A comparative study of inflow conditions for two- and three-dimensional spatially developing mixing layers using large eddy simulation

A series of spatially developing mixing layers are simulated using the large eddy simulation (LES) technique. A hyperbolic tangent function and data derived from boundary layer simulations are used to generate the inflow condition, and their effects on the flow are compared. The simulations are performed in both two and three dimensions. In two-dimensional simulations, both types of inflow conditions produce a layer that grows through successive pairings of Kelvin-Helmholtz (K-H) vortices, but the composition ratio is lower for the hyperbolic tangent inflow simulations. The two-dimensional simulations do not undergo a transition to turbulence. The three-dimensional simulations produce a transition to turbulence, and coherent structures are found in the post-transition region of the flow. The composition ratio of the three-dimensional layers is reduced in comparison to the counterpart two-dimensional runs. The mechanisms of growth are investigated in each type of simulation, and amalgamative pairing interactions are found in the pre-transition region of the three-dimensional simulations, and throughout the entire computational domain of those carried out in two-dimensions. The structures beyond the post-transition region of the three-dimensional simulations appear to behave in a much different manner to their pre-transition cousins, with no pairing-type interactions observed in the turbulent flow. In order to accurately simulate spatially developing mixing layers, it is postulated that the inflow conditions must closely correspond to the conditions present in the reference experiment. Copyright (9 2007 John Wiley & Sons, Ltd.