In study of the train rollover or the slipstream.

In
recent decades, in railway transportation a trend towards higher train speeds
has evolved.

As
a result, the optimization of the aerodynamic performance of high-speed trains
has become one of the main objectives of the design process. The major reasons
to consider carefully the aerodynamic design of trains and other ground
vehicles are the increasing requirements for low energy consumption, high safety
and stability requirements, improved operational convenience and comfort and
low acoustic noise production. Among the different issues of aerodynamics considered
these days, side wind effects play an important role owing to their non-trivial
impact on the rollover stability of trains. Cruising in cross winds results in
the generation of aerodynamic forces and moments which tend to roll the train
of the tracks. Obviously, the resulting forces and moments depend strongly on
the speed of travel and consequently the threat from side winds becomes more
serious as the speed is increased. 1
With the increase of the train speed, aerodynamics has become an important key
in the rail vehicles field. The study of the aerodynamics of a train can lead
to substantial cost savings and more environmental friendly trains. The most
studied phenomenon in the realization of a train is the drag generated by the
displacement of the train in the air flow. Reducing the drag leads to a
reduction of the amount of energy needed. But some others aerodynamic phenomena
are of interests, such as the pressure variation while the train is driving in
a tunnel, the study of the consequences of a crosswind, the study of the train
rollover or the slipstream. 2
In the last two decades and with the introduction of high-speed trains,
aerodynamic effects became a major issue for both train operators and train
manufacturers. When a train moves in open-air, it generates regions of highly turbulent
flow known as a slipstream. The slipstream is generally associated with high
air velocities and rapidly-changing pressure fields. These two effects can
create significant problems for passengers on platforms, trackside workers and
also for the stability of high-speed trains if running in strong side winds.
However, when a train passes a confined space such as a tunnel, additional
aerodynamic issues appear. This can be explained through the underlying
phenomena such as the compressibility of air around high-speed trains due to
running in a confined space, which produces pressure transients along the
tunnel. Three main aerodynamic phenomena occur when a train enters a tunnel:

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1.      generation
of pressure waves inside the tunnel

2.      propagation
of these waves along the tunnel length and the

3.      Reflection
of the waves back along the tunnel length with micro pressure waves that
eventually escapes from the tunnel exit.

The
pressure transients caused by trains passing through tunnels are significantly
larger than those in the open air and can cause passenger discomfort,
especially in poorly-sealed trains. Moreover, velocity, pressure variation and
direction of the flow inside tunnels are different than those around a train in
open air. These differences can be related to the cross section and the length
of the tunnel and can result in a number of problems. In addition to passenger
discomfort issues due to pressure fluctuations, the pressure difference between
the outside and the interior of vehicles can load and stress the components of
vehicle body. Also the radiation of pressure waves at tunnel exit generates
sonic boom, which is greater for slab track rather than for ballasted track. This
can cause inconvenience for workers and people living or passing in proximity of
the exits of tunnels. 3