Study On Aerodynamic Shape Optimization Design Of High-speed Trains

With an increase in train running speeds,the aerodynamic problems of high-speed trains become more and more prominent,which significantly influence the operational safety,stability and environmental friendliness of trains.The aerodynamic performance of the high-speed train is greatly influenced by its aerodynamic shape.Thus,the aerodynamic shape optimization design of high-speed trains is of great importance,such as the head shape,the shape of the devices over the train,etc.For the optimization issue of the head shape of high-speed trains,the multi-objective optimization design method of the head shapes of high-speed trains running in the open air with and without crosswinds was established,and the optimization design of the high-speed train head shapes was performed.The optimization design method of high-speed trains with different head shapes was established to reduce the aerodynamic drag and noise,and the influence law of the key design parameters of the head shapes on the aerodynamic performance of the trains was studied.For the shape optimization issue of the devices over the train,the fixing position of pantographs was optimized using the optimal selection method,and the aerodynamic optimization design of a pantograph fairing shape was performed to reduce its aerodynamic drag and noise.Based on the theory of computational fluid dynamic(CFD)and multi-objective genetic algorithms,a multi-objective optimization design method of a high-speed train head shape was established.The aerodynamic drag force of the head coach and the aerodynamic lift force of the tail coach were set to be the optimization objectives,and the automatic multi-objective optimization design of the train head was carried out.A parametric model of the high-speed train was established by the CATIA software,and seven design variables of the head shape were selected.The automatic deformation of the train head was achieved through MATLAB code and CATIA script file.The script files of the software of ICEM CFD and FLUENT were integrated into the multi-discipline optimization software ISIGHT,and the automatic meshing of the external flow filed and the automatic calculation of the train aerodynamics could be obtained.The design variables were automatically updated through the multi-objective genetic algorithm NSGA-II,and the automatic multi-objective optimization of the aerodynamic head shape of the high-speed train was obtained.After optimization,the correlations between the design variables and the optimization objectives were analyzed,and the key design variables which influenced the optimization objectives were obtained.A series of Pareto optimal head shapes were obtained through the multi-objective optimization.The two optimization objectives can not reach their minimums at the same time.Compared with the original train,for the optimized train which has the minimum drag force of the head coach,the aerodynamic drag force of the head coach is reduced by 3.15%,and the aerodynamic lift force of the tail coach is reduced by 11.94%.While,for the optimized train which has the minimum lift force of the tail coach,the aerodynamic drag force of the head coach is reduced by 2.45%,and the aerodynamic lift force of the tail coach is reduced by 17.05%.To reduce the time cost in the re-meshing and the calculation of train aerodynamics in the optimization process,an approximate model between the optimization objectives and the design variables was constructed based on the Kriging method.After then,the automatic optimization of the high-speed train head shape was performed through the multi-objective genetic algorithm NSGA-II.In order to study the influence law of the key design parameters of the head shape on the train aerodynamic performance,the multi-objective optimization design models of high-speed train head shapes were established based on three typical high-speed trains.Taking the total aerodynamic drag force and the maximal surface sound power of the head coach as the optimization objectives,the automatic multi-objective optimizations of the train head shapes were carried out.After optimization,the influence law of the key design parameters of the head shape on the optimization objectives was analyzed,which included the analysis of correlation,contribution,main effect and interaction effect.The results show that the correlation between each design variable and the two optimization objectives are basically the opposite,and the most correlated variable is the portion angle of the bogie area.The interaction effect between the design variables has an influence on the optimization objectives,and it should be considered in the optimization design of train head shape.The aerodynamic models of a high-speed train running in different road conditions(ground,embankment and viaduct)under crosswinds were established.The flow field around the train was numerically calculated using detached eddy simulation(DES)method,and the characteristics of the unsteady aerodynamic loads acting on the train running in different road conditions were obtained,which included the characteristics of the time domain,the frequency domain and the unsteady flow structures around the train.The results show that obvious unsteady characteristics exist for the aerodynamic loads acting on trains under crosswinds,and the lateral aerodynamic performance of the train running on the leeward side of double-track embankment is the worst.For the most critical condition,the total aerodynamic drag force,the aerodynamic lift and side force of the head coach were set as the optimization objectives,and the multi-objective optimization design method of the high-speed train head shape under crosswinds was established.The free form deformation(FFD)method was used to obtain the morphing of the original train mesh,and the time cost in the remodeling and re-meshing was reduced.The design variables of the head shape in the sample points were obtained using the optimal Latin hypercube sampling(opt.LHS)plan,and the aerodynamic performances of the trains in the sample points were calculated by the CFD method.A Kriging approximate model which met the engineering design requirement was constructed based on the design variables and optimization objectives in the sample points.After then,the multi-objective optimization design of the high-speed train head shape was performed based on the multi-objective genetic algorithm NSGA-? combing the Kriging approximate model.After optimization,a series of Pareto optimal head shapes were obtained,and an optimal head shape was picked out through the minimum distance method for comparison with the original train.Compared with the original train,the total aerodynamic drag force of the train with the optimal head shape is reduced by 2.63%,the aerodynamic lift force of the head coach is reduced by 9.51%,the aerodynamic side force of the head coach is reduced by 2.06%,and the unsteady aerodynamic performance of the train with the optimal head shape is also improved.Based on the adjoint method and the mesh deformation technique using radial basic functions(RBF),the optimization design method of a high-speed train head shape was proposed combing the CFD method.The total aerodynamic drag force and the aerodynamic lift force of the tail coach were set to be the optimization objectives,and the multi-objective optimization design of the high-speed train head shape was carried out.The sensitivity between the objective functions and the head shape was calculated through the adjoint method.This method did not need to define any design variables of the head shape,and the influence of the artificial defined design variables on the optimization results could be avoided.The mesh deformation technique combining RBF was introduced to avoid repeated generations of the mesh in the optimization of the train head.Thus,the optimization efficiency had been improved.The results show that the adjoint method can be efficiently applied to the optimization of high-speed train head.After optimization,the total aerodynamic drag force is reduced by 2.83%,and the aerodynamic lift force of the tail coach is reduced by 25.86%.The reduction of the aerodynamic drag force is mainly located at the streamlined part of the head coach and tail coach,and the aerodynamic drag forces of the middle coach and the train body of the head coach and the tail coach basically keep unchanged.The reduction of the aerodynamic lift force of the tail coach is mainly located at the streamlined part,and the absolute value in the down lift force of the train body of the tail coach decreases slightly.Based on the theory of CFD,the aerodynamic models of a high-speed train with pantographs fixed on different positions were established,and the pantograph fixing position was optimized using the optimal selection method.The flow field around the high-speed train with pantographs fixed on different positions was numerically calculated using the DES method,and the characteristics of the unsteady aerodynamic forces acting on the pantographs were obtained,which included the characteristics of the time domain,the frequency domain and the unsteady flow structures around the pantographs.It is concluded that when the pantograph fixing position moves backward along the longitudinal direction of the train,the time-averaged values of the aerodynamic drag and lift force of the pantograph tend to decrease.When the pantographs are fixed on the front end of the sixth middle coach and the lift pantograph is in the knuckle-downstream orientation,the amount and the size of the eddies detached from the pantographs are smaller than those in other conditions,and the aerodynamic performance of the pantographs is the best.Meanwhile,in order to improve the aerodynamic performance of a pantograph fairing of a high-speed train,the aerodynamic optimization design method of the pantograph fairing shape was established based on multi-objective genetic algorithms and CFD theory.The aerodynamic drag force and the maximal surface sound power of the fairing were set to be the optimization objectives,and the automatic multi-objective optimization of the pantograph faring shape was carried out.The batch commands and script files were used to achieve the automatic deformation of the pantograph fairing and the automatic calculation of the external flow field around the train.The design variables were updated by the multi-objective genetic algorithm NSGA-?,and the automatic optimization of the pantograph fairing shape could be obtained.Compared with the original train,the aerodynamic drag force of the middle coach with the optimized fairing is reduced up to 4.21%,and the maximal surface sound power of the fairing is reduced up to 7.38%.