Optical Velocimeters

Abstract

<p>Three types of optical velocimeters are investigated in this thesis. The first, which employs a laser source, is the differential heterodyne laser Doppler velocimeter (DLDV). The second, which employs a conventional light source, is what we refer to as the white light fringe image velocimeter (WFIV). The third, which employs a laser source. we refer to as Fourier image velocimeter (FIV). The latter two techniques are new and were developed during the course of this work.</p> <p>The systems parameters for all three velocimeters are investigated both theoretically and experimentally. The main objective was to determine optimum operating conditions for achieving a high signal-to-noise ratio (SNR) and good spectral quality.</p> <p>In the cases of the DLDV and WFIV the research included extensive investigations of the effects of receiver aperture size, particle number density (n), and detection angle, on the SNR. Theoretical derivations of the SNR, based on optical heterodyning and the Mie scattering theory for the WFIV, are found to have good agreement with carefully conducted experiments.</p> <p>In particular it was observed that with the WFIV the SNR is proportional to n for n values corresponding to less than one particle in the probe volume. The SNR remains constant for large n values. In the case of the DLDV, the SNR was always found to be proportional to n when the coherent condition was satisfied. Consequently, it is concluded that the DLDV signal results almost entirely from coherent scattering, even when the number of particles in the probe volume is as low as 0.02 ( the lower limit of the measurements).</p> <p>The FIV is designed to provide a large probe depth. A derivation of the self-imaging property of Fourier images is presented in Chapter 8. Fourier images are a result of Fresnel diffraction from a periodic object, and have the properties of reproducing the intensity distribution of the object. There is excellent agreement between theory and experiment on the functional form of the diffraction patterns at and near the Fourier image. The phenomenon of repair of errors in the object is also observed.</p> <p>Preliminary applications of the two new techniques (WFIV and FIV) to the measurement of particle velocities have been carried out using a range of particle sizes, both on rotating disks and in fluid flow. In addition, velocity profile measurements of water flow in a square duct were carried out employing the WFIV. In all cases, comparable measurements were carried out employing a DLDV. It is found that under certain conditions the techniques are comparable both in accuracy and sensitivity.</p>