Range Of Excitation Frequencies Research Articles

Thermoplastic Dielectric Elastomer of Triblock Copolymer with High Electromechanical Performance.

Dielectric elastomer (DE) actuators have been shown to have promising applications as soft electromechanical transducers in many emerging technologies. The DE actuators, which are capable of large actuation strain over a wide range of excitation frequencies, are highly desirable. Here, the first single-component DE of a triblock copolymer with attractive electromechanical performance is reported. Symmetric poly(styrene-b-butyl acrylate-b-styrene) (SBAS) is designed and synthesized. The SBAS actuator exhibits about 100% static actuation area strain and excellent dynamic performance, as evidenced by a wide half bandwidth of 300 Hz and a very high specific power of 1.2 W g-1 within the excitation frequency range of 300-800 Hz.

The effort of the dynamic simulation on the fatigue damage evaluation of flexible mechanical systems loaded by non-Gaussian and non stationary loads

Even if in fatigue application it is common to assume stationary and Gaussian excitation, the impact of non-Gaussian and non-stationary loadings on the service life of a mechanical component is known. Non-Gaussian and non-stationary excitations are generally observed in several industrial applications (i.e. automotive, aeronautical, etc.) and for this, the assessment of the effect of such loads results necessary. From this assumption, the activity herein presented starts from experimental results, previously obtained, that analysed the influence of non-Gaussianity (generally evaluated by kurtosis) and of non-stationarity of inputs on the fatigue life of an Y-shaped specimen. In the present paper the finite element model of the sample and its full validation obtained by numerical/experimental comparison is presented. Moreover, due to the relevant effect of the system’s dynamics on the stress/strain response previously observed, a wider assessment of non-Gaussianity and non-stationarity influence on the fatigue life has been numerically analysed together with the influence of the multi modal behaviour of the component by adopting an excitation frequency range that excites two modes of the model.

Non-linear methods based on ultrasonic waves to analyse disbonds in single lap joints

Adhesive bonded lap joints are widely used in the aerospace field and non-destructive testing (NDT) techniques are critical in evaluating the quality of adhesion before and during use. Two types of bonded samples have been experimentally investigated in order to verify the reliability of non-linear elastic wave spectroscopy (NEWS) based on the use of ultrasound. Piezoelectric sensors have been attached to the samples and used as generators and receivers. Both the samples have shown non-linearities in their dynamic behaviour. Non-linear metrics have been applied to their structural responses over an assigned range of excitation frequencies based on higher order harmonic analysis in order to evaluate the degree of non-linearity of the samples. Possible interpretations of the experimental behaviour are provided in the paper based also on tomographic testing of the adhesive layer that showed the presence of microbubbles in the bond due to manufacturing process.

Modal parameter identification of a long-span footbridge by forced vibration experiments

Performing forced vibration tests on full-scale structures is the most reliable way of determining the relevant modal parameters in structural dynamics, such as modal frequencies, mode shapes, modal damping, and modal masses. This study describes the modal identification of a double-level curved cable-stayed bridge with separate deck systems for pedestrians and vehicles via forced vibration tests. The steady-state structural responses to sinusoidal excitations produced by an electrodynamic shaker are recorded under varying excitation frequencies, and the frequency response functions are established. The measured frequency response functions are curve fitted to estimate the modal parameters. The numerical simulation of frequency response function–based modal parameter identification of an elastically multi-supported continuous beam structure is carried out, and the emphasis has been placed on the evaluation of the effect of an additional shaker mass, excitation frequency step and range, multi-mode vibration, and noise on identification results. Finally, the modal parameters for the first lateral mode of a double-level curved cable-stayed bridge are identified by forced vibration experiments, and the results are compared with those from ambient vibration tests and free vibration tests. The effect of the unmeasured wind excitation on identification is discussed. It is shown that the effect of ambient vibration is minor for wind velocity of 3–5 m/s. The damping ratios identified by forced and free vibration tests are comparable, while those from ambient vibration are subject to large variations. The modal mass obtained from forced vibration tests is in good agreement with finite element prediction, which provides design basis for mass-type dampers.

A novel approach for the design and analysis of nonlinear dampers for automotive suspensions

This paper proposes an analytical technique for frequency analysis and the design of nonlinear dampers to further improve ride dynamics performance of vehicle suspensions over a wide range of excitation frequencies. Using the energy balance method (EBM), the proposed methodology estimates the equivalent linear damping coefficient of any nonlinear passive damper whose force is a general function of the damper’s relative displacement and relative velocity. Knowing the equivalent linear damping coefficient makes it possible to perform a frequency analysis of the suspension ride performance with any nonlinear damper. Some specific criteria are defined to design the desired form of equivalent linear damping coefficient which provides a high/small damping ratio at low-/high-frequency excitations, so the corresponding nonlinear damping force required to obtain improved ride performance of the suspension using a 1-degree-of-freedom quarter car model is also defined. A sensitivity analysis is then performed to provide a design guideline. The results show that the dependency of the equivalent damping coefficient either relative to the velocity of the suspension (velocity-dependent damper) or the relative displacement of the suspension (position-dependent damper) could provide a variable damping ratio leading to better vibration isolation over the excitation frequency. A noticeable ride dynamic performance can be reached over the entire range of the excitation frequency by designing a nonlinear damper such that its equivalent linear damping ratio becomes a desired function of both its relative displacement and relative velocity (position-velocity-dependent damper).

The Impact of Reciprocating Compressor Pulsations on the Surge Margin of Centrifugal Compressors

Pressure pulsations into a centrifugal compressor can move its operating point into surge. This is concerning in pipeline stations where centrifugal compressors operate in series/parallel with reciprocating compressors. Sparks (1983, “On the Transient Interaction of Centrifugal Compressors and Their Piping Systems,” ASME Paper No. 83-GT-236); Kurz et al. (2006, “Pulsations in Centrifugal Compressor Installations,” ASME Paper No. GT2006-90700); and Brun et al. (2014, “Impact of the Piping Impedance and Acoustic Characteristics on Centrifugal Compressor Surge and Operating Range,” ASME J. Eng. Turbines Power, 137(3), p. 032603) provided predictions on the impact of periodic pressure pulsation on the behavior of a centrifugal compressor. This interaction is known as the “compressor dynamic response” (CDR) theory. Although the CDR describes the impact of the nearby piping system on the compressor surge and pulsation amplification, it has limited usefulness as a quantitative analysis tool, due to the lack of prediction tools and test data for comparison. Testing of compressor mixed operation was performed in an air loop to quantify the impact of periodic pressure pulsation from a reciprocating compressor on the surge margin (SM) of a centrifugal compressor. This data was utilized to validate predictions from Sparks’ CDR theory and Brun’s numerical approach. A 50 hp single-stage, double-acting reciprocating compressor provided inlet pulsations into a two-stage 700 hp centrifugal compressor. Tests were performed over a range of pulsation excitation amplitudes, frequencies, and pipe geometry variations to determine the impact of piping impedance and resonance responses. Results provided clear evidence that pulsations can reduce the surge margin of centrifugal compressors and that geometry of the piping system immediately upstream and downstream of a centrifugal compressor will have an impact on the surge margin reduction. Surge margin reductions of over 30% were observed for high centrifugal compressor inlet suction pulsation.

Aerodynamic response of internal passages to pulsating inlet supersonic conditions

Abstract Supersonic flow passages benefit from high specific mass-flow rate, which can potentially be used in fluid machinery with the highest power density. Currently, the maximization of the propulsive efficiency, on forthcoming power plant systems, requires reduced engine cores, while for distributed local-power plant units, small gas turbine engines are ideal. Hence, ultra-compact fluid machinery is an enabling technology for future propulsion and power generation applications. In this context, the use of supersonic flow within internal passages allows the downsizing of the turbomachinery. However, the efficient operation of supersonic internal passages is a challenge due to shock losses and a constrained operability. Therefore, fully supersonic internal passages are typically discarded, and their unsteady operation scarcely documented. This paper presents a detailed characterization of the unsteadiness experienced across internal flow passages exposed to pulsating supersonic conditions. The analysis was performed in a wide range of excitation frequencies. To generalize the results the fluctuations are expressed in terms of a non-dimensional reduced frequency, i.e. ratio of excitation period to the propagation wave time. This aerodynamic characterization was performed with three-dimensional unsteady Reynolds-Averaged Navier Stokes simulations, revealing a hysteresis like behavior and amplitude modulation between the inlet and the outlet distinct from the steady prediction. The paper provides a numerical approach, based on a one-dimensional Euler solver that can predict the unsteady response of internal passages to the inlet varying conditions.

Spin-wave propagation in ultra-thin YIG based waveguides

Spin-wave propagation in microfabricated 20 nm thick, 2.5 μm wide Yttrium Iron Garnet (YIG) waveguides is studied using propagating spin-wave spectroscopy (PSWS) and phase resolved micro-focused Brillouin Light Scattering (μ-BLS) spectroscopy. We demonstrate that spin-wave propagation in 50 parallel waveguides is robust against microfabrication induced imperfections and extract spin-wave propagation parameters for the Damon-Eshbach configuration in a wide range of excitation frequencies. As expected from its low damping, YIG allows for the propagation of spin waves over long distances; the attenuation lengths is 25 μm at μ0H = 45 mT. Moreover, direct mapping of spin waves by μ-BLS allows us to reconstruct the spin-wave dispersion relation and to confirm the multi-mode propagation in the waveguides, glimpsed by propagating spin-wave spectroscopy.

Electromechanical modeling and power performance analysis of a piezoelectric energy harvester having an attached mass and a segmented piezoelectric layer

Conventional vibration-based piezoelectric energy harvesters (PEHs) have advantages including the ubiquity of their energy source and their ease of manufacturing. However, they have a critical disadvantage as well: they can produce a reasonable amount of power only if the excitation frequency is concentrated near a natural frequency of the PEH. Because the excitation frequency is often spread and/or variable, it is very difficult to successfully design a conventional PEH. In this paper, we propose a new cantilevered PEH whose design includes an attached mass and a segmented piezoelectric layer. By choosing a proper size and location for the attached mass, the gap between the first and second natural frequencies of the PEH can be decreased in order to broaden the effective excitation frequency range and thus to allow reasonable power generation. Especially, the output power performance improves significantly around the second natural frequency of the PEH since the voltage cancellation effect can be made very weak by segmenting the piezoelectric layer at an appropriate location. To investigate the power performance of the new PEH, herein a reduced-order electromechanical analysis model is proposed and the accuracy of this model is validated experimentally. The effects of variable load resistance and piezoelectric layer segmentation location upon the power performance of the new PEH are investigated by means of the reduced-order analysis model.

Influence of the excitation frequency on the density of helium metastable atoms in an atmospheric pressure dielectric barrier discharge

Diffuse dielectric barrier discharges in atmospheric-pressure helium can be sustained over a wide range of excitation frequencies (from, but not restricted, 25 kHz to 15 MHz). The aim of the present paper is to identify the specific characteristics of the discharge modes that can be sustained in this frequency range, namely, the atmospheric-pressure Townsend-like discharge (APTD-L) mode, the atmospheric-pressure glow discharge (APGD) mode, the Ω mode, the hybrid mode, and the RF-α mode. This is achieved experimentally, by measuring the density of helium metastable atoms, which are known to play a driving role on the discharge kinetics. This density is measured by means of two absorption spectroscopy methods, one using a spectral lamp and the other one using a diode laser as a light source. The first one provides the time-averaged atom densities in the singlet He(21S) and triplet He(23S) metastable states, while with the second one we access the time-resolved density of He(23S) atoms. Time-averaged measurements indicate that the He(23S) density is relatively low in the APTD-L, the Ω and the RF-α modes (<4×1016 m−3) slightly higher in the APGD mode (2–7×1016 m−3), and still higher (>1×1017 m−3) in the hybrid mode. The hybrid mode is exclusively observed for frequencies from 0.2 to 3 MHz. However, time-resolved density measurement shows that at 1 MHz and below, the hybrid mode is not continuously sustained. Instead, the discharge oscillates between the Ω and the hybrid mode with a switching frequency about the kilohertz. This explains the significantly lower power required to sustain the plasma as compared to above 1 MHz.

Vibration transmission through an impacting mass-in-mass unit, an analytical investigation

Impacting events are discontinuous and non-smooth in nature; thus, resulting in various forms of complex nonlinear dynamics. A numerical algorithm has been developed based on the analytical solution to distinguish different classes of impacting responses. The main advantages of the solver are that it can either stop the integration process after automatically identifying the steady state solution or continue until the maximum time is reached in case of the chaotic type response. To identify the frequency of higher periodic response, a periodicity coefficient has been defined as the frequency ratio of excitation and the system's response. The effect of coefficient of restitution on the different dynamic responses is also discussed within the scope of the paper. The amplitude of the vibration of the main mass is reduced due to the presence of the multi-periodic and chaotic impacting responses for a wide range of excitation frequencies. These characteristics make impact dampers ideal for applications in wideband vibration insulation and a unit of wideband nonlinear metamaterial.

Important role of magnetization precession angle measurement in inverse spin Hall effect induced by spin pumping

Here, we investigate the spin Hall angle of Pt in a Ni80Fe20/Pt bilayer system by using a broadband spin pumping and inverse spin Hall effect measurement. An out-of-plane excitation geometry with an application of external magnetic field perpendicular to the charge current direction is utilized in order to suppress unwanted galvanomagnetic effects. Magnetization precession angle (θC) on ferromagnetic resonance for wide excitation frequency range (4–14 GHz) is estimated from the rectification voltage of anisotropic magnetoresistance (AMR) and a conventional method of using the microwave power in a coplanar waveguide. A marked difference in θC profiles for the different methods is observed, resulting in the large variation in estimated values of spin current density at Ni80Fe20/Pt interface. The frequency dependence of the spin current density estimated using the AMR effect is found to be similar to that of the inverse spin Hall voltage. We obtain the frequency-invariant spin Hall angle of 0.067 ± 0.002.

Modeling and Characterization of a Curved Piezoelectric Energy Harvester for Smart Paver Tiles

Abstract: The transformation of vibrations into low-power electricity has received growing attention over the past couple of decades. The goal in this research field is to power wireless sensors and other small electronic components by harvesting ambient mechanical energy. Piezoelectricity is arguably the most popular transduction mechanism for converting mechanical vibrations into electrical energy. As a promising concept of piezoelectric energy conversion, smart paver tiles are proposed to harness human steps for energy harvesting and a wide range of critical sensing applications ranging from security to healthcare. In this work, we explore the use of a curved piezoelectric transducer (specifically a THUNDER® configuration) for smart paver tiles both analytically and experimentally. Following the description of the smart paver tiles developed, an analytical model is developed and closed-form electromechanical frequency response functions are obtained. Experiments are performed for a range of excitation frequencies (1-10 Hz ), electrical load resistance values, and low force levels. It is shown that the first-order electromechanical model successfully represents the governing dynamics.

Parameters identification of fractional models of viscoelastic dampers and fluids

An identification method for determination of the parameters of the rheological models of dampers made of viscoelastic material is presented. The models have two, three or four parameters and the model equations of motion contain derivatives of the fractional order. The results of dynamical experiments are approximated using the trigonometric function in the first part of the procedure while the model parameters are determined as the solution to an appropriately defined optimization problem. The particle swarm optimization method is used to solve the optimization problem. The validity and effectiveness of the suggested identification method have been tested using artificial data and a set of real experimental data describing the dynamic behavior of damper and a fluid frequently used in dampers. The influence of a range of excitation frequencies used in experiments on results of identification is also discussed.

Nonlinear Analysis of Two-layer Fluid Sloshing in a Rectangular Tank Subjected to Width Direction Excitation

This paper describes a nonlinear theory to describe oscillations of two liquid layers formed in a rectangular tank. Nonlinear equations based on the variational principle and the Galerkin method are used, which are written in a form of direct expanding in eigenmodes. Analysis using the equations is described under the experimental conditions reported in a previous study. Both the experiment and the analysis demonstrate a “peculiar oscillation” in which the interface of the two liquids oscillates at 1/5 to 1/7 of the excitation frequency in a particular excitation frequency range. By observing the nonlinear force time series, four eigenmodes are identified to be mainly relevant to the oscillation. An amplitude equation analysis was applied to the four relevant eigenmodes. It was found that this “peculiar oscillation” is one of summed and differential harmonic oscillations (combination oscillation) in which one asymmetric and one symmetric eigenmode excite each other through the mediation of other two asymmetric eigenmodes.

Robust identification of the mechanical properties of viscoelastic non standard materials by means of frequency domain experimental measurements

This work presents an identification procedure for the constitutive model of viscoelastic non-standard materials, such as Functionally Graded Materials (FGM). A generalized Kelvin model of arbitrary order is selected, making it possible to define the material constitutive relationship by means of the ratio of polynomials in the frequency domain. Since ill-conditioning may occur at the numerical identification stage of high order model parameters, a novel approach to identify the model optimal order and parameters, mainly employing a orthogonal polynomial basis, is proposed in this paper. Least square error fitting techniques employing a classic monomial and a Forsythe orthogonal polynomial basis are compared by starting from numerically estimated measurements with noise. The selected approach is used to fit dynamical measurement data obtained from real test specimens in a wide excitation frequency range. The optimal model results obtained by fitting real experimental data are presented and discussed.

On the refined modeling of the force distribution in a bistable magnetoelastic energy harvesting system due to a magnetic field

AbstractAbstract: The problem of a bistable magnetoelastic beam under base excitation was discussed in [1] under the aspect of chaotic behaviour in mechanical systems. Three decades later the system was used in [2] to design an energy harvesting system which performs well under harmonic excitation for a broad range of excitation frequencies due to its bistability. The initial modeling was tailored to obtain a model with one degree of freedom based on the assumption that the magnetic force acts on the beam tip only. A more appropriate model can be found when considering a distributed force along the beam.The authors present the force distribution on a ferromagnetic beam due to the magnetic field of two permanent magnets. A semi‐analytic method is used to compute the magnetic field. The force distribution can in future be used to derive a refined nonlinear dynamical model for the ferromagnetic elastic beam. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analytical assessment of elevated tank natural period considering soil effects

The main purpose of current study is to find the soil effects on natural period of elevated tank. The coupled analytical method is used to assess in this study. The current study presented models which are capable to consider the soil dynamic stiffness changes and fluid- structure interaction effects on natural period of elevated tanks. The basic of mentioned models is extracted from elastic beam and lumped mass theory. The finite element is used to verify the results. It is observed that, external excitation can change the natural period of elevated tanks. Considering the increase of excitation frequency, the natural period will be decreased. The concluded values of natural period in case of soft and very soft soil are more affected from excitation frequency values. The high range of excitation frequency may reduce the natural period values. In addition it is observed that the excitation frequency has no significant effect on convective period compare with impulsive period.

Steady-State Analysis of Cable with Nonlinear Damper via Harmonic Balance Method for Maximizing Damping

AbstractSteady-state solutions represent a significant subset of dynamics of a nonlinear system. Herein, they are concerned for a taut cable damped by a nonlinear damper near one cable end, for the purpose of maximizing cable damping. Periodic vibrations of the cable-damper system are efficiently analyzed using the multiharmonic balance method. Furthermore, a continuation method is employed to predict system responses to a range of excitation frequencies near the cable eigenfrequencies. Control effect is then evaluated and optimized based on the frequency response functions, and optimal damper parameters are determined meanwhile. The proposed procedure is implemented for studying two common structural dampers in cable vibration control, i.e., nonlinear viscous damper and friction damper. In both cases, alternating time/frequency domain strategy is integrated for evaluating the nonlinear force and pertinent stiffness, and analytical formulations are obtained, ensuring that strong nonlinear behavior such as...

Ground-borne vibration control in a building-like structure using multi-positive position feedback combined with sliding mode control

This work deals with the structural and dynamic analysis of a building-like structure consisting of a three-story building with one type of actuators, that is, a passive/active electromechanical actuator. The base of the structure is perturbed using an electromagnetic shaker, which provides forces with a wide range of excitation frequencies, including noisy excitations emulating traffic, underground railways and earthquakes, common in Mexico City. The overall mechanical structure is modeled using Euler-Lagrange method and validated using experimental modal analysis techniques. The active controls laws are synthesized to actively attenuate the noise and vibration system response caused by noise excitation forces acting on the base of the structure. The control scheme is obtained using multi-positive position feedback (MPPF) combined with sliding mode (SM) to improve the closed-loop system response, which is able to simultaneously attenuate the three vibration modes of the primary system. Some simulation and experimental results are included to illustrate the overall system performance.