30. Mechanisms of flow-induced vibrations of trains running in tunnels

Although variation in air pressure has already been found to be the likely cause for train oscillation in tunnels, the mechanisms generating air pressure variation are not yet understood.

First, a simple model was built of a 6-car high speed train fitted with small scale helical-groove airflow meters on the underbody of the cars, and used to carry out numerical simulations of the simple phenomenon of airflowing around carbodies on a train running along a stretch of open track.

Results of this test showed that underbody airflows were slower than airflows along the side of the train. The difference in airflow speed produced large Karman vortex-like vortices (staggered Karman vortex street), which in turn caused meandering airflows beneath the train (horizontal left-to-right moving airflow) (Figure 1).

When the same numerical analysis was applied to a tunnel run, the meandering airflows followed the tunnel wall side and rose up the side of the model train. This was identified as being the possible cause for train oscillation in tunnels (Figure 2).

It was confirmed that the predicted meandering airflow found through numerical simulation also occurred in actual wind tunnel tests.

The next step in this research will be to use more realistic model trains in further tests to gather deeper insight into the phenomenon.