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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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 shape, and the speed of the truck. The basic geometry used in the simulation is a 1:8 scale Ground Transportation System (GTS). The effects of vehicle geometry, frontal shape, vehicle speed, and the gap size were investigated and the drag coefficients were computed. The effect of add-on devices such as aerodynamic boat-tail plates at the rear of the trailer on the aerodynamic drag of the truck has also been analyzed. In particular, the effects of the gap between tractor and trailer, and the gap between trailers on the aerodynamic drag of tractor and two trailers configuration were determined. The feasibility of Reynolds-Average Navier-Stokes (RANS) κ-ε model in the prediction of aerodynamic drag at various Reynolds numbers has also been studied. The CFD software from CD-adapco together with an expert tool, es-aero, was used for all the analyses reported in this paper. CFD simulations for tractor and single trailer configuration were performed for various Reynolds numbers. The simulation results were validated with available experimental data and good agreements were found. Validation of numerical results for tractor and single trailer with the experimental data formed the basis for analyzing tractor and double trailer configuration. The tractor and two trailers configuration for different gap sizes between the trailers at various speeds were analyzed. 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 33 percent. Drag coefficient also reduced by 46 percent by introducing smooth frontal fillets in case of tractor and single trailer. The study also emphasizes flow structures around the vehicle that contribute to the total drag.

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