Many scientific and engineering applications require the solution of large systems of initial value problems arising from method of lines discretization of partial differential equations. For systems with widely varying time scales, or with complex physical dynamics, implicit time integration schemes are preferred due to their superior stability properties. However, for very large systems accurate solution of the implicit terms can be impractical. For this reason approximations are widely used in the implementation of such methods. The primary focus of this work is on the development of novel ``lightly-implicit'' time integration methodologies. These methods consider the time integration and the solution of the implicit terms as a single computational process. We propose several classes of lightly-implicit methods that can be constructed to allow for different, specific approximations. Rosenbrock-Krylov and exponential-Krylov methods are designed to permit low accuracy Krylov based approximations of the implicit terms, while maintaining full order of convergence. These methods are matrix free, have low memory requirements, and are particularly well suited to parallel architectures. Linear stability analysis of K-methods is leveraged to construct implementation improvements for both Rosenbrock-Krylov and exponential-Krylov methods. Linearly-implicit Runge-Kutta-W methods are designed to permit arbitrary, time dependent, and stage varying approximations of the linear stiff dynamics of the initial value problem. The methods presented here are constructed with approximate matrix factorization in mind, though the framework is flexible and can be extended to many other approximations. The flexibility of lightly-implicit methods, and their ability to leverage computationally favorable approximations makes them an ideal alternative to standard explicit and implicit schemes for large parallel applications. / Ph. D.

This thesis presents an investigation into sources of variability in activating and processing numerical information. Chapter 1 provides an overview of research literature exploring the ways in which magnitude information can be represented, and how models relating to number information have developed. These theoretical models are addressed in relation to the neural representation of number, and the range of behavioural markers which suggest an association between spatial and numerical processing. Chapter 2 using a dual-task paradigm investigated whether magnitude information is accessed on perceiving numbers, or if this information is linked to response selection or execution. Previous research studies investigating this question produced inconsistent findings (Oriet, Tombu & Jolicoeur, 2005; Sigman & Dehaene, 2005) with regard to the locus of magnitude processing; the findings of Experiments 1-3 reliably support access to magnitude information during response selection. Chapter 3 explored the activation of spatial-numerical response associations, where response-irrelevant magnitude information was not represented by a single stimulus (i.e. an Arabic digit) but by a numerosity representation. Experiments 4-7 found a strong association between spatial-orientation processing and numerical magnitude, but no association with perceptual-colour processing, extending previous work by Fias, Lammertyn and Lauwereyns (2001) regarding the neural overlap between the attended and irrelevant stimulus dimensions. However the strength of this association was found to be inconsistent across the number range. Chapter 4 investigated the impact of healthy aging on the presence of neural-overlap in processing spatial-numerical information, further developing the paradigm used in Chapter 3, and addressed direct predictions from the literature as to how age should influence these associations (Wood, Willmes, Nuerk & Fischer, 2008). Experiments 8-11 found evidence for spatial-numerical associations across the lifespan, but that the strength of these effects were moderated by 5 task instruction. Chapter 5 was designed to assess aging differences in numerical and spatial processing with a battery of tests and the extent to which other sources of individual difference (sex, embodiment) have a measureable impact. A range of standardised measures were used to assess verbal ability, mathematical processing, and spatial working memory alongside behavioural measures of spatial numerical associations. Experiment 12 provided evidence of aging and sex differences in different cognitive tasks and a marginal impact of embodiment on spatial-numerical processing; however the effect of embodiment was not supported in a larger more homogenous sample in Experiment 13. Chapter 6 reflects on the current findings and provides contextual information on how they align with previous research, outlining how evidence from the thesis extend current research paradigms and provides new evidence regarding the maintenance of spatial-numerical associations in healthy aging. Methodologies developed in the thesis are considered with relation to how they may be applied to assess individual differences in early number acquisition in children. Finally the discussion outlines methods and controversies within the field of numerical cognition, with consideration of new methods for measuring the strength of spatial-numerical associations (Pinhas, Tzelgov, & Ganor-Stern, 2012), alongside the potential application of modelling techniques to investigate individual differences in task performance.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

In our everyday life, turbulence is an omnipresent phenomenon and yet remains poorly understood. Its random and chaotic nature makes it a subject almost impossible to treat from the mathematical point of view and, at present, there is no real prospect of a simple analytic theory. Scientists have therefore regarded the numerical simulation as an alternative to compute the relevant properties of turbulent flows. In this context, our thesis aims at developing and using accurate computational methods, namely pseudo-spectral methods, for studying hydrodynamic (1st part) and magnetohydrodynamic (2nd part) turbulence. In the hydrodynamic part, Chapter I introduces the governing equations of fluid mechanics as well as the main issues related to the numerical study of turbulent flows. In particular, the Direct Numerical Simulations (DNS) of turbulence, in which accurate numerical solutions of the Navier-Stokes equations are obtained, are shown to be limited to moderately turbulent flows. Chapter II introduces the Large Eddy Simulation (LES) technique which aims at simulating highly turbulent flows and which is based on a separation of scales. In practice, it consists of simulating the large - resolved - scales of the flow explicitly while modelling the small - unresolved - scales. Two different approaches for modelling the kinetic energy of the unresolved scales are proposed and their respective advantages and drawbacks are discussed. Chapter III is devoted the study of the mixing-layer using both DNS and LES. It consists of an inhomogeneous turbulent flow which has been studied experimentally and for which well-documented measurements are available. A highly accurate DNS mimicking the same experiment has been produced. It allows to study the inhomogeneity and anisotropy properties of this flow. Also, LES of the same flow, using different models, have been evaluated. In Chapter IV, we explore a pseudo-spectral method to investigate turbulence in a pipe. In this case, the method has to take into account two additional difficulties: i) the presence of the boundary and ii) the axis singularity. We detail how to circumvent these issues. The second part of the thesis is devoted to magnetohydrodynamic (MHD) turbulence. It concerns phenomena where electrically conducting flows interact with electromagnetism and for which governing equations are derived in Chapter V. In Chapter VI, a detailed analysis of the energy transfers between the magnetic and velocity fields is performed thanks to a high resolution database of homogeneous MHD turbulence. It provides some insights to understand the physics of the nonlinear interactions and is also a valuable diagnostic in the framework of LES modelling. Finally, the inhomogeneous configuration studied in Chapter III has been extended to MHD. Several statistics related to the kinetic and magnetic energies are measured and LES of this flow are performed and presented in Chapter VII.

<p>Multi-scale wave propagation problems are computationally costly to    solve by traditional techniques because the smallest scales must be    represented over a domain determined by the largest scales of the    problem.  We have developed new numerical methods for multi-scale wave    propagation in the framework of heterogeneous multi-scale methods.  The    numerical methods couples simulations on macro and micro scales with    data exchange between models of different scales.  With the new method    we are able to consider a general class of problems including some    problems where a homogenized equation is unknown.  We show that the    complexity of the new method is significantly lower than that of    traditional techniques.  Numerical results are presented from problems    in one, two and three dimensional and for finite and long time.  We also    analyze the method, in one and several dimensions and for finite time,    using Fourier analysis.</p>

This thesis uses numerical simulations to assess the most suitable model type for simulating dispersive and non dispersive tsunami wave propagation over a range of bathymetries. These simulations are presented in two parts. The first part highlights differences between results as predicted by a fully nonlinear Boussinesq model (with its ability to predict dispersion) and a non dispersive, linear or weakly nonlinear model, for simulations of a dispersive wave incident at various idealized bathymetric features. The second part determines the efficacy in a real world application of the Boussinesq model as opposed to a nonlinear shallow water model. In addition, a discussion on the geophysical parameters which influence the choice of numerical model for simulating tsunami propagation in a particular bathymetric region is provided.

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