Experimental measurements of pressure, temperature, and density using high-resolution N(2) coherent anti-Stokes Raman scattering

Mean and instantaneous measurements of pressure, temperature, and density have been acquired in an optically accessible gas cell and in the flowfield of an underexpanded sonic jet using the high-resolution N₂ coherent anti-Stokes Raman scattering (CARS) technique. This non-intrusive method resolves the pressure- and temperature-sensitive rotational transitions of the ν = 0 $\to$ 1 N₂ Q-branch to within Δω = 0.10 cm -1. To extract thermodynamic information from the experimental spectra, theoretical spectra, created by an N₂ spectral modeling program, are fit to the experimental spectra in a least-squares manner, leaving pressure and temperature as adjustable parameters in the fitting process. Density is then determined from the best-fit pressure and temperature values using an equation of state.; Single-shot and time-averaged CARS spectra were acquired in the gas cell over a 0.10–5.0 atm pressure range. The CARS-measured pressures compare favorably with the transducer-measured gas-cell pressures. The precision and accuracy of both the time-averaged and single-shot CARS pressure measurements increase at sub-atmospheric conditions. Moreover, this technique is able to generate single-shot CARS spectra at low molecular number-density conditions (N = 0.003 kmol/m3). At high pump-laser intensities, Stark broadening and stimulated Raman pumping altered the rotational transitions in the ν = 0 $\to$ 1 and ν = 1 $\to$ 2 N₂ CARS manifolds. Unlike Stark broadening, however, stimulated Raman pumping did not affect the pressures extracted from the ν = 0 $\to$ 1 N₂ CARS spectra.; In the underexpanded jet flowfield, the experimental P/T/p measurements are compared to similar quantities extracted from a RANS CFD simulation. The agreement between the mean CARS measurements and CFD predictions along the jet centerline and radial traverses is generally excellent. This CARS technique is able to capture the low-pressure and low-temperature conditions of the M = 3.4 flow entering the Mach disk, as well as the conditions immediately downstream of this normal shock. Further downstream, both the CARS and CFD temperature distributions corroborate the existence of concentric inner and outer shear layers. The former evolves from a slip line that separates the subsonic inner jet fluid from the surrounding annulus of supersonic fluid. Similarly, the existence of counter-rotating streamwise-oriented vortices may explain the slight deviations of the CARS pressure measurements from the CFD pressure distribution in the outer compressible shear layer.