The effect of geometrical configurations on flows in idealised and realistic vascular geometries

Abstract

This thesis reports the use of computational fluid dynamics (CFD) to investigate geometrical effects on flows in idealised non-branching double curved geometries (Study A) and in realistic distal anastomoses where geometries have been determined in vivo using magnetic resonance imaging (Study B). The purpose of this research is to improve understanding of the effects of geometrical configurations, especially curvature and non-planarity, on steady flow in idealised non-branching double curved geometries typical of arteries such as the aortic arch, the right coronary artery or the femoral arteries and on pulsatile flow in realistic distal anastomosis geometries. It is explained that the further knowledge gained from these idealised geometries can be useful to understand flows in anatomically correct geometries in order to optimise the design of end-to-side bypass graft vessels in clinical surgery. In the Study A, three-dimensional computations of steady flows in planar and non-planar double bends with 8 = 0.25 (curvature ratio) at Reynolds numbers of 125 and 500 were performed using the Navier-Stokes solver called Nektar that is based on spectral/hp element methods. The numerical haemodynamics analysis is presented in terms of the various mechanical factors which primarily involve axial velocity, transverse flows, vorticity, coherent vertical structure and wall shear stress. From these results, we can anticipate the wall shear stress distributions and secondary flow patterns in various double bend geometries with different non-planarity at low Reynolds numbers. Non-planarity plays a significant role on the wall shear stress distribution and the mixing properties of flow in the double bends, both of which are believed to be important factors for the patency of bypass grafts. In the Study B, Three sets of MRI data from patients undergoing tunnelled or superficial femoral bypass surgery were processed to give input velocity waveforms and geometries. From the latter and using the same Navier Stokes solver, detailed patient-specific pulsatile haemodynamics were calculated. Wall shear stress and velocity are influenced by the anatomical geometry, and wall shear stress distributions in pulsatile flow are compared with those in steady flow. The correlation is discussed between the haemodynamics and the remodelling of the vessel following bypass surgery determined in follow-up studies carried out a few months or a few years later. The results suggest that better designs should be possible for bypass grafts and indicate how inflow conditions can affect the flow field. The findings imply that proper anastomotical configurations might, induce haemodynamics environments that may prevent cardiovascular disorders or delay the progression of vascular disease.