Characteristic And Theoretical Study Of Train Piston Wind In Railway Tunnels

The piston wind in train running through a tunnel is one of the phenomenons of tunnel aerodynamics and also the main component part to solve the tunnel ventilation problems, especially to one-way long and special long tunnel. Taking into account the current conditions of application and investigation, general and systematic research in this paper provides the basis for considering and using the piston wind reasonably and effectively in tunnel ventilation engineering.Making use of the fundamentals and equations of fluid mechanics, according to the structure of flow and the principle of action of different flow sections, the train action segment is regarded as "press source" of piston wind in this paper to demonstrate the mechanism of piston wind and advance the calculation method of piston wind pressure. The theoretical study results fully show the influence of every main action term of tunnels and trains on piston wind and derive the overall characteristics and variation law through dynamic analysis of the main variables. On the basis of the pipe turbulence half-experience theories and related transformation, the velocity distribution relationship of annulus flow which is the main component part of the functional transformation between train and airflow is established and the main properties of flow field are analyzed.Through analysis and demonstration in this paper, the fundamental theoretical model for studying piston wind is determined, namely the steady flow of incompressible fluid, and the expressions of piston wind's pressurization which is composed of propulsive force of train nose, traction force of train tail and shearing force of annulus are put forward. According to the relative movement principles and basis equations, the train nose and tail's formulas of pressurization which is related to blockage ratio a, behavior of flow section and coefficient of resistance in the flow around nose are set up. According to the principles of momentum balance and pipe turbulence half-experience theories, the annulus homogeneous section's relationship of pressurization which is caused by shearing stress of moving (train) and static (tunnel) walls and related to the geometrical characteristics, wall resistance properties and features of velocity distribution of annulus are built. To annulus, these flows always include pressure flow which is relative to the static wall and drag force flow which is caused by the moving wall. On this basis, the annulus flow is viewed as the combination of two kinds of flows, namely the Poiseuille turbulence and the Couette turbulence. At same time, taking advantage of the mirror principle and flow transformation, the essential relationship of the Couette turbulence is established; by the pipe turbulence half-experience theories and method of coordinate transformation, the velocity relationship and characteristic velocity location relationship of annulus combination flow are put forward. comprehensive evaluation method not only can avoid the disadvantages brought by the single evaluation method, but also integrate different advantages. And then detailed discussed the basic principle of DEA and TOPSIS, and improved existing theory and method. Based on this, this paper presents the evaluation model combined the DEA with the TOPSIS which contraposes and compares the service quality of the PDL in different sections. Finally, this paper takes the PDL in four sections as the research objects and verifies the scientificity and the practicality of this model. Also, it puts forwards the countermeasures and Suggestions which is used to improve service quality in PDL through the analysis of the results in the evaluation.