Unsteady Blade-Element Momentum Theory

Wind turbine rotors primarily operate in a pseudo-periodic flowfield. Transient conditions stemming from wake dynamics and blade/wake interactions, however, introduce significant unsteadiness to the flowfield, complicating the calculation of expected loads and moments acting on a wind turbine. This complexity is increased when attempting to simulate a wind turbine operating on a floating offshore platform. Blade element moment (BEM) theory is able to yield good preliminary predictions of load and performance with minimal computational effort in steady axisymmetric flows, but it breaks down when applied to more unsteady, complex flow regimes like those encountered in the aforementioned aperiodic conditions. Free vortex wake models (FVM) are able to address unsteady flows by modeling the turbine wake as vortical elements; the strengths of these elements may be used to compute the induced velocity field of the rotor, and in turn, the loads acting on it. FVM can, however, be computationally intensive and solutions may be numerically unstable. This work focuses on marrying elements of FVM to BEM theory to create an unsteady BEM method that can accurately simulate wind turbines performance and loads across a wide range of complex, varying flowfields.