Masanobu Iida

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 Greens 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.

Optimization of Train Nose Shape for Reducing Micro-pressure Wave Radiated from Tunnel Exit

When a compression wave generated by a high-speed train entering a tunnel propagates through the tunnel and arrives at the tunnel exit, an impulsive pressure wave (micro-pressure wave) is radiated from the tunnel exit. Improving the train nose shape is one of the techniques for suppressing the micro-pressure wave. Furthermore, tunnel entrance hoods are required for long concrete slab tunnels in order to suppress the micro-pressure wave. The effect of the tunnel entrance hood on the compression wave generated by the train can be evaluated by means of a rapid computational scheme devised and validated experimentally by Howe et al. In this study, the optimal longitudinal distribution of the cross-sectional area of the train nose shape was determined by using the rapid computational scheme and a genetic algorithm. The effect of the nose shape optimization was confirmed through experiments using scale models.

Aeroacoustics of a tunnel-entrance hood with a rectangular window

An analytical and experimental investigation is made of the compression wave generated when a train enters a tunnel fitted with a long, uniform hood with a rectangular window. The window is situated at the junction of the hood and tunnel, which are taken to have the same uniform cross-sectional area. An understanding of the mechanics of this canonical configuration is important for the design of tunnel entrance hoods for new high-speed trains, with speeds in excess of 300 km h -1 . The compression wave is formed in two stages: as the train nose enters the hood and as it passes the window. The elevated pressure within the hood produces a flow of air from the window in the form of a high-speed jet, whose inertia generates an additional rise in pressure that propagates into the tunnel as a localized pulse. Multiple reflections from the window and the hood portal cause the temporary trapping of wave energy within the hood (prior to its radiation into the tunnel). All of these aspects of the flow are modelled analytically and the results are found to be in good accord with new model-scale measurements and flow visualization studies reported in this paper.

Influence of a Scarfed Portal on the Compression Wave Generated by a High- Speed Train Entering a Tunnel

An optimally designed entrance portal must be capable of minimizing the maximum growth rate of the compression wave generated when a high-speed train enters a tunnel. A theoretical and experimental investigation has been made to determine the changes in compression wave characteristics produced when the portal is ‘scarfed’ with tapering side walls. It is concluded that portal modifications of this type are unlikely to produce a significant reduction in the maximum compression wave growth rate. Small decreases in growth rate are possible (up to about 15%) for scarf walls extending a distance beyond the tunnel entrance of the order of the tunnel height, but little or no additional improvement is achieved with longer walls.

Field Measurement of Wayside Low-Frequency Noise Emitted from Tunnel Portals and Trains of High-Speed Railway:

In order to determine the actual circumstances of wayside low-frequency noise and infrasound generated by high-speed trains (Shinkansen), field measurements were performed at two sites, one near a tunnel portal and the other in a fully open section. The measurements were based upon the manual issued by the Ministry of the Environment of Japan in October 2000 and conducted to obtain G-weighted SPL, 1/3-octave band spectra, velocity dependence and distance attenuation of SPL. The measured results show that major components of the low-frequency noise from the tunnel portal are impulsive micro-pressure waves and continuous pressure waves, while those in the open section are near-field hydrodynamic pressure variations and far-field acoustic pressure waves.

Model Experiments on the Tunnel Compression Wave Using an Axisymmetric and Three-Dimensional Train Model

Generation of the compression wave by a train entering a tunnel is investigated by model experiments. In the model experiments, the train and the tunnel are represented by an axisymmetric model, a three-dimensional mirror image model and a three-dimensional model. The experimental results indicate that the effect of the three-dimensionality of the train nose shape is approximately 2 % for the pressure gradient of the compression wavefront when the train nose is streamlined, hence without large flow separation around the train nose. Furthermore, the relationship between the pressure gradient of the compression wavefront and the train position in a cross-section at the tunnel portal is clarified.

Distortion of Compression Wave Propagating through Shinkansen Tunnel

When a compression wave generated by a train entering a tunnel is propagated through the tunnel and reaches the exit portal, a pressure pulse (‘micro-pressure wave’) is radiated from the exit portal. The distortion of the compression wave during propagation in the tunnel is important for estimating the magnitude of the micro-pressure wave radiated from the exit portal. In this paper, the dependence of the distortion of the compression wave on the initial waveform at the tunnel entrance is investigated. First, a simple system of equations concerning the wave propagation in the tunnel is described. Secondly, theoretical and numerical investigations into the effects of nonlinearity and unsteady friction on the tunnel wall on the distortion of the compression wave are made.

Propagation characteristics of tunnel compression waves with multiple peaks in the waveform of the pressure gradient: Part 2: Theoretical and numerical analyses

This paper is the second part of a two-part study on the propagation characteristics of compression waves generated by a train entering a long slab track tunnel with a tunnel entrance hood, which generates a tunnel compression wave with multiple peaks in the waveform of its pressure gradient. In Part 1, we described field measurements on the propagation characteristics of the compression waves generated by Shinkansen trains in a 9.7-km-long tunnel, and we compared the results with those of simulations. It was shown that initial waveforms whose pressure gradient waveforms have shallower valleys tend to steepen more easily, and a mathematical model of the distortion based on the field measurements using a quasi-laminar friction model was proposed. This follow-up report describes the theoretical and numerical analyses conducted on the basis of the mathematical model. Initial waveforms of the pressure gradient that have no valleys and are higher on their right-hand side grow up easily during propagation; this is due to the unsteady friction being small in the region where the magnitude of the second time derivative of the pressure is small. This results in a dependence of the propagation characteristics on the initial waveform of the compression wave.

1117 Micro-pressure wave emitted from slits of a shelter linking neighboring tunnels

Takashi FUKUDA, Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji-shi, Tokyo 185-8540 Sanetoshi SAITO, Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji-shi, Tokyo 185-8540 Masanobu IIDA, Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji-shi, Tokyo 185-8540 Takeshi KURITA, East Japan Railway Company, 2-479 Nisshin-cho, Kita-ku, Saitama-shi, Saitama 331-8513 Satoru OZAWA, Professor emeritus, Tokyo University of Technology

Continuous Pressure Waves Generated by a Train Running in a Tunnel

This research describes several characteristics of tunnel continuous waves (TCW), which are a kind of low-frequency pressure wave radiated from railway tunnel portals. Several theoretical results are presented and measurement data obtained in field tests. Ratios of the amplitude and the frequency of the TCW radiated in the forward direction to those radiated backward have been obtained. Also described is a method for separating an incident wave from pressure data measured in a tunnel and this method is applied to data obtained in field tests. As a result, the separation method is shown to be applicable to the field test data.