1. 42-45 provide comprehensivereviewsof thePIV/DPIV technique.
2. A generic DPIV system is shown in figure 9. Typical DPIV operation consists of utilizing the beam from a multiple-pulse (e.g., double-pulse) Nd:YAG laser system to form a thin sheet of light which is directed into the flow within the field-of-view of the DPIV camera system. Particles are illuminated by a series of short (nominally 10 nS) pulses of light with a fixed inter-pulse time delay. The recording and laser systems are synchronized such that each camera frame obtains one exposure per laser pulse. These camera frames may be acquired using a frame-straddling camera where one camera captures multiple light sheet pulses on different frames. Conversely, as was the case for the present experiment, multiple cameras may be employed with polarization used to direct the scattered light from particles to the various cameras. Analysis of the recorded images to extract velocity information consists of segmenting the images into small, square interrogation regions typically no more than 32 - 64 pixels wide. Corresponding interrogation regions from pairs of laser exposures are cross correlated to provide an indication of the two-dimensional particle movement within the plane of the light sheet. Flow velocities are then extracted by dividing the physical measure of the particle movementbythelaserpulseseparation time.
3. DGVis capable of measuring the three-componentflow velocity within a measurement plane defined by a laser light sheet46-47. The key element in the DGV approach is the use of the absorption characteristics of Iodine vapor, figure 10, to determine the absolute optical frequency of Doppler shifted laser light scattered by small particles passing through a laser light sheet. Since the optical frequency of the scattered light is measured directly, the resolution of light scattered from a single particle is notnecessary. Further, sinceoptical Figure9: DPIV concept. 600 mJ,532 nm
4. Figure 11 shows the DGV configuration for measurement of a single flow velocity component. A three-component DGV system consists of a single frequency laser, either Argon ion48-51or frequencydoubled Nd:YAG52-59, three receiver systems each consisting of two video cameras and an Iodine vapor cell, and a laser frequency monitor - a fourth receiver system used to monitor the optical frequency of the laser. The laser beam is expanded into a light sheet and oriented to the desired measurement plane. Each receiver system is placed about the light sheet to yield the maximum common viewing area with each out-ofplane angle set greater than 30-degrees to minimize viewing pixel overlap errors. The receivers are also set to positions that will yield component velocity vectors that are greater than 45-degrees from each other. The laser optical frequency is then tuned to a point midway along the side of the absorption line, figure 10. When small particles pass through the light sheet, the scattered laser light is Doppler shifted based on their direction and velocity. When a portion of this scattered light is collected and directed through the Iodine vapor, some of the light energy will be absorbed by the vapor. The amount of light energy reduction will be greater (or less, depending on the direction of the Doppler shift) than that lost if a portion of the original laser beam were directed through the cell. Since the optical power passing through the Iodine vapor is also dependent on the particle number density, particle size, and laser intensity profile, a second or reference video camera is used to obtain a map of the collected scattered light intensities. A beam splitter is placed in front of the Iodine vapor cell to direct a portion of the collected scattered light toward the reference camera via a mirror, figure 11. This configuration is used to maintain the same optical axis and image orientation for the two cameras. Normalizing the signal image by this reference image yields the desired velocity-dependent transfer function imposed by the Iodine vapor. The ratio-to-frequency calibration of each Iodine vapor cell and Doppler shift equation are then used to determine the velocity of the flow passing through the light sheet at every pixel viewing point. An example velocity map oftheflow aboveadeltawingisshowninfigure12. Figure 10: Iodine vapor absorption line transfer function,514.5nm Darker