Near-to-Far Wake Characteristics of Road Vehicles Part 2: Influence of Cross Winds and Free-Stream Turbulence
Abstract:
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 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.
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.