| The development of economy and society has brought about the overall speed increase of traffic.But due to the influence of the surface atmosphere,it is difficult to exceed 400km/h of the velocity of the ground vehicles.However the limit velocity of ETT vehicles can be greatly increased when driving in the low pressure tube,but the aerodynamic problems will be more complex.In transonic or even supersonic driving,the common aerodynamic problems of spacecraft will be magnified in the tube,and will appear repeatedly.For the phenomenon in the atmospheric environment,we should also observe whether it is suitable for ETT system.Therefore,the aerodynamic analysis of ETT system is necessary.This paper selects a suitable numerical analysis method based on the theory of computational fluid dynamics（CFD）.The phenomena of piston wind and shock wave in ETT system are simulated by using the SST k-ωand DDEs coupled turbulence model with the self-adaptive dynamic mesh and the mesh updating method of layering-method for two-dimensional and three-dimensional models.The paper,improves by parameterization the ETT geometric model in the early stage of the research group,adopts the partition hybrid mesh generation method,proposes the idea of sharing the moving area of unstructured grid and the static area of structured grid to greatly reduces the modeling time.Researching the structure evolution and crash effect of shock wave.There are attachment,bow,reflection and normal shock wave in the front of the head part,reflection shock wave and rhombus shock string structures in the middle,and attachment,bow（detached）,reflection,lamda and rhombus shock string structures in the rear.There can find regular and Mach reflection of shock wave,boundary layer separation induced by shock wave,interaction among shock waves,vortexes and other shock waves.The shock wave can impact the tube wall and the body periodically.The high pressure area and the low pressure area are equal and expand linearly with the ETT vehicles running,and there is the lowest pressure area which does not change with time.Analyzing the piston wind under different tube blocking ratio,environmental pressure and driving velocity,and discussing the relationship between shock wave and piston wind.Following the increase of tube blockage ratio,the velocity and pressure of piston wind rapidly raise,the constant-velocity region in the annular gap decrease,and the total drag（F）and drag coefficient（Cd）of the body increase.Accompanied by environmental pressure increasing,the piston wind pressure raises exponentially,the piston wind velocity is almost unchanged,the back-flow velocity of the annular gap increases and the area expands,the F increases proportionally,and the Cddecreases in inverse proportion.There is a critical velocity v0,the piston wind velocity,pressure and F increase with the increase of driving velocity under less than v0.The piston wind velocity,pressure,annular flow velocity and F decrease significantly when the driving velocity is greater than v0.At this time,the vehicle will break through the Kantrowitz limit,and the tube blocking effect disappears.The flow field of annular gap is similar to Couette flow.The influence of shock wave on piston wind is mainly manifested in the oscillation of piston wind,and the enhancement of blocking effect.The change of shock wave structure and piston wind in the three-dimensional model is consistent with that in the two-dimensional model,which shows that the two-dimensional model has a good simulation effect on the three-dimensional model.However,the change of shock wave and piston wind in the three-dimensional model is slower than that in the corresponding two-dimensional model due to the strong three-dimensional effect.This paper mainly discusses the change law of shock wave and piston wind in the ETT when the vehicle is running at high velocity,finds the shock wave structure and evolution law in the tube,the periodic impact effect of shock wave on the vehicle body and tube wall,and obtains the transient and steady piston wind change law,as well as the influence of different blocking ratio,vacuum degree and running velocity on the piston wind,which can provide a reference. |
