| The increasing energy demands promote the development of the offshore wind power industry.With the development of the offshore wind power towards deep sea,floating wind turbines have become the preferred structure to exploit the deep ocean wind energy.Semi-submersible floating wind turbine platforms are a kind of important ocean platform that supports wind turbine system.They have some advantages,for example,good stability,large range of the suitable depth of the sea,and so on.Therefore,they have been widely concerned by researchers all over the world and have been applied in some floating offshore wind farms.Flow past cylinders is an important issue in marine engineering.When fluids flow around the cylinders of the ocean platform,the periodical vortex shedding occuring around the cylinders leads to large reciprocating motion of the ocean platform,which may aggravate fatigue damage and thus cause reduction of fatigue life of the moorings.In addition,the reseach on flow past cylinders involves many theoretical issues,such as flow separation,vortex shedding,the mechanical behavior of cylinders,information and structures of flow field,flow pattern of wake and wake interference with each other.Specially,compared with the study on flow past single cylinder,in the study on flow past multiple cylinders,due to the wake interference with each other,the mechanical characteristics of the cylinders will become more complex.What's more,the information and structures of flow field also become more complex.Therefor,it is of great engineering significance and theoretical value to carry out the study on flow around a semi-submersible floating fan platform.In this thesis,in order to study the mechanical characteristics of flow past multi-column floating wind turbine platform,understand the mechanism of wake interference clearly,master the distribution characteristics of fluid velocity and its evolution laws and reflect the movement characteristics of the flow field structures better,three-dimensional numerical simulation was performed by using the Delayed Detached Eddy Simulation(DDES)to study the issue of flow past semi-submersible floating wind turbine platform,the research results provide a theoretical reference for understanding the phenomenon of flow past multi-column offshore platform structure.The main contents and results of this thesis are as follows:(1)The effects of headings and current velocities on the mechanical characteristics of semi-submersible floating wind turbine platform were investigated.The results indicate that:(a)the averaged drag force coefficient of downstream column has a large fluctuation with the change of current velocity;(b)under 30°and 60°heading,the root mean square of the lift force coefficient of downstream column shows the phenomenon of jumping fluctuation;(c)there are three patterns of vortex shedding behind columns of the platform.(2)The coherent structures of flow field were studyed,and the mechanism were revealed from the coherent structure of flow field,and the patterns of wake interference with each other were proposed.The results demonstrate that:(a)coherent structures are multiscale and have various types,the main coherent structures are streamwise vortex and hairpin vortex;(b)the interaction between coherent structures result in wake interference;(c)wake interference exists three patterns.(3)The velocity information of flow field was extracted,in addition,the distribution feature of mean value of velocity and its evolution were analyzed comprehensively,which included the distribution feature and its evolution of mean value of velocity in the longitudinal direction along downstream dirention in the center plan of wake,as well as the distribution feature and its evolution of mean value of velocity in the longitudinal,transvers,vertical dirention along transvers direction,respectively,in the horizontal plan and vertical plan.The results indicate that the mean value of velocity in the longitudinal direction in the wake backflow area is distributed in a “V” shape along the downstream direction,and there are three stages in the change along the downstream direction.The evolution of distribution pattern of the mean value of velocity along the transverse flow direction in the horizontal plane is as follows:(a)firstly,the distribution pattern of the velocity in the longitudinal along the transverse flow direction changes from “V” shape into “U” shape,secondly,it becomes the approximate quadratic parabola,which calls middle pattern,finally,it becomes a gentle curve gradually.(b)the distribution pattern of the velocity in the transverse direction along the transverse flow direction presents a continuous double “V” shape with opposite opening direction,lastly,it changes into a gentle curve gradually.(c)firstly,the distribution pattern of the velocity in the vertical direction along the transverse flow direction presents “trapezoid”,secondly,it changes into a continuous double inverted “V” shape,finally,it also turns into a gentle curve.The evolution of distribution pattern of the mean value of velocity along the transverse flow direction in the vertical plane is as follows:(a)the distribution pattern of mean value of velocity in the longitudinal direction along transverse flow direction presents the shape that a “U” shape is in the middle of double “V” shape,and as the height increases,the “V” shape disappears gradually,thus,it turn into “U” shape finally.(b)The distribution pattern of mean value of velocity in the transverse direction along transverse flow direction presents double inverted “V” shape connected by a connection section,with the height increasing,the length of the connection section grow longer and longe.(c)The mean value of velocity in the vertical direction changes greatly,therefore,the distribution pattern presents irregular shape,meanwhile,its evolution with the height increasing doesn't exist.(4)The one-dimensional spatial correlation of velocity in the longitudinal,transvers,vertical direction was analyzed respectively.On the base of the analysis,a calculation method to analyze the two-dimensional spatial correlatin was proposed.Next,the two-dimensional spatial correlation of velocity in the longitudinal,transvers,vertical dirention in a horizontal plan by using that method was studied respectively.The results demonstrate that: both the results from the analysis of the one-dimentional spatial correlation and the two-dimentional spatial correlation can reflect the characteristic of flow field: the vortex structures of flow field maintain a high level of organization and regularity when they develop forwards.Moreover,the results of analysis of two-dimentional spatial correlation can reflect that characteristic better and more directly.(5)The relationship between turbulent energy and wavenumber was studied by the analysis of wavenumber energy spectra.The results show that when the wavenumber is larger than the peak wavenumber,the relationship between turbulent energy and wavenumber confirms to the-5/3 law of wavenumber energy spectra in isotropic turbulence. |
PDF Full Text Request