Separation Flow Study Based On RANS Method And RANS/LES Hybrid Method

The phenomenon of separation flow is a common phenomenon of turbulence in our daily life,both widespread in nature and in engineering.People in order to find some orderly movement from the irregular vortex shedding movement,the RANS/LES hybrid method has been proposed in recent years and has received wide interest,which can improve the prediction accuracy of RANS and reduce the computational complexity of LES simultaneously.Through the method of RANS/LES,the unsteady flow information of interest can be obtained and it should be very meaningfully in prospect of numerical simulation for engineering applications.The purpose of this paper is to study the separation flow phenomenon based on the traditional RANS method and the RANS/LES hybrid method.The different turbulence model development by the RANS method or the RANS/LES hybrid method are compared by different separation flow examples,and then,conducting a comprehensive evaluation about the merits and demerits of these turbulence models,and the applicability of respect model as well.What's more,a comprehensive analysis is made that the factors can affect the large separation flow of airfoil at the large angle of attack by combining with the advantages of the RANS/LES hybrid method.Firstly,the numerical simulation by the RANS method is carried out for the two models of ONERA M6 wing and DLR-F6 wing-body combination.The results show that the RANS method is less accurate for the separation induced by the shock wave on the ONERA M6 wing and the separation zone at the DLR-F6 wing-body combination.Then,the paper compares the RANS method with the RANS/LES hybrid method using the classical example of circular flow around the flow field with Re = 3900 and concludes that the SST turbulence model can calculate the excess eddy viscosity in the flow field,which will "suppress" the formation and development of three-dimensional vorticity structure.However,the RANS/LES hybrid method can well calculate the three-dimensional vorticity structure.And the error of DDES model calculation is below 5%,which is better than other models.Finally,the large separation flow of NACA0021 airfoil at large attack angle is further carried out in this paper,and a study of the selection of the turbulence model based the RANS/LES hybrid method is made.The smaller the grid scale is,the more vorticity structure is captured in the flow field,and the vorticity can be kept far away from the airfoil surface.And,with the increasing of the characteristic length,lift coefficient is closer to experimental results,the three-dimensional flow effect also will be more obvious.The results show that the relative precision of DDES and IDDES model in RANS/LES hybrid method can meet the simulation needs of engineering application.The error of the lift coefficient of the IDDES model is only 0.42%.