# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 3 Results and Discussion
### Surface Pressure  
The air resistance of a driven vehicle is mainly determined by the forward positive pressure and the rear negative pressure. To investigate why the drag reduction mechanism was induced by the new design of the cab shape, the pressure distribution of the baseline model and the parametric cab design tractor-trailer needed to be analyzed.  
Figure 7 displays the pressure contour comparison of the forebody of the cab between the baseline model and the 500 mm-125 degrees new design cab. As shown in Figure 7a, in the baseline model, a stagnant region can be observed on the front surface of the cab; there was a large area of positive pressure, and the front of visor and both sides of fairing also had positive pressure zones, which were the main sources of the aerodynamic resistance. The transition region between the 'front face' and the lateral part of cab showed negative pressure zones, indicating that there was a clear separation of the airflow. Both the positive and negative pressure zones extended from the bottom of the wheelhouse to the top of the front face. After modification of the design, as shown in Figure 7b, the upper surface of the protruding part of cab had a positive pressure area, compared with the baseline model; the positive pressure area of the front windscreen was smaller, and the size and area of the positive pressure at the fairing and visor were all reduced. At the same time, for the parametric design cab, the negative pressure zone of the transition position between the front windscreen and the side window had become smaller, but the value slightly increased, and the value of the negative pressure zone between the engine and side girth was reduced.  
In the baseline model as shown in Figure 8a, the pressure distribution of the front area of trailer was mainly a positive pressure zone. The obviously high pressure area that existed at the leading edge of the top and the front of the trailer, and the transition region between the front and both sides of trailer, were both striped. The shape of the cab influenced the flow conditions in front of the trailer; when the shape of cab was changed, the subsequent turbulence had changed too, and the  
Figure 7: Pressure contour comparison of the front surface of the cab between (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of the tractor-trailer.  
pressure distribution of the front facade of the trailer was also affected. After the parametric cab design modification, as shown in Figure 8b, the positive pressure area of the trailer was reduced, where the high-pressure region at the front edge and both sides of the transition areas all became a thin sliver region. At the same time, the negative pressure zone at the front surface of the trailer model was significantly increased, whilst its location rose and was closed to the front edge of the trailer.  
Figure 9a shows the pressure value comparison at the forebody between the baseline cab and the parametric design cab along the longitudinal symmetry plane. It can be seen from the line frame area (for the parametric design cab) at the position that near the protruding part, the positive pressure value was less than the baseline model; meanwhile, at the position above the protruding part, such as forward windshield and visor, the surface pressure has also been reduced too. Figure 9b shows the pressure comparison at the forebody of the trailer between different cabs; it was also seen from the line frame area that after redesign, the positive pressure value decreased and the negative pressure value increased. From the comparison of the trend of the curves, the phenomenon of negative pressure area that had arisen was obvious, which corresponds to the display of the pressure contour (Figure 8).  
Figure 8: Pressure distribution comparison of the leading position of trailer between (**a**) baseline model and (**b**) 500 mm-125 degrees parametric cab design of the tractor-trailer.  
Figure 9: _Cont._