lec -1- introduction & fluid properties.pptx

8/9/2019 lec -1- Introduction & Fluid Properties.pptx

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AN INTRODUCTIONFluid Mechanics


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,i&ni-cance of Fluid Mechan

Analysis of Fluid Flow is essential in numerous daily encounters. e/&

0 Desi&nin& of aero%lanes/

0 Desi&nin& irri&ation canals. dams. and water su%%ly systems/

0Desi&nin& turbines. heat e1chan&ers. and +uid coolin& systems/

0 Desi&nin& of Air 2Conditionin& ,ystem/


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)ranches Of Fluid Mechanics

Followin& are some maor Di*isions of Fluid mechanics0 Fluid statics0 )ranch of +uid mechanics which deals with res%onse of +uid at rest only

0 Fluid 3inematics0 )ranch of +uid mechanics that deals with res%onse of +uid when they are in motion withou

ener&ies and forces in them/

0 4ydrodynamics0 )ranch of +uid mechanics that deals with res%onse of +uid when they are in motion consid

and forces in them

0 4ydraulics0 )ranch which deals with res%onse of water either in rest or in motion/


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Fluid6

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Di'erence )etween a solid 9 Fluid (8i:uid or ;as!

,hearstress

,hearstress

A%%lied shear stress%roduces a%ro%ortionaldeformation (strain!/

A%%lied shear stress%roduces a %ro%ortionalcontinuously increasin&deformation (strain!/

,olids Fluid


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Com%arison

8I=UID,

o It can +ow at normal tem%erature/

oMolecules are distinctly a%art/

oRelati*ely less molecular attraction/

o,li&htly com%ressible/

o 8i:uids con-rm sha%e of container/

o 8i:uids cannot sustain shear force/

,O8ID

oIt cannot +ow at normal tem%erature/

oMolecules are close to each other/

oMore molecular attraction/

o4i&hly com%ressible/

oIt is not %ossible in case of solid because solid hassha%e/

o,olid can sustain shear force/

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Com%arison

8I=UID,

o 8i:uids ha*e de-nite *olume at any tem%erature and %ressure/

oMolecules are close to&ether/

oMore molecular attraction/

o 8ess com%ressible

o 8ess di'usion

;A,?,

o;ases do not ha*e de-nite *olume/

oMolecules are far a%art/

o8ess molecular attraction/

o4i&hly com%ressible/

o4i&h di'usion/

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5ro%erties of +uids There are two ty%es of +uidso 8i:uids e1hibit a free surface/

o;asses need a lid to be contained/

)ut same %rinci%les often a%%ly to both/

Followin& are some im%ortant %ro%erties/oDensity

o,%eci-c *olume

o,%eci-c wei&ht

o,%eci-c &ra*ity

oCom%ressibility

oBiscosity

o,urface tension

o 5ressure

o)uoyancy

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,%eci-c Bolume

,%eci-c *olume is de-ned as the ratio of the Bolume of a substanceMass/

Mathematically &i*en as

7here is a ;ree3 letter termed as JKNu’’.

Its units are mEL&. ftELslu&. cmEL&/

Relation between s%eci-c *olume 9 density is &i*en as

Find out the s%eci-c *olume. for the %re*ious stated e1am%le/

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,%eci-c wei&ht

The s%eci-c wei&ht (wei&ht! of a li:uid may be de-nes as the wei&*olume (at standard %ressure 9 tem%erature!. mathematically stat

For all %ur%oses the s%eci-c wei&ht of the water is considered to beNLmE/

Example in an e1%eriment. the wei&ht of / mE of a certain li:uid

found to be #@/> KN, -nd its s%eci-c wei&ht and density/ Bolume G /mE . wei&ht G #@/> H #E

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Com%ressibilityCom%ressibility of a li:uid may be de-ned as the "ariation in its "!ith "ariation o pressure#

A li:uidKs com%ressibility is measured in terms of its )ul3 Modulus oelasticity/ Com%ressibility de-ned as

The bul3 modulus of elasticity is the % is its reci%rocal/

The *ariation in the *olume of water. with the *ariation of %ressuresmall that for all %ractical %ur%oses it is ne&lected/ Thus water is coto be incom%ressible +uid/

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BiscosityConsider an e1%eriment in which you ha*e four ars -lled with followin&o7atero ,yru%

o4oney

oOil

oNow if we in*ert all of the four ars. what will be the rate at which the +uid +ows out

oIs it same or all o them or it "aries&

oIf we analyQe we -nd out that there is some +uid characteristic that controls the +ow

as "iscosity# And it is clear from the abo*e e1%eriment that the denser the +uid theBiscosity/

o There are two assum%tionso There is no relati*e motion between the solid boundary and adacent +uid layer/

o ,hear stress between two adacent layers is directly %ro%ortional to the rate of shear in the %er%endicular to that of motion/

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NewtonKs 8aw of *iscosityConsider two 5lates. one -1ed and other mo*able.-lled with +uid/ A force F bein& a%%lied on u%%er%late. mo*in& it with uniform *elocity/

?1%eriments ha*e shows . .

Abo*e e:uation is termed as NewtonKs law of*iscosity/ Followin& NewtonKs law i/e/

inematic *iscosity. ratio bLw absolute *iscosity (!and density (!. mathematically &i*en as

)A4AUDDIN SAARIA UNIB?R,IT 8A4OR? CAM5U,

7here.

A G conta G &a% b

F G a%%lieU G *eloc


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Ba%our 5ressure Consider a +uid. a %ortion. (as shown in the -&ure!

,ome of the molecules (more ener&etic! 3ee% on lea*in&the surface/ Formin& *a%ours/

Now consider some water in a closed container/ 5articlescontinue to brea3 away from the surface of the li:uid 2 butthis time they are tra%%ed in the s%ace abo*e the li:uid/*ressure exerted )y these "apors is termed as "aporpressure/

As the &aseous %articles bounce around. some of them willhit the surface of the li:uid a&ain. and be tra%%ed there/

There will ra%idly be an e:uilibrium set u% in which thenumber of %articles lea*in& the surface is e1actly balancedby the number reoinin& it/

In this e:uilibrium. there will be a -1ed number of the&aseous %articles in the s%ace abo*e the li:uid/ 7henthese %articles hit the walls of the container. they e1ert a%ressure/ This %ressure is called the saturated *a%or

%ressure/ )A4AUDDIN SAARIA UNIB?R,IT 8A4OR? CAM5U,


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,aturated *a%our 5ressure 9)oilin& 5oint

A li:uid boils when its saturated *a%or %ressure becomes e:ual to te1ternal %ressure on the li:uid/ 7hen that ha%%ens. it enables bub*a%or to form throu&hout the li:uid 2 those are the bubbles you seeli:uid boils/

If the e1ternal %ressure is hi&her than the saturated *a%or %ressurebubbles are %re*ented from formin&. and you ust &et e*a%oration surface of the li:uid/ If the li:uid is in an o%en container and e1%osnormal atmos%heric %ressure. the li:uid boils when its saturated *a

%ressure becomes e:ual to # atmos%here (or ##E 5a or ##/E>C/ 7hene*er we ust tal3 about Vthe boilin& %ointVwe always assume that it is bein& measured at e1actly # atmos%he%ressure/ In %ractice. of course. that is rarely e1actly true/

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5ressureFluid %ressure P at a %oint is the ratio of normal force to area as thea%%roaches to a small *alue/

Units are Psi or N/m2 as explained in the picture.

Unli3e stress howe*er. %ressure acts in all directions.

not +ust the direction o the applied orce#

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5ressure 3ey %oints

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5ressure ey 5oints (contd/!

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)uoyancyAn obect submer&ed in li:uid feels an u%ward force called buoyan

The buoyancy force is e1actly e:ual to the wei&ht of li:uid dis%laceobect/

Obects sin3 if the buoyancy force is less than their own wei&ht/

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=U?,TION,6

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lec -1- introduction & fluid properties.pptx

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