Choked Flow Characteristics And Aerodynamic Heating Effect In Low-Evacuated Tube Of High-Speed Trains

With the development of high-speed railways,the further speed-up of high-speed trains in dense atmosphere is restricted by aerodynamic problems such as high drag and high noise,as well as complex railway line.The tube train run at low drag and high speed by extracting air and maintaining a low-pressure environment in the tube,and is not disturbed by the external environment.It has become one of the key reserve technologies for future high-speed rail transit.The high-speed airflow generated by the train running in the tube with large blockage ratio causes multi-scale,unsteady flow heating and heat transfer problems.The gas in the tube is compressed and rubbed,forming a strong aerodynamic heating.Under the interference of shock waves,the flow field fluctuates frequently and violently.The deterioration of the local flow field causes the deformation of the vehicle body surface,reduce the aerodynamic performance of the vehicle body,and threaten the driving safety.The study on the flow field characteristics and aerodynamic heating effects of tube train is in its infancy,and the study on the flow field structure and aerodynamic heating characteristics is quite scarce.It is urgent to carry out the research on the evolution characteristics of the flow field.Therefore,establishing a correct aerodynamic thermal effect prediction model,improving the prediction ability of the flow structure,and studying the evolution characteristics of the flow field are very important for this research.Based on the traditional high-speed railway aerodynamics,combined with the relevant theories and calculation methods in the aerospace field,this study carried out research work on grid technology,turbulence model,grid division strategy,simulation method,etc.,and obtained a numerical calculation method for flow and heat transfer around tube train.Combined with the wind tunnel model test data,the accuracy of the calculation method is verified.Aiming at the complex flow phenomenon in the tube,the flow structure inside the tube under different flow phenomena is studied.The research shows that there are two main phenomena: choked and unchoked.The condition of the throat reaching the sound speed is the basis for judging the choked flow phenomenon.When the flow in the tube is choked,the flow field in the annular space is affected by the flow characteristics of the variable cross-section area.The overall temperature/pressure distribution from the train head to train tail is high,and the shock waves at the shoulder and train tail will cause local flow field fluctuations.The temperature rise of the local flow field in the front of the train head exceeds 50 K,the temperature rise of the stagnation point of the train head exceeds 88 K,and the pressure of the flow field is about 1.7 times of the initial pressure in the tube.When the flow in the tube is unchoked,there are differences in the distribution of the flow field corresponding to different incoming Mach numbers.When the incoming flow is supersonic,the flow field in the tube maintains supersonic speed,and a bow-shaped shock wave is formed near the train head.Affected by the shock wave or expansion wave,the local flow field has large fluctuations in temperature and pressure.When the incoming flow is subsonic,the flow field in the tube maintains subsonic speed,and there is no obvious wave system structure.Based on the wave structure characteristics in supersonic flow,combined with the temperature/pressure characteristics of the flow field and the continuous compression effect in the choked flow,the formation and evolution mechanism of the wave system in the flow field were studied.The study found that the moving normal shock wave,oblique shock wave and expansion fan structure formed in the tube,which have reflection and interaction in the flow field.The reflection and interaction of the wave system form a diamond-shaped shock train in the wake.The irregular reflection of the shock wave induces turbulent shedding to form an unstable shear layer,which acts together with the shock wave in the wake to aggravate the fluctuation of the flow field.The temperature difference of the tube wall is close to 130 K,the temperature gradient of the flow field near the central axis of the tube is large,and the temperature rise exceeds 99 K.On the whole,the train body and the wake area are greatly disturbed by the wave system.The strong stagnation aerodynamic heating effect and the shock wave separation-attached aerodynamic heating effect will cause a local high temperature area,and the local temperature rise can reach 100 K.A high-pressure area appears near the train head.Based on the research on the identification of the aerodynamic heating area,the sensitivity analysis of characterization parameters for the aerodynamic heating effect is carried out.The results of local sensitivity analysis show that with the increase of the blockage ratio,the temperature growth rate slows down;with the increase of the vacuum degree,the pressure increase rate decreases.Using the regression analysis method and the approximate model method,the global sensitivity analysis was carried out,and it was found that the prediction results of the two methods on the sensitivity of the effect parameters were consistent.The lowest;the interaction between Mach number and vacuum and blockage ratio has a great influence on the pressure in the aerodynamic heating effect.Aiming at the potential safety hazard caused by the aerodynamic heating effect of the tube train,a multi-parameter coupling relationship model of the tube train with the temperature/pressure in the aerodynamic heating effect as the target is established.The matching combination set of the main parameters to minimize the temperature/pressure is determined.From the perspective of comprehensive consideration of constraints such as construction cost and geographical conditions,the required scheme can be determined in the combination set for reference in the aerodynamic design of the tube train.The research results show that for the multivariate parameter combination,the model prediction levels under different algorithms are equivalent,and the error does not exceed 1%;A numerical simulation model is established,it is found that the prediction accuracy of the temperature is relatively high,and the prediction difference of the pressure is large,but the error does not exceed 4%.