The alleviation of the aerodynamic drag and wave effects of high-speed trains in very long tunnels

The design of new high-speed railway lines requires longer and more numerous tunnel sections, where aerodynamic effects limit the maximum allowed train velocity for a given tunnel cross-section area. These effects influence the train power requirement, the traction energy costs and the pressure wave amplitude: the knowledge of the unsteady aerodynamic field around the train is therefore essential to the optimum choice of a tunnel configuration, and mainly of the cross-section diameter and of the presence and position of pressure relief ducts. In this paper, the aerodynamic phenomena generated by a train traveling at high speed through a long tunnel of small cross-section are analyzed by means of quasi one-dimensional numerical simulations of the air flow induced by a train traveling at 120 m/s in a tunnel connecting two stations 60 km apart. Several tunnel configurations at high blockage ratio are discussed, together with the positive and negative effects of pressure relief ducts and of partial air vacuum. Aerodynamic phenomena are evaluated in terms of drag, pressure wave amplitude and shock wave onset on the train tail. Results suggest that configurations consisting of twin tunnels connected by pressure relief ducts near stations and operated under partial vacuum should be preferred.