Wind turbines are already among the largest man-made objects. It is obvious that interactions take place between the motion of this flexible structure and the surrounding air. Because of the continuing trend towards larger rotor diameters, which are accompanied by slimmer and more flexible rotor blades, it can be assumed that this interaction will intensify and thereby increase the likelihood of vibrations and at worse the risk of aeroelastic instabilities. Simulations are indispensable for the design of the turbines and the evaluation of the vibration behavior in order to ensure safe operation. In other words, if we want to increase the robustness and safety of next generation turbines, we have to increase the confidence level in our simulations.
Simulations use numerical models as a representation of reality which are a result of simplifying assumptions. This allows the reduction of the complex reality into a manageable mathematical formulation. In engineering, these assumptions are made based on the laws of physics. However, the identification of the input parameters of such models is usually not as easy as it may seem in the models. The structural and aerodynamic properties of a turbine will not be a single deterministic value, but a distribution of possible values, due to deviations in manufacturing processes, inaccurate measurements or degradation over the turbines’ lifetime. With the help of statistical methods it is, however, possible to determine the influence of such uncertainties on the simulation results.
In the QuexUS project, the DLR Institute of Aeroelasticity, together with the Institute for Wind Energy Systems at Leibniz Universität Hannover and the wind turbine manufacturer Nordex, has addressed the question how blade parameter uncertainties can be considered in the evaluation of wind turbine vibrations. The result is a framework (wtuq) which bridges the gap between existing stochastic uncertainty analysis methods and simulation tools. This allows the uncertainty quantification of models with relevant complexity and computational demand. Key elements are pre- and postprocessing functionalities to map uncertain blade parameters with modifiable spanwise distributions to model inputs and to analyze vibrational characteristics from time domain simulations. The framework was put to the test by a study on the influence of uncertainties in the structural models of wind turbine blades on the damping of critical vibrations in different simulation tools.
The framework wtuq has been published as open source software and can be used and further developed by other scientists. It will be used at DLR in future research projects for further analyses and will be extended accordingly. Realistic input and validation data will be provided by the Research Wind Farm (WiValdi), where several thousand sensors provide a unique research infrastructure to measure wind turbine vibrations on operating turbines.
Author: Hendrik Verdonck, Department Aeroelastic Simulation, DLR-Institute of Aeroelasticity