Study On Aerodynamic Characteristics Of Evacuated Tube High Speed Train

The faster,safer,more environmentally friendly and more energy-saving mode of transportation has always been the goal of mankind.However,if the train speed exceeds 400km/h,the aerodynamic noise and aerodynamic drag induced by the traditional high-speed wheel-rail train will increase sharply.Excessive aerodynamic noise can cause environmental pollution and excessive aerodynamic drag can lead to increasing energy loss.The traditional wheel-rail high-speed train is hard to meet the requirements of higher speed travel,and the combination traffic mode of evacuated tube and high-speed train has gradually become a research hotspot.Due to the higher running speed of high-speed train in the evacuated tube,new aerodynamic problems such as choked flow,shock wave and aerodynamic heat occur.Therefore,it is of great significance to study the aerodynamic characteristics of evacuated tube high-speed train for the development of evacuated tube transportation system in the future.On the basis of summarizing the existing computation methods of the flow field generated by the evacuated tube train,this thesis studies the aerodynamic characteristics of evacuated tube train in different spatial dimensions.The main contents and conclusions are as follows.The quasi 1-D adiabatic viscous unsteady flow model of evacuated tube high speed train is established,the critical chocked state of airflow in the tube is analyzed,and the flow in the tube is classified based on the flow mechanism.Meanwhile,the corresponding Mac-Cormack difference scheme is used to study the effects of train speed,ambient pressure,blockage ratio and other parameters on the aerodynamic characteristics in the tube.The results show that as the train operates at transonic speed,the chocked flow occurs,a normal shock wave(NSW)is formed near the TC(tail car)nose,and the overflow effect at the boundary inlet is obvious.As the train operates at supersonic speed,the flow congestion in the tube is aggravated,NSW near the TC nose moves downstream,meanwhile the overflow effect is enhanced.The environment change has little effect on the relative distribution of aerodynamic parameters in the tube.Comparing the three streamlined styles,namely,cone,ellipse and parabola,the elliptical streamlined shape helps NSW near TC move downstream.The blockage ratio has little effect on the position of NSW in wake,but with the blockage ratio increase,the overflow effect and NSW strength are enhanced.The quasi 2-D steady flow model of evacuated tube high speed train is established.The aerodynamic viscosity changing with temperature is modeled by the Sutherland equation,and the turbulence motion is predicted by SST k-ω model.The effects of blockage ratio and train speed on the aerodynamic characteristics in the tube are studied.The results show that the boundary layer distribution around TC is closely related to the full development of the shock wave and expansion wave system in the wake area.If the wake wave system is fully developed,increasing blockage ratio or decreasing operation Mach number is conducive to delaying the boundary layer separation.Under different blockage ratios,the turning point of the maximum and minimum values of the vehicle drag and surface pressure changing with operation Mach number is also related to the full development of wake wave system.The variation trend of the maximum or minimum pressure on the tube wall is consistent with that on the train surface.Different from the maximum or minimum pressure,the maximum or minimum temperature of train surface is not only affected by shock waves,expansion waves,but also related to the temperature boundary layer.The variation trend of the maximum or minimum temperature of the tube wall is consistent with that of the maximum or minimum pressure of the tube wall,meanwhile there is a same turning point.The quasi 2-D unsteady flow model of evacuated tube high speed train with moving boundary is established.The shock motion is captured by adaptive grid method,and the train motion is realized by dynamic stratification method.The influence of blockage ratio on the aerodynamic characteristics of train in transonic pipe is studied.The results show that the larger the blockage ratio is,the earlier the complete wave system appears in the tube.With the movement of the train,the reflected shock waves near the head car(HC)and the mid car(MC)experience the process of formation,development and disappearance.In the shock wave reflection process,the regular reflection appears at the tube wall,while the Mach number reflection appears at the axis and the head car wall.The velocity of NSW in front of the train has experienced rapid growth stage,first steady stage,continuous fluctuation stage and final steady stage,while its intensity has experienced rapid growth stage,first steady stage,sudden weakening stage and final steady stage.Compared with NSW in front of the train,the strength of NSW in wake is lower,but its change is more complex due to the influence of reflected shock waves and reflected expansion waves.The expansion wave thins the temperature boundary layer of TC,which increases the train wall temperature.After the boundary layer separation,the oblique shock wave increases the temperature at the TC nose.The 3-D steady flow model of evacuated tube high speed train is established,the airflow physical properties are constructed by molecular dynamics theory model,the SST k-ω transition model is used to predict boundary layer transition,and the influence of suspension gap on three typical flow states is studied.As the train speed is 500,600 and 800km/h,respectively,the flow in the tube is in three different flow states: no congestion,just congestion and complete congestion.As the flow begins to be chocked,an obvious NSW occur in the TC streamline transition area,and the boundary layer separation point moves upstream compared with the unchoked flow state.As the flow is completely chocked,an oblique shock wave occurs near the TC cab,resulting in the reflection of oblique shock wave and expansion wave in the wake.The boundary layer forms separation bubble at the TC cab,and the steady and closed downwash vortex occur near the TC nose,forming the impact on the ground.The separated flow is often accompanied by the boundary layer transition.The boundary layer transition mainly occurs in the head shape transition area,suspension gap inlet area,TC nose and other separated flow locations.Due to no slip ground wall and the small suspension gap,there is an obvious ground effect between vehicles and ground,producing the obvious temperature spatial gradient.As the flow begins to be chocked,the local supersonic region where the flow Mach number increases with train speed occurs due to the ground effect in the suspension gap inlet area.In most areas of the suspension space,the flow Mach number decreases gradually along the downstream,and the temperature on the ground and train bottom wall increases gradually along the downstream.Reducing the suspension height is conducive to increasing the maximum temperature on the train surface,ground and tube wall,and increasing the train speed is conducive to reducing the minimum temperature on the ground and tube wall.The 3-D unsteady flow model of evacuated tube high speed train is established.Combined with DDES model,the turbulent dissipation term of SST k-ω transition model is modified to study the unsteady aerodynamic characteristics under three typical flow states.The results show that compared with the steady flow,the unsteady flow has a greater impact on the fluctuation of train lift and less on that of train drag.Compared with the 3-D steady flow,the time-averaged distribution trends of wall load and slip velocity are generally consistent.Before the boundary layer is completely separated,there is little difference between the instantaneous boundary layer and the time-averaged boundary layer.After the boundary layer is separated,the boundary contour distribution is greatly affected by the wake vortex.Whether the flow is chocked or not,the main frequency of train aerodynamic change is in the low frequency band.Compared with not completely chocked flow,as the completely chocked flow causes that the main frequency of pressure fluctuation caused by the wake vortex is lower,the main amplitude of pressure fluctuation in most areas of the wake is smaller,and the aerodynamic fluctuation of the downstream suspension space is faster and more.The 3-D unsteady flow model of evacuated tube high speed train with moving boundary is established.The train movement is realized by the dynamic layering method.The influence of suspension height on the aerodynamic characteristics of transonic train is studied.The results show that the influence of suspension height on the train bottom is obvious due to the ground effect.The suspension height decrease is conducive to expanding the low-pressure area at the HC bottom and the high-temperature area at the train bottom.On the longitudinal symmetry line of the train,the suspension height decrease has little effect on the pressure and significantly increases the temperature.Based on the pressure or temperature distribution on the longitudinal symmetry line of the tube wall,the pressure or temperature distribution in the tube can be divided into five sections.The suspension height decrease has almost no effect on the wall pressure and temperature,but shortens the formation time of NSW.The suspension height decrease is conducive to enhance the ground effect and its impact on the ground.Different computation methods have different advantages and disadvantages.Generally speaking,the steady flow computation method cannot consider the wave propagation characteristics in the pipe and cannot accurately predict the aerodynamic heat distribution on the train surface,but it has certain rationality in pressure distribution and aerodynamic calculation.Compared with the high-dimensional model,the low-dimensional model has the advantage of fast calculation speed and can predict the change trend of train pressure and aerodynamic force in the pipe,but it loses the actual geometric characteristics of the train and cannot consider the ground effect and accurately predict the distribution of train surface load.