Phenomenology of current-induced dynamics in antiferromagnets

We derive a phenomenological theory of current-induced staggered magnetization dynamics in antiferromagnets. The theory captures the reactive and dissipative current-induced torques and the conventional effects of magnetic fields and damping. A Walker ansatz describes the dc current-induced domain-wall motion when there is no dissipation. If magnetic damping and dissipative torques are included, the Walker ansatz remains robust when the domain wall moves slowly. As in ferromagnets, the domain-wall velocity is proportional to the ratio between the dissipative torque and the magnetization damping. In addition, a current-driven antiferromagnetic domain wall acquires a net magnetic moment.     

Shear and compressional damping effects of constrained layered beams

A theory is formulated for combined shear and compressional damping effects of contrained layered beam structures with complicated cross section areas. The theory is applied to some selected theoretical examples. The calculation results indicate that the loss factor values of these beams are larger over a wider frequency range than could be expected from corresponding shear damping effect only or compressional damping treatment only.     

On the description of noise in quantum systems

A formalism of probability operators which generalizes the notion of density operator is introduced into the theory of noisy quantum systems. The Markov property and the connexion between Heisenberg and Schrödinger picture for systems undergoing an irreversible change are discussed in detail. The probability-operator treatment of noise is related to the Langevin method discussed byLax through a generalized Einstein-relation. The master equation for the quantum mechanical oscillator with linear damping is written down in a Fokker-Planck-type approximation. By means of the Einstein-relation the coefficients in the Fokker-Planck-equation are related to the parameters in the phenomenological equations.     

数字锁相检测在合肥光源逐圈测量系统的应用

从其原理出发,分析了数字锁相检测在逐圈束流位置测量系统中用于束流振荡阻尼率(增长率)计算的可行性,并在Matlab中对其进行了仿真计算。将该方法应用到合肥光源逐圈测量系统中,进行了Beta振荡的增长和阻尼时间的计算,结果显示振荡增长时间约为0.26ms,阻尼时间为1.2ms(反馈系统调试时)。仿真和实验结果都表明,数字锁相检测可以有效用于逐圈测量系统中阻尼率的计算。     

A solvable model of microscopic frequency modulation: II. Rigorous treatment of the damping operator

The solvable model of quantum-statistical mechanical frequency modulation, which was proposed previously by the authors, is solved exactly by rigorous treatment of the damping operator for the cases of boson and two-state irrelevant subsystems to obtain the intensity distribution and the response function for a certain operator of the relevant subsystem. The results are compared with the previous ones which were obtained within the conventional treatment of the damping operator.     

An analysis of the distortion effects of Coulomb damping on the vector plots of lightly damped systems

An analysis of the vector plot responses of lightly damped single degree-of-freedom systems with Coulomb damping has been made. The vector plots, as derived by using both an exact and an approximate method (the method of harmonic balance) are compared and it is shown that the distortion of the normally circular vector locus is due to the Coulomb damping. Although the vector plots of such systems are distorted it is also shown that the frequency gradient criterion is still applicable for location of a natural frequency even when the frictional force levels approach the excitation force levels. To permit estimation of the modal damping of these systems a criterion by means of which the limits of the useful frequency range can be specified is suggested. The criterion, which is based upon the quadrature input power necessary to excite the mode of vibration, is found to be equivalent to that obtained from the half power point theory when applied to linear systems.     

The effect of damping and mass ratio on the stability of a tube bank

Experiments have been conducted in a wind tunnel on a parallel triangular tube array with a pitch ratio of 1·375. The aerodynamic component of damping was determined as a function of flow velocity. This damping was found to increase linearly with flow to about 50% of the stability threshold and then to decrease. Additionally, six different sets of tubes were tested to examine the effect of mass ratio on fluid-elastic stability. The results were compared with similar experiments in which damping alone was varied. It is seen that damping and mass ratio do not appear to be linearly dependent parameters which is contrary to the assumption used commonly in the stability analysis of tube banks. More importantly, however, the stability threshold is substantially less dependent on both damping and mass ratio than indicated by conventional theory.     

位阻效应对受阻酚杂化体系阻尼机理的影响

针对尚未解决的受阻酚结构变化与杂化体系阻尼机理间关系的问题,本文采用分子动力学模拟方法构建了三种受阻程度不同的受阻酚/聚合物杂化体系,从理论上探讨了位阻效应对阻尼机理的影响.对体系氢键相互作用、结合能、相对自由体积及扩散系数进行模拟分析表明,位阻效应对受阻酚分子内氢键相互作用有显著的弱化效果,可减少小分子团聚倾向,有利于小分子与聚合物分子间氢键相互作用的形成.但是,过高的位阻对小分子运动有阻碍作用,不利于小分子与聚合物形成强烈的氢键键合,也即不利于杂化体系阻尼性能的提高.因此,如何选择受阻程度适中的受阻酚是制备高阻尼杂化材料的一关键要素.     

A perturbation technique for gyroscopic systems with small internal and external damping

The response of linear damped gyroscopic systems can be obtained by means of techniques of linear systems theory, which involves the computation of the transition matrix. The response is in terms of complex quantities, which is likely to cause computational difficulties as the order of the system increases. In the absence of damping, it is possible to derive the response of a linear gyroscopic system with relative ease by working with real quantities alone. When damping is small, one can use a perturbation approach to produce the response by regarding the undamped gyroscopic system as the unperturbed system. In a previous paper, a perturbation analysis was used to derive the response of a gyroscopic system with small internal damping. This paper extends the approach to the case of external damping, which is characterized not only by symmetric coefficients multiplying velocities but also by skew symmetric coefficients multiplying displacements, where the latter terms are known as circulatory. A numerical example is presented.     

The modulation instability revisited

The modulational instability (or “Benjamin-Feir instability”) has been a fundamental principle of nonlinear wave propagation in systems without dissipation ever since it was discovered in the 1960s. It is often identified as a mechanism by which energy spreads from one dominant Fourier mode to neighboring modes. In recent work, we have explored how damping affects this instability, both mathematically and experimentally. Mathematically, the modulational instability changes fundamentally in the presence of damping: for waves of small or moderate amplitude, damping (of the right kind) stabilizes the instability. Experimentally, we observe wavetrains of small or moderate amplitude that are stable within the lengths of our wavetanks, and we find that the damped theory predicts the evolution of these wavetrains much more accurately than earlier theories. For waves of larger amplitude, neither the standard (undamped) theory nor the damped theory is accurate, because frequency downshifting affects the evolution in ways that are still poorly understood.     

Identification of the dominant precession-damping mechanism in Fe, Co, and Ni by first-principles calculations

The Landau-Lifshitz equation reliably describes magnetization dynamics using a phenomenological treatment of damping. This Letter presents first-principles calculations of the damping parameters for Fe, Co, and Ni that quantitatively agree with existing ferromagnetic resonance measurements. This agreement establishes the dominant damping mechanism for these systems and takes a significant step toward predicting and tailoring the damping constants of new materials.     

Thermoelastic vibrations in micro-/nano-scale beam resonators with voids

In this paper the closed form expressions for the transverse vibrations of a homogenous isotropic, thermoelastic thin beam with voids, based on Euler-Bernoulli theory have been derived. The effects of voids, relaxation times, thermomechanical coupling, surface conditions and beam dimensions on energy dissipation induced by thermoelastic damping in (micro-electromechanical systems) MEMS/(nano-electromechanical systems) NEMS resonators are investigated for beams under clamped and simply supported conditions. Analytical expressions for deflection, temperature change, frequency shifts and thermoelastic damping in the beam have been derived. Some numerical results with the help of MATLAB programming software in case of magnesium like material have also been presented. The computer simulated results in respect of damping factor and frequency shift have been presented graphically.     

Mn-Cu系高阻尼合金的凝固组织控制及阻尼特性的改善

Mn—Cu系阻尼合金兼有较高的阻尼特性和良好的力学性质，因而具有较大的实用前景。这类合金通常经过铸造，塑性加工和热处理来获得要求的性能和组织。加工后的Mn—Cu合金存在一个特定温度，在此温度以上合金的阻尼性能将会消失，因而影响了合金的在较高温度下的使用。该温度和合金的Mn含量有关，然而提高合金的Mn含量有会降低合金的加工性及力学性能。一般Mn-Cu合金的热处理都是利用400℃附近的时效来获得相分解后的局部高Mn组织。但是目前的时效处理后的Mn—Cu阻尼合金的最高使用温度只在80℃以下。为解决Mn—Cu阻尼合金使用温度的局限性，本研究选用凝固过程控制的方法在铸造组织中来获得较大幅度的Mn含量分布。从而在Mn-Cu合金得到较高的高阻尼特性温度。本工作利用铸型温度控制的方法，将M2052（Mn-20Cu-5Ni-2Fe）合金在250～0．1K／s冷却速度范围内控制凝固。随凝固冷却速度的降低在合金的铸态组织中观察到二次枝晶间距和晶粒尺寸的明显增大。同时还发现缓冷凝固的合金的成分比快冷凝固有较大的分布幅度。铸态下的合金阻尼性能评价也证实了凝固冷却速度对合金的凝固组织有很大的影响。尽管铸态组织的合金的高温阻尼性能并没有很大的改善，然而通过对铸态组织实施时效处理后发现缓冷凝固合金的高温阻尼性能有很大的改善。凝固冷却速度对时效处理后合金的阻尼性能有明显的影响。实验结果表明0．1K／s的冷却速度下缓慢凝固的合金在时效处理后高阻尼特性可持续高达120℃．     

On eigenproblem solution of damped vibrations associated with gyroscopic moments

A new efficient approach is presented for solving the quadratic eigenvalue problem of weakly, nonproportionally damped vibration systems. In the analysis of these systems, gyroscopic moments and external damping are both considered. Traditional restriction of symmetry of inertia, damping and stiffness matrices is slightly relaxed. A second-order perturbation theory is developed such that the perturbed solution is based on the eigensolution of an unperturbed subproblem. This subproblem considers the unperturbed system in two different forms: (i) a conservative, gyroscopic part of an original problem, or (ii) a nonconservative, gyroscopic part of an original problem that is proportionally damped. To cope with asymmetry of the system matrices, a Duncan's like state formulation is used to bring these matrices into a suitable form for perturbations. Two numerical examples are introduced for explaining the detailed implementation of the presented approach. Additionally, a practical problem of rotor supported by two tilting pad-bearings is investigated. The eigensolutions obtained by the current approach match, to a great extent, other solutions obtained by time-consuming exact methods. The investigation procedure given here gives a framework to handle vibration problems of weakly nonproportional damping and/or weakly asymmetric inertia, damping and stiffness matrices.     

FINITE ELEMENT FORMULATION AND ACTIVE VIBRATION CONTROL STUDY ON BEAMS USING SMART CONSTRAINED LAYER DAMPING (SCLD) TREATMENT

This work deals with the active vibration control of beams with smart constrained layer damping (SCLD) treatment. SCLD design consists of viscoelastic shear layer sandwiched between two layers of piezoelectric sensors and actuator. This composite SCLD when bonded to a vibrating structure acts as a smart treatment. The sensor piezoelectric layer measures the vibration response of the structure and a feedback controller is provided which regulates the axial deformation of the piezoelectric actuator (constraining layer), thereby providing adjustable and significant damping in the structure. The damping offered by SCLD treatment has two components, active action and passive action. The active action is transmitted from the piezoelectric actuator to the host structure through the viscoelastic layer. The passive action is through the shear deformation in the viscoelastic layer. The active action apart from providing direct active control also adjusts the passive action by regulating the shear deformation in the structure. The passive damping component of this design eliminates spillover, reduces power consumption, improves robustness and reliability of the system, and reduces vibration response at high-frequency ranges where active damping is difficult to implement. A beam finite element model has been developed based on Timoshenko's beam theory with partially covered SCLD. The Golla-Hughes-McTavish (GHM) method has been used to model the viscoelastic layer. The dissipation co-ordinates, defined using GHM approach, describe the frequency-dependent viscoelastic material properties. Models of PCLD and purely active systems could be obtained as a special case of SCLD. Using linear quadratic regulator (LQR) optimal control, the effects of the SCLD on vibration suppression performance and control effort requirements are investigated. The effects of the viscoelastic layer thickness and material properties on the vibration control performance are investigated.     

Measurement of Chemical Exchange Rate Constants with Solvent Protons Using Radiation Damping

When chemical exchange occurs between a concentrated and a diluted species like the proton exchange between water and proteins in biochemical systems, it is difficult to use the conventional methods for a quantitative dynamical study. In this paper, a new approach which utilizes radiation damping of the strong water magnetization exchanging with the weak resonance of the mobile protons in biomolecules is proposed. It is demonstrated with a dilute guanidine solution, using the simple inversion-recovery pulse sequence, that the exchange rate constants between water and guanidine protons are evaluated by solving the Bloch equations modified with both the radiation damping and the chemical exchange effects.     

核磁共振中的量子控制

近年来, 随着量子信息科学的发展, 对由量子力学原理描述的微观世界的主动调控已成为重要的前沿研究领域. 为构造实际的量子信息处理器, 一个关键的挑战是: 如何对处于噪声环境下的量子体系实现一系列高精度的任意操作, 以完成目标量子信息处理任务. 为此, 人们将经典系统控制论的思想方法延伸到量子体系的领域, 提出了大量的量子控制方法以及相关的数值技术(如量子优化控制、量子反馈控制等), 并取得了丰富的研究成果. 核磁共振自旋体系具备成熟的系统理论和操控技术, 为量子控制方法的实用性研究提供了优秀的实验测试平台. 因此, 基于核磁共振的量子控制成为量子控制领域的重要方向. 本文简要介绍了量子控制的基本概念和方法; 从系统控制论的角度对核磁共振自旋体系的基本原理和重要控制任务做了阐述; 介绍了近些年来在该领域发展的相关控制方法及其应用; 对基于核磁共振体系的量子控制的进一步的研究做了几点展望.     

Swept sine testing of rotor-bearing system for damping estimation

Many types of rotating components commonly operate above the first or second critical speed and they are subjected to run-ups and shutdowns frequently. The present study focuses on developing FRF of rotor bearing systems for damping estimation from swept-sine excitation. The principle of active vibration control states that with increase in angular acceleration, the amplitude of vibration due to unbalance will reduce and the FRF envelope will shift towards the right (or higher frequency). The frequency response function (FRF) estimated by tracking filters or Co-Quad analyzers was proved to induce an error into the FRF estimate. Using Fast Fourier Transform (FFT) algorithm and stationary wavelet transform (SWT) decomposition FRF distortion can be reduced. To obtain a theoretical clarity, the shifting of FRF envelope phenomenon is incorporated into conventional FRF expressions and validation is performed with the FRF estimated using the Fourier Transform approach. The half-power bandwidth method is employed to extract damping ratios from the FRF estimates. While deriving half-power points for both types of responses (acceleration and displacement), damping ratio (ζ) is estimated with different approximations like classical definition (neglecting damping ratio of order higher than 2), third order (neglecting damping ratios with order higher than 4) and exact (no assumptions on damping ratio). The use of stationary wavelet transform to denoise the noise corrupted FRF data is explained. Finally, experiments are performed on a test rotor excited with different sweep rates to estimate the damping ratio.     

Vibrational resonance in Duffing systems with fractional-order damping

The phenomenon of vibrational resonance (VR) is investigated in over- and under-damped Duffing systems with fractional-order damping. It is found that the factional-order damping can induce change in the number of the steady stable states and then lead to single- or double-resonance behavior. Compared with vibrational resonance in the ordinary systems, the following new results are found in the fractional-order systems. (1) In the overdamped system with double-well potential and ordinary damping, there is only one kind of single-resonance, whereas there are double-resonance and two kinds of single-resonance for the case of fractional-order damping. The necessary condition for these new resonance behaviors is the value of the fractional-order satisfies α?>?1. (2) In the overdamped system with single-well potential and ordinary damping, there is no resonance, whereas there is a single-resonance for the case of fractional-order damping. The necessary condition for the new result is α?>?1. (3) In the underdamped system with double-well potential and ordinary damping, there are double-resonance and one kind of single-resonance, whereas there are double-resonance and two kinds of single-resonance for the case of fractional-order damping. The necessary condition for the new single-resonance is α?相似文献