Vortex Evolution in Unsteady Aerodynamics

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The proposed project addresses basic mechanisms of vortex growth and detachment under unsteady conditions, in particular when lifting surfaces are moving. Such conditions exist not only in nature (flapping flight), but also in the aerodynamics of helicopters or wind turbines, the latter occurring during gusty conditions. Motivation for this study arises from the well-known influence of leading-edge vortices on lift hysteresis, also known as dynamic stall, which can be used advantageously if properly controlled, but otherwise can be detrimental in many respects through fluctuating aerodynamic loads (fatigue) and due to acoustic excitations. The overriding goal of the project is to secure a deeper understanding of the flow physics and dependencies of leading-edge vortex dynamics and to use these in formulating concepts for modeling and influencing these flows.The study proposes an experimental investigation of the above phenomena, capturing the flow field using time-resolved Particle Image Velocimetry simultaneous to direct force measurements, from which the inception, growth and detachment of the leading and trailing-edge vortex can be visualized topologically and quantified in terms of circulation. The experimental facility is designed to offer a unique parameter space, allowing single governing parameters (Reynolds number, Strouhal number and reduced frequency) to be varied, while other parameters are kept constant. Kinematically, pure pitch, pure plunge and other periodic motions are achievable. The experiments are conducted on the basis of working hypotheses derived from preliminary work and from open literature and which postulate the changing role of viscosity, depending on Reynolds number. Having identified the main mechanisms involved in the leading-edge vortex evolution, specific concepts for influencing the duration and strength of this vortex will be tested.