Molecular vs. Continuum
Classical fluid mechanics (i.e., A&AE 511) treats a gas as an
infinitely divisible substance, a continuum.
As a consequence of continuum assumption, each fluid property
is assumed to have a definite value at each point in space.
Density, temperature, velocity, and so on, are considered to be
functions of position and time.
For most of gas dynamics phenomena experienced by humans (1
atm pressure, room temperature, length scales ~ 1 m), continuum
hypothesis is valid.
However, many modern engineering applications (space flight,
micro-electro-mechanical devices) occur at conditions where
more detailed molecular description of the structure of gaseous
matter is necessary.