Concentration measurements in simulations of the variable density mixing layer

In this paper we analyse the mixing in a plane turbulent mixing layer of variable density. The mixing layers originate from initially laminar conditions. Local Reynolds numbers, based on the velocity difference across the layer and its visual thickness, of up to 250,000 are achieved. The purpose of this study is to assess the effect of the streamwise vortex structure on the mixing statistics of the flow. In the near-field the stationary streamwise vortices substantially alter the mean concentration statistics, and the associated scalar probability density function distributions. In the far-field, the weakening of the stationary streamwise structure results in a less marked spanwise variation in the concentration statistics. High density ratio mixing layers display a non-marching scalar probability density function, whereas low density ratio mixing layers display hybrid probability density functions. At a given streamwise location the local entrainment is influenced by the action of the streamwise vortices, leading to spanwise variations in the entrainment ratio. The entrainment data are compared to both experimental and theoretical data.