© Christopher Earls Brennen


``Thank you, Jones. You were in university ? ....''
``Yes, sir.'' .... ``California Institute of Technology. Five semesters completed, A average. I didn't finish.''
``Why did you leave ?''
Jones smiled. ``Well sir, you gotta understand that Caltech is, well, kinda a funny place. I played a little trick on one of my professors. He was working with strobe light for high speed photography, and I rigged a little switch to work the room lights off the strobe. Unfortunately, there was a short in the switch and it started a little electrical fire which burned out a lab destroying three months of data and fifteen thousand dollars of equipment. That broke the rules.''

from ``The Hunt for Red October'' (1984) by Tom Clancy.

I had been working away on my research at NPL's Ship Division for about a year when a professor at the California Institute of Technology (Caltech) came to the lab to visit several of the senior staff members. The professor's name was Ted Wu and he had made a number of outstanding contributions to the published literature on cavitation, several of which I had studied during my Ph.D. research. So it was that I knew of Caltech and, in particular, its contributions to the state of knowledge of cavitation through the work of faculty members such as Robert Knapp, Milton Plesset, Anatol Roshko, and Allan Acosta as well as Ted Wu. I think that my senior colleagues at NPL, felt that Ted might enjoy hearing of my research efforts and therefore made room for me to spend time with him during his visit. I well remember that hour spent with Ted in my office, telling him of my findings and seeking his input. He was clearly very interested and, as he was leaving, expressed the hope that I might visit Caltech at some point in the future. It seemed like a pleasantry and so I did not think much about his remark. However, I had started to think about the next step in my career and so I allowed myself to daydream of the possibility of travelling to California at some point in the future. While I was enjoying my time at N.P.L. I did recognize the need to move onward, to point toward a more permanent and better paid job, perhaps in academia. I had started to apply for a more permanent position at N.P.L. but without much enthusiasm. Then, several weeks after Ted Wu's visit I received the letter that was to change my life. Ted wrote asking whether I might be interested in coming to Caltech for a one-year post doctoral appointment. I almost flew home that day to tell Doreen of this amazing and incredibily exciting development. Ted did also write that he would like me to explore the possibility of obtaining some ancilliary funding from foundations in the United Kingdom and so, before the arrangement could be finalized I spent a number of days in central London going from one foundation headquarters to another exploring the possibilities. In the end only one such opportunity came to fruition. I was awarded a Fulbright Scholarship that paid for my travel to and from California. The money was not much, but the prestige of being a Fulbright Scholar helped greatly to seal the deal with Caltech and with the Office of Naval Research, who were to support my visit through their research contract with Ted. There were many other minor arrangements to be made before we could fly off to a new adventure. Doreen and I managed to scrape together the funds for her airfare and that of the children, mostly by cashing in the retirement funds I had accumulated at N.P.L. We managed to give away the old Hillman car that had been secreted in our garage throughout our stay in Shepperton. We crated up our few household belongings and placed the crate in storage in London in the anticipation of returning to England in about a year (that crate was actually recovered and disposed of about a year later during a brief visit I made to England).

So it was that Doreen, Dana, Kathy and I left Shepperton for good just before Christmas 1968. We travelled back to Northern Ireland to spend Christmas there and make our preparations for our trip to California. On New year's Eve we were taken to Belfast Airport by my father and boarded a flight to Glasgow where we transferred to Prestwick Airport for a night flight to New York. I remember how stormy the weather was and how dark the night for our landing and take-off in Scotland and the trepidation that engendered in me. We were met in New York by some very good friends from our days in Oxford who had emigrated to the United States. They took us to their home in Queens where we were able to get our bearings and to recover from the trip. After a brief visit to friends in Boston, we continued our air journey to Los Angeles, landing in the bright sunshine in the land that was to be my home for the rest of my life (though we little realized that at the time). We had some apprehension about what we would find in California and at Caltech but the welcome we received exceeded our most optimistic expectations. Two of Ted's graduate students, Mike Wilson and Art Whitney, met us at the airport and drove us to Pasadena where Ted and his staff (Barbara Hawk and Cecilia Lin) had arranged an apartment for us. Cecilia had even stocked the fridge with some basic foodstuffs, an extraordinary act of kindness that was typical of her.

* * *

Thus began a fabulous first year at Caltech. In the lab I became deeply involved with the research projects of Ted Wu's group. We examined non-linear effects and devised numerical methods to investigate tsunami waves. In addition I was drawn into Ted's new investigation of the locomotion of small organisms, prokaryotic (bacteria) and eukaryotic (large organisms). Among other discoveries we were involved in showing how and why bacteria use flagella that continuously rotate relative to their heads - unlike eukaryotic propulsion in which a similar propulsive effect is produced by propagating waves along the flagella. In addition to all of these new adventures, I continued my interest in cavitation and conducted experiments on cavitating wedges in the Caltech High-Speed Water Tunnel. All this led to a renewal of my research fellowship for a second year.

Professor Wu's group in 1969: CEB, Prof. Acosta and students in 1975: Phil Engelhauf,
Allen Chwang, Mike Wilson, Art Whitney, CEB, David Braisted, Allan Acosta.
Ted Wu, Ed James.

On the personal side, Doreen, the girls and I enjoyed a much better quality of life than we had seemed destined to have in England. We bought camping equipment and began to explore the southwest, camping at many of its National Parks. Dana and Kathy learnt to swim and spent a great deal of time at the Caltech pool. We made many friends within the Caltech community and began to think of an extended career at that marvellous institution.

In the 40 years that followed I feel I was privileged to enjoy a truly extraordinary career at Caltech. The initial appointment as a Research Fellow expanded into a total of seven years on the research faculty as a Research Fellow (1969-1972), Senior Research Fellow (1972-75) and Research Associate (1975-76). When I look back I recognize that those years allowed me to establish a sound foundation for my scientific career without the obligations of teaching and administration that young tenure-track faculty face. During the first few years I formed close collegial and research bonds that lasted throughout my career, most notably with Allan Acosta and Rolf Sabersky. When I first arrived at Caltech, I was given an office in the Karman Building that adjoined Allan's office. We rapidly became friends and that friendship led to a career-long collaboration in cavitation research and turbomachinery research.

In the 1930s Caltech had begun many decades of leading-edge research into high-speed liquid flows and particularly the phenomenon of cavitation. The very first high-speed water tunnel was designed and built at Caltech by Robert Knapp with the support of Theodore von Karman. Moreover, observation of these high-speed flows was made possible through the development by Albert Ellis and others of remarkable cameras with framing rates as high as a million frames per second. These observations motivated pioneering programs of experimental and theoretical research into cavitation by Blaine Parkin, Milton Plesset, Allan Acosta and Ted Wu among others. It was inevitable that I would become entrained into this inspiring effort and in the years that followed I was able, with the support of the US Office of Naval Research, to pursue many of the leading research questions associated with cavitation. Specific projects included (1) studies of the interactions between cavitation bubbles and the flow and the implications for cavitation noise and damage (2) the population dynamics of cavitation nuclei of microbubbles and the relation to cavitation event rates (3) studies of the dynamics and acoustics of clouds of cavitation bubbles (4) the dynamics of cavitating propellers. The results of these studies and much more were incorporated in my books "Cavitation and Bubble Dynamics" and "The Fundamentals of Multiphase Flow".

Strange Cavitation Bubbles. With Steve Ceccio.

Cavitating Propeller.

In the early 1970s, during the design of the Space Shuttle, NASA became determined to try to avoid an endemic problem that had plagued many earlier liquid-propelled rockets, an instability called POGO after the childs toy of the same name. This problem was characterized by longitudinal oscillations of the rocket that would create pressure and flow rate oscillations in the fuel and oxidizer feedlines to the engine that would then produce an oscillating thrust that would further amplify the longitudinal vibrations. The key to understanding and treating this problem lay in understanding the relation between the flow rate and pressure oscillations in the flow entering the high-speed pumps at the heart of the engine and the oscillations exiting the pumps. At the time little was known of this relation since it was clearly complicated by the fact that the cavitation within the pumps would also oscillate in volume causing instantaneous differences between the entering and exiting flow rates (and pressures). It was essential to understand those differences and to model them in order to devise ameliorative measures. I remember the day when Allan and I flew back to Huntsville, Alabama, at the request of the staff at the NASA George Marshall Space Flight Center to explore whether we could help them with this issue. We soon recognized that progress would require a controlled and instrumented laboratory investigation of the relation between the oscillating pressures and flow rates at inlet to a cavitating pump and the same quantities at discharge, a relation that could be represented by a "dynamic transfer matrix".

Dynamic Pump Test Facility.

S.L.Huang. SSME Model Cavitating.

We proposed to NASA that we would build a facility called the "Dynamic Pump Test Facility (DPTF)" at Caltech in order to measure these transfer matrices on a small scale pump, pumping water rather than liquid oxygen or hydrogen. This would allow us to explore the nature of these transfer matrices and how they varied with the frequency of the oscillations and the extent of cavitation. It was by far the most ambitious experimental facility I had ever been involved in building. Initially we did not even know whether it would be possible to measure the oscillating flow rates with the necessary accuracy in the presence of a highly turbulent flow. In the end we succeeded and the knowledge of the transfer matrices that we obtained allowed accurate prediction of liquid-propelled rocket engine dynamics for the first time. The Space Shuttle thus avoided any POGO instability problems and our expertize continues to be used around the world more than 30 years later. Many other laboratories attempted similar measurements without success before confirmatory measurements were finally obtained in the 2010s using modern computing power and modern instrumentation. When the DPTF reached the end of its useful life at Caltech around the turn of the millenium, NASA disassembled it and transported it back to Huntsville, Alabama. For Allan and I (and the many graduate students who worked on the project) the program had been a great success and the papers we published were among our proudest achievements.

Parenthetically I should add that the approach and instrumentation we used in the DPTF had much value in other technological contexts such as the instabilities that could occur in power-generation cooling systems (including nuclear power systems). For a time I worked as a consultant to the Nuclear Regulatory Agency in efforts to understand instabilities in emergency cooling systems.

Three-phase Flow Facility.Robert Bernier.

The success of the DPTF project motivated NASA to call on our help with another major SSME problem, one that arose during early testing of the engines. The design of the Space Shuttle Main Engine (SSME) pushed well beyond the boundaries of many known technologies. The rotating speeds and operating conditions of the high speed liquid oxygen and liquid hydrogen turbopumps at the heart of the engines were extreme indeed and early testing revealed a whirl instability whose magnitude exceeded expectations and allowable limits. Rotordynamic instabilities in machines rotating at nearly 40,000rpm in cryogenic liquids are clearly very serious in any context and potentially catastrophic in rocket engines. Early on it was suspected that fluid-induced rotordynamic effects might be a contributing factor and yet very little was known of such phenomena for it had historically been erroneously assumed that the fluid would simply damp out the whirl motions. NASA's response to this crisis was to launch several parallel research investigations designed to provide some long-term fundamental understanding of these fluid-induced rotordynamic effects. Dara Childs, who had already done important work on the effects in hydrodynamic seals, was to expand his program at Texas A&M to research that aspect of the problem. At Caltech, Allan and I were asked to examine the effects that might result from the main flow through the pump impeller itself. In the year that followed we built an add-on to the DPTF that helped define a new set of fluid-structure interaction phenomena in turbomachines, namely fluid-induced rotordynamic forces and instabilities. The new add-on facility allowed us to identify and measure fluid-induced rotordynamic forces that can cause instability in high-speed turbomachinery. Rotordynamic coefficients were measured for a wide range of pump designs and operating conditions. In addition, ameliorative measures such as the installation of anti-swirl vanes in the discharge-to-suction leakage path were also identified. This second contribution to the SSME program in particular and to pump dynamics in general represented another major internationally-recognized success and one whose results were used throughout the world.

The results of these two NASA-sponsored research programs received much international recognition. They were included as a large part of my widely-distributed book on the ``Hydrodynamics of Pumps'', a treatise that was subsequently translated into Japanese, Chinese and Farsi. It became an essential reference work for researchers and pump engineers around the world and earned me much international recognition.

At some point in the 1970s both Allan and I moved our offices to the Thomas Building on the Caltech campus. There I also developed a close relationship with Rolf Sabersky who interested me in the brand new research area of the mechanics of granular material flow. As the years rolled on this became an area to which I devoted a significant effort. Experimentally, we studied granular flows in hoppers and down chutes; we also developed computer simulation methods whose results, when compared with the experiments, allowed us to verify and improve our understanding of the grain-level mechanics and modelling techniques. While the knowledge of these flows made imprtant contributions to industrial equipment transporting bulk materials, it also had relevance to many geological processes. Thus we became involved in several geological investigations including the intriguing phenomenon of ``booming dunes''.

Hydraulic Jump of Granular Material.At the "booming" Dumont Dunes.

In working with Allan Acosta, I believe I had the privilege to work with the best fluids engineer in the world. Somehow we managed a chemistry that was exceptionally productive. Allan is also a great friend who helped me survive the tragedies life threw at me. I have also been singularly fortunate in my younger colleagues and have so enjoyed my work with Melany Hunt and Tim Colonius among others. Another great group of academic friends were the young post-doctoral researchers who made extended visits to Caltech among them Yoshi Tsujimoto and Kenjiro Kamijo from Japan. Yoshi in particular became a great, lifelong friend with whom I shared many adventures. Next to these faculty colleagues, I feel deeply blessed that I had the honor and privilege to work with an outstanding group of graduate students as well as some undergraduates. About 35 young people obtained their PhDs under my supervision and most of them have gone on to outstanding careers. Quite a few are now professors at institutions around the world including MIT, University of Michigan, University of Southern California, Purdue University, Cambridge University, University of Pisa, Yeditepe University in Istanbul, National Cheng Kung University in Taiwan, National Autonomous University of Mexico, and Keio University in Tokyo. Others have become movers and shakers in companies like Chevron, Schlumberger, Boeing, TRW, Exxon Mobil and Dow Chemical. Others have made major contributions to small companies and start-ups and several have worked at government laboratories including Sandia National Laboratories and the Institute for Defence Analysis. One is now an orthopaedic surgeon and another became a NASA astronaut. I have kept in touch with almost all of these academic children and take great pride in their accomplishments. One of great side benefits was the opportunity to get to know many of these very talented young people outside the classroom and to have their company on some marvelous wilderness adventures. To all of these intellectual explorers I am deeply grateful for their company on so many adventures.

Hiking with graduate students. Left: Carl Fischer, Tricia Waniewski, Carl Wassgren,
Garrett Reisman, Sudipto Sur, CEB and Frank Liu.    Right: Garrett Reisman,
Tricia Waniewski, CEB and Fabrizio d'Auria.

My early research accomplishments eventually persuaded Caltech to appoint me to the tenure-track and in 1976 I became an Associate Professor of Mechanical Engineering. Thus I was never an Assistant Professor, something that would be most unusual today. Later I was given tenure and promoted to full Professor in 1982; and finally, made the Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering.

Reading names at graduation. Urging restraint after 9/11.

In parallel with these faculty appointments, I also served in four different positions in the Caltech Administration, and I tell the story of these in a different chapter. The first of these four year appointments was as Master of Student Houses, a position I held from 1983 to 1987; then, after a one year break I served as Dean of Students from 1988 to 1992. After these eight years interacting with the students I was fairly certain that was the end of that phase of my career. I returned to the Mechanical Engineering Department where I served as the Executive Officer for Mechanical Engineering from 1993 to 1997. However, with the arrival of a new president, David Baltimore, I was persuaded to rejoin the administration as the Vice-President for Student Affairs, a position I held from 1997 until 2002. During my years in the administration of Caltech I had the opportunity to meet some extraordinary people, not just remarkable scientists but also visitors from all walks of life and corners of the globe. It gave me particular pleasure to host two remarkable men from near the village of Magherafelt where I grew up, the 1995 Nobel Prize winner for Literature, Seamus Heaney, and the 1998 Nobel Peace Prize winner, John Hume.

Tsujimoto-sensei, Osaka University. Doreen and I with Seamus Heaney.

Last updated 8/1/01.
Christopher E. Brennen