1.National Defense Key Laboratory of Underwater Acoustic Technology, Harbin Engineering University, Harbin 150001, China 2.Key Laboratory of Marine Information Acquisition and Security Ministry of Industry and Information, Harbin Enhineering University, Harbin 150001, China 3.College of Underwater Acoustic Engineering, Harbin Engineering University, Harbin 150001, China
Abstract:Inhomogeneity in a medium will cause wave scattering, influencing the transfer of energy or information. However, it is possible to prepare a prescribed wavefront which propagates through an inhomogeneous medium with unity flux-transmittance. This phenomenon is first predicted in the context of mesoscopic electron transport. Another remarkable phenomenon is the bimodal distribution of the transmission singular values, which implies that in a lossless medium the full solution space in the scattering region can be spanned only by open channels, which are completely transmitted, and closed channels, which are completely reflected. In mesoscopic physics, random-matrix theory is usually utilized to deal with the statistical properties of matrices with randomly distributed entries since the medium is assumed to be randomly fluctuating. In this paper, we propose a method of systematically studying the maximal flux transmission through an inhomogeneous acoustic waveguide. The model is chosen to be a waveguide with varying cross-sections and a penetrable scatterer, and the method is based on the coupled mode theory. This method can be used to analyze the frequency of nearly complete transmission for an arbitrary incident wave, and to analyze the incident wave that is able to generate the maximal flux-transmittance for any given frequency. We construct the transmission matrix and the horizontal wavenumber matrix by using orthonormal basis functions, and give the expression of flux-transmittance. Then the optimal incident wave which brings the maximal transmittance through the scattering region is derived based on singular value decomposition. The optimal incident waves are independent of the evanescent modes since evanescent modes do not transfer any energy. But the evanescent modes can give rise to the multivaluedness of wave solutions with complete flux transmission. Considering the fact that acoustic waveguides can naturally resist the influence of highly oscillating perturbations since most of them correspond to evanescent modes), the maximal flux transmission in waveguide is thus found to be highly robust. Especially at a specific frequency, the complete wave transmission has perfect robustness. This proposed method can be generalized to any other frequency, to other types of scatterers, or to other kinds of boundary conditions, and can provide guidance in designing acoustic metamaterials and in highly efficient communication. Keywords:inhomogeneous waveguides/ maximal flux transmission/ complete wave transmission/ optimal incident wave
其中向量${\boldsymbol{p}}$和${\boldsymbol{s}}$中元素分别为${p_n}\left( x \right)$和${s_n}\left( x \right)$, 附加量${s_n}\left( x \right)$与${p_n}\left( x \right)$满足关系:
(7)式中${A_{mn}}$与${B_{mn}}$分别为矩阵${\boldsymbol{A}}$与${\boldsymbol{B}}$中的元素, 假设散射体的上、下边界参数分别为$[ y = \beta \left( x \right),\; $$ y = \alpha \left( x \right)]$, ${A_{mn}}$与${B_{mn}}$的表达式为[36]
${E_{{\rm{ft}}}} = \frac{1}{{2\omega {\rho _0}}}{\boldsymbol{p}}_{\rm{t}}^{\rm{H}}\left( L \right){\rm{Re}} \left( {{\boldsymbol{K}}\left( L \right)} \right){{\boldsymbol{p}}_{\rm{t}}}\left( L \right).$
使用相同的波导结构, 本文选取图2中平面波能流透射率较小的频率$k = 1.45{\text{π}} $, 研究能流的最大透射率、对应的最佳入射波以及对应声场. 结果如图5所示. 图5(a)给出选定频率时, 矩阵${\boldsymbol{M}}\left( L \right){\boldsymbol{T}}{{\boldsymbol{M}}^{ - 1}}\left( 0 \right)$的奇异值平方分布. 在当前频率下, ${N_{\rm{p}}}\left( 0 \right) = {N_{\rm{p}}}\left( L \right) = 2$, 只存在两阶可传播模态, 图5(a)表现出了典型的双峰分布[5,6], 即奇异值平方非零即1. 根据前述分析可知, 奇异值为1代表对应入射波可实现能流的全透射, 奇异值为0代表实现能流的零透射(这里等价于全反射). 图5(a)表明, 与光学或介观物理中的高频情况条件对比, 即使对于仅有两阶可传播模态的低频条件, 波在非均匀波导中依然有存在全透射的可能性. 需要注意的是, 对于不同的波导结构, 奇异值可能表现出多种的分布特性, 不局限于双峰分布现象. 图5(b)画出产生能流最大透射的最佳入射波幅值分布, 图5(c)给出最佳入射波的模态展开系数, 其中${v_{T1 n}}$为${{\boldsymbol{M}}^{ - 1}}\left( 0 \right){{\boldsymbol{v}}_1}$的前${N_{\rm{p}}}\left( 0 \right)$个元素. 可以看出, 最佳入射波由可传播的第零阶模态(平面波)和第一阶模态共同决定. 图5(d)画出最佳入射波产生的声场, 此时声波近乎实现能流的全透射, 声场亦近似表现出关于$x = {L / 2}$的轴对称分布. 另外, 对于本文中所使用的波导结构, 当频率选取在$k \in \left( {0, {\text{π}} } \right)$范围时, 波导内仅存在一阶可传播模态, 即平面波. 此时最大声透射问题与平面波透射问题等价, 平面波的能流透射率即为能流最大透射率, 因此${G_{\max }}\left( k \right)$曲线与图2中的$G\left( k \right)$在$k \in \left( {0, {\text{π}} } \right)$区间内完全重合. 图 5 (a) 奇异值平方分布; (b) 最佳入射波幅值分布; (c) 最佳入射波的模态展开系数; (d) 产生能流最大透射的声场. 波导参数与图2中使用的一致, 频率$k = 1.45{\text{π}} $ Figure5. (a) Distribution of squares of singular values; (b) modulus of the optimal incident wave; (c) expansion coefficients of the optimal incident wave; (d) wave field with the maximum energy flux transmittance. The geometry of the waveguide is same as that in Fig. 2, and the frequency is $k = 1.45{\text{π}} $.
4.衰逝模态的影响及最大声透射的鲁棒性根据(14)式可知, 衰逝模态不传播能量, 所以不影响能流的大小. 但是衰逝模态会影响波导中的声场结果, 尤其是水平变化区域的近场结果[44]. 本节将考虑衰逝模态对最大声透射的影响并分析最大声透射的鲁棒性. 图6(a)给出当图3(a)中入射平面波叠加衰逝模态时产生的声场, 图6(b)给出当最佳入射波(图5(b))叠加衰逝模态时产生的声场. 图6(a)中的入射波为 图 6 (a) 图3(a)情况下平面波叠加衰逝模态后的声场; (b) 图5(d)情况下最佳入射波叠加衰逝模态后的声场 Figure6. (a) Wave field generated by a plane wave mixed by evanescent modes in the case of Fig. 3(a); (b) wave field generated by the optimal incident wave mixed by evanescent modes in the case of Fig. 5(d).