Sub-laminar surface drag due to the “roller-bearing effect” over butterfly scale inspired grooves

This experimental study investigates the fluid dynamic function of butterfly scales to reduce friction drag below that experienced in a laminar boundary layer. A previous study on flight-testing live monarch butterflies showed that the removal of the scales from the wings had a significant effect on reducing flying efficiency. Monarch butterfly scales have a typical size of 0.1 mm in length, where the tips of the scales extend slightly upward to create microscopic grooves within a roof-shingle patterned surface. The hypothesis was tested that flow passing transverse to the grooves, for very low Reynolds number flow (Red < 10, based on cavity depth) as would occur on a butterfly wing, would result in the formation of cavity vortices that result in a partial slip condition to the outer boundary layer flow, resulting in reduced surface or skin friction drag. This flow phenomenon is designated the “roller-bearing effect.” Tow tank studies, using both water and high viscosity mineral oil to vary the Reynolds number, measured the flow over various butterfly scale inspired grooves, or transverse cavity, models. A flow visualization study first proved the presence of an entrapped vortex inside each cavity that can act as a fluidic bearing. Drag measurements, using a momentum analysis of the velocity field obtained by digital particle image velocimetry, confirmed the presence of sub-laminar drag at low Reynolds numbers for multiple groove geometries. Results also confirmed that drag reduction is lost, as the Reynolds number increases an order of magnitude, due to instabilities developing for the embedded cavity flow.