On the alignment of strain, vorticity and scalar gradient in turbulent, buoyant, nonpremixed flames

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 Psi is used to identify these regions. Examination of the vorticity field and the vorticity production in these different regions indicates that Psi and consequently the alignment properties near the flame surface identified by the mixture fraction band F approximate to F-st differ from those in the fuel region, F>F-st and the oxidizer region, F<F-st. The F approximate to F-st band shows strain-dominance resulting in vorticity/alpha alignment while F>F-st (and F< F-st for the vertical flame) band(s) show(s) vorticity/beta alignment. The implication of this result is that the scaler dissipation, epsilon(F), attains its maximum value always near F approximate to F-st. 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. (C) 1998 American Institute of Physics.