Low Vibrational Modes Research Articles

Negative Thermal Expansion of GaFe(CN)6 and Effect of Na Insertion by First-Principles Calculations**Supported by the National Natural Science Foundation of China under Grant Nos 11874328 and 11372283.

We study the negative thermal expansion (NTE) properties and effect of Na insertion on the NTE of the framework material GaFe(CN)6 by first-principles calculations based on density functional theory within the quasi-harmonic approximation. The calculated results show that the material exhibits NTE due to the low transverse vibrational modes of the CN groups. The modes demonstrate larger negative values of the mode Grüneisen parameters. Once Na is introduced in the framework of the material, it prefers to locate at the center of the quadrates of the framework material and binds to the four N anions nearby. As a consequence, the transverse vibrational mode of the CN group is clearly hindered and the NTE of the material is weakened. Our theoretical calculations have clarified the mechanisms of NTE and the effect of the guest Na on the NTE of the framework material.

Sensitivity analysis of an open TBM gripping-thrusting-regripping mechanism based on the natural characteristics

The gripping-thrusting-regripping mechanism, as the core transmission components of the open TBMs, is always subjected to the strong and low frequency distributed impacts in tunneling operations, causing the severe vibration and unexpected damage of the key components. It is therefore crucial to gain deep insight into the basic vibration characteristics. With the consideration of the stiffness of the actuating hydraulic cylinders, coupling of the generalized coordinates and elasticity of main beam, etc., the dynamic model is established by Lagrangian method to analyze the lower order natural frequencies and relevant vibration modes, and it is validated by the finite element model developed in SAMCEF software. The effects of the stiffness, inertial and dimensional parameters on natural frequencies as well as their sensitivity are investigated in detail. The results indicate that the 1st mode shape is the rotation around z-axis of cutter-head, main beam and carrier, and the 2-4th mode shapes are the translations at y, x and z directions; the parameters of cutter-head support unit and propel cylinder are the main factors affecting the natural characteristics. These findings can provide some references to design the mechanical structure and to select the duty parameters in engineering applications.

Ta4C3 MXene as supercapacitor electrodes

MXenes, 2D layered solids is a hot topic in 2D materials proving its potential for various niche applications. The present work focuses on the synthesis of tantalum carbide MXene sheets by etching the intermediate “aluminium” from the parental Ta4AlC3 MAX phase using hydro fluoricacid. The synthesized MXene' were analyzed by X-ray diffraction for its structural analysis, FE-SEM, TEM for morphological studies and Raman spectroscopy for vibrational analysis. XRD analysis detailed on the crystal structure revealing that the tantalum carbide MXene is a hexagonal crystal lattice. FESEM revealed that the morphology of the synthesized tantalum carbide were layered solid structures and micro Raman vibrational studies reveal the lower vibrational modes, which attributes to the intermediate “A” element, which were not visible due to the absence of aluminium after etching. Furthermore, the synthesized MXene were tested for its electrochemical behavior towards electrochemical energy storage applications using techniques like cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). Characterization and electrochemical testing showed promising performance in electrochemical charge storage applications.

Modal parameters estimation of an electrical submersible pump installed in a test well using numerical and experimental analysis

Modal parameters of an Electrical Submersible Pump (ESP) installed in a test well were numerically and experimentally identified. The numerical model was developed using the finite element method. All the masses and stiffness elements necessary to evaluate the lateral vibration of the equipment were considered, as well as the hydrodynamic added mass effect. In the experimental test, an impact hammer was used for exciting the ESP structure near the wellhead, in the production pipe, and not directly at equipment frame. Vibration signals along the ESP were acquired and Eigensystem Realization Algorithm (ERA) method was used to identify the modal properties. Results showed the impact test as a useful method for modal parameters identification of ESP's installed in a test well. However, due to the location where the impacts were applied, the lower vibration modes, outside the operating range, were not excited. On the other hand, the natural frequencies numerically identified were overestimated in comparison with those obtained experimentally.

Effect of column size on the seismic capacity of elevated reinforced concrete water tank

Elevated reinforced concrete water tanks are one of the most essential structure to major cities and also in rural areas before, during and after a disaster such earthquake. It is important to prevent elevated reinforced concrete water tank from collapse so the water supply can be maintained. One of the methods used to evaluate the seismic capacity of elevated reinforced concrete water tank is the pushover analysis. Pushover analysis is based on the assumption that structure oscillate predominantly in the first mode or in the lower modes of vibration during a seismic event. The objective of this project is to study the effect of column size on the seismic capacity of elevated reinforced concrete water tank. A total number of 10 models of elevated reinforced concrete water tanks consist of 4 storey and 7 storey has been used for this project. All models have been designed repeatedly to 5 different size of column for each storey, where the beam size is fixed for each model. All models have been designed based on BS8110 to represent the existing elevated reinforced concrete water tanks. Then the pushover analysis has been conducted on all models to study the seismic capacity of elevated reinforced concrete water tank. An adequate information on seismic demands imposed on the structural system and its components by the designed ground motion will be provided from the pushover analysis. Based on the pushover analysis conducted in this study, the elevated RC water tank with larger size of column tend to have higher value of force at yield limit state and ultimate limit state.

Direct excitation of molecular vibrational motion by intense broadband THz pulses

We theoretically investigate molecular vibrational dynamics driven by intense and broadband terahertz (THz) pulses. The light–nuclei interaction is derived from a fully quantum-mechanical minimal coupling Hamiltonian to analyse the direct excitation of vibrational modes by the THz pulses. We show that for diatomic molecules a ‘vibrational moment’ can be defined in a manner similar to the dipole moment and that it has non-zero values only for heteronuclear diatomic molecules. Taking ultra-cold heteronuclear diatomic molecules as examples, we show that vibrational-mode excitation by THz pulse irradiation occurs in an almost stepwise manner, especially for low vibrational modes. A coherent control method using a frequency-controlled THz pulse sequence is also proposed.

Vibration measurement and structural performance evaluation of a 17th century timber guest house in Japan

AbstractTo clarify the actual structural performance of an existing traditional timber structure in Japan, vibration measurements were carried out at a 17th century timber guest house (Gekkouden of Gokokuji temple). The results of the micro tremor measurement, earthquake monitoring, and analysis are discussed in this paper. For minute angles of less than 1/5000 rad, it was revealed through earthquake monitoring that each dominant frequency was strongly dependent on the vibration amplitude. It was also confirmed that the dominant frequencies of the lower vibration modes in 1/1000 rad declined to 65%‐75% of the frequencies obtained through micro tremor measurement. We propose a structural model that consists of a joint spring, column, beam, and hanging wall, based on previously reported experiments. In this study, it was clarified that the dominant frequencies determined by the analysis of the proposed structural model were in good agreement with the frequencies obtained through earthquake monitoring.

Vibration behavior of metallic sandwich panels with Hourglass truss cores

Exploring lightweight sandwich structures with excellent load-bearing and vibration damping performances is one of the important topics in structural and functional applications. The aim of the present work is to design, fabricate novel metallic sandwich structures with Hourglass truss cores and investigate their modal characteristics and vibration isolation performances by comparing with the traditional pyramidal sandwich structure experimentally and numerically. It is shown that the natural frequencies of the Hourglass sandwich structures are much higher than that of the pyramidal sandwich structures under free-free boundary condition and equal relative density of the truss cores. The torsional modes and transverse bending modes of the Hourglass sandwich structures separately play a dominant role in lower and higher modes of vibration, which are opposite to the pyramidal sandwich structures. From the results of the acceleration frequency responses and vibration level difference (VLD), it is indicated that the Hourglass sandwich structures exhibit better vibration isolation performance than the pyramidal sandwich structures. Furthermore, experimentally validated finite element analysis (FEA) models are developed to study the influences of truss inclination angles and boundary conditions on the modal characteristics of the present sandwich structures. Some conclusions are summarized, which may be useful for understanding the vibration behavior of such kinds of lattice sandwich structures.

The Energy Flow Analysis as a Tool for Identification of Damping in Tall Buildings Subjected to Wind: Contributions of the Foundation and the Building Structure

In this paper, the energy dissipated in a tall building is identified by means of the energy flow analysis. This approach allows assessing energy dissipation within a specific domain or element of the structure. In this work, the focus is placed on the superstructure, which is the part of the building above the ground, and on the foundation. Damping operators for the superstructure and the foundation are formulated based on the identified energy dissipation in these parts of the building. The obtained damping operators are used to compute the modal damping ratios in a simplified model of the building. The modal damping ratios of the three lowest modes of vibration are compared to those identified in full-scale measurements by means of the half-power bandwidth method.

The Structure of [HSi9 ]3- in the Solid State and Its Unexpected Highly Dynamic Behavior in Solution.

We report on the first unambiguous detection of the elusive [HSi9 ]3- anion in solutions of liquid ammonia by various 29 Si and 1 H NMR experiments including chemical exchange saturation transfer (CEST). The characteristic multiplicity patterns of both the 29 Si and 1 H resonances together with CEST and a partially reduced 1 H,29 Si coupling constant indicate the presence of a highly dynamic Si8 entity and a Si-H moiety with slow proton hopping. Theoretical calculations corroborate both reorganization of Si8 on the picosecond timescale via low vibrational modes and proton hopping. In addition, in a single-crystal X-ray study of (K(DB[18]crown-6))(K([2.2.2]crypt))2 [HSi9 ]⋅8.5 NH3 , the H atom was unequivocally localized at one vertex of the basal square of the monocapped square-antiprismatic cluster. Thus experimental studies and theoretical considerations provide unprecedented insight into both the structure and the dynamic behavior of these cluster anions, which hitherto had been considered to be rigid.

Arresting "Loose Bolt" Internal Conversion from -B(OH)2 Groups is the Mechanism for Emission Turn-On in ortho-Aminomethylphenylboronic Acid-Based Saccharide Sensors.

Different mechanisms for the emission turn-on of ortho-aminomethylphenylboronic acids with appended fluorophores in response to saccharide binding in aqueous media have been postulated, such as photoinduced electron transfer (PET), "pKa switch", and disaggregation. However, none of the hypotheses is consistent with all the data for boronic acid-based sensors. To create a unifying theory that can explain the data, we performed a series of experiments to explore the origin of the emission turn-on with several boronic-acid based sensors upon binding fructose. First, we showed that the receptors and their complexes with fructose are solvent-inserted, with no B-N interactions. Second, we verified that the sensors are not aggregated. Third, in pure methanol, that exchanges -B(OH)2 to -B(OMe)2 groups, we found no fluorescence response upon binding fructose. We propose this occurs via lessening of internal conversion mechanisms. To investigate this proposal further, we performed a solvent isotope effect study. The fluorescence of the probes in D2O (-B(OH)2 → -B(OD)2) does not change upon fructose binding. It is well accepted that -OD oscillators are less efficient energy acceptors due to their lower frequency vibrational modes. Thus, our studies reveal that modulating the -B(OH)2-induced internal conversion (an example of a "loose bolt effect") explains how potentially all ortho-aminomethylphenylboronic acid-based fluorescence sensors signal the presence of sugars.

Understanding global infrared opacity and hot bands of greenhouse molecules with low vibrational modes from first-principles calculations: the case of CF4.

Fluorine containing molecules have a particularly long atmospheric lifetime and their very big estimated global warming potentials are expected to rapidly increase in the future. This work is focused on the global theoretical prediction of infrared spectra of the tetrafluoromethane molecule that is considered as a potentially powerful greenhouse gas having the largest estimated lifetime of over 50 000 years in the atmosphere. The presence of relatively low vibrational frequencies makes the Boltzmann population of the excited levels important. Consequently, the "hot bands" corresponding to transitions among excited rovibrational states contribute significantly to the CF4 opacity in the infrared even at room temperature conditions but the existing laboratory data analyses are not sufficiently complete. In this work, we construct the first accurate and complete ab initio based line lists for CF4 in the range 0-4000 cm-1, containing rovibrational bands that are the most active in absorption. An efficient basis set compression method was applied to predict more than 700 new bands and subbands via variational nuclear motion calculations. We show that already at room temperature a quasi-continuum of overlapping weak lines appears in the CF4 infrared spectra due to the increasing density of bands and transitions. In order to converge the infrared opacity at room temperature, it was necessary to include a high rotational quantum number up to J = 80 resulting in 2 billion rovibrational transitions. In order to make the cross-section simulation faster, we have partitioned our data into two parts: (a) strong & medium line lists with lower energy levels for calculation of selective absorption features that can be used at various temperatures and (b) compressed "super-line" libraries of very weak transitions contributing to the quasi-continuum modelling. Comparisons with raw previously unassigned experimental spectra showed a very good accuracy for integrated absorbance in the entire range of the reported spectra predictions. The data obtained in this work will be made available through the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru) that contains ab initio born line lists and provides a user-friendly graphical interface for a fast simulation of the CF4 absorption cross-sections and radiance under various temperature conditions from 80 K to 400 K.

Interrogation of a ring-resonator ultrasound sensor using a fiber Mach-Zehnder interferometer

We experimentally demonstrate an interrogation procedure of a ring-resonator ultrasound sensor using a fiber Mach-Zehnder interferometer (MZI). The sensor comprises a silicon ring resonator (RR) located on a silicon-oxide membrane, designed to have its lowest vibrational mode in the MHz range, which is the range of intravascular ultrasound (IVUS) imaging. Ultrasound incident on the membrane excites its vibrational mode and as a result induces a modulation of the resonance wavelength of the RR, which is a measure of the amplitude of the ultrasound waves. The interrogation procedure developed is based on the mathematical description of the interrogator operation presented in Appendix A, where we identify the amplitude of the angular deflection Φ0 on the circle arc periodically traced in the plane of the two orthogonal interrogator voltages, as the principal sensor signal. Interrogation is demonstrated for two sensors with membrane vibrational modes at 1.3 and 0.77 MHz, by applying continuous wave ultrasound in a wide pressure range. Ultrasound is detected at a pressure as low as 1.2 Pa. Two optical path differences (OPDs) of the MZI are used. Thus, different interference conditions of the optical signals are defined, leading to a higher apparent sensitivity for the larger OPD, which is accompanied by a weaker signal, however. Independent measurements using the modulation method yield a resonance modulation per unit of pressure of 21.4 fm/Pa (sensor #1) and 103.8 fm/Pa (sensor #2).

A spectral-velocity-based combination-type earthquake intensity measure for super high-rise buildings

A spectral-velocity-based combination-type ground motion intensity measure (IM), which is inspired by the superior sensitivity of spectral velocity to structural response compared with other two spectral quantities, is proposed for super high-rise building structures with the consideration of the characteristics of ground motions and structures themselves. Two super high-rise buildings with typical frame/core-tube/outrigger lateral resisting system and a wide range of structural height (H = 258 m~660 m) are deliberately selected to identify the correlation between the maximum inter-story drift ratio and nineteen IMs developed in recent decade together with the proposed IM, with sixty chosen far-field and near-field pulse-like ground motions. With the suggested optimal number of lower vibration modes and corresponding combination factors, the efficiency, sufficiency and the scaling robustness of the proposed IM is further demonstrated. In addition, the relative sufficiency measures of other IMs with respect to the proposed IM are investigated. The results from the study indicate that, the proposed IM is believed, from the standpoint of efficiency, sufficiency, relative sufficiency measure and scaling robustness, to be a more specialized and desirable tool for super high-rise buildings, either for far-field or near-field ground motions.

Stepwise relaxation and stochastic precession in degenerate oscillators dispersively coupled to particles

By numerical integration, we study the relaxation dynamics of degenerate harmonic oscillator modes dispersively coupled to particle positions. Depending on whether the effective inertial potential induced by the oscillators keep the particles confined, or if the particle trajectories traverse the system, the local oscillator energy dissipation rate changes drastically. The inertial trapping, release and retrapping of particles results in a characteristic step-wise relaxation process, with alternating regions of fast and slow dissipation. To demonstrate this phenomenon we consider first a one-dimensional minimal prototype model which displays these characteristics. We then treat the effect of dispersive interaction in a model corresponding to an adsorbate diffusing on a circular membrane interacting with its three lowest vibrational modes. In the latter model, stepwise relaxation appears only in the presence of thermal noise, which also causes a slow-in-time stochastic precession of the mixing angle between the degenerate eigenmodes.

Coupling of multi-vibrational modes in bacteriochlorophyll a in solution observed with 2D electronic spectroscopy

Low vibrational modes in a range of 80–400cm−1 for bacteriochlorophyll a are excited and observed as beating dynamics in two-dimensional electronic spectra. A coupled multi-vibrational mode displaced oscillator model is proposed to account for the vibronic coherence. We found that these low frequency vibrational modes are coupled. By comparing the fitted lifetime of the vibrational modes appearing in the beating dynamics for bacteriochlorophyll a and a protein-bound bacteriochlorophyll a dimer B820 probed by transient grating method, it is suggested that the protein scaffold provides a protection effect on the vibronic coherence where no excitonic coherence has be excited.

Structural, intramolecular hydrogen bonding and vibrational studies on 3-amino-4-methoxy benzamide using density functional theory

An extensive theoretical study on the molecular structure and vibrational analysis of 3-amino-4-methoxy benzamide (3A4MBA) was undertaken using density functional theoretical (DFT) method. The possibility of formation of intramolecular hydrogen bonding was identified from structural parameter analysis and confirmed with the natural bond orbital (NBO), molecular electrostatic potential (MEP) and natural charge analysis. The harmonic oscillator model of aromaticity (HOMA) index elucidated the impact of hydrogen bonding in the ring. Intramolecular hydrogen bonding energy has been calculated from topological study. The low wavenumber vibrational modes obtained from experimental FT-Raman spectrum also supported the presence of hydrogen bonding. Bands are assigned for vibrational frequencies using DFT/B3LYP/6-311 ++G** level of theory. An extensive theoretical study on the molecular structure and vibrational analysis of 3-amino-4-methoxy benzamide was undertaken using density functional theoretical (DFT) method. The formation of intramolecular hydrogen bonding was identified from structural parameter analysis and confirmed with the natural bond orbital (NBO), molecular electrostatic potential (MEP) and natural charge analysis.

Full 3D nonlinear time history analysis of dynamic soil–structure interaction for a historical masonry arch bridge

Historical masonry arch bridges, which represent the richness of cultural heritage of the country (Anatolia, Turkey), should be protected against detrimental effects and transferred well to next generations with a thorough effort given for the accurate estimations under the seismic responses affected by the nonlinear soil and structural characteristics. In this viewpoint, a relatively comprehensive method, full 3D nonlinear time history analysis of soil–structure interaction (SSI) (using direct method of configuration), has been employed in this work through the understanding of SSI effect well specifically investigated for a historical masonry stone arch bridge (Mataraci Bridge, Trabzon). For this purpose, the SSI model of bridge–substructure soil was built with the finite elements in 3D using the solid element, and then figured out in detail based on the results of seismic responses due to earthquake excitation. It is found from the results that in comparison with the fixed base solution, the influence of SSI becomes relatively prominent on the responses of displacement, acceleration, rotation, frequency (at lower modes of vibration), modal shapes, base shear, and overturning moment, while the stresses almost remain unchanged. The effort given in this study could be beneficial for other historical structures in the country for accurate estimations of the responses when seismically considered.

A new approach for structural damage detection exploring the singular spectrum analysis

This article presents a novel approach for damage detection applied to structural health monitoring systems exploring the residues obtained from singular spectrum analysis. In this technique, a lead zirconate titanate patch acting as actuator excites the structure, and three other patches are used as sensors to receive the structural responses. This method is based on a high-frequency excitation range in order to overcome the problem caused when the low-vibration modes are excited. In this method, a wideband chirp signal, with low amplitude and variable frequency, is used to excite the structure. The response signals are acquired in the time domain, and the singular spectrum analysis procedure is performed. The residues obtained between the reconstructed and original time series are used to compute statistical metrics. The residues calculated from singular spectrum analysis are used to compute the root mean square deviation and correlation coefficient deviation metric indices, rendering the damage detection approach more reliable. Tests were carried out on an aluminum plate, and the results have demonstrated the effectiveness of the proposed method making it an excellent approach for structural health monitoring applications. The results exploring different numbers of components used during the reconstruction process of time series are obtained, and the highlights are presented.

Suppressing self-excited vibrations of mechanical systems by impulsive force excitation

In this contribution, self-excited mechanical systems subjected to force excitation of impulsive type are investigated. It is shown that applying force impulses which are equally spaced in time, but whose impulsive strength depends in a certain manner on the state-variables of the mechanical system, results in a periodic energy exchange between lower and higher modes of vibration. Moreover, in the theoretical case of Dirac delta impulses, it is possible that no energy crosses the system boundary while energy is transferred across modes, i.e. neither external energy is fed to the mechanical system, nor energy is extracted from the mechanical system. Shifting energy to higher modes of vibration, whose natural damping is larger compared to lower ones, results in a faster dissipation of energy. An analytical stability investigation is presented using the assumption of impulsive forcing of Dirac delta type, which allows deciding easily about the stability by evaluating the eigenvalues of the coefficient matrix of a corresponding set of difference equations. It is shown that the developed impulsive forcing concept is capable to suppress self-excited vibrations of mechanical systems. Some numerical results of a simple mechanical system with two degrees of freedom underline the presented approach.