Aerodynamic Force Acting on a High Speed Train at Tunnel Entrance

Abstract

When a train enters into a tunnel, at first the pressure of the air immediately before the train head rises and the portion of the air with raised pressure spreads forward with a velocity nearly equal to that of sound. In other words the wave front, the boundary surface between the undisturbed air and the compressed air, moves forward. And it is reflected at the exit of the tunnel and comes back to the train head. In the present paper the aerodynamic phenomena during the time interval from the instant of the train head entering into the tunnel to the instant of either its tail end coming into the tunnel or the wave front being reflected back to its head, whichever occurs earlier, are treated theoretically. The air is assumed to be compressible and inviscid. The velocities and pressures of air in each portion of the tunnel are calculated. The pressure drag of the train is also obtained. It is interesting to note that the effect of compressibility of air is remarkable in spite of a comparatively low speed of train.