Surface topology of a buoyant turbulent nonpremixed flame

The temporal evolution of the three-dimensional stoichiometric mixture fraction F-st isosurface is investigated using direct numerical simulations of a turbulent nonpremixed flame with and without buoyancy. After an initial transient, the surface area of the F-st isosurface increases monotonically in time. The rate of area increase of the buoyant flame is larger than that of the nonbuoyant flame. The stretch rate of the buoyant flame surface indicates that the tangential strain rate S-t is dominant and positive at the troughs, whereas the relative propagation velocity term, u(n)del.n, is dominant and negative at the crests of the F-st isosurface. Thus, the local surface area of the crests decreases in time while that of the troughs increases, leading to the steep ridge topology. The strain rate field generated by the oppositely signed vortices saddling the F-st isosurface is responsible for this topology. The reaction rate in the buoyant flame is largest at the troughs where the scalar dissipation is maximum. (C) 2000 American Institute of Physics. [S1070-6631(00)02408-9].