Critical evaluation of numerical techniques for highly non-linear field diffusion modelling

Various numerical techniques have been applied to multidimensional field diffusion problems with front-type behaviour, moving boundaries and non-linear material properties. Advantages and implementation challenges of the methods are discussed with special attention paid to conservation properties of the algorithm and achieving accurate solutions close to the moving boundaries. The techniques are evaluated using analytical solutions of diffusion problems with cylindrical symmetry

Modelling of impulse loading in high-temperature superconductors. Assessment of accuracy and performance of computational techniques.

Purpose – The aim of this paper is to access performance of existing computational techniques to model strongly non-linear field diffusion problems. Design/methodology/approach – Multidimensional application of a finite volume front-fixing method to various front-type problems with moving boundaries and non-linear material properties is discussed. Advantages and implementation problems of the technique are highlighted by comparing the front-fixing method with computations using fixed grids. Particular attention is focused on conservation properties of the algorithm and accurate solutions close to the moving boundaries. The algorithm is tested using analytical solutions of diffusion problems with cylindrical symmetry with both spatial and temporal accuracy analysed. Findings – Several advantages are identified in using a front-fixing method for modelling of impulse phenomena in high-temperature superconductors (HTS), namely high accuracy can be obtained with a small number of grid points, and standard numerical methods for convection problems with diffusion can be utilised. Approximately, first order of spatial accuracy is found for all methods (stationary or mobile grids) for 2D problems with impulse events. Nevertheless, errors resulting from a front-fixing technique are much smaller in comparison with fixed grids. Fractional steps method is proved to be an effective algorithm for solving the equations obtained. A symmetrisation procedure has to be introduced to eliminate a directional bias for a standard asymmetric split in diffusion processes. Originality/value – This paper for the first time compares in detail advantages and implementation complications of a front-fixing method when applied to the front-type field diffusion problems common to HTS. Particular attention is paid to accurate solutions in the region close to the moving front where rapid changes in material properties are responsible for large computational errors. Keywords - Modelling, Numerical analysis, Diffusion, High temperatures, Superconductors Paper type - Research pape

Preliminary Investigation into Modeling The Damage to Carbon Fibre Composites Due to the Thermo-electric Effects of a Lightning Strikes

The impact of a lightning strike causes a short high electrical current burst through Carbon Fibre Composites (CFC). Due to the electrical properties of CFC the large current leads to a rapid heating of the surrounding impact area which degrades and damages the CFC. It is therefore necessary to study in detail the thermal response and possible degradation processes caused to CFC. The degradation takes place in two ways, firstly via direct mechanical fracture due to the thermal expansion of the CFC and secondly via thermo-chemical processes (phase change and pyrolysis) at high temperatures. The main objective of this work is to construct a numerical model of the major physical processes involved, and to understand the correlation between the damage mechanisms and the damage witnessed in modern CFC. For this work we are only considering the thermo-chemical degradation of CFC. Bespoke numerical models have been constructed to predict the extent of the damage caused by the two thermo-chemical processes separately (e.g. a model for phase change and a model for pyrolysis). The numerical model predictions have then been verified experimental by decoupling of the damage mechanisms, e.g. the real Joule heating from a lightning strike is replaced by a high power laser beam acting on composite surface. This was done to simplify the physical processes which occur when a sample is damaged. The experimentally damaged samples were then investigated using X-ray tomography to determine the physical extent of the damage. The experimental results are then compared with the numerical predictions by considering the physical extent of the polymer removal. The extent of polymer removal predicted by the numerical model, solving for pyrolysis, gave a reasonable agreement with the damage seen in the experimental sample. Furthermore the numerical model predicts that the damage caused by polymer phase change has a minimal contribution to the overall extent of the damage

Arc modelling for switching performance evaluation in low-voltage switching devices

Arc modelling is an efficient tool for predicting the switching performance of low-voltage switching devices (LVSDs)prior to testing real products. Moreover, it offers a valuable design aid in the improvement and optimization of LVSDs. This paper focuses on the investigation of evaluators that predict reignition phenomena and the numerical simulation of arc characteristics in LVSDs. It is found that the probability of reignition depends strongly on the ratio of the system voltage to the exit-voltage. The implemented 3-D arc model is based on conventional magnetohydrodynamics theory and takes into account the properties of air that vary with temperature and pressure, motion of the contact, arc root formation and plasma radiation. The simulated results are compared with experimental data to validate the proposed arc model and the voltage trends show agreement

Assessing the dispersion of nanoinclusions in nanoreinforced CFRP laminates using electrical resistance measurements

In this work electrical resistance measurements were utilized as a mean to assess the dispersion of Graphene Oxide nanoinclusions into CFRP laminates. This approach involves resistance measurements in the through-thickness and transverse directions of the laminates. Measurements were conducted in CFRP laminates containing Graphene Oxide dispersed into the epoxy matrix with filler contents of 1wt% and 5wt%. The morphology of the samples was examined by means of optical microscopy and scanning electron microscopy. The obtained results suggest that at low filler contents, 1wt%, the dispersion is uniform into the bulk of the sample, while at higher filler loadings, 5wt%, there are indications of non-uniformity due to the formation of areas with higher filler concentration

Estimation of contact resistivity in lightning protection equipotential Bonding joints of wind turbine blades

Modern lightning protection systems for wind turbine blades with conducting structural elements, e.g., carbon fiber reinforced polymer (CFRP) spar caps, contain equipotential bonding joints to prevent sparking during strikes. Significant current levels are experienced through the joints and the characterization of the electrical contact at the bonding regions is essential for reliable protection. Therefore, this article aims to characterize the contact resistivity of several equipotential bonding joints. The proposed methodology first measures the total resistance of the samples, and then the bulk resistance of the conductive elements is computed using the finite-element method. The latter is required to predict the spreading effects in CFRP components due to the strong anisotropic nature of such materials. After that, the contact resistance is calculated by subtracting the predicted bulk resistances from the measured total resistances. The developed procedure was applied to three typical equipotential bonding materials: expanded copper foil (ECF), biaxial (BIAX) CFRP, and unidirectional (UD) CFRP. Both ECF and BIAX CFRP showed superior contact quality than the UD CFRP, with one to two orders of magnitude smaller contact resistivity.</p