The effect of inflow conditions on the transition to turbulence in large eddy simulations of spatially developing mixing layers

Abstract

Large Eddy Simulations (LES) of spatially developing turbulent mixing layers have been performed for flows of uniform density and Reynolds numbers of up to 50,000 based on the visual thickness of the layer and the velocity difference across it. On a fine LES grid, a validation simulation performed with a hyperbolic tangent inflow profile produces flow statistics that compare extremely well with reference Direct Numerical Simulation (DNS) data. An inflow profile derived from laminar Blasius profiles produces a flow that is significantly different to the reference DNS, particularly with respect to the initial development of the flow. When compared with experimental data, however, it is the boundary layer-type inflow simulation produces the better prediction of the flow statistics, including the mean transition location. It is found that the boundary layer inflow condition is more unstable than the hyperbolic tangent inlet profile. A suitably designed coarse LES grid produces good predictions of the mean transition location with boundary layer inflow conditions at a low computational cost. The results suggest that hyperbolic tangent functions may produce unreliable DNS data when used as the initial condition for studies of the transition in the mixing layer flow.

Organised large structure in the post-transition mixing layer. Part 2. Large-eddy simulation

“…The one of greatest concern in the present context is its prediction of a non-zero eddy viscosity in regions of laminar flow. As has been shown in previous work (McMullan et al 2009), this can have the effect of damping the instability modes in the pre-transition part of the mixing layer and delaying the transition itself.…”

“…In earlier work by the present authors (McMullan 2005;McMullan et al 2009) comparisons have also been made with the results of a DNS and of a simulation performed with a 'structure-function' SGS model (Metais & Lesieur 1992). These comparisons showed no evidence of any significant backscatter from the unresolved to the resolved scales (Piomelli et al 1991) in these threedimensional spatially evolving mixing layers with the inflow fluctuation levels of interest here.…”

“…It has also required realistic evolution of the inflected velocity profile through the progressive merging of the laminar boundary layers formed on the two sides of the splitter plate. As has been demonstrated elsewhere (McMullan et al 2007(McMullan et al , 2009, if a symmetrical inflected velocity profile (e.g. one of hyperbolic-tangent form) is taken instead as the inlet condition, not only is the pre-transition development of the flow altered but an unrealistically symmetrical balance of entrainment from the two free streams persists into the post-transition flow.…”

Initial condition effects on large scale structure in numerical simulations of plane mixing layers

“…Initially-laminar spatial mixing layer simulation inflow conditions are typically produced from a base mean velocity profile onto which pseudo-random white noise fluctuations are superimposed at each time step. 21,23,24,25 In the pre-transition region the nature of the imposed disturbances influences the nature of the laminar vortices. Three-dimensional random disturbances result in structures which undergo localised pairings, whilst highly two-dimensional random disturbances produce concentrated streamwise vortices between the primary rollers.…”

Late-Time Mixing Sensitivity to Initial Broadband Surface Roughness in High-Energy-Density Shear Layers

“…In particular, this sensitivity has been demonstrated to depend on whether or not the fluid flowing into the mixing layer's splitter-plate geometry was itself initially either laminar or turbulent. In addition, McMullan et al [13] recently showed that initial conditions may even affect direct numerical simulations, the benchmark for turbulence calculations. What we present here shows that in our shock-driven HED mixing layer, which has a more than adequately developed dynamic Reynolds number (Re d ¼ 10 7 ) to have evolved to be a fully turbulent mixing layer, the initial scale lengths set by the shocksurface interactions play the role of inflow condition scale lengths and influence the intermediate late time mixing of the developed flows.…”