Current Research | Renewable Energy

A Bio-Inspired Study on Tidal Energy Extraction with Flexible Flapping Wings

(Supported by Lloyd's Register Group Limited)

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 pre-determined 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 degrees, 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.

We have performed a turbulent CFD simulation of an ocean tidal energy extraction device consisting of an oscillating/flapping flexible single and twin wing. The concept is inspired by the flexible wings of natural flying insects/birds and swimming fish. Particularly, the LEC, TEC and a further developed integrated model combining the best features of the above two models.

The simulation shows that the chord-wise deformation causes a remarkable increase in the local AoA, leading to the enhanced power efficiency of a flexible wing device compared to a rigid wing. The hydrodynamic performance of the wing is affected not only by the increased instantaneous lift and moment amplitude due to the deformation, but also by the phase shift among lift and heaving-velocity and moment and pitching-velocity, by initiating an earlier development of LEV. The contribution from the peak force and phase shift to the overall cycle-mean power efficiency very much depends on the specific models, i.e. whether the deformationmainly occurs in the vicinity of the trailing edge like a hawkmoth wing or near the LE as with a trout ray fin, as well as the degree of flexibility. Our systematic simulation results find that, with the new proposed integrated model, the power efficiency reaches a 7.68% enhancement relative to a rigid wing, which is associated with a nominal effective AoA of 10° at f∗ = 0.15. A dramatic increase of efficiency (about six times that of a rigid wing) is obtained for a nominal effective AoA at 0°. One striking finding is that, with such a flexible wing, the pitching amplitude can be profoundly reduced when the wing operates at a feathering condition.

Studies on a parallel-arranged twin-wing configuration for various nominal effective AoAs show that twin wings generate much more power than a single wing. A relatively small gap between the two wings (Sf = 2.0) enriches the vortex interaction between the gap, and thus improves the energy extraction ability.

Finally, in the present study the flexible structure of wings is predetermined. In reality, insects and fish with different wing/fin stiffness and mass ratios could achieve their best performance by passive deformation. Performing a fully coupled FSI analysis to account for wing passive torsion and bending will be our next task in the future. However, it is reasonable to believe that the present work will produce a similar behaviour if we allow wing structural dynamic properties to be determined beforehand. In that case, the results of the present study could provide vital guidance for industry design on similar flapping wing energy devices.

Please refer to:

Wendi Liu, Qing Xiao and Fai Cheng. "A bio-inspired study on tidal energy extraction with flexible flapping wings" Bio-inspiration & Bio-mimetics 8 (2013) 036011 pp 1-16.

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