Fluid flow Doppler velocimetry using adaptive photo-induced electromotive force detectors

Two mutually coherent beams are intersected on the surface of a photo-sensitive crystal producing an interference grating. Free charges produced in the bright regions produce a space-charge electric field. When there is a frequency difference between the two beams the grating moves in a predictable direction, consequently setting the charges in motion even in the absence of an applied field. By measuring the magnitude and direction of this current, the frequency difference can be found. In this work, photo-emf detectors are used to find Doppler frequency shifts scattered from a moving target. A rotating disk is used to characterize and quantify several parameters and different configurations that could make this device feasible for instrumentation. Then, the system is tested with a seeded subsonic free jet. The sensor response shows a combination of currents from frequency-dependent (photo-emf) and intensity-dependent (photovoltaic) effects. It has been found that for a number of experimental conditions the device can be calibrated so that the these effects are decoupled. A double-electrode sensor was tested for instantaneous calibration of photovoltaic effect at any velocity, and has been demonstrated to work for the rotating disk experiment. This arrangement could be programmed for real-time velocity measurements. Also, a reference-less configuration is demonstrated that has the potential of becoming a versatile velocimetry instrument. Average velocity uncertainty was estimated to be slightly above 10%. The device could also be used in instantaneous measurements, however random electronic noise must be further reduced.