Demonstration and characterization of filtered Rayleigh scattering for planar velocity measurements

Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering