The measurement of dynamic strain and resonant frequency for three-dimensional solids partially immersed in water using free-edge bonded fiber Bragg grating sensors

In this work, the feasibility of liquid level detection based on structural vibration modes is investigated. We specifically consider impact-induced vibration modes of an aluminum solid partially immersed in a water tank. A free-edge bonded fiber Bragg grating (FBG) sensor and a corresponding high-speed FBG filter-based demodulation system are employed to measure the transient impact responses. To perform more accurate in vacuo finite element analysis (i.e., for the mode shapes, resonant frequencies, and the impact responses), elastic constants of the aluminum solid and impact loading history of a steel ball are obtained beforehand respectively according to the elastic wave theory and with a polyvinylidene fluoride (PVDF)-based technique. Our experimental results show good correspondences of intensities and shifts of the impact-induced resonant modes with liquid levels. The modes of the same type (e.g., double modes in out-of-plane or in-plane dominated modes) are shown to possess similar behaviors in relationships between shifts of the resonant modes and the liquid level. This work also provides related experimental evidence for future researchers interesting in simulation of complicated fluid-structure interaction (FSI) problems for three-dimensional structures.