Collective light scattering and instantaneous fluid velocity measurement

Collective Light Scattering (CLS) from gas density irregularities is analysed using electromagnetic theory and fluid mechanics. The scattered signal is shown to provide three decisive pieces of information.: (1) the signal frequency spectrum gives the Doppler shifted velocity probability distribution; (2) new processing of the modulus and phase spectra of the complex signal provides the velocity fluctuations frequency spectrum; (3) appropriate filtering of the signal phase time derivative provides the real time instantaneous convection velocity. This information is illustrated and extracted from CLS signals obtained in a turbulent mixing layer experiment.