friction (and shear)
DESCRIPTION
Friction (and Shear). Gas Origin of Viscosity Mix of gases Liquid Origin of Viscosity Effect of foreign materials Dilute vs Concentrated (sol-gel) Non-newtonian Fluids Concentrated Effect of non-s pherical dispersed materials Presence of structure. Gas. 3. 2. Gas - PowerPoint PPT PresentationTRANSCRIPT
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Friction (and Shear)
Gas Origin of Viscosity Mix of gases
Liquid Origin of Viscosity Effect of foreign materials
Dilute vs Concentrated (sol-gel) Non-newtonian Fluids
Concentrated Effect of non-spherical dispersed materials Presence of structure
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Gas
123
VY
X
Gas Kinetic Theory of gas Non polar, low density
22
33
2dTm
Mean Free Path is large Molecular movement between 1 and 2 (and 2 and 1, etc) Momentum Transfer between planes ==> viscosity Increase Temp ==> Increase velocity, Viscosity
Rigid Spheres
dydVx
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Gas Accounting for van der Waals attractive force Lennard-Jones potential
26107.2
MT Sigma- collision dia
omega- collision integral M -molecular wt
Mix of gases2
41
21
21
\
118
1
i
j
j
i
j
iij M
MMM
iji
iimix x
x
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Liquids Theory is not as well developed
Eyring’s Theory Inter-molecular forces cause viscosity (NOT moving molecules) Temp increase ==> more energy for molecule ==> less viscosity
Similar to reaction equilibrium
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Liquid Viscosity
State
Energy A
B
C
To go from A to C, the particle should have energy EAct(Activation Energy)
Energy released is heat of reaction ERxn
ActE
RxnE
For Liquid movement EA and EC are same Application of stress shifts A up
and C down ==> Movement from left to right
State
Energy A
B
CActE
RxnE A’ C’
Force
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Dilute solutions Assume
No interaction between particles Spherical, uncharged
Liquid velocity on particle surface = particle surface velocity
fractionvoleff .5.21 Newtonian behavior Emulsions will show lower viscosity
particles do not shear, emulsions will surface contamination will increase emulsion viscosity
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Non newtonian fluids When one or more of the assumptions are violated Usually heterogenous
Higher concentration (eg 40% of blood has red blood cells in plasma) ==> interaction between particles Non spherical particles Electrically charged (not discussed here)
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Non newtonian fluids High concentration (high is relative) Interaction, structure formation
Structural viscosity
Application of shear stress breaks structure over time ==> thixotropic breaks structure quickly, more stress ==> more disintegration ==> pseudoplastic alternate: cylinders, ellipses align better with flow under higher shear ==> pseudoplastic thixotropic (60 sec) --> pseudoplastic
L
D
Axis Ratio = L/D
DLfeff
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Non newtonian fluids Dilatant: Mostly solids with some fluid in between
Low stress ==> lubrication and less viscosity higher stress ==> insufficient lubrication, more viscosity
Stress
Strain
Newtonian
DilatantPseudo plastic
Bingham Plastic
Bingham Plastic Minimum yield stress Newtonian
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Non newtonian Fluids: Models
dxdVNewtonian :
n
dxdVKDilatentticPseudoplas
:,
factoryconsistencKindexlawpowern
dxdVPlasticBingham 0:
stressyield0
dependenttimecThixotropielasticVisco ,
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Non Newtonian Fluids:Models
Viscoelastic: usually coiled or connected structure stretched (not broken) by stress recoil after stress is released normal stress on pipe != 0 eg. Pull back after the applied force is removed
Non-newtonian != high viscosity Many polymers added to reduce friction in water
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Fluid flow in a pipe Hagen-Poiseuille’s law
Momentum balance
Assumptions Laminar flow steady state no-slip incompressible
xr
r
Vols
VoldVt
dAnVVF )(.
rrxrrrxxxx dxrdxrdrrPdrrPF 2222
022. xxxxxxxs
VrrVVrrVdAnVV Pressure drop = friction
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Fluid flow in a pipe xr
r
rCr
dxdP
rx1
2
0
drrd
dxdPr rx
finiter ,0 2r
dxdP
rx
Newtonian xrx
dVdr
2
32DV
xP avg
Flow Rate Average Velocity
22
14 R
rRdxdPVx
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Non newtonian: power law fluidFluid flow in a pipe
n
dxdVK
n
nn
nn
x rRnn
dxdP
KV
111
121
Flow Rate Average Velocity
Double the pressure != double velocity
2
0 0
,R
Q V r r dr d
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Non newtonian: Bingham PlasticFluid flow in a pipe
Flow Rate Average Velocity
Double the pressure != double velocity
dxdVPlasticBingham 0:
arfor
220 1
4 RrR
dxdPrRVx
0xV arfor
dxdP
a 02
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Flow between plates
Micro fluidics Identification of DNA fragments (for example) Flow rate depends on
Viscosity Surface Tension Sample movement rate depends on affinity
Sample 1
Sample 2
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Flow between plates
X
Y
Z
Steady state Incompressible Laminar flow no-slip
Element of width length X, height Y and width (or depth) of 1 unit
Vols
VoldVt
dAnVVF )(.
yyyxyyxxxx XXYPYPF
2b
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Flow between plates
0
dyd
dxdP yx
1CydxdP
yx
By symmetry, at the center, shear stress =0 ydxdP
yx
Newtonian
dydVx
yx
1
2
21 Cy
dxdPVx
Flow rate Average velocity
byVx @,0
2
1 2ybdxdPVx
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Flow between plates Non newtonian : Power law fluids
n
dxdVK
1
11
11
1
C
n
ydxdP
K
dxdPKV
n
w
byVx @,0
0@, yw
Flow rate Average velocity
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Examples Pipe flow Fluid flow ~= Current flow P = Voltage, Vavg = current avgV
DxP 2
32
Resistance Non-newtonian fluid: non-linear relation
between P and Vavg
Newtonian fluid: easier prediction of results of changing one or more parameters
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Example Non newtonian : Bingham Plastic
42
31
341
32mm
xPDVavg PD
Lm
04 1m
smkg /2.0Pa200 3/2000 mkg
H=10 m
L=20m/5m
D=0.1m082.04 0
PDLm
89.0factor
smVavg /6.13
02.04 0
PDLm
97.0factor smVavg /4.59
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Example Find the time taken to drain the tank
1
221 A
VAVdtdH
H=10 m
L=20m/5m
D=0.1m
V2 is a function of H
Tank will not drain completely!
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Example Non newtonian : Power law fluid
nn
avg xKPD
nn
xKPDV
12
4314
32
smVmkgn
mskgKcmDP
avg /1,2000,5.0
,/5.2,1?,
3
23
MPaP 79.0
1m/s
25 m Long, 1cm dia
MPaP 12.1
If flow rate has to be doubled, pressure needed
navg PCV
1
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Example Pbm. 8.2 Given, L12=22 km, L23=18 km, Q, P known
Consider this as resistance model
2
32DV
xP avg
L12 L23
4
128DQ
2312 LLafterbefore PPPP
413
DQLK
P beforebefore 4
1212 D
QLKP after 4
2323 2
1DQLK
P after
31/40
223
12
13
LL
LQQ
before
after
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Viscometers Tube,Cone&Plate,Narrow gap cylinder, infinite gap
cylinder
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Viscometers Cone and Plate Viscometer Ref: BSL, pbm 2B.11
0
22 0 tofromchangecan
drdr
X
TorquevelangularRGiven ,,
)2
tan(,)sin( 000
hyrVx
YA
rV
Goal: Shear Stress, Velocity Profile, Torque
Fluid between two plates, linear profile
0
2
rV
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Viscometers Shear stress vs Velocity: Spherical Co-ordinates
Vr
Vr sin
1sin
sin
22
2
sincossin
sinsin
rVV
r
Shear Stress at cone: )tan(,2 0
0
, , ,independent of r z
Force, Torque drdrF
rdrdrTorque
R
drdrTorque0
2
0
2
0
0
3
32
RTorque
Practical 0 ~ 1o
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Viscometers Cylindrical viscometer
rgapplatesparallelgapNarrow ,
r
rr
rV
drdV
rrLrFTorque 2
Vary , obtain Torque and velocity gradients for plots
Torque
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Differential momentum balance:Navier-Stokes Equation
Newtonian Fluids ONLY Assumptions/applicability:
Isothermal conditions both Compressible/Incompressible both laminar/turbulent Stokes assumption for bulk-viscosity (needed for
compressible fluids)
0.,2 VifVPgDtDV
0. VDtD Continuity (Velocity Divergence)
0, ifPgDtDV
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Appendix Pbm. 8.6 Given, L=8m,P=207kPa, d=.635 cm, Find velocity for no friction vs friction
22
22
11
21
22ghPvghPv
PVhhV
2,0 2211
Frictional effects
LPDV
32
2
2
2.112322
cos
PD
LV
V
viscous
ityVisNo
1 2
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IIT-Madras, Momentum Transfer: July 2005-Dec 2005
Appendix: Blood Flow in Arteries 40% red blood cells in plasma, non-newtonian Pulsating motion, varying pressure Re = 600, during exercise 6000 Blood vessel dilation , short term, long term Shear stress vs platelet activation (wound vs
stenosis); ultrasonic detection Tensile vs compressive stress; structure of blood
vessel Collapse of vessel during BP measurement Collapse near stenosis and cardiac arrest Mass & heat transport