Degroote, Joris

Peeters, Mathijs

Santo, Gilberto

Van Paepegem, Wim

English

Ghent University Academic Bibliography

Proceedings of the 7th GACM Colloquium on Computational Mechanicsfor Young Scientists from Academia and IndustryOctober 11-13, 2017 in Stuttgart, GermanyTransient aeroelastic simulations of windturbines with composite blades.Gilberto Santo1*, Mathijs Peeters2, Wim Van Paepegem2 and Joris Degroote1Micro AbstractA fluid-structure interaction model is employed to numerically investigate the interaction of windflow and blade structures of a horizontal axis wind turbine. On the fluid side, the atmosphericboundary layer is included and sliding interfaces are adopted to handle the rotation of the rotor. Onthe structural side, a detailed model of each blade composite structure is used. The two modelsare coupled in a transient simulation where stresses, loads and deformations of the blades are monitored.1Dpt. of Flow, Heat and Combustion Mechanics, Ghent University, Gent, Belgium2Dpt. of Materials, Textiles and Chemical Engineering, Ghent University, Gent, Belgium*Corresponding author: gilberto.santo@ugent.be1 1 IntroductionSince the dimensions of horizontal axis wind rotor tend to increase, the slenderness of their bladesis also sensibly increasing. The deflection of the blades due to the wind load can reach peaks of15% [1]. This leads to a fully coupled fluid-structure interaction (FSI) problem where the windflow and the flexible blades influence each other and continuously interact. This work aims atsimulating the FSI on a full scale horizontal axis wind turbine employing accurate computationalfluid dynamics (CFD) and computational structural mechanics (CSM) models. The atmosphericboundary layer (ABL) is taken into account in detail. The resulting oscillating loads and stresseson each blade are analyzed. On the structural side, a complete and accurate model reproducingthe complex composite nature of each blade is employed. The implicit coupling between the flowand the structural models is guaranteed by the in-house code Tango, resulting in a segregatedapproach [2].2 1 CFD modelGiven the relative motion of the modeled wind turbine, two different domains (a rotating and astationary one) are adopted, separated by sliding interfaces. The layout of the complete meshand the imposed boundary conditions are shown in Fig. 1. The rotor, whose radius R is 50 m,Figure 1. (left) Layout of the simulations and (right) detail of the rotating domainis embedded in the rotating cylindrically-shaped domain (marked in yellow in Fig. 1, which hasa length of 12 m and a radius of 52 m. The mesh is fully hexahedral and obtained by meansof multi-block strategy. The rotating domain consists of 3 M cells, while the stationary one of10 M cells.The turbulence model is chosen to be the k − ε (unsteady RANS) model. The atmosphericboundary layer inlet conditions first proposed by Richard and Hoxey [5] are employed inorder to replicate the neutral ABL conditions in the numerical domain, with friction velocityu∗ = 0.79196 m/s and aerodynamic roughness length y0 = 0.5 m. The modified wall functionsproposed by Parente and Benocci [3] are employed on the ground wall to correctly preserve theABL profiles throughout the domain.3 1 The CSM modelThe analyzed blade is entirely made of composite material, with a total weight exceeding 9 tons.Only shell elements are employed and composite layups are defined to reproduce the compositelayering. The elements are positioned on the outer mold layer (OML) with material offsettowards the inside. Different layups are assigned to different regions of the structure, mimickingits real composition. Each layup is composed of a varying number of plies (up to 127). For eachply a material and a thickness are assigned, together with the necessary orientation to fullydefine the characteristics of layers made of anisotropic materials. The shear webs and the shearcaps are modeled using the same strategy. The mesh is created according to the process outlinedand discussed in [4]. Following this procedure, a mesh composed of 64000 three-dimensionalshell elements is obtained.4 1 CouplingThe two outlined models are coupled by an in-house code named Tango [2]. The Gauss-Seidelcoupling algorithm is chosen in order to enforce equilibrium at the fluid-structure interfacewithin every time step. The displacements prescribed by the CSM model are applied on theCFD mesh where the diffusion method based on boundary distance is chosen to deform only therotating domain mesh.5 1 ResultsThe results of a fully coupled FSI simulation are compared with the ones coming from a purelyCFD simulation, thus considering rigid blades. In both cases the rotor spins at 1.3 rad/s anda full revolution is simulated in 120 time steps. The logics illustrated in Fig. 2 will be usedto define the azimuth angle of each blade and the sign of the radial and tangential forces andvelocities.Figure 2. Definition of the blade azimuth angle and components of forces and velocities.Furthermore, as usually done, the torque (T) and the forces (F) acting on the blades are madenon-dimensional by means of the formula: cT = T/(12ρv2AR) and cF = F/(12ρv2A) where v isthe wind velocity at the hub height (10 m/s), ρ the air density and A is the frontal area of therotor. The velocity distribution induced by the ABL leads to higher angles of attack on theblade span when the it points upwards (positive azimuth angles) and lower angles of attackwhen it points downwards (negative azimuth angle). As a result, the torque contribution followsthe same trend, as illustrated by Fig 3.Figure 3. Single blade contribution to the torque as a function of the azimuth angle.No difference is reported in the average value of the single blade contribution to the torque(about 0.01) while a reduction in the peak-to-peak amplitude of the oscillation (-3.8%) is visiblein the FSI case. A delay is also reported in the FSI case. These differences are linked to theaxial oscillation of the tip in the FSI case, showed in Fig. 4 in terms of axial displacement andaxial velocity. A positive tip axial velocity leads to a reduction of the angle of attack at the tipFigure 4. (left) Tip axial displacement and (right) axial velocity in the FSI case.and, consequently, to a local decrease of the performance. Similarly, a negative tip axial velocityleads to an increase of the tip angle of attack and thus to an increase in the local performance.A comparison of Fig. 4 and Fig. 3 shows that when the tip speed is positive, the FSI torque islower than the CFD torque, confirming the reasoning just outlined. Fig. 5 compares the totalradial force induced by the wind load on the blade in the CFD and FSI cases: it can be seenthat the radial aerodynamic force is centrifugal in the CFD case but becomes centripetal in theFSI case.Focusing on the internal stresses, most of the load acting on the blade is counteracted by itsinner shear webs, which are always solicited by bending moment during the revolution. Inparticular, the pressure side experiences traction longitudinal stress, while the suction side issubject to compression stress. Fig. 6 monitors the longitudinal stresses in the maximal tractionand maximal compression points in the shear webs.Figure 5. (left) Deformed blade on top of undeformed one and (right) radial aerodynamic force on the blade.Figure 6. Stress evolution in the maximal traction and maximal compression points of the shear webs.6 1 ConclusionsThe FSI model described was successfully used to investigate the mutual interaction of windflow and structural response of a modern horizontal axis wind turbine. The effect of the ABLwas also highlighted in both CFD and FSI cases. A noticeable effect of the tip deflection on theperformance of the turbine was addressed.1 References[1] Y. Bazilevs, M. Hsu, J. Kiendl, R. Wu¨chner, and K. Bletzinger. 3D simulation of windturbine rotors at full scale. Part II: Fluid-structure interaction modeling with compositeblades. Int. journal for numerical methods in fluids, Vol. 65:p. 236–253, 2011.[2] J. Degroote. Partitioned simulation of fluid-structure interaction: Coupling black-box solverswith quasi-Newton techniques. Archives of computational methods in engineering, Vol. 20(3):p.185–238, 2013.[3] A. Parente, C. Gorle´, C. van Beeck, and A. Benocci. Improved k–epsilon model and wallfunction formulation for the RANS simulation of ABL flows. Journal of wind engineeringand industrial aerodynamics, Vol. 99:p. 267–278, 2011.[4] M. Peeters and V. P. W. Development of automated high fidelity finite element models forlarge wind turbine blades. In 16th European Conference on Composite Materials: 22-26 June2014, Sevilla, Spain., Sevilla, 2014.[5] P. Richard and R. Hoxey. Appropriate boundary conditions for computational wind engineer-ing models using the k-epsilon turbulence model. Journal of wind engineering and industrialaerodynamics, Vol. 46:p. 145–153, 1993.

Transient aeroelastic simulations of wind turbines with composite blades

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Transient aeroelastic simulations of wind turbines with composite blades

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