ON THE ALIGNMENT OF STRAIN, VORTICITY AND SCALAR GRADIENT IN TURBULENT, BUOYANT, NONPREMIXED FLAMES

The alignment of vorticity and scalar gradient with the eigendirection s of the rate of strain tensor is investigated in turbulent buoyant no npremixed 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 th ese regions. Examination of the vorticity field and the vorticity prod uction in these different regions indicates that Psi and consequently the alignment properties near the flame surface identified by the mixt ure 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) vor ticity/beta alignment. The implication of this result is that the scal er dissipation, epsilon(F), attains its maximum value always near F ap proximate to F-st. These results are also discussed within the framewo rk 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 attrac ting solution are also consistent with our results of buoyant turbulen t nonpremixed flames. (C) 1998 American Institute of Physics.