High-enthalpy models for boundary-layer stability and transition
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The prediction of the transition dynamics of high-enthalpy boundary-layer flows requires appropriate thermodynamic and transport models. This work quantifies the influence of transport, diffusion, collision, equilibrium, and chemical-kinetics modeling on the stability characteristics and the estimated transition-onset location of canonical boundary layers. The computed behavior of second-mode instabilities is consistently highly dependent on the base-flows boundary-layer height. The Blottner-Eucken-Wilke transport model is seen to underpredict the boundary-layer height, hence overpredicting the growth-rate distribution and forecasting the transition onset to occur 38% sooner. The other low-order transport models (Brokaw and Yos) returned very close results to the most-accurate Chapman-Enskog model. The use of Gupta et al.s collisional data instead of Wright et al.s more accurate data is also seen to predict the transition onset to occur 8% closer to the leading edge. The modeling of mass diffusion and the chemical-equilibrium constant is observed to have a negligible influence on the boundary-layer height and transition-onset-location estimations (less than 5% and 2%, respectively). For the analyzed conditions, all chemical models predict the same transition-onset location (1%); since at the streamwise positions where perturbations have reached sufficiently large amplitudes, the flow is close to equilibrium and thus independent of the reaction rates. The use of different transport models for the perturbation terms, while maintaining the same model for the basic state, leads to negligible differences in the predictions. This further reinforces the thesis that the boundary-layer height calculation is paramount to the simulation of the development of second-mode instabilities.
