Damping Levels Research Articles

Design of a Super Multispan Continuous Girder Bridge Using Enhanced Seismic Response Modification Technologies

This paper reports the state-of-theart seismic design concept utilizing an extensive combined application of seismic response modification devices, including elastomeric bearings, seismic dampers, and low-friction slide bearings with a particular layout for the super multispan continuous girder bridges, used in the Imagiregawa Bridge project in Japan. The baseline of the design concept is the reduction of the resilient force introduced by the bearing system, while a high level of structural damping is introduced with the use of high-capacity seismic dampers to utilize the seismic force reduction effect of the seismic isolation concept. There are other great advantages in the present design concept for long super multispan continuous girder bridges: statically indeterminate forces that act on multispan continuous girders supported by elastomeric bearings induced by temperature changes, desiccation shrinkage, and creep can be reduced so that any unfavorable effects to the seismic response are reduced. It also eliminates the necessity for adjustment of elastomeric bearing installation with the “post slide” procedure during construction. Since the elastomeric bearings and seismic dampers are installed on a limited number of piers, 22% reduction of the projected construction cost of the substructures is achieved as compared with conventional seismic isolation design.

Parametric study on acceleration-based design of low-rise base isolated systems

The dynamic response of a series of base isolated systems subjected to ground motions that were recorded in firm soils of the Mexican Pacific Coast was estimated in order to evaluate the influence of the structural properties of these systems in their floor acceleration demands. While the super-structures were assumed to remain elastic and to exhibit 2% of critical damping, the isolation systems were assumed to exhibit linear behavior with viscous damping ranging from 10% to 30% of critical damping. The different damping levels assigned to the super-structures and isolation systems were taken into account through a non-classical damping approach. After identifying in general terms the structural properties of the systems that are able to better control their floor acceleration demands, it is concluded that flexible isolation systems with low levels of damping coupled with stiff super-structures result in substantial reductions of the participation of upper modes to the global dynamic response of isolated structures. Within this context, an equivalent single-degree-of-freedom system that can be used within an acceleration-based format to conceive base isolation systems is formulated, and implications for its practical use discussed.

Pemberian Kalium pada Beberapa Kelembaban Tanah terhadap Pertumbuhan dan Produksi Jagung Manis (Zea mays saccharata Sturt)

The objective of this research was to know influence of interaction between the potassium dosage and soil dampness level, and also influence of potassium dosage and soil dampness level toward sweet corn growth and production. It was conducted from May to July 2006. The result of the research can be summarized as follow: (1) Interaction between potassium dosage and soil dampness level had significant effect on cob weight without coat and dry cob weight, and the best combination was 225 KCl/hectare with 85% soil dampness level; (2) Potassium dosage and soil dampness level also had significant effect on cob weight without husk, which was 64,29% higher than those without potassium at similar soil dampness level. This was the result for 225 KCl/hectare and soil dampness of 85%.

Analysis and Development of a Semiactive Damper for Compliant Actuation Systems

The development of actuation systems with intrinsic compliance has recently gained increased attention due to the benefits that compliance brings with respect to interaction, safety, and energy efficiency. However, the incorporation of compliance into the actuation system results in systems with more complex dynamics that are not trivial to control due to the induced oscillations and the decreased mechanical bandwidth. To assist the control of robots powered by compliant actuators, this study proposes the use of physical damping in the structure of the compliant actuator. A motivation study is presented to demonstrate the improvements in compliant joint performance gained by introducing physical damping. The variable physical damping actuator (VPDA) aims to improve the control of compliant actuators with the provision of a desired physical damping level on demand. A single degree-of-freedom compliant joint is used to evaluate the ability of the VPDA in replicating different values of physical damping when the link of the compliant joint is perturbed. The performance of the unit is experimentally verified, showing that the VPDA system (mechanism and its control) is capable of generating desired viscous damping levels with good fidelity assisting to regulate the generated oscillations.

Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, Part II: Effects of surface waves, external damping and draft condition

Aiming to complete the results presented before by Gonçalves et al. (2012d. Ocean Eng. 54, 150–169) the present work brings new experimental results on VIM of a semi-submersible platform with four square columns, particularly concerning changes in three different aspects: simultaneous presence of current and surface waves in the same direction, external damping level, and draft conditions. The VIM tests were performed in the presence of regular and irregular waves, both conditions with simultaneous current presence, to understand the wave effects. Considerable differences between the presences of regular and irregular waves were observed. The motion amplitudes in the transverse direction, in the tests with regular waves, were markedly lower than those with irregular waves, and the VIM behavior was not observed. In the sea state tests, the amplitudes are lower than current-only ones, yet a periodic motion characterized by VIM was observed. Furthermore, the effects of the lower draft condition and damping level were addressed, showing they are important for model tests because they contribute to decreasing VIM amplitudes.

Elastic and Large-Strain Nonlinear Seismic Site Response from Analysis of Vertical Array Recordings

AbstractStrong ground motions from the Mw=6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array at a nuclear power plant. Site conditions consist of about 70 m of medium-dense sands overlying clayey bedrock, with groundwater located at 45 m. Ground shaking at the bedrock level had a geometric mean peak acceleration of 0.55g, which reduced to 0.4g at the ground surface, indicating nonlinear site response. One-dimensional ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification levels, provided small-strain damping levels somewhat larger than those from laboratory tests are applied. Nonlinear ground response analyses of strong-motion data using laboratory-based modulus reduction and damping relations valid up to moderate strain levels (<∼0.5%) produce unrealistic strain localization at a velocity contrast. A procedure is presented to more realistically represent the large-strain por...

Towards improved floor spectra estimates for seismic design

Current codes incorporate simplified methods for the prediction of acceleration demands on secondary structural and non-structural elements at different levels of a building. While the use of simple analysis methods should be advocated, damage to both secondary structural and non-structural elements in recent earthquakes have highlighted the need for improved design procedures for such elements. In order to take a step towards the formation of accurate but simplified methods of predicting floor spectra, this work examines the floor spectra on elastic and inelastic single-degree of freedom systems subject to accelerograms of varying seismic intensity. After identifying the factors that appear to affect the shape and intensity of acceleration demands on secondary structural and non-structural elements, a new series of calibrated equations are proposed to predict floor spectra on single degree of freedom supporting structures. The approach uses concepts of dynamics and inelasticity to define the shape and intensity of the floor spectra at different levels of damping. The results of non-linear time-history analyses of a series of single- degree of freedom supporting structures indicate that the new methodology is very promising. Future research will aim to extend the methodology to multi-degree of freedom supporting structures and run additional verification studies.

Development and validation of a composite finite element with damping capability

This paper presents development and validation of a new composite Finite Element (FE) with damping capability. The damping capability is added to the composite shell element with physical drilling degrees of freedom by means of complex element stiffness matrix of the individual layers. The formulation is implemented in a FE code that allows modelling damped structures and performing modal analysis of the resulting complex eigenvalue problem. Some test cases are prepared and frequency response functions are measured in order to validate the predictions. The material properties used in the sample structures are identified experimentally using the Oberst Beam Method. Furthermore, the results obtained using the proposed composite FE are compared with those based on modal strain energy method. Comparisons of the experimental and the theoretical results confirm that the damping levels and the natural frequencies of damped structures can be predicted with high accuracy using the proposed formulation.

TURBINEâˆ'CHAMBER COUPLING IN AN OWC WAVE ENERGY CONVERTER

Oscillating Water Column (OWC) systems are one of the most popular technologies for wave energy conversion. Their main elements are the chamber with the water column and the air turbine. When studying the performance of an OWC system both elements should be considered together, for they are effectively coupled: the damping exerted by the air turbine affects the efficiency of the conversion from wave power to pneumatic power in the OWC chamber, which in turn affects the air flow driving the turbine. The optimum level of damping is that which maximizes the efficiency of the conversion from wave to pneumatic power. In this work the turbine-chamber coupling is studied through a combination of physical and numerical modeling.

A fundamental mechanism of legged locomotion with hip torque and leg damping

New models and theories of legged locomotion are needed to better explain and predict the robustly stable legged locomotion of animals and some bio-inspired robots. In this paper we observe that a hip-torque and leg-damping mechanism is fundamental to many legged robots and some animals and determine its affect on locomotion dynamics. We discuss why this hip-torque-and-leg-damping mechanism is not so easily understood. We investigate how hip-torque and leg-damping affect the stability and robustness of locomotion using a mathematical model: First, we extend the canonical spring-loaded-inverted-pendulum model to include constant hip torque and leg damping proportional to leg length speed. Then, we calculate the stability and robustness of locomotion as a function of increasing levels of torque and damping, starting from zero—the energy conserving and marginally stable special case—to high levels of torque and damping. We find that the stabilizing effects of hip-torque and leg-damping occur in the context of the piecewise-continuous dynamics of legged locomotion, and so linear intuition does not apply. We discover that adding hip torque and leg damping changes the stability of legged locomotion in an unexpected way. When a small amount of torque and damping are added, legged locomotion is initially destabilized. As more torque and damping are added, legged locomotion turns stable and becomes increasingly more stable and more robust the more torque and damping are added. Also, stable locomotion becomes more probable over the biologically-relevant region of the parameter space, indicating greater prediction and explanatory capabilities of the model. These results provide a more clear understanding of the hip-torque-and-leg-damping mechanism of legged locomotion, and extend existing theory of legged locomotion towards a greater understanding of robustly stable locomotion.

A method for damping matrix identification using frequency response data

Accurate modeling of damping in structures is of great importance for vibration response analysis and control. This paper addresses the issue of identification of damping matrix of a structure by posing it as a finite element damping matrix updating problem. Many of the current updating approaches, dealing with updating of damping matrix, perform simultaneous updating of mass, stiffness and damping matrices. However, such a strategy is faced with numerical problems in practical implementation, since the magnitude of stiffness and mass matrix elements is generally much more than that of the damping matrix elements causing difficulties in accurate identification of the damping matrix. Some other approaches divide the process of updating of the mass and stiffness matrix and the damping matrix into two stages, but their application is restricted to structures with low levels of damping. This paper addresses these issues by developing an updating formulation that seeks to separate updating of the damping matrix from that of updating of the stiffness and the mass matrix. The proposed damping matrix updating method utilizes the concept of normal frequency response functions (FRFs) available in the literature. The method is formulated so as to reduce the difference between the complex FRFs, which can be measured in practice, and the normal FRFs, whose estimates can be obtained from the measured complex FRFs. The effectiveness of the proposed method is demonstrated through a numerical study on a simple but representative beam structure. The issue of coordinate incompleteness and robustness of the method under presence of noise is investigated. It is found that the proposed method is effective in the accurate identification of the damping matrix in cases of complete, incomplete and noisy data and is not limited by the level of damping in the structure.

Comparative Analysis of Numerically Computed Chaos Diagrams in Duffing Oscillator

<b> </b>This study utilised optimum fractal disk dimension algorithms to characterize the evolved strange attractor (Poincare section) when adaptive time steps Runge-Kutta fourth and fifth order algorithms are employed to compute simultaneously multiple trajectories of a harmonically excited Duffing oscillator from very close initial conditions. The challenges of insufficient literature that explore chaos diagrams as visual aids in dynamics characterization strongly motivate this study. The object of this study is to enable visual comparison of the chaos diagrams in the excitation amplitude versus frequency plane. The chaos diagrams obtained at two different damp coefficient levels conforms generally in trend to literature results[1] and qualitatively the same for all algorithms. The chances of chaotic behaviour are higher for combined higher excitation frequencies and amplitudes in addition to smaller damp coefficient. Fourth and fifth order Runge-Kutta algorithms indicates respectively 62.3% and 53.3% probability of chaotic behaviour at 0.168 damp coefficient and respectively 77.9% and 78.9% at 0.0168 damp coefficient. The chaos diagrams obtained by fourth order algorithms is accepted to be more reliable than its fifth order counterpart, its utility as tool for searching possible regions of parameter space where chaotic behaviour/motion exist may require additional dynamic behaviour tests.

The Use of Perforated Damping Liners in Aero Gas Turbine Combustion Systems

This paper considers the use of perforated porous liners for the absorption of acoustic energy within aero style gas turbine combustion systems. The overall combustion system pressure drop means that the porous liner (or “damping skin”) is typically combined with a metering skin. This enables most of the mean pressure drop, across the flame tube, to occur across the metering skin with the porous liner being exposed to a much smaller pressure drop. In this way porous liners can potentially be designed to provide significant levels of acoustic damping, but other requirements (e.g., cooling, available space envelope, etc) must also be considered as part of this design process. A passive damper assembly was incorporated within an experimental isothermal facility that simulated an aero-engine style flame tube geometry. The damper was therefore exposed to the complex flow field present within an engine environment (e.g., swirling efflux from a fuel injector, coolant film passing across the damper surface, etc.). In addition, plane acoustic waves were generated using loudspeakers so that the flow field was subjected to unsteady pressure fluctuations. This enabled the performance of the damper, in terms of its ability to absorb acoustic energy, to be evaluated. To complement the experimental investigation a simplified one-dimensional (1D) analytical model was also developed and validated against the experimental results. In this way not only was the performance of the acoustic damper evaluated, but also the fundamental processes responsible for this measured performance could be identified. Furthermore, the validated analytical model also enabled a wide range of damping geometry to be assessed for a range of operating conditions. In this way damper geometry can be optimized (e.g., for a given space envelope) while the onset of nonlinear absorption (and hence the potential to ingest hot gas) can also be identified.

On linear modeling of interface damping in vibrating structures

Dissipation of mechanical vibration energy at contact interfaces in a structure, commonly referred to as interface damping, is an important source of vibration damping in built-up structures and its modeling is the focus of the present study. The approach taken uses interface forces which are linearly dependent on the relative vibration displacements at the contact interfaces.The main objective is to demonstrate a straightforward technique for simulation of interface damping in built-up structures using FE modeling and simple, distributed, damping forces localized to interfaces where the damping occurs.As an illustration of the resulting damping the dissipated power is used for evaluation purposes. This is calculated from surface integrals over the contact interfaces and allows for explicit assessment of the effect of simulated interface forces for different cases and frequencies. The resulting loss factor at resonance is explicitly evaluated and, using linear simulations, it is demonstrated that high damping levels may arise even though the displacement differences between contacting surfaces at damped interfaces may be very small.

Vibration Characteristics of Rotating Thin Disks—Part I: Experimental Results

Analysis of the linear vibration characteristics of unconstrained rotating isotropic thin disks leads to the important concept of “critical speeds.” These critical rotational speeds are of interest because they correspond to the situation where a natural frequency of the rotating disk, as measured by a stationary observer, is zero. Such speeds correspond physically to the speeds at which a traveling circumferential wave, of shape corresponding to the mode shape of the natural frequency being considered, travel around the disk in the absence of applied forces. At such speeds, according to linear theory, the blade may respond as a space fixed stationary wave and an applied space fixed dc force may induce a resonant condition in the disk response. Thus, in general, linear theory predicts that for rotating disks, with low levels of damping, large responses may be encountered in the region of the critical speeds due to the application of constant space fixed forces. However, large response invalidates the predictions of linear theory which has neglected the nonlinear stiffness produced by the effect of in-plane forces induced by large displacements. In the present paper, experimental studies were conducted in order to measure the frequency response characteristics of rotating disks both in an idling mode as well as when subjected to a space fixed lateral force. The applied lateral force (produced by an air jet) was such as to produce displacements large enough that non linear geometric effects were important in determining the disk frequencies. Experiments were conducted on thin annular disks of different thickness with the inner radius clamped to the driving arbor and the outer radius free. The results of these experiments are presented with an emphasis on recording the effects of geometric nonlinearities on lateral frequency response. In a companion paper (Khorasany and Hutton, 2010, “Vibration Characteristics of Rotating Thin Disks—Part II: Analytical Predictions,” ASME J. Mech., 79(4), p. 041007), analytical predictions of such disk behavior are presented and compared with the experimental results obtained in this study. The experimental results show that in the case where significant disk displacements are induced by a lateral force, the frequency characteristics are significantly influenced by the magnitude of forced displacements.

Multi-objective optimal control of vibratory energy harvesting systems

This article examines the use of actively controlled electronics to maximize the energy harvested from a stationary stochastic disturbance. In prior work by the authors, it has been shown that when the harvester dynamics are linear and the transmission losses in the electronics are resistive, the optimal feedback controller is the solution to a nonstandard linear-quadratic-Gaussian optimal control problem. This article augments the theory in the following three distinct ways: (a) It illustrates how to use linear matrix inequalities to balance the objective of energy harvesting against other response control objectives (such as minimum requirements on closed-loop damping and maximum levels of voltage response), in the synthesis of the optimal feedback law; (b) it establishes a more realistic characterization of the transmission losses in the actively controlled power electronics used to regulate the extraction of power; and (c) it illustrates how the optimal control theory for resistive loss models can be extended to accommodate the more realistic loss models. The theory is illustrated in the context of a piezoelectric energy harvesting model, and an example is used to illustrate that the theory can be used to simultaneously optimize the feedback law, together with the switching frequency and storage bus voltage of the power electronics.

On the beneficial effect of rotor suspension anisotropy on viscous-dry hysteretic instability

The destabilizing influence of the internal friction on the supercritical rotor whirl can be efficiently counterbalanced by other external dissipative sources and/or anisotropic suspension systems. The theoretical approach may take the internal dissipation into consideration either by dry or viscous models. Nevertheless, several numerical results and a new perturbation technique of the averaging type prove that similar rotor motions and stability limits are achievable by both models, whence the linear viscous assumption appears preferable. Thus, the internal hysteretic force may be expressed by the product of an equivalent viscous coefficient and the rotor centre velocity relative to a reference frame rotating with the shaft end sections. After calculating the natural frequencies and the response to dynamic imbalances, the stability of the steady motion is checked by the Routh-Hurwitz criterion, focusing the analysis on the individual influence of several characteristic properties, like the gyro structure, the stiffness anisotropy of the supports and their asymmetry, and searching for the external damping level needed for stability. A fairly interesting result is that the benefit of the suspension anisotropy is most effective for a symmetric rotor mounted at the shaft mid-span and decreases significantly on increasing the configuration asymmetry, even moderately. It is also observed how the stability may somehow be associated with the coupling between progressive and retrograde precession motions.

A novel parametrically excited non-linear energy harvester

In this study, a method utilizing parametrical resonance for harvesting energy from low-frequency vibrations was investigated. A pendulum-type architecture with electromagnetic induction as the energy conversion mechanism was proposed. A prototype device with a natural frequency of approximately 2Hz was fabricated and evaluated experimentally. The performance of the device was studied theoretically and numerically. The results were validated by experiments. It was found that the numerical results were consistent with the experimental measurements, while the theoretical analysis slightly overestimated the power output. In the cases considered, the proposed device demonstrated qualitatively different behaviors as compared to a conventional linear device. Such difference, e.g. broadened bandwidth and increased power output at higher damping levels, may be advantageous in certain applications.

Wave motion and dispersion phenomena: Veering, locking and strong coupling effects

The dispersion curves describe wave propagation in a structure, each branch representing a wave mode. As frequency varies the wavenumbers change and a number of dispersion phenomena may occur. This paper characterizes, analyzes, and quantifies these phenomena in general terms and illustrates them with examples. Two classes of phenomena occur. Weak coupling phenomena-veering and locking-arise when branches of the dispersion curves interact. These occur in the vicinity of the frequency at which, for undamped waveguides, the dispersion curves in the uncoupled waveguides would cross: if two dispersion curves (representing either propagating or evanescent waves) come close together as frequency increases then the curves either veer apart or lock together, forming a pair of attenuating oscillatory waves, which may later unlock into a pair of either propagating or evanescent waves. Which phenomenon occurs depends on the product of the gradients of the dispersion curves. The wave mode shapes which describe the deformation of the structure under the passage of a wave change rapidly around this critical frequency. These phenomena also occur in damped systems unless the levels of damping of the uncoupled waveguides are sufficiently different. Other phenomena can be attributed to strong coupling effects, where arbitrarily light stiffness or gyroscopic coupling changes the qualitative nature of the dispersion curves.

Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (FPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6 mm (6.716 in) diameter seals for rotor speeds up to 15 kprm, inlet air pressure of 6.9 bar (100 psi), ambient back pressure, and with inlet gas preswirl. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250 Hz. Three different test methods were employed to give confidence in the rotordynamic coefficients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefficients for all seal configurations while paying special attention to the crossover frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswirl, where the straight-through FPDS possessed the lowest cross over frequency of 40 Hz at 15 krpm. The testing also revealed that the preswirl parameter had significantly more influence on effective damping levels and crossover frequencies when compared to rotor speed.