Wake Velocity Characteristics Of Horizontal-axis Tidal-current Turbine

The research of turbine wake is important for the acquisition of tidal current,the analysis of flow field and the research of load and hydrodynamic performance.The thesis focuses on the improvement of the efflux velocity equation of the wake of horizontal-axis tidal-current turbine using both theoretical method and CFD simulation.An improved method for the efflux velocity equation presented by Lam is proposed,considering the limitation of the axial momentum theory.The DES model is used for the three-dimensional simulation of wake.By comparing the CFD results with previous experimental results and theoretical predictions,the velocity characteristics of axial,tangential and radial velocity component are studied.The improved method for the efflux velocity equation is that the energy coefficient of turbine is defined to study the influence of tip speed ratio and solidity on the efflux velocity,and finally some semiempirical efflux velocity equations with different tip speed ratio and solidity can be proposed for the horizontal-axis tidal-current turbine.A classic experiment is simulated using the three-dimensional CFD simulation to evaluate the influence of grid and turbulence model on the simulation results.An optimal numerical simulation method for tidal current turbine is proposed.The numerical simulation can be conducted by using structured grid with a resolution of 3 mm,sliding mesh model and DES-SA turbulence model.The velocity distribution of axial,tangential and radial velocity component is analyzed by comparing the CFD simulation results with experimental and theoretical results.Along the axial direction,the axial velocity component of turbine wake is continuously recovered.Based on the position of minimum axial velocity,turbine wake can be divided into two zones: zone of flow establishment and zone of established flow.In the zone of flow establishment,two velocity valleys of the axial velocity distribution appear along the radial direction,the minimum axial velocity occurs at these positions.These two valleys are combined to one valley at the rotation axis in the zone of established flow,and the minimum axial velocity occurs at rotation axis.The tangential velocity component is the second largest velocity component of turbine wake and the radial velocity component accounts for only a small proportion.Both of tangential and radial velocity decreases along the axial direction.Two velocity peaks of both tangential and radial velocity appear along the radial direction.Finally,based on the proposed method to predict the efflux velocity of tidal turbine wake,the energy coefficients with different tip speed ratio and solidity are determined using the efflux velocity obtained by CFD simulation.The curve of the energy coefficient with the tip speed ratio is given and some semi-empirical efflux velocity equations are proposed.The application of these equations in the prediction of wake flow and the power calculation of tidal turbine is also introduced in this thesis.