The Role of Preen Oil on Aerodynamic Efficiency as a Drag Reduction Agent.

Energy consumption is a critical aspect in the aviation industry. Some organisms form a list of morphological formations adapting to efficient locomotion, for example, using passive flow control elements to reduce drag. Some of these draw the attention to reach energy-saving goals in human-made machines. A number of birds, to well preserve their feathers and plumage, evolved into developing the uropygial gland exclusively to self-produce preen wax (oil). This oil is assumed to also serve for waterproofing, UV protection and chemo-signaling. In order to examine whether the preen oil have also an aerodynamic function (i.e.: drag reduction), controlled experiments were performed in a wind tunnel using 3D particle tracking velocimetry to measure the near wake flow of a preen oil coated 3D-printed NACA wing. The experiments included a comparison to a non-coated wing over a range of angles of attack at a Re=10⁵. Subsequent wake characteristics have been compared to examine the coated wing aerodynamics. Furthermore, to characterize the flow and shed light on the impact of oil on the wake flow mechanisms, turbulent parameters including Reynolds stresses, energy terms and spectral analysis have been determined.