Piston effect refers to the forced air flow inside a tunnel caused by moving vehicles[1].
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Air in a tunnel is confined by the tunnel walls, hence the movement of air is restricted. In open air, when a vehicle travels along, air is being pushed and can move to any directions except into the ground. But inside a tunnel, air cannot move through the tunnel walls. Air has to be pushed 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. The situation is similar to the piston inside of an engine, hence the name 'piston effect'. This effect is analogous to the pressure fluctuation generated by the flow of waste water in drainage pipes, the air in front of the waste water is being pushed and the air behind is being dragged. In addition, because of fluid viscosity, the surface of the vehicle also drags the air to flow with 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 the 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 piston effect can exert large force 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.[3]
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 in air to the tunnel from the closest ventilation shaft behind it. Piston effect can also assist the ventilation in the road vehicle tunnel.
In high speed railway tunnels, piston effect can cause the so-called sonic boom, or micro-pressure wave, to occur. The compression wave generated travels along the tunnel and when it flow out the portal, noise is generated. The ventilation system has to be capable of venting this compression wave.