带有复杂齿条结构的弦杆绕流和涡激振动机理研究

As the core component of truss leg of jack-up platform, chord bears jacking loads, environmental loads and secondary Loads caused by deformation. Currently, the wave-current load calculation of chord is basically solved by semi-theoretical and semi-empirical method. Few studies have been carried out on the flow field mechanism of chord-like blunt bodies. This project intends to study the chord flow field and explore the mechanism of VIV, so as to meet the theoretical and applied needs of the pile leg design of jack-up platform. Based on the N-S governing equation, the flow field numerical simulation method suitable for analysis of chord-bar flow field is discussed. Combining with the results of physical model experiment, a stable, convergent and high-precision analysis system of chord-bar flow field is obtained. Starting with changing the structural attributes and geometrical forms of chord, the flow characteristics of chord under different mass ratio and different damping ratio are analyzed; the flow field parameters near chord are changed, and the flow characteristics of chord under different Reynolds number and reduced speed are analyzed. A method to determine the resistance coefficient Cd of chord with the change of environmental parameters and chord parameters is proposed, and the mechanism of chord vibration response is solved. The research achievements of this project will improve the accuracy of practical engineering design, and change the status that only empirical formulas can be used to calculate chord drag coefficient and the analysis of chord vortex-induced vibration for the lack of theoretical basis have to be ignored.

弦杆作为自升式平台桁架式桩腿的核心部件，承受着平台升降负荷、环境载荷和变形引起的二次载荷等。弦杆环境载荷的计算通常采用半理论半经验的方法，鲜有针对此类钝体的流场机理进行研究。本项目拟对弦杆的绕流流场进行研究，探索弦杆涡激振动机理，使之满足自升式平台桩腿设计中的理论和应用需要。基于N-S控制方程，探讨适合弦杆绕流流场数值模拟的研究方法，结合物理模型实验的结果，得到稳定、收敛和高精度的弦杆绕流流场分析体系。从改变弦杆结构属性和几何形式入手，研究在不同质量比、不同阻尼比下的弦杆绕流特征；改变弦杆附近的流场参数，研究在不同雷诺数、不同约化速度下的弦杆绕流特征，提出弦杆阻力系数Cd随外界环境参数和弦杆参数变化的确定方法，解决弦杆涡激振动响应的机理问题。本项目的研究成果将弥补实际工程设计中因缺乏理论依据而只能采用经验公式计算弦杆阻力系数并忽略弦杆涡激振动分析评估的现状，提高设计的精确度。

带有复杂齿条结构的弦杆是自升式平台桁架式桩腿的核心构件，弦杆周围海水质点的绕流流场水动力及可能产生的涡激振动将直接影响到弦杆乃至整条桩腿的强度和稳定性。本项目基于计算流体力学方法分析研究了带有复杂齿条结构的弦杆绕流和涡激振动现象，通过改变齿条相对尺寸、流场参数，得到了弦杆绕流流场规律和涡激振动特性。基于以上思路，以模型实验结果为参照，探讨了适合弦杆流场数值分析的方法，得到了弦杆绕流流场分析的研究体系；设置具有不同尺高比、齿宽比的系列弦杆，分析了在不同Re数、KC数或约化速度Ur下的弦杆绕流流场，给出弦杆阻力系数Cd和升力系数Cl的数据图谱；建立了低质量比、低阻尼比的弦杆涡激振动分析模型，研究结构的运动轨迹、尾流场的漩涡泄放模式以及振动响应特性等，为细长钝体结构的安全评价和工程应用提供了理论依据。进一步提出了涡激的抑制方法和利用方法，为海洋工程广泛应用于海洋能源的开采、利用奠定了理论基础。