Piston effect
Encyclopedia
Piston effect refers to the forced air flow inside a tunnel
caused by moving vehicles.
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
tunnel
s, 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.
ventilation. In railway
tunnel
s, 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
tunnel
s, 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.
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
caused by moving vehicles.
Cause of the Effect
Air in a tunnelTunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
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
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...
is created and air is pulled to flow into the tunnel. The situation is similar to the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
inside of an engine
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
, hence the name 'piston effect'. This effect is analogous to the pressure fluctuation generated by the flow of waste water
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
in drainage pipes, the air in front of the waste water
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
is being pushed and the air behind is being dragged. In addition, because of fluid viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, the surface of the vehicle also drags the air to flow with vehicle.
The piston effect is very pronounced in railway
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...
tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
s, because the cross sectional area of train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
is large and almost completely fills the whole tunnel cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
. The wind felt by the passengers on underground train station platforms
Railway platform
A railway platform is a section of pathway, alongside rail tracks at a train station, metro station or tram stop, at which passengers may board or alight from trains or trams. Almost all stations for rail transport have some form of platforms, with larger stations having multiple platforms...
(that do not have platform screen doors
Platform screen doors
Platform screen doors and platform edge doors at train or subway stations screen the platform from the train. They are a relatively new addition to many metro systems around the world, with some platform doors retrofitted rather than installed with the metro system itself. They are widely used in...
installed) when a train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
is approaching is the air flow from the piston effect. The effect is less pronounced in road vehicle tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
, as the cross-sectional area of vehicle is small compared to the total cross-sectional area of the tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
.
Air flow caused by piston effect can exert large force on the installations inside the tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
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.
Applications
Piston effect is used in tunnelTunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
ventilation. In railway
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...
tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
s, the train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
pushes out the air in front of it toward the closest ventilation shaft
Ventilation shaft
In subterranean civil engineering, ventilation shafts, also known as airshafts or vent shafts, are vertical passages used in mines and tunnels to move fresh air underground, and to remove stale air....
in front, and sucks in air to the tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
from the closest ventilation shaft
Ventilation shaft
In subterranean civil engineering, ventilation shafts, also known as airshafts or vent shafts, are vertical passages used in mines and tunnels to move fresh air underground, and to remove stale air....
behind it. Piston effect can also assist the ventilation in the road vehicle tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
.
In high speed railway
High-speed rail
High-speed rail is a type of passenger rail transport that operates significantly faster than the normal speed of rail traffic. Specific definitions by the European Union include for upgraded track and or faster for new track, whilst in the United States, the U.S...
tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
s, piston effect can cause the so-called sonic boom, or micro-pressure wave, to occur. The compression wave generated travels along the tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
and when it flow out the portal
Portal (architecture)
Portal is a general term describing an opening in the walls of a building, gate or fortification, and especially a grand entrance to an important structure. Doors, metal gates or portcullis in the opening can be used to control entry or exit. The surface surrounding the opening may be made of...
, noise is generated. The ventilation system has to be capable of venting this compression wave.