Confined Transonic Aerodynamics: From Rifle Bullets to Hyperloop Vehicles

The Hyperloop concept, which uses partially evacuated tubes to transport passenger pods at high speeds, has attracted significant attention in recent years as a potential future mode of travel. Hyperloop presents a unique aerodynamics problem as it sits at the intersection of transonic, low Reynolds number and confined/choked flow. Consequently, very little experimental research has been conducted on the aerodynamics of Hyperloop-type systems, making validation of simulation methods very difficult. Here, we draw attention to a preexisting experimental data set for a transonic bullet, travelling at Mach 1.1 in groundeffect. This is the basis for validating Computational Fluid Dynamics (CFD) simulations and these are shown to accurately replicate pressure distributions. Next, the computational approach is extended, culminating in aerodynamic simulations of a generic Hyperloop system. This is achieved by successively making the following changes to the validated projectile model, while keeping all other variables and settings consistent: (i) scale, (ii) operating pressure, (iii) full-tube confinement, and (iv) tail geometry. Flow choking is found to dominate the flow characteristics within the tube, however, extremely low aerodynamic drag levels are predicted, underlining the clear potential of the fundamental concept. It is envisaged that the generic Hyperloop pod design presented here will form the basis for a benchmark case to assist wider research efforts in this area.