| While automobiles are running in a highway tunnel, the air in the tunnel is driven to flowalong with the direction of marching, this phenomenon is called “the piston effect”,and theaircurrent is called “the piston wind”. Making clear of the piston effect can not only optimizethe designing project, but also save the expenses and energy consumption of mechanicalventilation when the tunnel is engaged into operation.This dissertation is focused on the numerical simulation of the piston wind in highwaytunnels,by using a CFD software——CFDesign.First, this article introduces the necessary basic hypothesis of analyzing the piston windmechanism and the fluid mechanics governing equation it complies with. At the same time,the employed numerical simulation method and the corresponding model equation are alsoput into discussion.Second, it describes the procedure of the finite element method applied to current CFDsoftware to resolve computational fluid mechanics issues. It emphasizes the content aboutscattering, solution, convergence of the governing equation, the mesh generating and erroranalyzing.Afterwards, according to the tunnel aerodynamics, the next chapter gives the specificderiving formula to the speed equation and pressure equation of the piston wind, under twocertain conditions——the tunnel with vertical shaft and the tunnel without vertical shaft.Then, we discuss several factors that the piston effect is influenced by, which includetraffic volume, driving speed, tunnel length, traffic composition, running interval and thedesign parameter of vertical shaft. Yet, we compare the wind speed of piston wind andrequired wind, finding out the critical value of switching on the mechanical ventilation.At last, this dissertation discusses how the piston wind affect the distribution of velocityfield and pressure field of the airflow in the highway tunnels, under transient state and steadystate, as well as the flow distribution around the time when two opposite cars meet. |
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