| 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, the presence of gaps in truck body, and the speed of the truck. The basic geometry used in the simulation is 1:8 scale Ground Transportation System (GTS). The GTS model has a simplified geometry with a cab-over engine design with either one or two trailers. 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-ons device such as boat-tail plates has also been analyzed. In particular, the effects of the gap between the tractor and the trailer, and the gap between trailers on the aerodynamic drag were determined. The feasibility of Reynolds-Average Navier-Stokes (RANS) standard k-epsilon and RNG k-epsilon turbulence models in the prediction of aerodynamic drag at various Reynolds numbers had also been studied. The CFD software STAR-CD from CD-adapco together with an expert tool, es-aero, was used for all analyzes reported in this study. The simulation results for tractor and a single trailer were validated with available data, and good agreements were found over the low and high ranges of Reynolds numbers. Drag coefficient decreased 46 percent by introducing smooth frontal fillets in the case of a tractor and a single trailer. Validation of results for the tractor and a single trailer formed the basis for the analysis of doubles, a tractor and two trailers configuration. The tractor and two trailers configuration for different gap sizes between trailers at various speeds were analyzed. The results showed that closing the gaps and incorporating boat-tails at the rear of the trailer and reducing the ground clearance from 24 inches to 18 inches could reduce the drag by as much as 36 percent, which will result in substantial savings in fuel and transportation costs. The study also emphasized on flow structures around the vehicle that contribute to the total drag. |
