THE COMPRESSION WAVE GENERATED BY A HIGH-SPEED TRAIN AT A VENTED TUNNEL ENTRANCE

An analytical investigation is made of the compression wave generated when a high-speed train enters a long tunnel with distributed venting. The compression wave amplitude is determined by train speed and the a rea ratio of the train and tunnel, but its rise time depends principal ly on the geometry of the tunnel entrance. Vented tunnel entrance ''ho ods'' are frequently used to increase the rise time, in order to reduc e the impact of the micro-pressure pulse radiated from the tunnel exit when the compression wave arrives at the far end of the tunnel. Appro ximate calculations are performed to determine the initial rise time f or a tunnel of rectangular cross section with a continuously variable vented roof near the entrance, for train Mach numbers less than about 0.2 (similar to 150 mph). The distribution of venting apertures can be optimized to maximize rise time, and a sixfold increase is shown to b e possible when the aperture distribution decreases exponentially with distance into the tunnel. The method of this paper is applicable also to more general tunnel entrance geometries, and for higher train Mach numbers. (C) 1998 Acoustical Society of America. [S0001-4966(98)01408 -8].