Numerical analysis of piston effect in one of the tunnels of Shiraz subway system including the effect of fans

The flow field around each moving vehicle experiences the pressure and velocity changes. The pressure in front of the vehicle increases while the pressure behind it decreases. This phenomenon, which is more pronounced in closed space like tunnels, is called piston effect. When the train enters the tunnel a compression wave would be generated and propagates along the tunnel which causes the most and the first pressure rise along the tunnel. The compression wave along with an expansion wave of the train tail severely affects the pressure and velocity distributions in the tunnel and around the train. In this research, firstly the results were compared with experimental data available in the literatures to verify the accuracy of the present numerical model. Then the flowfield around the train in different time steps was obtained.
The results show that, in each time step, the most pressure and velocity changes along the tunnel, occurs around the train body. The maximum velocity magnitude is around the train rear and the highest pressure magnitude is around the train head except at the time of train negative acceleration, which the pressure magnitude is maximum at the rear of the train.
Finally, the interaction of the fans near the tunnel inlet and outlet and train movement was investigated which results show that when the fan is in operation, the separation bubble on the train top would be larger which leads to increase in the first pressure rise due to train entry to the tunnel. In addition the inlet and outlet flow to the tunnel from stations change remarkably due to train movement.