A laser optical method for the direct measurement of normal propagation velocities of phase objects

Abstract

An argument, based on the equivalence of ‘Doppler velocimetry’ and fringe anemometry, shows how orthogonal velocity components can be measured directly when each part of a moving phase object, such as a flame, is displayed as an equi-spaced fringe pattern parallel to its local refractive index contours. A schlieren system with a special aperture which produces this type of interference is devised for the purpose. When the reactants are in flow, in the case of a flame, direct measurement of burning velocities is made possible by allowing the interference pattern due to a small particle to interact with that of the front. The method is thus applicable whether the flame (or other propagating front) is stationary, moving or fluctuating. It has been tested on moving diffraction gratings, particles and flames stabilized on a burner and propagating in a tube.