fluid structure interaction in abdominal aortic aneurysm using ansys workbench
DESCRIPTION
Fluid structure interaction in abdominal aortic aneurysm using ANSYS WorkbenchTRANSCRIPT
Fluid structure interaction in abdominal aortic aneurysm using
ANSYS Workbench
byFlorentina Ene
Outline
• Objectives• Abdominal aortic aneurysm (AAA)• Computational methods (CM)
Finite Element Analysis (FEA) Computational Fluid Dynamics (CFD) Fluid Structure Interaction (FSI)
• Validation of CM• Comparison of CM• Haemodynamics and mechanical factors
Objectives
• To simulate the interaction between the blood flow and the diseased aneurismal wall by– Computational simulation– Experimental testing
for the study of abdominal aortic aneurysm (AAA)
• To investigate the influence of certain haemodynamics factors
Abdominal Aortic Aneurysm (AAA)
• AAA - a localised abnormal dilatation of the abdominal aorta
• Diameter - 1.5 times larger than the nominal diameter
• Causes - primarily atherosclerosis
• Population - 4:1 ratio male to female, 75% over 60 years old
• Risk - a high risk of sudden rupture
• Rupture - 3:1 female to male risk rupture
http://www.emedicine.com/MED
Physics of AAA
• Rupture of AAA– Surgical criterion: max diameter 5-5.5 cm
– Maximum wall stress (Raghavan, 1996)(Raghavan, 1996)
– Asymmetry influence (Vorp, 1998; Scotti,2005)(Vorp, 1998; Scotti,2005)
– Intraluminal thrombus (ILT) ((Wang, 2002)Wang, 2002)
– Pulsating interaction (DiMartino, 2001)(DiMartino, 2001)
Hemodynamics in AAA
• Low flow
• Recirculation regions
• Secondary flow
• Low mean wall shear stress
• Temporal oscillations in shear
(Moore,1992; Moore, 1994; Taylor,1998; Taylor, 2002; Long,1998; Tang, 2006)(Moore,1992; Moore, 1994; Taylor,1998; Taylor, 2002; Long,1998; Tang, 2006)
ANSYS Workbench
• ANSYS Workbench– ANSYS ICEM
Mesh– ANSYS Simulation (ANSYS Structural) FEA– ANSYS CFX CFD
Computational Methods for AAA
• Structural Pressure Analysis (FEA)– Static (sFEA)– Transient (tFEA)
• Computational Fluid Dynamics (CFD)– Steady flow (sCFD)– Pulsating flow (tCFD)
• Fluid-Structure Interaction (FSI)– Steady FSI (sFSI)– Pulsating FSI (tFSI)
Computational Methods for AAA
• FEA evaluates rupture potential– Deformations – Stresses
• CFD evaluates unfavourable flow conditions– Velocity distribution– Pressure distribution– Wall shear stress
• FSI evaluates rupture potential due to extra loading of unfavourable flow conditions
Steps of Computational Methods
Solver
Post-processor
Pre-processor
• Geometry• Mesh (Elements)• Materials• Boundary conditions
• Convergence• Solution monitor and control
• Independence analysis• Validation• Comparison
2
3
1
Realistic Aorta Model withAneurysm from CT Scan
IdealisedRealistic
Mimics,Materialise
1
AAA Geometry
Realistic AAA model with/without ILT
Idealised AAA model with/without ILT
1
Meshing1
• Multiblocking technique with O-grid strategy
Meshing
• Hexahedral – fluid volume - Blood• Quadratic – shell element - Wall• Tetrahedral – solid volume - ILT
1
Material and Boundary Conditions
Fluid domainCFD simulation
Solid domainFEA simulation
Material properties
Material properties
Simulationparameters
Boundaryconditions
Simulationparameters
Boundaryconditions
Solver
FEA CFD
•Linear elastic•E=2.7 MPa•ν=0.45•ρ=2000 kg/m3
•Newtonian homogenous incompressible laminar•ρ=1055 kg/m3
•µ=0.0035 Pa s•Ui=0,i=x,y,z@Si,So•FSI interface
@wall(Pressure from
CFD)
• Time step 0.01s• 5 pulse cycles
• Time step 0.01s• 5 pulse cycles
Geometry&
Mesh
1
-0.05
0.05
0.15
0.25
0 0.4 0.8 1.2
Time (s)
Vel
oci
ty (
m/s
)
-10
0
10
20
30
40
Pre
ssu
re (
mm
Hg
)
Velocity inlet Pressure outlets
Solver
FLUID governing equations• The continuity equation
• Navier-Stokes momentum equation
SOLID governing equations• The motion equation• The equilibrium equation • The constitutive equation
FSI• Fluid: Force send as load to solid• Solid: Displacement send as BC to fluid
2
)(in Skl tCijklij
)(on S ttn iiij )(in S
, taf iijij
Fluid
Solid
0u
uuuu 2)/(/)(
ffpt
Postprocessor
• Vector/Countours/Streamlines/Animation/GraphsResults for every domain’s element
• Is the solution valid?– Engineering criteria– Compare to analytical solutions– Compare to experimental solutions– Compare to previous studies– Compare to similar applications– Mesh check
3
Independence Tests
Independence tests Determine
Mesh type independence 1 mesh type
Mesh independence 1 mesh density
Convergence criteria 1 convergence value
For time-dependent analysis
Pulse cycle independence No of pulse cycles
Timestep independence 1 timestep size
3
Verification and Optimisation
Analytical solutions
Solution type Wall type Numerical technique
Laplace Pressure in thin wall Elastic CSD (FEA)
Hagen-Poiseuille’s Steady flow Rigid CFD (FVM)
Womersley Pulsatile flow Rigid CFD (FVM)
Womersley Pulsatile flow Elastic FSI (FEA, FVM)
Inlet surface
FLUID DOMAIN
Wall surface
Outlet surface
Inlet edge
SOLID DOMAIN
Wall surface
Outlet edge
Solid domain Fluid domain
Young’s modulus E=2.7MPa Density ρ=1055kg/m3
Poisson ratio υ=0.45 Viscosity µ=0.0034 Pas
Density ρ=2000kg/m3
Thickness (SHELL181) h=0.002m
3
Validation with Analytical Solutions
Steady flow in rigid tube
3
0
0.1
0.2
-0.01 -0.005 0 0.005 0.01
Radial distance (m)
Ve
loc
ity
(m
/s)
Theory CFD
0
0.04
0.08
0.12
-1 -0.5 0 0.5 1
Diameter ratio
Velo
cit
y w
(m
/s)
Theory FSI0.25s
Pulsatile flow in elastic tube
Pulsatile flow in rigid tube
0.25 sec
0
0.04
0.08
0.12
-1 -0.5 0 0.5 1
Diameter Ratio
Ve
loc
ity
(m
/s)
Theory CFD
Validation with Experimental/Published Results
Pulsatile pressure in compliant AAA model
3
Static pressure in compliant AAA model
0.00
0.50
1.00
1.50
0 5 10 15 20 25
Axial length from maximum diameter (mm)
Ra
dia
l de
form
ati
on
(m
m)
10
20
30
Ra
diu
s (
mm
)
FEA w/ILT EXP w/ILT FEA w/outEXP w/out Outer radius Inner radiusILT radius
0
0.4
0.8
1.2
0 0.4 0.8 1.2
Time (s)
Ch
an
ge
in d
iam
ete
r(m
m)
Position 1 FEAPosition 2 FEAPosition 3 FEAPosition 4 FEAPosition 1 EXPPosition 2 EXPPosition 3 EXPPosition 4 EXP
Cp1 - rest
Validation with Experimental/Published Results3
Pulsatile flow in rigid wall bifurcation
(Morris, 2004)Steady flow in rigid wall bifurcation
(Walburn & Stein, 1981)
10 mm
-0.05
0.05
0.15
0.25
0.35
0.45
-1 -0.5 0 0.5 1Diameter ratio
Ve
loc
ity
(m
/s)
Walburn & Stein CFDPosition 2
0
0.02
0.04
0.06
0.08
-1 -0.5 0 0.5 1
Diameter ratio
Ve
loc
ity
(m
/s)
LDA CFD
Ultrasound Flow Visualisation3
Comparison of CM
• 6 numerical methods– sFEA/tFEA– sCFD/tCFD– sFSI/tFSI
• 3 models Idealised Realistic Realistic with ILT
3
Comparison in Realistic AAA3
Comparison in Realistic AAA
1
2
3
4
1 - AAA without ILT
2 - AAA with ILT
POSITIONS
3
Comparison in Realistic AAA
• Deformations & Von Mises Stresses– Max 5% difference between sFEA and tFSI
• Pressure– Max difference 2%
• Velocity– tCFD>tFSI by max 40%
• Wall shear stress– tCFD>tFSI by max 20%
• Computational effort– tFSI - 10 x sFEA sFEA may be used as clinical tool– tFSI – 10 x sCFD
3
Haemodynamics & Mechanical Factors
• Non-planarity effect• Aortic arch• Wall thickness• ILT presence
• ILT properties Exp• Wall curvature
A1
A2
A3
A4
A5
A6
FSI in realistic aorta model with aneurysmA1
FSI in realistic aorta model with aneurysmA1
Aortic Arch Velocity (tCFD)
Max acceleration
T=0.15s
Max velocityT=0.25s
Max deceleration
T=0.35s
A2
Wall Thickness
– AAA – thinning of wall
– 3 models of varying wall thickness
Aorta
H1 & H2: 2 mm
AAA
H1 : 2 mm
H2: 1 mm
Iliac arteries:
H1 & H2: 2 mm
0
0.001
0.002
0.00 0.05 0.10 0.15 0.20
Axial length (m)
Th
ick
ne
ss
(m
)
0
0.02
0.04
Dia
me
ter
(m)Thickness Diameter
Aorta AAA Iliac
arteries
H3
A3
Wall ThicknessD
efo
rma
tio
n (
m)
AAA without ILTH1 H2 H3
Vo
n M
ise
s s
tre
ss
(P
a)
Vo
n M
ise
s s
tre
ss
(P
a)
De
form
ati
on
(m
)
AAA with ILTH1 H2 H3
A3
AAA With and Without ILTV
elo
cit
y (
m/s
)
Wa
ll s
he
ar
str
es
s (
Pa
)
De
form
ati
on
(m
)
Pre
ss
ure
(P
a)
-ILT +ILT -ILT +ILT
A4
Influence of ILT Properties &Wall Curvature
A5
• Wall curvature– ↑ peaks in diametral strains and compliance– ↑ high stress
1: No ILT2: ILT E=0.05 MPa3: ILT E=0.1 MPa4: ILT E=0.2 MPa
Conclusions
• ANSYS was proven to be an efficient and accurate tool to analyse haemodynamics and mechanical factors influencing AAA
• Mesh independence and pulse cycle independence should be optimised
• Validation of FEA/CFD/FSI was obtained with analytical and experimental results
Conclusions
• AAA rupture could be predicted using computational simulation
• Effects of patient specific geometry are important on haemodynamics in AAA
• Wall thickness and ILT presence are essential in evaluating AAA rupture potential
Thank you!