Other Techniques

Schlieren and shadowgraph are the other two techniques besides interferometry, that  can be used to form images of the thermal field using refractive index changes. The advantages of these methods are a simpler optical configuration, better spatial resolution since the image is an intensity distribution rather than fringes, and suitability for large thermal gradients. The disadvantage arises from the fact that the intensity variation is related to the derivatives of the refractive index field, rather than the refractive index itself. Specifically, a schlieren system can capture the temperature gradient, while a shadowgraph yields the second derivative of the temperature distribution.  These distributions can be evaluated in three dimensions, once again by using principles of tomography. The temperature field in principle, can be determined by integrating the gradient field. In practice this can be cumbersome and these two methods continue to be in use primarily for flow visualization. In some problems, the temperature gradient itself may be primary quantity of interest, for example the local Nusselt number on a solid wall. The schlieren image generates a visual picture of the local heat transfer rates, and hence, on occasion can be a superior measurement technique in comparison to interferometry. A review of schileren nand shadowgraph techniques can be seen in the edited books of Goldstein [4] and Mayinger [6].
The application of the schlieren  technique to problems of practical importance have been reported by the following authors. Kosugi  et al. [70] have experimentally recorded gas temperature profiles in the shock region of excimer lasers and correlated them to the xenon concentration in a helium gas. Koreeda et al. [71] have studied shock structure in air at very high Mach numbers (up to 35) using schlieren signals recorded by a split photodiode.  Agrawal et al. [72] have developed a color schlieren technique coupled with tomography for measurement of temperature in gas flows. Bystrov et al. [73] report extraction of the density and temperature fields in shock tube experiments. using schlieren signals.
Coherent structures in a mixing layer were visualized using shadowgraphy by Brown and Roshko [74] in an early pioneering experiment. Images of shocks in high-speed gas flow can be seen in the exhaustive compilation of Van Dyke [75].  Convective flow in a water-filled square cavity with differentially heated side walls has been visualized by a shadowgraph technique by Schopf et al. [76]. The images have been quantitatively evaluated by comparison to a numerical solution of an identical problem.
Schlieren and shadowgraph are popular measurement techniques in the aerospace industry where qualitative yet reliable information is required on shocks, boundary-layers, and their interaction.

Text Organization

In the following sections, principles of laser interferometry, image processing, data analysis, and computerized tomography have been discussed. Interferometric experiments of buoyancy-driven flow have been separately discussed in detail. The discussion in the following sections contains material drawn from Darbhe and Muralidhar [77], Muralidhar et al. [10],  Mishra et al. [78], and Mishra et al. [79-82].