Page 165 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 152

152 CHAPTER SIX

Compressible Air

Because of the small forces involved at low speeds we said that air is
to be considered an incompressible fluid. If we were to flow such a
fluid through a pipe with a constriction in it, what would we see? We
know the answer from the discussion in Chapter 1. As the fluid comes
to the constriction, the velocity increases while the static pressure
(measured perpendicular to the flow) decreases. So as velocity goes
up pressure goes down, and because the fluid is incompressible the
density and temperature remain the same. In general, for an incom-
pressible fluid the velocity and pressure change in opposite directions
and the density and temperature remain constant.
As the speed of the air increases, the forces become significant and
the compressibility of the air becomes significant. Now the density of the
air can change and thus its volume no longer remains constant. So let us
look at what happens when a compressible fluid such as air flows
through a pipe with a constriction in it. At the constriction, or venturi,
things look quite similar to the incompressible case, but now the density
and temperature can change. As long as the air is traveling at
In general, at high speeds where less than Mach 1 at the venturi, the velocity increases and the
the compressibility of air static pressure decreases, and density and temperature decrease.
becomes significant, the velocity If the air is traveling greater than Mach 1 entering the
of the air changes in the venturi, the velocity decreases and the static pressure, density,
opposite direction to that of the and temperature increase. In general, at high speeds where
pressure, density, and the compressibility of air becomes significant, the velocity of
temperature. the air changes in the opposite direction to that of the
pressure, density, and temperature.
An analogous situation to the flow of compressible air is that of
traffic. As the density of cars increases, speeds decrease. At the
maximum density we have a traffic jam and a minimum in speed. As
the density decreases again and the distance between vehicles
increases, so do speeds.
As we discuss later in this chapter and in greater detail when
discussing supersonic wind tunnels, there is one big difference in the
behavior of supersonic and subsonic air. The difference is that
supersonic air cannot communicate upstream (forward in the rest