With a globally increasing usage of renewable energy resources, wind engineering technology has been continuously investigated and developed over the past several decades. In order to meet the increased demand for power production, floating platforms with of different mooring configurations have been fielded, accommodating large-scale offshore wind turbines in deep water areas. These systems experience wave loads as well as high wind loads that can lead to system level vibrations, and component fatigue and failure.

OpenFAST is a computational framework for multidisciplinary wind turbine simulations developed by National Renewable Energy Laboratory (NREL), in the United States. It serves as an open platform that connects interdependent modules responsible for aerodynamics, hydrodynamics, structural load analysis, etc., and provides a versatile platform for simulating coupled dynamic responses of horizontal axis wind turbine (HAWT) components.

The aerodynamics module currently available in OpenFAST is based on low-fidelity classical blade element-momentum theory (BEMT). In the present study, a one-way coupling technique that incorporates rotor blade dynamics data from OpenFAST into a state-of the-art in-house computational fluid dynamics (CFD) solver has been developed to improve prediction accuracy for large-scale offshore wind turbines with floating foundation, especially at high-wind conditions where significant tidal effects are expected.

The in-house CFD code in this study employs a hybrid method which combines a conventional practice of numerically solving Reynolds-averaged Navier-Stokes (RANS) in the near-field region around a rotor blade, with a Lagrangian wake representation technique in the far-field region. Compared to computationally more expensive wake-capturing method used in conventional CFD solvers, the hybrid method has proven economical yet accurate in terms of computing aerodynamic performance of wind turbine rotors. It has been validated for small scale turbines as well as very large scale 5MW class wind turbines. Good comparisons for power production as well as coefficient of power have been obtained (see ASME GT India, December 2019; TSME-ICoME, December 2019).

Coupling of the structural/aerodynamic motion time history with the CFD analysis is done using an open File I/O process. A stand-alone script is being used, which may be easily modified for future versions of OpenFAST. The file contains ASCII date for the three linear motions and three angular rotations at several radial locations on the rotor blade. As needed, the computed air loads from the CFD solver may be fed back into OpenFAST using a similar file I/O, leading to a two-way coupled CFD-CSD analysis. The graphical features of OpenFAST are exploited to plot aerodynamic and structural loads as a function of time and radial location. These coupled simulations, are currently in progress for a number of floating off-shore configurations. Comparisons of the aerodynamic loads and structural loads will be presented, and compared against the baseline OpenFAST simulations with classical blade element-momentum theory.


Development of a CFD-CSD Coupling Technique for Large Scale Offshore Wind Turbines

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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