Thermographic detection and localisation of dynamic stall on wind turbine rotor blades

• Applicants: Professor Dr.-Ing. Andreas Fischer; Professor Dr.-Ing. Wolfgang Schröder
• Subject Area: Fluid Mechanics ; Measurement Systems
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 420278089

Dynamic stall on wind turbines results in time-varying structural loads, an aerodynamic performance loss and an increased noise emission of the rotor blades. However, an in-process capable measurement method for the detection and localization of dynamic stall on wind turbines is currently missing. In order to investigate and understand the underlying dynamic stall mechanisms and ultimately minimize the occurrence of this phenomenon, the realization of an in-process capable measurement method for the detection and localization of dynamic stall is an important task. The working hypothesis pursued by the applicants and substantiated in the preliminary work is, thatthermal as well as acoustic signatures enable a contactless detection of dynamic stall on wind turbine rotor blades. Since the measurement method of thermographic flow visualization provides a high spatial resolution, but is limited with respect to the temporal resolution due to the thermal inertia of the rotor blade, a complementary acoustic measurement approach will be pursued in order to increase the temporal resolution. The aim is to identify thermal and acoustic dynamic stall signatures and to realise a spatiotemporally resolved detection and localization of dynamic stall on operating wind turbines using a hybrid thermographic-acoustic measurement approach. With harmonized experimental and numerical basic tests on a 2D rotor blade profile under laboratory conditions, the thermodynamics and fluid dynamics as well as the aeroacoustic mechanisms in theoccurrence of dynamic stall are first clarified. Based on this, the capabilities and limitations of a hybrid thermographic-acoustic measurement approach are investigated and an inverse measurement model is derived. The characterization of the measurement approach involves, for example, determining the time scale on which dynamic stall are thermally and acoustically resolvable and to which extent different degrees of stall, i.e. different detachment positions along the blade chord, are distinguishable. Furthermore, the effect of the radial extent of the dynamic stall regions on the acoustic signal is clarified. In addition, the influence of variable measurement conditions on wind turbines, such as different inflow and thermal boundary conditions, as well as systematic cross-influences, such as thermal interactions between the rotor blade surface and structure, on the measurement result will be investigated. Finally, the realized thermographic-acoustic measurement approach is applied to a real, non-scaled research wind turbine in order to determine the achievable quality of the detection and the spatial-temporal localization of dynamic stall in field measurements.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Matthias Meinke