Blind hood effects on the compression wave generated by a train entering a tunnel

The running of high-speed trains on railway networks creates strong transient flows in tunnels. A large number of solutions have been proposed to reduce the amplitude of the pressure gradients in tunnels and one of the most efficient solutions consists in the addition of a hood before the tunnel entrance. The present paper provides a detailed experimental study on the effects of blind hood (hood without any perforation) with constant section on the generation of a compression wave. An axisymmetric reduced scale (1/140th) experimental apparatus was used and the train Mach number was in the range from 0.06 to 0.15. Results showed that the initial compression front can be split into several smaller fronts in the presence of a blind hood. The number of fronts depends on the respective length of the hood to the train nose length and only slightly on the train Mach number. The characteristics of each of these fronts (amplitude and gradient) can be predicted for given train, tunnel and hood. Furthermore the present analysis showed that, for a fixed train/tunnel blockage ratio, there exists a unique optimum hood for which the pressure gradient can be minimized. For the usual train/tunnel blockage ratio (around 0.2), the maximum reduction of the pressure gradient was proved not to exceed 3. The exact value of the optimum pressure gradient depends on both the train/tunnel blockage ratio and the train nose geometry. (C) 2001 Elsevier Science Inc. All rights reserved.