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排序方式: 共有1083条查询结果,搜索用时 343 毫秒
71.
Higuti PT Adamowski JC 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2002,49(9):1260-1268
This work presents recent improvements in a density measurement cell with a double-element transducer that can eliminate diffraction effects. A new mechanical design combined with the use of more appropriate materials has resulted in better parallelism between interfaces, more robust assembly, and chemical resistance. A novel method of signal processing, named the energy method, is introduced to obtain the reflection coefficient, reducing sensitivity to noise and improving accuracy. The measurement cell operation is verified both theoretically, using an acoustic wave propagation model, and experimentally, using homogeneous liquids with different densities. The accuracy in the density measurement is 0.2% when compared with the measurements made with a pycnometer. 相似文献
72.
Julio Garrido Campos Author Vitae Martin Hardwick Author Vitae 《Computer aided design》2006,38(5):540-551
This paper proposes an information model for tracing CNC manufacturing operations. The objective of the model is to assure that traceability data is comprehensive and available for every CNC machined product, independent of the relationship between the subcontractor and the contractor. The prominent feature of the model is a link between CNC report data for a product instance, the CAD design and the CAM data. This link enables users to browse the traceability data and understand the relationships between the manufacturing process, the CAD design and the CAM data. The link is independent of the systems used to build the CAD and CAM data, and allows the users to be sure the manufactured product contains the required characteristics. Then if a product instance fails, this linkage will make possible to analyze the trace data and identify any exceptions or unusual conditions in the manufacturing process. 相似文献
73.
Our main purpose in this paper is to further address the global stabilization problem for affine systems by means of bounded feedback control functions, taking into account a large class of control value sets: p, r ‐weighted balls ??m r (p), with 1<p?∞, defined via p, r ‐weighted gauge functions. Observe that p=∞ is allowed, so that m‐dimensional r ‐hyperboxes ??m r (∞)?[?r1?,r1+]×???×[?rm?,rm+], rj±>0 are also considered. Working along the line of Artstein–Sontag's approach, we construct an explicit formula for a one‐parameterized family of continuous feedback controls taking values in ?? r m(p) that globally asymptotically stabilize an affine system, provided an appropriate control Lyapunov function is known. The designed family of controls is suboptimal with respect to the robust stability margin for uncertain systems. The problem of achieving disturbance attenuation for persistent disturbances is also considered. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
74.
Incidence on power system dynamics of high penetration of fixed speed and doubly fed wind energy systems: study of the Spanish case 总被引:1,自引:0,他引:1
Rodriguez J.M. Fernandez J.L. Beato D. Iturbe R. Usaola J. Ledesma P. Wilhelmi J.R. 《Power Systems, IEEE Transactions on》2002,17(4):1089-1095
In this paper, a preliminary analysis of the impact of high wind power penetration in the planning and operation of the Spanish power system is presented. The problems studied in the paper are those related to the stability of the power system. This key subject could be deeply influenced by the installation of up to 15000 MW of wind power generation as it has been planned for the next six years. The analysis presented here is based on the results of dynamic simulations. Dynamic models of the induction generator (squirrel cage and doubly fed) and models of the wind farms have been developed to make possible the simulations. From the simulation results some conclusions and recommendations have been extracted, which would contribute to the appropriate integration of the new wind generation foreseen for the Spanish power system. 相似文献
75.
Mantas Ruiz Jose M. Lopera Julio Ortega Carrillo de la Plata Jose A. 《International journal of parallel programming》2002,30(2):99-148
A component-based methodological approach to derive distributed implementations of parallel ODE solvers is proposed. The proposal is based on the incorporation of explicit constructs for performance polymorphism into a methodology to derive group parallel programs of numerical methods from SPMD modules. These constructs enable the structuring of the derivation process into clearly defined steps, each one associated with a different type of optimization. The approach makes possible to obtain a flexible tuning of a parallel ODE solver for several execution contexts and applications. Following this methodological approach, a relevant parallel numerical scheme for solving stiff ODES has been optimized and implemented on a PC cluster. This numerical scheme is obtained from a Radau IIA Implicit Runge–Kutta method and exhibits a high degree of potential parallelism. Several numerical experiments have been performed by using several test problems with different structural characteristics. These experiments show satisfactory speedup results. 相似文献
76.
Huedo-Medina Tania B.; Sánchez-Meca Julio; Marín-Martínez Fulgencio; Botella Juan 《Canadian Metallurgical Quarterly》2006,11(2):193
In meta-analysis, the usual way of assessing whether a set of single studies is homogeneous is by means of the Q test. However, the Q test only informs meta-analysts about the presence versus the absence of heterogeneity, but it does not report on the extent of such heterogeneity. Recently, the I2 index has been proposed to quantify the degree of heterogeneity in a meta-analysis. In this article, the performances of the Q test and the confidence interval around the I2 index are compared by means of a Monte Carlo simulation. The results show the utility of the I2 index as a complement to the Q test, although it has the same problems of power with a small number of studies. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
77.
Kenneth S. Denison Charles E. Hamrin Julio C. Diaz 《Computers & Chemical Engineering》1982,6(3):189-196
A combination of the Newton-like S-method of Bank and Rose[1] and the conjugate gradient method for nonsymmetric matrices described by Axelsson[7] with appropriate pre-conditioning is used in the numerical solution of a system of nonlinear elliptic partial differential equations. The S-method is used to linearize the equations and provides quadratic convergence. The preconditioned conjugate gradient algorithm is used to solve the system of N2 linear equations in far fewer than N2 iterations without destroying the sparsity of the matrix. The methods can be applied to a large class of problems. A two-dimensional, nonlinear, steady-state diffusion/reaction problem is solved to illustrate the use of the methods. The problem describes diffusion and reaction in a catalyst slab applicable to a two-phase, cross-flow reactor. Results for extreme problems are presented to demonstrate the usefulness and efficiency of the methods. 相似文献
78.
Epidemiological studies show that populations consuming a predominantly plant-based Mediterranean-style diet exhibit lower
incidences of chronic diseases than those eating a northern European or North American diet. This observation has been attributed
to the greater consumption of fruits and vegetables and the lower consumption of animal products, particularly fat. Although
total fat intake in Mediterranean populations can be higher than in other regions (ca. 40% of calories), the greater proportion
is derived from olive oil and not animals. Increased olive oil consumption is implicated in a reduction in cardiovascular
disease, rheumatoid arthritis, and, to a lesser extent, a variety of cancers. Olive oil intake also has been shown to modulate
immune function, particularly the inflammatory processes associated with the immune system. Olive oil is a nonoxidative dietary
component, and the attenuation of the in-flammatory process it elicits could explain its beneficial effects on disease risk
since oxidative and inflammatory stresses appear to be underlying factors in the etiology of these diseases in man. The antioxidant
effects of olive oil are probably due to a combination of its high oleic acid content (low oxidation potential compared with
linoleic acid) and its content of a variety of plant antioxidants, particularly oleuropein, hydroxytyrosol, and tyrosol. It
is also possible that the high oleic acid content and a proportionate reduction in linoleic acid intake would allow a greater
conversion of α-linolenic acid (18∶3n−3) to longer-chain n−3 PUFA, which have characteristic health benefits. Adoption of
a Mediterranean diet could confer health benefits in high-risk populations. 相似文献
79.
Normalized cuts in 3-D for spinal MRI segmentation 总被引:1,自引:0,他引:1
Segmentation of medical images has become an indispensable process to perform quantitative analysis of images of human organs and their functions. Normalized Cuts (NCut) is a spectral graph theoretic method that readily admits combinations of different features for image segmentation. The computational demand imposed by NCut has been successfully alleviated with the Nystr?m approximation method for applications different than medical imaging. In this paper we discuss the application of NCut with the Nystr?m approximation method to segment vertebral bodies from sagittal T1-weighted magnetic resonance images of the spine. The magnetic resonance images were preprocessed by the anisotropic diffusion algorithm, and three-dimensional local histograms of brightness was chosen as the segmentation feature. Results of the segmentation as well as limitations and challenges in this area are presented. 相似文献
80.
Guizar-Sicairos M Gutiérrez-Vega JC 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(9):1682-1688
We obtain a Fourier transform scaling relation to find analytically, numerically, or experimentally the spectrum of an arbitrary scaled two-dimensional Dirac delta curve from the spectrum of the nonscaled curve. An amplitude factor is derived and given explicitly in terms of the scaling factors and the angle of the forward tangent at each point of the curve about the positive x axis. With the scaling relation we determine the spectrum of an elliptic curve by a circular geometry instead of an elliptical one. The generalization to N-dimensional Dirac delta curves is also included. 相似文献