Numerical Analysis Of Compression Wave Deformation And Preliminary Acoustic Characteristics Of Micro Pressure Wave

A micro-pressure wave will be generated when the compression wave in the tunnel propagates to the tunnel exit,in some cases,the will be a sonic boom produced by the micro-pressure wave.With a higher train speed,the phenomenon of compression wave generated when the train enters into the tunnel and micro-pressure wave acoustic problem will be more obviously.The major work in this paper was divided into two parts,the distortion law of compression wave in the long high speed railway tunnel and the acoustic analysis of the micro-pressure at the tunnel exit.During the calculation and analysis of the propagation law of compression wave in long tunnel,a finite volume method based on the three-dimensional,unsteady,compressible fluid flow control equation,and combining with the two equation turbulence k-ε model was used to simulate the flow field inside the tunnel,the distortion processes of compression wave were simulated,the variations of the maximum pressure gradient of compression wave calculated,and the attenuation processes of compression wave at the tunnel exit were analyzed.During the analysis of acoustic characteristics of micro-pressure wave,the LES simulation method was used to calculate the parameter in the micro-pressure flow field,the finite model based on the Lighthill analogy theory was established,and the acoustic field of micro-pressure was calculated.The main conclusions achieved in this paper are as follows:(1)The deformation of the compression wavefront is not only related to friction between air and walls of tunnel and the inertial motion of the compression wave,but also related to the energy input recharged by the movement of the train in the railway tunnel.The maximum pressure gradient of compression wavefront increases with the inertia movement of compression wave during propagation and external energy recharge caused by the motion of train in the tunnel,and decreases with impact of turbulent kinetic energy dissipation and friction effects.Through the calculation of multi conditions,the variation law of the maximum pressure gradient and its appearance position relating to train speeds and to tunnel blockage ratios in long railway tunnel are obtained.(2)When the compression wave propagates near the tunnel exit,there will be a significant decrease of the maximum value of compression wave amplitude and wave front pressure gradient.The calculation results show that the location of the falling point is within 100 meters from the exit in the tunnel.The pressure gradient at the tunnel exit will be the largest if the tunnel length is equal to location where the pressure gradient occurs in the long enough tunnel.In order to reduce the pressure gradient at the exit of the tunnel effectively,the length of the tunnel should avoid the most unfavorable tunnel length based on the maximum amplitude of the micro-pressure wave.(3)According to Lighthill’s acoustic analogy theory,the aerodynamic acoustic source of micro pressure wave at the tunnel exit can be analogous to the quadrupole source.In the railway tunnel with blockage ratio of 0.2 and with length of 500 m,when the speed of the train speed is greater than 300km/h,there will be sonic boom of the micro-pressure wave at the tunnel exit,according to the human auditory threshold curve.