Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Sergio GONZALEZ HORCAS (1) F P François DEBRABANDERE (1) Benoît TARTINVILLE (1) Charles HIRSCH (1) Grégory COUSSEMENT (2)
(1) NUMECA International, Brussels, Belgium (2) UMONS, Mons, Belgium
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Mr. Gonzalez Horcas holds a Bachelor in Aerospace Engineering from UPC (Spain), and a Master Degree in Aerospace Mechanics and Avionics from ISAE (France), with specialization in Advanced Fluid Engineering.
In 2013, he joined NUMECA International as a Computational Fluid Dynamics (CFD) software developer. He also started a PhD concerning the computational study of aeroelastic and hydroelastic effects of floating offshore wind turbines at UMONS (Belgium).
Previously, Mr. Gonzalez Horcas worked as a developer of other engineering oriented models for floating offshore wind turbines prediction, aiming to study their complex non-linear dynamics.
A new, high fidelity offshore wind turbines aeroelasticity prediction method with considerable CPU time reduction
Industry standards for Offshore Wind Turbines (OWTs) aeroelastic simulations are based on the Blade Element Momentum Theory (BEM).
BEM method offers a very good computational efficiency and an acceptable flow response, thanks to the introduction of additional sub-models. However, the accuracy of this approach is limited when dealing with OWT rotors, due to the existence of highly skewed flows, heavy dettachements and important blade deflections.
Hence, the use of more sophisticated Computational Fluid Dynamics (CFD) techniques is justified. Even though, the introduction of these approaches in the market is facing an important bottlenck: required engineering and computational time.
Unsteady aeroelastic simulations were performed using the commercial CFD package FINETM /Turbo.
In order to drastically reduce required computational time, flow model was based on Non-linear Harmonic (NLH) approach presented by Vilmin et al. (2006). Since rotor dynamics is mainly driven by periodic motions, flow equations were solved in frequency domain in order to speed-up our simulations.
In a first approach OWT structure was linearized, characterizing it by a set of natural frequencies and deformed shapes.
Main body of abstract
To illustrate the performances of the developed methodology, DTU-10MW Reference Wind Turbine was studied (Bak et al. 2013). A structured mesh of the whole OWT assembly (rotor, nacelle and tower) was automatically generated with Autogrid5TM. Included wizard based interface allowed to produce a high quality mesh in a very short time.
The aeroelastic analysis of the DTU-10MW was performed in two steps:
• Firstly, NLH computations were performed in order to assess unsteady flow effects in a rigid configuration. Steady simulations results compiled by Horcas et al. (2014) served as a basis for this evaluation.
• Secondly, described structure model was included in the assembly in order to take into account elasticity effects. A flutter analysis was performed by exciting our structure at different natural frequencies.
 C.Bak, F.Zahle, R.Bitsche, T.Kim, A.Yde, L.C.Henriksen, M.H.Hansen, J.P.A.A.Blasques, M.Gaunaa, A.Natarajan, The DTU 10-MW Reference Wind Turbine. Danish Wind Power Research 2013, Fredericia, Denmark (2013). Related data publicly available at http://dtu-10mw-rwt.vindenergi.dtu.dk/
 S.G.Horcas, F.Debrabandere, B.Tartinville, Ch.Hirsch, G.Coussement, Hybrid mesh deformation tool for offshore wind turbines aeroelasticity prediction. 6th European Conference on Computational Fluid Dynamics, Barcelona (2014)
 S.Vilmin, E.Lorrain, Ch.Hirsch, M.Swoboda, Unsteady flow modeling across the rotor/stator interface using the nonlinear harmonic method, ASME Turbo Expo, GT2006-90210 (2006)
This work represents the first time that NLH is used in the framework of Wind Energy aeroelasticity predictions. As also observed for the Turbomachinery field, this approach allows to reduce the computational time of unsteady flow simulations. Engineering time of the whole modeling process was also minimized via automatic mesh generation tools.
Future works will be devoted to extend the possibilities of the developed model. The structural response will be assessed together with fluid excitations (two-way coupling). Additionally, we will consider replacing the actual linear structural model by a full Finite Element representation.
This paper reviews the potential of CFD-based tools in Offshore Wind Energy sector. Both computational and engineering times were considerable reduced thanks to the use of innovative numerical methods. The set-up of a high fidelity fluid model allowed us to identify the complex 3D mechanisms driving rotor flutter.
The authors acknowledge the European Commission (EC) for their research grant under the project FP7-PEOPLE-2012-ITN 309395 MARE-WINT, see: http://marewint.eu/.