Improvement And Application Of RANS/LES Turbulence Model Based On Turbulent Flow Field With Curvature And Rotation Effect

Turbulent flow field affected by curvature and rotation is a typical flow in centrifugal pumps and other rotating machinery.Curvature effect caused by channel bending and rotation effect caused by system rotation make the flow affected by centrifugal force and Coriolis force,which will lead to the increase of flow instability and obvious transient characteristics.The RANS/LES model,which compromises computational accuracy and engineering application efficiency,is called the substitution model of the Large Eddy Simulation?LES?before its wide application.It is a potential fine simulation method of complex flow in engineering application.However,the governing equation of Reynolds-averaged Navier-Stokes?RANS?is based on the time-averaged instantaneous velocity and linear assumption of turbulent viscosity,and the sub-grid model of LES in the existing RANS/LES model is also based on the isotropic assumption of small vortices,it is difficult for the current RANS/LES model to accurately simulate the turbulent flow field with centrifugal force and Coriolis force.In addition,the RANS/LES switching function of RANS/LES model determines the switching position of RANS and LES models,which affects the simulation accuracy of the whole flow.However,the RANS/LES switching function still needs further study.Thus,it is of great academic and engineering significance to study the switching function of the RANS/LES model and further to develop the RANS/LES model considering the curvature and rotation effects.In this thesis,the methods of theoretical derivation,numerical calculation and experimental verification are used to study the RANS/LES model.The current RANS/LES model is improved from RANS/LES switching function and curvature rotation,and a new RANS/LES hybrid model for the turbulence flow field with curvature and rotation effect is eventually proposed—CRQM-DES.The verification of the improved model was performed based on two classical examples of the inertial force flow field—the curved duct and the rotating channel,and the model was finally applied to the centrifugal pump simulation.The main work and achievements of this thesis are as follows:1.Introduce the numerical research status of turbulent flow field under inertial force and the research and application status of RANS/LES model.Summarize the existing improvement progress of RANS/LES hybrid model and explain the research background of this thesis.2.Elaborate the basic theory of two kinds of RANS/LES hybrid model,and deduce and summarize the general construction method of RANS/LES hybrid model based on two-equation SST k-?.On this basis,two kinds of models are applied to study the flow in 90°elbow and rotating straight channel.Based on the experimental results,the simulation results of the two kinds of hybrid models are evaluated comprehensively.The DES model is chosen as the more suitable hybrid model for the simulation of flow field under inertial force.3.The scale of filtering mesh in the RANS/LES switching function of DES hybrid model is extended by the definition of LES,while the DES boundary layer mesh is anisotropic mesh which is quite different from the LES isotropic mesh.Aiming at the anisotropy of DES boundary layer meshes,a new method of defining filtering mesh scale of DES model is proposed.The influence of DES boundary layer mesh on the working mode of DES model is studied,and the length-width-ratio characteristics of typical three-dimensional mesh size in DES boundary layer mesh are obtained.The disadvantageous influence of existing filter mesh scale on DES simulation is analyzed,and three new defining methods of filter scale which consider the size of three-dimensional mesh are proposed:?_AM??_GM??_QM.Based on OpenFOAM platform,the improved scheme is programmed.The results show that the results of QM-DES which adopts?_QM are the most accurate.4.In view of the inherent defect that DES model can not accurately simulate the anisotropic turbulence characteristics in flow field with curvature and rotation effect,Spalart-Shur curvature correction method and additional rotation term method were applied to DES model for the first time,and CRQM-DES model was obtained.In this process,the original improvement terms were transformed and transplanted into the control equation of QM-DES model.The results show that the modified model can improve the simulation accuracy of the turbulent flow field under the effect of centrifugal force and Coriolis force.CRQM-DES was then used to study the flow in curved duct under four curvatures and in rotating channels under five rotational numbers,and the effects of curvature and rotation on the internal flow were obtained respectively.5.Based on the multi-node parallel computation of Tianhe-2,the internal flow law of a centrifugal pump with a specific speed of 66.7 was studied by using CRQM-DES model.The flow structure,pressure and velocity distribution of different impeller passage cross sections along flow direction were analyzed.Finally,the influence of curvature and rotation on the flow development of streamwise cross sections in impeller was obtained.