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In semi-active synchronized switch damping (SSD) approaches for structural vibration control, the damping effect is achieved by properly switching the voltage on the piezoelectric actuators. Unsymmetrical SSD switch circuit has been designed in the previous paper to increase the effective voltage range on the PZT actuator for improvement of the control performance. In this study, analysis and experimental validation of control performance of a synchronized switch damping system based on the unsymmetrical switch circuit are carried out. First the model of an unsymmetrical SSD system is presented and the working principle is introduced. The general expression of the switched voltage on the piezoelectric actuator is derived. Based on its periodicity in steady-state control, the harmonic components of the actuator voltage are derived using Fourier series expansion. Next, the displacement response of the system is derived under combined actions of the excitation and switched voltage. Finally, a setup of a flexible beam with unsymmetrical switch circuit is used to demonstrate the control performance under different voltage sources and to verify the theoretical results. The results show that the control performance mainly depends on the voltage range on the PZT. A higher effective voltage range can be generated in unsymmetrical SSDV than in symmetrical SSDV and better control performance can be achieved at the same negative actuator voltage. The unsymmetrical SSDV makes better utilization of the actuator capability. [ABSTRACT FROM AUTHOR]

Abstract: The synchronized switch damping (SSD) approaches based on piezoelectric actuators have been successfully used in multimode vibration control of structures. By suppressing voltage inversion at some displacement extrema, the control performance of SSD approaches can be significantly improved. In this study, conversion of energy from mechanical to electrical in two-mode control is analyzed for different amplitude ratios and frequency ratios. The results show that the converted energy in two-mode control with voltage inversion at every extremum is smaller than the sum of the energies of the two modes in single-mode control, but the converted energy of the second mode in two-mode control may exceed that in single-mode control when the amplitude ratio of the second mode to the first mode is small enough. The influence of voltage threshold used to suppress voltage inversion on energy conversion of the system and each individual mode is also investigated. The converted energy of the first mode in two-mode control may be much larger than that in single-mode control when voltage inversion is suppressed suitably. The analytical results can successfully explain the phenomenon in the previous control experiments and can also be extended to multimode control. [Copyright &y& Elsevier]

Abstract: In this paper, a new switch control strategy based on an energy threshold is proposed for the synchronized switch damping techniques in multimode control. This strategy is derived from the total converted energy of a synchronized switch damping (SSD) system in a given time window. Using the new strategy the voltage is inverted only at those extrema where the effective distance, which is proportional to the converted energy between two neighboring extrema, exceeds the threshold. The new switch control strategy is used in both the synchronized switch damping on inductor (SSDI) technique and the synchronized switch damping on voltage source (SSDV) technique, which are applied to the two-mode control of a composite beam. Their control performances are compared with those of the single-mode control and those of classical SSDI and SSDV techniques in two-mode control. The experimental results show that voltage inversion is prevented at some of the displacement extrema to increase the total converted energy, and exhibit better global damping effect than classical SSDI and classical SSDV, respectively. In single mode, the best control performance is achieved when the voltage is inverted at every extremum. But in multimodal control, the total converted energy in a given time window is increased and the control performance is improved when some extrema are skipped. [Copyright &y& Elsevier]

Acoustic black hole (ABH) structures have been exploited to manipulate the flexural wave propagation with results showing a great potential for structural vibration damping enhancement and suppression of free-field acoustic noise radiation. In the present paper, ABHs are used to reduce the noise inside a cavity bounded by a flexible plate with multiple two-dimensional (2-D) ABH indentations. The interior sound field is generated by and fully coupled to the vibration of the flexible plate subject to a point force excitation. A refined numerical finite element model considering the plate-cavity coupling is established and validated by experiments. Both the simulation and experimental results show a significant noise reduction inside the cavity in a relatively wide frequency range through embedding the 2-D ABHs into the flexible plate. Analyses on the underlying mechanisms show a dual physical process of the ABH effects: the first being the direct consequence of the vibration reduction of the plate as a result of ABH-induced damping enhancement, whilst the second one being caused by a reduction in the coupling strength between the plate and the cavity. This ABH-specific decoupling phenomenon is characterized by the spatial coupling coefficients, which depend on the degree of morphological matching between structural modes and acoustic modes over the plate-cavity interface. The reported phenomenon of the impaired structural-acoustic coupling reveals a new ABH-specific feature which enriches the existing knowledge on ABH structures. [ABSTRACT FROM AUTHOR]

An Acoustic Black Hole (ABH) indentation embedded in thin-walled structures has been proved remarkably useful for broadband flexural wave focalization, in which the phase velocity of the flexural waves and the refractive index of the media undergo gradual changes from the outside towards the center of the indentation. A generalized two-dimensional ABH indentation can be defined by three geometric parameters: a power index, an extra thickness and a radius of a plateau at the indentation center. The dependence of the energy focalization on these parameters as well as the energy focalization process is of paramount importance for the understanding and design of effective ABH indentations. This work aims at investigating the energy focalization characteristics of flexural waves in such generalized ABH indentations. The calculation of the flexural ray trajectories is conducted to reveal and analyze the wave propagation features through numerical integration of the eikonal equation from the Geometric Acoustics Approximation (GAA). The theoretical results are verified by both experiment using wave visualization technique based on laser acoustic scanning method and finite element (FE) simulations. Finally, the influence of the geometric parameters on the flexural wave focalization characteristics in ABH indentations is discussed in detail. [ABSTRACT FROM AUTHOR]

Semi-passive vibration control with the Synchronized Switching Damping on Inductor (SSDI) technique has attracted significant attention in recent years. However, the damping performance of the SSDI technique will be decreased by the lag in detection of switching time. In order to detect switching time without lag, the paper proposes a self-powered semi-passive vibration damping system based on self-sensing approach. In the proposed technique, the switching control signal is obtained by properly processing the voltage of one piezoelectric element which is also used for vibration damping at the same time. Compared with other self-powered SSDI techniques, the proposed approach can detect switching time without lag, while keeping simplicity of implementation with a small number of piezoelectric elements. The proposed technique is also experimentally implemented by a self-powered circuit in the paper. The theoretical analysis, the experimental validation and the drawback of the proposed technique are discussed in the paper. [ABSTRACT FROM AUTHOR]

• A new method to suppress the panel flutter by using add-on acoustic black hole (AABH). • Numerical simulations investigate the interaction between the panel and supersonic flow. • The energy flow between panel and AABH are calculated. • AABH can increase the critical flutter velocity and reduce flutter amplitude validly. • AABH can transfer and dissipate energy from the supersonic flow effectively. Panel flutter is a critical challenge for supersonic aircraft, impacting their structural integrity and safety. This study introduces a new method to suppress panel flutter in supersonic flow using an add-on acoustic black hole (AABH). Numerical simulations investigate the interaction between the panel and supersonic flow, comparing the panel only, panel with AABH, and panel with equivalent mass. Results show that the AABH increases the critical flutter velocity by 33.9%, outperforming the equivalent mass configuration (18.7%). The AABH acts as a linear energy sink with high damping and rich modal characteristics, transferring and dissipating energy from the supersonic flow effectively. This capability stabilizes the panel and reduces flutter amplitude, demonstrating the AABH's potential for supersonic flutter suppression. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical and experimental investigation on active flutter suppression of a swept-back cantilevered lifting surface using piezoelectric (PZT) actuation. A finite element method, a panel aerodynamic method, and the minimum state–space realization are involved in the development of the equation of motion in state–space, which is efficiently used for the analysis of the system and design of control laws with a modern control framework. PZT actuators, bonded symmetrically on the plate, are optimally grouped into two equivalent actuator sets using genetic algorithms to enhance controllability. H 2- and μ-synthesized control laws are designed and the flutter suppression performance is evaluated via wind tunnel testing. In the μ-synthesis design, a simple parametric uncertainty model is used to take into account the system changes with respect to airflow speed. Both controllers show comparable flutter suppression performance around the flutter point. However, the μ-synthesized controller shows improved behavior over a wide flow speed range. [Copyright &y& Elsevier]

Abstract The acoustic black hole (ABH) effect produces drastic wave compression and energy trapping in thin-walled structures. Recent research has shown that the reflected wave in laboratory ABH structures can be reduced by attaching a thin absorbing layer in specific wave concentration areas. In this paper, a novel design and implementation of one-dimensional (1D) functionally graded acoustic black hole (FG-ABH) is presented, as an alternative to achieve low reflection effects. Two configurations of the FG-ABHs are demonstrated, i.e., an axially graded ABH and a thickness graded ABH. The FG-ABH beams are manufactured by 3D printing technology using an Objet Connex 500 printer. Materials with varying levels of viscoelastic characteristics are produced using two base materials called TangoPlus and VeroWhitePlus. The FG-ABH beams have both diminishing thickness and elastic modulus in the tip of the ABH wedge. Numerical investigation of wave propagation, attenuation and reflection are conducted utilizing a viscoelastic code: University of Michigan's Local Interaction Simulation Approach (UM/LISA). The damping effect of the materials is included based on Kelvin-Voigt viscoelastic theory. Finally, the process of wave propagation in FG-ABHs, traditional ABH and uniform beam are experimentally investigated using a Scanning Laser Doppler Vibrometer (SLDV) system. The reflection coefficients, i.e. the ratios between the amplitude of reflected wave and incident wave, are computed. Results indicate that the FG-ABHs enhance the ABH effect, and lower reflection is observed when compared to a traditional ABH structure. [ABSTRACT FROM AUTHOR]

• The bandgap of a periodic ABH strip is dominated by two flexural wave modes. • There are two types of bandgaps with one satisfying Bragg condition. • The mechanism of the attenuation band splitting is clarified. • The method of tuning attenuation band splitting is investigated. Acoustic black holes (ABH) lattice is an effective way to realize the bandgaps (BGs) through lightweight design. The predicted BGs using infinitely large periodic structures are commonly believed to enable corresponding attenuation bands (ABs) even the structure only contains a limited number of unit cells. This paper reports an unusual splitting phenomenon in the ABs of a finite plate strip with two-dimensional (2D) ABHs. A wide AB predicted by BG is split into two ABs in the finite strip separated by a high resonance peak, which challenges the aforementioned common belief. Results show that the increasing number of periodic unit cells has a negligible effect on the frequency and amplitude of the resonance peak. Its underlying mechanism is clarified by investigating the relationship between the BGs and ABs, obtained from infinite and finite periodic plate strips with the same 2D ABHs respectively. Complex wavenumber analysis results show the existence of two flexural wave modes in the considered frequency range due to the finite width of the strip. Two types of BGs, in which one satisfies the Bragg condition, are found when both wave modes fail to propagate. Analysis of structural modes further uncover the physical reason for the emerging AB splitting: a certain natural frequency of the boundary portion of the 2D ABHs strip falling into the theoretical BG. Due to the boundary reflection and energy concentration, this structural portion can loosely be regarded as a clamped-free plate strip. It is demonstrated that the resonance peak splitting the AB becomes tunable via disrupting the structural details of finite ABH strips. In particular, extending the terminal end of a finite ABH strip allows the change in boundary portion, which is conducive to eliminating the splitting peak. The reported study is helpful for the design of finite periodic structures with broadband attenuation bandgap. [ABSTRACT FROM AUTHOR]

Acoustics Black Hole (ABH) effect shows promising features for potential vibration control and energy harvesting applications. The phenomenon occurs in a structure with diminishing thickness which gradually reduces the phase velocity of flexural waves. The coupling between the tailored ABH structure and the damping layer used to compensate for the adverse effect of the unavoidable truncation is critical and has not been well apprehended by the existing models. This paper presents a semi-analytical model to analyze an Euler-Bernoulli beam with embedded ABH feature and its full coupling with the damping layers coated over its surface. By decomposing the transverse displacement field of the beam over the basis of a set of Mexican hat wavelets, the extremalization of the Hamiltonian via Lagrange׳s equation yields a set of linear equations, which can be solved for structural responses. Highly consistent with the FEM and experimental results, numerical simulations demonstrate that the proposed wavelet-based model is particularly suitable to characterize the ABH-induced drastic wavelength fluctuation phenomenon. The ABH feature as well as the effect of the wedge truncation and that of the damping layers on the vibration response of the beam is analyzed. It is shown that the mass of the damping layers needs particular attention when their thickness is comparable to that of the ABH wedge around the tip area. Due to its modular and energy-based feature, the proposed framework offers a general platform allowing embodiment of other control or energy harvesting elements into the model to guide ABH structural design for various applications. [ABSTRACT FROM AUTHOR]

• A novel synchronized switch damping technique based on energy injection strategy is proposed. • The injection energy can be easily adjusted through changing the duty cycle of the corresponding switch control signal. • Low voltage source might lead to high piezoelectric driven voltage in the proposed technique. • Piezoelectric structural damping characteristic can be effectively enhanced or timely adjusted. • The stability problem of the controlled system can be easily solved and implemented. Synchronized switch damping (SSD) principle and derived techniques are a classical semi-active vibration control method based on piezoelectric elements. This paper presents a novel energy injection based SSD technique which has better effect of vibration reduction but with simpler implementation of electronic circuitry. The proposed circuit is composed of a flyback transformer, an ordinary low voltage source and two kinds of synchronized switches. Through controlling the synchronized switches, the electrical energy is extracted from the voltage source to the transformer, and then injected to the piezoelectric elements, increasing the piezoelectric voltage as needed. Moreover, this injection energy can be easily adjusted by changing the duty cycle of the corresponding switch control signal. The operation principle of this technique is introduced, the analytical expression of vibration attenuations is also derived for a typical electromechanical model. Finally, theoretical predictions confirmed by experimental results show that the proposed SSD technique provides an adaptive structural damping and enhances the effect of vibration attenuation. [ABSTRACT FROM AUTHOR]

As a passive damping technique for vibration and noise mitigation, acoustic black hole (ABH) structures have been drawing an increasing attention because of their easy-to-realize and broadband wave focusing and energy dissipation characteristics. Structures with embedded ABHs, however, inevitably compromise the overall structural stiffness and strength, which hampers their use as critical structural components. As an alternative, this paper proposes a new type of device, i.e. a two-dimensional circular ABH-based dynamic vibration absorber (2D ABH-DVA), as an auxiliary component to be added to an existing structure for vibration suppressions. Using a plate as benchmark structure, finite element (FE) simulation results show a systematic reduction of its resonant peaks over a broad frequency range upon the deployment of the ABH-DVA. Analyses uncover two underlying mechanisms which dominate the physical process: dynamic interaction with the host structure and damping enhancement as a result of ABH-specific energy trapping. This is warranted by an effective dynamic coupling between the primary structure and the add-on ABH-DVA, which can be quantified by a mode-specific and location-dependent coupling coefficient defined in the paper. It is further demonstrated that, despite the rich modal contents of the ABH-DVA, strong coupling with the primary structure only takes place through a few DVA modes. Analyses also lead to a simple linear relationship relating the overall system damping with the properties of the damping material over the ABH-DVA. Finally, the broadband vibration suppression ability of the proposed 2D ABH-DVA is verified through experiments. The study demonstrates the unique coupling features between the DVA and the host structure, which provides design guidelines for unsymmetrical 2D or other ABH-DVA designs in the future. [ABSTRACT FROM AUTHOR]

Acoustic black hole (ABH) effect in thin-walled structures provides drastic wave focalization. In a practical ABH structure with a tapering thickness profile, the concentrated wave energy can be efficiently dissipated by introducing a damping layer to the local high energy density area. In this paper, simultaneous optimization of the geometric parameters of the ABH profile and the topology of the damping layer covering the ABH area is investigated based on the former study of wave focalization. To optimize the ABH structure for achieving maximum energy dissipation, parametric modelling is implemented, and the dissipated energy is computed by the commercial finite element software Abaqus. The optimal geometric parameters of the ABH thickness profile are obtained by the method of moving asymptotes (MMA). The topological optimization problem of the damping layer is solved by using optimality criteria (OC). These two algorithms are nested alternatively in the main optimization procedure. The influence of mesh size and direction of wave incidence on the optimization is discussed. Numerical results show that the proposed approach is efficient in finding the optimal design for the ABH structure to achieve maximum wave energy dissipation. In addition, the influence of coupling between the host ABH plate and the layer on the optimization is investigated. Superior performance of energy dissipation is achieved by the proposed method comparing to the results before the optimization. [ABSTRACT FROM AUTHOR]