Final report for WP4.3: Enhancement of design methods ... - Upwind

UPWIND WP4: Offshore Support Structures and Foundations
8.1 Aerodynamic theories
All of the design codes currently capable of performing integrated modelling of floating wind turbines use combined
Blade Element Momentum theory to calculate aerodynamic forces on the wind turbine rotor. However the
large low-frequency platform motions experienced by floating offshore wind turbines result in flow conditions
which are considerably more complex than those experienced by conventional onshore or fixed-bottom offshore
wind turbines. Extensive load-case simulations performed by Jonkman and Matha [107] have shown,
particularly for catenary moored floating wind turbines, that significant rotational pitch motions occur at approximately
the incident-wave frequency. For the investigated OC3 Hywind spar buoy featuring the NREL 5MW
baseline turbine, pitch motions of up to 8° for production design load cases (DLCs) and 14° for extreme DLCs
have been found. In addition to significant pitch motions, large lower frequency translational surge motions are
also predicted. These rotational and translational motions cause the rotor to operate often in non-axial flow
conditions and lead to a change in the interaction between the rotor and wake, with the rotor in some cases
traversing back over its own wake. The transitions between windmill and propeller states where the rotor interacts
with its own wake cannot be accurately modelled using traditional BEM theory with common corrections,
as Sebastian and Lackner [100] have shown in a preliminary study. BEM cannot model the resulting development
of a turbulent region behind the rotor leading to a toroidal recirculation normal to the rotor blade, most
significantly expected at the blade tips, where the effect on loads is most significant. Sebastian and Lackner
identify this transitional aerodynamic phenomenon as vortex ring state (VRS), and conclude that momentum
equations used in typical BEM analysis methods break down and yield unrealistic results for the axial and the
rotational motion of the rotor.
Since wind turbine airfoils in production up to rated power usually operate close to their maximum lift coefficient
to generate maximum power, the described pitch motions also cause the airfoil to more often operate in stalled
condition, increasing the importance of dynamic stall models. Common dynamic stall models such as Beddoes-
Leishman are semi-empirical and their applicability for the dynamic stall effects occurring on floating wind turbines
has yet to be investigated.
For floating support platform designs with relatively little yaw stiffness, yawed inflow conditions also occur more
often. Yawed inflow is also likely to occur frequently for proposed downwind floating concepts without tower-top
mounted yaw drive (such as Sway’s concept). BEM theory originally assumes flow perpendicular to the rotor
plane and the common skewed wake correction models introduced have been shown to give unreliable load
predictions, with deviations increasing with higher yaw errors. The proposed downwind FOWT configurations,
combined with the increased WT motions, also increase the importance of tower dam and shadow models as
well as improved models for aerodynamic blade and tower interaction.
While beyond the scope of this section, the aero-elastic representation of the rotor blade is also important when
investigating the aforementioned effects and has significant influence on the resulting loads and deflections
CFD
To model these aerodynamic effects, several possibilities exist. The computationally most demanding, but
physically most accurate approach is to solve the Navier-Stokes Equations (NSE) with a CFD solver. In CFD, to
model turbulent flows with their large range of length scales, three basic approaches exist. Direct Numerical
Simulation (DNS), where the full Navier Stokes Equations are solved, needs all relevant length scales to be
resolved by the computational grid, resulting in extremely large grids. With current available computational
power, DNS is therefore not applicable to structures such as wind turbines with large Reynolds numbers. Large
Eddy Simulation (LES), where larger turbulent structures are resolved, is well applicable for the turbine wake.
To resolve the smaller turbulent structures in the boundary layer of the blades with LES is also currently very
difficult to apply due to the very large number of necessary grid cells. The least computationally expensive
method to model turbulent flows in CFD is the Unsteady Reynolds Averaged Navier Stokes (URANS) method.
URANS uses a time-average formulation of the NSE. The occurring nonlinear Reynolds stress term requires
the introduction of turbulence models (for example 2-equation models like k-ω) to close the RANS equations.
These turbulence models have large effects on the CFD solution and need to be selected carefully. A combination
of URANS and LES, using URANS for the flow around the blades, and LES for the wake, is also possible
and could provide a good solution for floating wind turbine modelling.
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