AME 60676 Biofluid & Bioheat Transfer 4. Mathematical Modeling.
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Transcript of AME 60676 Biofluid & Bioheat Transfer 4. Mathematical Modeling.
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AME 60676Biofluid & Bioheat Transfer
4. Mathematical Modeling
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Objectives
• Applications of hydrostatics and steady flow models to describe blood flow in arteries
• Unsteady effects:– pressure pulse propagation through arterial wall– Effects of inertial forces due to blood
acceleration/deceleration– Effects of artery distensibility on blood flow
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Outline• Steady flow considerations and models:
– Hydrostatics in circulation– Rigid tube flow model– Application of Bernoulli equation
• Unsteady flow models:– Windkessel model for human circulation– Moens-Korteweg relationship (wave propagation, no viscous effects)– Womersley model for blood flow (wave propagation, viscous effects)– Wave propagation in elastic tube with viscous flow (wave
propagation, viscous effects)• Bioheat transfer models:
– Pennes equation– Damage modeling
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Damage modelingPennes equationComplete model
1. Hydrostatics
Womersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics
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Hydrostatics in the Circulation
• Blood pressure in the “lying down” position– Arterial: 100 mmHg– Venous: 2 mmHg
• Distal pressure is lower
Hydrostatic pressure differences in the circulation
“lying down” position
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics
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Hydrostatics in the Circulation
• Blood pressure in the “standing up” position– Head artery: 50 mmHg– Leg artery: 180 mmHg– Head vein: -40 mmHg– Leg vein: 90 mmHg
• Pressure differences due to gravitational effects Hydrostatic pressure differences
in the circulation“standing up” position
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics
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Hydrostatics in the Circulation
• Bernoulli equation:
• Tube of constant cross section:
• Effects of pressure on vessels:– Arteries are stiff: pressure does
not affect volume– Veins are distensible: pressure
causes expansion
Hydrostatic pressure differences in the circulation
“standing up” position
2
2
V pz const
g g
p g z
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics
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2. Rigid Tube Flow Model
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Hagen Poiseuille Model
• Assumptions:– incompressible– steady– laminar– circular cross section
• From exact analysis:
2
2
4 4z
p dv r r
L
L
r
z
4
128
p dQ
L
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Hagen Poiseuille Model
• Assumptions:– incompressible– steady– laminar– circular cross section
• From control volume analysis:
L
r
z1P 2P
w
w
Control volume
4 wLp
d
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Hagen Poiseuille Model
• Validity considerations– Newtonian fluid: reasonable• Casson model: linear at large shear rate
– Laminar flow: reasonable• Average flow: Re=1500 (< Recr=2100)• Peak systole: Re = 5100
– Blood vessel : compliance– Flow measurements: no evidence of sustained turbulence
– No slip at vascular wall: reasonable• Endothelial cell lining
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Hagen Poiseuille Model
• Validity considerations– Steady flow: not valid for most of circulatory system
• Pulsatile in arteries– Cylindrical shape: not valid
• Elliptical shape (veins, pulmonary arteries)• Taper (most arteries)
– Rigid wall: not valid• Arterial wall distends with pulse pressure
– Fully developed flow: not valid• Finite length needed to attain fully developed flow• Branching, curved walls
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Blood Vessel Resistance
• On time-average basis:
– p: time-averaged pressure drop (mmHg)– Q: time-averaged flow rate (cm3/s)– R: resistance to blood flow in segment (PRU,
peripheral resistance unit)
4
128
p LR
Q d
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Blood Vessel Resistance
• Series connection:
• Parallel connection:
31 21 2 3
pp ppR R R R
Q Q Q Q
31 2
1 2 3
1 1 1 1QQ QQ
R p p p p R R R
R3R2R1
R1
R2
R3
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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Transition Flow in Pipes• Entrance
– V=0 at wall – Velocity gradient in radial direction
• Downstream– Fluid adjacent to wall is retarded – Core fluid accelerates
• Viscous effects diffuse further into center
U
region dominated by viscous effects
region dominated by inertial effects
parabolic velocity profile
Entrance region Fully developed flow region
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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BL Thickness and Entrance Length
• Balance inertial force and viscous force:
• Entrance length definition:
– Re > 50:
– Re 0:
1
2~
xx
U
L R ReL kD k = 0.06
0.65L D
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsHydrostaticsRigid tube flow model
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3. Application of Bernoulli Equation
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Bernoulli Equation
• Assumptions:– Steady– Inviscid– Incompressible– Along a streamline
2
2
V pz const
g g
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Stenosis
• Narrowing of artery due to:– Fatty deposits– Atherosclerosis
• Effects of narrowing:– p1, V1, V2: known
a1
p1, V1
δ
p2, V2
a2
2 22 1 1 22
p p V V
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Stenosis
• Arterial flutter– Low pressure at
contraction– Complete obstruction
of vessel under external pressure
a1
p1, V1
δ
p2, V2
a2
– Decrease in flow velocity– Increase in pressure– Vessel reopening (cycle)
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Aneurysm
• Definition: Arterial wall bulge at weakening site, resulting in considerable increase in lumen cross-section
• Characteristics:– Elastase excess in blood– Decrease in flow velocity– Limited increase in pressure (<5 mmHg)– Significant increase in pressure under exercise– Increase in wall shear stress– Bursting of vessel
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Heart Valve Stenoses
• Flow through a nozzle
– Flow separation recirculation region– Fluid in core region accelerates– Formation of a contracted cross section: vena
contracta
2
1 2 2 20
1
2 d
Qp p
A C
Cd: discharge coefficient (function of nozzle, tube, throat geometries)
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Heart Valve Stenoses
• Effective orifice area:
• Gorlin equations (clinical criteria for surgery):
0 2d
QEOA A
C p
Q: mean flow rate (CO)
For aortic valve: Q: mean systolic flow rate
44.5
MSFAVA
p
AVA: aortic valve area (cm2)MVA: mitral valve area (cm2)MSF: mean systolic flow rate (cm3/s)MDF: mean diastolic flow rate (cm3/s)p: mean pressure drop across valve (mmHg)
31.0
MDFMVA
p
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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Heart Valve Stenoses
• Effects of flow unsteadiness and viscosity:
2dQp A BQ CQ
dt Young, 1979
p = temporal acceleration
convective acceleration
viscous dissipation+ +
pp
EOA KQp
rmsm
EOA KQp
based on mean values based on peak-systolic values
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegWindkessel modelRigid tube flow modelHydrostatics Bernoulli applications
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4. Windkessel Models for Human Circulation
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegBernoulli applicationsRigid tube flow modelHydrostatics Windkessel model
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Windkessel Theory
• Simplified model
• Arterial system modeled as elastic storage vessels
• Arteries = interconnected tubes with storage capacity
Unsteady flow due to pumping of heart
Steady flow in peripheral organsAttenuation of unsteady
effects due to vessel elasticity
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegBernoulli applicationsRigid tube flow modelHydrostatics Windkessel model
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Windkessel Theory
Definitions:• Inflow: fluid pumped intermittently by ventricular ejection• Outflow: calculated based on Poiseuille theory
inflow outflow
Q(t)p(t), V(t), Di
RS
Windkessel chamber
Variables Definition
p Windkessel chamber pressure
V Windkessel chamber volume
DiChamber distensibility
RSPeripheral resistance
Q Ventricular ejection flow rate
pVVenous pressure
pV
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegBernoulli applicationsRigid tube flow modelHydrostatics Windkessel model
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Windkessel Solution• Pressure pulse solution– Systole (0 < t < ts):
– Diastole (ts < t < T):
• Stroke volume
0 0 0S i
t
R DS Sp t R Q R Q p e
S i
T t
R DTp t p e
p0: pressure at t=0pT: pressure at t=T
Windkessel (left) vs. actual (right) pressure pulse
00
St
S SV Q t dt Q t Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegBernoulli applicationsRigid tube flow modelHydrostatics Windkessel model
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Windkessel Theory Summary
• Advantages:– Simple model– Prediction of p(t) in arterial system
• Limitations:– Model assumes an instantaneous pressure pulse
propagation (time for wave transmission is neglected)
– Global model does not provide details on structures of flow field
Damage modelingPennes equationComplete modelWomersley modelMoens-KortewegBernoulli applicationsRigid tube flow modelHydrostatics Windkessel model
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5. Moens-Kortweg relationship
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Wave Propagation Characteristics
• Speed of transmission depends on wall elastic properties
• Pressure pulse:– depends on wall/blood interactions– Changes shape as it travels downstream due to
interactions between forward moving wave and waves reflected at discontinuities (branching, curvature sites)
Need for model of wave propagation speed
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Moens-Korteweg Relationship
• Speed of pressure wave propagation through thin-walled elastic tube containing an incompressible, inviscid fluid
• Relationship accounts for:– Fluid motion– Vessel wall motion
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Problem Statement
z
rR
h
Vz(r, z, t)
Vr(r, z, t)
flow
Infinitely long, thin-walled elastic tube of circular cross-section
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Derivation Outline
Equations of fluid motion in infinitely long, thin-walled elastic
tube of circular cross section
Equations of vessel wall motion(inertial force neglected on wall)
Simplified Moens-Korteweg relationship
Equations of vessel wall motion(with inertial force on wall)
Moens-Korteweg relationship
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Simplified Moens-Korteweg relationship• Reduced Navier-Stokes equations:
• Inviscid flow approximation:
2
2
2
2
10
1
1
r z
r r r rr z r
z z z z zr z
rv v
r r z
v v v v pv v rv
t r z r r r z r
v v v v v pv v r
t r z r r r z z
2
z
zr
v p
t z
vRv R
z
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Moens-Korteweg relationship
• Tube equation of motion:
• Coupling with fluid motion (without inertial effects):
2
2t
dhRd Rpd hd
dt
2 2
2 2 20
1p p
z c t
20 constant
2
hEc
R where
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Moens-Korteweg relationship
• Tube equation of motion:
• Coupling with fluid motion (with inertial effects):
2
2t
dhRd Rpd hd
dt
2 2
2 2 20
1p p
z c t
2 220 1
2t RhE
cR E
where
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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Experimental vs. Theoretical c0
Damage modelingPennes equationComplete modelWomersley modelWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Moens-Korteweg
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6. Womersley model for blood flow
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Problem Statement
z
rR
h
Vz(r, z, t)
Vr(r, z, t)
flow
Infinitely long, thin-walled elastic tube of circular cross-section
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Problem Assumptions
• Flow assumptions:– 2D– Axisymmetric– No body force– Local acceleration >> convective acceleration
• Tube assumptions:– Rigid tube– No radial wall motion ( no radial fluid velocity)
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Equation of Motion
• Pressure gradient:
• Axial flow velocity:
• Axial flow velocity magnitude:
i tpt Ae
z
, i tzv r t w r e
2 22
2
1d w dw ARi w
dr r dr
where: R
rr
R
Womersley number
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Examples of Womersley Number
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Flow Solution
• Flow solution:
20
02
410
102
sin
sin
z
A R Mv t
A R MQ t
0
0
3 20
0 3 20
3 21
0 3 2 3 20
1
21
i
i
J i rM e
J i
J iM e
i J i
where:
kJ x : Bessel function of order k
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Flow Solution
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Application• Flow rate calculation for complex (non-
sinusoidal) pulsatile pressure gradients
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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Flow Solution• Time history of the
axial velocity profile (first 4 harmonics)
• Characteristics:– Hagen-Poiseuille parabolic
profile never obtained during the cardiac cycle
– Presence of viscous effects near the wall makes the flow reverse more easily than in the core region
– Main velocity variations along the tube cross section are produced by the low-frequency harmonics
– High-frequency harmonics produce a nearly flat profile due to absence of viscous diffusion
Damage modelingPennes equationComplete modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Womersley model
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7. Complete model:Wave propagation in elastic tube with viscous
flow
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Elastic Tube Equations of Motion
Stresses on a tube element
2
2
2
2
t rr
zt rz
hh S
t R
Sh h
t z
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Elastic Tube and Fluid Stresses
• Tube stresses (from Hooke’s law)
• Fluid stresses (cylindrical coordinates)
2
2
2
2
2
1
1
1
z
z
dR dzS E
ddz RS E
d ddS dz R dzEdz
2 rrr
z rrz
vp
rv v
r z
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Elastic Tube Equations of Motion
• Equations of motion for tube and flow must be solved simultaneously to obtain solutions for:
2
2 2 2
2 2
2 2 2
21
1
rt
r R
z rt
r R
v hEh p
t r R R z
v v hEh
t r z z R z
Governing equations of motion for elastic tube
, , , ,z rv v p
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Flow Equations
• Non-linear inertial terms can be neglected (see order-of-magnitude study performed in Moens-Korteweg derivation)
• Along with the 2 tube equations, we obtain a set of 5 equations with 5 unknowns
2 2
2 2 2
2 2
2 2
1
1
10
r r r r r
z z z z
r z
v v v v vp
t r r r r z r
v v v vp
t z r r r z
rv v
r r z
Governing flow equations
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Flow Equations
• Boundary conditions at fluid-tube interface:
r r Rv
t
z r Rv
t
No penetration No slip
2 2
2 2 2
2 2
2 2
1
1
10
r r r r r
z z z z
r z
v v v v vp
t r r r r z r
v v v vp
t z r r r z
rv v
r r z
Governing flow equations
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Solutions• All variables are, at least, functions of z and t Seek for solutions that vary as:
where:k1 : wave number (= 1/)k2 : damping constant (decay along z)
• Solutions:
i kz te
1 2k k ik
1
1
i kz t
i kz t
i kz t
e
e
p Pe
1
1
i kz tr r
i kz tz z
v v r e
v v r e
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Solutions• After performing an order-of-magnitude study,
the problem statement reduces to:
11 1
22
112
21 1
1 2
11
1
1
10
z
r R
z
r R
z zz
rz
viEkR rhk
vEh iP
R kR r
v vi v ikP
r r r
rvikv
r r
1 1
1 1
z
r
v R i
v R i
BCs:
Equation for 1
Equation for 1
NS / z
continuity
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Solutions• By combining the 4 equations and applying the
BCs, the problem can be expressed as a system of 2 equations with 2 unknowns:
1 2 1 22
1 1 11 2 1 22
1 2
1 2 1 2
1 0 122
2
12
2
1
k Eh k i k iA J R J R
R i iR
ii
kEh i iA J R J R
EkR R ihk R
1 2
11 2
iJ R
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model
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Solutions• Non trivial solutions if and only if determinant
of the system = 0• If (k/ω) = φ is the root of the determinant:
1 2
1
2
Re
Im
k ik
k
k
Damage modelingPennes equationWomersley modelMoens-KortewegWindkessel modelBernoulli applicationsRigid tube flow modelHydrostatics Complete model