## 3 Results
### Flow around a Simplified DrivAer Model
The region between the wheelhouses is characterized by an almost constant \(c_{P}\). The smooth underbody behaves like a diffuser, generating a downforce effect. Figure 10 show the \(c_{P}\) distribution on the plane \(z=0.06\) for the low-Reynolds number case. The \(c_{P}\) distribution shows some discrepancies with experimental data, motivated by the absence of the front rotating wheels, which alter the flowfield near the doors region. The rear part seems to be similar to the experimental data, suggesting a lower influence  
Figure 8: Simplified DrivAer model—Polynomial degree distribution \(\mathbb{P}^{n}\) of the solution in the low-Reynolds number case far from the model (**left**) and near to the model (**right**). Figure 7: Simplified DrivAer model—Detail of the mesh near the side pillars (**left**) and the rear part of the body (**right**). of the rear wheels, but the flow separation at the end of the model is more pronounced. Figures 11 and 12 show the pressure coefficient distributions for the high-Reynolds number case at the same sections described above (\(y=0\) and \(z=0.06\)). In addition, in this case the \(c_{P}\) distribution is in good agreement with references, i.e., the experimental data of [PERSON] et al. [11] and the LES results [PERSON] et al. [13]. An in depth comparison between low and high Reynolds results shows some differences. In particular, Figure 9 (left) shows at \(x/L\simeq 0.15\) a pressure drop that corresponds to a laminar separation bubble. This difference can be explained by the lower Reynolds number, i.e., by the lower velocity approaching the car model. In fact, in the hood region, characterised by an adverse pressure gradient, the lower velocity of the flow promotes the onset of a laminar separation bubble. As expected, the predicted results for the higher Reynolds number on the symmetry plane for the body (see left side of Figure 11) are very similar to the reference data and demonstrate that the wheels do not influence significantly the pressure distribution. On the contrary, the results for the underbody (see right side of Figure 11) still show the detachment after the radiator grid and an underestimation of the \(c_{P}\) in the doors region. However, starting from the rear wheels, the \(c_{P}\) distribution is almost coincident with the experimental data, suggesting a lower influence of the rear wheels on the predicted flow field. Finally, on the plane \(z=0.06\) (see Figure 12) the discrepancy in the doors region is still evident, confirming also in this case the importance of considering the frontal wheels to preserve the simulation accuracy. After the rear wheel, the pressure increase is correctly predicted, but as for the low-Reynolds case, the final separation is larger. Figure 13 show the time-averaged pressure contours (top-left), the time-averaged pressure coefficient distribution on the windshield (top-right), the front side window (bottom-left) and the back window (bottom-right) for the low-Reynolds number case. The views and the scales of the pressure coefficient contours are the same used by [PERSON] et al. [11] to ease the comparison. It can be observed that the pressure coefficient distribution on the windshield and back window are similar to the RANS simulations by [PERSON] et al. [11], while some discrepancies are evident in the zone near the pillars. The side window shows a more pronounced low pressure region, probably due to a not correct prediciton of the A-pillar vortices. As demonstrated in Section 3.2, this difference can be ascribed to the absence of the wheels. The predicted drag and lift coefficients for the low- and high-Reynolds number cases are summarised in Table 1 and compared with experimental and numerical results of [PERSON] et al. [11]. The negative value of \(c_{L}\) confirms that the underbody acts like a diffuser. The \(c_{D}\) slightly changes increasing the Reynolds number, confirming the experimental findings [11].