Scielo RSS <![CDATA[Journal of the Brazilian Society of Mechanical Sciences and Engineering]]> http://www.scielo.br/rss.php?pid=1678-587820040003&lang=pt vol. 26 num. 3 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <![CDATA[<B>The height of maximum speed-up in the atmospheric boundary layer flow over low hills</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300001&lng=pt&nrm=iso&tlng=pt In this paper, we study the height of the maximum speed-up, l, for atmospheric boundary layer flows over low hills in a neutral atmosphere. A recent analytically-derived expression for l is compared to the results of several other expressions available in the literature. A critical analysis of all these equations is presented and a new constant obtained from field data is proposed for one of them. We find that the new expression describes observational data better than the others. Through an order of magnitude analysis, we also show that the inner layer depth, calculated as the height where inertia and turbulent forces dominate the other terms and balance each other in the x-momentum equation, can also be used to estimate the height of maximum speed-up. Starting from four analytical speed-up profiles available in the literature, we calculate l by searching for the critical points of these speed-up functions, resulting in new equations for l. All these equations are analysed and our results suggest which one of them performs better when compared to field and wind tunnel data. <![CDATA[<B>Model predictive disturbance rejection during cooperative mobile robot assembly tasks</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300002&lng=pt&nrm=iso&tlng=pt This paper addresses the problem of disturbance compensation for the successful assembly of structures by mobile field robots. A control architecture, consisting of a linear PID joint controller with model predictive feed-forward compensation for mobile base motions and interactive-force disturbance rejection is discussed. Object insertion is achieved by predicting environment-object contact states and motions are planned in the direction of least resistance. Issues addressed include dynamic modeling of multiple cooperative robots, control architecture design and stability analysis, and environment-object contact state prediction. Simulation results show the effectiveness of the control architecture. <![CDATA[<B>Seeding of Görtler vortices through a suction and blowing strip</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300003&lng=pt&nrm=iso&tlng=pt The resulting wavelength of Görtler vortices in boundary layers over concave surfaces is determined by the upstream history of the flow and by wall disturbances such as roughness, heating/cooling strips or suction and blowing. In isotropic disturbance conditions, the predominant wise wavelength corresponds to the strongest growing vortex mode predicted by the linear stability theory. If the disturbance environment is not isotropic, vortices with wavelength different from the one with the highest growth rate may emerge. The present investigation considers the wavelength selection when Görtler vortices are excited by a suction and blowing strip at the wall. The study is based on numerical simulations of the vorticity transport equations derived from the Navier-Stokes equations. They are solved using a compact high-order finite difference technique. The results show that, when the vortices are excited by suction and blowing at the wall, their wise wavelength does not necessarily correspond to the imposed wavelength. Curves of streamwise development of the disturbance energy for different harmonics are presented, showing the evolution of the dominant modes. Isolines of streamwise velocity in the wise plane are also presented, showing how the higher harmonics distort the characteristic mushroom structures. <![CDATA[<B>Detection and treatment of faults in manufacturing systems based on Petri Nets</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300004&lng=pt&nrm=iso&tlng=pt This paper introduces a methodology for modeling and analyzing fault-tolerant manufacturing systems that not only optimizes normal productive processes, but also performs detection and treatment of faults. This approach is based on the hierarchical and modular integration of Petri Nets. The modularity provides the integration of three types of processes: those representing the productive process, fault detection, and fault treatment. The hierarchical aspect of the approach permits us to consider processes on different levels of detail (i.e. factory, manufacturing cell, or machine). Case studies considering detection and treatment of faults are presented, and a simulation tool is applied for verifying the models. <![CDATA[<B>Normal force calculations for rocket-like configurations</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300005&lng=pt&nrm=iso&tlng=pt Transonic and supersonic flow simulations over typical launch vehicle configurations are presented. A 3-D finite difference numerical code, written for general, curvilinear, body-conforming coordinate systems, is used. The code solves the thin-layer approximation for the laminar Navier-Stokes equations. Simulations are performed for a launcher and a sounding rocket configurations, currently under development at Instituto de Aeronáutica e Espaço. Calculations consider cases at angle of attack and at various freestream Mach numbers. Normal force coefficients are obtained such that the loads required for the design phase can be determined. Computational results are compared to available experimental data. In general, good results within engineering error margins are obtained. <![CDATA[<B>A method for the automatic identification of contacts in assemblies of cylindrical parts</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300006&lng=pt&nrm=iso&tlng=pt A significant amount of research has been made on determining the optimum assembly sequence, assembly planning, etc. However, those research works consider that the necessary information for the analysis (e.g. the contacts among the parts in the assembly) is already available, which is not usually the case. This paper describes a method for extracting information within the design, aiming at supporting applications of assembly identification, which is needed for assembly planning. The system determines the surfaces to be machined and the dimensions to receive tighter tolerances, which are important for process and inspection planning. Initially, the group of parts that compose the product is input, and then the procedure for automatic identification of assembled parts begins for the given assembly. The final analysis is done by an expert system that checks the conditions of the assembled parts. The system is applied to cylindrical parts, in a design by features environment. <![CDATA[<B>The phase distribution of upward co-current bubbly flows in a vertical square channel</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300007&lng=pt&nrm=iso&tlng=pt In this work one shows experimental data and numerical results of the void fraction distribution in vertical upward air-water bubbly flows in a square cross-section channel. To measure the void fraction distribution one used a single wire conductive probe. The averaged void fraction ranged from 3.3% to 15%; the liquid and the gas superficial velocities varied from 0.9 m/s to 3.0 m/s and 0.04 m/s to 0.5 m/s, respectively. The experimental results for the void fraction distribution were compared with numerical calculation performed by an Eulerian-Eulerian implementation of the Two-Fluid Model. In this work one performs the turbulence modeling with three approaches: using an algebraic model, the k-epsilon two-phase model and the k-epsilon two-phase two-layer model. Comparisons between the experimental and numerical data revealed, in general, good agreement. <![CDATA[<B>The effect of wollastonite on operational characteristics of AWS E6013 electrodes</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300008&lng=pt&nrm=iso&tlng=pt The main objective of this work was to assess the operational behavior of ANSI/AWS A5.1-91 E6013 type electrodes when 0, 8 and 16 % of quartz (100 % SiO2) is replaced with wollastonite (Calcium Silicate, 50 % SiO2 - 50 % CaO) in the coating composition. The electrodes were tested through bead-on-plate welds in flat position on DC, both polarities, and on AC currents. Arc stability, fusion rate and deposition rate were used as operational characteristics evaluation criteria. The results suggested that the replacement of quartz with wollastonite, that increased slag basicity, kept, or even improved, the typical excellent operational characteristics of E6013 type electrodes. <![CDATA[<B>Mixed convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300009&lng=pt&nrm=iso&tlng=pt A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated independently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary condition of uniform wall temperature in the flow direction is considered. A special discritization method is employed to solve the problem associated with near boundary grid points. Results for mean and total mean Nusselt numbers for various values of Reynolds number ,Re; Rayleigh number , Ra ; Geometric ratio ,r g ; Aspect ratio, rA ; Radiation-Conduction parameter , . ; Optical thickness, <FONT FACE=Symbol>J</FONT> ; Rotational Reynolds number ,Ro and Emissivity, epsilon ; are presented. For the range of parameters considered, results show that radiation and rotation enhance heat transfer. It is also indicated in the results that heat transfer from the surface of the circle exceeds that of the rectangle. Optimum heat transfer and fluid bulk temperature are attained when the duct is vertically positioned. The Parameter ranges of 0.2 # r g # 0.84, 0 # PeRa # 7.3 x 10(5) and r g rA # 1 demarcate the extent of the validity of the numerical solution. <![CDATA[<B>Inexact Newton-type methods for non-linear problems arising from the SUPG/PSPG solution of steady incompressible navier-stokes equations</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300010&lng=pt&nrm=iso&tlng=pt The finite element discretization of the incompressible steady-state Navier-Stokes equations yields a non-linear problem, due to the convective terms in the momentum equations. Several methods may be used to solve this non-linear problem. In this work we study Inexact Newton-type methods, associated with the SUPG/PSPG stabilized finite element formulation. The resulting systems of equations are solved iteratively by a preconditioned Krylov-space method such as GMRES. Numerical experiments are shown to validate our approach. Performance of the nonlinear strategies is accessed by numerical tests. We concluded that Inexact Newton-type methods are more efficient than conventional Newton-type methods. <![CDATA[<B>Reduction of squeal noise from disc brake systems using constrained layer damping</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782004000300011&lng=pt&nrm=iso&tlng=pt Squeal noise generation during braking is a complicated dynamic problem which automobile manufacturers have confronted for decades. Customer complaints result in significant yearly warranty costs. More importantly, customer dissatisfaction may result in rejection of certain brands of brake systems. In order to produce quality automobiles that can compete in today's marketplace, the occurrence of disc brake squeal noise must be reduced. The addition of a constrained layer material to brake pads is commonly utilized as a means of introducing additional damping to the brake system. Additional damping is one way to reduce vibration at resonance, and hence, squeal noise. The simulation of braking events in dynamometers has typically been the preferred insulator selection process. However, this method is costly, time consuming and often does not provide an insight into the mechanism of squeal noise generation. This work demonstrates the use of modal analysis techniques to select brake dampers for reducing braking squeal. The proposed methodology reduces significantly the insulator selection time and allows an optimized use of the brake dynamometer to validate selected insulators.