| Most of the train-related flow phenomena are turbulent flows.Therefore,when discussing the numerical simulation of the air flow field around the train,the problem of how to simulate the turbulence phenomenon is naturally indispensable.Especially for many complex air flow fields around trains,such as boundary layer separation and wake flow with obvious separation characteristics,besides demanding high-precision calculation methods and more suitable grids,the most important and key problem is which turbulence model to simulate properly.Turbulent flow is a common phenomenon in nature.It is a highly non-linear and complex flow.However,some numerical methods have been used to simulate turbulence,and the results are in good agreement with the reality.For turbulent motion,the numerical methods adopted can be divided into three categories: direct numerical simulation,large eddy simulation and Reynolds time-averaged equation method.In present work,STAR-CCM+,a computational fluid dynamics code with high accuracy and wide applications,was used to carry out simulation methods study.Each research object was calculated with different grids and turbulence models.In each case,it should be compared with the possible analytical results or published experimental results and numerical simulation results.The main research contents and conclusions are as follows:(1)When the air flows around the ciucular cylinder,there exists friction and positive pressure gradient,which causes the boundary layer on the surface of the cylinder to fall off and form eddy flow.The pressure drops behind the cylinder and the pressure distribution becomes different,resulting in positive resistance.The characteristics of velocity and pressure distribution and air resistance of two-dimensional ciucular cylinder under different turbulence models were studied,and the time-consuming differences of solution were analyzed.(2)The characteristics of velocity and pressure distribution,air drag and wake shape in three-dimensional rectangular channel were obtained under different turbulence models.In addition,polyhedral meshes can produce high quality results with fewer mesh elements and shorter running time than the other two mesh types.The numerical results of hexahedron,tetrahedron and polyhedron grids were in good agreement with the experimental data in the case of obstacle plate flow.But when approaching the ground,the calculated results of the three grids were quite different from the experimental results.Among them,the error of tetrahedron grids was the largest,and basically did not reflect the characteristics of backflow side reflux zone.The polyhedral mesh error was the smallest among the lattices.In addition,the comparison of different turbulence models on the same grid showed that DES and LES could reveal the unsteady characteristics of turbulence.(3)The calculation results of the RANS method were basically consistent with those of the URANS method.The main reason was that the theoretical ideas of the two methods were basically the same,and only the average information of the whole flow field was provided.The calculation of the single time step of the URANS method was essentially no different from that of the RANS method.In addition,the inflow velocity of ICE2 model was too small,which made the pressure and velocity of the whole flow field very small,and made the difference between the two methods smaller.(4)Compared with the RANS method and URANS method,DES and LES method gave more abundant flow information,especially LES method,in terms of velocity and pressure distribution characteristics,air drag coefficient,lateral force coefficient and lift coefficient of the flow field around ICE2 high-speed train,which can be used for large separation flow and train wall surface.The structure of nearby vortices was revealed more clearly. |
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