共查询到20条相似文献,搜索用时 31 毫秒
1.
由于异步电动机的数学模型具有高阶次、强非线性和多变量耦合的特点,所以要直接得到效率最优控制方程的清晰表达式是十分困难的。针对风机泵类、恒转矩和恒功率负载,利用电动机线性化公式导出了效率最优控制方程的表达式及其约束条件。并分别给出新方程和基于模型的效率优化方法的仿真计算结果。结果表明:两者的控制效果十分近似,而新方程却具有简单、实用的突出特点,可直接用于实际变频驱动系统的设计和应用。 相似文献
2.
O. AKSOGANA.H. SOFIYEV 《Journal of sound and vibration》2002,254(4):693-702
In this study, the dynamic buckling of an elastic cylindrical shell with variable thickness, subject to a uniform external pressure which is a power function of time, has been considered. Initially, the fundamental relations and Donnell-type dynamic buckling equation of an elastic cylindrical shell with variable thickness have been obtained. Then, employing Galerkin's method, these equations have been reduced to a time-dependent differential equation with variable coefficients. Finally, applying a special Ritz-type method, the critical static and dynamic loads, the corresponding wave numbers and dynamic factor have been found analytically. Using those results, the effects of the variation of the thickness with a linear, a parabolic or an exponential function in the axial direction and the effect of the variation of the power of time in the external pressure expression are studied using pertinent computations. It is observed that these effects change appreciably the critical parameters of the problem. The present method has been verified, comparing the results of the present work and those of previous works in the literature, for a shell with constant thickness subject to a uniform external pressure varying linearly with time. 相似文献
3.
The mathematical modeling of the laser cutting of steel plates is considered here by implementing the model proposed by Niziev and Nesterov, as formulated in [1]. The 3D-cutting front is described, according to this model, by a highly nonlinear partial differential equation. A number of simplifying assumptions can be formulated, however, so that this complex equation can be handled more easily. This enables us to concentrate on the physics of the model, rather than having to struggle with its mathematical manipulations. This simple model confirms in a capturing way the conjecture originally launched in [1] that cutting speed can be increased with a factor of about 1.5 to 2 by switching over from circular to radial polarization. As a further consequence, the model predicts that a similar improvement is also found regarding the plate thickness at a constant cutting speed. 相似文献
4.
提出了金属薄膜厚度对薄膜中自由电子的平均自由程影响的物理模型,并给出了薄膜中自由电子的平均自由程的修正公式.理论研究表明:当膜厚小于自由电子的平均自由程时,薄膜中电子平均自由程随膜厚的减小而减小|当膜厚大于或等于自由电子的平均自由程时,薄膜中电子的平均自由程与块状材料一样.利用薄膜中电子平均自由程的计算公式,修正了薄膜导电率的基本理论表达式,再利用金属薄膜的反射率与薄膜导电率的关系,得出金属薄膜厚度对其光反射率的影响.计算机模拟表明:当薄膜厚度小于电子自由程时,金属薄膜反射率随薄膜厚度变化而呈非线性关系. 相似文献
5.
基于电磁模式的色散方程,研究了有损金属圆波导中电磁模式的传输问题.在考虑了损耗层厚度影响的情况下,得到了各模式传输常数的近似解析表达式,此式适用于截止频率附近.通过解析表达式和波导模式色散方程,对衰减常数和相位常数进行了大量数值计算,得到了它们随损耗层厚度、频率以及损耗层电导率的变化规律,两种方法所获得的结果有较好的一致性.研究表明,损耗强度和损耗层厚度的变化会改变波导中电磁模式的色散和简并特性,对于改善高功率回旋行波放大器中注波互作用带宽和模式竞争非常有益.
关键词:
有损圆波导
相位常数
衰减常数
色散方程 相似文献
6.
A new analytical model of high voltage silicon on insulator (SOI) thin film devices 总被引:2,自引:0,他引:2 下载免费PDF全文
A new analytical model of high voltage silicon on insulator (SOI)
thin film devices is proposed, and a formula of silicon critical
electric field is derived as a function of silicon film thickness by
solving a 2D Poisson equation from an effective ionization rate,
with a threshold energy taken into account for electron multiplying.
Unlike a conventional silicon critical electric field that is
constant and independent of silicon film thickness, the proposed
silicon critical electric field increases sharply with silicon film
thickness decreasing especially in the case of thin films, and can
come to 141V/μm at a film thickness of 0.1μm which is
much larger than the normal value of about 30V/μm. From the
proposed formula of silicon critical electric field, the expressions
of dielectric layer electric field and vertical breakdown voltage
(VB,V) are obtained. Based on the model, an ultra thin film
can be used to enhance dielectric layer electric field and so
increase vertical breakdown voltage for SOI devices because of its
high silicon critical electric field, and with a dielectric layer
thickness of 2μm the vertical breakdown voltages reach 852
and 300V for the silicon film thicknesses of 0.1 and 5μm,
respectively. In addition, a relation between dielectric layer
thickness and silicon film thickness is obtained, indicating a
minimum vertical breakdown voltage that should be avoided when an
SOI device is designed. 2D simulated results and some experimental
results are in good agreement with analytical results. 相似文献
7.
S. V. Ghaisas 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(4):557-562
Growth of interfaces during vapor deposition is analyzed on a discrete lattice. It leads to finding distribution of local
heights, measurable for any lattice model. Invariance in the
change of this distribution in time is used to determine the cross over
effects in various models. The analysis is applied to the discrete linear growth equation
and Kardar-Parisi-Zhang (KPZ) equation. A new model is devised that shows early convergence to the KPZ dynamics. Various known
conservative and non conservative models are tested on a one
dimensional substrate by comparing the growth results with the exact KPZ and linear growth equation results. The comparison
helps in establishing the condition that determines the presence of cross over effect for the given model. The new model is
used in (2+1) dimensions to predict close to the true value of roughness constant for KPZ equation. 相似文献
8.
To describe the motion of the film flowing downward the vertical wall in the mode of condensation or evaporation into the surrounding space, the model proposed in [1] is used. It is reduced to one equation for the film thickness. The model comprises two governing parameters. The first one is proportional to the difference in the wall temperature, assumed to be constant, and the saturation temperature, and the second is proportional to the surface tension coefficient. In a series of publications [1–4] the authors studied solutions of thementioned equation at zero surface tension. Their characteristic feature is the presence of strong and weak discontinuities of layer thickness. In this paper we studied the regularizing effect of surface tension on the film flow structure with phase transitions. Numerical and asymptotic analysis of the resulting structures is carried out. Also, the case where the wall temperature is an arbitrary function of time is considered. 相似文献
9.
在结合应变Si,高k栅和SOI结构三者的优点的基础上,提出了一种新型的高k栅介质应变Si全耗尽SOI MOSFET结构.通过求解二维泊松方程建立了该新结构的二维阈值电压模型,在该模型中考虑了影响阈值电压的主要参数.分析了阈值电压与弛豫层中的Ge组分、应变Si层厚度的关系.研究结果表明阈值电压随弛豫层中Ge组分的提高和应变Si层的厚度增加而降低.此外,还分析了阈值电压与高k栅介质的介电常数和应变Si层的掺杂浓度的关系.研究结果表明阈值电压随高k介质的介
关键词:
应变Si
k栅')" href="#">高k栅
短沟道效应
漏致势垒降低 相似文献
10.
A. M. Basharov 《Journal of Experimental and Theoretical Physics》2018,126(3):310-320
The Markov model of spontaneous emission of an atom localized in a spatial region with a broadband electromagnetic field with zero photon density is considered in the conditions of coupling of the electromagnetic field with the broadband field of a neighboring space. The evolution operator of the system and the kinetic equation for the atom are obtained. It is shown that the field coupling constant affects the rate of spontaneous emission of the atom, but is not manifested in the atomic frequency shift. The analytic expression for the radiative decay constant for the atom is found to be analogous in a certain sense to the expression for the decay constant for a singly excited localized ensemble of identical atoms in the conditions when the effect of stabilization of its excited state by the Stark interaction with the vacuum broadband electromagnetic field is manifested. The model is formulated based on quantum stochastic differential equations of the non- Wiener type and the generalized algebra of the Ito differential of quantum random processes. 相似文献
11.
We consider the problem of propagation of acoustic-gravity waves in the atmosphere with a constant temperature gradient in the near-surface layer. The assumption of linear temperature dependence on height allowed us to reduce the wave equation to the hypergeometric form, regardless of the compressibility of the medium. The solution of this equation is represented in terms of degenerate hypergeometric functions. To analyze the obtained solution, we consider a two-layer model of a half-bounded atmosphere with a height-independent background temperature in the upper layer. The results are studied in detail under the approximation of an incompressible medium. For the model specified above, we find analytical expressions for the perturbation fields and obtain a characteristic equation whose solution allows us to calculate wave dispersion characteristics at frequencies close to the Brunt-Väisälä frequency for large horizontal scales as compared to the layer thickness. 相似文献
12.
Samad Khakshournia 《General Relativity and Gravitation》2008,40(8):1791-1796
Within our thick brane approach previously used to obtain the cosmological evolution equations on a thick brane embedded in
a five-dimensional Schwarzschild Anti-de Sitter spacetime it is explicitly shown that the consistency of these equations with
the energy conservation equation requires that, in general, the thickness of the brane evolves in time. This varying brane
thickness entails the possibility that both Newton’s gravitational constant G and the effective cosmological constant Λ4 are time dependent. 相似文献
13.
We study the propagation of thickness-twist (TT) waves in a crystal plate of AT-cut quartz with periodically varying, piecewise constant thickness. The scalar differential equation by Tiersten and Smythe is employed. The problem is found to be mathematically equivalent to the motion of an electron in a periodic potential field governed by Schrodinger’s equation. An analytical solution is obtained. Numerical results show that the eigenvalue (frequency) spectrum of the waves has a band structure with allowed and forbidden bands. Therefore, for TT waves, plates with periodically varying thickness can be considered as phononic crystals. The effects of various parameters on the frequency spectrum are examined. 相似文献
14.
Radiative heat transfer in an absorbing, emitting, anistropically-scattering, one-dimensional medium is analyzed. Unlike many of the existing works, the present analysis does not require a known temperature distribution within the medium. Assuming a model of linear anistropic scattering, the transfer equation and the energy equation are solved simultaneously by utilizing a recently developed successive approximation technique. Closed-form approximate solutions and accurate higher-order results are both presented. Calculations show that the relative importance of the anistropic scattering effect generally decreases with decreasing wall emissivity and decreasing optical thickness. For radiative equilibrium without internal heat generation, it is demonstrated that the anistropic-scattering heat-transfer results can be approximated quite adequately by the isotropic-scattering result with the introduction of the concept of an effective optical thickness. For media with internal heat generation, an interesting effect of the scattering albedo is observed. It is established that, in the limit of a large scattering albedo, the temperature of the medium approaches a constant value that is independent of anistropic-scattering effects and wall emissivity. The exact limiting expressions for the temperature and apparent emissivity of an isothermal slab are found. 相似文献
15.
Effective dielectric constant model of electromagnetic backscattering from stratified air–sea surface film–sea water medium 下载免费PDF全文
Studies of surface film medium on the sea surface are carried out in this paper for developing the technology to automatically detect and classify sea surface films, and an effective dielectric constant model of electromagnetic backscattering from a stratified air–ocean interface. Numerical results of the new model show the characteristics of effective dielectric constants for the air–sea surface film–sea water medium as follows. The effective dielectric constants decrease with increasing relative dielectric constants of the sea surface films. The effective dielectric constants decrease in horizontal polarization(abbr. HH polarization) and increase in VV vertical polarization(abbr. VV polarization) with increasing radar incident angle. Effective dielectric constants vary with relative sea surface film thickness as a cosinusoidal function of sea surface film thickness. Effective dielectric constant of VV polarization is larger than that of HH polarization. Two potential applications are found with our model, i.e., the retrieval of dielectric constants from the sea surface film, and the film thickness retrieval with our model. Our model has a highly significant influence on improving the technology related to the remote sensing of sea surface films. 相似文献
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Two-dimensional thermoelasticity analysis of functionally graded thick beams is presented using the state space method coupled
with the technique of differential quadrature. Material properties vary continuously and smoothly through the beam thickness,
leading to variable coefficients in the state equation derived from the elasticity equations. Approximate laminate model is
employed to translate the state equation into the one with constant coefficients in each layer. To avoid numerical instability,
joint coupling matrices are introduced according to the continuity conditions at interfaces in the approximate model. The
differential quadrature procedure is applied to discretizing the beam in the axial direction to make easy the treatment of
arbitrary end conditions. A simply-supported beam with exponentially varying material properties is considered to validate
the present method. Numerical examples are performed to investigate the influences of relative parameters. 相似文献
20.
This paper considers a new model of individual displacement, based on fish motion, the so-called Persistent Turning Walker
(PTW) model, which involves an Ornstein-Uhlenbeck process on the curvature of the particle trajectory. The goal is to show
that its large time and space scale dynamics is of diffusive type, and to provide an analytic expression of the diffusion
coefficient. Two methods are investigated. In the first one, we compute the large time asymptotics of the variance of the
individual stochastic trajectories. The second method is based on a diffusion approximation of the kinetic formulation of
these stochastic trajectories. The kinetic model is a Fokker-Planck type equation posed in an extended phase-space involving
the curvature among the kinetic variables. We show that both methods lead to the same value of the diffusion constant. We
present some numerical simulations to illustrate the theoretical results. 相似文献