Wind flow transfers forces to the wind turbine's rotor blades. These then set the rotor in motion. The hub and the gearbox, where present, transfer this rotational energy to the generator for conversion into electrical power. All the rotating components have significant mass and are located at the head of a slender, elastic load‐bearing tower in which they induce dynamic effects. The resulting vibrations, generated at the upper end of the tower, are modified by the dynamic properties of the tower structure and pass through the foundations into the ground. Broadband seismometers record these ground vibrations not only directly adjacent to the wind turbine but also at greater distances of (up to) several kilometers from the turbine. We are aware that local residents and opponents of wind power consider that these vibration phenomena bear potential negative health effects. In the context of this paper, seismic vibrations were measured at the foundation of a 2 MW reference turbine. These seismic signals were compared to numerical simulations. Based on this, we explain the physical background. In the past, any ground vibrations measured have usually been attributed exclusively to the excitation frequencies from the rotor. However, the investigations presented here show that the structural properties of the tower structure significantly influence the type and intensity of the vibrations induced in the ground and dominate the ground motion amplitudes. Finally, we show that the targeted use of absorbers can significantly reduce the vibrations induced in the ground.
This work compares continuous seismic ground motion recordings over several months on top of the foundation and in the near field of a wind turbine (WT) at Pfinztal, Germany, with numerical tower vibration simulations and simultaneous optical measurements. We are able to distinguish between the excitation of eigenfrequencies of the tower‐nacelle system and the influence of the blade rotation on seismic data by analyzing different wind and turbine conditions. We can allocate most of the major spectral peaks to either different bending modes of the tower, flapwise, and edgewise bending modes of the blades or multiples of the blade‐passing frequency after comparing seismic recordings with tower simulation models. These simulations of dynamic properties of the tower are based on linear modal analysis performed with finite beam elements. To validate our interpretations of the comparison of seismic recordings and simulations, we use optical measurements of a laser Doppler vibrometer at the tower of the turbine at a height of about 20 m. The calculated power spectrum of the tower vibrations confirms our interpretation of the seismic peaks regarding the tower bending modes. This work gives a new understanding of the source mechanisms of WT‐induced ground motions and their influence on seismic data by using an interdisciplinary approach. Thus, our results may be used for structural health purposes as well as the development of structural damping methods, which can also reduce ground motion emissions from WTs. Furthermore, it demonstrates how numerical simulations of wind turbines can be validated by using seismic recordings and laser Doppler vibrometry.
Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of internal defects on the fatigue resistance of cast steel components. Defects with varying sizes, geometric shapes and distribution were enforced by influencing the solidification and recorded by computer tomography (CT). Besides the characteristics of the detected cavities, the surrounding fungoid microstructure is classified and evaluated. Later the specimens were tested under cyclic tension and S/N-curves are derived. These data form the basis for extensive numerical simulations of the damage process and the crack growth of every individual specimen. Both processes are affected by the local multiaxial stress states and have their origin in the inside of the specimen. For validation, knowledge of the crack initiation time and propagation properties are essential. Therefore, all specimens respectively the properties of the internal defects are monitored during testing with three different state-of-the-art non-destructive testing (NDT) methods. Background and application of these NDT techniques are described within this paper. Finally, fracture surface analyses show different failure modes and provide further information for model validation.
Durch die Windanströmung werden auf die Rotorblätter von Windenergieanlagen (WEA) Kräfte ausgeübt, die den Rotor in Rotation versetzen. Über die Nabe und ggf. ein Getriebe wird diese Drehbewegung auf den zur Stromerzeugung notwendigen Generator übertragen. Alle rotierenden Komponenten weisen erhebliche Massen auf, sitzen am Kopf einer schlanken, elastischen Turmtragstruktur und rufen in dieser dynamische Effekte hervor. Die dynamisch angeregte Turmstruktur leitet diese durch die Kopfanregung provozierten Schwingungen modifiziert über die Gründung in den Baugrund ein. Mithilfe von Breitbandseismometern können Bodenschwinggeschwindigkeiten nicht nur im direkten Anlagenumfeld, sondern auch in größeren geometrischen Abständen aufgezeichnet werden. Windenergiegegner sehen in der Bodenanregung ein Potenzial für gesundheitsbeeinträchtigende Schwingungseffekte. Im Rahmen dieses Aufsatzes werden die am Fundament einer 2‐MW‐Windenergie‐Beispielanlage gemessenen seismischen Anregungen mithilfe von numerischen Simulationen analysiert und deren physikalische Hintergründe erklärt. Bislang werden die gemessenen Bodenschwinggeschwindigkeiten meist ausschließlich den Anregungsfrequenzen aus dem Rotor zugeschrieben. Die hier vorgestellten Untersuchungen verdeutlichen, dass die strukturellen Eigenschaften der Turmstruktur die Art und Intensität der in den Boden eingebrachten Schwingungen erheblich beeinflussen und die in den Baugrund eingebrachte Bodenbewegungsamplitude dominieren. Abschließend wird gezeigt, dass ein gezielter Einsatz von Schwingungstilgern eine Reduktion der in den Baugrund übertragenen Schwingungsamplituden bewirken kann.
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