Mechanism of expanded equal-section inclined hood to reduce initial compression wave by high-speed maglev passing through the tunnel

The initial compression wave is generated when the high-speed rail vehicle enters the tunnel. The compression wave propagates to the exit of the tunnel at the speed of sound and radiates outward to form a micropressure wave, which brings serious environmental problems. Using the three-dimensional unsteady, compressible flow N–S equation and the SST k–ω turbulence model, and taking the maglev train with a speed of 600 km/h as the research object, the initial compression wave generated by maglev train entering the tunnel with extended equal-section hood, extended equal-section oblique hood and no hood was simulated. The mitigation effect and mechanism of the inclined end and the oblique angle of the hood on the initial compression wave were analyzed. The following conclusions are mainly drawn: the formation of the maximum pressure gradient of the initial compression wave is directly related to the entrance of the part of the train into the tunnel/hood where the change rate of the cross-sectional area of the train head is the maximum, which corresponds to the maximum change rate of the flow in the tunnel. The maximum gradient of the compression wave can be greatly reduced by setting the extended constant section hood, and the relief rate is 49.92%. Changing the vertical port of the expanded isocross section hood to the positive oblique port can further improve the mitigation rate. When the oblique angle is 10°, 20°, 30° and 39°, the increase of the mitigation rate is 12.93%, 10.32%, 8.18% and 6.28%, respectively. It is suggested that the oblique hood has the most obvious effect on the peak pressure gradient of the initial compression wave when the oblique angle is 10°, and the total relief rate is 62.85%. In this paper, the coupling analysis method of the change rate of the cross section area of the head, the air flow rate and the compression wave at the observation point, and the mutual mapping relationship between the head shape and the air flow rate, which affect the maximum pressure gradient of the initial compression wave, can reasonably explain the mechanism of the hood at the entrance to the cave to reduce the initial compression wave. It provides a new method for further optimization of the train head shape and design of different types of hood and analysis of aerodynamic effects.