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The mechanisms associated with an owls ability to fly silently have been thesubject of scientific interest for many decades and a source of inspiration inthe context of reducing noise in both flapping and non-flapping flight. Here,we characterize the near wake dynamics and associated flow structures that areproduced by flying owls. The goal is to shed light on unique flow features thatresult from the owls wing morphology and its motion during forward flappingflight. We study the wake of the southern boobook owl (Ninox boobook); amid-sized owl, which shares the common feature of stealthy flight. Threeindividual owls were flown, separately, in a climatic avian wind tunnel attheir comfortable speed. The velocity field in the wake was sampled usinglong-duration highspeed Particle Image Velocimetry (PIV) while the wingskinematics were imaged simultaneously using high speed video. The time seriesof velocity maps that were acquired over several consecutive wingbeat cyclesenable us to characterize the wake patterns and associate them with the variousphases of the wingbeat cycle. Results reveal that the owls wake issignificantly different compared with other birds (western sandpiper, Calidrismauri; European starling, Strunus vulgaris). The near wake of the owl did notexhibit any apparent shedding of organized vortices. Instead, a more chaoticwake pattern is observed, in which the characteristic scales of vorticity(associated with turbulence) are substantially smaller in comparison to otherbirds. Estimating the pressure field developed in the wake depicts that the owlreduces the pressure to approximately zero. It is therefore conjectured thatowls manipulate the near wake to suppress the aeroacoustic signal bycontrolling the size of vortices generated in its wake, which are associatedwith noise reduction through suppression of the pressure field

Unique flow features of the silent southern Boobook owl wake during flapping flight