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iscosity of Fluids

By Muhammad Usman

(08-chem-02-B)

12-10-2009

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History

Word Viscosity is derived from aLatin word Viscum for mistletoe(Hemi-parasitic plants).

A viscous glue was made frommistletoe berries and used for lime-twigs to catch birds.

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Viscosity (General Term)

Generally viscosity is known asthickness offluids.

Example:

Compound Generalobservation

Viscosity

water Thin Low viscosity

Honey Thick High viscosity

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VISCOSITY

All fluids offer resistance to any

force tending to cause one layer to

move over another. Viscosity is the fluid propertyresponsible for this resistance. Sincerelative motion between layers

requires the application of shearingforces, that is, forces parallel to thesurfaces over which they act, theresisting forces must be in exactlythe opposite direction to the applied

shear forces and so they too are

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- Apparent Viscosity

T h e sh e a r ra te d e p e n d e n ce o f -ca te g o rize s n o n N e w to n ia n flu id s in to

.severaltypes:Po w e r La w Flu id s

: Pse u d o p la stic ( )viscosity decreases as shear

( )ra te in cre a se s sh e a r ra te th in n in g : D ila ta n ts ( )viscosity increases as shear rate

( )in cre a se s sh e a r ra te th icke n in g

:B in g h a m P la stics (depends on a criticalshear stress 0 ) and then

b ecom es con stan t

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Pete 314 6

Viscosity (resistance to flow)

Shearstress

=

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Pete 314 7

sft

lbm106.72cP1

sPa0.001cP1

scm

g0.01cP1

4-

=

=

=

( )cP0100smkg1sPa1=

=

Units of viscosity

The viscosity is the slope of the line of shear stress versusshear rate so its SI unit is 1 Pa / (1/s) = 1 Pa s

The customary unit of viscosity is thepoise , howeverit istoo large a unit for most common fluids.

By sheer coincidence the viscosity of pure water at about is0.01 poise; for that reason the common unit of viscosityin the US is the centipoise.

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Coefficients of viscosity

1.Dynamic viscosity (Absoluteviscosity)

2.Volume viscosity (Bulk viscosity)

3.Shear viscosity

4.Extensional viscosity

5.Kinematic viscosity

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Coefficients of viscosity

Dynamic viscosity (Absoluteviscosity):

Determines the dynamics of an incompressible

Newtonian fluid Volume viscosity (Bulk viscosity): Determines the dynamics of a compressible

Newtonian fluid

Shear viscosity: Is the viscosity coefficient when the applied stress

is a shear stress (valid for non-Newtonian fluids)

Extensional viscosity: Is the viscosity coefficient when the applied stress

-

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Kinematic viscosity

Dynamic viscositydivided by thedensity for a Newtonian fluid

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Eddy viscosity

As we know that shear stress () for alaminar flow is

Where is the absolute viscosity.

Ev is analogous to absolute viscosity

(), called Eddy Viscosity

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Eddy diffusivity

As we know that kinematicviscosity is

similarly Eddy Diffusivity is

Eddy Diffusivity is analogous tokinematic viscosity

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Total shear stress

Total shear stress in a turbulentflow is the sum ofviscous stress &turbulent stress.

Similarly

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Viscosity

Liquids

Viscosities of liquidsare very much

greater than gases. Viscosity decrease

with temperatureincrease. e.g.viscosity of waterfalls from 1.79cp (at0C) to 028cp (at100C)

Gases

Viscosities of gasesare very much lower

than liquids (due to farspaces between molecules)

Viscosity increase withtemperatureincrease

approximatelyaccording toequation

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Viscosity

Liquids

Viscosity increase withgreat increase in

pressure.

Gases

Viscosity of gas isalmost independent

of pressure in theregions where gaslaws are applicable

At high pressure,viscosity increasewith pressure,especially nearcritical point

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Fluid

A fluid is a substance that does notpermanently resist distortion.

Study of viscosity is called

Rheology. Types of Fluid:

Whenever a shear force is applied

onto a fluid its velocity gradient hasa relationship with it involving aconstant quantity called viscosity init. Whenever their behavior is

studied on this basis, they are

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Newtonian fluids

Shear stress is related to shear rate by thisrelationship

here shows the shear stress arising

from viscous or laminar flow.

is the symbol ofviscosity here that is aconstant no. in case of Newtonianfluids.

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Pete 314 20

Newtonian vs. non-Newtonian rheology)( f=

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Non- Newtonian Fluids

A non- Newtonian fluid is one inwhich the viscosity is a functionsome mechanical variable like

shear stress or time. Non- Newtonian fluids that change

over time are said to have a

memory. There are different types of non-

Newtonian fluids based upon the

variation from linear behavior. They

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Shear thinning fluids

Shear-thinning fluids can be classified intoone ofthree general groups.

A material that has a viscosity that decreasesunder shear stress but stays constant

over time is said to be Pseudoplastic. A material that has a viscosity that decreasesunder shear stress and then continues todecrease with time is said to bethixotropic.

If the transition from high viscosity (or nearlysemisolid) to low viscosity (or essentiallyliquid) takes place only after the shearstress exceeds some minimum value, thematerial is said to be a Bingham plastic.

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Pete 314 24

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Examples of shear thinningfluids

Toothpaste is another example of amaterial whose viscosity decreasesunder stress. Toothpaste behaves likea solid while it sits at rest inside the

tube. It will not flow out spontaneouslywhen the cap is removed, but it willflow out when you put the squeeze onit. Now it ceases to behave like a solid

and starts to act like a very thick liquid.when it lands on your toothbrush, thestress is released and the toothpastereturns to a solid (or at least asemisolid) state. You do not have to

worry about it flowing off the brush as you

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Shear thickening fluid

They're ideal candidates for bodyarmor and protective sportspadding.

A bulletproofvest or a kneepadmade of shear-thickening materialwould be supple and yielding to the

mild stresses of ordinary bodymotions, but would turn rockhard in response to the

traumatic stress imposed by a

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Types

Shear-thickening fluids are also divided intotwo groups.

Those with a time-dependent viscosity (memory

materials) and those with a time-independent viscosity

(non-memory materials). Rheopectic:

If the increase in viscosity increases overtime, the material is said to be rheopectic.

Dilatants: If the increase is roughly directly

proportional to the shear stress and doesnot chan e over time, the material is said

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Bibliography Nevers, d. fluid mechanics for chemical

engineers. mcGrawhill. physics hypertext. (n.d.). Retrieved

from http://physics.info/viscosity/ warren l. Mccabe, j. c. unit operations of

chemical engineering. mcGraw - hill. wikipedia: the free encyclopedia. (n.d.).

Retrieved from www.wikipedia.org:http://en.wikipedia.org/wiki/Viscosity