Influence of spanwise domain on the Large Eddy Simulation of an idealised mixing layer

In this paper a series of planar mixing layers are simulated using Large Eddy Simulation. This study focuses on the influence of the spanwise domain on the development of the mixing layer, and the evolution of the coherent structures in the flow. The mixing layers originate from initially laminar conditions, and an idealised inflow condition is found to produce accurate flow predictions when the spanwise computational domain extent is sufficient to avoid confinement effects. An extremely narrow computational domain is found to inhibit the transition to turbulence, and to restrict the flow to develop in a manner similar to a mixing layer confined to a two-dimensional box. The reason for this restriction is that the formation of secondary, streamwise vortices is inhibited in the initial region of the mixing layer. With a sufficiently wide computational domain, the secondary streamwise vortices form and the flow undergoes a transition to turbulence. Once the secondary structure has formed, spanwise domain confinement results in a reduction in the growth rate of the integral scale of the mixing layer. A criterion for the specification of the minimum spanwise domain for a spatially-developing mixing layer is proposed.