Another lecture from a “turbulent” mechanicist – Grigory I. Barenblatt

Again from iMechanica.org (http://imechanica.org/node/198) and this time the Timoshenko medal lecture of Barenblatt

… great mentor Andrey Nikolaevich Kolmogorov, whose name is known to everybody in this audience, said: “I have lived being guided by a principle that the truth is a blessing, and our duty is to find it and to guard it.”

and the anecdote is:

obviously the elegant lady who starts and finishes her days by using the flow in pipes should be interested in such work. And I did my best to present our results in the short time given to me. The Beauty – Consul – looked at me (with her wonderful dark blue eyes!) and said: “Professor, of course, what you said is interesting, even exciting. However, frankly speaking, I am astonished. When we have some problems with pipes, we address a plumber, not a professor with a world-wide reputation!” I was ashamed, and up to now I have a feeling of personal guilt. Indeed, now we know the structure of remote stars better than the strength of a shuttle or a dam and contrary to astronomers and astrophysicists how little we do to explain in particular to younger generations the fundamental depth and beauty of our profession and to popularize it.

However, the common opinion of the layman, even scientific and engineering laymen, is that nowadays there are no such problems of the scale of the Manhattan Project whose importance for the nation and the world is understood by everybody. This is deeply wrong! Such problems do exist, and they can be understood by everybody. First of all, among these problems are large-scale natural disasters, and energy problems. I will present several examples.
Tropical hurricanes. The scales of these disasters are huge, and the morale and material losses are formidable. I want to emphasize here that in fact hurricanes present a fascinating problem of applied mechanics. And, in general, Sir James Lighthill, one of the first winners of the Timoshenko Medal, considered natural disasters, in particular, hurricanes, as problems of first importance for applied mathematics and mechanics.

As far as hurricanes are concerned, the situation is as follows. As a preliminary note, I want to mention a simple calculation, by which A.N. Kolmogorov started his course on turbulence at Moscow State. He asked the listeners: What will the velocity be at the surface of the river Volga in Russia (close by its parameters to the Mississippi in this country) if by some miracle it becomes laminar ? The answer was striking: hundreds of thousands of miles per hour! Why then is it kept so slow ? The reason is that the flow is turbulent: it is stuffed with turbulent vortices, and these vortices play the role of brakes, slowing the flows. An analogy: moving along mountain slopes, drivers use chains to cover the wheels – the vortices play the role of such chains.

Our group (Professor A.J. Chorin, Dr. V.M. Prostokishin and myself) considered, under some natural assumptions, turbulent flow of ocean spray. The general theory of turbulent flows carrying heavy particles, developed earlier by A.N. Kolmogorov and myself, at that time his graduate student, was used in this consideration. It happened that the droplets reduce turbulence intensity, because turbulent vortices spend a significant part of their energy for suspension of droplets. Returning to the analogy with wheel chains – the chains that are worn out become weaker. The flow accelerates under the same pressure drop. Our calculations showed that this acceleration can be very large, reaching velocities of large tropical hurricanes.

Let me for a moment to enter a word about dilute polymer solutions – I have not theory (yet) but the principle is similar – the ‘chains’ spend a great part of their energy on ‘stretching’ the polymers and therefore become weaker.

And I didn’t know this story – amazing:

Another very important matter. I think that an honest analysis, deeply based on scientific consideration of natural and technogenic disasters can be not less but very often more exciting and important than great projects like Manhattan and all these cosmology and particle acceleration enterprises. There is a difference. Money, and even Big Money, cannot prevent such analysis. But Very Big Money plus politics can do it, and in these cases a chain reaction of disasters started. An example: “Titanic”. In 1913 fundamental engineering and scientific analysis of this disaster was not performed; only much later it was understood what had happened there – the temperature was lower than the temperature of the steel embrittlement, and the vessel’s body became brittle. Twenty-seven years later: 24 May 1941 at 5:52 a.m. the HMS battle-cruiser “Hood”, the flagship of the fleet chasing the German battleship “Bismarck” made a first volley. “Bismarck” answered by a shot of a small antiaircraft gun. And at 6:00 a.m. “Hood” sank; fifteen hundred people perished, only four were saved! (The steel was supplied by the same firm as for the “Titanic” .) Thinking about this case I was astonished: 24 May, spring – it should not be cold! But read Volume I11 of Churchill’s “The Second World War” – 24 May was an extremely cold day at the place.. .clearly the temperature of embrittlement again was crossed. And again: no competent engineering and scientific analysis! Only later when the welded “Liberty” ships started to break in two halves in the North Sea (tens of thousands of people perished), such analysis was performed, and Fracture Mechanics was created. George Irwin, later a Timoshenko Medal winner, was the leader. I also participated in this work. Fracture Mechanics is now as a charming lady in her forties: a remarkable past and a lot in the future. A wonderful branch of mechanical engineering and applied mechanics! Each fracture surface can tell you a lot about both the material and the loading: those who are really interested in what happened can achieve it (of course, only if they will be allowed to obtain the fractographs!)!

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