Discrete element model for general polyhedra

We present a version of the Discrete Element Method considering the particles as rigid polyhedra. The Principle of Virtual Work is employed as basis for a multibody dynamics model. Each particle surface is split into sub-regions, which are tracked for contact with other sub-regions of neighboring particles. Contact interactions are modeled pointwise, considering vertex-face, edge-edge, vertex-edge and vertex-vertex interactions. General polyhedra with triangular faces are considered as particles, permitting multiple pointwise interactions which are automatically detected along the model evolution. We propose a combined interface law composed of a penalty and a barrier approach, to fulfill the contact constraints. Numerical examples demonstrate that the model can handle normal and frictional contact effects in a robust manner. These include simulations of convex and non-convex particles, showing the potential of applicability to materials with complex shaped particles such as sand and railway ballast. © 2021, The Author(s)

WOVEN FABRICS COMPUTATIONAL SIMULATION USING BEAM-TO-BEAM CONTACTS FORMULATION

This paper presents the study of the woven fabric mechanical behavior using computational simulations. For that, the finite element analysis is used, where the woven weft and warps are modeled by three-dimensional beams and the interaction among them are performed by frictionless contact models. Therefore, the methodology for the woven fabrics computational simulation is developed, such as the constitution of these numerical models and computational tests techniques are presented. For it, super elliptical cross sections are considered for warp and weft textile fibers and the beam-to-beam contact formulations are assumed in the problem solution, where parametrized surfaces represent the boundaries of the beams that are candidate to contact. The result of the computational simulations is the possibility to study the textile mechanical behavior, where their properties can be obtained for woven fabrics samples under several load cases

COMPUTATIONAL MODEL TO EVALUATE ACTIONS IN WHEELRAIL CONTACT INTERACTION

Railway traffic is a big source of vibrations. Over the years, with higher freight capacity and speeds getting higher, excessive stresses are experienced by the subgrade. Then, stress waves propagate through the ground until buildings on the surroundings. This leads to considerable negative impacts on the neighborhood. On this context, this work is immersed in a broader scope of proposing an approach to create a model to simulate railway systems, with the main objective of predicting the vibration and stresses at the subgrade, in different situations, such as to predict the contact forces between the wheels and the rails. In this model, the dynamic characteristics of the vehicles and the contact between their wheels and the track are considered. The numerical model was solved using the software Giraffe. To simulate the traffic of a vehicle, a two-car train composition was modeled. One of the cars is loaded by an imposed torsion moment, applied on the axle of the wheels. A time-varying torsional moment was considered, so the simulation could reproduce an accelerating vehicle, the same vehicle in constant speed, as well as braking. The model, as it was proposed, represented the global mechanical behavior of the cars and it was possible to obtain the contact forces between wheels and a surface that represents the rails

Silicon-on-Insulator RF Filter Based on Photonic Crystal Functions for Channel Equalization

International audienceA compact silicon-on-insulator 2-tap interferometer is demonstrated as a channel equalizer. The radiofrequency filter is reconfigurable thanks to thermally-controlled photonic crystal couplers and delay lines. The channel fading of a dispersive fiber link supporting a directly modulated telecommunication signal is successfully compensated for using the interferometer, leading to eye diagram opening and the possibility to recover the bit-error-rate of a reference signal with less than 1-dB power penalty

Structural stability of the static configuration of catenary risers.

Risers em configuração de catenária podem apresentar nível de tração muito baixo próximos ao fundo do mar. Isso pode ocorrer em algumas condições de lançamento, em que sua estrutura pode se configurar de forma quase vertical. Quando se trata de tubos flexíveis ou cabos umbilicais, a composição interna do riser contém armaduras helicoidais de tração. Essas podem induzir a ocorrência de giro axial quando o tubo é solicitado à tração. Se esse movimento não for permitido, surgirá um momento de torção na estrutura. O baixo nível de tração da configuração de catenária combinado com o momento de torção surgido durante o lançamento do riser pode levar a uma forma de instabilidade estrutural que culmina na formação de um laço. Isso é indesejável uma vez que, se existe o laço, dependendo dos esforços submetidos à linha, é possível que o laço se transforme em uma dobra, danificando a estrutura. O presente trabalho analisa as condições de formação de laços em configurações de catenária. Para isso, foram utilizados critérios de estabilidade aplicados a um modelo de elementos finitos, que leva em conta as não linearidades geométrica e de contato entre o riser e o solo. Foi utilizada uma formulação cinematicamente exata de elemento de viga através de uma descrição lagrangiana atualizada, que pode tratar de forma correta as grandes rotações que são impostas ao riser para induzir o surgimento do momento de torção. É mostrado que uma expressão analítica baseada na Fórmula de Greenhill pode prever o fenômeno com boa concordância com os resultados numéricos, mesmo considerando-se fenômenos como contato unilateral com atrito e correntezas marítimas. Além disso, foi feita uma análise paramétrica para prever a formação do laço para diversas geometrias de catenária, procurando generalizar as conclusões obtidas.Catenary risers can show a very low tension level close to the seabed. This can occur in some launching conditions, in which the structure can be almost vertical. When dealing with flexible pipes or umbilical cables, their internal composition includes the helical tension armors. These can induce an axial rotation to the riser when it is tensioned. If this movement is constrained, a torsion moment will act upon the structure. The low tension level present in the catenary configuration combined with the torsion moment that occurs during the riser launching can lead to a structural instability, resulting in a loop formation. This is undesirable since, if the loop is present, depending on the loads applied to the riser it is possible that it turns into a kink, causing damage. The present work analyzes the loop formation conditions in catenary riser. For that, stability criteria were applied to a finite element model that takes into account the geometric nonlinearities and also the contact between the riser and the seabed. A kinematically-exact beam formulation was adopted, dealing correctly with the large rotations that are imposed to the riser, in order to induce the torsion moment. It is showed that an analytical expression based on Greenhills formula can predict the phenomenon with a good agreement with numerical results, even including phenomena as frictional unilateral contact and sea currents. Furthermore, a parametric analysis was done to predict the loop formation to some catenary geometries, in order to generalize the obtained conclusions

OMAE2009-79507 A COMPARATIVE BUCKLING STUDY FOR THE CARCASS LAYER OF FLEXIBLE PIPES

ABSTRACT When there are some failures on the external plastic layer of a flexible pipe, a high value of hydrostatic pressure can be transferred to its interlocked carcass layer, maybe causing a collapse. So it is necessary to predict on the design of a flexible pipe the maximum value of pressure that would be acceptable to avoid collapse of the carcass layer. That value depends on the imperfections on the internal diameter due to fabrication uncertainties. To study that problem, two numerical finite element models were created and used to simulate external pressure loading condition. The first model is a full 3D approximation, composed by solid elements. The second one is a 3D ring approximation, still made by solids. An analytical model using an equivalent thickness approach for carcass was done. A good correlation between analytical and numerical models was achieved for pre-buckling behavior, but analytical buckling behavior was not the same as numerical values predictions. Discussions about these differences are done. INTRODUCTION Flexible pipes are compound by many layers, with different structural and operational functions. These layers are usually made of different materials, including metal and plastic. A typical example of a pipe internal structure is shown o

BUCKLING OF PIPELINES DUE TO INTERNAL PRESSURE

The pipelines used to transport oil and gas tend to expand due to high temperature and pressure conditions. If this expansion is inhibited, a compressive axial force arises. The pipeline can relieve the stresses by lateral or upheaval buckling. The objective of the present work is to analyze the instability of pipelines due to internal pressure, experiencing different boundary conditions and imperfection magnitudes. It aims at discussing the equivalence between approaches that involve the application of the load as the internal pressure and as an equivalent compression with follower and non-follower characteristics, besides discussing the influence of using static or dynamic analysis for such approaches. The methodology involves the development of geometrically-simple Timoshenko beam structural models. To perform the simulations, Giraffe finite element software is used for nonlinear analysis. The study presents comparisons between critical forces and post-buckling configurations for the different boundary conditions, imperfections, load types and analysis methods considered,  as well as comparisons between numerical and analytical solutions. Through the study, it is concluded that the equivalence in results between the distinct approaches depends on the nature of boundary conditions

WOVEN FABRICS COMPUTATIONAL SIMULATION USING BEAM-TO-BEAM CONTACTS FORMULATION

This paper presents the study of the woven fabric mechanical behavior using computational simulations. For that, the finite element analysis is used, where the woven weft and warps are modeled by three-dimensional beams and the interaction among them are performed by frictionless contact models. Therefore, the methodology for the woven fabrics computational simulation is developed, such as the constitution of these numerical models and computational tests techniques are presented. For it, super elliptical cross sections are considered for warp and weft textile fibers and the beam-to-beam contact formulations are assumed in the problem solution, where parametrized surfaces represent the boundaries of the beams that are candidate to contact. The result of the computational simulations is the possibility to study the textile mechanical behavior, where their properties can be obtained for woven fabrics samples under several load cases

Structural stability of flexible lines in catenary configuration under torsion

Catenary risers can present during installation a very low tension close to seabed, which combined with torsion moment can lead to a structural instability, resulting in a loop. This is undesirable once it is possible that the loop turns into a kink, creating damage. This work presents a numerical methodology to analyze the conditions of loop formation in catenary risers. Stability criteria were applied to finite element models, including geometric nonlinearities and contact constraint due to riser-seabed interaction. The classical Greenhill's formula was used to predict the phenomenon and parametric analysis shows a “universal plot” able to predict instability in catenaries using a simple equation that can be applied for typical risers installation conditions and, generically, for catenary lines under torsion.The authors acknowledge FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for the support under the grants 2006/06277-0 (PhD), 2012/09912-0 and 2012/21167-8 (PostDoc). The second author acknowledges the support by CNPq (Conselho Nacional de Desenvolvimento Cientí fi co e Tec- nológico) under the grant 310105/2009-9

A Comparative Wet Collapse Buckling Study for the Carcass Layer of Flexible Pipes

When there is a failure on the external sheath of a flexible pipe, a high value of hydrostatic pressure is transferred to its internal plastic layer and consequently to its interlocked carcass, leading to the possibility of collapse. The design of a flexible pipe must predict the maximum value of external pressure the carcass layer can be subjected to without collapse. This value depends on the initial ovalization due to manufacturing tolerances. To study that problem, two numerical finite element models were developed to simulate the behavior of the carcass subjected to external pressure, including the plastic behavior of the materials. The first one is a full 3D model and the second one is a 3D ring model, both composed by solid elements. An interesting conclusion is that both the models provide the same results. An analytical model using an equivalent thickness approach for the carcass layer was also constructed. A good correlation between analytical and numerical models was achieved for pre-collapse behavior but the collapse pressure value and post-collapse behavior were not well predicted by the analytical model. [DOI: 10.1115/1.4005185]FAPESPFAPESP [2006/06277-0]CNPq [310105/2009-9]CNP