The Physics of H-Darrieus Turbines Self-Starting Capability: Flapping-Wing Perspective

Abstract

It has been widely reported that Darrieus turbines cannot self-start and that they require external assistance to accelerate to their operating tip speed ratios. However, recent experiments have shown conclusively that H-Darrieus rotors with fixed-pitch blades that employ a symmetrical aerofoil can reliably self-start in steady controlled environments. Previous attempts have also been made to model the starting characteristics but there still exists a significant discrepancy between the experimental data and model prediction, suggesting that our understanding of this starting characteristic remains weak. The investigation and explanation of the starting characteristics is the focus of this paper. The investigation was made through a careful analysis of aerofoils that undergo Darrieus motion, giving some insights on how the blade experiences different flow conditions and how driving force is developed over the flight path. The analysis reveals that the aerofoil in Darrieus motion is analogous to flapping wing mechanism; the mechanism that fish and birds employ to generate propulsion. The explanation of flow physics and torque development can then be made through a simple pitch-heave concept. The investigation using this concept together with observations of flapping creatures suggests that the key feature that promotes driving torque generation and the ability to self-start is the unsteadiness associated with the rotor. This unsteadiness is related to chord-to-diameter ratio. This, together with blade aspect ratio, and number of blades, is the reason why H-Darrieus turbines that employ a symmetrical aerofoil can self-start.

The Physics of H-Darrieus Turbine Starting Behavior

“…Though clearly these results depend on the choice of aerofoil for the turbine. Woransinchai [8] observed that many of the early Darrieus turbines were constructed with low c/D ratios and the lack of unsteady thrust explains their inability to self-start.…”

“…However the curve is not perfectly sinusoidal suggesting that the motion can be decomposed into further components and not only pitch. The decomposition into pitch and plunge angles was described in detail by [8] and is shown in Fig. 5.…”

“…Based upon the analogy and the decomposition of Darrieus incidence angle [8], the Darrieus flight path can be divided into an effective upstroke and downstroke which is shown in Fig. 6 and a flapping motion can be plotted.…”

“…To address this issue, a physical explanation of starting behaviour based on an analogy between the aerofoil in Darrieus motion and flapping-wing mechanisms was proposed by Worasinchai et al [8]. The investigation involved a decomposition of the Darrieus incidence angle into two separate angles; a pitch angle variation and a plunge angle variation.…”

“…This discrepancy suggested that the modelling assumptions oversimplified the physical mechanisms inherent in the machine. The fundamental physics of starting must be explored in order that proper modelling assumptions can be made.To address this issue, a physical explanation of starting behaviour based on an analogy between the aerofoil in Darrieus motion and flapping-wing mechanisms was proposed by Worasinchai et al [8]. The investigation involved a decomposition of the Darrieus incidence angle into two separate angles; a pitch angle variation and a plunge angle variation.…”