Aerodynamic Drag Reduction of a Platoon of Generic Tractor-Trailers equipped with Base Flaps under Realistic Boundary Conditions

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The major part of CO2 emitted by road traffic within the European Union is ascribed to heavy-duty vehicles which generate about 30 % of the greenhouse gases although they represent only 4 % of all road vehicles. This situation justifies the common interest of politics and economy to increase the fuel efficiency of those vehicles. On highways the aerodynamic drag contributes to about 40 % of the overall losses of a standard 40 t European tractor-trailer combination. One major source represents the large separation area downstream of the vehicle rear end. Many efforts have been undertaken particularly in the USA during the last 10 years to develop commercial solutions such as base flaps which significantly reduce the aerodynamic drag. These add-on devices were object of several fundamental studies within the last decades. The achievable benefits are, therefore, well known for single trucks. Recently, the legislation of the EU has been modified allowing the application of add-on devices with a total length of 0.5 m to the rear end of trucks. Additionally, recent progress in automated driving opens new ways for drag reduction through platooning with very small inter-vehicle spacing. However, the application of inclined base flaps completely changes the flow field within the wake and its underlying dynamics. Consequently, the incident flow conditions of following vehicles will significantly deviate compared to vehicles in isolation. The flow field modifications through the base flaps may change the optimal distance to following vehicles associated in a platoon as well as the optimal flap angle. Therefore, the efficacy of base flaps may negate the benefits of platooning. As a result the combination of both approaches (base flap application and platooning) needs to be considered in detail. The proposed project focuses on the elaboration of guidelines based on the fundamental understanding of the flow dynamics of a platoon of generic tractor-trailer models with and without base flaps to achieve the maximum drag reduction for the involved vehicles as well as the complete platoon. Therefore, the influence of the flap inclination angle, the vehicle distance, and the number of vehicles comprising the platoon on the flow field and the related drag values will be investigated in this study. Especially for road vehicles an important aspect is the ground effect. In many studies this effect was represented by a stationary ground and in some cases as a moving belt. In this study a particular experimental setup will be used: A 250 m water towing tank facility of the TU Berlin enabling realistic boundary conditions and high Reynolds numbers by moving the submerged models over a stationary ground. Based on the anticipated results, industry and academia can develop and realize innovative technologies including adaptive base flaps and automated driving to significantly reduce the aerodynamic drag and thus the CO2 emissions of future heavy-duty vehicles.