Fluids are the generic name given to two states of matter characterized
by a lack of long range order and a high degree of mobility at the
molecular scale. Fluids have the following properties:
They usually assume the shape of any vessel they are placed in
(exceptions are associated with surface effects such as surface tension
and how well the fluid adheres to the surface in question).
They are characterized by a mass per unit volume density
.
They exert a pressure (force per unit area) on
themselves and any surfaces they are in contact with.
The pressure can vary according to the dynamic and static
properties of the fluid.
The fluid has a measure of its ``stickiness'' and resistance to
flow called viscosity. Viscosity is the internal friction of a
fluid, more or less. We will treat fluids as being ``ideal'' and ignore
viscosity in this course.
Fluids are compressible - when the pressure in a fluid is
increased, its volume descreases according to the relation:
(212)
where is called the bulk modulus of the fluid (the equivalent
of a spring constant).
Fluids where is a large number (so large changes in pressure
create only tiny changes in fractional volume) are called incompressible. Water is an example of an incompressible fluid.
Below a critical speed, the dynamic flow of a moving fluid tends
to be laminar, where every bit of fluid moves parallel to its neighbors
in response to pressure differentials and around obstacles. Above that
speed it becomes turbulent flow. Turbulent flow is quite difficult to
treat mathematically and is hence beyond the scope of this introductory
course - we will restrict our attention to ideal fluids either static
or in laminar flow.