Numerical study on choking onset of Hyperloop systems: Effect of blockage ratio

This study performs a numerical investigation into the choking onset of Hyperloop systems. As a pod travels through a near-vacuum tube, the bypass flow between the pod and tube walls accelerates, eventually reaching a choked state at the minimum bypass area (throat). Using a two-dimensional axisymmetric model, the choking onset—defined by the first occurrence of sonic flow at the throat—is analyzed across a blockage ratio (BR) range of 0.16–0.64. The numerical results indicate that the idealized isentropic limit underestimates the critical pod Mach number at the choking onset, with the discrepancy intensifying as BR increases. To account for this discrepancy, two effects are systematically analyzed: the upstream state change induced by the leading shock wave (LSW) ahead of the pod and the reduction in bypass area caused by boundary layer displacement on the pod and tube walls. At sufficiently low BR, the modified correlation incorporating only the LSW-induced upstream state change aligns reasonably well with the numerical results. However, the discrepancy between this LSW-only correlation and the numerical results intensifies as BR increases. Specifically, at the highest BR, the maximum deviation of −38% for the isentropic limit is reduced to +14.4% by accounting for the LSW effect and is ultimately reduced to +3.8% when the reduction in bypass area is additionally incorporated. These findings demonstrate that both effects must be considered to accurately predict the choking onset across a broad range of BR in Hyperloop systems.