Vibrations In Different Directions Research Articles

Numerical simulation of flow-induced vibration of a circular cylinder close to a free surface at low Reynolds number

The flow-induced vibration of a two-degree-of-freedom cylinder near a free surface at low Reynolds number (Re = 200) is numerically investigated using the lattice Boltzmann method and a free-surface model. The effects of the Froude number and submergence depth on the characteristics of the flow field and cylinder vibration are discussed. With the approach of the free surface, the wake behind the cylinder interacts with the free surface, resulting in a switch in the vortex shedding mode and the fluctuation of the free surface, which in turn affects the vibration of the cylinder. Additionally, a decrease in the normalized submergence depth h ∗ results in a decrease in the transverse amplitude of vibration and an increase in the streamwise amplitude. The topology of the cylinder vibration trajectory changes from a curve of eight to a teardrop, half-moon, or ellipse shape and gradually loses symmetry. Seven characteristic parameters are extracted from the trajectory of the cylinder, among which the amplitude of the vibration frequency and the phase difference of the vibration in different directions mainly affect the symmetry and topological structure of the trajectory.

Vibration phase difference analysis of long-span suspension bridge during flutter

The research on vibration characteristics of long-span bridges is of great importance to flutter analysis, yet the literature is limited which concerns the phase difference characteristic of the full bridge. This study thus aims to investigate the vibration phase difference characteristics and mechanism along the main span during flutter. The time-domain flutter analysis was conducted in a long-span suspension bridge. The results manifest that the vibration in different directions at the same position and the vibration in the same direction at different positions of the main girder and the main cable all exist phase difference, which varies along the main span. The deduced phase difference equation reveals that the variable of the distance from mid-span is in cosine term of the denominator, making the increasing of the distance resulting in the growth of phase difference. Due to the variation of phase difference, the vibration of the main girder and main cable along the span is asynchronous. The motion trajectories of individual points are elliptic and vary along the main span. These findings can help understand the flutter performance and mechanism of long-span bridges.

Impact of Blasting Construction of Tunnel Crossing Expressway

In this study, we monitored the blasting process for constructing a water-conveying tunnel under an expressway and analyzed the dynamic response of the existing structures along the road resulting from the blasting process. The vibration comfort index of adjacent structures was evaluated. The results show that the amplitude of the vibration velocity of existing structures along the line resulting from the blasting process ranges from 0.4 to 71.8 mm/s, and the amplitude of the vibration frequency ranges from 22.7 to 166.7 Hz. More attention should be paid to structures with natural vibration frequency in this range. The maximum vibration velocity of the measuring points on the shoulder of the expressway and near rural roads was recorded in the vertical direction, whereas that of the measuring points on the slope protection and dam mainly appeared in the horizontal direction. The effect of vibration in different directions should be considered in different construction structures. The deformation of expressway pavement, slope protection, and reservoir dam caused by the blasting process is small, which meets the safety requirements. DIN4150-2 can be used for vibration comfort control in blasting construction, but the vibration comfort index determined by DIN4150-2 is inconsistent with personnel’s perception of horizontal vibration comfort. Therefore, it is necessary to further quantify the comfort evaluation index of adjacent structures in the tunnel blasting process.

Investigation on coupled vibration of machine tool table system with position deviations

The contact state and stiffness of linear guide are affected by the complex external loads, and the vibrations in different directions are coupled with each other. However, existing dynamic models are inaccurate because they ignore displacement coupling and do not consider the position deviation of guide rail. This paper presents a five degree-of-freedom dynamic model for table system that incorporates the effects of the displacement coupling and position deviation of guide rail. The ball-to-groove contact model and position deviation model are established by using the Hertz contact theory and homogeneous coordinate transformation, respectively. Dynamic equations are solved by combining the Monte Carlo and Newmark methods to determine the displacements of table system, and the accuracy of proposed dynamic model is verified through experimental method. The results show that the vibrations of table system are coupled with each other, and the position deviation of guide rail only affects the vibration amplitudes in the region other than the resonance regions. Proposed model is thus suitable for improving the machining accuracy and stability of machine tools.

Vibration coupling effects and machining behavior of ultrasonic vibration plate device for creep-feed grinding of Inconel 718 nickel-based superalloy

Ultrasonic vibration-assisted grinding (UVAG) is an effective and promising method for machining of hard-to-cut materials. This article proposed an ultrasonic vibration plate device enabling the longitudinal full-wave and transverse half-wave (L2T1) vibration mode for UVAG. The characteristics of two-dimensional coupled vibration in different directions were analyzed on the basis of apparent elastic method and finite element method. Furthermore, a correction factor was applied to correct the frequency error caused by the apparent elastic method. Finally, the comparative experiments between the conventional creep-feed grinding and UVAG of Inconel 718 nickel-based superalloy were carried out. The results indicate that the apparent elastic method with the correction factor is accurate for the design of plate device under the L2T1 vibration mode. Compared with the conventional creep-feed grinding, the UVAG causes the reduction of grinding force and the improvement of machined surface quality of Inconel 718 nickel-based superalloy. Furthermore, under the current experimental conditions, the optimal ultrasonic vibration amplitude is determined as 6 μm, with which the minimum surface roughness is achieved.

Scale model test study of influence of joints on blasting vibration attenuation

Ground vibrations are an integral part of the process of rock blasting. The analysis of blasting vibration attenuation is the basis for a blasting risk assessment. To study the influence of rock joints on the blasting vibration attenuation, an autoclaved aerated concrete block was used as a similar material of rock in model tests of blasting vibration propagation. The attenuation process of the blasting vibration was physically simulated. The attenuation behaviors of the blasting vibrations in different directions to joint strike were fitted. Additionally, a positive correlation between the attenuation parameters and the directions was obtained. Furthermore, the Hilbert–Huang transform (HHT) based on the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) was used to analyze the vibration velocity signals. The characteristics and variation of the energy distribution were studied in the time–frequency domain. The results showed that the energy amplitude decreased as the propagation distance increased. The energy distribution gradually evolved from concentrated to discrete in the frequency domain. The low-frequency components were always present in the velocity signals at each measurement point, but the high-frequency components constantly changed due to the reflection and transmission of the waves after encountering the joint surfaces. The results of this study provide a reference for the prediction and control of the blasting vibration effect in jointed rock masses.

Experimental study on behavior of time-related preload relaxation for bolted joints subjected to vibration in different directions

The effect of preload relaxation caused by the mechanical vibration on loosening of bolted joints with self-locking capability has been widely observed. This paper proposed a combination testing strategy involving tightening and relaxation to elucidate the loss mechanism of bolt preload. A logarithmic relationship between torque coefficient and tightening torque is reported based on the results of fastening tests. Two types of specimen were designed to evaluate the preload relaxation under axial and transverse vibration. A time-varying model is proposed for predicting the residual preload of bolts. An orthogonal test involving tightening torque, amplitude, and frequency was conducted. The results show that the relaxation evolution of bolted joints subjected to vibration is similar and can be divided into two stages.

On the gyroscopic and centrifugal effects in the free vibration of rotating beams

This paper addresses the gyroscopic and centrifugal effects on vibrations of rotating Bernoulli–Euler beams. The gyroscopically coupled system is established as a complex eigenvalue problem. It is concluded that there exist three types of velocity-dependent terms that contribute to the natural frequencies with different effects. In particular, the contribution of gyroscopic terms and static and dynamic centrifugal terms to the natural frequencies are discussed for free vibrations of rotating beams with constant angular velocity. On the other hand, the vibrations in different directions coupled through the gyroscopic terms are investigated. Some numerical examples for the elliptic vibration modes in the rotating plane are presented and the results reveal the actual vibration contour of the rotating beams.

Vibration Decoupled Modeling and Robust Control of Redundant Cable-Driven Parallel Robots

Trajectory tracking and vibration control of redundant cable-driven parallel robots (CDPR) attracted the attention of different researchers; however, the effects of the CDPRs end-effectors disturbing force/moment on the undesired vibrations and minimization of such effects has received few attention. A new general dynamic modeling and robust control structure for redundant CDPR is proposed in this paper, which evaluates such factors and tries to minimize their effects on the performance of CDPRs. Considering the effects of redundant cables on the CDPRs’ stiffness improvement, we neglect the second and higher order terms of motion errors that helps to separate the moving platform's undesired vibration equation from its desired (nominal) equation of motion. The obtained vibration model forms a linear parametric variable (LPV) dynamic system that lets us investigate the effects of external disturbances on the undesired vibration in different directions, and use a wide class of well-established robust and optimal vibration control techniques. Consequently, in order to minimize the effect of disturbances on the moving platform trajectory tracking performance, a robust and optimal vibration compensator is designed using the $LPV-H_\infty$ control design techniques. Based on the platform's desired trajectory, the proposed control scheme calculates the cables’ nominal tension required for trajectory tracking and further generates the auxiliary signals to augment these tensions to counteract the undesirable vibrations. Based on the obtained dynamic model, the developed approach is simplified for kinematically-constrained redundant CDPRs as a new actuation method with restricted rotational degrees of freedom. Via numerical analysis and experimental tests of a planar kinematically-constrained CDPR, the proposed modeling and robust control design approach are examined and compared with other common control design methodologies. Moreover, the effectiveness of kinematically-constrained actuation method in vibration elimination and control design simplification of CDPRs is demonstrated.

Fatigue Life Analysis of the Proton Exchange Membrane Fuel Cell Stack

AbstractProton exchange membrane fuel cell (PEMFC) stacks usually undergo vibrations during packing, transportation and serving time, in particular for those used in automobiles and portable equipment. Using finite element method (FEM), the response stress of the stack under random vibration is analyzed in the present paper. Based on the Miner fatigue damage theory, the fatigue lifes of fuel cell components (such as bolts, gaskets and PEMs) are assessed. Then the component fatigue life contours under four working conditions, i.e., the three single‐axial (in x‐, y‐ and z‐axis separately) and multi‐axial random vibrations are obtained. Accordingly, the component damage under vibrations in different directions is evaluated. The stress distribution on gasket and PEM will greatly affect their fatigue lifes. Moreover, comparing the fatigue lifes of 4‐bolt‐ and 6‐bolt‐clamping stacks under the same total clamping force, it is found that increasing the bolt number could improve the bolt fatigue lifes.

Energy harvesting characteristics of PVDF layers on thin-walled structures with curved surface

Energy harvesting properties of PVDF layers partially covered on double-curved shallow shells are studied. Firstly, the analytical expressions of voltage on the PVDF layers induced by shell vibration are derived; then the contribution to the voltage from vibrations in different directions is explored and also the effect of geometric parameters is analyzed. The results obtained theoretically indicate that: (1) The PVDF layers used on curved surface possess higher energy harvesting capabilities than those used on planar surface. (2) With the decreasing of radius of the shell, the energy harvesting capability of the PVDF layers is improved. (3) The energy harvesting ability of the laminated PVDF layers is better than that of single layer when laminated layers are connected in series mode. To validate the analytical results, energy harvesting experiments of PVDF layers on a straight beam and a curved beam are carried out comparatively. The voltage induced by vibration of the curved beam is much higher than that produced by the straight beam and the energy harvesting rate of PVDF layers on the curved beam is more rapidly than that on the straight beam. Furthermore, the voltage on the laminated PVDF layers in series connection is almost the summation of the voltages of PVDF layers which work separately. The results obtained experimentally are in agreement with those obtained analytically.

Thermal conductivity of silicene calculated using an optimized Stillinger-Weber potential

Silicene, the silicon-based counterpart of graphene with a two-dimensional honeycomb lattice, has attracted tremendous interest both theoretically and experimentally due to its significant potential industrial applications. From the aspect of theoretical study, the widely used classical molecular dynamics simulation is an appropriate way to investigate the transport phenomena and mechanisms in nanostructures such as silicene. Unfortunately, no available interatomic potential can precisely characterize the unique features of silicene. Here, we optimized the Stillinger-Weber potential parameters specifically for a single-layer Si sheet, which can accurately reproduce the low buckling structure of silicene and the full phonon dispersion curves obtained from ab initio calculations. By performing equilibrium and nonequilibrium molecular dynamics simulations and anharmonic lattice dynamics calculations with the new potential, we reveal that the three methods consistently yield an extremely low thermal conductivity of silicene and a short phonon mean-free path, suggesting silicene as a potential candidate for high-efficiency thermoelectric materials. Moreover, by qualifying the relative contributions of lattice vibrations in different directions, we found that the longitudinal phonon modes dominate the thermal transport in silicene, which is fundamentally different from graphene, despite the similarity of their two-dimensional honeycomb lattices.

Thermal Conductivity of Zeolitic Imidazolate Framework-8: A Molecular Simulation Study

While metal–organic frameworks have been extensively investigated for gas storage and separation, their thermal properties are scarcely studied and largely unknown. We report here a molecular simulation study to examine the thermal conductivity of zeolitic imidazolate framework-8 (ZIF-8). From equilibrium molecular dynamics simulation, the thermal conductivity of ZIF-8 is predicted to be about 0.165 W/mK. The low thermal conductivity is due to the short mean free path of the phonon in ZIF-8, which is estimated to be less than two unit cells. With increasing temperature from 300 to 1000 K, the thermal conductivity increases from 0.165 to 0.190 W/mK. The temperature effect is attributed to the enhanced overlap in the vibrational density of states between Zn and N atoms. The contributions of lattice vibrations in different directions to heat flux are examined from nonequilibrium molecular dynamics simulation. It is found that the longitudinal vibration contributes 60% to thermal transport in ZIF-8; in contrast, transverse vibration contributes 40%. Furthermore, the contributions from different forces to heat flux are analyzed. While the stretching and Lennard-Jones components have 31% contribution each, the bending and Coulombic components contribute 17% and 21%, respectively. However, the contribution of the torsional component is nearly zero. This simulation study provides quantitative understanding of the thermal conductivity of ZIF-8 and microscopic insight into the mechanism of heat transfer.

The vibration discomfort of standing people: Relative importance of fore-and-aft, lateral, and vertical vibration

Few studies have compared the discomfort caused by vibration in different directions, and few have investigated the vibration discomfort of standing people. This study was designed to compare the discomfort experienced by standing people exposed to sinusoidal vibration in the fore-and-aft, lateral, and vertical directions. Using the method of magnitude estimation, 12 subjects estimated the discomfort caused by 4-Hz sinusoidal vibration at 10 different magnitudes. At 4 Hz, subjects were less sensitive to lateral vibration than to fore-and-aft vibration (Ky/Kx = 0.71), and more sensitive to vertical vibration than to horizontal vibration (Kz/Kx = 1.95; Kz/Ky = 2.77). Previous findings showing how the discomfort of standing people depends on the frequency of fore-and-aft, lateral, and vertical vibration were used to define frequency weightings that reflect relative sensitivity to vibration in each direction. The frequency weightings differ from those appropriate for seated people, and differ from the weightings for standing people in current standards that were mostly derived from understanding of the discomfort of seated people.

Indefinite permittivity in uniaxial single crystal at infrared frequency

We propose a kind of indefinite media at infrared frequency without artificial structures. Natural existing indefinite media are realized based on strong anisotropy in polar lattice vibrations in different directions, parallel or perpendicular to the crystal axis. As an example, the dielectric dispersions of α-Al2O3 single crystal with different polarizations of electric field are measured, the regions of indefinite permittivity are demonstrated and an all-angle negative refraction is also shown based on the simulated results. This mechanism to realize indefinite medium will supply a promising way to explore natural existing negative refraction materials.

Interionic potentials and localized vibrations in Al2O3 crystals with vacancies

Empirical parameters of interionic interactions in α-Al2O3 crystals were determined in terms of the shell model. The lattice dynamics of α-Al2O3 crystals with vacancies in different charge states was simulated using these parameters and the recursive method. Frequencies of defect-induced resonance vibrations in different directions were determined. Main features specific to vibration spectra of anion-nonstoichiometric α-Al2O3 crystals, which were due basically to changes in the effective force of interaction between vacancies and the nearest environment, were analyzed and explained.

The three-dimensional equivalent circuit and the natural frequencies of rectangular piezoelectric ceramic resonators

The three-dimensional coupled vibration of the piezoelectric ceramic resonator is analyzed when the shearing stress and strain are ignored. Three mechanical coupling coefficients are introduced and the coupled vibration is reduced to three equivalent one-dimensional extensional vibrations. On the basis of the analysis, the three-dimensional equivalent circuit is derived. From the equivalent circuit, it can be seen that as the three equivalent one-dimensional vibrations are coupled to each other, a new kind of electromechanical transformation force arises in the circuit. The natural frequency equation is derived and the natural frequencies of some rectangular piezoelectric ceramic resonators are computed. As the mechanical coupling between the vibrations in different directions and the piezoelectric effect are considered, the computed resonance frequencies are in good agreement with the measured results.

Relative Importance of the Various Vibration Transmitting Mechanisms in Hoses in Typical Hydraulic Systems

The relative magnitudes of vibration in different directions and pressure fluctuations generated by typical hydraulic pumps are examined in conjunction with the extent to which they are transmitted along different types of hose to the subsequent circuit. By using a vibratory power flow approach, and looking at trends in the absence of resonances, it is possible to compare the importance of the different excitations and of their transmission by axial, bending and torsional waves along the hose. The predominant effect of pressure fluctuations from the pump is revealed.