V2控制Buck变换器分岔与混沌行为的机理及镇定

V²控制的Buck变换器在反馈放大系数变化的情况下表现出丰富的非线性行为. 本文建立了V²控制Buck变换器的离散迭代模型, 利用单值矩阵方法研究了系统不稳定行为. 随着反馈放大系数的增大, 变换器从稳定的周期一状态发生一系列的倍周期分岔现象进入周期二、周期四, 不断倍化直至混沌态. 同时其单值矩阵的最大特征值也沿着实负轴穿越单位圆, 从而从稳定性的角度揭示了系统发生一系列倍周期分岔的机理. 基于单值矩阵理论, 利用正弦电压补偿方法镇定了系统的分岔和混沌行为, 得到了镇定后系统的稳定边界. 仿真和实验结果证明了本文分析方法和结论的正确性. 关键词： V²控制')" href="#">V²控制 Buck变换器 分岔 镇定控制     

振动筛系统的两类余维三分岔与非常规混沌演化

建立了振动筛系统的动力学模型,推导出了其周期运动的Poincaré 映射,基于Poincaré 映射方法着重研究了系统Flip-Hopf-Hopf余维三分岔、三次强共振条件下的Hopf-Hopf余维三分岔以及三种非常规的混沌演化过程.研究结果表明,此两类余维三分岔点附近的动力学行为变得更加复杂和新颖,在分岔点附近出现了三角形吸引子、3T²环面分岔以及“五角星型”、“轮胎型”概周期吸引子,揭示了环面爆破、环面倍化以及T²环面分岔向混沌演化的过程,这些结果对于振动筛系统的动力学优化设计提供了理论参考. 关键词： 余维三分岔 非常规混沌演化 T²环面分岔')" href="#">T²环面分岔     

线性延时反馈Josephson结的Hopf分岔和混沌化

考虑线性延时反馈控制下电阻-电容分路的Josephson结,运用非线性动力学理论分析了受控系统平凡解的稳定性.理论分析表明,随着控制参数的改变,系统的稳定平凡解将会通过Hopf分岔失稳,并推导了发生Hopf分岔的临界参数条件.对不同参数条件下受控系统的动力学进行了数值分析.结果显示,系统由Hopf分岔产生的稳定周期解,将进一步通过对称破缺分岔和倍周期分岔通向混沌. 关键词： 约瑟夫森结 线性延时反馈 Hopf分岔 混沌     

开关变换器倍周期分岔精细层次结构及其普适常数研究

讨论了开关变换器倍周期分岔级联的精细层次结构和标度不变性;数值仿真得到一维、二维开关变换器倍周期分岔及混沌带合并序列的普适常数;系统谱结构可以进一步判断解的性态,显示解的结构,同时开关变换器混沌层次结构的尺度差别也反映在谱结构的精细特征上;最后实验研究了开关变换器倍周期分岔通向混沌的过程,验证了数值仿真的结论. 关键词： 开关变换器 倍周期分岔 普适常数 标度不变性     

频率扫描法研究倍周期分岔与混沌现象

在一类非线性系统中,应用频率控制方法,对倍周期分岔与混沌行为进行了研究。在V₀-ω外控参数平面上,频率扫描显示了分岔与混沌的整体结构:正的和逆的倍周期分岔序列的对称性;分岔收敛于一点的封闭性。本文中所建议的方法,将是一种研究分岔与混沌现象有效而快速的手段。它不仅能定量测量收敛比δ和标度因子α,分段展开还能定性地观察阵发混沌和嵌套在混沌带中的各种窗口等。分岔与混沌是一类非线性系统的频率响应。 关键词：     

可兴奋性细胞混沌放电区间的识别机理

在神经起步点记录到加周期分岔过程的生理实验数据，在对此分岔过程中位于周期n爆发 和周期(n+1)爆发之间的混沌的峰峰间期数据检测不稳定的周期轨道时，发现从靠近周期 n爆发的混沌的峰峰间期数据中，可以检测出不稳定的周期n轨道；而从靠近周期(n+1)爆 发的混沌的峰峰间期数据中，不仅可以检测出不稳定的周期(n+1)轨道，还可以检测出不稳 定的周期n轨道.针对该现象，借助于Sherman建议的胰腺β细胞模型，从非线性动力 学角度给出了理论解释.指明了由鞍结分岔和倍周期分岔分别产生第一类阵发和第三类阵发 为出现该 关键词： 峰峰间期 不稳定的周期轨道 鞍结分岔 倍周期分岔     

分段线性电路切换系统的复杂行为及非光滑分岔机理

分析了分段线性电路系统在周期切换下的复杂动力学行为及其产生的机理. 基于平衡点分析, 给出了两子系统Fold分岔和Hopf分岔条件. 考虑了在不同稳定态时两子系统周期切换的分岔特性, 产生了不同的周期振荡, 并揭示了其产生的机理. 在不同的周期振荡中, 切换点的数量随参数变化产生倍化, 导致切换系统由倍周期分岔进入混沌. 关键词： 分段线性电路 切换系统 非光滑分岔     

周期脉冲作用下Logistic映射的复杂动力学行为及其分岔分析

研究了两种周期脉冲作用下Logistic映射的复杂动力学行为. 随着参数的变化, 该系统产生平衡解、周期解、混沌等现象, 且该系统可经级联倍周期分岔到达混沌. 通过构造Poincaré 映射, 对周期脉冲作用下Logistic映射进行了分岔分析. 最后基于Floquet理论揭示了该系统周期解的分岔机理. 关键词： Logistic映射 脉冲 周期解 分岔机理     

耦合电路中的复杂振荡行为分析

讨论了两个非线性电路适当连接后的耦合系统随耦合强度变化的演化过程.给出了两子系统各自的分岔行为及通向混沌的过程,指出原子系统均为周期运动时,耦合系统依然会由倍周期分岔进入混沌,同时在混沌区域中存在有周期急剧增加及周期增加分岔等现象.而当周期运动和混沌振荡相互作用时,在弱耦合条件下,受混沌子系统的影响,原周期子系统会在其原先的轨道邻域内作微幅振荡,其振荡幅值随耦合强度的增加而增大,混沌的特征越加明显,相反,周期子系统不仅可以导致混沌子系统的失稳,也会引起混沌吸引子结构的变化. 关键词： 非线性电路 耦合强度 分岔 混沌     

Josephson结的动力学行为(Ⅰ)

本文对射频电流驱动下的包含干涉项电流的Josephson结方程进行了大量的数值研究工作。我们发现,对应于不同的参数范围,分别出现混沌行为、倍周期分岔序列、混沌带的反序列以及阵发混沌现象。我们也计算了2ⁿP序列的收敛因子δ_n和功率谱中2^k与2^k+1分频的平均峰高之比Φ(k)/Φ(k+1)。另外,还研究了周期解的对称性以及它与通向混沌途径的关系。 关键词：     

Magnetism,spin relaxation and hyperfine interactions of rare earths in RM6Al6(M = Cr,Mn, Cu)

The systems RM₆Al₆ (R = rare earth or Y, M = Cr, Mn, Cu, Rh) were studied by magnetization measurements and by Mossbauer spectroscopy of ¹⁵⁵Gd, ¹⁶¹Dy, ¹⁶⁶Er and ¹⁷⁰Yb. The magnetization studies show weak R-R antiferromagnetic exchange interactions in RCu₆Al₆(T_n)Gd) = 21 K, less than 4 K for all other R and strong crystalline field effects. Similar phenomena are observed in RMn₆Al₆ and RCr₆Al₆, however, due to the presence of a Mn or Cr local moment the systems order ferrimagnetically. In RCr₆Al₆the order temperatures are low T_c ～ 25 K, yet T_c(GdCr₆Al₆) = 170 K. The Mossbauer studies observations are consistent with the magnetiza results. In the case where Er and Yb are not ordered at 4.1 K, the spectra still show magnetic hyperfine structure however of paramagnetic nature. The spectra yield the hyperfine interaction spin Hamiltonian parameters and the spin relaxation rates. These turn out to be extremely slow (1O⁸–1O⁹ sec^?1, a very uncommon phenomenon for a concentrated Er or Yb metallic system.     

Numerical study of the directed polymer in a 1 + 3 dimensional random medium

The directed polymer in a 1+3 dimensional random medium is known to present a disorder-induced phase transition. For a polymer of length L, the high temperature phase is characterized by a diffusive behavior for the end-point displacement R² ∼L and by free-energy fluctuations of order ΔF(L) ∼O(1). The low-temperature phase is characterized by an anomalous wandering exponent R²/L ∼L^ω and by free-energy fluctuations of order ΔF(L) ∼L^ω where ω∼0.18. In this paper, we first study the scaling behavior of various properties to localize the critical temperature T_c. Our results concerning R²/L and ΔF(L) point towards 0.76 < T_c ≤T₂=0.79, so our conclusion is that T_c is equal or very close to the upper bound T₂ derived by Derrida and coworkers (T₂ corresponds to the temperature above which the ratio remains finite as L ↦ ∞). We then present histograms for the free-energy, energy and entropy over disorder samples. For T ≫T_c, the free-energy distribution is found to be Gaussian. For T ≪T_c, the free-energy distribution coincides with the ground state energy distribution, in agreement with the zero-temperature fixed point picture. Moreover the entropy fluctuations are of order ΔS ∼L^1/2 and follow a Gaussian distribution, in agreement with the droplet predictions, where the free-energy term ΔF ∼L^ω is a near cancellation of energy and entropy contributions of order L^1/2.     

Temperature characteristics of heat exchange due to thermal conductivity of three hollow cylinders in their contact planes

For the case where a system of three hollow cylinders, having contact along their ends, are heated the problem on the determination of the thermal characteristics in the contact planes is solved analytically. Numerical calculations and the construction of plots of the temperature variation, thermal fluxes, and the heating and stabilization times in the contact planes are carried out on the basis of the formulas so obtained.Notation _j = (T_j–T₀)/(T₀–T_c) dimensionless temperature - T₀ initial temperature - T_c temperature of the medium - T_j temperature at an arbitrary point of the j-th cylinder - Fo_j a_jt/R ₁ ² - K_a a₂/a₁ - d R₂/R₁ - b R₁/l - _j, a_j thermal and thermiometric conductivities, respectively - R₂, R₁ respectively, the outer and inner radii of the cylinders - t time - X, Z dimensionless cylindrical coordinates Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 80–84, July, 1972.     

2T1 pumping of the R lines in ruby

We compare the effects of excitations in the ²T₁ lines and ⁴T₂ broad band in ruby. The ratios of R₂ and R₁ emissions are compared before and during bottleneck. The results lead to an approximately equal feeding ratio when the excitation takes place in the ²T₁ levels although it is strongly different for the ⁴T₂ excitation. The transition time from the ²T₁ level to the ²E level is estimated to be 2.5 ns.     

Antiferroelectric and ferroelectric phase transitions of the displacive and order-disorder type in PbZrO3 and PbZr1-xTi x O3 single crystals

For PbZrO₃ and PbZr_0.99Ti_0.01O₃ single crystals the first order Raman light scattering spectra in the paraelectric phase have been analysed. In lead zirconate two kinds of crystals, with and without a transient phase, were investigated. It was found that temperature dependence of the defect-induced soft mode frequency in the paraelectric phase has a negligible contribution to the low-frequency dielectric response. From the point of view of strong dielectric relaxation in the range of 10⁶–10⁹ Hz, which originates in a disorder of the lead sites, crossover of the order-disorder and displacive type of antiferroelectric phase transition in lead zirconate is considered: the phase transition mechanism is neither purely displacive nor order-disorder. Contribution of physical phenomena responsible for the ε(ω,T) dielectric response at low frequencies of an external electric field are described.     

Critical points and points of the bifurcation of the rotating magnetized newtonian polytropes with a 1 ⩽ n ⩽ 1.6 index

The presence of critical points and the bifurcation points of rotating Newtonian polytropes with an index of 1 ? n ? 1.6 has been shown for the first time in this paper. The symbolic-numerical calculation error in metric L ₂ has reached the size of the 10^?4 order. The approximate analytical solution to the problem to the abovementioned accuracy has been set forth. A critical value of the polytropes index n = n _k = 1.51025 has been calculated which is the highest among the critical points and bifurcation points. The value n _k corresponds to the infinitely slow polytropes rotation. Furthermore, in this paper the presence of the period jump at the bifurcation point T _b has been predicted and the relative value of this jump ΔT _b /T _b ～ B _0in ^4/3 has been estimated.     

EFFICIENCY OF β-NUCLEATING AGENTS IN PROPYLENE/α-OLEFIN COPOLYMERS

Crystallization and melting behavior of β-nucleated propylene/1-pentene random copolymers were studied. Samples with different comonomer content were nucleated with the calcium salt of suberic acid. The WAXS, differential scanning calorimetry (DSC), and polarized light microscopy (PLM) measurements showed that morphological behavior of the β-nucleated copolymers is similar to that of the homopolymer, but the rate of crystallization and the characteristic temperatures decrease with increasing pentene content. Crystallization at low temperatures and lower chain regularity lead to the formation of imperfect crystal structures with high instability, which enables the study of structural perfection processes like ββ′- or βα-recrystallization during partial melting. The PLM studies proved that the critical crossover temperature T(βα) decreases with increasing pentene content. The effect of comonomer content on the critical recooling temperature ( T _R ^* ), as well as on the equilibrium melting point of the β-form ( T _m ⁰) was also investigated. However, in our case, the linear Hoffman–Weeks method proved to be unsuitable for the accurate determination of T _m ⁰.     

Hall effect of a linear-chain metal: NbSe3

We present measurements of the low-field Hall constant R_H as a function of field and temperature in the linear-chain metal NbSe₃, The data range from 2 K to 200 K in temperature and up to 15 kG in field. At the two phase transitions T₁ (142 K) and T₂ (58 K) R_H shows an abrupt increase in magnitude but no change in sign. Below 30 K R_H becomes strongly field dependent. The zero-field-limit R_H changes from n-type to p-type at 15 K. It is argued that the results are consistent with a two-band model in which the difference in population p - n is equal to 3 × 10¹⁸ cm^-3 below T₂. The hall data are also consistent with a model in which both a loss of carriers and a drastic change in the conductivity anisotropy occur at T₁ and T₂ and lend support to the charge-density-wave hypothesis which has been used to characterize the two transitions.     

The relaxivity of Gd-EOB-DTPA and Gd-DTPA in liver and kidney of the Wistar rat

The NMR relaxivities of Gd-EOB-DTPA and Gd-DTPA were determined in the kidney and liver of intact male Wistar rats immediately following sacrifice and in vitro in solutions and gels, at 1.5 T using a clinical MR scanner. T₁ and T₂ values of tissue samples were derived from spin-echo image sequences. Tissue gadolinium concentrations were determined by radioassay of Gd¹⁵³. Gd-EOB-DTPA T₁ and T₂ relaxivities, R₁ and R₂ (s⁻¹ mmole⁻¹ kg), were found to be 10.7 ± 0.5 and 22.5 ± 3.2, respectively, for liver, 2.4 ± 0.2 and 12.1 ± 1.7 for kidney cortex, 2.7 ± 0.2 and 14.5 ± 1.9 for kidney outer medulla, 2.0 ± 0.2 and 11.4 ± 2.1 for kidney inner medulla. Gd-DTPA R₁ and R₂ were found to be 4.8 ± 0.4 and 14.5 ± 3.7 for liver, 1.2 ± 0.1 and 7.9 ± 0.8 for kidney cortex, 1.6 ± 0.1 and 10.2 ± 1.4 for kidney outer medulla, 1.3 ± 0.1 and 10.2 ± 1.2 for kidney inner medulla. Gd-EOB-DTPA and Gd-DTPA R₁ was increased in liver compared to agarose gels at 38°C (4.49 ± 0.03 and 3.47 ± 0.06), but reduced in kidney tissues. All R₂ were elevated compared to agarose gels at 38°C (5.72 ± 0.12 and 4.12 ± 0.03). Elevated R₂ and R₁ (expressed in terms of the concentration of gadolinium per kg of tissue) can be accounted for in part by the lower water content of tissues compared with gels or solutions, increased microviscosity and binding to macromolecules. In addition, susceptibility effects may give rise to further increases in R₂. By contract, the reduced R₁ observed in kidney may be the result of compartmentalization of the magnetopharmaceuticals. Statistically improved fits were obtained for T₁ recovery curves for liver in the presence of Gd-EOB-DTPA when a dual exponential model was used. Assuming in vitro values for the relaxivities of these artificial contrast agents will lead to inaccuracies when relating observed signal enhancement factors to tissue gadolinium concentration.