## 1 Introduction
However, in the context of the present paper, amongst these proposed solutions, the most interesting one is the proposal of using a compressor [20,21]. It shows that by implementing an air bypass system such as a compressor that crosses the pod, the air passing through the area between the pod and the tube will be reduced, and therefore the velocity of the pod could be increased [22,23]. Moreover, the larger the flow that the compressor is able to handle, the greater the drag reduction. However, within the found literature, the problem of the adaptation of a compressor in the internal aerodynamic system has not been addressed, as these works [20, 21, 22, 23] consider the compressor and the nozzle by means of a simple model in which some mass flow is swallowed at the vehicle front and discharged at its rear part. The current work therefore predicts for the first time the flow field in the compressor and the downstream geometry in the framework of a hyperloop system. Then, once the solution of using a compressor is exposed, the next step would be its implementation. Nevertheless, this brand-new idea leads to new problems: both the lack of compressors for cases with these characteristics, as they are deeply developed in Appendix A, and the lack of completely designed pods adapted to have a compressor that would allow a test of this concept in wind tunnels. On the other hand, the main advances in CFD calculations make the characterization of the compressor map and performance possible [24], potentially overcoming the second problem. With respect to the lack of ad hoc compressors, this present paper proposes the adaptation of existing compressors to the original hyperloop flow properties. Because of the characteristics of the incoming flow, an axial compressor typical of air engines is the best option. As a result, this paper addresses the new problem of the design of a mixer downstream of the compressor, which allows the implementation of an axial compressor by maintaining, within the possibilities, the achieved compression and mix the two currents in order to, later, being expelled by the nozzle. To the author's knowledge, such a core-to-bypass flow mixer represents a new concept for the hyperloop, and due to its novelty, it has not been investigated. In order to overcome these problems, several hypotheses are taken, and a suitable optimization strategy is proposed in several stages. In conclusion, the outlined challenges shown in Section 1, together with the appendix, serve as a motivation for the investigations in the next sections. In Section 2, a general view of the paper is provided, including the system description and the optimization strategy. In Section 3, the numerical model is developed, discussing the CFD methodology, the physical models that govern the case and the assumptions made for the feasibility of the optimization process. In Section 4, the chosen optimization process for both the 2D and 3D cases is theoretically developed by exposing not only the method itself, but the reasons of its choice. In Section 5, the results of the optimization process are shown. Once again, the 2D and the 3D are justified separately and, finally, a simulation of the whole compressor and mixer system is used to complete the process. Finally, Section 6 tries to synthesize the main conclusions of the present paper, as well as present future work.