Stall in yawed flow conditions : a correlation of blade element momentum predictions with experiments

Abstract

Yawed flow conditions introduce unsteady loads in a wind turbine that affect generated

power quality and fatigue life. An unsteady phenomenon of special concern is dynamic

stall, due to the significant load fluctuations associated with it. Although the assumptions

underlying BEM models are totally inadequate in yawed flow conditions, these models,

modified with engineering models, are still widely used in industry. It is therefore relevant

to assess the capabilities of BEM models in predicting the location of dynamic stall on the

blade for a rotor in yawed flow conditions. A rotor model is placed in an open jet wind

tunnel and tested in yawed flow conditions. The locations of dynamic stall on the blade of a

rotor model as a function of the blade position are observed. Two experimental techniques

are used; tufts glued to the blade and hot-film anemometry in the near wake. The results

from the two techniques are compared and possible causes for differences are identified.

Furthermore, the rotor model in yaw is modelled with a simple BEM model, utilizing a

Gormont dynamic stall model. The regions of dynamic stall on the blades predicted by the

BEM model are compared with the experimental results. The BEM model seems capable

of a crude prediction of the dynamic stall locations found for the rotor model in yawed flow

conditions.