José C. Páscoa, Francisco P. Brójo, Fernando C. Santos and Paulo O. Fael
The Journal of Mechanical Science and Technology, vol. 26, no. 6, pp.1663-1670, 2012

Abstract : Ground vehicle drag coefficient is herein obtained using an unconventional on-road test in real scale. At low-Re numbers, and as a function of velocity variations, transition introduces changes on the vehicle’s drag coefficient. Therefore, the drag coefficient must be obtained as a function of velocity. Traditionally, only an average drag coefficient value is usually obtained using the coast down method. To obtain the on-road, velocity dependent, drag coefficients we introduce a new approach. The aerodynamic resistance coefficient is obtained by towing the vehicle with and without an aerodynamic shield, in order to eliminate the rolling resistance component. A detailed description of the method, its associated techniques, and related errors is presented. We conclude that the present experimental procedure is needed when comparing the experimental drag coefficient against computational results, since numerical computations are usually performed in a velocity dependent framework. Further, the same on-road test procedure is herein used to obtain the rolling and aerodynamic drag coefficient for a prototype vehicle working in the transition regime.

Keyword : Ground vehicle; On-road test; Experimental method; Aerodynamics