Abstract:
Although the hypothesis of statistically local isotropy has been extensively applied to turbulence theory and modeling, the heat release near turbulent premixed flames can significantly change the fluid density and viscosity, leading to anisotropic statistics in turbulent flow fields. We extend the vortex-surface field (VSF) to characterize anisotropic vortex surfaces in turbulent premixed combustion. We find that a tangle of twisted vortex tubes on the unburnt side are stretched along the streamwise direction near the flame front owing to the thermal expansion, and the small-scale vortex tubes gradually merge into large-scale bulky structures on the burnt side. By analyzing the enstrophy transport equation and the Lumley triangle of Reynolds stresses, we find that the anisotropy of velocity fields increases from the unburnt to the burnt side. Thus, the variation of the geometry of vortex surfaces near the flame front is highly correlated to the anisotropic statistics of the fluctuating velocity.