Wake dynamics and three-dimensional shock wave morphology in an evacuated tube transportation system under choked flow via proper orthogonal decomposition with domain partitioning

The wake in an evacuated tube transportation system under choked flow is a complex, unstable, three-dimensional structure composed of trailing vortices, shear layers, shock waves, and their interactions, which may threaten the stability and safety of train operations. In this study, the aerodynamic performance of an evacuated tube transportation system at 1000 km/h is investigated using the Improved Delayed Detached Eddy Simulation (IDDES) method with the shear stress transport k-ω model. The numerical algorithm and mesh strategy are validated through in-pipe projectile experiments. The instantaneous and time-averaged vortex topologies are compared, with the time-averaged flow field revealing the interaction between the shock waves and the vortex pairs. Using the proper orthogonal decomposition (POD) technique, both spanwise and streamwise wake dynamic characteristics are analyzed, and the primary oscillation modes of the wake flow are identified. The modes with significant energy contributions correspond to the oscillation of shock wave intensity and the motion of the trailing vortices. The POD technique with domain partitioning is effectively employed to characterize the three-dimensional shock wave morphologies. The oblique shock S1 originates from the tail surface and develops inward in a curved manner, resembling two half-cones. The morphology of the trailing normal shock wave is closely related to the tube cross section and deforms at interaction points with the vortex pairs, exhibiting varying degrees of deformation over time.