## 3 Results
### Flow around the DrivAer Model
In the lower part there is a region with positive \(x\)-velocity coming from the underbody that is not yes mixed with the wake. Figure 19 shows the details of the time-averaged velocity vectors behind the front wheels, in the gap between the cowl top and the windshield and on the rear slant. In the wheel region the horseshoe vortices are evident, which are created just ahead of the wheel  
Figure 17: DrivAer model (**top,left)**—Time-averaged \(x\)-velocity contours at different slices: \(x/L=0.173\) (**top,right**), \(x/L=0.273\) (**bottom,left**), and \(x/L=0.423\) (**bottom,right**). Figure 18: DrivAer model (**top,left**)—Time-averaged \(x\)-velocity contours at different slices: \(x/L=0.88\) (**top,right**), \(x/L=0.98\) (**bottom,left**), and \(x/L=1.10\) (**bottom,right**). and are convected downstream. The two counter rotating vortices can be observed also in the LES simulations of [PERSON] et al. (2013). Moreover, the recirculating zone in the gap between the cowl top and the windshield can be observed, while the recirculation on the rear slant can be noticed only near the symmetry plane, as shown in Figure 19 (bottom-left), where the discontinuity between the roof and the rear slant is higher. Moving towards the side, the discontinuity between roof and the rear slant is not enough to promote separation, as shown in Figure 19 (bottom-right). The predicted drag coefficient is \(c_{D}=c_{D,\textit{turbed}}+c_{D,\textit{hydy}}=0.2556\), where \(c_{D,\textit{turbed}}=0.0327\) and \(c_{D,\textit{body}}=0.2229\) are the wheels and the body contributions to the drag, respectively. The drag can be decomposed in the skin friction, \(c_{D,\textit{v}}=0.02c_{D}=0.0048\), and pressure, \(c_{D,\textit{P}}=0.98c_{D}=0.2505\) contributions. The results are in good agreement with the reference data, as reported in Table 2. In particular, the predicted \(c_{D}\) value overestimates the experimental data, but this behaviour is confirmed also by most numerical references, e.g., [PERSON] et al. (2012), [PERSON] et al. (2016) and [PERSON] et al. (2014), which predict a drag coefficient larger than 2.5. As expected, the presence of the wheels heavily influences the aerodynamic performance of the car model. In fact, the down force predicted without the wheel becomes a lift force. The predicted lift coefficient is \(c_{L}=c_{L,\textit{turbed}}+c_{L,\textit{body}}=0.0345\), where \(c_{L,\textit{turbed}}=-0.1560\) and \(c_{L,\textit{body}}=0.1912\) are the wheels and the body contributions to the lift, respectively. The lift can be decomposed again in the skin friction, \(c_{L,\textit{v}}=0.0011\), and pressure, \(c_{L,\textit{P}}=0.0334\), contributions. Figure 20 shows the A-pillar vortex (the streamlines are coloured with the \(x\)-component velocity for the time-averaged solution), which develops close to the root of the A-pillar and detaches before the roof. The vortex is convected downstream and merges with the C-pillar vortex. Figures 21 and 22 compare the time-averaged streamlines coloured with the time-averaged \(x\)-component velocity contours obtained with the present and the simplified car model. In both cases the flow accelerates from the stagnation point in the front bump area and moves toward the bonnet and the wheel region. Along the sides of the bonnet, a high pressure region (see Figure 16) redirects the flow downward. Moreover, the high pressure zone on the windshield pushes the flow towards the A-pillar. The vortex created at the A-pillar root is convected downstream and merges into the wake. The presence of the wheels  
Figure 19: DrivAer model—Details of the time-averaged velocity vectors behind the front wheels (**top,left**), in the gap between the cowl top and the windshield (**top,right**) and on the rear slant (**bottom**).