On the aerodynamic loads when a high speed train passes under an overhead bridge

Abstract

The aerodynamic loads on the overhead bridge bottom surface induced by train passage are reported in this paper. Both moving model test and numerical simulation approaches at the 1:20 scale are used. The numerical work is validated through both mesh independence tests and comparison with experimental data. Typical pressure variation curves are plotted and compared with previous studies. The peak pressure values’ dependence on the Reynolds number is considered through four sets of experiments with different train running speeds. The peak pressure coefficient distribution law for the bridge bottom surface is presented. Differences in the pressure distribution in different bridge bottom areas are explained based on more detailed flow field information. The influence of the bridge height on the aerodynamic load magnitude and time interval is presented. Moreover, the application of the CEN Standard to practical engineering issues is discussed.

Introduction

With the increasing need for passenger transportation, railway transportation has gradually played a more important role with its features of rapid and convenient transportation. However, with the increasing speed of passenger trains, aerodynamic issues that threaten the security of line side facilities, buildings and pedestrians have become more common. The train-induced pressure fluctuation, as a main aerodynamic issue caused by train passage, has been proven to be strongly related to the safe operation of line side facilities (Tian, 2007; Baker, 2010, 2014a). Thus, the regularity of this transient load on line side structures caused by train passage should be understood at a deeper level.

For a train running through a tunnel, as the air inside the tunnel will be compressed or expanded by the entrance of the train head or tail, a pressure wave will travel back and forth inside the tunnel, thus causing pressure variation. This phenomenon is called the ‘pulsation effect’ (Gilbert et al., 2012; Sajben, 1997; Howe, 1998). Recent studies, including Gilbert et al. (2013), Chen et al. (2017), Niu et al. (2017a) and Wang et al. (2018) have discussed the aerodynamic effect of trains running inside tunnels under several conditions. For trains passing a line side structure in open air, no evident pulsation effect exists because the space is not completely confined, and the air density will not greatly change.

The pressure variation on line side structures will be more influenced by the natural pressure distribution around the moving train in the open air situation, which involves high positive and negative amplitude pressure pulses around the train head and tail and low amplitude pressure fluctuations around the train middle and in the wake area behind the tail (CEN European Standard, 2013; Hemida et al., 2014; Zhang et al., 2018). The magnitude of the transient aerodynamic loads is normally proportional to the square of the train speed (Tian, 2007; Zhou et al., 2014), and the time interval of the load is inversely proportional to the train speed (CEN European Standard, 2013; Xiong et al., 2018). Thus, increasing train speed significantly increases the impact of transient aerodynamic loads. Several previous studies have been carried out to study the transient pressure fluctuation on line side structures in open air. Baker (2010, 2014a, 2014b) studied the line side pressure fluctuation via a 1/25 scale moving model test rig. The aerodynamic loads on noise barriers, bridges, platform roofing and station platforms induced by three different types of trains were reported and discussed. The research findings were applied to complement the EU railway standard and the 2013 ​EN (CEN European Standard, 2013). In Zhou et al. (2014), the pressure variations on a platform screen door caused by a train passing through a station were studied with a 1/20 scale moving model test; the situation of two trains passing each other in the station was also included. Additionally, the distribution of positive and negative pressure peak values on the screen door was discussed. More recently, Xiong et al. (2018) carried out a full-scale test on a bridge to test the pressure transient on the bridge noise barrier, and a comparison between the result and CEN European Standard (2013) was carried out. These studies have provided an adequate source of data for the assessment of the safe operation of line side facilities and fundamentals for future research.

Recently, with the development of the railway industry in China, an increasing number of railway stations have been built. Overhead bridges, as an important medium for connecting trains, stations, and passengers, are widely used at numerous railway stations. However, a train running on the main line at a high speed would have a considerable influence under this circumstance. The transient pressure loads induced by train passage will cause the vibration of the overhead bridge, which would greatly influence the comfort of pedestrians and even threaten the structural safety of the bridge. Thus a deeper understanding of the aerodynamic loads induced by train passage is demanded for lateral researches against overhead bridge in terms of structural assessment and optimization. This subject has been covered in CEN European Standard (2013) as introduced above. Also in Yang et al. (2015), an overhead bridge was utilized as the research target for transient pressure. Both full-scale tests and unsteady Reynolds-averaged Navier-Stokes (URANS) simulations were used to obtain the pressure variation in the train passage process. The pressure distributions on the bridge bottom and windward surface were discussed, together with the influence of the bridge height on the pressure peak magnitude. However, existing researches related to this subject are still insufficient to take fully consideration of this phenomenon. In CEN European Standard (2013), the aerodynamic load is characterized as distributed loads ​+ ​Cp and –Cp, each 5 ​m long and up to 20 ​m wide, centred on the track centre and moving at the speed of the train. This description of aerodynamic load distribution failed to include the pressure decay in the transversal direction. In Yang et al. (2015), pressure distribution along different directions have been presented. But the numerical data of the bridge bottom surface presented in Yang et al. (2015) failed to be validated directly through test results as no pressure sensors have been arranged there – a pressure sensor installed on bridge bottom surface in field test environment would certainly introduce safety concerns. This would have considerable influence on the accuracy and reliability of the data. Also only several measuring points in the numerical work of Yang et al. (2015) are used for the illustration of pressure distribution on the bridge surface, which is inadequate to make the full description. Moreover, in CEN European Standard (2013), the horizontal structure above the track was described as a flat plate. While in Yang et al. (2015), the beam structure located on the bridge bottom surface was also neglected. As the beam structure is normally the bearing structure of a bridge, and its existence would have significant influence on the pressure distribution of bridge bottom surface. Thus the pressure loads and the application of the CEN Standard in this more realistic situation should be further discussed and assessed. Based on the situation introduced above, researches with higher reliability and more detailed information should be further pursued to enrich this field, which motivate the current study.

As introduced above, constraints related to safety issues would exist in field test situation. The application of moving model facility will overcome this point and pressure transient data of bridge bottom surface will be directly accessed for lateral research. So in this paper, results obtained from both moving model tests and numerical simulations will be reported and discussed. The results of the moving model tests will also be used to validate the numerical simulations, which are employed to provide more field information. Through this study, the effect of train passage under an overhead bridge will be introduced at a more detailed level, and the application of the CEN Standard will be further discussed.

The paper is organized as follows: In Section 2, the experimental arrangement and setup are presented. Section 3 gives the numerical method and validation. In Section 4, the numerical together with the experimental results are presented and discussed, and Section 5 presents the conclusion of the research.