Heavy-Duty Vehicles (HDVs) are responsible for a measurable and growing portion of the worldwide greenhouse gas emissions. As aerodynamic drag constitutes the single biggest source of engine power demand for HDVs at highway speeds, aerodynamic improvements are an effective measur
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Heavy-Duty Vehicles (HDVs) are responsible for a measurable and growing portion of the worldwide greenhouse gas emissions. As aerodynamic drag constitutes the single biggest source of engine power demand for HDVs at highway speeds, aerodynamic improvements are an effective measure to reduce fuel consumption and thus greenhouse gas emissions. Automated close-distance following of two or more HDVs, better known as “platooning”, can lead to lower drag through beneficial aerodynamic interference effects. While the influence of longitudinal separation distance between HDVs on the aerodynamic drag of the vehicles has been investigated in several studies, considerably less research has been done on the influence of relative lateral positioning or “misalignment”. This report covers a wind-tunnel experiment with two simplified models, which are known as squareback Ahmed bodies, in several longitudinal and lateral positions. Pressure measurements were done on the rear face of the leading models, and force measurements were done on both models utilizing integrated load cells. Coaxial Volumetric Velocimetry, a large-scale Particle Image Velocimetry technique, was used for reconstruction of the mean three-dimensional velocity field in the gap between the models. Three separation distances were tested: 20%L, 40%L, 60%L, where L is model length. For each longitudinal separation distance five lateral positions were tested: -25%W, -15%W, 0%W, 15%W, 25%W where W is model width. Based on the force readings it has been found that, for zero lateral offset, the leading model benefits of reduced drag coefficients for all separation distances. Pressure measurements confirm that the pressure across the base area is increased relative to the vehicle in isolation. The trailing model, on the other hand, suffers from increased drag coefficients at the tested separation distances. The positive effects of driving in a low-momentum wake are negated by secondary aerodynamic effects that increase the pressure on the nose section of the trailing model. The introduction of lateral stagger generally increases the drag coefficient, relative to the drag coefficient when aligned, for all models and all separation distances. The magnitude of the drag coefficient increase is dependent on the separation distance. The smaller the separation distance is, the more negative impact lateral offset has. Pressure measurements show that areas of low pressure occur on the base of the leading model at the side to which the trailing model moves. That is: if the trailing model is offset to the left of the leading model, then an area of lower pressure exists across the left part of the base area of the leading model. Time-averaged flow field visualizations show that for a platoon at 0.6L separation, the near wake flow topology remains largely unchanged when a lateral offset is introduced. At 0.2L separation, however, the wake topology changes considerably with lateral offset. When aligned, a toroidal vortex exists in the gap between the models. The toroidal structure changes in a horseshoe vortex when the platoon is laterally misaligned. The topological change is accompanied by strong flow curvature and leads to regions of low pressure across the base of the leading model.