# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 3 Results and Discussion
### Impact Factors of the 'Vortex Cushion'
When the length of the protrusion was smaller, this value relative to the whole size of the tractor was not obvious, and the cab could still be regarded as a blunt body without a 'car nose'; the separation of the airflow here was still similar to that of the flat head cab. However, when the protrusion length was increased, its proportion relative to the cab was raised too, and the effect on the separation of the airflow could not be ignored. The cab was somewhat similar to the design of the \"wedge\" car. It can be clearly seen from Figure 16 that with the increase of the protrusion length, the separation of the flow tended to be gentler at the angle that at between the protruding part and the front windshield, and the area of the separation region was gradually decreased. From Figure 17, it can be seen that the change of the turbulent kinetic energy at the lower part of the cab could also directly capture the characteristics of this 'wedge' design, similarly to the function of the car engine bay that inserts it into the flow and guides it to move to the rear. Figure 16: Pressure distribution contrast along longitudinal symmetry plane of tractor-trailer cabs with different parametric designs: (**a**) 125 degrees design schemes and (**b**) pressure measurement position with 400 mm-125 degrees scheme as an example. Figure 17: Comparison of the velocity contour at the angle between the protruding part and the front windshield between (**a**) 400 mm-125 degrees, (**b**) 500 mm-125 degrees, and (**c**) 600 mm-125 degrees parametric cab design of the tractor-trailer. It was found from Table 4 that when the protruding part was 400 mm and the angle between the protruding part and the A pillar was 135 degrees, the \(\mathbf{C}_{d}\) of the tractor-trailer did not decrease as expected, but increased by 7.67%. However, when the protruding length was 500 mm and 600 mm, the aerodynamic drag was still reduced. Therefore, it was also necessary to analyze the cases when the angle was larger, as well as the relationship between the protruding part length and the outflow structure of the parametric cab design tractor-trailer. Figure 19 shows the velocity pathlines diagram of the longitudinal symmetry plane of the cab when the angle was 135 degrees. It is interesting to note that the vortex that acted as a 'vortex cushion' disappeared when the length of protruding part was 400 mm. At the same time, when the extended length was 500 mm and 600 mm, compared with other two cases of the same length of 125 degrees and 130 degrees, the size of the vortex was obviously smaller. The appearance of this vortex not only depended on the blocked windshield by the front, but also on the friction between the airflow and the 'cab nose' surface. When the angle was too large, the frictional force was affected when the airflow travelled on a tilted slope, and meanwhile the blocking effect of the front windshield was weakened. When the design length was 400 mm, this length was not enough to form the corresponding vortex, and at the same time, with the rounded corners of the leading edge of the protruding part, the incoming airflow could not discriminate the angle, instead turning it 'as a big blunt form' and continuing to flow upward. This is one of the main reasons for the significant increase of aerodynamic drag in the 400 mm-125 degrees new cab design. Compared to the 500 mm and 600 mm cases, there was enough length to form a vortex, but the area of the vortex was significantly reduced. The 'vortex cushion' effect also existed, but the effect only to led the airflow flow to the upper part of the windshield, which had no fundamental influence on the outflow field structure. The characteristics of the vortex and its 'vortex cushion' effect were also influenced by the angle between the protruding part and the A pillar.