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Internal Flows, Metros and Underground Systems

Reasons for this note

This note sets out the aerodynamic issues associated with operating trains in metros and

underground systems. There are few formal rules in standards which apply directly to

the aerodynamic environments in the tunnel networks and stations which comprise

such systems. Nevertheless, there is value in having a general overview of the

phenomena of relevance in these systems.

In particular, the following will be discussed: train-generated pressures and airflows,

aerodynamic drag and generated heat, ventilation, platform barriers and fire safety in

general terms for underground systems. Where relevant standards or recommended

limits exist, they will be included for reference. Finally, methods for the calculation of

aerodynamic phenomena in underground systems will be briefly described.

General Description

For all trains entering, travelling through and exiting tunnels, aerodynamic pressures and

airflows are set up by the movement of the trains. Depending on the layout of the

underground system, this may lead to large volumes of air being pushed into the station

regions. These may in turn generate high air speeds on the station platforms or in the

platform approach tunnels which, unless controlled, could potentially reach dangerous

speeds for station users.

The air within the system will be heated up by the train traction and braking energy

dissipated as heat, heat from on-board train services or waste heat extracted by air

conditioning from within the train carriages. This heat has to be managed to avoid

unpleasant conditions for passengers on the trains or waiting in stations. One potential

remedial action is the inclusion of airshafts in the system and the passive use of the train

movements to expel hot air from the tunnels or ingest cooler air into the system.

Artificial ventilation may also supplement the passive ventilation of the system, and is

often fitted to control air and smoke movements in the event of a train or station fire.

Other methods to control heat build-up include the use of infiltrating groundwater to

carry away heat or chilled water cooling like that used in the Channel Tunnel.

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Pressures

Just as in mainline railways, when trains enter and leave tunnels they generate pressure

changes, also called pressure waves, a few percent above or below ambient atmospheric

pressure. The strength of these waves depends on the train entry and exit speed, among

a number of other influencing parameters. These pressure waves move along the tunnel

at the speed of sound, reflecting at any interface with the open air or large volumes of

air, such as at portals or at the top of airshafts and station voids. On reflection, the

pressure waves are modified with waves above ambient pressure returning back into

the tunnel as waves below ambient pressure and vice versa. This means that a train

moves through a complex superposition of pressure changes as it travels through the

tunnel.

For mainline railways, these waves can superimpose to give relatively large positive or

negative pressure transients. However, for underground railway systems, trains are

travelling at much lower speeds and are generally either accelerating from station stops

or decelerating into stations. This results in much smaller generated pressure waves. In

addition, there are usually many different paths that pressure waves can take through

the system, which tend to dissipate the pressures. The nett result is that pressure

changes are generally very small and are not likely to be felt by train passengers or

people waiting on the station platforms. Figure 1 shows predicted tunnel pressures in a

London Underground Ltd (LUL) station and between two consecutive stations for a

stopping strain travelling at a maximum speed of 80 km/h.

Figure 2 shows the predicted pressures at the front and at the rear of a LUL 1967 tube

stock train. The train leaves a first station at time 0 seconds, accelerates at 1 m/s2 until

reaching 80 km/h at about 22.5 seconds, then travels at constant speed until 53 seconds

before decelerating at 1.15 m/s2 and stopping at about 72 seconds in the next station. It

then accelerates out of the station after a 30 second dwell time.



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Figure 1: Predicted pressures in a LUL station (top) and in the tunnel between two

consecutive stations (bottom).

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Figure 2: Predicted pressures alongside a LUL tube stock train - A front, B rear.

One of only two known pressure limit criteria for underground rapid transit systems was

formulated by the USA’s Urban Mass Transportation Administration (UMTA), and is

shown below. This consists of a maximum pressure change criterion and a second

criterion related to the rate of change of pressures.

A.1.3 - U.S. UMTA - Underground rapid transit systems

Max. Change of pressure = 700 Pa within a 1.7 second period

Max. Rate of change of pressure = 410 Pa/s (as an average rate over longer periods than

1.7 second)

Operating conditions:

- Low-speed operations: 80-100 km/h,

- Unsealed rolling stock,

- Tight bore tunnels,

- Regular commuter customers.

LUL [1] specifies that passengers should not be subject to pressure pulses exceeding 3

kPa and where pulses occur at regular intervals of less than 10 seconds, the maximum

pulse should not exceed 0.45 kPa.

Pressure variations caused by worst case combinations of train movements must

nevertheless be considered in the design of fan ventilation systems.



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Air flows

The most noticeable aerodynamic effects caused by trains in underground systems are

air flows. In single bore tunnels connecting stations air is pushed ahead of the train as it

progresses through the tunnel, the so-called ‘piston effect’. Some of the air, and how

much depends on the relative size of the train cross-sectional area to the tunnel cross-

sectional area, passes back alongside the train towards the tail. Air is then drawn along

behind the train in the direction of train travel.

Figure 3 shows a plan view of a computer model of a LUL station; (the vertical scale is

exaggerated). Points C-F are located on the station platform, point G and H are located

in passages connecting the two station platforms, point I is at the base of one escalator

shaft and point J is located halfway up a second escalator. The approaching train in the

tunnel is indicated by points A and B. The circles at each end of the platforms are air

shafts.

Figure 3: Prediction points within a model LUL station

Figure 4 shows predictions of the air speeds at the points shown in Figure 3 within the

station for the train scenario described for Figure 2. Points C-F are shown in the upper

diagram, points G-J in the lower.

It can be observed that as the train leaves the previous station air begins to flow along

the station platform, with the highest air speeds occurring closest to the entrance to the

station on the left (point C). The air speeds briefly drop to zero as the train enters the

station and comes to a halt, before increasing to previous levels. The air speeds then

decay as the train waits in the station, before increasing again as the train accelerates

out of the station, this time the highest air speeds occur near the exit tunnel (point F).

Meanwhile air escapes from the platform bore through into the other station platform

bore, with quite high air speeds occurring in the cross passage close to the station

entrance (point G). Air is drawn down one escalator (point J), but forced up the other

(point I). At the latter point there is a peak in the airspeed as the train enters the station.

There tend to be reversals of the flow directions once the train has entered the station

and then leaves again. The air speeds in these passages are of a similar magnitude to

those on the platform.

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Figure 4: Predicted air flows within a LUL station

Figure 5 shows the air speed at the mid-point between the two stations. The air speed

gradually increases in the direction of train travel as the train enters and moves through

the connecting tunnel, reaching a steady value of 15.8 m/s before the train passes,

when it drops to about 3.9 m/s as the train passes. The air speed then increases again,

becoming constant once more, until the train enters and stops in the station, when the

air speed begins to decay towards zero.



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Figure 5: Predicted air flows at the tunnel mid-point between two stations

LUL [1] specifies that air velocities experienced by passengers during normal train

operations should not exceed 5 m/s, and should be further limited to 3 m/s in stations

areas where retail outlets are located.

Aerodynamic drag and heat

Trains experience increased aerodynamic drag as they travel through tunnels due to the

work needed to overcome the air movements noted in Section 4, and the energy to

overcome drag has to come from the traction energy of the train. Coupled with normal

traction requirements to overcome mechanical resistance, and due to inefficiencies in

the traction motors, this leads to the generation of heat. However, the frequent braking

of the trains in the system tends to add considerably more heat into the system than

that arising from just aerodynamic resistance.

Figures 6 and 7 show the speed profile and predicted aerodynamic drag of the first train

in a flight of six new design metro trains entering and passing eastwards at a maximum

speed of 100 km/h through an underground system. A similar flight of trains enters the

line from the westerly direction at the same time. There are five station stops during the

journey. In this configuration the trains run in separate bores between the stations.

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Figure 6: First train speed profile

The leading train of the flight experiences the highest aerodynamic drag, (a comparison

with the other trains is not shown), and the highest values occur between the stations,

where the train reaches maximum speed.

Figure 7: First train aerodynamic drag

Ways in which the aerodynamic drag can be reduced include the use of double bore

tunnels or connecting single bore tunnels with cross-passages or installation of airshafts.

The first two permit the air to move more easily from the front to the back of the train,

so reducing the overall aerodynamic drag of the trains. A disadvantage of these

arrangements is that hot air tends to be retained in the system, gradually increasing in

temperature during the day as the number of trains that have travelled in the system

increases. Airshafts reduce drag by allowing air to be pushed out of or drawn into the

system rather than through the system, and beneficially may aid cooling.



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Other sources of heat in underground metro systems include:

he passengers

rain auxiliary equipment (including air conditioning if fitted)

Services such as escalators and other equipment, commercial units and lighting in

stations,

The surface air in hot countries or in summer time when this air is drawn into the

system. (In all cases, seasonal variations of ambient air temperatures have to be

considered).

In some parts of the world, e.g. Switzerland, there may also be geothermal heat to

contend with.

On the other hand, infiltrating water and the train structures themselves can provide

mechanisms for the removal of heat, depending on their temperatures relative to the air

in the underground system. The use of regenerative braking also reduces the amount of

heat generated during braking.

There are two timescales over which the heat balance in the tunnel system is important.

One is the daily and seasonal temperature variations as they affect the passenger

environment in the stations. The second is the long-term, (perhaps twenty to thirty

years), when the local rock through which the system has been built gradually warms up

before reaching a steady state temperature, reducing its capacity to take heat from the

system.

An analysis of the annual temperature variations in the tunnels of a particular

underground system scheme showed that the approximate proportions of heat transfer

were as follows:

1. Conduction to rock: 7%

2. Convection to the air: 65%

3. To train structure: 28%.

An alternative heat balance is given in [2] for London Underground, and is shown in

Table 1.

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Table 1: Heat balance for London Underground [2].

Here the largest heat sink, loss of heat into the tunnel walls, is gradually reducing as the

surrounding clay heats up. The large differences in the heat balance illustrated above

indicate the complexity of heat management in underground systems, where local

factors have to be taken into account and can have large effect.

Ventilation

Ventilation of underground systems is required to control the heat generated in the

system for the reasons described above, and also to ensure the air quality for

passengers at platforms and, in fire emergencies, help to control the movement of

smoke. The latter will be dealt with in Section 8.

6.1 Normal ventilation

LUL [1] requires that air temperatures in passenger areas should not exceed 30°C under

normal operating conditions. LUL has relied on the use of airshafts located at stations

and between stations for ventilation. The movement of trains in the system expels

heated air above ground ahead of the trains, and then draws fresh and usually cooler air

down the shafts into the system once the trains have passed the shafts. Fan ventilation

is also applied to purge hot air from parts of the system.

One potential difficulty with fan ventilation is the associated noise, and silencers may

have to be fitted to control noise levels in tunnels or at ground level.

Large scale air-conditioning of London Underground is not practical due to the cost and

other methods have been suggested including cooling using ingress water and re-

engineering the existing ventilation system [2].

The heat in the tunnels is largely generated by the trains, with a small amount coming

from station equipment and passengers. So addressing the heat generated by the trains

at source can be very beneficial, as it is often much easier to manage the generation of

heat than to remove it. Accelerating more rapidly and then using controlled coasting to

the next station optimises energy usage. Controlled and firmer braking maximises

energy recovery from regenerative brakes.



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Usually, air conditioning of underground stations is neither a realistic nor economic

method of cooling due to the large air volumes therein and the movement of air caused

by the trains. However, local heat removal using under- or over-platform exhausts is

often used to extract heat from auxiliary equipment on train roofs or from the braking

units.

6.2 Trains stopped in tunnels

Trains may be halted between stations because of congestion or operational difficulties.

In these cases, heat is transferred to the tunnel from traction units, auxiliary equipment

and passengers. Heat loss is at a peak when the trains first stop and the temperatures of

the traction units are highest.

For stopped trains the internal environment can rapidly become uncomfortable and

unpleasant for passengers due to heat build-up, air quality degeneration, (in particular,

carbon dioxide build-up from the respiration of the passengers themselves), and

increase in relative humidity. There are a number of criteria and limits for these factors,

either singly or in combination, and include the dry bulb temperature, the Relative

Warmth Index, the Effective Temperature and the Wet Bulb Global Temperature. These

criteria go beyond aerodynamic considerations and will not be considered further here,

however, Pope et al [3] and Eckford[4] et al provide further details of these and some

comparison between various of the criteria.

Ventilation may be required to provide an appropriate airflow past a stationary train, for

example using a push-pull technique. This is when fans in a shaft on one side of the

affected train draw air downwards into the affected tunnel, whilst fans in another shaft

located on the other side of the train suck air from the tunnel upwards.

Platform barriers

Platform barriers are increasingly used in underground rail systems to ensure the safety of passengers on the station platform who are waiting to board vehicles. The operation of the barriers is synchronised with the opening and closing of train doors and can assist in the management of station dwell times, permitting higher entry speeds for trains arriving at the stations. In tunnel networks, barrier installations may also be part of a continuous partition between the running tunnel and the station areas for the purposes of:

Fire safety (including smoke management)

Tunnel and station ventilation

Trackside noise reduction

Passenger comfort at climate controlled stations.

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Having full height platform barriers means air will be pushed ahead of the trains in the

running bores until the barriers are opened when the trains have stopped. In this way

hot air is prevented from entering the stations in such large quantities, allowing the use

of air conditioning on the platforms. Figure 8 shows full height platform barriers on the

Shanghai Metro at University station.

Platform barriers can also assist with the control of smoke in the event of a fire on a

train and delay migration of smoke into the station platform and access passageways.

Reduced height platform barriers are usually introduced for passenger control during

overcrowding or to keep people away from the sides of trains and platform edge more

generally, and are therefore safety devices. Depending on the details of their design,

these barriers could have some beneficial aerodynamic effects, but the effects will be

very much less than for full height barriers.

Fire safety

The issue of passenger safety in the event of fires in underground systems is treated

very seriously during the design of metros. Consideration has to be taken of: the size

and location of the fire (on a train/ in the running tunnels/ in the station areas); the

procedures to evacuate passengers safely; ventilation and smoke control; train

operations after the fire starts.

There is evidence in the literature [5] that a flow speed of 3 m/s past a train on fire

should be sufficient to keep smoke moving in the same direction as the ventilation flow.

Local meteorological effects have to be considered to ensure that a forced ventilation

system delivers the required air flow. Another consideration is to ensure that airflows

generated in passenger walkways by the forced ventilation system do not exceed safe

levels.

It can be easily seen that a full fire safety analysis of an underground system is a large

task.

Figure 8: Full height platform screen doors on the Shanghai Metro



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Prediction methods

The previous sections demonstrate the extent of aerodynamic effects that have to be

considered when designing and operating an underground system. Due to the

complexity of such systems, they can only be analysed using computer programs. There

are two main methods, one-dimensional analysis and computational fluid dynamics

(CFD).

9.1 One-dimensional analyses

One-dimensional analysis is extensively used in tunnel ventilation design. The software

tools allow integrated modelling of the aerodynamic, thermodynamic and fire scenarios

associated with underground system.

One example is the Subway Environment Simulation (SES) program, developed by the

United States Department of Transportation, Federal Transit Administration. This

software allows the dynamic simulation of trains (piston effect), and predicts air

velocity, temperature and humidity. The program also computes cooling and heating

capacities required to satisfy environmental criteria, and the long-term effect of the

system on the temperature of the surrounding strata.

Ventilation systems consisting of fans, tunnels, stations and dampers can be modelled

using the SES program. Input to the model includes details of plant, civil engineering

details and the loads to be controlled by the ventilation system. Outputs from the model

include air volume flow rates, pressure profiles and changes in air temperature caused

by the heat output of the loads to be controlled.

This program has been used extensively in the design of metro systems around the

world.

The Mott Macdonald rail tunnel aerodynamic and thermodynamic programs [6], using

one dimensional analyses, form a suite of tools that have been used extensively by the

company for designing metros systems around the world. The aerodynamic program

models airflows using unsteady, compressible equations. Flows can be simulated in

complex tunnel networks, with stations, airshafts and cross passages. Multiple train

movements with force ventilation can also be modelled.

The thermodynamics program uses outputs from the aerodynamic program to

determine temperatures throughout the modelled tunnel network. Heat inputs from

trains and other sources are included, together with any heat removal mechanisms.

Time varying ambient conditions can also be taken into consideration.

A third example of programs based on one-dimensional analysis is the THERMOTUN©

software developed by Professor Vardy of Dundee University, [7]. This is based on a one-

dimensional representation of unsteady, homentropic, compressible flow. Parameters

are considered constant across tunnel, station bore and passenger walkway cross-

sections.

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Trains are regarded as impervious cylinders with constant cross-sectional areas. Local

pressure loss coefficients can be specified at the front and rear of the trains to allow for

flow separations or regions of different cross-sectional area, (e.g. locomotives or leading

vehicle shaping). The values of such losses have to be determined experimentally from

full-scale test or moving model tests.

In open tunnel and in regions alongside trains, the equations of continuity, momentum

and energy are combined and expressed in Characteristic form to enable the passage of

pressure wave fronts to be simulated.

Along each tunnel section account can be taken, if necessary, of variations in cross-

sectional area and elevation. There can be airshafts, with or without fans, and adjacent

tunnels can be linked by cross-passages. Frictional effects due to tunnel walls and train

surfaces are computed using friction laws for steady flow.

Figure 9 shows a THERMOTUN© model of a typical underground station in plan, (the

vertical scale is exaggerated), and in side elevation.

Predictions can be made of pressures, air speeds, aerodynamic drag, pollution levels and

temperatures anywhere on the trains and in the underground system,

The main benefit of this program is the ease with which complex underground networks

with multiple trains can be set up, and the speed with which the simulations run. A

comparison of predictions with specially commissioned test pressure measurements

made in the Victoria Line in London [8] is shown in Figure 10; the agreement is generally

very good.

Figure 9: Figure 9 – THERMOTUN© underground station model



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Figure 10: Figure 10 – Comparison of predicted and measure pressures in a LUL station:

P8 on station platform, P9 in northbound running tunnel, P10 in southbound running

tunnel.

9.2 CFD methods

CFD software is increasingly being used for aerodynamic predictions in railway tunnels

as computer power increases and the cost of such applications reduces.

CFD solves the fundamental equations of heat and mass transfer by approximating them

by algebraic equations valid in very small volumes defined within the tunnel system. The

latter equations are solved iteratively in each volume until solutions are found for the

whole computational domain. Nevertheless, the complexity of underground systems

and the requirement to solve transient equations to capture the effect of the moving

trains means that only limited scenarios are currently modelled.

References

1. London Underground Ltd. ‘Design of station and tunnel ventilation’, LUL

Engineering Standard E 4064 A3, March 1998.

2. ‘Cooling the tube’, Rail Engineering, November/December 2007.

3. Pope, C.W., Pope, D.J. & Barrow, H. ‘Review of the environmental conditions

inside an immobilized train in a tunnel’. International Railtech Congress ’98,

Seminar: Tomorrow’s World, I Mech E Seminar Publication, National Exhibition

Centre, Birmingham, UK, 24-26 November 1998.

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4. Eckford, D.C. & Pope, C.W. ‘Cooling of underground railways’. Proc. 12th Int.

Symposium on the Aerodynamics and Ventilation of Vehicle Tunnels, Portoroz,

Slovenia, 11-13 July 2006, pp 409-424. Organised by BHRG, UK,

5. Henson, D.A. & Bell, R.J.W. ‘Planning and design of the ventilation system for the

Vancouver advanced light rapid transit system’. Proc. 5th Int. Symposium on the

Aerodynamics and Ventilation of Vehicle Tunnels. (Lille, France 20-22 May

1985), Cranfield UK, BHRA Fluid Engineering 1985, Paper K4, pp 605-618.

6. Henson, D.A. & Fox, J.A. ‘Transient flows in tunnels of the type proposed for the

Channel Tunnel’. Papers 1 and 2, Proceedings of the Institution of Mechanical

Engineers, Vol 188, 1974.

7. http://www.thermotun.com/

8. Vardy, A.E. 'Unsteady airflows in rapid transit systems'. Proc IMechE, 194(32),

341-356, (1980).

Bibliographic note

The BHRA Group publish the ‘Aerodynamics and Ventilation of Vehicle Tunnels’

Conference Series, which contain many useful references to technical issues related to

underground tunnel systems. Unfortunately, these are only available as bound symposia

proceedings and individual papers are not available on the internet. More information

can be found at http://www.bhrgroup.co.uk/confsite/infobook.htm.

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