High-speed PIV measurements of the near-wall flow field over hairy surfaces

Abstract

The geometry of the barn owl wing, that is, the planform, the camber line, and the thickness distribution, differs significantly from the wing geometry of other bird species of comparable weight and size. Moreover, the owl wing possesses special features like a velvet-like surface, fringes on the trailing edge, and a serrated leading edge. The influence on the flow field of one of the specific adaptations of the owl wing, namely the velvet-like surface structure on the suction side, was analyzed via high-speed particle-image velocimetry. Measurements were performed in a Reynolds number range of 40,000 ≤ Re _c  ≤ 120,000 based on the chord length and angles of attack of 0° ≤ α ≤ 6°. As a reference, a clean wing model which possesses the geometry of a natural owl wing with its distinct nose region and large thickness in conjunction with a small chordwise position of the maximum thickness was measured. A separation bubble on the suction side of the wing was found to be the dominant flow feature. The results were compared with measurements performed with the same model geometry covered with two artificial surface structures that resemble the surface of the natural wing to investigate the influence of these surfaces on the flow field. The first artificial textile, referred to as velvet 1, was selected to imitate the filament length, density, and thus the softness of the natural surface. Velvet 2, the second artificial texture, possesses longer, softer filaments and a preferred filament direction. A strong influence of the surface structures on the flow field was found for both velvet structures. The velvet seems to force the transition process in the wall-bounded shear layer at higher Reynolds numbers by redistributing the turbulent kinetic energy and thus enables the flow to reattach earlier. This leads to a stabilization and in some cases even to a reduction of the size of the separation bubble on the suction side of the wing.