Study On Characteristics And Flow Control Of High-speed Train Wake

Because the air is viscous,when the train runs at high speed on the railway,it will drive the surrounding air to move with it,forming a wake behind the train.The wake of a high-speed train is a very complex turbulent motion,and the turbulent vortex formed plays an important role in aerodynamic problems.The turbulent motion of the wake will endanger the safety of passengers and workers near the track,and cause problems such as aerodynamic drag,tail car vibration and aerodynamic noise.To solve the above problems and improve the aerodynamic performance and environmental friendliness of the high-speed train,the basic research on wake characteristics and control methods will have important theoretical guiding significance and broad engineering application prospects.Based on this,this paper makes an in-depth study on the motion characteristics and formation mechanism of the high-speed train wake under the influence of Reynolds number and bogie surrounding structures,as well as the wake passive control method based on nonsmooth surface and the wake active control method based on synthetic jet.The main research contents and conclusions are as follows:For the research on numerical simulation and wind tunnel test verification of high-speed train wake,firstly,the numerical calculation method of wake is introduced,then the basic theories of flow separation,vortex motion and turbulent wake are deeply studied,and then the wake flow is numerically simulated,Finally,the grid convergence is studied,and the correctness of the numerical simulation results is verified by the results of wind tunnel test.The results show that the grid selected in this paper has sufficient calculation accuracy and can be used to simulate the flow around the high-speed train.In addition,the numerical simulation results are in good agreement with the wind tunnel test results,which proves the correctness of the numerical simulation in this paper.The effects of five Reynolds numbers and three structures around bogie on wake characteristics are studied by numerical simulation,the characteristics of wake are revealed,and the formation mechanism of wake is explored.The results show that the large-scale vortices of wake at different Reynolds numbers are similar.However,many small-scale vortices will be formed at high Reynolds number,and the vortex tube size will become larger,and the vorticity of the vortex in the wake area will increase.The flow mechanism on the train surface and the cause of wake formation are not changed under different Reynolds numbers,but the separation point will move forward with the increase of Reynolds number.The Strouhal number St defined by the vortex shedding frequency f does not change with the Reynolds number Re,which shows that the flow mechanism is the same in the range of Reynolds number selected in this paper.The existence of bogie and installation cabin makes the flow separation and vortex on the side of the tail car,which has a great impact on the wake: Firstly,under the action of the vortex on the side of the tail car,the wake vortex has a trend of spanwise movement.Secondly,more complex vortices are formed at the rear of the train.These vortices increase the momentum loss of the wake and form a wide range of negative pressure near the nose of the train,which will increase the differential pressure drag of the whole vehicle.Finally,the bogie and installation cabin increase the degree of velocity fluctuation and wake instability in the wake area.The bogie and its surrounding structures do not affect the basic components of the velocity in the wake area.The velocity component in the wake area mainly comes from the vortex shedding on the surface of the tail car,and the vortex caused by the bogie and surrounding structures on the side of the tail car only amplifies the spanwise motion of the wake area and does not introduce new velocity components.A passive flow control method based on non-smooth surface is selected to suppress the vortex motion.The control effect is analyzed by studying the wake vortex structure topology,wake region velocity and aerodynamic characteristics.Finally,the control mechanism is discussed by analyzing the flow characteristics around the tail car.The results show that the arrangement of non-smooth surfaces at the side of the train can effectively delay the separation of boundary layer,inhibit the formation of wake vortex,weaken the motion intensity of wake vortex,and reduce the influence range of wake vortex.After the control of non-smooth surface,the amplitude of velocity fluctuation decreases,but the dominant frequency of velocity does not change significantly.After the control of non-smooth surface,the aerodynamic drag of the train is reduced,which will greatly reduce the energy consumption of train operation.For the frequency domain characteristics of aerodynamic drag,the dominant frequency of aerodynamic drag coefficient does not change significantly after control.The active flow control mechanism of synthetic jet for the wake is studied by numerical simulation,and the effects of synthetic jet momentum coefficient,excitation frequency and jet angle on the control performance of wake are analyzed.The results show that the mechanism of synthetic jet controlling wake is to delay the flow separation on the surface of the tail car,and then alleviate the formation and shedding of vortices on the side of the tail car.A variety of synthetic jet control parameters selected in this paper can realize the control of wake.Specifically,in the wake area,the peak value and pulsation value of slipstream velocity are reduced,the aerodynamic drag of tail car is reduced,and the pulsation degree of aerodynamic transverse force is reduced.With the gradual increase of the momentum coefficient of the synthetic jet,the maximum slipstream velocity and the train aerodynamic drag show a gradual decreasing trend.On the contrary,the variation trend of the pulsation degree of the aerodynamic transverse force is opposite,and the vertical and transverse influence range of the vorticity in the area far from the tail of the train decreases gradually.With the gradual increase of synthetic jet excitation frequency,the maximum slipstream velocity and train aerodynamic drag show a trend of first decreasing and then increasing.On the contrary,the variation trend of train aerodynamic lift is opposite.The transverse and vertical influence range of vorticity in wake area also shows a trend of decreasing first and then increasing.The influence of synthetic jet angle on train wake is as follows: when the synthetic jet angle is-10°and 0°,a small maximum peak of slipstream velocity is formed in the wake area,and the pulsation of tail car aerodynamic drag and transverse aerodynamic force increases first,then decreases and then increases with the change of synthetic jet angle from negative maximum to positive maximum.On the basis of the wake open-loop control law,the adaptive extremumseeking control algorithm and ergodic optimization control algorithm can realize the adaptive adjustment of the control parameters of the high-speed train wake closed-loop control system and achieve good control performance.