Visualization of jet flow generated by a flat-fronted train entering a tunnel

Abstract

When a train enters a tunnel, a compression wave is generated and propagates through the tunnel towards the other side of the tunnel portal, emitting a micro-pressure wave. The amplitude of the compression wave generated by a flat-fronted train is generally much larger than that by a streamlined train probably due to flow separation at the front end of the flat-fronted train. Flow separation at the front ends of flat-fronted trains of meter gauge railway lines results in increasing the aerodynamic drag and pressure fluctuations in tunnels, which may cause serious environment-related issues. Real train experiments have been conducted to explore pressure fluctuations in tunnels but details of spatial flow field around front end of cars are still unknown. In this study, model experiments for two varieties of rounded corners of the fronts of circular cylindrical trains entering a circular tube are carried out to visualize the unsteady flow with oil mist and tufts by using a high-speed camera as well as to measure the compression wave in the tube. It is shown that the pressure waves in the tunnel and the unsteady flows emitted from the tunnel entrance are considerably different between square- and round-ended trains. A vortex ring is generated by the round-ended train but not clearly by the square-ended train. A turbulent vortex ring may be generated by the square-ended train. The difference of amplitude of the compression waves generated by flat-fronted and streamlined trains can be attributed to the difference of structure of the jet flows generated by the trains.