SPECTRAL-ANALYSIS OF A SIMULATED PREMIXED FLAME SURFACE IN 2 DIMENSIONS

This paper presents two-dimensional direct numerical simulations of a passive flame surface passing through homogeneous isotropic turbulence . The flame was represented by a field variable, G(x, t), whose isocon tours constitute flame surfaces. One well known complication in analyz ing premixed combustion in a homogeneous environment is decoupling the effect of the decaying turbulent velocity field from the dynamics of the flame surface. To overcome this, the velocity field was made stati onary by introducing a random forcing term into the Navier Stokes equa tions. Forcing was done over two different ranges of wavenumbers (k(f) = 10-14, and k(f) = 80-84) thus creating turbulence with different le ngth scales and inertial range power laws. By comparing the response o f the flame to the two types of turbulence it was possible to determin e the effect the spectral distribution energy has on the surface topol ogy and mean rate of propagation. Indeed, the flames were found to be remarkably sensitive to the spectral distribution of the turbulent ene rgy, and not just its magnitude. Furthermore, a k(-5/3) inertial range was shown to produce a flame surface that was preferentially wrinkled at intermediate to small scales for purely geometric reasons. By defi ning a surface area spectrum it was possible to rationalize this resul t by recognizing that flame surface area is closely related to the dis sipation spectrum of the scalar field. Collectively the results sugges t that knowledge of the energy spectrum al a minimum is required to pr edict a turbulent flame speed under general circumstances.