次表层剪切流改变的表面波模式的观测
Observation of surface wave patterns modified by sub-surface shear currents
摘要：
我们报告两个典型的表面波模式的实验观测-船波和环波-歪斜的次表面剪切，从而证实了最近的理论预测的效果。观测到的环波在一个静止的表面与次表面剪切流是惊人的不对称，强烈的各向异性波色散的影响。静止水面上的船波运动穿过次表层流时，船波关于船的运动线表现出惊人的不对称性，上游与下游运动的尾流角和横向波长存在很大的差异，所有这些都与理论预测吻合得很好。这两种现象在水深均匀的海流中都不会发生。一个环波的测量与预测的平均相移的定量比较是严重错误的预测无剪切理论，但在良好的协议与预测测得的剪切电流。对于所有剪切流，在上游与下游方向上观察到环形波包内的波频率存在明显差异，而正如预期的那样，它符合静水垂直于剪切流传播的理论。所测得的2维傅立叶谱的峰值船波同意以及与预测标准的固定船波，除了在某些情况下，由于有限的波数分辨率，瞬态效应和/或实验噪声的结果是不完善的。实验进行了两种不同的方式，与一个弯曲的网格，和下面的阻塞停滞表面流创建的控制剪切流。速度分布测量粒子图像测速仪，表面波与合成纹影法。我们的观察提供了强有力的经验支持最近的预测，船舶和结构物上的波浪力可以很大程度上影响河口和潮汐水域的剪切。
英文摘要：
We report experimental observations of two canonical surface wave patterns --- ship waves and ring waves --- skewed by sub-surface shear, thus confirming effects predicted by recent theory. Observed ring waves on a still surface with sub-surface shear current are strikingly asymmetric, an effect of strongly anisotropic wave dispersion. Ship waves for motion across a sub--surface current on a still surface exhibit striking asymmetry about the ship's line of motion, and large differences in wake angle and transverse wavelength for upstream vs downstream motion are demonstrated, all of which in good agreement with theoretical predictions. Neither of these phenomena can occur on a depth-uniform current. A quantitative comparison of measured vs predicted average phase shift for a ring wave is grossly mispredicted by no-shear theory, but in good agreement with predictions for the measured shear current. A clear difference in wave frequency within the ring wave packet is observed in the upstream vs downstream direction for all shear flows, while it conforms with theory for quiescent water for propagation normal to the shear current, as expected. Peak values of the measured 2-dimensional Fourier spectrum for ship waves are shown to agree well with the predicted criterion of stationary ship waves, with the exception of some cases where results are imperfect due to the limited wave-number resolution, transient effects and/or experimental noise. Experiments were performed on controlled shear currents created in two different ways, with a curved mesh, and beneath a blocked stagnant-surface flow. Velocity profiles were measured with particle image velocimetry, and surface waves with a synthetic schlieren method. Our observations lend strong empirical support to recent predictions that wave forces on vessels and structures can be greatly affected by shear in estuarine and tidal waters.
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