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Program on Physics of Hydrodynamic Turbulence (Jan 31 - Jun 30, 2000)
Coordinators: P. Constantin, I. Procaccia, B. Shraiman, and K. Sreenivasan

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Water flowing from a slightly open faucet is smooth and steady. As the faucet opens up more, the water flow transforms itself into a turbulent state: one sees an abundance of swirling eddies distributed irregularly in space, interacting with each other and evolving in a complex manner.

Turbulent flows are common in technology. Without turbulence, the mixing of air and fuel in an automobile engine would not occur on useful time scales; the transport and dispersion of heat, pollutants and momentum in the atmosphere and the oceans would be far weaker; in short, life as we know on the earth would not be possible. However, turbulence also has several undesirable consequences: it enhances energy consumption of pipelines, aircraft and ships, and automobiles; it makes an air traveller occasionally queasy and worse; it distorts the propagation of electromagnetic signals; and so forth. A major goal of a turbulence practitioner is the prediction of the effects of turbulence and control them---suppress or enhance them, as circumstances dictate---in various applications such as industrial mixers and burners, nuclear reactors, aircraft and ships, and rocket nozzles.

While the practical importance of turbulence has long been appreciated, less well appreciated has been its intellectual richness and the central place it occupies in modern physics. Looking into the problem, we are immediately faced with an apparent paradox. Even with the smoothest and most symmetric boundaries possible, flowing fluids---except when their speed is very low---assume the irregular state of turbulence. This feature, though not fully understood, is now known to bear some connection with the occurrence of dynamical chaos in nonlinear systems. In fact, until the 1960's, turbulence was the paradigm system in which the excitation of many length scales was recognized as centrally important. The powerful notions of scaling and universality, which matured when renormalization group theory was applied to critical phenomena, had already manifested in turbulence a couple of decades earlier. Turbulence and critical phenomenon share the feature that a continuous range of scales is excited in both; however, they are different in that the fluctuations in turbulence are strongly coupled and there exists no small parameter. It is a paradigm in nonequilibrium statistical physics, in which fluctuations and macroscopic space-time structure coexist. It is an example like no other of spatially extended dissipative systems.

It can thus be said that turbulence is central to flow technology as well as modern statistical and nonlinear physics. However, the problem has not yet been mastered despite serious scientific study for over hundred years. Much qualitative, and very useful, progress has been made but large gaps exist in our understanding. The subject is at once very old and very new.

The diverse clientele the subject enjoys---such as astrophysicists, atmospheric physicists, aeronautical, mechanical and chemical engineers, to name but a few---has different needs and espouses correspondingly different approaches and emphases. This makes it difficult to mount a focused frontal attack on a single aspect of the problem. It has also often made the communication among the different segments of the community somewhat difficult. These aspects have compounded to some degree the extraordinary complexity already inherent to the subject. Fortunately, the rate of progress in the subject has increased in recent years, chiefly due to the increasing interaction among experiment, theory and simulations.

Arising from such considerations, ITP has initiated a broad-based Program with the following premises: that the Program can help accelerate progress by encouraging serious dialogue among physicists, mathematicians, engineers and other practitioners of the subject; that research on the fundamental and practical aspects of the problem would benefit from each other; that the influx of talent into the subject from neighboring areas would be beneficial overall. Its focus at different times will be on different subtopics, but an integration of experiment, theory and simulations is an intended goal.

While more details of the Program will be announced shortly, some of its elements are the following: It is expected that the Program will last between January 31, 2000 and June 30, 2000. It intends to incorporate two major conferences. It will house long-term and short-term visitors, and encourages all forms of collaborations; both senior and junior scholars are encouraged. ITP provides office and computing facilities on its site at UC Santa Barbara, overlooking the Pacific Ocean, and also provides living accommodations. Some level of financial support will be available, depending on the needs of the participants and the overall availability of funds.

Two other concurrent Programs of potential interest are "Astrophysical Turbulence" and "Galaxy and Evolution."

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For further information, please contact: Sharon O'Ran.