Aerodynamics of trains in tunnels

A train moving through air generates a turbulent flow around it called a slipstream. The slipstream is associated with high air velocities and rapidly-changing pressure fields. When a train passes through a tunnel the air velocity, pressure variation and direction of the flow inside tunnels is different to the slipstream in open air. These differences depend on the size of the tunnel (cross section and length of the tunnel) and the shape and speed of the train. In this paper, the aerodynamic behaviour of a generic train and a simplified ICE2 train passing a tunnel has been studied. The investigation uses computational fluid dynamics techniques (CFD), in which a full-scale generic train with a speed of 70 m/s and a 1/25th model of the ICE2 train with a speed of 32 m/s are used. The results are compared with previous CFD results and moving train rig experiments. The air velocity and pressure at different locations of the tunnels are analysed and conclusions are drawn. It is found that the pressure inside the tunnel increases significantly when the train enters the tunnel and decreases behind the train once the tail of the train enters the tunnel, which results in a complicated flow and pressure fields in the gap between the train and the tunnel walls. A reverse flow has been generated around the train due to the differences in the front and wake pressures.