  Cross-flow, or vertical-axis, turbines are a promising technology for
capturing kinetic energy in wind or flowing water and their inherently unsteady
fluid mechanics present unique opportunities for control optimization of
individual rotors or arrays. To explore the potential for beneficial
interactions between turbines in an array, coherent structures in the wake of a
single two-bladed cross-flow turbine are examined using planar stereo particle
image velocimetry in a water channel. First, the mean wake structure of this
high chord-to-radius ratio rotor is described, compared to previous studies,
and a simple explanation for observed wake deflection is presented. Second, the
unsteady flow is then analyzed via the triple decomposition, with the periodic
component extracted using a combination of traditional techniques and a novel
implementation of the optimized dynamic mode decomposition. The latter method
is shown to outperform conditional averaging and Fourier methods, as well as
uncover frequencies suggesting a transition to bluff-body shedding in the far
wake. Third, vorticity and finite-time Lyapunov exponents are then employed to
further analyze the oscillatory wake component. Vortex streets on both sides of
the wake are identified, and their formation mechanisms and effects on the mean
flow are discussed. Strong axial (vertical) flow is observed in vortical
structures shed on the retreating side of the rotor where the blades travel
downstream. Time-resolved tracking of these vortices is performed, which
demonstrates that vortex trajectories have significant rotation-to-rotation
variation within one diameter downstream. This variability suggests it would be
challenging to harness or avoid such structures at greater downstream
distances.
