As an emerging material, metasurfaces open the door to a wide range of applications based on ultrasonic and elastic waves. However, most of the existing elastic metasurfaces have a fixed functionality and operate at a fixed frequency, which make them difficult to adapt to changing working requirements and/or environments. Although several recently proposed reconfigurable designs can relax this limitation, they either need laborious tuning or complex active control systems. In this work, by encoding multiple functionalities onto a single metasurface through operation frequencies, passive metasurfaces with switchable multifunctionalities are proposed and realized. Different wave manipulation functions of a single metasurface can be switched from one to another simply by changing the operation frequency. The design of these metasurfaces is realized using a systematic design approach based on topology optimization. Frequency-coded multiple elastic wave manipulation functions, including wave beam steering and focusing, are numerically and experimentally demonstrated. Good agreement between the numerical simulations and experimental measurements is achieved. The proposed design strategy significantly enhances the functionalities and adaptivity of metasurfaces, moving them closer towards real-world applications.
Purpose -The purpose of this paper is to develop an easy-to-implement and accurate fast boundary element method (BEM) for solving large-scale elastodynamic problems in frequency and time domains. Design/methodology/approach -A newly developed kernel-independent fast multipole method (KIFMM) is applied to accelerating the evaluation of displacements, strains and stresses in frequency domain elastodynamic BEM analysis, in which the far-field interactions are evaluated efficiently utilizing equivalent densities and check potentials. Although there are six boundary integrals with unique kernel functions, by using the elastic theory, the authors managed to accelerate these six boundary integrals by KIFMM with the same kind of equivalent densities and check potentials. The boundary integral equations are discretized by Nyström method with curved quadratic elements. The method is further used to conduct the time-domain analysis by using the frequency-domain approach. Findings -Numerical results show that by the fast BEM, high accuracy can be achieved and the computational complexity is brought down to linear. The performance of the present method is further demonstrated by large-scale simulations with more than two millions of unknowns in the frequency domain and one million of unknowns in the time domain. Besides, the method is applied to the topological derivatives for solving elastodynamic inverse problems. Originality/value -An efficient KIFMM is implemented in the acceleration of the elastodynamic BEM. Combining with the Nyström discretization based on quadratic elements and the frequencydomain approach, an accurate and highly efficient fast BEM is achieved for large-scale elastodynamic frequency domain analysis and time-domain analysis.
The frequency-domain approach (FDA) to transient analysis of the boundary element method, although is appealing for engineering applications, is computationally expensive. This paper proposes a novel adaptive frequency sampling (AFS) algorithm to reduce the computational time of the FDA by effectively reducing the number N c of sampling frequencies. The AFS starts with a few initial frequencies and automatically determines the subsequent sampling frequencies. It can reduce N c by more than 2 times while still preserving good accuracy. In a porous solid model with around 0.3 million unknowns, 4 times reduction of N c and the total computational time is successfully achieved.
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