The Structure of Inhomogeneous Turbulence in Variable Density Nonpremixed Flames

Abstract. Some observations concerning the structure of turbulence associated with an exothermic nonpremixed reacting flow are presented. Direct numerical simulations (DNS) with a resolution of 1283 grid points and an initial Reynolds number R~ = 33 provide data for the analysis. In these simulations the density varies with temperature and the resulting flow field is inhomogeneous. Conditional probabilities of the vorticity and rate-of-strain, three-dimensional visualization, and topological characteristics are presented and compared with those of a constant density flow. Initially, thermal expansion causes significant changes in the small-scale statistics. As the development continues, the statistics reflect the competing mechanisms of vortex stretching, dilatation, and baroclinic torque. Preferential alignment of the vorticity with the eigenvector associated with either the intermediate or most extensional principal strain is observed depending on the value of the local mixture fraction. Intermittent vortex structures tend to exist as sheets or ribbons rather than tubes due to the diminishing levels of vorticity and a change in the distribution and preferential orientation of the principal strains. Topological characteristics not present in constant density flows are observed. However, as the flow develops and the divergence decreases, the topology becomes similar to those of incompressible turbulence.