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An Unsteady Prescribed Wake Model for Vertical Axis Wind Turbines

This paper describes the development of a fully unsteady three-dimensional aerodynamic performance prediction scheme for vertical axis wind turbines (VAWTs). The method is based on the prescribed wake approach of Basuno in which the turbine is represented by a series of bound vortex segments from which emanates a wake consisting of shed and trailing vorticity filaments. Unlike a free vortex method, these wake filaments are constrained to follow a predetermined path, the shape of which is dictated by momentum considerations. By coupling this technique with the unsteady aerofoil performance method of Leishman and Beddoes, a fully unsteady V AWT model is developed. The difficulties associated with coupling the two techniques are discussed and a new formulation for the tangential blade force is introduced. The resulting calculation scheme is shown to agree well with field data from a straight bladed vertical axis wind turbine, even at very low tip speed ratios.