Imperial College London

ProfessorSpencerSherwin

Faculty of EngineeringDepartment of Aeronautics

Professor of Computational Fluid Mechanics
 
 
 
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Contact

 

+44 (0)20 7594 5052s.sherwin Website

 
 
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Location

 

313BCity and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

332 results found

Sharma AS, Abdessemed N, Sherwin S, Theofilis Vet al., 2008, Optimal growth of linear perturbations in low pressure turbine flows, IUTAM Symposium on Flow Control and MEMS, Publisher: SPRINGER, Pages: 339-+

Conference paper

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.

Journal article

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

Journal article

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.

Journal article

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.

Conference paper

Bruno S Carmo, Spencer J Sherwin, Peter W Bearman, Richard HJ Willdenet 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.

Journal article

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

Journal article

Alastruey J, Parker KH, Peiro J, Sherwin SJet 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

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.

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

Sharma AS, Abdessemed N, Sherwin SJ, Theofilis Vet al., 2008, Optimal Growth of Linear Perturbations in Low Pressure Turbine Flows, Pages: 339-343

Conference paper

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

Alastruey J, Moore SM, Parker KH, David T, Peiro J, Sherwin SJet 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

Journal article

A Miliou, A De Vecchi, SJ Sherwin, JMR Grahamet 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

Journal article

Denis Doorly, Spencer Sherwin, 2007, Geometry and Flow, Cardiovascular Mathematics, Editors: Quarteroni, Formaggia, Veneziani, Publisher: Springer

Book chapter

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

Journal article

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

Conference paper

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

Journal article

van Loon R, Anderson PD, van de Vosse FN, Sherwin SJet al., 2007, Comparison of various fluid–structure interaction methods for deformable bodies, Computers and Structures, Vol: 85, Pages: 833-843, ISSN: 0045-7949

Journal article

KELee, KH Parker, CG Caro, SJ Sherwinet al., 2007, The spectral/hp element modelling of steady flow in non-planar double bends, Int. J. Num. Meth. Fluids, Vol: in press

Journal article

Matthys KS, Alastruey J, Peiro J, Khir AW, Segers P, Verdonck PR, Parker KH, Sherwin SJet al., 2007, Pulse wave propagation in a model human arterial network: Assessment of 1-D numerical simulations against in vitro measurements, J. Biomech., Vol: 40, Pages: 3476-3486

A numerical model based on the nonlinear, one-dimensional (1-D) equations of pressure and flow wave propagation in conduit arteries is tested against a well-defined experimental 1:1 replica of the human arterial tree. The tree consists of 37 silicone branches representing the largest central systemic arteries in the human, including the aorta, carotid arteries and arteries that perfuse the upper and lower limbs and the main abdominal organs. The set-up is mounted horizontally and connected to a pulsatile pump delivering a periodic output similar to the aortic flow. Terminal branches end in simple resistance models, consisting of stiff capillary tubes leading to an overflow reservoir that reflects a constant venous pressure. The parameters required by the numerical algorithm are directly measured in the \emph{in-vitro} set-up and no data fitting is involved. Comparison of experimental and numerical pressure and flow waveforms shows the ability of the 1-D time-domain formulation to capture the main features of pulse wave propagation measured throughout the system test. As a consequence of the simple resistive boundary conditions used to reduce the uncertainty of the parameters involved in the simulation, the experimental set-up generates waveforms at terminal branches with additional non-physiological oscillations. The frequencies of these oscillations are well captured by the 1-D model, even though amplitudes are overestimated. Adding energy losses in bifurcations and including fluid inertia and compliance to the purely resistive terminal models does not reduce the underdamped effect, suggesting that wall visco-elasticity might play an important role in the experimental results. Nevertheless, average relative root-mean-square errors between simulations and experimental waveforms are smaller than 4% for pressure and 19% for the flow at all 70 locations studied.

Journal article

Dongbin Xiu, Spencer Sherwin, 2007, Parametric uncertainty analysis of pulse wave propagation in a model of a human arterial network, J. Computational Physics, Vol: 226, Pages: 1385-1407

Journal article

Abdessemed N, Sherwin SJ, Theofilis V, 2006, Linear stability of the flow past a low pressure turbine blade, Pages: 1496-1510

The present work is concerned with linear BiGlobal instabilities of flow in a cascade of Low-pressure turbine (LPT) blades. In particular the three-dimensional instability of two-dimensional time-periodic basic states past a T106/3 blade is investigated using Floquet theory and a high-order spectral/hp-element scheme, used for both the basic flow and the instability calculations. The two-dimensional bifurcation of steady to time-periodic flow has been identified in the framework of earlier work on the present project.1 The present Floquet analysis shows that the time-periodic flow becomes three-dlmensionally unstable soon after the two dimensional transition. Two modes of significance have been identified, associated to the separation bubble and the wake of the basic state. Three-dimensional DNS initialized on the identified Floquet eigenmodes at low amplitudes confirms the results of linear stability analysis. Besides the completion of the eigenvalue-based analyses, first-ever transient-growth results of an essentially nonparallel flow have been obtained. In particular, the pseudospectrum of the flow under consideration has been identified and new mechanisms have been revealed, demonstrating the potential for three-dimensional transition as consequence of transient growth.

Conference paper

Alastruey J, Parker KH, PeirĂ³ J, Sherwin SJet al., 2006, Can the modified Allen's test always detect sufficient collateral flow in the hand? A computational study., Comput Methods Biomech Biomed Engin, Vol: 9, Pages: 353-361, ISSN: 1025-5842

Blood flow in the largest arteries of the arm up to the digital arteries is numerically modelled using the one-dimensional equations of pressure and flow wave propagation in compliant vessels. The model can be applied to different anatomies of arterial networks and can simulate compression of arteries, these allowing us to simulate the modified Allen's test (MAT) and to assess its suitability for the detection of sufficient collateral flow in the hand if radial blood supply is interrupted. The test measures blood flow in the superficial palmar arch before and during compression of the radial artery. The absence of reversal flow in the palmar arch with the compression indicates insufficient collateral flow and is referred to as a positive MAT. This study shows that small calibres of the superficial palmar arch and insufficient compression of the radial artery can lead to false-positive results. Measurement of the drop in digital systolic pressures with compression of the radial artery has proved to be a more sensitive test to predict the presence of sufficient ulnar collateral flow in networks with small calibres of the superficial palmar arch. However, this study also shows that digital pressure measurements can fail in detecting enough collateral flow if the radial artery is insufficiently compressed.

Journal article

Kirby RM, Sherwin SJ, 2006, Aliasing errors due to quadratic nonlinearities on triangular spectral /hp element discretisations, JOURNAL OF ENGINEERING MATHEMATICS, Vol: 56, Pages: 273-288, ISSN: 0022-0833

Journal article

Eskilsson C, Sherwin SJ, Bergdahl L, 2006, An unstructured spectral/hp element model for enhanced Boussinesq-type equations, COASTAL ENGINEERING, Vol: 53, Pages: 947-963, ISSN: 0378-3839

Journal article

Sherwin SJ, Kirby RM, Peiro J, Taylor RL, Zienkiewicz OCet al., 2006, On 2D elliptic discontinuous Galerkin methods, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Vol: 65, Pages: 752-784, ISSN: 0029-5981

Journal article

Kirby RM, Sherwin SJ, 2006, Stabilisation of spectral/hp element methods through spectral vanishing viscosity: Application to fluid mechanics modelling, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, Vol: 195, Pages: 3128-3144, ISSN: 0045-7825

Journal article

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