| Summary: | In this thesis, a numerical investigation of Fluid Structure Interaction (FSI) problems has been carried out. Such Fluid Structure Interactions (FSI) have been the subject of interest in many studies as they are encountered in almost all practical engineering applications. In particular Vortex Induced Vibrations (VIV) are of great interest, as the oscillation of a structure due to the flow can lead to fatigue and even failure. The fluid flow and the motion of the body are strongly coupled because the fluid exerts forces on the body, which in turn affects the flow field. The flow exerts periodic forces on the body due to the vortex shedding and the body tends to oscillate. The wake behind the body changes considerably depending on the lift and drag force coeffcients, amplitude and frequency of oscillation. Thus, understanding the behavior of the turbulent wake behind the body is crucial to the understanding of any VIV problem.
In the present study, the methodology for analyzing the turbulent energy budget in the wake of structures immersed in a fluid flow has been developed. The analysis of a periodically excited turbulent mixing layer using Large Eddy Simulations (LES) provides a validation test case to assess the accuracy of this methodology. The determination of large scale quasi-organized coherent structures were done by phase averaging. Then, turbulent energy production and their mutual interactions between the mean flow, the coherent structures, and the random flow is computed using the triple decomposition approach to understand the energy transfer between organized and stochastic turbulent structures.
Next, numerical investigations of an elastically mounted circular cylinder undergoing VIV have been carried out. The cylinder motion is restricted to oscillate only in the transverse direction of flow with one degree of freedom. The Immersed Boundary Method (IBM) is used to simulate the moving boundaries, thereby requiring a single cartesian mesh for all time steps. Simulations are carried out at moderate Reynolds number of Re = 8000 with a low mass-damping parameter at different reduced velocities and damping ratios. The response of self-regulating cylinder in cross-flow were compared with the results of another numerical study.
The analysis is focused on a cylinder vibrating only in the synchronization region where the frequency of vortex shedding is locked to the frequency of oscillation of the cylinder as this is the most challenging and most hazardous regime. The results give very interesting insight into the wake dynamics of the cylinders undergoing VIV and the flow physics in such Fluid-Structure Interaction problems. The results show that the maximum amplitude of a VIV is limited by the ability of the mean flow to feed energy to the wake vortices.
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