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.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsfs.2016.0078

Features of owl wings that promote silent flight.

This thesis provides a detailed account of an experimental investigation into the effects of leading edge sinusoidal protrusions (tubercles) on the performance of airfoils. The leading edge geometry was inspired by the morphology of the Humpback whale flipper, which is a highly acrobatic species. The aim of this study is to investigate the potential advantages and disadvantages of incorporating tubercles into the leading edge of an airfoil. Specific parameters have been varied to identify an optimum tubercle configuration in terms of improved lift performance with minimal drag penalties. The investigation has shown that for all tubercle arrangements investigated, increased lift performance in the post-stall regime comes at the expense of degraded lift performance in the pre-stall regime. However, it has also been noted that through optimizing the amplitude and wavelength of the tubercles, pre-stall lift performance approaches the values attained by the unmodified airfoil and post-stall performance is much improved. In general, the configuration which demonstrates the best performance in terms of maximum lift coefficient, maximum stall angle and minimum drag has the smallest amplitude and wavelength tubercles. A new alternative modification has also been explored, whereby sinusoidal surface waviness is incorporated into the airfoil, giving a spanwise variation in local attack angle. Results indicate that optimisation of this configuration leads to similar performance advantages as the best-performing tubercle configuration. It is believed that the flow mechanism responsible for performance variation is similar to tubercles. The deterioration in pre-stall performance for airfoils with tubercles in the current study has been explained in terms of Reynolds number effects and also the relatively weak spanwise flow in the boundary layer. In swept and tapered wings such as the Humpback whale flipper, spanwise flow occurs along the entire span, so the effect of tubercles can be expected to be much larger. Surface pressure measurements have indicated that the region of separation and reattachment for airfoils with tubercles is restricted to the trough between the tubercles rather than extending across the entire span. Hence, leading-edge separation is initiated at the troughs but occurs at a higher angle of attack for other locations, leading to a delayed overall stall for airfoils with tubercles. In addition, integration of the surface pressures along the airfoil chord has indicated that lift, and hence circulation, varies with spanwise position, providing suitable conditions for the formation of streamwise vorticity. A spanwise variation in circulation is also predicted for the wavy airfoil since the relative angle of attack varies along the span. Counter-rotating streamwise vortices have been identified in the troughs between tubercles using particle image velocimetry in a series of cross-streamwise, crosschordwise planes which have not been investigated previously using this technique. The associated peak primary…

https://digital.library.adelaide.edu.au/dspace/bitstream/2440/79211/9/01front.pdf

Effect of leading edge tubercles on airfoil performance.

This study investigates the aerodynamics of the falcon Falco peregrinus while diving During a dive peregrines can reach velocities of more than 320 km h⁻¹ Unfortunately, in freely roaming falcons, these high velocities prohibit a precise determination of flight parameters such as velocity and acceleration as well as body shape and wing contour Therefore, individual F peregrinus were trained to dive in front of a vertical dam with a height of 60 m The presence of a well-defined background allowed us to reconstruct the flight path and the body shape of the falcon during certain flight phases Flight trajectories were obtained with a stereo high-speed camera system In addition, body images of the falcon were taken from two perspectives with a high-resolution digital camera The dam allowed us to match the high-resolution images obtained from the digital camera with the corresponding images taken with the high-speed cameras Using these data we built a life-size model of F peregrinus and used it to measure the drag and lift forces in a wind-tunnel We compared these forces acting on the model with the data obtained from the 3-D flight path trajectory of the diving F peregrinus Visualizations of the flow in the wind-tunnel uncovered details of the flow structure around the falcon's body, which suggests local regions with separation of flow High-resolution pictures of the diving peregrine indicate that feathers pop-up in the equivalent regions, where flow separation in the model falcon occurred

/pdf/diving-flight-aerodynamics-of-a-peregrine-falcon-falco-1olmy36qyk.pdf

Diving-flight aerodynamics of a peregrine falcon (Falco peregrinus).

Biologic flight has undoubtedly intrigued man for thousands of years, yet it has been only the last 100 years or so that any serious challenge has been mounted to the pre-eminence of birds. Although present-day large-scale aircraft are now clearly able to fly higher, faster and farther than any bird or insect, it is obvious that these biological creatures have a mastery of low Reynolds number, unsteady flows that is unrivaled by man-made systems. This paper suggests that biological flight should be examined for mechanisms that may apply to engineered flight systems, especially in the emerging field of small-scale, uninhabited aerial vehicles (UAV). This paper discusses the kinematics and aerodynamics of bird and insect flight, including some aspects of unsteady aerodynamics. The dynamics of flapping wing flight is briefly examined, including gait selection, flapping frequency and amplitude selection, as well as wing planform and angle-of-attack dynamics. Unsteady aerodynamic mechanisms as practiced by small birds and insects are reviewed. Drag reduction morphologies of birds and marine animals are discussed and fruitful areas of research are suggested.

/pdf/biomimetic-flow-control-55210w9fn8.pdf

Biomimetic Flow Control

The feathers of owls possess three adaptations that are held responsible for their quiet flight. These are a comb-like structure at the leading edge of the wing, fringes at the trailing edge, and a...

Silent owl flight: The effect of the leading edge comb

: A study was made of the aerodynamic function of the comblike fixtures found on the leading edge of owl wings. Microphotographs of an owl's wing showed the comb to resemble a row of spanwise twisted airfoils oriented to form a cascade. Smoke flow visualization tests on an owl wing showed that the comb acts as a cascade which turns the flow close to the wing leading edge in a spanwise direction. Flow visualization experiments were run using flat plate and cambered airfoils with combs in a low speed three-dimensional wind tunnel. Results showed that the leading edge comb produced a stationary spanwise vortex that delays flow separation at high angles of attack. The high lift device was related to the vortex lift phenomena observed on delta wing aircraft. The comb's small relative size, simple structure, and lack of moving parts may make it attractive for aircraft use.

/pdf/an-experimental-investigation-of-a-high-lift-device-on-the-2szqkk9neq.pdf

George W. Anderson

Papers

An Experimental Investigation of a High Lift Device on the Owl Wing