High-resolution N₂ CARS measurements of pressure, temperature, and density using a modeless dye laser

In this study, high-resolution N₂ CARS measurements have been made using a modeless dye laser as the Stokes beam source to reduce the presence of mode noise. Along with this modification to the CARS technique, a new spectra fitting procedure was developed to avoid a starting-point bias in the leastsquares fitting results. Time-averaged and single-shot measurements of pressure were made in a static pressure vessel over the range of 0.1 to 4.0 atm to test the pressure sensitivity of the technique. In addition, the precision of single-shot measurements in the pressure vessel is indicative of the baseline pressurefluctuation detection limit of the system. The precision uncertainty of the measurements was studied to investigate the possibility of making property fluctuation measurements in high-speed flows. Centerline measurements of pressure and temperature in an underexpanded jet (M_j = 1.85) were also used to determine the performance of the technique in a compressible flowfield. Improvements in accuracy for time-averaged and single-shot mean measurements and increased precision were found for pressure levels above 1.0 atm. For the subatmospheric pressure levels that are important in high-speed flows, the results indicated that the current method is incapable of making fluctuation measurements due to limited precision. Nevertheless, the increased precision above 1.0 atm indicates that fluctuation measurements may be possible with further modifications.