Mode Coupling Effects Research Articles

Phonon localization transition in relaxor ferroelectric PZN-5%PT

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn1/3 Nb2/3)O3]–0.05PbTiO3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increase in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. We also model the effects of damped mode coupling on the observed phonons and phonon localization structure.

Active Vibration Suppression of a Motor-Driven Piezoelectric Smart Structure Using Adaptive Fuzzy Sliding Mode Control and Repetitive Control

In this paper, we report on the use of piezoelectric sensors and actuators for the active suppression of vibrations associated with the motor-driven rotation of thin flexible plate held vertically. Motor-driven flexible structures are multi-input multi-output systems. The design of active vibration-suppression controllers for these systems is far more challenging than for flexible structures with a fixed end, due to the effects of coupling and nonlinear vibration behavior generated in structures with poor damping. To simplify the design of the controller and achieve satisfactory vibration suppression, we treated the coupling of vibrations caused by the rotary motion of the thin flexible plate as external disturbances and system uncertainties. We employed an adaptive fuzzy sliding mode control algorithm in the design of a single-input–single-output controller for the suppression of vibrations using piezoelectric sensors and actuators. We also used a repetitive control system to reduce periodic vibrations associated with the repetitive motions induced by the motor. Experimental results demonstrate that the hybrid intelligent control approach proposed in this study can suppress complex vibrations caused by modal excitation, coupling effects, and periodic external disturbances.

Features of the energy structure of acoustic fields in the ocean with two-dimensional random inhomogeneities

The paper is devoted to the analytic study and numerical simulation of mid-frequency acoustic signal propagation in a two-dimensional inhomogeneous random shallow-water medium. The study was carried out by the cross section method (local modes). We present original theoretical estimates for the behavior of the average acoustic field intensity and show that at different distances, the features of propagation loss behavior are determined by the intensity of fluctuations and their horizontal scale and depend on the initial regular parameters, such as the emission frequency and size of sound losses in the bottom. We establish analytically that for the considered waveguide and sound frequency parameters, mode coupling effect has a local character and weakly influences the statistics. We establish that the specific form of the spatial spectrum of sound velocity inhomogeneities for the statistical patterns of the field intensity is insignificant during observations in the range of shallow-water distances of practical interest.

Dynamics and Stability Prediction of Five-Axis Flat-End Milling

The tool orientation of a flat-end cutter, determined by the lead and tilt angles of the cutter, can be optimized to increase the machining strip width. However, few studies focus on the effects of tool orientation on the five-axis milling process stability with flat-end cutters. Stability prediction starts with cutting force prediction, and the cutting force prediction is affected by the cutter-workpiece engagement (CWE). The engagement geometries occur between the flat-end cutter and the in-process workpiece (IPW) are complicated in five-axis milling, making the stability analysis for five-axis flat-end milling difficult. The robust discrete vector method (DVM) is adopted to identify the CWE for flat-end millings, and it can be extended to apply to general cutter millings. The milling system is then modeled as a two-degrees-of-freedom spring-mass-damper system with the predicted cutting forces. Thereafter, a general formulation for the dynamic milling system is developed considering the regenerative effect and the mode coupling effect simultaneously. Finally, an enhanced numerical integration method (NIM) is developed to predict the stability limits in flat-end milling with different tool orientations. Effectiveness of the strategy is validated by conducting experiments on five-axis flat-end milling.

A review of the most important failure, reliability and nonlinearity aspects in the development of microelectromechanical systems (MEMS)

PurposeThe purpose of this paper is to provide an overview of the major reliability issues of microelectromechanical systems (MEMS) under mechanical and environmental loading conditions. Furthermore, a comprehensive study on the nonlinear behavior of silicon MEMS devices is presented and different aspects of this phenomenon are discussed.Design/methodology/approachRegarding the reliability investigations, the most important failure aspects affecting the proper operation of the MEMS components with focus on those caused by environmental and mechanical loads are reviewed. These studies include failures due to fatigue loads, mechanical vibration, mechanical shock, humidity, temperature and particulate contamination. In addition, the influence of squeeze film air damping on the dynamic response of MEMS devices is briefly discussed. A further subject of this paper is discussion of studies on the nonlinearity of silicon MEMS. For this purpose, after a description of the basic principles of nonlinearity, the consequences of nonlinear phenomena such as frequency shift, hysteresis and harmonic generation and their effects on the device performance are reviewed. Special attention is paid to the mode coupling effect between the resonant modes as a result of energy transfer because of the nonlinearity of silicon. For a better understanding of these effects, the nonlinear behavior of silicon is demonstrated by using the example of Si cantilever beams.FindingsIt is shown that environmental and mechanical loads can influence on proper operation of the MEMS components and lead to early fracture. In addition, it is demonstrated that nonlinearity modifies dynamic response and leads to new phenomena such as frequency shift and mode coupling. Finally, some ideas are given as possible future areas of research works.Originality/valueThis is a review paper and aimed to review the latest manuscripts published in the field of reliability and nonlinearity of the MEMS structures.

Anti-Unwinding Attitude Control with Fixed-Time Convergence for a Flexible Spacecraft

This paper investigates the fixed-time attitude tracking control problem for flexible spacecraft with unknown bounded disturbances. First, with the knowledge of norm upper bounds of external disturbances and the coupling effect of flexible modes, a novel robust fixed-time controller is designed to deal with this problem. Second, the controller is further enhanced by an adaptive law to avoid the knowledge of norm upper bounds of external disturbances and coupling effect of flexible modes. This control law guarantees the convergence of attitude tracking errors in fixed time where the settling time is bounded by a constant independent of initial conditions. Moreover, the proposed controllers can prevent the unwinding phenomenon. Simulation results are presented to demonstrate the performance of the proposed control scheme.

Channel impulse response equalization scheme based on particle swarm optimization algorithm in mode division multiplexing

Mode division multiplexing (MDM) technique has been introduced as a promising solution to the rapid increase of data traffic. However, although MDM has the potential to increase transmission capacity and significantly reduce the cost and complexity of parallel systems, it also has its challenges. Along the optical fibre link, the deficient characteristics always exist. These characteristics, damage the orthogonality of the modes and lead to mode coupling, causing Inter-symbol interference (SI) which limit the capacity of MDM system. In order to mitigate the effects of mode coupling, an adaptive equalization scheme based on particle swarm optimization (PSO) algorithm has been proposed. Compared to other traditional algorithms that have been used in the equalization process on the MDM system such as least mean square (LMS) and recursive least squares (RLS) algorithms, simulation results demonstrate that the PSO algorithm has flexibility and higher convergence speed for mitigating the effects of nonlinear mode coupling.

Ocean surface wave effect on the spatial characteristics of ambient noise

The ocean ambient noise field experiences a stochastic process of many such noise sources and the respective interactions of their wave fields with the waveguide boundaries. At frequencies of about 1 kHz and higher, forward scattering from surface wave can strongly affect shallow water sound propagation. However, most of the available ambient forecasting models do not consider the effects of multiple forward scattering from surface wave. Therefore, there is a need for an accurate method of predicting ambient noises at middle and high-frequency which can account for surface scatterings. Aiming at such a requirement, a propagation model based on transport theory method is described which yields the second-order moment of the acoustic field. Monte Carlo simulations of acoustic propagation loss are employed to validate the transport theory method. The mode number dependence of mode coupling phenomenon is demonstrated at 1000 Hz via the competing effects of mode coupling and attenuation ranges. Low and middle propagating modes are seen to have a smaller coupling range than the attenuation range, allowing mode coupling effects to take precedence over attenuation effects. The mode energies and the coherences are also examined, and it is found that the mode coupling rate for surface wave is significant, but strongly dependent on mode number. Mode phase randomization by surface waves is found to be dominated by coupling effects. On the basis of transport theory propagation model, connecting with the properties of ambient noise sources, a spatial characteristic model for ambient noise under surface wave is presented. Further, the effects of surface wave on ambient noise intensity, vertical correlation and vertical directionality are analyzed. Simulation results show that the surface wave may result in energy transfer from medium modes to low modes and high modes, the rate of energy transfer depends on the mode energy difference. Since the medium mode plays an important role in noise intensity, the noise intensity decreases with the increase of surface wave. In addition to noise intensity, the vertical correlation of ambient noise also decreases due to mode phase randomization by surface wave. Besides, mode coupling can also lead to a change of vertical beam intensity distribution, positive high-angle beams associated with direct, surface, and bottom-surface-bounced rays become weaker, while negative high-angle beams associated with bottom bounced rays become stronger. Since the vertical directionality is sensitive to surface wave, the model can be applied to ocean surface parameter inversion. In summary, the model provided in this paper is closer to actual ocean waveguide and has future prospect in ocean acoustic engineering application.

Nonlinear vibration energy harvesting with adjustable stiffness, damping and inertia

A novel nonlinear structure with adjustable stiffness, damping and inertia is proposed and studied for vibration energy harvesting. The system consists of an adjustable-inertia system and X-shaped supporting structures. The novelty of the adjustable-inertia design is to enhance the mode coupling property between two orthogonal motion directions, i.e., the translational and rotational directions, which is very helpful for the improvement of the vibration energy harvesting performance. Weakly nonlinear stiffness and damping characteristics can be introduced by the X-shaped supporting structures. Combining the mode coupling effect above and the nonlinear stiffness and damping characteristics of the X-shaped structures, the vibration energy harvesting performance can be significantly enhanced, in both the low frequency range and broadband spectrum. The proposed 2-DOF nonlinear vibration energy harvesting structure can outperform the corresponding 2-DOF linear system and the existing nonlinear harvesting systems. The results in this study provide a novel and effective method for passive structure design of vibration energy harvesting systems to improve efficiency in the low frequency range.

Effect of the cutter path and the workpiece clamping position on the stability of the robotic milling system

The milling robots have many advantages compared with the traditional numerical control machine, but the chatter phenomenon in the robotic milling obstructs its popularization. Therefore, the chatter problem in the planar milling by robot is discussed in the way of cutter path and clamping position to give directions for chatter suppression and quality improvement. Firstly, the dynamic behaviour of the robotic milling system is studied by the models of the chatter stability domain analysis, which include robot stiffness model and dynamic milling force model. The mode coupling effect is considered in chatter stability lobe diagram of robotic milling, which is different with the machine tool. Then, the robotic milling experiments are conducted at six different clamping positions relative to robot by two ways of cutter path. Based on the obtained modal parameters under the different milling conditions, the corresponding chatter stability lobe diagrams are plotted. By the comparative analysis about the experiment results, it is found that the surface quality of workpiece with the feed motion of milling tool along the Y-axis direction of robot world coordinate system is better than the X-axis direction and the chatter is more likely to occur in the down milling than in the up milling. Furthermore, a proper workpiece clamping position can availably prevent the chatter in the robotic milling. These conclusions can be references for designing a suitable machining scheme in the robotic milling to ensure the application of robot in the metal cutting.

Generalized design method of highly-varied-helix end mills for suppression of regenerative chatter in peripheral milling

Peripheral finishing of hardened steel by milling has recently been introduced to the machining field, but often causes severe chatter vibration due to high specific cutting force and low stiffness of slender end mills. It was shown in the previous papers that highly-varied-helix end mills were effective for the suppression of regenerative chatter, and a low radial immersion was applied to avoid the effect of mode coupling. Especially for the highly-varied-helix end mills, it was proved experimentally that they were effective across a wide range of cutting conditions, making it possible for cutting with a relatively large axial depth of cut to be carried out and thus realizing a high surface generation rate in peripheral milling of hard materials. However, the milling tests were carried out under just one specific condition, i.e., tool diameter 8 [mm], projection length 36 [mm], and workpiece of 60 HRC hardened steel, etc., and the helix angles of the tool were varied large enough to suppress regenerative chatter but with no quantitative discussion for their determination. In this paper, a generalized design method for varied-helix end mills to suppress regenerative chatter is proposed. Namely a design index apalim, in which ap is the distance between 2 adjacent regenerative effect cancellation lines and alim is the asymptotic stability limit in the targeted cutting process, is introduced to design the helix angle difference of a tool, and milling experiments are carried out in several conditions to verify the validity of the proposed index.

Robust adaptive attitude manoeuvre control with finite-time convergence for a flexible spacecraft

This paper investigates a finite-time attitude manoeuvre control problem for a flexible spacecraft subject to bounded external disturbances. A robust discontinuous finite-time controller with terminal sliding mode control is designed to solve this problem provided that the disturbances and the coupling effect of flexible modes are bounded with a known boundary. The controller is further enhanced by an adaptive scheme to deal with the more practical case that the boundary is unknown. The enhanced version is continuous and chattering-free. The results are rigorously proved using the Lyapunov stability theory. The effectiveness and robustness of the proposed controllers are demonstrated by numerical simulation.

Effects of mode coupling between low-mode radiation flux asymmetry and intermediate-mode ablator roughness on ignition capsule implosions

The low-mode shell asymmetry and high-mode hot spot mixing appear to be the main reasons for the performance degradation of the National Ignition Facility (NIF) implosion experiments. The effects of the mode coupling between low-mode P2 radiation flux asymmetry and intermediate-mode L = 24 capsule roughness on the implosion performance of ignition capsule are investigated by two-dimensional radiation hydrodynamic simulations. It is shown that the amplitudes of new modes generated by the mode coupling are in good agreement with the second-order mode coupling equation during the acceleration phase. The later flow field not only shows large areal density P2 asymmetry in the main fuel, but also generates large-amplitude spikes and bubbles. In the deceleration phase, the increasing mode coupling generates more new modes, and the perturbation spectrum on the hot spot boundary is mainly from the strong mode interactions rather than the initial perturbation conditions. The combination of the low-mode and high-mode perturbations breaks up the capsule shell, resulting in a significant reduction of the hot spot temperature and implosion performance.

Analysis of the arriving-angle structure of the forward scattered wave on a vertical array in shallow water.

The arriving-angle structure for the forward scattered wave on a vertical line array is obtained upon a modified scattering model in the Pekeris waveguide. The structure is investigated and interpreted by the array invariant theory combined with target induced modal coupling effect. Compared with that of the direct blast, the arriving-angle structure of the forward scattering wave owns multi-striations as well as an increased vertical array invariant. The forward scattered angle structure is dependent on the target position on the source-receiver line. Simulations indicate a potential separation for the forward scattered wave overwhelmed by the direct blast.

Hydrodynamic flow amplitude correlations in event-by-event fluctuating heavy-ion collisions

The effects of event-by-event fluctuations in the initial geometry of the colliding nuclei are important in the analysis of final flow observables in relativistic heavy-ion collisions. We use hydrodynamic simulations to study the amplitude correlations between different orders of event-by-event fluctuating anisotropic flow harmonics. While the general trends seen in the experimental data are qualitatively reproduced by the model, deviations in detail, in particular for peripheral collisions, point to the need for more elaborate future calculations with a hybrid approach that describes the late hadronic stage of the evolution microscopically. It is demonstrated explicitly that the observed anti-correlation between $v_2$ and $v_3$ is the consequence of approximately linear hydrodynamic response to a similar anti-correlation of the corresponding initial eccentricities $\epsilon_2$ and $\epsilon_3$. For $n{\,>\,}3$, the hydrodynamic correlations between $v_{2,3}$ and $v_n$ deviate from the rescaled correlations among the corresponding initial eccentricities, demonstrating nonlinear mode coupling effect in higher order flows.

Dominant mode selection and modal coupling analysis for wind-induced response of long-span roofs

Mode selection and modal coupling analysis are important to estimate wind-induced structural response of long-span roof structures. This article presents a framework for predicting wind-induced structural response of long-span roof structures based on modal analysis. This framework first identifies the dominant modes according to the correlation between the mode shape and the wind load spatial distribution on the structure as well as a proposed “modal participation coefficient.” Second, the concept of modal strain energy is introduced and a modal coupling coefficient is defined, based on which the dominant coupling modes are determined. A modified square root of the sum of the squares methodology is then developed to account for the modal coupling effects of the background and the resonant response components. The total responses can be obtained by combining the contributions of the dominant coupling modes and the square root of the sum of the squares results from the dominant modes. This avoids the use of the computation expensive complete quadratic combination method. Finally, an illustrative example of wind-induced response analysis of the China National Stadium roof structure is provided to demonstrate the effectiveness of the proposed framework.

Mode-coupling effects in anisotropic flow in heavy-ion collisions

Higher-order anisotropic flows in heavy-ion collisions are affected by nonlinear mode coupling effects. It has been suggested that the associated nonlinear hydrodynamic response coefficients probe the transport properties and are largely insensitive to the spectrum of initial density fluctuations of the medium created in these collisions. To test this suggestion, we explore nonlinear mode coupling effects in event-by-event viscous fluid dynamics, using two different models for the fluctuating initial density profiles, and compare the nonlinear coupling coefficients between the initial eccentricity vectors before hydrodynamic expansion and the final flow vectors after the expansion. For several mode coupling coefficients we find significant sensitivity to the initial fluctuation spectrum. They all exhibit strong sensitivity to the specific shear viscosity at freeze-out, but only weak dependence on the shear viscosity during hydrodynamic evolution.

Modal testing of epoxy carbon–aramid fiber hybrid composites reinforced with silica nanoparticles

In the current paper, the modal characterisation of aramid–carbon fiber hybrid composites (ACFRP) and ACFRP reinforced with silica nanoparticles (nACFRP) is investigated through the analytical-experimental transfer function method. The modal properties, such as resonant frequencies and modal loss factors, are measured by vibrating cantilever beam specimens with an impact hammer, while the vibratory response is detected through an acceleration transducer. The procedure for the identification of analytical-experimental transfer functions is carried out using a genetic algorithm by minimising the difference between the measured response from tests and the calculated response, which is a function of the modal parameters. The analytical transfer functions provide a substructuring process to identify modes, as a function of damped natural frequencies and loss factors of a complex structure, and it is insensitive to experimental noise as well as the modal coupling effect. The validation of the proposed method is verified with 10 degrees of freedom mass-spring dashpot model. The effectiveness of the proposed method is demonstrated by investigating the static and dynamic behaviour of the ACFRP and nACFRP specimens. Results indicate that the inclusion of nanosilica particles increase the stiffness of the ACFRP, although the damping response of the reinforced specimens is moderately improved.

Modal testing of nanocomposite materials through an optimization algorithm

An efficient identification method for modal testing of viscoelastic composite materials is demonstrated in this paper, through the analytical–experimental transfer function method. The procedure for the identification of analytical–experimental transfer functions is carried out using a genetic algorithm (GA) by minimizing the difference between the measured response from tests and the calculated response, which is a function of the modal parameters. The analytical transfer functions provide a sub-structuring process to identify modes, as a function of damped natural frequencies and loss factors of a complex structure and it is insensitive to experimental noise as well as the modal coupling effect. The proposed method is verified with the calculation of the elastic modulus and modal properties of a cantilever steel beam with the FEM and compared with the identification results of the proposed algorithm. The effectiveness of the proposed method is demonstrated by investigating the static and dynamic behavior of epoxy cantilever beam specimens reinforced with silica nanoparticles. Analytical–experimental transfer functions accurately identified the viscoelastic and dynamic response of the studied specimens, while the results indicated that the inclusion of nanosilica particles increased the stiffness of the epoxy network and the damping response of the reinforced specimens is improved.

Multilevel finite element modeling and coupling vibration analysis of the drum in blade–disk–drum assembly

Blade–disk–drum (BDD) assemblies are commonly used in rotors of aeroengine. This paper investigates the vibration characteristics of the drum in the BDD assembly. In the analysis, multilevel modeling method based on ANSYS software is proposed. Free vibrations of single drum, disk-drum assembly, and the BDD assembly are calculated and carefully examined to discover the special modal shapes and coupling effects. Then according to the working environment, equivalent aerodynamic loads are imposed on blades and the drum respectively, to explore the vibration behaviors of the drum in the BDD assembly. Results reveal that, vibration of the drum, which may result in rub-impact or/and fatigue failure, can be induced by coupling vibration result from the aerodynamic loads on blades and drum, as well as the drum-dominant mode triggered by the aerodynamic loads on drum.