T.N. Croft

Automatic Stream Surface Seeding: A Feature Centered Approach

The ability to capture and visualize information within the flow poses challenges for visualizing 3D flow fields. Stream surfaces are one of many useful integration based techniques for visualizing 3D flow. However seeding integral surfaces can be challenging. Previous research generally focuses on manual placement of stream surfaces. Little attention has been given to the problem of automatic stream surface seeding. This paper introduces a novel automatic stream surface seeding strategy based on vector field clustering. It is important that the user can define and target particular characteristics of the flow. Our framework provides this ability. The user is able to specify different vector clustering parameters enabling a range of abstraction for the density and placement of seeding curves and their associated stream surfaces. We demonstrate the effectiveness of this automatic stream surface approach on a range of flow simulations and incorporate illustrative visualization techniques. Domain expert evaluation of the results provides valuable insight into the users requirements and effectiveness of our approach.

Computational modelling of metal extrusion and forging processes

The computational modelling of extrusion and forging processes is now well established. There are two main approaches: Lagrangian and Eulerian. The first has considerable complexities associated with remeshing, especially when the code is parallelised. The second approach means that the mould has to be assumed to be entirely rigid and this may not be the case. In this paper, a novel approach is described which utilises finite volume methods on unstructured meshes. This approach involves the solution of free surface non-Newtonian fluid flow equations in an Eulerian context to track the behaviour of the workpiece and its extrusion/forging, and the solution of the solid mechanics equations in the Lagrangian context to predict the deformation/stress behaviour of the die. Test cases for modelling extrusion and forging problems using this approach will be presented.

Computational Modelling of Multi-Physics and Multi-Scale Processes in Parallel

This paper provides an overview of the developing needs for simulation software technologies for the computational modelling of problems that involve combinations of interactions amongst varying physical phenomena over a variety of time and space scales. Computational modelling of such problems requires software technologies that enable the mathematical description of the interacting physical phenomena together with the solution of the resulting suites of equations in a numerically consistent and compatible manner. This functionality requires the structuring of simulation modules for specific physical phenomena so that the coupling can be effectively represented. These multi-physics and multi-scale computations are very compute intensive and the simulation software must operate effectively in parallel if it is to be used in this context. An approach to these classes of multi-disciplinary simulation in parallel is described, with some key examples of application to challenging engineering problems.

Computational modeling of mold filling and related free-surface flows in shape casting: An overview of the challenges involved

Accurate representation of the coupled effects between turbulent fluid flow with a free surface, heat transfer, solidification, and mold deformation has been shown to be necessary for the realistic prediction of several defects in castings and also for determining the final crystalline structure. A core component of the computational modeling of casting processes involves mold filling, which is the most computationally intensive aspect of casting simulation at the continuum level. Considering the complex geometries involved in shape casting, the evolution of the free surface, gas entrapment, and the entrainment of oxide layers into the casting make this a very challenging task in every respect. Despite well over 30 years of effort in developing algorithms, this is by no means a closed subject. In this article, we will review the full range of computational methods used, from unstructured finite-element (FE) and finite-volume (FV) methods through fully structured and block-structured approaches utilizing the cut-cell family of techniques to capture the geometric complexity inherent in shape casting. This discussion will include the challenges of generating rapid solutions on high-performance parallel cluster technology and how mold filling links in with the full spectrum of physics involved in shape casting. Finally, some indications as to novel techniques emerging now that can address genuinely arbitrarily complex geometries are briefly outlined and their advantages and disadvantages are discussed.

The Variation in Wake Structure of a Tidal Stream Turbine with Flow Velocity

A combined Blade Element Momentum—Computational Fluid Dynamics (BEM-CFD) model is applied to a 10 m diameter tidal stream turbine blade and the supporting nacelle and tower structure in a 700 m long rectangular channel. The modelling approach is computationally efficient and is suitable for capturing the time-averaged influence of the turbine on the flow. A range of simulations are conducted for the purpose of undertaking a comparative study of the influence of the turbine on mean flow characteristics. Variations in flow structure around the turbine for different flow conditions were evaluated.

An alternative mixed Eulerian Lagrangian approach to high speed collision between solid structures on parallel clusters

The conventional approach to the analysis of collision problems, where a projectile penetrates a structure, involves a Lagrangian-Lagrangian contact driven methodology. Over the years there has been an enduring interest in collision type problems. However, since the events of 11th September 2001 (9/11) there has emerged a particular interest in projectile-structure collision events which simultaneously involve combustion, significant heat transfer and melting. These latter aspects are conventionally modelled using an Eulerian approach with computational fluid dynamics (CFD) software technology. Thus to model high speed collision in a comprehensive manner, it is necessary to take full advantage of the wide range of physics represented by CFD codes and explicit dynamic structural FE codes, which is not a trivial matter. The strain rates are so high in the neighbourhood of the collision that in representing the material behaviour as plastic one can formulate a model in an Eulerian manner. Thus, if the projectile-structure interaction can be captured adequately by an Eulerian approach, then one could use conventional CFD technology to model the whole spectrum of physics where combustion is simultaneously involved. As such, the prime objective of this paper is to implement and evaluate the use of conventional Eulerian CFD technology using well established free surface algorithms to capture the multi-material behaviour in a fixed grid environment and to evaluate the performance and whether the parallel scalability can be preserved. The paper is completed by a preliminary evaluation of whether compatible Eulerian and Lagrangian code modules can be coupled to capture the elastic behaviour of the structure far from the collision site.

Multi-component free surface flows and rotating devices in the context of minerals processing

In analysing the treatment and transport of slurries (i.e. particle loaded fluids) in minerals processing, it is common to come up against significant challenges from the perspective of the computational fluid dynamics (CFD) modelling, especially in trying to optimise their transport – to maintain uniformity of particle distribution or minimise their abrasive effects. These flows are essentially multi-component, non-Newtonian and their context is such that they may well involve complex free surfaces and also be in rotating equipment, as well, of course, involving rather complex geometrical configurations. Here we describe CFD models of some key slurry transport processes using a finite volume unstructured mesh-based code using a range of numerical procedures – algebraic slip models for capturing the particulate behaviour, scalar equation algorithms for the free surfaces and source-sink algorithms for the flow through rotating machinery. Applications of the above phenomena coupled are described together with some of the challenges in configuring CFD models.

Domain Decomposition Using a 2-Level Correction Scheme

The PHYSICA software was developed to enable multiphysics modelling allowing for interaction between Computational Fluid Dynamics (CFD) and Computational Solid Mechanics (CSM) and Computational Aeroacoustics (CAA). PHYSICA uses the finite volume method with 3-D unstructured meshes to enable the modelling of complex geometries. Many engineering applications involve significant computational time which needs to be reduced by means of a faster solution method or parallel and high performance algorithms. It is well known that multigrid methods serve as a fast iterative scheme for linear and nonlinear diffusion problems. This papers attempts to address two major issues of this iterative solver, including parallelisation of multigrid methods and their applications to time dependent multiscale problems.