# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 3 Results and Discussion
### Change of Flow Field
Figure 12 shows the contrast of the longitudinally symmetric turbulent cloud diagram between the baseline model and the new design cab model; the results obtained earlier (Figures 10 and 11) are also reflected in this picture. The flow separation appeared at the bottom of the original cab and the turbulence intensity was also larger there. At the leading edge of the fairing, the flow was blocked after it impinged on the mounting base of the fairing, but most parts of the flow still moved along the curved surface of the fairing to the rear until they collided with the front of the trailer. Figure 11: Flow field comparison of the gap between (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of the tractor-trailer. Figure 10: Flow field structure of protruding part of 500 mm-125 degrees parametric cab design of the tractor-trailer. The airflow separation at the bottom of the new design of the cab changed greatly, and the turbulence intensity at this point was obviously smaller than that of the original model. This means that after the parametric cab design, the air flow was smoother, and the shunting action of the cab's protruding 'cab nose' began to be highlighted. At the same time, the obvious change of the turbulence intensity at the gap between the cab and trailer was also caused by the 'vortex cushion' effect of the vortex above the protruding 'cab nose'. From functional viewpoint, this vortex acted as a 'vortex cushion'; it even could be seen as a part of the cab, and combined with its shape, it also could be considered that the cab has formed a certain streamline design here. The existence of the 'vortex cushion' effect made the flow over the front and top of the cab smoother, and the diversion effect of the fairing on the cab was more pronounced. Figure 13 shows the wake structure comparison between the baseline model and the parametric cab design tractor-trailer. As shown in Figure 13b, after the cab redesigned, there were also two vortices that occurred at the rear of the trailer; one was higher up and the other one was lower down. Compared to the baseline model, in the wake flow of the 500 mm-125 degrees model, the position of the upper vortex was obviously closer to the rear of the vehicle and had moved upwards, and its vortex area was smaller. Accordingly, the form of the lower vortex in the wake flow was more complete, its height was reduced, and the core of the lower vortex was closer to the vehicle too. From comparison of the wake structure of the different tractor-trailers after the new cab was designed, the separation area of the wake flow showed a decreasing trend, and its location was concentrated more at the rear of the trailer. Figure 14 shows the turbulent intensity contour comparison of wake flow of different tractor-trailer models. The parametric design of cab also had a certain influence on the tail flow structure. Comparing the different tractor-trailers, it was interesting to note that the turbulence intensity of the parametric cab design vehicle was obviously smaller than that of the baseline model, as shown in Figure 14. The 'vortex cushion' of the 'cab nose' not only impacted the flow structure of the cab and the gap but also influenced the vortex structure of the boundary layer at the bottom of the trailer, and this meant that the dynamic exchange of the upper and lower layers of the boundary layer were weakened and the momentum transport was smoother; finally, the aerodynamic drag force of the tractor-trailer was decreased. Figure 12: Turbulence intensity comparison of (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of tractor-trailer.