悬跨海底管道绕流三维特性的大涡数值模拟研究

Large Eddy Numerical Simulation of Flow Characteristics around Submarine Pipeline with Free Span

自由悬跨是海底管道安全的风险之一,涡激振动(VIV)引起的疲劳是自由悬跨海底管道的主要失效模式之一。悬跨高度是影响海底管道涡激振动频率和载荷大小的一个重要因素。采用大涡数值模拟方法(LES)研究了不同悬跨间隙比(G/D)海底管道绕流的流场特性,分析了悬跨的间隙比对海管绕流场剪切层相互作用、升阻力系数和旋涡脱落等的影响规律。结果表明:与湍流模型的雷诺平均方法(RANS)相比,大涡数值模拟方法能够得到更准确和详细的近壁面圆柱绕流结果。在G/D小于0.8以后升阻力系数有明显变化,升力系数的均方根值随着G/D的减小而减小。当G/D大于0.8,升力系数的均值和均方根值基本不变,此时悬跨高度对绕流不再有明显影响。因此,可将G/D等于0.8作为临界悬跨间隙比,大于临界悬跨高度时的疲劳分析可等同于孤立管道。当G/D小于0.3以后,由于旋涡发放被抑制,升力系数的均方根值基本保持在很小的值,因此疲劳失效的风险大幅降低。而在中间过渡阶段的间隙比情况下,可以参考获得的升力系数值做更为精确的疲劳分析。

Free span is one of safety risks of submarine pipelines with free span. Fatigue caused by vortex-induced vibration （VIV）is one of the main failure modes of free-span submarine pipelines.The height of free span is an important factor that influences the VIV frequency and magnitude of load. The large eddy simulation turbulence model （LES） was used to study the characteristics of flow around submarine pipeline with different free span height ratio （G/D）,and analyze the influence rules of G/D on the interaction of shear layers of flow field around submarine pipeline,the lift and drag coefficient and the vortex shedding,etc. The results show that compared with turbulence model of Reynolds-averaged Navier–Stokes equations （RANS）,more accurate and detailed results of the flow around the cylinder near the wall can be obtained with the large eddy numerical simulation method. When the G/D value is less than 0.8,there is a significant change in the lift and drag coefficient,and the root mean square of the lift coefficient （Clrms） decreases with the decrease of the G/D value.When G/D value is greater than 0.8,the mean value and the root mean square value of the lift coefficient are basically unchanged,which means the height of the free span no longer has significant influence on the flow around the cylinder. Therefore,G/D=0.8 can be adopted as the critical height of the free span. The fatigue analysis when the height of span is greater than the critical height can be equivalent to the isolated pipeline.When G/D value is less than 0.3,the root mean square value of lift coefficient is basically kept at a very small value due to the suppression of vortex shedding,so the risk of fatigue failure is greatly reduced. Whereas in the case of intermediate gap ratio,the more accurate fatigue analysis can be made by reference to the lift coefficient value obtained.