Abstract. Detailed 3D finite-element simulations are state of the art for structural analyses of wind turbine rotor blades. It is of utmost importance to validate the underlying modeling methodology in order to obtain reliable results. Validation of the global response can ideally be done by comparing simulations with full-scale blade tests. However, there is a lack of test results for which also the finite-element model with blade geometry and layup as well as the test documentation and results are completely available. The aim of this paper is to validate the presented fully parameterized blade modeling methodology that is implemented in an in-house model generator and to provide respective test results for validation purpose to the public. This methodology includes parameter definition based on splines for all design and material parameters, which enables fast and easy parameter analysis. A hybrid 3D shell/solid element model is created including the respective boundary conditions. The problem is solved via a commercially available finite-element code. A static full-scale blade test is performed, which is used as the validation reference. All information, e.g., on sensor location, displacement, and strains, is available to reproduce the tests. The tests comprise classical bending tests in flapwise and lead–lag directions according to IEC 61400-23 as well as torsion tests. For the validation of the modeling methodology, global blade characteristics from measurements and simulation are compared. These include the overall mass and center of gravity location, as well as their distributions along the blade, bending deflections, strain levels, and natural frequencies and modes. Overall, the global results meet the defined validation thresholds during bending, though some improvements are required for very local analysis and especially the response in torsion. As a conclusion, the modeling strategy can be rated as validated, though necessary improvements are highlighted for future works.
Abstract. Detailed 3D finite element simulations are state of the art for structural analyses of wind turbine rotor blades. It is of utmost importance to validate the underlying modelling methodology in order to obtain reliable results. Validation of the global response can ideally be done by comparing simulations with full scale blade tests. However, there is a lack of test results for which the blade data are completely available. The aim of this paper is to validate one particular blade modelling methodology that is implemented in an in-house model generator, and to provide respective test results to the public. A hybrid 3D shell/solid element model is created including the respective boundary conditions. The problem is solved via a commercially available finite element code. A full scale blade test is performed as the validation reference, for which all relevant data are available. Some data have been measured prior to or after the test in order to account for manufacturing deviations. The tests comprise classical bending tests in flap-wise and lead-lag direction as well as torsion tests. For the validation of the modelling methodology, global blade characteristics from measurements and simulation are compared. These include the overall mass and centre of gravity as well as their distributions along the blade, deflections, strain levels, and natural frequencies and modes. Overall, good agreement is obtained, though some improvements might be required for the response in torsion. As a conclusion, the modelling strategy can be rated as validated.
Rotor blades of wind turbines are subjected to high-cycle fatigue during their lifetime of 20-25 years. To show that a blade design fulfils the normative requirements, the test campaign usually consists of two consecutive cyclic tests, i.e., in the flapwise and lead-lag directions. The fatigue test excites the blade close to its corresponding natural frequency. Combining the two uniaxial tests into one biaxial test excites the blade in both directions simultaneously. Using the same excitation frequency for both directions results in the blade cross-sections describing an elliptical deflection path. To realize such a test while still exciting close to resonance, the natural frequencies for the two directions need to be equalized with the aid of decoupled masses and stiffness elements. This approach reduces the testing time, and induces a more realistic loading which is comparable with field conditions while keeping the energy consumption of the hydraulic actuation low. This work describes the concept of the elliptical biaxial rotor blade fatigue test with resonant excitation using a commercial blade design. To this end, the design model, which uses both a transient and a harmonic simulation, is validated with the experimental results of the test. The simulation model and the experiment agree well with each other in terms of displacements and loads along the blade.
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.
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