Validation of a hybrid modeling approach to floating wind turbine basin testing

Abstract

Hybrid modeling combining physical tests and numerical simulations in real time opens new opportunities in floating wind turbine research. Wave basin testing is an important validation step for floating support structure design, but current methods are limited by scaling problems in the aerodynamic loadings. Applying wind turbine loads with an actuation system controlled by a simulation that responds to the basin test offers a way to avoid scaling problems and reduce cost barriers for floating wind turbine design validation in realistic coupled conditions. In this work, a cable-based hybrid coupling approach is developed and implemented for 1:50-scale wave basin tests with the DeepCwind semisubmersible floating wind turbine. Tests are run with thrust loads provided by a numerical wind turbine model. Matching tests are run with physical wind loads using an above-basin wind maker. When the numerical submodel is set to match the aerodynamic performance of the physical scaled wind turbine, the results show good agreement with purely physical wind-wave tests, validating the hybrid model approach. Further hybrid model tests with simulated true-to-scale dynamic thrust loads and wind turbulence show noticeable differences and demonstrate the value of a hybrid model approach for improving the true-to-scale realism of floating wind turbine basin tests. KEYWORDS basin test, floating, hybrid model, hybrid testing, wind turbine 1 INTRODUCTION Scale-model testing of floating wind turbines, often considered an essential validation step in the design process, faces a difficult scaling problem. Two different fluid-structure-interaction domains need to be scaled simultaneously: the hydrodynamics of the floating support structure and the aerodynamics of the wind turbine. The dominance of different nondimensional ratios in each makes scaling without compensation cause discrepancies in the results. For example, a scaled-down floating wind turbine may require higher wind speeds and blade pitch adjustments to achieve the correct thrust force. 1 On top of this, there is the challenge of recreating a wind turbine's mechanical properties at small scale. For less advanced facilities, even providing controlled wind conditions above the wave basin may not be feasible. All these factors contribute to the scaling difficulty in floating wind turbine experiments. A number of approaches have arisen in recent years to counter some of these challenges, especially to do with the turbine aerodynamics. Müller et al review many of the challenges and options for floating wind turbine basin testing. 2 The most direct approach is to compensate for performance reductions through changes to the rotor geometry. Martin et al explored enlarging the blades, using special airfoils, and adding leading edge roughness to counter the lowered Reynolds number. 3 Research at the University of Maine and the Maritime Research Institute of the Netherlands(MARIN) showed that using low-Reynolds number airfoils with enlarged chord lengths can come close to matching true-to-s...

A review of modelling techniques for floating offshore wind turbines

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“…As the physical testing of FOWTs is relatively new, a standardised approach has yet to emerge and thus a quantification of experimental uncertainty is often absent from the relevant literature. For example, in [8][9][10][11], experimental uncertainty was not addressed, whilst in [12][13][14][15][16][17], comparisons between numerical and physical data were presented, which can be considered a measure of experimental accuracy, but with no assessment of experimental precision.…”