Study On Aerodynamic Characteristics Of A High-Speed Evacuated Tube Train Under Different Running States

As a new means of transportation,the high-speed evacuated tube train has the advantages of high speed,low energy consumption and environmental protection.However,its low-pressure closed operating environment and wide cruising range from subsonic to supersonic make the aerodynamic research of high-speed evacuated tube train more challenging than that of traditional transportation vehicles.And its aerodynamic design is still in the conceptual development stage,so it is urgent to conduct comprehensive and in-depth research.The steady and unsteady aerodynamics and impact of geometric feature parameters of a high-speed evacuated tube train has been investigated under different running states by adopting the combination of theoretical analysis and numerical simulation method.This thesis mainly explores the correlation of the wave structures and its evolution characteristics with the aerodynamics of a tube train from the choking range to the supersonic unchoked range.The main contents and conclusions are as follows:1.The impact of the cruising speed,system pressure and ambient temperature on steady aerodynamics of high-speed evacuated tube train is studied.The results show that with the increase of cruising speed(from subsonic to supersonic speed),the drag of a tube train shows different trends,and the pressure drag is dominant.When choking occurs,the pressure of the head rises significantly and the drag increases sharply.The relationship between aerodynamic drag and system pressure is nearly linear,which is mainly attributed to the fact that it does not change the dominant flow structure around the tube train.With the decrease of ambient temperature,the aerodynamic drag of the train increases,which is more obvious at higher speed.2.Based on the isentropic limit and Kantrowitz limit,the lateral study with variable speed and longitudinal study with variable blockage ratio were carried out to explore the difference of steady aerodynamics of the tube train in three different flow state ranges(R1: subsonic unchoked range,R2: choked range and R3: supersonic unchoked rang).The results show that with the increase of cruising speed(R1 ～ R2 ～R3 range),the aerodynamic drag increases first,then decreases and increases again.The local minimum of aerodynamic drag in the supersonic unchoked range of R3 is mainly attributed to the disappearance of choking and the relatively weak oblique shock wave attached to the train head.In the study of variable blockage ratio,compared with the monotonically increasing trend of aerodynamic drag under subsonic velocity,the different phases of the reflected shock waves at the rear edge of the train carriage under supersonic velocity leads to non-monotonic changes in aerodynamic drag with the increase of blocking ratio(the local minimum appears at the critical blockage ratio in this study).3.Based on overlapping grid technology,the unsteady aerodynamics under two motion conditions,i.e.uniform velocity(400 m/s under choking)and uniform acceleration(100 ～ 600 m/s),were studied by means of the unsteady numerical simulation method of improved delayed eddy simulation.The focus is placed on the evolution characteristics of the flow and wave structure around the tube train in the process of the transition between the three flow states in the acceleration condition.The results show that the near-field flow of the train is relatively stable in the uniform motion of the train under the condition of choking,while the far-field shows the unidirectional propagation of the waves away from the train,which has almost no effect on the instantaneous aerodynamic drag of the train.The instantaneous aerodynamic drag of the tube train increases monotonically from subsonic unchoked to choked state.From the choked state to the supersonic unchoked state,the front shock wave of the tube train exhibits the unsteady wave evolution characteristics with return,attachment,and deformation,leading to the non-monotonic change of the instantaneous aerodynamic drag.4.The study on the influence of geometric characteristic parameters of tube train was carried out under three different flow state ranges(R1,R2 and R3).The impact of three key parameters on the aerodynamics of the tube train,namely,the length of the train head,tail and carriage(R3 only),was explored.The results show that the aerodynamic drag reduction is most obvious in the R3 range(the drag is even lower than the R2 choked range after optimization)with the increase of the length of the head,which is mainly attributed to the decrease of pressure drag by the decrease of the shock angle of the attached shock wave in front of the head.With the increase of the tail length,the aerodynamic drag reduction is most obvious in the R1 range,which is mainly attributed to the reduction of the outer folding angle of the flow at the tail leading edge.This weakens the separation of airflow at the tail resulting in a significant reduction of the pressure drag at the tail.The aerodynamic drag reduction by increasing the length of head and tail simultaneously is approximately a linear superposition of the drag reduction by increasing the head and tail alone.With the increase of carriage length,the different phases of the reflected shock wave at the rear edge of the carriage lead to nonmonotonic changes in the aerodynamic drag of the tail,and even cause a local minimum drag.In this thesis,the differences in aerodynamics of the tube train under three different flow states(R1,R2 and R3)divided by isentropic limit and Kantrowitz limit were firstly explored,especially the change law of aerodynamic drag with the change of crusing speed,which first increases and then decreases.Then,the steady wave structure and unsteady evolution characteristics of the tube train from choked to supersonic unchoked state are clarified,and the correlation between the steady wave system and the aerodynamic drag of the tube train is established.Finally,the influence of the key geometrical parameters of a tube train on aerodynamic characteristics under three different flow states is further studied,and the correlation between the phase of reflected shock wave at the back edge of the carriage and the non-monotonic variation of aerodynamic drag at the rear of the train under the supersonic unchoked state is established.This study has an important reference significance for fully understanding the complex flow and wave structure characteristics of the high-speed evacuated tube train in the whole range from subsonic to supersonic speed and the evolution characteristics of the flow field across the flow state.Meanwhile,it can provide certain guidance for the aerodynamic design of the high-speed evacuated tube train in the future.