Effects of compressible flow phenomena on aerodynamic characteristics in Hyperloop system

In this study, numerical simulations were conducted at various pod speeds (vpod= 100–350 m/s) using an unsteady, compressible solver with the Reynolds-averaged Navier–Stokes model to analyze the aerodynamic characteristics and pressure wave behavior in the Hyperloop system. Furthermore, the aerodynamic drag and pressure wave behavior were theoretically predicted based on quasi-one-dimensional assumptions. The flow around the pod is classified into three regimes according to the pod speed based on the compressible flow phenomena. In regime 1 (vpod= 100–170 m/s), the compression waves develop into normal shock waves even without the occurrence of choking at the throat. In regime 2 (vpod= 180–230 m/s), choking occurs at the throat and an oblique shock wave appears within the pod tail section. In regime 3 (vpod=240–350 m/s), a trailing shock wave propagates at the end of the oblique shock wave. Due to fully accelerated flow in this regime, the Mach number of the flow behind the pod in a pod-fixed coordinates remains constant as 2.1, regardless of the pod speed. This constant Mach number causes the drag coefficient to decrease while the pod speed increases. Although non-isentropic conditions such as the formation of a boundary layer and flow separation cause variation between the theoretical prediction and simulation results, the predicted properties of the pressure waves and the aerodynamic drag of the pod concur with the simulation results. Because the boundary layer along the pod is thinner at a higher pod speed, the difference between the theoretical and simulation values of the leading shock pressure decreases from 9.71% to 2.83% and that of the leading shock speed decreases from 4.39% to 1.30% as the pod speed increases from 180 to 350 m/s. Additionally, the theoretically predicted drag of the pod shows good agreement with the simulation results with an error of around 6%.