3DOF galloping analysis based on quasi-steady theory with reference to the aerodynamic stiffness

Abstract eng:
Galloping has been a continuous problem for power transmission lines, bridge towers and cable stays. Although the classical Den Hartog criterion can provide reasonable predictions of galloping and is still being used in a variety of practical situations, it is limited to only acrosswind galloping, which is not always realistic. Recently, there is increasing awareness of the importance of including all three degrees of freedom (3DOF-sway, heave and torsion) for galloping analysis. Therefore, it is necessary to take into account the couplings between the degrees of freedom, especially when the structural natural frequencies are close to each other. A conventional way of conducting stability analysis about the static equilibrium for a 3DOF system is to solve an eigenvalue problem, where the coupling terms can be derived by linearising the change of forces due to the motion. There are two aerodynamic couplings, namely the aerodynamic damping and stiffness. While aerodynamic damping is widely acknowledged to play a pivotal role in galloping, the aerodynamic stiffness has been normally neglected since it is generally considered to be insignificant compared to the structural stiffness, especially for structures with a compact section. However, there seems to be limited research to confirm this impression. Thus, the present work will focus on the significance of the aerodynamic stiffness for a 3DOF system with perfectly tuned and detuned structural natural frequencies. Firstly, a numerical eigenvalue approach will be employed to show the effects of including aerodynamic stiffness along with aerodynamic damping, the results of which will be compared with those of the same 3DOF system but coupled only by aerodynamic damping. Secondly, a case study will be given to stress the necessity of including the aerodynamic stiffness using a full-scale galloping incident reported in the literature.

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