Experimental investigation of high-enthalpy effects on attachment-line boundary-layer transition

A series of experiments was performed on high-enthalpy effects on hypersonic boundary-layer transition using nitrogen, air, and carbon dioxide as test gases in the T5 hypervelocity shock tunnel. Previous experiments on a sharp cone showed significant high enthalpy effects. This series concerns the attachment line boundary layer on swept cylinders with sweep angles of 60 and 45 deg. The observed trend of transition Reynolds number with enthalpy, which is found to be similar to that in the cone results, shows strong transition delay at the larger sweep angle for carbon dioxide, whereas no significant effect is observed in nitrogen air. The acoustic wave absorption rate due to relaxation shows a quite similar trend in enthalpy dependence to transition Reynolds number for the carbon dioxide case. This suggests that the dominant effect in delaying transition in the case is vibrational relaxation. The comparisons between the magnitude of the strongest amplification rate from an inviscid linear stability analysis and the absorption rate due to relaxation show that they are of the same order of magnitude for carbon dioxide and that absorption is not significant for nitrogen or air, which supports the stated understanding of the effect of relaxation on transition.