Understanding Aerodynamics: Arguing from the Real Physics

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From Elementary Particles to Aerodynamic Flows

Step back for a moment to consider the really big picture and ponder how aerodynamics fits into the whole body of modern physical theory. The tour I'm about to take you on will be superficial, but I hope it will help to put some of the later discussions in perspective.

First, consider some of the qualitative features of the phenomena we commonly deal with in aerodynamics. Even in flows around the simplest body shapes, there is a richness of possible global flow patterns that can be daunting to anyone trying to understand them, and most flows have local features that are staggeringly complex. There are complicated patterns in how the flow attaches itself to the surface of the body and separates from it (Figure 2.1a, b), and these patterns can be different depending on whether you look at the actual time-dependent flow or the “mean” flow with the time variations averaged out. Even in flows that are otherwise steady, the shear layers that form next to the surface and in the wake are often unsteady (turbulent). This shear-layer turbulence contains flow structures that occur randomly in space and time but also display a surprising degree of organization over a wide range of length and time scales. Examples include vortex streets in wakes and the various instability “waves,” “spots,” “eddies,” “bursts,” and “streaks” in boundary layers. Examples are shown in Figure 2.1c–f, and many others can be found in Van Dyke (1982). Such features ...