Aero-elastic loads on a 10 MW turbine exposed to extreme events selected from a year-long Large-Eddy Simulation over the North Sea

In this presentation the aero-elastic loads on a 10 MW turbine in response to unconventional wind conditions selected from a year long Large Eddy Simulation are evaluated. Thereto an assessment was made of the practical importance of these wind conditions within an aero-elastic context. Moreover the accuracy of BEM based methods for modelling such wind conditions was assessed.

The study is carried out in a joint effort by the Energy Research Centre of the Netherlands ECN.TNO and the Dutch meteorological consultancy company Whiffle within the project DOWA.

Whiffle uses a Large Eddy Simulation (LES) model that is based on the Dutch Atmospheric Large Eddy Simulation (DALES) [1]. The LES code runs on Graphics Processing Units (GPUs) and is therefore referred to as GRASP: GPU-Resident Atmospheric Simulation Platform [2]. GRASP can be run with boundary conditions from a large scale-weather model as described in [3]. For this study, GRASP has been run for the location of the Meteo Mast IJmuiden in the Dutch North Sea area with ERA5 boundary conditions. A full year of LES runs of 24 hours each has been performed on a resolution of 20m. From the results, a number of extreme wind events have been identified, including Low Level Jets, high shear, high veer, strong gusts, fast ramps and high turbulence cases. These cases have been re-run and used as boundary conditions for a higher resolution run in the concurrent precursor setting described in [4].

These extreme events are fed as a wind field to the AeroModule of ECN.TNO[5], i.e. an aerodynamic model coupled to a structural solver(PHATAS). The unique feature of the AeroModule lies in the easy switch between a BEM based model (i.e. an industrial design model) and a physical more accurate (but also more time consuming) free vortex wake (FVW)  method. In this way the aero-elastic response is calculated with aero-models of different levels of fidelity where the same input assures that differences are caused by the aerodynamic modelling.

The turbine on which the load calculations are performed is the AVATAR Reference Wind Turbine[6]. This a 10 MW turbine with a diameter of 206 meter, designed in the EU project AVATAR. All design data of this turbine are publicly available. The wind fields generated from Grasp have a period of 10 minute and a temporal resolution of 0.1 seconds and a spatial resolution of 5 meter. For the present turbine size and operational conditions this yields 20 elements per blade and an azimuthal resolution of approximately 6 degrees, i.e. a resolution which is sufficiently fine for aero-elastic calculations.

The loads from the present study  are compared with the  loads of the original design load spectrum which are known from the AVATAR project.  

Finally a comparison is made between the aero-elastic loads calculated with the AeroModule BEM model and the AeroModule FVW model. This could lead to recommendations on how to model extreme events like Low Level Jets with BEM based models, similar to the recommendations given in [3] where the comparison between AeroModule BEM and AeroModule FVW led to insights  on the modelling of the induced velocity for extreme shear situations

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