Introduction The necessity for improved vehicle fuel economy is a major motivator in the attempt to better understand ground vehicle aerodynamics. Drag@ lift@ and stability are three concepts that constitute the cornerstone in the study of ground vehicle aerodynamics. The consideration of stability typically influences the design of control surfaces@ a common feature in race cars. Though lift is a desirable occurrence in flight@ groundedness@ an inverse of lift@ is the goal in ground vehicle design. This negative lift is often referred to as downforce in ground vehicle aerodynamics. Perhaps the single largest aerodynamic consideration in the design of ground vehicles is drag. A major goal of the effort to better understand ground vehicle aerodynamics is the optimum reduction of aerodynamic drag. The benefits of a successful drag reduction are instant. They include better fuel efficiency@ improved vehicle performance@ and increased passenger comfort. What really is drag? To answer this question@ perhaps we should first treat the question@ What really is aerodynamics?@ and@ by extension@ address the concept of ground vehicle aerodynamics. Aerodynamics is simply the study of the forces involved in the movement of an object through the air. Various objects??airplanes@ cars@ trains@ footballs@ cricket balls@ tennis balls@ baseballs@ feathers@ even agbada (Fig. 1.1)??interact with and are therefore affected by the dynamics of the surrounding air. Because of the general implication of the term vehicle@ vehicle aerodynamics narrows our consideration to transportation. Ground vehicle narrows it even further to vehicles that make contact with the ground throughout their movement. So ground vehicle aerodynamics is the aerodynamic study of cars@ trains@ trucks@ trailers@ motorcycles@ carts@ bicycles@ and@ lest we forget@ human beings??walking@ running@ or crawling. Air molecules bombard everything that they encounter and flow against or past it. When the air molecules are not constrained@ the flow is generally termed incompressible@ and when this incompressible flow travels past a cooperative object such as an airfoil@ we have a streamline of the flow. Streamlines are typically laminar flow. As flow speed increases rapidly or as obstruction to flow increases in the form of friction or in the form of a blunt obstacle@ the tendency for turbulence increases. The streamlines soon give way to micro flow reversals and the build-up of the shear layer at the incipience of turbulence. Be it laminar or turbulent flow@ drag is generated as a reaction to flow@ and all objects struggle to overcome this drag@ which is either in the form of pressure or friction. The pressure form of drag is dominant in flow against blunt objects such as a flat plate or a bluff body positioned head-on against the flow. Drag in the form of friction is the most common form of drag experienced by moving bodies@ the latter having evolved to minimize pressure drag. Although the airfoil is considered the most efficient form of design for drag reduction@ ironically@ protrusions from an almost-airfoil design actually assist in reducing the drag on the body@ especially at high speed. Just like the spinning stitches of a baseball assist the ball in its movement through the air by breaking up the air@ the extruded parts of a vehicle??the door handle and mirror@ for example??also help to break up the air and reduce air resistance. Drag reduction@ which is the primary aerodynamic design goal of a vehicle@ is therefore not merely a straightforward horizontal tear-drop design exercise but a comprehensive approach in which particular attention is paid to parts of the vehicle at different locations such that the collective is well integrated into the aerodynamic big picture. Written for senior-level undergraduate students and graduate students@ this book serves well the practicing engineer@ the aerodynamicist@ the vehicle designer@ and the vehicle rendering artist. It consists of 13 chapters@ some of which can be skipped without loss of continuity. Chapter 1 is primarily aerodynamic review. Naturally@ it starts with the treatment of the subject of drag. It goes on to describe the triggers and the consequences of drag in general and in ground vehicles in particular. Chapter 1 may be skipped if the reader already has an aerodynamic background. Chapter 2 discusses the effects of noise and vehicle soiling on ground vehicles@ particularly on cars. Passenger comfort is of a greater interest (than aerodynamics) in this chapter. Chapters 3 discuses wind tunnel testing as well as track and road testing@ both experimental methods for vehicle testing. Wind tunnel types and functions are introduced. Inherent and potential errors in wind tunnel testing are addressed@ and the correction methods are presented. Design methods of rigs for road tests and data collection methods for road tests are presented. Chapter 4 is an introduction to numerical methods. A case is made for CFD over wind tunnel and road test methods in particular settings. Various types of computational models are listed with their respective benefits and deficiencies. The sequence of numerical solution from model building through boundary layer prescription to solution and result interpretation is treated. In chapter 5@ topics of vehicle stability and control and vehicle performance are treated. Primary control surfaces such as spoilers@ winglets@ and endplates are presented. Secondary control surfaces and media such as vehicle underbody and cross wind are treated. A detailed treatment of truck-car interaction is presented based on extensive studies by the author. Chapter 6 brings together background knowledge from earlier chapters into the design of an aerodynamically sound car design@ with the car broken into front@ mid-@ and rear assemblies. The hood@ windscreen@ nose@ and fascia assembly make up the front section. The roof and the cabin-sustaining posts (A@ B@ C@ and sometimes D) make up the mid-section@ and the trunk and rear bumper constitute the rear section. Chapter 7 may be considered as an introduction to the aerodynamics of large vehicles. From light trucks to trailers and buses@ aerodynamic factors in the vehicle design are presented. A detailed aerodynamic analysis of side skirts??a recent add-on to large trucks??is presented. The topic of car-truck interaction begun in chapter 5 is given further treatment due to a crucial safety issue that the author feels is at stake in highway car-truck common environment. If chapter 7 is an introduction to large vehicle aerodynamics@ chapter 8 is a continuation of large vehicle aerodynamics@ with a focus on trains. Low-speed@ high-speed@ and very high speed passenger trains are discussed. Vibration is a current problem in high-speed trains as they travel through tunnels. The forces and moments that trigger such vibration are presented as well as the design features that have the potential to ameliorate them. Chapters 9 and 10 are at opposite ends of the aerodynamic spectrum. Severe service and off-road vehicles are treated in chapter 9@ while chapter 10 looks at race cars. Even though aerodynamics is not what comes to mind in the design of severe service and off-road vehicles@ there are inexpensive modifications that can be made with a resulting improvement in fuel efficiency. An example of such improvements is found in the aerodynamic profiling of the underbody of severe service vehicles. Race cars and sports cars are the epitome of aerodynamic ground vehicles. It is recommended to review chapter 5 before embarking on chapter 10. Motorcycle aerodynamics is introduced in chapter 11. The same features that have been discussed in earlier chapters are here applied to motorcycles. The seriousness of the safety issue in motorcycle-truck interaction is underscored by the attention given to it in the treatment that follows up on chapters 5 and 7. Chapter 12 treats the subject of internal aerodynamics and cooling system flow in ground vehicles. Although some aspects of this chapter are treated in chapters 5 and 6@ the chapter@ like chapter 2@ can be studied independently. Chapter 13 is an openended chapter. Rather than the title@ Concept Ground Vehicles@ it may very well be titled@ The Quest Continues.