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.

Crop lodging

In this blog post I want to introduce the work that I, together with a number of colleagues, are carrying out on the phenomenon known as crop lodging. First I guess it is actually necessary to define what the word “lodging” means. In simple terms, lodging is the failure of crops due to stem breakage or uprooting during periods of high winds and/ or heavy rainfall. I need to make the point very firmly right at the start that it has got absolutely nothing at all to do with crop circles!  It does however have significant economic consequences, with yield losses in winter wheat resulting in costs to growers of the order of £100m in the UK in a high lodging season. Some pictures of lodging are given in figure 1 below.

Figure 2. Lodging in coral crops

Our work on this issue goes back in one form or another over a period of 30 years. It all began in in 1987 when I was an academic at the University of Nottingham. After the Great Storm of that year wreaked havoc with the tree stock in the south of the country, I still remember a colleague (Andrew Dawson) putting his head around the door of my office and saying “I have an idea for a research grant….”. This led to a grant from the Science Research Council to investigate the aerodynamics of urban trees – and we thoroughly enjoyed ourselves making measurements of the mechanical and aerodynamic properties of trees on and around the University campus, evolving an experimental technique that we named tree-twanging – pulling trees with a winch and then releasing them to measure the frequency of the oscillations. One of the less successful parts of that work was the initial development of a mechanical model of trees in high winds, which tried to represent trees in engineering terms. At the time this didn’t progress very far, but a few years later, in the early 1990s, I was approached by a colleague from the University School of Agriculture at the Sutton Bonnington Campus (Prof. Keith Scott) to help with a project that was investigating the lodging of winter wheat, and in particular to help supervise the PhD research of two students – John Griffin and Pete Berry. Perhaps the most challenging part of this work, both for me and staff and students at Sutton Bonnington was the need to learn to speak the vocabulary of anther discipline. This collaboration led to me doing some serious work on analytical model development that produced a reasonably robust description of the mechanical behavior of plants, and in particular winter wheat, in high winds and heavy rainfall. 

The next phase of this work began in 1998, when we (myself and colleagues at Sutton Bonnington and ADAS) obtained a grant from the Biology and Biotechology Research Council (BBSRC) to investigate lodging of winter wheat in some detail, to identify those plant characteristics that resulted in an increase in lodging risk. This date also coincided with my move from Nottingham to the University of Birmingham.  This work involved an extensive series of field trials at ADAS to measure characteristics of plants relevant to the lodging process, and we at Birmingham were responsible for developing a model of the lodging process and for carrying out experiments to calibrate the model. By this time (Dr.) Pete Berry was working for ADAS, so the collaboration with him thus continued. The Research Fellow appointed at Birmingham for this work was Dr. Mark Sterling, who had recently graduated from there with a PhD in open channel flow. We built the lodging model on the basis of the earlier modeling work, but needed a variety of aerodynamic information to calibrate this. Normally in engineering terms, this would have been obtained through wind tunnel tests – it is however not easy to put a representative section of a wheat field into a wind tunnel. The solution was to take a wind tunnel into the field – see the picture below. This proved to be more than a little challenging, but during the course of the experiments we were able to obtain the very first video footage of lodging actually taking place – this usually occurs in high winds and heavy rain and more often than not in the middle of the night, so the use of a portable wind tunnel, difficult as it was, was actually the most straightforward way of doing this. I am told by Mark that fixing strain gauges to wheat stems in the field to measure the displacement was one of the most entertaining tasks that he has ever been faced with.

Overall the project was very successful and enabled us to learn a great deal about the mechanics of root and stem lodging, to provide solid scientific information that cut through much of the hearsay that was around in the industry at the time about lodging, and to provide robust agronomical advice for farmers for techniques to avoid lodging. The collaboration between the University and ADAS was vital in this regard.

Over the next few years, work continued at a lower level, with the production of a few collaborative review papers, and the application of the lodging model to barley. However by the start of the current decade it was becoming clear that the model as it stood, whilst perfectly acceptable for wheat crops where the plants were essentially isolated throughout the growing season, was not really applicable to a range of crops for which, late in their growing season, individual plants interlocked to produce a much denser canopy. Thus we (myself, Pete Berry and Mark Sterling, by now the Head of Civil Engineering at Birmingham and thus my boss) began work on the development of a generalized lodging model that could allow for plant interlocking. Whilst the modeling was quite complex, it resulted in the relatively simple pictorial representation shown below in figure 2, where regions of stem lodging and root lodging were defined in terms of the daily rainfall rate and the hourly wind speed. The various velocities and rainfalls shown on this figure are all (rather complex) functions of plant and soil parameters and can, once the model is calibrated, be fairly easily specified. In principle this graph can be used with a representation of wind and rainfall probabilities to determine the risk of lodging occurring for any set of plant and soil parameters, and mitigation methods taken if this risk is deemed to be too high. In the peer review process, one of the reviewers of the paper acknowledged the elegance of the model, but made the comment that it would never find a practical outcome. We were to prove him wrong!

Figure 2. Lodging regions in the rainfall / windspeed plane

Over the last few years the work on lodging has grown very significantly, and we now have three projects underway. The first was funded by Teagasc in Ireland, to investigate methods to reduce oat lodging. We used the model described above and the work included a series of experiments in Ireland to measure, the behavior of oats in high winds.. The second project was funded by BBSRC under the SARIC (Sustainable Agriculture Research and Innovation Club) scheme, with myself and Mark, working again with Pete at ADAS. This used the same set of techniques to investigate the lodging of Oil Seed Rape. The unique aspect of this project however was a collaboration with Dr. Alan Blackburn and his colleagues at the University of Lancaster who are experts in Earth Observation and Remote Sensing, and the local modeling of lodging is being embedded in a much wider scheme to integrate spatial, topographic and meteorological data sets to predict the risk of lodging for individual crops and fields, and to identify those soil, plant and weather characteristics that cause lodging. The final project was also funded by BBSRC, but this time through the Global Challenges Research Fund which directs research funds to the problems of developing countries. We used a similar approach to the SARIC project, but this time directed towards maize and rice, working again with ADAS and Lancaster University, and also with colleagues at the Chinese Agricultural University and with CIMYTT in Mexico, who work in a large range of countries in the developing world.  The potential significance of this project is huge – lodging causes yield losses of up to 40% in rice and maize, reduces grain quality, increases time to harvest, increases grain drying costs and increases health damaging micro-organisms on grain. It is estimated that lodging in rice and maize reduces crop production in China and Mexico alone by $1500 million per year. 

All this research has all developed from a chance conversation and some early blue-sky research on trees over 30 years ago – and now has the possibility of producing results that will have a major effect on crop productivity around the world. In these days when funding for such fundamental research is under increasing pressure, this is perhaps worth remembering. But for now, these are exciting times – watch this space for future updates. 

Scruton Lecture 2017

In 2017 I delivered the UK Wind Engineering Society’s bi-annual Scruton Lecture, which was instituted in 1991 in honour of Kit Scruton of the National Physical Laboratory, who carried out significant work on dynamic structures. My lecture was entitled Wind Engineering for Serviceability and Resilience and described in rather a broad way my wind engineering work in this area over the previous three decades. The slides can be found here. Note that the file is very large and may take some time to download.