On the Growth of a Plane Mixing Layer From Laminar or Turbulent Initial Conditions

In this paper the effect of the state of the separating high-speed boundary layer on the spatially developing mixing layer is analysed through Large Eddy Simulation. The initially-laminar mixing layer has an inflow condition obtained from simple Blasius profiles, whilst the initially-turbulent flow obtains a turbulent high-speed side boundary layer from a recycling and rescaling method. The recycling and rescaling method produces reasonable turbulent statistics, given the uncertainty in the reference experimental data. The simulations demonstrate that the rate of growth of the integral scale in the initially-turbulent mixing layer is significantly reduced when compared with the initially laminar flow. The simulations also provide evidence for a relaxation in the growth rate of the initially-turbulent mixing layer towards that of the idealised initially-laminar flow. Flow visualisation demonstrates that coherent structures of the Brown-Roshko form are present in both the initially-turbulent and initially-laminar mixing layers, with the emergence of the structures in the initially-turbulent flow occurring at a considerable distance downstream of the trailing edge of the splitter plate. Recommendations for the documentation of initial conditions in mixing layer experiments are made, such that more accurate numerical simulations of the flow type can be performed.