Picosecond Unstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED-CARS) is used for the measurement of nitrogen Q-branch (ΔJ = 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 CO2, NO, and H2 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 N2, or N2 with 0.25 torr CO2 injection, no vibrational relaxation is observed behind the bow shock.
Experiments in Fluids – Springer Journals
Published: Dec 21, 2012
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