Dynamic stall models applied to deep stall models

Abstract

Wind turbines experience a variety of different operating conditions during their operation. As a result, sections of their blades experience a range of angles of attack. Additionally, several commonly used modelling methods use the sectional loading of the blade directly. To adapt this sectional loading for higher angles of attack dynamic stall models are often used. However, these models are generally based in empirical results from pitching airfoils at moderate angles of attack. In this context moderate angles of attack are considered to be below 30 degrees. As a result, their applicability to stationary airfoils at much larger angles of attack is not known. Placing an airfoil at such angles causes it to enter deep stall conditions. During deep stall, vortex shedding becomes a dominant effect on the loading of the airfoil. This behaviour can greatly affect the loading on the turbine and thus should be modelled accurately.

Based on these observations, the presented work aims to investigate the applicability of current dynamic stall models to deep stall conditions. Additionally, an adapted dynamic stall models is created. This model was designed to improve the prediction of the vortex shedding frequencies for stationary airfoils in deep stall. In this process, three different experiments were used for comparison to experimental results.

Three previously developed dynamic stall models were considered, which all showed different responses in terms of vortex shedding. These responses ranged from being completely aperiodic to undamped oscillations. However, none of the models was able to accurately predict the vortex shedding frequencies of the considered experiments. Therefore, the current dynamic stall models were found not to be applicable to the deep stall regime.

Therefore, an adapted model was constructed and calibrated based on the experimental results. This model greatly improves the prediction of vortex shedding frequencies in all considered experiments. Additionally, the accuracy of this model when applied to pitching airfoils at moderate angles of attack was examined. When applied to those conditions the model performed only slightly worse compared to a reference dynamic stall model. Hence, the presented models shows promising results for extending current dynamic stall models to the deep stall conditions.

Finally, it should be noted that the general validity of the adapted model remains to be investigated further. For this purpose, more experiments are to be considered. However, the approach outlined in this research has been shown to provide good results for creating a dynamic stall model which includes vortex shedding in deep stall conditions. Therefore, the presented method can be used to develop a validated dynamic stall model for such conditions provided more experimental results are available.