In this paper, we study flexural vibrations of two thin beams that are coupled through an otherwise quiescent viscous fluid. While most of the research has focused on isolated beams immersed in placid fluids, inertial and viscous hydrodynamic coupling is ubiquitous across a multitude of engineering and natural systems comprising arrays of flexible structures. In these cases, the distributed hydrodynamic loading experienced by each oscillating structure is not only related to its absolute motion but is also influenced by its relative motion with respect to the neighbouring structures. Here, we focus on linear vibrations of two identical beams for low Knudsen, Keulegan-Carpenter and squeeze numbers. Thus, we describe the fluid flow using unsteady Stokes hydrodynamics and we propose a boundary integral formulation to compute pertinent hydrodynamic functions to study the fluid effect. We validate the proposed theoretical approach through experiments on centimetre-size compliant cantilevers that are subjected to underwater base-excitation. We consider different geometric arrangements, beam interdistances and excitation frequencies to ascertain the model accuracy in terms of the relevant nondimensional parameters.

In this paper, we investigate underwater energy harvesting of a parallel array of nominally identical ionic polymer metal composites (IPMCs) subjected to low frequency base excitation in water. The IPMCs are connected in parallel and shunted with a varying resistor. We model the IPMCs as slender beams with uniform cross section undergoing small oscillations in an otherwise quiescent viscous fluid. We utilize a boundary element approach to compute the hydrodynamic loading on each structure, which is due to the oscillations of the whole array. Leveraging recent findings on sensing in ionic polymer metal composites, we propose a coupled electromechanical model for predicting energy harvesting as a function of the IPMCs' impedance and the base excitation. To validate our theoretical predictions, we perform experiments on an in-house-fabricated array of five centimeter-size composites, which we characterize on a dedicated test rig. We experimentally determine the power harvested by varying the excitation frequency in the broad range 2-35 Hz and the shunting resistance from 1 to 1000 .