IMPACT OF GEOMETRY ON BLOOD FLOW PATTERNS IN ABDOMINAL AORTIC ANEURYSMS ...

110 Pages·2011·9.79 MB·English

IMPACT OF GEOMETRY ON BLOOD FLOW PATTERNS IN ABDOMINAL
AORTIC ANEURYSMS
by
VASHISHTH PATEL
A Thesis submitted to the
Graduate School-New Brunswick
Rutgers, The State University of New Jersey
and the
Graduate School of Biomedical Sciences
UMDNJ – Robert Wood Johnson Medical School
in partial fulfillment of the requirements
for the degree of
Master of Science
Graduate Program in Biomedical Engineering
written under the direction of
Professor John K-J. Li
and approved by
________________________
________________________
________________________
New Brunswick, New Jersey
October, 2011
ABSTRACT OF THE THESIS
Impact of Geometry on Blood Flow Patterns in Abdominal Aortic Aneurysms
By VASHISHTH PATEL
Thesis Director: Professor John K-J. Li
The rupture of abdominal aortic aneurysm (AAA) is the 13th leading cause of death in the
US. The dilation of aneurysm and the risk of rupture depend on hemodynamic stresses
on the wall and changes in wall mechanics, which are both impacted by blood flow
patterns. Flow patterns in AAA are affected by the changes in the geometry of aneurysm
bulge and underlying vessels. Thus the flow patterns pertaining to various geometrical
aspects of AAA need to be characterized in order to better understand AAA etiology and
physiological impacts before rupture. Using computational fluid dynamics (CFD), the
impact of geometrical parameters in combination with boundary conditions on the flow
patterns and pressure distribution in AAA models was evaluated in this study. It is found
that bifurcation angle in combination with asymmetry has significant impact on blood
flow patterns and pressure distribution in the aneurysm bulge. Increase in aneurysm bulge
size leads to increased turbulence and instability in the flow characterized by complex
flow patterns in both symmetric and asymmetric models. Larger asymmetric aneurysms
are more prone to increased wall stress distributions, high wall shear stresses and
increased overall pressure in the aneurysm bulge making it more likely to rupture. The
rupture risk for large asymmetric aneurysm is highest at the posterior distal end of the
aneurysm bulge due the complex flow patterns and high fluid shear stresses generated in
this region. The neck angle dramatically changes the flow patterns in both the symmetric
and asymmetric aneurysms leading to aggravating blood flow patterns with irregular
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vortices formation that lead to high wall shear stresses and stress concentrations at the
proximal neck and distal end of the aneurysm bulge. The increase in Reynolds number
leads increased turbulent flow and high fluid shear stresses near the distal end of the
aneurysm bulge. Change in outlet pressure at the iliac arteries does not impact either the
flow patterns or pressure distribution patterns in aneurysms. Thus the current study
provides a new approach to characterize AAA complementing fluid-structure interaction
models for clinical applications.

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ACKNOWLEDGEMENTS
First and foremost, I would like to thank my thesis advisor, Dr. John Li for his
indispensable guidance during my project. I am very grateful for his cooperation and
accommodation with my work schedule and extensive traveling for work. I would also
like to acknowledge the thesis committee members – Dr. George Shoanne and Dr. Gary
Drzewiecki.
I would like to thank my family for their unconditional support to finish my thesis and
help me achieve everything that I have so far in my life. And I would like to thank
Steven Nguyen for helping and guiding me to balance work and academic activities
during the period of thesis completion.

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DEDICATION

The thesis is dedicated to my family for their unconditional love and support

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TABLE OF CONTENTS
ABSTRACT OF THE THESIS ........................................................................................... ii
ACKNOWLEDGEMENTS ................................................................................................ iv
DEDICATION ..................................................................................................................... v
TABLE OF CONTENTS .................................................................................................... vi
LIST OF FIGURES .......................................................................................................... viii
LIST OF TABLES ............................................................................................................. xii
SYMBOLS AND ABBREVIATIONS ............................................................................. xiii
1 INTRODUCTION AND BACKGROUND ..................................................................... 1
1.1 Abdominal Aortic Aneurysm (AAA) ........................................................................ 1
1.1.1 Disease Condition ............................................................................................... 1
1.1.2 Symptoms ........................................................................................................... 2
1.1.3 Current Treatment Options ................................................................................. 3
1.1.4 Need of Predictive Methods for Rupture ............................................................ 5
1.2 Rupture Predictive Critical Parameters and their Inter Relation: Past and Current
Approaches ...................................................................................................................... 6
1.3 Rational for Proposed Approach to Focus on Flow Patterns in Simple Predictive
Geometry Models........................................................................................................... 13
2 AIMS AND SIGNIFICANCE OF THE THESIS ........................................................... 17
3 METHODS ..................................................................................................................... 20
3.1 Models and Parameters Selection ............................................................................ 20
3.2 Geometry Creation ................................................................................................... 24
3.3 Mesh Generation ...................................................................................................... 37
3.4 Flow physics, Parameters and Boundary Conditions .............................................. 39
3.5 Solver ....................................................................................................................... 41
4 RESULTS ....................................................................................................................... 43
4.1 Impact of Bifurcation Angle .................................................................................... 43
4.1.1 Impact of Bifurcation Angle on Control Models .............................................. 43
4.1.2 Impact of Bifurcation Angle on Axi-symmetric Aneurysms ............................ 44
4.1.3 Impact of Bifurcation Angle on Asymmetric Aneurysms ................................ 46
4.2 Impact of Size .......................................................................................................... 49
4.2.1 Impact of Size on Axi-symmetric Aneurysms .................................................. 49
4.2.2 Impact of Size on Asymmetric Aneurysms ...................................................... 51
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4.3 Impact of Asymmetry .............................................................................................. 54
4.3.1 Impact of Asymmetry on Pressure Distribution ............................................... 54
4.3.2 Impact of Asymmetry on Flow Patterns ........................................................... 56
4.4 Impact of Number of Aneurysms ............................................................................ 59
4.4.1 Impact of Double Aneurysms on Flow Patterns ............................................... 59
4.4.2 Impact of Double Aneurysms on Pressure Distribution ................................... 61
4.5 Impact of Neck Angle .............................................................................................. 64
4.5.1 Impact of Neck Angle on Flow Patterns ........................................................... 64
4.5.2 Impact of Neck Angle on Pressure Distribution ............................................... 68
4.6 Impact of Re and Inlet Velocity Profile ................................................................... 70
4.6.1 Impact of Re and Inlet Velocity Profile on Flow Patterns ................................ 70
4.6.2 Impact of Re and Inlet Velocity Profile on Pressure Distribution .................... 75
4.7 Impact of Outlet Pressure Condition ........................................................................ 78
4.7.1 Impact of Outlet Pressure on Flow Patterns ..................................................... 78
4.7.2 Impact of Outlet Pressure on Pressure Distribution .......................................... 81
4.8 Impact of Geometry on Wall Shear Stress (WSS) Distribution .............................. 83
5 DISCUSSION AND SUGGESTIONS FOR FUTURE RESEARCH ............................ 86
CONCLUSION .................................................................................................................. 93
REFERENCES .................................................................................................................. 95

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LIST OF FIGURES
Figure 1-1 Anatomy of Abdominal Aortic Aneurysm ........................................................ 1
Figure 1-2 Types of Aneurysms ......................................................................................... 2
Figure 1-3 CFD flow vectors in a patient specific aneurysm model under steady flow with
Re 550 (left) and pulsatile flow (right) [7].......................................................................... 9
Figure 1-4 Velocity vectors in asymmetric aneurysm model with Re 300 in lateral and A-
P plane under pulsatile flow condition [15] ........................................................................ 9
Figure 1-5 Pressure (wall) distribution (left) and Shear stress (wall) distribution for
asymmetric aneurysm model Re 300 under pulsatile flow conditions [15] ...................... 10
Figure 1-6 Velocity vectors (left) and Pressure (right) distribution in A-P plane of
asymmetric aneurysm FSI model with pulsatile flow conditions [20] ............................. 12
Figure 1-7 Pressure and wall shear stress distribution for patient specific aneurysm model
with peripheral branches under steady flow conditions using CFD [25] .......................... 15
Figure 2-1 Schematic diagram showing the combination of the geometrical features of the
aneurysm (bulge) model and boundary conditions ........................................................... 19
Figure 3-1 Schematics of AAA morphology (asymmetric) [2] ........................................ 20
Figure 3-2 Control models [Ctrl 1 and Ctrl 2] geometry .................................................. 25
Figure 3-3 Bifurcation angle and iliac artery geometry [Ctrl1 and Ctrl2 models] ........... 26
Figure 3-4 Axi-symmetric model A (bottom) with maximum bulge size D=3d and
associated cosine curve (top) ............................................................................................ 27
Figure 3-5 Section view of axi-symmetric models with D=3d: A (left) with 60 degree and
B (right) 80 degree bifurcation angles .............................................................................. 28
Figure 3-6 Axi-symmetric models C (bottom-left) and D (bottom-right) with bulge size
D=2.5d and associated cosine curve (top) ........................................................................ 29
Figure 3-7 Section view of axi-symmetric models with D=2.5d: C (left) with 60 degree
and D (right) 80 degree bifurcation angles ....................................................................... 30
Figure 3-8 Asymmetry relation (top) [20] and plotted deformation curves used to create
models E and F with D=3d (middle) and models G and H with D=2.5d (bottom) .......... 31
Figure 3-9 Asymmetric deformations in AAA models representative of models E/F (left)
and models G/H (right) ..................................................................................................... 32
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Figure 3-10 Profile curve used to create the kink and angle in the proximal neck of the
aneurysm bulge ................................................................................................................. 33
Figure 3-11 Model K: Lateral plane section view (left) and A-P plane view (right) with
axi-symmetric aneurysm bulge (D=3d) and bifurcation angle (80 degrees) .................... 34
Figure 3-12 Model L: Lateral plane section view (left) and A-P plane view (right) with
asymmetric aneurysm bulge (D=3d) and bifurcation angle (80 degrees) ......................... 34
Figure 3-13 Cosine curve for the aneurysm bulges for double aneurysm models ............ 35
Figure 3-14 Double aneurysm models I (left) and J (right) with D =2d and D =3d ........ 36
1 2
Figure 3-15 Volumetric mesh elements for model A ....................................................... 39
Figure 3-16 Parabolic inlet velocity profile ...................................................................... 40
Figure 3-17 CFX-Solver screen shot of the real time solution convergence monitor
showing converged solution for Ctrl1 model.................................................................... 42
Figure 4-1 Velocity vectors in lateral plane for models Ctrl 1 (left) and Ctrl 2 (right) .... 44
Figure 4-2 Velocity vectors in lateral plane for models A (left) and B (right) ................. 45
Figure 4-3 Pressure distribution in lateral plane model A (a) and B (b) with normal
velocity profile and model B (c) with parabolic inlet velocity ......................................... 46
Figure 4-4 Flow patterns in lateral plane for asymmetric aneurysm model E with 60
degree bifurcation angle .................................................................................................... 48
Figure 4-5 Flow patterns in lateral plane for asymmetric aneurysm model F with 80
degree bifurcation angle .................................................................................................... 48
Figure 4-6 Pressure distribution in axi-symmetric bulge models B (D=3d) and D
(D=2.5d) ............................................................................................................................ 49
Figure 4-7 Flow patterns in axi-symmetric models B (D=3d): Lateral plane (left) and A-P
plane (right) ....................................................................................................................... 50
Figure 4-8 Flow patterns in axi-symmetric models D (D=2.5d): Lateral plane (left) and
A-P plane (right) ............................................................................................................... 51
Figure 4-9 Pressure distribution in asymmetric aneurysm models E (D=3d) and G
(D=2.5d) ............................................................................................................................ 52
Figure 4-10 Flow patterns in asymmetric models E (D=3d): Lateral plane (left) and A-P
plane (right) ....................................................................................................................... 53
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Figure 4-11 Flow patterns in asymmetric models G (D=2.5d): Lateral plane (left) and A-P
plane (right) ....................................................................................................................... 54
Figure 4-12 Overall pressure distribution for model B (axi-symmetric) and F
(asymmetric) ..................................................................................................................... 55
Figure 4-13 Overall pressure distribution for model D (axi-symmetric) and H
(asymmetric) ..................................................................................................................... 56
Figure 4-14 Flow patterns in A-P plane for model A (axi-symmetric) and model E
(asymmetric) ..................................................................................................................... 57
Figure 4-15 Flow patterns in A-P plane for model B (axi-symmetric) and model F
(asymmetric) ..................................................................................................................... 57
Figure 4-16 Flow patterns in A-P plane for model D (axi-symmetric) and model H
(asymmetric) ..................................................................................................................... 58
Figure 4-17 Velocity vectors for double aneurysm model I (60 degree bifurcation angle)
........................................................................................................................................... 60
Figure 4-18 Velocity vectors for double aneurysm model J (80 degree bifurcation angle)
........................................................................................................................................... 60
Figure 4-19 Velocity vectors for double aneurysm model J with parabolic inlet velocity
profile ................................................................................................................................ 61
Figure 4-20 Pressure distribution at lateral plane section view for double aneurysm
models I and J ................................................................................................................... 63
Figure 4-21 Overall pressure distribution for double aneurysm models I and J ............... 63
Figure 4-22 Flow patterns in lateral plane for models K and L with neck angle before
aneurysm bulge ................................................................................................................. 65
Figure 4-23 Flow patterns in A-P plane for models K and L with neck angle before
aneurysm bulge ................................................................................................................. 65
Figure 4-24 Flow patterns in aneurysm bulge at A-P plane for models K and L ............. 67
Figure 4-25 Flow patterns in aneurysm bulge at lateral plane for models K and L ......... 67
Figure 4-26 Overall pressure distribution for models K and L with neck angle .............. 69
Figure 4-27 Pressure distribution on lateral section plane for models K and L ............... 69
Figure 4-28 Flow patterns in aneurysm bulge in lateral plane for model F with varying
inlet velocity conditions .................................................................................................... 71
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