Low-frequency mode conversion of elastic waves enabled by coiling-up structures

Elastic wave mode conversion is a fundamental mechanism in wave manipulation. A key challenge is to achieve efficient longitudinal-to-transverse mode conversion at low frequencies. In this work, we numerically demonstrate that a pair of coiling-up structures embedded in a solid medium can induce nearly complete mode conversion at extremely low frequencies, where the wavelength is more than 50 times the structural period. Parametric studies and modeling reveal that the outermost opening of the coiling-up structure plays a dominant role in determining the conversion frequency. This structural detail has been largely overlooked in previous studies. A simplified Helmholtz resonator model is introduced to explain the observed effect. The proposed design offers a geometrically induced resonant yet structurally compact approach for low-frequency elastic wave control.