The entrainment and large structure in turbulent mixing layers

The definition of entrainment rate is reviewed, and it is found that a definition which takes account of the intermittent structure of the flow is useful. Entrainment rates are determined from measurements in mixing layers having various velocity and density ratios. The most striking conclusions drawn are that the total rate at which the two fluids are entering the mixing layer depends on the difference in velocity between the two streams but the dependence on density ratio appears to be weak, and that the proportion of this total rate with which each fluid is entrained depends critically on its relative density and not specifically on whether it is on the high speed or low speed side of the layer. A model is proposed which appears to account for the observed spreading rates and entrainment rates and provides new insight into the basic similarity properties and flow structure behind the mechanics of the whole flow and particularly the entraining process.