Experimental Study of CD Variation With Aspect Ratio 1999-01-0649

There is little information in the technical literature about the dependence of drag coefficient, C_D, on aspect ratio (height/width) for car and truck aerodynamics. Some of the information suggests that C_D 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 streamwise development of the wake structures is presented in the companion paper, SAE Report 1999-01-0648. The study results indicate that the commercial van measurements are correct - C_D decreases with aspect ratio. An empirical drag build-up analysis of the van measurements is used to illustrate that underbody, wheel, and wheel-well drag forces are essentially unchanged as roof height is increased. C_D comparisons can be influenced by body proportions. A definition of aspect ratio for vehicle aerodynamics is proposed.