Owls are known for their silent flight. Even though there is some information available on the mechanisms that lead to a reduction of noise emission, neither the morphological basis, nor the biological mechanisms of the owl's silent flight are known. Therefore, we have initiated a systematic analysis of wing morphology in both a specialist, the barn owl, and a generalist, the pigeon. This report presents a comparison between the feathers of the barn owl and the pigeon and emphasise the specific characteristics of the owl's feathers on macroscopic and microscopic level. An understanding of the features and mechanisms underlying this silent flight might eventually be employed for aerodynamic purposes and lead to a new wing design in modern aircrafts. A variety of different feathers (six remiges and six coverts), taken from several specimen in either species, were investigated. Quantitative analysis of digital images and scanning electron microscopy were used for a morphometric characterisation. Although both species have comparable body weights, barn owl feathers were in general larger than pigeon feathers. For both species, the depth and the area of the outer vanes of the remiges were typically smaller than those of the inner vanes. This difference was more pronounced in the barn owl than in the pigeon. Owl feathers also had lesser radiates, longer pennula, and were more translucent than pigeon feathers. The two species achieved smooth edges and regular surfaces of the vanes by different construction principles: while the angles of attachment to the rachis and the length of the barbs was nearly constant for the barn owl, these parameters varied in the pigeon. We also present a quantitative description of several characteristic features of barn owl feathers, e.g., the serrations at the leading edge of the wing, the fringes at the edges of each feather, and the velvet-like dorsal surface. The quantitative description of the feathers and the specific structures of owl feathers can be used as a model for the construction of a biomimetic airplane wing or, in general, as a source for noise-reducing applications on any surfaces subjected to flow fields.

The influence of coupled heave and pitch oscillations on the transonic flowfield over a supercritical DRA 2303 laminar-type airfoil in buffet conditions is analyzed using a combination of time-resolved stereo particle-image velocimetry and unsteady pressure measurements. Three flow configurations at a Mach number of ${Ma}_{\infty }=0.73$ and at an angle of attack of $\alpha =3.5\mathrm{deg}$ are analyzed to show the impact of the fluid-structure interaction on the overall flow structure: the flow over a fixed DRA 2303, the flow over a DRA 2303 undergoing flow-induced motion, and the flow over a DRA 2303 experiencing a forced sinusoidal heave/pitch motion at frequencies in the range of the natural buffet frequency of the airfoil. The results are analyzed regarding the origin and nature of the unsteady shock-boundary-layer interaction, the fluid-structure response behavior, and the influence of the structural motion on the shock oscillation. For the buffet flow, the shock motion is determined by the sound pressure level generated at the trailing edge. At buffet conditions, the frequency of the self-sustained airfoil motion is in the same range as the natural buffet frequency. At forced oscillation at excitation frequencies in the buffet range, the shock oscillation locks into the excitation frequency indeed, but the sound-wave-based feedback loop is hardly influenced by the airfoil motion.