In Section 2.1, we encountered two fundamental forces: (1) the force perpendicular to a surface due to normal stress, or pressure; and (2) the tangential force due to viscous-shearing stress. These two forces are clearly related to the lift and drag on a body. That is, integration of the differential force contributions over the wetted surface of a body moving through air leads to the force system discussed in Chapter 1. The integrated force components can be resolved into lift and drag and moments about the center of gravity, as previously defined. The emphasis in Section 2.1 was on associating these forces with processes occurring at the molecular level. The benefits of using a macroscopic point of view based on a continuum model were also described. This section explores this concept in more detail. Rather than concentrating on microscopic interactions, we attempt to discover how much can be learned by applying the simple ideas of dimensional analysis. In this process, we identify important properties, both physical and geometrical, that influence the generation of the force system on a body moving through a fluid.
Many benefits accrue from the process. For instance, we discover key scaling parameters, or similarity parameters. Understanding the physical content of these parameters is a major step in classifying types of aerodynamic problems in ways that simplify the modeling process and lead to approximate formulations of practical flow problems that are both accurate and easy to use. The results also have an acritical application in guiding the design of experimental procedures and in the correct interpretation and correlation of experimental measurements in aeronautics as well as other fields.