Experimental Characterization of the Flow around a Bat Wing

Understanding the flow in the boundary layer around a bat’s wing is essential for elucidating the relationship between its aerial agility and wing kinematics. The flow field near the surface of a dissected bat wing is studied in a wind tunnel using stereo-PIV measurement. Imaging of the particles near the surface is challenging due to reflections from the wing features, such as bones (digits) and hairs. To resolve this problem, the tracer particles, aerosols of Polyethylene glycol generated by a Laskin Nozzle, are mixed with fluorescent Rhodamine 6G dye, and the images are recorded through a high-pass filter that removes most of the green light reflected from the surface but transmits the yellow- orange fluorescence emitted from the tracers. Settling of the aerosols on the surface, particularly the hair, over time eventually causes fluorescence from the surface, degrading the image quality, but also allows detection of the hair motion. Data are acquired at flow speeds ranging from 4 to 10 m/s, and incidence angles of 0^o to 20^o, in a series of streamwise-wall normal planes covering the entire wingspan. The PIV results highlight the impact of wing curvature on the flow structure and the effect of digits in creating region of low momentum and flow separation.