Experimental investigation in nonlinear aerodynamic characteristics of a double-layer truss girder

This study investigated the nonlinear aerodynamic characteristics of a double-layer truss girder at various angles of attack (AoAs) through section model wind tunnel tests. The free vibration test revealed a divergent hard flutter of the girder at 0° AoA and a self-limiting soft flutter of the girder at +3° and +5° AoAs, with responses primarily dominated by the fundamental frequency. The forced vibration test detected significant high-order harmonic components in the aerodynamic forces. Analysis of the hysteresis loops of each decomposed high-order harmonic affirmed the modification of these components on the system's energy exchange behavior over time, but refuted their impact on the occurrence of a limit cycle oscillation (LCO). Conversely, the consistency in the sign changes of the dimensionless aerodynamic work and the phase lag of the linear self-excited-moment to the torsional motion underscores the critical role of the phase difference in the occurrence of soft flutter and LCO. At last, the macroscopic energy distribution of the girder in different degrees of freedom (DOF) varies with amplitude and wind speed was quantified in terms of the three-dimensional (3D) energy map, which confirmed the consistency between the free and forced vibration tests on the section. This also revealed the fundamental mechanisms behind instability hysteresis and aerodynamic discrepancies under different DOF and AoAs.