Piston effect

Piston effect refers to the forced-air flow inside a tunnel or shaft caused by moving vehicles. It is one of numerous phenomena that engineers and designers must consider when developing a range of structures.

In open air, when a vehicle travels along, air pushed aside can move in any direction except into the ground. Inside a tunnel, air is confined by the tunnel walls to move along the tunnel. Behind the moving vehicle, as air has been pushed away, suction is created, and air is pulled to flow into the tunnel. In addition, because of fluid viscosity, the surface of the vehicle drags the air to flow with vehicle, a force experienced as skin drag by the vehicle. This movement of air by the train is analogous to the operation of a mechanical piston as inside a reciprocating compressor gas pump, hence the name 'piston effect.' The effect is also similar to the pressure fluctuations inside drainage pipes as waste water pushes air in front of it.

The piston effect is very pronounced in railway tunnels, because the cross sectional area of train is large and almost completely fills the whole tunnel cross section. The wind felt by the passengers on underground train station platforms (that do not have platform screen doors installed) when a train is approaching is air flow from the piston effect. The effect is less pronounced in road vehicle tunnel, as the cross-sectional area of vehicle is small compared to the total cross-sectional area of the tunnel. Single track tunnels experience the maximum effect but clearance between rolling stock and the tunnel as well as the shape of the front of the train affect its strength.

Air flow caused by the piston effect can exert large forces on the installations inside the tunnel and so these installations have to be carefully designed and installed properly. Non-return dampers are sometimes needed to prevent stalling of ventilation fans caused by this air flow.

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Wikipedia