Numerical Study of Owls’ Leading-Edge Serrations

Author

Asif Shahriar Nafi, Coastal Carolina University

Date of Award

1-15-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Roi Gurka

Second Advisor

Angelos Hannides

Third Advisor

George Hitt

Additional Advisors

Elias Balaras; Nikolaos Beratlis

Abstract

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's chord and oriented at an angle relative to the freestream airflow. In order to shed light on the flow dynamics associated with owls' 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.

Recommended Citation

Nafi, Asif Shahriar, "Numerical Study of Owls’ Leading-Edge Serrations" (2023). Electronic Theses and Dissertations. 183.
https://digitalcommons.coastal.edu/etd/183