Preface – Optical Methods for Data Processing in Heat and Fluid Flow

Preface

It is difficult to overstate the importance of metrology in science, it provides the data on which theories are accepted or rejected and can force a rethinking of ideas formerly held true for many centuries. This book contributes to the science of measurement and is concerned with the use of optical techniques and data processing to quantify fluid mechanic and heat transfer properties as well as closely related topics. The range and diversity of fluid and heat applications necessitates a correspondingly large range of measurement techniques and devices, some of which are not optically based but in the current context provide useful data for comparison purposes. Examples of both optical and non-optical techniques and devices include, thermocouples, strain-gauges, pitot-static tubes, piezoelectric transducers, infrared thermal imaging, laser Doppler velocimeters, hot-wire probes holography, schlieren, nuclear magnetic resonance, and condenser microphones.

The rapid development of these techniques over the last few decades ensures the list of methods continually increases and, of course, provides the raison d'etre of this current collection of Chapters. Optical methods have been used for centuries to qualitatively examine flows. The sketches of Leonardo da Vinci capturing the three-dimensional swirls of flowing water provides proof of this in a most aesthetically pleasing way. Why then the huge increase in fluid and heat measurement over the last three or four decades? The advent of sophisticated numerical techniques, particularly in the area of image processing and the rapid advances in the associated hardware, have undoubtedly served as a catalyst. However, prior to this, the development of the laser and laser technology was perhaps even more fundamental.

In so many optical measuring techniques applied to fluid and heat measurement: laser Doppler velocimeters, laser speckle techniques, particle tracking velocimeters, holography, fluorescence-based techniques, and infrared techniques, the laser is a fundamental tool. It is perhaps initially surprising that the use of this ubiquitous tool in fluid measurement came about in an almost inadvertent way. Cummins, Knable, and Yeh (1) were investigating the Brownian motion of a colloidal suspension by observing the broadening of the laser light spectrum. In doing this they observed a net shift in the frequency of the light, generated by small convection currents set-up in their measurement volume. Yeh and Cummins (2) then proceeded with this concept, directly using it to measure fluid velocities and hence the field of laser Doppler velocimetry (LDV) was born. The fact that this was a non-intrusive technique, it provided an unambiguous measurement of one or more of the components of the velocity vector and it was largely unaffected by the thermophysical properties of the fluid ensured its rapid growth in the fluid measurement community.

Almost in parallel with this, starting with Burch and Tokarskii experiments (3) on fringe formation from multiple scattering sources in 1968, speckle metrology developed, and over the next two decades the field grew and subdivided, creating several new measurement techniques such as particle image velocimetry (PIV) and dynamic speckle methods. More recently with the advances in CCD technology and computer hardware, digital PIV, DPIV, and holographic PIV, HPIV have emerged.

Processing the raw experimental data has also seen dramatic changes in the last few decades. For example, in early studies using PIV, the data recorded on photographic negatives was optically correlated based on a Young's Fringes method. This has now largely been replaced by numerical correlation schemes applied directly to the raw data retrieved from CCD sensors. The development of sophisticated analysis schemes has now become a large research field with the difference in the methods of numerical analysis often largely differentiating between schemes. For example PIV and particle tracking velocimetry (PTV) can use identical recording techniques however the analysis differs. In PIV a statistical correlation is performed yielding an average particle displacement, whereas particle tracking attempts to identify and track individual particles from frame to frame. These analysis techniques are increasingly borrowing from the fields of image processing and are used, for example, to differentiate between the phases in multiphase flows, to examine droplet formation, and to analyse flame phenomena. The harnessing of optical phenomena for heat and temperature measurement continues apace ranging from infrared emissions to the temperature dependence of the phospherence of certain materials. This collection brings together a selection of Chapters detailing some of the latest advances in the many differing aspects of laser measurement.

Section 1 is concerned with investigations using measurement techniques based on the Doppler principle. These include methods of improving signal analysis and velocity measurements of rotating and transonic flows as well as flows in combustion engines. An interesting application of phase Doppler anemometry (PDA) to the sizing of droplets in a combustion process is detailed. The laser speckle techniques of Section 2 indicate how these are playing increasingly useful roles in the important field of medical diagnostics. Holographic methods are also presented here with the very interesting application to the automated classification of small marine organisms, as well as the use of holography for the measurement of the three-dimensional positions of particles. In Section 3 an indication of the ever broadening use of techniques, based on fluorescence and phospherence, is given with applications to pollutant dispersion due to ocean wave action, an excellent Chapter on state-of-the-art temperature measurement using thermographic phosphor thermometry and the use of fluorescence in the studies of acoustic phenomena. Two interesting liquid crystal applications are also included in this section. It becomes evident from a perusal of Section 4 that PIV is now well established as a powerful velocity measurement technique in the engineering world with applications to the wake and vortex formation of helicopters and ships. The final section of the book indicates, perhaps the most difficult, problems from a measurement point of view and is titled by flow type, i.e multi-phase flow, rather than measurement technique, since in these challenging flows more the one technique, either on the optical or the analysis side, has to be employed to separate the phases. Thus Section 5 includes a gas–liquid mixing application that combines PIV, LIF, and mie-scattering diffusion, image processing techniques to enhance flame visualization, detonation, turbulence, combustion applications, and mass transfer measurements.

(1) Cummins, H. Z., Knable, N., and Yeh, Y. ‘Observation of Diffusion Broadening of Rayleigh Scattered Light’, Physical Review Letters, Vol. 12, pp. 150–153, 1964.

(2) Yeh, Y. and Cummins, H. Z. ‘Localized Fluid Flow Meeasurements with a He-Ne Laser Spectrometer’, Applied Physics Letters, Vol. 4, pp. 176–178, 1964.

(3) Burch, J. M. and Tokarskii, J. M. J. ‘Production of Multiple Beam Fringes from Photographic Scatterers’, Optica Acta, Vol. 15 (2), pp.101–111, 1968.