The Next Generation of Rear Drag Reduction Devices for Road Vehicles - The Multi Cavity Concept

• The multi cavity device reduces drag by utilising a downstream high-pressure bubble. • The outer cavity relocates the high-pressure bubble closer to the vehicle base. • Inner cavities extract and redistribute pressure energy from the high-pressure bubble. • Internal ventilation enables a strong backflow, generating thrust within the device. • Applied to road vehicles, the multi cavity concept achieves drag reductions over 20%. This study explores the new multi cavity rear drag reduction concept, evaluating methods to enhance its performance. Initially, a simplified body study was utilised to examine different configurations for optimal performance followed by a full-scale study on road-driven vehicles. The drag reduction mechanism is based on harnessing a downstream high-pressure bubble that forms where the free shear layers converge. The outer cavity brings this bubble closer to the vehicle, increasing its intensity, while the inner cavities extract and redistribute this pressure energy on the vehicle’s base, reducing drag. Findings indicate that internal ventilation within the multi cavity significantly improves performance, enhancing the energy transfer from the high-pressure bubble. The internal flow mechanism promotes a strong backflow within the inner cavity, which stagnates on the vehicle’s base, generating a substantial thrust peak. The inclusion of end caps on the inner cavity were found to improve performance, whereas features such as slots and mounds on the outer cavity had a detrimental effect. Overlapping the cavities provided further benefits. The study identified a triple multi cavity design with an angled outer cavity and two straight inner cavities, as the most effective device. Full-scale implementation on real, road-driven vehicles was carried out, which included built prototypes for a passenger car and van. Drag reductions exceeding 20% were reported for the triple cavity devices. The findings suggest that the multi cavity device outperforms existing rear drag reduction technologies. Recommendations for refining its design and performance are presented, supporting future optimisation studies on the concept.