A bio-inspired study on tidal energy extraction with flexible flapping wings

Abstract

Previous research on the flexible structure of flapping wings has shown an improved propulsion performance in comparison to rigid wings. However, not much is known about this function in terms of power efficiency modification for flapping wing energy devices. In order to study the role of the flexible wing deformation in the hydrodynamics of flapping wing energy devices, we computationally model the two-dimensional flexible single and twin flapping wings in operation under the energy extraction conditions with a large Reynolds number of 106. The flexible motion for the present study is predetermined based on a priori structural result which is different from a passive flexibility solution. Four different models are investigated with additional potential local distortions near the leading and trailing edges. Our simulation results show that the flexible structure of a wing is beneficial to enhance power efficiency by increasing the peaks of lift force over a flapping cycle, and tuning the phase shift between force and velocity to a favourable trend. Moreover, the impact of wing flexibility on efficiency is more profound at a low nominal effective angle of attack (AoA). At a typical flapping frequency f * = 0.15 and nominal effective AoA of 10°, a flexible integrated wing generates 7.68% higher efficiency than a rigid wing. An even higher increase, around six times that of a rigid wing, is achievable if the nominal effective AoA is reduced to zero degrees at feathering condition. This is very attractive for a semi-actuated flapping energy system, where energy input is needed to activate the pitching motion. The results from our dual-wing study found that a parallel twin-wing device can produce more power compared to a single wing due to the strong flow interaction between the two wings.

Energy harvesting of a heaving and forward pitching wing with a passively actuated trailing edge

“…At a similar Strouhal number, 0.033 in this study versus 0.050 by Wu et al (2015), the mean value of the power coefficient, C p , is 0.144 in this study versus 0.032 for Wu et al This indicates the advantage of the added component due to angular motion induced by the present study. Similarly, at a reduced frequency of 0.048 in the present study versus 0.05 used by Liu et al (2013), the mean value of the power coefficient in the present study is 0.12, compared with that of Liu et al of 0.0625. So in both of these cases, a significant increase in the mean power coefficient is obtained for equivalent measures of Strouhal number and reduced frequency for a rigid wing undergoing the off-chord pitching motion.…”

“…On the contrary, the effect of wing flexibility on energy harvesting is not well understood. Liu et al (2013) showed, using 2D computational modeling, that a deforming trailing edge of a wing is beneficial to energy extraction by increasing the instantaneous lift force. On the other hand, a flexible leading edge has the ability to manipulate the phase shift between the instantaneous forces and the wing motion.…”

The function of resilin in honeybee wings

“…8C) to show the influence of wing flexibility on hovering, which demonstrates that aerodynamic performance can be enhanced by wing flexibility. Additionally, the flexible structure is conducive to reinforcing load-lifting capacity, power efficiency and wing propulsion efficiency (Mountcastle and Combes, 2013;Liu et al, 2013;Zhu, 2007), and deformations of insect wings bring about a camber effect that may regulate the magnitude of lift and drag ratio and control the alteration of aerodynamic forces Zhao et al, 2010). Therefore, we believe that resilin, largely by determining the flexibility of the wings, is a principal reason why the honeybee and even insects in general are able to excellently perform flapping flight.…”

Systematic investigation of the flow evolution and energy extraction performance of a flapping-airfoil power generator

“…Compared with a traditional NACA0012 airfoil, an optimal airfoil shape increased the output power and efficiency by 6%. More recently, an investigation into the effects of flexible flapping wings on power generation was conducted by Liu et al [18]. A similar study on the flexible airfoil effects can also be fond in the work by Hoke et al [19].…”