Road vehicles have been shown to experience measurable changes in aerodynamic performance when travelling in everyday safe-distance driving conditions, with a major contributor being the lower effective wind speed associated with the wakes from forward vehicles. Using a novel traffic-wake-generator system, a comprehensive test program was undertaken to examine the influence of traffic wakes on the aerodynamic performance of heavy-duty vehicles (HDVs). The experiments were conducted in a large wind tunnel with four primary variants of a high-fidelity 30%-scale tractor-trailer model. Three high-roof-tractor models (conventional North-American sleeper-cab and day-cab, and a zero-emissions-cab style) paired with a standard dry-van trailer were tested, along with a low-roof day-cab tractor paired with a flat-bed trailer. Amongst these, trailer variants provided a total of 10 HDV configurations that were tested in uniform turbulent flow over a range of freestream yaw angles between ±15°, and with wake effects over a range of yaw angles between -2° and +11°. Up to 53 specific wake-flow conditions were applied to each HDV configuration. Wind-load and surface-pressure measurements were acquired and provide indicators of the manner in which the aerodynamic performance of the HDV models are influenced by traffic wakes.
Drag-coefficient reductions up to 17% for individual drag-coefficient values and up to 9% for wind-averaged values were observed. Wakes from adjacent-lane vehicles were observed to have comparable, or sometimes greater, influence to those from safe-distance same-lane vehicles. The wakes influence primarily the forward-facing surfaces of the HDV, resulting in performance changes associated with tractor modifications being affected more than for trailer modifications. These results represent the first comprehensive study of traffic-wake effects on HDVs at safe inter-vehicle distances in highway-driving conditions, and highlight potential differences in real-world aerodynamic performance relative to the standard wind-averaged uniform-flow metrics used for fuel/energy-use and emissions predictions.
