# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 2 Research Approach
### CFD Simulation Set up  
Both the baseline model and the tractor-trailer models with parametric design cabs were drawn using UG (10.0, Siemens PLM Software Company, Plano, TX, USA, 2014), and imported into ICEM (16.0, ANSYS Company, Canonsburg, PA, USA, 2015) to produce dividing grids. Taken the baseline model as an example and shown in Figure 4, a box was created around the models to define the computation domain. The established principle of the computational domain in CFD is the boundary, which does not affect the flow field around the vehicle; meanwhile, the range is be minimized [30; 31; 32]. In this paper, the clearance between the inlet and the front of the cab was 3 L, and the clearance between the outlet and the back of the trailer was 7 L, in which L was the length of the baseline model; a clearance of 5 W was used between the side of the box and the model on both sides, and a clearance of 4 H was used between the top of the box and the model, in which W was the width of the model and the H was the height. The block ratio of this study was 2.27%, it satisfied the experiment requirements. For every parametric cab design tractor-trailer model, one difference was the increased length of protruding part of cab; thus, the length of computing domain increased with the cab length, and the clearance between the inlet and the front of the parametric designed cab kept 3 L; the other parameters remained.  
As shown in Figure 5, after some tentative studies, the tetrahedral meshes were used to create an unstructured grid system in the whole calculation domain, and the triangular surface meshes were used to better fit the shape of the model body. To catch the boundary layer from the model surface, there were six prism layers in the boundary, and the thickness was 0.4 mm. Three layer density boxes were set up outside the baseline mode body, of which the minimum size was 8 mm; there were more  
Figure 4: Numerical simulation of the calculation domain.  
Figure 3: **(a)** External forehead shape of a cheetah; **(b)** two modeling design variables of new cab design, L (400 mm, 500 mm, and 600 mm), 0 (120 degrees, 125 degrees, 130 degrees, and 135 degrees); and **(c)** partial cab design schemes.  
than ten million meshes for the case. The mesh and physics parameters used in ICEM software have been summarized in Table 2.  
The numerical simulation experiment was carried out by FLUENT (16.0, ANSYS Company, Canonsburg, PA, USA, 2015), which is based on the finite volume method. A steady and free-stream velocity of U = 25 m/s was set at the inlet; the Reynolds number based on the hydraulic diameter was 1.95 \(\times\) 10\({}^{6}\), and the turbulence intensity was 2.6%. Through some tentative studies and consideration of the characteristics of the numerical simulation of vehicle external flow field [33; 34; 35], the Shear Stress Transport k-omega (SST k-\(\omega\)) turbulence model was adopted in this paper; meanwhile, convection term was solved by second-order upwind scheme, and coupling numeration of velocity field and stress was based on SIMPLEC scheme. The boundary condition of the outlet was pressure-outlet; no-slip condition was used on the lateral walls, roof, and the tractor-trailer baseline model surface.