How essential non‐linear stiffness affects the adaptation of flow energy converter based on fully‐passive oscillating‐foil?

Abstract

An essential non-linear flow-energy converter based on fully-passive oscillating-foil is proposed to improve the adaptation under varied flow conditions. The converter consists of a C-shape foil elastically supported by two springs, an arm that transfers the plunging motion into swing motion and two dampers to absorb mechanical energy. For the purpose of improving performance within a broader range of exciting frequency under varied flow speeds the authors employ two essential non-linear springs in the converter. The adaptation and performance of an essential non-linear converter were assessed numerically. The effects of non-linear stiffness coefficients, varied damping factors, and speed ratios are investigated in detail. The results indicated that the effective range of speed ratio (range of synchronisation) is broadened from (0.4, 2.0) to (0.2, 4.0) with appropriately tuned non-linear stiffness, so the adaptation of the flow energy converter based on oscillating-foil is improved, rendering it more suitable for varied flow conditions and real applications. Besides, performance gains at the design point can be realised as compared with the linear converter. It is found that two fundamental mechanisms are advantageous to enhance the adaptation and performance of the converter: the increased effective stiffness with oscillating amplitude and the non-sinusoidal swing motion.