Experimental study on the flapping dynamics for a single flexible filament in a vertical soap film

A flexible filament with one end fixed in a flowing soap film may show sustained flapping. For a certain filament, this flapping occurs only when the flow velocity exceeds a threshold value, which is called critical velocity. A previous study with silk filament demonstrated the following three stable dynamical states: stretched-straight, flapping, and bistable. In the present study, experiments involving filaments made of various materials (spandex, polyester, silk), with different diameters and lengths were tested in the vertical soap film. The effects of some representative parameters including length, diameter, and bending stiffness of the filament, inflow velocity on the amplitude, and frequency of the filament flapping were examined to reveal the flapping dynamics of the filament. Additionally, the critical velocity was specifically determined. The results in two-dimensional fluids showed a discrepancy with previous 3D studies. This discrepancy was mainly because filaments were not entirely immersed in the soap film. Modified models were raised based on previous 3D theories and concurred with our experimental results as well as previous works. The underlying mechanism of flag-in-wind problems was able to be revealed by the two-dimensional experiments and modified models in this study.