恒定导通时间控制buck变换器多开关周期振荡现象分析

研究了恒定导通时间(constant on-time,COT)控制buck变换器的多开关周期振荡现象,揭示了buck变换器输出电容等效串联电阻(equivalent series resistance,ESR)对COT控制buck变换器控制性能的影响.研究结果表明,输出电容等效串联电阻是buck变换器发生多开关周期振荡现象的关键因素,并给出了COT控制buck变换器发生多开关周期振荡的临界ESR值.当输出电容ESR值小于临界值时,发生多开关周期振荡现象;而当ESR大于临界值时,多开关周期振荡现象消失.最后 关键词： 恒定导通时间控制 buck变换器 振荡 等效串联电阻     

A quantile-based Time at Risk: A new approach for assessing risk in financial markets

In this paper, we provide a new measure for evaluation of risk in financial markets. This measure is based on the return interval of critical events in financial markets or other investment situations. Our main goal was to devise a model like Value at Risk (VaR). As VaR, for a given financial asset, probability level and time horizon, gives a critical value such that the likelihood of loss on the asset over the time horizon exceeds this value is equal to the given probability level, our concept of Time at Risk (TaR), using a probability distribution function of return intervals, provides a critical time such that the probability that the return interval of a critical event exceeds this time equals the given probability level. As an empirical application, we applied our model to data from the Tehran Stock Exchange Price Index (TEPIX) as a financial asset (market portfolio) and reported the results.     

False Vacuum Decay with Gravity in a Critical Case

The vacuum decay in a de Sitter universe is studied for the class of effective inflaton potentials that curvature at the top is less than as well as greater than a critical value determined previously. By comparing the actions of the Hawking - Moss instanton and the Coleman - de Luccia instanton(s) the mode of vacuum decay is determined in this critical situation.     

换流变分接头及直流控制器极限诱导分岔分析

本文研究了直流子系统整流站定电流逆变站定电压控制方式下逆变站换流变变比和熄弧角达到限值时交直流互联系统发生极限诱导分岔的情况.对系统遭遇硬限制时不会发生奇异诱导分岔的原因作了详细的探讨;发现系统发生极限诱导分岔时换流变变比需大于临界值并给出了不同类型极限诱导分岔临界值的计算公式.根据临界值对参数灵敏度的信息,可通过调整参数来消除分岔点以增大稳定域.分析了发电机励磁电压极限对换流变变比临界值及系统稳定域的影响.给出了发生此类极限诱导分岔的机理解释:换流变无功消耗的增加,换流站无功补偿的下降及控制方式的转换导致系统因无功缺乏失稳.     

On the critical diameter and hotspot combustion of a double-base propellant

An analysis of experimental data on the critical diameter for burning of nitroglycerin-based propellant NB and on the temperature profiles in the combustion wave on a propellant sample in a heat-absorbing confinement was performed. It was demonstrated that, for cylindrical samples placed into a heat-absorbing confinement, there are two values of the critical diameter. The smallest value of the critical diameter (lower extinction limit) is associated with heat losses from the heat conduction zone and reaction zone of the condensed phase. The largest value of the critical diameter (upper extinction limit) is determined by heat losses from the condensed and gas phases. It was demonstrated that the value of the critical diameter is controlled by the maximum size of hotspots on the burning surface. For example, the mean critical diameter equals approximately twice the maximum hotspot size. Critical diameters characteristic of the range between the lower and upper limits range from a quarter to a half of the maximum hotspot size, with the process of extinction in this interval being of stochastic character.     

The nonequilibrium phase transition and the symmetry revival induced by time delay in an asymmetric bistable system with correlated noises

The nonequilibrium phase transition and the symmetry revival induced by time delay in a bistable system are investigated. The stationary probability distribution function (SPDF) of the bistable system with time delay and correlated noises are calculated by an analytical method and stochastic simulation respectively. The analytical and simulative results indicate that: (1) There is a certain value of λ (λ denotes the strength of correlations between the multiplicative and additive noises) to make the SPDF symmetric under some time delay; however, above or below the given value, the symmetry will be broken; (2) With the monotonic change of λ , the unimodal peak structure of SPDF becomes bimodal at the beginning, then it becomes unimodal again; this means that there is a reentrance phenomenon in the process; (3) There is a critical value of delay time, which makes the lower peak of SPDF equal to the higher one under the critical condition. This means that the symmetry revival phenomenon emerges.     

自旋S=1座稀释蜂窝格子的伊辛模型

本文对于自旋S=1的淬灭座稀释蜂窝格子的伊辛模型,在约束条件exp(K)cosh(J)=1下,求得了系统的临界温度和磁化强度与自旋浓度之间的关系。 关键词：     

Effects of time delay on stochastic resonance of the stock prices in financial system

The effect of time delay on stochastic resonance of the stock prices in finance system was investigated. The time delay is introduced into the Heston model driven by the extrinsic and intrinsic periodic information for stock price. The signal power amplification (SPA) was calculated by numerical simulation. The results indicate that an optimal critical value of delay time maximally enhances the reverse-resonance in the behaviors of SPA as a function of long-run variance of volatility or cross correlation coefficient between noises for both cases of intrinsic and extrinsic periodic information. Moreover, in both cases, being a critical value in the delay time, when the delay time takes value below the critical value, reverse-resonance increases with the delay time increasing, however, when the delay time takes value above the critical value, the reverse-resonance decrease with the delay time increasing.     

Sharp signature of a dx2-y2 quantum critical point in the Hall coefficient of cuprate superconductors

We study the behavior of the Hall coefficient, R(H), in a system exhibiting dx(2)(-y(2)) density-wave order in a regime in which the carrier concentration, x, is tuned to approach a quantum critical point at which the order is destroyed. At the mean-field level, we find that n(Hall)=1/R(H) evinces a sharp signature of the transition. There is a kink in n(Hall) at the critical value of the carrier concentration, x(c); as the critical point is approached from the ordered side, the slope of n(Hall) diverges. Hall transport experiments in the cuprates, at high magnetic fields sufficient to destroy superconductivity, should reveal this effect.     

Kinetic Gaussian Model with Long-Range Interactions

In this paper dynamical critical phenomena of the Gaussian model with long-range interactions decaying as 1/r^d+δ (δ>0) on d-dimensional hypercubic lattices (d=1, 2, and 3) are studied. First, the critical points are exactly calculated, and it is found that the critical points depend on the value of δ and the range of interactions. Then the critical dynamics is considered. We calculate the time evolutions of the local magnetizations and the spin-spin correlation functions, and further the dynamic critical exponents are obtained. For one-, two- and three-dimensional lattices, it is found that the dynamic critical exponents are all z=2 if δ>2, which agrees with the result when only considering nearest neighboring interactions, and that they are all δ if 0<δ<2. It shows that the dynamic critical exponents are independent of the spatial dimensionality but depend on the value of δ.     

Time-Inhomogeneous Fokker-Planck Equation for Wave Distributions in the Abelian Sandpile Model

The time and size distributions of the waves of topplings in the Abelian sandpile model are expressed as the first arrival at the origin distribution for a scale invariant, time-inhomogeneous Fokker-Plank equation. Assuming a linear conjecture for the time inhomogeneity exponent as a function of a loop-erased random walk (LERW) critical exponent, suggested by numerical results, this approach allows one to estimate the lower critical dimension of the model and the exact value of the critical exponent for LERW in three dimensions. The avalanche size distribution in two dimensions is found to be the difference between two closed power laws.     

磁绝缘传输线振荡器中辐射场的非线性行为

 导出了磁绝缘传输线振荡器(MILO)中辐射场的非线性演化方程，并讨论了在临界值点附近可能出现非线性不稳定解的条件。结论是：(1)非线性增长速率与线性增长速率的比值g<1.2，且远离g+γ=1(γ为非线性相位增长率与线性相位增长率的比值)点时，出现非稳定解的失谐量临界值很小，而线性增长速率临界值临界值很大，容易出现非稳定解；(2)当g≥1.2时，任意小的失谐量都可以使场出现非稳定解；(3)线性增长速率越大，越不容易出现非稳定解。     

磁绝缘传输线振荡器中辐射场的非线性行为

导出了磁绝缘传输线振荡器 (MILO)中辐射场的非线性演化方程 ,并讨论了在临界值点附近可能出现非线性不稳定解的条件。结论是 :(1)非线性增长速率与线性增长速率的比值g <1.2 ,且远离 g γ =1(γ为非线性相位增长率与线性相位增长率的比值 )点时 ,出现非稳定解的失谐量临界值很小 ,而线性增长速率临界值临界值很大 ,容易出现非稳定解 ;(2 )当 g≥ 1.2时 ,任意小的失谐量都可以使场出现非稳定解 ;(3)线性增长速率越大 ,越不容易出现非稳定解     

Characterization of Critical Values of Branching Random Walks on Weighted Graphs through Infinite-Type Branching Processes

We study the branching random walk on weighted graphs; site-breeding and edge-breeding branching random walks on graphs are seen as particular cases. Two kinds of survival can be identified: a weak survival (with positive probability there is at least one particle alive somewhere at any time) and a strong survival (with positive probability the colony survives by returning infinitely often to a fixed site). The behavior of the process depends on the value of a certain parameter which controls the birth rates; the threshold between survival and (almost sure) extinction is called critical value. We describe the strong critical value in terms of a geometrical parameter of the graph. We characterize the weak critical value and relate it to another geometrical parameter. We prove that, at the strong critical value, the process dies out locally almost surely; while, at the weak critical value, global survival and global extinction are both possible.     

Chaotic advection in a 2-D mixed convection flow

Two-dimensional numerical simulations of particle advection in a channel flow with spatially periodic heating have been carried out. The velocity field is found to be periodic above a critical Rayleigh number of around 18 000 and a Reynolds number of 10. Particle motion becomes chaotic in the lower half plane almost immediately after this critical value is surpassed, as characterized by the power spectral density and Poincare section of the flow. As the Rayleigh number is increased further, particle motion in the entire domain becomes chaotic. (c) 1995 American Institute of Physics.     

Influence of induced uniaxial anisotropy on the domain structure and phase transitions of yttrium-iron garnet films

The effect of induced uniaxial anisotropy on the properties and parameters of the domain structure and phase transitions in yttrium-iron garnet (YIG) films is investigated. Based on the measurements and the derived formulas we determine the difference between the magnetization and the uniaxial anisotropy field for each of the films. We have also measured the parameters of the domain structures and phase transitions of the films for the magnetization parallel and perpendicular to the projections of the [111] crystallographic axes onto the plane of the film. We find that films of pure YIG films grown in (111) are characterized by the existence of some critical value of the uniaxial anisotropy field. It is found that for films in which the uniaxial anisotropy field is larger than this critical value and films in which it is less than this critical value, such parameters of the domain structures as the ratio of the width of the domains to the film thickness, the orientation of the magnetization of the domains, the orientation of the domain boundaries, and the magnitudes of the phase transition fields differ substantially. Fiz. Tverd. Tela (St. Petersburg) 41, 2034–2041 (November 1999)     

Thermal Entanglement in XXZ Heisenberg Model for Coupled Spin-Half and Spin-One Triangular Cell

In this paper, we investigate the thermal entanglement of two-spin subsystems in an ensemble of coupled spin-half and spin-one triangular cells, (1/2, 1/2, 1/2), (1/2, 1, 1/2), (1, 1/2, 1) and (1, 1, 1) with the XXZ anisotropic Heisenberg model subjected to an external homogeneous magnetic field. We adopt the generalized concurrence as the measure of entanglement which is a good indicator of the thermal entanglement and the critical points in the mixed higher dimensional spin systems. We observe that in the near vicinity of the absolute zero, the concurrence measure is symmetric with respect to zero magnetic field and changes abruptly from a non-null to null value for a critical magnetic field that can be signature of a quantum phase transition at finite temperature. The analysis of concurrence versus temperature shows that there exists a critical temperature, that depends on the type of the interaction, i.e. ferromagnetic or antiferromagnetic, the anisotropy parameter and the strength of the magnetic field. Results show that the pairwise thermal entanglement depends on the third spin which affects the maximum value of the concurrence at absolute zero and at quantum critical points.     

A circular zone counting method of identifying a Duffing oscillator state transition and determining the critical value in weak signal detection

Identifying state transition and determining the critical value of the Duffing oscillator are crucial to indicating external signal existence and have a great influence on detection accuracy in weak signal detection.A circular zone counting(CZC) method is proposed in this paper,by combining the Duffing oscillator's phase trajectory feature and numerical calculation for quickly and accurately identifying state transition and determining the critical value,to realize a highefficiency weak signal detection.Detailed model analysis and method construction of the CZC method are introduced.Numerical experiments into the reliability of the proposed CZC method compared with the maximum Lyapunov exponent(MLE) method are carried out.The CZC method is demonstrated to have better detecting ability than the MLE method,and furthermore it is simpler and clearer in calculation to extend to engineering application.     

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function $K$ (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function $K$ for plane Couette flow is derived. The result indicates that $K$ reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high $\operatorname{Re}$ for given same level of normalized perturbation in the domain. The critical value of $K$ at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of $K$ at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case).     

Instability of Waves in Magnetized Vortex-Like Ion Distribution Dusty Plasmas

A modified Zakharov-Kuznetsov equation for small but finite dustic waces in a magnetized votex-like ion distribution dusty plasma is obtained in this paper.It seems that there are instability for a soliton under higher-order transverse perturbations in this system. There is a certain critical value 4λ0. If the ratio of the wave length of the higher-order perturbations to the width of the soliton is larger than this critical value, the solitary wave is unstable, otherwise it is stable.