Damping Levels Research Articles

Non-Linear Motion Characteristics of a Shallow Draft Cylindrical Barge Type Floater for a FOWT in Waves

A shallow draft cylindrical barge type floater with footing close to the water surface was experimentally evaluated in waves to investigate non-linear motion characteristics. The floater was designed to be used as an option for FOWT—floating offshore wind turbines. The non-linear mechanism can be promoted due to the viscous force that acts on the footing edges and the footing interaction with the free surface. In general, the observed non-linear viscous damping is modeled as a force proportional to the square of the relative velocity between the floater and the water. Therefore, the viscous damping levels is expected to increase, and the response in waves, to decrease. However, an increase in motion responses was observed for a broad range of wave periods. An attempt was made to clarify the hydrodynamic mechanism by comparing wave tank experiments, numerical calculations by CFD—computational fluid dynamics codes, and linear potential theory codes. Regular wave tests for three different wave height conditions were carried out, including free decay tests in still waters. For CFD simulations, the OpenFOAM code was selected. For potential theory simulations, the WAMIT code was chosen. As a result of the research, three points could be highlighted and discussed: first, the hydrodynamic phenomenon that contributed to the non-linear motion of the floater was identified; second, the increase and coupling of the motions response of heave and pitch motions; and finally, the phenomenon that the footing held water mass and lifted it to the water surface. The CFD calculations were able to get good qualitative results compared with the experiments and confirmed the use of CFD as a useful tool to capture the non-linear hydrodynamic phenomenon. The linear potential theory was not able to capture the phenomenon discussed herein.

МЕХАНИЗМ ОБРАЗОВАНИЯ ТРЕЩИН В КЛАДКЕ ЛИЦЕВОГО СЛОЯ НАРУЖНЫХ СТЕН ПРИ ТЕМПЕРАТУРНЫХ ВОЗДЕЙСТВИЯХ

On the basis of the carried out experimental and numerical studies, the mechanism of crack formation in the masonry of the face layer with flexible ties under temperature influences has been established. Experimental studies were carried out for five years on a floor-high fragment of a building formed by external three-layer walls resting on a monolithic reinforced concrete slab. Numerical studies were carried out using the author's specialized program for calculating masonry structures, which implements the finite element method, taking into account structural nonlinearity. The program was verified by comparing the calculation results with the experimental data, as well as at the stage before the formation of cracks using the LIRA-SAPR program. It is shown that, depending on the ratio of the tensile strength of the masonry along the vertical joints and the cut along the horizontal mortar joints, different variants of crack development are possible. Vertical cracks, like expansion joints, reduce tensile and shear stresses. Cutting the masonry along the horizontal mortar joint at the base of the wall leads to a reduction in the length of the area of influence of the floor slab. Together, this leads to a decrease in the level of tensile and tangential stresses and damping of the crack propagation process. At the top of the wall, the effect of the floor slab on the stress-strain state of the masonry is relatively small and the temperature deformations there are close to free. This made it possible to apply a simplified methodology for assessing the crack opening width. A method is proposed for calculating the facing layer masonry for temperature effects and for assigning distances between vertical expansion joints.

Laboratory experimental study of contact interaction between cut shells and resilient bodies

The study presented herein describes promising designs of shell vibration isolators. The feature of the proposed designs is the cut thin-walled shell usage as the main bearing link. These resilient elements have high load capacity and, on the other hand, can provide the desired level of damping. From the point of view of mechanics, shell resilient elements are considered as the deformable systems with dry friction. When simulating these systems, structurally nonlinear non-conservative mixed contact issues of cut shell – resilient body frictional interaction arise. In order to take into account all essential options of the aforementioned issues and specify shell resilient element peculiarities of behavior under operational loads, the authors used the method of laboratory experiments for research. We considered two different contact systems. The first one is a cylindrical shell cut along its generatrix, which contacts a deformable filler. The second system is a cylindrical shell with several incomplete slots interacting with the elastic filler. The stress state and radial displacements of the shells, pliability of the resilient elements, and energy dissipation in the contact systems were time-tracked. As a result, we obtained relations for monitored options of the contact bodies and deformation diagrams for different physical-mechanical and geometrical options of the systems It was found that for a fixed cycle asymmetry coefficient with an increase in the friction coefficient between the shell and the filler, the amount of energy dissipated per cycle gradually decreases. The idea of optimizing shell vibration protection devices according to the criterion of maximum absorption of energy from external influences by determining the required tribological properties of contacting pairs is declared.

Influence of Hydro Turbine Supervision and Control System on Frequency Characteristics of High Proportion Hydropower System

With the construction of interconnected power grid and the rapid development of clean energy, the frequency problem of power system is becoming increasingly prominent. The problem of ultra-low frequency oscillation affects the stability of system frequency. In order to analyze the frequency regulation characteristics of the system, it is necessary to study the local active power control link and stability calculation model of the hydropower plant computer monitoring system. In this paper, the influence of the local active power control link of the monitoring system on the system frequency small signal stability is studied, and the model is built in the simulation program. Through the simulation of 4-machine-2 regional power grid, the influence characteristics of local active power control on frequency dynamic damping level are verified. In the example of large power grid, the coordination between the local active power control of the monitoring system and the speed regulation system of hydropower units is comprehensively considered, the suggestion of coordination of multi-level frequency modulation resources to system frequency regulation is put forward.

Simulation assessment of the half-power bandwidth method in testing shock absorbers

AbstractThe work deals with usability of the half-power bandwidth method in the diagnostic testing of automotive shock absorbers. In all the simulation tests, the front and rear suspension system of a present-day medium-classmotor car was considered. At the first stage, calculations were made in the frequency domain for a linear “quarter-car” model with two degrees of freedom; then, simulations were carried out in the time domain with using a similar but strongly nonlinear model. In the latter case, actual characteristics (corresponding to those obtained from test rig measurements) of shock absorber damping, suspension and tire elasticity, sliding friction in the suspension, and “wheel hop” were considered. The calculations were carried out every time for twelve levels of viscous damping in the suspension system, which made 48 calculation series in total. The factors of gain in the vertical force between the tester’s vibration plate and the vehicle tire (relative to the input force applied) and the dimensionless coefficients of viscous damping in the suspension system, determined by the half-power bandwidth method, were thoroughly analyzed. The calculation results were presented in graphical form. Attention was also paid to the distortions caused by the force of inertia of the tester’s vibration plate.

Hybrid Low-Viscous-Damping Inelastic Displacement Ratios for Seismic Design and Assessment of Structures

ABSTRACT Until recently, the viscous damping ratio pervasively assumed has been . However, the dynamic identification of the experimental response recorded in recent large-scale shake-table tests shows that lower levels of viscous damping are more appropriate when material nonlinearities are explicitly accounted for. This paper investigates a hybrid constant-ductility inelastic displacement ratio, , which identifies the inelastic demand for a viscous damping ratio when the elastic demand for is known. A comprehensive statistical analysis is developed, and analytical estimates are provided, for inelastic single-degree-of-freedom systems with a wide variation of the constitutive parameters and a large earthquake database.

An assessment of damper placement methods considering upfront damper cost

Previous studies have shown that earthquake repair costs can be minimised by using large levels of supplemental damping and uniform damper placement. However, it is not always feasible to achieve this damping due to structural or financial restraints. Iterative damper placement methods may be able to achieve higher levels of damping than simple methods such as uniform damping for the same total damper cost. In this work, six damper placement methods were assessed based on structural and non-structural repair costs. The total damper cost was constrained to be the same in each case. The scope of work was limited to linear fluid viscous dampers, concentric braced frames and regular structures. The iterative methods were found to provide a greater total damping coefficient to the structures than the simple methods. This resulted in a higher supplemental damping ratio and lower repair costs. If upfront funds are limited, or if architectural constraints prevent the placement of dampers in lower storeys, then iterative methods provide the most favourable total-building seismic performance. However, the conclusions should be extended cautiously. Although iterative methods are favourable when the upfront damper investment is strictly limited, in terms of total-building seismic performance, it is advantageous to provide a large damping ratio using uniform damping.

Passive vibration damping of a cantilever plate

Vibration control of mechanical systems divided into two types active and passive, this case study illustrates other approach of passive control of the cantilever plate by using double layers of the viscoelastic material (Dyad 606) one in the upper face of the Al substrate (5051) and the second one in the lower face. This layers are affected by axial uniform distributed force by applying tension load, the damping level depend on the value of this axial force where the vertical component of this force consistently the other way of the vibrating element that reduce vibration amplitude in this direction by about (95%) of the original amplitude without damping layers. Computational analysis using ANSYS program are used firstly to verify that the results from the software are compatible with that obtained from analytical and experimental evaluation by comparing the values of natural frequencies. Then using ANSYS model to calculate the reduction occurs in the lateral vibration with changing of the axial loads on the viscoelastic plates. The benefits of this model are enhancing the reliability and reducing the cost.

Cross-axis control system design for borehole drilling based on damping optimum criterion and utilization of proportional-integral controllers

This paper presents a control system design methodology for the drill-string rotary drive and draw-works hoist system aimed at their coordinated control for the purpose of establishing a fully-automated mechatronic system suitable for borehole drilling applications. Both the drill-string rotary drive and the draw-works hoist drive are equipped with proportional-integral (PI) speed controllers, which are readily available within modern controlled electrical drives. Moreover, the rotary speed control system is equipped with torsional active damping system and drill-string back-spinning prevention scheme for the case of stuck drill-bit scenario, whereas the draw-works-based drill-bit normal force control system is extended with the auxiliary control system aimed at timely prevention of the drill-string torsional overload. The design of proposed control systems has been based on suitable reduced-order control-oriented process models and a practical tuning methodology based on the damping optimum criterion aimed at achieving the desired level of closed-loop system damping. The functionality of the proposed cross-axis control system has been systematically verified, first by experimental tests of individual rotary/vertical axis control systems on a downscaled laboratory experimental setup, followed by thorough simulation study of the overall control system for realistic scenarios encountered in the field.

A simplified model to evaluate peak amplitude for vertical vortex-induced vibration of bridge decks

A simplified model is developed to conveniently evaluate the peak vortex-induced vibration (VIV) amplitudes of a bridge deck at various mass-damping conditions. As a simplified form of the describing function-based model previously developed by the authors, the key innovation of the new model is the introduction of an envelope curve of the aerodynamic describing functions (or amplitude-dependent flutter derivatives), which determines the maximum negative aerodynamic damping versus vibration amplitude in the lock-in range. Based on the envelope curve, the peak VIV amplitudes at different mass-damping conditions can be obtained directly without calculating all the VIV amplitudes in the entire lock-in range. The new model is more practical for engineers in the bridge engineering community since it only contains a single group of aerodynamic parameters which don't vary with the reduced wind speed. The envelope curve can be either identified based on the VIV decay-to-resonance and/or grow-to-resonance signals at a single mass-damping condition, or based on the VIV steady amplitudes at different mass-damping conditions. Numerical examples involving the VIV analyses of a rigid rectangular cylinder and a bridge deck sectional model are utilized to validate the simulation accuracy of the proposed model, and the model is applied to calculate the peak VIV amplitudes of two flexible bridge decks at various mechanical damping levels. The proposed model is capable of accurately and conveniently predicting the peak VIV amplitudes of a bridge deck sectional model at various mass-damping conditions. For a flexible bridge deck, the peak VIV amplitude calculated by the proposed model is slightly conservative due to the overestimated negative aerodynamic damping at some span-wise segments of the bridge deck. The superiority of the proposed model relative to the conventional van der Pol-type model is also demonstrated.

Analysis of Energy Flow Method for Oscillation Source Location

Sudden occurrence of a large forced oscillation (FO) is a challenging problem for power system operators, especially when the forced oscillation is resonating with a natural system mode. Among the methods for oscillation source location, the energy flow method has shown promise for detecting the source of a natural oscillation as well as a forced oscillation. In this paper, we analyze the performance of the energy flow method in depth by evaluating the expressions of incremental energy in one period of the forced oscillation and by looking at a wide range of forced oscillation frequencies. We show that the energy flow based method can misidentify sources of forced oscillations. That is, the method may not be able to locate the true forced oscillation source as well as identify a non-source device to be the source. Examples are shown for systems with different damping levels of system modes and for different load composition types.

ANLYTICAL SOLUTION OF THERMOELASTIC DAMPING IN RECTANGULAR MINDLIN MICRO PLATES

Analytical solution of thermoelastic damping for rectangular Mindlin micro plate with the four edges simply supported is presented for the first time. Based on the Mindlin plate theory and the one-way coupled heat conduct theory, governing differential equations for thermo-elastically coupled free vibration of the micro plate are formulated. Ignoring the in-plane variation of the temperature gradient, analytical solution of temperature field in terms of the kinematic parameters is obtained under the adiabatic boundary conditions at the top and bottom surfaces. Furthermore, the equations of structural vibration including the thermal bending moment are transformed into a fourth-order partial differential equation only in terms of the deflection. By using the mathematical similarity between the eigenvalue problems under the simply supported boundary conditions, analytical solution of the complex natural frequency of the Mindlin plate is expressed in terms of the frequency of isothermal Kirchhoff plate. Then the inverse quality factor which represents the level of the thermoelastic damping is obtained. Finally, the effects of the shear deformation, the material and the geometry parameters on the thermoelastic damping are examined in detail by the numerical results. The numerical results show that thermoelastic damping estimated by Mindlin plate theory is less than that by Kirchhoff plate theory. The difference between the values evaluated by the two plate theories becomes very significant near the critical thickness. Moreover, along with the increase of the side-to-thickness aspect ratio,the maximum of the thermoelastic damping in micro Mindlin plate increase monotonically, however, that of micro Kirchhoff plate keeps constant.

A novel concept for adaptive friction damper based on electrostatic adhesion

This work presents a novel concept for an adaptive friction damper based on electrostatic adhesion for medium to small industrial applications and investigates the effects of electrostatic tuning on the energy dissipated by the damper under quasi-static as well as dynamic conditions. The novel damper has a low mechanical complexity. The amount of energy dissipated can be controlled through an electric voltage and be easily adapted to different working conditions. The concept is based on a stack of electrostatic clutches hinged around a common axis, which can dissipate energy through Coulomb friction. An electric field is used to vary the maximum shear stress transferred by the clutches and, consequently, to control the damping level. A prototype of the damper was manufactured and tested. The experimental results demonstrate the increase in the resistive moment of the clutches with increasing electric field and display the capability of the concept to adapt the damping level of a single-degree-of-freedom resonating system. The presented concept might replace linear dampers in rotating machines operating at different regimes, in road vehicles as well as in aircrafts.

Velocity, acceleration, jerk, snap and vibration: forces in our bodies during a roller coaster ride

Changing acceleration and forces are part of the excitement of a roller coaster ride. According to Newton’s second law, , every part of our body must be exposed to a force to accelerate. Since our bodies are not symmetric, the direction of the force matters, and must be accounted for by ride designers. An additional complication is that not all parts of the body accelerate in the same way when the acceleration is changing, i.e. when there is jerk. Softer parts of the body provide varying levels of damping, and different parts of the body have different frequency responses and different resonance frequencies that should be avoided or reduced by the roller coaster designer. This paper discusses the effect of acceleration, jerk, snap and vibration on the experience and safety of roller coaster rides, using authentic data from a dive coaster as an example.

Effect of Water-Cement Ratio on Mix Design and Mechanical Strength of Copper Slag Aggregate Concrete

In concrete, the distinct influence on most or the entirety of the properties is the measure of water utilised in the matrix. In concrete mix design, the water-cement ratio is responsible for binding all constituents of concrete together. If the water-cement proportion is higher, it brings about wider spacing between the cement aggregates and thus, influences the compaction. Correspondingly, concrete’s durability and compressive strength are decreased due to increased dampness levels. Especially with added fine aggregates the water demand increases because of the increased surface area of concrete which prompts a higher water-cement ratio. Wider spaces are formed between the aggregate materials with the addition of extra water and when the moisture vaporises, voids are filled up with air. The subsequent insufficient compaction lessens the concrete’s strength. Sets of basic experimental investigations were conducted to examine the impact of various water-cement proportion on mix design and mechanical strength of concrete incorporated with fine aggregate in terms of copper slag. Natural sand replaced with copper slag with four different percentage and correlation coefficients between mechanical properties of copper slag concrete were also evaluated.

Poles and Stability of Full-Wave PEEC

Time-domain simulation of full-wave partial element equivalent circuit (PEEC) often encounters stability problems. In the past, various approaches have been proposed to determine the stability of a given model. Most of these methods, however, have low efficiency and scalability. In this article, we propose a highly efficient method to obtain a particular group of the poles of the model; such information then serves as a necessary-condition test for stability. Furthermore, the method is extended to include the effects of damping resistors. From the root locus plots of the poles, we clearly see the unstable poles moving left as damping is added in, and by the time they move into the left-half-plane, we can quantitatively determine the minimum damping level required to achieve a stable model. Simulation results are provided to verify the proposed method.

Aerodynamic damping models for vortex-induced vibration of a rectangular 4:1 cylinder: Comparison of modeling schemes

Accurate modeling of vortex-induced force acting on a structure is of great significance to precisely evaluate its vortex-induced vibration (VIV). This paper systematically compares the capabilities of five aerodynamic damping models in simulating the VIV amplitudes of a line-like structure at various mass-damping conditions. These models are categorized into two types. Scanlan’s model, the polynomial model, and the describing function-based model, as the first type, can be utilized to calculate the VIV amplitudes in the entire lock-in range. The second type, including Larsen’s model and a newly-developed aerodynamic envelope model, that is designed to directly obtain the peak VIV amplitude. The relationships between aerodynamic damping of the aeroelastic systems and aerodynamic parameters of five models are established to convenience the identification of aerodynamic parameters. The aerodynamic parameters can be either identified based on the VIV decay-to-resonance and/or grow-to-resonance displacement signals at a single mass-damping condition, or based on the VIV steady amplitudes at various mass-damping conditions. Numerical examples involving the VIV analyses of a rigid rectangular 4:1 cylinder and two flexible cylinders are presented to compare the predictive capabilities of various models. The polynomial model and the describing function-based model can satisfactorily predict the VIV amplitudes at various mass-damping conditions, while the predicative capabilities of Scanlan’s model, Larsen’s model, and the aerodynamic envelope model may be comparatively lower. The underlying reasons for the different levels of accuracy of various models are discussed. Furthermore, it is demonstrated the mode shape correction factor of a flexible cylinder may be dependent not only on the mode shape, but also on other parameters such as the mechanical damping level.

Numerical study of acoustic transmission across heavily damped plate using hybrid Ray-Tracing-SEA method

Energy transmission across lightly damped structures has been well studied including the approved success of statistical energy analysis in mid and high frequency bands. For heavily damped elements, the diffuse field theory, which is used in computing coupling loss factors, tends to fail. Energy attenuation with distance becomes more significant for such elements and hence the energy is less likely to be evenly distributed within those elements. A ray tracing algorithm is developed taking account of this phenomenon by tracking the travel history of a great number of discrete rays. The predicted transmitted energy is used in a modified statistical energy analysis model to calculate energy level difference between different subsystems. Numerical validation and comparison on a concrete five-plate system are conducted in both lightly damped and heavily damped cases. Both the classic and the hybrid models show good agreement for lightly damped system and differ for heavily damped system. The difference tends to become larger with increasing frequency and internal damping level. The parameter “effective length ratio” is proposed to describe the phenomena of energy concentration along the edge and as in indicator of whether the application of diffuse field theory is appropriate.

Design and Analysis of a Geometrically Nonlinear Dynamic Vibration Absorber

AbstractThe design and optimization of a nonlinear dynamic vibration absorber based on a snap-through truss geometry is investigated. The effect of absorber's parameters on primary system (PS) vibration amplitude reduction and frequency range of operation is analyzed. Within parametric analyses of the absorber, a methodology was proposed to tune the absorber's stiffness. Results show that the nonlinear vibration absorber may be substantially more effective than its linear counterpart both in terms of vibration amplitude reduction and absorption frequency range. Possible difficulties and/or limitations caused by the nonlinearity induced by the absorber are analyzed and, for the studied case, do not diminish the advantages of the nonlinear absorber (NLAbs). The effect of absorber's damping on the vibration reduction performance was also analyzed indicating that the NLAbs outperforms its linear counterpart even for higher damping levels.

Analysis of power split vibration absorber performance in heavy-duty truck powertrains

The current development of more efficient combustion engines leads to an increase in engine torsional vibrations; therefore, new technology is needed for reducing the vibrations transmitted from the engine to the driveline. In this article, the concept of power split vibration absorber is evaluated. A mathematical model of the power split vibration absorber is presented, and an analytical study shows how different design parameters affect the power split vibration absorber performance. Numerical simulations with models representing typical heavy-duty truck powertrains are used in the evaluations. It is concluded that for a low level of damping, the power split vibration absorber can provide significantly lower vibration amplitudes than a corresponding dual mass flywheel within a limited speed range. If the power split vibration absorber is optimised for the critical low engine speeds, an overall decrease in the level of vibration can be obtained, but a larger installation space than with a conventional dual mass flywheel would probably be required.