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

TL;DR

This study numerically investigates intracycle angular velocity control in cross-flow turbines, revealing significant power gains at low TSR by modulating blade speed, and analyzing flow mechanisms behind efficiency changes.

Contribution

It introduces a comprehensive analysis of intracycle control effects across a range of TSR, highlighting optimal conditions and flow mechanisms for performance enhancement.

Findings

Power increased up to 71% at TSR < 2 with control.

Intracycle control benefits are linked to boundary layer reattachment.

Control at TSR ≥ 2 degrades performance.

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.