347 results found
Comerford A, Chooi KY, Nowak M, et al., 2014, A combined numerical and experimental framework for determining permeability properties of the arterial media, Biomechanics and Modeling in Mechanobiology, accepted
Ekelschot D, Biotto C, Peiro J, et al., 2013, P-adaption for compressible flows, Pages: 215-220
We present a p-adaptive method which takes advantage of the ability of a discontinuity sensor used to quantify the difference between the actual solution and a projected reduced order solution in order to vary the polynomial resolution in an element.
Peiffer V, Sherwin SJ, Weinberg PD, 2013, Computation in the rabbit aorta of a new metric - the transverse wall shear stress - to quantify the multidirectional character of disturbed blood flow, JOURNAL OF BIOMECHANICS, Vol: 46, Pages: 2651-2658, ISSN: 0021-9290
Spatial variation of the haemodynamic stresses acting on the arterial wall is commonly assumed to explain the focal development of atherosclerosis. Disturbed flow in particular is thought to play a key role. However, widely-used metrics developed to quantify its extent are unable to distinguish between uniaxial and multidirectional flows. We analysed pulsatile flow fields obtained in idealised and anatomically-realistic arterial geometries using computational fluid dynamics techniques, and in particular investigated the multidirectionality of the flow fields, capturing this aspect of near-wall blood flow with a new metric – the transverse wall shear stress (transWSS) – calculated as the time-average of wall shear stress components perpendicular to the mean flow direction. In the idealised branching geometry, multidirectional flow was observed downstream of the branch ostium, a region of flow stagnation, and to the sides of the ostium. The distribution of the transWSS was different from the pattern of traditional haemodynamic metrics and more dependent on the velocity waveform imposed at the branch outlet. In rabbit aortas, transWSS patterns were again different from patterns of traditional metrics. The near-branch pattern varied between intercostal ostia, as is the case for lesion distribution; for some branches there were striking resemblances to the age-dependent patterns of disease seen in rabbit and human aortas. The new metric may lead to improved understanding of atherogenesis.
Peiffer V, Sherwin SJ, Weinberg PD, 2013, Does low and oscillatory wall shear stress correlate spatially with early atherosclerosis? A systematic review, CARDIOVASCULAR RESEARCH, Vol: 99, Pages: 242-250, ISSN: 0008-6363
Blackburn HM, Hall P, Sherwin SJ, 2013, Lower branch equilibria in Couette flow: the emergence of canonical states for arbitrary shear flows, JOURNAL OF FLUID MECHANICS, Vol: 726, ISSN: 0022-1120
Assi GRS, Bearman PW, Carmo BS, et al., 2013, The role of wake stiffness on the wake-induced vibration of the downstream cylinder of a tandem pair, JOURNAL OF FLUID MECHANICS, Vol: 718, Pages: 210-245, ISSN: 0022-1120
When a pair of tandem cylinders is immersed in a flow the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. Assi, Bearman & Meneghini (J. Fluid Mech., vol. 661, 2010, pp. 365–401) concluded that the WIV excitation mechanism has its origin in the unsteady vortex–structure interaction encountered by the cylinder as it oscillates across the wake. In the present paper we investigate how the cylinder responds to that excitation, characterising the amplitude and frequency of response and its dependency on other parameters of the system. We introduce the concept of wake stiffness, a fluid dynamic effect that can be associated, to a first approximation, with a linear spring with stiffness proportional to Re and to the steady lift force occurring for staggered cylinders. By a series of experiments with a cylinder mounted on a base without springs we verify that such wake stiffness is not only strong enough to sustain oscillatory motion, but can also dominate over the structural stiffness of the system. We conclude that while unsteady vortex–structure interactions provide the energy input to sustain the vibrations, it is the wake stiffness phenomenon that defines the character of the WIV response.
Fogell NA, Sherwin S, Cotter CJ, et al., 2013, Fluid-structure iInteraction simulation of theinflated shape of ram-air parachutes, 22nd AIAA Aerodynamic Decelerator Systems Technology Conference, Daytona Beach, Florida [Best Student Paper Award]
Mao X, Blackburn HM, Sherwin SJ, 2013, Calculation of global optimal initial and boundary perturbations for the linearised incompressible Navier-Stokes equations, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 235, Pages: 258-273, ISSN: 0021-9991
Peiffer V, Bharath AA, Sherwin SJ, et al., 2013, A Novel Method for Quantifying Spatial Correlations Between Patterns of Atherosclerosis and Hemodynamic Factors, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 135, ISSN: 0148-0731
Steinman DA, Hoi Y, Fahy P, et al., 2013, Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: The ASME 2012 Summer Bioengineering Conference CFD Challenge, JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 135, ISSN: 0148-0731
Cohen J, Moxey D, Cantwell C, et al., 2013, Nekkloud: A Software Environment for High-order Finite Element Analysis on Clusters and Clouds, 2013 IEEE INTERNATIONAL CONFERENCE ON CLUSTER COMPUTING (CLUSTER), ISSN: 1552-5244
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.
, 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
Peiffer V, Rowland M, Weinberg PD, et al., 2011, Database of rabbit aortic geometries for use in computational flow studies, Pages: 19-20
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
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