# A Numerical Aerodynamic Analysis on the Effect of Rear Underbody Diffusers on Road Cars
## 4 Conclusions  
The initial goal of the present manuscript was the study of the effect of rear underbody diffusers on road cars. The understanding of the flow behaviour and the generation of downforce are the two key elements of this work. We presented the novelty of applying slow directly to a realistic road car geometry as opposed to the majority of the available literature, which has used the Ahmed body. Front diffusers research is also rarely reported in the literature; therefore, the results presented here could potentially be valuable for further research. The design and modification of the rear diffuser of the self-designed baseline model were performed according to the literature review and were based on already published numerical analyses. However, front diffuser design and sealing skirts adaptation were based on the authors' expertise in the field. Consequently, four different configurations were studied, and their aerodynamic coefficients were evaluated. It was observed that the configuration that produced the highest downforce coefficient was the one composed of a \(7^{\circ}\) venturi diffuser equipped with diagonal sealing skirts, achieving a \(C_{L}\) value of \(-0.887\), which represents an increase of around \(1780\%\) with regard to the baseline model. This same configuration also had the highest aerodynamic efficiency \(E\). The other two diffuser configurations (venturi diffuser and venturi frontal diffusers) were stuck between the highest and the lowest downforce scenarios. Additionally, the two and three-dimensional contours allow for a great understanding of the flow behaviour around the different regions of the car. Studying the wake effects, pressure distribution, and vorticity map could represent a separate line of research, as all this is an extremely complex matter. Figure 22: Vorticity map on the XZ-plane of (**A**) baseline, (**B**) Venturi diffuser, (**C**) Venturi skirts, and (**D**) Venturi frontal configurations. Units are s\({}^{-1}\). The wake behaviour of the diffuser configurations in the velocity field has a smoother and cleaner appearance when compared to the baseline model. The pressure distribution and the pressure coefficient (Cp) displayed a non-subtle change when both front and rear diffusers were employed, including a pronounced spike in low pressure at the starting point of the sealing skirts. It is important to note that the addition of the diffuser and other aerodynamic devices also contribute to a slightly higher drag coefficient. This finding was clearly observed in the vorticity map and when studying the sizes of the eddies. This must be kept in consideration, although the underbody aerodynamics are cleaner in terms of wake generation. Future work could involve experimental validation of the presented numerical results or extending the current geometric analysis with new, untested aerodynamic concepts. Unsteady simulations of the same problem would also be useful to test the performances of the solutions we achieved. The authors are aware that some sources of error derived from the numerical RANS approach must have been inherently introduced. [PERSON] and [PERSON] defined and conceptualized the CAD geometry. [PERSON] configured and performed the numerical simulations and the analysis of the results. [PERSON] contributed to the literature review and writing of the manuscript. [PERSON] reviewed the paper and supervised the work. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: