An experimental decomposition of nonlinear forces on a surface-piercing column: Stokes-type expansions of the force harmonics
An experimental decomposition of nonlinear forces on a surface-piercing column: Stokes-type expansions of the force harmonics
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DOI：
10.1017/jfm.2018.339
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发表时间：
2018-06
影响因子：
3.7
通讯作者：
L. Chen;J. Zang;P. Taylor;L. Sun;G. Morgan;J. Grice;J. Orszaghova;M. Tello Ruiz
中科院分区：
文献类型：
--
作者：
L. Chen;J. Zang;P. Taylor;L. Sun;G. Morgan;J. Grice;J. Orszaghova;M. Tello Ruiz
海洋结构物上的波浪载荷是此类结构物设计中要考虑的主要外力。波浪载荷的非线性高阶分量的精确预报一直是一个尚未解决的具有挑战性的问题。本文对作用在底部直立圆柱上的水动力的非线性谐波分量进行了实验研究。作为欧洲合作研究项目的一部分，在丹麦水力研究所浅水波浪池的圆柱上进行了大量的实验，包括平床和斜床。已收集了各种波浪条件下的高质量聚焦波组数据集。高次谐波力分量的分离，应用“相位反转”的方法来测量力的时间历程的波峰聚焦波组和相同的波组倒置。这种分离方法被发现工作良好，即使是局部激烈的近破波形成的双向波对。它还发现，$n$次谐波力尺度与包络线的线性未扰动自由表面高程和线性力分量的时间变化和振幅的$n$次幂。这允许仅根据线性分量的知识来估计高阶谐波形状和时间历史。实验还表明，谐波结构的波加载在圆柱上几乎是不变的引入一个倾斜的床，仅取决于当地的波在圆柱的属性。此外，我们的新实验结果表明，对于某些波浪情况，线性载荷实际上小于总波浪载荷的40%，而高次谐波贡献了超过60%的载荷。这一引人注目的新结果的意义在于，它揭示了高阶非线性波浪载荷对海洋结构物的重要性，并意味着在设计中应考虑此类载荷。
Wave loading on marine structures is the major external force to be considered in the design of such structures. The accurate prediction of the nonlinear high-order components of the wave loading has been an unresolved challenging problem. In this paper, the nonlinear harmonic components of hydrodynamic forces on a bottom-mounted vertical cylinder are investigated experimentally. A large number of experiments were conducted in the Danish Hydraulic Institute shallow water wave basin on the cylinder, both on a flat bed and a sloping bed, as part of a European collaborative research project. High-quality data sets for focused wave groups have been collected for a wide range of wave conditions. The high-order harmonic force components are separated by applying the ‘phase-inversion’ method to the measured force time histories for a crest focused wave group and the same wave group inverted. This separation method is found to work well even for locally violent nearly-breaking waves formed from bidirectional wave pairs. It is also found that the $n$ th-harmonic force scales with the $n$ th power of the envelope of both the linear undisturbed free-surface elevation and the linear force component in both time variation and amplitude. This allows estimation of the higher-order harmonic shapes and time histories from knowledge of the linear component alone. The experiments also show that the harmonic structure of the wave loading on the cylinder is virtually unaltered by the introduction of a sloping bed, depending only on the local wave properties at the cylinder. Furthermore, our new experimental results reveal that for certain wave cases the linear loading is actually less than 40 % of the total wave loading and the high-order harmonics contribute more than 60 % of the loading. The significance of this striking new result is that it reveals the importance of high-order nonlinear wave loading on offshore structures and means that such loading should be considered in their design.