Control of the nonlinear flutter of a rectangular cylinder using auxiliary flexible films

Wind tunnel experiments were conducted to test the effects of auxiliary flexible films (FFs) attached at the leading edges of a rectangular cylinder on its nonlinear flutter. The aspect ratio (B/D) of the tested cylinder was 5, with the cylinder height D of 70 mm. The FFs with length of 0.3 D were arranged along span of the tested cylinder. Both free- and forced-vibration tests were carried out with the reduced velocity (U_r) ranging from 1.0 to 9.2, to reveal the control effects and mechanism of the FFs. It was found that the flapping FFs significantly increase the flutter critical wind speed (U_cr) of the tested cylinder by 93%. Moreover, the FFs have reasonable control robustness under external excitations. The forced-vibration results show that the flapping FFs successfully reduce the energy absorption of the tested cylinder from surrounding flow, because of the reducing of both aerodynamic torque and the phase lag between the aerodynamic force and torsional vibration angle. According to the variation of aerodynamic force, flutter derivative and energy map, it seems that the auxiliary FFs make the rectangular cylinder behaves more like a streamlined body, rather than a bluff one.