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Time-resolved reconstruction of the full velocity field around a dynamically-scaled flapping wing

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Abstract

The understanding of the physics of flapping flight has long been limited due to the obvious experimental difficulties in studying the flow field around real insects. In this study the time-dependent three-dimensional velocity field around a flapping wing was measured quantitatively for the first time. This was done using a dynamically-scaled wing moving in mineral oil in a pattern based on the kinematics obtained from real insects. The periodic flow is very reproducible, due to the relatively low Reynolds number and precise control of the wing. This repeatability was used to reconstruct the full evolving flow field around the wing from separate stereoscopic particle image velocimetry measurements for a number of spanwise planes and time steps. Typical results for two cases (an impulsive start and a simplified flapping pattern) are reported. Visualizations of the obtained data confirm the general picture of the leading-edge vortex that has been reported in recent publications, but allow a refinement of the detailed structure: rather than a single strand of vorticity, we find a stable pair of counter-rotating structures. We show that the data can also be used for quantitative studies, such as lift and drag prediction.

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Notes

  1. It should be noted that there are alternative definitions in the literature for the Reynolds number of a flapping wing (e.g., using the mean tip velocity, instead of the maximum). Due to the nature of the wing kinematics chosen in this study, the choice for the maximum tip velocity will become obvious.

  2. An a posteriori analysis using the obtained out-of-plane motion made evident that the out-of-plane tracer displacement was always smaller than a quarter of the light sheet thickness for the chosen delay times.

  3. Passive deformation refers to deformation by aerodynamic or inertial forces, i.e., not due to actual control by muscles.

  4. The chord length is relatively constant over the span of the wing, except for the tip region, corresponding to roughly the last 10–20% of the wing. For simplicity only one value is used here.

  5. While this value is rather arbitrary, the results did not change significantly with different threshold values.

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Acknowledgments

Support from The Netherlands Organization for Scientific Research (NWO) in the form of a ‘Talent’ Fellowship is gratefully acknowledged (CP). Additional support was provided by a grant from the National Science Foundation, the Packard Foundation and the Office of Naval Research (MHD).

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Correspondence to C. Poelma.

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Poelma, C., Dickson, W.B. & Dickinson, M.H. Time-resolved reconstruction of the full velocity field around a dynamically-scaled flapping wing. Exp Fluids 41, 213–225 (2006). https://doi.org/10.1007/s00348-006-0172-3

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  • DOI: https://doi.org/10.1007/s00348-006-0172-3

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