  This research computationally investigates the complex dynamic stall
phenomena of a cross-flow turbine blade utilizing modal analysis to identify
pertinent events within the cycle. The blade rotation perpendicular to the
freestream generates a curved relative flow, a non-sinusoidal variation of
relative flow speed and angle of attack, and the necessity of travelling
through its own wake. These complexities have challenged traditional predictors
of dynamic stall such as pitch rate, pitching moment, or relative angle of
attack. To investigate these phenomena, aerodynamic loads and flow fields on
the blade from large-eddy simulations are examined across two tip speed ratios.
Proper orthogonal decomposition of the velocity fields is employed to analyze
the spatio-temporal evolution of the dominant flow features. The modes' time
development coefficients reveal a stronger representation of the flow at the
higher rotation rate, capturing the trend of relative flow velocity magnitude
and lift generation on the blade, along with critical events such as vortex
formation and detachment. Additionally, mean power generation is enhanced by
40\% by applying a non-constant rotation rate (intracycle control or angular
velocity control). The flow fields, supported by corresponding changes in the
modal analysis, demonstrate that a delayed stall behavior is responsible for
the additional power extraction. Finally, flow curvature, history effects, and
induced flow are identified as significant factors that modify the dynamic
stall onset and resulting force and moment curves as compared to non-rotating
pitching or plunging foils.
