Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

2020 
The effects of body forces (alternatively, the Froude number) on both vorticity and enstrophy evolutions within the flame brush have been analyzed using direct numerical simulation data of freely propagating statistically planar turbulent premixed flames subjected to different turbulence intensities. The turbulence parameters are taken to represent the thin reaction zone regime of premixed turbulent combustion. The enstrophy has been found to decay significantly from the unburned to the burned gas side of the flame brush for high turbulence intensities, and this trend becomes particularly prominent with the strengthening of the body force promoting unstable stratification. However, local instances of enstrophy generation have been observed, and in some cases, the decay of enstrophy is arrested across the flame brush for small turbulence intensities. This trend strengthens with the increasing magnitude of the body force promoting stable stratification. The enstrophy generation due to the baroclinic torque is primarily responsible for this local enstrophy generation for small turbulence intensities especially under the body force promoting stable stratification. This baroclinic torque contribution is also found to be responsible for anisotropic behavior of vorticity components within the flame brush. The vortex stretching and viscous dissipation terms have been found to be the leading order source and sink terms, respectively, in the enstrophy transport for high turbulence intensities especially in the case of body force promoting unstable stratification. However, baroclinic torque and the sink term due to dilatation rate continue to play significant roles even for high turbulence intensity cases considered here, but their relative importance increases with a decreasing turbulence intensity especially under the body force that promotes stable stratification. The surface-weighted entrainment velocity has been found to be mostly unaffected by the body force in this analysis, and a minor influence can be discerned in the case of small turbulence intensities where an unstable stratification tends to promote high negative values of entrainment velocity only toward the unburned gas side of the flame brush.
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