On the stress-strain alignment in premixed turbulent flames

The interaction of large and small scale structures is fundamental to the energy cascade in turbulent flows. The correct representation of this interaction becomes important in the context of large eddy simulation (LES), where the response of small-scale structures to the resolved quantities, or large-scale structures, must be parametrised. This challenging task becomes more demanding when LES of premixed flames are considered, as heat release affects the interaction of turbulence and chemistry occurring at the unresolved scales. In this work, the influence of sub-grid scale (SGS) stresses on the kinetic energy budget of the resolved velocity field in turbulent premixed flames is investigated. In this spirit, the alignment between the SGS stresses and the resolved strain rate has been analysed by interrogating a direct numerical simulation (DNS) database of statistically planar premixed flames subjected to forced isotropic turbulence. It has been found that the alignment between the eigenvectors of the SGS stresses and the resolved strain rate changes across the flame brush and this change is dependent on the level of turbulence experienced by the flame. The influence of different turbulence intensities and different filter widths along with the implications of this misalignment on the SGS modelling are discussed in detail in the paper.