346 results found
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
Zhang Y, Zaki TA, Sherwin SJ, et al., 2011, Nonlinear response of a laminar boundary layer to isotropic and spanwise localized free-stream turbulence
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
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
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 S, 2010, Spectra of Swirling Flow, Journal of Fluid Mechanics
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
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
Alastruey J, Parker K, Peiro J, et al., Analysing the pattern of pulse waves in arterial networks: A time Domain Study, Journal of Engineering Mathematics, Vol: Submitted
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
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