Pressure, Temperature, and Velocity Measurements in Underexpanded Jets Using Laser-Induced Fluorescence Imaging
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We report measurements of pressure, temperature, and velocity in an underexpanded jet using planar laser-induced fluorescence of nitric oxide. Ultraviolet transitions near 44,097.5 cm -1 in the A-X (0,0) system of nitric oxide were excited using narrow linewidth laser radiation generated from an optical parametric system, injection seeded using a distributed feedback diode laser. Planar laser-induced fluorescence images with excellent spatial resolution and signal-to-noise ratio were acquired by tuning the frequency of the laser radiation over the nitric oxide absorption line. The images were corrected on a shot-to-shot basis for fluctuations in the laser spatial profile. Line shapes constructed from the corrected planar laser-induced fluorescence images were used to determine pressure and temperature values along the centerline of the jet. Good agreement between laser-induced fluorescence and previously reported N 2 coherent anti-Stokes Raman scattering measurements was observed. The laser-induced fluorescence measurements also compared well with calculations of pressure and temperature using computational fluid dynamics codes. Velocity was measured in supersonic regions of the flowfield on the basis of the Doppler shift in the nitric oxide absorption lines. Planar laser-induced fluorescence images were acquired using laser sheets propagating at 90 and 45 degrees with respect to the flow direction; velocity was determined from the frequency shift of the absorption lines for these two shifts. The laser-induced fluorescence technique can potentially be applied to obtain instantaneous measurements of thermodynamic properties and multiple velocity components in high-speed turbulent flows.
