Vibration Reduction by a Partitioned Dynamic Vibration Absorber with Acoustic Black Hole Features

Vibration quality is a vital indicator for assessing the progress of modern equipment.The dynamic vibration absorber(DVA)based on the acoustic black hole(ABH)feature is a new passive control method that manipulates waves.It offers efficient energy focalization and broad-spectrum vibration suppression,making it highly promising for applications in large equipment such as aircraft,trains,and ships.Despite previous advancements in ABH-DVA development,certain challenges remain,particularly in ensuring effective coupling with host structures during control.To address these issues,this study proposes a partitioned ABH-featured dynamic vibration absorber(PABH-DVA)with partitions in the radial direction of the disc.By employing a plate as the host structure,simulations and experiments were conducted,demonstrating that the PABH-DVA outperforms the original symmetric ABH-DVA in terms of damping performance.The study also calculated and compared the coupling coefficients of the two ABH-DVAs to uncover the mechanism behind the enhanced damping.Simulation results revealed that the PABH-DVA exhibits more coupled modes,occasionally with lower coupling coefficients than the symmetric ABH-DVA.The influence of frequency ratio and modal mass was further analyzed to explain the reasons behind the PABH-DVA's superior damping performance.Additionally,the study discussed the impact of the number of slits and their orientation.This research further explains the coupling mechanism between the ABH-DVA and the controlled structure,and provides new ideas for the further application of ABH in engineering.

Regular black holes with improved energy conditions and their analogues in fluids

On the premise of the importance of energy conditions for regular black holes,we propose a method to remedy those models that break the dominant energy condition,e.g.,the Bardeen and Hayward black holes.We modify the metrics but ensure their regularity at the same time,so that the weak,null,and dominant energy conditions are satisfied,with the exception of the strong energy condition.Likewise,we prove a no-go theorem for conformally related regular black holes,which states that the four energy conditions can never be met in this class of black holes.In order to seek evidences for distinguishing regular black holes from singular black holes,we resort to analogue gravity and regard it as a tool to mimic realistic regular black holes in a fluid.The equations of state for the fluid are solved via an asymptotic analysis associated with a numerical method,which provides a modus operandi for experimental observations,in particular,the conditions under which one can simulate realistic regular black holes in the fluid.

Statistical Entropy of an Acoustic Black Hole in Bose–Einstein Condensates

The entanglement entropy of an acoustic black hole in a Bose-Einstein condensates （BEC） is derived, which is associated with the phonons generated via the Hawking mechanism in a sonic hole. Considering the dispersion relation of a BEC, we recalculate the entanglement entropy of the acoustic black hole by means of statistical method in two limits. We find that the entropy is still proportional to the area of event horizon, but with a coefficient dependent on the infrared cutoff.

内嵌异质声学黑洞阵列板结构的阻尼演化机制研究

声学黑洞是一种有效的被动减振结构.这种结构的特点在于可以俘获结构中的弯曲波形成高能量密度区域,从而通过附加阻尼材料来实现高效率的振动能量耗散.基于这种声学黑洞效应,本文通过有限元仿真与实验探究了单个声学黑洞与声学黑洞阵列的动力学性能.研究发现,声学黑洞阵列可以有效地提高整体板结构的模态密度和模态损耗因子,从而在较宽的频率范围下实现更好的减振性能.为了进一步揭示声学黑洞结构的阻尼损耗机理,建立了声学黑洞与阻尼材料的耦合模型并进行了参数化讨论.结果表明,系统的模态损耗因子不仅与阻尼材料的性能有关,而且还取决于阻尼材料所覆盖的结构模态动能的大小.同时,模态密度则由结构的几何尺寸决定.此外,本文对声学黑洞阵列中不同单元的作用也进行了探讨,并分析了不同类型声学黑洞阵列结构的动力学性能.结果表明,异质声学黑洞阵列比传统的均匀阵列在宽频范围内具有更稳定的动力学性能.这些研究有利于声学黑洞阵列在减振降噪方面的实际工程应用.

Sound radiation attenuation by circular total-reflection elastic metasurface composed of subunits with cubic profiles

Utilizing metamaterials or acoustic black holes(ABHs)to control wave propagation and then to realize vibration control and sound radiation attenuation is a hot topic in recent years.However,using elastic metasurfaces that possess similar wave manipulation abilities with metamaterials and ABHs to attenuate sound radiation has not been reported yet.In this paper,a circular total-reflection elastic metasurface(CTREM)composed of subunits with cubic profiles similar with ABHs is proposed to realize vibration isolation and achieve broadband sound radiation attenuation of a plate below the cut-on frequency of the ABH.Compared with the corresponding bare plate and the plate containing a single ABH with a conventional design,the sound radiation efficiencies of the CTREM plate within and outside the vibration isolation band are both substantially attenuated.This phenomenon can be attributed to two distinct mechanisms:the total reflection of flexural waves caused by vibration isolation,and the local resonances of subunits.Analyses of the wavenumber spectra obtained from normal vibration velocities of the CTREM plate,both experimentally and numerically,along with the supersonic intensity patterns,reveal that the confined vibration energies are subsonic components localized within ineffective sound radiation areas.This,in turn,reduces the coupling strength of sound and vibration,thereby significantly attenuating sound radiation efficiency.The proposed CTREM provides a lossless and lightweight method for sound radiation attenuation.

Improving the performance of structure-embedded acoustic lenses via gradient-index local inhomogeneities

We investigate the use of graded inhomogeneities in order to enhance the focusing and collimation performance of structure-embedded acoustic metamaterial lenses.The type of inhomogeneity exploited in this study consists in axial symmetric exponential-like gradients of either material or geometric properties that create gradient-index inclusions able to bend and redirect propagating waves.In particular,we exploit the concept of gradient index inclusions to achieve focusing and collimation of ultrasonic beams created by embedded drop-channel lenses in both bulk and thin-walled structures.In the latter,the implementation is possible thanks to geometric exponential tapers known as Acoustic Black Holes(ABH).ABH tapers allow accurate control of the characteristics of the acoustic beam emanating from the lens channel which in the conventional design is severely affected by diffraction.The concept of beam control via graded inclusions is numerically illustrated and validated by using a combination of methodologies including geometric acoustics,finite difference time domain,and finite element methods.