John A. Boles

Large Eddy/Reynolds-Averaged Navier-Stokes Simulation of a Mach 5 Compression-Corner Interaction

Simulations of Mach 5 turbulent flow over a 28-deg compression corner are performed using a hybrid large-eddy/ Reynolds-averaged Navier-Stokes method. The model captures the mean-flow structure of the interaction reasonably well, with observed deficiencies relating to an underprediction of the displacement effects of the shock-induced separation region. The computational results provide some support for a recent theory concerning the underlying causes of low-frequency shock-wave oscillation. In the simulations, the sustained presence of a collection of streaks of fluid with lower/higher momentum than the average induces a low-frequency undulation of the separation front. Power spectra obtained at different streamwise stations are in good agreement with experimental results. Downstream of reattachment, the simulations capture a three-dimensional mean-flow structure, dominated by counter-rotating vortices that produce wide variations in the surface skin friction. Predictions of the structure of the reattaching boundary layer agree well with experimental pitot pressure measurements. In comparison with Reynolds-averaged model predictions, the hybrid large-eddy/Reynolds-averaged Navier-Stokes model predicts more amplification of the Reynolds stresses and a broadening of the Reynolds stress distribution within the boundary layer that is probably due to reattachment-shock motion.

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

Computational predictions of transverse injection of air, helium, and ethylene into a Mach 1.98 crossflow of air are presented. A hybrid large-eddy simulation/Reynolds-averaged Navier―Stokes turbulence model is used. A blending function, dependent on modeled turbulence variables, is used to shift the turbulence closure from the Menter t-ω model near solid surfaces to a Smagorinsky subgrid model in the outer part of the incoming boundary layer and in the jet mixing zone. The results show reasonably good agreement with time-averaged Mie-scattering images of the plume structure for both helium and air injection and with experimental surface pressure distributions, even though the penetration of the jet into the crossflow is slightly overpredicted. Predictions of ethylene mole fraction at several transverse stations within the plume are in good agreement with time-averaged Raman-scattering mole-fraction data. The model results are used to examine the validity of the commonly used assumption of the constant turbulent Schmidt number in the intense mixing zone downstream of the injection location. The assumption of a constant turbulent Schmidt is shown to be inadequate for jet mixing dominated by large-scale entrainment.

Simulations of High -Speed Internal Flows using LES/RANS Models

Computational stud ies of transverse sonic injection of ethylene into a Mach 1.98 cro ss flow and Mach 5 flow of air into a subscale inlet / isolator configuration are presented . A hybrid large -eddy simulation / Reynolds -averaged Navier -Stokes (LES/RANS) turbulence model is used , with the two -equation Menter -BSL closure for the RANS part of the flow and a Smagorinsky -type model for the LES part of the flow . A time -dependent blending function, dependent on modeled turbulence variables, is used to shift the closure from RANS to LES. Turbulent st ructures are sustained through the use of a ‘random -walk’ recycling / rescaling technique . The ethylene injection results using the hybrid model show s very good agreement with the Raman scattering data collected at the Air Force Research Laboratory . The LE S/RANS database is used to examine the validity of the commonly -used assumption of a constant Schmidt number in the intense mixing zone downstream of the injection location . Predictions of Mach 5 flow into the inlet / isolator are compared with particle imaging velocimetry (PIV) and wall pressure data obtained at the University of Texas . Preliminary computational results are presented for two cases involving shock -train propagation within the isolator, one of which leads to inlet unstart. Here, the compu tational method appears to predict more flow separation than indicated in the experiment, leading to stronger shock trains that are not stabilized at the correct position.

Multi-wall Recycling / Rescaling Method for Inflow Turbulence Generation

A recycling / rescaling technique is proposed for Large Eddy Simulations (LES) and hybrid LES/Reynolds-Averaged Navier Stokes (RANS) applications. This method is a generalization of the flat plate technique of Edwards et al. (2008) to a flow affected by multiple walls. Mach 3.5 flow in a square duct is simulated to test the new method, and results are compared to the experimental data of Davis and Gessner (1986). The impact of using a spanwise shifting technique and a quadrant averaging technique within the new recycling / rescaling technique is shown. As a further proof of concept, simulations of flow within a hypothetical isolator geometry are presented.

Hybrid LES / RANS Simulation of a Mach 5 Compression- Corner Interaction

*† ‡ Simulations of Mach 5 turbulent flow over a twenty-eight degree compression corner are performed using a hybrid large-eddy / Reynolds-averaged Navier-Stokes (LES/RANS) method. The model captures the mean-flow structure of the interaction reasonably well, with observed deficiencies relating to an under-prediction of the displacement effects of the shock-induced separation region. The computational results provide some support for a recent theory concerning the underlying causes of low-frequency shock wave oscillation. In the simulations, the sustained presence of a collection of streaks of fluid with lower / higher momentum than the average induces a low-frequency undulation of the separation front. Power spectra obtained at different streamwise stations are in good agreement with experimental results, indicating that the hybrid LES/RANS model is capable of predicting both low- and high-frequency dynamics of the interaction. Downstream of re-attachment, the simulations capture a three-dimensional mean-flow structure, dominated by counterrotating vortices that produce wide variations in the surface skin friction. Predictions of the structure of the re-attaching boundary layer agree well with experimental Pitot pressure measurements. In comparison with Reynolds-averaged model predictions, the hybrid LES/RANS model predicts more amplification of the Reynolds stresses and a broadening of the Reynolds stress distribution within the boundary layer that is probably due to reattachment shock motion.

Hybrid LES / RANS Simulation of Transverse Sonic Injection into a Mach 2 Flow

A computational study of transverse sonic injection of air and helium into a Mach 1.98 cross-flow is presented. A hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) turbulence model is used, with the two-equation Menter baseline (Menter-BSL) closure for the RANS part of the flow and a Smagorinsky-type model for the LES part of the flow. A time-dependent blending function, dependent on modeled turbulence variables, is used to shift the closure from RANS to LES. Turbulent structures are initiated and sustained through the use of a recycling / rescaling technique. Two higher-order discretizations, the Piecewise Parabolic Method (PPM) of Colella and Woodward, and the SONIC-A ENO scheme of Suresh and Huyhn are used in the study. The results using the hybrid model show reasonably good agreement with time-averaged Mie scattering data and with experimental surface pressure distributions, even though the penetration of the jet into the cross-flow is slightly over-predicted. The LES/RANS results are used to examine the validity of commonly-used assumptions of constant Schmidt and Prandtl numbers in the intense mixing zone downstream of the injection location.

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.

LES/RANS Simulation of a Supersonic Reacting Wall Jet

This work presents results from large-eddy / Reynolds-averaged Navier-Stokes (LES/RANS) simulations of the well-known Burrows-Kurkov supersonic reacting wall-jet experiment. Generally good agreement with experimental mole fraction, stagnation temperature, and Pitot pressure profiles is obtained for non-reactive mixing of the hydrogen jet with a non-vitiated air stream. A lifted flame, stabilized between 10 and 22 cm downstream of the hydrogen jet, is formed for hydrogen injected into a vitiated air stream. Flame stabilization occurs closer to the hydrogen injection location when a three-dimensional combustor geometry (with boundary layer development resolved on all walls) is considered. Volumetric expansion of the reactive shear layer is accompanied by the formation of large eddies which interact strongly with the reaction zone. Time averaged predictions of the reaction zone structure show an under-prediction of the peak water concentration and stagnation temperature, relative to experimental data and to results from a Reynolds-averaged Navier-Stokes calculation. If the experimental data can be considered as being accurate, this result indicates that the present LES/RANS method does not correctly capture the cascade of turbulence scales that should be resolvable on the present mesh. Instead, energy is concentrated in the very largest scales, which provide an over-mixing effect that excessively cools and strains the flame. Predictions improve with the use of a low-dissipation version of the baseline piecewise parabolic advection scheme, which captures the formation of smaller-scale structures superimposed on larger structures of the order of the shear-layer width.

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.