Harnessing Hydro-kinetic Energy from Wake-Induced Vibration (WIV) of Bluff bodies

Abstract

In this dissertation, the application Wake-Induced Vibration (WIV) of a bluff body for harnessing the kinetic energy of a fluid flow is presented. WIV arises when a body undergoes vibrations in the wake of an upstream body. This project investigates the WIV of a bluff body (circular cylinder), constrained to vibrate in the transverse direction, operating in the wake produced by a stationary and upstream bluff body. The upstream body serves as an energy concentrator and increases the oscillations experienced by the downstream body. An efficient coupling of the spatially and temporally concentrated energy from the upstream body and the downstream and vibrating body will result in WIV being considered as a viable form of renewable energy. 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 wake behind a bluff body augments the hydro-kinetic energy in space as well as time, in the form of a vortex street. Firstly, the kinetic energy distribution of a bluff body (circular cylinder) wake is characterized using numerical modelling, in order to identify the form and density of the available energy. Secondly, the spatial and temporal energy in the wake from different bluff bodies is investigated experimentally to identify a flow energy concentrator that is more suitable for WIV than the circular cylinder. The semicircular, straight-edged triangular, convex-edged triangular and trapezoidal cylinders were chosen for this analysis where the semicircular and convex-edged triangular cylinders were found to augment more temporal energy compared to the circular cylinder. Thirdly, experiments were performed in the water channel to investigate the effects of Reynolds number and separation gaps for the different cross-sections of upstream cylinders. The results indicated that an upstream semicircular cylinder produces more efficient WIV in a downstream circular cylinder compared to an upstream circular cylinder. In addition, both numerical and experimental results indicated that a staggered arrangement with 3 ≤ 𝑥/D ≤ 4 and 1 ≤ 𝑦/D ≤ 2 (here, D is the diameter of the cylinder, and x and y 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 33%. This was achieved due to the favourable phase lag between the velocity of the cylinder and force imposed by the fluid. Finally, the effect of mass and damping ratio of the downstream cylinder is investigated to optimize the vibration efficiency of the staggered semicircular-circular cylinder WIV system. The results of this test showed that a lower damping ratio results in lower impedance of the system and hence a larger vibration response. The vibration response was also inversely proportional to the mass ratio, however, a mass ratio of 2 – 3 proved to be the most efficient for the WIV system resulting in a maximum efficiency of 49%.