BASIC TERMINOLOGY Before embarking on a discussion about the aerodynamics of race cars or even of simple wings, I must explain some of the most basic terms in the professional jargon. I'll start with the observation that, due to a vehicle's forward motion, the otherwise still air is set into motion. In order to visualize this air motion, I must discuss the resulting airflow directions, the magnitude of velocity and pressure fields, and a few other basic terms. These basics will establish the relation between airspeed and pressure. Understanding the pressure distribution over a vehicle's body is, of course, one of the primary objectives of this discussion, since the collective effect of the small differences in the pressure around the vehicle's body are responsible for aerodynamic loads such as lift and drag. Some patience and dedication is needed for the next few sections because an initial "load" of definition is required before we can better relate to the new information and relate its significance to the main scope: the effect on vehicle performance. STREAMLINES, ATTACHED AND SEPARATED FLOWS Let us begin with one of the simplest definitions. It is related to the frequently shown smoke traces in the airflow near cars being tested in wind tunnels. These streamlines are the curves associated with a pictorial description of a fluid motion. If our vehicle is moving forward at a steady speed, the flow is then called steady-state flow. In this case the air particles will move along the streamlines (lines which are parallel to the local velocity direction). Fig. 2-1 demonstrates the shape of such streamlines as formed near an airfoil. Flow visualization of the streamlines can be obtained in a wind tunnel by injecting smoke or, in a water tunnel (which is usually used with smaller-scale models), by the injection of colored dye. However, if the injected fluid has different density than the fluid, it may not follow the streamlines exactly. Therefore, the coloring material has to be selected very carefully, and in the case of automotive wind tunnel testing, the injection of smoke (which fairly close density to air) is widely used.