## 5 Conclusions  
After comparing the thermodynamic behavior of a capsule inside a low-pressure tunnel using and not using a compressor, the conclusions can be presented. The addition of the compressor to the simulation leads to a reduction in the required power to maintain a constant speed for high blockage ratios. At low BRs (around 0.2), it can even be beneficial to avoid using the compressor, as the flow is entirely subsonic. However, as the tunnel gets smaller and transonic and supersonic effects start to appear, the power consumed by the _base_ case grows exponentially. In those cases, the compressor consumes considerably less power. This reduction grows asymptotically to a value around 70% at a BR between 0.35 and 0.5. Higher blockage ratios do not increase the improvement of the compressor. The capsule and the tunnel behave similarly to a converging-diverging nozzle, in which, once critical conditions are reached, the flow becomes supersonic downstream, enforcing the appearance of obliques shock waves. The analysis of the flow patterns has shown that the addition of the compressor can remove these waves of the vehicle in all of the simulated cases, even if they reach sonic conditions. Thus, the flow is qualitatively independent of the size of the tunnel. Not only is the mitigation of shock waves an effect of the turbomachinery, but also, for the geometry studied, the downstream separation is removed due to the nozzle that substitutes the wall at the tail of the capsule. The delay in the transonic effects is also evident when analyzing the induced speed present on the inlet of the domain. The _compressor_ case can maintain the inlet speed equal to the capsule speed, especially at 500 km/h, as the case is always below the Kantrowitz limit. For higher speeds, the induced speed is still negligible compared to the _base_ case, in which the induced effect increases linearly with the BR independently of the reference speed. This is because this effect is mainly dependent on the geometry. The benefits of the installation of the compressor can also be evaluated in terms of the point at which the critical Mach number is reached. The turbomachinery enables the vehicle to operate subsonic in a wider range, which is what explains the reduction in power consumption. For the _base_ case, for 700 km/h, the limit is at \(\beta=0.18\), while with the compressor it is increased somewhere in between 0.5 and 0.6. It is important to remember that the reference value of the blockage ratio for an HSR is 0.23, which means that a conventional train will operate above the Kantrowitz limit if running at this speed. With regard to 500 km/h, the _base_ case blocks at \(\beta=0.36\), while the _compressor_ never does  
Figure 16: Equivalent pressure level for the _base_ case that requires the same power as the _compressor_ case at 0.1 atm for the different blockage ratios and speeds. in the range studied. Moreover, it is remarkable how the isentropic approximation follows accurately the trend of the CFD simulation when evaluating the Mach number on the passage, although a Mach number slightly greater than 1 have been achieved on the throat. In that sense, an equivalent tube has been compared between both cases. The main conclusion is that, until the _compressor_ case blocks, it can operate with tunnels of 2.8 times lower cross-sectional area for the same capsule section, independently of the speed. This is a considerable reduction on the tunnel size, and so, on the infrastructure cost. If future ground transport is to overcome 500 km/h, a new concept where ambient pressure can be decreased must be proposed. This allows decreasing the drag of the capsuled linearly with this parameter, leading to acceptable air resistance even at very high speeds and small tunnels. Moreover, the system performance improves considerably when installing a compressor on the vehicle due to the power consumption reduction and the chance to use considerably smaller tunnels. [PERSON], [PERSON] and [PERSON] have equally contributed to the development of this work.