Aerodynamics of vehicle platooning

Doctoral thesis, 2023

Many factors are pushing automotive manufacturers to increase the efficiency of their fleets; some of these are legislative (requiring reduced greenhouse gas emissions) as well as ensuring sufficient range and power consumption in electric vehicles. An important part of improving the energy efficiency of road vehicles is reducing their aerodynamic drag. Much effort has gone into improving the aerodynamics of trucks, cars, and buses. There are, however, limits on the attainable aerodynamic performance due to several different factors.

This thesis focuses on a relatively unutilized way of reducing the aerodynamic drag of vehicles, that is, vehicles driving in close proximity, or platooning. Although such a solution has long been envisioned as a way of reducing drag, it is only now becoming possible with advancements in vehicle automation and communication. Platooning is, however, regularly, and successfully used in many sports.

Although there have been many studies on the topic, the focus has mostly been on the differences in drag. This thesis attempts to improve the understanding of the observed changes in drag for a cab over engine style tractor-trailer, a passenger car, and a coach bus. The work herein was performed using both CFD simulations and wind tunnel experiments. The aerodynamics of platoons were investigated for separation distances between 0.5m and 30m, 0m and 0.5m lateral offsets, and 0°, 5°, and 10° yaw angles, to emulate wind conditions. The effects for different vehicle types are generally similar, although with varying magnitudes depending on the combination.

The results showed that the aerodynamic phenomena of the leading vehicle are, in most cases, fairly straightforward, with a base pressure increase due to the pressure field emanating from the trailing vehicle’s stagnation area. This causes a reduction in drag with a decreased distance and a reduction of the relative savings under yaw conditions. The effects on the trailing vehicle are more complex as they are dominated by changes to the flow field. Many of the changes are generated by the leading vehicle slowing the flow before the trailing vehicle, as well as slight changes to the flow angularity. This typically causes the relative pressure on the front to trend toward zero and the pressure in the tractor-trailer gap to increase. At yaw, similar effects remain, but the trailing vehicle also experiences a decrease in the effective yaw angle, similar to that of a lower yaw angle. The effects on the leading and trailing vehicles generally combine to reduce the drag of the entire platoon as the separation distance decreases. Yawed flow tends to reduce the relative savings for the platoon as a whole, while a lateral offset can recover some of it.