External Dynamic Excitations Research Articles

Investigations on the dynamic snap-through of MFC bonded self-resetting bistable laminates

Unsymmetric composite laminates having two stable equilibrium configurations have been studied extensively in the recent past due to their potential applications in morphing structures. Surface bonded Macro Fiber Composites (MFC) actuators have been considered as a viable solution to trigger the snap-through transition in bistable laminates. Although MFC bonded bistable laminates are widely used in morphing applications, they might require considerably high voltage inputs to achieve the required levels of actuation control during the shape transition. As a solution, other possible energy sources can be combined with active MFC patches to reduce the snap-through energy required from the single source of MFC actuation. In this work, we examine the dynamic behavior of bistable composite plates actuated using MFC actuators, where external vibration energy has been used to assist with the MFC-controlled actuation between stable states. A refined semi-analytical model based on the Rayleigh–Ritz formulation has been proposed, where the membrane energy and the bending energy are separately evaluated. Bending components are directly evaluated using the approximated transverse displacement functions, whereas the membrane components are evaluated separately by combining compatibility conditions and equilibrium equations. Results from the proposed semi-analytical framework are compared with a full geometrically nonlinear finite element framework and necessary experimental observations. The results show a significant reduction in the snap-through energy demand on MFC layers where external dynamic excitation assists the snap-through process. Additionally, a parametric study is performed using variable stiffness (VS) fiber orientation parameters, achieving bistable laminate-MFC configurations that lower snap-through requirements through the proposed morphing strategy. Thus, the study offers to aid a multi-efficient snap-through strategy for the morphing of multistable composite structures.

Vibration control of offshore wind turbines with a novel energy-adaptive self-powered active mass damper

Slender and flexible offshore wind turbines (OWTs) are vulnerable to external dynamic excitations, and passive tuned mass dampers (TMDs) have been widely used to control excessive vibrations of OWTs under harsh marine environments (e.g., strong winds and irregular sea waves). However, TMDs are only effective in the vicinity of the controlled frequency, i.e., in a narrow frequency band. Compared to passive TMDs, active control methods are normally considered to possess better control performances but at the cost of a large amount of external energy input. To this end, the present study proposes a novel energy-adaptive self-powered active mass damper (SPAMD) to mitigate the responses of OWT towers. The proposed control device can harvest energies from OWTs and then use them as the power to drive an active mass damper for structural vibration control. Specifically, a representative OWT is selected as a prototype structure and its tower is modeled as a multi-degree-of-freedom system by simplifying the rotor-nacelle assembly as a lumped mass and moment of inertia. The dynamic characteristics (mainly natural frequency and mode shape) of the tower obtained by the developed model are validated against a finite element model. Subsequently, the system configuration and working mechanism of SPAMD are introduced and SPAMD is incorporated into the developed model to simultaneously harvest energy and mitigate the fore-aft responses of the tower under wind and sea wave loads. The control effectiveness of SPAMD is further compared to the traditional TMD. Results show that SPAMD has a superior effect over TMD in controlling OWT responses.

The Flexible Cable Deformation Effect of a Prestressed Tuned Mass Damper on Vibration Control for Wind Turbine Towers

Larger megawatt wind turbines are of significant height. The wind turbine tower (WTT) can suffer excessive vibration under external dynamic excitation, so an additional vibration control device is needed. Taking a novel prestressed tuned mass damper (PS-TMD) as the research object, its nonlinear vibration control performance induced by a flexible cable deformation effect was investigated. The dynamic coefficient amplitudes at two fixed points were derived based on the principle of virtual work, and results showed that the vibration amplitude considering flexible cable deformation is smaller than that of the linear PS-TMD system. For harmonic response, the numerical simulation of a 3.2 MW WTT indicated that vibration migration performance of the nonlinear PS-TMD is better than that of the linear PS-TMD. The Wilson-[Formula: see text] method was applied to analyze the vibration control effect under three fatigue and two ultimate wind loads. Results showed that the nonlinear PS-TMD can decrease the vibration amplitude more than the linear PS-TMD, and the vibration mitigation performance under the fatigue wind loads is slightly better than ultimate wind loads.

Mathematical modeling of the response of the human locomotor system subject to the action of disturbing mechanical vibrations

In this paper, we will follow the configuration and development of a physical model in accordance with the main objectives of the study. The model must be useful in estimating the essential dynamic parameters, specific to the response of a multi-body system under the action of intense and varied external dynamic excitations, such as mechanical vibrations produced by different technical systems, indirect and direct interaction with the locomotor system of the human body. In the first stage, a basic configuration will be considered, which will take into account the functionality of the leg - calf- thigh assembly. In the second stage, the model will be harmonized in accordance with the additional restrictions and demands imposed by the muscular system on the skeletal structure of the modeled assembly. The third stage will consist of the comparative evaluation of the simulated answer, with the data obtained that experimental, in order to validate the conformity of the structure and parameters of the proposed model.

Nonlinear Frequency Response Analysis of Double-helical Gear Pair Based on the Incremental Harmonic Balance Method

The torsional dynamic model of double-helical gear pair considering time-varying meshing stiffness, constant backlash, dynamic backlash, static transmission error, and external dynamic excitation was established. The frequency response characteristics of the system under constant and dynamic backlashes were solved by the incremental harmonic balance method, and the results were further verified by the numerical integration method. At the same time, the influence of time-varying meshing stiffness, damping, static transmission error, and external load excitation on the amplitude frequency characteristics of the system was analyzed. The results show that there is not only main harmonic response but also superharmonic response in the system. The time-varying meshing stiffness and static transmission error can stimulate the amplitude frequency response of the system, while the damping can restrain the amplitude frequency response of the system. Changing the external load excitation has little effect on the amplitude frequency response state change of the system. Compared with the constant backlash, increasing the dynamic backlash amplitude can further control the nonlinear vibration of the gear system.

A Novel Fatigue Crack Monitoring Method Based on Nonlinear Dynamic Characteristics of Strain

This paper presents a novel fatigue crack monitoring method for steel specimens based on the smoothness priors method (SPM) and Tsallis entropy (TE) of strain measurements. The aim of the study is to detect initiation of a crack in steel specimens and subsequently to monitor its propagation under the fatigue load, based on real-time strain measurements. The nonlinear dynamic response of the structure was exploited since it degrades due to the initiation and subsequent propagation of the crack under the external dynamic excitation. The proposed method was experimentally validated. Here, the SPM is applied to decomposing the structural strain response into a nearly-stationary (NS) component and a low frequency aperiodic trend (LFAT) component. Features associated with crack initiation can be extracted from the NS component. The LFAT component, on the other hand, can be used to identify crack propagation. To tackle the singularity of the structural responses associated with a crack, the TE of the NS component was used in detection and monitoring of the crack in the steel specimen. Two other techniques, namely, acoustic emission (AE) sensor and crack opening displacement (COD) gauge were used for the purpose of calibration and comparison. The results show remarkable resemblance in terms of crack initiation and propagation identification exhibited by all three types of sensors, highlighting the potential of the proposed method for real-time detection and subsequent monitoring of crack propagation in steel structures.

Structural Acoustics and Vibration: Which came first, the sound or the vibration?

Structural acoustics may be defined as a multidisciplinary coupled field of physics usually referring to the characterization of either (1) the sound power emitted by a vibrating structure subjected to external dynamic excitation or (2) the vibrational response of structures excited by incident sound fields or fluid excitation. The Structural Acoustics and Vibration (SAV) Technical Committee is a diverse group of scientists and engineers sharing a common interest and shared stewardship in the health and advancement of the SAV technical discipline. In addition to the fundamental scientific study of the underlying SAV physics which is important to all SAV practitioners, there is also interest on the practical real-world, application of the SAV science to predict, control, and potentially reduce the vibroacoustic response of given structural acoustic systems. This paper begins by providing a fundamental definition of the underlying physics governing the SAV technical discipline. The numerous categories and subdivisions within the general SAV area are then addressed, along with illustrations of the broad array of scientific and engineering real-world applications in the field. Following that, examples of interesting career options as well as exciting new research areas in SAV are presented. Finally, this paper concludes with information regarding the makeup, functioning, and administrative philosophy of the SAVTC.

Exact Solutions for Free Vibration of Cylindrical Shells by a Symplectic Approach

In engineering applications, the vibration analysis of cylindrical shells is highly significant for the design of engineering structures subjected to external dynamic excitations. In practice, the explicit expression would make a better and more quick design comparing to the numerical results. However, the studies on analytical method were concentrated on the semi-inverse method. To overcome the limitation, we introduce a new analytical treatment for the free vibration of cylindrical shells. Under the framework of the Hamiltonian system, an analytical symplectic approach is introduced into the free vibration of cylindrical shells. In the symplectic space, exact solutions are obtained in a rational and systematic way without any trial functions. Analytical frequency equation for arbitrary boundary conditions is derived. Highly accurate natural frequencies and analytical vibration mode functions are obtained simultaneously. In this paper, the governing equations for Donnell’s and Reissner’s shell theories can be successfully transformed into the Hamiltonian form by introducing a full state vector. The free vibration of cylindrical shells is reduced into a symplectic eigenproblem. Numerical results are compared with those previously published in literature and good agreement is observed. In addition, the proposed method can be extended to micro/nanoscale cylindrical shells.

Introduction to the structural acoustics and vibration technical committee

The Structural Acoustics and Vibration (SAV) Technical Committee is a broad group of scientists and engineers sharing a common interest and shared stewardship in the health and advancement of the SAV technical discipline. Structural acoustics may be defined as a multidisciplinary coupled field of physics usually referring to the characterization of either (1) the sound power emitted by a vibrating structure subjected to external dynamic excitation or (2) the vibrational response of structures excited by incident sound fields or fluid excitation. While the fundamental scientific study of the underlying SAV physics is certainly important to SAV practitioners, there is also interest on the practical application of the prediction, control, and potential reduction of the vibroacoustic response of given structural acoustic systems. This paper first provides a fundamental definition of the underlying physics behind the SAV technical discipline. The many categories and subdivisions within the general SAV area are then addressed, along with illustrations of the broad array of scientific and engineering real-world applications in the field. Following that, examples of interesting career options as well as exciting new research areas in SAV are presented. Finally, the paper concludes with information regarding the makeup, functioning, and administrative philosophy of the SAVTC.The Structural Acoustics and Vibration (SAV) Technical Committee is a broad group of scientists and engineers sharing a common interest and shared stewardship in the health and advancement of the SAV technical discipline. Structural acoustics may be defined as a multidisciplinary coupled field of physics usually referring to the characterization of either (1) the sound power emitted by a vibrating structure subjected to external dynamic excitation or (2) the vibrational response of structures excited by incident sound fields or fluid excitation. While the fundamental scientific study of the underlying SAV physics is certainly important to SAV practitioners, there is also interest on the practical application of the prediction, control, and potential reduction of the vibroacoustic response of given structural acoustic systems. This paper first provides a fundamental definition of the underlying physics behind the SAV technical discipline. The many categories and subdivisions within the general SAV area are...

New Bridge Weigh‐in‐Motion System Using Piezo‐Bearing

The traditional BWIM (bridge weigh‐in‐motion) system measures the deformation of the bridge by means of sensors and uses these measurements to estimate the characteristics of passing traffic by means of dedicated algorithms. However, the application of the BWIM system is subordinated to the type of superstructure, composition, geometry, materials, and shape of the bridge, the location of the strain sensors used in the system, and the need to calibrate the measured strain curve and of a precise model of the structure at hand. In order to be free from these constraints, this paper proposes a simpler BWIM system using the bridge bearings as a weighing scale to measure the reaction forces at the supports resulting from the passing traffic. To that goal, the piezocomposite element known for its durability and responsiveness to external loading is embedded appropriately in the bridge bearing to achieve the piezo‐bearing. This paper presents the BWIM system constituted by the so‐called piezo‐bearing, the results of a series of tests conducted to verify the responsiveness of the system to external dynamic excitation, and a numerical example to validate the feasibility of the proposed BWIM system. The numerical example shows that the identification of the characteristics of the vehicle crossing the bridge can be realized based simply upon the theory of mechanics using the time histories of the measured reaction forces instead of the deformation of the bridge.

Dynamic Response Analysis of Gearbox Housing System Subjected to Internal and External Excitation in High-speed Train

In order to study the dynamic characteristics of gearbox housing in high-speed train, the internal and external dynamic excitations of gear transmission system in high-speed train are calculated by numerical simulation at first, and a multi-body dynamics model of motor car including the driving system subjected to internal and external excitation is established. In this model, the internal excitations include the time-varying stiffness, damping and transmission error, the external excitations include the harmonic torque of asynchronous motor and rail irregularity, then the dynamic forces generated in the gear mesh are calculated when the train running with constant power. What is more, the dynamic forces of housing bearing seats are obtained and loaded to the finite element model of gearbox housing, the dynamic response of housing is carried out using a direct integrate method and harmonic response analysis are also finished. The results show that the high frequency vibration of the gear transmission system can only be acquired when the gear meshing is considered in the dynamics model. The gearbox housing structure design is sufficient for operation requirement. Frequency of harmonic torque and the gear mesh frequency are also appeared in the dynamic response of gearbox housing. During the structure design of gearbox, it should pay attention that the gearbox mode natural frequencies need to avoid the meshing frequency and external excitation frequency, so as to prevent the resonance.

Capillary effect on the sloshing of a fluid in a rectangular tank submitted to sinusoidal vertical dynamical excitation

Surface tension effects on fluid sloshing in a tank subjected to external excitation has been less studied. This work aims at understanding this phenomenon in order to derive practical solutions to problems faced in several engineering. A tank containing a fluid with a free surface is submitted to gravity and capillary forces and subject to external dynamic excitation. Introduction of vertical sinusoidal dynamical excitation leads to a problem of paramtric oscillations governed by the Mathieu equation. Analysis of the Mathieu equation shows the existence of stable and unstable regions in the stability diagram. Some results induced by harmonic excitations on the fluid sloshing are presented. When the external dynamical excitation amplitude ɛ is small, periodic solutions appear in stable regions and when ɛ increases, the fluid behavior is not perfectly periodic and the amplitudes are not regular. Nonlinear effects make the behavior of the fluid complicated and render it almost unpredictable. In stable regions, the solution remains bounded at any time. When changing the perturbation parameter ɛ, the phase difference increases and also with the increase of the surface tension.

Dynamics in lithographic projection objectives

High-performance optical systems like lithography objectives are very sensitive to external dynamic excitations, which may occur both during operation and out of operation, i.e., during handling and transport. Lithography objectives are typical opto-mechanical systems. For the non-operating state, the focus of dynamical investigations is on avoiding damage, whereas during operation, the preservation of a good imaging behavior is of interest. In the first part, some out-of-operation situations are introduced, namely handling, transport, and earthquakes. In this context, excitation models are discussed and also applied to a nonlinear model. In the second part, an overview of techniques for modeling interfaces between dynamics and optics is given regarding lens systems. The interfaces are summarized in the lens kinematics. Lens kinematics is comprised of rigid lens motion, surface deformations, and refraction index variations due to stresses. According to these issues, some examples are provided.

Elasto‐aerodynamic lubrication analysis of a self‐acting air foil journal bearing

ABSTRACTNowadays, air foil bearings find widespread use in very high speed, lightly loaded oil‐free rotating turbomachineries such as compressors and microgas turbines because they have theoretically no speed limitations and they are environmentally benign. In the design of such bearings, it is of cardinal importance to enhance their steady‐state and dynamic performance characteristics for the safety operation, especially against the external dynamic excitations. Most of elasto‐aerodynamic approaches under dynamic conditions proposed in the technical literature include only the static pressure induced deformation of foils. This paper presents a theoretical investigation on the effects of both static and dynamic deformations of the foils on the dynamic performance characteristics and stability of a self‐acting air foil journal bearing operating under small harmonic vibrations. For the dynamic deformations of foils to be taken into account, the perturbation method is used for determining the gas‐film stiffness and damping coefficients for given values of excitation frequency, compressibility number and compliance factor of the bump foil. The rotor‐dynamic coefficients serve as input data for the linear stability analysis of rotor‐bearing system. The nonlinear stationary Reynolds' equation is solved by means of the Galerkin's finite element formulation, whereas the finite differences method are used to solve the first‐order complex dynamic equations resulting from the perturbation of the transient compressible Reynolds' equation. As a first approximation, the corrugated subfoil is modelled as a simple elastic foundation, i.e. the stiffness of a bump is uniformly distributed throughout the bearing surface. It was found that the dynamic properties and stability of the compliant finite length journal bearing are significantly affected by the compliance of foils especially when the dynamic deformation of foils is considered in addition to the static one by applying the principle of superposition. Copyright © 2012 John Wiley & Sons, Ltd.

Numerical modeling of sloshing motion in a tuned liquid damper outfitted with a submerged slat screen

AbstractTuned liquid dampers (TLDs) are among the most economical and effective passive damping devices. They have been increasingly used to reduce dynamic response and protect structures from failure due to external dynamic excitations. Slat screens are one of the most effective devices used to increase the inherent damping of a TLD, and to reduce the non‐linearity of the free surface motion. A numerical algorithm has been developed to solve the complete non‐linear, moving boundary flow problem in a TLD outfitted with slat screens. The model has been developed to handle conditions leading from small to large interfacial deformations without imposing any linearization assumptions. The numerical algorithm is based on the finite‐difference method. The free surface has been determined using the volume‐of‐fluid method and the donor–acceptor algorithm. The effect of the slat screens has been modeled explicitly by using the partial‐cell treatment method. The present algorithm has been validated against experimental data. The results indicated that the present algorithm is capable of providing accurate details of the flow field inside the TLD and through the screens. These details are essential to improve our understanding of the important parameters governing the performance of a TLD, and hence, to enhance our ability to design better TLDs. Copyright © 2010 John Wiley & Sons, Ltd.

Localization of a Breathing Crack Using Super-Harmonic Signals due to System Nonlinearity

In this paper, a new damage detection technique able to identify the location of a breathing crack in an isotropic rod, relying only on real-time measurements, is proposed. The detection algorithm exploits the phase information associated with the superharmonic components produced, in the Fourier spectrum, by the nonlinear dynamic response of this kind of defect under the influence of an external dynamic excitation. The validity of the proposed algorithmforaweaklynonlinearsystemissupportedbyananalyticalsolutionforacrackedbeamobtainedthrough the harmonic balance approach. A numerical investigation is conducted by means of a finite element model of an isotropic beam integrating nonlinear contactelements in the damaged area and solved for the steady-state response. Threedifferentpostprocessingapproaches,incorporatingtheproposeddamagedetectionalgorithm,areformulated andcomparedtoassessthecapabilityofthecurrentmethodology.Resultsfromthecrackedbeammodelclearlyshow thegenerationofthesuperharmonicsasaresultofthenonlineardynamicbehaviorofthebreathingcrack.Thephase associated with the superharmonic components is then processed through the detection algorithm and the predicted location is compared with the actual position of the defect to assess the performances of the methodology.