Doctor of Philosophy
Civil and Environmental Engineering
Kopp, Gregory A.
This study investigates the flutter mechanism of a 5,000m bridge with a wide-slotted deck. Preview full aeroelastic model testing indicated that the flutter of this deck was self-sustained rather than violently destructive. Follow-up sectional model tests confirmed that aerodynamic nonlinearity is the cause of the self-sustained flutter. The study aims to understand the mechanisms behind this nonlinearity and develop computational methods to analyze nonlinear aerodynamics.
To understand the mechanisms, CFD and PIV were used to collect data on the flow fields, pressure distribution, aerodynamic forces, and dynamic responses at 85m/s. The PIV phase-averaged flow fields revealed that flutter is not caused by vortex travel. Instead, it is linked to periodic changes in the separation scale of the windward deck, variations in the inflow conditions of the leeward deck, and alterations in the streamline pattern within the wake. The CFD results showed that self-sustained flutter is influenced by nonlinear aerodynamic forces, particularly the highly nonlinear moments. The third-order frequency plays a crucial role. The windward moment acts as the driving force, while the leeward moment acts as a stabilizing factor that limits the vibration amplitude. The combination of PIV flow fields and CFD pressure distribution revealed that the aerodynamic nonlinearity of the windward deck is caused by the periodic changes in the leading-edge separation scale. The aerodynamic nonlinearity of the leeward deck is attributed to the inflow conditions, which are affected by the periodic changes in the height difference between the two decks, resulting in additional wind loading.
The second part of the study addressed the limitations of existing state-space methods for calculating nonlinear aerodynamics. An updated nonlinear form was proposed, maintaining structural similarity. The polynomial expansion was employed to represent nonlinear hysteresis, and the B-curve fitting method was used to account for the amplitude dependence. The study combined the L-M algorithm and the RK-4 method to develop a novel approach for the nonlinear fitting of aerodynamic forces. By substituting the identified coefficients into the dynamic equations, the study demonstrated that this new method has the potential to accurately predict the self-sustained flutter response under various wind speeds.
Summary for Lay Audience
Long-span bridges are susceptible to flutter-induced failures, similar to the Tacoma Narrows Bridge, when subjected to wind loads. However, in certain bridge sections, flutter can manifest as nonlinear self-sustained oscillations rather than the typical linear divergence. This doctoral dissertation focuses on a suspension bridge with a main span of 5,000m and reveals the occurrence of self-sustained flutter at high wind speeds. To investigate this phenomenon, a combination of computational fluid dynamics (CFD) simulations and experimental methods, specifically, Particle Image Velocimetry (PIV) to analyze flow field distribution during flutter, was employed. The research explored various aspects, including aerodynamics, surface pressures, and dynamic flow fields, to comprehend the underlying causes of this nonlinear behavior. Additionally, a mathematical model was proposed to predict this self-sustained flutter phenomenon.
Given the growing attention to self-sustained flutter in bridge research, the findings of this study establish crucial theoretical foundations and provide engineering references for similar scenarios. The research outcomes significantly contribute to the comprehension and prediction of self-sustained flutter in practical bridge studies, offering valuable insights for the design and analysis of large-span bridges subject to wind-induced vibrations.
XIA, JINLIN, "Flow mechanism and force modelling of a central-slotted bridge's self-sustained flutter" (2023). Electronic Thesis and Dissertation Repository. 9847.
Available for download on Thursday, May 29, 2025