Extrapol360: Development of a new method for extrapolation of 360° polars for use in BEM

When designing the aerodynamics of wind turbines, the force coefficients of aerodynamic 2D profiles, so-called polars, are applied. These are measured and simulated in the range of the mostly used, low angle of attack. However, when turbines are in operation, the blades are often exposed to forces from completely different directions than those intended. In such cases, the angles of attack are much greater. Such large angles of attack have only been measured very rarely and barely simulated to date as this is involves a complex procedure.

Instead, simplified extrapolation models for higher angles of attack are used. (Extrapolation refers to the process of estimating beyond the original observation range). The methods used to date (e.g., Viterna, Montgomerie) are based in part on theories for thin profiles which are extremely rare in wind turbines, even at the outer edge of the blade. Here, the blades have a relative thickness of 15%-18%. The profiles become thicker in the middle and inside areas of the blades (the middle blade area has a relative thickness of around 24%-26%) and cannot be correctly extrapolated using the existing methods. With new, very long blades in particular, a growing tendency for thicker profiles can be seen.

This is why the Extrapol360 project is developing a method for extrapolating the polars of thick profiles in an angle range of 360°. To this end, wind tunnel tests for thick profiles are planned in combination with eddy resolving, hybrid RANS-LES current simulations.

Improved models for the 360° extrapolation of profile polars will be derived from this. In larger turbines this leads to a reduction in uncertainty in terms of designs for extreme loads. The aim of these measures is to reduce safety factors and, in turn, reduce the costs of turbines.

In this project, Fraunhofer IWES will lay the foundation for modeling with its simulations. The development of the model will be done in collaboration with the project partner.