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

346 results found

Moxey D, Hazan M, Sherwin SJ, Peiro Jet al., 2015, Curvilinear mesh generation for boundary layer problems, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Vol: 128, Pages: 41-64, ISSN: 1612-2909

© Springer International Publishing Switzerland 2015. In this article, we give an overview of a new technique for unstructured curvilinear boundary layer grid generation, which uses the isoparametric mappings that define elements in an existing coarse prismatic grid to produce a refined mesh capable of resolving arbitrarily thin boundary layers. We demonstrate that the technique always produces valid grids given an initially valid coarse mesh, and additionally show how this can be extended to convert hybrid meshes to meshes containing only simplicial elements.

Journal article

Moura RC, Sherwin S, Peiro J, 2015, Modified Equation Analysis for the Discontinuous Galerkin Formulation, 10th International Conference on Spectral and High-Order Methods (ICOSAHOM), Publisher: SPRINGER-VERLAG BERLIN, Pages: 375-383, ISSN: 1439-7358

Conference paper

Peiro J, Moxey D, Jordi B, Sherwin SJ, Nelson BW, Kirby RM, Haimes Ret al., 2015, High-order visualization with ElVis, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Vol: 128, Pages: 521-534, ISSN: 1612-2909

© Springer International Publishing Switzerland 2015. Accurate visualization of high-order meshes and flow fields is a fundamental tool for the verification, validation, analysis and interpretation of high-order flow simulations. Standard visualization tools based on piecewise linear approximations can be used for the display of highorder fields but their accuracy is restricted by computer memory and processing time. More often than not, the accurate visualization of complex flows using this strategy requires computational resources beyond the reach of most users. This chapter describes ElVis, a truly high-order and interactive visualization system created for the accurate and interactive visualization of scalar fields produced by high-order spectral/hp finite element simulations. We show some examples that motivate the need for such a visualization system and illustrate some of its features for the display and analysis of simulation data.

Journal article

Cohen J, Cantwell C, Moxey D, Austing P, Darlington J, Sherwin S, Nowell J, Guo Xet al., 2015, TemPSS: A service providing software parameter templates and profiles for scientific HPC, IEEE International Conference On eScience, Publisher: IEEE, Pages: 78-87, ISSN: 2325-372X

Conference paper

Burovskiy P, Grigoras P, Sherwin S, Luk Wet al., 2015, Efficient Assembly for High Order Unstructured FEM Meshes, 25th International Conference on Field Programmable Logic and Applications, Publisher: IEEE, ISSN: 1946-1488

Conference paper

Xu H, Sherwin SJ, 2015, On a mechanism of delaying laminar-turbulent transition

© TU Delft. In this paper we investigate the boundary layer flows over a flat plate on which smooth localized imperfections are located. The localized imperfections have the width scale (d) comparable with the wavelength (λTS) of the Tollmien-Schlichting (T-S) waves and the height scale (h) less than the boundary layer thickness δ99. The existence of the localized imperfection gives rise to the change of the instability property of the boundary layer. The investigations are focused on the interaction between the T-S waves and the base flows distorted by smooth forward-facing steps and aim to forge links between the localized imperfections and the mechanisms of stabilizing the T-S waves. Numerical investigations show that isolated smooth forward-facing steps can perform as a robust strategy of delaying laminar-turbulent transition. Finally, direct numerical simulations are implemented to validate the strategy.

Conference paper

Cohen J, Moxey D, Cantwell C, Austing P, Darlington J, Sherwin Set al., 2015, POSITION PAPER: Ensuring an Effective User Experience when Managing and Running Scientific HPC Software, IEEE ACM 1st International Workshop on Software Engineering for High Performance Computing in Science (SE4HPCS), Publisher: IEEE, Pages: 56-59

Conference paper

Moxey D, Feldman Y, Perdikaris P, Insley J, Yakhot A, Sherwin S, Karniadakis Get al., 2015, Proper orthogonal decomposition analysis of a puff in a pipe

We have performed direct numerical simulations of transitional turbulence in pipe flow for Re=2,250. The results confirm the existence of a spatio-temporal intermittency when turbulence is localized in a puff convected downstream. To analyze the turbulence, we follow a turbulent puff by a 3D moving-window centered at the location of the maximum total energy of transverse (turbulent) motion. The flow field data collected over 6,000 time instances (snapshots) have been analyzed by Proper Orthogonal Decomposition (POD) and used for identifying vortical structures. The presence of large-scale structures in a puff has been found by time-averaging of the cross-sectional turbulent velocity field and confirmed by POD analysis and by applying the Q- and λ2-criteria.

Conference paper

Mohamied Y, Rowland EM, Bailey EL, Sherwin SJ, Schwartz MA, Weinberg PDet al., 2015, Change of Direction in the Biomechanics of Atherosclerosis, ANNALS OF BIOMEDICAL ENGINEERING, Vol: 43, Pages: 16-25, ISSN: 0090-6964

Journal article

Xu H, Sherwin SJ, 2015, On a mechanism of delaying laminar-turbulent transition

© TU Delft. In this paper we investigate the boundary layer flows over a flat plate on which smooth localized imperfections are located. The localized imperfections have the width scale (d) comparable with the wavelength (λTS) of the Tollmien-Schlichting (T-S) waves and the height scale (h) less than the boundary layer thickness δ99. The existence of the localized imperfection gives rise to the change of the instability property of the boundary layer. The investigations are focused on the interaction between the T-S waves and the base flows distorted by smooth forward-facing steps and aim to forge links between the localized imperfections and the mechanisms of stabilizing the T-S waves. Numerical investigations show that isolated smooth forward-facing steps can perform as a robust strategy of delaying laminar-turbulent transition. Finally, direct numerical simulations are implemented to validate the strategy.

Conference paper

Moxey D, Green MD, Sherwin SJ, Peiro Jet al., 2015, An isoparametric approach to high-order curvilinear boundary-layer meshing, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, Vol: 283, Pages: 636-650, ISSN: 0045-7825

Journal article

Mohri Z, Rowland EM, Clarke LA, De Luca A, Peiffer V, Krams R, Sherwin SJ, Weinberg PDet al., 2014, Elevated Uptake of Plasma Macromolecules by Regions of Arterial Wall Predisposed to Plaque Instability in a Mouse Model, PLoS One, Vol: 9, ISSN: 1932-6203

Atherosclerosis may be triggered by an elevated net transport of lipid-carryingmacromolecules from plasma into the arterial wall. We hypothesised that whetherlesions are of the thin-cap fibroatheroma (TCFA) type or are less fatty and morefibrous depends on the degree of elevation of transport, with greater uptake leadingto the former. We further hypothesised that the degree of elevation can depend onhaemodynamic wall shear stress characteristics and nitric oxide synthesis. Placinga tapered cuff around the carotid artery of apolipoprotein E -/- mice modifiespatterns of shear stress and eNOS expression, and triggers lesion development atthe upstream and downstream cuff margins; upstream but not downstream lesionsresemble the TCFA. We measured wall uptake of a macromolecular tracer in thecarotid artery of C57bl/6 mice after cuff placement. Uptake was elevated in theregions that develop lesions in hyperlipidaemic mice and was significantly moreelevated where plaques of the TCFA type develop. Computational simulations andeffects of reversing the cuff orientation indicated a role for solid as well as fluidmechanical stresses. Inhibiting NO synthesis abolished the difference in uptakebetween the upstream and downstream sites. The data support the hypothesis thatexcessively elevated wall uptake of plasma macromolecules initiates thedevelopment of the TCFA, suggest that such uptake can result from solid and fluidmechanical stresses, and are consistent with a role for NO synthesis. Modificationof wall transport properties might form the basis of novel methods for reducingplaque rupture.

Journal article

Xu H, Hall P, sherwin S, 2014, Effect of curvature modulation on Gortler vortices in boundary layers, 67th Annual Meeting of the APS Division of Fluid Dynamics

Conference paper

Burovskiy PA, Girdlestone S, Davies C, Sherwin S, Luk Wet al., 2014, Dataflow acceleration of Krylov subspace sparse banded problems, 24th International Conference on Field Programmable Logic and Applications (FPL), 2014, Publisher: IEEE

Most of the efforts in the FPGA community related to sparse linear algebra focus on increasing the degree of internal parallelism in matrix-vector multiply kernels. We propose a parametrisable dataflow architecture presenting an alternative and complementary approach to support acceleration of banded sparse linear algebra problems which benefit from building a Krylov subspace. We use banded structure of a matrix A to overlap the computations Ax, A2x,..., Akx by building a pipeline of matrix-vector multiplication processing elements (PEs) each performing Aix. Due to on-chip data locality, FLOPS rate sustainable by such pipeline scales linearly with k. Our approach enables trade-off between the number k of overlapped matrix power actions and the level of parallelism in a PE. We illustrate our approach for Google PageRank computation by power iteration for large banded single precision sparse matrices. Our design scales up to 32 sequential PEs with floating point accumulation and 80 PEs with fixed point accumulation on Stratix V D8 FPGA. With 80 single-pipe fixed point PEs clocked at 160Mhz, our design sustains 12.7 GFLOPS.

Conference paper

Ferrer E, Moxey D, Willden RHJ, Sherwin SJet al., 2014, Stability of Projection Methods for Incompressible Flows Using High Order Pressure-Velocity Pairs of Same Degree: Continuous and Discontinuous Galerkin Formulations, COMMUNICATIONS IN COMPUTATIONAL PHYSICS, Vol: 16, Pages: 817-840, ISSN: 1815-2406

Journal article

De Grazia D, Mengaldo G, Moxey D, Vincent PE, Sherwin SJet al., 2014, Connections between the discontinuous Galerkin method and high-order flux reconstruction schemes, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Vol: 75, Pages: 860-877, ISSN: 0271-2091

Journal article

Roney CH, Cantwell C, Qureshi NA, Ali RL, Chang ETY, Lim PB, Sherwin SJ, Peters NS, Siggers JH, Ng FSet al., 2014, An Automated Algorithm for Determining Conduction Velocity, Wavefront Direction and Focal Source Location using a Multipolar Catheter, IEEE Engineering in Medicine and Biology Conference

Determining locations of focal arrhythmia sources and quantifying myocardial conduction velocity (CV) are two major challenges in clinical catheter ablation cases. CV, wavefront direction and focal source location can be estimated from multipolar catheter data, but currently available methods are time-consuming, limited to specific electrode configurations, and can be inaccurate. We developed automated algorithms to rapidly identify CV from multipolar catheter data with any arrangement of electrodes, whilst providing estimates of wavefront direction and focal source position, which can guide the catheter towards a focal arrhythmic source. We validated our methods using simulations on realistic human left atrial geometry. We subsequently applied them to clinically-acquired intracardiac electrogram data, where CV and wavefront direction were accurately determined in all cases, whilst focal source locations were correctly identified in 2/3 cases. Our novel automated algorithms can potentially be used to guide ablation of focal arrhythmias in real-time in cardiac catheter laboratories.

Conference paper

King J, Yakovlev S, Fu Z, Kirby RM, Sherwin SJet al., 2014, Exploiting Batch Processing on Streaming Architectures to Solve 2D Elliptic Finite Element Problems: A Hybridized Discontinuous Galerkin (HDG) Case Study, JOURNAL OF SCIENTIFIC COMPUTING, Vol: 60, Pages: 457-482, ISSN: 0885-7474

Journal article

Bolis A, Cantwell CD, Kirby RM, Sherwin SJet al., 2014, From h to p efficiently: optimal implementation strategies for explicit time-dependent problems using the spectral/hp element method, International Journal for Numerical Methods in Fluids, Vol: 75, Pages: 591-607, ISSN: 1097-0363

We investigate the relative performance of a second-order Adams–Bashforth scheme and second-order andfourth-order Runge–Kutta schemes when time stepping a 2D linear advection problem discretised using aspectral/hp element technique for a range of different mesh sizes and polynomial orders. Numerical experimentsexplore the effects of short (two wavelengths) and long (32 wavelengths) time integration for sets ofuniform and non-uniform meshes. The choice of time-integration scheme and discretisation together fixes aCFL limit that imposes a restriction on the maximum time step, which can be taken to ensure numerical stability.The number of steps, together with the order of the scheme, affects not only the runtime but also theaccuracy of the solution. Through numerical experiments, we systematically highlight the relative effects ofspatial resolution and choice of time integration on performance and provide general guidelines on how bestto achieve the minimal execution time in order to obtain a prescribed solution accuracy. The significant roleplayed by higher polynomial orders in reducing CPU time while preserving accuracy becomes more evident,especially for uniform meshes, compared with what has been typically considered when studying this typeof problem.

Journal article

Xu H, Sherwin S, Hall P, 2014, Transmission coefficient of Tollmien-Schlichting waves undergoing small indentation/hump distortion, The 29th Congress of the International Council of the Aeronautical Sciences

Conference paper

Warboys CM, de Luca A, Amini N, Luong L, Duckles H, Hsiao S, White A, Biswas S, Khamis R, Chong CK, Cheung W-M, Sherwin SJ, Bennett MR, Gil J, Mason JC, Haskard DO, Evans PCet al., 2014, Disturbed Flow Promotes Endothelial Senescence via a p53-Dependent Pathway, ARTERIOSCLEROSIS THROMBOSIS AND VASCULAR BIOLOGY, Vol: 34, Pages: 985-995, ISSN: 1079-5642

Journal article

Jordi BE, Cotter CJ, Sherwin SJ, 2014, Encapsulated formulation of the selective frequency damping method, Physics of Fluids, Vol: 26, ISSN: 1089-7666

We present an alternative “encapsulated” formulation of the selective frequencydamping method for finding unstable equilibria of dynamical systems, which isparticularly useful when analysing the stability of fluid flows. The formulation makesuse of splitting methods, which means that it can be wrapped around an existingtime-stepping code as a “black box.” The method is first applied to a scalar problemin order to analyse its stability and highlight the roles of the control coefficient χand the filter width in the convergence (or not) towards the steady-state. Then thesteady-state of the incompressible flow past a two-dimensional cylinder at Re = 100,obtained with a code which implements the spectral/hp element method, is presented.

Journal article

Bailey EL, Peiffer V, Mohamied Y, Chooi KY, Rowland EM, Sherwin SJ, Weinberg PDet al., 2014, SPATIAL CORRELATION BETWEEN PATTERNS OF WALL PERMEABILITY AND PATTERNS OF MULTIDIRECTIONAL DISTURBED BLOOD FLOW IN RABBIT AORTA, Autumn Meeting of the British-Atherosclerosis-Society (BAS), Publisher: ELSEVIER IRELAND LTD, Pages: E2-E2, ISSN: 0021-9150

Conference paper

Cantwell CD, Yakovlev S, Kirby RM, Peters NS, Sherwin SJet al., 2014, High-order spectral/hp element discretisation for reaction-diffusion problems on surfaces: Application to cardiac electrophysiology, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 257, Pages: 813-829, ISSN: 0021-9991

We present a numerical discretisation of an embedded two-dimensional manifold using high-order continuous Galerkin spectral/hp elements, which provide exponential convergence of the solution with increasing polynomial order, while retaining geometric flexibility in the representation of the domain. Our work is motivated by applications in cardiac electrophysiology where sharp gradients in the solution benefit from the high-order discretisation, while the compu- tational cost of anatomically-realistic models can be reduced through the surface representation. We describe and validate our discretisation and provide a demonstration of its application to modeling electrochemical propagation across a human left atrium.

Journal article

Mengaldo G, De Grazia D, Peiro J, Farrington A, Witherden F, Vincent PE, Sherwin SJet al., 2014, A guide to the implementation of boundary conditions in compact high-order methods for compressible aerodynamics

The nature of boundary conditions, and how they are implemented, can have a significant impact on the stability and accuracy of a Computational Fluid Dynamics (CFD) solver. The objective of this paper is to assess how different boundary conditions impact the performance of compact discontinuous high-order spectral element methods (such as the discontinuous Galerkin method and the Flux Reconstruction approach), when these schemes are used to solve the Euler and compressible Navier-Stokes equations on unstructured grids. Specifically, the paper will investigate inflow/outflow and wall boundary conditions. In all studies the boundary conditions were enforced by modifying the boundary flux. For Riemann invariant (characteristic), slip and no-slip conditions we have considered a direct and an indirect enforcement of the boundary conditions, the first obtained by calculating the flux using the known solution at the given boundary while the second achieved by using a ghost state and by solving a Riemann problem. All computations were performed using the open-source software Nektar++ (www.nektar.info).

Conference paper

Moxey D, Ekelschot D, Keskin U, Sherwin SJ, Peiro Jet al., 2014, A thermo-elastic analogy for high-order curvilinear meshing with control of mesh validity and quality, 23rd International Meshing Roundtable (IMR), Publisher: ELSEVIER SCIENCE BV, Pages: 127-135, ISSN: 1877-7058

Conference paper

Loh SA, Blackburn HM, Sherwin SJ, 2014, Transient growth in an airfoil separation bubble

In this work, transient growth in a laminar separation bubble in a low-to-moderate Reynolds number airfoil flow is investigated. Optimal two and three-dimensional initial perturbations are computed for the flow over a NACA 0012 airfoil at angle-of-attack α = 5° with chord based Re = 5 × 104. At this Reynolds number, the pressure surface boundary layer remains attached over the length of the airfoil while the suction surface boundary layer undergoes separation before becoming two-dimensionally unstable leading to periodic vortex shedding and reattachment. Transient growth analysis shows that this flow supports large energy growth for both two and three-dimensional perturbations. Optimal growth values and flow topologies show that two-dimensional mechanisms are the primary energy growth mechanisms. Upstream of vortex shedding, the primary growth mechanism appears to be the Orr mechanism while a separate mechanism appears to drive further energy growth downstream in the base flow shed vorticity.

Conference paper

, 2014, A guide to the implementation of boundary conditions in compact high-order methods for compressible aerodynamics

The nature of boundary conditions, and how they are implemented, can have a significant impact on the stability and accuracy of a Computational Fluid Dynamics (CFD) solver. The objective of this paper is to assess how different boundary conditions impact the performance of compact discontinuous high-order spectral element methods (such as the discontinuous Galerkin method and the Flux Reconstruction approach), when these schemes are used to solve the Euler and compressible Navier-Stokes equations on unstructured grids. Specifically, the paper will investigate inflow/outflow and wall boundary conditions. In all studies the boundary conditions were enforced by modifying the boundary flux. For Riemann invariant (characteristic), slip and no-slip conditions we have considered a direct and an indirect enforcement of the boundary conditions, the first obtained by calculating the flux using the known solution at the given boundary while the second achieved by using a ghost state and by solving a Riemann problem. All computations were performed using the open-source software Nektar++ (www.nektar.info).

Conference paper

Ali RL, Cantwell CD, Roney CH, Qureshi NA, Lim PB, Siggers JH, Sherwin SJ, Peters NSet al., 2014, A novel method for quantifying localised correlation of late-gadolinium intensity with conduction velocity, Computing in Cardiology (CinC) 2014, Pages: 193-196

Patient-specific computer models of the human atria have the potential to aid clinical intervention in the treatment of cardiac arrhythmias. However, quantifying and integrating the heterogeneous qualities of the myocardium through imaging is particularly challenging due to the unknown relationship between voxel intensity and tissue conductivities. We establish a method to determine the relationship between local conduction velocity and scar density, extracted through the analysis of late-gadolinium enhanced magnetic resonance imaging data.

Conference paper

Comerford A, Chooi KY, Nowak M, Weinberg PD, Sherwin SJet al., 2014, A combined numerical and experimental framework for determining permeability properties of the arterial media, Biomechanics and Modeling in Mechanobiology, accepted

Journal article

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