Thermal fatigue analysis of solar panel structure for micro-satellite applications

Thermal fatigue analysis based on 2D finite difference and 3D finite element methods is carried out to study the performance of solar panel structure during micro-satellite life time. Solar panel primary structure consists of honeycomb structure and composite laminates. The 2D finite difference (I-DEAS) model yields predictions of the temperature profile during one orbit. Then, 3D finite element analysis (ANSYS) is applied to predict thermal fatigue damage of solar panel structure. Meshing the whole structure with 2D multi-layer shell elements with sandwich option is not efficient, as it misses thermal response of the honeycomb structure. So we applied a mixed approach between 3D solid and 2D shell elements to model the solar panel structure without the sandwich option.

Thermal Deformation Analysis of Micro satellite Structure

Mounting accuracy of satellite payload and ADCS (attitude determination and control subsystem) seats is one of the requirements to achieve the satellite mission with acceptable performance. Components of mounting inaccuracy are technological inaccuracies, residual plastic deformations after loading (during transportation and orbital insertion), elastic deformations, and thermal deformations during orbital operation. This paper focuses on estimation of thermal deformations of satellite structure. Thermal analysis is executed by applying finite-difference method (IDEAS) and temperature profile for satellite components case is evaluated. Then, Perform thermal finite-element analysis applying the finite-difference model results as boundary conditions; and calculate the resultant thermal strain. Next, applying the resultant thermal strain, perform finite-element structure analysis to evaluate structure deformations at the payload and ADCS equipments seats.

Delamination threshold load for dynamic impact on plates

A criterion is derived for delamination onset in transversely isotropic laminated plates under small mass, high velocity impact. The resulting delamination threshold load is about 21% higher than the corresponding quasi-static threshold load. A closed form approximation for the peak impact load is then used to predict the delamination threshold velocity. The theory is validated for a range of test cases by comparison with 3D finite element simulation using LS-DYNA and a newly developed interface element to model delamination onset and growth. The predicted delamination threshold loads and velocities are in very good agreement with the finite element simulations. Good agreement is also shown in a comparison with published experimental results. In contrast to quasi-static impacts, delamination growth occurs under a rapidly decreasing load. Inclusion of finite thickness effects and a proper description of the contact stiffness are found to be vital for accurate prediction of the delamination threshold velocity

Tip-enhanced Raman microscopy: practicalities and limitations

The feasibility of apertureless scanning near-field Raman microscopy, exploiting the local enhancement in Raman scattering in the vicinity of a silver or gold tip, was investigated. Using the finite difference time domain method we calculated the enhancement of electric field strength, and hence Raman scattering, achieved through the resonant excitation of local modes in the tip. By modelling the frequency-dependent dielectric response of the metal tip we were able to highlight the resonant nature of the tip-enhancement and determine the excitation wavelength required for the strongest electric field enhancement, and hence Raman scattering intensity, which occurs for the excitation of modes localized at the tip apex. It is demonstrated that a peak Raman enhancement of 10(7)-fold should be achievable with <5 nm spatial resolution. We show that surface-enhanced Raman scattering from carbon contamination on a silver or gold tip can be significant. However, we find for a tip of radius of curvature 20 nm that the Raman enhancement should decay totally within 20 nm from the tip. Hence withdrawal of the tip by this distance should lead to the disappearance of the tip-enhanced signal, leaving only that from carbon contamination on the tip itself and the intrinsic signal from the sample. Copyright (C) 2003 John Wiley Sons, Ltd.

An investigation into the acceleration response of a damaged beam-type structure to a moving force

In recent years there have been a growing number of publications on procedures for damage detection in beams from analysing their dynamic response to the passage of a moving force. Most of this research demonstrates their effectiveness by showing that a singularity that did not appear in the healthy structure is present in the response of the damaged structure. This paper elucidates from first principles how the acceleration response can be assumed to consist of ‘static’ and ‘dynamic’ components, and where the beam has experienced a localised loss in stiffness, an additional ‘damage’ component. The combination of these components establishes how the damage singularity will appear in the total response. For a given damage severity, the amplitude of the ‘damage’ component will depend on how close the damage location is to the sensor, and its frequency content will increase with higher velocities of the moving force. The latter has implications for damage detection because if the frequency content of the ‘damage’ component includes bridge and/or vehicle frequencies, it becomes more difficult to identify damage. The paper illustrates how a thorough understanding of the relationship between the ‘static‘ and ‘damage’ components contributes to establish if damage has occurred and to provide an estimation of its location and severity. The findings are corroborated using accelerations from a planar finite element simulation model where the effects of force velocity and bridge span are examined.

Fabrication and optical properties of gold nanotube arrays

Arrays of gold nanotubes with polypyrrole cores were grown on glass substrates by electrodeposition into thin film porous alumina templates. Measurements of optical transmission revealed strong extinction peaks related to plasmonic resonances, which were sensitive to the polarization state and angle of incidence. On prolonging the electrodeposition of gold, the polypyrrole core became fully encapsulated and this had a dramatic effect on the optical properties of the arrays, which was rationalized by finite element simulation of the local field intensities resulting from plasmon excitation.

Manipulating plasmon modes in tip-enhanced Raman microscopy

The influence of polarization on the plasmon modes excited in tip-enhanced near-field optical microscopy have been investigated using the Finite Difference Time Domain Method. Analysis of the calculated results have laid particular emphasis on the ability to align local field enhancements with the orientation of molecules in order to optimize Raman signals, with particular relevance to recent experimental work on carbon nanotubes.

On the numerical solution of the 2d wave equation with compact fdtd schemes

This paper discusses compact-stencil finite difference time domain (FDTD) schemes for approximating the 2D wave equation in the context of digital audio. Stability, accuracy, and efficiency are investigated and new ways of viewing and interpreting the results are discussed. It is shown that if a tight accuracy constraint is applied, implicit schemes outperform explicit schemes. The paper also discusses the relevance to digital waveguide mesh modelling, and highlights the optimally efficient explicit scheme.

Grid structure impact in sparse point representation of derivatives

In the Sparse Point Representation (SPR) method the principle is to retain the function data indicated by significant interpolatory wavelet coefficients, which are defined as interpolation errors by means of an interpolating subdivision scheme. Typically, a SPR grid is coarse in smooth regions, and refined close to irregularities. Furthermore, the computation of partial derivatives of a function from the information of its SPR content is performed in two steps. The first one is a refinement procedure to extend the SPR by the inclusion of new interpolated point values in a security zone. Then, for points in the refined grid, such derivatives are approximated by uniform finite differences, using a step size proportional to each point local scale. If required neighboring stencils are not present in the grid, the corresponding missing point values are approximated from coarser scales using the interpolating subdivision scheme. Using the cubic interpolation subdivision scheme, we demonstrate that such adaptive finite differences can be formulated in terms of a collocation scheme based on the wavelet expansion associated to the SPR. For this purpose, we prove some results concerning the local behavior of such wavelet reconstruction operators, which stand for SPR grids having appropriate structures. This statement implies that the adaptive finite difference scheme and the one using the step size of the finest level produce the same result at SPR grid points. Consequently, in addition to the refinement strategy, our analysis indicates that some care must be taken concerning the grid structure, in order to keep the truncation error under a certain accuracy limit. Illustrating results are presented for 2D Maxwell's equation numerical solutions.

Influence of the Human Head in the Radiation of a Mobile Antenna

The big proliferation of mobile communication systems has caused an increased concern about the interaction between the human body and the antennas of mobile handsets. In order to study the problem, a multiband antenna was designed, fabricated and measured to operate over two frequency sub bands 900 and 1800 MHz. After that, we simulated the same antenna, but now, in the presence of a human head model to analyze the head's influence. First, the influence of the human head on the radiation efficiency of the antenna has been investigated as a function of the distance between the head and the antenna and with the inclination of the antenna. Furthermore, the relative amount of the electromagnetic power absorbed in the head has been obtained. In this study the electromagnetic analysis has been performed via FDTD (Finite Difference Time Domain).

Bi-Dimensional Characterization of a Wimax Radio Channel at 3.5GHz

This paper presents the characterization of an indoor Wimax radio channel using the Finite-Difference Time-Domain (FDTD) [1] method complemented with the Convolutional Perfect Matched Layer (CPML) technique [2]. An indoor 2D scenario is simulated in the 3.5GHz band (IEEE 802.16d-2004 and IEEE 802.16e-2005 [3]). In this study, we used two complementary techniques in both analysis, technique A and B for fading based on delay spread and technique C and D for fading based on Doppler spread. Both techniques converge to the same result. Simulated results define the channel as flat, slow and without inter-symbolic interference (ISI), making the application of the spatial diversity the most appropriate scheme.

Estudo de Ferramentas FEA Comerciais na Simulação Numérica de Processos de Conformação de Chapas Metálicas. Aplicação à Indústria Automóvel.

Os componentes obtidos através da conformação plástica de chapas têm uma grande importância, tanto na etapa de concepção do produto como na etapa de produção na indústria automóvel. Isto comprova-se pelo facto de, em média, cada automóvel integrar cerca de 500 componentes estampados para construir o chassis e a carroçaria [Alves 2003]. Deste total de componentes, 50 são de grandes dimensões (portas, tejadilho, painéis inferior e laterais, entre outros) e necessitam, em média, de cinco ferramentas para o seu fabrico, sendo o custo estimado para cada ferramenta de 230 000 € [Col 2000, Alves 2003]. Para além da indústria automóvel, a conformação plástica de chapas metálicas é um processo tecnológico presente nas indústrias relativas à aeroespacial, petrolífera, decoração, alimentar, entre outras. Do ponto de vista do enquadramento económico, cerca de 20% do custo total de um automóvel novo é devido à incorporação de componentes metálicos estampados. [Alves 2003]. A pressão do “Mercado Global” faz com que os custos relativos à matéria-prima, energia e mão-de-obra sejam uma constante em termos de redução do seu impacte no orçamento das empresas fornecedoras destes produtos. É neste contexto que surge a necessidade da realização deste estudo de Benchmark de Softwares, tornando-se bastante importante, quer ao nível da competitividade industrial, quer ao nível da inovação para novos produtos. A análise por elementos finitos desempenha um papel primordial no tryout virtual e otimização das ferramentas e processos de conformação plástica. Os objetivos principais deste estudo de simulação numérica são a identificação e comparação dos resultados obtidos pelo AUTOFORM e pelo PAMSTAMP, para cada uma das variáveis identificadas como as mais influentes na robustez dos processos de estampagem de chapa metálica. Estas variáveis identificadas são: consumo de material (Draw-in) após conformação; forças de conformação; valores de variação de espessura e dos valores de extensão e resultados de Springback. Os resultados obtidos são comparados com os resultados experimentais e, desta forma, avalia-se a capacidade inovadora e a eficácia de cada um dos softwares, obtendo-se assim, uma orientação mais real para o software mais indicado aos objetivos impostos pela indústria automóvel. Para este efeito, a indústria automóvel, como maior impulsionador e motor da investigação na área da simulação numérica aplicada aos processos de estampagem, tem aderido em peso ao Benchmarking. Um exemplo disto, é o que acontece nas conferências Numisheet. O Benchmark #2 da conferência Numisheet 2008 é analisado pormenorizadamente e os resultados numéricos e experimentais são comparados e apresentados. Dois materiais distintos (aço HC260LAD e liga de alumínio AC170), assim como três modelos com geometrias diferentes (com e sem freios) são apresentados neste relatório. Com vista à redução dos ciclos tentativa-erro, tem-se adotado ciclos virtuais ou numéricos e tem-se incrementado a interatividade entre as fases de concepção e projeto, num conceito muito próprio, mas cada vez mais abrangente, denominado “produção virtual”. É nesta filosofia que se insere a simulação numérica dos processos de conformação de chapa.

La méthode IIM pour une membrane immergée dans un fluide incompressible

La méthode IIM (Immersed Interface Method) permet d'étendre certaines méthodes numériques à des problèmes présentant des discontinuités. Elle est utilisée ici pour étudier un fluide incompressible régi par les équations de Navier-Stokes, dans lequel est immergée une membrane exerçant une force singulière. Nous utilisons une méthode de projection dans une grille de différences finies de type MAC. Une dérivation très complète des conditions de saut dans le cas où la viscosité est continue est présentée en annexe. Deux exemples numériques sont présentés : l'un sans membrane, et l'un où la membrane est immobile. Le cas général d'une membrane mobile est aussi étudié en profondeur.

Modélisation d'arythmies auriculaires modulées par le système nerveux autonome

La fibrillation auriculaire (FA) est la forme d’arythmie la plus fréquente et représente environ un tiers des hospitalisations attribuables aux troubles du rythme cardiaque. Les mécanismes d’initiation et de maintenance de la FA sont complexes et multiples. Parmi ceux-ci, une contribution du système nerveux autonome a été identifiée mais son rôle exact demeure mal compris. Ce travail cible l’étude de la modulation induite par l’acétylcholine (ACh) sur l’initiation et le maintien de la FA, en utilisant un modèle de tissu bidimensionnel. La propagation de l’influx électrique sur ce tissu est décrite par une équation réaction-diffusion non-linéaire résolue sur un maillage rectangulaire avec une méthode de différences finies, et la cinétique d'ACh suit une évolution temporelle prédéfinie qui correspond à l’activation du système parasympathique. Plus de 4400 simulations ont été réalisées sur la base de 4 épisodes d’arythmies, 5 tailles différentes de région modulée par l’ACh, 10 concentrations d’ACh et 22 constantes de temps de libération et de dégradation d’ACh. La complexité de la dynamique des réentrées est décrite en fonction de la constante de temps qui représente le taux de variation d’ACh. Les résultats obtenus suggèrent que la stimulation vagale peut mener soit à une dynamique plus complexe des réentrées soit à l’arrêt de la FA en fonction des quatre paramètres étudiés. Ils démontrent qu’une décharge vagale rapide, représentée par des constantes de temps faibles combinées à une quantité suffisamment grande d’ACh, a une forte probabilité de briser la réentrée primaire provoquant une activité fibrillatoire. Cette activité est caractérisée par la création de plusieurs ondelettes à partir d’un rotor primaire sous l’effet de l’hétérogénéité du gradient de repolarisation causé par l’activité autonomique.

Modélisation mathématique de la propagation de la malaria.

Un modèle mathématique de la propagation de la malaria en temps discret est élaboré en vue de déterminer l'influence qu'un déplacement des populations des zones rurales vers les zones urbaines aurait sur la persistance ou la diminution de l'incidence de la malaria. Ce modèle, sous la forme d'un système de quatorze équations aux différences finies, est ensuite comparé à un modèle analogue mais en temps continu, qui prend la forme d'équations différentielles ordinaires. Une étude comparative avec la littérature récente permet de déterminer les forces et les faiblesses de notre modèle.