Newtons law L3.ppt

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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Newtons Law of Viscosity

Velocity Gradient

= DU/DY


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Velocity gradient


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Shear stress acts tangentially to thesurface (F=tangential force).

F

FA

areatangential

forceshear

DX


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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


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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.mov


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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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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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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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Liquid Atoms can move around

but are attracted together

Cold liquid Hot liquid


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Gases Large spaces between atoms

Cold gas Hot gas


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Gases fill the whole space


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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.

Newtons law L3.ppt

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