Research And Application Of LRN Turbulence Model Based On Transition And Separation Mechanism Of Flow Around Automobiles

Nowadays,for the accuracy of the numerical simulation of flow around automobiles,the calculated aerodynamic drag has been close to engineering needs,but there is room for improvement.And,the accuracy of the calculated aerodynamic lift could hardly meet engineering needs.Research shows that unreasonable turbulence model is the greatest source of the numerical error,and the proportion reached 15%.Currently,no turbulence model is universal,which should be constructed based on the deep understanding of the specific flow characteristics.However,the three-dimensional flow around the automobile is particularly complex.On the one hand,the lack of complete experimental methods makes it impossible to reveal the flow mechanism of automobiles.On the other hand,there is no enough computing resource to carry on the direct numerical solution.Therefor,the flow mechanism of automobiles has not been understood completely,and the appropriate turbulence model has not been constructed.Usually used turbulence models in the automobile aerodynamics are constructed based on common criteria,which is obviously inappropriate.Therefor,the turbulence model based on the deeply study of the flow mechanism of automobiles are proposed in this paper,and the precise numerical simulation strategy is constructed.Reliable theoretical basis and effective study methods for the automobile aerodynamics are provided in this study.The main contents of this paper are as follows:1.Typical local flow around automobiles is studied to reveal its mechanism.The basic theory of fluid mechanics and construction idea of commonly used turbulence models are introduced.The existing problems of commonly used turbulence models in automobile aerodynamics are concluded,especially the flow separation problem.And,the assumption that the capacity of the turbulence model to capture flow transition is the key to accurately simulate the flow around the automobile is proposed.Through the construction of reasonable calculating environment,the local flow of the automobile is regarded as the flow around a simple model.And,the mechanism of the typical local flow is studied in detail with the LES model.At last,the assumption is verified by comparing with the results achieved by the commonly used turbulence models.2.For steady issues of the flow around automobiles,a modified LRN k-? model which could accurately capture the transition phenomenon is proposed.The transition mechanisms of flow around automobiles are analyzed,and the research situation of numerical simulation is summarized in this paper.Then,the stream curvature factor and response threshold are introduced into the LRN k-?(Jones and Launder)model to solve its problems of the excessive dependence on turbulence dissipation and underestimation of the total stress.Finally,the modified LRN k-? model is achieved.The modified model is used to simulate the flow around the turbine blades,plates,cylinder and the approximate simple models.And,the results show that the predicted value of the modified LRN k-? model is close to the experimental data under different Reynolds number.3.For transient issues of the flow around automobiles,a CLES model which could capture the transition phenomenon accurately is proposed.Based on the analysis of the commonly used RANS/LES turbulence models,including zoning and non-zoning method,a modified CLES model is proposed based on the modified LRN k-? model.In comparison with the experimental data,the capacity of the modified model to capture the transition phenomenon is verified by simulating the flow around the sylinder,tunnel,wing and cavity.4.For the automobile aerodynamics,the precise numerical simulation strategy is established.The numerical error in computational fluid mechanics is introduced.Then,the empirical and discrete errors in the numerical simulation of the flow around automobiles are summarized.Considering the mechanism of the flow around automobiles,for all processes,including the automobile model,calculation domain,boundary condition,grids and discrete format,the discrete and empirical errors are minimized by the discussion of the calculation efficiency and precision.Finally,the precise numerical simulation strategy based on the modified LRN k-? model and the transition CLES model is established.5.Verification based on the complex automobile models.Local flow around diferent parts of a complex automobile model could interfere with each other,and the overall aerodynamic charactristics of the automobile is the center of attention,which makes the calculation more complex.For this reason,the Ahmed model,the complex automobile model and the wiper model are introduced to verify the accuracy of the numerical simulation strategy,and commonly used turbulence models are also introduced as the contrast.The results show that the aerodynamic characteristics obtained by the numerical simulation strategy established in this paper are in great agreements with the experimental data achived in the wind tunnel test and road test,especially the accuracy of the calculated aerodynamic lift,which has been greatly improved.In conclusion,the precise numerical simulation strategy proposed in this paper,which is based on the mechanism of the flow around automobiles,provides a reliable theoretical basis and effective numerical solution for the study of the automotive aerodynamics.