Research On Hydrodynamic Characteristics And Harvesting Energy In Ocean Current Power Generation Drived By VIV

Using new renewable energy to replace fossil energy has been attracted more and more attention when non-renewable energy is gradually exhausted and the environment is polluted gradually serious while energy demand grows continuously today. Tidal current energy as a kind of renewable energy with high energy density and abundant reserves has broad prospects for development. Ocean current energy with low velocity is one of tidal current energies, its energy density is propotional to the cube of velocity. While traditional underwater turbine is difficult to realize energy conversion as effectively as wind turbine at low current velocity. Consequently, the driving mode of vortex-induced vibration based on fluid mechanics becomes a kind of choices. Vortex-induced vibration is based on resonance mechanism induced by the vortex shedding when current flows around cylinder to extract current energy, it is a kind of driving power generation methods that is suitable for low speed current environment. This paper mainly studied the parameter range of vortex-induced vibration and the current environment where harvesting energy was maximum. Through the numerical methods of fluid-structure interaction, vortex-induced vibration was numerically studied and the optimal parameters which affected energy extracting which vortex-induced vibration was analyzed. Designed mult-cylindrical oscillation systems as energy conversion devices with different spacing ratios in flow and transverse direction and different arrangements, analyzed detailedly the relationship between the system parameters with vortex-induced vibration responses, hydrodynamic characteristics, vortex model, harvesting energy and energy density. The main research and achievements of this thesis are as follows:(1) Used a kind of two-way fluid-structure coupling numerical method that could better simulate vortex-induced vibration With Arbitrary Lagrangian-Eulerian (ALE) dynamic grid mechanism, fluid equation used incompressible Reynolds Average Navier-Stokes(RANS) equation that was closed by SST k-co turbulence mode. Based on FCBI-C grid cell developed by ADINA system to discrete computation area of fluid, realized efficient numerical simulation which wake vortexes were formed in vortex-induced vibration when cylinder vibrated with high amplitude. Respond branches, amplitudes, frequencies, phase angles, flow field information and vortex shedding characteristics in each response branch at different reduced velocities were obtained. By comparing above results with references, it was confirmed that the numerical method put forward in this thesis was reasonable and reliable and can be used to simulate the vortex-induced vibration phenomenon of sigle cylinder or muti-cylinder.(2) Proposed four kinds of parameters in the harvesting energy from vortex-induced vibration to analyze the influence of mass ratio m*, damping ratio l mass-damping ratio m*l and natural frequency fn,w in water on the vortex-induced vibration response and the energy conversion efficiency ?. The results showed that mass ratio m* and damping ratio l had impact on the energy conversion efficiency ?and effective velocity range of extracting energy. There is an optimal mass-damping ratio value m*l that can obtain maximal energy conversion efficiency ?max and maximal average energy conversion efficiency ?p. Selecting an appropriate value fn,w, can not only make the cylinder's harvesting energy efficiency is maximal but also has a wide range of velocity and reduced velocity.(3) For rectangular arrangement four-cylindrical vibration system, studied the influence of spacing ratio on its hydrodynamic characteristics and extracting energy efficiencies. If extract energy from ocean current on a large scale, energy converter should be multi-cylindrical structure. This paper proposed a rectangular arrangement of mechanical coupling four-cylinder vibration system with a combination of tandem and side-by-side configuration. In order to describe the dynamic characteristics of the system, the concept of mechanical coupling was put forward for the first time. Regarded the whole four-cylinder system as a mass-spring-damper system and the relative position between each cylinder remained the same at any moment to get the best hydrodynamic characteristics. In addition, put forward the concept of combination spacing ratio LH/D2 because the spacing ratios of the four-cylinder vibration system were different in flow and transverse direction. Through numerical simulation of free vortex-induced vibration of the four-cylinder system at different spacing ratios in flow and transverse direction, studied the influence of combined spacing ratios on vibration response, lift and pressure characteristic, wake vortex structure and harvesting energy of the four-cylinder vibration system. It was found that there was a reasonable combination spacing ratio that the four-cylinder vibration system could extract maximal energy from the ocean current and had maximal energy density.(4) The hydrodynamic characteristic and energy efficiency of five-cylinder oscillating system with staggered arrangement were studied and analyzed the feasibility of harvesting energy on a large scale for this multi-cylinder system with staggered arrangement. Combinating tandem, side-by-side and staggered configuration on the basis of the mechanical coupling four-cylinder vibration system, proposed another mechanical coupling vibration system that was composed of five cylinders with staggered arrangement and obtained its results of vibration responses, hydrodynamic characteristics, wake vortex structure and harvesting energy at different spacing ratios in flow and transverse direction. The results showed that it was same as the four-cylinder oscillating system, there was an optimal combination spacing ratio at which five-cylinder oscillating system could extract the maximum energy from ocean current and had maximal energy density. Comparing the energy density of the two muti-cylindrical vibration systems at their optimal combination spacing ratios showed that staggered arrangement was better than rectangular arrangement. With the increase of combination spacing rato, two rows of vortex Street gradually appeared in the wake. There were "2 S" mode, "S+C" mode and "2 C" mode gradually appeared in the inside of the wake vortex when spacing ratios in transverse direction were increased.Analyzing the values of lift spectrum peak and harvesting energy of the five-cylinder and sigle cylinder vibration system showed that using the muti-cylinder vibration systems proposed in this paper as an energy converter and cooperating with proper driving transmission device was effective and feasible to harvest ocean current energy on a large scale.