Scielo RSS <![CDATA[Journal of the Brazilian Society of Mechanical Sciences and Engineering]]> vol. 27 num. 4 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<B>Experimental investigation of turbulent boundary layers over steep two-dimensional elevations</B>]]> This work presents a laboratory study on the behavior of turbulent boundary layers over steep topographic elevations. Two main topics of interest are addressed here: (i) to investigate and characterize the separated flow region that is formed on the leeside of a steep elevation, and (ii) to evaluate the effects of flow stability conditions on the properties of boundary layers also subject to surface changes in elevation. To carry out this task, water channel and wind-tunnel investigations were conducted. For the former research topic, a neutrally stratified boundary layer was simulated in the water-channel of the Hydraulics Laboratory of FEUP. Mean and turbulent velocities were measured through laser-Doppler anemometry. Results provided a thorough description of the inner layers along the hill and inside the recirculation region. The refined near-wall data has contributed to the calculation of the friction velocity along the hill through different methods. For the latter topic, neutral, stable and unstable boundary layers were simulated in a wind-tunnel in the Laboratory of Turbulence Mechanics of COPPE/UFRJ. Simultaneous velocity and temperature fields were measured with the aid of thermal anemometry. These results allowed the characterization of the effects of the stratification on the speed-up factor, i.e. the maximum acceleration of the flow on hilltop. The present paper has introduced the concept of the heat up/down factor, in order to study the behaviour of the temperature field on the crest of the elevation. <![CDATA[<B>Differential kinematics of serial manipulators using virtual chains</B>]]> This paper presents a new approach to calculate the direct and inverse differential kinematics for serial manipulators. The approach is an extension of the Davies method for open kinematic chains based on a virtual kinematic chain concept introduced in this paper. It is a systematic method that unifies the kinematics of serial manipulators considering the type of kinematics and the coordinate system of the operational space and constitutes an alternative way to solve the differential kinematics for manipulators. The usefulness of the method is illustrated by applying it to an industrial robot. <![CDATA[<B>A method to determinate the thickness control parameters in cold rolling process through predictive model via neural networks</B>]]> The single stand rolling mill governing equation is a non-linear function on several parameters (input thickness, front and back tensions, yield stress and friction coefficient among others). Any alteration in one of them will cause alterations on the rolling load and, consequently, on the outgoing thickness. This paper presents a method to determinate the appropriate adjustment for thickness control considering three possible control parameters: roll gap, front and back tensions. The method uses a predictive model based in the sensitivity equation of the process, where the sensitivity factors are obtained by differentiating a neural network previously trained. The method considers as the best control action the one that demands the smallest adjustment. One of the capital issues in the controller design for rolling systems is the difficulty to measure the final thickness without time delays. The time delay is a consequence of the location of the outgoing thickness sensor that is always placed to some distance to the front of the roll gap. The proposed control system calculates the necessary adjustment based on a predictive model for the output thickness. This model permits to overcome the time delay that exists in such processes and can eliminate the thickness sensor, usually based on X-ray. Simulation results show the viability of the proposed technique. <![CDATA[<B>Retrograde orbits perturbed by a third-body</B>]]> This paper develops a semi-analytical study of the perturbation caused to a spacecraft by a third body involved in the dynamics. There are several important applications for this research, such as to calculate the effect of lunar and solar perturbations on high-altitude Earth satellites. In the present research the goal is to study the evolution of retrograde orbits around the Earth. There is a special interest to see under which conditions a near-circular orbit remains near-circular. The existence of circular, equatorial and frozen orbits are also considered, The results are valid for any system of primaries by making a time transformation that depends on the masses of the bodies involved. Several plots will show the time-histories of the Keplerian elements of the orbits involved. <![CDATA[<B>Numerical simulation of two dimensional compressible and incompressible flows</B>]]> In this article, we make use of a stabilized Finite Element method to solve the complete set of Navier-Stokes equations. The methodology adopted is such that it allows for the use of different sets of variables, particularly the so called conservative and pressure variables. A space-time formulation using a simple augmented SUPG stabilizing term is proposed for the particular case of pressure variables. Comparison with data published in the available literature is done and a reasonably good agreement is obtained. <![CDATA[<B>A mathematical method for solving mixed problems in multislab radiative transfer</B>]]> In this article, we describe a mathematical method for solving both conservative and non-conservative radiative heat transfer problems defined on a multislab domain, which is irradiated from one side with a beam of radiation. We assume here that the incident beam may have a monodirectional (singular) component and a continuously distributed (regular) component in angle. The key to the method is a Chandrasekhar decomposition of the (mathematical) multislab problem into an uncollided transport problem with singular boundary conditions and a diffusive transport problem with regular boundary conditions. Solution to the uncollided problem is straightforward, but solution to the diffusive problem is not so. For then we make use of a recently developed discrete ordinates method to get an angularly continuous approximation to the solution of the diffusive problem. We suitably compose uncollided and diffuse solutions, and the task of generating an approximate solution to the original multislab radiative transfer problem is complete. We illustrate the accuracy of the proposed method with numerical results for a test problem in shortwave atmospheric radiation, and we conclude this article with a discussion. <![CDATA[<B>The use of artificial neural network in the classification of pulse-echo and TOFD ultra-sonic signals</B>]]> The present work evaluates the application of artificial neural networks for pattern recognition of ultrasonic signals using pulse-echo and TOFD (Time of Flight Diffraction) techniques in weld beads. In this study pattern classifiers are implemented by artificial neural network of backpropagation type using MATLABĀ®. The ultrasonic signals acquired from pulse-echo and TOFD were introduced, separately, in the artificial neural network with and without preprocessing. The preprocessing was only used to smoothen the signal improving the classification. Four conditions of weld bead were evaluated: lack of fusion (LF), lack of penetration (LP), porosity (PO) and non-defect (ND). The defects were intentionally inserted in a weld bead of AISI 1020 steel plates of 20 mm thickness and were confirmed using radiographic tests. The results obtained show that it is possible to classify ultrasonic signals of weld joints by the pulse-echo and TOFD techniques using artificial neural networks. The results showed a performance superior a 72% of success for test. Although the preprocessing of the signal improved the classification performance of the signals acquired by the TOFD technique considerably, the same didn't happen with the signals acquired by the pulse-echo technique. <![CDATA[<B>Numerical simulation of heat transfer during the solidification of pure iron in sand and mullite molds</B>]]> Many complex phenomena favoring the solidification of metal that occur during the casting process, such as cast metal flow, thermal gradient and heat transfer between the cast metal and the mold. The grain size and mechanical properties of cast metal are defined by both these phenomena, and by the geometrical characteristics and thermo-physical properties of the metal and the mold. Heat loss from the mold to the environment through convection can also affect the mechanical properties of cast metal. In this study reported, two-dimensional numerical simulations were made of pure iron solidification in industrial AI 50/60 AFS greensand and mullite molds, using the finite element technique and the ANSYS software program. For this purpose, the iron's thermo-physical properties were considered dependent with temperature, while for sand and mullite these properties were considered constant, and the convection phenomenon was also considered on the mold's external surface. Metallurgical characteristics, such as the attack zone in the feed head and hot top were not taken into account in this study, since they are irrelevant the behavior of heat transfer of the metal to the mold. Owing to the iron's temperature-dependent thermo-physical properties, this type of problem is of nonlinear characteristic. The results of the heat transfer are shown in 2D, as well as, the thermal flux, the thermal gradient and the convergence curves that control the feasibility of the Newton-Raphson algorithm calculation process. The cooling curves at various points of the solidified specimen, and the heating and cooling curves in the mold were also shown. These results were considered relevant. <![CDATA[<B>An axisymmetric finite volume formulation for the solution of heat conduction problems using unstructured meshes</B>]]> In this work, a finite volume formulation developed for two-dimensional models is extended to deal with axisymmetric models of heat conduction applications. This formulation uses a vertex centered finite volume method and it was implemented using an edge-based data structure. The time and domain discretization using triangular meshes is described in details, including the treatment of boundary conditions, source terms, and domains with multiple materials. The proposed formulation is validated and proves to be effective and flexible through the solution of simple model problems. <![CDATA[<B>Modeling and analysis of an auto-adjustable stroke end cushioning device for hydraulic cylinders</B>]]> This paper describes the theoretical-experimental study of an auto-adjustable stroke end cushioning device utilized in hydraulic cylinders, focusing the characterization of the bush geometry effect on the cushioning achieved. A nonlinear model is presented which includes the physical phenomena that exert a significant influence on the performance of this hydraulic component, such as: friction, fluid compressibility and pressure energy loss in the cushioning section. The model is validated through the comparison between theoretical and experimental results, under different conditions of load, supply pressure and piston speed. From this point it is possible to obtain a model applicable for the design of stroke end cushioning devices in hydraulic cylinders. Consequent contributions related to proportional directional valves modeling are also presented. <![CDATA[<B>Numerical and experimental analysis of tube drawing with fixed plug</B>]]> Numerical simulation of manufacturing processes has become in the last years an important tool to improve these processes reducing lead times and try out, and providing products free of defects and with controlled mechanical properties. Finite Element Method (FEM) is one of the most important methods to simulate metal forming. In tube drawing with fixed plug both the outer diameter and the inner diameter of the tube are properly defined if correct process conditions are chosen for the die angle, drawing speed, lubrication and area reduction per pass. These conditions have great influence on drawing loads and residual stresses present in the product. In this work, the cold drawing of tubes with fixed plug was simulated by FEM with the commercial software MSC.Superform to find the best geometry of die and plug to reduce the drawing force. The numerical analysis supplied results for the reactions of the die and plug and the stresses in the tube, the drawing force and the final dimensions of the product. Those results are compared with results obtained from analytic models, and used tooling design. Experimental tests with a laboratory drawing bench were carried out with three different lubricants and two different lubrication conditions. <![CDATA[<B>Multi-dimensional discretization error estimation for convergent apparent order</B>]]> This work presents procedures for estimating the error of numerical solutions of multi-dimensional problems. It is considered that: the numerical error is caused only by truncation errors; error estimations are based on the Richardson extrapolation; and numerical approximations are one-dimensional over uniform grids in each dimension. Two cases are analyzed: when grids are simultaneously refined in all four dimensions (x,y,z,t); and when grid refinement in each dimension is separate from the remaining ones. Examples of uses are presented for problems involving heat transfer and fluid mechanics, which are solved by the finite difference and finite volume methods. It was found that, for the situation in which the apparent order of the estimated error is a monotone convergent one, two values of estimated error can be calculated, which bound the true error. <![CDATA[<B>Multiple phase silicon in submicrometer chips removed by diamond turning</B>]]> Continuous chips removed by single point diamond turning of single crystal silicon have been investigated by means of Scanning Electron Microscopy/Transmission Electron Microscopy and micro-Raman Spectroscopy. Three different chip structures were probed with the use of electron diffraction pattern: (i) totally amorphous lamellar structure, (ii) amorphous structure with remnant crystalline material and, (iii) partially amorphous together with amorphous with remnant crystalline material. Furthermore, micro-Raman spectroscopy from the chips left in the cutting tool rake face showed different silicon phases. We have found, from a detailed analysis of the debris, five different structural phases of silicon in the same debris. It is proposed that material removal mechanisms may change along the cutting edge from shearing (yielding lamellar structures) to extrusion. Shearing results from structural changes related to phase transformation induced by pressure and shear deformation. Extrusion, yielding crystalline structures in the chips, may be attributed to a pressure drop (due to an increase in the contact area) from the tool tip towards the region of the cutting edge where brittle-to-ductile transition occurs. From this region upwards, pressure(stress) would be insufficient to trigger phase transformation and therefore amorphous phase would not form integrally along the chip width. <![CDATA[<B>Dynamic positioning system of semi-submersible platform using fuzzy control</B>]]> The purpose of this paper is the study of fuzzy control applied to a Dynamic Positioning System (DPS) of semi-submersible platforms. A numerical simulator program in time domain was coded using mathematical models of the floating platform dynamics and the external forces (wind, current, wave and thruster) that act on the platform. Subsequently, a fuzzy controller applied to DPS was developed. The Fuzzy controller and the Proportional-Integral-Derivative (PID) controller were then subjected to the same environmental conditions in order to compare their performance. <![CDATA[<B>The boundary element method applied to incompressible viscous fluid flow</B>]]> An Integral equation formulation for steady flow of a viscous fluid is presented based on the boundary element method. The continuity, Navier-Stokes and energy equations are used for calculation of the flow field. The governing differential equations, in terms of primitive variables, are derived using velocity-pressure-temperature. The calculation of fundamental solutions and solutions tensor is showed. Applications to simple flow cases, such as the driven cavity, step, deep cavity and channel of multiple obstacles are presented. Convergence difficulties are indicated, which have limited the applications to flows of low Reynolds numbers. <![CDATA[<B>Use of quality maps in reservoir management</B>]]> The definition and management of a production strategy for petroleum fields is one of the most important tasks in reservoir engineering. It is a complex process due to the high number of parameters, operational restrictions and objectives involved, and due to uncertainties in geological and economic scenarios. This work shows an important tool to improve the performance of such a process, called quality map. Quality map is a tool that indicates the production potential of each place of the reservoir, combining several parameters that influence oil recovery efficiency. It serves as a visualisation tool and as a quality index distribution allowing the automation of production strategy definition. The case studies presented in this work involve numerical simulation of horizontal wells in offshore reservoir models. It is observed that quality maps constitute a powerful tool that can be used (1) to locate wells and (2) to speed up the optimisation process by efficiently allowing the analysis and quantification of several parameters and their influence on the reservoir exploitation. <![CDATA[<B>Verification of the pressure equalisation inside the satellite compartment of the Brazilian satellite launch vehicle</B>]]> During the atmospheric flight of the Brazilian satellite launch vehicle the pressure inside the satellite compartment should be equalised with the atmospheric pressure. This becomes necessary due to the high pressure differences which result when the vehicle reaches high altitudes with decreasing atmospheric pressures, generating high loads acting on the internal surface of the fairing. The equalisation is achieved through venting holes placed around the fairing of the satellite compartment. The design of the venting orifices should be constrained to the restriction of constant evacuation of the compartment, so that, at any time of the trajectory, the pressure difference is minimal. Meeting this constraint becomes complex due to the flight environment of the vehicle which is characterised by very high acceleration levels. So, the flow around the fairing undergoes constant variation of the velocity field going from subsonic to hypersonic velocities. The position, size and number of venting orifices were determined using gas dynamic analysis and calculations and later validated through flight tests. The article describes the mean features of the design process, discusses the venting criterion, and shows the flight results.