# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 3 Results and Discussion
### Impact Factors of the 'Vortex Cushion'  
In Section 3.2, it was found that a vortex formed above the protruding part and obviously influenced the pressure distribution and the flow field of the tractor-trailer. As shown in Table 4, when the angle between the protruding part and the A pillar was greater than 125 degrees, with the protruding part length increased, the \(\mathbf{C}_{d}\) decreased firstly then increased subsequently. Therefore, the formation of this 'useful vortex' depended on the friction of the protruding part surface and the  
Figure 14: Turbulence intensity contour comparison of rear part: (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of the tractor-trailer. Figure 13: Flow field comparison of (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of the tractor-trailer. blocking effect of the front windscreen, and must have been affected by the two design variables: the length of the protruding part and the angle between this part and the A column. As shown in Figure 15a, when the extended length was 400 mm, it was slightly protruded, and the above vortex area was obviously smaller than other two cases. At the position of the include angle between protruding part and A pillar, the flow velocity was smallest. Owing to the small vortex volume, a part of the incoming flow contacted the vortex above the 'cab nose' first and then continued to move forward and impinge on the front windshield. The other part of the flow went down to the sun shading board, and the movement was blocked. At this time, the effect of 'vortex cushion' was not obvious. When the extended length was 500 mm, as shown in Figure 5b, the vortex was fully developed, its size was larger than 400 mm case, and the shape was even rounder and fuller. At this moment, at the include angle, the flow velocity was faster, and the bearing role of this vortex began to be highlighted. The friction between the airflows was apparently smaller than the friction between the airflow and the cab surface; therefore, compared with the 400 mm-125 degrees case and the baseline model, the value of \(C_{d}\) decreased. When the extended length was 600 mm, the vortex did not increase linearly, but also kept a size similar to the 500 mm case, and the effect of 'vortex cushion' was not enhanced too. There was still a certain length at the front corner of the protruding 'cab nose' that was not occupied by the vortex. The incoming air flow to the cab was first affected by the surface of the protruding position, and then it impinged on the vortex. Some of the airflow was blocked by the vortex, and the velocity was reduced; the other part continued to flow upward with the lifting of the vortex and at a later point the flow impinged on the cab's sunshade. Meanwhile, the friction between flow and vehicle surface was greater than 500 mm case, and the value of \(C_{d}\) increased. Therefore, for such a forward protruding 'cab nose' type of cab shape design, the protruding length needs to refer to the overall size and proportional relationship of the cab. The same results also could be obtained in Figure 16; as shown in Figure 16a, the area of gray dash wire frame was the surface of the protruding part, and the value of pressure of 500 mm case was less than 400 mm case clearly and close to the value of the 600 mm case. In the area of purple dot dash line, the surface pressure of 500 mm case was obviously less than other two cases; correspondingly, the value of \(C_{d}\) decreased first and then increased as the length of protruding part increased. Figure 15: Flow field of different new designs of the cab: (**a**) 400 mm-125 degrees, (**b**) 500 mm-125 degrees, and (**c**) 600 mm-125 degrees parametric cab design of the tractor-trailer. Figures 17 and 18 show the velocity and turbulent kinetic energy contour at the same position when the included angle was 125 degrees and with different extended lengths of the cab.