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 CO 2 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 CO 2 . 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 CO 2 asymmetric stretch vibrational mode. The results show that rapid V–V′ energy transfer to CO 2 , followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO 2 and their V–T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO 2 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 CO 2 injection flow are detected when the source of N 2 vibrational excitation is turned on. CO 2 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 CO 2 , 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.