On the alignment of the α-strain and vorticity in turbulent nonpremixed flames

Evolution of the strain and vorticity fields in a turbulent nonpremixed flame is examined using direct numerical simulations. The results show that the flow is strain dominated for both the buoyant and nonbuoyant cases. The evolution equation for the cosine of the angle between the vorticity and the principal strain directions is derived. It is shown that the strain dominance causes vorticity to align with the α-strain, in contrast to the β-strain/vorticity alignment in constant density flows. The divergence term in the evolution equation changes the time-scale of the process. The rotation term (due to the strain coordinates rotation) and the baroclinic term reduce the vorticity/α-strain alignment.