Mode-transition phenomena are explored through three-dimensional, unsteady RANS simulations of the HIFiRE-2 scramjet flowpath. Using a validated reduced-order combustion model in a mature code, the results are first compared to steady-state experimental data from the ground tests. From this basis, the methods and assumptions involved in the transient simulations are discussed, and the results are presented for mode-transition simulations of the ground and flight tests. The simulations indicate that interactions of adverse pressure gradient separated supersonic boundary layers, shock / boundary layer interactions, and jet-in-crossflow barrel shock / flameholding phenomena all play an important role in the process of mode-transition. The transition event, wherein the character of the flow changes in a rapid manner to delineate dual and scramjet-mode phenomena is generally similar in the flight and ground test simulations. However, some noteworthy difference are present due to the shock system generated by the inlet of the flight test article. Finally, observations of numerical hysteresis are discussed, and the performance of the turbulence and combustion models is evaluated.
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