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223.00-1
Fluid Mechanics - Course 223
VISCOSITY
There are many factors which affect the manner in which afluid
flows and, of all these factors, viscosity requires thegreatest
consideration.
Viscosity may be defined as the fluid property whichcauses the
fluid to have resistance to shear. The shearstress in a fluid is
directly proportional to viscosity.
Let's consider lubricating oil for a moment. We all knowthat in
the mid winter the oil is reluctant to flow and in thesummer flows
more freely. What causes this change? There isno significant volume
change and thus the density, ie, mass!unit volume doesn't change.
This change in behaviour is dueto the change in viscosity, due to
temperature.
VISCOSITY depends only upon TEMPERATURE
For liquids, viscosity decreases as temperature
increases.Viscosity is a property concerned with flow and has no
effecton fluids in a static situation.
In a lubrication application, where one body is movingrelative
to another, separated by an oil film, as in a shaftand bearing
application, a shearing action of the oil filmoccurs. The force
required to shear the oil film depends uponthe viscosity and thus
on the temperature.
For interest, a definition of viscosity is,
the shear stress in a liquidthe rate of shear strain in a
liquid
This ratio is 'dynamic viscosity' denoted by ~ (mu) and
di-mensionally has units of
There are two basic types of flow which may occur,laminar and
turbulent. The criterion for the change from onetype of flow to the
other is a function of the dynamic vis-cosity divided by the
density. This ratio is called the'kinematic viscosity' denoted by v
{nu~.
Thus
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223.00-1
Kinematic viscosity is used in many ways when consideringfluid
flow calculations. Figures 1.1 and 1.2 show how thedynamic
viscosity varies with temperature for a variety offluids. From
Figure 1.2 we can see that the dynamic viscosityof water is almost
100 times that for air. You can see thatgases do not change
viscosity in the same manner as liquids
..
-
.r"1~
'"~~o ~
.-
...,~!fa
&"'e -f"o.
~~tilo~ ........ "'j..
"........ "-
-.---is'..,~.?o~'=~ ~
E f'..~Ip"-I !'...I 1-... "j...."""J I--
- ~::=-:-- ___::;.-C-
~-...-~~~u"'.. ~+l 5.10..-l000000
-.-1>U
-r-ISttlC>t
.Jl.Cl I" 10
-~5ttJO
80 90 100 110 120 130 140 150 160 170 180 190 200 210
220Temperature. t of
Figure 1.1
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223.00-1
-I5)( 10
100 150"lempe:-atun'! in Fahrp.nhtit
"I'- ~
r-,
'" "~ ~~ ...
'"",,-",C/~
'"~ ""~"'- ~-- Olionoilr-- -- I "'l;;;:" ..... ~- -r--- -- I'...
.......
"'-
-1--- -- - f--- f':: ~ ~I----- r---.... .........'"
1__ f."...-
......
r--...P'- .....I ~-~-! i. c_ -.-- -- I----. I f'... N,AEIQI I I
!i j f'.... to-
~~+-~s~",,--....r~~~-r-- Mercunj-----+--- ,_- ~ , J-t ~
-'--t--I r-t+-~N- r"/"r"ntiM~ ,enzrn t Waf..,.t--t---.i:--
1----r-----=F---I---~- -,--1------+ -- ~~--; -- --
,._-._- f--
-t Gasofirre1----- '-- -- I------- ~ -1__ 'Sro'68J
-t iI II i j
-- -~--= _.~ __ .J .J..._ ~1--- - --- 1=:F...:.:t:---- f-----
r---- --l--- I---- - HeL,m
Carbm diO,fi(k- Ai,."I..f*'a~
I ,1
, I Hvr~Tl I I.
-f2.S>e /0 -
'"E:"
r.n J: _/ 10-S..)
z
>-.,.~lI.I00rn.~:> -u..5)( .!~u.~
~rtjs::>-c
.~ )I 10~
'iil!rollity graph.
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223.00-1
ASSIGNMENT
1. Describe 'viscosity' in your own words
2. What are the units of dynamic viscosity?
3. Why is 'kinematic viscosity' used in flow calculations?
4. Which of the following oils will absorb the lowest energywhen
lubricating a journal bearing?(a) SAE 40 at 200 0 P(b) SAE 20 at
130 0 P
5. What factors affect viscosity?
6. How would you expect a multigrade oil SAE 20/50 tobehave?
J. Irwin-Childs
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