Study on the train-induced unsteady airflow in a metro tunnel with multi-trains

Super-long metro tunnels, which allow two trains to simultaneously drive in the same tunnel direction, have become increasingly important in urban areas where construction is occurring at a rapid pace. The tunnel environment is significantly affected by the phenomenon of piston wind. Based on the unsteady flow theory of Bernoulli equation, the piston effect of metro tunnel caused by single train and multiple trains passing through the tunnel was mathematically modeled in this study. The accuracy and feasibility of the proposed theoretical method is verified by establishing a theoretical model based on a highly cited small-scale experimental study. Based on the verified theoretical method, the influence of various factors on the development law of the piston wind is analysed in detail. The results indicate that the maximum piston wind speed, v(max2), generated by double trains is greater than the maximum piston wind speed, v(max1), generated by a single train, and v(max2) is approximately 1.24 times the value of v(max1). According to the fitting results of five parameters (i.e., the train speed, train length, tunnel length, blockage rate, and frictional resistance coefficient) on the maximum piston wind speed, a unified formula for predicting the maximum piston wind speed with the consideration of various influencing factors is proposed.