Finite element analysis of steady-state natural convection problems in fluid-saturated porous media heated from below

In this paper, a progressive asymptotic approach procedure is presented for solving the steady-state Horton-Rogers-Lapwood problem in a fluid-saturated porous medium. The Horton-Rogers-Lapwood problem possesses a bifurcation and, therefore, makes the direct use of conventional finite element methods difficult. Even if the Rayleigh number is high enough to drive the occurrence of natural convection in a fluid-saturated porous medium, the conventional methods will often produce a trivial non-convective solution. This difficulty can be overcome using the progressive asymptotic approach procedure associated with the finite element method. The method considers a series of modified Horton-Rogers-Lapwood problems in which gravity is assumed to tilt a small angle away from vertical. The main idea behind the progressive asymptotic approach procedure is that through solving a sequence of such modified problems with decreasing tilt, an accurate non-zero velocity solution to the Horton-Rogers-Lapwood problem can be obtained. This solution provides a very good initial prediction for the solution to the original Horton-Rogers-Lapwood problem so that the non-zero velocity solution can be successfully obtained when the tilted angle is set to zero. Comparison of numerical solutions with analytical ones to a benchmark problem of any rectangular geometry has demonstrated the usefulness of the present progressive asymptotic approach procedure. Finally, the procedure has been used to investigate the effect of basin shapes on natural convection of pore-fluid in a porous medium. (C) 1997 by John Wiley & Sons, Ltd.

Experimentally tracking unstable steady states by large periodic modulation

Experimental suppression of chaos has been achieved in an optically pumped far-infrared (NH3)-N-15 laser which displays Lorenz-like chaos. The method of control involves the application of a large amplitude slow (i.e., nonresonant) modulation of the pump power. This may be related to a delayed bifurcation to chaos observed when the pump power is ramped at a constant late.

Analysis of small signal stability margins using genetic optimization

Power system small signal stability analysis aims to explore different small signal stability conditions and controls, namely: (1) exploring the power system security domains and boundaries in the space of power system parameters of interest, including load flow feasibility, saddle node and Hopf bifurcation ones; (2) finding the maximum and minimum damping conditions; and (3) determining control actions to provide and increase small signal stability. These problems are presented in this paper as different modifications of a general optimization to a minimum/maximum, depending on the initial guesses of variables and numerical methods used. In the considered problems, all the extreme points are of interest. Additionally, there are difficulties with finding the derivatives of the objective functions with respect to parameters. Numerical computations of derivatives in traditional optimization procedures are time consuming. In this paper, we propose a new black-box genetic optimization technique for comprehensive small signal stability analysis, which can effectively cope with highly nonlinear objective functions with multiple minima and maxima, and derivatives that can not be expressed analytically. The optimization result can then be used to provide such important information such as system optimal control decision making, assessment of the maximum network's transmission capacity, etc. (C) 1998 Elsevier Science S.A. All rights reserved.

Preaortic Iliac Confluence (Marsupial Vena Cava): Report of 4 Cases

Congenital anomalies of the inferior vena cava (IVC) call represent a difficult for abdominal surgeries, and the radiologist must be aware even of the less common of these anatomical variations. Preaortic iliac venous confluence, also known as marsupial vena cava, is a rare congenital anomaly of the development of the IVC in which the IVC or the left common iliac vein is located anterior to the aortic bifurcation or the right common iliac artery. We report 4 cases of marsupial vena cava detected on multidetector computed tomography examinations in asymptomatic patients and discuss that this congenital anomaly can be recognized more frequently with the use of this new technique based on thinner images.

Malformations of the index nails

We report a 52-year-old woman with micronychia of the index fingers. Radiographic examination revealed a Y-shaped bifurcation of the distal phalanx of both index fingers. She was diagnosed with congenital onychodysplasia of the index fingers (COIF) or Iso-Kikuchi syndrome. COIF is a rare condition characterized by a variety of nail dysplasia of the index fingers. Five criteria characterize COIF: congenital occurrence, unilateral or bilateral index finger involvement, variability in nail appearance, hereditary involvement and frequently associated bone abnormalities. Micronychia, polyonychia, anonychia, hemionychrogryphosis and malalignment are the observed index finger defects. Most cases have been described in Japan, and to our knowledge, this is the first case of COIF reported in South America.

An anatomical study on the lingual-facial trunk

Purpose We quantified variations of the lingual artery origin, measured the lingual artery origin distance from clinical relevant landmarks and compared the lingual artery diameters with normal and variable origin. Methods Forty-two formalin fixed male cadavers were bilaterally evaluated. Measurements were performed with the aid of an electronic digital caliper. Results The origin distances from the common carotid artery bifurcation was 1.05 +/- 0.11 and 1.02 +/- 0.11 cm for the right and left lingual arteries respectively with no differences compared to the lingual-facial trunks. The diameters of the lingual arteries were 0.25 +/- 0.01 and 0.26 +/- 0.01 cm for the right and left sides, respectively, while the lingual facial trunks showed diameters of 0.21 +/- 0.02 and 0.24 +/- 0.02 cm for the right and left sides, respectively. Conclusions The present study adds information on the lingual artery diameter and its anatomical relation to clinically useful landmarks.

Dorsal root ganglionectomy for the diagnosis of sensory neuropathies. Surgical technique and results

Background: Inflammatory diseases stand out among sensory neuronopathies because, in their active phase, they can be treated with immunosuppressive agents. Immunosuppressive therapy may present severe adverse effects and requires previous inflammatory activity confirmation. Sensory neuronopathies are diagnosed based on clinical and EMG findings. Diagnostic confirmation and identification of inflammatory activity are based on sensory ganglion histopathological examination. We describe the surgical technique used for dorsal root ganglionectomy in patients with clinical/EMG diagnosis of sensory neuronopathies. Methods: The sensory ganglion was obtained from 15 patients through a small T7-T8 hemilaminectomy and foraminotomy to expose the C7 root from its origin to the spinal nerve bifurcation. In 6 patients, the dural cuff supposed to contain the ganglion was resected en bloc; and in 9 patients, the ganglion was obtained through a longitudinal incision of the dural cuff and microsurgical dissection from the ventral and dorsal roots and radicular arteries. All ganglia were histopathologically examined. Results: No ganglion was found in the dural cuff in 2 patients submitted to en bloc removal, and the ganglion was removed in all patients who underwent microsurgical dissection. All but 2 patients that had ganglion examination presented a neuronopathy of nerve cell loss, 3 with mononuclear inflammatory infiltrate. These patients underwent immunosuppressive therapy, and 2 of them presented clinical improvement. No surgical complications were observed. Conclusions: Microsurgical dorsal root ganglionectomy for diagnosing inflammatory sensory ganglionopathies was effective and safe. Although safe, en bloc resection of the proximal dural cuff was not effective for this purpose. (c) 2008 Published by Elsevier Inc.

The anatomy of two-rooted mandibular canines determined using micro-computed tomography

P>Aim To investigate the internal and external anatomy of extracted human mandibular canines with two roots and two distinct canals using micro-computed tomography (mu CT). Methodology Fourteen two-rooted human mandibular canines were scanned using a high-resolution mu CT system (SkyScan 1174v2; SkyScan N.V., Kontich, Belgium). The images were processed to evaluate the size of the roots, the furcation regions, the presence of accessory canals, the mean distances between several anatomical landmarks, the position of the apical foramina, the direction of root curvatures, the cross-sectional appearances (SMI index), the volume and surface areas of the root canals. Results Root bifurcation was located in both apical (44%, n = 6) and middle (58%, n = 8) thirds of the root. The size of the buccal and lingual roots was similar in 29% of the sample. From a buccal view, no curvature towards the lingual or buccal direction occurred in either roots. From a proximal view, no straight lingual root occurred. In both views, S-shaped roots were found in 21% of the specimens. Location of the apical foramen varied considerably, tending to the mesio-buccal aspect of both roots. Lateral and furcation canals were observed mostly in the cervical third in 29% and 65% of the sample, respectively. The structure model index (SMI) index ranged from 1.87 to 3.86, with a mean value of 2.93 +/- 0.46. Mean volume and area of the root canals were 11.52 +/- 3.44 mm3 and 71.16 +/- 11.83 mm2, respectively. Conclusions The evaluation of two-rooted mandibular canines revealed that bifurcations occurred in the apical and middle third. S-shaped roots were found in 21% of the specimens. Mean volume, surface area and SMI index of the root canals were 11.52 mm3, 71.16 mm2 and 2.93, respectively.

Control of chaotic motion in a dual-spin spacecraft with nutational damping

Control of chaotic vibrations in a dual-spin spacecraft with an axial nutational damper is achieved using two techniques. The control methods are implemented on two realistic spacecraft parameter configurations that have been found to exhibit chaotic instability when a sinusoidally varying torque is applied to the spacecraft for a range of forcing amplitudes and frequencies. Such a torque, in practice, may arise under malfunction of the control system or from an unbalanced rotor. Chaotic instabilities arising from these torques could introduce uncertainties and irregularities into a spacecraft's attitude motion and, consequently, could have disastrous effects on its operation. The two control methods, recursive proportional feedback and continuous delayed feedback, are recently developed techniques for control of chaotic motion in dynamic systems. Each technique is outlined and the effectiveness on this model compared and contrasted. Numerical simulations are performed, and the results are studied by means of time history, phase space, Poincare map, Lyapunov characteristic exponents, and bifurcation diagrams.

Control of chaotic instability in a dual-spin spacecraft with dissipation using energy methods

Control of chaotic instability in a rotating multibody system in the form of a dual-spin spacecraft with an axial nutational damper is achieved using an algorithm derived using energy methods. The control method is implemented on two realistic spacecraft parameter configurations which have been found to exhibit chaotic instability when a sinusoidally varying torque is applied to the spacecraft for a range of forcing amplitudes and frequencies. Such a torque, in practice, may arise under malfunction of the control system or from an unbalanced rotor. Chaotic instabilities arising from these torques could introduce uncertainties and irregularities into a spacecraft's attitude and consequently impair pointing accuracy. The control method is formulated from nutational stability results derived using an energy sink approximation for a dual-spin spacecraft with an asymmetric platform and axisymmetric rotor. The effectiveness of the control method is shown numerically and the results are studied by means of time history, phase space, Poincare map, Lyapunov characteristic exponents and Bifurcation diagrams.

Control of chaotic instabilities in a spinning spacecraft with dissipation using Lyapunov's method

Control of chaotic instability in a simplified model of a spinning spacecraft with dissipation is achieved using an algorithm derived using Lyapunov's second method. The control method is implemented on a realistic spacecraft parameter configuration which has been found to exhibit chaotic instability for a range of forcing amplitudes and frequencies when a sinusoidally varying torque is applied to the spacecraft. Such a torque, may arise in practice from an unbalanced rotor or from vibrations in appendages. Numerical simulations are performed and the results are studied by means of time history, phase space, Poincare map, Lyapunov characteristic exponents and bifurcation diagrams. (C) 2002 Elsevier Science Ltd. All rights reserved.

Entanglement in the steady state of a collective-angular-momentum (Dicke) model

We model the behavior of an ion trap with all ions driven simultaneously and coupled collectively to a heat bath. The equations for this system are similar to the irreversible dynamics of a collective angular momentum system known as the Dicke model. We show how the steady state of the ion trap as a dissipative many-body system driven far from equilibrium can exhibit quantum entanglement. We calculate the entanglement of this steady state for two ions in the trap and in the case of more than two ions we calculate the entanglement between two ions by tracing over all the other ions. The entanglement in the steady state is a maximum for the parameter values corresponding roughly to a bifurcation of a fixed point in the corresponding semiclassical dynamics. We conjecture that this is a general mechanism for entanglement creation in driven dissipative quantum systems.

Postbuckling of piezoelectric FGM plates subject to thermo-electro-mechanical loading

In this paper, we examine the postbuckling behavior of functionally graded material FGM rectangular plates that are integrated with surface-bonded piezoelectric actuators and are subjected to the combined action of uniform temperature change, in-plane forces, and constant applied actuator voltage. A Galerkin-differential quadrature iteration algorithm is proposed for solution of the non-linear partial differential governing equations. To account for the transverse shear strains, the Reddy higher-order shear deformation plate theory is employed. The bifurcation-type thermo-mechanical buckling of fully clamped plates, and the postbuckling behavior of plates with more general boundary conditions subject to various thermo-electro-mechanical loads, are discussed in detail. Parametric studies are also undertaken, and show the effects of applied actuator voltage, in-plane forces, volume fraction exponents, temperature change, and the character of boundary conditions on the buckling and postbuckling characteristics of the plates. (C) 2003 Elsevier Science Ltd. All rights reserved.

Dynamic stability of laminated FGM plates based on higher-order shear deformation theory

This paper conducts a dynamic stability analysis of symmetrically laminated FGM rectangular plates with general out-of-plane supporting conditions, subjected to a uniaxial periodic in-plane load and undergoing uniform temperature change. Theoretical formulations are based on Reddy's third-order shear deformation plate theory, and account for the temperature dependence of material properties. A semi-analytical Galerkin-differential quadrature approach is employed to convert the governing equations into a linear system of Mathieu-Hill equations from which the boundary points on the unstable regions are determined by Bolotin's method. Free vibration and bifurcation buckling are also discussed as subset problems. Numerical results are presented in both dimensionless tabular and graphical forms for laminated plates with FGM layers made of silicon nitride and stainless steel. The influences of various parameters such as material composition, layer thickness ratio, temperature change, static load level, boundary constraints on the dynamic stability, buckling and vibration frequencies are examined in detail through parametric studies.

Path-dependence, expectativas e regulação econômica : elementos de análise a partir de uma perspectiva pós-keynesiana

O artigo busca fazer uma relação entre a ocorrência de path-dependence, a natureza expectacional do investimento e as flutuações econômicas, a partir de um referencial pós-keynesiano de análise. Em um ambiente não-ergódico, a fim de que o sistema tenha uma relativa estabilidade no tempo, a regulação econômica requer muito mais do que simples ajustamentos incrementais. Tornam-se necessárias, e possíveis, modificações qualitativas do sistema econômico. Tais modificações são analisadas a partir do conceito de bifurcação.