Control of vibrational distribution functions in nonequilibrium molecular plasmas and high-speed flows

The control of the vibrational distribution of nitrogen by energy transfer to CO2 is studied in two closely related experiments. In the first experiment, the time-resolved N-2(v = 0-3) vibrational level populations and temperature in the afterglow of a diffuse filament nanosecond pulse discharge are measured using broadband coherent anti-Stokes Raman spectroscopy. The rotational-translational temperature in the afterglow is inferred from the partially rotationally resolved structure of the N-2(v = 0) band. The measurements are performed in nitrogen, dry air, and their mixtures with CO2 center dot N-2 vibrational excitation in the discharge occurs by electron impact, with subsequent vibration-vibration (V-V) energy transfer within the N-2 vibrational manifold, vibration-translation (V-T) relaxation, and near-resonance V-V' energy transfer from the N-2 to CO2 asymmetric stretch vibrational mode. The results show that rapid V-V' energy transfer to CO2, followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO2 and their V-T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO2 into a supersonic flow of vibrationally excited nitrogen demonstrates the effect of accelerated vibrational relaxation on a supersonic shear layer. The nitrogen flow is vibrationally excited in a repetitive nanosecond pulse/DC sustainer electric discharge in the plenum of a nonequilibrium flow supersonic wind tunnel. A transient pressure increase as well as an upward displacement of the shear layer between the supersonic N-2 flow and the subsonic CO2 injection flow are detected when the source of N-2 vibrational excitation is turned on. CO2 injection leads to the reduction of the N-2 vibrational temperature in the shear layer, demonstrating that its displacement is caused by accelerated N-2 vibrational relaxation by CO2, which produces a static temperature and a pressure increase in the test section. This demonstrates the significant potential of accelerated vibrational relaxation for nonequilibrium flow control, by injection of rapid 'relaxer' species at a desired location, resulting in the rapid thermalization of vibrational energy in nitrogen and air flows, and producing a significant effect on the flow field.