Total Damping Ratio Research Articles

Data driven damping separation method for the Mega-offshore wind turbine tower

This study proposes a novel damping separation method based on operating state-measured data collected from a 6.7 MW offshore wind turbine tower to provide various damping ratios caused by different loads. First, the operating frequencies and total damping ratios for various operating states are identified using the dynamic responses under normal operating conditions. Subsequently, the natural and harmonic frequencies caused by the rotation of the blades are distinguished using rotor speed data and the Campbell diagram. Finally, to determine the various damping ratios of different modal orders of offshore wind turbines under multiple load conditions using only actual response data, a new damping equation based on different damping sources is proposed. Filtering strategies for total damping ratios corresponding to different damping sources with varying vibration amplitudes and wind speed thresholds were proposed. The outlier elimination method was introduced to remove error-scattered points, and the average value calculation was used to identify various damping ratios. The actual measured data of a 6.7 MW offshore wind turbine tower was studied, and the results show that the proposed method can effectively separate various damping ratios.

Flow-Induced Motion and Energy Conversion of the Cir-T-Att Oscillator in a Flow Field with a High Reynolds Number

The present study aims to systematically investigate the effects of a high Reynolds number on the flow-induced motion and energy conversion of the Cir-T-Att oscillator. Experiments are conducted in six Reynolds number ranges (2.89 × 104~6.51 × 104 < Re < 7.71 × 104~15.21 × 104) in regimes of TrSL2, TrSL3 and TrBL0. The system total damping ratio (ζtotal) is adjusted by varying excitation voltages (VB) with a controllable magnetic damping system. The VB is varied from 0 V to 165 V, corresponding to 0.082 ≤ ζtotal ≤ 1.153. The amplitude, frequency, fluid force, spectral content, active power, and upper limit of power output are analyzed. The results show that the Reynolds number affects both amplitude and global response characteristics. The active power increases with an increasing Reynolds number within the upper branch. The maximum power output of the Cir-T-Att oscillator reaches 10.43 W, appearing at Re = 14.67 × 104 (D = 0.16 m, ζtotal = 0.468, Ur = 6.34), while the maximum upper limit of power output is 17.80 W at Re = 15.21 × 104 (D = 0.16 m, ζtotal = 0.678, Ur = 6.57).

Hydrokinetic energy harvesting from flow-induced motion of oscillators with different combined sections

Flow-Induced Motion (FIM) of elastically-supported oscillators with "circle-polygon-attachments"sections are experimentally investigated in a water channel to examine the effects of combined sections on hydrokinetic energy harnessing. The incoming flow velocity considered is U = 0.55 – 1.35 m/s, corresponding to reduced velocity Ur = 5.3 –13. A controllable magnetic damping system is applied to change the total damping ratio (ζtotal) of the flow-induced motion energy conversion system (FIMECS) by varying excitation voltages VB (VB = 0 – 108 V, ζtotal = 0.037 – 0.398). Particularly, to calculate fluid force with experimental results, a torque sensor is introduced into FIMECS. The amplitude, frequency, fluid force, active power and efficiency are analyzed. Results suggest that an oscillator with symmetric sharp attachments and no vortex reattachment is conducive to galloping with self-excitation under large damping, and the circle-T-attachments oscillator is optimum. Its best branch for energy conversion is the galloping branch, the peak of active power is Pharn is 19.28 W and the peak of efficiency is ηharn is 26.22% (VB = 99 V, Ur = 11.28). Under the same condition, the circular-T-attachments oscillator has a better energy conversion capacity than the existing ones (triangular prism).

Active Metadamping: A phenomenon of damping enhancement in metamaterial via feedback control

Metadamping is defined as the enhancement of dissipation (or the damped response) of an acoustic metamaterial over an equivalent phononic crystal for the same amount of prescribed damping.The emergence of metadamping and decay coefficient in conjunction with active feedback control is the prime novelty of this paper. The wave-number-dependent active metadamping is defined as the difference between the damping ratio of the active-controlled and that of the uncontrolled metamaterial to indicate its improved energy dissipation. The controller design is based on the Linear Quadratic Regulator with output feedback of velocity, and displacement as well as velocity. The damping emergence metric over the entire Brillouin zone is obtained from the dispersion relationship by applying Bloch’s theorem, and a parametric study is performed to get an insight into the metadamping phenomenon. Velocity feedback control (VC) provides the highest value of total damping ratio when compared to the case of displacement- and velocity-based feedback control (DVC) for an identical long wave sound speed; however, at the expense of reduced attenuation bandwidth. The decay coefficient of DVC is higher compared to that of VC up to a threshold of long wave speed of sound. Beyond that threshold, both VC and DVC provide almost equal decay. We identified the possibility of obtaining high damping in a stiff metamaterial via active control, which will be useful for applications in the field of vibrations and acoustics, impact mitigation, shock absorption, and vehicle dynamics.

Effects of Parameters on Non-Linear Seismic Analysis of Fixed Offshore Platform

Fixed offshore structures in seismically active zones are likely to have deformations large enough to exceed the elastic limits of materials under strong ground motions. Therefore, investigation of nonlinear response of offshore structures under dynamic loadings is very interesting to structural engineers. Nonlinear behavior of structures is highly related with the soil properties and soil behavior. The main purpose of this study is to investigate and show the methodology for inelastic dynamic analyses of fixed-offshore structures under the 10,000-year earthquake loadings. The investigation in this study includes the effect of soil radiation damping, soil-pile gapping, and soil degradation on fixed offshore structures. Additionally, the effect of the direction of earthquake is included in this study. An offshore structure model ‘Platform A’ is modeled in CAPFOS. 10,000-year earthquake events are considered and for this purpose, two different earthquakes are used: Hector Mine Earthquake (1999) and Landers Earthquake (1992). These ground motion records are scaled to have a resulting earthquake that has a return period of 10,000-year. CAPFOS software is used to complete time-history analyses with these earthquakes. Results indicate that: (i) Considering the soil radiation damping negligible is not correct, because without the radiation damping, the total damping ratio is estimated as % 3, while including radiation damping raises the ratio to % 4.5, (ii) soil-pile gapping and soil radiation damping generate adverse effects on analyses outcome, and (iii) using high values of soil degradation factors is not always conservative.

Experimental investigation of downstream cylinder vibrations caused by FIV in the presence of upstream stationary cylinder

The mechanism of flow induced vibrations (FIV) exists in many engineering applications and plays a significant role. In offshore engineering, the risers subjected to flow induced vibrations. In this study, two circular cylinders of different diameters are arranged in tandem manner. In cross-flow transverse direction, the upstream cylinder is fixed, while the downstream cylinder is subjected to vibrations. This study carries out experiments in wind tunnel, which focuses on significant vibration-galloping. Variable mass-damping ratio is used to achieve the critical velocity corresponding to onset galloping vibration. In the present study, the diameter ratio is 0.4 and the gap L is 1.5D, and 3.5D. In the wind tunnel experiments, by changing the mass of the vibrating cylinder, the corresponding values of damping ratio, results in variable mass damping ratio m*ζ. For each value of m*ζ, the amplitude, frequency at every reduced velocity will be obtained at first, the critical velocity and stability map will be summarized. At static area between lock-in and galloping (desynchronization branch), a uniform initial displacement was set to get the total damping ratio. This study also investigates the relationship among the total damping ratio, m*ζ and critical velocity.

Aero-elastic behavior of open-type one-way tensioned membrane structure models

Wind tunnel tests were conducted on an aero-elastic model of an open-type one-way tensioned membrane to investigate the aero-elastic instability mechanism of membrane structures. The structural response and wind velocities above the membrane were measured and analyzed. Complementary simulations were carried out to explain the interaction between the wind and the structure. The results indicate that the aero-elastic instability of the membrane is a kind of vortex induced vibration. This kind of instability is characterized by a sudden increase in the amplitude, or a sudden change in the dominant vibration mode, when the coming flow velocity exceeds a certain value. The vortices generated around the time-averaged deformation of the membrane lead to vortex induced vibration when the dominant frequency of the vortices is close to a certain natural frequency of the membrane. When the aero-elastic instability occurs, the frequency of the vortices is locked in by the structural vibration within a certain coming flow velocity range, and the total damping ratio of the vibration decreases dramatically and becomes nearly zero. Finally, the critical wind velocities are defined, and the reduced critical wind velocities corresponding to the first and second aero-elastic instabilities are 0.84 and 2.3 approximately.

In-Plane Nonlinear Vibration of Overhead Power Transmission Conductors With Coupling Effects of Wind and Rain

In this paper a small sag-to-span ratio conductor, belonging to overhead power transmission conductors, is studied. The single conductor is assumed to withstand a prescribed motion at a single conductor ends caused by suspended insulator swing and placed in the simultaneous wind and rain condition. It is modeled as a planar, nonlinear, one-dimensional continuum, and its ends are assumed to undergo the suspended insulator swing of given amplitude and frequency. Raindrops are blowing by uniformed wind flow, hitting the single conductor and may form rivulets on its surface. Besides rain-wind flow induces aerodynamic instability, the motions of the suspended insulator swing contribute to external and parametric excitation. Vertical motion equation of the single conductor is discretized with Galerkin method of which first order in-plane symmetric mode is took as trial function. Multiple-scale perturbation technique and computation methods are introduced to study nonlinear dynamic responses of the single conductor under parametric excitation or force excitation, or both excitations. Effects of key parameters, such as total damping ratio, upper rivulet angle, suspended insulator swing amplitude and frequency on dynamic characteristics of the single conductor are discussed. The results indicated that those key-parameters have obviously affected the dynamic characteristics of high-voltage conductor.

The architecture and dynamic properties of young Salix purpurea trees in the context of riparian engineering systems

Soil and Water Bioengineers using plants for civil engineering purposes are faced with challenges to determine the interrelation of dynamic and architectural properties of riparian trees. This study aims to improve the characterization of young riparian trees (Salix purpurea L.) in the framework of such an eco-engineering approach. Dimensioning the mechanical effects of a complex multilevel system in a temporal and spatial context is a key issue understanding the plant-fluid interaction. We investigated the natural frequency (fn), the damping ratio (ζ) and the architectural properties of three 4 year old purple willow specimens. From a tree architectural point of view, we found a clear distinction of the specimens in terms of the distribution of the projected area along the tree height, and for the slenderness ratio over the axes orders. The foliage of the specimens had a significant impact on the damping ratio depending on the tree architecture. A high slenderness ratio across the tree axes increased the flexibility of the branching system, and reduced the share of the foliage impact on the damping ratio. In comparison, a low slenderness ratio led to a stiffer tree structure, and therefore the damping ratio, due to the aerodynamic drag of the leaves, had received a higher share of the total damping ratio. Linking tree architectural analysis with its dynamic properties proved to be helpful in advancing the physical characterization of complex riparian trees.

Air-induced nonlinear damping and added mass of vertically vibrating bridge deck section models under zero wind speed

The damping ratio and frequency of vertically vibrating bridge deck section models under zero wind speed, extracted by the proposed piecewise fitting method from both vibration tests and fluid-structure interaction simulations, vary exponentially with vibration amplitude. In vibration tests, the damping ratio of a tri-box section model will be doubled when the top and/or bottom of the two gaps between the three boxes are covered by grilles. Air-induced damping is firstly identified as an important source of damping of a section model. The air-added mass also has a notable influence on the vibration frequency of section models. For three section models with shapes of tri-box, grille-covered tri-box, and closed single-box, the amplitude-averaged air damping ratios account for 48.5%, 67.5% and 55.1% of the total damping ratios of vibrating systems, respectively; and the amplitude-averaged added masses account for 3.8%, 4.3% and 12.8% of the model masses, respectively. Explanations for the dramatically different air damping and added masses of the section models are given based on the flow patterns of the surrounding air. A dynamic equation including nonlinear air damping and added mass is presented to accurately reproduce the whole procedure of vibration tests.

Modeling and experimental validation of a multiple-mass-particle impact damper for controlling stay-cable oscillations

Impact dampers are often used in the field of civil, mechanical, and aerospace engineering for reducing structural vibrations. This research is motivated by a practical problem, that is, the reduction of wind-induced and rain–wind-induced vibrations in long, flexible and low-damping stay-cables. A new concept of distributed-mass impact damper has been proposed, designed, fabricated, and tested experimentally at Vienna University of Technology by using a 31.2-m actual stay. The results of the full-scale experiments (free-decay tests) are illustrated in this study. Because the performance of the new device was very promising, with total damping ratio three to 10 times larger than the original damping in the stay, modeling of the damper device was needed to ensure adequate practical application in the future. This study examines three different reduced-order models, derived in an attempt to characterize the complex behavior of the new device. Verification and validation of the models are carried out by comparison with the experimental results. Finally, selection of an adequate model, based on both experiments and simulations, is described and discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Assessment of the Viscoelastic and Oscillation Properties of a Nano-engineered Multimodality Contrast Agent

Combinations of microbubbles (MBs) and superparamagnetic iron oxide nanoparticles (SPIONs) are used to fabricate dual contrast agents for ultrasound and MRI. This study examines the viscoelastic and oscillation characteristics of two MB types that are manufactured with SPIONs and either anchored chemically on the surface (MBs-chem) or physically embedded (MBs-phys) into a polymer shell. A linearized Church model was employed to simultaneously fit attenuation coefficients and phase velocity spectra that were acquired experimentally. The model predicted lower viscoelastic modulus values, undamped resonance frequencies and total damping ratios for MBs-chem. MBs-chem had a resonance frequency of approximately 13 MHz and a damping ratio of approximately 0.9; thus, MBs-chem can potentially be used as a conventional ultrasound contrast agent with the combined functionality of MRI detection. In contrast, MBs-phys had a resonance frequency and damping of 28 MHz and 1.2, respectively, and requires further modification of clinically available contrast pulse sequences to be visualized.

Characteristics of UV Irradiated Waste Biopolymer from Renewable Resources (Part 2)

In the developed countries, waste cooking oil has proven to be a problematic material since people usually pour waste cooking oil into the kitchen drainage because they unaware that this waste can be recycled. In this study, the waste cooking oil was converted into polymer, fabricated by using hot compression machine and named as waste biopolymer (WB). WB characteristics before and after UV-irradiated such as vibration damping, density and surface morphology were analysed in this study. Total damping ratio of WB changes more than 5% after 1000 hour UV-irradiation. UV-irradiation does not give major influence to the WB morphology since there is no big change for overall structure of UV-irradiated WB after 1000 hours of UV. Even though the density of UV-irradiated WB is decreasing with increasing exposure time to UV, but it gives good influence to the damping ratio. WB is expected to have a good service life even in a harsh UV environment thus it can be applied in many applications especially in automotive field and manufacturing packaging.

Modeling and Simulation of Linear and Nonlinear MEMS Scale Electromagnetic Energy Harvesters for Random Vibration Environments

The simulation results for electromagnetic energy harvesters (EMEHs) under broad band stationary Gaussian random excitations indicate the importance of both a high transformation factor and a high mechanical quality factor to achieve favourable mean power, mean square load voltage, and output spectral density. The optimum load is different for random vibrations and for sinusoidal vibration. Reducing the total damping ratio under band-limited random excitation yields a higher mean square load voltage. Reduced bandwidth resulting from decreased mechanical damping can be compensated by increasing the electrical damping (transformation factor) leading to a higher mean square load voltage and power. Nonlinear EMEHs with a Duffing spring and with linear plus cubic damping are modeled using the method of statistical linearization. These nonlinear EMEHs exhibit approximately linear behaviour under low levels of broadband stationary Gaussian random vibration; however, at higher levels of such excitation the central (resonant) frequency of the spectral density of the output voltage shifts due to the increased nonlinear stiffness and the bandwidth broadens slightly. Nonlinear EMEHs exhibit lower maximum output voltage and central frequency of the spectral density with nonlinear damping compared to linear damping. Stronger nonlinear damping yields broader bandwidths at stable resonant frequency.

Energy-based numerical evaluation for seismic performance of a high-rise steel building

As an alternative to current conventional force-based assessment methods, the energy-based seismic performance of a code-designed 20-storey high-rise steel building is evaluated in this paper. Using 3D nonlinear dynamic time-history method with consideration of additional material damping effect, the influences of different restoring force models and P-<TEX>${\Delta}/{\delta}$</TEX> effects on energy components are investigated. By combining equivalent viscous damping and hysteretic damping ratios of the structure subjected to strong ground motions, a new damping model, which is amplitude-dependent, is discussed in detail. According to the analytical results, all energy components are affected to various extents by P-<TEX>${\Delta}/{\delta}$</TEX> effects and a difference of less than 10% is observed; the energy values of the structure without consideration of P-<TEX>${\Delta}/{\delta}$</TEX> effects are larger, while the restoring force models have a minor effect on seismic input energy with a difference of less than 5%, but they have a certain effect on both viscous damping energy and hysteretic energy with a difference of about 5~15%. The paper shows that the use of the hysteretic energy at its ultimate state as a seismic design parameter has more advantages than seismic input energy since it presents a more stable value. The total damping ratio of a structure consists of viscous damping ratio and hysteretic damping ratio and it is found that the equivalent viscous damping ratio is a constant for the structure, while the equivalent hysteretic damping ratio approximately increases linearly with structural response in elasto-plastic stage.

Energy Dissipation in Nearly Saturated Poroviscoelastic Soil Columns during Quasi-Static Compressional Excitations

AbstractThis paper presents a theoretical investigation on the energy dissipation in a nearly saturated poroviscoelastic soil column under quasi-static compressional excitations. Different components of the energy dissipation are evaluated and compared. The magnitude of fluid- induced energy dissipation is primarily a function of a normalized excitation frequency Ω. For small values of Ω, a drained soil column is fully relaxed and essentially behaves as a dry column with negligible pore pressure. For such a soil column, fluid-induced energy dissipation is negligible, and the total damping ratio of the column is essentially the same as that of the solid skeleton. For very high values of Ω, a drained soil column is fully loaded and the excitation-generated pore pressure decreases as the fluid becomes more compressible. For such a soil column, the fluid pressure gradient only exists in a thin boundary layer near the drainage boundary, where drainage occurs and fluid induces energy dissipation, whereas the re...

Effect of Soil Conditions on the Seismic Response of Spatial Structure

This paper is to study the seismic response of spatial structure incorporating the soil structure interaction (SSI). The SSI model for the long-span spatial structure with supporting substructure is proposed and the relevant soil parameters according to the IBC code are presented. A procedure for calculating the total damping ratios for one DOF system is used in the seismic analysis of spatial structure including SSI. The displacement and the axial force in the member under different soil conditions are compared and some important conclusions are drawn.

Control of kinetic energy of a one-dimensional structure using multiple vibration neutralizers

There are two cases presented in this paper: the effects of multiple vibration neutralizers on the kinetic energy of a continuous structure, all tuned to a particular natural frequency, and when the tuning ratio of each neutralizer is optimally adjusted at each excitation frequency using quadratic minimization technique. In the first case, theoretical formulations are developed to calculate the width of the separation between the new resonances and the kinetic energy reduction. The width between the new resonances is found to be a function of the total mass of the neutralizers and the modal amplitudes at the neutralizer's location, whereas the kinetic energy reduction is determined by their total damping ratios, total mass and also the modal amplitudes at their respective locations. In the second case, it is found that the reduction of the kinetic energy in the frequency range of interest increases with the number of optimally detuned neutralizers, and the reduction is comparable to that of the feedforward active control method. Simulations are presented to facilitate better understanding on the control effects using multiple vibration neutralizers.

The half-octave temporary threshold shift

A model is proposed which provides a simple explanation for the noise-induced half-octave temporary threshold shift. The cochlear partition is modeled as a series of lightly coupled mechanical oscillators in which the outer hair cells are displacement responsive and the inner hair cells are velocity responsive. The outer hair cells are assumed to take an active role in providing negative damping in a region of stimulation of the cochlea at low-sound pressure levels. In the model, each oscillator is linear to very-high-sound-pressure levels except in a region of stimulation where the damping ratio varies with sound-pressure level. The model predicts that the passive viscous damping ratio of each oscillator is 1/√2. At low-sound-pressure levels negative structural damping reduces the total damping ratio to about 0.055. At high-sound-pressure levels negative damping ceases and the response in a region of high-level stimulation is then passive. The variable damping ratio provides an explanation for all cases of nonlinear cochlear response which have been investigated.

On the Mechanism of Fluidelastic Instability of a Tube Placed in an Array Subjected to Two-Phase Crossflow

In this work, the fluidelastic instability of a tube placed in an array subjected to two-phase crossflow has been studied. For the determination of fluidelastic instability, acrylic tubes 2.2 cm or 2.37 cm in diameter and 20 cm in length were arranged in a triangular array with pitch to diameter ratio of 1.4 and 1.3. The test tube was flexibly supported with two cantilever beams. By installing cantilever beams horizontally and vertically, drag and lift direction tube vibration were studied. The total damping ratio is found to increase as the liquid gap velocity or void fraction increases. It has been found that the effective spring constant decreases rapidly with increase of liquid gap velocity or void fraction just prior to the onset of instability. For the range of parameters studied in this work, the onset of fluidelastic instability is associated with a rapid decrease of the effective spring constant because for a given applied force the amplitude of the tube vibration is inversely proportional to the spring constant.