D1.1 Hybrid Eulerian-Lagrangian Aerodynamic Model

This document is a deliverable of the FLOATECH project, funded under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007142.

The design of a wind turbine requires a detailed mechanical analysis of the entire turbine system. The turbine must be capable of safely operating within a broad load envelope, in a number of operating states while consistently generating an optimal electrical output. These loads vary greatly as a function ofturbine size, environmental conditions and turbine architecture. The current interest in floating offshore architecturesimpliesthat the complexity of the simulated systems increases. The dynamics of the turbine are significantly more complex than the equivalent onshore or fixed bottom-offshore scenarios as the entire turbine assembly becomes mechanically coupled with the sea state and dynamics of the floater assembly and mooring lines. These couplings influence the dynamical response of the turbine. A holistic treatment of the problem is required in order to ensure the correct estimation of load envelopes and safety margins.

The inherent complexity of this dynamic system quickly limits the applicability of closed-form expressions and necessitates simulation with numerical methods. This has given rise in recent decades to a range of simulation tools and software packages aimed at addressing this issue. The state-of-the-art of current simulation packages must be advanced in order to allow for simulation of floating offshore architectures to ensure that turbine design iterations can be carried out efficiently. These advances must occur in a range of disciplines including hydrodynamic, structural and aerodynamic modelling. This report focusses on the latter, and introduces a new numerical method for simulating the aerodynamics of wind turbines.

The simulation of aerodynamic loads acting on a floating offshore device introduces unique challenges. The relative motion of the floating turbine system can give rise to a stronger interaction of the turbine rotor with its own wake. Furthermore, the influence of environmental conditions in offshore environments are markedly different to equivalent onshore scenarios as a result of the fundamentally different behaviour of the atmospheric boundary layer and surface roughness effects. The turbine wake can remain stable for a greater downstream distance in comparison with onshore scenarios [64], potentially increasing aerodynamic interaction between turbines. A detailed knowledge of the evolution of the wake is therefore required. Pursuant to this goal, this report introduces a new aerodynamic model, built into the open-source simulation suite QBlade Ocean (hereafter QB).

The model makes use of a hybrid Eulerian-Lagrangian formulation of the flow using vortex particles and is accelerated by applying the vortex-particle multilevel (VPML) method. The method is capable of simulating not only the flow in the near-wake of the turbine, but also the evolution of the far-wake. The method has been validated against an experimental turbine and comparable results using higher-order aerodynamic models. Numerous cases have been simulated which illustrate the applicability of the method to more complicated aerodynamic scenarios. The solver makes use of the graphical processor unit (GPU) to optimise calculations and significantly reduce the computation time of large aerodynamic problems.