The calculation of train slipstreams using Large-Eddy Simulation techniques

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1 The calculation of train slipstreams using Large-Eddy Simulation techniques Abstract Hassan Hemida, Chris Baker Birmingham Centre for Railway Research and Education, School of Civil Engineering, University of Birmingham, Birmingham, B15 2TT, UK This paper describes the results ofnumerical work to determine the flow structures of the slipstream and wake of a high speed train in an open air and on platforms with different heights using Large- Eddy Simulation (LES). The simulations were carried out on a 1/20th scale model of a simplified fivecoach train and were carried out at a Reynolds number of , based on the speed and height of the train. The simulations were performed on a fairly fine mesh consists of 18 million nodes and the LES results were validatedagainst experimental data and good agreement was obtained. A number of different flow regions were observed: upstream region, nose region, boundary layer region, intercarriage gap region, tail region and wake region. Localized velocity peaks were obtained near the nose of the train and in the near wake region. Maximum and minimum pressure values were also noticed near to the nose tip. Coherent structures were formed at the nose, roof and inter-carriage gaps of the train. These structures extended for a long distance behind the train in the far wake flow. Large turbulent intensity was found in the near wake flow. The slipstream velocity and pressure obtained from different platform heights were compared and the results showed a significant effect of the platform height on the slipstream structures. 1. Introduction The effects of train slipstreams have become of increasing concern in recent years with regards to the safety of waiting passengers on platforms and of trackside workers, the stability of pushchairs and baby carriers and the forces imposed by the transient pressures and velocities on trackside and station structures. Existing safety practices for people on platforms and staff at the trackside depend on maintaining particular safe clearances, which essentially are based on generalised pragmatic judgements. There is pressure internationally to tie them to measurable quantities. This in turn produces a need to understand and quantify the physical processes and reactions of people to slipstream disturbances. Moreover,the assessment of train slipstream behaviour is now part of the train acceptance procedure through the Technical Standards for Interoperability (TSI)process[1]. As the magnitude of aerodynamic forces broadly increases with the square of train speed, these effects can be expected to become of more significance as train speeds become higher. There are two current approaches to the measurement of train slipstreams either at full scale[2][2] or at model scale[3][5][6][7]. These approaches are fully described in [2] and[2]. These papers describe a variety of measurements reduced scale measurements made around a generic train on a moving model rig as part of an UK Research Council grant, further reduced scale measurements on the same rig made in the EU sponsored RAPIDE project, full scale measurements made in the RAPIDE project, around a variety of different ICE configurations, and full scale measurements made around UK container trains. However, since the train slipstream is a transient phenomena associated with a highly turbulent flow, a large number of realizations is needed in order to obtain adequate results for the ensemble average and standard deviations of time histories. This makes the two experimental techniques very intensive in terms of time and financial resources. For this reason, Gil et al[5] used a rotating rig with a diameter of 3.61 m to measure the slipstream velocity. The new experimental technique made it possible to measure the assemble average of the velocity at certain points along

2 the length of the train. However, the complete picture of the flow field and the coherent structures of the slipstream as well as the pressure field were still missing and out of the range of their experimental investigations. Hemida and Baker [7]numerically studied the slipstream around the model of Gil et al [5] using LES and they found out that the rotation of the model has a significant effect on the slipstream velocity and pressure. Some theoretical modelling of the nosepressure pulse caused by passing vehicles and the effect on pedestrians has beencarried out by Sanz-Andres and Santiago-Prowald[3]and, despite thesimplicity of the model used, comparison with experiments was encouraging. Gerhardt and Kramer [8]have made some measurements of the train driven pressure transients andwind movement in railway stations due to the passage of high speed trains. However, the effect of the platform on the slipstream is missing in the previous studies. Current methods for assessing train slipstream behaviour, however, require measurements to be made of slipstream velocities at trackside and on a platform. Platform height varies considerably around the world and it is not immediately clear how the slipstream velocity varies as platform height changes. Whilst in principle such information could be obtained from full scale measurements, such measurements are very difficult to carry out as multiple train passes are required to enable sufficient representative data to be obtained. To study this effect in detail, computational fluid dynamics (CFD) calculations have been carried out using the Large-Eddy Simulation (LES) techniques that can predict the unsteady flows around trains, which is not possible using the simpler Reynolds Averaged Navier- Stokes CFD methods. Calculations have been carried out to predict the flow around a simplified fivecoach ICE2 train shape for platform heights of 0.3 m, 0.6 m and 0.9 m and for the no platform configuration. These calculations demonstrate a novel method for the prediction of train slipstreams that has potential for use in the train acceptance process and will enable infrastructure managers to properly address a number of safety issues. The open source CFD solver OpenFOAM[9] is used in this work together with the meshing technique SnappyHexa mesh that is implemented in OpenFOAM v Train model and computational domain The train used in this investigation is a 1/20 scale of a simplified ICE2 train. It consists of five coaches, inter-carriage gaps and bogies with four wheels each as shown in Fig.1. For simplicity, the height of the model is denoted by H as shown in Fig.1.a. The total length of the model is 37.5H as shown in Fig.1.b. The train is running on a rail of height 0.035H. The computational domain extends 10H ahead of the train nose and 30H from the train tail to the exit of the computational domain. The roof of the computational domain is at a distance 10H from the bottom of the rail and the sides are at a distance of 10H from the centre of the train as shown in Fig.1.b. (a) (b) Figure 1 (a) Train model. (b) Computationaldomain

3 3. Numerical Details The slipstream velocity has been obtained using one of the most accurate CFD techniques; largeeddy simulation (LES). In LES, the large scale motions are resolved directly while the influences of the small scaleson the large scales are modelled. The standard Smagorinsky model is used to model the sub-grid scales in this work. This model is commonly used with LES in flow around trains [7]andgives good results compared to experiments. The open source CFD solver OpenFOAM[9] version 1.6 has been employed to solve the LES equations governing the air flow around the train. The SnappyHexMesh utility provided with the standard installation of OpenFOAMv1.6 has been used to generate the required meshes for the LES. This utility provides an automatic mesh generation with reasonable control of the mesh resolution and density in different places in the computational domain. (a) (b) (c) Figure 2 Computational mesh. (a) Mesh around the train model, (b) Mesh distribution around the inter-carriage gap and (c) Mesh on the surface of the first car and floor. To resolve the boundary layer around the train a prism layer of 10 cells has been created in a belt of thickness 1.0 mm around the model. The total number of cells in the computational domain is about 18 million nodes. About 90% of the cells are concentrated in a region that extends about two times the train height from the sides and roof of the train along the train length as most of the variations of the flow velocity is expected in this region. The region also extends all the way from the train tail to the exit section of the computational domain in order to resolve the wake flow. Figure 3 showsthe slipstream velocity, obtained from our LES, around the modelat a plane passing through the train mid height. The time-average slipstream velocity shown in Fig. 3.a and the instantaneous slipstream velocity shown in Fig. 3.b demonstrate that, at the simulation Reynolds number, the variation of the slipstream velocity extends about half the height of train distance from the side of the train. (a) (b) Figure 3 Slipstream velocity around the train model scaled with train velocity. (a) Time-averaged flow and. (b) Instantaneous flow. The train is kept stationary in the LES simulations while the air moves. The flow enters the computational domain with a uniform velocity. The turbulent intensity of the inlet air was 5% and no side winds were simulated. Periodic boundary conditions were employed on the sides of the

4 computational domain. Zero pressure boundary condition is employed at the roof of the computational domain. At the exit of the computational domain, the convective boundary condition is used to allow the convection of the wake vortices. A no-slip boundary condition combined with a damping function is used on the surface of the train. In order to simulate the relative motion of the train the ground, rail and platform were giving a velocity equal to that of the inlet air and hence no boundary layer was formed ahead of the train. All the simulations have been performed at Reynolds number 300,000 based on the height of the train and the upstream velocity. One LES simulation has been made for the flow around train on platforms of heights 30, 60 and 90cm and also around train running in the open air. In every LES simulation, the computational domain is initialized with the inlet velocity. The central difference second-order scheme is used to discretized the convective and diffusive terms in the LES equations while the Crank-Nicolson scheme is used to discretized the time derivative terms. The time step has been chosen to be low enough to maintain the Courant-Friedrichs-Lewy(CFL) number less than one in each time step. The averaging process is turned on when the flow is full turbulent around the train model. This has been guaranteed by monitoring the slipstream velocity at different points around the train. These simulations consume a great deal of computational resource (the run time is around six weeks using 80 processors on the University of Birmingham Central Computing facilities), but do enable the unsteady nature of the train slipstreams to be studied in detail. 4. Results This section gives the time-averaged and instantaneous flow of the slipstream around the train at the four different running configurations.the component of the velocity in the direction of train travel is used in the calculation of the slipstream. The slipstream is normalized using the train speed unless otherwise stated. At least 30 sec actual time is used to calculate the time-averaged flow. 4.1 Time-averaged slipstream velocity Figure 4 shows the time-averaged slipstream velocity obtained from the simulation of the flow around the train without platformin lines parallel to the length of the train at different distances from the centre of rail (COR)and at 1.2 m from the top of rail (TOR). Figure 4 Trackside: time-averaged slipstream at 1.2 m from the TORat different distances from the COR parallel to the train length.

5 Figure 5 Slipstream around a full scale using 15 vehicle [2] Figure 4 shows different regions in the slipstream velocity: the upstream region (Region I in Fig.4), the nose region (Region II in Fig.4), the boundary layer region (Region III in Fig.4), the inter-carriage gap regions (Region IV in Fig.4), the tail region (Region V in Fig.4) and the wake region (Region VI in Fig.4). The train pushes the air upstream and the effect of the speed of the train is noticed ahead of the train in a distance equal to about half wagon length. Due to the high stagnation pressure on the nose of the train, the air moves around the nose generating what is called the noseregion. This region starts with a sudden increase of the slipstream velocity followed by a sudden decrease in the air velocity and in some parts the air moves in reverse to the train travel direction. The region in which the air moves backward, however,is small close to the side of the train and extends for a long distance along the lengthaway from the sides of the train.similar results have been obtained in the slipstream around a rotating train in a previous LES work by the authors [7]and in the work of Muld et al [5]. This region is not shown in the experiment of Sterling et al[2] in figure 5 and in the work of Gil et at [6]. However, the Cobra probe used in measuring the slipstream velocity in this work is not capable of measuring reverse flows. Along the length of the train a boundary layer grows on the vehicle sides. Variations across the boundary layer are shown in Fig.4. Variations due to the inter-carriage gaps between the vehicles can also be seen. Similar behaviours are shown in the experiments in Fig.5. However, the LES slipstream is smoother than that in the experiment in Fig.5 as Fig.5 represents an ensemble average of limited number of realizations whilst Fig.4effectively represents the timeaveraged slipstream of a sufficiently large number of realizations. Our LES results, however, are comparable with the experiments in Fig.5. Figure 4 demonstrates a drop in the slipstream velocity close to the tail due to the suction pressure in the wake behind the train.this is followed by large slipstream velocity in the near wake. Figure 4 shows also that the largest slipstream velocity is in the boundary layer close to the surface of the train while the near wake region represent the largest slipstream velocity away from the surface of the train. Figure 6 Trackside: time-averaged slipstream velocity at 2 m from the CORat different heights from the TOR parallel to the train length. Figure 6 shows the slipstream velocity at three different heights from the TOR and at 2m from the COR. The slipstream velocity is large close to the TOR while its value decreasesfurther up the train side. This can be attributed to the underbodyflows. This is clearly shown in the near wake flow as the

6 slipstream velocity at 0.2m from TOR is about twice that at 1.58m from the TOR. Figure 6 demonstrates also that there is a larger increase in the slipstream velocity in the near wake flow close to the TOR than that close to the roof of the train. This is due to the two trailing vortices generated in this region. Figure 7 Platform height 30 cm: time-averaged slipstream velocity at 1.2 m from the platform at different distances from the COR parallel to the train length. Figure 8Platform height 30 cm: time-averaged slipstream velocity at 2 m from thecorat different heights from the platform parallel to the train length. Figure 9 Platform height 60 cm: time-averaged slipstream velocity at 1.2 m from the platform at different distances from the COR parallel to the train length.

7 Figure 10Platform height 90 cm: time-averaged slipstream velocity at 1.2 m from the platform at different distances from the COR parallel to the train length. Figure 11Platform height 90 cm: time-averaged slipstream velocity at 2 m from thecorat different heights from the platform parallel to the train length. Figures from 7 to 11 show the slipstream velocities around a train on platforms of heights 30, 60 and 90 cm.close to the train, there is a significantly higher slipstream velocity in standing at the platform compared to the trackside especially around the first vehicle.conversely, a significant decrease in slipstream velocity (and therefore risk) is achieved byincreasing distance from the side of the train. There is also a significant increase of the slipstream velocity by increasing the height of the platform as the largest slipstream velocity is obtained on the platform with 90cm. Figure 12 Time averaged boundary layer at half the train length and 1.2 m from the TOR in the case of trackside measurements and 1.2 m from the platform in the case of train on platform measurements.

8 Figure 13 Time-averaged boundary layer on the ground and platform at half the train length and 2 m from the COR. Figure12 shows the boundary layer on the side of the train at half of its length and 1.2 m from the TOR in case of sidetrackand 1.2m from the platform in case of a train in a platform. There is a significant increase in the slipstream velocity in the boundary layer in case of platform height 90 cm. Figure 12 shows also that at the simulation Reynolds number and at about 4 m from the COR there is no significant slipstream velocity in all cases. The slipstream velocity obtained from the simulation around platform 60 cm is comparable to that of the platform 30 cm and both are less than that of the trackside. Figure 13, however, demonstrates the variation of the slipstream velocity along the height of the train by showing the boundary layer along a line at half the length of the train and 2m away from the COR. The trackside simulation shows largest slipstream velocity along the bottom half of the train height. This is due to the underneath flow and the influences of bogies and wheels on the slipstream velocities. There is a drop in the slipstream velocity at about 1m from the TOR followed by a slight increasedue to the boundary layer on the side of the train. Figure 13 shows also that there a decrease in the slipstream velocity in the direction of the roof of the train. The same behaviour can be noticed on the slipstream velocity on the 30 cm platform height where the effect of the underneath complexitiesis shown as an increase in the slipstream velocity close to the platform. However, with increasing the height of the platform, the platform blocks the underneath flows and hence there is no significant effect of the underneath complexitieson the slipstream velocity. 4.2 Time-averaged slipstream pressure Figure 14 shows the time-averaged pressure coefficient, Cp, on a line parallel to the train length at mid height of the train and 2m from COR, where Cpis defined as: Here is the upstream pressure, the time-averaged static pressure, is the air density and is the train speed. There is a significant increase in the static pressure on the nose of the train due to the stagnation pressure in this region followed by a significant drop around the nose as the velocity of air increases in this area. The pressure along the train length and within the slipstream boundary layer is in general slightly lower that the upstream pressure except around the inter-carriage gaps where an increase in the static pressure has been notices.

9 Figure 14 Time-averaged pressure coefficient on a line at half the train height and 2m from the COR along the train length. There is also a decrease in the static pressure around the tail of the train. The pressure builds up in the circulation region behind the train and a pressure increase is shown in the near wake region. The pressure decreases to the upstream value in about two vehicles length in the wake. Figure 14 shows also that the height of the platform has negligible effect on the slipstream pressure. 4.3 Instantaneous and wake flow Although the atmospheric air around the train has low turbulence intensity in these simulations, the nose of the train and the inter-carriage gaps are generating vortices in regular fashions. Figure 15 shows the vortices around these regions using the iso-surface of the second invariant of velocity gradient technique. These vortices together with the new vortices generated on the surface of the train spread around to form the train slipstream. Figure 16 shows small turbulent structures in the slipstream by visualizing the flow using, again,the iso-surface of different values of the second invariant of velocity gradient technique. Figure 15Iso-surface of the second invariant of velocity gradient around the train nose and inter-carriage gaps Figure 16Iso-surface of the second invariant of velocity gradient around the train

10 The boundary layer separates from the train surface at the tail and low pressure region is formed in the near wake. This low pressure region forces the flow to move towards the near wake. This flow interacts with the flow underneath the train to form two strong vortices behind the train, as shown in Fig.17. Figure 17 shows also that the near wake region is dominated by a complex three-dimensional turbulent flow with larger turbulent structures compared to those in the boundary layer. Figure 17 Wake vortices Figure 18 Trackside: time-averaged rmsof slipstream velocity at 2 m from thecorat different heights from TOR parallel to the train length. The large wake vortices are highly unsteady, shed away from the train tail and fluctuate on both sides of the train. Figure 18 shows the root-mean square (rms) of the slipstream along three lines parallel to the train length at 2m from the COR and at three different heights 0.2 m, 1.2 m and 1.58 m from the TOR. After the first vehicle and along the length of the train the turbulent intensity is nearly constant. However, there is a significant increase in the turbulent intensity in the near wake. This large increase in the turbulent intensity can be attributed to the fluctuation of the large turbulent structures in the near wake.

11 5. Conclusion Large eddy simulations were made on the flow around a five coach train on platforms of different heights and without a platform. Large slipstream velocity was found on the trackside on the region from the top of rail to about one third of the train height. Similar behaviour was found in the slipstream on a platform of height 30 cm above which the platform blocks the effect of the under bodycomplexitiesin the slipstream velocity. However, large slipstream velocity was noticed on the platform with height 90 cm as the space in which the air it allowed to move was decreased. A large pressure was formed on the front of the train followed with a sharp decrease. Slightly lower pressure than the upstream pressure was obtained along the length of the train. The inter-carriage gaps tend to slightly increase the static pressure in the slipstream. A significant drop in the static pressure was obtained close to the train tail followed by a high pressure region in the near wake flow. In general, the LES results showed that the platform height has a negligible effect on the static pressure. The instantaneous flow demonstrated that the near wake flow is highly unsteady three-dimensional turbulent flow dominated by large structures and the large turbulent intensity was obtained in the near wake flow.there may be also a possibility of a further full scale comparison using data from the currently running AEROTRAIN project. Acknowledgment This work was sponsored by theepsrc Rail Research UK, project number RRUKA5. The authors would like to acknowledge the computer resources provided by the Birmingham Environment for Academic Research, BlueBEAR. References [1] TSI. Technical Specification for Interoperability of high speed rolling stock (TSI HS RST), Official Journal of the European Union, L64 of 7/3/2008. [2] Sterling M., Baker C. J., Jordan S. C. and Johnson T. A study of the slipstreams of high-speed passenger trains and freight trains. Proceedings of the Institution of Mechanical Engineers F: Journal of Rail and Rapid Transit. Vol. 222, pp , [3] Baker C. J., Dalley S. J., Johnson T., Quinn A., and Wright N. G. The slipstream and wake of a high speed train. Proceedings of the Institution of Mechanical Engineers: part F, Rail and Rapid Transit, Vol. 215, pp 83-99, [4] Sanz-Andres A, Santiago-Prowald J (2002) Train inducedpressuresonpedestrians, Journal of WindEngineering and Industrial Aerodynamics 90, [5] Muld T., Efraimsson G., Henningson D., Herbst A. and Orellano A. Detached eddy simulation and validation on the aerodynamic train model. EUROMECH COLLOQUIUM, Berlin, Germany, March 24-25, [6] Gil N., Baker C.J., Roberts C. Themeasurement of trainslipstreamcharacteristicsusing a rotating rail rig. BBAA VI International Colloquiumon Bluff BodiesAerodynamics&Applications Milano, Italy, [7] H. Hemida, N. Gil and C. Baker. Large-Eddy Simulation of Train Slipstream. J. Fluids Eng. Vol. 132, Issue 5, , doi: / , (2010) [8] Gerhardt H J, Kramer O (1998) Wind and train driven air movements in train stations, Journal of Wind Engineering and Industrial Aerodynamics [9] OpenCFD website: