Density ratio effects on the simulated plane turbulent mixing layer

Abstract

We present the results from a large eddy simulation (LES) study of the plane turbulent mixing layer, at various values of density ratio, and velocity ratio, between the constituent freestreams. The inflow conditions are representative of those found in laboratory experiments with clean laminar upstream initial conditions. A low-Mach number approximation solver is used to perform the LES, and an inflow condition generator produces upstream conditions which mimic those found in laboratory experiments. The simulated mixing layers display modifications to the visual thickness growth rate as a function of density and velocity ratio that are in excellent agreement with experimental observations. The mean flow statistics are biased toward the low-density stream, and the magnitude of the turbulent statistics display a functional dependence on the velocity ratio, for non-uniform density flows. The “Brown–Roshko” structure is observed for all values of density ratio. It is shown that the mean structure spacing is a function of both density ratio and velocity ratio, with the mean structure spacing scaling universally as approximately 1.5 times the local visual thickness. The evolution of the streamwise structure is functionally the same for all values of density ratio, and the streamwise structures lose their coherence downstream of a non-dimensional pairing parameter greater than 16, corresponding to the third generation of interactions between primary spanwise structures. The influence of baroclinic torque on the variable density flow is examined. The results of the study are discussed in the context of the archival literature.