【作者】 李平；

【导师】 康庄；

【作者基本信息】 哈尔滨工程大学 ， 船舶与海洋结构物设计制造， 2017， 硕士

【摘要】 随着海洋油气装备技术的迅速更新,愈来愈多的深水资源被勘测和开发出来。不管使用什么类型的浮式装置,立管系统在海洋工程装备中始终是关键的连接部件。在CFD领域,对立管等细长结构的绕流及涡激振动的数值模拟研究得到了学者的广泛关注。而随着近年来绕流的三维效应逐渐为人所知,所以愈来愈多的研究开始往三维数值模拟这方面发展。基于此,本文将对三维细长结构进行了绕流特性数值研究。论文的主要工作包括:第一、对OpenFOAM软件环境下的绕流并行技术进行了研究;第二、探讨了细长体在均匀来流以及剪切来流条件下的绕流特点;第三、对三维模型的VIV进行了数值分析。首先,论文分析了不同处理器数量、不同区域分解方法以及网格数量对并行性能的影响。发现了最适合本实验室计算机硬件条件下的并行处理方法,从而得到较高的并行计算效率。本文还利用OpenFOAM具有开源性的特点,基于MPI + OopenMP的混合并行模型,重新编写了并行计算的改进模型,并验证了其可行性。其次,对不同细长比的三维圆柱在均匀流中的绕流进行数值计算,发现:三维模型的阻力系数CD、升力系数CL和斯特劳哈尔数St都小于二维绕流结果;当细长比a*= 30时,其数值结果已经十分接近试验值;且均匀流的尾涡总体上显示为2S模式;同时观察到沿展向分布的条形状的涡管。通过对圆柱尾流场的时均速度分析还发现,尾流在流场较远处受到回流的影响较小。此外,分离点会随着雷诺数的增大而往后移,而且回流区长度则会随着细长比的增大而减小。然后,对三维模型(a*= 30)在剪切来流作用下进行了绕流分析,发现:不同流速的截面会受到其它相邻截面的影响,从而导致模型表面的受力比均匀流(相同Re)作用下的结果要大,这说明了剪切流在整个轴向是存在耦合作用的。剪切流尾涡在流速较大处出现2S模式,而在同一截面较远处还存在2P模式,这与均匀流中尾流整体表现为2S模式是完全不同的现象。另外,剪切流也出现了明显的垂向涡管;并且随着轴向速度由上往下逐渐变小化,涡管现象越显著,同时在轴向整体呈现三角形状。最后,对三维圆柱模型的VIV问题进行了数值计算,经过对比三维和二维的VIV结果,发现:在相同来流条件下,三维模型的最大幅值和锁定区间结果比二维的模拟结果要更加接近试验值。此外,在3D-VIV振幅较小的初始分支,其尾涡表现出2S模式;而处在振幅较大的上端分支的尾涡则出现2P模式。总之,本文通过CFD软件对三维细长体的绕流特性进行了多方面的研究和总结,为后续对细长体的大规模并行计算及涡激振动研究提供了参考。更多还原

【Abstract】 With the rapid updating of marine oil and gas equipment technology,more and more deepwater resources are being investigated and developed.No matter what types of floating device are used,the riser system is always the key connecting component in the marine equipment.In the field of CFD,the numerical simulation of flows and vortex-induced vibrations of slender structures such as risers has been widely concerned by researchers.In recent years,the three-dimensional effect of the flow around is gradually known,so more and more research begin to develop 3D numerical simulation.Based on this,the numerical investigation of flow characteristics around three-dimensional slender structures is carried out.The main work includes:1st,the parallel technology of flow is studied in OpenFOAM software.2nd,the flow characteristics of slender bodies under uniform flow and shear flow conditions are analyzed.3rd,the vortex induced vibration of 3D model is calculated numerially.Firstly,the effects of the number of processors,the decomposition methods of different regions and the number of grids on the parallel performance are analyzed.This paper finds out the most suitable parallel processing method for our laboratory computer hardware.This paper also establishes a hybrid parallel model of MPI +openMP,and verifies its feasibility.The results show that the parallel computing technology using mixed programming can accelerate the computation speed.Secondly,the numerical calculation of the flow around a 3D cylinder with different slender ratios is carried out.We find that the drag coefficient,lift coefficient and Strouhal number of the 3D model are less than the 2D results.When the slender ratio is 30,the numerical results are very close to the experimental values.And the tail vortex of uniform flow is generally shown as 2S mode.Besides.We also find that the wake is less affected by the circumfluence in the flow field.In addition,the separation point will move with the increase of Reynolds number,and the recirculation zone length will decrease with the increase of slender ratio.Then,we analyze the flow around a three-dimensional model under shear flow,and the force situations of shear flow are larger than uniform flow.The shear flow wake vortex appears 2S mode and 2P mode.Besides,the shear flow also has obvious vertical vortex tube,and with the axial velocity gradually reduced from top to bottom,the vortex tube phenomenon is more significant.Finally,the VIV problem of three-dimensional cylinder model is numerically calculated.We find that under the same flow condition,the maximum amplitude and lock-in interval of the 3D model are closer to the experimental values than the 2D simulation results.In addition,the tail vortex exhibits 2S mode in the initial branch with smaller amplitude,while the tail vortex in the upper branch with larger amplitude appears 2p mode.In a word,the flow characteristics of 3D slender body are studied and summarized in detail by CFD software,which provides a reference for the large-scale parallel computation and vortex-induced vibrations study of slender body.更多还原