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WP 3

Aero-elastic analysis of the Wind Turbine (W/T)

Objectives: To adapt existing simulations tools towards a fully coupled hydro-aero-elastic analysis of the whole system and design verification

Task 3.1: Adaptation of simulation tools for the aero-elastic analysis of the W/T

Objective: Adaptation of simulation tools

Description: Adaptations will be carried out on GAST, LA’s design tool. GAST^[15]  is one of the two most advanced aero-elastic simulation tools for wind turbine systems; the other is HAWC developed at RISOE-DTU^[16]. GAST has two options for the aerodynamic modeling: the blade-element-momentum model (similar to HAWC and many others) and a 3D free-wake flow model ^[17]. GAST is also equipped with several structural models: a non-linear 2^nd-order beam structural model ^[18] and a sub-element Timoshenko linear model (similar to HAWC). In GAST, the flow equations are solved in full coupling with the dynamic equations of the structure. Viscous effects are added as corrections to the aerodynamic loading by means of semi-empirical corrections. GAST has been extensively validated for on-shore and recently has been extended to sea-bed founded wind turbine as well as simple floating wind turbines. In its present form GAST can handled only single spar-type (cylindrical) floaters with simple spring moorings. Within the present task, the floater part of the code will be upgraded so as to treat different types of floaters (multi-cylindrical floaters, barge type floaters etc.) plus a state-of-art mooring model.

In addition to the above adaptations, GAST will be also properly modified in order to also simulate vertical axis wind turbines (VAWTs). VAWTs represent a viable alternative to the commercially successful and widely used horizontal axis wind turbines. Their most important feature is that the heavy drive train can be accommodated at the tower bottom and therefore reduce the dynamic of the floating system. One of the difficulties in analyzing the behavior of VAWTs is the strong blade-vortex interactions that take place. In this connection, the free-wake aerodynamic model already implemented in GAST is most suitable.

Task 3.2: Reduced order models for the aero-elastic behaviour of the W/T

Objective: To implement the necessary interfaces towards to the coupling with the floater dynamics.

Description: The coupling with the floater will be formulated within the context of multi-body dynamics in the WP3. Since the solution of the resulting equations in their full form is cost demanding, the formulation of reduced order models is necessary. Here, the modal approach option will be adapted. Modal representation will be defined with respect to the most important modes. To this end the system is linearized with respect to a reference state and then an eigenvalue analysis will be carried out. Out of the resulting modes, the most important will be retained. The selection of modes will be made based on the eigenfrequency and the energy content they represent. Usually 10-20 modes would be enough. Based on the selected modes, the inertia, damping the stiffening loads in the form of relevant mass, damping and stiffness matrices will be evaluated. They represent the necessary interface for the coupling between the W/T and the rest of the system and they will be used as input in the T2.3 in order for the modal equations of the W/T together with the dynamic equations of the floater and the moorings are solved both in the frequency and the time-domain. In the case of the fully non-linear equations, the loading terms induced by the W/T are expressed with respect of the local degrees of freedom on the tower.

Task 3.3: Design verification of the coupled system in full scale

Objective: To verify the full-scale design with reference to the International Energy Agency (IEA) standard.

Description: Simulations for the full system will be carried out in Task 2.3 aiming at evaluating the couple hydro-aero-elastic behavior of the system, while in the present task, the simulations will comply with the IEA standard in view of estimating the extreme and fatigue loads of the system.

 

[15] Riziotis V., Voutsinas S. 1997. “GAST: A General Aerodynamic and Structural Prediction Tool for Wind Turbines”, Proc. EWEC’97, Dublin.

[16] Larsen T., Hansen A. 2007. “How 2 HAWC2: User’s Manual”, Risø-R-1597(EN) (ver. 3-1).

[17] Voutsinas S. 2006. “Vortex Methods in Aeronautics: How to make things work”, Int.J.CFD, 20(1).

[18] Hodges D., Yu W. 2007. “A rigorous, Engineer-friendly approach for modeling realistic composite rotor blades”, Wind Energy, 10, 179-193.