Hans W. Liepmann, 94

Hans Wolfgang Liepmann, a pioneering researcher and passionate educator in fluid mechanics, passed away at the age of 94. Liepmann, the Theodore von Kármán Professor of Aeronautics, Emeritus, at the California Institute of Technology (Caltech), passed away June 24 at his home in La Canada Flintridge. Widely honored for his contributions to aeronautics, Liepmann came to Caltech in 1939 and was the third director of Caltech's Graduate Aeronautical Laboratories (GALCIT), from 1972 to1985.

Liepmann, known for his sharp wit and distinctive accent, was a noted teacher who mentored more than 60 PhD students and hundreds of undergraduates during his career at GALCIT. Through his students and colleagues at GALCIT, Liepmann was highly influential in spreading the fundamental research approach and rigorous curriculum of GALCIT. His students became leaders in the aerospace industry as well as universities around the world.

Liepmann was born in Berlin in 1914 and grew up surrounded by the political turmoil and liberal Berlin society of the 1920s.  His father—a well-known physician and hospital director—had a passion for the humanities and an abhorrence of mathematics.  Insisting that Hans have a classical education despite his interest in physics, his father nearly ended his son's scientific career before it began.  Looking back, Liepmann once observed, "of my 10 years in high school I can remember no more than maybe three teachers who were more than drillmasters." Those experiences likely contributed to Liepmann's passion for teaching at Caltech.

Liepmann's early years in Berlin came to a close shortly after graduating from high school and a stint in the Siemens factory as an apprentice. His father decided to emigrate following the rise of the Nazi government and the infamous Reichstag fire in 1933. Liepmann joined his family in Turkey in 1934 after his father was invited to be head of the gynecology department at the University of Istanbul. He enrolled in the university to study physics, mathematics, astrophysics, and mechanics. The classes were taught in a mix of German, French, and Turkish, under the numerous German expatriates that found Turkey more welcoming than Germany under Hitler. 

After a year in Istanbul and an unproductive term in Prague, Liepmann traveled to Switzerland and found academic success in the physics department at the University of Zürich. His talent as an experimenter was immediately recognized, leading to an invitation to pursue his doctoral studies on low-temperature physics under Richard Bar.  Liepmann's scientific temperament was strongly influenced by the exciting physics scene of 1930s Zürich and the teaching style of Gregor Wentzel, a student of Arnold Sommerfeld, whom many consider the father of modern physics. Throughout his life, Liepmann maintained the perspective of a physicist and emphasized to his students the importance of a scientific approach.

Liepmann came to the U.S. in 1939 after impulsively expressing an interest in "hydrodynamics" during a drinking party at the successful conclusion of his PhD defense. An offer from Theodore von Kármán led to a research position in experimental fluid mechanics at GALCIT, where Kármán was the first director. Liepmann's first experiments, on boundary layer instability and transition to turbulence, were followed by investigations of various turbulent flows that are relevant to engineering application—a recurring theme throughout his career. With the entry of the U.S. into World War II, he began research on problems associated with high-speed flight, including transonic flight phenomena and interaction of shock waves with boundary layers on aerodynamic surfaces. This also marked the beginning of a longtime interaction with the Southern California aircraft industry. With Allen Puckett, he organized short wartime courses on high-speed aerodynamics for working engineers, resulting in their pioneering textbook, "The Aerodynamics of Compressible Flow." It was followed in 1956 by "Elements of Gasdynamics," with Anatol Roshko, which impacted a broader, mainly graduate-student following, and was translated into Russian, Spanish, and Japanese.

In the rapid expansion of scientific and applied research that followed World War II, Liepmann emerged as a respected and influential contributor to the aeronautics scene and to physics of fluid flow. By 1949 he had advanced to professor of aeronautics at Caltech and had developed a vigorous program of research around his group of PhD students and visiting postdoctoral fellows as well as senior scientists, many of them seeking a change from their work in postwar Europe. Believing strongly that experimental research must relate to theoretical foundations and questions, Liepmann sought association for his group with applied mathematicians, visitors as well as Caltech faculty. An outgrowth of this was the establishment, in 1967, of the applied mathematics option at Caltech. As if tying up loose ends, he was also instrumental, along with Caltech's Amnon Yariv, Martin and Eileen Summerfield Professor of Applied Physics and professor of electrical engineering, and Roy W. Gould, Simon Ramo Professor of Engineering, Emeritus, in the establishment of the applied physics option, in 1974.

The work of his group was distinguished by its innovation in experimental apparatus and instrumentation, often designed for the specific needs of particular problems. Pioneering contributions were made to a wide range of topics that frequently anticipated future technology. These include flow instability and transition, turbulent shear flow, transonic flow, shock wave-boundary layer interaction, turbulent skin friction at supersonic speeds, aircraft buffeting, rarefied gas flow, magnetohydrodynamics, plasma physics, fluid mechanics of liquid helium, chemistry of turbulent mixing, and flow control.

Another strongly held principle was that teaching is vital, even in a research-oriented institution like Caltech.  Throughout his career, up to retirement, Liepmann was devoted to teaching both graduate and undergraduate courses. The enthusiasm, clarity, and teaching effectiveness of his lectures are legendary.
 
In recognition of his accomplishments, Liepmann was elected a member of the National Academy of Engineering and the National Academy of Sciences.  He was a recipient of the National Medal of Technology and the Ludwig Prandtl Ring—the highest honor conferred by the German Society for Aeronautics and Astronautics. In 1986, President Ronald Reagan awarded Liepmann the National Medal of Science.

Liepmann leaves behind sons Dorian, Till, Christopher, and Paul, and two grandchildren. His wife, Dietlind, passed away in 1990.

A memorial service for Liepmann will be announced at a later date.