ICFLOAT · Coupled fluid-solid numerical modelling for deep-water and far-offshore floating wind turbines using an adaptive finite element method

Coupled fluid-solid numerical modelling for deep-water and far-offshore floating wind turbines using an adaptive finite element method

Executive Summary: The proposed research aims to numerically simulate the dynamics of floating wind turbines placed in deep waters. Offshore wind turbines offer several advantages over those onshore. For example, they can generate more power, because offshore winds are stronger and steadier, and they reduce the impact on human activities. However, mounting wind turbines on the seabed is expensive in deep seas. Instead, wind turbines could float kept upright by ballast and moored to the seabed. Floating wind turbines do not yet exist on the market, but prototypes are flourishing. Before commercialisation, ensuring safety, minimal environmental impact, and competitive energy production costs are key challenges. Computer models are attractive to design floating wind turbines because they can analyse several different configurations, while limiting expensive laboratory or onsite testing. Reproducing numerically the effect of the ocean on the turbine movement, and vice-versa, is however difficult. It requires coupling the resolution of both the fluid- and solid- dynamics equations, either in a single numerical model or using separate fluid- and solid- dynamics models. The project delivered an open-source code to model the mutual interactions between multiple fluids and floating solids. In this work, the fluid-dynamics model computes the fluids motion across the entire domain, while the solid-dynamics model calculates the motion of the wind turbine. Since fluids and solids interact, information has to be exchanged between both models at every time interval. Importantly, certain conditions need to be ensured at the fluid-solid interface. For example, the action-reaction principle states that the total force acting on the fluids should equal the total force acting on the solids, the two forces being oriented in opposite directions. Also, the fluid and solid velocities should be identical at the fluid-solid interface. The project yielded a new coupling algorithm to ensure spatial conservation of the forces, independently of the mesh resolutions and the order of representation of the forces in each model. The models were applied to the simulation of a floating pile and a wind turbine represented as an actuator disc. In addition to targeting floating wind turbines, the present work directly impacts on other types of marine renewable energy devices (e.g. tidal turbines, wave energy converters). This work can also be applied to a wide range of engineering disciplines. Interactions between fluid flows and solids are ubiquitous. The air flowing around an airplane, the blood circulating in arteries, or the water currents navigated by a robotic fish are just a few examples. Although different applications have different requirements in terms of numerical techniques, the present framework of modelling fluid-structure interactions remains applicable. The proposed tool is capable of: (i) analysing the limits of stability and resistance of the floating system to different weather conditions, (ii) estimating the power extracted, and (iii) accurately modelling waves interacting with the floating structure. Key components are: (a) a numerical wave basin that uses unstructured meshing, coupled to (b) a discrete element method for the dynamics of solids with (c) finite element modelling of stresses and (d) dynamic mesh adaptivity.

Data: CORDIS, © European Union