Picosecond CARS measurements of nitrogen rotational/translational and vibrational temperature in a nonequilibrium Mach 5 flow

Picosecond Unstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED-CARS) is used for the measurement of nitrogen Q-branch (Δ J = 0) spectra in the subsonic plenum and supersonic flow of a highly nonequilibrium Mach 5 wind tunnel. Spectra are processed to infer rotational/translational ( T rot ) and first-level vibrational ( T vib ) temperatures in the 200–370 torr plenum simultaneously. Operation of the nominally high reduced electric field ( E / n peak ~ 500 Td), nsec pulsed discharge alone results in fairly significant vibrational loading, T vib ~ 720 K/ T rot ~ 380 K; addition of an orthogonal low E / n (~10 Td) DC sustainer discharge produces substantial vibrational loading, T vib ~ 2,000 K/ T rot ~ 450 K. Effects of injection of CO 2 , NO, and H 2 downstream of the pulser–sustainer discharge are examined, which result in vibrational relaxation accompanied by simultaneous gas heating, T vib ~ 800–1,000 K/ T rot ~ 600 K. CARSk measurements within very low-density flows in the Mach 5 expansion nozzle are also performed, with T vib measured in both the supersonic free-stream and downstream of a bow shock created by a 5-mm-diameter cylindrical test object in the Mach 5 flow. Measurements within 300 μm of the cylinder leading edge show that for pure N 2 , or N 2 with 0.25 torr CO 2 injection, no vibrational relaxation is observed behind the bow shock.