Repositório RCAAP

The aim of this work was the development of a robust multiobjective optimization algorithm by using as a reference the Fireflies Colony Algorithm associated with the concept of Effective Mean. The main operators for the extension of the algorithm for multiobjective case were the ordering of Pareto curves by means of ranking procedures and the truncation of solutions through the crowding distance operator. To insert robustness to the optimization process, the mean effective definition was used instead the commonly used expectancy measures, as suggested by the literature. The proposed methodology was tested in mathematical problems whose nominal and robust Pareto curves were known. In addition, so as to evaluate the quality of the algorithm proposed metrics of convergence and diversity were taken into account. The proposed algorithm proved to be very efficient with respect to convergence and diversity of solutions. The methodology was also applied to design classical mechanical systems, including the design a flexible rotor with respect to the position of the critical speeds. The main contributions of this thesis was the development of a computational tool for the treatment of robust multi-objective optimization problems, the analysis and interpretation of the influence of robustness parameter on robust Pareto curves compared with the nominal curves and the formation of a bank data for future comparisons with other robust multi-objective optimization strategies. The results indicate that the approach proposed arises as an interesting strategy for the robust design of engineering systems.

The present work proposes to extend the IMERSPEC methodology to solve two-phase flows. This methodology based on the coupling of Fourier psedospectral method (FPSM) and immersed boundary method (IBM). The main features of the FPSM are the high rate of numerical convergence, high numerical accuracy combined with high computational efficiency because it uses the Fast Fourier Transform algorithm, and beside that, in order to solve the Navier-Stokes equations, solving linear system for pressure-velocity coupling is not necessary. To deal with two-phase flows with moving and deformable geometry, two methods were coupled the IMERSPEC methodology: the Front-Tracking method (FT) and Volume of Fluid method (VOF). The FT method works with two domains, the Eulerian, where the equations for the fluid are evaluated and the Lagrangian for modeling the interfaces. In the FT, both domains are coupled by using interpolation and distribution process, with no restriction on the movement and deformation of the dispersed phase of the Lagrangian mesh over Eulerian domain. The disadvantages of the FT method are a possible mass loss and the need of a fragmentation and coalescence bubble model. The VOF method defines the surface by the volume fractions which allows the fragmentation and coalescence of bubbles without the requirement of implementing a specific model, however it may present numerical instabilities. The results obtained by evaluating the spurious currents, mass conservation and analysis through numerical experimentation of bubbles rise show that IMERSPEC-FT can be considered validated and promising, while the IMERSPEC-VOF presents promising results for cylindrical bubble regime.

The aim of this work was to study the behavior of ferritic stainless steels in abrasive-corrosive environment of 10%wt SiO2 + 1N H2SO4. It was developed an apparatus and a methodology to perform the tests. The equipment has a load cell to measure the normal and friction forces during the test, an original device due to the difficulty to use it without damaging in abrasivecorrosive environment. It was used samples of ferritic stainless 11Cr, 11CrTi, 16Cr, 16CrNb (Nb stabilized) and, for comparative purposes, the stainless steels 18Cr8Ni (austenitic), 17CrTiNb (ferritic) and the carbon steel A36. It was proceeded the mechanical characterization (hardness and tensile testing), chemical and metallographic test (grain size) of these samples. For 16Cr and 16CrNb steels the crystallographic texture was analyzed, by EBSD. It has been realized corrosion test at turbulent and aerated environment, abrasion and abrasion-corrosion test. So it was analyzed the polarization curves, the wear coefficient k and the samples, by SEM. It was clearly observed at the corrosion tests the increasing at corrosion resistance with the increasing of Cr content in the samples. Although the 18Cr8Ni was the hardest material, it had worse performance on the abrasion tests. In this abrasive system studied there was a wear resistance increased after prior cold deformation. For the abrasioncorrosion tests the 18Cr8Ni had the best performance, although it was the worse in abrasion test. The materials were ranked by the higher wear resistance due to the increasing Cr content that was associated to the corrosive influence on the total wear in this abrasive-corrosive environment. In abrasive-corrosive medium, the wear was lower when compared to the abrasive medium for all materials mainly due to the friction coefficient reduction. For the steels 16CrNb and 16Cr, it was observed a slight influence of crystallographic texture on the corrosion resistance in turbulent medium and abrasive wear. However, this distinction does not exist for the abrasion-corrosion tests. It was also noticed an influence of niobium carbides in reducing the wear coefficient, even in a soft matrix as 16CrNb steel.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Numerical simulations to predict erosion in cyclone separators due to the impact of particles were accomplished in this work. The predictions were performed through Computational Fluid Dynamics methods. The geometry investigated was similar to that of a second stage cyclone of a fluidized catalytic cracking unit. The numerical results were compared to experimental results available in the literature. The cyclone walls were made of acrylic with multiple coatings of drywall in the experiments. However, the implemented models to predict the erosion were developed for metallic materials. In this context, the validation was performed with cases in which the materials involved were the same as that used in the implemented models. The influence of the two-phase models, turbulence modelling, mesh resolution, dipleg presence and the models of particle/wall collision in erosion were evaluated after the validation. It was found that the turbulence modelling and mesh resolution were the most relevant factors in the erosion prediction, at least in the studied cases. Another relevant parameter is the friction factor, whose value significantly modifies the erosion rate. It was noticed that the interaction between the fluid and the particles reduces the erosion rate, even at low concentrations, as well as the interparticle collisions. Generally, the eroded regions were observed to match those from the experiments.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

In this work small and large-scale wear tests of polyurethane (PU) sliding against polyamide (PA) were carried out. In the large-scale tests, PU and PA samples were submitted to conditions similar to those found in oil and gas platform operations. The wear of these samples was measured using a coordinate measuring machine. Using the small-scale test configuration, the influence of contact geometry, sliding velocity, sliding stroke and contact pressure at the coefficient of friction (COF) and on the wear mechanisms was evaluated. In both test scales, the wear mechanisms of PA and PU samples were analyzed using a scanning electron microscope. The thermal behavior of PA samples was investigated using differential scanning calorimetry. The PA physic-chemical analysis was performed using X-ray diffraction and Raman spectroscopy and infrared spectroscopy for PU. In large-scale tests was observed that the outer cover of flexible riser pipes had a maximum wear rate of 0.016 mm/km, while in the liner this wear rate was 0.264 mm/km. It was also observed that variations in the hardness of PU liner lead to an important change on the wear mechanisms and, consequently, on the wear rate. The wear rate at large-scale was also strongly affected by the contact temperature. Due to the reduction of the contact temperature, a sharp reduction on the liner wear rate was observed. However, no significant variation in the rate of wear of PA riser cover was noticed. In small-scales tests, under the same sliding velocity and contact pressure observed in the field, the tribological wear behavior was changed. It was also observed that increasing the contact pressure or reducing the sliding stroke leads to a decrease the COF, while the sliding velocity only induced a change in this parameter if the contact temperature was significantly increased. Using test parameters in small scale which lead to a temperature rise in the tribocontact, it was possible to reproduce the tribological behavior observed in large-scale test.

The determination of the dynamic parameters of great or complex structures can be made using modal synthesis methods subdividing the complete structure in substructures. The employment of this method may be done by using analytical or experimental procedures. Generally speaking, poor eigenvalues and eigenvectors may be identified using experimental modal synthesis methods due to a normalization deficient process of the modal bases and to the low orthogonality condition of the identified bases. The contribution of this current work is about the improvement of the great or complex structures dynamic identification process by using the experimental modal synthesis method. The basis of this research methodology is the SMFR (Modal Synthesis with Residual Flexibilities) method and an identification of the physical matrices of the system by using the experimental FRF (Frequency Response Function), called ACS (Simultaneous Curve Fitting) method. These matrices contribute to the improvement of the orthogonality conditions and normalization of the experimental modal bases. Simultaneously, two new methods (CSME and CSMF) for the automatic choice of the used substructures modal bases in the modal synthesis process have been developed. The validation procedures of these methodologies were developed by using examples of numerical simulation and experimental models. Using CSMF method, it was possible to improve the modal choice process, automatizing and minimizing the interference of the user in the modal synthesis method. In the case of experimental data with high level noise, the recommended is the iterative method.

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The present work aims to present a model of fluid-structure interaction, which was implemented in IMERSPEC2D code, that is a numerical code that solves problems of flow over immersed bodies by using the pseudospectral Fourier method coupled with the immersed boundary method. Initially, it has been proposed a comparative analysis between two numerical methods: Finite Volume Method (FVM) and Fourier Pseudospectral Method (MPEFO) with and without the use of the Immersed Boundary Method (MFI). This first analysis includes verification and validation of both numerical codes, in order to determine the advantages and disadvantages of MPEFO compared to MVF. The second part is the implementation of a fluid structure interaction model in IMERSPEC2D code, this approach takes account the partitioned interaction model, by using the fourth order optimized Runge-Kutta method in both time advanced of the fluid and of the structure. Furthermore, it is proposed to non-dimensionalization of the equations that models the structural movement. These implementations yield results with more accuracy and low computational cost, by proving the applicability and potentiality of IMERSPEC methodology. Lastly, several applications have been held in drilling and extraction oil problems. These simulations are still two-dimensional, however, it is possible to observe interesting flow patterns into extraction pipes for different aspect ratios and Reynolds numbers.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Universidade Federal de Uberlândia

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior