Resumo em Inglês:

The granular media is of great importance in our quotidian, and their transport by a fluid flow is frequently found in nature and in industry. When the shear stresses exerted by the fluid flow on a granular bed are bounded to some limits, a mobile granular layer known as bed-load takes place in which the grains stay in contact with the fixed part of the granular bed. Under these conditions, a flat granular bed may be unstable, generating ripples and dunes. In a recent article (Franklin, 2010), the mechanisms of this instability were explained and a linear stability analysis was presented, in which a scaling between the fluid flow conditions and the typical length of the initial bed-forms was proposed. The present paper proposes a nonlinear stability analysis (weakly nonlinear approach) applicable to sheared granular beds, shedding light on the evolution of the bed-forms after their initial phase. The scope of the nonlinear analysis is the same as that of Franklin (2010): granular beds under turbulent liquid flows and in the presence of bed-load. It is shown here that, in this case, the initial instabilities saturate (supercritical bifurcation). Also, a discussion is made on some published experimental data.

Resumo em Inglês:

A mathematical model is developed for a liquid flow on solid particles in a trickle bed reactor. A mathematical formulation is followed based on the liquid-solid model approach where the liquid phase with the (KCl) tracer is treated as a continuum. The physical modeling is discussed, including the formulation of initial and boundary conditions and the description of the solution methodology. Results of mathematical model are presented and validated. The model is validated through comparison using three experimental cases. The optimized values of the axial dispersion (Dax), liquid-solid mass transfer (kLS), and partial wetting efficiency (F M) coefficients are obtained simultaneously using the objective function. The behavior of Dax, kLS, and F M is analyzed by the empirical correlations.

Resumo em Inglês:

Electronic components are usually assembled on printed circuit boards cooled by forced airflow. When the spacing between the boards is small, there is no room to employ a heat sink on critical components. Under these conditions, the components’ thermal control may depend on the conductive path from the heater to the board in addition to the direct convective heat transfer to the airflow.The conjugate forced convection-conduction heat transfer from a two-dimensional strip heater flush mounted to a finite thickness wall of a parallel plates channel cooled by a laminar airflow was investigated numerically. A uniform heat flux was generated along the strip heater surface. Under steady state conditions, a fraction of the heat generation was transferred by direct convection to the airflow in the channel and the remaining fraction was transferred by conduction to the channel wall. The lower surface of the channel wall was adiabatic, so that the heat conducted from the heater to the plate eventually returned to the airflow. A portion of it returned upstream of the heater, preheating the airflow before it reached the heater surface. Due to this, it was convenient to treat the direct convection from the heater surface to the airflow by the adiabatic heat transfer coefficient. The flow was developed from the channel entrance, with constant properties.The conjugate problem was solved numerically within a single solution domain comprising both the airflow region and the solid wall of the channel. The results were obtained for the channel flow Reynolds number ranging from about 600 to 1900, corresponding to average airflow velocities from 0.5 m/s to 1.5 m/s. The effects of the solid wall to air thermal conductivities ratio were investigated in the range from 10 to 80, typical of circuit board materials. The wall thickness influence was verified from 1 mm to 5 mm. The results indicated that within these ranges, the conductive substrate wall provided a substantial enhancement of the heat transfer from the heater, accomplished by an increase of its average adiabatic surface temperature.

Resumo em Inglês:

Active-Passive Piezoelectric Networks (APPN) integrate active voltage sources with passive resistance-inductance shunt circuits to a piezoelectric patch. This technique allows to simultaneously passively dissipate vibratory energy through the shunt circuit and actively control the structural vibrations. This work presents an analysis of active-passive damping performance of beams with extension and shear APPN. A coupled finite element model with mechanical and electrical degrees of freedom is developed and used to design passive and active control parameters. Then, stochastic modeling and analyses of two cantilever beam configurations, with extension and shear APPN, are performed to evaluate the effect of uncertainties in circuit components on passive and active-passive vibration control. Results show that active-passive shunt circuits can be very interesting since they may combine an adequate passive control performance with an increase of active control authority when a control voltage is applied to the circuit. For the extension configuration, vibration amplitude reductions of up to 22 dB and 28 dB are obtained for passive and active-passive cases, respectively. Considering relative dispersions of 10% for the resistance and inductance values, the passive and active-passive amplitude reductions are found to be in the ranges 16-24 dB and 27-28 dB, respectively. For the shear configuration, increases in the active control authority of up to 29 dB due to a properly tuned resonant circuit are observed. When subjected to uncertainties in the resistance and inductance values, with 10% relative dispersions, the control authority increase is in the range of 6-29 dB.

Resumo em Inglês:

The goal of the present study is the development of a spectral method to obtain the frequency response of the half-vehicle subjected to a measured pavement roughness in the frequency domain. For this purpose, a half-vehicle dynamic model with a two-point delayed base excitation was developed to correlate with the spectral density function of the pavement roughness, to obtain the system spectral transfer function, in the frequency domain. The vertical pavement profile was measured along two roads sections. The surface roughness was here expressed in terms of the spectral density function of the measured vertical pavement profile with respect to the evenness wave number of the pavement roughness. A frequency response analysis was applied to obtain the vertical and angular modal vehicle dynamic response with the excitation of the power spectral density (PSD) of the pavement roughness. The results show that at low speed, the vehicle suspension mode is magnified due to the unpaved track signature. At 120 km/h in an undulated asphalted road, the first vehicle vibration mode has a significant motion amplification, which may cause passenger discomfort.

Resumo em Inglês:

Passive vibration isolators are usually made of viscoelastic materials. These materials have non-linear characteristics that change their dynamical properties with temperature, frequency and strain level. The vibration isolator’s mathematical modeling and optimal design requires the prior knowledge of the stiffness and damping of the applied viscoelastic material. This work presents a dynamical characterization methodology to identify the stiffness and damping of three samples of viscoelastic rubber with hardness of 25, 33 and 48 SHORE A. The experimental apparatus is a one-degree of freedom vibratory mechanical system coupled to the viscoelastic damper. Sweep sine excitations are applied to the system and the resulting forces and vibration levels are measured. The amplitude of the excitation is controlled to achieve a constant RMS level of strain in the viscoelastic samples. The experimental results are obtained for conditions of no pre-strain and with a 10% of pre-strain. The time domain data is post-processed to generate frequency response functions that are used to identify the damping and stiffness properties of the viscoelastic damper.

Resumo em Inglês:

This work presents a mathematical modeling to study the ester oil ISO VG-10-refrigerant R134a mixture two-phase flow with foam formation through a 3.22 mm ID tube. Based on experimental visualization results, the flow is divided into three regions: a single phase flow at the inlet of the tube; an intermediary bubbly flow region; and a foam flow region at the end of the tube. Numerical results for mass flow rate, pressure and temperature distributions along the flow were compared with experimental data available in literature, showing good agreement. The major discrepancy between the mass flow rate data was about 21%. These results show that the mathematical modeling worked well for predicting the overall characteristics of the flow and can be generically used to other oil-refrigerant mixtures.

Resumo em Inglês:

This study presents mathematical modeling and calculation procedure for problems of electromagnetic forming of thin circular metal sheets using a flat spiral coil as actuator. The methodbased on the Biot-Savart Law focuses specifically on the calculation of the electromagnetic field generated by the flat coil and analysis of the circuit that models the electromagnetic forming system to the initial time, before the plastic deformation of the sheet. The solution of magnetic induction integral equations is performed by numerical methods specifically with the use of Matlab® software, providing important information that serves as feedback for system design. Free bulging experiments were performed to demonstrate a good relationship with the mathematical model predictions for electrical discharge current in the coil and induced currents in the metal sheet, behavior of the transient electromagnetic force between coil and workpiece, and distribution of magnetic field, electromagnetic density force along the coil.

Resumo em Inglês:

Use of conventional flexible polymeric mould materials yields to longer solidification time of (wax/plastic) patterns in soft tooling process, thereby reducing the rapidity of the process to a great extent, which is not desirable in present competitive market. In this work, approach of particle-reinforcement with mould materials is introduced to reduce the cycle time of Soft tooling process and the resulting cooling time is experimentally investigated in considering a case of manufacturing of a typical wax pattern with aluminium particle filled polyurethane. It is observed that cooling time is significantly reduced particularly with higher loading condition of aluminium filler. This happens due to the increase of effective thermal conductivity of mould material. However, it is also found that the stiffness of mould becomes simultaneously high due to increase of effective modulus of elasticity of mould material. Realizing these facts, an extensive study is carried out to find the effect on equivalent thermal properties and modulus of elasticity of polyurethane composite mould materials with the reinforcement of aluminium and graphite particles independently through rigorous experimentation and correlation of experimental findings with the models cited in literatures.

Resumo em Inglês:

The needs to comply with an increasingly competitive international market lead industries to some innovative solutions, such as the use of robotic arms as machine tools. Although these solutions present some well known drawbacks, there are some advantages and niches of application where success is possible. The present work investigates the use of such pieces of equipment to machine aluminum alloys AA2024 applying high speed machining (HSM) technique, assessing surface finishing as a function of different orientation angles between end mill and machined surface. It also tests the best condition to machine foam for prototyping applications. Results indicate that the directions close to the normal are the best compromises because of dynamic stability of the robot arm structure and roughness as low as 4 µm Ra are possible to be achieved in aluminum alloys. A complex shape such as a semi sphere can be easily machined in foam for rapid and accurate prototype machining. Surface finishing can be very smooth and well suitable for industrial applications in such materials.

Resumo em Inglês:

A theoretical model of a porous-inclined slider bearing lubricated with magnetic fluid has been considered together with slip velocity boundary condition. Our aim is to study the influence of various dimensionless parameters arising out of the analysis of the model. By assuming the viscosity µ = µ0 exp [-β(t m - t0)] of magnetic fluid, the expressions for mean temperature and load capacity have been obtained. It has been observed that both mean temperature field and load capacity are the functions of slip parameter, magnetic parameter, thermal parameter and permeability parameter. The dependence of the mean temperature field as well as of load capacity on these parameters has been seen graphically.

Resumo em Inglês:

This note focuses attention on a novel approach to disturbance rejection when the µ-synthesis control procedure is applied to Unmanned Underwater Vehicles (UUVs). Environmental external disturbances simplify to ocean current for a totally submerged vehicle and greatly contributes for hydrodynamical loads and the tether cable disturbance. Our case scenario deals with the incorporation of the sea current disturbance to the plant model employed for control design. In the proposed design method, we substitute the structured unmodeled dynamics uncertainty, which is generally difficult to come up with and eventually utilized to represent external disturbances, by parametric uncertainty, relatively easier and straightforward to come by. The sea-current load parameters are, therefore, treated as parametric uncertainty and fit in the µ design framework. Assuming that both vehicle motion and current direction lie in the horizontal plane, the incoming (to vehicle) current vector sets a horizontal circumference sector in which it may vary. When in the 3D space, current uncertainty renders a cone in space. For validation purposes, the linear controller is simulated with the nonlinear vehicle model.

Resumo em Inglês:

The prosthetic leg (PL) is a typical human-machine system in which the dynamic interaction between the human body and PL (machine) determines a high requirement of ergonomics design for PL and consequently needs to consider an indicator of usability indicating the performance of gait biomechanics. How to evaluate the indexes of usability for PL products is critical to the design technology of PL based on ergonomics. The gait symmetry of PL products, which is a core usability index, was experimentally analyzed by using a specially designed testing device in this paper. The test results show that the swing speed symmetry for intelligent prosthetic leg (IPL) is high up to 96.5%, which indicates a high performance for gait tracking. Then, a comprehensive evaluation was made for the usability of PL products with the analysis method of Grey Correlation Degree. Three types of PL product, including ordinary PL, IPL and Gait-following IPL (GL-IPL), were used in the evaluation. The evaluation results show that the GL-IPL product is the best in usability while both of IPL and GF-IPL products based on ergonomics are obviously better than the ordinary PL. The method of usability evaluation studied here is expected to help directing the design of prosthetic leg products.

Resumo em Inglês:

In many situations analysts use incomplete models to design a system, or to make decisions. These models are incomplete due to unmodeled phenomena, which means that some features are not included in the model (either because it was not previously thought about or because it would be too expensive to include). These uncertainties related to the model are difficult to take into account. In this paper, the nonparametric probabilistic approach is used to investigate model uncertainties in the problem of structures excited by internal flow. A reference model is constructed, where an Euler-Bernoulli beam is used to model the structure, and the fluid is added to the model by means of a constant mass, damping and stiffness. Then, an incomplete model of the reference model is considered. In it the influence of the fluid stiffness is not taken into account, hence, the uncertainty is related to this unmodeled feature (fluid stiffness). The incomplete model is then used together with the nonparametric probabilistic approach to infer the behavior of the reference model. Besides, a procedure is proposed to calibrate the dispersion parameter of the probabilistic model.