Indirect assessment of local surface defects on wind turbine rotor blades

• Applicants: Professor Dr. Marc Avila Canellas; Professor Dr.-Ing. Andreas Fischer
• Subject Area: Measurement Systems ; Fluid Mechanics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540652448

During the operation under the harsh conditions of the atmospheric boundary layer, the rotor blades of wind turbines are directly exposed to mechanical influences such as raindrops, sand grains or insects. As a result, the leading edge of the rotor blades suffers from erosion damages and contamination, which lead to aerodynamic performance losses of up to 25 % and premature fatigue failures. For this reason, a condition monitoring of rotor blades with an early detection and characterisation of initial surface defects on wind turbines in operation is needed. The working hypothesis pursued by the applicants and substantiated in the preliminary work is, that even small overflow surface defects in the millimeter range induce characteristic turbulent structures, which lead to defect-dependent, thermographically measurable thermal signatures. Our aim is to make use of the effect of a local surface defect (as a flow disturbance element) on the boundary layer flow as a measuring principle. By means of thermographic flow visualisation, an indirect, non-invasive measurement of the defect properties type (additive/subtractive), size, shape (aspect ratio) and position is reached. Thus, the characterization of incipient surface defects, which cannot be measured with a direct imaging approach due to the large measuring distance to the ground, will become possible. To realise the measurement approach, the fluid-mechanical interactions between the desired defect properties and the measurable thermal signatures are investigated using numerical and experimental methods. Subsequently, an inverse measurement model is created. The data and knowledge base required for the modeling is achieved by a large number of experiments with thermographic measurements, as well as with detailed insights into the flow physics by means of direct numerical simulations of the wake flow fields. In this way, the understanding of the physical processes in critical and fully turbulent flows is fundamentally expanded, a solution of the inverse metrological problem is made possible and the fundamental potential of thermographic flow visualisation for the indirect measurement of the geometric properties of the surface defects is clarified. For example, the parameter ranges that can be resolved by the model are determined by investigating the minimum resolvable element height and the thermographic distinguishability between the defect types as well as different positions and aspect ratios of the elements. Furthermore, in addition to the evaluation of individual thermograms, an evaluation of flow fluctuations is conducted using image series. As a result, the achievable measurement uncertainties of the indirect measurement approach are determined, for wind tunnel experiments under ideal measurement conditions as well as for field experiments on operating wind turbines.

• DFG Programme: Research Grants