In Chapter 7, we applied analysis methods to flows inside pipes and other closed conduits. We started with a practical challenge of estimating the extent of a home flood and developed our solution method by thinking about that problem from various angles (Figure 8.1). We first decided on the goal of our analysis; then, star ting with the simplest models, we systematically investigated flows of increasing complexity until we found a solution to the burst-pipe problem through dimensional analysis and data correlations. This protocol is general, and it can be applied to other flows, as demonstrated in this chapter.
We turn now to external flows. External flow is a term used to describe flows over or around obstacles. The wind blowing on a skyscraper is an example of an external flow (see Example 2.5), as is an electric fan cooling a printed circuit board in a computer or a cleaning jet directed past the fender of a freshly painted automobile. Objects moving through fluids also create external flows (see Figure 2.11). Ships on the ocean, mixing blades in viscous liquids, and skydivers (Figure 8.2) are all operating in external flows. External flows are not unidirectional, steady flows; thus, both inertia and viscosity affect flow behavior.