Low wall shear stress predicts subsequent development of wall hypertrophy in lower limb bypass grafts

Background: Venous grafts commonly develop myointimal hyperplasia, which can lead to stenoses and, ultimately, with expression of adhesion molecules, lumenal occlusion. The aim of the present study was to investigate whether wall shear stress measured post-operatively would predict subsequent myointimal hypertrophy in lower limb venous bypass grafts. Methods: Magnetic resonance imaging and ultrasound were performed in a cohort of patients following lower limb venous bypass graft surgery for peripheral arterial disease at baseline (1-2 weeks) and at follow-up (9-12 months). Wall shear stress was determined at baseline using computational fluid dynamics techniques and intima-media thickness along the length of the graft was measured by ultrasound at baseline and follow up. Results: Complete follow-up was possible in eight patients, in whom low wall shear stress at baseline predicted high intima-media thickness. The relationship between wall shear stress (WSS) and intima-media thickness (IMT) was curvilinear with IMT increasing sharply at lower levels of WSS (IMT >1.0 mm at <0.3 Pa). Conclusions: Low wall shear stress is associated with subsequent increase in myointimal thickness in lower limb venous bypass grafts. This is believed to be the first prospective study in humans to demonstrate the relationship between low wall shear stress and myointimal thickening and indicates a likely causative role for low wall shear stress in the development of myointimal hyperplasia. © 2009 Association for Research into Arterial Structure and Physiology.Published versio

CFD analysis of microchannel emulsification: Droplet generation process and size effect of asymmetric straight flow-through microchannels

This is the author’s version of a work that was accepted for publication in Chemical Engineering Science. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Chemical Engineering Science, Volume 66, Issue 22, DOI 10.1016/j.ces.2011.07.061Asymmetric straight flow-through microchannel (MC) arrays are high-performance MC emulsification devices for stable mass production of uniform droplets. This paper presents computational fluid dynamics (CFD) simulation and analysis of the generation of soybean oil-in-water emulsion droplets via asymmetric straight flow-through MCs, each consisting of a microslot and a narrow MC. We also used CFD to investigate the effects of the channel size and the flow of the dispersed phase on MC emulsification using asymmetric straight flow-through MCs with a characteristic channel size of 5 to 400 μm. The overall shape of an oil-water interface and the time scale during droplet generation via a control asymmetric straight flow-through MC were appropriately simulated. Better insight was obtained on the flow profile of the two phases and the internal pressure balance of the dispersed phase during droplet generation. Comparison of the CFD and experiment results also provided insight into dynamic interfacial tension during droplet generation. Successful droplet generation was observed below a critical dispersed-phase velocity. In this case, the resultant droplet size was proportional to the channel size and was not sensitive to the dispersed-phase velocity applied. The maximum droplet generation rate per channel was inversely proportional to the channel size, unless the buoyancy force did not promote droplet detachment. The maximum droplet productivity per unit area of an asymmetric straight flow-through MC array was estimated to be constant, regardless of channel size

Computational modelling of metal extrusion and forging processes

The computational modelling of extrusion and forging processes is now well established. There are two main approaches: Lagrangian and Eulerian. The first has considerable complexities associated with remeshing, especially when the code is parallelised. The second approach means that the mould has to be assumed to be entirely rigid and this may not be the case. In this paper, a novel approach is described which utilises finite volume methods on unstructured meshes. This approach involves the solution of free surface non-Newtonian fluid flow equations in an Eulerian context to track the behaviour of the workpiece and its extrusion/forging, and the solution of the solid mechanics equations in the Lagrangian context to predict the deformation/stress behaviour of the die. Test cases for modelling extrusion and forging problems using this approach will be presented