Rotational Non-Equilibrium CARS Measurements in a Hypersonic Boundary Layer

This work presents measurements of rotational population distributions using hybrid fs/ps Coherent Anti-Stokes Raman Scattering (CARS) in a hypersonic testing facility. Wind tunnel runs were conducted at Mach 5.5 with pure N2 as the test gas. On separate runs, experimental CARS spectra were measured from locations in the freestream and in the boundary layer. The data were fitted using synthetic CARS spectra to determine the rotational temperature in the N2 gas flow. Hybrid CARS was successfully used to determine the rotational temperature in the freestream flows under both steady and unsteady conditions. In the freestream, rotational temperature was measured to range from 121 to 143 K. In the boundary layer, a non-Boltzmann rotational population distribution was observed during the steady-flow regime, while during the unsteady portion of the run, the retrieved temperatures were consistent with the freestream values.