**Preamble**

In January 2020 and January 2021, I posted quite lengthy blog posts that attempted to collate all the published papers in train aerodynamics over the previous year – see here for the 2020 post and here and here for the two part 2021 post. . These were intended as supplements to the book “Train Aerodynamics – Fundamentals and Principles” published in 2019. These blog posts have been quite widely read. At the time of writing (mid-January 2022) the 2020 post has had 190 views and the two parts of the 2021 post 129 and 70 views. It had been my intention to do something similar for the papers published in 2021. However, I have changed my mind on this, and instead will take a different approach in this post. My reasons for this are twofold.

- The number of papers in the field continues to proliferate and, quite frankly, many of them are of poor quality. This seems to be driven by the need, in some jurisdictions, for research students to publish papers in order to be awarded a PhD. This inevitably encourages a low standard of output. Also, I have noticed an increasingly disturbing trend, whereby when a paper is rejected by one of the higher quality journals, it is submitted in much the same form to other journals with less impact. I have seen a number of such papers sent to me to review by different journals – and on two occasions in 2021 I have been sent the same paper by three journals. Obviously I have little influence on how researchers submit papers, other than through the normal reviewing process, but there seems to me no reason to give such papers the benefit of a mention in any comprehensive annual compilation.
- The use of CFD techniques in train aerodynamics, which is proliferating at the same rate as the number of papers, is giving me increasing concern. CFD techniques ranging from RANS to LES are exceptionally useful tools in all fluid dynamics research and the same applies in the train aerodynamics field. But they are as much tools as any physical model tests and need to be used and interpreted very carefully. There are many investigators who do just that, including colleagues in my own institution. However, I fear that that is not always the case Specifically, the use of such techniques is in many circumstances becoming divorced from practical reality. There is a tendency to apply quite high level, but inflexible, CFD methods (such as IDDES) to look at quite trivial problems where much simpler methods could have given equivalent answers over a wider parameter range. And in the consideration of the results from these calculations, there is often little appreciation of the uncertainty that is attached both to the CFD results themselves (for example I have seen the percentage changes in predicted drag given to two decimal places) or in relation to full scale reality, where the uncertainties are multiplied by an order of magnitude or more. Further the results of the CFD calculations are often massively over-analysed. For example, in studies of cross wind effects on trains, I have come across papers where the predicted wake systems are analysed in very great detail, with little realisation that any such systems cannot exist in reality due to the (unsimulated) large scale turbulence in the approach flow field – as of course is the case with many wind tunnel tests. The same can be said of the analysis of many other applications. Again, there is little I can do to influence these trends, but I see no reason to publicise such work any further in blog posts.

In the light of such developments, in this post I will not present a comprehensive compilation of all the train aerodynamics papers from 2021 but will rather choose a much smaller number (ten in total) which I believe are of particular significance and likely to influence the field in the future. These are spread across the range of train aerodynamic applications including train drag studies, trains in tunnels, crosswind effects and emerging issues. The choice of what to include is of course to some degree subjective and mirrors my own interests, but I hope that readers find it of interest.

**Train drag studies**

*On the influence of Reynolds number and ground conditions on the scaling of the aerodynamic drag of trains**. Tschepe et al (2021)*

Very often the effect of Reynolds number on train drag measurements or calculations is broadly ignored provided that the Reynolds number is “high enough”. This is of course not adequate, as the skin friction component of drag must vary with Reynolds number throughout the parameter range – see for example my historically rather crude analysis of the problem from 1991. This paper, drawn from the doctoral work of Tschepe) presents the results of a thorough experimental and analytical investigation into this effect, using the results from water towing tank experiments. These experiments are quite novel and deserving of attention in their own right. The three-dimensional nature of the train boundary layer is clear, and the effect of ground roughness (ie sleepers and ballast) is shown to be of some importance (see also my blog post here). A simple analytical approach, based on flat plate theory, allows a correction method to be developed for extrapolating low Reynolds number results to full scale conditions.

*A field study on the aerodynamics of freight* tra*ins Quazi ei al (2021)*

This paper presents the results of full-scale measurements of the pressure drag of a freight container during a typical journey. As such it provides a basic benchmark for further studies. The technique is of interest in its own right, but the basic result, that, despite the container not having other containers immediately in front and behind it, the drag coefficient is much lower than that found in other full-scale, physical model and numerical calculations is of considerable interest. The authors suggest that this is because of the container position much further down the train than in other measurements, as well as other modelling issues. The results perhaps give pause for thought about the measurement of train drag from wind tunnel tests or CFD calculations.

**Tunnel aerodynamics**

*Influence of air chambers on wavefront steepening in ra*ilw*ay tunnels. Liu et al (2021)*

The phenomenon of micro-pressure waves (sonic booms) emitted from tunnel portals has been much studied in recent years. These are caused by the steepening of the train nose pressure wave as it passes along the tunnel, resulting in a steep wave at the tunnel exit that is not wholly reflected with some energy being transmitted out of the tunnel in the audible frequency range. The standard method for the amelioration of such effects is through the use of tapering tunnel entry portals, that reduce the initial (and thus the final) steepness of the waves. Such portals can be quite long and extend some way out of the tunnel, and indeed can be quite expensive. This paper investigates an alternative to such portals – the distribution of air chambers along the length of the tunnel that in principle reduces the steepening of the pressure wave. Using a relatively straight forward gas dynamics analytical model, the authors show that suitably designed chambers can remove the dependence of the exit wave on the steepness of the inlet wave. Guidance is given for appropriate chamber volumes and the resistance of the connectors between the chambers and the tunnel. Overall, the method has much potential for future tunnel design.

*Virtual homologation of high-speed trains in railway tunnels: A new iterative numerical approach for train-tunnel pressure signature. Brambilla et al (2021)*

The standard methodology to investigate the passage of pressure waves along tunnels is to use full-scale measurements to measure the pressure wave system on train entry, and then to use data from those measurements to predict the pressure wave along the length of the tunnel using one dimensional gas dynamics methods. The latter can be run many thousands of times to investigate a range of operational conditions. Clearly the required full-scale tests are expensive and complex. Recently some full CFD calculations of the flow along tunnels have been published using sliding grids, which are again highly complex and computer resource requirements limits their use to just one or two conditions. This paper presents a combined methodology where CFD calculations using a standard fixed grid are carried out to measure the pressure characteristics at train inlet to the tunnel, and these are then used in one dimensional methods. The methodology has been validated against an extensive full scale data set. Its relative cheapness and flexibility means that it has the potential to become widely used within the industry.

*Semi-empirical model of internal pressure for a high-speed train under the excitation of tunnel pressure waves. Chen et al (2021)*

This paper looks in detail at the development of internal pressures within train cabins in tunnels. Using a combination of commercial CFD and finite element analysis, together with simple models of internal ventilation flow, the authors looked at pressure changes due to body deformation, pressure transmission through gaps in the train envelope and transmission through the air ducts of HVAC systems. Body deformation has little effect (unsurprisingly in my view) with the balance between gap and duct transmission varying depending on the degree of opening of the latter. Whilst the analysis is complex, the results should be of interest in describing a methodology that could ultimately be applied quite straightforwardly in design.

*Pressure fluctuation and a micro-pressure wave in a high-speed railway tunnel with large branch shaft. Okubo et al (2021)*

This paper describes an extensive experimental programme using a moving model facility that looked at the micro-pressure waves that occur as a result of the junction between the main tunnel and large branch tunnels with similar diameter (which would be used for passenger evacuation). The results are skillfully interpreted through the use of analytical models and show that in some instances the micro pressure wave emitted from the branch tunnel can be of greater magnitude than that omitted from the main tunnel. Both the physical and analytical modelling methodology have potential use for the design of complex branching tunnel systems.

**Trains in crosswinds**

*Influence of the railway vehicle properties in the running safety against crosswinds. Heleno et al (2021)*

I include this paper with some temerity, as I am named as an author – albeit the last one. However, my role was very minor, and mainly involved discussions on some technical details and proof reading the final draft (although they all contribute to my long term aim of getting to 200 journal publications before my demise!). This paper considers the effect of various railway vehicle properties on the overturning risk of a rail vehicle. It uses realistic vehicle dynamic and track roughness models and generates realistic time series of wind speed from wind statistical parameters. It is more rigorous in its modelling than the current method used in the CEN code, which uses a very simplified wind gust model. A thorough parametric analysis of the various vehicle parameters is carried out. In my view the major point to emerge is the lack of sensitivity of the calculated overturning wind speeds and safety risk to variations in the train suspension parameters. In principle this could lead to much simpler models for the CEN safety assessment than are used at present, where full dynamic modelling is required. This is personally satisfying as I have been arguing this very point for the last 10 to 15 years – see the discussion in this post from 2020.

**Emerging issues**

*CWE study of wind flow around railways: Effects of embankment and track system on sand sedimentation. Horvat et al (2021)*

I include this paper because it contributes to what I believe to be an important emerging issue as railways are developed in arid conditions – sand sedimentation over railway tracks. It is a straightforward CFD study of flow patterns over different railway track geometries that calculates wall shear stresses and used these to define potential regions of erosion and sedimentation. It lays the foundation for future work – possibly to integrate sediment modelling into the CFD calculations.

*Investigation on flow field structure and aerodynamic load in vacuum tube transportation system. Zhong et al (2021)*

This paper is a detailed CFD analysis of the flow around vacuum tube vehicles using IDDES techniques. Because of the enclosed nature of the vehicles and the well-defined geometry, this is a case where one would expect good accuracy from such calculations. Also of course the issues cannot be easily addressed by physical modelling techniques. Both subsonic and supersonic flows are considered, the nature of the flow field elucidated, and vehicle drag calculated. The results form a useful addition to the publicly available body of knowledge about the flows around such vehicles that can be used in further development of the concept. That being said, it is my firm view that, fascinating as the aerodynamics of the system might be, vacuum tube systems will not meet with wide adoption due to simple operational constraints – primarily the low capacity in comparison to conventional high speed rail systems.

**General**

*Railway applications – Aerodynamics – Part 7: Fundamentals for test procedures for train-induced ballast projection. CEN (2021)*

This is not a paper, but rather the latest offering from the CEN working group on Aerodynamics that looks at the issue of ballast flight beneath high-speed trains. It contains a wealth of information of the issues involved, economic aspects of the damage caused by ballast flight, current national practices and possible ways forward in terms of homologation. It is well put together and forms a very useful basis for further work in the field.

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