Properties of Fluids 
Properties of Fluids
The properties outlines below are general properties of fluids which are of
interest in engineering. The symbol usually used to represent the property is
specified together with some typical values in SI units for common fluids.
Values under specific conditions (temperature, pressure etc.) can be readily
found in many reference books. The dimensions of each unit is also give in the
MLT system (see later in the section on dimensional analysis for more details
about dimensions.)
Density
The density of a substance is the quantity of matter contained in a unit
volume of the substance. It can be expressed in three different ways.
Mass Density
Mass Density,
,
is defined as the mass of substance per unit volume.
Units: Kilograms per cubic metre,
(or
)
Dimensions:
Typical values:
Water = 1000,
Mercury = 13546
Air = 1.23,
Paraffin Oil = 800.
(at pressure =1.013and
Temperature = 288.15 K.)
Specific Weight
Specific Weight
,
(sometimes , and sometimes known as specific gravity) is defined
as the weight per unit volume.
or
The force exerted by gravity, g, upon a unit volume of the substance.
The Relationship between g and
can be determined by Newton's 2^{nd} Law, since
weight per unit volume = mass per unit volume g
Units: Newton's per cubic metre,
(or
)
Dimensions:.
Typical values:
Water =9814
,
Mercury = 132943
,
Air =12.07
,
Paraffin Oil =7851
Relative Density
Relative Density,
,
is defined as the ratio of mass density of a substance to some standard
mass density.
For solids and liquids this standard mass density is the maximum mass
density for water (which occurs at
c)
at atmospheric pressure.
Units: None, since a ratio is a pure number.
Dimensions: 1.
Typical values: Water = 1, Mercury = 13.5, Paraffin Oil =0.8.
Viscosity
Viscosity, m, is the property of a fluid, due
to cohesion and interaction between molecules, which offers resistance to
sheer deformation. Different fluids deform at different rates under the same
shear stress. Fluid with a high viscosity such as syrup, deforms more slowly
than fluid with a low viscosity such as water.
All fluids are viscous, "Newtonian Fluids" obey the linear relationship
given by Newton's law of viscosity.
,
which we saw earlier.
where
is the shear stress,
Units
;
Dimensions.
is
the velocity gradient or rate of shear strain, and has
Units:
,
Dimensions
is the "coefficient of dynamic viscosity"  see below.
Coefficient of
Dynamic Viscosity
The Coefficient of Dynamic Viscosity,
,
is defined as the shear force, per unit area, (or shear stress
),
required to drag one layer of fluid with unit velocity past another
layer a unit distance away.
Units: Newton seconds per square metre,
or
Kilograms per meter per second,.
(Although note that
is often expressed in Poise, P, where 10 P = 1
.)
Typical values:
Water =1.14
,
Air =1.78
,
Mercury =1.552
,
Paraffin Oil =1.9
.
Kinematic
Viscosity
Kinematic Viscosity,
,
is defined as the ratio of dynamic viscosity to mass density.
Units: square metres per second,
(Although note that n is often expressed
in Stokes, St, where
St
= 1
.)
Dimensions:
.
Typical values:
Water =1.14
,
Air =1.46
,
Mercury =1.145
,
Paraffin Oil =2.375
.

