Aerodynamic characteristics of offshore wind turbine

Onshore wind turbines are generally fixed on the ground which transfers the load-bearing demand to the bottom of the tower, so it will have an impact on the tower structure only without affecting the performance of the wind turbine. However, for a floating wind turbine, the situation is different. A six degrees of freedom motion (surge, sway, heave, roll, pitch, and yaw) caused by the joint action of wind, wave and current will change the relative velocity and orientation of the blades. Floating offshore wind turbines (FOWT) are subjected to a combination of aerodynamic, hydrodynamic and mooring line forces, which influence the performance of the rotor and of the aerofoil sections along the blade. Therefore, the aerodynamic environment of FOWT is considerably more complex than land-based onshore wind turbines. Several parameters, such as rotational speed, wind loading, wave loading, current velocity and the ensuing combined loading are investigated to study the dynamic behaviour of FOWT. This project focuses on the aerodynamic characteristics acting on NREL 5MW offshore wind turbine blades under different environmental conditions based on Computational Fluid Dynamics (CFD) and unsteady Blade Element Momentum (BEM). The main differences between floating and fixed offshore wind turbines and are elucidated and discussed. The Reynolds-Averaged Navier-Stokes method is applied, using the commercial software, Star CCM+, to model the complex flow around a wind turbine, simulating the wake at various tip speed ratios. Typical results indicate that: rapid diffusion of the wake appears to generate an over-prediction of power and thrust; the NREL 5MW rotor demonstrates significant inboard flow separation up to 30% of span; platform motions result in transient flow conditions leading to complex flow fields, which are fully captured by the CFD method used.

For more information about the project contact Prof. Dracos Vassalos (d.vassalos@na-me.ac.uk), Professor of Maritime Safety at the Department of Naval Architecture, Ocean and Marine Engineering at the University of Strathclyde.
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