Surface structure and dimensional effects on the aerodynamics of an owl-based wing model

A model wing based on the geometry of the wing of a barn owl was designed, in which the feather structure of the barn-owl wing is approximated by a velvet-like surface. The first objective of this paper is to investigate the impact of artificial surface filaments on the overall flow field of a quasi-2D configuration of the model 3D wing. Two velvet-like surfaces are used and the velocity field is measured by particle-image velocimetry in a chord-length based Reynolds number range 20, 000 <= Re-c <= 60, 000 at angles of attack 0 degrees <= alpha <= 6 degrees. An explanation of the mechanism that leads to the change in the near-wall flow field due to the surface structures is given. The second objective of the paper is the comparison of the 2D and the 3D results and the analysis of the impact of the three-dimensionality on the flow field. The first surface structure (velvet 1) mimics the length and density of the hairs and the softness of the natural owl-wing surface. It diminishes the size of the separation bubble or completely prevents separation. However, at three-dimensional flow the effect of the velvet 1 surface is clearly reduced. The velvet 2 surface consists of longer and thinner filaments than the velvet 1 surface. At the lower Reynolds numbers (Re-c <= 40, 000), the velvet 2 surface structure does not alter the near-wall flow field significantly. However, at Re-c > 40, 000 the velvet 2 surface structure serves as a distributed field of moving roughness elements such that the size of the separation bubble is reduced and becomes nearly independent of the angle of attack. When the three-dimensional flow field at the highest Reynolds number (Re-c = 60, 000) is considered it is evident that the velvet 2 surface yields the aerodynamically more stable flow field. (C) 2012 Published by Elsevier Masson SAS.