Theoretical and numerical study of choking mechanism of fluid flow in Hyperloop system

The Hyperloop is a proposed transportation system in which a high-speed vehicle travels in a near-vacuum tube. High-speed motion of an object (near Mach 1) in a confined space generates choked flow, endangering operational safety. This study investigates the physics and behaviour of choked flow in the Hyperloop system, and the corresponding law and influencing factors are theoretically analysed. A CFD method is used to simulate the compressible flow in the system. The different flow states caused by a pod travelling in the Hyperloop tube are numerically obtained, and the results concurred with the theoretical prediction. The results show that the choking strength gradually increases as the vehicle accelerates, reaching its maximum at Mach 1 and then gradually decreasing. When the blockage ratio reaches 0.4, nearly one-third of air hardly passes through the throat at most owing to the choking. The inflow velocity, choked flow density, and total pressure loss caused by the shock wave jointly affect the mass-flow limitation. In addition, the normal shock wave is the main characteristic of choked flow. The flow transitions from choked to unchoked regimes owing to the supersonic pod and forms a bow shock wave. These findings, including the equations describing the choking mechanism, can benefit various design aspects of the Hyperloop system.