# 11.1 What is Compressible Flow?

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This Chapters deals with an introduction to the flow of compressible substances (gases). The main difference between compressible flow and "almost'' incompressible flow is not the fact that compressibility has to be considered. Rather, the difference is in two phenomena that do not exist in incompressible The first phenomenon is the very sharp discontinuity (jump) in the flow in properties. Choking is referred to as the situation where downstream conditions, which are beyond a critical value(s), doesn't affect the flow. The shock wave and choking are not intuitive for most people. However, one has to realize that intuition is really a condition where one uses his past experiences to predict other situations. Here one has to build his intuition tool for future use. Thus, not only engineers but other disciplines will be able use this "intuition'' in design, understanding and even research.