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.)
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,
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.
The force exerted by gravity, g,
upon a unit volume of the substance.
The Relationship between g and
can be determined by Newton's 2nd Law, since
weight per unit volume =
mass per unit volume g
Units: Newton's per cubic metre,
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
Units: None, since a ratio is a
Typical values: Water = 1, Mercury
= 13.5, Paraffin Oil =0.8.
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
All fluids are viscous, "Newtonian
Fluids" obey the linear relationship
given by Newton's law of viscosity. ,
which we saw earlier.
is the shear stress,
the velocity gradient or rate of shear strain, and has
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 .)
, 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 .)
, Air =1.46
, Mercury =1.145
Paraffin Oil =2.375