On the design of a tunnel-entrance hood with multiple windows

An analysis is made to determine the optimal distribution of ‘windows’ in the wall of a long tunnel-entrance hood used to suppress the micro-pressure wave produced when the compression wave generated by an entering high-speed train reaches the far end of the tunnel. An ideally designed hood causes the pressure to rise linearly across a compression wavefront of thickness equal approximately to the ratio of the hood length to the train Mach number. At moderate train Mach numbers the hood length can be assumed to be ‘acoustically compact’, and the initial form of the compression wave can then be expressed in terms of an equivalent source distribution representing the train and the compact Greenʹs function for sources in the hood. The requirement that the wavefront profile be linear imposes certain conditions on the Greenʹs function that permits the determination of the hood window dimensions. Our calculations take no account of the influence on compression wave formation of separated air flow over the train, from the hood portal and from the peripheries of the windows. The explicit predictions of the optimum window sizes given in this paper may therefore be too small, and should be refined by using model scale tests that allow window dimensions to be varied from their predicted optimal values to incorporate the effects of flow separation.