tant amplitude. A feedback circuit (a phase where material probes cannot endure, e. g. bubbles in water, where the velocities of locked loop) adjusts frequency and phase in flames, explosions, combustion, acids. both components may be determined sepa­ of this oscillator to track that of the receiv­ Besides the optical systems discussed — rately, flames (with graphite particles), ed photodetector signal. This involves reference beam, dual focus, cross-beam — sediment transport in rivers and oceans, some inertia to overcome intervals of In­ there are several others, Including tele­ powdered materials in air and others. adequate received signal, but a continuous scopic with up to 1 km range and glass Another field is the study of turbulence output is available in terms of the Doppler fibre systems capable of measuring inside which is now possible with a higher degree frequency and analog voltage. blood vessels. Other refinements in optical of sophistication. An area that has at­ systems allow the flow direction to be dis­ tracted the curiosity of many physicists is Very Few Photons: the Digital Cor­ tinguished (referring to equation (4), posi­ that of hydrodynamic instabilities4).In relator tive and negative ωD cannot be distinguish­ some well-defined geometries, the transi­ The Digital Correlator works well even ed experimentally) and the simultaneous tion from laminar to turbulent flow occurs with very low received light levels, fairly ac­ measurement of two or three components. via several intermediate modes of fluid curate velocity estimates being obtained Velocities measured range from 0.1 µm/s flow, characterized by well-defined fre­ from as little as 50 detected photons. A fur­ to 1000 m/s. Once assembled, equipment quencies and wave numbers. A first step ther advantage is its "democratic" opera­ works reliably and can be operated by un­ towards interpreting these phenomena is a ting principle, i.e. all received photons are skilled labour. theoretical model due to Landau which is treated alike. Hence statistical and informa­ On the other hand, further development formally analogous to the mean-field theo­ tion analysis may be applied to obtain the may still be required if one wants to ry of continuous phase transition due to limiting accuracy of results. By compari­ measure at very large/very small dimen­ Landau and Lifschitz. son, threshold devices which are used in sions, if extremely high/extremely low the counter and tracker introduce a degree velocities are expected, if high accuracy of arbitrariness, e. g. in the level in (relative error smaller than 10-3) or good BIBLIOGRAPHY threshold devices, which makes a similar data on the statistics of turbulence are ask­ 1. Durst F., Melling A. and Whitelaw J.H., Prin­ assessment of results difficult. ed for, or if flow at not easily accessible ciples and Practice of Laser Doppler Anemome­ Photomultiplier pulses, each one repre­ locations is to be studied. Examples are try (Academic Press) 1976. senting a single detected photon, are con­ measurements in the boundary layer at the 2. Durrani T.S. and Greated C.A., Laser Sys­ verted to pulses of standard amplitude and wing of airborne aircraft, near the propeller tems in Flow Measurement (Plenum Press) duration, and the number registered in of a ship while moving on the ocean, or 1977. equal time slots is recorded. Using a shift between blades of high-speed turbines. 3. Cummins H.Z. and Pike E.R., (Eds.) Photon register delay line, many multipliers and Not surprisingly, laser Doppler anemo­ Correlation Spectroscopy and Velocimetry (Ple­ stores, the digital, discrete, truncated form metry has revitalized several areas of fluid num Press) 1977. of the intensity autocorrelation function is mechanics research. Besides the topics 4. Swinney H.L. and Gollub J.P. "The Transi­ accumulated in the stores. If I(t) is light in­ mentioned already, it provides a means of tion to Turbulence", Physics Today 31 (1978) 8, tensity, the function so obtained is, except studying two-phase flows, for example p. 41. for unessential factors, an estimate of the autocorrelation function : (9) A Breadth of Vision By the Wiener-Khintchine theorem, the Fourier transformed of G(τ) is the spec­ Wolfgang Gentner, founder member of — tracer methods, thermally induced pho­ trum. In the present case, the spectrum EPS and member of the first Executive toluminescence, track sensitive methods, contains a maximum at the most prominent Committee, has died at the age of 74. A etc., to the solution of archaeological Doppler frequency ωD. In addition, there is keen promoter of European co-operation in puzzles. He was able to trace the origins of instrumental broadening due to the finite physics, Gentner's influence has been felt the silver to be found in old coins and by width of laser beams and possibly further in the development of a number of Euro­ applying tracer techniques to identifying broadening due to velocity fluctuations, pean scientific collaborations. the origins of pottery, proved that trade in i.e. turbulence. Basically a nuclear physicist, he was one ancient times was much freer than had A trade-off is possible between measure­ of the first in the Federal Republic of Ger­ been supposed. ment time and accuracy. For best ac­ many to look towards higher energies, star­ With his breadth of interests, he was an curacy, one uses a long accumulation time, ting the first cyclotron there of 20 MeV at energetic promoter of the inter-disciplinary perhaps many seconds, to overcome the Heidelberg. He was a vigorous supporter of and cultural aims of the European Science effects of digitalization and genuine noise. CERN and for five years headed the Divi­ Foundation. He was closely involved in the On the other hand, fair estimates of veloci­ sion that brought the CERN 600 MeV foundation of the Laue-Langevin Institute ty may be obtained after short accumula­ sychro-cyclotron into operation and laun­ in Grenoble and equally enthusiastic over tion, e. g. over a fraction of a microsecond. ched the programme of physics research. the creation of Emblab. He had special con­ A number of algorithms exist to extract He served on the CERN Scientific Policy nexions with Israel and was his country's velocity information out of correlograms in Committee and as delegate to Council, be­ representative on the Minerva collaboration various circumstances. This is not restric­ ing elected President from 1972-74. His with Rehovot. ted to average velocity but includes higher particular interest was in highly unstable Founder of the Max-Planck-Institut fur moments of the velocity distribution as nuclei and excited states. Kernphysik in 1958, he was until 1972 its well. He was also interested in geology and in Executive Director. In all his activities, he the isotopic concentrations of matter and was particularly well-known for his en­ the clues they give to the origin of the couragement of young physicists, open to State of the Art Universe. With his collaborators, he was the unconventional approach and welcom­ Laser Doppler anemometry offers a non- the first in Europe to study moon rocks. He ing personalities of widely different intrusive method of measuring fluid veloci­ became an expert on dating and was active background. His breadth of vision will re­ ties, suitable for hostile environments in introducing modern physical techniques main an inspiration. 11