Damping Forces Research Articles

A floating system integrating a wind turbine with a steel fish farming cage: Experimental validation of the hydrodynamic model

This paper is dedicated to comparing the numerical results of the dynamic responses of a floating offshore wind turbine integrated with a steel fish farming cage (FOWT-SFFC) under wind and waves against the experimental data. FOWT-SFFC's numerical model is built with OrcaFlex where the potential flow theory and quadratic drag formula are used to calculate the hydrodynamic loads on the structural components of the floater, while Morison-type formulation is employed to model the force on fish nets. The numerical model is tuned in terms of the measured data. The simulated motion response amplitude operators and stochastic responses under irregular waves are compared with the experimental results to fathom the influence of drag's modeling on the simulation outcome. The effects of additional damping and 2nd-order wave forces are also investigated to determine the optimal simulation method. With the selected numerical model, the responses of FOWT-SFFC with and without nets under 100-year extreme waves, combined wind and irregular waves are calculated. The simulated response time histories, spectra and statistics are in good agreement with the experimental data. Besides, it is revealed that the effects of 2nd-order wave forces are important for the accurate simulation of low-frequency pitch motion, but unfavorable to the prediction of other responses.

Impact of loading rate on the mechanical behavior of jointed rock

The impact of loading rate is a vital issue in the study of the time-dependent behavior of rock masses. A sample containing a single inclined joint is represented by the particle flow code and used as an analog to examine the rate-dependent behavior of jointed rock. A series of numerical triaxial compression tests are completed at various loading rates on specimens containing this single joint inclined at 30°, 45°, and 60° with respect to the orientation of the confining stress of 5 MPa. Observations are recovered for the evolution of stress–strain, deformation and energy release together with resulting failure mode. Rate sensitivities of four parameters defining a smooth joint model (normal stiffness, shear stiffness, stiffness ratio and friction coefficient) are used to represent observed response. We find that competition between mechanical damping and inertial force results in the strain rate effect and that the peak strength of the rock specimens increases with increasing loading rate. The step-wise form of both the stress–strain and kinetic energy can be used as indicators of the onset of dynamic failure. These numerical experiments are consistent with observations from laboratory experiments on identical samples where rate effects in the jointed rock are accentuated over those apparent in intact rocks. Change in the loading rate has a significant effect on the failure mode for specific joint angles relative to the confining stress. The joint friction coefficient is the main rate dependent factor controlling behavior and is an important factor in defining the significance of loading rate effects.

Mechanical Properties of Improved Multiple-Layers Viscoelastic Damper

Viscoelastic dampers (VEDs) have been implemented successfully to reduce structural vibrations due to earthquakes and wind events. Conventional VEDs consist of two viscoelastic (VE) layers chemically bounded at their entire contact surface to three steel plates. This configuration has proven to be efficient in controlling the structure vibration. It has also been reported that VEDs can dissipate energy at any level of vibration, producing damping forces. However, when very large damping forces are required, the shear area of the VE layers and the steel plates should be increased to reach the target damping force since the energy is dissipated by hysteretic shear deformation developed in the VE material. Two main issues are associated with this large area. First, the steel plates are more susceptible to experiencing buckling due to out-of-plane deformations. Second, the overall size of the damper becomes larger which is not desirable from the architectural perspective and sometimes from the space usage perspective. As a solution, this study proposes an innovative VED so-called multiple-layers viscoelastic damper (MLVED) able to produce larger damping forces. The proposed MLVED consists of four VE layers bounded between five steel plates. The dynamic mechanical properties of MLVED are initially investigated through a full-scale test. With the test results, the finite element model is developed and calibrated using the commercial software ABAQUS. At a later stage, the calibrated model was used to investigate numerically the mechanical performance of MLVED if different VE layers areas and thicknesses are considered. Results indicated that the proposed MLVED possesses good energy dissipation capacity and its mechanical properties are strongly influenced by strain amplitude rather than loading frequency. Numerical results also showed that the damper is effective in dissipating energy even if different VE layers areas and thicknesses are considered. However, an optimal combination between the area and thickness of the VE layers needs to be found to maximize the damper performance.

Ride comfort analysis of vehicle seat suspension systems: A co-simulation study

In this study, the vehicle seat suspension system installed with a semi-active magnetorheological (MR) damper is excited by the power spectral density (PSD) and its vibration control performance is evaluated using finite element analysis (FEA). The control system receives the velocity of the sprung and unsprung mass from the seat mounting locations and calculates the desired controllable damping forces available from MR damper. The effectiveness of the control systems is demonstrated by adopting ride quality evaluation method. To predict a better ride comfort and ride quality, co-simulation methodology is utilized considering the dynamic behavior of the real-world seat system. In this case, the multi-body dynamics and control system are coupled by solving the mechanical equations and the control logic. Then, the computed damping force is exchanged between the dynamics and controller in an iterative manner by passing state variables. The simulation results achieved from co-simulation methodology associated with the controller are analyzed by comparing with ISO 2631-1. The co-simulation results studied using vibration dose value (VDV), crest factor (CF) and seat effective amplitude transmissibility (SEAT) in terms of percentage improvement were found to be 44.33%, 26.66% and 17.65% respectively which were better than that for the passive suspension for a random rough road profile. Modified skyhook controller delivers superior performance for vibration suppression of the vehicle seat suspension as compared to other control policies. The application of co-simulation methodology can reduce time and cost for the development of a semi-active seat suspension system.

A coupled isogeometric/multi-sphere discrete element approach for the contact interaction between irregular particles and structures

An isogeometric/multi-sphere discrete-element coupling method is presented to model the contact or impact between structures and particles with complex shape. This coupling method takes advantages of the multi-sphere discrete element method for particles to provide the high computational efficiency and excellent robustness of their contact modelling. The advantage of isogeometric analysis (IGA) for continuous solid material, e.g. the exact geometric description, is also taken to achieve a more accurate contact interaction with an excellent time continuity. In the coupling procedure, the CGRID method is used for the global searching. The exact contact situation of the discrete element and the IGA element surface is further determined in the local searching by solving non-linear equations numerically. Then, the normal contact force between a sphere and an IGA element is calculated using a penalty based Hertz-Mindlin contact model, and damping and friction forces are also considered. Both the accuracy and validity of the coupling method are examined by comparing the numerical results of an example with one particle impacting on a quarter of a cylinder, with those of the FEM model where the particle is modelled as a rigid body. Two additional examples involving particles impacting onto a corrugated plate and particles of different shapes impacting on a chute, are simulated to further assess the applicability and robustness of the proposed method.

Numerical Investigation of Double Transonic Dip Behaviors in Supercritical Airfoil Flutter

This numerical study examines the behavior of the double transonic dip with a supercritical airfoil. A two-dimensional model of a wing section with two degrees of freedom was used for the investigation. The flowfield was modeled by solving the unsteady Reynolds-averaged compressible Navier–Stokes equations using the Spalart–Allmaras turbulence model, and the equations of motion were solved to determine the structural dynamics. The present model successfully captured the behavior of the double transonic dip on the flutter boundary of the supercritical airfoil, which contrasts with the well-known behavior of the single transonic dip exhibited by conventional symmetric airfoils. Although the mechanism of the first dip at a lower Mach number corresponded to that of the well-known conventional transonic dip, the second dip at a higher Mach number was uniquely observed for the supercritical airfoil. The analysis established that, for the supercritical airfoil, the motion of the shock wave over the upper surface was significantly affected by the behavior of the boundary layer around the highly cambered aft region of the lower surface during flutter. The behavior of the boundary layer involving the separation and reattachment over the lower surface caused the unusual shock wave motion over the upper surface under the Mach number condition at the bottom of the second dip. This motion exerted negative damping forces on the motion of the airfoil, thereby becoming the primary contributor to generating the second dip experienced by the supercritical airfoil.

Vibration isolation characteristics of magnetorheological damper for the large wind turbine gearbox

ABSTRACT The gearbox vibration isolation theory in the large wind turbine transmission system is analyzed. The host frame is flexible, and the vibration isolation system is approximately equivalent to a two-degree-of-freedom system. According to large wind turbine gearbox isolation requirement, and a vibration isolation device of magnetorheological damper of the large wind turbine gearbox is designed, the magnetic field characteristics inside the magnetorheological damper are simulated and calculated by using COMSOL, when 0.5, 1, 1.5and 2 A currents are passed through the magnetorheological damper coil, the magnetic induction intensity of the damping gap and the output force of the magnetorheological damper are calculated, it can reach 1300N-5400N, which meets the gearbox vibration isolation requirements. It designed the on-off current control method of the magnetorheological damper. The prototype is tested on a certain type of offshore wind turbine, and the acceleration spectrum signals of the gearbox in the Z direction are collected under the working conditions of 990, 1200 and 2000 r/min. According to the collected data values, the vibration isolation characteristics of the designed magnetorheological damper are analyzed; the data show that the vibration level drop is about 20 dB, and the vibration isolation effect is good. By using the coherence coefficient method to process the measured data, the modified transfer function is obtained, which is consistent with the theoretical transfer function of the vibration model of two-degree-of-freedom system, so it is proved to be reasonable. The vibration isolation characteristics of rubber and the magnetorheological damper are compared at the rated speed, and it shows that the maximum vibration acceleration of the gearbox under the magnetorheological damper reduces from 0.375 m/s2 to 0.175 m/s2, which verify the damping effect of the magnetorheological damper.

Active control for vehicle suspension using a self-powered dual-function active electromagnetic damper

Concerning vehicle suspension techniques, active suspension tends to offer the most precise control performance compared to passive and semi-active alternatives. However, due to its high-power consumption, its practical use is often limited. This study aims to develop an innovative self-powered active suspension by assessing the performance and feasibility of a dual-function active electromagnetic damper (DF-AEMD), which is capable of providing simultaneous force tracking and energy harvesting functions. In addition to outlining its working mechanism, the DF-AEMD is utilized to track the full-loop target control force calculated from the classic linear quadratic regulator in a self-powered manner. From the force tracking standpoint, the DF-AEMD can not only provide damping forces, but also generate actuation forces, and thus achieve better tracking accuracies of target control force and control performances for vehicle suspension than the traditional semi-active damper; while from the energy harvesting aspect, sufficient energy can be harvested simultaneously to realize the self-powered active control. Systematic simulations are conducted to discuss the relationship between control performance and energy harvesting efficiency of the DF-AEMD.

Theoretical model and experimental study on working characteristics of electromagnetic damper

An analysis model for electromagnetic damper is proposed in order to obtain the working characteristics of electromagnetic damper under different working conditions. Firstly, the inertia force and the electromagnetic force of electromagnetic damper are calculated, and the friction force is tested. Secondly, by combining the theoretical results and the test data, the working characteristic analysis model for electromagnetic damper is established. Finally, the damping characteristics and energy harvesting characteristics of the electromagnetic damper under multiple working conditions are discussed by using the present model. The working condition characteristic surface of the electromagnetic damper is obtained by using the recovered energy and equivalent damping coefficient as the coordinate plane. The results show that the error between the calculated results and the test results is below 10%. The damping force and output voltage of the electromagnetic damper are positively correlated with excitation frequency and amplitude. The excitation amplitude and frequency influence the energy harvesting characteristic and damping property deeply, respectively. The increase in load resistance reduces the energy regenerative capacity and damping capacity of the electromagnetic damper.

Analysis on the split absorber integrated with taut-moored floating turbine

A new wave energy converter is proposed in this paper, consisting of three split heave point absorbers, combined with a taut-moored floating turbine. It is adapted to the waves in the China Sea area, which are characterized by short periods and small amplitudes. Based on a series of physical model tests in regular, irregular, and extreme waves, the hydrodynamic performance of the integrated device is systematically investigated under different damping forces and incident wave directions. The experimental results reveal that the split point absorber presents new hydrodynamic characteristics and that the wave energy capture efficiency of the new device is greatly improved for the short-period waves in low sea states. Moreover, due to the out-of-phase heave motion, as well as the induced shallow water effect, the submerged platform makes a contribution to improving the energy capture efficiency of the split floater, particularly pronounced in the case of a high damping force in the power takeoff system. Under the condition of incident wave direction being coincident with the horizontal projection of mooring lines, the energies of pitch motion and mooring force of the integrated system are increased as a result of the high-frequency oscillation, which needs to be solved by further optimizing the taut mooring system.

Coupled numerical simulation of liquid sloshing dampers and wind–structure simulation model

Long-span bridges, high-rise buildings, chimneys, or wind turbines are often susceptible to wind-induced vibrations due to their high flexibility and lightweight. Therefore, these structures should be carefully designed to sustain the wind loads. Employing robust solutions for vibration mitigation, such as tuned liquid dampers (TLDs), enables energy dissipation at reduced structural and maintenance costs. This study presents a numerical coupling methodology for evaluating the wind-induced response of slender vertical structures that include a TLD. The methodology is based on coupling a Computational Fluid Dynamics (CFD) model of a TLD, a semi-analytical aerodynamic force model, and a structural model. The coupling is realized in a time-domain fashion by continuously exchanging sloshing forces, aerodynamic forces, and structural response. The highlight is the use of the volume of fluid (VOF) method as a CFD strategy to represent the sloshing damping forces, including an experimental validation. The methodology is applied to (i) a chimney-type structure to mitigate the buffeting and vortex-induced vibrations and (ii) a high-rise building to reduce the acceleration buffeting response for human comfort. In both cases, strategy for TLD predesign is employed based on free- and forced-vibration analysis, an equivalent tuned mass damper model, and wind vibrations, respectively. The results of the dynamic analyses show that there is a significant reduction in structural response for both cases. The outcome of this study is intended to showcase the applicability of CFD for TLD predesign and modeling nonlinear sloshing forces during dynamic analysis as an alternative to experiments. Thus, it is intended to serve academics as well as practitioners.

Innovative nonlinear vibration control of beam structures using shear thickening fluid dampers

This paper investigates the use of shear thickening fluid (STF) dampers for reducing vibrations in beam-based structures. Designing and analyzing these systems is challenging because STF’s viscosity changes as the fluid’s vibration frequency changes. To understand the impact of key design parameters, the behavior of STF dampers in various configurations of supports is analyzed. The governing equations are derived, considering the STF dampers as nonlinear damping forces, and solved/validated using Galerkin/Hencky’s discretization methods. An energy reduction factor is introduced to compare the performance of different configurations in the frequency domain to obtain the optimal design. The results show that although the performance of STF dampers is highly dependent on the excitation frequency (with high efficiency in reducing vibrations near the resonance frequencies), the STF dampers fail to dissipate vibrations in some frequency regions. Surprising ranges of dynamic behavior, from Quasi-periodic to chaotic, are detected and analyzed. The findings also reveal that by analyzing only the shape of force–velocity curves of the STF damper, one can accurately predict the system’s performance, without the need for a full system analysis.

Seismic response control of core wall structures using tuned viscous mass damper (TVMD) outriggers

This paper presents research on the seismic response control of core wall structures through the installation of innovative damped outriggers known as tuned viscous mass damper (TVMD) outriggers. In this novel system, the TVMDs are arranged vertically between the outrigger trusses/beams and perimeter columns. A simplified model based on the distributed-parameter system was developed to represent a core wall structure equipped with TVMD outriggers. The dynamic properties and seismic responses of high-rise buildings were calculated through a dynamic analysis of the simplified model in MATLAB. The results indicate that TVMD outriggers designed by using fixed-point theory not only suppress the dynamic responses for the target tuning mode, but also supplement additional damping to lower-order modes. When tuned to the second mode, TVMD outriggers with a mass ratio of 0.1 reduce the maximum inter-story drift and floor acceleration responses by 23.48% and 35.80%, respectively. In addition, the control advantages of TVMD outriggers were demonstrated through energy dissipation analysis and damper force decomposition. The vibration control performance of multiple TVMD outriggers was also investigated and was found to achieve superior seismic control with a significantly reduced demand on the TVMD parameters.

Oscillation decay of a pendulum by an air jet

A heavy pendulum oscillating in a vertical air jet is studied in this short communication. Different jet Reynolds number and initial inclination angle of the pendulum are considered. As the jet Reynolds number increases, it is always observed that the time taken by the pendulum to reach the final equilibrium vertical state reduces. The suction pressure induced by the streamlines of the air jet deflected by the pendulum aids the gravitational restoring force. The subsequent increase in the frictional damping force from the pivot of the pendulum is shown to be responsible for this behavior. A theoretical model along with numerical results corroborate this conclusion.

A Novel Design Concept of a Magnetorheological Fluid-Based Damper Utilizing the Porous Medium for Implementation in Small-Scale Applications

Magnetorheological (MR) dampers have a virtue over conventional dampers, where their damping properties can be adjusted using a magnetic field. However, MR dampers have been barely implemented in small vibratory systems, in which the modal mass and stiffness are relatively small. This is due to two major reasons, namely its high parasitic damping force and big moving mass. When such an MR damper is installed in a small vibratory system, the system‘s default damping ratio is increased and therefore its dynamic is reduced. Here, a new concept of an MR damper utilizing the porous medium and shear operating mode together with an external non-moving electromagnet is proposed. This combination results in an MR damper with a low parasitic damping force and a small moving mass. For comparison purposes, a benchmark MR damper with comparable geometry is constructed. The proposed MR damper possesses a passive friction force that is 8× smaller and OFF-state passive viscous damping that is 10–20× smaller than the benchmark MR damper. An investigation of the proposed MR damper performance in a test vibratory system shows almost no reduction of the system dynamic. Therefore, this proposed MR damper configuration can be suitable for applications in small vibratory systems.

Longitudinal displacement control of a long-span suspension bridge under random traffic loads using a polyimide friction damper

This paper proposes a novel method to control the cumulative displacement of the girder end of a long-span suspension bridge caused by traffic loads using friction dampers. Friction dampers have Coulomb damping characteristics and high initial stiffness, providing a significant damping force at low speed to suppress the longitudinal deformation of the girder end. The working mechanism of the friction damper is explained, and the performance of the proposed polyimide (PI) friction damper is tested. A random traffic load model of a long-span suspension bridge is established, and the longitudinal displacement, reaction force time history, longitudinal peak displacement, and cumulative longitudinal displacement (CLD) under different yield forces of the PI friction damper are analyzed. We propose the excess ratio δ of the yield displacement to determine the optimum yield force. The influences of different vehicle speeds and traffic flows on the performance of the PI friction damper are discussed. The results show that the PI friction damper substantially reduces the longitudinal displacement, longitudinal peak displacement, and CLD of the girder end; the reduction rate of the CLD exceeds 57% when the yield force of the PI friction damper is 1000 kN. The vehicle speed and traffic flow have negligible influences on the CLD of the long-span suspension bridges equipped with PI friction dampers in the normal working range.

Design, Modeling and Experiments of a Variable Stiffness Soft Robotic Glove for Stroke Patients With Clenched Fist Deformity

Soft wearable devices for hand rehabilitation can assist activities of daily living. While existing soft robotic gloves can help with finger flexion, they are less suitable for patients who cannot extend their fingers due to poststroke increased muscular tension. In this letter, we proposed a variable stiffness pneumatic actuator that effectively increases the elasticity of the actuator to facilitate finger extension. The actuator consists of a fiber-reinforced actuator and a multi-stage articulated elastomer. It bends and extends by pneumatic actuation, with multi-stage articulated elastomer providing stiffness adjustment to accommodate varying joint resistance. The variable stiffness mechanism was achieved by the multi layered discrete metal plates which can gracefully fit the unique structure of human fingers. Analysis models were established to quantify the actuator's air pressure, vacuum pressure, and bending angle, also to predict the actuator's stiffness under different vacuum pressures. Experiments and finite element simulation were carried out to evaluate the bending performance of the actuator. Meanwhile, the output force and variable stiffness characteristic of the actuator were tested. The results show that the actuator can achieve 1.54 N fingertip output force and 2.4 times stiffness change, which satisfy the requirement of stroke patient. In addition, a clenched artificial hand with similar damper forces to human hand was applied to further verify the extending effect of the variable stiffness actuator. Finally, a rehabilitation glove integrated with multiple actuators was tested to assess typical rehabilitation gestures and the ability to grasp daily objects.

A Meta-Heuristic Optimization Procedure for the Identification of the Nonlinear Model Parameters of Hydraulic Dampers Based on Experimental Dataset of Real Working Conditions

Abstract Hydralic dampers are widely implemented in railway vehicle suspension stages, especially in high-speed passenger trains. They are designed to be mounted in different positions to improve comfort, stability, and safety performances. Numerical simulations are often used to assist the design and optimization of these components. Unfortunately, hydraulic dampers are highly nonlinear due to the complex fluid dynamic phenomena taking place inside the chambers and through the by-pass orifices. This requires accurate damper models to be developed to estimate the influence of the nonlinearities of such components during the dynamic performances of the whole vehicle. This work aims at presenting a new parametric damper model based on a nonlinear lumped element approach. Moreover, a new model tuning procedure will be introduced. Differently from the typical sinusoidal characterization cycles, this routine is based on experimental tests of real working conditions. The set of optimal model parameters will be found through a metaheuristic iterative approach able to minimize the differences between numerical and experimental damper forces. The performances of the optimal model will be compared with the ones of the most common Maxwell model generally implemented in railway multibody software programs.

PERFORMANCE EVALUATION OF A SMALL-SCALE MAGNETORHEOLOGICAL DAMPER FOR CIVIL ENGINEERING APPLICATIONS

Magnetorheological dampers (MRDs) are devices that adjust their damping properties in response to an external magnetic field. Large-scale MRDs have been successfully used as vibration control devices in structures. This study focuses on modelling and optimizing an MRD using COMSOL Multiphysics. Various parameters, such as coil turns and current, are optimized to achieve the maximum flux value in the MRD. The simulation yielded a maximum magnetic flux of 0,44 T with 500 coil turns. Based on the optimized MRD parameters, a numerical equation is then used to calculate the total damping force. The maximum numerical and experimental damping forces corresponding to a 2,0 A current were 989,39 and 1004,63 N, respectively. The numerical damping force is then compared to the experimental results to validate the accuracy of the model. The MRD is integrated into a scaled-down reinforced concrete frame and subjected to a cyclic loading test for performance evaluation. The results show that the MR dampers improve the performance of the frame structure, increasing its load-carrying capacity and energy dissipation by 19,45 % and 20,43 %, respectively. The findings of the study provide valuable insights into the behaviour of MRDs and their optimization using numerical simulations, as well as highlight the importance of experimental validation for accurate prediction of the performance of MRDs in practical civil engineering applications.

Reprint of: Physical modeling for digital twin of Continuous Damping Control damper

Continuous Damping Control (CDC) damper is a kind of active damper with adjustable damping coefficients, which has a broad application prospect for vibration attenuation in robots and manufacturing systems. To develop a digital twin of CDC damper for monitoring its working conditions, it is necessary to develop a physical model describing the characteristics of CDC damper. However, the existing physical models of CDC damper have some disadvantages such as expensive computing costs and low accuracy, which make it difficult to apply them in the digital twin. In order to solve these issues, this paper proposes a hybrid model which can accurately describe the physical characteristics of CDC damper by combining a lumped parameter model and a neural network model. First, the physical characteristics of a CDC damper are measured on a test rig under harmonic excitations and random excitations. Then the modeling and parameter identification methods for hybrid models are proposed to obtain model parameters. Finally, the output force of the CDC damper under random excitation is calculated using the hybrid model, and the accuracy of the hybrid model is validated after comparing the calculated results with the measured results. The proposed hybrid model has high accuracy and low computational cost, and is applicable to developing digital twins of CDC dampers which require high calculation accuracy and efficiency.