Forced Vibration Technique Research Articles

Extraction of aerodynamic damping and prediction of vortex-induced vibration of bridge deck using CFD simulation of forced vibration

This study presents computational fluid dynamics (CFD) simulation of forced vibration of a bridge deck for extraction of amplitude-dependent aerodynamic damping in the vicinity of vortex lock-in region. The coordinate system fixed on the oscillating body is used to bypass the requirement of mesh updating. The three-dimensional flow around the vibrating deck is assumed to be homogeneous in spanwise direction and simulated using the efficient high-order spectral element/Fourier method. The aerodynamic damping is extracted from the simulated lift and then used to estimate the steady-state vortex-induced vibration (VIV) amplitude for given wind velocity and structural parameters. A good match of VIV amplitudes between simulations and wind tunnel tests validates the accuracy of simulation. The VIV mechanisms are discussed from the vortex shedding modes, distributions of dynamic pressure and energy transfer from flow to bridge deck. The leading-edge vortex shedding rather than Karman vortex shedding is identified to be responsible for the VIV. The hysteresis of VIV is observed which is related to the mode change of impinging leading-edge vortex and leading-edge vortex shedding. Finally, a novel forced vibration technique with a varying amplitude is proposed that permits extraction of aerodynamic damping by a single CFD run with improved efficiency.

Experimental investigation of self-damping characteristics of the ACSR Beresford and TERN transmission line conductors using log-decrement and quality factor methods

This work considered the experimental investigation of self-damping characteristics of ACSR Beresford and TERN transmission line conductors using the free and forced vibration techniques. When subjected to a load, self-damping is the power dissipated per unit length of the conductor as it vibrates. ACSR Beresford and TERN conductors are the two most commonly used conductors in the South African transmission network. The paper has revealed that for all the tests, the damping factors for the forced vibration on the TERN conductor was ζ ≤ 0.0307, and for the free vibration was, ζ ≤ 0.057, whereas the damping factors for the ACSR Beresford (forced vibration) were found to be ζ ≤ 0.0103, and for the ACSR Beresford (free vibration) were found to be ζ ≤ 0.056.The damping factors obtained in both conductor-tests were not sufficient to suppress Aeolian vibrations. Results obtained by Mokeretla (2011), that the internal self-damping of TERN transmission lines could not suppress Aeolian vibrations, have been confirmed, due to the damping values being much less than critical damping (ζ ≪ 1). An additional conductor was tested, the ACSR Beresford conductor, whose self-damping factor was also not sufficient to suppress Aeolian vibration.

Seismic vulnerability assessment of existing school buildings

This paper presents a methodology to perform the seismic vulnerability assessment of existing buildings. It starts with the acquisition of structural data from available construction drawings and field investigations to create a preliminary finite element model. Then, a wireless sensor network is used to collect the structural response at different locations. The sensors are connected and synchronized to each other to download and process data in real time. Modal identification methods, such as output-only and forced vibration techniques, are used to determine the modal characteristics and consequently calibrate the structural model for the subsequent vulnerability assessment. The proposed methodology is applied to a reinforced concrete school building in Italy. The seismic vulnerability is evaluated using a variety of alternative formulations. In particular, material nonlinearities and contact interaction at the structural joints are considered.

Measurement of unsteady aerodynamic force on a galloping prism in a turbulent flow: A hybrid aeroelastic-pressure balance

This paper proposes a novel hybrid aeroelastic-pressure balance (HAPB) technique for the measurement of unsteady aerodynamic force on a galloping prism. HAPB wind tunnel tests were performed to simultaneously observe the unsteady aerodynamic force and galloping response of a test model. The amplitude-dependent nonlinear damping and stiffness of the HAPB system that include the non-wind-induced aerodynamic force that is caused by the interaction between the oscillating model and its surrounding ‘still’ air were identified using a wavelet method. The non-wind-induced aerodynamic force was determined by a forced vibration technique. Subsequently, the galloping response was calculated by substituting the unsteady aerodynamic force and physical nonlinearities into the governing equation of motion. The results show that (1) the proposed HAPB technique is effective in obtaining unsteady crosswind forces; (2) the unsteady self-excited force with excluding the non-wind-induced aerodynamic force can accurately predict the galloping response of the test model whereas the classical quasi-steady theory fails for the prediction. This study has not only provided a HAPB used in wind tunnel, but also addressed shortcomings of the classical quasi-steady theory in predicting galloping instabilities of slender prisms.

Dynamic Identification as a Tool to Constrain Numerical Models for Structural Analysis of Historical Buildings

In recent few decades, operational modal analysis (OMA), also known as output-only or ambient vibration test (AVT), has become a powerful tool for a wide range of applications in the field of civil engineering. The output-only modal test represents an effective alternative to forced vibration techniques when historical structures are investigated. This method takes advantages from natural sources of vibration (wind, traffic, etc.) instead of having to shake the structure artificially. This paper shows how the dynamic characteristics of a building (i.e. frequency, modal shapes and damping ratios) can be automatically identified by enhanced frequency domain decomposition (EFDD) technique. The assessment of the dynamic characteristics is performed analyzing the Baptistery of San Giovanni in Firenze (Italy) where a network of seismic sensors was temporarily installed. The modal parameters are used to calibrate a 3D finite element (FE) numerical model, by using a genetic algorithm (GA). This procedure allows to obtain accurate and robust model capable to reproduce the actual dynamic behavior of the structure by minimizing the uncertainties related to the mechanical parameters. This paper, providing an illustrative case study in the field of health monitoring of monumental structures, confirms that the OMA technique is able to derive effective information on the dynamic behavior of the analyzed building, which is useful to tune reliable and robust numerical models to be employed for structural analysis.

Non-wind-induced nonlinear damping and stiffness on slender prisms: a forced vibration-pressure balance

Abstract The non-wind-induced additional nonlinear damping and stiffness of a spring-suspension system (under wind-off conditions) would significantly influence the prediction of aeroelastic response. This study proposes a forced vibration technique to identify the non-wind-induced nonlinear damping and stiffness of both vertical and inclined prisms. The forced vibration-pressure test was performed to observe the non-wind-induced pressure at first. It was observed that the pressure was remarkably affected by the amplitude of structural oscillation, and the corresponding non-wind-induced force is thus nonlinear. A procedure is then proposed to identify the non-wind-induced nonlinear damping and stiffness of the prisms from the tested nonlinear forces. The results yielded by the proposed procedure in terms of a vertical prism has been verified by comparing the response predicted by the unsteady aerodynamic force with/without the non-wind-induced nonlinearities with the measured response. Moreover, the non-wind-induced nonlinearities of inclined prisms have been tested by using the proposed forced vibration technique, which also suggests a mathematical model of the identified results for the purpose of convenience.

Vortex-induced vibration of bridge decks: Describing function-based model

A describing function (DF)-based model is introduced for the simulation of vortex-induced vibration (VIV) of bridge decks. Similar to the linear frequency response function, the DF is the complex ratio of the first-order component of the nonlinear output to the harmonic input. The DF can be either identified using the forced vibration technique or based on the VIV nonlinear response time history. An iterative procedure is accordingly developed to predict the VIV response with the DF-based model, and an equivalent-damping-ratio-based simplified method is further proposed to efficiently obtain the limit cycle oscillation (LCO) amplitude of VIV. It is demonstrated that the conventional van der Pol-type model is equivalent to a special case of the DF-based model for VIV. Three case studies involving various cross-sections are utilized to validate the simulation accuracy and efficiency of the proposed DF-based model for typical features at VIV lock-in such as LCO and hysteresis phenomena. The vertical VIVs can be well captured by the DF-based model, while its capability of simulating the torsional VIVs requires further improvement. Furthermore, the predictive capability of DF-based model for vertical VIVs of bridge decks within a wide range of mass-damping conditions is highlighted.

Processing, manufacturing, and characterization of vibration damping in epoxy composites modified with graphene nanoplatelets

This research aims to characterize the vibration and damping properties of epoxy composites modified with pristine and amino‐functionalized graphene nanoplatelets (GNPs) at four different nanofiller loadings (0.2, 0.4, 0.6, 1.2 wt%). The GNPs/acetone solution was mixed with epoxy through mechanical stirring. The mixture was then heated by means of a hot plate to evaporate the acetone. Once the mixture was cooled down, the hardener was added and the mixture injected into aluminum molds to form the nanocomposite test coupons. A shaker generated a periodic signal to excite the cantilever nanocomposite specimens at the fixed end. The frequency response functions (FRFs), damping ratios and natural frequencies measured using the forced vibration technique. The experimental results confirmed the beneficial effect of graphene nanoplatelets on the damping ratio of high content epoxy nanocomposites. POLYM. COMPOS., 40:3914–3922, 2019. © 2019 Society of Plastics Engineers

Fabrication and experimental evaluation of vibration and damping in multiscale graphene/fiberglass/epoxy composites

In this paper, the vibration and damping properties of multiscale laminated fiberglass/epoxy composites modified with a wide range of carbon nanofillers, including multiwalled carbon nanotubes, graphene oxide, reduced-graphene oxide and graphene nanoplatelets were examined for use in structural vibration applications. Simultaneous reinforcement of matrix and fibers was carried out via a novel method that combines a nanoparticle spraying process with nanoparticle/epoxy mixture to incorporate nanoparticles for the enhancement of vibration and damping properties of multiscale laminated fiberglass/epoxy composites. The vibration and damping properties as well as morphological, mechanical properties of the glass fiber-reinforced multiscale composites were investigated. Using a forced vibration technique, the frequency-response functions, natural frequencies and damping ratios of the nanocomposites were measured. The experimental results revealed that the damped natural frequencies of the nanocomposites increased with an increase in nanoparticle concentration. However, at higher contents of nanoparticles, the damped natural frequencies decreased and the damping ratio increased.

Patterned Magnetorheological Elastomer Developed by 3D Printing

This article delineates the characterization of the 3D printed MR elastomer through a forced vibration technique in the squeeze mode of operation. An anisotropic hybrid magnetorheological (MR) elastomer is developed via 3D printing. The 3D printed MR elastomer consists of three different materials; magnetic particles, magnetic particles carrier fluid, and an elastomer. MR fluid filaments are encapsulated layer-by-layer within the elastomer matrix using a 3D printer. When a moderately strong magnetic field is applied, the 3D printed MR elastomer changes its elastic and damping properties. The hybrid 3D printed MR elastomer also shows an anisotropic behavior when the direction of the magnetic field is changed with respect to the orientation of the printed filaments. The relative MR effect is higher when the applied magnetic field is parallel to the orientation of the printed filaments. The maximum change in the stiffness is observed to be 65.2% when a magnetic field of 500 mT is applied to the MR elastomer system. This result shows that the new method, 3D printing could produce anisotropic hybrid MR elastomers or possibly other types.

Effect of functionalization of carbon nanotubes on vibration and damping characteristics of epoxy nanocomposites

The vibration and damping characteristics of epoxy composites reinforced by pristine and functionalized multi-walled carbon nanotubes (MWCNTs) were investigated experimentally for potential use as integral passive damping elements in structural composite applications. The MWCNTs were introduced into the acetone solvent and then mixed with epoxy resin through a sonication process and mechanical stirring. The solvent was evaporated from the mixture by means of magnetic hot plate and the hardener was added once it was cooled down to room temperature. The MWCMTs/epoxy mixture was then injected into a mold to form the nanocomposite specimens. Nanocomposite specimens were fabricated for six different MWCNT loadings (0.02, 0.041, 0.061, 0.123, 0.25 and 0.37 wt%). Microstructural analysis, tensile and bending tests were carried out to examine the effect of pristine multi-walled carbon nanotubes (p-MWCNTs) and functionalized multi-walled carbon nanotubes (f-MWCNTs). The frequency response functions (FRFs), coherence and phase diagrams of nanocomposites were measured using a forced vibration technique. A periodic up-chirp signal was generated by a shaker to excite the cantilever nanocomposite specimen at the base. The damped natural frequencies and damping ratios were obtained for different loadings of MWCNTs. The experimental results indicated that the damped natural frequencies of p-MWCNTs/epoxy and f-MWCNTs/epoxy composites increased by adding MWCNTs up to 0.12 wt.%, and decreased at higher MWCNTs content. Addition of p-MWCNTs were improved the damping ratio of nanocomposites. While the damping ratios from f-MWCNTs at loadings of 0.02–0.06 wt.% were higher than those from p-MWCNTs, they did not increase at higher CNTs contents for the first mode of vibration.

Prediction and explanation of the aeroelastic behavior of a square-section cylinder via forced vibration

The work presented in this study concerns the existing link between fluctuating transverse force characteristics and aeroelastic instability phenomena, which is a step toward further understanding the complex nature of the interactions between square cylinders and air flow. First, by using experimental data from forced vibration, a predictive calculation of the free vibration of an elastically mounted cylinder is carried out. It is found that the prediction results appeared to be qualitatively in agreement with the experimental measurements. This agreement indicates that the measurements using the forced vibration technique is a valuable tool for analysis of free vibration behavior of an elastically mounted cylinder, particularly in cases where the quasi-steady theory is not entirely valid. Furthermore, a new and simple insight into the link between the unsteady force characteristics and the free vibration amplitudes is proposed in this study. Although this approach does not offer details about the nature of the separated flow field around the cross-sections of cylinders, certain complex free-vibration behaviors of square cylinders can be easily interpreted from this perspective.

Unsteady Galloping Force and Response Prediction of a Slender Prism Using a Novel Wind Tunnel Test Technique

This paper examines a new measure of the unsteady galloping force of a slender prism using a hybrid pressure-aeroelastic test (HPAT) technique. The HPAT was performed to simultaneously observe the unsteady crosswind force and response of a test model. The observed crosswind force contains amplitude-dependent non-windinduced aerodynamic force that was caused by the interaction between the oscillating test model and the surrounding air. The unsteady galloping force of the test model was therefore evaluated by removing the non-wind-induced aerodynamic force identified by using a forced vibration technique from the observed unsteady crosswind force. The amplitude-dependent mechanical nonlinearities (damping and stiffness) of the HPAT system were identified by using a wavelet method from a free decay response of the test model. By substituting the obtained unsteady galloping force and the mechanical nonlinearities into the governing equation of motion of the test model, the galloping responses of the test model were predicted. The results show that the galloping responses of the test model predicted by the identified unsteady galloping force are identical to the experimentally measured response, and

Unsteady pressure measurements on an oscillating slender prism using a forced vibration technique

The unsteady aerodynamic forces acting on a slender prism were investigated using a forced vibration technique. The prism was driven to oscillate by an actuator and the unsteady distributed pressures, under different wind velocities and oscillation amplitudes, were measured. The measurement was calibrated with respect to driving oscillation, aerodynamic force coefficient, as well as motion-induced force coefficient. Then, the generalized and local aerodynamic force coefficients and the motion-induced force coefficients of the prism, which are functions of reduced wind velocity and oscillation amplitude, were analyzed. It shows that the effects of structural motion on the coefficients are significant in the crosswind direction while the effects are slight in the along-wind direction. Furthermore, in the crosswind direction, the coefficients tend to increase with oscillation amplitudes at low wind speeds while they are at a quasi-steady state at high wind speeds. These characteristics were analyzed from the perspectives of generalized and pointwise spectra, force-response coherences and Strouhal numbers of the prism. The study advances the understanding of the effect of structural motion on three-dimensional prisms, which can be utilized to improve response predictions of the prisms.

Damping Evaluation of Linseed Oil-Based Engineering Elastomers by Vibration Response Method

A low cost experimental setup has been fabricated for evaluation of vibration damping properties of a variety of elastomers developed from linseed oil. Free and forced vibration principles were utilized in this experimental setup. Under forced vibration, the shear loss factor varies from 0.37 to 1.03 at 2nd bending vibration mode and 0.43 to 0.99 at 3rd bending vibration mode for different elastomers. The loss factor varies from 0.52 to 0.94 under free vibration. The loss factors determined by both forced and free vibration techniques are in good agreement with the loss factors obtained from dynamic mechanical analysis. The ultimate tensile strength and Young’s modulus of the elastomers vary from 0.37 to 3.71 MPa and 0.27 to 10.27 MPa, respectively, whereas these properties in compression are in the range of 3.1 to 72.9 MPa and 1.5 to 30.1 MPa, respectively. Thus, these elastomers are mechanically stable for vibration damping applications.

Characterizing vibration damping response of composite laminates containing silica nanoparticles and rubber particles

This research aims to characterize the damping properties of fiber/epoxy composites containing different degrees of silica nanoparticles and rubber particles. Conventionally, adding rubber particles into fiber/epoxy composites would lead to dramatic reduction of stiffness although the vibration damping could be improved accordingly. In order to enhance the damping properties of the fiber composites without sacrificing the stiffness, silica nanoparticles together with rubber particles were introduced into the epoxy resin through the sonication process. The epoxy resin was then treated as matrix and impregnated into the fiber layer by means of the vacuum hand lay-up process to form the composite laminates. The vibration damping as well as the flexural stiffness of the fiber composite was measured using the forced vibration technique together with the half-power method. In addition, the vibration damping of the composite laminates, consisting of silica nanoparticles and rubber particles, was characterized using the micromechanical analysis. The repeated unit cell with the fibers displaying randomly in the matrix was employed to represent the microstructures of the unidirectional composites. The loss factor as well as the moduli obtained from the micromechanical analysis were regarded as the effective properties homogenizing within the fiber composites. In conjunction with the modal shapes, the vibration damping of the composite laminates with stacking sequence of [0]10, [90]10, [±45]2s, and [90/0]2s was calculated using the finite element analysis. Experimental results indicated that with the incorporation of the silica nanoparticles together with the rubber particles, the reduction of flexural stiffness of fiber composites, especially for the [90]10 laminates, was diminished while the damping properties of laminates were improved. Moreover, it was found that the effect of the particles in the [0]10 laminates is relatively minimal. The vibration damping responses of composites laminates obtained using the micromechanical analysis together with the modal analysis exhibit an agreement with the experimental data.

Traditional Varnishes and Acoustical Properties of Wooden Soundboards

Background: The impact of three different coating formulations, used traditionally in Iranian musical instrument industry, on acoustical properties of wooden soundboard was investigated. Methods: Shellac, Opaque Polyester Varnish (OPV) and Sandrous powder solution (50 and 5% solutions in Ethanol), were applied on pine wood and then the samples were tested using forced vibration technique. The measured acoustical parameters were specific modulus of elasticity, acoustic coefficient and acoustical converting efficiency. Results: The results indicated that, shellac lacquer caused the least negative impact on the acoustical properties of wood. Conclusion: To find better lacquers to improve the acoustical qualities of wood it should be considered that this new lacquer must be lighter in densities, exhibit higher modulus of elasticity and/or lower internal friction compare to raw unpolished wood.

Identification of Rational Functions using two-degree-of-freedom model by forced vibration method

In prediction of self-excited forces and flutter instability of flexible structures, time domain method has distinct advantages. Rational Functions that are used to formulate self-excited aerodynamic forces in time domain were indirectly extracted from experimentally obtained flutter derivatives in the past. Recently, an algorithm was published to directly extract the Rational Functions from wind tunnel section model tests in free vibration. To overcome the limitations of free vibration technique, a new algorithm that is developed for direct extraction of the Rational Functions from section model tests using a forced vibration technique is presented here. The new algorithm can be used to extract all the Rational Functions associated with one, two or three degree-of-freedom motion (vertical, lateral and torsional) of a section model. To validate the new algorithm, forced vibration wind tunnel tests in two degrees of freedom (vertical and torsional) were performed on a streamlined bridge deck section model with width-to-depth ratio B/D=15:1 and also a bluff rectangular section model with B/D=5:1. This is a significant improvement from other forced vibration methods that require separate one-degree-of-freedom model tests which are dependent on phase angle difference between aerodynamic loads and displacements.

Investigating damping properties of nanocomposites and sandwich structures with nanocomposites as core materials

This research aims to investigate the damping responses of the epoxy-based nanocomposites as well as the composite sandwich structures with the nanocomposite as core materials. Both the silica nanoparticles and the rubber particles (CSR, CTBN) were introduced in the epoxy matrix through the sonication process to form the nanocomposites. Furthermore, the nanocomposites were sandwiched between the unidirectional composites laminates to fabricate the nanocomposite sandwich structures. The damping performances of the nanocomposites as well as their sandwich structures were determined from the forced vibration technique together with the half-power method. Meanwhile, the flexural stiffness of the material systems was evaluated by the resonance frequency obtained from the vibration tests. Results indicated that either silica nanoparticles or rubber particles can improve the damping responses of the epoxy-based nanocomposites. However, it was found that when the rubber particles were present alone, the stiffness of the nanocomposites was dramatically reduced. By introducing the hybrid material systems (10 wt% silica nanoparticles and 10 wt% rubber particles), the superior damping properties and flexural stiffness can be concurrently accomplished. Moreover, when the hybrid material system was employed as core materials in the sandwich structures, this enhanced damping property can also be observed in the vibration tests.

Identification of flutter derivatives of bridge decks with free vibration technique

The flutter derivatives of bridge decks can be determined in a unique manner on condition that the complex modal parameters of the system at one reduced frequency are obtained. Based on the idea, a new method of identifying the flutter derivatives of bridge decks is proposed and it can overcome some shortcoming of the existing method and extend the applicability of the free vibration technique at high wind velocity. The identified results have agreements with the target ones of an ideal thin-plate section and those of a thin-plate section measured by the forced vibration technique. The proposed method is reliable and effective to extract the flutter derivatives from coupled free vibration.