Transition within a hypervelocity boundary layer on a 5-degree half-angle cone in freestream air/CO$_{2}$ mixtures

The effect of freestream CO$_{2}$ content on transition in hypervelocity flow over a slender cone was investigated in experiments and computations. Experimental data were obtained in Caltech's T5 reflected shock tunnel. The model was a 5 degree half-angle sharp cone instrumented with thermocouples, providing heat transfer measurements from which transition locations were determined by comparison with laminar and turbulent heat flux correlations. Four carbon dioxide/air gas mixtures were tested at reservoir enthalpies from 7--10 MJ/kg and reservoir pressures from 55--60 MPa to attempt to reproduce the largest shift in transition location implied by computations using the semi-empirical e$^{N}$ approach. By mass fraction of carbon dioxide these mixtures were 0.0 (e.g. all air), 0.5, 0.75, and 1.0. For tests at an enthalpy of $\sim $9.2 MJ/kg, transition delays of up to 30{\%} in terms of x-location, 38{\%} in terms of edge Reynolds number, and 140{\%} in terms of the Reynolds number evaluated at reference conditions were documented for increasing CO$_{2}$ mass fractions compared with similar experiments in air.