The Aerodynamic Characteristics of Passenger Vehicles Operating in a Platoon

Ebrahim, Hesham (2019) The Aerodynamic Characteristics of Passenger Vehicles Operating in a Platoon. Doctoral thesis, Northumbria University.

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Abstract

Passenger vehicles operating in close proximity platoons have been consistently reported to yield aerodynamic drag reductions as a system, however, the current understanding of a vehicle’s pressure field and the associated flow structure remains vague. In the literature, researchers have demonstrated broadly similar trends of drag reduction, but the detailed differences between their results, even for a relatively simple parametric variation such as the inter-vehicle spacing reveal quantitative and qualitative differences that are unexplained. Recent and ongoing developments in vehicle intelligence and control have brought the potential to operate in platoon close to reality, and therefore improving and understanding the aerodynamic characteristics of each vehicle is the subject of the present work.
An idealised bluff body, the Ahmed model and a production vehicle, the Nissan Leaf, have been used to analyse the sources of drag reduction of individual vehicles in platoons of 2 and 3 vehicles by investigating the aerodynamic forces, pressure distributions, flow structures and sources of flow unsteadiness. Data were collected on-track to evaluate the concept in real world conditions and measurements were also made in simulation environments including a 3=4 open jet model wind tunnel and CFD simulations.
The new findings of this study showed that for a 2-vehicle platoon the overall drag reduction of the leading vehicle was attributed to a base pressure increase predominantly caused by lateral and vertical stretching of the near-wake structure. The pressure field acting on the trailing vehicle was significantly influenced by the flow trajectory off the leading vehicle. Some of the base geometry features that were optimised to minimise the drag of the vehicle-in-isolation such as tapering the sides and adding a diffuser, can have a detrimental effect on the following cars at platoon spacings of 0:25L and 0:5L for a 2-vehicle platoon. The resulting impingement effect on the trailing vehicle reduces as the spacing between the vehicles is increased above 0:5L, however this also reduces the overall drag reduction due to the base pressure decrease on the leading vehicle. In a 3-vehicle platoon, all the vehicles experience drag reduction as the wake dissipates and has lower impingement energy on the vehicles following. In general, increasing the number of vehicles in platoon and reducing the spacing between the vehicles improves the drag reduction. The intensity of flow unsteadiness was found to be related to vehicle spacing such that the areas identified with high instability appeared to shrink in size and increase in magnitude as the spacing was reduced. Low frequency (2-50Hz) oscillations were found to be responsible for the flow convection and impingement on the trailing vehicle that introduced stagnation point instabilities.
Overall, the results presented in this work help to explain why many previous studies have been seemingly contradictory as it identifies the sensitively of vehicle geometry and wake structure on the platoon performance. It also provides insight into the main parameters that influence the performance of the platoon that could in the future improve the design of autonomous vehicles to be more tailored for efficient platoon operation.

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