Abstract
Bluff bodies, including cylinders and prisms, are widely utilized in engineering structures as integral components of larger systems such as risers, heat exchangers, bridges, and offshore structures. However, these bluff bodies are susceptible to flow-induced vibrations (FIV), caused by fluid flow forces acting upon them. These vibrations can lead to structural fatigue and eventual failure. Understanding and investigating the response of bluff bodies under various operating conditions is crucial for preventing such failures. Among the different types of FIV, two major phenomena are vortex-induced vibrations (VIV) and transverse galloping. VIV occurs when the shedding frequency of vortices from the bluff body synchronizes with the body’s oscillation frequency, resulting in large amplitude oscillations within a specific range of flow velocities known as the lock-in region. Transverse galloping, on the other hand, involves large amplitude, low-frequency oscillations in the transverse direction, with the amplitude increasing indefinitely as flow velocity rises. The occurrence of VIV or galloping depends on factors like the geometry and angle of attack relative to the incoming flow. While most studies have focused on FIV in flexibly mounted rigid cylinders with oscillation restricted to a single degree of freedom (DoF), FIV is observed in multiple DoF in many real world applications such as mooring lines, heat exchangers, offshore wind turbines etc. For this reason, this thesis explores FIV in two DoF, which exhibit complex large amplitude oscillations at higher vibrational modes due to additional variables associated with the dynamics of flexible systems. The objective of this thesis is to enhance our understanding of complex FIV behavior in flexible structures by examining the effect of additional degrees of freedom on the FIV response. To achieve this, a systematic and controllable experimental setup was developed to facilitate changes in the system’s natural frequencies and structural excitation at single or multiple higher modes (single and 2 DoF). Cylinders with different cross-sections were mounted in a 2 DoF mass-spring-damper system, enabling oscillation in the first two vibrational modes in the crossflow direction. The experiments were conducted in a recirculating water tunnel, and the FIV response of the cylinders was assessed in terms of amplitude, oscillation frequency, modal contribution, and associated flow forces. Qualitative studies using hydrogen bubble flow visualization techniques were employed to investigate vortex dynamics in the wake of the structures. In the study of VIV in a circular cylinder with 2 DoF, the analysis focused on the impact of the eigenfrequency ratio between the first and second natural frequencies on the FIV response. It was observed that increasing the eigenfrequency ratio extended the width of the lock-in region, and a secondary lock-in region emerged at higher eigenfrequency ratios. The presence of an additional degree of freedom resulted in both mono and multimodal oscillations throughout the lock-in region. Furthermore, the FIV of a prism with a triangular cross-section was explored by varying the angle of attack while maintaining a constant ratio between the first and second natural frequencies. The investigation revealed the occurrence of both VIV and galloping responses, with higher harmonics in the flow forces playing a significant role in the FIV behavior of the triangular prism. Overall, the findings from this research can contribute to the development of more comprehensive strategies for mitigating FIV-induced structural failures in practical applications.