Model of Bird Alula Generates Arching Vortex on Finite Wings

The inimitable maneuvering capability of birds at high angles of attack is due in part to a miniature collection of feathers located at the bird's wrist termed the alula. The believed aerodynamic benefit of the alula stems from the vortex that it generates when deflected from the wing plane which facilitates the reattachment of flow over the wing. Further understanding of these physics of the alula specifically in the context of finite wings involving complex three-dimensional flow is needed to gauge its use in real world engineering applications. Towards this end, direct force/moment measurements alongside surface oil flow visualizations are conducted in a wind tunnel for which the alula and wing are modeled as rigid flat plates. Results indicate that the high-lift benefit of the alula stems from the formation of a vortex that arches from the leading edge of the wing, at the root of the deflected alula, toward the wing's side edge. The lift enhancement attributed to the alula increases as the alula is placed further from the wing's side edge until a critical distance for which the arching vortex is lost and lift enhancement is subsequently reduced.