Aerodynamic performance of a hovering hawkmoth with flexible wings: a computational approach. Proc R Soc B

Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 08/2011; 279(1729):722-31. DOI: 10.1098/rspb.2011.1023
Source: PubMed


Insect wings are deformable structures that change shape passively and dynamically owing to inertial and aerodynamic forces during flight. It is still unclear how the three-dimensional and passive change of wing kinematics owing to inherent wing flexibility contributes to unsteady aerodynamics and energetics in insect flapping flight. Here, we perform a systematic fluid-structure interaction based analysis on the aerodynamic performance of a hovering hawkmoth, Manduca, with an integrated computational model of a hovering insect with rigid and flexible wings. Aerodynamic performance of flapping wings with passive deformation or prescribed deformation is evaluated in terms of aerodynamic force, power and efficiency. Our results reveal that wing flexibility can increase downwash in wake and hence aerodynamic force: first, a dynamic wing bending is observed, which delays the breakdown of leading edge vortex near the wing tip, responsible for augmenting the aerodynamic force-production; second, a combination of the dynamic change of wing bending and twist favourably modifies the wing kinematics in the distal area, which leads to the aerodynamic force enhancement immediately before stroke reversal. Moreover, an increase in hovering efficiency of the flexible wing is achieved as a result of the wing twist. An extensive study of wing stiffness effect on aerodynamic performance is further conducted through a tuning of Young's modulus and thickness, indicating that insect wing structures may be optimized not only in terms of aerodynamic performance but also dependent on many factors, such as the wing strength, the circulation capability of wing veins and the control of wing movements.

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    • "The biological wings of vertebrates and insects are flexible, which allows the wings to deform during flapping as a result of aerodynamic forces and the inertia of the wing mass [9] [10] [11] [12] [13] [14]. Several previous studies have suggested that wing flexibility improves the generation of aerodynamic forces or the efficiency in flapping flight; such results have been demonstrated both experimentally [15] [16] [17] [18] and numerically [19] [20] [21]. Natural wing structures possess complex, multi-scale morphologies that can yield anisotropic flexural stiffness and tensile elasticity. "
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    ABSTRACT: Bio-inspired flapping wings with a wrinkled wing membrane were designed and fabricated. The wings consist of carbon fibre-reinforced plastic frames and a polymer film with microscale wrinkles inspired by bird feathers and the corrugations of insect wings. The flexural and tensile stiffness of the wrinkled film can be controlled by modifying the orientations and waveforms of the wrinkles, thereby expanding the design space of flexible wings for micro flapping-wing aerial robots. A self-organization phenomenon was exploited in the fabrication of the microwrinkles such that microscale wrinkles spanning a broad wing area were spontaneously created. The wavy shape of these self-organized wrinkles was used as a mould, and a Parylene film was deposited onto the mould to form a wrinkled wing film. The effect of the waveforms of the wrinkles on the film stiffness was investigated theoretically, computationally and experimentally. Compared with a flat film, the flexural stiffness was increased by two orders of magnitude, and the tensile stiffness was reduced by two orders of magnitude. To demonstrate the effect of the wrinkles on the actual deformation of the flapping wings and the resulting aerodynamic forces, the fabricated wrinkled wings were tested using a tethered electric flapping mechanism. Chordwise unidirectional wrinkles were found to prevent fluttering near the trailing edge and to produce a greater aerodynamic lift compared with a flat wing or a wing with spanwise wrinkles. Our results suggest that the fine stiffness control of the wing film that can be achieved by tuning the microwrinkles can improve the aerodynamic performance of future flapping-wing aerial robots.
    Full-text · Article · Aug 2015 · Bioinspiration & Biomimetics
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    • "The mechanics of flapping flight is essentially three-dimensional. The relative importance of chordwise, spanwise and twist deformations depends on the species and the overall effect of the wing flexibility varies [36] [3] [37]. However, in some cases, the problem can be simplified by only considering one type of deformations. "
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    ABSTRACT: We present a novel scheme for the numerical simulation of fluid–structure interaction problems. It extends the volume penalization method, a member of the family of immersed boundary methods, to take into account flexible obstacles. We show how the introduction of a smoothing layer, physically interpreted as surface roughness, allows for arbitrary motion of the deformable obstacle. The approach is carefully validated and good agreement with various results in the literature is found. A simple one-dimensional solid model is derived, capable of modeling arbitrarily large deformations and imposed motion at the leading edge, as it is required for the simulation of simplified models for insect flight. The model error is shown to be small, while the one-dimensional character of the model features a reasonably easy implementation. The coupled fluid–solid interaction solver is shown not to introduce artificial energy in the numerical coupling, and validated using a widely used benchmark. We conclude with the application of our method to models for insect flight and study the propulsive efficiency of one and two wing sections.
    Full-text · Article · Jan 2015 · Journal of Computational Physics
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    • "For example, Katz and Weihs [9] and Michelin and Llewellyn Smith [10] have used the potential flow theory to describe the interaction between an inviscid flow and a flexible flapping wing; whereas a reduced-order model has been used for the structures in other works [11] [12] [13]. With the availability of better computing power and more sophisticated numerical methods, simulations which include the interaction of viscous fluid and solid continuum were performed in some more recent studies [14] [15] [16] [17] [18] [19] [20] [21] [22]. "
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    ABSTRACT: The present study is a numerical investigation of the hydrodynamic effects of passive flexibility on a self-propelled plunging foil. In the model problem, the flow is two-dimensional, incompressible and laminar, while the flexible foil is treated as an inextensible filament. The leading-edge of the foil undergoes a prescribed harmonic oscillation in the vertical direction. In the horizontal direction, the foil is free to move and no constraint is imposed. The simulations are performed by using a solver which couples the immersed boundary method for the flow and the finite difference method for the structure. A systematic parametric study has been conducted to investigate the effects of flexibility on important physical quantities such as the cruising speed, swimming power and propulsive efficiency. It is found that optimal cruising speed is always achieved in foils with some passive flexibility and not the rigid ones. Another important finding is that optimum performance is always achieved at a forcing frequency much lower than the resonance point. Based on the simulation results, three dynamical states of a self-propelled foil have been identified with the increase of bending rigidity, i.e., non-periodic movement, periodic backward-movement and periodic forward-movement. For a flexible foil in forward movement, depending on the range of bending rigidity, either a deflected or a symmetric vortex street arises as the characteristic wake structure. It is found that moderate flexibility is beneficial to symmetry preservation in the wake, while excessive flexibility can trigger symmetry-breaking. The results obtained in the current work shed some light on the role of flexibility in flapping-based biolocomotion.
    Full-text · Article · May 2014 · Computers & Fluids
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