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Wind effects induced by high speed train pass-by in open air
Journal of Wind Engineering & Industrial Aerodynamics 173 (2018) 279–288
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Wind effects induced by high speed train pass-by in open air D. Rocchi *, G. Tomasini, P. Schito, C. Somaschini Department of Mechanical Engineering, Politecnico di Milano, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: High speed trains Full scale measurements Head pressure pulse Wind barrier Slipstream Ballast lifting
Structures and people, when standing along a railway line, are exposed to wind loads produced by train pass-by depending on their relative position, the object shape and dimensions, train speed and aerodynamic features. The design of railway equipment and the safety of passengers/workers along the line require specific investigation of the wind effects induced by the train transit, which are prescribed by standard specifications but are still a subject of ongoing research activities. This paper presents some results of an intensive full scale experimental investigation performed on high speed trains running on the Italian railway lines, considering train aerodynamic aspects in open air.
1. Introduction A passing- train produces an air movement that blows on people and objects disposed along the railway line, generating aerodynamic loads that are critical for the design of both structures and trains, and for the safety conditions of passengers and workers close to the tracks. Wind loads are a result of the air dragged by the train and the induced perturbation in the pressure field around the train itself. Looking at the flow from the train reference system, three distinct zones can be highlighted: the zone around the train nose which is mainly inviscid, the boundary layer around the train body that is fully turbulent and unsteady, and the wake behind the train tail that is dominated by large turbulent scales (Baker, 2014; Baker, 2010; Baker et al., 2014a; Baker et al., 2014b; Sterling et al., 2008; Baker et al., 2001). The interaction between these three zones and the railway structures and elements produces different kinds of excitation that have to be carefully considered to prevent serious damage and numerous accidents, and moreover to properly dimension the structures and the line. Awareness about the risks related to these aerodynamic effects in railway applications and operation has grown substantially in the last decades with the development of the high speed lines, the increase in the train operating speed, the increase of mixed traffic operating conditions, and the occurrence of unpredicted accidents (failure of noise barriers (Friedl et al., 2011), train door/window damage during train crossing, train and track damage due to ballast lifting (Quinn et al., 2009), unattended pushchairs on platform being sucked as a result of train pass-by (Hardy, 2007)). Knowledge about the aerodynamic problems has largely increased
thanks to several research activities performed by train manufacturers, railway lines operators and managers, universities and private and public research groups using both experimental and numerical investigations, starting from simplified situations up to full-scale complexity. Many European research projects gave a boost to problem analysis, starting from the TRANSAERO (Transient Aerodynamics for Railway) project in 1996, when full-scale tests and 3D panel methods were used to study the unsteady forces arising when two trains passed each other (Matschke et al., 1999), moving to the RAPIDE (Railway Aerodynamics of Passing and Interaction with Dynamic Effects) project, where the slipstream problem was studied through full-scale tests in Germany, complemented by moving scaled model slipstream measurements and CFD simulations (Schulte-Werning et al., 1999). Later, the AOA (Aerodynamics in Open Air) DeuFrako project studied the aerodynamics of the flow along the train underbody concerning the ballast lifting phenomenon through full scale tests, wind tunnel tests and CFD simulations (Kaltenbach et al., ) and, lastly, the AeroTRAIN project, where open air pressure pulses, aerodynamic loads on track and slipstream effects were studied using full-scale and model-scale tests, as well as numerical simulations (Sima et al., 2011). Parametric studies of the aerodynamic problem to analyse what the governing quantities are, and their influence on the results, were performed using numerical approaches or experimental tests on scaled models in wind tunnels or in moving model test rigs. The effects of the train speed, the train cross-sectional area, the distance between the passing train and the considered structure, the lateral clearance between trains, the ground clearance, the train nose length and the train and line geometry in general, were investigated. The results of these parametric
* Corresponding author. E-mail address: [email protected] (D. Rocchi). https://doi.org/10.1016/j.jweia.2017.10.020 Received 18 February 2017; Received in revised form 16 October 2017; Accepted 19 October 2017 0167-6105/© 2017 Elsevier Ltd. All rights reserved.
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Journal of Wind Engineering & Industrial Aerodynamics 173 (2018) 279–288
Fig. 1. Head pressure pulse set-up.
on the same infrastructures, with the same set-up and in the same operating condition. For sake of comparison and confidentiality, the results are presented in a non-dimensional form using the values of one of the trains as a reference to point out relative percentage differences.
studies made it possible to define the experimental procedures for performing full-scale tests that are unavoidably required to quantify the aerodynamic effects in real conditions because of the great complexity of the aerodynamic problem. The results of this process are summarised in national and international standards (EN 14067–2, 2003; EN 14067–4, 2013; EN, 1991-2, 2003), and technical specifications for the interoperability of rolling stock in different countries (TSI RST LOC&PAS, 2014; TSI INF, 2014), and, above all, are strongly considered in the design of railway structures and safe operating conditions. Nevertheless, the continuous evolution of the railway system towards higher speed and higher capacity requires investigations on this aspect to be continued, considering the impact they have on the new operating conditions. This is the case of the Italian High Speed (HS) lines where, in the last years, the capacity was increased with the introduction of new HS trains, reaching up to 7 HS trains/hour in the rush hours (a number that could be further increased in final conditions), the introduction of mixed traffic operating conditions in some parts of the HS line, and the plan to increase the current operating speed of 300 km/h up to 350 km/h. For these reasons, a survey of the present situation of the Italian HS speed line was performed, analysing the wind effects induced on all the structures and people around the track by HS train pass-by in open air through full scale experimental tests according to the EN-14067-4 standard procedures for the following aspects:
2. Experimental results 2.1. Head pressure pulses The first kind of interaction between railway structures and flow field induced by a passing train is the swift pressure variation produced by the transit of the nose and by the tail of the train. The magnitude of this pressure pulse determines the impact and fatigue structural loads on the structure. A high-pressure zone (where the train nose impacts the still air) moves together with the train just in front of the train nose followed by a low-pressure zone around it. The reverse situation occurs at the train tail, where the negative pressure zone just before the train tail is followed by a high-pressure zone immediately behind it, but in this case, the pressure variation is usually lower than the one produced by the train head passage. This pressure field travels at the train speed and it determines two pressure pulses (or even more, in multiple-unit compositions) on the structures around the line, whose magnitude decreases with the distance from the track. The corresponding aerodynamic excitation on structures, people and trains has impulsive characteristics and the produced effects depend on the dynamic response of the considered element. The dynamic response may produce dynamic amplification of the applied loads, and fatigue verifications must be carried out considering the number of cycles per passing train. Usually, the duration of the pressure pulses is too short to cause a person standing near the track to fall over, since the reaction time of people to wind gusts is around 0.3 s or more (people suffer much more from the train slipstream than the head pressure pulses). The principal impacts of the pressure pulses are therefore the potential damage to wayside structures and their effects on other trains. The duration and magnitude of the head pressure pulses are measured 2.5 m aside the centre of the track at 9 different heights from 0.9 m to 3.3 m above the top of the rail (TOR). Pitot tubes are mounted aligned with the train direction on a vertical beam, connected to a portal of the
Head pressure pulses; Aerodynamic loads on noise barriers; Slipstream effects beside the track; Aerodynamic loads on trackbed.1
Thanks to an extended experimental campaign, for all these aspects, a comparison between the results obtained considering three different Italian HS trains will be proposed and discussed. In particular, it was possible to consider the effect on the aerodynamic induced loads of different train geometries, by comparing experimental results, obtained
1 Aerodynamic loads on trackbed and the resulting issue of ballast lifting does not generally have a defined standard procedure, but a proposal for the experimental set-up is provided in Annex A of (EN 14067–4, 2013). The experimental set-up used in the present research is not fully compliant with the one suggested in Annex A because of the plates covering the ballast are not used.
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Fig. 2. Comparison between head pressure pulse of three different HS trains: time histories at 1.1 m above TOR (left) and vertical mean profiles of pressure jumps (right).
pressure absolute values is different from train to train in the range of 80–90%: 0.87 for Train A, 0.9 for Train B and 0.8 for Train C. Pressure pulse duration is related to the train nose length, as is well highlighted by the proposed non-dimensional representation. Since Train A has the shortest nose length, it produces the swiftest pressure pulse, while the other two trains with a similar nose length have similar pressure pulse duration. Considering the aerodynamic loading, the most important characteristics of the head pressure pulse are the duration and the maximum pressure jump Δp, defined as the difference between the maximum and the minimum peak pressure values. The value of the maximum pressure jump was computed for all the recorded train passages at the different heights from TOR. Even if small differences in the train speed were recorded, a scaling of the pressure values to a common reference train speed equal to 300 km/h was performed using the square of the ratio of the reference speed over the actual train speed. In Fig. 2b, the average value of the maximum pressure jump recorded at the different height for each train is plotted in non-dimensional form, taking the maximum value recorded for Train A at 1.8 m above TOR as a reference value. Looking at the values, Train A shows the largest values at all the considered heights, Train C has values 86% lower than the previous one and Train B, which takes advantage of the smaller absolute values of the minimum negative peak pressure, shows the smallest values (80% lower than Train A). Looking at the trend, Train A shows a maximum value at 1.8 m above TOR while Trains B and C reach the maximum value at 1.5 m. The reduction of the pressure jump with the height is similar for all the three trains if we move from the maximum value up to the higher levels, with a reduction on top of about 15% of the maximum. Moving towards the lower heights, Train A shows the largest reduction (5%), while the other two trains both have only a slight reduction of 2%. The new generation trains (Trains B and C), having a more refined aerodynamic shape, obtained from longer noses and a smoother variation of the geometry in the transition from the nose to the following carbody, show better aerodynamic behaviour in terms of head pressure pulse, with lower pressure jumps values and longer durations.
overhead line (Fig. 1). Differential pressure transducers measure the static pressure channel of each pitot tube with respect to the site reference pressure. The train speed is measured using 2 pairs of light barriers positioned at known distances. Fig. 1 shows the instrumentation layout and a picture of the experimental set-up that includes a weather station (4 m apart along the track and 2 m above the top of the rail) to record the environmental conditions during the tests. During the experimental campaign, the passages of all the 3 Italian HS trains (in the following labelled as Train A, Train B and Train C) were recorded for three days. A set of passages with an ambient wind speed of lower than 2 m/s was analysed containing: 13 Train A, 8 Train B and 11 Train C passages. A comparison of the head pressure pulse time evolution recorded 1.1 m above the TOR, for the three trains, is reported in Fig. 2a for passages at the same train speed. The time histories are plotted versus a non-dimensional length variable, obtained by multiplying the time scale by the train speed and dividing the result by the nose length of each single train (Train B and C have similar nose lengths, while Train A has a shorter nose length in the ratio of 2/3). The nose length is defined as the length of the leading car where there is a variation of the train cross sectional geometry. For sake of comparison, the origin of the x-axis (x ¼ 0 value) is positioned in correspondence with the maximum positive value of the pressure pulse that is close to the train head transit in front of the instrumentation. The vertical axis reports the non-dimensional pressure defined assuming the maximum negative value of Train A as reference (pRef) as a reference, so that it represents the percentage difference between the pressure recorded on Train B and Train C passages with respect to Train A values (Train A is the oldest train in operation). Thanks to the adoption of the chosen non-dimensional x-axis coordinate, the maxima and minima of the train head pressure pulses occur at the same location: close to x/LNose ¼ 0 the positive values (train head position) and close to x/LNose ¼ 1 for the negative values (1 nose length downstream). It is possible to observe that the pressure pulse produced by Train A has the largest values (both positive and negative) among the three trains. The pressure increase is already noticeable three nose lengths before the train head transit, but it is worth remembering that Train A has a shorter nose length, so in time the increase is contemporary to those of the other trains. Also, the pressure recovery after the negative peak occurs further downstream from the train head (3 nose lengths) but similar considerations can be performed. Trains B and C show the same value of positive peak pressure but different negative peak pressure values. For all the trains, the negative peak reaches larger absolute values than the positive peak, but the ratio between positive over negative
2.2. Wind loads on noise barriers When the air flow produced by the passing train is confined by solid walls disposed along the line, the pressure field is modified by the presence of the structure itself and cannot be inferred by the previously analysed results in open air. This is the case of noise barriers, that stand along the line at lateral distances which are greater than the 2.5 m from 281
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Fig. 3. Experimental setup for pressures and deformation measurements on the noise barrier: (left) lateral view, (right) along track view.
the instrumented panel and x/LTrain ¼ 1 the train tail position facing the instrumented panel. Passages of the 3 different trains at the same running speed are considered. To compare the different train results, pressure was plotted in nondimensional form by dividing it by the maximum absolute value measured for train A (pREF). When the train head arrives (x/LTrain¼0), according to what happens in open air, a strong overpressure is generated, followed by a nearly equivalent negative pressure that derives from the negative pressure on the train head. Also in this case, apart from Train B, the absolute values of the negative peaks were larger than those related to positive peaks, the ratio between the two being very similar to what was measured in open air (Train B in open air showed the most similar positive and negative values). After this first perturbation, the pressure fluctuations were limited, and mainly dependent on section changes of the train body (coaches and intercar-gaps may have the same or different cross-sections). The old Train A, having a less uniform cross section along the train axis (less aerodynamically optimised intercar-gap geometries), shows the largest fluctuations in this region. When the tail approaches (x/LTrain¼1) the pressure pulse is reversed, as happens in open air, and it is confirmed that the values are lower than those induced by the passage of the train head. The maximum pressure jump (ΔP ¼ PMAX - PMIN) is therefore produced by the train head passage and Fig. 4-d compares the maximum pressure difference observed for the different trains at different heights: ΔPREF value is the one computed at 0.5m above TOR for Train A. Differences in the ΔP values are mostly due to the negative values of the pressure jumps, since the positive ones are more similar among the considered trains. It is possible to see that the trend of the pressure load on the barrier generally has a similar shape for all trains: a larger pressure fluctuation was recorded in the lower part of the barrier where the air is trapped between the train and the vertical wall. This behaviour is not unexpected, since by increasing the height the pressure is not constrained by the barrier itself and tends towards the pressure variations in open air that are discussed in section 2.1. Comparing the vertical profile of pressure jumps in open air (Fig. 2)
the centre of the track where the head pressure pulses in open air are measured. It is therefore necessary to measure the aerodynamic effect directly on the structure that can interact with the flow field induced by the train in different ways, depending on the train-structure relative position, on the noise barrier height and geometry, and on the train geometry. Even if the time evolution of the pressure measured on the structure is similar to what is already presented in open air with two main pressure pulses produced by the passage of the train head and tail, the magnitude and spatial distribution of the pressure field have to be specifically investigated. To investigate these aspects, pressure time histories on the panels of a 4.2 m high vertical noise barrier positioned 3.15 m from the centre of the track are measured. The total length of the noise barrier is 705 m, and the distance of the sensors from the beginning of the structure is 68 m. The deformation at the base of some structural posts of the barrier is monitored as well, to measure the structure response and consider fatigue stresses that lead to maintenance and structural integrity problems (Friedl et al., 2011). For these very long structures, the wind-structure interaction, in fact, depends also on the train speed, since loading is no more than just a repetition of impulsive loads, but it is a travelling load that may produce dynamic amplification in the structural response. Fig. 3 sketches the instrument setup, showing the position of the 4 pressure taps on the noise barrier panels and of the 2 strain gauges on the posts. Pressure sensor positions are referred to the TOR, since the pressure distribution is related to the relative position of the train. Pressure measurements are performed using AMS4711 barometric sensors, with 700–1200 mbar range and an uncertainty of 10Pa. The dynamic response of the sensor was verified up to 200 Hz. For each pressure tap, a hole was drilled in the noise barrier panel and a metal pipe was used to cross the panel thickness and to connect the pressure sensor through a rubber tube. Strain gauges, at the connection point between post and foundation, measure the deformation at the base of the barrier post. Deformation data were filtered with a 6th order low-pass filter at 40 Hz, while pressure data were filtered with a 6th order low-pass filter at 75 Hz. Fig. 4-a-b-c reports the distribution of the pressure along the barrier, 1 m above the TOR, with x/LTrain ¼ 0 being the train nose position facing 282
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Fig. 4. Pressure horizontal distribution on the noise barrier (sensor P3 at 1 m above TOR) and deformation of the post at the base (sensor E1) at the passage of the 3 HS trains: (a) train A, (b) train B, (c) train C. Vertical pressure distribution on the noise barrier: (d) ΔP/ΔPREF for the different trains.
distributions, where differences are about 10%. For Train A and Train C, the ratio between the maximum negative and maximum positive deformation is around 0.65 and it can be related to the corresponding ratio between the positive and negative pressure peaks that have almost the same value for these two trains (around 0.87). For Train B, the ratio of the two deformation peaks is around 0.78 with a ratio of positive and negative pressure peaks of 1. Of course, the response of the post is not related only to the pressure load values, but also to the time evolution of the pressure load and its interaction with the structure dynamics. Looking at the deformation recorded during the train passage between the nose and the tail transit, and after the extinction of the initial decay, the train-flow-structure interaction is different among the 3 trains: small fluctuations in a wide frequency band for Train A and B and larger fluctuations with a narrower frequency band for Train C. The results are obviously dependent on the train speed and geometry, besides the natural frequency response of the structure, but results show the clear connections between the aerodynamic design of the train and the effects induced on the line.
with the one measured on the barrier (Fig. 4-d) it is possible to see how the maximum value recorded at 1.5–1.75 m above the TOR has now shifted to the lower instrumented level. The trend in reducing the pressure with the level above TOR ΔP/Δz is larger in open air than on the barrier and is strongly related to the noise barrier height and shape. It is interesting to examine the response of the barrier post to the passage of the different trains. The passage of the train head represents an impulsive load, therefore the dynamic response of the post shows the decay of the vibration amplitude at the natural frequency of the structure. Considering the nose train passage, the first deformation of the post is negative, since the positive pressure is pushing the noise barrier away from the train (see sign conventions of Fig. 3); when the negative pressure occurs, the post is pulled towards the track, and this is the highest deformation value that is observed. In Fig. 4-a-b-c the deformation measured by the strain gauge on the post base is reported. The quantity reported is ε/εREF, where ε is the deformation measured during the train passage, while εREF is the maximum deformation measured for train A. Considering the initial large negative value of deformation, values are similar among the 3 trains, with the largest value recorded for Train A since it has the largest positive pressure peak value. Considering the first large positive value of deformation, results are more different among the 3 trains, reaching a 30% difference between Train A and B that is not predictable by looking only at the ΔP vertical
2.3. Slipstream effects beside the track Together with the pressure field produced at train pass-by, there is also a velocity field that has important effects on railway structures (signal boards, equipment boxes and cabinets) and on people (passengers 283
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number of runs available for each considered train. Fig. 6 shows a comparison between the results obtained from the 3 different HS trains. It represents the local average (μ) of the magnitude of the horizontal in-plane wind velocity component U recorded by the anemometers during all Train C passages. The value was normalised by dividing it by the maximum value of the average evaluated for Train A (indicated as Uref in the following plots) to show the percentage difference among the results. On the x-axis, the non-dimensional x/LTrain coordinate was used to associate the results to the passage of the single train sections in front of the anemometer. A similar large deterministic peak was produced by the train nose passage (x/LTrain ¼ 0) independently from the train nose geometry. The increase of the boundary layer along the train (0 < x/LTrain <1) was much higher for Train A compared to the other two trains, which showed an almost identical behaviour. These two trains, belonging to modern aerodynamic design standards, are equipped with more refined aerodynamic solutions, such as the adoption of skirts in front of the bogies and gangways in the intercar gap to avoid strong variations in the cross section geometry in that region. The performance of similar devices in controlling the growth of the boundary layer around the lateral walls of the train was very evident in slipstream measurements. The passage of the train tail (x/LTrain ¼ 1) produced another evident and almost deterministic peak that was almost equal, in comparison with Train B and Train C, but it was not clearly visible in Train A because hidden by the large growth of the boundary layer mixing with the wake region. For Trains B and C, this tail peak in the average value of the in-line velocity was smaller than the one produced by the train head. Maximum values of the slipstream local average value were reached, in any case, in the near wake region just after the tail. Different maximum values were reached by the three trains: Train C reached 50% of the Train A value and Train B the 40%. From the analysis of the average flow field, it seems that Train A had a different wake structure compared to the other two trains: it was characterised by a sharp peak very close to the tail. A trend very similar to that found for train A is shown in (Baker et al., 2014a) for the ICE-1 and ICE-2 trains and the S-120 and S-130 trains which show a very large and sharp peak just after the tail. In (Baker et al., 2014a), this behaviour is associated to less rounded noses/tails. For the new generation of trains, the large local average values of the near wake region last longer downstream from the tail instead of starting to immediately decrease, as occurs with Train A. It is recognized that dangerous slipstream effects are related to the gustiness of the flow that may lead people to lose their equilibrium. An estimation of the possible gust speed distribution along the train, for the
Fig. 5. Ultrasonic anemometer installed at 0.2 m above TOR and 3 m from the track centre.
Table 1 The experimental database. Train type
# runs
Train A Train B Train C
57 24 21
and workers) standing along the line. Specifically, this second aspect is considered most at standard levels since it is related to safety. Slipstream effects induced by train pass-by were already being studied in the late seventies (Gawthorpe, 1978) and were intensively investigated in the European projects RAPIDE (Sterling et al., 2008; Hardy, 2007; Schulte-Werning et al., 1999) and AeroTRAIN (Baker et al., 2014a). Measurements of air speed during train passing were carried out by three-components ultrasonic anemometers (Fig. 5) positioned 0.2 m above the TOR and 3 m from the centre of the track, to study the slipstream on workers along the line. The train speed was measured indirectly using light barriers at known distances; temperature and ambient pressure where also measured to monitor ambient conditions. The presented data give a selection of the recorded train passages at the same train speed and in similar environmental conditions (in particular ambient wind speed lower than 2 m/s). Table 1 resumes the
Fig. 6. Normalized in-plane component U: local average values (μ) as a function of the normalized coordinate X. Comparison between the three trains. 284
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Fig. 7. Normalized in-plane component U: local peak velocity values (μþ2σ) as a function of the normalized coordinate X. Comparison between thethree trains.
Fig. 8. Normalized in-plane component U: maximum normalised velocity value for each train passage as a function of the normalized coordinate X.
the maximum values of slipstream velocity are gathered again in the near wake region, but also some extreme gust locations are visible in the region close to the train nose. These are relative to some records of particular windy days, that slightly afflicted test results. Finally, watching Train A, it was seen that the gusts were deterministically located just after the train tail, but their magnitudes were widespread along the vertical axis, reaching extreme values significantly higher than those of other trains. The found results, especially for Train A and Train C, are in line with those shown in (Baker et al., 2014b) for S103 and S100.
horizontal in-line velocity component, is obtained by summing, to the already described ensemble averaged values μ, two times the standard deviation σ of the instantaneous values measured during all the considered passages at each time step. The result (μþ2σ) is reported in Fig. 7 for each train, using the same previously adopted reference speed for normalization, for the sake of comparison with all the averaged values. As can be appreciated by the comparison of Figs. 6 and 7, the largest local peak values of the slipstream flow are more confined in the near wake region for all the considered HS trains, and also for Train B and C, which showed a more prolonged average value in the wake region. Train B and Train C are confirmed as having a very similar behaviour and may reach maximum local peak values in the near wake region that are three time higher than the average value in the same position, while for Train A the ratio is lower than 2.5. Local peak values in the middle of the train are better correlated to the variation of the train geometry, highlighting sharp regular peaks. Flow fluctuations are also present even if smaller, in correspondence with the head of the train. In order to understand where the gusts typically occur in terms of xposition, Fig. 8 shows the maximum normalized peak velocity values Ugust of the in-plane component, for each analysed time-history versus the normalized X-coordinate where the value was recorded. Regarding Train C, the gusts mostly fall in the region just after the train tail, with very few exceptions occurring in the mid-far and far wake regions. Train B shows high dispersions of data along the train length (X-Coordinate):
2.4. Aerodynamic loads on the trackbed The study of aerodynamic loads on the trackbed is necessary since it is strongly related to the issue of flying ballast, one of the major problems caused by increases in railway speed over 300 km/h in terms of safety and early deterioration of both rolling stock and track (Quinn et al., 2010; Saussine et al., 2014; Navarro-Medina et al., 2012). In simple terms, ballast particles are lifted due to the air flow driven by the underbody of a train. Generally, the ballast lifting phenomenon may arise also at low speeds, if external agents such as ice or other scraps on the track help stone dislodgment. On the other hand, the problem becomes extremely evident as the speed increases, when the ballast stones are lifted due to the pressure and velocity field generated by the train in the upper layer of ballast without the need for an external trigger. Recent studies suggest 285
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Fig. 9. Examples of experimental setups for the flow measurements.
of u ðustd Þ which is an easier to measure variable in full-scale tests (Weise et al., ). For this reason, in order to estimate the aerodynamic forces on the ballast, it is useful to measure the profile and intensity of the flow in the region closest to the trackbed. In particular, in some countries the experimental measurements of the flow above the trackbed have already been implemented in national standards, based on the estimate of the power of the flow (Lazaro, Rodriguez-Plaza; EPSF). To measure the flow velocity field between the train underbody and the ballast surface, different types of transducers can be used (Deeg et al., 2008) so as to measure also the different components of both the flow and the aerodynamic loads acting on the track. On the other hand, comparing the averaged values of the three components, it was found (Kaltenbach et al., ; Sima et al., 2011; Premoli et al., 2015) that the main component was by far the longitudinal one, since the lateral component was almost null, while the vertical component assumed negative values (that correspond to a suction component towards the train underbelly) but it remained about 10 times lower than the longitudinal one. This means that, to describe the train-induced aerodynamic loads on the trackbed, only the longitudinal flow speed in the gap between the track and the train underbelly can be measured, especially if the target is to compare different train-track configurations. The results that will be shown in this work were collected using a vertical array of a single pitot tube and 13 pressure probes (we used
that collisions begin to occur only at speeds of 270 km/h or higher because the transfer of energy needs to be big enough in order to cause the ballast to rise (Lazaro et al., 2011; Rocchi et al., 2016). The consequences of this phenomenon are different: on the safety of the people working along railway lines, on the running safety of the trains themselves, and, finally, on the extra costs associated with both the rolling stock and the infrastructure maintenance. Since the attempt to predict and control the phenomenon is probably the most important, as it is impossible to stop a stone once it is set in motion, research over the years has focused mainly on it. Although not all factors contributing to ballast lifting have been completely identified (infrastructure geometry, train underbelly design up to the track excitation during the train passage) it is well established that the key factor is the imposed aerodynamic load (Gawthorpe, 1978). Wall shear stress has been identified as the most appropriate measure of the aerodynamic load on the track (Weise et al., ). On the other hand, as soon as the stone rises slightly from its position, it runs into the flow, and the dynamic pressure, proportional to the square of the flow velocity u2 , becomes predominant. Furthermore, due to the many discontinuities both on the train and on the track geometries, this flow is characterised by a high turbulence index (Lazaro et al., 2011), therefore it is quite difficult to measure the shear stress in train operation conditions. Fortunately, the shear velocity near the ground is approximately proportional to the standard deviation
Fig. 10. Comparison between the averaged time histories of the normalized speed of the flow induced by three different trains at three different heights (27 mm, 180 mm and 230 mm with respect to the Top Of Rail). 286
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Fig. 11. Comparison between the averaged vertical air speed profiles of the three HS trains.
all the sensors of the vertical rake when calculating the mean vertical profile of the flow velocity longitudinal component, averaging the values of the Cu of the central section where the flow reaches steady conditions (the averages are calculated in the section between the two dashed lines of Fig. 10 omitting the initial and the final evolutions). In Fig. 11 the comparison of the results obtained for the three cases is shown. The flow profiles generated by the three different trains, although different for the absolute values, have almost equal shapes. The air speed profile of Train B has the lowest values in terms of both mean value and standard deviation (see Table 2), maybe due to the lower number of bogies per unit length, which results in a less generalized roughness of the underbody, while the Train A and the Train C are characterised by similar values, with the flow speed of the Train C being slightly lower than that of the Train A, probably as a result of the better aerodynamic design of the Train C underbelly. Finally, with regard to the maximum value measured in correspondence of the tail, all the trains are characterised by similar absolute values of 1.6 times the reference speed Uref. This means that, in the case of Train B, the tail peak is almost two times (1.86) the mean of the central section. This last peak is the highest velocity during the wall passage. Even though stones potentially moved by it cannot hit the train, they could become a safety problem for people and structures standing along the railway lines.
Table 2 Comparison between the statistical values of the speed measured by the upper probe.
Train A Train B Train C
#bogies/length
Mean
Std Dev
Tail Peak
0.08 0.06 0.08
1 0.85 0.96
0.048 0.030 0.041
1.57 1.59 1.53
simple tubes with the only total pressure tap, using as static pressure the one measured by the pitot) able to measure the vertical profile of the longitudinal component of the flow velocity (see Fig. 9). The pressure probes made it possible to study in detail the region close to the sleepers and to the trackbed where the flow is characterised by a high vertical gradient (Premoli et al., 2015). Also in this case, the results are presented in a non-dimensional manner taking as reference (Uref) the mean speed recorded by the highest pitot during the passages of train A. Fig. 10 compares the averaged temporal evolution of the flow speed coefficients measured through the pitot rake at three different heights during the passage of the three different trains (twenty different runs for each train were averaged). A first small peak of speed (corresponding to the overpressure in front of the train head at x ¼ 0) characterises the passing of the nose region. After it, the speed gradually increases and, after a couple of cars, it stabilises with periodic oscillations around a constant value until the tail of the train passes over the instrumented section and a second major peak occurs; finally, the speed reduces with some fluctuations due to the wake. The main periodic oscillation at low frequency caused by the passage of the coaches depends on the length of the coaches themselves and, considering a train speed of 300 km/h, the corresponding frequency lies at around 3–5 Hz. Comparing the averaged temporal evolution of the speed measured by the upper pitot it is possible to compare the flow induced by different trains from a qualitative point of view. The oscillations are strongly influenced by the shape of the underbody region; in the case of Train A, after two cars the flow is completely developed and oscillates around a constant value, while in the other two cases the behaviour is slightly different. Although about two cars are still required to reach a constant value, the flow does not oscillate with a regular pattern as occurs with Train A. Train B and Train C are new generation trains of the EMU type (Electric Multiple Unit); the airflows necessary for cooling the power components are partially blown below the trains and probably modify the flow in the underbody region. Considering now the dependence of the phenomenon on height, it can be noted that the behaviour is always comparable and the height only has the effect of scaling the mean values. It is therefore possible to consider
3. Conclusions Results of experimental campaigns performed to investigate different aspects of the aerodynamic loads induced by the passing of HS trains on structures, people and tracks are presented. In particular, the comparison between the aerodynamic effects induced by three HS trains running on the Italian HS line is proposed for head pressure pulses in open air, overpressures on noise barriers, slipstream flows and flow on the trackbed. The comparison highlights, in quantitative terms, the improvements in the aerodynamic design of HS trains (Train A vs Trains B and C) related to a more aerodynamic shape of the nose, and the adoption of aerodynamic devices such as bogie skirts and gangways in the inter-car gap. Differences in the aerodynamic behaviour of the 2 modern trains (Trains B and C) are presented and discussed in the paper, demonstrating that the aerodynamic design of HS train is an important and ongoing activity and that aerodynamic issues are becoming dominant in the recent development of HS railway lines aiming at increasing train speed and capacity.
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Acknowledgments
Lazaro, B.J., Gonzalez, E., Rodriguez, M., Rodriguez, M., Osma, S., Iglesias, J., 2011. Characterization and modelling of flying ballast phenomena in high-speed train lines. In: 9th World Congress on Railway Research. Tokyo. Lazaro B., Rodriguez-Plaza M., “Spanish Guideline for Ballast Impact Risk Assessment”. Matschke, G., Schulte-Werning, B., Gregoire, R., Malfatti, A., Mancini, G., 1999. TRANSAERO: Results of an European Research Project in Railway Aerodynamics. World Congress on Railway Research (WCRR), Tokyo, Japan. Navarro-Medina, F., Sanz-Andres, A., Perez-Grande, I., 2012. Gust wind tunnel study on ballast pick-up by high-speed trains. Exp. fluids 52 (1), 105–121. Premoli, A., Rocchi, D., Schito, P., Somaschini, C., Tomasini, G., 2015. Ballast flight under high-speed trains: wind tunnel full-scale experimental tests. J. Wind Eng. Ind. Aerod. 145, 351–361. Quinn, A.D., Hayward, M., Baker, C.J., Schmid, F., Priest, J.A., Powrie, W., 2009. A fullscale experimental and modelling study of ballast flight under high-speed trains. Proc. Instn Mech. Engrs, Part F J. Rail Rapid Transit 224, 61–74. Quinn, A.D., Hayward, M., Baker, C.J., Schmid, F., Priest, J.A., Powrie, W., 2010. A fullscale experimental and modelling study of ballast flight under high-speed trains. Proc. Inst. Mech. Eng. Part F J. Rail Rapid Transit 224, 61–74. Rocchi, D., Tomasini, G., Schito, P., Somaschini, C., Testa, M., Cerullo, M., Arcoleo, G., 2016. Ballast lifting: a challenge in the increase of the commercial speed of hs-trains. In: 11th World Congress on Railway Research (WCRR) Milan. Saussine, G., Quezada, J.C., Breul, P., Radjai, F., 2014. Railway ballast settlement: a new predictive model. In: Second International Conference on Railway Technology: Research, Development and Maintenance. Stirlingshire. Schulte-Werning, B., Matschke, G., Gregoire, R., and Johnson, T. RAPIDE: a project of joint aerodynamics research of the European high-speed rail operators. In Proceedings of theWorld Congress on Railway Research, ES1–3, Tokyo, 19–23 October, 1999 paper no. ES 1–3. Sima, M., Grappein, E., Weise, M., Paradot, N., Hieke, M., Baker, C., Licciardello, R., Couturier, M., 2011. Presentation of the EU FP7 AeroTRAIN Project and First Results. WCRR, Lille France. Sterling, M., Baker, C.J., Jordan, S.C., Johnson, T., 2008. A study of the slipstreams of high-speed passenger trains and freight trains. Proc. IMechE Part F J. Rail Rapid Transit 222, 177–193. TSI INF, 2014. EU Technical Specifications for Interoperability Relating to the ‘infrastructure’ Subsystem of the Rail System in the European Union. TSI RST LOC&PAS, 2014. EU Technical Specification for Interoperability Relating to the ‘rolling Stock — Locomotives and Passenger Rolling Stock’ Subsystem of the Rail System in the European Union. Weise M. Eisenlauer M., Hoefener L. and Sima M. “AeroTRAIN WP2 R2.3 Literature Survey”.
The authors are grateful to RFI for the support received during all the test campaigns. References Baker, C.J., 2010. The flow around high speed trains. J. Wind Engng Ind. Aerodyn. 98, 277–298. Baker, C.J., 2014. A review of train aerodynamics Part 1 – Fundamentals. Aeronautical J. 118 (1201). Baker, C.J., Dalley, S., Johnson, T., Wright, N.G., Quinn, A., 2001. The slipstreams and wake of a high-speed train. Proc. Instn Mech. Engrs, Part F J. Rail Rapid Transit 215, 83–99. Baker, C.cJ., Quinn, A., Sima, M., Hoefener, L., Licciardello, R., 2014. Full-scale measurement and analysis of train slipstreams and wakes. Part 1: ensemble averages. Proc IMechE Part F J Rail Rapid Transit 228 (5), 451–467. Baker, C.J., Quinn, A., Sima, M., Hoefener, L., Licciardello, R., 2014. Full-scale measurement and analysis of train slipstreams and wakes. Part 2: gust analysis. Proc. Inst. Mech. Eng. Part F J. Rail Rapid Transit 228 (5), 468–480. Deeg, P., Jonsson, M., Kaltenbach, H.J., Schober, M., Weise, M., 2008. Cross-comparison of measurement techniques for the determination of train-induced aerodynamic loads on the trackbed. In: Proceedings of the BBAA VI, pp. 20–24. EN 14067–2, 2003. Railway Applications - Aerodynamics - Part 2: Aerodynamics on Open Track. EN 14067–4, 2013. Railway Applications - Aerodynamics - Part 4: Requirements and Test Procedures for Aerodynamics on Open Track. EN 1991-2, 2003. Eurocode 1: Actions on Structures - Part 2: Traffic Loads on Bridges. EPSF “SAMX012 Envols de ballast” http://www.securite-ferroviaire.fr. Friedl H., Reiterer M., Kari H., Aerodynamic excitation of noise barrier systems at highspeed rail lines – Fatigue analysis. Proceedings of IOMAC'11 – 4th International Operational Modal Analysis Conference, 9-11 May 2011, Istanbul, Turkey. Gawthorpe, R., 1978. Aerodynamics of trains in the open air. Railw. Eng. Int. 3 (3), 7–20. Hardy, G.I., 2007. Effective management of risk from slipstream effects at trackside and platforms Appendix B: analysis of existing experimental data on train slipstreams including the effects on pushchairs. RSSB T425 Report. Kaltenbach H.-J., Gautier P. E., Agirre G., Orellano A., Schroeder-Bodenstein K., Testa M., Tielkes T. Assessment of the aerodynamic loads on the trackbed causing ballast projection: results from the DEUFRAKO project Aerodynamics in Open Air (AOA)..
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Wind effects induced by high speed train pass-by in open air D. Rocchi *, G. Tomasini, P. Schito, C. Somaschini Department of Mechanical Engineering, Politecnico di Milano, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: High speed trains Full scale measurements Head pressure pulse Wind barrier Slipstream Ballast lifting
Structures and people, when standing along a railway line, are exposed to wind loads produced by train pass-by depending on their relative position, the object shape and dimensions, train speed and aerodynamic features. The design of railway equipment and the safety of passengers/workers along the line require specific investigation of the wind effects induced by the train transit, which are prescribed by standard specifications but are still a subject of ongoing research activities. This paper presents some results of an intensive full scale experimental investigation performed on high speed trains running on the Italian railway lines, considering train aerodynamic aspects in open air.
1. Introduction A passing- train produces an air movement that blows on people and objects disposed along the railway line, generating aerodynamic loads that are critical for the design of both structures and trains, and for the safety conditions of passengers and workers close to the tracks. Wind loads are a result of the air dragged by the train and the induced perturbation in the pressure field around the train itself. Looking at the flow from the train reference system, three distinct zones can be highlighted: the zone around the train nose which is mainly inviscid, the boundary layer around the train body that is fully turbulent and unsteady, and the wake behind the train tail that is dominated by large turbulent scales (Baker, 2014; Baker, 2010; Baker et al., 2014a; Baker et al., 2014b; Sterling et al., 2008; Baker et al., 2001). The interaction between these three zones and the railway structures and elements produces different kinds of excitation that have to be carefully considered to prevent serious damage and numerous accidents, and moreover to properly dimension the structures and the line. Awareness about the risks related to these aerodynamic effects in railway applications and operation has grown substantially in the last decades with the development of the high speed lines, the increase in the train operating speed, the increase of mixed traffic operating conditions, and the occurrence of unpredicted accidents (failure of noise barriers (Friedl et al., 2011), train door/window damage during train crossing, train and track damage due to ballast lifting (Quinn et al., 2009), unattended pushchairs on platform being sucked as a result of train pass-by (Hardy, 2007)). Knowledge about the aerodynamic problems has largely increased
thanks to several research activities performed by train manufacturers, railway lines operators and managers, universities and private and public research groups using both experimental and numerical investigations, starting from simplified situations up to full-scale complexity. Many European research projects gave a boost to problem analysis, starting from the TRANSAERO (Transient Aerodynamics for Railway) project in 1996, when full-scale tests and 3D panel methods were used to study the unsteady forces arising when two trains passed each other (Matschke et al., 1999), moving to the RAPIDE (Railway Aerodynamics of Passing and Interaction with Dynamic Effects) project, where the slipstream problem was studied through full-scale tests in Germany, complemented by moving scaled model slipstream measurements and CFD simulations (Schulte-Werning et al., 1999). Later, the AOA (Aerodynamics in Open Air) DeuFrako project studied the aerodynamics of the flow along the train underbody concerning the ballast lifting phenomenon through full scale tests, wind tunnel tests and CFD simulations (Kaltenbach et al., ) and, lastly, the AeroTRAIN project, where open air pressure pulses, aerodynamic loads on track and slipstream effects were studied using full-scale and model-scale tests, as well as numerical simulations (Sima et al., 2011). Parametric studies of the aerodynamic problem to analyse what the governing quantities are, and their influence on the results, were performed using numerical approaches or experimental tests on scaled models in wind tunnels or in moving model test rigs. The effects of the train speed, the train cross-sectional area, the distance between the passing train and the considered structure, the lateral clearance between trains, the ground clearance, the train nose length and the train and line geometry in general, were investigated. The results of these parametric
* Corresponding author. E-mail address: [email protected] (D. Rocchi). https://doi.org/10.1016/j.jweia.2017.10.020 Received 18 February 2017; Received in revised form 16 October 2017; Accepted 19 October 2017 0167-6105/© 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Head pressure pulse set-up.
on the same infrastructures, with the same set-up and in the same operating condition. For sake of comparison and confidentiality, the results are presented in a non-dimensional form using the values of one of the trains as a reference to point out relative percentage differences.
studies made it possible to define the experimental procedures for performing full-scale tests that are unavoidably required to quantify the aerodynamic effects in real conditions because of the great complexity of the aerodynamic problem. The results of this process are summarised in national and international standards (EN 14067–2, 2003; EN 14067–4, 2013; EN, 1991-2, 2003), and technical specifications for the interoperability of rolling stock in different countries (TSI RST LOC&PAS, 2014; TSI INF, 2014), and, above all, are strongly considered in the design of railway structures and safe operating conditions. Nevertheless, the continuous evolution of the railway system towards higher speed and higher capacity requires investigations on this aspect to be continued, considering the impact they have on the new operating conditions. This is the case of the Italian High Speed (HS) lines where, in the last years, the capacity was increased with the introduction of new HS trains, reaching up to 7 HS trains/hour in the rush hours (a number that could be further increased in final conditions), the introduction of mixed traffic operating conditions in some parts of the HS line, and the plan to increase the current operating speed of 300 km/h up to 350 km/h. For these reasons, a survey of the present situation of the Italian HS speed line was performed, analysing the wind effects induced on all the structures and people around the track by HS train pass-by in open air through full scale experimental tests according to the EN-14067-4 standard procedures for the following aspects:
2. Experimental results 2.1. Head pressure pulses The first kind of interaction between railway structures and flow field induced by a passing train is the swift pressure variation produced by the transit of the nose and by the tail of the train. The magnitude of this pressure pulse determines the impact and fatigue structural loads on the structure. A high-pressure zone (where the train nose impacts the still air) moves together with the train just in front of the train nose followed by a low-pressure zone around it. The reverse situation occurs at the train tail, where the negative pressure zone just before the train tail is followed by a high-pressure zone immediately behind it, but in this case, the pressure variation is usually lower than the one produced by the train head passage. This pressure field travels at the train speed and it determines two pressure pulses (or even more, in multiple-unit compositions) on the structures around the line, whose magnitude decreases with the distance from the track. The corresponding aerodynamic excitation on structures, people and trains has impulsive characteristics and the produced effects depend on the dynamic response of the considered element. The dynamic response may produce dynamic amplification of the applied loads, and fatigue verifications must be carried out considering the number of cycles per passing train. Usually, the duration of the pressure pulses is too short to cause a person standing near the track to fall over, since the reaction time of people to wind gusts is around 0.3 s or more (people suffer much more from the train slipstream than the head pressure pulses). The principal impacts of the pressure pulses are therefore the potential damage to wayside structures and their effects on other trains. The duration and magnitude of the head pressure pulses are measured 2.5 m aside the centre of the track at 9 different heights from 0.9 m to 3.3 m above the top of the rail (TOR). Pitot tubes are mounted aligned with the train direction on a vertical beam, connected to a portal of the
Head pressure pulses; Aerodynamic loads on noise barriers; Slipstream effects beside the track; Aerodynamic loads on trackbed.1
Thanks to an extended experimental campaign, for all these aspects, a comparison between the results obtained considering three different Italian HS trains will be proposed and discussed. In particular, it was possible to consider the effect on the aerodynamic induced loads of different train geometries, by comparing experimental results, obtained
1 Aerodynamic loads on trackbed and the resulting issue of ballast lifting does not generally have a defined standard procedure, but a proposal for the experimental set-up is provided in Annex A of (EN 14067–4, 2013). The experimental set-up used in the present research is not fully compliant with the one suggested in Annex A because of the plates covering the ballast are not used.
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Fig. 2. Comparison between head pressure pulse of three different HS trains: time histories at 1.1 m above TOR (left) and vertical mean profiles of pressure jumps (right).
pressure absolute values is different from train to train in the range of 80–90%: 0.87 for Train A, 0.9 for Train B and 0.8 for Train C. Pressure pulse duration is related to the train nose length, as is well highlighted by the proposed non-dimensional representation. Since Train A has the shortest nose length, it produces the swiftest pressure pulse, while the other two trains with a similar nose length have similar pressure pulse duration. Considering the aerodynamic loading, the most important characteristics of the head pressure pulse are the duration and the maximum pressure jump Δp, defined as the difference between the maximum and the minimum peak pressure values. The value of the maximum pressure jump was computed for all the recorded train passages at the different heights from TOR. Even if small differences in the train speed were recorded, a scaling of the pressure values to a common reference train speed equal to 300 km/h was performed using the square of the ratio of the reference speed over the actual train speed. In Fig. 2b, the average value of the maximum pressure jump recorded at the different height for each train is plotted in non-dimensional form, taking the maximum value recorded for Train A at 1.8 m above TOR as a reference value. Looking at the values, Train A shows the largest values at all the considered heights, Train C has values 86% lower than the previous one and Train B, which takes advantage of the smaller absolute values of the minimum negative peak pressure, shows the smallest values (80% lower than Train A). Looking at the trend, Train A shows a maximum value at 1.8 m above TOR while Trains B and C reach the maximum value at 1.5 m. The reduction of the pressure jump with the height is similar for all the three trains if we move from the maximum value up to the higher levels, with a reduction on top of about 15% of the maximum. Moving towards the lower heights, Train A shows the largest reduction (5%), while the other two trains both have only a slight reduction of 2%. The new generation trains (Trains B and C), having a more refined aerodynamic shape, obtained from longer noses and a smoother variation of the geometry in the transition from the nose to the following carbody, show better aerodynamic behaviour in terms of head pressure pulse, with lower pressure jumps values and longer durations.
overhead line (Fig. 1). Differential pressure transducers measure the static pressure channel of each pitot tube with respect to the site reference pressure. The train speed is measured using 2 pairs of light barriers positioned at known distances. Fig. 1 shows the instrumentation layout and a picture of the experimental set-up that includes a weather station (4 m apart along the track and 2 m above the top of the rail) to record the environmental conditions during the tests. During the experimental campaign, the passages of all the 3 Italian HS trains (in the following labelled as Train A, Train B and Train C) were recorded for three days. A set of passages with an ambient wind speed of lower than 2 m/s was analysed containing: 13 Train A, 8 Train B and 11 Train C passages. A comparison of the head pressure pulse time evolution recorded 1.1 m above the TOR, for the three trains, is reported in Fig. 2a for passages at the same train speed. The time histories are plotted versus a non-dimensional length variable, obtained by multiplying the time scale by the train speed and dividing the result by the nose length of each single train (Train B and C have similar nose lengths, while Train A has a shorter nose length in the ratio of 2/3). The nose length is defined as the length of the leading car where there is a variation of the train cross sectional geometry. For sake of comparison, the origin of the x-axis (x ¼ 0 value) is positioned in correspondence with the maximum positive value of the pressure pulse that is close to the train head transit in front of the instrumentation. The vertical axis reports the non-dimensional pressure defined assuming the maximum negative value of Train A as reference (pRef) as a reference, so that it represents the percentage difference between the pressure recorded on Train B and Train C passages with respect to Train A values (Train A is the oldest train in operation). Thanks to the adoption of the chosen non-dimensional x-axis coordinate, the maxima and minima of the train head pressure pulses occur at the same location: close to x/LNose ¼ 0 the positive values (train head position) and close to x/LNose ¼ 1 for the negative values (1 nose length downstream). It is possible to observe that the pressure pulse produced by Train A has the largest values (both positive and negative) among the three trains. The pressure increase is already noticeable three nose lengths before the train head transit, but it is worth remembering that Train A has a shorter nose length, so in time the increase is contemporary to those of the other trains. Also, the pressure recovery after the negative peak occurs further downstream from the train head (3 nose lengths) but similar considerations can be performed. Trains B and C show the same value of positive peak pressure but different negative peak pressure values. For all the trains, the negative peak reaches larger absolute values than the positive peak, but the ratio between positive over negative
2.2. Wind loads on noise barriers When the air flow produced by the passing train is confined by solid walls disposed along the line, the pressure field is modified by the presence of the structure itself and cannot be inferred by the previously analysed results in open air. This is the case of noise barriers, that stand along the line at lateral distances which are greater than the 2.5 m from 281
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Fig. 3. Experimental setup for pressures and deformation measurements on the noise barrier: (left) lateral view, (right) along track view.
the instrumented panel and x/LTrain ¼ 1 the train tail position facing the instrumented panel. Passages of the 3 different trains at the same running speed are considered. To compare the different train results, pressure was plotted in nondimensional form by dividing it by the maximum absolute value measured for train A (pREF). When the train head arrives (x/LTrain¼0), according to what happens in open air, a strong overpressure is generated, followed by a nearly equivalent negative pressure that derives from the negative pressure on the train head. Also in this case, apart from Train B, the absolute values of the negative peaks were larger than those related to positive peaks, the ratio between the two being very similar to what was measured in open air (Train B in open air showed the most similar positive and negative values). After this first perturbation, the pressure fluctuations were limited, and mainly dependent on section changes of the train body (coaches and intercar-gaps may have the same or different cross-sections). The old Train A, having a less uniform cross section along the train axis (less aerodynamically optimised intercar-gap geometries), shows the largest fluctuations in this region. When the tail approaches (x/LTrain¼1) the pressure pulse is reversed, as happens in open air, and it is confirmed that the values are lower than those induced by the passage of the train head. The maximum pressure jump (ΔP ¼ PMAX - PMIN) is therefore produced by the train head passage and Fig. 4-d compares the maximum pressure difference observed for the different trains at different heights: ΔPREF value is the one computed at 0.5m above TOR for Train A. Differences in the ΔP values are mostly due to the negative values of the pressure jumps, since the positive ones are more similar among the considered trains. It is possible to see that the trend of the pressure load on the barrier generally has a similar shape for all trains: a larger pressure fluctuation was recorded in the lower part of the barrier where the air is trapped between the train and the vertical wall. This behaviour is not unexpected, since by increasing the height the pressure is not constrained by the barrier itself and tends towards the pressure variations in open air that are discussed in section 2.1. Comparing the vertical profile of pressure jumps in open air (Fig. 2)
the centre of the track where the head pressure pulses in open air are measured. It is therefore necessary to measure the aerodynamic effect directly on the structure that can interact with the flow field induced by the train in different ways, depending on the train-structure relative position, on the noise barrier height and geometry, and on the train geometry. Even if the time evolution of the pressure measured on the structure is similar to what is already presented in open air with two main pressure pulses produced by the passage of the train head and tail, the magnitude and spatial distribution of the pressure field have to be specifically investigated. To investigate these aspects, pressure time histories on the panels of a 4.2 m high vertical noise barrier positioned 3.15 m from the centre of the track are measured. The total length of the noise barrier is 705 m, and the distance of the sensors from the beginning of the structure is 68 m. The deformation at the base of some structural posts of the barrier is monitored as well, to measure the structure response and consider fatigue stresses that lead to maintenance and structural integrity problems (Friedl et al., 2011). For these very long structures, the wind-structure interaction, in fact, depends also on the train speed, since loading is no more than just a repetition of impulsive loads, but it is a travelling load that may produce dynamic amplification in the structural response. Fig. 3 sketches the instrument setup, showing the position of the 4 pressure taps on the noise barrier panels and of the 2 strain gauges on the posts. Pressure sensor positions are referred to the TOR, since the pressure distribution is related to the relative position of the train. Pressure measurements are performed using AMS4711 barometric sensors, with 700–1200 mbar range and an uncertainty of 10Pa. The dynamic response of the sensor was verified up to 200 Hz. For each pressure tap, a hole was drilled in the noise barrier panel and a metal pipe was used to cross the panel thickness and to connect the pressure sensor through a rubber tube. Strain gauges, at the connection point between post and foundation, measure the deformation at the base of the barrier post. Deformation data were filtered with a 6th order low-pass filter at 40 Hz, while pressure data were filtered with a 6th order low-pass filter at 75 Hz. Fig. 4-a-b-c reports the distribution of the pressure along the barrier, 1 m above the TOR, with x/LTrain ¼ 0 being the train nose position facing 282
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Fig. 4. Pressure horizontal distribution on the noise barrier (sensor P3 at 1 m above TOR) and deformation of the post at the base (sensor E1) at the passage of the 3 HS trains: (a) train A, (b) train B, (c) train C. Vertical pressure distribution on the noise barrier: (d) ΔP/ΔPREF for the different trains.
distributions, where differences are about 10%. For Train A and Train C, the ratio between the maximum negative and maximum positive deformation is around 0.65 and it can be related to the corresponding ratio between the positive and negative pressure peaks that have almost the same value for these two trains (around 0.87). For Train B, the ratio of the two deformation peaks is around 0.78 with a ratio of positive and negative pressure peaks of 1. Of course, the response of the post is not related only to the pressure load values, but also to the time evolution of the pressure load and its interaction with the structure dynamics. Looking at the deformation recorded during the train passage between the nose and the tail transit, and after the extinction of the initial decay, the train-flow-structure interaction is different among the 3 trains: small fluctuations in a wide frequency band for Train A and B and larger fluctuations with a narrower frequency band for Train C. The results are obviously dependent on the train speed and geometry, besides the natural frequency response of the structure, but results show the clear connections between the aerodynamic design of the train and the effects induced on the line.
with the one measured on the barrier (Fig. 4-d) it is possible to see how the maximum value recorded at 1.5–1.75 m above the TOR has now shifted to the lower instrumented level. The trend in reducing the pressure with the level above TOR ΔP/Δz is larger in open air than on the barrier and is strongly related to the noise barrier height and shape. It is interesting to examine the response of the barrier post to the passage of the different trains. The passage of the train head represents an impulsive load, therefore the dynamic response of the post shows the decay of the vibration amplitude at the natural frequency of the structure. Considering the nose train passage, the first deformation of the post is negative, since the positive pressure is pushing the noise barrier away from the train (see sign conventions of Fig. 3); when the negative pressure occurs, the post is pulled towards the track, and this is the highest deformation value that is observed. In Fig. 4-a-b-c the deformation measured by the strain gauge on the post base is reported. The quantity reported is ε/εREF, where ε is the deformation measured during the train passage, while εREF is the maximum deformation measured for train A. Considering the initial large negative value of deformation, values are similar among the 3 trains, with the largest value recorded for Train A since it has the largest positive pressure peak value. Considering the first large positive value of deformation, results are more different among the 3 trains, reaching a 30% difference between Train A and B that is not predictable by looking only at the ΔP vertical
2.3. Slipstream effects beside the track Together with the pressure field produced at train pass-by, there is also a velocity field that has important effects on railway structures (signal boards, equipment boxes and cabinets) and on people (passengers 283
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number of runs available for each considered train. Fig. 6 shows a comparison between the results obtained from the 3 different HS trains. It represents the local average (μ) of the magnitude of the horizontal in-plane wind velocity component U recorded by the anemometers during all Train C passages. The value was normalised by dividing it by the maximum value of the average evaluated for Train A (indicated as Uref in the following plots) to show the percentage difference among the results. On the x-axis, the non-dimensional x/LTrain coordinate was used to associate the results to the passage of the single train sections in front of the anemometer. A similar large deterministic peak was produced by the train nose passage (x/LTrain ¼ 0) independently from the train nose geometry. The increase of the boundary layer along the train (0 < x/LTrain <1) was much higher for Train A compared to the other two trains, which showed an almost identical behaviour. These two trains, belonging to modern aerodynamic design standards, are equipped with more refined aerodynamic solutions, such as the adoption of skirts in front of the bogies and gangways in the intercar gap to avoid strong variations in the cross section geometry in that region. The performance of similar devices in controlling the growth of the boundary layer around the lateral walls of the train was very evident in slipstream measurements. The passage of the train tail (x/LTrain ¼ 1) produced another evident and almost deterministic peak that was almost equal, in comparison with Train B and Train C, but it was not clearly visible in Train A because hidden by the large growth of the boundary layer mixing with the wake region. For Trains B and C, this tail peak in the average value of the in-line velocity was smaller than the one produced by the train head. Maximum values of the slipstream local average value were reached, in any case, in the near wake region just after the tail. Different maximum values were reached by the three trains: Train C reached 50% of the Train A value and Train B the 40%. From the analysis of the average flow field, it seems that Train A had a different wake structure compared to the other two trains: it was characterised by a sharp peak very close to the tail. A trend very similar to that found for train A is shown in (Baker et al., 2014a) for the ICE-1 and ICE-2 trains and the S-120 and S-130 trains which show a very large and sharp peak just after the tail. In (Baker et al., 2014a), this behaviour is associated to less rounded noses/tails. For the new generation of trains, the large local average values of the near wake region last longer downstream from the tail instead of starting to immediately decrease, as occurs with Train A. It is recognized that dangerous slipstream effects are related to the gustiness of the flow that may lead people to lose their equilibrium. An estimation of the possible gust speed distribution along the train, for the
Fig. 5. Ultrasonic anemometer installed at 0.2 m above TOR and 3 m from the track centre.
Table 1 The experimental database. Train type
# runs
Train A Train B Train C
57 24 21
and workers) standing along the line. Specifically, this second aspect is considered most at standard levels since it is related to safety. Slipstream effects induced by train pass-by were already being studied in the late seventies (Gawthorpe, 1978) and were intensively investigated in the European projects RAPIDE (Sterling et al., 2008; Hardy, 2007; Schulte-Werning et al., 1999) and AeroTRAIN (Baker et al., 2014a). Measurements of air speed during train passing were carried out by three-components ultrasonic anemometers (Fig. 5) positioned 0.2 m above the TOR and 3 m from the centre of the track, to study the slipstream on workers along the line. The train speed was measured indirectly using light barriers at known distances; temperature and ambient pressure where also measured to monitor ambient conditions. The presented data give a selection of the recorded train passages at the same train speed and in similar environmental conditions (in particular ambient wind speed lower than 2 m/s). Table 1 resumes the
Fig. 6. Normalized in-plane component U: local average values (μ) as a function of the normalized coordinate X. Comparison between the three trains. 284
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Fig. 7. Normalized in-plane component U: local peak velocity values (μþ2σ) as a function of the normalized coordinate X. Comparison between thethree trains.
Fig. 8. Normalized in-plane component U: maximum normalised velocity value for each train passage as a function of the normalized coordinate X.
the maximum values of slipstream velocity are gathered again in the near wake region, but also some extreme gust locations are visible in the region close to the train nose. These are relative to some records of particular windy days, that slightly afflicted test results. Finally, watching Train A, it was seen that the gusts were deterministically located just after the train tail, but their magnitudes were widespread along the vertical axis, reaching extreme values significantly higher than those of other trains. The found results, especially for Train A and Train C, are in line with those shown in (Baker et al., 2014b) for S103 and S100.
horizontal in-line velocity component, is obtained by summing, to the already described ensemble averaged values μ, two times the standard deviation σ of the instantaneous values measured during all the considered passages at each time step. The result (μþ2σ) is reported in Fig. 7 for each train, using the same previously adopted reference speed for normalization, for the sake of comparison with all the averaged values. As can be appreciated by the comparison of Figs. 6 and 7, the largest local peak values of the slipstream flow are more confined in the near wake region for all the considered HS trains, and also for Train B and C, which showed a more prolonged average value in the wake region. Train B and Train C are confirmed as having a very similar behaviour and may reach maximum local peak values in the near wake region that are three time higher than the average value in the same position, while for Train A the ratio is lower than 2.5. Local peak values in the middle of the train are better correlated to the variation of the train geometry, highlighting sharp regular peaks. Flow fluctuations are also present even if smaller, in correspondence with the head of the train. In order to understand where the gusts typically occur in terms of xposition, Fig. 8 shows the maximum normalized peak velocity values Ugust of the in-plane component, for each analysed time-history versus the normalized X-coordinate where the value was recorded. Regarding Train C, the gusts mostly fall in the region just after the train tail, with very few exceptions occurring in the mid-far and far wake regions. Train B shows high dispersions of data along the train length (X-Coordinate):
2.4. Aerodynamic loads on the trackbed The study of aerodynamic loads on the trackbed is necessary since it is strongly related to the issue of flying ballast, one of the major problems caused by increases in railway speed over 300 km/h in terms of safety and early deterioration of both rolling stock and track (Quinn et al., 2010; Saussine et al., 2014; Navarro-Medina et al., 2012). In simple terms, ballast particles are lifted due to the air flow driven by the underbody of a train. Generally, the ballast lifting phenomenon may arise also at low speeds, if external agents such as ice or other scraps on the track help stone dislodgment. On the other hand, the problem becomes extremely evident as the speed increases, when the ballast stones are lifted due to the pressure and velocity field generated by the train in the upper layer of ballast without the need for an external trigger. Recent studies suggest 285
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Fig. 9. Examples of experimental setups for the flow measurements.
of u ðustd Þ which is an easier to measure variable in full-scale tests (Weise et al., ). For this reason, in order to estimate the aerodynamic forces on the ballast, it is useful to measure the profile and intensity of the flow in the region closest to the trackbed. In particular, in some countries the experimental measurements of the flow above the trackbed have already been implemented in national standards, based on the estimate of the power of the flow (Lazaro, Rodriguez-Plaza; EPSF). To measure the flow velocity field between the train underbody and the ballast surface, different types of transducers can be used (Deeg et al., 2008) so as to measure also the different components of both the flow and the aerodynamic loads acting on the track. On the other hand, comparing the averaged values of the three components, it was found (Kaltenbach et al., ; Sima et al., 2011; Premoli et al., 2015) that the main component was by far the longitudinal one, since the lateral component was almost null, while the vertical component assumed negative values (that correspond to a suction component towards the train underbelly) but it remained about 10 times lower than the longitudinal one. This means that, to describe the train-induced aerodynamic loads on the trackbed, only the longitudinal flow speed in the gap between the track and the train underbelly can be measured, especially if the target is to compare different train-track configurations. The results that will be shown in this work were collected using a vertical array of a single pitot tube and 13 pressure probes (we used
that collisions begin to occur only at speeds of 270 km/h or higher because the transfer of energy needs to be big enough in order to cause the ballast to rise (Lazaro et al., 2011; Rocchi et al., 2016). The consequences of this phenomenon are different: on the safety of the people working along railway lines, on the running safety of the trains themselves, and, finally, on the extra costs associated with both the rolling stock and the infrastructure maintenance. Since the attempt to predict and control the phenomenon is probably the most important, as it is impossible to stop a stone once it is set in motion, research over the years has focused mainly on it. Although not all factors contributing to ballast lifting have been completely identified (infrastructure geometry, train underbelly design up to the track excitation during the train passage) it is well established that the key factor is the imposed aerodynamic load (Gawthorpe, 1978). Wall shear stress has been identified as the most appropriate measure of the aerodynamic load on the track (Weise et al., ). On the other hand, as soon as the stone rises slightly from its position, it runs into the flow, and the dynamic pressure, proportional to the square of the flow velocity u2 , becomes predominant. Furthermore, due to the many discontinuities both on the train and on the track geometries, this flow is characterised by a high turbulence index (Lazaro et al., 2011), therefore it is quite difficult to measure the shear stress in train operation conditions. Fortunately, the shear velocity near the ground is approximately proportional to the standard deviation
Fig. 10. Comparison between the averaged time histories of the normalized speed of the flow induced by three different trains at three different heights (27 mm, 180 mm and 230 mm with respect to the Top Of Rail). 286
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Fig. 11. Comparison between the averaged vertical air speed profiles of the three HS trains.
all the sensors of the vertical rake when calculating the mean vertical profile of the flow velocity longitudinal component, averaging the values of the Cu of the central section where the flow reaches steady conditions (the averages are calculated in the section between the two dashed lines of Fig. 10 omitting the initial and the final evolutions). In Fig. 11 the comparison of the results obtained for the three cases is shown. The flow profiles generated by the three different trains, although different for the absolute values, have almost equal shapes. The air speed profile of Train B has the lowest values in terms of both mean value and standard deviation (see Table 2), maybe due to the lower number of bogies per unit length, which results in a less generalized roughness of the underbody, while the Train A and the Train C are characterised by similar values, with the flow speed of the Train C being slightly lower than that of the Train A, probably as a result of the better aerodynamic design of the Train C underbelly. Finally, with regard to the maximum value measured in correspondence of the tail, all the trains are characterised by similar absolute values of 1.6 times the reference speed Uref. This means that, in the case of Train B, the tail peak is almost two times (1.86) the mean of the central section. This last peak is the highest velocity during the wall passage. Even though stones potentially moved by it cannot hit the train, they could become a safety problem for people and structures standing along the railway lines.
Table 2 Comparison between the statistical values of the speed measured by the upper probe.
Train A Train B Train C
#bogies/length
Mean
Std Dev
Tail Peak
0.08 0.06 0.08
1 0.85 0.96
0.048 0.030 0.041
1.57 1.59 1.53
simple tubes with the only total pressure tap, using as static pressure the one measured by the pitot) able to measure the vertical profile of the longitudinal component of the flow velocity (see Fig. 9). The pressure probes made it possible to study in detail the region close to the sleepers and to the trackbed where the flow is characterised by a high vertical gradient (Premoli et al., 2015). Also in this case, the results are presented in a non-dimensional manner taking as reference (Uref) the mean speed recorded by the highest pitot during the passages of train A. Fig. 10 compares the averaged temporal evolution of the flow speed coefficients measured through the pitot rake at three different heights during the passage of the three different trains (twenty different runs for each train were averaged). A first small peak of speed (corresponding to the overpressure in front of the train head at x ¼ 0) characterises the passing of the nose region. After it, the speed gradually increases and, after a couple of cars, it stabilises with periodic oscillations around a constant value until the tail of the train passes over the instrumented section and a second major peak occurs; finally, the speed reduces with some fluctuations due to the wake. The main periodic oscillation at low frequency caused by the passage of the coaches depends on the length of the coaches themselves and, considering a train speed of 300 km/h, the corresponding frequency lies at around 3–5 Hz. Comparing the averaged temporal evolution of the speed measured by the upper pitot it is possible to compare the flow induced by different trains from a qualitative point of view. The oscillations are strongly influenced by the shape of the underbody region; in the case of Train A, after two cars the flow is completely developed and oscillates around a constant value, while in the other two cases the behaviour is slightly different. Although about two cars are still required to reach a constant value, the flow does not oscillate with a regular pattern as occurs with Train A. Train B and Train C are new generation trains of the EMU type (Electric Multiple Unit); the airflows necessary for cooling the power components are partially blown below the trains and probably modify the flow in the underbody region. Considering now the dependence of the phenomenon on height, it can be noted that the behaviour is always comparable and the height only has the effect of scaling the mean values. It is therefore possible to consider
3. Conclusions Results of experimental campaigns performed to investigate different aspects of the aerodynamic loads induced by the passing of HS trains on structures, people and tracks are presented. In particular, the comparison between the aerodynamic effects induced by three HS trains running on the Italian HS line is proposed for head pressure pulses in open air, overpressures on noise barriers, slipstream flows and flow on the trackbed. The comparison highlights, in quantitative terms, the improvements in the aerodynamic design of HS trains (Train A vs Trains B and C) related to a more aerodynamic shape of the nose, and the adoption of aerodynamic devices such as bogie skirts and gangways in the inter-car gap. Differences in the aerodynamic behaviour of the 2 modern trains (Trains B and C) are presented and discussed in the paper, demonstrating that the aerodynamic design of HS train is an important and ongoing activity and that aerodynamic issues are becoming dominant in the recent development of HS railway lines aiming at increasing train speed and capacity.
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Acknowledgments
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