Nicholas V. Watkins

The Influence of Out-of-Plane Geometry on the Flow Within a Distal End-to-Side Anastomosis

This paper describes a computational and experimental investigation of flow in a proto-type model geometry of a fully occluded 45 deg distal end-to-side anastomosis. Previous investigations have considered a similar configuration where the centerlines of the bypass and host vessels lie within a plane, thereby producing a plane of symmetry within the flow. We have extended these investigations by deforming the bypass vessel out of the plane of symmetry, thereby breaking the symmetry of the flow and producing a nonplanar geometry. Experimental data were obtained using magnetic resonance imaging of flow within perspex models and computational data were obtained from simulations using a high-order spectral/hp element method. We found that the nonplanar three-dimensional flow notably alters the distribution of wall shear stress at the bed of the anastomosis, reducing the peak wall shear stress peak by approximately 10 percent when compared with the planar model. Furthermore, an increase in the absolute flux of velocity into the occluded region, proximal to the anastomosis, of 80 percent was observed in the nonplanar geometry when compared with the planar geometry.

Preliminary comparative study of small amplitude helical and conventional ePTFE arteriovenous shunts in pigs

Intimal hyperplasia (IH), which causes occlusion of arterial bypass grafts and arteriovenous (A-V) shunts, develops preferentially in low wall shear, or stagnation, regions. Arterial geometry is commonly three-dimensional, generating swirling flows, the characteristics of which include in-plane mixing and inhibition of stagnation. Clinical arterial bypass grafts are commonly two-dimensional, favouring extremes of wall shear. We have developed small amplitude helical technology (SwirlGraft) devices and shown them to generate physiological-type swirling flows. Expanded polytetrafluorethylene (ePTFE) grafts, although widely used as A-V shunts for renal dialysis access, are prone to thrombosis and IH. In a small preliminary study in pigs, we have implanted SwirlGraft ePTFE carotid artery-to-jugular vein shunts on one side and conventional ePTFE carotid artery-to-jugular vein shunts contralaterally. There was consistently less thrombosis and IH in the SwirlGraft than conventional shunts. At eight weeks (two animals), the differences were marked, with virtually no disease in the SwirlGraft devices and occlusion of the conventional grafts by thrombosis and IH. The study had limitations, but the lesser pathology in the SwirlGraft devices is likely to have resulted from their geometry and the associated swirling flow. The results could have implications for vascular biology and prolongation of the patency of arterial bypass grafts and A-V shunts.

Steady inspiratory flow in planar and non–planar models of human bronchial airways

Swirling flow associated with non–planar arterial geometry encourages interest in flow in larger human bronchial airways, where bifurcations are planar but consecutive bifurcation planes rotate by an angle (φ) of ca.90°. Steady ‘inspiratory’ flow has been investigated in a two–generation symmetrically bifurcating human bronchial airway model by studying reddening by acid vapour of a litmus–containing coating as an approximate indicator of relative local wall shear (Sw). The inlet tube Reynolds number (Reit) was 600 or 1800; the branching angle (θ) was 32.5° at first generation and 32.5° or 55° at second generation; φ was 0° or 90° between first and second generations; second–generation daughter tube volume flow rates were the same. With φ = 0°, Sw distribution between second–generation daughters was non–uniform. With φ = 90°, Sw distribution between second–generation daughters was uniform and flows were swirling with pitch λ With φ = 90° and Reit given, increase of θ reduced λ and increased Sw. With φ = 90° and θ given, increase of Reit reduced λ Inspiratory flow in larger human bronchial airways is expected to be asymmetric and swirling, with implications for all transport processes including those of particles. The study may have implications for the design of general piping systems.