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Sequence of instantaneous images showing a full wingbeat cycle of the boobook owl as the owl moves from right to left as it did in the wind tunnel.
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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 th...
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... as Gurka et al. (2017), which followed guidelines suggested by Wies-Fogh (1973), where the wingbeat cycle was divided into four distinct phases: upstroke (US), transition from US to downstroke (USDS), DS, and transition from DS to US (DSUS). Following an analysis of the owl wing's kinematics as described in the ''Materials and methods'' section, Fig. 3 presents images in sequential order from right to left as an owl flies through one full wingbeat cycle during forward flight (upstream, against the wind). We estimated that for the various flight durations, the average frequency was about 6 Hz (Fig. 4) at their characteristic Strouhal numbers allows owls to fly more slowly while still ...
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... to the measurement plane. Therefore, Taylor's hypothesis (1938) is applied, following the assumption that the flow remains relatively unchanged as it passes through the measurement plane. The utilization of the long-duration time-resolved PIV system enabled the reconstruction of the wake evolving behind the wings. The owls flew from right to left (Fig. 3); therefore, the downstream distance is measured as positive chord lengths. What appears as downstream essentially happened earlier while what appears as upstream happened later. Each wingbeat cycle corresponds to five to eight cord lengths for the various wakes analyzed. Each individual scene analysis corresponds to 0.5-2 wingbeat ...
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... characteristics, owl-inspired leading-edge serrations are of great research interest due to their potential as a passive flow control device. It had been hypothesized that owls' leading-edge serrations alter the adjacent flow field to some extent in order to suppress the aerodynamic noise, impact its aerodynamic performance, and function as a passive flow control mechanism (Graham, 1934;Kroeger et al., 1972;Lilley, 1998;Wagner et al., 2017;Jaworski and Peake, 2020;Lawley et al., 2019;and Nafi et al., 2020). Noise generation during gliding flights of a Florida barred owl was investigated by Kroeger et al. (1972). ...
... Traditional wind tunnel experiments, whether conducted with preserved owl wings or live birds, provide valuable insights; however, they do not isolate the specific flow dynamics associated with individual wing features, such as LE serrations, trailing-edge fringes (TE), and velvety surfaces, among others. This holistic perspective is exemplified by the comprehensive wake field data discussed by Lawley et al. (2019). ...
Owls' silent flight is commonly attributed to their special wing morphology combined with wingbeat kinematics. One of these special morphological features is known as the leading-edge serrations: rigid miniature hook-like patterns found at the primaries of the wings' leading-edge. It has been hypothesized that leading-edge serrations function as a passive flow control mechanism, impacting the aerodynamic performance. To elucidate the flow physics associated with owls' leading-edge serrations, we investigate the flow-field characteristic around a barn owl wing with serrated leading-edge geometry positioned at 20° angle of attack for a Reynolds number of 40 000. We use direct numerical simulations, where the incompressible Navier–Stokes equations are solved on a Cartesian grid with sufficient resolution to resolve all the relevant flow scales, while the wing is represented using an immersed boundary method. We have simulated two wing planforms: with serrations and without. Our findings suggest that the serrations improve suction surface flow by promoting sustained flow reattachment via streamwise vorticity generation at the shear layer, prompting weaker reverse flow, thus augmenting stall resistance. Aerodynamic performance is negatively impacted due to the shear layer passing through the serration array, which results in altered surface pressure distribution over the upper surface. In addition, we found that serrations increase turbulence level in the downstream flow. Turbulent momentum transfer near the trailing edge increased due to the presence of serrations upstream the flow, which also influences the mechanisms associated with separation vortex formation and its subsequent development over the upper surface of the wing.
... Lilley [2] studied the aerodynamic characteristics of the three owl-wing morphologies and provided a tentative explanation of the associated noise reduction mechanisms, i.e. the LE serrations/combs may suppress turbulent fluctuations in the vicinity of the TE, while the TE fringes reduce noise by mitigating edge scattering. Furthermore, it was also reported that the three unique owl wing morphologies can adapt their shapes during wing beats in owl flight, leading to reducing the length scales of the wake flow of owl flight, hence a substantial impact on the aeroacoustic performance of owls [25,26]. ...
As one of the unique owl-wing morphologies, trailing-edge (TE) fringes are believed to play a critical role in the silent flight of owls and have been widely investigated using idealized single/tandem airfoils. However, the effect of TE fringes and associated mechanisms on the aeroacoustics of owl wings, which feature curved leading edges, wavy TEs, and several feather slots at the wingtips, have not yet been addressed. In this study, we constructed two 3-D owl wing models, one with and one without TE fringes, based on the geometric characteristics of a real owl wing. Large-eddy simulations and the Ffowcs Williams‒Hawkings analogy were combined to resolve the aeroacoustic characteristics of the wing models. Comparisons of the computed aerodynamic forces and far-field acoustic pressure levels demonstrate that the fringes on owl wings can robustly suppress aerodynamic noise while sustaining aerodynamic performance comparable to that of a clean wing. By visualizing the near-field flow dynamics in terms of flow and vortex structures as well as flow fluctuations, the mechanisms of TE fringes in owl wing models are revealed. First, the TE fringes on owl wings are reconfirmed to robustly suppress flow fluctuations near the TE by breaking up large TE vortices. Second, the fringes are observed to effectively suppress the shedding of wingtip vortices by mitigating the flow interaction between feathers (feather-slot interaction). These complementary mechanisms synergize to enhance the robustness and effectiveness of the TE fringe effects in owl wing models, in terms of aerodynamic force production and noise suppression. This study thus deepens our understanding of the role of TE fringes in real owl flight gliding and points to the validity and feasibility of employing owl-inspired TE fringes in practical applications of low-noise fluid machinery.
... Observing how birds can fly has been the subject of scientific interests for many decades bio-inspired design to improve performances and reduce the noise of the wing devices. An interesting study (Lawley et al. (2019)) aimed at characterising the structure of real owls flying and the near wake dynamics by means of long-duration PIV in a wind tunnel at a speed of 8 m/s. The study revealed that the near wake of the owl compared to other birds did not exhibit any apparent shedding of organized vortices but more chaotic patterns. ...
This thesis aims at investigating efficient concepts for morphing wings design in different scales (reduced and near scale one), in the subsonic regime and at different stages of the flight. The study has been carried out by High-Fidelity numerical simulations using adapted turbulence modelling closures able to sensitise the coherent structures development. A special attention is paid to the fundamental mechanisms and physical understanding of the flow around the wing and their modification when the morphing is activated. Electroactive morphing concepts are implemented in the Navier Stokes MultiBlock (NSMB) solver and a large parametric study concerning the actuation frequencies and amplitudes has been accomplished in a similar way in synergy with the experimental studies in the H2020 European research project SMS, "Smart Morphing and Sensing for aeronautical configurations". The morphing concepts investigated in this thesis concern the effects of near trailing edge actuation and low deformation in the frequency range of (30 - 400) Hz by means of piezoactuators, as well as by high deformation cambering (as in experiments through Electromechanical or Shape Memory Alloys actuations), in low frequency (order of 1Hz) and finally, by associating both, in the context of hybrid electroactive morphing. The considered prototypes have been those of the SMS project, the Reduced scale (RS) and the Large Scale (LS) two element wing-flap high lift prototype of an A320 wing. Two- and three-dimensional simulations were carried out using adopted turbulence modelling approaches as the Organised Eddy Simulation OES model and Hybrid methods as the Detached Eddy Simulation, to reveal the surrounding turbulence and the wake's coherent structures as the Kelvin Helmholtz and von Kármán vortices leading to the alternating vortices in the wake using spectral analysis. These models have been able to capture the morphing effects through the actuations and to provide the aerodynamic performance increase. It has been shown that optimal trailing edge vibrations are able to suppress the three-dimensional motion in the wake and to lead to a lift increase of +4.28 % and lift-to-drag of +1.61 % on the RS prototype. An optimal vibration has been studied in case of the LS high lift prototype and provided a lift increase of +0.55 % together with noise sources reduction past the trailing edge of -15 to -20 dB. An optimal cambering system has been derived and studied for the LS prototype, being near full scale and leading to a lift-to-drag performance of +3.26 %. These benefits have been studied in the case of a full A320 aircraft and provided a +2.24 % lift-to-drag increase. Finally the simultaneous and bio-inspired association of the cambering and the trailing edge vibrations in the hybrid electroactive morphing context, revealed a -0.71 % of drag decrease, leading to a considerable augmentation of lift-to-drag ratio of +0.50 % compared to only cambering. These investigations have been carried out in the H2020 N° 723402 European Research programme SMS project.
... Direct measurements of aerodynamic forces during in vivo bird flight are very challenging. Lawley et al. (2019) and Nafi et al. (2020) reported near-wake flow measurements for a boobook owl freely flying in a climatic wind tunnel. The latter study verified that in comparison to other birds, the owl aerodynamic performance was low, while the former demonstrated strong turbulent suppression in the wake, which can potentially muffle its aeroacoustic (aerodynamic noise) signature. ...
... The near wake as manifested by the Q-isosurfaces in Fig. 13, and vorticity magnitude in Fig. 14 show a fair degree of coherence and high levels of turbulence and mixing. The strong growth of the wake in the vertical direction, resulting from the shedding of large vortical structures from the leading edge during the transition phase in the cycle, was not observed in experiments involving other owl species (i.e., Lawley et al. 2019). Given that the active or passive deformations of the base wing structure are captured by the proposed computational model we can attribute these differences to the micro-features suggesting that they may also contribute in confining the wake and thus, reducing its momentum. ...
Synopsis
The fluid dynamics of owls in flapping flight is studied by coordinated experiments and computations. The great horned owl was selected, which is nocturnal, stealthy, and relatively large sized raptor. On the experimental side, perch-to-perch flight was considered in an open wind tunnel. The owl kinematics was captured with multiple cameras from different view angles. The kinematic extraction was central in driving the computations, which were designed to resolve all significant spatio-temporal scales in the flow with an unprecedented level of resolution. The wing geometry was extracted from the planform image of the owl wing and a three-dimensional model, the reference configuration, was reconstructed. This configuration was then deformed in time to best match the kinematics recorded during flights utilizing an image-registration technique based on the large deformation diffeomorphic metric mapping framework. All simulations were conducted using an eddy-resolving, high-fidelity, solver, where the large displacements/deformations of the flapping owl model were introduced with an immersed boundary formulation. We report detailed information on the spatio-temporal flow dynamics in the near wake including variables that are challenging to measure with sufficient accuracy, such as aerodynamic forces. At the same time, our results indicate that high-fidelity computations over smooth wings may have limitations in capturing the full range of flow phenomena in owl flight. The growth and subsequent separation of the laminar boundary layers developing over the wings in this Reynolds number regime is sensitive to the surface micro-features that are unique to each species.
... All the studies observed that the presence of the flow separation and bubble formation improved on the lower side and moved to the upper side as the AOA increases. Lawley et al. (2019) showed that flow in the near wake region of a boobook owl was disorganized and characterized by small turbulent scales and reduced dynamic pressure field. To the best of our knowledge, no flow field characteristics around hawks during flight have been established. ...
... Therefore, the paradox is how does a more, turbulent wake suppress the level of aerodynamic noise which is a function of the turbulent activity? In our previous study (Lawley et al. 2019), we used PIV to compare the downstream wake flow characteristics of a freely flying boobook owl (Ninox boobook) with two other bird species: European starling (Sturnus vulgaris) and western sandpiper (Calidris mauri). We showed that flow in the near wake region was disorganized and characterized by small turbulent scales. ...
... The TKEP indicates the process of energy from the large scales (mean flow) to their smallest scales (turbulence); as this value increases, it suggests that turbulence increases and is transported toward the small scales. Our results are in agreement with our previous founding showing that the near wake of a boobook owl during flapping flight was governed by un-energized small scales, where we could not observe a distinct shedding at the vorticity field unlike other birds where organized shedding was observed (Lawley et al. 2019). One may assume that if the TKEP is high, given the energy balance equation for TKE, then the dissipation rate (Fig. 7b) will be high (see, e.g., figure 5.16; Pope 2000 depicting the energy budget components for a self-similar round jet). ...
Synopsis
Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris’s hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds’ flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds’ motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.
... The difference in vorticity magnitude among the birds' wakes may be related to their size and mass difference; which implies that the owl is generating more lift (and therefore also more vorticity) than the other two birds. During the upstroke phase (roughly the left half of each wake) it appears the starling generates large structures of vorticity that are accompanied with high strength, whereas the sandpiper produces smaller vortices with lower strength while the owl generates unorganized vortices with high strength (see for example table 4 in [43]). Such difference in the wake signature can be attributed to the different wing kinematics of the three birds during their upstroke phase. ...
... Experimental Thermal and Fluid Science 113 (2020) 110018 attributed to the morphological structure of the wing which plays a role in its stealth capabilities [42]. Presumably, the owl achieves this by alternating the flow developed over the wing which creates a quiet wake and less turbulent yet appear to be not organized in the region of interest; behind the mid-span region [43]. ...
The flapping wing is one of the most widespread propulsion methods found in nature. The current study focuses on the estimation of unsteady aerodynamic forces on freely flying birds through analysis of wingbeat kinematics and near wake flow measurements using long duration time-resolved particle image velocimetry. Three distinct bird species have been investigated: a shorebird (Western sandpiper, Calidris mauri), a songbird (European starling, Sturnus vulgaris) and a boobook owl (Australian boobook, Ninox boobook), corresponding to different flight strategies. Using long-time sampling data, several wingbeat cycles have been analyzed. Drag and lift were estimated using the momentum equation for viscous flows, revealing a highly unsteady behavior. The owl experienced the highest total drag over the cycle whilst the sandpiper was shown to exert minimal drag compared to the other two birds. The unsteady drag term was found to have a crucial role in the balance of drag (or thrust), particularly during the transition phases. The lift estimation presents a similar distribution over the wingbeat cycle, where both the owl and the sandpiper feature high values of unsteady lift compared to the starling. The large lift variation generated by the sandpiper is presumably used to achieve its high-performance migratory flight. For owls, the large variation of the lift appears to be essential for weight support while flying at slow speeds. The aerodynamic performance of the starling appears to be relatively low which matches their limited migratory behavior. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag and maximize lift during flapping flight by incorporating unsteady motion that interacts with the wake flow dynamics.
We studied the effects of leading-edge serrations on the flow dynamics developed over an owl wing model. Owls are predatory birds. Most owl species are nocturnal, with some active during the day. The nocturnal ones feature stealth capabilities that are partially attributed to their wing microfeatures. One of these microfeatures is small rigid combs (i.e., serrations) aligned at an angle with respect to the incoming flow located at the wings' leading-edge region of the primaries. These serrations are essentially passive flow control devices that enhance some of the owls' flight characteristics, such as aeroacoustics and, potentially, aerodynamics. We performed a comparative study between serrated and non-serrated owl wing models and investigated how the boundary layer over these wings changes in the presence of serrations over a range of angles of attack. Using particle image velocimetry (PIV), we measured the mean and turbulent flow characteristics and analyzed the flow patterns within the boundary layer region. Our experimental study suggests that leading-edge serrations modify the boundary layer over the wing at all angles of attack, but not in a similar manner. At low angles of attack (<20o), the serrations amplified the turbulence activity over the wing planform without causing any significant change in the mean flow. At 20oangle of attack, the serrations act to suppress existing turbulence conditions, presumably by causing an earlier separation closer to the leading-edge region, thus enabling the flow to reattach prior to shedding downstream into the wake. Following the pressure Hessian equation, turbulence suppression reduces the pressure fluctuations gradients. This reduction over the wing would weaken, to some extent, the scattering of aerodynamic noise in the near wake region.
Synopsis
Animal wings produce an acoustic signature in flight. Many owls are able to suppress this noise to fly quietly relative to other birds. Instead of silent flight, certain birds have conversely evolved to produce extra sound with their wings for communication. The papers in this symposium synthesize ongoing research in “animal aeroacoustics”: the study of how animal flight produces an acoustic signature, its biological context, and possible bio-inspired engineering applications. Three papers present research on flycatchers and doves, highlighting work that continues to uncover new physical mechanisms by which bird wings can make communication sounds. Quiet flight evolves in the context of a predator–prey interaction, either to help predators such as owls hear its prey better, or to prevent the prey from hearing the approaching predator. Two papers present work on hearing in owls and insect prey. Additional papers focus on the sounds produced by wings during flight, and on the fluid mechanics of force production by flapping wings. For instance, there is evidence that birds such as nightbirds, hawks, or falcons may also have quiet flight. Bat flight appears to be quieter than bird flight, for reasons that are not fully explored. Several research avenues remain open, including the role of flapping versus gliding flight or the physical acoustic mechanisms by which flight sounds are reduced. The convergent interest of the biology and engineering communities on quiet owl flight comes at a time of nascent developments in the energy and transportation sectors, where noise and its perception are formidable obstacles.
Synopsis
We raise and explore possible answers to three questions about the evolution and ecology of silent flight of owls: (1) do owls fly silently for stealth, or is it to reduce self-masking? Current evidence slightly favors the self-masking hypothesis, but this question remains unsettled. (2) Two of the derived wing features that apparently evolved to suppress flight sound are the vane fringes and dorsal velvet of owl wing feathers. Do these two features suppress aerodynamic noise (sounds generated by airflow), or do they instead reduce structural noise, such as frictional sounds of feathers rubbing during flight? The aerodynamic noise hypothesis lacks empirical support. Several lines of evidence instead support the hypothesis that the velvet and fringe reduce frictional sound, including: the anatomical location of the fringe and velvet, which is best developed in wing and tail regions prone to rubbing, rather than in areas exposed to airflow; the acoustic signature of rubbing, which is broadband and includes ultrasound, is present in the flight of other birds but not owls; and the apparent relationship between the velvet and friction barbules found on the remiges of other birds. (3) Have other animals also evolved silent flight? Wing features in nightbirds (nocturnal members of Caprimulgiformes) suggest that they may have independently evolved to fly in relative silence, as have more than one diurnal hawk (Accipitriformes). We hypothesize that bird flight is noisy because wing feathers are intrinsically predisposed to rub and make frictional noise. This hypothesis suggests a new perspective: rather than regarding owls as silent, perhaps it is bird flight that is loud. This implies that bats may be an overlooked model for silent flight. Owl flight may not be the best (and certainly, not the only) model for “bio-inspiration” of silent flight.
