Strong External Excitation Research Articles

Isolation Performance of the Quasi-Zero Stiffness Isolation System Enhanced by Mixed Tuned Inerter Damper

The low-frequency isolation performance of the transverse spring-rocker quasi-zero stiffness (QZS) isolation system has been demonstrated. However, the QZS isolation system could encounter a deterioration of the low-frequency isolation performance under strong external excitation, due to the hardening phenomenon of its amplitude-frequency response (AFR). To end this problem, a mixed tuned inerter damper (MTID) which combines the advantages of the tuned inerter damper (TID) and the tuned viscous mass damper (TVMD) is proposed in this research. The analytical solution of the AFR of the MTID-QZS isolation system under harmonic excitation is solved by the harmonic balance method (HBM) and verified by numerical method. Whereafter, the corresponding stability analysis of AFR is conducted, and the force transmissibility (FT) of the MTID-QZS is defined to qualify the isolation performance. The effects of the tuning frequency of MTID, the inertance-to-mass ratio, and damping ratios on AFR and FT are conducted, and optimized parameter combinations are suggested to obtain distinguished ultra-low-frequency isolation performance. The comparison among the MTID-QZS system, TID-QZS system, TVMD-QZS system, and QZS system shows that the MTID-QZS system has obvious advantages in suppressing the low-frequency and high-frequency vibration either under strong or weak external excitation.

Post-earthquake reliability assessment of segmental column structures based on nonlinear model updating

In this study, a Bayesian based nonlinear model updating approach is developed to enhance the post-earthquake reliability assessment of posttensioned segmental column structures. According to the proposed procedure, dynamic global sensitivity analysis (DGSA) approach is firstly performed to analyze the sensitivity of the model outputs to the model parameters, which aims to select the appropriate parameters for model updating, and then, the likelihood function of Bayesian estimation is constructed based on the residual errors between the measured and simulated dynamic responses of segmental column structures. Since the decomposed structural dynamic responses are conducted for model updating, the simulated error variance parameters are also considered as unknown. To simultaneously estimate the unknown model parameters and variance parameters, the maximum likelihood estimation approach is employed in this study. Then, the Cram-Rao lower bound (CRLB) analysis method is applied to estimate uncertainty of the identified model parameters that caused by measurement noise. Once nonlinear model updating is completed, the earthquake-induced failure probability of segmental column structure can be evaluated by performing reliability analysis. In numerical simulations, the feasibility of the proposed reliability assessment approach is validated by using a planar posttensioned segmental column model subjected to seismic loads. Furthermore, a scaled posttensioned segmental column structure subjected to bi-directional earthquake exactions is applied to verify the effectiveness of the proposed approach. Both numerical and experimental results have shown that the proposed strategy is accurate and effective for nonlinear model updating of segmental column structures, and the updated results are capable of using to evaluate the failure probability of such type of structures subjected to strong external excitations.

The performance of nonlinear vibration control via NiTiNOL–Steel wire ropes

Conventional nonlinear vibration absorbers must provide additional mass and motion space to achieve the expected vibration control effect, which will significantly alter the original design of the target structure. In this study, the shape memory NiTiNOL–steel (NiTi–ST) wire rope and composite plate are coupled to investigate the vibration control effect of such a configuration. The vibration response of the model is studied using the Galerkin truncation method (GTM) and the harmonic balance method (HBM), and the stability of the frequency response curve is determined using the linearized stability theory. NiTi–ST wire rope is found to have a good damping effect for both low- and high-order resonances. Under a strong external excitation, the appearance of the closed detached response (CDR) may cause system amplitude mutation; however, it can also achieve effective inhibition. Finally, the effect of the structural parameters on the vibration control is discussed. The nonlinear damping term in NiTi–ST determines the soft and hard characteristics of the system, while the equivalent linear damping term is mainly responsible for vibration suppression. Therefore, as a novel shock absorber, NiTi–ST can be applied to engineering structures in order to prevent vibration damage.

Wind-Induced Response Control of a Television Transmission Tower by Piezoelectric Semi-Active Friction Dampers

To be a typical flexible structure with small damping, a high-rise television (TV) transmission tower is sensitive to wind excitations. The response control of TV transmission towers by semi-active friction dampers under strong external excitations is still very limited and the parametric study on control performance has not been systematically investigated. To this end, the semi-active control of a wind-excited TV transmission tower by piezoelectric friction dampers is conducted in this study. A fine finite element (FE) model of a real TV transmission tower is constructed and then a two-dimensional (2D) dynamic model is also established. The analytical model of a semi-active friction damper based on piezoelectric actuators is then established with damper stiffness considered. The equations of motion of the tower with friction dampers are then established. The local feedback control algorithm based on nonlinear Reid damping mechanisms is used to command piezoelectric friction dampers for wind-excited towers. A high-rise TV transmission tower in China is used to investigate the validity of the proposed semi-active control approach and compared with that of passive control. Furthermore, a detailed parametric investigation is conducted to examine the influence of gain coefficient, damper stiffness, hysteresis loops, damper force, and wind loading intensity on control efficacy. The analytical results indicate that piezoelectric friction dampers with optimal parameters are beneficial to the vibration mitigation of TV transmission towers under different wind loading intensities.

Graphene–polyimide-integrated metasurface for ultrasensitive modulation of higher-order terahertz fano resonances at the Dirac point

The initial Fermi level of graphene is quite close to the Dirac point. It is ultrasensitive to external stimuli in the modulation of Fano resonance. Thus, the Fermi level directly skips the Dirac point during modulation with strong external excitation. Therefore, the modulation behavior of the Fano resonance is not easy to capture at the Dirac point. Here, we applied a laser pump with micropower densities and a mains with weak voltages to the terahertz metasurface and observed the modulation behavior of higher-order Fano resonance at the Dirac point. Accompanied by Fermi level shifts between the Dirac point, conduction band, and valence band, higher-order Fano resonances exhibited the process of disappearance, appearance, and re-disappearance. Concomitantly, the transmission amplitude underwent an increasing stage and a decreasing stage. More importantly, a 0.05 V change in the bias voltage or a 7.5 mW/cm2 change in the power density enabled a modulation depth of 90%. These findings could be potentially used for designing ultrasensitive terahertz metaphotonic devices.

Understanding the nature of mean-field semiclassical light-matter dynamics: An investigation of energy transfer, electron-electron correlations, external driving, and long-time detailed balance

Semiclassical electrodynamics is an appealing approach for studying light-matter interactions, especially for realistic molecular systems. However, there is no unique semiclassical scheme. On the one hand, intermolecular interactions can be described instantaneously by static two-body interactions connecting different molecules plus a classical transverse E-field; we will call this Hamiltonian #I. On the other hand, intermolecular interactions can also be described as effects that are mediated exclusively through a classical one-body E-field without any quantum effects at all (assuming we ignore electronic exchange); we will call this Hamiltonian #II. Moreover, one can also mix these two Hamiltonians into a third, hybrid Hamiltonian, which preserves quantum electron-electron correlations for lower excitations but describes higher excitations in a mean-field way. To investigate which semiclassical scheme is most reliable for practical use, here we study the real-time dynamics of a pair of identical two-level systems (TLSs) undergoing either resonance energy transfer (RET) or collectively driven dynamics. While all approaches perform reasonably well when there is no strong external excitation, we find that no single approach is perfect for all conditions. Each method has its own distinct problems: Hamiltonian #I performs best for RET but behaves in a complicated manner for driven dynamics. Hamiltonian #II is always stable, but obviously fails for RET at short distances. One key finding is that, under externally driving, a full configuration interaction description of Hamiltonian #I strongly overestimates the long-time electronic energy, highlighting the not obvious fact that, if one plans to merge quantum molecules with classical light, a full, exact treatment of electron-electron correlations can actually lead to worse results than a simple mean-field treatment.

Computation of quasi-periodic localised vibrations in nonlinear cyclic and symmetric structures using harmonic balance methods

In this paper we develop a fully numerical approach to compute quasi-periodic vibrations bifurcating from nonlinear periodic states in cyclic and symmetric structures. The focus is on localised oscillations arising from modulationally unstable travelling waves induced by strong external excitations. The computational strategy is based on the periodic and quasi-periodic harmonic balance methods together with an arc-length continuation scheme. Due to the presence of multiple localised states, a new method to switch from periodic to quasi-periodic states is proposed. The algorithm is applied to two different minimal models for bladed disks vibrating in large amplitudes regimes. In the first case, each sector of the bladed disk is modelled by a single degree of freedom, while in the second application a second degree of freedom is included to account for the disk inertia. In both cases the algorithm has identified and tracked multiple quasi-periodic localised states travelling around the structure in the form of dissipative solitons.

Identifying nonlinear characteristics of model-free MR dampers in structures with partial response data

MR dampers have been widely applied in passive and semi-active vibration control over the past decades. The modeling of hysteretic behavior of structural components is a prerequisite for optimal performances of MR dampers and the predication of nonlinear dynamic responses of structures subjected to severe dynamic loads. However, it is hard to establish proper mathematical models for the nonlinear hysteretic behaviors of MR dampers in service due to the complexities of nonlinearities and the degradations of the construction materials. Consequently, it is strongly desired to develop model-free methodologies for the nonlinear hysteretic performance identification with no assumption on the nonlinear hysteretic models of MR dampers. In this paper, a novel method is proposed for this purpose. Firstly, the MR dampers are in the linear state when the structure is under the weak external excitations, the restoring force of the MR dampers is only provided by linear stiffness and viscous damping of MR dampers, the structural physical parameters can be estimated based on the extended Kalman filter (EKF) approach. Then, MR dampers are in the nonlinear state when the structure is under the strong external excitations. The nonlinear restoring forces from MR dampers in a structure are treated as ‘unknown fictitious inputs’ to the corresponding structural systems without MR dampers. The recent Kalman filter with unknown input (KF-UI) algorithm developed by the authors is adopted for the simultaneous identification of the corresponding structural systems and the ‘unknown fictitious inputs’. No real information about the structure is needed, so the proposed method is capable of identifying hysteretic restoring forces of MR dampers by the direct use of partial structural dynamic response measurements, which are usually available from an employed structural health monitoring (SHM) system. To validate the performances of the proposed method, some numerical simulation examples of identifying nonlinear hysteretic restoring forces of MR dampers in different models are used.

Specific Features of Aerodynamic Journal Bearings with Elastically Supported Pads

Aerodynamic bearings with elastically supported tilting pads have operational properties comparable with widely-used foil journal bearings. They combine the excellent stability of tilting pad bearings, as a result of very small cross-coupling stiffness terms, with the positive properties of foil bearings, namely their ability to adapt to changing operating conditions and presence of additional damping due to friction between elastic members and bearing casings. Air cycle machines (ACMs) are used in the environmental control systems of aircrafts to manage the pressurization of the cabin. An ACM with the abovementioned type of bearings and an operational speed of 60,000 rpm was designed and successfully tested, even under conditions of strong external excitation. Some problems with rotor stability in certain operation regimes were encountered. Rotor relative vibrations measured at both bearing locations increased substantially when excitation frequency was close to the lowest rotor eigenvalues. In spite of that and the 1000 start/stop cycles passed by the end of the test, any traces of wear on the bearing sliding surfaces were negligible. When the bearing distance had to be shortened in order to insert the machine into the defined space, the rotor quickly became unstable at relatively low speeds. Although rotor stability reserve was reduced only slightly, the rotor had to be redesigned in order to achieve stability. Operation characteristics of aerodynamic bearings with elastically supported tilting pads are presented together with rotor dynamic analysis and validated with measured results.

Supercritical Nonlinear Vibration of a Fluid-Conveying Pipe Subjected to a Strong External Excitation

Nonlinear vibration of a fluid-conveying pipe subjected to a transverse external harmonic excitation is investigated in the case with two-to-one internal resonance. The excitation amplitude is in the same magnitude of the transverse displacement. The fluid in the pipes flows in the speed larger than the critical speed so that the straight configuration becomes an unstable equilibrium and two curved configurations bifurcate as stable equilibriums. The motion measured from each of curved equilibrium configurations is governed by a nonlinear integro-partial-differential equation with variable coefficients. The Galerkin method is employed to discretize the governing equation into a gyroscopic system consisting of a set of coupled nonlinear ordinary differential equations. The method of multiple scales is applied to analyze approximately the gyroscopic system. A set of first-order ordinary differential equations governing the modulations of the amplitude and the phase are derived via the method. In the supercritical regime, the subharmonic, superharmonic, and combination resonances are examined in the presence of the 2 : 1 internal resonance. The steady-state responses and their stabilities are determined. The various jump phenomena in the amplitude-frequency response curves are demonstrated. The effects of the viscosity, the excitation amplitude, the nonlinearity, and the flow speed are observed. The analytical results are supported by the numerical integration.

Forced response of quadratic nonlinear oscillator: comparison of various approaches

The primary resonances of a quadratic nonlinear system under weak and strong external excitations are investigated with the emphasis on the comparison of different analytical approximate approaches. The forced vibration of snap-through mechanism is treated as a quadratic nonlinear oscillator. The Lindstedt-Poincare method, the multiple-scale method, the averaging method, and the harmonic balance method are used to determine the amplitude-frequency response relationships of the steady-state responses. It is demonstrated that the zeroth-order harmonic components should be accounted in the application of the harmonic balance method. The analytical approximations are compared with the numerical integrations in terms of the frequency response curves and the phase portraits. Supported by the numerical results, the harmonic balance method predicts that the quadratic nonlinearity bends the frequency response curves to the left. If the excitation amplitude is a second-order small quantity of the bookkeeping parameter, the steady-state responses predicted by the second-order approximation of the LindstedtPoincare method and the multiple-scale method agree qualitatively with the numerical results. It is demonstrated that the quadratic nonlinear system implies softening type nonlinearity for any quadratic nonlinear coefficients.

Pilot Study for Investigating the Cyclic Behavior of Slit Damper Systems with Recentering Shape Memory Alloy (SMA) Bending Bars Used for Seismic Restrainers

Although the steel slit dampers commonly utilized for aseismic design approach can dissipate considerable energy created by the yielding of base materials, large residual deformation may happen in the entire frame structure. After strong external excitation, repair costs will be incurred in restoring a structure to its original condition and to replace broken components. For this reason, alternative recentering devices characterized by smart structures, which mitigate the damage for such steel energy dissipation slit dampers, are developed in this study. These devices, feasibly functioning as seismic restrainers, can be improved by implementing superelastic shape memory alloy (SMA) bending bars in a parallel motion with the steel energy-dissipating damper. The bending bars fabricated with superelastic SMAs provide self-centering forces upon unloading, and accordingly contribute to reducing permanent deformation in the integrated slit damper system. The steel slit dampers combined with the superelastic SMA bending bars are evaluated with respect to inelastic behavior as simulated by refined finite element (FE) analyses. The FE slit damper models subjected to cyclic loads are calibrated to existing test results in an effort to predict behavior accurately. The responses of the proposed slit damper systems are compared to those of the conventionally used slit damper systems. From the analysis results, it is concluded that innovative steel slit dampers combined with superelastic SMA bending bars generate remarkable performance improvements in terms of post-yield strength, energy dissipation, and recentering capability.

Observation of the fundamental Nyquist noise limit in an ultra-high Q-factor cryogenic bulk acoustic wave cavity

Thermal Nyquist noise fluctuations of high-Q bulk acoustic wave cavities have been observed at cryogenic temperatures with a DC superconducting quantum interference device amplifier. High Q modes with bandwidths of few tens of milliHz produce thermal fluctuations with a signal-to-noise ratio of up to 23 dB. The estimated effective temperature from the Nyquist noise is in good agreement with the physical temperature of the device, confirming the validity of the equivalent circuit model and the non-existence of any excess resonator self-noise. The measurements also confirm that the quality factor remains extremely high (Q > 108 at low order overtones) for very weak (thermal) system motion at low temperatures, when compared to values measured with relatively strong external excitation. This result represents an enabling step towards operating such a high-Q acoustic device at the standard quantum limit.

Nonlinear vibrations of axially moving Timoshenko beams under weak and strong external excitations

In this paper, nonlinear vibrations under weak and strong external excitations of axially moving beams are analyzed based on the Timoshenko model. The governing nonlinear partial-differential equation of motion is derived from Newton's second law, accounting for the geometric nonlinearity caused by finite stretching of the beams. The complex mode approach is applied to obtain the transverse vibration modes and the natural frequencies of the linear equation. The method of multiple scales is employed to investigate primary resonances, nonsyntonic excitations, superharmonic resonances, and subharmonic resonances. Some numerical examples are presented to demonstrate the effects of a varying parameter, such as axial speed, external excitation amplitudes, and nonlinearity, on the response amplitudes for the first and second modes, when other parameters are fixed. The stability of the response amplitudes is investigated and the boundary of instability is located.

External activity and the freedom to recode

Our hippocampal model depends on randomization. In principle, randomizations, e.g., chaotic activity fluctuations, quantal synaptic failures, or initial state randomization, can be overcome by strong external excitation. However, if external activity is too low, randomization will destroy the information transmitted by the inputs. Here, computer simulations of the transitive inference paradigm reveal an optimal range of external excitation. At lower activity levels, optimal performance occurs when the relative external excitation accounts for 35–40% of the total with activity, while at higher activity, external activity can be as low as 30% of the total.

Quenching of Luminescence Emission due to a Transient Bonding State of Donor-Acceptor Pairs at High Excitation Levels in GaAlAs Crystals Doped to Compensation

A new kind of a bound state has been identified at strong external excitation levels in semiconductors with high amphoteric doping, which is formed of a close donor-acceptor molecule and a neighboring second donor and/or acceptor. The recombination behavior of such a bound state is best described by AUGER-type transitions, reason for which this state is called an AUGER molecule. We have determined the existence region of such molecules in silicon-doped Ga1-xAlxAs with respect to excess carrier density and temperature by means of electron-beam excited luminescence spectroscopy at low and medium temperatures. The donor-acceptor pair radiation intensity is affected in a characteristic manner by the existence of the AUGER molecule state at elevated excitation levels and not too low temperatures. A computerized model calculation of corresponding rate coefficients turns out to be in good agreement with experimental results.

Nonparametric Identification of a Building Structure from Experimental Data Using Wavelet Neural Network

Abstract: This study presents a wavelet neural network‐based approach to dynamically identifying and modeling a building structure. By combining wavelet decomposition and artificial neural networks (ANN), wavelet neural networks (WNN) are used for solving chaotic signal processing. The basic operations and training method of wavelet neural networks are briefly introduced, since these networks can approximate universal functions. The feasibility of structural behavior modeling and the possibility of structural health monitoring using wavelet neural networks are investigated. The practical application of a wavelet neural network to the structural dynamic modeling of a building frame in shaking tests is considered in an example. Structural acceleration responses under various levels of the strength of the Kobe earthquake were used to train and then test the WNNs. The results reveal that the WNNs not only identify the structural dynamic model, but also can be applied to monitor the health condition of a building structure under strong external excitation.