Aerodynamics and ventilation in rail tunnels May 2015

The need for secure, fast and environmentally friendly passenger and freight transportation prompts a need for the planning and construction of new tunnels for rail transportation. As train speeds increase, the effects of tunnel aerodynamics on the design of tunnels and vehicles become more important. Aerodynamics, rather than the size of trains or the economics of construction, might become the determining factor for the size or layout of rail tunnels and are of increasing importance when designing and building tunnels.

Fig 1. Pressure patterns as measured in the Grauholz Tunnel in Switzerland and calculated by THERMOTUN (train at 200km/hr entering a 70m² tunnel)

Planning of aerodynamics and the related climate and ventilation requires interfacing with engineering disciplines such as civil design, technical equipment and railway technology. Aerodynamics, climate and ventilation of tunneling projects are important in projects characterized by the following:

• high-speed rail tunnels
• very long tunnels
• steep tunnels
• deep tunnels
• high-performance underground or metro systems
• high overburden resulting in high heat ingress from the ground
• tunnels exposed to extreme climate conditions (wind at portals, outside temperatures)

Civil measures can help eliminate non-acceptable aerodynamic conditions.

Aerodynamic issues related to tunnel engineering

When a train enters and travels through a tunnel, it behaves as a loosely fitting piston in a tube. At higher velocities, it becomes more noticeable that the pressure and airflow field within the tunnel is being formed by the movement of distinct pressure waves. These waves pass down the tunnel at the speed of sound, reflect from the portals, and then return to pass over the trains several times after successive reflections at the portals (and also at airshaft junctions and from other trains in the tunnel). The interfering pressure changes are the cause for several aerodynamic phenomena of engineering interest, including: pressure loads, pressure comfort, traction power, micro-pressure waves (sonic booms), tunnel climate, loads due to velocity, health limits related to pressure changes, and comfort and safety related to air velocities.

Pressure fluctuations

A train entering a tunnel displaces air and instantly increases the pressure in the tunnel region at the train nose. The overpressure leads to some air flowing back alongside the train and out of the entrance portal. The remainder passes down the tunnel behind a pressure wave front. The pressure wave propagates with the speed of sound as a compression wave (+) along the tunnel. At the opposite portal the pressure wave is reflected and changes from a compression wave into an expansion wave (-) propagating back into to the tunnel.

As the tail of the train enters the tunnel, a sudden pressure drop occurs behind the train. This second pressure wave propagates with the speed of sound as a decompression wave (-) along the tunnel. At the same time the resistance, or aerodynamic drag, that a moving vehicle experiences leads to a characteristic pressure distribution along the train.

As the train nose reaches a certain point in the tunnel, a pressure drop occurs. As the train passes this certain point, the pressure further drops due to the longitudinal friction along the train surface. Behind the train tail the pressure increases again. A comparison between measured pressure changes at a certain point in a tunnel and the results of a simulation program THERMOTUN (www.thermotun.com) is given in Fig 1.

Fig 4. Required power of ICE-3 train to run at 250km/hr from north to south through Gotthard Base Tunnel

Pressure loads

The magnitude of the pressure fluctuations in a tunnel is, among other factors, a result of the velocity, cross-section, length, shape and roughness of the train and the length, free cross-sectional area, roughness and civil construction type of the tunnel and the portals. The traversing pressure waves and pressure changes along a moving train will affect the:

• tunnel equipment and installations
• forces acting on the train's surfaces (windows, doors, air-conditioning system, etc.)
• pressure comfort for the train passengers and staff
• function of the drainage system of the tunnel
• functionality of the ventilation/cooling systems of the cross-passages
• forces acting on cross-passage doors and cabinets

Equipment can be designed for the elevated pressure fluctuations in a rail tunnel. Sliding doors are aerodynamically neutral since they remain in their open position independently of the direction of pressure gradients, i.e., they do not swing open or closed due to pressure loads. Civil measures, such as larger tunnel cross-sections, can help prevent extreme pressure fluctuations.

Pressure comfort

Sudden pressure changes might create discomfort to train passengers and staff. The pressure comfort problem is associated with the effects of pressure on the eardrum. Rapid and significant changes of pressure external to the eardrum that are not relieved by similar changes internal to the eardrum (within the middle ear) can give rise to discomfort. In extreme cases, the pressure fluctuations might lead to injury.

Fig 5. Specific investment and operation costs for rail tunnels with different sizes of cross-sectional areas (left); optimal size of cross-sectional area depending on costs for electrical power (right; principal relationship only)

Several studies with pressure chambers and additional statistical enquiries on rail tracks with various tunnels led to different comfort criteria. The International Union of Railway (UIC) harmonised some different national criteria and published two sets of four different criteria each^1,2. The UIC pressure comfort criteria specify the maximum acceptable pressure changes at given time intervals in a train, and are quite strict in the sense that even new high-speed trains have difficulties meeting the requirements, particularly in new single-track double-tube tunnels.

Modifications of the civil design of the tunnel can reduce pressure fluctuations (Fig 2). In all cases, the objectives of the measures are to reduce the amplitudes and the gradients of the pressure waves. The trumpet-like portal creates moderate amplitudes and gradients of entry/exit waves. An increased free cross-sectional area of the whole tunnel leads additionally to less frictional pressure losses along the train and in the tunnel. Open cross-passages and shafts lead to partial reflections and weakening of pressure waves. By implementing the shaft according Fig 3, the pressure comfort improved for tunnels with a free cross-sectional area of 76m². To obtain the same level of pressure comfort without shafts, the tunnels would have required a free cross-sectional area of about 105m².

Fig 6. Pressure relief ducts in Channel Tunnel to reduce traction power demand of trains (green)⁴

Traction power

Traction power is the power required at the wheel rim of the locomotive or trainset to overcome forces of resistance, including aerodynamic resistance. Aerodynamic resistance is a major contributor to the power consumption of the trains, both on open track and in the tunnel (Fig 4). In the tunnel, the aerodynamic resistance leads to significantly higher power requirements than on the open track particularly at high velocity (in the tunnel by a factor of two to three more than on open track). In order to reduce the required power, the following measures could be applied in tunnels: increase the free cross-sectional area, introduce draught relief shafts or pressure relief ducts and reduce the friction coefficient of the wall and trackway. The influence of the free cross-sectional area on investment costs and the costs for traction power are shown in Fig 5, which indicates that for given boundary conditions an optimal tunnel size might exist.

Pressure relief ducts are another measure to reduce the traction power demand in a tunnel (Fig 6). The ducts allow an air-exchange between the tubes. Trains push air though the tunnel due to the piston effect. Because of the pressure relief ducts, the air can bypass the train through the parallel tube. The air does not need to be moved through the whole tunnel. This reduces the power demand of the trains substantially. As a result, shuttle trains can still be operated with standard locomotives and traction power supply.

Table 1: Origin, impact and measures of non-acceptable micro-pressure waves

Micro-pressure waves (sonic boom)

The initial pressure wave generated by trains at the entrance portal steepens as the wave propagates through the tunnel. The steepening process happens if the entry pressure gradient is so high that the speed of sound differs significantly within the wave front and the dispersion and friction effects that usually counteract this phenomenon are sufficiently small. With unfavourable tunnel and train design, the pressure wave might detonate with a loud sound upon reaching the exit portal. In a more moderate form, micro-pressure waves might create vibrations of doors, windows and walls in the surrounding of portals. This phenomenon is called non­acceptable micro-pressure waves or a sonic boom.

Micro-pressure waves are produced upon trains leaving the tunnel as well. However, these waves are typically more moderate than the waves created by the entrance of a train. Every train entering and leaving a tunnel produces micro-pressure waves. However, it is only with certain combinations of parameters that they become noticeable. Certain minimum requirements must be fulfilled with respect to train velocity (e.g., > 200km/hr), blockage ratio (e.g., > 0.12), tunnel length (e.g., > 4km), surface quality of tunnel, etc.

In general, the probability of creating non-acceptable pressure fluctuations at the portals increases with smaller cross-sections at the opposite portal of train passage (high blockage ratios) and with slab track rather than ballast track. The amplitude of the pressure wave increases in a quadratic manner and the gradient of the pressure wave in a cubic manner with increasing train velocity. The magnitude of micro­pressure waves is assessed using an empirical Japanese acceptance criterion. There is no international standard. The acceptance criterion is applied in a distance of 20m and at an angle of 45 degrees from the portal (outside the tunnel). If the pressure fluctuation at this point is more than 20Pa, there will be a high risk of non-acceptable micro-pressure waves.

Table 1 lists possible measures to be applied at different locations of the tunnel in order to handle the effects of micro-pressure waves. Measures at the rolling stock need to focus on aerodynamic shape of the nose and tail of the train. Hoods at portals of rail tunnels have become a common measure to prevent non-acceptable micro-pressure waves at the portals. The hoods have a larger free cross-sectional area than the actual tunnel. Additionally, the hoods are equipped with openings. The hoods lead to less extreme gradients and amplitudes of the pressure waves.

Table 2: Possible measures to influence aerodynamics and their efficiency (see Fig 9 for illustration)

Climate

The temperature is the major parameter for tunnel climate in a narrow sense. Parameters for climate in a wider sense include humidity, air velocity, pressures and air-quality (pollution, odour, dust concentrations, etc.). The tunnel climate is the result of a complex interaction of various aero-thermal processes. Apart from the various heat and humidity sources, the train-induced air exchange in a tunnel is the major factor influencing the tunnel climate. The climate or the environmental conditions in a tunnel are strongly affected by the aerodynamic conditions in the tunnel and vice-versa.

The measures to affect the train-induced airflow and the climate might include shafts, portal design, gates, air locks and platform screen doors. As an example, Fig 7 shows the influence of the free cross-sectional area of shafts and the number of station exits on the temperature for selected reference cases of an underground system. The results of this design study show that train-induced airflow can be used to influence the temperature in station and tunnels to some degree.

The design of portals of a single-track double-tube tunnel system is one way to influence the tunnel climate by civil measures (Fig 8). The appropriate design of the tunnel portals prevents warm air being expelled from one tube to re-enter the tunnel in the parallel tube. Apart from the beneficial effect on the climate, the anti-recirculation design of the portal also prevents smoke re-entry in case of tunnel fires.

Fig 9. Illustration of the civil measures described in Table 2

Civil design measures

Train-induced pressure fluctuations lead to extreme fluctuation of air pressure in tunnels. Non­acceptable aerodynamic conditions can be eliminated by reasonable civil measures, as presented in Table 2 and illustrated by Fig 9. These measures ensure comfort and safety for the passengers and staff and, possibly, reduced cost compared to changes at vehicles or operation. The best solutions are generated by cooperation of those planning for the vehicle, the track and the tunnels together with an intensive exchange with the future operator.