Flow characteristics of aerial swimmers: flapping wing propulsion of tiny insects flying at extremely low Reynolds numbers

• Applicant: Professor Dr. Fritz-Olaf Lehmann
• Subject Area: Fluid Mechanics ; Sensory and Behavioural Biology ; Experimental Condensed Matter Physics ; Mathematics
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277483007

• Final Report Year: 2022

Final Report Abstract

Main goal of this project was to evaluate the aerodynamic principles of insect flight with bristled, oscillating wings. Insect wings are hybrid structures that are typically composed of veins and solid membranes. In some of the smallest flying insects, however, the wing membrane is replaced by hairlike bristles attached to a solid root. Bristles and membranous wing surfaces coexist in small but not in large insect species. There is no satisfying explanation for this finding as aerodynamic force production is always smaller in bristled than solid wings. To gain insights into aerodynamic mechanisms, we developed insect handling tools and studied wing motion in miniature insects during tethered and free flight. Extended kinematic and morphological measurements of miniature insects allowed estimations of air flows, aerodynamic force production and aerodynamic efficiency using computational fluid dynamics and robotic experiments. The experiments were performed at Reynolds numbers typical for flight of insects below one millimeter body length. Our findings highlight wing flapping aerodynamics at highly viscous flows that result in low lift coefficients, elevated drag coefficients, and aerodynamic Froude-efficiencies for flight below 10%. Data suggest that miniature insects may fly with bristled and solid wing surfaces at similar efficacy, while larger insects must use membranous wings for an efficient production of flight forces. In miniature beetles, the oscillating elytra serve as an inertial brake that stabilizes body motion of the insect. The above findings are significant for an in-depth understanding of locomotor propulsion in the smallest flying insects and may help to construct biomimetic air vehicles using flapping wings for propulsion.

Publications

• (2019). Local deformation and stiffness distribution in fly wings. Biol. Open 8, bio038299
  Wehmann, H.-N., Heepe, L., Gorb, S. N., Engels, T. and Lehmann, F.-O.
  (See online at https://doi.org/10.1242/bio.038299)
• (2020) Wing design in flies: properties and aerodynamic function. Insects 11, 466
  Krishna, S., Cho, M., Wehmann, H.-N., Engels, T. & Lehmann, F.-O.
  (See online at https://doi.org/10.3390/insects11080466)
• (2020). Aerodynamic performance of a bristled wing of a very small insect. Exp. Fluids 61, 1-13
  Kolomenskiy, D., Farisenkov, S., Engels, T., Lapina, N., Petrov, P., Lehmann, F.-O., Onishi, R., Liu, H. and Polilov, A. A.
  (See online at https://doi.org/10.1007/s00348-020-03027-0)
• (2020). Three-dimensional wing structure attenuates aerodynamic efficiency in flapping fly wings. J. R. Soc. Interface 17, 20190804
  Engels, T., Wehmann, H.-N. and Lehmann, F.-O.
  (See online at https://doi.org/10.1098/rsif.2019.0804)
• (2021) An experimental data-driven mass-spring model of flexible Calliphora wings. Bioinsp. Biomim. 17, 026003
  Truong, H., Engels, T., Wehmann, H., Kolomenskiy, D., Lehmann, F.-O. & Schneider, K.
  (See online at https://doi.org/10.1088/1748-3190/ac2f56)
• (2021). Flight efficiency is a key to diverse wing morphologies in small insects. J. R. Soc. Interface 18, 20210518
  Engels, T., Kolomenskiy, D. and Lehmann, F.-O.
  (See online at https://doi.org/10.1098/rsif.2021.0518)
• (2021). Vortex trapping recaptures energy in flying fruit flies. Sci. Reports 11, 1-7
  Lehmann, F.-O., Wang, H. and Engels, T.
  (See online at https://doi.org/10.1038/s41598-021-86359-z)
• (2022). Efficiency and aerodynamic performance of bristled insect wings depending on Reynolds number in flapping flight. In: Advances in Biological Flows and Biomimetics, Volume II. Fluids, 7, 75
  O'Callaghan, F., Sari, A., Ribak, G. and Lehmann, F.-O.
  (See online at https://doi.org/10.3390/fluids7020075)
• (2022). Flight activity and age cause wing damage in house flies. J. Exp. Biol. 225, jeb242872
  Wehmann, H.-N., Engels, T. and Lehmann, F.-O.
  (See online at https://doi.org/10.1242/jeb.242872)
• (2022). Novel flight style and light wings boost flight performance of tiny beetles. Nature, 602, 96–100
  Farisenkov, S. E., Kolomenskiy, D., Petrov, P. N., Engels, T., Lapina, N. A., Lehmann, F.-O., Onishi, R., Liu, H. and Polilov, A. A.
  (See online at https://doi.org/10.1038/s41586-021-04303-7)