Abstract:

In this dissertation, the application of Vortex-Induced Vibration (VIV) and Wake-Induced Vibration (WIV) of a bluff body for harnessing the kinetic energy of a fluid flow is presented. The application of induced vibration due to vortices in harnessing hydrokinetic energy of the fluid is relatively immature and this research work, which is written as a compilation of journal articles, attempts to address major scientific and technological gaps in this field. The project spans both VIV and WIV, with a particular attention to the development of a better understanding of the wake behaviour in a tandem configuration and the effect of boundary layers for harnessing the kinetic energy of the flow. Accordingly, two separate coupled test cases of tandem bodies comprising Coupled Circular-Cylinder (CCC) and Coupled Cylinder-Airfoil (CCA) configurations were proposed and investigated. In the first series of tests on the CCC, two circular cylinders were employed to investigate the unsteady wake interactions on the energy yield. The upstream cylinder was fixed, while the downstream one was mounted on a virtual elastic base with one degree of freedom. The virtual elastic system consisted of a motor and a controller, a belt-pulley transmission and a carriage. In the CCC, the influence of the Reynolds number, gap between cylinders and boundary layers on the dynamic response of the downstream cylinder were numerically and experimentally investigated. In a numerical analyse of the system, a dynamic mesh technique within the ANSYS Fluent package was utilized to simulate the dynamic response of the cylinder. The experimental tests confirmed the numerical outcomes and demonstrated that in the WIV mechanisms, a positive kinetic energy transfer from fluid flow to the cylinder was achieved. It is also observed that the dynamic response of the cylinder under the WIV mechanism differs from the dynamic response of VIV. In addition, both numerical and experimental results indicated that a staggered arrangement with 3.5 <= x0/D <= 4.5 and 1 <= y0/D <= 2 (here, D is the diameter of the cylinder, and x0 and y0 are the horizontal and vertical offsets, respectively) is the optimum arrangement among all test cases to harness the energy of vortices, resulting in a power coefficient of 28%. This was achieved due to the favourable phase lag between the velocity of the cylinder and force imposed by the fluid. The results revealed that for the staggered arrangement of the cylinders, the WIV responses can occur at frequencies outside the range in which VIV is observed. In the second series of tests utilizing a CCA, the downstream circular cylinder was replaced by a symmetric airfoil with two degrees of freedom; heave and pitch. The heave degree of freedom employed the same virtual elastic base used for the CCC experiments. The pitch angle of the foil was actively controlled, as opposed to using passive mechanical impedance, since this enables full control over the foil behaviour, thereby facilitating the adjustment of the angle of attack accurately and rapidly. The results of CCA show that both longitudinal and lateral distances play an important role in the Strouhal number, power density and, consequently, the heave response of the airfoil. In addition, it was shown that the circulation of the vortices was influenced by the gap spacing between the cylinder and the airfoil. Furthermore, it was found that an optimum angle of attack of  = 10 degree is the most efficient for harnessing the energy of vortices with a maximum power coefficient of 30% for cases with 3.5 <= x0/D <= 4.5 and 1 <= y0/D <= 1.5 arrangements. Such a range is narrower laterally when compared with the optimum arrangement of the CCC. This work provided the foundation for further work to utilize the potential of this technology and further explore the opportunity to harness the vortical power in shallow water and ocean currents.