Vertical axis wind turbines (VAWT) have a relatively simple, rugged construction compared to HAWTs. However, vertical-axis wind turbines have numerous challenges that may hinder their performance. For instance they are strongly affected by dynamic stall at low tip speed ratios. A significant part of the kinetic energy contained in the oncoming wind is lost in swirl and vortices. As a result, VAWTs have a lower power production and efficiency compared to HAWTs. In an effort to alleviate the adverse effects of dynamic stall phenomena, the present study explores the use of two-element airfoils. A comparative study of single element and dual element VAWT configurations for representative VAWT turbines is given. The benefits of dual-element configurations are analyzed through a detailed flow visualization study of the single and two-element VAWT configurations at various azimuthal locations for a representative tip speed ratio. Analysis of these qualitative phenomena is complemented by a discussion on quantitative data for torque, surface pressure distributions, and airloads.

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