Dynamics of intracycle angular velocity control applied to cross-flow turbines

Sara F. Hartke; Ari Athair; Owen Williams; Jennifer A. Franck

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arXiv · 2505.14651

Dynamics of intracycle angular velocity control applied to cross-flow turbines

Sara F. Hartke·Ari Athair·Owen Williams·Jennifer A. Franck

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Abstract

Understanding the intricate dynamics of cross-flow turbines (CFT) is critical to the improvement of performance and optimal control strategies. The current study numerically investigates intracycle control by modulating the angular velocity as a function of blade position for a 2-bladed NACA0018 turbine at a lab-scale chord-based Reynolds number of 45,000. Previous work has implemented intracycle control in attempts to improve turbine efficiency at the best performing tip-speed ratio (TSR). However, intracycle modulation of angular velocity simultaneously changes the time-averaged TSR, making it difficult to understand if the effects on performance are due to changes in mean TSR or imposed by the intracycle dynamics. Thus, this work explores a wider region of TSR across which intracycle control is applied, and assesses turbine performance with respect to time-averaged TSR. The effect of intracycle amplitude and phase shift of the velocity modulation is reported in terms of power generation and blade-level forces, and time-resolved flow fields reveal mechanisms behind changes in efficiency. For the 2-bladed turbine explored, the peak performance at constant angular velocity occurs at approximately TSR = 2. Intracycle control is found to be most beneficial at TSR < 2 where power is increased up to 71% over its constant speed baseline and 12% over the peak performance without control. This is accomplished through boundary layer reattachment through the acceleration portion of the stroke. In contrast, applying control when TSR $\ge 2$ is not beneficial due to degraded performance in the downstream portion of the stroke.

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Sara F. Hartke, Ari Athair, Owen Williams, Jennifer A. Franck. 2025-05-20. Dynamics of intracycle angular velocity control applied to cross-flow turbines. https://arxiv.org/abs/2505.14651

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