Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Filippo Campagnolo (1) F P Carlo Luigi Bottasso (1) Antonis Koumarianos (1)
(1) Technische Universität München, Garching b. München, Germany
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Dr. Campagnolo is currently research and teaching assistant at the Wind Energy Institute of the Technische Universität München. His research is mainly focused on the design and testing of wind turbine scaled models both in wind tunnels and wave tank.
FOWT wave-wind testing with non-Froude wind turbine scaled models
The understanding and simulation of the wind energy conversion process for Floating Offshore Wind Turbine (FOWT) requires the ability to model multiple complex interacting physical processes. Clearly, the ability to effectively design floating wind energy systems ultimately relies, apart from an appropriate knowledge of the physics, on the fidelity to reality of the mathematical models used in simulations, which must then be validated. However, testing and measurements conducted in the field, although invaluable, present some hurdles, like high cost and not complete and accurate knowledge of the environmental testing condition.
The use of scaled models can complement field testing but it is impossible matching all relevant physics due to limitations of the scaling conditions. This aspect is accentuated with FOWT. In fact, hydro forces are proportional to gravity acceleration and their scaling requires the exact enforcement, between full-scale F and scaled models M, of the same Froude values (Fr_F=Fr_M). On the other side, aerodynamic quality of the scaled wind turbine rotor is related to the Reynolds mismatch (Re_F/Re_M) between full-scale and scaled model, and this mismatch should be kept as lower as possible.
Main body of abstract
Many wind-wave tests are conducted with Fr-scaled models of FOWT which means, given n_l the geometry ratio, that Re_F/Re_M=n_l^(3/2). The major downside of this mismatch is that a model rotor, whose geometry is the exact scaling of the full-scale one, shows lower C_T and C_P coefficients. To overcome these drawbacks, experimenters use model rotors whose solidity is higher than full-scale and whose blade are equipped with low Re aerofoils. This allows getting desired thrust, but C_P is still quite lower than desired and the rotor aerodynamic kinematics, linked to TSR, is substantially different from full-scale ones. Moreover, C_T and C_P derivatives differ from full-scale ones, which affect aerodynamics damping.
The paper than discusses about wave-wind testing with models characterized by a lower Re mismatch and right kinematics, which translates into better aerodynamics. To reduce the Re mismatch, the model rotor rotates faster than Fr-scaled ones. This implies higher aerodynamic forces, i.e. higher platform displacements and rotations. Furthermore, model and full-scale system show different relative placement of platform natural frequencies and harmonic excitations due to rotor rotation.
In the paper, a way to overcome these drawbacks by modifying the model platform restoring is described. Simple math models of FOWT are used to provide scientific evidence to the proposed concept, based on suitably tuning the weight of the mooring lines, as well as suitably conceiving and designing additional up-ward mooring lines that extend vertically from the top of the model platform.
The concept is than applied to the 1/45 scaled model of the NREL 5 MW coupled with OC4 semi-submersible and results obtained simultating both full-scale and scaled models are compared. The results show satisfactory agreement between full-scale and scaled-model static and dynamic behaviour, both in terms of displacement/rotations at platform level, relative placement of platform natural frequencies and harmonic excitations as well as main platform RAOs.
Different approach than using Fr-scaled models when dealing with wave-wind testing of FOWT.