Aerodynamic Modelling of Floating Wind Turbines
Doctoral thesis
View/ Open
Date
2015Metadata
Show full item recordCollections
- Institutt for marin teknikk [3469]
Abstract
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.