Experimental measurements of pressure, temperature, and density in an underexpanded sonic jet flowfield

High-resolution N₂ CARS measurements were performed in the flowfield of art underexpanded sonic jet. By resolving the rotational structure of the U=0 —> 1 N₂ Q-branch to Δω - 0.10cm^-1, this laser-, based technique can be used to obtain spatially well- resolved, non-intrusive measurements of pressure, temperature, and density in this high-speed gas flow. A number of time-averaged CARS spectra, collected along the jet centerline, highlight the pressure- and temperature-sensitivity of the aforementioned rotational structure. The experimental P/T/p measurements are compared to similar quantities extracted from a RANS CFD simulation of the jet flowfield. The agreement between the mean CARS measurements and CFD predictions along the centerline and radial traverses in the jet flowfield is excellent This CARS technique is able to capture the low-pressure (0.12 atm) and low- temperature (98 K) conditions of the M=3.2 flow entering the Mach disk as well as the conditions (1.06 atm, 262 K) immediately downstream of it Further, in the downstream region, both the CARS and CFD temperature distributions corroborate the existence of concentric inner and outer shear layers. The former evolves from the slip line that separates the subsonic inner jet fluid from the surrounding annulus of supersonic fluid in the outer jet region. Slight deviations of the CARS pressure measurements from the CFD pressure distribution in the outer compressible shear layer suggest the existence of streamwise-oriented vortices.