Viscosity is the quantity that describes a fluid's resistance
to flow. It exists in both liquids and gases and is essentially a
friction force between different layers of fluid as they move past
one another. In liquids the viscosity is due to the cohesive forces
between the molecules. in gases the viscosity is due to collisions
between the molecules.
Slide 3
Viscosity contd. represented by the symbol "eta is the ratio of
the shearing stress (/A) to the strain rate (v/l). SI unit of
viscosity is the Pascal second [Pa s]. A common unit is the poise
(P) where 1 Pa.s = 10 P
Slide 4
Types of viscosity
Slide 5
Kinematic Viscosity a measure of the resistive flow of a fluid
under the influence of gravity. represented by the symbol "nu ratio
of the viscosity of a fluid to its density. Can be measured using
the Capillary Viscometer. SI unit of kinematic viscosity is the [m
2 /s]. Common unit used is the stroke (St) = [cm 2 /s] v=/
Slide 6
Factors affecting Viscosity 1. Viscosity is very temperature
dependent. Viscosity of a liquid decreases with increasing
temperature. Ex. It is easier to pour skin lotion during summer
than cold season. Different oils are used in hot and cold
countries. Viscosity of a gas increases with increasing
temperature.
Slide 7
Factors affecting Viscosity 2. Viscosity is normally
independent of pressure, but liquids under extreme pressure often
experience an increase in viscosity
Slide 8
Viscous Flow
Slide 9
Viscosity contd. Viscous fluids tend to cling to a solid
surface. Syrup and honey are more viscous than water. Grease is
more viscous than engine oils. Liquids are more viscous than gases.
Lava is an example of a very viscous material.
Slide 10
Equation of Continuity When fluids move through a full pipe,
the volume of fluid that enters the pipe must equal the volume of
fluid that leaves the pipe, even if the diameter of the pipe
changes. This equation says that as the cross-section of the pipe
gets smaller, the velocity of the fluid increases, and as the
cross-section gets larger, the fluid velocity decreases.
