Development of Aerodynamic Drag Reduction around Rear Wheel

Due to new CO₂ regulations and increasing demand for improved fuel economy, reducing aerodynamic drag has become more critical. Aerodynamic drag at the rear of the vehicle accounts for approximately 40% of overall aerodynamic drag due to low base pressure in the wake region. Many studies have focused on the wake region structure and shown that drag reduction modifications such as boattailing the rear end and sharpening the rear edges of the vehicle are effective. Despite optimization using such modifications, recent improvements in the aerodynamic drag coefficient (C_d) seem to have plateaued. One reason for this is the fact that vehicle design is oriented toward style and practicality. Hence, maintaining flexibility of design is crucial to the development of further drag reduction modifications.

The purpose of this study was to devise a modification to reduce rear drag without imposing additional design restrictions on the upper body. First, the modification concept was devised using CFD analysis focusing on the air flow around the rear wheel. The concept is to guide the underfloor flow toward the rear wheel inboard side to reduce the size of the vehicle wake region from the underfloor center region. Then, a production Prius was modified to verify the actual modification developed from the concept in the wind tunnel. The air flow channel toward the rear wheel inboard region was expanded by adding a cut-out to the undercover at the side of fuel tank to derive the desired flow. The force and total pressure distribution of the underfloor region behind the vehicle were measured in a wind tunnel for cases with and without the modification applied to verify its effect. The modification exhibited the desired effect of expanding the high total pressure region at the underfloor center, and a reduction in drag was confirmed.