A review of Hyperloop aerodynamics

Evacuated tube transport, also known as Hyperloop, is a proposed mode of ground transport that uses depressurised tubes to transport passengers and cargo at high-speeds. The aerodynamic flow regime of a Hyperloop system combines the characteristics of low Reynolds number, high Mach number, and confined/choked flow. This makes it unique compared to more commonly studied aerodynamic problems, and as such it is not yet well understood. This review aims to evaluate the current state of Hyperloop aerodynamics research. First, the effects of low Reynolds number and compressibility in confined flows are explored. Next, 1D analysis is used to determine the theoretical flow characteristics of the Hyperloop. Analytical expressions are derived for the isentropic and Kantrowitz limits, which divide the state-space of the flow into choked and unchoked regimes. The use of Computational Fluid Dynamics to model the flow in the system is then discussed. This is by far the most active area of Hyperloop research, and results from the literature are evaluated and combined here to give further predictions for the expected flow states, which depend on the blockage ratio and Mach number. Finally, aerodynamic design considerations are explored, including the pod and tube geometry, boundary layer transition, ground proximity, ambient temperature, tube breaches and mitigating flow choking using bleed systems. •Near-vacuum tube pressure allows rapid speeds by reducing aerodynamic drag.•Unique flow states exist in the subsonic, transonic and supersonic regimes.•Isentropic and Kantrowitz limits determine the states for a given blockage ratio.•Mitigating choked flow effects is the greatest challenge for Hyperloop design.