Page 345 - 360.revista de Alta Velocidad - Nº 5

Influence of Aerodynamics in Tunnels Design

speed of sound, and is affected by the inner structure of the tunnel: tracks, platform ...

• Radiation of part of the micro - pressure wave to the outside (sonic boom). As the velocity
of the train increases at the entrance to the tunnel, the amplitude of the pressure wave
grows cubically

The effect called "Sonic Boom" is characterized by a sound similar to an explosion in some cases.
It can be detected even at distances of 1km, and it can cause vibrations in doors and windows
of nearby buildings. In addition, the strong sound generated causes a generally unacceptable
noise pollution.

The maximum value of these micro - pressure waves is approximately proportional to the
pressure gradient of the wave arriving at the exit portal (generated by the entrance of the
train in the tunnel) and inversely proportional to the distance to which the train is located of
said portal.

Figure 5 Sonic Boom generation

Currently, from different countries, and mainly at European level, measures are contemplated
dissipating this effect once it has been proven to occur. However, there are still few actions that
arise at the construction project level, since this effect can not be reliably calculated with the
computer tools available today.
The most important measures used to mitigate this effect are detailed below. Some of them
can only be included in the drafting phase of the project since these are measures that affect
the construction of the tunnel. Other measures may be incorporated later, even if the tunnel
is in operation.
As a main measure to mitigate the effect of the "Sonic Boom" is considered the reduction of the
pressure gradient of the wave that reaches the exit portal of the tunnel once the train enters
the same. This is achieved mainly by reducing the speed at the entrance of the tunnel and
optimizing the aerodynamics of the nose of the train. These two measures can be implemented
even if the tunnel is in operation.
On the other hand, structural measures can be applied that help reduce the effects of the sonic
boom phenomenon. These measures could be:
• Increase the free cross- section of the tunnel which is only possible in the design phase.
• Modification of the design of the packaging: inclusion of "windows" or pores (also called
porous or perforated packaging). These "pores" act as pressure sinks. Another measure is
to modify the outer geometry of the portals making them flared, as well as to increase the
length of the same.
• Use of ballasted track instead of slab track, since ballast, not being a continuous medium,
has gaps between the edges that act as "pressure absorbers".
• Use of absorbent materials inside the lining (porous panels). This measure is very innovative
and it has detractors because of fire safety and durability.

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