Unsteady Simulation of Dual-Mode Transition Flows Using a Hybrid LES/RANS Approach with Improved Sub-Grid Scale Models

Hypersonic air-breathing propulsion technologies hold great promise for revolutionizing America’s means of accessing space. However, the successful design of hypersonic propulsion systems is hindered by inadequate simulation and modeling capabilities. This inadequacy is most problematic in the dual-mode transition regime, where a vehicle transitions from ramjet to scramjet operation. In this regime, the isolator and combustor flow fields typically contain a complex shock train, shock-boundary-layer interactions, and large regions of separated flow. Combustion processes are typically tightly-coupled to these fluid mechanics and are often mixing-limited. It is especially important that designers understand the effect of these phenomena on the flow and combustion physics in order to guarantee vehicle operability. Unfortunately, even today, standard computational fluid dynamics (CFD) approaches are unable to address these inherently unsteady phenomena to the degree required for engine operability analyses. Significant advancements to CFD numerics and combustion models are necessary to enable robust simulation of engineering-level engine concepts. Consequently, the current award supported the validation of advanced numeric and simulation techniques for the hybrid large eddy/Reynolds-averaged Navier Stokes (LES/RANS) simulation of supersonic cavity flows and the development of a compressible flamelet combustion model for application to RAS and LES of high-speed reacting flows. First, in an effort to characterize the numerical and physical limitations of current large-eddy simulation approaches for flows of engineering interest, simulations of a supersonic recessed cavity flow were performed using a hybrid large-eddy/Reynolds-averaged simulation approach with an inflow turbulence recycling procedure and hybrid advection scheme. Simulations were performed to assess grid sensitivity of the solution, efficacy of the turbulence recycling, and the effect of the shock sensor used with the hybrid advection scheme. Analysis of the turbulent boundary layer upstream of the rearward-facing step indicated excellent agreement with theoretical predictions. Mean velocity and pressure results were compared to Reynolds-averaged simulations and experimental data and indicated good agreement. Coarse grid simulations indicated strong grid density sensitivity and were performed with and without inflow turbulence recycling to isolate the effect of the recycling procedure, which was demonstrably critical to capturing the relevant shear layer dynamics. Shock sensor formulations of Ducros and Larsson were found to predict mean flow statistics equally well. Results of this study of a supersonic cavity will facilitate the application of large-eddy simulation as a tool for engineering design and development. Subsequent work was devoted to the a priori analysis and development of a compressible flamelet combustion model to make higher-fidelity reacting flow simulations more affordable while maintaining significant combustion physics. An a priori investigation of the applicability of flamelet-based combustion models to dual-mode scramjet combustion was performed utilizing a Reynolds-averaged Navier-Stokes (RANS) simulation. For this purpose, the HIFiRE Direct Connect Rig flow path, fueled with a JP-7 fuel surrogate and operating in dual- and scram-mode was considered. The chemistry of the JP-7 fuel surrogate was modeled using a 22 species, 18- step chemical reaction mechanism. Simulation results were compared to experimentallyobtained, time-averaged, wall pressure measurements to validate the RANS solutions. The analysis of the dual-mode operation of this flow path showed regions of predominately nonpremixed, high-Damkohler number, combustion. Regions of premixed combustion were also present but associated with only a small fraction of the total heat-release in the flow. This is in contrast to the scram-mode operation, where a comparable amount of heat is released from non premixed, low-Damkohler number and premixed combustion modes, respectively. Compressibility and wall heat transfer were shown to have a significant effect on the combustion for both dual-mode and scram-mode operation. Representative flamelet boundary conditions were estimated by analyzing probability density functions for temperature and pressure for fuel and oxidizer conditions. These results helped to guide the subsequent development of a compressible flamelet-model for application to high-speed reacting flows of engineering complexity. With the ultimate goal of making high-fidelity CFD simulations of hydrocarbon-fueled scramjet combustors affordable using realistic chemical reaction mechanisms, a compressible flamelet model was formulated that extends the incompressible flamelet model formulation to high-speed, compressible flows. The compressible model uses static pressure and sensible enthalpy to model the effects of compressibility and oxidizer temperature, respectively, on the combustion chemistry. The model was tested using a priori flamelet-modeled Reynolds-averaged Navier Stokes simulations of the HIFiRE engine combustor operating in a dual-mode configuration. These simulations were compared to Reynolds-averaged Navier-Stokes simulations using a reduced chemical mechanism, in addition to available experimental data and to a priori flamelet modeled simulations using an incompressible flamelet model formulation. Progress variable formulations were investigated for their ability to recover static temperature, chemical heat release, and major and minor product species. The effects of pressure on the tabulated progress variable source term were analyzed. Also, a novel model for the effects of oxidizer temperature variation was developed, in which static enthalpy serves as a proxy tracking scalar for oxidizer temperature. The final model formulation is used to parameterize all species mass fractions by mixture fraction, progress variable, pressure, and static enthalpy. A priori flamelet-modeled simulations show this model to be a significant improvement over standard incompressible flamelet models. A posteriori testing of the compressible flamelet model is underway, and final validation will be performed using the HDCR simulations studied previously during the a priori study.