Aerodynamics of two trucks driving in close-proximity

Licentiate thesis, 2021

There are several factors driving the development of vehicles with higher efficiency in society, chief among them are the emissions of greenhouse gases and their contribution to global warming. Therefore, there is a strong incentive to improve heavy-duty trucks' energy efficiency, supported by new legislation. This focus, combined with growing demands for transport, drives vehicle manufacturers to improve their vehicles. One of the important ways to increase the energy efficiency of vehicles is to reduce their aerodynamic resistance. There are, however, limitations on how much the aerodynamics of a vehicle can be refined. As the development of current technologies nears this limit, it becomes more expensive and difficult to further reduce drag. It is, therefore, fundamental to identify new ways to decrease the aerodynamic drag of vehicles. This, in conjunction with improvements in vehicle automation and sensor technology, has spurred a renewed interest in vehicles driving in close proximity to each other, also known as platooning.

Although there is interest in this topic, a lack of understanding remains concerning the phenomena that cause changes in drag when two or more vehicles are traveling in a platoon. Furthermore, the behavior of platooning cab over engine style trucks that are common in the European Union is not well understood. The focus of this work lies in understanding such a system’s behavior and producing data that shows the changes in drag observed for two cab over engine style trucks driving in close proximity.

The work was split into two different studies: the initial part was based on numerical results, and the second on experimental ones obtained in the Volvo Cars wind tunnel. The results show that the behavior of the leading truck is simple, where a closer inter-vehicle distance will mean a lower drag. The trailing truck has a more complex behavior, where there are both local minima and maxima with respect to the drag experienced as the separation distance between the vehicles is reduced. The effects that cause the changes in drag are many but can be generally defined as the leading truck mainly being affected by differences in pressure, and the trailing truck affected mainly by changes in the flow due to the wake of the leading vehicle. The combined changes in drag of the two vehicles result in a continuous decrease in drag with a reduced inter-vehicle distance.