Related Damping Research Articles

An efficient automatic modal identification method based on free vibration response and enhanced Empirical Fourier Decomposition technique

This paper presents an efficient yet practical approach for the automatic modal identification of structures based on their free vibration response. The proposed approach relies on the Empirical Fourier Decomposition (EFD) technique. It implements a new procedure to recognize automatically the number of modal components to be extracted from noisy data in such a way to prevent both mode-mixing and mode-splitting effects. A suitable strategy is adopted to improve the segmentation of the frequency spectrum of the free vibration response, so as to identify accurately the bounds of the frequency spectrum partition corresponding to each modal component. The related modal damping ratios are estimated by means of a robust area-based approach in order to mitigate the noise-induced disturbances whereas a time-domain method based on the phase shift of the free vibration response peaks is employed to identify the mode shapes.The proposed approach is first validated through the analysis of synthetic signals that embed closely spaced components and a lowly excited vibration mode. Finally, the proposed approach is applied to two real bridges. The first case-study deals with the identification of modal frequencies and damping ratios of the cables of a stay-cabled bridge. The second case-study involves the modal identification of a steel railway bridge deck that exhibits two closely spaced vibration modes. The outcomes obtained using the proposed approach based on EFD technique are compared with the results obtained by means of the Variational Mode Decomposition (VMD) technique as well as with those computed through classical operational modal analysis techniques. The consistent estimates produced by means of the proposed approach demonstrate its accuracy and robustness.

Time-harmonic analysis of acoustic pulsation in gas pipeline systems using the Finite Element Transfer Matrix Method: Theoretical aspects

Structural vibration of gas pipeline systems can be strongly affected by the response of the acoustic domain represented by the gas being transported through pressurized pipes, cylinders and other related components. The resultant Acoustically Induced Vibration (AIV) can be a cause of failure, for example, in reciprocating compressor systems, where the discharge and suction pulsations are the main source of excitation. The objective of this work is to explore and enhance the use of the Finite Element Transfer Matrix Method (FETM) for solving time-harmonic acoustic problems related to AIV, whose results can be used to analyze pulsation in piping systems. Four different damping models are explored and implemented (apart from the classical undamped model) resulting in five different acoustic elements for the quiescent fluid case. In addition, two different damping models are analyzed for the case of a fluid with mean flow, and implemented through two flow-acoustic elements. The physical behavior of certain typical junctions is also investigated, represented by acoustic and flow-acoustic length corrections, and an FETM formulation is proposed to account for local non-linear effects in transfer impedance elements. Also, a discussion related to the minimum and maximum FETM element sizes is presented. A critical analysis of the effectiveness of the whole FETM strategy is performed using a variety of representative gas pipeline geometries as application examples with a particular set of boundary conditions for quiescent and moving fluid medium. Nodal pressure frequency plots of cases considering all the presented acoustic and flow-acoustic elements, and related damping models, are presented. Coherent results were achieved using the procedure when frequency plots are compared with experimental data and 3D FEM equivalent models.

Multi-objective optimization design for a magnetorheological damper

As one of the most important components in the semi-active damping system, the performance of MR damper directly determines the damping capacity of the damping system. In order to make the damping system has excellent damping effect, it is necessary to optimize the working performance of the MR damper. Therefore, Non-Dominated Sorting Genetic Algorithm version II (NSGA-II) was applied to optimize the structure of MR dampers in this paper. Firstly, the structural scheme of MR damper was proposed. Secondly, the design principle of MR damper was described, and the magnetic circuit material and MR fluid were selected. Thirdly, taking the maximum dynamic range and the minimum number of turns of electromagnetic coil as the optimization objective, the structure of MR damper was optimized by NSGA-II. The structural parameters of MR damper were determined in the Pareto optimal solution set based on the principle of minimum mass. Finally, through the magnetic simulation and the performance testing of the MR damper, it was verified that the MR damper has reasonable magnetic circuit and excellent performance. And the design results meet the requirements. The proposed optimization method can provide a theoretical basis for the optimal design of related damping devices.

Anomalous changes in the acoustic properties and central peak behaviors of PbxBa1−xNb2O6 single crystals with x = 0.5 studied by Brillouin spectroscopy

The acoustic properties of tetragonal PbxBa1−xNb2O6 (PBN-x) single crystals with x = 0.5 were studied by Brillouin spectroscopy over a wide temperature range from room temperature to 600 °C. Anomalous changes in the Brillouin frequency shift and the half width of acoustic waves were observed at ∼350 °C that corresponds to the ferroelectric–paraelectric phase transition temperature of PBN-0.50. The longitudinal sound velocity and its associated elastic constant (C33) showed substantial softening in the paraelectric phase when the phonon propagated along the polar c axis, while their degree of softening for the phonon along the a axis was small, revealing clear acoustic anisotropy. It was associated with the strong polarization dependence of the quasi-elastic central peak. The relaxation time derived from the central peak seemed to follow the critical slowing-down behavior indicating order–disorder nature of the phase transition. Among C44 and C66, the former displayed substantial softening in the ferroelectric phase resembling the order parameter behavior. The longitudinal sound velocity and the related acoustic damping showed significant anomalies in the ferroelectric phase of unpoled PBN-0.5, which disappeared completely under poled condition. The possible origin for these unexpected acoustic anomalies was suggested.

Ultra mega power plant disturbance related oscillation detection in Indian grid using PMU data

AbstractThe power system oscillations in the grid are an issue of concern to the system operator. They may transform a stable system into an unstable system when the system is operating on the edge and contingency occurs. The oscillations developed in the system may grow and spread across the power grid if no timely, corrective action is taken to mitigate them. The paper presents the ultra-mega power plant generation complex related blackout analysis. The case study presented in this paper demonstrates the utilization of synchrophasor data to have a better realization about the grid disturbance. It covers some of the important dynamic simulation studies considered representative of various scenarios during this. The paper illustrates utilization of Prony analysis using multiple signals and Fast Fourier Transform methods in conjunction with frequency data obtained in real-time from PMUs installed in the western regional grid of India to determine the frequency of oscillations, its magnitude, and related damping during this disturbance. The focus of this study involves the application of these techniques to gain insight into the oscillatory behavior of the system during this disturbance associated with ultra-mega power plant having generation capacity greater than or equal to 4000 MW from the western part of Indian grid. Based on this, distinct categorization of oscillations into “local mode” and “inter-area mode” of oscillations was done for enhanced understanding about the type of oscillations getting excited during this disturbance.

Mechanical and free vibration properties of skin and core designed basalt woven intertwined with flax layered polymeric laminates

Design of fiber-reinforced polymer damping laminates has been attracting great interest in industrial sectors for lightweight structural damping applications. The present work investigates the impact of skin, core, neutral, and alternate intertwined basalt/flax fabric on the mechanical and vibrational properties of the newly designed polymeric laminate. The designed sequence structure was fabricated using a wet hand lay-up technique with hydraulic compression. Tensile and flexural strength of intertwined multilayer basalt/flax woven composite were studied. An effect of the intertwining on the tensile and flexural strength fractured surface of the composites has been further evaluated. Free vibration technique was used for recording vibration response and the related damping frequencies of intertwining composites. A cantilever mode impact hammer was used for generating periodic signals of the designed composite systems. Damping ratios and damped natural frequencies were calculated with several plies and sequence of flax/basalt woven in the composite. The experimental results revealed that the damped natural frequencies of class II, skin basalt layer intertwined seven core flax layered composite (B2F7) was high, followed by two skin basalt layers intertwined core flax layered composite (B4F5). The addition of the flax layer enhanced the natural frequency to the higher value. It was found that the skin basalt layer with intertwined flax layered B2F7 design structure exhibits maximum damping value with acceptable mechanical properties.

Equivalent linear systems of nonlinear systems

The dynamic behaviour of civil engineering structures is frequently nonlinear, but it is often engineering practice to assume linearity. Consequently, a link between the nonlinear and linear models is required to ensure appropriate correspondence between the true dynamic behaviour and the model, which can be achieved by enforcing an equivalence between the two. This paper focuses on estimating equivalent linear systems of simulated multi-degree-of-freedom systems with stiffness and damping nonlinearities subjected to broad-band or general excitation using three different approaches based on different equivalences and assumptions. The considered methods are: a method based on balancing the intensity of the excitation and the response in the frequency domain, a modal version of the so-called energy dissipation method, and an approach that estimates equivalent linear stiffness, viscous damping, and equivalent linear mode shapes directly through a least squares fit. Notably, the latter method enables the assessment of the common assumptions such as low damping, slowly varying amplitude and phase, invariant mode shapes, and proportional damping. Furthermore, using the aforementioned procedures, extensive numerical simulations enable a comparative assessment in terms of the related linear stiffness, damping, and mode shape estimation potential of nonlinear systems and the accompanying error of the mentioned assumptions. In general, it is concluded that the methods provide a reliable estimation and interpretation of equivalent linear systems.

Investigation of strut collision in tensegrity statics and dynamics

Strut collisions can occur in tensegrity structures and seriously change the general mechanical behaviors of the structures. For many years, there have been few investigations into this topic. This paper systematically addresses the issue of strut collision modeling in tensegrity mechanical analyses. A general and efficient rigid body model for strut collision analysis is presented. A two-strut collision model is formulated using contact force methods. The normal contact force and the tangential friction force are evaluated by using the Lankarani–Nikravesh model and the modified Coulomb's friction law, respectively. Exact formulations of the generalized force vectors and the related tangent stiffness and tangent damping matrices are analytically derived. Fundamental issues related to collision detection and stiffness coefficient determination, are also addressed. Two representative examples involving both classical and clustered tensegrity structures are presented to demonstrate the effectiveness of the general framework, as well as to make a detailed study into the influence of collisions effects. Outcomes presented in this work will broaden the research discipline on tensegrity systems.

Effects of structural-fluid coupling uncertainty on the dynamic behavior of nominally straight and uniform pipes conveying fluid: Modeling and numerical study

This investigation is focused on understanding and modeling the effects of specific structural uncertainties in nearly-straight and uniform pipes conveying fluid on the dynamic response and stability of these pipes. The structural uncertainties of interest here are those that affect directly the fluid flow and its feedback on the structural response, e.g., uncertainties on/variations of the inner cross-section and curvature of the pipe. Owing to the complexity of introducing such uncertainties directly in finite element models, it is desired to proceed directly at the level of modal models by randomizing simultaneously the appropriate mass, stiffness, and damping matrices. The maximum entropy framework is adopted to carry out the stochastic modeling of these matrices with appropriate symmetry constraints guaranteeing that the nature, e.g., divergence or flutter, of the bifurcation is preserved when introducing uncertainty.To support the formulation of this stochastic reduced order model (ROM), a series of finite element computations are first carried out for pipes with straight centerline but inner radius varying randomly along the pipe. The results of this numerical discovery effort demonstrate that the dominant effects originate from the variations of the exit flow velocity, induced by the change in inner cross-section at the pipe end, with the uncertainty on the cross-section at other locations playing a secondary role. Relying on these observations, the stochastic reduced order model is constructed to model separately the uncertainty in inner cross-section at the pipe end and at other locations. Then, the fluid related mass, damping, and stiffness matrices of this stochastic ROM are all determined from a single random matrix and one additional random variable. The predictions from this stochastic ROM are found to closely match the corresponding results obtained with the randomized finite element model. It is finally argued that this stochastic ROM also accounts for the small effects due to uncertainty in pipe curvature.

Acoustic Anomalies and Phase Transition Behaviors of Lead-Free Piezoelectric (Na1/2Bi1/2)TiO₃-xBaTiO₃ Single Crystals as Revealed by Brillouin Light Scattering.

The elastic properties of unpoled and prepoled (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) single crystals near the morphotropic phase boundary were investigated as a function of temperature using Brillouin light scattering. The acoustic mode frequency and the related acoustic damping of unpoled NBT-xBT showed very broad minimum and maximum, respectively, consistent with typical relaxor behaviors. The frequency softening of the longitudinal acoustic mode together with the increase in acoustic damping was largest along the <100> direction, indicating that polarization fluctuations were most substantial along this crystallographic direction. The difference in acoustic behaviors between the unpoled NBT-xBTs with x = 0.05 and 0.08 were negligible, which means that the NBT-xBT system exhibits typical relaxor properties over a certain composition range of at least 5~8%. The obtained relaxation time of polar nanoregions in the paraelectric phase showed a gradual slowing-down character without any critical divergent behavior. The prepoling of NBT-xBT along the <100> direction induced drastic changes in both mode frequency and damping at ~110 °C when the poling field was larger than 1.4 kV/mm, corresponding to the depoling process from macroscopic/mesoscopic ferroelectric order to ergodic relaxor state upon heating. Phase coexistence of ferroelectric and relaxor states was observed at the intermediate poling field of 1.4 kV/mm.

Тороидальные альфвеновские моды в плазме токамака Глобус-М

AbstractConditions of the excitation of toroidal Alfvén eigenmodes (TAEs) and their inf luence on the confinement of fast particles into the Globus-M spherical tokamak have been studied by KINX code calculations of the magnetohydrodynamic spectra of reconstructed divertor equilibrium configurations with stability margin q _0 &gt; 1 on the magnetic axis. The sensitivity of the frequencies of TAE modes with toroidal wavenumber n = 1 to the type of boundary conditions and choice of boundary surface has been studied. It has been established that the frequencies of modes with dominating poloidal harmonics m = 1 and 2 in the gap of continuum are significantly higher than those observed in the spectra of signals measured in the Mirnov coil probes, especially under the assumption of free plasma boundary with allowance for its compressibility. The TAE modes with lower frequencies and higher poloidal wavenumbers localized near the plasma boundary may be responsible for the oscillations observed in the experiment. However, these modes are characterized by the interaction with continuum and, probably, exhibit related damping.

Toroidal Alfvén Eigenmodes in Globus-M Spherical Tokamak Plasma

Conditions of the excitation of toroidal Alfven eigenmodes (TAEs) and their inf luence on the confinement of fast particles into the Globus-M spherical tokamak have been studied by KINX code calculations of the magnetohydrodynamic spectra of reconstructed divertor equilibrium configurations with stability margin q0 > 1 on the magnetic axis. The sensitivity of the frequencies of TAE modes with toroidal wavenumber n = 1 to the type of boundary conditions and choice of boundary surface has been studied. It has been established that the frequencies of modes with dominating poloidal harmonics m = 1 and 2 in the gap of continuum are significantly higher than those observed in the spectra of signals measured in the Mirnov coil probes, especially under the assumption of free plasma boundary with allowance for its compressibility. The TAE modes with lower frequencies and higher poloidal wavenumbers localized near the plasma boundary may be responsible for the oscillations observed in the experiment. However, these modes are characterized by the interaction with continuum and, probably, exhibit related damping.

Reduced Order Level Modeling of Structure-Based Uncertainty on Fluid Forces for the Dynamics of Nearly-Straight Pipes

This investigation is focused on the consideration of structural uncertainties in nearly-straight pipes conveying fluid and on the effects of these uncertainties on the dynamic behavior of the pipes. Of interest more specifically are the structural uncertainties which affect directly the fluid flow and its feedback on the structural response, i.e., uncertainties on/variations of the inner cross-section and curvature of the pipe. A finite element-based discovery effort is first carried out on randomly tapered straight pipes to understand how the uncertainty in inner cross-section affects the behavior of the pipes. It is found that the dominant effect originates from the variations of the exit flow speed, induced by the change in inner cross-section at the pipe end, with the uncertainty on the cross-section at other locations playing a secondary role. The development of a generic model of the uncertainty in fluid forces is next considered by proceeding directly at the level of modal models by randomizing simultaneously the appropriate mass, stiffness, and damping matrices. The maximum entropy framework is adopted to carry out the stochastic modeling of these matrices with appropriate symmetry constraints guaranteeing that the nature, e.g., divergence or flutter, of the bifurcation is preserved when introducing uncertainty. To achieve this property, it is proposed that the fluid related mass, damping, and stiffness matrices of the stochastic reduced order model (ROM) all be determined from a single random matrix and a random variable. The predictions from this stochastic ROM are found to closely match the corresponding results obtained with the randomized finite element model.

新型轻质复合材料夹芯结构振动阻尼性能研究进展

As a new generation of advanced super-strong lightweight structural materials, composite grid and lattice sandwich structures have received extensive attention from domestic and foreign scholars. An abundant amount of research on such novel sandwich structures focused on the manufacturing techniques and mechanical properties was carried out. However, it needs to be pointed out that, the research on the vibration and damping characteristics of such sandwich structures has just started, which is a new topic. Here a comprehensive review of the state of the research on vibration damping models was made for fiber reinforced resin matrix composite laminates and sandwich structures. Firstly, the related damping mechanisms were elaborated, and then an overview of micro- and macro-damping models for simple composite laminates, viscoelastic damping and novel sandwich structures was given. Finally, the results and shortcomings of the existing research on the vibration damping characteristics of such structures were summarized and the possible development in the future was predicated.

Towards the design of an enriched concrete with enhanced dissipation performances

The effect of micro-particle addition on internal dissipation of concrete was experimentally investigated. The idea was to increase the internal friction and improve the related damping performances of the material by allowing the presence of micro-particles inside the concrete pores, thus changing the contact interaction between internal surfaces of pores. However, a high percentage of micro-particle addition could either hinder the sliding or even degrade the mechanical strength of the material. Thus, the optimal percentage has to be experimentally determined for every mixtures one considers. In our study, specimens were tested under cyclic loading to evaluate energy dissipation. In some enriched mixtures presenting a not too low water/cement ratio a significant increase in damping capability is observed with respect to standard concrete. We used experimental data to perform a parameter identification and found that the employed micromorphic model is suitable to forecast the dissipative behavior of the material.

Effects of ionization and ion loss on dust ion-acoustic solitary waves in a collisional dusty plasma with suprathermal electrons

The combined effects of ionization, ion loss, and electron suprathermality on dust ion-acoustic solitary waves in a collisional dusty plasma are examined. Carrying out a small but finite amplitude analysis, a damped Korteweg-de Vries (dK–dV) equation is derived. The damping term decreases with the increase of the spectral index and saturates for Maxwellian electrons. Choosing typical plasma parameters, the analytical approximate solution of the dK-dV equation is numerically analyzed. We first neglect the ionization and ion loss effects and account only for collisions to estimate the relative importance between these damping terms which can act concurrently. Interestingly, we found that as the suprathermal character of the electrons becomes important, the strength of the collisions related dissipation becomes more important and causes the dust ion-acoustic solitary wave amplitude to decay more rapidly. Moreover, the collisional damping may largely prevail over the ionization and ion loss related damping. The latter becomes more effective as the electrons evolve far away from their thermal equilibrium. Our results complement and provide new insights into previously published work on this problem.

Damage detection on framed structures: modal curvature evaluation using Stockwell Transform under seismic excitation

The key parameters for damage detection and localization are eigenfrequencies, related equivalent viscous damping factors and mode shapes. The classical approach is based on the evaluation of these structural parameters before and after a seismic event, but by using a modern approach based on time-frequency transformations it is possible to quantify these parameters throughout the ground shaking phase. In particular with the use of the S-Transform, it is possible to follow the temporal evolution of the structural dynamics parameters before, during and after an earthquake. In this paper, a methodology for damage localization on framed structures subjected to strong motion earthquakes is proposed based on monitoring the modal curvature variation in the natural frequency of a structure. Two examples of application are described to illustrate the technique: Computer simulation of the nonlinear response of a model, and several laboratory (shaking table) tests performed at the University of Basilicata (Italy). Damage detected using the proposed approach and damage revealed via visual inspections in the tests are compared.

Automatic evaluation of the fundamental frequency variations and related damping factor of reinforced concrete framed structures using the Short Time Impulse Response Function (STIRF)

Structural Health Monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures and infrastructures. This paper presents an innovative strategy for automatic evaluation of the variable fundamental frequency and related damping factors of nonlinear structures during strong motion phases. In fact, most of methods for damage detection are based on the assessment of the variations related to dynamic parameters characterizing the monitored structure. In this case, a crucial aspect is a correct estimation of both structural eigenfrequencies and related damping factor also during the nonlinear behavior of a monitored structure. In this paper, nonlinear interferometric analyses combined with the Fourier Transform (FT) are proposed to evaluate the Short-Time Impulse Response Function (STIRF) able to characterize frequencies and damping variations of a monitored structure. Two application of the STIRF are proposed on both numerical and experimental models.

Short wavelength limits of current shot noise suppression

Shot noise in electron beam was assumed to be one of the features beyond control of accelerator physics. Current results attained in experiments at Accelerator Test Facility in Brookhaven and Linac Coherent Light Source in Stanford suggest that the control of the shot noise in electron beam (and therefore of spontaneous radiation and Self Amplified Spontaneous Emission of Free Electron Lasers) is feasible at least in the visible range of the spectrum. Here, we present a general linear formulation for collective micro-dynamics of e-beam noise and its control. Specifically, we compare two schemes for current noise suppression: a quarter plasma wavelength drift section and a combined drift/dispersive (transverse magnetic field) section. We examine and compare their limits of applicability at short wavelengths via considerations of electron phase-spread and the related Landau damping effect.

The effect of antiphase boundaries on the elastic properties of Ni–Mn–Ga austenite and premartensite

The evolution of elastic properties with temperature and magnetic field was studied in two differently heat-treated single crystals of the Ni–Mn–Ga magnetic shape memory alloy using resonant ultrasound spectroscopy. Quenching and slow furnace cooling were used to obtain different densities of antiphase boundaries. We found that the crystals exhibited pronounced differences in the c′ elastic coefficient and related shear damping in high-temperature ferromagnetic phases (austenite and premartensite). The difference can be ascribed to the formation of fine magnetic domain patterns and pinning of the magnetic domain walls on antiphase boundaries in the material with a high density of antiphase boundaries due to quenching. The fine domain pattern arising from mutual interactions between antiphase boundaries and ferromagnetic domain walls effectively reduces the magnetocrystalline anisotropy and amplifies the contribution of magnetostriction to the elastic response of the material. As a result, the anomalous elastic softening prior to martensite transformation is significantly enhanced in the quenched sample. Thus, for any comparison of experimental data and theoretical calculations the microstructural changes induced by specific heat treatment must be taken into account.