Effects of Tunnel and Its Ventilation Modes on the Aerodynamic Drag of a Subway Train

This paper reports an in situ measurement on the effects of a tunnel and its ventilation modes on the aerodynamic drag of a subway train with eight carriages during its routine operation. The train speed (V) varied continually from 0 to 22 m/s. Two modes of tunnel ventilation were examined, i.e., recirculation and free-cooling modes. The former mode is associated with pumping cooled air into the tunnel to provide extra cooling, while the latter is not. The friction coefficient C_f of the train surface was estimated using two hotwire probes mounted on the roof of the first and last carriages, respectively. The front- and rear-stagnation pressures (P_f and P_l) were measured using two pressure taps located at the center of the forward surface of the first carriage and the backward surface of the last carriage, respectively. It has been found that the presence of a tunnel significantly increases both C_f and P_f. For example, at V = 20.5 m/s, C_f and P_f were 30.2% and 24.5% higher, respectively, in the tunnel than their counterparts in open air. The tunnel ventilation mode also has remarkable effects on C_f. The recirculation mode resulted in 23.5% higher C_f than the free-cooling mode. On the other hand, the tunnel ventilation mode does not seem to have an appreciable effect on P_f. The physics behind these observations is also discussed.