Influence of an unvented tunnel entrance hood on the compression wave generated by a high-speed train

Abstract

A theoretical and experimental study is made of the compression wave generated when a train enters a nominally uniform tunnel with a long, unvented entrance hood. The purpose of the hood is to reduce as much as practicable the maximum gradient of the compression wave front. The pressure gradient can increase in a long tunnel as a result of nonlinear wave steepening, and thereby increase the impact on residential dwellings of the acoustic ‘boom’ (or micro-pressure wave) radiated from the far end of the tunnel when the compression wave arrives. Our experiments are conducted at model scale using axisymmetric ‘trains’ projected at speeds up to 350 kph along the axis of a cylindrical tunnel fitted with a cylindrical entrance hood. Theoretical predictions of the compression wave are made using the equation of aerodynamic sound containing a slender body approximation to the effective source representing the moving train, coupled with a small correction that accounts for the ‘vortex’ sources in the free shear layers in the exit flows from the hood and tunnel of the air displaced by the train. The compression wave is generated by the two successive interactions of the train nose with the hood portal and with the junction between the hood and tunnel. The interactions produce a system of compression and expansion waves, each having characteristic wavelengths that are much smaller than the hood length; the waves are temporarily reflected back and forth within the hood prior to transmission into the tunnel, and are resolved analytically by use of an approximate Green's function determined by the hood geometry. Theoretical predictions are found to be in excellent agreement with experiment, including in particular a detailed correspondence between measured and predicted interference patterns produced by the multiple reflections of waves in the hood.