Damping Levels Research Articles

Determinación del amortiguamiento independiente de la frecuencia en estructuras civiles incluso en la situación de modos de vibración muy próximos aplicando técnicas de optimización

Specific computer programs for identification in modal analysis of structures are not economical, and they may not always be available. This paper presents a technique for simple optimization, getting fast and quality results, in the estimation of the damping ratios for each separate mode, and that can be resolved with a demo version (downloadable for free from the internet) of commercial optimization software. It is suitable for low levels of structural damping (or non-frequency-dependent damping), which are those that appear in large structures. Likewise, it is not necessary for the modes to be separate, or even that the magnitude of the near modes be less than that of those with which we are working; within the algorithm movable residual complements optimize their position according to the frequency and vertical axes (the response magnitude), to take into account the influence of the modes outside the considered range. In addition, it is not very sensitive to the presence of noise in the signal.

Mechanism Analysis of PV Generation System for Damping Electromechanical Oscillations

For physically understanding the dynamic response of a photovoltaic (PV)-based integrated power system for electromechanical oscillation damping, firstly, this paper develops the linear mathematical model of a single machine infinite bus system integrated with PV in the electromechanical time scale. Then, based on the electric torque analysis method and the functional route of active power control (P-control) and reactive power control (Q-control), the key factors and influence laws of the power system inertial effect, damping level and synchronization capability are analyzed. The results show that when the PV operating mode utilizes the rotor speed as feedback signal, the PID controller parameters of P or Q-control loop can affect the damping level, synchronization capability and inertial effect of the power system, respectively. The accuracy of the physical mechanism analysis is verified by MATLAB.

Simplified procedures for seismic design verification and evaluation of lead rubber bearing base‐isolated buildings based on free‐vibration response

SummaryBase isolation has seen widespread application to buildings and infrastructures over the past four decades. However, there is a lack of methods for assessing the performance of a base‐isolated structure at the end of construction and during its service life. To this end, simplified methods are developed for verifying isolation design and evaluating seismic demands of rubber‐bearing‐supported base‐isolated buildings based on their free‐vibration response, which could be obtained using field (on‐site) testing. The base isolation layer consists of lead rubber bearings (LRBs) and linear natural rubber (LNR) bearings. For design verification purposes, analytical solutions are provided to benchmark the free‐vibration response of base‐isolated buildings, considering the general case of a multilinear hysteretic isolation response representing multiple LRBs with distinct mechanical specifications. In seismic demand evaluation, seismic capacity of an isolation system is estimated using free‐vibration response of various amplitudes that cover a range of expected seismic intensity of interest. Seismic demands are obtained when capacity coincides with an earthquake response spectrum at a compatible damping level. Procedures are developed for the potential use of snap‐back tests and verified using experimental and numerical data.

Development of Damping Support for a Turbine Pump Assembly of a Cryogenic Fuel Aviation Gas Turbine Engine

Theoretical studies and design studies have been conducted to demonstrate the possibility of using a long hydrodynamic damper in the supports of a hydrogen pump rotor was shown. The complex problem of rotor dynamics and design synthesis of a hydrodynamic damper for a turbopump assembly (TPA) was solved. The geometric parameters of the hydrodynamic damper are determined that ensure the necessary damping level, d = 3 × 103 N s/m. It is shown that at such a damping level, the amplitude of rotor oscillations does not exceed the value of clearances in supports and on blade shoulders.

Numerical Modeling of Resonant Vibrations of an Elongate Plate with an Integral Damping Coating

The classical methods of surface damping of bending vibrations of thin-walled structures and a promising integrated version with a damping coating consisting of two layers of a material with pronounced viscoelastic properties and an intermediate thin reinforcing layer of a high-modulus material are discussed. A four-layer finite element for an elongate plate with an integral damping coating is developed taking into account the lateral compression of the damping layers. A system of governing equations of the finite-element method is constructed for analyzing the dynamic response of the plate during its resonant vibrations. Iterative algorithms have been developed to take into account the amplitude dependence of the logarithmic decrements of vibrations of material of the damping layers when determining the damping properties of the plate and determining its vibration eigenmodes and eigenfrequencies with consideration of frequency dependence of the dynamic elastic moduli of the material. Numerical experiments were carried out to test the finite element developed and the iterative algorithms mentioned. The influence of aerodynamic resistance forces on the overall damping level of a cantilever plate with an integral damping coating is assessed.

Analysis of conditions for effective crack detection in a rod on two supports based on a change in damping

Existing experimental studies of the sensitivity of damping characteristics to the presence of cracks in structural elements are contradictory. Some studies declare high damping sensitivity but others conclude that a change in the dissipative ability of the structure is not enough for reliable crack diagnostics. This difference may be brought about by the influence of many factors on damping efficiency in relation to crack detection. To predict a possible change in the damping characteristic taking into consideration these factors, an experimental-analytical procedure based on the approaches of fracture mechanics was developed. This procedure has made it possible to identify conditions for reliable detection of an edge crack in a rod on two supports under transverse and longitudinal vibrations. It has been shown that the sensitivity of the damping characteristic to the presence of damage is inversely proportional to the damping level of an undamaged structure. Damping change is effective for diagnosing damages in relatively rigid structures. In this case, the stress level in the damaged area must be high enough so that the crack periodically opens or is constantly open. Based on the analysis of the study results, a condition was formulated that can help engineers easily determine the effectiveness of the damping characteristic for crack diagnostics. The damping characteristic is effective if the ratio of the energy dissipated in the crack to the two-fold potential energy of the structure deformation exceeds the product of the vibration damping characteristic of an undamaged structure by the relative error in its determination.

Experimental and numerical investigations of replaceable moment-resisting viscoelastic damper for steel frames

An innovative type of replaceable moment-resisting viscoelastic damper (RMVD) was developed to increase the level of inherent damping of steel moment frame buildings to control wind-induced and/or earthquake-induced dynamic vibrations. The RMVDs are installed in lieu of existing steel moment connections, which occupies no additional architectural space. The system level damping of the steel moment frames could be increased from the viscoelastic segment of the RMVDs. Besides, the proposed damper exhibits passively adaptive performance. The energy dissipation mechanism of the RMVD could be easily shifted between the viscoelastic segment and fuse segment under different levels of inter-story drifts. The sacrificial fuse segment of the damper, which could be easily replaced after an earthquake event, provides ductile and stable performance when the inter-story drifts exceed a predefined threshold. The paper first presents a systemic investigation of the performance of viscoelastic material. Then, a series of dynamic experimental studies were carried out on the viscoelastic segment when subjected to input motions that feature different frequencies and magnitudes, providing a comprehensive overview of the damper performance. Furthermore, the numerical simulation of the damper is provided at the component level. Finally, a steel portal frame is simulated as a case study to present the structural performance with the dampers under static pushover loads. The result of the study shows that the energy dissipation capacity of steel moment frames could be improved throughout the implementation of such devices.

Flexible Support for Gas-Lubricated Bearing Bushings

Herringbone-grooved journal bearings (HGJBs) for high-speed turbo-machinery are often supported on O-rings due to their favorable stiffness and damping characteristics for stability and alignment reasons. They yield limited lifetime at high temperatures, insufficient assembly repeatability, and dynamic characteristics that are difficult to measure and to tune, however. In order to overcome these shortcomings, a parametric model has been developed with the aim to design a flexible membrane bushing support that offers tunable tilting and radial stiffness and that can be coupled to various concepts to provide suitable damping levels to support a gas-lubricated bearing bushing. Three damping concepts have been investigated that offer independent tuning of the support damping coefficient in combination with the novel flexible bearing bushing support. The new flexible membrane support was manufactured and its stiffness in radial and tilting directions was measured before it was successfully implemented in a prototype. Test results obtained from the new prototype with membrane support show a significant improvement in bushing alignment quality compared to O-rings as well as stable operation up to the design rotor speed of 250 krpm.

Damage-Associated Molecular Patterns and Th-Cell-Related Cytokines Released after Progressive Effort.

Inflammation-induced processes commence with the activation of signalling pathways at the cellular level, which mobilize inflammatory cells and stimulate the secretion of chemokines, cytokines, and damage-associated molecular pattern molecules (DAMPs). Physical effort stimulates inflammation, contributing to muscle repair and regeneration. We have examined the impact of different protocols of progressive-effort tests on T-cell DAMP levels, extracellular cleavage products (fibronectin and hyaluronan), and Th-cell-related cytokine levels among soccer players. Thirty male soccer players with a median age of 17 (16–22) years performed different defined protocols for progressive exercise until exhaustion: (1) YO-YO intermittent recovery test level 1 (YYRL1, n = 10); (2) maximal multistage 20 m shuttle run (Beep, n = 10); and mechanical treadmill (MT, n = 10); and (3) shuttle-run test (n = 10). Blood samples were taken three times as follows: at baseline, post effort, and in recovery. Significantly higher post-effort concentrations of IL-4, IL-6, IL-10, and IFN-γ were observed in the Beep group, IL-4 in the YYRL1 group, and IL-6 and IFN-γ in the MT group as compared with the baseline values. Recovery values were significantly higher for concentrations of IL-4, IL-10, and IFN-γ in the YYRL1 group, only for IFN-γ in the Beep group, and for IL-6, IL-10, and INF-γ in the MT group as compared with the baseline values. Post-effort concentrations of DEFβ2, Hsp27, Fn, and UA in the Beep group and Hsp27 and HA in the YYRL1 group were significantly higher as compared with the baseline values. It seems the performed efficiency test protocols caused a short-term imbalance in Th1/Th2 cytokine levels without giving common molecular patterns. The rapidity of these changes was apparently related to specific physical movements and the type of running surface.

Numerical and experimental analyses of modal frequency and damping in tow-steered CFRP laminates

This present paper is devoted to the numerical and experimental investigation of the modal characteristics of composite laminates, with emphasis on damping. Complementing previous studies dedicated to conventional laminates, one considers variable-angle tow laminates, in which the fibers are deposited following curvilinear trajectories. The main objective is to characterize the influence of fiber steering on the damping levels, and evaluate the possibility of achieving increased damping. A dynamic model is derived by combining the semi-analytical Rayleigh-Ritz approach, the Classical Lamination Theory, and the Strain Energy Method. This later enables to estimate the specific damping capacity of each vibration mode. Based on this model, analytical developments are performed aiming at putting in evidence the contribution of each strain component in each layer of the laminate to the specific damping capacities. The results of numerical simulations are presented, enabling to compare the values of specific damping capacities and vibration natural frequencies obtained for variable-angle tow and conventional laminates in a variety of simulation scenarios. Some of the numerical results are validated by comparisons with experimental counterparts. The results confirm the effectiveness of design strategies intended to regulate and possibly increase the damping levels of composite laminates by exploring fiber steering.

Robust Stabilization and Weighted Damping of Bounded Disturbances in Descriptor Control Systems

We establish necessary and sufficient conditions for the existence of dynamic regulators guaranteeing the prescribed estimation of the weighted level of damping of bounded disturbances and the asymptotic stability of linear descriptor systems. An algorithm for the construction of these regulators in the problems of robust stabilization and generalized H∞-optimization is proposed for the descriptor systems with controlled and observed outputs. The main computational procedures of the algorithm are reduced to the solution of linear matrix inequalities with additional rank restrictions. The efficiency of the algorithm is demonstrated by an illustrative example of descriptor stabilization system with bounded disturbances.

Effects of Fish Nets on the Nonlinear Dynamic Performance of a Floating Offshore Wind Turbine Integrated with a Steel Fish Farming Cage

This paper aims to study the effects of fish nets on the nonlinear dynamic performance of a floating offshore wind turbine integrated with a steel fish farming cage (FOWT-SFFC). Fully coupled aero-hydro-servo-elastic numerical models of FOWT-SFFC, with and without nets, are constructed to probe the nonlinear time-domain stochastic response. The first-order potential flow model with quadratic drag forces is employed to calculate the hydrodynamic loading on the foundation. The effects of nets on the damping ratios of 6 degree-of-freedom motions and on their displacement response amplitude operators (RAOs) are respectively investigated in numerical decay tests and monochromatic regular waves. The results show that the nets help to increase the damping level for the whole system and reduce motion RAOs when wave periods are around the natural periods of motions, while nets play insignificant role in motions when wave periods are far away from motion natural periods. The dynamic performances of FOWT-SFFC with and without nets under random ocean waves, the combined random wind and random waves as well as current are comprehensively compared and discussed. The simulation results indicate that in wind-sea dominated conditions, the nets tend to slightly increase the dynamic responses of FOWT-SFFC, especially the components corresponding to natural periods. Nonetheless, under sea states that comprise both wind-sea waves and swell, nets help to reduce the dynamic responses of FOWT-SFFC by introducing additional damping.

An integrated damping system for tall buildings

SummaryThis paper proposes an integrated damping system that aims at providing relatively high damping levels through the mobilization of a proportion of the structure's own mass. This offers significantly higher mass levels and, consequently, considerably more damping compared to conventional tuned mass dampers. Fluid viscous dampers are used to control accelerations in parallel with springs to resist the static loads applied to the moving mass. The advantages of employing relatively large mass levels in achieving considerable damping and reducing sensitivity to tuning are first analyzed using an idealized two degree of freedom structural representation. This is then followed by a description of the proposed “integrated damping system,” which is illustrated through a case study of a 250‐m tall building. The benefits of the proposed damping system are demonstrated through several numerical parametric assessments, as well as a selected suite of earthquake records. For the adopted case study, it is shown that, besides reducing the level of perceivable accelerations, the use of the suggested arrangement can offer an equivalent damping exceeding 50% of the critical damping, resulting in more than 40% reduction in the wind loads as well as over 60% reduction in displacement and acceleration response under seismic excitations.

Mechanics of metadamping in flexural dissipative metamaterials: Analysis and design in frequency and time domains

Locally resonant metamaterials are engineered structures with intriguing acoustic mitigation properties which stem from an interplay between incident elastic waves and a set of internal resonances. Augmented with damping, such properties are shown to vary in unique and non-trivial ways which can amplify the wave attenuation capacity in certain frequency regions, as well as curtail it at times. Wave dispersion mechanics of dissipative metamaterials in literature are typically investigated in the context of wave propagation where the wavenumber is directly involved in band structure evaluations. In this paper, we show that it is possible to conduct such investigation in the context of structural vibrations and demonstrate how modal analysis of a given finite periodic structure is sufficient to assess the accuracy of the predictions obtained from a representative unit cell. The free wave approach is known to be associated with complex frequency solutions where attenuation due to dissipation takes place only temporally and as a consequence, frequency and damping ratio band structures can be constructed. In an attempt to establish direct correlations between infinite and finite medium predictions, it is shown that a combination of frequency and damping ratio band structures (i.e. frequency and damping ratio vs. wavenumber relations) for a given unit cell leads to a new band structure directly relating the damping ratio to wave frequency, which is capable of replicating the findings obtained from the corresponding finite structure. The different phenomena encountered at the fundamental unit cell level are validated via a series of numerical simulations of metamaterial beams with a damped host medium, damped resonators, as well as combinations thereof. The results reveal that the overall damping level as well as the damping source play key roles in dictating the metadamping emergence in locally resonant metamaterials as they compare to baseline designs including phononic and homogeneous structures of equal size and shape. Finally, the notion of positive and negative metadamping is introduced and phase diagrams are constructed to guide and inform the design of such metamaterials in the presence of viscous and structural losses.

Extraction of Nonlinear Aerodynamic Damping of Crosswind-Excited Tall Buildings from Aeroelastic Model Tests

AbstractThis paper presents a new approach for extracting nonlinear aerodynamic damping of crosswind-excited tall buildings from response statistics at different levels of structural damping obtain...

Inelastic floor spectra for designing anchored acceleration-sensitive nonstructural components

In this study, inelastic floor spectra are developed for designing acceleration-sensitive nonstructural components (NSCs). The parameters response modification (reduction) factor, $$ R_{\text{cc}} $$, and inelastic displacement ratio, $$ C_{\text{cc}} $$, are evaluated to quantify the effects of NSCs inelasticity on their seismic-induced force and displacement demands, respectively. The results of the conducted response history analyses illustrate that the inelastic behavior of NSCs can significantly de-emphasize the effects of their tuning period ratio and viscous damping ratio, and of the characteristics of the primary structure and ground excitation. Due to the quasi-harmonic characteristic of building floor motions, NSC inelasticity is more effective for NSCs attached to buildings than for those attached to the ground. NSC inelasticity is most effective for a low-damping roof-mounted NSC tuned to the first modal period of an elastic building (i.e., the most critical NSC from the design point of view). Adopting even a mild level of inelasticity for tuned NSCs not only decreases their seismic force demands significantly but also reduces their displacement demands. For non-tuning conditions, particularly for rigid NSCs, achieving even a relatively small $$ R_{\text{cc}} $$ (i.e., a small reduction in force demand) leads to a significant increase in NSC displacement and ductility demands suggesting that these NSCs should be designed to remain elastic. Results illustrate that the amplitude of $$ R_{\text{cc}} $$ and $$ C_{\text{cc}} $$ depends on the tuning ratio, viscous damping, and level of inelasticity of NSCs, and to a lesser extent, on the characteristics of the primary structure and ground motion. Simplified yet reliable equations are proposed for the estimation of the parameter $$ R_{\text{cc}} $$ for non-rigid NSCs with different levels of inelasticity and viscous damping.

Kullback–Leibler divergence and sample skewness for pathological voice quality assessment

This paper proposes new features aiming to improve the performance of an automatic voice pathology detection system. The features are designed precisely in terms of voice pathologies effects upon the speech signal. The system is intended to deliver high accuracy with a low number of parameters. Kullback–Leibler divergence (KLD) applied to consecutive frames of the speech signal provides a measure of voice instability. In this work, the KLD is applied to frame’s histogram and a modified form of its spectrum named higher amplitude suppression spectrum (HASS). The H-KLD (histogram KLD) and the HASS-KLD are two of the three features presently approached. An additional feature that provides the level of damping of the voice pitch period waveform is proposed, the short-term sample skewness of the signal. The H-KLD, the HASS-KLD, and the sample skewness are features employed along with mel-frequency cepstral coefficients (MFCC) in a voice pathology detection system. The system is composed of a Gaussian mixture models (GMM) classifier and two generalized extreme value (GEV) distribution classifiers. They are fused by means of a Gaussian naïve Bayes classifier. A standard subset of the Massachusetts Eye and Ear Infirmary (MEEI) voice disorders database is adopted for evaluating the system. The obtained global success rate of 99.55% shows that the proposed features are suitable for pathological voice quality assessment.

Performance Assessment of Steel Moment Connections Retrofitted with Various Reduced Section Patterns

It is of high importance in seismic retrofitting of lateral load-bearing systems to increase the connections performance. The crucial point in the steel frame retrofitting process is to create plastic hinges in these types of frames. The formation of plastic hinges in beams and near columns generates large strains on column flanges as well as welding metal and heated surroundings, which can lead to brittle failure. The connection should be designed in such a way as to allow plastic hinge formation at certain points of the beam. One such method suggested for retrofit connections is to reduce the beam section locally away from the connection zone. There are various patterns available to locally reduce the beam section, such as circular, elliptical, and symmetric/asymmetric. In recent years, different proposals have been presented to design these connections which vary from older instructions. For this study, radiused cuts in the flange and slotted holes in the web of connection beams were selected for retrofitting analysis. Cyclic behavior, energy damping levels, and ductility of these connections were studied and compared before and after the retrofit by using nonlinear dynamic analysis. The results showed that the symmetrical circular hole pattern in the beam flanges demonstrated reliable performance.

Seismic performance of multistory CBFs with novel recentering energy dissipative braces

For concentrically braced frames (CBFs) with conventional braces, although they can prevent collapse during earthquakes, people have to confront the costly repair work and long downtime induced business interruption, which is often attributed to excessive residual deformations. Shape memory alloys (SMAs) provide promising solutions to this problem. SMAs are gaining increasing favor by the community, owing to their excellent superelasticity which endows them with capabilities of recovering large deformation and dissipating input energy. Currently, the authors analyzed the seismic performance of CBFs with a novel brace termed recentering energy dissipative brace (REDB). The REDB is essentially a parallel combination of SMA bars and steel bending plates, the former primarily provides self-centering capability and the later is to enhance damping. This paper is to understand the seismic behavior of CBFs braced with REDBs, with an additional motivation to examine whether the REDBs can outperform the corresponding buckling-restrained braces (BRBs). Nonlinear static and time-history analyses are conducted for a prototype 6-story frame building. The corresponding results are compared against similar BRB frame. The basic comparison is made under the seismic scenarios associated with the design-basis earthquake hazard level. Through the comparative analysis, it is found that the properly designed REDBs are superior to the BRBs from the perspectives of generating identical demands in terms of maximum interstory drift ratios and maximum floor accelerations, and meanwhile eliminating residual interstory drift ratios. The confidence of the results is further gained by varying the seismic intensity and the assumed damping level.

High plasma level of S100A8/S100A9 and S100A12 at admission indicates a higher risk of death in septic shock patients

Biomarkers in sepsis for severity, prediction of outcome or reversibility of organ dysfunction are warranted. Measurements of plasma DAMP levels at admission can reflect the severity of cellular damage in septic shock, which might predict the prognosis and reduce the risk of overtreating patients with costly therapies. We measured plasma levels of two DAMPs, S100A8/S100A9 and S100A12 during the first 24 h of admission of septic shock patients. Forty-nine septic shock patients with a similar SOFA scores were selected from our sepsis database to compare a similar proportion of survivors and non-survivors. Plasma levels of S100A8/S100A9 and S100A12 were compared with healthy volunteers using in-house ELISA. Plasma levels of S100A8/S100A9 and S100A12 (5.71 [2.60–13.63] µg/mL and 0.48 [0.22–1.05] µg/mL) were higher in septic shock patients than in healthy volunteers (1.18 [0.74–1.93] µg/mL and 0.09 [0.02–0.39] µg/mL) (P < 0.0001 and P = 0.0030). Levels of S100A8/S100A9 and S100A12 in non-survivors at day 28 (11.70 [2.85–24.36] µg/mL and 0.62 [0.30–1.64] µg/mL) were significantly higher than in survivors (4.59 [2.16–7.47] µg/mL and 0.30 [0.20–0.49] µg/mL) (P = 0.0420 and P = 0.0248) and correlated well (Spearman r = 0.879, P < 0.0001). The high level of plasma calgranulins at admission in septic shock, were higher in non-survivors compared to survivors. These markers could indicate a higher risk of death when SOFA scores are similar and help the stratification of patients for improved care and therapy selection.