Undergraduate Research on Trailer-Truck Aerodynamic Drag

Abstract

The transportation industry is heavily dependent on ‘big rigs’ or semitrailers. Since its introduction during 1920s semitrailers have revolutionized the industry. However their geometrical designs have not evolved much to make them aerodynamically more streamlined, thus more fuel efficient. While over 5.6 million such commercial trailer trucks are registered in the country and with increasing diesel fuel prices, it is more important than ever to study their aerodynamics, redesign for reducing aerodynamic drag and help make these ‘big rigs’ more fuel efficient. Aerodynamic drag is the force that acts on a solid object moving in air due to difference in dynamic pressure developed around that object. Skin friction also causes resistance force which is small compared to pressure induced drag. Higher drag resistance, just like road and tire resistance, causes loss of energy and thereby lowers fuel mileage. Drag resistance is caused by both surface friction as well as air pressure difference around a moving object/vehicle. An ideal remedy is of course to completely redesign the shape and size of these semitrailers to conform to those with known low drag. Another intermediate approach would be to retrofit the existing semitrailers with devices that change the overall shape towards more aerodynamic ones. During the recent past a wide range of such add on devices have been introduced. Current research was directed in two fronts: CAD and Drag simulation as well as experimental drag testing. First a base CAD model and then various modifications were developed using an industry standard CAD package. These models were then imported into Computational Fluid Dynamics (CFD) software. These followed by modeling add-on devices to reduce drag. The simulations were repeated with various combinations of these add-on drag reducers. The areas targeted for drag reduction study included gap between tractor and trailer, lower sides of the trailer between front and rear wheel sets, and rear of the trailer. The results showed varying effectiveness of these add-on devices, individually and in combination. Scale models of the trailer truck were built using wood as well as Rapid Prototyping (RP) directly from CAD using polymer. These models were then tested in the wind tunnel at speeds between 35 and 75 miles per hour. The data and the trends in Cd values compared well with the simulated values. The overall CFD and scale model studies provided a comprehensive knowledge and understanding of the drag in semi-trailers and factors that affect it. Future studies may expand the varieties and locations of these devices as well as complete redesigns of the trailer-trucks.