Robert J. Yentsch

Unsteady Three-Dimensional Mode Transition Phenomena in a Scramjet Flowpath

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 general...

Numerical Investigation of Dual-Mode Operation in a Rectangular Scramjet Flowpath

Three-dimensional turbulent simulations with combustion modeling are employed to understand fundamental phenomena encountered in the Hypersonic International Flight Research Experimentation Flight 2 scramjet flowpath. Experimental data from the ground-test campaign as well as a grid resolution study are used to validate the results of the simulations. These confirmed results are then leveraged to explain the physics of dual- and scramjet-mode operation in a steady sense, including detailed examinations of shock structure, boundary-layer separation, and combustion chemistry. Simulations of the flight and ground tests are compared in order to characterize the effects of the inlet on the boundary-layer development and internal shock structures. Overall, agreement with ground-test data is excellent, and the simulations show highly complex three-dimensional structures in the flowpath. Boundary-layer separation induced shock–boundary-layer interactions dominate dual-mode operation, whereas at higher Mach number...

Numerical Investigation of the HIFiRE-2 Scramjet Flowpath

Three dimensional turbulent simulations with combustion modeling are employed to understand fundamental phenomena encountered in the HIFiRE-2 scramjet flowpath. Experimental data from the ground test campaign as well as a grid resolution study is used to validate the results of the simulations. These confirmed results are then leveraged to explain the physics of dual and scramjet-mode operation in a steady sense, including detailed examinations of shock structure, boundary layer separation, and combustion chemistry. Initial results of unsteady simulations are used to investigate the process of mode-transition, and a simplified heat release model is developed and tested. Simulations of the flight test are presented and contrasted with the ground test results to characterize the effects of the inlet on the boundary layer development and internal shock structures. Overall, agreement with ground test results is excellent, and the simulations show highly complex three dimensional structures to be present in the flow. Boundary layer separation induced shock / boundary layer interactions dominate dual-mode operation, while at higher Mach numbers a flameholding effect of the barrel shock from fuel injection is striking. The effects of the inlet are found to be of great importance to accurately model flight tests, however, the same macro-scale effects as observed in the ground test simulations are dominant.

Unsteady Three-Dimensional Phenomena in Mode-Transition Simulations of the HIFiRE-2 Scramjet Flowpath

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 modetransition 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.

Numerical Investigation of Hypersonic Phenomena Encountered in HIFiRE Flight 1

A hierarchy of computational techniques is employed to examine several external aeropropulsive phenomena of interest in hypersonic vehicles, including transitional flow on a forebody cone, shock wave/boundary layer interactions (SWBLI) and supersonic flow in a channel. Inviscid and turbulent analyses are performed at a laminar-to-turbulent transition point in the flight path of the HIFiRE-1. The baseline inviscid analysis is computed with Cart3D, while the viscous Reynolds averaged Navier-Stokes (RANS) simulations were performed using the commercial code GASP with ak ! turbulence model on a 17 million point structured mesh. The results of the simulations are compared to the experimental pressure and heat transfer data, and the performance of the turbulence model in the separated boundary layer is discussed. The analysis suggest that the nominal point in the flight path chosen for simulation represents a transitional point between dierent regimes, complicating accurate prediction. Preliminary results at a later point in the trajectory yield better agreement with the experimental data.

Performance of Turbulence Modeling in Simulation of the HIFiRE-1 Flight Test

Pressure and heat transfer data from the HIFiRE-1 flight test are employed to evaluate the performance of a k-ω turbulence model at different Mach and Reynolds numbers along the flight path. A sequence of three-dimensional structured meshes is employed to discretize the domain consisting of a cone (transition experiment) and a flare (shock–boundary layer interaction experiment) with channel cutouts (mass capture experiment). Variations in model parameters are also explored and the effects are discussed. The overall flow structure agrees with expectations and observations from earlier ground tests, including laminar-to-turbulent transition on the cone, strong shock–boundary layer interactions at the compression corner, and a complex flow in the channel. Trends in predicted surface pressure generally compare well to the flight test data, although the point of separation at the flare is upstream of observation by about 2% of body length, or 20% of separation length in all cases. Heat transfer agreement is go...

Comparison of Mode-Transition Phenomena in Axisymmetric and Rectangular Scramjet Flowpaths

Dual to scramjet mode-transition phenomena in rectangular and axisymmetric scramjet geometries are compared through three-dimensional, unsteady Reynolds Averaged Navier-Stokes simulations. The rectangular case geometry is taken from previous work on the HIFiRE-2 ground test, whereas the axisymmetric geometry is a hypothetical case which reformulates the cross sectional area ratios of the HIFiRE-2 into a round configuration to examine the effects of corner separation (exists in the rectangular but not axisymmetric configuration) on the mode-transition process. After describing the previously validated computational methodology, including meshing, theoretical model, and numerical procedure, the mode transition simulations are described for both the rectangular and round geometries. The adverse pressure gradient induced boundary layer separation forward of the primary injectors that dominates dual-mode operation in the rectangular geometry is muted in the axisymmetric geometry. However, the mode transition event in both cases is triggered by a loss of primary injector interaction induced boundary layer separation, i.e., the collapse of the flameholding from the primary injectors. Differences in flame holder and combustion patterns are described.