Contactless in-process measurement of separated flow on non-scaled rotor blades of wind turbines

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The visualization of flow separation is essential for a deeper understanding of the real flow behavior of wind energy rotor blades, for example. In contrast to wind tunnel experiments on scaled measurement objects, the required non-contact field measurements on the non-scaled rotor blade are particularly challenging due to the measurement object size (~100 m) and the associated difficult accessibility and the fluctuating in-process measurement conditions. Currently, flow separation measurements are carried out indirectly by means of markers such as wool tufts or pressure tubes which have to be mounted on or in the rotor blade. Therefore, these measurement methods are time-consuming to install and are usually very limited with respect to the spatial resolution and the measurement field. They are also invasive and thus influence the flow behavior to be measured.Therefore, the aim of the project is to validate and fundamentally characterize the non-invasive thermographic flow visualization for the identification and localization of flow separation based on a novel spatio-temporal evaluation for the use on real-scale operating wind turbines in the field. The focus of the project is the approach of a spatio-temporal signal/image evaluation of the thermographic images in order to achieve a sufficient contrast to noise ratio despite the lack of an active heating of the measurement object and without a modification of the rotor blade. The flow-induced temporal and spatial temperature fluctuations will be evaluated to achieve the required sensitivity improvement for the visualization of flow separations. In the context of the particular challenge of a field measurement, the measurement chain will be modelled, the dominant measurement uncertainty components identified and the fundamental measurement limits of the thermographic measurement approach are determined. In addition to a low signal-to-noise ratio, the visualization of flow separation is further complicated due to the multi-axis movement of the rotor blades. Therefore, a geometric assignment of the two-dimensional image to the three-dimensional, rotating rotor blade surface is planned. Based on a model-based geometric calibration, for example, the locally varying imaging scales will be compensated. The measurement method will be verified in wind tunnel experiments under defined conditions and subsequently validated and applied in the field on a non-scaled real wind turbine, at standstill as well as in operation.