The aerodynamic drag characteristics of a heavy duty truck with two configurations, a tractor and a single trailer, and a tractor and a tandem-trailer (two trailers), have been studied. The aerodynamic drag of a truck depends on geometry, frontal area, and the speed of the truck. The basic geometry used in the simulation is 1:8 scale Ground Transportation System (GTS). The present simulation model has a simplified geometry of GTS with a cab-over engine design with either one or two trailers. In particular, the effects of the gap between the tractor and the trailer, and the gap between the tandem trailers on the aerodynamic drag were determined. The effects of vehicle geometry, vehicle speed, and the gap size were investigated and the drag coefficients were computed. CFD software STAR-CD with an expert tool, es-aero, was used for all the analyses reported in this paper. The simulation results were validated with available experimental data and good agreements were found for vehicle speeds at highway and city limits. The results showed that closing the gaps and incorporating boat-tails at the rear of the trailer could reduce the drag by as much as 40 percent.

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