Meeting Abstract
71.6 Wednesday, Jan. 6 The effect of wingbeat frequency on aerodynamic force and wake structure using a bat-like mechanical flapper BAHLMAN, Joseph Wm*; SCHUNK, Cosima; SWARTZ, Sharon M.; BREUER, Kenneth S.; Brown University; Hochschule Bremen, University of Applied Sciences; Brown University; Brown University joseph_bahlman@brown.edu
The aerodynamic forces produced by a bat’s wings depends on its flapping kinematics. However, the relationship between flapping kinematics and aerodynamic force can be difficult to study in live animals because live animals typically change multiple kinematic parameters simultaneously, and because detailed force measurements cannot be made on a live flying animal. To help isolate the aerodynamic effects of wingbeat frequency, we built a mechanical flapper with bat-like wings. For this experiment, the flapper moved with a single degree of freedom at the shoulder, resulting in a vertical stroke plane with an amplitude of 42 degrees. The planform was modeled after the lesser dog-faced fruit bat, Cynopterus brachyotis . The wing skeleton was constructed from rigid plastic, and the wing membranes made from a deformable latex sheet (500 microns thick). The model was mounted in a wind tunnel on a six-axis load cell, and actuated with wingbeat frequencies ranging from 1 to 8 Hz at wind speeds ranging from 2.5 to 5.0 m/s (reduced frequencies, k, ranging from 0.1 to 0.5). Force measurements were taken simultaneously with particle image velocimetry (PIV). Force, measured with the load cell, increased to a maximum at mid-downstroke and decreased to a minimum at mid-upstroke. The magnitude of maximum and minimum forces increased with flapping frequency so that at 8 Hz, during the downstroke, the maximum lift reached nearly 4 times the amount of lift generated by non-flapping wings, and during the upstroke, the minimum lift was -3 times the non-flapping value. Comparison with the PIV data shows the wake transitioning from a nearly continuous tip vortex structure at 1 Hz to discreet structures alternating with counter-rotating vortices at 8 Hz.
