Resumo em Inglês:

We present a computational study of the solution of the Falkner-Skan equation (a thirdorder boundary value problem arising in boundary-layer theory) using high-order and high-order-compact finite differences schemes. There are a number of previously reported solution approaches that adopt a reduced-order system of equations, and numerical methods such as: shooting, Taylor series, Runge-Kutta and other semi-analytic methods. Interestingly, though, methods that solve the original non-reduced third-order equation directly are absent from the literature. Two high-order schemes are presented using both explicit (third-order) and implicit compact-difference (fourth-order) formulations on a semi-infinite domain; to our knowledge this is the first time that high-order finite difference schemes are presented to find numerical solutions to the non-reduced-order Falkner-Skan equation directly. This approach maintains the simplicity of Taylor-series coefficient matching methods, avoiding complicated numerical algorithms, and in turn presents valuable information about the numerical behaviour of the equation. The accuracy and effectiveness of this approach is established by comparison with published data for accelerating, constant and decelerating flows; excellent agreement is observed. In general, the numerical behaviour of formulations that seek an optimum physical domain size (for a given computational grid) is discussed. Based on new insight into such methods, an alternative optimisation procedure is proposed that should increase the range of initial seed points for which convergence can be achieved.

Resumo em Inglês:

An accurate knowledge of thermophysical properties is very important, for example, to optimize the engineering design and the development of new materials for many applications. Thermal effusivity is a thermal property which presents an increasing importance in heat conduction problems. This property indicates the amount of thermal energy that a material is able to absorb. The estimation can be done by simulating a transient heat transfer model. In this case a one-dimensional semi-infinite thermal model is used. A resistance heater in contact with the sample generates a heat pulse. Variations of temperature and heat flux are measured simultaneously on the top surface of the sample. In this work, thermal effusivity is estimated in time domain through the minimization of the objective function, defined as the square difference between experimental and theoretical temperatures. The golden section technique is used for minimizing this objective function. A sensitivity analysis and a comparison between the semi-infinite and the finite models were also done to define the number of points to be used in the estimation. Measurements were carried out with three different polymers: polymethyl methacrylate, polyvinyl chloride and polyethylene. In all cases studied the results are in good agreement with literature. In addition, an uncertainty analysis is also presented.

Resumo em Inglês:

In the present work the inverse problem of simultaneous heat and mass transfer modeled by Luikov equations is studied using a hybrid combination of the Levenberg-Marquardt (LM), Simulated Annealing (SA) and Artificial Neural Network (ANN) methods. The direct and inverse problems are described, formulated and solved. After the use of an experiment design technique, the hybrid combination ANN-LM-SA yielded good estimates for the heat and mass transfer problem of interest. The proper choice of the set of parameters to be estimated allowed the design of an experiment with higher sensitivity coefficients. One ANN was used to generate the initial guess for the LM, another one to approximate the gradient needed by LM, and, finally, the global minimum was searched using the SA. The experimental data considered in the inverse problem was generated using the solution for the direct problem with the addition of noise.

Resumo em Inglês:

The impulse response of the velocity potential and the discrete representation methods were used in order to model the acoustic field radiated by ultrasonic transducers and arrays. The first method deals with the calculation of the exact impulse response, in which solutions are possible only for simple geometries, such as the circular piston. The second method is an approximated solution based on the discretization of the acoustic aperture in small elementary areas, each of them radiating a spherical wave. By using circular transducers, which can be considered circular pistons, many simulations comparing the methods were carried out. The relation between the computational cost and the precision was analyzed, thus establishing the time and space discretization levels. The simulations were made using the Matlab software and the results were compared to experimental measurements showing good agreement. The experimental results were obtained using a scanning system. The acoustic field radiated from a 1 MHz circular transducer was measured as well as a 3.5 MHz array of 16 elements both immersed in water. The acoustic field radiated by the array was simulated and measured with focalization on a radius of 30 mm with deflections of 0º and 20º.

Resumo em Inglês:

The present work is aimed at building a computational model for a typical stranded cable based on the basic principles of Verification and Validation. The model calibration and model tracking are guided based on a pool of validation metrics suitable for data which are commonly used in structural dynamics. The estimator used for the associated inverse problem is the Maximum a Posteriori estimator and the parameter estimation process is performed sequentially over experiments. Experimental tests have been performed at CEPEL's (Electric Power Research Center) laboratory span with the overhead conductor Grosbeak in order to provide the measured data. The predictive capacity of the computational model is assessed by means of frequency-and time-domain validations through FRFs, band limited white-noise and sine sweep excitations. We also present novel and reliable estimates for the bending stiffness and damping parameters of a widely used transmission line conductor.

Resumo em Inglês:

Piezoelectric shunt damping is a well known structural vibration control technique that consists in connecting an electrical circuit to a piezoelectric transducer attached to the structure. In the case of a resonant shunt, the network consisting of an inductor-resistor network when combined with the capacitive nature of the piezoelectric transducer impedance can be designed to act as a tuned vibration absorber. This paper discusses a method for the design and online adaptation of multimodal piezoelectric resonant shunts. The method presented in this work is different from previously multi-modal shunting methods ("current blocking" and "current flowing") and implements the shunting network with a reduced number of discrete electrical components besides allowing for online tuning of the shunting parameters. The mathematical model of a structure with bonded piezoelectric transducers connected to a general electrical network is reviewed and the coupled equations of motion of a simply supported beam with piezoelectric elements and passive shunt networks are derived. The design of the multimodal shunt network is presented based on passive filter synthesis methods. The multimodal self tuning piezoelectric damper is demonstrated experimentally as a two-mode system applied to add damping to a cantilevered beam.

Resumo em Inglês:

Analytical models to evaluate vehicle dynamic handling properties are extremely interesting to the project engineer, as these can provide a deeper understanding of the underlying physical phenomena being studied. It brings more simplicity to the overall solution at the same time, making them very good choices for tasks involving large amounts of calculation iterations, like numerical optimization processes. This paper studies in detail the roll gradient, understeer gradient and steering sensitivity vehicle dynamics metrics, starting with analytical solutions available in the literature for these metrics and evaluating how the results from these simplified models compare against real vehicle measurements and more detailed multibody simulation models. Enhancements for these available analytical formulations are being proposed for the cases where the initial results do not present satisfactory correlation with measured values, obtaining improved analytical solutions capable of reproducing real vehicle results with good accuracy.

Resumo em Inglês:

The synthesis of a magneto-rheological vehicle suspension system built on the variable structure control approach is considered in the present work. The suspension is synthesized in order to improve the ride comfort obtained by a standard passive suspension. Although a nominal half-vehicle model with rigid body is considered in the synthesis of the suspension, phenomenological models for the MR dampers and for the seat-driver subsystem, along with the flexibility of the vehicle body, are considered in the performance assessment. For comparison purposes, active and magneto-rheological suspensions built on the optimal control approach and an active suspension built on the variable structure control approach are also considered. The numerical results show that the proposed suspension outperforms the passive suspension and presents a performance comparable to that of the active ones when the vehicle body may be assumed as rigid. Besides, when its flexibility is an important issue, a great performance drop may be observed, depending on the road quality, the damper characteristics and the adopted control strategy.

Resumo em Inglês:

The surface analysis of nanomachined AISI D2 tool steel materials is measured using atomic force microscopy. The surface roughness and fractal dimensional analysis are the important factors in nano tribology and evaluating the quality of nanomachined surface. Carbon nanotube increases the heat carrying capacity, thermal conductivity of the lubricating oil and thus prevents any damage to the work piece. The surface morphology of different machined surface was studied by Fractal Dimension analysis and roughness characterization was carried out. The results indicate that the fractal dimension changed according to the smoothness of the machined surface. The Power Spectrum Density (PSD) method based Root mean square (Rms) surface roughness was calculated for carbon nano tube based nanofluids in grinding process. The fractal dimension and roughness are decreased due to single wall carbon nano tube based nanofluids and smooth surface finish has been obtained.

Resumo em Inglês:

Time Petri Net (TPN) models have been widely used to the specification and verification of real-time systems. However, the claim that most of these techniques are useful for real-time system verification can be discussed, based on two assumptions: i) to be suitable for real-time systems verification, a technique must be able to check timing properties, both safe and behavioral, and ii) the underlying algorithm must be efficient enough to be applied to complex systems design. In this paper we will discuss the suitability of commonly accepted verification techniques that use model-checking as a verification framework and TPN as a description model. We present a new algorithmic approach that allows computation of end-to-end time between activities over an abstract state space model. The proposed approach needs to compute the abstract state space only once improving the efficiency of the verification process and turning it suitable for large problems. We also introduce a new sufficient condition for abstract states space to preserve the branching time properties that yields more compact graphs than the condition already used in actual approaches. The approach would fit a design environment also based on Petri Nets called GHENeSys (General Hierarchical Enhanced Petri Nets). The results obtained, using our verification approach are compared with similar available approaches.

Resumo em Inglês:

Numerical simulation of engineering problems has reached such a large scale that the use of a parallel computing approach is required to obtain solutions within a reasonable time. Recent efforts have been made to implement these large scale computational tasks on general-purpose programmable graphics hardware (GPGPU). The Graphics Processing Unit (GPU) is specially well-suited to address problems that can be formulated in form of data-parallel computations with high arithmetic intensity. This work addresses the implementation of the direct version of the Boundary Element Method (DBEM) on a complementary GPU-CPU system. In this article, constant elements were used for the solution of 2D potential problems. A serial implementation of the BEM was rewritten under the SIMT (Single Instruction Multiple Thread) parallel programming paradigm. The code was developed on an NVidiaTM CUDA programming environment. The efficiency of the implemented strategies is investigated by solving a representative 2D potential problem. The paper reviews in detail the classical BEM formulation in order to be able to address the possible parallelization steps in the numerical implementation. The article reports the performance of the GPU-CPU system compared to the classical CPU-based system for an increasing number of boundary elements.

Resumo em Inglês:

This paper presents the application of a proposed numerical approach, denoted as SAAFE Program (System for Advanced Analysis for Fire Engineering), developed to provide an inelastic analysis of steel and composite (steel-concrete) 2D semi-rigid framed-structures under fire conditions. The proposed structural model allows an accurate description of the structural non-linear response, with less computational effort when compared to the general FEM formulation. The method, although similar in concept to earlier plastic-hinge approaches, differs with regards to numerical implementation methodology and precision. The proposed plastic-hinge model is formulated in a succinct format based on the following characteristics: (i) a refined plastic lumped formulation with interaction surfaces, (ii) a tangent modulus model which includes both gradual inelastic loss of stiffness and ultimate strength of column members, (iii) a second-order large-displacement formulation, based on the Stability Functions concept, (iv) non-homogeneous temperature distribution over the cross-section, (v) a connection semi-rigid model. Obtained results for connection model calibration examples are compared to reported experimental data, showing reasonable agreement. In addition, results of a proposed case-of-study demonstrate the efficiency and robustness of the SAAFE model to perform inelastic analysis semi-rigid members, outlining the advantage of considering advanced analysis in the current fire-design practice of structures.

Resumo em Inglês:

This work presents a systematic comparison between several detailed chemical kinetic models recently developed and available experimental data. The aim is to assess the predictive capabilities of the combustion with air of the following fuels: hydrogen, methane, ethanol and liquefied petroleum gas, in a large range of equivalence ratio. The prevailing thermodynamical conditions range from ambient to the more stringent ones, such as high pressure combustion. In order to assess the predictive performance of the twelve chosen chemical kinetics models, the results of numerical simulations are compared with existing experimental data of the combustion process in two simplified physical systems: the perfectly stirred reactor and the freely propagating premixed laminar flame. When ambient conditions are considered, the comparisons reveal a good agreement among most of detailed kinetic mechanisms, on the prediction of thermochemical properties of practical interest with respect to the corresponding experimental data only, as far as lighter fuels such as hydrogen, methane and ethanol are considered. The chosen mechanisms are shown to meet with difficulties when mixtures of liquefied petroleum gas and air are considered, even in ambient conditions. The obtained results highlight the necessity for (i) updating the existing mechanisms with the use of recent experimental results and (ii) the development of new comprehensive models.