Application of laser bessel beams in velocity measurements

Abstract

In this thesis we explore the use of Laser Bessel beams in solid surface and fluid velocity measurements. We present a novel simple technique to measure two velocity components of a solid surface, using a Bessel beam Laser Doppler Velocimetry (LDV) system. The experiments examined the intersection of similar two Bessel beams to generate interference fringes at the intersection region. These fringes, along with the Bessel beam fringes can be used in a simple LDV system to measure two velocity components. We also explored the use of single Bessel beams for measurement of fluid flow velocity in a horizontal transparent smooth straight circular pipe using forward scattering Laser Bessel velocimetry (LBV). The measurements were validated using a commercial LDV system. In order to produce an acceptable spatial resolution for fluid flow measurements, we investigated experimentally and numerically the use of Durnin rings (annular slits) with finite width to produce nearly Bessel beams with limited transverse profile extent and depth of field (DOF). One purpose of the work was to develop the suitability of laser Bessel Velocimetry for flow conditions where velocity measurements by alternative instrumentation were not feasible or were subject to optical access limitation. We also demonstrated, a significant advantage in using a single Bessel beam to measure the total velocity of two-dimensional flows.