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Underground
tunnels are the life line of traffic movement in metropolitan
cities, world over. High speed trains running on the
tracks inside tunnels, produce fine metallic and other
pollutants which can get suspended in the air and can reach
unacceptable limits, at times. Also accidents, fires and explosions,
though rarely occurring, can be suffocating and threat to life.
Study of Underground tunnel ventilation and flow of air due to
movement of trains, specially at crosses and junctions, is essential
to prepare emergency evacuation plans, in such
situations.
Due to the frequent movement of high speed trains, the
velocity of air inside the tunnels becomes very high and turbulent.
Because of the trains coming from opposite directions, the flow of
air is often obstructed and modified. Apart from this, the
ventilation system of the tunnel, despite being robust, may not be
sufficient for accidental situations, like fire, explosion
etc.
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:
Train
vocabulary |
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Velocity Vector plot as the train crosses
station Pollutant field as the train crosses
station |
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Location of the sources
of
emission
View of the flow in the
stations
Isocontours of speeds in
Station |
The pollutants in
the form of fine particles can be produced from 3
sources:
1. Fine dust produced by friction of break
pads against wheels.
2. Fine dust produced by
friction of wheels against rails.
3. Fine dust
produced by the sliding action of the pantograph mechanism
over the catenaries.
PM10 Pollutant particles
emanate from two sources
1. Particles at the top (from
electric cable, catenaries and pantograph)
2. Particles at
the bottom (from rails, brake-pads, packing material of
brakes, wheels.)
There is no
emission, when the train is stopped. There are emissions
from cable and pantograph and wheels and
rails during the acceleration phase while all
sources emit during the deceleration phase. The distance
traveled by an average coach per month is nearly 11 000
km.
Covering the aforesaid
aspects, a number of studies were done by Fluidyn Engineers.
We are giving below the excerpts from one such study done for
a railway underground train station system.
Taking into
account the pistonnement, induced by
the passage of the trains in the stations and tunnels, the
same can be simulated by the technique of the sliding
grid. The technique of
the sliding grid and non matching mesh is used
in order to take into account the
displacement of an object in a fixed environment,
Thus, with
each cycle, the whole of the interfaces not in conformity is
recomputed to take into account flows crossing these
interfaces.
For taking into account of the crossing
passage of two trains, the technique used is the same
one: the grid was
thus extended on four zones (two zones on both sides of each
train.
The three-dimensional digital simulations
of the flows, as well as dispersion (gas and particles) were
carried out on a portion of tunnel, including the station and its
various accesses towards outside. The results of calculations,
were compared with the experimental values available.
A detailed and rigorous comparison of the
results obtained by simulation and the results obtained in
experiments is difficult to implement: the lack of
reproducibility of the experimental results available, the
fact that the flows in which measurements were carried out are
not perfectly stationary, the localization of the intake
points (always located near the walls of the tunnels), as well
as the insufficient information available on the
kinetics of dispersion (weak temporal sampling for gas
dispersion, concentrations realized in time for the dispersion
of particles) limit the possible
comparisons.
The comparison carried out for the test of
dispersion of particles is more delicate: although the flow
observed between two stations was considered stable (at the
moment of the velocity measurements) and was directed towards
the system of extraction, one station indeed seems to have the
behavior of an agitated engine (which causes to homogenize the
concentrations inside the station)
The results of the steady flow
obtained correspond to the flow pattern most frequently
observed during conduction of experimental measurements: all the air volume
displacements are made outside of the field towards the system
of extraction
The flow obtained in the access passages towards outside
are, then, also strongly dependent on the movement of the
trains in the station.
The field speed is not homogeneous in this
zone: thus, under
the effect of the reversal of the flow, coming from the
corridors from the station, the flow is faster in the
central part of the tunnel
The adjoining images present some
visualizations of the flow around the trains. One observes a zone of
overpressure (responsible for pushed masses of air just in
front of train,
and a depression (responsible for the aspiration of the masses
of air to the back of the train
It appears that the flow induced by the movement of the
trains in the network strongly influences the propagation of
the tracers.
It is
thus necessary to take into account this movement within the
framework of simulations of the dispersion of pollutants in
the subway in activity. |
 
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Position of
sensors. Contour plots for speed distribution near train &
line
Forthcoming Issue
Modeling
of Polluttion of River
Tarn
Rivers ,
estuaries and large water bodies are source of water supplies to
adjoining cities, towns, Also they are a graet factor in
ecological balance. Dispersion of pollutants, generated out of
industrial effluents, metropolitan sewage
stations, need a close watch. Also the sedimentation, in the river
beds due to flow of solid particles with effluent water,
errosion due to rain and the river ciurrent itself, can cause
fllooding and shifting of the main current. This will cause
suspended pollutants to drift towards stagnated flow areas and
increase the pollutant concentration, and thus disbalance the river
ecology. fluidyn-FLOWSOL family of software are customised to model the flows,
pollutant dispersion and sedimentation in rivers, water bodies and
estuaries. Fluidyn Engineers have done a number of such studies.
Coming issue of the News Letter will feature the study done for the
TARN River. |
