It is for me both a pleasure and an honour to write a Preface to this special edition of the Journal of Turbulence, celebrating Javier Jiménez 70th Birthday and his outstanding contributions to Fluid Mechanics as a Researcher and Professor. His very successful research and teaching work has been devoted not only to the basic aspects of Fluid Mechanics, but also to the most important and difficult problem of the multiscale analysis of turbulent flows. He has introduced seminal ideas, very deep and influential, which appear reflected in this Issue; and as an exceptional Teacher he has attracted an impressive number of very bright students and collaborators to work with him in his research projects, to follow later very successfully careers in research or in business.
In his analysis of the turbulent flows, he began using the available high-speed computers for pattern recognition techniques, in which he played an important role in their development, for many areas of research. He obviously used also the techniques to uncover the evolution of the large-scale structures of the turbulent mixing layers, based on the experimental results obtained by high speed photography. He has been using later these techniques, together with direct numerical simulations, with high speed computers and sophisticated analytical tools, to uncover the dynamics of the large and small scale structures of the homogenous turbulence and in the various regions of channel and boundary layer turbulent flows.
Let me now turn my attention to my privileged view of his less known early development as a researcher and teacher of Fluid Mechanics. My association with Javier Jiménez initiation and later work on Fluid Mechanics began when he took my one year course, where I stressed its important multiscale aspects. I was then a young Professor at our School of Aeronautical Engineering, and had the privilege of dealing with very bright students, where Javier was certainly a towering figure, in a class that participated actively without accepting any unjustified statement. Javier was a quiet observer most of the time, but I was impressed with his own unexpected ways to deal with the topics under discussion. The strong personal relation that we maintain began before the course end.
This time, around 1966, was an optimistic time everywhere, with drastic changes occurring in Spain. Our students participated actively, without leaving aside their studies and our seminars, to show that they wanted a change in our political system. Javier Jiménez and César Dopazo also organized a theatre group at our School, with Javier as director and César as an actor and musician. I had mixed feelings with this dispersion of interests of the students, enhanced when Javier chose as a final year project, the preliminary design and viability of a space probe to explore Jupiter.
Fortunately, when he finished in 1969 he got a NASA-ESRO Fellowship to follow graduate studies at Caltech; first at the Guggenheim Aeronautical Laboratories for a Master degree in 1970, and then at the Department of Applied Mathematics, linked to the Galcit, for his PhD thesis of 1973. There he worked, with the supervision of Gerald Whitman, on how to account for small viscous effects on Whitman's mean Lagrangian method for non-linear wave propagation in conservative systems. The second part of his thesis dealt with the theoretical and experimental evaluation of the amplitude and structure of the nonlinear acoustic waves in a tube, when excited by oscillations of a piston close to resonance frequency, so that shock waves were generated.
Javier Jiménez was preceded one year at Caltech by Manolo Rebollo, also a former student of our School, who was working under Anatol Roshko, on the experimental analysis of the turbulent mixing of two parallel streams of nitrogen and helium with argon. This project, of Brown and Roshko, was aimed at uncovering the reasons for the difficulties encountered in the supersonic combustion of hydrogen and hot air; a subject in which I was also working at INTA with AFOSR support. Manolo Rebollo developed an aspirating probe to measure the strong oscillations, in time and space, of the mass fractions in the mixing layer; he was also in charge of the visualization, without particle seeding, with shadow photographs at 8000 frames per second. The resulting observations were astonishing, showing a series of vortices with size growing with distance to the splitter plate, a consequence of the pairing of neighboring vortices. I visited often Caltech to follow the work of my former students, and thus was early witness of the discovery and of their enthusiasm when showing me the photographs of the mixing layer with the unexpected coherent structures, so important for combustion applications. Clearly this discovery led Javier Jiménez to accept the invitation of Caltech to extend his stay to work for one year in the analysis of these structures. Thus, turbulence became the main topic of his research work.
When, after his PhD, Rebollo returned to Spain, he was appointed in 1974 as director of a division of the recently created IBM Scientific Center at UAM, Universidad Autónoma de Madrid. The division was devoted to the computer analysis of massive experimental data and images. He asked Javier to join him in his division. Their task was to identify and help research groups in universities and research centres with tasks in which digital imaging, and their computer analysis, could be useful. As an example, Javier Jiménez identified a biology group of UAM, and helped them to reconstruct the image of a virus, which appeared in the cover of Science 232, 113-5 (1986), from the information obtained by electron microscopy.
Rebollo and Javier were named part time Professors in our School and, in addition to teaching turbulence and applied mathematics, set up a hydrodynamic channel with a PhD student R. Martínez Val, assigned to them, to analyse the mixing layer between two parallel streams of water with slightly different temperatures. With water they could reach the important high Reynolds numbers with moderate velocities, and measure the local temperature fluctuations with small hot plate sensors. The signal from the sensor was digitized, with the electronic equipment that they designed and built, to provide 50,000 data per second; which they stored to be later analysed at the IBM Center with their computers.
The pattern recognition techniques were adapted by Jiménez with his students Marta Cogollos and Miguel Hernán, for their seminal analysis of the coherent structures using the high speed films obtained by Rebollo at Caltech. Miguel went as a post-doc to Caltech, although soon ended at JPL as director of the Image Analysis Lab. There he had to deal with the reprogramming of the back-up computer of the Voyager II, travelling from Jupiter to Uranus and Neptune, so that they could retrieve efficiently the information gained when passing close to these planets; storing in the computer memory the photographs taken then, to be sent later at slow pace to the Earth.
During the stay, until 1986, of Javier Jiménez at the IBM Center he used with advantage the scholarships offered by IBM to attract very bright students for their PhD work with him. Thus, Roque Corral developed with Javier multi-grid methods for transonic flows; he was later hired by ITP for the use of advanced numerical techniques for turbine design. Juan Antonio Hernández was involved in the instability of multiphase flows, later a professor at our School. Other students began working with Javier as undergraduates, like Juan Carlos Agüi and Juan Zufiría. The first, to work on the reconstruction of the flows based on tracking the particles seeded in the gas for its visualization. He later went to Stanford to obtain a PhD on development of holographic techniques for three-dimensional flow visualization. Zufiría worked with Javier in the analysis of the flow under tectonic plates; and was also advised by Javier to go to Caltech for his PhD, to work under Philip Saffman on the Rayleigh-Taylor instabilities that plague the implosion stage in the development of nuclear fusion by inertial confinement. Like Agüi, Zufiría returned to enter IBM; where Zufiría is now Vice-president of IBM International.
An important change in the relation of Javier with our School and with the international turbulence community occurred in 1986, when we were able to bring him full time, using the special Chairs created to attract distinguished researchers to the universities. The idea was to facilitate the growth of their research effort, with positions requiring only research and teaching at the graduate level. Javier got one of the Chairs, which increased significantly the number of his PhD students from Spain and abroad.
Since then, he was able to accept his nomination as a Research Scientist at the CTR, of Stanford University and NASA-AMES, authorized by our University, for his extended summer stays, as well as his appointment as part-time Professor by the Ecole Politechnique. These appointments and, what is more important, the stubborn, rest-less, and enthusiastic way of carrying out his research work by Javier Jiménez and his students and collaborators, had led to significant progress in our understanding of Turbulence, which still needs their important contributions.