## 1 Introduction  
Aerodynamical design is one of the key parts in the development of racing cars. Its main objectives include providing sufficient negative lift force for racing cars, reducing aerodynamic drag as much as possible, and maintaining a reasonable center of pressure (CoP). Diffuser is the main mechanism to provide negative lift force for high-level formula racing cars with long wheelbase and cars with closed wheel arches. Taking Formula One racing cars as an example, the baseplate and diffuser contribute to approximately 50% of the negative lift to the whole vehicle. The acting point of the aerodynamic force of the diffuser is close to the mass center of the whole vehicle, which helps the car to obtain a more favorable CoP. However, the structures, such as tire and front wing, tend to result in unpalatable jet or turbulence for the short wheelbase open wheel race cars represented by formula SAE (FSAE) cars, thereby causing an adverse impact on the performance of diffuser [1]. Therefore, the short wheelbase open wheel race cars tend to use side wing structure to provide negative lift force for the mid-back of the vehicle. The performance of side wing depends on the pressure difference between its upper and lower surfaces. Although the airflow on the lower surface is usually less blocked, the upwash of the airflow by the front wing and the turbulence of the airflow through the front suspension system often make the working conditions above side wing extremely bad, and the flow field above side wing presents the characteristics of low pressure and turbulence. Therefore, combining the turbulence in front of side wing and increasing the pressure on the upper surface of side wing are the first choices to improve side wing efficiency. A whisker is a type of positive lift-producing device that is widely used in FSAE car design. It is usually a pair of upright airfoils without endplates installed behind or above the front wing and controls the tire wake by generating wingtip vortex. Thus, the upwash of front wing can obtain downward kinetic energy and out-shuffle to enhance the performance of side wing and mainly improve the rear wing. CoP can be adjusted backward to accommodate a more radical front wing design. Figure 1 demonstrates the first pair of whiskers used on the FSAE car of Rennteam Uni Stuttgart. A whisker can be arranged in various positions: a larger wingtip vortex can be produced if installed above the nosecone, better protection can be achieved if it is installed in front of the front suspension, and better comprehensive performance can be attained if it is installed above the front suspension. Different installing positions are designed for different functions. Designs, such as a whisker, mainly focus on global optimization. However, few studies are conducted on the improvement of side wing. Thus, devices specifically designed for side wing flow field, such as the shutter-like fairing structure in this study, are needed to improve the working condition of side wing. Louvres widely used in F1 and other high-speed racing cars are different from shutters mentioned in this study, and their mechanisms are extremely different. The former uses the high pressure in front of it and the airfoil fins to generate vortex opposite to that produced by rear wing to reduce the induced drag of the rear wing and increase the negative lift in a small range. The latter combines the turbulent flow and increases the pressure behind. The most obvious distinction between the two devices is that the high- and low-pressure zones are in the inversed position of the other. Figure 2 shows the TUfast racing car EB018, which is equipped with louvres rather than a shutter-like fairing structure, and it is also the first FSAE car to do so. Figure 1: Design for controlling the side air flow using a whisker. Figure 2: Louvres used on the endplate. The shutter-like fairing structure was first used for rotating air supply machinery, such as fans and air conditioners. Its main function is air flow guidance.