COLLECTIVE LIGHT-SCATTERING AS NON-PARTICLE LASER VELOCIMETRY

The collective light scattering (CLS) principle and method are extende d to the case of partial diffraction of light that is propagated throu gh a transparent medium of inhomogenous density. The physical principl es of this macroscopic scattering mechanism are presented. The scatter ed electromagnetic field is shown to be given by the spatial Fourier t ransform of the density fluctuations, evaluated at a wave vector defin ed by the optical geometry. For non-stationary media like fluids, the dynamic part of the detected signal is found to consist of two differe nt components: a 'convection' part formed by convected density fluctua tions, and an 'acoustic' part due to propagating sound waves. Each of these parts results in different lines in the signal frequency spectru m. The 'convection' line is a Doppler transform of the mass velocity p robability distribution. This is experimentally verified by observatio ns in a supersonic mixing layer. Simultaneous measurements were perfor med with a conventional laser Doppler velocimeter (LDV) and with a spe cially designed collective light scattering device. The LDV velocity h istograms and the CLS frequency spectra are compared at different posi tions in the mixing layer. In most cases, the information thus provide d on the density fluctuations, Mach number, and the velocity probabili ty distribution with its moments, is found to be in good agreement.