Frequency-dependent Damping Research Articles

Tuned medium-band UHF PD measurement method for GIS

SUMMARY In order to replace the lightning impulse test, a very sensitive PD-measurement is required for onsite tests of GIS [1]. The most sensitive UHF-PD-measurement technique consists of low-noise high –gain broadband amplifiers applied directly to the PD-sensors combined with the manual selection of possible resonant frequencies in the frequency spectrum for narrowband signal extraction with a bandwidth of some MHz. Further a correlation with the test voltage will lead to phase resolved signal display. The only disadvantage is the time consuming visual selection of the optimal centre frequency of the narrowband measurement system done sensor by sensor compared to broadband or fixed band measurement techniques. The broadband system design has the disadvantage of significantly reduced measurement sensitivity as soon as interfering frequencies are located within the measurement bandwidth. Beside this, in principle, the signal to noise ratio is lower for a broadband measurement system compared to a narrowband system. The narrowband system with fixed frequencies show poor or no sensitivity when the GIS resonant frequency caused by a PD source do not correspond with the narrow band measurement frequency or when interfering frequencies appear at the fixed measurement frequencies. The tuned medium band UHF PD measuring system design consists of one or more manually pretuned band-pass filters with a bandwidth of 50 ... 150 MHz applied in a frequency range of approx. 100 ... 2000 MHz. The selection of the centre frequencies should be based on the individual resonant frequencies of the PD-sensors determined by the CIGRE sensitivity check [2] on site. The medium bandwidth allows to integrate the individually shifted resonant frequencies of a PD-signal at a PDsensor within the measurement band, caused by different PD locations. Due to the wider bandwidth, the probability of missing PD-signals at a specific centre frequency is much lower than with the narrow band technique [3]. The evaluation of measurements carried out in different environment and different types of GIS showed a high possibility that a medium bandwidth is still narrow enough to avoid fix frequency disturbances by the use of a suitable centre frequency. Using several tuned frequency bands the probability of missing the resonance frequencies of a PD-signal is even lower and additionally allows to conduct a first coarse localization of the PD-source based on the frequency dependent damping of the PD-signal. The main advantage of the proposed design is the combination of simultaneous measurement with optimal sensitivity based on selective avoiding of fixed band interfering frequencies together with tuning into the most sensitive frequency band of each individual PD sensor of a GIS.

Spectral formulation of finite element methods using Daubechies compactly-supported wavelets for elastic wave propagation simulation

A novel and generic formulation of the wavelet-based spectral finite element approach, which is applicable to linear transient dynamics and elastic wave propagation problems, is presented in this paper. In order to spectrally formulate the variational form of governing equations in a temporally-decoupled manner, the wavelet-Galerkin discretization based on Daubechies compactly-supported wavelets is employed. The spectral formulation reduces the problem to a set of temporally independent equations which can be solved in parallel. It is demonstrated that for the spatial discretization, any class of standard finite element method can be adopted to facilitate capturing the complex geometries and boundary conditions. It is demonstrated that the wavelet-based temporal discretization is not influenced by the finite element mesh. Also frequency-dependent damping models are straight-forward to apply. These, along with the fact that the method is directly amenable to parallel computation make the approach very promising for wave propagation problems.To attain hp-refinement capability and spectral convergence properties for the spatial discretization, a higher-order Lagrangian FEM on the Gauss–Lobatto–Legendre grid, i.e. spectral element method, is adopted. For the sake of numerical investigation, a number of 2D and 3D examples including anisotropic and non-homogeneous structures are studied. The accuracy and the convergence rate of the method are evaluated and found to be superior to the explicit Newmark time integration. Possible approaches for speeding up the method are also discussed.

Nonlinear hydrodynamic damping of sharp-edged cantilevers in viscous fluids undergoing multi-harmonic base excitation

In this paper, we investigate finite amplitude polychromatic flexural vibration of a thin beam oscillating in a quiescent viscous fluid. We consider a cantilever beam with rectangular cross section undergoing periodic base excitation in the form of a triangular wave. Experiments are performed on centimeter-size beams in water to elucidate the effect of the amplitude and the frequency of the base excitation on the fluid structure interaction. The fundamental frequency of the excitation is selected to induce structural resonance and the shape of the cantilevers is parametrically varied to explore different flow regimes. Experimental results demonstrate the presence of a frequency-dependent nonlinear hydrodynamic damping which tends to enhance higher frequency harmonics as compared to the fundamental harmonic. Such filtering effect produced by the encompassing fluid increases with both the frequency and amplitude of the base excitation. Experimental results are interpreted through available theoretical models, based on the notion of the complex hydrodynamic function, and pertinent computational fluid dynamics findings.

Dynamic Response of Block Foundations Resting on Soil–Rock and Rock–Rock System Under Vertical Excitation

In the present study, the effect of various soil–rock and rock–rock foundation systems on dynamic response of block foundations of different mass and equivalent radius under vertical mode of vibration is investigated. The frequency dependent stiffness and damping of foundation resting on homogeneous soil and rocks are determined using the half-space theory. The dynamic response characteristics of foundation resting on the layered system considering soil–rock and rock–rock combinations are evaluated using finite element program with transmitting boundaries. The procedure to determine the frequency amplitude response of soil–rock and weathered rock–rock systems is discussed in detail and the equations are proposed for the same. The variation of natural frequency and resonant amplitude with shear wave velocity are investigated for different top layer thickness. The dynamic behaviour of foundation is also determined for different rock–rock systems by considering sandstone, shale and limestone underlain by basalt rock. The variation of the stiffness, damping and amplitudes of foundation with frequency are shown in this study for various rock–rock combinations. It has been observed that the natural frequency increases and the peak displacement amplitude decreases with increase in shear wave velocity ratio. The variation of natural frequency and peak displacement amplitude are also studied for different top layer thickness and eccentric moments. Finally the parametric study is carried out to determine the natural frequency and resonant amplitude for block foundations of size 4 m × 3 m × 2 m and 8 m × 5 m × 2 m using generalized relations proposed in the present study.

Tilting Pad Journal Bearings—A Discussion on Stability Calculation, Frequency Dependence, and Pad and Pivot

For several years, researchers have presented predictions showing that using a full tilting-pad journal bearing (TPJB) model (retaining all of the pad degrees of freedom) is necessary to accurately perform stability calculations for a shaft operating on TPJBs. This paper will discuss this issue, discuss the importance of pad and pivot flexibility in predicting impedance coefficients for the tilting-pad journal bearing, present measured changes in bearing clearance with operating temperature, and summarize the differences between measured and predicted frequency dependence of dynamic impedance coefficients. The current work presents recent test data for a 100 mm (4 in.) five-pad TPJB tested in load on pad (LOP) configuration. Measured results include bearing clearance as a function of operating temperature, pad clearance and radial displacement of the loaded pad (the pad having the static load vector directed through its pivot), and frequency-dependent stiffness and damping. Measured hot-bearing clearances are approximately 30% smaller than measured cold-bearing clearances and are inversely proportional to pad surface temperature; predicting bearing impedances with a rigid pad and pivot model using these reduced clearances results in overpredicted stiffness and damping coefficients that are several times larger than previous comparisons. The effect of employing a full bearing model versus a reduced bearing model (where only journal degrees of freedom are retained) in a stability calculation for a realistic rotor-bearing system is assessed. For the bearing tested, the bearing coefficients reduced at the frequency of the unstable eigenvalue (subsynchronously reduced) predicted a destabilizing cross-coupled stiffness coefficient at the onset of instability within 1% of the full model, while synchronously reduced coefficients for the lightly loaded bearing required 25% more destabilizing cross-coupled stiffness than the full model to cause system instability. The same stability calculation was performed using measured stiffness and damping coefficients at synchronous and subsynchronous frequencies. These predictions showed that both the synchronously measured stiffness and damping and predictions using the full bearing model were more conservative than the model using subsynchronously measured stiffness and damping, an outcome that is completely opposite from conclusions reached by comparing different prediction models. This contrasting outcome results from a predicted increase in damping with increasing excitation frequency at all speeds and loads; however, this increase in damping with increasing excitation frequency was only measured at the most heavily loaded conditions.

Viscoelasticity determined by measured wave absorption coefficient for modeling waves in soft tissues

This paper reports the construction of viscoelasticity models for simulation of wave propagation in soft tissues. Aided by the Carson transform, Fung’s model and Iatridis’s model are transformed into the frequency domain with the imaginary part of the transformed relaxation function shown to be related to wave absorption effect. Based on measured wave absorption coefficients, the viscoelasticity models in the time domain are determined. Results show that Fung’s model does not support the frequency-dependent damping effect, but Iatridis’ model does. To validate the approach, numerical simulation of propagating wavelets in a skeletal pig muscle is performed. The constructed relaxation function is discretized by parallelly connected Standard Linear Solid (SLS) models. The hereditary integration is then transformed into a set of partial differential equations by introducing the internal variables. Together, the governing equations include the equation of motion, the constitutive equation, and the equations of internal variables. Velocity, stress, and internal variables are the primary unknowns. The model equations are solved by using the space–time CESE method. The calculated wave absorption effect compares well with the measured data.

Modification of the loop filter design for a plucked string instrument

A modified loop filter design method for plucked string instruments is introduced. Previous loop filter designs do not properly represent the frequency-dependent damping of a silk stringed instrument. To solve this problem, the modified method evaluates how many harmonics are required and which portion of the sound should be chosen to effectively replicate the instrument. Then, the most reasonable filter parameters are determined based on the frequency signal-to-noise ratio of the synthesized sound.

An experimental comparative study on non-conventional surface and interface damping techniques for automotive panel structures

In order to increase the comfort of vehicle drivers, automotive panel structures are normally damped on their surfaces. Common surface damping treatments like free layer damping or constrained layer damping have the drawback of heavy weight and temperature as well as frequency dependent damping performance. New alternatives with simple structure, high robustness and light weight are sought. In this contribution, three different passive and active damping approaches were investigated: interface damping (ID), active constrained layer damping (ACLD) and particle damping (PD). They were surveyed under same boundary conditions and their performances were compared in terms of weight. The results show that ID strongly decouples the vibration from the source to the panel and provides steady performance over the whole frequency range. ACLD is a lightweight treatment with high damping capability for special mode shapes. PD is an effective simple damper providing excellent damping performance.

Relaxation mechanisms in magnetic colloids studied by stroboscopic spin-polarized small-angle neutron scattering

Stroboscopic small-angle neutron scattering by using polarized neutrons is presented as a technique for investigations of relaxation phenomena on a nanometer scale. Applying a cyclic perturbation to the sample by an oscillating magnetic field allows the polarization-dependent scattering cross-sections to be extracted as time-dependent response. The potential of this special type of ``physical'' contrast variation technique is demonstrated with three examples of ferrofluids of different particle sizes and viscosities. The dynamics of magnetic moment reorientations turned out to be governed by different time scales. For a temperature-dependent fraction of individual mobile particles, a frequency-dependent phase shift and damping of the intensity modulation have been precisely determined and quantitatively explained using a Debye-like relaxation model. The characteristic relaxation times, \ensuremath{\tau}, in the liquid state of all samples scale with the particle volume and viscosity confirming a Brownian-relaxation mechanism. For the remaining fraction of particles that form locally ordered domains the orientation distribution of the dynamically arrested static moments has been quantified by an order parameter ${S}_{1}$${}^{\mathrm{arr}}$, which is not accessible with nonpolarized neutrons. In a high viscous Co and a magnetite-based ferrofluid, ${S}_{1}$${}^{\mathrm{arr}}$ corresponds to maximal alignment of the static moments in the liquid state and to a gradual decrease at lower temperatures when the solvent is frozen. In a low viscosity Co-ferrofluid almost no preferred alignment of the arrested particle moments was observed. In the magnetite-based ferrofluid with the highest dipolar attraction energy an additional phase shift has been detected which increases with frequency and magnitude of the magnetic field. This indicates clearly that the set-up of local particle ordering in distorted hexagonal arrangement is delayed with respect to the moment alignment in the time varying magnetic field.

Study on Damping Capacities of Nodular Cast Iron Dense Bar Produced by Horizontal Continuous Casting

Damping capacities of the annealed nodular cast iron dense bar produced by horizontal continuous casting were measured by Dynamic Mechanical Analyzer. The relation of damping capacities with vibration amplitude, frequency and temperature was analyzed to investigate the damping mechanism of the alloy. The results show that the damping capacities increase with increasing temperature and frequency. The internal friction spectra exhibits two internal friction peaks at about 40°C and 150°C and caused by Snoek relaxation and Snoek-Köster relaxation, respectively. The maximum damping capacity can be obtained at about 63Hz. The damping is positive amplitude-dependent, whereas critical amplitude exists where the damping increases dramatically. The temperature-dependent damping results from the superposition effect of point-defect damping, grain boundary damping and interface damping, while dislocation damping is predominant in the frequency dependent damping. The amplitude dependent damping can be interpreted by G-L theory.

Stability enhancement of haptic interaction by frequency-dependent damping and its application to scaled teleoperation

This paper presents a frequency-dependent damping element, called an analog input shaper (AIS), to improve the stability of haptic interaction with virtual or real environments. High frequency inputs to a haptic interface, which usually occur in the collision with very stiff objects, can bring limit cycle oscillations and instabilities. In order to reduce these high frequency inputs and thus to improve the stability of haptic interaction, the AIS is added to the haptic system. Moreover, the AIS is applied to a passivity-based haptic stability control algorithm, called an energy-bounding algorithm (EBA), to increase the displayed impedance range by the EBA. Through scaled teleoperation experiments using an atomic force microscope (AFM), we show that the AIS can enhance stability during haptic interaction and consequently increase impedance range that a haptic interface can stably display. Also, we show that the AIS can alleviate conservativeness of the passivity-based haptic control algorithms.

개선된 발현악기의 루프 필터 설계 방법

본 논문에서는 발현악기 물리적 모델링에서의 개선된 루프필터 설계 방법을 제안한다. V<TEX>$\"{a}$</TEX>lim<TEX>$\"{a}$</TEX>ki가 제안한 기존의 루프필터 설계 방법은 악기의 음이 오래 지속되는 경우에는 타당하지만, 그렇지 못한 경우에는 악기 음의 주파수 의존 감쇠를 표현하지 못하는 문제점이 있다. 이를 해결하기 위해 녹음된 악기의 단위음에 대해 감쇠구간을 선택, 배음의 개수를 최소 5개부터 20개까지 변경하며 루프필터의 파라미터를 추정하고 이를 이용한 합성음과 원 신호 간 주파수 영역에서의 신호 대 잡음비가 가장 좋은 파라미터를 선택한다. 제안한 방법의 성능 검증을 위해 몸통의 구조와 현의 재질이 각각 다른 기타, 가야금, 거문고를 대상악기로 선정하였다. 제안한 방법은 배음의 지속시간에 상관없이 악기 음의 주파수 의존 감쇠를 잘 표현하는 루프필터 파라미터를 추정해 낼 수 있었다. This paper describes a development of a loop filter design in a physical modeling of the plucked string instrument. The conventional method proposed by V<TEX>$\"{a}$</TEX>lim<TEX>$\"{a}$</TEX>ki cannot estimate right parameters if a sound has either very short sustain or no sustain. In order to overcome this drawback, we propose the use of the decay region and 5 to 20 harmonics of the sound in the estimation of loop filter parameters. The most appropriate filter coefficient is chosen by frequency signal to noise ratio. To verify the performance of the proposed method, the guitar, gayageum and geomungo were selected as the target because they have different shape, structure, and material of strings. Regardless of the duration of harmonics, the proposed method was able to estimate the loop filter parameters representing frequency-dependent damping of harmonics.

A Digital Input Shaper for Stable and Transparent Haptic Interaction

This paper presents frequency-dependent digital damping referred to a digital input shaper (DIS). Compared to the previously proposed analog input shaper, the DIS makes the implementation of damping much more flexible and significantly reduces the electrical noise. Moreover, the DIS incorporates a position control method at the initial contact to improve the perceived contact hardness. Then, through a series of simulations and benchmark virtual wall experiments, we show that the DIS can significantly increase the impedance range that a haptic interface can stably display and that the initial contact hardness that a user perceives can be noticeably improved by using the proposed position control method. Finally, the DIS is applied to scaled teleoperation using an atomic force microscope and through contact experiments it is shown that it can indeed increase the scaled impedance of a real environment that can be stably displayed. ©

Along-Wind Aero-Elasticity of High-Rise Buildings by Using Indirect Forced Actuation Technique

The frequency-dependent aerodynamic damping and stiffness of high-rise buildings in along-wind motion have been systematically investigated and compared through wind tunnel tests under smooth wind flow. A novel identification scheme based on the indirect forced actuation technique was developed, involving only a simple curve-fitting technique on the frequency response function induced by the actuation. To ensure that global minimization in curve-fitting was achieved, a genetic algorithm and a conventional gradient search method were used in obtaining the final results. An alternative derivation of the frequency response function via the time-domain state space equation is also presented, which has the supporting advantage that the simulation of time history of the structural response becomes possible. To demonstrate the approach, various prism models representing different high-rise buildings with varied aspect ratios and height-width ratios were used in the experimental identification. A total of nine mo...

OBSERVATIONAL EVIDENCE OF RESONANTLY DAMPED PROPAGATING KINK WAVES IN THE SOLAR CORONA

In this Letter we establish clear evidence for the resonant absorption damping mechanism by analyzing observational data from the novel Coronal Multi-Channel Polarimeter (CoMP). This instrument has established that in the solar corona there are ubiquitous propagating low amplitude ($\approx$1 km s$^{-1}$) Alfv\'{e}nic waves with a wide range of frequencies. Realistically interpreting these waves as the kink mode from magnetohydrodynamic (MHD) wave theory, they should exhibit a frequency dependent damping length due to resonant absorption, governed by the TGV relation showing that transversal plasma inhomogeneity in coronal magnetic flux tubes causes them to act as natural low-pass filters. It is found that observed frequency dependence on damping length (up to about 8 mHz) can be explained by the kink wave interpretation and furthermore, the spatially averaged equilibrium parameter describing the length scale of transverse plasma density inhomogeneity over a system of coronal loops is consistent with the range of values estimated from TRACE observations of standing kink modes.

Near resonant bubble acoustic cross-section corrections, including examples from oceanography, volcanology, and biomedical ultrasound

The scattering cross-section sigma(s) of a gas bubble of equilibrium radius R(0) in liquid can be written in the form sigma(s)=4piR(0) (2)[(omega(1) (2)omega(2)-1)(2)+delta(2)], where omega is the excitation frequency, omega(1) is the resonance frequency, and delta is a frequency-dependent dimensionless damping coefficient. A persistent discrepancy in the frequency dependence of the contribution to delta from radiation damping, denoted delta(rad), is identified and resolved, as follows. Wildt's [Physics of Sound in the Sea (Washington, DC, 1946), Chap. 28] pioneering derivation predicts a linear dependence of delta(rad) on frequency, a result which Medwin [Ultrasonics 15, 7-13 (1977)] reproduces using a different method. Weston [Underwater Acoustics, NATO Advanced Study Institute Series Vol. II, 55-88 (1967)], using ostensibly the same method as Wildt, predicts the opposite relationship, i.e., that delta(rad) is inversely proportional to frequency. Weston's version of the derivation of the scattering cross-section is shown here to be the correct one, thus resolving the discrepancy. Further, a correction to Weston's model is derived that amounts to a shift in the resonance frequency. A new, corrected, expression for the extinction cross-section is also derived. The magnitudes of the corrections are illustrated using examples from oceanography, volcanology, planetary acoustics, neutron spallation, and biomedical ultrasound. The corrections become significant when the bulk modulus of the gas is not negligible relative to that of the surrounding liquid.

Optimal design and parameter estimation of frequency dependent viscoelastic laminated sandwich composite plates

Recent developments in optimization and parameter estimation of frequency dependent passive damping of sandwich structures with viscoelastic core are presented in this paper. A finite element model for anisotropic laminated plate structures with viscoelastic frequency dependent core and laminated anisotropic face layers has been formulated, using a mixed layerwise approach, by considering a higher order shear deformation theory (HSDT) to represent the displacement field of the viscoelastic core, and a first order shear deformation theory (FSDT) for the displacement fields of adjacent laminated face layers. The complex modulus approach is used for the viscoelastic material behaviour, and the dynamic problem is solved in the frequency domain, using viscoelastic material data for the core, assuming fractional derivative constitutive models. Constrained optimization of passive damping is conducted for the maximisation of modal loss factors, using the Feasible Arc Interior Point Algorithm (FAIPA). Identification of the frequency dependent material properties of the sandwich core is conducted by estimating the parameters that define the fractional derivative constitutive model. Optimal design and parameter estimation applications in sandwich structures are presented and discussed.

Comparison of external damping models in a large deformation problem

In many applications of flexible multibody dynamics, the magnitudes of damping forces are very small in comparison with the elastic and inertial forces. However, these small forces may have a very significant influence on responses near resonant frequencies. The role of damping is to remove the energy of a system by dissipation, and dissipative forces in structures can be the result of either internal or external damping. External damping includes aerodynamic and hydrodynamic drag and dissipation in the supports of structures, and internal damping is usually related to energy dissipation in materials. In large deformation problems, because of the flexibility of very thin structures, external damping is more important. Two types of damping models, proportional damping and quadratic damping, have been widely applied to flexible multibody dynamics. The advantages and weaknesses of the two damping models are considered in this study. To make up for the common drawbacks in these two models, a frequency-dependent generic damping model based on experimental modal analysis is proposed. The proposed damping model leads to a accurate correlation with experimental results because it directly uses the modal parameters of each mode obtained by experiment, and can represent exact high frequency behaviors simultaneously. To define and formulate a large deformation problem, the absolute nodal coordinate formulation (ANCF) was used, and computer simulations with the ANCF were compared to experimental results. Using the proposed experimental method, modal parameters and damping behaviors are extracted until 5th mode, which has a frequency of 89 Hz. It is shown that the common drawbacks of proportional and quadratic damping are complemented by the proposed generic damping model.

Frequency-dependent Drude damping in Casimir force calculations

The Casimir force is calculated between Au thin films that are described by a Drude model with a frequency dependent damping function. The model parameters are obtained from available experimental data for Au thin films. Two cases are considered; annealed and nonannealed films that have a different damping function. Compared with the calculations using a Drude model with a constant damping parameter, we observe changes in the Casimir force of a few percent. This behavior is only observed in films of no more than 300 Å thick.

Performance characteristics of prototype MR engine mounts containing LORD glycol MR fluids

LORD Corporation has recently developed glycol-based MR fluids for use in applications such as engine mounts and bushing, in which the MR fluid will contact rubber and other oil-sensitive elastomers. To demonstrate the performance characteristics of these fluids, prototype MR engine mounts were designed and their dynamic stiffness and damping were tested. In one configuration, the MR mount contained a simple MR valve and was filled with a glycol fluid containing 22% iron by volume. This mount had low dynamic stiffness and frequency-dependent damping in the off-state and higher dynamic stiffness with little damping in the on-state. The low-frequency stiffness and damping could be varied by adjusting the applied magnetic field. In a second mount configuration, the mount contained both an MR valve and an inertia track. Two such mounts were evaluated with MR fluids containing 22% and 36% iron, respectively. In the off-state, both mounts displayed low stiffness and damping as fluid flowed through the MR valve. In the on-state, the MR valve was closed and the inertia track was activated, giving characteristic frequency-dependent stiffness and damping that could be tuned by varying the strength of the magnetic field. These behaviors were observed at both low and high displacements of the mount.