# Effects of Tunnel and Its Ventilation Modes on the Aerodynamic Drag of a Subway Train
## 5 Conclusions  
The effects of the tunnel and also tunnel ventilation modes on the aerodynamic drag of a subway train of eight carriages at a relatively low speed are investigated. In situ measurements were conducted during a routine operation to estimate the surface shear stress, using a hotwire, and the pressure difference between the forward surface of the first carriage and the backward surface of the last carriage. Two tunnel ventilation modes were examined, i.e., the recirculation and the free-cooling modes. The following conclusions may be drawn based on the investigation:  
(1) The measured wall shear stress or the skin friction coefficient agrees with previous reports in terms of its value and dependence on the train speed, thus providing a validation for the present measurement. The magnitude of the stagnation pressure on the forward surface of the first carriage is about 10 times that on the backward surface of the tail carriage. As such, the pressure drag (\(D_{p}\)) on the train is largely determined by the pressure on its forward surface;  
(2) It has been found that the presence of the tunnel increases considerably both \(\tau_{\rm tau}\) and \(P_{f}\) on the train even at low speeds. The tunnel effect is more appreciable on \(\tau_{\rm tau}\) than on \(P_{f}\), and at a train speed \(V\) = 20.5 m/s the presence of the tunnel operated in the free-cooling mode increases \(\tau_{\rm tau}\) by 30.2% and \(P_{f}\) by 24.5%;  
Figure 12: The forward stagnation pressure \(P_{f}\) of the first carriage in the two ventilation modes and in the open air.  
(3) The tunnel ventilation mode has an appreciable effect on \(\tau_{uu}\), but not on \(P_{f}\). In the recirculation mode, the injection of cooled air raises the turbulence intensity inside the tunnel, thus resulting in an increase in the velocity gradient at the surface and hence a higher \(\tau_{uu}\). The effect of the ventilation mode on \(\tau_{uu}\) is enhanced with increasing \(V\). The increase in \(\tau_{uu}\), compared with the free-cooling mode, owing to the recirculation ventilation mode is negligibly small at a small \(V\), but reaches 27% at \(V=\) 20.5 m/s;  
(4) The friction coefficient \(C_{f}\) decreases with increasing \(V\) and reaches an asymptotic value of 0.0013, 0.0017 and 0.0021 in the open air, free-cooling and recirculation modes, respectively. \(C_{f}\) in the free-cooling and recirculation modes augments, compared with that in the open air, by 30.4% and 66.2% at \(V=\)11.5 m/s and by 27.9% and 76.5% at \(V=\) 20.5 m/s, respectively. The effect of the tunnel on \(C_{f}\) shrinks with decreasing \(V\) in the free-cooling mode, effectively vanishing at \(V=\) 2-6 m/s; this effect is however still appreciable in the recirculation mode even at \(V<\) 6 m/s.  
Conceptualization, Y.Z.; Methodology, H.W., Y.Z. and J.H.; Data curation, G.L.; Writing--original draft, H.W.; Writing--review & editing, Y.Z., H.T., J.D. and M.M.A.; Supervision, Y.Z.; Funding acquisition, Y.Z., H.T. and J.D. All authors have read and agreed to the published version of the manuscript. Y.Z. wishes to acknowledge support given to him from CRRC Qingdao Sifang Co., Ltd. through contract SF/JS-R-7-2021-239.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
This work was completed when authors worked at The Hong Kong Polytechnic University and would be impossible without the support, through Train Service Engineering Manager Ir [PERSON], of MTR Corporation Limited in Hong Kong, which not only funded the project but also provided all facilities essential for the measurements.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.