Near-to-Far Wake Characteristics of Road Vehicles Part 1: Influence of Ground Motion and Vehicle Shape 2021-01-0957

Conventional assessments of the aerodynamic performance of ground vehicles have, to date, been considered in the context of a vehicle that encounters a uniform wind field in the absence of surrounding traffic. Recent vehicle-platooning studies have revealed measurable fuel savings when following other vehicles at inter-vehicle distances experienced in every-day traffic. These energy savings have been attributed in large part to the air-wakes of the leading vehicles. This set of three papers documents a study to examine the near-to-far regions of ground-vehicle wakes (one to ten vehicle lengths), in the context of their potential influence on other vehicles.

Part one of this three-part paper documents principally the influence of vehicle shape on the development of its wake. A series of high-fidelity numerical simulations, based on a Lattice-Boltzmann approach, and a series of scaled-model wind-tunnel measurements are presented to examine the effects of four types of vehicles: a sedan, an SUV, a pickup truck, and a heavy-duty vehicle. The influence of using a stationary-ground-plane setup in the wind tunnel is examined using numerical simulations, to provide context for the wind-tunnel results.

The results of these investigations show that ground motion, or the lack thereof, has a greater influence on the wakes of the slant-back and step-back shapes than for a square-back shape due to an interaction of the wake vortex structures with a horseshoe vortex generated by the interaction of the vehicle pressure field with the oncoming boundary layer. The results also demonstrate two distinct types of wake regimes at low yaw angles for these different classes of vehicles shapes. Slant-back and step-back configurations like the car and pickup-truck models demonstrate the classic C-pillar vortex structure with central downwash, while the square-back shapes like the SUV model demonstrate a central upwash from a vortex pair of opposite sign. The mechanisms leading to these two opposing vortex pair orientations is examined. Drag reduction technologies applied to a heavy-duty-vehicle shape are shown to modify the wake structure such that slant-back and square-back wake characteristics can be generated.