AN EXPERIMENTAL STUDY OF FLAG FLUTTER

摘要

We present an experimental study of the flutter instability of a flag. Our experiments are performed in a low turbulence horizontal wind tunnel. The flags tested are made of paper or plastic of different flexural rigidities. One end of the flag is clamped to a profiled and vertical mast, the other end is left free. At low velocities, the flag is parallel to the flow and motionless. Above a critical wind velocity, the flag exhibits periodic flutter. The bifurcation is subcritical and the system shows a strong hysteresis cycle. We particulary focus on the variation of the instability threshold with respect to two non-dimensional parameters: the mass ratio and the aspect ratio. When the mass ratio increases, different flutter modes appear. We have studied systematically the effect of aspect ratio on the instability threshold. In agreement with a recent theoretical study , the critical velocity is a decreasing function of aspect ratio as long as three-dimensional effects are negligible (when the flag span is not too large).