Large eddy simulation of transitional boundary layers at high free-stream turbulence intensity and implications for RANS modeling

Large-eddy simulations of transitional flows over a flat plate have been performed for different sets of free-stream-turbulence conditions. Interest focuses, in particular, on the unsteady processes in the boundary layer before transition occurs and as it evolves, the practical context being the flow over low-pressure turbine blades. These considerations are motivated by the wish to study the realism of a RANS-type model designed to return the laminar fluctuation energy observed well upstream of the location at which transition. sets in. The assumptions underlying the model are discussed in the light of turbulence-energy budgets deduced from the simulations. It is shown that the pretransitional field is characterized by elongated streaky structures which, notwithstanding their very different structural properties relative to fully established turbulence, lead to the amplification of fluctuations by conventional shear-stress/shear-strain interaction, rather than by pressure diffusion, the latter being the process underpinning the RANS-type transitional model being investigated.