Flow visualization is one of the more important experimental tools for studying fluid flow and heat transfer. Its objective is to render a flow and/or temperature field visible in some way. (Many fluids are transparent, so motion and temperature are ordinarily invisible to the eye). But by using dye, tracer particles, and interference techniques it is possible not only to display these fields, but also to glean quantitative information about them. In this project we will infer heat transfer from heated blocks in a duct using the visualization technique holographic interferometry. In this technique, a laser apparatus produces interference patterns which are associated with density differences in the flow field. Since these density differences are directly related to temperature differences, the interference pattern--maybe time dependent--can be used to infer the temperature field in both steady and unsteady flows.

HOLOGRAPHY

Principles of holography

Holography is a procedure for recording and reconstructing images using coherent light waves. The image-containing light wave is called the object wave. All information is embedded in the complex amplitude, i.e., amplitude and phase, of the wave. Consequently, the reconstruction of this object wave requires the reproduction of the complex amplitude of the wave at some plane in space. Since conventional detectors and photographic film respond only to irradiance, it is necessary to have a technique for converting phase information into an irradiance pattern, so that the full information of the object wave may be captured. Such a technique--a method of optical recording--was developed by Gabor in 1949. This technique is called holography. Holography can be effected in several ways, but we will consider here the one most suitable for making measurements of heat transport--off-axis, interferometric holography. (The general theory of holography is very comprehensive and the reader is referred to the literature for details (Gabor (1948) and (1949), Vest (1979), Jones and Wykes (1983))).

In interferometric holography, two light beams--one, a reference, the other, the object beam--must be brought together at an image plane where their mutual interference may be observed or photographically recorded. For our experiment in heat transfer, the object beam passes through the test section of our heated duct and incurs phase changes depending on the distribution of air temperatures in the test section. A second beam (reference) from the same laser source is skirted around the experiment and reunited with the object beam. If the single source for these two light beams is coherent, e.g., a laser (in this case an Argon-Ion laser), then the two beams will form interference fringes at the image plane. A schematic of this arrangement is illustrated in Figure 1. The phase object to be recorded, in our case air in a of the light waves passing through the object. The goal is to record these distributions on photographic material in order to enable their reconstruction later. The recording of the amplitude presents no problem, as it has already been explained. In order to record the phase of the object wave, the effect of interference is used, and the phase information is converted into irradiance distribution. In order to achieve this, a reference wave is superimposed to the object wave. If coherent light, such as laser light, is used, an interference pattern is generated in the region where the object and reference beams overlap. The photographic plate illuminated in this way is called the hologram after the development and it contains complete information on the object wave. The phase distribution is recorded in the interference pattern and the amplitude in form of varying contrast. When illuminating the hologram by the original reference beam only, it will function as a diffraction grating with spatially varying diffraction constant and it will allow the viewing of the reference wave recorded earlier. The reconstruction of the reference state by illuminating the holographic plate with the reference beam is shown in Figure 1b. In the third step, when taking measurements, the investigated process is initiated, the copper blocks are heated and the air flow in the wind tunnel is generated by a blower. Due to the increase of temperature in the heated regions the refractive index of air changes and causes a change in the shape of the wavefronts as the object beam passes through the test section. The undisturbed parallel wavefronts, corresponding to the reference state and recorded earlier, are reconstructed with the reference beam. The waves featuring the reference state and the measurement state will interfere forming a fringe pattern, as illustrated in Figure 1c. The interferogram can then be recorded by a photographic or high-speed camera. Applications The holographic recording technique is extensively used in flow measurements. In single phase flow, it provides means to freeze the three dimensional picture of the flow. It can be combined with tracer methods, and, in this way, in the reconstructed image the position of the individual tracer particles can be analyzed. If the double-pulse technique is used, flow velocities can be evaluated by determining the vector change in tracer position. Also, three dimensional spatial positions and velocity fields of a particle can be obtained by measuring the radii of concentric interference fringes of the diffractive image of a single spherical particle. Different optical visualization systems can be combined with holography, in order to eliminate the temporal limitations in the analysis of high speed phenomena. The combination of holography with schlieren and shadowgraph techniques is used in flow visualization studies. In two phase flow, holography can be used instead of photography to determine aerosol, droplet and particle size and velocity. The interested reader can find details on the application of holography in flow visualization in the literature (Merzkirch (1987)).