  We study the dynamics and properties of a turbulent flame, formed in the
presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type
turbulence in an unconfined system. Direct numerical simulations are performed
with Athena-RFX, a massively parallel, fully compressible, high-order,
dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion
model represents a stoichiometric H2-air mixture. The system being modeled
represents turbulent combustion with the Damkohler number Da = 0.05 and with
the turbulent velocity at the energy injection scale 30 times larger than the
laminar flame speed. The simulations show that flame interaction with
high-speed turbulence forms a steadily propagating turbulent flame with a flame
brush width approximately twice the energy injection scale and a speed four
times the laminar flame speed. A method for reconstructing the internal flame
structure is described and used to show that the turbulent flame consists of
tightly folded flamelets. The reaction zone structure of these is virtually
identical to that of the planar laminar flame, while the preheat zone is
broadened by approximately a factor of two. Consequently, the system evolution
represents turbulent combustion in the thin-reaction zone regime. The turbulent
cascade fails to penetrate the internal flame structure, and thus the action of
small-scale turbulence is suppressed throughout most of the flame. Finally, our
results suggest that for stoichiometric H2-air mixtures, any substantial flame
broadening by the action of turbulence cannot be expected in all subsonic
regimes.
