Flutter Characterstics of a Flexible Filament and its Control

Flow induced flutter of a thin flexible filament attached to the trailing-edge of a NACA 0015 airfoil was studied experimentally in a low-speed wind tunnel. Hotwire and PIV measurements were carried out to study the nature of the flow field around the airfoil and the filament at different wind speeds. At a critical wind speed, the filament starts to flutter and achieves a mode of limit cycle oscillation (LCO). Beyond the critical wind speed, the flutter frequency of the filament increases with the increase in wind speed. Also, a change in flutter mode, from 2 to 3 mode LCO, was observed at a higher wind speed. A trip wire was introduced to the fore-body to control the flutter characteristics of the filament by modifying the boundary layer characteristics. Trip wire was attached to the airfoil surface (one or both sides of the airfoil) at different chord-wise locations. A delay in the mode shape transition from 2nd mode to 3rd mode LCO was observed due to the presence of the trip wire at certain locations. Trip wires attached to both sides of the airfoil, at a distance of 30% of the chord from the leading edge, was found to be the most effective in delaying the transition. Keywords— Flexible filament; Flow-induced flutter, Limit cycle oscillation; Flow control