La technique du contrôle non destructif (CND) par courants de Foucault (CF) est une solution envisageable pour caractériser l’état de contraintes dans un matériau magnétique. En effet, les propriétés magnétiques d’un matériau magnétique dépendent sensiblement de l’état de contraintes et, par ailleurs, les CF dépendent de la perméabilité magnétique. La technique des CF est potentiellement intéressante comparée à d’autres méthodes de CND, telles que la diffraction des rayons X, car elle est simple à mettre en œuvre, automatisable et peu coûteuse. Dans l'objectif de permettre in fine l’identification inverse de l’état de contraintes, un modèle prédisant l'évolution du signal fourni par un capteur à CF en fonction de l’état de contraintes du matériau ferromagnétique contrôlé a été élaboré dans cette thèse. Ceci implique la mise en place d’une double modélisation. D’une part, l’effet des contraintes sur la perméabilité magnétique a été modélisé par un modèle de comportement magnéto-élastique simplifié dérivé d'une approche multi-échelle. Cette approche permet de décrire la perméabilité magnétique d’un matériau soumis à un chargement multiaxial, en incluant notamment les effets d'anisotropie induite. D’autre part, un modèle reposant sur la méthode des éléments finis a été développé afin de prédire le signal fourni par un capteur à CF en fonction de la perméabilité anisotrope du matériau inspecté. Afin de valider la démarche de modélisation, un protocole expérimental de caractérisation magnétique et par CF a été mis en place. Les résultats de mesure obtenus présentent un bon accord qualitatif avec la modélisation, en l’absence de tout étalonnage. Une procédure d’étalonnage s'appuyant sur une mesure sous contraintes est nécessaire pour atteindre un accord quantitatif. Le modèle développé pourrait être utilisé pour concevoir des sondes à CF idoines et identifier les conditions opératoires optimales pour l'estimation de contraintes dans les matériaux magnétiques. Les procédures d'inversion à mettre en œuvre restent cependant un défi à relever / The non-destructive evaluation (NDE) technique by eddy current (EC) is a conceivable solution to characterize the stress state in magnetic materials. The approach relies on the high sensitivity of eddy current (EC) signals to the magnetic permeability, itself highly dependent on stress. The EC technique is potentially attractive compared to other NDE methods such as X-ray diffraction, due to its simple practical implementation, easiness of automation and low cost. In order to allow eventually the inverse identification of stress states in magnetic materials, a predictive model for the evolution of an EC probe signal as a function of stress has been developed during this thesis. The modelling is done in two steps. First, the effect of stress on the magnetic permeability is described using a simplified version of a multiscale model for magneto-elastic behaviour. This approach allows describing the effect of multiaxial mechanical loadings on the magnetic behaviour of materials including induced anisotropy effects. Then, the EC probe signal is determined as a function of the anisotropic permeability of the stressed material using the finite element method (FEM). In order to validate the modelling approach, an experimental setup for magnetic characterisation and EC measurements was developed. The measurements show a good qualitative accordance with the modelling results, in absence of any calibration. A calibration procedure based on a measurement under stress is necessary to obtain a quantitative agreement. The proposed model can be used to design efficient EC probes and to define optimal operating conditions to evaluate stress in magnetic materials. The development of inversion procedures, however, remains a challenge.

Manufacturers of cast metal parts are interested in the development of a feedback control system for use with the Precision Sand-Casting (PSC) process. As industry demands the ability to cast more complex geometries, there are a variety of challenges that engineers have to address. Certain characteristics of the mold, such as thick-to-thin transitions, extensive horizontal or flat surfaces, and sharp corners increase the likelihood of generating defective casts due to the turbulent metal-flow during fills. Consequently, it is critical that turbulent flow behavior within the mold be minimized as much as possible. One way to enhance the quality of the fill process is to adjust the flow rate of the molten metal as it fills these critical regions of the mold. Existing systems attempt to predict the position of the metal level based on elapsed time from the beginning of the fill stage. Unfortunately, variability in several aspects of the fill process makes it very difficult to consistently predict the position of the metal front. A better approach would be to embed a sensor that can detect the melt through a lift-off distance and determine the position of the metal-front. The information from this sensor can then be used to adjust the flow rate of the aluminum as the mold is filled. This thesis presents the design of a novel non-invasive sensor monitoring system. When deployed on the factory floor, the sensing system will provide all necessary information to allow process engineers to adjust the metal flow-rate within the mold and thereby reduce the amount of scrap being produced. Moreover, the system will exhibit additional value in the research and development of future mold designs.

A Corrente do Brasil (CB) é a Corrente de Contorno Oeste (CCO) que ocupa a parte mais externa da plataforma continental sudeste brasileira e o talude continental e está centrada entre as isóbatas de 200 e 1000 m (Stramma & England, 1999). A CB flui ao largo do sudeste brasileiro sustentando a característica das CCO\'s, que é apresentar intensa atividade de mesoescala (Campos, 1995). Dentre as feições formadas estão os Vórtices de Cabo de São Tomé (VCST) e Cabo Frio (VCF). As causas da grande atividade vortical da CB e processos envolvendo o crescimento dos meandros foram extensivamente estudados (Campos, 1995; Silveira et. al. 2004). No entanto, o conhecimento acerca dos padrões de ocorrência dessas feições, ainda não é completo. Esse é o objetivo desse estudo, no qual foram utilizados, dados hidrográficos e de corrente de três Comissões Oceanográficas para localizar o Eixo de Velocidade Máxima da CB, utilizado para nortear a digitalização manual dos VCF e VCST sobre de um conjunto de 8 anos de imagens MODIS de TSM no período de 2003-2010. As curvas digitalizadas foram analisadas estatisticamente. Dos 258 vórtices que foram digitalizados 101 são relativos ao VCST e 157 relativos ao VCF, o que representou uma ocorrência de 60,14% nas imagens analisadas. O VCF apresentou um padrão mais propagante no verão e padrão de crescimento estacionário com o desprendimento de alguns vórtices na direção sul no outono e inverno. Todas as estações evidenciaram que 24º S - 42º W é a região de formação do VCF. O VCST apresentou um padrão de crescimento estacionário para as quatro estações, com o desprendimento de alguns vórtices isolados principalmente no outono e inverno. A maior concentração de curvas se deu em 22,5º S - 40,5º W, evidenciando o seu sítio de formação. A primavera apresentou maior alternância de formação entre os dois vórtices. A maior simultaneidade foi encontrada no verão. A média de tamanho do eixo maior foi de 73,13 km para o VCST e 112,03 km para o VCF. No que tange à forma, ambos são preferencialmente elípticos, tendo o VCF direção de crescimento NE - SO e o VCST direção L - O. Os tamanhos máximos do eixo maior de ambos os vórtices (~200 km) foram encontrados em 2010, um ano aparentemente atípico. / The Brazil Current (BC) is the Western Boundary Current (WBC) flowing along the outer part of the Brazilian southeastern continental shelf and continental slope and is centered between the isobaths of 200 and 1000 m (Stramma & England, 1999). The BC flows off Brazilian Southeast Coast sustaining a feature of WBC\'s, which present intense mesoscale activity (Campos , 1995) . Among these features, the Cabo Frio Eddy (CFE) and Cabo de São Tomé Eddy (CSTE) are formed. The causes of the large eddy activity of the BC and the processes involving the growth of meanders have been extensively studied (Campos, 1995; Silveira et. al. 2004). However, the knowledge of the patterns of occurrence of these features is not complete yet. That is the goal of this study, in which were used, hydrographic and current data of three Oceanographic Cruises to locate the BC Maximum Velocity Axis, used to guide the manual scanning of the CFE and CSTE on a set of eight years (2003-2010) of MODIS SST images. The digitalized curves were analyzed statistically. Of the 258 vortices that have been scanned 101 are relative to CSTE and 157 related to the CFE, which represented 60.14 % of the occurrences in the total images analyzed. The CFE presented a propagating characteristic during the summer and steady growth with the release of some eddies southbound in the autumn and winter. All seasons showed that 24 º S - 42 º W is the region where the CFE is formed. The CSTE presented a steady growth for the four seasons, with the detachment of some isolated eddies mainly in autumn and winter. The largest concentration of curves occurred at 22.5 ° S - 40.5 ° W, showing that this is the site of its formation. The spring had the highest alternating formation between the two eddies. The most common concurrency was found in the summer. The average size of the major axis was 73.13 km for the CSTE and 112.03 km for the CFE. Regarding the form, both are preferably elliptical, with the CFE growth direction NE - SW and CSTE growth direction E - W. The maximum sizes of the major axis of both (~ 200 km) were found in 2010 , apparently an atypical year.

A Corrente do Brasil (CB) é a Corrente de Contorno Oeste (CCO) que ocupa a parte mais externa da plataforma continental sudeste brasileira e o talude continental e está centrada entre as isóbatas de 200 e 1000 m (Stramma & England, 1999). A CB flui ao largo do sudeste brasileiro sustentando a característica das CCO\'s, que é apresentar intensa atividade de mesoescala (Campos, 1995). Dentre as feições formadas estão os Vórtices de Cabo de São Tomé (VCST) e Cabo Frio (VCF). As causas da grande atividade vortical da CB e processos envolvendo o crescimento dos meandros foram extensivamente estudados (Campos, 1995; Silveira et. al. 2004). No entanto, o conhecimento acerca dos padrões de ocorrência dessas feições, ainda não é completo. Esse é o objetivo desse estudo, no qual foram utilizados, dados hidrográficos e de corrente de três Comissões Oceanográficas para localizar o Eixo de Velocidade Máxima da CB, utilizado para nortear a digitalização manual dos VCF e VCST sobre de um conjunto de 8 anos de imagens MODIS de TSM no período de 2003-2010. As curvas digitalizadas foram analisadas estatisticamente. Dos 258 vórtices que foram digitalizados 101 são relativos ao VCST e 157 relativos ao VCF, o que representou uma ocorrência de 60,14% nas imagens analisadas. O VCF apresentou um padrão mais propagante no verão e padrão de crescimento estacionário com o desprendimento de alguns vórtices na direção sul no outono e inverno. Todas as estações evidenciaram que 24º S - 42º W é a região de formação do VCF. O VCST apresentou um padrão de crescimento estacionário para as quatro estações, com o desprendimento de alguns vórtices isolados principalmente no outono e inverno. A maior concentração de curvas se deu em 22,5º S - 40,5º W, evidenciando o seu sítio de formação. A primavera apresentou maior alternância de formação entre os dois vórtices. A maior simultaneidade foi encontrada no verão. A média de tamanho do eixo maior foi de 73,13 km para o VCST e 112,03 km para o VCF. No que tange à forma, ambos são preferencialmente elípticos, tendo o VCF direção de crescimento NE - SO e o VCST direção L - O. Os tamanhos máximos do eixo maior de ambos os vórtices (~200 km) foram encontrados em 2010, um ano aparentemente atípico. / The Brazil Current (BC) is the Western Boundary Current (WBC) flowing along the outer part of the Brazilian southeastern continental shelf and continental slope and is centered between the isobaths of 200 and 1000 m (Stramma & England, 1999). The BC flows off Brazilian Southeast Coast sustaining a feature of WBC\'s, which present intense mesoscale activity (Campos , 1995) . Among these features, the Cabo Frio Eddy (CFE) and Cabo de São Tomé Eddy (CSTE) are formed. The causes of the large eddy activity of the BC and the processes involving the growth of meanders have been extensively studied (Campos, 1995; Silveira et. al. 2004). However, the knowledge of the patterns of occurrence of these features is not complete yet. That is the goal of this study, in which were used, hydrographic and current data of three Oceanographic Cruises to locate the BC Maximum Velocity Axis, used to guide the manual scanning of the CFE and CSTE on a set of eight years (2003-2010) of MODIS SST images. The digitalized curves were analyzed statistically. Of the 258 vortices that have been scanned 101 are relative to CSTE and 157 related to the CFE, which represented 60.14 % of the occurrences in the total images analyzed. The CFE presented a propagating characteristic during the summer and steady growth with the release of some eddies southbound in the autumn and winter. All seasons showed that 24 º S - 42 º W is the region where the CFE is formed. The CSTE presented a steady growth for the four seasons, with the detachment of some isolated eddies mainly in autumn and winter. The largest concentration of curves occurred at 22.5 ° S - 40.5 ° W, showing that this is the site of its formation. The spring had the highest alternating formation between the two eddies. The most common concurrency was found in the summer. The average size of the major axis was 73.13 km for the CSTE and 112.03 km for the CFE. Regarding the form, both are preferably elliptical, with the CFE growth direction NE - SW and CSTE growth direction E - W. The maximum sizes of the major axis of both (~ 200 km) were found in 2010 , apparently an atypical year.

In this dissertation we describe a newly discovered subsurface frontal meander of the North Brazil Undercurrent (NBUC), centered at about 4ºS, 36.5ºW, hereafter Potiguar Eddy (PE). The Potiguar Eddy is an elliptically-shaped eddy, with major and minor axes of approximately 330 and 130 km, with a subsurface signature. It extends vertically from 100 m to 400 m, with maximum velocities of 0.6 m.s-1 and recirculates about 2 Sv of waters from the NBUC. Despite the presence of the PE in subtermocline waters, the surface flow is northwestward throughout the year. The analysis of variability of mooring velocity data in the Potiguar Bight revealed two most important statistical modes of variability (EOFs); one with high amplitudes over most of the portion of the water column which corresponds to the PE, and another one, explaining a lower percentage of the variance, associated with upper-layer processes. The first EOF mode is found to be associated with baroclinic oscillations with periods of about 25-35 days. This signal is also seen in altimetric fields in the Atlantic Ocean. We characterize these signals as Rossby waves, and speculate that the trigger for their generation would be barotropic instability generated by the current shear in the Tropical Atlantic Ocean. Aditionally, with the intent of analyzing the generation process of the PE, we developed Feature Models of the NBUC and Deep Western Boundary Current (DWBC), to be used as basis for developing the initial field for numerical model simulations in a dynamical process-study approach. The Feature Models, developed from observed velocity data, with temperature calculated through the thermal wind equation, and salinity from WOA (2013) climatology, successfully capture the main attributes of the features of interest, and thus are suitable for the investigation of the main characteristics of the flow in the Potiguar Bight. Two numerical experiments were set up; (i) a NBUC-only experiment, with maximum depth of 1500 m, and (ii) a NBUC-DWBC experiment, with a maximum depth of 5500 m. The Potiguar Eddy was formed in both experiments, with maximum velocity, core depth and size consistent with synoptic observations. As a result, the PE can be generated by a velocity field containing only the NBUC; and the DWBC - induced vertical shear seems to play a part on the eddy\'s characteristics (vertical extent, maximum velocities, etc), since the PE had different formation processes in the two experiments. / Nesta tese descrevemos o Vórtice Potiguar (VP): um meandro frontal da Subcorrente Norte do Brasil (SNB) recentemente identificado na costa nordeste brasileira, com núcleo em aproximadamente 4ºS e 36,5ºW. O VP tem formato elíptico com maior e menor eixos medindo 330 e 130 km, aproximadamente, intensificado em subsuperfície. O vórtice se estende verticalmente de 100 a 400 metros, com velocidades máximas de 0,6 m/s-1, recirculando cerca de 2 Sv das águas da SNB. Apesar da presença do vórtice na termoclina, o fluxo em superfície é para noroeste durante todo o ano. A análise da variabilidade da velocidade coletada em fundeio na Bacia Potiguar revela dois principais modos estatísticos de variabilidade; um com maiores amplitudes na porção da coluna de água correspondente ao VP, e um outro modo, que explica menor porcentagem da variância da série de dados original, associado a processos ligados à camada de superfície. O primeiro EOF é associado a oscilações baroclínicas com períodos de 25 a 35 dias. Este sinal também é identificado em altimetria no Atlântico. Nós caracterizamos esses sinais como expressão de ondas de Rossby, e especulamos que o gatilho para a geração das mesmas seria a instabilidade barotrópica gerada pelo cisalhamento de correntes no Atlântico Tropical. Adicionalmente, com o intuito de analisar os processos de geração do VP, nós desenvolvemos modelos de feição da SNB e Corrente Profunda de Contorno Oeste (CCP), para serem usados como base para o desenvolvimento do campo inicial para simulações de modelagem numérica em uma abordagem de estudo de processo dinâmico. Os Modelos de Feição, desenvolvidos a partir de dados de velocidade, com densidade e temperatura calculados a partir da relação do vento térmico e salinidade da climatologia WOA (2013), capturam os principais atributos das feições de interesse com sucesso, sendo adequados para a investigação das principais características do campo de correntes na Bacia Potiguar. Dois experimentos numéricos foram realizados: (i) Somente SNB, com profundidade máxima de 1500 m, e (ii) SNB + CCP, com profundidade máxima de 5500 m. O VP foi gerado em ambos os experimentos, com velocidade máxima, profundidade do núcleo e tamanho consistentes com observações sinóticas. Assim, temos que o VP pode ser gerado em um campo de velocidades contendo somente a SNB, e que o cisalhamento vertical induzido pela presença da CCP parece ter um papel importante nas características do mesmo (extensão vertical, velocidades máximas, etc), uma vez que o VP teve um processo de formação diferente em cada experimento.

Aero-engine intakes play a critical role in the performance of modern high-bypass turbofan engines. It is their function to provide uniformly distributed, steady air flow to the engine fan face under a variety of flow conditions. However, during situations of high incidence, high curvature of the intake lip can accelerate flow to supersonic speeds, terminating with a shock wave. This produces undesirable shock wave boundary layer interactions (SWBLIs). Reynolds-Averaged Navier Stokes (RANS) turbulence models have been shown to be insensitive to the effects of boundary layer relaminarisation present in these highly-accelerated flows. Further, downstream of the SWBLI, RANS methods fail to capture the distorted flow that propagates towards the engine fan face. The present work describes simulations of a novel experimental intake rig model that replicates the key physics found in a real intake- namely acceleration, shock and SWBLI. The model is a simple geometric configuration resembling a lower intake lip at incidence. Simulations are carried out at two angles of attack, $\alpha=23^{\circ}$ and $\alpha=25^{\circ}$, with the more aggressive $\alpha=25^{\circ}$ possessing a high degree of shock oscillation. RANS, Large Eddy Simulations (LES) and hybrid RANS-LES are carried out in this work. Modifications to the one-equation Spalart-Allmaras (SA) RANS turbulence model are proposed to account for the effects of re-laminarisation and curvature. The simulation methods are validated against two canonical test cases. The first is a subsonic hump model where RANS modifications give a noticeable improvement in surface pressure predictions, even for this mild acceleration case. However, RANS is shown to over-predict the separation size. LES performs much better here, as long as the Smagorinsky-Lilly SGS model is not used. The $\sigma$-SGS model is found to perform best, and is used to run a hybrid RANS-LES that predicts a separation bubble size within $4\%$ of LES. The second canonical test case is a transonic hump that features a normal shockwave and SWBLI. RANS performs well here, predicting shock location, surface pressure and separation with good agreement with experimental measurements. Hybrid RANS-LES also performs well, but predicts a shock downstream of that measured by experiment. The use of an improved shock sensor here is able to maintain solution accuracy. Simulations of the intake rig are then run. RANS modifications provide a significant improvement in prediction of the shock location and lip surface pressure compared to the standard SA model. However, RANS models fail to reproduce the post shock interaction flow well, giving incorrect shape of the flow distortion. Further, RANS is inherently unable to capture the unsteady shock oscillations and related flow features. LES and hybrid RANS-LES predict the shock location and SWBLI well, with the downstream flow distortion also in very good agreement with experimental measurements. LES and hybrid RANS-LES are able to reproduce the time averaged smearing of the shock which RANS cannot. However, shock oscillations in the $\alpha=25^{\circ}$ case present a particular challenge for costly LES, requiring long simulation time to obtain time averaged flow statistics. Hybrid RANS-LES offers a significant saving in computational expense, costing approximately $20\%$ of LES. The work proposes recommendations for simulation strategy for intakes at incidence based on computational cost and performance of simulation methods.

In this dissertation we describe a newly discovered subsurface frontal meander of the North Brazil Undercurrent (NBUC), centered at about 4ºS, 36.5ºW, hereafter Potiguar Eddy (PE). The Potiguar Eddy is an elliptically-shaped eddy, with major and minor axes of approximately 330 and 130 km, with a subsurface signature. It extends vertically from 100 m to 400 m, with maximum velocities of 0.6 m.s-1 and recirculates about 2 Sv of waters from the NBUC. Despite the presence of the PE in subtermocline waters, the surface flow is northwestward throughout the year. The analysis of variability of mooring velocity data in the Potiguar Bight revealed two most important statistical modes of variability (EOFs); one with high amplitudes over most of the portion of the water column which corresponds to the PE, and another one, explaining a lower percentage of the variance, associated with upper-layer processes. The first EOF mode is found to be associated with baroclinic oscillations with periods of about 25-35 days. This signal is also seen in altimetric fields in the Atlantic Ocean. We characterize these signals as Rossby waves, and speculate that the trigger for their generation would be barotropic instability generated by the current shear in the Tropical Atlantic Ocean. Aditionally, with the intent of analyzing the generation process of the PE, we developed Feature Models of the NBUC and Deep Western Boundary Current (DWBC), to be used as basis for developing the initial field for numerical model simulations in a dynamical process-study approach. The Feature Models, developed from observed velocity data, with temperature calculated through the thermal wind equation, and salinity from WOA (2013) climatology, successfully capture the main attributes of the features of interest, and thus are suitable for the investigation of the main characteristics of the flow in the Potiguar Bight. Two numerical experiments were set up; (i) a NBUC-only experiment, with maximum depth of 1500 m, and (ii) a NBUC-DWBC experiment, with a maximum depth of 5500 m. The Potiguar Eddy was formed in both experiments, with maximum velocity, core depth and size consistent with synoptic observations. As a result, the PE can be generated by a velocity field containing only the NBUC; and the DWBC - induced vertical shear seems to play a part on the eddy\'s characteristics (vertical extent, maximum velocities, etc), since the PE had different formation processes in the two experiments. / Nesta tese descrevemos o Vórtice Potiguar (VP): um meandro frontal da Subcorrente Norte do Brasil (SNB) recentemente identificado na costa nordeste brasileira, com núcleo em aproximadamente 4ºS e 36,5ºW. O VP tem formato elíptico com maior e menor eixos medindo 330 e 130 km, aproximadamente, intensificado em subsuperfície. O vórtice se estende verticalmente de 100 a 400 metros, com velocidades máximas de 0,6 m/s-1, recirculando cerca de 2 Sv das águas da SNB. Apesar da presença do vórtice na termoclina, o fluxo em superfície é para noroeste durante todo o ano. A análise da variabilidade da velocidade coletada em fundeio na Bacia Potiguar revela dois principais modos estatísticos de variabilidade; um com maiores amplitudes na porção da coluna de água correspondente ao VP, e um outro modo, que explica menor porcentagem da variância da série de dados original, associado a processos ligados à camada de superfície. O primeiro EOF é associado a oscilações baroclínicas com períodos de 25 a 35 dias. Este sinal também é identificado em altimetria no Atlântico. Nós caracterizamos esses sinais como expressão de ondas de Rossby, e especulamos que o gatilho para a geração das mesmas seria a instabilidade barotrópica gerada pelo cisalhamento de correntes no Atlântico Tropical. Adicionalmente, com o intuito de analisar os processos de geração do VP, nós desenvolvemos modelos de feição da SNB e Corrente Profunda de Contorno Oeste (CCP), para serem usados como base para o desenvolvimento do campo inicial para simulações de modelagem numérica em uma abordagem de estudo de processo dinâmico. Os Modelos de Feição, desenvolvidos a partir de dados de velocidade, com densidade e temperatura calculados a partir da relação do vento térmico e salinidade da climatologia WOA (2013), capturam os principais atributos das feições de interesse com sucesso, sendo adequados para a investigação das principais características do campo de correntes na Bacia Potiguar. Dois experimentos numéricos foram realizados: (i) Somente SNB, com profundidade máxima de 1500 m, e (ii) SNB + CCP, com profundidade máxima de 5500 m. O VP foi gerado em ambos os experimentos, com velocidade máxima, profundidade do núcleo e tamanho consistentes com observações sinóticas. Assim, temos que o VP pode ser gerado em um campo de velocidades contendo somente a SNB, e que o cisalhamento vertical induzido pela presença da CCP parece ter um papel importante nas características do mesmo (extensão vertical, velocidades máximas, etc), uma vez que o VP teve um processo de formação diferente em cada experimento.

Tese de doutoramento. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 2009

When a ferromagnet is heated, it loses its magnetism. Stars and planets have magnetic fields, as does the Earth. But it is known that the center of the Earth is very hot. Therefore, to sustain the large magnetic field of a planet, we cannot look to simple ferromagnetism like that of a bar magnet, but we have to look at the movement of electric charges within the Earth’s molten core to generate magnetic field. This magnetic field sustainment against ohmic dissipation by turbulent flow is referred to as the turbulent dynamo effect. Theoretical research into the mechanisms that create the dynamo has been actively pursued for several decades, culminating recently in massive computer simulations of the Earth’s core. Most of these studies have employed the equations of magnetohydrodynamics (MHD), a nonlinear theory of electrically conducting fluids. The EDQNM (Eddy-Damped Quasi-Normal Markovian) closure is a statistical model designed so that the turbulence equations derived from Navier-Stokes dynamics can be closed and satisfy the realizability condition of positivity of the kinetic energy spectrum. In case of MHD turbulence, realizability requires more work. We have proved in an earlier work that equations analogous to those expected of the EDQNM closure for MHD without mean fields satisfy the appropriate realizability conditions (Turner and Pratt 1999). In this work, we discuss requirements needed to make the MHD equations realizable with mean fields, extending those of neutral fluid turbulence by Turner [1]. Finally, we discuss direct numerical simulations and the correspondence of the statistical theories with simulation results.

The primary aim of this study was to investigate the sedimentation processes within Buffalo Bay, particularly within and adjacent to the Whitianga tidal inlet, in order to ascertain reasons for the shoaling at both the inlet, and the identified shallow zone around Pandora Rock. Comparison of historic bathymetries suggests the ebb delta and ebb discharge channel of the Whitianga tidal inlet are rapidly accreting and the ebb tidal discharge channel is gradually migrating northeast towards Whakapenui Point. Accretion rates of up to 25 cm y-1 were calculated in the ebb delta and inlet discharge channel area between 1979 and 1995 and aerial photo comparisons suggest the ebb delta area had increased by 400 % between 1990 and 2002. Results of the hydrodynamic and sediment transport modelling suggest the rapid accretion in the ebb delta vicinity is likely to be caused by a combination of catchment estuary inputs, which are deposited on the ebb tide as the ebb flow decelerates over the ebb delta, and inputs that have been moved south along Buffalo Beach by flood currents and an eddy that forms landward of the ebb tidal discharge. Residual tidal velocities further suggest a deposition zone in the ebb delta vicinity resulting from opposing currents and the deceleration of currents. Hydrodynamic modelling results indicate the isolated shallow zone around Pandora rock appears to be caused by a transient eddy in the southern section of Buffalo Bay. The eddy is formed by the ebb tidal discharge from the inlet. Accretion probably occurs in the centre of the eddy which moves north as the ebb tide progresses. Results obtained from a current meter and sediment trap deployed in northern Buffalo Bay suggest suspended sediment transport is minimal in northern Buffalo Bay, only occurring with large wave activity. Results of the hydrodynamic and sediment transport modelling further demonstrate that this area experiences low flow velocities, and has little interaction with the rest of Buffalo Bay. The minimal sediment input to this area, combined with the occasional erosion of the seafloor, primarily by wave activity, is thought to have resulted in long term erosion of northwestern Buffalo Bay between 1938 and 1979. Although the beach and nearshore is eroding, it is likely the addition of sediment would act to stabilise this section of eroding beach. Renourishment material could be provided by the ebb delta, the southern tip of Buffalo Bay or the isolated sandbar northeast of the inlet entrance.