related to compressibility.
Compressibility and Incompressibility
The terms compressibility and incompressibility describe the ability
of molecules in a fluid to be compacted or compressed (made more
dense) and their ability to bounce back to their original density,
in other words, their "springiness." An incompressible
fluid cannot be compressed and has relatively constant density throughout.
Liquid is an incompressible fluid. A gaseous fluid such as air,
on the other hand, can be either compressible or incompressible.
Generally, for theoretical and experimental purposes, gases are
assumed to be incompressible when they are moving at low speeds--under
approximately 220 miles per hour. The motion of the object traveling
through the air at such speed does not affect the density of the
air. This assumption has been useful in aerodynamics when studying
the behavior of air in relation to airfoils and other objects moving
through the air at slower speeds.
However, when aircraft began traveling faster than 220 miles per
hour, assumptions regarding the air through which they flew that
were true at slower speeds were no longer valid. At high speeds
some of the energy of the quickly moving aircraft goes into compressing
the fluid (the air) and changing its density. The air at higher
altitudes where these aircraft fly also has lower density than air
nearer to the Earth's surface. The airflow is now compressible,
and aerodynamic theories have had to reflect this. Aerodynamic theories
relating to compressible airflow characteristics and behavior are
considerably more complex than theories relating to incompressible
airflow. The noted aerodynamicist of the early 20th century, Ludwig
Prandtl, contributed the Prandtl-Glaubert rule for subsonic airflow
to describe the compressibility effects of air at high speeds.
At lower altitudes, air has a higher density and is considered
incompressible for theoretical and experimental purposes.