###### Abstract  
One of the most restrictive conditions in ground transportation at high speeds is aerodynamic drag. This is even more problematic when running inside a tunnel, where compressible phenomena such as wave propagation, shock waves, or flow blocking can happen. Considering Evacuated-Tube Trains (ETTs) or hyperloops, these effects appear during the whole route, as they always operate in a closed environment. Then, one of the concerns is the size of the tunnel, as it directly affects the cost of the infrastructure. When the tube size decreases with a constant section of the vehicle, the power consumption increases exponentially, as the Kantrowitz limit is surpassed. This can be mitigated when adding a compressor to the vehicle as a means of propulsion. The turbomachinery increases the pressure of part of the air faced by the vehicle, thus delaying the critical conditions on surrounding flow. With tunnels using a blockage ratio of 0.5 or higher, the reported reduction in the power consumption is 70%. Additionally, the induced pressure in front of the capsule became a negligible effect. The analysis of the flow shows that the compressor can remove the shock waves downstream and thus allows operation above the Kantrowitz limit. Actually, for a vehicle speed of 700 km/h, the case without a compressor reaches critical conditions at a blockage ratio of 0.18, which is a tunnel even smaller than those used for High-Speed Rails (0.23). When aerodynamic propulsion is used, sonic Mach numbers are reached above a blockage ratio of 0.5. A direct effect is that cases with turbomachinery can operate in tunnels with blockage ratios even 2.8 times higher than the non-compressor cases, enabling a considerable reduction in the size of the tunnel without affecting the performance. This work, after conducting bibliographic research, presents the geometry, mesh, and setup. Later, results for the flow without compressor are shown. Finally, it is discussed how the addition of the compressor improves the flow behavior and power consumption of the case. CFD; evacuated tube; train; vacuum tube; hyperloop; aerodynamic propulsion +
Footnote †: journal: Medical  
tunnel), or the Swiss AlpTransit project with 120 km of tunnels. Moreover, in Japan, the new Maglev Yamanashi test line has 82 % of the total route (42.8 km) inside tunnels. One evolution of conventional trains is the Evacuated-Tube Train (ETT), in which the train, or capsule, runs insides a tunnel during the whole track. This closed environment enables it to reduce the operating pressure and therefore the aerodynamic resistance. Not only drag but also the impact of certain flow patterns present in HSR tunnels are minimized when decreasing the pressure. If reducing the pressure is proposed as a solution to surpass current speed limitations, which is one of the focuses of the present work, it is important to remark that construction costs become more significant as the whole route has to be confined inside a tube. However, an optimal cross-sectional design can reduce the investment considerably. [PERSON] in 2013 took the concept of vacuum train one step forward, naming it as the hyperloop on its document, the Hyperloop Alpha [5]. [PERSON] proposed the combination of levitating trains and a low-pressure environment. Although he planned on using air bearings for levitation, the concept could be adapted to use other types of levitation, such as magnetic ones. The same happens with propulsion, as he proposed the capsules to be driven by linear induction motors. However, other methods such as aerodynamic propulsion might be considered. The latter is preliminarily studied in the present work. Back to the aerodynamic problem, certain issues are peculiar to the movement of trains inside tunnels, and they do not appear in open air [4]. One of these problems is the compression and expansion waves generated when the train enters the tunnel. This scenario has to be taken into consideration during the design process of a standard HSR. Other authors have also researched this topic.