Toevaluate the influence of the velvet-like surface mimicking the structure on the suction side of barn owl wings on the flow field and on the aerodynamic performance of the wing, high-speed particle image velocimetry and time- resolved force measurements were performed. The Reynolds number was varied in the range of 40;000 ≤ Rec ≤ 120;000, and the range of the angles of attack was 0 ≤ α ≤ 6 deg for the particle image velocimetry measurements and −15 ≤ α ≤ ?20 deg for the force measurements. The flow over a clean-wing model whose geometry corresponds to that of the owl wing without any special adaptations was investigated as a reference case for the same Reynolds number and angle of attack range. This clean wing possesses a laminar separation bubble as the dominant flow feature. Two artificial surfaces were selected to mimic the natural surface concerning the length, density, and thickness of the filaments. The surfaces were able to reduce flow separation. Although this reduction of the separation region might have a positive influence on the pressure drag, the aerodynamic performance of the models with the artificial surfaces was significantly reduced due to the increased skin-friction drag. Furthermore, the models equipped with the velvets possessed a reduced susceptibility to changes in Reynolds number and angle of attack concerning the aerodynamic performance. It can be stated that the velvet surfaces stabilize the flow field at low Reynolds numbers, enabling the owl to fly more slowly and thus more silently.
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