Limitations of the flutter derivative model

Flutter derivatives identified from transient (free-decay) vibrations might not be suitable for the analysis of bridge flutter. The application of transient flutter derivatives in the flutter analysis relies on two assumptions: (1) transient flutter derivatives and steady-state flutter derivatives are equivalent and (2) aeroelastic effects are superposable. Both assumptions are challenged in this article. It is shown through transient vibration tests that (1) the aeroelastic-coupling between heaving and rotational motions may switch from one pattern to another as the wind speed varies and (2) some of the transient flutter derivatives may be time-varying. The former implies that the predicted flutter type based on transient flutter derivatives may not be unconditionally consistent with the experimentally observed flutter type; the latter implies the transient flutter derivatives may be physically different from the steady-state flutter derivatives. These two issues undermine the basic assumptions of the flutter analysis of bridges. A possible corollary to this study is that if free vibration is used to predict bridge flutter, we should resort to the steady-state (flutter state) vibration instead of the transient vibration of the sectional model. A revision to the aeroelastic force model is proposed to facilitate the discussion.

Stability Analysis of a Yawing Flat Plate Into the Water Current

The present study deals with the stability analysis of an oscillating flat plate into the water current. The flat plate, which is attached to a torsion spring and located vertically in the water current, has only 1 DOF that is yawing motion. The experiments have shown that as the current velocity exceeds a special threshold, the flat plate becomes unstable and begins to oscillate. This oscillation can be utilized to extract energy. A free vibration experimental technique is used in this study. The experimental results are analyzed using the flutter derivative theory, in which the flutter derivatives of the motion are extracted using GLS (General Least-Square) method. The results confirm that the flat plate becomes dynamically unstable. Also, there is a Liapunov stable fixed point on the origin at the phase portrait of the yawing motion.

Research on the Impact of Damping to the Flutter Derivatives of Steel Truss Suspension Bridge

The flutter derivative is the important basic tache of bridge flutter stability analysis. Taking the Liujiaxia Bridge in Gansu province as the research object, this dissertation studies the impaction of damping ratio on the flutter derivatives and the critical wind speed through different series of section model vibration test. The results showed that the change of vertical bending and torsional damping ratio have no obvious regular influence on the eight flutter derivatives. But the changing of vertical bending and torsional damping ratio have the greatly impact on the critical wind speed at 0° and -3° angle of attack. When it is at 0° angle of attack ,the vertical bending damping ratio ζh is increased by 23%,the torsional damping ratio is increased by 0.63%, the flutter critical wind speed is increased by 4%;when the ζh is increased by 1.04%, the ζα is increased by 0.87% , the flutter critical wind speed is increased by 7%. When it is at -3° angle of attack, the vertical bending damping ratio remained around 0.8%, the torsional damping ratio is increased from 0.65% to 1.05%, the flutter critical wind speed is increased by 14%; when the ζα is increased from 0.65% to 2.05%, the flutter critical wind speed is increased by 24 %.When it is at +3 °angle of attack, the vertical bending and torsion damping ratio have little effect on the flutter critical wind speed.

Research on the Influence of the Aerodynamic Measure on the Flutter Derivative of the Steel Truss Suspension Bridge

Flutter derivative is a significant index of the structure flutter stability. Identifying flutter derivative precisely contributes to the bridge flutter stability analyzing. In this paper, we take a research on the Liujiaxia Bridge in Gansu Province, China. Different flutter derivatives, which were got via segment model vibration tests with different aerodynamic measures, were classified, and made comparison in order to get the law of how different aerodynamic measures effect on the flutter derivative. The results show that, setting central stabilized plate, Build-in deflector, flange plate all affect flutter derivative significantly, which leads to changes in the flutter critical wind velocity of the structure. Setting central stabilized plate above the deck contributes to identify the flutter derivative of the 0° and positive attack angle, while setting central stabilized plate will contribute to flutter derivative identification at negative angles. It will make it difficult to identify the flutter derivative at 0° and -3° if the built-in deflector was set. Wind plate contributes to the identification of the flutter derivative at +3°, however, it will make it harder to identify the flutter derivative at 0° and -3°.