Model test research on pressure wave in the subway tunnel

7 The pressure wave is of crucial importance for subway development since it greatly influences 8 the comfort while taking. As the subway lines are rapidly developing in the cities, the pressure wave 9 in different subway tunnel constructures is urgently needed to be studied and receded. In this paper, a 10 subway tunnel pressure wave experimental system was designed, constructed, and tested. The 11 influence of train model head shape, train model speed, shaft number in the tunnel, and bypass 12 number in the tunnel on the pressure wave amplitude were experimented with and analyzed. The 13 results show that the train model head shapes significantly impact the amplitude of the initial 14 compression wave in the tunnel. The blunter train model head generates a greater amplitude of the 15 initial compression wave. When the train passes through a single-track tunnel, the maximum positive 16 pressure amplitude of the pressure wave in the tunnel is at the first compression wave at the tunnel 17 entrance. The maximum negative pressure value in the tunnel is at the superposition of the initial 18 compression wave reflected from the first time and the train's body, which is related to the length of 19 the train's body, tunnel length, train's speed, and sound speed. The shaft set in the tunnel decreases the 20 amplitude of the initial compression wave in the tunnel space behind, but it will increase the pressure 21 wave's amplitude reflected in the tunnel when the train passes through the shaft. After the bypass 22 tunnel is added, the initial compression wave propagation in the tunnel behind the bypass tunnel is 23 receded. Still, it also increases the negative pressure amplitude when the train passes.


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With the rapid development of the modern city scale, the number of urban populations is 29 increasing, and the ground traffic is becoming more and more congested. To alleviate the urban traffic 30 pressure, cities across the country are actively planning and building a more scientific and perfect 31 traffic network system, among which, subway, one of the effective tools to relieve urban traffic 32 pressure, has developed rapidly in recent years. By the end of 2019, 40 cities in China have opened 33 208 operating lines, with a total length of 6,736.2 kilometers. Besides, there are 279 subway lines 34 under construction nationwide, with a total length of 6,902.5 kilometers. 35 When the train enters the tunnel, the internal air will be strongly squeezed by the train due to the 36 closed space of the subway tunnel, and the gas at the tunnel entrance will be compressed, which leads 37 to rapidly rising pressure, forming a pressure pulse [1][2]. As is shown in Fig 1, the phenomenon 38 when the pressure pulse travels along the tunnel at speed close to that of sound is called compression tunnel [18][19]. The train's running resistance with a blunt head is the largest, and the longer the shape 48 of the head is, the smaller the resistance will be, but the slowing effect is gradually decreasing [20][21]. 49 The peak value and gradient of the initial compression wave generated by the common CRH3 and 50 CRH380A trains in China are also different at the same speed, the change of which in the CRH3 train 51 is more severe than that of the CRH380A train [22]. Also, the velocity of the train has a great 52 influence on the pressure wave. At present, most of the subway lines in China operate at 80 km/h. In 53 recent years, 100 km/h and more than 100 km/h subway lines have been gradually put into operation, 54 and many high-speed subways are under construction. After the subway speed reaches above 100km/h, 55 many aerodynamic problems, especially the tunnel pressure wave [12][13] to study the effect of increasing bypass tunnels on pressure waves in the tunnel.

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In this paper, the model test analyzes the influence of locomotive shape and running speed on the 72 pressure wave. The influence of the number of shafts in the tunnel and the bypass tunnel is also 73 studied. The second section of this paper introduces the working principle of the test system in detail.

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The third section analyzes the errors in the model test. The three-dimensional view of the model test bench is shown in Figure 2 The longitudinal section 1 of the tunnel when train model enters the tunnel is shown in Figure 3.  As is shown in Figure 4, the train operation system includes the launcher for train model, train 102 model, motor, drive wheel, driven wheel, traction rope, guide rope, tensioner wheel and other 103 equipment.

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The working principle of the model test stand is as follows. Firstly, set the test speed in the PLC

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control system before the test, then press the launcher to separate the traction rope from the train 106 model. Thirdly, control the motor to start working to drive the traction rope to accelerate it to the 107 required traction speed. At this point, the release launcher connects the traction rope with the train 108 model, driving the train model to run into the tunnel at speed required by the test in a short time. The 109 two sensors at the entrance of the tunnel, which is shown in Fig. 5, detect the speed at which the 110 model car enters the tunnel, and the sensor at the exit of the tunnel, which is shown in Fig. 6 The specific deceleration plan is as follows. As is shown in Fig. 6, the train's deceleration is 118 realized by the motor brake and the train's connection multi-stage buffer device. When the train leaves 119 the tunnel at high speed, the photoelectric switch, installed at the tunnel exit, senses and feeds back to 120 the PLC. In the early stage, active deceleration is achieved through motor reversal and energy 121 consumption braking given by frequency converter controlled by PLC, while in the later stage, the 122 train is decelerated passively by setting a multi-stage buffer sponge. Two kinds of deceleration not 123 only protect the body but also achieves an efficient speed reduction. When the test is completed, the 124 motor is reversed at low speed to allow the train to run from the tunnel exit to the tunnel entrance to 125 achieve rapid train recovery. being processed in the data acquisition system. can also press the stop button to stop the motor immediately. In this test, a total of 6 test sections were arranged along the tunnel, and the distance between each 172 was 3.8m. Specifically, Fig. 11 to the ambient temperature of the test site. So, when the temperature has less changes, ρ/μ can be 197 regarded as a fixed value. Therefore, V and L should be equal to make Re equal. If the Re is to be 198 equal, the speed of the train model should be higher than 20 times the actual train speed because the 199 size similarity ratio of the model is less than 1/20, which cannot be achieved in the model test.

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However, people found that in applying the dynamic similarity criterion, the flow field has a 201 "self-modeling region" [32]. The phenomenon that the flow state and velocity distribution are similar 202 to each other and not depend on the Re's change when it is in a certain range is called "self-modeling". In the test process, various links will produce certain errors, which will lead to deviations in the

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The model test bench is set up in an indoor laboratory to minimize the impact of the test 238 environment on this test. As shown in Table 1, the on-site ambient temperature is recorded before each 239 test, and the maximum change in each case doesn't exceed 10%. For the same test case, the 240 experimental data are obtained after repeated testing three times, the relative error of whose pressure 241 amplitude is within 3%.

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The pressure wave in the tunnel is affected by many factors. This paper mainly discusses the 245 influence of the shape of the train's head, train speed, the establishment of the shaft, and the bypass 246 tunnel in the tunnel on the pressure wave. in the model tunnel, respectively, for research, whose positions are shown in Figure 11(b) and Figure   310 11(c). During the test, the diameter of the shaft is 0.11 m, and the height is 2.6 m, and a model train 311 with a head shape of 60° was used, running speed of which was 80km/h, and no bypass tunnel was set 312 in the model tunnel.
313 Figure 18 shows the dynamic pressure curve of the initial compression wave at section 1 when pressure wave will be generated and superimposed with the pressure wave reflected in the tunnel so 336 that the pressure monitored at the measuring point will increase.

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It can be clearly seen from Fig. 18, Fig. 19, and Fig. 20 that when the train model passes through   Figure 23 shows the dynamic pressure change curve at the measuring points of the test section 6.

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It can be seen that the change rule of the initial compression wave is consistent with that of section 5.

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The increase of the bypass tunnel slows down the amplitude of the initial compression wave, and the 376 mitigation effect of two bypass tunnels is greater than that of one bypass tunnel. When the train model 377 passes through the measuring points of three different structure tunnels (no bypass tunnel, one bypass 378 tunnel, and two bypass tunnels), the negative pressure amplitudes generated are -227 Pa, -300 Pa, and

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-306 Pa, respectively. The results are the same as those on section 5 similar. The main reason is that 380 the air circulation area becomes larger after the bypass tunnel is added, and the drag coefficient encountered by the model car when traveling decreases, which will increase the air velocity and static 382 pressure at the section. (1) When the train runs at a speed of 80 km/h, the blunter the locomotive shape is, the larger the initial 390 compression is. The pressure amplitude generated by the locomotive angle of 75° is 43.6 % higher 391 than that generated by the 60° locomotive angle.

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(2) After the train enters the tunnel from outside, the place with the largest positive pressure amplitude 393 of the pressure wave is the tunnel entrance, and the amplitude will continuously attenuate along 394 the tunnel length. The maximum negative pressure amplitude is related to the length of the tunnel 395 and the train as well as the train itself. When the train encounters the expansion, a wave reflected 396 back and forth in the tunnel as the train is running, the negative pressure amplitude will increase 397 and may become the maximum one.

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(3) After the middle air shaft is set in the tunnel, the amplitude of the initial compression wave in the 399 tunnel space in front of the air shaft is less slow down, and the amplitude of the initial 400 compression wave in the space behind the air shaft will be slowed down, but the amplitude value 401 of the pressure wave propagating in the space behind the air shaft will be enhanced.

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(4) After the bypass tunnel is added to the tunnel, the positive pressure amplitude of the initial 403 compression wave will not be affected for the tunnel space in front of the bypass tunnel, but the 404 negative pressure amplitude will be affected due to the reflection of the initial compression wave 405 propagating to the bypass tunnel, and the specific impact is related to the train length, tunnel 406 length and train speed. For the tunnel space behind the bypass tunnel, the set of bypass the tunnel 407 will slow down the impact of the initial compression wave but will enhance the amplitude of The raw/processed data required to reproduce these findings cannot be shared at this time as the 412 data also forms part of an ongoing study.