Numerical Study of Owls’ Leading-Edge Serrations

The silent flight ability of owls is often attributed to their unique wing morphology and its interaction with their wingbeat kinematics. Among these distinctive morphological features, leading-edge serrations stand out – these are rigid, miniature, hook-like patterns located at the leading edge of the primary feathers of their wings. It had been hypothesized that these leading-edge serrations serve as a passive flow control mechanism, influencing the aerodynamic performance and potentially affecting the boundary layer development over the wing, subsequently influencing wake flow dynamics. Despite being the subject of research spanning multiple decades, a consensus regarding the aerodynamic mechanisms underpinning owls’ leading-edge serrations remains elusive. While the literature extensively explores the aerodynamic and aeroacoustic properties of serrated wing geometries, the predominant focus had been on owl-like serrations, including sawtooth patterns, wavy configurations, cylindrical shapes, and slitted variations. This emphasis has often overshadowed the authentic geometry of owl wing serrations, which are notably shorter than the wing\u27s chord and oriented at an angle relative to the freestream airflow. In order to shed light on the flow dynamics associated with owls\u27 leading-edge serrations, this study delves into numerically simulating the flow field surrounding an owl wing, meticulously replicating the serrated leading-edge geometry, at an intermediate chord-based Reynolds number (40000). A direct numerical simulation (DNS) approach is employed to simulate the fluid flow problem, where the Navier-Stokes equations for incompressible flow 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. Two wing planforms are considered for numerical analysis: one featuring leading-edge serrations and another without them. The 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, and thus augmenting stall resistance. However, 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. It is also found that serration increases turbulence level in the downstream flow. Turbulent momentum transfer near the trailing edge is significantly 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. Turbulent budget analysis at the leading-edge shear layer demonstrates that serration reduces turbulence production in the immediate vicinity; however, the reduction effect does not persist further downstream when the shear layer rolls up, and eventually merges with a large separation vortex. In the wake of the serrated wing, integral scale was found to be larger than the smooth wing which implies that serrations at the leading-edge does not promote scale reduction at the wake

Numerical Study of Owls\u27 Leading-edge Serrations

Owls\u27 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\u27 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\u27 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. This article was published as Open Access through the CCU Libraries Open Access Publishing Fund. The article was first published in Physics of Fluids: https://doi.org/10.1063/5.017414

The role of leading-edge serrations in controlling the flow over owls’ wing

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\u27 leading-edge region of the primaries. These serrations are essentially passive flow control devices that enhance some of the owls\u27 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, 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 ( \u3c 20° ), the serrations amplified the turbulence activity over the wing planform without causing any significant change in the mean flow. At 20° angle 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. This article was published as Open Access through the CCU Libraries Open Access Publishing Fund. The article was first published in the journal Bioinspiration & Biomimetics: https://doi.org/10.1088/1748-3190/acf54

DataSheet_2_On an adaptation of the Reynolds number, applicable to body-caudal-fin aquatic locomotion.pdf

The Reynolds number, which describes the relative importance of viscous and inertial contributions is commonly used to analyze forces on fish and other aquatic animals. However, this number is based on steady, time-independent conditions, while all swimming motions have a periodic component. Here we apply periodic flow conditions to define a new non-dimensional group, which we name the “Periodic Swimming Number, P”, which rectifies this lacuna. This new non-dimensional number embodies the periodic motion and eliminates the arbitrariness of choosing a length scale in the Reynolds number for Body –Caudal-Fin (BCF) swimming. We show that the new number has the advantage of compressing known data on fish swimming to two orders of magnitude, vs. over six required when using the existing Reynolds number and can point to a new comparison of swimming effectiveness for swimming modes.</p

DataSheet_1_On an adaptation of the Reynolds number, applicable to body-caudal-fin aquatic locomotion.pdf

The Reynolds number, which describes the relative importance of viscous and inertial contributions is commonly used to analyze forces on fish and other aquatic animals. However, this number is based on steady, time-independent conditions, while all swimming motions have a periodic component. Here we apply periodic flow conditions to define a new non-dimensional group, which we name the “Periodic Swimming Number, P”, which rectifies this lacuna. This new non-dimensional number embodies the periodic motion and eliminates the arbitrariness of choosing a length scale in the Reynolds number for Body –Caudal-Fin (BCF) swimming. We show that the new number has the advantage of compressing known data on fish swimming to two orders of magnitude, vs. over six required when using the existing Reynolds number and can point to a new comparison of swimming effectiveness for swimming modes.</p