Browsing by Author "Pina, H."
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- Advances in Engineering SoftwarePublication . Madeira, J.F. Aguilar; Camotim, D.; Basaglia, C.; Santos, J.V. Araujo dos; Lopes, H.M.R.; Topping, B. H. V.; Pina, H.; Poroseva, S. V.; Mota, Soares, C. M.; Araújo, A. L.; Moleiro, Duarte, F.; Sarrate, J.; Ledoux, F.; Kruis, J.; Dumont, S.; Lebon, F.
- An Improved Linear Model for Rotors Subject to Dissipative Annular FlowsPublication . Moreira, Miguel; Antunes, J.; Pina, H.In a previous paper, Antunes, Axisa and co-workers developed a linearized model for the dynamic of rotors under moderate fluid confinement, based on classical perturbation analysis covering two different cases: (i) dissipative motions of a centered rotor; (ii) motions of an eccentric rotor for a frictionless flow. Following the same procedures and assumptions, we derive here an improved model to cover the more general case of a dissipative linearized motion of an eccentric rotor. Besides the natural position variables, a new flow variable, which can be physically interpreted as the fluctuating term of average tangential velocity, was introduced, yielding an additional eigenvalue in the linear analysis. The new variable introduced, coupled with the rotor motions, is unavoidable when frictional effects are not neglected and yield a richer modal behavior which can be related with delay effects of the flow responses to the abovementioned rotor motions. Because system dynamics are strongly dependent on actual rotor eccentricity, the validity of this model (or other linear model) is dependent on an adequate estimation of this parameter.
- Analysis of the Nonlinear Orbital Motions of Immersed Rotors Using a Spectral/Galerkin ApproachPublication . Moreira, Miguel; Antunes, J.; Pina, H.Recently, we developed a symbolic-numerical formulation for the nonlinear planar motion of rotors under fluid confinement, based on a spectral/Galerkin approach, for gap geometries of about [equação] where H is the average annular gap and R is the rotor radius. Results showed a quite good agreement between the class of approximate models generated, the corresponding analytical exact planar model and experiments. Note that this methodology can be almost entirely automated on a symbolic computing environment. In the present paper this symbolic-numerical spectral/Galerkin procedure is extended in order to deal with nonlinear orbital motions - X (t) and Y (t) taking place in orthogonal directions. Numerical simulations performed over a centered rotor configuration maintained by non-isotropic supports [equação], which exhibit rich rotordynatnics, show a quite good agreement between this type of approximate models and the corresponding analytical exact (but quite involved) model developed in the past by the authors.
- Uma Aplicação do Teorema dos ResíduosPublication . Moreira, Miguel; Antunes, J.; Pina, H.O movimento rotativo de um rotor numa região confinada determina o escoamento do fluído envolvente e o desenvolvimento de forças de interacção fluído-estrutura, cujo conhecimento é essencial na previsão do comportamento dinâmico deste sistema. A determinação explícita das forças referidas a partir das equações de Navier-Stokes conduz à necessidade de resolução de integrais definidos do tipo [equação]. A aplicação de uma forma particular do teorema dos resíduos da análise complexa constitui a solução natural do problema anterior, concretizada recorrendo ao auxílio de um manipulador simbólico para fazer face à extensão das manipulações algébricas necessárias.
- Computers and StructuresPublication . Camotin, C.; Basaglia, C.; Madeira, J. F. Aguilar; Pina, H.; Ilanko, S.; Kennedy, D.; Rabczuk, T.; Soares, C. M. Mota; Araújo, A. L.; Duarte, F. M.; Pallares, L.; Pallares, F. J.; B. H. V.
- Modelação Computacional de um Dessalinizador com Evaporação por Spray para Dessalinização de Água do MarPublication . Moreira, Miguel; Joyce, A.; Pina, H.O desenvolvimento dum evaporador com evaporação por "Spray" motivou a elaboração dum seu modelo computacional. A comparação dos resultados da simulação com resultados experimentais revelou um idêntico comportamento qualitativo e erros relativos de 20%. Simulações computacionais posteriores evidenciam a possibilidade de atingir desempenhos energéticamente atractivos.
- Nonlinear Vibrations of Vertical Asymmetrically-Supported Rotors Under Fluid Confinement: Theoretical ResultsPublication . Moreira, Miguel; Pina, H.; Antunes, J.This paper is concerned about vertical rotors immersed in fluid annulus of moderate confinement. Such rotors are subjected to the dynamical effects of the fluctuating co-rotating flows. For high enough spinning velocities, the fluid-elastic forces become significant, and often lead to unstable dynamical regimes. These depend on the fluid gap and density, on the rotor eccentricity and spinning velocity, as well as structural properties. We developed an improved linear model for rotors under moderate fluid confinement as well as an exact model for the corresponding nonlinear rotordynamics. Recently a symbolic-numerical formulation based on a spectral/Galerkin approach was also developed by the authors. Numerical results showed a quite good agreement between exact solutions and these formulations and experimental validation of the theoretical model has been provided for symmetrically-supported rotors. Numerical simulations carried over immersed rotor configurations maintained by non-isotropic supports show that the rotor stability is affected by support stiffness-asymmetry. In this paper, we briefly summarize the theoretical approaches used in the numerical simulations and present an analysis of the linear rotor-dynamics, as a function of the support stiffness-asymmetry and of the rotor eccentricity. Theoretical stability domains are computed from the eigenvalues of the linearized model. Finally, we present time-domain numerical simulations of some stable solutions and nonlinear limit-cycles which stem from linearly-unstable solutions.
- Optimization of baffle configurations to prevent aeroacoustic instabilities in heat exchangers - preliminary experimentsPublication . Moreira, Miguel; Antunes, J.; Debut, V.; Pina, H.It is well known that gas heat exchangers are prone to aeroacoustic instabilities, which often lead to severe noise levels, structural vibrations and fatigue. These are unacceptable, as they threaten the component integrity and expose the plant workers to excessive noise levels. Such phenomenon is due to a cooperative interplay between the Karman vortices generated by the cross-flow and the heat exchanger acoustical modes (mainly those transverse to the tube banks). Energy exchanges are then such that, for certain operating velocities, self-excitation of one or more acoustical modes arises. Actually, this problem is solved by placing rigid baffles inside the container, which modify the acoustic modal fields and eventually inhibit the instability. However, an effective location of such baffles is more or less difficult depending on the system complexity and on the range of flow velocities of interest. For realistic industrial components - using a restricted number of acoustical baffles - their optimal location is a challenging problem, as trial and error experimentation is often a costly and frustrating procedure. In this paper we improve a recently proposed strategy for the optimal location of a given number of baffles, in order to inhibit instability of the acoustical modes in a given frequency range. Our approach is based on a stochastic global optimization technique. Some preliminary experiments are also performed and compared with the simulation results.
- O Problema da Escassez da Água Potável e a Questão da DessalinizaçãoPublication . Moreira, Miguel; Joyce, A.; Pina, H.No presente texto introduz-se o problema da escassez de água potável apontando-se algumas das soluções indicadas por diferentes autores. Os mais importantes métodos de dessalinização são resumidamente descritos e o seu desempenho é caracterizado e assinalado. O desenvolvimento de diferentes técnicas de dessalinização, impulsionado pela escassez deste recurso constitui igualmente a motivação para estudo, construção e desenvolvimento dum destilador com evaporação por " spray ", no departamento de Energias Renováveis do INETI cuja necessidade de otimização levou à elaboração do modelo computacional aqui descrito. A comparação dos resultados da simulação computacional com resultados de testes experimentais revelou um idêntico comportamento qualitativo (do modelo computacional e do protótipo) e erros relativos de cerca de 20% nos parâmetros de desempenho. A realização de simulações computacionais posteriores evidenciam a possibilidade de atingir desempenhos energeticamente atractivos e apontam direcções a seguir no desenvolvimento do sistema de dessalinização estudado.