Experimental study of the unsteady aerodynamic field outside a tunnel during a train entry

The compression wave generated by a train entering a tunnel has been widely studied. However; the resulting exit flow created at the tunnel portal has never been investigated. The experimental work reported in the present paper provides some insight into the structure of this flow and its main characteristics. Experiments were conducted with a 1/140th scale apparatus and the explored range of the train speed was 5-50 m/s (M less than or equal to 0.15). The study focused on the influence of the train speed and the train nose geometry on the flow. Unsteady velocity measurements were taken to attempt to clarify the influence of the train speed on the jet induced at the tunnel portal when the train enters. A mass balance was undertaken to compare the quantity of air ejected from the tunnel to that compressed inside (i.e. involved in the compression wave). The study revealed that, at low speed, the quantity of air compressed to that ejected is of the order 5:1, subject to boundary conditions. At high speed, the volume ejected is comparable to that compressed. Interestingly, the train nose geometry does not influence the mass balance. The jet momentum was also calculated and found to increase with the train speed and is insensitive to the train nose geometry. Careful discussions are provided in the paper on how these results can be extended to the full-scale case.