Hans Wolfgang Liepmann

Hans Wolfgang Liepmann, distinguished fluid dynamicist, outstanding teacher, and former director of the Graduate Aeronautical Laboratories at Caltech (GALCIT), passed away on 24 June 2009 at his home in La Cañada Flintridge near Pasadena, California.

Liepmann was born on 3 July 1914 in Berlin. After schooling in Germany, he moved to Istanbul, Turkey, where his father, a well-known physician, had accepted a professorship in order to escape the Nazis. A year later Liepmann went to Prague, Czechoslovakia, and quickly thereafter to Zürich, Switzerland. He obtained his PhD in physics from the University of Zürich in 1938 under Richard Bär. For his thesis, he used light-scattering methods to measure the speed of sound in liquid oxygen and other cryogenic fluids. Liepmann then accepted an invitation from Theodore von Kármán at Caltech and migrated to the US, joining GALCIT in 1939 just as the war that he had foreseen erupted in Europe. He took an immediate liking to Caltech and the US and stayed there for the rest of his life.

His research career at Caltech began with investigations of the stability of a laminar boundary layer. Liepmann confirmed Galen Schubauer’s detection of previously controversial instability waves at the National Bureau of Standards (now NIST) and reported interesting new results on curved surfaces. Much later, in 1982, he and Dan Nosenchuk would show, in an early example of active flow control, how those Tollmien-Schubauer waves (as Liepmann insisted on calling them) could be suppressed by suitably phased forcing.

As World War II was ending, Liepmann went on to look at transonic and supersonic flow. Pictures he and his students Anatol Roshko and Satish Dhawan took in his laboratory in 1952 were among the first to clearly demonstrate the nature of shock-boundary layer interaction. His spectral theory of transonic buffeting was a seminal effort on a real aeronautical problem. His enduring interest in turbulent flows began with a study of mixing layers, followed by careful experiments by students such as Stanley Corrsin, John Laufer, Roshko, and Donald Coles.

With the advent of the space era, Liepmann’s interests moved toward magneto-hydrodynamics and rarefied gas dynamics. In the early 1960s he worked with Moustafa Chahine and me on the structure of a shock layer, using the Bhatnagar-Gross-Krook model within the shock. That work provided the first solutions that spanned the whole range of flow regimes from the continuum limit to large departures from local equilibrium. Liepmann’s later studies included the fluid dynamics of liquid helium.

From his unpaid postdoctoral beginnings at Caltech in 1939, Liepmann became a professor in 1949 and director of GALCIT from 1970 to 1985, when he formally retired as Theodore von Kármán Professor of Aeronautics. During his time as director, GALCIT emerged as the liveliest center in the world for basic fluid-dynamics research with a distinctive aerospace flavor.

Liepmann’s teaching was legendary at Caltech. His enthusiasm, ability to make nonobvious connections, and insistence on teaching without notes—except for a few “emergency” index cards carried in his pocket—made his classes very special. With Allen Puckett he wrote the path-breaking Introduction to Aerodynamics of a Compressible Fluid (Wiley, 1947). A more formal and definitive volume, The Elements of Gasdynamics , coauthored with Roshko (Wiley, 1957), taught the subject to generations of young students across the world. He also wrote some very readable short reviews that reflected his own incisive views of a subject; an excellent example is “The Rise and Fall of Ideas in Turbulence,” published in 1979 in American Scientist. More than 60 PhD students carried some of the spirit of Liepmann’s approach to fluid dynamics across the US and around the world.

Liepmann loved to tell stories about both big and small people from all over the world. His famous wit owed part of its charm to phrases lifted from “proper” English, delivered in a thick German accent. I recall his shouting down the corridor at a retreating student, “Have fun!” (But he also meant “Better work hard.”) He had strong likes and dislikes within and outside science. Among his favorite global leaders was the unlettered, tough, liberal-eclectic Mughal emperor Akbar the Great; we once went together to see Akbar’s failed capital near Agra, India, to the utter disgust of our taxi driver at being unable to persuade his “ignorant” customers to see the nearby Taj Mahal instead.

Among the numerous honors Liepmann received were the 1968 Ludwig Prandtl Ring of the German Society for Aeronautics and Astronautics, the 1986 Daniel Guggenheim Medal, the 1986 National Medal of Science, and the 1993 National Medal of Technology.

The world of fluid dynamics will miss a respected figure with a sharp mind, who looked at the subject with the critical eyes of both physicist and engineer, upheld the highest standards of research, and inspired hundreds of students to investigate the basic fluid flow problems that lurked behind modern aerospace technology.