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Biblioteca Digital do IPB (53.442 recursos)

A Biblioteca Digital do IPB, tem por objectivo divulgar e permitir o livre acesso à produção científica produzida pela comunidade académica, promovendo a integração, partilha e a visibilidade da informação científica e garantindo a preservação da memória intelectual do Instituto Politécnico de Bragança.

DEMAT - Artigos em Revistas Indexadas

Mostrando recursos 1 - 17 de 17

  1. Angular momentum transport by internal waves in the solar interior

    Zahn, Jean-Paul; Talon, Suzanne; Matias, José
    The internal gravity waves of low frequency which are emitted at the base of the solar convection zone are able to extract angular momentum from the radiative interior. We evaluate this transport with some simplifying assumptions: we ignore the Coriolis force, approximate the spectrum of turbulent convection by the Kolmogorov law, and couple this turbulence to the internal waves through their pressure fluctuations, following Press (1981) and Garc´ıa L´opez & Spruit (1991). The local frequency of an internal wave varies with depth in a differentially rotating star, and it can vanish at some location, thus leading to enhanced damping (Goldreich & Nicholson 1989). It is this dissipation mechanism only that we...

  2. Interior point filter method for semi-infinite programming problems

    Pereira, Ana I.; Costa, M. Fernanda; Fernandes, Edite M.G.P.
    Semi-infinite programming (SIP) problems can be efficiently solved by reduction-type methods. Here, we present a new reduction method for SIP, where the multi-local optimization is carried out with a stretched simulated annealing algorithm, the reduced (finite) problem is approximately solved by a Newton’s primal–dual interior point method that uses a novel twodimensional filter line search strategy to guarantee the convergence to a KKT point that is a minimizer, and the global convergence of the overall reduction method is promoted through the implementation of a classical two-dimensional filter line search. Numerical experiments with a set of well-known problems are shown.

  3. Asymmetry of red blood cell motions in a microchannel with a diverging and converging bifurcation

    Leble, Vladimir; Lima, R.; Dias, Ricardo P.; Fernandes, Carla S.; Ishikawa, Takuji; Imai, Yohsuke; Yamaguchi, Takami
    In microcirculation, red blood cells (RBCs) flowing through bifurcations may deform considerably due to combination of different phenomena that happen at the micro-scale level, such as: attraction effect, high shear and extensional stress, all of which may influence the rheological properties and flow behavior of blood. Thus, it is important to investigate in detail the behavior of blood flow occurring at both bifurcations and confluences. In the present paper, by using a micro-PTV system, we investigated the variations of velocity profiles of two working fluids flowing through diverging and converging bifurcations - human red blood cells suspended in dextran 40...

  4. On the relative solvability of certain inverse monoids

    Cordeiro, Edite; Delgado, Manuel
    The notion of kernel of a finite monoid relative to a pseudovariety of groups can be used to define relative solvability of monoids in a similar way to the manner in which the notion of derived subgroup can be used to define solvable group. In this paper we study the solvability of certain inverse monoids relative to pseudovarieties of abelian groups.

  5. Solving myopia in real-time decision-making using Petri nets models’ knowledge for service-oriented manufacturing systems

    Leitão, Paulo; Alves, Joel; Pereira, Ana I.
    This paper introduces a novel approach to the real-time decision-making in service-oriented manufacturing systems, addressing the myopia problem usually presented in such systems. The proposed decision method considers the knowledge extracted from the Petri nets models used to describe the services process behavior, mainly the T-invariants, combined with a multi-criteria function customized according to the system’s particularities and strategies. An experimental laboratorial case study was used to demonstrate the applicability of the proposed real-time decision-making approach in service-oriented manufacturing systems, considering some productivity and energy efficiency criteria.

  6. Golden tilings

    Pinto, Alberto A.; Almeida, João Paulo; Portela, A.
    We introduce the notion of golden tilings and we prove a oneto- one correspondence between (i) smooth conjugacy classes of Anosov diffeomorphisms, with an invariant measure absolutely continuous with respect to the Lebesgue measure, (ii) affine classes of golden tilings and (iii) solenoid functions. The solenoid functions give a parametrization of the infinite dimensional space consisting of the mathematical objects described in the above equivalences.

  7. New plates for different types of plate heat exchangers

    Fernandes, Carla S.; Dias, Ricardo P.; Maia, João M.
    The first patent for a plate heat exchanger was granted in 1878 to Albretch Dracke, a German inventor. The commercial embodiment of these equipments has become available in 1923. However, the plate heat exchanger development race began in the 1930’s and these gasketed plate and frame heat exchangers were mainly used as pasteurizers (e.g. for milk and beer). Industrial plate heat exchangers were introduced in the 1950’s and initially they were converted dairy models. Brazed plate heat exchangers were developed in the late 1970’s. However, copper brazed units did not start selling until the early 80’s. Nickel brazing came to...

  8. Computing relative abelian kernels of finite monoids

    Cordeiro, Edite; Delgado, Manuel
    Let H be a pseudovariety of abelian groups corresponding to a recursive supernatural number. In this note we explain how a concrete implementation of an algorithm to compute the kernel of a finite monoid relative to H can be achieved. The case of the pseudovariety Ab of all finite abelian groups was already treated by the second author and plays an important role here, where we will be interested in the proper subpseudovarieties of Ab. Our work relies on an algorithm obtained by Steinberg.

  9. Relative abelian kernels of some classes of transformation monoids

    Cordeiro, Edite; Delgado, Manuel; Fernandes, V.H.
    We consider members of some well studied classes of finite transformation monoids and give descriptions of their abelian kernels relative to decidable pseudovarieties of abelian groups.

  10. A reduction method for semi-infinite programming by means of a global stochastic approach

    Pereira, Ana I.; Fernandes, Edite M.G.P.
    We describe a reduction algorithm for solving semi-infinite programming problems. The proposed algorithm uses the simulated annealing method equipped with a function stretching as a multi-local procedure, and a penalty technique for the finite optimization process. An exponential penalty merit function is reduced along each search direction to ensure convergence from any starting point. Our preliminary numerical results seem to show that the algorithm is very promising in practice.

  11. Friction factors of power-law fluids in chevron-type plate heat exchangers

    Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Maia, João M.
    In the present work laminar flows of Newtonian and power-law fluids through cross-corrugated chevron-type plate heat exchangers (PHEs) are numerically studied in terms of the geometry of the channels. The plates area enlargement factor was a typical one (1.17), the corrugation angle, β, varied between 30° and 60° and the flow index behaviour, n, between 0.25 and 1. Single friction curves fReg = K for both Newtonian and non-Newtonian fluids are proposed for each β by developing an adequate definition of the generalised Reynolds number, Reg. The coefficient K compares well with experimental data, for all (seven) values of β,...

  12. Starch analysis using hydrodynamic chromatography with a mixed-bed particle column

    Dias, Ricardo P.; Fernandes, Carla S.; Teixeira, José A.; Mota, Manuel; Yelshin, Alexander
    Columns packed with commercial glass beads 5 and 19 μm average size and a mixture of both (0.7 volume fraction of large particles) were used to analyse starch composition by hydrodynamic chromatography (HDC), applying water as mobile phase. To obviate retrogradation, experiments were carried out at column temperatures of 15 and 3 °C and several types of starch were assayed. In what concerns amylopectin and amylose separation, a better resolution and a lower pressure drop were obtained for the mixed binary packing when compared with the packing containing uniform 5 μm glass beads. A more efficient cooling of the mobile...

  13. Simulation of stirred yoghurt processing in plate heat exchangers

    Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.
    In the present work, simulations of stirred yoghurt processing in a plate heat exchanger were performed using computational fluid dynamics (CFD) calculations and the results compared with experimental data, showing a very good agreement. A Herschel–Bulkley model for the viscosity and an Arrhenius-type term for the temperature dependence were used to model the thermo-rheological behaviour of yoghurt. The heat exchanger used in this study operates in a parallel arrangement, thus simplifying the problem to the construction of a single complete 3D channel. After analysis of the velocity field and fanning friction factors, laminar flow was observed for all the operating conditions used and relations...

  14. Thermal behaviour of stirred yoghurt during cooling in plate heat exchangers

    Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.
    In this study, CFD calculations were made in order to analyse the thermal behaviour of yoghurt in a plate heat exchanger, the yoghurt viscosity being described by a Herschel-Bulkley model. The influence of Reynolds number on local Nusselt numbers was analysed as well as the influence of entry effects on average Nusselt number. Numerical runs considering and discarding the influence of temperature on the viscosity were performed and the impact on average Nusselt numbers was analysed. Transversal variations of viscosity were studied and thermal correlations including the ratio between bulk and wall viscosities are proposed. The obtained thermal correlations were compared with...

  15. Permeability and effective thermal conductivity of bisized porous media

    Dias, Ricardo P.; Fernandes, Carla S.; Mota, Manuel; Teixeira, José A.; Yelshin, Alexander
    In the region of minimum porosity of particulate binary mixtures, heat exchange and permeability were found to be higher than the ones obtained with a mono-size packing built with the same small size particles used in the binary packing. This effect was noticed in the range of the particles size ratio 0.1–1.0. The obtained improvement on thermal performance is related to the increase of effective thermal conductivity (ETC) in the binary packing and to the increase in transversal thermal dispersion due to the porosity decrease and tortuosity increase. Permeability can increase by a factor of two, if the size ratio between small...

  16. Permeability analysis in bisized porous media: wall effect between particles of different size

    Dias, Ricardo P.; Fernandes, Carla S.; Teixeira, José A.; Mota, Manuel; Yelshin, Alexander
    The permeability of binary packings of glass beads with different size ratio – 13.3, 20, and 26.7, was investigated. In the Kozeny–Carman equation, the dependence of the tortuosity s on the mixture porosity e(xD) was described according to s = 1/en for different volume fraction of large particles in the mixture, xD. Obtained data on packing permeability shows that the parameter n is a function of the volume fraction and particle size ratio, with values between 0.5 and 0.4. This can be explained by the wall effect resulting from the arrangement of the small particles occurring near the large particle surface. A model taking in account this...

  17. Laminar flow in chevron-type plate heat exchangers: CFD analysis of tortuosity, shape factor and friction factor

    Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Maia, João M.
    Liquid foodstuffs with high viscosity are usually processed in plate heat exchangers (PHEs) at laminar or low Reynolds numbers, Re, flows. The tortuosity coefficient is used by the manufacturers for the design and optimization of PHE geometries. Using the finite-element computational fluid dynamics program POLYFLOW®, fully developed laminar flows in double-sine chevron-type PHEs passages are analysed in this work. The corrugation angle and channel aspect ratio of the passages vary in a broad range, PHEs with common area enlargement factors and with high area density being studied. The tortuosity coefficient and the coefficient K (Kozeny´s coefficient in granular beds) from the friction...

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