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7/27/2019 Newtons law L3.ppt
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MPD/FFO/Lect_3
Newtons law of viscosity, Pressure and temperature dependence of
viscosity
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Viscosity Viscosity is a property that represents the
internal resistance of a fluid to motion.
Ethanol Water
Honey
Sludge
Slurry
Pastes
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MPD/FFO/Lect_3
Newtons Law of Viscosity
Velocity Gradient
= DU/DY
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MPD/FFO/Lect_3
Velocity gradient
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MPD/FFO/Lect_3
Newtons Law of Viscosity
Shear stress acts tangentially to thesurface (F=tangential force).
F
FA
areatangential
forceshear
DX
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MPD/FFO/Lect_3
Newtons Law of Viscosity
Newtons law of viscosity states that
Shear stress is directly proportional to velocitygradient
a DU/DY
mDU/DY
m = Viscosity of the fluid
Unit ofm
Kg/m.s
Poise
Pa.s1 Poise = 1g/cm. s
7/27/2019 Newtons law L3.ppt
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MPD/FFO/Lect_3
Role of Viscosity
Statics
Fluids at rest have no relative motion between
layers of fluid and thus du/dy= 0
Therefore the shear stress is _____ and is
independent of the fluid viscosity
Flows
Fluid viscosity is very important when the fluidis moving
zero
http://localhost/var/www/apps/conversion/tmp/scratch_4/Clips/V1_2.mov7/27/2019 Newtons law L3.ppt
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MPD/FFO/Lect_3
Kinematic viscosity n
The ratio m / rappears in many equations.
Kinematic viscosity n (pronounced: new)
rmn
Gases: Viscosity increases with
increasing temperature, why?
Liquids: Viscosity decreases with
increasing temperature, why?
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MPD/FFO/Lect_3
Kinematic viscosity
Units
m2/s
Stokes =cm2/s
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MPD/FFO/Lect_3
Temperature and pressure
dependency of viscosity
Viscosity will also change with pressure -
but under normal conditions this change is
negligible in gasses
High pressure can also change the
viscosity of a liquid. As pressure increases
the relative movement of molecules
requires more energy hence viscosityincreases
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MPD/FFO/Lect_3
Temperature and pressure
dependence of viscosity
Viscosity of Newtonian fluids depends only on
temperature and pressure
)PP(expTTTT
REexp)P,T( o
o
o0
Dmm
Wheremo:viscosity at To and Po
(reference temperature and pressure)
DE: activation energy for flow
R: gas constant
:material property [m2/N]
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MPD/FFO/Lect_3
Atoms
Molecules - Atomsbonded together
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Mixture Different atomsnot bonded together
Element One type of atom
Compound Different atoms
bonded together (molecules)
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Compound
Different atoms in fixed proportions
1Y 1R 1B2R 1G1Y 1G
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Element
Atoms of the
same type
GasLiquid
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MPD/FFO/Lect_3
Liquid Atoms can move around
but are attracted together
Cold liquid Hot liquid
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MPD/FFO/Lect_3
Gases Large spaces between atoms
Cold gas Hot gas
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MPD/FFO/Lect_3
Gases fill the whole space
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MPD/FFO/Lect_3
Liquids fill containers
from the bottom
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Next Lecture
Numerical based on viscosity of fluids
Come with calculator
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MPD/FFO/Lect_3
Boundary Layer Effects
As a fluid flows past the rigid walls of a channel or a over a bed it is affected
by retarding stresses exerted by the bounding surfaces. The region of the
flow in contact with the bed or walls of the channel is the boundary layer.