Flow features of the near wake of the Australian boobook owl (Ninox boobook) during flapping flight suggest an aerodynamic mechanism of sound suppression for stealthy flight

Jonathan Lawley, Hadar Ben-Gida, Krishnan Krishnamoorthy, Erin E. Hackett, Gregory A. Kopp, Gareth Morgan, Christopher G. Guglielmo, Roi Gurka

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

The mechanisms associated with the ability of owls to fly silently have been the subject of scientific interest for many decades and may be relevant to bioinspired design to reduce noise of flapping and nonflapping flying devices. Here, we characterize the near wake dynamics and the associated flow structures produced during flight of the Australian boobook owl (Ninox boobook). Three individual owls were flown at 8 ms-1 in a climatic avian wind tunnel. The velocity field in the wake was sampled at 500 Hz using long-duration high-speed particle image velocimetry (PIV) while the wing kinematics were imaged simultaneously using high speed video. The time series of velocity maps that were acquired over several consecutive wingbeat cycles enabled us to characterize the wake patterns and to associate them with the phases of the wingbeat cycle. We found that the owl wake was dramatically different from other birds measured under the same flow conditions (i.e., western sandpiper, Calidris mauri and European starling, Sturnus vulgaris). The near wake of the owl did not exhibit any apparent shedding of organized vortices. Instead, a more chaotic wake pattern was observed, in which the characteristic scales of vorticity (associated with turbulence) are substantially smaller in comparison to other birds. Estimating the pressure field developed in the wake shows that owls reduce the pressure Hessian (i.e., the pressure distribution) to approximately zero. We hypothesize that owls manipulate the near wake to suppress the aeroacoustic signal by controlling the size of vortices generated in the wake, which are associated with noise reduction through suppression of the pressure field. Understanding how specialized feather structures, wing morphology, or flight kinematics of owls contribute to this effect remains a challenge for additional study.

Original languageEnglish
Article numberobz001
JournalIntegrative Organismal Biology
Volume1
Issue number1
DOIs
StatePublished - 1 Jan 2019
Externally publishedYes

ASJC Scopus subject areas

  • Animal Science and Zoology
  • Ecology, Evolution, Behavior and Systematics
  • Plant Science

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