Laser cross-beam intensity-correlation spectrum for a turbulent flow.

The spectrum of the temporal correlation function of intensity (i.e., intensity correlation spectrum) is analyzed for a dual-beam, oss-beam light scattering experiment. The cross-beat intensity correlation signal resulting from individual particles and scattered off the interference fringes of the cross beam contains information of flow velocity as well as turbulence. The self-beat signal resulting from particle pairs and scattered off each laser beam contains information of turbulence also, although in most cases, the turbulent self-beat signal is overpowered by the effects of finite transit time. The relative strengths, coherence properties, and kinematic differences of the self-beat and cross-beat signals are compared to explore the means for studying the dynamical and statistical properties of the turbulent flow. The question of fluctuations in scatterer concentration is investigated, and it is found to have little practical importarce. The typical experimental spectrum is presented to illustrate the conclusions of the analysis.