Validation of unsteady flamelet models for non-premixed turbulent combustion with intermittency

Flamelets play an important role as subgrid models in large eddy simulations of turbulent flames: they are based on a one-dimensional steady asymptotic solution for the flame. The focus of the present study is to validate their use when unsteadiness and multidimensional effects are present, as to be expected for turbulent flows. To shortcut the prohibitively expansive step of solving the complete Navier-Stokes equations in the turbulent regime, a synthetic turbulentlike flow field is specified, which allows for extensive yet affordable simulations and analysis. The flow field consists of a simple steady horizontal shear with a time-periodic vertical sweep. Despite the simplicity of the flow field, the passive scalar response displays qualitatively many characteristics observed in experiments with fully turbulent flow, in particular, in terms of the strong departure from Gaussianity of its probability distribution function. The same set-up is utilized for the reactive case in order to generate challenging conditions to test the robustness of unsteady versions of the laminar flamelet models. We analyze the asymptotic behavior of the models for a large range of Damköhler and Peclet numbers in the presence of intermittency and confirm for those demanding test-cases the good performance of the models that had been observed for less-demanding onedimensional test-cases with smooth time behavior. In particular, the performance of the models is quite satisfactory in the intermediate regimes where neither the very fast nor the very slow chemistry asymptotic approximation would be appropriate.