Meeting Abstract

S11.11  Friday, Jan. 7  Swifts have an edge on drag reduction LENTINK, D*; DE KAT, R; Wageningen University; Delft University of Technology david.lentink@wur.nl

Bird wings are buildup by overlapping feathers, which gives the wing a wavy surface with distinct edges formed by the rachis. The aerodynamic function of this wavy surface roughness is unclear. To better understand its effect on the boundary layer flow over bird wings we measured the location of laminar-turbulence transition in the boundary layer of swift (Apus apus) wings. Next we approximated the wavy surface roughness, using strips of tape on model wings, to test its effect on laminar-turbulent transition. Counter-intuitively the wavy surface roughness delays transition to turbulence on our model wings at low glide speeds, near the lower limit that swifts can attain. At high glide speeds we find that wavy surface roughness promotes transition to turbulence. To better understand how wavy surface roughness can modify glide performance we measured the corresponding lift and drag forces. We found that wavy surface roughness significantly improves glide performance at low speed through delayed turbulence transition; yielding a more laminar flow over swift wings.