Page 22 - Aerodynamics for Engineering Students

Basic concepts and definitions 5

those of a normal gas and, although belonging to the third group, can be regarded
justifiably as a separate, distinct form of matter.
In a solid the intermolecular bonds are very rigid, maintaining the molecules in
what is virtually a fixed spatial relationship. Thus a solid has a fmed volume and
shape. This is seen particularly clearly in crystals, in which the molecules or atoms are
arranged in a definite, uniform pattern, giving all crystals of that substance the same
geometric shape.
A liquid has weaker bonds between the molecules. The distances between the
molecules are fairly rigidly controlled but the arrangement in space is free. A liquid,
therefore, has a closely defined volume but no definite shape, and may accommodate
itself to the shape of its container within the limits imposed by its volume.
A gas has very weak bonding between the molecules and therefore has neither
a definite shape nor a definite volume, but will always fill the whole of the vessel
containing it.
A plasma is a special form of gas in which the atoms are ionized, i.e. they have lost
one or more electrons and therefore have a net positive electrical charge. The
electrons that have been stripped from the atoms are wandering free within the gas
and have a negative electrical charge. If the numbers of ionized atoms and free
electrons are such that the total positive and negative charges are approximately
equal, so that the gas as a whole has little or no charge, it is termed a plasma.
In astronautics the plasma is usually met as a jet of ionized gas produced by passing
a stream of normal gas through an electric arc. It is of particular interest for the
re-entry of rockets, satellites and space vehicles into the atmosphere.

1.2.2 Fluids
The basic feature of a fluid is that it can flow, and this is the essence of any definition
of it. This feature, however, applies to substances that are not true fluids, e.g. a fine
powder piled on a sloping surface will also flow. Fine powder, such as flour, poured
in a column on to a flat surface will form a roughly conical pile, with a large angle of
repose, whereas water, which is a true fluid, poured on to a fully wetted surface will
spread uniformly over the whole surface. Equally, a powder may be heaped in
a spoon or bowl, whereas a liquid will always form a level surface. A definition of
a fluid must allow for these facts. Thus a fluid may be defined as ‘matter capable of
flowing, and either finding its own level (if a liquid), or filling the whole of its
container (if a gas)’.
Experiment shows that an extremely fine powder, in which the particles are not
much larger than molecular size, will also find its own level and may thus come under
the common definition of a liquid. Also a phenomenon well known in the transport
of sands, gravels, etc. is that they will find their own level if they are agitated by
vibration, or the passage of air jets through the particles. These, however, are special
cases and do not detract from the authority of the definition of a fluid as a substance
that flows or (tautologically) that possesses fluidity.

1.2.3 Pressure
At any point in a fluid, whether liquid or gas, there is a pressure. If a body is placed in
a fluid, its surface is bombarded by a large number of molecules moving at random.
Under normal conditions the collisions on a small area of surface are so frequent that
they cannot be distinguished as individual impacts. They appear as a steady force on
the area. The intensity of this ‘molecular bombardment’ force is the static pressure.