Understanding Aerodynamics: Arguing from the Real Physics

5

General Features of Flows around Bodies

In Chapter 4, we looked in considerable detail at the physics of the flow closest to the surface of a body, in the viscous boundary layer. In this chapter, we explore the generic characteristics of the entire flowfield in external flows around bodies, in preparation for more detailed discussion of specific aspects such as drag and lift in following chapters. While the basic physical laws (the NS equations: Navier-Stokes) govern all of the flow phenomena we'll look at, the equations by themselves won't help us much in our quest for intuitive understanding. As we saw in Chapters 2 and 3, there is just too much of a gap between the simplicity of the local physical laws and the potential complexity of the flow phenomena that they govern for us to be able to get much from the equations by mental exercise alone. Our discussion will therefore rely heavily on knowledge of phenomenology, that is, what actually happens in real flows. But we'll try to get beyond just describing what happens and understand why things happen the way they do. Some of the flow features we'll consider will seem at first to be trivial or obvious, but I'll ask you to bear with me. We'll find that explaining the “why” of even the most obvious flow features can be surprisingly tricky.

For purposes of this discussion, we'll put ourselves in the body-centered reference frame and assume the flow can be treated as steady. The unsteadiness in turbulent boundary layers and wakes ...