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Cause of the effect
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. 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', as well as to the pressure fluctuations inside drainage pipes as waste water pushes air in front of it. In addition, because of fluid viscosity, the surface of the vehicle also drags the air to flow with vehicle, a force experienced as skin drag by the vehicle.
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.
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.
Piston effect is used in tunnel ventilation. In railway tunnels, the train pushes out the air in front of it toward the closest ventilation shaft in front, and sucks air into the tunnel from the closest ventilation shaft behind it. Piston effect can also assist the ventilation in road vehicle tunnels.
In high speed railway tunnels, piston effect can cause a so-called sonic boom, or micro-pressure wave, to occur. The compression wave generated travels along the tunnel and when it flows out the portal, noise is generated. The ventilation system must be capable of venting this compression wave.
- Innovation and Advanced Technology - High Speed Train (Slide 7), Hitachi Brasil Ltd.
- Practical Railway Engineering, Clifford F. Bonnett, Imperial College Press, June 2005