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Cross-flow, or vertical-axis, turbines are a promising technology forcapturing kinetic energy in wind or flowing water and their inherently unsteadyfluid mechanics present unique opportunities for control optimization ofindividual rotors or arrays. To explore the potential for beneficialinteractions between turbines in an array, coherent structures in the wake of asingle two-bladed cross-flow turbine are examined using planar stereo particleimage velocimetry in a water channel. First, the mean wake structure of thishigh chord-to-radius ratio rotor is described, compared to previous studies,and a simple explanation for observed wake deflection is presented. Second, theunsteady flow is then analyzed via the triple decomposition, with the periodiccomponent extracted using a combination of traditional techniques and a novelimplementation of the optimized dynamic mode decomposition. The latter methodis shown to outperform conditional averaging and Fourier methods, as well asuncover frequencies suggesting a transition to bluff-body shedding in the farwake. Third, vorticity and finite-time Lyapunov exponents are then employed tofurther analyze the oscillatory wake component. Vortex streets on both sides ofthe wake are identified, and their formation mechanisms and effects on the meanflow are discussed. Strong axial (vertical) flow is observed in vorticalstructures shed on the retreating side of the rotor where the blades traveldownstream. Time-resolved tracking of these vortices is performed, whichdemonstrates that vortex trajectories have significant rotation-to-rotationvariation within one diameter downstream. This variability suggests it would bechallenging to harness or avoid such structures at greater downstreamdistances.

Near-Wake Dynamics of a Vertical-Axis Turbine