The wings of the Monarch butterfly exhibit scales with a morphological structure that has a characteristic size on the order of micrometers. This unique micro-patterning results in a surface drag alteration, and leads to an increase in thrust and lift during flapping and gliding flight at high Reynolds numbers (Re ~= 10e3- 10e4). This reduction in energy expended would be important to the Monarch which has the longest migration of any insect. Preliminary results performed by the PIs indicate: (1) flow passing transverse to the rows of scales can decrease the local surface drag by 40% at low Re via a roller bearing effect, (2) flow passing parallel to the rows of scales can increase local surface drag by over 100%, (3) higher drag differences occur in the very low Re regime (Re ~ 5), (4) leading edge vortex strength may vary based on surface drag alteration, and (5) initial flight tests of Monarch butterflies indicate a 10% increase in flapping frequency for those without scales to maintain similar energetic free flight. Proposed work will statistically determine the increased aerodynamic efficiency of butterfly wings with scales through flight testing of live Monarch specimens in a state-of-the-art autonomous tracking facility at the University of Alabama Huntsville; preliminary results have already shown the capability to obtain mm level tracking of body and wing motion at 370 fps. Autorotating drop tests of single Monarch forewings will also be performed by a REU student to measure increased aerodynamic performance with the presence of scales. A series of dynamically scaled tests carried out in high viscosity silicone oil at the University of Alabama will allow for models of butterfly-inspired geometries with an increase in 10 to 100 times in sizing from real scales. The proposed studies will (1) measure variation in the drag coefficient over streamlined models in drop tests with butterfly-inspired surface patterning, (2) utilize DPIV measurements of boundary layer formation in tow tank studies over flat plate models with butterfly-inspired surface patterning, and (3) evaluate leading edge vortex strength variation based on surface patterning inspired by butterfly scales in pitching plate experiments. The third set of experiments will utilize the TSI V3V system for full 3-D volumetric velocity flow measurements recently acquired by the PI through a NSF MRI-R2 award.
Intellectual Merit : The proposed collaborative effort will result in the potentially transformative discovery of a new and unique passive surface drag control methodology derived from butterfly scales functioning at the micro-scale level. Proposed work will test our working hypothesis that local surface drag alteration results in reduced energy requirements for butterflies in flapping and gliding flight. Additional work will better elucidate the surface drag alteration and corresponding vortex control for fluid dynamic confirmation of the hypothesized mechanisms for increased aerodynamic efficiency. This fundamental understanding will advance knowledge for the ultimate use of a butterfly-inspired surface patterning for other engineering applications.
Broader Impacts : Innovations in the field of boundary layer control are needed to provide efficient methodologies to decrease drag (resulting in increased payload, range or fuel savings) for MAVs as well as higher Re applications. A unique, bioinspired technology in the form of a passive microgeometry leading to drag reduction has the potential to impact this area of research with future applications with increased energy conservation and flow control. The discovery of the biological aerodynamic function of butterfly scales would also result. In addition, the proposed study involves a number of other beneficial outcomes including the training of students at all levels and broad dissemination of results in journals/conference proceedings and the public media (e.g. National Geographic Online). Undergraduate student involvement will take place through REU participation with a focus on involving underrepresented groups. Bio-inspired engineering is an excellent topic for public outreach, and butterflies generate a high level of interest.