Page 40 - 48Fundamentals of Compressible Fluid Mechanics

2 CHAPTER 1. INTRODUCTION
1.2 Why Compressible Flow is Important?

Compressible flow appears in many natural and many technological processes.
Compressible flow deals with more than air, including steam, natural gas, nitrogen
and helium, etc. For instance, the flow of natural gas in a pipe system, a common
method of heating in the u.s., should be considered a compressible flow. These
processes include the flow of gas in the exhaust system of an internal combustion
engine, and also gas turbine, a problem that led to the Fanno flow model. The
above flows that were mentioned are called internal flows. Compressible flow also
includes flow around bodies such as the wings of an airplane, and is considered
an external flow.
These processes include situations not expected to have a compressible
flow, such as manufacturing process such as the die casting, injection molding.
The die casting process is a process in which liquid metal, mostly aluminum, is
injected into a mold to obtain a near final shape. The air is displaced by the liquid
metal in a very rapid manner, in a matter of milliseconds, therefore the compress-
ibility has to be taken into account.
Clearly, Aero Engineers are not the only ones who have to deal with
some aspect of compressible flow. For manufacturing engineers there are many
situations where the compressibility or compressible flow understating is essential
for adequate design. For instance, the control engineers who are using pneu-
matic systems use compressed substances. The cooling of some manufactur-
ing systems and design of refrigeration systems also utilizes compressed air flow
knowledge. Some aspects of these systems require consideration of the unique
phenomena of compressible flow.
Traditionally, most gas dynamics (compressible flow) classes deal mostly
with shock waves and external flow and briefly teach Fanno flows and Rayleigh
flows (two kind of choking flows). There are very few courses that deal with isother-
5
mal flow. In fact, many books on compressible flow ignore the isothermal flow .
In this book, a greater emphasis is on the internal flow. This doesn’t in
any way meant that the important topics such as shock wave and oblique shock
wave should be neglected. This book contains several chapters which deal with
external flow as well.

1.3 Historical Background

In writing this book it became clear that there is more unknown and unwritten about
the history of compressible fluid than known. While there are excellent books about
6
the history of fluid mechanics (hydraulic) see for example book by Rouse . There
are numerous sources dealing with the history of flight and airplanes (aeronau-
5 Any search on the web on classes of compressible flow will show this fact and the undersigned can
testify that this was true in his first class as a student of compressible flow.
6 Hunter Rouse and Simon Inc, History of Hydraulics (Iowa City: Institute of Hydraulic Research,
1957)