Spatio-temporal dynamics in pipe flow

When fluid flows through a channel, pipe or duct, there are two basic forms of motion: smooth laminar flow and disordered turbulent motion. The transition between these two states is a fundamental and open problem which has been studied for over 125 years. What has received far less attention are the intermittent dynamics which possess qualities of both turbulent and laminar regimes. The purpose of this thesis is therefore to investigate large-scale intermittent states through extensive numerical simulations in the hopes of further understanding the transition to turbulence in pipe flow. We begin by reviewing the spectral-element code Semtex which is used to perform the simulations. We discuss modifications to this code to impose a constant flowrate to the flow through a pipe and to improve the computational efficiency on certain multicore architectures. We then move on to examine the reverse transition from turbulence to laminar flow in a long, 125 diameter periodic pipe, which unlike the forward transition does not depend on finiteamplitude perturbations to the flow and thus captures the natural dynamics contained within the transition. The Reynolds number Re is reduced from Re = 2,800 to Re = 2,250 over a long timescale, and by investigating the resultant spatio-temporal dynamics we discover that the transition can be characterised by three fundamentally different states separated by two Reynolds numbers. Below Rec <= 2,300, turbulence takes the form of equilibrium puffs which eventually decay. Above Rei = 2,600, flow remains uniformly turbulent throughout the domain. Between these two values, the dynamics are an intermitent mixture of both turbulent and laminar regimes which take the form of unsteady alternating laminar-turbulent bands. Finally, we concentrate on finding a more exact value for Rec, which marks the onset of sustained turbulence in pipe flow. We examine the process through which isolated turbulent puffs split and find that, like decay, this process is stochastic and memoryless. By drawing comparisons with other simple stochastically driven systems – in particular, directed percolation – we compare the timescales for decay and splitting, and ascertain that Rec = 2,040 +- 10

Volume/Outcome in CABG and PTCA: A Summary of the Literature

No abstract available

A comparison of interpolation techniques for non-conformal high-order discontinuous Galerkin methods

The capability to incorporate moving geometric features within models for complex simulations is a common requirement in many fields. Fluid mechanics within aeronautical applications, for example, routinely feature rotating (e.g. turbines, wheels and fan blades) or sliding components (e.g. in compressor or turbine cascade simulations). With an increasing trend towards the high-fidelity modelling of these cases, in particular combined with the use of high-order discontinuous Galerkin methods, there is therefore a requirement to understand how different numerical treatments of the interfaces between the static mesh and the sliding/rotating part impact on overall solution quality. In this article, we compare two different approaches to handle this non-conformal interface. The first is the so-called mortar approach, where flux integrals along edges are split according to the positioning of the non-conformal grid. The second is a less-documented point-to-point interpolation method, where the interior and exterior quantities for flux evaluations are interpolated from elements lying on the opposing side of the interface. Although the mortar approach has significant advantages in terms of its numerical properties, in that it preserves the local conservation properties of DG methods, in the context of complex 3D meshes it poses notable implementation difficulties which the point-to-point method handles more readily. In this paper we examine the numerical properties of each method, focusing not only on observing convergence orders for smooth solutions, but also how each method performs in under-resolved simulations of linear and nonlinear hyperbolic problems, to inform the use of these methods in implicit large-eddy simulations.Comment: 37 pages, 15 figures, 5 tables, submitted to Computer Methods in Applied Mechanics and Engineering, revision

Simplifying the Development, Use and Sustainability of HPC Software

Developing software to undertake complex, compute-intensive scientific processes requires a challenging combination of both specialist domain knowledge and software development skills to convert this knowledge into efficient code. As computational platforms become increasingly heterogeneous and newer types of platform such as Infrastructure-as-a-Service (IaaS) cloud computing become more widely accepted for HPC computations, scientists require more support from computer scientists and resource providers to develop efficient code and make optimal use of the resources available to them. As part of the libhpc stage 1 and 2 projects we are developing a framework to provide a richer means of job specification and efficient execution of complex scientific software on heterogeneous infrastructure. The use of such frameworks has implications for the sustainability of scientific software. In this paper we set out our developing understanding of these challenges based on work carried out in the libhpc project.Comment: 4 page position paper, submission to WSSSPE13 worksho

A semi-structured approach to curvilinear mesh generation around streamlined bodies

We present an approach for robust high-order mesh generation specially tailored to streamlined bodies. The method is based on a semi-sructured approach which combines the high quality of structured meshes in the near-field with the flexibility of unstructured meshes in the far-field. We utilise medial axis technology to robustly partition the near-field into blocks which can be meshed coarsely with a linear swept mesher. A high-order mesh of the near-field is then generated and split using an isoparametric approach which allows us to obtain highly stretched elements aligned with the flow field. Special treatment of the partition is performed on the wing root juntion and the trailing edge --- into the wake --- to obtain an H-type mesh configuration with anisotropic hexahedra ideal for the strong shear of high Reynolds number simulations. We then proceed to discretise the far-field using traditional robust tetrahedral meshing tools. This workflow is made possible by two sets of tools: CADfix, focused on CAD system, the block partitioning of the near-field and the generation of a linear mesh; and NekMesh, focused on the curving of the high-order mesh and the generation of highly-stretched boundary layer elements. We demonstrate this approach on a NACA0012 wing attached to a wall and show that a gap between the wake partition and the wall can be inserted to remove the dependency of the partitioning procedure on the local geometry.Comment: Preprint accepted to the 2019 AIAA Aerospace Sciences Meetin