Abstract
The alignment of vorticity and scalar gradient with the eigendirections of the rate of strain tensor is investigated in turbulent buoyant nonpremixed horizontal and vertical flames. The uniqueness of a buoyant nonpremixed flame is that it contains regions with distinct alignment characteristics. The strain-enstrophy angle Ψ is used to identify these regions. Examination of the vorticity field and the vorticity production in these different regions indicates that Ψ and consequently the alignment properties near the flame surface identified by the mixture fraction band F≈Fst differ from those in the fuel region, F>Fst and the oxidizer region, F<Fst. The F≈Fst band shows strain-dominance resulting in vorticity/α alignment while F>Fst (and F<Fst for the vertical flame) band(s) show(s) vorticity/β alignment. The implication of this result is that the scalar dissipation, εF, attains its maximum value always near F≈Fst. These results are also discussed within the framework of recent dynamical results [Galanti et al., Nonlinearity 10, 1675 (1997)] suggesting that the Navier-Stokes equations evolve towards an attracting solution. It is shown that the properties of such an attracting solution are also consistent with our results of buoyant turbulent nonpremixed flames.
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