An Experimental Study of Tractor Base Bleed for Heavy Vehicle Aerodynamic Drag Reduction

A significant contributor to heavy-vehicle aerodynamic drag is the tractor-trailer gap, especially when operating in a crosswind. At this condition, the freestream flow turns into the tractor-trailer gap, imparting a momentum exchange to the vehicle and subsequently increasing the aerodynamic drag. In common use today, tractor side-extenders provide significant drag reduction, but they are not without problems. Frequently damaged when the tractor pivots sharply with respect to the trailer, side extenders can incur additional costs for maintenance and repair. This issue can be alleviated by shortening extenders (thereby reducing their benefit) or devising an alternative drag-reduction concept. One such concept is tractor base bleed, in which air-flow is vented into the tractor-trailer gap through the back of the tractor. To study this concept, a wind-tunnel study was conducted for a generic 1:20-scale tractor-trailer configuration at width-based Reynolds number of 420,000. Delivered through a porous material embedded in the tractor base, the bleed flow was varied so as to generate velocities behind the tractor ranging from zero to 10% of the freestream velocity. Configurations were studied both with and without side extenders at two different tractor-trailer separation distances.