This paper compares approaches for small-scale design validation of floating wind turbines in terms of predicted loads. A range of options exists for how Froude-scaled wave basin testing for design validation is done, especially with regards to the turbine aerodynamics. Geometric scaling of the rotor results in reduced aerodynamic performance requiring higher wind speeds. Adjusting the rotor geometry to compensate allows for matching of thrust and wind speed but leaves discrepancies in rotor torque. Hybrid approaches which actuate correctly scaled, simulation-calculated wind forces onto the platform are limited in representing elasticity of the structure. This paper compares test data from each approach, all applied to the DeepCwind semisubmersible floating wind turbine at 1:50 scale. Wind-wave tests done in 2011 and 2013 provide the first two data sets while the third is provided by hybrid model tests conducted at the University of Maine in 2016. Results of the comparison are inconclusive due to differences in the test parameters.
To avoid this difficulty, simulations are run which enable a more controlled comparison between the three turbine scaling approaches. The simulations show very similar results for each case. This would suggest that matching mean thrust may be sufficient for loads prediction in basin testing. However, this is based on using a blade element momentum theory model, which has limited ability to represent aerodynamic damping. More work is needed to confirm the results using higher-fidelity models or carefully matched experiments.
When it comes to small-scale design validation of floating wind turbines, a range of options exists for how the wind turbine behaviour is represented. No single approach is clearly superior because compromises are involved in all cases. As reviewed by Muller et al. (2014), there is a fundamental scaling problem between the aerodynamics and hydrodynamics, so any solution method involves departures from a perfectly scaled experiment.
The first wave basin tests that resolved the individual rotor blade aerodynamics used a geometrically scaled wind turbine rotor under Froude- scaled winds created by an above-basin wind nozzle (Goupee et al., 2014). These tests encountered significant changes to the aerodynamics caused by the lowered Reynolds number. As detailed by Martin (2011), viscous effects are increasingly important to wind turbine aerodynamics in Froude-scaled testing scenarios, causing severe performance reductions in geometrically scaled turbine models.
