A Numerical Study On The Flow Field Of The Wind Turbine In Atmospheric Boundary Layers And The Improved Spectral Method Simulation

In order to alleviate the global warming phenomenon,the development and utilization of wind energy in recent years has received the attention of all countries in the world.In early stage of the wind power utilization,wind power equipments were mainly installed on the flat area.With the deeper knowledges of surface wind resources and the development of construction technology,newly built wind power equipments are distributed more frequently in mountains and seas with higher average wind speeds.The atmospheric boundary layers above different land-or seascapes have different properties,which pose challenges for wind resource assessment and wind-farm operational simulation.Based on spectral-method large-eddy simulation and actuator line model,a platform of wide scene and high fidelity for the numerical simulation of flow field in wind energy applications is developed in this dissertation.Flow characteristics around wind turbines in various atmospheric boundary layers above plain,hill and ocean,are systematically studied,which may provide reference for the design of efficient evaluation methods for wind energy potential in complex environments.The contents are briefed as follows:(1)Numerical simulation and boundary-layer turbulent inflow generation for wind field above flat ground.Based on Fourier-basis spectral method,fractional step method,dynamic sub-grid-scale stress model and logarithmic-law wall model,a large-eddy simulation numerical framework of the plain boundary layer wind field is established.Turbulent channel flow simulation under various Reynolds numbers was carried out to verify the accuracy of the numerical framework.Based on spectral tensor Fourier transform and Reynolds stress rescaling method,the artificial synthesis method of boundary-layer turbulent inflow is developed.The self-retainability of the boundary-layer turbulent inflow is studied using the proposed numerical framework.(2)Large-eddy simulation study on the mechanisms of self-similarity development in a turbine wake above flat ground.A mixed derivative method of the smooth spectral method and the finite difference method(s PSMFDM)is proposed to alleviate the numerical oscillation caused by local jump in the turbine-flow.Based on the large-eddy simulation numerical framework of the wind field above flat ground,a numerical simulation framework of the turbine-flow above flat ground is established by adding the actuatorline method,the nacelle and tower model,the inlet velocity relaxation zone,s PSMFDM and the high-order dealiasing method.With the theoretical solution of one-dimensional inviscid Burgers equation,s PSMFDM and the high-order dealiasing method are verified.Numerical simulation is carried out according to the wind tunnel experimental data of a small-scale turbine to verify the accuracy of the algorithm.By using the axisymmetric wake theory and the wake-meanding tip-vortices collision hypothesis,the characteristics and mechanism of self-similarity in wake of turbine above flat ground is revealed.(3)Large-eddy simulation study of the flow around turbine over hill.For the uneven surface pressure caused by the undulation of hill,an integral wall model for rough curvy surface that considers the surface roughness and the uneven pressure is proposed.Large eddy simulation,integral wall model of rough surface,Lagrangian-averaged scaleinvariant dynamic sub-grid-scale stress model,actuator line method and nacelle/tower model are used to establish the spectral method numerical simulation framework of turbine-flow over hill.The accuracy of wall model and sub-grid-scale stress model in the numerical framework was verified by turbulent channel flow simulation.Based on the numerical framework,the development characteristics of free wind and turbine-flow over hill are studied,and the interaction between hill wake and turbine wake is analyzed.The route of hill-turbine wake deficit is found to be consistent with the streamline passing through the turbine hub.The development characteristics of self-similarity in velocity of hill-turbine wake is acquired along that streamline.(4)Large-eddy simulation study of flow around the offshore turbine array.Based on the numerical simulation framework of the turbine-flow over hill,a numerical simulation framework of the offshore turbine-flow is established by adding the high-order spectral method wave simulation and the unsteady actuator line method.The accuracy of wave simulation is verified by the calculation of high-order stokes wave.Based on the numerical simulation framework of offshore turbine-flow,the influence of swell waves on the free wind field above was studied.The power generation capacity and flow field characteristics of fixed-pile and floating offshore turbine arrays are studied.Under the same velocity at the top of atmospheric boundary layer,the fixed-pile offshore turbine array harvests the most wind energy.The self-similarity is found in the wake flow of turbine array.