This document constitutes Deliverable ‘D6.3 Structural Dynamic Model Development” of the LiftWEC project. LiftWEC is a collaborative research project funded by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No 851885. It is the intention of the project consortium that the LiftWEC project culminates in the development of one or more promising configurations of a Wave Energy Converter operating through the use of one or more rotating hydrofoils that generate lift as the primary interaction with the incident waves.

In this report, a structural dynamic model is developed for LiftWEC to understand structural conditions that could amplify motions on the sub-structures and to evaluate the impact of these motions in the power performance of the device. To perform the analysis, LiftWEC is sub-divided into three sub-structures: the hydrofoils, the central shaft and the support structure. The support structure is modelled as a bottom mounted v-frame and each of the substructures is analysed individually and collectively.

The structure of deliverable is described briefly as follows. Firstly, the model is presented, and the corresponding equations of motion are introduced for each of the sub-structures, assuming two-dimensional flow. A separate single degree of freedom (DOF) is considered for the hydrofoils and the central shaft, namely with radial and rotational motion, respectively. Whilst a v-frame type of support structure with two DOFs (heave and surge) is considered. Secondly, the equations of motions are solved with a 4th order numerical integration scheme (Runge-Kutta) and the amplitude of the motions are obtained. Thirdly, instantaneous and some instances of frequency analysis are evaluated in order to obtain the relevant conclusions.

We note that in this analysis, the radial forces on the hydrofoils are the major drivers on the induced motions of the hydrofoils and support structure, whilst the tangential forces exert the highest influence in the central shaft. As such, the analytical tools provided by the consortium to model the loads on LifWEC have been utilised.

In this deliverable, it is found that passive control strategies, such as passive compliance can promote conditions that enhance the performance of the rotors. In particular, simple compliance of one of the foils seems to be beneficial in terms of power extraction. In contrast, it is found that compliance of the structure is not detrimental to the power output of the device.