Collective light scattering as non-particle laser velocimetry

Abstract

The collective light scattering (CLS) principle and method are extended to the case of partial diffraction of light that is propagated through a transparent medium of inhomogenous density. The physical principles of this macroscopic scattering mechanism are presented. The scattered electromagnetic field is shown to be given by the spatial Fourier transform of the density fluctuations, evaluated at a wave vector defined by the optical geometry. For non-stationary media like fluids, the dynamic part of the detected signal is found to consist of two different components: a 'convection' part formed by convected density fluctuations, and an 'acoustic' part due to propagating sound waves. Each of these parts results in different lines in the signal frequency spectrum. The 'convection' line is a Doppler transform of the mass velocity probability distribution. This is experimentally verified by observations in a supersonic mixing layer. Simultaneous measurements were performed with a conventional laser Doppler velocimeter (LDV) and with a specially designed collective light scattering device. The LDV velocity histograms and the CLS frequency spectra are compared at different positions in the mixing layer. In most cases, the information thus provided on the density fluctuations, Mach number, and the velocity probability distribution with its moments, is found to be in good agreement.