Study on non-linear structural effects in large and flexible wind turbine blades for aeroelastic analysis

Historically, the size of wind turbine blades is becoming larger in order to produce more power from the wind, and more flexible due to advanced material and optimised engineering technology. Therefore, accurate prediction of non-linear deformation for this gigantic wind turbine blade is needed to ensure safety in various operational conditions.

In this study, advanced and developed structural model is suggested as a novelty to improve aeroelastic analysis for very large and flexible wind turbine blades. Co-rotational method which is one of the famous methods for analysing non-linear geometry is adopted to investigate largely deformed beam and shell element. Structural model in co-rotational method has continuously changed local coordinate system on each element. This floating coordinate system is transformed to global coordinate system, and the overall structure deformation as a result in the global coordinate system can be obtained by considering this relationship. Co-rotational beam in this thesis evaluates tapered geometry, warping, and anisotropic effect with modified sectional stiffness matrix and integral formulation for stiffness matrix. This stiffness matrix is newly proposed in this thesis as the novel approach. Therefore, this developed co-rotational beam blade can be used properly to consider stronger non-linearity due to complicated geometry as well as anisotropic properties. Unsteady Blade Element Momentum Theory (UBEMT) is applied as external aerodynamic load. It has efficient approach to calculate aerodynamic load and can be expanded to reflect more realistic airflow near the wind turbine blade by additional methods such as dynamic wake and stall model. Aerodynamic load and structure responses are coupled to be interconnected in each time step. Blade section has several axes, which are not located on centre position. Off-diagonal terms in sectional matrix are added to reflect structural coupling effect from axis offset.

For aeroelastic analysis, several external loads including aerodynamic, gravity and centrifugal load are combined into co-rotational method. DTU 10MW Refence Wind Turbine (RWT) is employed as a reference model. It has complicated geometry such as pre-curved geometry and axis offsets as initial conditions. Structural velocity and other parameters in each element are continuously updated and used in next time step. This complicated algorithm including control system is established again to be used in software, MATLAB. Aerodynamic load distributions on the blade as well as total power and trust are analysed, Furthermore, variation of tip deformations according to time into flap- and edgewise directions with and without gravity load are investigated. They agree well comparing to the existing literatures and HAWC2, which is aeroelastic analysis tool.