Page 39 - 48Fundamentals of Compressible Fluid Mechanics

1.1 What is Compressible Flow ?
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This book 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
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two phenomena that do not exist in incompressible flow . The first phenomenon
is the very sharp discontinuity (jump) in the flow in properties. The second phe-
nomenon is the choking of the flow. Choking is when downstream variations don’t
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effect the flow . Though choking occurs in certain pipe flows in astronomy, there
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also are situations of choking in general (external) flow . Choking is referred to as
the situation where downstream conditions which are beyond a critical value do not
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 expe-
riences to predict other situations. Here one has to learn to use his intuition as a
tool for future use. Thus, not only aeronautic engineers, but other engineers, and
even manufacturing engineers will be able use this “intuition” in design and even
research.
1 This book gradually skipping to include more material that isn’t so introductory. But attempt is made
to present the material in introductory level.
2 It can be argued that in open channel flow there is a hydraulic jump (discontinuity) and in some
ranges no effect of downstream conditions on the flow. However, the uniqueness of the phenomena
in the gas dynamics provides spectacular situations of a limited length (see Fanno model) and thermal
choking, etc. Further, there is no equivalent to oblique shock wave. Thus, this richness is unique to gas
dynamics.
3 The thermal choking is somewhat different but similarity exists.
4 This book is intended for engineers and therefore no discussion about astronomical conditions is
presented.

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