Model-based displacement estimation of wind turbine blades using strainmodal data

For wind turbine rotor blades, the use of strain sensors is preferred over acceleration sensors for the purpose of permanent monitoring. Experimental modal analysis during operation is thus constrained to strain information, yielding strain modal data including strain mode shapes. For follow-up investigations such as aerodynamic load assessment or flutter monitoring it is however advantageous to have this information as displacement mode shapes or as displacements of the blade contour over time. This research applies a generic approach that converts strain mode shapes to displacement mode shapes utilizing an FE shell model as a basis for approximation. The accuracy of the approach is assessed by comparison with experimentally identified high-resolution displacement mode shapes which are acquired with accelerometers and serve as a reference. In the process the conversion procedure is illustrated with the help of strain data that has been obtained using a sensor instrumentation installed for certification testing of the blade. The requirements for successful usage of the employed conversion scheme and its suitability for rotor blade data are discussed

Experimental modal analysis of aeroelastic tailored rotor blades in different boundary conditions

In the present study results of a modal test campaign involving four identical rotor blades of 20 m length with geometric bend-twist coupling are reported. Design, realisation, and post-processing of the experiments have been carried out under careful consideration of a pre-existing FE shell model. Modal data is obtained at two different stages of the manufacturing process and for one blade in two separate boundary conditions, i.e. free-free in elastic suspensions and clamped to a test rig. Due to the high sensor density in both configurations, the identified normal modes do not only include coupled eigenforms but also mode shapes illustrating cross-sectional vibrations; the latter attributed to the deflection of the blade shells. The acquired dataset is found to be well-suited for the validation of the numerical model and representsa reliable basis for updating

Experimentelle Modalanalyse an einem aeroelastisch optimierten Rotorblatt mit Biege-Torsions-Kopplung im Projekt SmartBlades2

Im Rahmen des Verbundprojekts SmartBlades2 wurde in vorliegender Untersuchung ein aeroelastisch optimiertes Rotorblatt unmittelbar nach Fertigung je einem experimentellen Modaltest in elastischer Aufhängung (frei-frei) sowie in fester Einspannung am Blattprüfstand des Fraunhofer Instituts für Windenergiesysteme (IWES) in Bremerhaven unterzogen. Beide Testkonfigurationen zeichnen sich durch eine hohe Sensordichte aus und ermöglichen neben der Identifikation globaler Moden wie Schlag-, Schwenkbiegung und Torsion die Identifikation und räumliche Auflösung von höherfrequenten Eigenformen aufgrund eines entsprechend breit angeregten Frequenzbereichs. Die Eigenformen umfassen auch Moden, bei denen sich Blattquerschnitte wölben. Hierbei werden Methoden angewandt wie sie bei Ground Vibration Tests [1-4] an Verkehrsflugzeugen zum Einsatz kommen. Die identifizierten modalen Modelle der beiden Testkonfigurationen bieten die Grundlage für die Validierung und Anpassung des Finite-Elemente (FE) Modells des Rotorblatts unter verschiedenen Randbedingungen. Das angepasste FE-Modell wird anschließend im Verbundprojekt weiterverwendet z.B. für gekoppelte Rechnungen zur Gesamtanlagensimulation

A systematic investigation of common gradient based model updating approaches applied to high-fidelity test-data of a wind turbine rotor blade

Within the context of the "SmartBlades2"-project, a wind turbine rotor blade was designed and extensively tested. The rotor-blade uses a lightweight composite structure and bend-twist coupling. The bend-twist coupling facilitates a passive load-reduction by changing the angle of attack under load. Due to a high sensor-density of 265 accelerometers in the experimental modal test of the blade, the sophisticated structural dynamics of the model are captured. Apart from the commonly measured first flapwise-, edgewise-bending and torsion mode, 35 modes up to a frequency of 60 Hz are identified. Unlike in many other wind turbine rotor blade investigations, the Finite Element (FE) model uses shell elements instead of beam elements and is directly based on production drawings. This experimental and simulative setup is particularly relevant, since a significant number of mode shapes exhibit a distinct local behavior which was in previous studies not accounted for. The differences between experimental and simulated results are minimized using computational model updating procedures. In this case-study, two formerly underrepresented aspects of the updating of large-scale FE models are examined. One is the use of different parameterizations and the other is the possibility of insufficient experimental data. The parametrizations are based on well-established criteria like error-localization and sensitivity. Moreover, the updating is performed with different (i.e. reduced) subsets of the modal data and the results are then compared to the model updating results achieved with the entire dataset. This in-depth investigation of the model updating of a composite structure allows the deduction of general guidelines in the model updating of industrial-sized FE models

SELECTED RESULTS ON THE DEVELOPMENT AND TESTING OF SMART BLADES TECHNOLOGIES FOR WIND TURBINES

Within the frame of the Smart Blades and the SmartBlades2 projects, different technologies for developing smart rotor blades for wind turbines have been developed and are still being studied and tested. These cover the three following technologies: bend-twist coupled rotor blades; rotor blades with trailing edge flaps and rotor blades with leading edge slats. In addition, cross-technology topics that need to be considered for successfully implementing all three technologies as well as for evaluating their performance within a wind turbine system are being studied

German research wind farm WiValdi: Planning, installation and testing of innovative research instrumentation for six wind turbine rotor blades

Together with the partners from the German Research Alliance the German Aerospace Center is building the German research wind farm WiValdi (Wind Validation). The wind farm will consist of two standard wind turbines of type E-115 EP3 E4 and a smaller custom build research turbine. The setup of the wind farm is supported by national funding in the research project DFWind. Within DFWind the wind turbine rotor blade research instrumentation has been planned, installed during manufacturing and tested during rotor blade tests at IWES in Bremerhaven. The research instrumentation covers various fields of research from aeroelasticity, structural dynamics, structural health monitoring, vibroacoustic to loads and control. Different types of sensors and measurement systems have been integrated. The sensors vary from classical acceleration sensors over fiber optical acceleration, strain and temperature sensors to innovative continuous fiber optical strain sensors, ultrasonic sensor arrays and finally markers for digital image correlation from inside the blade and on the outer surface. The presentation will give insights in the sensor planning and testing, the instrumentation campaign during manufacturing process and the rotor blade tests before installation on the turbines