Steady and moving high-speed train crosswind simulations. Comparison with wind-tunnel tests.

Steady and moving high-speed train crosswind simulations. Comparison with wind-tunnel tests. Federico Cheli, Daniele Rocchi, Paolo Schito, Gisella Tomasini Dipartimento di Meccanica del Politecnico di Milano, via La Masa 1, 20156 Milano, Italy Abstract CFD simulations are used to investigate the effect of the relative motion between train and infrastructure in order to capture the differences between performing wind tunnel tests with static or moving vehicle to measure aerodynamic coefficient. Two infrastructure scenarios are considered: a single track ballast and rail and a 6m high embankment. CFD simulations are performed in the same geometrical scale of wind tunnel tests to allow a numerical validation against experimental data on still models, both in terms of global aerodynamic coefficients and surface pressure distribution, while the considerations on the relative motion effects will be drawn from the comparisons between numerical results obtained with moving and still models simulations. Keywords Cross wind, high speed trains, moving vehicle, CFD simulation, aerodynamic coefficients, surface pressure distribution Introduction In the present paper, the results of a CFD analysis on the effects of the train-infrastructure relative motion on the aerodynamic coefficients will be presented and discussed. The interest in such activity is related to the common practice, adopted in train aerodynamics studies, to perform wind tunnel tests on still models, reproducing the relative wind angle of attack in the horizontal plane by rotating both the train and the infrastructure models around the vertical axis. This situation is different from real one where the relative wind speed on the vehicle is due to the sum of the vectors of the train velocity and of the cross wind velocity, while the infrastructure, being still, is subjected to the absolute wind speed. In case of cross wind blowing perpendicular to the line, the infrastructure is actually run over by a perpendicular wind while the aerodynamic forces, acting on the train, are produced by the relative wind that is skewed in the horizontal plane. A scaled representation of the aerodynamic phenomena would require wind tunnel tests on moving models. The main drawbacks of this test methodology, relying on the experience of previous activities, performed also by the authors, are related to the test rig complexity, to the contemporary action of aerodynamic and inertial forces and to the difficulty to keep a steady state condition for a long time [1] [2] [3] [11]. Considering high speed train applications (train speed higher than 250 km/h), the interesting wind -train yaw angles are confined in a range between 0 and 30 deg (if cross wind speed higher than 25 m/s is taken into account). For still model teststs the flow over the infrastructure is therefore far from the real condition while for moving model tests this means that the train velocity vector representss the major contribution to the magnitude of the relative wind speed and the moving model in the wind tunnel has to move faster than the wind speed. Among the major difficulties in performing moving vehicle tests the most demanding are: the need to perform many tests since only few seconds of steady state conditions are available in each test run; and the need to deal with inertial forces and the dynamic behavior of the moving parts that may influence the train aerodynamics. In the authors experience, the comparison between experimental results on still and moving models is complicated by the uncertainties of all the mentioned aspects even if comparable results have been

obtained [3]. In the present study, CFD simulations have been performed to investigate how much the aerodynamic coefficient are modified by the relative motion between the train and the infrastructure. Numerical simulations are used as virtual experimental laboratory tests where the scaled model and the flow conditions are under control while the vehicle motion represents a simple change in the boundary conditions [10]. At the beginning, the same scaled testing conditions used in wind tunnel on still model have been reproduced to validate the CFD approach. Two infrastructure scenarios have been considered: a smaller one where just a single track with ballast and rails is modeled and a larger one where a full double track 6 m embankment is taken into account. They have been chosen since they are widely studied ([4], [5], [6]) because they represent two of the configurations considered in European standards [7] and technical specifications [8] on cross wind effects on trains and overall they represent two situationss where the infrastructure plays a small and largerulein the whole models geometry. Infrastructure scenarios with infinite length are considered in the CFD simulations by extruding the infrastructure geometry up to the numerical domain in order to investigatee just the relative motion effects while during wind tunnel tests a finite length infrastructure is used [3]. After the initial validation of the numerical results comparing the aerodynamic coefficients and the surface pressure distribution for different yaw angles on the still model with experimental results, CFD simulations have been used to perform moving train tests using the same mesh and simulation set-up with a moving reference frame approach [10]. Numerical results have been therefore compared and considerations have been proposed in the conclusions. Model Setup The analysis has been conducted on a 1:15 scaled model of an ETR500 train. The numerical setup has been prepared in order to be representative of the experimental wind tunnel test performed at the Politecnico di Milano Wind Tunnel [3]. The same CAD model that was used for the CNC machining of the wind tunnel modelis used to preparethe CFD mesh domain ( -a). The train convoy is made by a first vehicle that was instrumented for the wind tunnel tests with a six components balance to measure the forces and moments, and by a second vehicle, reproducing a part of a trailer car to correctly reproduce the boundary condition for the flow around the leading car. Surface pressure distribution was measured during wind tunnel tests on STBR scenario using 156 pressure taps distributed over 24 sections on the first vehicle, according to the scheme reported in Figure 2-c. Forces, moments and pressures are reported in non-dimensional form as coefficients, calculated according to the reference system of Figure 1-d as follows: C P p Fi CFi 1 2 1 2 V V A 2 2 M i CMi i x, y, z 1 2 V Ah 2 wherep is the static pressure, F i is the force projected along the i-th direction, M i the moment measured around the i-th axis with respect to the point located on the plane on the top of the rails in the middle of the rails and the boogies, ρ the air density, V the relative wind speed, A the train reference area (A=10m 2 full-scale) and h is the train reference height (h=3m full-scale).

(a) (b) (c) (d) 3.2 m 17 m 28 m The two considered infrastructure scenario are the Single Track Ballast and Rail (STBR) and the 6m high Embankment (EMBK) reported in. During wind tunnel tests on the STBR a splitter plate was used to cut-off the incoming boundary layer profile while tests on EMBK were performed positioning the infrastructure model directly on the wind tunnel floor. The splitter plate used in the wind tunnel tests is not reproduced in the numerical simulation but similar flow conditions are obtained through floor boundary condition. The numerical rectangular domain, whose main dimensions are reported in Figure 2 (for the STBR case), has been decomposed in different sub-volumes allowing for: the allowance to combine different volumes in different ways in order to obtain different scenarios without the need of further meshing, and a better control of the mesh quality, allowing for using a structured grid for most of the sub-domains. Where no structured grid could be realized (typically near the train) a tetrahedral unstructured grid with a refinement near the surface of the train has been created. The mesh has 8.5 million cells, there is no prism layer on the train surface and the non dimensional wall distance is y + >10. Steady state Reynolds Averaged Navier-Stokes (RANS) equations have been solved, using a shear-stress transport (SST) k-ω model [9]. Numerical simulations have been performed in nominal smooth flow conditions (turbulence intensity I U =0.2%) at the same Reynolds number of wind tunnel tests (Re 2.5e 5 ). The reproduction of the wind yaw angle β W is experimentally obtained rotating the train and the scenario with respect to the fixed direction of the air flow ( ), while in the CFD simulations the model is maintained aligned with the reference frame and the wind yaw angle is obtained imposing boundary condition to the vertical face of the domain.

CFD Validation The comparison between experimental and CFD results are carried out in terms of forces and moments acting on the first vehicle and on pressure distributions on the sections where the pressure taps are located. The analysis has been focused on wind incidence angles 0 <β<30, because these angles are typical of high speed trains. The results of the CFD simulations compared to the experimental data are reported in.in the same figure an histogram of the relative error, at each yaw angle, is reported.themaximum error (around 13%) is on the rolling moment fora yaw angle β W =15, while for the other yaw angles the error is generally lower than 10%. An additional comparison between CFD and wind tunnel results is reported in Figure 7 in terms of surface pressure distribution for different train sections (section 15 on the train head and section 19 and 20 on the train body) and different yaw angles (10 degrees and 25 degrees). A good agreement is visible on the pressure distribution on the train head, while some discrepancies are present just on the leeward side of section 19. The good numerical-experimental agreement on forces and pressure distributions means that CFD simulations are able to reproduce the main flow features of the train aerodynamics in the investigated yaw angles. Therefore, in the following, CFD simulation will be used to investigate the effects of the relative train-infrastructure motion on the definition of the aerodynamic coefficients.

a b c d e f. Moving train simulations The reproduction of the train movement has been performed through the imposition of proper boundary conditions on the entire domain. All the performed simulations are steady-state simulations, assuming a constant velocity for the train and using an inertial Moving Reference Frame (MRF) with a relative motion with respect to the absolute one. The equations of flow are solved in the Absolute Reference System (ARS), and a speed equal to the trainspeedis assigned to the entire cell domain as it were measured by

the Moving Reference Frame (MRF) linked to the train. The flow boundary conditions on the train surface is imposed as no slip condition in the MRF, while the flow boundary condition on the ground, on the ballast and on the rails is no slip condition in the ARS. To appropriately compare the wind yaw angle experienced by the train in the still and the moving condition, andto maintain the Reynolds similitude, the relative air-train speed is kept constant and equal to 15 m/s. For the MRF simulations the relative wind yaw angle and speed results from the combination of the train speed and the wind speed: fixing the absolute cross wind angle to 90 with respect to the train, the relative train-wind velocity vector is given by the sum of the train velocity vector and the wind velocity vector as indicated in. The combination of the two velocity vectors, in presence of an atmospheric boundary layer profile, results in a variation of the yaw angle with the elevation from the ground, as sketched in Figure 9. Figure 9comparesthe vertical profiles of the relative wind velocity and of the wind yaw angle along a vertical line located 0.5m upwind the leading vehicle in the middle of the rails for the two different infrastructure scenarios, and for moving and still train simulations.it is possible to observe that the main differences between the still and the moving case are located in the lower part of the profiles. The differences consist in a variation of both the relative wind speed and the wind yaw angle. When the train is moving the relative wind speed is obviously higher than for the still case in the underbody region, but the wind yaw angle is lower. The train running on the EMBK experiences higher relative wind speed for both still and moving calculations, if compared to the STBR case, due to the speed-up effect of the scenario. In the upper part the discrepancies between still and moving model simulations are negligible in terms of relative wind speed even if the moving vehicle on the EMBK scenario is run over by a more skewed relative wind. STILL TRAIN MOVING TRAIN

The aerodynamic coefficients computed by still train and moving train simulationson Single Track Ballast and Rail (STBR) scenario are reported in.

The larger effects of the relative motion between train and infrastructure are visible in the vertical force coefficients, while for the other two coefficients it can be assumed that the moving model coefficients are higher than the still model ones less than 6% at low yaw angles and less than 2-3% at high yaw angles. The larger variation in the vertical force coefficient can be related to the different flow field that is produced in the lower part of the train,in the two conditions. This is confirmed by the analysis of the pressure distribution reported in. The main differences are located essentially in the lower part of the train sections, close to the train head. The higher flow rate that is present when the train is moving results in a higher suction under the train body and in a larger negative lift force. Apart from the underfloor region, the pressure distribution is very similar between MRF and Still simulations, highlighting that the flow field around the still model is very similar to the one generated around the moving model since the infrastructure effect in the STBR case is very limited. Considering the 6 meters high embankment (EMBK) ( ), the differences in forces and pressure distribution between the still and moving train are more evident, but in any case the differences on the lateral force and overturning moment coefficients are lower than 13%for all the yaw angles, almost uniformly distributed on the whole range. For the EMBK scenario, higher variations between the steady and the moving case are expected, since the scenario has a higher influence on the incoming flow as already seen in the wind profiles of.

Aerodynamic coefficients on EMBK are larger than those on STRB since, starting from the same incoming wind conditions, the wind-infrastructure interaction produce a larger wind speed and a larger relative wind yaw angle in front of the train. Comparing the vertical relative wind profiles for the moving and still model simulations on EMBK it is possible to appreciate that the wind speed up is similar while a larger yaw angle is perceived by the train in case of moving model. The difference of the yaw angle is less than 1 degbut,considering the slope of the trend of the aerodynamic coefficient versus the yaw angle, it may justify the variation of the aerodynamic coefficients. Pressure distributions on the train sections for the EMBK case are reported in for the still and moving vehicle case and show more evident differences than in the STBR case. The expected differences on the lower part of the train are present for the EMBK case as well, but some differences are also present on the upper part of the train section. The larger suction of the under-floor region is related to the wind speed up and deflection induced by the wind-infrastructure interaction. The larger suction in the upper part is, on the contrary, mainly due to the different yaw angle seen in the two testing conditions. In order to separate the effects of the wind speed up and of the variation of the yaw angle on the pressure distribution the following analysis is proposed for both STBR and EMBK scenarios. The analysis has been conducted using the relative wind profiles measured 0.5m in front of the train, reported in, to compute the pressure coefficient distribution around the train. At each point the pressure coefficient is computed using not the reference wind speed, but the wind speed at the corresponding height. In this way, even if it is not completely correct to assume that the flow around the train is stratified, the speed up effect is filtered out from the coefficients and the pressure distribution discrepancies may be related to the variation of the yaw angle. The resulting normalized pressure coefficient distributions are reported in.

For the STBR case the differences in pressure distribution in the lower part of the train section observed in, can be imputed to the combined effect of a wind incidence angle and a relative wind speed: the moving train experiences higher pressures, but, normalizing the pressures with the incoming wind speed, the pressure coefficient becomes lower, as it can be expected for lower wind incidence angles. On the upper part there is no evident change in pressures, since the relative wind speed and the wind incidence angle are similar for the two cases. The EMBK scenario shows more differences, even if the comments already done for the STBR scenario are valid for the lower part of the EMBK case, on the higher part of the sections the moving train has higher normalized pressure coefficients: this effect can be imputed to the higher angle of attack that is observed in the moving case with respect to the still train.

Conclusions The comparison between the results of the simulations with still and moving models highlighted that considering the relative motion between the train and the infrastructure leads to larger aerodynamic coefficients. The variation is larger the larger is the infrastructure dimension with respect to the train dimension. In fact considering the small STBR scenario negligible effectsare present, in the range of the investigated yaw angles, on the lateral and rolling moment aerodynamic coefficients. Small differences appear in the surface pressure distribution, just in the region between train and infrastructure that influence mainly the vertical force coefficient. Considering the embankment scenario, the effect of neglecting the train motion leads to an underestimation of the lateral and rolling moment coefficients of the order of 10 % in the considered range of yaw angles.the deflection of the flow induced by the interaction with the infrastructure changes the incoming flow conditions on the front part of the train. This effect is less important in the rear part where the flow interaction with the train is predominant. Since the effect is limited to a small part of the whole train the impact on the aerodynamic coefficient is not so pronounced. Considering the overturning risk associated to cross wind, the underestimation of the lateral and momentaerodynamic coefficient will lead to an underestimation of the Characteristic wind curves that represent the limit conditions for the train overturning. It s worth saying that an underestimation of the aerodynamic coefficients don t means an equivalent variation of the threshold values since, for high speed trains, they are computed through non linear multibody simulations. A further study will be performed to evaluate the propagation of the variation of the aerodynamic coefficients on the CWC computation. In the framework of a simplification of the tests requirements for high speed train homologation that led in the past to the adoption of one single reference scenario for wind tunnel tests, the results of the present research support the adoption of still model tests on STBR scenario as representative of the train aerodynamics. Results on the embankment scenario furthermore highlighted that the aerodynamics of the train on the embankment is deeply influenced by the infrastructure and the extrapolation of the aerodynamic behavior starting from the STBR results needs careful investigation. The magnitude of the aerodynamic coefficients variation when train motion is considered is anyway of the same order of the measurement uncertainties that could be introduced when more complex test rig set-up and measurement systems are used for moving models wind tunnel tests. References [1] C.J. Baker and N.D. Humphreys, Aerodynamic forces and moments on containers on flat wagons in cross winds from moving model tests, Nottingham University, Department of Civil Engineering, Report Number FR91017 (1991). [2] C.J. Baker, Train aerodynamic forces and moments from moving model experiments, J. Wind Eng. Ind. Aerodyn. 24 (1986) 227 251. [3] M. Bocciolone, F. Cheli, R. Corradi, S. Muggiasca, G. Tomasini, Crosswind action on rail vehicles: Wind tunnel experimental analyses, Journal of Wind Engineering and Industrial Aerodynamics, Volume 96, Issue 5, May 2008, Pages 584-610 [4] F. Cheli, R. Corradi, D. Rocchi, G. Tomasini, E. Maestrini, Wind tunnel tests on train scale models to investigate the effect of infrastructure scenario, Journal of Wind Engineering and Industrial Aerodynamics, Volume 98, Issues 6-7, 6th International Colloquium on Bluff Body Aerodynamics and Applications, June-July 2010, Pages 353-362 [5] M. Schober, M. Weise, A. Orellano, P. Deeg, W. Wetzel, Wind tunnel investigation of an ICE 3 endcar on three standard ground scenarios, Journal of Wind Engineering and Industrial Aerodynamics, Volume 98, Issues 6-7, 6th International Colloquium on Bluff Body Aerodynamics and Applications, June-July 2010, Pages 345-352 [6] M. Suzuki, K. Tanemoto, T. Maeda, Aerodynamic characteristics of train/vehicles under cross winds, Journal of Wind Engineering and Industrial Aerodynamics, Volume 91, Issues 1-2, January 2003, Pages 209-218

[7] CEN, 2010. EN 14067 railway applications aerodynamics part 6: requirements and test procedures for cross wind assessment. European Norm, CEN/TC 256 [8] EC, 2006. TSI Technical Specification for Interoperability of the trans-european high speed rail system. European Law, Official Journal of the European Communities. [9] F.R. Menter, Two-Equation Eddy-Viscosity Turbulence Models for Engineering Applications, AIAA Journal, 32(8):1598-1605, August 1994. [10] C. Catanzaro, F. Cheli, D. Rocchi, P. Schito, G. Tomasini: Hi-speed train crosswind analysis: CFD study and validation with wind-tunnel tests, Proceedings of Aerodynamics of Heavy Vehicles III: Trucks, Buses and Trains, Potsdam, September 2010 [11] Li,Y.Hu,P.Zhang,M.,Liao,H.:Windtunneltestwithmovingvehiclemodelforaerody-namicforcesofvehicle- bridgesystemsundercrosswind.apcwe7theseventhasia- PacificConferenceonWindEngineering,Taiwan,(2009)