In this work, the impact of the isentropic and Kantrowitz limits on the aerodynamic behavior of evacuated tube transportation (ETT) was numerically explored. Two tube train systems with different blockage ratios ( β), that is, β = 0.09 and β = 0.25, were employed for the comparative study of aerodynamic drag and flow structure. The results revealed three distinct aerodynamic behaviors, corresponding to the three speed regions separated by the two critical Mach numbers. Furthermore, the influence of head and tail lengths on drag reduction was investigated in these three speed ranges. An increase in head length appeared to be more sensitive to drag reduction at a speed of 600 m/s, while a long tail was found to induce a pronounced drag reduction at 200 m/s. In addition, the combined effect of the head and tail lengths on drag reduction was close to the superposition of their individual optimization effect. Based on the results, this study concludes that the individual designs of the head and tail of ETT systems may be rather demanding to achieve the desired optimization when considering distinct cruising speeds.

In this study, the aerodynamic characteristics of the three-dimensional evacuated tube transportation (ETT) system based on the Reynolds-averaged Navier–Stokes κ−ω shear-stress transport turbulent model were investigated. The effects of two key parameters on the drag and flow topology of the ETT system, namely the travelling speed and ambient pressure in the tube, were studied. Compared with trains in the atmospheric environment without the tube (i.e., the open system), the ETT system shows considerable drag reduction with suitable operating parameters in the tube, particularly at a higher travelling speed range. The drag varying with the speed from subsonic to supersonic, shows various change trends at different speeds because of their distinct flow structures. The higher pressure in front of train head was observed to be reduced by choking, and a low pressure in the wake by expansion waves led to rapid increase in the drag and drag coefficient. The relationship between the drag and operating pressure was observed to be approximately linear for both the subsonic and supersonic speeds.