Energy-driven mechanism of a flat box girder in bending–torsional coupled flutter

Various nonlinear aerodynamic force frameworks have been proposed to explain the complicated nonlinear phenomena during bending–torsional coupled flutter. Whereas these frameworks are mainly applicable to their respective situations and lack universality. The present study aims to explore the energy-driven mechanism of a flat box girder in bending–torsional coupled flutter from the fundamental perspective of energy. Experimental investigations on a flat box girder were conducted using the forced vibration method to study nonlinear aerodynamic forces under various coupled motion parameters, such as bending and torsional motion amplitudes, as well as the phase lag between them. Based on energy input and dissipation by aerodynamic forces, the concept of dimensionless aerodynamic work was employed to comprehensively quantify the nonlinear aerodynamic effects on structural energy excitation. Nonlinear energy maps for bending–torsional coupled motion states were constructed, and a comparative analysis of the evolving trends in these maps under different motion parameters was performed. The role of phase lag between bending and torsional motions in regulating aerodynamic energy transferring effects is exploited from the phase lag relationships. The mutual energy input effect of the bending and torsional two-degrees-of-system on the occurrence and development of coupled flutter is clarified by energy maps. Finally, the sophisticated multiple limit cycle oscillation states and corresponding development paths are graphically represented utilizing the energy maps with aerodynamic damping form.