FE - DEM | Documentos por Auto-Depósito
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- A method for particle location and field interpolation on complex, three-dimensional computational meshesPublication . Oliveira, Paulo J.; Gosman, A. D.; Issa, R. I.A method for locating particles within arbitrary three-dimensional computational meshes is described. It is based on an iterative procedure which uses transformed coordinates defined by iso-parametric functions. The method also enables one to interpolate field values from the mesh nodes to the particle position. Example applications demonstrate how effective the method is. For very distorted computational cells special practices have to be introduced in order to keep the number of iterations to a minimum.
- Pressure drop coefficient of laminar newtonian flow in axisymmetric sudden expansionsPublication . Oliveira, Paulo J.; Pinho, FernandoLaminar flow of a Newtonian fluid in an axisymmetric pipe expansion has been studied numerically by means of the finite-volume approach. Predicted values of some of the overall flow characteristics, such as recirculation length, its strength, and centre location, were compared with available experimental data and correlations, and good agreement was found. The purpose of the work was to evaluate the pressure-loss coefficient Cl for a range of Reynolds numbers and to compare the results with existing simplified theory, which is based on a one-dimensional (1-D) overall balance of energy and momentum. Considerable differences were found, which lead us to formulate corrected theoretical equations in the scope of the 1-D approximation. These corrections were evaluated from the numerical results and accounted for three effects: (1) differing actual and fully developed wall friction; (2) distortion of velocity profiles from the parabolic shape at the sudden expansion section; and (3) nonuniformity of pressure at the expansion plane. Predicted values of the loss coefficient agreed to within 4% with the corrected theory and were found to be proportional to the inverse of the Reynolds number for Re ≤ 17.5 [with effect (3) above predominant and accounting for up to 85% of Cl] and approximately constant for Re > 17.5 [with effect (1) above predominant and accounting for 20% of Cl]. Finally, a correlation for calculating the local loss coefficient as a function of the Reynolds number for the 1:2.6 sudden expansion and fully developed conditions is proposed.
- Optimization Criteria for Cellular Planning of Mobile Broadband Systems in Linear and Urban CoveragesPublication . Velez, Fernando José; Correia, Luis M.This paper addresses the comparison of characteristics between the prospective allocated bands for MBS by the International Telecommunications Union, 40 and 60 GHz, and its consequences in the optimization criteria for the cellular design. The key difference between the two bands is the oxygen absorption attenuation which is negligible at 40 GHz, but presents high values at 60 GHz, larger than 10 dB for the typical reuse distance. At 60 GHz the carrier-to-interference ratio is larger, and depends on the coverage distance, while at 40 GHz this dependence does not exist. At 60 GHz lower values for the co-channel reuse factor are obtained and, as a consequence, a larger system capacity is achieved.
- A general correlation for the local loss coefficient in newtonian axisymmetric sudden expansionsPublication . Oliveira, Paulo J.; Pinho, Fernando; Schulte, A.Results from numerical simulations and guidance from an approximated corrected-theory, developed by Oliveira and Pinho (1997), have been used to arrive at a correlation expressing the irreversible loss coefficient for laminar Newtonian flow in axisymmetric sudden expansions. The correlation is valid for the ranges 1.5 < D2/D1 < 4 and 0.5 < Re < 200 with errors of less than 5%, except for 25 < Re < 100 where the error could be as much as 7%. The recirculation bubble length is also presented for the same range of conditions and the pressure recovery coefficient was calculated for Reynolds numbers above 15.
- Numerical Simulation of Non-linear Elastic Flows with a General Collocated Finite-Volume MethodPublication . Oliveira, Paulo J.; Pinho, Fernando; Pinto. G. A.This paper reports the development and application of a finite-volume based methodology for the calculation of the flow of fluids which follow differential viscoelastic constitutive models. The novelty of the method lies on the use of the non-staggered grid arrangement, in which all dependent variables are located at the center of the control volumes, thus greatly simplifying the adoption of general curvilinear coordinates. The pressure–velocity–stress decoupling was removed by the development of a new interpolation technique inspired on that of Rhie and Chow, AIAA J 82 (1982) 998. The differencing schemes are second order accurate and the resulting algebraic equations for each variable are solved in a segregated way (decoupled scheme). The numerical formulation especially designed for the interpolation of the stress field was found to work well and is shown to be indispensable for accurate results. Calculations have been carried out for two problems: the entry flow problem of Eggleton et al., J. Non-Newtonian Fluid Mech. 64 (1996) 269, with orthogonal and non-orthogonal meshes; and the bounded and unbounded flows around a circular cylinder. The results of the simulations compare favourably with those in the literature and iterative convergence has been attained for Deborah and Reynolds numbers similar to, or higher than, those reported for identical flow problems using other numerical methods. The application of the method with non-orthogonal coordinates is demonstrated. The entry flow problem is studied in more detail and for this case differences between Newtonian and viscoelastic fluids are identified and discussed. Viscoelasticity is shown to be responsible for the development of very intense normal stresses, which are tensile in the wall region. As a consequence, the viscoelastic fluid is more intensely decelerated in the wall region than the Newtonian fluid, thus reducing locally the shear rates and the role of viscosity in redeveloping the flow. A layer of high stress-gradients is formed at the wall leading edge and is convected below and away from the wall; its effect is to intensify the aforementioned deviation of elastic fluid from the wall.
- A qualitative assessment of the role of a viscosity depending on the third invariant of the rate-of-deformation tensor upon turbulent non-Newtonian flowPublication . Oliveira, Paulo J.; Pinho, FernandoThe numerical simulation of some non-Newtonian effects in wall and wall-free turbulent flows, such as drag reduction in pipe flows or the decrease in transverse normal Reynolds stresses, has been attempted in the past with a limited degree of success on the basis of modified wall functions applied to traditional turbulence models (k–epsilon), rather than through more realistic rheological constitutive equations. In this work, it is qualitatively shown that if the viscosity function of a generalised Newtonian fluid is assumed to depend on the third invariant of the rate of deformation tensor, there is an increase of the viscous diffusion terms, but especially, of the dissipation of turbulence kinetic energy by a factor equal to the Trouton ratio of the fluid, divided by the Trouton ratio of the solvent, thus indicating a possible way to improve rheological–turbulence modelling.
- Traffic from Mobility in Mobile Broadband SystemsPublication . Velez, Fernando J.; Correia, Luis M.Models allowing the study of the influence of coveragedistance and velocity on the supported traffic and on thenew calls traffic linear density are examined, and resultsare obtained for typical scenarios in a Mobile BroadbandSystem (MBS) with a linear coverage geometry. For systemswithout guard channels for handover, for a fixed boundingvalue for the blocking probability, the new calls traffic lin-ear density was analyzed, increasing with the decrease of themaximum coverage distance, R, being upper limited by avalue which depends on the characteristics of the mobilityscenario. However, call-dropping probability requirementsalso need to be fulfilled, leading to a new calls traffic densitythat only increases with the decrease of R down to an opti-mum value of R, and being lower for scenarios with highermobility. These optimum values of R are higher for scenar-ios with higher and higher mobility, leading to limitations insystem capacity, mainly for high mobility scenarios. Inorder to resolve these limitations, the use of guard channelsfor handover is studied, particularly for high mobilityscenarios. For these scenarios one concludes that there is adegradation in system capacity because, for the typicalcoverage distances foreseen for MBS, the new calls trafficlinear density is one order of magnitude below the valuesobtained for the pedestrian scenario (where it is approxi-mately 15 Erlang/km), decreasing from 2.47 Erlang/km, inthe urban scenario, down to 0.84 Erlang/km, in the highwayscenario, when two guard channels are used.
- Influence of Traffic from Mobility on the Microcellular Coverage Distance in Mobile Broadband SystemsPublication . Velez, Fernando J.; Correia, Luis M.Models allowing the study of the influence of coverage distance and velocity on the supported traffic and on the new calls traffic linear density are examined, and results are obtained for typical scenarios in a Mobile Broadband System (MBS) with a linear coverage geometry. For situations without using guard channels for handover, for a fixed bounding value for the blocking probability, the new calls traffic linear density has been analyzed, increasing with the decrease of the coverage distance R, and being upper limited by a value which depends on the characteristics of the mobility scenario. However, call-dropping probability requirements also need to be fulfilled, leading to a new calls traffic linear density that only increases with the decrease of R down to an optimum value of R, and being lower for scenarios with higher mobility. This situation leads to limitations in system capacity for lower values of the coverage distance, mainly for high mobility scenarios.
- Microcellular Design and System capacity determination for Outdoors Urban Mobile Broadband Communication Systems in the Millimetrewave BandsPublication . Velez, Fernando José; Brazio, José M.This paper addresses lhe cellular design of mobile microcellular communications systems operating in lhe millimetrewave bands in outdoor urban environments. ln such systems and environments lhe shape of cells and lhe interference among them are highly dependent on lhe geomctry of lhe surrounding obstacles (c.g., buildings) and lhe classical regular geametry based approaches to frequency reuse and capacity evaluation eannat be used. In lhe paper is described an approach to frequency reuse and system capacity evaluation where canclusions are drawn frem cellular layouts obtained, via an interactive computer graphical tool, for specific urban environments. Numerical examples are presented for coverage in lhe 40 and 60 GHz bands showing that, depending on lhe celI size, reuse factors in lhe range 5-11 are achievable. It is also concluded that, in interference límited scenarios, if cells with similar coverage length are used, equal values are obtained for lhe reuse factor and system eapacity in both bands.
- Capacity trade-offs in mobile broadband systems using guard channels for high mobility handoverPublication . Velez, Fernando J.; Correia, Luis M.Models allowing the study of the influence of coverage distance and mobile velocity on the supported traffic and on the new calls traffic linear density are examined, and results are obtained for typical scenarios in a mobile broadband system (MBS) with a linear coverage geometry. In order to cope with handover failure probability requirements, the use of guard channels for handover is assumed, mainly for high mobility scenarios. For these scenarios one concludes that there is a degradation in system capacity because, for the typical coverage distances foreseen for MBS, the new calls traffic linear density is one order of magnitude bellow the values obtained for the pedestrian scenario (where it is approximately 15 Erlang/km), decreasing from 2.47 Erlang/km, in the urban scenario, down to 0.84 Erlang/km, in the highway scenario, when two guard channels are used.