On the functional form of a nonlinear vibration absorber

Due to the frequency-energy dependence of nonlinear oscillations, nonlinear dynamical absorbers present interesting properties for mitigating unwanted vibrations in mechanical systems. Unlike the tuned mass damper, the functional form of a nonlinear absorber is not known a priori and must be determined. This short note addresses this issue when a light-weight nonlinear absorber is attached to a nonlinear primary structure. Numerical simulations demonstrate that the determination of an adequate functional form may be directly linked to the frequency-energy dependence of the primary structure.     

基于链码的减震器示功图故障检测算法

减震器示功图是测量减震器外特性的重要手段,快速准确的识别减震器故障类型对降低减震器的次品率和改善减震器的设计缺陷具有重要意义。为此,提出了基于链码的多特征模式匹配算法对故障示功图进行分类。首先,对示功图进行预处理并记录最大拉伸阻尼力和最大压缩阻尼力的值,对预处理后的示功图进行十字分割,分别求出分割后的示功图各部分的链码序列,然后求出其对称性、链码长度、面积这三个特征值,最后根据编码原则对特征参数进行编码,将其编码后的值与类型库的编码进行匹配,从而得出样本的故障类型。实验结果表明,该算法能够正确识别示功图的13类基本故障类型。     

Experimental study of targeted energy transfer from an acoustic system to a nonlinear membrane absorber

This paper deals with the application of the concept of targeted energy transfer to the field of acoustics, providing a new approach to passive sound control in the low frequency domain, where no efficient dissipative mechanism exists. The targeted energy transfer, also called energy pumping, is a phenomenon that we observe by combining a pure nonlinear oscillator with a linear primary system. It corresponds to an almost irreversible transfer of vibration energy from the linear system to the auxiliary nonlinear one, where the energy is finally dissipated. In this study, an experimental set-up has been developed using the air inside a tube as the acoustic linear system, a thin circular visco-elastic membrane as an essentially cubic oscillator and the air inside a box as a weak coupling between those two elements. In this paper, which mainly deals with experimental results, it is shown that several regimes exist under sinusoidal forcing, corresponding to the different nonlinear normal modes of the system. One of these regimes is the quasi-periodic energy pumping regime. The targeted energy transfer phenomenon is also visible on the free oscillations of the system. Indeed, above an initial excitation threshold, the sound extinction in the tube follows a quasi-linear decrease that is much faster than the usual exponential one. During this linear decrease, the energy of the acoustic medium is irreversibly transferred to the membrane and then damped into this element called nonlinear energy sink. We present also the frequency responses of the system which shows a clipping of the original resonance peak of the acoustic medium and we finally demonstrate the ability of the nonlinear absorber to operate in a large frequency band, tuning itself to any linear system.     

Amplitude reduction of parametric resonance by dynamic vibration absorber based on quadratic nonlinear coupling

The paper proposes an amplitude reduction method for parametric resonance with a new type of dynamic vibration absorber utilizing quadratic nonlinear coupling. A main system with asymmetric nonlinear restoring force and harmonic excitation causes parametric resonance in the system. In contrast with autoparametric vibration absorber, the natural frequency of the vibration absorber is tuned to be in the neighborhood of twice that of the main system. For such a vibration absorber, we investigate the effect on the amplitude reduction for a parametrically excited main system. Analytical results using the method of multiple scales show that the amplitude of parametric resonance is reduced by the effect of the vibration absorber. The experimental results by a simple apparatus indicate that the parametric resonance is stabilized by the effects of both vibration absorber and Coulomb friction of the main system. Moreover, numerical results considering the Coulomb friction of the main system show that the amplitude of parametric resonance becomes close to zero by the proposed vibration absorber.     

Using shock waves to improve the sound absorbing efficiency of closed-cell foams

Producing closed-cell foams is generally cheaper and simpler than open-cell foams. However, the acoustic and filtration efficiency of closed-cell foam materials is generally poor because it is very difficult for fluid or acoustic waves to penetrate into the material. A new method using shock waves to remove the membranes closing the cell pores (known as reticulation) and thus to improve the acoustic and filtration behavior of closed-cell foam material is presented. Various shock treatments have been carried out on polyurethane and polyimide foams and the following conclusions were drawn: (1) reticulation efficiency increased and thus the airflow resistivity and tortuosity decreased when increasing the amplitude of the shock treatment; (2) the rigidity of the foam is decreased; (3) the process is reliable and repeatable and (4) obtained acoustic performance is comparable to classical thermal reticulation.     

非线性超声束的冲击波形成研究

通过用频域方法数值求解KZK方程;以活塞声场为例,研究了非线性超声束的冲击波形成规律。结果表明:除了非线性强度、吸收外,衍射是另一个影响非线性超声束冲击波形成的重要因素。文章发现在不同的衍射条件下的活塞声场的冲击波形成距离有很大差异。最后,还比较了不同参数下冲击波形成时的声压波形和冲击波形成的空间分布。     

Use of ion processing to improve the quality of fullerene-coated field emitters

The efficiency of tip field emitters covered by activated fullerene coatings is studied in a wide range of emission currents and residual gas pressures. Main mechanisms behind the influence of the gaseous medium and ion bombardment on the emitter efficiency are determined. The feasibility of improving the homogeneity of the fullerene coating by potassium ion bombardment is demonstrated. From data on the emitter performance in a technical vacuum, a previously unknown effect is discovered: the structure of activated fullerene coatings is reproduced under intense ion bombardment. It is found that intense bombardment by residual gas ions increases a limiting current extracted from fullerene-coated tip field emitters.     

Enhanced optical limiting performance of a nonlinear absorber in a solution containing scattering nanoparticles

The optical limiting performance of a water-soluble porphyrin derivative is shown to be improved when small-sized (400-nm-diameter) nonabsorbing polystyrene spheres are added to the solution. The optical limiting enhancement is attributed to absorption of diffusive photons that experience longer light pathways than ballistic photons. The effect is demonstrated for nanosecond laser excitation at 532 nm.     

On nonlinear stability of general undercompressive viscous shock waves

We study the nonlinear stability of general undercompressive viscous shock waves. Previously, the authors showed stability in a special case when the shock phase shift can be determined a priori from the total mass of the perturbation, using new pointwise methods. By examining time invariants associated with the linearized equations, we can now overcome a new difficulty in the general case, namely, nonlinear movement of the shock. We introduce a coordinate transformation suitable to treat this new aspect, and demonstrate our method by analyzing a model system of generic type. We obtain sharp pointwise bounds andL ^p behavior of the solution for allp, 1p.     

Suppression of the primary resonance vibrations of a forced nonlinear system using a dynamic vibration absorber

In a single degree-of-freedom weakly nonlinear oscillator subjected to periodic external excitation, a small-amplitude excitation may produce a relatively large-amplitude response under primary resonance conditions. Jump and hysteresis phenomena that result from saddle-node bifurcations may occur in the steady-state response of the forced nonlinear oscillator. A simple mass-spring-damper vibration absorber is thus employed to suppress the nonlinear vibrations of the forced nonlinear oscillator for the primary resonance conditions. The values of the spring stiffness and mass of the vibration absorber are significantly lower than their counterpart of the forced nonlinear oscillator. Vibrational energy of the forced nonlinear oscillator is transferred to the attached light mass through linked spring and damper. As a result, the nonlinear vibrations of the forced oscillator are greatly reduced and the vibrations of the absorber are significant. The method of multiple scales is used to obtain the averaged equations that determine the amplitude and phases of the first-order approximate solutions to primary resonance vibrations of the forced nonlinear oscillator. Illustrative examples are given to show the effectiveness of the dynamic vibration absorber for suppressing primary resonance vibrations. The effects of the linked spring and damper and the attached mass on the reduction of nonlinear vibrations are studied with the help of frequency response curves, the attenuation ratio of response amplitude and the desensitisation ratio of the critical amplitude of excitation.     

Use of an anisotropic absorber for simulating electromagnetic scattering by a perfectly conducting wire

A perfectly matched anisotropic absorber (PMAA) is used as a new type of medium in a formalism previously developed for investigating diffraction from an inhomogeneous isotropic aperture in a thick metallic screen. In this paper we consider that the aperture consists of five homogeneous regions: PMAA, transparent dielectric, perfect conductor, transparent dielectric and PMAA. This configuration could allow us to simulate a conducting cylinder of rectangular section in open space and it sets the basis for further extensions of the method to other geometries. We present near and far field simulations for different geometrical and constitutive parameters of the PMAA regions. The results seem to indicate that this new medium could be used to represent a lateral open space, thus enhancing the modal method and making it suitable to simulate scattering problems of finite objects.     

The effect of hydrostatic pressure,temperature and magnetic field on the nonlinear optical properties of asymmetrical Gaussian potential quantum wells

In this study, simultaneous effects of hydrostatic pressure, temperature and magnetic field on the linear and nonlinear intersubband optical absorption coefficients (OACs) and refractive index changes (RICs) in asymmetrical Gaussian potential quantum wells (QWs) are theoretically investigated within the framework of the compact-density-matrix approach and iterative method. The energy eigenvalues and their corresponding eigenfunctions of the system are calculated with the differential method. Our results show that the position and the magnitude of the resonant peaks of the nonlinear OACs and RICs depend strongly on the hydrostatic pressure, temperature and external magnetic field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.     

Behavior of nonlinear fluid viscous dampers for control of shock vibrations

Fluid viscous dampers have been widely used for suppression of high velocity shocks. While linear fluid viscous dampers have been used for a long time, nonlinear fluid viscous dampers show considerable promise due to their superior energy dissipation characteristics and significant reduction in the damper force compared to a linear fluid viscous damper for the same peak displacement. This paper presents results from experimental study to characterize fluid viscous dampers when subjected to half-cycle sine shock excitation. The mathematical formulation and a numerical study to evaluate the relative performance of structures with fluid viscous dampers subjected to short-duration shock (impulse) loading are also discussed. The influence of damper nonlinearity (α) and the supplemental damping ratio (ξ_sd) on response has been investigated. The supplemental damping ratio of nonlinear fluid viscous dampers when subjected to shock excitation is found by equivalent linearization using the concept of equal energy dissipation. The paper also presents some design charts, which can be used for preliminary decisions on parameters of nonlinear dampers to be used in design.     

Counterpropagation of waves with shock fronts in a nonlinear tissue-like medium

A numerical model for describing the counterpropagation of one-dimensional waves in a nonlinear medium with an arbitrary power law absorption and corresponding dispersion is developed. The model is based on general one-dimensional Navier-Stokes equations with absorption in the form of a time-domain convolution operator in the equation of state. The developed algorithm makes it possible to describe wave interactions in the presence of shock fronts in media like biological tissue. Numerical modeling is conducted by the finite difference method on a staggered grid; absorption and sound speed dispersion are taken into account using the causal memory function. The developed model is used for numerical calculations, which demonstrate the absorption and dispersion effects on nonlinear propagation of differently shaped pulses, as well as their reflection from impedance acoustic boundaries.     

Electric field effect on the linear and nonlinear intersubband refractive index changes in asymmetrical semiparabolic and symmetrical parabolic quantum wells

By using the displacement harmonic variant method and the compact density matrix approach, the linear and nonlinear intersubband refractive index changes (RICs) in a semiparabolic quantum well (QW) with applied electric field have been investigated in detail. The simple analytical formulae for the linear and nonlinear RICs in the system were also deduced. The symmetrical parabolic QWs with applied electric fields were taken into account for comparison. Numerical calculations on typical GaAs QWs were performed. The dependence of the linear and nonlinear RICs on the incident optical intensity, the frequencies of the confined potential of the QWs and the strength of the applied electric field were discussed. Results reveal that the RICs in the semiparabolic quantum well system sensitively depend on these factors. The calculation also shows that the semiparabolic QW is a more ideal nonlinear optical system relative to the symmetric parabolic QW systems.