Effects of Polynomial Order and Stabilization Technique on the Large Eddy Simulation of an Axisymmetric Scramjet Combustor using High-Order Flux Reconstruction

The high-order flux reconstruction method is used to analyze the dual-mode operation of an axisymmetric inlet-isolator-combustor flowpath experimentally tested in the ACT-II arc-heated combustion tunnel at the University of Illinois at Urbana-Champaign. A 7-species/3-step reaction mechanism developed at the Air Force Research Laboratory is used in these calculations. Modifying the computation of the backwards rate was found to admit a favorable solution. An isothermal wall of 375 K is assumed. A baseline solution is obtained using a divergence-based filter for solution stabilization on a grid with 20 million solution points and cubic elements (P3, 4th order). This solution compares well with experimental wall pressure measurements, capturing the shock train location correctly while over-predicting the cavity pressure. The cavity pressure discrepancy is caused by the presence of a cavity-wake stabilized instead of a jet-wake stabilized flame. Next, a solution is obtained which uses linear, P1, elements while maintaining the same number of solution points and stabilization mechanism. The P1 solution was found to capture the inlet shocks better but was much more dissipative for unsteady flow features. It was unable to sustain a lit cavity without using ignition aids. Additionally, a stabilization scheme based on a local minimum entropy is implemented for non-reacting flow. This scheme admits a solution with lower dissipation than the divergence filter and requires minimal user-input. The choice of stabilization technique was found to have a much larger effect on the higher-order P3 solution than the 2nd order P1 solution.