Browsing by Author "Moreira, Miguel"
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- 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.
- Design Challenges of a Maritime Multipurpose Unmanned VehiclePublication . Lopes, Vasco Pereira; Silva, Paulo Alexandre Marques Pires da; Moreira, MiguelMilitary Navies have several missions in which due to their complex, dangerous, or disaster scenarios, unmanned vehicles are suitable to perform the necessary tasks. Besides other issues it is a current subject of research the trade-off between using many mission oriented unmanned vehicles or the use of less number of more robust and versatile multimission vehicles. Challenged by the technological advances achieved in mobile robotics and unmanned vehicles, and the need to develop in house know-how, this project aimed to put into practice the development of a concept design of a multimission unmanned surface vehicle, that can operate individually or cooperate with other systems whose development will be done in parallel on other projects. This unmanned vehicle, designated Unmanned Surface Vehicle for the Portuguese Navy - USV – PoN is part of project based oriented learning strategy in development in the Portuguese Naval Academy. It poses particular challenges related to the broad spectrum of the concept of operations, such as mine warfare operations, port surveillance and protection operations, search and rescue operations, hydrographic data collection operations. All of these operations demand different and challenging missions and tasks, to which the concept design of the USV-PoN will answer in terms of feasibility studies and possible configurations. The purpose of this work is to address the challenges that the concept of operations and corresponding technical specifications put into the design of the USV-PoN and show how to overcome them making the study and analysis necessary for the production of a prototype based on the concept design. Out of the scope of this work were aspects related to sensor development and integration as well as the command and control of the vehicle.
- Destilador com Evaporação por Spray para Dessalinização de Água do MarPublication . Joyce, A.; Loureiro, D.; Collares Pereira, M.; Moreira, MiguelO presente trabalho descreve um destilador para dessalinização de água do mar, em que a evaporação é efectuada através de um 'spray', com a finalidade de aumentar a produção diária de água. É apresentado o princípio de funcionamento do sistema, descrito o protótipo construído e apresentados e discutidos os resultados já obtidos durante os ensaios em laboratório. A produção aumenta com a temperatura, de uma forma quadrática, e com a diminuição quer do caudal de entrada quer do diâmetro das gotas produzidas pelos pulverizadores. A câmara pode ser associada a um sistema de Colectores Solares Planos ou Colectores CPC, sendo no entanto preferível um sistema que permita o seu funcionamento em contínuo como por exemplo os Lagos Solares, podendo, nestas condições obter-se produções de cerca de 40 kg/dia.
- Experimental Validation of Theoretical Models for the Nonlinear Vibrations of Immersed RotorsPublication . Moreira, Miguel; Tissot, A.; Antunes, J.Vibration of rotating shafts has been studied for different gap geometries, ranging from bearing configurations to pump systems. This paper deals with the rotor-flow dynamics of immersed shafts unde.r moderate confinement—clearance gap about [equação] (where H is the average gap and R is the rotor radius). Following simplified assumptions, analytical models for the linearized forces, for both centered and eccentric immersed rotors have been developed as well as a theoretical nonlinear model which fully describes the nonlinear flow terms. These models were supported by encouraging results from preliminary experiments. In the present paper, we discuss some recent and representative results of an extensive series of tests performed on a small-scale model, in order to assert the validity of our theoretical models. From the overall experimental programma, the following conclusions emerged • (1) The linearized bulk-flow model is adequate, provided the dissipative effects are duly accounted for using an empirical friction coefficient to empirically model the turbulent. stresses. Such predictions are quite accurate if the system is working at low rotor eccentricities and far from the instability boundaries. (2) However, for large rotor eccentricities and for dynamic regimes near the linear instability, the folly nonlinear model leads to better predictions. Obviously, these effects are instrumental to obtaining reasonable predictions for all post-stable motion regimes. (3) When discrepancies arise, the nonlinear model was usually found to be conservative.
- Finite length bearings and squeeze-film dampers: a two-dimensional dynamical solutionPublication . Antunes, J.; Moreira, Miguel; Piteau, P.In this paper we develop a non-linear dynamical solution for finite length bearings and squeeze-film dampers based on a Spectral-Galerkin method. En this approach the gap-averaged pressure is approximated, in the lubrication Reynolds equation, by a truncated double Fourier series. The Galerkin method, applied over the residuals so obtained, generate a set of simultaneous algebraic equations for the time-dependent coefficients of the double Fourier series for the pressure. In order to asse:t the validity of our 2D—Spectral-Galerkin solution we present some preliminary comparative numerical simulations, which display satisfactory results up to eccentricities of about 0.9 of the reduced fluid gap 1-1/R. The so-called long and short-bearing dynamical solutions of the Reynolds equation, reformulated in Cartesian coordinates, are also presented and compared with the corresponding classic solutions found on literature.
- Flow-acoustic interaction in corrugated pipes: Time-domain simulation of experimental phenomenaPublication . Debut, V.; Antunes, J.; Moreira, MiguelA phenomenological model based on the coupling between a set of “pressure vortex oscillators” and an acoustic field is considered to simulate the dynamical behaviour of a corrugated pipe. The effect of a random Gaussian dispersion of the Strouhal number is investigated by means of nonlinear numerical time-domain simulations and also in terms of the modeshapes of the linearized aeroacoustical model. The results demonstrate that such perturbations may qualitatively change the dynamics of the model and might cause localization phenomenon.
- 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 flow effects on immersed spent nuclear racksPublication . Moreira, Miguel; Antunes, J.Fluid-coupling effects lead to a complex dynamical behavior of immersed spent fuel assembly storage racks. Predicting their responses under strong earthquakes is of prime importance for the safety of nuclear plant facilities. In the near-past we introduced a simplified linearized model for the vibrations of such systems, in which gap-averaged velocity and pressure fields were described analytically in terms of a single space-coordinate for each fluid inter-rack channel. Using such approach it was possible to generate and assemble a complete set of differentialalgebraic equations describing the multi-rack fluid coupled system dynamics. Because of the linearization assumptions, we achieved computation of the flowstructure coupled modes, but also time-domain simulations of the system responses. However, nonlinear squeeze-film and dissipative flow effects, connected with very large amplitude responses and/or relatively small water gaps, cannot be properly accounted unless the linearization assumption is relaxed. Such was the aim of a previous paper. Here, our nonlinear model is introduced and tested in order to expose the significance of the squeeze-film and dissipative effects. The response of a fuel storage pool with several racks submitted to strong seismic excitations applied in different directions are also presented and discussed.