On A Window-embedded RANS/LES Method

The combination of RANS and LES is one feasible solution to unsteady flow prediction while ensuring both accuracy and efficiency. This thesis develops a Window-Embedded RANS/LES(WE-RANS/LES) method using explicit interface for integrated computationin different zones. To investigate and apply this multi-scale/multi-model method of turbulence simulaiton,focus is placed on its three components: RANS, LES and the interfacein between.The in-house CFD code NSAWET is improved in computational effienciency for massive high-resolution numerical simulations. Its accuracy of drag prediction is alsoverified. This lays a solid foundation for the current work on WE-RANS/LES.In the RANS zone, the k-kLtwo-equation turbulence model is selected. The realization correction is introduced to improvethe computational robustness. Itshows great advantages in predicting massively separated flows. Applied into WE-RANS/LES, this modelcan provideinformation on the length scales of unsteady flows, which facilitates the construction of synthetic turbulence fluctuations.An implicit LES method is presented combining the MDCD reconstruction and the SLAU numerical flux. In the validation offree decaying turbulence, internal flows and subcritical flows around circular cylinders, results agree well with the experimental or DNS data. The effects of grid density and numerical dissipation are discussed, and some recommendations are made on the grid cell scale. As a good choice for the LES part of WE-RANS/LES, the MDCD/SLAU scheme exhibits reasonable dissipation to reproduce the subgrid-scale flow behaviors.To bridge the spatial and temporal scale gap between RANS and LES, synthetic turbulence is generated near the interface. A window boundary condition considering two-way information exchange is developed by introducing a weight function. A unified digital filter is proposed to approximate the turbulence-like spectrum with a relatively simple formula.The yielding WE-RANS/LES method is validatedbyseveral testcases.Preliminary resultsconfirm its capacity and penitential in turbulence-resolving simulations.