Wavefront Evolution During The Propagation Of Compression Wave And Micro-Pressure Wave

Micro-pressure wave for High-speed train tunnel has a significant impact on the surrounding environment. It leads sonic booms in the victinity of tunnel exit and severe vibration from nrarby housing structure. These issues will become more prominent with increasing trsin speed.The intensity of micro-pressure wave is proportional to the maximum pressure gradient at exit portal, which denpends on non only the initial compression wave during train-entry to a tunnel but also the comperssion evolusion during propagation along thr tunnel. In the case of railway tunnels, by far the most common method of reducing MPWs radiating from tunnel exit portals is the construction of special extension regions at entrance portals. The purpose of these is to elongate the nose-entry wavefront sufficiently to ensure that, even after steepening during propagation along the tunnel, it is not too steep when it reaches the exit portal. However, it has some important limitations. So the research on wavefront exolution propagation along tunnel is essential. Moreover, entrance region counter-measures have no influence at all on wavefronts that are generated further along the tunnel, notably when a train crosses an airshaft or other branch feature in a tunnel. Therefor, new tunnel extended exit regions for micro-pressure wave is welcome.First, the evolution of wavefront during the propagation of a compression wave was studied numerically using a method, which was established by one-dimensional, unsteady and non-isentropic model and MUSCL scheme. Compared with filed data, the results show good agreement, which presumes that this method can well simulate the evolution of wavefront. Moreover, the theoretical formula of pressure gradient under the inviscid condiction was deduced based on one-dimensional plane wave equation and the flow characteristics of compression wave. The effects of propagation distance, the amplitude and the length of wavefront of initial compression wave on pressure gradient of different positions were analyzed. The results indicate that the pressure gradients vary nonlinearly with propagation distance, and the changes increase drastically. When the propagation distance is close to critical length, the pressure gradients appear abrupt change. With the same propagation distance, the pressure gradients increase along with the amplitude of initial compression wave and decrease with the length of wavefront of that.Second, the influence mechanisms of friction effects on wavefront during the propagation of a compression wave、evolution characteristics of initial compression waves with different shapes and the most unfavorable propagation distance were analyzed using MUSCL program. Moreover, the theoretical formula of pressure gradient under the inviscid condiction is revised using the nuermical results from steady friction effect. The results indicate that (1) the steady friction effect makes the amplitude of compression wave attenuate and pressure gradient decrease with the propagation distances increase; (2) the unsteady friction term presents the same effect with steady friction term, however, the unsteady effect mainly influences the wave shape of compression wave, while the steady effect mainly influences its amplitude; (3) the two effects are mutually affected, moreover, the bigger the steady friction coefficient and unsteady friction factor, the stronger the impact;(4)the evolution characteristics of compression waves not only depends on the amplitude and the maximum pressure gradient,but also is closely related to the wavefronts of initial compression wave;(5) the higher the train speed or the greater the steady friction coefficient and unsteady friction factor, the shorter the most unfavorable propagation distance.Based on the wave radition characteristics, CFD numerical method of micro-pressure waves is established. The flow characteristics in different radiation spaces and the mechanism that causes this behaviour is analyzed. Attention is focussed on a tunnel exit counter-measure comprising an extended region of tunnel with holes/slots along its length leading directly to the atmosphere and the influence of a perforated extension region on pressures radiated from the end of a duct is studied numerically. The results indicates the wave diffuse occurs when the wave propagates in two-dimensional space, so for the radiation of micro-pressure wave, the planar 2D geometry is limited.In this paper, The effectiveness of long, perforated exit regions in reducing pressure disturbances from railway tunnels is assessed using two-dimensional and axi-symmetric model.The mechanisms causing the disturbances are described and the potential effectiveness of exit regions as a counter-measure is demonstrated. It is shown that such regions can strongly reduce the pressures radiated from a duct outlet, but that this benefit is offset by pressures radiated directly from the holes along the perforated region itself, moreover, the effectiveness is sensitive to the number, size, distribution of pressure relief holes,slot height along the exit region and the overall dimensions of the exit regions (length, area), but that the most important parameter is the combined area of all of the holes. This parameter controls the balance between external disturbances alongside the perforated region and disturbances beyond the exit portal. It is also shown that the amplitudes of the external disturbances are strongly dependent upon the amplitude and duration of wavefronts arriving at the exit region as well as upon their steepness. This contrasts with the behaviour found for tunnels with simple exit portal regions.