# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 3 Results and Discussion
### Change of Flow Field  
The time-averaged flow fields around the parametric designed cab tractor-trailer models have been presented in this section. When the incoming flow reached the front of the baseline cab model, part of the flow was halted by the front face of the model. As a result, the flow stagnation region appears on the front surface of the baseline cab model. The flow above the stagnation point turned upwards and reached the top of the cab, and after turning upwards, the flow reached and expanded along the surface of the fairing and the roof of the cab. The flow then reached the rear end of the cab, and some of the flow entered the gap between the cab and the trailer; at the same time, the rest of the incoming flow crossed the gap and impinged on the front surface of the baseline trailer model. From Figure 10, it can be seen after the parametric designed case was applied (500 mm-125 degrees) that when the incoming flow reached the new design of the cab, the flow attacked the 'cab nose' first and immediately halted. Due to the friction of the 'cab nose' surface and the blocking effect of the front windscreen, a vortex formed here that rotated clockwise at the top of the 'cab nose'. Consequently, under the lift action from this vortex, the incoming flow above it changed direction and turned upward to the top of the cab's front face and the visor. Some of the airflow even went over the visor with the assistance of this vortex and directly reached the front edge of the cab. At the same time, because of the vortex, the upward incoming flow contacted the cab and developed relative motion instead of making surface contact with the cab body. Compared to the friction between the airflow and the cab surface, the friction between the airflows apparently decreased. Therefore, the vortex that existed above the 'cab nose' can be defined as it has a 'vortex cushion' effect; the actual function of this vortex was even like a diversion device. It not only effectively reduced the frictional resistance of the airflow, but it also acted as a lift to speed up the airflow. It was believed that the presence of the 'vortex cushion' on the 'cab nose' induced changes in the incoming flow filed, thus varying the pressure distribution and the aerodynamic drag force, corresponding to Figure 6b and Table 4, as shown. The structure of the flow that passed over the front face of the cab was changed obviously, and its subsequent flow was bound to be affected. As can be seen in Figure 11b, compared to the baseline model (Figure 11a), the position of the upper vortex core that was located in the gap region between the new design of the cab and trailer obviously lifted and moved closer to the rear edge of the fairing, whilst the horizontal position of this vortex was close to the front edge of the trailer. This phenomenon was the main cause of the elevation of the negative pressure zone, which was in front of the surface of the trailer, as shown in Figure 8b. Because of this upper vortex lift, the location of the upper backflow stagnated on the rear surface of the new design of the cab and was lifted accordingly; meanwhile, this left more space for the motion of the bottom vortex at the gap, and the shape of this bottom vortex was more complete. The variety in the flow structure at the gap correspond  
Figure 9: Pressure distribution contrast of tractor-trailers with different cab design along longitudinal symmetry plane between the (**a**) the fore body of the cab and (**b**) the forebody of the tailer; (**c**) pressure measurement position with baseline model as an example. The vehicle model was displayed in blue, longitudinal symmetry line along the driving direction was displayed in green. pressure distribution on the front surface of the trailer and resulted in the positive pressure decreasing and the negative pressure increasing; the aerodynamic pressure resistance of the whole tractor-trailer was reduced.