Filtered Rayleigh Scattering or FRS is a non-intrusive laser diagnostic technique that is utilized to observe and quantify various flow properties in high-speed gas flows. In this technique, a laser beam is used to irradiate a portion of the gas under investigation. The receiving gas molecules scatter the incident laser beam in all the directions. The scattered radiation from a single molecule stems from the induced oscillations of the electron cloud that acts as a classical dipole radiator. The intensity of the net scattered radiation from all the molecules that are in random motion depend on the spatio-temporal fluctuations of the dielectric permittivity of the gas. The thermodynamic properties of the gas govern the nature of these variations and the resulting spectra of the scattered radiation is referred to as Rayleigh Brillouin spectrum. In high-speed gas flows applications, the bulk velocity of the gas molecules causes the entire scattered spectrum to be Doppler shifted from the lasing frequency. The extent of this shift is very significant in supersonic flows and this enables the isolation of the scattered spectrum from the laser reflections using a molecular notch filter. Measurement of flow properties can be made based on the intensity of the RBS signal that is collected by a charged coupled device. The FRS technique is also utilized to perform mixing measurements in binary mixture of gases in high-speed flows. In non-reacting mixing studies, helium is often used as a passive scalar and simulant gas for hydrogen fuel. Helium concentration measurements in supersonic flows based on the FRS technique require two independent experiments. The first experiment involves injection of helium gas and the second separate experiment requires air injection. The helium mole-fraction is retrieved from the FRS imaging data under the key assumptions that the total number density profiles and the extent of the Doppler shift that is associated with each one of the two independent experiments at the measuring plane are identically the same. In this review, the theory governing Rayleigh scattering and the FRS technique to retrieve helium mole-fraction in supersonic flow will be discussed in detail. Measurement results from recent FRS studies on complex vortex interactions in supersonic flow will be also presented and discussed as well as the impact on the results of a departure from the key assumptions in FRS mixing studies will be analyzed for a canonical parallel injection using strut injector in supersonic flow.
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