Full scale measurements of train underbody flows and track forces
Introduction
Ballast projection or flight is the phenomenon by which ballast grains may become airborne during the passage of a high-speed train. Airborne ballast can cause extensive damage to the underbody of a train, and to the railhead if trapped between a wheel and the rail. Further evidence of ballast flight may include damaged wheel sets, broken glass in stations and damaged trackside acoustic screens. It has been conjectured that the phenomenon is caused by strong aerodynamic flows between the train underbody and the track, coupled with mechanical excitation during a train passage (Quinn et al., 2010).
Ballast flight seems to manifest itself in different ways in different countries. During ICE3 tests in France and Belgium in normal weather conditions in 2003 and 2004, large quantities of quite large ballast grains became airborne and caused extensive pitting of train under bodies (Kaltenbach, 2008) and similar incidents have been reported in Italy and Spain. Elsewhere in Europe (including the UK) and in the Far East, lumps of ice falling from trains can displace ballast, causing train and track damage (Shinojima, 1984). In the UK, the problem appears mainly to result from smaller ballast grains being lifted onto the track, where they are crushed by either the train that caused the ballast to lift or by a following train, leading to pitting of the wheel and rail and the need for more regular maintenance (Quinn et al., 2010).
The phenomenon of ballast flight has prompted a significant amount of research around the world, particularly within Europe through the Aerodynamics in the Open Air (AOA) (Kaltenbach, 2008) and AeroTRAIN projects (Sima et al., 2011). A number of investigators have measured aerodynamic flows beneath trains at full scale (Kwon and Park, 2006, Deeg et al., 2008, Quinn et al., 2010, Premoli et al., 2015) and at model scale (Kwon and Park, 2006, Kaltenbach et al., 2008a, Kaltenbach et al., 2008b, Ido et al., 2008, Ido and Yoshioka, 2009, Ido et al., 2013, Jonnson et al., 2012, Jonsson et al., 2013). CFD calculations have also been carried out (eg. Sima et al., 2008, Garcia et al., 2011).
Full-scale tests have been carried out in a number of countries (Korea, Japan, France, Germany, Italy, Spain, UK), with different track types (slab track and different sleeper/ballast configurations). Wind tunnel tests have been carried out over a range of scales, with and without simulation of the track bed itself. Tests using a model train propelled by a car have been reported by Ido et al., 2008, Ido and Yoshioka, 2009, Ido et al., 2013. In general, CFD calculations have used standard RANS methods, and some authors have carried out comparisons with equivalent experimental results. The overall trends from these measurements and calculations are clear, showing a highly sheared and turbulent flow near the ground and an indication that bogie cavities can increase both the magnitudes of the flow and its unsteadiness. The type of the sleeper and the height of the ballast above or below the sleeper top have significant effects on the apparent aerodynamic roughness of the track.
Ballast flight itself has been studied analytically (Sanz-Andres and Navarro-Medina, 2010, Quinn et al., 2010, Jing et al., 2012) and experimentally at both model scale using large scale wind tunnel rigs (Kwon and Park, 2006, Kaltenbach et al., 2008a, Kaltenbach et al., 2008b), and at full scale using instrumented ballast (Quinn et al., 2010, Premoli et al., 2015) and train borne microphones to assess impacts (Premoli et al., 2015). It has come to be generally accepted that the primary parameter in assessing ballast flight is a measure of the horizontal velocity beneath the train (i.e. drag/shear forces on the ballast); hence much effort has been put into determining these and correlating them with ballast movement. Although a number of authors have mentioned the possibility of track vibrations playing a role in ballast flight, this has only been studied in two investigations – Quinn et al. (2010), with acknowledged shortcomings in the instrumentation, and Premoli et al. (2015). The evidence here is somewhat contradictory, with early authors such as Luo et al. (1996) predicting track and ballast accelerations of the order of one g or more Conversely, the more recent measurements of Premoli et al. (2015) suggest much lower ballast accelerations of the order of 0.2g, leading them to conclude that track vibrations are not a relevant issue. This divergence of view will be addressed later.
A number of authors present models and methods for assessing the risk and mitigating the effects of ballast flight. With regard to risk assessment, Saussine et al., 2009, Saussine et al., 2013 developed an outline methodology involving the “stress” on the bed caused by the passage of the train (based on the velocity beneath the train), and the “strength” of the bed (defined by the mean and standard deviation of the number of ballast particles moved at a particular “stress”). The probability of ballast movement can then be calculated from a convolution of the two probability distributions. The “stress” and the “strain” are not as conventionally defined, the former being an observed function of the train type and the latter of the track characteristics.
Both track and train based methods of mitigation have been proposed. The most frequently advocated track based methods are the use of slab track (i.e. the complete removal of ballast), lowering the ballast to well below sleeper height, adjusting the sleeper shape and ballast gluing. In terms of train modifications, proposals have been made for shielding bogies and reducing the high mean velocities and turbulence levels in their vicinity, and for generally smoothing the undersurface of the train to reduce near-track velocities (Kaltenbach, 2008).
Finally, work is underway within CEN to develop a testing methodology that can be used in train homologation. The current proposal, based on the outcomes of the AeroTRAIN project, and contained in CEN (2013) as an informative annex), is for tests to be carried out to measure the flow velocities beneath a train on a section of track where a smooth surface has been created by covering the ballast (Weise and Sima, 2013) It is acknowledged that this method lacks realism, but it does allow a comparison to be made between different train types. The proposed methodology is still however complex and resource intensive, and agreement on its implementation has not yet been reached.
The work reported in this paper formed part of a project whose objectives were
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to investigate the interaction between track/ballast vibration effects and aerodynamic effects on the initiation of ballast flight; hence to address the uncertainties that have arisen from earlier work;
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to investigate the possibilities of using scale moving model tests and an advanced unsteady CFD methodology to predict the flow field beneath trains, hence to assess such methods as possible future options for the homologation of new trains, that would be less restrictive and resource intensive than field monitoring.
This paper presents the results of on-track monitoring in which concurrent measurements were made of the aerodynamic velocities and pressures, track displacements and ballast accelerations. These measurements have contributed to our understanding of the interaction between the aerodynamic and geotechnical aspects of ballast flight, and provide a detailed set of data for comparison with model test and CFD calculations. Other aspects of the project will be presented elsewhere. Section 2 of this paper describes the field monitoring carried out, and the analysis techniques used. The main results are presented in section 3. Section 4 discusses these results and analyses some aspects of the data in greater detail. In particular it investigates the conditions where ballast flight might be initiated. Concluding comments are given in section 5.
Section snippets
Test site
Field monitoring was carried out on a high speed railway line in the UK which has been the subject of previous aerodynamic and geotechnical investigations (Quinn et al., 2010). Fig. 1 shows a schematic plan of the site, with the locations of the instrumentation indicated. The site is situated on the down line of a relatively straight section of twin track. A schematic of the equipment set up is shown in Fig. 2. The aerodynamic measurement equipment was mounted on a gauge bar across the track,
Results
Fig. 4 shows the range of sleeper movements recorded at each geophone location for the 14 trains considered (Table 2). These are the peak to trough movements obtained by applying the appropriate frequency domain calibration, then filtering and integrating the geophone native velocity measurements. Fig. 4 is representative of typical ballasted railway track performance even on well maintained high speed lines, in that nearby sleepers and opposite ends of the same sleeper have significantly
Velocity profile analysis
Fig. 13 (a) and (b) shows the velocity and turbulence intensity profiles below the train at a number of sections along the track. These represent an average of the ensembles along the lengths of the train indicated. The turbulence intensities are simply the ensemble standard deviations of the velocities over the ensemble mean, this approach being appropriate for the fixed ground based frame of reference. The velocity profiles are of the expected form (Fig. 13a). The highest velocities occur
Conclusions
From the material presented in this paper, the following conclusions can be drawn.
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Measurements of track displacement and ballast acceleration were very consistent across all train runs, showing little variation.
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Aerodynamic measurements, particularly of velocity, indicated a much greater degree of variability (as would be expected), and ensemble averaging was needed to reveal fully their spatial and temporal variations.
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Ballast grain vertical accelerations were largely driven by track
Acknowledgments
The work in this paper was funded by EPSRC project EP/K037676/1. The authors would like to thank Network Rail High Speed for their assistance with the field monitoring and Eurostar International Ltd for their cooperation throughout the project.
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2023, Journal of Wind Engineering and Industrial AerodynamicsCitation Excerpt :Within the research program of Aerodynamics in the Open Air (AOA) in the frame of the official German-French cooperation “DEUFRAKO”, a full-scale track-side measurement campaign has been carried out in Italy for the ETR 500 train at about 280 km/h using ultra-sonic anemometer as well as rakes of Pitot tubes (Deeg et al., 2008). To assess the forces acting on the ballast which is hypothesized to be a new reason of railhead damage in Channel Tunnel Rail Link (CTRL) HSR track in the UK, full scale measurements were made of the flow velocities and pressures beneath the train (Quinn and Hayward, 2008; Quinn et al., 2010; Soper et al., 2017; Saussine et al., 2011) as well as sleeper displacements and ballast accelerations (Saussine et al., 2011). While the above principal studies on the flow field beneath high-speed trains mainly employed the full-scale testing method to understand the fundamental characteristics of the flow field, several numerical approaches have been followed for further understanding of the flow field as well as to acquire an efficient tool to explore the design spaces related to the flow.
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2020, Journal of Wind Engineering and Industrial AerodynamicsCitation Excerpt :However, according to the authors’ knowledge, there is as yet no explicit study on the effect of rails on HST slipstream characteristics, or even from the general aerodynamic viewpoint. Perhaps the most relevant previous research are the studies on train underbody flow and its loading on the track (Soper et al., 2017a, 2017b), which is also strongly correlated to the ballast flight phenomenon (Quinn et al., 2010). More recently, by using dynamic meshing, Paz et al. (Paz et al., 2017) modelled the sleepers in studying the HST underbody flow, and this improved the prediction for ballast flight.