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3-d Fiber Optic PDV

3-d Fiber Optic PDV

A diagnostic system has been developed to measure spatially-resolved velocities non-intrusively in aircraft engine exhausts. The method is called Planar Doppler Velocimetry, and uses fiber optic bundles to collect the scattered light at specific observation angles that yield three components of the velocity vector. This strategy yields a 3-velocity component measurement system with a total of two cameras and a single iodine filter. When collecting light scattered by particles in the measurement volume, each observation direction yields one velocity component as indicated in Figure 1. The Doppler shift, Δν, associated with this velocity component is given by:

where λ is the optical frequency of the laser, ▁a and ▁l are the unit vectors of the observation direction (i.e. position of fiber bundle) and laser transmission, respectively. ▁V is the object velocity vector, and c is the speed of light in a vacuum.

Figure 1. Vectorial representation of the measured velocity component
Figure 1. Vectorial representation of the measured velocity component

Thus, with three fiber bundles placed at different observation directions one measures three independent velocity components from which the velocity vector can be computed. Furthermore, three orthogonal components can be calculated from any three other components. The proposed strategy is illustrated schematically in Figure 2.

Figure 2 Schematic representation of a 3-component PDV system with 3 fiber bundles positioned at different angles
Figure 2 Schematic representation of a 3-component PDV system with 3 fiber bundles positioned at different angles

Figure 2 shows a special coherent imaging bundle with three branches converging into a single 3-into-1 coherent imaging combiner. Each of the branches collects the light scattered from particles in the measurement volume at a specific view, thus corresponding to a specific velocity component. That is, each view yields one velocity component. The bundles are coherent and may be several meters long. The 3-into-1 fiber bundle coherent combiner brings the collected signals to the measurement system, which includes an imaging lens, beam splitter, iodine cell and two cameras. Notice that the three velocity components are measured with a single iodine cell and two cameras. Thus each CCD camera captures three signals, one from each bundle. The PDV analysis compares the filtered and unfiltered signal associated from each bundle. This is shown schematically in Figure 3, which shows two sets of equivalent pixels from the two CCD cameras. A similar measurement strategy was successfully demonstrated by groups in England and Germany . MetroLaser recently demonstrated the first non-intrusive velocity measurements in an unseeded jet engine exhaust using this technique, and the results are shown in Figure 4.

Figure 3. Images from four square bundles on two CCD chips. The top right quadrant (shown in yellow) is an extra branch that may be used to calibrate the laser. The blank right portion is because the CCD chip is rectangular while the fiber bundle is square.
Figure 3. Images from four square bundles on two CCD chips. The top right quadrant (shown in yellow) is an extra branch that may be used to calibrate the laser. The blank right portion is because the CCD chip is rectangular while the fiber bundle is square.
Figure 4. Vector map of three-component velocities measured in a turbojet engine exhaust. Average of 100 frames, measurement area 5.8 cm x 5.8 cm.
Figure 4. Vector map of three-component velocities measured in a turbojet engine exhaust. Average of 100 frames, measurement area 5.8 cm x 5.8 cm.
[1] Nobes, D.S., Ford, H.D., and Tatam, R.P., 2004, Three Component Planar Doppler Velocimetry Using Imaging Fibre Bundles, Experiments in Fluids, 36, 1, 3-10.
[2] Willert, C., Stockhausen, G., Beversdorff, M., klinner, J., Lempereur, C., Barricau, P., Quest, J., and Jansen, U., 2005, Application of Doppler Global velocimetry in cryogenic wind tunnels, Experiments in Fluids, 39, 420-43.