A validated numerical investigation of the effects of high blockage ratio and train and tunnel length upon underground railway aerodynamics

• Validated CFD model of a high blockage ratio subway environment.
• Blockage ratio varied and impact on tunnel air velocities and train drag highlighted.
• Increase in blockage ratio of 30% increases air flow and drag by 40%.
• High blockage ratios restrict air flows during deceleration and increase drag by 300%.

In order to ensure the safety and comfort of passengers and staff, an underground railway requires an extensive ventilation and cooling system. One mechanism for underground railway ventilation is the movement of air induced by trains, termed the ‘piston effect’. This study investigated the effect of altering the blockage ratio of an underground train upon the ventilating air flows driven by a train. First a computational model was developed and validated with experimental data from literature. This model was scaled to represent an operational underground railway with high blockage ratio and the blockage ratio varied to evaluate the effects upon ventilation. The results of this study show that ventilating air flows can be increased significantly during periods of constant train motion and acceleration, by factors of 1.4 and 2 respectively, but that the train drag will increase at the same rate. During deceleration negligible increases in ventilation flows are found but drag increases by a factor of 4.