Train Aerodynamics Research in 2018 / 2019

 Introduction

The book “Train Aerodynamics – Fundamentals and Applications” (hereafter referred to as TAFA) was published in early 2019, but in reality took no account of any material published after June 2018. There has however been a significant number of studies published in the second half of 2018 and 2019, and it seemed worthwhile to try to collate these in some way, and this blog post attempts to do this. Selfishly such a collation might help for any second edition of TAFA that is produced (if the sales warrant it!) but more generally it is hoped that it may prove useful to all those involved in Train Aerodynamics in one way or another in signposting ongoing work around the world.

It should be emphasized at the outset that this collation cannot properly be described as a review. A review (as I have told my graduate students for the last three decades) needs some degree of synthesis of the various reports and papers discussed. This of course requires a number of papers addressing the same issue to be available to synthesise. Looking at papers from a short time period that cover a wide range of subject matter, this is not really possible, so what follows is essentially a brief description of the work that has been carried out in 2018 and 2019, with a few interpretive comments. 

We consider the various publications roughly in the order of the Applications described in Part 2 of TAFA – train drag, pressure transient loads, slipstream loads, ballast flight, OHL issues, crosswind studies, tunnel aerodynamics and emerging issues. A brief section is included on train and tunnel ventilation that was not considered in TAFA. Some concluding reflections are included at the end of the post.

In the text, published references are linked directly to their DOI, rather than to a reference list. Those references with no DOI (recent conference publications in the main) are given in a short list at the end of the post. A full reference list can be found here

Train drag

One of the major issues in both experimental and computational assessments of train drag is the simulation of the ground, as the nature of this simulation can have a significant effect on the measured force. Niu et al (2018b) present some CFD analyses that usefully address this issue for a number of ground and ballast representations. Overall they show that the effect on measured drag coefficients at zero yaw angle is small (of the order of 2 to 3%) and probably not significant in comparison with the large variation in drag measured at full scale, and in different types of CFD and physical model simulations. Nonetheless the results can give some guidance for the setting up of physical model tests and CFD trials. 

There have been a small number of train drag investigations looking at the effect of modifications to different parts of the train on overall drag – nose length, inter-car gaps, bogie position, roof equipment and for freight trains, container spacing. They will be considered in turn below. 

Chen et al (2019a), using IDDES, looked at effect of nose lengths of between 5 and 10m on drag on a five-car train. The drag was shown, unsurprisingly, to decrease with nose length. Li et al (2018b) used the k-omega method to investigate the effect of inter car gap length on drag, and showed that gap lengths of less than 80mm full scale had no effect on drag. The k-epsilon CFD work of Gao et al (2019) looked at the effect of changing bogie position on the leading car of a three-car high-speed train. The results indicate that moving the front bogie back 2m from its normal position can reduce the overall drag of the front car by 10% and the overall three-car drag by 6.5%. These drag reductions will of course be a smaller proportion of the overall drag for full-length trains.

Tschepe et al (2019) investigated the drag of roof-mounted insulators through wind tunnel experiments. They observed considerable Reynolds number effects and effects of insulator position on the measured drag (which in aerodynamic terms were in the critical range), and showed that overall drag due to roof elements could be about 5% of overall train drag. Interestingly they found that soft insulators displayed a tendency to flutter, resulting in higher drag than rigid insulators.  

Maleki et al (2019) carried out a CFD investigation of the flow around containers using LES. There were major changes in the wake of containers as container spacing increased. Above a certain spacing wake closure occurs with high speed flows impinging on downstream container, resulting in increased drag. They also present potentially useful results for the optimisation of single and double stack container positions for low -drag. 

Pressure transients and loads

A small number of investigations have been reported of the pressure transients and loads caused by passing trains. Soper et al (2019) made full scale experiments on the transient pressure loads caused by high speed trains on acoustic barriers, and were able to determine the effect of variations in track distance and the nature of the overall trackside infrastructure on the measured loads. The CEN load correlations were found to represent the results well. 

Huang et al (2019) added to the small amount of data available on the loads caused by passing trains through an experimental and compressible CFD study of high speed Maglev trains passing each other, including detailed calculations of the transient pressure field around the train. 

Munoz-Paniagua and Garcia (2019) developed an optimization methodology for nose shape to optimize (i.e. minimize) the pressure transients. This involved a large number of CFD runs for different geometries that were used to train a genetic algorithm. It was shown, perhaps unsurprisingly, that nose length and bluntness were the most important parameters in the optimization.  They also considered the optimization of nose shape for cross wind performance, which will be discussed further below. 

Slipstream velocities and loads

In 2018 and 2019 a significant number of papers have been published that use (in the main) CFD IDDES to calculate trains slipstream and wakes and to look at specific flow effects. These all give a great deal of information concerning the micro-nature of the flow field that it is not always easy to interpret of to put into a bigger picture. They are useful however in helping to build up a picture of the complexity of even the idealised CFD flow fields around trains. As with drag investigations described above, these studies were aimed at assessing the effects of slipstreams of changes to different parts of the train – nose and tail, gaps between double units and bogies. 

Chen et al (2019a) looked at the effect of different nose / tail lengths on drag and lift, but also looked at the effect of the slipstream behavior along and behind the train. They found that the TSI slipstream velocities decreased with tail length, with the longitudinal trailing vortices becoming weaker. In a further paper they extended this work to look at the effect of changes in nose length on the slipstreams and wakes in crosswinds (Chen et al, 2019b). Not surprisingly they found that the effect of nose length on the overall flow field was complex and quite difficult to quantify.  

Li et al (2019b) looked at the effect of the gap in double unit trains on the development of the slipstream and the wake and showed that the main effect was to increase the boundary layer and wake velocities downstream of the gap. 

Two papers from the same group in Changsha (Wang et al, 2019 and Dong et al, 2019) describe aspects of an IDDES investigation of flow around bogies. The first looks at the effect of bogie fairings on the slipstream and wake and, as might be expected, shows that bogie fairings reduce the velocities in the boundary layer and the strength of the longitudinal vorticity in the wake.  The second studies the effects of simplifying the geometry of train bogies in CFD simulations. It shows that the effects are mainly felt in the underbody flow region rather than in the wider flow around the train, and offers some suggestions for appropriate degrees of geometric simplification. 

Finally the IDDES modelling of Wang et al (2018c) should be mentioned. This is a fundamental study of the effects of bogies on the slipstreams of high-speed trains. The study shows that the generation of the strong spanwise oscillation of the wake, observed especially in the presence of bogies is due to the amplification of a natural instability of the time-mean pair of counter-rotating vortices. 

Ballast flight

The work in this area has focused on two aspects – the nature of the flow field around the train, and the accumulation of snow in the bogie area.

With regard to the first, two useful studies have been reported that address the issue of the train underbody flow, which of course controls the flight of ballast. The first of these is the thesis by Jönsson (2016), which is somewhat outside the publication time range considered here, but is nonetheless worth mentioning. The author carried out extensive measurements using PIV to measure the flow field beneath 1/50thscale model trains, with different underbody geometries and sleeper layouts. Comparisons were made with full scale and showed that the essential aspects of the underbody flow field could be reproduced. The tests also showed how important train underbody irregularities were in increasing velocities and thus the likelihood of ballast flight. The data was also used in a simple analytical framework similar to (and earlier than, so it takes academic priority!) that included in TAFA.  

Also with regard to the underbody flow, Paz et al (2019) consider the nature of ground simulation in CFD trials, and present a method for simulating track and ballast geometries in this region, using scanned profiles of real sleeper and ballast geometries. The results show significant differences between the simulation and the normal flat ground geometries, particularly close to the ballast where higher levels of turbulence were measured. Overall the methodology as set out is potentially of great use in simulations for train authorization purposes. 

Two CFD studies, both by the same group have also been carried out to address the related problem of snow accumulation around bogies – one using a discrete phase model and IDDES (Liu et al, 2018) and one using a discrete phase model and URANS (Wang et al, 2018b). A somewhat qualitative validation of the methodology against experiments was carried out. Similar results were obtained using both methods, but the URANS work used considerably less computer resource. Unsurprisingly snow was shown to accumulate in areas of low velocity in the bogie cavity. Overall the results give useful qualitative indications of those aspects of bogie design that could be altered to reduce snow accumulation.

Overhead and pantograph systems

Two interesting studies on the dynamics of pantograph and catenary systems were reported in 2018/2019. In the first, Li et al (2018a) describes DDES calculations of the flow around and forces on pantographs on a three car high speed train at yaw angles of 0, 20 and 30 degrees, thus representing a range of cross wind velocities. As might be expected, the flow around the pantographs becomes increasingly complex and turbulent as the yaw angle increases. The aerodynamic forces on the pantograph were found to oscillate around a mean value even at zero yaw, and as the yaw angle increased a range of different dominant frequencies appeared. Whilst these results are doubtless quite specific to the case being considered, and the simulation does not fully reproduce the range of turbulent fluctuations in the atmosphere, they do show the potential for high cross winds to excite a range of pantograph oscillations.  This is an area where further work would be of significant interest.

Secondly, Xie and Zhi (2019) report wind tunnel results for the dynamic behavior of catenary systems, including the effect of wire tension on the natural frequency and displacements of the contact wire in a range of cross wind conditions.  A large scale if somewhat crude simulation of the near ground atmospheric boundary layer was used. The authors discuss the possibility of resonant oscillations occurring between the train pantograph system and the overhead wire. Deflections of 6cm at mean wind speeds of 17m/s were measured. This is again a topic where further work would be useful – in particular a study of the interaction between pantographs and the overhead wires in high crosswinds would be very interesting and potentially very significant. 

Crosswind

The number of new studies on trains in crosswinds has increased significantly in recent years and shows no sign of slowing down. In 2018/19 results of studies have been published on wind conditions near the track, train aerodynamic forces in cyclonic winds, train aerodynamic forces in tornado winds, the effects of wind shelter and full-scale measurement of wheel unloading risk. These will be considered in turn below.  It is perhaps worth noting at this point however that many of the CFD calculations described below, even though they give a detailed description of unsteady flow fields, nonetheless do not fully simulate the turbulence structure of the oncoming atmospheric boundary layer. These results thus not fully represent reality, where the flow structures around the train can be expected to be disrupted by the oncoming turbulence. They are nonetheless useful in giving an overall impression of the flow field and the effects of different geometry changes for example. 

Zhang at al (2019b) used IDDES, calibrated against wind tunnel tests, to predict the flow speed up over embankments of varying geometry, to enable a rational siting  of warning anemometers. This work usefully adds to the data available for the wind speed up over railway embankments.  Hu et al (2019) also consider wind characteristics in terms of the nature of the wind relative to a moving vehicle. The analytical framework they have produced is more extensive and rigorous than those developed by earlier authors and has the potential to be used to generate realistic time series of velocity that could be used in  overturning calculations. 

A number of studies have been reported that enlarge the database of aerodynamic coefficients of trains in cross winds. Noguchia et al (2019) provide experimental and LES data for the crosswind forces and moments on a range of conventional train types on embankments.  Guo et al (2019) present the results of IDDES calculations that investigated the difference in cross wind pressures and forces for both a 6 car single unit and a 6 car double unit high-speed train. They also provide extensive discussions of the nature of the wake and the unsteady flow.  The effect of the gap between the two units was shown to have a significant effect on the crosswind forces on coaches in the centre of the formation, and also affected the primary frequencies of oscillation.  Lin et al (2019) report the results of a benchmark test to measure crosswind forces and moments with two different trains in three different wind tunnels, all of which were carried out to confirm to the CEN guidelines.  Systematic differences in results between nominally similar tests are observed, and seem to be associated with blockage and boundary layer effects in the wind tunnels. These results were presented in brief at a conference, and a full write up of the results should prove extremely interesting and is eagerly awaited by the author. 

The opimisation work of Munoz-Paniagua and  García (2019) has already been mentioned above. As with the pressure transients they found that nose length was the major factor determining the crosswind forces and moments.

The above studies were concerned with trains in cyclonic winds. A couple of studies have been reported where pressures on trains were measured using Tornado Vortex Generators – that of Bourriez et al (2019) with a moving model, and that of Cao et al (2019) with a stationary model. Both these studies have major scaling issues, where the train scale and tornado vortex scale do not match. Nonetheless they give interesting indicative results. Further developments can be expected in this field in the future. 

In view of the importance of reducing crosswind forces and moments, the literature describing the effect of wind barriers on trains in cross winds is surprisingly sparse. Recent studies have gone some way to remedy this – Mohebbi et al (2019), Niu et al (2018a), Hashmi et al (2019) used a variety of CFD techniques to investigate the effect of wind fences on train forces; Misu et al (2019) used equivalent wind tunnel tests; He et al (2018) and Flamand et al (2019) both considered train / bridge systems, where the cost to providing shelter on the train in increasing the loads on the bridge were considered. Wu et al (2019) looked at a case where wind shielding effects were undesirable, when a train runs in the wake of a bridge tower. Through the use of a simple low speed moving model rig to measure transient train forces, and a dynamic model of the wind / bridge / vehicle system they concluded that the shielding effect could have an adverse impact on both the running safety and riding comfort of the train.

Finally in terms of assessing safety and risk, two interesting full-scale experimental techniques have been developed. Wei et al (2018) report a method for measuring wheel unloading by making continuous measurement of the accelerations and displacements of the wheel set using relatively simple equipment, rather than the more conventional measurements made by instrumenting the wheels themselves (although such measurements are themselves quite innovative and difficult). The results from full-scale experiments on trains for the derailment and loading coefficients as they move in and out of the shelter provided by wind breaks are impressive and indicate that the methodology may be of some use in the future. Similarly, Lu et al (2019) use measurements from primary suspension. Good agreement with instrumented wheelset data was demonstrated provided a suitable calibration was carried out. 

Tunnels

In 2018 and 2019 a number of papers have been published on tunnel aerodynamics, addressing the issues of pressure transients and micro-pressure waves, tunnel velocities and structural loading. We will consider each of these issues in turn.

The work on pressure transients in tunnels has been carried out using both experimental and computational methods.  The experimental work of Heine et al (2019) using a moving model rig investigates the effect of wall cavities (for cross passages) on tunnel pressures. These cavities were installed to reduce the pressure load on interconnecting doors, but interestingly the results show that, by creating extra surfaces for pressure waves to reflect from, the pressure loading on the doors can actually increase under certain circumstances.  

Iliadis et al (2018) also used a moving model rig to look at pressure transients as blunt freight trains entered a tunnel, with measurements both on the tunnel wall and the train. The major point to emerge is that for certain freight train loading situations, gaps in the train formation can result in significant tunnel entry pressure transients, and the maximum pressure in the tunnel might not always be associated with the entry of the train nose as for passenger trains. 

Li et al (2019c) report a k-epsilon CFD investigation of the pressure waves in tunnels with variable cross section, but with sudden transitions between the sections. Unsurprisingly they show that a complex series of pressures results from these transitions, but on the whole the magnitudes of the pressures are reduced from the single area case.  Wang et al (2018) describe a similar investigation using k-omega CFD, but with gradually varying area rather than abrupt transitions. The pressure magnitudes are again reduced as would be expected. Both of these also showed that there was the potential for reducing the gradient of the initial pressure wave, which is the main parameter of importance in the generation of micro-pressure waves, using such approaches.

Micro-pressure waves were considered in more detail in the work of Saito (2019) who used the results of moving model rig experiments and a simple analytical formulation of tunnel entry pressures. He investigated the optimum area and length of unvented entrance hoods, and derived some useful design guidelines. 

Another method of reducing the strength of micro-pressure waves is to use ballast rather than slab track in tunnels. Fukuda et al (2019) report on the results of full-scale tests where experiments were measured before and after slab track in a tunnel was replaced by ballasted track. Significant reductions in pressure gradient were observed. A simple methodology for predicting these pressure gradient reductions has been derived. 

The propagation of micro-pressure waves themselves has been considered by Zhang et al (2018) and Zhang et al (2019a).  The former developed straightforward analytical models of pressure magnitudes around the tunnel exit portal, which were calibrated against experimental and CFD data.  The latter made measurements of the relatively small pressure amplitudes at the exit of a tunnel simulation using a moving model rig.

In some situations it is necessary to determine the velocity transients in tunnels as well as the pressure transients, particularly when loads on structures or people are required. These issues have been addressed by Jiang et al (2019) who carried out a URANS study of the slipstreams generated by different train types in a double track tunnel, providing a great deal of detailed information concerning the nature of the slipstream variation with height above the ground and distance along the tunnel. The work of Iliadis et al (2019) mentioned above for freight trains, also made such measurements. Kikuchi et al (2019) took a different approach and developed a simple calculation method based on unsteady incompressible flow for the flow over the roof of a train. This method takes into account the unsteady boundary layer development along the train roof, and allows the velocities to be determined that can be used to assess pantograph performance in confined tunnel situations. 

Finally a couple of papers have appeared that have addressed the issues of loads on trains and tunnel infrastructure directly. The first, by Lu et al (2018) looked at the fatigue loading caused by the pressure waves due to two high speed trains passing in a tunnel, and involved the use of unsteady k-epsilon CFD and a finite element model of the car body to determine the loads at specific points on the vehicle. Courtine at al (2019) carried out full scale and model scale experiments to analyse designs of “deflectors” to be placed around lorries in the Channel Tunnel, particularly with regard to the effect that they might have on the loads on the soft-sided trailers. 

Emerging Issues

The final chapter of TAFA briefly summarises a number of emerging issues. Of the ones discussed, there are two that have seen further work published in 2018/19 – evacuated tube transport, and snow drifting.

With regard to the former, two investigations have investigated the shock wave formation around such tube trains for very high speed vehicles using different methodologies. Zhou et al (2019) use the compressible Navier-Stokes equation for flow around an axisymmetric body in a tube, whilst Niu et al (2019) use a variety of different CFD methods, and also consider heating of the flow. Both reveal complex shock wave patterns, particularly behind the vehicle, and investigate the choked flow region in particular. Both sets of results serve to emphasise the complexity of the flows around such vehicles and indicate the formidable challenges that still remain before this type of transport can be implemented. 

The issue of wind blown sand around railways has been addressed by a group at Torino in Italy. The paper by Raffaele and Bruno (2018) presents an outline of a probabilistic method for assessing the accumulation of sand around railways, while the second by Bruno et al (2018) presents a thorough review of current literature and methodologies. Clearly it is not possible to easily summarise a review paper – suffice it to say that this provides and excellent starting point for those who wish to delve into this subject further. 

Ventilation

One area of study that was not included in TAFA, and in retrospect probably should have been, is that of ventilation of tunnels and stations, and the internal ventilation of the trains themselves. The need for such studies is becoming increasingly urgent as the poor air quality in underground stations and on trains becomes more and more apparent. Whilst no attempt will be made to present a full review of this subject here, it is worthwhile to set out the work that has been done in the 2018/19 period at least.

First of all let us consider the ventilation of tunnels and underground stations. Izadi et al (2019) present the results of a unsteady RANS calculation of velocities and pressures in a small number of stations connected by a single tunnel. Pressures and velocities were predicted for a variety of train operating scenarios using both a simple axisymmetric geometry and a more complex three-dimensional geometry. Various fan operating strategies were also discussed. Koc et al (2018) use a simpler one-dimensional approach, but applied to a more complex network of tunnels and stations. A methodology for using ANNs to predict pressures and velocities in complex situations is presented. Zarnaghsh et al (2019) use a finite volume technique to investigate the behavior of tunnel ventilation fans and in particular the interaction between the velocity field produced by trains and those produced by the fans themselves. It was shown that the passage of trains could result in the operating characteristics of the fans moving significantly away from their nominal operating point. 

Work on the ventilation of trains has also been reported. Abadi et al (2019) report a CFD k-omega study to improve the performance of a roof mounted ventilation system under crosswind conditions, by varying the inlet geometry. Li et al (2019a) report a DES similar study to investigate the performance of an ACU with roof inlets on a high speed train at different speeds.

Concluding reflections

Firstly some paper statistics are of interest, but note the figures that follow are somewhat arbitrary in many ways and reflect both the chosen publication dates and the sources consulted. There were 55 items accessed in total, all but one from between mid-2018 to the end of 2019.  Of these, 28 were from Chinese institutions, reflecting the fact that research there is driven by the rapid development of the Chinese high-speed rail network, with the rest being spread fairly evenly around Europe, the Middle East and the Pacific Rim. Of the Chinese papers, 18 were from the Central South University at Changsha, 7 from South West Jiaotong University at Chengdu and 3 from elsewhere (although there was some overlap of authorship). In terms of technique used, 30 of the papers were mainly based on CFD studies, although some included experimental verification of one sort or another.  Of the Chinese papers 21 were CFD studies, including 16 from Central South University. There was thus a significant imbalance between the use of CFD in China and elsewhere. 

The growing use of CFD techniques is thus notable, and in particular the IDDES technique seems to be becoming the most favoured. This growth is wholly understandable and will no doubt continue. Arising from this, it seems to the author that there is a growing need for a small set of freely available well documented validation cases, ideally from full scale experiments for a range of train types, that investigators can use routinely to prove their techniques. At the moment the validations used are somewhat ad hoc, and perhaps a more systematic approach would give greater confidence in the results, and also allow research papers to be reduced somewhat in length, as the details of the validation cases would not be required. It would be interesting to hear from CFD practitioners on such a possibility.

Finally it must be remembered that CFD simulations, in the same way as physical models, can only offer a simplified representation of the flow around full-scale trains, and need to be interpreted in this light. There is a tendency amongst some authors (and I name no names!) to quote numerical results to higher levels of accuracy than is either sensible or useful when the uncertainty of the full-scale situation is considered.  Just as with physical model tests, the role of the engineer in interpreting CFD results in terms of the reality of the operating railway is crucial. 

References without DOIs

Bourriez F, Soper D, Baker C and Sterling M (2019) Physical model measurement of tornado induced forces on trains, Proceedings of the 15thInternational Conference on Wind Engineering, Beijing

Cao J, Cao S, Ge Y (2019) Experimental investigations of wind load distributions on a high-speed train under tornado-like vortices, Proceedings of the 15thInternational Conference on Wind Engineering, Beijing

Courtine S, Aguinaga S, , Bouchet J-P, Brunel C (2019) Aerodynamic improvement of superstructure interacting with trucks carried in the Channel Tunnel, 15thInternational Conference on Wind Engineering, Beijing

Flamand O, Gattas M, van Stuijvenberg J, te Morsche N, Jongstr B, Knapp G, Benthem J (2019) Improving the wind protection of High Speed trains whilst decreasing wind loads on the supporting bridge, 15thInternational Conference on Wind Engineering, Beijing

Jönsson M (2016) Particle image velocimetry of the undercarriage flow of downscaled train models in a water-towing tank for the assessment of ballast flight, PhD Thesis, Technical University of Hamburg

Lin P, Brambilla E, Rocchi D, Tomasini G (2019) Measurement of the aerodynamic coefficients of high-speed railway vehicles: benchmark between different wind tunnels, 15thInternational Conference on Wind Engineering, Beijing

Misu Y, Takeda S, Nagumo Y, Doi K  (2019) Estimation Effectiveness of Windbreak Fences at Rolling Moment on Leeward Rails, 15thInternational Conference on Wind Engineering, Beijing

Raffaele L, Bruno L (2018) Probabilistic assessment of windblown sand accumulation around railways”, Proceedings Italian Conference on wind Engineering 

Wu J, Tang Q, Li X (2019) Shielding effect of bridge tower on aerodynamic characteristics and running safety of high-speed train with wind tunnel tests, 15thInternational Conference on Wind Engineering, Beijing

Aerodynamic effects on railway infrastructure

This post is based on a talk given by the author at the PWI Midland Technical Seminar – High Speed Rail: The Technical Challenges, on October 19th2017. 

Introduction

When the word “aerodynamics” is used within a railway context, most listeners would immediately think of trains with streamlined front ends whose function is to reduce aerodynamic resistance and thus fuel consumption. This indeed is an important issue for train manufacturers and operators, and, since train aerodynamic resistance varies with (train speed)2, has become of increasing importance in recent years as train speeds increase. However the most aerodynamic effects on the railway are not simply concerned with train design and operation, but are of significant importance to the railway infrastructure as well. These effects include the pressure waves that pass up and down tunnels and affect passengers aural comfort; the sonic booms that are emitted from the end of tunnels on high speed lines; the transient pressures that load trackside structures as trains pass; the high velocity slipstreams flows that can cause problems to passengers waiting on train platforms; the pickup of ballast beneath high speed trains; and a range of disruptive effects caused by high winds. 

In the next section, we will briefly consider the tools that are available to the engineer to investigate aerodynamic effects, and then we will proceed to give brief introductions to the effects outlined above. 

The available tools

There are essentially four different tools that can be used to investigate aerodynamic effects on train infrastructure – full scale testing; physical model scale testing; computational fluid dynamics (CFD); and the use of codes and standards. We will consider each of these in turn.

Clearly full scale measurements are the preferred type of testing, as these, properly conducted, will actually measure the reality of the situation. There are well established techniques for measuring wind velocities (using a range of anemometers of different types) and pressures (loads) on the infrastructure or the train (using pressure probes and pressure transducers) – see figure 1 for some slipstreams measurements made by BCRRE staff in Germany. However, there are serious practical issues. Setting up measurements requires close liaison with operators, development of safe working practices, possessions to install equipment etc., all of which can be costly and difficult to organize. Full scale tests are very vulnerable to weather conditions, and high winds, which unfortunately tend to come at the most inconvenient time, can make any test results very hard to interpret. To obtain representative data, usually around 20 passes of trains are required, with trains of the same configuration running at close to the same speed, which is of course not always achievable. And of course, very often, information is required for trains or structures that are not yet built, and so full scale testing is not possible. This technique remains the best to use, but these practical issues can often make it impossible to do so. 

Figure 1 Slipstream measurements in Germany

Physical model tests can take a variety of forms. The simplest (used to investigate cross wind effects) are standard low turbulence wind tunnel tests. However such tests do not properly reproduce the relative motion between the train and the ground. To model these effects properly it is necessary to use a moving model rig, such as the BCRRE TRAIN Rig based at Derby, which is a 150m long test track, capable of firing 1/25thscale model trains along two tracks at 75m/s (270 km/hr) – see figure 2 below. Such rigs can also be used to measure tunnel pressure transients (the TRAIN rig was originally built by BR Research in the 1980s to investigate transients in the proposed Channel Tunnel); loads on trackside structures; slipstream effects and (most recently) for the measurement of flow beneath trains in ballast pickup studies.

Figure 2 The BCRRE TRAIN Rig

CFD techniques have seen major advances in recent years, and there have been a large number of applications to investigate railway aerodynamic effects. Most investigators use steady flow methods (usually referred to as RANS – Reynolds Average Navier Stokes – methods), which can be run on desktop workstations with run times of the order of hours or a few days. These methods can be learnt quite easily, and can give an indication of the flow field around trains and railway infrastructure.  Whilst they are undoubtedly convenient, they do have significant limitations in that they cannot fully describe the highly transient an unsteady aerodynamic conditions that exist in the railway environment. Thus in recent years unsteady methods such as Detached Eddy Simulation (DES) and Large Eddy Simulation (LES) have come to be used more and more. Whilst these methods can produce a great deal of flow field and loading information (see figure 3 below for exapmle), they are extremely resource intensive. For example recent calculations at Birmingham to measure cross wind effects on trains have used computational grids with more than 50 million cells, which take weeks or months to run on the University supercomputer facility. Such methods are not yet practical for routine use in the train and infrastructure design, although they may well become so in the not too distant future. 

Figure 3 DES Simulation of the flow field around a Class 43 HST

 Finally we come to codes and standards. In recent years much work has been undertaken by CEN to produce a series of standards entitled “Railway Applications – Aerodynamics EN14067”. There are six parts to this code – Part 1 Symbols and Units; Part 2 Aerodynamics on the Open Track; Part 3 Aerodynamics in tunnels; Part 4 Requirements and test procedures for aerodynamics on open track; Part 5 Requirements and test procedures for aerodynamics in tunnels; Part 6 Requirements and test procedures for cross wind assessment. These standards have aimed to synthesise a great deal of work that has been carried out over recent years, and provide a very valuable resource.  At the time of writing revisions are underway on parts 4 to 6, and a further standard on ballast pickup assessment is in the very early stage of development. Various aerodynamic requirements are also set out in the Infrastructure and Rolling Stock TSIs, often referring back to the CEN standards for the methodology.

Tunnel pressure waves and sonic booms

The phenomenon of pressure changes in tunnels is well known. Essentially when a train enters a tunnel a compression wave (pressure increase) passes along the tunnel at the speed of sound. This reflects from the open end of the tunnel as an expansion wave (pressure decrease) and travels back along the tunnel. To complicate matters when the end of the train enters the tunnel this generates and expansion wave which again passes along the tunnel but reflects as a compression wave. These pressure waves can thus bounce from one end of the tunnel to another, resulting in extremely complicated pressure patterns both in the tunnel and on the train itself. The complexity can be compounded by the presence of airshafts which can also cause wave reflections, or by more than one train in the tunnel at the time. On unsealed trains this variable pressure can be very uncomfortable for passengers, and there are some tunnels on the GB network where aerodynamic speed limits are in operation. Most modern trains are sealed of course, and depending on the degree of sealing the pressure transients may or may not be felt by passengers. The CEN codes specify criteria for allowable pressure changes over different periods of time – for example 3kPa in 4 seconds in a single track tunnel. These transients can be quite accurately predicted using very simple one dimensional calculation methods, based on the equations of gas dynamics, and more complex CFD methods are only necessary for very detailed calculations around, say, the inlet to the tunnel. The advantage of the simple methods is that they can be run a large number of times to investigate a wide range of cases i.e. when trains enter the tunnels from both ends, the time lag between the first and the second entering the tunnel can be varied to investigate the worst case scenario.

An associated issue is that of the sonic booms that are emitted form the ends of tunnels on high-speed lines. This was first noticed in Japan in the 1980s as Shinkansen speeds increased through relatively small tunnels. Once observed, its cause was quite quickly established. The compression wave that results from train entry into the tunnel is not an instantaneous pressure rise, but rather a more gradual rise over a short period of time. The pressure wave thus has a spatial gradient. For long tunnels, particularly with ballasted track, the friction of the tunnel wall and track bed tends to steepen the wave significantly. When this wave arrives at the far end of the tunnel, most of the energy is reflected as an expansion wave, but some of the energy propagates outward as a sonic boom, with an intensity that is proportional to the pressure gradient at the exit. The method for minimizing sonic booms is straightforward, although perhaps not intuitive, and involves giving the inlet pressure wave as shallow a gradient as possible, so that even if it does steepen in the tunnel, its gradient at the outlet is still not sufficiently large to cause a strong sonic boom. This is achieved by the use of variable area hoods at the entrance to the tunnel – see figure 4 below for a hood that was tested on the TRAIN Rig at Birmingham as part of the initial HS2 studies.

Figure 4 Tunnel entrance hood to eliminate sonic booms

Aerodynamic loading of trackside structures

As the front of trains pass by trackside structures, they generate a transient pressure wave on the structure with the pressures first increasing and then decreasing. For example figure 5 shows the pressure on the underside of an overbridge as three different trains pass, plotted in dimensionless terms as a pressure coefficient. The peak to peak pressures are not in general that large (around 0.5 to 1kPa in dimensional terms) but, when they are repeated many thousands of times, can lead to fatigue failure of structures such as noise barriers, platform canopies etc. Guidance on allowing for these effects can be found in the CEN codes, based on full-scale tests carried out in Germany in the 1980s. These are of course not directly applicable to the GB situation because of the different loading gauge, so the RSSB have recently commissioned some tests on the TRAIN rig to measure the pressure transients on a variety of structures (hoardings, canopies, overbridges, tressle platforms) using 1/25thscale models, to provide information for a UK National Annex. The data shown in figure 5 is taken from those experiments and shows that the pressure transients caused by a Class 66 locomotive is very much greater than that caused by a Class 158 multiple Unit or the Class 390 Pendolino.

Figure 5 Pressure transients measured on a 10m wide bridge, 4.5 m above the track in TRAIN Rig experiments

Train slipstreams

As trains move along the track, not only do they generate pressure transients as outlined above, but they also generate high velocities. This velocity field is known colloquially as a slipstream. Along the side of the train (and on the roof) there is a region of flow where the air around the train is moving at the train speed next to the surface of the train (due to friction effects), decaying to zero at some distance away from the train. This region is known as the train boundary layer, and its thickness grows along the train. For passenger trains the boundary layer can be around 1m to 1.5 thick at the end of the train. For freight trains the boundary layer can be much thicker and extend 3 or 4m from the side of the train. In the wake of the train, the flow is very different for passenger and freight trains. For passenger trains the flow within the boundary layer rolls up into longitudinal vortex structures that can extend for tens or hundreds of metres behind the train – and indeed for such trains the highest airflow velocities are usually found in the wake and experienced by passengers on platforms when the train is disappearing into the distance. For freight trains these vortices do not seem to exist, and the flow velocities decay rapidly behind the train. This difference in the flow field is illustrated in figure 6, which shows the average velocities measured at trackside for around 20 train passes, as an ICE2 and a container freight train pass by. It can be seen that the highest velocities for the ICE2 occur in the wake of the train, but for the freight train these occur along the side of the train and the high velocities exists for much longer than for passenger trains. In this figure the air velocities are normalized by train speed, and the normalized freight train air velocities are much higher than those for the passenger train. This implies that the air flow velocities caused by freight trains at their normal operating speed (80 to 100km/h) can be higher than those caused by passenger trains moving much faster. This issue has come to prominence lately because of two incidents in the UK. In the first, at Twyford station, a disabled girl in a pushchair was sucked into the side of a hopper train, and, mercifully, simply bounced of with only a minor injury. In the second an empty pushchair was moved by the slipstream of a passing container train at Nuneaton station and hit the train before bouncing back onto the platform. There is potentially an issue here that needs to be addressed by the industry. 

Figure 6. Slipstream velocities measured for a high speed passenger train (ICE2) and a container freight train. (passenger train figure shows velocities at different distances from the side of the 270m long train; container train figure shows velocities for two different container consists for a 600m long train) 

As noted above, train slipstream velocities can be measured at full scale, with all the practical difficulties that involves. A methodology is given in the CEN standards, together with limits for allowable velocities at both  trackside and on platforms. Over recent years, it has been demonstrated that moving model tests an also be used for slipstream velocity measurements, although careful consideration of scaling effects is required. Standard RANS CFD techniques are not yet suitable for slipstream velocity assessments, because they do not capture the highly transient nature of the flow.

Ballast pickup

The problem of ballast flight beneath trains is one that has made itself felt very forcibly over recent years, with a variety of (usually unpublicised and unpublished) events occurring on high speed lines, where ballast has been lifted from the track, seemingly by aerodynamic effects, and caused considerable damage to train under bodies and tracks. The phenomenon seems to manifest itself in different ways in different countries. In parts of Europe incidents have occurred in normal weather conditions, where large quantities of quite large ballast have become airborne and caused extensive pitting of train under bodies. In particular this occurred during ICE3 tests in French and Belgium lines in 2003 and 2004, where major damage was caused to the trains, but other incidents have been reported in Italy and Spain. In other parts of France and the Far East, the problem seems to be due to ice particles falling from trains, that then displace ballast that then causes train and track damage. In the UK, the problem appears to be due to smaller ballast particles being lifted onto the track, where they are crushed by either the train that caused the ballast to lift or a following train, leading to pitting of the wheel and rail, and the need for more regular maintenance and shown below. Indeed discussion with operators in the UK suggest that the flight of ballast has been a problem for many years, with long term requirements for extra wheel maintenance through grinding, for the front and rear wheel sets, and regular observations of ballast high up in the underbody equipment during maintenance periods. But without a doubt the most severe issues occur on ballasted high-speed lines when operating around and above 300km/hr. 

The causes of ballast pickup are quite complex. Ballast experiences a number of different types of force as trains pass over – mechanical forces due to track vibration and aerodynamic pressure and friction forces. These forces all vary in different ways with train geometry speed, ballast size and packing etc., and these variations probably explain the different ways in which the phenomenon is manifested. This phenomenon has initiated a significant amount of research work around the world, particularly within Europe, through the Aerodynamics in the Open Air (AOA) and AeroTRAIN projects. The thrust of these investigations has been towards developing a method for the certification of new trains, through a standardised testing procedure, based on measuring the flow velocity beneath trains above a standardised track form.  More recent tests by the University of Birmingham and the University of Southampton on HS1 have measured mechanical effects and aerodynamic effects simultaneously during the passage of Eurostars in an attempt to understand more fully the physics behind ballast pick up. Southampton University developed the technique of using ballast with accelerometers, data loggers and batteries mounted within the ballast itself – with the data downloaded each evening via a USB port! Alongside these full scale tests, work was carried out to predict the flow beneath the train using physical models on the TRAIN Rig and advanced CFD techniques. The Eurostar model on the TRAIN Rig is shown in Figure 7. It can be seen that the train travelled along the rig upside down under a false floor, to make access easier for instruments. The physical model experiments were in excellent agreement with full-scale results and potentially offer a way for the certification of new train designs, although much further development work is required. 

Figure 7 Upside down Eurostar model on the TRAIN Rig for underbody flow measurements

High wind effects

The study of the effects of high winds on trains has a long history dating back to the risk assessment that was carried out to determine the risk of the Advanced Passenger Train blowing over on the West Coast Main Line in the late 1970s. The Rolling Stock TSI specifies that all new high speed trains have to demonstrate that they have sufficient cross wind stability (i.e. they will not overturn) and likewise the Infrastructure TSI requires that a suitable risk assessment is carried out before trains can be certified to run on specific routes. The methods for carrying out these assessments are quite complex and are set out in the TSIs and CEN standards. The risk assessment requires the exposure of routes to high winds to be assessed, and the risks of high wind gusts to be specified. 

However the effect of cross winds on railway operation is not of course confined to train design and route risk assessment. In the recent storms in Europe (early 2018), the German and Dutch railway authorities simply stopped train movement, because of worries about debris on the track or trees falling onto overhead wires – not because of worries about trains overturning, as the wind speeds were much too low for that. The same happens in the UK. When high winds are forecast Network Rail and the TOCs first impose a blanket regional 50mph speed limit, mainly so that trains have some hope of reducing speed when debris is blown onto the track. A major problem in this regard seems to be trampolines at the moment (!) and at higher predicted gust speeds of around 65mph, train operation is stopped completely. Also, very often, train movements are blocked by tree fall onto the overhead.  

Closing comments

It can be seen from the descriptions of the various issues set out above that there are a wide range of aerodynamic issues that need to be taken into account in railway infrastructure design and operation, as well as in the design of new trains and other rolling stock. One final point is worthy of mention however. It seems to the author that this range of issues is ill served by a simple split between train effects and infrastructure effects, as is, inevitably, fostered by the Rolling Stock and Infrastructure TSI. Aerodynamic effects on the railway are usually a combination of the effects of train movement around and through infrastructure – trackside structures, the track itself, tunnels and so on, and cannot easily be compartmentalized. This is perhaps what makes the subject such a fascinating one for study!

Trains in crosswinds

A blog from a previous version of this website – written in 2017

At the time this blog was written, Storm Friederike has just passed over Western Europe and has resulted in a number of deaths and considerable traffic chaos. The BBC reported that

Deutsche Bahn had already suspended rail traffic in North Rhine-Westphalia (NRW), neighbouring Rhineland-Palatinate state and Lower Saxony, when it announced a Germany-wide suspension of long-distance trains. Any regional trains still running have cut their speed because of the strong winds. 

A spokesman said it was the “right decision” due to the risk of trees falling on overhead wires and on tracks.

and

The Dutch Railways (NS) and operator ProRail said overhead power lines had been damaged by the wind, as well as some railway tracks. An alert on the NS website said that “at most, only a few trains” would run throughout the evening.

Trains do occasionally blow over. The first recorded incident was on the Leven viaductin south Cumbria in 1904 when a wooden bodied train blew over on the embankment on the approach to a viaduct. A number of other incidents have occurred around the world in recent years, the latest being in Switzerland where a video has been posted online of a train in the process of being blown over – here and here.  Clearly as accidents of this type can have potentially very severe consequences they need to be in some was taken into account by train builders and railway operators in design and practice.  

The effect of cross winds on trains (and lorries to some extent) is a research topic that has stayed with me throughout my career.  My first involvement with the issue was when I worked for BR Research in the early 1980s in looking at the effects of high winds on the Advanced Passenger Train. The issue arose again when the Channel Tunnel was opened as the very light lorry carrying vehicles were found to be at risk of blowing over in ports. Then, with the advent of high speed trains in the 1990s, considerable effort has been devoted to developing a methodology to ensure that cross wind effects are taken into account in both design and operation  – in Europe, Japan, Korea, and most recently in China.  There is broad agreement on the methodology that is to be used. It consists of three parts.

  • An assessment of the aerodynamic loads on the train – usually in the form of graphs of aerodynamic forces and moments against wind angle.
  • The use of this aerodynamic data in some sort of mathematical model of the effects of wind on the vehicle under consideration to determine a graph of wind speed that will cause and accident against vehicle speed – usually referred to as a Cross Wind Characteristic or Characteristic Wind Curve (CWC).
  • The use of this CWC together with weather, route and operating data to determine the risk that the train will blow over on the route under consideration.

The design of trains usually considers only the first two steps and the CWC that is obtained is compared with reference CWCs in the train certification process. Train operators clearly need to know the output of the third stage, so they can design suitable risk alleviation systems – eg. slowing trains down, providing protection such as wind breaks etc.  

Each of the above steps can have varying levels of complexity. 

  • The assessment of aerodynamic loads can involve physical model tests of different types – using standard low turbulence wind tunnel tests, wind tunnel tests with a simulation of atmospheric turbulence or moving model tests. Embankments and bridges may or may not be modeled. Alternatively the loads can be determined by CFD calculations, again of varying levels of complexity, from simple RANS calculations, through to complex (and resource hungry) DES and LES calculations. 
  • The calculation CWCs needs a simulation of the wind – that can either be the specification of a simple gust velocity, the specification of a spatially and temporally varying ideal gust, or the full specification of a wind time history; and also a simulation of the vehicle system – either a simple one, two or three mass model or the specification of the suspension system with varying levels of complexity. More recently some authors have even used calculations that are coupled with the track dynamics and with the dynamics of a bridge that the train passes over.
  • Finally the determination of the risk requires detailed wind statistical information that is not always easily available, together with route topographical information – embankment heights, bridge geometries etc. 

To my mind one of the most important things about this three part process, and one that is not always appreciated, is that each component has a very different level of precision. The aerodynamic forces and moments can probably be specified to within 5%. The calculation methodology for CWCs, given specific values of the forces and moments, is highly accurate (say to within 1%), whilst the calculation of risk has massive inbuilt uncertainties because of the uncertainties in the meteorological information. Thus usually the risk of a wind induced accident can only be specified to within an order of magnitude i.e. 10-8or 10-9. Thus highly accurate determinations of CWCs are really pointless when the uncertainties in the risk calculations are considered. 

Having spent the last 40 years involved with this problem to some degree or other, I would thus like to make the following reflections.

  • The different aspects of the subject – fluid mechanics, vehicle dynamics, meteorology etc. – make for a fascinating intellectual mix, and have led to the development of a range of complex modeling and analytical techniques. For an academic these challenges are fascinating – but these intellectual challenges can sometimes result in the end points of the process (train certification and risk specification) to be forgotten. I am as guilty of this as anyone of course. As an academic I can argue that my work in this area has enabled progress in other research areas, as indeed it has, but the end goal shouldn’t be forgotten.
  • The current train certification methodology in the CEN code is essentially a comparative one with CWCs for particular trains being compared with CWCs for trains that are considered safe. As such, accurate values of accident wind velocities are not required, as long as they were derived in the same way as for the reference safe vehicle. The CEN code sensibly goes down this route, and specifies a simple type of wind tunnel test to obtain the force coefficients. However it requires a full multi-body dynamic simulation with an artificial gust simulation, with a complexity that seems inconsistent with the accuracy with which the aerodynamic forces and moments can be specified. 
  • The above multi-body simulation technique can, and has in the recent past, result in game playing that has no relevance to train operation or safety – by marginally changing the suspension parameters in an arbitrary way in the dynamic calculations so that the CWC is above the reference value and thus allowing the train to be certified. There must be doubts about any methodology that allows such things to happen.
  • Taking the above considerations a little further, there is an increasing tendency in published papers in this area to include as many complications as possible – multiple degrees of freedom of train, track and (if appropriate) bridge; coupling of train movement with the aerodynamic coefficients; very high resolution (and resource usage) CFD calculations. In my view the proper way to use such techniques is to carry out studies to determine the effects of such complex methodologies on overall aerodynamic forces and CWCs (almost always second order) and then to develop a much similar methodology that allows for them in an approximate way that is consistent with the accuracy of the overall process. Just because it is possible, using modern numerical techniques, to make complex calculations, it is not always sensible, or a proper use of resources, to do so. 
  • Finally there is the effect of operation that needs to be taken into account, which brings us back to where we began. In the recent storms, the German and Dutch railway authorities simply stopped train movement, because of worries about debris on the track or trees falling onto overhead wires – not because of worries about trains overturning, as the wind speeds were much too low for that. The same happens in the UK. When high winds are forecast Network Rail and the TOCs first impose a blanket regional 50mph speed limit, mainly so that trains have some hope of reducing speed when debris is blown onto the track. A major problem in this regard seems to be trampolines at the moment – see figure 2 below – and at higher predicted gust speeds of around 65mph, train operation is stopped completely. Also, very often, train movements are blocked by tree fall onto the overhead. Operational reality takes precedence over all the wind tunnel tests, CFD calculations and MDS modeling we can conceive of doing.

The study of the effects of high winds on trains is fascinating and alluring academically, and allows the use of a whole range of fun physical model experiments and challenging computational techniques. But a sense of perspective is required I think – to keep the various methodologies simple enough for reasonably routine use in train certification and route risk assessment; and not to forget the overriding importance of train operational considerations.