Propagation In Tunnels Research Articles

Experimental study on the inundation characteristics of flooding in a long straight subway tunnel

In recent years, the increase in extreme urban flooding events has caused severe property damage and safety problems. Subway tunnels are particularly vulnerable to underground space flooding. There have been traditional hydraulic experiments conducted to investigate unsteady turbulence properties of free surface flow in open channels. However, most experiments in flumes and pipes do not focus on the early formation process of inundation considering the unique structural configurations of subway tunnels. In this study, the general pattern that floodwater intrudes into a subway tunnel was studied by a scaled model experiment. Under different conditions of tunnel slope and inlet water discharge, the flood flow pattern, the water elevation, and flow velocity were investigated and analyzed. The results show that the flooding domain could be divided into three regions, including a Forming Region, a Uniform Region, and a Front Region. The unsteady Front Region performs an exponential rise in water elevation, and the velocity of flood propagation along the tunnel is nearly constant. The shape of the water surface profile is mainly influenced by tunnel slope, and the effect has a transition at the critical tunnel slope which was measured to be around 3‰. The classifications of flooding behavior under different conditions are described in detail. An empirical nondimensionalized formula for the tunnel inundation process in both unsteady and steady stages was proposed. This model enables rapid calculation of water depths with time in a subway tunnel. This research could provide an understanding of flood propagation in subway tunnels and facilitate the study of flooding detection and warnings in underground space. It also provides reference for evacuation decisions and subway flood control design.

Comparative analysis of finite‐difference and split‐step based parabolic equation methods for tunnel propagation modelling

AbstractRadio wave propagation modelling in railway environments is of fundamental importance in designing reliable train communication systems. Parabolic equation (PE) methods have been widely applied to the modelling of wave propagation in tunnels due to their high computational efficiency and fidelity. The finite‐difference parabolic equation (FDPE) and the split‐step parabolic equation (SSPE) methods are two commonly used approaches to solve PE numerically. However, the relevant literature is still missing a comprehensive study of their performance, including the selection of parameters such as discretisation steps and the tradeoffs involved in terms of their accuracy and efficiency, especially as current wireless systems shift to high frequencies. In this study, a systematic analysis of the error and computational complexity of the FDPE and SSPE methods for radio wave propagation modelling in tunnels is provided. Guidelines for the choice of their parameters are provided, and their performance is demonstrated through both numerical examples and experimental measurements in actual tunnel cases.

Generalisable convolutional neural network model for radio wave propagation in tunnels

AbstractPropagation models are essential for the prediction of received signal strength and the planning of wireless systems in a given environment. The vector parabolic equation (VPE) method has been widely applied to the modelling of radio wave propagation in tunnels. However, carrying out simulations for large‐scale environments is still computationally expensive. A convolutional neural network (CNN)‐based propagation model, which can provide high‐fidelity received signal strength prediction based on results from low‐cost VPE simulations, is proposed. A thorough study of the generalisability, including both interpolation and extrapolation capabilities, of the proposed CNN model is conducted. It is found that the proposed model can achieve significant computational savings while maintaining acceptable accuracy, and its performance is validated in both simulations and actual tunnel cases.

Experimental and numerical simulation study on the relationship between cutting depth of high-pressure water jet with high traverse speed and disc cutter penetration of TBM in hard rock tunnel

The disc cutting load of a TBM is insufficient to induce crack propagation in hard rock tunnels due to the high strength of the rock. High-pressure water jets, considered an auxiliary rock-breaking method, enhance the disc cutter's capacity to penetrate cracks by creating pre-existing fissures. In order to examine the correlation between cutting depth achieved by a high-pressure water jet operating at high traverse speeds and the penetration capability of TBM disc cutters, we conducted rock cutting experiments employing varying traverse velocities for the water jet. Additionally, we performed linear disc cutting experiments using full-size disc cutters equipped with grooves. These experiments were conducted using high-strength granite from China's Bei Shan region. We utilized the Discrete Element Software PFC2D to construct a rock breaking model based on Tyson polygons, enabling us to investigate the microfine influence mechanism of grooves with symmetry. The experiments involving high-pressure water jet grooving at high traverse velocities revealed a logarithmic decrease in groove depth as the nozzle's traverse velocity increased. These findings suggest that when high-pressure water jets are employed as an auxiliary method in TBM operations, increased distance from the center of the disc hob leads to higher traverse velocities, resulting in less efficient rock breaking. Nonetheless, increasing the nozzle diameter can enhance the rock-breaking effect. Linear disc cutting experiments indicate that, at a penetration depth of 4 mm, grooves with depths less than 4 mm have minimal impact on assisting in disc cutter rock breaking. However, when the groove depth exceeds 4 mm, there is a noticeable improvement in the disc cutter's breaking performance. The numerical simulation results provide initial validation for the linear disc cutting experiment. Lastly, considering the specific conditions of the Beishan tunnel, we propose a design scheme that involves the zoned arrangement of water jets. Additionally, we recommend appropriately reducing the rotational speed when employing high-pressure water jets as an auxiliary method for rock breaking. The research findings presented in this paper can serve as valuable references for the design and implementation of high-pressure water jet-assisted TBM cutter rock breaking.

Experimental evaluation and real-time forecast of smoke propagation in tunnels under intervention of sub-critical longitudinal ventilation

Sub-critical longitudinal ventilation can restrict the back-layering length in the upstream region and maintain suitable smoke stratification in the downstream region of a tunnel fire. In this study, experiments using a 1/15 scale, 80-m long, 1.2-m wide, and 0.5-m high model tunnel were conducted to investigate the effects of sub-critical longitudinal ventilation velocity and heat release rate (HRR) on the key parameters describing smoke propagation. The smoke front temperature near the fire source decreased significantly as the longitudinal ventilation velocity increased. The longitudinal ventilation velocity and HRR were incorporated into a regression model describing the smoke front velocity, which was applied to the evaluated fire scenarios using an ensemble Kalman filter to account for intervention of sub-critical longitudinal ventilation. The smoke front temperatures at discrete locations were assimilated to estimate the heat exchange correction coefficient and air entrainment ratio of smoke front in real-time and dynamically correct the forecasted smoke front temperature and velocity accordingly. When sub-critical longitudinal ventilation intervened, the smoke front parameters could be forecasted with a maximum lead time of 60 s and an average error within 5%. Therefore, the proposed forecasting method can inform the selection of disposal measures in early tunnel fire emergencies.

UTD-Based Ray-Tracing MIMO Channel Modeling for the Next-Generation Communications Within Underground Tunnels

Limited work has previously been conducted into deterministic modeling of the 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> Generation (5G) millimeter (mm)-wave wireless communications within geometrically non-uniform tunnels. This study predicts and analyzes broadband Multiple–Input Multiple–Output (MIMO) propagation in underground mine tunnels for 5G communications and beyond. At extremely short wavelength, each rugged and uneven surface of the tunnel is modelled as a group of diffracting rectangular prism wedges. A deterministic Ray-Tracing (RT) MIMO model is developed based on the Uniform Theory of Diffraction (UTD). All predicted MIMO sub-channels have been successfully fitted with the experimental measurements. MIMO multipath fading of underground tunnels are found to be lognormally distributed and strongly depends on the tunnel’s size and shape.

Efficient physics‐based recurrent neural network model for radio wave propagation in tunnels at 2.4 GHz

AbstractThe vector parabolic equation (VPE) method has been widely applied to the modelling of radio wave propagation in tunnel environments. However, dense spatial discretizations are generally required for VPE simulations to achieve acceptable accuracy. This results in high computational costs for large‐size tunnels and high‐frequency wireless systems. This letter presents an efficient physics‐based recurrent neural network (RNN) model for modeling radio wave propagation in tunnels. The proposed RNN‐based model can leverage coarse‐mesh VPE to generate high‐fidelity received signal strength and achieve significant computational savings. The validity of the proposed approach is demonstrated against fine‐mesh VPE simulations in various tunnel cases.

Study on control of upstream smoke propagation in road tunnels by using a side-wall sprinkler system

Side-wall sprinkler systems used for fire suppression in road tunnels have been found to redistribute the longitudinal air flow within the tunnel, with the most notable feature being an acceleration of the airflow near the ceiling. The present work is driven by the motivation of utilizing the acceleration effect to block the upstream smoke propagation in cases when the ventilation rate is sub-critical. A nozzle prototype that has practical applications in road tunnels was selected for a side-wall water spray system in a 300 m road tunnel, and a total of 60 cases were modelled using the CFD code FDS (version of 6.7.3) to investigate its smoke blocking effect. Experiments and correlations in the literature have validated the accuracy of the FDS method. The confinement velocity reduction rate, which is determined as the reduction of the ventilation rate required to stop the smoke front point at a given position, was found to be positively affected by both the water spray rate and the ventilation rate and was successfully correlated using dimensionless momentums of these two factors. By comparing the confinement velocity reduction rates with the increases in the ceiling velocity in the absence of tunnel fire, utilization efficiencies of the spray-induced accelerations were further obtained.

Influence of air chambers on wavefront steepening in railway tunnels

The behaviour of wavefronts propagating in a tunnel with multiple, compact air chambers is investigated with particular reference to the suppression of unacceptable pressure disturbances radiating from exit portals. Attention is focussed primarily on chambers that respond in an over-damped manner to pressure variations in the tunnel. In the early part of the paper, comparisons are made between the qualitative behaviour of wavefront propagation in tunnels with and without air chambers. Thereafter, attention turns to a quantitative assessment of the influence of key design parameters, especially the chamber volume and properties of connectors between chambers and the tunnel. It is shown that the optimum type of connector depends upon the length of the tunnel along which the chambers exist. For short tunnels, relatively low-resistance connectors with linear pressure:flow rate characteristics are preferred, whereas for long tunnels, relatively high-resistance connectors with quadratic pressure:flow rate characteristics are likely to be more suitable. Brief attention is paid to the practical feasibility of creating suitable connectors.

Heuristic UTD Diffraction Coefficient for Three-Dimensional Dielectric Wedges

This article presents a heuristic diffraction coefficient for 3-D dielectric wedges of arbitrary angles. First, the proposed diffraction coefficient is derived for 2-D wedges with soft and hard polarization, by enforcing continuity of the total field at every shadow boundary associated with the multiply reflected fields inside the wedge. The accuracy of the proposed heuristic 2-D solution is verified comparing with the UTD solution derived from Maliuzhinets' exact integration. Then, it is extended to 3-D wedges for arbitrary polarizations and incident/scattered directions by transforming ray-fixed to edge-fixed coordinate systems. The total field predicted using the proposed heuristic diffraction coefficient is continuous for arbitrary angle 3-D dielectric wedges, including interior wedges, and is close to the geometrical optics field far from the shadow boundaries. The proposed method is validated against a Method-of-Moment solution. Finally, we show the method can significantly improve the accuracy of ray-tracing modeling of wave propagation in arched tunnels.

Shooting-Bouncing-Rays Technique to Model Mine Tunnels: Theory and Accuracy Validation

We present a shooting-bouncing rays technique for electromagnetic modeling of wireless propagation in long tunnels focusing on the accuracy of ray-tracing computation. The examples demonstrate excellent agreement with the traditionally more accurate but less efficient alternative ray-tracing approach using path corrections based on image theory and with a commercial solver.

Scaled Modeling and Measurement for Studying Radio Wave Propagation in Tunnels

The subject of radio wave propagation in tunnels has gathered attention in recent years, mainly regarding the fading phenomena caused by internal reflections. Several methods have been suggested to describe the propagation inside a tunnel. This work is based on the ray tracing approach, which is useful for structures where the dimensions are orders of magnitude larger than the transmission wavelength. Using image theory, we utilized a multi-ray model to reveal non-dimensional parameters, enabling measurements in down-scaled experiments. We present the results of field experiments in a small concrete pedestrian tunnel with smooth walls for radio frequencies (RF) of 1, 2.4, and 10 GHz, as well as in a down-scaled model, for which millimeter waves (MMWs) were used, to demonstrate the roles of the frequency, polarization, tunnel dimensions, and dielectric properties on the wave propagation. The ray tracing method correlated well with the experimental results measured in the tunnel as well as in a scale model.

Efficient solution of Noye–Hayman implicit finite‐difference method for modelling wave propagation in tunnels

Parabolic equation models discretised with the finite-difference method have been a research topic for a long time. Through the simulation of electromagnetic wave propagation characteristics in typical scenarios, it is pointed out that the fast algorithm needs to be developed for the solution of two-level Noye–Hayman (NH2) implicit method to meet the accuracy and computational requirements of electromagnetic wave propagation problems in three-dimensional environments. In this Letter, hierarchical skeletonisation is used for an efficient inversion of three-dimensional parabolic equation discretised by the NH2 implicit method which is the modification of conventional Crank–Nicolson (CN) method. Numerical results show that the NH2 implicit method is more accurate than the CN method at no additional cost for a given numerical example.

Artificial neural network models for radiowave propagation in tunnels

The authors present a machine learning approach for the extraction of radiowave propagation models in tunnels. To that end, they discuss three challenges related to the application of machine learning to general wireless propagation problems: how to efficiently specify the input to the model, which learning method to use and what output functions to seek. The input that any propagation modelling tool (be it a ray-tracer, a full-wave method or a parabolic equation solver) uses, can be considered as visual, in the form of an image or a point cloud of the environment under consideration. Therefore, they propose an artificial neural network structure that generalises well to various geometries. The desired output can be values of the electromagnetic field components across the channel or just a path loss model. They apply these ideas to the case of arched tunnels for the first time. They consider cases where the geometric parameters of the tunnel, the position of the receiver and the frequency of operation are parts of a model trained by a vector parabolic equation solver. The model is evaluated using solver-generated as well as measured data. The numerical results demonstrate that this approach combines computational efficiency with high accuracy.

Flexural wave propagation in periodic tunnels with elastic foundations

In order to meet the needs of vibration reduction in complex vibration environments, one-dimensional phononic crystals constituted with periodic tunnels on elastic foundations are proposed. The governing equations of the flexural wave in a periodic structure are established based on Euler beam theory and its band structure is calculated by the plane wave expansion method. The theoretical and numerical results show that there are distinct forbidden bands and pass bands when the flexural wave propagates in the periodic tunnel structure. The elastic foundation stiffness, elastic modulus ratio, and axial prestress have significant effects on the location and width of the band gap. The results of the paper could be used for the vibration mitigation of the tunnel structure.

Millimeter Wave Propagation in Long Corridors and Tunnels—Theoretical Model and Experimental Verification

The development of the Fifth-Generation (5G) of cellular communications considers bands in millimeter waves (MMW) for indoor, short-range links. The propagation of MMW is affected by atmospheric and weather conditions, specular reflections from surfaces, and the directivity of the antennas. The short wavelength enables utilization of a quasi-optical propagation model for the description of indoor multi-path scenarios. A study of MMW propagation in tunnels, long corridors, or canyons is carried out using ray-tracing to evaluate the link budget and group delay. The analysis considers radiation patterns of both transmitting and receiving antennas, deriving a criterion for the number of dominating rays. Error analysis demonstrates the convergence of the method, while using a finite number of reflected rays. Experiments in a small-scale tunnel model demonstrate the accuracy of the analysis.

Simulation of fracture propagation in fibre-reinforced concrete using FDEM: an application to tunnel linings

The application of the combined finite-discrete element method (FDEM) to simulate fracture propagation in fibre-reinforced-concrete (FRC)-lined tunnels has been investigated. This constitutes the first attempt of using FDEM for the simulation of fracture in FRC structures. The mathematical implementations of the new FDEM joint-element constitutive model are first introduced, and the numerical model is then validated comparing the results for plain and FRC beams with three-point bending experimental data. The code has also been applied to two practical tunnel design case studies, showing different behaviours depending on the type of concrete and shape of tunnel section. The FDEM simulations of the linings are also compared with results from a finite element code that is commonly used in the engineering design practise. These results show the capabilities of FDEM for better understanding of the fracture mechanics and crack propagation in FRC tunnels. A methodology for directly inferring the numerical parameters from three-point bending tests is also illustrated. The results of this research can be applied to any FRC structure.

Influence of winding wall on the entrainment characteristics of air jet in curved road tunnels

The interchange structure in tunnel complexes has the characteristics of small radius and multi-ramp compared with the general tunnel, and the design of the ventilation system needs further research and optimization. This paper studied the development characteristics of air jets in urban road tunnels with small radius through commercial software FLUENT. The purpose is to provide reference for ventilation design in curved tunnels. According to the Chinese code and the survey report on tunnel complexes published by the World Road Association (PIARC), six different tunnel radii were selected to carry out this research, and the feasibility of using the realizable k-ε model to simulate the development of air jets was verified by full-scale experiments. The results show that negative pressure and low velocity (NPLV) zones with different degrees are found in the curved tunnel and the wind speed will be reversed. For tunnels with the radii of 150 m and 200 m, the wind speed reverses more frequently, and the dimensionless pressure minimum value will move from the convex wall to the concave one of curved tunnels. When the tunnel radius is greater than or equal to 300 m, the minimum value of dimensionless pressure increases with the increase of radius, which illustrates that the flow field in the curved tunnel gradually becomes uniform with the increase of the radius. In addition, the pressure-rise coefficient will increase with the increase of the tunnel radius. Characteristics of contaminant propagation in tunnels with different radii will be investigated in the next stage.

Statistical Modeling of Electromagnetic Wave Propagation in Tunnels With Rough Walls Using the Vector Parabolic Equation Method

The vector parabolic equation (VPE) method is a very efficient methodology for high-fidelity modeling of electromagnetic wave propagation. While the VPE has been widely applied to model wave propagation in tunnel environments, most of the work involves only smooth surface walls and leaves out of consideration scenarios with rough surfaces. Although it is often disregarded, surface roughness, as an additional source of path loss, is very important in the accurate modeling of tunnel propagation. This paper presents a rigorous approach to incorporate surface roughness into VPE-based models. To that end, the rough surface is first parameterized using a Karhunen–Loeve expansion and then embedded into the VPE. The proposed approach can be applied to realistic tunnel geometries with arbitrary cross sections and curvature variations. Validations are provided by comparisons with experimental data in various practical tunnel cases.

Measurement of the Influence of Antennas on Radio Signal Propagation in Underground Mines and Tunnels.

This paper reports the influence of antennas on radio signal propagation in tunnels and underground mines. Radio signal propagation measurement results in a concrete tunnel and underground mines using antenna types with various radiation patterns, i.e., omnidirectional, Yagi, patch, and circular, are reported. Extensive measurements were taken in various scenarios which include vertical, horizontal, and circular polarization for line-of-sight (LoS) radio signal propagation at four frequencies (455, 915, 2450, and 5800 MHz) that are common to many voice and data transport radio systems used in underground mines. The results show that antenna pattern has a strong influence on the uniformity of radio signal propagation gain in the near zone and typically does not significantly influence behavior in the far zone, except for a constant gain offset.