## 2 Analysis of Side Wing Flow Field  
The main sources of negative lift for most racing cars are aerodynamic kits, such as front wing and rear wing, which can provide up to 70-80% of the overall negative lift [2]. However, the penalty of the huge negative lift of front and rear wings is due to the relatively long longitudinal distance between them and the center of mass, and any imbalance of front and rear negative lift produces serious pitching moment. Therefore, the longitudinal position of the front and rear wings is often subject to the demand of balancing pitching moment. The front wing is usually designed to control air flow to out-shuffle for enabling the rear wing to obtain more undisturbed air flow. However, out-shuffled air flow will \"overhead\" the flow field of side wing and place the side wing in the low-pressure turbulent  
Figure 4: Installation position of shutter-like fairing structure (orange). In our implementation, the shutters are installed between the front wheel and side wing leading edge to combine the low-pressure turbulent airflow above the side wing. Figure 3: Example of shutter-like fairing structure. flow field. As shown in Figure 5a, the yellow-green volume represents the space where the surface pressure is less than 0 (PA), and the blue-green area in Figure 5b denotes the area where the vorticity is between 75 and 150 (/s). The flow is more chaotic and difficult to harness compared with an undisturbed case. The side wing is placed in this low-pressure turbulent flow field. The low-pressure turbulent flow field is a turbulent region with low pressure compared with the reference pressure at infinite distance and significantly increased vorticity compared with undisturbed state. It is used for qualitative description without strict threshold definition. The low-pressure turbulent flow field will make a huge negative impact on the performance of any aerodynamic kits exposed to it. This negative effect can be suppressed by increasing the air pressure or reducing the vorticity. In the research of [PERSON] et al. (2020), they found that the diffuser efficiency can be greatly improved by increasing the number of channels and restraining the bottom cross flow. As shown in the surface pressure of wings in Figure 5a, pressure on the upper surface of side wing is significantly lower than that of the front wing and rear wing. In contrast, the negative pressure area on the lower surface of side wing is still an important source of the negative lift, although it is lower than that of the front and rear. At present, three technical routes are used for racing cars to increase the negative lift: negative lift wing technology, diffuser technology, and ground effect technology ([PERSON] et al., 2020). The side wing or sidepod can install a negative lift wing and can effectively increase the underbody area. Thus, reasonable design of the bottom shape can achieve better ground effect. Lightweight diffusers can be installed on side wings to deliver negative pressure to the rear wheels for reducing the aerodynamic drag of the rear wheels. Therefore, the side wing can make the front and rear of the whole vehicle more compact and can provide considerable negative lift with minimal cost of aerodynamic pitching moment. As shown in Figure 6, CFD simulation is performed on a prototype car after canceling the front wing and front wheel. Under the incoming flow of 15 m/s, the negative lift of its side wing reaches \(-\)253.43 N. More simulation boundary conditions can be found in Section 4 \"Establishment of CFD Model\". Figure 5: Distribution of low-pressure area and turbulence of the whole vehicle. (**a**) Side wing is exposed to the relatively low-pressure zone; (**b**) side wing is subjected to air flow with high vorticity.