Unsteady Three-Dimensional Mode Transition Phenomena in a Scramjet Flowpath

Abstract

The physical process of mode transition, in which an accelerating hypersonic propulsion ramjet progresses from dual to scramjet mode operation, is explored in the context of the HIFiRE-2 ground and flight test campaign using three-dimensional, unsteady Reynolds-averaged Navier–Stokes simulations with a validated combustion model. Key highlights of the numerical procedure, mesh development, experimental validation, and the asymptotic end states are first summarized from prior work. From this basis, the methods and assumptions involved in the transient simulations are discussed with a focus on computational tractability. The results indicate that several major interactions play a crucial role in the mode transition, including separation associated with unsteady shock/turbulent boundary layer interactions, jet-in-crossflow barrel shock/flameholding, and cavity dynamics. 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. As the freestream Mach number is increased under constant dynamic pressure and air-fuel ratio, shock interactions with the primary injectors result in loss of flameholding. The effects rapidly propagate downstream, altering the flow and combustion patterns in the cavity flameholder. Some noteworthy differences are present between the flight and ground test due to the shock system generated by the inlet of the flight article. These differences are highlighted to comment on the use of ground tests to predict flight performance. Details of the complex three-dimensional manner in which the combustion products and turbulence quantities vary during mode transition are also summarized.