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

Because of the efficiency lowering effect, boundary layer flow separation is an unwanted phenomenon on rotor blades of wind turbines. The separated flow also induces unsteady loads, reducing the lifespan of the structural material and causing sound emission. To achieve a better understanding of the boundary layer flow separation, a fast, non-invasive and contactless method for flow visualization is demanded to be capable of coping with the challenging requirements of in-process measurements on real wind turbines.

The aim of this project is thus to advance the existing thermographic flow visualization metrology for the detection of flow separation on wind turbines in operation. Compared to state-of-the-art methods with an invasive preparation of the blade surface, thermographic flow visualization uses the effect of different surface temperatures due to different heat transfer coefficients in the flow boundary layer to differentiate between different flow regimes without any contact and at a long working distance.

While the principle potential of thermography for detecting flow separation was already verified in wind tunnel application, the application to in-process measurements has yet to be made. Different spatiotemporal image processing methods are studied to increase the contrast between the flow separation and other flow states, while one open research question is how to record and evaluate image series on a moving rotor with unsteady inflow conditions. Additional tasks are to cope with a low spatial resolution due to high measuring distances and the overall non-reproducible, non-laboratory environmental circumstances. As a result of the project findings, the in-process flow visualization on wind turbines in operation will be fundamentally enhanced by the stall detection capability.

Funding authority:DFG - German Research Foundation
Funding ID:420278089