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There is little information in the technical literature about the dependence of drag coefficient, CD, on aspect ratio (height/width) for car and truck aerodynamics. Some of the information suggests that CD should increase with aspect ratio as the flow over the body becomes more two dimensional. Recent tests of candidate shapes for a commercial van with various roof heights suggested the opposite is true; the taller vans had lower drag coefficients. This report discusses the results of several experimental investigations to examine this relationship. Scale model and production drag measurements of commercial vans are presented along with drag measurements of simple shapes. The shapes consisted of eight radiused rectangular boxes of constant length and frontal area, but with different height/width ratios. The effects of underbody roughness and bumper presence were evaluated and are discussed.

The work presented here is a further step in a continuing effort by the authors to study and document the aerodynamic effect of varying aspect ratio, ground clearance, and underbody roughness for basic rectangular shapes with radiused edges in ground effect. Previous papers by the authors have presented extensive wake data for a range of aspect ratios and underbody roughness conditions. Force and moment data have been published for various aspect ratios, underbody roughness, and ground clearance. In this paper, additional wake data showing the effects of variation in ground clearance is presented. The focus of this paper is on analyses to make progress toward a rational correlation between body characteristics, the forces, and the measured wake properties. A parametric analysis is presented linking the effects of the parameters varied in the study to the changes in the aerodynamic forces and moments. Results show correlations between the wake flows and the force results.