An automobile diffuser is traditionally used as a downforce generator, but it can also be effective for drag reduction for some types of bodies. The overall idea and function are that a cross-sectional area increases in the flow direction, producing a decrease in fluid velocity from inlet to outlet; with a corresponding increase in static pressure. The increase in static pressure at the exit will lead to a higher base pressure that is desirable for drag reduction. The drag reduction is depending on several parameters and the length of the diffuser is one of the parameters.
A diffuser at the rear of the undertray, its dimensions are restricted from the rules which set a standard safe distance from the wheels. The maximum available space for the angle of the diffuser at the rear side of the chassis is also restricted from the bulkhead, so the maximum angle is settled at 8°. Six vertical flaps are placed along the exit of the diffuser to keep the flow attached and avoid a stall, while there is not any ground effect device used on the underside.
Along the sides of the undertray, side-skirts are placed to prevent air from escaping and as it can be seen from the floor plan there is a narrowing of the undertray which aims to guide all the incoming air directly to the diffuser. However, this type of undertray produces downforce but it reduces drag force due to the angle of the diffuser.
The airflow stays attached along the undertray’s surface. Although, the narrowing of the undertray seems to work and guide the air directly to the diffuser.