Inter-scale turbulence-chemistry couplings in premixed turbulent combustion for large-eddy simulations

With the ever-increasing computing power and advances in computational technologies, Large-Eddy Simulation (LES) has become a feasible tool for the design of aerospace and propulsion devices. In traditional LES, the filtered dynamical equations must be solved on a grid that resolves a large percentage of the variance associated with the turbulent fluctuations. As such, the evolution of turbulence motions and scalar quantities over these scales are primarily governed by the dynamical interactions explicitly resolved on the grid, with the effects of the unresolved, Sub-Filter-Scale (SFS) dynamics on the Resolved-Scales (RS) of higher order and to be modeled. However, in turbulent combustion systems, chemical reactions leading to heat release typically occur over scales unresolvable by any practical LES grid, which violates the assumption underlying traditional LES as discussed above. In this talk, we explore new modeling elements that can be potentially embedded in existing LES frameworks to capture dynamically-important inter-scale couplings between RS and SFS, necessary for more accurate predictions of the evolution of turbulent combustion events at RS. We first perform a scale-based decomposition of key primary variables, including momentum, energy, and chemical species. We then present the kinematic relationship between coherent structures in physical and scale spaces for these variables, characterized as SFS and RS contributions in the context of LES. Finally, we identify a subset of the SFS contributions that have substantial impact on the RS evolution through convective nonlinearities. These dynamically-dominant SFS modes are found to be localized near the SFS-RS interface in scale space and near the thin flame heat-release zone in physical space.