We report water wave experiments performed in a long tank where we consider the evolution of nonlinear deep-water surface gravity waves with the envelope in the form of a large-scale rectangular barrier. Our experiments reveal that, for a range of initial parameters, the nonlinear wave packet is not disintegrated by the Benjamin-Feir instability but exhibits a specific, strongly nonlinear modulation, which propagates from the edges of the wavepacket towards the center with finite speed. Using numerical tools of nonlinear spectral analysis of experimental data we identify the observed envelope wave structures with focusing dispersive dam break flows, a peculiar type of dispersive shock waves recently described in the framework of the semi-classical limit of the 1D focusing nonlinear Schr"odinger equation (1D-NLSE). Our experimental results are shown to be in a good quantitative agreement with the predictions of the semi-classical 1D-NLSE theory. This is the first observation of the persisting dispersive shock wave dynamics in a modulationally unstable water wave system.

中文翻译：

---

从调制不稳定性到聚焦水波溃坝

我们报告了在一个长水箱中进行的水波实验，在该水槽中，我们考虑了非线性深水表面重力波的演化，其包络形式为大型矩形屏障。我们的实验表明，对于一定范围的初始参数，非线性波包不会被本杰明·费尔不稳定性分解，而是会表现出特定的强非线性调制，该调制会以有限的速度从波包的边缘向中心传播。使用对实验数据进行非线性频谱分析的数值工具，我们可以确定观察到的包络波结构与聚焦色散坝破裂流，这是最近在一维聚焦非线性薛定od半经典极限框架内描述的一种特殊类型的色散冲击波方程（1D-NLSE）。结果表明，我们的实验结果与半经典1D-NLSE理论的预测具有良好的定量一致性。这是对调制不稳定水波系统中持续存在的色散冲击波动力学的首次观察。