This study investigates the nonlinear aerodynamic characteristics and amplitude-dependent mechanisms of a streamlined box girder during flutter. First, based on free-vibration wind tunnel tests of a sectional model at a 0° wind angle of attack, the weakening effect of initial perturbations on flutter performance and the critical wind speed is elucidated. Subsequently, a forced vibration approach combined with user-defined functions is employed within a computational fluid dynamics framework to simulate the self-excited forces under varying torsional amplitudes and reduced wind speeds. The results confirm the pivotal role of fluid memory effects in governing aerodynamic nonlinearity and the amplitude dependence of flutter derivatives. Through three-dimensional damping maps and energy exchange analyses, both macroscopic and microscopic mechanisms underlying aerodynamic amplitude dependence are explored. The findings reveal that uncoupled motion exhibits destabilizing effects only under conditions of high wind speeds and large amplitudes, whereas the aerodynamic damping (i.e., energy transfer) induced by coupled motion serves as the primary driving mechanism for perturbation-triggered divergent flutter. From the perspective of surface pressure distribution, the energy exchange is found to originate predominantly from the windward fairing and the leading edge of the upper deck, providing clear guidance for aerodynamic optimization of bridge deck sections. Finally, the potential flow mechanisms underlying aerodynamic amplitude dependence are preliminarily examined from the standpoint of sectional flow features.
Li et al. (Fri,) studied this question.