Date of Award

5-15-2022

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

Christopher G. Guglielmo

Third Advisor

Gregory A. Kopp

Additional Advisors

Christopher E. Hill; F. Javier Diez-Garias; Shaowu Bao

Abstract

Owls exhibit unique flight capabilities in the low Reynolds number flow regime which is prone to complex viscous flow phenomena. They possess unique feather features and flexible wing structures which are postulated to help them fly nearly silently and stably at low speeds in a complex flow setting. Understanding the aerodynamics of owls could pave the way to enhance the future designs of small flying vehicles. Though it has been a focus of research over multiple decades, no conclusive agreement has been attained on the aerodynamic mechanisms associated with owl flight. Particularly, the aerodynamics of flapping owl flight is severely understudied and there is a gap in the literature regarding the energetics of the owl’s flight. The major goal of this research is to advance the understanding of the owl’s aerodynamics by investigating the near-wake turbulent flow features, aerodynamic force characteristics and energy expenditure during free forward flight. Understanding the downstream wake-flow dynamics of owl flight can elucidate the aerodynamic mechanisms employed by owls during flight and provide insight into the fluid-owl interaction characteristics. A great horned owl (Bubo virginianus) was chosen for the investigation in the current research. A similar-sized bird of prey, Harris’s hawk (Parabuteo unicinctus) was also chosen to conduct a comparative analysis between two distinct raptors. Both raptors were trained to fly inside a large wind tunnel in a perch-to-perch flight style. The wake-flows behind the freely flying birds were sampled with a high-speed, long-duration time-resolved PIV system, while the kinematics of the birds were captured using multiple high-speed cameras, simultaneously. Multiple flights were conducted over a span of two days and large velocity and kinematics data sets were acquired. The kinematics images enabled us to estimate the flight speed of the birds during each flight. Kinematic analysis has been performed to compare the characteristics of wingbeat kinematics between both birds. Using the theoretical models concurrently with the experimental data, the total aerodynamic power output during a level flight and an intermittent flight have been estimated for both the birds. In this study, a primary attempt has been made to estimate the mechanical power output of a great horned owl during its typical forward flight speed and to establish the power-speed relationship traits. Compared to the hawk owl displayed higher aerodynamic power output during level flight at relatively lower flight speed. Though both birds had similar wingspan, frequency, and amplitude, the owl displayed relatively shorter downstroke. The effect of wing morphology and the other kinematics parameters on the aerodynamic power output has been highlighted. In order to examine the aerodynamic force characteristics during flapping flight, the sectional profile drag coefficient and lift coefficient are estimated over a single wingbeat cycle directly from the PIV wake velocity fields using viscous momentum equations and compared between both birds. The near wake-flow fields of both raptors have been described by mean velocity fields, vorticity field, and turbulent fluctuation fields. Besides, spatial and temporal correlation of the turbulent fluctuations, Reynolds stresses, turbulent kinetic energy, production, and dissipation terms have also been evaluated in the near wake-flow. It is found that the near wake of the owl is characterized by significantly higher turbulent activity than the hawk. To the best of our knowledge, it is observed for the first time that the turbulent eddies in the wake of the owl’s downstream flow field are characterized by small time scales (turnover time) which can have strong implications in its noiseless flight associated with turbulent generated noise.

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