High-speed PIV measurements of the near-wall flow field over hairy surfaces

Winzen, Andrea; Klaas, Michael; Schröder, Wolfgang

doi:10.1007/s00348-013-1472-z

Cited by 18 publications

(16 citation statements)

References 10 publications

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“…Interestingly, the start of the boundary layer on an owl‐based wing model was found slightly behind this chordwise position (Winzen et al. , , ,b). This leads to the speculation that the presence of a velvet‐like surface may be especially important in regions where flow separation may occur (see also below).…”

Section: Discussionmentioning

confidence: 96%

“…This was paralleled by an increase in the number of hook radiates for the most medial lengthwise position. Interestingly, the start of the boundary layer on an owlbased wing model was found slightly behind this chordwise position (Winzen et al 2012(Winzen et al , 2013(Winzen et al , 2014a. This leads to the speculation that the presence of a velvet-like surface may be especially important in regions where flow separation may occur (see also below).…”

Section: Distribution Of the Velvet-like Surface On An Owl Wingmentioning

confidence: 99%

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Distribution of the characteristics of barbs and barbules on barn owl wing feathers

Weger

Wagner

2017

Journal of Anatomy

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Owls are known for the development of a silent flight. One conspicuous specialization of owl wings that has been implied in noise reduction and that has been demonstrated to change the aerodynamic behavior of the wing is a soft dorsal wing surface. The soft surface is a result of changes in the shape of feather barbs and barbules in owls compared with other bird species. We hypothesized that as the aerodynamic characteristics of a wing change along its chordwise and spanwise direction, so may the shape of the barbs and barbules. Therefore, we examined in detail the shapes of the barbs and barbules in chordwise and spanwise directions. The results showed changes in the shapes of barbs and barbules at the anterior and distal parts of the wing, but not at more posterior parts. The increased density of hook radiates at the distalmost wing position could serve to stiffen that vane part that is subject to the highest forces. The change of pennulum length in the anterior part of the wing and the uniformity further back could mean that a soft surface may be especially important in regions where flow separation may occur.

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Section: Discussionmentioning

confidence: 96%

Section: Distribution Of the Velvet-like Surface On An Owl Wingmentioning

confidence: 99%

Distribution of the characteristics of barbs and barbules on barn owl wing feathers

Weger

Wagner

2017

Journal of Anatomy

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“…From a biological point of view, it would also be interesting to do more comparative work like that by Weger & Wagner [45] to find out which evolutionary forces drove the development of these adaptations. From an aerodynamic point of view, the investigation of real owl wings [59,60] has yielded interesting new insights into how the flexibility of the wing may influence the performance of a wing. More studies with real wings should be carried out.…”

Section: Discussionmentioning

confidence: 99%

“…We turn to the latter issue in the next section. Klän et al [47,58] and Winzen et al [59,60] conducted experiments with several artificial velvet-like surfaces that mimicked the length and the density of the natural pennula (figure 6a). The softness of the artificial material was also chosen to be similar to that of the natural owl-wing surface.…”

Section: Serrationsmentioning

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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“…Studies modeling the velvety feathers with an artificial surface that mimics the feathers (e.g., velvet) have shown that the separation bubble is reduced, delayed, or eliminated altogether ( Klän et al. 2009 , 2012 ; Winzen et al. 2013 ).…”

Section: Introductionmentioning

confidence: 99%

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

Lawley

Ben-Gida

Krishnan

et al. 2019

Integrative Organismal Biology

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Synopsis 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 bio-inspired design to reduce noise of flapping and non-flapping 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.

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High-speed PIV measurements of the near-wall flow field over hairy surfaces

Cited by 18 publications

References 10 publications

Distribution of the characteristics of barbs and barbules on barn owl wing feathers

Distribution of the characteristics of barbs and barbules on barn owl wing feathers

Features of owl wings that promote silent flight

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

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