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vol. 28 num. 3 lang. pt<![CDATA[SciELO Logo]]>http://www.scielo.br/img/en/fbpelogp.gif
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<![CDATA[<B>Heat transfer coefficient in a shallow fluidized bed heat exchanger with a continuous flow of solid particles</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300001&lng=pt&nrm=iso&tlng=pt
This work shows the experimental study of a continuous gas-solid fluidized bed with an immersed tube where cold water is heated by fluidized solid particles presenting inlet temperature from 450 to 700°C. Experiments were carried out in order to verify the influence of solid particle flow rate and distance between baffles immersed in a shallow fluidized bed. The solid material was 254µm diameter silica sand particles, fluidized by air in a 0.90m long and 0.15m wide heat exchanger. The measurements were taken at steady state conditions for solid mass flow rate from 10 to 100 kg/h, in a heat exchanger with the presence of 5 or 8 baffles. Bed temperature measurements along the length of the heat exchanger were experimentally obtained and heat balances for differential control volumes of the heat exchanger were made in order to obtain the axial profile of the bed-to-tube heat transfer coefficient. The results showed that heat transfer coefficient increases with the solid particle mass flow rate and with the presence of baffles, suggesting that these are important factors to be considered in the design of such heat exchanger.<![CDATA[<B>Critical flow regions in tissue artificial heart valve assessed by laser doppler anemometer in continuous flow</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300002&lng=pt&nrm=iso&tlng=pt
Flow diagnosis using non-invasive techniques such laser Doppler anemometer (LDA) is an important tool to improve the design of artificial heart valves. In the present study, an experimental protocol to obtain flow velocity field and colour coded maps of turbulent eddies dimensions using LDA measurements in a 25 mm bovine pericardium bio prosthesis valve is reported. A transparent Plexiglas chamber was specially designed to allow optical access to the flow passing through the valve. Experiments were conducted for non-pulsate flow (to study the valve performance in the peak flow) for the aorta Reynolds number ranging from 3300 to 6800. LDA interrogation volume visited five thousand and one hundred points along the flow (2500 points upstream and 2600 points downstream) for each Reynolds number. Post-processing methodology was employed to obtain haemolytic potential colour-coded maps, which were related to turbulent quantities. It was observed that haemolytic regions tend to move downstream the valve when the flow rate is increased.<![CDATA[<B>Unsteady MHD flow of a dusty non-Newtonian Bingham fluid through a circular pipe</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300003&lng=pt&nrm=iso&tlng=pt
In this paper, the transient magnetohydrodynamic (MHD) flow of a dusty incompressible electrically conducting non-Newtonian Bingham fluid through a circular pipe is studied taking the Hall effect into consideration. A constant pressure gradient in the axial direction and an uniform magnetic field directed perpendicular to the flow direction are applied. The particle-phase is assumed to behave as a viscous fluid. A numerical solution is obtained for the governing nonlinear equations using finite differences.<![CDATA[<B>Some observations on wear and damages in cemented carbide tools</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300004&lng=pt&nrm=iso&tlng=pt
Cutting tools are subjected to extremely unfavorable conditions during machining operations. High cutting temperatures, compressive and shearing stresses, chemical attacks, variable cyclic thermal and mechanical loads are some adverse conditions that wear and damage these tools. Therefore, it is crucial to understand the process of tool wear and damage and how the cutting parameters affect it in order to underpin decisions regarding the most favorable conditions to address the problem. This article treats on some forms and mechanism of wear and damage that cemented carbides can undergo during machining. Special attention was given to damages caused during interrupted cutting (e.g., milling), such as fracture, chipping and thermal fatigue. Experimental details and results of the latter phenomenon, which was studied under different cutting conditions, are discussed and confronted with literature.<![CDATA[<B>Influence of loading, contamination and additive on the wear of a metallic pair under rotating and reciprocating lubricated sliding</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300005&lng=pt&nrm=iso&tlng=pt
This paper deals with an experimental study of wear and friction responses from lubricated sliding. Tests were carried out using a tribometer having devices for both continuous and reciprocating motion. The tested specimens were pins of AISI 52100 steel and counter-faces of AISI 8640 steel. The lubricant was paraffin mineral oil, VI 100. The presence of additives and contamination in the lubricant was investigated under two mechanical loading levels, determined by the velocity/load relation. Wear was evaluated in terms of morphology of the worn surfaces and by dimensional analysis of worn area of the pins. It was possible to obtain a ranking of influences on wear of mechanical loading, mechanical motion, oil additive and contamination presence in oil.<![CDATA[<B>Sensitivity analysis of shallow water problems via perturbative methods</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300006&lng=pt&nrm=iso&tlng=pt
We applied the perturbative theory to perform sensitivity analysis of the shallow water equations. The numerical solution of these equations was found via the mass lumping finite element technique. Then, the adjoint system of the shallow water equations was derived for the one-dimensional case and the expression of the sensitivity coefficient of a generic functional with respect to a generic parameter (Chézy resistance coefficient, solitary wave amplitude and bed channel slope) was obtained, using the differential formalism. The sensitivity of the mean functional, representing the first approximation of the velocity and the depth, was analyzed with regard to these parameters. Results of the sensitivity coefficients obtained via the perturbative methodology satisfactorily matched the values computed by the direct method, i.e., by means of the direct solution of the shallow water equations changing the values of input parameters for each case considered.<![CDATA[<B>Multimodal vibration damping through piezoelectric patches and optimal resonant shunt circuits</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300007&lng=pt&nrm=iso&tlng=pt
Piezoelectric elements connected to shunt circuits and bonded to a mechanical structure form a dissipation device that can be designed to add damping to the mechanical system. Due to the piezoelectric effect, part of the vibration energy is transformed into electrical energy that can be conveniently dissipated. Therefore, by using appropriate electrical circuits, it is possible to dissipate strain energy and, as a consequence, vibration is suppressed through the added passive damping. From the electrical point of view, the piezoelectric element behaves like a capacitor in series with a controlled voltage source and the shunt circuit, commonly formed by an RL network, is tuned to dissipate the electrical energy, more efficiently in a given frequency band. It is important to know that large inductances are frequently required, leading to the necessity of using synthetic inductors (obtained from operational amplifiers). From the mechanical point of view, the vibration energy can be attenuated in a single mode, or in multiple modes, according to the design of the damping device and the frequency band of interest. This work is devoted to the study of passive damping systems for single modes or multiple modes, based on piezoelectric patches and resonant shunt circuits. The present contribution discusses the modeling of piezoelectric patches coupled to shunt circuits, where the basics of resonant shunt circuits (series and parallel topologies) are presented. Following, the devices used in passive control (piezoelectric patch and synthetic inductors) are analyzed from the electrical and experimental viewpoints. The modeling of multi-degree-of-freedom mechanical systems, including the effects of the passive damping devices is revisited, and, then a design methodology for the multi-modal case is defined. Also, it is briefly reviewed the optimization method used for design purposes, namely the LifeCycle Model. Finally, experimental results are reported, illustrating the success of using the methodology presented in passive damping applications applied to mechanical and mechatronic systems.<![CDATA[<B>Recent advances in multi-body dynamics and nonlinear control</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300008&lng=pt&nrm=iso&tlng=pt
This paper presents some recent advances in the dynamics and control of constrained multi-body systems. The constraints considered need not satisfy D'Alembert's principle and therefore the results are of general applicability. They show that in the presence of constraints, the constraint force acting on the multi-body system can always be viewed as made up of the sum of two components whose explicit form is provided. The first of these components consists of the constraint force that would have existed were all the constraints ideal; the second is caused by the non-ideal nature of the constraints, and though it needs specification by the mechanician who is modeling the specific system at hand, it nonetheless has a specific form. The general equations of motion obtained herein provide new insights into the simplicity with which Nature seems to operate. They are shown to provide new and exact methods for the tracking control of highly nonlinear mechanical and structural systems without recourse to the usual and approximate methods of linearization that are commonly in use.<![CDATA[<B>Structural optimization for statics, dynamics and beyond</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300009&lng=pt&nrm=iso&tlng=pt
Structural optimization has matured to the point that it can be routinely applied to a wide range of real design tasks. The purpose here is threefold. First, the general optimization task will be defined. Second, the state of the art in structural optimization will be reviewed. Finally, examples will be presented to demonstrate the level of sophistication possible in applying this technology. It is concluded that, while much research always remains, optimization technology has matured to the point where it can and should be used routinely for engineering design.<![CDATA[<B>Adaptive techniques applied to offshore dynamic positioning systems</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300010&lng=pt&nrm=iso&tlng=pt
Dynamic positioning systems (DPS) comprise the deployment of active propulsion to maintain the position and heading of a vessel. Several sensors are used to measure the actual position of the floating body, while a control algorithm is responsible for the calculation of forces to be delivered by each propeller, in order to counteract all environmental forces, such as wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous amount of power to be attenuated, possibly causing damage to the propeller system. That is the reason why a filtering algorithm is to be put in place to separate high-frequency components from the low-frequency ones, which are, then, fed into the control loop. Usual commercial systems apply Kalman filtering technique to perform such task, due to the smaller phase-lag introduced in the control loop compared to conventional low-pass filters. The Kalman filter draws on a model of the system to be controlled, which, in turn, depends on an unknown parameter, related to the wave frequency. Adaptive filtering is called upon with a view to perform an on-line estimation of such parameter. Most control algorithms, however, rely on fixed gains, thus making it possible for a noticeable performance degradation to take place in some situations, as those associated to mass variation during a loading operation. This paper presents the application of model-reference adaptive control (MRAC) technique to DPS's, cascaded with the commonly used adaptive Kalman filter. The model of a dynamically-positioned shuttle tanker exposed to waves and current is employed to highlight the advantages of the adaptive controller compared to commonplace fixed-gain controllers.<![CDATA[<B>Transient stability of empty and fluid-filled cylindrical shells</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300011&lng=pt&nrm=iso&tlng=pt
In the present work a qualitatively accurate low dimensional model is used to study the non-linear dynamic behavior of shallow cylindrical shells under axial loading. The dynamic version of the Donnell non-linear shallow shell equations are discretized by the Galerkin method. The shell is considered to be initially at rest, in a position corresponding to a pre-buckling configuration. Then, a harmonic excitation is applied and conditions to escape from this configuration are sought. By defining steady state and transient stability boundaries, frequency regimes of instability may be identified such that they may be avoided in design. Initially a steady state analysis is performed; resonance response curves in the forcing plane are presented and the main instabilities are identified. Finally, the global transient response of the system is investigated in order to quantify the degree of safety of the shell in the presence of small perturbations. Since the initial conditions, or even the shell parameters, may vary widely, and indeed are often unknown, attention is given to all possible transient motions. As parameters are varied, transient basins of attraction can undergo quantitative and qualitative changes; hence a stability analysis which only considers the steady-state and neglects this global transient behavior, may be seriously non-conservative.<![CDATA[<B>Identification of flutter parameters for a wing model</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300012&lng=pt&nrm=iso&tlng=pt
A flexible mounting system has been developed for flutter tests with rigid wings in wind tunnel. The two-degree-of-freedom flutter obtained with this experimental system can be described as the combination of structural bending and torsion vibration modes. Active control schemes for flutter suppression, using a trailing edge flap as actuator, can be tested using this experimental setup. Previously to the development of the control scheme, dynamic and aeroelastic characteristics of the system must be investigated. Experimental modal analysis is performed and modes shape and frequencies are determined. Then, wind tunnel tests are performed to characterize the flutter phenomenon, determining critical flutter speed and frequency. Frequency response functions are also obtained for the range of velocities below the critical one showing the evolution of pitch and plunge modes and the coupling tendency with increasing velocity. Pitch and plunge data obtained in the time domain during these tests are used to evaluate the ability of the Extended Eigensystem Realization Algorithm to identify flutter parameter with increasing velocity. The results of the identification process are demonstrated in terms of the evolution of frequency and damping of the modes involved in flutter.<![CDATA[<B>Analytical and numerical approaches to study the gravitational capture in the four-body problem</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300013&lng=pt&nrm=iso&tlng=pt
The objective of this paper is to study the problem of gravitational capture in the bi-circular restricted four-body problem. A gravitational capture occurs when a massless particle changes its two-body energy around one celestial body from positive to negative without the use of non-gravitational forces. It is mainly studied the effect of the fourth-body included in the dynamics. The Earth-Moon system with the inclusion of the Sun is used for the numerical simulations. The results show the savings obtained in this more realistic model when compared with the more traditional restricted three-body problem model. It is clear that large savings are obtained thanks to the effect of the Sun, if a proper geometry is used for the maneuver.<![CDATA[<B>Virtual kinematic chains to solve the underwater vehicle-manipulator systems redundancy</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300014&lng=pt&nrm=iso&tlng=pt
This paper addresses the kinematics of the Underwater Vehicle-Manipulator Systems (UVMSs). Due the adittional degrees of freedom (dofs) provided by the vehicle, such systems are kinematically redundant, i.e. they possess more dofs than those required to execute a given task, and need to be solved using some redundancy technique. We present an approach based on introducing kinematic constraints. The approach uses the screw representation of movement and is based on the so-called Davies method used to solve the kinematics of closed kinematic chains. We describe the vehicle-manipulator system as an open-loop chain and present a virtual kinematic chain concept that allows closing this open chain. Applying the Davies method to this resulting closed chain, the UVMS direct kinematic is solved. The inverse kinematics is obtained using the same approach, by introducing extra constraints derived from energy savings requirements. The proposed approach is compared to another redundancy resolution method to illustrate the ability of the proposed strategy.<![CDATA[<B>Effects of compressibility on aerodynamic surface quantities over low-density hypersonic wedge flow</B>]]>
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-58782006000300015&lng=pt&nrm=iso&tlng=pt
Hypersonic flow past truncated wedges at zero incidence in thermal non-equilibrium is investigated for a range of Mach number from 5 to 12. The simulations were performed by using a Direct Simulation Monte Carlo (DSMC) Method. The study focuses the attention of designers of hypersonic configurations on the fundamental parameter of bluntness, which can have an important impact on even initial design. Some significant differences between sharp and blunt leading edges were noted on the heat transfer, pressure and skin friction coefficients as well as on total drag. Interesting features observed in the surface fluxes showed that small leading edge thickness compared to the freestream mean free path still has important effects on high Mach number leading edge flows. The numerical results present reasonable comparison for wall pressure and heat transfer predictions with experiments conducted in a shock tunnel.