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 vehicle 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 trains is large and in many cases almost completely fills the tunnel cross section. The wind felt by the passengers on underground railway 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 tunnels, 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.
The piston effect has to be considered by building designers in relation to smoke movement within an elevator shaft. A moving elevator car forces the air in front of it out of the shaft and pulls air into the shaft behind it with the effect most apparent in elevator systems with a fast moving car in a single shaft. This means that in a fire a moving elevator may push smoke into lower floors.
The 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. The piston effect can also assist ventilation in road vehicle tunnels.
In underground rapid transit systems, the piston effect contributes to ventilation and in some cases provides enough air movement to make mechanical ventilation unnecessary. At wider stations with multiple tracks, air quality remains the same and can even improve when mechanical ventilation is disabled. At narrow platforms with a single tunnel, however, air quality worsens when relying on the piston effect alone for ventilation. This still allows for potential energy savings by taking advantage of the piston effect rather than mechanical ventilation where possible.
Tunnel boom is a loud boom sometimes generated by high-speed trains when they exit tunnels. These shock waves can disturb nearby residents and damage trains and tunnel structures. The perception of this sound by humans is similar to that of a sonic boom from supersonic aircraft, but unlike a sonic boom, tunnel boom is not caused by trains exceeding the speed of sound. Instead, tunnel boom results from the structure of the tunnel preventing the air around the train from escaping in all directions. As a train passes through a tunnel, it generates compression waves in front of it. These waves coalesce into a shock wave that generates a loud boom when it reaches the tunnel exit. The strength of this wave is proportional to the cube of the train's speed, so the effect is much more pronounced with faster trains.
Tunnel boom can disturb residents near the mouth of the tunnel, and it is exacerbated in mountain valleys where the sound can echo. Reducing these disturbances is a significant challenge for high-speed lines such as Japan's Shinkansen and the French TGV. Tunnel boom has become a principal limitation to increased train speeds in Japan where the mountainous terrain requires frequent tunnels. Japan has created a law limiting noise to 70 dB in residential areas, which applies to many tunnel exit zones.
Methods of reducing the phenomenon include making the train's profile highly aerodynamic, adding hoods to tunnel entrances, installing perforated walls at tunnel exits, and drilling vent holes in the tunnel. (This is somewhat like fitting a Silencer (firearms) on a firearm, but on a far bigger scale.)