Domain
Scientific Analysis  A description of the global behaviour of the flow in the tunnelA study on the effectiveness of the ventilation system installed on an underground system is undertaken. This case consideres a two carriage 40 [m] long and  train located at 120 [m] from an underground station where the considered ventilation system is installed. This simulation supposes that the middle portion of the train is on fire and sits inside the 5[m] radius tube on a single magnetic rail. For a given time, a plot of the streamlines is shown in Figure (\ref{479483}). This plot reveals how the flow is directed away from the station and exhibits two distinct behaviours. Inside the train station the flow is first seen to follow a circular motion along the walls. Then, the flow enters the tunnel where it quickly becomes parallel to the tunnel itself. The pressure is also shown to reach its peak values at the level of the station and to decrease its values along the tunnel as shown in Figure (\ref{161606}). The temperature inside the tunnel is plotted in Figure (\ref{659948}). It can be noticed that the temperature on the surface of the fire-hit middle section of the train peaks at 580 Kelvin.  However, the temperature on the lower section of the tunnel between the train and the station exhibitis temperatures around 290 Kelvin. The flow inside the train station in shown in figure (_???_) and (_???_); the flow injected from the ventilation system at a very high pressure has two main effects on the global flow: in the first place, it makes the pressure inside the station rise well above the average pressure in the tunnel, thus helping to generate the directional flow observed in the tunnel. In the second place, it creates vortices inside the tran station of the diameter of few meters. For what concerns the flow around the train, as shown in figure (_???_), the flow created by the section on fire is successfully deviated towards the end of the tunnel and away from the station. A vortex is created above the train close to the section on fire. As soon as the flow approaches the train from the station, the flow is deviated above the train (creating a safety zone from smoke in front of the train) and as soon as the flow pass over the train, the flow invades the whole tunnel causing a higher presence of smoke behind the train.A description of how the ventilation system helps the evacuation of passengersAs can be seen from Figure(\ref{233413}), the smoke is restricted to the upper 30% of the tunnel. This gives a roughly 3m high clear zone on either side of the train for passengers to evacuate from. Furthermore the ventilation system ensures that the smoke is sucked out of the tunnel, as can be seen from Figure (\ref{237423}) the smoke velocity is nearly 5 m/s.  The region of smoke to the front of the train is stagnant , as can be seen from Figure (\ref{237423}). But it does not obstruct the evacuation of the passengers towards the station.  The spread of the smoke towards the station could cause a problem as the vortical flow in the station may spread the smoke hroughout the station and tunnel. It can be seen from Figures (\ref{463230}) and (\ref{581264}), the temperature does not cross 300K near the regions where the passengers would be expected to evacuate from. However the simulation of the fire using a localized region of high temperature seems unrealistic. The simulation appears to ignore the spread of the fire along the train and subsequent change in smoke generation region. Also the relatively cool temperature within the tunnel, close to the fire, appears to be unrealistic. Figure(_???_).