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首页> 外文期刊>Journal of Physical Oceanography >Lagrangian Measurement of Steep Directionally Spread Ocean Waves: Second-Order Motion of a Wave-Following Measurement Buoy
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Lagrangian Measurement of Steep Directionally Spread Ocean Waves: Second-Order Motion of a Wave-Following Measurement Buoy

机译：陡向定向传播海浪的拉格朗日测量：随波测量浮标的二阶运动

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摘要

The notion that wave-following buoys provide less accurate measurements of extreme waves than their Eulerian counterparts is a perception commonly held by oceanographers and engineers (Forristall 2000, J. Phys. Oceanogr., 30, 1931-1943, ). By performing a direct comparison between the two types of measurement under laboratory conditions, we examine one of the hypotheses underlying this perception and establish whether wave measurement buoys in extreme ocean waves correctly follow steep crests and behave in a purely Lagrangian manner. We present a direct comparison between Eulerian gauge and Lagrangian buoy measurements of steep directionally spread and crossing wave groups on deep water. Our experimental measurements are compared with exact (Herbers and Janssen 2016, J. Phys. Oceanogr., 46, 1009-1021, ) and new approximate expressions for Lagrangian second-order theory derived herein. We derive simple closed-form expressions for the second-order contribution to crest height representative of extreme ocean waves-namely, for a single narrowly spread wave group, two narrowly spread crossing wave groups, and a single strongly spread wave group. In the limit of large spreading or head-on crossing, Eulerian and Lagrangian measurements become equivalent. For the range of conditions that we test, we find that our buoy behaves in a Lagrangian manner, and our experimental observations compare extremely well to predictions made using second-order theory. In general, Eulerian and Lagrangian measurements of crest height are not significantly different for all degrees of directional spreading and crossing. However, second-order bound-wave energy is redistributed from superharmonics in Eulerian measurements to subharmonics in Lagrangian measurement, which affects the "apparent" steepness inferred from time histories and poses a potential issue for wave buoys that measure acceleration.
机译：海洋学家和工程师普遍认为，跟波浮标所提供的对极端海浪的测量要比欧拉同行的精确度低（Forristall 2000，J. Phys.Oceanogr。，30，1931-1943，）。通过在实验室条件下对两种类型的测量值进行直接比较，我们检验了这种看法的一种假设，并确定了极端海浪中的测量浮标是否正确跟随陡峭的波峰并以纯拉格朗日方式运行。我们提出了在深水中陡峭的方向性传播和交叉波群的欧拉规范和拉格朗日浮标之间的直接比较。将我们的实验测量结果与精确值（Herbers and Janssen 2016，J.Phys.Oceanogr。，46，1009-1021，）和此处推导的拉格朗日二阶理论的新近似表达式进行比较。我们得出了代表极端海浪对波峰高度的二阶贡献的简单封闭式表达式，即，对于单个窄扩展波组，两个窄扩展交叉波组和单个强扩展波组。在大范围传播或正面穿越的极限下，欧拉和拉格朗日的测量变得等效。对于我们测试的条件范围，我们发现我们的浮标以拉格朗日方式运行，并且我们的实验观察结果与使用二阶理论所做的预测非常好。总的来说，欧拉和拉格朗日对波峰高度的测量对于定向扩展和交叉的所有程度都没有显着差异。然而，二阶束波能量从欧拉测量中的超谐波重新分配到拉格朗日测量中的次谐波，这会影响从时间历史推论得出的“表观”陡度，并给测量浮标的浮标带来潜在问题。

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