Geometrical properties of rigid frictionless granular packings as a function of particle size and shape

Three-dimensional discrete numerical simulation is used to investigate the properties of close-packed frictionless granular assemblies as a function of particle polydispersity and shape. Unlike some experimental results, simulations show that disordered packings of pinacoids (eight-face convex polyhedron) achieve higher solid fraction values than amorphous packings of spherical or rounded particles, thus fulfilling the analogue of Ulam's conjecture stated by Jiao and co-workers for random packings [Y. Jiao and S. Torquato, Phys. Rev. E $\textbf{84}$, $041309$ ($2011$)]. This seeming discrepancy between experimental and numerical results is believed to lie with difficulties in overcoming interparticle friction through experimental densification processes. Moreover, solid fraction is shown to increase further with bidispersity and peak when the volume proportion of small particles reaches $30\%$. Contrarywise, substituting up to $50\%$ of flat pinacoids for isometric ones yields solid fraction decrease, especially when flat particles are also elongated. Nevertheless, particle shape seems to play a minor role on packing solid fraction compared to polydispersity. Additional investigations focused on the packing microstructure confirm that pinacoid packings fulfill the isostatic conjecture and that they are free of order except beyond $30$ to $50\%$ of flat or flat \& elongated polyhedra in the packing. This order increase progressively takes the form of a nematic phase caused by the reorientation of flat or flat \& elongated particles to minimize the packing potential energy. Simultaneously, this reorientation seems to increase the solid fraction value slightly above the maximum achieved by monodisperse isometric pinacoids, as well as the coordination number. Finally, partial substitution of elongated pinacoids for isometric ones has limited effect on packing solid fraction or order.Comment: 12 figures, 12 page

Ion temperature gradient instability at sub-Larmor radius scales with non-zero ballooning angle

Linear gyro-kinetic stability calculations predict unstable toroidal Ion Temperature Gradient modes with normalised poloidal wave vectors well above one ($k_\theta \rho_i > 1$) for standard parameters and with adiabatic electrons. These modes have a maximum amplitude at a poloidal angle $\theta$ that is shifted away from the low field side ($\theta \ne 0$). The physical mechanism is clarified through the use of a fluid model. It is shown that the shift of the mode away from the low field side ($\theta \ne 0$) reduces the effective drift frequency, and allows for the instability to develop. Numerical tests using the gyro-kinetic model confirm this physical mechanism. It is furthermore shown that modes with $\theta \ne 0$ can be important also for $k_\theta \rho_i < 1$ close to the threshold of the ITG. In fact, modes with $\theta \ne 0$ can exist for normalised temperature gradient lengths below the threshold of the ITG obtained for $\theta = 0$

Mechanical tests for evaluation of the integrity of the implant abutment connection

Nos últimos trinta anos, a reabilitação oral por sistemas de implantes tornou-se uma das técnicas mais bem-sucedidas. Um sistema de implante tem habitualmente 2 corpos, um endosteal e um dispositivo de suporte para a prótese conectados por um parafuso. Ao longo desta interface, as microgaps podem se desenvolver. Estes espaços constituem um abrigo para bactérias, o que pode aumentar o risco de periimplantites, associadas a uma maior infiltração de células inflamatórias e perda de osso. Para reduzir esse risco e, portanto, o tamanho e número de microgaps, são realizados testes mecânicos para adquirir um maior conhecimentos sobre o desempenho de cada sistema de implantes. O comportamento dos dispositivos fora testados recorrendo a : análise microbiológica, extração de DNA, análise por estereomicroscópio, microscópio eletrônico de varredura, modelagem de elementos finitos, análise radiográfica e análise espectrofotométrica. Materiais e métodos : A pesquisa foi realizada entre novembro de 2016 e abril de 2017. Estudos experimentais, artigos de revisão e livros escritos em inglês com texto de versão de acesso completo, data de edição da publicação entre 2005 e 2017, e condições do teste mais realistas, como as condições na cavidade oral são critérios de inclusão. Os resultados foram posteriormente analisados, com o objetivo de avaliar a integridade da conexão de implante-pilar.Over the last thirty years, the oral rehabilitation by implants systems has become one of the most successful techniques. An implant system has two main parts, an endosteal fixture and a prosthesis-supporting abutment connected to fixture with a screw. Along this interface, microgaps can develop; they are defined as a microscopic space. This localisation play a role of safe house for bacteria, which can increase the risk of peri-implantitis, they are associated with a higher inflammatory cell infiltration and bone loss. To reduce this risk and therefore the size and number of microgaps, mechanical tests are carried out in order to develop knowledge about the capacities of each implant system. As tests, it was found: microbiological analysis, DNA extraction, analysis by stereomicroscope, scanning electron microscope, finite element modeling, radiographic analysis and spectrophotometric analysis. Material and methods: the research was done between November 2016 and April 2017. Experimental studies, review articles and books written in the English with a full access version text, the edition date of the publication between 2005 and 2017, and conditions of the test more realistic, like the conditions in oral cavity are inclusions criteria. Their results are subsequently analysed, for the aim of an evaluation of the integrity of the implant-abutment connection

Estimate of convection-diffusion coefficients from modulated perturbative experiments as an inverse problem

The estimate of coefficients of the Convection-Diffusion Equation (CDE) from experimental measurements belongs in the category of inverse problems, which are known to come with issues of ill-conditioning or singularity. Here we concentrate on a particular class that can be reduced to a linear algebraic problem, with explicit solution. Ill-conditioning of the problem corresponds to the vanishing of one eigenvalue of the matrix to be inverted. The comparison with algorithms based upon matching experimental data against numerical integration of the CDE sheds light on the accuracy of the parameter estimation procedures, and suggests a path for a more precise assessment of the profiles and of the related uncertainty. Several instances of the implementation of the algorithm to real data are presented.Comment: Extended version of an invited talk presented at the 2012 EPS Conference. To appear in Plasma Physics and Controlled Fusio

Mechanical tests for evaluation of the integrity of the implant abutment connection

Nos últimos trinta anos, a reabilitação oral por sistemas de implantes tornou-se uma das técnicas mais bem-sucedidas. Um sistema de implante tem habitualmente 2 corpos, um endosteal e um dispositivo de suporte para a prótese conectados por um parafuso. Ao longo desta interface, as microgaps podem se desenvolver. Estes espaços constituem um abrigo para bactérias, o que pode aumentar o risco de periimplantites, associadas a uma maior infiltração de células inflamatórias e perda de osso. Para reduzir esse risco e, portanto, o tamanho e número de microgaps, são realizados testes mecânicos para adquirir um maior conhecimentos sobre o desempenho de cada sistema de implantes. O comportamento dos dispositivos fora testados recorrendo a : análise microbiológica, extração de DNA, análise por estereomicroscópio, microscópio eletrônico de varredura, modelagem de elementos finitos, análise radiográfica e análise espectrofotométrica. Materiais e métodos : A pesquisa foi realizada entre novembro de 2016 e abril de 2017. Estudos experimentais, artigos de revisão e livros escritos em inglês com texto de versão de acesso completo, data de edição da publicação entre 2005 e 2017, e condições do teste mais realistas, como as condições na cavidade oral são critérios de inclusão. Os resultados foram posteriormente analisados, com o objetivo de avaliar a integridade da conexão de implante-pilar.Over the last thirty years, the oral rehabilitation by implants systems has become one of the most successful techniques. An implant system has two main parts, an endosteal fixture and a prosthesis-supporting abutment connected to fixture with a screw. Along this interface, microgaps can develop; they are defined as a microscopic space. This localisation play a role of safe house for bacteria, which can increase the risk of peri-implantitis, they are associated with a higher inflammatory cell infiltration and bone loss. To reduce this risk and therefore the size and number of microgaps, mechanical tests are carried out in order to develop knowledge about the capacities of each implant system. As tests, it was found: microbiological analysis, DNA extraction, analysis by stereomicroscope, scanning electron microscope, finite element modeling, radiographic analysis and spectrophotometric analysis. Material and methods: the research was done between November 2016 and April 2017. Experimental studies, review articles and books written in the English with a full access version text, the edition date of the publication between 2005 and 2017, and conditions of the test more realistic, like the conditions in oral cavity are inclusions criteria. Their results are subsequently analysed, for the aim of an evaluation of the integrity of the implant-abutment connection

Modelling the settling of suspended sediments for concentrations close to the gelling concentration

This paper deals with the sedimentation of highly concentrated sediment suspensions (cohesive as well as non-cohesive) and the beginning of the consolidation of cohesive sediments. Based on a comparison of existing empirical formulas and experimental data, the particle Reynolds number was shown to be of importance for the behaviour of particularly non-cohesive sediments. In addition it plays a role in determining whether one or two interfaces develop during the sedimentation phase. In the case of cohesive sediments, the estimation of the gelling concentration, although difficult, seems to be fundamental. Some suggestions on the estimation of the permeability coefficient and total settling function are then given in order to improve the modelling of the sedimentation and consolidation behaviour for concentrations close to the gelling concentration

Angular momentum transport modeling: achievements of a gyrokinetic quasi-linear approach

QuaLiKiz, a model based on a local gyrokinetic eigenvalue solver is expanded to include momentum flux modeling in addition to heat and particle fluxes. Essential for accurate momentum flux predictions, the parallel asymmetrization of the eigenfunctions is successfully recovered by an analytical fluid model. This is tested against self-consistent gyrokinetic calculations and allows for a correct prediction of the ExB shear impact on the saturated potential amplitude by means of a mixing length rule. Hence, the effect of the ExB shear is recovered on all the transport channels including the induced residual stress. Including these additions, QuaLiKiz remains ~10 000 faster than non-linear gyrokinetic codes allowing for comparisons with experiments without resorting to high performance computing. The example is given of momentum pinch calculations in NBI modulation experiments

Effect of confinement on dense packings of rigid frictionless spheres and polyhedra

International audienceWe study numerically the influence of confinement on the solid fraction and on the structure of three-dimensional random close-packed granular materials subject to gravity. The effects of grain shape (spherical or polyhedral), material polydispersity, and confining wall friction on this dependence are investigated. In agreement with a simple geometrical model, the solid fraction is found to decrease linearly for increasing confinement no matter the grain shape. Furthermore, this decrease remains valid for bidisperse sphere packings, although the gradient seems to reduce significantly when the proportion of small particles reaches 40% by volume. The confinement effect on the coordination number is also captured by an extension of the aforementioned model

Confined packings of frictionless spheres and polyhedra

International audienceBy means of numerical simulations, we study the influence of confinement on three-dimensional random close packed (RCP) granular materials subject to gravity. The effects of grain shape (spherical or polyhedral) and polydispersity on this dependence are investigated. In agreement with a simple geometrical model, the solid fraction is found to decrease linearly for increasing confinement no matter the grain shape. This decrease remains valid for bidisperse sphere packings although the gradient seems to reduce significantly when the proportion of small particles reaches 40% by volume. The aforementioned model is extended to capture the effect of the confinement on the coordination number

Conceptual Modelling of the Sediment Flux during a Flushing Event (Arc En Maurienne, France)

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv