A fluid is any substance that cannot
maintain its own shape. Fluid mechanics is
concerned with the static and dynamics of fluid. Fluid
static treats fluids in the equilibrium state. Fluid
dynamics is concerned with fluid in motion relative to
other parts. The following table summarizes the variables
needed to define a fluid and its environment.
Variables Needed to Define Fluid
Quantity 
Symbol 
Object 
Unit 
Pressure 
p 
Scalar 
N/m^{2} 
Velocity 
v 
Vector 
m/s 
Density 
r 
Scalar 
kg/m^{3} 
Viscosity 
m 
Scalar 
kg.m 
Force 
F 
Vector 
N/kg 
Time 
t 
Scalar 
s 
Fluids is one of the oldest disciplines
in physics and engineering. Ancient civilizations were
faced with the difficult tasks of controlling water for
agricultural development, water consumption, and travel.
Agricultural developments led to the construction of
irrigation channels, dams, weirs, pumps, and sprinkler
systems. Human water consumption led to building wells,
fountains, and water storage systems. The need for
travel helped the development of ships and aeroplanes.
Motion and behaviour of fluids is therefore critical to
improving the quality of human life, even to the point
of his survival.
Roots of fluids are extended to almost
every aspect of science and engineering. Civil
engineering, for example, is developed primarily from
the need for fluid systems and structures. Mechanical
engineering studies fluids in combustion, lubrication,
and energy systems. Aeronautical engineering studies gas
flow to produce energy and to provide lift on flying
structures. Electrical engineering uses fluids to cool
electronic devices with air flow. The study of fluids is
essential for the chemical engineer, because the
majority of chemical processing operations are conducted
either partially or totally in the fluid phase. Flow
processes in the human body, cardiac and cardiovascular
systems, blood flow and respiratory system are few
examples from the discipline of biofluid in the human
body.
When an object is submerged in a fluid
the fluid can only push on it, or compress it. The
compression force is always a normal force, that is, it
is always perpendicular to the surface of the object,
independent of the orientation of the object. As shown
in the following figure, the
weight of the liquid exerts a force on the bottom of the
container. This force produces a pressure on the bottom
of the container.
An important property of fluids is
pressure.
Fluid Pressure
Pressure is the quantity which causes
fluid flow or sustains the weight of a column of fluid.
Fluid pressure is defined as the normal force
exerted on a surface (real or imaginary) in a fluid per
unit area.
We define the pressure at a point
in a fluid as the force (F) per unit area on a
surface of area A
Also
we can take a very small area and take the limit as the
area goes to zero
If
we replace the force in the above equation with the
weight, we find that
Where
m is the mass of the liquid in kg, A is
the area of the bottom of the container in m^{2},
and g is the acceleration of gravity m/s^{2}.
Note that the pressure is a scalar quantity; it does not
have a direction.
The SI unit for pressure is the Newton
per square meter (N/m^{2}), which is named
pascal (Pa) after the French scientist Blaise Pascal
(16231662). Two other
commonly used pressure units are the bar,
1 bar = 100
kPa, and standard atmosphere, 1
atm = 101.325
kPa. The absolute pressure is measured relative
to absolute vacuum. Most pressuremeasuring devices,
however, are calibrated to read zero in the atmosphere,
and so they indicate the difference between the absolute
zero pressure and the local atmospheric pressure. This
difference is called the gauge pressure.
