Mark A. Hagenmaier

Numerical Investigation of a Supersonic Cavity Flameholder

Simulation results are presented for non-reacting flow within a supersonic cavity flameholder. The freestream is air at Mach 2. A case is simulated with no fuel injection, and two cases are simulated with different rates of ethylene fuel injected through holes located on the back face of the cavity. The simulations correspond to a series of experiments for which particle image velocimetry measurements of two velocity components were made within the cavity. The flow within the cavity is computed using unsteady hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation, as well as steady-state Reynolds-averaged NavierStokes simulations. A thorough grid resolution study is presented in which the resolution required to resolve the flow within the cavity is determined. The effects of wall temperature and the thickness of the oncoming turbulent boundary on the solutions is examined and found not to affect the mean velocity and turbulence, and to affect mean mixing only slightly. For steady-state Reynolds-Averaged Navier-Stokes simulations, several turbulence models are used and compared to the hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation results, and the effects of the turbulent Schmidt number, are investigated. The influence of the side walls are investigated by comparing simulations of the full-width duct to simulations of a partial-width duct that uses periodic boundary conditions. The results of the simulations are also compared to the velocity measurements from the experiments, and the hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation results are found to compare well with the experiment in most locations.

Dual-Mode Scramjet Combustor: Numerical Sensitivity and Evaluation of Experiments

Abstract : Experiments were performed at the Air Force Research Laboratorys Propulsion Directorate (AFRL/RZ) in Research Cell 22 (RC22). Twelve cases from the experiment were computationally analyzed and each case varied in either engine operating condition and/or combustor configuration. Initial computations were performed on all twelve cases to establish a baseline computational approach. Computations were performed on one of the cases to test sensitivity to turbulent Schmidt number, reaction rate, and grid resolution. Improvements to the baseline analysis using the results from the sensitivity analyses were extended to two additional cases. It was shown that adjustments in Schmidt number, reaction rate, or grid refinement improved the agreement with experimental data for two cases relative to the baseline results, but worsened agreement for the third case. It was left undetermined that grid refinement was a better approach to improving the baseline analysis as compared to calibrations in Schmidt number and/or reaction rate. Improvement to the grid using local refinement in regions with chemical reactions produced better results for one case and was computationally less expensive than globally refining the grid. Negligible differences were shown between results that were obtained using wall functions with Y+ value as high as 38 or results obtained using wall integration with Y+ values around one. Negligible differences were shown between periodic results that were obtained by averaging results using either a constant CFL or a constant time step. CFL-averaging a result using the constant CFL approach was 5.4 times less computationally expensive than using the constant time step approach. Computations showed that 2.53-lbm/sec. of air leaked into the exhauster housing at the exit of the combustor in RC22s test apparatus.

Hybrid Reynolds-Averaged and Large-Eddy Simulations of a Supersonic Cavity Flameholder

Simulation results are presented for non-reacting flow within a supersonic cavity flameholder. The freestream is air at Mach 2. A case is simulated with no fuel injection, and two cases are simulated with different rates of ethylene fuel injected through holes located on the back face of the cavity. The simulations correspond to a series of experiments for which particle image velocimetry measurements of two velocity components were made within the cavity. Reynolds-averaged Navier-Stokes simulations are used to examine the influence of the finite-width, low-angled slot used to seed the flow for the particle image velocimetry. The simulations indicate that the seeder has little influence on the flow within the cavity, allowing for the seeder to be neglected in the remainder of the simulations. The flow within the cavity is simulated using steady-state Reynolds-averaged Navier-Stokes simulations, as well as unsteady hybrid Reynolds-averaged Navier-Stokes and large-eddy simulations. A thorough grid resolution study is presented in which the resolution required to resolve the flow within the cavity is determined. The results of the simulations on the final grids are compared to the velocity measurements from the experiments and the hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation results are found to provide improved agreement with certain aspects of the flow. The simulation results are then used to investigate the mixing within the cavity, which was not measured in the experiments. The mixing information from the simulations provides further insight into the physics of the cavity flameholder flowfield.

Effect of Flow Distortion on Cavity-Assisted Fuel Injection

This paper shows simulations and analysis of a new cavity-assisted fuel injection experiment with an upstream shock generator. This simulation is meant to simulate the effects of shock distortion on mixing flows with a cavity flameholder. Hybrid LES/RANS is performed in order to show its ability to simulate a shock boundary layer interaction. Details of the computation and an overview of the experiment will be presented.

Flow Distortion: Computational Investigation of a Shocked Cavity Flameholder

Cavity flameholder experiments with incident shocks have recently been performed at the Air Force Research Laboratories (AFRL) Aerospace Systems Directorate (RQ) in Research Cell 19 (RC19). The incident shocks are intended to replicate flow distortion from an inlet. A Computational Fluid Dynamics (CFD) effort, described here, was performed for evaluation and assessment. The computations used the Reynolds-averaged-Navier Stokes (RANS) approach with a 2-equation turbulence model and a 22-species finite-rate kinetics model. In general, CFD results are in reasonably good agreement with the experiment. The analysis indicates that flow distortion has a significant impact on the cavity flowfield, which can lead to ignition failure as observed by the experiment for one of the configurations. Insights from the CFD were used to shed light into the ignition problems, revealing that conditions for the case that did not light were not favorable for flameholding.

Analysis of Hybrid LES / RANS Simulations of a Back-Pressured Supersonic Isolator

This paper shows the simulation and analysis of a back-pressured isolator using a hybrid LES / RANS method. These simulation were performed to analyze the ability of LES-type solvers to replicate the unsteady nature of shock trains in rectangular isolators. A partial width simulation is shown to determine the viability of using fewer computational cells to gain similar insights into the unsteady nature of shock trains as that of a full geometry simulation.

Hybrid LES / RANS Simulations of Shock-Distorted Injection Plumes

This paper shows the results of the simulation and analysis of a new injection experiment with shock impingement downstream of the injector. These simulations show the eects of shock distortion on mixing flows. Hybrid LES/RANS is shown to improve on the performance of steady-state RANS for these flows. A planar-averaged mixedness parameter was calculated to better quantify the impact of shock impingement. Details of the computation and experiment will be presented.

Hybrid Large-Eddy Simulation / Reynolds-Averaged Navier-Stokes Simulations of Sonic Injection into Mach 2 Crossflow

This paper presents numerical simulations of sonic injection of ethylene into a Mach 2 crossflow of air. Experimental Raman scattering imagery, pressure-sensitive paint data and NO-PLIF images are compared with hybrid LES/RANS simulations. The impact of a low-diusion method on the hybrid LES/RANS solutions is investigated. The eect of grid coarsening is also investigated. The low-dissipation approximation (LD-PPM) is shown to be less impacted by the grid coarsening, but it is numerically unstable at high injection pressures. A three-dimensional visualization of velocity tensor discriminant is used to show the dierences between the vortical structures of PPM and LD-PPM as well as the impact of grid coarsening on these structures. The implications of these findings on future research is also presented.

The Advantages and Costs of Higher-Fidelity Turbulence Modeling

This paper compares the results and the computational efficiency of a research code with that of a commercial code on the same problem. The research code (REACT-MB) is tested using its unsteady hybrid large-eddy simulation/Reynolds-averaged Navier-Stokes (LES/RANS) method as well as a more common steady-state Menter RANS method. CFD++ of Metacomp is tested on the same problem using its realizable k-ε turbulence model. Normal sonic ethylene injection through a circular injector into a Mach 2 cross-flow was simulated by each code. Time-averaged statistics of the hybrid LES/RANS computations and converged solutions from the RANS computations are compared with experimental contours of time-averaged mixture fraction. Scalability of the codes is also compared.

Uncertainty Quantification for a Scramjet Inlet Flow

A numerical framework is presented for CFD-based uncertainty quantification (UQ) involving propagation of combined aleatory and epistemic uncertainties to relevant output quantities, with intended application to scramjet inlet analysis. Latin hypercube sampling routines are used in combination with surrogate response surface methodology and nested Monte Carlo sampling to generate probability boxes and other UQ results based on CFD output data. For a demonstration case involving subscale Mach 6 semi-freejet testing of an inward turning inlet configuration, a total of 21 CFD simulations are performed, and probability boxes are computed for three output quantities through consideration of four different input uncertainties. Global sensitivity analysis calculations are also performed to assess relative contributions of each input uncertainty. For the current demonstration case, we find Mach number uncertainty from thermal expansion of the facility nozzle to be a dominant contributor to uncertainty in the inlet mass flow rate, while mounting angle uncertainties account for large contributions to uncertainty in wall pressure at the isolator throat.