Sub-laminar drag reduction over butterfly inspired grooves due to the roller-bearing effect

It has long been thought that the scales found on butterfly wings perform an aerodynamic function.  A motion capture flight test study of live Monarch specimens revealed that removing the scales covering their wings reduced their flight efficiency (measured as Joules per flap) by 38%.  One possible function of the scales is to reduce the skin friction of the air passing over their wings through the microscopic grooves found within the roof-shingle pattern formed by the scales.  Tow tank studies were performed to measure the flow over butterfly inspired grooved geometries embedded in a flat plate which confirmed the formation of trapped embedded vortices within the cavities that resulted in sub-laminar surface drag due to the roller-bearing effect.  In other words, at low enough Re (Re below 100 based on cavity depth) stable vortices form in the grooves that allow the outer boundary layer flow to pass over the grooved surface with lower drag due to the partial slip condition when passing over the embedded cavity vortices.  Various geometries were tested based on the butterfly scale geometry.  The highest drag reduction measured 26.3% over grooves with a 45 degree wall angle and 2:1 aspect ratio at a Re of 8.5, which is dynamically similar to the flow over butterfly wings.