Chapter 4 Molecular Tagging Velocimetry and Thermometry

The advent of the laser as a relatively common tool for flow visualization has stimulated the development of velocimetry techniques based on the use of photosensitive molecules. While a variety of different molecules have been employed for this purpose, they share the common attribute that laser excitation is used to produce a defined pattern of long-lived tracers which are embedded within the flow field. This process is often referred to as “tagging.” After a suitable time delay, a CCD (or other) camera is used to obtain an image of the displaced pattern (often termed “interrogation”). The observed displacement divided by the elapsed time is assumed to constitute a measurement of vector velocity field. Dependent upon the details of the optical processes and/or the nature of the tracer, the technique has alternately been referred to as Laser-Induced Photochemical Anemometry (Falco & Nocera, 1993), Flow Tagging Velocimetry (Lempert, 1995) and Molecular Tagging Velocimetry (Gendrich, Koochesfahani & Nocera, 1997). As will be shown, the properties of available photo-sensitive materials vary quite significantly, albeit it in somewhat subtle ways. The purpose of this chapter is to provide a framework sufficiently detailed to enable potential users to effectively match the diagnostic to their particular measurement environment. To avoid confusion, we shall adopt the terminology of Gendrich & Koochesfahani (1996), and use the term Molecular Tagging Velocimetry (MTV) to encompass