrich Wolfgang Bechert, retired chief scientist at the Institute for Turbulence Research of the German Aerospace Organization (now called DLR) in Berlin. Dietrich died on 1 December 2004 in Berlin of complications after a successful chemotherapy. His death was a shock because he had always downplayed the seriousness of his health problems.
Dietrich was open, caring, and sensitive, and had a great sense of humor. To those familiar with Berliner slang, his wit is preserved in cryptic acknowledgments in some of his scientific publications. Inspired by his superiors, particularly Eberhard Pfizenmaier, the director of his institute, he pursued his work with uncompromising perfectionism and persistence. Dietrich’s passionate dedication to his work and his refusal to grant requests for quick results made him one of the preeminent experimentalists in fluid mechanics; he was also well versed in theory.
I had the immense privilege of learning firsthand his exceptional qualities in the fall of 1987, when, equipped with water and cookies, Dietrich and I spent many nights in the basement of the Institute for Turbulence Research. There, we experimentally investigated the theoretical prediction that low-density jets develop self-excited oscillations below a critical density. Measurements were impossible during the day because of vibrations from scores of shopping buses from Poland that were parked, engines running, in front of the building.
Born in Munich on 7 August 1936, Dietrich received his diplom in mechanical engineering from the Technical University Darmstadt in 1962. He subsequently pursued his doctoral work at TU Berlin under Rudolf Wille, who directed both the university’s Hermann Föttinger Institute and DLR. Dietrich, who joined DLR in 1965 as a scientific collaborator, received his PhD in 1968 for his thesis on planar jets deviated by lateral flow. In 1986, TU Berlin awarded him the habilitation, and the DLR named him Seniorwissenschafter (chief scientist) in 1989.
For nearly 40 years, Dietrich worked at DLR, except for a brief stint at the Max Planck Institut für Strömungsforschung (Max Planck Institute for Fluid Dynamics; renamed the Max Planck Institute for Dynamics and Self-Organization in 2004) in Göttingen in 1971 and a sabbatical in the US at the University of Houston in 1980–81. The photo on page 66 shows him in 2000 next to a remaining section of the Berlin Wall, within which he had lived for 25 years.
Dietrich’s remarkable scientific achievements cover acoustics, flow instabilities, turbulence, and biology-inspired fluid mechanics. They include the definitive experimental test (in 1975) of the unsteady Kutta condition at a nozzle lip, followed in the early 1980s by the only fully quantitative experiment on the coupling between an acoustic field and Kelvin–Helmholtz instability waves at the origin of a planar two-stream mixing layer. Beginning in the mid-1970s, he used the directional-microphone-array technique to resolve the question of whether the dominant noise produced by fast trains was aerodynamic in nature or came from the wheels. That research prompted a long-term collaboration on railway noise between DLR and German Rail.
His most widely noted research concerned the drag-reducing properties of shark skin and other structured surfaces. In the late 1980s he built a unique oil channel and a novel differential balance to measure turbulent skin friction to within a fraction of a percent. In the mid-1990s, after years of painstaking improvements to that flow facility, he clarified what large-eddy break-up devices, or LEBUs, do to turbulent boundary layer flows.
Also during the mid-1990s, he optimized the so-called riblet surface—a surface with tiny ridges running in the direction of the flow—and achieved the world record for drag reduction by riblet surfaces: about 10% relative to a smooth surface. Dietrich worked with the 3M Co on a riblet foil that was later used on a prizewinning America’s Cup yacht (Cup rules have since barred its use) and tried on commercial airplanes. The foil did lead to substantial fuel savings, but its price and maintenance problems prevented its widespread use.
In later years, Dietrich became convinced that Nature had much to teach engineers; the difficulty was in seeing and understanding what Nature had to offer. Inspired by birds that raise their feathers on the suction side of their wings during a slow-landing approach, Dietrich started a research program on anti-return-current flaps in the late 1990s. That program successfully delayed and stabilized flow separation on a motorized glider plane. He was also closely involved in the German Research Foundation’s (DFG’s) priority program on control of complex turbulent shear flows.
Dietrich’s outstanding scientific achievements have been internationally recognized. In 1986, he won the DFG’s Ernst Mach Prize. The first Bionic Prize of the Society for Technical Biology and Bionics, Saarbrücken, and the Association for Bionics, Taufkirchen, was presented to him in 1992. He received the Philip Morris Research Prize in the transport and traffic category in 1998.
After he retired in 2001, Dietrich remained active on the lecture circuit and in the DFG’s priority program on flow control. His last major talk was the 2004 Lanchester Lecture “Aero-dynamics and Biology” at the Royal Aeronautical Society in London.
You are sorely missed, Dietrich, by your family, your friends, and the fluid mechanics community.
