Modelling Methods And Application Research Of Boundary Layer Transition In Multi-speed Range Based On RANS Equations

Laminar drag reduction has always been a key research topic in aircraft aerodynamic design.Because influencing factors of the laminar-to-turbulent transition in boundary layer are very complicated,the research of boundary layer transition prediction becomes hot and difficult.However,the stability theory is difficult to be compatible with large-scale parallel computing of modern CFD technology,and the traditional transition model has a variety of shortcomings and deficiencies.This thesis studies the transition mechanism of the boundary layer in depth with the analysis results of the stability theory and wind tunnel experimental data based on Reynolds average method.Then,the boundary layer transition prediction methods are developed for large-scale parallel computing of modern CFD technology,and the present methods can carry on the high-precision numerical simulation of boundary layer transition phenomenon in various speed domains.Therefore,the main contents of this thesis are listed as following:(1)A detailed theoretical analysis is carried out,which involves the modeling strategy of several commonly used turbulence-transition model,stability theory prediction method and the boundary layer similarity solution.Summing up the various types of transition prediction ideas and refining the essence of the transition prediction method,theoretical basis of the new transition models development are established.(2)Based on the new indicator of turbulence intensity and pressure gradient,a new formula of bypass transition prediction is proposed,and a three-equation k-?-?turbulence transition model is constructed coupling with intermittent factor equation.The results of the zero-pressure gradient plate,plate with pressure gradient and the turbine cascade flow in high turbulence intensity environment are predicted and validated,which prove the transition prediction model to be accurate and reliable in subsonic boundary layers.(3)Through laminar kinetic energy and stability theory,the analysis of Tollmien-Schlichting(TS)wave instability mode,cross flow(CF)stationary wave instability mode and laminar separation mode is presented.Based on the analysis results,a four-equation k-?-k_L-?turbulence transition model is developed,where kinetic energy transport equation is used to predict the trigger of transition and intermittent factor transport equation to describe the transition process.A variety of two-dimensional and three-dimensional TS-wave dominated transitions,laminar separation induced transition and CF stationary wave instability dominated transitions are selected to verify the present model.The satisfactory description of various transition phenomena in test cases proves the high accuracy of present transition model in subsonic and transonic boundary layer with low turbulence intensity environment.(4)By analyzing of the characteristic parameters of the cross-flow strength,the transport equation is constructed by calculating the cross-flow displacement thickness Reynolds number.The equations containing two sets of experimental data as transition criterion,crossflow Reynolds number criterion for judgment at the unstable position,and cross-flow displacement thickness Reynolds number to determine whether the cross-flow instability transition would be triggered.Test cases include infinite swept wing,three dimensional swept wing and inclined prolate spheroid are validated to prove high accuracy of present model for crossflow stationary waves instabilities induced transition in subsonic and transonic boundary layer with low turbulence intensity environment.(5)Based on the stability theory and analysis results of first and second modes in compressible boundary layer as well as the analysis of the similarity solution in compressible boundary layer,the transport equation of eddy-viscosity coefficient of laminar fluctuations is constructed to trigger transition and the intermittent factor transport equation is used to describe transition process.Flat plate,straight cone,flared cone and double ramp are selected as test cases proving the competence of this model predicting transition phenomena accurately for Mack first-mode and second-mode dominated transition in supersonic and hypersonic boundary layers.