Sub-laminar skin friction drag over butterfly inspired grooves due to the ‘roller-bearing effect’

The scales found on butterfly wings are known to perform fluid dynamic functions. This study investigates the aerodynamic function to reduce skin friction drag. A previous flight test study with live monarch butterflies showed that removing scales from the wings reduced flight efficiency (measured in energy change or Joules/flap) on average by 38%. Butterfly scales covering monarch wings have a typical size of 0.1 mm in length, where the tips of the scales curve upwards to create microscopic grooves within the roof-shingle patterned surface. This DPIV study tested the hypothesis that flow passing transverse to the grooves would result in the formation of trapped or embedded vortices that result in a partial slip condition to the outer flow passing over the wing surface. This flow control method to decrease skin friction drag is designated the 'roller-bearing effect'. Tow tank studies, using high viscosity mineral oil to lower the Re, measured the flow over various butterfly scale inspired groove, or transverse cavity, models. Five of the six models tested confirmed the presence of sub-laminar drag at low Reynolds numbers. A maximum value of 26.3% drag reduction was measured for the grooves with a 45-degree wall angle and 2:1 aspect ratio for a Re = 8.5, which is dynamically similar to the air flow over butterfly scales. Results showed that drag reduction is lost as the Re increases by one order of magnitude as the embedded vortices become unstable.