Woodruff School of Mechanical Engineering
Mechanical Engineering Seminar
Bio-Inspired Flow Control Derived from Shark Skin and Butterfly Scales
Dr. Amy Lang
University of Alabama, Aerospace Engineering & Mechanics
Monday, July 8, 2013 at 11:00:00 AM
MRDC Building, Room 4211
Dr. David Hu
Sharks and butterflies swim and fly in two completely different flow regimes, yet the structure of their surfaces interacting with the surrounding fluid look amazingly similar at first glance. Both are not smooth but have unique microgeometries that potentially help the animals move more efficiently through the surrounding fluid. The capability for shark skin to control flow separation has been theorized but not validated. Biological studies on specimens of the shortfin mako (Isurus oxyrinchus), considered to be one of the fastest and most agile marine predators, have determined the passive bristling capability of shark scales as a function of body location. Observations under a microscope found that scales can be easily manipulated to angles in excess of 50o in a region on the flank behind the gills; flexible scales were also found on the caudal and pectoral fin. High contragility, or the ability to change direction while already in a turn, necessitates minimal pressure drag and corresponding control of flow separation on body regions behind the point of maximum girth. Thus, these results coincide with regions on the body where separation of the boundary layer is most likely to occur during swimming maneuvers. SEM and histological evidence suggest that bristling capability results from a reduction of the size and change in shape of the scale base anchored into the skin, such that the base length has been reduced in the area that would protrude out of the skin as the scale pivots when bristled. It is proposed that flow reversal, first occurring inside the low speed streaks during incipient turbulent boundary layer separation, initiates this passive, flow-actuated separation control mechanism. The second study serves to quantify the drag over a bio-inspired surface patterning modelled after the scales observed on the Monarch butterfly (Danaus plexippus). Butterfly scales cover the wing in a roof shingle pattern and have a characteristic size of about 0.1 mm. High speed video of Monarch specimens in flight showed a distinct reduction in aerodynamic aptitude for those with the scales removed. Microscopic studies of the wing identified the microgeometry resulting from the scales, which protrude up and out from the wing surface, and a model of this geometry was used for both experimental and computational study. The drag induced over patterned surfaces was measured. Flow transverse, both forward and reverse, resulted in a 25-40% drag decrease while flow parallel to the rows increased the drag upwards of 100%. CFD analysis using Fluent yielded similar data for the transverse flow and attributed the drag reduction to the formation of embedded vortices resulting in a “roller bearing” effect. The effect of the scales should work to decrease net drag while potentially increasing thrust and thus overall lower power requirements for flapping flight.
Dr. Amy Lang graduated from Caltech in 1997 with a PhD in Aeronautics. She is currently at the University of Alabama as an Associate Professor in the Aerospace Engineering & Mechanics department. To support her experimental fluid dynamics research related to studying shark skin and butterfly scales as a means of boundary layer control, she has received funding to date from the National Science Foundation (including a collaborative grant with biologists), Lindbergh Foundation, NASA and the Army. She also has received funding to run a NSF Research Experience for Undergraduates program in fluid mechanics since 2008.