The calculation of train slipstreams using Large

Challenge B: An environmentely friendly railway
The calculation of train slipstreams using Large-Eddy Simulation
techniques
Hassan Hemida, Chris Baker
Birmingham Centre for Railway Research and Education,
School of Civil Engineering, University of Birmingham,
Birmingham, B15 2TT, UK
Abstract
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 LargeEddy 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 300000, 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
Challenge B: An environmentely friendly railway
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 NavierStokes 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
v1.6.
2. 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.
(b)
(a)
Figure 1 (a) Train model. (b) Computationaldomain
Challenge B: An environmentely friendly railway
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
Challenge B: An environmentely friendly railway
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.
Challenge B: An environmentely friendly railway
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
Challenge B: An environmentely friendly railway
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.
Challenge B: An environmentely friendly railway
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.
Challenge B: An environmentely friendly railway
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.
Challenge B: An environmentely friendly railway
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
Challenge B: An environmentely friendly railway
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.
Challenge B: An environmentely friendly railway
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
[3]
[4]
[5]
[6]
[7]
[8]
[9]
passenger trains and freight trains. Proceedings of the Institution of Mechanical Engineers F:
Journal of Rail and Rapid Transit. Vol. 222, pp 177-193, 2008.
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, 2001.
Sanz-Andres A, Santiago-Prowald J (2002) “Train inducedpressuresonpedestrians”, Journal of
WindEngineering and Industrial Aerodynamics 90, 1007-1015
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, 2009.
Gil N., Baker C .J., Roberts C. Themeasurement of trainslipstreamcharacteristicsusing a rotating
rail rig. BBAA VI International Colloquiumon Bluff BodiesAerodynamics&Applications Milano,
Italy, 2008.
H. Hemida, N. Gil and C. Baker.“Large-Eddy Simulation of Train Slipstream”. J. Fluids Eng. Vol.
132, Issue 5, 051103, doi:10.1115/1.4001447, (2010)
Gerhardt H J, Kramer O (1998) “Wind and train driven air movements in train stations”, Journal
of Wind Engineering and Industrial Aerodynamics 74-76 589-597
OpenCFD website: http://www.openfoam.com/