RCAAP Repository

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

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

A novel methodology combining Fourier pseudospectral and immersed boundary methods - IMERSPEC - has been developed for heat transfer problems, using Navier-Stokes, mass conservation and energy equations for incompressible flows. The numerical algorithm consists of a Fourier pseudospectral collocation method, used for flow solution and thermal heat transfer, while the immersed boundary method (multi-direct forcing method) is used only to apply the thermal boundary conditions. The IMERSPEC methodology has been developed in the fluid mechanic laboratory (MFLab) to solve isothermal fluid flows problems. In the present work, a new formulation for first (Dirichlet), second (Neumann) and third (Robin) boundary conditions types is proposed. The verification and validation of the methodology are presented for every boundary conditions and the results show a good agreement with the literature. Furthemore, the methodology convergence when applied to mathematical problems with synthesized analytical solutions shows accuracy machine and four order to rate of convergence for non coincidents nodes. Whilst for physical problems, the presented methodology provides second order rate of convergence for the energy equation and the Navier-Stokes equations. The computational cost has been analyzed and it is shown that runtime presents Nlog2N order, expected result to Fourier method

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Universidade Federal de Goiás - Goiânia

In this work, the radiative heat transfer was evaluated in corrugated cylindrical surfaces similar the surfaces employed in corrugated furnaces at a fire tube steam generator using the zonal and finite volume methods, with the modeling of corrugated surfaces by cubic spline interpolation. Associated with this study was realized the optimization of these corrugated surfaces using the methodologies of simplex and simulated annealing to determine the optimal configuration of these surfaces for maximum radiative heat transfer. The results indicated that the zonal and finite volume methods, were able to represent the radiative heat transfer in cylindrical and complex literature problems. In terms of the cases studied, the theoretical furnace and the combustion chamber with the changed cylindrical surfaces for corrugated surfaces, the results indicated that the methods of the zonal and of finite volume showed differences in the radiative heat flux to the simulation conditions with large amplitudes, due to absence of modeling the shaded areas of corrugated surfaces in the finite volume method. In the optimization, it was demonstrated that the optimization methodologies found the same solutions to the case study of the theoretical furnace with corrugated surface, highlighting the simplex method for the least amount of evaluations of the objective function. However, for combustion chamber with corrugated surfaces noted that the problem has become ill-posed due to inhomogeneous temperature profile, which led to the simplex method fails to get the optimal point and highlighted non-deterministic characteristics of the simulating annealing method to achieve best results of maximum heat flow. In this context, the simulations show that applying the optimization methodologies is possible to improve radiative heat transfer in the theoretical furnace with corrugated surfaces up to 16.74% and in the combustion chamber surfaces corrugated 7.79%.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Parallel manipulators are of great interest mainly because they present advantages in several applications, showing great resistance, positioning accuracy, load capacity larger than serial manipulators and they can be operated to high-speeds and accelerations. In the Laboratory of Robotics and Mechatronics in Casino, Italy, a parallel mechanism was created with three degrees of freedom, called CaPaMan (Cassino Parallel Manipulator). The main objective of this work is obtains the optimal trajectory of the CaPaMan parallel structure. The multi-objective optimization problem is written taking into account the mechanical energy of the actuators, the total traveling time and jerk. The trajectory is calculated assuming that the input angles are given by a function of the time, represented by a uniform B-splines. The kinematic modelling is obtained by deriving the trajectory equation according the time. The analytic model for the inverse dynamics of CaPaMan uses the equations of Newton-Euler. The peculiar kinematic chain and the properties of symmetry of the CaPaMan architecture are useful in this formulation, which allows, for each trajectory, to calculate the input torques and the mechanical energy. The multicriterion vector function is transformed to a scalar function by using the Weighting Objectives Method. The optimization problem is investigated by using genetic algorithms. Some numeric examples are presented for verification and validation of the methodology.

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conselho Nacional de Desenvolvimento Científico e Tecnológico

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