Vertical-axis wind turbines operate under unsteady aerodynamic conditions that induce dynamic stall. The resulting unsteady loads limit turbine performance and structural lifetime. In dynamic stall, the flow response lags behind variations in angle of attack, and separation and reattachment occur at angles different from static conditions. The delays associated with these transitions define the characteristic timescales of dynamic stall. This thesis investigates the timescales governing separation and reattachment across configurations of increasing complexity and provides a theoretical interpretation of the observed scaling.

The thesis first examines the reattachment process. Time-resolved velocity-field and surface-pressure measurements on a sinusoidally pitching airfoil reveal three successive stages: reaction delay, wave propagation, and relaxation. Reattachment begins when the leading-edge suction parameter exceeds a critical threshold, independent of pitch rate. The identification of these stages and the triggering condition supports the design of flow-recovery control strategies for the separated phase of vertical-axis wind turbine rotation. The onset delays of separation and reattachment are then measured as functions of pitch rate on a thick airfoil commonly used in vertical-axis wind turbines. This airfoil exhibits static hysteresis, and the critical separation and reattachment angles differ. When each delay is referenced to its corresponding critical angle, both follow the same power-law decay with pitch rate. With this common timescale, we extended the Goman-Khrabrov model to predict reattachment onset.

Vertical-axis wind turbine blades encounter turbulent inflow, have finite span, and follow nonlinear angle-of-attack variations. We examine the robustness of the delay scaling under each condition. Turbulent inflow and finite-wing effects shift the critical angles toward higher values, but the power-law scaling holds when delays are referenced to the shifted angles. For nonlinear pitching motions, the delays collapse onto the baseline scaling when the pitch rate is evaluated at the instant the critical angle is exceeded. The aerodynamic response within the delay depends on the motion profile. Accelerating motions generate higher loads, decelerating motions produce lower loads. A reformulation of the Goman-Khrabrov model captures these variations.

Across all configurations, the pitch rate at the critical angle governs the delay, and operating conditions shift the critical angle without changing the scaling. We establish the theoretical foundation by interpreting the dynamic stall delay as a slow passage through a fold bifurcation. The normal form predicts a delay exponent of -1/3, which agrees with the measured exponent of −0.34, demonstrating that the scaling is a consequence of the bifurcation structure governing the transition between attached and separated flow.

The bifurcation framework shows that the delay depends on the configuration-specific critical angle and the instantaneous pitch rate, both set by turbine geometry and operating conditions. Accurate estimation of the delay defines the time window for active flow control on vertical-axis wind turbine blades and supports the development of reduced-order models and pitch control strategies to improve turbine efficiency and structural lifetime.