Recent Studies of Train Slipstreams
T. Johnson, S. Dalley, and J. Temple
AEA Technology Rail, Derby, England
1 Introduction
When a train travels in the open air it displaces the air around and over it
forming a slipstream alongside the train and a wake behind. The air at the
surface of the train moves at the speed of the train, whilst far from the train
the air moves at the ambient air speed. Therefore, there is a region near the
train sides where the air can be moving at speeds comparable to that of the
train. In this region the air is very turbulent and, depending on the aerodynamic roughness of the train, may contain complex and interacting vortices.
After the train passes there is a wake flow, which decays as the train moves
away.
The turbulent and highly chaotic air flows in the train’s slipstream and wake
impact on people and objects near the passing train. The adverse effects increase with the aerodynamic roughness and speed of the train. Certain trains
can create airflows sufficiently large that they pose a danger to people either
working at the trackside or to passengers waiting on station platforms.
With increasing freight and passenger train speeds in Europe, there has been
a strong interest in understanding the nature of slipstreams with a view to
minimising the potential danger and producing common European standards
to control the risk. Recent studies of high-speed train slipstreams have been
carried out in a European collaboration and are briefly reviewed in this paper.
2 Slipstream Generation and Effects
The mainly longitudinal airflows created by the train slipstream can produce
hazardous aerodynamic effects at the trackside, both on trackside workers and
their equipment as well as on passengers with their belongings on station
platforms. Indeed, there have been a number of incidents caused by train slipstreams on station platforms in Britain since 1977:
• A luggage barrow was drawn by the train slipstream and hit the train side
before rebounding across the platform,
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• Children’s pushchairs, usually with luggage in and the brakes on, have been
drawn into the trains’ slipstreams and set into motion or destroyed by the
train.
• Three people have been almost knocked off their feet by passing trains.
The train slipstream is generated by the viscous shearing effect of the train
moving through the air. Gust components and turbulence are created by surface roughness elements and discontinuities in the train surface, such as exposed bogies, intercar gaps and gaps between containers. Aerodynamically
rough trains such as freight trains, especially freightliners and open car carriers,
generate very strong slipstream flows, which peak as the train is passing. For
streamlined trains the peak air speeds occur in the wake of the train after it has
passed.
Fig. 1 shows typical measured averaged resultant slipstream air speeds
caused by a German ICE high-speed passenger train passing a station platform.
The air speeds have been normalised by the train speed and the time base has
been adjusted to be a distance base by multiplying by train speed. Measurements were made using anemometers measuring the longitudinal and lateral
horizontal components of the air velocity at a height of 1.335 m above the
platform and at 0.5 m, 1.0 m and 1.5 m from the platform edge.
At each position, the resultant air speeds show similar characteristics:
• A sudden peak and subsequent fall in speed as the train nose passes (at about
50 m).
• A maximum value just after the tail of the train passes (just after 400 m).
• A steady rise in mean air speed between the nose and tail passing.
• A gradual decay in air speed after the train has passed for a distance similar to
that of the train length.
0.3
Normalized Velocity
0.25
Platform 1.5 m
from edge
0.2
Platform 1.0 m
from edge
0.15
Platform 0.5 m
from edge
0.1
0.05
0
0
100
200
300
400
500
600
700
800
Distance (m)
Fig. 1. Comparison of mean slipstream velocities on a platform at a height of 1.335 m
and at different locations from the platform edge
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417
0.6
Non-dimensionalised velocity
0.5
95th percentile
0.4
0.3
mean
0.2
0.1
5th percentile
0
300
400
500
600
700
800
Distance, m
900
1000
1100
1200
Fig. 2. Mean, 5th and 95th percentile slipstream velocities from full-scale tests
However, at the measurement position closest to the train, the peaks associated with the nose and tail passing are significantly greater than at the other
two positions. Also, the air speed as the train passes is much higher and exhibits larger fluctuations at the closest measurement position. The larger
fluctuations have been found to correlate with the passing of the coaches of the
train.
Fig. 2 shows the spread of the data around the mean slipstream history for a
typical series of data, consisting of 10 train passes and taken with very low ambient wind speeds. This demonstrates the wide spread of the data even when
ambient conditions are favourable.
The main factors influencing slipstream strengths are the train speed; the
distance away from the side of the train; the shape and surface finish of the
train and the ambient wind speed and direction. It has been observed in tests
that strong cross-winds significantly enhance slipstream effects on the lee-side
of the train.
It is worth noting that improvements in the design of trains from the point
of view of aerodynamic drag are usually also beneficial in reducing slipstream
effects.
Finally, a train slipstream is a truly unsteady and almost chaotic phenomenon when viewed from a stationary viewpoint. This has sometimes led to
confusing and contradictory results being obtained from full-scale test measurements in the past.
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T. Johnson, S. Dalley, and J. Temple
3 Safety Standards
Despite the concerns held by the European railway operators, there is currently
no single safety standard for train slipstreams in Europe. However, the European standardisation body, CEN, will shortly be working to determine one.
Nevertheless, each railway company has its own rules for controlling the risk
based on notional limit values. For instance, in the UK a maximum peak slipstream value of 17 m/s has been applied in the past, although more recently a
comparative air speed criterion has been applied on the basis that existing operations are ‘safe’. In France, the French national railways apply limits to the
maximum force exerted by a train’s slipstream on a cylindrical instrument; the
limit value being derived by comparison with exerting traffic. The German
railways also apply limits to maximum slipstream speeds using existing operations to determine limit values.
The risk from the slipstreams is then controlled by applying safe standing
distances for people near the track, indicated by a yellow line on the platform
for passengers; limiting train speeds or banning people from passing high
speed trains altogether. Safe standing distances vary from one European
country to another.
What is puzzling is that, although the physics of slipstreams must be invariant, there is no international consensus about the most important characteristics of the turbulent slipstream and wake as they affect people. This has led to
different characteristics being measured, i.e. air speeds or air forces on a representative body, rather different limit values being adopted and differing measures to control the risk.
There are several possible reasons for this lack of consensus.
1. There is a genuine uncertainty about which physical quantity is the most
important for the stability of people.
2. The use of limit values determined from the assumption that the current
situation is safe will highlight differences between national railways e.g. operating practices and train speeds, existing control measures, train patterns
etc.
3. Ensuring the safety of people from train slipstreams is not entirely an aerodynamics issue as there is a strong element of subjective response involved,
with other factors being involved such as the person’s perception of danger,
their preparedness for the event among several.
It is for the reasons discussed above the recent studies of train slipstreams
were made at a European level.
4 The RAPIDE Project
The RAPIDE Project (Schulte-Werning et al 1999) was a three year EC cofunded project under the Brite-Euram 3rd Framework with the following partners:
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419
• Deutsche Bahn AG, the German national railway company.
• Société Nationale des Chemins de Fer Français, the French national railway
company.
• Trenitalia, the former Italian national railway company.
• AEA Technology Rail, a British railway consultancy company.
• Motor Industries Research Association, a British motor research and development company.
• RUAG, a Swiss aircraft and systems company.
• Bombardier, one of the world’s leading railway manufacturers.
Part of the project programme of studies consisted of investigating the slipstreams of high-speed passenger trains and consisted of a series of tests at fullscale in Germany, complementary model scale measurements and CFD studies.
4.1 Full Scale Tests - HST
Full-scale tests were carried out in Germany in 2000 with German ICE highspeed passenger trains as part of the RAPIDE project. A variety of slipstream
measurements were made by the trackside and on a station. The along-track
and across-track components of the slipstream velocity were measured at a
number of lateral positions using ERA gust anemometers, (see Fig. 3). Other
measurements were made simultaneously: the aerodynamic forces on a cylinder (see Fig. 4) and on instrumented shop mannequins; the ambient conditions, temperature, pressure and wind speed. During the tests, data was collected for a number of passes of an ICE 2 test train and for normal service ICE
trains.
The analysis of the velocity data is described later in the paper.
Fig. 3. ERA gust anemometer array at the trackside during full-scale tests
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T. Johnson, S. Dalley, and J. Temple
Fig. 4. Force measurement cylinder at trackside. (Note the height of the ballast shoulder)
4.2 Model Scale Tests
Prior to the full-scale tests described in the last section, an extensive series of
model scale tests were carried out to systematically investigate the slipstream of
a high-speed train. The facility used was the AEA Technology Rail Moving
Model Rig (MMR).
4.2.1 The MMR
The MMR consists of two 150 m long tracks, along which train models can be
fired at full-scale train speeds. About a third of the track length is required for
the acceleration of the models, the central third comprises the test section and
the final third is needed to decelerate the trains. Fig.5 shows the model ICE
train on the MMR test section.
The MMR was originally conceived as a facility to study train-generated
pressures in tunnels and to ensure that the pressure waves are correctly phased
with the train movement, it is necessary to ensure the train Mach number
similarity with full-scale.
The power for the models is supplied by bungee rubbers using a catapult
principle and train speeds up to 270 km/h can be easily achieved. However,
gearing is necessary to prevent over-rapid acceleration and deceleration, which
potentially could damage the models. The train speed is nearly constant along
the test section if no tunnel is mounted. Braking is achieved by the model
picking up a link to a piston that is drawn into a deformable tube and the
model kinetic energy is thereby dissipated.
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Fig. 5. ICE train model on the AEATR Moving Model Rig
The MMR has been used extensively for determining train aerodynamic
characteristics in tunnels and in the open air (pressures). Pressure measurements in both types of tests have been validated against full-scale data.
Further information about the MMR can be found in Johnson and Dalley
1999.
4.2.2 Test Programme
A programme of studies was carried out on the MMR. A 1/25th scale model of
the leading and trailing end vehicles plus two intermediate vehicles of an ICE
train model was fired along the MMR. A large number of runs were undertaken with two nominal train speeds of 110 km/h and 180 km/h. Eight hot
film anemometers were used in a trackside rake, extending from 5-80 mm
from the train surface, to measure the slipstream velocity along the sides and
over the roof of the train. Both lateral and longitudinal components of velocity
were measured. The data was collected using 4000 Hz sampling with filtering
at 1000 Hz.
4.2.3 Data Analysis
Preliminary analysis of the data showed (Baker et al 2001) that it is necessary
to treat the time histories of slipstream velocity in a statistical manner. Even
when the ambient conditions were the same and the train speeds very similar,
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it was found that there was a large variability between runs in the time histories
of the velocity components at each measurement position. It was decided,
therefore, to ensemble-average the results.
The runs were sorted according to train speed, with two target values, and
the data grouped when the speed varied within +2 m/s of the group mean
speed. The time base was converted a distance base by multiplying by train
speed, and the slipstream velocity magnitudes formed and nondimensionalised by the train speed. The time series data was then aligned to
event markers, particularly the train nose passing. This enabled the data to be
meaningfully averaged at each measurement position.
Following detailed analysis, it was found that 10 runs were sufficient to
form stable ensemble averages of means, but at least 20 runs were needed to
form stable ensemble averages of standard deviations.
This result is important and explains some of the non-repeatability between
results observed in previous full-scale tests obtained with the few measurements that were practicable. This seems to imply that the slipstream is a chaotic process requiring a statistical description. This point will be returned to
later.
Nose - normalised velocities
Upstream - normalised velocities
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
-2
-1.5
-1
-0.5
0
0
0
0.05
0.1
t
0.15
0.2
0.25
t
Boundary layer - normalised velocities
Near wake - normalised velocities
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
0
1
2
t
3
4
0
2
4
6
8
10
t
Fig. 6. Ensemble-averaged measurements of the train model slipstream velocities. (Distances
from train side: dark line 5 mm, medium line 20 mm, light line 60 mm)
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Fig. 6 shows the ensemble-averaged slipstream velocities as the train passes.
At non-dimensional times t=0 and t=4, the nose and tail of the model respectively pass the measurement position. Measurements at 5 mm, 20 mm and
60 mm from the side of the train are shown. The measurement made at 5 mm
from the train side is within the train boundary layer.
It was concluded that:
1. Five flow regions describe the train slipstream: an upstream region, a nose
region, a boundary layer region as the train passes, a near wake region just
after the tail passes and a far wake region.
2. The effect of model speed is small if the results are suitably normalised.
3. Ensemble averaging of the velocity data is required to achieve some order to
the data (as stated above).
4. The train boundary layer reaches equilibrium (ie the ratio of the momentum and displacement thicknesses is constant) along the first carriage then
grows steadily after that.
4.3 Comparison of Full Scale and Model Scale Tests
The analysis of the full-scale data followed the methodology developed during
the analysis of the measurements from the model scale tests. The velocity data
was re-based to a distance rather than a time base and non-dimensionalised by
train speed. The full-scale test train speeds were approximately 280 km/h for
ICE2 test train and 250 km/h for the ICE service trains. The velocities were
ensemble-averaged, although there were only 9 suitable runs with the service
trains and 7 for the ICE2 test train, and the mean slipstream resultant velocities determined. The data from the MMR tests had also to be adjusted for
model scale.
One further process was necessary before comparison of the data from the
model and full-scale tests. The ICE train is 364 m long; the ICE2 train is
205 m long whilst the model was approximately 100 m long (full-scale value).
Therefore, it was necessary to cut the various velocity traces in order to align
the tail passing events. The comparison is shown in Fig. 7 for trackside measurements at approximately 1.5 m from the nearest rail. This shows reasonable
agreement in the magnitude of the velocity pulse produced by the passing of
the train nose.
The magnitude of the velocity pulse caused by the passing of the model
train tail is significantly larger that that measured at full scale for the service
trains, but is slightly smaller than that for the test train.
Between the passing of the train nose and tail there is a significant difference
in the velocity magnitudes measured on the model and those measured at fullscale. At full-scale, the velocity magnitudes climb steadily after the nose passes,
generally being above 10% of train speed. For the model measurements, however, it looks as if the velocity magnitude will remain well below 10% of train
speed after the nose passes. This is ascribed to the boundary layer around the
model train being thinner than that at full scale.
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T. Johnson, S. Dalley, and J. Temple
At the surface of the train the air will move at train speed and will reduce
with lateral distance from the train side. The train boundary layer thickness
may be defined by the lateral distance from the side of the train at which the
air velocity is 1% of train speed. In practice, it is the region of fast moving air
close to the train side which is on most interest for the safety of people at
trackside. For the MMR it appears as if this region of fast moving air is thinner than at full-scale. It is not clear if this is a Reynolds’ number effect, an artefact of the reduced train length used at model scale or caused because the
high track ballast shoulder at the full-scale test site was not simulated at model
scale.
Mean normalised slipstream velocity
0.35
0.30
Service ICE trains
0.25
ICE2 test train
0.20
0.15
0.10
0.05
MMR data
0.00
0
200
400
600
800
1000
Distance (m)
Fig. 7. Comparison of model slipstream measurements with full-scale results for the ICE service
trains and the ICE2 test train.
The slipstream velocities in the wake initially compare favourably, with the
decay well captured. However, in the far wake region there is a poorer comparison. This has been attributed to the use of a linear calibration for the hot
film sensors, which introduces an increasing error as velocities become smaller.
It was concluded that model tests are useful for examining train slipstreams,
but, until the discussed discrepancies are satisfactorily resolved, they should be
conservatively used for making comparative studies, eg freight trains with and
without streamlining devices, rather than absolute measurements.
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4.4 CFD Calculations
Deutsche Bahn carried out a transient analysis of the flow around the end vehicle and in the wake of an ICE train. Of interest originally, was the possibility
of aerodynamically induced oscillations of the train end-car when travelling in
tunnels.
Fig. 8. Transient flow around the rear section of an ICE2. The approximate time step between
frames is 0.15 seconds
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T. Johnson, S. Dalley, and J. Temple
This phenomenon has been observed in Japan and has led to complaints
from passengers of a type of motion sickness. For this application, however,
CFD was used to provide some insight into the transient nature of the slipstream and wake behind a high-speed passenger train.
The method used was a fully three-dimensional transient RANS approach,
with a RNG k~_ turbulence model. The method was embodied in the
FLUENT 5.4 CFD software.
The transient flow around a horizontal section through the ICE was examined. Fig. 8 shows a series of instantaneous solutions from the CFD simulation, depicting the flow pathlines. The periodic nature of the can be observed
in the pathline patterns.
It was observed by inspecting the evolution of the pathlines from the train
frame of reference, that the flow behind the train has a periodic nature and
vortices are shed from alternate sides of the train with a frequency of about
1.4 Hz. This means that when the train passes a single velocity measurement
device, it may see only one of a number of different velocity time-history possibilities. From the train frame of reference the predicted slipstream behaviour
is more orderly than appears from a fixed passed point. This goes some way to
explaining why such variability has been observed in slipstream velocity data
and why a statistical approach was necessary to analyse the data. It also implies
that the use of many along-track measurement positions, rather than multiple
train passes may be a practical way of capturing the data in future with the
consequent reduction in costs.
4.5 Human Response Modelling
It was stated in section 3 that the issue of people’s safety from slipstream effects is complicated by subjective response. Nevertheless, a model of human
response to the aerodynamic effects alone is useful to begin with. A simple
mechanical model was developed within the RAPIDE project which provides
some insights.
There is a wide body of published research into the effects of wind on people, primarily associated with wind speeds around the built environment (e.g.
Peters 1999, Poulton et al. 1975, Soligo et al. 1998).
Fukuchi (1961) examined human response to winds and conjectured that
muscular response begins after about 0.25 s following the application of an instantaneous force. After a further 0.125 s, 64% of muscular response is
achieved. After this it is assumed here that subjective response will begin,
which cannot be predicted in a mechanical way.
Thus, the model proposed in RAPIDE assumed that in the period for about
1/3 of a second following the application of a wind force, humans respond
mechanically ie like an object.
Fig. 9 shows an idealised body with an applied aerodynamic force, F(t),
generated by a wind. By applying conservation of energy, it is possible to relate the angular acceleration of the body to the angle of tilt, q(t):
Recent Studies of Train Slipstreams
I
2
d q
2 = F(t)( bcosq + 2asinq ) - mg(acosq - bsinq )
dt
427
(1)
where:
I is the body moment of inertia about the line of rotation
F(t) is the time-dependent aerodynamic force due to the wind load
a is the body half-width
b is the body half-height
m is the body mass
g is the acceleration due to gravity
t is time
The aerodynamic force is related to the wind speed Vw(t) in the usual way:
F(t) =
1
2
2
rAC DVw (t)
(2)
where r is the density of air, A is the area of the body facing the wind and CD
is the aerodynamic drag coefficient.
2a
F(t)
2b
mg
mg
Time t = 0
q(t)
Time t
Fig. 9. Idealised human body rotating under the influence of an aerodynamic force.
Using values of a, b and CD appropriate for human beings, a solution to (1)
can be obtained using a simple time-stepping method. An idealised slipstream
gust consisting of a mean wind speed with a sinusoidal pulse superimposed on
it is shown in Fig. 10. The amplitude and width of this pulse, (hence the gust
frequency), can be varied and used as an input to the model.
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T. Johnson, S. Dalley, and J. Temple
Wind speed, m/s
20
15
10
5
0
0
0.1
0.2
0.3
Time, s
Fig. 10. Wind pulse superimposed on a 5 m/s mean wind speed.
‘Falling down’ is defined as occurring when the body tilts to the angle at
which the mass moment acts to continue the rotation rather than resisting it
within 0.35 s. An examination was then made of when falling down occurs
with different gust amplitudes and pulse widths. Such an examination is
shown in Fig. 11 for an object with the characteristics of a typical man of
85 kg weight.
This example shows three curves corresponding to the falling down wind
speeds associated with three values of mean wind speed on which the wind
pulses have been superimposed. It shows how the combined wind pulse and
mean wind speed needed to cause falling down increases as the pulse width becomes smaller (effectively as its frequency increase), as might be expected. The
effect of the mean wind is to reduce the total wind speed needed to cause falling down, as it provides a pre-load to the body. However, the relative effect of
the pre-load is not large unless the pulse frequency is high.
This model appears capable of providing useful insights into the aerodynamic effects on people before they have a chance to respond, and it will be
used to compare with published results for human stability in natural winds
and in the building environment.
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29
Wind Speed. /(m/s)
27
25
23
21
19
17
15
0
0.1
0.2
0.3
0.4
Pulse Width, /s
Fig. 11. Wind speeds necessary to cause falling down for a man of weight 85 kg for different
wind pulse widths and mean wind speeds. (Heavy line, mean wind=0 m/s; chained line, mean
wind=5 m/s, dotted line, mean wind=10 m/s)
5 Conclusions
A brief review has been made of recent studies undertaken on the slipstreams
of high-speed passenger trains. This work was carried out in the European research project RAPIDE. The studies included:
• Full-scale test measurements of slipstream velocities generated by German
ICE trains.
• A model scale investigation of slipstream velocities at the side and above the
roof of a model ICE.
• CFD modelling of the flow around the rear of an ICE.
• Estimating the wind speeds necessary to make humans fall over before they
have time to react using a simple mathematical model.
The model tests showed that a stochastic approach is necessary for slipstream analysis. Single measurements of slipstream velocity give only one possibility, as the slipstream is highly transient and appears chaotic from the
ground frame of reference. It is necessary to have at least ten ensembleaveraged runs to obtain stable mean values of velocity, and about twenty for
stable standard deviations. This may be achieved either by having either multiple runs of the train and/or having multiple simultaneous measurements for a
single train pass.
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T. Johnson, S. Dalley, and J. Temple
Model tests may offer a cheaper approach to full scale testing, but some issues related to boundary growth on short train models have not been resolved.
CFD has shown that the flow structure behind a train may exhibit more orderly behaviour when viewed from the train reference frame. The use of CFD
as a useful flow visualisation tool has been demonstrated.
The simple mathematical modelling of wind speeds necessary to cause falling down shows some promise in helping to understand human behaviour
before subjective response takes over. However, more subjective data is needed
for track workers and people in stations, including the effects of; wind speed,
train noise, distraction and surprise. Until this information is available and
correlated with the physical effects, it is difficult to determine how much the
safety of people from slipstream effects depends solely on aerodynamics.
Another outstanding issue is that the effect of cross winds not been reliably
established, although there is some evidence that they can significantly augment slipstream velocities in the lee-side of trains.
A new European project is proposed to address some of these outstanding
issues, particular subjective response.
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