Publications
401 results found
Cantwell CD, Sherwin SJ, Kirby RM, et al., 2010, From h to p efficiently: Strategy selection for operator evaluation on hexahedral and tetrahedral elements, Computers and Fluids, Vol: 43, Pages: 23-28
Vincent PE, Sherwin SJ, Weinberg PD, 2010, The Effect of the Endothelial Glycocalyx Layer on Concentration Polarisation of Low Density Lipoprotein in Arteries, Journal of Theoretical Biology, Vol: 265, Pages: 1-17, ISSN: 0022-5193
It has been postulated that a flow-dependent (and hence spatially varying) low density lipoprotein (LDL) concentration polarisation layer forms on the luminal surface of the vascular endothelium. Such a layer has the potential to cause heterogeneity in the distribution of atherosclerotic lesions by spatially modulating the rate of LDL transport into the arterial wall. Theoretical analysis suggests that a transmural water flux which is spatially heterogeneous at the cellular scale can act to enhance LDL concentration polarisation in a shear dependent fashion. However, such an effect is only observed if a relevant Peclet number (i.e. the ratio of LDL convection to LDL diffusion) is of order unity or greater. Based on the diffusivity of LDL in blood plasma, such a Peclet number is found to be far less than unity, implying that the aforementioned enhancement and shear dependence will not occur. However, this conclusion ignores the existence of the endothelial glycocalyx layer (EGL), which may inhibit the diffusion of LDL near the luminal surface of the endothelium, and hence raise any Peclet number associated with the transport of LDL. The present study numerically investigates the effect of the EGL, as well as a heterogeneous transmural water flux, on arterial LDL concentration polarisation. Particular attention is paid to measures of LDL concentration polarisation thought relevant to the rate of transendothelial LDL transport. It is demonstrated that an EGL is unlikely to cause any additional shear dependence of such measures directly, irrespective of whether or not LDL can penetrate into the EGL. However, it is found that such measures depend significantly on the nature of the interaction between LDL and the EGL (parameterised by the height of the EGL, the depth to which LDL penetrates into the EGL, and the diffusivity of LDL in the EGL). Various processes may regulate the interaction of LDL with the EGL, possibly in a flow dependent and hence spatially non-uniform f
Vos PEJ, Sherwin SJ, Kirby RM, 2010, From h to p efficiently: Implementing finite and spectral/hp element methods to achieve optimal performance for low- and high-order discretisations, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 229, Pages: 5161-5181, ISSN: 0021-9991
Carmo BS, Meneghini JR, Sherwin SJ, 2010, Possible states in the flow around two circular cylinders in tandem with separations in the vicinity of the drag inversion spacing, PHYSICS OF FLUIDS, Vol: 22, ISSN: 1070-6631
Carmo BS, Meneghini JR, Sherwin SJ, 2010, Secondary instabilities in the flow around two circular cylinders in tandem, JOURNAL OF FLUID MECHANICS, Vol: 644, Pages: 395-431, ISSN: 0022-1120
Sharma AS, Abdessemed N, Sherwin SJ, et al., 2010, Transient growth mechanisms of low Reynolds number flow over a low-pressure turbine blade, Theoretical and Computational Fluid Dynamics, ISSN: 0935-4964
A direct transient growth analysis for three-dimensional perturbations to flow past a periodic array of T-106/300 low-pressure turbine fan blades is presented. The methodology is based on a singular value decomposition of the flow evolution operator, linearised about a steady or periodic base flow. This analysis yields the optimal growth modes. Previous work on global mode stability analysis of this flow geometry showed the flow is asymptotically stable, indicating a non-modal explanation of transition may be more appropriate. The present work extends previous investigations into the transient growth around a steady base flow, to higher Reynolds numbers and periodic base flows. It is found that the notable transient growth of the optimal modes suggests a plausible route to transition in comparison to modal growth for this configuration. The spatial extent and localisation of the optimal modes is examined and possible physical triggering mechanisms are discussed. It is found that for longer times and longer spanwise wavelengths, a separation in the shear layer excites the wake mode. For shorter times and spanwise wavelengths, smaller growth associated with excitation of the near wake are observed.
Alastruey J, Sherwin SJ, Parker KH, et al., 2010, Reply to 'Cord clamp insult may predispose to SIDS', EARLY HUMAN DEVELOPMENT, Vol: 86, Pages: 67-67, ISSN: 0378-3782
Mao X, Sherwin SJ, 2010, Spectra of Swirling Flow, Seventh IUTAM Symposium on Laminar-Turbulent Transition, Publisher: Springer Netherlands, Pages: 247-252, ISBN: 9789048137220
Clavica F, Alastruey J, Borlotti A, et al., 2010, One-dimensional computational model of pulse wave propagation in the human bronchial tree, 32nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Buenos Aires, Argentina
Alastruey J, Parker KH, Peiro J, et al., 2009, Analysing the pattern of pulse waves in arterial networks: a time-domain study, JOURNAL OF ENGINEERING MATHEMATICS, Vol: 64, Pages: 331-351, ISSN: 0022-0833
Alastruey J, Sherwin SJ, Parker KH, et al., 2009, Placental transfusion insult in the predisposition for SIDS: A mathematical study, EARLY HUMAN DEVELOPMENT, Vol: 85, Pages: 455-459, ISSN: 0378-3782
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- Citations: 6
Abdessemed N, Sherwin SJ, Theofilis V, 2009, Linear instability analysis of low-pressure turbine flows, JOURNAL OF FLUID MECHANICS, Vol: 628, Pages: 57-83, ISSN: 0022-1120
Kazakidi A, Sherwin SJ, Weinberg PD, 2009, Effect of Reynolds number and flow division on patterns of haemodynamic wall shear stress near branch points in the descending thoracic aorta., J R Soc Interface, Vol: 6, Pages: 539-548
Atherosclerotic lesions are non-uniformly distributed at arterial bends and branch sites, suggesting an important role for haemodynamic factors, particularly wall shear stress (WSS), in their development. The pattern of lesions at aortic branch sites depends on age and species. Using computational flow simulations in an idealized model of an intercostal artery emerging perpendicularly from the thoracic aorta, we studied the effects of Reynolds number and flow division under steady conditions. Patterns of flow and WSS were strikingly dependent on these haemodynamic parameters. With increasing Reynolds number, WSS, normalized by the fully developed aortic value, was lowered at the sides of the ostium and increased upstream and downstream of it. Increasing flow into the side branch exacerbated these patterns and gave rise to a reversing flow region downstream of the ostium. Incorporation of more realistic geometric features had only minor effects and patterns of mean WSS under pulsatile conditions were similar to the steady flow results. Aspects of the observed WSS patterns correlate with, and may explain, some but not all of the lesion patterns in human, rabbit and mouse aortas.
Grinberg L, Pekurovsky D, Sherwin SJ, et al., 2009, Parallel performance of the coarse space linear vertex solver and low energy basis preconditioner for spectral/hp elements, PARALLEL COMPUTING, Vol: 35, Pages: 284-304, ISSN: 0167-8191
Moore P, Barlis P, Spiro J, et al., 2009, A Randomized Optical Coherence Tomography Study of Coronary Stent Strut Coverage and Luminal Protrusion With Rapamycin-Eluting Stents, Journal of the American College of Cardiology Cardiovascular Interventions, Vol: 2, Pages: 437-444, ISSN: 1936-8798
Objectives We used optical coherence tomography, which has a resolution of <20 mu m, to analyze thin layers of neointima in rapamycin-eluting coronary stents.Background Lack of neointimal coverage has been implicated in the pathogenesis of drug-eluting coronary stent thrombosis. Angiography and intracoronary ultrasound lack the resolution to examine this.Methods We conducted a randomized trial in patients receiving polymer-coated rapamycin-eluting stents (Cypher, Cordis, Johnson & Johnson, Miami, Florida) and nonpolymer rapamycin-eluting stents (Yukon, Translumina, Hechingen, Germany) to examine neointimal thickness, stent strut coverage, and protrusion at 90 days. Twenty-four patients (n = 12 for each group) underwent stent deployment and invasive follow-up at 90 days with optical coherence tomography. The primary end point was binary stent strut coverage. Coprimary end points were neointimal thickness and stent strut luminal protrusion.Results No patient had angiographic restenosis. For polymer-coated and nonpolymer rapamycin-eluting stents, respectively, mean (SD), neointimal thickness was 77.2 (25.6) mu m versus 191.2 (86.7) mu m (p < 0.001). Binary stent strut coverage was 88.3% (11.8) versus 97.2% (6.1) (p = 0.030). Binary stent strut protrusion was 26.5% (17.5) versus 4.8% (8.6) (p = 0.001).Conclusions Mean neointimal thickness for the polymer-coated rapamycin-eluting stent was significantly less than the nonpolymer rapamycin-eluting stent but as a result coverage was not homogenous, with >10% of struts being uncovered. High-resolution imaging allowed development of the concept of the protrusion index, and >25% of struts protruded into the vessel lumen with the polymer-coated rapamycin-eluting stent compared with <5% with the nonpolymer rapamycin-eluting stent. These findings may have important implications for the risk of stent thrombosis and, therefore, future stent design. (An optical coherence tomography study to determine stent coverag
Vincent PE, Sherwin SJ, Weinberg PD, 2009, The Effect of a Spatially Heterogeneous Transmural Water Flux on Concentration Polarization of Low Density Lipoprotein in Arteries, Biophysical Journal, Vol: 96, Pages: 3102-3115, ISSN: 0006-3495
Uptake of low density lipoprotein (LDL) by the arterial wall is likely to play a key role in atherogenesis. A particular process that may cause vascular scale heterogeneity in the rate of transendothelial LDL transport is the formation of a flow-dependent LDL concentration polarization layer on the luminal surface of the arterial endothelium. In this study, the effect of a spatially heterogeneous transmural water flux (that traverses the endothelium only via interendothelial cell clefts) on such concentration polarization is investigated numerically. Unlike in previous investigations, realistic intercellular cleft dimensions are used here and several values of LDL diffusivity are considered. Particular attention is paid to the spatially averaged LDL concentration adjacent to different regions of the endothelial surface, as such measures may be relevant to the rate of transendothelial LDL transport. It is demonstrated in principle that a heterogeneous transmural water flux can act to enhance such measures, and cause them to develop a shear dependence (in addition to that caused by vascular scale flow features, affecting the overall degree of LDL concentration polarization). However, it is shown that this enhancement and additional shear dependence are likely to be negligible for a physiologically realistic transmural flux velocity of 0.0439 mu m s(-1) and an LDL diffusivity (in blood plasma) of 28.67 mu m(2) s(-1). Hence, the results imply that vascular scale studies of LDL concentration polarization are justified in ignoring the effect of a spatially heterogeneous transmural water flux.
Abdessemed N, Sharma AS, Sherwin SJ, et al., 2009, Transient growth analysis of the flow past a circular cylinder, PHYSICS OF FLUIDS, Vol: 21, ISSN: 1070-6631
de Vecchi A, Sherwin SJ, Graham JMR, 2009, Wake Dynamics of External Flow Past a Curved Circular Cylinder with the Free-Stream Aligned to the Plane of Curvature, IUTAM Bookseries, Pages: 175-185
The fundamental mechanism of vortex shedding past a curved cylinder has been investigated at a Reynolds number of 100 using three-dimensional spectral/hp computations. Two different configurations are presented herein:in both cases the main component of the geometry is a circular cylinder whose centreline is a quarter of a ring and the inflow direction is parallel to the plane of curvature. In the first set of simulations the cylinder is forced to transversely oscillate at a fixed amplitude, while the oscillation frequency has been varied around the Strouhal value. Both geometries exhibit in-phase vortex shedding, with the vortex cores bent according to the body's curvature, although the wake topology is markedly different. In particular, the configuration that was found to suppress the vortex shedding in absence of forced motion exhibits now a primary instability in the near wake. A second set of simulations has been performed imposing an oscillatory roll to the curved cylinder, which is forced to rotate transversely around the axis of its bottom section. This case shows entirely different wake features from the previous one: the vortex shedding appears to be out-of-phase along the body's span, with straight cores that tend to twist after being shed and manifest a secondary spanwise instability. Further, the damping effect stemming from the transverse planar motion of the part of the cylinder parallel to the flow is no longer present, leading to a positive energy transfer from the fluid to the structure.
Denis Doorly, Spencer Sherwin, 2009, Geometry and Flow, Cardiovascular Mathematics, Editors: Quarteroni, Formaggia, Veneziani, Publisher: Springer
Vincent PE, Hunt AAE, Grinberg L, et al., 2009, A Realistic Representation of the Rabbit Aorta for use in Computational Haemodynamic Studies, ASME Summer Bioengineering Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 985-986
Alastruey J, Parker K, Peiro J, et al., 2009, Analysing the pattern of pulse waves in arterial networks: A time Domain Study, Journal of Engineering Mathematics, Vol: Submitted
Cookson A, 2009, Computational Investigation of Helical Pipe Geometries From a Mixing Perspective
Recent research on small amplitude helical pipes for use as bypass grafts and arterio- venous shunts suggest that in-plane mixing induced by the geometry may help pre- vent occlusion by thrombosis. In this thesis, a coordinate transformation of the Navier-Stokes equations is solved within a spectral/hp element framework to study the flow field and mixing behaviour of small-amplitude helical pipes. An apparent discrepancy between the flow field and particle trajectories is observed, whereby particle paths display a pattern characteristic of a double vortex, though the flow field reveals only a single dominant vortex. It is shown that a combination of trans- lational and rotational reference frames changes resolves this discrepancy.It is then proposed that joining together two helical geometries, of differing helical radii, will enhance mixing, through the phenomenon of ‘streamline crossing’. An idealised prediction of the mixing is obtained by concatenating the velocity field solutions from the respective single helical geometries. The mixing is examined using Poincar ́e sections, residence time data and information entropy. The flow is then solved for those combined geometries showing the most improvement in mixing, with a 70% increase in mixing efficiency achieved, with only a small increase in pressure loss. It is found that although the true velocity fields vary significantly from the prediction, the overall mixing behaviour is captured, allowing the use of the idealised prediction for guiding future designs of combined geometries.
Vincent P, 2009, A Cellular Scale Study of Low Denisty Lipoprotein Concentration Polarisation in Arteries
Uptake of Low Density Lipoprotein (LDL) by the arterial wall is likely to play a key role in the process of atherogenesis, which occurs non-uniformly within the ar- terial vasculature. A particular process that may cause vascular scale heterogeneity in the rate of transendothelial LDL transport is the formation of a flow-dependent LDL concentration polarisation layer adjacent to the luminal surface of the arte- rial endothelium. In this thesis the effects of cellular scale endothelial features on such LDL concentration polarisation are investigated using an idealised theoretical model. Specifically, the effect of a spatially heterogeneous transmural water flux is considered (flowing only through intercellular clefts), as well as the effect of the endothelial glycocalyx layer (EGL). The idealised model is implemented using both analytical techniques and the spectral/hp element method. A range of scenarios are considered, including those were no EGL is present, those where an EGL is present but LDL cannot penetrate into it, and finally those where an EGL is present and LDL can penetrate into it.For cases where no EGL is present, particular attention is paid to the spatially averaged LDL concentration adjacent to various regions of the endothelial surface, as such measures may be relevant to the rate of transendothelial LDL transport. It is demonstrated, in principle, that a heterogeneous transmural water flux alone can act to enhance such measures, and cause them to develop a shear dependence (in addition to that caused by vascular scale flow features affecting the overall degree of LDL concentration polarisation). However, it is shown that this enhancement and additional shear dependence are likely to be negligible for a physiologically realistictransmural flux velocity of 0.0439μms−1 and an LDL diffusivity in blood plasma of 28.67μm2 s−1 .For cases where an EGL is present, measures of LDL concentration polarisation relevant to the rate of transendo
deVecchi A, Sherwin SJ, Graham JMR, 2009, Wake dynamics past a curved body of circular cross-section under forced cross-flow vibration, Journal of Fluids and Structures, Vol: 25, Pages: 721-730, ISSN: 0889-9746
Clavica F, Alastruey J, Khir AW, et al., 2009, One-dimensional modelling of pulse wave propagation in human airway bifurcations in space-time variables, 31st Annual International IEEE Engineering in Medicine and Biology Conference, Minneapolis, Minnesota, USA
Arimon JA, Balossino R, D'Angelo C, et al., 2009, Applications and test cases, CARDIOVASCULAR MATHEMATICS: MODELING AND SIMULATION OF THE CIRCULATORY SYSTEM, Editors: Formaggia, Quarteroni, Veneziani, Publisher: SPRINGER, Pages: 447-475, ISBN: 978-88-470-1151-9
de Vecchi A, Sherwin SJ, Graham JMR, 2009, Wake Dynamics of External Flow Past a Curved Circular Cylinder with the Free-Stream Aligned to the Plane of Curvature, IUTAM Symposium on Unsteady Separated Flows and their Control, Publisher: SPRINGER, Pages: 175-185
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- Citations: 1
Peiro J, Sherwin SJ, Giordana S, 2008, Automatic reconstruction of a patient-specific high-order surface representation and its application to mesh generation for CFD calculations, MEDICAL & BIOLOGICAL ENGINEERING & COMPUTING, Vol: 46, Pages: 1069-1083, ISSN: 0140-0118
de Vecchi A, Sherwin SJ, Graham JMR, 2008, Wake dynamics of external flow past a curved circular cylinder with the free-stream aligned to the plane of curvature, IUTAM Symposium on Unsteady Separated Flows and their Control, Publisher: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, Pages: 1262-1270, ISSN: 0889-9746
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- Citations: 14
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