Analysis of dynamic stall development on a cross-flow turbine blade

TL;DR

This study computationally analyzes the dynamic stall phenomena on a cross-flow turbine blade using modal analysis and large-eddy simulations, revealing how flow features and control strategies influence power generation and stall behavior.

Contribution

It introduces a modal analysis approach combined with large-eddy simulations to understand dynamic stall in rotating turbine blades, highlighting the effects of flow curvature and control strategies.

Findings

Flow at higher rotation rates better captures vortex formation and lift.

Applying non-constant rotation improves power output by 40%.

Delayed stall behavior enhances power extraction.

Abstract

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.