286 results found
Mengaldo G, De Grazia D, Moura RC, et al., 2018, Spatial eigensolution analysis of energy-stable flux reconstruction schemes and influence of the numerical flux on accuracy and robustness, Journal of Computational Physics, ISSN: 0021-9991
This study focusses on the dispersion and diffusion characteristics of high-order energy-stable flux recon-struction (ESFR) schemes via the spatial eigensolution analysis framework proposed in . The analysis isperformed for five ESFR schemes, where the parameter ‘c’ dictating the properties of the specific schemerecovered is chosen such that it spans the entire class of ESFR methods, also referred to as VCJH schemes,proposed in . In particular, we used five values of ‘c’, two that correspond to its lower and upper boundsand the others that identify three schemes that are linked to common high-order methods, namely theESFR recovering two versions of discontinuous Galerkin methods and one recovering the spectral differencescheme. The performance of each scheme is assessed when using different numerical intercell fluxes (e.g.different levels of upwinding), ranging from “under-” to “over-upwinding”. In contrast to the more commontemporal analysis, the spatial eigensolution analysis framework adopted here allows one to grasp crucialinsights into the diffusion and dispersion properties of FR schemes for problems involving non-periodicboundary conditions, typically found in open-flow problems, including turbulence, unsteady aerodynamicsand aeroacoustics.
Chooi KY, Comerford A, Sherwin SJ, et al., 2017, Noradrenaline has opposing effects on the hydraulic conductance of arterial intima and media, JOURNAL OF BIOMECHANICS, Vol: 54, Pages: 4-10, ISSN: 0021-9290
Ekelschot D, Moxey D, Sherwin SJ, et al., 2017, A p-adaptation method for compressible flow problems using a goal-based error indicator, COMPUTERS & STRUCTURES, Vol: 181, Pages: 55-69, ISSN: 0045-7949
Ghim M, Alpresa P, Yang S-W, et al., 2017, Visualization of three pathways for macromolecule transport across cultured endothelium and their modification by flow, AMERICAN JOURNAL OF PHYSIOLOGY-HEART AND CIRCULATORY PHYSIOLOGY, Vol: 313, Pages: H959-H973, ISSN: 0363-6135
Mao X, Zaki TA, Sherwin SJ, et al., 2017, Transition induced by linear and nonlinear perturbation growth in flow past a compressor blade, Journal of Fluid Mechanics, Vol: 820, Pages: 604-632, ISSN: 0022-1120
© 2017 Cambridge University Press. Flow past a NACA 65 blade at chord-based Reynolds number 138 500 is studied using stability analysis, generalized (spatially weighted) transient growth analysis and direct numerical simulations (DNS). The mechanisms of transition on various sections of the blade observed in previous work by Zaki et al. (J. Fluid Mech., vol. 665, 2010, pp. 57-98) are examined, with a focus on the pressure side around the leading edge. In this region, the linearly most energetic perturbation has spanwise wavenumber 40π (five boundary-layer thicknesses) and is tilted against the mean shear to take advantage of the Orr mechanism. In a DNS, the nonlinear development of this optimal perturbation induces Λ structures, which are further stretched to hairpin vortices before breaking down to turbulence. At higher spanwise wavenumber, e.g. 120π, a free-stream optimal perturbation is obtained upstream of the leading edge, in the form of streamwise vortices. During its nonlinear evolution, this optimal perturbation tilts the mean shear and generates spanwise periodic high- and low-speed streaks. Then through a nonlinear lift-up mechanism, the low-speed streaks are lifted above the high-speed ones. This layout of streaks generates a mean shear with two inflectional points and activates secondary instabilities, namely inner and outer instabilities previously reported in the literature.
Mengaldo G, Moura RC, Giralda B, et al., 2017, Spatial eigensolution analysis of discontinuous Galerkin schemes with practical insights for under-resolved computations and implicit LES, Computers and Fluids, ISSN: 0045-7930
© 2017 The Authors. The study focusses on the dispersion and diffusion characteristics of discontinuous spectral element methods - specifically discontinuous Galerkin (DG) - via the spatial eigensolution analysis framework built around a one-dimensional linear problem, namely the linear advection equation. Dispersion and diffusion characteristics are of critical importance when dealing with under-resolved computations, as they affect both the numerical stability of the simulation and the solution accuracy. The spatial eigensolution analysis carried out in this paper complements previous analyses based on the temporal approach, which are more commonly found in the literature. While the latter assumes periodic boundary conditions, the spatial approach assumes inflow/outflow type boundary conditions and is therefore better suited for the investigation of open flows typical of aerodynamic problems, including transitional and fully turbulent flows and aeroacoustics. The influence of spurious/reflected eigenmodes is assessed with regard to the presence of upwind dissipation, naturally present in DG methods. This provides insights into the accuracy and robustness of these schemes for under-resolved computations, including under-resolved direct numerical simulation (uDNS) and implicit large-eddy simulation (iLES). The results estimated from the spatial eigensolution analysis are verified using the one-dimensional linear advection equation and successively by performing two-dimensional compressible Euler simulations that mimic (spatially developing) grid turbulence.
Mohamied Y, Sherwin SJ, Weinberg PD, 2017, Understanding the fluid mechanics behind transverse wall shear stress, JOURNAL OF BIOMECHANICS, Vol: 50, Pages: 102-109, ISSN: 0021-9290
Moura RC, Mengaldo G, Peiró J, et al., 2017, An LES Setting for DG-Based Implicit LES with Insights on Dissipation and Robustness, Pages: 161-173, ISSN: 1439-7358
© 2017, Springer International Publishing AG. We suggest a new interpretation of implicit large eddy simulation (iLES) approaches based on discontinuous Galerkin (DG) methods by analogy with the LES-PLB framework (Pope, Fluid mechanics and the environment: dynamical approaches. Springer, Berlin, 2001), where PLB stands for ‘projection onto local basis functions’. Within this framework, the DG discretization of the unfiltered compressible Navier-Stokes equations can be recognized as a Galerkin solution of a PLB-based (and hence filtered) version of the equations with extra terms originating from DG’s implicit subgrid-scale modelling. It is shown that for under-resolved simulations of isotropic turbulence at very high Reynolds numbers, energy dissipation is primarily determined by the property-jump term of the Riemann flux employed. Additionally, in order to assess how this dissipation is distributed in Fourier space, we compare energy spectra obtained from inviscid simulations of the Taylor-Green vortex with different Riemann solvers and polynomial orders. An explanation is proposed for the spectral ‘energy bump’ observed when the Lax-Friedrichs flux is employed.
Moura RC, Mengaldo G, Peiro J, et al., 2017, On the eddy-resolving capability of high-order discontinuous Galerkin approaches to implicit LES / under-resolved DNS of Euler turbulence, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 330, Pages: 615-623, ISSN: 0021-9991
Moura RC, Peiro J, Sherwin SJ, 2017, On the accuracy and robustness of implicit LES/under-resolved DNS approaches based on spectral element methods
We present a study on the suitability of under-resolved DNS (uDNS)-also called implicit LES (iLES)-approaches based on spectral element methods (SEM), with emphasis on high-order continuous and discontinuous Galerkin (i.e. CG and DG) schemes. Broadly speaking, these are model-free eddy-resolving approaches to turbulence which solve the governing equations in unfiltered form and rely on numerical stabilization techniques for small-scale regularization. Model problems in 1D, 2D and 3D are used in the assessment of solution quality and numerical stability. A rationale for the excellent potential of these methods for transitional and turbulent flows is offered on the basis of linear dispersion-diffusion analysis.
Moxey D, Cantwell CD, Mengaldo G, et al., 2017, Towards p-Adaptive Spectral/hp Element Methods for Modelling Industrial Flows, Pages: 63-79, ISSN: 1439-7358
© 2017, Springer International Publishing AG. There is an increasing requirement from both academia and industry for high-fidelity flow simulations that are able to accurately capture complicated and transient flow dynamics in complex geometries. Coupled with the growing availability of high-performance, highly parallel computing resources, there is therefore a demand for scalable numerical methods and corresponding software frameworks which can deliver the next-generation of complex and detailed fluid simulations to scientists and engineers in an efficient way. In this article we discuss recent and upcoming advances in the use of the spectral/hp element method for addressing these modelling challenges. To use these methods efficiently for such applications, is critical that computational resolution is placed in the regions of the flow where it is needed most, which is often not known a priori. We propose the use of spatially and temporally varying polynomial order, coupled with appropriate error estimators, as key requirements in permitting these methods to achieve computationally efficient high-fidelity solutions to complex flow problems in the fluid dynamics community.
Serson D, Meneghini JR, Sherwin SJ, 2017, Direct numerical simulations of the flow around wings with spanwise waviness at a very low Reynolds number, COMPUTERS & FLUIDS, Vol: 146, Pages: 117-124, ISSN: 0045-7930
Serson D, Meneghini JR, Sherwin SJ, 2017, Direct numerical simulations of the flow around wings with spanwise waviness, JOURNAL OF FLUID MECHANICS, Vol: 826, Pages: 714-731, ISSN: 0022-1120
Serson D, Meneghini JR, Sherwin SJ, 2017, Extension of the Velocity-Correction Scheme to General Coordinate Systems, Pages: 331-342, ISSN: 1439-7358
© 2017, Springer International Publishing AG. The velocity-correction scheme is a time-integration method for the incompressible Navier-Stokes equations, and is a common choice in the context of spectral/hp methods. Although the spectral/hp discretization allows the representation of complex geometries, in some cases the use of a coordinate transformation is desirable, since it may lead to symmetries which allow a more efficient solution of the equations. One example of this occurs when the transformed geometry has a homogeneous direction, in which case a Fourier expansion can be applied in this direction, reducing the computational cost. In this paper, we revisit two recently proposed forms of extending the velocity-correction scheme to general coordinate systems, the first treating the mapping terms explicitly and the second treating them semi-implicitly. We then present some numerical examples illustrating the properties and applicability of these methods, including new tests focusing on the time-accuracy of these schemes.
Xu H, Lombard J-EW, Sherwin SJ, 2017, Influence of localised smooth steps on the instability of a boundary layer, JOURNAL OF FLUID MECHANICS, Vol: 817, Pages: 138-170, ISSN: 0022-1120
Xu H, Mughal SM, Gowree ER, et al., 2017, Destabilisation and modification of Tollmien-Schlichting disturbances by a three-dimensional surface indentation, JOURNAL OF FLUID MECHANICS, Vol: 819, Pages: 592-620, ISSN: 0022-1120
Xu H, Mughal SM, Gowree ER, et al., 2017, Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation, Journal of Fluid Mechanics, Vol: 819, Pages: 592-620, ISSN: 0022-1120
Bao Y, Palacios R, Graham M, et al., 2016, Generalized thick strip modelling for vortex-induced vibration of long flexible cylinders, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 321, Pages: 1079-1097, ISSN: 0021-9991
Bolls A, Cantwell CD, Moxey D, et al., 2016, An adaptable parallel algorithm for the direct numerical simulation of incompressible turbulent flows using a Fourier spectral/hp element method and MPI virtual topologies, COMPUTER PHYSICS COMMUNICATIONS, Vol: 206, Pages: 17-25, ISSN: 0010-4655
Chooi KY, Comerford A, Sherwin SJ, et al., 2016, Intimal and medial contributions to the hydraulic resistance of the arterial wall at different pressures: a combined computational and experimental study, JOURNAL OF THE ROYAL SOCIETY INTERFACE, Vol: 13, ISSN: 1742-5689
Lombard J-EW, Moxey D, Sherwin SJ, et al., 2016, Implicit Large-Eddy Simulation of a Wingtip Vortex, AIAA JOURNAL, Vol: 54, Pages: 506-518, ISSN: 0001-1452
Mengaldo G, De Grazia D, Vincent PE, et al., 2016, On the Connections Between Discontinuous Galerkin and Flux Reconstruction Schemes: Extension to Curvilinear Meshes, JOURNAL OF SCIENTIFIC COMPUTING, Vol: 67, Pages: 1272-1292, ISSN: 0885-7474
Moura RC, Sherwin SJ, Peiro J, 2016, Eigensolution analysis of spectral/hp continuous Galerkin approximations to advection-diffusion problems: Insights into spectral vanishing viscosity, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 307, Pages: 401-422, ISSN: 0021-9991
Moxey D, Cantwell CD, Kirby RM, et al., 2016, Optimising the performance of the spectral/hp element method with collective linear algebra operations, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, Vol: 310, Pages: 628-645, ISSN: 0045-7825
Moxey D, Ekelschot D, Keskin U, et al., 2016, High-order curvilinear meshing using a thermo-elastic analogy, COMPUTER-AIDED DESIGN, Vol: 72, Pages: 130-139, ISSN: 0010-4485
Serson D, Meneghini JR, Sherwin SJ, 2016, Velocity-correction schemes for the incompressible Navier-Stokes equations in general coordinate systems, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 316, Pages: 243-254, ISSN: 0021-9991
Turner M, Moxey D, Sherwin SJ, et al., 2016, Automatic generation of 3D unstructured high-order curvilinear meshes, Pages: 428-443
The generation of suitable, good quality high-order meshes is a significant obstacle in the academic and industrial uptake of high-order CFD methods. These methods have a number of favourable characteristics such as low dispersion and dissipation and higher levels of numerical accuracy than their low-order counterparts, however the methods are highly susceptible to inaccuracies caused by low quality meshes. These meshes require significant curvature to accuratly describe the geometric surfaces, which presents a number of difficult challenges in their generation. As yet, research into the field has produced a number of interesting technologies that go some way towards achieving this goal, but are yet to provide a complete system that can systematically produce curved high-order meshes for arbitrary geometries for CFD analysis. This paper presents our efforts in that direction and introduces an open-source high-order mesh generator, NekMesh, which has been created to bring high-order meshing technologies into one coherent pipeline which aims to produce 3D high-order curvilinear meshes from CAD geometries in a robust and systematic way.
Xu H, Mughal S, Gowree ER, et al., 2016, Effect of a 3D indentation on boundary layer instability
We are concerned about effect of a 3D surface indentation on instability and laminar-turbulent transition in a boundary layer. For natural transition in a boundary layer, the transition onset is dominated by growth of the Tollmien-Schlichting (TS) wave and its subsequent secondary instability. In the paper, both linear analysis and nonlinear calculations are carried out to address the 3D surface indentation effect on amplifying TS waves' amplitudes and prompting transition onset. By the linear analysis, we address sudden amplification of the TS modes by a separation bubble in a surface indentation region. The nonlinear calculations are implemented to validate the traditional transition criteria predicted by the linear theory when a 3D indentation is present. Finally, applicability of the traditional transition criteria is assessed.
Xu H, Sherwin SJ, Halle P, et al., 2016, The behaviour of Tollmien-Schlichting waves undergoing small-scale localised distortions, JOURNAL OF FLUID MECHANICS, Vol: 792, Pages: 499-525, ISSN: 0022-1120
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