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• Axial tension per unit width is related to the axial speed and acceleration. • The fluid has an effect on both the inertia term and the damping term. • The immersion depth has little effect on the natural frequency. • Truncation order of the Galerkin method afflects the stability. The vibration of axially moving panels immersed in fluid are widely concerned in engineering problems. In this paper, free vibration characteristics and dynamic stabilities of an axially moving viscoelastic panel immersed in fluid are investigated. The governing equation is given by applying the generalized extended Hamilton principle. The axial tension per unit width is related to the axial speed and the axial acceleration, changing continuously in the axial direction, which is the innovation point and key problem of this paper. However, they were independent of each other in most previous studies. The fluid is assumed to be an ideal liquid, that is, inviscid, homogeneous and incompressible. By using the velocity potential function, the Bernoulli equation, the dynamic pressure and the boundary conditions of the fluid are derived and incorporated into the governing equation in the form of an additional mass. The Galerkin method is used to discretize the governing equation and convert it into a matrix form. The influences of the truncation order numbers and the immersion depths on the first four natural frequencies are analyzed. We found an interesting phenomenon that the fluid has an effect on both the inertia term and the damping term of the system and the effect of damping force generated by fluid is greater than that of inertia. Another innovation is that the immersion depth has little effect on the natural frequency. Moreover, the subharmonic resonance and the combination resonance are analyzed by using the averaging method. The influences of system parameters including truncation order numbers, axially supported stiffness parameters, viscoelasticity coefficients, mean axial speeds, fluids and immersion depth on the stability boundaries are considered. The stability boundaries of three models, namely the vacuum model, the material derivative model, and the partial derivative model, are compared. The results show that the material derivative model is more reasonable. The results are verified by the Runge-Kutta algorithm. The results of the analytical and numerical methods of the subharmonic and combination resonances are in good agreement with each other. In this paper, the dynamic stability of an axially moving viscoelastic panel immersed in fluid is studied by means of the average method. It extends the application range of the averaging method and provides a reference for the study of axially moving structures immersed in fluid. [ABSTRACT FROM AUTHOR]

Exact dynamic stiffness (DS) models for three-dimensional (3D) cavities in cylindrical coordinates under any classical acoustic boundary conditions (BCs) are proposed in this paper. The paper first derives the exact general solutions that satisfy the governing differential equations. Then, these general solutions are used to develop the DS matrices for 3D cavities in cylindrical coordinates by substituting them into the acoustic pressure and normal velocity BCs. Subsequently, the Wittrick–Williams algorithm, along with the analytical expressions of the j 0 count for various shaped cavities, is applied to compute natural frequencies. Finally, reliable and efficient techniques for dynamic response solution are employed. The DSM provides benchmark solutions for various 3D cavities. The convergence, universality, and efficiency of the proposed method in modal and response analysis are illustrated by comparing the results obtained using the DSM and FEM. The calculation time when using the DSM is only 0.1% of that when using the commercial finite element method, while maintaining the same accuracy. This research not only provides an effective technique for analyzing the cylindrical acoustic pressure field in engineering, but also establishes a foundation for the analysis of structural–acoustic coupling in tubular structures. • Dynamic stiffness derived for circular/sectorial/annular/sectorial annular 3D cavities. • Different BCs can be modeled and non-uniform acoustic excitations can be applied. • Analytical j0 count of W–W algorithm in cylindrical coordinates is derived. • High efficiency and exactness is demonstrated for broadband modal/response analyses. • Benchmark solutions are provided for various 3D cavities subjected to different BCs. [ABSTRACT FROM AUTHOR]

• To establish band structures of phononic crystal metabeams with multiply local resonators. • To analyze the effects of external excitation sources on the bandgap. • To investigate the effects of resonator number and mass on the bandgap structures. • To actively control the bandgaps using piezoelectric springs. Acoustic metamaterials and bandgap engineering have been research areas that have drawn intensive and extensive research attention in recent years. The exploration of low-frequency bandgaps as well as the corresponding bandgap modulation has been a subject of much concern by numerous research scholars and engineers. In this paper, a new laminated beam metastructure which uses piezoelectric springs to connect local resonators to the laminated beam is proposed. Considering the intrinsic relationship of materials and loads due to resonator system, the Euler-Bernoulli beam theory is employed to derive the governing equation of the system, and the band structure is solved by the spectral element method. Subsequently, we solve and analyze the effects of external excitation sources including electric and magnetic fields on the bandgap, and also investigate the influence resonator number and mass. Finally, the modulation effect of piezoelectric spring stiffness on the bandgap is analyzed. The result of this paper provides a sound theoretical guide for the application and control of multi-frequency domain bandgaps. It also serves new ideas for vibration isolation and suppression of metabeam structures in real-life engineering applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The aerodynamic noise in the gas turbines primarily originates from the pulsations generated by the air within compressors, and it is a combination of single-tone noise caused by the pressure fluctuation of blades and broadband noise generated by interactions between turbulent fluid and blades. Therefore, the separation of single-tone noise is vital in predicting the contribution of each noise source for noise reduction and health monitoring. In current practice, the Vold-Kalman filter (VKF) is widely employed for single-tone noise separation. However, the optimal VKF bandwidth is often difficult to determine, thereby impeding the separation accuracy of single-tone noise. To solve this problem, this paper proposes a single-tone noise separation method by utilizing computed order tracking based on double criteria VKF (DC-VKF). Firstly, the order spectrum of the original signal is obtained by order analysis. Secondly, the relative error of order amplitude (REOA) and total relative standard deviation (TRSD) of the extracted signal are calculated under different bandwidths and corresponding curves are separately carried out. The optimal bandwidth is determined by identifying the first bandwidth that meets either the REOA or TRSD threshold, which are respectively set based on trend of curves. Finally, the corresponding order components are separated by VKF with optimized bandwidth. The effectiveness of DC-VKF is validated on simulation and gas turbine datasets. This paper provides an efficient extraction method for the separation of single-tone aerodynamic noise of gas turbines, and provides a scientific basis for noise monitoring, evaluation, and reduction. • Single-Tone aerodynamic noise source separation for gas turbines. • Double criteria vold-kalman filter for single-tone noise separation is proposed. • The optimal bandwidth is selected using double criteria of the order spectra. • Double criteria ensure that the optimal bandwidth is closer to the practical value. • The simulation and acoustic measurement data analysis are carefully carried out. [ABSTRACT FROM AUTHOR]

• The nonlinear dynamic models of riser under different suspension evacuation conditions are established. • The nonlinearity of the model takes into account the actual configuration of riser, VIV in cross-flow (CF) and in-line (IL) directions, and large deformation. • Experiments are designed and carried out to verify the nonlinear model. • The vibration frequency and mode shape of riser under different influence factors are analyzed. This paper establishes a nonlinear dynamic model for a suspended riser under the condition of considering realistic configurations, based on the principles of virtual work and Hamilton's variational principle. The nonlinearity of the model is manifested by considering vortex-induced vibration in the cross flow (CF) and in-line (IL) directions, as well as large deformations, taking into account the variable diameter of the riser in the compliant riser system. The model is discretized using the finite element method and solved using the Newmark-β method. Experimental validation is conducted to verify the effectiveness of the model. Utilizing the parameters of a riser in a well in the South China Sea, the paper explores the influence of different environmental and structural parameters on the VIV response characteristics of a compliant riser. Results show that the vibration frequency is controlled by both the bare tube and buoyant block when the buoyant block coverage rate is 25%, which can significantly inhibit VIV. With the increase of flow velocity, the VIV energy jumps from low order to high order, and the riser mode also transitions from low order to high order. The increase in suspension length transfers the vibration energy from high to low frequency, thus realizing energy transition and mode shape transformation. With the increase in LMRP weight, the vibration and natural frequencies of the riser increase synchronously, the vibration amplitude decreases, and the mode shape remains unchanged. [ABSTRACT FROM AUTHOR]

The use of Gas Foil Bearings (GFBs) can facilitate an environmentally friendly and oil-free alternative for high-speed rotating light machinery. The applicability is, however, drastically limited by their low load-bearing capability and inherently low damping. One possible way to mitigate these limitations is through hybridisation and active feedback control. This paper exemplifies the possibilities of using passive Hybrid Gas Foil Bearings (HGFBs) with radial gas injection to increase the load-bearing capacity and the stability of a rotor-bearing system. With hybridisation comes also the possibility of active feedback control, to this extent, the paper presents a step-by-step methodology for designing feedback control for an active HGFB. The non-linear set of differential equations comprising the system, pressure states, foil deflections, rotor movements, and actuator position, are solved simultaneously with control feedback. The hybrid system is presented for a yet-to-be-built version of an existing industrial-scale GFB test rig augmented with an existing injection system. The industrial-scale test rig mimics an 120 kW compressor weighing 21.17 kg used for water treatment. The presented results comprise pressure distributions, film height, foil deformation, Campbell diagrams, eccentricity diagrams, and waterfall plots. The augmentation with radial injection is seen to greatly improve the system's capabilities, i.e., a significant enlargement of operational speed range from 30 kRPM to 40 kRPM (33 %), and start-ups without dry friction between the rotor and top foil, and an increase in load capacity described through rotor position. • A coupled nonlinear GFB model extended with piezoelectric actuators. • Dynamically reduced linearised model applied to design an optimal LQR state controller. • Hybrid case offers a 20% increase in speed. • Feedback controlled case offers a 33% increase in speed. • Feedback controlled case ensures "lift-off" at start and stop conditions. [ABSTRACT FROM AUTHOR]

This paper explores the dynamic behavior of metamaterial-like structures by investigating the evolution of their band gap under the influence of geometrical nonlinearities in the large displacement/rotations field. The study employs a unified framework based on the Carrera Unified Formulation (CUF) and a total Lagrangian approach to develop higher-order one-dimensional beam theories that account for geometric nonlinearities. The axis discretization is achieved through a finite element approximation. The equations of motion are solved around nonlinear static equilibrium states, which are determined using a Newton–Raphson algorithm combined with a path-following method of arc-length type. The CUF approach introduces two key innovations that are highly suitable for the evolution of the band gap: (1) Thin-walled structures can be effectively represented using a single one-dimensional beam model, overcoming the common limitations of standard finite elements. This is crucial as three-dimensional solid elements would result in significant computational costs, and two-dimensional elements based on von Kármán theory pose limitations for this type of investigation. Moreover, employing one-dimensional finite elements usually requires a combination of elements, leading to additional mathematical complexities in their connections and lacking geometric precision. (2) CUF enables the use of the full Green–Lagrange strain tensor without the need for assumptions, as is the case with von Kármán nonlinearities. The paper specifically compares results obtained with linear and nonlinear stiffness matrices, highlighting the differences. Numerical investigations are conducted on thin-walled structures composed of repeatable cells, assessing mode changes under traction and compression loading. Additionally, the paper presents a more complex metamaterial-like structure. The findings emphasize that the band gap is an inherent property of the equilibrium state, underscoring the necessity of a proper nonlinear analysis for accurately evaluating frequency transitions. • Dynamics of metamaterial-like structures investigating the evolution of band gap. • Analysis of the mode and frequency change in the geometrical nonlinear field. • CUF for 1D Finite element model of thin-walled metamaterial-like structures. • Lagrange polynomials for the description of complex geometries. • Comparison between linear and nonlinear approaches. [ABSTRACT FROM AUTHOR]

• Novel CRB vibration model considering contact stiffness and natural frequency characteristics. • The outer raceway spalling fault with varying sizes is simulated in this paper, considering spalling defect extents and load distribution. • This paper showed that the effects of contact stiffness variation on natural frequency characteristics by the statistical distribution of fault feature signals. The modeling of early spalling defects in cylindrical roller bearings (CRB) is a challenging problem in fault diagnosis. Due to the absence of prominent "drop-in" excitation, accurately describing the signal characteristics of the early failure mechanism model for CRBs becomes difficult. In this paper, the failure of cylindrical roller bearings due to spalling caused by fatigue is a frequent occurrence. The previous defect models have been centered on sharp-edge impact between the roller and spalling defects. However, they have not provided an adequate depiction of the variation in contact stiffness caused by roller-fault interaction. In this paper, a novel CRB vibration model that takes into account contact stiffness and natural frequency characteristics is proposed. The model considers both the load redistribution and contact stiffness variation caused by spalling defects. This new model provides a spring-damper system based on contact stiffness between the roller and raceway, enabling an estimation of the CRB system's natural frequency characteristics. The outer raceway spalling fault with varying sizes is simulated in this paper, taking into account spalling defect extents and load distribution. The vibration response of the fault signal model is investigated. The simulation results show that the load and contact stiffness will change as the roller passes through the defect zone. Furthermore, the statistical distribution of fault feature signals demonstrates the effects of contact stiffness variation on natural frequency characteristics. The experiments given in this paper have verified the simulation results of the proposed signal model. [ABSTRACT FROM AUTHOR]

Abstract: In this note, in response to the previous note by Zinoviev and Bies [Author''s Reply to: F. Farassat, Comments on the paper by Zinoviev and Bies “On acoustic radiation by a rigid object in a fluid flow”, Journal of Sound and Vibration 281 (2005) 1224–1237], the present authors briefly discuss the assumptions used in the acoustic analogy and its intended applications. It is pointed out that the scattering problems discussed by Zinoviev and Bies do not fall within the intended applications of the Ffowcs Williams–Hawkings (FW–H) equation. However, the FW–H equation can be used to derive an integral equation for finding scattered pressure fields. We derive this integral equation and show the validity of the equation for some of the examples of Zinoviev and Bies. We respond to the other issues brought up by these authors in their notes. We believe that Zinoviev and Bies have misinterpreted the acoustic analogy and have not applied the Curle formula and the FW–H equation correctly. [Copyright &y& Elsevier]

• We propose a single-mode switch defect detection method based on the dominant mode. • The dominant mode and the excitation node are obtained according to the 3σ rule. • The dominant mode can be excited to detect the defect of the switch rail. • A new way for detecting the rail defect based on ultrasonic guided wave method. Guided wave structural health monitoring offers a promising way for continuous surveillance and identification of structural damage. However, due to the variable cross-section properties of the switch rail in the longitudinal direction, there are different modes present on different cross-sections, so the guided wave signals excited are very complex. The traditional baseline subtraction method can be used to extract defect echo features, but it is prone to false positives due to the influence of external factors such as field temperature change and environmental noise. This paper proposes a switch rail defect detection method based on the pulse reflection method of the dominant mode. Initially, eight typical cross-sections are chosen based on the cross-sectional dimensions of the switch rail. Semi-analytical finite element(SAFE) method is utilized to solve the dispersion curves and wave structure matrix of guided waves in the switch rail. According to the 3σ rule, the dominant mode whose node amplitude is much higher than other modes is selected, and the excitation signal is applied at the maximum value of the mode, which successfully excites the dominant mode. Through simulation and experimental validation, the reflected waves of the dominant mode at the defect are observable and visible, and the pulse reflection method can be used to detect the defects at the switch rail foot with a positioning error of less than 10 cm. The pulse reflection method of the dominant mode proposed in this paper, which has high detection efficiency and is not affected by the ambient temperature, provides a new idea and way to detect defects of the switch rail based on the ultrasonic guided wave method. [ABSTRACT FROM AUTHOR]

• This article introduced the stiffness characteristics of the joint surface between the mechanical connection structures of the spindle-tool holder, tool holder-spring clamp, and spring clamp-tool of the spindle system in the robot machining system into the prediction of machining stability. • This paper proposed a method for establishing a tool tip frequency response prediction model for robot milling systems, which improves the accuracy of theoretical prediction of the tool tip dynamic characteristics of robot milling systems under different postures, Compared to the method of obtaining tool tip frequency response of robot milling system through experiments to support milling stability prediction, the method proposed in this paper effectively improves the efficiency of robot milling stability prediction. • The prediction results of the proposed tool tip frequency response prediction model were imported, and a two-degree-of-freedom chatter prediction model considering the regenerative effect is established, and the robot milling chatter is predicted considering redundant degrees of freedom. The accuracy of prediction results is verified by milling experiment. The change of tool tip frequency response caused by the posture dependence of robot dynamic is one of the key problems that make it difficult to accurately predict the milling stability of robot. In this paper, a tool tip frequency response prediction method considering the interface stiffness characteristics of spindle-tool system is proposed for stability prediction of robotic milling under the condition of posture variation. Firstly, the interface stiffness models of spindle-toolholder, toolholder-spring clip and spring clip-tool are established based on Yoshimura's unit area method. Then, The dynamics model for the robot body and the interface stiffness models for spindle-tool system are imported into the finite element analysis model of the spindle system, so that the prediction of the tool tip frequency response is realized by harmonic response analysis. Compared with the experimental results, the maximum error of the natural frequency was not more than 2 %, and the maximum error of the amplitude was not more than 12 %. Finally, the 2 DOF robot milling stability prediction model is established. Then the robot milling chatter is predicted considering redundant degrees of freedom from the perspective of regenerative chatter prediction theory, and the accuracy of prediction results is verified by milling experiment. [ABSTRACT FROM AUTHOR]

Outdoor noise measurements made by a microphone mounted into or on an acoustically rigid thin board lying on the ground can be affected to some degree by diffraction from the edges of the board due to the thickness of the board or the finite impedance of the surrounding ground. This can result in the sound pressure level varying over the top surface of the board and deviating from the 6 dB increase (relative to the incident sound field) expected if the board was very thin and the surrounding ground was also acoustically rigid. In this paper, an experimental and numerical investigation is carried out to assess the variation in the sound pressure level over the top surface of the ground-board from sound incident at several different angles. The experiments utilise a probe microphone flush-mounted into a 0.4 m diameter circular thin metal ground-board slightly protruding from a simulated grass-covered field in an anechoic chamber. The experiments are repeated on an outdoor grass-covered field and compared. The effect of different installation methods such as the flush-mounted probe microphone, inverted microphone, and a side-set microphone are also investigated. The variation of the sound pressure level on the top surface of the ground-board due to diffraction from the edges of the board is shown. It is also shown that the presence of the microphone body begins to affect the noise measurements from a frequency of 3 kHz. The results presented in this paper suggest that narrow-band noise measurements using a microphone mounted on a circular ground-board installed on a grass-covered surface can deviate by more than 3 dB from the nominal 6 dB increase above the incident field level. [ABSTRACT FROM AUTHOR]

Impacting systems are widely used in many engineering applications, such as self-propelled robots, energy harvesting and percussive drilling, which exhibit rich and complex nonlinear phenomena. Among these applications, predicting nonlinearities and estimating system parameters are of great interest of nonlinear dynamics research community. Backbone curve is an analytical tool that captures the frequency–amplitude dependence of nonlinear systems. In this paper, we estimate the impacting stiffness of a single-degree-of-freedom non-smooth dynamical system qualitatively by using the backbone curve. It was found that an increase of the impacting stiffness may lead to lowering the backbone curve. An adaptive differential evolution algorithm with the Metropolis criterion is proposed to identify the parameters of the impacting system quantitatively using experimental data, which are consistent with our theoretical predictions. Finally, the identified parameters are verified, and the limitations of the backbone curve are drawn. The nonlinear characteristics identification method studied in this paper can be extended to other vibro-impact systems and is potentially applicable to structural health monitoring and robotic sensing. [Display omitted] • Nonlinear characteristics identification for an impacting system is studied. • Backbone curve is employed to identify the stiffness of the impacting spring. • Differential evolution algorithm with Metropolis criterion was also adopted. • Backbone curve can only estimate the impacting stiffness qualitatively. • The method is applicable to structural health monitoring and robotic sensing. [ABSTRACT FROM AUTHOR]

Current offshore wind turbine installation and positioning methods require mechanical equipment attached on the lifted components and human intervention. The present paper studies the development of a contactless motion compensation technique by investigating a magnetically controlled pendulum. The technique involves the interaction of a magnetic pendulum with an electromagnetic actuator. Two control modes are considered: the imposition of a desired motion to the mass and the motion attenuation of a prescribed pivot excitation. The numerical model is validated and calibrated against experiments and demonstrates excellent predictive capabilities. The control exerted is effective for a broad range of excitation frequencies and amplitudes. Important parameters associated with the performance of the technique such as the separation distance of the magnets and the saturation of the controller are identified. The controllability regions for effective control depending on the characteristics of the excitation are derived. The force amplitude of the contactless actuator is comparable to currently-used active tugger line control systems, but with the additional advantage of both attractive and repulsive forces. The findings of this paper illuminate the path for the further development of a non-contact control technique which has the potential to increase the efficiency of offshore wind installations. • Contactless position control for offshore wind turbines: a proof of concept. • Pendulum motion control via magnetic interaction. • Additional control authority with both repulsive and attracting forces. • Efficient control for the desired motion of a pendulum mass for a fixed or moving pivot. • Estimated force amplitude comparable to active tugger control systems. [ABSTRACT FROM AUTHOR]

• Acoustoelasticity theory for plane elastic waves and second harmonics. • Nonlinear wave equation integrated predeformation. • Acoustoelasticity affects wave vectors of P and S waves while not amplitudes. • Acoustoelasticity of free second harmonics follows similar rules of P and S wave. • Acoustoelasticity affects wave vectors and amplitudes of driven second harmonics. This paper presents a novel approach to investigating the acoustoelasticity of plane elastic waves and second harmonics within isotropic solid media. The new contributions of this approach consist of providing an intuitive understanding of the essence of acoustoelasticity, offering a more straightforward method to deduce the expressions for acoustic velocities and pre-strains in natural coordinates compared to conventional theory of acoustoelasticity, and introducing a novel methodology for investigating acoustoelasticity of plane elastic waves and second harmonics. The core idea of this approach can be explained as follows: by introducing an intermediate state for describing predeformation between the natural state and the final state, the predefomation can be integrated into the nonlinear wave equation, and acoustoelastic equations of plane elastic waves and second harmonics can be deduced consequently by decomposing this nonlinear wave equation under the second approximation of perturbation theory. When applying this approach to study the acoustoelasticity of P wave and S wave of plane elastic waves, the validity and practicality of this approach are confirmed by contrasting expressions for acoustic velocities and pre-strains in natural coordinates to those found in previous research [8] , which exhibit perfect consistency. Furthermore, this paper focuses on the acoustoelasticity of the free second harmonics and the driven second harmonics generated by self-excitation and interaction of plane elastic waves. The results concerning acoustoelasticity can be summarized as follows: firstly, pre-strain only influences wave vectors of plane elastic waves through the speed term in their expressions without altering their amplitudes; secondly, the acoustoelasticity of free second harmonics follows similar rules above; finally, Pre-strain affects the amplitudes and wave vectors of driven second harmonics through the speed terms in their expressions without changing the form of their expressions. [ABSTRACT FROM AUTHOR]

• A highly efficient structural dynamic analysis method with controllable accuracy is proposed. • It is applicable to a wide range of coupled and nonlinear dynamic systems with time-varying modal characteristics.. • It can attain high-precision solutions with a minimal number of elements. Matrix perturbation theory (MPT) is widely used in sensitivity analysis and reanalysis of structural eigenvalue problems when small changes are imposed on parameters. However, the conventional MPT based on the finite element method is only applicable to the cases with small parameter variations, and it is difficult to avoid cumulative errors caused by multi-step perturbations. A key issue for the application of MPT is how to evaluate the error of perturbation solution and correct the results during the process of multi-step perturbation solution. By invoking the dynamic stiffness method (DSM), Wittrick-Williams algorithm, and the precise integration method (PIM), in this paper, we propose an improved method with controllable accuracy, self-checking and elimination of accumulated errors, referred to as the dynamic stiffness matrix perturbation theory (DSMPT). In present method, the rationality of the results can be estimated preliminarily according to dynamic stiffness equation, which is not present in the original MPT, can greatly enhance the accuracy of the perturbation results and evaluate the errors. Case studies prove that the DSMPT combined with PIM provides a convenient way to realize accuracy self-inspection and rectification of the perturbation results, and the maximum relative error of eigenvalue and response can be controlled to be smaller than 0.2% and 0.025%, respectively. Besides, DSMPT can also be applied to the analysis of coupled and nonlinear dynamic system with time-varying modal characteristics. Finally, a thermally-induced vibration analysis of tensegrity modules during space deployment is taken as an example to further illustrate the advancements of the DSMPT in solving time-varying dynamic problems. Results show that the DSMPT effectuate an almost sixfold reduction in computational time. In this paper, an improved structural dynamic analysis method with controllable accuracy, self-checking and elimination of accumulated errors is proposed. It is applicable to a wide range of coupled and nonlinear dynamic systems with time-varying modal characteristics. [Display omitted] [ABSTRACT FROM AUTHOR]

Negative stiffness vibration isolation devices have attracted significant attention and research interest for their ability to enhance the low-frequency vibration isolation performance of passive vibration isolators. However, their narrow linear negative stiffness region has hindered their broader application. To address this limitation, a novel design method for vibration isolators is proposed, integrating the permanent magnetic negative stiffness device (NSD) with the displacement amplification mechanism (DAM). The paper begins by presenting a specific vibration isolator design method and analyzing the kinematic relationship. Next, a single-degree-of-freedom (SDOF) nonlinear equivalent model of the vibration isolator is established using the pseudo-rigid-body model (PRBM) approach. Numerical simulations are then conducted to obtain the transmissibility of the isolator. Furthermore, two isolators—one with the displacement amplification effect and another without—are manufactured, and experimental investigations are carried out. The experimental results confirm that the incorporation of the displacement amplification mechanism effectively expands the linear negative stiffness region of the negative stiffness magnet pair, enabling wide-frequency and large-amplitude vibration isolation. Notably, these experimental findings align well with the simulation results. In summary, this paper proposes a creative design method for wide frequency and large amplitude vibration isolators, and proves it through theoretical analysis and experimental validation. • A create negative stiffness isolator based on displacement amplification is proposed. • The linear region of the negative stiffness is significantly expanded by three times. • Employ the harmonic balance method for nonlinear equation solving and simulating. • Inspire the design of large displacement amplitude negative stiffness isolators. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive continuous Cosserat rod model is developed to study the dynamics of a drill-string. The model presents the following features: (1) it can simulate the 3-D dynamics of the system, including lateral, axial and torsional motion; (2) it includes damping effects by adopting a Kelvin–Voigt material; (3) it uses a velocity-independent bit-rock interaction formulation in which the forces and torques at the bit are obtained by considering the dynamics of the cutting blades and the evolution of the soil profile, which is simulated with the help of an advection equation; (4) lateral contact is also included. In this paper, two application cases are analysed. The first one deals with a vertical borehole. It includes four simulations that are used to further investigate the hypothesis that low-dimensional lumped formulations are not able to capture the dynamics of a drill-string and, while doing so, evaluate the effect of the damping in the system's response. The predictions are compared with those given by two other models: the first one considers a 2-degrees-of-freedom (DOF) formulation, with 1-axial and 1-torsional DOF; the second one is a semi-discrete approach, with 1 DOF for the axial dynamics and a continuous formulation for the torsional one. Divergences between the predictions are found, associated with the presence of higher frequencies in the signals obtained with the Cosserat model, which reassures the hypothesis that the new continuous approach can capture aspects of the dynamics that cannot be modelled with low-dimensional representations. The efficiency of the drilling is also assessed for the simulations obtained with the new model. The highest performance was observed in those simulations where only axial oscillations occurred, while a performance drop was seen when those vibrations were accompanied by torsional stick–slip, suggesting that an optimum damping value associated with the best drilling performance could be found, were damping controllable. Finally, the second application case is used to illustrate the full potential of the new model to tackle the 3-D lateral, axial, and torsional dynamics in a curved borehole. [Display omitted] • A Cosserat model is used. 3D axial, lateral and flexural motions allowed. • Lateral contact with a curved borehole geometry is considered in the model. • Damping is considered through the adoption of a Kelvin–Voigt constitutive relation. • The influence of damping in the penetration rate is also analysed. • Results are compared with 2-DOF and semicontinuous models in time and frequency. [ABSTRACT FROM AUTHOR]

Zimmerman and Weissenburger's flutter margin method is widely used to estimate the aeroelastic coalescence flutter speed. In contrast to aeroelastic decay rates, the flutter margin exhibits monotonic decay with respect to airspeed rendering it effective in extrapolating the flutter speed using flight test data conducted at pre-flutter airspeeds. This paper reports the generalization of the Bayesian formulation of the flutter margin method by Khalil et al. developed to tackle measurement and modeling uncertainties. This paper improves the predictive performance of the previous algorithm by incorporating the joint prior of aeroelastic modal frequencies and decay rates among airspeeds in order to better estimate the joint posterior of modal parameters using observational data. The modal parameter prior is constructed using the classical two-degree-of-freedom pitch–plunge aeroelastic model whose system matrices (e.g. structural stiffness and damping matrices) vary randomly. Such joint modal parameter prior enforces statistical dependence among posteriors of modal parameters and the associated flutter margins across airspeeds. Numerical studies demonstrate a considerable reduction of uncertainties on the predicted flutter speed obtained from the generalized Bayesian flutter margin method. This improved algorithm can cut cost by reducing the number of flight tests and better assess the uncertainty against aeroelastic flutter. • This paper reports the generalization of the Bayesian flutter margin method. • This algorithm includes joint prior of aeroelastic modal parameters in Bayesian setting. • This method improves the predictive performance of the previous algorithm. [ABSTRACT FROM AUTHOR]

This paper demonstrates a theoretical and experimental study on a universal vibro-impact system with bidirectional drift. With a dedicated experimental rig that is able to verify numerical simulations, the system has revealed various bifurcations from a piecewise-smooth dynamical model involving impact and dry friction. This experimental rig with a two-sided constraint is designed with adjustable configurations which can be used for experimental testing of versatile piecewise-smooth dynamical systems, e.g., the impact oscillator, the drifting oscillator and the capsule system. It is able to be customised to alter different system parameters, such as friction, number of constraints, mass ratio, impact gap and constraint stiffness, for developing and testing various control strategies. In this paper we particularly studied several types of bifurcations from our preliminary experimental results and discussed the dynamical responses of the system under different excitation frequencies and amplitudes. It was found that with a higher excitation frequency and amplitude, the vibro-impact dynamics on both ends of the constraints could result in a large amount of uncertainties, thus bi-stable and chaotic motions are likely to be observed. While being unstable, the system runs with a relatively high energy efficiency in this configuration. [Display omitted] • An experimental rig with a two-sided constraint and bidirectional drift is studied. • The rig is designed with adjustable configurations for testing versatile systems. • System parameters are identified and verified by numerical simulations. • Grazing and sliding bifurcations, bi-stability and chaotic motions are studied. • Control parameters that cause instability may lead to high energy efficiency. [ABSTRACT FROM AUTHOR]

• The study proposed a method to predict the complex subharmonic vibrations. • Local subharmonic vibrations of order-1/3 and −1/5 were obtained accurately. • Local "full tree" and "half tree" of subharmonic vibrations were predicted. • Strange vibrations in the real turbo-engine testings were explained. • Experiments were conducted for comparing with the theoretical predictions. In real turbomachinery testing, we always confront strange high order vibrations which scatter a few points or draw some local lines in the experimental bifurcation diagrams. Such vibrations maybe ignored due to the lack of experiences and knowledge while they can be important in design and operation due to the nonsmooth jumping and instability. This paper proposes a new method for predicting the complex high order subharmonic vibrations for a nonlinear rotor system with light rub. The nonlinearity is induced by the contact of the bristles and the rotor. A continuous nonlinear rotor system is established based on the geometric and bending force diagrams. We discretize the complex orbits and built implicit mappings between the adjacent nodes. A directional eigenfunctional matrix is constructed which maps each nodes and passes the perturbation along the orbits for semi-analytical prediction of the high order subharmonic vibrations. Stability criterion is defined. Numerical bifurcation diagrams and harmonic contours are presented for quick glance of the global subahrmonic vibrations. Two local subharmonic vibrations of odd orders are succesfuly predicted. Local high order subharmonic vibrations of "full tree" and "half tree" are predicted and achieved accurately. An experimental platform is established to compare the semi-analytical predictions with experimental results. The research delivers a new perspective for predicting and computing the high order subharmonic vibrations in nonlinear rotor system. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive experimental analysis of the noise characteristics of an isolated tilt-rotor system under edgewise flight conditions. The investigation explores the effect of rotor tilt on noise and aerodynamics using flow velocity, thrust, and far-field noise measurements. Flow field results show that as the tilting angles vary, there is a change in speed over the rotor blade surface, which influences vortex formation and wake attributes. The far-field noise spectra revealed a significant decrease in sound pressure level with increasing tilt angle, predominantly evident in both the high-amplitude tonal peaks and the broadband signal within the low-to-mid frequency range. The directivity patterns of the overall sound pressure level also demonstrated a reduction in magnitude with the tilting of the rotor. This reduction was observed consistently across all top-plane observation points, with a monopole directivity trend. Meanwhile, in the side plane array, the decrease was primarily evident above the plane of rotation with a dipole directivity pattern. The radiation direction at which the minimum sound pressure level occurs was identified near the rotation plane, irrespective of the tilt angle. Furthermore, the time-dependent noise analyses revealed dominant and persistent signal characteristics at the blade passing frequency, whereas the mid-frequency range exhibited more intermittent behaviour, and the high-frequency range indicated transient characteristics. • Experimental characterisation of tilt-rotor noise in edgewise flight. • Noise attenuation with rotor tilt in low-to-mid frequency bands is evident. • Visualised noise directivity on the rotor's top and side planes. • Persistent and transient characteristics observed in the time-dependent analysis. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive shunted electrodynamic metamaterial, for broadband robust vibration control. The study considers a unit cell of 12 miniature, low-cost proof-mass actuators for the control of vibration in a three degree-of-freedom structure subject to parametric uncertainty. In order to modify their dynamic responses, each actuator is connected to a shunt circuit consisting of a parallel resistor and a switched in/out inductor and capacitor. Provided the impedance of the actuator is cancelled out using a negative impedance, the shunt circuit is capable of tuning the resonance of the actuator up or down in frequency. An adaptive tuning approach is proposed, whereby the shunted actuator resonance frequencies are periodically switched to the centre frequencies of the highest magnitude bins of a real-time frequency analysis of the velocity measured on the structure. This approach is compared to a blind swept tuning method and a fixed-shunt tuning in terms of the robustness to parametric uncertainty, and in practical terms for realisation using analogue or digital shunt impedances. • An electrodynamic metamaterial is proposed. • The effect of a parallel RLC shunt is examined. • An adaptive tuning algorithm is proposed. • The adaptive approach is compared to a blind swept tuning approach. • The adaptive approach is shown to have greater robustness to structural uncertainties. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid wave and finite element/boundary element method for predicting the vibroacoustic characteristics of complex panels characterised by an irregular cross-sectional shape with a non-flat upper surface. In this approach, the panels are treated as waveguides and only a small segment of the panel is modelled using finite elements. The mass and stiffness matrices of the segment are then post-processed to project the motion onto its cross-sectional boundaries interfacing with the surrounding fluids. The acoustic pressures in the fluids are modelled by applying Fourier transforms to the Helmholtz boundary integral equations in the wavenumber domain. For each wavenumber component, the pressures acting on the segment are transformed into external nodal forces by evaluating the virtual work performed by the surface pressure on the segment. The wave and finite element formulations are subsequently coupled to the discretised boundary integral equations to determine the response of the panel to acoustic wave excitation. The hybrid method for predicting dispersion curves and sound transmission is validated through comparisons with predictions obtained from both an analytical model and a wave-based method found in the literature. To demonstrate the practical utility of the approach, the hybrid method is employed to predict sound transmission through a corrugated sandwich panel, where developing an analytical model is difficult. The method provides accurate predictions at a low computational cost. • Hybrid WFE and BE method is developed. • Address non-periodic structure and non-flat/non-cylindrical surfaces. • Application to corrugated sandwich panels with trapezoidal surfaces. • Accurately predict acoustic performance with low computational cost. [ABSTRACT FROM AUTHOR]

• Formulation of TMD-PSEH system attached to an undamped primary structure. • Extension of the fixed point theory to the TMD-PSEH system. • Close-form expression of optimal tuning, damping, and resistance ratios. • Insight into the power flow analysis. • Performance of the system along with attendant numerical simulations. A tuned mass damper with a piezoelectric stack energy harvester (TMD-PSEH) is a passive vibration control device for the dual purpose of absorbing vibration and harvesting energy. This paper develops the fixed point theory for the optimal design of a TMD-PSEH attached to an undamped primary structure under harmonic external excitation. It has been proven that the existence of two fixed points of the amplitude-frequency curve doest not depend on the damping of the primary structure. Based on the requirements for suppressing the vibration of the primary structure and enlarging the harvested power, the analytical expressions for the optimal tuning, damping, and resistance ratios are determined. It is found that the minimum damping ratio occurs in the vicinity of the electrical resistance ratio α = 1 for various values of the electromechanical coupling coefficient. Power flow analysis is carried out resulting in closed-form expressions of instantaneous and average powers. Numerical examinations of the system with the obtained optimal parameters are performed demonstrating a very good compatibility between theory and computation. In addition, the effect of damping in the primary structure is also investigated. It is shown that not only the vibration of the primary structure effectively reduces, but also a large amount of the harvested energy can be captured in the main resonance region. [ABSTRACT FROM AUTHOR]

Modal parameter estimation of operational structures is often a challenging task when confronted with unwanted distortions (outliers) in field measurements. Atypical observations present a problem to operational modal analysis (OMA) algorithms, such as stochastic subspace identification (SSI), severely biasing parameter estimates and resulting in misidentification of the system. Despite this predicament, no simple mechanism currently exists capable of dealing with such anomalies in SSI. Addressing this problem, this paper first introduces a novel probabilistic formulation of stochastic subspace identification (Prob-SSI), realised using probabilistic projections. Mathematically, the equivalence between this model and the classic algorithm is demonstrated. This fresh perspective, viewing SSI as a problem in probabilistic inference, lays the necessary mathematical foundation to enable a collection of new OMA approaches. To this end, a statistically robust SSI algorithm (robust Prob-SSI) is developed, capable of providing a principled and automatic way of handling outlying or anomalous data in the measured timeseries, such as may occur in field recordings, e.g. intermittent sensor dropout. Robust Prob-SSI is shown to outperform conventional SSI when confronted with 'corrupted' data, exhibiting improved identification performance and higher levels of confidence in the found poles when viewing stabilisation diagrams. Similar benefits are also demonstrated on the Z24 Bridge benchmark dataset, highlighting enhanced performance on measured systems. [Display omitted] • Novel probabilistic formulation of stochastic subspace identification. • Novel robust formulation of stochastic subspace identification. • Statistically robust to outliers in the time series responses. • Improvements to identification performance over Cov-SSI in such cases. • Demonstrated on Z24 data series. [ABSTRACT FROM AUTHOR]

A new experimental method is proposed to improve the force measurements during water impact experiments (e.g. water entry or wave impact tests) carried out with "pseudo-rigid" mock-ups. Despite the efforts of making the mock-up as stiff as possible, the impulsive nature of water impact loads may induce a transient response of the mock-up with a broad frequency content and a perturbation of the force measurements. Using the principles of momentum conservation, it can be shown that the load cell signal is the sum of the hydrodynamic forcing term and of an additional inertial term directly related to the vibrations of the structure. In the present paper, we suggest to estimate the inertial term using several accelerometers which record the response of the structure at different locations. Assuming that the structure response can be approximated by a set of natural modes, we show that it is possible to estimate the inertial term by a linear combination of the acceleration signals. The coefficients of the linear combination may be identified a priori by performing hammer tests, but in certain cases they can also be identified a posteriori using a segment of the signal time series. The performance of the method is demonstrated by considering two experimental test cases of increasing complexity. The first test case is a hydrofoil of constant section impacting water at constant speed which exhibits two-dimensional beam-like vibrations. The second test case is a segmented model of a vertical cylinder impacted by a breaking wave whose segments exhibit three-dimensional vibrations. The proposed method is efficient, conceptually simple and very simple to implement from a signal processing point of view, which makes it promising not only for water impact problems but also for other unstationary fluid–structure interaction problems. [Display omitted] • The response of the structure is recorded in order to reduce the oscillations induced by the vibrations on the force signal. • The response of the structure is approximated by its natural modes. • A set of accelerometers is used to estimate the second time-derivative of the modal coordinates. • The solid inertia contribution to the measured force is removed in order to find the hydrodynamic impact force. [ABSTRACT FROM AUTHOR]

Conventional tuned mass dampers are designed either for a local frequency response or for broadband applications. In either case, optimization of both the resonance amplitudes and the tuned frequency amplitude is not possible. This paper considers a novel friction damper, the tuned wedge damper, which utilizes piecewise-defined contact geometries to decouple vibration absorption from vibration damping. Consequently, a simultaneous optimization of the tuned frequency and the amplitude in the resonance regimes is possible. The damper is investigated via numerical and analytical methods. Experimental investigations validate the theoretical insights and offer the first proof of concept for the implementation of the tuned wedge damper. • Damper based on dry friction and piecewise-defined contact surfaces. • The decoupling of damping and absorption was achieved. • Damper optimization leads to low amplitude at resonance and at absorption frequency. • Numerical, analytical, and experimental analysis. • Theoretical behavior was verified by experiments. [ABSTRACT FROM AUTHOR]

• A method for interpolating the acoustic transfer function with sparse equivalent sources is presented. • An optimization reproduction model is proposed to find the sparse loudspeaker weights. • The effects of typical parameters of the model on the reproduction performance are analyzed. • Simulation and experimentation results validate the effectiveness of the reproduction method. Multizone sound field reproduction aims to create different acoustic environments in multiple spatial zones, allowing listeners to enjoy their individual sounds without being disturbed by sound from other zones. The conventional pressure matching method is used in practice to reproduce the desired sound field by minimizing the sound field error with a limited number of control points, whose reproduction performance decreases above the cutoff frequency. In this paper, a multizone sound field reproduction method based on a sparse equivalent source method is proposed to reproduce the sound field in the whole target region. The acoustic transfer function of each loudspeaker and the desired sound field are represented by the sparse equivalent sources. The whole target region is controlled by interpolating fine virtual control points within the region of the uncontrolled points, and the acoustic transfer functions between the virtual control points and the loudspeaker array are interpolated by the sparse equivalent sources. An optimization reproduction model with the objective of reproducing a desired sound field in the bright zone and constraining the acoustic energy in the dark zone is formulated to find the sparse loudspeaker weights. The simulation results and experimental results show that the proposed method achieves better performance than the conventional pressure matching method in free field and reverberant environments. [ABSTRACT FROM AUTHOR]

• Frequency assignment of an assembled structure via links between substructures. • Implementation on a test structure comprising two substructures and six links. • Discussion of challenges, e.g., rotational receptance estimation and model build-up. • Determination of links with the right cross sections as an optimisation solution. Structural modifications for assigning desirable natural frequencies and modes are an effective passive vibration control strategy. In many assembled structures, the substructures, which may come from different manufacturers/suppliers or have intended individual functions, do not allow modifications or are difficult to modify. In such common situations, designing proper links or joints used for assembling the substructures will be an effective solution to ensure that the assembled structure possesses the right modal properties. In this paper, a receptance-based frequency assignment approach is presented, which consists of a theoretical method of frequency assignment suitable for assembled structures, an indirect method for estimating rotational recpetances and a substructuring method. It is implemented on an assembled structure, which is a simplified model of a floating raft platform installed on a ship hull. The model comprises two substructures and six links. A number of experiments are carried out to measure the required receptances systematically, including rotational receptances which are measured with the help of an auxiliary structure and two rotational accelerometers. One and two frequencies are successfully assigned by using the links with the right cross-sections determined by solving an optimisation problem. Key challenges are also discussed. This work should be practically very useful in passive vibration control of assembled structures. [ABSTRACT FROM AUTHOR]

Acoustic Black Holes (ABH) embedded within structures can efficiently damp vibrations above the cut-on frequency but fails below it, particularly when the flexural wavelength exceeds the size of the ABH. Therefore, addressing low-frequency vibration mitigation becomes imperative. Periodic ABH structures and shunt damping have been employed individually and proved effective, yet their combined application has not been explored previously. This paper introduces a novel approach for broadband vibration mitigation. It combines a periodic ABH beam, damping layer, and a resonant element comprising a piezoelectric patch and an inductance–resistance shunting circuit (referred as lossy PZT-ABH metabeam). This design addresses the shortcomings in the attenuation performance within the pass bands of conventional periodic structures and allows for easy adjustment of the attenuated bands without the need for physical structural modification. To characterize this system, a semi-analytical model of the electromechanical problem is developed using the Rayleigh–Ritz method, and the complex dispersion curve is obtained to evaluate the mitigation performance. The obtained results are validated against Finite Element Method (FEM) simulations, demonstrating that the lossy PZT-ABH metabeam achieves broadband vibration mitigation covering the entire frequency spectrum with remarkable flexibility. To facilitate designs, a detailed analysis is conducted on the length and attached position of the piezoelectric patch, thickness of the damping layer and the value of inductance and resistance. Furthermore, a thorough investigation into the mechanism of electromechanical resonance (ER) is carried out to deepen our understanding of the ER attenuated band. • Achievement of broadband vibration mitigation covering the entire frequency range. • Semi-analytical model of Gaussian expansion method for the multi-physical problem. • Merging periodic ABH beam, damping layer and piezoelectric resonant element. • Flexibility for changing local attenuated band at arbitrary frequencies. [ABSTRACT FROM AUTHOR]

Due to harsh working conditions, rotating blades are prone to failures. Thus, it is urgently needed to monitor blade health. Blade tip timing (BTT) is a promising non-contact vibration technique for rotating blades owing to its high efficiency and long service time. However, due to severe undersampling, traditional spectrum analysis methods are ineffective in identifying characteristic parameters and recovering the power spectrum of vibrations for condition monitoring. To address this issue, this paper proposes a compressed covariance sensing-based BTT (CCS-BTT) method for power spectrum estimation. Specifically, we build a compressed covariance sensing representation model for BTT signal, where the covariance representation can be non-underdetermined owing to the Hermitian Toeplitz structure of the covariance matrix. Then, a universal covariance sampling pattern (UCSP) is derived to ensure the complete covariance information can be efficiently recovered without sparsity constraint. By utilizing the inherent periodicity, the derived UCSP significantly improves the efficiency of BTT measurement. Moreover, for practicality, we present a series of optimized layouts based on UCSP that cover almost all possible cases of probe numbers in BTT measurement. Finally, extensive simulations and experiments validate the effectiveness and performance of CCS-BTT. By shifting the perspective from the signal itself to its covariance, CCS-BTT opens many avenues for the development of BTT signal processing. In addition, the proposed method is expected to be used in deterministic compressive sampling of wide-sense stationary signals. • A compressed covariance representation for blade tip timing (BTT) signal is built. • A universal covariance sampling pattern is derived to ensure recoverability. • We improve the BTT measurement efficiency by utilizing the inherent periodicity. • The power spectrum of BTT signal can be estimated without sparsity constraint. • We provide a series of practical layouts that cover almost all cases in BTT. [ABSTRACT FROM AUTHOR]

• A novel beam-spring 2-DOF magnetically coupled bistable energy harvester is proposed. • The operating bandwidth is improved by approximately 60 % compared with a CBEH. • The subharmonic responses are characterized by the coexistence of two coupling modes. • Reasonable spring stiffness design is helpful to further enhance the bandwidth. • The excitation threshold peak effect of the 2-DOF coupled system is studied. Introducing the magnetic coupling effect with multiple degrees of freedom to a conventional bistable energy harvester is helpful to further enhance its operating bandwidth. However, it is not straightforward to design due to its extremely complicated dynamic characteristics, meanwhile, some underlying mechanisms also need to be explored ulteriorly. This paper proposes a novel beam-spring two-degree-of-freedom magnetically coupled bistable energy harvester, combining the advantages of multistable and multimodal harvesters. Based on the theoretical model validated by experiments, the complex nonlinear dynamic behaviors of the system are investigated in detail, as well as the effects of key parameters on power generation capacity. It is found that the working bandwidth is improved by more than 60 % because of the additional contribution from the 2nd resonance region. And there are abundant interwell ultraharmonic and subharmonic responses characterized by the coexistence of out-of-phase and in-phase coupling modes. Under appropriate spring stiffness or certain initial conditions, the snap-through motion can be successfully triggered to enter a high-energy orbit. The parameter study shows that the magnet distance and spring stiffness significantly affect the effective bandwidth. Furthermore, the excitation threshold peak effect that hinders the connection of the two resonant frequency bands is also observed. Fortunately, it can be eliminated by the reasonable design of the 1st and 2nd natural frequencies to achieve a consecutive and broad effective bandwidth. This research provides a theoretical case for designing and developing wide-band and low-threshold bistable energy harvester. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel nonlinear system identification method is proposed based on OKID. • Nonlinear coefficients can be estimated without system order determination. • Combined with ERA, nature frequencies of the system can be identified accurately. In this paper, a novel time-domain method for identifying nonlinear structural dynamical system with explicit model is proposed. By exploiting the observer/Kalman filter identification (OKID) and nonlinear feedback interpretation, the nonlinear parameters of the system are estimated accurately. The effectiveness of the proposed nonlinear OKID (NOKID) method is demonstrated through three numerical examples, namely a five degree-of-freedom system with one nonlinear spring, a three degree-of-freedom system with three nonlinear springs and a nonlinear rectangular membrane structure with all edges clamped. Comparing with the time-domain nonlinear subspace identification (TNSI) method, the proposed NOKID method can identify the nonlinear parameters without system order determination, and the identification results obtained by the proposed method are real numbers and have no dependency on frequency. [ABSTRACT FROM AUTHOR]

This paper develops an asymptotic method to predict the dispersion curves of a weakly disordered medium. The disordered coefficients of the original acoustic equation are related to periodic coefficients through a change of variables, assumed asymptotically close to the identity. At leading order in the amplitude of the disorder, the medium is driven by an acoustic equation with periodic coefficients, which can be analyzed through a classical Floquet–Bloch approach. At first order, with simple eigenvalues, a simple post-processing of the periodic eigenvalues and eigenfunctions of the leading order allows to account for the weakly disordered character of the original coefficients. In the case of repeated eigenvalues (for instance, at Dirac points), a residual is introduced, whose minimization allows to recover the opening of the band gaps with the loss of periodicity. Examples in 1D and 2D illustrate the validity of our asymptotic approach, by comparing its results with eigenmodes and dispersion curves computed over much larger periods for reference. • Dispersion curves and wave modes in weakly non-periodic media are analyzed. • An asymptotic method to predict them is developed. • Opening of the band gaps generated by the loss of periodicity is well reproduced. • Only cell-scale solutions of the periodic medium are needed. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to active chatter control in milling operations using a new concept called the virtual inerter-based dynamic vibration absorber (VIDVA). While passive control methods, such as tuned mass damper (TMDs), have their merits, they may not provide optimal performance and adaptability in certain scenarios. Moreover, the realisation of an idealised inerter-based absorber as a localisation addition can be a difficult task to achieve. In response of these challenges, the integration of the inerter concept into virtual passive absorber (VPA) control to improve chatter stability performance is proposed. Four IDVAs are numerically evaluated to enhance the absolute chatter stability limit, and the numerical results are experimentally validated using cutting tests with a proof-mass actuator providing the control force. The study also includes robustness and actuator saturation analysis to provide a comprehensive evaluation of the proposed virtual IDVA. The findings demonstrate that the virtual IDVA offers improved chatter suppression performance, making it a promising solution for active chatter control and application of IDVAs in milling operations. • A virtual passive inerter-based absorber is proposed for chatter suppression. • A robustness analysis is presented, applicable also passive control methods. • Actuator saturation is evaluated for chatter suppression. • Inerter-based absorbers are more preferable than TMDs in virtual active controls. [ABSTRACT FROM AUTHOR]

This paper develops a novel low-frequency vibration isolation device based on rigid body dynamics called a Rigid Elastic Vibration Isolator (REVI). The REVI model is realized by coupling four elastic beams with two rigid bodies monolithically made using 3D printing. The system operates based on the vibration of the intermediate rigid bodies, which leads to the anti-resonance phenomenon at low frequencies. The dynamic analysis of the proposed REVI system has been meticulously investigated through analytical methods and real-world experimentation. The analytical method uses the Spectral element method to obtain the dynamic response, which is also validated by the experimental findings. Furthermore, the REVI transmittance sensitivity analysis was conducted by adjusting the rigid mass and system load and exploring methods for generating wide low-frequency bandgaps. The inclination angle of the REVI system is also varied, revealing the bandgap characteristics regarding negative transmittance level. The parametric study varying the geometric properties of the REVI system enhances our understanding of the bandgap and attenuation characteristics within the attenuation band. The REVI mechanism is practical and easily implemented, allowing for accurate and repeatable modeling. Moreover, the analytical observations assist in refining the shape of the REVI mechanism to achieve the necessary bandgap for the desired transmittance. • A novel vibration isolator is fabricated using additive manufacturing technology. • The analytical model is developed with spectral element method and rigid body dynamics. • Analytical and experimental studies are done to calculate transmittance parameters. • The rationale behind the anti-resonance peak in transmittance is observed. • Variation of transmittance is observed by modulating rigid mass and system's load. [ABSTRACT FROM AUTHOR]

• A comprehensive stiffness model of intermediate bearing is presented. • A reduced model of dual rotor-bearing-casing coupling system is developed. • The effects of typical parameters on the vibration of the system are discussed. • The mechanism of the comprehensive stiffness and system vibration is revealed. This paper focuses on the vibrational properties of a dual rotor-casing coupled system with nonlinear clearance in the intermediate bearing, considering both the oil film stiffness and Hertzian contact stiffness between rolling elements and raceways. A comprehensive stiffness model for the intermediate bearing has been developed based on elastohydrodynamic lubrication (EHL) and Hertz contact theory. Subsequently, a dynamical model of the dual rotor-bearing-casing system is established using the finite element method, thereby verifying the feasibility of simulating the casing with beam elements. The dimensionality of the system equation is reduced using the fixed-interface component mode synthesis (CMS) method, and the resulting reduced-dimensionality equation is solved using the improved Newmark- β method. The effects of casing support, rotational speed, and radial clearance on the vibration characteristics of the system are investigated. Then, the comprehensive stiffness and coupled vibration characteristics of the system are analyzed to elucidate its intrinsic mechanism. Finally, the contact characteristics within the intermediate bearing under dynamic load are also studied. The results indicate that the support stiffness of the casing has a more significant impact on the second resonance caused by both the inner and outer rotors. The system is prone to experiencing varying compliance (VC) resonance at low rotational speeds. The excessive bearing clearance leads to complex vibrational characteristics of the system, making it challenging to predict its motion. The impact of minor changes in stiffness on system vibration is small when the comprehensive stiffness is large. At this time, the limited effect of oil film stiffness allows it to be disregarded in terms of comprehensive stiffness. Furthermore, the speed significantly influences the contact characteristics within the intermediate bearing under the unbalanced force of the rotor and gravity. [ABSTRACT FROM AUTHOR]

Duffing oscillators are increasingly explored for vibration and noise reduction, owing to their wideband properties and rich dynamic behavior. Applications involving piezoelectric treatments typically take advantage from using negative capacitances, as their effect is similar to enhanced electromechanical coupling. This paper integrates negative capacitance to a recently introduced synthetic impedance-based Duffing-type inductive shunt for frequency bandwidth enhancement. The frequency-domain numerical and experimental results show that, as the negative-to-piezoelectric capacitance ratio of the fully digital nonlinear shunt circuit approximates the stability limit, the in-phase and the out-of-phase modes of the Duffing-type shunt become increasingly separated from each other, which enables high-energy branches that are wider than the case without negative capacitance. Under the maximum capacitance ratio attained for two different setups, bandwidth enhancements around 65%–75% have been numerically predicted and experimentally confirmed. This nonlinear piezoelectric shunt with enhanced response is promising for low-level, wideband noise and vibration applications requiring precise parameter tuning, where the sort of input excitation can trigger its broadband, steady-state dynamic responses. • A fully-digital, Duffing-type piezoelectric shunt with enhanced bandwidth response is presented. • The fully digital nonlinear shunt circuit features a negative capacitor. • Bandwidth enlargements above 60% have been numerically and experimentally confirmed. [ABSTRACT FROM AUTHOR]

In earthquake-prone areas, seismic isolation systems are designed to preserve structural safety and prevent occupants' injury and properties' damage. However, seismically isolated structures can be subject to large displacements relative to the ground under earthquakes, which lead to define two problems, the first problem concerns to excessive displacements, whereas the second problem relates to excessive accelerations that occur when the displacements are limited by an obstacle with high relative stiffness. An arrangement that favors the solution of these problems is an appropriate interposition between the isolated structure and the obstacle of the shock absorbers or bumpers. The aim of this paper is to investigate, via numerical parametric analyses, the possibility to exploit the occurrence of impact in a two-sided vibro-impact isolated single-degree-of-freedom (SDOF) system under harmonic base excitation with beneficial effects. Through the analysis of the response scenarios which can occur varying the obstacles' parameters, that is the width of the gap and the mechanical (stiffness and damping) properties, the target is to reduce both the maximum value of the displacement and of the acceleration of the mass, compared to the free flight condition, without possibly reducing the vibration isolation frequency range. This research path leads to the definition of vibro-impacting control systems and to the free and constrained single and multi-objective optimal design of the control system consisting of the integrated assembly of the isolation system, bumpers and gaps. The main results obtained consist in having (i) found a unique optimality condition, (ii) calculated the dissipated energy and given an energetic interpretation to the optimality condition (iii) built maps, that allow to advance behavior predictions, provide indications on the design and allow to make evaluations of the performance of the vibro-impacting system as a function of the stiffness and damping parameters and of the width of the gap. The stiffness ratio and the damping ratio, which define the mechanical properties of the bumpers, are such that the dimensionless relaxation time is about 1, (iv) finally the results are compared to multi-objective optimal design. Ultimately, the advantages of this new isolation system are: (1) provide a physical limit switch to the isolation system in order to limit the maximum displacement of the mass; (2) reduce the effects of amplification dynamics in the resonance zone; (3) reduce the static displacement, all without possibly modifying the dynamic response in the isolation zone. • A new Vibro-Impact Control System integrating isolation, bumper and gap is defined. • Properly selecting parameters allows to beneficially exploit the occurrence of impact. • The study of the response scenarios is functional to identify optimal parameters. • The minimum acceleration criterion is proposed to select optimal parameters. [ABSTRACT FROM AUTHOR]

• A novel method (JSTB) was proposed to control flow-induced noise. • The control effect of JSTB on turbulent fluctuating pressure was studied. • The mechanism of JSTB in reducing flow-induced noise was revealed. • Noise reduction effects of JSTB were proved through simulation and experiment. At medium to high speeds, the flow-induced structural vibration noise generated by structural vibration under the excitation of the turbulent fluctuating pressure can seriously damage the acoustic performance of underwater vehicles. In this paper, we have proposed a new and noiseless method, called junction suction and trailing edge blowing (JSTB), to reduce the flow-induced structural vibration noise of the wing–plate model. We have investigated the effects of JSTB on the flow and sound fields through a combination of numerical simulations and experimental tests. The experiments are conducted in a low-noise gravity water tunnel. Numerical results show that JSTB significantly reduces the peak values of the internal self-noise at 70 Hz, 180 Hz, 225 Hz, and 265 Hz when the velocity is 6.87 m s − 1 . The model radiated noise reduction is more prominent in the 20–1000 Hz frequency range, with an optimal noise reduction level of 15.4 dB. The simulation results agree well with the experimental results. JSTB can effectively reduce the internal sound pressure level, turbulent fluctuating pressure, and radiated sound power of the model. This work provides a novel solution for the flow-induced structural vibration noise control of underwater vehicles. [ABSTRACT FROM AUTHOR]

Structural damage detection using wavelet transform is effective if a suitable wavelet function is selected. However, the selection of the appropriate wavelet function can be a challenging task. This paper introduces a novel and efficient criterion for selecting the proper wavelet function. It is emphasized that there is a relationship between the accuracy of damage detection using wavelet detail coefficients and the correctness of the approximation of the original function. Accordingly, we define a ratio function and then a novel wavelet selection criterion called WSC in this study. After applying the proposed WSC criterion for 128 wavelet functions, it is found that the wavelet functions selected via the proposed criterion (both in numerical and experimental scenarios) can detect the position of damages in the beam structure with high accuracy. [ABSTRACT FROM AUTHOR]

• Harnessing quasi-zero-stiffness properties and time-delayed control for vibration isolation. • Innovative introduction of the "retarded (delayed)" term as a substitute for the derivative term. • Comprehensive tuning strategies for impact, harmonic, and random excitations. This paper explores the control design and performance analysis of an active quasi-zero-stiffness vibration isolator utilizing the proportional-retarded (PR) control architecture. The PR architecture replaces the conventional "derivative" term in the proportional-derivative (or displacement-velocity) controller with the novel "retarded (time-delayed)" term, which is a new and effective way of active vibration suppression. Comprehensive gain-delay conjoint regulation strategies are developed for the strengthened isolation performance. For impact excitations, a complete stability analysis is conducted near the equilibrium position. Furthermore, a parameterized dominant pole placement algorithm is proposed to optimize the transient response of the PR-actuated isolator. Focusing on the rectangular pulse and long-duration step impacts, the algorithm adjusts the dominant poles to optimize the transient performance metrics, including the maximum displacement overshoot, steady-state displacement overshoot, and settling time. Regarding harmonic excitations, the delay-coupled effect is scrutinized for better low-frequency isolation performance, including the resonance frequency, nonlinear steady-state amplitude, multiple steady-state bands and nonlinearity elimination, equivalent active damping and stiffness, and absolute displacement transmissibility. In the case of random excitations, we demonstrate the superiority of the PR structure over the passive system. [ABSTRACT FROM AUTHOR]

• A novel model-free finite frequency band H ∞ controller is proposed for turning chatter suppression. • Users can specify frequency band for optimization according to actual requirements. • An iterative algorithm is used without a mathematical model of controlled target. • An off-policy Q -learning method is utilized to minimize manual parameter tuning. Regenerative chatter is an extremely harmful phenomenon in machining, which generally results in overcut and rapid tool wear. This paper presents the design, analysis, and verification of a novel model-free finite frequency band (MFFFB) H ∞ control algorithm, which is dedicated to chatter suppression of turning process. The proposed MFFFB H ∞ controller is established by utilizing the reinforcement Q -learning along with the generalized Kalman-Yakubovich-Popov (GKYP) lemma. Unlike available entire frequency domain (EFD) H ∞ controllers, the proposed approach facilitates a less conservative implementation since the H ∞ performance index is only designed for the concerned frequency band which implies the limited energies of the actuator are mainly focused on this band, and users can specify optimized frequency band according to the actual turning conditions. Furthermore, it permits the construction of H ∞ controller without any knowledge of the dynamic model of turning process, only the data measured from the system state and input are required. Besides, an iterative solution algorithm for H ∞ control on the basis of policy iteration is presented, which decreases the burden on manual parameter adjustment and capable of achieving the optimal solution. Simulation and experiment results demonstrate that the proposed MFFFB H ∞ controller generates significant improvement in chatter-free region compared with EFD H ∞ controller and conventional FFB H ∞ controller. [ABSTRACT FROM AUTHOR]

• A hybrid multi-scale/arbitrary Lagrangian-Eulerian finite element method is proposed. • The novel method is used for predicting nonlinear structural-acoustic responses. • The novel method offers valuable insights into structural-acoustic interaction mechanisms. This paper introduces a novel hybrid multi-scale/arbitrary Lagrangian-Eulerian finite element method (HMS/ALE-FEM) for addressing the nonlinear vibro-acoustic problem of a large-deformed beam in an infinite fluid. In the HMS/ALE-FEM, the vibrational response of the beam is tackled through modal superposition and a temporal multi-scale approach, while the acoustic wave emitted from the beam is addressed using the arbitrary Lagrangian-Eulerian finite element method (ALE-FEM). An alternating frequency/time domain technique is employed to handle the displacement and velocity of the moving mesh and the acoustic pressure on the beam surface. To validate the HMS/ALE-FEM, it is compared against the finite element method for beam response and the ALE-FEM for acoustic response, serving as a reference method. Taking the nonlinear vibro-acoustic problem of a buckled beam with 2:1 internal resonance as an example, the results of the HMS/ALE-FEM are compared with those of the reference method. The results show that the HMS/ALE-FEM is in good agreement with the reference method under different harmonic excitation amplitudes and frequencies. Due to the 2:1 internal resonance, double modes can be generated, and the second mode amplitudes of both beam displacement and acoustic pressure remain constant as the excitation amplitude varies. Notably, the HMS/ALE-FEM provides direct access to mode amplitude and phase information for beam displacement and acoustic pressure on the beam surface, offering valuable insights into fluid-structure interaction mechanisms in both single- and double-mode scenarios. [ABSTRACT FROM AUTHOR]

• A general expression for the ratio between amplitudes of the centrifugal inertia force and gravity force is obtained and used to determine inertia-force effect on the contact forces • It is demonstrated that, regardless of the contact formulation used (constraint or elastic), sum of the constraint and applied forces always has zero component along Frenet bi-normal vector. • Closed-form analytical solution of simple planar example is obtained to show the significant effect of the centrifugal inertia force on the constraint forces. • It is shown that the instantaneous motion plane and instantaneous zero-force axis orientations for non-centroidal trajectories depend on an inertia-force vector that does not appear in rigid-body Newton-Euler equations. • The paper demonstrates the effect of the prescribed kinematics, particularly the forward motion, on the inertia and contact forces highlighting issues related to using roller rigs in measuring wheel/rail contact forces In vehicle-motion scenarios, non-negligible centrifugal-force component that cannot be balanced by the gravity force may develop, leading to increase in wheel-contact forces. This inertia-force component, which depends on vehicle forward velocity and oscillation amplitude, can arise when the vehicle negotiates straight road or railroad track. An approach based on the Frenet geometry and recorded motion trajectory (RMT) is introduced in this investigation to examine forward-kinematics effect on the motion geometry and inertia forces and to obtain general expression of the ratio between the amplitudes of the centrifugal and gravity forces. While the two three-dimensional vectors of applied and constraint forces define Frenet osculating plane of mass-center trajectories; for non-centroidal points, the osculating plane is defined by two vectors; the resultant of the applied and constraint forces and an inertia-force vector that depends on the angular velocity and angular acceleration. The concepts of instantaneous motion plane (IMP) and instantaneous zero-force axis (IZFA) are used to convert three-dimensional Cartesian forces into two-dimensional Frenet forces, based on the Frenet-Newton equations. Because contact forces can be formulated using two fundamentally different approaches; constraint contact formulation (CCF) and elastic contact formulation (ECF), the effect of the contact formulation on the IMP and IZFA orientation is examined. The results obtained, using railroad suspended wheelset example, demonstrate the following: (1) limitations of roller rigs that do not properly account for forward-velocity effect on the inertia forces; (2) difference between IMP and intermediate wheelset plane defined by wheelset lateral axis and global vertical axis; (3) non-negligible inertia centrifugal-force component in the absence of physical curvature; and (4) need to distinguish between physical and motion geometries; the latter is one that directly enters into the definition of the inertia forces. Trajectories with oscillation frequency independent of the oscillation amplitude for a given forward velocity are also identified. [ABSTRACT FROM AUTHOR]

Linearising the dynamics of nonlinear mechanical systems is an important and open research area. A common approach is feedback linearisation, which is a nonlinear control method that transforms the input–output response of a nonlinear system into an equivalent linear one. The main problem with feedback linearisation is that it requires an accurate first-principles model of the system, which are typically hard to obtain. In this paper, we design an alternative control approach that exploits data-driven models to linearise the input–output response of nonlinear mechanical systems. Specifically, a model-based reference tracking architecture is developed for nonlinear feedback systems with output nonlinearities. The overall methodology shows a high degree of performance combined with significant robustness against imperfect modelling and extrapolation. These findings are demonstrated using large set of synthetic experiments conducted on a asymmetric Duffing oscillator and using an experimental prototype of a high-precision motion system. • Novel control method that linearises input–output dynamics of mechanical systems. • Nonlinear data-driven state-space models are used; linear part is preserved. • Nonlinear distortion analysis quantifies efficacy over frequency range of interest. • Simulations and experiments show high performance and robustness properties. [ABSTRACT FROM AUTHOR]

Uncontrolled beams and plates often perform poorly as sound radiating structures because of their resonant modes, creating a frequency response with sharp peaks and nulls as well as rapid changes in acoustic directionality. An active control technique known as Traveling Wave Control (TWC) has been previously demonstrated to perform well at forcing beams and plates to vibrate without modes, as though they were infinitely large, producing an improved acoustic response. This paper is specifically concerned with evaluating the interaction of piezoelectric actuators with simulated TWC systems and comparing the vibratory and acoustic responses as well as the derived filters to those of ideal point source actuators. Simulations demonstrate that piezoelectric actuators act in a very similar manner to point source actuators on simulated beams and plates, and that the bandwidth of control is proportional to the inverse distance between secondary actuators and edges. An experiment demonstrates that the TWC system implemented on a beam with both moving-coil and piezoelectric actuators is able to mitigate the modal response considerably. • TWC filters for piezoelectric actuators on beams are similar to those for point source actuators. • Control bandwidth for TWC systems on beams is proportional to secondary actuator placement. • Control bandwidth for TWC systems on plates is proportional to the number of TWC module pairs. • Experiments achieve similar results with both point-source and piezoelectric actuators. [ABSTRACT FROM AUTHOR]

The Equivalent Source Method (ESM) consists of modeling the acoustical field radiated or scattered from an object by a set of radiating monopole sources located below its surface. While highly efficient for modeling the scattered field by a smooth object, ESM becomes less effective in representing acoustic diffraction from sharp edges. In this article, ESM is understood as an approximation to the single layer integral equation. In this framework, ESM belongs to the family of Indirect Boundary Element Methods (BEM) with an implicit surface mesh slightly below the object surface, and a zero order integration scheme on each element of the mesh. This implicit mesh is well approximated by the Voronoi diagram associated to the source distribution. This formalism establishes a strong link between these two widely used methods (ESM and BEM) and allows the construction of hybrid methods that can combine performances of both methods. On the first hand, a new hybrid method able to model edge diffraction while remaining as computationally efficient as ESM is developed. In this method, labeled "BEM Voronoi", equivalent sources are positioned on the surface's object rather than below it and singularities are handled with the integral formulation. On the other hand, a set of new hybrid methods which combines ESM with higher order integration scheme is constructed. Performances of the new methods developed in this paper are numerically compared to each other and to Finite Element simulations (used as a reference) for modeling scattering from an object excited by a plane wave. Each hybrid model is tested on multiple configurations involving two sets of boundary conditions (rigid and elastic objects respectively) successively immersed in two different external environments (water and air) and for both smooth and a sharp-edged objects. The results show that the hybrid method "BEM Voronoi" better handles edge diffraction than conventional ESM, at a much lower computational cost (implementation of BEM Voronoi is 50 times faster than conventional BEM implementation). • Edge diffraction cannot be modeled by equivalent sources inside the object. • Using sources at the object's surface resolves the issue but introduces singularities. • Integral formulation on a Voronoi diagram solves the singularities. • This method is an efficient zero-order functional Boundary Element Method. • Equivalent sources and Boundary Element Method are equivalents. [ABSTRACT FROM AUTHOR]