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961.
Peter A. Orlin A. Louise Perkins George Heburn 《Numerical Methods for Partial Differential Equations》1997,13(5):549-560
We present a method for designing spatial derivative approximations that achieves a priori accuracy in the spatial frequency domain. We use a general, average value approximation with undetermined coefficients together with a set of constraints that ensure convergence and consistency to formulate a constrained optimal fitting problem. These constraints lead to a linear matrix formulation. We apply the method to the design of spatial approximations for simulating equations with wavelike solutions using both an explicit central difference approximation (which has no phase error) and an upwind design where the level of dissipation can be specified by the designer. © 1997 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 13: 549–560, 1997 相似文献
962.
A serendipitous finding during development of an automated “electronic flow chart” system to gather data on ICU patients [1] was the observation of low frequency oscillations in blood pressure that were not explained by systematic variability in the environment. [2] This finding has since been confirmed by others. [3,4] In the present report, hemodynamic data for critically ill surgical patients was continuously collected and visualized on a computer workstation to search for patterns not noted by standard monitoring. With this system, we observed low-frequency periodic oscillations in the cardiac output of ten patients, with regular periodicities of 4 to 280 minutes (average = 34 minutes). The mortality rate in these patients was 40%, while the mortality was only 10.8% in 83 similarly monitored intensive care unit (ICU) patients who did not develop regular oscillations in cardiac output. Interestingly, these oscillatory patterns appear to be associated with inadequate resuscitation of increased metabolic rates. The mathematical definition of “chaos” refers to irregular behavior that appears to be random but is actually deterministic. [5] A surprising finding concerning transitions between states of apparent randomness and order in nonlinear systems is that many systems become more organized after being disturbed. Chaotic behavior in biological systems may represent a normal physiologic state, while the loss of chaotic behavior may herald a pathophysiologic state. [6] The mechanism of the regular low frequency oscillations in cardiac output remains to be determined, but the high mortality rate suggests that it is a pathophysiologic marker, perhaps due to inadequate oxygen delivery in under-resuscitated shock. © 1997 John Wiley & Sons, Inc. 相似文献
963.
鉴于传统电力电子相关课程实验既无法直观了解电路内部构造,也不能有效提升实际动手能力的情况,设计了一种基于实验室环境的开放式创新型感应电机变频调速系统,改造了主电路和控制电路的电源,在适应实验室用电环境的同时,能实现主电路低压可测量和提升学生实验的安全性;并设计了采样电路和保护电路,能加深学生对电路的了解,增强实际动手能力.经实验验证,该系统能完成多段速运动控制、模拟量与数字量给定调速等相关实验. 相似文献
964.
R. PrezFeito R. DíazCalleja J. Guzmn E. Riande 《Journal of Polymer Science.Polymer Physics》1999,37(17):2486-2498
A comparative study on the mechanical and dielectric relaxation behavior of poly(5‐acryloxymethyl‐5‐methyl‐1,3‐dioxacyclohexane) (PAMMD), poly(5‐acryloxymethyl‐5‐ethyl‐1,3‐dioxacyclohexane) (PAMED), and poly(5‐methacryloxymethyl‐5‐ethyl‐1,3‐dioxacyclohexane) (PMAMED) is reported. The isochrones representing the mechanical and dielectric losses present prominent mechanical and dielectric β relaxations located at nearly the same temperature, approximately −80°C at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the neighborhood of 27, 30, and 125°C for PAMMD, PAMED, and PMAMED, respectively, at the same frequency. The values of the activation energy of the β dielectric relaxations of these polymers lie in the vicinity of 10 kcal mol−1, ∼ 2 kcal mol−1 lower than those corresponding to the mechanical relaxations. As usual, the temperature dependence of the mean‐relaxation times associated with both the dielectric and mechanical α relaxations is described by the Vogel–Fulcher–Tammann–Hesse (VFTH) equation. The dielectric relaxation spectra of PAMED and PAMMD present in the frequency domain, at temperatures slightly higher than Tg, the α and β relaxations at low and high frequencies, respectively. The high conductive contributions to the α relaxation of PMAMED preclude the possibility of isolating the dipolar component of this relaxation in this polymer. Attempts are made to estimate the temperature at which the α and β absorptions merge together to form the αβ relaxation in PAMMD and PAMED. Molecular Dynamics (MD) results, together with a comparative analysis of the spectra of several polymers, lead to the conclusion that flipping motions of the 1,3‐dioxacyclohexane ring may not be exclusively responsible for the β‐prominent relaxations that polymers containing dioxane and cyclohexane pendant groups in their structure present, as it is often assumed. The diffusion coefficient of ionic species, responsible for the high conductivity exhibited by these polymers in the α relaxation, is semiquantitatively calculated using a theory that assumes that this process arises from MWS effects, taking place in the bulk, combined with Nernst–Planckian electrodynamic effects, due to interfacial polarization in the films. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2486–2498, 1999 相似文献
965.
966.
967.
利用X射线衍射谱(XRD)和X射线光电子谱(XPS)研究了热处理对AgxO样品的结构及成份的影响.研究结果表明所有制备的AgxO样品基本为无定型,并且AgO和Ag2O两种成份共存;两组具有代表性的AgxO样品经过高温热处理后分别呈现了(Ag+Ag2O)和Ag2O的多晶结构,结构及成份的巨大差异与样品制备条件息息相关;AgO和Ag2O两种成份的热分解临界温度分别为200℃和300℃;热处理过程中,伴随着AgxO的热分解及体内的氧原子向样品表面的扩散过程,并且Ag2O具有相对致密的结构. 相似文献
968.
研究了无电感补偿和有电感补偿的一级和两级电阻分压器的幅频特性-3 dB频率与阶跃响应10%~90%上升时间的关系。无电感补偿一级分压器的-3 dB频率与阶跃响应上升时间之积为常数0.350;对无电感补偿两级分压器,该乘积在0.349附近很小范围内变动;对电感补偿一级分压器,该乘积由过冲决定,当过冲在0~10%范围内变化时,该乘积在0.35~0.29之间线性变化;对电感补偿两级分压器,该乘积随过冲和分压器参数变化,在不大于10%的确定过冲下,变化范围约为±10%;当两级分压器第一级的时间常数远大于第二级的时间常数时,可能难以在第二级进行有效的电感补偿。 相似文献
969.
970.
针对沙漠环境实地链路存在的温度变化大、室外风力、地表振动等多种复杂噪声来源,通过对系统反馈补偿带宽、反馈补偿强度、光功率等时频传递系统关键参数的优化配置,研究了不同反馈补偿参数下复杂链路噪声的有效抑制技术.全链路的频率传递稳定度8×10~(-14)@1s,1×10~(-16)@1000 s,千秒尺度下时间信号传递的时间方差仅为1.2 ps.实现了氢钟信号在200 km量级沙漠环境实地链路的无损传输.该验证实验在基于短基线干涉测量的卫星测轨系统中发挥了重要作用. 相似文献