Scaling of Vertical Axis Wind Turbine Dynamic Stall

Vertical axis wind turbines (VAWT) are an alternative to horizontal axis wind turbines (HAWT) for wind and tidal energy harvesting applications. Studies of vertical axis wind turbines have heavily invested in understanding the behaviour of the complete turbine, but often with only cursory assessment of the contributions of unsteady aerodynamics of individual blades. This paper experimentally explores the process of unsteady flow separation, or dynamic stall, of the flow around the blades of a gyromill-style vertical axis wind turbine, and the factors upon which its appearance and evolution rely. Introduction Wind and tidal energy harvesting have long been mainstays of a growing renewable energy generation industry. Horizontal axis wind turbines (HAWT) have been the dominant turbine form for decades, but there has at times been an interest in alternative configurations. Amongst these, the vertical axis wind turbine (VAWT) presents a number of potential advantages, but also significant technical challenges. Chief amongst the aerodynamic challenges posed by vertical axis wind turbines is the dynamic stall phenomenon. This unsteady flow separation is thought to induce undesirable structural vibrations, contribute to the production of noise, and have a deleterious effect on the efficiency of the turbine, and is a crucial problem to understand and address for more widespread adoption of vertical axis wind turbine technology. The history of dynamic stall research is rich, but the highly nonlinear nature of the phenomenon, and the large parameter space within which the experimentalist must work, mean that previous results must be considered with caution when approaching the question of dynamic stall in a new application or context. Much previous work has been published on dynamic stall of helicopter blades, and of flapping wings or oscillating fins [1, 2, 3], but the combination of rotation and rapid blade incidence angle changes particular to vertical axis wind turbines warrants greater attention. Insufficient study of the relative importance of the kinematic parameters involved has been performed. Buchner and Soria [4] addresses the dynamic stall’s dependence on Reynolds number, while Ferreira et al [5] studies the effect of tip speed ratio, and also touches on the dependence of the flow on Reynolds number, but the dimensionless pitching rate, and its relevance to the onset and evolution of the dynamic stall are not addressed. Dynamic Stall and its Governing Parameters The bulk of the literature regarding the aerodynamics of vertical axis wind turbine blades considers the effect of tip speed ratio λ,