Aerodynamic characteristics of leakage-induced pressure waves in an evacuated tube transportation system

This study experimentally investigated the aerodynamic characteristics of pressure waves induced by sudden air leakage in a vacuum tube by simulating an evacuated tube transportation system failure. Using a scaled-down test section, leakage scenarios with various orifice diameters and initial pressures were analyzed. The flow field was classified into two distinct regimes based on the interaction between the underexpanded jet and the tube wall: supersonic core impingement (SCI) and decayed jet impingement (DJI). A dimensionless geometric criterion, 0.62<yM/dt<0.77, was identified as the transition threshold between these two regimes. The SCI regime generated severe localized impact pressures, whereas the DJI regime exhibited turbulent fluctuations. Despite the complex near-field structures, the flow rapidly reorganized into a one-dimensional planar shock wave within a short axial distance (x/dt≈3.23). In the far field, the shock wave demonstrated linear attenuation driven by turbulent boundary layer growth, governed by the Reynolds analogy. Furthermore, empirical power-law correlations for shock intensity and decay rate were derived to predict the local shock wave intensity. These findings provide fundamental insights into transient compressible flows in confined geometries and offer essential design criteria for the safety of vacuum-tube transportation systems.