Particle-image velocimetry and force measurements of leading-edge serrations on owl-based wing models

Winzen, Andrea; Roidl, Benedikt; Klän, Stephan; Klaas, Michael; Schröder, Wolfgang

doi:10.1016/s1672-6529(14)60055-x

Cited by 37 publications

(43 citation statements)

References 11 publications

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“…Further aerodynamic analysis is needed to understand why performance differences are small. The fact, however, that such rough wings perform equivalent to smooth wings, similar to findings for barn owls (Winzen et al, 2014), is an important insight specific for low Reynolds numbers. Our detailed high-resolution PIV study supports earlier qualitative measurements showing that the boundary layer stays laminar over rough swift wings at low angles of attack, a remarkable feat for 2% rough wings, but becomes turbulent at high angles of attack.…”

Section: Discussionsupporting

confidence: 64%

“…Based on recently published aerodynamic measurements on prepared hummingbird wings (Kruyt et al, 2014), we know the leading edge of a hummingbird hand wing (Calypte anna) is thinner than the 3D printed hummingbird wing of Elimelech and Ellington (2013). The leading edge shape of the avian hand wing matters for aerodynamic performance, as reported earlier for swift wing models, which need a sharp leading edge to generate a leading edge vortex (Videler et al, 2004;Videler, 2006), and for gliding barn owls, for which the serration of their leading-edge primary feather helps reduce flow separation (Winzen et al, 2014).…”

Section: Discussionmentioning

confidence: 77%

“…The reduction of flow separation induced by leading edge serration has also been found on the hand wing of gliding barn owls, which operate at much lower Reynolds numbers (Winzen et al, 2014). The serrations reduce the length of the separation bubble, but remarkably, force measurements did not show a corresponding drag reduction (Winzen et al, 2014), likely an effect due to low Reynolds number.…”

Section: Introductionmentioning

confidence: 73%

“…At the high Reynolds numbers of the pectoral fins of humpback whales, fin serrations at the leading edge generate chordwise vortices that are much larger than the thickness of the boundary layer and mix them effectively, which reduces flow separation and delays stall (Fish and Battle, 1995;Miklosovic et al, 2004;Pedro and Kobayashi, 2008;van Nierop et al, 2008). The reduction of flow separation induced by leading edge serration has also been found on the hand wing of gliding barn owls, which operate at much lower Reynolds numbers (Winzen et al, 2014). The serrations reduce the length of the separation bubble, but remarkably, force measurements did not show a corresponding drag reduction (Winzen et al, 2014), likely an effect due to low Reynolds number.…”

Section: Introductionmentioning

confidence: 94%

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Feather roughness reduces flow separation during low Reynolds number glides of swifts

Bokhorst

Kat

Elsinga

et al. 2015

Journal of Experimental Biology

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Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar-turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small ( proto) birds with an evolutionary window to high glide performance.

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Section: Discussionsupporting

confidence: 64%

Section: Discussionmentioning

confidence: 77%

Section: Introductionmentioning

confidence: 73%

Section: Introductionmentioning

confidence: 94%

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Feather roughness reduces flow separation during low Reynolds number glides of swifts

Bokhorst

Kat

Elsinga

et al. 2015

Journal of Experimental Biology

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“…In contrast with other results [47][48][49], no change in aerodynamic performance was observed by these authors. Furthermore, at high angles of attack (248), Geyer et al [51] observed a reduction of 5 dB in gliding-flight noise in one of two barn owl wings tested.…”

Section: Serrationscontrasting

confidence: 99%

Features of owl wings that promote silent flight

et al. 2017

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Owls are an order of birds of prey that are known for the development of a silent flight. We review here the morphological adaptations of owls leading to silent flight and discuss also aerodynamic properties of owl wings. We start with early observations (until 2005), and then turn to recent advances. The large wings of these birds, resulting in low wing loading and a low aspect ratio, contribute to noise reduction by allowing slow flight. The serrations on the leading edge of the wing and the velvet-like surface have an effect on noise reduction and also lead to an improvement of aerodynamic performance. The fringes at the inner feather vanes reduce noise by gliding into the grooves at the lower wing surface that are formed by barb shafts. The fringed trailing edge of the wing has been shown to reduce trailing edge noise. These adaptations to silent flight have been an inspiration for biologists and engineers for the development of devices with reduced noise production. Today several biomimetic applications such as a serrated pantograph or a fringed ventilator are available. Finally, we discuss unresolved questions and possible future directions.

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Unsteady bio-fluid dynamics in flying and swimming

Kolomenskiy

Nakata

et al. 2017

Acta Mech. Sin.

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Particle-image velocimetry and force measurements of leading-edge serrations on owl-based wing models

Cited by 37 publications

References 11 publications

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Features of owl wings that promote silent flight

Unsteady bio-fluid dynamics in flying and swimming

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