Hybrid reynolds-averaged/large-eddy simulation of a cavity flameholder: Modeling sensitivities

Abstract

Steady-state and scale-resolving simulations have been performed for flow in and around a model scramjet combustor flameholder. The cases simulated corresponded to those used to examine this flowfield experimentally using particle image velocimetry. A variety of turbulence models were used for the steady-state Reynolds-averaged simulations, which included both linear and nonlinear eddy viscosity models. The scale-resolving simulations used a hybrid Reynolds-averaged/large-eddy simulation strategy that is designed to be a large-eddy simulation everywhere except in the inner portion (log layer and below) of the boundary layer. Hence, this formulation can be regarded as a wall-modeled large-eddy simulation. This effort was undertaken to formally assess the performance of the hybrid Reynolds-averaged/large-eddy simulation modeling approach in a flowfield of interest to the scramjet research community. The numerical errors were quantified for both the steady-state and scale-resolving simulations before making any claims of predictive accuracy relative to the measurements. The hybrid Reynolds-averaged/large-eddy simulation results were also carefully scrutinized to ensure that even the coarsest grid had an acceptable level of resolution to meet accepted guidelines for large-eddy simulation and that the time-averaged statistics were acceptably accurate. The autocorrelation and its Fourier transform were the primary tools used for this assessment. Both simulation strategies accurately predicted the mean streamwise velocity distribution within the cavity, although the Reynolds-averaged simulations that used a linear eddy viscosity model tended to overpredict the strength of the primary cavity recirculation zone. Second-order moments of the velocity field were found to be highly sensitive to the turbulence model chosen for the Reynolds-averaged simulations, with all models overpredicting the intensity of the velocity fluctuations within the cavity flameholder. The hybrid Reynolds-averaged/large-eddy simulation results also overpredicted the velocity variances and covariances, unless a filtering operation was applied using a filter size that matched the control volume used to process the particle image velocimetry measurements. This observation suggests that a significant fraction of the turbulence energy was not resolved by the measurements. Taking this uncertainty into account, the second-order statistics extracted from the hybrid simulation strategy could not be shown to be any more accurate than the "best" Reynolds-averaged result.