Particle-image velocimetry investigation of the fluid-structure interaction mechanisms of a natural owl wing

Abstract

The increasing interest in the development of small flying air vehicles has given rise to a strong need to thoroughly understand low-speed aerodynamics. The barn owl is a well-known example of a biological system that possesses a high level of adaptation to its habitat and as such can inspire future small-scale air vehicle design. The combination of the owl-specific wing geometry and plumage adaptations with the flexibility of the wing structure yields a highly complex flow field, still enabling the owl to perform stable and at the same time silent low-speed gliding flight. To investigate the effects leading to such a characteristic flight, time-resolved stereoscopic particle-image velocimetry (TR-SPIV) measurements are performed on a prepared natural owl wing in a range of angles of attack 0° ≤ α ≤ 6° and Reynolds numbers 40,000 ≤ Re(c) ≤ 120,000 based on the chord length at a position located at 30% of the halfspan from the owl's body. The flow field does not show any flow separation on the suction side, whereas flow separation is found on the pressure side for all investigated cases. The flow field on the pressure side is characterized by large-scale vortices which interact with the flexible wing structure. The good agreement of the shedding frequency of the pressure side vortices with the frequency of the trailing-edge deflection indicates that the structural deformation is induced by the flow field on the pressure side. Additionally, the reduction of the time-averaged mean wing curvature at high Reynolds numbers indicates a passive lift-control mechanism that provides constant lift in the entire flight envelope of the owl.

Distribution of the characteristics of barbs and barbules on barn owl wing feathers

“…(Winzen et al. , ; Winzen & Roidl, ) that demonstrate an even better aerodynamic behavior of natural owl wings than of artificial wings equipped with a velvet‐like surface.…”

“…, ; Winzen et al. , , ; Winzen & Roidl, ), the shapes of the barbs and barbules that form the velvet‐like surface may do so, too. Thus, here we tested the hypothesis that the characteristics of the velvet‐like surface change along the chordwise and spanwise directions of the T. furcata pratincola wing.…”

Features of owl wings that promote silent flight

“…Winzen et al [43,44] used time resolved particle-image velocimetry to investigate the fluid-structure interaction of a barn owl wing. They found no flow separation on the suction side for Reynolds numbers between 40 000 and 120 000 at spanwise positions of 0.3, 0.5, 0.7 and 0.9 and angles of attack, corresponding to the flight envelope of the barn owl, varying between 08 and 68.…”

A Wind Tunnel Setup for Fluid-Structure Interaction Measurements Using Optical Methods

“…An example is the interaction of a heart valve with the surrounding blood, where different pressure distributions change the shape and position of the valve [1]. The interaction is bidirectional in other examples, such as flapping owl wings or wind turbines [2,3]. The fluid modifies the wing/blade and the deformation leads to additional turbulences.…”