Sammendrag
The aim of the present paper is to demonstrate the capability of medium fidelity
modelling of wind turbine component fatigue loading, when the wind turbines are subjected to
wake affected non-stationary flow fields under non-neutral atmospheric stability conditions.
To accomplish this we combine the classical Dynamic Wake Meandering model with a
fundamental conjecture stating: Atmospheric boundary layer stability affects primary wake
meandering dynamics driven by large turbulent scales, whereas wake expansion in the
meandering frame of reference is hardly affected. Inclusion of stability (i.e. buoyancy) in
description of both large- and small scale atmospheric boundary layer turbulence is facilitated
by a generalization of the classical Mann spectral tensor, which consistently includes buoyancy
effects. With non-stationary wind turbine inflow fields modelled as described above, fatigue
loads are obtained using the state-of-the art aeroelastic model HAWC2.
The Lillgrund offshore wind farm (WF) constitute an interesting case study for wind farm
model validation, because the WT interspacing is small, which in turn means that wake effects
are significant. A huge data set, comprising 5 years of blade and tower load recordings, is
available for model validation. For a multitude of wake situations this data set displays a
considerable scatter, which to a large degree seems to be caused by atmospheric boundary
layer stability effects. Notable is also that rotating wind turbine components predominantly
experience high fatigue loading for stable stratification with significant shear, whereas high
fatigue loading of non-rotating wind turbine components are associated with unstable
atmospheric boundary layer stratification.
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