# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 2 Research Approach
### Validation of the Computational Model and CFD Procedure  
The purpose of the wind tunnel test carried out in this paper was to validate the correctness of the numerical simulation. Experiments were conducted in a closed-type reflux low-speed wind \begin{table}
\begin{tabular}{c c} \hline \hline
**Mesh Property** & **Parameter** \\ \hline Surface mesh & Triangular mesh \\ Volume mesh & Tetrahedral mesh \\ Boundary layers & Prisms \\ Number of prism layers & 6 \\ Prism layers stretching & 1.1 \\ Prism layers thickness & 0.4 mm \\ Minimum cell size & 1 mm \\ \hline
**Physics Properties** & **Parameter** \\ \hline Flow characteristics & Steady, stationary, incompressible, \\ Turbulence model & **Shear Stress Transport k-omega** \\ Flow velocity & 25 m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mesh and physics parameters. Figure 5: Numerical computation meshes: (**a**) surface mesh; (**b**) body surface boundary layer grid, and (**c**) density box of body. Each color shown different position of the cab body in ICEM software. tunnel at Shenyang Aerospace University, and the test section had the dimensions of 3500 mm (length) \(\times\) 1200 mm (width) \(\times\) 1000 mm (height); the wind speed was varied in the range of 10-50 m/s, the contraction rate was 8:1, and the turbulence intensity of the freestream flow was less than 0.14%. For the wind tunnel test, the cab of the tractor-trailer baseline model was made of wood, and the trailer and the remainder of the model were made of aluminum. The wheels were bolted to the model, and the cab roof fairing and both side deflectors were also constructed and bonded to the cab. In this paper, the free stream velocity was fixed at U = 25 m/s. The vehicle models installed in the test section had an insignifigant effect on the pressure distribution and the drag coefficient. The details of this wind tunnel test are shown in Figure 6.  
The aerodynamic forces and moments of the tractor-trailer baseline test model were measured using a six-component force balance system. The drag force in the direction of the wind was determined by statistically averaging the data obtained from three independent measurements. The drag coefficient was calculated using the following equation:  
\[\mathbf{C_{d}}=2\cdot\frac{F}{\rho\cdot A\cdot V^{2}}\]  
in which \(F\) is the drag force measured in the flow direction, \(\rho\) is the fluid density, \(V\) is the wind speed, and \(A\) is the orthographic projection area of the tractor-trailer model. As a first step towards an improved approach, only a zero-yaw angle was tested in this study. In subsequent research, the aerodynamic drag coefficient (\(\mathbf{C_{d}}\)) of the baseline model was needed to compare the value of different parametric cab design tractor-trailer models to make the expression more explicit; the aerodynamic drag coefficient of baseline model was set as \(\mathbf{C_{d\text{-}baseline}}\). During the numerical simulation of the baseline model, the aerodynamic drag coefficient was found to be stable after 1000 steps. The difference of the average value between 2000 and 2500 steps and 3501 and 4000 step was in the order of \(10^{-4}\). Thus, in order to reduce the amount of calculation, the baseline model scheme was iterated by 3000 steps, and the average value of last 500 steps result was taken as the simulation value. The \(\mathbf{C_{d\text{-}baseline}}\) values obtained by wind tunnel test and numerical simulation were 0.4507 and 0.4613, respectively, as shown in Table 3. The relative error of the \(\mathbf{C_{d}}\) was 2.36%; this estimation of the numerical number obtained from the simulation agreed well with the result measured in the wind tunnel experiments. This validated the grid division, the boundary conditions, and the physical model adopted for the simulation of the baseline model in this paper correctly, and has provided strong evidence for the validity and accuracy of the results obtained from the simulation.