Drag Measurements over Embedded Cavities Modeled after Butterfly Scales in Low Reynolds Number Couette Flow

Recent research has shown that symmetric, embedded square cavities can reduce the net drag acting on a surface through the formation of embedded vortices. It is hypothesized that the scales on butterfly wings (approximately 100 microns in length), though asymmetric, may act in a similar way resulting in greater flying efficiency. In this experimental study, cavities were modeled based on the geometry observed for bristled butterfly scales. Plates were designed to have parallelogram-shaped embedded cavities with an approximate 2:1 length to depth aspect ratio. The plates were suspended in high viscosity mineral oil above a rotating belt to generate a Couette flow condition such that the cavity Re was maintained in a similar regime as that occurring for the flow over butterfly scales. The net drag forces were measured with a force gauge and compared to flat plate measurements in the same facility. The variation in drag over a range of Reynolds numbers was analyzed.