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  • Conference paper
    Peiffer V, Sherwin SJ, 2012,

    CFD CHALLENGE: SOLUTIONS USING AN IN-HOUSE SPECTRAL ELEMENT SOLVER, NEKTAR

    , ASME Summer Bioengineering Conference (SBC), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 145-146
  • Journal article
    Vincent PE, Plata AM, Hunt AAE, Weinberg PD, Sherwin SJet al., 2011,

    Blood Flow in the Rabbit Aortic Arch and Descending Thoracic Aorta

    , Journal of the Royal Society Interface, Vol: 8, Pages: 1708-1719, ISSN: 1742-5689

    The distribution of atherosclerotic lesions within the rabbit vasculature, particularly within the descending thoracic aorta, has been mapped in numerous studies. The patchy nature of such lesions has been attributed to local variation in the pattern of blood flow. However, there have been few attempts to model and characterize the flow. In this study, a high-order continuous Galerkin finite-element method was used to simulate blood flow within a realistic representation of the rabbit aortic arch and descending thoracic aorta. The geometry, which was obtained from computed tomography of a resin corrosion cast, included all vessels originating from the aortic arch (followed to at least their second generation) and five pairs of intercostal arteries originating from the proximal descending thoracic aorta. The simulations showed that small geometrical undulations associated with the ductus arteriosus scar cause significant deviations in wall shear stress (WSS). This finding highlights the importance of geometrical accuracy when analysing WSS or related metrics. It was also observed that two Dean-type vortices form in the aortic arch and propagate down the descending thoracic aorta (along with an associated skewed axial velocity profile). This leads to the occurrence of axial streaks in WSS, similar in nature to the axial streaks of lipid deposition found in the descending aorta of cholesterol-fed rabbits. Finally, it was observed that WSS patterns within the vicinity of intercostal branch ostia depend not only on local flow features caused by the branches themselves, but also on larger-scale flow features within the descending aorta, which vary between branches at different locations. This result implies that disease and WSS patterns in the vicinity of intercostal ostia are best compared on a branch-by-branch basis.

  • Conference paper
    Peiffer V, Rowland M, Weinberg PD, Sherwin SJet al., 2011,

    Database of rabbit aortic geometries for use in computational flow studies

    , Pages: 19-20
  • Journal article
    Kazakidi A, Plata AM, Sherwin SJ, Weinberg PDet al., 2011,

    Effect of reverse flow on the pattern of wall shear stress near arterial branches

    , JOURNAL OF THE ROYAL SOCIETY INTERFACE, Vol: 8, Pages: 1594-1603, ISSN: 1742-5689
  • Journal article
    Alastruey J, Khir AW, Matthys KS, Segers P, Sherwin SJ, Verdonck PR, Parker KH, Peiro J, Alastruey J, Khir AW, Matthys KS, Segers P, Sherwin SJ, Verdonck PR, Parker KH, Peiro Jet al., 2011,

    Pulse wave propagation in a model human arterial network: Assessment of 1-D visco-elastic simulations against in vitro measurements

    , Journal of Biomechanics, Vol: 44, Pages: 2250-2258, ISSN: 1873-2380

    The accuracy of the nonlinear one-dimensional (1-D) equations of pressure and flow wave propagation in Voigt-type visco-elastic arteries was tested against measurements in a well-defined experimental 1:1 replica of the 37 largest conduit arteries in the human systemic circulation. The parameters required by the numerical algorithm were directly measured in the in vitro setup and no data fitting was involved. The inclusion of wall visco-elasticity in the numerical model reduced the underdamped high-frequency oscillations obtained using a purely elastic tube law, especially in peripheral vessels, which was previously reported in this paper [Matthys et al., 2007. Pulse wave propagation in a model human arterial network: Assessment of 1-D numerical simulations against in vitro measurements. J. Biomech. 40, 3476–3486]. In comparison to the purely elastic model, visco-elasticity significantly reduced the average relative root-mean-square errors between numerical and experimental waveforms over the 70 locations measured in the in vitro model: from 3.0% to 2.5% (p<0.012) for pressure and from 15.7% to 10.8% (p<0.002) for the flow rate. In the frequency domain, average relative errors between numerical and experimental amplitudes from the 5th to the 20th harmonic decreased from 0.7% to 0.5% (p<0.107) for pressure and from 7.0% to 3.3% (p<10−6) for the flow rate. These results provide additional support for the use of 1-D reduced modelling to accurately simulate clinically relevant problems at a reasonable computational cost.

  • Journal article
    Mao X, Sherwin SJ, Blackburn HM, 2011,

    Transient growth and bypass transition in stenotic flow with a physiological waveform

    , THEORETICAL AND COMPUTATIONAL FLUID DYNAMICS, Vol: 25, Pages: 31-42, ISSN: 0935-4964
  • Journal article
    Plata AM, Sherwin SJ, Krams R, 2010,

    Endothelial Nitric Oxide Production and Transport in Flow Chambers: The Importance of Convection

    , ANNALS OF BIOMEDICAL ENGINEERING, Vol: 38, Pages: 2805-2816, ISSN: 0090-6964
  • Journal article
    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

  • Journal article
    Alastruey J, Sherwin SJ, Parker KH, Rubens DDet al., 2010,

    Reply to 'Cord clamp insult may predispose to SIDS'

    , EARLY HUMAN DEVELOPMENT, Vol: 86, Pages: 67-67, ISSN: 0378-3782
  • Conference paper
    Clavica F, Alastruey J, Borlotti A, Sherwin SJ, Khir AWet 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
  • Journal article
    Alastruey J, Parker KH, Peiro J, Sherwin SJet 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
  • Journal article
    Alastruey J, Sherwin SJ, Parker KH, Rubens DDet al., 2009,

    Placental transfusion insult in the predisposition for SIDS: A mathematical study

    , EARLY HUMAN DEVELOPMENT, Vol: 85, Pages: 455-459, ISSN: 0378-3782
  • Journal article
    Moore P, Barlis P, Spiro J, Ghimire G, Roughton M, Di Mario C, Wallis W, Ilsley C, Mitchell A, Mason M, Kharbanda R, Vincent P, Sherwin S, Dalby Met 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

  • Journal article
    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.

  • Journal article
    Alastruey J, Parker K, Peiro J, Sherwin Set al.,

    Analysing the pattern of pulse waves in arterial networks: A time Domain Study

    , Journal of Engineering Mathematics, Vol: Submitted
  • Conference paper
    Vincent PE, Hunt AAE, Grinberg L, Sherwin SJ, Weinberg PDet 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
  • Conference paper
    Clavica F, Alastruey J, Khir AW, Sherwin SJet 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
  • Journal article
    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
  • Journal article
    Khir AW, Sherwin SJ, 2008,

    Special issue on theoretical, computational, and experimental biofluid mechanics

    , PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART H-JOURNAL OF ENGINEERING IN MEDICINE, Vol: 222, Pages: I-I, ISSN: 0954-4119
  • Journal article
    Alastruey J, Parker KH, Peiro J, Sherwin SJ, J Alastruey, KH Parker, J Peiro, SJ Sherwinet al., 2008,

    Lumped parameter outflow models for 1-D blood flow simulations: Effect on pulse waves and parameter estimation

    , Commun. Comput. Phys., Vol: 4, Pages: 317-336

    Several lumped parameter, or zero-dimensional (0-D), models of the micro-circulation are coupled in the time domain to the nonlinear, one-dimensional (1-D) equations of blood flow in large arteries. A linear analysis of the coupled system, together with in-vivo observations, shows that: (i) an inflow resistance that matches the characteristic impedance of the terminal arteries is required to avoid non-physiological wave reflections; (ii) periodic mean pressures and flow distributions in large arteries depend on arterial and peripheral resistances, but not on the compliances and inertias of the system, which only affect instantaneous pressure and flow waveforms; (iii) peripheral inertias have a minor effect on pulse waveforms under normal conditions; and (iv) the time constant of the diastolic pressure decay is the same in any 1-D model artery, if viscous dissipation can be neglected in these arteries, and it depends on all the peripheral compliances and resistances of the system. Following this analysis, we propose an algorithm to accurately estimate peripheral resistances and compliances from in-vivo data. This algorithm is verified against numerical data simulated using a 1-D model network of the 55 largest human arteries, in which the parameters of the peripheral windkessel outflow models are known a-priori. Pressure and flow waveforms in the aorta and the first generation of bifurcations are reproduced with relative root-mean-square errors smaller than 3%.

  • Journal article
    Kazakidi A, Sherwin SJ, Weinberg PD, 2008,

    Effect of Reynolds number and flow division on patterns of haemodynamic wall shear stress near branch points in the descending thoracic aorta

    , Interface, Vol: Submitted

    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 idealised 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, normalised 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 stagnation region downstream of the ostium. Incorporation of more realistic geometric features had only minor effects. 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.

  • Journal article
    Vincent PE, Sherwin SJ, Weinberg PD, 2008,

    Viscous Flow Over Outflow Slits Covered by an Anistropic Brinkman Medium: A Model of Flow Above Inter-Endothelial Cell Clefts

    , Physics of Fluids, Vol: 20
  • Journal article
    Cookson AN, Doorly DJ, Sherwin SJ, 2008,

    Mixing through stirring of steady flow in small amplitude helical pipes

    , Ann. Biomed. Engrg., Vol: 37, Pages: 710-721

    In this paper we numerically simulate flow in a helical tube for physiological conditions using a co-ordinate mapping of the Navier–Stokes equations. Helical geometries have been proposed for use as bypass grafts, arterial stents and as an idealized model for the out-of-plane curvature of arteries. Small amplitude helical tubes are also currently being investigated for possible application as A–V shunts, where preliminary in vivo tests suggest a possibly lower risk of thrombotic occlusion. In-plane mixing induced by the geometry is hypothesized to be an important mechanism. In this work, we focus mainly on a Reynolds number of 250 and investigate both the flow structure and the in-plane mixing in helical geometries with fixed pitch of 6 tube diameters (D), and centerline helical radius ranging from 0.1D to 0.5D. High-order particle tracking, and an information entropy measure is used to analyze the in-plane mixing. A combination of translational and rotational reference frames are shown to explain the apparent discrepancy between flow field and particle trajectories, whereby particle paths display a pattern characteristic of a double vortex, though the flow field reveals only a single dominant vortex. A radius of 0.25D is found to provide the best trade-off between mixing and pressure loss, with little increase in mixing above R = 0.25D, whereas pressure continues to increase linearly.

  • Journal article
    Alastruey J, Moore SM, Parker KH, David T, Peiro J, Sherwin SJ, J Alastruey, SM Moore, KH Parker, T David, J Peiro, SJ Sherwinet al., 2008,

    Reduced modelling of blood flow in the cerebral circulation: Coupling 1-D, 0-D and cerebral auto-regulation models

    , Int. J. Numer. Meth. Fluids, Vol: 56, Pages: 1061-1067
  • Book chapter
    Denis Doorly, Spencer Sherwin, 2007,

    Geometry and Flow

    , Cardiovascular Mathematics, Editors: Quarteroni, Formaggia, Veneziani, Publisher: Springer
  • Journal article
    Blackburn HM, Sherwin SJ, 2007,

    Instability modes and transition of pulsatile stenotic flow: pulse-period dependence

    , JOURNAL OF FLUID MECHANICS, Vol: 573, Pages: 57-88, ISSN: 0022-1120
  • Conference paper
    Kazakidi A, Sherwin SJ, Weinberg PD, 2007,

    Reverse flow in arterial branches influences wall shear stress patterns around branch ostia

    , Joint Autumn Meeting of the British-Society-for-Cardiovascular-Research/British-Atherosclerosis-Society, Publisher: B M J PUBLISHING GROUP, ISSN: 1355-6037
  • Conference paper
    Vincent PE, Sherwin SJ, Weinberg PD, 2007,

    Computational investigation of a mechanism by which blood flow could control lipoprotein uptake by the arterial

    , Joint Autumn Meeting of the British-Society-for-Cardiovascular-Research/British-Atherosclerosis-Society, Publisher: B M J PUBLISHING GROUP, ISSN: 1355-6037
  • Conference paper
    Lee KE, Parker KH, Sherwin SJ, Car CGet al., 2007,

    The effects of geometrical configurations on steady flow in non-planar double bends

    , World Congress on Medical Physics and Biomedical Engineering, Publisher: SPRINGER-VERLAG BERLIN, Pages: 3461-+, ISSN: 1680-0737
  • Journal article
    Alastruey J, Parker KH, Peiro J, Byrd SM, Sherwin SJet al., 2007,

    Modelling the circle of Willis to assess the effects of anatomical variations and occlusions on cerebral flows

    , JOURNAL OF BIOMECHANICS, Vol: 40, Pages: 1794-1805, ISSN: 0021-9290

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