Aerodynamic Modelling of Floating Wind Turbines
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Engineering methods that are commonly used to estimate aerodynamic loads on the rotors of floating wind turbines were originally developed for the analysis of land based, or bottom fixed, wind turbines. In an offshore environment a wind turbine rotor is likely to undergo appreciably larger rigid body motions, and hence unsteady loading, if it is supported on a floating platform. The ever increasing interest in deploying wind turbines offshore on floating support structures, calls for an in depth study on the modelling of rotor aerodynamic loads to assess the applicability of onshore analysis tools, and to reduce uncertainty in aerodynamic load modelling. In this thesis, the influence of rigid body motions on rotor aerodynamic loads, induced velocities and wake geometry was investigated by implementing aerodynamic models that represent the rotor wake more explicitly and with fewer assumptions than in simplified engineering models. An axisymmetric moving actuator disc model was implemented in the Navier-Stokes solver FLUENT, and used to perform simulations of prescribed platform surge motion. The calculated rotor aerodynamic loads, induced velocities and wake geometries were compared to results predicted using a blade element momentum theory model including dynamic inflow correction. It was found that for surge motions comparable to those of current, realistic platform designs, the integrated thrust loads from the engineering models did not differ substantially from those calculated using the more advanced modelling approach - despite differences in wake structures and induced velocity distributions. To asses the influence of additional degrees of freedom, in a computationally efficient manner, a simplified free wake vortex ring model was developed. This model was based on a lifting line representation of the rotor blades and semi-infinite straight line vortices for the near wake that were concentrated into axisymmetric vortex rings in the far wake. This approach was followed so that reasonable wake geometry and -dynamics could be modelled using a minimal number of vortex elements, thereby minimizing the computational cost associated with the free wake method. The model was used to investigate basic wake geometric and -dynamic properties, and then thoroughly tested against experimental measurements and more advanced numerical simulations. With the exception of modelling a strongly sheared inflow, the free wake vortex ring model agreed closely with the reference results. Compared to simpler engineering models that depend on calibrated extensions, the simplified free wake model inherently described wake geometry and dynamics, making it a viable choice for aerodynamic modelling of floating wind turbines beyond the conservative scope of current designs. The original contributions of this thesis include the implementation of a moving actuator disc model that can be used for detailed study of the effect of platform surge motion on rotor aerodynamic loads, induced velocities and wake characteristics. Although the platform motion is limited only to surge, it is indicative of the unsteady effects that can be expected from other platform motions. Furthermore, the simplified free wake vortex ring model that was developed is capable of modelling the influence of platform motion in all six degrees of freedom. The main benefit of the free wake vortex ring model is that it provides a reasonable physical representation of the rotor wake, at moderate computational cost.