Transition delay in hypervelocity boundary layers by means of CO ₂/acoustic instability interactions

A novel method to delay transition in hypervelocity flows over slender bodies by injecting CO₂into the boundary layer of interest is investigated. The results presented here consist of both experimental and computational data. The experimental data was obtained at Caltech's T5 reflected shock tunnel, while the computational data was obtained at the University of Minnesota. The experimental model was a 5 degree sharp cone, chosen because of its relevance to axisymmetric hypersonic vehicle designs and the wealth of experimental and numerical data available for this geometry. The model was instrumented with thermocouples, providing heat transfer measurements from which transition locations were determined and the efficacy of adding CO₂ in delaying transition was gauged. For CO₂/N₂ freestream blends without injection, the transition Reynolds number more than doubled for mixtures with 40% CO₂ mole fraction compared to the case of 100% N₂. For the cases with injection, shadowgraph visualizations were obtained, allowing verification of the injection timing. The computations provide encouraging results that for the injection schemes proposed CO ₂ is reaching high enough temperatures to excite vibrational modes and thus delay transition.