<div class="section abstract"><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">Part two of this three-part paper documents the influence of the ambient winds on the development of the wake behind a vehicle. A series of scaled-model wind-tunnel measurements, supplemented by some high-fidelity numerical simulations, based on a Lattice-Boltzmann approach, are presented to examine the effects cross-wind magnitude, by means of changes in yaw angle, on the wakes behind four vehicle shapes: a sedan, an SUV, a pickup truck, a medium-duty vehicle and a heavy-duty vehicle. The influence of road-representative freestream turbulence is also examined.</div><div class="htmlview paragraph">The results of these investigations show that, under yaw conditions, the distinct differences between the wake structures of slant/step-back and square-back shapes, documented in Part 1, are eliminated. At yaw, the moderate-to-far wake region is dominated by a large vortex structure of similar size to the vehicle itself that generates significant sidewash, analogous to the downwash in the wake of a wing in pitch. All vehicle shapes studied demonstrate this vortex structure which increases in strength with yaw angle. For vehicles following in the wake, not only do they experience a wind-speed deficit associated with the wake, but they experience a twisted wind profile with reduced yaw angles near the ground. The introduction of freestream turbulence is shown to generate a large wake with reduced shear, but without changing the dominant flow characteristics.</div></div>

<div class="section abstract"><div class="htmlview paragraph">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 moderate-to-far regions of ground-vehicle wakes (one to ten vehicle lengths), in the context of their potential influence on other vehicles.</div><div class="htmlview paragraph">Part Three of this three-part paper documents the wake characteristics for multi-vehicle scenarios of two or three vehicles, in single-lane or two-lane arrangements. A set of high-fidelity numerical simulations, based on a Lattice-Boltzmann approach, and a series of scaled-model wind-tunnel measurements are presented to examine some multi-vehicle scenarios with four types of vehicle shapes: a sedan, an SUV, a medium-duty vehicle, and a heavy-duty vehicle. The wake-merging process of a two-lane longitudinally-staggered sedan-and-SUV configuration is examined in detail with and without cross winds. Experimental-results are presented only for the SUV, medium- and heavy-duty vehicle shapes due to challenges described in Part One with respect to the sedan-model and the stationary-floor arrangement.</div><div class="htmlview paragraph">Complex wake patterns emerge under multi-lane scenarios, particularly under cross-wind conditions, but the results suggest a level of predictability in the aggregate wake properties. Only for the cases with close lateral proximity (side-by-side configurations) do the wake patterns change substantially such that the characteristics of the individual wakes are not perceptible. Additionally, the merged wakes in cross winds are shown to potentially influence vehicles up to three or four lane widths away. The results suggest that superposition of moderate-to-far-wake profiles may be possible to generate on-road wind conditions associated with a range of traffic scenarios.</div></div>

<div class="section abstract"><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div></div>

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