Investigation Of The Inflow Condition For The Hybrid LES/RANS Method

Within recent thirty years, the development of high performance computer(HPC) and computational fluid dynamics(CFD) has opened up broad prospects for numerical simulation of turbulent flows. Among these turbulence simulation techniques, hybrid LES/RANS methods combine advantages of both and have been widely used in high-Reynolds number and massively separated flows. They maintain as RANS in boundary layer and switch into LES in regions where turbulent fluctuations are intense, and detached-eddy simulation method(DES) is a representative of them. In many engineering problems, research is only concerned with the fully developed turbulent stage and the simulation of transition process is a waste of computational resources, which overwhelms the advantages of hybrid methods. With the proper inflow turbulent boundary conditions, computational domains can be limited to turbulent regions, which can greatly decrease the amount of computational. In this thesis, some methods to generate appropriate inflow conditions for detached-eddy simulation are proposed. These methods are applied to three-dimensional flow case, which can promote hybrid LES/RANS methods applications in practical conditions of engineering.In the introduction, firstly, the importance and necessity for developing the proper methods to generate inflow conditions for hybrid LES/RANS methods are discussed. And then the art of the states of hybrid LES/RANS and inflow generation methods are introduced in detail. At the end, the primary works of this thesis are described briefly.In the second chapter, numerical methods applied in this study are presented in detail, which include the non-dimensional governing equations, boundary conditions, spatial discretization schemes, time integration methods, turbulence models with its solution method and detached-eddy simulation method.In the third chapter, firstly, the rescaling-recycling method(RRM) of Lund is introduced in detail. A series of inflow generation methods based on RRM, which can be applied in compressible flows or coupled with LES/RANS methods, are summarized. And then the synthetic eddy method(SEM) is introduced briefly. At the end, several new inflow generation methods, which combine advantages of RRM and SEM, are proposed.In the fourth chapter, a spatial-mode turbulent compressible flat-plate boundary layer with free-stream Mach number 2.99 is simulated with improved delayed detached-eddy simulation(IDDES) method, the inflow boundary conditions are generated using RRM and several new methods. Mean flow and turbulent statistical quantities are compared with DNS results and theoretical values. Inflow quality generated by RRM and the new methods are assessed through comparison of skin friction, mean velocity profiles, turbulence intensities and coherent structures.In the fifth chapter, RRM and the new inflow generation method are applied to three-dimensional flow cases through the simulation of compressible boundary layer over a blunt cone with free-stream Mach number 2.99. Mean flow quantities and flow structures are compared to test feasibility of the two inflow generation methods.In the sixth chapter, concluding remarks of all the works in this thesis are given. Some deficiencies and considerations on the further research work are listed briefly.