| Research of vibration and noise suppression is of great significance in aeronautic and astronautic structures for various purposes,e.g.preventing fatal accidents,improving the reliability of aerocraft,improving ride comfort and prolonging the service life of aerocraft.The fundamental nature of all vibration and acoustic phenomena in engineering structures,such as aircraft or spacecraft,is wave motion.Wave manipulation,such as change of propagation path,focusing,trapping and damping,is the foundation for vibration and noise suppression in these structures.Acoustic Black Hole(ABH)effect is a new concept for wave manipulation taking advantages of the propagation properties of the structure-borne waves in thin-walled structures.By reducing the wall thickness,the local phase(and the group)velocity of the flexural waves gradually reduces,resulting in energy accumulation and producing local area with high energy density.Efficient energy dissipation then can be achieved with a small amount of damping materials.Therefore,ABH structures offer great potentials for vibration and noise suppression in aerocraft.Research works in this thesis attempts to provide solutions to the crucial issues of wave manipulation by ABH structures,e.g.understanding the wave trapping process and characteristics of wave evolution in ABHs,providing assessment method of wave focalization,tracking and predicting wave propagation path,providing optimization design method and proposing novel ABH designs for improving ABH effect.From theoretical modelling,numerical simulation to experimental validation,the ABH effect has been thoroughly investigated.The main contributions and innovations of this thesis include:1.The general imperfect ABH thickness profile is proposed since the geometrical flaws are inevitable due to the limitation of machining capability.A general description of geometrical parameters of both ideal scenarios and imperfections is presented.It overcomes the problem of weak structural strength and guarantees the surface integrity.The new profile also enables a drastic increase of the energy density around the tapered area.However,the energy focalization phenomena are different from those of conventional ABH structures.The energy focalization point is offset from,and downstream of the indentation center,depending on the structural geometry.Features of wave focalization by the new indentation profile have been studied through numerical simulations in time domain.The influence of the structural parameters on ABH effect and on the focalization positions of the bending wave has also been analyzed.2.A quantitative assessment method and experimental verification of the wave focalization effect of flexural wave are proposed.Insight on energy focalization in ABH indentations is quantitatively analyzed by structural power flow.Numerical calculations on the structural power flow allow clear visualization of the energy transport and focalization of flexural wave in the vicinity of the indentation area.Wave focalization spot is defined and the focalization range in the indentation is demarcated.Through the integration of the power flow at different cross sections and the focalization spot,wave focalization effect is quantitatively evaluated.Furthermore,the phenomenon of flexural wave focalization is verified by experiments using laser ultrasonic scanning technique.3.Numerical integration model for solving flexural wave trajectory is proposed.Based on the eikonal equation from the Geometric Acoustics Approximation(GAA),the calculation of the flexural ray trajectories is solved by Taylor series expansion to reveal and analyze the wave propagation features in ABH indentations.The accuracy of the model is verified through comparison with experimental results and numerical power flow vector.The wave propagation path and wave focalization in imperfect two-dimensional ABHs are successfully tracked and predicted.The wave trajectory model greatly improves the efficiency of analyzing wave propagation characteristics in ABHs using the finite element method and experiments.Its flexibility makes it possible to be utilized in other medium with gradient refractive index.4.A general design methodology of Acoustic Black Hole(ABH)structures is proposed.Both the geometric parameters of the ABH profile and the topology of the damping layer coated on the ABH are simultaneously optimized.The influence of coupling between the host ABH plate and the damping layer on the energy focalization and dissipation effect is considered in the optimization process.Optimal solutions of the ABH design for achieving maximum energy dissipation is obtained.It reveals the underlying physical mechanisms of energy dissipation in the optimization evolution by the ABH structure.It also evokes particular attention to the design of imperfect ABHs,in which the energy focalization phenomenon is different from traditional ABHs and the energy dissipation is sensitive to both the layout of the damping layer and the geometric profile of the ABH.5.A novel design and implementation of a new configuration of ABH structures is proposed,by functionally grading the taper associated with acoustic black holes(FG-ABHs).With the ability to 3D print abundant and various digital materials,specimens of FG-ABHs are manufactured by an Objet Connex 500 printer.The Young’s modulus of the materials as well as thickness of the functionally graded taper gradually diminishes from the uniform part to the tip.This arrangement improves the energy focalization effect of ABH.In the meantime,efficient energy dissipation is realized due to the viscoelastic effect of the printed materials.Both numerical calculations and experimental studies are conducted to investigate the wave propagation and reflection properties of the presented ABH structures.The reflection coefficient based on the wave extraction and separation is determined allowing the evaluation of the reflection effect of the various ABH beams.The FG-ABHs enhance the ABH effect,and superior performance of low reflection is obtained without any additional layer. |
PDF Full Text Request