Micro-pressure Wave Research Articles

Parameter study and optimization design of a hat oblique tunnel portal

The present work focuses on the aerodynamic problems resulting from a high-speed train (HST) passing through a tunnel. Numerical simulations were employed to obtain the numerical results, and they were verified by a moving-model test. Two responses, [Formula: see text] (coefficient of the peak-to-peak pressure of a single fluctuation) and[Formula: see text] (pressure value of micro-pressure wave), were studied with regard to the three building parameters of the portal-hat buffer structure of the tunnel entrance and exit. The MOPSO (multi-objective particle swarm optimization) method was employed to solve the optimization problem in order to find the minimum [Formula: see text] and[Formula: see text]. Results showed that the effects of the three design parameters on [Formula: see text] were not monotonous, and the influences of[Formula: see text] (the oblique angle of the portal) and [Formula: see text] (the height of the hat structure) were more significant than that of[Formula: see text] (the angle between the vertical line of the portal and the hat). Monotonically decreasing responses were found in [Formula: see text] for [Formula: see text] and[Formula: see text]. The Pareto front of [Formula: see text] and[Formula: see text]was obtained. The ideal single-objective optimums for each response located at the ends of the Pareto front had values of 1.0560 for [Formula: see text] and 101.8 Pa for[Formula: see text].

A newly designed entrance hood to reduce the micro pressure wave emitted from the exit of high-speed railway tunnel

Micro-pressure waves (MPWs) emitting from the exit of a railway tunnel pose a serious environmental hazard causing vibration and rattling of nearby structures. Consequently, with the advent of ever increasing train speeds, it has become extremely important for tunnel engineers and designers to control the emission of MPWs from the exit of a railway tunnel. The most economical and effective method known till date to reduce the magnitude of MPWs is by installing an aerodynamic hood at the entrance of a tunnel. Therefore, a newly designed entrance hood with air-slits attached on each side is studied in the current work. The air-slits are designed to bio-mimic the ‘ram ventilation’ technique used for respiration by hunter shark gills. These air-slits reduce the maximum value of the pressure gradient of the compression wave generated near the entrance. Thereby, a subsequent reduction in the maximum magnitude of MPWs emitted from the exit of tunnel is observed. A prototype of this entrance hood has been employed on a 1/64.2 reduced scale single track model tunnel and experimental tests were conducted using a model train with the nose shape of EMU-250 at two entry speeds of 180 km/hr. and 250 km/hr.. Pressure on the tunnel wall were measured at six different locations using a piezo-resistive pressure transducer and MPW at the tunnel exit were captured at three locations using a low-frequency sound level meter. A reduction in the pressure gradient of the compression wave and magnitude of MPW of about 56.3% and 78.7% respectively was observed with the installation of this entrance hood at a train speed of 250 km/hr. The obtained pressure transient data and the corresponding MPW values for both the train speeds are presented in detail with clear elucidations on the flow phenomena. Further, an analytical model to predict MPWs was developed using the solution of a vibrating circular piston in an infinite baffle plate. The predicted results of the analytical model when compared with the experimental values show a reasonably good match for the peak pressure magnitude and waveform of the MPW.

Design of a tunnel entrance hood for high-speed trains

Micro-pressure waves (MPWs) are radiated from tunnel exits when trains enter the tunnels at high speeds. Because MPWs produce infrasound and audible noise around the tunnel portal, they should be suppressed to mitigate its environmental impact. Hood installation at the entrance portal of tunnels is a common MPW suppression method. However, an increase in train speed mandates a corresponding increase in hood length. This paper reports the development of a tunnel entrance hood that allows a decrease in the extension required at high speeds. Calculations based on the acoustic theory were used to investigate an economical hood-extension method, and model experiments were performed to validate predictions. Results revealed that increasing the cross-sectional area of the extended part of the hood over that of existing hoods can effectively shorten the overall hood length. As a result, the first optimum cross-sectional area of the hood was greater than that of the tunnel by a factor of 3.9, and the second optimum value demonstrated a corresponding factor of 3.2. Based on this finding, the proposed hood design can be more economical than existing hoods.

Aerodynamic effect of cross passages at the entrance section of a high-speed railway tunnel in a region with mountains and canyons

The environmental characteristics of difficult mountainous regions has ruled out the easy installation of buffer structures in tunnels. To address this problem, we proposed the installation of cross passages in an entrance section to alleviate the aerodynamic effects of a high-speed train entering the tunnel. In this study, a slide rail high-speed train experimental aerodynamic model was established based on the same principles. By combining the aerodynamic model experiments with a numerical simulation, the effect of cross passage on the generation and propagation of compression waves and micro-pressure waves was investigated. The simulation was capable of predicting the behavior of compression waves and verifying the correctness of the model at a low cost. The results showed that the installation of cross passages could reduce the peak pressure of the initial compression wave, modulate the pressure fluctuation in the tunnel and reduce the micro-pressure wave. For a train speed of 350 ​km/h and a tunnel cross-sectional area of 100 ​m2, the cross passage with the best modulation effect had a length of 10 ​m and a cross-sectional area of 20 ​m2. This cross passage was located at a distance of 10–20 ​m from the tunnel opening.

The Feasibility of Adapting Air Compressor to a High-Speed Train to Attenuate the Aerodynamic Problems in Tunnel

An air compressor has been adapted to a high-speed train to reduce the aerodynamic problems when passing through a tunnel. To evaluate the feasibility of the air compressor to attenuate the aerodynamic drag, pressure transient and the micro-pressure wave, the flow around the high-speed train has been simulated using the unsteady axisymmetric compressible Navier–Stokes equation. The simulation results by the dedicated CFD program for the train-tunnel aerodynamics named C-STA™ showed good agreements with the field test result of Korea Train eXpress train running through the Bok-An tunnel. By adapting a compressor with 20% of the train cross-sectional area, the simulation results showed 9% reduced average aerodynamic drag, 54% reduced peak pressure rise in tunnel, and 20% reduced micro-pressure wave, which prove that adapting a compressor can be a good measure to attenuate the tunnel aerodynamic problems for high-speed train.

압축기 적용 고속열차의 터널 통과 시 공기력 저감 효과 연구

A series of CFD simulation have been conducted to evaluate the effect of the Hyperloop-type compressor adapted to a high-speed train to reduce the aerodynamic drag, the pressure transient and the micro-pressure during tunnel passage. Unsteady axi-symmetric Navier-Stokes equation has been employed to calculate the unsteady flow field induced by the KTX-Shancheon train through the Honam high-speed line tunnel. By comparing the calculation results with and without compressor, the effect of compressor to the reduction of tunnel transient forces has been analyzed. It is concluded that the adaption of compressor of 30% inlet area relative to the train cross-sectional area reduces the mean aerodynamic drag to 15.9%, the pressure change in the cabin up to 24.9%, and the micro-pressure wave to 40.3%.

고속열차의 압축기 적용 효과 분석을 위한 무격자 방식의 유동해석 프로그램 개발

A computational fluid analysis program was developed to analyze aerodynamic phenomena as High-speed trains pass through tunnels. For robust and accurate analysis of moving object and thin boundary layers, a meshless method with least square method that satisfies geometric conservation law (GC-LSM) is introduced to analyze the flow field with only the connectivity between points without volume grid information. The developed program was validated through some problems and comparisons with other studies, confirming that the flow analysis was possible within the error (3.6%) similar to the existing flow analysis program. We also qualitatively analyze the effect of how aerodynamic problems are alleviated when compressors are applied to High-speed trains. The maximum intensity of pressure wave decreased by 15.6% and the intensity of the micro-pressure wave decreased by 13.5%, suggesting the possibility of excellent effect through the compressor.

고속철도 터널출구지형이 미기압파 방사특성에 미치는 영향

고속열차가 터널에 돌입 시, 터널입구에서 형성된 압축파는 음속으로 터널내부를 전파한다. 터널내부를 전파하는 압축파는 파의 비선형성에 의하여 약한 충격파와 같은 유한 진폭파로 되어 터널출구에 도달하게 되면, 그 일부가 충격성 소음과 저주파수 진동을 수반한 미기압파로 방사한다. 이 간헐적인 미기압파는 터널출구 주변 민가에 새로운 소음 공해의 원인이 되었다. 이러한 공학적인 문제를 해결하기 위하여 대부분의 연구는 터널내부에서 발생하는 압축파를 제어하여 미기압파의 크기를 줄이려고 시도하여 왔다. 본 연구에서는 고속철도 터널출구 지형들이 미기압파의 방사특성에 미치는 영향을 조사하기 위하여 3차원 수치해석을 수행하였다. 이러한 연구는 심각한 환경문제를 겪고 있는 터널출구 주변 민가의 방음대책 마련을 위한 기초 데이터를 제공한다.

고속열차 진입속도가 터널 미기압파에 미치는 영향

고속열차 진입속도가 터널 미기압파에 미치는 영향

Moving model tests on transient pressure and micro-pressure wave distribution induced by train passing through tunnel

Using a 1:20 scale moving model device, pressure distributions on train and tunnel surfaces, and the distribution of micro-pressure waves within 50 m of tunnel exits were investigated. In addition, the effects of train speeds on the transient pressures and micro-pressure waves were analyzed. The results revealed that the effect of the train speed on Pmin values was more significant than that on Pmax values, on the train surface. Significant differences between symmetrical measurement points located on the same cross section of the tunnel with double tracks could be observed. Moreover, similar differences between symmetrical points located on the same cross section in the streamlined zone of the model train were observed and analyzed. The pressure changes in the measurement points located on the same cross section of the train body, other than the streamlined zone, were approximately coincident. The differences between the Pmin values of measurement points located on different cross sections of the tunnel determined the differences between the ΔP values of these measurement points. The micro-pressure waves were approximately equal for measurement points located on the same cross section. Moreover, linearity decreased when the distance between the measurement points and the tunnel exit increased.

Influence of ballast quantity on compression wavefront steepening in railway tunnels

The impulsive pressure wave (micro-pressure wave) emitted from a tunnel portal is one of the environmental problems associated with high-speed railways. The principle underlying the countermeasures for this micro-pressure wave is to reduce the pressure gradient of the compression wavefront at the tunnel portal. This study investigated the influence of the ballast layer on the rate at which compression waves steepen in railway tunnels. Field measurements in an actual Shinkansen tunnel with slab tracks were recorded before and after installing ballast along most of the length of the tunnel. Previously published results on the spread of the compression wavefront in a ballasted track tunnel are also presented for comparison. A rapid method using an empirical equation is shown to accurately estimate the variation in the maximum pressure gradient of the compression wavefront in certain conditions. A more widely applicable estimation method using a numerical analysis is also demonstrated, which describes the distortion of the compression waveform. The results show that both of these methods can estimate the influence of the ballast quantity on mitigating the compression wavefront steepening in Shinkansen tunnels.

Influence of the enlarged portal length on pressure waves in railway tunnels with cross-section expansion

The train/tunnel aerodynamic effects still remain a major concern during high-speed railway construction. At present, different sizes of tunnels with constant cross-section are generally adopted based on train speed and blockage ratio. However, for tunnels with constant cross-section, narrower cross-section leads to worse aerodynamic effects while larger cross-section results in sharply increasing costs. Focusing on this problem, this paper proposes a new type of high-speed railway tunnel to mitigate the aerodynamic effects by enlarging the cross-section at both ends and reducing the cross-section in the middle portion of the tunnel. A three-dimensional, unsteady, compressible and RNG k-ε turbulence model is utilized to simulate the pressure waves induced by a high-speed train passing through such a tunnel. Three cases with different enlarged portal lengths are simulated. The pressure variations on the tunnel wall and train surface are compared with previous moving model tests to validate the numerical algorithm. The results show that the train-tunnel-entry induced pressure transients are greatly reduced compared with that of constant cross-section tunnel. In addition, the pressure gradient and micro-pressure wave (MPW) emitted from the tunnel exit are also considerably decreased. Due to the enlarged portal of the tunnel, pressure waves are repeatedly reflected, and a new compression wave is developed at the junction where the tunnel cross-section abruptly changes. As a result, the wave energy is dissipated, and its strength is reduced. However, it is revealed that the length of the enlarged portal has less influence on the maximum pressure gradient and the MPW.

Effect of two-step hood on micro-pressure wave

Effect of two-step hood on micro-pressure wave

Experimental study on the train noses optimized with the linear acoustic theory for the micro-pressure wave reduction

Experimental study on the train noses optimized with the linear acoustic theory for the micro-pressure wave reduction

Model Experiments on the Effect of a Tunnel Hood with Multi-step Cross-section for Reducing the Micro-pressure Wave

Model Experiments on the Effect of a Tunnel Hood with Multi-step Cross-section for Reducing the Micro-pressure Wave

Optimizing cross-sectional area of tunnel entrance hood for high speed rail

A micro-pressure wave (MPW) is radiated from the tunnel exit when a train enters a tunnel at high speed. The MPW causes infrasound and audible noise near the tunnel portal; hence, it must be reduced for environmental reasons. A hood with openings is a typical countermeasure. The openings must be suitably distributed to effectively reduce the MPW. The optimum distribution depends on the shape of the train nose and its velocity; therefore, it needs to be readjusted, and the hood sometimes needs to be extended when the train velocity increases or a new type train is put into operation. This study reports on numerical studies and model experiments to investigate the effective composition of an unvented hood because such a hood would eliminate the need to adjust the distribution of the openings according to the train characteristics. The results show that the optimum cross-sectional area of the hood is 2.6 times the cross-sectional area of the tunnel, and its optimum length is 1.8 times the tunnel diameter when the train velocity is 350 km/h, and the train nose length is three to six times the train radius. Finally, the unvented hood is economically advantageous compared with the currently used hood.

Prediction of micro-pressure waves generated at the exit of a model train tunnel

A micro-pressure wave is discharged from the exit of the train tunnel when a compression wave induced by a high-speed train moves outside the exit of the train tunnel. Micro-pressure waves generate sound noises, making serious environmental problems for local residents. In order to efficiently control the micro-pressure wave discharged from the exit of a train tunnel, numerical simulations investigating the generation and propagation of the micro-pressure wave were carried out. A 2-D axisymmetric model tunnel was simulated at different operating conditions. Different overpressure and pressure gradients across assumed compression waves were investigated and the effect of the length of the tunnel flange was also considered. Micro-pressure waves located in the near field, in the far field and at positions with different angles at a specific radius were studied in detail. In addition, the open-end correction and directivity coefficient affecting the magnitudes of micro-pressure waves at different locations were discussed in-depth as well. Magnitudes of micro-pressure waves induced by different pressure gradients across initial compression waves, magnitudes of micro-pressure waves at locations with different angles at a specific radius and the open-end correction were theoretically calculated and compared with obtained CFD results.

Full-Scale Measurement of Micropressure Waves in High-Speed Railway Tunnels

We aim to perform a series of field measurements for high-speed railway tunnels in China to obtain the micropressure wave (MPW) at the tunnel exit and the transient pressures near the tunnel portals. The relationship between the MPW and the nose-entry wave and the effects of train speed and the tunnel exit hole on the MPW are analyzed. The results show that the MPW decreases with increasing distance from the tunnel exit, but increases rapidly with increasing train speeds. Additionally, holes in the hoods near the tunnel exit could decrease the MPW near the tunnel exit by 10–20%.

Numerical studies on the radiation of train-tunnel impulse waves

An impulse wave is always discharged from the exit of a train tunnel when a weak compression wave induced by a moving high-speed train at the tunnel entrance propagates outside of the tunnel exit. An impulse wave is a micro-pressure wave, which leads to environment problems such as noise, vibration and other structural damages in the vicinity of the tunnel exit. Currently, due to the development of the optimization and acceleration systems of the high-speed train, the train speed becomes much higher than before and environmental problems become more serious as well. Therefore, it is significantly important to find effective methods to control and reduce magnitudes of impulse waves discharged from the tunnel exit. In the present study, numerical simulations were carried out to investigate the generation and propagation of impulse waves discharged from the exit of a model tunnel. Weak compression waves with different pressure gradients were assumed at the tunnel entry for investigating the relationship between the compression wave and the impulse wave radiated from the tunnel exit. Several tunnel exit geometries with and without a flange at the exit portal of the tunnel were simulated to check their effects on the radiation of impulse waves. Coefficient of effective radiation solid angle indicating the effects of the surrounding environment near the tunnel exit was discussed in-depth. The comparison was made in terms of the magnitudes of impulse waves and coefficients of effective radiation solid angle at different boundary conditions.

Wavefront evolution of compression waves propagating in high speed railway tunnels

The amplitude of micro-pressure wave emitted from the exit portal of a high-speed railway tunnel is approximately proportional to the pressure gradient of the internal wavefronts approaching the portal. For long tunnels, the evolution during propagation is an important factor for estimating the pressure gradient in the exit wavefront. Moreover, the relationship between the entrance and exit states depends upon the conflicting influences of inertia and damping along the propagation. In this paper, the investigation is done to study the evolution of wavefronts with different initial waveforms and to assess the influence of friction and train speed in the process. This work is based on numerical simulations of wavefront propagation using a third-order monotonic upwind scheme for conservation laws and backed up by field measurements. Numerical analyses show that the wavefront evolution not only depends on the pressure amplitude and the maximum pressure gradient of the initial wavefront but also depends strongly on the shape and timing of the peak steepness of ∂P/∂t. The predictions also demonstrate the existence of a critical tunnel length. For any particular train and tunnel entrance, the magnitudes of the micro-pressure waves increase with the tunnel length when this length is below the critical one and decrease with the tunnel length otherwise. The critical length decreases with the increase of the tunnel wall friction and the train speed.