Extension of the Non-Linear Harmonic method for the study of the dynamic aeroelasticity of horizontal axis wind turbines

In this paper an innovative methodology for the study of horizontal axis wind turbines dynamic aeroelasticity is presented. It can be understood as an extension of the Non-Linear Harmonic (NLH) method, an efficient computational approach for the analysis of unsteady periodic flows. A linearized model of the structure consisting of a set of mode shapes and natural frequencies was included. The aeroelastic equilibrium was ensured through a set of equations linking the structural displacements and the fluid loads for both the time-averaged and the harmonic contributions. First, the developed methodology is tested in the framework of a 2D cylinder mounted on a single degree of freedom elastic system and undergoing Vortex Induced Vibrations (VIV). The results are compared with previous experimental and computational studies, revealing the potential of the method for the prediction of both the shedding frequency and the aeroelastic response. Secondly, the dynamic aeroelasticity of the complete DTU 10MW RWT wind turbine (i.e. including the tower) is assessed. A nominal operating point is studied, and the rotor flexibility is considered via a blade structural model. The results of this Fluid-Structure Interaction (FSI) simulation are compared with two additional computations, both assuming rigid blades, that modeled the isolated DTU 10MW RWT rotor and the complete machine. This allowed to distinguish the impact of the blade flexibility on the rotor performance from the potential effects associated to the presence of the tower. In particular, the consideration of the aeroelasticity led to a decrease of the predicted time-averaged rotor loads and the corresponding amplitudes of oscillation. For its application on the DTU 10MW RWT, the developed methodology was found to be one order of magnitude faster than a standard time marching approach.