An Investigation Of Wake Characteristics Behind The 2D Airfoil And Vertical-axis Turbine

For the study of the vertical-axis tidal or wind turbine, the currently most widely used numerical simulation method is sufficient enough for the prediction of turbine's fluid dynamics,but incapable of predicting the mutual interference between each parameter. The establishment of the mathematical model of the vertical-axis turbine' wake can provide a more comprehensive understanding of details of the flow, which is significant for the design and optimization of the vertical-axis turbine. This work studied the wake of the vertical-axis turbine through the study of the vortex-shedding frequency, the geometrical structure and the distribution of circulation.The research is done through the respective study of the steady and unsteady airfoil, and the combination of each study' s conclusions.Through the study of the wake of steady airfoil,this work made a comprehensive study of both the overall features and the local characteristics of the wake region. The overall features of the wake include the behavior of the boundary layer on the suction surface, the vortex-shedding characteristics in the wake and the development of the instability of the shear layer.The local characteristics include the geometrical structure of the vortex street and the circulation-distributing law. Through the calculation of the flow around the 2D steady airfoil,this work studied the evolution process of the boundary layer behavior and the vortex-shedding mode with varying parameters. Based on the boundary layer theory and the. analysis of the calculation results, this work also achieved the variation of the vortex-shedding frequency with the boundary conditions. On the aspect of local characteristics, this work established a model describing the evolution process of the vortex circulation through the analysis of the acquired data based on the Oseen model, and obtained the method calculating the vortex street geometrical structure.With the study of unsteady airfoil wake, this work obtained the wake form of the turbine blade undertaking unsteady motion, including the generation of drag and thrust, vortex-shedding frequency, the wake evolution and the vortex-pairing phenomenon. Based on the vortex lock-in phenomenon,this work explained the multiple-vortex-per-half-cycle shedding of a flapping foil, and obtained an approximate vortex lock-in boundary on the h-k or a-k plane.In order to find out the cause of the various complicated wake forms in the harmonic region,this work developed a simple model that explains the harmonic shedding in the wake.According to the obtained results, this model is capable of depicting change of the vortex-shedding number with the flapping parameters. It's also found among the results that the 2P-2S wake transition should happen with the increase of flapping frequency and amplitude. This phenomenon is explained by analyzing the composition of the circulation in the wake, so that the key parameter that decides this transition is obtained.Based on the research of unsteady airfoil, this work obtained the method of calculating the vortex-shedding frequency of the vertical-axis turbine and the geometrical structure of the wake.Based on the study of steady airfoil, the distribution of circulation in the turbine wake is also obtained. On combination of the above methods of each aspect of the turbine wake, the simplified mathematical model of the vertical-axis turbine wake is eventually established. As is shown by the comparison with the numerical results, the established model is capable of depicting most of the wake's features.