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Physical Insight (physical + insight)
Selected AbstractsStability and accuracy analysis of a discrete model reference adaptive controller without and with time delayINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 9 2010Oreste S. Bursi Abstract Adaptive control techniques can be applied to dynamical systems whose parameters are unknown. We propose a technique based on control and numerical analysis approaches to the study of the stability and accuracy of adaptive control algorithms affected by time delay. In particular, we consider the adaptive minimal control synthesis (MCS) algorithm applied to linear time-invariant plants, due to which, the whole controlled system generated from state and control equations discretized by the zero-order-hold (ZOH) sampling is nonlinear. Hence, we propose two linearization procedures for it: the first is via what we term as physical insight and the second is via Taylor series expansion. The physical insight scheme results in useful methods for a priori selection of the controller parameters and of the discrete-time step. As there is an inherent sampling delay in the process, a fixed one-step delay in the discrete-time MCS controller is introduced. This results in a reduction of both the absolute stability regions and the controller performance. Owing to the shortcomings of ZOH sampling in coping with high-frequency disturbances, a linearly implicit L-stable integrator is also used within a two degree-of-freedom controlled system. The effectiveness of the methodology is confirmed both by simulations and by experimental tests. Copyright © 2009 John Wiley & Sons, Ltd. [source] Transport mechanisms and performance simulation of a PEM fuel cellINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 6 2008Geng-Po Ren Abstract A three-dimensional, gas,liquid two-phase flow and transport model has been developed and utilized to simulate the multi-dimensional, multi-phase flow and transport phenomena in both the anode and cathode sides in a proton exchange membrane (PEM) fuel cell and the cell performance with different influencing operational and geometric parameters. The simulations are presented with an emphasis on the physical insight and fundamental understanding afforded by the detailed distributions of velocity vector, oxygen concentration, water vapor concentration, liquid water concentration, water content in the PEM, net water flux per proton flux, local current density, and overpotential. Cell performances with different influencing factors are also presented and discussed. The comparison of the model prediction and experimental data shows a good agreement. Copyright © 2007 John Wiley & Sons, Ltd. [source] Nonlinear approaches to the design of microwave power amplifiersINTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 6 2004Paolo Colantonio Abstract Commonly used approaches for the design of power amplifiers (PAs), specifically in the microwave and millimeter-wave frequency range, are reviewed and discussed. Measurement-based techniques are compared with CAD-based approaches, stressing their relative strengths and weaknesses. Simplified techniques are also discussed, particularly addressing the preliminary evaluation of the power capabilities of a given device and to gather physical insight into the power-generating mechanisms. Finally, harmonic tuning for high-efficiency power amplifier design is outlined, together with its basic application rules. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14: 493,506, 2004. [source] Drying-induced birefringence of polyimide optical filmsAICHE JOURNAL, Issue 3 2010Po-Ju Chen Abstract Drying-induced birefringence of polyimide film was investigated. The polyimide solutions were coated and dried on two different types of substrates and then removed for optical measurements. The operating window, which was a region for stable and uniform film formation, was determined experimentally. The out-of-plane birefringence (OPBR) was found to increase with decreasing dry film thickness, and the increase became more significant for films less than 10 ,m thick. The experimental OPBR results were compared with the predictions of two theoretical models. The results agreed reasonably well with the one-dimensional model of Lei et al. (J Appl Polym Sci. 2001;81:1000-1013). On the other hand, a simple viscoelastic model, which is an extension of the elastic model of Croll (J Appl Polym Sci. 1979;23:847-858), could provide clear physical insight, but its applicability was somewhat limited. The effects of several variables such as elasticity number, solidification concentration, yield stress, and mass transfer rate on OPBR were examined. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] A simple model for vapor-moisture equilibrium in biomass subtratesAICHE JOURNAL, Issue 6 2009V. R. Vásquez Abstract Equilibrium moisture content (EMC) plays a fundamental role on the properties of bioderived solids such as food, waste biomass, seeds, and many others. For foods, the moisture content affects flavor, texture, and the way the food is stored. In other applications, moisture affects drying conditions and energy requirements for biomass processing. Adequate modeling of EMC in biomass as a function of the water activity is very important in many engineering applications. Most models used to correlate EMC with water activity are based on traditional adsorption models which take into account the adsorption energy of water with the substrate only and do not consider other nonadsorption interactions of water within the substrate such as mixing and swelling effects. Many other models are empirical, which are commonly used for interpolation. Although a detailed description of this type of substrate is very complex, we present a simple model based on standard molecular-thermodynamic models and standard statistical mechanics formulations to correlate the EMC with water activity that attempts to consider both adsorption and nonadsorption interactions of the water with the substrate. Although the model is basic, it captures the behavior of water in this type of systems fairly well and it can be used to fit experimental data with parameters that provide better physical insight on the nature of system. We suggest how to improve the model fundamentally if better knowledge of the water interactions in the substrate are available. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Light scattering from anisotropic particles: propagation, localization, and nonlinearityLASER & PHOTONICS REVIEWS, Issue 2 2010C.W. Qiu Abstract Plasmon resonances and extraordinary light scatterings of a nanoparticle with radial anisotropy are studied and summarized. The coupling between localized surface plasmons and far-field quantities is discussed. It is found that the presence of radial anisotropy redistributes the localization of plasmons and also results in certain novel phenomena in the far zone, which provide the possibility of scattering control such as electromagnetic transparency, enhanced scattering cross section, etc. The nonlinear optical response is explored in order to yield deeper physical insight into the interaction between plasmons and incident light. [source] Radiative torques: analytical model and basic propertiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007A. Lazarian ABSTRACT We attempt to get a physical insight into grain alignment processes by studying basic properties of radiative torques (RATs). For this purpose we consider a simple toy model of a helical grain that reproduces well the basic features of RATs. The model grain consists of a spheroidal body with a mirror attached at an angle to it. Being very simple, the model allows analytical description of RATs that act upon it. We show a good correspondence of RATs obtained for this model and those of irregular grains calculated by ddscat. Our analysis of the role of different torque components for grain alignment reveals that one of the three RAT components does not affect the alignment, but induces only for grain precession. The other two components provide a generic alignment with grain long axes perpendicular to the radiation direction, if the radiation dominates the grain precession, and perpendicular to magnetic field, otherwise. The latter coincides with the famous predictions of the Davis,Greenstein process, but our model does not invoke paramagnetic relaxation. In fact, we identify a narrow range of angles between the radiation beam and the magnetic field, for which the alignment is opposite to the Davis,Greenstein predictions. This range is likely to vanish, however, in the presence of thermal wobbling of grains. In addition, we find that a substantial part of grains subjected to RATs gets aligned with low angular momentum, which testifies that most of the grains in diffuse interstellar medium do not rotate fast, that is, rotate with thermal or even subthermal velocities. This tendency of RATs to decrease grain angular velocity as a result of the RAT alignment decreases the degree of polarization, by decreasing the degree of internal alignment, that is, the alignment of angular momentum with the grain axes. For the radiation-dominated environments, we find that the alignment can take place on the time-scale much shorter than the time of gaseous damping of grain rotation. This effect makes grains a more reliable tracer of magnetic fields. In addition, we study a self-similar scaling of RATs as a function of ,/aeff. We show that the self-similarity is useful for studying grain alignment by a broad spectrum of radiation, that is, interstellar radiation field. [source] Nonlinear piezoelectric properties of GaN quantum dots nucleated at the edge of threading dislocationsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007ewski Abstract It was observed experimentally by Rouviere et al. that GaN/AlN Quantum Dots (QDs) nucleate at the edge of threading dislocations (Appl. Phys. Lett. 75, 2632 (1999) [1]). The preferred nucleation of QDs in this way is generally assumed to be due to the influence of the stress/strain field around the dislocation core, which in turn, gives the chemical and geometric conditions for nucleation of the QDs. We solve the finite element problem for QDs situated at the edge of threading dislocations where different lattice parameters, piezoelectric and spontaneous polarisation coefficients are assumed for the QD and its matrix. By solving the elastic and electric equilibrium problems we obtain both the residual stress and electric fields. The computational scheme employed here was obtained by linking two previous finite element algorithms described inreferences (P. D,u,ewski et al., Comput. Mater. Sci. 29, 379 (2004) [2]) and (G. Jurczak et al., phys. stat. sol. (c) 2, 972 (2005) and S.P. ,epkowski et al., Phys. Rev. B 73, 245201 (2005) [3, 4], respectively). This approach allows us to get a deeper physical insight into the mechanics and electrical properties of QDs and ultimately determine the efficiency of light emission from these objects. [source] Numerical analysis on the propulsive performance and vortex shedding of fish-like travelling wavy plateINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2005Gen-Jin Dong Abstract Numerical analysis is carried out to investigate viscous flow over a travelling wavy plate undergoing lateral motion in the form of a streamwise travelling wave, which is similar to the backbone undulation of swimming fish. The two-dimensional incompressible Navier,Stokes equations are solved using the finite element technique with the deforming-spatial-domain/stabilized space,time formulation. The objective of this study is to elucidate hydrodynamic features of flow structure and vortex shedding near the travelling wavy plate and to get into physical insights to the understanding of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive performance. The effects of some typical parameters, including the phase speed, amplitude, and relative wavelength of travelling wavy plate, on the flow structures, the forces, and the power consumption required for the propulsive motion of the plate are analysed. These results predicted by the present numerical analysis are well consistent with the available data obtained for the wave-like swimming motion of live fish in nature. Copyright © 2005 John Wiley & Sons, Ltd. [source] Half metallic properties of LaSrVMoO6JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2010Weiyu Song Abstract The recent synthesized LaSrVMoO6 was speculated to be compensated half metal, i.e., half metal with zero magnetic moment. Based on the experimental structure, our first principles study indicates that it is ferrimagnetic and half metallic with the magnetic moment 2.0 ,B when the electron correlation of Mo 4d electrons is larger than 2.72 eV. This indicates the strong electron correlation effect of Mo 4d electrons. Nonetheless, the obtained large magnetic moment (2.0 ,B) contradicts with the experimental observed nearly zero magnetic moment. Although the large antisite defects of the experimental sample might be the reason to reduce the saturated magnetic moment, further physical insights need to be investigated. The spin-orbit coupling effect has minor effect on the studied properties. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Steady Flow of Power Law Fluids across a Circular CylinderTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2006Ram Prakash Bharti Abstract The momentum equations describing the steady cross-flow of power law fluids past an unconfined circular cylinder have been solved numerically using a semi-implicit finite volume method. The numerical results highlighting the roles of Reynolds number and power law index on the global and detailed flow characteristics have been presented over wide ranges of conditions as 5 , Re , 40 and 0.6 , n , 2. The shear-thinning behaviour (n < 1) of the fluid decreases the size of recirculation zone and also delays the separation; on the other hand, the shear-thickening fluids (n > 1) show the opposite behaviour. Furthermore, while the wake size shows non-monotonous variation with the power law index, but it does not seem to influence the values of drag coefficient. The stagnation pressure coefficient and drag coefficient also show a complex dependence on the power law index and Reynolds number. In addition, the pressure coefficient, vorticity and viscosity distributions on the surface of the cylinder have also been presented to gain further physical insights into the detailed flow kinematics. Les équations de mouvement décrivant l'écoulement transversal permanent de fluides de loi de puissance en aval d'un cylindre circulaire non confiné ont été résolues numériquement par une méthode de volumes finis semi-implicite. Des résultats numériques soulignant le rôle du nombre de Reynolds et de l'indice de loi de puissance sur les caractéristiques d'écoulement globales et détaillées sont présentés pour de vastes gammes de conditions, soit 5 , Re , 40 et 0,6 , n , 2. Le comportement rhéofluidifiant (n < 1) du fluide réduit la taille de la zone de recirculation et accroît également la séparation; d'autre part, les fluides rhéoépaississants (n > 1) montrent un comportement opposé. En outre, alors que la taille du sillage varie de manière non monotone avec l'indice de loi de puissance, elle ne semble pas influencer les valeurs du coefficient de traînée. Le coefficient de pression de stagnation et le coefficient de traînée montrent aussi une dépendance complexe envers l'indice de loi de puissance et le nombre de Reynolds. Les distributions des coefficients de pression, de la vorticité et de la viscosité sur la surface du cylindre sont également présentées afin de mieux comprendre les cinématiques d'écoulement détaillées. [source] | ||||||||||||||||