Search Results (10)

The dynamic characteristics of buildings and their behavior under various dynamic loads play a crucial role in civil engineering applications, particularly for earthquake-resistant structural design. Employing a precise mathematical model of the structural system makes it possible to accurately predict the actual structural performance under dynamic loads, such as winds and earthquakes. Given this perspective, finite element model-updating approaches in structural systems have gained significant attention in recent decades. This paper proposes a simulation-based model-updating technique that utilizes measured free vibration responses to the correct structural parameters of multi-degree-of-freedom systems. A five-degree-of-freedom building model is subjected to shaking table tests to demonstrate the effectiveness of the proposed method. The experimental data for this method consists of the dynamic behavior of the system under the seismic excitation of the El Centro 1940 earthquake and the results of the free vibration tests. The MATLAB/Simulink parameter estimation tool is employed to establish a correlation between the analytical model and the measured dynamic response from the building model. Compared to the measured structural responses, the updated analytical model, which incorporates the proposed simulation-based model-updating technique, demonstrates high accuracy in predicting the responses through effective corrections of stiffness and damping coefficients. Full article

The high-frequency vibration and resonance of armature assembly in the hydraulic servo valve are the main reasons for instability and failure. Magnetic fluid (MF) operating in the squeeze mode can be taken as an effective damper for resonance suppression in the servo valve. Due to excitation difficulty and the low signal-to-noise ratio of high-frequency vibration signals, the capability of MF to modify multiple-order modal damping ratios in a multi-degree-of-freedom system is still unclear. To reveal the mechanism of magnetic fluid for improving modal damping ratios, an algorithm for modal damping ratio identification is proposed. The modal damping ratios of the armature assembly with and without magnetic fluid are identified based on the tested resonance free decay responses. Four resonance frequencies of armature assembly are observed, and the corresponding damping ratios are identified. The equivalent modal damping ratios due to squeeze flow of MF are obtained. The results show that the proposed algorithm can identify damping ratios with an accuracy of up to 98.79%. The damping ratios are improved by double or more due to the magnetic fluid, and the maximum resonance amplitudes are significantly reduced by 65.2% (from 916.5 μm to 318.6 μm). Full article

For the free vibrations of multi-degree mechanical structures appeared in structural dynamics, we solve the quadratic eigenvalue problem either by linearizing it to a generalized eigenvalue problem or directly treating it by developing the iterative detection methods for the real and complex eigenvalues. To solve the generalized eigenvalue problem, we impose a nonzero exciting vector into the eigen-equation, and solve a nonhomogeneous linear system to obtain a response curve, which consists of the magnitudes of the n-vectors with respect to the eigen-parameters in a range. The n-dimensional eigenvector is supposed to be a superposition of a constant exciting vector and an m-vector, which can be obtained in terms of eigen-parameter by solving the projected eigen-equation. In doing so, we can save computational cost because the response curve is generated from the data acquired in a lower dimensional subspace. We develop a fast iterative detection method by maximizing the magnitude to locate the eigenvalue, which appears as a peak in the response curve. Through zoom-in sequentially, very accurate eigenvalue can be obtained. We reduce the number of eigen-equation to $n-1$ to find the eigen-mode with its certain component being normalized to the unit. The real and complex eigenvalues and eigen-modes can be determined simultaneously, quickly and accurately by the proposed methods. Full article

Many specific features of ultrafast electron transfer (ET) reactions in macromolecular compounds can be attributed to nonequilibrium configurations of intramolecular vibrational degrees of freedom and the environment. In photoinduced ET, nonequilibrium nuclear configurations are often produced at the stage of optical excitation, but they can also be the result of electron tunneling itself, i.e., fast redistribution of charges within the macromolecule. A consistent theoretical description of ultrafast ET requires an explicit consideration of the nuclear subsystem, including its evolution between electron jumps. In this paper, the effect of the multi-timescale nuclear reorganization on ET transitions in macromolecular compounds is studied, and a general theory of ultrafast ET in non-Debye polar environments with a multi-component relaxation function is developed. Particular attention is paid to designing the multidimensional space of nonequilibrium nuclear configurations, as well as constructing the diabatic free energy surfaces for the ET states. The reorganization energies of individual ET transitions, the equilibrium energies of ET states, and the relaxation properties of the environment are used as input data for the theory. The effect of the system-environment interaction on the ET kinetics is discussed, and mechanisms for enhancing the efficiency of charge separation in macromolecular compounds are analyzed. Full article

To reveal the nonlinear dynamic behavior of gear rattling vibration caused by gear backlash, a 2-DOF oscillator model with spring and damping elements was established. Based on the theory of discontinuous dynamical systems, the phase plane of gear motion was divided into three parts: the domain of tooth surface meshing motion, the domain of free motion and the domain of tooth back meshing motion. Introducing the global mapping and local mapping dynamics method, the process of gear teeth from impact to meshing and then impact and meshing was accurately described. The influence of different restitution coefficients on gear impact-meshing motion was studied by numerical simulation. The results showed that the grazing bifurcation caused by gear backlash will lead to complex mapping structures of the system and even chaos. The restitution coefficient directly affects the impact-meshing behavior. The introduction of meshing stiffness and restitution coefficient can reasonably characterize the elastic deformation and energy loss during gear meshing, which provides a theoretical model for the application of the theory of discontinuous dynamical systems to a more complex multi-degree of freedom flexible contact gear transmission system. Full article

This paper reports analyses of a 5-degrees-of-freedom (5-DOF) carbon fiber-reinforced polymer (CFRP) robot manipulator, which has been developed for farm applications. The manipulator was made of aluminum alloy (AA) and steel materials. However, to check the effectiveness of CFRP materials on the static and free-vibration performance of the manipulator, the AA parts were replaced with CFRP. For this purpose, the effects of various cross-sections and layups on three design criteria—deflection, load-carrying capacity, and natural frequency—were investigated. Two types of thin-walled laminated sections, specifically the I section and rectangular tubular sections, were used for the composite parts. These parts were made from three hollow square section (“SSS” section) beams and three I section (“III” section) beams. These multi-cell beams were modeled using the finite element (FE) method. Three configurations were selected for analysis based on the manipulator’s most common operating conditions. The results indicated that the use of CFRP increased the manipulator’s natural frequencies, increased the load-carrying capacity, and decreased the manipulator’s tip deflection when compared with its AA counterpart. An analysis showed that using CFRP in the manipulator’s structure could improve static and vibrational performances. It was observed that the “SSS” section beams were 1.17 times stiffer, could carry a 1.20 times higher load, and were 1.40 times heavier than the “III” section beams. Also, decreasing the fiber direction in angle-ply layups from 90° to 0° and adding 0° plies, while keeping the total number of layers constant, decreased the manipulator’s tip deflection and increased its natural frequencies. Full article

This paper proposes an accurate and stable gearbox fault diagnosis scheme that combines a localized adaptive denoising technique with a wavelet-based vibration imaging approach and a deep convolution neural network model. Vibration signatures of a gearbox contain important fault-related information. However, this useful fault-related information is often overwhelmed by random interference noises. Furthermore, the varying speed of gearboxes makes it difficult to distinguish the fault-related frequencies from the interference noises. To obtain a noise-free signal for extraction of fault-related information under variable speed conditions, first, a new localized adaptive denoising technique (LADT) is applied to the vibration signal. The new localized adaptive denoising technique results in optimized vibration sub-bands with negligible background noise. To obtain fault-related information, the wavelet-based vibration imaging approach (WVI) is applied to the denoised vibration signal. The wavelet-based vibration imaging approach decomposes the vibration signal into different time–frequency scales, these scales are reflected by a two-dimensional image called a scalogram. The scalograms obtained from the wavelet-based vibration imaging approach are provided as an input to the deep convolutional neural network architecture (DCNA) for extraction of discriminant features and classification of multi-degree tooth faults (MDTFs) in a gearbox under variable speed conditions. The proposed scheme outperforms the already existing state-of-the-art gearbox fault diagnosis methods with the highest classification accuracy of 100%. Full article

Vertical-axis wind turbines (VAWTs) are compact and efficient and have become increasingly popular for wind energy harvesting. This paper mainly focuses on free and forced vibration analysis of two different types of VAWTs, i.e., an H-type VAWT and a new hybrid VAWT. The H-type VAWT has a lower cost, while the hybrid VAWT has a better self-starting capability at a low wind velocity. Both of them can be used for wind energy harvesting. By using the assumed modes method, the two VAWTs are simplified by a single degree-of-freedom (SDOF) model. By utilizing the method of structural mechanics, a multi-degree-of-freedom (MDOF) model is developed for the two VAWTs and the turbines in them are reasonably simplified. Natural frequency analyses for the SDOF and MDOF models of the two VAWTs are conducted. A beam element model (BEM) of the two VAWTs is created to calculate their natural frequencies and mode shapes and to verify natural frequency results from the SDOF and MDOF models. By using the BEM of the two VAWTs, their amplitude-frequency responses are obtained from harmonic response analysis. To analyze forced vibrations of the two VAWTs, aerodynamic loads on the two VAWTs are obtained from computational fluid dynamics (CFD) simulation. By using solid element models of the two VAWTs, forced transient responses of the two VAWTs are calculated by using the aerodynamic loads from CFD simulation. Steady-state forced response amplitudes of the 1 m-mast hybrid VAWT are 23.8% and 20.5% smaller in X- and Y-directions than those of the 1 m-mast H-type VAWT, respectively. Frequency contents of the aerodynamic loads from CFD simulation are calculated, which confirm that they are periodic, and the power efficiency of the H-type VAWT is about 2.6% higher that of the hybrid VAWT. Full article

In the present study, as a novel and alternative form of foundation for offshore wind turbines, the air-floating characteristics of a large-diameter multi-bucket foundation (LDMBF) in still water and regular waves are investigated. Following the theory of single degree of freedom (DOF)-damped vibration, the equations of oscillating motion for LDMBF are established. The spring or restoring coefficients in heaving, rolling and pitching motion are modified by a dimensionless parameter $\vartheta$ related to air compressibility in every bucket with the ideal air state equation. Combined with the 1/25 scale physical model tests and the numerically simulated prototype models by MOSES, the natural periods, added mass coefficients and damping characteristics of the LDMBF in free oscillations and the response amplitude operator (RAO) have been investigated. The results shown that the added mass coefficients between 1.2 and 1.6 is equal to or larger than the recommended values for ship dynamics. The coefficient 1.2 can be taken as the lower limit 1.2 for a large draft and 1.6 can be taken as the upper limit 1.6 for a small draft. The resonant period and maximum amplitudes for heaving and pitching motions decrease with increasing draft. The amplitudes of heaving and pitching movements decrease to a limited extent with decreasing water depth. Full article

In this paper, a novel energy- and cost-efficient hybrid semi-active mass damper configuration for use in structural applications has been developed. For this task, an arrangement of both active and semi-active control components coupled with appropriate control algorithms are constructed and their performance is evaluated on both single and multi-degree of freedom structures for which practical constraints such as stroke and force saturation limits are taken into account. It is shown that under both free and forced vibrations, the novel device configuration outperforms its more conventional passive and semi-active counterparts, while at the same time achieving performance gains similar to the active configuration at considerably less energy and actuation demands, satisfying both strict serviceability and sustainability requirements often found to govern most modern structural applications. Full article