Abstract
This study examined the effect of a spanwise angle of attack gradient on the growth and stability of a dynamic stall vortex on a rotating blade. It was found that a spanwise angle of attack gradient induces a corresponding spanwise vorticity gradient, which, in combination with spanwise flow, results in a redistribution of circulation along the blade. Specifically, when replicating the angle of attack gradient experienced by a wind turbine at the 30% span position during a gust event, the spanwise vorticity gradient was aligned such that circulation was transported from areas of high circulation to areas of low circulation. This in turn increased the local dynamic stall vortex growth rate, which corresponds to an increase in the lift coefficient, and a decrease in the local vortex stability at this point. Reversing the relative alignment of the spanwise vorticity gradient and spanwise flow results in circulation transport from towards areas of high circulation generation, which acted to reduce local circulation and thereby stabilize the vortex. This circulation redistribution behaviour describes a mechanism by which the fluctuating loads on a wind turbine are magnified, which is detrimental to turbine lifetime and performance.
Highlights
1.1 Challenges in Wind Turbine Modelling and Design Modelling wind turbine performance, and subsequently determining design specifications for turbine blades, is most often performed using blade element models, such as those described by Glauert (1935)
The effect of an angle of attack gradient on the growth and stability of the dynamic stall vortex was explored through flow visualizations and three main integral parameters, consisting of the spanwise flow, the spanwise vorticity gradient, and the circulation
The flapping case was actuated such that the spanwise angle of attack gradient was equal in magnitude, and opposite in direction, relative to the spanwise flow velocity found at the 30% span of a wind turbine blade experiencing a transient gust event
Summary
1.1 Challenges in Wind Turbine Modelling and Design Modelling wind turbine performance, and subsequently determining design specifications for turbine blades, is most often performed using blade element models, such as those described by Glauert (1935). Due to the empirical, as opposed to predictive, nature of these methods, such correction factors can not account for the highly unsteady detached flows one expects in gusty conditions, such as the formation of a dynamic stall vortex. This results in large errors from these models when predicting aerodynamic loading on turbine blades, as discussed by Tangler and Kocurek (2005). The specific three-dimensional considerations observed in wind turbines, as opposed to other rotating systems Based on this established context, the problem formulation will be developed at the end of the chapter.
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