Abstract
Pulsatile flows in geometries of physiological relevance have been investigated.
Atherosclerotic plaques are (initiated) near junctions and bifurcations in larger arteries.
The flow in these regions is characterized by flow separation and unsteadiness, which indicates that local flow conditions contribute to atherogenesis.
Flows in curved and bifurcating pipes have been investigated over many years. However, details of dynamical patterns of pulsating flow, near wall effects, and differential diffusion effects are not well documented. The effect of wall elasticity on the flow has been assumed to be small but no quatification data exist.
There are same basic difficulties in studying physiological flow: The geometries have large interindividual variations. The mechanical properties of the vessels are unknown. Equally, the boundary conditions (temporal and spatial distribution of the blood velocity) are not know. Additional difficulties arise due to measuring difficulties both invivo and invitro. The flow itself may be rather complex (timedependent 3D, transitional with locally strong effects of viscosity and unsteadiness, leading to variable phase lag between pressure gradient and the local flow).
The aim of this study is to enhance understanding of the timedependent, physiologically relevant, flow field in bifurcations, and relate that to hypotheses of atherosclerotic disease. Additionally, an FSImodel has been developed with the purpose to model flow through elastic pipes, and to assess the effect of wall elasticity on the flow.
The investigations have shown clear patterns of wall shear stress (WSS) variations. Local regions of temporal and spatial variations of the WSS was found at sites usually referred to as risksites of atherosclerosis, but also at locations often referred to as ``safe''.
Some of the characteristics of the WSS are further related to changes in the secondary flow field. The secondary flow shows similar characteristics for an increased Reynolds number, although unsteady asymmetric patterns appear at peak flow, while a large Womersley number shows more simple secondary flow structures.
It is also shown that the effects of upstream geometrical variations on the flow field itself, are important mainly over one stage of arterial bifurcation. On the other hand, blood components (modeled as passive scalars with different values of Schmidt numbers) do exhibit upstream effects over a longer range.An important finding is that Schmidt number effects may lead to redistribution of the different scalars. The variations in the concentrations of the scalars are of the same order as the local concentration themselves.
The FSImodel developed combines an Immersed BoundaryFinite Difference code with a shell model for the arterial wall. The shell model is solved on a (surface 2D) using a Finite Element Method (FEM) code.
The structural solver is verified against an analytical expression for bending of a thinwalled pipe. The studies with respect to the importance of arterial wall elasticity on the flow, are not yet completed.
Atherosclerotic plaques are (initiated) near junctions and bifurcations in larger arteries.
The flow in these regions is characterized by flow separation and unsteadiness, which indicates that local flow conditions contribute to atherogenesis.
Flows in curved and bifurcating pipes have been investigated over many years. However, details of dynamical patterns of pulsating flow, near wall effects, and differential diffusion effects are not well documented. The effect of wall elasticity on the flow has been assumed to be small but no quatification data exist.
There are same basic difficulties in studying physiological flow: The geometries have large interindividual variations. The mechanical properties of the vessels are unknown. Equally, the boundary conditions (temporal and spatial distribution of the blood velocity) are not know. Additional difficulties arise due to measuring difficulties both invivo and invitro. The flow itself may be rather complex (timedependent 3D, transitional with locally strong effects of viscosity and unsteadiness, leading to variable phase lag between pressure gradient and the local flow).
The aim of this study is to enhance understanding of the timedependent, physiologically relevant, flow field in bifurcations, and relate that to hypotheses of atherosclerotic disease. Additionally, an FSImodel has been developed with the purpose to model flow through elastic pipes, and to assess the effect of wall elasticity on the flow.
The investigations have shown clear patterns of wall shear stress (WSS) variations. Local regions of temporal and spatial variations of the WSS was found at sites usually referred to as risksites of atherosclerosis, but also at locations often referred to as ``safe''.
Some of the characteristics of the WSS are further related to changes in the secondary flow field. The secondary flow shows similar characteristics for an increased Reynolds number, although unsteady asymmetric patterns appear at peak flow, while a large Womersley number shows more simple secondary flow structures.
It is also shown that the effects of upstream geometrical variations on the flow field itself, are important mainly over one stage of arterial bifurcation. On the other hand, blood components (modeled as passive scalars with different values of Schmidt numbers) do exhibit upstream effects over a longer range.An important finding is that Schmidt number effects may lead to redistribution of the different scalars. The variations in the concentrations of the scalars are of the same order as the local concentration themselves.
The FSImodel developed combines an Immersed BoundaryFinite Difference code with a shell model for the arterial wall. The shell model is solved on a (surface 2D) using a Finite Element Method (FEM) code.
The structural solver is verified against an analytical expression for bending of a thinwalled pipe. The studies with respect to the importance of arterial wall elasticity on the flow, are not yet completed.
Original language  English 

Qualification  Doctor 
Awarding Institution 

Supervisors/Advisors 

Award date  2009 Dec 1 
Publisher  
Print ISBNs  9789162879525 
Publication status  Published  2009 
Bibliographical note
Defence detailsDate: 20091201
Time: 10:15
Place: Room M:E, Mbuilding, Ole Römers väg 1, Faculty of Engineering, Lund University
External reviewer(s)
Name: Lundström, Staffan
Title: Professor
Affiliation: Luleå Tekniska Universitet

Subject classification (UKÄ)
 Energy Engineering
Keywords
 Wall shear stress
 Biofluid mechanics
 Pulsatile flow
 Atherosclerosis
 Flow in bifurcations