Fluent/ICEM 12.1
Computational fluid dynamics (CFD) employs numerical methods and algorithms to analyse and solve problems that involve fluid flow, traditionally determined through experimental testing. The CFD package, FLUENT (ANSYS Inc. Canonsburg, PA), utilises the finite control volume method to obtain a solution for the governing equations of fluid flow for both simplified and complex flow problems and is utilised extensively throughout CABER. Initially the fluid flow problem is characterised in ICEM (ANSYS, Inc. Canonsburg, PA) through the creation of physical bounds, referred to as the geometry. This geometry is then subdivided into a large number of discrete cells called a mesh or grid and boundary conditions are defined on the geometry. The fluid flow problem is then solved in FLUENT. FLUENT is an experimentally validated method of analysing fluid flow, pressure and mass transport problems and is widely used in the Bioengineering field of research. CABER has applied ICEM and FLUENT in a wide range of biomedical engineering analyses including the determination of the flow field and shear stress environment within the distal junction of peripheral by-pass grafts, the coronary arteries, arteriovenous (AV) fistulae and AV grafts, the cerebral vasculature, namely the circle of Willis, and within abdominal aortic aneurysms (AAA) both prior and post endovascular repair. CABER has also employed FLUENT to conduct mass transport investigations, primarily in the distal junction of peripheral bypass grafts.
Peripheral Bypass Grafts
CABER uses CFD to investigate the development of intimal hyperplasia in the downstream graft/artery junction of peripheral bypass graft due to its focal development at specific locations within the junction. The analysis has focused on variations in wall shear stress acting on the artery wall, the influence of such changes in wall stress (spatial and temporal) within the artery wall, and the possible role played by mass transport disturbances in the initiation and progression of the intimal hyperplastic response. Various by-pass geometries have been investigated, both idealised and realistic, including the standard by-pass graft, the Miller Cuff and Taylor Patch.
Figure 1: Idealised downstream graft/artery junction and path lines in an end-to-side geometry (Coloured by velocity magnitude).
Figure 2: In-plane velocity vectors and species mass fraction. End-to-side velocity vectors and associated wall shear stress distribution Sherwood number distribution at the bed of the junction
Coronary Arteries
Computational flow studies were conducted at CABER to establish the flow patterns present in the right (RCA) and left (LCA) coronary arteries. Atherosclerosis, a disease that frequently affects the coronary arteries, involves the development of fatty plaques on the inner wall of arteries. Atherosclerotic plaques have a tendency to develop in specific locations of the circulatory system that contain complex flow patterns, such as branching and curvature. This research focused on the haemodynamic environment, in particular wall shear stress (WSS), within the RCA and LCA. Strong counter rotating vortices were detected in regions of significant curvature in the coronary arteries, and bifurcations were found to skew the flow away from the flow divider. Regions experiencing the higher ranges of shear stress correlated closely to areas that have demonstrated a tendency to be protected from plaque development. This computational analysis was further employed to investigate if complex flow patterns, such as those found within the coronary arteries, could influence the distribution of monocytes in these vessels and result in higher levels of adhesion along the inner walls of curvature and outer wall of bifurcations.
Figure 3:WSS contours of the RCA
Arteriovenous (AV) Vascular Access
A CFD analysis was conducted at CABER to investigate the influence of the local haemodynamic environment on the development of venous neointimal hyperplasia (VNH) and the subsequent formation of stenotic lesions within the venous conduit of arteriovenous (AV) fistulae and AV grafts. Flow patterns within the vascular access junction of AV fistulae and AV grafts were established, both idealised and realistic. Significant blood flow skewness towards the venous floor, with associated flow stagnation, recirculation and flow impingement were evidenced along with counter rotating flow vortices within the vascular access junction and proximal vein, figure 5. This anomalous flow environment produced elevated WSS magnitudes and significant WSS gradients (WSSGs) on the floor of the junction area and efferent vein. The analysis further investigated the influence of such a non-physiological haemodynamic environment on the vessel wall. Another goal of this study was to investigate the influence of geometrical characteristics on the flow patterns within, and shear stress forces on the floor of, the VA junction and correlate this to the development of VNH. This research has led to the development of a novel AV Graft, the Prolong AV Graft, which produces improved haemodynamics within the VA junction and efferent vein through the elimination of the bed of the VA junction, figure 6.
Figure 4: (A) Velocity Contours and Vectors in the EV-SA AV fistula geometry (B) Venous floor WSS contours and corresponding WSS profile: EV-SA AV fistula geometry (Retrograde Flow)
Figure 5: (a)The Prolong AV Graft, (b) The graft-vein junction of the Prolong AV Graft, (c) Velocity Contours and Vectors within the graft-vein junction of the Prolong AV Graft
Cerebral Vasculature: Circle of Willis
The hemodynamics of the cerebral vasculature and in particular blood flow characteristics in the circle of Willis has been examined, using CFD, in the CABER research group. The circle of Willis is a key element in maintaining blood perfusion to the brain as it redistributes the oxygenated blood throughout the cerebral mass. However, the circle of Willis has a high degree of variability in relation to artery diameters and geometry. Furthermore, when a subject undergoes a preventive procedure such as carotid angioplasty or stent implantation, complications can occur due to the vascular variations within the circle of Willis. It is the effect of these variations, in conjunction with carotid angioplasty and stent implantations that have been accessed using FUENT to determine the flow characteristics within the brain.
Figure 6: 3-D Reconstructed image of velocity profiles within the Circle of Willis
Abdominal Aortic Aneurysms (AAA)
CFD has been used to investigate fluid flow and shear forces in abdominal aortic aneurysm stent-graft devices. Drag forces which are the sum of the pressure and viscous forces acting on the stent-graft have also been determined using FLUENT. These forces have been found to play a role in stent-graft migration. The effect of alternative stent-graft design on aneurysm haemodynamics has also been investigated. A tapered graft design reduces flow recirculation, flow skewing and wall shear stress in idealised cases but this does not extend to patient-specific cases. Figure 8 shows the complete shear stress profile that is developed in an arch stent graft model.
Figure 7: Wall shear stress (WSS) in a tapered graft and conventional graft.