A shallow water free-surface turbulent jet is investigated at Froude numbers variable between about 0.15 and 0.6 and a Reynolds number equal to 5000, with a jet height-to-width aspect ratio variable from 0.5 to 1.1. In these experimental conditions, the flow is unstable and develops local transverse instabilities, which are amplified to a global meandering motion, forcing the jet to oscillate orthogonally to its axis. Instantaneous and averaged velocity fields are obtained by means of high-density, correlation-based time-resolved Particle Tracking Velocimetry. From the average point of view, the present jet configuration resembles a confined jet condition similar to that of wall jets, but with additional relevant three-dimensional effects, retaining self-similar properties, with a net co-flow and some ambient fluid entrainment. For increasing Froude numbers, such configuration is moved downstream and the jet spreading is delayed. The high resolution in space and time of present measurements allows to locally detect and following perturbed patterns and to derive amplitude and frequency of oscillation of the global meandering motion, the former increasing with axial distance and decreasing with Froude number, the latter doing just the opposite. The velocity of propagation of perturbed patterns is also investigated in comparison to the jet mean velocity, the local celerity of propagation of fluctuating velocity fields being derived. Results indicate that axial fluctuations propagate downstream similarly to transverse fluctuations along the orthogonal direction, thus showing a close coupling among the two motions and the onset of a self-sustained mechanism driven by the mean flow, responsible for the generation of the macroscopic meandering motion. This effect is in agreement with numerical predictions and is emphasised as the Froude number increases.

中文翻译：

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自由表面浅水射流中扰动和曲折的传播

研究了浅水自由表面湍流射流，其弗劳德数在约 0.15 到 0.6 之间变化，雷诺数等于 5000，射流高宽纵横比从 0.5 到 1.1 变化。在这些实验条件下，流动是不稳定的，会产生局部横向不稳定性，这种不稳定性被放大为全局曲折运动，迫使射流与其轴垂直振荡。瞬时和平均速度场是通过高密度、基于相关性的时间分辨粒子跟踪测速法获得的。从平均的角度来看，当前的射流配置类似于类似于壁射流的受限射流条件，但具有额外的相关三维效果，保留自相似的特性，具有净协流和一些环境流体夹带。为了增加弗劳德数，这种配置向下游移动，射流扩散被延迟。当前测量的空间和时间高分辨率允许局部检测和跟踪扰动模式，并推导出全局曲折运动的振幅和振荡频率，前者随轴向距离增加而随 Froude 数减小，后者正好相反. 还研究了扰动模式的传播速度与射流平均速度的比较，得出了波动速度场的局部传播速度。结果表明轴向波动向下游传播，类似于沿正交方向的横向波动，因此显示出两种运动之间的紧密耦合以及由平均流动驱动的自持机制的开始，负责产生宏观蜿蜒运动。这种效应与数值预测一致，并且随着弗劳德数的增加而得到强调。