Improved prediction of low-pressure turbine wake mixing by Delayed Detached Eddy Simulation, including an algebraic model for bypass transition

We present a hybrid RANS-LES technique of the Delayed Detached Eddy Simulation (DDES) form for transitional flow through low-pressure turbine blade cascades. A first ingredient is representation in RANS-mode of the flow that approaches the blades and the flow near the blade surfaces. The objective is to provide the correct level of modelled turbulent kinetic energy to an algebraic model for bypass transition which is coupled to a turbulence model, and thus simulation of bypass transition in attached boundary layer state by a RANS-model component. A second ingredient is resolution of the Kelvin-Helmholtz instability of a laminar separated boundary layer in RANS-mode, with transition in RANS-mode by a model component or transition in LES-mode by resolved breakdown. The third ingredient is representation in LES-mode of a wake flow and a separated layer that enters a wake, such that the wake mixing is by resolved fluctuations. For realisation of the RANS-zones and the LESzones, we derive an appropriate sensor function. We demonstrate the good functioning of the technique for transitional flows of different types.