Theoretical And Numerical Studies On Aerodynmaics Of Subsonic Evacuated Tube Train Under Low Vacuum Condition

Subsonic evacuated tube maglev train is a new type of ground high-speed transportation in the future.It is of great significance to study the flow field in the tube and aerodynamic characteristics of the train for the tube design,energy saving and operation safety.Based on the one-dimensional isentropic gas theory and intake similarity theory,the ideal model of a subsonic evacuated tube train is established.The flow characteristics inside the tube is studied.And the formulas for solving the parameters of flow inside the evacuated tube are derived.Based on computational fluid dynamics(CFD),the flow characteristics inside the tube and the aerodynamic characteristics of the train are simulated.The differences in results obtained using different numerical approaches are compared.The relationship between the aerodynamic characteristics of the train in the evacuated tube and the blocking ratio,atmospheric pressure and train’s running speed is studied.The effect of the length of the streamlined nose on the aerodynamic characteristics in different flow states is analyzed.Firstly,the phenomenon of aerodynamic choking in the tube is clarified based on the onedimensional isentropic gas and intake similarity theory.And the flow chart of solving the flow parameters including pressure,density and temperature in the tube is established under different flow conditions.The results show that when the train’s running speed is subsonic and the corresponding relationship between running speed and blockage ratio exceeds Kantrowitz limit,a choked flow is shown in the tube.When the flow inside the tube is not choked,the whole tube can be divided into 3 flow regions including the undisturbed region in front of the train,low pressure region around the train and undisturbed region at the rear of the train.When the tube is choked,the whole tube can be divided into 7 flow regions including the undisturbed region in front of the train,high pressure region in front of the train,throat region around the train,region in front of the shock wave at the rear of the train,region behind the shock wave at the rear of the train,region in front of the expansion wave of the train and undisturbed region behind the train.Secondly,the aerodynamic numerical model of a subsonic evacuated tube train is established.The effects of non-viscous gas and viscous gas on the aerodynamic performance of the train are studied using the steady and transient numerical simulation methods,respectively.And the results are compared with the theoretical results.The results show that when the tube is non-choked and the internal gas is as assumed to be viscous,the aerodynamic characteristics of the train are basically the same using the steady or transient simulation.The error in the aerodynamic resistance of the train is small.And the flow parameters in the tube are basically consistent with the theoretical results.When the tube is choked and the gas is as assumed to be viscous,the aerodynamic resistance of the tail car solved by the steady simulation is slightly larger than that by the transient simulation.And the flow parameters in the tube are basically consistent with the theoretical results.Then,the aerodynamic numerical model of a 600 km/h maglev train running in an evacuated tube is established.The feasibility of the numerical method is verified by comparing the numerical results with the wind tunnel test data.The relationship between aerodynamic resistance of train and running speed,blockage ratio and atmospheric pressure is studied.The formula of the train’s aerodynamic resistance in the evacuated tube is obtained.The critical atmospheric pressure in the tube under different blockage ratios and running speed is determined by taking the aerodynamic resistance of the train with a speed of 600 km/h in open air as a reference value.The results show that the aerodynamic resistances of the head car,middle car and whole train increase continuously with the increase of train’s running speed when the tube is choked.And the aerodynamic resistance of the tail car firstly increases and then decreases.When the train’s running speed and the pressure in the tube are constant,the aerodynamic resistances of the front and middle cars increase with the increase of the blockage ratio.The aerodynamic resistance of the train is proportional to the atmospheric pressure inside the tube.Finally,the aerodynamic numerical models of evacuated tube trains with different streamlined nose length are established to study the effect of streamlined nose length on the aerodynamic characteristics of evacuated tube train.4 typical operation states are the open air operation,non-choked state,shock attached state and shock detached state,repsectively.The results show that,the aerodynamic resistance and lift force of the head and tail cars can be effectively reduced with the increase of the streamline nose length when the train is running at a speed of 600 km/h in the open air.When the train is running in a non-choked tube or choked tube with the(shock waves attached),the aerodynamic resistance decreases slightly with the increase of the streamline nose length.When the train is running in a choked tube with the shock waves separated,the reduction in the drag force is not obvious with the increase of the streamline nose length.