Vibration Modes Of Structure Research Articles

Topology optimization of dynamic acoustic–mechanical structures using the ersatz material model

Topology optimization of dynamic acoustic–mechanical structures is challenging due to the interaction between the acoustic and structural domains and artificial localized vibration modes of structures. This paper presents a floating projection topology optimization (FPTO) method based on the mixed displacement/pressure (u/p) finite element formulation and the ersatz material model. The former is able to release the need for tracking the interface boundaries explicitly between the structural and acoustic domains during the optimization process. The ersatz material model enables us to entirely avoid artificial localized vibration modes caused by the extremely high ratio between mass and stiffness. The floating projection simulates the original 0/1 constraints, and it gradually pushes the design variables toward 0 or 1 at the desired level so that the optimized element-based design can be accurately represented by a smooth design. Some 2D and 3D numerical examples, including minimizing sound pressure at the designated domain, restraining structural vibration, and maximizing sound transmission loss, are presented to demonstrate the effectiveness of the proposed topology optimization algorithm. The optimized solutions achieve the consistency of the objective function between the element-based design using the mixed formulation and the smooth design using the segregated formulation. The study suggests that the FPTO method using the ersatz material model is a promising approach for optimizing dynamic acoustic-mechanical structures.

Forced Oscillations of a Finite Cylindrical Shell Excited by Discrete Forces

In this paper, we propose a numerical-analytical method for calculating forced oscillations of a shell structure composed of a set of finite elastic cylindrical shells and elastic rings, to which discrete disturbing forces are applied. Examples of calculating the amplitude–frequency characteristics and vibration modes of the shell structure are given.

A Numerical Approach to Estimate Natural Frequency of Trees with Variable Properties

Free vibration analysis of a Euler-Bernoulli tapered column was conducted using the finite element method to identify the vibration modes of an equivalent tree structure under a specified set of conditions. A non-prismatic elastic circular column of height L was analysed, taking distributed self-weight into account. Various scenarios were considered: column taper, base fixity, radial and longitudinal stiffness (E) and density (ρ) and crown mass. The effect on the first natural frequency was assessed in each case. Validation against closed form solutions of benchmark problems was conducted satisfactorily. The results show that column taper, base fixity and E/ρ ratio are particularly important for this problem. Comparison of predictions with field observations of natural sway frequency for almost 700 coniferous and broadleaved trees from the published literature showed that the model worked well for coniferous trees, but less well for broadleaved trees with their more complicated crown architecture. Overall, the current study provides an in-depth numerical investigation of material properties, geometric properties and boundary conditions to create further understanding of vibration behaviour in trees.

Experimental and numerical analysis on serviceability of cantilevered floor based on human-structure interaction

To evaluate the vibration serviceability of structure under human-induced excitation, experimental and numerical analysis were conducted on a cantilevered floor of a gymnasium, which was assumed as a case study. A series of field tests were performed using the ambient excitation and the human excitation approaches on the floor to obtain the fundamental vibration characteristics and human-induced response, respectively. Finite element models (FEMs) of the cantilevered floor were established, modified and validated according to the field test results and the weak constraint effect of glass curtain walls was proposed in this paper. The numerical analysis of human-induced vibration was conducted by considering the pedestrian load as the combination of the Fourier series load model and the mass-spring-damper (MSD) human dynamic model. To better simulate the human-structure interaction (HSI), both the main-harmonics and sub-harmonics load spectra were considered for the equivalent load model due to the characteristics of narrow bands. The results showed that both structural vibration modes and human-induced acceleration responses were in good agreement compared with experimental results. The weak constraint effect of glass curtain walls was then validated. The numerical results were more accurate when considering the influence of HSI on the structural vibration serviceability in practical engineering applications.

Novel low-permittivity microwave dielectric ceramics in garnet-type Ca4ZrGe3O12

Novel low-permittivity microwave dielectric ceramics Ca4ZrGe3O12 with garnet structure were fabricated by a traditional solid-state route. A part of the Ca2+ cations are located at the octahedral sites, a quite unusual coordination for this cation in oxygen compounds. The structural characteristics and vibrational modes for the ceramics were studied based on structure refinement and Raman spectra analysis. Owning to the octahedral site is occupied by equivalent Ca2+ and Zr4+ simultaneously, the deviation between εtheo and εr could be related to the rattling effect of smaller Zr4+ in the octahedron. Optimum microwave dielectric properties were obtained for Ca4ZrGe3O12 ceramic sintered at 1340 °C with εr = 10.68, Q × f = 76900 GHz, and τf = −41.3 ppm/°C.

Wave Superposition Method Applied To Calculate the Radiation Sound Power of a Stiffened Plate

An indirect boundary element method using discrete sound field of virtual sound source is used to calculate the acoustic and vibration coupling response of the stiffened plate in water. In this paper, the relationship between the structural vibration mode coefficient and the virtual sound source intensity is established by using the superposition principle through Fourier transform using structural dynamics equations and boundary compatibility condition satisfying the function of virtual sound source intensity. Once the virtual sound source intensity is obtained, the radiation power can be determined. Taking the simply supported rectangular thin plate as an example, the sound radiation power is calculated on the condition that surface velocity is unknown, and the results are compared with the results of the analytical method, which shows that the method has high accuracy.

Improved pushover analysis for underground large-scale frame structures based on structural dynamic responses

This paper proposes an improved pushover analysis for underground large-scale frame structures (ULSFSs) that includes structural dynamic responses. The ULSFS employed is used in underground supermarket construction, and it has an extremely large-scale in two horizontal directions. Traditional pushover analysis for underground structures (UPA) is used for structures with small-scale, but results show that it is not sufficiently accurate for use with ULSFSs (either with and without structural joints), as structural vibration is neglected. Therefore, the structural dynamic coefficient of ULSFSs relative to the structural vibration mode is obtained, and an improved pushover analysis (ULPA) is proposed that considers both structural vibration and site characteristics. The applicability of using the ULPA is verified by comparing the seismic response with a dynamic time history analysis, and the seismic responses of ULSFS under combined horizontal and vertical earthquakes are also investigated using the ULPA. The results show that the ULPA is more highly reliable than the UPA for ULSFSs, particularly for structures with structural joints, and it is feasible for use under both unidirectional and bidirectional earthquakes.

A vibration mode-based adhesion impulse characterization technique

In this paper a technique for the characterization of small impulses produced by adhesion force is described. The need to address this topic arose in the frame of the ground testing of a space mechanism dedicated to the precise injection of a body into a geodesic trajectory, both as a reference for the spacecraft navigation and for a scientific measurement. The experimental facility consists of a proof mass suspended by a simple pendulum, and a second actuated body which can be moved and quickly retracted from the adhesive contact. At the retraction, the motion of the proof mass is measured to estimate the impulse (and its time duration) produced by the adhesive bonds arose between the two bodies, up to separation. In the current set-up, systematic effects (elastic energy stored in the system), structural vibration modes and measurement noise affect the displacement signal of the proof mass, dominating the adhesion contribution under study. However, the information on the force impulse and time duration – even if with a low signal to noise ratio – is available in the different response of the vibration modes to the same input. An innovative technique is developed to estimate the adhesion impulse and time duration from the amplitudes of oscillation of the vibration modes of the proof mass in a relevant point of the modal shapes. Since no modification nor installation of additional sensors on the sensing body are required, the method can be applied also to systems with limited access and/or possibility of interaction. The vibration modes of the proof mass are calculated by means of FEA allowing for an accurate modal decomposition of the measured signal, focusing on the bottom end of the relevant bandwidth. The amplitudes of oscillation of each mode, even if affected by systematic effects and strong disturbances, are estimated by means of optimal filtering of the dynamic response. By means of the vibration mode-based mathematical model the adhesion impulse and its time duration are calculated from the estimated oscillation amplitudes.

Crystal structure, Raman spectra and microwave dielectric properties of novel temperature-stable LiYbSiO4 ceramics

Olivine−type structure microwave dielectric ceramics LiYbSiO4 with near-zero τf were fabricated by solid-state reaction process for the first time. The relationships among structural parameters, sintering behavior, vibrational modes and microwave dielectric properties for the ceramics were studied. The variation in εr could be correlated with Raman shift. The variation in Q×f values was inversely correlated to FWMH and average cation covalency. The τf values were explained with the bond valence sum of cations. Single phase LiYbSiO4 ceramics could be obtained at the range of 1100–1140 °C and showed promising microwave dielectric properties with εr = 7.36 – 7.42, Q×f = 19081 – 25276 GHz and τf = +4.52 – +8.03 ppm/°C.

Remote vibrometry recognition of nonlinear eigen-states for object coverage of randomly large size

For objects of “large” vibration size such as waves on the sea surface, the choice of measurement method can create different understandings of system behavior. In one case, laser vibrometry measurements of a vibrating bar in a controlled laboratory setting, variation in probe spot size can omit or uncover crucial structural vibration mode coupling data. In another case, a finite element simulation of laser vibrometry measures a nonlinearly clattering armor plate system of a ground vehicle. The simulation shows that sensing the system dynamics simultaneously over the entire structure reveals more vibration data than point measurements using a small diameter laser beam spot, regardless of the variation of footprint (coverage) boundaries. Furthermore, a simulation method described herein allows calculation of transition probabilities between modes (change-of-state). Wideband results of both cases demonstrate the 1/f trend explained within – that the energy of discrete structural vibration modes tends to decrease with increasing mode number (and frequency), and why. These results quantify the use of less expensive non-imaging classification systems for vehicle identification using the remote sensing of surface vibrations while mitigating spectral response distortion due to coverage variation on the order of the structural wavelength (spectral reduction or elimination).

Shot-Noise-Limited Two-Color Stimulated Raman Scattering Microscopy with a Balanced Detection Scheme

Stimulated Raman scattering (SRS) microscopy has been considered a useful technique for investigating chemical components by selectively targeting the vibration mode of chemical structures. Its practical application to the observation of molecular structures and dynamics in complicated biological environments requires broad spectral coverage with both high resolution and a high signal-to-noise ratio. Here, we demonstrate a two-color SRS microscopy employing a balanced detection scheme and a spectral focusing method. Two different SRS signals are generated with pump and Stokes laser pulse pairs in perpendicular polarization, where each of them acts as an intensity reference for the other, significantly reducing the background noise level close to the shot-noise limit even with a fiber-based femtosecond laser system. The high spectral resolution comparable to that of spontaneous Raman scattering spectroscopy is achieved with the spectral focusing method. The two-color SRS images are obtained for a mixture of polymer beads and for the distributions of lipids and proteins in U2OS cells.

Forced Vibrations of a Cylindrical Shell Immersed in Liquid

A numerical-analytical method is proposed for calculating the forced vibrations of a shell structure immersed in liquid. The structure is a set of finite elastic cylindrical shells and elastic rings to which concentrated discrete forces are applied. Examples of a comparative calculation of the frequency response and vibration modes of the shell structure in a vacuum and in liquid are given.

A root-to-foliage tree dynamic model for gusty winds during windstorm conditions

Improving forest wind risk models relies on a better understanding of the tree response to damaging gusts. However, our empirical knowledge of the tree dynamic response to wind is limited by the feasibility of field experiments at recording the dynamics of both, the tree aerial structure and the tree root-system, under windstorm conditions. Here, we present the first complete root-to-foliage tree dynamic model simulating the tree aerial structure and root-soil system responses to turbulent winds, including the possibility of tree breakage and tree uprooting. This model allowed us to investigate the impact of both (1) various anchorage conditions, related to the soil humidity or root rigidity, and (2) the history of gusts experienced by the tree, on the dynamics and windthrow vulnerability of a Pinus pinaster tree. The model suggests a negligible sensitivity of the upper tree swaying dynamics to the anchorage condition as opposed to the stem base movement. The low-frequency motions and natural vibration modes of the tree aerial structure in response to the wind are felt by the roots, with a propagation intensity depending on the root-soil system properties. Only the extremities of the transverse roots appear weakly impacted by the tree aerial motion. A study of the tree uprooting to the wind load demonstrates the importance of the gustiness history experienced by the tree in weakening the tree anchorage, and thus in progressively enhancing the tree vulnerability to uprooting. This result suggests that the critical overturning bending moment predicted by static tree-pulling experiments may overestimate the ultimate critical bending moment in situations where trees are damaged progressively by a succession of wind gusts. This novel model represents a promising tool for better understanding the tree response to gusty winds for various plant anchorage conditions.

Determination of satellite structural modes of vibration using machine vision

A machine vision method based on an eigenvalue realization algorithm was proposed to determine the FY-4 satellite structural modes of vibration. An experiment was designed to prove the feasibility of the proposed method. The experiment results show that the first mode 0.32 Hz and the second mode 1.57 Hz of the FY-4 satellite structure were both clearly distinguished from the background by the proposed method. The comparison of the results of the proposed method and that of simulation and gyroscope testing shows that the error rate of the proposed method is below 0.1% for the first mode and below 6% for the second mode, which is feasible for engineering applications.

Effect of large displacements on the linearized vibration of composite beams

Natural frequencies and mode shapes are functions of the equilibrium state. In the large displacement regime, pre-stresses may modify significantly the modal behaviour of structures. In this work, a geometrical nonlinear total Lagrangian formulation that includes cross-sectional deformations is developed to analyse the vibration modes of composite beams structures in the nonlinear regime. Equations of motion are solved around nonlinear static equilibrium states, which are identified using a Newton–Raphson algorithm along with a path-following method of arc-length type. Different boundary conditions and stacking sequences are analysed. It is shown that vibration modes are strongly modified by nonlinear phenomena. Moreover, models that do not describe those effects accurately may results in misleading results, especially if compression is dominant. In fact, results show a crossing phenomenon in the post-buckling regime of an asymmetric cross-ply beam, whereas it is completely unforeseen by the linearized analysis.

Structural health monitoring and performance analysis of a 12-story recycled aggregate concrete structure

To encourage the application of recycled coarse aggregates (RCA) produced from construction and demolition (C&D) waste in newly-built structures, this study investigates the dynamic behaviors of a 12-story recycled aggregate concrete (RAC) in the operational environment through structural health monitoring (SHM), and compares with an adjacent 12-story natural aggregate concrete (NAC) structure with similar arrangements of components and structural construction. The SHM has undergone 12 months, and has provided a wealth of data detailing the dynamic behaviors with the variation in the operational environment as well as significant structural responses excited by typhoons and earthquakes. Firstly, the first 5 vibration modes of both structures are identified, and the effects of the ambient temperature, hourly mean wind speed and structural response amplitude are analyzed. A lower natural frequency and a larger damping ratio of the fundamental vibration mode of the RAC structure are found than those of the NAC structure. For both the RAC and NAC structures, the fundamental frequency increases with the increasing ambient temperature, the decreasing hourly mean wind speed, and the decreasing structural response amplitude. The opposite trend of the damping ratios is identified. Secondly, the stick-slip model is used to explain the differences between the modal parameters of the RAC and the NAC structures, and to discover the mechanism of the effects of the ambient temperature, the hourly mean wind speed, and the structural response amplitude. Thirdly, the serviceability of the RAC structure is evaluated based on the full-scale measurements during typhoons. Finally, the artificial neural network (ANN) is validated to have the capacity for predicting the modal parameters precisely, and the model error of the ANN has the potential as an indicator for detecting abnormal structural performances.

Cell alignment and accumulation using acoustic nozzle for bioprinting

Bioprinting could spatially align various cells in high accuracy to simulate complex and highly organized native tissues. However, the uniform suspension and low concentration of cells in the bioink and subsequently printed construct usually results in weak cell-cell interaction and slow proliferation. Acoustic manipulation of biological cells during the extrusion-based bioprinting by a specific structural vibration mode was proposed and evaluated. Both C2C12 cells and human umbilical vein endothelial cells (HUVECs) could be effectively and quickly accumulated at the center of the cylindrical tube and consequently the middle of the printed construct with acoustic excitation at the driving frequency of 871 kHz. The full width at half maximum (FWHM) of cell distributions fitted with a Gaussian curve showed a significant reduction by about 2.2 fold in the printed construct. The viability, morphology, and differentiation of these cells were monitored and compared. C2C12 cells that were undergone the acoustic excitation had nuclei oriented densely within ±30° and decreased circularity index by 1.91 fold or significant cell elongation in the printing direction. In addition, the formation of the capillary-like structure in the HUVECs construct was found. The number of nodes, junctions, meshes, and branches of HUVECs on day 14 was significantly greater with acoustic excitation for the enhanced neovascularization. Altogether, the proposed acoustic technology can satisfactorily accumulate/pattern biological cells in the printed construct at high biocompatibility. The enhanced cell interaction and differentiation could subsequently improve the performance and functionalities of the engineered tissue samples.

3D structured laser engraves decorated with gold nanoparticle SERS chips for paraquat herbicide detection in environments

Fabrication of SERS-active substrates is a thrust area of research in the field of SERS. Here, we report the fabrication of a new SERS-active substrate composed of Au nanostructures grown on aluminum sheets marked with laser lines (L-Mark SERS chips). The SERS activity of these SERS chips exploited with environmentally important herbicide, paraquat, and dye molecule of methylene blue. Density functional theory (DFT) calculation was performed to theoretically study the molecular structure and Raman vibrational modes of the paraquat. A detailed discussion has been devoted to uniformity, reproducibility, and practical utility of the fabricated substrates. Especially, great amount of discussion has been given to paraquat detection in soil extracts. The results emanate the ultra-sensitive detection of paraquat, uniform enhanced signals over the substrates, and high reproducibility. The report successfully demonstrated the detection of paraquat in drinking water, tap water, agri-water, environmental water, and soil extracts. Liquid chromatography-mass spectrometry (LCMS) measurements were also performed to demonstrate the realiability of the SERS method. The L-Mark SERS chips based paraquat detection is simple, rapid, and sensitive. We propose that the present sensors hold the potential to be used in herbicide detection and monitoring in-the-field experiments.

Study on membrane roof schemes of annular crossed cable-truss structure

Based on annular crossed cable-truss structure model with diameter of 17.15 m, the type of membrane roof was first studied. Two membrane roof schemes, cable-supported membrane roof and skeleton-supported membrane roof, were proposed. The form-finding analysis of two types of membrane roof structures was carried out. By comparative study, the strengths and weaknesses of two schemes were given, and the feasibility of skeleton-supported membrane roof was demonstrated. Second, skeleton-supported membrane roof was studied by collaborative form-finding and non-collaborative form-finding. The form-finding method being suitable for the structure was given. Finally, load analysis, parametric analysis, and dynamic characteristics of membrane and non-membrane roof structures were carried out. The results show that skeleton-supported membrane roof has the strength to resist to external loads. Poisson’s ratio and self-weight have little influence on membrane roof, and arch height and pretension of membrane surface have great influence on membrane roof. The vibration modes of structure are mainly manifested as up and down vibrations accompanying with local torsion of planar cable-truss frame, which is not bad for overall structure. Meanwhile, film-covering effects of annular crossed cable-truss structure are obvious. The study contents have promoted the application of annular crossed cable-truss structure in practical engineering.

Structural, magnetic, grain and grain boundary mediated conduction features of low dimensional LaFeO3 nanoparticles

A detailed investigation of crystal structure, magnetic and electrical conduction properties of a low dimensional LaFeO3 is reported. The sample is synthesized by Sol-Gel method with a low preparation temperature. Synchrotron x-ray diffraction, Raman and FTIR spectroscopy methods were used to establish the structural phase and vibrational modes present in the sample. The low dimensionality and morphology of the prepared sample is probed through transmission electron microscopy. From the explicitly field and temperature dependent magnetization, the magnetic phase exhibited by the nanoparticle is weak ferromagnetic which is further evidencing from the Fe57 Mossbauer spectroscopy. To gain further understanding of electrical conduction mechanism and related features of AC conductivity, impedance spectroscopy techniques are used. It is noticed that, the grain effect is dominated while the electrode effect is suppressed with temperature. The activation energies due to grain and grain boundary effect are found to be 0.0780 eV, 0.175 eV for T < 90 °C and 0.451 eV, 0.525 eV for T > 90 °C respectively. Finally, jump relaxation model and Jonscher’s power law are used to explain the frequency-dependent conduction behavior in the system.