Statistical Behavior and Modeling of the Flame Normal Vector in Turbulent Premixed Flames.

An understanding of the flame normal vector and the relationships between its components is important in flamelet modeling of premixed turbulent combustion. The product of flame normal components N i N j is closely related to the magnitude of the tangential strain rate acting on the flame. In the context of flame surface density (FSD)-based modeling, an accurate estimate of the mean or filtered values is necessary for the evaluation of the mean tangential strain rate in Reynolds-average Navier-Stokes (RANS) calculations and the resolved tangential strain rate in large-eddy simulation (LES). In the present work, the mutual interaction between local flame normal components is studied using three-dimensional compressible direct numerical simulations (DNS) of statistically planar turbulent premixed flames. Detailed statistics of the flame normal vector are presented. The probability density functions (pdfs) of combinations of the flame normal components reveal the predominance of locally cylindrical curvature on the three-dimensional wrinkled flame surface. This is confirmed by the pdf of the curvature shape factor, which is found to be consistent with previous DNS studies. The fluctuations of the normal components are shown to be anisotropic in nature, which is in accordance with previous experimental results obtained in two dimensions. Three different approaches currently in use for modeling < N i N j > s in the RANS context are extended in order to provide new models for in LES, and the predictions of these new models are assessed in comparison to values of obtained from LES-filtered DNS data. [ABSTRACT FROM AUTHOR]