Simulating Piston Effect in Underground Metro Tunnel- Ansys CFX Tutorial
 

CFD analysis of Piston effect simulation in underground metro tunnel, using ANSYS CFX 19

This tutorial simulates piston effect in a underground metro tunnel. I have included following steps-
1. Design Modeler Modeling- create 3D model using Ansys design modeler.
2. Meshing- create mesh using Ansys meshing, adding inflation.
3. CFX Pre setup- configuring transient setup, immersed solid and boundary conditions.
4. Solution- solve using CFX solve manager.
5. Results Post processing- post processing using CFD post, create and save time-step animations of pressure, velocity and streamlines. creating variable chart XY transient or sequence.

Parameters of simulation as as-
Initial velocity of train 10 m/s
start/ stop acceleration- (120/360) m/s2
t arrive = 22 s
t depart = 40 s


What is Piston effect-

Piston effect refers to the forced-air flow inside a tunnel or shaft caused by moving vehicles.

In open air, when a vehicle travels along, air pushed aside can move in any direction except into the ground. Inside a tunnel, air is confined by the tunnel walls to move along the tunnel. Behind the moving vehicle, as air has been pushed away, suction is created, and air is pulled to flow into the tunnel. In addition, because of fluid viscosity, the surface of the vehicle drags the air to flow with vehicle, a force experienced as skin drag by the vehicle. This movement of air by the vehicle is analogous to the operation of a mechanical piston as inside a reciprocating compressor gas pump, hence the name 'piston effect.' The effect is also similar to the pressure fluctuations inside drainage pipes as waste water pushes air in front of it.
Air flow caused by the piston effect can exert large forces on the installations inside the tunnel and so these installations have to be carefully designed and installed properly. Non-return dampers are sometimes needed to prevent stalling of ventilation fans caused by this air flow.