THE INFLUENCE OF THE NOSE SHAPE OF HIGH SPEED TRAINS ON THE AERODYNAMIC COEFFICIENTS

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1 THE INFLUENCE OF THE NOSE SHAPE OF HIGH SPEED TRAINS ON THE AERODYNAMIC COEFFICIENTS Heine Ch. and Matschke G. Deutsche Bahn AG, Research and Technology Centre Voelckerstrasse 5, D Munich, Germany ABSTRACT Increasing velocity combined with decreasing mass of the leading vehicle of modern high speed trains poses the question of the influence of strong side winds on the vehicle dynamics. Deutsche Bahn has undertaken experiments in an automobile wind tunnel at 1:10 scale. 14 different nose shapes were investigated, and all six aerodynamic coefficients were measured at yaw angles up to 90. The contours of the wind tunnel models were chosen following a parametric approach. The models differ from the existing DB vehicle ICE2 in one distinct feature: One set of models differ in nose length, others differ in nose width or vehicle height. The crosswise curvature of the roof and the underfloor spacing were investigated as well. The results can be directly compared with the ICE2 model which serves as the reference case. Additionally two very innovative nose shapes were mounted on the wind tunnel balance: A model, where a large portion of the front has a concave contour an a model entitled flat-iron. The tested models differ significantly in some of the aerodynamic forces and moments, especially the lift appears to be very sensitive to changes in nose shape. The results offer a starting point to judge future train concepts in terms of aerodynamic forces in the presence of side wind. 1 INTRODUCTION The least generation of high speed trains reach speeds of 300km/h in regular operation, e.g. the German ICE3, the French TGV Duplex or the Italian ETR500. At these speeds aerodynamic forces and moments are becoming more and more important for the running performance of the train. The energy consumption is by almost 90% covered by the aerodynamic drag, the lift forces and yawing moment influence the running behaviour and riding comfort of the vehicles and strong wind may effect the running stability via the lift, side force, pitching, yawing and 1

2 rolling moment. Because of the asymmetrical flow around the nose of the train the car in leading position is the most effected and therefore in the centre of interest. For the optimisation of future train layout clear knowledge of the influence of different design parameters on the aerodynamic characteristics of the train are mandatory. Modern train concepts cover a wide range of design variants: Streamlined noses like the German ICE3, extremely long noses in Japan, wide body concepts, double-deck trains, or relatively bluff designs like early TGV trains. For most of these trains the results of computational investigations or wind tunnel tests are not published. Even if they were, so many parameters differ between the designs, that a isolated view on each of these parameters is not possible. The aim of the project was consequently to investigate the influence of different design parameters like height, width, roof corner radii etc. with only one parameter being changed at a time. 2 MODELS AND WIND TUNNEL TECHNIQUE 14 different models of train heads were built in the scale 1:10. The basis formed the head of an ICE2 driving trailer with a length of 26,4m and a operational speed of 280km/h. Some of the most drastic configurations can be seen in figure 1. Figure 1: Long + slender nose, long nose, duck nose and sharp nose 2

3 The models were build modularly on a wooden basis with interchangeable heads. The noses, roofs and bottom sections were made of high density foam, which was CNC shaped. The construction allowed to save wind tunnel time and costs by a very quick change of the design variants within 2-3 minutes without needing to dismantling the model from the wind tunnel balance. Each model consisted of the leading car, which was mounted on the wind tunnel balance and one half of an intermediate car which was streamline shaped at it s rear to ensure realistic flow conditions around the leading vehicle. On the basis of the ICE2 driving trailer the following parameters were changed iteratively (see Table 1): An overview of some configurations can be seen in figure 2. Table 1: List of the design variants Parameter Variation 1 Variation 2 Nose length X nose =3 X ICE2 X=1/3 X ICE2 Nose width Y nose =3 Y ICE2 Y=1/3 Y ICE2 height 0.5 m lower than ICE2 0.5 m higher than ICE2 Roof radius R = 0.2 m R = 1.0 m Lower car bottom 15 cm less than ICE2 Nose curvature convex (sharp nose, flat iron ) Concave ( duck nose ) combinations X nose =3 X ICE2 long + Y nose =1/3 Y ICE2 narrow X nose =1/3 X ICE2 short + Y nose =3 Y ICE2 wide Figure 2: Sketch of configurations The test were carried out in a large German automotive wind tunnel. The wind tunnel is equipped with an under floor 6D external balance, where normally the car tires are mounted on 3

4 special pads. Two of these pads were chosen to fix the train model on the balance. The wind tunnel is of the Göttingen type with open test section (closed plenum) and closed return circuit. It s maximum flow speed is 200km/h when the blockage is small. Some of the basic wind tunnel parameters are listed in table 2. Table 2: Basic wind tunnel parameters Flow speed Cross section (nozzle) Dimensions of test section Blockage ratio during tests 210km/h m_ 12m 6.2m 3.6m 0.5 %(ß=0 )... 6,5% (ß=90 ) 3 RESULTS Respecting the height of rail and sleepers the model was mounted with a distance of 18mm over the wind tunnel floor. During the tests the boundary layer suction capability of the wind tunnel was used, were the boundary layer can be held constant at about 20mm on the wind tunnel floor by distributed holes. The results are presented in the form of the non- dimensionalised aerodynamic coefficients. The forces F i and moments M i in x,y and z direction are represented by F / 2 u c i and i c i 2 i x,y, z mi 2 i x, y, z A M / 2 u A l with = (kg/m_), u = flow speed, A = 10m_ (ref. area), l=3m (ref. length). The moments were calculated relative to a point in the mid of the first coach on rail level. A definition of the co-ordinate system is given in figure 3. 4

5 Figure 3: Definition of coordinate system (shown on no.7 sharp nose -config.) The yawing angle was varied between in steps of ß =5. The symmetry of the results in the relevant coefficients c y, c z and c mx is for most configurations very good and in a range of c i <3% at =30. All tests were carried out with a maximum flow speed of 200km/h leading to a Reynolds number of Re model = (relative to the hydraulic diameter of the train), which is one order of magnitude below the full scale value. Comparative test runs at lower flow speed have shown, that nevertheless the results for the investigated configurations can be regarded as independent from the Re number at Re> 5*10 5. All results are compared relative to the ICE2 configuration. Influence on the drag force Typically the rolling resistance of a modern high speed train is at speeds above 250km/h dominated by the aerodynamic drag by more than 80%. It has been shown, that the head of the train participates to the overall drag with a share of ca. 10% [1]. In Germany the typical mean wind speeds are in the magnitude of 4m/s, resulting for a representative train operation in yawing angles of ß<5. Figure 4 shows the comparison for these conditions for the different train configurations. 5

6 Drag force Cx 0,250 0,200 Drag force Cx [- ] 0,150 0,100 0,050 1-ICE2 2-short nose 3-long nose 4-wide nose 5-narrow nose 6-duck shape 7-sharp nose 8-sharp edge 9-round edge 10-short+wide 11-long+narrow 12-low bottom 13-low roof 14-high roof , yawing angle ß [deg] Figure 4: Comparison of drag force for ß=0 and ß=5 The aerodynamic drag is for most configurations slightly higher than the drag of the ICE2. Only for the configurations 3,11(long noses) and 13 (low car body) significant reductions of around c x 5% can be found, whereas the low car bottom increases the drag by c x 28%. Also a rise is observed for the configurations 2, 5, 10 an 14 (high roof and short nose configurations) with c x 5%. Influence on the side force The side wind behaviour of the train is dominated by side and lift forces acting on the car body. Under extreme wind conditions these forces may be strong enough to exceed limiting values for vehicle dynamics (wheel unloading or flange climbing) or track parameters (track shift). The proof of safe operation under side winds is, as a standard procedure, for DB part of the homologation of the vehicle [2,3]. The simulation of the dynamic vehicle behaviour shows, that tolerable wind speeds for a high speed train are typically between 25m/s and 35m/s, resulting in yawing angles in the range of 20 <ß<30. Among all other forces the side force has the biggest effect on the running stability of the vehicle. The comparison for the different train configurations is given in figure 5. Again the difference between the different configurations is relatively small. Only for the configurations 3, 11 (long noses), 7 (sharp nose) and 13 (low roof) a significant reduction relative to the ICE2 can be observed. The effect of the long and the sharp nose is moderate with reductions of the side force between 2%< c y < 6%. The biggest effect of c y 20% can be gained lowering the roof of the train by 0.5m. Corresponding the side force is increased by the same extent when rising the car body height. 6

7 Side force Cy Side force Cy [- ] 6,000 5,000 4,000 3,000 2,000 1-ICE2 3-long nose 5-narrow nose 7-sharp nose 9-round edge 11-long+narrow 13-low roof 2-short nose 4-wide nose 6-duck shape 8-sharp edge 10-short+wide 12-low bottom 14-high roof 1, , yawing angle ß [deg] Figure 5: Comparison of side force for ß=20 and ß=30 Influence on the lift force The lift force has it s share on the running behaviour of the train causing under high yawing angles and high driving speed undesired unloading of the wheels in the order of several kn. The main results are summarised in figure 6. Lift force Cz Lift force Cz [- ] 3,500 3,000 2,500 2,000 1,500 1,000 1-ICE2 3-long nose 5-narrow nose 7-sharp nose 9-round edge 11-long+narrow 13-low roof 2-short nose 4-wide nose 6-duck shape 8-sharp edge 10-short+wide 12-low bottom 14-high roof 0,500 0, yawing angle ß [deg] Figure 6: Comparison of lift force for ß=20 and ß=30 Also for the lift force the configurations 3, 11 (long nose) and 13 (low roof) show the best results with relative reductions of the lift of about c z 20%. The highest reduction however 7

8 of c z 30% can be gained with the duck nose shape of configuration 6. Also with the sharp roof radii of configuration 8 a significant lift reduction is reached at high yawing angles forcing the flow to separate at the edges of the roof. The configurations 7 (sharp nose), 12 (low bottom) and 13 (high roof) show the worst behaviour with a relative increase of the lift force between 10%< c z <25%. With increasing yawing angle also the effects on the lift force increase. Influence on the rolling moment The biggest effect on the vehicle dynamics is caused by the rolling moment as a resultant of lift and side forces. Several accidents in the recent years on narrow gauge track in Japan have shown, that extreme wind gusts may lead to an overturning of the train even at moderate driving speeds. Particular attention must therefore be laid on the minimisation of the rolling moment. Figure 7 shows the rolling moment for the different head shapes. Rolling moment Cmx [- ] 3,000 2,500 2,000 1,500 1,000 1-ICE2 3-long nose 5-narrow nose 7-sharp nose 9-round edge 11-long+narrow 13-low roof Rolling moment Cmx 2-short nose 4-wide nose 6-duck shape 8-sharp edge 10-short+wide 12-low bottom 14-high roof 0,500 0, yawing angle ß [deg] 30 Figure 7: Comparison of the rolling moment for ß=20 and ß=30 As one could expect the most relevant parameter on the rolling moment is the vehicle height, reducing to rolling moment by c mx 25% for the low roof and causing an increase of c mx 35% for the high roof. As a rule of thumb the critical wind speed for a leading vehicle in the relevant range is in- or decreased by 10m/s when the rolling moment changes by c mx 30%. This in turn may cause extreme efforts for measures on the line or may restrict the operation of the train to an unacceptable level. It is not surprising, that again the configurations with the long noses are capable of reducing the rolling moment by c mx 10%. The slenderness of the nose (config. 5 and 11 res. 4) brings only little additional effect relative to the normal configurations. The sensitivity to side winds increases by c mx 8% when the nose is very short (config 2 and 10, lokomotive ) or when the roof radii are sharp (no. 8, c mx 6%). Here the benefit 8

9 from low lift is compensated by a rise of the point of attack of the side force. Exotic shapes like the duck nose have only little effect. 4 CONCLUSIONS A parametric study has been carried out on the influence of shape parameters of the head of high speed trains on the aerodynamic loads and moments of the leading vehicle. Extensive tests were therefore performed in a large automotive wind tunnel on 14 configurations of train models. Due to the modularised construction of the models the tests were successfully carried out in relatively short time. The train variants respected changes in nose length and shape, car body height and roof corner radii. The results show, that drag can be significantly reduced with long and slender noses as well as low-rise car bodies. These configurations are at the same time the most effective ones for improving the side wind stability. The rolling moment is most effectively decreased by over 25% when reducing the height of the car body by 0.5m (e.g. Talgo intermediate coaches). This corresponds to an increase of the critical wind speed of about 10m/s. The next effective variant is the elongation of the nose to 15m (like in the Japanese Shinkansen 500 train) leading to a reduction in rolling moment of ca. 10%. Adverse effects are of course gained with bluff nose shapes of high car bodies (double deckers). The slenderness of the nose and exotic shapes like the duck nose showed only little effect. Together with other measures like intelligent weight management and bogy optimisation the results will be very helpful for the construction low-drag trains which are un- sensitive to external wind loads. The next steps will be the simulation of the wind tunnel tests with CFD methods and further validation of the codes for the use in the early design process [4]. REFERENCES [1] Peters J.-L., Aerodynamics of very high speed trains and maglev vehicles: State of the art and future potential, Int. J. of Vehicle Design, Special Publications SP3, pp (1983) [2] Matschke G. and Schulte-Werning B., Measures and Strategies to Minimise the Effect of Strong Cross Winds on High Speed Trains, Proc. of the World Congress of Railway Research WCRR 97, Florenz, Italy, Nov (1997) [3] Matschke G. et.al., Nachweis der Sicherheit im Schienenverkehr bei extremem Seitenwind- Die Richtlinie Ril 401 der DB, Dynamik von Fahrzeug und Fahrweg, VDI- Tagung Kassel , VDI- Berichte 1568, VDI-Verlag (2000) [4] Matschke G., Validation of Computational Prediction of Side Wind Effects, Proc. of ECCOMAS 2000, Barcelona, Spain, Sept (2000) 9