332 results found
Alastruey J, Parker KH, Sherwin SJ, 2012, Arterial pulse wave haemodynamics, Pages: 401-442
The shape of the arterial pulse wave is intimately related to the physical properties of the cardiovascular system. Understanding the mechanisms underlying this relation is clinically relevant, since pulse waveforms carry valuable information for the diagnosis and treatment of disease. We overview some numerical, theoretical and experimental efforts (using in vivo and in vitro data) made in this field of research, focusing on the physical aspects of arterial pulse wave propagation in the systemic circulation. The mathematical and numerical tools that we describe are based on the one-dimensional formulation in the time-domain. © BHR Group 2012 Pressure Surges 11.
Mao X, Blackburn HM, Sherwin SJ, 2012, Optimal inflow boundary condition perturbations in steady stenotic flow, JOURNAL OF FLUID MECHANICS, Vol: 705, Pages: 306-321, ISSN: 0022-1120
Peiffer V, Rowland EM, Cremers SG, et al., 2012, Effect of aortic taper on patterns of blood flow and wall shear stress in rabbits: Association with age, ATHEROSCLEROSIS, Vol: 223, Pages: 114-121, ISSN: 0021-9150
Mao X, Sherwin SJ, Blackburn HM, 2012, Non-normal dynamics of time-evolving co-rotating vortex pairs, JOURNAL OF FLUID MECHANICS, Vol: 701, Pages: 430-459, ISSN: 0022-1120
Mao X, Sherwin SJ, 2012, Transient growth associated with continuous spectra of the Batchelor vortex, JOURNAL OF FLUID MECHANICS, Vol: 697, Pages: 35-59, ISSN: 0022-1120
Kirby RM, Sherwin SJ, Cockburn B, 2012, To CG or to HDG: A Comparative Study, JOURNAL OF SCIENTIFIC COMPUTING, Vol: 51, Pages: 183-212, ISSN: 0885-7474
Alastruey J, Siggers JH, Peiffer V, et al., 2012, Reducing the data: Analysis of the role of vascular geometry on blood flow patterns in curved vessels, PHYSICS OF FLUIDS, Vol: 24, ISSN: 1070-6631
De Luca A, Warboys CM, Amini N, et al., 2012, IMAGE-BASED COMPUTATIONAL HEMODYNAMICS AND MICROARRAY ANALYSIS OF THE PORCINE AORTIC ARCH REVEALS A CORRELATION BETWEEN SHEAR STRESS AND ENDOTHELIAL CELL APOPTOSIS, PROCEEDINGS OF THE ASME SUMMER BIOENGINEERING CONFERENCE, PTS A AND B, Pages: 923-924
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
Peiffer V, Rowland EM, Cremers SG, et al., 2012, AGE-RELATED DIFFERENCES IN HAEMODYNAMICS OF THE RABBIT AORTA AND COMPARISON WITH AVERAGE MAPS OF ATHEROSCLEROTIC LESION PREVALENCE, ASME Summer Bioengineering Conference (SBC), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 495-496
Peiffer V, Weinberg PD, Sherwin SJ, 2012, THE WALL SHEAR STRESS VECTOR: METHODS FOR CHARACTERISING TRULY DISTURBED FLOW, ASME Summer Bioengineering Conference (SBC), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 21-22
Markall GR, Slemmer A, Ham DA, et al., 2012, Finite element assembly strategies on multi- and many-core architectures, International Journal for Numerical Methods in Fluids
Vincent PE, Plata AM, Hunt AAE, et 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.
Peiffer V, Rowland M, Weinberg PD, et al., 2011, Database of rabbit aortic geometries for use in computational flow studies, Pages: 19-20
Zhang Y, Zaki TA, Sherwin SJ, et al., 2011, Nonlinear response of a laminar boundary layer to isotropic and spanwise localized free-stream turbulence, 6th AIAA Theoretical Fluid Mechanics Conference
This paper is concerned with the nonlinear response of a pre-transitional flat-plate boundary layer to isotropic and spanwise localized free-stream turbulence (FST). The turbulence is represented as a superposition of Fourier modes and the displacement effect of the boundary layer on FST is taken into consideration. The responses of the boundary layer to FST are low-frequency streamwise streaks, and their development is obtained by numerically solving the nonlinear unsteady boundary-region (NUBR) equations. Direct numerical simulations (DNS) are carried out to validate the results. Nonlinearity is stabilizing in that it reduces the root mean square (rms) of the perturbation velocity in the boundary layer for small FST Reynolds number RL11, while it is destabilizing for large RL11. The issue of upstream-downstream versus top-down mechanisms is investigated. Streaks primarily develop from the upstream forcing; the top-down forcing plays a minor role. The numerical calculations for isotropic FST are compared with DNS results of Ovchinnikov et al. and experimental data of Roach & Brierley. The computed disturbances do not reach the levels in the DNS and experiment. However, good quantitative agreement is obtained when the anisotropy of FST induced by the blunt leading edge is accounted for. The results suggest that the blunt leading edge can play a key role in explaining the large amplitudes of streaks in that it leads to the deviation from pure isotropy of the FST. The numerical calculation for spanwise localized FST is compared with experimental data of Westin et al. Agreement is obtained except for the amplitude of the disturbances, which is due to the lack of the velocity spectral information of FST in experiment. The viscous secondary instability analysis indicates that there is strong instability in the streaky boundary layer before bypass transition. The maximum growth rate of the unstable modes is larger than that of Tollmien-Schlichting (T-S) waves in the B
Kazakidi A, Plata AM, Sherwin SJ, et 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
Alastruey J, Khir AW, Matthys KS, et 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.
Mao X, Sherwin S, 2011, Continuous spectra of the Batchelor vortex, JOURNAL OF FLUID MECHANICS, Vol: 681, Pages: 1-23, ISSN: 0022-1120
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
Carmo BS, Sherwin SJ, Bearman PW, et al., 2011, Flow-induced vibration of a circular cylinder subjected to wake interference at low Reynolds number, JOURNAL OF FLUIDS AND STRUCTURES, Vol: 27, Pages: 503-522, ISSN: 0889-9746
Gonzalez L, Theofilis V, Sherwin SJ, 2011, High-order methods for the numerical solution of the BiGlobal linear stability eigenvalue problem in complex geometries, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Vol: 65, Pages: 923-952, ISSN: 0271-2091
Vos PEJ, Eskilsson C, Bolis A, et al., 2011, A generic framework for time-stepping partial differential equations (PDEs): general linear methods, object-oriented implementation and application to fluid problems, INTERNATIONAL JOURNAL OF COMPUTATIONAL FLUID DYNAMICS, Vol: 25, Pages: 107-125, ISSN: 1061-8562
Cantwell CD, Sherwin SJ, Kirby RM, et al., 2011, From h to p Efficiently: Selecting the Optimal Spectral/hp Discretisation in Three Dimensions, MATHEMATICAL MODELLING OF NATURAL PHENOMENA, Vol: 6, Pages: 84-96, ISSN: 0973-5348
Sharma AS, Morrison JF, McKeon BJ, et al., 2011, Relaminarisation of Re=100 channel flow with globally stabilisinglinear feedback control, Physics of Fluids, Vol: 23, Pages: 125105-1-125105-17, ISSN: 1070-6631
Waters SL, Alastruey J, Beard DA, et al., 2011, Theoretical models for coronary vascular biomechanics: Progress & challenges, Progress in Biophysics and Molecular Biology, Vol: 104, Pages: 49-76
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
Hall P, Sherwin S, 2010, Streamwise vortices in shear flows: harbingers of transition and the skeleton of coherent structures, J. Fluid Mechanics, Vol: 661, Pages: 178-205, ISSN: 0022-1120
The relationship between asymptotic descriptions of vortex-wave interactions and more recent work on ’exact coherent structures’ is investigated. In recent years immense in- terest has been focused on so-called self-sustained processes in turbulent shear flows where the importance of waves interacting with streamwise vortex flows has been eluci- dated in a number of papers. In this paper it is shown that the so-called ’lower branch’ state which has been shown to play a crucial role in these self-sustained processes is a finite Reynolds number analogue of a Rayleigh vortex-wave interaction with scales ap- propriately modified from those for external flows to Couette flow the flow of interest here. Remarkable agreement between the asymptotic theory and numerical simulations is found even down to relatively small Reynolds numbers thereby suggesting the possible importance of vortex-wave interaction theory in turbulent shear flows. The relevance of the work to more general shear flows is also discussed.
Cookson AN, Doorly DJ, Sherwin SJ, 2010, Using coordinate transformation of Navier–Stokes equations to solve flow in multiple helical geometries, Journal of Computational and Applied Mathematics, Vol: 234, Pages: 2069-2079
Recent research on small amplitude helical pipes for use as bypass grafts and arterio-venous shunts, suggests that mixing may help prevent occlusion by thrombosis. It is proposed here that joining together two helical geometries, of different helical radii, will enhance mixing, with only a small increase in pressure loss. To determine the velocity field, a coordinate transformation of the Navier–Stokes equations is used, which is then solved using a 2-D high-order mesh combined with a Fourier decomposition in the periodic direction. The results show that the velocity fields in each component geometry differ strongly from the corresponding solution for a single helical geometry. The results suggest that, although the mixing behaviour will be weaker than an idealised prediction indicates, it will be improved from that generated in a single helical geometry.
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
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