Nonlinear aerodynamic and energy input properties of a twin-box girder bridge deck section

By means of computational fluid dynamics (CFD), the nonlinear aeroelastic properties of a bridge deck configuration is investigated in this study in terms of amplitude-dependent flutter derivatives and indicial functions. The results are partially compared with experimental results. It shows that the concerned properties exhibit significant dependence on the motion amplitudes. Moreover, based on flutter derivatives, the nonlinear aerodynamic properties can be divided into two groups: the group with torsional amplitudes less or equal than 10°, and the one with amplitudes larger than 10°. Flow patterns around the section of the two groups differ substantially; one group remains an overall streamlined pattern with locally distributed vortices and detached flow, while the other shows fully detached flow with large vortices emerging and developing drastically. Dynamic load coefficients indicate that, as the motion amplitude increases, the smoothness of the hysteresis loops decreases, suggesting irregular fluctuations of the loads resulted from signature turbulence, which becomes progressively prominent. Energy trapping properties derived from indicial functions are expressed in terms of dimensionless coefficients, of which the results indicate there is no possibility of single-DOF flutter, and coupling between vertical and torsional motions is necessary for flutter instability. Moreover, by the analysis of the phase angles involved in coupling, it is indicated that symmetricity of vertical motion has to be consistent with that of torsional motion in the event of a coupled flutter.