Investigations and Improvement of Robustness of Reduced-Order Models of Reacting Flow

Abstract

The impact of chemical reactions on the robustness and accuracy of projection-based reduced-order models (ROMs) of fluid flows is investigated. Both Galerkin and least squares Petrov–Galerkin ROMs are shown to be less robust in reacting flows as compared with nonreacting flows. In particular, reacting flow ROMs show a strong sensitivity to resolution and are often unstable. To identify the main underlying causes, a representative problem that contains the essential physics encountered in typical combustion dynamics problems is chosen. Comparisons with nonreacting solutions are used to assess the impact of reactions. Investigations are focused on three potential areas of significance: 1) preservation of conservation laws; 2) loss of dissipation; and 3) existence of unphysical local phenomena. Results indicate that conservation is relatively well controlled and the global dissipation in the ROMs is actually larger than that in the underlying computational fluid dynamics solutions. Spurious local phenomena are, however, highly deleterious. Specifically, the steep temperature gradients that characterize combustion can lead to oscillations in local temperatures even in the absence of reactions. Representative calculations with physics-based temperature constraints verify that eliminating such excursions results in considerable improvement in both stability and future-state prediction capability.