Aerodynamic effects of varying aspect ratio of flapping wings at low Reynolds numbers relevant to the flight of tiny insects

A significant transition occurs between chord-based Reynolds number (Re) of approximately 30 and 60 for revolving elliptical wings at constant angle of attack. While significant spanwise flow, attached leading edge vortex (LEV), and separated trailing edge vortex (TEV) are observed at higher Re, diminished spanwise flow and attached LEV and TEV are observed at lower Re. The effects of aspect ratio (AR), defined herein as the ratio of wingspan squared to total wing area, have not been examined at low Re relevant to the flight of tiny insects. We experimentally measured time-varying lift and drag forces on elliptical wing models with AR of 3, 5 and 7 (identical in wing span) at Re=20, with each wing revolving through 180 degrees about a horizontal stroke plane at a constant angle of attack. The results showed that angle of attack had minimal effect on the lift coefficients. Drag coefficient increased with decreasing AR, particularly for high angles of attack. Flow field data will be presented using 2D particle image velocimetry to further investigate the effects of AR.