Recent studies have shown that molecular vibrational relaxation leads to transition delay at hypervelocity conditions. The objective of this study is to examine the role of vibrational non-equilibrium on re-laminarization. Specifically, Radio-Frequency-plasmas were used to impose thermal non-equilibrium onto a low Reynolds number (Ret = 49) turbulent channel flow. Experiments were conducted at two plasma settings, 150W and 300W, which led to vibrational excitation of the nitrogen, in air, to Tvib ~1240K and 1550K, respectively. Oxygen was not significantly excited, and the rotational/translational temperatures were only slightly elevated above room temperature. Statistical flow properties were quantified using particle image velocimetry, two-line laser induced fluorescence, coherent anti-Stokes Raman spectroscopy, and emission spectroscopy. Qualitatively, the effect of the RF-plasma was increased stabilization of the turbulence in both the shear and zero-shear regions of the flow. The peak axial turbulence intensities in the shear layers were reduced by 15% and 30% moving across the plasma for the 150 and 300W cases, respectively. The plasma did not alter the transverse intensities. The Reynolds shear stresses were reduced by 30% and 50% for the 150 and 300W cases. The corresponding Reynolds shear stress correlation coefficient was reduced suggesting diminished coherent structure. The plasma also enhanced the turbulence decay in the zero shear region, where the power-law, t-1/n, exponential factor n decreased from 1.0 to 0.8. Integral conservation law balance and second order transport analyses demonstrated that the plasma induced thermal non-equilibrium, thermal heating, flow accelerations and wall heating. Large-scale computations, also show the importance of increased dissipation.
Dr. Bowersox is a Professor and Department Head of Aerospace Engineering at Texas A&M University. His research interests include (1) high-speed flows with mechanical and thermochemical non-equilibrium effects, with an emphasis on turbulent and transitional boundary layers and their surface interactions, (2) specialized hypersonic facilities and high-energy laser based optical diagnostics, and (3) flow control methods for high-speed and combusting flows. Dr. Bowersox is a fellow of the ASME, associate fellow of the AIAA, member of the American Chemical Society, American Physical Society, and the Optical Society of America.