10 Aerodynamics of Road Vehicles THOMAS MOREL I. INTRODUCTION Vehicle aerodynamics concerns the effects arising due to motion of the vehicle through, or relative to, the air. Its importance to road vehicles became apparent when they started to achieve higher speeds. The automobile as we know it came onto the scene in the last decade of the nineteenth century. Its beginnings roughly coincided with the advent of powered flight, and perhaps for this reason, it became of interest to aerodynamicists right from the start. One of the first attempts to apply aerodynamic principles to road vehicles was the streamlining given to the first holder of the land speed record, a car named Jantaud driven by Gaston Chaseloup-Laubat (Fig. 1). This vehicle held the record several times, culminating with 93 kmIhr (58 mph) achieved in 1899. The early interest in vehicle drag continued and an early paper published in 1922 by Klemperet1) (apparently the very first paper on the subject) already reported actual wind tunnel studies on several then current automobile shapes and also on one low-drag shape. The work was done in the wind tunnel of the Zeppelin Company which was involved in the development and construction of dirigibles. The influence of dirigibles showed in the shape of the low-drag model, which had a streamlined teardrop shape and no wheels. The drag coefficient of this model was 0.15, and it is interesting to note that this value is still among the lowest obtained for a vehicle-shaped body near the ground. The thrust of the original work during the first few decades of the twentieth century was toward the reduction of drag to increase the top speed of road vehicles. That objective became less important with the steady improvements and increasing power of automobile engines, and with the increasing efficiency THOMAS MOREL. Integral Technologies Incorporated, Westmont, Illinois 60559. 335 J. C. Hilliard et al. (eds.), Fuel Economy © Springer Science+Business Media New York 1984 336 Thomas Morel FIGURE 1. First land speed record holder Jantaud, driven !>Y Gaston Chaseloup-Laubat. of the whole power-train system. Although the fuel costs were never negligible, efficiency improvement was not considered a pressing issue and, consequently, aerodynamics was often subjugated to other objectives. The original need for it, related to top speed, remained an issue somewhat longer with racing cars (but even there the increasing engine power shifted the emphasis) and land speed record cars. Examples of the latter are the Goldenrod, (2) holder of the world speed record for wheel-driven automobiles set in 1965 with 664 kmlh (413 mph), and the rocket-propelled Blue Flame, (3) record holder in the unlimited class with 1001 kmlh (622 mph) set in 1970. The drag coefficient claimed for the pencil­ shaped Goldenrod is CD = 0.12. This is the lowest value reported for any full­ scale driveable vehicle. Practical road vehicles are much less slender and so comparison to Goldenrod as the ideal is probably not appropriate. With the more recent emphasis on fuel economy, aerodynamics has regained some of its former importance. The need for increased fuel economy, dictated by the realities of the marketplace, has led to a climate in which aerodynamics is being asked to contribute its share to the attainment of the highest possible efficiency of the road vehicle as a system. This renewed interest shows up in the number of meetings organized in recent years, at which developments in the area of vehicle aerodynamics have been discussed. (4-10) There is a commonly held belief that the principles of low-drag vehicle design are well understood. It is, after all, fairly widely known that the lowest drag body at subsonic speeds is one with a teardrop shape, already considered by Klemperer. An optimized teardrop in free flight (away from the ground) is usually assumed to have a drag coefficient as low as 0.04, but, somewhat surprisingly, the actual proportions of an optimum teardrop and their dependence on Reynolds number and yaw angle are in fact not that well established. Also, it is in fact not necessary to have the body taper at the end to a point, but it may be abruptly terminated well before the point would be reached, with no increase in drag as was determined experimentally in the 1930s. This idea is credited to Kamm, and the resulting shape is referred to as Kamm-back. These subtleties notwithstanding, it is a fact that current production automobiles have drag coefficients an order of magnitude larger than tear-shaped bodies in free flight and several times larger than shapes like Klemperer's. They range from Aerodynamics of Road Vehicles 337 0.8 for the open-roof early automobiles to today's typical cars averaging 0.45, to the best current designs quoted at approximately 0.35. The major reason for the difference between the ideal and the current state of the art is that the vehicle shapes are subject to many constraints, which will be discussed in the next section. Most wind tunnel investigations of the flow around road vehicles have concentrated on the measurements of forces and moments, while the details of the flow have not been studied in any great detail. This is mostly because of the complexity of vehicle shapes, which is the direct consequence of the practical constraints which influence and dictate various aspects of vehicle shape. An important consequence of the complexity of the vehicle shapes is that the resulting flow fields are also complex. They are turbulent, three-dimensional, they have numerous regions of separated flow, typically have strong streamwise vortices in the wake, and are influenced by the proximity of the ground. Some of these features are graphically illustrated in the sketch in Fig. 2. As a result of this complexity, the work in the area of vehicle aerodynamics was not well focused until recently, being fragmented into many parallel inves­ tigations often involving the development of a particular vehicle, with not enough emphasis being given to the general discipline. An additional contributing factor was that aerodynamics had not been considered an essential and integral part of vehicle design, but had been included into the process after the fact. The vehicle shape was designed by the stylists, and the aerodynamicists who followed tried to make the most out of what they were given. The final decision whether to incorporate any changes proposed by aerodynamicists rested with the stylists, and this situation did nothing to encourage fundamental research in aerodyn­ amics. The literature related to vehicle aerodynamics is very extensive. No effort to compile all of these numerous publications is being made here, but interested readers may consult a paper by McDonald(1) which includes a list of over 100 references. In addition to these studies one may consider the large amount of FIGURE 2. Schematic view of some of the complex features of vehicle flows (from Ref. 8). 338 Thomas Morel work done on bluff or nonstreamlined body aerodynamics related to other fields. Its relevance to road vehicles was reviewed during a symposium at the General Motors Research Laboratories in 1976. (8) Most of the available bluff body work may be classified into several categories: fundamental--concerning simple shapes studied to provide basic understanding of separated flows; aeronautical and bal­ listic--concentrating on airplanes, projectiles, and missiles; and architectural­ related to flows over buildings and structures. Since all this literature is available, how much can be learned and transferred from it to road vehicles? The last category, architectural aerodynamics, is not particularly relevant as it deals with bodies placed directly on the ground, immersed in boundary layers whose thickness is often larger than the body height. The resulting flow fields are very different from road vehicle flow fields. The other two categories have more in common with road vehicles, and thus they might be expected to be a valuable source of information and ideas. As will be discussed later in a section dealing with the mechanisms of drag generation, the great majority of that literature deals with two-dimensional and axisymmetric bodies, whose flow fields are by and large not too relevant to the complex three-dimensional flows of road vehicles. In this context it is worthwhile to bring out one of the differences between the aeronautical and vehicle aerodynamics, which should be kept in mind when viewing the presented results, and this concerns the coordinate sys­ tem. In consequence of the positive contact of the vehicle with the road, all forces and moments are referred to body axes, rather than to the relative wind direction as is done for free-flying objects. The objective of this review is to attempt to address a number of key questions concerning road vehicle aerodynamics: 1. How does aerodynamics fit into the context of a practical vehicle? 2. How much does aerodynamics contribute to fuel consumption? 3. How is drag generated? 4. What is the current state of the art of low-drag design? 5. What is the future outlook for low-drag design and what is the practical lower limit of CD for production vehicles? These questions guided the outline of the review, with separate sections devoted to each of them. II. BASIC CONSIDERATIONS INFLUENCING VEHICLE SHAPE As pointed out in the introduction, current production automobiles have drag coefficients an order of magnitude larger than tear-shaped bodies in free flight and several times larger than idealized shapes like Klemperer's. An im­ portant reason for this discrepancy is that vehicle shapes are subject to many Aerodynamics of Road Vehicles 339 practical constraints from the point of view of intended vehicle purpose and use, vehicle safety, maintenance, and cooling and product identity.