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-flow’s 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.

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Advancing towards the correct modeling and subsequent understanding of laminar-toturbulent transition during atmospheric reentry is paramount for the future of aerospace technology. The coexistence of multiple physical phenomena and the grand amount of conditioning factors require the progressive extension of the applicability capabilities of the theoretical models. Past efforts have been mostly dedicated to investigate high-temperature and non-equilibrium effects using parallel stability theories. However, the implications of coupling these thermochemical phenomena with non-parallelism remains uncertain. Advanced state of the art thermodynamic and transport models are employed both in parallel and weakly non-parallel stability theories (LST and LPSE). A parametric study about the influence of nonlocal effects under different re-entry conditions and flow assumptions (i.e. CPG, TPG, CNE and LTE) showed that non-parallel effects stabilize/destabilize the boundary-layer, depending on the altitude and independently from the gas model employed. Particularly, they lead to a stronger destabilization of the 2 nd Mack mode at the earliest points of the atmospheric re-entry flight envelope, reducing their effect until being weakly stabilizing at the lowest altitudes. Drastic N factor increments occurred assuming LTE, due to the presence of unstable supersonic modes, promoted by the boundary-layer cooling, caused by the intense chemical activity. Nomenclature Acronyms BEW Blottner-Eucken-Wilke transport model CE Chapman & Enskog's transport model

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