Reconfigurable Design Of Broadband Metasurfaces And Continuously Tunable Control Of Reflected Acoustic Waves

As a low-dimensional family of acoustic metamaterials,acoustic metasurfaces(AMs)are composed of discrete artificial micro-units arranged in a certain order.They make it possible to effectively regulate the propagation and attenuation of sound waves and achieve a variety of novel acoustic functions with the total thickness in subwavelength.Therefore,AMs have shown the promising applications in the spatial sound field reconstruction and low frequency noise reduction.However,the conventional AMs and their units are particularly predefined by the geometric structure and cannot be changed once the devices were manufactured.This results in a fixed operating frequency and a single acoustic function,or a narrow band near the targeted frequency.Besides,in the small frequency range,the corresponding performance of the metasurfaces will be weakened by deviating from the targeted frequency.Unit design and structural assembly must be redone if the operating frequency or acoustic function is to be altered.In order to break through the above limitations,the present thesis performs the structural designs of several continuously reconfigurable AM units based on the matched helical mechanism,and then assembles them into two-dimensional(2D)flat or curved forms and three-dimensional(3D)circular or square arrays to realize broadband tunability and multifunctionality.The main contents and achievements include:(1).A continuously reconfigurable AM unit is designed by rotating a matched screw into a helical channel from bottom to top.The finite element numerical model and equivalent theoretical model of the helical unit are established,and then the circular samples are fabricated.The broadband tunability and multifunctionality of a flat AM structure are discussed by full-wave numerical simulations and experimental measurements.The analysis of the system sensitivity to the inevitable tuning error is presented.The influence of geometric parameters on the phase response is discussed.The results show that the phase shift can cover the complete 2? range by changing the helical depth.The numerical,theoretical and experimental results of the phase response are in good agreement.A sample of the flat metasurface is manufactured to demonstrate the continuously tunable multifunction,including anomalous reflection,arbitrary focusing and self-bending beams.(2).The above reconfigurable helical units are applied to design a curved AM with the continuous tunable property.The phase distribution functions for realizing carpet cloak and ground illusion are derived by using an arc-shaped AM.The numerical simulations and experimental measurements are presented to demonstrate the performance of the cloaking and illusion.A 3D cloak of a hemispherical shape is designed.A conceptual design of self-feedback automatic control for a programmable AM are proposed.The results show that the arc-shaped AM can achieve an excellent effect to restore the disturbed reflective field or to mimic an arbitrarily shaped ground by adjusting the phase delay or compensation.It can also be able to operate for a wide-angle detection with oblique incidence.The cloak and illusion functions at different frequencies can be achieved in simulation and experiment by configuring one tunable curved structure.The influence of the thermoviscous loss on the helical unit and the tunable acoustic performance can be ignored.The broadband real-time adjustable operation of cloak and illusion can be realized by the programmable conceptual design.(3).A continuously tunable AM vortex generator is designed by rotating 24 matched screws into the corresponding helical channels with the compact unit arrangement.The generated acoustic vortices propagating stably in a circular waveguide system with a certain radius are analyzed theoretically.The theoretical and numerical models of the three row units in the radial direction are constructed.Numerical simulations and experimental measurements of acoustic vortex fields are carried out.In addition,the effect of thermal viscosity is also discussed.The results show that the proposed helical units have the capability of continuously broadband phase modulation.The limiting frequencies corresponding to multiple order vortex modes are obtained.Acoustic vortices carrying different orders of the orbital angular momentum at different frequencies and the multiple vortex modes at a single frequency can be realized by the reconfiguration of the tunable AM.Especially,compared to the vortices in an infinite free space,acoustic vortices generated in a waveguide system with a suitable wave number might be more beneficial to the remote signal propagation.(4).A tunable lossy AM without the foamed materials is designed by using a two-parameter reconfigurable unit,which is composed of a grating channel and an internal absorber.The physical mechanism of the independent amplitude and phase modulations in a simple lossy unit is analyzed.By selecting different geometrical parameters,the reflected amplitude and phase response of the tunable unit are measured experimentally.The broadband tunability and multifunctionality of a 3D lossy AM structure are discussed.The results show that the scattering-free anomalous refection,the multi-plane acoustic hologram and the broadband holographic image are demonstrated to perform the excellent tunable multifunction based on the independent amplitude and phase modulations.By quantitatively evaluating the image quality of each complex image at different frequencies,it is illustrated that the designed AM can exhibit the high-fidelity hologram effect in broadband.The conceptual design of an AM holography system with programmable automatic control function provides an enlightening way for the broadband real-time adjustable operation of arbitrary acoustic holographic images.Therefore,the present research on the reconfigurable design of broadband AMs and their continuously tunable control of the reflected acoustic waves extends the application fields of traditional metasurface structures.It provides an important theoretical guidance for the fine modulation of spatial acoustic field and the design basis for the performance improvement of new acoustic functional devices.