Development of high-resolution N₂coherent anti-Stokes Raman scattering for measuring pressure, temperature, and density in high-speed gas flows

Mean and instantaneous measurements of pressure, temperature, and density have been acquired in an optically accessible gas cell and in the flow field of an underexpanded sonic jet by use of the highresolution N₂coherent anti-Stokes Raman scattering (CARS) technique. This nonintrusive method resolves the pressure- and temperature-sensitive rotational transitions of the v = 0 → 1 N₂Q-branch to within Δω = 0.10 cm⁻¹. To extract thermodynamic information from the experimental spectra, theoretical spectra, generated by a N₂spectral modeling program, are fit to the experimental spectra in a least-squares manner. In the gas cell, the CARS-measured pressures compare favorably with transducer-measured pressures. The precision and accuracy of the single-shot CARS pressure measurements increase at subatmospheric conditions. Along the centerline of the underexpanded jet, the agreement between the mean CARS P/T/ρ measurements and similar quantities extracted from a Reynolds-averaged Navier-Stokes computational fluid dynamic simulation is generally excellent. This CARS technique is able to capture the low-pressure and low-temperature conditions of the M 5 3.4 flow entering the Mach disk, as well as the subsonic conditions immediately downstream of this normal shock.