Laser Strophometry

Abstract

This is an excellent small book with an unfamiliar title. The word `strophometry' was certainly unknown to me and it would have been interesting to know its origins. However, the sub-title `High Resolution Techniques for Velocity Gradient Measurements in Fluid Flows' says it all. The book is part of the Springer series on Lecture Notes in Physics and, in some sense, this dictates its style, which is generally clear, complete and informative. There is a brief Introduction that lays the foundations of the method, though I would have liked this to be expanded with a little more explanation. Although it is hinted at, it is only in Chapter 4 that the physical basis is clearly revealed. Essentially this is that velocity differences between particles cause phase variations in their scattered light contributions. This results in `boiling' of the speckle pattern reflected by the particle cloud. The velocity gradients are revealed through the rate of change of the speckle pattern, and in turn are quantified via correlations in the scattered light. Such an explanation would have clarified the path taken by the book. The second chapter sets the scene by explaining why velocity gradients are important with a good, brief discussion of fluid mechanics. It then reviews previous methods for measuring them. Inevitably these involve the determination of velocity at two points separated in space and taking the difference. This limits the spatial resolution of these techniques. The potential advantage of strophometry is that it measures gradient directly and is capable of very high resolution. Beyond this point the book sets about its teaching task with an excellent chapter on the mathematical background. This covers statistics, correlation functions and the necessary properties of tensors in a very compact and readable way. This chapter would be a valuable aid even in isolation. The next two chapters set the physical basis of the method, describing the behaviour of the speckle pattern and the foundation and requirements of the basic experimental method. These are followed by three chapters describing particular techniques that have been applied and their results. The requirements of the experiments are discussed carefully, though they do leave a feeling that they are not necessarily very easy to apply. No doubt development will take care of this and we may well see instruments on the market at reasonable cost in years to come. Finally, there is a chapter on correlations between velocity gradients and applications to turbulence. In summary this is an interesting and well-written work on a potentially powerful measurement tool. It contains a number of short reviews that are valuable in their own right, and is a valuable addition to the literature. A R Jones