Research Objectives
Nettuno is a very ambitious project, aiming at:
1. Produce measurements of wakes in conditions representative of normal operation of floating wind turbines.
The lack of open-access experimental data concerning floating wind turbines remains one of the crucial aspects currently ampering the progress of research. The measurements performed in the wind tunnel of Politecnico i Milano will provide a useful benchmark for the validation and development of numerical codes specifically tuned for floating wind turbines.
2. Evaluate the impact of the wake of a floating offshore wind turbine on a downstream machine.
To minimize costs floating wind turbines will be installed in clusters. However, the effect of the interaction between wakes and downstream machines is still unclear. NETTUNO aims to provide further insight into this issue, shedding light into possible differences between wake development in fixed-bottom or floating machines. Ideally, results will show any critical aspects specific to floating machines, like increased fatigue loading, which could lead to increased loads and consequently increased costs.
3. Perform code-to-experiment validation for simulation of floating wind farms.
To minimize design costs industry takes advantage of numerical codes to design the novel floating wind turbines. Previous numerical models, developed for fixed-bottom machines have been used for this task, but it is still unclear whether these methods can be directly applied to floating wind turbines or if additional tuning is required. The comparison between simulations and experiments will provide insight into the capability of the available numerical codes to correctly capture the performance of the novel floating wind turbines.
Methodology
The project objectives will be fulfilled with two actions:
Performing high quality wind tunnel tests
Of two model floating wind turbines in the wind tunnel of POLITECNICO di Milano.
The upstream machine will be mounted on a hexapod structure with 6 degrees of freedom. This will replicate the motion laws typical of a FOWT in open sea, thus generating a wake having a realistic behavior in terms of steering, meandering and wake deficit.
The unsteady rotor forces will be measured on a downstream turbine, which will be positioned directly behind the upstream machine as well as at an offset. This will help understand FOWT wake behavior and its impact on downstream turbines in terms of performance and loads.
Additionally wake measurement will be performed to understand the frequency signature of FOWT wakes, and whether wind farm designers should be concerned about wake interactions exciting low-frequency platform natural frequencies. In this framework, high frequency hot wire anemometers and PIV will be used to characterize the wake over a set of representative working conditions, providing a useful dataset for validation of numerical models.
Performing high quality wind tunnel tests
of the investigated test cases to understand which physical phenomena need to be modeled accurately to improve numerical tool predictiveness.
Ideally, this project will also help define appropriate guidelines for the set-up of medium and low fidelity engineering models, with great impact on FOWT design and optimization. The numerical methods employed will span from lower fidelity approaches, like Blade Element Momentum (BEM) and Lifting Line methods, up to blade-resolved CFD simulations, providing further insight into the physics governing the aerodynamic
response of a floating wind turbine.
Additionally, the limitations of each method will be investigated by comparing the numerical and experimental results.
