Single-point Excitation Research Articles

Method and Experimental Study of Oscillator Frequency Optimization of Distributed Tuned Mass Dampers for Broadband Multimodal Vibration Mitigation of Reinforced Concrete Wall

Distributed tuned mass dampers (dTMD) can effectively mitigate the broadband vibration of a structure. However, when the vibration frequency in question reaches several hundred hertz, traditional optimization methods represented by fixed point theory are difficult to apply due to dense modal density, complex boundary conditions, and vibration inputs. This paper proposes the minimax method based on modal damping to optimize the oscillator’s frequency. Two typical wall panel specimens are tested to evaluate the proposed method. The mode shape of the uncontrolled wall and the vibration mitigation effect of the stacked sandwich-damped TMD under single-point bidirectional excitation is tested. The correlation between the modal damping and the vibration mitigation effect is evaluated. The results show that the RC wall panel has a dense mode when the frequency of interest reaches 300 Hz and above; the distributed stacked sandwich-damped TMDs can effectively mitigate the vibration of the RC wall panel in the frequency range of 200~450 Hz; and that the idea of optimizing the frequency of dTMD based on modal damping is feasible.

Vibration behaviours of foam-filled grille composite sandwich cylindrical shells

In this study, a novel lightweight composite sandwich cylindrical shell, namely an all-composite sandwich cylindrical shell (ACSCS) with a hollow grille core (HGC) filled with polyurethane foam (PF), is designed and prepared, and its free and forced vibration behaviours are investigated for the first time. First, an analytical model of the PFHGC-ACSCSs subjected to both base excitation and single-point pulse excitation loads is developed by employing the shear deformation theory enriched by the layerwise method, virtual artificial spring technique, Rayleigh-Ritz approach, modal superposition principle, Newmark-Beta approach, and improved cross-fill equivalent theory. Furthermore, the solution procedures for the natural frequencies, mode shapes, and vibration responses in the time and frequency domains of the PFHGC-ACSCSs are defined. Convergence investigations are performed to determine the appropriate truncation numbers and stiffness values for the virtual artificial springs used in the prediction model. Both literature and experimental validations are conducted on the current model, the results of which indicate that it is reliable for predicting the concerned dynamic parameters. Finally, the effects of the critical parameters on the free and forced vibrations of the PFHGC-ACSCSs are studied. The results suggest that the vibration suppression capability could be significantly strengthened by adopting a large core-to-skin thickness ratio, high ratio of the circumferential length of the hollow grille ribs to the whole thickness, large axial and circumferential number of the hollow grille ribs, high ratio of the wall thickness to the circumferential length of the hollow grille ribs, and type of foam with a high elastic modulus. The modelling and solving techniques, preparation processes, testing methods, and conclusions of this study pave the way for the application of such shell structures.

Modal Test and Model Modification of the Rotating Parts of a Self-propelled Antiaircraft Gun

The rotating parts of a self-propelled antiaircraft gun is taken as the research object, the single point excitation and multi-point response frequency response function method were used to test the experimental mode. At the same time, the finite element simulation model is established, and the frequency and mode of vibration are obtained. Combined with the modal test and analysis of the rotating parts, the stiffness of the connection between the rotating parts and the turret is selected as the optimization parameter. The numerical simulation model was optimized and modified by comparing the experimental and simulation modes and combining with the sensitivity analysis of the connection parameters, the correction variable with the higher sensitivity of the parameters to the results was selected. After optimization, the error between the first order calculation frequency and the test frequency is reduced from 8.61% to 1.13%, it is shown that the simulation model optimized by parameters can better reflect the physical characteristics of the real structure.

Developing coexisting traveling and standing waves in Euler-Bernoulli beams using a single-point excitation and a spring-damper system

In the inner ear, the Basilar Membrane (BM) and inner hair cells work to transduce acoustic waves into electrical signals; of these, the BM is the structural membrane that carries the acoustic information as traveling structural waves. These waves propagate from the base of the cochlea towards its apex. The helicotrema is the main component of the cochlea's apex, which prevents the waves from traveling back from the apex to the base. Due to this unique characteristic of the BM, humans are able to hear continuous acoustic signals without any reflection and overlap of acoustics. The work herein is inspired by this biological behavior and seeks to understand and replicate such behavior in a passive manner in basic structures.This feature of the inner ear leads us to study the dynamics of a uniform beam connected to a spring-damper system as a passive absorber, in order to understand some of the observed phenomenological behaviors of the basilar membrane. The location of the spring-damper system divides the beam into two dynamic regions: one which exhibits non-reflecting traveling waves and the other with standing waves. In this paper, traveling waves co-existing with standing waves in the two distinct regions of the structure and are realized analytically, numerically, and experimentally. The results of this work can be used in various applications such as vibration energy harvesting, particle transportation, vibration suppression and control, and in exploring possible explanations of the functionality of the helicotrema in the cochlea.

Optimizing hyperparameters of Data-driven simulation-assisted-Physics learned AI (DPAI) model to reduce compounding error

In this paper, we propose the study of optimizing the hyperparameters of deep learning Data-driven simulation-assisted-Physics learned AI (DPAI) model to simulate the ultrasonic wave propagation for extended depth with a lower error. DPAI model has layers of encoder–decoder structure with modified convolutional long short-term memory (ConvLSTM). DPAI model is trained using the finite element (FE) simulations dataset of distributed single-point to multi-point excitation sources in the 2D domain. The DPAI is the data-driven approach to apprehending the underlying physics of elastodynamic wave propagation. Six different combinations of hyperparameters (hidden dimensions, kernel size, batch size) are used in the DAPI model to study parameter optimization for lowering compounding error. The effectiveness of the trained DPAI models with varying hyperparameters is demonstrated to reduce the compounding error for modeling the deeper simulations of the single-point excitation and multi-point excitation sources. The maximum MAE on amplitude is 5.0×10-2, and MAPE is 2.64% on time of flight (TOF) between DPAI and FE simulations.

Local deformation on damping performance of integral-forming aluminum foam sandwich

Damping performances of porous metals are generally measured by dynamic mechanical analysis (DMA) method which has strictly dimensional requirements on specimens especially on their thickness. In this work, based on sinusoidal excitation and Fourier transform, a new set of test equipment suitable for evaluating damping performances of large-scale porous metals was constructed. On this basis, the influence of local compression deformation on the damping performances of the newly developed integrated-forming aluminum foam sandwich (IFAFS) was investigated by means of single-point excitation method. Damping performances of different regions were obtained through partition measurement and the reasons were discussed which are beneficial to evaluate damping performances of IFAFS after local deformation.

A Theoretical Evaluation of the Efficiencies of Metal-Free 1,3,4-Oxadiazole Dye-Sensitized Solar Cells: Insights from Electron–Hole Separation Distance Analysis

Herein, some novel metal-free 1,3,4-oxadiazole compounds O1–O7 were evaluated for their photovoltaic properties using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to determine if they can serve as metal-free organic dyes in the use of dye-sensitized solar cells (DSSCs). To understand the trends in the relative efficiencies of the investigated compounds as dyes in DSSCs, their electron contributions, hole contributions, and electron–hole overlaps for each respective atom and fragment within the molecule were analyzed with a particular focus on the electron densities on the anchoring segments. As transition density matrices (TDM) provide details about the departure of each electron from its corresponding hole during excitations, which results in charge transfer (CT), the charge separation distance (Δr) between the electron and its corresponding hole was studied, in addition to the degree of electron–hole overlap (Λ). The latter, single-point excitation energy of each electron, the percentage electron contribution to the anchoring segments of each compound, the incident-photon-conversion-efficiency (IPCE), charge recombination, light harvesting efficiency (LHE), electron injection (Φinj), and charge collection efficiency (ncollect) were then compared to Δr to determine whether the expected relationships hold. Moreover, parameters such as diffusion constant (Dπ) and electron lifetime (t), amongst others, were also used to describe electron excitation processes. Since IPCE is the key parameter in determining the efficiency, O3 was found to be the best dye due to its highest value.

Implementing Data-Driven Approach for Modelling Ultrasonic Wave Propagation Using Spatio-Temporal Deep Learning (SDL)

In this paper, we proposed a data-driven spatio-temporal deep learning (SDL) model, to simulate forward and reflected ultrasonic wave propagation in the 2D geometrical domain, by implementing the convolutional long short-term memory (ConvLSTM) algorithm. The SDL model learns underlying wave physics from the spatio-temporal datasets. Two different SDL models are trained, with the following time-domain finite element (FE) simulation datasets, by applying: (1) multi-point excitation sources inside the domain and (2) single-point excitation sources on the edge of the different geometrical domains. The proposed SDL models simulate ultrasonic wave dynamics, for the forward ultrasonic wave propagation in the different geometrical domains and reflected wave propagation phenomenon, from the geometrical boundaries such as curved, T-shaped, triangular, and rectangular domains, with varying frequencies and cycles. The SDL is a reliable model, which generates simulations faster than the conventional finite element solvers.

An Intelligence Method for Recognizing Multiple Defects in Rail.

Ultrasonic guided waves are sensitive to many different types of defects and have been studied for defect recognition in rail. However, most fault recognition algorithms need to extract features from the time domain, frequency domain, or time-frequency domain based on experience or professional knowledge. This paper proposes a new method for identifying many different types of rail defects. The segment principal components analysis (S-PCA) is developed to extract characteristics from signals collected by sensors located at different positions. Then, the Support Vector Machine (SVM) model is used to identify different defects depending on the features extracted. Combining simulations and experiments of the rails with different kinds of defects are established to verify the effectiveness of the proposed defect identification techniques, such as crack, corrosion, and transverse crack under the shelling. There are nine channels of the excitation-reception to acquire guided wave detection signals. The results show that the defect classification accuracy rates are 96.29% and 96.15% for combining multiple signals, such as the method of single-point excitation and multi-point reception, or the method of multi-point excitation and reception at a single point.

Nonlinear modal testing of damped structures: Velocity feedback vs. phase resonance

In recent years, a new method for experimental nonlinear modal analysis has been developed, which is based on the extended periodic motion concept. The method is well suited to experimentally obtain amplitude-dependent modal properties (modal frequency, damping ratio and deflection shape) for strongly nonlinear systems. To isolate a nonlinear mode, the negative viscous damping term of the extended periodic motion concept is approximated by ensuring phase resonance between excitation and response. In this work, an alternative approach to isolate a nonlinear mode is developed and analyzed: velocity feedback. The accuracy of the extracted modal properties and robustness of velocity feedback is first assessed by means of simulated experiments. The two approaches phase resonance and velocity feedback are then compared in terms of accuracy and experimental implementation effort. To this end, both approaches are applied to an experimental specimen, which is a cantilevered beam influenced by a strong dry friction nonlinearity. In this work, the discussion is limited to single-point excitation. It is shown that a robust implementation of velocity feedback requires the measurement of several response signals, distributed over the structure. An advantage of velocity feedback is that no controller is needed. The accuracy of the modal properties can, however, suffer from imperfections of the excitation mechanism such as a phase lag due to exciter–structure interactions or gyroscopic forces due to single-point excitation.

Study on target energy transfer of 3D acoustic cavity - plate coupling system with the membrane nonlinear energy sink

The target energy transfer (TET) between a membrane nonlinear energy sink (NES) and the acoustic medium inside a rectangular cavity is studied. The acoustic medium is interacted with a plate and multi-order modes coupling of the 2 structure is considered. Based on the modal expansion approach, with Green's function, Helmholtz equation and the boundary conditions of the acoustic medium and the plate, the coupling coefficient matrix of the mode of 2 structures is derived. The equations of the membrane NES, multi-order modes of the acoustic medium and multi-order modes of the plate are established, and numerical analysis is used to investigate the TET phenomenon. The results show that in condition of a single-point excitation to the plate, under a certain range of excitation levels, the membrane can be seen as a kind of NES, and the energy in the acoustic medium can be unidirectionally transmitted to the membrane NES and attenuated, reducing the sound pressure level in the cavity. At the same time, it is found that the NES can suppress multi-order sound pressure of the acoustic medium at the same time, and realize the control of cascaded resonance noise.

High-resolution seismic processing technique with broadband, wide-azimuth, and high-density seismic data — A case study of thin-sand reservoirs in eastern China

Seismic processing and interpretation techniques provide important tools for oil and gas exploration in the Songliao Basin in eastern China, which is dominated by terrestrial facies. In the Songliao Basin, a large number of thin-sand reservoirs are widely distributed and they are the primary targets of potential oil and gas exploration and exploitation. An important part of exploration in the Songliao Basin is to accurately describe the distribution of these thin-sand belts and the sand-body shapes. However, the thickness of these thin-sand reservoirs is generally below the resolution of conventional seismic processing. Most reservoirs are thin interbeds of sand and mudstones with strong vertical and lateral variations. This makes it difficult to accurately predict the vertical and horizontal distribution of the thin-sand bodies using conventional seismic processing and interpretation methods. In addition, these lithologic traps are difficult to identify due to the complex controlling factors and distribution characteristics and the strong concealment. These challenges motivate us to improve the seismic data quality to help delineate thin-sand reservoirs. We have used the broadband, wide-azimuth, and high-density integrated seismic exploration technique to help delineate thin reservoirs. We first use field single-point excitation and single-point receiver acquisition to obtain seismic data with wide frequency bands, wide azimuth angles, and high folds, which contain rich geologic information. Next, we perform near-surface Q compensation, viscoelastic prestack time migration, seismic attributes, and seismic waveform indication inversion on the newly acquired seismic data. The 3D case study indicates the benefits of improving the imaging of thin-sand body and the accuracy of inversion and reservoir characterization using our method.

Transforming Method of Torsional Tuned Liquid Column Damper-Eccentric Structure to Torsional Tuned Mass Damper-Eccentric Structure for Vibration Control

The performance of torsional tuned liquid column dampers (TTLCDs) for mitigating coupled lateral and torsional vibrations of an eccentric structure, which can be modeled as a 3N degree-of-freedom structure, is theoretically investigated. Biaxial horizontal earthquakes are applied at the rigid foundation as a single-point excitation. The equations of relative fluid motion in a TTLCD with different reference points A and inclined angles β of upright columns are derived. Moreover, the interaction forces and moments between the absorber and structure are determined. On the basis of the coupled equations of motion of the main system with the TTLCD, a TTLCD is tuned with respect to a selected natural frequency of the main structure through a geometric transformation in analogy to a torsional tuned mass damper (TTMD). An equivalent structure mass ratio renders the mass of the TTMD and the changed structural modal mass. Thus, the easy implementation of the simulated TTMD by Ansys software can analyze the vibration control of an eccentric structure installed with TTLCDs. Numerical simulations show that a transforming method with an explicit physical definition is feasible and accurate, and can be directly used.

Challenging an experimental nonlinear modal analysis method with a new strongly friction-damped structure

In this work, we show that a recently proposed method for experimental nonlinear modal analysis based on the extended periodic motion concept is well suited to extract modal properties for strongly nonlinear systems (i.e. in the presence of large frequency shifts, high and nonlinear damping, changes of the mode shape, and higher harmonics). To this end, we design a new test rig that exhibits a large extent of friction-induced damping (modal damping ratio up to 15%) and frequency shift by 36% (considered the first bending mode). The specimen, called RubBeR, is a cantilevered beam under the influence of dry friction, ranging from full stick to mainly sliding. With the specimen's design, the measurements are well repeatable for a system subjected to dry frictional force. Then, we apply the method to the specimen and show that single-point excitation is sufficient to track the modal properties even though the deflection shape changes with amplitude. Computed frequency responses using a single nonlinear-modal oscillator with the identified modal properties agree well with measured reference curves of different excitation levels, indicating the modal properties' significance and accuracy.

Analysis of Vibration Harvesting Properties and Fracturing Structures of Soapberry (Sapindus Mukorossi)

HighlightsAnalyzing the fruit removal force and harvesting time of Sapindus fills a gap of Sapindus research.A dual excitation vibration method for harvesting Sapindus and the best harvesting parameters were obtained.The microstructure of the different fracturing locations was analyzed for the first time. Abstract.To address the issue of labor intensity in the harvest of Sapindus fruit, it is necessary to understand the harvesting properties of Sapindus mukorossi. In this study, a fruit removal force (FRF) test platform was used to analyze the relationship between the FRF and maturation time. An eccentric vibration harvesting test was conducted to obtain the frequency, acceleration, and other parameters of the Sapindus tree. According to the different falling locations of fruit, the fracturing locations were observed by a scanning electron microscope (SEM). The results revealed that the optimal time (9 to 12 November) to harvest is in the middle of the harvest period. Additionally, the shear force from 1.0N to 5.2 N was less than the tensile force from 2.7 to 10.2 N. Therefore, back and forth swinging is the major fracturing form of the fruit. Moreover, dual excitation is preferable to single-point excitation, and the acceleration root mean square (RMS) of the dual excitation was two to three times the acceleration RMS of single point excitation. The harvesting efficiency of dual excitation was 92.3% at 20 Hz. Additionally, there existed four fracturing locations for harvested fruit, as the frequency increased. Furthermore, the fracturing in the ear and stem was the most important form of fracturing, and microscopic observations revealed that it had a thinner and tidier cell wall and cell cavity. Keywords: Sapindus, Fruit removal force, Vibration, Microstructure.

An Experimental Study on Dynamic Model Groove Under Shock Excitation

Introduction: Variation laws of dynamic response and settlement deformation at different depths of soil roadbed were investigated and summarized in this study through a simulation test with a dynamic model groove in soft clay foundation of low road embankment. Materials and Methods: In this test, the traffic load was simulated by single-point and double-point shock excitation. Besides, loading frequency, intensity and duration were adjusted by a single variable control method. Research results show that peak values of soil pressure and pore water pressure decrease with the increase of buried depth, but they are unrelated to loading intensity and frequency. Conclusion: Nevertheless, settlement deformation does not increase linearly with the increase of frequency or time difference. The accumulative settlement at a depth varies as the loading time increases.

Influence of a tamping operation on the vibrational characteristics and resistance-evolution law of a ballast bed

A ballast bed provides a stable foundation and a sufficient vibration-damping performance for a railway line due to its loose and porous structure. Tamping operations change the compactness of ballast stacks and improve and maintain the state of ballast beds, which also affect the vibration-transfer characteristics and resistance performance of ballast beds. In the present study, impact excitation technology was used to test and analyze the influence of the number of tamping operations performed by a tamping car on the damping and stiffness of a ballast bed. Additionally, the influence of different tamping operations on the longitudinal vibration-transmission characteristics of the ballast bed was obtained by the methods of “single-point excitation and multi-point pick-up”. The mode of analysis of the railway structure was carried out by numerical simulation, and the resistance test on site was used to study the evolution law of the longitudinal and lateral resistance of the ballast bed during three tamping operations. The results showed that a large machine tamping operation changed the values of the two dominant resonances (140.75 Hz to 202.35 Hz and 381.25 Hz to 418.75 Hz) of track structure in the range of 0–600 Hz. The tamping operation also affected the resistance, damping, and stiffness of the ballast bed (the longitudinal and lateral resistances decreased by 17.39% and 20.34%, respectively, and the damping and stiffness increased by 204.07% and 148.80%, respectively), which suggests that the longitudinal/lateral resistances, damping, and stiffness of the ballast bed were strongly negatively correlated with one another. The modal shapes of the track structure caused by the first tamping and the second tamping were obviously different, and the positions and values of the maximum displacement changed (1.92 mm to 1.71 mm). The tamping also affected the longitudinal vibration transmission between the sleepers, and the maximum attenuation rate between adjacent sleepers was 81.25%. The change of the resistance work of the ballast bed with the displacement was more significant than the change of the resistance of the ballast bed with the displacement (when the ballast bed resistance decrement rates were 4.34%, 16.19%, and 20.34%, the track bed resistance work decrement rates were 13.14%, 28.15%, and 39.35%, respectively). Thus, the resistance work was more applicable when describing the variational degree of the stability of the ballast bed.

Spatial distribution of the phononic crystal modes excited by a moving laser source

In this paper, we have presented the dispersion curve of Lamb waves in an aluminum plate with a periodic rectangular corrugation experimentally. A mode selection method was applied to generate Lamb waves in the region of the corrugated periodic structure using a moving continuous wave laser, and the signal can be detected at any point in the excitation region. The relationship between the phase velocity and the frequency is observed. It is found that the intensity distribution of the dispersion curves in the experimental results is related to the slope of the dispersion curves as well as the selection of the detection point. Vibrations at a series of locations were detected, reflecting the change in energy that Lamb waves undergo in the process of propagation through the periodic structure, which is different from the existing method of single-point excitation and detection. The spatial distribution of the mode amplitude at the resonances points, in particular induced by the Bragg reflection, has been studied.

Single-Layer Series-Fed Planar Array With Controlled Aperture Distribution for Circularly Polarized Radiation

We propose a compact circularly polarized series-fed patch array with enhanced radiation performance at $S$ -band. To our best knowledge, no similar single-layer structure has been designed, measured, and reported in the literature with equivalent radiation performances in terms of reduced sidelobe level (SLL) and aperture efficiency as well as compactness and simplicity. The planar array consists of a 50- $\Omega $ microstrip single feed point, offering uniform and efficient excitation of all its elements that enable a broadside beam with high-gain and low sidelobes. A feeding network for such aperture control is designed and optimized to provide good impedance matching with a two-point excitation per radiating element, which is based on 0° and 90° meander lines to enable circularly polarized radiation. A structure made by a $4\!\!\times \!\!1$ arrangement of square patches is simulated, optimized, and manufactured, providing a realized gain of about 10 dBic and an SLL below −15 dB. Very good axial ratios and high efficiencies are obtained. The proposed antenna may be of interest for next-generation far-field wireless power transmission systems and other applications including target tracking, radar, and internet of things technologies requiring efficient circularly polarized radiation.

Structural damage detection with distributed long-gauge FBG sensors under multi-point excitations

Distributed strain sensing techniques using long-gauge fiber Bragg grating sensors have been widely applied in health monitoring of civil engineering structures. Developing an effective damage identification method based on the acquired macro-strain is a crucial issue for real applications. In this paper, based on the previous studies on modal macro-strain vector (MMSV) for damage identification under single-point excitation, the damage detection approach under multi-point excitations is further studied. An effective damage index is further proposed as an indicator of the structural damage and the frequency domain decomposition (FDD) technique is introduced to accurately extract the MMSV. The sensitivity and effectiveness of the proposed method for damage detection of a cantilever beam are verified in detail with both numerical simulations and experiments. Results show that both the single and multi damages with different damage extents can be effectively detected under either single point excitations or the wind load excitations. It shows that the proposed approach can do the work well and could be applied for the structural health monitoring of real structures.