# Evaluation of the Aerodynamic Effect of a Smooth Rounded
## 3 Results
### Pressure Distribution  
Mean pressure distributions on the surface of the train for all the eleven tapping loops are presented in Figure 6. The results correspond to the wind tunnel test with wind yaw angle of 65\({}^{\circ}\), being the angle at which the biggest difference in force aerodynamic coefficients is observed. At 65\({}^{\circ}\), for tapping loops 1-5, no significant differences are observed between the results at 30 m/s and 50 m/s. The pressure distribution shows a positive pressure on the windward side and a uniform suction around the upper part and leeward side of the train nose. The presence of a sharp edge on the upper windward corner of the nose locks up the flow separation point, whose position is independent of the wind speed. On the contrary, for tapping loops 6 to 8, two different flow distributions depending on the wind speed may be observed. For the wind speed equal to 30 m/s, the behaviour of the flow is in line with the behaviour found for sections 1 to 5: the flow over the roof separates early, and thus a uniform pressure distribution is observed on the roof and on the leeward side of the train body. On the contrary, when the wind speed is 50 m/s, the flow remains attached to the roof and a large suction peak over the windward roof corner is observed. Figure 6: Aerodynamic force coefficients as a function of yaw angle. Wind tunnel test: wind speed 30 m/s and 50 m/s; train: ETR1000; scenario: STBR. Pressure coefficient in tapping loops 9, 10 and 11 have a similar behaviour for both wind speeds, but the suction zone in the windward corner of the roof is smaller and less pronounced for the test at 30 m/s. The uniform pressure distribution observed around the whole roof zone when wind speed is set to 30 m/s, typical of a completely separated flow over the roof, is consistent with the important reduction in lift force observed from the corresponding coefficient. On the contrary, the typical suction peak, observed at the windward corner of the roof zone in the central portion of the car body leads to an increase in the lift, as shown in Figure 6. Moreover, the suction zone over windward corner of the roof, not only increases lift, but also reduces side force. As outlined by pressure distributions displayed in Figure 7, the gaps observed over 65\({}^{\circ}\) in force aerodynamic coefficients at two wind speeds, are mainly due to the different behaviour of the flow at the windward roof corner, in correspondence of the region just behind the train nose (sections 6, 7 and 8), where the roof profile is completely rounded. To verify the behaviour at 90\({}^{\circ}\), Figure 8 shows the pressure coefficient around sections 6, 7, and 8 obtained for a wind tunnel test with wind yaw angle set to 90\({}^{\circ}\): it is possible to see that at this angle, the pressure distributions obtained at the two velocities are equivalent. A small difference on the magnitude of the suction zone over the roof is observed for tapping loop 6, that can be responsible for the very small difference in vertical force observed in Figure 6.  
Figure 7: Pressure coefficient around the surface of the train. Wind tunnel test: wind speed 30 m/s and 50 m/s; wind yaw angle = 65\({}^{\circ}\); train: ETR1000; scenario: STRR. In conclusion, it is interesting to note that the double trend found in the force aerodynamic coefficients over 65\({}^{\circ}\) is to be attributed to a different behaviour of the flow and, correspondingly, of the position of the separation point, which, however, occurs only in a well-defined portion of the carriage, i.e., just after the nose, where the roof profile is completely rounded. In the opinion of the authors, the double trend found in the force aerodynamic coefficients is not only due to the Reynolds number, because the gap between the two trends is not constant in the range 65\({}^{\circ}\)-90\({}^{\circ}\) (although in this range the car behaves as a bluff body) and because differences in the pressure distribution take place only in some portions of the train.