341 results found
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
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
Barkley D, Blackburn HM, Sherwin SJ, 2008, Direct optimal growth analysis for timesteppers, 14th International Conference on Finite Elements in Flow Problems, Pages: 1435-1458
Methods are described for transient growth analysis of flows with arbitrary geometric complexity, where in particular the flow is not required to vary slowly in the streamwise direction. Emphasis is on capturing the global effects arising from localized convective stability in streamwise-varying flows. The methods employ the 'timestepper's approach' in which a nonlinear Navier-Stokes code is modified to provide evolution operators for both the forward and adjoint linearized equations. First, the underlying mathematical treatment in primitive flow variables is presented. Then, details are given for the inner level code modifications and outer level eigenvalue and SVD algorithms in the timestepper's approach. Finally, some examples are shown and guidance provided on practical aspects of this type of large-scale stability analysis. Copyright (C) 2008 John Wiley & Sons, Ltd.
D Barkley, HM Blackburn, SJ Sherwin, 2008, Direct optimal growth analysis for timesteppers, International Journal for Numerical Methods in Fluids, Vol: 57, Pages: 1435-1458, ISSN: 0271-2091
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
Pedley TJ, 2008, Special issue on theoretical, computational, and experimental biofluid mechanics. Foreword., Proc Inst Mech Eng H, Vol: 222, Pages: i-iv, ISSN: 0954-4119
Broadhurst MS, Sherwin SJ, 2008, Helical instability and breakdown of a batchelor trailing vortex, 14th European Conference for Mathematics in Industry, Publisher: SPRINGER-VERLAG BERLIN, Pages: 191-195
Sharma AS, Abdessemed N, Sherwin S, et al., 2008, Optimal growth of linear perturbations in low pressure turbine flows, IUTAM Symposium on Flow Control and MEMS, Publisher: SPRINGER, Pages: 339-+
Khir AW, Sherwin SJ, Pedley TJ, 2008, Guest Editorial, Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, Vol: 222, Pages: i-iv, ISSN: 0954-4119
Over the last 50 years, developments in medical imaging, experimental techniques, and computational power have continued to advance research into physiological fluid dynamics. Almost all the topics covered by the papers in this Special Issue would have been recognizable to the researchers forty years ago, but in (almost all) cases the methods used would not, both for the technical reasons referred to above and for genuine advances in scientific insight. For example, the first two papers are concerned with the arterial pressure and both show, rather surprisingly, that Otto Frank's 1899 Windkessel model is still the best way of relating diastolic arterial blood pressure to the flowrate through the microcirculation; however, the first paper shows that more insight into pulse propagation can be gained from Parker's wave impulse analysis than from the traditional Fourier analysis. It appears that, despite its longevity, pulse wave analysis is not dead and is still capable of providing new insights.
de Vecchi A, Sherwin SJ, Graham JMR, 2008, FLOW PAST A CURVED PIPE IN DIFFERENT IN-LINE CONFIGURATIONS UNDERGOING FORCED TRANSVERSE OSCILLATIONS, 9th International Conference on Flow-Induced Vibration, Publisher: ACAD SCI CZECH REPUBLIC, INST THERMOMECHANICS, Pages: 413-418
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
Vos PEJ, van Loon R, Sherwin SJ, 2008, A comparison of fictitious domain methods appropriate for spectral/hp element discretisations, Symposium on the Immersed Boundary Method and its Extensions held at the 7th World Congress on Computational Mechanics, Pages: 2275-2289
The fictitious domain, finite cell and fat boundary method, which are commonly adopted for fluid-structure interaction (FSI) problems, have been and implemented in a spectral/hp element code for a 1D test problem. The finite cell and fat boundary method are extended for the general case of a deformable solid in a fluid, and analysed. Furthermore, a new implicit variation of the fat boundary method is proposed. The 1D framework allows for a clear and detailed analysis of the different approaches and to highlight the similarities and differences between the methods. Exponential p-convergence, which is typical for smooth solutions discretised with spectral/hp elements, is demonstrated for all methods except the classic fictitious domain formulation. This overview can be considered as a starting point to study the capability, advantages and disadvantages of these methods for fluid-structure interaction problems in a more realistic setting and in higher dimensions. (C) 2007 Elsevier B.V. All rights reserved.
Alastruey J, Moore SM, Parker KH, et 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
Blackburn HM, Sherwin SJ, Barkley D, 2008, Convective instability and transient growth in steady and pulsatile stenotic flows, Journal of Fluid Mechanics, Vol: 607, Pages: 267-277, ISSN: 0022-1120
We show that suitable initial disturbances to steady or long-period pulsatile flows in a straight tube with a 75%-occlusion axisymmetric stenosis can produce very large transient energy growths. The global optimal disturbances to an initially axisymmetric state found by linear analyses are three-dimensional wave packets that produce localized sinuous convective instability in extended shear layers. In pulsatile flow, initial conditions that trigger the largest disturbances are either initiated at, or advect to, the separating shear layer at the stenosis in phase with peak systolic flow.
Vos PEJ, van Loon R, Sherwin SJ, 2008, A comparison of fictitious domain methods appropriate for spectral/hp element discretisations, Comp. Meth. Appl. Mech. Engrg, Vol: 197, Pages: 2275-2289
Bruno S Carmo, Spencer J Sherwin, Peter W Bearman, et al., 2008, Wake Transition in the flow around two circular cylinders in staggered arrangements, Journal of Fluid Mechanics, Vol: 597, Pages: 1-29
The wake transition of the flow around two circular cylinders placed in staggered arrangements with fixed streamwise separation of 5D and cross stream separation varying from 0D to 3D has been studied. The wake transition is compared to that of a single isolated cylinder. Linear stability analysis utilising Floquet theory and direct numerical simulations using a spectral/hp element spatial discretisation were carried out. The unstable modes that first appear in the wake transition of the flow around a single cylinder, which are the long spanwise wavelength mode A and the short spanwise wavelength mode B, are also found in the flow around the staggered arrangements. However, a third mode, referred to as mode C, is also present in the wake transition of the flow around staggered arrangements, depending on the relative positioning of the cylinders. This mode has an intermediate spanwise wavelength and period-doubling character. The structure and onset characteristics of mode C are analysed and the non-linear character of the bifurcation for this mode is investigated.
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.
Sharma AS, Abdessemed N, Sherwin SJ, et al., 2008, Optimal Growth of Linear Perturbations in Low Pressure Turbine Flows, Pages: 339-343
Alastruey J, Parker KH, Peiro J, et 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%.
Michael S Broadhurst, Spencer J Sherwin, 2008, The Parabolised Stability Equations for 3D-Flows: Implementation and Numerical Stability, Applied Numerical Mathematics, Vol: 58, Pages: 1017-1029
The numerical implementation of the parabolised stability equations (PSE) using a spectral/hp-element discretisation is considered, and the numerical stability of the governing equations is presented. Analogous to the primitive variable form of the two-dimensional PSE, the equations are ill-posed; although choosing an Euler implicit scheme in the streamwise z-direction yields a stable scheme for sufficiently large step sizes (Δz>1/|β|, where β is the streamwise wavenumber). The source of the instability is a residual ellipticity that remains in the equations, and presents itself as an upstream propagating acoustic wave. Neglecting this term relaxes the lower limit on the step-size restriction. The θ-scheme is also considered, allowing the step-size restriction of the scheme to be determined. The explicit scheme is always unstable, whereas neglecting the pressure gradient term shows stable eigenspectra for θ>=0.5
Blackbrun HM, Barkley D, Sherwin SJ, 2008, Convective instability and transient growth in flow over a backwards-facing step, J. Fluid Mechanics., Vol: 603, Pages: 271-304
Transient energy growths of two- and three-dimensional optimal linear perturbations to two-dimensional flow in a rectangular backwards-facing-step geometry with expansion ratio two are presented. Reynolds numbers based on the step height and peak inflow speed are considered in the range 0--500, which is below the value for the onset of three-dimensional asymptotic instability. As is well known, the flow has a strong local convective instability, and the maximum linear transient energy growth values computed here are of order 80 x 10^3 at Re=500. The critical Reynolds number below which there is no growth over any time interval is determined to be Re=57.7 in the two-dimensional case. The centroidal location of the energy distribution for maximum transient growth is typically downstream of all the stagnation/reattachment points of the steady base flow. Sub-optimal transient modes are also computed and discussed. A direct study of weakly nonlinear effects demonstrates that nonlinearity is stablizing at Re=500. The optimal three-dimensional disturbances have spanwise wavelength of order ten step heights. While they have slightly larger growths than two-dimensional cases, they are broadly similar in character. When the inflow of the full nonlinear system is perturbed with white noise, narrowband random velocity perturbations are observed in the downstream channel at locations corresponding to maximum linear transient growth. The centre frequency of this response matches that computed from the streamwise wavelength and mean advection speed of the predicted optimal disturbance. Linkage between the response of the driven flow and the optimal disturbance is further demonstrated by a partition of response energy into velocity components.
A Miliou, A De Vecchi, SJ Sherwin, et al., 2007, Wake dynamics of external flow past a curved circular cylinder with the free-stream aligned to the plane of curvature, Journal of Fluid Mechanics, Vol: 592, Pages: 89-115
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
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
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
Alastruey J, Parker KH, Peiro J, et 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
Lee KE, Parker KH, Sherwin SJ, et 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
van Loon R, Anderson PD, van de Vosse FN, et al., 2007, Comparison of various fluid–structure interaction methods for deformable bodies, Computers and Structures, Vol: 85, Pages: 833-843, ISSN: 0045-7949
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