Low Re flow in butterfly inspired grooves, a potential passive drag reduction method

Butterfly wings are covered with minute inclined scales (for the Monarch these measure around 100 µm with angle of inclination 22°) that align to form rows of roof-like shingles with cavities between them. Flow in the longitudinal direction to the scales encounters increased surface area which increases the laminar drag. However, when the flow is transverse to the scales at low Re = 5-25, a single vortex is formed inside each of the cavities. These embedded vortices act as a “roller bearing effect” to the outer boundary layer. Therefore, when compared to a flat plate a sub-laminar drag is possible. As the flow Re is increased beyond a critical number, the vortex ejects out of the groove causing the vortex to mix with outer boundary layer flow. Since the low momentum vortex mixes with the higher momentum flow, this exchange of momentum results in an increase in drag. Experiments are performed using DPIV in a tow tank facility to determine the critical Re where the embedded vortex ejects out of the cavity to mix with the outer boundary layer flow. For flow below critical Re, comparison of skin friction drag between the grooved and flat plate demonstrates that the butterfly scales can lead to drag reduction.